Metal immobilization and soil amendment efficiency at a contaminated sediment landfill site: a field study focusing on plants, springtails, and bacteria

Recent progress on emerging technologies for trace elements-contaminated soil remediation

Abiotic stresses from potentially toxic elements (PTEs) have devastating impacts on health and survival of all living organisms, including humans, animals, plants, and microorganisms. Moreover, because of the rapid growing industrial activities together with the natural processes, soil contamination with PTEs has pronounced, which required an emergent intervention. In fact, several chemical and physical techniques have been employed to overcome the negative impacts of PTEs. However, these techniques have numerous drawback and their acceptance are usually poor as they are high cost, usually ineffectiveness and take longer time. In this context, bioremediation has emerged as a promising approach for reclaiming PTEs-contaminated soils through biological process using bacteria, fungus and plants solely or in combination. Here, we comprehensively reviews and critically discusses the processes by which microorganisms and hyperaccumulator plants extract, volatilize, stabilize or detoxify PTEs in soils. We also established a multi-technology repair strategy through the combination of different strategies, such as the application of biochar, compost, animal minure and stabilized digestate for stimulation of PTE remediation by hyperaccumulators plants species. The possible use of remote sensing of soil in conjunction with geographic information system (GIS) integration for improving soil bio-remediation of PTEs was discussed. By synergistically combining these innovative strategies, the present review will open very novel way for cleaning up PTEs-contaminated soils.

Nano hydroxyapatite pre-treatment effectively reduces Cd accumulation in rice (Oryza sativa L.) and its impact on paddy microbial communities

Cadmium (Cd) contamination in paddy soil has become a worldwide concern and severely endangered human health. Nano hydroxyapatite (n-HAP) is a practical material to manage paddy Cd pollution, but its dosage should not be excessive. Based on previous studies, we validated the effect of n-HAP pre-treatment on rice Cd uptake in pot and field experiments. The results indicated that n-HAP pre-treatment effectively restricted Cd translocation in the soil-rice system. In pot experiment, when soil n-HAP concentration was 5000 mg/kg, the Cd content in the grains of n-HAP pre-treated rice was 0.171 mg/kg, decreased by 29.3% compared with normal rice (0.242 mg/kg). In field experiment, when soil n-HAP concentration was 20,000 mg/kg, the Cd content in the grains of n-HAP pre-treated rice was 0.156 mg/kg, decreased by 35.3% compared with normal rice (0.241 mg/kg). The primary mechanism was that n-HAP pre-treatment altered the formation and composition of iron plaque and therefore enhanced the Cd binding ability of iron plaque. The available N and P content and urease activity in paddy field were increased. We further investigated the impact of n-HAP on the diversity and structure of paddy microbial communities. The Chao1 and Shannon diversity indices showed no significant difference. The relative abundance of Actinobacteria and Proteobacteria was significantly decreased by n-HAP, indicating that Cd pollution might be alleviated. Desulfobacterota, Gemmatimonadota, and Geobacteraceae were significantly enriched by n-HAP. The declining relative abundance of Basidiomycota and the increasing relative abundance of other fungal taxa also suggested that n-HAP could alleviate Cd toxicity in soil.

A systematic review on the bioremediation of metal contaminated soils using biochar and slag: current status and future outlook

Heavy metals contaminated soils are posing severe threats to food safety worldwide. Heavy metals absorbed by plant roots from contaminated soils lead to severe plant development issues and a reduction in crop yield and growth. The global population is growing, and the demand for food is increasing. Therefore, it is critical to identify soil remediation strategies that are efficient, economical, and environment friendly. The use of biochar and slag as passivators represents a promising approach among various physicochemical and biological strategies due to their efficiency, cost-effectiveness, and low environmental impact. These passivators employ diverse mechanisms to reduce the bioavailability of metals in contaminated soils, thereby improving crop growth and productivity. Although studies have shown the effectiveness of different passivators, further research is needed globally as this field is still in its early stages. This review sheds light on the innovative utilization of biochar and slag as sustainable strategies for heavy metal remediation, emphasizing their novelty and potential for practical applications. Based on the findings, research gaps have been identified and future research directions proposed to enable the full potential of passivators to be utilized effectively and efficiently under controlled and field conditions.

