Biochar and bacteria inoculated biochar enhanced Cd and Cu immobilization and enzymatic activity in a polluted 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.

Effects of Phosphate, Red Mud, and Biochar on As, Cd, and Cu Immobilization and Enzymatic Activity in a Co-Contaminated Soil

Arsenic (As), cadmium (Cd), and copper (Cu) are the primary inorganic pollutants commonly found in contaminated soils. The simultaneous stabilization of the three elements is a preferred approach for mixture-contaminated soils which has received extensive research attention. However, few studies have focused on the immobilization efficiency of a single amendment on the three elements. In this study, phosphate, red mud, and biochar were used to remediate As (237.8 mg kg−1), Cd (28.72 mg kg−1), and Cu (366.5 mg kg−1) co-contaminated soil using a 180-day incubation study. The BCR (European Community Bureau of Reference) extraction method, NH4H2PO4–extractable As, and diethylenetriamine penta-acetic acid (DTPA)–extractable Cd and Cu were analyzed at different time intervals. The results indicated that the application of red mud and biochar significantly reduced soil DTPA–Cd and Cu concentrations during the incubation, while the decrease in soil NH4H2PO4–As was much less than that of soil DTPA–Cd and Cu. After 180 days of incubation, the concentrations of NH4H2PO4–As in red mud and biochar treatments decreased by 2.15~7.89% and 3.01~9.63%, respectively. Unlike red mud and biochar, phosphate significantly reduced the concentration of soil DTPA–Cd and Cu, but failed to lower that of As. The BCR extraction method confirmed that red mud and biochar addition increased the reducible fraction of As due to the surface complexes of As with Fe oxide. Canonical correspondence analysis (CCA) demonstrated that soil pH in addition to available As, Cd, and Cu concentrations were the primary factors in driving the changes in soil enzymatic activity. Soil pH showed positive correlation with soil urease and catalase activities, while negative correlation was observed between soil-available As, Cd, and Cu, and soil enzyme activities. This study revealed that it is difficult to simultaneously and significantly reduce the bioavailabilities of soil As, Cd, and Cu using one amendment. Further research on modifying these amendments or applying combined amendments will be conducted, in order to develop an efficient method for simultaneously immobilizing As, Cd, and Cu.

Sustainable biochar: A facile strategy for soil and environmental restoration, energy generation, mitigation of global climate change and circular bioeconomy

The increasing agro-demands with the burgeoning population lead to the accumulation of lignocellulosic residues. The practice of burning agri-residues has consequences viz. Release of soot and smoke, nutrient depletion, loss of soil microbial diversity, air pollution and hazardous effects on human health. The utilization of agricultural waste as biomass to synthesize biochar and biofuels, is the pertinent approach for attaining sustainable development goals. Biochar contributes in the improvement of soil properties, carbon sequestration, reducing greenhouse gases (GHG) emission, removal of organic and heavy metal pollutants, production of biofuels, synthesis of useful chemicals and building cementitious materials. The biochar characteristics including surface area, porosity and functional groups vary with the type of biomass consumed in pyrolysis and the control of parameters during the process. The major adsorption mechanisms of biochar involve physical-adsorption, ion-exchange interactions, electrostatic attraction, surface complexation and precipitation. The recent trend of engineered biochar can enhance its surface properties, pH buffering capacity and presence of desired functional groups. This review focuses on the contribution of biochar in attaining sustainable development goals. Hence, it provides a thorough understanding of biochar's importance in enhancing soil productivity, bioremediation of environmental pollutants, carbon negative concretes, mitigation of climate change and generation of bioenergy that amplifies circular bioeconomy, and concomitantly facilitates the fulfilment of the United Nation Sustainable Development Goals. The application of biochar as seen is primarily targeting four important SDGs including clean water and sanitation (SGD6), affordable and clean energy (SDG7), responsible consumption and production (SDG12) and climate action (SDG13).

Long-term and high-bioavailable potentially toxic elements (PTEs) strongly influence the microbiota in electroplating sites

Multiple potentially toxic elements (PTEs) wastes are produced in the process of electroplating, which pollute the surrounding soils. However, the priority pollutants and critical risk factors in electroplating sites are still unclear. Hence, a typical demolished electroplating site (operation for 31 years) in the Yangtze River Delta was investigated. Results showed that the soil was severely polluted by Cr(VI) (1711.3 mg kg^-1), Ni (6754.0 mg kg^-1) and Pb (2784.4 mg kg^-1). The spatial distribution of soil PTEs performed by ArcGIS illustrated that the soil pollution varied with plating workshops. Hard Cr electroplating workshops (HCE), decorative Cr electroplating workshops (DCE) and sludge storage station (SS) were the hot spots in the site. Besides, the toxicity characteristic leaching procedure (TCLP) - extractable Cr and Ni contents in different workshops were significantly related (P < 0.05) to their bioavailable fractions (exchangeable fraction (F1) + bound to carbonate fraction (F2)), which pose potential risk to humans. Although the soil total Pb concentration was high, its mobility was very low (<0.007%). Moreover, the soil microbial community dynamics under the stress of long term and high contents of PTEs were further revealed. The soil microbiota was significantly disturbed by long term and high concentration of PTEs. A bit of bacteria (Caulobacter) and fungi (Cladosporium and Monocillium) showed tolerance potential to multiple metals. Furthermore, the canonical correspondence analysis (CCA) showed that the bioavailable fractions (F1 + F2) of Cr and Ni were the most critical environmental variables affecting microbiota. Therefore, remediation strategies are required urgently to reduce the bioavailability of soil Cr and Ni. The results of this study provide an overview of the pollution distribution and microbial dynamics of a typical plating site, laying a foundation for ecological remediation of electroplating sites in Yangtze River Delta of China.

Citric-Acid-Assisted Preparation of Biochar Loaded with Copper/Nickel Bimetallic Nanoparticles for Dye Degradation

Immobilization of nanocatalysts on biochar is receiving unprecedented interest among material and catalysis scientists due to its simplicity, versatility, and high efficiency. Herein, we propose a new direct approach to obtain bimetallic copper/nickel nanoparticles loaded on olive stone biochar. The bimetallic-coated biochar and the reference materials, namely bare biochar, copper rich-loaded biochar, and nickel-loaded biochar, were prepared by pyrolysis from olive pit powder particles impregnated first with citric acid (CA) and then with copper and nickel nitrates at 400 °C under nitrogen flow. We employed citric acid in the process in order to examine its effect on the structural and textural properties of biochar supporting the metallic nanoparticles. Surprisingly, citric acid induced the formation of agglomerated or even raspberry-shaped bimetallic copper/nickel nanoparticles. Large 450–500 nm agglomerates of ~80 nm bimetallic CuNi NPs were noted for B-CA@CuNi. Interestingly, for biochar material prepared with initial Cu/Ni = 10 molar ratio (B-CA@CuNi10/1), the bimetallic NPs formed unusual nanoraspberries (174 ± 8 nm in size), which were agglomerates of individual 10–20 nm CuNi10/1 nanoparticles. The B-CA@CuNi and reference materials were characterized by Raman spectroscopy, scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and magnetometry. The B-CA@CuNi and B-CA@Ni materials could be efficiently attracted with a magnet but not B-CA@CuNi10/1 due to the low nickel loading. B-CA@CuNi was tested as a catalyst for the degradation of methyl orange (MO). Discoloration was noted within 10 min, much faster than a similar material prepared in the absence of CA. B-CA@CuNi could be recycled at least 3 times while still exhibiting the same fast catalytic discoloration performance. This paper stresses the important role of citric acid in shaping bimetallic nanoparticles loaded in situ on biochar during the slow pyrolysis process and in enabling faster catalytic discoloration of organic dye solution.

