Effect of Fe-functionalized biochar on toxicity of a technosol contaminated by Pb and As: sorption and phytotoxicity tests

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.

Natural ageing of biochar improves its benefits to soil Pb immobilization and reduction in soil phytotoxicity

Amendments are good tools for immobilizing metal(loid) and improving phytoremediation success. However, the amendment effect is variable and depends on multiple parameters, including amendment type and ageing. Such an ageing effect is rarely studied. Our study is one of the first focusing on how biochar storage affects its effect on soil properties and metal(loid) immobilization, when biochar was applied alone or in combination with green manure. To answer this, a 33-day pot incubation experiment was set up using contaminated soil, amended with two biochars (differing in ages: old (Bo) and new (Bn)) and/or two green manures (leaves of clover or poplar) and sown with Phaseolus vulgaris (bioindicator plant). Soil pore waters, plant growth and metal(loid) accumulation were evaluated. Biochar reduced soil acidity (Bn: + 0.75 pH unit, Bo: + 0.72 unit) and Pb mobility (Bn: - 42%, Bo: - 50%), while green manures acidified the soil (- 0.30 pH unit) and immobilized Pb only after 10 days (- 44%). All amendments reduced soil phytotoxicity. Moreover, the biochar stored at room temperature for a few years demonstrated better abilities to improve soil properties, particularly for Pb immobilization, than the biochar freshly prepared. Finally, as mixtures maturated, soil parameters changed until about ten days, then tended to stabilize. Therefore, it can be concluded that (1) biochar storage will affect its chemical properties and ameliorate its effects, (2) biochar can ameliorate soil properties and immobilize metal(loid)s, while green manures tended to have adverse effects at first, and (3) soil/amendment mixtures should be left to mature about two weeks before potential plant implementation.

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.

The reduction of the As and Pb phytotoxicity of a former mine technosol depends on the amendment type and properties

In remediation of metal(loid) polluted soils, it is crucial to improve soil conditions and reduce metal(loid) toxicity to permit plant growth. To do that, amendments, such as biochar, activated carbon, and redmud, can be applied to the soil. Their effects are dependent on their type and properties. The aims of this study were thus to evaluate the potential of diverse biochars, activated carbons, and redmuds to reduce phytotoxicity of a former mine technosol polluted with As and Pb. Two pots experiments were set up. The first one applied on Pontgibaud technosol ten biochars, eight activated carbons, and three redmuds, at 2% for the biochars and activated carbons and 1% for the redmud. Soil pore water properties (pH, electrical conductivity), metal(loid) mobility, and Phaseolus vulgaris growth were monitored. In a second experiment, the five best amendments, one redmud associated with two biochars and two activated carbons, selected based on their ability to improve soil conditions, immobilize metal(loid)s and improve plant growth, were applied. The same plant species was used and soil and plant parameters were measured. Results demonstrated that not all amendments were capable of ameliorating soil conditions and reducing soil phytotoxicity. Moreover, the five selected amendments (biochars from oak bark sapwood and bamboo, activated carbons from vegetal feedstock chemically activated and physically activated, modified redmud) showed good sorption capacity towards Pb, with maximum sorption capacity between 63 and 217 mg g^-1, depending on the amendment, and their combined application led to better soil properties improvement than the single amendments. However, plant growth was only ameliorated further than a single application in the redmud-biochar combination but not in the association of redmud with activated carbon. This study is one of the first to deliver a rapid phytotoxicity test screening demonstrating that redmud associated with particular biochar could be beneficial in reducing the phytotoxicity of technosol polluted with As and Pb and thus allow plant growth and a phytomanagement process.

Toxicity of biochar influenced by aging time and environmental factors

The extensive application of biochar has drawn more attentions on its potential risk to aquatic organisms. However, the influence of environmental factors (i.e. pH, HA, SDBS and aging time) after they discharged into environment on their toxicity have not been clarified. Here, we synthesized biochar with local pine needles via pyrolysis, and then aged in different media. Followed, the toxicity of pristine and aged biochar was checked with Scenedesmus obliquus (S. obliquus). Our investigation showed that the toxicity of biochar was mitigated when aged in different pH levels or SDBS, while it was opposite in the presence of HA. The increment of pH decreased the toxicity of both the pristine and the aged biochar, while the presence of HA did same impact on the pristine biochar. The presence of SDBS decreased the toxicity of pristine biochar but increased that of aged biochar. Meanwhile, we showed these environmental factors (pH, HA, SDBS and aging time) influenced the biochar toxicity may be due to the adjustment of the aggregation and adhesion of biochar on cell surfaces or the intracellular oxidative stress. Further, the PFRs contained in biochar did influence the toxicity, along with the physicochemical properties of biochar (i.e. carbon structure, functional group or surface charge). Our results aimed to reflect the toxicity profile of biochar in the natural aquatic environment, without misunderstanding of potential ecological risk of biochar in the future application.

