Phytostabilization of Mine Tailings in Arid and Semiarid Environments—An Emerging Remediation Technology

1Mine tailings disposal sites from either inactive or abandoned mine sites are prevalent in arid and semiarid regions throughout the world. Major areas include northern Mexico and the Western United States, the Pacific coast of South America (Chile and Peru), southwestern Spain, western India, South Africa, and Australia. The global impact of such mine tailings disposal sites is enormous, as unreclaimed mining sites generally remain unvegetated for tens to hundreds of years, and exposed tailings can spread over tens of hectares via eolian dispersion and water erosion.

2Mine tailings, or mill tailings, are the materials remaining after extraction and beneficiation of ores. What prevents the natural revegetation of mine tailings? It is generally a combination of factors beginning with metal toxicity. Tailings are characterized by elevated concentrations of metals such as arsenic, cadmium, copper, manganese, lead, and zinc (1–50 g/kg). Further, tailings contain no organic matter or macronutrients, and usually exhibit acidic pH, although some tailings may be alkaline. For these reasons, tailings remain without normal soil structure and support a severely stressed heterotrophic microbial community. Hence, the microbial community is extremely low in species richness and carbon utilization diversity compared with uncontaminated soil. Furthermore, autotrophic iron- and sulfur-oxidizing bacteria dominate the microbial community in mine tailings and are associated with plant death in acidic tailings.

3In arid and semiarid regions, plant establishment on mine tailings is further impeded by a number of physicochemical factors including extreme temperatures especially at the tailings surface, low precipitation, and high winds. These factors contribute to the development of extremely high salt concentrations ranging up to 22 dS/m due to high evaporation and low water infiltration.

4Disposal of mine wastes historically involved either returning the materials to the mining site; dumping into the ocean, a stream, or lake; or placing them into a receiving pond. Today, surface containment of tailings within embankments remains a commonly used approach. In 1995 it was estimated that on an annual basis over 700 million kg of metals in mine tailings were disposed on land. Alternatively, tailings may be returned to the mine (in-pit storage or backfilling) or mixed with coarse mine waste (co-disposal). In arid and semiarid regions, dry-stacking facilities are most common wherein tailings are dried, spread out, and compacted. However, they remain unstable and subject to eolian dispersion and water erosion with the potential to contaminate nearby communities and environmentally sensitive areas. Some countries mandate mining companies to remediate or contain tailings piles, whereas others have no such requirements and still allow dumping of mine tailings into bodies of water, thus escalating the existing problem of thousands of abandoned mine tailings sites worldwide.

5The construction of above-ground impoundments for mine tailings storage is increasingly problematic in arid and semiarid regions. Prevention of wind erosion by surface-wetting is not practical in such environments, especially after closure of mining operations. Therefore, tailings are a significant source of air pollution in the form of particulate matter measured in fractions of ≤ 10 μm (PM10) and ≤ 2.5 μm (PM2.5) in aerodynamic diameter. Short-term exposure to particulates (PM2.5/10) can lead to illness and even premature death in people with heart or lung disease, respiratory conditions, and decreased lung function, whereas long-term exposure to fine particles can accelerate lung cancer and cause chronic respiratory disease in children. Measurements of PM2.5/10 are monitored in recently established mining operations in some countries but are not monitored for abandoned mine tailings disposal sites despite a link to respiratory health problems and the proximity of disposal sites to human populations.

6Conventional technologies for remediation of mine tailings have focused on physical and chemical stabilization. Physical stabilization entails covering mine waste with an innocuous material, generally waste rock from mining operations, gravel, topsoil from an adjacent site, or a clay capping, to reduce wind and water erosion. These solutions are often temporary in nature because of the impermanence of the capping process. For example, clay caps in arid and semiarid environments crack from the wetting–drying cycles and poor consolidation of the tailings due to their high salinity. Chemical stabilization aims to prevent wind and water erosion using a chemical agent such as a lignin sulfonate or a resinous adhesive to form a crust over the tailings, also a temporary stabilization technique, as these crusts can eventually fail. Recently, reprocessing historic tailings materials using more advanced technologies to reduce metal concentrations and toxicity has been considered and is economical in some cases. However, the tailings material is still left after reprocessing and must be stabilized in some way. In general, traditional remediation techniques range from approximately US$1.50–450 per m3 for mine tailings contained by waste rock or cemented backfilling. An emerging remediation technology, phytostabilization, can reduce this cost to an estimated US$0.40–26 per m3 for revegetation alone or for lined and revegetated repositories.

