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Article
Nitrate Leaching from Sand and Pumice Geomedia
Amended with Pyrogenic Carbon Materials
Jihoon Kang 1, *    ID
                         , Marissa Davila 2 , Sergio Mireles 1 and Jungseok Ho 3
 1   School of Earth, Environmental and Marine Sciences, University of Texas Rio Grande Valley, Edinburg,
     TX 78539, USA; sergio.mireles01@utrgv.edu
 2   Harlingen Waterworks System, Harlingen, TX 78550, USA; mldavilaz13@gmail.com
 3   Civil Engineering, University of Texas Rio Grande Valley, Edinburg, TX 78539, USA; jungseok.ho@utrgv.edu
 *   Correspondence: jihoon.kang@utrgv.edu; Tel.: +1-956-665-3526

 Received: 1 September 2017; Accepted: 30 September 2017; Published: 3 October 2017

 Abstract: There is increasing interest in using pyrogenic carbon as an adsorbent for aqueous
 contaminants in stormwater. The objective of this study was to investigate pyrogenic carbon
 materials as an amendment to geomedia to reduce nitrate leaching. Batch adsorption and column
 experiments were conducted to evaluate the performance of a commercial activated carbon and two
 biochars incorporated (5% by weight) into sand and pumice columns. The batch adsorption with
 50 mg L−1 of nitrate solution showed that only activated carbon resulted in a substantial adsorption
 for nitrate up to 41%. Tested biochars were not effective in removing aqueous nitrate and even
 released nitrate (
Nitrate Leaching from Sand and Pumice Geomedia Amended with Pyrogenic Carbon Materials - MDPI
Environments 2017, 4, 70                                                                                                2 of 7
Environments 2017, 4, 70                                                                                               2 of 7

nutrients
 (nutrient (nutrient
            retention)retention)     or even providing
                        or even providing     nutrients tonutrients
                                                            plants withto plants  with
                                                                            increase  in increase
                                                                                          microbialinactivity
                                                                                                       microbial are activity
                                                                                                                     claimed
are  claimed
 to be         to be the
        the benefits      benefits of commercially
                      of commercially                    available
                                          available biochars      soldbiochars
                                                                        in USA.sold in USA.
      There
       Therehave
               havebeen
                      beenincreasing
                             increasing adoptions
                                           adoptionsof raingardens,
                                                         of raingardens, bioswales,   and infiltration
                                                                              bioswales,                  basinsbasins
                                                                                             and infiltration      in urban in
landscapes,     which which
 urban landscapes,      are designed      to slow
                                 are designed       and and
                                                to slow    filterfilter
                                                                    stormwater
                                                                        stormwaterrunoff
                                                                                       runoffas as
                                                                                                a part
                                                                                                    a partofoflow
                                                                                                                low impact
                                                                                                                      impact
development
 development(LID)(LID)practices
                         practices[11].
                                     [11].For
                                           Forexample,
                                               example,the theLower
                                                                LowerRio RioGrande
                                                                              GrandeValley
                                                                                         Valley(LRGV)
                                                                                                (LRGV)in     inSouth
                                                                                                                SouthTexas
                                                                                                                        Texas
has
 has increasingly
      increasingly adopted
                      adopted bioswale
                                 bioswale systems
                                             systemsandand more
                                                             more are are expected
                                                                          expected to to be
                                                                                          be built
                                                                                              built in
                                                                                                     in parking
                                                                                                         parking lots
                                                                                                                    lots and
                                                                                                                          and
driveways
 drivewaysas  as aa green
                    green infrastructure
                           infrastructure projects
                                              projects [12].
                                                        [12]. The
                                                               The objective
                                                                     objectiveofof this
                                                                                    this study
                                                                                          study was
                                                                                                 was to to investigate
                                                                                                            investigate thethe
performance     of  pyrogenic     carbon  materials  (AC    and    biochars)   as an   additive
 performance of pyrogenic carbon materials (AC and biochars) as an additive to selected geomedia  to  selected    geomedia
(sand
 (sand and
         and pumice)
              pumice) forfor reducing
                              reducing nitrate
                                          nitrate leaching.
                                                  leaching. Specific
                                                                Specific objectives
                                                                           objectives were
                                                                                         were to
                                                                                               to (1)
                                                                                                   (1) determine
                                                                                                       determine nitrate
                                                                                                                       nitrate
adsorption
 adsorption capacity
               capacity ofof the
                              the carbon
                                   carbon materials,
                                            materials, and
                                                        and (2)
                                                              (2) evaluate
                                                                   evaluate nitrate
                                                                              nitrate retention
                                                                                        retention inin sand
                                                                                                       sand andand pumice
                                                                                                                     pumice
columns
 columns amended
            amended with
                       with thethe carbon
                                    carbon materials
                                            materials (5%
                                                       (5% byby weight).
                                                                  weight).

