Enhanced Pb immobilization via the combination of biochar and phosphate solubilizing bacteria

Efficient cadmium-resistant plant growth-promoting bacteria loaded on pig bone biochar has higher efficiency in reducing cadmium phytoavailability and improving maize performance than on rice husk biochar

Green agriculture faced challenges due to the shortage of efficient cadmium (Cd)-resistant plant growth-promoting bacteria (CdR-PGPB) and their low survival rate and activity during application. In this study, a diverse range of efficient CdR-PGPB were isolated from the rhizosphere soil of Desmodium elegans, especially those with high phosphate-solubilizing capabilities (272.87-450.45 mg L-1). Two highly efficient CdR-PGPB namely, XH1 and XH3 were loaded on to rice husk biochar (RHB) and pig bone biochar (PBB), labelled as RHBM and PBBM respectively. This study aimed to explore their effectiveness and mechanisms in promoting maize growth in a Cd-contaminated planting system. Results showed that PBBM performed best among all treatments. It significantly decreased soil phytoavailable Cd by 53.19 % and Cd content in maize shoot by 85.89 %. It also increased soil available phosphorus by 145.72 %, soil alkaline phosphatase activity by 76.34 %, maize shoot/root biomass by 47.06 %/67.98 %, Chlorophyll (a/b) content by 66.80 %/134.13 % and peroxidase activity by 171.96 %. These results were achieved through the synergistic action of efficient CdR-PGPB and PBB. Therefore, PBBM proved to be a promising and innovative application technique for sustainable agricultural development in Cd-contaminated farmland ecosystems.

Mechanistic insights into tris(2-chloroisopropyl) phosphate biomineralization coupled with lead (II) biostabilization driven by denitrifying bacteria

The ubiquity and persistence of organophosphate esters (OPEs) and heavy metal (HMs) pose global environmental risks. This study explored tris(2-chloroisopropyl)phosphate (TCPP) biomineralization coupled to lead (Pb2+) biostabilization driven by denitrifying bacteria (DNB). The domesticated DNB achieved synergistic bioremoval of TCPP and Pb2+ in the batch bioreactor (efficiency: 98 %).TCPP mineralized into PO43- and Cl-, and Pb2+ precipitated with PO43-. The TCPP-degrading/Pb2+-resistant DNB: Achromobacter, Pseudomonas, Citrobacter, and Stenotrophomonas, dominated the bacterial community, and synergized TCPP biomineralization and Pb2+ biostabilization. Metagenomics and metaproteomics revealed TCPP underwent dechlorination, hydrolysis, the TCA cycle-based dissimilation, and assimilation; Pb2+ was detoxified via bioprecipitation, bacterial membrane biosorption, EPS biocomplexation, and efflux out of cells. TCPP, as an initial donor, along with NO3-, as the terminal acceptor, formed a respiratory redox as the primary energy metabolism. Both TCPP and Pb2+ can stimulate phosphatase expression, which established the mutual enhancements between their bioconversions by catalyzing TCPP dephosphorylation and facilitating Pb2+ bioprecipitation. TCPP may alleviate the Pb2+-induced oxidative stress by aiding protein phosphorylation. 80 % of Pb2+ converted into crystalized pyromorphite. These results provide the mechanistic foundations and help develop greener strategies for synergistic bioremediation of OPEs and HMs.

Effects of phosphorus-solubilizing bacteria and biochar application on phosphorus availability and tomato growth under phosphorus stress

BACKGROUND

Phosphorus-solubilizing bacteria (PSB) are vital in converting insoluble phosphorus into a soluble form that plants can readily absorb and utilize in soil. While previous studies have mainly focused on the extracellular secretion of microorganisms, few have explored the intricate intracellular metabolic processes involved in PSB-mediated phosphorus solubilization.

RESULTS

Here, we uncovered that Ca3(PO4)2 could serve as a source of insoluble phosphorus for the PSB, Pseudomonas sp. NK2. High-performance liquid chromatography (HPLC) results indicated higher levels of organic acids released from insoluble phosphorus compared to a soluble phosphorus source (KH2PO4), with acetic acid released exclusively under insoluble phosphorus condition. Moreover, non-target metabolomics was employed to delve into the intracellular metabolic profile. It unveiled that insoluble phosphorus significantly enhanced the tricarboxylic acid cycle, glycolysis, glyoxylic acid metabolism, and other pathways, leading to the production of acetic acid, gluconic acid, oxalic acid, and citric acid for insoluble phosphorus solubilization. In our quest to identify suitable biochar carriers, we assessed seven types of biochar through the conjoint analysis of NBRIP medium culture and application to soil for 30 days, with cotton straw-immobilized NK2 emerging as the most potent phosphorus content provider. Lastly, NK2 after cotton straw immobilization demonstrated the ability to enhance biomass, plant height, and root development of Solanum lycopersicum L. cv. Micro Tom.

CONCLUSIONS

Pseudomonas sp. NK2 with cotton straw biochar could enhance phosphorus availability and tomato growth. These findings bear significant implications for the practical application of phosphorus-solubilizing bacteria in agricultural production and the promotion of environmentally sustainable farming practices.

Mechanistic insight into the effects of interaction between biochar and soil with different properties on phenanthrene sorption

Soil composition varies considerably in nature, so it is vital to investigate the mechanism of the effect of various soil parameters on biochar sorption capacity. In this study, two biochars (W4 and W7) were derived from wheat straw at temperatures of 400 and 700 °C and were incubated with three different soils. Changes in biochar surface features by aging in the soils and the consequent impact on phenanthrene sorption were examined. The results showed that the effect of adding biochar on phenanthrene sorption capacity (Koc) varied by soil. When biochar was freshly mixed with soil, the Koc value in soil with higher clay content was more dramatically altered by biochar, which is due to clay particles adhering to the biochar surface. Moreover, the Koc value was significantly decreased by the addition of W4 but increased by the addition of W7 in general. After aging, most of the Koc value decreased. The greatest decrease in Koc value was observed in biochar and soil composed with the highest clay content for W4 (24-63%), as well as soil composed with the highest organic matter content for W7 (46-64%). This is because the surface polarity and micropores of biochar dropped the most rapidly in these mixes, resulting in a significant decrease in hydrophobic and pore-filling properties. The results revealed that the impact of biochar-soil interactions on phenanthrene sorption is related to not only biochar properties but also soil clay particles, soil organic matter content and pH. The findings of the study can be utilized to assess the efficacy of biochar application in soil remediation for various features.

Remediation of Cr(VI)-contaminated soil by CS/PPy coupling with Microbacterium sp. YL3

In this research, a new material, chitosan/polypyrrole (CS/PPy), was synthesized and linked with the Cr(VI)-reducing bacterial strain YL3 to treat Cr(VI)-polluted soil. The findings demonstrated that the synergistic application of strain YL3 and CS/PPy achieved the greatest reduction (99.6 %). During the remediation process, CS/PPy served as a mass-storage and sustained release agent in the soil, which initially decreased the toxic effects of high concentrations of Cr(VI) on strain YL3, thereby enhancing the Cr(VI) reduction efficiency of strain YL3. These combined effects significantly mitigated Cr(VI) stress in the soil and restored enzyme activities. Furthermore, wheat growth in the treated soil also significantly improved. High-throughput sequencing of the microorganisms in the treated soil revealed that CS/PPy was not only effective at removing Cr(VI) but also at preserving the original microbial diversity of the soil. This suggests that the combined treatment using strain YL3 and CS/PPy could rehabilitate Cr(VI)-contaminated soil, positioning CS/PPy as a promising composite material for future bioremediation efforts in Cr(VI)-contaminated soils.

Comparative assessment of pantothenic, aspartic, ascorbic and tartaric acids assisted Pb-phytoextraction by sunflower (Helianthus annuus L.)

Phytoextraction of lead (Pb) is a challenging task due to its extremely low mobility within soil and plant systems. In this study, we tested the influence of some novel chelating agents for Pb-phytoextraction using sunflower. The Pb was applied at control (0.0278 mM) and 4.826 mM Pb as Pb(NO3)2 through soil-spiking. After 10 days of Pb addition, four different organic ligands (aspartic, ascorbic, tartaric, and pantothenic acids) were added to the soil at 1 mM concentration each. respectively. In the absence of any chelate, sunflower plants grown at 4.826 mM Pb level accumulated Pb concentrations up to 104 µg g-1 DW in roots, whereas 64 µg g-1 DW in shoot. By contrast, tartaric acid promoted significantly Pb accumulation in roots (191 µg g-1 DW; + 45.5%) and shoot (131.6 µg g-1 DW; + 51.3%). Pantothenic acid also resulted in a significant Pb-uptake in the sunflower shoots (123 µg g-1 DW; + 47.9%) and in roots (177.3 µg g-1 DW; + 41.3%). The least effective amongst the chelates tested was aspartic acid, but it still contributed to + 40.1% more Pb accumulation in the sunflower root and shoots. In addition, plant growth, biochemical, and ionomic parameters were positively regulated by the organic chelates used. Especially, an increase in leaf Ca, P, and S was evident in Pb-stressed plants in response to chelates. These results highlight that the use of biocompatible organic chelates positively alters plant physio-biochemical traits contributing to higher Pb-sequestration in sunflower plant parts.

