Elsevier

Chemosphere

Volume 58, Issue 4, January 2005, Pages 439-447
Chemosphere

Electrochemical detoxification of four phosphorothioate obsolete pesticides stocks

https://doi.org/10.1016/j.chemosphere.2004.09.037Get rights and content

Abstract

The phosphorothioate pesticides are widely used for crop production and fruit tree treatment, but their disposal causes serious environmental problems. Four commercial phosphorothioate pesticides (Demeton-S-methyl, Metamidophos, Fenthion and Diazinon) were treated by an electrolysis system using Ti/Pt as anode and stainless steel 304 as cathode. A number of experiments were run in a laboratory scale pilot plant and the results are presented. For Fenthion the achieved reduction was over 60%, while for Demeton-S-methyl, Metamidophos and Diazinon was more than 50%. Diazinon had the lowest energy demand. The COD/BOD5 ratio was improved considerably after electrolysis for all four pesticides examined. As a conclusion, electrochemical oxidation could be used as a pretreatment method of the pesticides detoxification.

Introduction

The disposal of pesticides can cause serious problems due to the chemical nature of the active ingredients in pesticide formulation and due to the large quantities of the unwanted products. These products undergo physical and chemical alterations either due to extended storage, beyond the recommended expiry date, or due to storage under improper conditions (high humidity and temperature). In many countries, large quantities of pesticides have accumulated since they have lost their desirable characteristics. Pesticides that have passed their self-life can be included in this category. Although, these products are not suitable for use, they still contain toxic compounds. In addition, many surplus pesticides, still within their expiration limits, may become useless, when their future use is prohibited due to toxicological or environmental concerns. FAO (Food and Agricultural Organization of the United Nations) estimated that more than 400 000 tonnes of obsolete pesticides are stocked worldwide (FAO Pesticide Disposal Series, 2000).

The biological degradation of pesticides is generally difficult due to their high content in toxic matter (Felsot, 1996; Zaleska and Hupka, 1999). An ideal treatment method for pesticide surplus should be non-selective, should achieve rapid and complete mineralization, and should be suitable for small-scale wastes (Krueger and Seiber, 1984; Bourke et al., 1991). Today the main disposal method of obsolete pesticide stock is incineration, an impractical and expensive procedure. High-temperature incineration in dedicated hazardous waste incinerators is the currently recommended method for obsolete pesticide treatment. However, sophisticated incinerators do not exist in developing countries (FAO Pesticide Disposal Series, 2000).

However, safety on environmental and processing grounds may be questioned in many cases. For products for which a suitable inactivation method does not exist, long time storage in concrete tombs is recommended (Zaleska and Hupka, 1999). However, oil–water emulsions and/or organic solvents from the solvent-based preparations may destroy the bituminous coatings on the concrete walls of the tomb. It is possible that the unprotected concrete walls might be corroded and that toxic chemicals may subsequently migrate to the surrounding area (Zaleska and Hupka, 1999; FAO Pesticide Disposal Series, 2000).

Various innovative technologies have been proposed for methyl-parathion treatment. These include the use of UV and hydrogen peroxide (Pignatello and Sun, 1995; Chen et al., 1998), ultrasonic radiation (Kotronarou et al., 1992) or mercury-promoted hydrolysis (Zeinali and Torrents, 1998). The major disadvantage of these technologies is that they are designed for decontamination of aqueous solutions with a very low active ingredient content and are not suitable for the higher concentrations of unwanted pesticides.

Recently, there has been increasing interest in the use of electrochemical methods for the treatment of recalcitrant toxic wastes. The organic and toxic pollutants present in such wastes, such as phenols which are present in many pesticides, are usually destroyed by anodic oxidation as a result of the production of oxidants such as hydroxyl radicals, ozone, etc. (Comninellis and Pulgarin, 1991; Comninellis, 1992, Comninellis, 1994; Comninellis and Nerini, 1995). These methods are environmentally friendly and do not produce new toxic wastes. Electrochemical methods have been successfully applied in the purification of domestic sewage (Della Monica et al., 1980; Vlyssides et al., 2001), landfill leachate (Chang and Wen, 1995), tannery wastes (Vlyssides and Israilides, 1997), olive oil wastewaters (Israilides et al., 1996) textile wastes (Vlyssides et al., 1999), etc.

