Removal of chromium from industrial waste by using eucalyptus bark
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
Bioresource Technology 97 (2006) 15–20
Removal of chromium from industrial waste by using eucalyptus bark
Vikrant Sarin, K.K. Pant *
Department of Chemical Engineering, Indian Institute of Technology, Hauz Khas, New Delhi 110 016, India
Received 16 November 2004; received in revised form 31 January 2005; accepted 18 February 2005
Available online 7 April 2005
Abstract
Several low cost biomaterials such as baggase, charred rice husk, activated charcoal and eucalyptus bark (EB) were tested for
removal of chromium. All the experiments were carried out in batch process with laboratory prepared samples and wastewater
obtained from metal finishing section of auto ancillary unit. The adsorbent, which had highest chromium(VI) removal was EB.
Influences of chromium concentration, pH, contact time on removal of chromium from effluent was investigated. The adsorption
data were fitted well by Freundlich isotherm. The kinetic data were analyzed by using a first order Lagergren kinetic. The Gibbs free
energy was obtained for each system and was found to be 1.879 kJ mol1 for Cr(VI) and 3.885 kJ mol1 for Cr(III) for removal
from industrial effluent. The negative value of DG0 indicates the feasibility and spontaneous nature of adsorption. The maximum
removal of Cr(VI) was observed at pH 2. Adsorption capacity was found to be 45 mg/g of adsorbent, at Cr(VI) concentration in
the effluent being 250 mg/l. A waste water sample containing Cr(VI), Cr(III), Mg, and Ca obtained from industrial unit showed
satisfactory removal of chromium. The results indicate that eucalyptus bark can be used for the removal of chromium.
2005 Elsevier Ltd. All rights reserved.
Keywords: Adsorption; Eucalyptus bark (EB); Hexavalent chromium; Lagregren kinetic; Freundlich isotherm
1. Introduction over the world. Most of the tanneries in India adopt
the chromium tanning process because of its processing
Water pollution by chromium is of considerable con- speed, low costs, and light color of leather and greater
cern, as this metal has found widespread use in electro- stability of the resulting leather. In the chromium tann-
plating, leather tanning, metal finishing, nuclear power ing process, the leather takes up only 60–80% of applied
plant, textile industries, and chromate preparation. chromium, and the rest is usually discharged into the
Chromium exists in two oxidation states as Cr(III) and sewage system causing serious environmental impact.
Cr(VI). The hexavalent form is 500 times more toxic Chromium ion in liquid tanning wastes occurs mainly
than the trivalent (Kowalski, 1994). It is toxic to micro- in trivalent form, which gets further oxidized to hexava-
organism plants, animals and humans. Human toxicity lent Cr(VI) form, due to the presence of organics.
includes lung cancer, as well as kidney, liver, and gastric The maximum levels permitted in wastewater are
damage (US Department of Health and Human Ser- 5 mg/L for trivalent chromium and 0.05 mg/L for hexa-
vices, 1991; Cieslak-Golonka, 1995). The tanning pro- valent chromium (Acar and Malkoc, 2004). With this
cess is one of the largest polluters of chromium all limit, it is essential for industries to treat their effluents
to reduce the Cr to acceptable levels. Due to more strin-
gent environmental regulations, most of the mineral
*
Corresponding author. Tel.: +91 11 26596172; fax: +91 11 2652
processing plants, metal-finishing industries are facing
1120. nowadays the difficult problem of disposal of waste-
E-mail address: kkpant@chemical.iitd.ac.in (K.K. Pant). water produced in huge quantities, laden with Cr.
0960-8524/$ - see front matter 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.biortech.2005.02.01016 V. Sarin, K.K. Pant / Bioresource Technology 97 (2006) 15–20
Chromium metal ions are usually removed by precip- grounded to small particles of size 120 < dp < 500 lm.
itation (Patterson, 1977), although ion exchange (Tirav- It was washed with deionized water and then dried. To
anti et al., 1997) and adsorption (Dahbi et al., 1999; avoid, the release of color by bark in to the aqueous
Orhan and Buyukgangor, 1993) are also used for its re- solution during adsorption, it was treated with formal-
moval. The hydroxides of heavy metals are usually dehyde (Randall et al., 1976). For this 5 mL of aqueous
insoluble, so lime is commonly used for precipitating formaldehyde was added to 100 mL of 0.1 M H2SO4
them. The most important factor in precipitation of and then 10 g of grounded and washed bark was added
heavy metal is the valence state of metal in water. Cr to this solution. The final mixture was stirred and heated
whose hexavalent form, chromate ðCrO2 4 Þ, is consider- at 50 C for 24–48 h till the mixture became thick slurry.
