Phosphate rock solubilization and the potential for lead immobilization by a phosphate-solubilizing bacterium (Pseudomonas sp.).

Lead (Pb) pollution is getting more and more serious in phosphate mining wastelands recently. However, seldom studies focused on the bioremediation of Pb pollution in phosphate mining wastelands by phosphate-solubilizing bacterium (PSB). In this study, a PSB named LA with high Pb tolerance was isolated from a phosphate mining wasteland. Based on its cell morphology, physiology, and phylogenetic analysis, it was identified as Pseudomonas sp. Its capabilities to solubilize mid-low-grade phosphate rock (PR) and immobilize Pb were assessed in this study. It was found that LA could effectively solubilize PR on PKO culture medium and release soluble phosphate in the culture medium. PR solubilization and Pb immobilization were investigated at different initial Pb concentrations and pH levels. The results showed that soluble phosphate was highly effective in immobilizing Pb and that when the initial concentration of Pb2+ was 100 mg/L, the immobilization rate of Pb was enhanced. Further, the mechanisms underlying solubilization of PR and biomineralization of Pb ions in LA were evaluated by Fourier transform infrared spectroscopy and X-ray diffraction. The results showed that some functional groups on the PR surface and LA were altered, and LA could form hydroxyapatite and pyrophosphate with Pb ions.

Increased growth and root Cu accumulation of Sorghum sudanense by endophytic Enterobacter sp. K3-2: Implications for Sorghum sudanense biomass production and phytostabilization.

Endophytic bacterial strain K3-2 was isolated from the roots of Sorghum sudanense (an bioenergy plant) grown in a Cu mine wasteland soils and characterized. Strain K3-2 was identified as Enterobacter sp. based on 16S rRNA gene sequence analysis. Strain K3-2 exhibited Cu resistance and produced 1-aminocyclopropane-1-carboxylate (ACC) deaminase, indole-3-acetic acid (IAA), siderophores, and arginine decarboxylase. Pot experiments showed that strain K3-2 significantly increased the dry weight and root Cu accumulation of Sorghum sudanense grown in the Cu mine wasteland soils. Furthermore, increase in total Cu uptake (ranging from 49% to 95%) of the bacterial inoculated-Sorghum sudanense was observed compared to the control. Notably, most of Cu (83-86%) was accumulated in the roots of Sorghum sudanense. Furthermore, inoculation with strain K3-2 was found to significantly increase Cu bioconcentration factors and the proportions of IAA- and siderophore-producing bacteria in the root interiors and rhizosphere soils of Sorghum sudanense compared with the control. Significant decrease in the available Cu content was also observed in the rhizosphere soils of the bacterial-inoculated Sorghum sudanense. The results suggest that the endophytic bacterial strain K3-2 may be exploited for promoting Sorghum sudanense biomass production and Cu phytostabilization in the Cu mining wasteland soils.

Wide Riparian Zones Inhibited Trace Element Loss in Mining Wastelands by Reducing Surface Runoff and Trace Elements in Sediment.

The diffusion of trace elements in mining wastelands has attracted widespread attention in recent years. Vegetation restoration is an effective measure for controlling the surface migration of trace elements. However, there is no field evidence of the effective riparian zone width in mining wastelands. Three widths (5 m, 7.5 m, and 10 m) of Rhododendron simsii/Lolium perenne L. riparian zones were constructed in lead-zinc mining wastelands to investigate the loss of soil, cadmium (Cd), copper (Cu), arsenic (As), lead (Pb), and zinc (Zn). Asbestos tiles were used to cut off connections between adjacent plots to avoid hydrological interference. Plastic pipes and containers were used to collect runoff water. Results showed that more than 90% of trace elements were lost in sediment during low coverage and heavy rainfall periods. Compared with the 5 m riparian zone, the total trace element loss was reduced by 69-85% during the whole observation period in the 10 m riparian zone and by 86-99% during heavy rain periods in the 10 m riparian zone, which was due to reduction in runoff and concentrations of sediment and trace elements in the 10 m riparian zone. Indirect negative effects of riparian zone width on trace element loss through runoff and sediment concentration were found. These results indicated that the wide riparian zone promoted water infiltration, filtered soil particles, and reduced soil erosion and trace element loss. Riparian zones can be used as environmental management measures after mining areas are closed to reduce the spread of environmental risks in mining wastelands, although the long-term effects remain to be determined.