Mixed bacteria-loaded biochar for the immobilization of arsenic, lead, and cadmium in a polluted soil system: Effects and mechanisms

The present study explored the immobilization of mixed bacteria-loaded biochar on As, Pb, and Cd was explored. Physisorption and sodium alginate encapsulation were used to synthesize two kinds of mixed bacteria-loaded biochars, referred to as BCM and BCB. The observations of Scanning electron microscope, X-ray diffraction, and Fourier transform infrared spectroscopy distinctly demonstrated the colonization of mixed bacteria on biochar. Besides, the addition of BCM and BCB could increase soil pH with increasing incubation time. The residual fraction of heavy metals and soil dehydrogenase activities were also enhanced after 28 days of incubation. Pb was mainly immobilized by co-precipitation, which meant that Pb could be converted into a consistent crystalline form such as Pb₅(PO₄)₃OH. The X-ray photoelectron spectroscopy and X-ray diffraction analyses of materials identified the formation of Ca₂As₂O₇ and the presence of oxidation from trivalent arsenic to pentavalent arsenic. Cd was adsorbed by forming precipitations (CdCO₃) and exchanging ions with the BCM and BCB. Synergistic reactions between anions and cations also contributed to the immobilization of heavy metals, such as the formation of PbAs₂O₆ and Cd₃(AsO₄)₂. These results confirmed that mixed bacteria-loaded biochar was a feasible technology for the remediation of heavy metals contamination in site soils.

Microbial investigations of new hydrogel-biochar composites as soil amendments for simultaneous nitrogen-use improvement and heavy metal immobilization

Agricultural sustainability is challenging because of increasingly serious and co-existing issues, e.g., poor nitrogen-fertilizer use and heavy metal pollution. Herein, we introduced a new poly(acrylic acid)-grafted chitosan and biochar composite (PAA/CTS/BC) for soil amendment, and provided a first microbial insight into how PAA/CTS/BC amendment simultaneously improved nitrogen cycling and immobilized heavy metals. Our results suggest that the PAA/CTS/BC amendment significantly promoted soil ammonium retention, and reduced nitrate accumulation, nitrous oxide emission and ammonia volatilization during the rice cultivation. The availability of various heavy metals (Fe, Mn, Cu, Zn, Ni, Pb, Cr, and As) markedly decreased in the PAA/CTS/BC amended soil, thereby reducing their accumulation in rice root. The PAA/CTS/BC amendment significantly altered the structure and function of soil microbial communities. Importantly, the co-occurrence networks of microbial communities became more complex and function-specific after PAA/CTS/BC addition. For example, the keystone species related to organic matter degradation, denitrification, and plant resistance to pathogen or stresses were enriched within the network. In addition to direct adsorption, the effects of PAA/CTS/BC on shaping microbial communities played dominant roles in the soil amendment. Our findings provide a promising strategy of simultaneous nitrogen-use improvement and heavy metal immobilization for achieving crop production improvement, pollution control, and climate change mitigation.

The Effects of Rabbit Manure-Derived Biochar on Soil Health and Quality Attributes of Two Mine Tailings

Biochar amendment is becoming a promising technology for mining soil restoration. The addition of biochar can improve soil microbiological parameters related to soil quality, such as enzyme activities. The aim of the present research was to evaluate the effect of rabbit manure (RM) and two rabbit manure biochars prepared at two pyrolysis temperatures (300 and 600 °C) on the biochemical properties of two mining soils in the Portman area (Spain) in the presence or absence of vegetation. Soils were amended with the RM, the two biochars and a mixture of the rabbit manure and biochars (50/50 w/w) at a rate of 10% in a mesocosms experiment to study the changes in dehydrogenase, phosphomonoesterase, β-glucosidase activities, geometric mean of enzyme activities (GMea) and soil microbial biomass (SMB). Changes in individual enzyme activities were not always consistent. However, when using the GMea as a measure of soil quality, our results showed an increase in the GMea (217–360 times) after the addition of rabbit manure to mining soils, while this increase was from 81–270 times following the addition of rabbit manure with biochar prepared at 300 °C. Therefore, the use of biochar prepared at low temperatures could be a promising direction for the improvement of soil quality and soil carbon sequestration.

Effective lead passivation in soil by bone char/CMC-stabilized FeS composite loading with phosphate-solubilizing bacteria

Bioremediation by phosphate-solubilizing bacteria (PSB) has attracted extensive attentions due to its economical and eco-friendly properties for lead (Pb) passivation in soil. Herein, bone char (BC) supported biochemical composite (CFB₁-P) carrying advantages of BC, PSB, iron sulfide (FeS) and carboxymethyl cellulose (CMC) was designed and applied to Pb passivation. The composite at a mass ratio of BC:CMC:FeS = 1:1:1 possessed high passivation efficiency (65.47%), and has been demonstrated to offer appropriate habitat environment for PSB to defend against Pb(II) toxicity, thus enhancing the phosphate-solubilizing amount of PSB to 140.72 mg/L for passivating Pb(II). Batch experiments showed that the CFB₁-P possessed excellent adsorption properties with maximal monolayer Pb(II) uptake of 452.99 mg/g during an extensive pH range of 2.0-6.0. Furthermore, by applying CFB₁-P dosage of 3% into Pb-contaminated soil, the labile Pb fractions were reduced from 29.05% to 6.47% after simulated remediation of 10 days, and converted into steady fractions. The CFB₁-P was demonstrated to achieve high Pb(II) passivation through combined functions of chemical precipitation, complexation, electrostatic attraction and biomineralization, accompanied by the formation of more stable crystal structures, for instance, Pb₅(PO₄)₃OH, Pb₃(PO₄)₂ and PbS. These results suggested CFB₁-P as a potential alternative for efficient remediation of Pb-contaminated soil.

Migration of heavy metals in the soil-grape system and potential health risk assessment

The accumulation of heavy metals in soil may introduce them to the food chain and cause health risks for humans. In the present study, 43 pairs of soil and grape samples (leaf and fruit) were collected form vineyards in the suburbs of Kaifeng city (wastewater-irrigated area in Henan Province, China) to assess the heavy metal (Pb, Cd, Cu, Zn and Ni) pollution level in soil, heavy metal accumulation in different grape tissues and the potential health risk via consumption of grapes. The results showed that the average contents of Pb, Cd, Cu, Zn and Ni in vineyard soil were 42.27, 3.08, 62.33, 262.54 and 26.60 mg/kg, respectively. Some of these soil samples were severely contaminated with Cd and Zn, with an average pollution index (P_i) of 5.14 and 0.88, respectively. Most of these soil samples were severely polluted by heavy metals, with an average Nemerow integrated pollution index (P_N) of 3.77. The bioavailable heavy metals were negatively correlated with soil pH and positively correlated with soil organic matter (OM). In addition, heavy metals were more likely to accumulate in grape leaves, and their contents in grape pulp were all within the maximum permissible limit set by China (GB 2762-2017). The average bioaccumulation factors (BFs) of Pb, Cd, Cu, Zn and Ni in grape pulp were 0.007, 0.096, 0.160, 0.078 and 0.023, respectively. Health risk assessment indicated that there was no noncarcinogenic risk for grape consumers (adults and children). However, the carcinogenic risk (CR) ranged from 4.95 × 10^-7 to 2.17 × 10^-4, and the CR value of three grape samples was higher than 10^-4, indicating that a probability of carcinogenic disease existed for humans who regularly consumed the grapes from this region.

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.

Biochar as environmental armour and its diverse role towards protecting soil, water and air

Biochar has been of considerable importance for various environmental applications in recent years. It has exhibited substantial advantages like favourable structural and surface properties, easy process of preparation and widely available feedstocks. These set of exceptional properties make it an efficient, cost-effective and environment friendly source for diversified elimination of pollutants. The heterogeneity of physico-chemical properties offers a possibility for biochar to optimize its efficacy for targeted applications. This review aims to highlight the critical role that biochar plays in various environmental applications, be it in soil, water or air. In particular the article offers a comprehensive review of the recent research findings and updates related to the diversified role of biochar. Also, the interaction of pollutants with biochar functional groups and the impact of variation of parameters on biochar attribute relevant to specific pollutant removal, modifications, mechanisms involved and competence for such removal has been discussed. Different technologies for production of biochar have also been summarized with an emphasis on post treatment of biochar, such as modification and doping. In addition to this, the underlying gaps in the studies carried out so far and recommendations for future research areas in biochar have also been deliberated.

