Selenium (selenium)

Selenium (Se) is a metalloid that is an essential trace nutrient at tiny doses but toxic at slightly higher levels — the gap between deficiency and toxicity is unusually narrow. Critically, it bioaccumulates in aquatic food chains, concentrating up through fish and birds, which is why its effluent limit is very tight at 0.05 mg/L for inland surface water and public sewers.

Selenium enters effluent from coal combustion and ash, copper and metal refining, electronics and semiconductor manufacture, glass production, and certain mining. Selenium is used in electronics, photovoltaics, rectifiers, pigments and glass.

For recyclers, selenium is relevant mainly to e-waste recycling, where it appears in older electronic components (selenium rectifiers, photocopier drums, some semiconductors and glass), and potentially in copper-refining and metal-recovery streams, since selenium is associated with copper ores and concentrates. It is a trace contaminant rather than a major stream, but its tight limit and bioaccumulation hazard mean even small quantities matter.

The practical relevance is that selenium is a trace heavy-metalloid contaminant to watch in e-waste and copper-recovery effluent, controlled to a strict 0.05 mg/L. It is more difficult to remove than common heavy metals because conventional hydroxide precipitation is less effective for selenium; removal may require specialised methods (chemical reduction to elemental selenium, ferrihydrite adsorption, or biological reduction). For e-waste and copper recyclers, the actionable point is to monitor selenium where the feedstock or process could introduce it, and to recognise that meeting its tight limit can need dedicated treatment beyond the standard metal-precipitation step — a detail easily overlooked until a discharge test flags it.