INTRODUCTION:

Chromium salts are highly toxic^1-3. But they are extensively used in various chemical processes in a number of industries. Major consumptions of Chromium salts are in leather industry as tanning agent¹ and in metallurgical industries to provide corrosive resistance and smooth and shining surface to metal alloys. Other places of its utility are: in the manufacture of paints, inks, wood preservatives and rubber ceramics and in fungicides; in Photography and textiles industry^1-3.

It is reported in Literature that more than 1,70,000 tonnes of Chromium wastes are being discharged to the environment annually as a consequence of industrial manufacturing activities^4,5 and if proper care is not envisaged in the treatment and disposal methods of wastes, the problem of contamination of natural waters occurs leading to significant threat to public health.

This is due to the fact that Chromium ions are non-degradable and persistent in nature and moreover, they undergo the biomagnifications and get accumulated in the food chain in unspecific compounds inside the cells of living organisms causing toxicity at cellular level^3-6

Chromium exists in trivalent and hexavalent states and of these, hexavalent ion is 500 folds more toxic than trivalent ion ⁶and the Chromium (VI) is hazardous to health when its limit in potable waters exceeds 0.05 ppm ^2,3,6 and it is attributed to its high oxidation potential and because of it , it can easily penetrate in to biological membranes and cause health hazards⁶. Chromium (VI) causes irritation, corrosion of skin and respiratory track and lung carcinoma. Chromium (VI) poisoning results in acute tubular necrosis of the kidney and consequent death^{2, 3,6}. Prolonged contact with certain Chromium compounds may produce allergic reaction and dermatitis in some individuals².The United Nations Food and Agriculture Organization recommended maximum level for irrigation waters is 100 µg/ml and US. EPA primary drinking water standard is 0.1 ppm of total Chromium⁶.

Traditionally, Chromium is removed by reducing Chromium (VI) to Chromium (III) using some reducing agents such as SO₂, Ferrous Sulphate or Sodium bisulphate and then precipitating Cr (III) as Chromium Hydroxide by the addition of bases^7-9. The major problem with this type of treatment is the disposal of the precipitated Chromium Hydroxide. It is generally done by the land filling and this involves high costs and further, there exists a great danger of groundwater contamination. From the environmental point of view, these methods do not solve the problem but it transfers it from one phase to another phase.

Various methods for the removal of Chromium from wastewaters have been reported in literature.^5-37. They include Chemical reduction ^5,7-9 Flocculation⁷, Electrolysis and Electroplating^10,11, Nanofiltration ¹², bioaccumulation¹³, ion exchange¹⁴ ,adsorption on silica composites ^15,16, activated carbons^17-19, fly Ash ²⁰, modified zeolites^21,22 and bone charcoal ^23,24. A number of patents also are existing in this regard ²⁵. These techniques, apart from being economically expensive have disadvantages like incomplete metal removal, high reagent and energy requirements and generation of toxic sludges or other waste products that require disposal. Hence efficient and environmentally friendly methods are thus needed.

Increasing interest is seen in using Lignocelluloses materials such as leaves, barks, biomasses, and Agricultural wastes in controlling pollution problems^26-37. The biological approaches for the removal and accumulation of Chromium from aqueous solutions during the recent past have shown interesting results,^18,19,26-37, which have stimulated continuous and expanding research in this field and these methods offers a potential alternative to the existing methods of detoxification and the recovery of toxic and valuable ions from industrial discharges/ polluted waters. Some Researchers have tried the use coconut fiber²⁸, eucalyptus bark³⁰, maple sawdust³¹, Hevea brasilinesis sawdust activated carbon³², waste tealeaves and rice husk ³³and Neem Sawdust³⁵. Comparative studies on the removal of Chromium using different absorbents has also been made recently^36,37. Asha Latha Singh reviewed the various methods available in literature in removing Chromium from waste waters using microbes ^24..

The present work aims to explore the sorption abilities of powders of barks and their ashes of some of the plants in effecting the Chromium (VI) removal from polluted waters.

