Sistema híbrido eletroquímico–fotoquímico para a mineralização do herbicida tebutiuron
Abstract
An electrochemical-photochemical system (EC-PC)
using a dimensionally stable anode (DSA®) was used to oxidize and mineralize
tebuthiuron (TBT) herbicide, which is a potential contaminant to the surface and
ground water. The electrochemical process was conducted using a filter-press flow
cell and the photochemical one by irradiation of the solution inside the reservoir using
Hg vapor lamps (type UVC). The studied variables were: i) nominal power of Hg
lamps (5, 9, 80, and 125 W), ii) pH (3, 7, 11, and no control), iii) NaCl concentration
(0, 1, 2, and 4 g L–1), and iv) current density (10, 20, and 30 mA cm–2). Other
parameters were kept constant, such as the flow rate (420 L h–1) and the solution
temperature (25 °C). The electrolyzed solution was composed of 100 mg L–1 TBT
and 0.1 mol L–1 Na2SO4, as supporting electrolyte. The oxidation and mineralization
performances of the TBT and its intermediates using the hybrid process were
assessed through high performance liquid chromatography coupled to mass
spectrometry (HPLC-MS/MS), depending on the experiment, and total organic carbon
(TOC) analyses. The experiment using a 9 W Hg lamp led to the complete oxidation
and mineralization of TBT and its intermediate compounds in the presence of NaCl
from acidic to neutral solutions, including the one with no control, and independently
of the applied electrical current density. High CO2 conversions were attained after 6 h
using the combined EC-PC process, due to the generation of hydroxyl radicals (HO•)
from HOCl homolysis. The HO• also favored the formation of many hydroxylated
organic compounds. Despite the formation of an organochlorine byproduct, this
compound was completely eliminated after 3 h treatment. Moreover, no toxicity was
observed when using the Artemia salina microcrustacean. Similar removal rates of
TBT were attained when only using an electrochemical method with boron-doped
diamond as anode. In addition, the EC-PC process showed a slightly higher electrical
energy consumption for the removal of organic matter in comparison to the
electrochemical one. Finally, only a chlorinated carboxylic acid was detected;
however, this compound was completely eliminated in the final stages of the EC-PC
treatment. All these features enables the classification of this method as an advanced
oxidation process.