Impact of Biochar Application on Ammonia Volatilization from Paddy Fields under Controlled Irrigation

Ammonia volatilization is an important nitrogen loss pathway in the paddy field ecosystem which leads to low nitrogen-utilization efficiency and severe atmospheric pollution. To reveal the influence and the mechanism of biochar application on ammonia volatilization from paddy fields under controlled irrigation, field experiments were conducted in the Taihu Lake Basin in China. The experiment consisted of three levels of biochar application (0, 20, and 40 t·ha−1) and two types of irrigation management (controlled irrigation and flood irrigation). Increasing ammonia volatilization occurred after fertilization. Biochar application reduced the cumulative ammonia volatilization from controlled-irrigation paddy fields, compared with non-biochar treatment. The cumulative ammonia volatilization in controlled-irrigation paddy fields with 40 t·ha−1 biochar application was reduced by 12.27%. The decrease in ammonia volatilization was related to the change in soil physical and soil physical–chemical properties and soil microbial activities. The high biochar application (40 t·ha−1) increased the NH4+-N content in soil (p < 0.01) and soil solution (p <0.05), increased by 64.98% and 19.72%, respectively. The application also increased the soil urease activity (p < 0.01), and high biochar application (40 t·ha−1) increased soil urease activity by 33.70%. Ammonia volatilization from paddy fields was significantly correlated with the nitrogen concentration (p < 0.01) in the soil solution and soil urease activity (p < 0.05). Meanwhile, the abundance of ammonia monooxygenase subunit A (AOA) and ammonia-oxidizing bacteria (AOB) with biochar application under controlled irrigation showed an increasing trend with rice growth. The long-term application of biochar may have a relatively strong potential to inhibit ammonia volatilization. In general, the combined application of controlled irrigation and biochar provides an eco-friendly strategy for reducing farmland N loss and improving paddy field productivity.

Transcriptional insights into Cu related tolerance strategies in maize linked to a novel tea-biochar

One-third of maize cultivation in Turkey has been performed in nutrient-rich soils of the coastal agricultural lands of the Black Sea Region, which is among the country's granaries. However, the yield of this chief crop is affected by Cu toxicity due to a decades-long abandoned opencast Cu-mine. As part of the modern agenda, against this problem, we valorized one of the region's signature plant waste by synthesizing a tea-derived biochar (BC) and evaluated for remediation effect on maize Cu tolerance. Among other rates (0%, 0.4%, 0.8%, 1.6%), maximum Cu absorption (168.27 mg kg^-1) was found in the 5%BC in in-vitro spiking experiments where natural Cu contamination levels were mimicked. Obvious increasing trends in both root and shoot tissues of maize plantlets growing in Cu-spiked soil (260.26 ± 5.19 mg Cu kg^-1) were recorded with proportionally increasing BC application rates. The black tea waste-BC (5%) amendment remarkably reduced the Cu uptake from Cu spiked-soil and showed no phenotypic retardation in maize. Accordingly, it boosted the metabolic and transcriptomic profile owing to up-regulation in the aquaporin and defense genes (PIP1;5 and POD1) by 1.31 and 1.6 fold. The tea-BC application also improved the soil-plant water relations by minimizing cytosolic volume changes between 85 and 90%, increasing chlorophyll intactness (65%) and membrane stability up to 41%. The tea-BC could be a strong agent with potential agronomic benefits in the remediation of the cationic Cu toxicity that occurred in the mining-contaminated agricultural soils.

Cd diminution through microbial mediated degraded lignocellulose maize straw: Batch adsorption and bioavailability trails

Lignocellulose degraded maize straw (LMS) was prepared with the interaction of soil-indigenous microorganisms and further deployed to attenuate the Cd contamination in polluted soil. The Lignocellulose degrading ratio was determined and results revealed the significant degradation of cellulose, hemicellulose and lignin by 33.03, 26.7 and 15.97% respectively as compared to pristine maize straw (PS). Moreover, LMS was also categorized through FE-SEM, FTIR, BET analysis, elemental analysis and XPS technique and the analytical results indicated that lignocellulose structure in maize straw was successfully degraded and was involved in metal-ion complexation. Batch sorption trials revealed that Cd²⁺ sorption onto LMS was explained well by Langmuir isotherm and pseudo-second-order kinetic model. The LMS showed maximum adsorption capacities (9.84 mg g^-1) for Cd²⁺ as compared to PS (3.30 mg g^-1). Moreover, the soil incubation trials (60 days) depicted the availability of Cd decreased by 11.03 and 34.7% with PS and LMS application respectively. The addition of LMS significantly decreased the exchangeable fractions of Cd and ensued an increase in organic matter and Fe-Mn oxides bound fractions. This work clarified the LMS as a promising amendment for effective remediation of Cd-contaminated matrices.

Biochar application for the remediation of trace metals in contaminated soils: Implications for stress tolerance and crop production

In modern agriculture and globalization, the release of trace metals from manufacturing effluents hinders crop productivity by polluting the atmosphere and degrading food quality. Sustaining food safety in polluted soils is critical to ensure global food demands. This review describes the negative effects of trace metals stress on plant growth, physiology, and yield. Furthermore, also explains the potential of biochar in the remediation of trace metal's contaminations in plants by adoption of various mechanisms such as reduction, ion exchange, electrostatic forces of attraction, precipitation, and complexation. Biochar application enhances the overall productivity, accumulation of biomass, and photosynthetic activity of plants through the regulation of various biochemical and physiological mechanisms of plants cultivated under trace metals contaminated soil. Moreover, biochar scavenges the formation of reactive oxygen species, by activating antioxidant enzyme production i.e., ascorbate peroxidase, catalase, superoxide dismutase, peroxidase, etc. The application of biochar also improves the synthesis of stressed proteins and proline contents in plants thus maintaining the osmoprotectant and osmotic potential of the plant under contaminates stress. Integrated application of biochar with other amendments i.e., microorganisms and plant nutrients to improve trace metal remediation potential of biochar and improving crop production was also highlighted in this review. Moreover, future research needs regarding the application of biochar have also been addressed.

Study on the physicochemical properties changes of field aging biochar and its effects on the immobilization mechanism for Cd2+ and Pb2

It has been widely reported that biochar can be used as a cost-effective amendment to immobilize of heavy metal contaminants in soil. While less research has been conducted on effect of biochar long-term field aging on its properties and the adsorption capability. In this study, the characteristics of aged biochar were investigated by comprehensive characterization to elucidate its mechanism transformation for heavy metal immobilization. Our results showed that, compared to fresh biochar, the relative content of C of aged biochar was reduced by 34.12%, while O was increased by 8.79%. Additionally, the specific surface area, pore volume, pore size and oxygen-containing functional groups of aged biochar were significantly increased compared to the fresh biochar. Batch adsorption experiment indicated that the maximum adsorption for Cd2+ (Qm = 32.157 mg/g) and Pb2+ (Qm = 39.216 mg/g) on aged biochar surface was much larger than that of Cd2+ (Qm = 7.573 mg/g) and Pb2+ (Qm = 8.134 mg/g) on fresh biochar. The underlying adsorption mechanisms for Cd2+ and Pb2+ on fresh biochar were dominated by coprecipitation, cation exchange and cation-π interaction, whereas surface complexation and cation exchange appeared to be more vital for aged biochar, as more active adsorption sites and Oxygen-containing functional groups were formed on its surface during aging, which was well explained by BET, XPS, FTIR and Elemental Analysis. Our study found that the physicochemical properties of biochar changed significantly during field aging. Although these changes increased the adsorption of heavy metals by biochar, the reduced stability of biochar to passivated heavy metal ions.

Ozonized biochar filtrate effects on the growth of Pseudomonas putida and cyanobacteria Synechococcus elongatus PCC 7942

BACKGROUND

Biochar ozonization was previously shown to dramatically increase its cation exchange capacity, thus improving its nutrient retention capacity. The potential soil application of ozonized biochar warrants the need for a toxicity study that investigates its effects on microorganisms.

RESULTS

In the study presented here, we found that the filtrates collected from ozonized pine 400 biochar and ozonized rogue biochar did not have any inhibitory effects on the soil environmental bacteria Pseudomonas putida, even at high dissolved organic carbon (DOC) concentrations of 300 ppm. However, the growth of Synechococcus elongatus PCC 7942 was inhibited by the ozonized biochar filtrates at DOC concentrations greater than 75 ppm. Further tests showed the presence of some potential inhibitory compounds (terephthalic acid and p-toluic acid) in the filtrate of non-ozonized pine 400 biochar; these compounds were greatly reduced upon wet-ozonization of the biochar material. Nutrient detection tests also showed that dry-ozonization of rogue biochar enhanced the availability of nitrate and phosphate in its filtrate, a property that may be desirable for soil application.

CONCLUSION

Ozonized biochar substances can support soil environmental bacterium Pseudomonas putida growth, since ozonization detoxifies the potential inhibitory aromatic molecules.

