Effect of Co-existing Heavy Metals and Natural Organic Matter on Sorption/Desorption of Polycyclic Aromatic Hydrocarbons in Soil: A Review
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Pollution, 6(1): 1-24, Winter 2020
DOI: 10.22059/poll.2019.284335.638
Print ISSN: 2383-451X Online ISSN: 2383-4501
Web Page: https://jpoll.ut.ac.ir, Email: jpoll@ut.ac.ir
Effect of Co-existing Heavy Metals and Natural Organic Matter
on Sorption/Desorption of Polycyclic Aromatic Hydrocarbons in
Soil: A Review
Saeedi M.1*, Loretta Y. Li1 and John R. Grace2
1. Department of Civil Engineering, University of British Columbia, 6250
Applied Science Lane, Vancouver, British Columbia, Canada V6T 1Z4
2. Department of Chemical and Biological Engineering, University of British
Columbia, 2360 East Mall, Vancouver British Columbia, Canada V6T 1Z3
Received: 14.02.2019 Accepted: 12.09.2019
ABSTRACT: Polycyclic aromatic hydrocarbons (PAHs), abundant in mixed
contaminant sites, often coexist with heavy metals. The fate and remediation of PAHs
depend heavily on the sorption and desorption behavior of these contaminants. The
sorption behavior can in turn be highly affected by certain soil components and
properties, such as soil organic matter (SOM) and the presence of heavy metals. Through
review of the literature focused on research from 2006 to 2018, this paper discusses
interactions, challenges, influencing factors and potential synergies in sorption/desorption
of mixed PAHs and heavy metal contamination of soil. The presence of either natural
organic matter or heavy metals can enhance the sorption capability of fine soil, retarding
the PAHs in the solid matrix. The co-existence of SOM and heavy metals has been
reported to have synergistic effect on PAHs sorption. Enhanced and surfactant desorption
of PAHs are also affected by the presence of both SOM and metals. Remediation
techniques for PAHs removal from soil, such as soil washing, soil flushing and
electrokinetics, can be affected by the presence of SOM and heavy metals. More detailed
studies on the simultaneous effects of soil components and properties on the
sorption/desorption of PAHs are needed to enhance the effectiveness of PAHs
remediation technologies.
Keywords: PAHs sorption/desorption; soil organic matter; heavy metal effect.
INTRODUCTION carcinogenicity, birth defects, and skin and
Polycyclic aromatic hydrocarbons (PAHs) liver damage. There is human health risk in
constitute a group of chemical compounds both short and long-term exposure to PAHs
containing benzene rings in their molecular (ATSDR, 2009; ATSDR, 1995).
structure. PAHs are naturally present in oil The U.S. Environmental Protection
and bitumen and can form during incomplete Agency has selected sixteen PAHs as
combustion (Omores et al., 2017). Certain priority pollutants (USEPA, 2008; USEPA,
PAHs, such as benzo[a]pyrene, can interact 1993). PAHs molecular structures, can affect
with DNA and are genotoxic (Ewa and human health (Ewa and Danuta, 2017). For
Danuta, 2017; Jiao et al., 2017). Other example, structural features formed between
adverse health effects of PAHs include aromatic rings named as “fjord” or “bay”
regions (Fig. 1) are important in determining
* Corresponding Author Email: moh1349@mail.ubc.ca
1Saeedi M., et al.
the health effects of PAHs. Molecules HMs is naturally present in soil as
contain “fjord” regions (such as background content, they are present at
dibenzo[a,l]pyrene) prefer to bind to adenine much higher concentrations in heavy-
nucleotides, whereas PAHs having a “bay” metal-contaminated soils.
region (such as benzo[a]pyrene) bind to The major sources of PAHs in the
guanine nucleotides. PAHs contain a “fjord” environment are oil and gas and related
region are less capable of being metabolized products production, storage and
to form DNA-damaging adducts (Lakshman transportation, and spills (Choi et al., 2009:
et al., 2000; Muñoz and Albores, 2011). CCME, 2010a). There are some natural
PAHs containing both four or more rings and sources, such as volcanoes and forest fires as
a “bay” region in their structure can induce well. Many of these sources release both
gene mutations that initiate cancer whereas PAHs and toxic heavy metals into the
others with 2 to 4 aromatic rings are known environment. Therefore, due to their
as co-carcinogens, affecting cancer common sources, PAHs coexist with heavy
promotion or progression (Bostrom et al., metals in many contaminated soils (Fazeli et
2002; Baird et al., 2005). al., 2019; Marija et al., 2017; Wang et al.,
2004; Morillo et al., 2008; Thavamani et al.,
2011; Thavamani et al., 2012).
There are thousands of contaminated
sites around the world. There were
estimated to be 294,000 in the US
(USEPA, 2004). 1337 superfund sites are
listed in a national priorities list of the
United States (USEPA, 2019). In Canada,
there are 22, 000 federal contaminated sites
(FCSI, 2018). These numbers are likely to
increase as contaminated sites continue to
be identified. About 29% of contaminated
sites in the US contain a single
contaminant (metals, PAHs, PCBs or
VOCs), whereas 71% are contaminated by
two or more types of contaminant
(USEPA, 2018). Among the contaminated
Fig. 1. Bay and fjord regions in PAHs molecular
sites, 12.3 and 3.7% are contaminated by
structures mixed heavy metals and PAHs in the US
and Canada, respectively (USEPA, 2018;
Table 1 and Fig. 2 provide the names, FCSI, 2018). Many other sites contain
molecular structures, properties and other types of contaminants in addition to
chemical structures of the 16 USEPA HMs and PAHs (Afkhami et al., 2013).
priority PAHs. Because of toxicity, Table 2 summarizes the proportions of
ecosystem life cycling, and non- sites contaminated solely by metals, PAHs
degradability of heavy metals (HMs), they or both in the US, Canada and Europe.
are of environmental concern. Remediating these mixed-contaminated
Accumulation of HMs in the environment, sites poses great challenges, because of the
particularly in soil and sediment, is a different physicochemical properties and
common problem in many countries (Su et the broad ranges of contaminants, each
al., 2014); with negative impact on people requiring different technologies (Eghbal et
on a global scale (UNEP, 2012). Although al., 2019).
2Pollution, 6(1): 1-24, Winter 2020
Table 1. USEPA’s 16 priority pollutant PAHs and selected properties (Wick et al., 2011; CCME, 2010b)
Water solubility
Vapor pressure
atm-m3 mol-1
Henry's law
PAH name
Number of
Molecular
Log Koc***
Log Kow**
constant
G mol-1
Weight
mmHg
mg l-1
rings
Acenaphthene 3 154.2 1.93 3.98 3.66 4.47×10-3 7.91× 10-5
Acenaphthylene 3 152.2 3.93 4.07 1.40 0.029 1.45×10-3
Anthracene 3 178.2 0.076 4.45 4.15 1.7× 10-5 1.77× 10-5
Benzo[a]anthracene* 4 228.2 0.01 5.61 5.30 2.2 × 10-8 1 × 10-6
Benzo[a]pyrene* 5 252.3 2.3× 10-3 6.06 6.74 5.6× 10-9 4.9× 10-7
Benzo[b]fluoranthene* 5 252.3 0.0012 6.04 5.74 5× 10-7 1.22× 10-5
Benzo[k]fluoranthene* 5 252.3 7.6× 10-4 6.06 5.74 9.59× 10-11 3.87× 10-5
Benzo[g,h,i]perylene* 6 276.3 2.6× 10-4 6.50 6.20 1.03× 10-10 1.44× 10-7
Chrysene* 4 228.3 2.8× 10-3 5.16 5.30 6.3× 10-7 1.05× 10-6
5× 10-4 1× 10-11 7.3× 10-8
*
Dibenz[a,h]anthracene 6 278.3 6.84 6.52
Fluoranthene 4 202.3 0.2-0.26 4.90 4.58 5× 10-6 6.5× 10-6
Fluorene 3 166.2 1.68-1.98 4.18 3.86 3.2× 10-4 1× 10-4
Indeno[1,2,3-c,d]pyrene* 6 276.3 0.062 6.58 6.20 1× 10-10 6× 10-8
Naphthalene 2 128.2 39.06 - - - -
Phenantherene 3 178.2 1.20 4.45 4.15 6.8× 10-4 2.56× 10-5
Pyrene 4 202.3 0.077 4.88 4.58 2.5× 10-6 1.14× 10-5
* The USEPA has classified these PAH as possible human carcinogens
** Octanol-Water partition coefficient
*** Organic carbon-water partition coefficient
Fig. 2. Molecular structures of 16 PAHs in priority list of the USEPA
3Saeedi M., et al.
