Cl-mediated electrochemical oxidation for treating an efﬂuent using platinum and diamond anodes

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Introduction
The day-to-day human and industrial activities have influenced the flow and storage of water and the quality of available fresh water [1].The remediation of urban and industrial wastewater containing organic pollutants can be carried out by different methods, including chemical, physicochemical and biological treatments [2,3].The most method used is the conventional-biological treatment, but it is time consuming, need large operational area and is not completely effective for effluents containing with biorefractory pollutants [4].
Physical-chemical methods (filtration, coagulation, adsorption, and flocculation), chemical oxidation (use of chlorine, ozone, hydrogen peroxide, wet air oxidation), and advanced oxidation processes (AOP) (Fenton's reaction, ozone + UV radiation, photochemistry) are currently used to treat industrial effluents [3,5].However, all these methods have some major drawbacks.For example, filtration and adsorption are not always sufficient to achieve the discharge limits [6]; coagulation and flotation generate a large amount of sludge; chemical oxidations have low capacity rates and need transportation and storage of dangerous reactants; and advanced oxidation processes require high investment costs.Consequently, an urgent challenge is the development of new environmentally benign technologies able to mineralize completely non-biodegradable organic matter and eliminate pathogens.
In this context, oxidative electrochemical technologies offer an alternative solution to several environmental problems regarding the wastewater treatment, because electrons provide a versatile, efficient, cost-effective, easily automation, and clean reagent [2][3][4][5].Some years ago, the effective application of electrochemical technologies for the treatment of organic pollutants has been relatively small [2,3].But nowadays, thanks to intensive investigations that have improved the electrocatalytic activity, stability of electrode materials as well as optimized reactor geometry; electrochemical technologies have reached a promising state of development and can be effectively used for disinfection and purification of wastewater polluted with organic compounds [7][8][9][10][11][12][13][14].
The main object of the electrochemical wastewater treatment is the complete oxidation of organics to CO 2 or, at least, their conversion to biodegradable compounds [7,8].In this frame, several organic substrates have been considered as target pollutants for direct and indirect electrochemical oxidations by using different experimental conditions and anode materials [3][4][5][6][7][8][9][10][11].Regarding the electrodes, a high number of them have been tested [3,5,7] including polypyrrole, granular activated carbon, ACF, glassy carbon, graphite, Pt/Ti and Pt, doped-PbO 2 and mixed metal oxides of Ti, Ru, Ir, Sn, Ta and Sb.However, a large variety of studies have demonstrated the efficiency of non-active anodes [7,8,11], such as PbO 2 and BDD electrodes by their better oxidative performance due to accumulation of • OH radicals at their surfaces.
On the other hand, the most popular method of electrochemical treatment is electro-chlorination (based on indirect electrochemical oxidation).Its main advantage is the on-site generation of disinfectants, thus avoiding the problems of common chlorination such as transport and storage of dangerous chlorine [3,5,9].There are two types of electro-chlorination procedures involving either the synthesis of free chlorine from brine in an electrolytic generator or the direct production of oxidants from the water to be treated through the electrolyser.
Active chlorine species such as Cl 2 , HOCl, ClO − and ClO 2 have been widely recognized as key oxidants responsible for degrading organic pollutants or inactivating cells in electro-chlorination [9,15].These species can be produced at the anode via the following total reactions [9]: Some researchers have pointed out that the disinfecting efficacy of this method is much higher than chlorination due to the competitive electrogeneration of other oxidants [11].Recent studies have also attributed the higher disinfecting power of electrochlorination to the additional oxidant role of reactive oxygen species (ROS) such as hydroxyl radical ( • OH), atomic oxygen ( • O), hydrogen peroxide and ozone, which can be generated from water discharge at the anode, as follows [16][17][18]: The most used types of electrode for this electrochemical approach are the metal oxide electrodes that, generally, are the derivatives of four metal oxides, SnO 2 , PbO 2 , RuO 2 and IrO 2 [19].However, Pt/Ti (active) and BDD (non-active) anodes are also able to produce appreciable amounts of ROS and other oxidizing species such as active chlorine [9], peroxodisulfate, peroxodicarbonate and peroxodiphosphate coming from the oxidation of ions present in the solution, also allowing a fast and permanent disinfection.
For this reason, taking into consideration the above information, our investigation focusses to study the role of chloride respect to the oxygen evolution reaction by using BDD and Pt/Ti anodes to better understand the effects during direct and indirect electrochemical incineration of a real sample of wastewater, with lower organic load, generated at Federal University of Rio Grande do Norte (UFRN).

