Sustainable Synthesis and Dual-Function Sorption of Carbonated Hydroxyapatite for Cadmium and Nitrate Removal

Nitrate (NO3−) and cadmium (Cd2+) are common water pollutants with distinct chemical behaviors, often requiring different removal strategies. This study presents a low-cost synthesis of carbonated hydroxyapatite nanopowder (cHA), Ca5(PO4)3-y(CO3)y(OH) (y = 0.13–0.17), using eggshell waste as a calcium precursor, aimed at removing both NO3− and Cd2+ from wastewater. SEM and TEM analyses revealed a porous nanostructure with an average particle size of 13.53 ± 6.43 nm and a specific surface area of 7.568 m2/g. Adsorption experiments were conducted under varying conditions, including contact time (0.3–3 h), dosage (0.3–2 g/L), initial concentrations (10–100 mg/L for NO3−; 5–15 mg/L for Cd2+), and temperature (22 and 50 ± 2 °C). Cd2+ removal reached up to 99% at pH 2–4.5, while NO3− removal peaked at 38% in competitive systems, within 30 min. In single-ion systems, maximum nitrate uptake was 19.14 mg/g at 50 °C. Characterization using FT-IR, EDS, and XRD (with Rietveld refinement) confirmed carbonate B-type substitution and structural changes due to ion exchange and chemisorption. The results demonstrate that cHA derived from food waste is an efficient and sustainable sorbent, particularly for cadmium removal in contaminated water.