Removal of arsenite by reductive precipitation in dithionite solution activated by UV light.

This study investigates the removal of arsenite (As(III)) from water using dithionite activated by UV light. This work evaluated the removal kinetics of As(III) under UV light irradiation as affected by dithionite dose and light intensity, and characterized the nature of the precipitated solids using XPS and SEM-EDS. Photolysis of dithionite was observed by measuring dithionite concentration using UV absorbance at 315nm. This study also investigated the effect of UV light path length on soluble As concentrations to understand resolubilization mechanisms. Total soluble As concentrations were observed to decrease with reaction time due to reduction of arsenite to form solids having a yellow-orange color. The removal mechanism was found to be reductive precipitation that formed solids of elemental arsenic or arsenic sulfide. However, these solids were observed to resolubilize at later times after dithionite had been consumed. Resolubilization of As was prevented and additional As removal was obtained by frequent dosing of dithionite throughout the experiment. As(III) removal is attributed to photolysis of dithionite by UV light and production of reactive radicals that reduce As(III) and convert it to solid forms.

Spectroscopic study of Se(IV) removal from water by reductive precipitation using sulfide.

This study investigates the removal of selenium (IV) from water by reductive precipitation using sodium sulfide at neutral pH. Also, it examines the application of UV light as an activating method to enhance reductive precipitation. Furthermore, this work evaluates the effects of sulfide dose and solution pH on behavior of Se(IV) reduction. Selenium was effectively removed in sulfide solution at both neutral and acidic pH. UV irradiation did not enhance removal efficiency of Se(IV) at conditions tested, but it affected solids morphology and composition. SEM/EDS and XPS results showed that selenite was reduced to elemental Se or Se-S precipitates (e.g. SenS8-n) in sulfide solution. High resolution S 2p XPS spectra suggested the presence of sulfur-containing anions (e.g. S2O3(2-), HSO3(-), etc.) or elemental S (S(0)), monosulfide (S(2-)), and polysulfides (Sn(2-)), which could be produced from sulfide photolysis or reaction with Se. In addition, large aggregates of irregular shape, which suggest Se-S precipitates or elemental sulfur, were found more prominently at pH 4 than at pH 7, and they were more noticeable in the presence of UV with longer reaction times. In addition, XRD patterns showed that gray elemental Se solids were dominant in experiments without UV, whereas Se-S precipitates (Se3S5) with an orange color were found in those with UV.