Synergistic effects (adsorption and biodegradation) of Streptomyces hydrogenans immobilization on nano-reed biochar for further application in upflow anaerobic sludge blanket

Water pollution due to wastewater is a serious issue that needs to be studied as many organic compounds are released from wastewater, affecting the ecosystem. Therefore, appropriate treatment methods should be used to prevent these effects. Phragmites australis biochar immobilized with bacteria was prepared in this study for use as an adsorbent in a pilot-scale up-flow anaerobic sludge blanket (UASB) to remove organic matter from wastewater in a buffalo farm. Combining reed biochar and immobilized Streptomyces hydrogenans introduces a synergistic effect: reed biochar serves as a substrate for microbial colonization and provides a conducive environment for microbial growth while Streptomyces hydrogenans, immobilized on the biochar, enhances the degradation of organic matter through its metabolic activities. Suitable techniques were employed, including infrared spectroscopy (FTIR) to determine the functional groups before and after adsorption, scanning electron microscopy (SEM) to determine the morphology of the composite before and after adsorption, X-ray diffraction (XRD) to examine the mineralogical changes through reflectometry, high-resolution diffraction and Brunauer-Emmett-Teller (BET) analyses to determine the surface area that always carried out by nitrogen adsorption/desorption technique based on the BET isotherm. Two-level factorial design experiments optimized using biochar, immobilized with bacteria, were employed to enhance the UASB performance. Chemical oxygen demand (COD) removal and biogas production were studied as a function of four experimental parameters: biochar dose, buffalo sludge dose, pH, and bacteria type. The buffalo sludge (manure) dose negatively affected the model's performance. The results showed better COD removal with Streptomyces hydrogenans S11 inoculation. The optimum biochar dose, buffalo sludge dose, and pH were 20 g L-1, 0%, and 7.5, respectively. The COD removal efficiency under these experimental conditions reached 92.70% with a biogas production of 5.0 mL. The experimental results of a validated point from the model were 90.80% for COD removal ratio and 4.80 mL for biogas production at 2 g L-1 biochar dose, 0% buffalo sludge dose, and pH 7.5 using Streptomyces hydrogenans S11 bacteria. A buffalo wastewater (BWW) anaerobic digestion experimental model was best fitted to the data under optimal conditions. This study aligns with the United Nations Sustainable Development Goals (SDGs), specifically SDG 6 (clean water and sanitation) and SDG 12 (responsible consumption and production). The implications of our work extend to large-scale applications, promising a greener and more sustainable future for wastewater treatment.

Application of phosphogypsum and phosphate-solubilizing fungi to Pb remediation: From simulation to in vivo incubation

Phosphogypsum (PG) is the produced solid waste during phosphorus (P) extraction from phosphate rocks. PG is featured by its abundant PO43- and SO42-. This study investigated the utilization of PG as a material for lead (Pb) remediation, with the assistance of functional fungus. Aspergillus niger (A. niger) is a typical phosphate-solubilizing fungi (PSF), which has high ability to secret organic acids. Oxalic acid is its major secreted organic acid, which is often applied to enhance the P release from phosphate minerals. In this study, synthetic oxalic acid increased the immobilization rate of Pb2+ up to >99 % with the addition of PG. Then, it was observed that biogenic oxalic acid from A. niger can achieve comparable remediation effects. This was due to that PG could provide sufficient P for fungal growth, which allowed sustainable remediation. Subsequently, oxalic acid secreted by A. niger significantly increased the release of active P from PG, and then induced the formation of PPb minerals. In addition, other metabolites of A. niger (such as tyrosine-like substance) can also be complexed with Pb2+. Simultaneously, A. niger did not induce evidently elevation water-soluble fluorine (F) as PG contained abundant Ca2+. Moreover, this study elucidated that oversupply of PG promoted the formation of anglesite (Ksp = 1.6 × 10-8, relatively unstable), whereas the formation of lead oxalate (Ksp = 4.8 × 10-10, relatively stable) was reduced. This study hence shed a bright light on the sustainable utilization of PG for fungus-assisted remediation of heavy metals.

Removal of Pb pollution using alginate-coupled magnetic sludge biochar: Solidification and stabilization behavior and electron promotion mechanisms

Environmental and human health problems caused by Pb pollution have attracted much attention, and solidification and stabilization are effective means for its remediation. Improving the ability of biochar to remediate heavy metals through modification is the focus of current biochar research. This study used calcium-alginate gel (GB) and Fe3+ (magnetic) to encapsulate and improve sludge biochar (SB), and explored the adsorption behavior and passivation mechanism of Pb2+ on it from outside to inside. The magnetic-biochar (MB) in magnetic-biochar-gel microspheres (MBGB) showed a homogeneous dispersion and part of the Fe ion was detached from the MB into the three-dimensional pores of the gel. The results of kinetic, isothermal and pH adsorption experiments showed that the MBGB has 108.4 % and 200 % higher Pb2+ adsorption capacity and rate than SB and can be applied to pH 3-9. The adsorption of Pb2+ by MBGB is a multilayer adsorption with both physical and chemical mechanisms. Mineralogical and electrochemical results demonstrate that the cross-linking of the gel with magnetic-biochar (MB) can provide a directional diffusion channel for Pb2+ from the outside to the inside. The electron transfer rate of MBGB was significantly higher than that of SB (222.2 %) after the reaction. The dissolved cations and electrons on the MB guide Pb2+ from the MBGB surface to the internal MB quickly via accelerating the electron transfer and migration rate between Pb2+ and MB. Subsequently, the abundance of PO43- on the MB ensures stable mineral precipitation (Pyromorphite). Moreover, four-step extraction analysis confirmed that most of Pb2+ in MBGB was stable (36.2 % acid-soluble and 47.6 % non-bioavailable). Meanwhile, the Pb adsorption efficiency of MBGB was still >93.0 % after three cycles of adsorption-desorption. Excellent reuse performance and stability guarantee the environmental security of MBGB. The results of the study provide theoretical support for the efficient treatment of Pb2+ polluted water assisted by gel materials.

Knowledge graph and development hotspots of biochar as an emerging aquatic antibiotic remediator: A scientometric exploration based on VOSviewer and CiteSpace

As an emerging material in the field of environmental remediation, biochar produced by carbonisation of organic solid waste has been widely used in the remediation of antibiotic wastewater due to its environmental friendliness and excellent adsorption properties. This study analyses the current literature in the field in a comprehensive and scientific manner using CiteSpace and VOSviewer technologies. Between 2011 and 2023, a total of 1162 papers were published in this domain, spanning three distinct stages: applied methods, mechanism investigation, and enhanced improvement. The results of keyword clustering indicate that the remediation of antibiotics complexed with multiple pollutants by biochar is the main research topic, followed by the remediation of antibiotics by biochar in combination with other technologies. Furthermore, drawing from current research hotspots in antibiotic remediation using biochar, this study identified the pivotal mechanisms involved: (1) The primary mechanisms by which raw biochar remediates antibiotics include π-π electron donor-acceptor interactions, hydrophobic interactions, electrostatic interactions, hydrogen-bonding, and pore filling. (2) Steam activation, acid/base, metal salt/metal oxide, and clay mineral modification can improve the physical/chemical properties of biochar, enhancing its adsorptive removal of antibiotics. (3) Biochar activated persulfate and degraded antibiotics via free radical pathways (SO4-•, •OH and O2-•) as well as non-free radical pathways (1O2 and electron transfer). In addition, the challenge and prospect of biochar engineering applications for antibiotic remediation lies in improving the main mechanism of antibiotic remediation by biochar. The prospective utilization of biochar in enhancing the remediation of antibiotic-related pollutants holds tremendous value for the future.

Phosphate-solubilizing bacteria improve the antioxidant enzyme activity of Potamogeton crispus L. and enhance the remediation effect on Cd-contaminated sediment

Phosphorus-solubilizing bacteria (PSB) assisted phytoremediation of cadmium (Cd) pollution is an effective method, but the mechanism of PSB-enhanced in-situ remediation of Cd contaminated sediment by submerged plants is still rare. In this study, PSB (Leclercia adecarboxylata L1-5) was inoculated in the rhizosphere of Potamogeton crispus L. (P. crispus) to explore the effect of PSB on phytoremediation. The results showed that the inoculation of PSB effectively improved the Cd extraction by P. crispus under different Cd pollution and the Cd content in the aboveground and underground parts of P. crispus all increased. The μ-XRF images showed that most of the Cd was enriched in the roots of P. crispus. PSB especially showed positive effects on root development and chlorophyll synthesis. The root length of P. crispus increased by 51.7 %, 80.5 % and 74.2 % under different Cd pollution, and the Ca/Cb increased by 38.9 %, 15.2 % and 8.6 %, respectively. Furthermore, PSB enhanced the tolerance of P. crispus to Cd. The contents of soluble protein, MDA and H2O2 in 5 mg·kg-1 and 7 mg·kg-1 Cd content groups were decreased and the activities of antioxidant enzymes were increased after adding PSB. The results showed that the application of PSB was beneficial to the in-situ remediation of submerged plants.

