Pb-contaminated Soil Research Articles

Long-Term Performance of Cement-Stabilized/Solidified Pb-Contaminated Soil under Simulated Erosive Environments

The performance of stabilization/solidification(S/S) monoliths is vital for the long-term effectiveness of potentially toxic-element-contaminated soil treatments, which arevulnerable to environmental conditions (e.g., strongly acidic or alkaline conditions). This study numerically investigated the long-term performance of S/S monolith materials in erosive environments with pHs ranging from 2.65 to 10.00 over three years. It was found that the leachability and availability of Pb decreased continually over the three years due to the transformation of the chemical fractions of Pb from acid-soluble fractions to reducible and oxidizable fractions. Environmental pH greatly affected the strength and permeability of the S/S monoliths, which increased in weakly acidic and alkaline conditions (e.g., pH 5.00–10.00), while they were significantly reduced in strongly acidic conditions (e.g., pH 2.65–3.65). The strength of the S/S monolith decreased from 19.7 MPa to 0.2 Mpa after three years when the environmental pH was 2.65. A model was established to predict the long-term strength evolution of the S/S monolith. The dissolution of hydration products in the S/S monolith materials caused by acid erosion directly attenuated the strength and permeability of the S/S-treated soils. Long-term exposure to acidic environments could lead to the failure of S/S treatments. Thus, more attention should be paid to the long-term effectiveness of S/S monoliths in erosive environments.

ZmbZIP54 and ZmFDX5 cooperatively regulate maize seedling tolerance to lead by mediating ZmPRP1 transcription

Extensive lead (Pb) accumulation in plants exerts toxic effects on plant growth and development and enters the human food chain. Combining linkage mapping, transcriptome analysis, and association studies, we cloned the ZmbZIP54 transcription factor, which confers maize tolerance to Pb. Combined overexpression and knockdown confirmed that ZmbZIP54 mitigates Pb toxicity in maize by alleviating Pb absorption into the roots. Yeast one-hybrid and dual-luciferase assays revealed that ZmbZIP54 binds to the ZmPRP1 promoter and promotes its transcription. Yeast two-hybrid and bimolecular fluorescence complementation assays indicated that ZmFdx5 interacts with ZmbZIP54 in the nucleus. ZmFdx5 acts as a switch that controls the regulation of ZmPRP1 expression by ZmbZIP54 when maize encounters Pb stress. Furthermore, we revealed that variation in the 5′-UTR of ZmbZIP54 affects its expression level under Pb stress and contributes to the difference in Pb tolerance among maize lines. Finally, we proposed a model to summarize the role of ZmbZIP54 in Pb tolerance, which involves the cooperative effect of ZmbZIP54 and ZmFdx5 on the ZmPRP1 transcription in maize response to Pb. This study provides novel insights into the development of Pb-tolerant maize varieties and bioremediation of Pb-contaminated soils.

Arbuscular Mycorrhiza Alters Metal Uptake and the Physio-biochemical Responses of Glycyrrhiza glabra in aLead Contaminated Soil.

Arbuscular mycorrhizal (AM) fungi can affect the host’s ability to cope with several environmental stresses, such as heavy metal stress. Therefore, an experiment was conducted to assess the effect of the Funneliformis mosseae inoculation on growth and physio-biochemical parameters and lead (Pb) accumulation in liquorice (Glycyrrhiza glabra L.) under Pb stress. A factorial experiment was performed with the combination of two factors, fungi (inoculated and non-inoculated (NM)) and soil Pb levels (0, 150, 300, and 450 mg kg−1 soil) with four replicates. In the presence of Pb, symbiosis with F. mosseae exert positive effect on growth parameters, which was more significant in shoots than roots. Mycorrhization improved fresh and dry weights and length in shoot by 147, 112.5 and 83%, respectively, compared to NM plants at Pb150 level. Moreover, F. mosseae significantly increased tolerance index and the concentrations of soluble sugars and flavonoids in shoots and proline, phosphorus, potassium, calcium, zinc and manganese in shoots and roots but decreased their malondialdehyde concentrations under Pb stress. The Pb concentrations, transfer and bioaccumulation factors of mycorrhizal plants were less than non-mycorrhizal ones. A positive correlation was also observed between glomalin secretion and colonization rate in Pb treated soils. These results indicate the importance of mycorrhizal colonization in alleviating the Pb-induced stress in liquorice, mainly through improving the nutrition, modifying reactive oxygen species detoxifying metabolites and reducing the translocation of Pb to shoots. Observations revealed that mycorrhization of liquorice would be an efficient strategy to use in the phytoremediation practices of Pb-contaminated soils.