7Phytostabilization creates a vegetative cap for the long-term stabilization and containment of the tailings. The plant canopy serves to reduce eolian dispersion whereas plant roots prevent water erosion, immobilize metals by adsorption or accumulation, and provide a rhizosphere wherein metals precipitate and stabilize. Unlike phytoextraction, or hyperaccumulation of metals into shoot/root tissues, phytostabilization primarily focuses on sequestration of the metals within the rhizosphere but not in plant tissues (Figure 1). Consequently, metals become less bioavailable and livestock, wildlife, and human exposure is reduced.

8Although phytostabilization of mine tailings sites in arid and semiarid regions has been experimented with by mining companies, documentation of this remediation technology only occasionally appears in published literature, so general understanding of this technology is limited. In this review we address the current knowledge of phytostabilization in arid and semiarid environments as well as potential problems that impact the long-term success of this technology.

9Phytostabilization of mine tailings in arid and semiarid environments involves the use of drought-, salt-, and metal-tolerant plants for immobilization of heavy metals in the tailings substrate. In theory, metal bioavailability (and hence toxicity) will decrease as plants facilitate the precipitation of metals to less soluble forms, for example, metal sulfides or metal carbonates, complex metals with organic products, sorb metals onto root surfaces, and accumulate metals into root tissues. Furthermore, the presence of plants in mine tailings enhances the heterotrophic microbial community, which may, in turn, promote plant growth and participate in metal stabilization. The ultimate objective for successful phytostabilization is the long-term succession of the plant community in mine tailings to promote soil development processes, microbial diversity, and finally, to restore soil ecosystem functions to a state of self-sustainability.

10Phytostabilization of mine tailings in arid and semiarid environments requires establishing a diverse plant community by including drought-, metal-, and salt-tolerant plants that do not hyperaccumulate metals of concern into shoot tissues. Candidates for phytostabilization ideally should be native to the area in which the mine tailings are found, as they have evolved survival mechanisms appropriate to the harsh climate of arid and semiarid environments. A secondary but also important consideration is that the use of native plants avoids introduction of nonnative and potentially invasive species that may result in decreasing regional plant diversity. To date, many field trials have not taken advantage of native plant diversity, resulting in poor plant colonization and soil conditions.

11Selection of a variety of perennial grasses, shrubs, and trees for revegetation of mine tailings is important for phytostabilization. Grasses provide a quick ground cover and temporarily limit eolian dispersion of tailings, whereas shrubs and trees become established. Shrubs and trees provide an extensive canopy cover and establish a deeper root network to prevent erosion over the long term. Furthermore, inclusion of the various growth habits maintains species and functional diversity. Shrubs or trees provide a high nutrient environment for grasses while reducing moisture stress and improving soil physical characteristics in arid and semiarid climates. Additionally, the establishment of different functional groups increases plant productivity and yield. Although a few plants may eventually dominate the ecosystem because of selective pressures, the presence and effect of less abundant species is still significant in promoting a self-sustainable ecosystem.

12Arid and semiarid soils are often saline, as evaporation rates exceed rainfall and natural salts originate from saline rainfall, unweathered minerals, and fossil salts. In mine tailings operations within arid and semiarid environments, saline groundwater is often used in the beneficiation process, which becomes hypersaline as it is recycled throughout ore processing. As tailings dry, salt crusts form on the surface. Therefore, halophytes (salt-tolerant plants) are especially valuable in phytostabilization. Members of the Chenopodiaceae family, specifically Atriplex spp., are highly salt tolerant and serve as pioneer species on mine tailings in semiarid western Australia and are used in revegetation of mine tailings in the Western United States. Other halophytic shrubs recommended in the Western United States are creosote bush (Larrea tridentate DC., Zygophyllaceae) and desert broom (Baccharis sarothroides Gray, Asteraceae). Also, leguminous trees that serve as a nitrogen supply such as Acacia spp. and Prosopis spp. have been reported as successfully colonizing mine tailings in the Western United States.

13Plants used for phytostabilization must be metallophytes (metal-tolerant plants) but ones that do not accumulate or limit metal accumulation to root tissues. Although metallophytes have developed mechanisms to impede translocation of metals in the above-ground plant mass, there may still be an excessively high concentration of metals in the shoot material. There are several ways to measure and express metal accumulation in plants. These include a) bioconcentration factor (BF) or accumulation factor (AF) = (total element concentration in shoot tissue ÷ total element concentration in mine tailings) and b) translocation factor (TF) or shoot:root (S:R) ratio = (total element concentration in shoot tissue ÷ total element concentration in the root tissue).