ably more soluble than trivalent form, Cr(III). In this The slurry (treated bark) was washed with deionized
case, the chromate, in which Cr is present as Cr(VI) water until the pH of the filtrate was more than 4.5.
must be reduced usually with SO2 available from sodium Finally the bark was air-dried and sieved. Particles in
metabisulphite at low pH for removal of chromium as the range of 120–500 lm size were collected as the final
Cr(III) by precipitation process. Another aspect of adsorbent. Surface area of the sorbent was determined,
precipitation process is the zeta potential of the initial using BET apparatus, using liquid nitrogen as
heavy metal colloidal precipitate. In many plants where adsorbent.
heavy metals are being removed, one of the principal Further, ultimate and proximate analysis of the EB
problems in reaching the desired effluent limits is the col- adsorbent was also carried out. The detailed characteris-
loidal state of precipitated materials—they have not tics of EB obtained are shown in Table 1.
been properly neutralized, coagulated and flocculated.
A final aspect of heavy metals is the possible formation 2.3. Determination of chromium content
of complex ions, which is common when dealing with
wastewaters containing ammonia, fluoride, or cyanide The chromium concentration in raw and treated efflu-
ions along with heavy metals. Because of these impor- ent was determined by UV (Varian, Australia) spectro-
tant aspects in the precipitation of heavy metals, there photometer. The wavelength of operation was kept at
is no way to predict the best solution of a specific prob- 540 nm. For this purpose, K2Cr2O7 solutions of different
lem without undergoing a series of bench tests to evalu- concentrations were prepared and their absorbance re-
ate the alternative available (Kemmer, 1988). corded by using a UV-spectrophotometer. A calibration
The present study is aimed at selection of a low cost plots for Cr(VI) were drawn between Ô%Õ absorbance
biosorbent, which can adsorb chromium from the waste- and standard Cr(VI) solutions of various strengths
water. Detailed batch studies with the selected adsor- (APHA, 1992). Runs were made in triplicate. Cr(III)
bent, eucalyptus bark has been carried out in the concentration was determined by measuring the differ-
present investigation. The effect of pH, contact time, ence between total chromium concentration and Cr(VI)
adsorbent concentration, thermodynamics study, and concentration. Total Cr concentration was determined
metal ion/adsorbent ratio were also investigated. by oxidizing Cr(III) to Cr(VI) using KMnO4 and then
determining final Cr(VI) content in the sample (APHA,
1992).
2. Methods
2.4. Experimental
2.1. Materials
Stock solution of 1000 ppm of Cr(VI) was prepared
All the chemicals used were of analytical grade. by dissolving K2Cr2O7 (AR grade), in deionised,
K2Cr2O7, HCHO, NaOH, diphenyl carbizide, KMnO4, double-distilled water. All the batch adsorption studies
HNO3 and H2SO4 were procured from Merck. The
adsorbents selected for the preliminary study were bag- Table 1
gase, charred rice husk, activated charcoal, and eucalyp- Characteristics of eucalyptus bark (EB) adsorbent
tus bark (EB). These were grounded and washed with Characteristics Values
deionized water. The adsorbents were dried at room 2
Surface area (m /g) 0.59 ± 0.05
temperature, (32 ± 1 C) till a constant weight of the Bulk density (g/cm3) 0.25 ± 0.02
adsorbent was achieved. A uniform particle size of the Moisture content (%) 10.1 ± 0.3
Ash content (%) 19.0 ± 0.5
adsorbent was maintained between 120 and 500 lm.
