Xiaofang Li

Resistance and resilience of Cu-polluted soil after Cu perturbation, tested by a wide range of soil microbial parameters

Copper (Cu)-polluted and unpolluted soils were used to study the effect of initial pollution on soil biological resistance and resilience by measuring the responses to perturbation using different parameters. Microbial biomass carbon, substrate-induced respiration and copy numbers of 16S rRNA gene were grouped as general parameters, while potential ammonia oxidation rate and copy numbers of amoA gene were grouped as specific functions. In addition, to illustrate how initial pollution affects soil biological resistance and resilience following secondary perturbation, the microbial community structure, together with free Cu(2+) activities ([Cu(2+)]) in soil pore water and soil pH were also measured after secondary perturbation. Results showed that general parameters were more stable than specific ones. High [Cu(2+)] and low pH in soil pore water induced by Cu addition may lead to apparently low resistance and resilience, whereas the formation of a tolerant community after Cu pollution, secondary perturbation and Cu aging may contribute to resistance and resilience. Analysis of the phospholipid fatty acids profile showed that microbial community structure shifted along with the [Cu(2+)] gradient. The microbial community structure of the control soil was both resistant and resilient to 400 mg kg(-1) Cu perturbation, whereas other treatments were neither resistant nor resilient.

Toward a new paradigm for tailings phytostabilization – nature of the substrates, amendment options and anthropogenic pedogenesis

Base metal tailings (BMTs) are normally sulfidic and contain high abundance of residue metals. Their adverse impacts on the environment can last for decades to centuries if without appropriate stabilization. While in situ phytostabilization has been thought to be a promising approach to stabilize surface tailings, few studies have reported success in constructing a sustainable plant community in BMTs so far, implying that a new paradigm involving a sophisticated understanding of the nature of BMTs is needed for BMTs phytostabilization. Using a property database of BMTs worldwide built in this study as a backdrop, this review explores how BMTs are different from normal soils and how these differences influence the strategies of BMTs phytostabilization. It is found that BMTs are mineralogically and chemically different from natural soils, which endows BMTs with unstable geochemistry and inherent extreme toxicity. Studies have documented that amendment options and soil development in BMTs phytostabilization are largely constrained by these abiotic factors. From a viewpoint of pedogenesis, BMTs can be seen as novel parent materials rather than soil. Accordingly, we propose that in BMTs phytostabilization, extensive engineering efforts are required to increase the biocapacity of tailings (i.e., anthropogenic pedogenesis) rather than focus on the selection and establishment of plants.

From lithotroph- to organotroph-dominant: directional shift of microbial community in sulphidic tailings during phytostabilization

Engineering microbial diversity to enhance soil functions may improve the success of direct revegetation in sulphidic mine tailings. Therefore, it is essential to explore how remediation and initial plant establishment can alter microbial communities, and, which edaphic factors control these changes under field conditions. A long-term revegetation trial was established at a Pb-Zn-Cu tailings impoundment in northwest Queensland. The control and amended and/or revegetated treatments were sampled from the 3-year-old trial. In total, 24 samples were examined using pyrosequencing of 16S rRNA genes and various chemical properties. The results showed that the microbial diversity was positively controlled by soil soluble Si and negatively controlled by soluble S, total Fe and total As, implying that pyrite weathering posed a substantial stress on microbial development in the tailings. All treatments were dominated by typical extremophiles and lithotrophs, typically Truepera, Thiobacillus, Rubrobacter; significant increases in microbial diversity, biomass and frequency of organotrophic genera (typically Nocardioides and Altererythrobacter) were detected in the revegetated and amended treatment. We concluded that appropriate phytostabilization options have the potential to drive the microbial diversity and community structure in the tailings toward those of natural soils, however, inherent environmental stressors may limit such changes.

