Simultaneous immobilization of lead and arsenic and improved phosphorus availability in contaminated soil using biochar composite modified with hydroxyapatite and oxidation: Findings from a pot experiment

Controlling As, Cd, and Pb bioaccumulation in rice under different levels of alternate wetting and drying irrigation with biochar amendment: A 3-year field study

One challenging task to produce rice that comply with the increasing demanding regulations, is to reduce, simultaneously, grain bioaccumulation of As, Cd, and Pb. A 3-year field experiment was conducted in a Mediterranean environment, to evaluate the effects on As, Cd, and Pb bioaccumulation in rice grain, of the adoption of two levels of alternate wetting and drying (AWD) irrigation conditions: moderate and intensive (reflooding at -20 kPa and -70 kPa soil matric water potential, respectively), relative to the traditional permanent flood irrigation. Plots were prepared with or without a one-time holm oak biochar application (35 Mg ha-1), in the first year of the study. Arsenic bioaccumulation decreased in rice grain in the AWD systems, both total and inorganic (AsInorg), with the lower values reached in the intensive AWD irrigation (0.131-0.151 mg kg-1 dry weight), when the drying conditions were more intense. For As, biochar contributed to a further reduction in the bioaccumulation in the first two years but lost its efficacy with the field aging after three years of its application. However, the transition to AWD irrigation led to a significant increase in Cd bioaccumulation in rice grain (21-fold increase in the more intensive system, whose values reached up to 0.127 mg kg-1), which can be counteracted by biochar application, to values statistically similar to those of permanent flooding. Contrariwise, the effects on Pb bioaccumulation were not so significant, but decreased with the transition to ADW irrigation, and with biochar application, relatively to the non-amended counterparts. Therefore, the implementation of intensive AWD with biochar represents a potentially fruitful strategy to enhance food safety of rice production, controlling, simultaneously, As, Cd, and Pb bioaccumulation. Nevertheless, new approaches need to be developed to attend the limits established for AsInorg to produce food for infants, even in uncontaminated soils.

Lead/cadmium impacts on zeolite-tobermorite: Nutrient release and sediment stability

To realize the comprehensive utilization of large amounts of high-ash coal slime and comprehensively understand the excellent performance of nutrient release and lead and cadmium adsorption of high-ash coal slime silicon composite materials, green and safe mild hydrothermal conditions (200 °C) were used to prepare the rich-rich coal slime. Zeolite/tobermorite composites (Z-TOBs) were used in this study. Batch adsorption tests and repeated extraction tests were used to determine whether silicon, potassium, and calcium nutrients of Z-TOBs have sustained release properties and are affected by pH. Through temperature gradient analyses, batch adsorption experiments at the water-soil interface, and density functional theory (DFT) calculations, the nutrient release mechanisms and the passivation of lead (Pb) and cadmium (Cd) by Z-TOBs were comprehensively investigated. The findings indicated that the release of silicon, calcium, and potassium nutrients from Z-TOBs was influenced by both ambient temperature and Pb and Cd concentrations. As the ambient temperature increased, the release patterns of different nutrients from Z-TOBs varied significantly, and the release of Pb and Cd was enhanced; however, the adsorption capacity for Pb consistently exceeded that for Cd. The passivation effect of soil amended with Z-TOBs on Pb remained stronger than that on Cd, significantly impacting the release of silicon. Characterization results revealed that silicon participated in the formation of silicon-containing compounds such as Cd2SiO4, CdAl2Si3O12, and Pb3Si2O7. The skew density calculated using DFT indicated that the silicic acid compounds formed with Pb and Cd exhibited greater stability than those formed with CO32- and SO42- groups, with Pb compounds demonstrating superior stability compared to Cd compounds. This study offers both practical and theoretical insights for the comprehensive utilization of high-ash coal slime in mild environments, presenting an alternative pathway for sustainable agricultural development.