Bioaugmentation and bioaugmentation-assisted phytoremediation of heavy metal contaminated soil by a synergistic effect of cyanobacteria inoculation, biochar, and purslane (Portulaca oleracea L.)

In recent decades, soil contamination with heavy metals has become an environmental crisis due to their long-term stability and adverse biological effects. Therefore, bioremediation is an eco-friendly technology to remediate contaminated soil, which the efficiency requires further research. This study was designed to comparatively investigate two strategies: bioaugmentation by using a cyanobacterial species (Oscillatoria sp.) and bioaugmentation-assisted phytoremediation by using Oscillatoria sp. and purslane (Portulaca oleracea L.) for the bioremediation of soil contaminated by heavy metals (Cr (III), Cr (VI), Fe, Al, and Zn). Various quantities of biochar (0.5, 2, and 5% (w/w)) were used as an amendment in the experiments to facilitate the remediation process. The results of the bioaugmentation test showed that applying biochar and cyanobacteria into contaminated soil significantly increased the chlorophyll a, nitrogen, and organic carbon contents. In contrast, the extractable fractions of Cr (III), Cr (VI), Zn, Al, and Fe declined compared with those of the control treatment. The highest reduction content (up to 87 %) in the extractable portion was obtained for Cr (VI). The development of longer root and hypocotyl lengths and vigour index from lettuces and radish seeds grown in the remediated soil confirmed the success of remediation treatments. Moreover, the findings of the bioaugmentation-assisted phytoremediation test displayed a reduction in the bioavailable fraction of Cr (III), Cr (VI), Zn, Al, and Fe. Cr (III) presented the highest reduction (up to 90 %) in metal bioavailability. With cyanobacteria inoculation and biochar addition, the shoot and root lengths of purslane grew 4.6 and 3-fold while the heavy metal accumulation decreased significantly. Besides, these treatments enhanced the tolerance index (TI) quantities of purslane whereas diminished its bioaccumulation coefficient (BAC) and bioconcentration factor (BCF) values. For all heavy metals (except Zn), translocation factor (TF) and BAC values were found to be less than 1.0 at all treatments, indicating the successful phytoextraction by the purslane. These results suggest that the purslane can be considered an excellent phytoextracting agent for soils contaminated with heavy metals.

A comprehensive assessment on bioavailability, leaching characteristics and potential risk of polycyclic aromatic hydrocarbons in biochars produced by a continuous pyrolysis system

Biochar application as a soil amendment has attracted worldwide attention. Nevertheless, polycyclic aromatic hydrocarbons (PAHs) formed during biochar production might enter into ecosystems and threaten human health after application to soil. Continuous pyrolysis systems tend to cause an accumulation of PAHs in biochar owing to short residence time and rapid cooling. This study conducted a comprehensive assessment regarding potential risk of PAHs in biochars produced by a continuous pyrolysis system based on bioavailability, leaching behavior, toxic equivalent quantity, health risk and phytotoxicity of PAHs. Results showed that the concentrations of total PAHs in biochars were in the range of 93.40-172.40 mg/kg, exceeding the European Biochar Certificate standard. 3-rings PAHs were the predominant groups. The percentages of total freely dissolved and leachable PAHs were lower than 1%. RH contained the least bioavailable and leachable PAHs concentration and phytotoxicity compared with CS and PS, which might attribute to the characteristic of three biochars. CS and PS were acidic and exhibited high levels of DOC and VFAs, while RH was strongly alkaline and presented greater aromaticity and higher surface area, which might have resulted in high adsorptive capacity and decreased bioavailability of PAHs. When the biochar application rate was higher than 0.6 t/ha, the incremental lifetime cancer risk value for human exposure to biochar-borne PAHs through the biochar-amended soil was over 10^-6, suggesting carcinogenic risks. Germination index values of biochars ranged from 25.66 to 88.95%. Phytotoxicity mainly was caused by bioavailable PAHs and dissolved organic compounds. Overall, these findings highlighted that although the percentage of bioavailable PAHs was low, the potential health risk and phytotoxicity of PAHs in biochars produced by a continuous pyrolysis system was of a great concern. High biochar application rates should be avoided without processing both for soil safety and human health.

Efficient reclaiming phosphate from aqueous solution using waste limestone modified sludge biochar: Mechanism and application as soil amendments

A novel limestone-modified biochar derived from sewage sludge was prepared to reclaim phosphorus (P) from aqueous solution, and the potential application of P-laden biochar as soil amendments was also investigated. The limestone-modified biochar demonstrated excellent performance on phosphate recovery from aqueous solution in a wide range of pH (2.0-11.0), with maximum adsorption capacity of the biochar (Limestone/sludge mass ratio of 3:1) up to 231.28 mg P/g, which was 10.7 times that of the original sludge biochar. The adsorption was well described by the pseudo second-order model and Langmuir isotherm model. According to the adsorption thermodynamic parameters, the phosphate adsorption was spontaneous (ΔG⁰ < 0) and endothermic (ΔH⁰ > 0) so that increasing the temperature was beneficial to adsorption. Characterization analysis by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscope-energy dispersive spectrometer (SEM-EDS) proved that electrostatic attraction, surface complexation and brushite (CaHPO₄^.2H₂O) precipitation were the dominant mechanism. The P-laden biochar exhibited an excellent ability to be reused as a new slow-release P fertilizer for soil. Pot experiment results showed that the treatment of P-laden LB 3:1 (P content of 22.8%) addition (1 wt%) significantly promoted Indian Lettuce germination (increasing by 14.4%), plant height (increasing by 18.6%), and dry biomass (53.0%) compared with the control, though it underperformed compared to commercial fertilizer.

Nitric acid-modified hydrochar enhance Cd2+ sorption capacity and reduce the Cd2+ accumulation in rice

Remediating the agricultural soil polluted by cadmium (Cd) is a serious issue in China. Hydrochar showed its potential to purify Cd-contaminated water and improve Cd-contaminated soil due to its vast amounts of macro- and microporous structures. In this study, three concentration gradients of nitric acid (HNO₃, mass fraction: 5%, 10%, 15%) were implemented to age pristine wheat straw hydrochar (N0-HC) aiming to improve surface physiochemical properties. Four HNO₃-aging hydrochars named N0-HC, N5-HC, N10-HC, N15-HC were used to both remove Cd²⁺ from aqueous solution and improve soil properties. Results showed that HNO₃-aging significantly improved the Cd²⁺ adsorption capacity by 1.9-9.9 folds compared to crude hydrochar due to the increased specific surface area (by 1.5-6.5 folds) and oxygen-containing functional group abundance (by 4.5-22.1%). Besides, initial solution pH of 8 or environmental temperature of 318.15 K performed the best Cd²⁺ adsorption capacity. Furthermore, the process of Cd²⁺ adsorption was fitted best to pseudo-second-order (R² = 0.95) and Langmuir models (R² = 0.98), respectively. Nanjing 46 (Oryza sativa L) and HNO₃-aging hydrochars were furtherly applied into Cd-contaminated paddy soil to investigate the mitigation of Cd translation from soil to rice. N15-HC-1% (w/w) performed the best effect on reducing cadmium accumulation in various parts of rice plants. Overall, this research provided an approach to improve hydrochar capacity to remove Cd²⁺ from aqueous solution and mitigate Cd translation from soil to rice.

Bioengineered biochar as smart candidate for resource recovery toward circular bio-economy: a review

Biochar's ability to mediate and facilitate microbial contamination degradation, as well as its carbon-sequestration potential, has sparked interest in recent years. The scope, possible advantages (economic and environmental), and future views are all evaluated in this review. We go over the many designed processes that are taking place and show why it is critical to look into biochar production for resource recovery and the role of bioengineered biochar in waste recycling. We concentrate on current breakthroughs in the fields of engineered biochar application techniques to systematically and sustainable technology. As a result, this paper describes the use of biomass for biochar production using various methods, as well as its use as an effective inclusion material to increase performance. The impact of biochar amendments on microbial colonisation, direct interspecies electron transfer, organic load minimization, and buffering maintenance is explored in detail. The majority of organic and inorganic (heavy metals) contaminants in the environment today are caused by human activities, such as mining and the use of chemical fertilizers and pesticides, which can be treated sustainably by using engineered biochar to promote the establishment of a sustainable engineered process by inducing the circular bioeconomy.