Effects of biochar-based materials on the bioavailability of soil organic pollutants and their biological impacts

Motivated by the unique structure and superior properties, biochar-based materials, including pristine biochar and composites of biochar with other functional materials, are considered as new generation materials for diverse multi-functional applications, which may be intentionally or unintentionally released to soil. The influencing mechanism of biochar-based material on soil organisms is a key aspect for quantifying and predicting its benefits and trade-offs. This work focuses on the effects of biochar-based materials on soil organisms within the past ten years. 206 sources are reviewed and available knowledge on biochar-based materials' impacts on soil organisms is summarized from a diverse perspective, including the pollutant bioavailability changes in soil, and potential effects of biochar-based materials on soil organisms. Herein, effects of biochar-based materials on the bioavailability of soil organic pollutants are detailed, from the perspective of plant, microorganism, and soil fauna. Potential biological effects of pristine biochar (PBC), metal/metal compounds-biochar composites (MBC), clay minerals-biochar composites (CMBC), and carbonaceous materials-biochar composites (CBC) on soil organisms are highlighted for the first time. And possible mechanisms are presented based on the different characters of biochar-based materials as well as various environmental interactions. Finally, the bottleneck and challenges of risk assessment of biochar-based materials as well as future prospects are proposed. This work not only promotes the development of risk assessment system of biochar-based materials, but broadens the strategy for the design and optimization of environmental-friendly biochar materials.

Effect of Biochar Application Depth on a Former Mine Technosol: Impact on Metal(Loid)s and Alnus Growth

The contamination of soil by potentially toxic elements (PTEs) is a problem resulting from various anthropic activities including the exploitation of mines, which determines an accumulation of metal(loid)s in the surrounding area. It is therefore necessary to use remediation techniques to prevent the potential damage to human health and the ecosystem. One of these techniques is phytoremediation, which involves the revegetation of contaminated areas in such a way as to reduce the spread of contaminants and entry into the groundwater by stabilizing the metal(loid)s in the soil, decreasing their mobility. To increase the ability of plants to grow under the extreme conditions of contaminated soils, it is necessary to use amendments, which can also intervene directly in reducing the mobility of contaminants. In this study, an open-field mesocosm was set up using a former mining technosol contaminated mainly by As. A biochar produced from hardwood was added at two different depths to evaluate the effectiveness of these application modalities for an overall observation duration of 17 months. Iron sulphate was also applied in both non-biochar and biochar amended conditions. In addition, trees of Alnus sp. were planted to examine the effectiveness of these plants for their use in soil remediation and the effect of the treatments used. The results showed an increase in soil pH induced by the biochar, which decreased over time. During the period examined, the application of biochar in the deepest layer was able to retain As more effectively. The Alnus sp. showed similar growth rates among the various treatments, resulting from its tolerance towards arsenic.

Physiological and molecular responses of flax (Linum usitatissimum L.) cultivars under a multicontaminated technosol amended with biochar

Soil pollution is a worldwide issue and has a strong impact on ecosystems. Metal(loid)s have toxic effects on plants and affect various plant life traits. That is why metal(loid) polluted soils need to be remediated. As a remediation solution, phytoremediation, which uses plants to reduce the toxicity and risk of polluted soils, has been proposed. Moreover, flax (Linum usitatissimum L.) has been suggested as a potential phytoremediation plant, due to its antioxidant systems, which can lower the production of reactive oxygen species and can also chelate metal(loid)s. However, the high metal(loid) toxicity associated with the low fertility of the polluted soils render vegetation difficult to establish. Therefore, amendments, such as biochar, need to be applied to improve soil conditions and immobilize metal(loid)s. Here, we analyzed the growth parameters and oxidative stress biomarkers (ROS production, membrane lipid peroxidation, protein carbonylation and 8-oxoGuanine formation) of five different flax cultivars when grown on a real contaminated soil condition, and in the presence of a biochar amendment. Significant correlations were observed between plant growth, tolerance to oxidative stress, and reprogramming of phytochemical accumulation. A clear genotype-dependent response to metal(loid) stress was observed. It was demonstrated that some phenylpropanoids such as benzoic acid, caffeic acid, lariciresinol, and kaempferol played a key role in the tolerance to the metal(loid)-induced oxidative stress. According to these results, it appeared that some flax genotypes, i.e., Angora and Baikal, could be well adapted for the phytoremediation of metal(loid) polluted soils as a consequence of their adaptation to oxidative stress.

