Natural Arsenic Attenuation via Metal Arsenate Precipitation in Soils Contaminated with Metallurgical Wastes: I. Wet Chemical and Thermodynamic Evidences

Abstract

Arsenic from natural and anthropogenic sources is a worldwide contaminant of aqueous environments, such as groundwater and soils. The present investigation was performed on Mexican soils contaminated with residues from metallurgical processes that have shown a natural As attenuation. Experimental aqueous arsenic extractions in these were successfully simulated for almost half of the soil samples using a database updated for all known metal arsenate formation constants, revealing the predominance of solubility-controlled As mobility via Pb, mixed Pb-Cu, and Ca arsenate solid formation. The relatively low total Fe/As ratios (2-13 w/w) present in the soils studied, together with the high and equivalent contents of As, Pb, and Cu in these, favor the precipitation process over As(V) adsorption to Fe oxides, despite a 2% average Fe content in the soils studied. Under these conditions bicarbonate was found to be a highly unsuitable extractant due to its indirect As release from the solid arsenates, via heavy metal carbonate precipitation processes.

Arsenate and cadmium co-adsorption and co-precipitation on goethite

“…The involvement of AsO 4 3− in precipitation reduces its truly adsorption (As(V)-goethite complexation) on goethite, reflected by the evidence of less As(V) remaining in the solid phase. In a series of studies on metal arsenate precipitation, increasing pH slightly favors the adsorption process, and high As/Fe ratio favors the precipitation process [41,42]. The Pb arsenate precipitation is the dominant process and no surface site saturation with As(V) occurs even at the high As/Fe ratio [42].…”

Approaching the geochemical complexity of As(V)-contaminated systems through thermodynamic modeling

“…However, recent evidence for the formation of highly insoluble metal arsenate minerals (mainly of lead and copper) was found as an alternative natural attenuation process for As(V) in soils contaminated with mine residues (Gutierrez-Ruiz et al, 2005;Villalobos et al, 2010), apparently competing against the sorption mechanism described above.…”

“…Modeling was performed using an ideal goethite with a high SSA (94 m 2 g − 1 ) as the iron (hydr-)oxide source (VacaEscobar et al, 2012), and was validated with experimental wet chemical data generated for this simple system. The applied model coupled the previous As(V) surface complexation model (Salazar-Camacho and Villalobos, 2010) with lead(II) arsenate minerals solubility [with updated solubility product constants (Gutierrez-Ruiz et al, 2005;Villalobos et al, 2010)], and with all available equilibria in the aqueous phase.…”

“…Log K f taken fromVillalobos et al (2010) and added to the Visual Minteq Database(Gustafsson, 2010).…”

Modeling the additive effects of Pb(II) and Cu(II) on the competitive attenuation of As(V) through solid precipitation versus adsorption to goethite

“…However, there is evidence that in situations where the metal contents that accompany As(V) are high (as in smelting, mining and metallurgical wastes), formation of (highly insoluble) heavy metal arsenates occurs, such as duftite, mimetite, hydroxymimetite and bayldonite, making precipitation the predominant immobilization mechanism over the adsorption process (Gutierrez-Ruiz et al, 2005;Villalobos et al, 2010;Drahota and Filippi, 2009;Vaca-Escobar et al, 2012). For example, Villalobos et al (2010) reported various As-contaminated soils with pH values between 4.5 and 10.2, As/Fe molar ratios of 0.03 -2.5, As/Pb molar ratios of 0.53 -300, and As/Cu molar ratios of 0.44 -32, in which the presence of mixed heavy metal arsenates was identified.…”