State of the science review: Potential for beneficial use of waste by-products for in situ remediation of metal-contaminated soil and sediment

Ineffectiveness of phosphorus-containing amendments to reduce Pb bioaccessibility in an urban alkaline soil

Urban soils contaminated by historical and current anthropogenic activities present an alarming human health risk requiring redress. Federal and state governments continue to lower residential soil lead (Pb) screening standards, which will likely require new risk-based approaches to address urban soil Pb contamination. Phosphorus (P) soil amendments have long been presented as a solution to sequester Pb, thereby reducing exposure risk. In this study, P-containing sources (biosolids incinerator ash, poultry litter, biosolids compost, and triple superphosphate) of varying solubilities were assessed as soil amendments to reduce Pb bioaccessibility and serve as an inexpensive remediation strategy for urban soil. Contaminated soil (1624 mg kg-1 Pb, pH 7.43) from Cleveland, OH, was treated with the four P-containing soil amendments at a 1:5 Pb:P molar ratio and two combination treatments at 1:10 Pb:P molar ratio and incubated for 3 months. A batch equilibration analysis was also conducted to assess reduction in in vitro bioaccessible Pb (IVBA Pb). Pb bioaccessibility was evaluated using US EPA Method 1340 at pH 1.5 and the Physiologically Based Extraction Test pH 2.5 at 1 and 3 months. In general, treatments were ineffective in reducing IVBA Pb regardless of IVBA extraction method, incubation duration, batch equilibration analyses, or P source. The results of this study suggest P-containing amendments are not suitable to address Pb exposure in the study soil. Site-specific efficacy testing to determine reductions in IVBA Pb from P-containing amendments should be performed before making recommendations for remediation of Pb-contaminated urban soil.

Evaluation of a battery of biotests to improve waste ecotoxicity assessment (HP 14), using incineration bottom ash as a case study

The assessment of waste ecotoxicity (hazardous property HP14 in the European Union) is fundamental for proper waste classification and safe application/disposal. Biotests are relevant for evaluating waste complex matrices, but their efficiency is crucial to encourage their adoption at the industrial level. This work aims at evaluating possibilities of improving the efficiency of a biotest battery previously suggested in the literature, regarding test selection, duration, and/or laboratory resources optimization. Fresh incineration bottom ash (IBA) was the case study. The test battery analysed included standard aquatic (bacteria, microalgae, macrophytes, daphnids, rotifers, fairy shrimp) and terrestrial (bacteria, plants, earthworms, collembolans) organisms. The assessment followed an Extended Limit Test design (three dilutions of eluate or solid IBA) and the Lowest Ineffective Dilution (LID-approach) for ecotoxicity classification. The results emphasize the importance of testing different species. It was also evidenced that tests with daphnids and earthworms may be shortened to 24 h; the miniaturization of tests is suitable as e.g. differential sensitivity of microalgae and macrophytes was captured with low variability; alternative testing kits can be used when methodological difficulties are found. Microalgae were more sensitive than macrophytes. Similar results were found for the Thamnotoxkit and daphnids test for eluates with natural pH, so the former may be used as an alternative. B. rapa was the most sensitive organism, suggesting that it may be tested as the only terrestrial plant species and that minimum test duration is appropriate. F. candida does not appear to add information to the battery. The differences in sensitivity of A. fischeri and E. fetida compared to the remaining species were not significant enough to exclude them from the battery. Thus, this work suggests a biotest battery to test IBA comprising aquatic tests - Aliivibrio fischeri, Raphidocelis subcapitata (miniaturised test), and Daphnia magna (24 h when clear deleterious effects are observed) or Thamnocephalus platyurus (toxkit) - and terrestrial tests - Arthrobacter globiformis, Brassica rapa (14 d), and Eisenia fetida (24 h). Testing waste with natural pH is also recommended. The Extended Limit Test design considering the LID-approach seems useful in waste testing, particularly for the industry, involving low effort, test material requirements, and few laboratory resources. The LID-approach allowed for differentiating ecotoxic from non-ecotoxic effects and captured different sensitivities between species. Ecotoxicological assessment of other waste may benefit from these recommendations, but caution should be taken given the properties of each waste type.