Phytoremediation of cadmium-contaminated soil by Bidens pilosa L.: impact of pine needle biochar amendment

The objective of the present study was to evaluate the feasibility of pine needle biochar as a soil amendment to promote the growth of Bidens pilosa L. and enhance its ability to phytoextract the cadmium from soil. Pot experiments (50 d) were designed as control experiment (C); metal treatment (MT), 20 mg Cd kg^-1; biochar treatment (BT₁₀₀ or BT₂₀₀), 100 or 200 mg kg^-1; and metal-biochar treatment (MBT₁₀₀ or MBT₂₀₀), 20 mg Cd kg^-1 and 100 or 200 mg biochar kg^-1. The Cd (20 mg kg^-1) or biochar treatment (100 mg kg^-1) increased the dry weight and root length of B. pilosa. The biochar amendment enhanced the metal concentration in root and shoot of the plant. The plant could accumulate 39.47±5.44 mg Cd kg^-1 in shoots (MT), which increased to 45.96±17.3 mg Cd kg^-1 and 55.01±5.65 mg Cd kg^-1 under biochar treatment sets MBT₁₀₀ and MBT₂₀₀, respectively. The Cd uptake by B. pilosa in MT, MBT₁₀₀, and MBT₂₀₀ treatments were 67.81 μg/plant, 78.58 μg/plant, and 76.13 μg/plant, respectively. The biochar amendments increased the proline concentrations while decreased the chlorophyll content in leaves indicating the stress on the plant. Overall, the result indicates that soil amended with pine needle biochar at 100 mg kg^-1 increased the phytoremediation ability of B. pilosa.

Selenium improved the combined remediation efficiency of Pseudomonas aeruginosa and ryegrass on cadmium-nonylphenol co-contaminated soil

Most chemical plant wastewater contains both organic and inorganic pollutants, which are easy to diffuse along with surface runoff. The combined pollution of nonylphenol (NP) and cadmium (Cd) in soil is a serious problem that has not attracted enough attention. Based on the effects of selenium (Se) and Pseudomonas aeruginosa (P. aeruginosa) on plant and soil microbial communities, we speculated that the application of Se and P. aeruginosa in soil could improve the phytoremediation efficiency of ryegrass on contaminated soil. In this study, pot experiments with Cd and NP co-contaminated soil were conducted, and the results showed that application of P. aeruinosa alone could improve the removal rates of NP and Cd by ryegrass, and the supplementary of Se further enhanced the effect of micro-phyto remediation, with the highest removal rates of NP and Cd were 79.6% and 49.4%, respectively. The application of P. aeruginosa plus Se reduced the adsorption of Cd and NP through C-O and Si-O-Fe of the soil, changed the enzyme activity, and also affected the changing trend of the microbial community in soil. Pseudomonas, Sphingomonadales, Nitrospira, and other beneficial bacteria were enriched after a 60-day period with P. aeruginosa and Se treatment, thus promoting the removal of NP and Cd. In light of the above results, we suggest that P. aeruginosa application can efficiently facilitate the phytoremediation of ryegrass on Cd-NP co-contaminated soil, and Se supplementation in soil showed the synergistic effect on the remediation.

Effects of Biochar Application on Enzyme Activities in Tea Garden Soil

Animal-manure biochar used as a sustainable amendment to garden soil has been widely applied, and the animal-manure pyrolysis temperatures would also have a regulatory effect on soil functions because of their affections on biochar physio-chemical properties. Here we studied the effects of different dosages of swine-manure biochar on tea garden soil functions, with the swine-manure pyrolysis temperature differed at 350 and 500°C. The results showed that the improvement of soil microbial biomass carbon and nitrogen and enzyme activities was closely related to the addition of 0.5-2% (biochar wt/soil wt) swine-manure biochar. Under different conditions of different carbon application rates and carbon type, the addition of 2% swine-manure biochar pyrolyzed at 350°C showed the best effects on soil enzyme activities and microbial biomass carbon and nitrogen contents. Compared to the control, after the addition of 2% swine-manure biochar, sucrase, phosphatase, catalase, and urease activities increased by 63.3, 23.2, 50.3, and 27.9%, respectively. Microbial biomass carbon and nitrogen contents also increased by 36.4 and 34.3%, respectively. Our study indicated that the effectiveness of using animal-manure swine-manure biochar as a sustainable amendment to soil would provide evidence of tea garden soil improvement and the environmental response to the usage of biochars.

Enhanced Cd phytostabilization and rhizosphere bacterial diversity of Robinia pseudoacacia L. by endophyte Enterobacter sp. YG-14 combined with sludge biochar

Robinia pseudoacacia L., a pioneer woody legume grown in mining areas, has been recognized as a remarkable accumulator of various heavy metals. Compared with other hazardous heavy metals (HMs), it is of low capacity in accumulating Cd, which, as a result, may hinder the phytoremediation efficiency. To enhance R. pseudoacacia's uptake efficiency of Cd, the individual effects of various rhizobia and arbuscular mycorrhizal fungi have been reported, however, the combined influence of endophytes and biochar receives little attention. In the current study, a Cd-adsorbing endophyte Enterobacter sp. YG-14 was inoculated to R. pseudoacacia, and its extraordinary effect on increasing R. pseudoacacia's Cd uptake was found, which was ascribed to the reinforced root Cd chelation by the strain through secreting siderophores/LMWOAs. Further, P-enriched sludge biochar was applied along with YG-14 to form a combined biochar-endophyte-accumulator system, in which biomineralization were reinforced (i.e. CdCO₃ and Cd₂P₂O₇ were generated), as the total and acid-soluble Cd in rhizosphere were reduced by 61.75% and 69.01% respectively, and soil's bacterial diversity was further improved with diversified N₂-fixing microbial biomarkers. Multiple synergistic effects (E > 0) were also found, with the optimum performance on plant growth parameters (increased by 39.61%-561.91%) in comparison to the control group. Moreover, the system exhibited a preferable Cd phytostabilization capacity with the highest increase (81.42%) in Cd accumulation and a significant reduction (72.73%) in Cd root-to-shoot translocation.

The use of mercapto-modified palygorskite prevents the bioaccumulation of cadmium in wheat

In this study, the mechanism by which mercapto-modified palygorskite (MPAL) mediates Cd and Mn absorption by wheat was elucidated. In the aqueous phase, MPAL can react with Cd to form Cd-thiol complexes and CdO and with Mn to form MnO. In the wheat-soil system, 0.1-0.3% MPAL application increased the biomass of wheat by 18.6-29.4% and decreased the Cd concentration in shoots and roots by 19.4-51.8% and 35.9-64%, respectively; however, MPAL application did not decrease the diethylenetriaminepentaacetic acid (DTPA)-extracted Cd concentration in soil, probably because the formed Cd-thiol complexes and CdO could not be taken up by plants but could be extracted by DTPA. MPAL appeared to increase the Mn concentration in plants and the DTPA-extracted Mn concentration in soil, possibly because of the reduction in soil Mn oxides to more soluble Mn(Ⅱ) by the thiol groups in MPAL. MPAL enriched plant growth-promoting rhizobacteria and Cd-immobilizing bacteria and strengthened the sulfate reduction metabolism in rhizosphere soil, which partly contributed to the improvement in plant growth and the reduction in Cd bioaccumulation in wheat. These findings highlight the importance of the thiol group in MPAL and the regulation of the rhizosphere bacterial community in mediating Cd and Mn bioaccumulation in wheat.

Effect of biochar-immobilized Sphingomonas sp. PJ2 on bioremediation of PAHs and bacterial community composition in saline soil

This study aimed to investigate the bioremediation efficiency and bacterial regulation mechanism of biochar-immobilized bacterium (BM) in polycyclic aromatic hydrocarbon (PAH)-contaminated saline soil by conducting pot experiments. In BM treatment, PAH-degrading strain Sphingomonas sp. PJ2 was inoculated into biochar produced at 400 °C and 600 °C using the pine needles (BM400 and BM600). The removal rates of PAHs, soil physicochemical properties, abundance of PAH-ring hydroxylating dioxygenase (PAH-RHD), and bacterial community composition were determined. After 60 days of bioremediation, BM treatment significantly (P < 0.05) increased the removal rate of PAHs compared with biochar and PJ2 alone (15.94% and 37.3%, respectively). BM treatment clearly improved the physicochemical properties of saline soil. Moreover, the amount of Gram-positive PAH degraders increased in BM-treated soils compared with other treatments, and their gene abundance had a strong positive correlation with the removal rates of PAHs in soils (r = 0.896; P < 0.01). Furthermore, BM treatment increased the abundance of Sphingomonas genus, indicating that the strain PJ2 could survive and colonize in PAH-contaminated saline soil under the protection of biochar. This study provided an effective and green approach for the remediation and improvement of the PAH-contaminated saline soil.