METIRIALS AND METHODS:

All chemicals used were of analytical grade. 50 ppm stock solution of Chromium (VI) was prepared using A.R. Potassium Dichromate in double distilled water. 6N Sulphuric acid and 0.25% of Diphenyl carbazide in 50% of acetone were employed.

(A)Adsorbents:

Barks powders and their ashes of various plants were tried for the removal of Chromium from synthetically prepared polluted waters by optimizing various physicochemical parameters viz., pH, conc. of sorbent and time of equilibration. It has been observed that the barks powders of Ficus benghalensis, Tamarindus indica and Acasia nilotica Indica, have affinity towards the Chromium (VI) ions.

Ficus benghalensis

Tamarindus indica

Acasia nilotica Indica

Ficus benghalensis, the banyan, is a large and extensive growing tree of the Indian subcontinent having propagating roots which grow downwards as aerial roots and once these roots reach the ground, they grow into woody trunks that can become indistinguishable from the main trunk. It belongs to Moraceae famlily. Tamarindus indicia is a long-lived tropical evergreen tree with a spreading crown and feathery evergreen foliage and fragrant flowers yielding hard yellowish wood and long pods with edible chocolate-colored acidic pulp and it belongs to Fabaceae family and is cultivated in tropical regions.. Acasia nilotica Indica is one of the species of acacia belongs to Fabaceae and is a medium sized deciduous tree with crooked and forked truck and is grown well in South Asia.

The Barks of Ficus benghalensis , Tamarindus indica and Acasia nilotica Indica , were scraped , washed with tap water followed by distilled water and then sun dried. The dried materials were powdered to a fine mesh of size: <75µ and activated at 105^oC in an oven and then employed in this study. Further, these Barks were burnt to ashes and these ashes were also used in this work.

(B) Adsorption experiment:

Batch system of extraction procedure was adopted ^[7-9]. Carefully weighted quantities of adsorbents were taken into previously washed 1 lit/500 ml stopper bottles containing 500ml/250ml of Potassium Dichromate solution of predetermined concentrations. The various initial pH values of the suspensions were adjusted with dil HCl or dil NaOH solution using pH meter. The samples were shaken vigorously in mechanical shakers and were allowed to be in equilibrium for the desired time. After the equilibration period, an aliquot of the sample was taken for Chromium determination. Chromium (VI) was determined Spectrophotometrically by using “Diphenyl Carbazide” method ³⁸.

Estimation of Chromium (VI): An aliquot amount of Chromate sample was taken in a 50 ml volumetric flask. To it 1ml of 6N Conc. H₂SO₄ solution and 1ml of Diphenyl Carbazide solution were added successively and the solution was then diluted to the volume and mixed well. Then O.D. of the developed color was measured against blank at 540 nm using U.V. and Visible Spectrometer.

Thus obtained O.D Value was referred to a standard graph (drawn between O.D and Concentration) prepared with known amounts of Chromium by adopting the method of Least Squares to find concentration of Chromium in unknown solutions.

The sorption characteristics of the said adsorbents were studied with respect to the time of equilibration, pH and sorbent dosage. At a fixed sorbent concentration, the % removal of Chromate was studied with respect to time of equilibration at various pH values. The results obtained were presented in the Graph Nos. A: 1-6 and B: 1and2. To fix the minimum dosage needed for the maximum removal of the Chromate ions for a particular sorbent at optimum pH and equilibration times, extraction studies were made by studying the % of extraction with respect to the sorbent dosage. The results obtained were presented in the Graph Nos. C: 1 and 2.

(C) Effect of Interfering Ions:

The interfering ions chosen for study are the common ions present in natural waters, viz., Sulphate, Nitrate, Chloride, Phosphate, Fluorides, Carbonate, Calcium, Magnesium, Copper , Zinc and Nickel. The synthetic mixtures of Chromium (VI) and one of the interfering ions were so made that the concentration of the interfering ions was maintained at ten fold excess than the Chromium (VI) ion concentration. 500 ml of these solutions were taken in stopped bottles and then correctly weighed optimum quantities of the promising sorbents were added. Optimum pH was adjusted with dil. HCl or dil. NaOH using pH meter. The samples were shaken in shaking machines for the desired optimum periods and then the samples were filtered and analyzed for Chromium (VI). % of extraction was calculated from the data obtained. The results are presented in the Table No. 1.

