Effects of hematite on the stabilization of copper, cadmium and phosphorus in a contaminated red soil amended with hydroxyapatite

Remediation of copper and lead contaminated sediments using iron-based granule biochar: mechanisms and enzyme activity

In recent years, there has been a growing concern about heavy metal contamination in sediments. In this study, iron-based granular biochar (MGB) is prepared to remediate Cu and Pb contaminated sediments. Characterizations via scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) show that the rough surface of MGB with abundant pore structures and a large number of oxygen-containing functional groups that facilitate stabilization of Cu and Pb in sediments. Potential mobility and bioavailability of Cu and Pb are investigated using BCR sequential extraction in the 35 day remediation experiment. The XPS results indicate that FeOOH and C-OH play a crucial part in stabilizing heavy metals. Large affinity of FeOOH for Pb allows it to occupy a proportion in F₂ while C-OH is attractive to Cu. Changes of pH, organic matter (OM), and available phosphorus (AP) in sediments after adding MGB as well as the relationship between changes and the stable solidification of Cu and Pb are explored. The stable solidification of heavy metals effectively reduces the available phosphorus in sediments. Magnetic and particle properties of the material are used to reduce the impact of MGB aging on sediment environment and separate it from the remediated sediment. Finally, 3% of MGB significantly enhanced the sediment catalase activity in the biological enzyme activity experiment. All findings indicate that MGB is a green and environmentally friendly sediment remediation material with satisfactory potential in synergistically stabilizing heavy metals and phosphorus.Highlights The complexation of FeOOH with Pb on the surface of MGB fixes it to the reduced stateThe C-OH on the surface of MGB is more attractive to Cu than PbMGB effectively mitigates the release of bioavailable phosphorus from sediments to overlying water.

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.

Immobilization of Pb and Zn in Contaminated Soil Using Alumina-Silica Nano-Amendments Synthesized from Coal Fly Ash

To apply coal fly ash to the remediation of heavy-metal-contaminated soil, an alumina-silica nano-amendment (ASNA) was synthesized from coal fly ash and was used for the immobilization of lead and zinc in contaminated soil. The investigation on the synthesis of the ASNA shows that the ASNA can be obtained under a roasting temperature of 700 °C, a ratio of alkali to coal fly ash of 1.2:1, and a molar ratio of silicon to aluminum of 1:1. The ASNA could increase the soil pH and cation exchange capacity (CEC) and decrease the bioavailability of Pb and Zn. When the ASNA addition increased from 0 to 2%, the bioavailability (extracted by CaCl₂) of Pb and Zn decreased by 47% and 72%, respectively. Moreover, the addition of the ASNA facilitated the transformation of Pb from a reducible fraction to oxidizable and residual fractions and Zn from an exchangeable fraction to a residual fraction. The correlation analysis and cluster analysis verify that the ASNA modulates the chemical speciation of heavy metals by increasing the soil's CEC and pH, thereby immobilizing heavy metals. It is expected that this study can provide a new method for the remediation of Pb- and Zn-contaminated soil.

Effects of phosphorus-containing material application on soil cadmium bioavailability: a meta-analysis

Diverse phosphorus-containing materials (PCMs) were widely applied in remediation of cadmium-contaminated soils, and their effects on the change of soil cadmium availability (SCA) varied with their physicochemical characteristics and environmental conditions. Investigation on the effect of various PCMs on reducing SCA under different conditions favors the safe utilization of Cd-contaminated soil. Herein, a meta-analysis of literature published before August 2021 was carried out. A total of 342 independent observations were obtained from 42 published papers which included 9 factors that may affect the passivation effect of fertilizer content: phosphorus type, phosphorus application rate, soil pH, soil CEC, soil organic matter, experiment type, and time. Results of boosted regression tree analysis showed that the application rate is the most important factor contributing to the SCA, followed by soil pH and duration. Results of this meta-analysis showed that medium P input shows potential for reactivating the SCA. Under alkaline soil conditions and high soil CEC values, PCM input can better deactivate SCA. In addition, the difference from the previous understanding is that under the medium input of phosphorus-containing fertilizer (90-500 mg P∙kg^-1), it will significantly increase the content of available cadmium in soil. In addition, future recommendation for exploring novel PCMs and suitable strategies for controlling the SCA though PCM application were also proposed. Our works may promote the interpretation of the interference factors on the SCA changes and fill the research gaps on utilization of PCM in Cd-polluted soil remediation.