Rice husk biochar resuscitates the microecological functions of heavy-metal contaminated soil after washing by enriching functional bacteria

Biochar has great potential for simultaneously improving soil ecological functions and eliminating environmental pollutants. However, studies on this strategy in the restoration of ecological functions in chelator-washed soil are lacking, and the effect of biochar on the structure, functions, and microbial interactions of washed soil microbiomes are unclear. Hence, the effect of rice husk biochar (RHB, 2 %) on the physicochemical properties, heavy metal fractions, and microbial community structure of glutamate-N, N-diacetic acid (GLDA)- and ethylenediaminetetraacetic acid (EDTA)- washed remediated soil were investigated. Results showed that the RHB addition restored the washed soil physical structure (pores and agglomerates) and meanwhile, the soil colloidal sheet sweeps increased by 20.49 % and 102.07 % in the z-axis, respectively. Additionally, RHB significantly increased washed soil pH (P < 0.05) and alkaline phosphatase and urease activities, while decreased acid phosphatase and glucosidase activities. The Observed-species and Shannon index were significantly higher in soil treated by RHB combined with GLDA and EDTA than those treated with GLDA and EDTA alone (P < 0.05). GLDA washing coupled RHB treatment enriched key bacterial groups such as MND1, Chelativorans, and Ellin6067, while EDTA washing coupled RHB treatment enriched Sreroidobacter, Micromonospora, and Reyranella, that both related to C-, N-, and P- cycles. Importantly, RHB addition could enrich functional bacteria by increasing bacterial resistance, including glucose metabolic homeostasis and metal ion resistance. The observed enrichment of functional bacteria provided evidence for the enhancement of soil nutrient cycles, indicative of improved soil functions by combination of chelator washing and biochar amended.

Synergistic effect of biochar and intercropping on lead phytoavailability in the rhizosphere of a vegetable-grass system

The effects of engineered steam exploded biochar on the phytoavailability of toxic elements in the shared- and nonshared-rhizosphere of vegetable-grass intercropping system have not been investigated yet. Therefore, we explored and elucidated the synergistic effect of pristine rape-straw biochar (BC), steam exploded BC (BCSE), KMnO4-modified BCSE (BCSEMn), and hydroxyapatite-modified BCSE (BCSEHA) on the solubility, fractionation and phytoavailability of lead (Pb) in a vegetable-grass intercropping system. In a rhizosphere box, Brassica chinensis L. (pakchoi; PC, as a vegetable) and Pennisetum polystachion L. (mission grass; MG, as a Pb hyperaccumulator), were grown in the biochar treated soil with (non-shared rhizosphere) or without (shared rhizosphere) root separation. Addition of BCSEMn and BCSEHA, particularly BCSEMn, significantly improved plant growth, photosynthetic pigment levels, and positively influenced the gas exchange attributes by suppressing oxidative stress and boosting antioxidant enzymes activities. Both biochars altered a proportion of Pb in the acid soluble to the immobile fraction and thus significantly decreased its leachability (TCLP-Pb) and bioavailability (CaCl2-extrcated Pb) by 32.7 %-33.9 % and 48.5 %-53.5 %, respectively, as compared to the control. Both biochars, particularly BCSEMn, reduced significantly the Pb content in shoots and roots of PC and MG with a significantly higher efficiency in the PC than in the MG; this was the case more in the shared than in the non-shared rhizosphere. These findings indicate the synergistic effect of BCSEMn and BCSEHA and intercropping for enhancing the grass phytostabilization capacity for Pb and reducing its uptake by edible plants in a vegetables-grass system, which could be used as a promising approach for the phytomanagement of Pb contaminated soils.

Simultaneous immobilization strategy of anionic metalloids and cationic metals in agricultural systems: A review

Concurrent heavy metals remediation in natural environments poses significant challenges due to factors like metal speciation and interactions with soil moisture. This review focuses on strategies for immobilizing both anionic and cationic metals simultaneously in soil-crop systems. Key approaches include water management, biochar utilization, stabilizing agents, nanotechnology, fertilization, and bioremediation. Sprinkler or intermittent irrigation combined with soil amendments/biochar effectively immobilizes As/Cd/Pb simultaneously. This immobilization occurs through continuous adsorption-desorption, oxidation-reduction, and precipitation mechanisms influenced by soil pH, redox reactions, and Fe-oxides. Biochar from sources like wine lees, sewage sludge, spent coffee, and Fe-nanoparticles can immobilize As/Cd/Pb/Cr/Co/Cu/Zn together via precipitation. In addition, biochar from rice, wheat, corn straw, rice husk, sawdust, and wood chips, modified with chemicals or nanoparticles, simultaneously immobilizes As and Cd, containing higher Fe3O4, Fe-oxide, and OH groups. Ligand exchange immobilizes As, while ion exchange immobilizes Cd. Furthermore, combining biochar especially with iron, hydroxyapatite, magnetite, goethite, silicon, graphene, alginate, compost, and microbes-can achieve simultaneous immobilization. Other effective amendments are selenium fertilizer, Ge-nanocomposites, Fe-Si materials, ash, hormone, and sterilization. Notably, combining nano-biochar with microbes and/or fertilizers with Fe-containing higher adsorption sites, metal-binding cores, and maintaining a neutral pH could stimulate simultaneous immobilization. The amendments have a positive impact on soil physio-chemical improvement and crop development. Crops enhance production of growth metabolites, hormones, and xylem tissue thickening, forming a protective barrier by root Fe-plaque containing higher Fe-oxide, restricting upward metal movement. Therefore, a holistic immobilization mechanism reduces plant oxidative damage, improves soil and crop quality, and reduces food contamination.