Volatile matter (%) 65.7 ± 2.0
Fixed carbon (%) 15.3 ± 0.5
2.2. Preparation of eucalyptus bark adsorbent Carbon (%) 43.68 ± 1.3
Hydrogen (%) 8.14 ± 0.25
Eucalyptus bark of Eucalyptus globulus tree species Nitrogen (%) 0.43 ± 0.01
Oxygen (%) 47.75 ± 1.4
was collected from the local area. The bark wasV. Sarin, K.K. Pant / Bioresource Technology 97 (2006) 15–20 17
were carried out using 100 mL of solution of appropri- The adsorptive properties of activated carbon are due
ate concentration as desired by dilution of the stock to its porous nature. Over 99% of the active sites for
solution. Requisite quantity of adsorbent was added to adsorption in GAC are located in the interior of the
250 mL plastic reagent bottles containing 100 mL of particle. Activated carbon particles have macropores
synthetic effluent of Cr(VI). The bottles were placed in having diameters 30 to 100,000 Å and the micropores
a shaker at 32 ± 1 C, for 24 h. The speed of shaker having diameters in the range of 10 to 30 Å (Weber,
was kept at 100 rpm. After 24 h the bottles were 1967). Results of our investigation revealed that euca-
removed and the content of the bottles was filtered lyptus bark has highest percent removal and sorption
through a filter paper. The filtrate was analyzed for capacity. Further investigations were made with this
pH and final chromium concentration using UV Spec- sorbent. A comparison of the sorbent capacity with var-
trophotometer. The removal of Cr(VI) was studied by ious sorbents studied in literature is given in Table 2
using various adsorbents such as baggase, charred rice and compared with EB.
husk, activated charcoal, and eucalyptus bark. For all
these runs the adsorbent dose was kept at 5 g L1 of syn- 3.2. Effect of pH
thetic effluent of Cr(VI) and Cr(VI) concentration was
kept at 50 ppm at pH of 5.2. Further studies on chro- Effect of solution pH on removal of Cr was studied
mium removal were carried out using adsorbent as using EB as sorbent. As the pH of the solution was in-
EB. This involved, varying initial Cr(VI) concentration creased from 1.5 to 9 the adsorption of Cr(VI) de-
ranging from 50 to 250 ppm. The pH was varied from creased. Increasing pH from 1.5 to 5, percent removal
1.5 to 9 of with different initial concentrations. The of Cr(VI) decreased 99 to 93, whereas as the pH was
contact time in batch was varied from 0.25 h to 24 h. increased from 5 to 9 the % removal decreased signifi-
The studies were also carried with industrial effluent cantly from 93 to 63. It was observed that the maximum
obtained from metal finishing industry (Automobile percentage of removal of Cr(VI) was at pH 2. Almost
ancillary unit, manufacturing brake shoes, Sahibabad, 100% of Cr(VI) removal was observed at this pH at
U.P). The characteristics of industrial effluent is as 50 ppm Cr(VI) concentration. Dominant form of Cr(VI)
follows: Cr(VI) concentration 200 mg/L, Cr(III) concen- at initial pH of 2 is HCrO 4 (Namasivayam and Yam-
tration 44.5 mg/L, total dissolved solids 780 mg/L, Ca una, 1995). Increase in pH shifts concentration of
concentration 135 mg/L and Mg concentration 92 mg/ HCrO 2
4 to other forms, CrO4 and Cr2 O7 . It can be
L. The samples were characterized by standard APHA concluded that the active form of Cr(VI) that can be ad-
method (APHA, 1992). sorbed by EB was HCrO 4 . Further it was observed that
there was an increase in pH during adsorption. The
increase in pH with contact time explained by hydrolysis
3. Results and discussion of the adsorbent in water, which will create positively
charged sites. Upon adsorption of HCrO 4 , a net
3.1. Performance of various adsorbents for Cr removal production of hydroxide ions will occur as shown below
(Saliba et al., 2002).
The performances of these sorbents were evaluated
for the percent removal of chromium. The maximum OHþ þ
2 þ HCrO4 $ OH2 ðHCrO4 Þ ð1Þ
(87.4%) removal of chromium was achieved with EB. Every mole of HCrO 4 adsorbed results in the release of
The percent chromium removal with other three sorbents two moles of hydroxyl ions in the solution, which raises
were significantly low as compared to EB (charred rice the solution pH (Namasivayam and Yamuna, 1995).
husk 36%, activated carbon 9% and bagasse 35%) there- This change in pH at lower initial pH is very small since
fore not considered for further investigations. The varia- the solutions at lower pH are well buffered by the acids
tion in the sorption capacity between the various used in this pH range.
adsorbents could be related to the nature and concentra-
tion of surface groups responsible for interaction with 3.3. Effect of contact time
the metal ions. The selected adsorbents were cellulose
based plant fibers having many hydroxyl groups that Fig. 1 shows the effect of contact time. Increasing
may bind the Cr(VI) ion. Formaldehyde pretreatment contact time from 0.25 h to 3 h increases % Cr removal.
