Kinetic, Equilibrium and Isotherm Studies of Cadmium Removal from Aqueous Solutions by Oxidized Multi-Walled Carbon Nanotubes and the Functionalized Ones with Thiosemicarbazide and Their Toxicity Investigations: A Comparison

The multi-walled carbon nanotubes (MWCNTs) were first oxidized by nitric acid to form a MWCNT-COOH. Then, it was modified with thiosemicarbazide to produce MWCNT-semi. Thus, these carbon materials, MWCNTs, MWCNT-COOH and MWCNT-semi, have been used as efficient adsorbents for the removal of cadmium from aqueous solutions. The influence of variables including pH, concentration of the cadmium, amount of adsorbents and contact time was investigated by the batch method. The kinetic studies were carried out using different kinetic models such as pseudo-first-order, pseudo-second-order, and intra-particle diffusion models. The sorption process with each adsorbent was well described by pseudo-second-order kinetics which was agreed well with the experimental data. The values of regression coefficient of various adsorption isotherms like Langmuir, Freundlich and Tempkin to obtain the characteristic parameters of each model have been carried out. The results showed that the Langmuir isotherm for all adsorbents and Tempkin model for MWCNT-COOH and MWCNT-semi were found to best represent the measured sorption data. Toxicity of these samples was evaluated in human embryonic kidney HEK293 and human breast cancer SKBR3 cell lines after culturing cells for 24 h. Cellular investigations showed that the modified MWCNTs in particular MWCNT-semi have considerably significant toxic impact on SKBR3 as compared to HEK293 at concentration 3 mu g/mL.