Mobility of Pb, Zn, Ba, As and Cd toward soil pore water and plants (willow and ryegrass) from a mine soil amended with biochar

Fabrication of engineered biochar for remediation of toxic contaminants in soil matrices and soil valorization

Biochar has emerged as an efficacious green material for remediation of a wide spectrum of environmental pollutants. Biochar has excellent characteristics and can be used to reduce the bioavailability and leachability of emerging pollutants in soil through adsorption and other physico-chemical reactions. This paper systematically reviewed previous researches on application of biochar/engineered biochar for removal of soil contaminants, and underlying adsorption mechanism. Engineered biochar are derivatives of pristine biochar that are modified by various physico-chemical and biological procedures to improve their adsorption capacities for contaminants. This review will promote the possibility to expand the application of biochar for restoration of degraded lands in the industrial area or saline soil, and further increase the useable area. This review shows that application of biochar is a win-win strategy for recycling and utilization of waste biomass and environmental remediation. Application of biochar for remediation of contaminated soils may provide a new solution to the problem of soil pollution. However, these studies were performed mainly in a laboratory or a small scale, hence, further investigations are required to fill the research gaps and to check real-time applicability of engineered biochar on the industrial contaminated sites for its large-scale application.

Heavy metals immobilization and bioavailability in multi-metal contaminated soil under ryegrass cultivation as affected by ZnO and MnO2 nanoparticle-modified biochar

Pollution by heavy metals (HMs) has become a global problem for agriculture and the environment. In this study, the effects of pristine biochar and biochar modified with manganese dioxide (BC@MnO2) and zinc oxide (BC@ZnO) nanoparticles on the immobilization and bioavailability of Pb, Cd, Zn, and Ni in soil under ryegrass (Lolium perenne L.) cultivation were investigated. The results of SEM-EDX, FTIR, and XRD showed that ZnO and MnO2 nanoparticles were successfully loaded onto biochar. The results showed that BC, BC@MnO2 and BC@ZnO treatments significantly increased shoots and roots dry weight of ryegrass compared to the control. The maximum dry weight of root and shoot (1.365 g pot-1 and 4.163 g pot-1, respectively) was reached at 1% BC@MnO2. The HMs uptake by ryegrass roots and shoots decreased significantly after addition of amendments. The lowest Pb, Cd, Zn and Ni uptake in the plant shoot (13.176, 24.92, 32.407, and 53.88 µg pot-1, respectively) was obtained in the 1% BC@MnO2 treatment. Modified biochar was more successful in reducing HMs uptake by ryegrass and improving plant growth than pristine biochar and can therefore be used as an efficient and cost effective amendment for the remediation of HMs contaminated soils. The lowest HMs translocation (TF) and bioconcentration factors were related to the 1% BC@MnO2 treatment. Therefore, BC@MnO2 was the most successful treatment for HMs immobilization in soil. Also, a comparison of the TF values of plant showed that ryegrass had a good ability to accumulate all studied HMs in its roots, and it is a suitable plant for HMs phytostabilization.

Si-enriched biochars improved soil properties, reduced Cd bioavailability while enhanced Cd translocation to grains of rice

Biochar and silicon (Si) have been widely considered to play an important role in mitigating cadmium (Cd) toxicity. In this study, wild-type rice (WT, high-Si) and Si-deficient mutant rice (lsi1, low-Si) were used as raw materials to prepare biochar at 500℃; the Si concentrations of high- and low-Si biochar were 15.9% and 5.3%, respectively. The impacts of different application rates (0%, 2%, 4%) of high- and low-Si biochars on soil chemical properties, Si and Cd fractions and availability, Cd absorption, and translocation were investigated. The results showed that both types of biochars increased soil pH, soil available nitrogen, and available phosphorus and potassium; and promoted Si uptake and plant growth of rice. Soil available Si, CaCl2-Si, acetic-Si, H2O2-Si, oxalate-Si, and Na2CO3-Si were also increased by biochar supply, especially for high-Si biochar treatments. In addition, both types of biochars had no effects on soil total Cd, but reduced soil available Cd by 2-17% in early season 2022, and reduced oxidizable Cd and residual Cd. Biochar application did not influence Cd concentrations in roots, stems, and leaves, but significantly increased Cd uptake and transport from stems and leaves to grains. The results suggested that Si-rich biochar could improve soil nutrients, change soil Si/Cd fractions and availability, promote rice growth but increase the risk of Cd toxicity in grains, indicating the complex of straw biochar in remediating Cd-contaminated paddy soil.

Metal behavior and soil quality changes induced by the application of tailor-made combined biochar: An investigation at pore water scale

The remediation performance of biochar varies based on the biomass used for its production. Further innovation involves developing tailor-made biochar by combining different raw materials to compensate for the limitations of pure biochar. Therefore, tailor-made combined biochar produced from the co-pyrolysis of pig manure and invasive Japanese knotweed (P1J1), as well as biochars produced from these feedstocks separately, i.e., pure pig manure (PM) and pure Japanese knotweed (JK), were applied to Pb and As contaminated soil to evaluate the biochar-induced changes on soil properties, microbial activity, DOM, and metal and metalloids solubility at the soil pore water scale. Biochar application reduced soluble Pb, whereas enhanced the As mobility; the increased soil pH after biochar addition played a fundamental role in reducing the Pb solubility, as revealed by their significant negative correlation (r = -0.990, p < 0.01). In contrast, the release of dissolved P strongly influenced As mobilization (r = 0.949, p < 0.01), especially in P-rich PM and P1J1 treatments, while JK showed a marginal effect in mobilizing As. Soils treated with PM, P1J1, and JK mainly increased Gram-negative bacteria by 56 %, 52 %, and 50 %, respectively, compared to the control. Fluorescence excitation-emission matrix spectroscopy combined with parallel factor analysis identified three components in pore water DOM, C1 (long wavelength humic-like), C2 (short wavelength humic-like), and C3 (protein-like), which were dominant respectively in the P1J1, JK, and PM-added soil. A principal component analysis (PCA) confirmed that the PM and P1J1 had similar performance and were more associated with releasing P and Mg and specific DOM components (C1 and C3). Meanwhile, P1J1 supplemented soil OM/OC and K, similar to JK. The results of this study suggest that combined biochar P1J1 can comprehensively enhance soil quality, embodying the advantages of pure PM and JK biochar while overcoming their shortcomings.

