Acid free Cr(Vi) Reduction in Contaminated Ground Water Collected from COPR Dump Site

Introduction

Industrial pollution is one of the evils that challenge the earth, air, water and land. Thus there is a striving need for efforts to manage its natural resources, eco system and bio-diversity from the dumping of hazardous waste on land. Many of the countries in the world are prone to land and ground water pollution that endures from Industrial activity. Developed countries like USA, Spain, Italy, France, UK, and developing countries like India, Chine, Pakistan also have COPR dump site¹.

Improper dumping of untreated residue obtained from Chromite ore processing industries significantly contaminate the soil and ground water by leaching Cr(VI)  and cause environmental impact to the aquifers and surface water and health problems to the surrounding inhabitants^2-3. Much studies have already been done to remediate Cr(VI) contaminant in ground water by physical remediation⁴, chemical⁵, electrochemical⁶ and biological transformation⁷. Adsorbents synthesized from groundwater treatment residuals, eucalyptus bark, activated carbon manufactured from palm shell with polyethyleneimine, agricultural waste biomass, distillery sludge were admitted for the removal of Cr(VI)^8-9.  Elimination of Cr(VI) has also been carried out by Phytoremediation¹⁰, constructed wetland¹¹ and electrochemical methods¹².

The most commonly used technology is the precipitation of metal ions as hydroxides, under appropriate pH conditions¹³. Precipitation of toxic Cr(VI) by converting it in to Cr(III) using reducing agents like ferrous sulphate heptahydrate and  monohydrate, Sodium meta bisulphite, Sodium sulphite, Sodium dithionite, hydrazine, hydroxylamine   and Ferrous sulphate have been used^14-15. Addition of 1.0% of tin chloride reduces 25mg/L Cr(VI), present in the hydrated cement¹⁶.

Industries manufacturing glass use Tin chloride as reducing agent ^17-18. Colour pigments, sensitized paper, mordant agent in dyeing unit, soldering flux, plastic based electronic components and tin chemicals manufacturing utilizes tin chloride as one of the reactants. Radiopharmaceuticals used as identifying lyophilized kits contain 99mTc-labelled tracers are being prepared with tin chloride. Visualization of heart, blood, lung and bone using nuclear  medicine as diagnostic agents also uses  tin chloride ^19-21. Preservative and colour retention agent existing in food additives contains tin (II) chloride ²².

The main objective of the present contribution is to find out the effectiveness of SnCl₂ for reduction of Cr(VI) to zero level in the contaminated groundwater at TCCL study area located at Ranipet, Vellore, Tamilnadu, India.  Findings explore a green chemistry approach for the complete eradication of Cr(VI)  using Tin Chloride.

Methods and Materials

Laboratory Studies

The analytical grade SnCl_2, HCl and  NaOH   procured from E-Merck India Ltd., were used as such. High purity distilled water was used for analysis. Adjustment of pH was carried out using analytical grade HCl and NaOH (pH meter 240 Elico L1614). Absorbance measurements were carried out in the spectrophotomter UV – 3200, Lab India at 540nm using diphenylcarbazide and heavy metals were determined on an Atomic Absorption Spectrometer (Shimadzu 6800).

The percentage removal of hexavalent chromium was calculated as

Cr(VI)=[( C_i – C_f)/C_i] × 100

Chromium Contaminated Groundwater

The samples of chromium contaminated groundwater (CGW) was collected from the bore wells located at the TCCL site, Ranipet, Vellore, Tamil Nadu, India. The samples were collected in polypropylene containers. The amount of Cr(VI) was 1976 mg/L. The other parameters  measured were turbidity – 25.4NTU, total dissolved solid – 9960 mg/L, electrical conductivity – 15,659 μs/cm, Sodium – 125-140 mg/L, Ammonium -1.5 – 4 mg/L, Sulphate – 2500-2860 mg/L, Hardness of calcium  310-350 mg/L, Hardness of magnesium – 1820-1910 mg/L, Chloride – 595 mg/L and  Nitrate – 210mg/L.

Results and Discussion

Treatment of Synthetic Cr(VI) contaminated Water

The concentration of Cr(VI) in the sample was brought to 1976 mg/L by dissolving  5.588 g of K₂Cr₂O₇ in 1000 ml of distilled water²³. The pH and reducing agents²⁴  are the two important variables in removing Cr(VI) by reduction and precipitation method. When the two variables are combined together, the net result shows the synergy effect of the reducing agent and pH in the conversion of Cr(VI) to Cr(III).  In order to ascertain the individual efficiency of reducing agent and the effect of pH in the elimination of Cr (VI) , the studies were adopted  by varying the concentration of reducing agent with and  without  adjusting the pH externally. The results obtained are presented in Table-1.

