Olive mill waste biochar: a promising soil amendment for metal immobilization in contaminated soils

Sustainable soil management under drought stress through biochar application: Immobilizing arsenic, ameliorating soil quality, and augmenting plant growth

Rise in climate change-induced drought occurrences have amplified pollution of metal(loid)s, deteriorated soil quality, and deterred growth of crops. Rice straw-derived biochars (RSB) and cow manure-enriched biochars (CEB) were used in the investigation (at doses of 0%, 2.5%, 5%, and 7.5%) to ameliorate the negative impacts of drought, improve soil fertility, minimize arsenic pollution, replace agro-chemical application, and maximize crop yields. Even in soils exposed to severe droughts, 3 months of RSB and CEB amendment (at 7.5% dose) revealed decreased bulk density (13.7% and 8.9%), and increased cation exchange capacity (6.0% and 6.3%), anion exchange capacity (56.3% and 28.0%), porosity (12.3% and 7.9%), water holding capacity (37.5% and 12.5%), soil respiration (17.8% and 21.8%), and nutrient contents (especially N and P). Additionally, RSB and CEB decreased mobile (30.3% and 35.7%), bio-available (54.7% and 45.3%), and leachable (55.0% and 56.5%) fractions of arsenic. Further, pot experiments with Bengal gram and coriander plants showed enhanced growth (62-188% biomass and 90-277% length) and reduced arsenic accumulation (49-54%) in above ground parts of the plants. Therefore, biochar application was found to improve physico-chemical properties of soil, minimize arsenic contamination, and augment crop growth even in drought-stressed soils. The investigation suggests utilisation of cow manure for eco-friendly fabrication of nutrient-rich CEB, which could eventually promote sustainable agriculture and circular economy. With the increasing need for sustainable agricultural practices, the use of biochar could provide a long-term solution to enhance soil quality, mitigate the effects of climate change, and ensure food security for future generations. Future research should focus on optimizing biochar application across various soil types and climatic conditions, as well as assessing its long-term effectiveness.

Biochar and soil properties affect remediation of Zn contamination by biochar: A global meta-analysis

Zinc (Zn) is one of the most common heavy metals that pollute soils and can threaten both environmental and human health. Biochar is a potential solution for remediating soil Zn contamination. This meta-analysis investigates the effect of biochar application on the remediation of Zn-contaminated soils and the factors affecting the remediation efficiency. We found that biochar application in Zn-contaminated soils reduced Zn bioavailability by up to 77.2% in urban soils, 55.1% in acidic soils, and 50.8% in coarse textured soils. Moreover, the remediation efficiency depends on the biochar production condition, with crop straw and sewage sludge feedstocks, high pyrolysis temperature (450-550 °C), low heating rate (<10 °C min-1), and short residence time (<180 min) producing high performing biochars. Biochar affects soil Zn bioavailability by changing soil pH and organic carbon, as well as through its high surface area, ash content, and O-containing surface functional groups. Our findings highlight the role of biochar as a promising and environmentally friendly material for remediating Zn contamination in acidic and/or coarse textured soils. We conclude that soil properties must be considered when selecting biochars for remediating soil Zn contamination.

Garbage to Gains: The role of biochar in sustainable soil quality improvement, arsenic remediation, and crop yield enhancement

Threats of soil quality deterioration and metal pollution have inflicted several parts of the world, apart from the need for surplus crop production. The investigation used biochar prepared from waste biomasses such as wheat and rice straw, kitchen waste, leaf litter, Lantana camara, orange peel, and walnut shell to improve soil quality, reduce As pollution, and enhance plant growth. Biochars were amended at doses of 0%, 2.5%, 5%, and 7.5% and conditioned for 3 months. At a 7.5% dose, the maximum improvements in cation exchange capacity (a 62% increase), anion exchange capacity (a two-fold increase), bulk density (a 31% decrease), porosity (a 32% increase), water holding capacity (an 86% increase), soil respiration (a 32% increase), total carbon (a two-and-a-half-fold increase), total nitrogen (an eleven-fold increase), total phosphorus (3 times rise), total potassium (a two-and-a-half-fold increase), mobile As (a 38% decrease), leachable As (a 53% decrease), and bio-available As (a 56% decrease) were observed. Further, pot experiments revealed augmented biomass growth (61% and 177%), increased length (71% and 209%), and decreased As accumulation (56% and 55%) in the above-ground parts of Bengal gram and coriander plants, respectively. Therefore, the application of biochar was found to enhance the physico-chemical properties of soil, reduce As contamination levels, and improve crop growth. The study recommends using waste biomasses to prepare eco-friendly biochars, which could contribute to advancing sustainable agriculture and the circular economy.

Influence of biochar produced from negative pressure-induced carbonization on transformation of potentially toxic metal(loid)s concerning plant physiological characteristics in industrially contaminated soil

Soil contamination and its subsequent impact on the food chain is a pressing challenge in the present day. The application of biochar has demonstrated a significant and positive effect on soil health, thereby enhancing plant growth and development. However, the application of biochar (BC) produced from negative pressure-induced carbonization to mitigate metal(loid) contamination is a new strategy that has been studied in current research. Results depicted that the application of biochar derived from the negative pressure carbonization (vacuum-assisted biochar (VBC) has a significant (p ≤ 0.05) positive impact on plant growth and physiological characteristics by influencing immobilization and speciation of metal(loid) in the soil system. Moreover, the interactive effect of VBC on physiological characteristics (photosynthesis, gas exchange, and chlorophyll contents) and antioxidant activities of maize (Zea mays L.) was significantly (p ≤ 0.05) positive by confining the translocation and movement of metal(loid)s to the aerial part of the maize plant. X-ray diffraction (XRD) provided information on the structural and chemical changes induced by the VBC-500 °C explaining metal(loid) adsorption onto mineral surfaces and complexation that can affect their mobility, availability, and toxicity in the contaminated soil. Fourier transform infrared spectroscopy (FTIR) further provided a more detailed understanding of the metal(loid)s and biochar complexation mechanisms influenced by VBC-based functional groups -OH, C-Hn, -COOH, CO, C-O-C, CC, C-O, C-H, OH, and C-C in the binding process. These results suggest that the application of biochar prepared at 500 °C under negative pressure-induced carbonization conditions to the soil is the most efficient way to reduce the uptake and transfer of metal(loid)s by influencing their mobility and availability in the soil-plant system.