Critically raw materials as potential emerging environmental contaminants, their distribution patterns, risks and behaviour in floodplain soils contaminated by heavy metals

The expanding demand for new critical raw materials can lead to their increased release to the environment in the form of emerging environmental contaminants (EECs). However, there has never been a comprehensive study that takes into account the total EEC content, the content of various EEC fractions, their behaviour in floodplain soils, and potential ecological and human health risks. The occurrence, fractions, and influencing factors of the seven EECs (Li, Be, Sr, Ba, V, B, Se) originating from historical mining in floodplain soils of various ecosystems (arable lands, grasslands, riparian zones, contaminated sites) were investigated. Based on the evaluation of the overall levels of EECs (potentially toxic elements) in comparison to the soil guideline values set by European legislation for Be, Ba, V, B, and Se, it was found that only Be did not exceed the recommended limits. Among the elements analyzed, Li had the highest average contamination factor (CF) of 5.8, followed by Ba with 1.5 and B with 1.4. Particularly concerning was the discovery of a potential serious health risk associated with Li exposure for children, as indicated by hazard quotients ranging from 0.128 to 1.478. With the exception of Be and Se, the partitioning of the EECs into the different fractions revealed that the EECs are primarily bound with the residual fraction. Be (13.8%) had the highest percentage of exchangeable fraction as the most bioavailable in the first soil layer, followed by Sr (10.9%), Se (10.2%), Ba (10.0%), and B (2.9%). The most frequently observed correlations were between EEC fractions and pH/KCl, followed by soil organic carbon and manganese hydrous oxides. Variance analyses confirmed the impact of different ecosystems on EEC total content and fractions.

Importance of Application Rates of Compost and Biochar on Soil Metal(Loid) Immobilization and Plant Growth

In this study, we investigated the effect of different rates of compost (20%, 40%, 60% w/w) in combination with biochar (0%, 2%, 6% w/w) on soil physiochemical properties and the mobility of arsenic (As) and lead (Pb), in addition to the ability of Arabidopsis thaliana (ecotype Columbia-0) to grow and accumulate metal(loid)s. All modalities improved pH and electrical conductivity, stabilized Pb and mobilized As, but only the mixture of 20% compost and 6% biochar improved plant growth. Plants in all modalities showed a significant reduction in root and shoot Pb concentrations compared to the non-amended technosol. In contrast, As shoot concentration was significantly lower for plants in all modalities (except with 20% compost only) compared to non-amended technosol. For root As, plants in all modalities showed a significant reduction except for the mixture of 20% compost and 6% biochar. Overall, our results indicate that the mixture of 20% compost with 6% biochar emerged as the optimum combination for improving plant growth and As uptake, making it the possible optimum combination for enhancing the efficiency of land reclamation strategies. These findings provide a foundation for further research on the long-term effects and potential applications of the compost-biochar combination in improving soil quality.

Enrichment and health risks associated with trace elements in medicine food homology teas

Owing to the irreplaceable role of traditional Chinese medicine in the history of human resistance to diseases, medicine food homology teas (MFHTs) have emerged as a widely-consumed daily drink, although they may contain toxic or excessive trace elements. This study aims to determine the total and infused concentrations of nine trace elements (Fe, Mn, Zn, Cd, Cr, Cu, As, Pb, and Ni) in 12 MFHTs collected from 18 provinces in China, to evaluate their potential risks to human health, and to explore the factors affecting the trace element enrichment in traditional MFHTs. The exceedances of Cr (82%) and Ni (100%) in 12 MFHTs were higher than those of Cu (32%), Cd (23%), Pb (12%), and As (10%). The high values of the Nemerow integrated pollution index of dandelions and Flos sophorae (25.96 and 9.06, respectively) indicate severe trace metal pollution. The health risk assessment results showed that As, Cr, and Mn in the 12 types of MFHTs posed high non-carcinogenic risk. Honeysuckle and dandelion teas may be hazardous to human health through trace element exposure when consumed daily. The enrichment of Cr, Fe, Ni, Cu, Zn, Mn, and Pb in MFHTs is influenced by the MFHT type and producing area, whereas As and Cd are mainly controlled by the MFHT type. Environmental factors such as soil background values, rainfall, and temperature also affect the enrichment of trace elements in MFHTs collected from different producing areas.

Phytoavailability and uptake of arsenic in ryegrass affected by various amendments in soil of an abandoned gold mining site

Abandoned gold mining spoils pose socio-environmental, human, and animal health impacts and threaten sustainability of mineral extraction. Green trials and ecological solutions are required to effectively remediate these contaminated soils and mitigate the associated risks. Here, we carried out a pot experiment using a highly contaminated soil (mean total As = 5104.0 mg/kg) collected from an abandoned mine spoil in Ghana. We aimed to quantify the impacts of compost, iron oxide, and poultry manure on the mobilization, fractionation, and uptake of As by ryegrass (Lolium perenne). The soil amendments were applied at a rate of 5% (w/w) each, separately or in combination. We extracted the mine spoil soil readily-bioavailable As and specific-sorbed As, and determined the As contents in plant and the uptake after harvest. The plant transfer indices for soil-to-root (bioconcentration factor, BCF), soil-to-shoot (bioaccumulation concentration- BAC), and root-to-shoot (translocation factor- TF) were also calculated. Addition of manure increased the mining readily-bioavailable As by 243% and specific-sorbed As by 38%, as compared to the control. Manure addition further aided root As-uptake by 134%, whilst its combination with compost increased uptake by 101%. Lone addition of manure and in combination with compost resulted in BCF above 1, indicating increased As-phytostability. The presence of carbon and iron in the roots of the ryegrass sorbed or precipitated As limited its soil-to-shoot and root-to-shoot transfer. These findings indicate that manure alone and in combination with compost can be used to augment the phytoremediation efficiency of ryegrass in the As-contaminated spoil.