The electrochemical reactions, which take place during the electrolysis of a chloride-containing solution (brine solution), are complicated and not entirely known. The electrochemical oxidation of aqueous solutions, which contain organic matter, by the use of Ti/Pt anode, proceeds in two steps (Comninellis, 1992). The first step is the anodic discharge of the water, forming hydroxyl radicals which are absorbed on the active sites of the electrode surface M[ ].H2O+M[]M[OH-]+H++e-After this the absorbed hydroxyl radical oxidizes the organic matter.R+M[OH-]M[]+RO+H++e-where RO represents the oxidized organic matter which can be produced continuously by the hydroxyl radicals which are also continuously formed, since the anodic discharge of the water goes on. The radicals radical dotOH, Oradical dot and HClOradical dot have a very short life-time due to their high oxidation potential and are either decomposed to other oxidants (such as Cl2, O2, ClO2, O3, and H2O2) or oxidize organic compounds (i.e. direct oxidation). The primary (Cl2, O2) and secondary (ClO2, O3, and H2O2) oxidants that are produced from the destruction of radicals have quite a long life-time and are diffused into the area away from the electrode, thus continuing the oxidation process (indirect oxidation). Effective pollutant degradation is based on the direct electrochemical process (that takes place in a closed area around the electrode) because the secondary oxidants are not able to convert totally all the organic species into water and carbon dioxide. From previous investigation (Comninellis, 1994), using acid solutions, oxygen, free chlorine and maybe some ozone and chlorine oxides are the main secondary oxidants, by-products of the direct oxidation process.

The mechanism of electrochemical incineration is already a complex one in the case of so called “direct” process, where extensive oxygen-transfer stages eventually lead to the mineralization of organic substrates. In the case of chloride mediated processes, the complication is of course greater, essentially because of the complex electrochemistry of chlorinated inorganic species, which may act both at the electrode surface and in the bulk of the solution and or in a reaction cage nearby the electrode surface itself. An explanation of the mediating role of chloride ions has been proposed by Bonfatti et al. (2000). The presence of relatively small amounts of chloride ions seems to inhibit the oxygen evolution reaction, causing an increase of the anode potential and therefore a higher reactivity of adsorbed hydroxyl and chloride-oxychloride radicals. Increasing the chloride concentration above a certain critical value would then cause a potentiostatic buffering by the chlorine redox system, and consequently a decrease of the anode potential. Moreover, experimental evidences have been interpreted as a possible mediation explicated by hypochlorous acid generated at the electrode surface (by interaction between hydroxyl and chloride radicals). Another possibility could be the reciprocal inhibiting effect of the OH and HOCl species, adsorbed at the electrode surface, with respect to the parasitic reactions of oxygen and chlorine evolution, respectively. The balanced presence of the two radical species allows the electrochemical process to be carried out with high Faradaic yields (Bonfatti et al., 2000).

This paper deals with the treatment of the following four commercial phosphorothioate pesticides by an electrochemical method, using a Ti/Pt electrode:

Section snippets

Laboratory pilot plant for the electrolysis

The experimental plant is shown in Fig. 1. The electrolytic cell was a cylindrical vessel (V), which contained 6 l of brine solution (H2O + NaCl). A Ti/Pt cylindrical electrode (14 cm long × 1.5 cm diameter) was used as anode. It was covered by platinum alloy foil approximately 0.22 mm thick. The electrode was located inside a perforated stainless steel 304 cylinder (14 cm long × 8 cm diameter) which served as cathode. This construction ensured homogenous dynamic lines between anode and cathode and

COD reduction

In Fig. 2 the % COD reduction for the four pesticides is presented. It is observed that the application of electrolysis in these pesticides has the ability to highly reduce the COD. For Fenthion the achieved reduction was over 70%, while for Metamidophos Demeton-S-methyl, and Diazinon was more than 55%.

BOD5 reduction

In Fig. 3 the % BOD5 reduction for the four pesticides is presented. It is observed that the application of electrolysis two of the pesticides has the ability to reduce the BOD5 in considerable

Conclusions

This work studied the efficiency of an electrochemical oxidation system for the treatment of organophosphoric pesticides. Electrochemical oxidation is a method that has never been applied for the treatment of this type of wastes.

Applying electrolytic oxidation, the two obsolete organophosphoric pesticides stocks cannot be treated effectively in terms of COD or BOD5 reduction. Nevertheless, the efficiency of the electrolysis system in terms of consumed energy (kWh kg CODr−1) was low in both cases.

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