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P. Sethuraman et.al. / International Journal of Engineering Science and Technology Vol. 2(6), 2010, 1811-1825

Removal of Cr(VI) from aqueous solution using Bacillus subtilis, Pseudomonas aeruginosa and Enterobacter cloacae P. Sethuraman1, N. Balasubramanian2* 1

Department of Chemical Engineering, A.C. Tech Campus, Anna University Chennai, Chennai-600 025, India

2

Department of Chemical Engineering, A.C. Tech Campus, Anna University Chennai, Chennai-600 025, India (E-mail: N. Balasubramanian* [email protected])

Abstract The objective of this study is to investigate the removal efficiency of Cr(VI) by Bacillus subtilis, Pseudomonas aeruginosa and Enterobacter cloacae from aqueous solution under different process conditions. Batch mode experiments were carried out as a function of solution pH, biosorbent dosage, Cr(VI) concentration and contact time. The FT-IR spectra and SEM analysis of the biosorbent were recorded to analyse the number and position of the functional groups available for the binding of Cr(VI) ions and to study the morphology of biosorbent. The batch isothermal equilibrium data were analyzed with Freundlich and Langmuir isotherm models. The kinetic models were examined with pseudo first order and pseudo second order kinetics. The results revealed that the Cr(VI) is considerably adsorbed on bacterial biomass and it could be an economical method for the removal of Cr(VI) from aqueous solution. Keywords: Biosorption, Cr(VI) removal, Bacillus subtilis, Pseudomonas aeruginosa, Enterobacter cloacae, Waste water treatment. 1. Introduction Heavy metal pollution holds a threat for human health and as such life in general. The disposal of heavy metals is a consequence of industrial activities like chemical manufacturing, painting and coating, mining, extractive metallurgy, nuclear and other industries. These metals exert a deleterious effect on the flora and fauna that grow in lakes and streams [1]. Chromium, a highly reactive element with an oxidation state of 6 exhibits stability as Cr(III) and Cr(VI). But Cr(VI) is more toxic to living organisms than the Cr(III) [2]. Furthermore, Cr(III) has limited hydroxide solubility making it relatively immobile and less available for biological uptake. Cr(VI) being powerfully carcinogenic, modifies DNA transcription process thereby causing important chromosomic aberration as quoted by The International Agency for Research on Cancer [3]. The Cr(VI) has also been classified as a group A carcinogen by USEPA based on its chronic effects [4]. Strong exposure of Cr(VI) causes cancer in the digestive tract and lungs [5] and may cause epigastric pain, nausea, vomiting, severe diarrhoea and hemorrhage [6]. Chromium has adverse impacts on aquatic species. It accumulates in fish tissues and at higher concentration causes reduction in fish production [7]. Chromium has wide spread applications in the making of alloys, chrome plating, leather tanning, batteries, dyes, paints, welding, catalysis and wood preservatives [8]. Chemical reduction [9], nanofiltration [10], ion exchange [11], adsorption on silica composites [12], and on activated carbon [13] are the most preferred methods to remove Cr(VI). Such trivial methods have several disadvantages as it requires more energy and chemical requirements. They also have low yield efficiency, lower cost effectiveness and are difficult to implement. Bioaccumulation/Biosorption process uses the fuctional groups present in bacteria, algae and fungi to form complexs with metal ions & thereby aid in the removal, has recovered greater significances [14,15].

