Acidic Tailings Research Articles

Analyzing variation of the water table level with three-dimensional numerical simulations to assess reclamation techniques for an acidic tailings impoundment

Tailings with sulphide minerals exposed to oxygen and water can oxidize and produce acid mine drainage (AMD). This study evaluated the impact of natural conditions and of a thin cover deposition on the water table level, with respect to selecting the reclamation technique to control AMD on the elevated portion of an abandoned tailings impoundment. The reactive tailings were partly covered with alkaline tailings transported as a pulp from a nearby mine. A three-dimensional numerical variably saturated groundwater flow model of the tailings impoundment and its surrounding area was built. The simulation results reproduced well the field observations before, during, and after the deposition of a thin layer of tailings. The calibrated model was then used to forecast the evolution of the water table position for the next 100 years under various site conditions, including a period of 5 years with dry summers. The results show that the water table levels are on average several metres below the interface between the reactive tailings and the cover, thus failing to meet the target criterion to control the production of AMD determined from previous column tests. The results are analyzed and discussed in terms of the site configuration and atmospheric recharge.

Characteristics of an Iron-Reducing, Moderately Acidophilic Actinobacterium Isolated from Pyritic Mine Waste, and Its Potential Role in Mitigating Mineral Dissolution in Mineral Tailings Deposits.

Reactive pyritic mine tailings can be populated by chemolithotrophic prokaryotes that enhance the solubilities of many metals, though iron-reducing heterotrophic microorganisms can inhibit the environmental risk posed by tailings by promoting processes that are the reverse of those carried out by pyrite-oxidising autotrophic bacteria. A strain (IT2) of Curtobacterium ammoniigenes, a bacterium not previously identified as being associated with acidic mine wastes, was isolated from pyritic mine tailings and partially characterized. Strain IT2 was able to reduce ferric iron under anaerobic conditions, but was not found to catalyse the oxidation of ferrous iron or elemental (zero-valent) sulfur, and was an obligate heterotrophic. It metabolized monosaccharides and required small amounts of yeast extract for growth. Isolate IT2 is a mesophilic bacterium, with a temperature growth optimum of 30 °C and is moderately acidophilic, growing optimally at pH 4.0 and between pH 2.7 and 5.0. The isolate tolerated elevated concentrations of many transition metals, and was able to grow in the cell-free spent medium of the acidophilic autotroph Acidithiobacillus ferrooxidans, supporting the hypothesis that it can proliferate in acidic mine tailings. Its potential role in mitigating the production of acidic, metal-rich drainage waters from mine wastes is discussed.

Microcosm-scale biogeochemical stabilization of Pb, As, Ba and Zn in mine tailings amended with manure and ochre

Mine tailings are major sources of metals and metalloids in the environment, making the physical and geochemical stabilization of tailings a serious environmental challenge. With a view to facilitate the development of covering vegetation and of decreasing the mobility of Pb in the acid tailings of a former Ag–Pb mine, laboratory microcosm experiments were performed to enable comparison of the effectiveness of several treatments. Tailings were mixed with 5% by weight of ochre, an iron-rich material produced during the treatment of a coal mine water, and with cow manure (0, 0.15, 1 and 2% by weight), either solely or in combination. They were then submitted to weekly watering over 84 days. All treatments raised the pH values from 4 to values between 7 and 8 and induced a strong decrease in the total dissolved Pb concentration in the percolating water (from 13 to 15 mg.L−1 to less than 0.5 mg.L−1). Several processes seemed to be involved in the immobilization of Pb by the amendments: precipitation as hydroxide, sulfate, carbonate and phosphate, and adsorption on iron hydroxides. A transient increase was observed in both Pb mobility and functional microbial diversity with 1% and 2% manure, with a peak after 28 days of incubation. This peak corresponded to an Average Well Color Development (AWCD) in Biolog™ Ecoplates increase from 0.5 to 0.8 with 1% manure and from 0.6 to 1.5 with 2% manure. However, at the end of experiment, Pb immobilization was strengthened by 2% manure and microbial functional biodiversity fell back, with AWCD values of 0.5 and 0.8 for 1% and 2% manure, respectively. Other toxic elements present in the tailings, namely As, Zn and Ba, were not strongly mobilized by the treatments, although cow manure slightly increased the leaching of Ba and As, which maximum concentrations in the leaching water reached 65 μg.L−1 Ba and 9 μg.L−1 As. All amendments improved the growth of ryegrass, which maximum dry biomass ranged from 38 mg/microcosm without amendment to 155 mg/microcosm with 0.15% manure. The results provide key information about the biogeochemical processes driving the mobility of Pb, As, Zn and Ba in acid mine tailings during the first 84 days following their amendment with iron-rich ochre and manure.