An integrated characterisation of incineration bottom ashes towards sustainable application: Physicochemical, ecotoxicological, and mechanical properties

Environmental protection is a central concern regarding municipal solid waste incineration bottom ash (IBA) management, but the assessment of waste Hazardous Property HP14 (ecotoxicity) is still under debate. Civil engineering applications may be a suitable management strategy. This work aimed at evaluating IBA regarding mechanical behaviour and environmental hazardous potential, including a biotest battery for ecotoxicity assessment (comprising miniaturised tests), to explore its potential for safe utilization. Physical, chemical, ecotoxicological (Aliivibrio fischeri, Raphidocelis subcapitata, Lemna minor, Daphnia magna, Lepidium sativum), and mechanical (one-dimensional compressibility, shear strength) analyses were performed. The low leaching for potentially toxic metals and ions complied with European Union (EU) limit values for non-hazardous waste landfills. No relevant ecotoxicological effects were found. The biotest battery seems suitable for ecotoxicological assessment in the aquatic ecosystem, providing wide information on waste impact on different trophic/functional levels and chemical uptake routes, simultaneously involving short-duration tests and reduced amounts of waste. IBA presented more compressibility than sand, but its mixture with sand (30%:70%) was closer to sand compressibility. IBA (lower stresses) and the mixture (higher stresses) showed slightly higher shear strength than sand. Overall, IBA presented the potential for valorisation as loose aggregates from an environmental and mechanical viewpoint in a circular economy framework.

High performance self-assembled nano-chlorapatite in the presence of lactonic sophorolipid for the immobilization of cadmium in polluted sediment

A novel lactonic sophorolipid (LS) self-assembled nano-chlorapatite (LS-nClAP) was prepared for the immobilization of severe cadmium (Cd) in sediment. The experimental results indicated that the introduction of LS not only improved the dispersed performance of chlorapatite, but also brought massive hydroxyl and carboxyl groups, which significantly improved the immobilization efficiency of Cd and reduced its eco-toxicity in sediment. LS can significantly increase the effective utilization rate of phosphorus in chlorapatite, and reduce the content of available phosphorus (AP) by half after remediation compared with ClAP. Additionally, the participation of LS possessed a significant impact on the enzyme activities in the sediment, especially for urease, which was closely related to the effective stability of Cd and the introduction of LS. All experimental results of this study provided new insights into the possible effects of Cd immobilization by chlorapatite in contaminated sediments, demonstrating great application potential for sediment remediation in the future.

Recent Progress on Ex Situ Remediation Technology and Resource Utilization for Heavy Metal Contaminated Sediment

Sediment is an important part of aquatic systems, which plays a vital role in transporting and storing metals. Due to its abundance, persistence, and environmental toxicity, heavy metal pollution has always been one of the hot spots in the world. In this article, the state-of-art ex situ remediation technology for metal-contaminated sediments is elaborated, including sediment washing, electrokinetic remediation (EKR), chemical extraction, biological treatment, as well as encapsulating pollutants by adding some stabilized/solidified materials. Furthermore, the progress of sustainable resource utilization methods, such as ecosystem restoration, construction materials (e.g., materials fill materials, partition blocks, and paving blocks), and agriculture use are reviewed in detail. Finally, the pros and cons of each technique are summarized. This information will provide the scientific basis for selecting the appropriate remediation technology in a particular scenario.

