Biosorption of Trivalent Chromium from Wastewater

Research Article

James Kanagaraj Thamizharasan Senthilvelan Ramesh C. Panda Rathinam Aravindhan Asit B. Mandal Leather Processing Division, CSIR – Central Leather Research Institute, Adyar, Chennai, India.

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Biosorption of Trivalent Chromium from Wastewater: An Approach towards Green Chemistry Chromium is one of the metals used in industry that create environmental toxicity and pollution problems. In the present study, chromium in wastewater was removed by a biosorption method using a bacterial culture of Bacillus pumilus. A mathematical model representing the biosorption kinetics was formulated, supporting the Langmuir isotherm for the adsorption of chromium. Fourier transform infrared and scanning electron microscopy studies were used to characterize the adsorption of chromium by the bacterial biomass. The analysis of pollution parameters (chemical oxygen demand, total organic content, total dissolved solid, total suspended solid) suggest a significant reduction of the pollution load. The results show that the method developed here is very effective for the removal of chromium from an aqueous environment. Keywords: Bacillus pumilus, Biosorption, Chromium, FT-IR, Pollution reduction, Scanning electron microscopy Received: December 19, 2012; revised: May 20, 2014; accepted: July 09, 2014 DOI: 10.1002/ceat.201200716

1

Introduction

Chromium is an essential micronutrient required for the growth of many organisms. However, at high concentrations it is toxic, carcinogenic, and teratogenic. Chromium has been designated as a priority pollutant by the US Environmental Protection Agency (EPA) [1]. It is released into the environment by a large number of industrial operations such as electroplating, chromate manufacturing, dye and pigment manufacturing, and wood preservation, in the leather tanning industry and the manufacture of alloys, and as corrosion inhibitor in conventional and nuclear power plants [2, 3]. Chromium(III) is used in the leather industry as a tanning agent, for converting hide/skin into leather by reacting it with collagen fibers and thus stabilizing it [4–7]. In chrome tanning, the sectional stream contains as much as 1500–3000 ppm of chromium. The environmental regulating agencies are very stringent on the discharge of chromium. As per the stipulated norms for the discharge of tannery effluents into the inland water bodies, the total chromium concentration should not exceed 2 ppm. This has called for technological interventions in meeting the standards for the discharge of tannery effluents containing chromium [8–10].

– Correspondence: Dr. James Kanagaraj ([email protected]), Leather Processing Division, CSIR – Central Leather Research Institute, Adyar, Chennai 600020, India.

Chem. Eng. Technol. 2014, 37, No. 10, 1741–1750

Conventional methods for the removal of chromium from aqueous solution include precipitation, lime coagulation, ion exchange, reverse osmosis, and solvent extraction. All these methods have disadvantages such as incomplete metal removal, high reagent and energy requirements, and the generation of toxic sludge or other waste products which, in turn, require careful disposal. Therefore, new technologies such as biosorption are of interest. In biosorption, highly toxic pollutants, especially from industrial and tannery waste, are separated and converted or degraded into less toxic innocuous substances, by using biological materials such as plants and microorganisms [11–14]. Microorganisms can physically remove heavy metals from aqueous solution through either bioaccumulation or biosorption. In bioaccumulation, metals are transported from the outside of the microbial cell through the cellular membrane, whereby the metal is separated. In biosorption, the biological material, by using its functional groups, forms complexes with the metal ions; the mechanism of biosorption usually involves the formation of a chemical link between the functional groups on the biosorbent and the metal ions [15, 16]. Bacteria have a high surface area to volume ratio and provide a large contact interface; this allows interaction with metals from the surrounding environment [17]. Macromolecules such as polysaccharides, proteins, humic substances, and uronic acid, collectively called extracellular polymeric substances or exopolysaccharides (EPS), are produced by the bacteria as metabolic products. These metabolic products (the EPS) contain several functional groups such as carboxyl, phosphoric, amine, and hydroxyl groups which are negatively charged and aid in binding metal ions.