Silicon and aluminium leaching kinetics from acidic gold mine tailings

Si and Al were leached from gold mine tailings (GMT) in order to determine if the GMT could be used as precursors for the synthesis of geopolymers. Possible synthesis of geopolymers from GMT would help reduce their potential pollution effects through heavy metal immobilisations. The effect of type of alkali (NaOH/KOH), alkali concentration, temperature, milling and calcination of GMT and solid liquid ratio (S/L) on the leaching of Si and Al was investigated. The optimum conditions were 10 M KOH, S/L of 0.5% m/v and a leaching temperature of 95°C. The leaching obeyed the shrinking core model with the surface chemical reaction being the controlling step. The leaching of Si was linked to the Al leaching since the Si/Al ratio of the leachate was around 2 irrespective of leaching conditions. KOH yielded more Si and Al. as compared to NaOH, with 54.7 and 26.7 Si and Al leached respectively from as received GMT. Calcination at 900°C resulted in 18% and 22 % increase in Si and Al yield respectively for KOH based leaching. The successful leaching of Si and Al from GMT using KOH provides opportunities for the geopolymerisation of GMT thereby allowing the minimisation of their pollution effects.

Successful isolation of a tolerant co-flocculating microalgae towards highly efficient nitrogen removal in harsh rare earth element tailings (REEs) wastewater

Acidic rare earth element tailings (REEs) wastewater with high nitrogen and low COD is the most serious and yet unsolved environmental issue in the rare earth mining industry. The effective and cheap remediation of NH4+-N and NO3−-N from the REEs wastewater is still a huge challenge. This harsh wastewater environment results in the difficulty for common microbes and microalgae to be survived. In this work, a novel highly tolerant co-flocculating microalgae (the combination of Scenedesmus sp. and Parachlorella sp.) was successfully isolated from the rare earth mine effluent through three-year cultivation. The removal efficiency of total inorganic nitrogen (TIN) by the co-flocculating microalgae cultivation was as high as 90.9%, which is 1.9 times than the average removal efficiency (47.9%) of previously-reported microalgae species in the wastewater with COD/N ratio ranging from 0 to 1. Thus, the residual concentrations of NH4+-N and TIN could reach the Emission Standards of Pollutants from Rare Earths Industry (GB 26451-2011). Along with the high N removal performance, other related characteristics of the co-flocculating microalgae were also revealed, such as high tolerance towards high NH4+-N and strong acid, rapid growth and sedimentation, and simultaneous removal of NH4+-N and NO3−-N. These algae characteristics were ascribed to the specific co-flocculating community structure covered by extracellular polymeric substances.

Cross-Layer Leaching of Coal Fly Ash and Mine Tailings to Control Acid Generation from Mine Wastes

Coal fly ash can be used in in various configurations (e.g. as cap, bottom liner, or blending) at a mine site, but comparative studies investigating their capacity to control acid mine drainage are limited. Batch and column leaching experiments were conducted to investigate the effects of fly ash-mine tailings mix ratios and system configurations on leachate chemistry. Acidic mine tailings (pH 2.72) were obtained from waste piles at a former gold and pyrite mine. Mixing the fly ash with the tailings in a 1:1 (w/w) ratio decreased Zn, Ni, Fe, Mn, Pb, and Cu leaching by 90 ± 6%, increased the pH from extremely acidic (2.9) to alkaline (8.0), and decreased electrical conductivity from 4 to 2.5 mS cm−1 due to solute precipitation. Using the fly ash as a ‘chemical liner’ beneath the tailings, applying the fly ash as both a cap and bottom liner, or blending the fly ash with tailings produced significantly less acidity, salinity, and metal leaching than using the fly ash as a cap. The capacity of fly ash to control acid generation is attributed to its acid neutralizing capacity and high pH (11.1).

