Evaluation of hydroxyapatite derived from flue gas desulphurization gypsum on simultaneous immobilization of lead and cadmium in contaminated soil

Magnetized bentonite modified rice straw biochar: Qualitative and quantitative analysis of Cd(II) adsorption mechanism

Industrialization has caused a significant global issue with cadmium (Cd) pollution. In this study, Biochar (Bc), generated through initial pyrolysis of rice straw, underwent thorough mixing with magnetized bentonite clay, followed by activation with KOH and subsequent pyrolysis. Consequently, a magnetized bentonite modified rice straw biochar (Fe3O4@B-Bc) was successfully synthesized for effective treatment and remediation of this problem. Fe3O4@B-Bc not only overcomes the challenges associated with the difficult separation of individual bentonite or biochar from water, but also exhibited a maximum adsorption capacity of Cd(II) up to 241.52 mg g-1. The characterization of Fe3O4@B-Bc revealed that its surface was rich in C, O and Fe functional groups, which enable efficient adsorption. The quantitative calculation of the contribution to the adsorption mechanism indicates that cation exchange and physical adsorption accounted for 65.87% of the total adsorption capacity. In conclusion, Fe3O4@B-Bc can be considered a low-cost and recyclable green adsorbent, with broad potential for treating cadmium-polluted water.

Field application of hydroxyapatite and humic acid for remediation of metal-contaminated alkaline soil

The quality of soil is essential for ensuring the safety and quality of agricultural products. However, soils contaminated with toxic metals pose a significant threat to agricultural production and human health. Therefore, remediation of contaminated soils is an urgent task, and humic acid (HA) with hydroxyapatite (HAP) materials was applied for this study in contaminated alkaline soils to remediate Cd, Pb, Cu, and Zn. Physiochemical properties, improved BCR sequential extraction, microbial community composition in soils with superoxide dismutase (SOD), peroxidase (POD), and chlorophyll content in plants were determined. Among the studied treatments, application of HAP-HA (2:1) (T7) had the most significant impact on reducing the active forms of toxic metals from soil such as Cd, Pb, Cu, and Zn decreased by 18.59%, 9.12%, 11.83%, and 3.33%, respectively, but HAP and HA had a minor impact on metal accumulation in Juncao. HAP (T2) had a beneficial impact on reducing the TCleaf/root of Cd, Cu, and Zn, whereas HAP-HA (T5) showed the best performance for reducing Cd and Cu in EFleaf/soil. HAP-HA (T5 and T7) showed higher biomass (57.3%) and chlorophyll (17.9%), whereas HAP (T4) showed better performance in POD (25.8%) than T0 in Juncao. The bacterial diversity in soil was increased after applying amendments of various treatments and enhancing metal remediation. The combined application of HAP and HA effectively reduced active toxic metals in alkaline soil. HAP-HA mixtures notably improved soil health, plant growth, and microbial diversity, advocating for their use in remediating contaminated soils.

Keystone taxa of phoD-harboring bacteria mediate alkaline phosphatase activity during biochar remediation of Cd-contaminated soil

Phosphorus (P)-modified biochar can efficiently remediate cadmium (Cd)-contaminated soil. However, the mechanisms of responses of alkaline phosphatase (ALP) and phoD-harboring microorganisms, which are notably sensitive to Cd and P, are not clear during the remediation process. In this study, apple (Malus domestica) tree branches were co-pyrolyzed with tripotassium phosphate (K3PO4) to prepare P-modified biochar, which was used to remediate Cd-soil contaminated soil collected near a mine site. The effect of P-modified biochar on the composition of the phoD-harboring microbial community and its mechanism of interacting with ALP were analyzed. The results showed that the application of P-modified biochar to Cd-contaminated soil promoted the co-precipitation of Cd and phosphate and reduced the content of bioavailable Cd by 69.77 %. P-modified biochar improved the complexity and stability of the soil phoD-harboring microbial community. Furthermore, this study clarified that ALP activity was not completely regulated by the abundance of phoD, but Priestia and Massilia that contain phoD genes dominated the activity of ALP in rhizosphere and bulk soils, respectively. It is notable that bioavailable Cd significantly stimulated Priestia, Massilia, and ALP activity. These findings provide a theoretical basis for the application of P-modified biochar to the remediation of soil contaminated with Cd with respect to P functional microorganisms.

