Formation of active Fe(OH)3 in situ for enhancing arsenic removal from water by the oxidation of Fe(II) in air with the presence of CaCO3

The wide-spread arsenic contamination is considered one of the most global serious environmental issues in the world. In this work, enhanced arsenic removal from water by the oxidation of FeSO4 with CaCO3 in air was investigated and the mechanisms were discussed. Oxidation of Fe(II) into Fe(OH)3 in air became possible in the presence of CaCO3, of which the hydrolyzed OH− from CaCO3 changed the conversion route of Fe(II) to Fe(OH)3. Freshly generated Fe(OH)3 in situ presented high activity for arsenic removal, with Fe/As molar ratio at 2 giving the removal rate of 99.93% and low remaining concentration of 0.035 mg/L from 50 mg/L initial concentration. Precipitation products were characterized by a set of analytical methods including XRD, XPS and Raman analyses to confirm the existence of ferric arsenate phase. Different from using Ca(OH)2 with enough OH− group, the slow hydrolysis of CaCO3 resulted in a continuous generation of fresh Fe(OH)3 for arsenate precipitation and could avoid its rapid transformation into stable FeOOH of large particle size to lose the activity. Aging the ferric compositions by prolonged agitation of Fe(II) and CaCO3 before with arsenate solution confirmed the growth of FeOOH and resulted in a rapid decrease in the capacity for arsenic removal. The observed phenomena were interpreted based on the difference between the precipitation by fresh Fe(OH)3 and the adsorption by large crystalline particles of FeOOH. Controlling the forming speed of ferric composition from Fe(II) with the addition of solid CaCO3 to maintain a fresh state as an easy operation could enhance tremendously its efficiency for As removal, with the ever highest capacity of 248 mg/g among iron absorbents.