Reagents
Ultrapure water was obtained using a water purification system (MilliQ).The chemical reagents used were of the highest quality available on the market, without additional purification.NaCl and Na 2 SO 4 were obtained from Fluka.

Electrochemical measurements
Electrochemical analyses were performed with an Autolab model PGSTAT320N (Metrohm).Quasi-steady polarization curves were carried out at a scan rate of 5 mV s −1 and with a 0.45 mV step potential, in solutions of NaCl at different concentrations, using Na 2 SO 4 to further increase the electrolyte conductivity.Experiments were carried out in a conventional three-electrode system, and measurements were performed between 0 and 2.5 V. Pt/Ti and BDD, with an exposed geometric area of ca.0.75 cm 2 , were used as the working electrode, while a platinum wire and an Ag/AgCl (KCl 3 mol dm −3 ) electrode were employed as the auxiliary and reference electrodes, respectively.

Electro-oxidation experiments
The electro-oxidation experiments were made in a single compartment using an electrolytic flow cell with parallel plate electrodes for treating 2 dm 3 of wastewater (Fig. 1).Disk anodes (Pt/Ti and BDD with 10 cm in diameter) were used, exposing to the aqueous solution a nominal surface area of 63.5 cm 2 .Pt/Ti or BDD were used as anodes, and titanium as the cathode.Ti-supported Pt anode was supplied by Industrie De Nora S.p.A. (Milan, Italy).BDD electrode was supplied by Adamant Technologies (Switzerland) and it was synthesized as described in previous works [20] maintaining the quality parameters (single-crystal with a thickness of 1 m (±5%) and a resistivity of 15 m cm (±30%) with a boron concentration of 5000 ppm, p-silicon wafers (1-3 m cm and 1 mm thick)).In order to stabilize its surface (hydrophilic nature) and to obtain reproducible results, the BDD electrode was pre-treated at 25 • C by anodic polarization in 1 M HClO 4 at 10 mA cm −2 for 30 min [21].
In order to understand the effect of current density during direct and indirect treatment, experiments were performed applying 2.5 and 5.0 mA cm −2 during 2 h.In the case of indirect electrochemical treatment by electro-chlorination, it was evaluated by adding 1.25 g dm −3 of NaCl in order to compare with the electrochemical depollution of effluent as received.

Analytical methods
Decontamination of real effluent was monitored from the abatement of its COD.Values were obtained, using a HANNA HI 83099 spectrophotometer after digestion of samples in a HANNA thermoreactor.From these data, the percentage of COD decay is estimated from the following equation: where COD 0 and COD f represent the values before and at the end of the electrolysis, respectively.Total current efficiency (TCE, in %) for direct and indirect electrochemical oxidations of the effluent was estimated by using the initial and final COD values, following relationship: where I is the current (A), F the Faraday constant (96,487 C mol −1 ), V is the electrolyte volume (dm 3 ), 8 is the oxygen equivalent mass (g eq.−1 ) and t is the electrolysis time, allowing for a global determination of the overall efficiency of the process.Additionally, the limiting current was estimated from the value of COD using Eq. ( 12) for electrochemical treatment of a real wastewater, as indicated by Panizza and Cerisola [21]: where I lim (t) is the limiting current (A) at a given time t, 4 the number of exchanged electrons, A the electrode area (m 2 ), F the Faraday's constant, k m the average mass transport coefficient in the electrochemical reactor (m s −1 ) and COD (t) the chemical oxygen demand (mol O 2 m −3 ) at a given time t.
The energy consumption per volume of treated effluent was estimated and expressed in kWh m −3 .The average cell voltage during the electrolysis (cell voltage is reasonably constant with just some minor oscillations, for this reason is calculated the average cell voltage), is taken for calculating the energy consumption by expression: where t is the time of electrolysis (h); Ec (V) and I (A) are the average cell voltage and the electrolysis current, respectively; and V s is the sample volume (m 3 ).

