Adsorption of textile dyes on hydroxyapatite based adsorbent: A review of surface functionality and adsorption mechanism

Contamination of water bodies due to industrial effluents is prohibiting its potability. To address this global challenge, the United Nations (UN) Sustainable Development Goal 6 (SDG-6) emphasizes obtaining pure and safe water resources. Access to pure water will guarantee SDG-3, which ensures healthy lives and promotes well-being for all. Adsorption is an abundantly adopted technique to remove contaminants from waterbodies. The interfacial interaction between the adsorbate and adsorbent, plays an important role in binding. Therefore, adsorbents are judiciously designed to generate the selective and specific sites to bind the adsorbate. Tuning of the surface functionalities of adsorbent eventually enhances its selectivity toward the adsorbate and adsorption capacity. This review attracts attention towards strategically improvised HAp-based adsorbents, which are tested with several textile dyes like Direct Red, Congo Red, Brilliant Green, Coomassie Brilliant Blue, Methylene Blue and Brilliant Yellow. HAp as a composite material, nanoparticles, microspheres, sodium metasilicate-treated, with modulated surface functional groups, offers remarkable surface area, pore volume and density, facilitate adsorption capacity and impart thermal stability. The surface morphologies are analyzed with X-ray Photoelectron Spectroscopy (XPS), Energy Dispersive X-ray (EDX), Scanning Electron Microscopy (SEM), X-ray Fluorescence (XRF), Fourier Transform Infrared (FTIR), Raman spectroscopy, Thermogravimetry. Notably, the analysis of surface, sheds light on the role of pH in ionizing the surface, which is evident from zeta potential measurement. The adsorption involves interactions like hydrogen bonding, electrostatic interaction, Vander Waals interaction, chelation, ion exchange, n-π interaction, etc.; such interactions are important where the adsorbent picks the dyes from contaminated aqueous medium.