Evaluation of biochar pyrolyzed from kitchen waste, corn straw, and peanut hulls on immobilization of Pb and Cd in contaminated soil

Advancements in Biochar for Soil Remediation of Heavy Metals and/or Organic Pollutants

The rapid industrialization and economic growth have exacerbated the contamination of soils with both heavy metals and organic pollutants. These persistent contaminants pose substantial threats to ecosystem integrity and human health due to their long-term environmental persistence and potential for bioaccumulation. Biochar, with its high specific surface area, well-developed pore structure, and abundant surface functional groups, has emerged as a promising material for remediating soils contaminated by heavy metals and organic pollutants. While some research has explored the role of biochar in soil remediation, several aspects remain under investigation. Fully harnessing the potential of biochar for soil contamination remediation is of critical importance. This review provides an overview of the preparation methods and physicochemical properties of biochar, discusses its application in soils contaminated by organic compounds and/or heavy metals, and examines the mechanisms underlying its interaction with pollutants. Additionally, it summarizes the toxicity assessments of biochar during soil remediation and outlines future research directions, offering scientific insights and references for the practical deployment of biochar in soil pollution remediation.

Effects of Biochar and Sepiolite on Pb and Cd Dynamics in Contaminated Soil with Different Corn Varieties

Biochar can stabilize heavy metals in soil and inhibit their accumulation in plants as a soil amendment. Sepiolite has also shown good effects in the remediation of soil heavy metal pollution. In this study, biochar, sepiolite, and biochar-sepiolite combined amendments were used to evaluate the accumulation of cadmium (Cd) and lead (Pb) in soil by 29 corn varieties. The concentrations of Cd and Pb in corn fruits were the lowest (Pb: 0.11 mg/kg, Cd: 0.06 mg/kg). There was a significant difference (p < 0.05) in Pb and Cd accumulation in the roots, stems, leaves, and fruits in the 29 corn varieties. The BCF and TF values of Pb and Cd in the 29 corn varieties were different, and Pb is more likely to accumulate in the roots, Cd is more likely to accumulate in the leaves, and neither heavy metal is easily translocated to the corn fruits. The combination of biochar and sepiolite creates an environment conducive to the retention of heavy metals in the root zone, effectively reducing the risk of heavy metal contamination in the edible parts of the plants. After considering various factors, such as environmental adaptability, we recommend using sepiolite and biochar combined as a soil amendment material and planting the WG1790 variety. Field experiments are needed to verify the effects. These results provide scientific evidence and new strategies for the selection of corn varieties and soil amendments.

Sustainable valorisation of kitchen waste through greenhouse solar drying and microwave pyrolysis- technology readiness level for the production of biochar

This study proposes an integrated and sustainable approach for the effective conversion of kitchen waste into valuable products through a two-step process. The primary step involves the implementation of greenhouse solar drying to reduce the moisture content of kitchen waste. The secondary step implies microwave pyrolysis for effective degradation of kitchen waste to biooil, biogas and biochar. Biooil and biogas can be used as renewable fuel source. Biochar can be used as soil amendment. Selection of atmospheric conditions for biochar preparation is discussed, highlighting its crucial role in biochar characteristics. This article highlights, technology readiness level of biochar production from kitchen waste to assess the economic viability for the scalability of the process. In this entirety, the conversion of kitchen waste to valuable products through microwave pyrolysis has significant potential to address the challenges posed by high moisture content and heterogenous nature. With continued research and innovation, it is possible to develop a wide array of value-added products from kitchen waste, ultimately leading to a more eco-friendly and economic approach to waste management.

Graphical Abstract

Biochar co-compost increases the productivity of Brassica napus by improving antioxidant activities and soil health and reducing lead uptake

Lead (Pb) is a serious toxic metal without any beneficial role in the biological system. Biochar (BC) has emerged as an excellent soil amendment to mitigate Pb toxicity. The impact of BC co-compost (BCC) in mitigating the toxic impacts of Pb has not been studied yet. Therefore, this study aimed to evaluate the potential of BC and BCC in improving the growth, physiological, and biochemical traits of Brassica napus and soil properties and reducing health risks (HR). The study was comprised of different Pb concentrations (control and 100 mg kg-1) and organic amendments (control, BC, compost, and BCC). The results indicated that Pb stress reduced the growth, photosynthetic pigments, seed yield, and oil contents by increasing hydrogen peroxide (H2O2) production and Pb uptake and accumulation in plant tissues and decreasing photosynthetic pigment and nutrient availability. The application of BCC alleviated the adverse impacts of Pb and improved seed production (40.24%) and oil yield (11.06%) by increasing chlorophyll a (43.18%) and chlorophyll b (25.58%) synthesis, relative water content (23.89%), total soluble protein (TSP: 23.14%), free amino acids (FAA: 26.47%), proline (30.98%), APX (40.90%), CAT (32.79%), POD (24.93%), and SOD (33.30%) activity. Biochar co-compost-mediated increase in seed and oil yield was also linked with a reduced accumulation of Pb in plant parts and soil Pb availability and improved the soil-available phosphorus, potassium, total nitrogen, soil organic carbon (SOC), and microbial biomass carbon (MBC). Furthermore, BCC also reduced the bioaccumulation concentration, daily metal intake, hazard index, and target hazard quotient. In conclusion, application of BCC can increase the growth, yield, and oil contents of Brassica napus by improving the physiological and biochemical traits and soil properties and reducing the Pb uptake.

Bio-derived carbon-based materials for sustainable environmental remediation and wastewater treatment

Biosynthesized nanocomposites, particularly those incorporating carbon-based materials, exhibit exceptional tunability and multifunctionality, surpassing the capabilities of conventional materials in these aspects. Developing practical solutions is critical to address environmental toxins from pharmaceuticals, heavy metals, pesticides, and dyes. Biomass waste is a readily available carbon source, which emerges as a promising material for producing biochar due to its inherent advantages: abundance, low cost, and environmentally friendly nature. This distribution mainly uses carbon-based materials (CBMs) and biomass waste in wastewater treatment. This review paper investigates several CBM types, including carbon aerogels, nanotubes, graphene, and activated carbon. The development of bio-derived carbon-based nanomaterials are discussed, along with the properties and composition of carbon materials derived from biomass waste and various cycles, such as photodegradation, adsorption, and high-level oxidation processes for natural remediation. In conclusion, this review examines the challenges associated with biochar utilization, including cost, recovery, and practical implementation.

Effect of biochar and iron ore tailing waste amendments on cadmium bioavailability in a soil and peanut seedling system

Biochar and iron ore tailing waste have been widely separately applied for remediation of various contaminants, but the remediation effect of their combination on cadmium (Cd) pollution is unclear. In this study, the peanut biochar (BC), thermally activated iron ore tailing waste (TS), and the products of the co-pyrolysis of peanut shell and iron ore tailing waste (TSBC) were prepared for stabilizing Cd and reducing its bio-accessibility in soil and peanut seedling system. Present amendments enhanced soil pH, cation exchange capacity, electrical conductivity, and organic carbon content. The application of BC, TS, and TSBC led to decreases in acid-extractable Cd proportion of 2.2-8.81%, 2.43-7.20%, and 7.84-11.57%, respectively, and increases in the residual Cd proportion of 3.48-8.33%, 3.27-11.50%, and 9.02-13.45%, respectively. There were no significant differences in Cd accumulation in peanut roots due to three amendments treatments, especially at low Cd concentrations (i.e., Cd concentration of 0, 1, and 2 mg·kg-1), and with a relatively small reduction (2.16-9.05%) in root Cd accumulation under the high Cd treatments of 5 and 10 mg·kg-1. The Cd concentrations in seedling roots were significantly positively related to the acid-extractable Cd fraction, with a Pearson correlation coefficient of r = 0.999. The maximum toxicity mitigating effects were found in TSBC treatment, with increases in the ranges of 9.80-17.58% for fresh weight, 5.59-14.99% for dry weight, 5.16-10.17% for plant height, 5.96-10.34% for root length, 5.43-21.67% for chlorophyll a content, 17.17-71.28% for chlorophyll b content, and 13.11-39.60% for carotenoid content in peanut seedlings. Therefore, TSBC is a promising amendment for minimizing Cd contamination in peanut crops and utilizing industrial solid waste materials efficiently.

