Adsorption of humic acid onto nanoscale zerovalent iron and its effect on arsenic removal

Batch experiments were performed to investigate the feasibility of humic acid (HA) removal by synthetic nanoscale zerovalent iron (NZVI) and its interaction with As(III) and As(V), the most poisonous and abundant of groundwater pollutants. High-resolution transmission electron microscopy (HR-TEM) and X-ray diffraction (XRD) were used to characterize the particle size, surface morphology of the pristine NZVI and HA-treated NZVI (NZVI-HA), and the zero valence state of the pristine NZVI. It was determined that HA was completely removed by NZVI (0.3 g/L) within a few minutes, at a wide range of initial pH values (similar to 3.0-12.0). Fourier transform infrared (FTIR) and laser light scattering (zeta potential measurement) studies confirmed that NZVI-HA forms inner-sphere surface complexation at different initial pH conditions. The effects of competing anions showed that there was complete removal of HA in the presence of 10 mM NO3- and SO42- whereas HA removal was observed 0%, 18% and 22% in presence of 10 mM H2PO42-, HCO3- and H4SiO40, respectively. However, the presence of 2 mM Ca2+ and Mg2+ enhanced HA removal from 17 mg g(-1) to 76 mg g(-1) and 55 mg g(-1), respectively. Long-term time-resolved studies of XRD and field emission scanning electron microscopy (FE-SEM) with energy-dispersive X-ray (EDX) revealed the formation of various types of new iron oxides (magnetite, maghemite, and lepidocrocites) during the continuous reaction of HA in the presence of water and NZVI at 1, 30, 60, and 90 days. In addition, the surface-area-normalized rate constant (k(sa)) of adsorption of As(III) and As(V) onto NZVI was reduced in the presence of HA (20 mg L-1), from 100% to 43% and 68%, respectively. Our results show the potential use of NZVI in removing HA and its possible effects on arsenic removal during the application of NZVI in groundwater remediation.