Removal of arsenic(III) and arsenic(V) ions from aqueous solutions with lanthanum(III) salt and comparison with aluminum(III), calcium(II), and iron(III) salts

Interactions of arsenic(III) and arsenic(V) ions with a lanthanum salt were studied with the aim of developing a new precipitation method for removal of arsenic from aqueous solutions. Performance was compared to those of aluminum? polyaluminum chloride (PAC), calcium, and iron(III) salts. Arsenic(III) was removed by iron(LII) and lanthanum in a narrow pH range with less than 60% removal. Arsenic(V) was removed more efficiently by aluminum, PAC, iron(III), and lanthanum. Lanthanum was most effective, meeting Japanese effluent and drinking water standards by adding three times as much lanthanum as arsenic(V). The stoichiometry and X-ray diffraction measurement showed that the precipitation reactions are La3+ + H2AsO4- --> LaASO(4) + 2H(+) (pH 5) (1) and La3+ + HAsO4- --> LaAsO4 + H+ (pH 9) (2) The solubility product of lanthanum arsenate, LaAsO4, was calculated to be 1.07 +/- 0.03 x 10(-21).