Adsorption of Phenol from Wastewater Using Calcined Magnesium-Zinc-Aluminium Layered Double Hydroxide Clay

The presence of priority and emerging aromatic-based pollutants in water sources is of growing concern as they are not bioavailable and are present in reuse plant feed streams. These pollutants have known mutagenic and carcinogenic effects and must therefore be removed. Adsorption has been widely accepted as a suitable remediation technology due to its simplicity. Clay-based adsorbents have attracted significant attention due to their low cost, environmentally benign properties and regeneration potential. The present work focused on the thermal modification of a commercial Layered Double Hydroxide (LDH) clay and its subsequent effectiveness as an adsorbent in the removal of phenol from wastewater. Calcination of the neat clay resulted in the formation of metal oxides with varying phases and crystallinity depending on the treatment temperature. The BET surface area increased by 233% upon calcination at 500 °C. The highest phenol removal (85%) was observed in the clay calcined at 500 °C compared to 10% for the neat clay. Optimization studies revealed a maximum adsorption capacity of 12 mg/g at an adsorbent loading of 10 g/L at pH 7. Phenol adsorption was postulated to occur via a two-stage intercalation and surface adsorption mechanism. The equilibrium data were best fitted on the Freundlich isotherm model which describes heterogeneous adsorption. The adsorption kinetics followed a pseudo-second-order kinetic model with rate constants of 4.4 x 10 −3 g/mg.h for the first 12h and 6.1 x 10 −3 g/mg.h thereafter.