Ecotoxicological Evaluation of Methiocarb Electrochemical Oxidation

At a Glance

Metadata	Details
Publication Date	2020-10-22
Journal	Applied Sciences
Authors	Annabel Fernandes, Christopher Pereira, Susana Coelho, Celso Afonso Ferraz, Ana C. A. Sousa
Institutions	University of Évora, University of Aveiro
Citations	5
Analysis	Full AI Review Included

Executive Summary

This study evaluates the efficiency of Electrochemical Oxidation (EO) using a Boron-Doped Diamond (BDD) anode for degrading the highly hazardous pesticide Methiocarb (MC) and reducing its acute ecotoxicity in aqueous solutions.

Core Value Proposition: EO with BDD provides a suitable and highly efficient solution for the complete elimination of MC and its undesirable ecological effects from contaminated industrial or agricultural wastewaters.
Toxicity Reduction: The optimal treatment reduced acute toxicity towards Daphnia magna by over 200x, dropping from a highly toxic classification (370.9 Toxic Units, TU) to near non-toxic levels (1.6 TU).
Electrolyte Influence (NaCl vs. Na₂SO₄): The presence of NaCl significantly accelerated MC degradation and toxicity reduction due to indirect oxidation by active chlorine species generated at the BDD anode.
Kinetic Performance: Degradation followed first-order kinetics, with the fastest rate constant (k) reaching 1.7 x 10^-3 s^-1 in the chloride medium (300 A m^-2).
Nitrogen Removal: Only the chloride-containing solutions achieved substantial Total Nitrogen (TN) removal (>50%), facilitated by the active chlorine species oxidizing ammonium (NH₄⁺) to N₂ gas and nitrate.
Current Density Trade-offs: Increasing the applied current density (j) enhanced TN removal but generally decreased energy efficiency for TOC removal and, in chloride solutions, risked increasing final ecotoxicity due to the formation of perchlorate (ClO₄^-) and organochlorinated compounds.

Technical Specifications

Parameter	Value	Unit	Context
Anode Material	Si/BDD	N/A	Commercial Boron-Doped Diamond
Electrode Area	10	cm²	Immersed area for both anode and cathode
Initial MC Concentration	20	mg L^-1	Concentration used in all EO assays
Supporting Electrolyte Conc.	250	mg L^-1	Minimum required concentration (NaCl or Na₂SO₄)
Applied Current Density (j)	100 and 300	A m^-2	Experimental variables
Optimized Applied Charge	1.8	kC	5 h treatment at 100 A m^-2 (NaCl)
Initial Acute Toxicity (NaCl)	370.9	TU	Highly toxic classification
Final Acute Toxicity (Optimized)	1.6	TU	Achieved after 1.8 kC treatment (NaCl)
Fastest Kinetic Constant (k)	1.7 x 10^-3	s^-1	NaCl, 300 A m^-2 (First-order MC decay)
Slowest Kinetic Constant (k)	0.28 x 10^-3	s^-1	Na₂SO₄, 100 A m^-2
Maximum TN Removal	>50	%	Achieved using NaCl electrolyte
Toxicity Test Organism	Daphnia magna	N/A	Freshwater crustacean (model species)

Key Methodologies

The electrochemical oxidation (EO) experiments were conducted in a batch setup to evaluate the degradation kinetics and toxicity reduction of Methiocarb (MC) solutions.

Electrochemical Setup: Experiments utilized an open, undivided, cylindrical glass cell (250 mL capacity) containing 200 mL of solution, maintained at room temperature (22-25 °C) with stirring (250 rpm).
Electrode Configuration: A commercial Si/BDD anode and a stainless-steel cathode, each with an immersed area of 10 cm², were placed in parallel with an inter-electrode gap of 0.3 cm.
Solution Preparation: MC solutions (20 mg L^-1) were prepared with ultrapure water and supplemented with 250 mg L^-1 of either NaCl or Na₂SO₄ to ensure adequate electrical conductivity (approx. 500 µS cm^-1).
Electrolysis Parameters: Assays were run at two applied current densities (j): 100 A m^-2 and 300 A m^-2. Total applied charges ranged from 1.8 kC (5 h at 100 A m^-2) to 3.24 kC (3 h at 300 A m^-2).
Analytical Characterization: Samples were analyzed hourly for MC concentration (HPLC), Total Organic Carbon (TOC), Total Nitrogen (TN), Chemical Oxygen Demand (COD), pH, and conductivity.
Ecotoxicity Assessment: Acute toxicity was measured using the freshwater crustacean Daphnia magna (neonates <24 h old) following OECD Guideline 202. The median effective concentration (EC₅₀) was calculated after 48 h exposure.

Commercial Applications

The use of Boron-Doped Diamond (BDD) anodes in Electrochemical Oxidation (EO) is highly relevant for treating complex, biorefractory, and toxic industrial effluents.

Agricultural Wastewater Treatment: Direct application for treating runoff or concentrated wash waters contaminated with highly toxic carbamate pesticides like Methiocarb.
Pesticide Manufacturing Effluents: Remediation of high-concentration wastewater streams generated during the synthesis and formulation of hazardous chemicals.
Industrial Water Reuse: Pre-treatment step for industrial water recycling systems, ensuring that persistent organic pollutants (POPs) are mineralized before water is reused or discharged.
Pharmaceutical and Chemical Processing: Degradation of other emerging contaminants (ECs) and biorefractory compounds that are resistant to conventional biological or chemical treatments.
Advanced Oxidation Processes (AOPs): Implementation in modular, decentralized WWT systems where high oxidation power and minimal sludge production are required.

View Original Abstract

The ecotoxicity of methiocarb aqueous solutions treated by electrochemical oxidation was evaluated utilizing the model organism Daphnia magna. The electrodegradation experiments were performed using a boron-doped diamond anode and the influence of the applied current density and the supporting electrolyte (NaCl or Na2SO4) on methiocarb degradation and toxicity reduction were assessed. Electrooxidation treatment presented a remarkable efficiency in methiocarb complete degradation and a high potential for reducing the undesirable ecological effects of this priority substance. The reaction rate followed first-order kinetics in both electrolytes, being more favorable in a chloride medium. In fact, the presence of chloride increased the methiocarb removal rate and toxicity reduction and favored nitrogen removal. A 200× reduction in the acute toxicity towards D. magna, from 370.9 to 1.6 toxic units, was observed for the solutions prepared with NaCl after 5 h treatment at 100 A m−2. An increase in the applied current density led to an increase in toxicity towards D. magna of the treated solutions. At optimized experimental conditions, electrooxidation offers a suitable solution for the treatment and elimination of undesirable ecological effects of methiocarb contaminated industrial or agricultural wastewaters, ensuring that this highly hazardous pesticide is not transferred to the aquatic environment.

Tech Support

Original Source

DOI: https://doi.org/10.3390/app10217435

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