Insights into cellulose/pectin effect in tobermorite/mandarin orange peel composite hydrogels and rehabilitation effect on Cd- and Pb-contaminated slopes.

The cadmium and lead contamination in soil poses a threat to ecological security. This work compared the adsorption performance and density functional theory (DFT)-calculated adsorption energies of three hydrogels for Cd/Pb: Tobermorite (TOB) hydrogel (T2-CH), pure cellulose-TOB composite hydrogel (FT2-CH), and mandarin orange peel (MOP)-TOB composite hydrogel (M6T2-CH), elucidating the synergistic mechanism of MOP key components (cellulose, pectin) in enhancing Cd/Pb adsorption. The results showed that the addition of cellulose promoted the adsorption rate and adsorption capacity of the FT2-CH for Cd/Pb, but the promoting effect was not as good as that of the addition MOP. Characterizations along with DFT calculations revealed that the components in MOP led to M6T2-CH having more carboxyl groups and an excellent surface morphology. M6T2-CH effectively promoted plant growth and net photosynthetic rate (Pn) in Astragalus sinicus L., while reducing heavy metal content across sandy loam, sandy clay loam, and sandy clay soils-with maximum decreases of 73.06% (Cd) and 70.86% (Pb). 16S rRNA sequencing of the three soil types revealed that M6T2-CH addition enhanced species diversity, facilitating soil carbon and nitrogen cycling and Cd/Pb stabilization. Slope erosion tests demonstrated that M6T2-CH addition notably enhanced erosion resistance in silty loam and silty clay loam planted with Astragalus sinicus L., reducing sand yield by 95.35% and 94.05%, respectively. This study unveils the action mechanism of key MOP components in composite hydrogels, demonstrating their potential as soil conditioners for Cd- and Pb-contaminated slopes, fostering plant growth, enhancing soil quality, and mitigating slope erosion.