The oxidation of arsenite (As(III)) to arsenate (As(V)) has received significant attention because it helps mitigate the hazardous and adverse effects of As(III) and subsequently improves the effectiveness of arsenic removal. This study developed an efficient freezing technology for the oxidative transformation of As(III) based on iodide (I-). For a sample containing a very low concentration of 20 μM As(III) and 200 μM I- frozen at -20 °C, approximately 19 μM As(V) was formed after reaction for 0.5 h at pH 3. This rapid conversion has never been achieved in previous studies. However, As(V) was not generated in water at 25 °C. The acceleration of the oxidation of As(III) by I- in ice may be attributed to the freeze-concentration effect. During freezing, all components (i.e., As(III), I-, and protons) are highly concentrated in the ice grain boundary regions, resulting in thermodynamically and kinetically favorable conditions for the redox reaction between As(III) and I-. The efficiency of the oxidation of As(III) using I- increased at high I- concentrations and low pH values. The low freezing temperature (below -20 °C) hindered the oxidative transformation of As(III) by I-. The efficiency of the oxidation of As(III) significantly increased using a fixed initial concentration of I- by subjecting the system to six freezing-melting cycles. The outcomes of this study suggest the possibility of the self-detoxification of As(III) in the natural environment, indicating the potential for developing an eco-friendly method for the treatment of As(III)-contaminated areas in regions with a cold climate. It also demonstrates radical remediation to almost completely remove a very small amount of As(III) that was input in As(III)-contaminated wastewater detoxification, a benchmark that existing methods have been unable to achieve.
Keywords: Arsenite; Freeze concentration effect; Freezing–melting cycle; Ice chemistry; Iodide.
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