2. Materials
2. Materialsand
             andMethods
                Methods

2.1. Filter
2.1. FilterMedia
            Mediaand
                  andPyrogenic
                     PyrogenicCarbon
                               Carbon Materials
                                      Materials
      Commerciallyavailable
      Commercially       availableplay
                                     play sand
                                           sand (Quikrete
                                                     (Quikrete International
                                                                InternationalInc.,
                                                                                 Inc., Atlanta,
                                                                                       Atlanta, GA,
                                                                                                  GA, USA)
                                                                                                        USA) and
                                                                                                               and pumice
                                                                                                                    pumice
 (Nature’sFootprint,
(Nature’s    Footprint,Inc.,
                          Inc.,Bellingham,
                                 Bellingham,      WA,WA,  USA)
                                                        USA)     were
                                                              were      used
                                                                     used      in this
                                                                           in this studystudy    as geomedia
                                                                                           as geomedia           (Figure
                                                                                                           (Figure        1).
                                                                                                                    1). The
 The  play  sand   was  determined      to be   fine   sand  having  97.5%  sand,   1.8%  silt, and
play sand was determined to be fine sand having 97.5% sand, 1.8% silt, and 0.7% clay according to     0.7%  clay according
 to the
the      particle
     particle  sizesize analysis
                     analysis   byby   hydrometer
                                   hydrometer            method
                                                       method     [13].The
                                                                [13].   Thepumice
                                                                             pumicewas  wasaafelsic
                                                                                                felsicvolcanic
                                                                                                       volcanicrock
                                                                                                                  rock with
                                                                                                                       with
 highly vesicular
highly   vesicular rough
                     rough texture
                            texture andand itit isisoften
                                                    oftenfound
                                                           foundtotofloat
                                                                     floaton
                                                                           onthe
                                                                               thebeaches
                                                                                    beachesofofthetheTexas coast
                                                                                                       Texas  coastdue to to
                                                                                                                     due  its
 low  density   (0.2 to 0.6 g cm  −3 ) [14]. No textural analysis was performed with pumice as it contained
its low density (0.2 to 0.6 g cm ) [14]. No textural analysis was performed with pumice as it contained
                                    −3

 grain sizes
grain   sizes coarser
              coarser than
                        than 22 mm
                                mm in in diameter.
                                         diameter. Both  Both materials
                                                              materials were
                                                                          were air-dried
                                                                                air-dried before
                                                                                           before use.
                                                                                                    use.

                                        Figure
                                         Figure1.1.Geomedia
                                                   Geomediatested
                                                            testedin
                                                                   inthis
                                                                      thisstudy.
                                                                           study.

      Three    pyrogeniccarbon
       Three pyrogenic     carbonmaterials—a
                                     materials—a     commercial
                                                  commercial   AC ACand and    two biochar
                                                                         two biochar           products,
                                                                                       products,   Hoffman Hoffman
                                                                                                              biochar
biochar    (HB)    and   Wakefield   biochar   (WB)—were      used   as  amendments
 (HB) and Wakefield biochar (WB)—were used as amendments to the sand or pumice geomedia  to  the   sand  or  pumicein
geomedia      in this study  (Figure  2). The  AC   (CR610A,   Carbon    Resources   LLC,   Oceanside,
 this study (Figure 2). The AC (CR610A, Carbon Resources LLC, Oceanside, CA, USA) was a granular          CA,  USA)
was   a granular
 product    sold forproduct  sold for water
                      water purification   in purification
                                              aquaculturein   aquaculture
                                                            and              and it was
                                                                 it was produced     fromproduced     fromby
                                                                                           selected coal    selected
                                                                                                               a high
coal  by  a  high   temperature   steam   activation  process  according    to the manufacturer.
 temperature steam activation process according to the manufacturer. The HB (A.H. Hoffman             The  HB  (A.H.
                                                                                                                 Inc.,
Hoffman      Inc., Lancaster,  NY,  USA)   was  a  pelletized charcoal   product   sold as  a soil  conditioner
 Lancaster, NY, USA) was a pelletized charcoal product sold as a soil conditioner and it was claimed to          and
itimprove
   was claimed      to improve
             drainage            drainage
                        and adsorb   harmfuland  adsorb harmful
                                               impurities  accordingimpurities   according toThe
                                                                       to the manufacturer.     the WBmanufacturer.
                                                                                                          (Wakefield
The WB (Wakefield Agricultural Carbon LLC, Columbia, MO, USA) was a USDA-certified soil
 Agricultural Carbon LLC, Columbia, MO, USA) was a USDA-certified soil conditioner and it was
conditioner and it was mainly claimed to contribute to healthier soils and improve drought resistance
 mainly claimed to contribute to healthier soils and improve drought resistance according to the
according to the manufacturer. The WB was not a pelletized product and it was much finer than other
 manufacturer. The WB was not a pelletized product and it was much finer than other two carbon
two carbon products. All carbon materials were analyzed for pH (1:80 solid to solution ratio) using a
 products. All carbon materials were analyzed for pH (1:80 solid to solution ratio) using a pH meter
pH meter (Extech Instruments, Waltham, MA, USA) and elemental composition via Energy-
 (Extech Instruments, Waltham, MA, USA) and elemental composition via Energy-dispersive X-ray
dispersive X-ray spectroscopy equipped in ZEISS EVO LS10 Scanning Electron Microscope (Hitachi,
 spectroscopy equipped in ZEISS EVO LS10 Scanning Electron Microscope (Hitachi, Japan). The results
Japan). The results for elemental composition were expressed in weight % (Table 1).
 for elemental composition were expressed in weight % (Table 1).
Environments 2017, 4, 70                                                                                                           3 of 7

