Effect of the direct use of biomass in agricultural soil on heavy metals __ activation or immobilization?

Effect of straw decomposition on hexavalent chromium removal by straw: Significant roles of surface potential and dissolved organic matter

Mobility and bioavailability of hexavalent chromium (Cr(VI)) in agricultural soils are affected by interactions between Cr(VI) and returned crop straws. However, the effect of straw decomposition on Cr(VI) removal and underlying mechanisms remain unclear. In this study, Cr(VI) removal by pristine and decomposed rice/rape straws was investigated by batch experiments and a series of spectroscopies. The results showed that straw decomposition inhibited Cr(VI) removal, regardless of straw types. However, the potential mechanisms of the inhibition were distinct for the two straws. For the rice straw, a lower zeta potential after decomposition suppressed Cr(VI) sorption and subsequent reduction. In addition, less Cr(VI) was reduced by the decomposed rice straw-derived dissolved organic matter (DOM) than the pristine one. In contrast, for the rape straw, due to the increased zeta potential after decomposition, the decreased Cr(VI) removal was mainly ascribed to less Cr(VI) reduction by the rape straw-derived DOM. These results emphasized the significant roles of straw surface potential and DOM in Cr(VI) removal, depending on straw types and decomposition, which facilitate the fundamental understanding of Cr(VI) removal by straws and are helpful for predicting the environmental risk of Cr and rational straw return in Cr(VI)-contaminated fields.

Application of soil amendments to reduce the transfer of trace metal elements from contaminated soils of Lubumbashi (Democratic Republic of the Congo) to vegetables

The extraction of copper and cobalt from mines has led to the contamination of agricultural soils by trace metal elements (TMEs) (e.g. Cu: 204 to 1355 mg/kg). The mining industry is one of the sources of metal discharges into the environment, contributing to water, soil, and air contamination and causing metabolic disorders in the inhabitants of the city of Lubumbashi (R.D. Congo). This study assessed the effectiveness of organocalcareous soil improvers applied to TME-contaminated soils to reduce their transfer to plants. Following a factorial design, increasing doses of organic soil improvers (chicken droppings and sawdust) and agricultural lime were applied to the soils of three market gardens (high, medium, and low Cu contamination). The experiment was monitored for 60 days. Soil physicochemical properties (pH, TOC, and total and available copper, cobalt, lead, cadmium, and zinc (mg/kg)) were determined for the three gardens and in the vegetable biomass. The daily consumption index of the vegetables was determined based on total TME content. The results show that organocalcareous soil improvers did not promote plant growth and survival on soils with high and medium levels of copper contamination. However, on soils with low copper content, organocalcareous soil improvers improved germination and plant survival and reduced the transfer of metals from the soil to the plants. The best germination and plant survival rates were obtained with the lightly contaminated market garden. In addition, the organo-limestone amendments applied to the soils slightly increased the soil pH from acidic to slightly acidic, with pH values ranging from (5.43 ± 0.07 to 7.26 ± 0.33). The daily vegetable consumption index obtained for cobalt in the low-contaminated garden ranged from (0.029 to 0.465 mg/60 kg/day), i.e. from 0.5 to 8.45 times higher than the FAO/WHO limit, unlike the other trace metals (Cd, Cu and Pb) for which the daily consumption index found was lower than the FAO/WHO limit. Organocalcareous soil improvers can only be applied to soils with low levels of TME contamination, but for soils with medium to high levels of metal contamination, new soilless production techniques such as hydroponics or bioponics are needed.

The critical role of organic matter for cadmium-lead interactions in soil: Mechanisms and risks

Understanding the interaction mechanisms between complex heavy metals and soil components is a prerequisite for effectively forecasting the mobility and availability of contaminants in soils. Soil organic matter (SOM), with its diverse functional groups, has long been a focal point of research interest. In this study, four soils with manipulated levels of SOM, cadmium (Cd) and lead (Pb) were subjected to a 90-day incubation experiment. The competitive interactions between Cd and Pb in soils were investigated using Fourier transform infrared spectrometer (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and X-ray adsorption near-edge structure (XANES) analysis. Our results indicate that Pb competed with Cd for adsorption sites on the surface of SOM, particularly on carboxyl and hydroxyl functional groups. Approximately 22.6 % of Cd adsorption sites on humus were occupied by Pb. The use of sequentially extracted exchangeable heavy metals as indicators for environment risk assessments, considering variations in soil physico-chemical properties and synergistic or antagonistic effects between contaminants, provides a better estimation of metal bioavailability and its potential impacts. Integrating comprehensive contamination characterization of heavy metal interactions with the soil organic phase is an important advancement to assess the environmental risks of heavy metal dynamics in soil compared to individual contamination assessments.

Degradation of Three Herbicides and Effect on Bacterial Communities under Combined Pollution

Pesticide residues in soil, especially multiple herbicide residues, cause a series of adverse effects on soil properties and microorganisms. In this work, the degradation of three herbicides and the effect on bacterial communities under combined pollution was investigated. The experimental results showed that the half-lives of acetochlor and prometryn significantly altered under combined exposure (5.02-11.17 d) as compared with those of individual exposure (4.70-6.87 d) in soil, suggesting that there was an antagonistic effect between the degradation of acetochlor and prometryn in soil. No remarkable variation in the degradation rate of atrazine with half-lives of 6.21-6.85 d was observed in different treatments, indicating that the degradation of atrazine was stable. 16S rRNA high-throughput sequencing results showed that the antagonistic effect of acetochlor and prometryn on the degradation rate under combined pollution was related to variation of the Sphingomonas and Nocardioide. Furthermore, the potential metabolic pathways of the three herbicides in soil were proposed and a new metabolite of acetochlor was preliminarily identified. The results of this work provide a guideline for the risk evaluation of combined pollution of the three herbicides with respect to their ecological effects in soil.

Investigating the carcinogenic and non-carcinogenic health hazards of heavy metal ions in Spinacia oleracea grown in agricultural soil treated with biochar and humic acid

This study addressed the bioaccumulation and human health risk among the consumption of Spinacia oleracea grown in agricultural soil treated with humic acid (189-2310 ppm) and biochars (0.00-5.10%.wt). The biochars came from two local feedstocks of rice-husk (RH) and sugar-beet-pulp (SBP) pyrolyzed at temperatures 300 and 600 °C. Total concentrations of Cu, Cd, and Ni found in both the soil and biomass/biochar exceeded global safety thresholds. The bioaccumulation levels of HMs in spinach leaves varied, with Fe reaching the highest concentration at 765.27 mg kg-1 and Cd having the lowest concentration at 3.31 mg kg-1. Overall, the concentrations of Zn, Cd, Pb, and Ni in spinach leaves exceeded the safety threshold limits, so that its consumption is not recommended. The assessment of hazard quotient (HI) for the HMs indicated potential health hazards for humans (HI > 1) from consuming the edible parts of spinach. The biochar application rates of 4.35%wt and 0.00%.wt resulted in the highest (3.69) and lowest (3.15) HI values, respectively. The cumulative carcinogenic risk (TCR) ranged from 0.0085 to 0.0119, exceeding the cancer risk threshold. Introducing 5.10%wt biomass/biochar resulted in a 36% rise in TCR compared to the control. The utilization of humic acid alongside HMs-polluted biochars results in elevated levels of HMs bioaccumulation exceeding the allowable thresholds in crops (with a maximum increase of 49% at 2000 ppm humic acid in comparison to 189 ppm). Consequently, this raised the HI by 46% and the TCR by 22%. This study demonstrated that the utilization of HMs-polluted biochars could potentially pose supplementary health hazards. Moreover, it is evident that the utilization of HMs-polluted biochars in treating metal-contaminated soil does not effectively stabilize or reduce pollution.

Bioleaching of Cd from contaminated Helianthus annuus L. stalk and the safe utilization of its byproducts by Aspergillus niger

Disposal and recycling of heavy metal-enriched biomass is the key to measure the success of phytoremediation. This study employed innovative approach to use Aspergillus niger (A. niger) for the treatment of Cd-contaminated Helianthus annuus L. (sunflower) stalk after phytoremediation. Single-factor results showed that the removal of Cd at an initial pH of 3 was superior to sucrose and inoculation amount. 67.67% of Cd was removed by A. niger leaching system after 11 days based on response surface methodology optimum conditions (sucrose: 76.266 g L-1; inoculation amount: 10%; initial pH: 3), while the concentrations of nitrogen, phosphorus and potassium (N, P and K) of sunflower stalk were unaffected. While physicochemical pretreatment effectively enhanced the bioleaching efficiency, it also resulted in significant loss of P and K elements, thereby reducing the value of biomass for recycling and utilization. Therefore, the direct A. niger leaching method without pretreatment is more advantageous for the safe treatment and recycling of Cd-contaminated sunflower stalks.