[Analysis on the Distribution Characteristics and Influence Mechanism of Migration and Transformation of Heavy Metals in Mining Wasteland].

In order to explore the distribution characteristics and the influence mechanism of migration and transformation of heavy metals in mining wasteland, soil and tailings samples were collected from the mining wasteland in the Dabaoshan Mining area, Guangdong Province, and the morphological characteristics of heavy metals were analyzed. At the same time, the pollution sources of the mining area were analyzed using Pb stable isotope analysis, and the characteristics and influencing factors of heavy metal migration and transformation in the mining area were expounded by combining X-ray diffraction analysis, transmission electron microscope-energy spectrum analysis (TEM-EDS), and Raman analysis of typical minerals in the mining area, as well as laboratory-simulated leaching experiments. Morphological analysis showed that the forms of Cd, Pb, and As in the soil and tailings samples in the mining area were mainly the residual phase, accounting for 85%-95% of the total, followed by the iron and manganese oxide-bound form (1%-15%). The main mineral types in the soil and tailings in the Dabaoshan Mining area were pyrite (FeS2), chalcopyrite (CuFeS2), and metal oxides, as well as a small amount of sphalerite (ZnS) and galena (PbS). Acidic conditions (pH=3.0) were beneficial to the release and migration of Cd and Pb from soil, tailings, and minerals (pyrite, chalcopyrite) and from the residual phase to the non-residual phase. Lead isotope analysis showed that the lead in the soil and tailings mainly came from the release of metal minerals in the mining area, and the contribution of diesel in the mining area was less than 30%. Multivariate statistical analysis showed that Pyrite, Chalcopyrite, Sphalerite, and Metal oxide were the main sources of heavy metals in the soil and tailings in the mining area, in which Cd, As, and Pb were mainly contributed by sphalerite and metal oxide. The form change in heavy metals in the mining wasteland was easily affected by environmental factors. The form characteristics and migration and transformation factors of heavy metals should be considered in the source control of heavy metal pollution in mining wasteland.

Contamination and Health Risk Assessment of Heavy Metals in Soil and Ditch Sediments in Long-Term Mine Wastes Area.

The ecological and health risks posed by wastes discharged from mining areas to the environment and human health has aroused concern. 114 soil samples were collected from nine areas of long-term mine waste land in northwestern Yunnan to assess the pollution characteristics, ecological and health risks of heavy metals. The result revealed that the geo-accumulation indexes were Cd (4.00) > Pb (3.18) > Zn (1.87) > Cu (0.25). Semi-variance analysis revealed that Cd and Cu showed moderate spatial dependency, whereas Pb and Zn showed strong spatial dependency. Cd posed an extreme potential ecological risk. Slopes and ditches were extreme potential ecological risk areas. Non-carcinogenic risk to children from Pb and Carcinogenic risk to adult and children from Cd was non-negligible and direct ingestion was the major source. This study provided a scientific basis for policymakers in management and exposure reduction.

[Characteristics of Plant Diversity and Heavy Metal Enrichment and Migration Under Different Ecological Restoration Modes in Abandoned Mining Areas].