Polarization curves in the presence of halide ion
Prior to direct and indirect electrochemical oxidations, the possible effect of halide on the oxygen evolution reaction (o.e.r.) was studied.Quasi-steady polarization curves were recorded in background solutions containing 0.25 mol dm −3 Na 2 SO 4 , in the absence and in the presence of different concentrations of Cl − (1 × 10 −3 mol dm −3 to 0.4 mol dm −3 ).The results obtained in the presence of chloride ions, at both anode materials, are shown in Fig. 2. In the case of the Pt/Ti anode, polarization curve concerning to the o.e.r. is modestly shifted to more positive potentials (Fig. 2a), when a little increasing in the chloride concentration is attained (0.001 mol dm −3 ); above this value, a reversal of the trend is observed.The inversion of the trend upon increasing chloride concentration above 0.01 mol dm −3 is due to the increase of the importance of the Cl 2 /H 2 O system.Under these conditions, a fast incineration of a number of organic substrates can be favored due to the production of reactive hydroxyl radicals, in concomitance with active chlorine species, as intermediates in the chlorine evolution reaction.
Similar experiments were carried out using BDD anode in the presence of Cl − , as shown in Fig. 2b, employing the same range of Cl − concentrations.In that case, at very small NaCl concentration (0.001 mol dm −3 ), a relevant shift of J/E curves in the positive direction is observed.Above 0.02 mol dm −3 , the anode potential becomes increasingly buffered by the halide electroactivity.This behavior can be attributed to an interaction between hydroxyl radicals (physisorbed) and Cl − to form active chlorine species (e.g., Cl − + • OH → ClO − + H + + e − ) on BDD surface [22,23].It may justify the formation of many new types of oxidants (desirable and undesirable [9,11], such as Cl • , Cl 2 , ClO 2 − and ClO 3 − , ClO 4 − , respectively) with this non-active material.After that, when an increase on the NaCl concentration was attained (from 0.4 mol dm −3 to 0.6 mol dm −3 ), polarization curves are shifted to less positive potentials (Fig. 2b).
Based on the results obtained, the concentration of halide in solution increases the importance of Cl 2 /oxy-chloro radicals system depending on the electrocatalytic material used and this behavior plays an important role in relation with the o.e.r.[24], influencing on the efficiency of electrochemical approach adopted [25,26].

COD removal by electrochemical treatment
Fig. 3a shows % of COD removal using Pt/Ti anode by applying 2.5 and 5.0 mA cm −2 after 2 h of electrolysis.Results clearly showed that at Pt/Ti electrode, a modest COD removal was obtained independent on the current density (2.5 and 5.0 mA cm −2 ), corresponding to 25.2% and 30.49%, respectively.Under the same experimental conditions, using BDD electrode, it occurred a small COD decay (1% and 4.5% at 2.5 and 5.0 mA cm −2 , respectively).These results clearly indicate that, the low conductivity, salts content and organic matter dissolved in the effluent complicate the depuration treatment [11].A higher charge is consumed for complete mineralization during the electrochemical process because a relative greater amount of • OH is wasted in parasite non-oxidizing reactions such as oxygen evolution [21].It can be confirmed from the current efficiencies (TCE, in %) obtained for each current density applied under these conditions (Fig. 3b).If this behavior is attained, it is frequently characteristic of electrolysis under mass transport control when the electrolysis is performed applying a current higher than the limiting one, as already indicated by other authors [8,21,27].
For a recirculation rate of 250 dm 3 h −1 , the mass transfer coefficient was 2.5 × 10 −5 m s −1 and consequently, the limiting current (I lim ) was approximately 0.78 A, according Eq. ( 12).However, this current is higher than the currents applied (I appl ) in this work (0.16 and 0.32 A), indicating that the preliminary assumption is incorrect.
As a common condition, when I appl is minor than I lim ; the current efficiency is close to 100%, and the COD decreases linearly with time, suggesting that the oxidation under these conditions could be occurring under current control [8].However, lower TCE values (Fig. 3b) indicate that the last condition was not attained.This behavior could be due to the use of applied current to favor other electrochemical processes, such as the production of oxidants and the oxidation/reduction of ions [22].In fact, the existing concentrations of Cl − , NO 3 − and SO 4 2− in the effluent (as-obtained) promote the occurrence of parallel reactions, consuming the applied current.In the particular case of Cl − , initial concentration of 208.6 mg/L (0.00356 mol dm −3 ) is enough to shift the o.e.r. to positive potentials at both electrodes, being more evident this behavior to BDD anode.It suggests that other electrochemical reactions can take place while anodic oxidation occurs.
Under these conditions, although the applicability of this treatment seems feasible, long times would be required to complete organic matter removal.For this reason, new set of experiments was performed to generate efficiently reactive oxidant species (active chlorine), plus hydroxyl radicals.