Successful Conversion of Pb-Contaminated Soils to Low-Bioaccessibility Plumbojarosite Using Potassium-Jarosite at Ambient Temperature.

Methods promoting lead (Pb) phase transformation in soils are essential for decreasing Pb bioaccessibility/bioavailability and may offer an in situ, cost-efficient process for mitigating contaminant exposure. Recent plumbojarosite (PLJ) conversion methods have shown the greatest potential to reduce soil Pb bioaccessibility, an in vitro bioaccessibility assay measurement of the proportion of Pb solubilized under gastric chemical conditions. Soils tested utilizing the recent PLJ method were found to have a Pb bioaccessibility of <1%, compared to original soils possessing bioaccessibility of >70%. However, this technique requires heat (95-100 °C) to promote mineral transformation. Jarosite-group minerals may incorporate multiple interlayer cations; therefore, we probed the potential for jarosite to remediate Pb via intercalation by reacting presynthesized potassium (K)-jarosite with aqueous Pb and/or Pb-contaminated soil at room temperature. Both K-jarosite and heated PLJ-treated samples were investigated by pairing bioaccessibility analyses with advanced bulk and spatially resolved X-ray absorption spectroscopy analyses. Samples treated with K-jarosite promoted Pb transformation to low-bioaccessibility (<10%) PLJ, with soil being converted to 100% PLJ using both heated and nonheated techniques. μ-X-ray fluorescence (μ-XRF) and μ-X-ray absorption near-edge structure (μ-XANES) showcase significant differences between elemental interactions for heated and nonheated PLJ-treated samples with anglesite impurities being found on the microscale. Although further development is necessary to accommodate for suitable field conditions, results indicate, for the first time, that K-jarosite may successfully convert soil Pb to PLJ without high-temperature conditions. The newfound utility of K-jarosite is expected to be key to future jarosite-based soil Pb remediation method development.

Synergistic solidification of lead-contaminated soil by magnesium oxide and microorganisms

Although microbially induced carbonate precipitation (MICP) technology effectively promotes the remediation of heavy metal contaminated soils in low concentrations, the high concentration of heavy metals has a toxic effect on microorganisms, which leads to the decline of carbonate yield and makes the soil strength and environmental safety after remediation no up to the standard. This study describes the synergistic curing effect of MgO and microorganisms on soil contaminated with high concentrations of heavy metals. The experimental results with MgO showed 2–6 times increase in unconfined compressive strength (UCS) compared to bio-cemented samples without MgO. Toxicity characteristic leaching procedure experiments indicated that Pb-contaminated soil at 10,000 mg/kg with quantitative MgO for synergistic solidification could meet the international solid waste disposal standards, which leachable Pb2+ are less than 5 mg/L. In addition, the microscopic results showed that the introduction of MgO promoted the formation of magnesium calcite and dolomite, improved the solidification efficiency of heavy metal contaminants, and demonstrated the presence of Pb2+ in carbonate minerals. This study suggests that MgO and microorganisms have broad application prospects for synergistic solidification of Pb2+ soil.

