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

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 (FeAsO 4 ·2H 2 O) 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.

The Effect of Heterogeneous Seed Crystals on Arsenite Removal as Biogenic Scorodite

“…17) In another study targeting a low initial As(III) concentration of 4.7 mM, the advantages in feeding biogenic-seeds, rather than chemically-synthesized-seeds, was found. 11) The morphological and structural differences between biogenic and chemically-synthesized scorodite-seeds significantly affected the formation process of biogenic scorodite; feeding the former was found more effective not only in accelerating the reaction but also in forming more recalcitrant products. 11) By feeding homogeneous scorodite-seeds, such complex interactions consequently accelerated the reaction kinetics by shortening the induction period during the biogenic scorodite crystallization process (Appendix A1).…”

“…10) In our biogenic studies, the positive effect of scoroditeseeds was also noted. 11,17) Since biomineralization reactions involve the interactions between the mineral, microbial cells (physicochemical as well as enzymatic effects) and reactant ions, the function of seed minerals within the biomineralization process can be complex. In one study, the scoroditeseeds directly influenced to push the microbial (enzymatic) activity; inhibition of microbial Fe 2+ -and As(III)-oxidizing abilities by co-existing Cu 2+ ions was readily alleviated by feeding scorodite-seeds.…”

“…1) The concentrated soluble As(V) (³hundreds of millimolar) can be chemically mineralized into scorodite via hydrothermal 2,3) or atmospheric reactions 410) at temperatures mostly 95160°C. 11) Nonetheless, residual As ions after such chemical reactions (in the form of As(V), as well as As(III) which had persisted the pre-oxidation step) yet remain to be treated. Some metallurgical operations also produce As(III) solutions at more dilute concentrations (³25 mM), at which chemical reactions become less effective.…”

“…Some metallurgical operations also produce As(III) solutions at more dilute concentrations (³25 mM), at which chemical reactions become less effective. 11) In order to realize scorodite crystallization at such thermodynamically less feasible As concentrations under even milder conditions (e.g., lower temperatures, the lower doses of chemical reagents), microbiological approaches were proposed. 1118) Our previous studies demonstrated the simultaneous microbial Fe 2+ and As(III) oxidations by the thermo-acidophilic archaeon Acidianus brierleyi (70°C), leading to one-step biogenic scorodite crystallization without adding chemical oxidants.…”

“…18) Unlike in abiotic studies targeting high As(V) concentrations where the pH ini generally set at around 1.0, 510) the necessity was indicated of starting the biogenic reaction at a relatively high pH ini of 1.5, in order to precipitate As from dilute solutions. 11) This is due to the biogenic scorodite crystallization proceeding via the twostage As precipitation (at pH ini 1.5), driven by the SO 4…”