Remediation of lead and cadmium co-contaminated mining soil by phosphate-functionalized biochar: Performance, mechanism, and microbial response

Phosphorus-loaded magnetic biochar for remediation of cadmium contaminated paddy soil: Efficacy and identification of limiting factors

Alleviating cadmium (Cd) risk in paddy soils is a global research hotspot. Although biochar reduces Cd mobility, a holistic perspective on the effects of biochar on Cd fraction distribution in rice rhizosphere and its immobilization mechanisms is lacking. Here, we developed a pathway model that links soil physicochemical properties, IP formation, enzyme activity, microbial biomass, porewater nutrients, and soil Cd fractions to fill knowledge gaps. Results revealed that phosphorus-loaded magnetic biochar (PMLB) application increased soil pH, available phosphorus (AP), total phosphorus (TP), microbial biomass, and TP and Fe contents in porewater while inhibiting soil enzyme activities. Compared with the control, 0.2 %-1 % w/w PMLB treatment reduced soil acetic acid-extractable Cd (Aci-Cd) content during the tillering, filling, and maturity periods by 23.71-32.92 %, 25.45-37.33 %, and 7.39-18.40 %, respectively. Cd content in brown rice was reduced by 44.02-47.86 %. Soil pH, AP and urease activity were the primary drivers of soil Aci-Cd reduction. Soil microbial biomass contributed most to reducing Cd content in rice tissues (total path coefficient: -0.48), followed by enzyme activity and IP. Additionally, PMLB promoted IP formation and altered the immobilization methods of Cd by IP, from coprecipitation with iron (hydr)oxides and phosphate to ternary complex formation with phosphate as a bridge to band Cd and iron (hydr)oxides.

Phosphate-modified biochar attenuates cadmium availability in contaminated soil and reduces its transfer to tomato fruits

Cadmium (Cd) is a toxic heavy metal for plant growth and development. Biochar has been proposed as an effective approach to increase immobilized cadmium fractions in soil and reduce its availability and accumulation by plants. In the present study, the enrichment of biochar with water-soluble polyphosphate (PLB) was tested to assess its ability to modify cadmium immobilization capacities in soils compared with standard biochar (SB) and orthophosphate enriched biochar (OLB) under Cd stress. In a pot experiment, the impact of these three biochar forms (SB, PLB, and OLB) on soil properties, soil Cd fractions, photosynthesis, tomato plant growth, yield, fruit quality, and nutrient uptake was assessed under two levels of Cd (0, and 5 mg kg-1). The obtained results showed that the enrichment of biochar with both forms of phosphorus fertilizers (orthophosphate and polyphosphate) significantly and positively impacted P and K contents in the final enriched biochar compared to SB. Results demonstrate that applying PLB under cadmium stress significantly increased phosphorus availability in soil (+32 %) and reduced Cd exchangeable fraction compared to the SB and OLB treatments. These findings suggest that, under cadmium stress, the slow-release properties as well as the organometallic chelation and precipitation capacities of the PLB can help in improving phosphorus, calcium, and iron uptake by plants and reducing Cd uptake and translocation to the shoot tissues, which resulted in significant enhancement of plant photosynthesis efficiency (PIabs: +144 %), shoot dry biomass (+117 %), and fruit yield (308 %) and quality, compared to standard biochar. Therefore, the enrichment of biochar with polyphosphate fertilizers could be proposed as an efficient strategy to enhance plant growth, physiology, and nutrition and mitigate cadmium stress and toxicity in contaminated soils.

Organic acid release and microbial community assembly driven by phosphate-solubilizing bacteria enhance Pb, Cd, and As immobilization in soils remediated with iron-doped hydroxyapatite