Mechanistic investigation of Pb2+ adsorption on biochar modified with sodium alginate composite zeolitic imidazolate framework-8

For the serious situation of heavy metal pollution, the use of cheap, clean, and efficient biochar to immobilize heavy metals is a good treatment method. In this paper, SA@ZIF-8/BC was prepared for the adsorption of Pb2+ in solution using sodium alginate (SA) and zeolitic imidazolate framework-8 (ZIF-8) modified corn cob biochar. The results showed that the specific surface area of modified biochar was greatly improved, with good adsorption capacity for Pb2+, strong anti-interference ability, and good economy. At the optimal adsorption pH of 5, the adsorption model of Pb2+ by SA@ZIF-8/BC was more consistent with the pseudo-second-order kinetic model and Langmuir isotherm model. This indicates that the adsorption of Pb2+ by SA@ZIF-8/BC is chemisorption and monolayer adsorption. The maximum adsorption of modified biochar was 300 mg g-1, which was 2.38 times higher than that of before modified BC (126 mg g-1). The shift in binding energy of functional groups before and after adsorption of SA@ZIF-8/BC was studied by XPS, and it was found that hydroxyl and carboxyl groups played an important role in the adsorption of Pb2+. It was demonstrated that this novel adsorbent can be effectively used for the treatment of Pb pollution in wastewater.

Comparison of bio-beads combined with Pseudomonas edaphica and three phosphate materials for lead immobilization: Performance, mechanism and plant growth

Phosphate materials (PMs) combine with phosphate solubilizing bacteria play an essential roles in lead (Pb) immobilization, but their resulting ability to reduce Pb bioavailability may vary depending on PMs used. In this study, Pseudomonas edaphica GAU-665 and three PMs: tricalcium phosphate, calcium phytate and nano-hydroxyapatite were respectively encapsulated into bio-beads by sodium alginate, which immobilization efficiency of Pb2+ were 99.11%, 97.76% and 99.02% at initial Pb2+ concentration of 200 mg L-1, respectively. The Pb2+ immobilization performance of bio-beads under different conditions and their organic acids secreted were examined. Most Pb2+ was immobilized by bio-beads through combined functions of adsorption, precipitation, ion exchange and biomineralization, accompanied by the formation of more stable compounds such as Pb3(PO4)2, Pb5(PO4)3OH and Pb5(PO4)3Cl. Meanwhile, pot experimental results indicated that the inoculation of CPhy (calcium phytate) bio-beads with PSB have highest biomass and root growth of oat (Avena sativa L.) in Pb-stressed compared with CK, which increased the content of chlorophyll b (167.51%) in shoot. In addition, the CPhy bio-beads enhance the peroxidase, catalase activities and reduce the malondialdehyde content to alleviating lead physiological toxicity in oat, which reductions the Pb accumulation in shoot (52.06%) and root (81.04%), and increased the residual fraction of Pb by 165.80% in soil. These findings suggest the bio-beads combined with P. edaphica GAU-665 and calcium phytate is an efficient Pb immobilization material and provided feasible way to improve safety agricultural production and Pb-contaminated soil remediation.

Mechanism and effectiveness of enzymatically induced phosphate precipitation (EIPP) in stabilizing coexisting lead, zinc, and cadmium in tailings

Lead-zinc (Pb-Zn) tailings ponds carry the risk of multiple heavy metals (HMs) contamination and pile destabilization. This poses requirements for in-situ applicable, low-distribution, and effective stabilization/solidification (S/S) methods. For this, the novel enzymatically induced phosphate precipitation (EIPP) method was implemented in this study. Its mechanism and performance on stabilization of composite Pb, Zn, and cadmium (Cd) in tailings were explored and evaluated under typical erosion conditions for the first time. Results show that the EIPP stabilized HMs by chemically transforming the unstable carbonate-bound HMs to stable phosphate precipitates and by physically encapsulating tailings particles with newberyite precipitates. The stabilization effect on the three HMs was ranked as Pb > Zn > Cd. Comparing magnesium resources for the EIPP reactants, the EIPP utilizing Mg(CH3COO)2 was more effective at decontamination than MgCl2 because its special pre-activation and re-precipitation function enhanced the chemical transformation function of EIPP. The EIPP stabilization was confirmed to reduce simulated acid rain-leachable and bio-extractive HMs by about 90% and 60%, respectively. Under the prolonged acid attack, treated HMs were ultimately leached through the dissolution mechanism. Zn exhibited significant instability in highly acidic conditions (pH = 2.5-3.5), where its cumulative leaching toxicity after long-term dissolution warrants attention. Overall, EIPP presents a novel and effective strategy for on-site mitigation of composite HMs pollution.

Gel-Embedded Biochar and Hydroxyapatite Composite for the Improvement of Saline-Alkali Soil and Plant Growth Promotion

Soil amendments play a crucial role in modern agriculture, as they effectively enhance the planting environment. This study innovatively proposes the use of gel as a crosslinking agent to embed biochar and hydroxyapatite (HAP), thereby preparing a novel soil amendment. Furthermore, this study investigates the soil improvement effects of this amendment as well as its influence on plant growth. This study employed a hydrothermal method to combine corn stalk (CB) or sludge (SB) biochar with HAP at different ratios (0-20%). Subsequently, sodium alginate gel (SA) was utilized to encapsulate the biochar and minerals, successfully forming a ternary composite gel material (corn stalk biochar/sludge biochar-sodium alginate gel-hydroxyapatite: CB/SB-SA-HAP). Finally, the practical effectiveness of this amendment was verified through potted soil experiments. The results indicate that the CB/SB-SA-HAP composite materials exhibited a micrometre-scale spherical structure with well-developed micropores and possess the functional groups of CB/SB, SA, and HAP, along with unique mineral properties. Through pot experiments, it was verified that the composite material effectively enhances multiple soil properties. After 21 days of cultivation, the soil pH values stabilized within the neutral range (pH = 7 ± 0.3) across all treatment groups. Except for the CB0 (CB:HAP = 1:0) and CB2.0 (CB:HAP = 1:2) treatments, the remaining treatments significantly reduced the soil EC values by 3.27% to 47.92%. All treatments significantly increased the contents of alkali-hydrolysable nitrogen (AHN) (34.89~57.91%), available phosphorus (AP) (35.93~56.55%), and available potassium (AK) (36.41~56.80%) in the soil. In comparison, although the SB treatment was more effective in regulating the pH and electrical conductivity (EC) of saline-alkali soil than the CB treatment, it was less effective in promoting plant growth in the short term. Through correlation analysis and redundancy analysis, a significant positive correlation was found between soil pH and ryegrass germination rate and plant height, particularly with the most pronounced impact on soil pH observed in the CB1.0 and SB0 (SB:HAP = 1:0) treatments. This study underscores the potential of CB/SB-SA-HAP composite materials in soil improvement and plant growth promotion, providing valuable insights for soil remediation, enhancement, and plant cultivation advancements in the agricultural sector.

Waste derived modified biochar as promising functional material for enhanced water remediation potential

The waste management and water purification are daunting environmental challenges. Biochar, a carbonaceous material prepared from diverse organic waste (agricultural, household residues and municipal sewage sludge) has garnered substantial attention due to its excellent attributes, including carbon content, cation exchange efficacy, ample specific surface area, and structural robustness. Thus, the present review comprehensively analyzes bio waste-derived biochar with a particular emphasis on water remediation applications. This article primarily delves into various strategies for modifying biochar, elucidating the underlying mechanisms behind these modifications and their potential for bolstering pollutant removal efficiency. Furthermore, it addresses the impact of functionalization on both biochar stability and cost for commercialization. Lastly, the article outlines key developments, SWOT analysis, and future prospects, offering insights into the practical execution of biochar applications at a larger scale. Therefore, this article paves the way for future research to deepen the understanding of modified biochar with mechanisms for exploring water remediation applications in a more sustainable manner.

Biochar loaded with ferrihydrite and Bacillus pseudomycoides enhances remediation of co-existed Cd(II) and As(III) in solution

Bioremediation is one of the effective ways for heavy metal remediation. Iron-modified biochar (F@BC) loaded with Bacillus pseudomycoides (BF@BC) was synthesized to remove the coexistence of cadmium (Cd) and arsenic (As) in solutions. The results showed that B. pseudomycoides significantly increased the removal rate of Cd(II) by enhancing the specific surface area and Si-containing functional groups of biochar (BC). The surface of F@BC was enriched with Fe-containing functional groups, significantly improving As(III) adsorption. The combination of ferrihydrite and strains on BF@BC enhanced the removal of Cd(II) and As(III). It also promoted the oxidation of As(III) by producing an abundance of hydroxyl radicals (·OH). The maximum saturated adsorption capacity of BF@BC for Cd(II) and As(III) increased by 52.47% and 2.99 folds compared with BC, respectively. This study suggests that biochar loaded with Fe and bacteria could be sustainable for the remediation of the coexistence of Cd(II) and As(III) in solutions.

