Humification is one of the critical processes in the ecological restoration of bauxite residue deposit areas. Straw addition is widely used strategy to increase organic carbon in bauxite residue. ...However, the effect of straw application on the humic carbon fractions in bauxite residue is largely unknown. In this study, the accumulation of humic fractions and associated microbial communities in bauxite residue following straw application were evaluated by humus fractionation and high-throughput sequencing technology. The results showed that straw application significantly increased humic carbon fractions (humic acid and fulvic acid) and humification degree in bauxite residue. The content of humic acid and fulvic acid increased by 27.1% and 22.9% in straw-amended bauxite residue after phosphogypsum addition, respectively. The glucosidase, cellulolytic enzyme, polyphenol oxidase and peroxidase increased by 7.15–8.76 times, 5.64–7.12 times, 2.69–4.57 times and 2.59–4.24 times following the straw application. High-throughput sequencing results indicated that the operational taxonomic unit (OTU) numbers and Shannon index of both bacterial and fungal communities significantly increased following co-application of straw and phosphogypsum. In addition, co-application of straw and phosphogypsum significantly increased the relative abundance of
Devosiaceae, Rhizobiaceae, Flavobacteriaceae, Caulobacteraceae
, and
Cellvibrionaceae
in bauxite residue. These findings provide us with a biological perspective of straw on the humification process in bauxite residue.
Bauxite residue, the by-product of the alumina industry, is mainly stored in land-based bauxite residue disposal areas (BRDAs). Environmental concern has been raised due to the large volumes in ...stockpile, the high alkalinity of the material, as well as the presence of elevated concentrations of trace elements. If not adequately managed, BRDAs can act as a source of pollution. In order to minimize the environmental risk, revegetation is implemented to stabilize the residue against water and wind erosion. Currently, two main approaches are used: the use of amendments or the installation of a capping layer. However, few studies evaluating the long-term success and self-sustainability of the rehabilitation programs have been published.
A series of field-established rehabilitation strategies reflecting both direct revegetation and revegetation on capping layer were assessed in terms of both soil and plant quality. Soil physico-chemical properties, including pseudo-total and plant-available fractions of nutrients and trace elements, were determined over a summer and winter seasons and aerial portions of vegetation were analysed for nutrients and trace elements.
Failure to adequately lower alkalinity remains the major constraint to long-term rehabilitation success of bauxite residue. This is evidenced from poor soil properties in unamended residue and in residue capped with a shallow soil layer, as well from vegetation displaying excessive concentrations of certain elements. Certain elements exceeded typical ranges for non-contaminated soils (i.e. Cr, Fe, Na, Ni and V), with some showing excessive plant-available fractions (i.e. of Al, As, Cr, Hg and V). Vegetation analysis found excessive uptake of some elements (i.e. of Al, Na, Fe, Cr and V). Future attempts for bauxite residue rehabilitation should include both gypsum and organic amendments, while a capping layer may only be effective if either a deep layer (>1 m) is installed or if the underlying residue is sufficiently treated prior to capping.
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•Bauxite residue disposal and environmental risk requires robust rehabilitation strategies•Different rehabilitation strategies were assessed for long term performance of direct revegetation and capping approaches•Capping is an insufficient approach where thin layers are applied over alkaline and saline residue•Direct revegetation following amendment of residue can promote suitable soil conditions and plant growth
Around 3 billion tonnes of bauxite residue (BR), the by-product of alumina extraction, have been produced and stockpiled worldwide, representing a potential risk for the environment due to the high ...alkalinity and the presence of relatively high concentrations of trace elements. Phytoremediation (or simply revegetation) is regarded as the most promising in situ remediation option to mitigate the environmental risk that might arise from the land-disposal of BR. Rehabilitation strategies (including the incorporation of amendments such as gypsum and organic matter) have been employed to address the main limitations to plant establishment and growth on BR, typically the high alkalinity, salinity and sodicity. However, the potential for trace element uptake and phytotoxicity have been largely unreported in revegetated BRs.
In order to assess the ecotoxicological risk, samples of previously revegetated BR were collected from the field, characterized in the laboratory, and used to conduct ex-situ plant bioassays (Phytotoxkit™ and the RHIZOtest). Without rehabilitation, fresh BR severely inhibits seed germinationand root/shoot development in test species Lepidium sativum, Sinapis alba and Sorghum saccharatum. Plant uptake for Al, As, Cr, V was assessed with RHIZOtest bioassay trials with Lolium perenne and demonstrated that plants exposed to fresh BR take up and translocated trace elements to their shoots at concentrations (As = 4.13 mg/kg dm; Cr = 3.29 mg/kg dm; V = 85.66 mg/kg dm) exceeding phytotoxic levels (vanadium) or maximum levels specified for animal feed (arsenic), showing visible stress symptoms in the seedlings.
Conversely, revegetated BR show improved chemical properties, allow seed germination, and permits seedling growth with no evidence of trace element phytotoxicity. However, Na can be taken up at concentrations that could elicit phytotoxicity and impair the success of revegetation. For future rehabilitation programmes, direct revegetation on BR after the incorporation of amendments such as gypsum and organic matter is recommended.
