1. INTRODUCTION:

Some metal ions excess to their permissible limits in drinking water may cause adverse effect to our health. Some metals, even in trace amounts may cause serious problems to human lives. Copper ion is important for digestion of food within the permissible limit (0.05 mg L^-1), but its excess amount causes metabolic disorders¹, while chromium in its + 6 state (one of the common states) is more toxic and carcinogenic than in other oxidation states². Copper may produce symptoms similar to those of silicosis and allergic contact dermatitis. Chronic copper poisoning causes hemolytic anemia, neurological abnormalities and corneal opacity. Some sources of copper in effluents include mining, fertilizer manufacturing, petroleum refining, paints and pigments, steel works, electroplating etc.³ Cr(VI) may also come from various industrial sources like metal finishing wastes, leather tanning, textiles, electroplating etc.⁴. The treatment of drinking water containing metal ions is a challenging problem. Various techniques like ion exchange, solvent extraction, chemical precipitation etc. are available for removal of metal ions from water, but these are found to be cost intensive and unaffordable for large quantities of water.

Removal of Cu (II) from aqueous solutions through adsorption process using zeolite, modified rice husk and industrial wastes as adsorbents have been reported by different workers^3,5,6. On the other hand, removal of chromium (III) ions using silica sand was studied to optimize the physiochemical conditions for maximum removal ⁷. Precipitation process has also been reported as an effective method for removal of Cr (III) from aqueous solutions⁸. There are more reports of different methods for removal of metal ions from water by various solid adsorbents^8-12. Removal of different heavy metals in most of these reported works is not based on studies on drinking water contamination but on industrial wastewater. In the present work, a detail analysis of drinking water of tribal dominated Chirang District of Assam has been carried out. Based on these results inexpensive rice husk has extensively been studied as adsorbent for adsorption of Cu (II) and Cr (III) ions from solutions containing these two ions with concentrations ten times higher than that found in these drinking water samples. Langmuir’s and Freundlich’s adsorption isotherms are drawn on the basis of these results.

2. MATERIALS AND METHODS:

2.1. Preparation of the adsorbents:

Rice husk of Malsara rice, a local sali rice species found in Bongaigaon area was cleaned, washed with de-ionized water and burnt under normal conditions. The residue was dried at 110⁰C in an oven for a period of 12 hours before use.

2.2. Characterization of rice husk:

Chemical composition of rice husk was determined by XRF (PANalytical, Model EXIOS Fluorescence Spectrometer). The material was characterized by Powdered XRD, FTIR and N₂ adsorption. XRD patterns were obtained in a Rigaku Miniflex Table Top XRD (copper source, wavelength, λ= 1.54 nm, K_α radiation) and FTIR (Perkin-Elmer spectrum RXIFT-IR system). Nitrogen adsorption measurements were carried out using a MICROMERITUS TRISTAR 3000 instrument at 77 K. Prior to nitrogen adsorption, samples were degassed for 2 h at 323 K.

2.3. Preparation of solutions:

13 sources of drinking water consisting of ring wells, tara pumps, tube wells and deep tube wells covering the Chirang district of Assam, have been analyzed by standard methods^13,14. The highest concentration (described as highest field concentration or HFC) of each of Cu (II) and Cr (III) was found to be 0.08 mgL^-1(Table.1). A stock solution with concentration 20 times more than that of HFC of Cu (II) and Cr (III) in drinking water samples of the district was prepared using de-mineralized water. This solution was termed as 20×HFC solution. Another solution of concentration equals to ten times more than the highest field concentration was prepared from this solution and termed as 10×HFC. These solutions were then acidified with 4-5 drops of concentrated nitric acid to bring the metal completely into aqueous medium.

Table1. Field concentrations of Cu²⁺ and Cr³⁺species found in the samples collected from different sites of Chirang district of Assam.

Sl. No.	Site	Source	Conc. of Cr³⁺ions mgL^-1	Conc. of cu²⁺ionsmgL^-1
1	Kajolgaon	DTW	0.03	BDL
2	Kajolgaon	RW	0.04	BDL
3	Kajolgaon	TP	BDL	0.01
4	Sidli	TP	BDL	0.30
5	Sidli	RW	0.006	BDL
6	Kashikotra	TW	0.08	0.03
7	Basugaon	DTW	BDL	BDL
8	Dangtola	DTW	BDL	0.02
9	Dangtola	RW	BDL	0.01
10	Chapaguri	TW	BDL	0.08
11	Dhaligaon	DTW	BDL	BDL
12	Manglagaon	TP	0.05	BDL
13	Manglagaon	RW	0.01	BDL

DTW=Deep Tube Well, RW= Ring Well, TP=Tara Pump, TW=Tube Well, BDL=below detection level.