Table 2. Sites contaminated solely by metals, PAHs or both in the US, Canada and Europe (% of total
contaminated sites in each)
Solely Solely PAH Mixed contamination by
Region Reference
HM contamination contamination PAH+ HMs
USA 16.8% 4.3% 12.4% USEPA (2018)
Canada 34.1% 2.6% 3.7% FCSI (2018)
Europe 34.8% 10.9% _ Panagos et al. (2013)
Since HMs are neither biodegraded nor sorption of PAHs onto soil, organic matter
transported by air in soil matrices, a content seems to be the most important one.
technology for simultaneous remediation of From the remediation of PAH-contaminated,
PAHs co-existing with HMs in soil should be co-existing heavy metals point of view, the
either based on desorbing and solubilizing effect of heavy metals on the
them into a liquid to carry the mixed sorption/desorption of PAHs in soil is an
contaminants, or solidifying/stabilizing them essential, but less studied, subject. Therefore,
into the soil matrix. However, the main objective of this chapter is to
sorption/desorption behavior and reaction review and evaluate the probable effect of
kinetics influence the mobility and subsequent soil organic matter and co-existing HMs on
removal of the contaminants in soil. sorption/desorption of PAHs, based on
Except for naphthalene, because of low previously published literature. The contents
solubility and high sorption affinity to soil of this chapter should help researchers find
and organic matter of PAHs, the chemical gaps in the field of sorption/desorption of
fate of PAHs is predominantly controlled PAHs in soil, taking into account the effects
by sorption and desorption processes in of co-existing HMs and SOM.
soil environments. From an enhanced soil
washing/flushing remediation point of METHOLOGY
view, sorption/desorption and mobility of In this study literature on the sorption and
contaminants influence the remediation desorption of PAHs in soil is reviewed
reaction/retention times, clean-up with emphasis on the effect of soil organic
efficiency and cost of remediation. matter and co-existing heavy metals on the
Given that in the soils of many PAH process. References are used to explain
contaminated sites, there are co-existing principles of PAHs sorption and affecting
heavy metals, there is a need to study factors. More recent published research
sorption/desorption of PAHs co-existing (from 2006 to 2018) relevant to the aim of
with heavy metals in soil. Among soil this chapter were collected and reviewed to
characteristics, organic matter content is be summarize and presented in the
vital to sorption/desorption, ultimate fate following sections; i) sorption of PAHs in
and mobility of both PAHs and HMs in soil, ii) effect of soil organic matter on
soil (Kaschl et al., 2002). PAHs and HMs PAHs sorption, iii) effect of co-existing
both interact with organic matter in soil- heavy metals, iv) other factors related to
water systems (Gao et al., 2006; Zhang and PAH sorption, v) desorption of PAHs.
Ke, 2004). Therefore, sorption/desorption Based on the review, conclusions and
of PAHs can be influenced by the presence future research needs are identified.
of heavy metals in the system (Walter and
Weber, 2002). Understanding these Sorption of PAHs in soil
The term "sorption" is used to cover the
mechanisms for PAHs in soil is vital to
adhesion of contaminants to solid particles
better assess soil pollution risks, as well as
by absorption, adsorption, ion exchange and
to design remediation systems based on
partitioning mechanisms. Sorption can occur
enhanced soil washing/flushing.
by several mechanisms, including absorption
Among the soil properties influencing
4Pollution, 6(1): 1-24, Winter 2020
into soil natural organic matter (partitioning); most often used to describe the sorption of
adsorption to mineral surfaces via van der contaminants in soil environments. The
Waals/London dispersion forces; adsorption Freundlich isotherm equation is
through electrostatic attractive forces to sites Cs= Kf (Ce)n (2)
with opposite charge on solid surfaces; and
chemical sorption (covalent bonding to whereas the Langmuir isotherm equation is
surface functional groups) on the soil solid Cs= (bkCe)/(1+kCe) (3)
particles (Hemond and Fechner-Levy, 2015).
If the adsorbed molecules interact with Kf is the Freundlich constant, n a
solid surface physically, the process is called constant reflecting the degree of non-
physisorption. On the other hand, if there is linearity, Cs is the concentration of sorbed
chemical bonding between adsorbed contaminant in soil (M M-1) and Ce is the
molecules and the solid surface, the aqueous equilibrium concentration of
adsorption process is called chemisorption. sorbate (M L-3); b and k are Langmuir
Chemisorption is mostly irreversible and constants, with k reflecting the maximum
occurs only as a monolayer. Depending on sorption capacity of a sorbent.
the sorbent, sorbate and the environmental Table 3 summarizes research on the
conditions, physisorption and chemisorption sorption of PAHs onto soil/minerals since
may occur simultaneously or alternatively. 2006. Because sorption is a key process in
For hydrophobic organic contaminants, the fate, transport and remediation of PAHs
partitioning into soil organic matter in soil- in contaminated soil, this information can
water system often takes place. also be useful for such studies in
Solids capable of sorbing PAHs in soil contaminated sites. It can be concluded that
include minerals, particularly clay parameters, test conditions, contaminants and
minerals, and natural organic matter. Many results of published studies on sorption of
sorption reactions are completely or partly PAHs onto soil/minerals are too scattered for
reversible. Depending on the conditions, there to be simple plain rules. Most previous
nature and composition of the sorbent and studies on PAH sorption are conducted
sorbate chemical, the relationship between through batch tests, with phenanthrene,
the amounts of dissolved contaminant in pyrene and naphthalene receiving the
the aqueous phase and the amount of greatest attention. The best-fitted isotherm
sorbed (partitioning), may be linear or non- models have been reported to be those of
linear. The distribution coefficient (Kd) Freundlich and Langmuir, in that order.
represents a ratio, indicating the amount of Linear behavior is reported for sorption of
sorbed contaminant to its aqueous phase benzo(a)pyrene onto minerals, phenanthrene
equilibrium concentration: onto quartz (Muller et al., 2007),
phenanthrene onto poor-organic content soils
Kd= (Cs/Ce) (1) and phenanthrene sorption from dilute
where Kd is distribution coefficient (L3 M-1), solutions (< 100 μg L-1) (Sun et al., 2010).
