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 Fe 2+ , simultaneous oxidation of the two progressed by MnO 2 and the resultant As(V) and Fe 3+ triggered the formation of crystalline scorodite (FeAsO 4 •2H 2 O). At 0.5% or 0.25% MnO 2 , 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% MnO 2 , 0.78 « 0.05 mg/L at 0.25% MnO 2 . For the oxidation of As(III) and Fe 2+ , 54% (at 0.5% MnO 2 ) or 14% (at 0.25% MnO 2 ) of initially added MnO 2 remained undissolved and the rest dissolved in the post As(III) treatment solution. For the Mn recycling purpose, the combination of Mn 2+ -oxidizing bacteria and biogenic birnessite (as homogeneous seed crystal) was used to recover up to 99% of dissolved Mn 2+ as biogenic birnessite ((Na, Ca)0.5(Mn IV , Mn III ) 2 O 4 •1.5H 2 O), 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.