Soil amendments alter cadmium distribution and bacterial community structure in paddy soils

Soil amendments play a pivotal role in ensuring the safety of food production by inhibiting the transfer of heavy metal ions from soils to crops. Nevertheless, their impact on soil characteristics and the microbial community and their role in reducing cadmium (Cd) accumulation in rice remain unclear. In this study, pot experiments were conducted to investigate the effects of three soil amendments (mineral, organic, and microbial) on the distribution of Cd speciation, organic components, iron oxides, and microbial community structure. The application of soil amendments resulted in significant reductions in the soil available Cd content (16 %-51 %) and brown rice Cd content (16 %-78 %), facilitating the transformation of Cd from unstable forms (decreasing 10 %-20 %) to stable forms (increasing 77 %-150 %) in the soil. The mineral and organic amendments increased the soil cation exchange capacity (CEC) and plant-derived organic carbon (OC), respectively, leading to reduced Cd accumulation in brown rice, while the microbial amendment enhanced OC complexity and the abundances of Firmicutes and Bacteroidota, contributing to the decreased rice Cd uptake. The synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectroscopy indicated that soil amendments regulated soil Cd species by promoting iron oxides and OC coupling. Moreover, both organic and microbial amendments significantly reduced the diversity and richness of the bacterial communities and altered their compositions and structures, by increasing the relative abundances of Bacteroidota and Firmicutes and decreasing those of Acidobacteria, Actinobacteria, and Myxococcota. Soil microbiome analysis revealed that the increase of Firmicutes and Bacteroidota associated with Cd adsorption and sequestration contributed to the suppression of soil Cd reactivity. These findings offer valuable insights into the potential mechanisms by which soil amendments regulate the speciation and bioavailability of Cd, and improve the bacterial communities, thereby providing guidance for agricultural management practices.

Recent advances in phyto-combined remediation of heavy metal pollution in soil

The global industrialization and modernization have witnessed a rapid progress made in agricultural production, along with the issue of soil heavy metal (HM) pollution, which has posed severe threats to soil quality, crop yield, and human health. Phytoremediation, as an alternative to physical and chemical methods, offers a more cost-effective, eco-friendly, and aesthetically appealing means for in-situ remediation. Despite its advantages, traditional phytoremediation faces challenges, including variable soil physicochemical properties, the bioavailability of HMs, and the slow growth and limited biomass of plants used for remediation. This study presents a critical overview of the predominant plant-based HM remediation strategies. It expounds upon the mechanisms of plant absorption, translocation, accumulation, and detoxification of HMs. Moreover, the advancements and practical applications of phyto-combined remediation strategies, such as the addition of exogenous substances, genetic modification of plants, enhancement by rhizosphere microorganisms, and intensification of agricultural technologies, are synthesized. In addition, this paper also emphasizes the economic and practical feasibility of some strategies, proposing solutions to extant challenges in traditional phytoremediation. It advocates for the development of cost-effective, minimally polluting, and biocompatible exogenous substances, along with the careful selection and application of hyperaccumulating plants. We further delineate specific future research avenues, such as refining genetic engineering techniques to avoid adverse impacts on plant growth and the ecosystem, and tailoring phyto-combined strategies to diverse soil types and HM pollutants. These proposed directions aim to enhance the practical application of phytoremediation and its integration into a broader remediation framework, thereby addressing the urgent need for sustainable soil decontamination and protection of ecological and human health.

Sustainable management of campus fallen leaves through low-temperature pyrolysis and application in Pb immobilization

Realizing campus sustainability requires the environmental-friendly and economical treatment of tremendous fallen leaves. Producing fallen leaf biochar at a low temperature is a candidate approach. In this study, six common types of fallen leaves on the campus were pyrolyzed at 300 °C. The obtained biochars were characterized and the adsorption mechanisms of lead (Pb) by the fallen leaf biochars were investigated. The adsorption capacity of leaf biochar for Pb was relatively high, up to 209 mg/g (Yulania denudata leaf biochar). Adsorption of Pb onto active sites was the rate-limiting step for most leaf biochars. But for Platanus leaf biochar, intraparticle diffusion of Pb2+ dominated owing to the lowest adsorption capacity. However, the highest exchangeable Pb fraction (27%) indicated its potential for removing aqueous Pb2+. Ginkgo and Prunus cerasifera leaf biochar immobilized Pb by surface complexation and precipitation as lead oxalate. Hence, they were suitable for soil heavy metal remediation. This study shed the light on the sustainable utilization of campus fallen leaves and the application of fallen leaf biochars in heavy metal remediation.

Quantifying the mitigating effect of organic matter on heavy metal availability in soils with different manure applications: A geochemical modelling study

Manure is one of the main sources of heavy metal (HM) pollution on farmlands. It has become the focus of global ecological research because of its potential threat to human health and the sustainability of food systems. Soil pH and organic matter are improved by manure and play pivotal roles in determining soil HM behavior. Geochemical modeling has been widely used to assess and predict the behavior of soil HMs; however, there remains a research gap in manure applications. In this study, a geochemical model (LeachXS) coupled with a pH-dependent leaching test with continuously simulations over a broad pH range was used to determine the effects and pollution risks of pig or cattle manure separate application on soil HMs distribution. Both pig and cattle manure applications led to soil pH reduction in alkaline soils and increased organic matter content. Pig manure application resulted in a potential 90.5-156.0 % increase in soil HM content. Cattle manure did not cause significant HM contamination. The leaching trend of soil HMs across treatments exhibited a V-shaped change, with the lowest concentration at pH = 7, gradually increasing toward strong acids and bases. The dissolved organic matter-bound HM content directly increased the HM availability, especially for Cu (up to 8.4 %) after pig manure application. However, more HMs (Cr, Cu, Zn, Ni) were in the particulate organic matter-bound state than in other solid phases (e.g., Fe-Al(hydr) oxides, clay minerals), which inhibited the HMs leaching by more than 19.3 % after cattle manure application. Despite these variations, high HM concentrations introduced by pig manure raised the soil contamination risk, potentially exceeding 40 times at pH ±1. When manure is returned to the field, reducing its HM content and mitigating possible pollution is necessary to realize the healthy and sustainable development of circular agriculture.

A review of solid wastes-based stabilizers for remediating heavy metals co-contaminated soil: Applications and challenges

The remediation of heavy metals/metalloids (HMs) co-contaminated soil by solid wastes-based stabilizers (SWBS) has received major concern recently. Based on the literature reported in the latest years (2010-2023), this review systematically summarizes the different types of solid wastes (e.g., steel slag, coal fly ash, red mud, and sewage sludge, etc.) employed to stabilize HMs contaminated soil, and presents results from laboratory and field experiments. Firstly, the suitable solid wastes for soil remediation are reviewed, and the pros and cons are presented. Thereafter, the technical feasibility and economic benefit are evaluated for field application. Moreover, evaluation methods for remediation of different types of HMs-contaminated soil and the effects of SWBS on soil properties are summarized. Finally, due to the large specific surface, porous structure, and high reactivity, the SWBS can effectively stabilize HMs via adsorption, complexation, co/precipitation, ion exchange, electrostatic interaction, redox, and hydration process. Importantly, the environmental implications and long-term effectiveness associated with the utilization of solid wastes are highlighted, which are challenges for practical implementation of soil stabilization using SWBS, because the aging of soil/solid wastes has not been thoroughly investigated. Future attention should focus on modifying the SWBS and establishing an integrated long-term stability evaluation method.