Cs is the concentration of sorbed Most tests are performed at room
contaminant in soil (M M-1) and Ce is the temperature or 20-25 ºC, but some studies
equilibrium concentration of sorbate in have examined the sorption of PAHs at 10
aqueous phase (M L-3). There are many and 15ºC (Ping et al., 2006; Kaya et al.,
non-linear equations, called isotherm 2013) and 40 and 50ºC (Rivas et al., 2008),
equations, expressing the relationship in addition to 20ºC. Overall, findings indicate
between the sorbed contaminants and that sorption is higher at lower temperatures.
sorbate concentration. Among these, the Other test conditions and results are very
Freundlich and Langmuir isotherms are scattered. Contact times tested have varied
5Saeedi M., et al.
from 20 min to more than 48 h. Times varied widely. Therefore, it is not surprising
needed to reach sorption equilibrium have that the results on PAHs sorption amounts
been reported to be from 4 h to more than 34 and coefficients have varied greatly from
h. However, it seems that in most cases there study to study. For example, Kf values in
is rapid sorption in the first 10 to 90 min different studies for phenanthrene have
during which about 90% of the overall ranged from 0.21 to 220 L kg-1 for different
sorption takes place (Muller et al., 2007; soils and minerals under different test
Rivas et al., 2008; Sun et al., 2010; Osagie conditions. In addition, sorption amounts of
and Owabor, 2015 a,b). Initial concentrations phenanthrene have been reported to range
of PAHs in solution were in the wide range from three to 400 mg kg-1 for soils and
of 0.0006 to 400 mg L-1. Even sorbent-to- minerals with different compositions.
solution ratio and amounts used in tests have
Table 3. Summary of research studies and results for sorption of PAHs onto soils/minerals
Sorbent Sorption model Comments/
PAHs Sorption test conditions Sorption capacity/property c Highlighted findings Reference
material (isotherm) Limitations
Batch 1 mg L-1 PHE mixed with HA Highest adsorption was at pH
and FA 0-10 g kg-1 soil for pH range
3 for HA Sorption was higher Ping et al.
PHEa HAb and FA 3-6 for 24 h Freundlich
Kf values for HA: 221 L kg-1 at 15ºC than at 25ºC (2006)
Temperature effect examined at pH 3
for FA: 176 L kg-1
for temperatures of 15 and 25ºC
Equilibrium time: 4
Batch tests Kf values; PHE: 0.21-2.46 L PHE and PYR
quartz, goethite- Freundlich: hInstantaneous kinetics
From 4 to 20 h at 8 time steps kg-1 showed the weakest
PHE, coated quartz, PHE and PYR Cation-π interactions seemed
Initial concentration: 10% of each PYR: 0.36-3.43 L kg-1 sorption onto quartz- Muller et al.
PYRd, and quartz- Linear: PHE to an important sorption and
PAH aqueous solubility Kd values; montmorillonite (2007)
BAPe montmorillonite quartz retention mechanism for low
Sorbent to solution ratio: 1:5 (15 g 75 BAP: 48-154 L kg-1 mixture in spite of the
mixture Linear: BAP organic matter soil
mL) PHE to quartz: 0.2 L kg-1 greatest surface area
environments
Three PAHs except for
acenaphthene showed
trapping of PAHs
anomalous isotherms when
by SOM released to
tested individually
f water is the main
ANA Batch As mixture non showed
Oswin, cause
PHE, Temperature abnormal sorption trend Rivas et al.
Natural soil Freundlich No information
ANg, 298, 313, Sorption kinetics was quite (2008)
and Caurie given on soil
FLANh and 333 K fast (10-20 min)
properties other
Sorption amounts order is
than SOM and
the same order as the
particle size
solubility of PAHs (ANA
> PHE > FLAN > AN)
Freundlich sorption
coefficients, correlated
well with the soil organic
carbon contents
Sorption predictions based
24 h Batch Sorption could not
on organic matter and one
PHE, 8 European Initial concentrations: PYR: 3-4.5 mg kg-1 be predicted from De Jonge et
Freundlich additional simple soil
PYR soils PYR: 0.0009 to 0.1 mg L-1 PHE: 30-50 mg kg-1 soil organic carbon al. (2008)
parameter (e.g., clay
PHE: 0.0006 to 1.1 mg L-1 contents alone
content, cation exchange
capacity, or pH) yielded
accurate sorption
predictions of Freundlich
coefficient
Equilibrium time: 8 h
Linear in OM-
(90% of sorption in first
Different poor soil
1.5 h)
natural soil Linear at < 100 Soil samples
Sorbent to solution: 3.5 g: 210 < 50 μg g-1 in soils at < The more organic content
samples: top, μg L-1 of PHE texture and
mL 1.5% OM the more time to reach Sun et al.
PHE aeration zone, in solution compositions not
From 20 to 480 min in 7 time 90-100 μg g-1 in soils at > sorption equilibrium (2010)
and aquifer Nonlinear at > determined
steps 1.5% OM linear relation between
zone soil 100 μg L-1 of Single contaminant
organic fraction of the soil
samples PHE in
and Freundlich sorption
solution
coefficient
Kf values for Ca, K and Na
Cation Batch modified
modified Initial concentration: 0.025-1.0 Smectite: 20.72, 14.65,
clay minerals mg L-1 13.72 L kg-1 Zhang et al.
PHE Freundlich
(smectite, Minerals modified by Na, K and for kaolinite: 7.39, (2011)
kaolinite, Ca cations 6.83, 6.61 L kg-1
vermiculite) Contact time: 48 h at 25ºC for vermiculite: 7.22, 7.8
9,6.94 L kg-1
Batch
Sorbent to solution: 0.05: 50 g Kf values;
i j k mL-1 Freundlich, for NB: 0.397-0.706 L g-1 Kaya et al.
NAP NB and HB 22.45 mg g-1
Initial concentration: 7.5-20 mg Langmuir for HB: (2013)
L-1 1.58-3.66 L g-1
At 10, 15 and 20ºC
1st-order Lagergren and
Bangham equation
describes sorption rate.
FL, Column test Clay minerals responsible Types and content
PHE, Test duration: 34 d for sorption because of of soil clay Yang et al.
Silty-clay soil Langmuir
FLAN, Constant flow rate: 200 mL d-1 low soil SOM minerals not (2013)
PYR Initial concentrations: 100 µg L-1 Sorption is not completely determined
reversible.
Sorption kinetics follows a
2nd-order kinetics model.
6Pollution, 6(1): 1-24, Winter 2020
Table 3. Summary of research studies and results for sorption of PAHs onto soils/minerals (continued)
Sorbent Sorption model
PAHs Sorption test conditions Sorption capacity/property c Highlighted findings Comments/ Limitations Reference
material (isotherm)
SOM is predominant in
Two types of Batch
PHE PAHs sorption
natural soil; PAH solution concentrations: Yu et al.
and Freundlich 350-400 mg kg-1 More SOM led to more
sandy loam 50-400 mg L-1 (2014)
PYR sorption and less
and clay loam contact time: 24 h
desorption
Correlation between
model and observed
data was lower for
Sandy soil sandy soil than for
Equilibrium time: 20 and
(sieved to < Batch Freundlich for clay soil Osagie
24 h for sandy and clay
2.5 mm) solution/sorbent: 500 mL:100 g clay soil Correlations for and
NAP 5.73 soil
Clay soil PAH solution concentration Langmuir for kinetics model and Owabor
Sorption kinetics fits
(sieved to < range: 50-250 sandy soil observed data were (2015a)
pseudo-2nd order model
220 µm) much lower in clay
soil than sandy soil
Single contaminant
study
Single contaminant
Correlations between
Sorption kinetics fits
kinetics model and
Sandy soil pseudo-2nd order model
observed data were
(sieved to < Batch Equilibrium time: 28 and Osagie
lower for clay soil
2.5 mm) solution/sorbent: 500 mL:100 g 0.286 mg g-1 for clay 34 h for sandy and clayey and
PYR Langmuir than sandy soil
Clay soil PAH solution concentration 0.225 mg g-1 for sandy soil soil Owabor
Correlation between
(sieved to < range: 50-250 Pseudo-2nd order rate (2015b)
sorption isotherm
220 µm) constant: 0.00088 for clay
model and observed
and 0.00085 for sandy soil
data were not very
high
Batch
Initial concentration of each
Kf values: phenanthrene:
PHE, PAH: 10-100 mg L-1
River 88.54 L g-1 Wang et
PYR, Mixing at 25ºC Freundlich
sediment fluorene: 188.71 L g-1 al. (2015)
FLAN for 8 h pyrene: 91.46 L g-1
sediment size with highest
sorption: < 38 μm.
