Abstract:The effect of alkali metal ions of Na+ and K+ on phase composition of As-bearing precipitate in hydrothermal arsenic solidization system is remarkable. In addition to co-precipitation with As(Ⅴ) to form scorodite (FeAsO4?2H2O) and sub-hydrous ferric arsenate (FeAsO4?0.75H2O), Fe(Ⅲ) will also competitively precipitate as Na-jarosite(NaFe3(SO4)2(OH)6), K-jarosite(KFe3(SO4)2(OH)6) or basic ferric sulfate Fe(OH)SO4, thereby affect the stability of As-bearing precipitate. The effect of Na+ and K+ on precipitation rate of arsenic and iron, composition of precipitate, transformation of mineral phase, and the stability of precipitated were studied in the system of Na2SO4-(K2SO4)-FeSO4-H3AsO4-H2O. The results show that with the co-existing of Na+ and K+, the precipitation rate of As and Fe is 98.6% and 94.5% respectively, and the As-bearing precipitate mainly composed of scorodite, Na-jarosite, K-jarosite, and is concomitant with a small amount of basic ferric sulfate. The precipitate contains As 21.2%, Fe 27.1%, Na 0.2%, K 2.0% and S 4.84%. The formation of metastable phase of jarosite causes As concentration exceeds the standard limit (5 mg/L) in the leaching toxicity test of As-bearing precipitate residue. Increasing reaction temperature and prolonging reaction time can inhibit the formation of jarosite phase or promote its transformation into scorodite.