Speciation, leachability, and phytoaccessibility of heavy metals during thermochemical liquefaction of contaminated peanut straw

In this study, the speciation, leachability, phytoaccessibility, and environmental risks of heavy metals (Cd, Zn, and Cu) during liquefaction of contaminated peanut straw in ethanol at different temperatures (220, 260, 300, 340, and 380 °C) were comprehensively investigated. The results showed that elevated temperatures facilitated heavy metal accumulation in the biochar. The acid-soluble/exchangeable and reducible fraction percentages of heavy metals were substantially reduced in the biochar after liquefaction as the temperature increased, and the oxidizable fraction became the dominant heavy metal fraction, accounting for 44.14-78.67%. Furthermore, although an excessively high liquefaction temperature (380 °C) increased the residual fraction percentages of Zn and Cu, it was detrimental to Cd immobilization. The acid-soluble/exchangeable Cd in the contaminated peanut straw readily migrates to the bio-oil during liquefaction, with the highest concentration of 1.60 mg/kg at 260 °C liquefaction temperature, whereas Zn and Cu are predominantly bound to the unexchangeable fraction in the bio-oil. Liquefaction inhibited heavy metal leachability and phytoaccessibility in biochar, the lowest extraction rates of Cd, Zn, and Cu were 0.71%, 1.66% and 0.95% by diethylenetriamine pentaacetic acid, respectively. However, the leaching and extraction concentrations increased when the temperature was raised to 380 °C. Additionally, heavy metal risk was reduced from medium and high risk to no and low risk. In summary, liquefaction reduces heavy metal toxicity and the risks associated with contaminated peanut straw, and a temperature range of 300-340 °C for ethanol liquefaction can be considered optimal for stabilizing heavy metals.

Effect and potential mechanisms of sludge-derived chromium, nickel, and lead on soil nitrification: Implications for sustainable land utilization of digested sludge

Increasing occurrence of heavy metals (HMs) in sewage sludge threatens its widespread land utilization in China due to its potential impact on nutrient cycling in soil, requiring a better understanding of HM-induced impacts on nitrification. Herein, lab-scale experiments were conducted over 185-day, evaluating the effect of sludge-derived chromium (Cr3+), nickel (Ni2+), and lead (Pb2+) on soil nitrification at different concentrations. Quantitative polymerase chain reaction and linear regression results revealed an inhibitory sequence of gene abundance by HMs' labile fraction: ammonia-oxidizing bacteria (AOB)-ammonia monooxygenase (amoA)> nitrite oxidoreductase subunit alpha (nxrA)> nitrite oxidoreductase subunit beta (nxrB). The toxicity of HMs' incremental labile fraction decreased in the order of Ni2+>Cr3+>Pb2+, with respective threshold values of 5.01, 24.03 and 38.42 mg·kg-1. Furthermore, extending incubation time reduced HMs inhibition on ammonia oxidation, mainly related to their fraction bound to carbonate minerals. Random Forest analysis, variation partitioning analysis, and Mantel test indicated that soil physicochemical properties primarily affected nitrification genes, especially in the test of Cr3+ on AOB-amoA, nxrA, nxrB, Ni2+ for complete ammonia-oxidizing bacteria-amoA, and Pb2+ for nxrA and nxrB. These findings underline the importance of labile HMs fractions and soil physicochemical properties to nitrification, guiding the establishment of HM control standards for sludge utilization.

Eco-friendly management of Meloidogyne incognita in cadmium-contaminated soil by using nematophagous fungus Purpureocillium lavendulum YMF1.683: Efficacy and mechanism

Root-knot nematodes (RKNs) are distributed globally, including in agricultural fields contaminated by heavy metals (HM), and can cause serious crop damages. Having a method that could control RKNs in HM-contaminated soil while limit HM accumulation in crops could provide significant benefits to both farmers and consumers. In this study, we showed that the nematophagous fungus Purpureocillium lavendulum YMF1.683 exhibited a high nematocidal activity against the RKN Meloidogyne incognita and a high tolerance to CdCl2. Comparing to the P. lavendulum YMF1.838 which showed low tolerance to Cd2+, strain YMF1.683 effectively suppressed M. incognita infection and significantly reduced the Cd2+ uptake in tomato root and fruit in soils contaminated by 100 mg/kg Cd2+. Transcriptome analyses and validation of gene expression by RT-PCR revealed that the mechanisms contributed to high Cd-resistance in YMF1.683 mainly included activating autophagy pathway, increasing exosome secretion of Cd2+, and activating antioxidation systems. The exosomal secretory inhibitor GW4869 reduced the tolerance of YMF1.683 to Cd2+, which firstly demonstrated that fungal exosome was involved in HM tolerance. The up-regulation of glutathione synthesis pathway, increasing enzyme activities of both catalase and superoxide dismutase also played important roles in Cd2+ tolerance of YMF1.683. In Cd2+-contaminated soil, YMF1.683 limited Cd2+-uptake in tomato by up-regulating the genes of ABCC family in favor of HM sequestration in plant, and down-regulating the genes of ZIP, HMA, NRAMP, YSL families associated with HM absorption, transport, and uptake in plant. Our results demonstrated that YMF1.683 could be a promising bio-agent in eco-friendly management of M. incognita in Cd2+ contaminated soils.

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.

Biochar application for the remediation of soil contaminated with potentially toxic elements: Current situation and challenges

Recently, biochar has garnered extensive attention in the remediation of soils contaminated with potentially toxic elements (PTEs) owing to its exceptional adsorption properties and straightforward operation. Most researchers have primarily concentrated on the effects, mechanisms, impact factors, and risks of biochar in remediation of PTEs. However, concerns about the long-term safety and impact of biochar have restricted its application. This review aims to establish a basis for the large-scale popularization of biochar for remediating PTEs-contaminated soil based on a review of interactive mechanisms between soil, PTEs and biochar, as well as the current situation of biochar for remediation in PTEs scenarios. Biochar can directly interact with PTEs or indirectly with soil components, influencing the bioavailability, mobility, and toxicity of PTEs. The efficacy of biochar in remediation varies depending on biomass feedstock, pyrolysis temperature, type of PTEs, and application rate. Compared to pristine biochar, modified biochar offers feasible solutions for tailoring specialized biochar suited to specific PTEs-contaminated soil. Main challenges limiting the applications of biochar are overdose and potential risks. The used biochar is separated from the soil that not only actually removes PTEs, but also mitigates the negative long-term effects of biochar. A sustainable remediation technology is advocated that enables the recovery and regeneration (95.0-95.6%) of biochar from the soil and the removal of PTEs (the removal rate of Cd is more than 20%) from the soil. Finally, future research directions are suggested to augment the environmental safety of biochar and promote its wider application.

Straw incorporation induces rice straighthead disease in As-contaminated paddy soil

Rice straw incorporation is globally recognized as a viable alternative to incineration. However, it might lead to arsenic (As) methylation in soils, resulting in increased accumulation of methylated As in rice plants, potentially contributing to the emergence of rice straighthead disease. To evaluate the effect of straw incorporation on the As transformation in the paddy field system, we conducted a pot experiment for rice cultivation in two paddy soils with different As background levels and also characterized the response of the soil microbial community to straw incorporation. The results showed that straw incorporation elevated the total and methylated As concentration within the soil solution and rice plants, which in turn reduced rice seed setting rate and yield, and caused straighthead disorder in rice cultivated in soils with high As levels. 16S rRNA-based sequencing demonstrated reduced abundance and diversity of microorganisms upon adding straw. Notably, the dominant phylum, Bacteroidetes, exhibited a significant increase in abundance due to straw integration, while the abundance of Proteobacteria and Acidobacteria decreased. At the family level, the prevalence of Rikenellaceae increased only in soils contaminated with As following straw incorporation. Redundancy analysis showed positive associations between Rikenellaceae and levels of methylated As present in both soil porewater and rice husks, suggesting a potentially pivotal role of Rikenellaceae in the As methylation process after straw integration. These findings collectively emphasize that including straw can reshape the soil's microbial community and amplify As methylation in the soil, thereby promoting the uptake and accumulation of methylated As in rice and inducing straighthead disease in As-contaminated soil.

A combined passivator of zeolite and calcium magnesium phosphate fertilizer: Passivation behavior and mechanism for Cd (II) in composting

Passivation of heavy metals is one of the most efficient techniques to improve the quality of compost. Many studies confirmed the passivation effect of passivators (e.g., zeolite and calcium magnesium phosphate fertilizer) on cadmium (Cd), but passivators with single component could not effectively passivate Cd in the long-term operation of composting. In the present study, a combined passivator of zeolite and calcium magnesium phosphate fertilizer (ZCP) was used to explore its impacts of adding at different composting periods (heating period, thermophilic period, cooling period) on the Cd control, compost quality (e.g., temperature, moisture content and humification), microbial community structure as well as the compost available forms of Cd and addition strategy of ZCP. Results showed that Cd passivation rate could be increased by 35.70-47.92% under all treatments in comparison to the control treatment. By altering bacterial community structure, reducing Cd bioavailability and improving the chemical properties of the compost, the combined inorganic passivator could achieve high efficiency for Cd passivation. To sum up, the addition of ZCP at different composting periods has effects on the process and quality of composting, which could provide ideas for the optimization of the passivators addition strategy.