14Ideally these values would be << 1, but they should not exceed a ratio of 1, which would indicate that the plant is useful for phytoextraction (accumulation of metals in shoot tissue) but should not be used in phytostabilization. Surveys of native plants colonizing mine tailings have provided promising information, especially plant families of colonizers with relatively low metal accumulation in above-ground tissues (Table 1).

15In addition to the above-mentioned metal accumulation ratios, several metal concentration guidelines can be used to help evaluate metal toxicity issues that may arise during phytostabilization (Table 2). The first is soil plant toxicity levels, which can provide a guide in evaluating metal tolerance. The second is plant leaf tissue toxicity limits, which can help assess the long-term potential for plant establishment. Last, domestic animal toxicity limits can be compared with above-ground metal accumulation, as foragers, including cattle and other wildlife, may consume these plants. Unfortunately, metal accumulation in field trials has not been thoroughly documented. Thus, identification of suitable phytostabilization plant candidates and understanding their metal accumulation patterns are areas in which additional research is critically needed for practitioners in the field.

16In general, direct seeding produces a more patchy vegetation cover than using transplants. However, although the use of transplants produces better results, this approach is more labor intensive, and this is one of the factors that must be taken into consideration for each site. Further, seeding can be successful as demonstrated by a tin mine tailings site in Zimbabwe in which 40% vegetation cover was achieved with a mixture of grasses, herbaceous legumes, and trees and a single treatment of NPK fertilizer.

17One recent study examined metal uptake by Lygeum spartum grown from seeds or rhizomes in tailings in a greenhouse study. The plants grown from seeds took up significantly more metal than plants grown from rhizomes. However, plants collected from the tailings site itself showed one order of magnitude less metal accumulation. The authors point out that greenhouse conditions (higher moisture content and homogeneity of the tailings) likely influenced the results of the study.

18Because the addition of topsoil amended with organic matter and nutrients is not economical for extensive mine tailings sites, organic amendments are generally used as a substitute. Organic amendments help to immediately decrease metal bioavailability, provide a slow-release fertilizer, and serve as a microbial inoculum. In addition, organic matter improves soil structure, reduces erosion, and increases infiltration. The organic matter may be composed of wood chips, straw, biosolids, composted municipal waste, or manure. The carbon-to-nitrogen (C:N) ratio of the organic amendment should range from 12:1 to 20:1 to prevent high rates of organic matter decomposition and nitrogen consumption by the microbial community. For example, uncomposted organic amendments such as wood-chips contain a high C:N ratio; therefore, nitrogen may become immobilized and impede long-term plant establishment. The addition of commercial compost to mine tailings has been shown to enhance plant growth in greenhouse trials. As well as pH levels increasing with compost amendment, there is a decrease in the number of iron- and sulfur-oxidizers attributed to acid production and vegetation death in pyritic tailings. Furthermore, compost can increase the water-holding capacity and cation exchange capacity of mine tailings.

19Biosolids have also been used as an amendment to ameliorate the harsh conditions of mine tailings. For example, biosolids successfully increased plant growth in a gold mine tailings field trial in New Zealand. Anaerobically digested biosolids are preferred to aerobically treated biosolids because of higher nitrogen content and a greater enhancement of plant growth as tested in a copper mine tailings site. However, biosolids may contain phytotoxic levels of metals, depending on the source of the material. Also, biosolids amendment is not generally desirable in ecologically sensitive areas.

20One area that requires additional research is the potential for the acceleration of metal leaching through the addition of compost or biosolids. Laboratory studies have shown the potential for enhanced leaching of Zn from mine tailings after addition of model organic acids, for example, oxalic or citric acid, that are found in the rhizosphere. A second study examined leaching of Cu, nickel, and Zn from biosolid-amended tailings from four sites in southern Arizona. This study was performed under laboratory conditions, and results showed that for three of the tailings, which were circumneutral in pH, metal leaching increased only slightly. For the fourth, acidic tailing, metal leaching increased up to 3-fold for Cu and up to 100-fold for Zn. It should be noted that both these studies were performed over the short term and in the absence of plants.