Table 1: Reduction of Cr(VI)  without adjusting pH in SCW

Dosage of SnCl2 (mg/L)	pH after addition of SnCl2	Concentration of Cr(VI) after 15min (mg/L)	% reduction after 15min
200	4.6	1429	27.68
400	3.4	1070	45.85
600	2.88	676.8	65.74
800	2.75	507.5	74.31
1000	2.7	308.76	83.87
1200	2.66	148.55	92.48
1400	2.68	0	100

 

Results implied, incremental effect of reducing agent on reduction of Cr(VI). Additional increase in dosage after 1400mg/L, displayed no significant change in the percentage reduction. Allowing the treated sample for long duration did not show any appreciable change in the conversion of Cr(VI).

Earlier report²⁵ recommended pH 2 as the convenient pH for the reduction of Cr(VI) to Cr(III). Based on this, an approach was made at pH 2 by adding HCl along with SnCl₂. The results presented in Table-2 reveals that the presence of acid did not alter appreciably the reducing power of stannous chloride. This is in agreement with the earlier report by Marks Neidle et.al.,²⁶.

Table 2: Conversion of Cr(VI) at pH-2 in SCW

Dosage of SnCl2 (mg/L)	Amount of Cr(VI) after 15min (mg/L)	% conversion after 15min
200	1287.5	34.84
400	1046.5	47.03
600	614.0	68.92
800	410.0	79.25
1000	220.8	88.82
1200	90.8	95.40
1400	0	100

 

Comparison of the results presented in Table-1, Table-2 and Fig-1 reveals that the present observation took forward the precipitation studies towards green chemistry approach.

Figure 1: Conversion of Cr(VI)  with and without addition of acid in SCW Click here to View figure

 

Treatment on contaminated Cr(VI) groundwater.           

The concentration of hazardous Cr(VI) in  groundwater indicates not only the toxicity of the solution but also the oxidizing power of the contaminated water. Based on the results obtained from synthetic Cr(VI) water, an experiment on real contaminated Cr(VI) ground water was executed at the optimized concentration of reducing  agent.

Table 3: Reduction of Cr(VI) without external addition of acid  in CGW (1976mg/L).

Dosage of SnCl2(mg/L)	pH	Cr(VI) Concentration after reduction	% reduction
200	5.2	1504	23.88
400	4.2	1220	38.25
600	3.61	986	50.10
800	3.34	608	69.23
1000	3.102	383.5	80.59
1200	3.06	85	95.69
1400	2.769	0	100

 

Table 4: Reduction of Cr(VI) without external addition of acid  in CGW(1271 mg/L).

Dosage of SnCl2 (mg/L)	pH	Cr(VI) Concentration after reduction	% reduction
200	6.2	730	42.56
400	4.6	429.8	66.18
600	3.4	193	84.81
800	2.9	52.7	95.85
1000	2.6	0	100

 

Comparison of   Table-3, Table-4 and Figure-2 shows that the amount of stannous chloride reduces with reduction in concentration of Cr(VI) present in untreated water.

Figure 2: Conversion of Cr(VI)  without addition of acid in SCW and CGW Click here to View figure

 

The treated water was analysed for other important parameters and the results obtained are given in Table-5. The amount of Cr(VI) and total Cr in the treated water fall within the regulatory level.

Table 5: Results of the various parameters before and after treatment for CGW at pH-9 precipitation.

Parameters	CGW	Treated CGW	Disposal standard
pH	6.22	7.1	5.5-9.5
Cr(VI) (mg/L)	1976	0.00	0.05
Total Chromium (mg/L)	2399.9	0.0256	2.00
Chloride( mg/L)	135	5494	1000
Sulphate (mg/L)	2526	1139	1000
BOD (mg/L)	53	13	30
Cadmium (mg/L)	0.0549	0.0142	2.0
Nickel (mg/L)	0.2737	0	3.0
Copper (mg/L)	0.1200	0.0230	3.0
Lead (mg/L)	0.6236	0.052	0.1
Zinc (mg/L)	2.8893	0.042	1.0
Total Dissolved Solids (mg/L)	4200	10000	2100

 

Conclusion

The observations in the above results reveal that the elimination of Cr(VI) can be achieved without adding acid externally. This observation has not been reported elsewhere. Further, the level of Cr(VI),  total Cr and pH of the discharge water are within the recommended level. Shortest time duration reported in this paper is the first observation reported so far. The high value of TDS can be reduced by Reverse Osmosis Process. To handle the sludge generated in this process, solidification²⁷ method is under investigation.

Acknowledgement

The authors gratefully acknowledge M/s. Tamilnadu Waste Management Limited, certified by National Accreditation Board for Testing and Calibration Laboratory, SIPCOT Industrial Estate, Gummudipoondi – 601201, Tamilnadu, India for their kind permission to carry out all the analytical studies on free of cost.

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