Table No: 1: Effect of interfering Ions on the Extractability of Chromates with different Bio-sorbents:

*Sl..No*	*Interfering ions :* *Ten fold excess*	*% of maximum extractability in synthetic water samples containing 50.0 ppm of Chromium (VI)*
		*Barks of Fichus benglensis*		*Barks of Tamarindus indica*		*Barks of Acasia indica*
		Powders (mesh:75 µ) :pH:2;2.5 hrs and 2.0 gms/lit	Ashes pH:2;2.0 hrs and 1.5 gms/lit	Powders (mesh:75 µ) pH:2;2.5 hrs and 2.0 gms/lit	Ashes pH:2;2.0 hrs and 1.5 gms/lit	Powders (mesh:75 µ) pH:2;2.5 hrs and 2.0 gms/lit	Ashes pH:2;1.5 hrs and 1.5 gms/lit
1.	Without interfering ions:	90.4%;	99.8%	92.5%	99.2%	95.2%	98.0%;
2.	SO₄^2-	88.4 %	95.3%	91.7%	74.6%	90.6%	82.1%
3.	NO₃^2-	86.5%	98.7%	90.4%	94.2%	94.7%	96.4%
4.	Cl^–	83.2%	96.2%	90.9%	93.5%	93.9%	97.0%
5.	PO₄^3-	75.0%	90.4%	89.6%	91.2%	95.0%	78.3%
6.	F^-	89.3%	89.7%	84.3%	96.1%	90.2%	90.6%
7.	CO₃^2-	90.0%	98.6%	86.1%	92.3%	88.0%	94.8%
8.	Ca²⁺	87.2%	97.8%	84.3%	95.4%	86.4%	97.1%
9.	Mg²⁺	84.9%	94.2%	89.8%	98.8%	89.8%	93.9%
10.	Cu²⁺	83.9%	93.6%	90.3%	99.0%	91.7%	92.8%
11.	Zn²⁺	85.3%	91.3%	91.6%	98.9%	92.6%	98.0%
12.	Ni²⁺	89.9%	92.6%	92.0%	97.4%	90.8%	90.4%

Table No: 2: Extractability of Chromium (VI) in Different Industrial and Natural Samples using Bio-sorbents

samples collected at different places

Cr(VI) in the Sample

(found on analysis)

% of Maximum extractability

Barks of Ficus benglensis

Barks of Tamarindus indica

Barks of Acasia indica

Powders (mesh:<75 µ)

:pH:2;2.5 hrs

and 2.0 gms/lit

Ashes

pH:2;2.5 hrs

and 1.5 gms/lit

Powders

(mesh:<75 µ)

pH:2;2.5 hrs

and 1.5 gms/lit

Ashes

pH:2;2.0 hrs

and 1.5 gms/lit

Powders

(mesh:<75 µ)

pH:2;2.5 hrsand

2.0 gms/lit

Ashes

pH:2;1.5 hrs

and 1.5 gms/lit

Tannery Industry Effluents:

12.5 ppm

21.5 ppm

14.5 ppm

89.6%

82.6%

89.5%

96.6%

93.6%

89.5%

90.0%

91.5%

78.3%

96.1%

89.5%

93.8%

93.2%

78.5%

88.3%

95.5%

79.6%

89.2%

Chromate Plating Industry Effluents:

21.5 ppm

24.5 ppm

15.8 ppm

79.5%

89.8%

76.5%

91.5%

89.8%

89.5%

79.9%

85.3%

90.2%

79.5%

90.5%

85.3%

82.3%

91.3%

79.6%

76.2%

86.5%

Natural Lake Samples

(fed with known amounts of Chromates)