Almond shell-derived biochar decreased toxic metals bioavailability and uptake by tomato and enhanced the antioxidant system and microbial community

The effectiveness of almond shell-derived biochar (ASB) in immobilizing soil heavy metals (HMs) and its impact on soil microbial activity and diversity have not been sufficiently studied. Hence, a pot study was carried out to investigate the effectiveness of ASB addition at 2, 4, and 6 % (w/w) on soil biochemical characteristics and the bioavailability of Cd, Cu, Pb, and Zn to tomato (Solanum lycopersicum L.) plants, as compared to the control (contaminated soil without ASB addition). The addition of ASB promoted plant growth (up to two-fold) and restored the damage to the ultrastructure of chloroplast organelles. In addition, ASB mitigated the adverse effects of HMs toxicity by decreasing oxidative damage, regulating the antioxidant system, improving soil physicochemical properties, and enhancing enzymatic activities. At the phylum level, ASB addition enhanced the relative abundance of Actinobacteriota, Acidobacteriota, and Firmicutes while decreasing the relative abundance of Proteobacteria and Bacteroidota. Furthermore, ASB application increased the relative abundance of several fungal taxa (Ascomycota and Mortierellomycota) while reducing the relative abundance of Basidiomycota in the soil. The ASB-induced improvement in soil properties, microbial community, and diversity led to a significant decrease in the DTPA-extractable HMs down to 41.0 %, 51.0 %, 52.0 %, and 35.0 % for Cd, Cu, Pb, and Zn, respectively, as compared to the control. The highest doses of ASB (ASB6) significantly reduced the metals content by 26.0 % for Cd, 78.0 % for Cu, 38.0 % for Pb, and 20.0 % for Zn in the roots, and 72.0 % for Cd, 67.0 % for Cu, 46.0 % for Pb, and 35.0 % for Zn in the shoots, as compared to the control. The structural equation model predicts that soil pH and organic matter are driving factors in reducing the availability and uptake of HMs. ASB could be used as a sustainable trial for remediation of HMs polluted soils and reducing metal content in edible plants.

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

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

Role of Woody Biomass Ash Material in Immobilization of Cadmium, Lead and Zinc in Soil

Nowadays, we have observed the dynamic development of bio-heating plants that use wood biomass for heating or energy purposes. The result of this process is a reduction in carbon dioxide emissions as well as in the production of biomass ash (BA). Despite the waste nature of BA, it should be carefully analyzed and assessed for various applications, including environmental ones. Due to the features attributed to BA, including its alkaline reaction, the high capacity of its sorption complex, relatively low salinity, and significant content of macro- and microelements, a hypothesis was put forward in this work undertaken about the positive role of BA as an immobilizing factor for Cd-, Pb-, and Zn-contaminated soils. This research was based on a pot experiment in which four series were considered: (1) BA; (2) BA + Cd; (3) BA + Pb; and (4) BA + Zn. BA was used at doses of 30, 60, and 90 mg pot-1, and metals at doses of 2 mg Cd, 100 mg Pb, and 300 mg Zn kg-1 of soil. The test plant was corn grown for green mass. The study took into account the influence of BA on the content of the total forms of heavy metals (Metot) and their available forms (Meav). In the soil without the addition of metals, a significant increase in the content of Cdtot and Cdav, and a decrease in the content of Zntot were observed due to the application of BA. The addition of metals against the background of the BA used resulted in a significant increase in Cdtot, Pbtot, and Zntot, as well as an increase in the available forms of Pbav but a decrease in Znav. However, there was no significant increase in the Cdav content. The obtained results may indicate the potentially immobilizing role of BA only in the case of zinc. They may constitute the basis for further, more detailed research aimed at determining the role of BA in the immobilization of various metals in soil.

Respective evolution of soil and biochar on competitive adsorption mechanisms for Cd(II), Ni(II), and Cu(II) after 2-year natural ageing

To reveal the respective evolution of soil and biochar on competitive heavy metal adsorption mechanisms after natural ageing, three soils and two biochars were tested in this study. The soil-biochar interlayer samples were buried in the field for 0.5, 1, and 2 years, for which competitive adsorption characteristics and mechanisms of soils and biochars in four systems (Cd, Cd+Ni, Cd+Cu, and Cd+Ni+Cu) were investigated. Results showed that physicochemical properties, adsorption capacity and mechanisms of soils and biochars all changed the most in the first 0.5 years. The properties and adsorption capacity of biochars gradually weakened with the ageing time, meanwhile, those of soils gradually enhanced. After co-ageing with acidic soil for 0.5 years, the Cd(II) adsorption capacity of modified biochar decreased by 86.59% in the ternary system; meanwhile, that of acidic soil increased by 65.52%. The contributions of mineral mechanisms decreased significantly, while non-mineral mechanisms were slightly affected by ageing. This study highlighted that when using biochar to remediate heavy metal-contaminated soils, biochar should be applied at least half a year in advance before planting crops so that biochar can fully contact and react with the soil.

Conversion of locally available materials to biochar and activated carbon for drinking water treatment

For environmental sustainability and to achieve sustainable development goals (SDGs), drinking water treatment must be done at a reasonable cost with minimal environmental impact. Therefore, treating contaminated drinking water requires materials and approaches that are inexpensive, produced locally, and effortlessly. Hence, locally available materials and their derivatives, such as biochar (BC) and activated carbon (AC) were investigated thoroughly. Several researchers and their findings show that the application of locally accessible materials and their derivatives are capable of the adsorptive removal of organic and inorganic contaminants from drinking water. The application of locally available materials such as lignocellulosic materials/waste and its thermo-chemically derived products, including BC and AC were found effective in the treatment of contaminated drinking water. Thus, this review aims to thoroughly examine the latest developments in the use of locally accessible feedstocks for tailoring BC and AC, as well as their features and applications in the treatment of drinking water. We attempted to explain facts related to the potential mechanisms of BC and AC, such as complexation, co-precipitation, electrostatic interaction, and ion exchange to treat water, thereby achieving a risk-free remediation approach to polluted water. Additionally, this research offers guidance on creating efficient household treatment units based on the health risks associated with customized adsorbents and cost-benefit analyses. Lastly, this review work discusses the current obstacles for using locally accessible materials and their thermo-chemically produced by-products to purify drinking water, as well as the necessity for technological interventions.

Iron-modified biochar inhibiting Cd uptake in rice by Cd co-deposition with Fe oxides in the rice rhizosphere

Fe-enriched biochar has proven to be effective in reducing Cd uptake in rice plants by enhancing iron plaque formation. However, the effect of Fe on biochar, especially the biochar with high S content, for Cd immobilization in rice rhizosphere was not fully understood. To obtain eco-friendly Fe-loaded biochar at a low cost, garlic straw, bean straw, and rape straw were chosen as the feedstocks for Fe-enhanced biochar production by co-pyrolysis with Fe2O3. The resulting biochars and Fe-loaded biochars were GBC, BBC, BRE, GBC-Fe, BBC-Fe, and BRE-Fe, respectively. XRD and FTIR analyses showed that Fe was successfully loaded onto the biochar. The pristine and Fe-containing biochars were applied at rates of 0% (BC0) and 0.1% in pot experiments. Results suggested that BBC-Fe caused the highest reduction in Cd content of rice grain, and the reductions were 67.9% and 31.4%, compared with BC0 and BBC, respectively. Compared to BBC, BBC-Fe effectively reduced Cd uptake in rice roots by 47.5%. The exchangeable and acid-soluble fraction of Cd (F1-Cd) in soil with BBC-Fe treatment was 37.6% and 63.7% lower than that of BC0 and BBC, respectively. Compared to BC0, soil pH was increased by 0.53 units with BBC-Fe treatment. BBC-Fe significantly increased Fe oxides (free Fe oxides, amorphous Fe oxides, and complex Fe oxides) content in the soil as well. DGT study demonstrated that BBC-Fe could enhance the mobility of sulfate in the rhizosphere, which might be beneficial for Cd fixation in the rhizosphere. Moreover, BBC-Fe increased the relative abundance of Bacteroidota, Firmicutes, and Clostridia, which might be beneficial for Cd immobilization in the rhizosphere. This work highlights the synergistic effect of loaded Fe and biochar on Cd immobilization by enhancing Cd deposited with Fe oxides.