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•Two main rehabilitation strategies on bauxite residue were assessed for their ability to allow seed germination and seedling development•Plant uptake of trace elements from unrehabilitated and rehabilitated bauxite residue was assessed using Rhizotest with Lolium perenne as test species•When residue is unrehabilitated there is potential for increased uptake of Al, As, Cr, Na and V•Amendment of residue allows seed germination and permits seedling growth with no evidence of trace element phytotoxicity•Well weathered residue can produce similar plant element transfer results as those obtained following amendment, but this option does not fulfil the wider legislative (and environmental) requirements
An inherent property of bauxite refining residue is high pH and associated elevated levels of solubilised aluminium. High solubilised Al content of the residue can inhibit plant growth and under ...natural conditions significant reduction of aluminium levels is extremely slow. Various procedures have been employed globally to reduce the alkaline properties of bauxite residue. At the Aughinish Alumina Ltd. refinery gypsum is applied as an amendment to reduce pH and sodicity of the residue for restoration. An additional benefit of this procedure is a significant decrease in plant available Al in the residue and low herbage concentrations. Furthermore, deficiencies of nutrients such as Ca, a symptom of Al toxicity, are not evident. To avoid excessive Al uptake in restored residue it is recommended that adequate calcium and other nutrients are added to the residue and nutrient cycling is encouraged as part of the restoration programme.
Field studies in post-mining sites on epigeic invertebrate groups are scarce despite their importance in pedogenesis and ecosystem multifunctionality. This research investigated the diversity, ...abundance and succession of aboveground invertebrates in a rehabilitated Pb/Zn tailings chronosequence of 5, 20 and 35 years. The study also explored the trophic relationship of selected arthropods and characterized the nitrogen (N) cycle using stable isotope measurements. The abundance and species richness in most investigated groups and the dominance index Berger–Parker (BP) of aboveground invertebrates increased with rehabilitation age from 0.17 BP in early to 0.31 BP in late stage. Elemental and stable isotope ratio analysis showed that N and C soil content increased and the C/N ratio decreased with age, yet despite this increased N availability in the system, the maturing N-cycle used N efficiently. The lack of large N losses from the system despite substantial atmospheric deposition inputs was indicated by the fact that N isotope ratios (δ15N) in plants and animals became significantly more negative with rehabilitation age, −6.0 δ15N for plants, −5.0 δ15N for herbivores and 3.0 δ15N for carnivores. The length of the invertebrate food chain expanded by more than half a trophic level (2.7‰ δ15N) for top predator Coleoptera from early to late stage, probably reflecting more complex food webs including intra-guild predation in older communities. In conclusion, δ15N measurements in plants and animals provided novel insights into the N-cycle, accumulative N flows and the trophic position in post-mining sites. It is proposed that isotope ratio measurements could be used as easy-to-measure, integrating indicators of nutrient cycling and the soil food web complexity of rehabilitated mine tailings and similar soil ecosystems.
Increasing volumes of bauxite residues and their associated leachates represent a significant environmental challenge to the alumina industry. Constructed wetlands have been proposed as a potential ...approach for leachate treatment, but there is limited data on field-scale applications. The research presented here provides preliminary evaluation of a purpose-built constructed wetland to buffer leachate from a bauxite residue disposal site in Ireland. Data collected over a 1-year period demonstrated that the pH of bauxite residue leachates could be effectively reduced from ca. pH 10.3 to 8.1 but was influenced by influent variability and temporal changes. The wetland was also effective in decreasing elemental loading, and sequential extractions suggested that the bulk of the sediment-bound metal inventory was in hard-to-leach phases. Elemental analysis of
Phragmites australis
showed that although vegetation displayed seasonal variation, no trace elements were at concentrations of concern.
Bauxite residue is typically alkaline, has high sodium content and elevated concentrations of trace elements. Effective rehabilitation strategies are needed to mitigate potential environmental risks ...from its disposal and storage. Increasingly, the importance of viable soil faunal populations as well as establishment of vegetation covers is recognized as key components of successful rehabilitation. Inoculation with earthworms is a strategy for accelerating mine site rehabilitation, but little is known on the effects of bauxite residue properties on earthworm survival and viability. In the current study, earthworms (Eisenia fetida) were exposed for 28 days to a series of bauxite residue/soil treatments (0, 10, 25, 35, 50, 75 and 100% residue) to evaluate possible toxic effects on earthworms, investigate the bioavailability of relevant elements (e.g. As, Cr, V), and assess the risk of element transfer. Results showed that soil containing ≥25% residue (pH ≥ 9.8; ESP ≥ 18.5%; extractable Na ≥ 1122 mg/kg) significantly impacted survival (mortality ≥28%) and reproduction (cocoon production inhibition ≥76%) of the exposed earthworms. Alkalinity, sodicity and bioavailable Na were identified as major factors causing toxicity and some earthworms were observed to adopt compensative response (i.e. swollen body) to cope with osmotic stress. Conversely, soil containing 10% residue (pH = 9.1; ESP = 9.2%; extractable Na = 472 mg/kg) did not elicit significant toxicity at the organism level, but biomarker analysis (i.e. superoxide dismutase and catalase) in earthworm coelomocytes showed an oxidative stress. Furthermore, earthworms exposed to soil containing ≥10% residue took up and accumulated elevated concentrations of Al, As, Cr and V in comparison to the control earthworms. We concluded that earthworm inoculation could be used in future rehabilitation programmes once the key parameters responsible for toxicity are lowered below specific target values (i.e. pH = 9.1, ESP = 18.5%, extractable Na = 1122 mg/kg for Eisenia fetida). Nonetheless, trace element uptake in earthworms should be regularly monitored and the risk to the food chain further investigated.