Impact of physiochemical properties, microbes and biochar on bioavailability of toxic elements in the soil: a review

The pollution of toxic elements (TEs) in the ecosystem exhibits detrimental effects on the human health. In this paper, we debated remediation approaches for TEs polluted soils via immobilization methods employing numerous amendments with reverence to type of soil and metals, and amendment, immobilization competence, fundamental processes and field applicability. We argued the influence of pH, soil organic matter, textural properties, microbes, speciation and biochar on the bioavailability of TEs. All these properties of soil, microbes and biochar are imperative for effective and safe application of these methods in remediation of TEs contamination in the ecosystem. Further, the application of physiochemical properties, microbes and biochar as amendments has significant synergistic impacts not only on absorption of elements but also on diminution of toxic elements.

Jack Bean Development in Multimetal Contaminated Soil Amended with Coffee Waste-Derived Biochars

Coffee waste-derived biochar was found to immobilize heavy metals in contaminated soil, although there are few studies involving these materials. Given the large amount of waste generated in the coffee industry, this presents a relevant opportunity to contribute to the circular economy and environmental sustainability. Therefore, the objective of this study was to evaluate the effects of the application of biochars derived from coffee grounds and coffee parchment in the remediation of a Cd, Zn and Pb contaminated soil and at the development of jack beans (Canavalia ensiformis) in this area’s revegetation. The biochars were pyrolyzed at 700 °C, and the treatments were: contaminated soil (CT); contaminated soil + calcium carbonate (CaCO3); contaminated soil + 5% (weight (w)/weitght (w)) coffee ground biochar and contaminated soil + 5% (w/w) coffee parchment biochar. These treatments were incubated for 90 days, followed by the cultivation of jack beans for 60 days. Soil samples, soil solution and plants were analyzed for nutrients and heavy metals. The addition of coffee grounds and coffee parchment biochars significantly reduced the contents of heavy metals in the soil compared to the Control (32.13 and 42.95%, respectively, for Zn; 26.28 and 33.06%, respectively, for Cd and 28.63 and 29.67%, respectively, for Pb), all of which had a superior performance than the CaCO3 treatment. Thus, following the observed reduction in the soil soluble fraction of metals, its uptake by the plants was also reduced, especially limiting Cd and Pb accumulation in plant dry matter. In addition, coffee parchment biochar promoted a greater accumulation of nutrients in the shoots, i.e., for K and P (1450 and 21.5 mg pot−1, respectively, dry matter basis) compared to the control (54.4 and 9.3 mg pot−1, respectively). Therefore, coffee parchment biochar use in association with jack beans may represent a viable tool for the remediation of metal contamination concomitantly with revegetation of the contaminated area.

Temporal evolution of surface and sub-surface geochemistry and microbial communities of Pb-rich mine tailings during phytostabilization: A one-year pilot-scale study

Old mine waste repositories can present health and/or environmental issues linked to their erosion, inducing dissemination of metals and metalloids in air and water that can be attenuated through phytostabilization. Here, the effect of this widespread phytomanagement option on the biogeochemistry of a Pb-rich mine waste was evaluated with a laboratory pilot-scale experiment giving access to the non-saturated and saturated zones below the rhizosphere compartment. Amendment of the tailings surface with biochar, manure and iron-oxide-rich ochre promoted growth of the seeded Agrostis capillaris plants. These events were accompanied by an increase of pH and a decrease of Pb concentration in pore water of the surface layer, and by a transient increase of Pb, Zn, and Ba concentrations in the deeper saturated levels. Macroscopic and microscopic observations (SEM) suggest that Pb was immobilized in A. capillaris rhizosphere through mechanical entrapment of tailing particles. Microbial taxonomic and metabolic diversities increased in the amended phytostabilized surface levels, with a rise of the proportion of heterotrophic micro-organisms. Below the surface, a transient modification of microbial communities was observed in the non-saturated and saturated levels, however 11 months after seeding, the prokaryotic community of the deepest saturated zone was close to that of the initial tailings. pH and water saturation seemed to be the main parameters driving prokaryotic communities' structures. Results obtained at pilot-scale will help to precisely evaluate the impacts of phytostabilization on the temporal evolution of reactions driving the fate of pollutants inside the tailings dumps.

Fertilizer Efficiency and Risk Assessment of the Utilization of AOD Slag as a Mineral Fertilizer for Alfalfa (Medicago sativa L.) and Perennial Ryegrass (Lolium perenne L.) Planting

Argon oxygen decarburization (AOD) slag is the by-product of the stainless steel refining process, which has caused considerable environmental stress. In this work, the utilization of AOD slag as mineral fertilizer for alfalfa (Medicago sativa L.) and perennial ryegrass (Lolium perenne L.) planting were investigated by pot experiments. The morpho-physiological parameters of biomass, plant height, root morphology and the biochemical parameters of malondialdehyde (MDA) content, superoxide dismutase (SOD) activity, catalase (CAT) activity, peroxidase (POD) activity, and chlorophyll were measured. The accumulation of chromium in plants was also determined for an environmental safety perspective. It was found that low rates (≤0.5 wt.% for alfalfa and ≤2 wt.% for perennial ryegrass) of AOD slag fertilization are beneficial to the growth of these two plants. However, the soil enrichment with higher AOD slag amounts resulted in the reduction of biomass, plant height, and root growth. Compared with the alfalfa, the perennial ryegrass showed higher tolerance for AOD slag fertilization. The toxicity of the utilization of AOD slag as mineral fertilizer for perennial ryegrass planting is slight. Health risks induced by the consumption of the alfalfa grown on the soil with high AOD slag rates (≥8 wt.%) were detected.