A double-edged sword: Reductive soil disinfestation changes the fates of trace metal elements in soil

Although the effects of reductive soil disinfestation (RSD) in soil sterilization have been proven in several countries, the potential risks of trace metal elements (TMEs) caused by RSD require further assessment. Here, freshly Cd-spiked soil and historically contaminated greenhouse soil were exposed to RSD and the fates of TMEs, Cd, Co, Cu, Ni, Pb, and Zn, were investigated. All RSD treatments lasted for 21 days and subsamples were collected at different time intervals. Samples were open-air incubated for another 7 days until day 28 to simulate the situation after drainage. The bioavailability and geochemical fractionation of TMEs were investigated based on single and sequential extraction procedures and the environmental risks were assessed. The results showed that RSD increased the relative abundance of Firmicutes and Bacteroidetes, and the content of functional groups, including Fe, Mn, and S compounds respirations increased after RSD, highlighting the possible reductive dissolution of FeMn oxides and precipitation of TMEs. The dissolution decreased the reducible fractions of TMEs and increased the acid-soluble fractions of Co, Ni, Pb, and Zn, in the European Community Bureau of Reference results, reflecting the activation of TMEs in soils. However, the precipitation of sulfate resulted in the stabilization of Cd and Cu in two types of soils, increased their residual fractions, and decreased their acid-soluble fractions and bioavailabilities. After drainage, because the influence caused by precipitation rapidly disappeared and the impact of FeMn oxides dissolution remained, the acid-solubility of TMEs was greater than their initial status in the two soils. Furthermore, as a highly toxic metal, the activation of Cd at 28 days caused the rapid increase of ecological risks, which is particularly concerning. The results suggest that RSD temporarily increases the potential risks of TMEs and that certain measures must be taken.

Green synthesized hydroxyapatite for efficient immobilization of cadmium in weakly alkaline environment

The development of cost-effective passivators for the remediation of heavy metal-contaminated soils has been a research hotspot and an unsolved challenge. Herein, a novel hydroxyapatite (GSCH) was synthesized by co-precipitating distiller effluent-derived Ca with (NH₄)₂HPO₄ using straw-derived dissolved organic matter (S-DOM) as the dispersant. Batch adsorption experiments and soil incubation tests were performed to assess the immobilization efficiency of GSCH for Cd in weakly alkaline environments. As a result, GSCH showed an excellent adsorption efficiency to Cd with a maximum adsorption amount of ∼222 mg g^-1, which was fairly competitive compared to other similar previously materials reported. The kinetic data indicated that the adsorption of Cd on GSCH was a chemical and irreversible process, while the thermodynamic data revealed a spontaneous (ΔG° < 0) and endothermic (ΔH° > 0) adsorption process. Based on mechanism analysis, both physisorption (e.g., electrostatic attraction and pore filling) and chemisorption (e.g., ion exchange and complexation) were responsible for Cd adsorption on GSCH. Particularly, the incorporated S-DOM and hydroxyapatite phase in GSCH acted synergistically in the adsorption process. The incubation results showed that GSCH application could significantly reduce the bioavailability, phytoavailability and bioaccessibility of Cd in soil by 48.4%-57.8%, 20.4%-28.6% and 12.6%-24.0%, respectively. Moreover, GSCH application also improved soil bacterial communities and enhanced soil nutrient availability. Overall, this is the first study to demonstrate the potential application value of GSCH in Cd immobilization, providing promising insights into the development of green and cost-effective hydroxyapatite-based passivators for the remediation of heavy metal-contaminated soils.

Variability of Micro- and Macro-Elements in Anaerobic Co-Digestion of Municipal Sewage Sludge and Food Industrial By-Products

The main aim of this study was to evaluate the effect of the addition of selected industrial food wastes on the fate of micro- and macro-elements within an anaerobic digestion process (AD), as well as define the relationship between their content and AD efficiency. Orange peels, (OP), orange pulp (PL) and brewery spent grain (BSG) were used as co-substrates, while municipal sewage sludge (SS) was applied as the main component. The introduction of co-substrates resulted in improvements in feedstock composition in terms of macro-elements, with a simultaneous decrease in the content of HMs (heavy metals). Such beneficial effects led to enhanced methane production, and improved process performance at the highest doses of PL and BSG. In turn, reduced biogas and methane production was found in the three-component digestion mixtures in the presence of OP and BSG; therein, the highest accumulation of most HMs within the process was also revealed. Considering the agricultural application of all digestates, exceedances for Cu, Zn and Hg were recorded, thereby excluding their further use for that purpose.

Humic Substances as a Versatile Intermediary

Humic substances are organic ubiquitous components arising in the process of chemical and microbiological oxidation, generally called humification, the second largest process of the carbon cycle. The beneficial properties of these various substances can be observed in many fields of life and health, whether it is the impact on the human organism, as prophylactic as well as the therapeutic effects; animal physiology and welfare, which is widely used in livestock farming; or the impact of humic substances on the environment and ecosystem in the context of renewal, fertilization and detoxification. Since animal health, human health and environmental health are interconnected and mutually influencing, this work brings insight into the excellence of the use of humic substances as a versatile mediator contributing to the promotion of One Health.

CaFe-layered double hydroxide corn straw biochar reduced heavy metal uptake by Brassica campestris L. and Ipomoea aquatic F.: Rhizosphere effects and oxidative stress alleviation

In the present study, CaFe-layered double hydroxide corn straw biochar (CaFe-LDH@CSB) was applied to the rhizosphere soil of both pakchoi (Brassica campestris L. ssp. Chinensis Makino, B. campestris L.) and water spinach (Ipomoea aquatic F., I. aquatic F.) to explore and clarify the potential mechanism by which CaFe-LDH@CSB helps vegetables reduce heavy metal (HM) uptake and alleviate oxidative stress. Pot experiments were conducted with CaFe-LDH@CSB applied at four levels: control (CK), T1 (5 g kg^-1), T2 (10 g kg^-1) and T3 (20 g kg^-1). The results indicated that the application of CaFe-LDH@CSB significantly increased pH and decreased the acid-soluble forms of Cd, Pb, Zn and Cu in the rhizosphere soil of both B. campestris L. and I. aquatic F.; decreases of 39.4%, 18.0%, 10.0% and 33.3% in B. campestris L. and of 26.6%, 49.1%, 13.2% and 36.8% in I. aquatic F., respectively, were observed at the T3 level. Moreover, CaFe-LDH@CSB application reduced HM uptake by B. campestris L. and decreased HM-induced oxidative stress through the regulation of soil physicochemical properties and microbial abundance. For B. campestris L., variations in Sordariomycetes helped alleviate the accumulation of HMs in the aerial part, while GSH and -SH from the nonenzymatic system played an important role in scavenging H₂O₂ in leaves, thus helping B. campestris L. alleviate HM-induced oxidative stress. For I. aquatica F., variations in Vicinamibacteria and Mortierellomycetes helped alleviate the accumulation of HMs in plants, while GSH and PCs from nonenzymatic systems played an important role in removing ·O₂^- in leaves, thereby helping I. aquatica F. alleviate HM-induced oxidation stress. Our study indicated that the application of CaFe-LDH@CSB improved the rhizosphere soil environment and rebuilt the soil microbial community, helping B. campestris L. and I. aquatica F. alleviate HM-induced oxidative stress and promoting the growth of both vegetables.

Metal- and antibiotic-resistant heterotrophic plate count bacteria from a gold mine impacted river: the Mooi River system, South Africa

The Wonderfonteinspruit, South Africa, is highly impacted by a century of gold mining activities. The aim of this study was to investigate the physico-chemical properties of the Wonderfonteinspruit and the receiving Mooi River system, the levels of antimicrobial (metals and antibiotics) resistance characteristics and heterotrophic bacteria levels in these water systems. Various physico-chemical parameters were determined. R2A agar and R2A agar supplemented with antimicrobials were used to enumerate heterotrophic bacteria. Morphologically distinct antimicrobial-resistant isolates were purified and screened for antibiotic susceptibility by a disc diffusion method. Selected isolates were identified, and minimum inhibitory concentration ranges determined. Among the antimicrobial resistant isolates, 87% were resistant to at least one antibiotic. Of these, almost 50% were resistant to more than 3 antibiotic classes. A large proportion was resistant to all 7 antibiotics tested. Phyla detected were Proteobacteria, Firmicutes and Bacteriodetes. High MIC levels for metals and antibiotics were detected among all the genera. Results demonstrate potential impacts of physico-chemical properties on levels of antimicrobial-resistant bacteria. Metal-resistant bacteria were also resistant to multiple antibiotics, suggesting that metal pollution from mining may be responsible for co-selection and maintenance of antibiotic-resistant bacteria in this aquatic system.

