Lead immobilization using phosphoric acid in a smelter-contaminated urban soil

Cadmium, lead, and zinc immobilization in the soil using a phosphate compound with citric acid present

Low molecular weight organic acids (LMWOAs) are common in rhizospheric soil and may impede the interaction between phosphate and metals. Thus, studying how phosphate compounds impact metal immobilization in rhizospheric soil using LMWOAs is crucial. An incubation experiment examined the effects of NaH2PO4 (a P compound) (3%), various concentrations of citric acid (CA), and combinations of P and CA, on soil cadmium (Cd), lead (Pb), and zinc (Zn) immobilization using the European Community Bureau of Reference (BCR) sequential extraction method, CaCl2 extraction method, zeta potential, fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The P, low CA (2 mmol kg-1 soil) (CA2), and P-CA2 treatments reduced acid-soluble and CaCl2-extractable Cd, Pb, and Zn, indicating metal immobilization, with the P-CA2 treatment being the most effective. High CA (>5-20 mmol kg-1 soil) or a P with high CA reversed prior patterns, suggesting metal mobilization. The zeta potential study indicated that when pH increased, treatments became more negative, notably P-CA2 followed by P, suggesting that electrostatic adsorption was the predominant metal immobilization mechanism, especially in P-CA2. XRD tests, however, showed that the P treatment alone produced Cd phosphate, pyromorphite, and hopeite, indicating that sorption and precipitation were the main metal immobilization processes in the P treatment alone. In conclusion, P-CA2 was found to be the most efficient metal immobilization and redistribution treatment for contaminated soils. Rhizospheric CA may alter Cd, Pb, and Zn mineral stability. Therefore, when treating Cd, Pb, and Zn-contaminated soils with a P compound, CA should be addressed.

Acidified biochar improves lead tolerance and enhances morphological and biochemical attributes of mint in saline soil

Lead (Pb) toxicity is a significant environmental issue, especially in areas with a past of industrial activities and mining. The existence of Pb in the soil can have negative impacts on plant growth and development, and it can also pose a risk to human health through the food chain. Acidified carbon has shown promise as an effective management technology for mitigating Pb toxicity. This study provides important insights into the potential of acidified biochar as a low-cost and eco-friendly method for managing Pb-contaminated soils. The current study explores the effectiveness of acidified biochar (AB) in alleviating Pb stress in mint. The study involved two levels of Pb (0 = control and 200 mg/kg Pb) and four levels of AB as treatments (0, 0.45, 0.90, and 1.20%). Results indicate that 1.20% AB was the most effective treatment, significantly decreasing root and shoot Pb concentration while enhancing shoot and root fresh and dry weight, shoot and root length, and shoot and root N, P, and K concentration. Moreover, a significant decrease in MDA (0.45AB, 0.90AB, and 1.20AB caused a decline in MDA content by 14.3%, 27.8%, and 40.2%, respectively) and an increase in ascorbic acid (0.45AB, 0.90AB, and 1.20AB led to an increase in ascorbic acid content of 1.9%, 24.8%, and 28.4%, respectively) validated the effectiveness of 1.20% AB compared to the control. Adding 0.45AB, 0.90AB, and 1.20AB led to an increase in soluble sugar content of 15.6%, 27.5%, and 32.1%, respectively, compared to the treatment without AB. Further investigations at the field level are suggested to confirm the efficacy of 1.20% AB as the best treatment against Pb toxicity in saline soil conditions.

Enhanced immobilization of lead, cadmium, and arsenic in smelter-contaminated soil by sulfidated zero-valent iron

Soils contaminated with multiple heavy metal(loid)s (HMs) such as lead (Pb), cadmium (Cd), and arsenic (As) are of great concern in many countries. In this study, taking three lead-zinc smelter soils, the performance of sulfidated zero-valent iron (S-ZVI) toward Pb, Cd, and As immobilization was systemically investigated. Results showed that more than 88% of water-extractable Pb and Cd could be immobilized and transformed into reducible, oxidizable, and/or reducible forms by S-ZVI within 3 h, whereas only 3-56% of them could be immobilized by unsulfidated ZVI even after 72 h. Meanwhile, the phytoavailability of the tested HMs could be effectively reduced by 79% after S-ZVI amendment. More importantly, anoxic/oxic incubation tests revealed that the dissolved concentrations of HMs were much lower in S-ZVI-treated soils than in the untreated or unmodified ZVI-treated soils. Speciation analysis further suggested that unmodified ZVI seemed to reduce the long-term soil stability by changing the residual HMs species to mild-acid soluble and/or reducible ones. In contrast, S-ZVI could effectively alleviate the remobilization of HMs under the changeover of soil redox environments. All these findings indicate that S-ZVI may be a promising amendment for the immobilization of Pb, Cd, and As in smelter-contaminated soil.

Lead remediation is promoted by phosphate-solubilizing fungi and apatite via the enhanced production of organic acid

Lead (Pb) is one of the most common heavy metal pollutants in the environment, which can indirectly or directly threaten human health. Lead immobilization by apatite can reduce the effectiveness of Pb cations via the formation of pyromorphite (Pyro). However, the formation of Pyro is always depending on the release of phosphorus (P) from apatite. Phosphate-solubilizing fungi (PSF) can secrete large amounts of organic acid to promote the release of P from apatite. Although the combination of PSF and apatite has shown a huge potential in Pb remediation, this pathway needs to be more attention, especially for organic acid secretion by PSF. This research mainly reviews the possible pathway to strengthen Pb immobilization by PSF and apatite. Meanwhile, the limitation of this approach is also reviewed, with the aim of a better stabilizing effect of Pb in the environment and promoting the development of these remediation technologies.

Performance of oxalate-doped hydroxyapatite as well as relative contribution of oxalate and phosphate for aqueous lead removal

An oxalate-doped hydroxyapatite (O-HAP) was hydrothermally synthesized for aqueous lead (Pb) removal based on the solubility-limiting ability of oxalate and phosphate over pH range 4-9. Free Pb²⁺ activities in oxalate and/or phosphate systems were controlled by oxalate to form soluble ion pairs Pb-Ox (aq) and Pb-Ox₂^2- at pH 4-7 while in preference to persist as PbHPO₄ (aq) when pH ≥ 8. Both phosphate and oxalate exhibited excellent efficiency in reducing Pb solubility, causing over 99 % of Pb precipitated from solution following oxalate < oxalate-phosphate < phosphate. The Visual MINTEQ model overestimated dissolved Pb and free Pb²⁺ in nearly all of the reaction systems due to the ill-defined stability constants and solubility products for Pb ion-pair formation. The addition of phosphate acting as a buffer in Pb-oxalate systems tended to lessen the spontaneous pH shifts within 24 h to equilibrate proton release from Pb precipitation and hydrolysis, indicating lower solubility products and faster kinetics of Pb-phosphate mineral formation. The TEM-EDS, FTIR and XRD identified a block-shaped Pb-oxalate mineral phase as the only precipitate at acidic pH while substituted by phosphate to form rod-shaped Pb₅(PO₄)₃OH and Pb₃(PO₄)₂ precipitates as pH increased. The optimum hydrothermal conditions of O-HAP were 433 K, pH 9 and P/Ox doping ratio of 0.5 for 24 h. Batch experiments revealed the endothermic process of O-HAP toward Pb with the maximum adsorption capacity reaching 2333 mg/g at a pH of 7, reaction time of 12 h, initial Pb concentration of 600 mg/L and temperature of 308 K, which were best fitted with the pseudo-second-order kinetic model and Langmuir isotherm. The synergetic mechanisms of O-HAP for Pb removal involved dissolution-precipitation, adsorption and ion exchange. This study provides an insight in developing effective remediation strategies for heavy metal contamination by interacting between low-molecular-weight organic acids and secondary mineral phases.

