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Minerals 2018, 8(1), 23; https://doi.org/10.3390/min8010023

Factors to Enable Crystallization of Environmentally Stable Bioscorodite from Dilute As(III)-Contaminated Waters

Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
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Received: 22 December 2017 / Revised: 11 January 2018 / Accepted: 12 January 2018 / Published: 15 January 2018
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Abstract

Applicability of the bioscorodite method (use of the thermo-acidophilic Fe(II)-oxidizing archaeon Acidianus brierleyi for arsenic (As) oxidation and immobilization at 70 °C) was tested for synthetic copper refinery wastewaters of a wide range of dilute initial As(III) concentrations ([As(III)]ini = 3.3–20 mM) with varying initial [Fe(II)]/[As(III)] molar ratios ([Fe(II)]ini/[As(III)]ini = 0.8–6.0). Crystallization of scorodite (FeAsO4·2H2O) tends to become increasingly challenging at more dilute As(III) solutions. Optimization of conditions such as initial pH, seed feeding and initial [Fe(II)]/[As(III)] molar ratio was found critical in improving final As removal and product stability: Whilst setting the initial pH at 1.2 resulted in an immediate single-stage precipitation of crystalline bioscorodite, the initial pH 1.5 led to a two-stage As precipitation (generation of brown amorphous precursors followed by whitish crystalline bioscorodite particles) with a greater final As removal. The formation process of bioscorodite precipitates differed significantly depending on the type of seed crystals fed (bio- versus chemical- scorodite seeds). Feeding the former was found effective not only in accelerating the reaction, but also in forming more recalcitrant bioscorodite products (0.59 mg/L; Toxicity Characteristic Leaching Procedure (TCLP) test). Under such favorable conditions, 94–99% of As was successfully removed as crystalline bioscorodite at all dilute As(III) concentrations tested by setting [Fe(II)]ini/[As(III)]ini at 1.4–2.0. Providing an excess Fe(II) (closer to [Fe(II)]ini/[As(III)]ini = 2.0) was found beneficial to improve the final As removal (up to 98–99%) especially from more dilute As(III) solutions. View Full-Text
Keywords: Acidianus brierleyi; thermo-acidophilic archaeon; arsenic; scorodite; As(III) and Fe(II) oxidation Acidianus brierleyi; thermo-acidophilic archaeon; arsenic; scorodite; As(III) and Fe(II) oxidation
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Tanaka, M.; Okibe, N. Factors to Enable Crystallization of Environmentally Stable Bioscorodite from Dilute As(III)-Contaminated Waters. Minerals 2018, 8, 23.

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