The present study reports the chemical modification of agricultural waste (rice straw) with urea using microwave radiation and the efficiency evaluation of this modified rice straw for the adsorption of a toxic heavy metal, cadmium. The elemental analysis of urea modified rice straw affirmed urea grafting on rice straw, and FTIR spectra of chemically benign modified adsorbent showed the presence of hydroxyl, carbonyl, and amino functional groups. Effects of process parameters (adsorbent dosage, contact time, agitation speed, pH, and temperature) were studied in batch mode. Parameters were optimized for the equilibrium study, and adsorption mechanism was elucidated using five mathematical models (Langmuir, Freundlich, Temkin, Harkin-Jura, and Dubinin-Radushkevich). Binding of Cd(II) ions on modified adsorbent followed Langmuir model, and the maximum uptake capacity was found to be 20.70？mg g？1. Kinetic modeling was done using six different kinetic models. The process was considered physisorption according to the obtained activation energy value. Thermodynamic parameters confirmed the process to be favorable and feasible. Exothermic nature of adsorption of Cd(II) ions on urea modified rice straw was confirmed by the negative value of ΔH°. 1. Introduction Rapid pace of industrialization has resulted in a number of problems among which water pollution is considered to be one of the serious problems. Industrial processes discharge huge amounts of untreated wastewater daily into the surrounding environment, leading to detrimental effects on aquatic, plant, and human life. Heavy metal such as lead, cadmium, chromium, and copper is regarded as major pollutants in wastewater. These contaminants are of major concern because they do not degrade naturally [1]. Cadmium has attracted wide attention of environmental chemists as one of the most toxic metals and has been categorized as a human carcinogen by USEPA (United States Environment Protection Agency), WHO (World Health Organization), and NTP (National Toxicology Program) [2]. It is a nonessential and nonbiodegradable metal which slowly accumulates in the human body, usually from food chain. The permissible limits for cadmium by WHO and USEPA are 3？μg？L？1 and 5？μg？L？1, respectively. It affects lungs, liver, and pancreas and disturbs the human DNA repair system. Chronic cadmium poisoning causes characteristic yellow pigmentation of teeth (the yellow ring of cadmium) [2]. It is important to treat contaminated waters on a continuous basis due to need of hour. A number of technologies are available with varying
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