Biochar impacts on carbon dioxide, methane emission, and cadmium accumulation in rice from Cd-contaminated soils; A meta-analysis

Climate change and cadmium (Cd) contamination pose severe threats to rice production and food security. Biochar (BC) has emerged as a promising soil amendment for mitigating these challenges. To investigate the BC effects on paddy soil upon GHG emissions, Cd bioavailability, and its accumulation, a meta-analysis of published data from 2000 to 2023 was performed. Data Manager 5.3 and GetData plot Digitizer software were used to obtain and process the data for selected parameters. Our results showed a significant increase of 18% in soil pH with sewage sludge BC application, while 9% increase in soil organic carbon (SOC) using bamboo chips BC. There was a significant reduction in soil bulk density (8%), but no significant effects were observed for soil porosity, except for wheat straw BC which reduced the soil porosity by 6%. Sewage sludge and bamboo chips BC significantly reduced carbon dioxide (CO2) by 7-8% while municipal biowaste reduced methane (CH4) emissions by 2%. In the case of heavy metals, sunflower seedshells-derived materials and rice husk BC significantly reduced the bioavailable Cd in paddy soils by 24% and 12%, respectively. Cd uptake by rice roots was lowered considerably by the addition of kitchen waste (22%), peanut hulls (21%), and corn cob (15%) based BC. Similarly, cotton sticks, kitchen waste, peanut hulls, and rice husk BC restricted Cd translocation from rice roots to shoots by 22%, 27%, 20%, and 19%, respectively, while sawdust and rice husk-based BC were effective for reducing Cd accumulation in rice grains by 25% and 13%. Regarding rice yield, cotton sticks-based BC significantly increased the yield by 37% in Cd-contaminated paddy soil. The meta-analysis demonstrated that BC is an effective and multi-pronged strategy for sustainable and resilient rice cultivation by lowering greenhouse gas emissions and Cd accumulation while improving yields under the increasing threat of climate change.

Effect of different production methods on physicochemical properties and adsorption capacities of biochar from sewage sludge and kitchen waste: Mechanism and correlation analysis

Different pyrolysis methods, parameters and feedstocks result in biochars with different properties, structures and removal capacities for heavy metals. However, the role of each property on adsorption capacity and corresponding causal relationships remain unclear. Here, we investigated various physicochemical properties of biochar produced via three different methods and two different feedstocks to clarify influences of biomass sources and pyrolysis processes on biochar properties and its heavy metal adsorption performance. Experimental results showed biochars were more aromatic and contained more functional groups after hydrothermal carbonization, while they had developed pores and higher surface areas produced by anaerobic pyrolysis. The inclusion of oxygen resulted in more complete carbonization and higher CEC biochar. Different biochar properties resulted in different adsorption capacities. Biochar produced by aerobic calcination showed higher adsorption efficiency for Cu and Pb. Correlation analysis proved that pH, cation exchange capacity and degree of carbonization positively affected adsorption, while organic matter content and aromaticity were unfavorable for adsorption. Microstructure and components determined biochar macroscopic properties and ultimate adsorption efficiency for metal ions. This study identifies the degree of correlation and pathways of each property on adsorption, which provides guidance for targeted modification of biochar to enhance its performance in heavy metal removal.

Iron-modified biochar improves plant physiology, soil nutritional status and mitigates Pb and Cd-hazard in wheat (Triticum aestivum L.)

Environmental quality and food safety is threatened by contamination of lead (Pb) and cadmium (Cd) heavy metals in agricultural soils. Therefore, it is necessary to develop effective techniques for remediation of such soils. In this study, we prepared iron-modified biochar (Fe-BC) which combines the unique characteristics of pristine biochar (BC) and iron. The current study investigated the effect of pristine and iron modified biochar (Fe-BC) on the nutritional values of soil and on the reduction of Pb and Cd toxicity in wheat plants (Triticum aestivum L.). The findings of present study exhibited that 2% Fe-BC treatments significantly increased the dry weights of roots, shoots, husk and grains by 148.2, 53.2, 64.2 and 148%, respectively compared to control plants. The 2% Fe-BC treatment also enhanced photosynthesis rate, transpiration rate, stomatal conductance, intercellular CO2, chlorophyll a and b contents, by 43.2, 88.4, 24.9, 32.5, 21.4, and 26.7%, respectively. Moreover, 2% Fe-BC treatment suppressed the oxidative stress in wheat plants by increasing superoxide dismutase (SOD) and catalase (CAT) by 62.4 and 69.2%, respectively. The results showed that 2% Fe-BC treatment significantly lowered Cd levels in wheat roots, shoots, husk, and grains by 23.7, 44.5, 33.2, and 76.3%. Whereas, Pb concentrations in wheat roots, shoots, husk, and grains decreased by 46.4, 49.4, 53.6, and 68.3%, respectively. Post-harvest soil analysis showed that soil treatment with 2% Fe-BC increased soil urease, CAT and acid phosphatase enzyme activities by 48.4, 74.4 and 117.3%, respectively. Similarly, 2% Fe-BC treatment significantly improved nutrients availability in the soil as the available N, P, K, and Fe contents increased by 22, 25, 7.3, and 13.3%, respectively. Fe-BC is a viable solution for the remediation of hazardous Cd and Pb contaminated soils, and improvement of soil fertility status.

Remediation of cadmium contaminated soil using K2FeO4 modified vinasse biochar

The remediation of cadmium (Cd) contaminated soil is challenging for agricultural practices. In this study, a novel vinasse biochar modified by potassium ferrate (K₂FeO₄) was synthesized to immobilize Cd in agricultural soil. Three biochars [i.e., vinasse biochar (BC), KMnO₄ modified vinasse biochar (MnBC), and K₂FeO₄ modified vinasse biochar (FeBC)] were applied to compare their efficiencies of Cd immobilization. The results showed that the orders of pH, ash content, and functional groups in different biochar were the same following BC < MnBC < FeBC. Scanning electron microscope images showed that the FeBC has more micropores than MnBC and BC. X-ray diffraction identified manganese oxides and iron oxides within MnBC and FeBC, indicating that Mn and Fe were well loaded on the biochar. In the soil-based pot experiment, both MnBC and FeBC significantly reduced soil available Cd by 23-38% and 36-45% compared with the control, respectively (p < 0.05). In addition, the application of BC, MnBC, and FeBC significantly increased the yield, chlorophyll, and vitamin C of Chinese cabbage (p < 0.05), and decreased its Cd uptake compared with the control. Notably, shoot Cd significantly reduced when 2% FeBC was applied (p < 0.05). Overall, using K₂FeO₄ to modify vinasse biochar enriched the surface functional groups and minerals as well as reduced Cd availability in soil and its uptake by the plant. Our study showed that K₂FeO₄ modified vinasse biochar could be used as an ideal amendment for the remediation of Cd-contaminated soil.

Effects of biochar on the physiology and heavy metal enrichment of Vetiveria zizanioides in contaminated soil in mining areas

The effects of biochar addition on the physiological and biochemical characteristics of Vetiveria zizanioides, and the enrichment of heavy metals, were studied herein. The aim was to provide a theoretical reference for biochar to regulate the growth of V. zizanioides in the heavy metal-contaminated soil of mining areas and the enrichment capacity of Cu, Cd, and Pb. The results showed that the addition of biochar significantly increased the contents of various pigments in the middle and late growth stages of V. zizanioides, reduced the contents of malondialdehyde (MDA) and proline (Pro) in each growth period, weakened the peroxidase (POD) activity during the entire growth period; superoxide dismutase (SOD) activity decreased in the initial stages and substantially increased in the middle and late stages. The addition of biochar reduced the enrichment of Cu in the roots and leaves of V. zizanioides, while the enrichment of Cd and Pb increased. In conclusion, it was found that biochar could reduce the toxicity of heavy metals in contaminated soil in the mining area, affect the growth of V. zizanioides and its accumulation of Cd and Pb, and is, therefore, beneficial to the restoration of contaminated soil and the overall ecological restoration of the mining area.