15.0 ppm

20.0 ppm

25.0 ppm

85.1%

81.6%

88.3%

85.1%

91.6%

88.0%

74.9%

76.6%

85.8%

93.4%

95.5%

86.7%

94.6%

92.5%

85.1%

95.5%

93.8%

79.6%

RESULTS AND DISCUSSION:

The barks and their ashes of Ficus benghalensis, Tamarindus indica, and Acasia nilotica Indica have been found to have affinity towards Chromates. The sorption characteristics of these adsorbents have been studied with respect to various physicochemical parameters such as pH, time of equilibration and sorption concentration. The results are presented in the Graph No. A: 1-6; B: 1and2; and C: 1and2. The following observations are significant:

1. Time of equilibration: % of extractability increases with time for a fixed sorbent and at fixed pH and after certain time, the extractability remains constant i.e. an equilibrium state has been reached (Graph Nos. A: 1-6). In other words, there will not be any further adsorption after certain time of equilibration time. As for example, in the case of bark powders of Ficus benghalensis, at pH:2, % of extraction is 60.1% at 0.5 hrs, 64.0% at 1.0 hr, 76.0% at 1.5 hrs, 88.0% at 2.0 hrs, 90.4% at 2.5 hrs ,90.4% at 2.5 hrs and 90.4% above 2.5 hrs.(vide Graph No.A:1). The same trend is noticed in the case of other sorbents.

2. pH: % of extraction is found to be pH sensitive. As pH decreases, % of extraction is found to be increasing (Vide Graph: B: 1and2). As for example, with the bark powders of Ficus benghalensis, % of extractability, is found to be 20.0% at pH: 10, 30.0% at pH: 8, 58.0% at pH:6, 69.0% at pH:4 and 90.4% at pH: 2 after an equilibration time of 2.5 hrs. In the case of ashes of Ficus benghalensis, the extraction of Chromium (VI) is found to be 40.0% at pH: 10, 46.0% at pH:8 , 59.0% at pH:6, 70.0% at pH:4 and 99.8% at pH:2 after an equilibration time of 2.0 hrs. Similarly, with the bark powders of Tamarindus indica, % of extractability is found to be 26.0 % at pH:10, 34.0% at pH:8, 38.0% at pH:6, 54.0% at pH:4 and 92.5% at pH: 2 after an equilibration time of 2.5 hrs while with the ashes of barks of Tamarindus indica, % of extractability is found to be: 40.0% at pH:10, 54.0% at pH:8, 64.0% at pH:6, 76.0% at pH:4 and 99.2% at pH:2 after an equilibration time of 2.0 hrs. In the case of bark powders of Acasia nilotica Indica, % of extractability is found to be 34.0% at pH: 10, 40.0% at pH: 8, 76.0% at pH: 6, 84.0% at pH: 4 and 95.2% at pH: 2 after an equilibration time of 2.5 hrs while with its ashes, % of extractability is found to be 38.0%, 60.0% 64.0%, 84.0% and 98.0% respectively after an equilibration time of 1.5 hrs.

3. The maximum % of extractability is found to be marginally more with bark ashes than with raw powders of barks.

4. In most of the sorbents, time of equilibration needed for maximum extractability of Chromate is found to be less for the ashes than with the raw powders of barks. With bark powders of Ficus benghalensis , Tamarindus indica and Acasia nilotica Indica , the equilibration time needed for maximum extraction is found to be 2.5 hrs at optimum pH: 2 while with their ashes, the optimum equilibration times is 2.0 hrs in the case of Ficus benghalensis and Tamarindus indica and 1.5 hrs with Acasia nilotica Indica (vide Graph Nos. A:1-6).

5. Sorbent Concentration: The sorbent dosage needed for the maximum extraction of Chromate is found to be less in the case of ashes of bark powders than with raw bark powders. Sorbent concentration for maximum extraction at optimum conditions of pH and equilibration time is found to be 2.0 gram/lit for the bark powders of Ficus benghalensis while with its ashes optimum sorption concentration has been reduced to 1.5 gms/lit. Similarly, with the bark powders of Tamarindus indica optimum sorbent concentration is 2.0 gm/lit but with their ashes, 1.5 gm/lit is adequate. With bark powders of Acasia nilotica Indica, the sorbent concentration needed is found to be 2.0 gms/lit while with its ashes, 1.5 gms/lit. is sufficient.