Hydroxyapatite: A journey from biomaterials to advanced functional materials

Hydroxyapatite (HAp), a well-known biomaterial, has witnessed a remarkable evolution over the years, transforming from a simple biocompatible substance to an advanced functional material with a wide range of applications. This abstract provides an overview of the significant advancements in the field of HAp and its journey towards becoming a multifunctional material. Initially recognized for its exceptional biocompatibility and bioactivity, HAp gained prominence in the field of bone tissue engineering and dental applications. Its ability to integrate with surrounding tissues, promote cellular adhesion, and facilitate osseointegration made it an ideal candidate for various biomedical implants and coatings. As the understanding of HAp grew, researchers explored its potential beyond traditional biomaterial applications. With advances in material synthesis and engineering, HAp began to exhibit unique properties that extended its utility to other disciplines. Researchers successfully tailored the composition, morphology, and surface characteristics of HAp, leading to enhanced mechanical strength, controlled drug release capabilities, and improved biodegradability. These modifications enabled the utilization of HAp in drug delivery systems, biosensors, tissue engineering scaffolds, and regenerative medicine applications. Moreover, the exceptional biomineralization properties of HAp allowed for the incorporation of functional ions and molecules during synthesis, leading to the development of bioactive coatings and composites with specific therapeutic functionalities. These functionalized HAp materials have demonstrated promising results in antimicrobial coatings, controlled release systems for growth factors and therapeutic agents, and even as catalysts in chemical reactions. In recent years, HAp nanoparticles and nanostructured materials have emerged as a focal point of research due to their unique physicochemical properties and potential for targeted drug delivery, imaging, and theranostic applications. The ability to manipulate the size, shape, and surface chemistry of HAp at the nanoscale has paved the way for innovative approaches in personalized medicine and regenerative therapies. This abstract highlights the exceptional evolution of HAp, from a traditional biomaterial to an advanced functional material. The exploration of novel synthesis methods, surface modifications, and nanoengineering techniques has expanded the horizon of HAp applications, enabling its integration into diverse fields ranging from biomedicine to catalysis. Additionally, this manuscript discusses the emerging prospects of HAp-based materials in photocatalysis, sensing, and energy storage, showcasing its potential as an advanced functional material beyond the realm of biomedical applications. As research in this field progresses, the future holds tremendous potential for HAp-based materials to revolutionize medical treatments and contribute to the advancement of science and technology.

Bioremediation system consisted with Leclercia adecarboxylata and nZVI@Carbon/Phosphate for lead immobilization: The passivation mechanisms of chemical reaction and biological metabolism in soil

Bioremediation is one of the most promising strategies for heavy metal immobilization. A new remediation system was demonstrated in this research, which combined phosphate solubilizing bacteria (PSB) with nZVI@Carbon/Phosphate (nZVI@C/P) composite to remediate lead contaminated soil. Experimental results indicated that the new system (nZVI@C/P + PSB) could effectively convert the labile Pb into the stable fraction after 30 days of incubation, which increased the maximum residual fraction percentage of Pb by 70.58%. The characterization results showed that lead may exist in the forms of Pb₅(PO₄)₃Cl, PbSO₄ and 3PbCO₃·2Pb(OH)₂·H₂O in the soil treated with nZVI@C/P + PSB. Meanwhile, soil enzyme activities and Leclercia abundance were enhanced in the treated soil compared with CK during the incubation time. In addition, the specialized functions (e.g. ABC transporters, siderophore metabolism, sulfur metabolism and phosphorus metabolism) in PSB and nZVI@C/P + PSB group were also enhanced. These phenomena proved that the key soil metabolic functions may be maintained and enhanced through the synergistic effect of incubated PSB and nZVI@C/P. The study demonstrated that this new bioremediation system provided feasible way to improve the efficacy for lead contaminated soil remediation.