Phosphate solubilizing microorganisms: a sustainability strategy to improve urban ecosystems

Intensification of urban construction has gradually destroyed human habitat ecosystems. Plants, which serve as the foundation of ecosystems, require green, low-cost, and effective technologies to sustain their growth in stressful environments. A total of 286 keywords and 10 clusters from the bibliometric analysis of 529 articles (1999-2023) indicate the increasing importance of research on microbial functionality in landscape ecosystems. Phosphate solubilizing microorganisms (PSMs) also improve plant disease resistance, adaptability, and survival. PSMs are widely used to promote plant growth and improve ecological quality. They can increase the availability of phosphorus in the soil and reduce the dependence of plants on chemical fertilizers. Microorganisms regulate phosphorus as key tools in landscape ecosystems. Most importantly, in urban and rural landscape practices, PSMs can be applied to green spaces, residential landscapes, road greening, and nursery planting, which play significant roles in improving vegetation coverage, enhancing plant resistance, improving environmental quality, and mitigating the heat island effect. PSMs are also helpful in restoring the ecological environment and biodiversity of polluted areas, such as brownfields, to provide residents with a more liveable living environment. Therefore, the multiple efficacies of PSM are expected to play increasingly important roles in the construction of urban and rural landscape ecosystems.

Potential bioremediation of lead and phenol by sunflower seed husk and rice straw-based biochar hybridized with bacterial consortium: a kinetic study

Environmental pollution is a global phenomenon and troublesome fact that poses a grave risk to all living entities. Via coupling carbonaceous feedstocks with outstanding microbial activity, kinetic experiments were established using the consortium of Proteus mirabilis and Raoultella planticola, biochar-derived sunflower seed husk (SHB) and rice straw (RSB), and their composites, which investigated at 30 °C (150 rpm) to eliminate 700 mg L-1 lead (120 h) and phenol (168 h) from synthetic wastewater. The derived biochars physicochemical properties of were studied. According to adsorption capacity (qe), consortium-SHB composites and consortium-RSB composites removed lead completely (70 mg g-1) within 48 h and 66 h, respectively. Besides, phenol was remediated entirely after 42 h and 48 h by both composite systems (69.90 mg g-1), respectively, comparing with bacterial consortium only or parent SHB and RSB. Moreover, four kinetic models were studied to describe the bioremediation process. Fractional power and Elovich models could be recommended for describing the adsorption kinetics for lead and phenol removal by the studied biomaterials with high correlation coefficient (R2 ≥ 0.91 for Pb2+ and ≥ 0.93 for phenol) and lower residual root mean square error (RMSE) and chi-square (X2). Overall, bacterial consortium-biochar composites exhibited greater remediation of lead and phenol than the sum of each single bacterial consortium and biochar systems; reflecting synergistic interaction of adsorptive capability of biochar and metabolic performance of bacterial consortium, as denoted by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The current study addressed the successful design of employing functional remediating consortium immobilized on waste biomass-derived biochar as a conducive alternative eco-sorbent and economic platform to detoxify organic and inorganic pollutants.

Phosphate-solubilizing bacteria: Their agroecological function and optimistic application for enhancing agro-productivity

Phosphorus (P) is a limiting nutrient in the soil-plant nutrient cycling. Although the exogenous application of chemical P fertilizers can satisfy crop P requirements during critical growth phases. While excessive P fertilizers use results in low phosphorus acquisition efficiency (PAE), it has serious environmental consequences and hastens the depletion of P mineral reserves. Phosphate-solubilizing bacteria (PSB) have the potential to make insoluble phosphate available to plants through solubilization and mineralization, increasing crop yields while maintaining environmental sustainability. Existing reviews mainly focus on the beneficial effects of PSB on crop performance and related mechanisms, while few of them elucidate the action mechanisms of PSB in soil-microbe-plant interactions for crop cultivation with high yield efficiency. Hence, this study provides a comprehensive review of the physicochemical and molecular mechanisms (e.g., root exudates, extracellular polysaccharides, organic acids, phosphatases, and phosphate-specific transport systems) of PSB to facilitate the P cycle in the soil-plant systems. Further, the potential of commercial applications of PSB (e.g., genetic engineering, seed priming and coating) are also discussed in order to highlight their contribution to sustainable agriculture. Finally, existing challenges and future prospects in agricultural applications are proposed. In conclusion, we firmly believe that PSB represent a highly significant biotechnological tool for enhancing agricultural productivity and offers a wide range of extensive potential applications.

Phosphogypsum/titanium gypsum coupling for enhanced biochar immobilization of lead: Mineralization reaction behavior and electron transfer effect

The hazards caused by Pb pollution have received worldwide attention. Phosphogypsum (PG) and titanium gypsum (TG) have the disadvantage of limited adsorption capacity and poor dispersion when used as heavy metal adsorbents on their own. The excellent pore and electron transfer capacity of biochar makes it possible to combine with PG and TG to solidify/stabilize Pb2+. In this study, the mechanism of Pb2+ adsorption/immobilization by rice husk biochar (BC) combined with PG/TG was investigated in terms of both mineral formation and electron transfer rate. The removal rate of Pb2+ by BC composite PG (BC/PG-Pb) or TG (BC/TG-Pb) was as high as 97%-98%, an increase of 120.9% and 122.5% over BC. Adsorption kinetics and mineral precipitation results indicate that the main removal of Pb2+ from BC/PG-Pb and BC/TG-Pb is achieved by PG/TG induced Pb-sulfate and Pb-phosphate formation. The addition of PG/TG significantly enhances the formation of stable Pb-minerals on the biochar surface, with the proportion of non-bioaccessible forms exceeding 50%. The four-step extraction results confirm that P and F in PG/TG are key in facilitating the conversion of Pb minerals to pyromorphite. The rich pore structure of biochar not only disperses the easily agglomerated PG/TG onto the biochar surface, but also attracts Pb2+ for uniformly dispersed precipitation. Furthermore, the excellent electrical conductivity and smooth electron transfer channels of biochar facilitate the reaction rate of Pb2+ mineralization. Overall, the use of biochar in combination with PG/TG is a promising technology for the combination of solid waste resourceisation and Pb remediation.

Enhanced removal efficiency of bensulfuron-methyl by a novel boron doping biochar-based Acinetobacter YH0317 at a lower temperature

Biochar-based bacteria are regarded as an efficient strategy for remediating organic pollutants in aquatic environments. Herein, a strain named Acinetobacter YH0317 that could degrade bensulfuron-methyl (BSM) at a lower temperature (15 °C) was isolated from a paddy rice field with long-term BSM application. Then Acinetobacter YH0317 was loaded on unmodified biochar (BC) and boron doping biochar (BBC). Results showed that BBC-based YH0317 significantly enhanced the removal efficiency of BSM (71.8-99.1%) compared with BC-based YH0317 (41.9-44.0%) and YH0317 alone (18.1-20.7%) in 24 h. BBC promoted the growth of YH0317 and secretion of extracellular secretions by providing a carrier and shelter for YH0317. The electrochemical analysis suggested BBC improved the electron transfer rate, which ultimately facilitated the removal of BSM. Hydroponic experiments indicated that BBC-based YH0317 effectively improved the growth of soybean. This work reports a novel BBC-based Acinetobacter YH0317 that could effectively remediate BSM contamination in the water environment.

Microbial functionalities and immobilization of environmental lead: Biogeochemical and molecular mechanisms and implications for bioremediation

The increasing environmental and human health concerns about lead in the environment have stimulated scientists to search for microbial processes as innovative bioremediation strategies for a suite of different contaminated media. In this paper, we provide a compressive synthesis of existing research on microbial mediated biogeochemical processes that transform lead into recalcitrant precipitates of phosphate, sulfide, and carbonate, in a genetic, metabolic, and systematics context as they relate to application in both laboratory and field immobilization of environmental lead. Specifically, we focus on microbial functionalities of phosphate solubilization, sulfate reduction, and carbonate synthesis related to their respective mechanisms that immobilize lead through biomineralization and biosorption. The contributions of specific microbes, both single isolates or consortia, to actual or potential applications in environmental remediation are discussed. While many of the approaches are successful under carefully controlled laboratory conditions, field application requires optimization for a host of variables, including microbial competitiveness, soil physical and chemical parameters, metal concentrations, and co-contaminants. This review challenges the reader to consider bioremediation approaches that maximize microbial competitiveness, metabolism, and the associated molecular mechanisms for future engineering applications. Ultimately, we outline important research directions to bridge future scientific research activities with practical applications for bioremediation of lead and other toxic metals in environmental systems.

The potential of biochar as a microbial carrier for agricultural and environmental applications

Biochar can be an effective carrier for microbial inoculants because of its favourable properties promoting microbial life. In this review, we assess the effectiveness of biochar as a microbial carrier for agricultural and environmental applications. Biochar is enriched with organic carbon, contains nitrogen, phosphorus, and potassium as nutrients, and has a high porosity and moisture-holding capacity. The large number of active hydroxyl, carboxyl, sulfonic acid group, amino, imino, and acylamino hydroxyl and carboxyl functional groups are effective for microbial cell adhesion and proliferation. The use of biochar as a carrier of microbial inoculum has been shown to enhance the persistence, survival and colonization of inoculated microbes in soil and plant roots, which play a crucial role in soil biochemical processes, nutrient and carbon cycling, and soil contamination remediation. Moreover, biochar-based microbial inoculants including probiotics effectively promote plant growth and remediate soil contaminated with organic pollutants. These findings suggest that biochar can serve as a promising substitute for non-renewable substrates, such as peat, to formulate and deliver microbial inoculants. The future research directions in relation to improving the carrier material performance and expanding the potential applications of this emerging biochar-based microbial immobilization technology have been proposed.