Although iron-doped hydroxyapatite (Fe-HAP) and its composites have been reported to immobilize arsenic (As), lead (Pb), and cadmium (Cd), its practical application is limited by the inefficient release of iron and phosphate. In this study, Ochrobactrum anthropic, a phosphate-solubilizing bacterium isolated from a lead-zinc smelting site, was employed to enhance multi-heavy metal immobilization in Fe-HAP-amended soils. O. anthropic secreted low-molecular-weight organic acids, promoted phosphate (25.6 mg/L) and iron (14.2 mg/L) release from Fe-HAP, and minimally disrupted native bacteria. Compared to CK, the combination of 2 % O. anthropic (v/w) and Fe-HAP (Fe-to-HAP ratio of 1:1) significantly increased the residual fractions of Cd, Pb, and As by 109.09 %, 49.21 %, and 25.00 %, respectively. The combined treatment also improved available phosphorus, available nitrogen, and acid phosphatase activity by 233.24 %, 196.55 %, and 246.45 %, respectively. Furthermore, O. anthropic facilitated the recruitment of phylum Firmicutes and genera Acidovorax, Sedimentibacter, and Brevundimonas, shifting the bacterial community from specialists to generalists. Positive correlations were observed between residual fractions of Pb, Cd, As, well-crystallized As, and the abundance of Firmicutes, Acidovorax, and Sedimentibacte. These findings demonstrate the potential of an O. anthropic-driven Fe-HAP remediation strategy for the eco-friendly restoration of barren and polymetallic-contaminated soils.

Phosphate-enhanced Cd stabilization in soil by sulfur-doped biochar: Reducing Cd phytoavailability and accumulation in Brassica chinensis L. and shaping the microbial community

To explore the potential of livestock manure-derived biochar for the remediation of Cd-contaminated soil, a pot experiment was conducted to explore the stabilization efficiency of cattle manure biochar (T2, BC), sulfur-doped biochar (T3, SBC), and SBC combined with phosphate (T4, SBC-PF) on Cd in contaminated soil and their effects on Cd accumulation in Chinese cabbage (Brassica chinensis L.) and soil microorganisms. The results showed that soil available phosphorus (AP), available potassium (AK), and organic matter (OM) significantly increased in T3 and T4, and the biomass of Chinese cabbage also increased from 0.46 g/pot to 0.57 and 1.05 g/pot, respectively. The DTPA-extractable Cd in T3 and T4 dramatically reduced by 78.6% and 91.4% (p < 0.05); the acid-soluble Cd decreased by 11.3% and 13.2%; and the residual Cd increased by 30.0% and 10.0%. Most importantly, the Cd contents in T2, T3, and T4 decreased by 2.2%, 89.7%, and 93.1% in the shoots of Chinese cabbage and 21.3%, 82.2%, and 86.2% in the roots of Chinese cabbage, respectively. Moreover, SBC-PF obviously changed the bacterial community and enhanced the interactions among microbes in the soil. Structural equation modeling revealed that microbial interspecific mutualistic relationships were the key factor in the pathway for reducing Cd phytoavailability. Mantel tests and random forest analyses further revealed that biochar enhanced the interactions among microorganisms by increasing the AP content in the soil. These findings demonstrated that SBC combined with phosphate is appropriate for stabilizing Cd and improving soil quality.

Synergistic effect between biochar and nitrate fertilizer facilitated arsenic immobilization in an anaerobic contaminated paddy soil

Nitrate nitrogen fertilizer was usually used to mitigate arsenic (As) release and mobilization in the anaerobic contaminated paddy soil. However, the effect of the interplay between nitrate fertilizer and biochar on As availability as well as the involved mechanism were poorly understood. Herein, the effects and mechanisms of biochar, nitrate fertilizer, and biochar-based nitrate fertilizer on the availability of As in the contaminated paddy soil were investigated via a microcosm incubation experiment. Results indicated that the application of biochar-based nitrate fertilizer significantly lessened the available As concentration in the contaminated paddy soil from 3.01 ± 0.03 (control group) to 2.24 ± 0.08 mg kg-1, which presented an immobilization efficiency of 26.6 % better than those of individual biochar (13.5 %) and nitrate fertilizer (17.6 %), exhibiting a synergistic effect. Moreover, the biochar-based nitrate fertilizer also facilitated the transformation of more toxic arsenite in the contaminated soil to less toxic arsenate. Further, biochar-based nitrate fertilizer increased soil redox potential (Eh), dissolved organic carbon, organic matter, and nitrate yet decreased soil pH and ammonium, which changed the microbial community in the soil, enhancing the relative abundance of Bacillus, Arthrobacter, and Paenibacillus. These functional microorganisms drove the coupled transformation between nitrate denitrification and Fe(II) or As(III) oxidation, favoring As immobilization in the anaerobic paddy soil. Additionally, the co-application of biochar offset the negative effect of single nitrate fertilizer on microbial community diversity. Overall, biochar-based nitrate fertilizer could be a promising candidate for the effective immobilization of As in the anaerobic paddy soil. The current research can provide a valuable reference to the remediation of As-contaminated paddy soil and the production of safe rice.