ª 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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There are different methods available to treat the chromium waste generated from the chrome tanning process [18–22]. However, the chrome recovery and reuse method, which uses magnesium oxide, is one of the most popular methods adopted by tanners. The disadvantage of this method is that chrome recovery is a slow process and leads to the formation of more neutral salts. Several articles are also available explaining how to precipitate chromium by using lime, sodium hydroxide, and sodium bicarbonate [23–25]. They all precipitate chromium through direct precipitation by altering the pH and the basicity of basic chromium sulfate (BCS). Researchers [26] have demonstrated the potential of a chromate-reducing strain of Enterobacter cloacae to remove toxic chromate from wastewaters. They claim that this organism is resistant to high levels of chromate and that it can work in aerobic as well as in anaerobic conditions. Recently, the removal of carcinogenic Cr(VI) using mixed Pseudomonas strains which were isolated from a marshy land and were able to anaerobically reduce Cr(VI) to Cr(III) was reported [27]. But, the presence of sulfates and polyphenols inhibited the metabolic activity of the species. The objective of the present work is to study the biosorption of chromium from tannery wastewater using bacterial biomass and to understand the mechanism of chromium removal through the adsorption kinetics. Also, the aim is to find possibilities of reusing and recycling chromium adsorbed by microbial biomass and to make the present method an effective and more economic alternative to conventional remediation.

2

Materials and Methods

2.1 Materials Nutrient agar, nutrient broth, BCS, minimal medium (g L–1: K2HPO4 4.21, NaH2PO4 3.22, NH4Cl 1.1, MgSO4 6H2O 0.912, pH 7.0), Simmon citrate agar medium, phenolphthalein phosphate agar, gelatin mannitol salt agar, mannitol salt agar medium, sodium carbonate, and potassium bromide (KBr) were used. All microbiological media and chemicals used in this study were purchased from Hi-media and Aldrich Chemicals.

2.2 Isolation of the Microorganism The organism was isolated from chromium-contaminated tannery effluent soil. The isolated organism was inoculated in nutrient agar medium containing Cr(III) to test the chromium tolerance. The organism was subcultured in nutrient agar medium and stored at –4 C. Biochemical tests and staining techniques were carried out for identifying the organism [28].

mal medium is as follows (g L–1): K2HPO4 4.21, NaH2PO4 3.22, NH4Cl 1.1, MgSO4 6H2O 0.912, pH 7.0). A trace of salt solution was added at a concentration of 1 mL L–1; the salt solution was prepared separately to a volume of 100 mL as follows (g): CaCl2 2H2O 4.25, FeSO4 7H2O 0.32, CoCl2 6H2O 0.31, MnCl2 4H2O 0.11. Minimal medium together with Cr(III) was used for the biosorption studies. Separately, sterilized glucose solution was added to the sterilized minimal medium to give an appropriate final concentration.

2.4 Biosorption of Chromium(III) All biosorption experiments were carried out in 250-mL Erlenmeyer flasks containing 50 mL minimal medium and different amounts of chromium(III) ranging from 1 to 5 mg. The bacterial culture was inoculated into the flasks, which were incubated at 32 C, and samples were withdrawn at regular time intervals and centrifuged (10 000 rpm for 10 min). After centrifugation, the supernatant was taken for chromium(III) estimation by a UV-vis spectrophotometer at a wavelength of 372 nm. Each experiment was carried out for a period of 96 h. The entire experiment was carried out under different pH and temperature conditions.

2.5 Fourier Transform Infrared Analysis The biomass samples were dried for a period of 2 h at 800 C and ground in a mortar for a minimum of 5 min. Dilution and homogenization to 0.01 wt % with KBr (spectroscopic grade) were carried out with additional grinding. The discs were pressed in a hydraulic KBr press. The Fourier transform infrared (FT-IR) transmission spectra were then recorded using a Perkin-Elmer Spectrum RX I FT-IR system between 400 and 4000 cm–1.

2.6 Scanning Electron Microscopy with Energy-Dispersive X-Ray Spectroscopy The control and experimental samples were subjected to scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) studies to assess the effects of biosorption of chromium. Both control and experimental samples were gold-coated for 3 s and magnified in different magnification ranges. A JEOL JSM 5300 scanning electron microscope with EDX was used to study the experimental samples.

2.7 Measurement of the Pollution Load 2.3 Culture Medium The minimal medium was dissolved in distilled water and the pH was adjusted to 7.0 with 2 M NaOH. The trace salt solution was prepared separately in distilled water and was stored in a dark bottle for 6–8 weeks. Chromium was added to the minimal medium after sterilization. The composition of the mini-

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The pollution load was estimated before and after biosorption of chromium(III) samples. The chemical oxygen demand (COD), total organic content (TOC), total dissolved solid (TDS), and total suspended solid (TSS) were analyzed by standard methods.