Combination of Biochar and Trichoderma harzianum Can Improve the Phytoremediation Efficiency of Brassica juncea and the Rhizosphere Micro-Ecology in Cadmium and Arsenic Contaminated Soil

Phytoremediation is an environment-friendly method for toxic elements remediation. The aim of this study was to improve the phytoremediation efficiency of Brassica juncea and the rhizosphere soil micro-ecology in cadmium (Cd) and arsenic (As) contaminated soil. A field experiment was conducted with six treatments, including a control treatment (CK), two treatments with two contents of Trichoderma harzianum (T1: 4.5 g m-2; T2: 9 g m-2), one biochar treatment (B: 750 g m-2), and two combined treatments of T1B and T2B. The results showed Trichoderma harzianum promoted the total chlorophyll and translocation factor of Brassica juncea, while biochar promoted plant biomass compared to CK. T2B treatment showed the best results, which significantly increased Cd accumulation by 187.49-308.92%, and As accumulation by 125.74-221.43%. As a result, the soil's total Cd content was reduced by 19.04% to 49.64% and total As contents by 38.76% to 53.77%. The combined amendment increased the contents of soil available potassium, phosphorus, nitrogen, and organic matter. Meanwhile, both the activity of glutathione and peroxidase enzymes in plants, together with urease and sucrase enzymes in soil, were increased. Firmicutes (dominant bacterial phylum) and Ascomycota (dominant fungal phylum) showed positive and close correlation with soil nutrients and plant potentially toxic elements contents. This study demonstrated that phytoremediation assisted by biochar and Trichoderma harzianum is an effective method of soil remediation and provides a new strategy for enhancing plant remediation efficiency.

Biochar-immobilized Bacillus megaterium enhances Cd immobilization in soil and promotes Brassica chinensis growth

Phosphate solubilizing bacteria (PSB) has been considered an environmental-friendly phosphate fertilizer without cadmium (Cd) input into soils, but its possibility of Cd fixation in soil needs to be explored. Since direct inoculation results in a rapid decline of the population and activity, we immobilized Bacillus megaterium with maize straw biochar (B-PSB) and investigated its feasibility in remediating Cd-contaminated soil. Pot experiments showed that the application of B-PSB significantly ameliorated the growth of Brassica chinensis under Cd stress, with a fresh weight increased by 59.08% compared to the Cd-control. B-PSB reduced Cd accumulation in Brassica chinensis by 61.69%, and promoted the uptake of P and N by 134.97% and 98.71% respectively. Microbial community analysis showed B-PSB recruited more plant growth-promoting bacteria in near-rhizosphere soil, which provides a favorable microenvironment for both PSB and crops. Column leaching experiments verified that B-PSB achieved the dissolution of stable P while fixing Cd. Batch tests further revealed that biochar served as a successful carrier facilitating the growth of B. megaterium and Cd immobilization. Given the widespread Cd contamination in agricultural soils, our results indicate that B-PSB is a promising soil amendment to secure food safety.

Biochar assists phosphate solubilizing bacteria to resist combined Pb and Cd stress by promoting acid secretion and extracellular electron transfer

Microorganisms have difficulty surviving and performing remediation functions in mixed systems with high concentrations of Pb and Cd. Biochar has the potential to assist microorganism remediation as an excellent adsorbent for heavy metals. In this study, pig manure biochar (PMB) was used to assist phosphorus solubilizing bacteria (PSB) to explore the mineralization protection and biofeedback mechanism of biochar on PSB under mixed stress of 1000 mg/L Pb²⁺ and 500 mg/L Cd²⁺. The adsorption results showed that the removal of Pb²⁺ and Cd²⁺ by PMB+PSB was 148.77% and 72.27% higher than that by PSB. Meanwhile, the non-bioavailable fraction of Cd²⁺ and acid-soluble fraction of Pb²⁺ in PMB+PSB were increased by 9% and 3%, respectively. Mineralogical and microbial secretion results confirm that showed that the acidic soluble fraction and non-bioavailable fraction were mostly Pb/Cd-carbonate and Pb/Cd-phosphate. The pore adsorption and precipitation (carbonate) of biochar were able to reduce the exposure of PSB to Pb/Cd and the background stress concentration, thus stimulating the biological positive feedback effect of PSB and forming a microenvironment in the cell periphery. The vesicle detoxification and extracellular polymeric substance protection mechanism of PSB were improved under biochar protection, and the individual size and activity of PSB cells were enhanced. Besides, citric acid release from PSB (28.85% increase) accelerated the dissolution of unstable Cd-carbonate, thereby releasing a large amount of Cd²⁺ to compete with Pb²⁺ for PO₄^3-. Thus, the protection of biochar and the positive feedback effect of PSB could reduce the biotoxicity of Cd²⁺ in the stress system by preferentially forming a stable Cd-phosphate. In addition, the excellent electrical conductivity and organic material adsorption of biochar increased the extracellular electron transport rate of microorganisms, which further accelerated the mineralization and immobilization of Pb²⁺ and Cd²⁺, so as to ensure the repair effect of PSB on heavy metals.

Assessment of the properties of aging biochar used as a substrate in constructed wetlands

Biochar has gained global recognition as an effective tool for environmental remediation, and is increasingly being used as an alternative substrate in constructed wetlands (CWs). While, most studies have focused on the positive effects of biochar for the pollutant removal in CWs, less is known about aging and longevity of the embedded biochar. This study investigated the aging and stability of biochar embedded in CWs post-treating the effluent of a municipal and an industrial wastewater treatment plant. Litter bags containing biochar were inserted into two aerated horizontal subsurface flow CWs (350 m² each), and retrieved on several dates (8-775 days after burial) for assessment of weight loss/gain and changes in biochar characteristics. Additionally, a 525-day laboratory incubation test was conducted to analyze biochar mineralization. The results showed that there was no significant biochar weight loss over time, but a slight increase in weight (2.3-3.0%) was observed at the end, likely due to mineral sorption. Biochar pH remained stable except for a sudden drop at the beginning (8.6-8.1), while the electrical conductivity continued to increase (96-256 μS cm^-1) throughout the experiment. The sorption capacity of the aged biochar for methylene blue significantly increased (1.0-1.7 mg g^-1), and a change in the biochar's elemental composition was also noted, with O-content increasing by 13-61% and C content decreasing by 4-7%. Despite these changes, the biochar remained stable according to the criteria of the European Biochar Foundation and International Biochar Initiative. The incubation test also showed negligible biochar mass loss (<0.02%), further validating the stability of the biochar. This study provides important insights into the evolution of biochar characteristics in CWs.

Phosphorus solubilizing microorganisms: potential promoters of agricultural and environmental engineering

Phosphate solubilizing microorganisms (PSMs) are known as bacteria or fungi that make insoluble phosphorus in soil available to plants. To date, as beneficial microbes, studies on PSMs indicated they have potential applications in agriculture, environmental engineering, bioremediation, and biotechnology. Currently high cost and competition from local microbe are the most important factors hindering PSMs commercialization and application as for instance biofertilizer, soil conditioner or remediation agent, etc. There are several technical strategies can be engaged to approach the solutions of these issues, for instance mass production, advance soil preparation, genetic engineering, etc. On the other hand, further studies are needed to improve the efficiency and effectiveness of PSMs in solubilizing phosphates, promoting plant growth, soil remediation preferably. Hopefully, PSMs are going to be developed into ecofriendly tools for sustainable agriculture, environment protection and management in the future.

Phosphorus-rich biochar modified with Alcaligenes faecalis to promote U(VI) removal from wastewater: Interfacial adsorption behavior and mechanism

Phosphorus-rich biochar (PBC) has been extensively studied due to its significant adsorption effect on U(VI). However, the release of phosphorus from PBC into solution decreases its adsorption performance and reusability and causes phosphorus pollution of water. In this study, Alcaligenes faecalis (A. faecalis) was loaded on PBC to produce a novel biocomposite (A/PBC). After adsorption equilibrium, phosphorus released into solution from PBC was 2.32 mg/L, while it decreased to 0.34 mg/L from A/PBC (p < 0.05). The U(VI) removal ratio of A/PBC reached nearly 100%, which is 13.08% higher than that of PBC (p < 0.05), and it decreased only by 1.98% after 5 cycles. When preparing A/PBC, A. faecalis converted soluble phosphate into insoluble metaphosphate minerals and extracellular polymeric substances (EPS). And A. faecalis cells accumulated through these metabolites and formed biofilm attached to the PBC surface. The adsorption of metal cations on phosphate further contributed to phosphorus fixation in the biofilm. During U(VI) adsorption by A/PBC, A. faecalis synthesize EPS and metaphosphate minerals by using the internal components of PBC, thus increasing the abundance of acidic functional groups and promoting U(VI) adsorption. Hence, A/PBC can be a green and sustainable material for U(VI) removal from wastewater.