Microbial inoculation accelerates rice straw decomposition by reshaping structure and function of lignocellulose-degrading microbial consortia in paddy fields

Inoculating lignocellulose-degrading microorganisms can accelerate straw decomposition in paddy field; however, the relationship between indigenous and inoculated microorganisms remains unclear. This study explored the effects of microbial inoculation on straw decomposition, microbial community, lignocellulose-degrading consortia, and associated functional genes. After inoculation, straw degradation rate increased by up to 4.9 %, and the rice yield increased by 790 kg/ha. Microbial inoculation restructured soil microbial community, influencing key taxa and interactions within the microbial network. A lignocellulose-degrading consortia consisting 37 genera was established, with a notable increase in the relative abundance of lignocellulose-degrading bacteria following inoculation. Among them, Pseudarthrobacter, with high lignin-degrading enzyme activity, emerged as a key genus after inoculation. Additionally, the abundance of lignin-degrading enzyme genes also increased significantly after inoculation. These findings offer new insights into how microbial inoculation accelerates the in situ decomposition of rice straw by reshaping the structure and function of lignocellulose-degrading consortia within the soil ecosystem.

Reclamation of co-pyrolyzed dredging sediment as soil cadmium and arsenic immobilization material: Immobilization efficiency, application safety, and underlying mechanisms

The safe management of toxic metal-polluted dredging sediment (DS) is imperative owing to its potential secondary hazards. Herein, the co-pyrolysis product (DS@BC) of polluted DS was creatively applied to immobilize soil Cd and As to achieve DS resource utilization, and the efficiency, safety, and mechanism were investigated. The results revealed that the DS@BC was more effective at reducing soil Cd bioavailability than the DS was (58.9-73.2% vs. 21.8-27.4%), except for the dilution effect, whereas the opposite phenomenon occurred for soil As (25.5-35.7% vs. 35.7-42.8%). The DS@BC immobilization efficiency was dose-dependent for both Cd and As. Soil labile Cd and As were transformed to more stable fractions after DS@BC immobilization. DS@BC immobilization inhibited the transfer of soil Cd and As to Brassica chinensis L. and did not cause excessive accumulation of other toxic metals in the plants. The appropriate addition of the DS@BC (8%) sufficiently alleviated the oxidative stress response of the plants and enhanced their growth. These findings indicate that the DS@BC was safe and effective for soil Cd and As immobilization. DS@BC immobilization decreased the diversity and richness of the rhizosphere soil bacterial community because of the dilution effect. The DS@BC immobilized soil Cd and As via direct adsorption, and indirect increasing soil pH, and regulating the abundance of specific beneficial bacteria (e.g., Bacillus). Therefore, the use of co-pyrolyzed DS as a soil Cd and As immobilization material is a promising resource utilization method for DS. Notably, to verify the long-term effects and safety of DS@BC immobilization, field trials should be conducted to explore the effectiveness and risk of harmful metal release from DS@BC immobilization under real-world conditions.

Characterization of phosphate modified red mud-based composite materials and study on heavy metal adsorption