of eucalyptus bark led to crosslinking of compounds in Maximum Cr removal was observed with in first 2 h.
the bark to form a phenol–formaldehyde copolymer that The kinetic data was fitted to the Lagergren equation
preserved high capacity of the support towards the (Singh and Pant, 2004).
adsorption of cations. This can be explained by the inter-
logðxe xÞ ¼ log xe K ads t=2:303 ð2Þ
actions in the solutions between the cations and the water
extracted moieties, leading to complexities that precipi- x = the amount of solute, Cr(VI), (mg/g of adsorbent)
tate on the support surface (Saliba et al., 2002). removed at time t, xe = amount removed at equilibrium18 V. Sarin, K.K. Pant / Bioresource Technology 97 (2006) 15–20
Table 2
Adsorption capacity of various adsorbents as reported in literature
Adsorbent Maximum Optimum pH Maximum References
adsorption concentration
capacity (mg/L) C0 (mg/L)
Saw dust 39.7 2.0 1000 Sharma and Foster (1994)
Coconut husk fibers 29.0 2.05 – Huang and Wu (1977)
Sugar cane bagasse 13.4 2.0 500 Sharma and Foster (1994)
Sugar beet pulp 17.2 2.0 500 Sharma and Foster (1994)
Palm pressed fibers 15.0 2.0 – Tan et al. (1993)
Activated carbon (Filtrasorb-400) 57.7 – – Huang and Wu (1977)
Biogas residual slurry 5.87 2.0 40 Namasivayam and Yamuna (1995)
Wool 8.66 2.0 100 Dakiky et al. (2002)
Pine needles 5.36 2.0 100 Dakiky et al. (2002)
Eucalyptus bark 45.00 2.0 250 Present study
120 2.5
100
Chromium(VI) removal (%)
2
80
1.5
60 log(Xe - X)
1
40
pH - 2
I.E., Cr(VI), at pH - 3.41
pH - 3
I.E., Cr(III), at pH - 3.41 0.5
20 pH - 4.7
pH - 2
I.E., Cr(VI), at pH - 3.41
pH - 3
pH - 4.7 I.E., Cr(III), at pH - 3.41
0 0
0 15 30 45 60 75 90 105 120 150 180 210 0 20 40 60 80 100 120
Time (min) Time (min)
Fig. 1. Effect of contact time on removal of Cr(VI) by eucalyptus bark Fig. 2. Lagergren plot for the adsorption of Cr(VI) by eucalyptus bark
from synthetic effluent (S.E.) having Cr(VI) 200 ppm, pH 4.7 and adsorbent from synthetic effluent having Cr(VI) 200 ppm, pH 4.7 and
industrial effluent (I.E.) having Cr(VI) 200 ppm, Cr(III) 44.5 ppm and industrial effluent having Cr(VI) 200 ppm, Cr(III) 44.5 ppm and pH
pH 3.41. Adsorbent dosage was 5 g/L. 3.41. Adsorbent dosage was 5 g/L.
and Kads = the rate constant of adsorption (1/min). The
Table 3
effect of contact time was studied for removal of Cr from Adsorption rates constant for EB for various systems
effluent containing 200 ppm of Cr(VI) at 32 ± 1 C,
Cr(VI) concentration pH Particle Rate constant R2
pH 2, pH 3, pH 4.7. Experiments were also carried out (mg/L) size (lm) kads (min1)
industrial effluent containing Cr(VI) 200 ppm and
200 ppm Cr(VI) 2.0 120–500 1.9806 · 102 0.9723
Cr(III) 44 ppm. For EB the contact time of 3 h was 200 ppm Cr(VI) 3.0 120–500 1.2206 · 102 0.9718
needed to establish equilibrium. The kinetic on different 200 ppm Cr(VI) 4.7 120–500 1.0133 · 102 0.9640
solution of Cr(VI) at different pH with EB as adsorbent 200 ppm (Ind effluent) 3.41 120–500 5.758 · 103 0.9662
was found to follow the first order rate. Fig. 2 depicts Cr(VI)
the Lagergren plots with a regression coefficient more 200 ppm (Ind effluent) 3.41 120–500 1.4040 · 103 0.9723
Cr(III)
than 0.9. Adsorption rate constant is given in Table 3.
