Preparation of Alkali–Fe-Modified Biochar from Agricultural Waste for Remediation of Cadmium-Contaminated Soil and Water

Remediating cadmium (Cd) contamination in aquatic and terrestrial environments has become an urgent environmental priority. Biochar has been widely employed for heavy metal removal due to its wide availability, strong adsorption capacity, and potential for recycling agricultural waste. In this study, samples of alkali–Fe-modified biochar (Fe@NaOH-SBC, Fe@NaOH-HBC, and Fe@NaOH-MBC) were prepared from agricultural wastes (ginger straw, Sichuan pepper branches, and kiwi leaves) through NaOH and FeCl 3 ·6H 2 O modification. A comprehensive characterization confirmed that the alkali–Fe-modified biochar exhibits a higher specific surface area, richer functional groups, and successful incorporation of the iron oxides Fe 3 O 4 and α-FeOOH. The fitting parameter qmax from the Langmuir model indicates that the alkali–Fe modification of carbon significantly enhanced its maximum capacity for Cd 2+ adsorption. Furthermore, a synergistic effect was observed between iron oxide loading and alkali modification, outperforming alkali modification alone. Furthermore, a 30-day soil incubation experiment revealed that the application of alkali–Fe-modified biochar significantly increased soil pH, SOM, and CEC while reducing the available cadmium content by 13.34–33.94%. The treatment also facilitated the transformation of highly bioavailable cadmium species into more stable, less bioavailable forms, thereby mitigating their potential entry into the food chain and the associated human health risks. Moreover, short-term spinach seed germination experiments confirmed that treatments with varying additions of alkali–Fe-modified biochar mitigated the inhibition of seed physiological processes by high concentrations of available cadmium to varying degrees. Overall, this study provides a sustainable and effective strategy for utilizing agricultural waste in the remediation of cadmium-contaminated water and soil systems.