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.

Application of humic acid and hydroxyapatite in Cd-contaminated alkaline maize cropland: A field trial

Soil quality is critical to the quality and safety of agricultural products, and remediation of heavy metal contaminated soils is an urgent task to be implemented. This study applied hydroxyapatite (HAP) and humic acid (HA) as remediation materials to Cd-contaminated alkaline cropland. Data on soil pH, electrical conductivity (EC), cation exchange capacity (CEC), soil organic matter (SOM), diethylenetriamine pentaacetic acid (DTPA) extraction, and improved BCR sequential extraction were obtained for different periods. The joint application of HAP and HA enhanced the soil's buffering capacity. During the experiment, treatment groups CK, H1, H2, H3, and H4 showed changes in pH of 0.29, 0.28, 0.21, 0.24, and 0.32, respectively, and changes in the conductivity of 341.4, 183.0, 133.1, 104.6 and 320.2 μS/cm. Soil organic matter had a positive effect on soil's effective phosphorus content. HAP and HA both reduced the BCF_grain/soil of Cd for the maize, but the impact of HA was more substantial (20.19 % reduction compared to CK). HA increased the yield of maize by 44.20 %. The combination of HA and HAP was recommended.

Negatively charged nano-hydroxyapatite can be used as a phosphorus fertilizer to increase the efficacy of wollastonite for soil cadmium immobilization

Improper application of phosphorus (P) fertilizer during soil cadmium (Cd) immobilization reduces the efficiency of fertilizer and Cd remediation. In this study, we synthesized three types of nano-hydroxyapatite (NHAP) with different surface charges as slow-release P fertilizers during Cd immobilization. We also evaluated the effects of wollastonite application with or without NHAP addition, in comparison with triple superphosphate (TSP) or bulk hydroxyapatite, on Cd accumulation in Amaranthus tricolor L. The results showed that adding wollastonite significantly reduced P availability (23.5%) in the soil, but it did not inhibit plant P uptake. In wollastonite-amended soil, the application of negatively/positively charged NHAP significantly increased plant biomass by 643-865% and decreased Cd uptake by 74.8-75.1% compared to the unamended soil as well as showed greater efficiency than those with TSP. This was ascribed to the increased soil pH (from 3.94 to 6.52-6.63) and increased abundance of organic acids (including citric acid, malic acid, lactic acid, and acetic acid) secreted by plants. In addition, the P-preferring bacterial class Bacteroidia was specific to soils amended with both wollastonite and NHAP-. These results suggest that NHAP- may be an appropriate P fertilizer for soil Cd immobilization using wollastonite.

Supplying amendments alleviates aluminum toxicity and regulates cadmium accumulation by spinach in strongly acidic soils

Al toxicity and Cd pollution are key limiting factors for agricultural production in the acidic soils in China. The application of amendments is an effective and promising measure for remediating strongly acidic Cd-contaminated soils. However, the information on applying amendments for alleviating Al toxicity and regulating plant Cd accumulation is still rare. Here, oyster shell (OS), red mud (RM), hydroxyapatite (HAP), and biochar (BC) at 30 g kg^-1 were investigated for alleviating Al toxicity and decreasing Cd accumulation in spinach plants. The results showed that four amendments significantly increased soil pH, and reduced soil exchangeable Al³⁺ and DTPA-Cd, promoted spinach growth (P < 0.05). Al(OH)₃⁰ and Al-HA were the main forms of active Al in soil. The BC and OS were more effective to alleviate Al toxicity but significantly (P < 0.05) increased Cd accumulation in spinach. RM and HAP effectively reduced the uptake of Cd by spinach plants as well as alleviated Al toxicity (P < 0.05). Bivariate correlation analysis and the partial least squares path modeling analysis indicated that soil exchangeable Al³⁺ was the main limiting factor for biomass production. Our study demonstrated that HAP could significantly alleviate Al toxicity, promote spinach growth, and decrease Cd accumulation in strongly acidic Cd-contaminated soils. Besides, OS and BC effectively alleviated soil Al toxicity leading to promoting the growth of spinach. Compared with CK, RM treatment significantly reduced soil Cd bioavailability (61.2%) and decreased Cd concentration and uptake of spinach plants by 90.0% and 50.7%. These results indicated that RM could be used as an efficient amendment in Cd contaminated.