Biochar application for greenhouse gas mitigation, contaminants immobilization and soil fertility enhancement: A state-of-the-art review

Rising global temperature, pollution load, and energy crises are serious problems, recently facing the world. Scientists around the world are ambitious to find eco-friendly and cost-effective routes for resolving these problems. Biochar has emerged as an agent for environmental remediation and has proven to be the effective sorbent to inorganic and organic pollutants in water and soil. Endowed with unique attributes such as porous structure, larger specific surface area (SSA), abundant surface functional groups, better cation exchange capacity (CEC), strong adsorption capacity, high environmental stability, embedded minerals, and micronutrients, biochar is presented as a promising material for environmental management, reduction in greenhouse gases (GHGs) emissions, soil management, and soil fertility enhancement. Therefore, the current review covers the influence of key factors (pyrolysis temperature, retention time, gas flow rate, and reactor design) on the production yield and property of biochar. Furthermore, this review emphasizes the diverse application of biochar such as waste management, construction material, adsorptive removal of petroleum and oil from aqueous media, immobilization of contaminants, carbon sequestration, and their role in climate change mitigation, soil conditioner, along with opportunities and challenges. Finally, this review discusses the evaluation of biochar standardization by different international agencies and their economic perspective.

Mitigation of OMW toxicity toward Enchytraeus albidus with application of additives

The olive oil industry generates considerable amounts of olive mill wastewater (OMW) which is treated and used in agriculture, energy production, or discharged into evaporating ponds where OMW contaminated soil (OMWS) is formed. Due to the extremely high phenol content, untreated OMWS is not suitable for plants and soil organisms. This study aimed to determine the adverse effects of OMWS on Enchytraeus albidus and whether the addition of clay and diatomaceous earth can reduce these effects. Without additives, reproduction was reduced up to 35%, with high number of unhatched cocoons, indicated hatching impairment. Furthermore, acetylcholinesterase (AChE) activity decreased significantly at the highest OMWS ratio (40%), as did glutathione-S-transferase (GST) activity at two ratios (40%), indicating neurotoxic effects and oxidative stress. The application of additives (clay and diatomaceous earth) decreased phenol concentration up to 35 and 45%, respectively. Consequently, the number of juveniles increased significantly compared to the control and AChE and GST activities were not affected. However, an increased number of unhatched cocoons and lipid peroxidation were observed after diatomaceous earth treatment, indicating the occurrence of stress. Although additives can bind phenols, neutralize harmful effects and allow the use of OMWS as a fertilizer in agriculture, they should be carefully selected taking into account soil organisms as well. The use of additives to reduce the adverse effects of OMWS can be a turning point in the remediation of olive contaminated soil.

Effects of compost, biochar and ash mixed in till soil cover of mine tailings on plant growth and bioaccumulation of elements: A growing test in a greenhouse

Mine closures necessitate vegetation restoration to cover tailings fields and reduce environmental risks. Sole use of forest soil as growth medium provides only low fertility and slow plant growth especially in the harsh boreal climate conditions. This preliminary study examines the feasibility of recyclable waste materials added to forest till soil for improving vegetation success on reclaimed mine tailings. One compost type, three biochar types (Bc1-3) and two ash types (Ash1-2) were studied for physical and chemical properties as well as their effects on the growth and element accumulation of timothy (Phleum pratense L.), white clover (Trifolium repens L.) and Scots pine (Pinus sylvestris L.) during one growing period in a greenhouse. Oxidized surface tailings soil and Ash2 were the finest media components while compost and Ash1 were the coarsest. Tailings soil also had the highest salt contents and electrical conductivity, while in till soil they were at the lowest levels. Timothy and white clover germinated well in moist pure tailings soil but grew poorest in it. White clover grew poorly also in pure till soil. Best biomass growth was in the mixture of till, compost and Bc2 (from sewage sludge and woodchips). Planted pine seedlings grew relatively well in all media during the first growing season but Ash1 (from wood and peat) tended to promote height growth and pure till soil root biomass. In media containing Ash1, pine tissues accumulated B, Ca, Mg, K, Na and S. Elevated As content in tailings soil accumulated in plant shoot tissues slightly; only in the old needles of pine were As levels elevated. The results suggest that till and tailings media with compost added as a nitrogen source can promote adequate plant growth during initial growing seasons. Suitable types of biochar and ash amendments can further expedite plant establishment.

Comparative efficacy of raw and HNO3-modified biochar derived from rice straw on vanadium transformation and its uptake by rice (Oryza sativa L.): Insights from photosynthesis, antioxidative response, and gene-expression profile

Low concentrations of vanadium (V) are essential for various plant species but it becomes toxic to plants, animals, and humans at high levels. A significant amount of V is currently being emitted into the atmosphere due to intensified industrial processing. Therefore, this study aimed at evaluating the effect of raw (BC) and HNO₃-modified biochar (OBC) derived from rice straw on growth, photosynthetic assimilation, relative chlorophyll content, SPAD index, ion leakage, enzyme activities, hydrogen peroxide (H₂O₂), bioavailability and V uptake by rice in a laboratory-scale experiment. Characterization of OBC and BC by FTIR (Fourier transform infrared spectroscopy), SEM (scan electron microscopy), BET (Brunauer-Emmett-Teller), elemental analysis, and z-potential revealed a substantial difference between both of them. The V-stress significantly reduced the rice plant growth, biomass yield, chlorophyll parameters, root length and surface area. Under V-stress conditions, root accumulated more V than shoots and OBC significantly improved the above-mentioned parameters, while, decreasing hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) levels in plants. The antioxidant function and gene expression levels induced by V-stress and OBC application further increased the expression profile of three genes (SOD, POD, and CAT) encoding antioxidant enzymes and one metal-tolerant conferring gene (OsFSD1). In summary, these results demonstrated the critical role of OBC in mitigating the detrimental effects of high V-stress on rice growth and enhancing plant defence against V-stress.

Effect of Commercial Amendments on Immobilization of Arsenic, Copper, and Zinc in Contaminated Soil: Comprehensive Assessing to Plant Uptake Combined with a Microbial Community Approach

Identifying the proper chemical and biological materials as soil amendments is a great concern because they replace soil properties and subsequently change the soil quality. Hence, this study was conducted to evaluate the effects of a diverse range of soil amendments including bentonite (B), talc (T), activated carbon (AC), and cornstarch (CS) in form of sole and composite on the immobilization and bioavailability of As, Cu, and Zn. The amendments were characterized by SEM, FT-IR, and XRF, and applied at 2% (w/w) in the experimental pots with an Asteraceae (i.e., lettuce) for 45 days to monitor plant growth parameters and soil microbial community. Soil pH from 6.1 ± 0.02 significantly increased in the amended soils with the maximum value found for TAC (7.4 ± 0.04). The results showed that soil amendments reduced easily in an exchangeable fractionation of As, Cu, and Zn with the maximum values found for BAC by 66.4%, AC by 84.2%, and T by 89.7% respectively. Adding B, T, AC, and their composites induced dry biomass of lettuce >40 wt.%, while CS and its composites did not affect the dry biomass of the plant. The average content of Cu and Zn in plant tissues decreased >45 wt.% in B, AC, and their composites amended soils; meanwhile, AC and its composites mitigated As uptake by >30 wt.% in lettuce. The results of Biolog Ecoplate showed that the amending soils improved the microbial community, especially for composites (e.g., TCS). The results demonstrated that adding composites amendments provided an efficient method for the immobilization of metals and metalloids, and also induced plant growth parameters and microbial community.

