Drought-resistant and water-retaining tobermorite/starch composite hydrogel for the remediation of cadmium-contaminated soil

The objective of this work is utilizing fly ash to synthesize tobermorite (TOB) with a higher specific surface area and layered structure, and incorporating it into the starch/acrylic acid network to boost the drought resistance, water retention and heavy metal adsorption properties. The water absorption and water retention performance and cadmium adsorption characteristics of tobermorite/leftover rice-based composite hydrogel (TOB@LR-CH) were evaluated by water absorption swelling test, soil evaporation test and batch adsorption experiment. By adjusting the addition of TOB and other synthesized conditions, the swelling property (from 114.80 g/g to 322.64 g/g), water retention (71.80 %, 144 h) and Cd2+ adsorption characteristics (up to 591.36 mg/g) were significantly enhanced. Adding a moderate amount of TOB (2 wt%) provided the most uniform tobermorite dispersion during synthesis, and TOB2@LR-CH exhibited the most stable three-dimensional network and highest proportion of effective TOB. The adsorption behavior of cadmium on TOB2@LR-CH was more consistent with the pseudo-second-order kinetics and Langmuir isotherm models. Additionally, the regeneration test results displayed that the adsorption removal rate of cadmium by TOB2@LR-CH adsorbent remained stable after 5 cycles. This study demonstrates that TOB@LR-CH has good water absorption and water retention potential in arid and semi-arid soils, and also has potential application prospects in remediating Cd(II)-contaminated soil.