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P. Sethuraman et.al. / International Journal of Engineering Science and Technology Vol. 2(6), 2010, 1811-1825 In live bacterial cells besides surface accumulation, metal ions have the possibility to enter into the cytoplasm through the specific carrier system and thus transport processes in bacteria can been studied [16,17]. Bacteria are able to synthesis macromolecules such as polysaccharides, protein, nucleic acid, humic substances and uronic acid usually called extracellular polymeric substances (EPS). EPS are metabolic products of bacteria and they originate from their lysis or hydrolysis [18] and they restrain the respective functional compounds like carboxyl, phosphoric, amine and hydroxyl groups. Extracellular polymeric substances react with the biological matter in the effluent that is to be treated [19]. EPS involved in cell aggregation [20], produce a protective fence for cells to keep at bay harmful substances and accept the accumulation of inorganic ions from the environment [21]. In existing researches, several microorganisms can reduce Cr(VI) concentration, including genus of Pseudomonas, Bacillus, Enterobacter, Escherichia coli, Shewanella, and several other bacterial isolates were also reported. This work aims to investigate the biosorption behavior of B.subtilis, P.aeruginosa and E.cloacae. These biomass growths were taken to treat Cr(VI) ion present in aqueous solutions. The effect of solution pH, contact time & biosorbent dosage on the removal of Cr(VI) ion were studied. The experimental data were analysed by fitting it into Langmuir and Freundlich adsorption isotherm models and pseudo first order and pseudo second order kinetic models. The Cr(VI) samples were characterized by Atomic absorption spectroscopy (AAS). The presence of functional compounds in the biomass that may have a role in the biosorption process was confirmed by Fourier Transform Infrared spectroscopy (FTIR) and Scanning Electron Microscopic (SEM) investigations. 2. Materials and Methods 2.1. Microorganism growth and preparation for biosorption Bacterial cultures of B.subtilis (MTCC-121), P.aeruginosa (MTCC-424) and E.cloacae (MTCC-509) were received from Microbial Type Culture Collection (MTCC) Chandigarh, India. The nutrient broth was prepared using the prescribed growth medium containing beef extract 1.0g, yeast extract 0.1g, peptone 5.0g, sodium chloride 5.0g and distilled water 1.0 litre. The bacterial culture was sterilized in an autoclave maintained at 15 lbs for 15 minutes and maintained as per the guidelines of MTCC.

2.2. Preparation of samples Synthetic Cr(VI) solution was prepared using potassium dichromate salt of Cr(VI). All chemicals used in the study are of analytical grade and were obtained from Ranbaxy Fine Chemicals Ltd., India. The potassium dichromate (K2Cr2O7) solutions were prepared using double distilled water. Cr(VI) solution of varying concentrations were acquired by diluting the stock solution. 1N Sodium hydroxide (NaOH) and 1N hydrochloric acid (HCl) solutions were used to adjust the solution pH. Characterization of the biosorbent was carried out using Scanning electron microscope (SEM) and FT-IR studies. Scanning electron microscopic (SEM; Model 6360-JSM; JEOL, Japan) study was also conducted to observe the surface texture and porosity of biosorbent. Fourier transform infrared spectroscopy (FTIR; Model Tensor 27, Bruker Optic GmbH, Germany) spectrometer was used to determine the type of functional groups in bacteria responsible for the Cr(VI) metal bioadsorption. Atomic absorption spectrophotometer (AAnalyst 800; PerkinElmer, USA) was used for the determination of Cr(VI) content in standard and treated solutions respectively. The pH of the solution was measured using a pH Meter (L1 120; Elico - India) containing standard buffer solutions. Centrifuge (R-24, Research Centrifuge. REMI- India) was used for the biosorbent centrifugation. Incubated shaker (Scingenic Biotech/ORBITEK- with temperature mode) at a constant speed of 150 rpm was used for different biosorbents.

2.3. Batch biosorption of Cr(VI) Batch experiments were conducted with solutions prepared from stock solution. A known quantity of metal concentrate solution was taken in several flasks and biomass was added. The flasks were stirred at a constant speed of 150 rpm at room temperature for 6 hrs. Test samples were collected at regular intervals of time, centrifuged and filtrated for the estimation of Cr(VI) concentration. Experiments were carried out over a wide range of operating conditions & the percentage of Cr(VI) removal, i.e., R(%) was calculated using the following equation:

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R (%) 