Photooxidation of Fe(II) to schwertmannite promotes As(III) oxidation and immobilization on pyrite under acidic conditions

Pollution of arsenic (As) in acid mine drainage (AMD) is a universal environmental problem. The weathering of pyrite (FeS₂) and other sulfide minerals leads to the generation of AMD and accelerates the leaching of As from sulfide minerals. Pyrite can undergo adsorption and redox reactions with As, affecting the existing form and biotoxicity of As. However, the interaction process between As and pyrite in AMD under sunlight radiation remains unclear. Here, we found that the oxidation and immobilization of arsenite (As(III)) on pyrite can be obviously promoted by the reactive oxygen species (ROS) in sunlit AMD, particularly by OH. The reactions between hole-electron pairs and water/oxygen adsorbed on excited pyrite resulted in the production of H₂O₂, OH and O₂^-, and OH was also generated through the photo-Fenton reaction of Fe²⁺/FeOH²⁺. Weakly crystalline schwertmannite formed from the oxidation of Fe²⁺ ions by OH contributed much to the adsorption and immobilization of As. In the mixed system of pyrite (0.75 g L^-1), Fe²⁺ (56.08 mg L^-1) and As(III) (1.0 mg L^-1) at initial pH 3.0, the decrease ratio of dissolved total As concentration was 1.6% under dark conditions, while it significantly increased to 69.0% under sunlight radiation. The existence of oxygen or increase in initial pH from 2.0 to 4.0 accelerated As(III) oxidation and immobilization due to the oxidation of more Fe²⁺ and production of more ROS. The present work shows that sunlight significantly affects the transformation and migration of As in AMD, and provides new insights into the environmental behaviors of As.

Legacy Lead in Urban Garden Soils: Communicating Risk and Limiting Exposure

Lead (Pb) exposure has long been recognized as a hazard to human health. Urban garden soils often contain elevated levels of Pb, mainly from legacy sources, which is a main barrier for urban gardening. The capacity of gardeners to access, understand, and act on scientific data related to soil contamination is also variable. This synthesis paper briefly summarizes the current scientific knowledge on soil Pb in urban gardens. Our objective is to produce clear recommendations about assessing actual risks and limiting exposure. First, we synthesize the nature and extent of soil contamination with Pb, and then describe how the bioavailability and risk of this contamination to humans is assessed. We then go on to potential exposure pathway through plants and remediation methods to improve soil health and reduce human exposure. We have developed best management practices for practitioners that include: (1) urban soil testing should be prioritized because of the high probability of Pb contamination, and urban gardening should not begin until thorough testing or remediation has been done; (2) documentation of land-use history should be required in all property transactions so that the potential for soil (and other) contamination can be clearly identified; (3) amendments cannot be relied upon as a treatment for contaminated soils to reduce risk to gardeners because they do not always make contaminants less harmful; (4) certain crops (such as fruiting vegetables) are much less susceptible to contamination than others and thus should be prioritized in urban gardens; (5) wherever feasible, raised beds filled with upcycled local mineral and organic materials are the preferred substrate for urban gardening. Further monitoring of potentially contaminated and remediated soils as well as effective communication with the public are necessary to ensure human safety.

Simultaneous mitigation of Cd and As availability in soil-rice continuum via the addition of an Fe-based desulfurization material

The simultaneous mitigation of toxic arsenic (As) and cadmium (Cd) in rice grain remains a global challenge. Passivation with natural or artificially modified materials has shown great potential to simultaneously reduce the bioavailability of As and Cd in paddy soils. To date, however, limited materials have are available, with unclear underling mechanisms. Here, a natural iron-based desulfurization material is hypothesized to simultaneously mitigate As and Cd availability in paddy soil-rice continuum, since it is rich in calcium (Ca), iron (Fe), Silicon (Si), manganese (Mn), and sulfur (S). The addition of the proposed material promoted rice growth and reduced soil availability of Cd (extracted with 0.01 mg·L^-1 of CaCl₂) by 88.0-89.6% and As (extracted with 0.5 mg·L^-1 of KH₂PO₄) by 37.9-69.9%. Grain Cd was reduced by 26.4-51.6%, whereas that of inorganic As (iAs) by 33.3-42.7%. The increased Fe (by 44.2%) and Mn (by 178.6%) in iron plaque on the root surface were conducive to the reduction of grain Cd and iAs after application. Furthermore, the maximum adsorption capacities of the proposed material for Cd and As(III) reached 526.31 and 2.67 mg·g^-1, respectively. The coprecipitation with Cd(OH)₂ as a product, Fe-As and Ca-As complexation, and ion exchange of Fe²⁺ released by the material with Cd²⁺ are involved in the mechanisms underlying the available As and Cd reduction. Combining the safety, low-cost, and high accessibility, Fe-based desulfurization material showed great potential for future safe-utilization of As-Cd contaminated paddy soil via passivation.