High-efficiency stabilization of lead in contaminated soil by thermal-organic acid-activated phosphate rock

Phosphate rock powder (PR) has been shown to possess the potential to stabilize lead (Pb) in soil. Most of the phosphorus (P) minerals in the world are low-grade ores, making it difficult to achieve the expected stabilization effect on heavy metals. This study compared the changes in the phase composition and structure of PR and three kinds of activated phosphate rock powder (APR) (organic acid-activated PR, thermal-activated PR, and thermal-organic acid-activated PR). The stabilization effectiveness of APR on Pb-contaminated soil was evaluated by toxicity leaching procedure; the Pb products adsorbed on APR and stabilization mechanism of APR on Pb were analyzed. The results demonstrated that APR showed decreased crystallinity and 3.4-fold increase in specific surface area, and a 53.07% and 49.32% increase in soluble P content in oxalic acid-activated PR and citric acid-activated PR, respectively, when compared with those of PR. These changes improved the stabilization effect of APR on Pb-contaminated soil, in which oxalic acid-600 °C-activated PR showed the best effect, presenting 94.0-99.8% reduction in Pb leaching concentration following addition of 2-10% modifier. Product characterization after Pb adsorption on APR showed that Pb was adsorbed onto APR by forming fluoropyromophite precipitation with APR.

MgO-loaded nitrogen and phosphorus self-doped biochar: High-efficient adsorption of aquatic Cu2+, Cd2+, and Pb2+ and its remediation efficiency on heavy metal contaminated soil

In this study, the MgO-loaded fish scale biochar (MgO-FB) was synthesized by impregnation of MgCl₂ using fish scales as precursor, and the modified biochar was applied for the adsorption of aquatic Cu²⁺, Cd²⁺, and Pb²⁺, and the immobilization of heavy metals in soils. The maximum adsorption capacities of MgO-FB for Cu²⁺, Cd²⁺, and Pb²⁺ were 505.8, 327.2, and 661.2 mg/g, respectively. In addition, MgO-FB can keep the excellent adsorption capacity under various environmental disturbances including pH, humic acid, and high ionic strength. Multiple characterizations and comparative experiments have demonstrated that the hydroxyapatite components and the MgO micro-nanoparticles on MgO-FB enhanced the adsorption capacity for Cu²⁺, Cd²⁺, and Pb²⁺ through ion-exchange and precipitation process. The metal-π electron coordination and complexation with oxygen-, nitrogen-, and phosphorus-containing groups were also responsible for the removal of heavy metal ions. Besides, MgO-FB also performed excellently for the immobilization of Cu, Cd, and Pb in soils, and the contents of available Cu, Cd, and Pb were reduced by 84.2%, 74.2%, and 53.7% respectively with the addition of MgO-FB. In general, these results show that waste fish scales co-pyrolysis with MgCl₂ impregnation can be considered as a promising and efficient material for the remediation of heavy metal contaminated waters and soils.

Sequential Application of Column Leaching and Plant Uptake Tests to Assess the Effect of Various Commercial Amendments on Cu Immobilization in Ultra-High Cu-Contaminated Soil