3.4. Adsorption isotherm
X = x/m, where ÔxÕ is in mg the amount of solute ad-
Adsorption isotherms, which are the presentation of sorbed, ÔmÕ is unit gram of adsorbent, Ce is the equilib-
the amount of solute adsorbed per unit of adsorbent, rium concentration of solute (mg L1); Xm is the
as a function of equilibrium concentration in bulk solu- amount of solute adsorbed per unit weight of adsorbent
tion at constant temperature, were studied. The equilib- required for monolayer coverage of the surface also
rium data obtained were fitted to Langmuir and called monolayer capacity and b is a constant related
Freundlich isotherms. to the heat of adsorption.
Linear form of Langmuir equation, Freundlich equation indicates the adsorptive capacity
or loading factor on the adsorbent, x/m is a function of
1=X ¼ 1=X m þ ð1=C e Þð10 =b X m Þ ð3Þ the equilibrium concentration of the solute. It can beV. Sarin, K.K. Pant / Bioresource Technology 97 (2006) 15–20 19
used for calculating the amount of adsorbent required to Table 4
reduce any initial concentration to predetermined final Thermodynamic parameters for the adsorption of Cr(VI) by EB
concentration. Effluent Effluent pH Equilibrium Gibbs free
The Freundlich equation is expressed linearly as: concentration constant Kc energy
of Cr DG0 kJ mol1
log x=m ¼ log K f þ 1=n log C e ð4Þ Pure solution 250 Cr(VI) 2 9.0 5.572
The thermodynamic equilibrium constant for vari- ðK 0c Þ Pure solution 200 Cr(VI) 3 4.95 4.057
Pure solution 200 Cr(VI) 4.7 1.36 0.795
ous systems using EB as adsorbent was obtained at Industrial 200 Cr(VI) 3.41 2.10 1.884
32 ± 1 C. effluent
Ca Industrial 44.5 Cr(III) 3.41 4.60 3.872
K 0c ¼ ; ð5Þ effluent
Ce
where Ca is concentration of Cr(VI) on the adsorbent at The Gibbs free energy (DG0) for the adsorption pro-
equilibrium in mg L1 and Ce is the equilibrium concen- cess for each effluent was obtained using the formula:
tration of Cr(VI) in solution in mg L1.
The initial Cr(VI) concentrations tested were 200 ppm DG0 ¼ RT ln K 0c ð6Þ
of synthetic effluent and true industrial effluent having 0
Values of DG and thermodynamic constant ÔK 0c Õ
for var-
Cr(VI) as 200 ppm and Cr(III) as 44.5 ppm at an adsor- ious systems are shown in Table 4.
bent dosage of 5 g L1. The adsorption followed Fre- The Gibbs free energy indicates the spontaneity of the
undlich isotherm. Freundlich plot is shown in Fig. 3. adsorption process, where higher negative values reflect
Linearity of these plots shows that first order mecha- a more energetically favorable adsorption process. The
nism is followed in this process. The Kf and n values negative DG0 values obtained for various systems in this
as calculated from the Fig. 3 for synthetic effluent hav- study confirm the feasibility of the adsorbent and spon-
ing 200 ppm of Cr(VI) and pH 4.7 was found out to taneity of adsorption. The studies further confirm that
be as 6.74 mg/g and 4.66, respectively. Industrial effluent as the pH of the system is reduced the adsorption of
having 200 ppm of Cr(VI) and 44.5 ppm of Cr(VI) at Cr increases. With all the industrial samples there was
3.41 pH had Kf and n values as 21.69 mg/g and 9.8 for more than 90% of Cr removal without any significant
Cr(VI), and 18.26 mg/g and 7.88 for Cr(III), respec- interference of other metal ions. This indicates that EB
tively. A higher than 1 value of n indicates that the has higher affinity towards Cr adsorption.
adsorption on EB is favorable and capacity is only
slightly reduced at the lower equilibrium concentrations.