Plumbojarosite formation in contaminated soil to mitigate childhood exposure to lead, arsenic and antimony

In this study, a novel method for lead (Pb) immobilization was developed in contaminated soils using iron (III) (Fe³⁺) in conjunction with 0.05 M H₂SO₄. During method optimization, a range of microwave treatment times, solid to solution ratios, and Fe₂(SO₄)₃/H₂SO₄ concentrations were assessed using a mining/smelting impacted soil (BHK2, Pb: 3031 mg/kg), followed by treatment of additional Pb contaminated soils (PP, Pb: 1506 mg/kg, G10, Pb: 2454 mg/kg and SoFC-1, Pb: 6340 mg/kg) using the optimized method. Pb bioaccessibility was assessed using USEPA Method 1340, with Pb speciation determined by X-ray Absorption (XAS) spectroscopy. Treatment efficacy was also validated using an in vivo mouse assay, where Pb accumulation in femur, kidney and liver was assessed to confirm in vitro bioaccessibility outcomes. Results showed that Pb bioaccessibility could be reduced by 77.4-97.0% following treatment of soil with Fe₂(SO₄)₃ (0.4-1.0 M), H₂SO₄ (0.05 M) at 150 °C for 60 min in a closed microwave system. Results of bioavailability assessment demonstrated treatment effect ratio of 0.06-0.07 in femur, 0.06-0.27 in kidney and 0.06-0.11 in liver (bioavailability reduction between 73% and 93%). Formation of plumbojarosite in treated soils was confirmed by XAS analysis.

Cadmium Immobilization in the Rhizosphere and Plant Cellular Detoxification: Role of Plant-Growth-Promoting Rhizobacteria as a Sustainable Solution

Food is the major cadmium (Cd)-exposure pathway from agricultural soils to humans and other living entities and must be reduced in an effective way. A plant can select beneficial microbes, like plant-growth-promoting rhizobacteria (PGPR), depending upon the nature of root exudates in the rhizosphere, for its own benefits, such as plant growth promotion as well as protection from metal toxicity. This review intends to seek out information on the rhizo-immobilization of Cd in polluted soils using the PGPR along with plant nutrient fertilizers. This review suggests that the rhizo-immobilization of Cd by a combination of PGPR and nanohybrid-based plant nutrient fertilizers would be a potential and sustainable technology for phytoavailable Cd immobilization in the rhizosphere and plant cellular detoxification, by keeping the plant nutrition flow and green dynamics of plant nutrition and boosting the plant growth and development under Cd stress.

Efficacy of Enzymatically Induced Calcium Carbonate Precipitation in the Retention of Heavy Metal Ions

