Hydroxyapatite incorporated metakaolin/sludge based geopolymer adsorbent for copper ions and ciprofloxacin removal: Synthesis, characterization and mechanisms

The efficacy of copper Cu(II) adsorption is significantly affected by the presence of antibiotics, such as ciprofloxacin (CIP). Therefore, researchers are highly interested in conducting extensive investigations on the simultaneous adsorption of Cu(II) and CIP. However, most of the adsorbents exhibited low adsorption capacity of CIP with increasing Cu(II) concentration due to the competition for adsorption sites. Hence, the integration of various adsorbents into a single composite could be an effective way to increase the adsorption sites. Thus, this study aims to incorporate hydroxyapatite (Hap) into metakaolin/sludge based geopolymer adsorbent for simultaneous adsorption of Cu(II) and CIP. The effect of different filler loading of Hap (1–3 %) on the metakaolin/sludge geopolymerization and also on the removal efficiency of Cu(II) and CIP were studied in a single and binary system. Moreover, the effects of varied concentrations of Cu(II) (0–100 mg/L) on the removal efficiency of CIP were investigated. Characterization techniques such as x-ray diffraction (XRD), fourier-transform infrared spectrometry (FTIR), scanning electron microscopy (SEM), brunauer-emmett-teller (BET) and neutron tomography imaging were employed to characterize the physicochemical properties of the synthesized geopolymer. It was found that the Hap content has a significant impact on the removal efficiency of CIP and Cu(II). The addition of 2 % Hap providing more nucleation site for the increasing geopolymerization (C-A-S-H) and silicoalumino phosphate gel (SAP) leading to the formation of highly cross-linked geopolymer network and abundant active sites which would favour the adsorption. Moreover, the removal efficiency of CIP by 2 % Hap-geopolymer increased (25.6 % to 61.51 %) with increasing Cu(II) concentration by the complexation and bridging effect between Cu(II) and CIP resulting in the formation of GMK25S1-2Hap-Cu(II)-CIP complexes. Therefore, the hybrid method of geopolymer and Hap is an exceptionally efficient approach for the treatment of wastewater that comprises Cu(II) and CIP.