Depending upon the industrial source, selenium occurs in either the particulate or the soluble form. For the former, physical treatment processes such as sedimentation or filtration are quite effective. Lime precipitation of soluble selenium has been shown to be ineffective. The most efficient process for soluble selenium appears to be anion exchange. Preoxidation of the selenite to selenate is reported to enhance exchange capacity.
Selenium dioxide is formed during combustion of elemental selenium present in fossil fuels.