a b c
PHE: Phenanthrene, HA: Humic acid, FA: Fulvic acid, Kf: Freundlich sorption coefficient, d PYR: Pyrene, e BAP:
Benzo(a)pyrene, f ANA: Acenaphthene, g AN:Anthracene, h
FLAN: Fluoranthene, i NAP: Naphthalene, j NB: Natural
bentonite, k HB: Modified bentonite
Effect of soil organic matter assumption that soil organic matter is
Natural organic compounds entering mainly responsible for PAH sorption,
natural systems undergo complex methods have been developed for
microbial and abiotic transformations that approximating Kd, taking both sorbate and
produce what is called natural organic sorbent properties into consideration. Since
matter (NOM). NOM contains fulvic, PAHs are mostly non-polar hydrocarbons,
humic and humin materials. In general, they have large octanol-water partitioning
NOM refers to thousands to tens of coefficient (Kow) values, showing less
thousands of different complex chemical tendency to dissolve in water and more
structures, such that no two molecules can tendency to sorb onto organic matter of the
be found to be identical (Hayes et al., soil. Larger PAH molecules with higher
1990; Suffet and MacCarthy, 1989). Kow are more likely to sorb onto soil
While recent research (Saeedi et al., particles. To predict Kd values for a given
2018a; Zhang et al., 2010; Zhang et al., PAH in soil, the fraction of organic carbon
2011) has shown that PAHs can adsorb of the soil (foc) and the Kow of the PAH are
onto clay minerals in soil matrix, it is used. In this method, it is assumed that the
widely believed that the partitioning sorption occurs as soil organic matter
(absorption) into the natural organic partitioning. Kd is then calculated as;
matters of soil (SOM) is the primary
Kd= foc . Koc (4)
sorption mechanism of PAHs (Ping et al.,
2006). Polarity, aromatic carbon and where foc is the organic carbon fraction in
aliphatic groups of organic matter are the soil, and Koc (chemical concentration
believed to control the PAHs sorption onto sorbed to organic carbon/chemical
soil. Aromatic domains of organic matter concentration in water) is the organic
have also been reported to play an carbon-water partition coefficient.
important role in sorption PAHs to soil and Based on laboratory data, several
sediment (Ahangar, 2010). Based on the regression equations have been reported in
7Saeedi M., et al.
the literature for different organic chemicals features that can enhance the sorption of
to predict Koc as a function of Kow (Lyman polar organic compounds, HOCs such as
et al., 1990). Based on this approach, one PAHs, and ionic contaminants such as
can calculate the Kd (sorption or distribution heavy metals, in soils containing NOM
coefficient) values for a given PAH using compared to the same soil without any
Kow of that PAH and the measured organic NOM.
carbon of the soil. The chemical functionality of NOM is
De Jonge et al. (2008) reported that Kd also important in its effect on the sorption
values of a given PAH with different of different contaminants in soil
sorbents were proportional to the organic environments. The main functional groups
carbon contents of the soil. Important observed and identified in NOM are
characteristics of organic matter in the summarized in Table 4. Water molecules in
sorption of PAHs are chemical properties, soil can form hydrogen bonds with charged
molecular sizes, and functional groups. and polar functional groups. The density of
Fig. 3 depicts the two representative these groups results in hydrophobic
structures of NOM, with (a) showing the character to organic matter on a
structure of fulvic acid and (b) a macroscopic scale (Laird and Koskinen,
conceptualized NOM structure (Dunnivant 2008). On the other hand, aliphatic groups
and Anders, 2005). As shown in Fig. 3, may form localized hydrophobic regions
there are numerous hydrophobic centers on a molecular scale. Therefore, NOM in
and ionic sorption sites in such large soil can enhance sorption of both heavy
organic matter molecules. These are key metals and PAHs.
Fig. 3. Structures of natural organic matter (Dunnivant and Anders, 2005), a) representative of fulvic
acid, b) a conceptualized NOM structure
8Pollution, 6(1): 1-24, Winter 2020
Table 4. Functional groups identified and observed in different NOM (Dunnivant and Anders, 2005;
Killops and Killops, 1993)
Symbol/molecular structure Name Resulting compound
Alcohol
ROH Hydroxyl
Phenol
Aldehyde
Carbonyl Ketone
Quinone
Carboxyl Carboxylic acid
Oxo Ether
Amino Amine
Amido Amide
Thio
Thiol
R=CH2 Indenyl
Indene
R= O Furanyl
Furan
R= NH Pyrryl
Pyrrole
R= S Thiophenyl
Thiophene
R= CH Phenyl Benzene
R= N Pyridinyl Pyridine
Pyranyl Pyran
LAW H2LAW
Quinone Hydroquinone
Iron (II) Porphyrin Iron (II) Porphyrin
Several researchers (Hassett et al., 1980; removing NOM from the soil by extractants
Means et al., 1980; Delle Site, 2001; Ping or by oxidation, then investigating the
et al., 2006) have shown that there are sorption process (Yaron et al., 1967; Wolcott
good correlations between the sorption of et al., 1970; Wahid and Sethunathan, 1979).
PAHs and other hydrophobic organic They concluded that, in addition to NOM,
compounds (HOCs), and SOM. Hassett et other soil properties appear to be responsible
al. (1980) showed that factors such as total for sorption, but their role could be masked
clay content and mineralogy, cation by the SOM effect. For "organic poor" soils,
exchange capacity, surface area and pH at no or low organic matter content, sorption
were not significant compared to organic coefficients (Kd) are expected to be much
matter of the soil in sorption of lower than in the same soil at high OM
dibenzothiophene by soil and sediment. contents (Schwarzenbach and Westall,
Other researchers have demonstrated the 1981).
role of SOM in sorption of HOCs by On the importance of the OM in
9Saeedi M., et al.
controlling sorption of HOCs with water Table 5 summarizes recent studies on
solubility less than 10-3 M (all USEPA-listed the effect of soil organic matter on sorption
PAHs except naphthalene, acenaphthene and of PAHs onto soil. These studies confirm
acenaphtylene), the following constants have the enhancing effect of SOM on sorption of
been defined (Delle Site, 2001): PAHs. However, reports show that the
composition of SOM is important in the
Kom=Kd / fom or Koc=Kd / foc (5)
sorption of PAHs. Ping et al. (2006)
where fom and foc denote the fractions of reported that fulvic acids do not enhance
organic matter and organic carbon (OC) in the sorption of phenanthrene at low pH.
the soil, respectively. Kom and Koc are Wu et al. (2011) also found that pH had no
expressed in cm3 g-1 or dm3 kg-1 of organic significant effect on the sorption of
matter and organic carbon, respectively. The phenanthrene onto organic matter-added
association of HOCs with SOM is montmorillonite. While PAHs tend to sorb
attributable to hydrophobic bonding because onto fulvic and humic acids rather than
of combined van der Waals forces humins in soil, the fraction of larger PAH
thermodynamic gradient that drives non- molecules sorbed onto humins is greater
polar organics out of aqueous solution. The than the fraction sorbed onto lower
interactions of non-polar organic compounds molecular weight PAHs (Yang et al.,
with SOM are thermodynamically preferred 2010). On the other hand, Ukalska-Jaruga
to those of compound-water or compound- et al. (2018) reported that low-molecular-
compound (Gauthier et al., 1987; Weber et weight (two and three rings) PAHs had
al., 1991). Some previous researchers have stronger bonds with humins in
provided a “rule of thumb” to determine if contaminated natural soil. They found no
SOM or clay minerals are responsible for significant relationship between humic and
sorption of HOCs in a given soil. HOCs fulvic acids for PAHs accumulation in
sorption on clay surfaces dominates the contaminated soil. Yang et al. (2010) also
organic matter partitioning if the clay/OC noted that with increasing SOM, most
ratio is high (Karickhoff, 1984; Weber et al., PAHs tend to accumulate in fulvic acids
1992). A ratio of (clay mineral)/(organic rather than humins.