Bioaccessibility of Pb in health-related size fractions of contaminated soils amended with phosphate

Lead (Pb) contamination is one of the most significant exposure hazards to human health. Contaminated soil particles may be eroded and transferred either to the atmosphere (<10 μm) or to streams; or they may be incidentally ingested (<200 μm). Among strategies for the long-term management of this risk, one of the most cost-effective is the reduction of Pb mobility and bioavailability via amendment with phosphorus-containing materials. To clarify the effectiveness of P amendment in reducing Pb mobility and bioaccessibility in different soil size fractions, an experiment was performed by adding a soluble P compound to a historically contaminated urban soil (RO), a mining soil (MI), and an uncontaminated spiked soil (SP) at different P:Pb molar ratios (2.5:1, 5:1, and 15:1). In the <10 μm fraction of soils, P addition reduced bioaccessible Pb only in the SP soil at the highest dose, with little to no effect on RO and MI soils. Similarly, in the coarse fraction, Pb was immobilized only in the SP soil with all three P doses. These results were probably due to the higher stability of Pb in historically contaminated soils, where Pb dissolution is the limiting factor to the formation of insoluble Pb compounds. The bioaccessible proportion of Pb (using SBET method) was higher than 70 % of the total Pb in all soils and was similar in both fine and coarse particle fractions. Due to the enrichment of Pb in finer particles, this implies possible adverse effects to the environment or to human health if these particles escape from the soil. These results call for increasing attention to the effect of remediation activities on fine soil particles, considering their significant environmental role especially in urban and in historically low or moderately contaminated areas.

Using phosphate amendments to reduce bioaccessible Pb in contaminated soils: A meta-analysis

Measuring the reduction of in vitro bioaccessible (IVBA) Pb from the addition of phosphate amendments has been researched for more than 20 years. A range of effects have been observed from increases in IVBA Pb to almost 100% reduction. This study determined the mean change in IVBA Pb as a fraction of total Pb (AC) and relative to the IVBA Pb of the control soil (RC) with a random effects meta-analysis. Forty-four studies that investigated the ability of inorganic phosphate amendments to reduce IVBA Pb were identified through 5 databases. These studies were split into 3 groups: primary, secondary, and EPA Method 1340 based on selection criteria, with the primary group being utilized for subgroup analysis and meta-regression. The mean AC was approximately -12% and mean RC was approximately -25% for the primary and secondary groups. For the EPA Method 1340 group, the mean AC was -5% and mean RC was -8%. The results of subgroup analysis identified the phosphorous amendment applied and contamination source as having a significant effect on the AC and RC. Soluble amendments reduce bioaccessible Pb more than insoluble amendments and phosphoric acid is more effective than other phosphate amendments. Urban Pb contamination associated with legacy Pb-paint and tetraethyl Pb from gasoline showed lower reductions than other sources such as shooting ranges and smelting operations. Meta-regression identified high IVBA Pb in the control, low incubated soil pH, and high total Pb with the greater reductions in AC and RC. In order to facilitate comparisons across future remediation research, a set of minimum reported data should be included in published studies and researchers should use standardized in vitro bioaccessibility methods developed for P-treated soils. Additionally, a shared data repository should be created for soil remediation research to enhance available soil property information and better identify unique materials.

Correlating soil nutrient test lead with bioaccessible lead in highly-contaminated soils receiving lead-immobilizing amendments

Lead (Pb) is one of the most common metals exceeding human health risk guidelines for soil concentrations worldwide. Pb bioaccessibility is known to vary depending on soil physiochemical characteristics and, as a result, in vitro and in vivo tests exist that are used to estimate bioaccessible Pb in contaminated soils. Although in vitro tests such as the relative bioaccessibility leaching procedure (RBALP) present simpler and more cost-effective risk assessments than in vivo methods, soil tests such as Mehlich-3, Modified Morgan, and ammonium bicarbonate-diethylenetriamine pentaacetate (AB-DTPA) extractions are extremely routine and even more cost-effective. Currently, there are few comparisons examining the viability of common soil nutrient tests for assessing Pb bioaccessibility in soils from contaminated sites with extremely high total Pb concentrations or for sites that have received amendments, such as those containing compost, iron, and/or phosphorus, intended to immobilize Pb. Here, we examine the correlation between RBALP Pb and Pb as determined using three commonly utilized soil tests, Mehlich-3, Modified Morgan, and AB-DTPA, in archived samples from one Pb-contaminated site receiving compost amendment (Seattle, WA, USA) and one extremely Pb-contaminated site receiving mixtures of compost, P, and Fe (Joplin, MO, USA). At both the Seattle and Joplin sites separately, RBALP Pb was significantly correlated with all three soil nutrient test values, regardless of soil amendment. However, RBALP was only significantly correlated with Modified Morgan and total Pb when examining the Joplin and Seattle data together, likely resulting from different factors controlling Pb solubility at the two sites. These findings suggest that a diverse suite of relatively inexpensive and accessible soil nutrient test methods correlate with bioaccessible Pb at a specific site, regardless of whether Pb-immobilizing amendments have been used.

Immobilization of high-Pb contaminated soil by oxalic acid activated incinerated sewage sludge ash

Identifying effective and low-cost agents for the remediation of Pb-contaminated soil is of great importance for field-scale applications. In this study, the feasibility of reusing incinerated sewage sludge ash (ISSA), a waste rich in phosphorus, under activation by oxalic acid (OA) for the remediation of high-Pb contaminated soil was investigated. ISSA and OA were mixed at different proportions for the treatment of the high-Pb contaminated soil (5000 mg/kg). The Pb immobilization efficacy was further examined by both the standard deionized water leaching test and the toxicity characteristic leaching procedure (TCLP). The overall results showed that the use of the ISSA alone and an appropriate mixture of the ISSA and OA could effectively reduce the leachability of Pb from soil. 20% ISSA together with 30% OA (0.2 mol/L) reduced leached Pb concentration by 99%. The main stabilization mechanisms were then explored by different microstructural and spectroscopic analytical techniques including SEM, XRD and FTIR. Apparently, OA released phosphate from the ISSA and Pb from soil via acid attack, which combined and precipitated as stable lead phosphate minerals. However, excessive OA could cause high leaching of phosphate and zinc from the ISSA. Overall, this study indicates that ISSA could be used together with OA to remediate high-Pb contaminated soil, but careful design of mix proportions is necessary before practical application to avoid excessive leaching of phosphate and zinc from the ISSA.

P-enriched hydrochar for soil remediation: Synthesis, characterization, and lead stabilization

One-step synthesis of multifunctional materials using biomass waste for environmental remediation is a current research hotspot. In this study, a novel P-enriched hydrochar was obtained by co-hydrothermal treatment of biomass (bamboo or hickory) with concentrated H₃PO₄ (biomass: H₃PO₄ = 1:4) at 200 °C for 7 h. The characteristics of the P-enriched hydrochar were determined and its effect on the stabilization of Pb in soils was investigated. Compared to pristine hydrochar, the weight yield of the P-enriched hydrochar was greater (by over 2 times). This was due to the enrichment of P (over 20% by weight), as the C, N, and H weight content was reduced. Moreover, the aromaticity, thermal stability, and surface functionality of P-enriched hydrochar were all higher than that of pristine hydrochar. Addition of the pristine hydrochar to a simulated 1300 mg·kg^-1 Pb-contaminated soil at 3% (w/w) resulted in a 20%-40% reduction in leached Pb only after 4 weeks, compared to the control without hydrochar amendment. However, addition of the P-enriched hydrochar to the spiked Pb-contaminated soil reduced Pb leaching by about 60% after only 1 week and about 90% after 3 weeks. Besides, using a real Pb-contaminated soil (149,000 mg·kg^-1 Pb), P-enriched hydrochar addition at 5% (w/w) resulted in a 100% decrease in Pb leaching in the first week and maintained leached Pb levels at <2 mg L^-1, meeting U.S.-E.P.A. standards. Thus, P-enriched hydrochar stabilized Pb in both simulated and real Pb-contaminated soil quickly and efficiently. Hence, the potential of one-step co-hydrothermal carbonization of biomass with H₃PO₄ to produce a novel and sustainable P-enriched hydrochar with properties suitable for environmental remediation of cationic metals.

Restoration and risk reduction of lead mining waste by phosphate-enriched biosolid amendments

Lead (Pb) contamination in environment has been identified as a threat to human health and ecosystems. In an effort to reduce the health and ecological risks associated with Pb mining wastes, a field study was conducted to stabilize Pb using phosphate (P)-enriched biosolid amendments in the contaminated mining wastes (average of 1004 mg Pb kg⁻¹) located within the Jasper County Superfund Site, southwest Missouri. Experiments consisted of six biosolid amendment treatments, including Mizzou Doo compost (MD); Spent mushroom compost (SMC); Turkey litter compost (TLC); Composted chicken litter (CCL); Composted sewage sludge (CSS); and Triple superphosphate (TSP). Kentucky tall fescue seeds were planted following the treatments, and soil and plant samples were collected and analyzed 8–10 years post treatment. Results indicated that, in all cases, the biosolid treatments resulted in significant reductions in bioaccessible Pb (96.5 to 97.5%), leachable Pb (95.0 to 97.1%) and plant tissue Pb (45.5 to 90.1%) in the treated wastes, as compared with the control. The treatments had no significantly toxicological effect to soil microbial community. Analysis of the Pb fractionation revealed that the Pb risk reduction was accomplished by transforming labile Pb fractions to relatively stable species through the chemical stabilization reactions as induced by the treatments. The solid-phase microprobe analysis confirmed the formation of pyromorphite or pyromorphite-like minerals after the treatment. Among the six biosolid amendments examined, SMC and MD treatments were shown most effective in the context of Pb stabilization and risk reduction. This field study demonstrated that the treatment effectiveness of Pb stabilization and risk reduction in mining wastes by P-enriched biosolid amendments was long-term and environmental-sound, which could be potentially applied as a cost-effective remedial technology to restore contaminated mining site and safeguard human health and ecosystems from Pb contamination.