Converting food waste into soil amendments for improving soil sustainability and crop productivity: A review

One-third of the annual food produced globally is wasted and much of the food waste (FW) is unutilized; however, FW can be valorized into value-added industrial products such as biofuel, chemicals, and biomaterials. Converting FW into soil amendments such as compost, vermicompost, anaerobic digestate, biofertilizer, biochar, and engineered biochar is one of the best nutrient recovery and FW reuse approaches. The soil application of FW-based amendments can improve soil fertility, increase crop production, and reduce contaminants by altering soil's chemical, physical, microbial, and faunal properties. However, the efficiency of the amendment for improving ecosystem sustainability depends on the type of FW, conversion method, application rate, soil type, and crop type. Engineered biochar/biochar composite materials produced using FW have been identified as promising amendments for soil remediation, reducing commercial fertilizer usage, and increasing soil nutrient use efficiency. The development of quality standards and implementation of policies and regulations at all stages of the food supply chain are necessary to manage (reduce and re-use) FW.

Soil pH restricts the ability of biochar to passivate cadmium: A meta-analysis

Soil acidification is the main cause for aggravation of soil cadmium (Cd) pollution. Biochar treatment can increase the soil pH and decrease the Cd availability in soils. However, there is limited information in literature on the comprehensive assessment of the response of Cd fractions to biochar. Therefore, in the present meta-analysis study, we evaluate the response of Cd fractions to biochar application in soils with different pH and to further examine the effect of physicochemical properties of biochar on Cd. Results from the overall analysis indicated that biochar treatment increased the soil pH by 7.0%, thereby decreasing the amount of available Cd (37.3%). In acidic soil, biochar significantly reduced the acid-soluble fraction (Acid-Cd) of Cd by 36.8%, while Oxidizable fraction of Cd (Oxid-Cd, 20.9%) and Residual fraction of Cd (Resid-Cd, 22.2%) were significantly increased. In neutral soils, only Acid-Cd was significantly reduced (33.0%) in the presence of biochar. In alkaline soils, biochar caused significant reduction in Acid-Cd of 12.4% and an increase in Oxid-Cd and Resid-Cd of 26.6% and 47.8%, respectively. Further, our findings showed that biochar with cation exchange capacity >100 cmol⁺/kg effectively decreased Acid-Cd (32.4%), while biochar with the percentage of hydrogen <2% was more contributory in increasing Resid-Cd (64.3%). These results demonstrate the importance of soil pH in regulating the biological effectiveness of Cd in soil and the complexation between the functional groups of biochar and Cd, and provide key information for the remediation of Cd pollution in soils with different pH by biochar.

Effects of biochars derived from four crop straws on a Cd-polluted cinnamon soil

Crop straw biochar is an efficient and low-cost alternative amendment for heavy metal immobilization in acidic soil. However, reports on the effect of these biochars on the amendment of actual Cd-polluted calcareous soil are limited. Therefore, four biochars, derived from peanut, rice, maize, and wheat straws, were applied to determine the changes in the chemical properties of alkaline cinnamon soil and effects on Cd immobilization. The results showed that the cation exchange capacity and the contents of organic C, Mehlich-3 K, and Mehlich-3 P in the biochar-amended soil increased by 4.87-22.02%, 68.78-218.83%, 1.9-10.3 times, and 19.18-74.40%, respectively, indicating the potential high performance of biochar in improving soil fertility and productivity. The Community Bureau of Reference sequential extraction results showed that decrease in acid-extractable Cd resulted in a reduced availability of Cd. Thus, crop straw biochar could be a promising alternative for soil Cd decontamination and fertilization.

Responses of aquatic vegetables to biochar amended soil and water environments: a critical review

Aquatic vegetables, including lotus root, water spinach, cress, watercress and so on, have been cultivated as commercial crops for a long time. Though aquatic vegetables have great edible and medicinal values, the increasing demands for aquatic vegetables with high quality have led to higher requirements of their soil and water environments. Unfortunately, the soil and water environment often face many problems such as nutrient imbalance, excessive fertilization, and pollution. Therefore, a new cost-effective and eco-friendly solution for addressing the above issues is urgently required. Biochars, one type of pyrolysis product obtained from agricultural and forestry waste, show great potential in reducing fertilizer application, upgrading soil quality and remediating pollution. Application of biochars in aquatic vegetable cultivation would not only improve the yield and quality, but also reduce its edible risk. Biochars can improve the soil micro-environment, soil microorganism and soil enzyme activities. Furthermore, biochars can remediate the heavy metal pollution, organic pollution and nitrogen and phosphorus non-point source pollution in the water and soil environments of aquatic vegetables, which promotes the state of cultivation conditions and thereby improves the yield and quality of aquatic vegetables. However, the harmful substances such as heavy metals, PAHs, etc. derived from biochars can cause environmental risks, which should be seriously considered. In this review, the application of biochars in aquatic vegetable cultivation is briefly summarized. The changes of soil physicochemical and biological properties, the effects of biochars in remediating water and soil environmental pollution and the impacts of biochars on the yield and quality of aquatic vegetables are also discussed. This review will provide a comprehensive overview of the research progress on the effects of biochars on soil and water environments for aquatic vegetable cultivation.

Biochar alleviating heavy metals phytotoxicity in sludge-amended soil varies with plant adaptability

Recycling sewage sludge (SS) to soil potentially causes soil heavy metal (HM) pollution and plant phytotoxicity. Biochar plays an important role in alleviating HM phytotoxicity, and responses vary with the feedstocks and usage of biochar. However, the effect of plant adaptability on biochar-mediated alleviation is poorly understood. Here, SS-derived biochar (SB) and rice straw-derived biochar (RB) applied at rates of 1.5% and 3% (W/W, SB1.5, SB3, RB1.5, and RB3) were used to improve the properties of soil amended with SS at 50% (W/W). Alleviation of phytotoxicity by biochar was further analyzed with SS-sensitive plant Monstera deliciosa and SS-resistant plant Ruellia simplex. Results revealed that both SB and RB significantly decreased the soil's bulk density and increased water retention. They also changed soil organic matter content and HMs fractionation. The addition of SB or RB alleviated the SS phytotoxicity, and they significantly promoted the growth and the root morphology and physiological index of M. deliciosa. But for R. simplex, these significant changes only synchronously occurred in SB3 treatment. The alleviation in M. deliciosa was more prominent and more closely connected with soil property changes than in R. simplex. Also, more soil property predictors were observed to play an important role in M. deliciosa growth than in R. simplex growth. These results indicated that biochar alleviating HMs phytotoxicity in SS-amended soil is associated with the changes of soil property. Moreover, the alleviation varies more prominently with plant adaptability than with biochar feedstocks and usage.

Long-term immobilization of cadmium and lead with biochar in frozen-thawed soils of farmland in China

The problem of potentially toxic elements (PTEs) in farmland is a key issue in global pollution prevention and control and has an important impact on environmental safety, human health, and sustainable agricultural development. Based on the climate background of high-latitude cold regions, this study simulated freeze-thaw cycles through indoor tests. Different initial conditions, such as biochar application rates (0%, 1%, 2%) and different initial soil moisture contents (15%, 20%, 25%), were set to explore the morphological changes in cadmium (Cd) and lead (Pb) in soil and the response relationship to the changes in soil physicochemical properties. The results indicate that soil pH decreases during freeze-thaw cycles, and soil alkalinity increases with increasing biochar content. Freeze-thaw cycles caused the total amount of PTEs to have a U-shaped distribution, and the amount of PTEs in the soluble (SOL) and reducible (RED) fraction increased by 0.28-56.19%. Biochar reduced the amount of Cd and Pb migration in the soil, and an increase in soil moisture content reduced the availability of Cd and Pb in the soil. Freezing and thawing damaged the soil structure, and biochar reduced the fractionation of small particle aggregates by enhancing the stability of soil aggregates, thereby reducing the soil's ability to adsorb Cd and Pb. In summary, for farmland soil remediation and pollution control, the application of biochar has a certain ability to optimize soil properties. Considering the distribution of PTEs in the soil and the physicochemical properties of the soil, the application of 1% biochar to soil with a 20% moisture content is optimal for regulating seasonally frozen soil remediation.

