Selenite Stable Isotope Fractionation during Abiotic Reduction by Sodium Sulfide

Area of Science:

• Environmental chemistry
• Geochemistry
• Biogeochemistry

Background:

• Sulfur-reducing bacteria (SRB) facilitate selenium removal from groundwater.
• Selenium removal occurs via direct respiration or hydrogen sulfide (H₂S_(g))-mediated abiotic reduction.
• Understanding abiotic selenium reduction is crucial for distinguishing microbial and non-microbial selenium cycling.

Purpose of the Study:

• To investigate the selenium isotope fractionation during abiotic reduction of selenium (IV) by H₂S_(g).
• To compare abiotic fractionation with microbial selenium reduction processes.
• To determine if abiotic and microbial selenium reduction can be differentiated based on isotopic signatures.

Main Methods:

• Increasing concentrations of sodium sulfide (Na₂S) were added to selenium (IV) solutions (SeO₃^2-) to induce reduction and precipitation.
• Precipitates were analyzed using Powder X-ray Diffraction (PXRD) to identify mineral phases.
• Selenium isotope ratios (δ⁸²Se) in residual dissolved selenium were measured.

Main Results:

• Three distinct colored precipitates were observed: orange (SeS_8-), red (Se(0)), and yellow (S(0)).
• δ⁸²Se values of residual dissolved selenium increased as aqueous selenium concentration decreased.
• Sulfur to selenium ratio in solution influenced isotopic fractionation, with higher ratios yielding lower fractionation.

Conclusions:

• Abiotic reduction of selenium (IV) by H₂S_(g) results in significant selenium isotope fractionation.
• The observed fractionation is dependent on the S/Se ratio in the solution.
• Distinguishing between abiotic and microbial selenium reduction may be possible by analyzing selenium isotope fractionation patterns.