The presence of copper (Cu)-contaminated soil has increased recently due to agricultural and industrial activities. Immobilization techniques using soil amendments have attracted significant research because of their cost-effectiveness, eco-friendliness, and community acceptance. This study used various commercial amendments, including magnetite (M), talc (T), activated carbon (AC), and cornstarch (CS), to immobilize Cu in soil contaminated by acidic waste materials with Cu in Korea (9546 ± 5 mg/kg). To evaluate the immobilizing effect of these amendments, this study applied a sequential process of column leaching and plant uptake tests to observe the ability of Cu to remain in soil with and without amendments through the Cu removal rate. The amendments were characterized by SEM, XRD, and specific surface area and applied to the soil at a rate of 2% (w/w). The first stage of evaluation, i.e., the column leaching test, was conducted by continuously pumping distilled water (DW) for 28 days, and the second stage of evaluation, i.e., the plant uptake test, was started immediately after by planting 10-day-old lettuce seedlings for 28 days. The experimental results showed that all of the amendments had a significant effect on Cu immobilization Cu in soil (p < 0.05), and the T treatment showed the highest efficiency in Cu immobilization, with only 47.0% Cu loss compared to 73.5% in the control soil when assessed by sequential column leaching and plant uptake tests. In conclusion, this study provides an effective assessment method to evaluate the effect of amendments on Cu immobilization in soil, as well as providing feasible options to immobilize Cu using commercial amendments.

Combined application of ferrihydrite and hydroxyapatite to immobilize soil copper, cadmium, and phosphate under flooding-drainage alternations

Hydroxyapatite (HAP) can effectively immobilize soil heavy metals, but excess phosphate would be released to aquatic ecosystem, resulting in eutrophication. This study investigated the effects of ferrihydrite (FH) on the HAP immobilization of copper (Cu) and cadmium (Cd) and their reduction of phosphorus release under flooding-drainage alternation conditions. Results showed that the incorporation of HAP and FH significantly increased soil solution pH and decreased Cu²⁺ and Cd²⁺ concentrations. Applications of FH, HAP, and FH-HAP (FH and HAP combination) can all enhance soil pH and reduce CaCl₂-extractable and exchangeable Cu and Cd, but HAP addition increased soluble phosphate by 6.60-7.77 times compared to control. However, FH-HAP application can significantly reduce phosphate release by 92.7-99.7% compared to HAP application. FH-HAP was the most effective to reduce exchangeable Cu and Cd by 49.8-93.4% and 50.9-88.8% and decreased labile and moderately labile phosphorus by 34.0-74.4% and 13.5-18.6%, respectively, while increased stable phosphorus by 22-45.1% than single HAP. All FH treatments significantly increased amorphous iron oxides by the factors of 4.66-20.8, but only 3% and 5% of FH applications slightly enhanced crystal iron oxides by the factors of 0.81-1.27. The major implication is that the combination of FH and HAP can not only immobilize of Cu and Cd, but also reduce the risk of phosphate release by HAP addition.

Effects of nanometer alumina and humic acid on the retention and transport of hexavalent chromium in porous media

Column experiments were conducted to investigate the effects of ion type, ion strength, humic acid (HA), and nanometer alumina (NA) particles on the transport of hexavalent chromium (HC) in saturated porous media. A one-dimensional model is developed to simulate the migration of HC affected by NA particles. The results show that nano-alumina particles would enhance the mobility of HC in saturated porous media. However, the influence of NA on the migration of HC in porous media is complex. When the concentration of NA reaches 30 mg/L, HC has minimum retention parameter and best mobility. The transport of HC also is affected by ion strength and ion type. Higher ionic strength would decrease the retention of HC and enhance its mobility. Compared with sodium ion, calcium ion has larger effects on the transport of HC. Moreover, HA can improve the mobility of HC in saturated porous media, but the corresponding promoting effect decreases with the increase of HA concentration. As nanometer contaminants and HC come into the subsurface environment, findings from this study elucidate the key factors and processes controlling the transport of HC in porous media, which can promote the prediction and assessment of HC in the groundwater system.

