Comparison kinetics studies of Cu(II) adsorption by multi-walled carbon nanotubes in homo and heterogeneous systems: Effect of nano-SiO2

• Cu(II) adsorption kinetics by CNTs and CNTs–SiO2 mixture were investigated.
• Cu(II) adsorption kinetic data were well described by pseudo-second-order kinetics.
• Hydroxy CNTs, with higher k2, adsorb Cu(II) ion much faster than carboxyl CNTs.
• The addition of nano-SiO2 accelerates the Cu(II) adsorption.
• This effect is related to adsorption mechanisms of Cu(II) by CNTs and SiO2.

The adsorption kinetics of Cu(II) in homogeneous systems of CNTs and heterogeneous systems of CNTs and nano-SiO2 were investigated using ion selective electrode potentiometry. Particular attention was paid on the role of nano-SiO2 in the adsorption kinetics of Cu(II) by CNTs. It was found that the Cu(II) adsorption by CNTs and CNTs–SiO2 mixture was well described by pseudo-second-order kinetics. In homogeneous systems, the equilibrium time of Cu(II) adsorption decreases and the pseudo-second-order rate constant (k2) increases with increasing CNTs concentrations. Comparing the different types of CNTs, hydroxy CNTs (CNTs-OH), with higher k2 values, adsorb Cu(II) ion much faster than carboxyl CNTs (CNTs-COOH). This may be ascribed to their different adsorption mechanisms. The electrostatic attraction is the primary mechanism of Cu(II) adsorption by CNTs-OH, while Cu(II) adsorbed on CNT-COOH through complexing with the carboxyl groups on the surface of CNTs besides electrostatic attraction. In heterogeneous systems of CNTs and nano-SiO2, the Cu(II) adsorption reaches equilibrium much faster than in homogeneous systems of CNTs, indicating nano-SiO2 accelerates the Cu(II) adsorption. This acceleration effect is possibly related to the adsorption mechanism (electrostatic attraction) and higher BET surface area of nano-SiO2. FTIR and XPS spectra demonstrate the binding of nano-SiO2 and CNTs, and CNTs and Cu(II). The present results highlight the significant influence of nano-SiO2 on the Cu(II) adsorption by CNTs. Hence, it is critical to consider these effects in modeling and predicting the adsorption kinetics of metals by CNTs in heterogeneous systems.