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•Fresh unrehabilitated bauxite residue is hostile to earthworms (Eisenia fetida) displaying a LC50 (28-day) = 37 ± 3.6%;•No significant effects earthworm survival, growth and reproduction are elicited when key properties are lowered (pH ≤ 9.1; ESP ≤ 9.2%; extractable Na ≤ 472 mg/kg);•Exposed earthworms adopt a compensative response to cope with osmotic stress suffered in bauxite residue when pH ≥ 9.8; ESP ≥ 18.5%; extractable Na ≥ 1122 mg/kg);•Biomarker analysis shows an oxidative stress in the exposed earthworms;•A potential risk for the food chain is anticipated for As due to the high bioaccumulation factors and internal accumulated concentrations
It is recognised that the establishment and function of soil biota is critical for successful mine residue rehabilitation. Bauxite residues are alkaline, saline and sodic and, whilst methods for ...establishing vegetation are well studied, little is known about key soil fauna such as earthworms. At a bauxite residue disposal area in Ireland, a 12-year-old rehabilitated residue was examined for evidence of earthworm populations. Five species of earthworm, dominated by
Allolobophora chlorotica
, were recorded in the rehabilitated residue representing the endogeic, epigeic and epi-anecic ecological groups. To further understand the potential for rehabilitated residues to support earthworm communities, a series of exposure tests was conducted. Whilst unamended residues (pH 10.2, EC 0.629 mS cm
−1
, ESP 54) was hostile to
A. chlorotica
survival, 100% survival was observed after 90 days for gypsum and organic-amended residue at salinity of up to 2.9 mS cm
−1
, possibly due to calcium becoming the dominant cation. Survival of earthworms at salinities higher than anticipated tolerance levels suggests that specific ion dominance plays a role in earthworm survival in saline soils. Percent mass change was negatively correlated with pH, EC and sodium content of the residues. Residue from the 12-year-old site also supported the anecic species
Aporrectodea longa
over 100 days. Percent mass change in residue samples retrieved from the 12-year-old site was significantly greater (
p
< 0.05) to that observed for a control soil. Capability of the rehabilitated residue to support earthworm populations indicates the development of a functioning soil system in rehabilitated residues.
The current study was undertaken to evaluate the success of a revegetation program on three sites within the Bauxite Residue Disposal Area at the Aughinish Alumina Ltd. refinery. This was achieved by ...determining botanical diversity, substrate conditions, and plant uptake. Two sites revegetated in 1999, with and without the use of gypsum, were assessed and compared to a site revegetated in 1997. Compared to an initial 6 species used in seeding, a total of 47 species were recorded growing on the three sites with greatest diversity on the 1997 site. There was limited variation in the residue properties of the three treatments indicating that diversity was most influenced by succession and not substrate conditions. Limited available manganese was found in all treatments and significantly lower exchangeable magnesium in the gypsum-amended treatment. Exchangeable sodium, aluminum, and pH in the substrate were not at levels of concern. Appreciable nitrogen, phosphorus, and potassium were found as a result of a fertilizing program. Dominant species in the 1999 treatments, Holcus lanatus and Trifolium pratense, were analyzed for elemental composition. Compared to previous studies, foliar nitrogen, phosphorus, potassium, and calcium were adequate and sodium levels were low. Manganese and magnesium levels were low, and availability should be assessed as part of the monitoring program. Furthermore, the effect of a fertilizing regime on plant uptake and substrate conditions needs to be assessed.
Leachates emanating from bauxite residue disposal areas are alkaline and require neutralisation prior to discharge. The use of passive technologies such as constructed wetlands has received ...increasing interest as possible treatments for alkaline leachates, including bauxite residues. Mechanisms proposed for wetland effectiveness have included calcite precipitation but it is not clear if such a pathway is feasible in the relatively low Ca residue leachates. Carbonation of Ca-spiked residue leachate treatments was conducted to observe rates of pH decrease and precipitate formation. For all treatments, carbonation effectively decreased pH to ca. 10.5 which remained stable following aeration. Decreases in Al content of 83–93% were also observed. Precipitates retrieved from carbonation experiments and from a constructed wetland trial were characterised using XRD, SEM, XPS and EDX. Calcium carbonates formed in Ca-spiked treatments and dawsonite precipitation occur in the absence of Ca. Rinsing of precipitates removes surface calcium indicating soluble forms adsorbed on precipitates. The results demonstrate that carbonation of bauxite residue leachate is an important component of passive treatments and neutralisation.
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