Electrocoagulation (EC) using electrodes made from iron scrap is a novel and promising strategy for arsenic (As) contaminated ground water remediation. In synthetic groundwater, amorphous hydrous ferric oxide (HFO) precipitates formed by the rapid dissolution of a sacrificial iron scrap anode adsorbed As very effectively. A competitive adsorption model developed in this study with parameters fitted for As and other coexisting anions (phosphate and silicate) was in good agreement with the observed results. It is indicated that the maximum adsorption capacity of HFO generated in the EC system was similar to 0.70 mol/mol. Reducing the As concentration in water from 500 mu g/L As(V) and As(III) to below 50 mu g/L (local drinking water standard in Bangladesh) required similar to 8 mg/L and similar to 32 mg/L iron respectively (pH = 7.1 +/- 0.1, charge dosage rate = 3 coulomb/L/min). It was found that coexisting cations (Ca2+ and Mg2+) neutralized the HFO surface charge, promoted aggregation and resulted in greater As removal. The presence of humic acid exhibited a negligible effect on As removal and HFO precipitate settling. Jar tests showed that the turbidity of the solution could be reduced to <1 NTU with the addition of 2 mg/L Al3+. (c) 2013 Elsevier B.V. All rights reserved.