Influences of modified biochar on metal bioavailability, metal uptake by wheat seedlings (Triticum aestivum L.) and the soil bacterial community

A 6 weeks pot culture experiment was carried out to investigate the stabilization effects of a modified biochar (BCM) on metals in contaminated soil and the uptake of these metals by wheat seedlings. The results showed that the application of BCM significantly increased the soil fertility, the biomass of wheat seedling roots increased by more than 50%, and soil dehydrogenase (DHA) and catalase (CAT) activities increased by 369.23% and 12.61%, respectively. In addition, with the application of BCM, the diethylenetriaminepentaacetic acid extractable (DTPA-extractable) Cd, Pb, Cu and Zn in soil were reduced from 2.34 to 0.38 mg/kg, from 49.27 to 25.65 mg/kg, from 3.55 mg/kg to below the detection limit and from 4.05 to 3.55 mg/kg, respectively. Correspondingly, the uptake of these metals in wheat roots and shoots decreased by 62.43% and 79.83% for Cd, 73.21% and 66.32% for Pb, 57.98% and 68.92% for Cu, and 40.42% and 43.66% for Zn. Furthermore, BCM application decreased the abundance and alpha diversity of soil bacteria and changed the soil bacterial community structure dramatically. Overall, BCM has great potential for the remediation of metal-contaminated soils, but its long-term impact on soil metals and biota need further research.

Sustainable stabilization/solidification of the Pb, Zn, and Cd contaminated soil by red mud-derived binders

Red mud and phosphogypsum are voluminous industrial by-products worldwide. They have long been disposed of in landfills or open storage, leading to a waste of resource and environmental pollution. This study provides a novel approach to recycle these industrial by-products as sustainable red mud-phosphogypsum-Portland cement (RPPC) binders for stabilization/solidification (S/S) of multimetal-contaminated soil. The physical strength, metal leachability and microstructure of S/S soil were investigated after 7-day and 28-day curing, as well as freezing-thawing (F-T) cycle and wetting-drying (W-D) cycle. The results show that the strength of soil treated by all binders fulfilled the uniaxial compressive strength requirement (350 kPa) of S/S waste in landfills. Microstructural analyses show that the main hydration products of the RPPC S/S soil are ilmenite, ettringite, anhydrite and hydrated calcium silicate. The 10% and 15% RPPC binders have a competitive metal immobilization ability compared with 10% PC, but the immobilization priority is different: Pb > Zn > Cd in RPPC system and Zn > Cd > Pb in PC system, respectively, probably due to the precipiataion of Pb²⁺ with the abundant SO₄^2- in phosphogypsum in RPPC system. The strength of RPPC and PC treated soil was still higher than 350 kPa except for RPPC7.5 after 10 freeze-thaw or 10 wetting-drying cycles. The RPPC binder performed worse than PC binder after both freeze-thaw and wetting-drying cycles, especially at a lower dosage. Only the metal leaching concentrations of samples treated by RPPC15 and PC10 could fulfil the Chinese standards for hazardous wastes.

Synergistically promoting plant health by harnessing synthetic microbial communities and prebiotics

Soil-borne diseases cause serious economic losses in agriculture. Managing diseases with microbial preparations is an excellent approach to soil-borne disease prevention. However, microbial preparations often exhibit unstable effects, limiting their large-scale application. This review introduces and summarizes disease-suppressive soils, the relationship between carbon sources and the microbial community, and the application of human microbial preparation concepts to plant microbial preparations. We also propose an innovative synthetic microbial community assembly strategy with synergistic prebiotics to promote healthy plant growth and resistance to disease. In this review, a new approach is proposed to improve traditional microbial preparations; provide a better understanding of the relationships among carbon sources, beneficial microorganisms, and plants; and lay a theoretical foundation for developing new microbial preparations.

Biochar and urban solid refuse ameliorate the inhospitality of acidic mine tailings and foster effective spontaneous plant colonization under semiarid climate

Phytomanagement is considered a suitable option in line with nature-based solutions to reduce environmental risks associated to metal(loid) mine tailings. We aimed at assessing the effectiveness of biochar from pruning trees combined with compost from urban solid refuse (USR) to ameliorate the conditions of barren acidic (pH ~5.5) metal(loid) mine tailing soils (total concentrations in mg kg^-1: As ~220, Cd ~40, Mn ~1800, Pb ~5300 and Zn ~8600) from Mediterranean semiarid areas and promote spontaneous plant colonization. Two months after amendment addition were enough to observe improvements in chemical and physico-chemical tailing soil properties (reduced acidity, salinity and water-soluble metals and increased organic carbon and nutrients content), which resulted in lowered ecotoxicity for the soil invertebrate Enchytraeus crypticus. Recalcitrant organic carbon provided by biochar remained in soil whereas labile organic compounds provided by USR were consumed over time. These improvements were consistent for at least one year and led to lower bulk density, higher water retention capacity and higher scores for microbial/functional-related parameters in the amended tailing soil. Spontaneous growth of native vegetation was favored with amendment addition, but adult plants of remarkable size were only found after three years. This highlights the existence of a time-lag between the positive effects of the amendment on tailing soil properties being observed and these improvements being translated into effective spontaneous plant colonization.

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

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

Mechanisms of Enterobacter bugandensis TJ6 immobilization of heavy metals and inhibition of Cd and Pb uptake by wheat based on metabolomics and proteomics

Microbial passivation remediation of heavy metal-contaminated farmland has attracted increasing attention. However, the molecular mechanism by which heavy metal-immobilizing bacteria inhibit the uptake of Cd and Pb by wheat is not clear. Herein, a heavy metal-immobilizing bacterium, Enterobacter bugandensis TJ6, was used to reveal its immobilization mechanisms of Cd and Pb and inhibition of Cd and Pb uptake by wheat using metabolomics and proteomics. Compared with the control, strain TJ6 significantly reduced (44.7%-56.6%) the Cd and Pb contents of wheat roots and leaves. Strain TJ6 reduced the Cd and Pb concentrations by adsorption, intracellular accumulation, and bioprecipitation in solution. Untargeted metabolomics showed that strain TJ6 produced indole-3-acetic acid (IAA), betaine, and arginine under Cd and Pb stress, significantly improving the resistance of strain TJ6 and wheat to Cd and Pb. Label-free proteomics showed that 143 proteins were upregulated and 61 proteins were downregulated in wheat roots in the presence of strain TJ6. The GO items of the differentially expressed proteins (DEPs) involved in protein-DNA complexes, DNA packaging complexes, and peroxidase activity were enriched. In addition, the ability of wheat roots to synthesize abscisic acid and jasmonic acid was improved. In conclusion, strain TJ6 reduced Cd and Pb uptake in wheat through its own adsorption of Cd and Pb and regulation of wheat root DNA repair ability, plant hormone levels, and antioxidant activities. These results provide new insights and a theoretical basis for the application of heavy metal-immobilizing bacteria in safe wheat production.

Effects of magnetic biochar-microbe composite on Cd remediation and microbial responses in paddy soil

There is growing global interest in the bioremediation of cadmium (Cd) using combinations of biochar and microorganisms. However, the interactions among biochar, introduced and indigenous microorganisms remain unclear. Accordingly, a 90 day microcosm experiment was conducted to investigate this by adding Bacillus sp. K1 strain inoculated rice straw biochar (SBB) and magnetic straw biochar (MBB) into a Cd contaminated paddy soil from Hunan, China. All treatments were incubated aerobically (60% water holding capacity) or anaerobically for 90 d. During both soil incubations, Bacillus sp. K1 successfully colonized in soil with composites applications. Soil pH was significantly increased from acid to neutral, and available Cd decreased with the addition of both composites. The better remediation efficiency of MBB than SBB under anerobic conditions was attributed to the transformation of acetic acid-extractable Cd into the residual fraction, caused by Cd²⁺ bonding with crystal Fe₃O₄. The application of the two kinds of composites caused similar changes to both microbial communities. There was a slight decrease in indigenous microbial alpha diversity with the MBB aerobic application, while the total population number of bacteria was increased by 700%. Both the redundancy analysis and Mantel analyses indicated that pH and microbial biomass C contributed to the colonization of Bacillus sp. K1 with SBB under aerobic conditions, and with MBB under anerobic conditions, respectively. The research provides a new insight into interactive effects and investigates immobilization mechanisms involved of bacterial/biochar composites in anaerobic and aerobic soils.