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Results and Discussion

Table 1. Biochemical characteristics of the isolated bacterial strain.

3.1 Organism Isolation Soil sample containing chromium was collected from a tannery effluent treatment plant and cultures of chrome-tolerant bacteria were isolated as follows. The collected soil samples were serially diluted in test tubes using sterile distilled water, and further serially diluted samples were inoculated by the spread plate method on petri dishes containing nutrient agar medium. Ten predominant bacterial colonies were found on the petri dishes, whereby all the colonies were tested for chromium removal. Among the ten bacterial strains, one showed better chromium removal when compared to the other strains. Hence, this particular isolated bacterial strain was used for the present study and the culture was further characterized by staining techniques and biochemical tests. This culture was examined with various biochemical tests for its suitability in this work. The bacterial culture was also subjected to morphological and biological tests and the results are presented in Tab. 1. The microorganism was positive for gram staining and motility; it was rod shaped and yellow in color. It also showed positive results in the citrate test, the Vogues Proskauer test, the triple sugar iron agar test, the mannitol motility test, the catalase test, and the methyl red test, while the results of the indole test, the oxidase test, and the urease test were negative. The indole test is an important test for identifying Bacillus sp. The culture produces pink color in the medium, which is due to the hydrolysis of tryptophan by the enzyme tryptophanase; the subsequent release of indole, pyruvic acid, and ammonia into the medium produces the pink color. Further to this, the culture was inoculated onto petri dishes containing blood agar medium; a transparent clear zone formed around the culture, indicating the ability of the organism to break down the heme group from hemoglobin present in blood. In addition to this, the growth rate of the organism was observed in the presence of NaCl in the fermentation medium, using UV-vis spectrophotometry to assess the optical density (OD) at 600 nm. Rapid growth was observed at 10 % NaCl concentration. However, the growth rate showed a decline beyond the concentration of 10 % NaCl. The biochemical tests and staining technique results revealed 90 % similarity with Bacillus pumilus. So, the isolated organism is identified as Bacillus pumilus.

3.2 Bacterial Growth Fig. 1 shows the growth rates of the bacteria at different pH values, temperatures, and incubation times. The bacterial growth increases with increasing pH from 4 to 4.8 and decreases beyond that. Maximum growth was recorded at pH 4.8. The activity increases with increasing temperature and shows a maximum at 32 C. The bacterial growth was studied for different incubation periods from 0 to 72 h. Maximum activity was recorded at 48 h. Similar observations were made by other researchers [29–31], establishing the validity of the present findings.


Biochemical test

Result

Indole

negative

Citrate

positive

Urease

negative

Triple sugar iron agar

positive

Mannitol motility

positive

Catalase

positive

Oxidase

negative

Methyl red

positive

Vogues Proskauer

negative

Glucose (acid production)

positive

Sucrose

positive

Lactose

positive

Xylose (acid production)

positive

Mannose (acid production)

positive

Hydrolysis of tyrosine

negative

Gram staining

positive

Nitrate reduction

negative

Starch hydrolysis

negative

Motility

positive (motile, non-branching, spore-forming rods)

Hemolysis

positive

Growth in 7 % NaCl

positive

Anaerobic growth

negative

3.3 Removal of Chromium The adsorption of chromium(III) onto bacterial biomass was investigated as a possible alternative method for its removal from aqueous solutions. The adsorption data were obtained from a batch study and were found to fit the Langmuir adsorption isotherm well. The effect of the pH on the adsorption isotherm was investigated at pH values of 2.5–5.5. It was found that, at pH values below 2.5, Cr(III) was not adsorbed, and at pH value above 5.5, Cr(III) was precipitated as Cr(OH)3. Maximum adsorption occurred at a pH of 4.5 [32–34]. The pH plays a very important role in the adsorption of Cr(III) since Cr(III) can form different complexes in aqueous solutions. The adsorption capacity was increased by about 20 % as the temperature was raised from 22 to 42 C. It was concluded that Cr(III) is adsorbed to an appreciable extent on activated carbon and that the adsorption is highly dependent on the pH.


