This study investigated the remediation effects of iron-modified biochar (FeBC-1 and FeBC-2) on arsenic (As) and cadmium (Cd) co-contaminated paddy soil and elucidated the underlying mechanisms from the perspectives of rhizosphere microbial ecology and plant As and Cd accumulation. A pot experiment with rice was conducted, comprising a control (CK) and iron-modified biochar treatments (FeBC-1 and FeBC-2). Parameters such as As and Cd speciation in rhizosphere soil, bacterial community composition, and the abundance of As-related functional genes were analyzed. The results demonstrated that iron-modified biochar reduced As and Cd accumulation in rice grains by promoting the formation of iron plaques on root surfaces. Meanwhile, the iron-modified biochar significantly enhanced the alpha diversity of bacterial communities and altered their composition. Quantitative analysis of functional genes revealed that the abundance of the As oxidase gene (aioA) increased from 3.54 × 10⁵ in the CK treatment to 7.20 × 10⁵ in FeBC-1 and 7.14 × 10⁵ in FeBC-2, and the abundance of the As efflux gene (arsA) decreased in the biochar-treated groups. These results indicate reduced As bioavailability in the rhizosphere and enhanced transformation of As(III) to As(V).

Biochar and phosphate fertilizer are commonly employed for the mitigation of soil cadmium (Cd) contamination. Nevertheless, there is a dearth of research regarding the mechanism behind their joint implementation. In this study, a combination of corn straw biochar (0 (C₀), 5 (C₅), and 10 (C₁₀) g kg⁻¹) and calcium superphosphate (0 (P₀), 0.1 (P₁), 0.2 (P₂), 0.5 (P₅), and 1.0 (P₁₀) g kg⁻¹) was applied in pot experiments, and the effects of the combined application on Cd bioavailability and its uptake by cabbage were investigated in Cd-contaminated soils. The results demonstrated that the combined treatment of applying biochar and Ca(H₂PO₂)₂ yielded a significant decrease in the uptake of Cd by cabbage in alkaline soil, in contrast to the individual treatments of biochar or Ca(H₂PO₂)₂. Compared to the CK treatment (C₀P₀), the Cd content in the shoots decreased by 46.26% and in the roots decreased by 24.81%, while the biomass of the cabbage demonstrated a noteworthy increase in C₅P₁₀ treatment. Compared to the CK treatment, the content of available phosphate (AP) in the soil increased by 17.57 mg kg⁻¹, residual Cd increased by 22.02%, the exchangeable Cd decreased by 45.86%, and carbonate-bound Cd decreased by 20.55% in the C₅P₁₀ treatment. Therefore, it is advisable to use a combination of 5 g kg⁻¹ biochar and 1 g kg⁻¹ Ca(H₂PO₂)₂ for the restoration of soil contaminated with Cd.

The use of biodegradable chelating agents and fertilizer to improve phytoremediation is a cost-effective and environmental-friendly method for remediation of copper (Cu)- and cadmium (Cd)-polluted agricultural soil. A pot experiment was conducted to investigate the effects of phosphorus (P) fertilizer and the chelator ethylenediamine disuccinic acid (EDDS), both individually and in combination, on the phytoremediation efficiency of castor plants. The experiment included six treatments with three replicates, which were as follows: control (no P or EDDS), EDDS alone, P at 100 mg kg⁻¹, P at 300 mg kg⁻¹, P at 100 mg kg⁻¹ + EDDS, and P at 300 mg kg⁻¹ + EDDS. The results demonstrated that phosphorus significantly promoted the growth of castor plants. In the treatment in which 300 mg kg⁻¹ P₂O₅ and 5.0 mmol kg⁻¹ EDDS were added, the shoot dry weight and root dry weight increased by 42.0% and 67.6%, respectively, when compared to the treatment only applying EDDS, and this treatment significantly promoted the absorption of Cd by shoots of castor. In the absence of phosphorus application, EDDS significantly diminished the dry weight of castor roots by 27.3%. Nevertheless, it improved the concentrations of Cu in the shoots and roots of castor plants, which were 3.43 times and 3.27 times higher than those of the control, respectively. Furthermore, when combined with phosphorus fertilizers, EDDS further promoted the absorption of Cu and Cd in the shoots of castor, which significantly increased by 13.34 times and 0.47 times, respectively, with addition of 100 mg kg⁻¹ phosphorus and 5.0 mmol kg⁻¹ of EDDS compared with the control. Phosphorus and EDDS synergistically decreased the activity of POD enzymes in leaves and roots compared with those treated with only EDDS and alleviated the toxicity of EDDS and heavy metals to castor plants. These findings provide scientific evidence for the use of agronomic measures and chelators to optimize phytoremediation efficiency in Cu and Cd co-contaminated soils.

Three biodegradable organic acids, citric acid (CA), malic acid (MA), and oxalic acid (OA), were evaluated for their ability to remove cadmium (Cd), lead (Pb), and copper (Cu) from contaminated soils. The effects of organic acid concentration, solution pH, and treatment time on metal removal were systematically investigated. Response surface methodology (RSM) was used to optimize these parameters. Sequential extraction was performed to track changes in heavy metal speciation. Under single-factor conditions (75 mmol/L CA, pH 5.0, 60 min), the removal efficiencies were 12.81% for Cd, 10.36% for Pb, and 14.94% for Cu, respectively. Under the optimized conditions (70 mmol/L, pH 5.0, 100 min), the removal efficiencies were further enhanced. The organic acids preferentially targeted bioavailable fractions (water-soluble, exchangeable, and carbonate-bound), which lowered ecological risk. Although CA was less efficient than chemical chelators such as EDTA, it caused much less nutrient loss. Organic acids, especially CA, provide an environmentally friendly alternative for heavy metal extraction with minimal side effects on soil fertility. They represent a promising low-impact option under the tested laboratory conditions. Nevertheless, the absolute removal values in a single washing step remained below 20% for all three metals, indicating that while the method is sustainable and eco-friendly, it is not suited for heavily contaminated soils as a standalone treatment.