SYNTHESIS OF CROSS-LINKED CARBOXYMETHYL STARCH FROM KWAKIL BEANS (Phaseolus vulgaris L.) FOR REMOVAL OF SELECTED HEAVY METALS FROM AQUEOUS SOLUTIONS

Abstract

Kwakil beans (Phaseolus vulgaris L.) starch and its derivatives were studied as potential adsorbents for heavy metals from aqueous solutions. The extracted starch was chemically modified through cross-linking using phosphorus oxychloride (POCl3), sodium trimetaphosphate (STMP) and epichlorohydrin (ECH), followed by carboxymethylation, to produce POCl3 cross-linked carboxymethyl (PCCS), STMP cross-linked carboxymethyl (SCCS), and ECH cross-linked carboxymethyl (ECCS) starch derivatives respectively. Degree of substitution (DS) on carboxymethylated starches was studied, while Fourier transformed infrared (FTIR), Scanning Electron Micrograph (SEM) and Electron Dispersion X-ray (EDX) characteristics of the native and modified starches were evaluated before and after the preliminary batch adsorption experiments which precedes further equilibrium, kinetic, isotherm, thermodynamic and regeneration studies. DS on derivatives followed the order: SCCS˃ECCS˃PCCS, while changes in functionality, surface morphology, and elemental compositions indicated successful modifications, and adsorption of ions on the derivatives. Adsorption on native starch was remarkably increased (˃˃ 60%) after crosslinking, while carboxymethylation further increased adsorption capacity/stability. Adsorption of Pb2+ was higher than Cd2+, and the amount of Pb2+ adsorbed increased with DS and Na+ released into the aqueous solution. Adsorption of Pb2+ and Cd2+ were not favourable at pH below 5, equilibrium was attained within 5, 15 and 30 min on PCCS, ECCS and SCCS respectively, and adsorption efficiency was higher at lower concentrations (≤ 100 ppm), and temperature ( 30 °C). Kinetic parameters showed that, Cd2+ data are best fitted to pseudo-first order model, while Pb2+ data are best fitted to pseudo-second order model on all the derivatives. Isotherm parameters showed that, Cd2+ data are best fitted to the Freundlich model, while Pb2+ data are best fitted to the Langmuir vii model on all the derivatives. Thermodynamic studies showed that the overall adsorption process on all the adsorbents was exothermic. Corresponding values of enthalpy change (ΔH) for Pb2+ and Cd2+ showed that stronger (chemisorption) bonds are involved in the adsorption of Pb2+ than Cd2+. ΔH values on PCCS (-15.51 kJ.mol-1 and -9.79 kJ.mol-1), also indicated that the overall adsorption process on the derivative is controlled by physisorption, ECCS (-48.67 kJ.mol-1 and -39.58 kJ.mol-1) is controlled by chemisorption and SCCS (-70.33 kJ.mol-1 and -52.82 kJ.mol-1) is more significantly controlled by chemisorption mechanisms. Adsorbents were successfully regenerated with efficient ion recovery (≥ 97 %). Cd2+ was more rapidly desorbed than Pb2+ and desorption time ranged between 5 – 20 min. The rate of desorption on the derivatives also followed the order PCCS˃ECCS˃SCCS, while reuse efficiency followed order SCCS˃ECCS˃PCCS. Cross linked carboxymethyl starch adsorbents synthesised from kwakil beans in this study were used in the removal of heavy metals from wastewaters; this showed that the adsorbents are of good potentials in the remediation of wastewater.