Extremely High Phosphate Sorption Capacity in Cu-Pb-Zn Mine Tailings

Elevated inorganic phosphate (Pi) concentrations in pore water of amended tailings under direct revegetation may cause toxicity in some native woody species but not native forbs or herb species, all of which are key constituents in target native plant communities for phytostabilizing base metal mine tailings. As a result, Pi sorption capacity has been quantified by a conventional batch procedure in three types of base metal mine tailings sampled from two copper (Cu)-lead (Pb)-zinc (Zn) mines, as the basis for Pi-fertiliser addition. It was found that the Pi-sorption capacity in the tailings and local soil was extremely high, far higher than highly weathered agricultural soils in literature, but similar to those of volcanic ash soils. The Langmuir P-sorption maximum was up to 7.72, 4.12, 4.02 and 3.62 mg P g-1 tailings, in the fresh tailings of mixed Cu-Pb-Zn streams (MIMTD7), the weathered tailings of mixed Cu-Pb-Zn streams (MIMTD5), EHM-TD (fresh Cu-stream, high magnetite content) and local soil (weathered shale and schist), respectively. Physicochemical factors highly correlated with the high Pi-sorption in the tailings were fine particle distribution, oxalate and dithionite-citrate-bicarbonate extractable Fe (FeO and Fed), oxalate-extractable Al and Mn, and the levels of soluble Cd and Zn, and total S and Fe. Large amounts of amorphous Fe oxides and oxyhydroxides may have been formed from the oxidation of pyritic materials and redox cycles of Fe-minerals (such as pyrite (FeS2), ankerite (Ca(Fe Mg)(CO3)2 and siderite (FeCO3), as indicated by the extractable FeO values. The likely formation of sparingly soluble Zn-phosphate in the Pb-Zn tailings containing high levels of Zn (from sphalerite ((Zn,Fe)S, ZnS, (Zn,Cd)S)) may substantially lower soluble Zn levels in the tailings through high rates of Pi-fertiliser addition. As a result, the possibility of P-toxicity in native plant species caused by the addition of soluble phosphate fertilizers would be minimal.

Understanding the salinity issue of coal mine spoils in the context of salt cycle

Coal mine spoils (CMSs), the solid wastes originated from the rock formations and soil cover overlying or interbedded with coal seams, are a worldwide environmental management challenge. Previous studies have shown that salinity is of most concern among the CMSs’ environmental impacts, especially in Australia. With increasing concerns from both the governments and communities, there is a real need for the coal mining industry to understand the source, dynamics and management options of CMS salinity. We reviewed the general properties of CMSs from coal mine sites worldwide and the current understanding of the CMS salinity, which are in a limited number of available published reports. Properties (e.g., pH, electrical conductivity and hydraulic conductivity) of studied CMSs varied largely due to its complex lithological origination. A conceptual model was proposed to illustrate the origin, dispersion paths and transformations dynamics of salts in spoils, taking the scenario of a coal mine in Australia as an example. The major factors governing the salt dynamics in CMSs are summarized as mineral weatherability and salt leachability of the spoils. Management of CMS salinity is still a vague area awaiting more extensive studies. Three topics related to the management were explored in the review, which are pre-mining planning, spatial variability of spoil properties and remediation including electrokinetics and phytoremediation. Particularly, based on the geological classification of CMSs and the leachate chemistry of spoils of various sources, a clear relationship between salinity and geounits was established. This association has a potential application in pre-mining planning for the management of salinity from coal mine spoils.

Copper toxicity thresholds in Chinese soils based on substrate-induced nitrification assay.