Microbial metabolic limitation and carbon use feedback in lead contaminated agricultural soils

Potentially toxic elements (PTEs) pollution causes a great threat to microbial metabolism, which plays a vital role in studying soil nutrient cycling and predicting carbon (C) storage in agroecosystems. However, the responses of microbial metabolism characteristic to heavy metal contamination and the mechanisms through which microbial metabolism mediate nutrient cycling and C dynamics in contaminated soil remain elusive. Here, we performed an incubation experiment over 80 days to investigate the variations in microbial metabolic limitation under various Pb levels (0, 100, 500, 800, 1500, 2000, and 3000 mg Pb kg−1 dry soil) in cropland soil using extracellular enzymatic stoichiometry, and to reveal the impact of Pb stress on soil C storage by associating with microbial metabolic quotients (qCO2) and C use efficiency (CUE). The results showed microbial relative C limitation and phosphorus (P) limitation were observed in Pb-contaminated soils. Pb addition enhanced the microbial relative C limitation by approximately 7.3%, while decreasing the P limitation by approximately 12.3%. Furthermore, Pb addition led to higher qCO2 (from 8.75 to 108 μg C kg−1 MBC−1 d−1) duo to the increase of microbial relative C limitation, suggesting that the more CO2 was released of per unit of microbial biomass C. The increase of microbial relative C limitation reduced CUE (from 0.35 to 0.10) because of the change in microbial metabolism from growth to respiration maintenance under Pb stress. Consequently, the CUE and qCO2 together determined the loss of soil C. Our study reveals that microbial relative C limitation is the dominant driver of soil C loss and provides important knowledge of microbial metabolic limitation regulating soil C turnover in PTEs-contaminated agricultural soils.

Ingestion of remediated lead-contaminated soils affects the fecal microbiome of mice.

The relationship between ingestion of diets amended with a Pb-contaminated soil and the composition of the fecal microbiome was examined in a mouse model. Mice consumed diets amended with a Pb-contaminated soil in its native (untreated) state or after treatment for remediation with phosphoric acid or triple superphosphate alone or in combination with iron-waste material or biosolids compost. Subacute dietary exposure of mice receiving treated soil resulted in modulation of the fecal intestinal flora, which coincided with reduced relative Pb bioavailability in the bone, blood and kidney and differences in Pb speciation compared to untreated soil. Shifts in the relative abundance of several phyla including Verrucomicrobia, Tenericutes, Firmicutes, Proteobacteria, and TM7 (Candidatus Saccharibacteria) were observed. Because the phyla persist in the presence of Pb, it is probable that they are resistant to Pb. This may enable members of the phyla to bind and limit Pb uptake in the intestine. Families Ruminococcaceae, Lachnospiraceae, Erysipelotrichaceae, Verrucomicrobiaceae, Prevotellaceae, Lactobacilaceae, and Bacteroidaceae, which have been linked to health or disease, also were modulated. This study is the first to explore the relationship between the murine fecal microbiome and ingested Pb contaminated soils treated with different remediation options designed to reduce bioavailability. Identifying commonalities in the microbiome that are correlated with more positive health outcomes may serve as biomarkers to assist in the selection of remediation approaches that are more effective and pose less risk.

Treating Pb-contaminated clay slurry by three curing agents.

Each year, extensive dredged clay slurries containing heavy metals need to be treated before being reused; in such contaminated slurries, lead (Pb) is frequently identified. Quicklime (CaO)-activated ground granulated blast-furnace slag (GGBS), magnesium (MgO)-activated GGBS, and ordinary Portland cement (OPC) are usually used to remediate the lead (Pb)-contaminated soil; nevertheless, using these curing agents (or binders), particularly CaO-GGBS and MgO-GGBS, to treat Pb-contaminated slurry with high water content is rarely reported. Moreover, inconsistent results were obtained from previous studies in terms of the mechanical and leaching performance of Pb-contaminated soils with the three binders. Based on the above-mentioned reasons, this study used CaO-GGBS, MgO-GGBS, and OPC to treat the Pb-contaminated clay slurry, and compared the effectiveness of the three binders in improving the mechanical and leaching properties of the slurry. Laboratory tests were performed to examine the leaching, strength, mineralogical, and micro-structural performance of treated clay slurries. The results showed that GGBS-based binders were more effective than OPC in improving the strength and Pb leachability of contaminated slurries. When suitable ratios between activators (CaO and MgO) and GGBS were used, a similar or even higher UCS was produced by CaO-GGBS than MgO-GGBS. Similar leachate pH and Pb leachability could be achieved between CaO-GGBS- and MgO-GGBS-treated contaminated clay slurries. Therefore, it is not rigorous to state that MgO-GGBS is better in improving the strength and leachability of Pb-contaminated soils than CaO-GGBS only by comparing the two GGBS-binders based on the same activator/GGBS ratio, as reported in some previous studies. The leachability of Pb was affected by the pH, but the addition of GGBS facilitated the decrease of Pb leachability in slurries. The XRD result showed the formation of CSH and Pb(OH)2, which facilitated the reduction of Pb leachability.