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Table 2. Reduction in chromium levels after 48 h of growth of B. pumilus bacteria at 32 C in minimal medium containing different nitrogen sources.

a)

b)

Nitrogen source

pH (initial)

pH (final)

Cr reduction [mg L–1]

Peptone

3

5.1

5

Yeast extract

3.2

4.9

6.60

Ammonium chloride

3.5

5.3

7.40

Ammonium sulfate

3.7

5.4

6.98

Ammonium nitrate

3.1

5.0

3.58

Urea

3

5.4

2.70

and yeast extract were also better, but ammonium nitrate and urea were not as effective as ammonium salts. Thus, it could be inferred that the pH plays a vital role in the effective removal of chromium by the bacteria [35–39]. By using ammonium salts, peptone, and yeast extract, the pH of the culture medium was maintained at 4.8, which is the optimum pH for growth. Owing to the increased growth of bacteria, the removal of chromium was also better in the presence of these nitrogen sources. Tab. 3 shows the growth of the bacteria in the presence of various carbon sources such as dextrose, maltose, and glycerol, under optimal temperature and pH conditions. It can be seen from the table that chromium reduction is higher in the presence of starch as compared to the other carbon sources. This is due to the fact that starch enhances the bacterial activity and in turn aids in a better removal of chromium.

c)

Table 3. Reduction in chromium levels after 48 h of growth of B. pumilus bacteria at 32 C in minimal medium containing different carbon sources. Carbon source

pH (initial)

pH (final)

Cr reduction [mg L–1]

Dextrose

3

5.5

0.50

Starch

3.2

5.3

1.40

Sucrose

3.5

5

0.80

Maltose

3.7

5.4

1.3

Glycerol

3.1

5.5

0.75

Figure 1. Culture growth vs. (a) pH, (b) temperature [C], and (c) duration [h].

3.4 Effect of Different Carbon and Nitrogen Sources on the Removal of Chromium Experiments were carried out using the optimized growth conditions to identify the effects of different nitrogen sources on the biosorption of chromium. Tab. 2 shows the growth of the bacteria in media containing different nitrogen sources and also the percentage of chromium reduction. The presence of ammonium salts led to a better removal of chromium. Peptone

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Fig. 2 shows the effect of the chromium concentration on the bacterial growth under optimal conditions. It was observed that, under the given experimental conditions, maximum growth of the bacteria occurred at chromium concentrations of 0.04 mg mL–1 (40 ppm). With further increases in the concentration of chromium, the growth of the bacteria was significantly affected. It is therefore inferred that a chromium concentration of 40 ppm results in better bacterial activity, and subsequent studies were carried out using this concentration of chromium. The cell biomass growth at a chromium concentration of 40 ppm is shown in Tab. 4. The growth in terms of cell



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Figure 2. Optimization of the initial chromium concentration in the biosorption process (duration 96 h).

density increases exponentially and attains a peak at 96 h (Tab. 4). Table 4. Biomass production and cell density at a chromium concentration of 0.04 mg mL–1. Time [h]

Cell biomass [g L–1]

0

0

24

0.197

48

0.431

72

0.735

96

1.421

3.5 Effect of Different Inoculum Sizes for the Removal of Chromium Further, experiments were carried out to assess the effect of the culture concentration on the removal of chromium. The obtained results are shown in Tab. 5. It can be observed that, with increasing culture concentration, the amount of chromium removed by the bacterial species is increased. It can also be seen from the table that, by using a culture concentration of 5 mL L–1, the chromium concentration in the supernatant is reduced to 16.0 ppm, while the remaining 24.0 ppm of chromium is present in the biomass. Similarly, the concentrations of chromium present in the supernatant and the biomass are 7.8 and 32.2 ppm, respectively, for a culture concentration of 10 mL L–1. The present findings are in close agreement with earlier observations reported by various researchers [40–44]. When the culture concentration was increased to 20 mL L–1, the concentration of chromium in the supernatant was found to be less than 2 ppm, which is in accordance with the standard discharge norms. When the culture concentration was further increased to 25 mL L–1, the chromium amount present in the


Table 5. Reduction of chromium at different biomass concentrations (B. pumilus). Chromium concentration [ppm]

Concentration of the culture [mL L–1]

Chromium present in the supernatant [ppm]

Chromium % Reduction present in the biomass [ppm]

16

24

60

40

5

40

10

8.0

32

80.5

40

15

5.0

35

88

40

20

2.0

38

95.5

40

25

1.0

39

99.5

supernatant was negligible. Similar results in the literature [45–48] confirm the validity of the present investigation.

