Electrochemical oxidation of the GH herbicide

We began our investigation by carrying out the electrochemical oxidation of an standard GH sample. Once the most efficienty electrolysis conditions had been established, the electrooxidation of a commercial formulation (Roundup®) was also investigated. Cyclic voltammograms in both the absence and presence of GH were conducted. Electrochemical characterization showed that GH is not electroactive in the potential window 0.2 -1.2 V vs. SCE, so its oxidation hindered by OER. This is a very common characteristic of DSA-type anodes once they are very active for OER and this reaction occurs simultaneously with the oxidation of organic compounds in aqueous medium. The competition between the oxidation of the organic compound and OER is responsible for a significant reduction in the efficiency of the electrochemical process (Aquino Neto & De Andrade, 2009a).

Establishment of the best degradation conditions

Aiming at finding the best conditions for the electrolysis, the preliminary stage of the investigation consisted of selecting the most suitable experimental setup, so that the highest rate possible of electrochemical degradation would occur. In this step, the evaluation of pH, current density, and supporting electrolyte should be performed. There are many ways to measure the real efficiency of a treatment technology. In general, both energy consumption and organic combustion are evaluated. An easy way for judging the performance of DSA® anodes in electrochemical degradation studies is to determinate the current that is effectively used for oxidation of the organic compound. The instantaneous current efficiency (ICE) is obtained considering that during electrochemical incineration two parallel reactions (organic compound oxidation and OER) takes place. So, ICE is defined as the current fraction used for the organic oxidation (Comninellis & Pulgarin, 1991; Pacheco et al., 2007) and was calculated considering the values of chemical oxygen demand (COD) of the wastewater before and after the electrolysis, using the relation

8I At where F is the Faraday constant (C mol-1), V is the volume of the electrolyte (m3), I is the applied current (A), and (COD)t and (COD)t + At are the chemical oxygen demand (g O2 m-3) at times t and t + At (s), respectively.

After 4h of electrolysis at a constant current density of 50 mA cm-2 in Na2SO4 medium, 24% of the starting material (1000 mg L-1) was oxidized, and the mineralization rate reached c.a. 16%. When one compares this value with the rate reported for the degradation of phenol (Comninellis & Pulgarin, 1991), which is a compound generally referred as a model for organic degradation, we can confirm the recalcitrant behavior of herbicides in aqueous solution. Due to the low degradation rate of GH, the ICE in these conditions was very low, less than 5 %, indicating that OER is an important side reaction in the electrochemical process. The difference between the data obtained from spectrophotometric methods (24%) and TOC removal (16 %) has been explained by us previously (Aquino Neto & De Andrade, 2009a) and is related to the formation of recalcitrant intermediate products such as AMPA (metabolite aminomethylphosphonic acid) and sarcosine (n-methylglycine). To understande the degrability of GH as a function of time, long-term electrolyses (12 h in Na2SO4 medium, pH 3, at 50 mA cm-2) were performed. The results of GH degradation as a function o time showed that after 12 h of electrolysis only 43 % GH had been oxidized. In order to improve the degradation rate, pH and concentration effects must be investigated. The best results for the electrochemical oxidation of GH were found in acidic medium (Aquino Neto & De Andrade, 2009a). The low oxidation rates obtained in Na2SO4 medium can be explained by the general mechanism of organic compound oxidation (Comninellis, 1994). Briefly, the oxidation power of the anode is directly related to the overpotential for oxygen evolution. For DSA-like anodes, the •OH radicals strongly bind to the surface, eventually leading to the indirect oxidation of organics via formation/decomposition of an oxide of higher valence (De Oliveira et al., 2008). In order to increase the oxidation rate of DSA-kind materials, different approaches have been proposed in the literature, such as the use of PbO2 (Cestarolli & De Andrade, 2003; Aquino et al., 2010; Panizza et al., 2008a) and BDD (Panizza et al., 2008b), and changes in the supporting electrolyte (Aquino Neto & De Andrade, 2009b).

