Synthesis of alkali-activated volcanic scoria and rice husk ash based composite materials for adsorptive removal of crystal violet: Optimization, kinetics, isotherms and mechanism

In this study, central composite design (CCD) was used to optimize the porosity structure of NaOH-activated volcanic scoria (VSc) and rice husk ash (RHA) composite materials. The synthesis conditions, namely VSc and RHA fractions by mass and NaOH concentration were used as the input (independent) variables and the porosity parameters, iodine index and methylene blue index, as the output variables. The optimum conditions that gave the material, CM, with the highest iodine index and methylene blue index, and hence porosity structure, were attained at 0.67 VSc (2g)/RHA (3g) ratio by mass and 5 M NaOH. The CM and precursors were characterized using FTIR, SEM, pHpzc, Methyl Blue index, and Iodine index. The CM and the precursor VSc were used to sorb crystal violet (CV) dye from synthetic wastewater. The pseudo-second-order kinetic and Langmuir models best-described CV adsorption by both materials. The adsorption mechanism was controlled by Coulombic interactions. At an initial CV concentration of 20–60 mg/L, 0.1g/30 mL adsorbent dosage, and pH of 6 at 25 °C, the CM had a slightly higher (∼1.13 times) adsorption capacity (11.06 mg/g) for the uptake of crystal violet dye than the precursor VSc (9.75 mg/g) consistent with their methylene blue indices. Electrostatic interactions and ion exchange are proposed dominant adsorption mechanisms. The porosity and adsorption performance of the VSc-RHA alkali-activated materials are shown to be strongly and significantly dependent on the RHA fraction and NaOH concentration relative to VSc.