Adsorption of phenol over bio-based silica/calcium carbonate (CS-SiO ₂ /CaCO ₃ ) nanocomposite synthesized from waste eggshells and rice husks

A bio-based Silica/Calcium Carbonate (CS–SiO 2 /CaCO 3 ) nanocomposite was synthesized in this study using waste eggshells (ES) and rice husks (RH). The adsorbents (ESCaCO 3 , RHSiO 2 and, CS-SiO 2 /CaCO 3 ) characterized using XRD show crystallinity associated with the calcite and quartz phase. The FTIR of ESCaCO 3 shows the CO −2 3 group of CaCO 3, while the spectra of RHSiO2 majorly show the siloxane bonds (Si–O–Si) in addition to the asymmetric and symmetric bending mode of SiO 2 . 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–SiO 2 /CaCO 3 nanocomposite shows the spectra pattern associated with ESCaCO 3 and RHSiO 2. The FESEM micrograph shows a near monodispersed and spherical CS–SiO 2 /CaCO 3 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–SiO 2 /CaCO 3 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 ( R 2 ) of 0.99 was observed in CS-SiO 2 /CaCO 3 nanocomposite, followed by RHSiO 2 and ESCaCO 3 . The result shows the equilibrium data for all the adsorbents fitting well to the Langmuir isotherm model, and follow the trend CS-SiO 2 /CaCO 3 > ESCaCO 3 > RHSiO 2 . The Langmuir equation and Freundlich model in this study show a higher correlation coefficient ( R 2 = 0.9912 and 0.9905) for phenol adsorption onto the CS–SiO 2 /CaCO 3 nanocomposite with a maximum adsorption capacity ( q m ) of 14.06 mg/g compared to RHSiO 2 (10.64 mg/g) and ESCaCO 3 (10.33 mg/g). The results suggest good monolayer coverage on the adsorbent’s surface (Langmuir) and heterogeneous surfaces with available binding sites (Freundlich).