Enhanced removal of zinc and cadmium from water using carboxymethyl cellulose-bridged chlorapatite nanoparticles

Zinc (Zn²⁺) and cadmium (Cd²⁺) in water pose serious threats to human health and the environment. In search for a more effective treatment technology, we prepared a type of carboxymethyl cellulose (CMC) bridged chlorapatite (CMC-CAP) nanoparticles and tested the material for removal of Zn²⁺ and Cd²⁺ from water. CMC macromolecules were attached to CAP by bidentate bridging and hydrogen bonding, preserving the high adsorption capacity of CAP nanoparticles while allowing for easy gravity-separation of the nanoparticles. CMC-CAP showed rapid adsorption kinetics and 22.8% and 11.2% higher equilibrium uptake for Zn²⁺ and Cd²⁺, respectively, than pristine CAP. An extended dual-mode isotherm model, which takes into account both sorption and chemical precipitation, provided the best fits to the sorption isotherms, giving a maximum Langmuir sorption capacity of 141.1 mg g^-1 for Zn²⁺ and 150.2 mg g^-1 for Cd²⁺ by CMC-CAP. Na⁺ at up to 5 mM showed modest effects on the uptake of the heavy metals, while 2-5 mM of Ca²⁺ exerted notable inhibitive effects. Dissolved organic matter (up to 5 mg L^-1 as TOC) inhibited the Zn²⁺ uptake by 16.5% but enhanced the Cd²⁺ removal by 8.6%. Material characterizations and surface binding analyses revealed that ion exchange, surface precipitation, and surface complexation were the removal mechanisms for the heavy metals. This study demonstrates stabilizer bridging may serve as a convenient strategy to facilitate water treatment uses of nanoparticles.

Determining the fate of lead (Pb) and phosphorus (P) in alkaline Pb-polluted soils amended with P and acidified using multiple synchrotron-based techniques

The effect of acidification on lead (Pb) and phosphorus (P) speciation in alkaline Pb-polluted soils that are amended with P to stabilize Pb is still unclear. It was studied in three alkaline Pb-polluted soils containing specific amounts of Soil Organic Matter (SOM), using multiple synchrotron-based techniques, i.e. bulk X-ray Absorption Fine Structure (XAFS) spectroscopy at Pb LIII- and P K-edges, micro-X-ray Fluorescence (μ-XRF), and micro-X-ray Diffraction (μ-XRD). These techniques provided unambiguous evidences that the formation of pyromorphite, i.e. the desired Pb stabilized chemical form, was severely limited in the acidified soil samples amended with fish bones or phosphoric acid (H₃PO₄). Most Pb present in the H₃PO₄-amended soil samples did not convert to pyromorphite due to Pb and P leaching and PbSO_4(s) formation. In contrast, most Pb present in the fish bone-amended soil samples was unaffected by acidification and did not convert to pyromorphite as it was inaccessible to soil solution or retained by SOM, similarly to P. Additionally, Pb-SOM association increased with increasing SOM content. Results had important implications on the applicability of the P-based method to stabilize Pb within the first centimeters below surface of Pb-polluted alkaline soils, which potentially represent the most hazardous part of these soils.

Effect of struvite and organic acids on immobilization of copper and zinc in contaminated bio-retention filter media

Few studies have been carried out to connect nutrient recovery as struvite from wastewater and sustainable utilization of the recovered struvite for copper and zinc immobilization in contaminated soil. This study revealed the effect of struvite on Cu and Zn immobilization in contaminated bio-retention soil in the presence of commonly exuded plant organic acids. The research hypothesis was that the presence of both struvite and organic acids may influence the immobilization of Cu and Zn in soil. The outcome of this research confirmed that more than 99% of Cu and Zn was immobilized in bio-retention filter media by struvite application. Water-soluble Cu and Zn concentrations of struvite treated soil were less than 1.83 and 0.86 mg/kg respectively, and these concentrations were significantly lower compared to the total Cu and Zn content of 747.05 mg/kg in the contaminated soil. Application of struvite to Cu- and Zn-contaminated soil resulted in formation of compounds similar to zinc phosphate tetrahydrate (Zn₃(PO₄)₂•4H₂O) and amorphous Cu and Zn phases. Struvite was effective in heavy metal remediation in acidic soil regardless of the presence of Ca impurities in struvite and the presence of plant organic acids in soil. Overall, this study revealed that struvite recovered from wastewater treatment plants has potential for use as an amendment for heavy metal remediation in contaminated bio-retention soil.

Effects of compaction on lead availability in contaminated soils with contrasting texture

The effects of soil compaction on porosity (α), bulk density (ρs), and saturated hydraulic conductivity (Ksat) can create a physical barrier in the soil, reducing the vertical movement of toxic elements in the soil profile. However, the indirect effects of compaction in altering the forms and availability of heavy metals in soil have not been well-studied. This study examined the influence of compaction on forms of lead (Pb) in soils with contrasting texture. Four levels of compaction were imposed on a sandy loam and a clayey soil, which were artificially contaminated based on their maximum Pb adsorption capacity. Compaction had different effects on Pb forms depending on soil texture. In the sandy loam soil, compaction had a dual beneficial effect in mitigating the impact of Pb contamination, since it decreased Ksat, reducing metal transport to deeper soil layers, and also prevented transformation to more available Pb forms (soluble and exchangeable). Instead, there was an increase in the most environmentally stable forms of Pb (inner sphere adsorption on iron and manganese oxides). In the clayey soil, compaction caused a significant increase in soluble and exchangeable Pb, accompanied by a significant reduction in environmentally stable Pb (inner sphere adsorption on gibbsite and kaolinite). In addition, studies about Pb contents under compacted soil layers should be investigated, mainly in clayey soils with edible crops, and environmental remediation practices that involve the machines traffic (for example, phytoremediation-successive cultivation of Pb-hyperaccumulating plants) should be used with care to minimise the compaction of clayey soils.

Transformation of Pb, Cd, and Zn Minerals Using Phosphates

Heavy metal contamination in soils has become one of the most critical environmental issues. The most efficient in-situ remediation technique is chemical immobilization that uses cost-effective soil amendments such as phosphate compounds to decrease Pb, Cd and Zn accessibility in the contaminated soils. The present study examined the effectiveness of KH2PO4 in immobilizing Pb, Cd and Zn in three samples of contaminated soils collected from ZGH “Bolesław” (Mining and Smelting Plant “Bolesław”). Effectiveness was evaluated using the following methods: a toxicity characteristic leaching procedure (TCLP)-based experiment, sequential extraction, X-ray diffraction analyses (XRD), and scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS). The most efficient percentage reduction of total leachable metal concentration assessed by TCLP was observed for lead (50%–80%), and the least reduction was observed for zinc (1%–17%). The most effective immobilization of stable compounds assessed by sequential extraction was noted for lead, while the weakest immobilization was noted for cadmium. New insoluble mineral phases were identified by SEM-EDS analysis. Cd, Zn, and Pb formed new stable mineral substances with phosphates. The predominant crystal forms were dripstones and groups of needles, which were easily formed by dissolved carbon rock surfaces containing zinc ions. The alkaline nature of the soil and a large number of carbonates mainly influenced the formation of new structures.

