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Solidification and stabilization (S/S) is one of the primary technologies for the treatment of heavy metals in soil. It is particularly important to strengthen the concept of solidification and stabilization materials in the research and development of materials. The mechanisms of solidification and stabilization materials on soil heavy metals include precipitation, adsorption, coordination, organic complexation, and redox reactions. This paper mainly focuses on the systematic study of materials in soil heavy metal solidification and stabilization, proposes research & development methods for soil heavy metal solidification and stabilization, including screening and comparison, optimization and combination, surface modification, surface loading, and bridging composite methods. The basic research on the application of solidification materials includes five aspects of the framework, solidification and stabilization materials-heavy metals-water bodies, solidification and stabilization materials-soil-heavy metals, solidification and stabilization materials-soil-plant-heavy metals, solidification and stabilization materials-field-crop-heavy metals, and the evaluation of environmental safety and application effect on heavy metal treatment. For each aspect, the main content and key linked points of the research are explained. This holds importance for promoting the study of materials in soil heavy metal solidification and stabilization, and strengthening the application based on this process. [ABSTRACT FROM AUTHOR]

Ningxia Helan County was taken as the research area in this paper to study the distribution characteristics and ecological risk of heavy metals in farmland soil in northwest China. 140 topsoil samples were collected aiming to analyze and measure the concentration of Cr, Ni, Cu, Zn, As, Cd and Pb belonging to heavy metal elements. The distributions, soil environment status and ecological risk of the seven heavy metals were studied by Nemerow pollution index and Håkanson potential ecological risk index methods. The results showed that concentrations of Cr, Ni, Cu, Zn, As and Pb were all lower than the second-level of Environmental Quality Standard for Soils of China except that concentrations of Cd of 1.42% samples were appeared to exceed the standard. Compared with the background values, seven heavy metals of all samples showed different degree accumulation, in which Cd, Ni elements beyond the background value ratio reached 100% and 85%. Spatial distribution of the concentrations of the seven heavy metals had obvious accordance. The content of each element was evenly distributed. The high content of each element was concentrated in the central and eastern part of Helan County, which was related to human activities. The calculation results of single-factor pollution index showed that among the 140 soil samples, 0.71% of Cd were moderate contamination, 7.14% of Ni and 5% of As were slight clean, Cr, Cu, Zn, Pb of all samples were all clean. According to the individual potential ecological risk index（Ei r）of heavy metals, the average potential ecological risk index of Cd element was 97.68, which indicated that Cd had reached the serious risk level, becoming the main element of potential ecological risk. All in all, the quality of farmland soil in Helan County of Ningxia is good, but it is noteworthy that the concentration of Cd, Ni has a serious accumulation. [ABSTRACT FROM AUTHOR]