In this paper, Bayer red mud (RM) and lotus leaf powder (LL) were used as the main materials, and KH2PO4 was added to modify the material. Under the condition of high-temperature carbonization, RMLL was prepared and phosphate modified red mud matrix composite (PRMLL) was prepared based on KH2PO4 modification, which can effectively remove Pb2+ from water. The optimum preparation and application conditions were determined through orthogonal experiment: dosage 0.1g, ratio 1:1, and temperature 600 °C. The effects of pH, dosage, and initial concentration on the adsorption of Pb2+ were studied. The pseudo-first-order, pseudo-second-order, and Elovich kinetic models were fitted to the experimental data. It was found that RMLL and PRMLL were more consistent with the pseudo-second-order kinetic model and chemisorption. Langmuir, Freundlich, Timkin, and Dubinin-Radushkevich isothermal adsorption models were used to fit the experimental data. It was found that RMLL and PRMLL were more consistent with Langmuir model. In addition, the maximum adsorption capacity of RMLL and PRMLL was 188.1 mg/g and 213.4 mg/g, respectively. It is larger than the adsorption capacity of their monomers. Therefore, the use of RMLL and PRMLL as the removal of Pb2+ from water is a potential application material.

Kitchen compost-derived humic acid application promotes ryegrass growth and enhances the accumulation of Cd: An analysis of the soil microenvironment and rhizosphere functional microbes

Phytoremediation is an environmentally friendly and safe approach for remediating environments contaminated with heavy metals. Humic acid (HA) has high biological activity and can effectively complex with heavy metals. However, whether HA affects available Cd storage and the Cd accumulation ability of plants by altering the soil microenvironment and the distribution of special functional microorganisms remains unclear. Here, we investigated the effects of applying kitchen compost-derived HA on the growth and Cd enrichment capacity of ryegrass (Lolium perenne L.). Additionally, the key role of HA in regulating the structure of rhizosphere soil bacterial communities was identified. HA promoted the growth of perennial ryegrass and biomass accumulation and enhanced the Cd enrichment capacity of ryegrass. The positive effect of HA on the soil microenvironment and rhizosphere bacterial community was the main factor promoting the growth of ryegrass, and this was confirmed by the significant positive correlation between the ryegrass growth index and the content of SOM, AP, AK, and AN, as well as the abundance of rhizosphere growth-promoting bacteria such as Pseudomonas, Steroidobacter, Phenylobacterium, and Caulobacter. HA passivated Cd and inhibited the translocation capacity of ryegrass. The auxiliary effect of resistant bacteria on plants drove the absorption of Cd by ryegrass. In addition, HA enhanced the remediation of Cd-contaminated soil by ryegrass under different Cd levels, which indicated that kitchen compost-derived HA could be widely used for the phytoremediation of Cd-contaminated soil. Generally, our findings will aid the development of improved approaches for the use of kitchen compost-derived HA for the remediation of Cd-contaminated soil.

Quantification of the effect of biochar application on heavy metals in paddy systems: Impact, mechanisms and future prospects

Biochar (BC) has shown great potential in remediating heavy metal(loid)s (HMs) contamination in paddy fields. Variation in feedstock sources, pyrolysis temperatures, modification methods, and application rates of BC can result in great changes in its effects on HM bioavailability and bioaccumulation in soil-rice systems and remediation mechanisms. Meanwhile, there is a lack of application guidelines for BC with specific properties and application rates when targeting rice fields contaminated with certain HMs. To elucidate this topic, this review focuses on i) the effects of feedstock type, pyrolysis temperature, and modification method on the properties of BC; ii) the changes in bioavailability and bioaccumulation of HMs in soil-rice systems applying BC with different feedstocks, pyrolysis temperatures, modification methods, and application rates; and iii) exploration of potential remediation mechanisms for applying BC to reduce the mobility and bioaccumulation of HMs in rice field systems. In general, the application of Fe/Mn modified organic waste (OW) derived BC for mid-temperature pyrolysis is still a well-optimized choice for the remediation of HM contamination in rice fields. From the viewpoint of remediation efficiency, the application rate of BC should be appropriately increased to immobilize Cd, Pb, and Cu in rice paddies, while the application rate of BC for immobilizing As should be <2.0 % (w/w). The mechanism of remediation of HM-contaminated rice fields by applying BC is mainly the direct adsorption of HMs by BC in soil pore water and the mediation of soil microenvironmental changes. In addition, the application of Fe/Mn modified BC induced the formation of iron plaque (IP) on the root surface of rice, which reduced the uptake of HM by the plant. Finally, this paper describes the prospects and challenges for the extension of various BCs for the remediation of HM contamination in paddy fields and makes some suggestions for future development.