Enhanced arsenite removal from water by Ti(SO4)2 coagulation

• Ti(SO4)2 performed much better than Fe2(SO4)3 for As(III) removal at pH 4.0–9.0.
• Ti(SO4)2, 50% lower dosage than Fe2(SO4)3, could achieve similar As(III) removal.
• The combined use of Ti(SO4)2/Fe2(SO4)3 may be a good choice for As(III) removal.
• As(III) was removed by forming bidentate binuclear complexes with Ti(IV) hydroxides.

Coagulation with the conventional coagulants such as ferric and aluminum salts is not efficient for As(III) removal. In this study Ti(SO4)2 was employed for enhanced As(III) removal and Fe2(SO4)3 was used as a reference. The removal efficiencies of As(III) by Ti(SO4)2 at pH 4.0–9.0 were greater than that by Fe2(SO4)3 by 7.39–32.8% and 3.14–48.1% for coagulants dosed at 8.0 mg/L and 12.0 mg/L, respectively. The advantage of Ti(SO4)2 over Fe2(SO4)3 for As(III) removal was more significant at lower pH, which may be ascribed to the more negatively charged surface of Ti(IV) hydroxides. To reduce As(III) from 0.2 mg/L to 10 μg/L, the necessary dosage of Ti(SO4)2 was only ∼50% of that of Fe2(SO4)3. The adsorption capacity of As(III) on Ti(IV) hydroxides formed in-situ was greater than that on Fe(III) hydroxides formed in-situ by ∼100 mg/g and several times higher than the adsorption capacities of TiO2 for As(III) reported in the literature. The presence of competing anions, silicate, phosphate and humic acid, did not alter the advantage of Ti(SO4)2 over Fe2(SO4)3 for arsenite removal. Replacing partial Ti(SO4)2 with Fe2(SO4)3 (same dosage) and applying them sequentially could achieve similar As(III) removal efficiency as single Ti(SO4)2, which could thus reduce the chemical cost. The extended X-ray absorption fine structure (EXAFS) spectroscopy indicated that As(III) form bidentate binuclear surface complexes with Ti(IV) hydroxides as evidenced by As(III)-Ti bond distances of 3.33–3.35 Å. This study revealed that Ti(SO4)2 may be an alternative coagulant for efficient As(III) removal.

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