                Environments 2017, 4, 70                                                                                  3 of 7

                                           Figure 2. Pyrogenic carbon materials tested in this study.
                                    Figure 2. Pyrogenic carbon materials tested in this study.
                                Table 1. pH and elemental composition (weight %) of pyrogenic carbon materials

                 Table 1. pH andCarbon Material
                                elemental       pH C(weight
                                           composition (%) O (%)
                                                             %) ofFepyrogenic
                                                                     (%) Al (%)
                                                                              carbon materials.
                                  Activated carbon            8.03    73.1      11.6   13.7        1.6
                                  Hoffman biochar             7.88    86.7      13.3    0           0
                       Carbon Material        pH                   C (%)          O (%)           Fe (%)         Al (%)
                                  Wakefield biochar          10.23 89.8         10.2    0           0
                      Activated carbon            8.03              73.1            11.6            13.7          1.6
                      Hoffman
                2.2. Batch       biochar
                           Adsorption Experiment 7.88               86.7            13.3             0             0
                      Wakefield biochar          10.23              89.8            10.2             0             0
                    Batch sorption experiments were conducted in 50-mL centrifuge tubes at room temperature.
              Nitrate solution was prepared in deionized (DI) water containing 50 mg L−1 as potassium nitrate
              (KNO3). The
2.2. Batch Adsorption        carbon materials (0.45 g) were weighted to the tubes and equilibrated with 36 mL of the
                           Experiment
              nitrate solution. The concentration of nitrate at 50 mg NO3− L−1 were equivalent to 11.3 mg L−1 NO3-N
              in elemental
      Batch sorption         basis. Samples were
                           experiments       with sand  or pumice only
                                                     conducted              were included
                                                                       in 50-mL              as experimental
                                                                                      centrifuge     tubes control.
                                                                                                               at room Thetemperature.
              tubes were shaken for 1 h on an end-to-end shaker at 90 oscillations min−1, and supernatants − 1        were
Nitrate solution     was prepared in deionized (DI) water containing 50 mg L as potassium nitrate
              filtered through 0.2-µm membrane filters. It is important to note that the 1 h reaction time in this
(KNO3 ). Thestudycarbon     materials
                      may have            (0.45
                                 not reached     g) were
                                             a chemical      weighted
                                                         equilibrium         to the
                                                                        of nitrate     tubes onto
                                                                                   adsorption   andtheequilibrated      with 36 mL of
                                                                                                        carbon materials.
              Our previous
the nitrate solution.     Thezinc   (Zn) adsorption of
                                 concentration       experiment
                                                       nitrate with
                                                                 at 50biochars
                                                                           mg NO   up to− 48-h
                                                                                           L −1equilibration
                                                                                                 were         showed that
                                                                                                        equivalent      to 11.3 mg L−1
                                                                                      3
              54–98% of Zn adsorption occurred after 1 h relative to the amount of Zn adsorbed after 48 h [15]. The
NO3 -N in elemental        basis. Samples with sand or pumice only were included as experimental control.
              filtrates were analyzed colorimetrically using a HACH UV–vis spectrophotometer (DR3900,
The tubes were      shaken
              Loveland,    CO, for
                                USA)1according
                                       h on antoend-to-end
                                                    dimethylphenol shaker
                                                                       methodat  for90  oscillations
                                                                                     nitrate            min−
                                                                                             (HACH method
                                                                                                                 1 , and supernatants
                                                                                                               10206). The
were filteredamount
                through of nitrate adsorbed
                              0.2-µm        by the testedfilters.
                                         membrane          materialsIt(q)is
                                                                          was  calculated byto
                                                                            important         Equation (1): the 1 h reaction time
                                                                                                 note that
in this study may have not reached a chemical              q = (C0Vequilibrium
                                                                    – CV)/M          of nitrate adsorption onto      (1) the carbon
materials. Our
             where previous      zinc (Zn)ofadsorption
                     C0 is the concentration     nitrate in inputexperiment
                                                                  solution (mg L−1with     biochars
                                                                                   ), V is the volume ofup   to (L),
                                                                                                          liquid 48-hC isequilibration
showed thatthe   concentration
              54–98%      of Znofadsorption
                                   nitrate in solution after 1-h equilibration,
                                                   occurred                     and M isto
                                                                 after 1 h relative       drythe
                                                                                               weight of carbon
                                                                                                  amount      ofmaterial
                                                                                                                 Zn adsorbed after
             (kg). Nitrate adsorption (removal) capacity of the carbon materials was calculated in % relative to the
48 h [15]. The    filtrates   were    analyzed       colorimetrically
             initial amount of aqueous nitrate added (1.8 mg NO3 ).   −      using    a  HACH       UV–vis      spectrophotometer
(DR3900, Loveland, CO, USA) according to dimethylphenol method for nitrate (HACH method 10206).
The amount 2.3.   Column Experiment
             of nitrate     adsorbed by the tested materials (q) was calculated by Equation (1):
                    A total of 16 columns (metal cylinder in 7.62-cm diameter by 7.62-cm depth) were packed to a
             depth of 4.56 cm with sand or pumice. Sixteen columns corresponded to our experimental treatments
                                                    q = (C0 V − CV)/M                                                           (1)
             consisting of two geomedia and four different carbon amendments, including control in duplicates.
             During our initial trials, maximum amounts of sand and pumice needed for packing the columns
where C0 is the
             (i.e.,concentration
                    control columns) of nitrate
                                      were       in input
                                           determined       solution
                                                      to be 318         (mg
                                                                g and 165      L−1 ), V isThese
                                                                          g, respectively. the given
                                                                                                volume          were (L), C is the
                                                                                                           of liquid
                                                                                                     amounts
concentrationdetermined
                of nitrateto inhave  a bulk density
                                  solution          of 1.67
                                             after 1-h      g cm for sand
                                                                  −3
                                                        equilibration,     and columns
                                                                                  M is and
                                                                                        dry 0.86 g cm−3offor
                                                                                             weight          pumicematerial (kg).
                                                                                                          carbon
             columns. Sand or pumice with and without carbon amendment (5% by weight) was dry-packed into
Nitrate adsorption     (removal) capacity of the carbon materials was calculated in % relative to the initial
             the columns (Table 2). The bottom of each column was covered with cheese cloth to prevent the
amount of aqueous
             geomedianitrate
                         loss.     added (1.8 mg NO3 − ).

