Removal of herbicide (diquat dibromide) from aqueous solution by using carbon nanotube based adsorbents

In this work, the adsorption of diquat dibromide which is a toxic herbicide and commonly found in wastewater, from aqueous solution onto carbon nanotube based adsorbents was studied. Firstly, oxidized multiwalled carbon nanotube (OMWCNT), magnetic oxidized multiwalled carbon nanotube (OMWCNT-Fe3O4) and magnetic oxidized multiwalled carbon nanotube-κ-carrageenan (OMWCNT-κ-carrageenan-Fe3O4) nanocomposite adsorbents were prepared. The effect of contact time, initial herbicide concentration and temperature on the adsorption process was investigated and an increase in the amount of herbicide adsorbed by the carbon nanotube based adsorbents was occurred with an increase in these parameter values. In all the systems studied, adsorption equilibrium was obtained at the end of 300 min contact time. The fit of experimental adsorption kinetic data with pseudo-first order, pseudo-second order and Elovich kinetic models was analyzed and it was found that the experimental kinetic data of the adsorption of diquat dibromide onto carbon nanotube based adsorbents followed the pseudo-second order kinetic model. Experimental isotherm data obtained at 25, 35 and 45 °C exhibited the best fitting with Langmuir isotherm model. As a result of the studies, it was seen that the adsorbent having the highest diquat dibromide adsorption capacity was OMWCNT. At 25 °C, maximum diquat dibromide adsorption capacity values were found to be 16.09x10-5 mol.g-1 for OMWCNT, 5.784x10-5 mol.g-1 for OMWCNT-Fe3O4 and 2.954x10-5 mol.g-1 for OMWCNT-κ-carrageenan-Fe3O4. The results of thermodynamic studies showed that adsorption processes are favorable, spontaneous and endothermic.