Calcium-enriched biochar modulates cadmium uptake depending on external cadmium dose

The impact of calcium-enriched biochar (BC, containing Ca, Al, Fe and P as dominant elements in the range of 6.9-1.3% with alkaline pH) obtained from sewage sludge (0.1 or 0.5% in the final soil) on cadmium-induced toxicity (final dose of 1.5 mg Cd/kg in control and 4.5 or 16.5 mg Cd/kg soil in low and high Cd treatment) was tested in medicinal plant Matricaria chamomilla. Low Cd dose had typically less negative impact than high Cd dose at the level of minerals and metabolites and the effect of BC doses often differed. Contrary to expectations, 0.5% BC with a high Cd dose increased Cd accumulation in plants about 2-fold. This was reflected in higher signals of reactive oxygen species, but especially the high dose of BC increased the amount of antioxidants (ascorbic acid and non-protein thiols), minerals and amino acids in shoots and/or roots and usually mitigated the negative effect of Cd. Surprisingly, the relationship between BC and soluble phenols was negative at high BC + high Cd dose, whereas the effect of Cd and BC on organic acids (mainly tartaric acid) differed in shoots and roots. Interestingly, BC alone applied to the control soil (1.5 mg total Cd/kg) reduced the amount of Cd in the plants by about 30%. PCA analyses confirmed that metabolic changes clearly distinguished the high Cd + high BC treatment from the corresponding Cd/BC treatments in both shoots and roots. Thus, it is clear that the effect of biochar depends not only on its dose but also on the amount of Cd in the soil, suggesting the use of Ca-rich biochar both for phytoremediation and safer food production.

First "unsaturated soils" view towards quantitative adsorption and immobilization mechanisms of Cd by biochar in soils during aging

Instead of traditional batch and column experiments with large water-soil ratios, this study investigated the behaviors and mechanisms of Cd adsorption and immobilization by biochar in unsaturated soils, in which the soil moisture conditions were closer to those in the actual field. The transport, transformation, and immobilization of cadmium (Cd) by pristine and KMnO₄-modified biochars in unsaturated soils were investigated during a 48-week mild aging process. Biochar acidified with HCl solution was employed to quantify the contributions of mineral and non-mineral components in biochar to Cd adsorption and immobilization in unsaturated soils with a three-layer mesh method. The behaviors and mechanisms of Cd adsorption by biochar in unsaturated soils significantly differed from those in aqueous solutions. The equilibrium times of Cd adsorption by biochar in unsaturated soils (weeks) were much longer than those in aqueous solutions (hours). The percentages of the Cd adsorbed by pristine and modified biochar remained relatively constant relative to the total Cd in unsaturated soils, which accounted for 39.50-49.39 % and 57.35-68.94 %, respectively. The contribution of mineral components to Cd adsorption dominated in both unsaturated soils (45.00-94.09 %) and aqueous solutions (70.73-95.51 %). The process of Cd immobilization in unsaturated soils was that biochar firstly adsorbed the exchangeable Cd from the soil, and then converted it to relatively stable Cd. After aging for 48 weeks, the contributions of non-mineral components to Cd immobilization dominated in unsaturated soil with a low concentration (1.23 mg·kg^-1), and the contributions of mineral components to Cd immobilization dominated in unsaturated soil with medium-high concentrations (4.08-51.26 mg·kg^-1).

Biochar raw material selection and application in the food chain: A review

As one of the largest carbon emitters, China promises to achieve carbon emissions neutrality by 2060. Various industries are developing businesses to reduce carbon emissions. As an important greenhouse gas emissions scenario, the reduction of carbon emissions in the food chain can be achieved by preparing the wastes into biochar. The food chain, as one of the sources of biochar, consists of production, processing and consumption, in which many wastes can be transferred into biochar. However, few studies use the food chain as the system to sort out the raw materials of biochar. A systematic review of the food chain application in serving as raw materials for biochar is helpful for further application of such technique, providing supportive information for the development of biochar preparation and wastes treating. In addition, there are many pollution sources in the food production process, such as agricultural contaminated soil and wastewater from livestock and aquatic, that can be treated on-site to achieve the goal of treating wastes with wastes within the food chain. This study focuses on waste resource utilization and pollution remediation in the food chain, summarizing the sources of biochar in the food chain and analyzing the feasibility of using waste in food chain to treat contaminated sites in the food chain and discussing the impacts of the greenhouse gas emissions. This review provides a reference for the resource utilization of waste and pollution reduction in the food chain.

Efficient Remediation of Cadmium Contamination in Soil by Functionalized Biochar: Recent Advances, Challenges, and Future Prospects

Heavy metal pollution in soil seriously harms human health and animal and plant growth. Among them, cadmium pollution is one of the most serious issues. As a promising remediation material for cadmium pollution in soil, functionalized biochar has attracted wide attention in the last decade. This paper summarizes the preparation technology of biochar, the existing forms of heavy metals in soil, the remediation mechanism of biochar for remediating cadmium contamination in soil, and the factors affecting the remediation process, and discusses the latest research advances of functionalized biochar for remediating cadmium contamination in soil. Finally, the challenges encountered by the implementation of biochar for remediating Cd contamination in soil are summarized, and the prospects in this field are highlighted for its expected industrial large-scale implementation.

Biochar rebuilds the network complexity of rare and abundant microbial taxa in reclaimed soil of mining areas to cooperatively avert cadmium stress

Understanding the interactions between the soil microbial communities and species is critical in the remediation of heavy metal-contaminated soil. Biochar has been widely applied as a stabilizer in the in situ remediation of cadmium (Cd)-contaminated soils in mining areas. However, the rebuilding of the microbial taxa of rare and abundant species by biochar and their cooperative resistance to Cd stress remains elusive. In this pursuit, the present study envisaged the effects of two types of biochars viz., poplar bark biochar (PB) and thiourea-modified poplar bark biochar (TP) on the rare and abundant bacterial and fungal taxa by using pot experiments. The results demonstrated that the PB and TP treatments significantly reduced the leached Cd content, by 35.13 and 68.05%, respectively, compared with the control group (CK), in the reclaimed soil of the mining area. The application of biochar significantly improved the physicochemical properties like pH and Soil Organic Matter (SOM) of the soil. It was observed that TP treatment was superior to the PB and CK groups in increasing the diversity of the soil abundant and rare species of microbial taxa. Compared with the CK group, the application of PB and TP enhanced and elevated the complexity of the microbial networks of rare and abundant taxa, increased the number and types of network core microorganisms, reshaped the network core microorganisms and hubs, and boosted the microbial resistance to Cd stress. Our results indicate the response of rare and abundant microbial taxa to biochar application and the mechanism of their synergistic remediation of Cd-contaminated soil, thereby providing technical feasibility for in situ remediation of Cd-contaminated soil in mining areas.

Effect of slow release nitrogenous fertilizers and biochar on growth, physiology, yield, and nitrogen use efficiency of sunflower under arid climate

Sunflower plants need nitrogen consistently and in higher amount for optimum growth and development. However, nitrogen use efficiency (NUE) of sunflower crop is low due to various nitrogen (N) losses. Therefore, it is necessary to evaluate the advanced strategies to minimize N losses and also improve sunflower productivity under arid climatic conditions. A field trial was conducted with four slow release nitrogenous fertilizers [SRNF (bacterial, neem, and sulfur-coated urea and N loaded biochar)] and three N levels (100% = 148 kg N ha-1, 80% = 118 kg N ha-1, and 60% = 89 kg N ha-1) of recommended application (100%) for sunflower crop under arid climatic conditions. Results showed that neem-coated urea at 148 kg N ha-1 significantly enhanced crop growth rate (CGR) (19.16 g m-2 d-1) at 60-75 days after sowing (DAS); leaf area index (2.12, 3.62, 5.97, and 3.00) at 45, 60, 75, and 90 DAS; and total dry matter (14.27, 26.29, 122.67, 410, and 604.33 g m-2) at 30, 45, 60, 75, and 90 DAS. Furthermore, higher values of net leaf photosynthetic rate (25.2 µmol m-2 s-1), transpiration rate (3.66 mmol s-1), and leaf stomatal conductance (0.39 mol m-2 s-1) were recorded for the same treatment. Similarly, neem-coated urea produced maximum achene yield (2322 kg ha-1), biological yield (9000 kg ha-1), and harvest index (25.8%) of the sunflower crop. Among various N fertilizers, neem-coated urea showed maximum NUE (20.20 kg achene yield kg-1 N applied) in comparison to other slow release N fertilizers. Similarly, nitrogen increment N60 showed maximum NUE (22.40 kg grain yield kg-1 N applied) in comparison to N80 and N100. In conclusion, neem-coated urea with 100% and 80% of recommended N would be recommended for farmers to get better sunflower productivity with sustainable production and to reduce the environmental nitrogen losses.