Evolution of redox activity of biochar during interaction with soil minerals: Effect on the electron donating and mediating capacities for Cr(VI) reduction

Biochar in soil is susceptible to natural aging along with soil minerals, which might alter its electrochemical properties and redox reactions with contaminants. In this study, soluble mineral salts (FeCl₃, MnCl₂, AlCl₃, CaCl₂) and clay mineral (kaolinite) were selected to investigate the impact of co-aging with soil minerals on the redox activity of peanut-shell biochar for Cr(VI) reduction. Natural aging for 3-month induced oxidation of biochar with the decrease of reducing moieties, i.e., ‒C‒OH from 26.8-43.7% to 18.4-24.1%. Co-aging with minerals except for Mn(II) further decreased the proportion of ‒C‒OH to 6.94-22.2% because of the interaction between mineral ions and biochar, resulting in the formation of mineral-biochar complex and new minerals, e.g. β-FeOOH. Due to its reductivity, Mn(II) presented the least decrease or even slight increase of ‒C‒OH while itself was oxidized to Mn(III) and Mn(IV). The decline of ‒C‒OH caused the decrease of Cr(VI) reduction rate constant from 2.18 to 2.47 × 10^-2 h^-1 for original biochars to 0.71-1.95 × 10^-2 h^-1 for aged ones, of which co-aging with Fe(III) showed the lowest reduction rate constant among all minerals. The electron mediating capacity of biochar also decreased after aging alone or co-aging with Al, Ca, and kaolinite, while co-aging with Fe(III) and Mn(II) facilitated the electron transfer process, increasing the rate constant by 219.3-1237% due to electron mediation through valence transformation of Fe(III)-Fe(II) and Mn(II)-Mn(III). Given the abundance of soil minerals, it was essential to consider this crucial factor for redox reactions when applying biochar for soil remediation.

Study of soil microorganisms modified wheat straw and biochar for reducing cadmium leaching potential and bioavailability

The application of crops straw and biochar in trace metals remediation from the contaminated environment attracted more and more attention during the past decade. Although there has been some review work on the mechanism of trace metals stabilization by crops straw, the effects and mechanisms of interaction among soil indigenous-microbes and crops-straw for trace metal adsorption and stabilization is still unclear. In this study, the dynamic effects along with potential mechanisms of wheat-straw (WS), wheat-straw biochar (WBC) and biologically modified wheat-straw (BMWS) were conducted to investigate the adsorption, leaching behaviour, chemical fractions and bioavailability of cadmium (Cd). The results showed that the biosorption capacity (q_e) was most elevated in the BMWS treatment (14.42 mg g^-1) as compared to WBC (6.28 mg g^-1) and WS (4.20 mg g^-1). The application of BMWS, WBC and WS at the rate of 3% significantly reduced Cd concentration in leachate to 53, 45 and 21% respectively, as compared to control. The addition of BMWS reduced the exchangeable Cd fraction resulted an increase in organic matter and carbonate bound Cd fraction in the soil. The DTPA extractable Cd was significantly decreased by 31.2 and 28.6% with the application of BMWS and WBC at 3% w/w respectively as compared to control. The research results may provide a novel perceptive for the development of functional materials and strategies for eco-friendly and sustainable trace metal remediation in contaminated soil and water by combination of straw and soil-indigenous microorganisms.

Removal Mechanisms of Slag against Potentially Toxic Elements in Soil and Plants for Sustainable Agriculture Development: A Critical Review

Potentially toxic element (PTE) pollution is a major abiotic stress, which reduces plant growth and affects food quality by entering the food chain, and ultimately poses hazards to human health. Currently, the use of slag in PTE-contaminated soils has been reported to reduce PTEs and toxicity in plants. This review highlights the role of slag used as a fertilizer for better crop production and sustainable agricultural development. The application of slag increased the growth, yield, and quality of crops under PTE toxicity. The mechanisms followed by slag are the immobilization of PTEs in the soil, enhancement of soil pH, changes in the redox state of PTEs, and positive changes in soil physicochemical and biological properties under PTE toxicity. Nevertheless, these processes are influenced by the plant species, growth conditions, imposition length of stress, and type of slag used. The current review provides an insight into improving plant tolerance to PTE toxicity by slag-based fertilizer application and highlights the theoretical basis for applying slag in PTE-contaminated environments worldwide.

Recent advances in biochar engineering for soil contaminated with complex chemical mixtures: Remediation strategies and future perspectives

Heavy metal/metalloids (HMs) and polycyclic aromatic hydrocarbons (PAHs) in soil have caused serious environmental problems, compromised agriculture quality, and have detrimental effects on all forms of life including humans. There is a need to develop appropriate and effective remediation methods to resolve combined contaminated problems. Although conventional technologies exist to tackle contaminated soils, application of biochar as an effective renewable adsorbent for enhanced bioremediation is considered by many scientific researchers as a promising strategy to mitigate HM/PAH co-contaminated soils. This review aims to: (i) provide an overview of biochar preparation and its application, and (ii) critically discuss and examine the prospects of (bio)engineered biochar for enhancing HMs/PAHs co-remediation efficacy by reducing their mobility and bioavailability. The adsorption effectiveness of a biochar largely depends on the type of biomass material, carbonisation method and pyrolysis conditions. Biochar induced soil immobilise and remove metal ions via various mechanisms including electrostatic attractions, ion exchange, complexation and precipitation. PAHs remediation mechanisms are achieved via pore filling, hydrophobic effect, electrostatic attraction, hydrogen bond and partitioning. During last decade, biochar engineering (modification) via biological and chemical approaches to enhance contaminant removal efficiency has garnered greater interests. Hence, the development and application of (bio)engineered biochars in risk management, contaminant management associated with HM/PAH co-contaminated soil. In terms of (bio)engineered biochar, we review the prospects of amalgamating biochar with hydrogel, digestate and bioaugmentation to produce biochar composites.

Pristine and iron-engineered animal- and plant-derived biochars enhanced bacterial abundance and immobilized arsenic and lead in a contaminated soil

In this study, typical animal- and plant-derived biochars derived from pig carcass (PB) and green waste (GWB), and their iron-engineered products (Fe-PB and Fe-GWB) were added at the dose of 3% (w/w) to an acidic (pH = 5.8) soil, and incubated to test their efficacy in improving soil quality and immobilizing arsenic (As = 141.3 mg kg^-1) and lead (Pb = 736.2 mg kg^-1). Soil properties, microbial activities, and the geochemical fractions and potential availabilities of As and Pb were determined in the non-treated (control) and biochar-treated soil. Modification of PB (pH = 10.6) and GWB (pH = 9.3) with Fe caused a decrease in their pH to 4.4 and 3.4, respectively. The application of PB and GWB significantly increased soil pH, while Fe-PB and Fe-GWB decreased soil pH, as compared to the control. Application of Fe-GWB and Fe-PB decreased the NH₄H₂PO₄-extractable As by 32.8 and 35.9%, which was more effective than addition of GWB and PB. However, PB and GWB were more effective than Fe-PB and Fe-GWB in Pb immobilization. Compared to the control, the DTPA-extractable Pb decreased by 20.6 and 21.7%, respectively, following PB and GWB application. Both biochars, particularly PB significantly increased the 16S rRNA bacterial gene copy numbers, indicating that biochar amendments enhanced the bacterial abundance, implying an alleviation of As and Pb bio-toxicity to soil bacteria. The results demonstrated that pristine pig carcass and green waste biochars were more effective in immobilizing Pb, while their Fe-engineered biochars were more effective in As immobilization in co-contaminated soils.

