Kinetic study of defluoridation of drinking water by electrocoagulation/electroflotation in a stirred tank reactor and in an external-loop airlift reactor

A kinetic study of defluoridation of drinking water was carried out using the electrocoagulation/electroflotation process in two batch reactors of identical volume (20 L): a stirred tank reactor (STR) and an external-loop airlift reactor (ELALR). When the evolution of fluoride content was independent of stirring speed, experimental results showed that the kinetics of fluoride removal could be modelled using a variable-order-kinetic (VOK) approach coupled with a Langmuir–Freundlich adsorption model in the STR. Conversely, when mixing was less efficient, which is the case in the ELALR, experimental data could be fitted adequately only using a pseudo-first-order model. This constitutes however only an empirical approach based on a lumped parameter that accounts simultaneously for mass transfer, adsorption and electrochemical steps. In this case, only regression analysis could be used to establish a quantitative relationship between the kinetic constant and the operating conditions, such as current density and initial fluoride concentration.