Comparison of the effectiveness of biochar vs. magnesite amendments to immobilize metals and restore a polluted soil

Here we addressed the remediation of a soil severely contaminated by Cu, Cd, Pb and Zn. In this regard, we tested the capacity of magnesite and biochar, inorganic and organic soil amendments, respectively, to reduce metal availability and improve soil properties. To this end, 1-kg pots containing the polluted soil were amended with either magnesite or biochar. Metal availability and soil properties were then measured at days 15 and 75. Also, to evaluate the impact of the two treatments on plant growth, we conducted experimental trials with Brassica juncea L. and compost addition. Both amendments, but particularly magnesite, markedly decreased metal availability. Soil properties were also enhanced, as reflected by increases in the cation exchangeable capacity. However, plant growth was inhibited by magnesite amendment. This observation could be attributable to an increase in soil pH and cation exchange capacity as well as a high Mg concentration. In contrast, biochar increased biomass production but decreased the quantity of metals recovered when the plants are harvested. In conclusion, on the basis of our results, we propose magnesite as a suitable approach for stabilizing contaminated soils (or even spoil heaps) where revegetation is not a priority. In contrast, although biochar has a lower, but still significant, capacity to immobilize metals, it can be used to restore natural soil properties and thus favor plant growth.

Simultaneous reduction and immobilization of Cr(VI) in seasonally frozen areas: Remediation mechanisms and the role of ageing

Among the toxic metals, hexavalent chromium [Cr(VI)] has attracted much attention due to its high mobility and toxicity, rendering considerable challenges for long-term remediation. In this study, the soil was collected from a dichromate contaminated industrial site in Liaoning Province, a seasonally frozen area in northern China, and subjected to frequent freeze-thaw cycles. Three additives, including (i) ferrous sulfate; (ii) calcium polysulfide; and (iii) combined biochar and calcium polysulfide were applied to reduce and immobilize Cr(VI) in the soils. The samples underwent 28 days of incubation followed by 16 freeze-thaw cycles. The toxicity characteristic leaching procedure (TCLP) and simulated acid rain leaching were adopted to test the remediation performances. It was observed that all three treatments can significantly reduce and immobilize Cr(VI) after short-term incubation, while biochar with abundant functional groups could adsorb and reduce Cr(VI) effectively. Notably, the concentration of Cr(VI) in TCLP leachates after incubation in combined treatment decreased by 67.87% and 37.27%, respectively, compared with the application of ferrous sulfate or calcium polysulfide alone. Freeze-thaw cycles induced the disintegration of soil particles and increased the risk of contaminant mobilization. Conversely, biochar particles has become finer and even produced nanoparticles with ageing, accompanied by the increase in oxygen-containing surface functional groups. Additionally, the specific surface area increased with the pyrolysis of biochar, which further enhanced the retention of soil colloidal particles and suppressed the migration of contaminants. Therefore, the cumulative release of Cr(VI) in the combined treatment (i.e., 10.97 ~ 32.97 mg/kg) was much lower than that of the other two treatments after freeze-thaw ageing. Overall, the combination of biochar and calcium polysulfide displayed advantages in the reduction and immobilization of Cr(VI), and offered a long-term, effective strategy for the remediation of Cr(VI) contaminated soils in cold regions.

Towards a Soil Remediation Strategy Using Biochar: Effects on Soil Chemical Properties and Bioavailability of Potentially Toxic Elements

Soil contamination with potentially toxic elements (PTEs) is considered one of the most severe environmental threats, while among remediation strategies, research on the application of soil amendments has received important consideration. This review highlights the effects of biochar application on soil properties and the bioavailability of potentially toxic elements describing research areas of intense current and emerging activity. Using a visual scientometric analysis, our study shows that between 2019 and 2020, research sub-fields like earthworm activities and responses, greenhouse gass emissions, and low molecular weight organic acids have gained most of the attention when biochar was investigated for soil remediation purposes. Moreover, biomasses like rice straw, sewage sludge, and sawdust were found to be the most commonly used feedstocks for biochar production. The effect of biochar on soil chemistry and different mechanisms responsible for PTEs' immobilization with biochar, are also briefly reported. Special attention is also given to specific PTEs most commonly found at contaminated soils, including Cu, Zn, Ni, Cr, Pb, Cd, and As, and therefore are more extensively revised in this paper. This review also addresses some of the issues in developing innovative methodologies for engineered biochars, introduced alongside some suggestions which intend to form a more focused soil remediation strategy.

Interactive assessment of lignite and bamboo-biochar for geochemical speciation, modulation and uptake of Cu and other heavy metals in the copper mine tailing

This study was designed to examine the combined effect of bamboo-biochar (BC) and water-washed lignite (LGT) at copper mine tailings (CuMT) sites on the concentration of Cu and other metals in pore water (PW), their bioavailability, and change in geochemical speciation. Rapeseed (first cropping-season) and wheat (second cropping-season) were grown for 40-days each and the influence of applied-amendments on both cropping seasons was observed and compared. A significant increase in pH, water holding capacity (WHC), and soil organic carbon (SOC) was observed after the applied amendments in second cropping-seasons. The BC-LGT significantly reduced the concentration of Cu in PW after second cropping seasons; however, the concentration of Pb and Zn were increased with the individual application of biochar and LGT, respectively. BC-LGT and BC-2% significantly reduced the bioavailability of Cu and other HMs in both cropping seasons. The treated-CuMT was subjected to spectroscopic investigation through X-ray photoelectron spectroscopy (XPS), Fourier transform Infrared spectroscopy (FTIR), and X-ray powder diffraction (XRD). The results showed that Cu sorption mainly involved the coordination with hydroxyl and carboxyl functional groups, as well as the co-precipitation or complexation on mineral surfaces, which vary with the applied amendment and bulk amount of Mg, Mn, and Fe released during sorption-process. The co-application of BC-LGT exerted significant effectiveness in immobilizing Cu and other HMs in CuMT. The outcomes of the study indicated that co-application of BC-LGT is an efficacious combination of organic and inorganic materials for Cu adsorption which may provide some new information for the sustainable remediation of copper mine tailing.

