INTRODUCTION:

Ever increasing industrial activities are responsible for the increasing load of toxic heavy metals in water bodies. Although many water treatment processes like metal precipitation¹, ion exchange, electro dialysis, cementation², solvent extraction, reverse osmosis³ and adsorption are followed, adsorption stands out to be the best of all⁴. Activated carbons are widely used as adsorbents as it is very effective in removing heavy metal ions from aqueous solution. The cost involved in the utilization of commercially available activated carbons is very high and this resulted in the search for newer adsorbents. Thorough study of the literature reveal that there are several methods available to develop low cost adsorbents from natural resources like coconut shell⁵, coir pith⁶, almond shell⁷, rice husk⁸ etc., About 7.5% rural population is involved in the sugar production in our country. Sugar industry is the second largest agro industry in India and a large amount of bagasse is obtained as a waste material. In the absence of co power generation units, bagasse is used as a fuel in jaggery making or as a starting material in hand paper production. Valix et al⁹developed activated carbons with high ash content from sugarcane bagasse by low temperature carbonization.

It was shown to be effective for removing acid dye from aqueous solution. Researchers^10, ¹¹ found the possibility of using low cost adsorbents such as sugarcane bagasse and other biomass to remove heavy metal ions like Cr (VI), Ni (II), Cd (II) and Cr (III) from wastewater. Whereas, amidoximated bagasse was found to adsorb Hg (II), Ni (II), Cr (III), Cu (II) and Pb (II) ions¹². They investigated adsorbents selectivity towards these metal ions. Brazil nutshells and saw dust proved as better adsorbents than sugarcane bagasse for Cd (II) ions whereas, it fared poor in the case of Ni (II) ions¹³. Sugarcane bagasse was treated with succinic anhydride and this incorporated carboxylic acid functions. These newer materials were used to anchor polyamines¹⁴ and put to use for the adsorption of Cu (II), Cd (II) and Pb (II) ions from aqueous single metal solutions. Gurgel et al¹⁵treated non mercerized sugarcane bagasse and twice mercerized sugarcane bagasse with succinic anhydride to get better adsorbents. Sulfurized steam activated carbons were prepared from sugarcane bagasse pith in a single step¹⁶ in the presence of SO₂ and H₂S at 400°C. They found that the capacity to remove metals were better in single metal solutions than in multi metal solutions. Johns et al¹⁷ and Girgis et al¹⁸ prepared activated carbons from sugarcane bagasse. The second group treated sugarcane bagasse with mineral acids and carbonized at 500°C. According to them the metal ions uptake was in the order Pb (II) > Hg (II) > Cd (II) > and Co (II). Higher temperature carbonization yielded carbons with higher surface area and mesopores. Amin¹⁹ prepared different activated carbons from sugarcane bagasse pith by chemical activation with H₃PO₄ and ZnCl₂. The third activated carbon was prepared by pyrolysis and physical activation of the carbon samples obtained in the earlier step at 600°C. Javier et al²⁰ developed activated carbons from sugarcane bagasse by phosphoric acid activation followed by carbonization. Temperature (300 - 600°C), weight ratio of phosphoric acid amount to sugarcane bagasse (R = 1 - 2.5) and carbonization time (0 – 3 h) were applied to obtain different types of activated carbons. According to them temperature higher than 500°C, impregnation ratio higher than 2 and prolonged carbonization resulted in less porous adsorbent. It’s clearly evident from the available literature that carbonization was carried at temperatures above 300°C and lower carbonization temperature yielded carbons with high ash content and mesopores. Hence the thought to investigate the possibility of utilizing sugarcane bagasse as a precursor to get adsorbents by carbonizing with H₂SO₄ at temperature much lower than 300°C initiated this study. The surface modifications of unmodified sugarcane bagasse adsorbent (UM-carbon) was achieved by treating with 2M HCl, 2M NaOH and oxidizing with 50% (v/v) HNO_3.

It is hoped that this study would throw more light on the effects of surface treatment of sugar carbon on the adsorption of two common wastewater contaminants, lead and copper from aqueous solution.

MATERIALS AND METHODS:

Chemicals:

Lead nitrate, Copper nitrate, Sulphuric acid and Hydrochloric acid used in this study were procured from Ranbaxy India Limited and were of analytical grade. Stock solution and working standards were prepared with deionized water.