Enhanced multi-metals stabilization: Synergistic insights from hydroxyapatite and peroxide dosing strategies

Sediment contamination by heavy metals is a pressing environmental concern. While in situ metal stabilization techniques have shown promise, a great challenge remains in the simultaneous immobilization of multi-metals co-existing in contaminated sediments. This study aims to address this challenge by developing a practical method for stabilizing multi-metals by hydroxyapatite and calcium peroxide (HAP/CaO2) dosing strategies. Results showed that dosing 15.12 g of HAP/CaO2 at a ratio of 3:1 effectively transformed labile metals into stable fractions, reaching reaction kinetic equilibrium within one month with a pseudo-second-order kinetic (R2 > 0.98). The stable fractions of Nickel (Ni), Chromium (Cr), and lead (Pb) increased by approximately 16.9 %, 26.7 %, and 21.9 %, respectively, reducing heavy metal mobility and ensuring leachable concentrations complied with the stringent environmental Class I standard. Mechanistic analysis indicated that HAP played a crucial role in Pb stabilization, exhibiting a high rate of 0.0176 d-1, while Cr and Ni stabilization primarily occurred through the formation of hydroxide precipitates, as well as the slowly elevated pH (>8.5). Importantly, the proposed strategy poses a minimal environmental risk to benthic organisms exhibits almost negligible toxicity towards Vibrio fischeri and the Chironomus riparius, and saves about 71 % of costs compared to kaolinite. These advantages suggest the feasibility of HAP/CaO2 dosing strategies in multi-metal stabilization in contaminated sediments.

Efficient stabilization of arsenic migration and conversion in soil with surfactant-modified iron-manganese oxide: Environmental effects and mechanistic insights

The use of iron-manganese oxide (FMO) as a promising amendment for remediating arsenic (As) contamination in soils has gained attention, but its application is limited owing to agglomeration issues. This study aims to address agglomeration using surfactant-modified FMO and investigate their stabilization behavior towards As and resulting environmental changes upon amendments. The results confirmed the efficacy of surfactants and demonstrated that cetyltrimethylammonium-bromide-modified FMO significantly reduced the leaching concentration of As by 92.5 % and effectively suppressed the uptake of As by 85.8 % compared with the control groups. The ratio of the residual fraction increased from 30.5-41.6 % in unamended soil to 67.9-69.2 %. The number of active sites was through the introduction of surfactants and immobilized As via complexation, ion exchange, and redox reactions. The study also revealed that amendments and the concentration of As influenced the soil physicochemical properties and enriched bacteria associated with As and Fe reduction and changed the distribution of C, N, Fe, and As metabolism genes, which promoted the stabilization of As. The interactions among cetyltrimethylammonium bromide, FMO, and microorganisms were found to have the greatest effect on As immobilization.

The application of P-modified biochar in wastewater remediation: A state-of-the-art review

Phosphorus modified biochar (P-BC) is an effective adsorbent for wastewater remediation, which has attracted widespread attention due to its low cost, vast source, unique surface structure, and abundant functional groups. However, there is currently no comprehensive analysis and review of P-BC in wastewater remediation. In this study, a detailed introduction is given to the synthesis method of P-BC, as well as the effects of pyrolysis temperature and residence time on physical and chemical properties and adsorption performance of the material. Meanwhile, a comprehensive investigation and evaluation were conducted on the different biomass types and phosphorus sources used to synthesize P-BC. This article also systematically compared the adsorption efficiency differences between P-BC and raw biochar, and summarized the adsorption mechanism of P-BC in removing pollutants from wastewater. In addition, the effects of P-BC composite with other materials (element co-doping, polysaccharide stabilizers, microbial loading, etc.) on physical and chemical properties and pollutant adsorption capacity of the materials were investigated. Some emerging applications of P-BC were also introduced, including supercapacitors, CO2 adsorbents, carbon sequestration, soil heavy metal remediation, and soil fertility improvement. Finally, some valuable suggestions and prospects were proposed for the future research direction of P-BC to achieve the goal of multiple utilization.