2.3. Column Experiment
       A total of 16 columns (metal cylinder in 7.62-cm diameter by 7.62-cm depth) were packed to a
depth of 4.56 cm with sand or pumice. Sixteen columns corresponded to our experimental treatments
consisting of two geomedia and four different carbon amendments, including control in duplicates.
During our initial trials, maximum amounts of sand and pumice needed for packing the columns
(i.e., control columns) were determined to be 318 g and 165 g, respectively. These given amounts
were determined to have a bulk density of 1.67 g cm−3 for sand columns and 0.86 g cm−3 for pumice
columns. Sand or pumice with and without carbon amendment (5% by weight) was dry-packed
into the columns (Table 2). The bottom of each column was covered with cheese cloth to prevent the
geomedia loss.
Environments 2017, 4, 70                                                                                                   4 of 7

      Table 2. The amount of geomedia used for column packing with and without carbon amendment
      (5% by weight).
      Environments 2017, 4, 70                                                                                    4 of 7

                                 Column                                 Sand or Pumice (g)        Amendment (g)
            Table 2. The amount of geomedia used for column packing with and without carbon amendment (5%
            by weight)     Sand only                      318                           0
                 Sand + amendment                302                       16
                     Pumice Column
                            only           Sand or
                                                 165Pumice (g) Amendment    0 (g)
                           Sand only
                Pumice + amendment                 318
                                                 157                     0 8
                       Sand + amendment            302                  16
                          Pumice only              165                  0
      Each column received 500
                      Pumice    mL of DI water for initial
                             + amendment           157     flushing on the
                                                                        8 first day (day 1). The water
was irrigated by 100 mL increments manually through a circular plastic pan that was placed on the
top of column.   The plastic
          Each column          pan500
                         received    hadmLmultiple  small
                                           of DI water   forholes,
                                                             initial allowing
                                                                     flushing on uniform
                                                                                   the first dripping
                                                                                             day (day 1).of The
                                                                                                            the water
                                                                                                                water and
     was irrigated
minimizing          by disturbance
              surface  100 mL increments
                                       on themanually
                                               geomedia.through
                                                            On the a circular
                                                                       secondplastic   pan that
                                                                                day (day          was placed
                                                                                             2), columns       on the
                                                                                                             were  leached
     top of column.  The  plastic  −
                                  pan1 had multiple  small  holes,  allowing   uniform   dripping
with nitrate solution (50 mg L ) five times by 100 mL increments, totaling 500 mL of aqueous nitrateof the  water and
     minimizing
throughput.    Thesurface
                    amount disturbance
                               of nitrateonapplied
                                            the geomedia.
                                                     was 25 Onmg theinsecond
                                                                         total.day
                                                                                On(day
                                                                                     the 2), columns
                                                                                          third         were leached
                                                                                                  day (day    3), columns
     with nitrate solution (50 mg L−1) five times by 100 mL increments, totaling 500 mL of aqueous nitrate
were leached again with DI water only (100 mL increments, totaling 500 mL of DI water) to assess
     throughput. The amount of nitrate applied was 25 mg in total. On the third day (day 3), columns
desorption of the nitrate. Collected column effluents were analyzed for nitrate in the aforementioned
     were leached again with DI water only (100 mL increments, totaling 500 mL of DI water) to assess
method.    The effluent
     desorption           samples
                 of the nitrate.       werecolumn
                                 Collected   also determined
                                                   effluents were  foranalyzed
                                                                        turbidityforusing
                                                                                     nitrateNEP
                                                                                              in the260  turbidity probe
                                                                                                     aforementioned
(McVan   Instruments,
     method.            Melbourne,
               The effluent  samples Australia)    in nephelometric
                                        were also determined               turbidity
                                                                  for turbidity   usingunits
                                                                                         NEP(NTU)       and forprobe
                                                                                                260 turbidity    pH using
a portable
     (McVan pH  meter (Extech
              Instruments,        Instruments,
                            Melbourne,           Waltham,
                                          Australia)           MA, USA).
                                                     in nephelometric     turbidity units (NTU) and for pH using
      a portable pH meter (Extech Instruments, Waltham, MA, USA).
3. Results and Discussion
      3. Results and Discussion
3.1. Batch Adsorption of Nitrate
      3.1. Batch Adsorption of Nitrate
     Only AC had ability to adsorb nitrate up to 41% (Figure 3) while biochars showed none
          Only AC(negative
to even release     had ability%)to adsorb  nitrate
                                    of nitrate      up to from
                                                 (0.93%   41% (Figure
                                                                  HB and3) while
                                                                             0.13% biochars
                                                                                      from showed
                                                                                            WB). DInone     to even
                                                                                                         water   extracts
     release (negative  %) of nitrate (0.93%  from   HB  and  0.13% from   WB).  DI  water extracts  (1:80
(1:80 solid-to-solution ratio) with the carbon materials (i.e., DI water only with no aqueous nitrate)     solid-to-
     solution ratio) with the carbon materials (i.e., DI water only with no aqueous nitrate)       confirmed that
confirmed that an appreciable concentration was found for HB (1.46                 mg NO3 − L−1 ) and WB (1.55 mg
     an  appreciable  concentration   was   found   for HB   (1.46 mg  NO   3− L−1) and WB (1.55 mg NO3− L−1).
NO3 − L−1 ). Commercial pumice for gardening use is often claimed to hold nutrients but it was not
     Commercial pumice for gardening use is often claimed to hold nutrients but it was not the case in the
the case in adsorption
     batch  the batch adsorption      with 1time,
                        with 1 h reaction     h reaction   time,for
                                                   accounting     accounting
                                                                    only 1.3% for    only 1.3%
                                                                                of nitrate       of nitrate
                                                                                           adsorption.   Sandadsorption.
                                                                                                                also
Sandshowed
      also showed
              limitedlimited  adsorption
                       adsorption            ability
                                   ability (1.2%),    (1.2%), indicating
                                                   indicating              a lacksurfaces
                                                               a lack of reactive   of reactive  surfaces for nitrate.
                                                                                            for nitrate.

