Combined Application of Commercial Hydroxyapatite and a Straw-Derived Organic Fertilizer Immobilizes Cadmium in an Alkaline-Contaminated Soil

A highly effective and economical method to immobilize cadmium in alkaline agricultural soil is urgently needed. Using adsorption kinetic and isotherm experiments, soil incubation tests, and cadmium leaching assays, this study aimed to evaluate the applicability of hydroxyapatite–organic fertilizer composite amendment (HO), individual hydroxylapatite (HA), individual organic fertilizer (OF), sepiolite (SP), and diatomite (DE) to passivate soil cadmium and their passivating effect. In the aqueous phase, HO successfully adsorbed Cd²⁺ onto the surface and has superior potential Cd²⁺ adsorption capacity than OF, DE, and SP, with its adsorption capacity closely approaching that of HA, enabling its use as a passivator in field Cd-contaminated soils. In Cd-contaminated soil, HO effectively lowered the pH from 9.22 to 8.59 at a 5% application rate and changed the aggregate-size distribution of the soil. The increase in the amount of passivator also significantly increased the soil aggregate size. Moreover, the addition of HO significantly improved the extractable contents of Cd in the soil. Compared with the control, the combined amendment decreased TCLP (toxicity leaching procedure test)-extractable Cd by 30.95%, 42.86%, 59.52%, and 69.05% at application rates of 0.5%, 1%, 3%, and 5% (w/w), respectively. These results demonstrate that HO is a highly efficient and low-cost organic–inorganic composite passivator for cadmium-contaminated soils.