The rhizosphere of Salix viminalis plants after a phytostabilization process assisted by biochar, compost, and iron grit: chemical and (micro)-biological analyses

Amendments, such as biochar, compost, and iron grit, used in phytostabilization studies, showed positive effects on soil physico-chemical properties, plant growth, and the microbial community. However, assisted phytostabilization studies do not always focus on the rhizosphere area where soil, plants, and microorganisms are affected by the amendments and plants and microorganisms can also interact with each other. The aims of this study were to evaluate the effects of amendment application on the exudation of organic acids by Salix viminalis plant roots, as well as the effects of amendments and plant development on the soil CHNS contents and the microbial community activity and diversity, assessed by measuring enzyme activities and using Biolog EcoPlates^TM tests and next-generation sequencing analyses. The results of the mesocosm experiment showed that soil C, H, and N contents were increased by amendment application, especially biochar and compost, while the one of S decreased. Enzyme activities, microbial activity, and diversity were also increased by the addition of amendments, except iron grit alone. Finally, the quantity of organic acids exuded by roots were little affected by amendments, which could in part explain the reduced effect of plant development on soil chemical and microbiological parameters. In conclusion, this study showed in particular that biochar and compost were beneficial for the soil CHN contents and the microbial community while affecting poorly Salix viminalis root exudates.

Combined passivators regulate the heavy metal accumulation and antioxidant response of Brassica chinensis grown in multi-metal contaminated soils

Passivation of heavy metals is one of the most efficient techniques to remediate soil pollution. However, passivators with single component are usually unsatisfactory in the case of multi-metal contaminated soils. To resolve this problem, a series of combined passivators containing different ratios of Fe-Mn ore, Fe powder, zeolite, bentonite, etc. were designed and used to study their effects on the growth, heavy metal accumulation, and the antioxidant response of Chinese cabbage (Brassica chinensis L.) as well as the soil available forms of heavy metals in a copper refinery's multi-metal (As, Cd, Pb, Cu) contaminated yellow-brown soil and an artificially contaminated (As, Cd, Pb, Cu) calcareous alluvial soil. The results showed that compared with the control, the addition of combined passivators significantly promoted cabbage growth, with the biomass increase up to 1.77 and 3.54 times in yellow-brown soil and calcareous alluvial soil, respectively. The activity of antioxidant enzymes (SOD, CAT, POD) and the content of malondialdehyde (MDA) and glutathione (GSH) decreased, while the chlorophyll content increased significantly, as compared with no passivators. In addition, passivator application decreased As, Cd, Pb, and Cu contents in shoots and roots by 34.8%, 45.6%, 34.9%, and 11.1% and 49.2%, 63.8%, 38.6%, and 46.4% in yellow-brown soil and by 29.8%, 27.3%, 26.8%, and 25.5% and 45.8%, 55.2%, 61.8%, and 5.7% in calcareous alluvial soil, respectively. Besides, the content of soil available heavy metals was reduced by 8.0-17.1% in yellow-brown soil and 3.3-19.1% in calcareous alluvial soil after the application of passivators. The results indicated that the combined passivators formulated in this experiment could efficiently reduce the content of the multi-metals in cabbage and relieve the oxidant stress and could be used as a way to remediate multi-metal polluted soils.

Microbial diversity and activity assessment in a 100-year-old lead mine

Mining activities frequently leave a legacy of residues that remain in the area for long periods causing the pollution of surroundings. We studied on a 100 year-old mine, the behavior of potentially toxic elements (PTEs) and their ecotoxicological impact on activity and diversity of microorganisms. The PTEs contamination assessment allowed the classification of the materials as highly (reference- and contaminated-samples) and very highly polluted (illegal spill of olive mill wastes (OMW), tailings, and dumps). OMW presented the lowest enzymatic activities while tailings and dumps had low dehydrogenase and arylsulfatase activities. All the α-diversity indices studied were negatively impacted in dumps. Tailings had lower Chao1 and PD whole tree values as compared to those of reference-samples. β-diversity analysis showed similar bacterial community composition for reference- and contaminated-samples, significantly differing from that of tailings and dumps. The relative abundance of Gemmatimonadetes, Bacteroidetes, and Verrucomicrobia was lower in OMW, tailings, and dumps as compared to reference-samples. Fifty-seven operational taxonomic units were selected as responsible for the changes observed between samples. This study highlights that assessing the relationship between physicochemical properties and microbial diversity and activity gives clues about ongoing regulating processes that can be helpful for stakeholders to define an appropriate management strategy.

Mechanism of cadmium removal from soil by silicate composite biochar and its recycling

Biochar added to the soil is generally difficult to separate. In order to solve the problem of separating biochar from soil, this paper applies a hydraulic silicate gel material to the preparation of biochar. Non-magnetic silicate bonded biochar (SBC) and magnetic silicate bonded biochar (MSBC) with hydraulic properties were prepared. The new silicate bonded biochar has good adsorption performance, separation and recovery characteristics. The findings are as follows: (1) after three times of soil remediation, the silicate bonded biochar still had good mechanical properties, and the compressive strength was not attenuated, remaining between 210 and 270 N. (2) After three times of SBC and MSBC remediation, total Cd in soil decreased by 29.33% and 31.82% respectively, and available Cd decreased by 60.82% and 62.74% respectively. (3) After three cycles, the recovery rates of SBC and MSBC both exceeded 94.88%, and the highest adsorption regeneration rates of SBC and MSBC reached 83.09% and 92.06%, respectively. (4) The Cd content of wheat after SBC and MSBC repair was reduced by 29.67-37.36% and 47.25-63.74%, respectively.

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.

Soil Enzyme Activity and Microbial Metabolic Function Diversity in Soda Saline–Alkali Rice Paddy Fields of Northeast China

Western Jilin province has the most serious area of soda salinization in Northeast China, which affects and restricts the sustainable development of agriculture. The effects of physico-chemical properties of rhizosphere and non-rhizosphere soil on soil microbial diversity and enzyme activities (polyphenol oxidase, catalase, invertase, amylase) were evaluated in typical soda saline-alkali paddy field. Community-level physiological profile (CLPP) based on Biolog-ECO plates was used to assess the functional diversity of soil microorganisms. Exchangeable sodium percentage (ESP) and pH were negative correlated with the microbial activity (AWCD), soil enzyme activities (amylase, sucrose, and catalase, except for polyphenol oxidase) in rice rhizosphere and non-rhizosphere soil (P < 0.05). The indexes of microbial diversity in rice rhizosphere soil were significantly higher than that of non-rhizosphere soil. The utilization of amino acids by rice rhizosphere microorganisms was relatively high, while non-rhizosphere soil had relatively high utilization of carboxylic acid, phenolic acid, and amine. Among the selected physico-chemical properties, soil organic carbon (SOC) and soil water content (SWC) had the greatest influence on the variation of microbial diversity indexes and enzyme activities in rhizosphere soil. ESP and pH showed a significant positive correlation with carbon source utilization, especially for amine (AM) and phenolic acid (PA) carbon source utilization (P < 0.05) by means of RDA, and the utilization rate of AM and PA carbon sources by rice rhizosphere and non-root soil microorganisms was P1 < P2 < P3.

