Solid phase extraction of hazardous metals from aqua system by nanoparticle-modified agrowaste composite adsorbents

This study reports for the first time the adsorption of Cd(II) and Hg(II) ions from simulated wastewaters onto Nauclea diderrichii agrowaste (NDS) modified with mesoporous SiO2 (MND) and mesoporous SiO2 + graphene oxide (GND). Modification of NDS with mesoporous SiO2 and graphene oxide improved its adsorption capacity for Cd(II) and Hg(II) in aqueous solution with surface area increasing from 5.4 m2/g (in NDS) to 209 m2/g (in MND) and 267 m2/g (in GND) respectively. The modification of NDS also improved its rate of uptake of both metal ions with uptake rate of Hg(II) being more than that of Cd(II). Kinetic data obtained gave better fit to the pseudo-second order kinetic model than to pseudo-first order, Elovich, modified pseudo-first order, ion exchange or Weber–Morris intraparticle diffusion kinetic models. Data obtained from the Weber–Morris intraparticle diffusion model suggests that pore diffusion mechanism may have played a significant role in the adsorption process. Thermodynamic data showed negative and positive values for ?G° and ?H° respectively for the adsorption systems, which supports the fact that the adsorption of both Cd(II) and Hg(II) by MND and GND adsorbents was feasible, spontaneous and endothermic in nature.