Carbon Samples:

Sugarcane bagasse obtained from Morinda sugar mill, Punjab (India) was dried in the sun for 2 days. Later, it was cooled to room temperature and shredded in to small pieces. About 100 grams of this material was added to 25 ml of conc. H₂SO₄ acid and heated in an electric oven maintained at 110°C for 24 hours. This mixture was stirred once an hour to get maximum carbonization. Slurry was washed repeatedly with distilled water till it was free from H₂SO₄. This carbon sample was oven dried at 90⁰C for 6 h. Activation of sugar carbon was achieved by chemical treatment with 2M HCl, 2M NaOH at 25°C and with 50% HNO₃ (v/v) at 80°C. These three treated carbon samples were washed several times with distilled water until they were free from CI^-, OH^- and NO₃ ^–ions respectively. These samples were dried in an electric oven at 80^oC for 6 h. Samples were cooled and stored in nitrogen flushed airtight containers. The unmodified sugar carbon, HCl, NaOH and HNO₃ treated sugar carbons are hereafter referred to as UM carbon, SM-A carbon, SM-B carbon and SM-O carbon respectively.

Surface treatment of carbon samples:

(i) HCl Treatment: Exactly 100ml of 2M HCl was added to 10g of UM sugar carbon sample and the suspension was shaken occasionally for 8 h. The carbon sample was washed several times with hot deionized water until the filtrate is free from Cl^- ions. The acid treated carbon sample was then dried in an electric oven for 8h and stored in airtight containers flushed with nitrogen

(ii) NaOH Treatment: Exactly 100ml of 2M NaOH was added to 10g of UM sugar carbon sample and the suspension was shaken occasionally for 8 h. The carbon sample was washed several times with hot deionized water until the filtrate is free from OH^- ions. The base treated carbon sample was dried in an electric oven for 8h and stored in airtight containers flushed with nitrogen.

(iii) HNO₃ Oxidation: Exactly 100ml of 1:1 HNO₃was added to 10 g of UM sugar carbon and the suspension was heated on a water bath at 80°C till the volume is reduced one half. The carbon sample was washed several times with hot deionized water until the filtrate is free from NO₃^- ions. The HNO₃oxidized carbon sample was dried in an electric oven for 8h and stored in airtight containers flushed with nitrogen.

Characterization of carbon samples:

Sugar carbon samples were characterized by FTIR (Perkin Elmer), XRD (XPERT-PRO model of PAN analytical) and SEM (JSM - JEOL) studies. Microprocessor based pH meter (Century 931, India) was used to measure pH of solutions.

Adsorption Studies:

Exactly 1g of adsorbent samples were added to 20 ml of either lead nitrate or copper nitrate solution of concentration ranging from 25-250 mgL^-1(copper) or 50-500 mgL^-1(lead ). Solution pH was maintained at 5 by addition of HCl or NaOH. The suspensions were shaken occasionally in a thermostated water bath at 25⁰C for 24 h. Adsorbent was separated by centrifugation and filtrate was analyzed for Pb or Cu concentration using an Atomic Absorption spectrophotometer (AAS 4127- Electronics Corporation of India).

RESULTS AND DISCUSSION:

Chemical characteristics of activated carbon surface is defined by the presence of heteroatom like O, N and S. Oxygen is an important heteroatom that generally occurs in the form of carboxyl acid groups, phenolic or enolic hydroxyl groups and quinone carbonyl groups^{21, 22, 23 and 24}. The carbon precursor and activation conditions affect activated carbon pore structure and surface chemistry²⁵. HCl treatment/ removes ash from carbons. NaOH treatment of carbons on the other hand decreases carboxyl groups and increases lactonic groups on the surface. Activated carbon assumes acidic character when oxidized with nitric acid in solution phase²². Increase in acidity of the carbon sample is primarily explained by the formation of carboxylic acid and phenol hydroxyl groups²¹ ^and
23. For all carbon samples, a broad O-H stretching vibration was observed in FTIR spectra (cf. figures 1and 2). Functional group assignments are presented in Table-1.