The effect of brewery sludge biochar on immobilization of bio-available cadmium and growth of Brassica carinata

Biochar has gained an attention in reducing the bio-availability of toxic heavy metals and minimize threat of entering into food chain from contaminated soil. This study was aimed at evaluating the potential use of brewery sludge biochar (BSB) as a soil amendment for reducing cadmium bio-availability and uptake by Brassica carinata in a pot experiment. In this pot experiment, artificially cadmium spiked, moderately fertile, and slightly basic silty-loam soil was used. The biochar was produced by pyrolyzing of the brewery sludge at 500 °C. The obtained biochar was sieved with 0.5 mm mesh size and applied at the rate of 4 % (w/w) on the Brassica carinata grown cadmium spiked soil. The additions of BSB to the soil contributed a significant reduction of the bio-availability of cadmium in the soil and its accumulation in the shoot of Brassica carinata by 86% and 93%, respectively. Besides, it remarkably increased the dry weight of the edible part of Brassica carinata by 228%. The results revealed that BSB is very effective additive in cadmium immobilization, in turn, significantly (p-value = 0.00) promoting vegetable (Brassica carinata) growth. Therefore, BSB can be used as agricultural soil remedy for cadmium contamination and as safe disposal of brewery sludge.

Performance of the emerging biochar on the stabilization of potentially toxic metals in smelter- and mining-contaminated soils

Soil potentially toxic metals (PTMs) pollution caused by anthropogenic activities has become serious concern with respect to the crop safety production. In this study, an emerging biochar derived from kiwi pruning branches waste was employed as amendment aiming to evaluate its remediation potential on smelter- and mining-contaminated soils. The effect of biochar on the soil physicochemical properties, leachability, and chemical fractions acted on stabilization practice of PTMs in soil was investigated. The results showed that the addition of biochar increased the soil pH, cation exchange capacity, organic matter, and enzymatic activities (dehydrogenase, urease, and sucrase) but reduced the extraction toxicity of PTMs in both smelter (Fengxian, FX) and mining (Tongguan, TG) soils. The fraction analysis showed that the maximum reduction of exchangeable fraction of Cd, Zn, and Pb in the 4% biochar amended soils decreased by 11.1, 13.3, and 24.7% in FX soil and 7.67, 22.8, and 7.89% in TG soil, respectively, in comparison with to control (no biochar added). Additionally, the residual fraction of Cd, Zn, and Pb increased by 55.9, 7.14, and 11.0% in FX soil and 23.7, 5.86, and 10.0% in TG soil, respectively. The further greenhouse experiment showed that the Indian mustard (Brassica juncea) production increased with the increasing application dosages of biochar, while the PTMs uptakes in plant notably decreased after amendments. Conversion of kiwi pruning branches waste into emerging biochar benefits the agricultural waste recycling utilization and enhances PTMs-contaminated soil remediation in practice. Graphical abstract.

Differential effects of three amendments on the immobilisation of cadmium and lead for Triticum aestivum grown on polluted soil

Conventional chemical soil amendments and novel material biochars have been widely reported for the immobilisation of cadmium (Cd) and lead (Pb) in polluted soil. However, information regarding their comparative effectiveness is poor. In the present study, rice husk biochar (RHB) was compared with two chemical soil amendments including hydroxyapatite (HAP) and hydrated lime (HDL) for their effectiveness to enhance plant growth and the reduction of Cd uptake and translocation by Triticum aestivum L. grown in heavy-metal-polluted soil. Compared with control and two chemical soil amendments, RHB rapidly improved wheat growth. The HAP, HDL, and RHB treated plants retained Cd and Pb in roots and restricted their translocation. The RHB treatment had the best effect on growth, yield promotion and the reduction of Cd and Pb in wheat grain. Furthermore, the soils treated with RHB and HAP showed lower DTPA-extracted Cd concentrations, and the maximum reduction was observed in HAP-amended soil. However, the DTPA-extracted Pb concentration was not significantly decreased after the application of two chemical soil amendments for 40 days; the maximum reduction was found in soil treated with RHB for 80 days. In all treatments, Cd in post-harvest soil was mainly present in exchangeable, carbonate bound, and Fe-Mn oxide Cd, while the dominant chemical form of Pb was Fe-Mn oxide Pb. Three soil amendments application decreased exchangeable and organic bound- Cd and Pb levels. HAP and RHB displayed significantly immobilisation for soil Cd and reduced translocation of heavy metal as well as its availability in soil, but the HAP had significant inhibition on growth of wheat in contaminated soil. Therefore, RHB shows a promising potential for the reduction of Cd and Pb bioaccumulation in grains from wheat grown on heavy-metal-polluted soils.

Stabilization of heavy metal-contaminated soils by biochar: Challenges and recommendations

Various types of biochar have been widely used to remediate soil contamination from heavy metals (HMs) and to reduce HM mobility and bioavailability in soils in recent years. Most researchers have paid attention to the beneficial effects of biochar during the remediation process, but few have emphasized their negative effects and the challenges for their application. In this review, the negative effects and challenges of applying biochar for the remediation of HM-contaminated soils are thoroughly summarized and discussed, including the changeable characteristics of biochar, biochar over-application, toxic substances in biochar, activation of some HMs in soils by biochar, nonspecific adsorption, and the negative influences of biochar on soil microorganisms and plants. In addition, further research directions and several recommendations (standardization, long-term field experiments, mechanisms research and designer biochars) were also proposed to enable the large-scale application of biochar for the remediation of HM-contaminated soils.

Long-term stabilization of Cd in agricultural soil using mercapto-functionalized nano-silica (MPTS/nano-silica): A three-year field study

Cadmium (Cd) contamination in agricultural soil is a worldwide environmental problem. In situ stabilization has been considered an effective approach for the remediation of Cd-contaminated agricultural soil. However, information about the long-term effects of amendment on soil properties and stabilization efficiency remains limited. In the present study, mercapto-functionalized nano-silica (MPTS/nano-silica) was used to stabilize Cd in contaminated agricultural soil under field conditions for three years (with application rates of 0%, 0.2%, 0.4%, 0.6%, 0.8% and 1.0%). The application of MPTS/nano-silica reduced the soil aggregate stability (PDA_0.25) (14.8%) and available K (24.9%) and significantly increased the soil dehydrogenase (DHA) (43.4%), yield of wheat grains (33.5%) and Si content in wheat tissues (55.2% in leaf, 50.4% in stem, and 37.7% in husk) (p < 0.05). More importantly, MPTS/nano-silica decreased the leachability (36.0%) and bioavailability (54.3%) of Cd in the soil and transformed Cd into a more stable fraction. The content of Cd in wheat grains decreased by 53.9%, 61.9% and 54.1% in 2017, 2018 and 2019, respectively, in comparison with the control. These results indicated that MPTS/nano-silica has long-term stabilization effects on Cd in agricultural soil and is a potential amendment for the remediation of Cd-contaminated agricultural soils.