Cadmium long-term immobilization by biochar and potential risks in soils with different pH under combined aging

Cd long-term immobilization by biochar and potential risk in soils with different pH were quantified under a combined artificial aging, which simulated five years of aging in the field based on local climate. Two biochars (original and KMnO₄-modified) and five soils with different pH were tested, and an improved three-layer mesh method was employed in this study. Five aging cycles were carried out (Cycle 1-Cycle 5), and each aging cycle quantitatively simulated 1 year of natural aging. As the aging time increased, Cd leaching loss in all soils gradually increased from Cycle 1 to Cycle 5; for relatively stable Cd fraction, it decreased firstly and then stabilized in acidic and neutral soils (S1-S4), while it decreased firstly and then increased in alkaline soil (S5). Biochars significantly promoted Cd immobilization in strongly acidic soil (S1) by increasing relatively stable fractions and decreasing leaching loss. For weakly acidic and neutral soils (S2-S4), although biochars still had positive effects, the immobilization effects were weakened to certain extents compared with S1. The percentage of Cd leaching loss decreased by 19.12% in strongly acidic soil (S1) and by 1.12-11.35% in weakly acidic and neutral soils (S2-S4) after modified biochar treatment. For alkaline soil (S5), the application of biochars had negative effects on Cd immobilization by decreasing relatively stable fractions and increasing leaching loss, and posed risks to the environment. For strongly acidic soil (S1) and weakly acidic and neutral soils (S2-S4), the percentages of relatively stable fractions increased from 6.09-19.93% to 24.98-36.70% after modified biochar treatment. However, for alkaline soil, the percentage of relatively stable fractions decreased from 55.27% to 53.93% after biochar treatment. The more acidic the soil, the more effective the Cd immobilization by biochar. Biochars with high pH level are not suitable for the remediation of alkaline Cd contaminated soil.

A review of pristine and modified biochar immobilizing typical heavy metals in soil: Applications and challenges

In recent years, the application of biochar in the remediation of heavy metals (HMs) contaminated soil has received tremendous attention globally. We reviewed the latest research on the immobilization of soil HMs by biochar almost in the last 5 years (until 2021). The methods, effects and mechanisms of biochar and modified biochar on the immobilization of typical HMs in soil have been systematically summarized. In general, the HMs contaminating the soil can be categorized into two groups, the oxy-anionic HMs (As and Cr) and the cationic HMs (Pb, Cd, etc.). Reduction and precipitation of oxy-anionic HMs by biochar/modified biochar are the dominant mechanism for reducing HMs toxicity. Pristine biochar can effectively immobilize cationic HMs. The commonly applied modification method is to add substances that can precipitate HMs to the biochar. In addition, we assessed the risks of biochar applications. For instance, biochar may cause the leaching of certain HMs; biochar aging; co-transportation of biochar nanoparticles with HMs. Future work should focus on the artificial/intelligent design of biochar to make it suitable for remediation of multiple HMs contaminated soil.

Sustainable biochar effects on the remediation of contaminated soil: A 2-crop season site practice near a lead-zinc smelter in Feng County, China

It is of great significance to explore the remediation pattern in actual heavy metal (HM) contaminated sites. The field trial was carried out to research the remediation effect of biochar near a lead-zinc smelter in Feng County, China, under the rotation condition of different crops. This kind of cultivation mode is very representative in northern of China. And the pattern of production and restoration is suitable for scarce land resources and large food demand in China. The changes of soil physiochemical properties with the biochar addition, crop growth and the accumulated HMs by crops were focused on. The results showed the biochar application was excellent in improving soil nutrient elements and crop growth. The contents of TK were more obvious than those of TN and TP, with an increase of 2.6%-28.2% compared with the controls (without the addition of biochar). The yield of first season crops, i.e., soybean and corn, increased by 30%-42% and 34%-61%, respectively, and the second season crops, i.e., rape and wheat, with the increment of 25%-41% and 9%-29%, respectively. The availability forms of Cd and Pb decreased by 1.07-10.0% and 2.92-8.35%, respectively. While the improvement on the status of the HMs accumulated by crops was disappointing. The contents of HMs accumulated by crops increased to varying degrees (e.g., Pb and Zn in root, Cu and Pb in grain, and Cd in stems and leaves). Moreover, the concentrations of HMs in seeds of crops were higher than the limited levels given by the Chinese directive. Considering the results of the study and food safety, it is suggested to change the nature of the land around the smelter into woodland or construction land to prohibit the cultivation of food crops in this area.

Application fields of kitchen waste biochar and its prospects as catalytic material: A review

In China, a large amount of kitchen waste (KW) is generated each year, and the resource utilisation of the KW has become a problem. KW has a high carbon content and can be used as a raw material for biochar. Kitchen waste biochar (KWB) can be used to prepare adsorption materials, soil amendments, energy materials, carbon quantum dots, and electrode materials. However, few studies have used KWB as a raw material for catalytic materials. The application of sulfur (S) and nitrogen (N) doped biochar in the field of catalysis has proved effective and feasible. KWB contained a certain mass percentage of N and S elements, which has good application potential for use in the field of catalysis by KWB. In the process of preparing KWB by KW, keeping S and N as much as possible and converting them into pyridine N and thiophene S benefit the application of catalysis. This review provides a reference for the future application of KWB in China.

Effects of different feedstocks-based biochar on soil remediation: A review

As a promising amendment, biochar has excellent characteristics and can be used as a remediation agent for diverse types of soil pollution. Biochar is mostly made from agricultural wastes, forestry wastes, and biosolids (eg, sewage sludge), but not all the biochar has the same performance in the improvement of soil quality. There is a lack of guidelines devoted to the selection of biochar to be used for different types of soil pollution, and this can undermine the remediation efficiency. To shed light on this sensitive issue, this review focus on the following aspects, (i) how feedstocks affect biochar properties, (ii) the effects of biochar on heavy metals and organic pollutants in soil, and (iii) the impact on greenhouse gas emissions from soil. Generally, the biochars produced from crop residue and woody biomass which are composed of lignin, cellulose, and hemicellulose are more suitable for organic pollution remediation and greenhouse gas emission reduction, while biochar with high ash content are more suitable for cationic organic pollutant and heavy metal pollution (manure and sludge, etc.). Additionally, the effect of biochar on soil microorganisms shows that gram-negative bacteria in soil tend to use WB biochar with high lignin content, while biochar from OW (rich in P, K, Mg, and other nutrients) is more able to promote enzyme activity. Finally, our recommendations on feedstocks selection are presented in the form of a flow diagram, which is precisely intended to be used as a support for decisions on the crucial proportioning conditions to be selected for the preparation of biochar having specific properties and to maximize its efficiency in pollution control.

A critical review of biochar-based materials for the remediation of heavy metal contaminated environment: Applications and practical evaluations

The contamination of heavy metals (HMs) in the environment has aroused a global concern. The valid remediation of HM contaminated environment is a highly significant issue. As alternative to carbon materials, biochar has been vastly documented for the remediation of HM contaminated environment. However, there are some possible imperfections to meet the actual remediation tasks as the finite properties of raw biochar, and the remediation process is complex and unexpectedly. This review focuses on the progress made on environmental HM remediation by biochar-based materials within the past six years. The property analysis and key modifications of biochar are summarized inspired by their applicability or necessity for HM decontamination, and the environmental remediation as well as the implicated mechanisms are thoroughly elaborated from multiple pivotal sides. The evaluations of practical application associated with biochar amendment are also presented. Finally, some pertinent improvements and research directions are proposed. To our knowledge, this article is the first time to make a systematic summary on the reliability and practicability of biochar-based materials for environmental HM remediation, and critically pointed out the existing issues to facilitate the judicious design of biochar-based materials and understanding the research trends. It is also aims to provide reference for subsequent research and propel the practical applications.

