Fe(II)/Sulfite Process with Rapid Fe(II) Oxidation Enhances p-Arsanilic Acid Oxidation and Simultaneous Arsenic Adsorption

Most studies focus on the oxidation capability of the Fe(II)/sulfite (Fe(II)/S(IV)) process for organic pollutants but overlook the phenomenon of rapid iron particle formation. The coupling of oxidation and in situ adsorption by iron particulates holds great potential for the deep removal of organometallic compounds. Herein, the removal of an organoarsenic compound of p-arsanilic acid (p-ASA) by Fe(II)/S(IV) was investigated. This process demonstrated efficient and rapid p-ASA degradation, achieving 95% p-ASA removal (10 μM) within 3 min, which is comparable to those of the Fe(II)/PDS and Fe(II)/PMS processes. Various evidence indicated that SO₅^•– was the primary radical driving p-ASA degradation, selectively attacking the amino group with As(V) as the ultimate byproduct. Ultrafast Fe(II) oxidation and iron particle formation were observed, with over 90% of Fe(II) ions converted into insoluble iron particles within 30 s. However, the Fe(II)/PDS and Fe(II)/PMS processes generated very few iron particles. It was found that S(IV)’s proton-taking feature increased the pH, significantly promoting Fe(II) oxidation, hydrolysis, and precipitation. The formed iron particles were amorphous ferric(oxyhydr)oxides, and showed great efficiency for As(V) adsorption during their formation. As a result, the Fe(II)/S(IV) process showed superior efficiency for total arsenic removal. In addition, the process remained effective under real water matrices. Overall, this study offers theoretical and data-driven guidance for applying this promising one-step procedure for effectively treating organic arsenic in water.