                                                           50

                                                           40
                                  Nitrate adsorption (%)

                                                           30

                                                           20

                                                           10

                                                            0

                                                           -10
                                                                 Sand   Pumice    AC         HB     WB

           Figure
      Figure      3. Adsorption
             3. Adsorption       from
                              from      aqueousnitrate
                                    aqueous     nitrate affected
                                                         affected by
                                                                  by geomedia
                                                                     geomediaand
                                                                              andcarbon materials.
                                                                                   carbon          ErrorError
                                                                                          materials.     bars bars
           represent standard error of the mean.
      represent standard error of the mean.

            It is often claimed that biochar can be used as a soil amendment to retain nutrients in soils [16,17].
     It is often the
     However,    claimed   that
                     biochar    biochartested
                              materials   can be  usedstudy
                                               in this   as a showed
                                                              soil amendment      to retain
                                                                      no adsorption    (evennutrients
                                                                                             release) ofinnitrate
                                                                                                            soils in
                                                                                                                  [16,17].
However,    the biochar
     a batch-scale        materialsOur
                     experiment.    tested   in this
                                        result  is instudy   showed
                                                       agreement      noYao
                                                                   with    adsorption   (evenwho
                                                                               et al. (2012)   release)
                                                                                                   foundoflimited
                                                                                                             nitrate in a
     adsorption
batch-scale       of nitrateOur
              experiment.    (
Environments 2017, 4, 70                                                                                                                                                           5 of 7

