Electrochemical removal of Terbuthylazine:Boron-Doped Diamond anode coupled with solid polymer electrolyte

doi:10.1016/j.envpol.2019.04.134

Environmental Pollution

Volume 251, August 2019, Pages 285-291

https://doi.org/10.1016/j.envpol.2019.04.134 Get rights and content

Highlights

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Pesticide contamination of water resources is a serious environmental problem.
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Terbuthylazine and desethylterbuthylazine are chemicals of emerging concern.
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Electrochemical oxidation by SPE and BDD anode is effective on terbuthylazine.
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Removal rate depends on the applied current and terbuthylazine concentration.
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Up to 92% of terbuthylazine is removed in 30 min by electrochemical oxidation.

Abstract

Terbuthylazine (TBA) has replaced atrazine in many EU countries, becoming one of the most frequently detected pesticides in natural waters. TBA is a compound of emerging concern, due to its persistence, toxicity and proven endocrine disruption activity to wildlife and humans. Techniques applied in water treatment plants remove only partially this herbicide and poor attention is given to the generation and fate of by-products, although some of them have demonstrated an estrogenic activity comparable to atrazine. This paper summarizes the environmental occurrence of TBA and its main metabolite desethylterbuthylazine and reports the performance of an innovative electrochemical cell equipped with a solid polymer electrolyte (SPE) sandwiched between a Ti/RuO₂ cathode and a Boron-Doped Diamond anode, operating at constant current, in the treatment of an aqueous solution of TBA. The herbicide removal in the first 30 min of treatment increases from 42% to 92% as the applied current is increased from 100 to 500 mA. The rate of degradation at 500 mA decreases between 30 and 60 min, with a final abatement of 97%. An 89% removal was reached at 100 mA when the initial TBA concentration was raised from 0.1 to 4 mg L⁻¹ and less than 1% of the herbicide was converted in desethylterbuthylazine and minor metabolites. No chemicals are needed, no sludge is produced. Further research is encouraged, as this technology may be promising for the achievement of a zero-discharge removal of different emerging pollutants as pesticides, pharmaceuticals and personal care products.

Graphical abstract

Environmental Pollution Electrochemical Removal of Terbuthylazine: BDD Anode coupled with Solid Polymer Electrolyte.

Introduction

The ubiquitous occurrence of pesticides and their metabolites in the environment is widely recognized. Notwithstanding the endocrine disruption potential of many of these chemicals is confirmed or still unknown, most of them are still not included in drinking water monitoring programs (Odendaal et al., 2015). Atrazine is a pre- or post-emergence broad spectrum herbicide introduced in 1957 for the control of broadleaf and grass weeds. It has been used worldwide in agriculture and forestry, sold as the active ingredient of several powder and water-dispersible granular formulations. Due to the extensive use, persistence and high solubility, this herbicide has been widely detected in the aquatic environment for decades (Du Preez et al., 2005). Atrazine and terbuthylazine (N2-tert-butyl-6-chloro-N4-ethyl-1,3,5-triazine-2,4-diamine) are members of the chlorotriazine family of pesticides. Terbuthylazine (TBA) is authorized for use in 20 countries of the EU and replaced atrazine in most of them (Glæsner et al., 2010), becoming one of the most frequently detected pesticides in continental, coastal and marine waters, while its deethylated metabolite desethylterbuthylazine (DET) is nowadays one of polar metabolites most detected in EU aquifers (Tasca et al., 2018). In Italy, Spain and many other European countries a growing number of aquifers is polluted by terbuthylazine and DET, with concentrations exceeding 0.1 μg L⁻¹, which is the standard set by the EC drinking-water legislation (EC 98/83EEC) (Grenni et al., 2009). Moreover, in the very recent years TBA and DET have been detected in tap water samples collected in different European countries (Tasca et al., 2018).