Accumulation and subcellular distribution of cadmium in rygegrass induced by Aspergillus niger TL-F2 and Aspergillus flavus TL-F3

Although plant growth-promoting fungi can greatly accelerate the ryegrass bioaccumulation of cadmium (Cd), the underlying mechanisms are not yet well documented. Therefore, we performed a 20-days hydroponic experiment to investigate the effects of Aspergillus niger TL-F2 (A. niger TL-F2) and Aspergillus flavus TL-F3 (A. flavus TL-F3) on accumulation/subcellular distribution of Cd by annual ryegrass Dongmu 70 at different Cd concentrations (0, 2.5, and 5 mg L^-1). Results indicated that both fungal strains promoted ryegrass biomass/growth by about 60%. Furthermore, we found that ryegrass roots (17.8-37.1 μg pot^-1) had a significantly higher capability for Cd uptake than the shoots (1.66-5.45 μg pot^-1) (p < 0.05). Of total Cd in ryegrass plants, 44-67% was in soluble form, 24-37% was in cell wall, and 8.5-25.5% was in organelles. Compared with non-fungus ryegrass, cell wall and soluble Cd fractions in fungus-inoculated roots increased and decreased by 13.5-44% and 21.5-26.4%, respectively. Besides, fungus inoculation generally increased the content of cell wall and soluble Cd fractions in ryegrass shoots. Altogether, the study concludes that inoculation of fungus in ryegrass is a promising approach to improve phytoremediation of Cd contaminated environments.Novelty statement Previous study by Han et al. (2018) examined the resistance of ryegrass plant to Cd stress after its inoculation with Aspergillus aculeatus. In this study, using a hydroponic experiment, we examined the effects of co-application of two species of Aspergillus fungi. i.e. A. niger TL-F2 and A. flavus TL-F3 on ryegrass growth/biomass, Cd absorption by ryegrass shoots and roots, and subcellular distribution of Cd in ryegrass roots and shoots.

Effects of biochar, ochre and manure amendments associated with a metallicolous ecotype of Agrostis capillaris on As and Pb stabilization of a former mine technosol

Metal(loid) soil pollution is a major environmental and health issue, requiring these areas to be remediated, for example through phytoremediation processes. In order to allow proper plant establishment and growth, amendments must be applied to highly contaminated and poorly fertile soils. Amendments are diverse, but many studies have shown the beneficial effects of biochar, manure and ochre, although studies on their combined use are scarce. Moreover, no studies have evaluated the effect of these combined amendments on endemic plant growth. Endemic plants growing on contaminated soils showed higher tolerance toward pollutants compared to plants coming from unpolluted areas. Therefore, the aim of the present study was to evaluate both the effect of amendments (single or combined) on the physicochemical properties of a former mining technosol, and the growth and metal(loid) accumulation ability of endemic Agrostis capillaris plants. This study revealed an improvement in the soil physicochemical properties following the application of amendments, with combined amendments showing better results than the application of just one. On top of this, Agrostis plants performed better on the amended technosols, especially the ones receiving manure, due to its high nutrient content. Finally, based on soil properties, plant growth and the metal(loid) accumulation profile, the use of biochar combined with manure seems to be the most appropriate treatment. Indeed, this treatment showed an improvement in both soil fertility and plant growth. Moreover, Agrostis plants grown in these conditions were among those showing higher root metal(loid) concentration associated with a lower translocation toward aerial parts.

Contrasted tolerance of Agrostis capillaris metallicolous and non-metallicolous ecotypes in the context of a mining technosol amended by biochar, compost and iron sulfate

Metal(loid) contamination of soil, resulting from the mining activities, is a major issue worldwide, due to its negative effects on the environment and health. Therefore, these contaminated soils need to be remediated. One realistic method is the assisted phytostabilization, which aims at establishing a vegetation cover on the soil that will reduce metal(loid) bioavailability and spreading through the prevention of wind erosion and water leaching. In addition, amendments are applied to improve soil conditions and ameliorate plant growth. In this goal, biochar and compost showed good results in terms of amelioration of soil fertility and reduction in lead bioavailability. However, they usually have a negative effect on arsenic. On the contrary, iron sulfate showed capacity to reduce arsenic mobility through interaction with its iron hydroxides. Finally, the choice of the appropriate plant species is crucial for the success of assisted phytostabilization. One good option is to use endemic species, adapted to the metal(loid) stress, with a fast growth and large shoot and root systems. The aims of this study were to (1) evaluate the effects of applying biochar, compost and iron sulfate, alone or combined, to a former mine soil on the soil properties and Agrostis capillaris growth, and (2) assess the difference between two Agrostis capillaris ecotypes, an endemic metallicolous ecotype and a non-metallicolous ecotype. Results of the mesocosm experiment showed that amendment application improved soil properties, i.e., reduced soil acidity, increased nutrient availability and lower metal(loid) stress, the best being the combination biochar-compost-iron sulfate. These ameliorations allowed a better plant growth. Finally, the metallicolous ecotype performed better in terms of growth than the non-metallicolous one and could thus be used in an assisted phytostabilization process on the former mine site.

Does biochar inhibit the bioavailability and bioaccumulation of As and Cd in co-contaminated soils? A meta-analysis

Biochar, an effective and low-cost amendment for immobilizing heavy metals, has been extensively studied. However, the simultaneous inhibition effects of biochar on the plant uptake for arsenic (As) and cadmium (Cd) in co-contaminated soils are still ambiguous due to their distinct environmental behaviors. A meta-analysis was conducted to quantitatively assess the effects using 1030 individual observations from 52 articles. On average, biochar application significantly decreased the bioavailability of Cd in soils by 50.12%, while slightly increased the bioavailability of As in soils by 2.39%. The more instructive result is that biochar application could also simultaneously reduce the concentration of As and Cd in plants by 25.48% and 38.66%, respectively. The orders of the decreased percentage of As and Cd in various tissues were root < stem< leaf < grain, and root < leaf < stem < grain, respectively. According to the analysis of critical factors, manure biochar, low pyrolysis temperature (at <400 °C), low application rate (<2%), and high SOC (>30 g/kg) were more conducive to reduce the bioaccumulation of As and Cd simultaneously in co-contaminated soils. Pristine and modified biochar could inhibit As and Cd accumulation in crops, but their efficiencies need to be further improved to ensure the safety of crop productions. Overall, the meta-analysis suggests that biochar has the potential to remedy the As and Cd co-contaminated soils.

