Degradação do Herbicida Diuron em um Reator Tubular Eletroquímico

Hydrodynamics, mass transfer [1] and reactions kinetics [2] studies become necessary to optimize the cost of liquid wastes treatment in a reactor. With this in mind, diuron was utilized in this work because this herbicide is often employed for Brazilian farmers. Unfortunately, its persistence in soils and waters is noticed: its natural degradation by non-specific decomposing environmental microorganisms is very slow [3]. By the way, a concentric cylindrical reactor with tangential entrance of 0.54 L and 12 cm high is performed in this work. It possesses an outer three-dimensional Inox 316® steel cathode of 8 mm of external diameter and 603 cm2 of electroactive area (Ae) and an inner two-dimensional DSA® anode made of Ti/Ru0.36Ti0.64O2 of 6 cm of internal diameter and 226 cm2 of Ae. Consequently, a geometric value for the specific area (As) was fixed at 0.42 cm-1. After that, spectrophotometric determinations using the Black 5 dye as a tracer leaded to the hydrodynamics characterization. The residence time distributions (DTR) provided values for Péclet number (Pe) of 3.7 for 10 L/h and 5.2 for 50 L/h. Concerning to the mass transfer, linear sweep voltammograms were applied over a ferricyanide/ferrocyanide system and current versus potential graphs were constructed. Mass transfer coefficients (Km) of 1∙10-6 m/s for 10 L/h and 2∙10-6 m/s for 50 L/h were calculated from the limiting current. Additionally, an empiric equation Sh = 0.7Re0.41Sc0.33 was obtained for working with an incompressible and Newtonian fluid. Sh, Re and Sc are Sherwood, Reynolds and Schmidt numbers respectively. Thus, kinetics research required 4 L effluent containing commercial diuron Nortox® 216 mg/L flowed at 10 and 50 L/h for 180 min in the presence of NaCl 0.05 M as the supporting electrolyte in a closed circuit. pH 3 and room temperature were established too. Then, a density of current of 30 mA/cm2 was applied to degrade diuron and total organic carbon (TOC) was followed. These experimental kinetics data were analyzed theoretically with the help of As, DTR and Km values indicated above. In the end, a plug flow reactor with a large dispersion model would be closer to the reality of the present reactor, a laminar regime may be deduced as well from the empiric equation and TOC measurements suggested that mineralization was incomplete although diuron conversion was concluded at 3 h.