A risk management framework for Gentle Remediation Options (GRO)

Gentle Remediation Options (GRO) are remediation measures involving plants, fungi, bacteria, and soil amendments that can be applied to manage risks at contaminated sites. Several studies and decision-support tools promote the wider range of benefits provided by GRO, but there is still skepticism regarding GRO implementation. Key issues that need to be better communicated are the various risk mitigation mechanisms, the required risk reduction for an envisioned land use, and the time perspective associated with the risk mitigation mechanisms. To increase the viability and acceptance of GRO, the phytomanagement approach implies the combination of GRO with beneficial green land use, gradually reducing risks and restoring ecosystem services. To strengthen the decision basis for GRO implementation in practice, this paper proposes a framework for risk management and communication of GRO applications to support phytomanagement strategies at contaminated sites. The mapping of the risk mitigation mechanisms is done by an extensive literature review and the Swedish national soil guideline value model is used to derive the most relevant human health exposure pathways and ecological risks for generic green land use scenarios. Results indicate that most of the expected risk mitigation mechanisms are supported by literature, but that knowledge gaps still exist. The framework is demonstrated to support the identification of GRO options for the case study site given two envisioned land uses: biofuel park and allotment garden. A more easily understandable risk management framework, as proposed here, is expected to act as a communication tool to educate decision-makers, regulatory bodies and other stakeholders for better understanding of risk mitigation mechanisms and preliminary timeframes of various GRO, particularly in the early stages of a brownfield redevelopment project.

Seasonal fluctuations of Zn, Pb, As and Cd contents in the biomass of selected grass species growing on contaminated soils: Implications for in situ phytostabilization

Phytostabilization aims to immobilize contaminants at the rhizosphere level using the root system of adapted plants. In order to exploit wild grasses with potential for phytostabilization, a screening throughout the year was conducted at a site contaminated by Pb and Zn. Three plant species were chosen: Agrostis capillaris, Arrhenatherum elatius and Calamagrostis epigeios. Rhizospheric soil and biomass was used for chemical characterization. Above- and below-ground was analyzed. For each sample, arbuscular mycorrhiza fungi colonization was determined. The highest concentrations of Pb were found in the A. capillaris rhizosphere (3417 mg kg^-1), and in A. elatius for Zn (3876 mg kg^-1). CaCl₂-extractable Zn in the rhizosphere of C. epigeios was the lowest and Pb was lower for A. elatius. CaCl₂-extractable Cd was neither species-dependent nor time-dependent. Arsenic was not species-dependent. The fractionation of target elements did not show differences between separate sampling campaigns and Pb was the only element that showed differences during the year. A. capillaris showed the best capacity to take up elements. The colonization by AMF did not show significant differences for different sampling times, or interactions between time and species, however differences were found for different species, i.e., C. epigeios showed significantly lower colonization by arbuscular mycorrhiza fungi. Our results indicate that A. capillaris appears to be a good indigenous candidate for phytostabilization.

Effect of vegetables and nano-particle hydroxyapatite on the remediation of cadmium and phosphatase activity in rhizosphere soil through immobilization

Nano-hydroxyapatite (n-HAP) and two types of plants (rape plant and cabbage) were used in this study to investigate their combined remediation effect on the removal of heavy metal contaminated soil. Different n-HAP concentrations (5, 10, 15, 20, and 25 g kg^-1) with two kinds of plants were used in the experiments and compared with two control groups without adding n-HAP. Compared with the control group, the total Cd concentration increased and the amount of available Cd decreased significantly in soil (p < 0.05) in text groups. Cd toxicity decreased, and the rape plant and cabbage biomass increased after adding n-HAP. Cd accumulation in rape plant and cabbage decreased when more n-HAP was added; Cd accumulated less in rape plant than in cabbage. The phosphatase activity in soil also increased after adding n-HAP. Results showed that the total Cd concentration was positively correlated with phosphatase activity, the concentration of the available Cd and Cd accumulated in root has a significant effect on phosphatase activity in Cd-contaminated soil (p < 0.05). Cd-contaminated soil could be remediated effectively with rape plant and n-HAP with a concentration of 15 g kg^-1.