These values are comparable with several published 4. Conclusion
literature reported for various sorbents (Sharma and
Foster, 1994; Namasivayam and Yamuna, 1995; Dakiky Removal of poisonous hexavalent form of chromium
et al., 2002). Significantly higher values of adsorption from solutions was possible using selected adsorbents.
capacity obtained with eucalyptus bark indicate that it Eucalyptus bark (EB) was the most effective for which
can be used for the treatment of chromium waste. the removal reached more than 99% for Cr(VI) at con-
centration of 200 ppm and at pH 2. Increase in the dose
of adsorbent, initial concentration of Cr(VI) and in-
1.6 crease in contact time upto 2 h are favorable for all
1.4 increase the adsorption of Cr(VI). The kinetic of the
Cr(VI) adsorption on EB was found to follow first order
1.2
mechanism. The Gibbs free energy was obtained for
1 each system. It was found to be 1.884 kJ mol1 for
log x/m
0.8 Cr(VI) and 3.872 kJ mol1 for Cr(III) for removal
from industrial effluent. The adsorption data can be sat-
0.6
isfactorily explained by Freundlich isotherm. Higher
0.4 Cr(VI) of I.E. sorption capacity of this sorbent indicates that eucalyp-
0.2
Cr(III) of I.E. tus bark can be used for the treatment of chromium
S.E. of Cr(VI) effluent.
0
-3 -2 -1 0 1 2 3
log Ce
References
Fig. 3. Freundlich plot for the adsorption of Cr(VI) from synthetic
effluent having Cr(VI) 200 ppm and pH 4.7 and industrial effluent Acar, F.N., Malkoc, E., 2004. The removal of chromium(VI) from
(I.E.) having Cr(VI) 200 ppm, Cr(III) 44.5 ppm and pH 3.41 at 32 C. aqueous solutions by Fagus orientalis L. Bioresource Technol. 94,
Adsorbent dosage was 5 g/L. 13–15.20 V. Sarin, K.K. Pant / Bioresource Technology 97 (2006) 15–20 APHA, 1992. Standard Methods for the Examination of Water and Patterson, J.W., 1977. Waste Water Treatment. Science Publishers, Wastewater, 18th ed. APHA, Washington, DC. New York. Cieslak-Golonka, M., 1995. Toxic and mutagenic effects of chro- Randall, J.M., Hautala, E., Waiss, A.C., 1976. For. Prod. J. 26, 46. mium(VI). Polyhedron 15, 3667–3689. Saliba, R., Gauthier, H., Petit-Ramel, M., 2002. Adsorpt. Sci. Technol. Dahbi, S., Azzi, M., de la Guardia, M., 1999. Removal of hexavalent 20 (2), 119–129. chromium from wastewaters by bone charcoal. Fresenius J. Anal. Sharma, D.C., Foster, C.F., 1994. A Preliminary examination into the Chem. 363, 404–407. adsorption of hexavalent chromium using low-cost adsorbents. Dakiky, M., Khamis, M., Manassra, A., MerÕeb, M., 2002. Selective Bioresource Technol. 47, 257–264. adsorption of chromium(VI) in industrial wastewater using low- Singh, T.S., Pant, K.K., 2004. Equilibrium, kinetic and thermody- cost abundantly available adsorbents. Adv. Environ. Res. 6, 533– namics studies for adsorption of As(III) on activated alumina. Sep. 540. Pur. Technol. 36, 139–147. Huang, C.P., Wu, M.H., 1977. The removal chromium(VI) from Tan, W.T., Ooi, S.T., Lee, C.K., 1993. Removal of chromium(VI) dilute aqueous solution by activated carbon. Water Res. 11, 673– from solution by coconut husk and palm pressed fibers. Environ. 679. Technol. 14, 277–282. Kemmer, N.F., 1988. Precipitation, 10.18–10.20, Nalco Water Hand- Tiravanti, G., Petrluzzelli, D., Passino, R., 1997. Pretreatment of book, Publisher McGraw Hill (Chapter 10). tannery wastewaters by an ion exchange process for Cr(III) Kowalski, Z., 1994. Treatment of chromic tannery wastes. J. Hazard. removal and recovery. Water Sci. Technol. 36, 197–207. Mater. 37, 137–144. US Department of Health and Human Services, 1991. Toxicological Namasivayam, C., Yamuna, R.T., 1995. Adsorption of Chromium(VI) Profile for Chromium. Public Health Services Agency for Toxic by a low cost adsorbent: biogas residual slurry. Chemosphere 30, substances and Diseases Registry, Washington, DC. 561–578. Weber Jr., W.J., 1967. Sorption from solution by porous carbon. In: Orhan, Y., Buyukgangor, H., 1993. The removal of heavy metals by Faust, S.D., Hunter, J.V. (Eds.), Principles and Applications of using agricultural wastes. Water Sci. Technol. 28, 247–255. Water Chemistry. John Wiley & Sons, New York (Chapter 5).
You can also read