removing cationic species from solution as most biochars were found to have a net negative surface
charge [15]. While both AC and biochars have carbonaceous materials as feedstock materials and
convert them to stable carbon through pyrolysis, AC typically receives an additional activation process
where it undergoes oxidation to increase adsorptive capacities [19].
     It is important to note that feedstock materials for the biochars were plant-based while
         Environments 2017, 4, 70                                                                                5 of 7
                                                                                                                        the AC
was produced from selected coal according to manufacturer. Detailed information on the feedstock
types andnegative    surface chargeprocesses
              manufacturing           [15]. While of
                                                   both AC and
                                                      these      biochars have
                                                             commercial         carbonaceous
                                                                            products          materials as feedstock
                                                                                         are proprietary     and unknown.
         materials    and convert   them   to stable carbon through  pyrolysis,  AC typically receives
However, it was notable that the AC contained substantial amount of Fe (13.7%) and Al (1.6%) in its    an  additional
         activation process where it undergoes oxidation to increase adsorptive capacities [19].
elemental composition (Table 1), indicating mineral inclusion of Fe and Al and possibly leading to
               It is important to note that feedstock materials for the biochars were plant-based while the AC
better adsorption
         was produced    offrom
                             nitrate.   Thecoal
                                  selected    Feaccording
                                                  mineralstosuch    as hematite,
                                                               manufacturer.       goethite,
                                                                             Detailed          and on
                                                                                       information  iron-oxide     have been
                                                                                                       the feedstock
found to   increase
         types           oxyanion adsorption
                 and manufacturing       processes ofsuch
                                                       theseascommercial
                                                               arsenite and   arsenate
                                                                          products        in the Fe-impregnated
                                                                                    are proprietary  and unknown. carbon
materials   thoughit surface
         However,       was notablecomplexation      betweensubstantial
                                       that the AC contained    the Fe-O surface
                                                                           amount ofand   oxyanion
                                                                                      Fe (13.7%) and[20,21].
                                                                                                      Al (1.6%) in its
         elemental composition (Table 1), indicating mineral inclusion of Fe and Al and possibly leading to
3.2. Nitrate
        betterLeaching in Column
               adsorption of nitrate.Experiment
                                      The Fe minerals such as hematite, goethite, and iron-oxide have been
         found to increase oxyanion adsorption such as arsenite and arsenate in the Fe-impregnated carbon
     Overall   turbidity
        materials   though in     effluent
                               surface        samples between
                                        complexation     were 3- the
                                                                   to 4-times      greater
                                                                        Fe-O surface    and in    pumice
                                                                                             oxyanion       columns (17 to 27 NTU)
                                                                                                          [20,21].
compared to sand columns (5 to 7 NTU) (Table 3). Coarse grains in pumice were likely to contribute to
        3.2. Nitrate
the higher   turbidityLeaching    in Column
                            by eluting     fineExperiment
                                                  suspended solids from pumice and the carbon materials. The pH
values in column
              Overall effluents
                         turbidity inranged
                                         effluentfrom    7.6 were
                                                   samples   to 8.73- and   the columns
                                                                       to 4-times    greater inamended         with WB
                                                                                                   pumice columns          showed
                                                                                                                       (17 to 27    the
highestNTU)
         pH (7.9compared
                     to 8.7),toindicating
                                   sand columns the (5 to 7 NTU)
                                                    presence        (Table 3).ash
                                                                 of mineral       Coarse   grains
                                                                                      in the   WBin[22].pumice were likely to
        contribute
     Under            to theinput
              the pulse         higherof turbidity  by eluting
                                          nitrate solution    (50fine
                                                                   mgsuspended
                                                                        NO3 − L−solids        from in
                                                                                      1 ), nitrate    pumice
                                                                                                         columnandeffluents
                                                                                                                     the carbon
                                                                                                                              increased
        materials. The pH values in column effluents ranged from 7.6 to 8.7 and the columns amended             −  1        with
with nitrate solution throughput (leaching event 1–5 in Figure 4) up to 11 mg L in both sand and
        WB showed the highest pH (7.9 to 8.7), indicating the presence of mineral ash in the WB [22].
pumice columnsUnder the(Figure
                            pulse 4).   The
                                    input      lowestsolution
                                           of nitrate   concentration
                                                                (50 mg NOwas       found with sand column amended with
                                                                             3− L−1), nitrate in column effluents increased
AC (7 mg       − 1                                                                        L−to1 ), suggesting
        withLnitrate
                   ) and     pumice
                        solution         column(leaching
                                    throughput      amended  eventwith   AC
                                                                    1–5 in      (8 mg
                                                                            Figure    4) up       11 mg L−1 in bothACsand
                                                                                                                        outperformed
                                                                                                                            and
biochars  in thecolumns
        pumice      flow-through
                              (Figure 4). experiment      as well. Under
                                            The lowest concentration      wasthe     desorption
                                                                                 found   with sand stage
                                                                                                       column(leaching
                                                                                                                amended event
                                                                                                                            with 6–10),
        AC (7
the nitrate      mg L−1) and pumice
              concentrations                 columndecreased
                                      gradually        amended withwithAC  AC (8 amendment
                                                                                  mg L−1), suggestingin bothAC geomedia
                                                                                                                 outperformed being the
        biochars in the   − flow-through experiment as well. Under the desorption stage (leaching event 6–10),
                            1
lowest (1 to 3 mg L ) suggesting that AC amendment was more effective than biochar amendment in
        the nitrate concentrations gradually decreased with AC amendment in both geomedia being the
retaining  nitrate during desorption stage.
        lowest (1 to 3 mg L−1) suggesting that AC amendment was more effective than biochar amendment
         in retaining nitrate during desorption stage.
      Table 3. Average pH and turbidity (mean ± standard error) in column effluents over the entire
      leachingTable
               events.
                    3. Average pH and turbidity (mean ± standard error) in column effluents over the entire
                            leaching events
                                                    Column a                                 pH                                      Turbidity (NTU)
                                                           Column a         pH         Turbidity (NTU)
                                                                           7.84 ± 0.18
                                                Sand only
                                                          Sand only     7.84  ± 0.18      7.29 ± 0.85 7.29 ± 0.85
                                                Sand + AC                  7.92 ± 0.06                 5.38 ± 0.59
                                                Sand + HB Sand + AC     7.92  ± 0.06
                                                                           7.93 ± 0.08    5.38 ± 0.59 7.43 ± 0.56
                                               Sand + WB Sand + HB      7.93  ± 0.08
                                                                           8.73 ± 0.16    7.43 ± 0.56 7.35 ± 1.72
                                               Pumice onlySand + WB        7.60
                                                                        8.73    ± 0.08
                                                                              ± 0.16      7.35 ± 1.72 23.86 ± 5.57
                                              Pumice + ACPumice only 7.60  7.50 ± 0.06
                                                                              ± 0.08     23.86 ± 5.57 24.61 ± 4.22
                                              Pumice + HB                  7.58 ± 0.05                26.80 ± 3.80
                                                        Pumice + AC 7.50
                                              Pumice + WB
                                                                              ± 0.06
                                                                           7.91 ± 0.17
                                                                                         24.61 ± 4.22 17.41 ± 2.46
                                                        Pumice + HB 7.58 ± 0.05          26.80 ± 3.80
                                            a AC = activated carbon; HB = Hoffman biochar; WB = Wakefield biochar.
                                                        Pumice + WB 7.91 ± 0.17          17.41 ± 2.46
                                                a   AC = activated carbon; HB = Hoffman biochar; WB = Wakefield biochar.
                             12                                                                                   12
                                                                            Sand only                                                                           Pumice only
                             10
                                      (a)                                   Sand + AC                             10
                                                                                                                           (b)                                  Pumice + AC
                                                                DI          Sand + HB                                                                           Pumice + HB
            NO3- (mg L-1)

