Phytostabilization of mine tailings using compost-assisted direct planting: Translating greenhouse results to the field

Abstract

Standard practice in reclamation of mine tailings is the emplacement of a 15 to 90 cm soil/gravel/rock cap which is then hydro-seeded. In this study we investigate compost-assisted direct planting phytostabilization technology as an alternative to standard cap and plant practices. In phytostabilization the goal is to establish a vegetative cap using native plants that stabilize metals in the root zone with little to no shoot accumulation. The study site is a barren 62-hectare tailings pile characterized by extremely acidic pH as well as lead, arsenic, and zinc each exceeding 2000 g kg−1. The study objective is to evaluate whether successful greenhouse phytostabilization results are scalable to the field. In May 2010, a 0.27 hectare study area was established on the Iron King Mine and Humboldt Smelter Superfund (IKMHSS) site with six irrigated treatments; tailings amended with 10, 15, or 20% (w/w) compost seeded with a mix of native plants (buffalo grass,arizona fescue, quailbush, mesquite, and catclaw acacia) and controls including composted (15 and 20%) unseeded treatments and an uncomposted unseeded treatment. Canopy cover ranging from 21 to 61% developed after 41 months in the compost-amended planted treatments, a canopy cover similar to that found in the surrounding region. No plants grew on unamended tailings. Neutrophilic heterotrophic bacterial counts were 1.5 to 4 orders of magnitude higher after 41 months in planted versus unamended control plots. Shoot tissue accumulation of various metal(loids) was at or below Domestic Animal Toxicity Limits, with some plant specific exceptions in treatments receiving less compost. Parameters including % canopy cover, neutrophilic heterotrophic bacteria counts, and shoot uptake of metal(loids) are promising criteria to use in evaluating reclamation success. In summary, compost amendment and seeding, guided by preliminary greenhouse studies, allowed plant establishment and sustained growth over four years demonstrating feasibility for this phytostabilization technology.

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

“…The concentration of arsenic in the IKMHSS tailings in the top half-meter is 53 mmol kg −1 , while the bulk density of the IKMHSS tailings is about 1.5 kg dm −3 . 4,10 Therefore, the total arsenic that would be contained in the live mesquite root growth volume determined by Ansley et al 51 represents ca. 0.001% of the total arsenic in the bulk 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.…”

“…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. 10–15 Compost-assisted direct planting during tailings phytostabilization has the goals of immobilizing contaminants against leaching or off-site particulate transport, establishing a positive feedback to improved soil health and fertility while decreasing contaminant leaching to groundwater. 16,17 However, the changes in metal(loid) speciation that occur as a result of root proliferation in the porous tailings media, and that control stabilization at the molecular scale, remain poorly resolved.…”

Bacterial Rhizoplane Colonization Patterns of Buchloe dactyloides Growing in Metalliferous Mine Tailings Reflect Plant Status and Biogeochemical Conditions

“…The tailings are characterized by low organic carbon content (0.14 g kg −1 ), hypersalinity (EC 6.5–9.0 ds m −1 ), poor substrate structure [6,7], and the complete absence of vegetation. Both a controlled 1-year greenhouse mesocosm study [7,34] and a 7-year field trial [31] have been conducted to develop phytostabilization strategies for heavy metal containment at this site. This study examines plant-microbe relationships for buffalo grass plants grown in IKMHHS tailings from both the mesocosm study and field trial, as described below.…”

“…In this study we evaluate rhizoplane and rhizosphere bacterial communities of buffalo plants grown in pyritic tailings from the Iron King Mine and Humboldt Smelter Superfund (IKMHSS) site in Dewey-Humboldt, AZ from previously described mesocosm [7] and field trial [31] experiments. We contend that understanding the factors controlling rhizoplane microbial colonization patterns will provide important insights into the significance of plant-microbe associations to plant survival during phytostabilization of acidic mine tailings.…”