carbon) = 30 (corresponding to a clay/OM Overall, although it is well known that
ratio of 15) was proposed by Karickhoff SOM enhances PAHs sorption, there seems
(1984) as a threshold. Therefore, regardless to be a minimum level of SOM in soil for
of the mineral content, if the mass ratio of them to predominate. Although threshold
clay mineral to OM in soil < 30, mineral ratios and relationships like equation 2.6
contributions in sorption is masked by have been suggested for the minimum
organic matter. The ratio, however, might level, it appears to be interrelated with the
depend on the hydrophobicity of the sorbate , properties and composition of both SOM
type of minerals, and functional groups and and other components of soil, as well as
chemical composition of OM. mineralogy. Most previous studies indicate
The critical organic carbon fraction (foc), that the Freundlich isotherm is commonly a
below which mineral adsorption dominates good model for sorption of PAHs in soil
over OM partitioning of HOCs, was containing organic matter. It has also been
suggested by McCarty et al. (1981) to be: found that increasing SOM content slows
foc= SA/[200 (Kow)0.84] (6) the rapid phase of PAH sorption towards
reaching equilibrium.
where SA is the specific surface area of the
mineral.
10Pollution, 6(1): 1-24, Winter 2020
Table 5. Summary of recent research results on the effect of soil organic matter on PAHs sorption and retention
Soil Types of PAHs Description of the study Highlighted findings Reference
Humic acid and fulvic acid showed
Isotherm tests conducted at initial different effects on adsorption.
concentrations of 0.025, 0.050, 0.100,
0.25, 0.50, and 1.0 mg L-1; 15 and 25ºC Adsorption increased at first, then
temperatures tested, test duration: 24 h decreased with increasing fulvic acid
Agricultural top soil treated At pH=3, Adsorption in FA treated
Exogenous humic acids (HAs) were
with humic acid (HA) and Phenanthrene soil was lower than untreated and HA Ping et al. )2006(
added to soil to enhance the amount of
fulvic acid (FA) treated soil
soil organic carbon (SOC) by 2.5, 5.0,
and 10.0 g kg-1
Sorption fitted the Freundlich
Sorption and desorption for pH of 3 and isotherm
6 and at 15 and 25ºC were evaluated, HA increased Freundlich sorption
coefficients
Freundlich sorption coefficients
correlated well with soil organic
8 European soils with carbon content. De Jonge et al.
phenanthrene, pyrene 24 h Batch tests
different properties Soil organic carbon content alone is )2008(
not adequate for sorption behavior
quantitative predictions.
Humins sorbed fraction of PAHs
increased with increasing PAH
acenaphthene,
molecular weight
Five surface soil samples fluorene,
Sorption of PAHs increased with
from Shisanlin, Beijing, with phenanthrene, Studied impact of SOM on distribution Yang et al .
increasing SOM content in soil.
different SOM and humic anthracene, of PAHs in soils. )2010(
With increasing SOM, fraction of
acids, fulvic acids and humins fluoranthene
each individual PAH in fulvic acids
and pyrene
increased, whereas the fraction in
humins decreased.
Sorption isotherm fitted by Freundlich
isotherm
Sorption capacity of organic matter
added montmorillonite was greatly
enhanced compared to MMT.
Effects on sorption could be related to
changes in surface structure, specific
surface area, hydrophobicity and the
average pore size of MMT.
Montmorillonite (MMT) and Sorption is exothermic and decreased
Batch sorption tests were carried out as a
dissolved organic matter with temperature in both MMT and
function of contact time, temperature,
(DOM) modified Phenanthrene DOC-MMT. Wu et al. )2011(
and adsorbent dose.
montmorillonite (DOM- pH had no significant effect on
Sorption at 25, 35 and 45ºC was tested
MMT) sorption.
Adsorption capacity of
DOM–MMT decreased with
increasing adsorbent dose.
Sorption of PHE on complex was
rapid; and the kinetics could be
described well by both pseudo-first-
order and pseudo-second-order
models, with an equilibrium time of
120 min.
Commercial HAs, as well as
those obtained from Umbric Relationship between the chemical Positive relationship
Andosol, Dystric Cambisol, properties of various types of humic between Kdoc and aromaticity Kobayashi and
Pyrene
and cow manure compost and acids sorption ability was investigated in Factors other than aromaticity may Sumida )2015(
fractionated Aldrich humic batch experiments. also influence the sorption of pyrene.
acid (AHA).
Positive relations between PAH
concentrations and TOC content
Links between humins and PAHs
were stronger for low-molecular
weight (two and three rings) PAHs.
to evaluate the mutual relations between
Relations among PAHs, humins and
Potentially contaminated the soil organic matter fractions: fulvic
black carbon were statistically Ukalska-Jaruga
natural soils from 16 PAHs acids, humic acids, humins, black carbon
significant only in soils with TOC et al. (2018)
southwestern Poland and the PAHs accumulation
content ≥ 12 g kg−1.
level in agricultural soils
Black carbon fraction showed higher
correlations with high-molecular-
weight PAHs.
Fulvic acids and humic acids showed
no significant relationship with PAHs.
11Saeedi M., et al.
Effect of co-existing heavy metals on capacity to sorb PAHs in adsorbed organic
PAH sorption matter, than the original soil organic matter.
Recent studies demonstrate that the Tao et al. (2013) reported that the sorption of
coexistence of heavy metals and other phenanthrene onto biomass depended on the
metals can also contribute to sorption of type and concentration of heavy metals
PAHs to soil. It is believed that this effect present in the system.
is greater in low organic content soils or Table 6 summarizes some previously
for high metal concentrations in soil published work on the effect of co-existing
(Obuekwe and Semple 2013; Zhang et al., metals on PAH sorption. Overall, the few
2011; Hwang and Cutright, 2004; Walter studies published on the effect of co-existing
and Weber, 2002). Some of the reports soil heavy metals show that for all PAHs/soil
speculate on probable causes and types/minerals tested, co-existence of heavy
mechanisms for this effect. metals in soil enhances the sorption of PAHs.
