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.