Fe-sulfide Coating For Arsenic Removal From Groundwater Under Reducing Condition

The widespread high arsenic groundwater has posed a serious threat on the safety of drinking water. In order to treat this globally natural problem, some in-situ remedia-tion technologies for high arsenic concentration have been proposed. Given the factor that high arsenic groundwater generally occurs under reducing condition in nature, the application of in-situ remediation technology still face many challenges to deal with some unexpected problems, for instance, the potential transformation among arsenic species.Based on these issues, an in situ Fe-sulfide coating approach was developed for As-contaminated groundwater remediation under reducing condition. Preparation and optimization of the Fe-sulfide coating material and arsenic removal capacity and effi-ciency have been studied through indoor experiments. And the materials were further characterized by XRD, SEM, and FT-IR to identify the arsenic removal mechanisms. The main results are as follows:1. A stable and uniform distribution of Fe-sulfide coating was achieved through a cyclical four-step procedure alternatively injecting FeSO4 and Na2S solution. At a rate of 4 mL/min,1 min 5 mmol/L FeSO4 solution and 1 min 4 mmol/L NaS2 solution were injected alternatively, flushing by 4 mL of deoxygenated deionized water between the solutions. Recycling the process 120 h can get the arsenic removal material with a po-rosity of 22.30%. The original Fe-sulfide minerals coated on sand are pyrite and amor-phous.2. When Na2HAsO3 solution (As(Ⅲ):1000 μg/L) flows through the Fe-sulfide coated quartz sand column, the breakthrough time is about 100 h, while the break-through time of inert tracer (fluorescein sodium:90mg/L) is 1.25 h. The surface com-plex and redox reactions are the main mechanism for arsenic removal by Fe-sulfide coating. Besides, arsenic can react with Fe-sulfide directly, resulting in the precipitation of arsenic-sulfide complex. The dynamic retention capacity is 44.94 mg As(Ⅲ)/g Fe.3. A cyclical four-step procedure with alternatively injecting 5 mmol/L 1000μg/L Na2HAsO3 and 4 mmol/L Na2S solution were further conducted to simulate in-situ arsenic removal from groundwater. The simulation lasted 800 hours, and the re-sults showed of the efficient removal of fluid As(Ⅲ), which were related to the for-mation of arsenopyrite by co-precipitation of iron, arsenic and sulfide, the interaction between As(Ⅲ) and S2- producing As-sulfides, and surface complex reaction.It has been fairly well found through experimental work above that fluid As(Ⅲ) can be efficiently removed by FeSm coating materials under reducing condition and the prospect of in-situ application would provide a cost-effective technology for the treat-ment of high arsenic groundwater.