First introduced in the 1990s, Neonicotinoids are popular due to properties like broad-spectrum insecticidal activity, persistence, and systemic nature,which make them the most widely used insecticides today. Thiamethoxam (TMX) was the first second-generation neonicotinoid registered for use. TMX is highly effective at low concentrations, can be applied in various ways, has long-lasting residual effects, and its chemical structure gives high water solubility.
Concerns over water contamination from pesticides have spurred development of treatment technologies like advanced oxidation processes (AOPs). AOPs degrade pollutants rather than concentrating or transferring them. Among AOPs, electrochemical oxidation uses no chemicals. New anode materials like boron-doped diamond (BDD) enable oxidation of many pollutants. BDD is non-active, has high overpotential for oxygen evolution, and weakly absorbs electrogenerated hydroxyl radicals (•OH). So, it readily degrades organics to mineralization with high current efficiency.
Experiments used BDD anode and stainless steel cathode in 150 mL cell, with magnetic stirring. Sulfuric acid (H2SO4) or sodium hydroxide (NaOH) are used to adjust pH value; sodium sulfate (Na2SO4) or sodium chloride (NaCl) were supporting electrolyte. Constant 0.2 A current was applied. Anode material influences mechanism, selectivity, and efficiency.
Platinum (Pt), stainless steel (SS) and BDD anodes were compared for oxidizing 2 mg/L TMX in 0.1 M Na2SO4 at 16 mA/cm2. SS and Pt were slightly more effective at higher PH. Pt was only 16% more active than SS after 30 min. But BDD was most efficient, generating •OH radicals more effectively than higher oxides (MOx+1) from Pt. Beyond chemical stability and low background current, BDD has higher oxygen overpotential. BDD effectively degraded and mineralized 10 mg/L lindane than Pt, so BDD was used thereafter. Current density controls •OH generation rate.
•OH forms on BDD via:
BDD + (H2O) → BDD(•OH) + H+ + e−
TMX reduction was 64% and 91% after 30 and 120 min at 2 mg/L, but 49% and 76% at 10 mg/L, as with TMX concentration. BDD effectively degraded TMX formulations. Conditions like current density and electrolyte concentration positively impacted kinetics, up to 40 mA/cm2 and 0.1 M Na2SO4. BDD also mineralized to CO2 and H2O. Using high pollutant concentrations introduces “matrix effects.” TMX degradation rate constant decreased 10-fold as concentration increased 10-fold. Anodic oxidation shows promise for water/wastewater treatment without chemicals. Non-active anodes like BDD fully oxidize organics, avoiding secondary pollution. Depending on economics, electrochemical treatment could oxidize most of the persistent pollutants into more biodegradable byproducts for biological treatment.
In summary, BDD electrochemical oxidation effectively degrades and mineralizes TMX and its formulations under appropriate conditions without additional chemicals, though economics may favor partial oxidation for biological post-treatment. Reaction rates slow at higher concentrations due to matrix effects. Anodic oxidation with BDD is a promising technology for eliminating aqueous pesticide contamination.
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