Stabilisation/solidification of lead-contaminated soil by using ladle furnace slag and carbon dioxide

Global sustainable development faces challenges in greenhouse gas emission, consumption of non-renewable resource and energy, waste landfilling, and environmental pollution. Geotechnical engineering also faces similar challenges; for example, the use of cement and lime for stabilisation/solidification (S/S) of contaminated soil is associated with carbon dioxide (CO2) emission and consumption of limestone and energy. In this context, this study introduces a sustainable S/S method by using an industrial waste, ladle furnace slag (LFS), and a greenhouse gas, CO2, to replace common additives for S/S of lead (Pb)-contaminated soil. LFS was first mixed with Pb-contaminated soils and then CO2 was introduced into the soil for carbonation. The results showed that LFS-stabilised Pb-contaminated soils could achieve CO2 uptake up to 18% of LFS mass. After carbonation, the concentration of leached Pb from contaminated soils was reduced by three orders of magnitude than that without carbonation, demonstrating higher S/S efficacy than cement, lime, and magnesia. Additionally, this method can improve soil strength, as well as reduce non-renewable resource consumption, energy use, LFS landfilling, and additive cost.

Home and community composts in Nantes city (France): quality and safety regarding trace metals and metalloids.

Home and community composting are key strategies for local organic waste management. The quality and safety of industrial composts are controlled, but those of home and community composts are not, and this could make them unsafe for use in kitchen gardens. Home (n = 20) and community (n = 41) composts, from urban and suburban areas including mildly Pb-contaminated allotment gardens, were analyzed for quality and safety regarding trace metals and metalloids (TMM) using mid-infrared Fourier transform spectrometry (FT-MIR) and portable X-ray fluorescence spectrometry, respectively. Home composts had a significantly higher Pb content (98mg.kg-1 ± 10mg.kg-1) than community composts (21mg.kg-1 ± 2mg.kg-1). Numerous home composts (85%) and a few community composts (17%) exceeded the organic farming thresholds for Pb (45mg.kg-1) and Zn (100mg.kg-1). The high mineral matter content and the relative abundance of chemical functions attributable to silicates (up to 35%) highly paralleled with TMM contents, mostly concentrated in the fine fraction. Co-inertia analysis highlighted strong and significant links between TMM contents and the whole chemical signature delivered by FT-MIR spectrometry. Pb-contaminated soil could be carried into home compost by green waste or by voluntary addition. Covariance analyses indicated that mineral matter and chemical functions only partly explained the variability in Pb content, suggesting a more complex combination of drivers. Community composting appears as a suitable local solution resulting in high-quality compost that complies with European organic farming regulations, while home composting from allotment gardens should be seriously evaluated to comply with such safety requirements.

Remediation of Pb-contaminated soil by magnetic micro-nano size composite MFH

Remediation of Pb-contaminated soil by magnetic micro-nano size composite MFH

Effects of composite environmental materials on the passivation and biochemical effectiveness of Pb and Cd in soil: Analyses at the ex-planta of the Pak-choi root and leave