Electrolysis in chloride medium

The electrolyses in chloride medium were performed as a function of chloride concentration. The assays were carried by varying the amount of chloride from 200 to 3500 mg L-1. An increase in the initial concentration of chloride ion leads to a significant enhancement in the rate of the oxidation reaction. In the case of GH oxidation, there is an increase of 42% PO4-3 release and 53% GH removal even at a very low NaCl concentration (220 mg L-1). When a high concentration of chloride ions is employed (1000 mg L-1), over 80% PO4-3 release is obtained (Aquino Neto & De Andrade, 2009b). It is noteworthy that as the medium becomes more active toward organic compound oxidation, as in the case of chloride medium, there is no significant influence of the anode composition or current density on the oxidation rate. Therefore, one can improve the electrolysis by changing the supporting electrolyte, which culminates in less drastic conditions. This procedure offers two main advantages, namely a decrease in total energy consumption and maximized oxidation rate and larger electrode lifetime, which both contribute to diminishing the cost of the electrolytic system. Figure 4 shows the electrochemical oxidation profile of standard GH and of a commercial formulation of this herbicide as a function of time.

Fig. 4. Linearized removal (A) and COD removal (B) as a function of electrolysis time. Electrode composition Ti/Ru0.30Ti0.70O2, i = 30 mA cm-2, [Cl-] = 2662 mg L-1, p = 1.5 (Na2SO4 + NaCl, pH 3). ■ = standard GH sample; • = commercial GH formulation

The kinetic data reveal a complex oxidation profile. This behavior can be explained considering the competition between the oxidation of the starting material and of the byproducts formed within the first minutes of electrolysis. However, a linear decay is obtained within the first 60 min of electrolysis, as depicted in Fig. 4A and Fig. 4B. A pseudo first-order kinetic behavior is achieved, so assuming that this is the case in the first 60 min (Pelegrino et al., 2002), the oxidation rate can be written as:

where C(0) corresponds to the initial GH concentration and k is the constant of velocity of GH oxidation. The k value is obtained by the following equation:

where V is the solution volume (m3) and A is the electrode area (m2). In the same way, the kinetic constant for COD removal was obtained. Table 2 summarizes the kinetic constants for standard GH and its commercial formulation:

Herbicide

k GH / 10-3 m s-1

k cod / 10-3 m s-i

GH commercial formulation

3.48 ± 0.10

2.40 ± 0.12

GH standard sample

5.75 ± 0.11

1.37 ± 0.15

Table 2. Kinetic constants of standard GH and its commercial formulation for the first 60 min of electrolysis. Composition Ti/Ir0,30Sn0.70O2, i = 30 mA cm-2, [Cl-] = 2662 mg L-1, ^ = 1.5 (Na2SO4 + NaCl, pH 3)

Table 2. Kinetic constants of standard GH and its commercial formulation for the first 60 min of electrolysis. Composition Ti/Ir0,30Sn0.70O2, i = 30 mA cm-2, [Cl-] = 2662 mg L-1, ^ = 1.5 (Na2SO4 + NaCl, pH 3)

Data from Table 2 demonstrate the higher oxidation rate of the active ingredient in relation to its commercial formulation (the kinetic constant is 1.6 times larger). Considering the COD decay, the commercial formulation displays the largest kinetic constant, because of the higher organic load of commercial formulations, which, apart from the active ingredient, present "inert compounds" such as carriers, wetting agents, antifreezes, and other compounds employed to facilitate handling and application. Most of the components of commercial formulations are surfactants that increase the spreading and the penetration power. Taking into account the kinetic data it can be inferred that these compounds are much less recalcitrant than the active principle. Also, these data provide some interesting information concerning GH oxidation compared to the electrochemical oxidation of other organic compounds. The kcoD values of GH presented here are far superior to the ones found for the electrochemical oxidation of phenols (Coteiro & De Andrade, 2007) particularly, 4-chlorophenol (Alves et al., 2004). It is clear that some experimental conditions must also be considered, in order to evaluate the real efficiency of the electrode material. However, the data presented here demonstrate that the electrochemical process is really satisfactory for the treatment of organic pollutants in chloride medium. Finally, the anodic mineralization of organic pollutants in chloride medium perhaps may open the possibility of using DSA-type materials under mild oxidation conditions for the treatment of organic waste in water.

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