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

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

Chemical speciation and release kinetics of Ni in a Ni-contaminated calcareous soil as affected by organic waste biochars and soil moisture regime

Biochars vary widely in properties and have been shown to have variable effects on potentially toxic element(s) stabilization in soil. This is the first study to examine the interaction effects of biochar and soil moisture regime on Ni stabilization in a Ni-contaminated calcareous soil. Three different organic waste (cow manure, municipal compost and licorice root pulp) biochars produced at two temperatures (300 and 600 °C) were applied (3% wt.) to a Ni-contaminated calcareous soil and incubated at field capacity and saturated conditions for 70 d. Sequential chemical fractionation and Ni release kinetics were then performed. All applied biochars, especially the high-temperature biochars, were significantly able to enhance Ni stabilization in the studied soil. In particular, the biochars significantly decreased Ni content in the water-soluble and exchangeable fractions (10-42% decrease), while increasing the immobile residual fraction (13-38% increase). The biochars also significantly decreased the rate and cumulative amount of EDTA-extractable Ni from the calcareous soil. Among the studied biochars, the cow manure and municipal compost biochars produced at 600 °C were the most effective at reducing Ni mobility factor (27-28% decrease) and initial release rate (42-49% decrease), likely due to their high ash content and pH, which promotes Ni sorption in soil. Soil moisture regime was not found to significantly affect the Ni mobility factor or rates of Ni release from the calcareous soil but did, however, affect certain soil Ni chemical fractions. Soil water saturation significantly decreased Ni in the Mn (4%) and non-crystalline Fe oxides (17%) fractions, while increased the crystalline Fe oxide fraction (3%), attributed to reductive dissolution of Mn and Fe oxide crystallinity enhancement. Saturation also significantly enhanced Ni in the residual fraction (4%), attributed to the associated pH increase and potential sulfide formation. The results of this study demonstrate that high temperature, ash-rich, and alkaline biochars are most effective at Ni immobilization, and that soil water saturation can further enhance Ni in the residual fraction.

Effects of antibiotics on anaerobic digestion of sewage sludge: Performance of anaerobic digestion and structure of the microbial community

As a common biological engineering technology, anaerobic digestion can stabilize sewage sludge and convert the carbon compounds into renewable energy (i.e., methane). However, anaerobic digestion of sewage sludge is severely affected by antibiotics. This review summarizes the effects of different antibiotics on anaerobic digestion of sewage sludge, including production of methane and volatile fatty acids (VFAs), and discusses the impact of antibiotics on biotransformation processes (solubilization, hydrolysis, acidification, acetogenesis and methanogenesis). Moreover, the effects of different antibiotics on microbial community structure (bacteria and archaea) were determined. Most of the research results showed that antibiotics at environmentally relevant concentrations can reduce biogas production mainly by inhibiting methanogenic processes, that is, methanogenic archaea activity, while a few antibiotics can improve biogas production. Moreover, the combination of multiple environmental concentrations of antibiotics inhibited the efficiency of methane production from sludge anaerobic digestion. In addition, some lab-scale pretreatment methods (e.g., ozone, ultrasonic combined ozone, zero-valent iron, Fe³⁺ and magnetite) can promote the performance of anaerobic digestion of sewage sludge inhibited by antibiotics.

Evaluation and analysis of the toxicity of mercury (Hg2+) to allophycocyanin from Spirulina platensis in vitro

As a global environmental pollution problem, heavy metal pollution has brought great harm to human beings. In this work, we studied the toxicity of Hg²⁺ on allophycocyanin (APC) at the molecular level. Firstly, APC was extracted and purified from Spirulina platensis mud and its purity (A₆₅₀/A₂₈₀) reached 3.75. In addition, the fluorescence intensity of APC decreased with increasing Hg²⁺ concentration from 0 to 5 × 10^-6 mol L^-1. The theoretical calculation and experimental results showed that the fluorescence quenching of APC by Hg²⁺ was static and had a good linear relationship. Moreover, the UV-Vis spectra of APC showed a significant decrease at 200 nm and 650 nm with the increase of Hg²⁺ concentration from 0 to 5×10^-6 mol L^-1, and a red-shift at 200 nm, which indicated that Hg²⁺ not only affected the structure of APC but also affected the light absorption and photosynthetic function of APC. Furthermore, the results of molecular simulation demonstrate that Hg²⁺ combinations with the Met77, Cys81 in the α chain and the Arg77, Cys81 in the β chain, which interact between the peptide chain and the chromophore, and Hg²⁺ forms a Hg-S bond with -SH. This study provides new insights into the structure and how Hg²⁺ effect the optical properties of APC.

Heavy Metal Pollution and Source Contributions in Agricultural Soils Developed from Karst Landform in the Southwestern Region of China

Heavy metal pollution of soil in agricultural areas is the most prominent environmental pollution problem in China, seriously affecting human health and food security. It has become one of the environmental problems to which all sectors of society attach great importance. Soil heavy metals in the weathering area of hazardous geological bodies in southwest China have naturally high background attributes. Therefore, ecological risk assessment and analysis of potential sources of soil heavy metals in southwest China is of great significance for soil health management, soil heavy metal pollution control and territorial spatial planning. In this study, we collected 787 soil samples (0-20 cm) in Xuanwei County in China and analyzed the concentrations of As, Cd, Cr, Cu, Hg, Ni, Pb and Zn in soils. Igeo, RI, HI and CR were used to calculate the pollution levels, ecological risks and human health risks. Additionally, the PMF model and one-way ANOVA were used to identify the potential sources and discuss the factors affecting the enrichment of heavy metals. The results showed that the mean contents of the surface soils were 1.190 (Cd), 139.4 (Cr), 96.74 (Cu), 0.081 (Hg), 56.97 (Ni), 46.66 (Pb) and 130.1 (Zn) mg/kg. All heavy metals exceeded the background values of the A layer soil in Yunnan Province. The I_geo showed that Cd was the most hazardous element in the study area, followed by Cu, Cr, As, Ni and Pb. The RI showed that low ecological risks, moderate ecological risks, considerable ecological risks and strong ecological risks accounted for 3.81%, 55.27%, 37.74% and 3.18%, respectively, of the total samples, and Cd was the main dominant element. The HI values of the As element in children were greater than 1, indicating a non-carcinogenic risk, and other elements' risks were acceptable. The CR values of Cr and Ni were higher than their limits (1 × 10^-4), and both had carcinogenic risks in children and adults, as did As in children. According to the PMF model, four heavy metals sources were identified: geological sources (32%), sources from mining activities (19.38%), atmospheric deposition sources (17.57%) and agricultural sources (31.05%). Thereinto, As and Pb were mainly derived from agricultural sources, Cd and Cr were mainly associated with geological sources, Cu was largely from mining activity sources, Hg was mainly from atmospheric deposition sources and Ni and Zn were mainly from geological sources, mining activities and agricultural activities. The parent material has a significant influence on the enrichment of heavy metals in the soil, and the heavy metals are significantly enriched in the carbonate parent material and quaternary parent material. Topography also plays a role in heavy metal accumulation; Cd, Cr, Cu, Ni and Zn gradually decreased with the increase in altitude, and As and Pb increased with the increase in altitude. Mn-oxide played a crucial part in the enrichment of Cu and Zn, while SOC, K₂O and pH had little influence on the accumulation of heavy metals.

Review on fate and bioavailability of heavy metals during anaerobic digestion and composting of animal manure

Anaerobic digestion and composting are attracting increasing attention due to the increased production of animal manure. It is essential to know about the fate and bioavailability of heavy metals (HMs) for further utilisation of animal manure. This review has systematically summarised the migration of HMs and the transformation of several typical HMs (Cu, Zn, Cd, As, and Pb) during anaerobic digestion and composting. The results showed that organic matter degradation increased the HMs content in biogas residue and compost (with the exception of As in compost). HMs migrated into biogas residue during anaerobic digestion through various mechanisms. Most of HMs in biogas residue and compost exceeded relevant standards. Then, anaerobic digestion increased the bioavailable fractions proportion in Zn and Cd, decreased the F4 proportion, and raised them more than moderate environmental risks. As (III) was the main species in the digester, which extremely increased As toxicity. The increase of F3 proportion in Cu and Pb was due to sulphide formation in biogas residue. Whereas, the high humus content in compost greatly increased the F3 proportion in Cu. The F1 proportion in Zn decreased, but the plant availability of Zn in compost did not reduce significantly. Cd and As mainly converted the bioavailable fractions into stable fractions during composting, but As (V) toxicity needs to be concerned. Moreover, additives are only suitable for animal manure treated with slightly HM contaminated. Therefore, it is necessary to combine more comprehensive methods to improve the manure treatment and make product utilisation safer.