Soil Microbiome Dynamics During Pyritic Mine Tailing Phytostabilization: Understanding Microbial Bioindicators of Soil Acidification

Challenges to the reclamation of pyritic mine tailings arise from in situ acid generation that severely constrains the growth of natural revegetation. While acid mine drainage (AMD) microbial communities are well-studied under highly acidic conditions, fewer studies document the dynamics of microbial communities that generate acid from pyritic material under less acidic conditions that can allow establishment and support of plant growth. This research characterizes the taxonomic composition dynamics of microbial communities present during a 6-year compost-assisted phytostabilization field study in extremely acidic pyritic mine tailings. A complementary microcosm experiment was performed to identify successional community populations that enable the acidification process across a pH gradient. Taxonomic profiles of the microbial populations in both the field study and microcosms reveal shifts in microbial communities that play pivotal roles in facilitating acidification during the transition between moderately and highly acidic conditions. The potential co-occurrence of organoheterotrophic and lithoautotrophic energy metabolisms during acid generation suggests the importance of both groups in facilitating acidification. Taken together, this research suggests that key microbial populations associated with pH transitions could be used as bioindicators for either sustained future plant growth or for acid generation conditions that inhibit further plant growth.

Trend evolution of physicochemical parameters and metals mobility in acidic and complex mine tailings long exposed to severe mediterranean climatic conditions: Sidi Driss tailings case (NW-Tunisia)

Sidi Driss initial waste of ore valorization unity (WOVU) and mine tailings (acidic wastes) are interesting examples to study the evolution of mining wastes which had a relatively complex mineralogy and were exposed for a long time to severe erosion and dispersion. The evolution of physicochemical parameters and the concentrations and mobility of Cr, Ni, Cu, Pb, Mn, Fe and Zn were studied. Between 90 and 98% of metals in WOVU were bound to residual fractions. The organic matter in WOVU mineralization, direct and indirect oxidation of pyrite, hydrolysis of Fe3+ and SO2 and jarosite formation contributed to pH decrease and metal mobility increase. The oxidation-reduction environments in mine tailings mobilized between 42 and 69% of Pb, Zn and Mn versus the initial quantities of stable metals. The mechanical dispersion caused by the torrential water runoff and the wind, and the metal mobility contributed to the liberation of 7–19% of total metal quantities. The calculated Redistribution (Utf) and Relative Binding Intensity (IR) indexes showed that the Sidi Driss mine tailings released from 1.4 times (Fe) to 88.8 (Cu) times more available metal quantities and Mn and Pb (76.1 ≤ IR ≤ 77.1) significantly had the lowest binding intensity (more mobile), when compared to other metals (86.3 < IR < 96.5) (less mobile). Thus, it is urgent to make a correct plan and to stop all available metal fractions and elucidate all possible risks associated to tailings.

Effect of potassium silicate and aluminate on the stabilisation of gold mine tailings

In this study, acidic gold mine tailings were alkali activated using potassium hydroxide (KOH). The effect of potassium silicate (KS) and potassium aluminate (KA) on the strength and durability of the synthesised geopolymers was investigated. An optimisation experimental programme of various conditions was used to get the best geopolymers in terms of strength and metal leachability. A 1·1 mass/mass (m/m) potassium silicate : potassium hydroxide activation solution yielded a geopolymer with an unconfined compressive strength (UCS) of 9·94 MPa after curing for 5 d at 80ºC. A 2·8 m/m potassium aluminate : potassium hydroxide activation solution yielded a geopolymer with a UCS of 18·10 MPa after curing for 5 d at 100ºC. The potassium aluminate-based geopolymer was more durable for use as a backfill material as, after 10 wet and dry cycles, it had a UCS of 4·8 MPa whereas the potassium silicate-based geopolymer had a UCS of 1·23 MPa. The potassium aluminate-based geopolymer resulted in over 85% reduction in leachability of heavy metals from the cured geopolymer. This study showed that potassium aluminate activation of acidic gold mine tailings is an attractive route to stabilise/solidify hazardous material. The use of elevated temperature to achieve high strength for the geopolymer pales in comparison to the energy requirements of cement production.