Effect of a low-cost and highly efficient passivator synthesized by alkali-fused fly ash and swine manure on the leachability of heavy metals in a multi-metal contaminated soil

Soil pollution, caused by heavy metals, is an environmental problem that requires an urgent solution in China. Chemical passivation is a technology that uses various passivators to reduce the availability of heavy metals in soil and realize the remediation of contaminated soil. In this study, we examined the effects of fly ash (FA), alkali-fused fly ash (AFFA), swine manure biochar (SB), and modifying biochar (MB) on the leachability of Cu, Zn, Pb, and Cd via soil culture experiments. The results showed that the addition of AFFA, SB, and MB significantly reduced the extractable contents of Cu, Pb, and Cd in the soil. AFFA and MB had the best passivation effect, followed by SB and FA. The passivation effect on Pb was the best, followed by that on Cu and Cd. AFFA modification significantly improved the passivation effect of MB on Cu, Pb, and Cd in composite contaminated soil. With the addition of 3% MB, the Pb, Cu, and Cd extracted by TCLP decreased by 95.7, 74.1, and 59.1%, respectively. Correlation analysis, Fourier transform infrared spectrometry, scanning electron microscopy, and energy-dispersive X-ray spectroscopy showed that the passivation mechanism is mainly due to an increase in the soil pH, silicate content, and cohesiveness. The soil culture experiments in this study proved that MB is a low-cost and highly efficient organic-inorganic composite passivator for multi-metal contaminated soils.

Review of organic and inorganic pollutants removal by biochar and biochar-based composites

Biochar (BC) has exhibited a great potential to remove water contaminants due to its wide availability of raw materials, high surface area, developed pore structure, and low cost. However, the application of BC for water remediation has many limitations. Driven by the intense desire of overcoming unfavorable factors, a growing number of researchers have carried out to produce BC-based composite materials, which not only improved the physicochemical properties of BC, but also obtained a new composite material which combined the advantages of BC and other materials. This article reviewed previous researches on BC and BC-based composite materials, and discussed in terms of the preparation methods, the physicochemical properties, the performance of contaminant removal, and underlying adsorption mechanisms. Then the recent research progress in the removal of inorganic and organic contaminants by BC and BC-based materials was also systematically reviewed. Although BC-based composite materials have shown high performance in inorganic or organic pollutants removal, the potential risks (such as stability and biological toxicity) still need to be noticed and further study. At the end of this review, future prospects for the synthesis and application of BC and BC-based materials were proposed. This review will help the new researchers systematically understand the research progress of BC and BC-based composite materials in environmental remediation.

Optimization of biochar production based on environmental risk and remediation performance: Take kitchen waste for example

Two major obstacles that need to be addressed for environmental application of biochar include its environmental risk and remediation performance for target pollutants. In this study, kitchen waste was taken as an example to optimize the pyrolysis temperature for biochar production based on its heavy metal risk and Cd(II) remediation performance. The results showed that the pH and ash content of kitchen waste biochar (KWB) increased; however, the yield, H/C, and N/C decreased with increasing pyrolysis temperature. Total content of heavy metals in KWB got enriched after pyrolysis, while heavy metals' risk was reduced from moderate to low due to the transformation of directly toxic heavy metal fractions into potentially and/or non-toxic fractions. The equilibrium adsorption capacities of biochar for Cd(II) ranked as follows: 49.0 mg/g (600 °C), 46.5 mg/g (500 °C), 23.6 mg/g (400 °C), 18.2 mg/g (300 °C). KWB pyrolyzed at 500 °C was found to be the most suitable for green, efficient, and economic remediation of Cd(Ⅱ) contaminated water. SEM-EDS and XPS characterization results indicated that KWB removed Cd(II) via precipitation, complexation with carboxyl/hydroxyl, ion exchange with metal cations, and coordination with π-electrons. This study puts forward a new perspective for optimizing biochar production for environmental application.

Preparation and characterization of boron-doped corn straw biochar: Fe (Ⅱ) removal equilibrium and kinetics

Nowadays, iron ions as a ubiquitous heavy metal pollutant are gradually concerned and the convenient and quick removal of excessive iron ions in groundwater has become a major challenge for the safety of drinking water. In this study, boron-doped biochar (B-BC) was successfully prepared at various preparation conditions with the addition of boric acid. The as-prepared material has a more developed pore structure and a larger specific surface area (up to 897.97 m²/g). A series of characterization results shows that boric acid effectively activates biochar, and boron atoms are successfully doped on biochar. Compared with the ratio of raw materials, the pyrolysis temperature has a greater influence on the amount of boron doping. Based on Langmuir model, the maximum adsorption capacity of 800B-BC_1:2 at 25 °C, 40 °C, 55 °C are 50.02 mg/g, 95.09 mg/g, 132.78 mg/g, respectively. Pseudo-second-order kinetic model can better describe the adsorption process, the adsorption process is mainly chemical adsorption. Chemical complexation, ions exchange, and co-precipitation may be the main mechanisms for Fe²⁺ removal.

A new method for the treatment of kitchen waste: Converting it into agronomic sprayable mulch film

This report describes a new method for converting kitchen waste (KW) with high water content into mulch films that can be applied in agricultural production. Specifically, the KW was emulsion polymerized with acrylic monomers (in various proportions) to obtain a series of sprayable bio-based KW-acrylate mulch films. The films' properties and performances were evaluated, and the experimental results showed that the KW-acrylate mulch films exhibited excellent sprayability, membrane formation, and mechanical properties, which allowed them to effectively reduce soil water evaporation (by 13-50%) and increase soil temperature (by 1.9-6.7%). Compared with bare soil, the soil treated with the optimized KW-acrylate mulch film increased the germination rate of rapeseeds by 20% and the yield of Chenopodium album by 0.7 times; the mulch film biodegradation then reached 70% after 100 days.

Influence of biochar and soil properties on soil and plant tissue concentrations of Cd and Pb: A meta-analysis

The application of biochar to soils contaminated with potentially toxic elements (PTEs) has received particular attention due to its ability to reduce PTE uptake by the plants. Therefore, we conducted a meta-analysis to identify Cd and Pb concentrations in plant shoots and roots in response to biochar application and soil properties. We collected data from 65 peer-reviewed journal articles published from 2009 to 2020 in which 66% of manuscripts were published from 2015 to 2020. The data were processed using OpenMEE software. The results pinpointed that addition of biochar to soil caused a significant decrease in shoot and root Cd and Pb concentrations as compared to untreated soils with biochar (control), and the reduction rate was affected by plant types and both biochar and soil properties. The biochar size less than 2 mm, biochar pH higher than 10, pyrolysis temperature of 401-600 °C, and the application rate higher than 2% appeared to be effective in reducing shoot and root Cd and Pb concentration. Soil properties such as pH, SOC, and texture influenced the efficiency of biochar for reducing plant Cd and Pb uptake. Biochar application increased SOC (54.3%), CEC (48.0%), pH (0.08), and EC (59.4%), and reduced soil extractable Cd (42.1%) and Pb (47.1%) concentration in comparison to control. A detailed study on the rhizosphere chemistry and uptake mechanism will help to underpin the biochar application rates and their efficiency reducing PTE mobility and plant uptake.

Efficiency of different types of biochars to mitigate Cd stress and growth of sunflower (Helianthus; L.) in wastewater irrigated agricultural soil

Cadmium contamination in croplands is recognized one of the major threat, seriously affecting soil health and sustainable agriculture around the globe. Cd mobility in wastewater irrigated soils can be curtailed through eco-friendly and cost effective organic soil amendments (biochars) that eventually minimizes its translocation from soil to plant. This study explored the possible effects of various types of plants straw biochar as soil amendments on cadmium (Cd) phytoavailability in wastewater degraded soil and its subsequent accumulation in sunflower tissues. The studied biochars including rice straw (RS), wheat straw (WS), acacia (AC) and sugarcane bagasse (SB) to wastewater irrigated soil containing Cd. Sunflower plant was grown as a test plant and Cd accumulation was recorded in its tissues, antioxidant enzymatic activity chlorophyll contents, plant biomass, yield and soil properties (pH, NPK, OM and Soluble Cd) were also examined. Results revealed that addition of biochar significantly minimized Cd mobility in soil by 53.4%, 44%, 41% and 36% when RS, WS, AC and SB were added at 2% over control. Comparing the control soil, biochar amended soil effectively reduced Cd uptake via plants shoots by 71.7%, 60.6%, 59% and 36.6%, when RS, WS, AC and SB. Among all the biochar, rice husk induced biochar significantly reduced oxidative stress and reduced SOD, POD and CAT activity by 49%, 40.5% and 46.5% respectively over control. In addition, NPK were significantly increased among all the added biochars in soil–plant system as well as improved chlorophyll contents relative to non-bioachar amended soil. Thus, among all the amendments, rice husk and wheat straw biochar performed well and might be considered the suitable approach for sunflower growth in polluted soil.