(C0  Ce )  100 C0

(1)

where C0 and Ce represent initial and final Cr(VI) concentration. The biosorption capacity can be estimated as

qe 

C0  Ce   V M

(2)

where qe is the amount of adsorbed metal ion onto the biomass at equilibrium (mg/l), M represents the amount of biomass in the suspension (L), and V is the volume of the suspension (L). 3. Result and Discussion This work, studied the effect of Cr(VI) ions on the growth rate of B.subtilis, P.aeruginosa and E.cloacae cells and the biosorption properties of Cr(VI). 3.1. Bacterial Growth curve The growth rate of B.subtilis, P.aeruginosa and E.cloacae in Nutrient medium under aseptic conditions is studied. It can be noticed from Fig. 1 the growth rate of bacteria has four distinct phases, such i) lag phase, ii) log phase, iii) stationary phase and iv) death phase. In lag phase, bacteria adapt to the environment & begin to grow. In this phase, the bacteria are not yet able to divide. The bacterial growth cycle, synthesis of RNA, enzymes and protein molecules may occur. Exponential phase or log phase is characterized by doubling of cell. During stationary phase, the growth rate slows down as a result of nutrient depletion and adsorption of toxic products. During death phase, due to lack of nutrients, nonavailability of space and Oxygen lead to the decay of bacteria. The optical density recorded at 600nm taken for B.subtilis, P.aeruginosa and E.cloacae shows good exponential phase within 4 hrs. Maximum biomass growth occurred in 20 hrs for B.subtilis, while it took 8 hrs to 16 hrs for P.aeruginosa and E.cloacae to show good exponential growth. The biomass for biosorption process for Cr(VI) removal was taken at the highest exponential phases for all the bacteria.

Fig. 1. Growth curve of B.subtilis, P.aeruginosa and E.cloacae on nutrient medium. pH = 6.6, dosage = l g, agitation rate = 150 rpm, temperature = 27 ºC.

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3.2. Effect of initial pH In these batch biosorption experiments, the influence of pH on Cr(VI) biosorption was studied using synthetic solutions. In synthetic solution, Cr(VI) biosorption strongly depends on initial solution pH Fig. 2. The effect of pH in batch system was studied by varying the pH from 2 to 9. Due to the activity of hydrogen ions, at low pH values the Cr(VI) uptake was observed to be on the higher side in B.subtilis and E.cloacae. As the pH increases to 6 the amount of Cr(VI) uptake also increases in P.aeruginosa. At lower pH values, the functional groups transfer H+ ions which indicates that majority of the binding sites were occupied. When the pH increases, the concentration of H+ ions decrease and negatively charged biomass surface can interact with the positively charged metal ions. At different pH condition the plenty of carboxyl groups of alginate, sulfonate groups of fucoidan and hydroxyl groups in other polysaccharides are found to play an important role in metal binding [22]. Fig. 2, very clearly shows that E.cloacae has a higher percentage removal of Cr(VI) (94.9%) compared to B.subtilis (37.5%) and P.aeruginosa (67.9%).

Fig. 2. Effect of initial pH on the biosorption of Cr(VI) ion (conditions: in B.subtilis; biosorbent dosage = 0.2 g, contact time = 6 h, temperature = 27 ºC, concentration = 100 mg/l, agitation rate = 150 rpm, the same parameter condition were kept as a constant for P.aeruginosa and E.cloacae.

3.3. Effect on biomass dosage The effect of biomass dosage on the biosorption of Cr(VI) ions in batch system was examined by varying biomass dosages from 0.2 to 1 g. Fig. 3 shows the results obtained by varying B.subtilis, P.aeruginosa and E.cloacae dosages during biosorption. This study noticed that removal efficiency increased with increase in biomass dosage. Increase in biomass concentration generally increases the level of biosorption of Cr(VI) ions because of an overall increase in surface area of the biosorbent, which in turn increases the number of binding sites [23]. The metal uptake decreases by increasing the biosorbent dosage, which can be explained due to the interaction of metal ion which influence the high biosorbent dosages available in metal ions are insufficient to cover all the exchangeable sites on the biosorbent, usually resulting in low metal uptake. The interference into binding sites due to increased biosorbent dosages cannot be overruled, which results in low specific uptake rate [24].

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Fig. 3. Effect of Biomass dosage on biosorption of Cr(VI) ion (conditions: in B.subtilis and E.cloacae; pH = 2, time = 6 h, concentration = 100mg/l, temperature = 27 ºC, agitation rate = 150 rpm. P.aeruginosa; (pH-6 and other parameter constants are same as the B.subtilis).

