The Elimination of Selenium from Contaminated Water using Sulfide or Dithionite Activated by UV Light
Authors: Jung Bahngmi Angela , Safan Aya , Abdel-Wahab Ahmed 
Published: July 2019

Selenium (Se) is an essential trace element for animals and humans because it functions as an antioxidant and catalyst for the production of active thyroid hormone. However, higher intake of Se can cause disease and death to humans. This study investigates the removal of inorganic Se (selenite, Se (IV)) from contaminated water which is known as less bioavailable and more toxic than organic forms. Several technologies exist for Se removal such as lime neutralization, bacterial reduction, ion exchange membranes, electrocoagulation, sorption, and hybrid processes such as coagulation/filtration. However, these techniques were demonstrated to be effective only at low Se concentrations levels (below 5 mg/L). This study employs an advanced reduction process (ARP) that combines a reducing agent and an activation method and investigates its applicability for Se removal at high concentrations (∼10 mg/L).

The goal of this research is to investigate the effectiveness of ARP for reducing Se (IV) to elemental Se or stable solids composing of Se and S (e.g. SeS or SeS2) in water using a combination of dithionite or sulfide as reducing agents and UV light as an activating method. Three different UV light sources including monochromatic 254 nm UV-L, UV-M with primary energy peak at 365 nm, and 312 nm UV-B were evaluated. This work studied the effects of operating conditions such as concentrations of reducing agents, solution pH, and initial selenium concentration for the optimum combination that achieves the efficient reduction of Se (IV).

In industrial applications, sulfur dioxide has been used as the reactant to reduce Se (IV) to elemental Se, but it was reported that SO2 was ineffective for Se (IV) reduction for waters with low Se concentration at room temperature. In this study, sodium dithionite and sodium sulfide were used as a strong reducing agent of interest. Sodium dithionite was proven to be an effective reductant and produce active reducing radicals when activated by UV light. It has been shown to be very effective in reducing chlorate, trichloroethylene or 1,2-dichloroethane in our earlier studies. It is known that dithionite undergoes decomposition reactions which is strongly dependent on the pH and is rapid in the acidic environment and its major decomposition products are thiosulfate (S2O3
2–) and bisulfite (HSO3–). Decomposition reaction is very complicated because its reaction products can decompose and interact with each other and can be activated by UV irradiation at various rates, which will result in the production of elemental sulfur or selenium, or precipitates of Se-S at different stages. In Se-S solution, the compounds having a general formula, SeSn (n:1–7) can be produced and it can form crystalline or amorphous composition. Therefore, we investigated the chemistry of dithionite and sulfide in Se-S water system and the behavior Se removal under UV irradiation.

Screening tests of various combinations of reducing agents and UV light types showed that the greatest Se removal (86.4%) was found with dithionite irradiated by UV-L lamp (254 nm) at around pH 5. Sulfide as a reducing reagent at neutral pH resulted in selenium removal percentages of 83.5% and 92.5% without and with UV-L irradiation, respectively. Maximum removal efficiency of Se was observed when sulfide or dithionite molar concentration was 40 times the initial Se molar concentration under UV light at given conditions. A typical color of the Se-dithionite mixture solution was milky yellow during UV-L irradiation.

Yellow-colored solids were precipitated. The combination of dithionite and UV-B (major peak at 312 nm) showed orange-colored solid in 3 hours of irradiation. It is expected that dithionite or dithionite decomposition products will produce active radicals dependent on UV light types, which may change Se-S solid chemistry. To investigate surface chemistry of solids and sulfur-containing species, X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy/energy dispersive spectrometry (SEM/EDS) were utilized. Also, incremental reducing agent solution injection or electrocoagulation method was utilized to reduce final Se concentration to below the maximum contaminant level.