Fe(III)-As(V) precipitates obtained from a sulphate system and their leach stability

Fe(III)-As(V) precipitates were synthesised from the Fe(II)-As(V)-SO42--H2O system at a temperature of 90 degrees C and a constant pH maintained at 1.50.05. The precipitates obtained were characterised by X-ray diffraction (XRD), chemical composition analysis, scanning electron microscope (SEM) and Raman spectrum, thermogravimetric analyzer (TGA) and electron probe micro-analyzer (EPMA). The precipitates were in irregular aggregation of about 1-4m in size. The precipitates consisted of scorodite, ferric arsenate and an amorphous ferric hydroxide sulphate formulated as Fe(OH)(x)(SO4)(y). The precipitates were stable in modified Toxicity Characteristic Leaching Procedure (TCLP) tests at pH 4.93 for 60h. Arsenic concentrations in the leaching solutions of 0.27mgL(-1) and 0.59mgL(-1) were obtained for the precipitates prepared initial Fe(II)/As(V) molar ratios of 4.0 and 5.0, respectively. Significantly more iron than arsenic was dissolved with up to 280mgL(-1) of iron reporting to solution. Long-term stability tests of the precipitates were carried out by leaching them for 40 days at 25 degrees C under various media of pH between 9.50 and 10.57. The results showed that the precipitates tested in this study were more stable than those by previous researchers owing to a preferential dissolution of the amorphous ferric hydroxide sulphate.