Simultaneous elimination of black-odor and stabilization of heavy metals in contaminated sediment using calcium peroxide/hydroxyapatite: Microbial responses and ecotoxicological effects

In this study, laboratory-scale experiments were conducted to investigate the feasibility of the combined use of calcium peroxide and hydroxyapatite (CaO₂/HAP) for simultaneous black-odor sediment remediation and heavy metal stabilization. The ecotoxicological effects of remediated sediment were also evaluated based on biological toxicity. Results showed that CaO₂/HAP effectively eliminated the black-odor and simultaneously stabilized heavy metals in the sediment. Under the optimal dosage ratio of CaO₂/HAP (1:2), the acid volatile sulfides decreased to approximately 20 mg/kg (dry weight, dw) and oxidation-reduction potential increased from - 165 mV to approximately - 90 mV. The leaching of heavy metals meets the strictest standards (Level I) of the "Technical Specification for Output Disposal of Contaminated Sediment Treatment Plant of River and Lake" (SZDB/Z 236-2017). The indigenous microbial community succession occurred (p < 0.01), Proteobacteria and Firmicutes accounting for 75.54% and 20.19%, respectively, were the predominant bacteria in the remediated sediment. Additionally, CaO₂/HAP remediated sediments were safer and more environmentally friendly than raw sediments, and were not biotoxic to the benthic environment (p < 0.01). This study provides new insights into the combined use of the beneficial amendments remediating heavy metal-contaminated black-odor river sediment.

Aging reduces the bioavailability of copper and cadmium in soil immobilized by biochars with various concentrations of endogenous metals

Biochar is widely used for environmental remediation. However, the effects of aging on the bioavailability of trace metals in biochar-amended soil remain largely unknown, especially for the biochars with various concentrations of endogenous metals. In this study, three biochars marked as BB, MB, and HB were produced from the straws of Pennisetum sinese grown in background soil, moderately-, and highly-polluted soils by trace metals, respectively. We distinguished the effects of dry-wet (DW) and freeze-thaw (FT) aging on the bioavailability of copper (Cu) and cadmium (Cd) from soil particles, the biochar interior, and the surface of biochar. The adsorption capacities of Cu²⁺ and Cd²⁺ followed the order of BB > MB > HB. DW and FT aging both increased the adsorption capacity of Cu²⁺, but decreased that of Cd²⁺ in the three biochars, resulting in a reduction in Cu bioavailability and increase in Cd bioavailability in the biochars after the saturated adsorption of Cu²⁺ and Cd²⁺. The incorporation of the three biochars decreased Cu bioavailability compared to the control after incubation for 30d, while the addition of MB increased Cd bioavailability. DW and FT aging decreased Cu bioavailability in biochar-amended soil by decreasing the bioavailability of Cu adsorbed on the biochar surface and immobilized by soil particles. Meanwhile, aging decreased Cd bioavailability by decreasing the bioavailability of Cd immobilized by soil particles. Overall, environmental risk would be increased by the application of biochars with high endogenous Cd. The major implications are that biochar dosage and environmental risk should be carefully assessed before large-scale, continuous application, especially for biochars containing high contents of endogenous trace metals.

Stabilization of lead in polluted sediment based on an eco-friendly amendment strategy: Microenvironment response mechanism

Stabilization is the most important remediation mechanisms for sediment polluted heavy metals. However, little research has been done on the identification of microenvironmental response and internal correlation, as well as synergistic mechanisms during heavy metal remediation. This study aims to investigate the inner response mechanisms of microenvironment after the lead (Pb) are gradually stabilized in sediment. An eco-friendly amendment strategy which firstly used 100% biodegradable sophorolipids (SOP) to modify chlorapatite (ClAP) for the fabrication of SOP@nClAP was applied in this study. The stabilization efficiency of Pb was significantly improved by SOP@nClAP compared with ClAP. Most importantly, the high-throughput sequencing showed that the dominant species in the sediment changed with the stabilization of Pb. The decrease of Proteobacteria and increase of Firmicutes, especially the Sedimentibacter within the phylum Firmicute directly suggested that large amounts of Pb were stabilized. This research is not only devoted to stabilize Pb in sediment by eco-friendly amendment strategy, but also keep a watchful eye on microenvironment response mechanisms during the Pb stabilization in sediment. Therefore, this study lays a foundation for the future application of more heavy metal amendment strategies in the sediment environment and improves the possibility of large-scale site amendment.

Cadmium Immobilization in the Rhizosphere and Plant Cellular Detoxification: Role of Plant-Growth-Promoting Rhizobacteria as a Sustainable Solution

Food is the major cadmium (Cd)-exposure pathway from agricultural soils to humans and other living entities and must be reduced in an effective way. A plant can select beneficial microbes, like plant-growth-promoting rhizobacteria (PGPR), depending upon the nature of root exudates in the rhizosphere, for its own benefits, such as plant growth promotion as well as protection from metal toxicity. This review intends to seek out information on the rhizo-immobilization of Cd in polluted soils using the PGPR along with plant nutrient fertilizers. This review suggests that the rhizo-immobilization of Cd by a combination of PGPR and nanohybrid-based plant nutrient fertilizers would be a potential and sustainable technology for phytoavailable Cd immobilization in the rhizosphere and plant cellular detoxification, by keeping the plant nutrition flow and green dynamics of plant nutrition and boosting the plant growth and development under Cd stress.