Deconstructing efficiency and mechanisms of ferric(FeIII) and ferrous (FeII)/oxidation coagulation in arsenic-laden water treatment

Coagulation/flocculation-sedimentation (CFS) is an important treatment process for removing arsenic from wastewater, yet its optimizing and underlying mechanism remain unexplored. This study comprehensively evaluated factors influencing As(V) sequestration through various CFS strategies: Fe III C, Fe II S/NaClO, and Fe II S/aeration. At 0.1 mM initial arsenic concentration, the residual arsenic concentration in the effluent water was reduced to 0.010 mg/l (Fe III C) and 0.191 mg/l (Fe II S/aeration), at a relative Fe dose (molar ratio of Fe dose to initial As) of 5.0. At the initial arsenic concentration 1.0 mM, the removal percentage of arsenic was immediately increased to 94 (Fe III C), 88 (Fe II S/NaClO), and 93% (Fe II S/aeration) at relative Fe dose of 2.0. The theoretical sequestering efficiency of Fe to arsenic remained to be 0.48 (Fe III C), 0.38 (Fe II S/NaClO), and 0.46 (Fe II S/aeration), resulting the coagulation efficiency of both Fe III C and Fe II S greater than 95% at all relative Fe doses. Contrary to the effective sequestration of arsenic with immediate and once dosing of coagulants, arsenic coagulation with gradual and continuous spiking of Fe was not very efficient. The overall results indicated that direct precipitation of insoluble arsenic-iron compounds (e.g., FeAsO₄) and coprecipitation via inclusion or occlusion contributed significantly to the overall performance in arsenic-CFS removal, especially at relatively higher initial arsenic concentrations.