3.4. Effect of initial metal concentration To investigate the effect of initial metal concentration, the experimental results for the biosorption of Cr(VI) ions onto B.subtilis, P.aeruginosa and E.cloacae. The initial Cr(VI) ion concentrations were changed between 25-200 mg/l values in each experiment set shown in Fig. 4. In the given time duration of 6 hrs the maximum Cr(VI) removal percentage in B.subtilis was (63 % at 100 mg/l) while in as P.aeruginosa it was (77.9 % at 25 mg/l) and in E.cloacae it was (86.2 % at 25 mg/l). In 200 mg/l B.subtilis gave the best removal rate compared to other bacteria, but initially in the lag phase growth removal was less in B.subtilis. The maximal removal percentage noted in 25 mg/l was P.aeruginosa (78%), E.cloacae (86%) and B.subtilis (33.5%) respectively. This figure clearly shows that as concentration increases there is a gradual decrease in the percentage removal in all bacteria in their biosorption capacities. In the metal ion adsorption due to increasing electrostatic interactions, present on the site affinity for metal ions gradually moves downward [25].

Fig. 4. Effect of Concentration on biosorption of Cr(VI) ion (conditions: B.subtilis; biosorbent dosage = 1g, pH = 2. P.aeruginosa; biosorbent dosage = 1 g, pH = 6. E.cloacae; biosorbent dosage = 0.2 g, pH = 2 for all bacteria other parameter conditions are same such as agitation rate = 150 rpm, contact time = 6 h, temperature = 27 ºC.

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P. Sethuraman et.al. / International Journal of Engineering Science and Technology Vol. 2(6), 2010, 1811-1825 3.5. Biosorption isotherms In biosorption, the metal ions adsorbed at the surface of bacterium cell wall & can well be represented by conventional isotherms. An attempt is made to test for the Langmuir and Freundlich isotherms models on the experimental data. Biosorption process availing batch technique needs an understanding of the interaction between metal ions and biosorbent. The Langmuir and Freundlich isotherm models are first applied to these interactions. In this study, the Scatchard plot has been used in order to obtain more compact information about the interaction between metal ions and the biosorbent [26]. 3.5.1. Freundlich isotherm The Freundlich isotherm is an empirical model that relates the adsorption intensity of the sorbent to the biosorbent. The isotherm is adopted to describe reversible adsorption and is not restricted to monolayer formation. The mathematical expression of the Freundlich isotherm model can be given as

Inqe  InK F  bF InCe

(3)

A plot of Inqe versus InCe gives a straight line with slope KF and intercept bF. The values of KF and bF along with the linear regression co-efficient (R2) for the present experimental conditions have been obtained and are given in Table 1. Fig. 5, the best correlation coefficient (R2) was observed from Freundlich isotherm model on P.aeruginosa (0.998) that matches satisfactorily with the experimental observation.

Langmuir constant Bacteria

Freundlich constant

KL

b

R2

KF

bF

R2

B.subtilis

10.526

0.0874

0.997

2.7179

1.9503

0.972

P.aeruginosa

3.496

0.2977

0.988

15.299

E.cloacae

25.641

0.0030

0.999

366.437

9.5499 6.9444

0.998 0.911

Table 1. Langmuir and Freundlich biosorption isotherm for Cr(VI) on B.subtilis, P.aeruginosa and E.cloacae.

Fig. 5. Freundlich isotherm models for B.subtilis, P.aeruginosa and E.cloacae on Cr(VI) ion.

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P. Sethuraman et.al. / International Journal of Engineering Science and Technology Vol. 2(6), 2010, 1811-1825 3.5.2. Langmuir isotherm The Langmuir isotherm assumes monolayer deposition of adsorbents on homogenous biosorbent can be given as

qe 

K L bC e 1  bC e

(4)

The binding constant (KL,) and the sorbent capacity (b) are estimated by plotting Ce/qe against Ce. The model simulations along with experimental observations for Cr(VI) with the experimental values of KL and b along with the linear regression co-efficient (R2) are given in Table 1 and Fig. 6 respectively. The B.subtilis (0.997), E.cloacae (0.999) showed a best correlation coefficient (R2) respectively.