Effects of Cap-and-Trade Mechanism and Financial Gray Rhino Threats on Insurer Performance

This paper develops a capped barrier option model to examine how a cap-and-trade mechanism affects an insurer’s guaranteed rate-setting behavior and policyholder protection in a financial gray rhino environment. Toward sustainability, the insurer explicitly captures the credit risk from the borrowing firms, participating in the cap-and-trade scheme to reduce carbon emissions, an essential issue of carbon emission and environmental protection when facing gray rhino threats. In addition, the energy economics and policy analysis are from the fund-providing insurer’s perspective. Green lending policies and life insurance policy loans (i.e., disintermediation related to insurance stability) are crucial to managers and regulators, particularly bridging the borrowing-firm carbon transactions for carbon emission reductions toward sustainability. We show that the shrinking regulatory cap of the cap-and-trade scheme harms policyholder protection, adversely affecting insurance stability. The harm becomes more serious when the gray rhino threat on borrowing firms becomes significant. An increase in policy loans decreases the insurer’s interest margin and policyholder protection. However, increasing the gray rhino threat decreases life insurance policies at a reduced guaranteed rate but increases policyholder protection, contributing to insurance stability. Therefore, the government can use the cap-and-trade scheme to control carbon emissions and improve the environment, but it harms policyholder protection. We suggest that, for example, the government should subsidize the insurer for green lending, affecting insurance stability.

Dissolved organic matter (DOM) was detected in MSWI plant: An investigation of DOM and potential toxic elements variation in the bottom ash and fly ash

The content of dissolved organic matter (DOM) and potentially toxic elements (PTEs) were investigated in the bottom ash (BA) and fly ash (FA) of different sections of the municipal solid waste incineration (MSWI) plant. BA and FA were collected from the dry (BA1-BA2), burn (BA3-BA4), and burn-out (BA5) sections of the grate incinerator; FA was collected after denitration (DNFA), and from the deacidification tower (FA1) and bag-type dust remover (FA2), respectively. The DOM concentration in BA was higher than that in FA, the highest concentration was in BA3 (556.18 mg/kg), while the lowest concentration was in DNFA (17.53 mg/kg). DOM in BA was mainly composed of protein-like, fulvic-like, tryptophan-like, and humic-like substances, of which humic-like substances accounted for more than 40%. DOM in FA consisted of tryptophan-like and humic-like substances, of which humic-like substances accounted for more than 80%. DOM still existed in BA which may be related to the incomplete combustion, and the influence of microbes, while DOM was increased in FA1, which might be due to the addition of lime slurry. PTEs were analyzed by the Tessier extraction method, Fe-Mn hydroxide-bound fraction of PTEs increased in FA1 in which DOM concentration (137.22 mg/kg) was 7.83 times that in DNFA. The increase of DOM may lead to a higher risk of PTEs in FA. FTIR results indicated that DOM can bond to PTEs in BA and FA. The contents of humus-like substances in DOM were positively correlated with the effective fraction of As, Cu, Pb, Cr, and Cd. This paper investigated the risk of DOM existing in BA and FA in MSWI plant, which can provide a new perspective on how to deal with BA and FA, and reduce their environmental risks.

Multidisciplinary Approach to Agricultural Biomass Ash Usage for Earthworks in Road Construction

Agricultural biomass has great bioenergy potential due to its availability, and it is a carbon-free energy source. During biomass incineration, biomass ash is formed, which is still considered as a waste without proper disposal and management solutions. Various biomass ash utilization options were investigated, mainly concerning engineering issues (the mechanical characterization of newly produced building materials or products), and there is a lack of knowledge of environmental issues arising from this “waste” material utilization in civil engineering practice. The main aim of this research is discussion of a different agricultural biomass characteristics as a fuel, the impact of agricultural biomass ashes (ABA) on the mechanical properties of stabilized soil with a particular emphasis on the environmental impacts within this kind of waste management. The results of this study indicate improved geotechnical characteristics of low-plasticity clay stabilized by lime/ABA binder. In addition to mechanical characterization for materials embedded in road embankments and subgrades, appropriate environmental risk assessment needs to be performed, and the results of this study indicate that the amount of ABAs added to the soil for roadworks should not have adverse effects on the soil fauna in the surrounding environment.

Effect of Sample Sources on Heavy Metal Concentration Measured in Beta Vulgaris Organs

Aim

Heavy metal concentration [mg/dL, MP] in soil and the transfer to vegetable organs may have a sampling effect. We compared the [MP] in soil and organ samples of Beta vulgaris collected in sites with socioeconomic differences potentially inducing phytotoxicity.

Materials and Methods

Samples of Beta vulgaris and soils (n = 4 per sample of soil and plant material) were randomly collected from two distant geographic areas (Mosquera and Sibaté, Cundinamarca, Colombia). We determined the [MP] using acid digestion of HCl : HNO₃ [1 : 1]; the [MP] was obtained by atomic absorption in Varian AA-140 and Shimadzu AA-7000 equipment. A two-way ANOVA estimated the effect (partial η2) of the sampling site and metal type on the [MP] and transfer to the vegetable.

Results

In Sibaté, the means (SD) of As_1.44 (0.18), Co_1.09 (0.51), Cr_6.21 (0.33), Ni_0.22 (0.02), and Pb_4.17 (0.87) were higher than in Mosquera (As_1.06 (0.21), Co_0.81 (0.19), Cr_3.72 (0.51), Ni_0.13 (0.04), and Pb_1.69 (0.40)) (p value <0.05). The effect of the interaction between the metal type and Beta vulgaris organs on the [MP] (0.801) in Sibaté was more meaningful than in Mosquera (0.430). Additionally, there was a strong correlation (Spearman's ρ > 0.8, p value <0.001) between [MP_soil] and [MP_plants] and between the transfer of metals to the plant and to the leaves. Discussion. The sampling location has a differential effect on the [MP] in soil and the transfer to Beta vulgaris. Given the differential effect described, the monitoring and phytoremediation strategies must be adjusted to scenarios with potentially phytotoxic conditions.

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

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

Speciation of heavy metals in soils and their immobilization at micro-scale interfaces among diverse soil components

Heavy metal (HM) pollution of soils is a globally important ecological and environmental problem. Previous studies have focused on i) tracking pollution sources in HM-contaminated soils, ii) exploring the adsorption capacity and distribution of HMs, and iii) assessing phyto-uptake of HMs and their ecotoxicity. However, few reviews have systematically summarized HM pollution in soil-plant systems over the past decade. Understanding the mechanisms of interaction between HMs and solid soil components is consequently key to effectively controlling and remediating HM pollution. However, the compositions of solid soil phases are diverse, their structures are complex, and their spatial arrangements are heterogeneous, all leading to the formation of soil micro-domains that exhibit different particle sizes and surface properties. The various soil components and their interactions ultimately control the speciation, transformation, and bioavailability of HMs in soils. Over the past few decades, the extensive application of advanced instrumental techniques and methods has greatly expanded our understanding of the behavior of HMs in organic mineral assemblages. In this review, studies investigating the immobilization of HMs by minerals, organic compounds, microorganisms, and their associated complexes are summarized, with a particular emphasis on the interfacial adsorption and immobilization of HMs. In addition, methods for analyzing the speciation and distribution of HMs in aggregates of natural soils with different particle sizes are also discussed. Moreover, we also review the methods for speciating HMs at mineral-organic micro-scale interfaces. Lastly, developmental prospects for HM research at inorganic-organic interfaces are outlined. In future research, the most advanced methods should be used to characterize the interfaces and in situ characteristics of metals and metal complexes. In particular, the roles and contributions of microorganisms in the immobilization of HMs at complex mineral-organic interfaces require significant further investigation.