Vanadium and chromium-contaminated soil remediation using VFAs derived from food waste as soil washing agents: A case study

Food waste (FW) is environmentally unfriendly and decays easily under ambient conditions. Vanadium (V) and chromium (Cr) contamination in soils has become an increasing concern due to risks to human health and environmental conservation. Volatile fatty acids (VFAs) derived from FW was applied as soil washing agent to treat V and Cr-contaminated soil collected from a former V smelter site in this work. The Community Bureau of Reference (BCR) three-step sequential extraction procedure was used to identify geochemical fractions of V and Cr influencing their mobility and biological toxicity. Optimal parameters of a single washing procedure were determined to be a 4 h contact time, liquid-solid ratio of 10:1, VFAs concentration of 30 g/L, and reaction temperature of 25 °C, considering for typical soil remediation projects and complete anaerobic fermentation of FW. Under the optimal conditions, butyric acid fermentation VFAs attained removal rates of 57.09 and 23.55% for extractable fractions of V and Cr, respectively. Simultaneously, a multi-washing process under a constant liquid-solid ratio using fresh and recycled VFAs was conducted, which led to an improvement on the total removal efficiency of toxic metals. The washing procedure could reach the pollution thresholds for several plants, such as of S. viridis, K. scoparia, M. sativa, and E. indica. This strategy enhances the utilization of VFAs derived from food waste, has a positive effect on V and Cr-contaminated soil remediation, wastewater control of soil washing and FW disposal.

Metal accumulation in populations of Calamagrostis epigejos (L.) Roth from diverse anthropogenically degraded sites (SE Europe, Serbia)

Heavy metal accumulation is recognized as a very important global pollution problem in the last decades. Plant species have been recognized as natural bioindicators of environmental pollution, especially the amount of heavy metals in soils. Moreover, only a limited number of plant species can survive in highly contaminated soils. It is also known that metal accumulation can vary greatly among different populations of the same species. This study examines the chemical composition and accumulation potential of the expansive clonal grass Calamagrostis epigejos at five localities exposed to different levels of anthropogenic pressure. Considerable differences were observed between uptake, translocation, and accumulation of total and available heavy metals, such differences corresponding to soil physico-chemical characteristics and the level of site pollution. The results indicate that Calamagrostis epigejos uptakes a significant portion of the available fraction of heavy metals in the soil and stores it in the roots, thereby exhibiting a certain potential for metal phytostabilization.

Soil and brownfield bioremediation

Soil contamination with petroleum hydrocarbons, persistent organic pollutants, halogenated organic chemicals and toxic metal(loid)s is a serious global problem affecting the human and ecological health. Over the past half‐century, the technological and industrial advancements have led to the creation of a large number of brownfields, most of these located in the centre of dense cities all over the world. Restoring these sites and regeneration of urban areas in a sustainable way for beneficial uses is a key priority for all industrialized nations. Bioremediation is considered a safe economical, efficient and sustainable technology for restoring the contaminated sites. This brief review presents an overview of bioremediation technologies in the context of sustainability, their applications and limitations in the reclamation of contaminated sites with an emphasis on brownfields. Also, the use of integrated approaches using the combination of chemical oxidation and bioremediation for persistent organic pollutants is discussed.

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

Metal and metalloid contamination of soil and sediment is a widespread problem both in urban and rural areas throughout the United States (U.S. EPA, 2014). Beneficial use of waste by-products as amendments to remediate metal-contaminated soils and sediments can provide major economic and environmental advantages on both a site-specific and national scale. These waste by-products can also reduce our need to mine virgin materials or produce synthetic materials for amendments. Waste by-products must not be hazardous or pose unacceptable risk to human health and the environment, and should be a suitable replacement for virgin and synthetic materials. This review serves to present the state of science on in situ remediation of metal-contaminated soil and sediment and the potential for beneficial usage of waste by-product materials. Not all unintended consequences can be fully understood or predicted prior to implementing a treatment option, however some realized, and potentially unrealized, benefits and unintended consequences are explored.