The Potential of Clover Green Amendment, Associated with Biochar, Activated Carbon or Ochre, for the Phytoremediation, Using Populus x. canescens, of a Former Mine Technosol

Metal(loid) soil pollution resulting from past and present mine activities is a serious environmental and health issues worldwide. Therefore, the remediation of those polluted areas has been a growing research interest over the last decades, especially the assisted phytoremediation. In this study, a pot experiment was set up, using a former mine technosol, highly polluted by As and Pb, to which biochar, activated carbon, or ochre was applied, alone or in combination to clover green amendment. Following amendment application, Populus x. canescens cuttings were planted. Results showed that all four amendments reduced soil acidity. However only the first three amendments immobilized As and Pb, while the green amendment drastically mobilized those two pollutants and none of the amendments improved plant growth. In conclusion, the association of clover green amendment to biochar, activated carbon, or ochre did not appear as an efficient remediation strategy in this case; although the aging of the amendments and degradation of the green amendment in the soil with time could have positive outcomes.

Effect of biochar, iron sulfate and poultry manure application on the phytotoxicity of a former tin mine

In phytomanagement approach the application of a combination of amendments is an option for remediating arsenic polluted areas and valorized biomass obtained. Various amendments can be used. Biochar has been shown to reduce metal(loid) availability, and increase soil fertility, while iron sulfate has a considerable As binding capacity, and poultry manure is a source of nutrients. A phytotoxicity test was performed by applying the three amendments (2% biochar, 0.15%, 0.30% and 0.45% iron sulfate and 0.4% poultry manure) to a former tin mine technosol, to investigate their effects on (i) soil pore water properties, (ii) metal(loid) immobilization and (iii) Phaseolus vulgaris L. growth, used as a bioindicator. Biochar addition alone did not affect soil properties or plant parameters. However, the addition of iron sulfate acidified the soil, decreased soil pore water As concentrations, and increased the ones of Fe and Pb. It also improved plant growth, and reduced As and Pb aerial and root concentrations. Finally, the addition of poultry manure had no effect on soil and plants. Based on our results, the combination of iron sulfate with biochar may be a solution for reducing soil toxicity of the Abbaretz mining technosol, improving its fertility, and thus ameliorating plant growth.Novelty statement:The work presented in this manuscript describes the effect of amendment application, i.e., biochar, chicken dung and/or iron sulfate, on soil properties, metals availability and dwarf bean growth, plant used as bioindicator.Our results showed that the combination of a low amount of iron sulfate with biochar is the strategy to reduce soil toxicity, improved its fertility and consequently authorizes plant growth.This study is one of the first describing the effects of combined amendments on a mining soil properties with focusing on metal(loid) mobility.

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.

Application of amendments for the phytoremediation of a former mine technosol by endemic pioneer species: alder and birch seedlings

Metal(loid) pollution of soils has important negative effects on the environment and human health. For the rehabilitation of these soils, some eco-innovative strategies, such as phytoremediation, could be chosen. This practice could establish a plant cover to reduce the toxicity of the pollutants and stabilize the soil, preventing soil erosion and water leaching; this technique is called phytoremediation. For this, plants need to be tolerant to the pollutants present; thus, phytoremediation can have better outcomes if endemic species of the polluted area are used. Finally, to further improve phytoremediation success, amendments can be applied to ameliorate soil conditions. Different amendments can be used, such as biochar, a good metal(loid) immobilizer, compost, a nutrient-rich product and iron sulfate, an efficient arsenic immobilizer. These amendments can either be applied alone or combined for further positive effects. In this context, a mesocosm experiment was performed to study the effects of three amendments, biochar, compost and iron sulfate, applied alone or combined to a former mine technosol, on the soil properties and the phytoremediation potential of two endemic species, Alnus sp. and Betula sp. Results showed that the different amendments reduced soil acidity and decreased metal(loid) mobility, thus improving plant growth. Both species were able to grow on the amended technosols, but alder seedlings had a much higher growth compared to birch seedlings. Finally, the combination of compost with biochar and/or iron sulfate and the establishment of endemic alder plants could be a solution to rehabilitate a former mine technosol.

