Arsenic (As) is a plentiful element ranked 14th in abundance in seawater and 20th in the Earth’s crust.[1–4] More than 320 naturally occurring minerals contain arsenic as the major component.[5] These As-enriched minerals include arsenates, arsinides, arsenites, sulfides, sulfo-salts, silicates, and oxides.[6] The primary source of As in the environment is actually found through natural desorption and dissolution of these minerals.[7–10] However, arsenic is also released into the environment through human vectors such as mining, coal combustion, wood preservatives, herbicides, fungicides, insecticides, and effluents from industrial sources.[5, 7, 8, 11]
There are four major valence states of As: arsenate (+5), arsenite (+3), arsenic (0), and arsine (-3).[12] The two most dominant species of arsenic found in natural environments are arsenate [As(V)] and arsenite [As(III)]. This is because arsenic rapidly oxidizes in oxygenated environments.[13] These cations readily associate with 3 or 4 oxygen atoms to form AsO33− or AsO43−.[13] These anionic states can also become protonated resulting in the acid forms: arsenous acid (H3AsO3) and arsenic acid (H3AsO4).[14] Arsenate is the most thermodynamically stable form in water. Arsenite is more commonly found in reduced redox environments.[15–17] Other forms of arsenic can be present in the environment such as monomethylarsonic acid, dimethylarsonic acid, arsenobentaine, arsenocholine, arsenosugars, etc.[18, 19]
Arsenic has been identified as a Group I human carcinogen by the World Health Organization and is a highly toxic non-essential element that is often categorized alongside mercury, lead, and cadmium.[6, 20] Inorganic arsenic-containing compounds are generally more toxic than organic compounds and typically, As(III) is more toxic than As(V).[21–23] Additionally, no chemical or biological pathways to degrade inorganic arsenics into harmless small molecules exist. They can be transformed into other compounds that are less toxic than their parent compounds.[14] Remediation of arsenics from groundwater often involve oxidation, adsorption, coagulation-flocculation, ion-exchange, and membrane processes.[24]
It has been reported that As(V) containing compounds react preferentially with Al3+, Fe3+, Mn2+, and Mn4+ containing compounds that can often be found as metal oxides in sediment deposits.[13] Arsenate containing compounds have been shown to be removed in some amounts from aquatic environments via soil adsorption.[25] Therefore, in addition to being generally less toxic, As(V) is shown to be less mobile than As(III) in regards to water transport.[3] It is for this reason that much attention has been drawn to the oxidation of As(III) compared to As(V).[9, 26, 27]
Of the As(III) containing compounds, arsenous acid (H3AsO3) is one of the most widely studied compounds.[14] It has been shown that H3AsO3 can be oxidized to H3AsO4 utilizing ozone, molecular oxygen, activated H2O2, photochemical oxidation, permanganate, Mn(III/IV) oxide, iron oxides, and ferrate, among others.[14] It has been reported that the oxidation of As(III) proceeds through the formation of an unstable As(IV) complex before achieving the As(V) oxidation state.[28]
While arsenite containing compounds can be readily oxidized to the less toxic arsenate compounds; computational studies have focused on arsenous acid and its anion (AsO33−). Sodium arsenite (NaAsO2) is another arsenite-containing compound that has been used as an herbicide, rodenticide, and insecticide; however, little has been published with regards to the oxidation of NaAsO2.[9, 14, 29] However, a rate constant for the oxidation of the arsenite anion from this compound by hydroxyl radical (OH• + AsO2− → As(IV)) was found to be 9.0 x 109 L mol−1 s−1.[30, 31] In order to investigate this process and compare to available experimental data we have performed density functional theory calculations to determine the mechanism of oxidation for sodium arsenite via hydroxyl radical.