Passivation of soil heavy metals using environmental materials is an important method or important in situ remediation measure. There are more studies on inorganic environmental materials for heavy metal passivation, but not enough studies on organic and their composite environmental materials with inorganic ones. In order to reveal the passivation effect of coal-based ammoniated humic acid (CAHA), biochemical humic acid (BHA), biochar (BC) and other organic types and inorganic environmental materials such as zeolites (ZL) on soil heavy metals and their biological effectiveness. The microstructures of these materials were analyzed by Scanning electron microscope (SEM). The main components of the environmental materials were analyzed by Energy dispersive spectrometer (EDS), Fourier transforms infrared spectroscopy (FT-IR) and X-ray diffraction spectrum (XRD) to elucidate the mechanism of passivation of heavy metals in soil by these environmental materials. The study was conducted to investigate the effects of different types of environmental materials and their combinations on the passivation effect and biological effectiveness of Pb and Cd complex contamination in soil by means of soil incubation and pot experiments using single-factor and multifactor multilevel orthogonal experimental designs. Soil incubation experiments proved that the effective state of soil Pb and Cd in T7 was reduced by 13.40% and 11.07%, respectively. The extreme difference analysis determined the optimized formulation of soil lead and cadmium passivation as BHA: CAHA: BC: ZL = 3.5:5:20:10. The pot experiment proved that the application of composite environmental materials led to the reduction of lead and cadmium content and increase of biomass of Pak-choi, and the optimal dosage of optimized composite environmental materials was 23.1 g/kg.

Effect of Low-Molecular-Weight Organic Acids on Migration Characteristics of Pb in Reclaimed Soil.

The effect of low-molecular-weight organic acids (citric acid and malic acid) on the migration characteristics of Pb in contaminated soils was explored in this study. Reclaimed soil was collected from the coal gangue hill area of the Panyi mine in Huainan City (China). The effect of citric acid and malic acid on the form of Pb present in the reclaimed soil was analyzed by spiking soil samples and simulating Pb-contaminated soil. The results indicate the following. 1) With increased concentration of exogenous Pb, the activity of Pb in the reclaimed soil was effectively improved. 2) The addition of citric acid and malic acid both resulted in an increased fraction of exchangeable Pb in the soil, which effectively promoted the active Pb fraction. As the concentrations of citric acid and malic acid increased, the active Pb fraction of the reclaimed soil increased accordingly. The Pb activation effect of citric acid was observed to be greater than that of malic acid. 3) With extended soil aging time, the activation effect of organic acids on Pb weakened, with the loosely bound Pb gradually transforming into strongly bound Pb. Chelating agents can activate heavy metals in soil, mainly through the combination of chelating agents and heavy metal ions in the soil solution to form soluble metal chelates, so as to increase the bioavailability of heavy metals in soil to plant roots. Therefore, adding citric acid can be considered as a strategy to enhance the efficiency of reclaimed soil remediation because of the ability of Pb activation.

Diversity, function and assembly of the Trifolium repens L. root-associated microbiome under lead stress

Root-associated microbial layers provide unique niches that drive specific microbe assemblies. While the rhizosphere microbiome has long received much attention, endophytic microbes remain largely elusive. Characterizing metal-tolerant plants’ strategies for assembling different root-associated microbial layers is important for optimizing phytoremediation. Here, a pre-stratified rhizo-box assay was conducted with Trifolium repens L. under greenhouse conditions with artificial Pb-contaminated soil. Cultivation compensated for the pollution-driven loss of soil microbial biomass carbon, enzyme activities and abundance. The acid-soluble Pb proportion increased in the rhizosphere (from 6.5–13.7% to 7.1–18.0%) compared with bulk soil. Under stress, root-layer variants were a considerable source of variation in the microbiome, with the endosphere representing a unique and independent niche. A core set of root microbes were selected by T. repens, with Proteobacteria and Actinobacteria composed of diverse plant-growth-promoting bacteria (PGPBs) and metal-tolerant members. Cluster analysis revealed endosphere-enriched genera, with Rhizobium, Nocardioides, Novosphingobium, Phyllobacterium, and Sphingomonas being the most dominant. Finally, inferred microbial metabolic pathways suggested that these potential metal-tolerant PGPB species provide critical services to hosts, enabling them to tolerate and even flourish in contaminated soil. Our results provide novel insights for understanding how root-associated microbes help metal-tolerant plants cope with abiotic stress.