Lead immobilization processes in soils subjected to freeze-thaw cycles

Soil freeze-thaw cycles (FTCs) change the physical and chemical properties of soils; however, information is limited about the consequences for heavy metal sorption and desorption. Lead (Pb) sorption isotherms and successive desorption tests were measured for three soils from North China (Chestnut, Lou and Black), following multiple freeze-thaw cycles (0, 1, 3, 6 and 9 FTCs) of -5 °C for 12 h and then +5 °C for 12 h. Lead adsorption dominated the sorption processes for all soils, and sorption capacity increased with additional FTCs. The Freundlich affinity parameter of soils for Pb sorption (i.e. A; L^β mmol^1-β kg^-1), was linearly correlated with carbonate content for soils with multiple FTCs. The effects of FTCs on lead adsorption may be more dependent on carbonate and clay contents than organic matter (OM), cation exchange capacity (CEC) and amorphous iron content. Repeated FTCs increased the pH of soil solutions at applied Pb concentrations >1.4 mmol L^-1, which could facilitate formation of inner-sphere complexes of Pb in studied soils. Cation exchange, a weak association, could occupy specific adsorption sites with increasing Pb doses in soils and it can also be facilitated by FTCs. Our results demonstrate the great potential for increasing Pb immobilization with repeated FTCs, by facilitating the formation of both inner-sphere and outer-sphere complexes. Hence, these findings provide useful information on Pb immobilization in contaminated soils that undergo frequent FTCs and offer an additional insight into predicting Pb behavior in cold and freezing environments like the polar regions.

Utilization of nanomaterials for in-situ remediation of heavy metal(loid) contaminated sediments: A review

Heavy metal(loid)s are toxic and non-biodegradable environmental pollutants. The contamination of sediments with heavy metal(loid)s has attracted increasing attention due to the negative environmental effects of heavy metal(loid)s and the development of new remediation techniques for metal(loid) contaminated sediments. As a result of rapid nanotechnology development, nanomaterials are also being increasingly utilized for the remediation of contaminated sediments due to their excellent capacity of immobilizing/adsorbing metal(loid) ions. This review summarizes recent studies that have used various nanomaterials such as nanoscale zero-valent iron (nZVI), stabilizer-modified nZVI, nano apatite based-materials including nano-hydroxyapatite particles (nHAp) and stabilized nano-chlorapatite (nCLAP), carbon nanotubes (CNTs), and titanium dioxide nanoparticles (TiO₂ NPs) for the remediation of heavy metal(loid) contaminated sediments. We also review the analysis of potential mechanisms involved in the interaction of nanomaterials with metal(loid) ions. Subsequently, we discuss the factors affecting the nanoparticle-heavy metal(loid)s interaction, the environmental impacts resulting from the application of nanomaterials, the knowledge gaps, and potential future research.

Responses of soil aggregates and bacterial communities to soil-Pb immobilization induced by biofertilizer

The objective of this study was to investigate how soil aggregates and bacterial communities responded to soil-lead (Pb) immobilization induced by biofertilizer. Wheat (Triticum spp.) was planted in Pb-polluted soil. The re-distribution of Pb in soil aggregates and change of soil microbial communities due to biofertilizers were believed to be responsible for immobilizing soil Pb and alleviating its phytotoxicity. Adding biofertilizer promoted the formation of large aggregates (0.20-2.0 mm) with more mass loading of Pb, and increased soil bacterial diversity and the abundance of beneficial taxa such as those from the phyla Bacteroidetes, Actinobacteria, and Proteobacteria. In addition, there was significant alleviation of Pb availability as indicated by decreases in the values of bioconcentration factors (BCF) (up to 35.7% and 42.3% for roots and shoots, respectively) of wheat and DTPA-extractable Pb in soil (up to 34.4%) receiving fertilizer treatments compared with the CK (no treatment). Similar bacterial community structures and alpha diversities for the biofertilizer treatments and their autoclaved controls were observed, suggesting that physicochemical properties drove the structure of the soil bacterial community. This study introduced a new idea for development of effective strategies to control or reduce soil Pb risks.

Effects of soil amendments applied on cadmium availability, soil enzyme activity, and plant uptake in contaminated purple soil

Soil Cd pollution resulting from human activities has become a serious food safety concern. This study was to investigate the impacts of eight soil amendments including a humic acid material (XZ), three organic-inorganic combined materials (FY, PX and PLY) and four inorganic materials (FS, LIME, PLL and PL) on soil Cd availability, enzyme activity, and the uptake by vegetables under incubation, greenhouse and field conditions. Results showed that soil exchangeable Cd (EX-Cd) concentrations under four amendments (FY, XZ, PX, FS) significantly decreased by 12.10-13.59% after 20d treatments, and both PX and PLY resulted in reduced soil EX-Cd fraction while increased fractions of Fe-Mn oxides (OX-Cd), organic matter (OM-Cd) and residual (Res-Cd) in the Cd-contaminated purple soil. PX significantly reduced soil EX-Cd and carbonate-bound (CB-Cd) fractions by 4.4% and 11.4%, and decreased vegetable Cd uptake by 38.8% and 49.1% in greenhouse and field experiments, respectively. Moreover, PX elevated the activities of soil catalase, urease and saccharase by 15.7%, 53.6% and 48.2% in pot soil, which were ~4, ~5 and ~14 times higher those in field soil, respectively. This research demonstrated that PX could be an effective soil amendment to reduce the Cd health and ecological riskthe 1s in the Cd-contaminated purple soil.

Long-Term in Situ Reduction in Soil Lead Bioavailability Measured in a Mouse Model

Effects of different treatments on the bioavailability of lead (Pb) in soil from a smelter emission contaminated site in Joplin, Missouri, were evaluated in a mouse model. Similar estimates of relative bioavailability for Pb in untreated or treated soil were obtained in mice and in the well-established juvenile swine model. In the mouse model, treatments that used phosphate (phosphoric acid or triple superphosphate) combined with iron oxide or biosolids compost significantly reduced soil Pb bioavailability. Notably, effects of these remediation procedures were persistent, given that up to 16 years had elapsed between soil treatment and sample collection. Remediation of soils was associated with changes in Pb species present in soil. Differences in Pb species in ingested soil and in feces from treated mice indicated that changes in Pb speciation occurred during transit through the gastrointestinal tract. Use of the mouse model facilitates evaluation of remediation procedures and allows monitoring of the performance of procedures under laboratory and field conditions.

Remediation of lead-contaminated sediment by biochar-supported nano-chlorapatite: Accompanied with the change of available phosphorus and organic matters

Some rivers in China have been seriously contaminated due to the discharge of lead (Pb) smelting wastewater. In this study, biochar-supported nano-chlorapatite (BC-nClAP) was synthesized to immobilize Pb in contaminated sediment. The remediation effect of BC-nClAP on Pb-contaminated sediment was evaluated through batch experiments and the materials were characterized by x-ray diffraction, scanning electron microscope, Brunner-Emmet-Teller and electronic differential system. It was found that BC-nClAP can transform Pb effectively from labile fraction into stable fraction with a maximum transformation efficiency increasing to 94.1% after 30 days of treatment, and the stabilization efficiency of toxicity characteristic leaching procedure reached 100% only after 16 days of treatment. The content of available phosphorus (AP) in the sediments treated by BC-nClAP was much less than that treated by nClAP, which indicated a lower risk of eutrophication and suggested the dissolution-precipitation mechanism involved in Pb immobilization. BC-nClAP presented the best immobilization efficiency of Pb and the content of organic matters in BC-nClAP treated samples increased the most, thus the OM might play an important role during the Pb immobilization.

Comparison of palygorskite and struvite supported palygorskite derived from phosphate recovery in wastewater for in-situ immobilization of Cu, Pb and Cd in contaminated soil

A phosphorus-bearing product S-PAL obtained from nutrient-rich wastewater was reused as ameliorant for Cu, Pb and Cd immobilization in contaminated soil with three different rates (1%, 5% and 10% w/w). The 0.01 mol/L CaCl₂ -extractability of metals significantly reduced with increasing rate of PAL and S-PAL in the first 7-day immobilization and insignificantly changed after 14-day immobilization. Compared with PAL, the lower metal extractability was observed after S-PAL addition. The BCR sequential extraction results showed that both of amendments were beneficial to transform acid soluble fraction to residual fraction. The XRD patterns of soil samples after immobilization evidenced that the formation of metal-bearing phosphate precipitates and the combination between functional groups such as Si-OH and metals played a key role for metal immobilization by S-PAL and PAL. Dominant phyla across all samples were Fusobacteria, Proteobacteria and Actinobacteria, and the relative abundance of Fusobacteria decreased under S-PAL treatment. The pH-dependent leaching test indicated that the metal release at a defined pH was not affected by the presence of PAL. Compared with S-PAL, the metals amended by PAL in soil were easier to release at acidic pH since the combination between functional groups and metals instead the formation of new metal-bearing precipitate.