3.6 FT-IR Analysis of Chromium Absorbed on Cell Biomass The mechanism of the interaction of chromium with bacterial biomass is of interest and was studied using FT-IR. The spectrum of the chromium-treated biomass (Fig. 3) confirmed the functional groups responsible for metal binding. A band peak at 3200–3500 cm–1 can be observed from the figure and this may be due to the stretching of the hydroxyl O–H bond and the N–H bond of amino groups. The peak observed at 2900– 3000 cm–1 in the bacterial biomass is due to asymmetric stretching of the C–H bond of alkanes. A peak ranging from 1640 to 1650 cm–1 corresponds to the amide bond and the C=O chelate stretch of the -COOH group. The peak at 1404 cm–1 corresponds to symmetric vibrations of the C=O of COO- and peaks in the range of 1300–1067 cm–1 are related to


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Figure 3. FT-IR spectra for studying the interaction of chromium with the bacterial biomass.

carboxyl and phosphate groups. A shift and slight changes in the above-mentioned peak positions in the spectrum of the chromium-treated bacterial biomass sample suggest an interaction of chromium with the functional groups of the bacterial biomass. The main functional groups responsible for the removal of chromium are hydroxyl, carboxyl, carbonyl, sulfonate, amide, imidazole, phosphate, and phosphodiester groups. Thus, it could be inferred that chromium binds to the reactive sites present in the bacteria. The more reactive sites are present, the better is the uptake. This can also be confirmed by the pH, temperature, and culture concentration studies, where under optimum pH and temperature conditions more growth of bacteria resulted in a better uptake of chromium. Similarly, when suitable nitrogen and carbon sources were used, better growth of the bacteria was observed, resulting in increased uptake of chromium. The FT-IR analysis confirmed that the removal of chromium is due to adsorption, which needs to be studied in a detailed manner. Adsorption is a viable method for the removal of chromium and other inorganic and organic chemicals. Adsorbent materials are basically derived from organic wastes, inert materials, and sometimes activated carbon prepared from various raw materials such as sawdust, nut shells, coconut shells, etc. [49–51]. It is a complex process affected by several factors such as the selection of the adsorbent material, particle size, surface area, porosity, etc. [52, 53]. If the adsorbent is not a good one, this may lead to inefficient adsorption. In that case, the adsorbent will not completely remove the compound from the medium. Other limitations are the disposal and high cost of the adsorbents as a recurring problem. The above limitations have forced the researchers to carry out research on this topic. The

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present investigation aims to overcome these limitations by choosing a proper adsorbent for chromium biosorption, such as B. pumilus. The removal of chromium from the medium by the bacterial species is due to adsorption. As the metal molecules bind to active sites of the microbial species, the kinetics of biosorption need to be studied with mathematical modeling. Low et al. [54] have derived mathematical models for the removal of chromium from a synthetic solution and tannery waste by sorption, using moss. From isotherm studies, they found that the sorption of Cr(III) by moss follows the Langmuir isotherm and the maximum sorption capacities are at 15.4 and 13.7 mg g–1 for the synthetic solution and the tannery waste, respectively. Sharma and Goyal [55] proposed mathematical models for the adsorption kinetics of chromium(III) from tannery effluent by using microbial biomass. They compared their kinetic models and correlation coefficient (R2 = 0.998) with that of Ho and Mckay [56] and concluded that it correlates well with the experimental kinetic data. Chromium adsorption from tannery wastewater has not been modeled to understand the adsorption kinetics [57].

3.7 Mathematical Modeling It is assumed that adsorbent particles (microbes) constitute a homogeneous medium. The adsorbate molecules adhere to the surface of the adsorbent particles, slowly enter through diffusion and are also transferred through the adsorbent particles by ‘‘creeping’’ from one adsorption site to another on the solid sur-



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face. Diffusion takes place by Fick’s second law and, slowly, an equilibrium is established between the solid phase and liquid phase concentrations at the surface of the adsorbent particles. Let C0 be the initial concentration of heavy metals (chromium) to be removed and Ce its equilibrium concentration. The adsorbent particles are of radius R, and let r be a variable distance along the radius. The development of a transient mass balance around the adsorbent particles is needed. If q is the dimensionless normalized concentration of the adsorbate inside the adsorbent, then a material balance for this kind of diffusion inside the particle can be written as ¶q 1 ¶ ¶q ¼ 2 Ds r 2 for 0 £ r £ R and t ‡ 0 ¶t r ¶r ¶r

