Harnessing of Carboxymethylated Rice Straw Biowaste as Anionic Adsorbent for Cationic Dye: Isotherm, Kinetics, Thermodynamics, and Reaction Mechanism

Abstract

Rice straw (RS) biowaste was turned into a high-performance anionic adsorbent for basic red 46 (cationic dye) by treating it with monochloroacetic acid via carboxymethylation utilizing a microwave heating method. For this objective, five levels of carboxymethylated RS (CMRS) with varying degrees of carboxymethylation expressed as carboxyl content, denoted as (CMRS-I to CMRS-V), were synthesized and characterized using FTIR, SEM, XRD, zeta potential, and BET analysis. Different parameters impacting cationic dye adsorption represented as adsorption capacities, such as pH, carboxymethylation extent, treatment duration, and cationic dye and RS concentrations, were thoroughly investigated. Various kinetic models (pseudo-first-order and pseudo-second-order), isothermal (Langmuir, Freundlich, and Temkin), and thermodynamic investigations were used. The results demonstrated that (a) FTIR, SEM, XRD, zeta potential, and BET analysis confirmed the formation of carboxyl groups onto the CMRS; (b) the adsorption capacity increased by increasing the pH value from 1 to 10, the extent of carboxymethylation from 37.9 to 189.7 meq-COOH/100 g sample, cationic dye concentration from 10 to 100 mg/L, and the adsorption time up to 180 min then leveled off afterward; (c) maximum adsorption capacity was determined to be 31.2 mg/g at pH 10, 50 mg/L dye concentration, 180 min adsorption period, and 0.1 g adsorbent; and (d) kinetic and isothermal experiments demonstrated the superiority of the pseudo-second-order rate equation and Langmuir model with higher R² values. The computed thermodynamic characteristics showed that the adsorption process was feasible, spontaneous, and exothermic at 298–318 K. Finally, a comparison with other adsorbents, regeneration, and adsorption mechanisms has been proposed.