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Magnetic calcium silicate hydrate–oyster shell waste nanocomposite for phosphate removal and recovery: RSM-based optimization, mechanism, and real water application
Author(s)
Date Issued
September 1, 2025
Type
Article
Abstract
Magnetic nanocomposites offer an effective strategy for phosphate removal from wastewater, preventing eutrophication and enabling phosphate recovery for reuse as fertilizer—supporting a zero-waste approach. In this study, a novel hybrid nanocomposite composed of magnetized calcined oyster shell waste and calcium silicate hydrate (M-COS-CSH) was synthesized through a simple process completed within 5.2 h. Response Surface Methodology was employed for optimization: a Central Composite Design determined the optimal FeCl₃ and COS ratios, while a Box–Behnken Design optimized adsorption conditions including adsorbent dose (20–100 mg), initial phosphate concentration (10–90 mg L−1), contact time (15–75 min), and pH (3−11). Under optimal conditions (60 mg adsorbent, 10 mg L−1 phosphate, 45 min, pH 6.28), M-COS-CSH achieved a predicted maximum removal efficiency of 97.30 %. An experimental removal efficiency of 98.06 % ± 0.19 % was obtained under the same conditions without pH adjustment (pH 6.84), offering a cost advantage. The adsorption process closely followed the Langmuir isotherm model (R2 = 0.9963), with a maximum adsorption capacity of 161.29 mg g−1, and was best described by the pseudo-second-order kinetic model (R2 = 1.0000). Characterization suggested a mechanism involving surface microprecipitation and inner-sphere complexation. Thermodynamic analysis confirmed the process to be endothermic and spontaneous (ΔG°: −9.97 to −10.84 kJ mol−1; ΔH°: 3.00 kJ mol−1; ΔS°: 43.52 J mol−1 K−1). M-COS-CSH achieved phosphate removal ranging from 70.81 % ± 2.47 % to 94.24 % ± 0.39 % in real water samples. Even in the presence of competing anions at fivefold phosphate concentration, removal efficiency remained high (93.58 % ± 0.32 %), confirming the material's strong selectivity and suitability in complex matrices.
Citation
Journal of Water Process Engineering, 77, 2025