Characterization of aqueous Pb2+ adsorption onto cross-linked-carboxymethyl legume starch phosphate using FTIR and SEM-EDX

Sodium trimetaphosphate (STMP) cross-linked starch (SCS) and the cross-linked-carboxymethyl starch (SCCS) derivatives were synthesized using native starch (NS) from under-utilized cultivar of red kidney beans. Phosphorous content (Pc) and degree of carboxymethyl substitution (DS) of modified starches range between 0.06 and 0.08% and 0.015 and 0.074 respectively, and the modifications influenced amylose content, solubility, and the granular size of the derivatives. Modifications improved starch stabilit in aqueous solution, and increased the efficiency of aqueous Pb2+ removal from 14.34 to 97.77% on NS and SCCS respectively. The FTIR spectra of SCCS derivative (Pc: 0.8%, DS: 0.068) with optimum Pb2+ removal efficiency showed shifts/disappearance of -OH, -COO-, and P-O(- )peaks, the SEM image showed disappearance of fissures on granular surfaces, while the EDX spectra showed the emergence of the Pb2+ peaks. These indicated interactions between Pb(2+ )and SCCS. Adsorption reached equilibrium within 20 min, at pH 6, and at lower initial concentrations (5 - 50 ppm). Adsorption data fit pseudo-second-order kinetic and Langmuir isotherm models. Pb2+ was efficiently recovered within 20 min, and adsorption efficiency remains good (88.78%) till the fifth reuse cycle.