Sustainable soil rehabilitation with multifunctional mandarin orange peel/tobermorite composite hydrogels: Water retention, immobilization of heavy metals, fertilizer release and bacterial community composition

Abstract

Traditional hydrogel applied in soil tends to be unacceptable due to the high cost of synthesis raw materials and difficulty in degradation. To enhance water holding, adsorption of heavy metals and substrate fertility, this work developed a novel composite hydrogel (MxTy-CH) prepared from mandarin orange peel (MOP) and tobermorite (TOB). By arranging the enhancement of MOP and TOB, the water absorbency (from 171.23 g/g to 204.29 g/g), water holding (66.77 %, 144 h), adsorption characteristics (Cd²⁺ 339.93 mg/g; Pb²⁺ 1743.44 mg/g) and fertilizer slow-release performance (0.938 % released in 6 days) were significantly enhanced. The results indicated that the addition of MOP and TOB contributed to an enrichment in the number of –COOH and –OH groups contained in the system. Additionally, the adsorption behavior of cadmium/lead on MxTy-CH was more consistent with the pseudo-second-order kinetics and Langmuir isotherm models. The regeneration test results suggested that the adsorption removal rate of cadmium/lead by MxTy-CH remained stable after 5 cycles. Fish toxicity tests indicated that MxTy-CH was biocompatible and would not cause obvious physiological toxicity to fish organisms. The MxTy-CH treatment group performed better than control group in Cd(II) and Pb(II) passivation performance in soil with mung beans. Changes in the microbial community composition and structure after MxTy-CH treatment in soil provided demonstration for stimulated C and N cycling in soil, suggestive of enhanced soil quality. The abovementioned further proves the considerable application potential of MxTy-CH in plant growth and rehabilitation of cadmium and lead contaminated soil