Effect of a low-cost and highly efficient passivator synthesized by alkali-fused fly ash and swine manure on the leachability of heavy metals in a multi-metal contaminated soil

Soil pollution, caused by heavy metals, is an environmental problem that requires an urgent solution in China. Chemical passivation is a technology that uses various passivators to reduce the availability of heavy metals in soil and realize the remediation of contaminated soil. In this study, we examined the effects of fly ash (FA), alkali-fused fly ash (AFFA), swine manure biochar (SB), and modifying biochar (MB) on the leachability of Cu, Zn, Pb, and Cd via soil culture experiments. The results showed that the addition of AFFA, SB, and MB significantly reduced the extractable contents of Cu, Pb, and Cd in the soil. AFFA and MB had the best passivation effect, followed by SB and FA. The passivation effect on Pb was the best, followed by that on Cu and Cd. AFFA modification significantly improved the passivation effect of MB on Cu, Pb, and Cd in composite contaminated soil. With the addition of 3% MB, the Pb, Cu, and Cd extracted by TCLP decreased by 95.7, 74.1, and 59.1%, respectively. Correlation analysis, Fourier transform infrared spectrometry, scanning electron microscopy, and energy-dispersive X-ray spectroscopy showed that the passivation mechanism is mainly due to an increase in the soil pH, silicate content, and cohesiveness. The soil culture experiments in this study proved that MB is a low-cost and highly efficient organic-inorganic composite passivator for multi-metal contaminated soils.

Zinc and cadmium adsorption from wastewater using hydroxyapatite synthesized from flue gas desulfurization waste

The purpose of this work is to produce an alternative cost-effective adsorbent to remove zinc and cadmium from wastewater using hydroxyapatite (HAP) synthesized with hydrothermal method from FGD (Flue gas desulfurization) waste generated by two different coal power plants. The effects of FGD type (Cayırhan and Orhaneli) and molar ratio (H₃PO₄/CaSO₄) (0.6-4.79) on HAP synthesis were investigated. Afterwards, effects of the adsorbent dose (1-2 g/L), heavy metal concentration (30, 40, 50 mg/L) and contact time (1, 2, 3, 4 h) on zinc and cadmium adsorption yield from synthetic wastewater using produced HAP were examined. FGD waste and synthesized FGD-HAP were characterized by X-Ray Diffraction (XRD), Fourier Transformed Infrared Spectroscopy (FT-IR), Scanning Electron Microscope (SEM) and Brunauer-Emmett-Teller (BET) instruments. The zinc and cadmium concentration was studied by Inductively coupled plasma atomic emission spectroscopy (ICP-AES). Maximum zinc adsorption capacity of the Cayırhan FGD-HAP was 49.97 and 49.99 mg/L, Orhaneli FGD-HAP was 49.96 and 49.99 mg/L, for 1 g/L and 2 g/L adsorbent dose, respectively, for 50 mg/L heavy metal concentration and 4 h contact time. Maximum cadmium adsorption capacity of the Cayırhan FGD-HAP was 39.98 and 39.99 mg/L, Orhaneli FGD-HAP was 40 and 39.99 mg/L, for 1 g/L and 2 g/L adsorbent dose, respectively, for 40 mg/L heavy metal concentration and 4 h contact time. Adsorption yields were calculated between 98.53% and 100%. The adsorption data were well explained by a second-order kinetic model, and the Freundlich isotherm model fits the equilibrium data. The adsorption results demonstrated that FGD's waste is an effective source to synthesize HAP, which is used as an adsorbent for zinc and cadmium removal from wastewater due to high adsorption capacity.

Effect of modified fly ash on environmental safety of two soils contaminated with cadmium and lead

In this study, a low-temperature roasting and hydrothermal methods were used to modify the fly ash resulting in two new types of adsorption materials - modified fly ash (MFA) and artificial zeolite (ZE). These modified fly ashes, as well as a natural zeolite (ZO) were applied to two types of contaminated soils to explore their effects and mechanisms on the behavior of Cd and Pb through leaching column experiments. The bioavailable of Pb, Cd, pH, dissolved organic carbon (DOC), organic matter, as well as the microbial community changings were detected. The results showed that, 2% ZE has a significant stabilizing effect on Cd and the bioavailable fraction contents in Guanzhong (GZ) and Hunan (HN) soils decreased by 40.5% and 53.2%, respectively. However, for Pb, the 2% MFA showed a better result than that of ZE and ZO; the contents of bioavailable Pb in HN and GZ decreased by 48.3% and 30%, respectively. Furthermore, based on the Illumina NovaSep sequencing platform, 18 soil samples of GZ and HN were sequenced for microbial communities. As compared to the control blank(CK) treatment, the abundance of soil microbial communities was significantly improved in the amended soils.