Brownfields to green fields: Realising wider benefits from practical contaminant phytomanagement strategies

Gentle remediation options (GROs) are risk management strategies or technologies involving plant (phyto-), fungi (myco-), and/or bacteria-based methods that result in a net gain (or at least no gross reduction) in soil function as well as effective risk management. GRO strategies can be customised along contaminant linkages, and can generate a range of wider economic, environmental and societal benefits in contaminated land management (and in brownfields management more widely). The application of GROs as practical on-site remedial solutions is still limited however, particularly in Europe and at trace element (typically metal and metalloid) contaminated sites. This paper discusses challenges to the practical adoption of GROs in contaminated land management, and outlines the decision support tools and best practice guidance developed in the European Commission FP7-funded GREENLAND project aimed at overcoming these challenges. The GREENLAND guidance promotes a refocus from phytoremediation to wider GROs- or phyto-management based approaches which place realisation of wider benefits at the core of site design, and where gentle remediation technologies can be applied as part of integrated, mixed, site risk management solutions or as part of "holding strategies" for vacant sites. The combination of GROs with renewables, both in terms of biomass generation but also with green technologies such as wind and solar power, can provide a range of economic and other benefits and can potentially support the return of low-level contaminated sites to productive usage, while combining GROs with urban design and landscape architecture, and integrating GRO strategies with sustainable urban drainage systems and community gardens/parkland (particularly for health and leisure benefits), has large potential for triggering GRO application and in realising wider benefits in urban and suburban systems. Quantifying these wider benefits and value (above standard economic returns) will be important in leveraging funding for GRO application and soft site end-use more widely at vacant or underutilized sites.

Trace element bioavailability, yield and seed quality of rapeseed (Brassica napus L.) modulated by biochar incorporation into a contaminated technosol

BACKGROUND AND AIMS

Rapeseed (Brassica napus L.) is a Cd/Zn-accumulator whereas soil conditioners such as biochars may immobilize trace elements. These potentially complementary soil remediation options were trialed, singly and in combination, in a pot experiment with a metal(loid)-contaminated technosol.

METHODS

The technosol [total content in mg kg(-1) Zn 6089, Cd 9.4, Cu 110, and Pb 956] was either amended (2% w/w) or not with a poultry manure-derived biochar. Rapeseed was cultivated for both soil treatments during 24 weeks up to harvest under controlled conditions.

RESULTS

Biochar incorporation into the technosol promoted the As, Cd, Cu, Mo, Ni, Pb and Zn solubility. It decreased foliar B, Cu and Mo concentrations, and Mo concentration in stems, pericarps and seeds. But, it did not impact neither the biomass of aerial rapeseed parts (except a decrease for seeds), nor their C (except a decrease for stems), seed fatty acid, seed starch and soluble sugar contents, and antioxidant capacity in both leaves and seeds. Biochar amendment increased the phytoextraction by aerial plant parts for K, P, and S, reduced it for N, Ca, B, Mo, Ni and Se, whereas it remained steady for Mg, Zn, Fe, Mn, Cu, Cd and Co.

CONCLUSIONS

The biochar incorporation into this technosol did not promote Cd, Cu and Zn phytoextraction by rapeseed and its potential oilseed production, but increased the solubility of several metal(loid)s. Here Zn and Cd concentrations in the soil pore water were decreased by rapeseed, showing the feasibility to strip available soil Zn and Cd in combination with seed production.

Potential of Ranunculus acris L. for biomonitoring trace element contamination of riverbank soils: photosystem II activity and phenotypic responses for two soil series

Foliar ionome, photosystem II activity, and leaf growth parameters of Ranunculus acris L., a potential biomonitor of trace element (TE) contamination and phytoavailability, were assessed using two riverbank soil series. R. acris was cultivated on two potted soil series obtained by mixing a TE (Cd, Cu, Pb, and Zn)-contaminated technosol with either an uncontaminated sandy riverbank soil (A) or a silty clay one slightly contaminated by TE (B). Trace elements concentrations in the soil-pore water and the leaves, leaf dry weight (DW) yield, total leaf area (TLA), specific leaf area (SLA), and photosystem II activity were measured for both soil series after a 50-day growth period. As soil contamination increased, changes in soluble TE concentrations depended on soil texture. Increase in total soil TE did not affect the leaf DW yield, the TLA, the SLA, and the photosystem II activity of R. acris over the 50-day exposure. The foliar ionome did not reflect the total and soluble TE concentrations in both soil series. Foliar ionome of R. acris was only effective to biomonitor total and soluble soil Na concentrations in both soil series and total and soluble soil Mo concentrations in the soil series B.