Rhamnolipid stabilized nano-chlorapatite: Synthesis and enhancement effect on Pb-and Cd-immobilization in polluted sediment

Phosphate (P) compounds are usually used as chemical amendment for in situ remediation of heavy metal polluted sediment. However, the low deliverability, weak utilization and potential risk of eutrophication inhibit the application of most P materials. Therefore, rhamnolipid (Rha), a kind of anionic biosurfactant which has algicidal activity, was employed in this study to synthesize a new kind of nano-chlorapatite (nClAP) for Pb and Cd immobilization. Characterization results showed that the Rha stablized nClAP (Rha-nClAP) was uniformly distributed in suspensions within about 5nm. Experimental data demonstrated that the combination of Rha and nClAP could greatly enhance the Pb- and Cd-immobilization efficiencies, promoting their transformation from labile fractions to stable fractions through precipitation or adsorption processes, especially when the Rha approached to its critical micelle concentration. And Rha-nClAP could also decrease both the TCLP-leachable Pb and Cd with maximum reduction efficiencies of 98.12% and 96.24%, respectively, which also presented concentration dependence of Rha. Changes of available phosphorus implied the dissolution of nClAP during the treatment and the detection of organic matter demonstrated that the microorganisms may involve in the remediation.

Effect of lead speciation on its oral bioaccessibility in surface dust and soil of electronic-wastes recycling sites

We measured bioaccessible lead (Pb) in simulated gastrointestinal fluids containing Pb-contaminated soil or dust from electronic waste (e-waste) recycling sites to assess the risk of Pb ingestion. The physiologically based extraction test (PBET) was used as in vitro bioaccessibility assay. Pb speciation was determined using X-ray absorption spectroscopy. The total Pb concentrations in dusts (n=8) and soils (n=4) were in the range of 1630-131,000 and 239-7800mg/kg, respectively. Metallic Pb, a common component of e-waste, was ubiquitous in the samples. We also found Pb adsorbed onto goethite and as oxides and carbonate, implying soil mixing and weathering influences. Pb phosphate and organic species were only found in the soil samples, suggesting that formation was soil-specific. We identified other Pb compounds in several samples, including Pb silicate, Pb chromate, and Pb(II) hydrogen phosphate. A correlation analysis indicated that metallic Pb decreased bioaccessibility in the stomach, while a Pb speciation analysis revealed a low bioaccessibility for Pb phosphates and high bioaccessibility for organic Pb species. The health risk based on bioaccessible Pb was estimated to be much lower than that of total Pb due to the lower concentrations.

Soil solution interactions may limit Pb remediation using P amendments in an urban soil

Lead (Pb) contaminated soils are a potential exposure hazard to the public. Amending soils with phosphorus (P) may reduce Pb soil hazards. Soil from Cleveland, OH containing 726 ± 14 mg Pb kg^-1 was amended in a laboratory study with bone meal and triple super phosphate (TSP) at 5:1 P:Pb molar ratios. Soil was acidified, neturalized and re-acidified to encourage Pb phosphate formation. PRSTM-probes were used to evaluate changes in soil solution chemistry. Soil acidification did not decrease in vitro bioaccessible (IVBA) Pb using either a pH 1.5, 0.4 M glycine solution or a pH 2.5 solution with organic acids. PRSTM-probe data found soluble Pb increased 10-fold in acidic conditions compared to circumnetural pH conditions. In acidic conditions (p = 3-4), TSP treated soils increased detected P 10-fold over untreated soils. Bone meal application did not increase PRSTM-probe detected P, indicating there may have been insufficient P to react with Pb. X-ray absorption spectroscopy suggested a 10% increase in pyromorphite formation for the TSP treated soil only. Treatments increased soil electrical conductivity above 16 mS cm^-1, potentially causing a new salinity hazard. This study used a novel approach by combining the human ingestion endpoint, PRSTM-probes, and X-ray absorption spectroscopy to evaluate treatment efficacy. PRSTM-probe data indicated potentially excess Ca relative to P across incubation steps that could have competed with Pb for soluble P. More research is needed to characterize soil solutions in Pb contaminated urban soils to identify where P treatments might be effective and when competing cations, such as Ca, Fe, and Zn may limit low rate P applications for treating Pb soils.

Synthesis and evaluation of a new class of stabilized nano-chlorapatite for Pb immobilization in sediment

During the past years, efforts have been made to deal with the Pb contaminated sediment in Xiawangang River in Hunan province, China, but it remains a serious problem since the smelting pollutants were accumulated. According to previous studies, phosphate showed an effective ability to transfer labile Pb to pyromorphite (Pb₅(PO₄)₃X, X=F, Cl, Br, OH) but its application was limited by its solubility and deliverability. Hence a new class of nano-chlorapatite was synthesized by using sodium dodecyl sulfate (SDS) as a stabilizer and characterized by TEM, FESEM, DLS, FTIR, and EDAX. Results demonstrated that the SDS stabilized nano-chlorapatite (SDS-nClAP) was in spherical or spheroidal shape with a hydrodynamic diameter of 40.4nm. Experimental data suggested that SDS-nClAP was effective in transforming labile Pb to stable fraction with a maximum increase of 38.3%, also the reduction of TCLP-leachable Pb from 0.30 to 0mg/L after 45-d treatment. The increase of available phosphorus in both SDS-nClAP and ClAP treated sediment samples verified dissolution-precipitation mechanism involved in Pb immobilization. Additionally, the increment of organic matter in 10:1 treated samples was approximately 5-fold than that in 2:1 treated samples, which revealed that the micro-organisms may play an important role in it.

Environmental application of nanotechnology: air, soil, and water

Global deterioration of water, soil, and atmosphere by the release of toxic chemicals from the ongoing anthropogenic activities is becoming a serious problem throughout the world. This poses numerous issues relevant to ecosystem and human health that intensify the application challenges of conventional treatment technologies. Therefore, this review sheds the light on the recent progresses in nanotechnology and its vital role to encompass the imperative demand to monitor and treat the emerging hazardous wastes with lower cost, less energy, as well as higher efficiency. Essentially, the key aspects of this account are to briefly outline the advantages of nanotechnology over conventional treatment technologies and to relevantly highlight the treatment applications of some nanomaterials (e.g., carbon-based nanoparticles, antibacterial nanoparticles, and metal oxide nanoparticles) in the following environments: (1) air (treatment of greenhouse gases, volatile organic compounds, and bioaerosols via adsorption, photocatalytic degradation, thermal decomposition, and air filtration processes), (2) soil (application of nanomaterials as amendment agents for phytoremediation processes and utilization of stabilizers to enhance their performance), and (3) water (removal of organic pollutants, heavy metals, pathogens through adsorption, membrane processes, photocatalysis, and disinfection processes).

Evaluation of metal mobility from copper mine tailings in northern Chile

This work shows the results obtained on a copper mine tailing in the Antofagasta Region, Chile. The tailing was classified as saline-sodic with high concentrations of metals, especially Cu and Fe, with pH 8.4. Our objectives were to (1) compare the physicochemical properties of the tailing with surrounding soils of the mine under study, and (2) evaluate the effect of two amendments (CaCO3 and compost) and their mixtures on Cu(2+), Mn, Fe, Zn, Mg(2+), and K(+) and Ca(2+), SO4 (2-), NO3 (-), and PO4 (3-) leaching. The data obtained were submitted to variance and covariance analysis. The results from the comparison between both substrates showed that in general, the tailing presented greater content of metals. Regarding tailing leaching, pH, electrical conductivity (EC), and concentration of the elements of interest were measured. The statistical analysis showed that Cu(2+) leaching and immobilization of Fe occurred to the greatest extent with compost. The EC decreased throughout the experiment with irrigation and increased upon treatment with compost. The major interactions found among the chemical parameters were (1) tailings without treatment, Cu(2+)/Fe and NO3 (-)/SO4 (2-); (2) tailings treated with CaCO3, Cu(2+)/K(+); (3) tailings treated with compost, NO3 (-)/SO4 (-2) and EC/Cu(2+); and (4) tailings treated with both amendments, EC/Fe and Cu(2+)/Fe. The ANOVA showed that the number of irrigations and the amendments statistically significantly affected the copper mobility and the organic amendment significantly influenced the iron mobility.

Remediation of Cd(ii)-contaminated soil by three kinds of ferrous phosphate nanoparticles

Three kinds of iron phosphate nanoparticles were synthesized and they could reduce leachability and bioaccessibility of Cd effectively.