Biochar aided aromatic grass [Cymbopogon martini (Roxb.) Wats.] vegetation: A sustainable method for stabilization of highly acidic mine waste

Dumping of acidic mine waste poses severe threats to the ecosystem due to high acidity, nutrient deficiency and mobility of toxic metals. The present study has been undertaken on phytoremediation by amending the acidic soil/mine waste with biochar (BC) and plantation of palmarosa (Cymbopogon martini (Roxb.) Wats. A greenhouse experiment in different combinations of biochar and acidic mine waste was conducted to assess the phytoremediation efficiency of palmarosa by BC amendments. Results indicate that the palmarosa tolerates multiple stresses effectively with a 54 % metal tolerance index (MTI) and capable of reducing acid production from the acidic mine waste alone. BC incorporation in the mine waste and soil treatments significantly enhanced the palmarosa biomass (1.11-3.3 times) and oil content by liming the acid, immobilization of metals and improving the soil quality. BC addition in highly acidic mine waste amplified the phytoremediation efficiency and mitigates abiotic oxidative stress on plants (MTI 84 % to >100 %). BC aided palmarosa plantation shifted the soil from high-risk assessment code (RAC) to low RAC for vegetation. Biochar amendments along with palmarosa plantation offer a sustainable technology for phytostabilization of highly acidic mine waste along with the production of industrially important essential oil.

Enhancing Cadmium Tolerance and Pea Plant Health through Enterobacter sp. MN17 Inoculation Together with Biochar and Gravel Sand

Contamination of soils with heavy metals, particularly cadmium (Cd), is an increasingly alarming environmental issue around the world. Application of organic and inorganic immobilizing amendments such as biochar and gravel sand in combination with metal-tolerant microbes has the potential to minimize the bioavailability of Cd to plants. The present study was designed to identify the possible additive effects of the application of Enterobacter sp. MN17 as well as biochar and gravel sand on the reduction of Cd stress in plants and improvement of growth and nutritional quality of pea (Pisum sativum) plants through the reduction of Cd uptake. Pea seeds were surface sterilized then non-inoculated seeds and seeds inoculated with Enterobacter sp. MN17 were planted in artificially Cd-polluted soil, amended with the immobilizing agents biochar and gravel sand. Application of biochar and gravel sand alone and in combination not only improved the growth and nutritional quality of pea plants by in situ immobilization but also reduced the uptake of Cd by plant roots and its transport to shoots. However, microbial inoculation further enhanced the overall plant health as well as alleviated the toxic effects of Cd on the pea plants. These soil treatments also improved rates of photosynthesis and transpiration. The combined use of biochar and gravel sand with bacterial inoculation resulted in an increase in plant height (47%), shoot dry weight (42%), root dry weight (57%), and 100 seeds weight (49%) as compared to control plants in Cd contaminated soil. Likewise, biochemical constituents of pea seeds (protein, fat, fiber, and ash) were significantly increased up to 41%, 74%, 32%, and 72%, respectively, with the combined use of these immobilizing agents and bacterium. Overall, this study demonstrated that the combined application of biochar and gravel sand, particularly in combination with Enterobacter sp. MN17, could be an efficient strategy for the remediation of Cd contaminated soil. It could support better growth and nutritional quality of pea plants.

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.

Biochar alleviates Cd phytotoxicity by minimizing bioavailability and oxidative stress in pak choi (Brassica chinensis L.) cultivated in Cd-polluted soil

The production of leafy vegetables such as Brassica chinensis L. in cadmium (Cd)-polluted soil causes serious threats to human health and food safety around the globe. A pot culture was established to examine the efficacy of rice-straw induced biochar (applied to soil at the rate of 0%, 2.5% and 5%, w/w) on growth, gaseous exchange attributes, antioxidative capacities and Cd uptake in pak choi (Brassica chinensis L.), when soil was spiked with Cd (CdCl₂) at 0, 5, 10 and 20 mg kg^-1 soil. The results revealed that Cd stress significantly (P < 0.05) reduced plant biomass and physiological attributes, and accumulated higher Cd concentrations in plant tissues with the increasing rate of Cd concentration in the soil. However, incorporation of biochar at 5% application rate prominently increased the shoot (98.27%) and root (85.96%) dry biomass, net photosynthesis (45.52%), transpiration rate (161.34%), stomatal activity (111.76%) and intracellular CO₂ concentration (32.25%) when Cd was added at 20 mg kg^-1 soil, relative to the respective treatment without biochar. Whereas, incorporation of biochar at 5% significantly reduced the bioavailable Cd by 16.64% under 20 mg kg^-1 soil, compared to respective Cd treatment without biochar.Similarly, Cd accumulation in shoots and roots was decreased by 42.49% and 29.23%, and thereby reduced leaf MDA and H₂O₂ contents by 21.45% and 31.28%, respectively, at 20 mg Cd kg^-1 spiked soil relative to without biochar amended soil. An increment was noticed in the activities of guaiacol peroxidase (POD), superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT) and glutathione (GSH) by 37.31%, 66.35%, 115.94%, 122.72% and 59.96%, respectively, with 5% biochar addition in 20 mg kg^-1 Cd spiked soil. Moreover, biochar induced a synergistic impact on plants by increasing soil alkalinization and thereby reducing Cd phytotoxicity throughimmobilization. Overall, results proposed that rice-straw biochar has an ability to restore Cd polluted soil and increased pak choi production and thereby reduced food security risks in polluted soil.

Effects of biochar-immobilized bacteria on phytoremediation of cadmium-polluted soil

This work is the first report of the ability of biochar-immobilized cadmium-resistant bacteria (CRB) on promoting the efficiency of cadmium phytoextraction by Chlorophytum laxum R.Br. The survival of CRB immobilized on biochar in cadmium-contaminated soil at a concentration of 75.45 mg kg^-1 was studied. The results found that both CRB, namely Arthrobacter sp. TM6 and Micrococcus sp. MU1, can survive and grow in cadmium-contaminated soil. To study phytoextraction in the pot experiments, 2-month-old C. laxum was individually planted in cadmium-contaminated soil and divided into four treatments, including (i) untreated control, (ii) biochar, (iii) biochar-immobilized (BC) Arthrobacter sp., and (iv) BC-Micrococcus sp. The results found that biochar-immobilized CRB did not cause any effect to the root lengths and shoot heights of plants compared to the untreated control. Interestingly, inoculation of biochar-immobilized CRB significantly increased cadmium accumulation in the shoots and roots compared to the untreated control. In addition, the highest cadmium content in a whole plant, best phytoextraction performance, and greatest bioaccumulation factor was found in plant inoculated with BC-Micrococcus sp., followed by BC-Arthrobacter sp. In conclusion, inoculation of biochar-immobilized CRB enhanced cadmium accumulation and translocation of cadmium from the roots to shoots, suggesting further applying biochar-immobilized CRB in cadmium-polluted soil for promoting cadmium phytoextraction efficiency of ornamental plants. Graphical abstract.