Biochar and nano biochar: Enhancing salt resilience in plants and soil while mitigating greenhouse gas emissions: A comprehensive review

Salinity stress poses a significant challenge to agriculture, impacting soil health, plant growth and contributing to greenhouse gas (GHG) emissions. In response to these intertwined challenges, the use of biochar and its nanoscale counterpart, nano-biochar, has gained increasing attention. This comprehensive review explores the heterogeneous role of biochar and nano-biochar in enhancing salt resilience in plants and soil while concurrently mitigating GHG emissions. The review discusses the effects of these amendments on soil physicochemical properties, improved water and nutrient uptake, reduced oxidative damage, enhanced growth and the alternation of soil microbial communities, enhance soil fertility and resilience. Furthermore, it examines their impact on plant growth, ion homeostasis, osmotic adjustment and plant stress tolerance, promoting plant development under salinity stress conditions. Emphasis is placed on the potential of biochar and nano-biochar to influence soil microbial activities, leading to altered emissions of GHG emissions, particularly nitrous oxide(N2O) and methane(CH4), contributing to climate change mitigation. The comprehensive synthesis of current research findings in this review provides insights into the multifunctional applications of biochar and nano-biochar, highlighting their potential to address salinity stress in agriculture and their role in sustainable soil and environmental management. Moreover, it identifies areas for further investigation, aiming to enhance our understanding of the intricate interplay between biochar, nano-biochar, soil, plants, and greenhouse gas emissions.

Foliar spraying of lanthanum activates endocytosis in lettuce (Lactuca sativa L.) root cells, increasing Cd and Pb accumulation and their bioaccessibility

Cadmium (Cd) and lead (Pb) accumulate easily in leafy vegetables and can harm human health. Lanthanum (La) have been used to improve agricultural yield and quality, but the effect of La application on Cd/Pb enrichment in leafy vegetables remains incomplete currently. A previous study reported that the endocytosis in lettuce leaf cells can be activated by La, leading to an increase in Pb accumulation in lettuce leaves. However, it has not been investigated whether foliar application of La enhances root cellular endocytosis and promotes its uptake of Cd and Pb. In this study, the influence of La on the uptake of Cd and Pb, Cd bioaccessibility, and the safety risks of cultivating lettuce under Cd and Pb stress were explored. It was found that La increased Cd (16-30 % in shoot, 16-34 % in root) and Pb (25-29 % in shoot, 17-23 % in root) accumulation in lettuce. The increased accumulation of Cd and Pb could be attributed to La-enhanced endocytosis. Meanwhile, La enhanced the toxicity of both Cd and Pb, inhibited lettuce growth, and aggravated the damage to the photosynthetic and antioxidant systems. Finally, gastrointestinal simulation experiments showed that La increased the Cd bioaccessibility in both gastric and intestinal phase by 7-108 % and 9-87 %, respectively. These results offer valuable insights into the safety of REEs for agricultural applications.

Phosphorus Footprint in the Whole Biowaste-Biochar-Soil-Plant System: Reservation, Replenishment, and Reception

Two phosphorus (P)-rich biowastes, sewage sludge (SS) and bone dreg (BD), were selected to clarify P footprints among biowaste, biochar, soil, and plants by introducing a novel "3R" concept model. Results showed that pyrolysis resulted in P transformation from an unstable-organic amorphous phase to a stable-inorganic crystalline phase with a P retention rate of 70-90% in biochar (P reservation). In soil, SSBC released more P in acid red soil and alkaline yellow soil than BDBC, while the opposite result appeared in neutral paddy soil. The P released from SSBC formed AlPO4 by combining with Al in soil, whereas P from BDBC transformed into Ca5(PO4)3F(or Cl) in conjunction with Ca in the soil (P replenishment). Various plants exhibited an uptake of approximately 2-6 times more P from biochar-amended soil than from the original soil (P reception). This study can guide the application of biochar in various soil-plant systems for effective nutrient reclamation.