21Addition of inorganic fertilizers should be limited, as native vegetation used for phytostabilization of mine tailings in arid and semiarid environments tends to be adapted to low nutrients and tends to respond differently to fertilizer inputs. Furthermore, if organic amendments are added, there is likely a sufficient or near sufficient concentration of nutrients already present. One exception to this is phosphorous fertilizers, which may be necessary to alleviate phosphorous deficiency due to the formation of insoluble metal-phosphates; thus it is important to consider phosphorous fertilizers to alleviate phosphate deficiency. However, the addition of phosphorous amendments can increase arsenic uptake into plants as well as leaching in mine tailings because phosphate behaves chemically similar to arsenate. In extremely acidic mine tailings, lime may be required to neutralize acidification; however, without organic matter addition, the site may require continuous inputs of lime to maintain a pH > 5.

22Although drought-tolerant plants must be used in phytostabilization, initial irrigation is usually required to aid plant establishment. If seeds are directly sown into mine tailings, irrigation is especially crucial. However, with exceptional planning, remote locations may be seeded by taking advantage of seasonal rain. Drip irrigation for at least 3–6 months or until plants become established has proven to be successful in revegetation of mine tailings. However, irrigation should be limited for both cost and dependence of the plant community on the availability of water.

23The majority of phytostabilization studies in arid and semiarid environments have focused on plant growth variables such as plant biomass and percent cover; however, other methods of evaluating successful revegetation must be taken into consideration for long-term rehabilitation of mine tailings (Appendix 1). For example, plants should be able to self-propagate successfully with no additional inputs. Also, plant species that were not seeded or transplanted should begin to colonize the area. As previously mentioned, the above-ground plant biomass may be a source of metal exposure for foraging animals. Therefore, domestic animal toxicity limits must be observed to prevent further contamination of the ecosystem. In addition, microbial communities have been largely ignored in field studies within arid and semiarid environments. Yet, the heterotrophic microbial community can be linked to plant establishment, and plant–microbe interactions are important for promoting nutrient cycling, soil aggregation, and plant nutrient uptake. Heterotrophic microbial numbers must increase, replacing the dominant presence of autotrophic iron- and sulfur-oxidizing bacteria, a community of microorganisms that plays a critical role in acidification of the tailings. Further, soil structure should improve with a recovery of the heterotrophic microorganisms and recycling of organic matter.

24One critical aspect largely missing from the published literature is information about the long-term success of phytostabilization. Most studies are terminated after 1–2 years. Although organic amendments are favored for their ability to immediately decrease metal bioavailability, weathering and decomposition of organic residues may ultimately enhance metal mobility. For example, examined both circumneutral and acidic copper mine tailings from Arizona in a simulated weathering study. Addition of biosolids decreased concentrations of copper, nitrate, and sulfate in leachate from the acidic tailings sample but slightly increased copper and arsenic in leachate from the circumneutral sample compared with concentrations in unamended tailings. Further, in short-term studies, metal availability based on plant metal accumulation may be deceiving. For example, a study of a 20-year rehabilitated uranium site in Australia indicated an increase in metal mobility in the soil surface and an increase in plant metal accumulation. As these studies suggest, additional long-term studies on the chemical state of tailings and plants are essential for determining phytostabilization success.

25Existing phytostabilization studies in arid and semiarid environments are limited and have not addressed several important issues. For example, plant metal accumulation has not been documented in the majority of field studies. This is especially important in determining the long-term fate of plant establishment as well as the metal contaminants. In addition, easily accessible information is needed on ranges of metal tolerance in metallophyte species suitable for growth in mine tailings; minimum amounts of organic amendments required for successful plant establishment; and minimum water requirements necessary for successful plant establishment. It is likely that a small pilot study, performed to optimize choice of candidate species and amounts of growth amendments for each specific tailings site, may be necessary to ultimately reduce costs and make implementation of phytostabilization successful.

26Finally, the long-term fate of metals in revegetated tailings has not been explored thoroughly. Such information is needed to help evaluate the efficacy of phytostabilization in permanently reducing metal toxicity, in promoting plant succession, and in promoting the formation of soil structure and properties in tailings materials.

27In summary, phytostabilization of mine tailings in arid and semiarid regions has promising potential. Studies have indicated that plant establishment on mine tailings is possible, and when successful, helps to reduce erosion processes and enhance soil formation properties. Although it may not be possible to create an ecosystem equivalent to the surrounding uncontaminated area, successful phytostabilization can create a self-sustaining biological cap with an attendant ecosystem that more closely resembles a healthy soil-plant environment. Defining and understanding this process will require long-term studies that explore interactions between plants and the microbial and physical–chemical characteristics of the tailings as they undergo the revegetation process.