This study evaluated the efficacy of enzyme induced calcite precipitation (EICP) in restricting the mobility of heavy metals in soils. EICP is an environmentally friendly method that has wide ranging applications in the sustainable development of civil infrastructure. The study examined the desorption of three heavy metals from treated and untreated soils using ethylene diamine tetra-acetic acid (EDTA) and citric acid (C6H8O7) extractants under harsh conditions. Two natural soils spiked with cadmium (Cd), nickel (Ni), and lead (Pb) were studied in this research. The soils were treated with three types of enzyme solutions (ESs) to achieve EICP. A combination of urea of one molarity (M), 0.67 M calcium chloride, and urease enzyme (3 g/L) was mixed in deionized (DI) water to prepare enzyme solution 1 (ES1); non-fat milk powder (4 g/L) was added to ES1 to prepare enzyme solution 2 (ES2); and 0.37 M urea, 0.25 M calcium chloride, 0.85 g/L urease enzyme, and 4 g/L non-fat milk powder were mixed in DI water to prepare enzyme solution 3 (ES3). Ni, Cd, and Pb were added with load ratios of 50 and 100 mg/kg to both untreated and treated soils to study the effect of EICP on desorption rates of the heavy metals from soil. Desorption studies were performed after a curing period of 40 days. The curing period started after the soil samples were spiked with heavy metals. Soils treated with ESs were spiked with heavy metals after a curing period of 21 days and then further cured for 40 days. The amount of CaCO3 precipitated in the soil by the ESs was quantified using a gravimetric acid digestion test, which related the desorption of heavy metals to the amount of precipitated CaCO3. The order of desorption was as follows: Cd > Ni > Pb. It was observed that the average maximum removal efficiency of the untreated soil samples (irrespective of the load ratio and contaminants) was approximately 48% when extracted by EDTA and 46% when extracted by citric acid. The soil samples treated with ES2 exhibited average maximum removal efficiencies of 19% and 10% when extracted by EDTA and citric acid, respectively. It was observed that ES2 precipitated a maximum amount of calcium carbonate (CaCO3) when compared to ES1 and ES3 and retained the maximum amount of heavy metals in the soil by forming a CaCO3 shield on the heavy metals, thus decreasing their mobility. An approximate improvement of 30% in the retention of heavy metal ions was observed in soils treated with ESs when compared to untreated soil samples. Therefore, the study suggests that ESs can be an effective alternative in the remediation of soils contaminated with heavy metal ions.

Application Research of Biochar for the Remediation of Soil Heavy Metals Contamination: A Review

Soil contamination by heavy metals threatens the quality of agricultural products and human health, so it is necessary to choose certain economic and effective remediation techniques to control the continuous deterioration of land quality. This paper is intended to present an overview on the application of biochar as an addition to the remediation of heavy-metal-contaminated soil, in terms of its preparation technologies and performance characteristics, remediation mechanisms and effects, and impacts on heavy metal bioavailability. Biochar is a carbon-neutral or carbon-negative product produced by the thermochemical transformation of plant- and animal-based biomass. Biochar shows numerous advantages in increasing soil pH value and organic carbon content, improving soil water-holding capacity, reducing the available fraction of heavy metals, increasing agricultural crop yield and inhibiting the uptake and accumulation of heavy metals. Different conditions, such as biomass type, pyrolysis temperature, heating rate and residence time are the pivotal factors governing the performance characteristics of biochar. Affected by the pH value and dissolved organic carbon and ash content of biochar, the interaction mechanisms between biochar and heavy metals mainly includes complexation, reduction, cation exchange, electrostatic attraction and precipitation. Finally, the potential risks of in-situ remediation strategy of biochar are expounded upon, which provides the directions for future research to ensure the safe production and sustainable utilization of biochar.

Alkalinity neutralization and structure upgrade of bauxite residue waste via synergistic pyrolysis with biomass

Bauxite residues, a large volume solid waste, are in urgent need of effective disposal and management. Especially, strategies to alleviate the high alkalinity of bauxite residue remain a big challenge. Here, we developed a synergistic pyrolysis to neutralize the alkalinity of bauxite residue and upgrade the structure of biomass simultaneously. By cooperating the catalytic feature from bauxite residue, rice straw, a cellulose-enriched biomass, could prefer to produce acidic components under a hypothermal pyrolysis temperature (below 250 °C) and partial oxygen-contained atmosphere as evidenced by the synchronous TGA-FTIR analysis. In return, these in-situ produced acidic components neutralized the bauxite residue profoundly (pH decreased from 11.5 to 7.2) to obtain a neutral product with long-term water leaching stability. Also, a higher pyrolysis temperature led to neutral biochar-based products with well-defined carbonization characteristics. Thus, the biomass-driven pyrolysis strategy provides a potential to dispose the alkalinity issue of bauxite residue and further opportunities for the sustainable reuse and continuing management of bauxite residue.