Remediation of Lead-Contaminated Soil by Using Saponin Derived from Sapindus Mukorossi

Lead (Pb) contamination in soils is a major global concern for both humans and the environment. Saponin, a plant-based based and environment-friendly surfactant was used for remediation of Pb-spiked garden soil to test its effectiveness and suitability for Pb removal. A Box-Behnken design was designed to optimise the washing conditions using the following variables: soil type (100% topsoil; 50% topsoil/50% sand; 20% topsoil/ 80% sand); pH of the washing solution (2; 3.5; 5); and saponin concentration (1%; 3%; 5%). The results showed that optimum removal efficiency of 58.69% was achieved with saponin, while distilled water removed 3.92% Pb in a single wash. Saponin was found to be statistically more effective than distilled water (P&lt; 0.05); an optimum pH of 3.5 significantly increased Pb removal efficiency (P&lt;0.05). The washing process was found to cause slight surface erosion and weathering of the soil grains.

Understanding the mechanisms of zeolite in inhibiting Pb accumulation in different rice cultivars (Oryza sativa).

Zeolite is one of the potential passivating amendments for the immobilization of lead (Pb) in contaminated farmland soils. In this study, pot experiments were carried out to investigate the effects and the mechanisms of zeolite on Pb accumulation in two rice cultivars grown in a slightly Pb-contaminated soil. Results showed that Pb content in grains of Zheyou 18 (ZY-18) decreased by the addition of 6g zeolite kg-1 soil (E6), which can be attributed to the reduction in soil Pb availability, dissolved organic carbon (DOC), water-soluble iron (Fe) and manganese (Mn), and the transfer factor from soil to grain (TFsoil-grain). These reductions were mainly resulting from the significant increase in soil pH, glutathione (GSH), phytochelatins (PCs), and non-protein (NPT) content in rice root, and the decrease in soil redox potential (Eh), due to zeolite addition. Pb content in brown rice of DL-5 was not significantly affected with E6 treatment, whereas it was raised by applying 12g zeolite kg-1 soil (E12). The increase of Pb content of Donglian 5 (DL-5) grains with E12 treatment can be attributed to more Pb uptake by the root, higher Pb transfer factors (TFs) between various parts of rice, and significant decrease in GSH, PCs, and NPT contents in the root. It is concluded that a suitable rate of zeolite addition can immobilize Pb in slightly Pb-contaminated acidic soil. However, the final immobilization effect also depends on rice cultivars.

Coupling phytoremediation of Pb-contaminated soil and biomass energy production: A comparative Life Cycle Assessment

Phytoremediation is an in-situ remediation technology based on the ability of plants to fix pollutants from the soil. In this sense, plants such as Festuca arundinacea are a promising for heavy metal removal in contaminated soils. The present work studies phytoremediation for Pb removal from a contaminated soil located in Spain using F. arundinacea by applying the Life Cycle Assessment (LCA) approach. Two different options for biomass management were assessed: direct disposal in a security landfill (case 1A) and energy recovery (case 1B). For the latter option, cogeneration was simulated using SuperPro Designer 9.5. In addition, traditional treatments such as soil washing (case 2) and excavation + landfill (case 3) were evaluated in terms of environmental impacts by LCA. The former was simulated using SuperPro Designer 9.5, whereas data from literature were used for the latter to perform the LCA. Results showed that biomass disposal in a landfill was the most important contributor to the overall impact in case 1A. In contrast, biomass conditioning and cogeneration were the main steps responsible for environmental impacts in case 1B. Comparing cases 1A and 1B, the energy recovery from biomass was superior to direct landfill disposal, reducing the environmental impacts in most of the studied categories. Regarding the rest of the treatments, chemical production and soil disposal presented the most critical environmental burdens in cases 2 and 3, respectively. Finally, the comparison between the studied cases revealed that phytoextraction + energy recovery was the most environmentally friendly option for the studied conditions, reducing impacts by 30–100%.

Is the harvest of Salix and Populus clones in the growing season truly advantageous for the phytoextraction of metals from a long-term perspective?

