Arsenic and iron speciation and mobilization during phytostabilization of pyritic mine tailings

Abstract

Particulate and dissolved metal(loid) release from mine tailings is of concern in (semi-) arid environments where tailings can remain barren of vegetation for decades and, therefore, become highly susceptible to dispersion by wind and water. Erosive weathering of metalliferous tailings can lead to arsenic contamination of adjacent ecosystems and increased risk to public health. Management via phytostabilization with the establishment of a vegetative cap using organic amendments to enhance plant growth has been employed to reduce both physical erosion and leaching. However, prior research suggests that addition of organic matter into the oxic weathering zone of sulfide tailings has the potential to promote the mobilization of arsenate. Therefore, the objective of the current work was to assess the impacts of phytostabilization on the molecularscale mechanisms controlling arsenic speciation and lability. These impacts, which remain poorly understood, limit our ability to mitigate environmental and human health risks. Here we report on subsurface biogeochemical transformations of arsenic and iron from a three-year phytostabilization field study conducted at a Superfund site in Arizona, USA. Legacy pyritic tailings at this site contain up to 3 g kg −1 arsenic originating from arsenopyrite that has undergone oxidation to form arsenate-ferrihydrite complexes in the top 1 m. Tailings were amended in the top 20 cm with 100, 150, or 200 g kg −1 (300-600 T ha −1 ) of composted organic matter and seeded with native halotolerant plant species. Treatments and an unamended control received irrigation of 360 ± 30 mm y −1 in addition to 250 ± 160 mm y −1 of precipitation. Cores to 1 m depth were collected annually for three years and sectioned into 20 cm increments for analysis by synchrotron iron and arsenic X-ray absorption spectroscopy (XAS) coupled with quantitative wet chemical and mass balance methods. Results revealed that > 80% of arsenic exists in ammonium oxalateextractable and non-extractable phases, including dominantly ferrihydrite and jarosite. Arsenic release during arsenopyrite oxidation resulted in both downward translocation and As (V)

Metal Lability and Mass Transfer Response to Direct-Planting Phytostabilization of Pyritic Mine Tailings

“…Abandoned, metalliferous, sulfide mine tailings pose a major human health and environmental hazard due to anthropogenic disturbance and the associated introduction of contaminant-bearing minerals into the environment [ 1 – 3 ]. Legacy tailings exhibit nutrient deficiency (C, N, and P), acidic pH, elevated salt levels, and high concentrations of hazardous metals and metalloids such as lead, zinc, copper, mercury, cadmium, and arsenic [ 4 ], Together, these characteristics make tailings an inhospitable environment for plant and heterotrophic microbial growth [ 5 – 8 ], which keeps them devoid of vegetation establishment for decades to centuries following mining cessation, particularly in (semi-) arid regions. As a result, tailings are highly susceptible to fugitive dust emissions and rainwater runoff erosion, leading to the exposure of wildlife and humans to toxic elements [ 5 , 7 , 9 – 11 ].…”

“…Ferric (sulf)hydroxide crusts, which have high affinity for metal(loid) sorption, can form on root surfaces in mine tailings and sequester labile metal(loids) [ 12 ]. Additionally, roots provide a supportive scaffold that reduces wind and water erosion, and transpiration from vegetation diminishes contaminant leaching [ 4 ], Many areas experiencing heavy mining activity are located in (semi-) arid regions where water throughflux is limited, and therefore, so is the production of acid mine drainage (AMD). Dry conditions lead to the persistence of secondary weathering minerals, metal contaminants, and acidity in tailings media, preventing natural plant establishment [ 3 , 15 , 16 ].…”

“…The wind-driven erosion of the IKMHSS surface tailings is known to transport toxic metal(loid)-containing particulates to the surrounding community [ 5 , 8 , 10 , 35 , 36 ], Toxic metal(loid) lability and transport are of particular concern at this site due to arsenic concentrations being measured above the allowed state limit for residential soils (10 mg kg −1 ) [ 37 ], The field-scale (appx. 1 ha) phytostabilization of the IKMHSS tailings, initiated in 2010, established the long-term feasibility and efficacy of the compost-assisted, direct planting of a vegetation cap to reduce off-site dust emissions and sequester toxic metalloids in place [ 4 , 38 ], Previous studies at this site showed that phytoremediation was especially effective in reducing emissions of fine particulate matter (e.g., PM 2.5 ), which have the greatest health risks and the highest potential for long-distance transport [ 38 ], The IKMHSS tailings remedial-strategy investigation of the subsurface fate and mobility of hazardous metals is a case study of the fate and mobility of metal contaminants in acidic sulfide mine waste in (semi-) arid regions worldwide.…”

Nano Zero Valent Iron (nZVI) as an Amendment for Phytostabilization of Highly Multi-PTE Contaminated Soil

“…These include phytoextraction, phytostabilization, phytodegradation, phytovolatilization, and rhizodegredation [ 10 ]. Phytostabilization relies on contaminated areas being colonized by plants which tolerate high concentrations of PTEs; it also prevents the spread of contaminants throughout the environment [ 11 ]. Aided phytostabilization is a relatively newly applied technique for the bioremediation of the environment.…”

Biochar-templated surface precipitation and inner-sphere complexation effectively removes arsenic from acid mine drainage

“…Containment is an effective management strategy at point sources of contamination, and successful efforts to amend mine tailings and AMD in-place to decrease risks of human health include excavation, capping, phytostabilization, and permeable reactive barriers (Hammond et al 2018 ; Hu et al 2019 ; Li et al 2018 ; Mendez and Maier 2008 ; Valentin-Vargas et al 2013 ). However, costs and unanticipated knock-on effects of remedial strategies compel further investigations into effective health risk mitigation (Hammond et al 2020 ).…”