Phosphorus Amendment Efficacy for In Situ Remediation of Soil Lead Depends on the Bioaccessible Method

A validated method is needed to measure reductions of in vitro bioaccessible (IVBA) Pb in urban soil remediated with amendments. This study evaluated the effect of in vitro extraction solution pH and glycine buffer on bioaccessible Pb in P-treated soils. Two Pb-contaminated soils (790-1300 mg Pb kg), one from a garden and one from a city lot in Cleveland, OH, were incubated in a bench scale experiment for 1 yr. Six phosphate amendments, including bone meal, fish bone, poultry litter, monoammonium phosphate, diammonium phosphate, and triple superphosphate, were added to containers at two application rates. Lead IVBA was assessed using USEPA Method 1340 and three modified versions of this method. Modifications included using solutions with pH 1.5 and 2.5 as well as using solutions with and without 0.4 mol L glycine. Soil amendments were ineffective in reducing IVBA Pb in these soils as measured by pH 1.5 with glycine buffer. The greatest reductions in IVBA Pb, from 5 to 26%, were found using pH 2.5 extractions. Lead mineral results showed several soil amendments promoted Pb phosphate formation, an indicator of remediation success. A significant negative linear relationship between reduction in IVBA Pb and Pb-phosphate formation was found only for pH 2.5 without glycine extraction solution. A modified USEPA Method 1340 without glycine and using pH 2.5 has the potential to predict P soil treatment efficacy and reductions in bioavailable Pb.

Chemical stabilisation of lead in shooting range soils with phosphate and magnesium oxide: Synchrotron investigation

Three Australian shooting range soils were treated with phosphate and magnesium oxide, or a combination of both to chemically stabilize Pb. Lead speciation was determined after 1 month ageing by X-ray absorption spectroscopy combined with linear combination fitting in control and treated soils. The predominant Pb species in untreated soils were iron oxide bound Pb, humic acid bound Pb and the mineral litharge. Treatment with phosphate resulted in substantial pyromorphite formation in two of the soils (TV and PE), accounting for up to 38% of Pb species present, despite the addition of excess phosphate. In MgO treated soils only, up to 43% of Pb was associated with MgO. Litharge and Pb hydroxide also formed as a result of MgO addition in the soils. Application of MgO after P treatment increased hydroxypyromorphite/pyromorphite formation relative to soils teated with phosphate only. X-ray diffraction and Scanning electron microscopy revealed PbO precipitate on the surface of MgO. Soil pH, (5.3-9.3) was an important parameter, as was the solubility of existing Pb species. The use of direct means of determination of the stabilisation of metals such as by X-ray absorption spectroscopy is desirable, particularly in relation to understanding long term stability of the immobilised contaminants.

Using soil properties to predict in vivo bioavailability of lead in soils

Soil plays a significant role in controlling the potential bioavailability of contaminants in the environment. In this study, eleven soils were used to investigate the relationship between soil properties and relative bioavailability (RB) of lead (Pb). To minimise the effect of source of Pb on in vivo bioavailability, uncontaminated study soils were spiked with 1500 mg Pb/kg soil and aged for 10-12 months prior to investigating the relationships between soil properties and in vivo RB of Pb using swine model. The biological responses to oral administration of Pb in aqueous phase or as spiked soils were compared by applying a two-compartment pharmacokinetic model to blood Pb concentration. The study revealed that RB of Pb from aged soils ranged from 30±9% to 83±7%. The very different RB of Pb in these soils was attributed to variations in the soils' physico-chemical properties. This was established using sorption studies showing: firstly, Freundlich partition coefficients that ranged from 21 to 234; and secondly, a strongly significant (R(2)=0.94, P<0.001) exponential relationship between RB and Freundlich partition coefficient (Kd). This simple exponential model can be used to predict relative bioavailability of Pb in contaminated soils. To the best of our knowledge, this is the first such model derived using sorption partition coefficient to predict the relative bioavailability of Pb.

In vitro bioaccessibility of lead in surface dust and implications for human exposure: A comparative study between industrial area and urban district

In this study, ground surface dust samples from two contrasting areas, a former zinc smelting area in Guizhou Province and a common urban district in Wuhan city, Hubei Province, China, were assessed for in vitro Pb bioaccessibility using a physiologically based extraction test (PBET). Extremely elevated concentrations of Pb (220-6348 mg/kg) and other trace metals were observed in the zinc smelting area. While moderate high metal concentrations (79-1544 mg/kg of Pb) in the urban dusts were attributed to various urban activities, coal combustion and traffic emissions. Lead bioaccessibility in the stomach-phase varied from 17.6 to 76.1% and no significant difference was found between industrial and urban dust samples. Compared with the stomach-phase, Pb bioaccessibility in the more alkaline intestinal-phase was considerably lower (1.2-21.8%). A significantly negative correlation was found between dust Ca concentrations and Pb bioaccessibility in the intestinal-phase, suggesting that Ca plays an important role in reducing the bioaccessible Pb in the intestinal-phase. The estimated Pb exposure based on gastric bioaccessible Pb was 13.9 and 1.8 μg/kg day for children living in the industrial and urban areas, respectively, accounting for 85% and 41% of their corresponding total Pb exposure.

Phosphate Treatment of Lead-Contaminated Soil: Effects on Water Quality, Plant Uptake, and Lead Speciation

Water quality threats associated with using phosphate-based amendments to remediate Pb-contaminated soils are a concern, particularly in riparian areas. This study investigated the effects of P application rates to a Pb-contaminated alluvial soil on Pb and P loss via surface water runoff, Pb accumulation in tall fescue ( Schreb; Kentucky 31), and Pb speciation. An alluvial soil was treated with triple superphosphate at P to Pb molar ratios of 0:1 (control), 4:1, 8:1, and 16:1. After a 6-mo reaction period, rainfall simulation (RFS) studies were conducted, followed by tall fescue establishment and a second set of RFS studies (1 yr after treatment). Results from the first RFS study (unvegetated) demonstrated that the total Pb and P concentrations in the effluents of 8:1 and 16:1 (P:Pb molar ratio) treatment levels were significantly greater ( < 0.05) than the control. One year after P treatment and 6 mo after vegetation establishment, total P and Pb concentrations of the effluents from a second RFS decreased by one to three orders of magnitude. Total and dissolved P concentration in runoff from the 16:1 P:Pb treatment remained significantly greater than all other treatments. However, total Pb concentration in the runoff was comparable among the treatments. Phosphorus treatment also reduced Pb uptake into tall fescue by >55%. X-ray absorption near-edge structure spectroscopy data showed that pyromorphite [Pb(PO)OH,Cl,F] abundance ranged from 0% (control) to 32% (16:1 P:Pb; 1 yr after treatment) of the total soil Pb. Although P treatment stimulated pyromorphite formation, pyromorphite abundance was comparable between the P-treated soils. These findings suggest that a 4:1 (P:Pb molar ratio) P treatment may be a sufficient means of reducing Pb bioavailability while minimizing concerns related to P loss in an alluvial setting.

Removal of lead and phosphate ions from aqueous solutions by organo-smectite

Smectite has been modified using hexadecyltrimethyl ammonium bromide in an amount of double cationic exchange capacity. This alteration makes it possible to use organo-smectite as a sorbent to remove anionic forms. The experiment consisted of the interchangeable sorption of phosphate(V) and lead(II) by organo-smectite. Research was carried out with varying pH (2-5) and various concentrations (0.1-5 mmol/L). Organo-smectite with previously adsorbed lead ions removed more phosphate than the untreated organo-smectite. Experimental data show that lead is more likely to absorb on the organo-smectite than on the organo-smectite with previously adsorbed phosphate ions. It follows that the most effective use of the organo-smectite is through the sorption of first - Pb cations and then PO4 anions. With an increasing concentration of Pb(II) or P(V), the sorption efficiency increases. The maximum sorption efficiency of lead and phosphate ions is observed at pH 5. This enables the removal of harmful lead and phosphorus compounds from waste water and immobilizes them on the sorbent's surface. The alternating reactions of lead and phosphorus ions result in the crystallization of brompyromorphite Pb5(PO4)3Br.

