Arsenates are salts of arsenic acid. They are oxidizing agents. Sometimes, not all of the protons are taken away from the arsenic acid and then hydrogen arsenate salts are formed. Their chemical formulas are HAsO42- or H2AsO4-.
Arsenate can replace phosphate in the cells but it does not work like a phosphate does. The cell dies, and that is why arsenates are toxic. Some bacteria can use arsenate instead of phosphate, though. They are called arsenic bacteria.
Arsenates occur naturally in a variety of minerals. Those minerals may contain hydrated or anhydrous arsenates. Unlike phosphates, arsenates are not lost from a mineral during weathering. Examples of arsenate-containing minerals include adamite, alarsite, annabergite, erythrite and legrandite.