Dependency of biological nitrogen fixation on organic carbon in acidic mine tailings under light and dark conditions

Several ecological restoration techniques have been used to enhance the levels of available organic matter in mine tailings. However, the effect of organic carbon on nitrogen fixation remains poorly understood. In this study, nitrogen fixation in two mine tailings treated with glucose was analyzed using 15N isotope labeling, pyrosequencing and DNA-based stable isotope probing (SIP) to identify nitrogen fixers and their activity under organic carbon addition (0.8 mg glucose-C/g soils). Following glucose addition, the total nitrogen content increased significantly (1.6–5.4% of initial N content), and the 15N content increased from 0.89 ± 0.20 mg·kg−1 to 37.08 ± 3.34 mg·kg−1 during a 28-day incubation period. In addition, glucose-stimulated nitrogen fixation in dark conditions was 8.53-fold (Shuimuchong) and 1.22-fold (Yangshanchong) higher than that in light conditions. At the same time, the addition of glucose led to distinctly different phylotypes of the bacterial communities and an increase in the abundance of the nifH gene in the two mine tailings. 13C-glucose SIP showed that the highest copy number of the 16S rRNA and nifH genes in the 13C-glucose-labeled treatment peaked in the heavy fraction, with a buoyant density of 1.737 g·mL−1. However, the pyrosequencing results revealed that the vast majority of the nifH genes were unique and belonged to yet uncultured bacteria, while the potentially known groups were associated with Azotobacter, acidophilic Leptospirillum and Pseudanabaena, accounting for 13.83% of all the total nifH genes. These results indicate that the addition of organic carbon may increase the nitrogen content by stimulating nitrogen fixers, highlighting the vital roles of organic carbon in increasing nitrogen accumulation during the restoration of mine tailings.

Integrated Management of Pig Residues and Copper Mine Tailings for Aided Phytostabilization.

There is high demand for identifying socio-environmentally sound ways to dispose of large quantities of offensive odor-generating organic residues produced by intensive livestock farming. The use of these residues as amendments at large-scale mine tailings storage facilities for in situ plant-based reclamation may be a useful alternative. We performed a greenhouse assay to evaluate effectiveness of pig slurries and the solid organic fraction of aerobic-aerated pig slurries, both treated and not treated for odor emission, as amendments for copper tailings. Different doses of slurries or the solid organic fraction of the slurries, untreated and treated with either ozone or a commercial product used to manage strong odors (Just a Drop), were incorporated into tailings and stabilized (15 d) under greenhouse conditions. L. (ryegrass) was used as bioindicator for Cu and Zn toxicity. Plant performance (shoot and root dry biomass, Cu and Zn content in leaves) and general physicochemical characteristics of substrates were evaluated. Our results showed that odor management of pig residues was possible with commercial products, and their incorporation into tailings rapidly and effectively reduced odor emission. The solid organic fraction of the slurries neutralized acidic tailings, and both pig residues increased organic matter and nutrient content in tailings while reducing extractable Cu. As a result, ryegrass productivity was significantly improved by incorporation of either slurries or their solid organic fraction into tailings in a dose-dependent form. Foliar Cu content in ryegrass was not affected but foliar Zn content increased; however, Zn remained within acceptable ranges. Both the pig slurries and their soil organic fractions proved to be valuable residues for tailings reclamation.

Glucose-induced changes in the bacterial communities of mine tailings at different acidification stages

Ecological restoration technologies applied to tailings can influence the associated bacterial communities. However, it is unknown if the shifts in these bacterial communities are caused by increased organic carbon. Glucose-induced respiration and high-throughput sequencing were used to assess the microbial activity and bacterial communities, respectively. Glucose addition increased the microbial activity, and glucose + ammonium nitrate addition resulted in slightly higher CO2 emission than did glucose addition alone, suggesting that carbon and nitrogen limited microbial community growth. In neutral pH tailings, the bacterial taxa that increased by glucose addition were assigned to the phyla Proteobacteria, Acidobacteria, Actinobacteria, Bacteroidetes, Firmicutes, and Planctomycetes. However, the bacterial taxa that increased by glucose addition in acidic tailings only belonged to the phylum Actinobacteria (maximum increase of 43.78%). In addition, the abundances of the total nitrogen-fixing genera and of the genus Arthrobacter (representing approximately 97.89% of the total nitrogen-fixing genera) increased by glucose addition in acidic tailings (maximum increase of 46.98%). In contrast, the relative abundances of the total iron- and (or) sulfur-oxidizing bacteria decreased (maximum decrease of 10.41%) in response to the addition of glucose. These findings indicate that the addition of organic carbon is beneficial to the development of bacterial communities in mine tailings.