Effect of different tissue biochar amendments on As and Pb stabilization and phytoavailability in a contaminated mine technosol

Phytomanagement of metal(loid) contaminated soils is an important study of research nowadays. However, such process often requires the application of amendments, i.e. biochar, to improve soil condition and thus permit plant establishment and growth. However, biochar properties and effects on the soil and plants depend on several parameters, for example: feedstock type, particle size, pyrolysis conditions, and application rate. The aim of this study was to assess which tissue from the oak trunk (bark, sapwood, heartwood) was responsible for the positive effects observed in previous studies on biochars derived from wood. A mesocosm experiment was thus set up using a former mine soil, amended or not, using 2% biochars produced from three oak tissues (bark, sapwood, heartwood) and with three particle sizes (0.2-0.4 mm, 0.5-1 mm, 1-2.5 mm). Phaseolus vulgaris plants were used as indicators of toxicity, and were grown for 14 days. Results of soil pore water (SPW) physico-chemical parameters, and plant growth and metal(loid) (As and Pb) accumulation showed a highly significant feedstock effect but no particle size effect. Among the three feedstocks, bark biochars induced greater improvements in the different SPW parameters whereas it was the only tissue increasing plant growth. Therefore, bark seems to be the best trunk part to produce a biochar that will immobilize mainly Pb compared to As.

Potential use of biochar, compost and iron grit associated with Trifolium repens to stabilize Pb and As on a multi-contaminated technosol

Vegetation cover can be used in the phytomanagement of polluted areas by adding value to abandoned sites and reducing the dispersion of pollutants by erosion. Appropriate amendments, that allow both efficient plant growth and the immobilization of contaminants in the soil must be chosen in order to optimize the efficiency of this process. We used a mining technosol mainly contaminated by arsenic (1068 mg kg^-1) and lead (23387 mg kg^-1) to study the effect of three amendments (biochar, compost and iron grit) on (i) physico-chemical properties of the soil and soil pore water, (ii) metal(loid) mobility, bioavailability and bioaccessibility (CaCl₂ and Simple Bioaccessibility Extraction Test (SBET)), and (iii) the capability of Trifolium repens to germinate and grow. All the amendments used increased the pH and electrical conductivity of the SPW, resulting in a 90% decrease in the concentration of lead in the soil pore water (SPW). We also demonstrated a decrease in Pb phytoavailability. The amendments allowed the establishment of a plant cover, although the addition of iron grit alone did not allow any clover germination. For the Pontgibaud technosol, the combination of the three amendments resulted in a significant decrease in As and Pb concentrations in clover tissues, mainly in the aerial organs. The amendments also made it possible for some of them to halve the phytoavailable fraction of arsenic. However, for compost, both the As concentrations in the SPW, and the bioavailable fraction of As increased. All the amendments used had contrasting effects on the bioaccessible fractions of metal(loid)s. The most efficient amendment combination was the addition of 5% biochar and 5% compost.

Capability of amendments (biochar, compost and garden soil) added to a mining technosol contaminated by Pb and As to allow poplar seed (Populus nigra L.) germination

The germination capacity of poplar seeds has never been studied in the context of metal(loid)-contaminated soils, even though poplars are present over a vast geographical area. In this study, black poplar seeds from the Loire Valley (France) were grown for 28 days in mesocosm on a heavily polluted soil that was subjected to different amendments. This phytomanagement process aimed to allow the revegetation of an As and Pb-contaminated mining soil by adding appropriate amendments, resulting in metal(loid) soil stabilisation and efficient plant growth. The objectives were to evaluate the effect of three amendments (garden soil, compost and biochar) when added alone or combined to a technosol on (i) the soil physicochemical properties, (ii) the mobility of As and Pb in the soil pore water (SPW), (iii) the capacity of poplar seeds to germinate and to grow and (iv) the metal(loid) distribution within the plant organs. The addition of amendments alone or combined allowed a 90% decrease in SPW Pb concentrations, while the arsenic concentrations were between 18 and 416 times higher. However, we were only able to obtain seed germination and plant growth on amended soils. These promising results will allow us to explore the use of such amendments in rehabilitating areas that are sources of significant metal(loid) dissemination, as well as allowing a natural plant recolonisation of these sites by seeds from the surrounding environment.