Adsorption of Humic Acid onto\nNanoscale Zerovalent Iron and Its\nEffect on Arsenic Removal

Abstract

Batch experiments were performed to investigate the\nfeasibility of humic acid (HA) removal by synthetic nanoscale\nzerovalent iron (NZVI) and its interaction with As(III) and\nAs(V), the most poisonous and abundant of groundwater\npollutants. High-resolution transmission electron microscopy\n(HR-TEM) and X-ray diffraction (XRD) were used to\ncharacterize the particle size, surface morphology of the\npristine NZVI and HA-treated NZVI (NZVI-HA), and the zero\nvalence state of the pristine NZVI. It was determined\nthat HA was completely removed by NZVI (0.3 g/L) within\na few minutes, at a wide range of initial pH values (∼3.0−12.0). Fourier transform infrared (FTIR) and laser light scattering\n(zeta potential measurement) studies confirmed that NZVI-HA forms inner-sphere surface complexation at different\ninitial pH conditions. The effects of competing anions showed\nthat there was complete removal of HA in the presence\nof 10 mM NO^3- and SO₄^2- whereas HA removal was observed\n0%, 18% and 22% in presence of 10 mM H₂PO₄^2-, HCO₃^-\nand H₄SiO₄⁰, respectively. However, the presence of 2 mM\nCa²⁺ and Mg²⁺ enhanced HA removal from 17 mg g^-1 to\n76 mg g^-1 and 55 mg g^-1, respectively. Long-term time-resolved\nstudies of XRD and field emission scanning electron\nmicroscopy (FE-SEM) with energy-dispersive X-ray (EDX)\nrevealed the formation of various types of new iron oxides\n(magnetite, maghemite, and lepidocrocites) during the\ncontinuous reaction of HA in the presence of water and\nNZVI at 1, 30, 60, and 90 days. In addition, the surface-area-normalized rate constant (<i>k</i>_sa) of adsorption of As(III)\nand As(V) onto NZVI was reduced in the presence of HA\n(20 mg L^-1), from 100% to 43% and 68%, respectively.\nOur results show the potential use of NZVI in removing\nHA and its possible effects on arsenic removal during the\napplication of NZVI in groundwater remediation.