Remediation of cadmium contaminated soil by composite spent mushroom substrate organic amendment under high nitrogen level

Cadmium (Cd) contamination in soil poses a serious threat to ecological environment and crop quality, especially under high nitrogen level. Here, the efficiency of composite organic amendment (spent mushroom substrate and its biochar) on remediation of Cd contaminated soil under high nitrogen level has been studied through a 42 days' soil incubation experiment. The results showed: (i) the application of composite organic amendment minimized the repercussions of high nitrogen and significantly reduced the exchangeable Cd proportion by 28.3%-29.5%, especially for Ca(NO₃)₂ treatment; (ii) the application of composite organic amendment improved the physicochemical properties of soil, such as pH, CEC and organic matter content increased by 0.63-0.99 unit, 39.69%-45.00% and 7.77%-11.47%, and EC decreased by 16.21%-44.47% compared with non-amendment Cd-contaminated soil, respectively; (iii) the application of composite organic amendment significantly increased the soil enzyme activities and microbial biomass, among which urease activity was increased most by 12.06-16.42 mg·g^-1·d^-1, and the copy number of AOA was decreased by 30.6%- 92.0%, and the copy number of AOB was increased most by about 45 times. In brief, the composite organic amendment can alleviate the adverse effects of Cd and nitrogen on the soil, but its long-term efficacy needs to be verified in further field study.

Biotechnological potential of rumen microbiota for sustainable bioconversion of lignocellulosic waste to biofuels and value-added products

Lignocellulosic biomass is an abundant resource with untapped potential for biofuel, enzymes, and chemical production. Its complex recalcitrant structure obstructs its bioconversion into biofuels and other value-added products. For improving its bioconversion efficiency, it is important to deconstruct its complex structure. In natural systems like rumen, diverse microbial communities carry out hydrolysis, acidogenesis, acetogenesis, and methanogenesis of lignocellulosic biomass through physical penetration, synergistic and enzymatic actions enhancing lignocellulose degradation activity. This review article aims to discuss comprehensively the rumen microbial ecosystem, their interactions, enzyme production, and applications for efficient bioconversion of lignocellulosic waste to biofuels. Furthermore, meta 'omics' approaches to elucidate the structure and functions of rumen microorganisms, fermentation mechanisms, microbe-microbe interactions, and host-microbe interactions have been discussed thoroughly. Additionally, feed additives' role in improving ruminal fermentation efficiency and reducing environmental nitrogen losses has been discussed. Finally, the current status of rumen microbiota applications and future perspectives for the development of rumen mimic bioreactors for efficient bioconversion of lignocellulosic wastes to biofuels and chemicals have been highlighted.

Phytoextraction of heavy metals after application of bottom ash and municipal sewage sludge considering the risk of environmental pollution

Waste management to reduce the loss of natural resources has become a basis of sustainable development and a circular economy. When using waste, the heavy metal (HM) concentration must be taken into account since HMs can be potentially released to the environment, posing a toxicity threat. The aim of the study was thus to estimate the availability for plants of Cr, Ni, Cu, Zn, Cd, and Pb introduced into the soil with waste. We hypothesized that the prepared waste mixtures containing coal or biomass ash and municipal sewage sludge would reduce the environmental risk compared to the studied waste used separately. The research was conducted during a 6-year field experiment with grasses and legumes. HM concentration in soil, waste, and plant biomass; tolerance index; and uptake of HMs by plants were measured. The ash-sludge mixtures had a more favourable effect on the soil in terms of pH_KCl, TOC, total nitrogen, and total exchangeable bases than the waste used separately. This provided beneficial conditions for plant growth and development. Consequently, the ash-sludge mixtures increased the plant yield as compared to ash alone, while the mixture containing the biomass ash also enhanced the yield in relation to the sewage sludge. The study showed that the mixtures allowed for a reduction of environmental risk arising from the HM input with waste to the soil. It was proven that HM availability for plants could be beneficially modified by mixing waste. Combining the coal ash with the sewage sludge is particularly recommended, owing to the unfavourable properties of coal ash for plants. The application of the higher dose of the coal ash-sludge mixture showed a better effect than the lower dose, while the influence of both doses of the biomass ash-sludge mixture was similar. Under the ash-sludge treatment, plants took up more HM than under the ash used separately, and the HM concentration in the obtained biomass did not generally exceed that observed under single wastes. This should reduce the accumulation of HMs in the soil during a long-term use of the waste and facilitates the utilisation of the produced biomass.

Slaked lime improves growth, antioxidant capacity and reduces Cd accumulation of peanut (Arachis hypogaea L.) under Cd stress

Slaked lime has been used to remediate contaminated agricultural soils as an in situ chemical immobilization amendment for a long time. However, the effects of slaked lime on peanut and soil cadmium (Cd) levels remain poorly understood with respect to remediating Cd-contaminated soil. In this study, six rates of slaked lime (e.g., 0, 300, 600, 900, 1200 and 1500 kg ha-1) were applied to evaluate the effects of slaked lime treatments on soil pH and the growth, Cd accumulation and physiology characteristics of peanut, which were in Cd-contaminated soil, and 0 kg ha-1 was taken as the control. The results indicated that slaked lime application significantly increased soil pH and reduced total Cd contents in peanut tissues at all growth stages. As the rates of slaked lime were increased, kernel biomass increased in the maturity stage, which increased peanut yields. The irregular variations in catalase, peroxidase, and superoxide dismutase activities and chlorophyll and malondialdehyde contents that were observed at all growth stages may be due to the interactions among soil pH, Ca nutrients and Cd, etc. In summary, slaked lime is suitable as an in situ chemical immobilization amendment to increase Cd immobilization and peanut yields in Cd-contaminated soil.

Bio-Based Waste’ Substrates for Degraded Soil Improvement—Advantages and Challenges in European Context

The area of degraded sites in the world is constantly expanding and has been a serious environmental problem for years. Such terrains are not only polluted, but also due to erosion, devoid of plant cover and organic matter. The degradation trends can be reversed by supporting remediation/reclamation processes. One of the possibilities is the introduction of biodegradable waste/biowaste substrates into the soil. The additives can be the waste itself or preformed substrates, such composts, mineral-organic fertilizers or biochar. In EU countries average value of compost used for land restoration and landfill cover was equal 4.9%. The transformation of waste in valuable products require the fulfillment of a number of conditions (waste quality, process conditions, law, local circumstances). Application on degraded land surface bio-based waste substrates has several advantages: increase soil organic matter (SOM) and nutrient content, biodiversity and activity of microbial soil communities and change of several others physical and chemical factors including degradation/immobilization of contaminants. The additives improve the water ratio and availability to plants and restore aboveground ecosystem. Due to organic additives degraded terrains are able to sequestrate carbon and climate mitigate. However, we identified some challenges. The application of waste to soil must comply with the legal requirements and meet the end of use criteria. Moreover, shorter or long-term use of bio-waste based substrate lead to even greater soil chemical or microbial contamination. Among pollutants, “emerging contaminants” appear more frequently, such microplastics, nanoparticles or active compounds of pharmaceuticals. That is why a holistic approach is necessary for use the bio-waste based substrate for rehabilitation of soil degraded ecosystems.

Performance of biochar-supported nanoscale zero-valent iron for cadmium and arsenic co-contaminated soil remediation: Insights on availability, bioaccumulation and health risk

Simultaneous stabilization of cadmium (Cd) and arsenic (As) in co-contaminated soil is challenging in environmental remediation because of their opposite properties. In this study, biochar-supported nanoscale zero-valent iron (nZVI-BC) was designed for simultaneously decreasing the soil availability of Cd and As and their bioaccumulation in vegetables. It was found that nZVI-BC exhibited remarkable performance for the stabilization of Cd and As in soil, and their availability decreased by 34.93% and 32.64% compared to the control sample, respectively, under 1.00% nZVI-BC treatment. The increase of soil pH and complexation dominated the Cd remediation process, while the formation of precipitation together and surface complexes transformed labile As into stable forms. Pot experiments showed that nZVI-BC application inhibited the bioaccumulation of Cd and As in vegetables by 23.63-36.48% and 43.09-45.10%, respectively, and hence effectively decreased the cancer risks by 38.19-42.93% related with vegetable consumption (P < 0.05). This study revealed that nZVI-BC is a promising amendment for achieving the simultaneous remediation of Cd and As co-contaminated farmland soil.