2.4. Adsorption study:

A bed of glass-wool was made in the lower part of a uniform glass tube fitted with a stop cock. Two grams of rice husk adsorbent was placed on the bed of adsorbent and then 60 mL of the solution (10 ×HFC) was allowed to pass through it for single elution. The process was repeated with the same volume of the solution for 2, 3 and 4 elutions taking two grams of the adsorbent separately. Identical methods were applied using 1, 3 and 5 g of the same adsorbent for 1, 2 and 3 elutions and 4 g of the adsorbent for 1 elution. All the solutions were analyzed by Atomic Adsorption Spectrophotometer (AAS) before and after passing through the adsorbent to investigate the extents of adsorption of Cu ²⁺ and Cr³⁺ions present in the water samples.

3. RESULTS AND DISCUSSIONS:

3.1. Adsorbent characteristics:

The chemical composition of rice husk ash on dry basis is given in Table 2.It is observed that silica concentration is the maximum followed by iron (III) oxide and alumina. XRD patterns of the rice husk ash show that all the components are in amorphous state (Fig.1). FTIR spectra of rice husk ash are shown in Fig. 2. Pore size distribution of rice husk ash from BJH adsorption data is depicted in Fig.3. There are two types of pores one having size less than 4 nm and the other falls in the mesopore region of 60 to 200 nm. The surface area and pore volume the rice husk sample has been found to be 132.77 m²g^-1 and 0.060597 cm³g ^-1 respectively [Figs. 4(a) and (b)].

Fig. 1 XRD of Rice husk ash

Fig. 2. FTIR of Rice husk ash

Fig. 3 Pore size distribution of rice husk as obtained from BJH Adsorption data

Fig. 4(a) BET surface area plot

Fig. 4(b) BJH adsorption cumulative pore volume plot

Table 2: Chemical composition of rice husk ash on dry basis

Serial no.	Constituent	Mass fraction (%)
1	Silica	38.2
2	Alumina	11.0
3	Ferric oxide	18.9
4	Potassium oxide	3.4
5	Sodium oxide	3.1
6	Calcium oxide	0.1
7	Manganous oxide	-
8	Loss on ignition (L.O.I.)	25.3

3.2. Adsorption isotherm:

3.2.1. Langmuir Isotherm:

The Langmuir adsorption model was applied to study the adsorption equilibrium of Cu (II) and Cr (III) on rice husk ash .The Langmuir adsorption may be expressed as:

C_e/x/m =1/Q₀ b+ C_e / Q₀

Where, C_eis the residual concentration ofCu (II) and Cr (III) at equilibrium in mgL^-1, x is the amount of Cu (II) and Cr (III) adsorbed in mgL^-1, m is the weight of the adsorbent in gram, Q₀and b are the Langmuir constants related to adsorption capacity and rate of adsorption respectively¹⁵. The linear plot of C_e/x/m versus C_eindicates that the adsorption follows the Langmuir adsorption model for Cu (II) and Cr (III) adsorption in the present study (Fig. 5(a) and (b)).The values of Q₀and b were calculated from the slope and the intercept of the plot and were found to be 3.212 mg/g and 5.48 l/g respectively for Cu (II) while the respective values for Cr (III) are 4.016 mg/g and 4.109 l/g .

Fig. 5(a) Langmuir plot for Cu (II) adsorption

Fig. 5(b) Langmuir plot for Cr (III) adsorption

Table 3(a): Adsorption of Cu (II) from drinking water samples using 1, 2, 3, 4 and 5 g of rice husk adsorbent

Solution	Amount of adsorbent	Concentration of Cu(II) ion in mgL^-1		Percentage of adsorption	No. of elution
Solution	Amount of adsorbent	Before adsorption	After adsorption	Percentage of adsorption	No. of elution
10xHFC	1g	0.8	0.481	40	1
10xHFC	1g	0.8	0.372	54	2
10xHFC	1g	0.8	0.284	65	3
10xHFC	2g	0.8	0.354	57	1
10xHFC	2g	0.8	0.233	71	2
10xHFC	2g	0.8	0.155	81	3
10xHFC	2g	0.8	0.054	93	4
10xHFC	3g	0.8	0.127	84	1
10xHFC	3g	0.8	0.072	91	2
10xHFC	3g	0.8	0.059	93	3
10xHFC	4g	0.8	0.073	91	1
10xHFC	5g	0.8	0.065	92	1
10xHFC	5g	0.8	0.029	96	2
10xHFC	5g	0.8	BDL	100	3

