Occurs in solutions and refers to a minor or trace element (such as arsenic) adsorbing onto or absorbing within the developing or fresh precipitates of other chemical species. Although sometimes difficult to distinguish, sorption involves the incorporation of contaminants onto or within preexisting solids (sorbents), whereas coprecipitation occurs as or shortly after the host solids precipitate from solution, such as arsenic coprecipitating with iron (oxy)(hydr)oxides in acid mine drainage. Coprecipitation might also involve arsenic-bearing colloids or other fine-grained particles becoming trapped (absorbed) in the interiors of precipitating compounds (compare with precipitation and sorption).