Shi, and 5 co-authors, reported early 2020 that ferric iron (Fe(III)) and arsenate (As(V)) generally co-exist in natural waters. Also, Fe(III) chemicals are widely used for treatment of As(V) in water. This paper reported the existence of Fe(III)–As(V) complexes, which are molecular clusters containing only one iron atom with sizes of less than 1 nm, typically considered as soluble species. The formation of Fe(III)–As(V) complexes varies with the change of pH. Specifically, the soluble Fe(III)–As(V) complex formed at acidic conditions (pH <2) with a Fe:As mole ratio of 1:1.
When the pH increased to 2–5, Fe(III) and As(V) formed precipitate rather than a soluble complex, which led to a reduced solubility of Fe(III) at pH 2–5. The soluble Fe(III)–As(V) complex again appeared at pH >5 with the Fe:As mole ratio of 1:2. DFT calculation identified that these soluble Fe(III)–As(V) complexes were in a monodentate configuration. The chloride was found to be coordinated with the Fe(III)–As(V) complexes, which improved their thermodynamic stability. The existence of these soluble complexes promoted the solubility of Fe(III) at neutral pH (i.e., total soluble Fe(III) increased to 0.73 mM (∼41 mg L−1) with an initial 0.1 M Fe(III)). The findings in this study shed new light on the understanding of the transport and fate of Fe(III) and As(V) in natural water systems, as well as the As(V) leaching from the sludge produced in the Fe(III)- based As(V) removal techniques.
The authors concluded that the discovery of the existence of Fe(III)–As(V) complex is important to understand the fate and transport of the Fe(III)
and As(V) in natural water systems. In addition, since Fe(III) based technologies (i.e., nano- to mill-meter scale iron oxides adsorbents) are extensively used in treating As(V) contaminated water, the findings in this study is important to understand the role of As(V) on the dissolution of ferric oxides and the possible leaching of As(V) from sludge. This study evidenced the formation of Fe(III)–As(V) complex at neutral pH with a size of 1 nm, providing new insights into the existence of nano-sized iron oxides particles, which could be considered as the soluble complex.
The existence of such a complex has a significant effect on the fate of nano- to micro-scale iron oxides particles, i.e., rethinking their solubility constants in the presence of As(V). The existence of Fe(III)–As(V) complex should be considered in fate and transport modeling studies regarding the iron oxides particles in natural systems, particularly when As(V) is present. Moreover, because the As(V)–Fe(III)–As(V) complex is soluble at neutral pH, it should be considered in the design of arsenate containing water treatment processes by the Fe(III) based nanotechnologies, as well as the disposal of treatment sludges.
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