Extreme enrichment of arsenic and rare earth elements in acid mine drainage: Case study of Wiśniówka mining area (south-central Poland)

The Wiśniówka rock strip mining area (south-central Poland) with quartzite quarries, acid water bodies and tailings piles is one of the most unique acid mine drainage (AMD) sites throughout the world. This is due to the occurrence of enormous amounts of pyrite unknown in sedimentary formations worldwide. Of the two mineralization zones, one that is the most abundant in arsenical pyrite occurs in the lowermost Upper Cambrian formation of the Podwiśniówka quarry. The As-rich pyritiferous clastic rocks are exposed as a result of deep quartzite extraction during 2013–2014. In addition, the clayey-silty shale interbeds are enriched in rare earth element (REE) minerals. The mining operation left an acidic lake with a pH of about 2.4–2.6 and increased contents of sulfates, metal(loid)s and REE. The Podwiśniówka pyrite-rich waste material was stacked up in many places of the mining area giving rise to strongly acidic spills that jeopardized the neighboring environment. One of these unexplored tailings piles was a source of extremely sulfate- and metal(loid)-rich pools with unusual enrichments in As (up to 1548 mg L−1) and REE (up to 24.84 mg L−1). These distinctly exceeded those previously reported in the Wiśniówka area. A broad scope of geochemical, mineralogical and petrographic methods was used to document these specific textural and mineralogical properties of pyrite facilitating its rapid oxidation. The pyrite oxidation products reacted with REE-bearing minerals releasing these elements into acid water bodies. Statistical methods were employed to connect the obtained tailings pool hydrogeochemical data with those derived from this and the previous studies of the Podwiśniówka and Wiśniówka Duża acid pit lakes. In contrast to metal(loid) profiles, the characteristic shale-normalized REE concentration patterns turned out to be more suitable for solving different AMD issues including provenance of mine waste material in the tailings pile examined.

Uranium adsorption and subsequent re-oxidation under aerobic conditions by Leifsonia sp. - Coated biochar as green trapping agent

It has generally been assumed that the immobilization of U(VI) via polyphosphate accumulating microorganisms may present a sink for uranium, but the potential mechanisms of the process and the stability of precipitated uranium under aerobic conditions remain elusive. This study seeks to explore the mechanism, capacity, and stability of uranium precipitation under aerobic conditions by a purified indigenous bacteria isolated from acidic tailings (pH 6.5) in China. The results show that over the treatment ranges investigated, maximum removal of U(VI) from aqueous solution was 99.82% when the initial concentration of U(VI) was 42 μM, pH was 3.5, and the temperature was with 30 °C much higher than that of other reported microorganisms. The adsorption mechanism was elucidated via the use of SEM-EDS, XPS and FTIR. SEM-EDS showed two peaks of uranium on the surface. A plausible explanation for this, supported by FTIR, is that uranium precipitated on the biosorbent surfaces. XPS measurements indicated that the uranium product is most likely a mixture of 13% U(VI) and 87% U(IV). Notably, the reoxidation experiment found that the uranium precipitates were stable in the presence of Ca2+ and Mg2+, however, U(IV) is oxidized to U(VI) in the presence of NO3− and Na+ ions, resulting in rapid dissolution. It implies that the synthesized Leifsonia sp. coated biochar could be utilized as a green and effective biosorbent. However, it may not a good choice for in-situ remediation due to the subsequent re-oxidation under aerobic conditions. These observations can be of some guiding significance to the application of the bioremediation technology in surface environments.

