Influence of Humic Acid on Nanoscale Zerovalent Iron Reaction in Oxic Water: Structural Evolution, ^•OH Generation, and As(III) Sequestration

The reactivity of nanoscale zerovalent iron (nZVI) in practical application is influenced by ubiquitous humic acid (HA), with its structural evolution and synchronous hydroxyl radical (^•OH) generation behaving as closely interrelated processes that govern contaminant fate under dynamic redox conditions. In this study, the influence of different HA concentrations on the structural evolution of nZVI, synchronous ^•OH generation, and arsenite [As(III)] sequestration was investigated. While HA did not alter the types of evolutionary products, it accelerated zerovalent iron (Fe⁰) corrosion. Extended X-ray absorption fine structure and density functional theory calculation revealed that adsorbed-HA extended the Fe-O bonds in the nZVI surface, causing the compact passivation layer to rupture and the inner Fe⁰ to expose. This structural destabilization, along with reduced nZVI aggregation induced by HA, promoted the continuous corrosion of Fe⁰. Furthermore, quantitative analysis by high performance liquid chromatography demonstrated that HA enhanced ^•OH production in the nZVI system by up to 7-fold, due to the increased Fe(II)/Fe(II)-HA concentrations in solution. In addition, rapid Fe⁰ corrosion facilitated more ^•OH-mediated oxidation of As(III) to arsenate [As(V)] at pH 6.0, whereas at pH 9.0, it resulted in faster As(III) sequestration by generating new adsorption sites and enhancing As-Fe coprecipitation. These findings provide valuable insights into the roles of HA in modulating nZVI performance in environmental applications and As(III) fate.