In order to select plant materials suitable for the ecological restoration of abandoned mining land, ecological restoration experiments were set up in landfills. The effects of different ecological restoration measures on plant diversity and heavy metal enrichment and migration characteristics were studied. The results showed that under different ecological restoration measures, a total of eight families and 10 species of surviving plants appeared, most of which were herbs. The restoration effect of the arbor shrub and grass plots was the best; the plant coverage reached 100%, and the survival rate was over 69%. The diversity index of the plant community was as follows:arbor shrub grass > shrub grass > joe grass > grass. The dominant plants under different ecological restoration measures were Pinus elliottii, Ligustrum lucidum, Boehmeria nivea, Lagerstroemia indica, and Plantago asiatica. The contents of most heavy metals in plants were higher than the normal values. Among them, the enrichment and transfer coefficients of the leaf stems of P. elliottii and L. indica were close to or greater than 1, showing a strong ability of enrichment and transfer to Cd; these two plants belong to Cd-enrichment type plants. The roots of L. lucidum Ait and P. asiatica, which are root-hoarding plants, had a high retention rate of heavy metals. Boehmeria nivea was a heavy metal-avoidance plant with a low content of heavy metals in each part and had the ability to fix and repair potential soil heavy metal pollution. Considering the diversity of plants and the ability of plant enrichment and migration, the combination of trees, shrubs, and grasses was the best restoration mode. Pinus elliottii, L. indica, and P. asiatica can be the target plants for ecological restoration in the abandoned land of mining areas.

Enhanced reduction of lead bioavailability in phosphate mining wasteland soil by a phosphate-solubilizing strain of Pseudomonas sp., LA, coupled with ryegrass (Lolium perenne L.) and sonchus (Sonchus oleraceus L.).

Due to ecologically unsustainable mining strategies, there remain large areas of phosphate mining wasteland contaminated with accumulated lead (Pb). In this study, a Pb-resistant phosphate-solubilizing strain of Pseudomonas sp., LA, isolated from phosphate mining wasteland, was coupled with two species of native plants, ryegrass (Lolium perenne L.) and sonchus (Sonchus oleraceus L.), for use in enhancing the reduction of bioavailable Pb in soil from a phosphate mining wasteland. The effect of PbCO3 solubilization by Pseudomonas sp. strain LA was evaluated in solution culture. It was found that strain LA could attain the best solubilization effect on insoluble Pb when the PbCO3 concentration was 1% (w/v). Pot experiments were carried out to investigate the potential of remediation by ryegrass and sonchus in phosphate mining wastelands with phosphate rock application and phosphate-solubilizing bacteria inoculation. Compared to the control group without strain LA inoculation, the biomass and length of ryegrass and sonchus were markedly increased, available P and Pb in roots increased by 22.2%-325% and 23.3%-368%, respectively, and available P and Pb in above-ground parts increased by 4.44%-388% and 1.67%-303%, respectively, whereas available Pb in soil decreased by 14.1%-27.3%. These results suggest that the combination of strain LA and plants is a bioremediation strategy with considerable potential and could help solve the Pb-contamination problem in phosphate mining wastelands.

Sludge Biochar Amendment and Alfalfa Revegetation Improve Soil Physicochemical Properties and Increase Diversity of Soil Microbes in Soils from a Rare Earth Element Mining Wasteland.

Long-term unregulated mining of ion-adsorption clays (IAC) in China has resulted in severe ecological destruction and created large areas of wasteland in dire need of rehabilitation. Soil amendment and revegetation are two important means of rehabilitation of IAC mining wasteland. In this study, we used sludge biochar prepared by pyrolysis of municipal sewage sludge as a soil ameliorant, selected alfalfa as a revegetation plant, and conducted pot trials in a climate-controlled chamber. We investigated the effects of alfalfa revegetation, sludge biochar amendment, and their combined amendment on soil physicochemical properties in soil from an IAC mining wasteland as well as the impact of sludge biochar on plant growth. At the same time, we also assessed the impacts of these amendments on the soil microbial community by means of the Illumina Miseq sequences method. Results showed that alfalfa revegetation and sludge biochar both improved soil physicochemical properties and microbial community structure. When alfalfa revegetation and sludge biochar amendment were combined, we detected additive effects on the improvement of soil physicochemical properties as well as increases in the richness and diversity of bacterial and fungal communities. Redundancy analyses suggested that alfalfa revegetation and sludge biochar amendment significantly affected soil microbial community structure. Critical environmental factors consisted of soil available K, pH, organic matter, carbon⁻nitrogen ratio, bulk density, and total porosity. Sludge biochar amendment significantly promoted the growth of alfalfa and changed its root morphology. Combining alfalfa the revegetation with sludge biochar amendment may serve to not only achieve the revegetation of IAC mining wasteland, but also address the challenge of municipal sludge disposal by making the waste profitable.