Arsenate

Arsenate is arsenic in the +5 oxidation state, commonly found in water as oxyanions such as H₂AsO₄⁻ and HAsO₄²⁻. Its resemblance to phosphate lets it enter biological systems and interfere with phosphate-dependent chemistry.

Arsenic acid: The acid H₃AsO₄, whose successive deprotonations produce arsenate oxyanions. Its dissociation determines which arsenate species dominate at different water pH levels.
Arsenic contamination of groundwater: Elevated arsenic in groundwater caused by natural geochemistry or human activity. Arsenate is a major dissolved form of arsenic in oxygen-rich groundwater.
Arsenite: Arsenic in the +3 oxidation state, commonly present in water as arsenous acid and its ions. It is the reduced counterpart of arsenate and often behaves differently in water and organisms.
Oxidative phosphorylation: Cellular ATP production driven by electron transport and a proton gradient across a membrane. Arsenate can interfere with phosphate-dependent energy metabolism, though it does not simply replace phosphate in this process.
Phosphate: Phosphate is phosphorus in the +5 oxidation state, commonly present as PO₄³⁻ and its protonated forms. Its close structural and chemical resemblance lets arsenate compete for phosphate-binding sites.
Arsenic speciation: The identification and measurement of distinct chemical forms of arsenic in a sample. Total arsenic alone cannot show whether contamination is present as arsenate, arsenite, or other forms.
Redox potential: A measure of a chemical environment’s tendency to accept or donate electrons. It helps determine whether arsenic occurs as arsenate or the more reduced arsenite.
Substrate-level phosphorylation: ATP formation by direct transfer of a phosphate group from a metabolic intermediate. Arsenate can substitute in some reactions, producing unstable arsenate esters that waste metabolic energy.
Phosphate transporters: Membrane proteins that move phosphate ions into or out of cells. Some transporters admit arsenate because they recognize its phosphate-like shape and charge.
Adsorption: The accumulation of atoms or molecules on the surface of a solid. Iron oxides and other sorbents can bind arsenate during water treatment.

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