Alleviation of lead toxicity and phytostimulation in perennial ryegrass by the Pb-resistant fungus Trichoderma asperellum SD-5

Lead (Pb), a highly toxic metal ion, is detrimental to plants and humans. Existing botanical techniques for Pb-contaminated soil remediation are limited in their efficiency. Here, we investigated the use of the fungus Trichoderma asperellum Samuels, Lieckf & Nirenberg SD-5, which we identified previously as being Pb-resistant, for phytoremediation and for its effects on plant growth, Pb adsorption, and physiological responses in perennial ryegrass (Lolium perenne L. 'Lark'). We set up four soil treatments: CK (uncontaminated by Pb), T1 (1000 mg kg-1 Pb), T2 (1:9 ratio of sawdust to T1), and T3 (T2 inoculated with T. asperellum SD-5). A pot experiment revealed that the addition of the Pb-resistant microorganism promoted growth and increased biomass in ryegrass under Pb stress, in addition to significantly enhancing photosynthesis by increasing the leaf chlorophyll content and improving the total protein content and expression of the pAPX, POD, SOD, and GPX genes, evidence of an improved antioxidant system and the alleviation of Pb stress. We demonstrated that Pb-resistant microorganisms can enhance Pb extraction from the soil, thus improving remediation. Mitigation mechanisms operating at the physiological and gene expression levels were also determined, providing a scientific basis for the role of combined plant-microorganism methods in remediating Pb-contaminated soil.

Biochar assisted phytoremediation and biomass disposal in heavy metal contaminated mine soils: a review

Mining activities causes heavy metal pollution and adversely affect the ecological safety and human well-being. Phytoremediation-biochar synergy can effectively remediate mine spoils contaminated with heavy metals (HM). A review which focuses exclusively on the application of biochar assisted phytoremediation in HM contaminated mine spoil is lacking. Mechanisms of metal immobilization by biochar, potential plants and contaminated biomass disposal methods has also been reviewed. Availability of biochar feedstock and production conditions, optimization of application rate, application techniques, selection of suitable hyperaccumulators and cost optimization of bulk biochar production are the key to a successful biochar-based HM remediation of mine tailings and coalmine spoil. Presently, herbs and shrubs are mostly used as phytoremediators, use of woody trees would encourage a long-term metal sequestration which would reduce the cost of biomass disposal. Also, use of non-edible plants would prevent the plants from entering the food chain. For a holistic biochar-phytoremediation technique, incineration and pyrolysis can effectively dispose contaminated biomass. From the economical viewpoint, the environment cost-benefit analysis should be considered before considering the feasibility of a technology.HighlightsMass scale in-situ biochar production and economics are keys issues.Biochar assisted phytoremediation for HM contaminated mine spoils.Long term studies using woody biomass needs attention.Disposal of contaminated biomass by pyrolysis method.

Impacts of bamboo biochar on the phytoremediation potential of Salix psammophila grown in multi-metals contaminated soil

We investigated the effects of bamboo biochar (BBC) as soil amendment on growth and phytoremediation potential of Salix psammophila in soil heavily polluted by Cd and Zn. Bamboo biochar was added to soil at ratios ranging from 1 to 7% (w/w), which significantly increased the organic matter, available potassium (K) content, while decreased the hydrolyzable nitrogen (N) content and the levels of total and bioavailable HMs in soil. The BBC amendment at ratios of 1% to 5% showed little effect on growth of plant, whereas at 7% ratio significantly decreased biomass compared to the control. BBC amendment stimulated the accumulation of Cu, Cd and Zn in plant tissues, meanwhile, Cd and Zn accumulation were more evident, especially in the BBC-3% treatment. BBC amendment improved the TF and BCF values of Cd, Zn and Cu compared to control. Higher BCF for Cd (BCF >1) and TF for Zn (TF >1) values indicate Salix psammophila have considerable potential for phytoremediation efficiency in BBC amended soil treatment. This study provides practical evidence of the efficient BBC-assisted phytoremediation capability of Salix psammophila and highlights its potential as a viable and inexpensive approach for in situ remediation.

Can biochar reclaim coal mine spoil?

Surface coal mining activities completely destroy vegetation cover, soil and biodiversity. The aftermaths include huge coal mine spoil dumps, changed topography, drainage and landscape, deteriorated aesthetics and increased pollution load. These coal mine spoils are characterised by high rock fragments, extremely low water holding capacity, compacted and high bulk density, lack of organic carbon and plant nutrients, low cation exchange capacity, acidic pH and toxic metal contamination, which poses difficulties in reclamation. An array of studies has been focused on the sustainable use of biochar for restoration of degraded agricultural soil by improving the soil physicochemical, nutritional and biological properties. Although a volume of studies has been done on biochar application, its specialised application in reclamation of coal mine spoils is still atypical, also a systematic review on the mechanism by which biochar amends the mine spoil is lacking. This review focuses on i) factors affecting the biochar properties, ii) the mechanism involved in altering the physical, chemical and biological properties by biochar, (iii) remediation of potentially toxic elements in soil and restoration of degraded land using biochar, and, iv) highlighting the important aspects to be considered while using biochar for reclamation of coal mine spoil. Biochar prepared at 450 °C from a lignocellulosic rich biomass can be an alternative for reclamation for coal mine spoil. Review also suggested suitable methodologies for bulk production, application and economics of biochar in coal mine spoil reclamation.