Fate of cadmium in the rhizosphere of Arabidopsis halleri grown in a contaminated dredged sediment

In regions impacted by mining and smelting activities, dredged sediments are often contaminated with metals. Phytotechnologies could be used for their management, but more knowledge on the speciation of metals in the sediment and on their fate after colonization by plant roots is needed. This work was focused on a dredged sediment from the Scarpe river (North of France), contaminated with Zn and Cd. Zn, Cd hyperaccumulating plants Arabidopsis halleri from metallicolous and non-metallicolous origin were grown on the sediment for five months in a pot experiment. The nature and extent of the modifications in Cd speciation with or without plant were determined by electron microscopy, micro X-ray fluorescence and bulk and micro X-ray absorption spectroscopy. In addition, changes in Cd exchangeable and bioavailable pools were evaluated, and Cd content in leachates was measured. Finally, Cd plant uptake and plant growth parameters were monitored. In the original sediment, Cd was present as a mixed Zn, Cd, Fe sulfide. After five months, although pots still contained reduced sulfur, Cd-bearing sulfides were totally oxidized in vegetated pots, whereas a minor fraction (8%) was still present in non-vegetated ones. Secondary species included Cd bound to O-containing groups of organic matter and Cd phosphates. Cd exchangeability and bioavailability were relatively low and did not increase during changes in Cd speciation, suggesting that Cd released by sulfide oxidation was readily taken up with strong interactions with organic matter and phosphate ligands. Thus, the composition of the sediment, the oxic conditions and the rhizospheric activity (regardless of the plant origin) created favorable conditions for Cd stabilization. However, it should be kept in mind that returning to anoxic conditions may change Cd speciation, so the species formed cannot be considered as stable on the long term.

Agronomic Practices for Improving Gentle Remediation of Trace Element-Contaminated Soils

The last few decades have seen the rise of Gentle soil Remediation Options (GRO), which notably include in situ contaminant stabilization ("inactivation") and plant-based (generally termed "phytoremediation") options. For trace element (TE)-contaminated sites, GRO aim to either decrease their labile pool and/or total content in the soil, thereby reducing related pollutant linkages. Much research has been dedicated to the screening and selection of TE-tolerant plant species and genotypes for application in GRO. However, the number of field trials demonstrating successful GRO remains well below the number of studies carried out at a greenhouse level. The move from greenhouse to field conditions requires incorporating agronomical knowledge into the remediation process and the ecological restoration of ecosystem services. This review summarizes agronomic practices against their demonstrated or potential positive effect on GRO performance, including plant selection, soil management practices, crop rotation, short rotation coppice, intercropping/row cropping, planting methods and plant densities, harvest and fertilization management, pest and weed control and irrigation management. Potentially negative effects of GRO, e.g., the introduction of potentially invasive species, are also discussed. Lessons learnt from long-term European field case sites are given for aiding the choice of appropriate management practices and plant species.

Developing and validating a practical decision support tool (DST) for biomass selection on marginal land

Marginal, often contaminated, sites exist in large areas across the world as a result of historic activities such as industry, transportation and mineral extraction. Remediation, or other improvements, of these sites is typically only considered for sites with high exploitation pressure and those posing the highest risks to human health or the environment. At the same time there is increasing competition for land resources for different needs such as biofuel production. Potentially some of this land requirement could be met by production of biomass on brownfield or other marginal land, thereby improving the land while applying the crop cultivation as part of an integrated management strategy. The design and decision making for such a strategy will be site specific. A decision support framework, the Rejuvenate DST (decision support tool) has been developed with the aim of supporting such site specific decision making. This tool is presented here, and has been tested by applying it to a number of case study sites. The consequent SWOT (strength, weakness, opportunities and threats) analysis is discussed and evaluated. The DST was found to be systematic, transparent, and applicable for diverse sites in France, Romania and Sweden, in addition to the sites to which it was applied through its development. The DST is regarded as especially useful if applied as a checklist in an iterative way throughout the decision process, from identifying potential crops to identifying knowledge gaps, working/non-working management strategies and potential risks. The DST also provides a structure promoting effective stakeholder engagement.