Evaluation of the performance of chemical extractants to mobilise metals for remediation of contaminated samples

The main objective was to evaluate the efficiency of chemical extractants to mobilise metals from contaminated samples under different land uses: industrial, agricultural and forest soils, and acidic and basic tailings ponds. Four extractants (citric, oxalic, Diethylenetriamine pentaacetate - DTPA - and Ethylenediaminetetraacetic - EDTA - acids) at different concentrations were tested. Extractants efficiency was dependent on sample type and specific metal as influenced by sample characteristics, mineralogy and speciation. We found that EDTA was the most efficient extractant for all metals in the forest soil; DTPA for Cu, Pb and Zn, and EDTA for Cd in the agricultural soil; EDTA for Pb and Zn, DTPA for Cu and oxalic acid for Cd in the industrial soil; EDTA for Cd and Zn in the acidic tailings with the other extractants showing similar performance; DTPA for Pb, DTPA/EDTA for Zn, citric acid for Cd and Cu in the basic tailing. The optimum extractant concentration across samples was the maximum tested for Pb (40 mM), 20 mM for Zn and 5–10 mM for Cu and Cd. Metal mobilisation varied widely across samples (maximum percentage of extraction shown from total metal amount): 27–81% for Cd, 3–55% for Cu, 1–24% for Pb and 2–16% for Zn. Soil pH was the most influential factor to explain the amount of metals extracted. This study evidenced the effectiveness of organic acids to mobilise metals, the possibility of using natural/nontoxic weak acids for some samples and metals and the convenience of performing preliminary tests with different extractants and concentrations as a previous necessary step before large scale remediation projects.

Phytostabilization of polymetallic contaminated soil using Medicago sativa L. in combination with powdered marble: Sustainable rehabilitation

ABSTRACTThis study uses an ecotoxicology approach to evaluate the effectiveness of combining powdered marble as an amendment, with phytostabilization by Medicago sativa L. on the neutralization of acidic mine tailings, and the stabilization of heavy metals. The mine tailings were collected from an abandoned polymetallic mine in Southern Morocco, and mixed with powdered marble as the following proportions, 25%, 50%, and 75%. Laboratory immobilization/stabilization tests showed that the application of powdered marble in the treatments led to a significant increase in pH, and significant reductions of Cu, Zn (99%), Pb (98%), and Fe (45%). Greenhouse experiments showed that plant growth in all treatments was significantly (p ≤ 0.05) less than growth in agricultural soil. Plant growth significantly (p ≤ 0.05) decreased as the proportion of powdered marble increased. The concentration of metals in plant roots were significantly (p ≤ 0.05) higher than those of shoots. Combining immobilization by powdered marble with phytostabilization by M. sativa L. could represent a viable method of rehabilitating acidic polymetallic mine tailings.

Mineral sequestration of CO 2 using saprolite mine tailings in the presence of alkaline industrial wastes

Abstract This study investigate the feasibility of sequestering CO2 via mineral carbonation of calcium-rich alkaline industrial wastes, such as steel slag (SS) and coal fly ash (FA) in acidic mine tailings. Mine tailings are treated with SS and FA at various liquid-to-solid ratios (15 and 30 ml/g) to extract metal components present in FA and SS to supplement the Ca2+ and Mg2+ content of the tailings and to partially neutralize the mine tailings. This remediation step has the benefit of reducing the consumption of neutralizing reagent, such as NaOH, required for pH adjustment to induce mineral carbonation. Compared with FA, it is found that SS with higher MgO and CaO content shows greater neutralizing effect. The ICP-AES analysis of the treated tailing shows its Ca2+ content significantly increased from 111 mg L−1 to 1100 mg L−1; which can proportionally increase the CO2 capture capacity. The highest carbonation efficiencies are observed at carbonation pH of 9 as confirmed by Total Carbon (TC) analyzer and TGA analysis. Mineralogical phase analysis of the carbonated products using TGA-DTG, XRD, and FTIR shows the carbonated solids precipitated at pH 7 and 8 are mainly nesquehonite, while hydromagnesite is the main constituent of carbonates that precipitated at pH 9. The insights from this study direct effort towards sustainable process development focused on remediation of alkaline industrial wastes and the utilization of anthropogenic CO2 emissions.

