Comparing Schwertmannite and Hydrobasaluminite Dissolution in Ammonium Oxalate (pH 3.0): Implications for Metal Speciation Studies by Sequential Extraction

Abstract

The “poorly crystalline iron oxy-hydroxides” are one of the most reactive and environmentally important fractions in soils and sediments due to the association of many toxic elements associated with these minerals. The metal content of this fraction in sequential extraction procedures is usually evaluated by dissolution in ammonium oxalate ([NH4]2C2O4·H2O) at pH 3.0 and 25 ᵒC [1–12]. Such chemical treatment, however, may also dissolve other mineral phases of comparable reactivity, which can lead to wrong interpretations of mineral carriers for specific metals. In this study, we compare the dissolution kinetics of schwertmannite and hydrobasaluminite, two minerals of comparable crystallinity and reactivity that play a major role in the mobility of many trace metals in waters and sediments affected by acid mine drainage (AMD). We first synthesized these two minerals in the laboratory by partial neutralization of two different metal-rich mine waters, and then we applied the standard protocol of ammonium oxalate dissolution to different specimens; the solutions were periodically sampled at intervals of 2, 5, 10, 15, 30 and 60 min to compare (i) the kinetics of mineral dissolution, and (ii) the metals released during dissolution of these two minerals. The results indicate a very similar kinetics of mineral dissolution, though hydrobasaluminite exhibited a faster rate. Some toxic elements such as As, Cr or V were clearly bonded to schwertmannite, while many others such as Cu, Zn, Si, Co, Ni and Y were clearly linked to hydrobasaluminite. These results suggest that studies linking the mobility of many elements with the Fe cycle in AMD-affected soils and sediments could be inaccurate, since these elements could actually be associated with Al minerals of poor crystallinity. The step of ammonium oxalate dissolution in sequential extraction studies should be best described with a more general term such as “low-crystallinity oxy-hydroxides”.

The use of operationally-defined sequential Fe extraction methods for mineralogical applications: A cautionary tale from Mössbauer spectroscopy

“…Such interpretations ignore the operational nature of the selective extractions and cannot assume the presence of any particular Fe mineral phase in the leachate. There has been a long-standing concern with the use of unverified extraction results to identify specific minerals, or mineral groups, in modern soils and sediments (Sanchez-Espana et al, 2019;Adamo et al, 2018;Schröder et al, 2016;Hass and Fine, 2010;Sutherland, 2010, Zimmerman andWeindorf, 2010;Bacon and Davidson, 2008;Ryan et al, 2008;Gómez-Ariza et al, 1999;Whalley and Grant, 1994;Kheboian and Bauer, 1987).…”

Coprecipitation of Co2+, Ni2+ and Zn2+ with Mn(III/IV) Oxides Formed in Metal-Rich Mine Waters

“…Cobalt and nickel concentrations in these waters can also reach extreme values of 47 mg/L and 17 mg/L, respectively, though the average concentrations of these metals are 3 mg/L for Co and 1 mg/L for Ni [50]. Unlike many other toxic elements which are naturally attenuated downstream of mine sites by coprecipitation with iron and/or aluminum oxy-hydroxides (e.g., As, Cr, Pb, Cu, U) [51][52][53][54][55], manganese remains virtually unaffected by precipitation or sorption because the pH of these waters never exceeds values of 3.5 until they reach the Tinto and Odiel estuary [50]. In a study conducted in 2003-2004, it was calculated that between 1000 and 2600 kg/day (365-950 tons/year) of manganese had been transported to the Atlantic Ocean as dissolved load by tens of mine effluents flowing from the IPB district to the Huelva estuary [50].…”

“…The adsorption and coprecipitation of less soluble trace metals and toxic elements (e.g., As, Cr, Pb, Cu, U) during iron and aluminum oxy-hydroxide is well-known [16,38,[51][52][53][54][55] and are being conventionally applied in many neutralization plants installed in mine settings worldwide [65,66]. But new treatment facilities could be designed to remove the most soluble trace metals (including Mn, Co, Ni, and Zn) from these mine waters by favoring oxygenation in aeration ponds at near-neutral pH.…”