Total nitrogen removal in biochar amended non-aerated vertical flow constructed wetlands for secondary wastewater effluent with low C/N ratio: Microbial community structure and dissolved organic carbon release conditions

Biochar was utilized to intensify constructed wetland (CW) for further organic and nitrogen removal from secondary wastewater. Four sets of non-aerated biochar amended vertical flow CW (VFCW) were developed to investigate the synergistic effects of biochar and microbes on pollutant removal. Results showed that the average COD and nitrogen removal efficiencies of VFCW1 (with 1% w/w biochar with microbe and plants) achieved 89.1 ± 5.6% and 90.2 ± 3.1% respectively, and their corresponding removal rates of 10.2 ± 0.8 mg-COD/(m³.d) and 3.57 ± 0.3 mg-TN/(m³.d) which were 35 and 52.3% higher than control. The biochar's dissolved organic carbon release in VFCWs indicated that water and acidic media portray the optimum conditions for nitrogen removal. The 16S RNA gene sequencing analysis indicated that in the biochar-amended VFCWs, bacterial phylum Proteobacteria (24.13-51.95%) followed by Chloroflexi (5.64-25.01%), Planctomycetes (8.48-14.43%), Acidobacteria (2.29-11.65%) were abundantly enhanced. Conclusively, incorporating biochar in non-aerated VFCWs is an efficient technique for enhancing nitrogen removal from secondary effluent.

Prospective usage of magnesium potassium phosphate cement combined with Bougainvillea alba derived biochar to reduce Pb bioavailability in soil and its uptake by Spinacia oleracea L

Combining biochar (BR) with other immobilizing amendments has additive effects on Pb immobilization and been recognized to be effective for the restoration of Pb polluted soils. However, the impacts of different proportions between BR and a highly efficient Pb immobilizing agent called "magnesium potassium phosphate cement (MC)" have never been earlier investigated. This work aimed to investigate the consequences of BR and MC alone and their mixtures of 25:75, 50:50, and 75:25 ratios on Pb bioavailability, Pb immobilization index (Pb-IMMi), and enzymatic activities in Pb polluted soil. Furthermore, amendments effects on Pb distribution in spinach, growth, antioxidant capacity, biochemical, and nutritional spectrum were also investigated. We found that MC alone performed well to immobilize Pb in soil and reducing its distribution in shoots, but was less efficient to improve soil enzymatic activities and plant attributes. Conversely, the application of BR alone stimulated soil enzymatic activities, plant growth, and quality but was less effective to immobilize Pb in soil and reducing shoot Pb concentrations. The combinations of BR and MC of various ratios showed variable results. Interestingly, the most promising outcomes were obtained with BR50%+MC50% treatment which resulted in enhanced Pb-IMMi (73%), activities of soil enzymes, plant growth and quality, and antioxidant capacity, compared to control. Likewise, significant reductions in Pb concentrations in shoots (85%), roots (78%), extractable Pb (73%) were also obtained with BR50%+MC50% treatment, compared to control. Such outcomes point towards a cost-effective approach for reducing Pb uptake by the plants via using MC and BR at a 50:50 ratio.

Effect of aging on stabilization of Cd and Ni by biochars and enzyme activities in a historically contaminated alkaline agricultural soil simulated with wet-dry and freeze-thaw cycling

Natural aging alters the surface physicochemical properties of biochars, which can affect the retention of heavy metals. This work investigated the effect of biochar aging on stabilization of heavy metals (Cd and Ni) and soil enzyme activities simulated with laboratory wet-dry (WD) and freeze-thaw (FT) cycling. A wheat straw (WS) biochar and a corn straw (CS) biochar were subjected to 30 WD or FT cycles, and Cd- and Ni-contaminated alkaline soils amended with the two fresh biochars (at 5% w/w) were subjected to 30-day constant moisture incubation and 30 WD or FT cycles. WD and FT aging caused slight reduction in the pH of the biochars, significant increases in their O contents and surface areas, and formation of new carbonate minerals. WS biochar was more effective than CS biochar at reducing the phytoavailable Cd in the soil, with reduction of 12.1%, 14.6%, and 12.9% under constant moisture incubation, WD aging, and FT aging, respectively. Reduction in phytoavailability of Ni by the addition of biochars was observed only under WD aging, by 17.0% and 18.5% in the presence of WS and CS biochars, respectively. Biochar amendment also reduced the distribution of Cd in the acid soluble and reducible fractions in all aging regimes. The addition of biochars decreased catalase activity in almost all aging regimes and invertase activity under FT aging, but increased urease activity under FT aging. Comparison of the enzyme activities in the soils amended with biochars under constant moisture and accelerated aging conditions indicates WD aging significantly decreased the activities of catalase, invertase, and urease in all treatments, while FT aging significantly increased urease activity in all treatments. These findings suggest that biochars can stabilize Cd in alkaline soils under changing environmental conditions, although the activities of some soil enzymes could be negatively impacted.

The Prediction of Calorific Value of Carbonized Solid Fuel Produced from Refuse-Derived Fuel in the Low-Temperature Pyrolysis in CO2

The decrease in the calorific value of refuse-derived fuel (RDF) is an unintended outcome of the progress made toward more sustainable waste management. Plastics and paper separation and recycling leads to the overall decrease in waste’s calorific value, further limiting its applicability for thermal treatment. Pyrolysis has been proposed to densify energy in RDF and generate carbonized solid fuel (CSF). The challenge is that the feedstock composition of RDF is variable and site-specific. Therefore, the optimal pyrolysis conditions have to be established every time, depending on feedstock composition. In this research, we developed a model to predict the higher heating value (HHV) of the RDF composed of eight morphological refuse groups after low-temperature pyrolysis in CO₂ (300–500 °C and 60 min) into CSF. The model considers cardboard, fabric, kitchen waste, paper, plastic, rubber, PAP/AL/PE (paper/aluminum/polyethylene) composite packaging pack, and wood, pyrolysis temperature, and residence time. The determination coefficients (R²) and Akaike information criteria were used for selecting the best model among four mathematical functions: (I) linear, (II) second-order polynomial, (III) factorial regression, and (IV) quadratic regression. For each RDF waste component, among these four models, the one best fitted to the experimental data was chosen; then, these models were integrated into the general model that predicts the HHV of CSF from the blends of RDF. The general model was validated experimentally by the application to the RDF blends. The validation revealed that the model explains 70–75% CSF HHV data variability. The results show that the optimal pyrolysis conditions depend on the most abundant waste in the waste mixture. High-quality CSF can be obtained from wastes such as paper, carton, plastic, and rubber when processed at relatively low temperatures (300 °C), whereas wastes such as fabrics and wood require higher temperatures (500 °C). The developed model showed that it is possible to achieve the CSF with the highest HHV value by optimizing the pyrolysis of RDF with the process temperature, residence time, and feedstock blends pretreatment.

Remediation of Soil Polluted with Cd in a Postmining Area Using Thiourea-Modified Biochar

Cadmium presence in soil is considered a significant threat to human health. Biochar is recognized as an effective method to immobilize Cd ions in different soils. However, obtaining effective and viable biochar to remove elevated Cd from postmining soil remains a challenge. More modifiers need to be explored to improve biochar remediation capacity. In this investigation, pot experiments were conducted to study the effects of poplar-bark biochar (PBC600) and thiourea-modified poplar-bark biochar (TPBC600) on Cd speciation and availability, as well as on soil properties. Our results showed that the addition of biochar had a significant influence on soil properties. In the presence of TPBC600, the acid-soluble and reducible Cd fractions were transformed into oxidizable and residual Cd fractions. This process effectively reduced Cd bioavailability in the soil system. Compared to PBC600, TPBC600 was more effective in improving soil pH, electrical conductivity (EC), organic matter (SOM), total nitrogen (TN), ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3−-N), available potassium (AK), available phosphorus (AP), and available sulfur (AS). However, this improvement diminished as incubation time increased. Results of Pearson correlation analysis, multivariate linear regression analysis, and principal component analysis showed that soil pH and available phosphorus played key roles in reducing the available cadmium in soil. Therefore, TPBC600 was shown to be an effective modifier that could be used in the remediation of soil polluted with Cd.