Performance and retention mechanisms of corn silk to atmospheric heavy metal lead

Atmospheric heavy metals are seriously harmful, and porous materials have unique advantages in the control of air pollutants. However, the direct use of plant porous materials to purify atmospheric heavy metals are rare. So agricultural waste corn silk with porous structures was selected to analyze the retention capability and mechanism to the atmospheric lead. The results show that the corn silk can effectively retain atmospheric lead in natural growing state or fixed experimental conditions. The analysis of a total of 765 corn silk samples from 17 different regions in Shandong province, China, shows that atmospheric lead is the main source of lead in the corn silk, and corn silk can be used for biological monitoring of atmospheric lead to some extent. Based on the analysis with different techniques including scanning electron microscope (SEM), energy spectrum dispersive X-ray spectrometer (EDS), Fourier infrared spectrometer (FTIR) and Zeta potential, the effective retention of lead by corn silk is due to a variety of mechanisms, including physical adsorption, electrostatic adsorption, complexation, chelation and ion exchange. So agricultural waste corn silk has great potential in the application of biosorption or preparation of porous materials in purification of atmospheric heavy metals.

Fluorescent dissolved organic matter facilitates the phytoavailability of copper in the coastal wetlands influenced by artificial topography

Dissolved organic matter (DOM) is a crucial driver in ecosystem services and a central part of the carbon transport and biological cycle in land-sea interaction. DOM exhibits characteristic environmental behavior in the coastal zone, but its sustainability is affected by expanding artificial topography (AT) construction. It requires combining analyses on AT-induced response of field fluorescent DOM (fDOM) and its quenching pattern under metal-complexation. Herein, we conducted systemic investigations into the spatiotemporal dynamics of fDOM compositions with further in-lab verification to study its Cu-binding capacity. We detected three humid-like fDOM components sensitive to AT. The total fDOM intensity was positively correlated with low molecular weight organic acid (LMWOA) extractable Cu and the Cu pools in above-ground biomass. The enriched fDOM serves as an ecological engineer by increasing the Cu mobility, confirmed by an in-lab fluorescence titration. The application of LMWOA greatly enhanced the intensity of one fDOM component, elevated its conditional stability constant, and decreased its quenched proportion, implying that LMWOA might extract part of Cu from fDOM complexation. The present work provides an "fDOM-LMWOA pump" explanation to suggest that fDOM is a novel ecological regulator on vegetation growth under the AT-induced matter accumulation.

Lead immobilization in soil using new hydroxyapatite-like compounds derived from oyster shell and its uptake by plant

Protecting the natural environment and ecological systems from the inorganic pollutants such as lead (Pb) has highlighted the urgent need to develop new and effective approaches for this substance's immobilization in soil. In this study, new, low-cost, and eco-friendly hydroxyapatite (HAp)-like compounds were prepared by reacting oyster shell (Oys) with diammonium phosphate ((NH₄)₂HPO₄) (DAP) and calcium hydroxide (Ca(OH)₂) at 25-28 °C (OyOHr) and 100 °C (OyOHh). Furthermore, OyOHr and OyOHh were assessed for their effectiveness to immobilize Pb in soil and suppress Pb uptake by Indian spinach (Basella Alba L.). Application of 0.5% OyOHr and OyOHh to soil (by weight) reduced Pb concentration in the shoots by 76.9-78.0% compared to control (CK), to a level that was slightly higher (by 15.5-21.5%) than the recommended food safety level (2 mg kg^-1) suggested by WHO. The changes in Pb fractions revealed that the total contents of oxidizable and residual forms in OyOHr or OyOHh after harvest was >415.0 mg kg^-1, which indicated that >92% of Pb when added to the soil, was immobilized and not able to be taken up by plants. The proposed Pb immobilization mechanism might be the dissolution of OyOHr or OyOHh followed by hydroxypyromorphite (Pb₁₀(PO₄)₆(OH)₂) (HP) formation. Due to their facile preparation and eco-friendly and excellent Pb immobilizing characteristics, OyOHr or OyOHh could be readily integrated into current farming systems to mitigate the risk of Pb transferring to plants. However, OyOHr seemed a better immobilizing agent correspond to OyOHh in terms of cost and efficiency.

Speciation and environmental risk of heavy metals in biochars produced by pyrolysis of chicken manure and water-washed swine manure

This study was conducted to investigate the speciation, bioavailability and environmental risk of heavy metals (HMs) in chicken manure (CM) and water-washed swine manure (WSM) and their biochars produced at different pyrolysis temperatures (200 to 800 °C). As the pyrolysis temperature increased, the remaining proportion, toxicity characteristic leaching procedure (TCLP), HCl and diethylenetriamine pentaacetic acid (DTPA) of HMs gradually declined. This result proved that the speciation of HMs in chicken manure biochars (CMB) and water-washed swine manure biochars (WSMB) was influenced by pyrolysis temperature. The proportions of stable fractions were enhanced with increased pyrolysis temperature and weakened the HM validity for vegetation at 800 °C. Finally, the results of the risk assessment showed that the environmental risk of HMs in CMB and WSMB decreased with increasing pyrolysis temperature. Therefore, pyrolysis at 800 °C can provide a practical approach to lessen the initial and underlying heavy metal toxicity of CMB and WSMB to the environment.

Steel mill waste effects on rice growth: comparison of chemical extractants on lead and zinc availability

Zinc deficiency is widespread in cultivated soils, limiting the grain crop production and the adequate human nutrition. Several wastes from metallurgical activity can be used as Zn source, but these materials generally also have other potentially toxic elements, such as Pb, that can be highly toxic for plants and humans. This study aimed to evaluate the efficiency of five chemical extractors (water, citric acid, DTPA, Mehlich 1, and USEPA 3051A) in better correlating with the bioavailable contents of Zn and Pb in soils treated with steel mill wastes (metallurgic press residue (MPR), filter press mud (FPM), and phosphate mud (PM)). Rice plants were cultivated in pots with 4 kg of a Haplic Eutrophic Gleisol and steel mill wastes were applied in soil at increasing doses (0, 0.5, 1, 2, 4, 8, and 16 t ha^-1). The availability of the potentially toxic elements Zn and Pb was assessed as total contents in rice shoots, grains, husks, and roots. The results showed that the USEPA 3051A method extracted greater contents of Zn and Pb from soil compared with other extractants. Due to their greater natural Pb and Zn contents, MPR and PM promoted higher contents of these elements in soils, respectively. Doses of PM influenced Zn contents in grains. After adding 16 t ha^-1 of PM, Zn content in rice grains was 0.1 mg kg^-1. However, at doses 0.5, 1, 2, and 4 t ha^-1, the average concentration of Zn in the grains was 40 mg kg^-1. The wastes MPR and FPM at 16 t ha^-1 promoted Zn concentration in grains of 42 and 45 mg kg^-1, respectively. The greatest contents of Pb in grains were found after addition of FPM at doses 0.5, 1, and 2 t ha^-1: 6.67, 4.96, and 0.45 mg kg^-1, respectively, and above 4 t ha^-1 (4, 8, and 16 t ha^-1); Pb content in grains was less than 0.3 mg kg^-1. The content of Pb in roots at 16 t ha^-1 of PM, MPR, and FPM was 18, 25, and 155 mg kg^-1, respectively, and for Zn, under the same conditions, 100, 255, and 813 mg kg^-1 for MPR, FPM, and PM, respectively. USEPA 3051A can be used to assess Pb and Zn available contents, and positive correlations with bioavailable contents of these elements in roots prove its feasibility. Further studies are necessary to state the safety of using steel mill application, including the use of other crop species, but PM is a promising waste for soil Zn fertilization.

Can the Application of Municipal Sewage Sludge Compost in the Aided Phytostabilization Technique Provide an Effective Waste Management Method?

(1) Background: sewage sludge is a by-product of wastewater treatment, which needs to be managed appropriately, e.g., in composting processes. The application of municipal sewage sludge composts (MSSCs) as a soil amendment is a potential way to effectively manage sewage sludge. (2) Methods: this paper presents the results of a vegetation pot experiment undertaken to assess the suitability of Dactylis glomerata L. and MSSC in the aided phytostabilization technique when applied on soils from an area effected by industrial pressure; this is characterized by high levels of heavy metal (HM). The contents of HMs in the test plant (the roots and above-ground parts), as well as in the soil and MSSC, were determined via an atomic spectrometry method. (3) Results: the application of MSSC positively contributed to an increased production of plant biomass and an increase in the pH in the soil. Concentrations of Cu, Cd, Pb, Zn, and Cr were higher in the roots than in the above-ground parts of Dactylis glomerata L. The addition of MSSC contributed most significantly to the considerable reduction in Ni, Pb, and Zn contents in the soil after the experiment. (4) Conclusions: MSSC can support the phytostabilization of soils contaminated with high levels of HMs.