                                                                water       Sand + WB                                                                           Pumice + WB
                                                                                                  NO3- (mg L-1)

                              8                                                                                    8
                                                                                                                                                 DI water
                              6                                                                                    6
                                                                                                                                     50 mg L-1
                              4                                                                                    4

                              2                     50 mg L-1                                                      2

                              0                                                                                    0
                                  0         2          4        6       8       10      12                             0         2     4         6          8        10       12

                                                       Leaching event                                                                   Leaching event

                    Figure Figure 4. Concentrations of nitrate (NO3−))in
                                                                       incolumn effluents: (a) sand and (b) pumice.
                                                                                        −
                           4. Concentrations    of nitrate   (NO 3        column   effluents:   (a) sand  and (b) pumice.
Environments 2017, 4, 70                                                                                                                                                   6 of 7

     Environments 2017, 4, 70                                                                                                                                    6 of 7
      Cumulative amount of nitrate leached (Figure 5) showed a clear separation of AC-amended sand
           Cumulative amount of nitrate leached (Figure 5) showed a clear separation of AC-amended sand
or pumice geomedia being the lowest (3.6 to 4 mg as NO3− − ) after the entire leaching event. In both
      or pumice geomedia being the lowest (3.6 to 4 mg as NO3 ) after the entire leaching event. In both
sand and pumice columns, HB resulted in higher nitrate leaching (6.3 mg) than control columns
      sand and pumice columns, HB resulted in higher nitrate leaching (6.3 mg) than control columns (5.2–
(5.2–5.3  mg)while
      5.3 mg)  while    WB-amended
                     WB-amended            columns
                                      columns   showedshowed     similarofamount
                                                        similar amount                of nitrate
                                                                             nitrate leached withleached    with control
                                                                                                  control columns.
columns.
      The higher amount of nitrate leached from the columns amended with HB was in agreementagreement
            The higher    amount    of  nitrate  leached from   the  columns    amended     with HB   was   in  with
with the
      thegreater
           greaterrelease
                   release ofof nitrate
                              nitrate    from
                                      from      batch
                                            batch     adsorption
                                                   adsorption       (Figure
                                                              (Figure         3). There
                                                                        3). There  was nowas  no substantial
                                                                                           substantial          difference
                                                                                                       difference  in
in thethe
        amount
          amountof ofnitrate
                      nitrate leached    betweensand-only
                              leached between       sand-only
                                                            (5.3(5.3
                                                                  mg)mg)
                                                                       andand    pumice-only
                                                                            pumice-only         columns
                                                                                          columns           (5.2 mg).
                                                                                                    (5.2 mg).

                              7                                                                                7
                                      Sand only
                              6       Sand + AC
                                                                                (a)                            6
                                                                                                                       Pumice only                              (b)
                                                                                                                       Pumice + AC
        NO3- leached (mg)

                                                                                           NO3- leached (mg)
                                      Sand + HB                                                                        Pumice + HB
                              5       Sand + WB                                                                5
                                                                                                                       Pumice + WB
                              4                                                                                4

                              3                                                                                3

                              2                                                                                2
                                                            DI water                                                                        DI water
                              1                                                                                1
                                              50 mg   L-1                                                                       50 mg L-1
                              0                                                                                0
                                  0      2        4         6          8   10         12                           0      2          4      6          8   10         12

                                                  Leaching event                                                                     Leaching event

          Figure
      Figure     5. Cumulative
             5. Cumulative     amount
                            amount ofof
                                      nitrate   (NO33−))leached
                                        nitrate(NO       leachedinin
                                                                   column effluents:
                                                                     column
                                                                                 −   (a) sand
                                                                             effluents:        and (b)
                                                                                          (a) sand andpumice.
                                                                                                       (b) pumice.

