A mechanistic and thermodynamics insight of cadmium adsorption on sulfur-functionalized rice husk biomass

Rice husk, a waste biomass, was chemically modified with sulfur-containing groups to enhance cadmium ions (Cd²⁺) removal from aqueous solution. New rice husk adsorbents were prepared by modifying with sodium diethyldithiocarbamate (RH-SDC) and carbon disulfide (RH-X and RH-XDMF). The introduction of sulfur groups on rice husk was confirmed through Fourier transform infrared spectroscopy (FT-IR), which revealed peaks at 1157 cm⁻¹ and 1030 cm⁻¹ attributed to the C = S and C-S stretching vibrations, respectively. Batch adsorption experiments were performed to assess the adsorption capabilities of the modified rice husk, taking into account factors such as pH, contact time, Cd²⁺ ion concentration and temperature. Maximum adsorption occurred at pH 7 for all the adsorbents. The concentration of Cd²⁺ ions after adsorption was determined by Atomic Absorption Spectroscopy (AAS). The incorporation of sulfur groups enhanced the maximum adsorption capacity of the Cd²⁺ ion, increasing it from 7.169 mg/g to 24.449 mg/g. The adsorption data aligned well with the Langmuir isotherm model and followed the pseudo-second-order kinetics. The enthalpy (ΔH) of adsorption was endothermic, the entropy (ΔS) was positive while ΔG was negative suggesting the adsorption process was spontaneous and became favorable with increasing temperatures. The activation energy of adsorption (E_a) was the lowest for RH-X. Modifying cellulosic waste by-products with sulfur groups to enhance the adsorption of Cd ions from aqueous solution is a sustainable strategy for environmental remediation.