Accumulation of sorbed heavy metals However, the extent of enhancement differs
on soil (particularly clay) particle surfaces for different physical/chemical conditions.
can make cation-π-bonds with PAHs (Zhu Zhang et al. (2011) studied the effects of pH,
et al., 2004). These authors introduced a SOM and Pb on sorption of phenanthrene
new perspective on the sorption of PAHs onto the clay content of soil and reported that
onto mineral surfaces. They reported that the presence of Pb and SOM increases
exchangeable cations on clay mineral phenanthrene sorption in soil. They also
interlayers could condition mineral reported that changes in pH affect soil
surfaces, opening more adsorption sites for properties, influencing the capacity for
strong cation-π-bonds with PAHs. phenanthrene sorption. Fonseca et al. (2011)
Nguyen et al. (2013) reported that larger reported that co-existence of phenanthrene
size pores, clay fraction aggregation and could prolong the Pb retention in soil.
more hydrophobicity are some of the As discussed before, in soil-water
effects of metal cations in clay minerals systems PAHs interact with soil organic
and soils, causing more PAH sorption matter and heavy metals cations to form
(Zhang et al., 2011). Heavy metals on clay complexes with organic molecules in soil
surfaces could play a bridging role between solids (Gao et al., 2003; Zhang and Ke,
PAHs and clay minerals. 2004; Gao et al., 2006), enhancing the
Gao et al. (2006) showed that sorption of sorption of PAHs onto soils. Moreover,
phenanthrene was significantly increased by Saeedi et al. (2018a) showed that the
the presence of heavy metals (Cu, Pb and presence of both heavy metals and organic
Zn). They also found that phenanthrene matter in soil can enhance PAHs sorption
sorption was intensified when concentrations to an even greater extent than the total of
of metals in soil increased. They suggested the separate effects of metals and SOM. In
that the bridging effects of heavy metals other words, co-existence of SOM and
could transform some dissolved organic heavy metals causes synergistic effects on
matter (DOM) in the system to particulates the sorption of PAHs onto soil and clay
and to the adsorbed phase in soil (soil mineral mixtures (Saeedi et al. 2018a).
organic matter), resulting in more They reported 10 to 48% increments in
phenanthrene sorption (mainly partitioning) PAH sorption enhancement by both SOM
by the solid phase. Reducing the dissolved and heavy metals compared to summation
organic matter could also decrease the of their individual effects for different
dissolved organic complexes containing PAHs and soils. Their results showed that
phenanthrene. They also mentioned that the this effect increased with increasing PAH
presence of heavy metals creates more molecular size and aromaticity.
12Pollution, 6(1): 1-24, Winter 2020
Metal cations can form bonds with PAHs to DOM, and consequently increase
organic matter functional groups in soil- the accumulation of PAHs in the
water systems. This connects the organic particulate phase (Obuekwe and Semple,
molecules creating hydrophobic sites for 2013).
PAH molecules sorption. Heavy metals Table 7 summarizes the reported
also bridge the dissolved organic matter in mechanisms that enhance PAH sorption
the system, changing them into the solid when heavy metals and soil organic matter
phase of the soil. This could increase soil co-exist, with categorization of the type of
organic matter, reduce the association of metal cation function.
Table 6. Summary of previous work on the effect of heavy metals on sorption of PAHs
Sorption
PAHs and metal co- Sorption
Soil/Mineral coefficients Sorption coefficient result Reference
contaminants Isotherm
studied
Only PHE: 1130
Montmorillonite PHE + Pb Linear Kd (L kg-1)
PHE+Pb: 1352
Only PHE: 531 Zhang et
olinite PHE + Pb Linear Kd (L kg-1)
PHE+Pb : 572 al. )2011(
Only PHE: 1130
Montmorillonite PHE + Cd Linear Kd (L kg-1)
PHE +Cd : 1247
Only PHE:8.55 Saison et
Silty Clay PHE/PHE + Metals Freundlich Kf (L kg-1)
PHE+Metals: 21.48 al. (2004)
Koc values for metal spiked soils
were 24% larger than for unspiked
Natural soil spiked with ones. Gao et al.
PHE + Pb, Zn or Cu Linear Kd/Koc
heavy metals Increased SOM as a result of co- )2006(
existing metals enhanced
phenanthrene sorption.
NAP (Paddy Soil: 96.5)(Black Soil: 799)
NAP+Cu (Paddy Soil: 101)(Black Soil: 846)
NAP+Pb (Paddy Soil: 116)(Black Soil: 877)
NAP+Cr (Paddy Soil: 123)(Black Soil: 896)
PHE Kf (Paddy Soil: 33.7)(Black Soil: 246)
PHE+Cu (modified (Paddy Soil: 35.1)(Black Soil: 251) Liang et
Paddy and Black Soil Freundlich Freundlich
PHE+Pb (Paddy Soil: 40.7)(Black Soil: 270) al. (2016)
parameter)
PHE+Cr mg kg-1 (Paddy Soil: 41.5)(Black Soil: 307)
PYN (Paddy Soil: 12.6)(Black Soil: 178)
PYN+Cu (Paddy Soil: 15.6)(Black Soil: 186)
PYN+Pb (Paddy Soil: 19.5)(Black Soil: 215)
PYN+Cr (Paddy Soil: 26.1)(Black Soil: 236)
ANA, FL, FLAN ANA: 193, FL:213, FLAN: 2175
Kaolinite (ANA, FL, Linear
ANA:203, FL: 239, FLAN:2566
FLAN)+(Pb, Ni, Zn)
ANA, FL, FLAN ANA: 122, FL: 128, FLAN: 1714 Saeedi et
Sand+kaolinite (ANA, FL, Linear Kd (L kg-1) al.
ANA: 130, FL: 140, FLAN: 1872 (2018a)
FLAN)+(Pb, Ni, Zn)
ANA, FL, FLAN ANA: 140, FL: 152, FLAN: 1607
Sand+kaolinite+bentonite (ANA, FL, Linear
ANA: 150, FL: 159, FLAN: 1880
FLAN)+(Pb, Ni, Zn)
Table 7. Effect of metals on sorption of PAHs in minerals and organic matter contained soil
Cation functioning
Effect on PAH sorption Result of cation interaction Reference
mechanism
Creating and concentrating
Bridging among SOM Obuekwe and Semple,
Enhancement hydrophobic sites desirable
molecules )2013(
for PAH sorption
Transforms DOM into SOM
Jones and Tiller )1999(
resulting in reduction of
Bridging between DOM and Traina et al. )1989(
Enhancement mobile PAHs
soil Saison et al. )2004(
Increases SOM with greater
Gao et al. )2006(
sorption capacity
Bonds between soil adsorbed Cation-π bond beween metal Zhu et al. )2004(
Enhancement
metals and aromatic rings cation and PAH Zhang et al. )2011(
13Saeedi M., et al.
Other factors affecting PAH sorption the soil and organic particles surface.
Based on our review of previous research, Karickhoff et al. (1979) reported 15%
the distribution and partition coefficients of increase in phenanthrene sorption when it
PAHs can depend on multiple factors of was transferred from pure water to KCl
soil and environmental conditions. Because solution at 0.34 M concentration.
of the multiple factors affecting sorption of Inorganic ions can decrease the
PAHs in soil, there is considerable solubility of PAHs, leading them onto
uncertainty in predicting Kd for a given sorbent material surfaces (Weston et al.,
PAH. That is why for PAHs and some 2010). Increasing the sorption coefficient
other organic and inorganic chemicals, and amount of phenanthrene in water
sorption properties are usually determined containing higher ionic concentrations has
in laboratory experiments. been reported (Karickhoff et al., 1979; El-
Other factors affecting the sorption Nahhal and Safi, 2004).
process include temperature, pH, ionic
strength, soil particle size and surface area, Solubility
soil composition and mineralogy, initial The solubility of PAHs is inversely
concentration of PAHs, contact time and proportional to their molecular weight and
kinetics of sorption, and PAH properties Kow. Lower solubility and higher Kow
such as solubility. Some of these factors and indicate greater hydrophobicity of a PAH.
their effects on PAHs sorption in soil Higher hydrophobicity in turn leads to
environments are briefly described in this greater sorption and a higher partition
section. coefficient. Increased sorption capacity for
decreased solubility of PAHs has been
Surface area reported in most previous studies (Hu et al.,
The soil surface area is believed to be 2008; Balati et al., 2015). Solubility and Kow
inversely proportional to sorption capacity are among the main PAHs properties
of any sorbate (Wang et al., 2015). Since affecting sorption and desorption behavior.
adsorption is a surface phenomenon, the
extent of adsorption per mass of soil is Temperature
proportional to the surface area. Previous Most previous studies have reported
work on sorption behaviour of different increased sorption at lower temperatures and
particle sizes of adsorptive media has decreased sorption at higher temperatures
confirmed this general rule (Cornelissen (Table 2) (He et al., 1995; Podoll et al.,
and Gustafsson, 2004; Lemic et al., 2007; 1989). For example, Hiller et al. (2008)
Wang et al., 2015). However, since the reported decreased sorption of pyrene,
sorption is not exactly the same as phenanthrene and naphthalene with the
adsorption and includes other mechanisms temperature increased from 4 to 27ºC.
like ion exchange, the sorption amount and Generally, sorption is exothermic; therefore,
the behavior of PAHs may differ from higher temperatures result in lower sorption.
expectations based solely on the specific However, some studies have reported the
surface area of the sorbent. opposite effect, confirming an endothermic
reaction for sorption (Balati et al., 2015).
