Elevated arsenic(As)in groundwater poses a great threat to human health.Coagulation using iron(Fe)salts has becoming one of the most cost-effective processes for groundwater As removal.Since its development,coagulation has usually been evaluated from the macroscopic level,which provide less sufficient information to understand the coagulation process and As removal mechanism.The motivation of this study was to comparatively evaluate As removal from groundwater using Fe2(SO4)3(FS)and polyferric sulfate(PFS)from the microscopic level,and finally provide a practical solution in As geogenic area.As(Ⅲ)in groundwater was pre-oxidized with Ca(ClO)2 to achieve satisfactory As removal.The results show that 5 mg/L Ca(ClO)2 was sufficient to oxidize 305-918 μg/L As(Ⅲ)in groundwater within 10 min.PFS exhibited a lower dose(0.10 g/L,i.e.,22.0 mg Fe/L)than FS(0.12 g/L,i.e.,25.0 mg Fe/L)to reduce As(321 to 1067 μg/L)to less than 10 μg/L.Using the optimized dose of Fe salts and Ca(ClO)2,As coagulation results for ten As-tainted groundwater samples showed that all the filtrated samples using FS were below 10 μg/L As,whereas three samples after PFS slightly exceeded 10 μg/L As from 10.8 μg/L to 14.7 μg/L.The variation in As removal was related with the Fe/As molar ratio and Si concentration,as evidenced by our correlation analysis.Efficient As removal was achieved by a two-bucket system combining coagulation and sand filtration on site and the filter was backwashed once the filtration time exceeding 1.5 hour.The field experimental results showed that the treatment system can produce approximately 500 liters of water using FS and PFS during five treatment cycles.XRD and Fe k-edge XANES analysis showed that ferrihydrite was the major component in the treatment solid residues.As k-edge XANES analysis demonstrated As(Ⅴ)occupied 93-94%in the solid residue and EXAFS results indicated that As formed bidentate binuclear complex on ferrihydrite.Such a stable surface complex is beneficial for As immobilization in the solid residue,as confirmed by the much lower leachate As(0.0009-0.487 mg/L)than the EPA regulatory limit(5 mg/L).Furthermore,PFS resulted in a much less solid residue than FS due to its higher As removal efficiency and subsequently lower dose.In summary,the results highlight the microscopic level mechanism of Fe coagulation to remove As from groundwater,and propose a convenient two-bucket system for As-safe drinking water treatment using PFS.