Rice Husk Ash–Derived Silica Particles for the Adsorptive Removal of Acetylsalicylic Acid From Water: Synthesis, Characterization, Isotherm, Kinetic, and Thermodynamic Studies

ABSTRACT The aim of this work is to investigate the adsorption of Aspirin or Acetylsalicylic acid (ASA) present in an aqueous solution, on SiO 2 , TiO 2 , and TiO 2 /SiO 2 nanostructures. TiO 2 /SiO 2 nanostructures were synthesized using rice husk ash (RHA) as the SiO 2 source and titanium tetrachloride (TiCl 4 ) as a precursor for TiO 2 . Nanostructures of TiO 2 , SiO 2 , and TiO 2 /SiO 2 , using XRD, FT‐IR, BET, SEM, and Zeta Potential equipment, were characterized. The effect of pH (pH 1, 4, 7, 12), initial ASA concentration (3.5‐5.5 ppm), temperature (25°C, 35°C, 50°C), adsorbent dose (0.1‐3.0 g L −1 ), and adsorption time (0‐24 h) on the adsorption process has been examined. The most favorable adsorption of ASA was observed at acidic pH, that is especially pH 4. Using an initial concentration of 5.5 ppm ASA solution and a dose of 0.75 g L −1 SiO 2 , TiO 2 , and TiO 2 /SiO 2 nanostructures, the maximum removal of ASA was 38.08%, 21.89%, and 23.83%, respectively. Experimental data were fitted to the Langmuir (LIM), Freundlich (FIM), Tempkin (TIM), and Dubinin‐Radushkevic (DRIM) adsorption isotherm models. The two‐parameter isotherms that provide the best match and have the highest correlation coefficient are, in order of precision: LIM > FIM > TIM > DRIM. Maximum adsorption capacities ( q m ) of SiO 2 , TiO 2 , and TiO 2 /SiO 2 were calculated as 3.42, 2.81, and 3.35 mg g − 1 , respectively. To test the data obtained from these isotherm models, several error functions, that is Marquardt's Percent Standard Deviation (MPSD), Hybrid error function (HYBRID), Sum of the Absolute Errors (SAE), Average Relative Error percentage (ARE), and Sum of Square Error (SSE), were also used. As all error functions are compared, the Langmuir (LIM) adsorption isotherm model fits the experimental data quite well. Adsorption kinetics of ASA was performed with pseudo‐first‐order (PFOM), pseudo‐second‐order (PSOM), Elovich (EM), and Weber‐Morris (WMM) models. Pseudo‐second‐order model, PSOM (R 2 = 0.995‐0.998) is found to be the most compatible with the experimental data. Thermodynamic studies on ASA removal elucidated that ASA removal using TiO 2 as an adsorbent is an exothermic and spontaneous process, while it is endothermic and nonspontaneous when SiO 2 or TiO 2 /SiO 2 is used as an adsorbent.