Effects of Different In Situ Remediation Strategies for an As-Polluted Soil on Human Health Risk, Soil Properties, and Vegetation

The demand for soils for recreational uses, gardening, or others in urban and periurban areas is increasing, and thus the presence of polluted technosols in these areas requires nature-based in situ remediation technologies. In this context, the capacity of three amendments, namely zero valent iron nanoparticles (nZVI), compost and a mixture of compost and biochar, to immobilise As in a polluted technosol simultaneously cultivated with Lolium perenne L. were tested and compared. The characteristics of the soil were comprehensively characterised by chemical and X-ray analysis to determine As contents, distribution, and mineralogy. As mobility was evaluated by the RBA methodology and then potential human health risks, both carcinogenic and non-carcinogenic, were assessed in all treatments. The nZVI treatment reduced risks due to the As immobilisation obtained (41% As decrease, RBA test), whereas the organic amendments did not imply any significant reduction of the RBA values. As to soil properties, the organic treatments applied lowered the pH values, increasing cation exchange capacity, and carbon and nutrient contents. To determine impacts over plant production, fresh biomass, As, Ca, Fe, K, Mg, Na and P were measured in Lolium under the different treatments. Notably, organic amendments improved As extraction by plants (57% increase), as well as fresh biomass (56% increase). On the contrary, nZVI diminished As extraction (65% decrease) and promoted a fresh biomass decrease of 57% due to nutrients immobilisation (61% decrease of P in plants tissues).

The effect of combined ecological remediation (plant microorganism modifier) on rare earth mine wasteland

Due to the vegetation destruction and soil desertification caused by excessive exploitation at Ganzhou ion-type rare earth mine in the mid-1980s, it is essential to carry out ecological remediation. The symbiotic mycorrhiza formed by the developed perennial ryegrass (Lolium perenne L.) roots infected with arbuscular mycorrhizal fungi (AMF) can significantly improve the growth and resistance of plants. In this study, the combination of symbiotic mycorrhiza and soil modifier was used to construct the ryegrass-AMF-soil modifier combined remediation technology, which achieved effective ecological remediation of soil tailings. The orthogonal experiment of soil modifier showed that the most efficient formula for ryegrass biomass, soil organic matter, soil alkaline hydrolysis, soil available phosphorus, and soil pH was 5 g/kg sepiolite, 3 g/kg chicken manure, 2 g/kg humic acid, and 2 g/kg biochar (A₄B₃C₃D₃), and chicken manure (B), humic acid (C), and biochar (D) had significant effects on the improvement of ryegrass biomass, soil organic matter, soil alkaline nitrogen, and soil available phosphorus. Sepiolite (A) had a significant improvement in soil pH. Furthermore, the AMF infection results indicated that Glomus moss (G.m.) had higher affinity with ryegrass. The T4 treatment-combined remediation using G.m. inoculation had the most significant effect on ryegrass growth; plant height increased by 39.19% compared with T1 treatment-inoculation using G.m. Under combined remediation, soil pH, organic matter, alkali nitrogen, and effective phosphorus content also significantly improved after combined treatment. Under G.m. inoculation treatment (T4 treatment), the soil nutrient content reached the three criteria of the soil nutrient grading standard.

Contrasting effects of biochar and hydrothermally treated coal gangue on leachability, bioavailability, speciation and accumulation of heavy metals by rapeseed in copper mine tailings

The purpose of this research was to examine the influence of hydrothermally treated coal gangue (HTCG) with and without biochar (BC) on the leaching, bioavailability, and redistribution of chemical fractions of heavy metals (HMs) in copper mine tailing (Cu-MT). An increase in pH, water holding capacity (WHC) and soil organic carbon (SOC) were observed due to the addition of BC in combination with raw coal gangue (RCG) and HTCG. A high Cu and other HMs concentration in pore water (PW) and amended Cu-MT were reduced by the combination of BC with RCG and/or HTCG, whereas individual application of RCG slightly increased the Cu, Cd, and Zn leaching and bioavailability, compared to the unamended Cu-MT. Sequential extractions results showed a reduction in the exchangeable fraction of Cu, Cd, Pb, and Zn and elevation in the residual fraction following the addition of BC-2% and BC-HTCG. However, individual application of RCG slightly increased the Cu, Cd, and Zn exchangeable fractions assessed by chemical extraction method. Rapeseed was grown for the following 45 days during which physiological parameters, metal uptake transfer rate (TR), bioconcentration factor (BCF), and translocation factor (TF) were measured after harvesting. In the case of plant biomass, no significant difference between applied amendments was observed for the fresh biomass (FBM) and dry biomass (DBM) of shoots and roots of rapeseed. However, BC-2% and BC-HTCG presented the lowest HMs uptake, TR, BCF (BCF_root and BCF_shoot), and TF for Cu, Cd, Cr, Ni, Pb, and Zn in rapeseed among the other amendments compared to the unamended Cu-MT. Overall, these findings are indicative that using biochar in combination with RCG and/or HTCG led to a larger reduction in HMs leaching and bioavailability, due to their higher sorption capacity and could be a suitable remediation strategy for heavy metals in a Cu-MT.

Improving zinc phytoremediation characteristics in Salix pedicellata with a new acclimation approach

Some species of Salix sp. (willows) are a potential phytoremediator that can accumulate substantial contents of mineral elements and, therefore, to detoxify soils contaminated with pollutants and heavy metals such as the zinc (Zn). However, high concentrations of Zn inhibit plant growth and reduce biomass production in plants. In an attempt to overcome this inconvenience and to enhance plant tolerance to Zn toxicity, we tested a new tolerance induction approach by acclimation in two clones of Salix pedicellata, named SPK-12 and SP-K20. The approach comprises two successive phases. The first is a "tolerance induction phase" consisting of gradual exposure of plants to low concentrations of Zn sulfate (ZnSO₄) at regular intervals until reaching DI₁₀₀ (ZnSO₄ inhibitory concentration). And, the second is a "tolerance maintenance phase" to uphold the acquired tolerance to Zn toxicity. The SP-K20 clone was acclimated to DI₁₀₀ threshold over 33 days without noticeable symptoms of chlorosis or growth inhibition. Compared to controls, the SP-K20 clone was able to accumulate high concentrations of Zn, suggesting that phytoremediation abilities of S. pedicellata have been improved throughout the applied approach. Acclimated Salix plants might thus improve metal phytoextraction in heavily polluted soils without biomass growth inhibition.

