PMID: 18335091;PMCID: PMC2265025;Abstract:
Objective: Unreclaimed mine tailings sites are a worldwide problem, with thousands of unvegetated, exposed tailings piles presenting a source of contamination fox nearby communities. Tailings disposal sites in arid and semiarid environments are especially subject to eolian dispersion and water erosion. Phytostabilization, the use of plants for in situ stabilization of tailings and metal contaminants, is a feasible alternative to costly remediation practices. In this review we emphasize considerations for phytostabilization of mine tailings in arid and semiarid environments, as well as issues impeding its long-term success. Data sources: We reviewed literature addressing mine closures and revegetation of mine tailings, along with publications evaluating plant ecology, microbial ecology, and soil properties of mine tailings. Data extraction: Data were extracted from peer-reviewed articles and books identified in Web of Science and Agricola databases, and publications available through the U.S. Department of Agriculture, U.S. Environmental Protection Agency, and the United Nations Environment Programme. Data synthesis: Harsh climatic conditions in arid and semiarid environments along with the innate properties of mine tailings require specific considerations. Plants suitable for phytostabilization must be native, be drought-, salt-, and metal-tolerant, and should limit shoot metal accumulation. Factors for evaluating metal accumulation and toxicity issues are presented. Also reviewed are aspects of implementing phytostabilization, including plant growth stage, amendments, irrigation, and evaluation. Conclusions: Phytostabilization of mine tailings is a promising remedial technology but requires further research to identify factors affecting its long-term success by expanding knowledge of suitable plant species and mine tailings chemistry in ongoing field trials.
Desert mine tailings may accumulate toxic metals in the near surface centimeters because of low water through-flux rates. Along with other constraints, metal toxicity precludes natural plant colonization even over decadal time scales. Since unconsolidated particles can be subjected to transport by wind and water erosion, potentially resulting in direct human and ecosystem exposure, there is a need to know how the lability and form of metals change in the tailings weathering environment. A combination of chemical extractions, X-ray diffraction, micro-X-ray fluorescence spectroscopy, and micro-Raman spectroscopy were employed to study Pb and Zn contamination in surficial arid mine tailings from the Arizona Klondyke State Superfund Site. Initial site characterization indicated a wide range in pH (2.5 to 8.0) in the surficial tailings pile. Ligand-promoted (DTPA) extractions, used to assess plant-available metal pools, showed decreasing available Zn and Mn with progressive tailings acidification. Aluminum shows the inverse trend, and Pb and Fe show more complex pH dependence. Since the tailings derive from a common source and parent mineralogy, it is presumed that variations in pH and "bioavailable" metal concentrations result from associated variation in particle-scale geochemistry. Four sub-samples, ranging in pH from 2.6 to 5.4, were subjected to further characterization to elucidate micro-scale controls on metal mobility. With acidification, total Pb (ranging from 5 - 13 g kg(-1)) was increasingly associated with Fe and S in plumbojarosite aggregates. For Zn, both total (0.4 - 6 g kg(-1)) and labile fractions decreased with decreasing pH. Zinc was found to be primarily associated with the secondary Mn phases manjiroite and chalcophanite. The results suggest that progressive tailings acidification diminishes the overall lability of the total Pb and Zn pools.
Abstract:
Using the radiolabeled model pollutants 2,4-dichlorophenol (DCP) and 2,4,5-trichlorophenol (TCP) we demonstrated that brief UV (300-nm) photolysis greatly facilitates the removal of the two chlorophenols from sewage through accelerated mineralization and binding of polar products. The addition of 0.1 M H2O2 strongly accelerated the photolysis process resulting in half-lives of 1.68 and 0.87 min for DCP and TCP, respectively. In natural sunlight, half-lives of the chlorophenols were less than 1 day when H2O2 was present. During 4 days of incubation in activated sewage sludge, only 3% of unphotolyzed DCP and 1 % of unphotolyzed TCP were mineralized. Mineralization rose to 79 and 59%, respectively, after photolysis in the presence of H2O2. Photolysis without H2O2 resulted in removal of chlorophenols from solution chiefly by binding. Increased mineralization and binding were observed also upon incubation of photolyzed chlorophenols in soil. Disruption of carbon-halogen bonds by brief photolysis followed by traditional biological effluent treatment offers an alternative to activated charcoal treatment for removal of xenobiotics from industrial effluents. © 1988 American Chemical Society.
Abstract:
Column studies were used to investigate the fate of a representative nonaqueous-phase liquid (NAPL), hexadecane, with specific regard to (1) the effect of attached bacteria on the formation of residual saturation and (2) the role of biodegradation and biosurfactants on the removal of residual NAPL. Residual saturation of hexadecane was determined using sterile sand (40/50 mesh) columns and was found to be 19.0 ± 4.8% of the pore volume. Columns loaded with bacterial biomass (Pseudomonas aeruginosa ATCC 15442, 109 cells g-1) showed no difference in residual hexadecane formation as compared to sterile sand columns. In further column studies examining the effect of ATCC 15442 and biosurfactants on the removal of hexadecane residual, results showed that biodegradation alone removed approximately 50% of the [14C]hexadecane, in the form of 14CO2 and undefined cellular metabolites, during elution with at least 200 pore volumes of mineral salts medium. The columns were then eluted with 1 mM rhamnolipid biosurfactant, which increased total removal to 65%. Rhamnolipid addition resulted in (1) the mobilization of hexadecane free product and (2) a transitory 3-12-fold increase in the rate of hexadecane mineralization. In a separate study, the column was eluted from the beginning with a low (0.1 mM) concentration of rhamnolipid. This lower concentration of biosurfactant enhanced the removal of hexadecane by mobilization, but had no effect on the rate of biodegradation of residual hexadecane. Analysis of residual radioactivity within two columns revealed only 2% remaining as intact hexadecane. These results suggest that a combination of biodegradation and rhamnolipid treatment could be used to maximize the removal of residual NAPL from porous media.
Past mining activities in northern Mexico left a legacy of delerict landscapes devoid of vegetation and seasonal formation of salt efflorescence. Metal content was measured in mine tailings, efflorescent salts, soils, road dust, and residential soils to investigate contamination. Climatic effects such as heavy wind and rainfall events can have great impact on the dispersion of metals in semi-arid areas, since soils are typically sparsely vegetated. Geochemical analysis of this site revealed that even though total metal content in mine tailings was relatively low (e.g. Cu= 1000 mg kg(-1)), metals including Mn, Ba, Zn, and Cu were all found at significantly higher levels in efflorescence salts formed by evaporation on the tailings impoundment surface following the rainy season (e.g. Cu= 68,000 mg kg(-1)). Such efflorescent fine-grained salts are susceptible to wind erosion resulting in increased metal spread to nearby residential soils. Our results highlight the importance of seasonally dependent salt-formation and wind erosion in determining risk levels associated with potential inhalation or ingestion of airborne particulates originating from contaminated sites such as tailings impoundments. In low metal-content mine tailings located in arid and semi-arid environments, efflorescence salts could represent a human health risk and a challenge for plant establishment in mine tailings.
