With the rapid development of industry and agriculture, soil contamination has become a significant environmental issue, and the heavy metal contamination of soils is an important part of it. The main methods for the remediation of heavy metal-contaminated soils include physical methods, chemical methods, biological methods, and combined remediation methods have been proposed as research deepens. However, the standards and evaluation methods for the remediation of heavy metal-contaminated soils are still not well-established. This article discusses the sources and contamination status of heavy metals in soils, the advantages and disadvantages of remediation technology for heavy metal-contaminated soils, remediation standards, and post-remediation evaluation methods. It also proposes scientific issues to be addressed in future research and provides an outlook on future development, hoping to assist in subsequent remediation studies of heavy metal-contaminated soils.

Natural biomass ash of agricultural residuals was collected from a power plant and modified with hexagonal mesoporous silica and functionalized with 3-aminopropyltriethoxysilane. The physicochemical and morphological properties of the biomass ash were analyzed by ICP-OES, SEM, TEM-EDS, FTIR, and BET analysis. The adsorption behavior of the modified product for Cd²⁺ in aqueous solution was studied as a function of pH, initial metal concentration, equilibrium time, and temperature. Results showed that the specific surface area of the modified product was 9 times that of the natural biomass ash. The modified biomass ash exhibited high affinity for Cd²⁺ and its adsorption capacity increased sharply with increasing pH from 4.0 to 6.0. The maximum adsorption capacity was 23.95 mg/g in a pH 5 solution with an initial metal concentration of 50 mg/L and a contact time of 90 min. The adsorption of Cd²⁺ onto the modified biomass ash was well fitted to the Langmuir model and it followed pseudo-second-order kinetics. Thermodynamic analysis results showed that the adsorption of Cd²⁺ was spontaneous and endothermic in nature. The results suggest that the modified biomass ash is promising for use as an inexpensive and effective adsorbent for Cd²⁺ removal from aqueous solution.

Phytoremediation is a potential cost-effective technology for remediating heavy metal-contaminated soils. This method was used to evaluate the biomass and accumulation of copper (Cu) and cadmium (Cd) of plant species grown in contaminated soil and their biological and physical effects on the soil. In co-contaminated soils with copper (Cu) and cadmium (Cd), a three-year field experiment was conducted by planting four plant species in the co-contaminated acidic soil treated with hydroxyapatite. The four species produced different amounts of biomass in this order: Pennisetum sp. > Elsholtzia splendens > Setaria lutescens > Sedum plumbizincicola. Over three growing seasons, the best accumulators of Cu and Cd were Elsholtzia splendens and Sedum plumbizincicola, respectively. Overall, Pennisetum sp. was the best species for Cu and Cd removal when biomass was considered. The four plant treatments could improve the content of >0.25 mm mechanically stable (DR_0.25) and water-stable (WR_0.25) aggregates and significantly improve the aggregate mean mass diameter (MWD) and the geometric mean diameter (GMD). The largest increase was with the treatment of Pennisetum sinese, while the fractal dimension (FD) of mechanically stable aggregates could be significantly reduced by the treatment of Pennisetum sp. Hydroxyapatite and phytoremediation could improve the soil enzyme activity, and Elsholtzia splendens had the best effect in this respect. This study will provide a better understanding of the remediation of heavy metal contaminated soil.

With the rapid development of industry and agriculture, soil heavy metal contamination has become an important environmental issue faced today and has gradually attracted widespread attention. Finding a cheap, widely available, and biodegradable material that can promote crop growth and stabilize heavy metals has become a research focus. Crop straw biochar, due to its high specific surface area, rich surface functional groups, and high cation exchange capacity (CEC), has shown good effects on the remediation of inorganic and organic pollutants in the environment. This article reviews recent research on the use of crop straw biochar for soil heavy metal contamination remediation, providing a detailed analysis from the preparation, characteristics, modification of crop straw biochar, mechanisms for reducing the toxicity of heavy metals in soil, and its application and risks in remediating heavy metal-contaminated soils. It also comprehensively discusses the potential application of crop straw biochar in the remediation of heavy metal-contaminated soils. The results show that crop straw biochar can be used as a new type of immobilizing material for the remediation of heavy metal-contaminated soils, but there are issues with the remediation technology that needs to be optimized and innovated, which poses challenges to the widespread application of crop straw biochar. In the future, efforts should be strengthened to optimize and innovate the application technology of crop straw biochar, conduct research on the remediation effects of cheap modified crop straw biochar and the co-application of crop straw biochar with other immobilizing materials on heavy metal-contaminated soils, and carry out long-term monitoring of the effects of crop straw biochar in soil heavy metal remediation in order to achieve the goal of ensuring food safety and the rational use of solid waste.

Applying hydroxyapatite to immobilize heavy metals may cause a decline in soil quality, whereas phytoextraction has been shown to be a slow process under normal agronomic conditions. The benefits of combining hydroxyapatite application and phytoextraction are unclear for improving the soil ecosystem at the same time as remediating heavy metals. A three-year field experiment was conducted to test the combined effects of hydroxyapatite application and phytoextraction (Sedum plumbizincicola, Elsholtzia splendens, and Pennisetum sp.) on remediating cadmium (Cd) and copper (Cu) and improving the soil microbial community structure in a smelter-impacted site. Total Cd and Cu were fractionated using a sequential extraction procedure. Soil microbial biomass and community composition were measured by phospholipid-derived fatty acid (PLFA) analysis. The results showed that hydroxyapatite addition (1%) increased soil pH and decreased the exchangeable fraction of Cu and Cd compared with the control. Combining with hydroxyapatite application, the growth of phytoextractors increased organic matter content and decreased total Cu and Cd concentrations in the soil. Hydroxyapatite application disturbed the soil microbial PLFA composition by increasing fungal PLFA, whereas a combination with plant growth reduced the trend of soil fungal growth. Combining hydroxyapatite with Pennisetum sp. particularly improved Gram-negative and Gram-positive bacterial biomass and reduced the ratio of fungi to bacteria. In conclusion, combining hydroxyapatite with suitable phytoextractors is a promising approach for remediating heavy metals (e.g., Cu and Cd) and simultaneously improving soil microbial community structure and the ecological environment in contaminated areas.