Scorodite encapsulation by controlled deposition of aluminum phosphate coatings
Introduction
Arsenic is a major contaminant in the non-ferrous extractive metallurgical industry and the safe disposal of arsenic wastes constitutes an important environmental issue in many countries including Canada, Chile, Brazil and Japan [1]. Its removal and immobilisation from industrial effluents typically involves neutralisation with lime and coprecipitation with ferric ions [2]. This approach, however, is feasible only for the treatment of low arsenic concentration in aqueous effluents. In the case of arsenic-rich and iron-deficient effluents, such as acid plant effluents, or residues and dusts the production of crystalline scorodite (FeAsO4·2H2O) is preferred instead [3]. Crystallisation of scorodite from chloride and sulphate solutions under atmospheric-pressure conditions and temperatures below the water boiling point has been researched extensively since the supersaturation-controlled approach was developed by Demopoulos and co-workers in the 1990s [4], [5], [6], [7], [8], [9], [10]. Some advantages of processing scorodite production are its high arsenic content (∼30%) and ease of slurry dewatering.
Scorodite, however, is stable only under oxic and acidic conditions [11], [12], [13], [14]. According to a recently completed study [14] the incongruent dissolution of scorodite is very slow, leading to formation of a highly metastable nano-crystalline ferrihydrite phase and simultaneous release of arsenic into solution. The solubility of scorodite at 22 °C was determined to be in the order of 1 mg/L As(V) at pH 6 and 5.8 mg/L As(V) at pH 7. Higher solubility values were observed in the alkaline region [14]. This implies that the disposal of scorodite is environmentally feasible only at pH < 7.
A special issue, related to the stability of scorodite, is its decomposition under anaerobic conditions. Rochette et al. [15], reported that scorodite undergoes reductive break down, when eh < 100 mV at 5.5 < pH < 7 releasing As(III) to groundwater. Similarly, crystalline Fe(III)–As(V) precipitates (basic ferric arsenate sulphate, BFAS [16]), produced in an autoclave (190 °C) and stored in an impounded tailings pond have been found to undergo reductive dissolution releasing As(III) (and Fe(II)) [17]. Typically anaerobic conditions are known to develop in aqueous environments below about 2 m depth [18]. Hence it is of interest to explore means that can render scorodite immune to pH (alkaline) or redox potential (reducing) variations enhancing thereby its stability.
It is the scope of this paper to describe a new stabilization process for scorodite-type solids based on the concept of encapsulation by controlled deposition of mineral coatings resistant to pH or redox alteration. Evangelou [19] has described a similar concept in order to prevent pyrite–pyrrhotite oxidation and acid production in pyritic waste. One of the encapsulation methodologies involved coating pyrite with an iron phosphate layer. In the present work, an isostructural to scorodite mineral (hydrated aluminum phosphate) was selected as encapsulating material because of its chemical–structural compatibility that allows for heterogeneous nucleation and growth on the surface of scorodite, on one hand, and its resistance to reductive breakdown on the other. This paper, in particular, provides a description of the controlled deposition–encapsulation process and examines the stability of the encapsulated scorodite under simulated anoxic (anaerobic) and oxic (aerobic) environments.
Section snippets
Preparation of scorodite
The scorodite substrate material was produced via atmospheric precipitation by the method previously developed at McGill's Hydrometallurgy Laboratory [5], [7], [8]. The procedure involved placing 1.5 L of isomolar As(V)–Fe(III)–H2SO4 (CAs = CFe = 0.13 M) solution in a 3 L Applikon reactor equipped with pH, temperature and agitation speed controls. All the reagents and chemicals used were of analytical grade. The solution was heated to 95 °C under stirring and the pH was adjusted to 0.9. Under these
Scorodite substrate material
XRD analysis (not shown) confirmed the synthetic substrate material used in the encapsulation tests to be scorodite with good crystallinity, as presented in a previous work [14]. Fig. 1 shows SEM and backscattered electron images of the synthetic scorodite. The material consisted of dense agglomerated particles with average size in the order of 25 μm. The scorodite particles are seen to have grown through multiple surface deposition cycles. The lighter core observed in Fig. 1(b) represents the
Conclusions
Scorodite was for the first time encapsulated by direct deposition of AlPO4·1.5H2O (AlPO4-H3) under controlled supersaturation conditions from a sulphate-matrix solution with Al(III):P(V) molar ratio equal to one at pH 1.7 and 95 °C. The controlled deposition of AlPO4·1.5H2O on scorodite particles began with an induction period characterised by an “ion exchange” reaction between PO4 and AsO4. This “ion exchange” process led to the formation of a solid solution surface layer with the following
Acknowledgments
The support of this work by the Natural Sciences and Engineering Research Council (NSERC) of Canada via a strategic project grant is gratefully acknowledged. The National Council for Scientific and Technological Development (CNPq, Brazil) is also acknowledged.
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