Soil heavy metal pollution and food safety in China: Effects, sources and removing technology

Soil plays a fundamental role in food safety and the adverse effects of contaminants like heavy metal (loid)s on crop quality have threatened human health. Therefore, it is important to focus on the food safety and agricultural soil pollution by heavy metals, especially for China where the demand for food production is increasing. This review comprehensively introduced the current status of agricultural soil pollution by heavy metals in China, analyzed the main sources of contaminants, including the applications of pesticides and fertilizers, atmospheric deposition related to vehicle emissions and coal combustion, sewage irrigation and mining. Food safety and agricultural soil pollution by heavy metals, the removal technologies for soil remediation such as soil amendments, phytoremediation and foliar sprays were also introduced. The review can provide significant insights for policymakers, environmental engineers, and agro-technicians regarding soil contamination control and management strategies and technologies.

Application of olive mill waste-based biochars in agriculture: Impact on soil properties, enzymatic activities and tomato growth

The olive oil industry is an important economic sector in Mediterranean countries. However, oil production is unfortunately accompanied by the generation of huge amounts of olive mill solid wastes (OMSW) and olive mill wastewater (OMWW). In the present study, a strategy is proposed for converting these olive mill wastes into biochar through pyrolysis, for their later use as an organic amendment in agriculture. Specifically, two biochars were prepared from the pyrolysis of OMSW at 500 °C, either alone or impregnated with OMWW (OMSW-B and I-OMSW-B). The characterization of the OMSW and I-OMSW samples and their derived biochars showed that the fixed carbon and ash contents in the feedstocks increased by 38% and 11% respectively for OMSW-B, and by 37% and 12% respectively for I-OMSW-B. Interestingly, the impregnation process significantly increased Na, P, K, Ca and Fe contents in the produced biochars. The effect of OMSW-B and I-OMSW-B amendments at different application dose (1%, 2.5% and 5% wt/wt) on the enzymatic activity of an agricultural soil was performed at laboratory scale with a pot test. The experimental results showed that phosphatase and urease activity increased with biochar application rate; amendment with I-OMSW-B at 1%, 2.5% and 5% enhanced the phosphatase activity by 63%, 142% and 285% and urease activity by 50%, 116% and 149%, respectively. On the other hand, dehydrogenase and protease activities were higher for the application rate of 2.5% biochar. Biochar amendment promoted tomatoes seedling growth after 10 weeks, which was highest in the application rates of 2.5% and 5% for both OMSW-B and I-OSMW-B. Thus, the produced biochars had great potential to be used as biofertilizers in agriculture.

Optimization of biosurfactant production from Pseudomonas sp. CQ2 and its application for remediation of heavy metal contaminated soil

The present study was conducted to enhance the biosurfactant production yield of Pseudomonas sp. CQ2 isolated from the Chongqing oilfield (China). Besides, the capability of biosurfactant and underlying mechanism for remediation of heavy metal contaminated soil was also investigated. Our results suggested that maximum biosurfactant production (40.7 g/L) was attained at 35 °C by using soybean oil and ammonium nitrate as carbon and nitrogen sources with pH 7, rotational speed of 175 rpm and inoculation ratio of 3%). The removal efficiencies of 78.7, 65.7 and 56.9% for Cd, Cu and Pb respectively were achieved at optimized bioleaching conditions (pH: 11, soil/solution ratio: 30:1 and non-sterilized soil), comparative tests between common chemical surfactants (SDS, Tween-80) and biosurfactants demonstrated the larger removal capacity of biosurfactants. Through SEM-EDX, it was found that the granular material disappeared, the content of Cd, Cu and Pb decreased significantly, and the soil surface became smooth with hole formation after soil washing following bioleaching. ATR-FTIR results showed that the carboxyl functional groups in biosurfactants could chelate heavy metals. These results indicated that biosurfactants from Pseudomonas sp. CQ2 could effectively eliminate Cd, Cu, and Pb from soil.

Inoculation of Bacillus megaterium strain A14 alleviates cadmium accumulation in peanut: effects and underlying mechanisms

AIMS

A cadmium (Cd)-tolerant Bacillus megaterium strain A14 was used to investigate the effects and mechanisms of bacterial inoculation on peanut growth, Cd accumulation in grains and Cd fixation in Cd-contaminated soil.

METHODS AND RESULTS

Spectroscopic analysis showed that A14 has many functional groups (-OH, -NH2 and -COO et al.) distributed on its surface. The pot experiment indicated that compared to the Cd-contaminated soil alone treatment, inoculation with strain A14 increased shoot and root biomass by 59·93 and 58·31% respectively. The accumulation of Cd in grains decreased by 48·14%, while the proportion of exchangeable Cd in soil decreased from 40 to 26% in A14 inoculated soil.

CONCLUSIONS

Inoculation with B. megaterium A14 improved peanut plant growth via (i) adsorbing Cd²⁺ through functional groups on cell surface, (ii) immobilization of Cd in soil through extracellular secretions, (iii) scavenging the reactive oxygen species through production of antioxidant enzymes, and (iv) by reducing the phytoavailable Cd through regulation of Cd transport gene expression.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provided a new sight on microbial approach for the chemical composition transformation of soil Cd and associated food safety production, which pointed out an efficient way to improve peanut cultivation.

Microbial Cd(II) and Cr(VI) resistance mechanisms and application in bioremediation

The heavy metals cadmium (Cd) and chromium (Cr) are extensively used in industry and result in water and soil contamination. The highly toxic Cd(II) and Cr(VI) are the most common soluble forms of Cd and Cr, respectively. They enter the human body through the food chain and drinking water and then cause serious illnesses. Microorganisms can adsorb metals or transform Cd(II) and Cr(VI) into insoluble or less bioavailable forms, and such strategies are applicable in Cd and Cr bioremediation. This review focuses on the highlighting of novel achievements on microbial Cd(II) and Cr(VI) resistance mechanisms and their bioremediation applications. In addition, the knowledge gaps and research perspectives are also discussed in order to build a bridge between the theoretical breakthrough and the resolution of Cd(II) and Cr(VI) contamination problems.

Application of mixed bacteria-loaded biochar to enhance uranium and cadmium immobilization in a co-contaminated soil

This research explored the effect of biochar pyrolyzed from five different materials on U and Cd immobilization in soil. The results showed that all biochars improved the soil properties and microbial metabolic activities, and effectively immobilized U and Cd, especially corn stalk biochar. Subsequently, three strains Bacillus subtilis, Bacillus cereus, and Citrobacter sp. were mixed in a 3:3:2 proportion as a kind of mixed bacteria (MB9) that could adsorb U and Cd effectively. Two types of MB9-loaded biochar were synthesized by physical adsorption and sodium alginate embed method and referred to as AIB and EIB, respectively. MB9-loaded biochar showed superior U and Cd immobilization performance. At 75 d, the highest reduction in the DTPA- extractable U and Cd (69 % and 56 %) was achieved with the 3% AIB amendment. Additionally, compared to the addition of biochar or MB9 alone, AIB was more effective in promoting celery growth and reducing U and Cd accumulation. Finally, the microbial community structure analysis suggested that the relative abundance of Citrobacter genus and Bacillus genus was significantly increased, suggesting that the mixed bacteria MB9 was successfully colonized. These findings may provide a feasible technology for green and cost-effective remediation of heavy metal contamination in farmland soil.

Role of passivators for Cd alleviation in rice-water spinach intercropping system

Soil pollution with cadmium (Cd) has posed a threat to our food safety. And rice consumption is the main source of Cd intake in China. Rice intercropping with water spinach is an efficient way for crop production and phytoremediation in Cd-contaminated soil. However, few people work on the Cd remediation by a combination of the passivation and intercropping. In this study, two passivators (the Si-Ca-Mg ameliorant and the Fe-modified biochar with microbial inoculants) were used in the monoculture and intercropping systems to evaluate the potential of co-effect of passivators and cropping systems on the plant growth and Cd phytoremediation. Results showed that the highest rice biomass and rice yield were presented in the intercropping system with the passivator additions, however, relatively lower biomass was showed in water spinach due to the competition with rice. And more Cd accumulated in water spinach while lower Cd in that of different rice parts. The intercropping system with the addition of the Si-Ca-Mg ameliorant and the microbial Fe-modified biochar significantly reduced the Cd contents in brown rice by 58.86% and 63.83%, while notably enhanced the Cd accumulation of water spinach by 32.0% and 22.0%, compared with the monoculture without passivation, respectively. This probably due to the increased pH, the lowered Cd availability in soil, and the reduced TF and BCF values in rice plants with passivator applications. Collectively, this study indicated that rice-water spinach intercropping, especially with the passivator additions, may function as an effective way for Cd remediation and guarantee rice grain safety.