Mineral elements uptake and physiological response of Amaranthus mangostanus (L.) as affected by biochar

Amaranthus mangostanus L. (amaranth) was hydroponically grown in different concentrations of biochar amended nutrient solution to investigate the mineral elements migration and physiological response of amaranth as affected by biochar. Our results showed that exposure to 26.6 g/L of biochar greatly increased the root and shoot K, Na and Al content, while 2.6 g/L of biochar greatly increased the root Ca and Mg content. The uptake of K and Al notably altered other elements' accumulation in shoots and roots upon the biochar exposure. The ratio of Ca: K in shoots and Mg: K in roots were negatively correlated to the biochar concentrations, while the ratio of Al: Ca and Al: Mg in roots were positively related to the biochar concentrations. The Al: Fe ratio was also polynomial correlated to the concentrations of biochar. The addition of biochar beyond 2.6 g/L resulted in the cell membrane and DNA damages in roots. The activity of SOD and CAT in 6.7 g/L biochar treated roots was significantly elevated as compared to the ones in other biochar treatments and was almost 2-fold of the control. The photosynthetic F_v/F_m intensity and subcellular structure in leaves were also compromised upon exposure to 26.6 g/L biochar. Taken together, biochar could significantly alter the mineral migration in amaranth and physiologically damage in the plants. It is essential to study the effect of biochar within appropriate concentrations on plants prior to wide application in agriculture.

Effects of Passivating Agents on the Availability of Cd and Pb and Microbial Community Function in a Contaminated Acidic Soil

A 3-year pot experiment was carried out to investigate the efficiencies of hydroxyapatite (H), thiol-functionalized bentonite (T) and biochar (B) alone or in combination in remedying a Cd-Pb-contaminated soil. The application of passivating agents reduced the Cd and Pb mobility in acidic soil and enhanced soil microbial community function. The largest reductions in the Cd and Pb acid-soluble portions were observed under H (33.49%, 37.37%) and hydroxyapatite + thiol-functionalized bentonite + biochar (HTB, 36.70%, 37.31%), respectively. Biological analysis indicated that the AWCD (average well color development) of the B and HTB amendments was 1.42 and 1.51 times higher, respectively, than of untreated soil at 192 h. Moreover, the Shannon-Wiener, Simpson and Pielou indices were significantly increased in these two treatments relative to the values in the other amendment treatments. Therefore, combination amendments, such as HTB, which can reduce the bioavailability of both Cd and Pb and increase soil microbial activity, are recommended for practical applications.

Using enzyme activities and soil microbial diversity to understand the effects of fluoxastrobin on microorganisms in fluvo-aquic soil

Whether now or in the foreseeable future, agricultural production cannot do without chemical pesticides. Strobilurin fungicides, including mitochondrial inhibitors, can easily enter the soil environment and causing pollution. Fluoxastrobin is one of the top-selling strobilurin fungicides. Limited information can be found in the literature on the environmental toxicity of fluoxastrobin in soil. It is essential to understand the effects of fluoxastrobin on microorganisms in fluvo-aquic soil. The present study used a Biolog-EcoPlate method to analyze the effects of fluoxastrobin on soil microbial diversity in groups exposed to 0.1, 1.0, and 2.5 mg/kg fluoxastrobin and in control groups at exposure and on days 7, 14, 21, 28, and 48. The effects of fluoxastrobin on enzyme activities (β-glucosidase, dehydrogenase, and urease) in the soil were also evaluated. The results indicated that the three tested enzyme activities were lower at each dose to a varying degree than those of the controls. The soil microbial diversity was shown to be affected according to the results of average well color development (AWCD) experiments. The present study aimed to assess environmental risks to terrestrial ecosystems under the stress of pesticide use. These results also enrich the basic data on fungicide toxicity to the soil environment and have a guiding significance for the rational use of pesticides and the sustainable utilization of resources.

Biochar effect associated with compost and iron to promote Pb and As soil stabilization and Salix viminalis L. growth

Soil pollution by metal(loid)s is one of the most significant problems in Europe. To remediate and potentially rehabilitate these contaminated sites, phytoremediation procedures are being put into place, often using amendments to help offset the extreme conditions of such soils. The aim of this study was to define the best amendment to use on the field. This was done by studying how the addition of three different amendments (biochar, compost and iron grit), alone or in combination, could affect: (i) soil physico-chemical properties, (ii) Salix viminalis growth, and (iii) metal(loid) stabilization. A 69 day-mesocosm study was thus set up using a former mine technosol, the three amendments applied alone or combined, and S. viminalis cuttings. The results showed that biochar and/or compost improved the soil fertility and the soil pore water characteristics, with reductions of acidity, metal(loid) mobility and toxicity, while iron grit amendment presented negative effects on such parameters. Such ameliorations allowed better plant growth and higher biomass production. In addition, stress indicators (leaf pigment content and root guaiacol peroxidase activity) showed a reduction in plant stress following biochar and/or compost application. Finally, among the different treatments, the use of compost or a biochar-compost combination showed better results in terms of improvement of soil conditions, increase in plant growth and reduced translocation of metal(loid)s towards upper parts, making these two treatments a valuable option for a field trial.

Evaluating toxicity of 1-octyl-3-methylimidazolium hexafluorophosphate to microorganisms in soil

Ionic liquids (ILs) were widely applied because of their excellent properties. The present investigation studied the toxicity of the IL 1-octyl-3-methylimidazolium hexafluorophosphate ([Omim]PF₆) to the soil microbial population and community diversity with dose (1.0, 2.0, 4.0, 6.0, and 8.0 mg kg^-1) and exposure time (7, 10, and 13 d). The results show the IL was stable during the entire experimental period. The Biolog-ECO plate results indicated that the average well color development (AWCD) in the 6.0 and 8.0 mg kg^-1 treatments was lower than these in the other treatments. The diversity indices of the Biolog analysis were significantly reduced. The abundance of the ammonia-oxidizing archaea (AOA-) and the ammonia-oxidizing bacteria (AOB-) ammonia monooxygenase (amoA) genes was measured by the real-time polymerase chain reaction (RT-PCR). In the treatments of 4.0, 6.0 and 8.0 mg kg^-1, the abundance of amoA genes of the AOA- and AOB- were inhibited by IL [Omim]PF₆.

