Surficial weathering of iron sulfide mine tailings under semi-arid climate

Abstract

Mine wastes introduce anthropogenic weathering profiles to the critical zone that often remain unvegetated for decades after mining cessation. As such, they are vulnerable to wind and water dispersion of particulate matter to adjacent ecosystems and residential communities. In sulfide-rich ore tailings, propagation to depth of the oxidative weathering front controls the depth-variation in speciation of major and trace elements. Despite the prevalence of surficial mine waste deposits in arid regions of the globe, few prior studies have been conducted to resolve the near-surface profile of sulfide ore tailings weathered under semi-arid climate. We investigated relations between gossan oxidative reaction-front propagation and the molecular speciation of iron and sulfur in tailings subjected to weathering under semi-arid climate at an EPA Superfund Site in semi-arid central Arizona (USA). Here we report a multi-method data set combining wet chemical and synchrotron-based X-ray diffraction (XRD) and X-ray absorption near-edge spectroscopy (XANES) methods to resolve the tight coupling of iron (Fe) and sulfur (S) geochemical changes in the top 2 m of tailings. Despite nearly invariant Fe and S concentration with depth (130–140 and 100–120 g kg−1, respectively), a sharp redox gradient and distinct morphological change was observed within the top 0.5 m, associated with a progressive oxidative alteration of ferrous sulfides to (oxyhydr)oxides and (hydroxy)sulfates. Transformation is nearly complete in surficial samples. Trends in molecular-scale alteration were co-located with a decrease in pH from 7.3 to 2.3, and shifts in Fe and S lability as measured via chemical extraction. Initial weathering products, ferrihydrite and gypsum, transform to schwertmannite, then jarosite-group minerals with an accompanying decrease in pH. Interestingly, thermodynamically stable phases such as goethite and hematite were not detected in any samples, but ferrihydrite was observed even in the lowest pH samples, indicating its metastable persistence in these semiarid tailings. The resulting sharp geochemical speciation gradients in close proximity to the tailings surface have important implications for plant colonization, as well as mobility and bioavailability of co-associated toxic metal(loid)s.

Mechanisms of Arsenic Sequestration by Prosopis juliflora during the Phytostabilization of Metalliferous Mine Tailings

“…4,5,10 Briefly, the federal Superfund site is a pyritic tailings pile containing bulk arsenic concentrations of ca. 4 g kg −1 from arsenopyrite that has undergone oxidative weathering in the top 2 m over the past 50 years since deposition.…”

“…2100 mmol kg −1 Fe, 3100 mmol kg −1 S, and 40 mmol kg −1 As). 4,5 Under the oxidizing conditions of the surficial IKMHSS tailings, ferrous sulfides naturally weather to form ferric (oxyhydr)oxides and (hydroxy)sulfates. 4,5 These arsenic enriched secondary minerals have the potential for off-site transport as geo-dust in wind-driven erosion.…”

“…4,5 Under the oxidizing conditions of the surficial IKMHSS tailings, ferrous sulfides naturally weather to form ferric (oxyhydr)oxides and (hydroxy)sulfates. 4,5 These arsenic enriched secondary minerals have the potential for off-site transport as geo-dust in wind-driven erosion. 6–9 One potential low-cost, long-term remediation method proposed for such abandoned mine tailings involves phytostabilization, i.e., the establishment of a sustainable vegetation “cap” with which to effectively contain legacy tailings particles and the associated metal(loid)s including arsenic, thereby diminishing contaminant exposure to adjacent communities.…”

Experimental Simulation of Long Term Weathering in Alkaline Bauxite Residue Tailings

“…This is in sharp contrast with the weathering trajectories of tailings derived from sulfidic ores, in which initially low pH and high salinity are exacerbated during weathering due to oxidation of residual iron sulfides [2,4]. Some increases in EC and TA were observed in 100 °C and 135 °C treatments whereas EC and TA initially increased and then sharply decreased in the 165 °C, 200 °C, and 235 °C treatments.…”

“…Tailings have typically been treated solely as waste materials; however, tailings are increasingly being recognized as substrates for soil development [2][3][4]. The annual mass of tailings produced globally is approximately one-third of the mass of soil lost through erosion globally [1,5].…”