Synthesis, characterisation, optimisation, and adsorptive removal of methylene blue from aqueous solutions using hydroxyapatite from cow teeth waste

Dye-contaminated wastewater poses a serious environmental challenge owing to the persistence and poor biodegradability of synthetic dyes. This study investigated hydroxyapatite synthesised from Cow Teeth Waste (CTW), a sustainable adsorbent for Methylene Blue (MB) removal from aqueous solution. CTW were cleaned, dried, calcined at 900 °C for 2 h, milled, and sieved before characterisation. Scanning Electron Microscopy (SEM) revealed porous surface favourable for adsorption, and Fourier Transform Infrared Spectrophotometry (FTIR) confirmed the characteristic hydroxyl, carbonate, and phosphate functional groups of hydroxyapatite. Batch adsorption experiments evaluated the effects of pH, adsorbent dosage, initial dye concentration, contact time, temperature, and agitation speed. Maximum MB removal (90.20%) occurred at pH 6, increasing to 96.89% at 180 rpm and 94.65% at an adsorbent dosage of 1.0 g. Adsorption capacity remained nearly constant after 60 min (9.394–9.448 mg g⁻¹). Response surface optimisation using Box–Behnken design and reciprocal response transformation (1/qₑ) produced an excellent predictive model (R² = 0.9991) with non-significant lack of fit (p = 0.4713). Nonlinear Langmuir, Freundlich, and Temkin models showed comparable fits (R² = 0.927–0.930), indicating no dominant adsorption mechanism. Cow teeth-derived hydroxyapatite therefore represents a promising low-cost adsorbent for dye removal, although regeneration, phase verification, and real wastewater validation are required before large-scale application.