Biochars from olive mill waste have contrasting effects on plants, fungi and phytoparasitic nematodes

Olive mill waste (OMW), a byproduct from the extraction of olive oil, causes serious environmental problems for its disposal, and extensive efforts have been made to find cost-effective solutions for its management. Biochars produced from OMW were applied as soil amendment and found in many cases to successfully increase plant productivity and suppress diseases. This work aims to characterize biochars obtained by pyrolysis of OMW at 300 °C to 1000 °C using ¹³C NMR spectroscopy, LC-ESI-Q-TOF-MS and SEM (Scanning Electron Microscopy). Chemical characterization revealed that biochar composition varied according to the increase of pyrolysis temperature (PT). Thermal treated materials showed a progressive reduction of alkyl C fractions coupled to the enrichment in aromatic C products. In addition, numerous compounds present in the organic feedstock (fatty acids, phenolic compounds, triterpene acids) reduced (PT = 300 °C) or completely disappeared (PT ≥ 500 °C) in biochars as compared to untreated OMW. PT also affected surface morphology of biochars by increasing porosity and heterogeneity of pore size. The effects of biochars extracts on the growth of different organisms (two plants, one nematode and four fungal species) were also evaluated. When tested on different living organisms, biochars and OMW showed opposite effects. The root growth of Lepidium sativum and Brassica rapa, as well as the survival of the nematode Meloidogyne incognita, were inhibited by the untreated material or biochar produced at 300 °C, but toxicity decreased at higher PTs. Conversely, growth of Aspergillus, Fusarium, Rhizoctonia and Trichoderma fungi was stimulated by organic feedstock, while being inhibited by thermally treated biochars. Our findings showed a pattern of association between specific biochar chemical traits and its biological effects that, once mechanistically explained and tested in field conditions, may lead to effective applications in agriculture.

Response of soil microbial communities to red mud-based stabilizer remediation of cadmium-contaminated farmland

In this work, a field test was conducted to investigate the effects of heavy metal stabilizer addition on brown rice and microbial variables in a cadmium (Cd)-contaminated farmland from April to October in 2016. Compared with the control, red mud-based stabilizer (RMDL) effectively reduced the concentration of Cd in brown rice (with the removal rate of 48.14% in early rice, 20.24 and 47.62% in late rice). The results showed that adding 0.3 kg m^-2 RDML in early rice soil or soil for both early and late rice increased the microbial biomass carbon (MBC), the number of culturable heterotrophic bacteria and fungi, and the catalase activity in soil at different stages of paddy rice growth. Furthermore, there was no notable difference in the diversity of the bacterial species, community composition, and relative abundance at phylum (or class) or operational taxonomic unit (OTU) levels between the control and treatment (RMDL addition) groups. In a word, RMDL could be highly recommended as an effective remediation stabilizer for Cd-contaminated farmland, since its continuous application in paddy soil cultivating two seasons rice soil could effectively decrease the Cd content in brown rice and had no negative impact on soil microorganisms.

Potential of Cassia alata L. Coupled with Biochar for Heavy Metal Stabilization in Multi-Metal Mine Tailings

To explore the effect of different biochars on Cassia alata L. growth and heavy metal immobilization in multi-metal mine tailings, a 100-day pot experiment was conducted. Three biochars derived from Hibiscus cannabinus core (HB), sewage sludge (SB) and chicken manure (MB), were added to mine tailings at rates of 0.4%, 1% and 3% (w/w). The results showed that the root biomass, shoot biomass, plant height and root length were 1.2-2.8, 1.7-3.2, 1-1.5 and 1.6-3.3 times of those in the control group, respectively. Pb, Zn, Cu, Cd and As contents in the shoot decreased by 63.9-89.5%, 46.9-66.0%, 32.7-62.4%, 40.4-76.4% and 54.9-77.5%, respectively. The biochar significantly increased the pH and decreased the mild acid-soluble Pb and Cu concentrations in the mine tailings. Specifically, SB immobilized Pb and Cu better than MB and HB did, although it did not immobilize As, Zn or Cd. Meanwhile, more attention should be paid to the potential As release as the biochar application rate increases. In conclusion, Cassia alata L. coupled with 3% of SB could be an effective measure for restoring multi-metal mine tailings. This study herein provided a promising ecological restoration technique for future practice of heavy metal stabilization in mine tailings.

A critical review on effects, tolerance mechanisms and management of cadmium in vegetables

Cadmium (Cd) accumulation in vegetables is an important environmental issue that threatens human health globally. Understanding the response of vegetables to Cd stress and applying management strategies may help to reduce the Cd uptake by vegetables. The aim of the present review is to summarize the knowledge concerning the uptake and toxic effects of Cd in vegetables and the different management strategies to combat Cd stress in vegetables. Leafy vegetables grown in Cd contaminated soils potentially accumulate higher concentrations of Cd, posing a threat to food commodities. The Cd toxicity decreases seed germination, growth, biomass and quality of vegetables. This reduces the photosynthesis, stomatal conductance and alteration in mineral nutrition. Toxicity of Cd toxicity also interferes with vegetable biochemistry causing oxidative stress and resulting in decreased antioxidant enzyme activities. Several management options have been employed for the reduction of Cd uptake and toxicity in vegetables. The exogenous application of plant growth regulators, proper mineral nutrition, and the use of organic and inorganic amendments might be useful for reducing Cd toxicity in vegetables. The use of low Cd accumulating vegetable cultivars in conjunction with insolubilizing amendments and proper agricultural practices might be a useful technique for reducing Cd exposure in the food chain.

In situ immobilization of Cd by organic amendments and their effect on antioxidant enzyme defense mechanism in mung bean (Vigna radiata L.) seedlings

In situ immobilization of Cd is desirable due to the damaging effects of ex situ remediation techniques on soil. In this greenhouse study, the role of biochar (BC), chitosan (CH), and green waste (GW) was studied for in-situ Cd immobilization and alleviating Cd toxicity in mung bean seedlings. Amendments were applied at rates of 0.5% and 1% (w/w). The minimum mean value of Cd, in root, shoot, and soil (DTPA-Cd) (12.2, 4.7, and 0.7 mg kg^-1, respectively), occurred in the Cd + 1% CH treatment compared to all Cd amended treatments. Shoot dry weight (21%) increased significantly in Cd + 1% BC amended soil compared to the control. Reactive oxygen species were affected significantly, with the lowest increased value of hydrogen peroxide (4%) in the Cd + 1% CH treatment while the minimum increase in the value of superoxide (O₂^•-) occurred in the Cd + 1% BC soil compared to the control. Malondialdehyde (20%) increased lowest with Cd + 1% CH treatment. Protein, ascorbate (AsA) contents, and catalase (CAT) activity increased the most (3, 2, and 15%, respectively) in the Cd + 1% BC treatment while dehydroascorbate reductase (DHAR) and superoxide dismutase (SOD) activity increased the most (9 and 234%, respectively) in the Cd + 1% CH soil compared to the control. Glutathione reductase (GR), ascorbate peroxidase (APX), and glutathione peroxidase (GPX), activity were reduced the most in the Cd + 1.0% BC treatment while dehydroascorbate (DHA) and glutathione S-transferase (GST) activity decreased the most in the Cd + 1% CH soil. Overall, in situ immobilization by amendments improved growth and antioxidant defense mechanisms of mung bean seedlings and was reflected by tolerance to Cd-toxicity.

