Synthesis of Cross-Linked Carboxymethyl Legume Starch for Adsorption of Selected Heavy Metals from Aqueous Solutions

In this work, legume starch from Kwakil beans (Phaseolus vulgaris L.) was crosslinked with epichlorohydrin and carboxymethylated with different concentrations of sodium monochloroacetate (SMCA) (5-30% w/w of starch db). Degree of substitution (DS) of carboxymethyl group ranged between 0.02-0.063, and adsorption was enhanced as the DS increased. The native starch (NS), crosslinked starch (ECS), and the carboxymethyl derivative (ECCS-15%; DS=0.062) with optimum adsorption capacity in Pb^2+ (98.51%) and Cd^2+ (97.32%) aqueous solution were further characterized using FTIR and SEMEDX. Equilibrium studies revealed that the adsorption of Pb^2+ and Cd^2+ from aqueous solution by ECCS increased by enhancing the pH, contact time (up to equilibrium time at 15 min), initial concentration and adsorbent dosage, but decreased with increasing the temperature and interfering ion (Na^+ and Ca^2+) concentrations. Pb^2+ data fits pseudosecond order kinetic (R²=0.9999) and Langmuir isotherm models (R²=0.9983), and ΔH=-48.67 Kj/mol^-1. This signals a chemisorption process. Pb^2+ and Cd^2+ can be recovered efficiently (up to 99%), and the adsorbent can be reused up to 5 cycles with sorption capacity above 75%. On application, ECCS reveals good potentials in the purification of wastewater with low heavy metal ion concentrations.