The global research trend on cadmium in freshwater: a bibliometric review

Cadmium pollution turns out to be a global environmental problem. This study conducted a quantitative and qualitative bibliometric analysis based on 9188 research items from the Web of Science Core Collection published in the last 20 years (2000-2020), presenting an in-depth statistical investigation of global freshwater cadmium research progress and developing trend. Our results demonstrated that the researchers from China, the USA, and India contribute the most to this field. The primary sources of cadmium are mining, industry, wastewater, sedimentation, and agricultural activities. In developing countries, cadmium exposure occurs mainly through the air, freshwater, and food. Fish and vegetables are the main food sources of cadmium for humans because of their high accumulation capability. Source evaluation, detection, and remediation represent the main technologies used to clean up cadmium-contaminated sites. To mitigate the risk of cadmium contamination in freshwater, biomarker-based cadmium monitoring methods and integrated policies/strategies to reduce cadmium exposure merit further concern.

Simultaneous immobilization of Pb, Cd and As in soil by hybrid iron-, sulfate- and phosphate-based bio-nanocomposite: Effectiveness, long-term stability and bioavailablity/bioaccessibility evaluation

Here, the bio-nanocomposite (n-HFP + n-HFS)@An was developed to simultaneously immobilize Pb, Cd and As in the severely contaminated soil. The immobilization rates of diethylenetriaminepentaacetic acid (DTPA)/decarbonate-extracted bioavailable Pb, Cd and As were 59.87%, 31.28% and 62.30%, and the immobilization rates of their water-soluble forms were 63.12%, 60.02% and 89.39%, respectively. Moreover, the ten-year acid rain simulated leaching assay showed that the maximum cumulative release contents of Pb, Cd and As in the treated soil samples were decreased by 2.94, 2.46 and 40.60 times, comparing to the un-treated ones. Additionally, the results of SBRC (Solubility Bioaccessibility Research Consortium) revealed that the bioaccessible rates of the three metals in intestinal phase were lower than in gastric phase, and both of them decreased with increasing the immobilization time. The gastric bioaccessibility of Pb, Cd and As had a higher correlation with the contents of water-soluble forms, while the intestinal bioaccessibility was more strongly positively associated with the bioavailable forms.

Hydroxyapatite as a passivator for safe wheat production and its impacts on soil microbial communities in a Cd-contaminated alkaline soil

The remediation of Cd-contaminated alkaline soil plays a critical role in safe wheat production. In this study, hydroxyapatite (HAP), a functional environmental remediation material, was selected to investigate the effects of HAP on cadmium accumulation in winter wheat (Triticum aestivum L.), Cd bioavailability in alkaline soil moderately polluted with Cd (2.46 mg kg^-1) and the soil bacterial community via pot experiments. The results showed HAP effectively inhibited Cd accumulation in the grains of two investigated wheat cultivars by hindering root uptake. The Cd concentrations decreased by 49.9-81.9%, and 35.7-92.4% in the grains of Zhoumai-30 and Zhengmai-7698, respectively. HAP increased the soil pH and reduced the bioavailability of Cd. 16S rRNA sequencing analysis indicated that the changes of soil physicochemical properties changed the diversity and composition of the bacterial community by increasing the relative abundance of beneficial soil bacteria. These results demonstrated the application of 2.5% HAP combined with planting Zhengmai-7698 treatment was a potential remediation method for safe wheat production, and also benefited soil P and N cycling by increasing the relative abundance of beneficial bacteria. The good performance of HAP in inhabiting Cd accumulation in wheat grains indicated it is a promising material for safe wheat production.