The treatment of wastewater contaminated by arsenic (As) ion is an important issue as a lot of abandoned mines in Japan. Normally, the co-precipitation process with ferric/ferrous ions by neutralization is used to remove arsenic ion from mines wastewater by using CaCO₃, Ca(OH)₂, NaOH etc. However, the process has some problems: It needs a lot of alkali reagents and produces a lot of the neutralized sludge. Therefore, in this research the synthesis of iron hydroxide compound as absorbent for arsenic ion was developed in our laboratory. The adsorptive properties of the arsenic ion in artificial wastewater (incl. As and SO₄^2-) and practical mine wastewater from abandoned mine were investigated by using the iron hydroxide compound. The results are as follows:
(1) The iron hydroxide compound has a high adsorption capacity for the wastewater containing arsenic ion with pH 4 to 6. In case of practical mine wastewater, it could remove the arsenic ion under Japanese standard regulations (0.1 mg/l) even if 1 gram per liter adsorbent was added in the wastewater at pH 4. It indicated that the iron hydroxide compound could achieve high removal performance of arsenic in mine wastewater not only co-precipitation phenomena.
(2) The maximum arsenic ion adsorption capacity of iron hydroxide compound was 87.32 mg per gram. In addition, it was observed the arsenic is adsorbed on the surface of iron hydroxide compound. Moreover, the CaCO₃ has a feasibility to use as an alkali reagent, as it could remove the arsenic ion from practical mines wastewater, similar with NaOH.

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