Infrared and X-ray absorption spectroscopic studies on the mechanism of chromium(III) binding to alfalfa biomass

Previous studies have shown that alfalfa biomass possesses the potential to be a biosorbent for chromium(III) removal from contaminated and industrial wastewaters. However, the mechanism through which chromium(III) binds to alfalfa biomass has not been identified. Therefore, studies were conducted to determine how modification of chemical groups present on the alfalfa biomass affect the chromium(III) binding. Batch pH profile studies were performed on esterified, hydrolyzed and unmodified alfalfa biomasses. A comparison study with ion exchange resins containing carboxyl, thiol, amino, sulfonic, and phosphate groups were also performed. These studies showed that chromium(III) binds predominantly to the alfalfa biomass through carboxyl ligands and follows a binding trend similar to that of the carboxyl resin. In addition, Fourier transform infrared spectroscopy (FTIR) studies were conducted in order to better understand how the chemical modification affects the alfalfa biomass and the mechanism(s) by which chromium(III) binds to alfalfa. From these experiments, it was determined that chromium(III) may be coordinating with carboxyl ligands present on the surface of the alfalfa biomass through a bridging bidentate complex. X-Ray absorption spectroscopy (XAS) studies were also performed, which corroborate with batch modification and ion exchange resin comparison studies, further indicating carboxyl involvement in chromium(III) binding by inactivated alfalfa biomass.