Remediation of an urban garden with elevated levels of soil contamination

Urban gardening is popular in many cities. However, many urban soils are contaminated and pose risks to human health. This study was conducted in a highly publicized urban garden in Brooklyn, NY with elevated Pb and As levels. Our objectives were to: (1) assess the nature and extent of Pb and As contamination at this site; (2) evaluate the effectiveness of amendments on reducing the bioaccessibility and phytoavailability of Pb and As in soil; and (3) assess the potential exposure of children to Pb and As through direct and indirect exposure pathways. Field surveys of the site revealed that contamination was highly concentrated in one area of the garden associated with fruit tree production. Field plots were established in this area, with three different treatments (bone meal, compost, sulfur) and an unamended control. Bioaccessibility of Pb was significantly reduced by all three treatments compared to the control (33%): bone meal (24%), compost (23%), sulfur (24%). In this study, As bioaccessibility remained high (80-93%) with or without treatments. We found that the effectiveness of soil remediation with amendments is variable and often limited, and contaminated sites can still pose a significant risk to urban gardeners. The results of a simple assessment model suggested that Pb and As exposure was mostly from soil and dust ingestion, rather than vegetable consumption. This work is unique in that it evaluates actual elevated levels of contamination, in actively gardened urban soils, in a highly visible public context. It fills important gaps between basic research and analysis of human exposure to toxic trace metals that can be a constraint on a highly beneficial activity.

Impacts of biochar and oyster shells waste on the immobilization of arsenic in highly contaminated soils

Soil contamination is a serious problem with deleterious impacts on global sustainability. Readily available, economic, and highly effective technologies are therefore urgently needed for the rehabilitation of contaminated sites. In this study, two readily available materials prepared from bio-wastes, namely biochar and oyster shell waste, were evaluated as soil amendments to immobilize arsenic in a highly As-contaminated soil (up to 15,000 mgAs/kg). Both biochar and oyster shell waste can effectively reduce arsenic leachability in acid soils. After application of the amendments (2-4% addition, w/w), the exchangeable arsenic fraction decreased from 105.8 to 54.0 mg/kg. The application of 2%biochar +2% oyster shell waste most effectively reduced As levels in the column leaching test by reducing the arsenic concentration in the porewater by 62.3% compared with the treatment without amendments. Biochar and oyster shell waste also reduced soluble As(III) from 374.9 ± 18.8 μg/L to 185.9 ± 16.8 μg/L and As(V) from 119.8 ± 13.0 μg/L to 56.4 ± 2.6 μg/L at a pH value of 4-5. The treatment using 4% (w/w) amendments did not result in sufficient As immobilization in highly contaminated soils; high soluble arsenic concentrations (upto193.0 μg/L)were found in the soil leachate, particularly in the form of As(III), indicating a significant potential to pollute shallow groundwater aquifers. This study provides valuable insights into the use of cost-effective and readily available materials for soil remediation and investigates the mechanisms underlying arsenic immobilization in acidic soils.

Removal of Cu(II) in water by polymer enhanced ultrafiltration: Influence of polymer nature and pH

This study presents an efficient removal of Cu(II) in water using the polymer enhanced ultrafiltration (PEUF) method. Polymer of different molecular weight (MW) (polyethyleneimine (PEI), sodium lignosulfonates (SLS) and dextrans) were investigated to evaluate efficiency in removal of Cu(II) in water by the PEUF method. The decomposition of Cu(II)-polymer complex was also evaluated in order to reuse polymers. Cu(II) complexation depends on the MW of chelating polymer and the pH of feed solution. It was found that the Cu(II) rejection increased with the polymer dosage with high removal of Cu(II) when using PEI and SLS at a 10:20 (mg/mg) ratio ([Cu(II)]:[polymer]). It was found that the maximum chelating capacity was 15 mg of Cu(II) per 20 mg of PEI. The Cu(II)-PEI complex could be decomposed by acid addition and the polymer could be efficiently reused with multiple complexation-decomplexation cycles. A conceptual flow chart of the integrated process of efficient removal of Cu(II) by PEUF method is suggested.