Residual effects of tobacco biochar along with different fixing agents on stabilization of trace elements in multi-metal contaminated soils

The residual effect of tobacco biochar (TB ≥ 500°C) mono and co-application with Ca-hydroxide (CH), Ca-bentonite (CB) and natural zeolite (NZ) on the bio-availability of trace elements TE(s) in alkaline soils has not been deeply studied yet. A pot study that had earlier been investigated TB mono and blended with CH, CB and NZ on the immobilization of Pb, Cu Cd, and Zn by Chinese cabbage. Maize crop in the rotation was selected as test plant to assess the residual impact of amendments on stabilization of Pb, Cu Cd, and Zn in mine polluted (M-P), smelter heavily and low polluted (S-HP and S-LP, respectively) soils. The obtained results showed that stabilization of Pb, Cd, Cu and Zn reached 63.84% with TB + CB, 61.19% with TB + CH, 83.31% with TB + CH and 35.27% with TB + CH for M-P soil, 36.46% with TB + NZ, 38.46% with TB + NZ, 19.40% with TB + CH and 62.43% with TB + CH for S-LP soil, 52.94% TB + NZ, 57.65% with TB + NZ, 52.94% with TB + NZ, and 28.44% with TB + CH for S-LP soil. Conversely, TB + CH and TB alone had mobilized Pb and Zn up to 19.29% and 34.96% in M-P soil. The mobility of Zn reached 8.38% with TB + CB and 66.03% with TB for S-HP and S-LP soils. The uptake and accumulation of Pb, Cd, Cu and Zn in shoot and root were reduced in three polluted soils. Overall, the combination of TB along with CH, CB and NZ has been proven to be effective in Pb, Cd, Cu and Zn polluted mine/smelter soils restoration.

Low-cost alkaline substances for the chemical stabilization of cadmium-contaminated soils

Owing to poor waste management in zinc mining, toxic heavy metals, particularly cadmium, are released and contaminate the surrounding agricultural areas. Waterlogging, which is a common practice in rice vegetation, creates anaerobic conditions that result in the conversion of organic matter into acetic acid and the reducing phase. This accelerates the release of cadmium into the water, where it is absorbed into the cells of rice. Chemical stabilization methods can be used to treat cadmium-contaminated soil by reacting an alkaline substance with acetic acid and increasing the soil pH for cadmium immobilization. However, to date, few studies using limestone dust and corncob fly ash have been conducted, and no studies have focused on the neutralization of the produced acetic acid in the anaerobic zone of the soil. This study aims to determine the optimum conditions for cadmium stabilization using different types of low-cost alkaline substances (lime, limestone dust, and corncob fly ash). The effects of alkaline amount, soil moisture content, and reaction time on soil stabilization were investigated. Lime was the most suitable for stabilization among the tested alkaline substances, and increasing the amount of lime can effectively reduce the amount of exchangeable cadmium. At 25% w/w of lime/soil, the exchangeable cadmium can be reduced from 29.3 to 7.8 mg kg^-1. The stabilization efficiencies of limestone dust and corncob fly ash were much lower. The statistical analysis shows that the amount of alkaline substance is the main factor affecting the stabilization performance at a 95% confidence limit for all tested alkalines.

The combined effect of phytostabilization and different amendments on remediation of soils from post-military areas

Army bases and battle fields are areas of high pollution due to the chemicals released there. Soils in these areas suffer from these uses of the land, and ecosystem services are affected. Although, in the 20th century, the production of bombs and the locations of battle fields and army bases were widespread, there is little research on the impact of war on nature. Moreover, there is a need to rehabilitate the disturbed soils. The contents and ecological risks of Cu, Ni, Cd, Pb, Zn, and Cr in the topsoil from a post-military area (north-eastern Poland) were investigated. In addition, a vegetation experiment was performed with the technique of aided phytostabilization on soils from the study area. The novelty of this study is the assessment of the usefulness of soil amendments (chalcedonite, limestone, activated carbon) in heavy metal (HM) phytostabilization in contaminated soils from post-military areas. Soil samples were also examined for pollution quantification indicators, including the index of geoaccumulation (I_geo), contamination factor (CF), and degree of contamination, and subjected to the Ostracodtoxkit test. The mean contents of the tested HMs were higher than those stipulated in soil environmental quality standards. The highest I_geo (7.38) and CF (346) values were those of Cr and Zn, respectively. The highest increase in soil pH was observed after the application of limestone to the soil. The greatest reduction in Cu, Ni, and Cd contents was caused by addition of limestone. The contents of HMs in Festuca rubra were higher in its roots than in its above-ground parts.

Effects of soil amendments on the growth response and phytoextraction capability of a willow variety (S. viminalis × S. schwerinii × S. dasyclados) grown in contaminated soils

This study was conducted to evaluate the effects of lime and bisphosphonates (BPs) such as N10O chelate amendment on the growth, physiological and biochemical parameters, and phytoextraction potential of the willow variety Klara (Salix viminalis × S. schwerinii × S. dasyclados) grown in soils heavily contaminated with copper (Cu), nickel (Ni) and zinc (Zn). The plants were irrigated with tap or processed water (mine wastewater). The results suggest that the combined effects of the contaminated soil and processed water inhibited growth parameters, gas exchange parameters and chlorophyll fluorescence (Fv/Fm) values. In contrast, malondialdehyde (MDA) content, organic acids, total phenolic and total flavonoid contents, and the accumulation of metals/metalloids in the plant tissues were increased compared to the control. When the soil was supplemented with lime and N10O; growth, physiological, biochemical parameters, and resistance capacity were significantly higher compared to unamended soil treatments, especially in the contaminated soil treatments. The combined lime‒ and N10O‒amended soil treatment produced higher growth rates, resistance capacity, photosynthesis rates and phytoextraction efficiency levels relative to either the lime‒amended or the N10O‒amended soil treatments. This study provides practical evidence of the efficient chelate‒assisted phytoextraction capability of Klara and highlights its potential as a viable and inexpensive novel approach for in situ remediation of Cu‒, Ni‒ and Zn‒contaminated soils and mine wastewaters.