The phytoextraction potential of short-rotation field coppice plantations was investigated on soil historically contaminated with risk elements (REs), especially Cd, Pb and Zn. The main objective of the study was to assess the effect of biomass harvest time (summer harvest versus traditional winter one) on biomass yield, REs accumulation and removal in the long-term study. The precise field experiment with two Salix clones and two Populus clones was established in April 2009. Shoots of all clones were harvested in February 2012 for the first time, and then identical branches and leaves were harvested every two years in September (summer harvest = SH) and the branches every two years in February (winter harvest = WH). The first summer harvest seemed to be more promising compared to the winter one, but the yields in the second and third harvests were inconsistent. The total phytoextraction efficiency of the SH declined in second and third harvests due to a decrease of leaves/aboveground biomass ratio, and the RE concentrations in leaves. Clonal Salix smithiana was the most productive one in the SH, with a dry matter yield of 15.1 t ha−1 year−1 and showed promising extraction ability for Cd (11.65 %) and Zn (4.59 %) over a 6-year field experiment. A lower portion of Cd (6.97 %) and Zn (2.38 %) was removed by this clone in the WH (calculated from the total soil content of REs). SH was more reasonable for phytoextraction compared to WH. Higher RE concentrations were found in leaves of Salix compared to Populus. Populus accumulated the highest Pb content in the branches; unfortunately, the Pb extraction was low, due to extremely high soil Pb contamination. Locally bred willows and poplars performed substantially better than internationally recognised ones, indicating the importance of phytoremediation optimisation, including clone selection, for suitable climatic conditions.

Performance of Red Mud/Biochar Composite Material (RMBC) as Heavy Metal Passivator in Pb-Contaminated Soil

Red mud/biochar composite material (RMBC), which was applied as heavy metal passivator in this research, was prepared with red mud (the bauxite residue) and cornstalk under anoxic sintering condition. Based on the batch experiments in Pb contaminated soil, the passivating properties of several materials, including red mud (RM), biochar (BC), RMBC and phosphate-containing RMBC (PRMBC), were investigated in comparison with each other. Some interesting results are as follows: through anoxic thermal activation, a rough and porous structure of RMBC was obtained. Substances such as Fe3O4 and metal-organic complexes generated in RMBC provided effective sites for Pb passivation; and the mechanisms were speculated as the precipitation between Pb2+ and the carbonate (or hydroxide), as well as the complexation reaction between Pb and metal organic complexes through ligand bonding. The pot experiments showed the promotion effects of four passivators on the growth of red onion were in the following order: PRMBC > RMBC > BC > RM. PRMBC stabilized Pb content in soil significantly due to the formation of insoluble substances, with the minimum transfer factor and bioconcentration factor for plant growth. The evidences above implied the composite materials (PRMBC and RMBC) would be potential passivators for heavy metal-contaminated soil.

Remediation of Pb-contaminated soil using modified bauxite refinery residue

This study examines amendment of Pb-contaminated soil with modified bauxite refinery residue (MBRR) to decrease soil Pb mobility and bioaccessibility. Amendment experiments were conducted using four soils contaminated with Pb from various sources, including smelting, shooting-range activities and Pb-based paint waste. Lead L3-edge X-ray absorption spectroscopy (XAS) indicated that Pb speciation in these soils was a mixture of Pb sorbed to Fe (hydr)oxide and clay minerals, along with Pb bound to organic matter. Amendment with MBRR decreased water-soluble Pb and/or Toxicity Characteristic Leachate Procedure (TCLP) Pb concentrations. Lead L3-edge XAS and X-ray diffraction (XRD) indicated that Pb retention by MBRR occurred via sorption to Fe- and Al-(hydr)oxides at low Pb loadings, in addition to formation of hydrocerussite (Pb3(CO3)2(OH)2) at high loadings. Soil amendment with MBRR had relatively little effect on gastric-phase Pb bioaccessibility; as quantified via the Solubility/Bioavailability Research Consortium, SBRC, in vitro assay. In contrast, amendment with MBRR caused substantial decreases in relative intestinal-phase Pb bioaccessibility (Rel-SBRC-I) due to increased Pb sorption by MBRR’s Fe- and Al-hydr(oxide) minerals as simulated GI tract conditions shifted from the gastric- to the intestinal-phase. These decreases in Rel-SBRC-I point to the potential efficacy of using amendment with MBRR to decrease soil Pb bioavailability.