Using bamboo biochar with compost for the stabilization and phytotoxicity reduction of heavy metals in mine-contaminated soils of China

Anthropogenic activities have transformed the global geochemical cycling of heavy metals (HMs). Many physical, chemical and biological methods are used to reduce the toxicity of HMs to humans, plants and environment. This study aimed to investigate the immobilization and phytotoxicity reduction of HMs after application of bamboo biochar (BB) in mine-polluted soil in Feng county (FC) and Tongguan (TG). The results showed that BB application to contaminated soil immobilized HMs (Zn, Pb, Cd and Cu). The soil pH and EC increased and the bioavailability of HMs decreased in FC and TG, whereas Pb and Cu increased in TG soil. The addition of BB reduced HMs uptake in the shoot/root of Brassica juncea. Physiological responses showed that BB application improved the shoot/root growth, dry biomass, and enhanced the chlorophyll (a and b) and carotenoid concentrations in Brassica. The incorporation of BB improved the soil health and accelerated enzymatic activities (β-glucosidase, alkaline phosphatase and urease) in HMs polluted soils. Antioxidant activities (POD, PPO, CAT and SOD) were also used as biomarkers to determine the negative effects of HMs on the growth of Brassica. Overall, the immobilization potential and phytotoxicity reduction of HMs were confirmed by BCF, TF and MEA for both soils.

Influence of pyrolytic and non-pyrolytic rice and castor straws on the immobilization of Pb and Cu in contaminated soil

Soil contamination with heavy metals has become a global environmental health concern. In the present study, European Community Bureau of Reference (BCR) sequential extraction and toxicity characteristic leaching procedure (TCLP) techniques were used to evaluate the Pb and Cu subsequent transformations, immobilizing impact of pyrolytic and non-pyrolytic rice and castor straws and their efficiency to reduce the metals mobility and leachability in the polluted soil. Obtained results highlight the potential of biochar over non-pyrolytic residues to enhance the immobilization of Pb and Cu in the soil. Castor leaves-derived biochar (CLB), castor stem-derived biochar (CSB), and rice straw-derived biochar (RSB) prominently decreased the mobility (acid-soluble fraction) of Pb 49.8%, 31.1%, and 31.9%, respectively, while Cu decreased 15.8%, 11.5%, and 12%, respectively, as compare to control. Sequential extraction showed that biochar treatments prominently modified the proportioning of Pb and Cu from acid soluble to a less bioavailable fraction and increased the geochemical stability in the polluted soil as compared to relative feedstocks as well as the controlled soil. Additionally, the soil pH increased markedly after the addition of biochar. Compared with control, the TCLP-extractable Pb and Cu were reduced to 29.2-41.4% and 5.7-22.8% from the soil respectively by the application of CLB. The immobilization and reduction in leachability of Pb and Cu were correlated with the soil pH. The biochar effect on the Pb immobilization was much better as compared to Cu in co-contaminated soil. Overall addition of CLB offered the best results and could be effective in both Pb and Cu immobilization thereby reducing their mobility and bioavailability in the co-contaminated soil.

Immobilization of Pb and Cu in polluted soil by superphosphate, multi-walled carbon nanotube, rice straw and its derived biochar

Lead (Pb) and copper (Cu) contamination in croplands pose severe health hazards and environmental concerns throughout soil-food chain transfer. In the present study, BCR, TCLP, CaCl2, and SBET techniques were employed to evaluate the simultaneous effectiveness of rice straw (RS) and its derived biochar (BC), multiwall carbon nanotube (MWCNT), and single superphosphate (SSP) to immobilize the Pb and Cu in co-contaminated soil. The BCR sequential extraction results suggested that with increasing BC and SSP amount, the acid-soluble fractions decreased while oxidizable and residual proportions of Pb and Cu were increased significantly. Compared to SSP, the application of BC amendment substantially modified partitioning of Cu from easily exchangeable phase to less bioavailable residual bound fraction. The immobilized Pb and Cu were mainly transformed to reducible forms. The TCLP and CaCl2-extracted Pb and Cu were reduced significantly by the addition of BC compared to RS and MWCNT, whereas the bio-accessibility of Pb significantly reduced with RS addition. SSP showed better results for Pb immobilization while marginal for Cu in co-contaminated soil. Overall, the addition of BC offered the best results and could be effective in both Pb and Cu immobilization thereby reducing their mobility and bioavailability in the co-contaminated soil.

Effects of biochar and alkaline amendments on cadmium immobilization, selected nutrient and cadmium concentrations of lettuce (Lactuca sativa) in two contrasting soils

To assess the efficiency of seven treatments including biochars produced from dried faecal matter and manures as stabilizing agents of cadmium (Cd)-spiked soils, lettuce was grown in glasshouse on two contrasting soils. The soils used were moderately fertile silty loam and less fertile sandy loam and the applied treatments were 7 % w/w. The reduction of bioavailable Cd (ammonium nitrate extractable) and its phytoavailability for lettuce were used as assessment criteria in the evaluation of stabilization performance of each treatment. Moreover, the agronomic values of the treatments were also investigated. Ammonium nitrate extraction results indicated that faecal matter biochar, cow manure biochar and lime significantly reduced bioavailable Cd by 84–87, 65–68 and 82–91 %, respectively, as compared to the spiked controls. Unpredictably, coffee husk biochar induced significant increment of Cd in NH₄NO₃ extracts. The immobilization potential of faecal matter biochar and lime were superior than the other treatments. However, lime and egg shell promoted statistically lower yield and P, K and Zn concentrations response of lettuce plants compared to the biochar treatments. The lowest Cd and highest P tissue concentrations of lettuce plants were induced by faecal matter and cow manure biochar treatments in both soils. Additionally, the greatest Cd phytoavailability reduction for lettuce was induced by poultry litter and cow manure biochars in the silty loam soil. Our results indicate that faecal matter and animal manure biochars have shown great potential to promote Cd immobilization and lettuce growth response in heavily contaminated agricultural fields.

Mechanisms of biochar-mediated alleviation of toxicity of trace elements in plants: a critical review

Trace elements (TEs) contamination is one of the main abiotic stresses which limit plant growth and deteriorate the food quality by their entry into food chain. In recent, biochar (BC) soil amendment has been widely reported for the reduction of TE(s) uptake and toxicity in plants. This review summarizes the role of BC in enhancing TE(s) tolerance in plants. Under TE(s) stress, BC application increased plant growth, biomass, photosynthetic pigments, grain yield, and quality. The key mechanisms evoked are immobilization of TE(s) in the soil, increase in soil pH, alteration of TE(s) redox state in the soil, and improvement in soil physical and biological properties under TE(s) stress. However, these mechanisms vary with plant species, genotypes, growth conditions, duration of stress imposed, BC type, and preparation methods. This review highlights the potential for improving plant resistance to TE(s) stress by BC application and provides a theoretical basis for application of BC in TE(s) contaminated soils worldwide.

Immobilization of copper and cadmium by hydroxyapatite combined with phytoextraction and changes in microbial community structure in a smelter-impacted soil

Soil heavy metal fraction and microbial community structure can be changed by combined remediation of hydroxyapatite and plants.