Cadmium(II) adsorption using functional mesoporous silica and activated carbon

Highlights: ► Mono-amino-functional groups were found better than di- and tri-amino for Cd(II) adsorption. ► Amino-functional group would serve as a basic adsorption site for heavy metal ions. ► Mercapto- and carboxyl-groups were found to behave as ion exchange sites for proton. ► Introduction of amino- and mercapto-groups onto silica was proven by XPS and FT-IR measurements. - Abstract: The role of surface functionality on silica and carbonaceous materials for adsorption of cadmium(II) was examined using various mesoporous silica and activated carbon. Silica surfaces were principally functionalized by mono-amino- and mercapto-groups, while carboxylic group was introduced to the activated carbons by oxidation. Functional groups on silica surface were formed using grafting and co-condensation techniques in their preparation. Mono-amino group was found more effective than di- and tri-amino groups for cadmium(II) adsorption on the grafted silica. Mono-amino groups prepared by co-condensation adsorbed cadmium(II) as much as 0.25 mmol/g compared to mercapto- and carboxyl-groups which adsorbed around 0.12 mmol/g, whereas Langmuir adsorption affinities were as strong as 50–60 L/mmol for all of the three functions. The working pH range was wider for mercapto- and carboxyl-functions than for amino-group. Basic site could be an adsorption center for amino-functional groups while ion exchange sites were found to work for the mercapto- and carboxyl-functions to adsorb cadmium(II) from aqueous phase. Based on the experimental results, surface functional groups rather than structure of silica and carbon seemed to play a decisive role for cadmium(II) adsorption.