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INTRODUCTION
Removal of Cu(II) from industrial effluents has become a major concern in the industrial sector due to its harmful effects on both human health and the environment. Among various types of heavy metal removal methods available in the treatment of industrial effluents, adsorption has shown many advantages (Priyantha & Bandaranayaka 2011; Bandara & Priyantha 2013). Among different types of adsorbents, activated carbon is widely used due to its strong metal ion adsorption properties. However, preparation of activated carbon is not economical, indicating the necessity of low-cost and environmentally friendly sorbents, such as rice husk, coir dust, sawdust, tea waste and some living organisms (Babel & Kurniawan 2003). Use of such biosorbents has become an attractive area of research in the field of environmental science. Biosorption occurs due to interactions between metal ions in solution and functionalities present in the adsorbent, such as carboxyl, hydroxyl, aceto, amino, phosphate and sulfhydryl groups (Višekruna et al. 2011). Metal trapping mechanisms in sorption processes include ion exchange, chelating, adsorption by physical or chemical forces, oxidation/reduction reactions and ion entrapment in polymerized structures (Volesky 2001). Low sorption capacity of some biosorbents has been enhanced via chemical modification, through introduction of new functional groups, increase in porosity, and removal of undesirable compounds, such as lignin and hemicelluloses (Wan Ngah & Hanafiah 2008; Ding et al. 2014).
Rice husk, which contains cellulose, hemicelluloses, lignin and mineral ash, is a byproduct of the rice processing industry. The composition of rice husk, having silica as the main constituent, qualifies it to be a strong adsorbent. The efficiency of rice husk as an adsorbent has been demonstrated by determining the extent of heavy metal removal under optimized conditions of experimental parameters, such as contact time, adsorbent dosage, solution pH and particle size (Kayal et al. 2010). Early studies with rice husk as an adsorbent involved its use without any modification step (Munaf & Zein 1997). It has been reported that some advantages associated with rice husk, such as high adsorption even at low metal concentration levels, no additional nutrients requirements, easy operation, strong affinity and high selectivity towards heavy metals due to the presence of binding groups on its surface can be further improved through chemical modification (Wan Ngah & Hanafiah 2008; Chauhan...