Influence of ageing on lead bioavailability in soils: a swine study

Aging is a time-dependent process that causes metal bioavailability to decrease with time. The current study investigated the bioavailability change of Pb in four contrasting soils over a time period until the Pb relative bioavailability (RB) levels achieved a steady state to assess the extent of the following: firstly, bioavailability change in each soil and secondly, correlation of these changes with the soil properties. Relative bioavailability of soils spiked with 1500 mg Pb/kg were measured in swine that were fed these soils, throughout an aging period (56 days) to investigate relationships between soil properties and in vivo bioavailability of Pb. Spiked soils were used to minimize the effect of varying sources of Pb on RB. The RB of Pb in GTA, IWA, and MLA decreased from their initial Pb RB values until a steady state RB of 34, 45, and 59 % was reached, respectively, by the 56th day. In contrast, however, to these RB decreases, NTA soil indicated no change in RB over the whole aging period of the experiment. The lack of change in RB in the NTA soil over time was attributed to it achieving a steady state RB within a very short time due to its comparatively high sorptive capacity (K d = 112).

Screening and assessment of solidification/stabilization amendments suitable for soils of lead-acid battery contaminated site

Lead exposure via ingestion of soil and dust generally occurs at lead-acid battery manufacturing and recycling sites. Screening solidification/stabilization (S/S) amendments suitable for lead contaminated soil in an abandoned lead-acid battery factory site was conducted based on its chemical forms and environmental risks. Twelve amendments were used to immobilize the Pb in soil and assess the solidification/stabilization efficiency by toxicity leaching tests. The results indicated that three amendments, KH₂PO₄ (KP), KH₂PO₄:oyster shell power=1:1 (by mass ratio; SPP), and KH₂PO₄:sintered magnesia=1:1 (by mass ratio; KPM) had higher remediation efficiencies that led to a 92% reduction in leachable Pb with the addition of 5% amendments, while the acid soluble fraction of Pb (AS-Pb) decreased by 41-46% and the residual fraction (RS-Pb) increased by 16-25%. The S/S costs of the three selected amendments KP, SPP, and KPM could be controlled to $22.3 per ton of soil when the Pb concentration in soil ranged from 2000 to 3000 mg/kg. The results of this study demonstrated that KP, SPP, and KPM can effectively decrease bioavailability of Pb. These findings could provide basis for decision-making of S/S remediation of lead-acid battery contaminated sites.

Lead immobilization and phosphorus availability in phosphate-amended, mine-contaminated soils

Over a century of mining activities in the Coeur d'Alene mining district in Idaho have contaminated soils of the downstream basin with lead, arsenic, zinc, and cadmium. Elevated soil-Pb levels are a significant hazard to the health of humans and wildlife in the region. One in situ treatment approach for remediating Pb-contaminated soils is application of phosphorus to promote the formation of lead phosphate minerals that have low solubility. However, this remediation strategy may result in excess P runoff to surface waters, which can lead to eutrophication, particularly when used in riparian areas. Research presented in this paper describes experiments in which monopotassium phosphate (KHPO) solution was applied to two Pb-contaminated soils from the Coeur d'Alene River valley to determine how P loading rates affect both Pb immobilization and P mobility and to determine if an optimal P amendment rate can be predicted. Toxicity characteristic leaching procedure extractions were used to assess changes in Pb availability for uptake by an organism or mobilization through the soil, and Bray extractions were used to assess P availability for leaching out of the soil system. For the two soils tested, increasing phosphate amendment caused decreasing Pb extractability. Phosphorus amendment rates above approximately 70 mg kg, however, did not provide any additional Pb immobilization. Phosphorus availability increased with increasing phosphate application rate. An empirical relationship is presented that predicts extractable Pb as a function of extractable P. This relationship allows for prediction of the amount of Pb that can be immobilized at specified P leaching amounts, such as regulatory levels that have been established to minimize risks for surface water degradation. Results suggest that phosphate can be used to immobilize Pb in contaminated wetland or riparian areas without posing risks of P loading to surface waters.

Stabilize lead and cadmium in contaminated soils using hydroxyapatite and potassium chloride

Combination of hydroxyapatite (HAP) and potassium chloride (KCl) was used to stabilize lead and cadmium in contaminated mining soils. Pot experiments of chilli (Capsicum annuum) and rape (Brassica rapachinensis) were used to evaluate the stabilization efficiency. The results were the following: (1) the optimal combination decreased the leachable lead by 83.3 and 97.27 %, and decreased leachable cadmium by 57.82 and 35.96% for soil HF1 and soil HF2, respectively; (2) the total lead and cadmium concentrations in both plants decreased 69 and 44 %, respectively; (3) The total lead and cadmium concentrations in the edible parts of both vegetables also decreased significantly. This study reflected that potassium chloride can improve the stabilization efficiency of hydroxyapatite, and the combination of hydroxyapatite and potassium chloride can be effectively used to remediate lead and cadmium contaminated mining soil.

Stabilization of simulated lead sludge with iron sludge via formation of PbFe₁₂O₁₉ by thermal treatment

This study investigated the feasibility of stabilizing lead sludge by reaction with iron sludge via the formation of PbFe12O19 through a thermal treatment process. Lead hydroxide was used to simulate lead-laden sludge and the sintering procedure was performed by firing a mixture of this simulated sludge together with iron sludge at a Fe/Pb molar ratio of 12 over the temperature range from 650 to 1400 °C. The accompanying phase transformations as well as the surface characteristic of sintered samples were observed by XRD and SEM, while the leaching behavior of the stabilized sludge in an acidic environment was evaluated by a modified Toxicity Characteristic Leaching Procedure (TCLP) test. The results confirmed that PbFe12O19 acts as a stabilization phase for lead, and showed that the formation of a PbFe12O19 phase began at 750 °C with the lead completely incorporated into the PbFe12O19 phase at 1050 °C. Above 1100 °C, the PbFe12O19 phase began to decompose, accompanied by the reappearance of Fe2O3. The volumes of compressed sludge samples were reduced significantly after thermal treatment, with accompanying volume reductions of 40% at 1050 °C. This study compared the leaching of lead from PbO and sintered sludge samples using a prolonged TCLP test, and the data showed that the PbFe12O19 phase was superior to the PbO and that the sintered sludge sample exhibited very high stability under acidic environments. These results suggest a promising and reliable method of reducing lead sludge mobility and toxicity has been identified.

Safety of gardening on lead- and arsenic-contaminated urban brownfields

Elevated levels of lead (Pb) and arsenic (As) are not uncommon for urban soils. Test plots were established at urban gardens in Tacoma and Seattle, WA. The Tacoma site was contaminated with Pb (51-312 mg kg) and As (39-146 mg kg), and the Seattle site had high Pb soil concentrations ranging from 506 to 2022 mg kg and As concentrations of <20 mg kg. The efficacy of biosolids mix and compost amendment in reducing Pb and As concentrations in three vegetables (carrots, lettuce, and tomatoes) and the bioaccessibility of soil Pb and As were evaluated. Food-chain transfer of Pb and As were evaluated by measuring plant Pb and As concentrations after kitchen-style washing, a laboratory cleaning procedure, or peeling. The experimental design was a randomized complete block with a split-plot arrangement. Tacoma site treatments included a Class A biosolids mix (TAGRO) with dolomite, and soil at the Seattle site was amended with Cedar-Grove compost (CGC) plus dolomite. TAGRO amendment diluted soil Pb by 10 to 23% and As by 12 to 25% at the Tacoma site, and CGC + dolomite resulted in 20 to 50% dilution in soil Pb at the Seattle site. Both amendments reduced Pb concentrations in vegetables by 50 to 71%, and As reductions ranged from 46 to 80%. At the Tacoma site, Pb concentrations (dry weight basis) in carrots, lettuce, and tomatoes ranged from 8.89 to 25.0, from 0.37 to 3.83, and from 0.54 to 1.24 mg kg, respectively. Plant As concentrations were below 703 μg kg (dry weight) for the vegetables and followed the order lettuce > carrot > tomato. Food-chain transfer of Pb and As in vegetables grown in contaminated urban soils were reduced by laboratory cleaning.

Fungal transformation of metallic lead to pyromorphite in liquid medium

Many approaches have been proposed to reduce the toxicity of hazardous substances such as lead in the environment. Several techniques using microorganisms rely on metal removal from solution by non-specific biosorption. However, immobilization of metals through formation of biominerals mediated by metabolic processes offers another solution but which has been given limited attention. In this work, we have investigated lead biomineralization by Paecilomyces javanicus, a fungus isolated from a lead-contaminated soil, in a liquid medium. P. javanicus was able to grow in the presence of metallic lead, supplied as lead shot, and secondary lead minerals were deposited on the lead surfaces as revealed by scanning electron microscopy. Energy dispersive X-ray analysis and X-ray powder diffraction revealed that pyromorphite was formed in the presence of the fungus, but not in abiotic controls. Our results clearly demonstrate that fungal activities can play an important role in lead biocorrosion and biomineralization in an aqueous environment. These findings are relevant to bioremediation approaches for liquid wastes contaminated with lead, or other metals, and also to the immobilization and biorecovery of rare or valuable elements. They also provide further understanding of microbial roles in environmental lead cycling.