V ðC Ce Þ ¼ Mqav

(8)

where qav is given by (1)

Here, the diffusivity Ds of the adsorbed molecules depends on the concentration. Assuming an average diffusivity (that does not change) along the diffusion path, and for a range of concentrations, Eq. (1) can have a solution. The initial and boundary conditions (BC) are given by q ¼ 0; 0 £ r £ R and t ‡ 0

(2)

¶q ¼ 0 for r ¼ 0 ð1st BCÞ ¶r

(3)

qe kL Ce ¼ for r ¼ R ð2nd BCÞ qm 1 þ kL Ce

by the Langmuir isotherm. This can also be replaced by the 1=n Freundlich isotherm as qs ¼ kL Cs . Eq. (6) represents the dynamics of the adsorbed phase concentration at the solution–adsorbent interface. The solid phase and liquid phase concentrations are in equilibrium at the surface of the adsorbent particle. The adsorption equilibrium is described here by the Langmuir isotherm. The overall mass balance for this closed batch experiment may be given by

(4)

R3 q ¼ 3 av

ZR

qr2 dr

(9)

0

and M is the mass of the adsorbent (g) and Ce is the equilibrium concentration of the metal in the liquid. Eq. (6) can be solved by numerical techniques. The analytical solution of Eq. (1) can be given as D t a i2 p2 s2 2 X ipr R sin qðr; tÞ ¼ e Rr i¼1 R 2 3 ipr ¢ ZR r¢q0 ðr¢Þ sin R 6 7 6 7 t 2 2 Ds 4 5 i R i p R2 l q ipD ð 1 Þ e ðlÞdl s s 0

(10)

0

In the above equation, subscript e is used for equilibrium, m is for maximum, L for liquid, and kL is the association equilibrium constant given by

qr ¼

kL ¼ eEabs =RT If a concentration gradient of metal ions exists outside the adsorbent particle, along the horizontal direction in solution, and considering external mass transfer resistance, Eq. (1) can be rewritten as ¶q 1 ¶ 2 ¶q ¼ 2 r Ds kL ðC1 Cs Þ ¶t r ¶r ¶r

(5)

where C1 is the concentration in the bulk and Cs is the concentration of metals at the surface. If the adsorbate concentration throughout the particle q is considered as constant qav, the material balance can be reduced to RrP ¶qav ¶ ¶q Ds rP kL ðC1 Cs Þ ¼ ¶r ¶r 3 ¶t

(6)

Similarly, the initial and boundary conditions are transformed to q ¼ qs ; C ¼ Cs and

where qs ðlÞ ¼ kC ðlÞ1=n with l as dimensionless time and

qs kL Cs ¼ qm 1 þ kL Cs

(7)

where the equilibrium concentrations become the surface concentration and are replaced in subscript. The 2nd BC is given


ðCs Ce ÞV W

(11)

Here, V is the volume of wastewater (mL), W is the mass of the dry adsorbent (g), and Cs and Ce are the initial and final concentrations of the metal in wastewater, respectively. Eq. (6) has two mass transfer resistance terms; one is Ds relating to surface diffusion and the other one is kf, the external mass transfer due to the concentration gradient in the liquid. The mass transfer mechanism can be described as follows: Initially, there is poor intraparticle diffusion while, with time, adsorption becomes predominant, with the external mass transfer resistance kf becoming poor. The latter part of the adsorption can be explained by the Langmuir isotherm in the present study. The adsorption isotherm explains the physics and kinetics behind the adsorption of adsorbate (Cr) molecules on the adsorbent (microbes). It helps to understand the mechanism of adsorption and the controlling steps, such as mass transfer, reactions, etc. The process may be external diffusion controlled or intraparticle pore diffusion controlled, a pseudo-first-order or pseudo-second-order reaction, etc. In this case, the process is an adsorption type with surface and pore diffusion. Langmuir, Freundlich, and Redlich–Peterson adsorption models were used to analyze the equilibrium data for adsorption. The observed qm values show that the maximum adsorption corresponds to a saturated monolayer of adsorbate molecules on the