Phosphate-solubilizing microorganisms regulate the release and transformation of phosphorus in biochar-based slow-release fertilizer

Coupling phosphate-solubilizing microorganisms (PSM) can improve the availability of phosphorous (P) in biochar-based slow-release P fertilizers (BPF). However, the mechanism in release and transformation of P in BPF regulated by PSM is still unclear. Herein, the biocompatibility and the adhesion behaviors of BPF and PSM (Enterobacter hormaechei Rs-198) in soil were firstly studied, and a 90 days' laboratory-scale soil incubation experiment of BPF and Rs-198 was performed to study the transformation of P of BPF. The results show that BPF has a good biocompatibility for Rs-198 due to its low aromaticity, graphitization and free radicals' content (0.084 mg/g). Rs-198 are adhered to the surface of BPF in soil due to the high negative secondary energy minimum and low total interaction energy between Rs-198 and BPF. Available P in the incubation of BPF and Rs-198 (BR treatment) is significantly higher than that of the incubation of BPF (BF treatment) at initial 60 days. However, the content of available P in BR treatment is much lower compared with that in BF treatment on day 90, which is attributed to the entrapment of released P from BPF by Rs-198 and the formation of polyphosphate (polyP) rather than bound with soil mineral. Overall, this study presents new insights into the transformation of P in BPF regulated by PSM.

Biochar-immobilized Bacillus spp. for heavy metals bioremediation: A review on immobilization techniques, bioremediation mechanisms and effects on soil

Heavy metals contamination present risks to ecosystems and human health. Bioremediation is a technology that has been applied to minimize the levels of heavy metals contamination. However, the efficiency of this process varies according to several biotic and abiotic aspects, especially in environments with high concentrations of heavy metals. Therefore, microorganisms immobilization in different materials, such as biochar, emerges as an alternative to alleviate the stress that heavy metals have on microorganisms and thus improve the bioremediation efficiency. In this context, this review aimed to compile recent advances in the use of biochar as a carrier of bacteria, specifically Bacillus spp., with subsequent application for the bioremediation of soil contaminated with heavy metals. We present three different techniques to immobilize Bacillus spp. on biochar. Bacillus strains are capable of reducing the toxicity and bioavailability of metals, while biochar is a material that serves as a shelter for microorganisms and also contributes to bioremediation through the adsorption of contaminants. Thus, there is a synergistic effect between Bacillus spp. and biochar for the heavy metals bioremediation. Biomineralization, biosorption, bioreduction, bioaccumulation and adsorption are the mechanisms involved in this process. The application of biochar-immobilized Bacillus strains results in beneficial effects on the contaminated soil, such as the reduction of toxicity and accumulation of metals in plants, favoring their growth, in addition to increasing microbial and enzymatic activity in soil. However, competition and reduction of microbial diversity and the toxic characteristics of biochar are reported as negative impacts of this strategy. More studies using this emerging technology are essential to improve its efficiency, to elucidate the mechanisms and to balance positive and negative impacts, especially at the field scale.

Improving radish phosphorus utilization efficiency and inhibiting Cd and Pb uptake by using heavy metal-immobilizing and phosphate-solubilizing bacteria

Phosphate-solubilizing bacteria play a key role in increasing plant growth as potential suppliers of soluble phosphorus and have great potential for the remediation of heavy metal-polluted soils. However, the soil and microbiological mechanisms by which phosphate-solubilizing bacteria prevent heavy metal absorption in radish have not been adequately studied. Here, the mechanisms of phosphorus solubilization, Cd and Pb immobilization, and the inhibition of heavy metal absorption by phosphate-solubilizing bacteria were studied in radish through solution adsorption and pot experiments. Two phosphate-solubilizing bacteria with high Cd and Pb removal rates (46.9-97.12 %), Klebsiella sp. M2 and Kluyvera sp. M8, were isolated. The soluble phosphorus content released by strains M2 and M8 was 265-277 mg L^-1, achieved by secreting oxalic acid, ascorbic acid, citric acid, and succinic acid in an inorganic phosphorus medium containing 3 mg L^-1 Cd and 5 mg L^-1 Pb. Furthermore, these two functional strains induced the formation of Pb₂(PO₄)₂, Cd(PO₃)₂, Fe₂Pb₃(PO₄)₂, CdS, and PbS precipitates that immobilized Cd and Pb in the solution. In general, strains M2 and M8 inhibited the absorption of Cd and Pb by radish by the following mechanisms: i) bacterial cell wall adsorption, ii) induction of Pb₂(PO₄)₂, Cd(PO₃)₂, Fe₂Pb₃(PO₄)₂, CdS, and PbS precipitation in the solution/soil, iii) increases in the Ca₂P and FeP contents in the radish rhizosphere, and iv) the promotion of bacterial community enrichment toward phosphorus-solubilizing and plant growth-promoting properties (Ramlibacter, Enterobacter, Bacillus, Gemmatimonas, and Lysinibacillusin) in the radish rhizosphere. These results provide bacterial resources and technical approaches to heavy metal pollution amelioration and efficient phosphorus fertilizer use in farmland.

Efficient lead immobilization by bio-beads containing Pseudomonas rhodesiae and bone char

Mineralization of lead ions (Pb²⁺) to pyromorphite using phosphorus-containing materials is an effective way to remediate lead (Pb) contamination. Bone char is rich in phosphorus, but its immobilization of Pb²⁺ is limited by poor phosphate release. To utilize the phosphorus in bone char and provide a suitable growth environment for phosphate-solubilizing bacteria, bone char and Pseudomonas rhodesiae HP-7 were encapsulated into bio-beads, and the immobilization performance and mechanism of Pb in solution and soil by bio-beads were investigated. The results showed that 137 mg/g of phosphorus was released from bone char in the presence of the HP-7 strain. Pb²⁺ removal efficiency reached 100 % with an initial Pb²⁺ concentration of 1 mM, bone char content of 6 g/L, and bio-bead dosage of 1 %. Most Pb²⁺ was immobilized on the surface of the bio-beads as Pb₅(PO₄)₃Cl. The soil remediation experiments showed a 34 % reduction in the acid-soluble fraction of Pb. The bio-beads showed good stability in long-term (30 d) soil remediation. The present study shows that bone char can be turned into an efficient Pb immobilization material in the presence of phosphate-solubilizing bacteria. Thus, bio-beads are expected to be used in the remediation of Pb-contaminated environments.

Machine learning: An effective technical method for future use in assessing the effectiveness of phosphorus-dissolving microbial agroremediation

The issue of agricultural pollution has become one of the most important environmental concerns worldwide because of its relevance to human survival and health. Microbial remediation is an effective method for treating heavy metal pollution in agriculture, but the evaluation of its effectiveness has been a difficult issue. Machine learning (ML), a widely used data processing technique, can improve the accuracy of assessments and predictions by analyzing and processing large amounts of data. In microbial remediation, ML can help identify the types of microbes, mechanisms of action and adapted environments, predict the effectiveness of microbial remediation and potential problems, and assess the ecological benefits and crop growth after remediation. In addition, ML can help optimize monitoring programs, improve the accuracy and effectiveness of heavy metal pollution monitoring, and provide a scientific basis for the development of treatment measures. Therefore, ML has important application prospects in assessing the effectiveness of microbial remediation of heavy metal pollution in agriculture and is expected to be an effective pollution management technology.

Periplasmic space is the key location for Pb(II) biomineralization by Burkholderia cepacia

Phosphate solubilizing bacteria (PSB) induced phosphate precipitation is considered as an effective method for Pb(II) removal through the formation of stable Pb(II)-phosphate compound, but the location of end-products is still unclear. Herein, the PSB strain of Burkholderia cepacia (B. cepacia) coupled with the hydroxyapatite (HAP) was used in this study to investigate the Pb(II) removal mechanism and the biomineralization location. The dissolving phosphate of three particle sizes of HAP and Pb(II) resistant capabilities, and the effect factors such as HAP dosage, initial concentrations of Pb(II), pH, temperature, and different treatments were determined. The results indicated that the highest soluble phosphate could reach 224.85 mg/L in a 200 nm HAP medium and the highest removal efficiency of Pb(II) was about 96.32 %. Additionally, it was interesting that Pb(II) was mainly located in the periplasmic space through the cellular distribution experiment, which was further demonstrated by scanning electron microscope (SEM) and transmission electron microscopy (TEM). Besides, the characterization results showed that the functional groups such as amide, hydroxy, carboxy and phosphate played an important role in Pb(II) biomineralization, and the free Pb(II) in aqueous solution could be transformed into pyromorphite through phosphate dissolution, extracellular adsorption/complexation, and intracellular precipitation.

Mechanism of lead adsorption by a Bacillus cereus strain with indole-3-acetic acid secretion and inorganic phosphorus dissolution functions

BACKGROUND

Heavy metal pollution has become a major source of environmental pollution because of increasing industrialization. Microbial remediation is a promising approach to remediate lead-contaminated environments owing to its cost-effective, environment-friendly, ecologically sustainable, and highly efficient properties. In this study, the growth-promoting functions and lead-adsorption ability of Bacillus cereus SEM-15 were examined, and the functional mechanism of the strain was preliminarily identified using scanning electron microscopy, energy spectrum, infrared spectrum, and genome analyses, providing theoretical support for utilization of B. cereus SEM-15 in heavy metals remediation.