Table 3(b): Adsorption of Cr (III) from drinking water samples using 1, 2, 3, 4 and 5 g of rice husk adsorbent

Solution	Amount of adsorbent	Concentration of Cr (III) in the solution		Percentage of adsorption	No. of elution
Solution	Amount of adsorbent	Before adsorption	After adsorption	Percentage of adsorption	No. of elution
10xHFC	1g	0.8	0.540	33	1
10xHFC	1g	0.8	0.503	37	2
10xHFC	1g	0.8	0.305	62	3
10xHFC	2g	0.8	0.410	49	1
10xHFC	2g	0.8	0.320	60	2
10xHFC	2g	0.8	0.225	72	3
10xHFC	2g	0.8	0.127	84	4
10xHFC	3g	0.8	0.249	63	1
10xHFC	3g	0.8	0.101	84	2
10xHFC	3g	0.8	0.138	83	3
10xHFC	4g	0.8	0.160	80	1
10xHFC	5g	0.8	0.050	84	1
10xHFC	5g	0.8	0.115	84	2
10xHFC	5g	0.8	0.123	85	3

3.2.2. Freundlich Isotherm:

The Freundlich adsorption model was also applied to study the adsorption equilibrium of Cu (II) and Cr (III) on rice husk ash .The Freundlich adsorption may be expressed as:

log x/m =log K_f+1/n log C_e

Where, x is the amount of Cu (II) and Cr (III) adsorbed in mg/100 mL. m is the weight of the adsorbent in gram and C_e isthe residual concentration of Cu (II) and Cr (III) at equilibrium in mg/100 mL¹⁶.

K_f and 1/n are Freundlich constants relating the adsorption capacity and adsorption intensity respectively and are evaluated from the log x/m vs. log C_eplot [Fig. 6(a) and (b)] with the slope 1/n and the intercept log K_f. The linearity of both the curves indicate the validity of Freundlich adsorption isotherms.

3.3. Effect of amount of adsorbent and number of elution

Adsorption of Cu (II) on rice husk ash against amount of adsorbent increases with the amounts of the adsorbent (Table 3(a)). This is naturally acceptable because of availability of more space for adsorption in larger amount of the adsorbent. In all the cases adsorption tends to equilibrate when more than 3 g of adsorbent are taken.

The maximum adsorption (100%) of the copper ion is observed for 5 g of adsorption when same solution is eluted 3 times. Rice husk can not remove 100% of the Cu cations from the solution for any amount less than 5g of the adsorbent per 60 mL for any elution under the present experimental conditions.

Fig. 6(a) Freundlich plot for Cu (II) adsorption

Investigation of adsorption of Cr (III) from water samples reveals that 33, 49, 63, 80 and 84 percents of the metal ion for 1, 2, 3, 4 and 5 g of rice husk ash respectively are possible for single elution (Table 3(b)). The trend is similar against different amounts of adsorbent for 2 and 3 time elutions. The adsorption of the metal ion increases with increase in the number of elution. However equilibrium appears to establish when more than 3 g of adsorbent are taken for 2 and 3 elutions. In case of single elution late arrival of equilibrium (after 4 g) is observed.

Fig. 6(b) Freundlich plot for Cr (III) adsorption

The maximum adsorption of the metal ion from 60 mL of water samples containing 0.8 g of Cr (III) was found to be 85 % in case of 5 g of the adsorbent for 3 elutions Adsorption appears to have a tendency of attaining equilibrium after 4 nos. of elution.

The study reveals that a better result for Cu (II) removal is observed from water in comparison to Cr (III).

4. CONCLUSION:

The study revealed that adsorption of Cu (II) and Cr (III) on rice husk ash follows Langmuir as well as the Freundlich adsorption models. Adsorptions of Cu (II) on rice husk ash was found to increase with the amounts of the adsorbent for a certain number elution and the maximum adsorption of Cu (II) was observed beyond 4 g of the adsorbent. The extent of adsorption of the metal increases with increase in the number of elution for fixed amount of the adsorbent. Complete removal of Cu (II) is applicable from drinking water if 60mL of water containing 0.8g of Cu (II) is passed through 5g of rice husk of 3 times. More or less same trend was observed for adsorption of Cr(III) from water solution when rice husk ash was used as adsorbent. The removal of Cr (III) from 60 mL of drinking water sample containing 0.8g of the metal was found to be maximum in case of 5 g of the adsorbent after 3 times elution. Further, according to the present study it was found that rice husk ash removed Cu(II) more efficiently than Cr(III) under similar conditions.

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Received on 10.02.2010 Modified on 19.02.2010

Asian J. Research Chem. 3(3): July- Sept. 2010; Page 626-630