Fig. 6. Langmuir isotherm models for B.subtilis, P.aeruginosa and E.cloacae on Cr(VI) ion.

3.5.3. Kinetics The prediction of batch kinetics is required for the design of industrial scale reactors. Kinetic models were employed to analyze the adsorption rates of chromium. The experimental batch biosorption kinetics data was modeled using pseudo first order and pseudo second order kinetics. The linear form of pseudo first and second order kinetic equations are given as,

log( qe  qt )  log( qe ) 

k1t 2.303

(5)

and

1 t t   2 qe q k 2 qe

(6)

where q and qe are the amount of metal adsorption per unit weight of biosorbent (mg/l) at time t, and at equilibrium respectively, and k1 and k2 are the adsorption rate constants. The pseudo first order and pseudo second order rate constant k1, k2 and qe were estimated from the model and are presented in Table 2-4 from B.subtilis, P.aeruginosa and E.cloacae with corresponding correlation coefficients respectively. The Fig. 7, E.cloacae with metal concentration shows time versus log (qe/qt) Fig. 8 shows time versus t/qt for E.cloacae at various initial concentrations. In this investigation it was noticed that the pseudo second order kinetics match satisfactorily with the experimental data.

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Pseudo first order

Pseudo second order

B.subtilis Concentration (mg/l)

qe

K1

R

qe

K2

R2

25

1.986

0.350

0.987

2.146

0.386

0.907

50

2.747

0.439

0.991

3.058

0.416

0.952

100

9.204

0.868

0.964

7.813

0.193

0.965

150

6.223

0.812

0.958

8.621

0.561

0.995

200

5.742

0.854

0.975

8.772

0.618

0.996

2

Table 2. Biosorption kinetics for Cr(VI) on Bacillus subtilis.

Pseudo first order

P.aeruginosa Concentration (mg/l)

qe

K1

R

25

2.372

0.688

50

2.958

100

2

Pseudo second order qe

K2

R2

0.992

2.353

0.349

0.901

0.739

0.972

2.849

0.543

0.958

4.246

0.905

0.974

4.046

0.565

0.982

150

3.845

0.787

0.927

3.584

0.568

0.972

200

2.249

0.796

0.971

3.076

1.487

0.994

Table 3. Biosorption kinetics for Cr(VI) on P.aeruginosa.

Pseudo first order

Pseudo second order

E.cloacae Concentration (mg/l)

qe

K1

R12

qe

K2

R22

25

8.791

0.304

0.958

9.615

0.108

0.938

50

13.77

1.358

0.975

16.129

0.101

0.967

100

11.66

0.412

0.830

22.727

0.276

0.997

150

13.61

0.412

0.963

17.857

0.125

0.979

200

7.227

0.398

0.642

20.408

0.801

0.999

Table 4. Biosorption kinetics for Cr(VI) on E.cloacae.

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Fig. 7. Pseudo first order plot of time versus t/qe/qt for the Cr(VI) on E.cloacae at various metal concentration

Fig. 8. Pseudo second order plot of time versus t/qt for the Cr(VI) on E.cloacae at various metal concentration

3.6. Characterization of the biosorbent and biosorption mechanism 3.6.1. FT-IR spectral analysis The IR spectra for B.subtilis, P.aeruginosa and E.cloacae are given in Fig. 9, Fig. 10 and Fig. 11 respectively. They are done in order to characterize the biosorbent. The FTIR spectra of Cr(VI) loaded & unloaded biosorbent in a range of 400–4000 cm−1 were analyzed in order to discover which functional groups are responsible for the biosorption process. IR spectra clearly showed the difference between the loaded and unloaded Cr(VI) metal ion in all biosorbent. It has intense absorption bands around 3500–3200 cm-1, which represent the stretching vibrations of amino groups in table 5 which clearly states the vibration peak. A very strong absorption peak was observed for both Cr(VI) loaded & unloaded biomass at about 2900 cm−1. The spectra of biomass also display absorption peaks at

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P. Sethuraman et.al. / International Journal of Engineering Science and Technology Vol. 2(6), 2010, 1811-1825 2915 and 2841 cm−1 [27] corresponding to stretching of the C-H bonds in the methyl and methylene groups present in the cell wall structure. The IR spectrum with loaded and unloaded biomass indicates that amine groups are present, and these are likely to be responsible for Cr(VI) binding.