6. The % of maximum extractability of Chromium (VI ) at optimum conditions of pH and equilibration time are found to be 90.4%, 92.5% and 95.2% in the case of bark powders Ficus benghalensis , Tamarindus indica and Acasia nilotica Indica respectively (vide Graph Nos.A:1-3).

With ashes of Ficus benghalensis , Tamarindus indica and Acasia nilotica Indica , % of maximum extractability is found to be 99.8%, 99.2% and 98.0% respectively at optimum conditions of pH: 2 and equilibration times of 2.0 hrs, 2.0 hrs and 1.5 hrs respectively (vide Graph Nos. A: 4-6).

7. Interfering Ions: The extractability of Chromate ions in presence of tenfold excess of common ions found in natural waters, namely, Sulphate, Nitrate, Chloride, Phosphate, Fluoride, Carbonate, Calcium, Magnesium, Copper, Zinc and Nickel ions, has been studied. The results are presented in Table No. 1.

· Cations envisaged marginal effect on the % extractability of Chromate with the sorbents of the present work at the optimum conditions of time of equilibration, pH and sorbent concentration. % of extraction of Chromate is found to be between 83.9% to 99.9% with bark powders of Ficus benghalensis and 92.6% to 97.8% with its ashes; 84.3% to 92.0% with bark powders of Tamarindus indica and 95.4% to 99.0% with its ashes; 86.4% to 92.6 % with bark powders of Acasia nilotica Indica and 80.4% to 98.0% with its ashes.

· Anions:

§ SO₄^2- is found to be interfering to with the ashes of Tamarindus indica and % of extractability decreases from 99.2% to 74.6% (vide S. No. 2 of Table Nos: 1) and in the reset of the sorbents, the extractability of Chromates is seldom effected.

§ Ten fold excess of NO₃^- , Chloride, Fluoride and Carbonate are found to have less interference with the % of extractability of Chromate in all the sorbents of interest (vide Sl. Nos. 3,4,6 and7 of Table No. 1).

§ Phosphate is found to be interfering with the extraction of Chromate with bark powders of Ficus benghalensis and in the bark ashes of Acasia nilotica Indica (Table No.1: Sl No.5). However, the extractability never comes down below 74.6% and with the remaining sorbents; the % of extractability is almost un-effected.

Applications:

The Applicability of the methodologies developed in this work have been tested with respects to the real samples of diverse nature, collected from the sewages/effluents of Tannery and Chrome plating industries and also in natural lakes (fed with known amounts of hexavalent Chromium). The results have been presented in the Table No: 2.

It is found that the sorbents developed in this work are remarkably successful in removing Chromates in all the samples studied at optimum conditions of pH, equilibration time and sorbent dosage. % removal of Chromates is found to be: 76.5% to 89.8% with bark powders of Ficus benghalensis; 74.9% to 91.5% with bark powders of Tamarindus indica; 78.5% to 93.2% with bark powders of Acasia nilotica Indica (vide Table No.2).

With the ashes of barks, the % of extractability of Chromium in nine different samples is found to be ranging from 85.1% to 96.6% with Ficus benghalensis; 79.5% to 96.1% with Tamarindus indica; 76.2% to 95.5% with Acasia nilotica Indica (vide Table No.2).

DISCUSSIONS:

The available data is in adequate to propose sound theoretical explanations for each observation made as it needs surface studies on the interactions of adsorbent and adsorbate using more sophisticated instruments and methodologies and it is beyond the aims of this work.

However, the observations may be accounted as follows:

· The bio-sorbents may be having some natural compounds which have affinity towards Chromate at low pH values.

