Surface charge engineering of phosphoric acid-activated geopolymers via hydroxyapatite incorporation for enhanced anionic water pollutant removal

Geopolymers have emerged as promising sorbents for environmental applications, particularly in removing cationic pollutants due to their negatively charged surfaces. However, their effectiveness for anionic pollutant removal, such as Congo red (CR) dye, remains limited. This study introduces an innovative approach by integrating hydroxyapatite (HAP) into phosphoric acid-activated geopolymers (PAGP) to optimize the surface charge and enhance the adsorption of anionic dyes. The effects of varying HAP loading (up to 50 wt%) on the structural, thermal, mechanical, and CR dye adsorption properties of PAGP were systematically investigated. The optimal formulation, M-50HAP (50 % HAP), exhibited a five-fold increase in specific surface area (from 14 to 72 m²/g), significantly improved structural integrity with a denser microstructure, and enhanced compressive strength (from 0.70 to 1.88 MPa). Zeta potential measurements confirmed that HAP addition reduced surface negativity, promoting anionic dye adsorption. The M-50HAP sorbent achieved a maximum CR dye adsorption capacity of 15.70 mg/g (pH 3, 100 mg/L initial concentration, 4 g/L adsorbent, 24 h contact time) and demonstrated excellent reusability over 12 cycles without performance degradation. Density functional theory (DFT) simulations revealed that CR protonation enhanced its positive charge distribution, facilitating adsorption onto M-50HAP via hydrogen bonding, electron interactions, and electrostatic forces. This study demonstrates that HAP-modified PAGPs are scalable, mechanically robust adsorbents with tunable surface charge, offering a significant advancement in sustainable wastewater treatment and anionic contaminant remediation. • Hydroxyapatite was incorporated into phosphoric acid-activated geopolymer. • 50 wt% HAP improved surface properties and compressive strength, surpassing PAGP. • HAP enhanced the composite adsorption effectiveness for Congo red dye removal. • Regeneration and reusability revealed high removal capacity up to 10 cycles. • DFT studies: protonated CR dye boosts reactivity, promoting adsorption.