The broad band at 3365-3399cm-¹ in all four carbons exhibited the presence of bonded O-H of carboxylic groups. A peak at 1380.9cm-¹ in nitric acid oxidized carbon is attributed to C-O stretching in O-C=O structures and C-N vibrations in heterocyclic structures. Another peak of the same carbon at 1576cm-¹ has contribution from the aromatic ring stretching mode and conjugated carbonyl groups such as quinine type C=O and or C=O ion or radical structures. Conjugated systems such as diketone, ketoester, and keto-enol structures resulted at 1576.5cm-¹. Acid treatment increased porosity of the adsorbent surface and it was confirmed by SEM figures of UM carbon and SM-A carbon (figures 3 and 4). Adsorption isotherms of Pb (II) ions and Cu (II) ions on UM and three modified sugar carbons are produced in figures 5 and 6. In the range of concentration of metal treated (50-500mg/L for Pb and 25-250mg/L for Cu), the amount of metal adsorbed increased with increase in the solution concentration. The increase was more rapid at lower concentrations and comparatively less at higher concentrations. This increase in metal adsorption is attributable to increase in electrostatic interactions relative to covalent interactions of the sites with lower affinity for metals²⁶. According to other workers²⁷, at low concentration of metal ions, the ratio of sorptive surface area to the total metal ions available is higher and therefore, there is a greater chance for metal removal. When the metal ion concentration is increased, binding sites become more quickly saturated as the amount of adsorbent concentration remained constant. HCl treatment of UM carbon decreased ash content and hence increased availability of micro pores for metal adsorption. NaOH treatment of carbon sample decreased carboxyl groups and increased lactone groups on the surface and thereby considerably decreasing the metal uptake. HNO₃ acid treatment is known to increase total surface acidity and to reduce mesopores to micro pores²². The increased metal adsorption may be attributed to these two reasons. Chemical activation is carried out at lower temperature than physical activation. Carbon produced at lower temperature is likely to possess an extensive surface area and a well-developed micro porosity¹⁸. Pb²⁺ ions are adsorbed at least three times as that of Cu²⁺ ions on nitric acid oxidized sugar carbon and this may be explained by the fact that carbon with high micro pores tend to adsorb more molecular weight compounds. XRD spectra of all four carbons confirmed the amorphous nature of all carbon samples.

Figure 3

Figure 4

Adsorption isotherms for the adsorption of Cu (II) ions and Pb (II) ions on all carbon samples are produced in figures 5 and 6. Applicability of Langmuir adsorption isotherm has also been analyzed by plotting C/x/m vs. C but the data were not found to be in good agreement. Langmuir and Freundlich constants are presented in Table 2. Linear Freundlich plots for the adsorption of Cu (II) and Pb (II) ions are shown in figures 7 and 8.

Table - 2

Sample	Langmuir constants			Freundlich constants
Sample	Xm	K	R²	1/n	k	R²
Pb
UM carbon	151.51	0.00318	0.9494	0.6703	0.1992	0.9612
SM-A carbon	-	-	-	0.7863	-0.061	0.9443
SM-B carbon	-	-	-	0.9775	-0.816	0.9426
SM-O carbon	-	-	-	0.8462	0.0596	0.9960
Cu
UM carbon	-	-	-	0.7261	0.6878	0.9849
SM-A carbon	30.86	0.0048	0.9106	0.737	-0.4987	0.9466
SM-B carbon	-	-	-	0.9615	-1.158	0.9716
SM-O carbon	-	-	-	0.9481	-0.5241	0.984

R² values of linear Freundlich plots obtained for the adsorption of Pb (II) and Cu (II) ions from aqueous solution on all four carbon samples are almost near to 1 (Figures 7 and 8) confirming multilayer coverage of metal ions on the adsorbent surface.Although same trend is followed in both metals the amount of metal ions sorbed on the carbon surfaces vary. Increased Pb(II) ions adsorption in SM-A and SM-O carbons may be due to the presence of micropores which are more accessible to these ions on the adsorbent surface. Hence, it is observed that low temperature (110°C) carbonization of the precursor material, sugarcane bagasse resulted in mesoporous adsorbent material. Nitric acid oxidation resulted in microporous structure and surface acidic groups formation favouring Pb(II) ions over Cu(II) adsorption.

CONCLUSIONS:

The unmodified sugar carbon surface was modified by three methods. Carbon samples were characterized by SEM, FTIR and XRD techniques. The results showed that:

1. Nitric acid oxidation of the original sugar carbon increased metal uptake by almost 100% and simple HCl treatment enhanced metal adsorption by about 10%.

2. Adsorption decreased due to reduction in the quantity of carbon – oxygen acidic surface groups during NaOH treatment.

3. Freundlich equation is the best fit for both metal ions based on the linear correlation coefficients.

4. Lead was adsorbed in more amounts than copper due to its bigger size and micro porous nature of sugar carbons.

5. Judicious selection of activation strategy can produce activated sugar carbon with desired surface functional groups and be put to use accordingly.

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Received on 05.07.2011 Modified on 14.08.2011

Asian J. Research Chem. 4(11): Nov., 2011; Page 1678-1684

Group or Functionality	Assignment region (cm-¹)	Reference
Carboxylic acids, esters, C-O-C stretching	1300-1000	Biniak et al²⁸ Lambert²⁹
-O-H	3600-3100	Lambert²⁹
C-C aromatic stretching	1580-1570	Jia and Thomas³⁰
Quinones	1680-1550	Biniak et al²⁸
Carboxylic acids	1200-1120, 1760-1665. 3300-2500	El-Hendawy³¹
Lactones	1370-1160, 1790-1675	El-Hendawy³¹