Chemical Engineering Journal 160 (2010) 129–137 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Microwave induced poly(acrylic acid) modification of Cassia javanica seed gum for efficient Hg(II) removal from solution Vandana Singh ∗ , Somit Kumar Singh, Sadhana Maurya Department of Chemistry, University of Allahabad, Allahabad 211002, India article info Article history: Received 28 December 2009 Received in revised form 8 March 2010 Accepted 9 March 2010 Keywords: Cassia javanica Seed gum Poly(acrylic acid) Grafting Microwaves Mercury Sorption abstract Microwave induced poly(acrylic acid) modification of Cassia javanica (CJ) seed gum furnished an efficient Hg(II) sorbent. Copolymer samples of different performances in terms of Hg(II) binding were synthesized by changing acrylic acid concentration at fixed microwave power and exposure time. The optimum sam- ple has been characterized using FTIR spectroscopy, X-Ray diffraction, BET and SEM analysis and using this sample adsorption of mercury (II) was studied as a function of pH, sorbent dose, initial Hg(II) con- centration, % grafting, temperature and ionic strength. Equilibrium isotherm data were analyzed using the Langmuir and Freundlich isotherms. The Langmuir model yielded a much better fit than the Fre- undlich model indicating unilayer sorption. Isotherms have also been used to obtain the thermodynamic parameters such as free energy, enthalpy, and entropy of sorption. The maximum sorption capacity was 135 mg g -1 at 30 ◦ C. In order to investigate the mechanism of sorption, kinetic data were modeled using the first order Lagergren, pseudo-second-order and intra-particle diffusion model where intra-particle diffusion and chemical reaction both seem relatively significant in the rate controlling step. The regen- eration experiments revealed that the CJ-graft-poly(acylic acid) can be successfully reused for six cycles without any significant loss in the sorption capacity. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Mercury (Hg) as one of the most toxic heavy metal is com- monly found in the global environment. Its toxicity is related to the capacity of its compounds to bioconcentrate in organisms and to biomagnify through food chain [1]. The toxicity of mercury depends strongly on its redox state [2]. The most toxic form of mercury is the highly reactive Hg 2+ which binds to the amino acid ‘cys- teine’ in proteins. In contrast, the danger of elemental mercury and organo-mercury compounds lies in their transport routes. Hg contamination can be much more widespread than that observed for other metals due to atmospheric transport [3,4] or to bio- magnification through the food chain, reaching fish and humans [5]. Removal of mercury ions from water and wastewater is very important and different technologies are available such as chemical precipitation, coagulation, ion exchange, membrane technologies, adsorption, etc. The latter is by far the most versatile and widely used and activated carbon is the most commonly used sorbent. However, the use of activated carbon is expensive, so there has been increased interest in the use of other adsorbent materials, par- ∗ Corresponding author. Tel.: +91 532 2461518; fax: +91 532 2540858. E-mail addresses: singhvandanasingh@rediffmail.com, vndsng@yahoo.com (V. Singh). ticularly low-cost adsorbents [6] derived from natural polymers, in particular chemically and physically modified polysaccharides [7] e.g. poly(acrylonitrile) and poly(acrylic acid) grafted cellulose [8,9] is reported to remove cadmium, copper and lead ions where the amount of metal ion adsorbed was dependent on metal type and the nature and level of the incorporated grafts in the polymer. Similarly poly(acrylic acid) grafted cotton cellulose [10], starch-g- poly(acrylic) acid copolymers [11] and poly(methylmethacrylate) grafted Cassia grandis seed gum [12] effectively removed heavy metal ions from the solutions. There has been recent interest in using microwave irradiation in the copolymer synthesis [13] as its use not only saves chemicals and reaction time but also furnishes copolymer samples that are more efficient sorbent materials than the conventionally synthesized ones. Cassia plants are non-conventional source of seed gums having structural resemblance with industrial gums like guar and locust bean gums. Non-conventional Cassia seed gums are galactoman- nans [14] and their mannose to galactose ratio and molecular weights vary from species to species. Thus they are renewable reservoir for obtaining polysaccharide materials that can be modi- fied to functional copolymers through vinyl grafting. Since primary structures and molecular weights of these Cassia seed gums are different from guar gum, on vinyl modification they are likely to furnish graft copolymers that are different from vinyl modified guar gum in terms of properties and performance. 1385-8947/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2010.03.020