Sulfur-impregnated porous carbon for removal of mercuric chloride: optimization using RSM

This study relates to optimization of process conditions for sulfur impregnation process on to a virgin steam-activated commercial carbon, with the objective of maximizing the mercuric chloride adsorption. Process optimization was performed using response surface methodology adopting Box–Behnken design. The chosen key process parameters being impregnation temperature (IT), carbon to sulfur ratio (CSR), and impregnation time (ITE), with lower and upper bounds for process conditions being 250–600 °C IT, 0.5–4 CSR, and 15–45 min ITE. The optimum conditions were identified to be an IT of 544 °C, CSR of 0.53 and ITE of 43 min with the HgCl₂ adsorption capacity of 85 mg/g. The optimized process conditions were further ensured with repeat runs with reproducible results within acceptable variation. Langmuir and Freundlich equilibrium adsorption isotherm models fitted well with the experimental data. The maximum monolayer adsorption capacity was found to be 294 mg/g which was quite high as compared with the maximum reported in literature. Further, the SEM–EDX analysis evidence the higher amount of sulfur and uniform distribution of sulfur over the surface of activated carbon.