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adsorbent surface, with constant energy and no transmission of adsorbate in the plane of the adsorbent surface. The observed KL values show that the adsorbent prefers to bind acidic ions and that speciation predominates in the sorbent characteristics, when ion exchange is the predominant mechanism taking place in the adsorption of chromium ions. The Langmuir constant (kL) is a measure of the amount of chromium adsorbed per unit weight of adsorbent when saturation is attained; it was found to be 10.98. A plot of qe versus time (Fig. 4) reveals that the adsorption reaches saturation after 120 h. A separation factor of RL = 0.9 suggests that (0 < RL < 1) adsorption is favorable, as it is also evident from Fig. 5. The straight-line nature of the curve confirms the monolayer adsorption of Cr molecules at the experimental temperature. With lowering of the temperature, multilayered adsorption may occur. The sorption takes place within particular sites of the adsorbent. Fig. 5 shows the validation of the present model (calculated values) with experimental data as it is evident from the R2 value (= 0.99). The Biot number (Bi = 993) was also calculated for the above case. The morphology of the bacterial biomass for the biosorption process was studied as explained below.

Figure 4. Adsorption kinetics of chromium on the microbial biomass, showing the concentration profile of the adsorbed mass of metal per unit mass of adsorbent with time.

3.8 SEM-EDX Studies Figure 5. Adsorption kinetics of chromium: Validation of the adsorption model/mechanism

SEM-EDX was carried out for the (Langmuir isotherm) with experimental data. chromium-loaded bacterial biomethod (Tab. 7) shows the percentage of reduction of TOC, mass. The SEM image of the chromium-treated biomass samTDS, and TSS at levels of 92, 90, and 93 % in comparison ple (Fig. 6 a–c) shows the deposition of chromium on the surto 94, 92, and 90 % with the chemical method [58–61]. It is face of the bacterial biomass. Biosorption of chromium on the clear from the tables that the present method shows a comsurface of the bacterial biomass is confirmed by the EDX specparable reduction of TOC, TDS, and TSS in the experimentrum (Fig. 6 d). Better binding of chromium onto the biomass tal sample. may be attributed to the functional groups such as hydroxyl, carboxyl, carbonyl, sulfate, amide, imidazole, phosphate, and phosphodiester groups. 4 Conclusions Tabs. 6 and 7 show the measurements of pollution loads like the COD, TOC, TDS, and TSS by the biosorption proChromium(III) was effectively removed from aqueous solution cess, using biological and chemical methods. Magnesium-oxby biosorption onto bacterial biomass. Maximum chromium ide based chemical precipitation of chromium was used for adsorption by the bacterial biomass takes place at pH 4.8, a comparison. From Tab. 6, it can be observed that between temperature of 32 C, and a duration of 48 h. An optimal chro24 and 120 h, the reduction in COD seems to be on par mium concentration of 40 ppm was found to be suitable for with the chemical method. Similarly, the present biological

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a)

b)

c)

d)

Figure 6. SEM-EDX images of (a) a chromium-loaded biomass sample, (b) the bacterial biomass, and (c) the biomass and chromium in the initial hour. (d) EDX spectrum of the chromium-loaded biomass.

growth in this study. A mathematical model representing the adsorption kinetics was formulated for this process. It was found that chromium (monolayer) adsorption follows the Langumuir isotherm and is predominantly supported by its correlation coefficient value (R2 value). It is evident from the study that 99.5 % chromium removal can be achieved using the pre-

sented method. The FT-IR results reveal that various functional groups are responsible for the binding of chromium to the reactive sites of the bacteria. SEM reveals that the chromiumtreated biomass sample shows crystalline deposition. The reductions in COD, TOC, TDS, and TSS were 92, 92, 90, and 93 %, respectively.

Acknowledgement

Table 6. COD measurements. Method

Sample

% COD reduction

0h

24 h

48 h

72 h

96 h

120 h

Biological method

11 660

6730

5560

4866

2328

610

95

Chemical method

11 047

6210

4971

4013

2745

650

94

The authors have declared no conflict of interest.

Table 7. Pollution load measurements. Method

TOC

TDS

TSS

% Reduction

Initial

Final

Initial

Final

Initial

Final

Biological method

352

28

1465

140

458

32

92

90

93

Chemical method

297

16

987

77

340

32

94

92

90


The authors thank the CSIR for funding under the Supra-institutional Project – S&T Revolution in Leather with a Green Touch (STRAIT), CSIR-CLRI communication no. 1084.


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