Simultaneous Arsenic and Iron Oxidation for One-Step Scorodite Crystallization Using Mn Oxide

Abstract

The necessity of arsenic (As) removal from metallurgical wastewaters is increasing. Despite its wide recognition as a natural oxidant, the utility of Mn oxide for scorodite production is mostly unknown. In acidic solutions containing both As(III) and Fe²⁺, simultaneous oxidation of the two progressed by MnO₂ and the resultant As(V) and Fe³⁺ triggered the formation of crystalline scorodite (FeAsO₄·2H₂O). At 0.5% or 0.25% MnO₂, 98% or 91% As was immobilized by day 8. The resultant scorodite was sufficiently stable according to the TCLP test, compared to the regulatory level in US and Chile (5 mg/L): 0.11 ± 0.01 mg/L at 0.5% MnO₂, 0.78 ± 0.05 mg/L at 0.25% MnO₂. For the oxidation of As(III) and Fe²⁺, 54% (at 0.5% MnO₂) or 14% (at 0.25% MnO₂) of initially added MnO₂ remained undissolved and the rest dissolved in the post As(III) treatment solution. For the Mn recycling purpose, the combination of Mn²⁺-oxidizing bacteria and biogenic birnessite (as homogeneous seed crystal) was used to recover up to 99% of dissolved Mn²⁺ as biogenic birnessite ((Na, Ca)0.5(Mn^IV, Mn^III)₂O₄·1.5H₂O), which can be utilized for the oxidation treatment of more dilute As(III) solutions at neutral pH. Although further optimization is necessary, the overall finding in this study indicated that Mn oxide could be utilized as a recyclable oxidant source for different As(III) treatment systems.

Fig. 4 XRD patterns and SEM images before (a, a′) and after (b–d, b′–d′) the scorodite precipitation reaction at different MnO₂ doses: 0.15% (b, b′), 0.25% (c, c′), 0.5% (d, d′). XRD peaks: M (ε-MnO₂; Akhtenskite, PDF No. 01-089-5171), S (scorodite; JCPDS 37-0468).