Phosphate-Mediated Remediation of Metals and Radionuclides

Worldwide industrialization activities create vast amounts of organic and inorganic waste streams that frequently result in significant soil and groundwater contamination. Metals and radionuclides are of particular concern due to their mobility and long-term persistence in aquatic and terrestrial environments. As the global population increases, the demand for safe, contaminant-free soil and groundwater will increase as will the need for effective and inexpensive remediation strategies. Remediation strategies that include physical and chemical methods (i.e., abiotic) or biological activities have been shown to impede the migration of radionuclide and metal contaminants within soil and groundwater. However, abiotic remediation methods are often too costly owing to the quantities and volumes of soils and/or groundwater requiring treatment. The in situ sequestration of metals and radionuclides mediated by biological activities associated with microbial phosphorus metabolism is a promising and less costly addition to our existing remediation methods. This review highlights the current strategies for abiotic and microbial phosphate-mediated techniques for uranium and metal remediation.

Micro-x-ray fluorescence, micro-x-ray absorption spectroscopy, and micro-x-ray diffraction investigation of lead speciation after the addition of different phosphorus amendments to a smelter-contaminated soil

The stabilization of Pb on additions of P to contaminated soils and mine spoil materials has been well documented. It is clear from the literature that different P sources result in different efficacies of Pb stabilization in the same contaminated material. We hypothesized that the differences in the efficacy of Pb stabilization in contaminated soils on fluid or granular P amendment addition is due to different P reaction processes in and around fertilizer granules and fluid droplets. We used a combination of several synchrotron-based techniques (i.e., spatially resolved micro-X-ray fluorescence, micro-X-ray absorption near-edge structure spectroscopy, and micro-X-ray diffraction) to speciate Pb at two incubation times in a smelter-contaminated soil on addition of several fluid and granular P amendments. The results indicated that the Pb phosphate mineral plumbogummite was an intermediate phase of pyromorphite formation. Additionally, all fluid and granular P sources were able to induce Pb phosphate formation, but fluid phosphoric acid (PA) was the most effective with time and distance from the treatment. Granular phosphate rock and triple super phosphate (TSP) amendments reacted to generate Pb phosphate minerals, with TSP being more effective at greater distances from the point of application. As a result, PA and TSP were the most effective P amendments at inducing Pb phosphate formation, but caution needs to be exercised when adding large amounts of soluble P to the environment.

Field evaluations on soil plant transfer of lead from an urban garden soil

Lead (Pb) is one of the most common contaminants in urban soils. Gardening in contaminated soils can result in Pb transfer from soil to humans through vegetable consumption and unintentional direct soil ingestion. A field experiment was conducted in 2009 and 2010 in a community urban garden with a soil total Pb concentration of 60 to 300 mg kg. The objectives of this study were to evaluate soil-plant transfer of Pb, the effects of incorporation of a leaf compost as a means of reducing Pb concentrations in vegetables and the bioaccessibility of soil Pb, and the effects of vegetable cleaning techniques on the Pb concentrations in the edible portions of vegetables. The amount of compost added was 28 kg m. The tested plants were Swiss chard, tomato, sweet potato, and carrots. The vegetable cleaning techniques were kitchen cleaning, laboratory cleaning, and peeling. Compost addition diluted soil total Pb concentration by 29 to 52%. Lead concentrations of the edible portions of vegetables, except carrot, were below the maximum allowable limits of Pb established by the Food and Agriculture Organization and the World Health Organization. Swiss chard and tomatoes subjected to kitchen cleaning had higher Pb concentrations than laboratory-cleaned plants. Cleaning methods did not affect Pb concentrations in carrots. Bioaccessible Pb in the compost-added soils was 20 to 30% less than that of the no-compost soils; compost addition reduced the potential of transferring soil Pb to humans via vegetable consumption and direct soil ingestion. Thorough cleaning of vegetables further reduced the potential of transferring soil Pb to humans.

Synthesis and characterization of a new class of stabilized apatite nanoparticles and applying the particles to in situ Pb immobilization in a fire-range soil

Phosphate compounds and the related materials are effective agents for in situ immobilization of heavy metals in contaminated soils. Problems associated with using these phosphate materials include difficulties in delivering the solid phosphate minerals to the deep contaminated zones or risks of eutrophication with applying soluble phosphates. Therefore, a new class of apatite nanoparticles was synthesized using carboxymethyl cellulose as a stabilizer in order to increase the dispersion rate of phosphate in soils but without introducing significant amount of soluble phosphate into the environment. The product was confirmed by XRD as chlorapatite (Ca5(PO4)3Cl) with poor crystallinity. TEM and SEM revealed that the particles were spherical or irregular in shape with sizes spanning from a few nm to around 200 nm. FTIR spectra suggested that Ca(II) cations formed outer-sphere bonds with carboxyl and hydroxyl groups in cellulose molecules, thus inhibiting further agglomeration of the particles. Dry combustion data supported a formula of [C6H7O2(OH)2OCH2COOCa5(PO4)3Cl]n for the nano-apatite composite. Laboratory tests showed that the nanoparticles could effectively decrease the TCLP-leachable Pb fraction in a Pb-contaminated soil from 66% to 10% after one-month amendment with a ratio of 2 mL solution to 1g soil and the resultant Pb content in the TCLP solution was reduced to 12 from 94 mg L(-1). When the amendment ratio was increased by 5 times, the leachable Pb was further reduced to 3.8 mg L(-1) with only about 3% of the soil Pb leachable. The soil sample, containing an average of 2.7×10(3)mg Pb kg(-1), was taken from a shooting-range in Southern USA.

Evaluating specificity of sequential extraction for chemical forms of lead in artificially-contaminated and field-contaminated soils

In the present study, we evaluated a commonly employed modified Bureau Communautaire de Référence (BCR test) 3-step sequential extraction procedure for its ability to distinguish forms of solid-phase Pb in soils with different sources and histories of contamination. When the modified BCR test was applied to mineral soils spiked with three forms of Pb (pyromorphite, hydrocerussite and nitrate salt), the added Pb was highly susceptible to dissolution in the operationally-defined "reducible" or "oxide" fraction regardless of form. When three different materials (mineral soil, organic soil and goethite) were spiked with soluble Pb nitrate, the BCR sequential extraction profiles revealed that soil organic matter was capable of retaining Pb in more stable and acid-resistant forms than silicate clay minerals or goethite. However, the BCR sequential extraction for field-collected soils with known and different sources of Pb contamination was not sufficiently discriminatory in the dissolution of soil Pb phases to allow soil Pb forms to be "fingerprinted" by this method. It is concluded that standard sequential extraction procedures are probably not very useful in predicting lability and bioavailability of Pb in contaminated soils.

Organic acids inhibit the formation of pyromorphite and Zn-phosphate in phosphorous amended Pb- and Zn-contaminated soil

Pyromorphite (PY) and some zinc phosphates (Zn-P) are very sparingly soluble minerals and hence can immobilize Pb and Zn in contaminated soils. However, mechanisms leading to the poor efficiency of PY and Zn-P formation in contaminated soils amended with P still remain unclear. We studied the influence of two low molecular weight organic acids (LMWOA) - oxalic acid and citric acid and diethylene triamine pentaacetic acid (DTPA) - in PY and Zn-P formation in a P-amended contaminated soil. Despite the high levels of metals (∼4% Pb and 21% Zn) in the study soil, the addition of up to 1% inorganic P transformed only up to 37% and 17% of the total Pb and Zn to PY and Zn-P, respectively. Semi-quantitative estimates from a linear combination fitting of X-ray absorption near edge spectra (LC-XANES fitting) showed that the formation of PY decreased from 37% to 3% of the total Pb in the presence of oxalic acid and the addition of 1% P. The reduced PY formation may be associated with the increase in organic-bound Pb from 9% to 54% and decrease in carbonate associated Pb from 42% to 12% with oxalic acid addition as indicated by a chemical sequential extraction (SE) technique. Citric acid seemed to have a less adverse effect in PY formation than oxalic acid. Our data also suggests both oxalic and citric acids have less adverse effects on the efficiency of Zn-P formation. From this study we conclude that the abundance of LMWOA in soil environments can be one factor contributing to the poor efficiency of P amendments practices to effectively immobilize Pb and Zn in metal contaminated soils.