RESULTS

B. cereus SEM-15 showed strong ability to dissolve inorganic phosphorus and secrete indole-3-acetic acid. The lead adsorption efficiency of the strain at lead ion concentration of 150 mg/L was more than 93%. Single factor analysis revealed the optimal conditions for heavy metal adsorption by B. cereus SEM-15 (adsorption time, initial lead ion concentration, pH, and inoculum amount were 10 min, 50-150 mg/L, 6-7, and 5 g/L, respectively) in nutrient-free environment, with the lead adsorption rate reaching 96.58%. Scanning electron microscopy of B. cereus SEM-15 cells before and after lead adsorption showed adherence of a large number of granular precipitates to the cell surface after lead adsorption. X-Ray photoelectron spectroscopy and Fourier transform infrared spectroscopy results indicated the characteristic peaks of Pb-O, Pb-O-R (R = functional group), and Pb-S bonds after lead adsorption, and a shift in the characteristic peaks of bonds and groups related to C, N, and O. Genome annotation results showed the presence of genes related to heavy metals tolerance and plant growth promotion in B. cereus SEM-15, providing a molecular basis for the strain's heavy metals tolerance and plant growth promotion functions.

CONCLUSIONS

This study analyzed the lead adsorption characteristics of B. cereus SEM-15 and the associated influencing factors, and discussed the adsorption mechanism and related functional genes, providing a basis for clarifying the underlying molecular mechanism and offering a reference for further research on plant-microorganisms combined remediation of heavy metals polluted environments.

Synergistic remediation of lead pollution by biochar combined with phosphate solubilizing bacteria

Pb(II) is extreme toxic to biological cells, which limits the restoration of Pb(II) by functional strains. This study examined a Pb(II)-tolerant phosphate solubilizing bacteria(PSB) Ochrobactrum sp. J023 combined with corn stover biochar to enhance the immobilization of Pb(II). The results showed that the removal rate of Pb(II) by biochar combined with phosphate-solubilizing bacteria was as high as 71.30 %. SEM-EDS showed that more disordered crystals appeared on the surface of biochar treated with bacteria. XRD analysis indicated that the mineralization products of Pb(II) in biochar combined strain system were mainly in Pb₅(PO₄)₃OH and Pb₅(PO₄)₃Cl. FT-IR analysis revealed that there were more phosphate groups involved in the mineralization process when biochar was added. XPS verified the formation of PbO or lead-containing precipitates in this system, and the amount of lead precipitates was larger. The mechanism of lead fixation by strain combined with biochar can be regarded that the strain regulates the microenvironment of the biochar surface, enhances the release of phosphate and promotes the generation of stable pyroxite. Moreover, biochar composition and porous structure not only provide nutrient elements for strains, but also protect and promote the metabolism of strains. Biochar adsorption also reduces the loss of available phosphorus, which helps PSB to fix Pb sustainably and effectively. This suggests that the synergistic effect of PSB-biochar can not only effectively reduce the mobility and bioavailability of Pb(II), but also increase the sustainability of remediation. Therefore, the combination of phosphate solubilizing bacteria and biochar is a promising approach to the remediation of heavy metal pollution.

Recent advances in biochar amendments for immobilization of heavy metals in an agricultural ecosystem: A systematic review

Over the last several decades, extensive and inefficient use of contemporary technologies has resulted in substantial environmental pollution, predominantly caused by potentially hazardous elements (PTEs), like heavy metals that severely harm living species. To combat the presence of heavy metals (HMs) in the agrarian system, biochar becomes an attractive approach for stabilizing and limiting availability of HMs in soils due to its high surface area, porosity, pH, aromatic structure as well as several functional groups, which mostly rely on the feedstock and pyrolysis temperature. Additionally, agricultural waste-derived biochar is an effective management option to ensure carbon neutrality and circular economy while also addressing social and environmental concerns. Given these diverse parameters, the present systematic evaluation seeks to (i) ascertain the effectiveness of heavy metal immobilization by agro waste-derived biochar; (ii) examine the presence of biochar on soil physico-chemical, and thermal properties, along with microbial diversity; (iii) explore the underlying mechanisms responsible for the reduction in heavy metal concentration; and (iv) possibility of biochar implications to advance circular economy approach. The collection of more than 200 papers catalogues the immobilization efficiency of biochar in agricultural soil and its impacts on soil from multi-angle perspectives. The data gathered suggests that pristine biochar effectively reduced cationic heavy metals (Pb, Cd, Cu, Ni) and Cr mobilization and uptake by plants, whereas modified biochar effectively reduced As in soil and plant systems. However, the exact mechanism underlying is a complex biochar-soil interaction. In addition to successfully immobilizing heavy metals in the soil, the application of biochar improved soil fertility and increased agricultural productivity. However, the lack of knowledge on unfavorable impacts on the agricultural systems, along with discrepancies between the use of biochar and experimental conditions, impeded a thorough understanding on a deeper level.

Improvement of fungal extraction of phosphorus from sewage sludge ash by Aspergillus niger using sludge filtrate as nutrient substrate

Fungal extraction is a promising approach for reclaiming phosphorus (P) from sewage sludge ash (SSA). However, this approach faces notable technical and economic challenges, including an unknown P speciation evolution and the addition of expensive chemical organic carbon. In this study, the use of an organic-rich effluent produced in sludge dewatering as nutrient source is proposed to initiate the fungal extraction of SSA-borne P with Aspergillus niger. The changes in P speciation in the ash during fungal treatment was analyzed by combined sequential extraction, solid-state ³¹P nuclear magnetic resonance, and P X-ray absorption near edge spectroscopy. Results showed that after 5 days of fungal treatment using sludge-derived organics, 85 % of P was leached from SSA. Dominantly, this considerable release of P resulted from the dissolution of Ca₃(PO₄)₂, AlPO₄, FePO₄, and Mg₃(PO₄)₂ in the ash, and their individual contribution rates to P released accounted for 28.0 %, 24.3 %, 20.6 %, and 18.8 %, respectively. After removal of metal cations (e.g., Mg²⁺, Al³⁺, Fe³⁺, and heavy metals) by cation exchange resin (CER), a hydroxyapatite (HAP) product with a purity of > 85 % was harvested from the extract by precipitation with CaCl₂. By contrast, without CER purification, a crude product of Ca/Mg-carbonates and phosphates mixture were obtained from this extract. A total of 73.2 wt% of P was ultimately recovered from SSA through integrated fungal extraction, CER purification, and HAP crystallization. These findings provide a mechanistic basis for the development of waste management strategies for improved P reclamation with minimal chemical organics consumption.

Isolation and screening of phosphorus solubilizing bacteria from saline alkali soil and their potential for Pb pollution remediation

The high pH and salinity of saline alkali soil not only seriously restrict the growth of crops, but also aggravate the pollution of heavy metals. The fixation of heavy metals and the regulation of pH by phosphorus solubilizing microorganisms may become a new way to repair heavy mental and improve saline alkali soil. In this study, a saline-alkali resistant bacteria (CZ-B1, CGMCC No: 1.19458) was screened from saline-alkali soil, and its tolerance to salt/alkali/lead stress was investigated by shaking flask experiment. The strain was identified as Bacillus amyloliquefaciens by morphology and 16S rRNA gene sequence analysis. The optimum growth temperature of CZ-B1 is about 35°C-40℃. The maximum salt stress and pH that it can tolerance are 100 g/L and 9 respectively, and its tolerance to Pb²⁺ can reach 2000 mg/L. The phosphorus release amount of CZ-B1 to Ca₃(PO₄)₂ within 72 h is 91.00-102.73 mg/L. The phosphate solubilizing index in PVK agar medium and NBRIP agar medium are more than 2, which can be defined as phosphate solubilizing bacteria. Moreover, the dissolution of CZ-B1 to phosphorus is mainly attributed to tartaric acid, citric acid and succinic acid in inorganic medium. In addition, the removal rate of Pb²⁺ by CZ-B1 can reach 90.38% for 500 mg/L. This study found that CZ-B1 can immobilize Pb through three biological mechanisms (organic acid, extracellular polymers and mineralization reaction). The release of succinic acid (10.97 g/L) and citric acid (5.26 g/L) may be the main mechanism to promote the mineralization reaction of CZ-B1 (phosphate and oxalate) and resistance to Pb stress. In addition, the high enrichment of Pb²⁺ by EPS can increase the rate of extracellular electron transfer and accelerate the mineralization of CZ-B1. The screening and domestication of saline-tolerant phosphorus-solubilizing bacteria not only help to remediate Pb contamination in saline soils, but also can provide P element for plant growth in saline soil.

