Heavy metal pollution, including that caused by cadmium (Cd), is a matter of increasing concern. Although in situ passivation remediation has been widely used to treat heavy metal-polluted soils, most studies have focused on acidic soils, while studies on alkaline soil conditions are scarce. In this study, the effects of biochar (BC), phosphate rock powder (PRP), and humic acid (HA) on Cd²⁺ adsorption were examined alone and in combination to choose an appropriate Cd passivation approach for weakly alkaline soils. Additionally, the combined impact of passivation on Cd availability, plant Cd absorption, plant physiology indexes, and soil microbial community was elucidated. BC had a higher Cd adsorption capacity and removal rate than those of PRP and HA. Moreover, HA and PRP enhanced the adsorption capacity of BC. A combination of biochar and humic acid (BHA), and biochar and phosphate rock powder (BPRP) significantly affected soil Cd passivation. BHA and BPRP decreased the plant Cd content and soil Cd-DTPA (diethylenetriaminepentaacetic acid) by 31.36 %, 20.80 %, 38.19 %, and 41.26 %, respectively; however, they increased the fresh weight by 65.64-71.48 % respectively, and dry weight by 62.41-71.35 %, respectively. Notably, only BPRP increased the node and root tip number in wheat. Total protein (TP) content increased in BHA and BPRP, with BHA showing lower TP than BPRP. BHA and BPRP showed a reduction in glutathione (GSH), malondialdehyde (MDA), H₂O₂, and peroxidase (POD); BHA showed a significantly lower GSH than BPRP. Additionally, BHA and BPRP increased soil sucrase, alkaline phosphatase, and urease activities, with BPRP showing considerably higher enzyme activity than BHA. Both BHA and BPRP increased the number of soil bacteria, altered the community composition, and critical metabolic pathways. The results demonstrated that BPRP could be used as a highly effective, novel passivation technique for the remediation of Cd-contaminated soil.

Cadmium (Cd) water pollution threatens environmental systems and human health. Adsorption is the preferred method for purifying water bodies polluted by Cd, and the development of effective adsorption materials is critical. The performance of original phosphate rock powder (PRP) as an adsorption medium for purifying water bodies polluted by Cd was compared with that of phosphate rock powder modified with fulvic acid, chitosan, MnO₂, and sulfhydryl, respectively, and their appearance and adsorption properties were investigated. The surface structures of all modified powders were rougher than the original, and their functional groups were richer. The greatest Cd²⁺ adsorption capacity, 1.88 mg g⁻¹, was achieved with chitosan-modified PRP (CMPRP). This was 106.59% greater than that of PRP. The capacities of fulvic acid and MnO₂ were 15.38% and 4.40% greater than that of the original, respectively. When the fulvic acid-modified PRP, CMPRP, and manganese dioxide PRP reached adsorption equilibrium, the removal rates of Cd²⁺ were 51.86%, 93.26%, and 46.70%, respectively. Moreover, the removal rate of Cd²⁺ by CMPRP was 104.43% higher than that of PRP. The main Cd²⁺ adsorption mechanisms for the MPRPs were electrostatic interactions, ion exchange, co-precipitation, and complexation. Moreover, the processing of the phosphate rock powder was straightforward, harmless to the environment, and could be effectively used for the removal of Cd. These results show that CMPRP is promising as a new adsorption material to treat Cd-contaminated water.

Cadmium (Cd) not only significantly threatens crop growth and food safety but also degrades soil quality and reduces microbial activity. Hydrochar is a carbon-rich material derived from biomass via hydrothermal carbonisation, due to its large specific surface area and abundant surface functional groups, shows great potential for immobilisation remediation of Cd-contaminated soils. We determined hydrochar’s (1) optimal application conditions for soil Cd immobilisation and (2) regulatory effects on foxtail millet growth and development, photosynthesis, antioxidant enzyme activity, resistance gene expression and Cd accumulation. Hydrochar was prepared at two temperatures (240 and 160 °C) and applied at two concentrations (1% and 2%). Hydrochar application reduced soil available Cd by 4.59–10.09% and shifted Cd fractions from acetic acid-extractable to residual forms. Soil organic matter (SOM) and microbial biomass carbon (MBC) increased. Hydrochar application increased plant antioxidant enzyme activity, downregulated SiNRAMP5 and SiHMA2 and upregulated SiHMA3, SiPCS1 and SiMT2. Hydrochar prepared at 240 °C and applied at a 2% concentration exhibited the highest efficiency in immobilising Cd, enhancing plant photosynthesis and antioxidant defence, activating soil microorganisms and increasing SOM and MBC, offering a promising remediation strategy for weakly alkaline Cd-contaminated farmland in northern China.

Passivation is a feasible approach for remediating heavy metal-contaminated soil. However, how passivation stability depends on material type, dosage, and plant cultivar remains unclear. Phosphate rock powder (PR) and bentonite (BN) were applied for passivation at three dosages, and the passivation activity was evaluated on high- and low-Cd-accumulating wheat cultivars. Phosphate rock powder (PR) and BN decreased the available Cd content in the soil by 11.52–26.65% and 11.08–35.00%, and in the wheat grains by 7.28–49.94% and 14.14–57.61%, respectively. PR₃ and BN₃ enhanced wheat yield by 12.77–21.31% and 12.23–18.67%, respectively. The passivation activity of both materials increased with increasing dosage. The optimal ranges for effective, stable Cd passivation were 0.29–0.61 and 3.85–8.46 t ha⁻¹ for PR and BN, respectively. Path analysis revealed that PR acts mainly through increases in soil available phosphorus and associated changes in Cd fractions, whereas BN acts primarily through the soil cation exchange capacity; grain Cd was chiefly associated with reactive Cd fractions. The different Cd accumulation capacities of wheat cultivars affected the passivation effects of PR and BN. The soil of Jimai22 showed significantly lower EXC-Cd and Carb-Cd and significantly higher FeMnOz-Cd than Zhoumai18. Moreover, soil pH was higher for Jimai22 than for Zhoumai18. These results suggest that combining the selection of suitable passivation materials, optimising the dosage and planting low-Cd-accumulating cultivars is an effective strategy for maintaining Cd passivation in weakly alkaline soils.