Mercury(II) removal from aqueous solutions and wastewaters using a novel cation exchanger derived from coconut coir pith and its recovery

Abstract

A new adsorbent (PGCP-COOH) having carboxylate functional group at the chain end was synthesized by grafting poly(hydroxyethylmethacrylate) onto coconut coir pith, CP (a coir industry-based lignocellulosic residue), using potassium peroxydisulphate as an initiator and in the presence of N,N′-methylenebisacrylamide as a cross-linking agent. The adsorbent was characterized with the help of infrared spectroscopy, powder X-ray diffraction, scanning electron microscopy, and potentiometric titrations. The ability of PGCP-COOH to remove Hg(II) from aqueous solutions was assessed using batch adsorption technique under kinetic and equilibrium conditions. Adsorbent exhibits very high adsorption potential for Hg(II) and more than 99.0% removal was achieved in the pH range 5.5–8.0. Adsorption process was found to follow first-order-reversible kinetics. An increase of ionic strength of the medium caused a decrease in metal removal, indicating the occurrence of outer-sphere surface complex mechanism. The equilibrium data were fitted well by the Freundlich isotherm model (R² = 0.99; χ² = 1.81). The removal efficiency was tested using chlor-alkali industry wastewater. Adsorption isotherm experiments were also conducted for comparison using a commercial carboxylate-functionalized ion exchanger, Ceralite IRC-50. Regeneration experiments were tried for four cycles and results indicate a capacity loss of <9.0%.

Introduction

Treatment of contaminated water to remove soluble heavy metals has been and continues to be a technical challenge due to very low concentrations established by current regulations. Mercury, which is widely used and extremely toxic in relatively low dosages is one of the principle heavy metals responsible for causing neurological and renal disturbances as it can easily pass through blood–brain barrier and affect the foetal brain. The main anthropogenic path way through which Hg(II) enters the water bodies is via wastes from industrial processes such as chlor-alkali, paper and pulp, oil refinery, mining, electroplating, paint, pharmaceutical and battery manufacturing. Among several chemical and physical methods, adsorption process is one of the effective techniques that has been successfully employed for metal removal from wastewater. A wide range of adsorbents has been used for removing Hg(II) from wastewaters and has been reviewed by Krishnan and Anirudhan [1]. Several recent publications utilized inexpensive naturally occurring lignocellulosic materials, e.g. wheat straw, peanut shell, moss peat, bagasse fly ash, tree fern, gram husk, coconut coir pith and saw dust for heavy metal removal [2], [3], [4], [5]. These materials were found to have good adsorption capacity due to substances inherently associated with cellulose such as lignin, tannin and pectin, which contains polyphenolic and aliphatic hydroxyl and carboxylic groups. The main disadvantages of these materials are their low resistance to abrasive forces in batch or column applications and leaching of some organics (water extractives) during adsorption. Many chemical modifications have therefore being thought of to improve the physical and chemical properties and also to increase the adsorption potential, which include esterification, graft copolymerization, cross-linking and quarternisation. Incorporation of different functional groups onto chemically modified lignocellulosic materials such as peanut shell, orange residue, sawdust and banana stalk has already been reported [6], [7], [8], [9], [10].

Coconut coir pith (CP) is widely available and abundant natural material, which basically contains cellulose and lignin. India is the market leader in the production of coconut fruit, producing 7.5 million tonnes of CP annually. After the separation of the fiber from the coconut husk, the CP has no use and is recognized as waste in the agricultural or industrial sector. Any attempt to reutilize the CP will be worthwhile. Efforts have already been made to recycle this solid waste by using it as precursors for the preparation of potential adsorbents in water purification [11], [12]. In these earlier studies, anion exchangers with different functionalities were prepared from CP, which effectively, and inexpensively decontaminates wastewater rich in heavy metals. In this study, a new adsorbent namely poly(hydroxyethylmethacrylate) grafted CP with carboxylate functionality was developed for the removal of Hg(II) from water and wastewater. The adsorption of Hg(II) onto modified CP was studied in batch system with respect to the initial pH, contact time, initial Hg(II) concentration, ionic strength, adsorbent dose and temperature.