     4. Conclusions
4. Conclusions
            Results in this study suggested that sand and pumice geomedia amended with biochars (5% by
      Resultswere
      weight)    in this
                       notstudy   suggested
                            as effective       thatamended
                                         as those    sand and   pumice
                                                              with  AC. Thegeomedia
                                                                              presenceamended
                                                                                         of Fe and Al  with  biochars
                                                                                                         in the  AC was (5% by
weight)    were
      likely       not as effective
              to promote               as those possibly
                             nitrate adsorption  amended      with AC.
                                                            through        Thecomplexation
                                                                      surface   presence of to   Fesome
                                                                                                     and Al   in the
                                                                                                           extent.    AC was
                                                                                                                   There
likelywas
        to promote     nitrate
            no substantial       adsorption
                              difference      possibly
                                          between   sandthrough
                                                          and pumicesurface  complexation
                                                                         columns                to some
                                                                                    in all carbon          extent.types.
                                                                                                     amendment      There was
      Our studydifference
no substantial      indicated that   net negatively
                                 between   sand andcharged
                                                      pumice surfaces
                                                                columnsofintheall biochars   may be attributed
                                                                                  carbon amendment          types.toOur
                                                                                                                      thestudy
      poor   adsorption    of nitrate anion  due  to electrostatic repulsion.  To   improve    its efficacy
indicated that net negatively charged surfaces of the biochars may be attributed to the poor adsorption     for aqueous
      nitrateanion
of nitrate     removal,
                      dueantoadditional  process
                                electrostatic     during biochar
                                               repulsion.          production
                                                             To improve         is desirable
                                                                            its efficacy    fortoaqueous
                                                                                                  create acid functional
                                                                                                             nitrate removal,
      groups (which have a positive charge) that can protonate surface –OH group in biochars [23,24]. It is
an additional process during biochar production is desirable to create acid functional groups (which
      important to note that while biochar may not be effective in reducing nitrate leaching, it can provide
have a positive charge) that can protonate surface –OH group in biochars [23,24]. It is important to
      other benefits such as increasing the N use efficiency by plants and decrease nitrous oxide (N2O)
note emission
      that while     biochar may not be effective in reducing nitrate leaching, it can provide other benefits
                  from N-rich soils. Our study included nitrate only under controlled leaching conditions and
suchfuture
       as increasing     the N
               study should       use efficiency
                               consider  evaluatingbymultiple
                                                        plants anions
                                                                and decrease      nitrous
                                                                        and/or cations    foroxide    (N2applications
                                                                                              the field   O) emission  of from
N-richgeomedia amended with pyrogenic carbon materials. In addition, future study investigating thestudy
         soils.  Our   study    included   nitrate  only   under   controlled   leaching     conditions    and   future
should    consider
      synergistic      evaluating
                    effect            multiple
                           of the carbon         anionsinand/or
                                          amendment        geomedia  cations  for with
                                                                       combined    the field
                                                                                        redoxapplications        of geomedia
                                                                                                 condition is needed   to
      advance
amended          ourpyrogenic
              with    understanding    of biotic
                                   carbon        and abiotic
                                            materials.        processes of
                                                          In addition,      nitrate
                                                                          future     retention
                                                                                   study         in stormwater
                                                                                            investigating     thecontrol
                                                                                                                   synergistic
effectmeasures.
        of the carbon amendment in geomedia combined with redox condition is needed to advance our
understanding of biotic and abiotic processes of nitrate retention in stormwater control measures.
    Acknowledgments: We gratefully acknowledge UTRGV graduate assistantship from the School of Earth,
    Environmental and Marine Sciences, UTRGV work study program, and US EPA Border 2020 Program for the
Acknowledgments: We gratefully acknowledge UTRGV graduate assistantship from the School of Earth,
    partial funding
Environmental   andofMarine
                     this project.
                              Sciences, UTRGV work study program, and US EPA Border 2020 Program for
the partial
     Author funding  of this project.
              Contributions:  Kang supervised the overall project and wrote the manuscript. Davila and Mireles
     collected
Author         column andKang
         Contributions:     batch supervised
                                  adsorption data,
                                               the respectively.
                                                    overall projectHo contributed
                                                                        and wrotetothe manuscript preparation
                                                                                          manuscript.    Davilaand  data
                                                                                                                  and Mireles
     analyses.
collected  column and batch adsorption data, respectively. Ho contributed to manuscript preparation and
data analyses.
     Conflicts of Interest: The authors declare no conflict of interest. The use of trade names in this publication does
     not imply
Conflicts      endorsement
          of Interest:      of the products
                       The authors   declarenamed or criticism
                                             no conflict       of similar
                                                          of interest. Theones
                                                                            usenot
                                                                                of mentioned by these
                                                                                   trade names        organizations.
                                                                                                 in this publication does
not imply endorsement of the products named or criticism of similar ones not mentioned by these organizations.
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