Ionic strength and salinity According to Kipling (1965), it is expected
Increasing the ionic strength and salinity of that compounds whose solubility increase
the solution has been found to increase the with increasing temperature will show lower
sorption of PAHs onto soil and sediment. sorption capacities. For compounds whose
Inorganic ions can bind water molecules solubility is inversely proportional to
into hydration shells (Weston et al., 2010) temperature, the lower the temperature, the
reducing the water solubility of PAHs onto more sorption takes place.
14Pollution, 6(1): 1-24, Winter 2020
pH unit of surface area was gibbsite > kaolinite
It seems that pH affects the sorption of > montmorillonite. They explained that the
PAHs due to a sorbent’s change in surface hydroxyl surfaces in gibbsite and kaolinite
charge distribution and functional groups could sorb more effectively than the oxygen
(Gao and Pedersen, 2005; Ping et al., 2006 surfaces in montmorillonite. However, for
Putra et al., 2009). However, some aromatic hydrocarbons such as benzene,
researchers (Mader et al., 1997; Raber et toluene and xylene, lower Kf values were
al., 1998; Zeledon-Toruno et al., 2007; Hu reported for kaolinite than for
et al., 2014) have reported that they did not montmorillonite and illite. This was
observe pH as a significant factor in the explained based on the clay mineral surfaces
sorption of PAHs onto solid media. of these clays: montmorillonite (8×106 m2
kg-1), illite (105 m2 kg-1) and kaolinite (3×104
Contact time and kinetics m2 kg-1) (Li and Gupta 1994). For minerals
Sorption of PAHs onto soil usually occurs in other than clays, it seems that sorption of
two stages. The first stage, physical PAHs does not occur significantly on
adsorption onto minerals and other mineral surfaces. Mader et al. (1997) showed
components of soil, is rapid. This is followed that PAHs interact weakly and non-
by a slower stage of chemical sorption onto specifically with some non-clay mineral
organic matter (Semple et al., 2007). Most surfaces (α-Fe2O3 and α-Al2O3). In summary,
previous work (Zeledon-Toruno et al., 2007; it is clear that clay minerals can be relatively
Awoyemi, 2011; Hu et al., 2014; Gupta, good sorbents of PAHs in soil environments.
2015) confirms this two-stage kinetics. The sorption capacity of different clay
Based on this, although a considerable minerals for PAHs seems to be proportional
portion of PAH sorption onto soil may occur to the surface area of the clay and to be
in the early hours, completion of the process dependent on the clay mineralogical and
may require much more time as it crystallographic structure in terms of the
approaches the maximum sorption capacity. number of layers and their expandability.
The equilibrium time for PAH sorption has
even been reported to be 34 h, but mostly Desorption of PAHs
less than 24 h (Table 3). The fate of PAHs in soil is mainly controlled
by sorption-desorption processes. Since
Clay and minerals some of the mechanisms that take place in
For fine soils that contain clay minerals, sorption process are equilibrium reactions,
recent research (Saeedi et al., 2018a; most factors affecting sorption also affect
Zhang et al., 2010; Zhang et al., 2011; desorption. Mobility of PAHs in soil can be
Hwang et al., 2003; Zhu et al., 2004) has influenced by soil properties, PAH
shown that PAHs can sorb onto the soil, physical/chemical properties and coexistence
even in the absence of soil organic matter. of other contaminants. As discussed above,
In clay minerals, surface area may be the soil organic matter and heavy metals are
main factor influencing the sorption of among the factors which play key roles in
PAHs. Hydrogen bonding between a PAH sorption-desorption processes. However,
and the oxygen of the clay mineral surfaces regarding hydrophobicity of PAHs, most
may take place. Nevertheless, this might not studies on the natural desorption and
be the only factor determining the sorption mobility of PAHs have shown that their
capacity of clay minerals for non-polar mobility is very low compared to the
organic compounds such as PAHs. Brindley sorption amounts (Saeedi et al., 2018b; Olu-
et al. (1963) surprisingly reported that the Owolabi et al., 2014; Yang et al., 2013;
order of sorption of acetoaceticethylester and Barnier et al., 2014). On the other hand,
β, β- oxydipropionitrile on clay minerals per
15Saeedi M., et al.
higher amounts of desorption are needed for for phenanthrene, fluoranthene and
soil washing/flushing and electrokinetic benzo[a]pyrene using Tween 80, Triton X-
remediation of PAH-contaminated soil. 100 and Brij 35 in soil washing (Cheng and
Application of surfactants/biosurfactants Wong, 2006; Ahn et al., 2008; Alcántara et
and some organic and weak acids to desorb al. 2008, 2009). Non-ionic surfactants
and extract PAHs or heavy metals from soil showed better efficiency in removing PAHs
has been widely investigated (Maceiras et al., than ionic ones (Lamichhane et al., 2017;
2018; Maturi and Reddy, 2006; Alcántara et Cheng et al., 2017). These surfactants have
al., 2008; Song et al., 2008; Cao et al., 2013; hydrophobic and hydrophilic groups in their
Saichek and Reddy, 2005; Saichek and structure. Hydrophilic groups of surfactants
Reddy, 2004; Jin et al., 2013; Sánchez- enter the aqueous phase and the lipophilic
Trujillo et al., 2013; Ling et al., 2015; Sales groups (tails) attach to soil particles and
and Fernandez, 2016; Greish et al., 2018). PAHs and soil organic matter (Mao et al.,
Lamichhane et al. (2017) reviewed 2015; Wan et al., 2011). Surfactants
previous studies on surfactant-enhanced accumulate at solid-liquid and solid-
remediation of PAHs. They reported that contaminant interfaces as monomers at low
factors such as surfactant type and concentrations. As the concentrations of
concentration, hydrophobicity of PAH surfactants increase, they replace water
molecules, temperature, pH, salinity, molecules, reducing the polarity of the
dissolved organic matter and microbial aqueous phase and the surface tension.
community could affect remediation of Surfactant molecules adsorbed on the solid
PAHs by surfactants. Cheng et al. (2017) phase creating repulsion between soil
reviewed the advantages and challenges of particles and hydrophobic groups then
Tween 80-surfactant enhanced remediation desorb PAHs from soil (Cheng et al., 2017).
for hydrophobic organic compounds. They If the concentrations of surfactants increase
summarized research on Tween 80 compared further, micelles start to form that can greatly
with other surfactants in remediation of improve the solubility and desorption of
hydrophobic organic compounds, such as PAHs (Yeom et al., 1996; Liang et al.,
PAHs. Research showed desorption and 2017). A schematic of surfactant enhanced
removal efficiencies in the range of 36-84% desorption of PAHs is presented in Fig. 4.
Fig. 4. Schematic of surfactant enhanced desorption of PAHs.