· Further, these natural substances have -OH/COOH groups and their dissociation is pH dependent and this imparts weak anion exchange ability at low pH values and weak cation exchange ability at high pH values as per the equilibrations:

At high pH values: Adsorbent-OH Adsorbent-O^-+ H⁺

Adsorbent-COOH Adsorbent-COO^- + H⁺

At low pH values: Adsorbent-OH + H⁺ Adsorbent-OH₂⁺

· In basic solutions, the hexavalent Chromium presents as tetrahedral Chromate ions CrO₄^2-‑, between pH: 2 to 6, the species: HCrO₄^- and the orange red dichromate ions Cr₂O₇^2- are in equilibrium and at pH values less than 1, the main species is H₂CrO₄.

· So, Chromate being an anion in the pH range: 2-6, is get sorbed by the leaves materials at low pHs due to weak anion exchange nature and thus results in higher % of removal at low pH values. As pH increases, the cation exchanging nature prevails and this results in low % removal of Chromate ions.

· Further, these materials may have naturally existing some divalent or trivalent metal ions that form sparingly soluble salts with of Chromate ^-which are gelatinous in nature and are being trapped in the matrixes of the bio-adsorbents and thus resulting in the increase of % removal of Chromate. This is more so in the case of ashes as adsorbents, which are oxides of some metal ions.

· Ashes are the oxides of some heavy metals containing large amounts of silica. These ashes, contains ‘-OH’ and ‘–O-’ groups. The observed behavior of increase in extractability with the decrease in pH may be understood in the same lines as described in the case of raw powders of leaves. In fact, the transition pH for silica from anion exchanging nature to cation exchanging nature is 3 ^39-41and this supports the proposed logic for the observed behavior.

· The decrease in the rate of adsorption with the progress in the equilibration time may be due to the more availability of adsorption sites initially and are progressively used up with time due to the formation of adsorbate (Chromate) film on the active sites of adsorbent and thus resulting in decrease in capability of the adsorbent.

· The observations made with respect to the foreign ions are also confirming as per the expected nature of extraction. % of extractability is less affected even in the presence of ten fold excess of cations chosen for study but some interference to some extent is found with some anions especially Sulphate and Phosphate.

CONCUSSIONS:

1. Barks Powder and their ashes of Ficus benghalensis , Tamarindus indica and Acasia nilotica Indica have been found to have strong affinity towards Chromate at low pH values.

2. % of removal of Chromate is pH sensitive and also depends on sorption concentration and time of equilibration

3. The conditions for the maximum extraction of Chromate at minimum dosage and equilibration time have been optimized.

4. Sorbent dosage and time needed for the maximum removal of Chromate is less for the ashes of barks than with the raw bark powders.

5. Ten fold excess of common cation ions present in natural waters, viz., Ca²⁺, Mg²⁺, Cu², Zn²⁺and Ni^{2 +}have less affected the % of extraction of Chromate at optimum conditions of pH, equilibration time and sorbent concentration. Nitrates, Chloride, Fluoride and Carbonate even when present in ten fold excess have showed marginal interference with all the bio-sorbents of the present study. Ten fold excess of Sulphates and Phosphates in one or two sorbents have showed interference to some extent but however, the extraction has never come down below 74.6%.

6. We claim 90.4%, 92.5%, and 95.2% of removal of Chromate from synthetic waters with the bark powders of Ficus benghalensis , Tamarindus indica and Acasia nilotica Indica respectively at pH:2 and at optimum equilibration time and sorbent concentrations. The % of removal is found to be 99.8%, 99.2%, and 98.0% in the case bark ashes of Ficus benghalensis , Tamarindus indica and Acasia nilotica Indica respectively at pH 2 and at optimum conditions of equilibration time and sorbent concentrations.

7. The suitability of the developed methodologies in this work is tested with respect to diverse waste water samples collected in nine different places. The developed methods have been found to be remarkably successful in removing the Chromate from industrial effluents and natural lake samples as detailed in Table No: 2.

ACKNOWLEDGEMENT:

The authors thank UGC for financial aid for conducting this research work.

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Received on 23.12.2011 Modified on 18.01.2012

Asian J. Research Chem. 5(2): February 2012; Page 225-233