Copper toxicity in 17 Chinese soils was screened using a substrate-induced nitrification assay to generate information for the development of a terrestrial biotic ligand model (tBLM). The leaching effect on the Cu toxicity thresholds was investigated. Both the total Cu-based median effective concentration (EC50) values (46.9-2726 mg/kg) and the solution Cu-based EC50 values (0.04-2.91 mg/L) in unleached soils varied substantially among the soils in the present study. For unleached soils, linear stepwise multiple regression analysis showed that total Ca and soil pH were the best predictors for total Cu-based EC50, while electrical conductivity (EC) and soil pH were the best predictors for solution Cu-based EC50. The variation in solution Cu-based EC50 was largely (R(2) = 0.75) explained by Mg but not Ca and H(+) concentration in soil solution at EC50, suggesting a protective effect of Mg(2+) against Cu toxicity in the test soils. Leaching impacted Cu toxicity differently among the soils and apparently reduced the variations of both the total Cu-based and the solution Cu-based EC50. The predictability of the Cu EC50 by empirical models was decreased after leaching. The leaching effect on Cu toxicity, indicated by a leaching factor, was not predicted by any soil properties. There is a need to investigate quantitatively the mechanisms for the leaching effect on Cu toxicity in soils.

Assessing the genetic diversity of Cu resistance in mine tailings through high-throughput recovery of full-length copA genes

Characterizing the genetic diversity of microbial copper (Cu) resistance at the community level remains challenging, mainly due to the polymorphism of the core functional gene copA. In this study, a local BLASTN method using a copA database built in this study was developed to recover full-length putative copA sequences from an assembled tailings metagenome; these sequences were then screened for potentially functioning CopA using conserved metal-binding motifs, inferred by evolutionary trace analysis of CopA sequences from known Cu resistant microorganisms. In total, 99 putative copA sequences were recovered from the tailings metagenome, out of which 70 were found with high potential to be functioning in Cu resistance. Phylogenetic analysis of selected copA sequences detected in the tailings metagenome showed that topology of the copA phylogeny is largely congruent with that of the 16S-based phylogeny of the tailings microbial community obtained in our previous study, indicating that the development of copA diversity in the tailings might be mainly through vertical descent with few lateral gene transfer events. The method established here can be used to explore copA (and potentially other metal resistance genes) diversity in any metagenome and has the potential to exhaust the full-length gene sequences for downstream analyses.

Phytostabilizing challenging mine wastes at the cost of phytostabilization

© The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. From time to time, tailings dam failures occur in most mining nations, such as the disaster in Minas Gerais, Brazil in 2015 (Science, 18 Dec 2015) and more recently the one in Luoyang in China (http://zy.takungpao. com/2016/0810/270511.html). This has raised a concern on the safe storage of mine tailings in tropical mining nations, where extreme weathers are inevitable, as well as in other parts of the world. Strengthened policing is obviously urgently needed in these areas to guide the design and monitoring of tailings storage facilities, and to balance the environmental protection and mining profit. Yet, we need to admit that it is still challenging worldwide to stabilize surface tailings, particularly those of base metal mines, at a reasonable cost, even if a safe geotechnical structure is achieved. One of such challenges facing restorationists is how to stabilize tailings using plants cost-efficiently (i.e. phytostabilization [1]). It is commonsense that a vegetation cover is the best option to minimize soil surface erosion by wind and rainfall. The advent of phytostabilization technologies is motivated by the fact that the volumes of legacy mine tailings have increased drastically in the past decades and the use of huge amount of capping soils by conventional dry cover technology has been impractical. In some developing countries like China, cost-effective phytostabilization technologies are more in need. Till 2012, China has accumulated more than 12,000 tailings impoundments, 3000 of which are with a volume greater than 106 m3. Unfortunately, most of the inactive impoundments, particularly the smaller ones, have not been properly closed. Many have even unknown liability subjects [2]. Worse, China’s tailings impoundments are mostly located in densely populated areas, e.g. the central China and northeast China, and many are just adjacent to arable land. This complicates the management, and the proper closure of these tailings ponds has been an economic burden to the nation. I thus propose that the development of cost-effective phytostabilization technologies (e.g. the technosol technologies [3]) can be a priority for the proper management of legacy mine tailings in China. The progress in technosol technologies may lead to the success of converting the troublesome mine tailings to valuable land resources, that is, phytostabilizing the mine wastes at the cost of phytostabilization in the near future.