16Pollution, 6(1): 1-24, Winter 2020
Table 8. Summary of recent studies on the effect of organic matter and heavy metals on PAHs desorption
Co-existing organic
Soil/material Type of PAHs Test details Highlighted findings Reference
matter/heavy metal
Batch sorption of PAHs Surfactants were more effective
followed by surfactants to desorb PAHs in soils with
(TX-100, TX-114 and TX- Organic matter; higher organic carbon content
Spiked three
Phenanthrene, 305) enhanced desorption measured as organic and lower clay content.
types of natural
fluorene, tests. carbon. Desorption by water was too Zhou and
soil from
acenaphthene, In batch desorption tests 2 Organic matter low except for naphthalene, Zhu (2007)
Hangzhou City,
naphthalene g of soils mixed with 20 ml contents were whereas the surfactants were
China
of surfactant solution at different in samples more effective for PAHs with
different concentrations for higher Kow than water insoluble
48 h ones
Sorption was affected by both
carbonate and organic matter.
Organic matter plays a more
important role for larger PAHs
Sorption-desorption (no
with greater Kows.
surfactant);
Phenanthrene, Lower-ring PAHs desorb faster
Sorption with solid to
acenaphthene, than PAHs having more rings;
liquid ratio of 1:100 (g mL- Natural organic
Sediment fluorine, 1 more tightly sorbed PAHs
), initial concentration: 10- matter contents in Wang et al.
(Yangtze fluoranthene, desorb with more difficulty
100 μg L-1, samples; measured (2008)
River) pyrene Organic matter may inhibit the
12 h test time then as TOC
(Multiple diffusion of PAHs from
desorption carried out by
sorbates) sediment to water.
replacing 96% supernatant
Desorption of PAHs into water
with water for 12 h test.
ranged from 0.25 to 8% of total
sorption, which was rather low.
However lighter PAHs showed
higher desorption amounts.
Batch tests to evaluate
Addition of phnanthrene to soil
desorption and desorption
reduced Pb desorption from
kinetics.
Natural 100% to 48%.
Soil-to-solution ratios of
contaminated Combined solutions enhanced
1:2.5, 1:5 and 1:10 were Fonseca et
soil sample Phenanthrene Lead solubilisation of mixed Pb and
tested for 24 h. al. (2011)
spiked with phenanthrene.
Combined and single
phenanthrene Desorption kinetics fitted
solutions of Tween 80, Brij
empirical power function and
35 and EDTA were used as
pseudo-second-order equation.
extractants.
Co-existing
phenanthrene enhanced Pb
sorption. Enhancement was
greater for humic acid-coated
Batch sorption and bentonite due to greater
desorption/extraction phenanthrene sorption.
experiments (no Phenanthrene sorption was also
surfactant); increased by the co-existing Pb.
0.1 g: 25 mL of solution Co-existing phenanthrene and
for 72 h. Pb redistributed each other
Bentonite and
Desorption/leachability from weak bonds to strongly
humic/ fulvic Lead, Cadmium, Zhang et
Phenanthrene then was assessed by bonded fractions. Humic acid
acid-coated humic/fulvic acid al. (2015)
deionized water, 0.1 M coating reduced this effect.
bentonite
NaNO3 for 60 min, Presence of Pb decreased
Modified Krauss sequential desorption of phenanthrene
extraction of phenanthrene from bentonite.
with methanol, and Tessier Presence of both humic acids
(1979) extraction of Pb and and Pb reduced the desorption
Cd. of phenanthrene to a greater
extent.
Desorption of phenanthrene to
water was very low compared
to sorption
Heavy metals and humic acid
decreased the desorption of
Batch desorption tests on PAHs.
spiked samples with added Humic acid had greater
Kaolinite,
and non-added heavy decreasing effect than heavy
Sand+
Acenaphthene, metals and humic acids. Humic acid, heavy metals.
kaolinite, Saeedi et
fluorene, Extracting solutions were metals (Pb, Ni and Desorption reducing effects
sand+ al. (2018b)
fluoranthene CaCl2 (0.01M), combined Zn) were greater for larger PAH
kaolinite+
EDTA (0.01M) and Tween than lighter ones.
bentonite
80, combined EDTA Greater clay content in the soil
(0.01M) and Triton X100 mixture led to more desorption
decrease by heavy metals and
humic acid.
17Saeedi M., et al.
Compared to extensive research on the extractability and desorption of PAHs. It
effect of surfactants in desorption and seems also that co-existence of PAHs and
solubilization of PAHs, pulished work on heavy metals affects the chemical bonds
the effect of soil organic matter and within the soil matrix. This can affect the
particularly heavy metals on desorption environmental and ecological risk of both
and removal efficiency of PAHs is limited. groups of contaminants. PAHs with higher
Obuekwe and Semple (2013) found that Kow (i.e. larger ones with more rings) are
the presence of Cu and Al enhanced the retained by SOM in soil more than smaller
extraction of 14C-phenanthrene by CaCl2 PAH molecules.
and hydroxypropyl-b-cyclodextrin Desorption kinetics of PAH removal
(HPCD). They concluded that high from a mixed-contaminated soil by
concentrations of metals could affect surfactants and enhancing solutions fit a
phenanthrene mobility/accessibility in soil. pseudo-second-order kinetic equation and
Saison et al. (2004) clearly demonstrated empirical function (Fonseca et al., 2011;
that the fate of PAHs in soil cannot be easily Wang et al, 2010), whereas the sorption of
predicted because it greatly depends on co- PAHs to soils, particularly at higher SOM
pollution with metals. Yang et al. (2001) contents, has been mostly reported to
showed that, under the effect of complexing follow linear kinetics.
agents (EDTA, citric acid), other metal ions Findings also show that application of
like Fe and Al are mobilized, loosening combined surfactants and chelating
humic-cation-mineral bonds and resulting in agents/organic acids is effective in
dissolution of some soil organic matter, in simultaneously desorbing PAHs and heavy
turn mobilizing the PAHs in soil. Table 8 metals for soil washing/flushing (Fonseca
summarizes recent studies on the effect of et al., 2011; Maturi and Reddy, 2008) and
soil organic matter and/or heavy metals on elecktrokinetic remediation methods.
the desorption of PAHs from soil.
Most previous work on co-existing CONCLUSION AND RESEARCH
contaminant has investigated either the effect NEEDS
of organic matter or heavy metals on Heavy metals and PAHs have totally
desorption of PAHs. There are very few different chemical properties in terms of
published papers on the mobility and sorption, desorption and remediation.
desorption of both PAHs and heavy metals in Sorption and desorption of PAHs in soil are
the presence of soil organic matter. There are the main processes affecting remediation
some discrepancies regarding the effect of technologies, particularly in fine soils and
heavy metals and SOM on the desorption of soils containing natural organic matter, heavy
PAHs, but this appears to be due to the metals or both. Given that most mixed-
number of affecting factors and variations in contaminated sites contain different types of
the composition of the soils tested in previous coexisting PAHs and heavy metals, it is
research. However, some consistent findings essential to study their combined behavior in
are important with respect to fate and/or fine and clayey soil environments in terms of
remediation of mixed-contaminated soils. sorption/desorption, as the main processes
Based on the chemical properties of the determining their environmental fate and
PAHs and findings of previous researchers, it transport, as well as remediation techniques.
is clear that sorbed PAHs in soil are not The presence of either natural organic
desorbed in great amounts by water. matter or heavy metals can enhance the
Previous findings also show that SOM sorption capability of fine soil, retarding
increases the retention of PAHs in soil. Co- the spread of PAHs in the solid matrix.
existence of heavy metals reduces the Although, enhancing agents and surfactants
have been reported to be able to increase
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