Introduction: The present study focuses on the biosorption of heavy metals in wastewater from various industrial and domestic wastes. When discharged to water bodies, these may be harmful to both human and aquatic life as they are nondegradable and thus persistent. Therefore, it is important to remove heavy metals prior to their discharge. Mercury (Hg) is one of the most hazardous pollutants, which acquires high toxicity even at low concentrations[1]. Adsorption represents an efficient and convenient method, which can separate low amounts of substances from large volumes of solution[2]-[5]. Biodegradable materials can prove to be a better adsorbent by their surface modification[6]-[9].
In this study, Methylmethacrylate is grafted onto Cassia grandis seed gum. The graft copolymer was characterized by FTIR, SEM and XRD and has been studied as an adsorbent with good metal-chelating properties for the removal of Hg (II) from aqueous solutions.
Materials and Methods: Cassia grandis seed gum(CG), Methylmethacrylate(MMA), potassium persulfate (KPS) and ascorbic acid (AA) were of analytical grade.
Synthesis of Cassia grandis gum-graft-polymethylmethacrylate (CG-g-PMMA)[10],[11]: CG, MMA and AA were added to water (25ml) and thermostated at 35°C. After 30 min calculated amount of KPS was added. Graft copolymerization was allowed for 1h and the copolymer was extracted with acetone and dried. The adsorbent with different %G were obtained by variations in concentrations of MMA, AA, KPS, CG, temperature and time. (Fig1.Table (1)
Hg (II) adsorption batch experiment: CG-g-PMMA with maximum %G was evaluated for Hg(II) removal from synthetic solution. The operating variables studied were pH, sorbent dosage, Hg concentration and temperature.
Desorption studies[12]: The copolymers loaded with mercury were placed in the 0.05N H2SO4 and stirred at 120 rpm for 4h at 30◦C and the final Hg(II) concentration was determined.
Results and Discussion: CG-g-PMMA was synthesized efficiently by thermal grafting method. The maximum %G(300%) and %E(70.69%) obtained was using [MMA]=17X10-2M, [KPS]=40X10-3, [AA]=2.3X10-2M, CG=4g/L (25 ml) for 1h. The samples were characterized using FTIR (Fig 1(1)), XRD (Fig 1(2)), SEM (Fig 1(3)). The results obtained confirm the synthesis of the graft copolymer.
Optimization of Adsorption Study: Maximum sorption condition was obtained at pH=6 (Fig. 4A), Hg conc.=100mg/L (Fig.4B), Adsorbent dose=50 mg (Fig.4C) and temperature=30oC (Fig.4D).
Sorption kinetics: The correlation coefficients R2 and the rate constants for CG-g- PMMA are summarized in Fig2(5)Table2.Kinetics of sorption was modeled by the first order Lagergren equation, pseudo-second-order equation and intra-particle diffusion model.
Desorption studies: Adsorption desorption cycles could be repeated for six times at adsorbent dose 250mg, pH=6, 120 rpm, contact time=4h. (Fig2(6))
Adsorption thermodynamics: It was observed that Adsorption of Hg(II) onto the CG-g-PMMA was increased at higher temperature. (Table (3), (Fig2(7)).
Conclusion: The optimum CG-g-PMMA was successfully synthesized and it proved to be an efficient mercury ion sorbent. The sorption by the copolymer was pH dependent and studies indicated sorption followed pseudo-second-order kinetic model.The sorbent could be successfully recycled for six consecutive cycles without much loss in its sorbing capacity. Thus, an adsorbent with good metal-chelating properties is obtained for the removal of Hg(II) from synthetic aqueous solutions.
University Grant Commission ,New Delhi, India
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