Utilization of multi-metal laden spent biosorbent for removal of glyphosate herbicide from aqueous solution and its mechanism elucidation

Extensive uses of glyphosate herbicide pose the environmental concerns of surface and ground water contaminations through leaching. The present investigation is based on potential reuse of multi-metal laden spent biosorbent for glyphosate removal. Our previous study showed effective removal of Ni(II), Co(II), Zn(II) and Cd(II) ions in multi-metal system using waste activated sludge from tannery industry as biosorbent. Glyphosate biosorption efficiency of the waste tannery sludge was enhanced from 26.18% to >92% after increase in the multi-metal loading on the sludge. Feasibility study for actual wastewater treatment showed about 91.7% removal of glyphosate in the simulated agricultural runoff. The glyphosate biosorption exhibited gradual removal process. The rapid 70–75% biosorption was attained in first 2 h of contact time and the final removal of 89–94% was achieved within 24–28 h of contact time. Equilibrium isotherm and kinetics data were obtained by adsorption experiments. Biosorbent characterization and removal mechanism were established. The formation of glyphosate-metal complexation was confirmed by the FTIR, XPS, XRD, XRF, FESEM-EDAX and elemental mapping results. Surface bound multi-metals of the biosorbent could predominantly form mononuclear, bi-, tri- or tetra-dentate complexes through phosphonate group of glyphosate molecule. Weak binding of glyphosate molecule with the carboxyl and amino groups also occurred. The glyphosate loaded biosorbent were successfully inertized up to 10 wt% in phosphate glass matrix as confirmed by XRD for safe disposal of the toxic sludge in the environment.