| Summary: | Abstract 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 ($ Se^{0} $) 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 $ Se^{0} $ was ≥ 95%, but ≤ 5% of Se(VI)(aq) was reduced to Se(IV)(aq) with no $ Se^{0} $. Thiourea was able to reduce ≤ 75% of Se(IV)(aq) to bulk $ Se^{0} $ 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 ($ S^{0} $) and $ S_{n} $$ Se_{8−n} $ phases. The reduction efficiency of Se(IV)(aq) to bulk $ Se^{0} $ upon the addition of AA+TH was ≤ 81% at low concentrations and ≥ 90% at higher concentrations. An inverse relation to what was observed with Se(IV)(aq) was found upon the addition of AA+TH with Se(VI)(aq). At low Se(VI)(aq) concentrations, AA+TH was able to reduce more effectively (≤ 61%) Se(VI)(aq)→Se(IV)(aq)→$ Se^{0} $, while at higher concentrations, it was ineffective (≤ 11%) and $ Se^{0} $, $ S^{0} $, and $ S_{n} $$ Se_{8−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 $ Se^{0} $ 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 Abstractᅟ
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