Primary and secondary processes conventionally applied in treatment plants do not entirely remove organic micropollutants, which are only partially degraded even when advanced techniques are used. Part of the removal is attained by transferring the pollutants into high volumes of sewage sludge, which becomes a source of secondary environmental pollution. A promising solution may be offered by the electrogeneration of efficient oxidants as hydrogen peroxide, ozone and hydroxyl radicals. Electrolytic production of the oxidants avoids the transportation and storage of the chemicals required for the generation of the molecules and reduces the equipment, as the oxidants are directly dissolved in the media as soon as they are formed. Moreover, the excellent performances of Boron Doped Diamond (BDD) anodes in the degradation of various bio-refractory organic pollutants is well recognized (Yu et al., 2014), while titanium coated with ruthenium dioxide (Ti/RuO₂) electrodes have recently emerged due to their low cost, dimensional stability, mechanical and chemical resistance (Kaur et al., 2019, 2018). However, the economic feasibility of electrochemical treatments is strictly related to the conductivity of the media; low conductivity of the solution is associated to high voltage losses and energy demand (Kraft et al., 2006). Current flux and low cell voltage drop can be provided even in a solution with low conductivity, by the use of Solid Polymer Electrolyte (SPE), a thin film placed between cathode and anode. SPE systems have been investigated for electrochemical ozone production (Arihara et al., 2007; Cui et al., 2009; Da Silva et al., 2010; Diniz et al., 2003; Honda et al., 2013; Okada and Naya, 2009), while they have been only barely explored as wastewater remediation technologies (Clematis et al., 2017; Grimm et al., 2000; Houk et al., 1998; Klidi et al., 2019; Simond and Comninellis, 1997).

This paper summarizes the occurrence in water resources of TBA and DET, then it investigates the performance of an innovative electrochemical cell equipped with a solid polymer electrolyte sandwiched between a Ti/RuO₂ cathode and a BDD anode, operating at constant current, in the treatment of an aqueous solution of TBA. Different herbicide concentrations and current intensity have been tested, and particular attention has been given to the metabolites formed.

Section snippets

Occurrence of terbuthylazine and desethylterbuthylazine in water resources

Pesticides are transported beyond source regions. The knowledge of their occurrence and behavior plays a relevant role in establishing efficacious national regulations, mitigation and removal technologies. The ubiquitous presence of TBA was confirmed in 2010 in coastal waters of Italy and Spain, as well as four years later in offshore areas of the western basin of the Mediterranean Sea. The herbicide was part of those with the highest concentrations and most frequently observed in the North Sea

Electrolytic system

Experiments were carried out in an open, undivided, cylindrical glass cell containing 300 mL solutions, maintained under stirring by a magnetic bar at a constant speed of 600 rpm. TBA dissolved in tap water (∼0.1 mg L⁻¹) have been treated at 100, 300 and 500 mA. Each run has been repeated under the same controlled conditions. Moreover, an aqueous solution of 4 mg L⁻¹ have been treated at 100 mA, with the aim to investigate the formation of minor metabolites.

Electrolysis is operating at

Results and discussion

The efficiency of the treatment significantly increases as the applied current increases (Fig. 2), due to a greater production of hydroxyl radicals, as recently observed for the electrochemical oxidation of the synthetic dye Safranin T (Clematis et al., 2017).

The oxidation of TBA and its metabolites is carried out mainly by the hydroxyl radicals electrogenerated on the anode surface (Panizza et al., 2001), as first step in the electrochemical generation of ozone:H₂O → •OH + H⁺ + e⁻

While the

Conclusions

Terbuthylazine and its metabolite desethylterbuthylazine are part of the most frequently detected pesticides in surface waters, groundwaters and marine environments. Monitoring programs and remediation technologies have to be improved with the aim to increase the quality of environmental waters, as well as to guarantee safe water supply (Tasca and Fletcher, 2017).

The efficiency of an innovative SPE electrochemical cell in the treatment of an aqueous solution of TBA is here confirmed. Up to 97%

Acknowledgments

We kindly thanks Matteo Serani and COTECA Srl for the support provided during the analyses.

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Electrochemical removal of Terbuthylazine:Boron-Doped Diamond anode coupled with solid polymer electrolyte☆

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Occurrence of terbuthylazine and desethylterbuthylazine in water resources

Electrolytic system

Results and discussion

Conclusions

Acknowledgments

Water Res.

Water Res.

Environ. Pollut.

Microchem. J.

J. Mar. Syst.

Sci. Total Environ.

Sci. Total Environ.

Water Res.

Electrochem. Commun.

Diam. Relat. Mater.

Environ. Pollut.

Sci. Total Environ.

Environ. Pollut.

Environ. Pollut.

Environ. Pollut.

Water Res.

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Agric. Ecosyst. Environ.

Sci. Total Environ.

Diam. Relat. Mater.

Sci. Total Environ.

Electrochim. Acta

Chemosphere

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Separ. Purif. Technol.

Sci. Total Environ.

Electrochem. Commun.

Atmos. Environ.

Sci. Total Environ.

Separ. Purif. Technol.

J. Electroanal. Chem.

Sci. Total Environ.