Study on the mechanism of biochar affecting the effectiveness of phosphate solubilizing bacteria

Low phosphorus utilization and phosphorus fertilizer pollution are serious issues primarily affecting soil health. To investigate the effects of biochar on the growth, phosphorus solubilization, and metabolites of phosphorus-solubilizing bacteria (PSB), rice husk biochar (RH) and rice straw biochar (RS) were incubated with Bacillus megatherium (BM1) and Bacillus mucilaginosus (BM2), respectively. The highest phosphorus solubilization was observed in BM2 following the addition of RS. The dissolved amount of phosphorus was 244.99 mg/L, which was 43.86% higher than that of the control group. Hence, biochar can improve the phosphorus solubilization capacity of PSB by affecting the organic acid and polysaccharide contents, and phosphatase activity secreted by the PSB, as the porous structure and surface characteristics of biochar ensured the adsorption of PSB. This study can help improve the functional activity of PSB and provide basis for improving the utilization of soil phosphorus, which in turn, aid in the development of biochar-based microbial fertilizers.

Optimistic contributions of plant growth-promoting bacteria for sustainable agriculture and climate stress alleviation

Global climate change is the major cause of abiotic and biotic stresses that have adverse effects on agricultural productivity to an irreversible level, thus threatening to limit gains in production and imperil sustainable agriculture. These climate change-induced abiotic stresses, especially saline, drought, extreme temperature, and so on affect plant morphological, physiological, biochemical, and metabolic characteristics through various pathways and mechanisms, ultimately hindering plant growth, development, and productivity. However, overuse and other inappropriate uses of agrochemicals are not conducive to the protection of natural resources and the environment, thus hampering sustainable agricultural development. With the vigorous development of modern agriculture, the application of plant growth-promoting bacteria (PGPB) can better ensure sustainable agriculture, due to their ability to improve soil properties and confer stress tolerance in plants. This review deciphered the underlying mechanisms of PGPB involved in enhancing plant stress tolerance and performance under various abiotic and biotic stresses. Moreover, the recent advancements in PGPB inoculation techniques, the commercialization of PGPB-based technology and the current applications of PGPB in sustainable agriculture were extensively discussed. Finally, an outlook on the future directions of microbe-aided agriculture was pointed out. Providing insights into plant-PGPB interactions under biotic and abiotic stresses and offering evidence and strategies for PGPB better commercialization and implementation can inspire the development of innovative solutions exploiting PGPB under climatological conditions.

Electron microscopic imaging and NanoSIMS investigation on physiological responses of Aspergillus niger under Pb(II) and Cd(II) stress

In the bioremediation process, coexistence of lead (Pb) and cadmium causes complex toxicity, resulting in the difficulty of bioremediation. This study investigated the physiological responses and bioaccumulation mechanisms of the typical filamentous fungus Aspergillus niger under the coexistence of Pb and Cd. Four treatments were set up, i.e., control, sole Pb, sole Cd, and coexistence of Pb and Cd. The morphology of A. niger were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. Then, nano-scale secondary ion mass spectrometry (NanoSIMS) was applied to accurately investigate the distribution of heavy metals in the fungal cells under the coexistence of Pb and Cd. Finally, the metallogenic process and mineral types were simulated by Geochemist's Workbench (GWB). The electron microscopic and NanoSIMS imaging showed that Pb and Cd were accumulated in both the extracellular and intracellular regions of the A. niger cells. In particular, the accumulated Pb content was ten times higher than that of Cd. However, Cd showed stronger toxicity than Pb to A. niger. Compared with the control treatment, Cd stress resulted in a two-fold increase of cell diameter and more extracellular substances, whereas the cell diameter increased nearly four times in the coexistence treatment. Moreover, the bioaccumulation of Pb was more intense than that of Cd during competitive sorption. The GWB simulation confirmed that Pb2+ can form multiple minerals (e.g., PbC2O4, PbHPO4, and Pb3(PO4)2, etc.), which significantly weakened its toxicity on the cell surface. This study elucidated the morphological characteristics of A. niger and competitive bioaccumulation under the coexistence of Pb and Cd, which would facilitate the application of microorganisms to the bioremediation of coexisted metals.

Effective Usage of Biochar and Microorganisms for the Removal of Heavy Metal Ions and Pesticides

The bioremediation of heavy metal ions and pesticides is both cost-effective and environmentally friendly. Microbial remediation is considered superior to conventional abiotic remediation processes, due to its cost-effectiveness, decrement of biological and chemical sludge, selectivity toward specific metal ions, and high removal efficiency in dilute effluents. Immobilization technology using biochar as a carrier is one important approach for advancing microbial remediation. This article provides an overview of biochar-based materials, including their design and production strategies, physicochemical properties, and applications as adsorbents and support for microorganisms. Microorganisms that can cope with the various heavy metal ions and/or pesticides that enter the environment are also outlined in this review. Pesticide and heavy metal bioremediation can be influenced by microbial activity, pollutant bioavailability, and environmental factors, such as pH and temperature. Furthermore, by elucidating the interaction mechanisms, this paper summarizes the microbe-mediated remediation of heavy metals and pesticides. In this review, we also compile and discuss those works focusing on the study of various bioremediation strategies utilizing biochar and microorganisms and how the immobilized bacteria on biochar contribute to the improvement of bioremediation strategies. There is also a summary of the sources and harmful effects of pesticides and heavy metals. Finally, based on the research described above, this study outlines the future scope of this field.

Potential of Biochar from Wood Gasification to Retain Endocrine Disrupting Chemicals

In this study, a biochar obtained from poplar wood gasification at a temperature of 850 °C was used to adsorb the xenoestrogens 4-tert-octylphenol (OP) and bisphenol A (BPA) and the herbicide metribuzin from water. Scanning electron microscopy (SEM-EDX) and Fourier-transform infrared (FTIR) spectroscopy were employed to investigate the surface micromorphology and functional groups composition of biochar, respectively. The study of sorption kinetics showed that all compounds achieved the steady state in less than 2 h, according to a pseudo-second order model, which denoted the formation of strong bonds (chemisorption) between biochar and the compounds. Adsorption isotherms data were described by the Henry, Freundlich, Langmuir and Temkin equations. At temperatures of 10 and 30 °C, the equilibrium data of the compounds were generally better described by the Freundlich model, although, in some cases, high correlation coefficients (r ≥ 0.98) were obtained for more than one model. Freundlich constants, K_F, for OP, BPA and metribuzin were, respectively, 218, 138 and 4 L g^-1 at 10 °C and 295, 243 and 225 L g^-1 at 30 °C, indicating a general increase of adsorption at higher temperature. Desorption of all compounds, especially OP and BPA, from biochar was slow and very scarce, denoting an irreversible and hysteretic process. Comparing the results of this study with those reported in the literature, we can conclude that the present biochar has a surprising ability to retain organic compounds almost permanently, thus behaving as an excellent low-cost biosorbent.

Microbiological inoculation with and without biochar reduces the bioavailability of heavy metals by microbial correlation in pig manure composting

Biochar provides a suitable microenvironment for the growth of microorganisms. It may directly or indirectly affect changes in the population of microorganisms, thus affecting heavy metal bioavailability. This study aims to explore the effects of microbiological inoculation with and without biochar on microorganisms and on the bioavailability of heavy metals during pig manure composting. Three composting experiments were conducted under various conditions including no treatment (CK), only microbiological inoculation (TA), and integration with biochar (TB). Compared with raw materials before compost, TA reduced the bioavailability of Cu by 25.1%, Zn by 25.64%, and both Pb and Cr by 1.75%. TB reduced the bioavailability of Cu by 35.38%, Zn by 19.34%, Pb by 0.81%, and Cr by 3.9%. Furthermore, correlation analysis demonstrated that Debaryomyces were the primary fungi, possibly controlling the passivation of Cr. Bacillus, Fusarium, Pseudogracilibacillus, Sinibacillus, and Botryotrichum were the primary bacteria and fungi potentially governing the passivation of Zn, Lastly, Debaryomyces and Penicillium were the primary bacteria and fungi potentially controlling the passivation of Pb and Cu, respectively. Overall, we demonstrated that pig manure added to the microbial inoculum and biochar effectively reduced the bioavailability of heavy metals, thereby offering an applicable technology for reducing heavy metal contamination during pig manure composting.

Substitution of manure for mineral P fertilizers increases P availability by enhancing microbial potential for organic P mineralization in greenhouse soil

The shortage of phosphorus (P) as a resource represents a major challenge for the sustainable development of agriculture. Manure has a high P content and is a potential substitute for mineral P fertilizers. However, little is known about the effects on soil P availability and soil microbial P transformation of substituting manure for mineral P fertilizers. In this study, variations in soil P availability and bacterial P mobilization were evaluated under treatment with manure as compared to mineral P fertilizers. In the greenhouse fruit and vegetable production system that provided the setting for the study, substitution of manure for mineral P (PoR treatment) resulted in a similar level of soil total P and a similar fruit and vegetable yield as compared to traditional fertilization, but a significantly increased level of soil available P. In addition, PoR treatment enhanced bacterial organic P mineralization potential and decreased inorganic P dissolution potential. These results demonstrate that manure application increases the availability of soil P primarily by enhancing soil microbial Po mineralization, indicating the potential feasibility of applying manure instead of mineral P fertilizers in greenhouse farming.