Wave numbers (cm-1) 3600-3750 3400-3550 3100-3500 2500-3400 2700-2950 1400-1660 1280-1430 1160-1420 900-1350 900-1380 800-880

Intensity shape Sharp Sharp Strong-broad Weak-broad Variable Variable Variable Variable Variable Variable Medium-strong

Assignment O-H stretching O-H stretching N-H stretching O-H stretching C-H stretching N-H bending C-H bending O-H bending C-N stretching C-O stretching N-H and C-H rocking

Table 5. Assignments of Infrared absorption bands

Fig. 9. FTIR Spectra of B.subtilis in Cr(VI), (a) with and (b) with out metal loaded

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Fig. 10. FTIR Spectra of P.aeruginosa in Cr(VI), (a) with and (b) with out metal loaded

Fig. 11. FTIR Spectra of E.cloacae in Cr(VI), (a) with and (b) with out metal loaded

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P. Sethuraman et.al. / International Journal of Engineering Science and Technology Vol. 2(6), 2010, 1811-1825 3.6.2. Scanning electron microscopy (SEM) analysis The biosorption of three different bacteria B.subtilis, P.aeruginosa and E.cloacae were analyzed by scanning electron microscopy to understand its surface morphological characterization of bacteria with the magnification of 10,000 x. An SEM micrograph of unloaded and loaded biomass is shown for B.subtilis in Fig. 12, P.aeruginosa in Fig. 13 and E.cloacae in Fig. 14. Over the biosorption period, the morphology of the bacteria had undergone remarkable physical disintegration. Fig. 12, represents the integrated and cluster arrangement that occurs before and after biosorption which clearly shows that metal particles adhere on the surface of the B.subtilis cell wall. Fig. 13, shows that flakes like morphology with inscribed cluster formation occurred, after usage of metal ion in biosorbent thus increasing the active surface area. Fig. 14, both loaded and unloaded metal ion on biosorbent, fly ash particles are covered by precipitated and complex formed by the heavy metal ions.

(a)

(b)

Fig. 12. SEM micrograph of (a) before and (b) after Cr(VI) loaded on B.subtilis

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(a)

(b)

Fig. 13. SEM micrograph of (a) before and (b) after Cr(VI) loaded on P.aeruginosa

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(a)

(b)

Fig. 14. SEM micrograph of (a) before and (b) after Cr(VI) loaded on E.cloacae

4. Conclusion The Biosorption experiments were carried out to remove Cr(VI) using B.subtilis, P.aeruginosa and E.cloacae in batch systems. The following conclusions can be made based on the analysis: 1. The investigation shows that B.subtilis, P.aeruginosa and E.cloacae are abundantly available and are cost effective. They can be used as biosorbent for the removal of Cr(VI) metal ion from aqueous solutions. 2. The biosorption performance is affected by various parameters, like pH, contact time, biosorbent dosage and initial metal ion concentrations.

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P. Sethuraman et.al. / International Journal of Engineering Science and Technology Vol. 2(6), 2010, 1811-1825 3. The kinetic and equilibrium data fitted well with the pseudo second order kinetic model and the Langmuir isotherm model, respectively. The biosorption of Cr(VI) onto B.subtilis, P.aeruginosa and E.cloacae was found as biosorption on the homogeneous surface via chemisorption process. 4. Of these three bacteria, the maximum biosorption capacity with good rate constant and correlation coefficient was obtained in E.cloacae. 5. The proposed biosorption system was successfully applied to real effluent with Cr(VI) metal ions.

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