title: Stability of arsenate-bearing Fe(III)/Al(III) co-precipitates in the presence of sulfide as reducing agent under anoxic conditions.
abstract: Currently, the co-precipitation of arsenate with ferric iron at molar ratios Fe(III)/As(V) ≥ 3 by lime neutralization produces tailings solids that are stable under oxic conditions. However not much is known about the stability of these hazardous co-precipitates under anoxic conditions. These can develop in tailings storage sites by the action of co-discharged reactive sulfides, organic reagent residuals or bacterial activity. The ferric matrix can then undergo reductive dissolution reactions, which could release arsenic into the pore water. Co-ions like aluminum could provide a redox-immune sink to scavenge any mobilized arsenic as a result of reduction of ferric. As such, in this work Fe(III)/As(V) = 4 and aluminum substituted Fe(III)/Al(III)/As(V) = 2/2/1 co-precipitates were produced in a mini continuous co-precipitation process circuit and subjected to excess sulfide addition under inert gas to evaluate their stability. It was found that the ferric-arsenate co-precipitate could retain up to 99% (30 mg/L in solution) of its arsenic content despite the high pH (10.5) and extremely reducing (Eh < -200 mV) environment. There was no significant reduction of arsenate and only 45% of ferric iron was reduced. Partial aluminum substitution was found to cut the amount of mobilized arsenic by 50% (down to 15 mg/L) hence mixed Fe(III)/Al(III)-arsenate co-precipitates may offer better resistance to reductive destabilization over the long term than all iron co-precipitates.
authors.affiliation: Department of Mining and Materials Engineering, McGill University, 3610 University Street, Montreal, QC, H3A 0C5, Canada.
authors.forename: Christoph
authors.initials: C
authors.lastname: Doerfelt
authors.affiliation: Department of Mining and Materials Engineering, McGill University, 3610 University Street, Montreal, QC, H3A 0C5, Canada.
authors.forename: Thomas
authors.initials: T
authors.lastname: Feldmann
authors.affiliation: Department of Chemical Engineering, McGill University, 3610 University Street, Montreal, QC, H3A 0C5, Canada.
authors.forename: Ranjan
authors.initials: R
authors.lastname: Roy
authors.affiliation: Department of Mining and Materials Engineering, McGill University, 3610 University Street, Montreal, QC, H3A 0C5, Canada. Electronic address: george.demopoulos@mcgill.ca.
authors.forename: George P
authors.initials: GP
authors.lastname: Demopoulos
mesh_terms: D000535:Aluminum / Q000737:chemistry*; D001149:Arsenates / Q000737:chemistry*; D017610:Calcium Compounds / Q000737:chemistry; D011232:Chemical Precipitation; D005290:Ferric Compounds / Q000737:chemistry*; D015386:Hazardous Substances / Q000737:chemistry*; D008906:Mining; D010084:Oxidation-Reduction; D010087:Oxides / Q000737:chemistry; D019163:Reducing Agents / Q000737:chemistry*; D013440:Sulfides / Q000737:chemistry*
chemical_list: D001149:Arsenates; D017610:Calcium Compounds; D005290:Ferric Compounds; D015386:Hazardous Substances; D010087:Oxides; D019163:Reducing Agents; D013440:Sulfides; C016538:lime; D000535:Aluminum
keywords: Anoxic environment; Arsenate; Reducing agent; Sulfide; Tailings stability