Design of a Geomembrane-Laminated Geosynthetic Clay Liner on the Slopes of a Mine Tailings Dam in Emet, Turkey

We evaluated the potential use of a geomembrane-laminated geosynthetic clay liner (GCL) along the slopes of a boric acid tailings dam in Emet, Turkey. Even though a compacted clay liner (CCL) had been used at the bottom of the tailings dam, it was not possible to place a CCL along the slopes of the dam due to their steepness. Triaxial permeability tests were conducted on the base GCL without a geomembrane and the results indicated that although the volumetric flow through a cross section of the GCL was measured to be very low initially, it increased after a while due to the interaction of the bentonite in the GCL and the mine leachate. For this reason, using a geomembrane-laminated GCL along the steep slopes was found to be an appropriate solution. The mechanical properties of the barrier material were evaluated by performing a parametric study, including a slope stability analysis and an anchorage design for the geomembrane-laminated GCL. Based on the results, a geomembrane-laminated GCL with appropriate mechanical and hydraulic properties was chosen.

Long term effects of Lespedeza bicolor revegetation on soil bacterial communities in Dexing copper mine tailings in Jiangxi Province, China

Soil microbial communities are important for ecological restoration and succession on mine tailings. In the present study, we performed Illumina sequencing to investigate the effects of long term Lespedeza bicolor revegetation on bacterial diversity and community structure in mine tailings under subtropical and moist climatic conditions. Microbial diversity indices (Shannon, OTU number and coverage estimator) of the revegetated soils were higher than that of the control, and increased over sampling period compared to the decreased pattern in the control. Species within known acid tolerant and nutrient regulated genera dominated both revegetated tailings and the control, and exhibited more abundant in the revegetated tailings. At the phylum level, percentage of the Proteobacteria and Actinobacteria were remarkedly higher in revegetated tailings than that in the control, while Chloroflexi performed reversely. Overall, this study found the positive role of L. bicolor revegetation in bacterial diversity development in the acidic mine tailings. Furthermore, the 30-year L. bicolor revegetation made the microbial community structure more homogenizable as a decrease of dissimilarity in tailings was identified over sampling times. Redundancy analysis at the OTU level indicated that Olsen-P and pH were the main regulators of microbial composition, suggesting that soil P and pH are determinant for ecological restoration and microbial community development in acidic mine tailings.

Use of Piptatherum miliaceum for the phytomanagement of biochar amended Technosols derived from pyritic tailings to enhance soil aggregation and reduce metal(loid) mobility

Tailings and overburdens pose high environmental and human health risks. The development of Technosols from anthropogenic materials and growth of native metal-tolerant plant species could be a sustainable strategy to minimize such risks. In this study, a 10-month pot experiment was carried out with Technosols developed from acidic tailings amended with marble waste (CaCO3) and two rates of biochar (10gCkg−1 and 20gCkg−1), with and without Piptatherum miliaceum. The main aims were to assess if biochar addition influenced plant metal uptake, increased soil aggregation, decreased metal mobility and enhanced plant growth. In addition, we assessed if the presence of P. miliaceum improved soil structure, stimulated microbial communities, and increased metal immobilization. Biochar and CaCO3 additions were the main factors responsible for acid neutralization. However, the growth of P. miliaceum also contributed to increase soil pH, likely due to increases in electrical conductivity. High organic matter recalcitrance with low content of labile and soluble organic compounds was observed in the developed Technosols. The presence of P. miliaceum positively stimulated microbial biomass and enzyme activities, and contributed to significantly increase the ratio recalcitrant C-to-recalcitrant N. This suggests an effect of rhizosphere in soil organic matter stabilization and degradation. Thus, microorganisms degrade N functional groups in biochar more easily than C functional groups. Greenhouse gas emissions were not increased by addition of biochar. Methane emissions decreased, with no effect of plant growth. The use of biochar and marble waste reduced the mobility of metal(loid)s due to increase in pH, and the presence of plant increased the fraction of Cd retained in Fe/Mn oxides. Plants showed low metal(loid) uptake in shoots, with no effect of biochar rate. Technosols showed higher proportion of macro and microaggregates than original tailings. The presence of plant significantly contributed to increase the proportion of microaggregates. The highest rate of biochar had a negative effect on P. miliaceum biomass which resulted in limited growth compared with the lower biochar rate.