Adsorption of phenol over bio-based silica/calcium carbonate (CS-SiO2/CaCO3) nanocomposite synthesized from waste eggshells and rice husks

A bio-based Silica/Calcium Carbonate (CS–SiO2/CaCO3) nanocomposite was synthesized in this study using waste eggshells (ES) and rice husks (RH). The adsorbents (ESCaCO3, RHSiO2 and, CS-SiO2/CaCO3) characterized using XRD show crystallinity associated with the calcite and quartz phase. The FTIR of ESCaCO3 shows the CO−23 group of CaCO3, while the spectra of RHSiO2 majorly show the siloxane bonds (Si–O–Si) in addition to the asymmetric and symmetric bending mode of SiO2. The spectra for Chitosan (CS) show peaks corresponding to the C=O vibration mode of amides, C–N stretching, and C–O stretching. The CS–SiO2/CaCO3 nanocomposite shows the spectra pattern associated with ESCaCO3 and RHSiO2. The FESEM micrograph shows a near monodispersed and spherical CS–SiO2/CaCO3 nanocomposite morphology, with an average size distribution of 32.15 ± 6.20 nm. The corresponding EDX showed the representative peaks for Ca, C, Si, and O. The highest removal efficiency of phenol over the adsorbents was observed over CS–SiO2/CaCO3 nanocomposite compared to other adsorbents. Adsorbing 84–89% of phenol in 60–90 min at a pH of 5.4, and a dose of 0.15 g in 20 ml of 25 mg/L phenol concentration. The result of the kinetic model shows the adsorption processes to be best described by pseudo-second-order. The highest correlation coefficient (R2) of 0.99 was observed in CS-SiO2/CaCO3 nanocomposite, followed by RHSiO2 and ESCaCO3. The result shows the equilibrium data for all the adsorbents fitting well to the Langmuir isotherm model, and follow the trend CS-SiO2/CaCO3 > ESCaCO3 > RHSiO2. The Langmuir equation and Freundlich model in this study show a higher correlation coefficient (R2 = 0.9912 and 0.9905) for phenol adsorption onto the CS–SiO2/CaCO3 nanocomposite with a maximum adsorption capacity (qm ) of 14.06 mg/g compared to RHSiO2 (10.64 mg/g) and ESCaCO3 (10.33 mg/g). The results suggest good monolayer coverage on the adsorbent’s surface (Langmuir) and heterogeneous surfaces with available binding sites (Freundlich).