Systematic study on the reduction efficiency of ascorbic acid and thiourea on selenate and selenite at high and trace concentrations.

Selenate (Se(VI)) and selenite (Se(IV)) are common soluble wastewater pollutants in natural and anthropogenic systems. We evaluated the reduction efficiency and removal of low (0.02 and 2 mg/L) and high (20 and 200 mg/L) Se(IV)(aq) and Se(VI)(aq) concentrations to elemental (Se0) via the use of ascorbic acid (AA), thiourea (TH), and a 50-50% mixture. The reduction efficiency of AA with Se(IV)(aq) to nano- and micro-crystalline Se0 was ≥ 95%, but ≤ 5% of Se(VI)(aq) was reduced to Se(IV)(aq) with no Se0. Thiourea was able to reduce ≤ 75% of Se(IV)(aq) to bulk Se0 at lower concentrations but was more effective (≥ 90%) at higher concentrations. Reduction of Se(VI)(aq)→Se (IV)(aq) with TH was ≤ 75% at trace concentrations which steadily declined as the concentrations increased, and the products formed were elemental sulfur (S0) and SnSe8-n phases. The reduction efficiency of Se(IV)(aq) to bulk Se0 upon the addition of AA+TH was ≤ 81% at low concentrations and ≥ 90% at higher concentrations. An inverse relation to what was observed with Se(IV)(aq) was found upon the addition of AA+TH with Se(VI)(aq). At low Se(VI)(aq) concentrations, AA+TH was able to reduce more effectively (≤ 61%) Se(VI)(aq)→Se(IV)(aq)→Se0, while at higher concentrations, it was ineffective (≤ 11%) and Se0, S0, and SnSe8-n formed. This work helps to guide the removal, reduction effectiveness, and products formed from AA, TH, and a 50-50% mixture on Se(IV)(aq) and Se(VI)(aq) to Se0 under acidic conditions and environmentally relevant concentrations possibly found in acidic natural waters, hydrometallurgical chloride processing operations, and acid mine drainage/acid rock drainage tailings. Graphical Abstract ᅟ.