Optimization of Au(III) adsorption by the Taguchi method using pyrogallol functionalized silica nanoparticles

Pyrogallol molecules were successfully immobilized onto aminopropyl molecule functionalized MCM41 nanoparticles to obtain a fast and high gold adsorption capacity. The Taguchi statistical method was used to determine the factors affecting the gold(III) adsorption efficiency. The effect of six factors, pH, rate, adsorbent mass, temperature, initial Au(III) concentration and time, each with 5 levels, on the adsorption capacity was investigated by forming an L25 orthogonal. The analysis of variance (ANOVA) of each factor showed that all factors had significant effects on adsorption. pH 5, 250 rpm stirring speed, 0.025 g adsorbent mass, 40 °C temperature, 600 mg L^-1 Au(III) concentration and 15 min time were determined to be the optimum adsorption conditions. The maximum Langmuir monolayer adsorption capacity of APMCM1-Py for Au(III) was calculated to be 168.54 mg g^-1 at 303 K. The adsorption mechanism fits the pseudo-second-order kinetic model assuming the formation of a single chemical adsorption layer on the adsorbent surface. The adsorption isotherms are best represented using the Langmuir isotherm model. It exhibits a spontaneous endothermic behavior. FTIR, SEM, EDX and XRD analyses showed that mostly phenolic -OH functional groups adsorb Au(III) ions on the APMCMC41-Py surface with their reducing character. These results enable the rapid recovery of gold ions from weakly acidic aqueous solutions by reduction of APMCM41-Py NPs.