Effect of NaCl dissolved in the effluent
COD decay was also monitored by applying 2.5 and 5.0 mA cm −2 of current density when an amount of 2.5 g of NaCl was dissolved in 2 dm 3 of the effluent (0.021 mol dm −3 ).Fig. 4 shows the influence of the current density on the percentage of COD removal during indirect electrochemical approach for treating a real wastewater using both anodes, at 25 • C. Results clearly demonstrated that Pt/Ti anode was more efficient than BDD electrode, achieving 63.7% and 60% for 2.5 and 5.0 mA cm −2 , respectively.Whereas at BDD electrode, % COD removals were about 15% and 47% under the same conditions.From these figures we can infer that, while at direct oxidation lower efficiencies were achieved (Fig. 3), more efficient process was performed when a considerable amount of Cl − was added to the effluent (Fig. 4), excepting for BDD at 2.5 mA cm −2 .
As already stated by other researchers, the change from Pt to BDD should not involve dramatic changes in the incineration mechanism because the oxidation reactions should be mainly a set of volume rather than surface reactions and it depends mainly on oxidant species produced [28], in the case of chloride mediation.However, restricting now our results to the potentiodynamic measurements, the process seems to be the consequence of some specific roles played by the halogen salt on the anode surface to generate active chlorine species.
For Pt/Ti, the effect of chlorides may be a hybrid of two mechanisms where the anion may partially change the stoichiometry and microstructure of the oxide film that grows on the Pt electrode surface at strongly positive potentials [24]; as a consequence, the o.e.r. is less favored and the electrochemical incineration is consequently privileged.In fact, the quasi-state polarization curves indicate that, between 0.001 and 0.01 mol dm −3 of Cl − concentrations, the o.e.r. is inhibited but the production of strong oxidants is modest.Then, if the electrogeneration of strong oxidants is partially attained, the oxidation occurs principally in the vicinity of the anode surface where the mixture of oxidants reacts with the organic pollutant, justifying the enhancement in the COD removal in chloride media.

Table 1
Energy consumption calculated from Eq. ( 13), per volume of treated effluent during direct and mediated oxidation of a real effluent.Conversely, at BDD anode, the potentiodynamic measurements support the idea that strong oxidants are generated in a first stage, and consumed in a solution region strictly confined around the electrode surface.The generated oxidants may be simply and HClO/ClO − (when lower Cl − concentration is used).But, for Cl − concentrations above 0.1 mol dm −3 , a more complex situation can be attained because the production of chloro and chloro-oxy radicals as well as their reactivity are favored due to the increase of the importance of Cl 2 /oxy-chloro radicals system at BDD surface.The occurrence of HClO can be considered, only within the Nernst layer as a consequence of a slight acidic condition at anode surface by the production of oxygen; and this active chlorine species participate in the oxidation of organic matter.However, the enhancement on the elimination of organic matter was attained at higher current density (5.0 mA cm −2 ), indicating that the active chlorine species are efficiently produced under these experimental conditions.This behavior explains the lower COD removal efficiency at 2.5 mA cm −2 .Conversely, at Pt/Ti electrode, higher efficiencies were attained by applying 2.5 and 5.0 mA cm −2 because an efficient production of active chlorine was attained.These assumptions are in accordance with the performances obtained during COD removal, at different applied current density.Another important feature was that, IC analysis showed that the concentration of some ions decreased after electrochemical treatment.For example, at Pt/Ti anode by applying 2.5 mA cm −2 at 25 • C, the concentrations were of 2.6 mg dm −3 of NO 3 − , 1.2 mg dm −3 of Ca 2+ , 4.1 mg dm −3 of Na + , 2.7 mg dm −3 of Mg 2+ and 12.6 mg dm −3 of SO 4 2− .

Energy consumption estimation
Based on the COD values obtained at different applied current densities, energy consumption was estimated by Eq. ( 13).Table 1 presents the electrical energy required per volume of treated effluent at both anodes.For example, in the case of Pt/Ti without addition of NaCl, it increases from 0.54 to 3.86 kWh m −3 of effluent treated when the current density passes from 2.5 to 5.0 mA cm −2 .Moreover, in the same conditions but with the addition of NaCl (1.25 mg m −3 ), it caused a slight increase in energy consumption from 0.54 to 0.75 kWh m −3 (Table 1), while in other case the NaCl addition resulted in a clear decrease of energy consumption.For BDD anode, the addition of NaCl in the effluent promotes an important decrease in the energy consumption, and consequently, a reduction on the costs.For example, 1.50 kWh m −3 are consumed in the absence of Cl − in the effluent, but it is reduced to 1.00 kWh m −3 when an amount of NaCl of 1.25 g dm −3 , was added.Comparing the electrical energy consumed by Pt/Ti and BDD anodes when 2.5 mA cm −2 are applied, in the absence or presence of NaCl in the effluent, the results clearly indicated that BDD is more efficient, but lower efficiencies on organic matter removal were achieved (see Fig. 4a).
Finally, taking into consideration an electrical energy cost of about R$ 0.3 (Brazilian price, taxes excluded) per kWh (Agência Nacional de Energia Elétrica, Brazil), the process expenditure was estimated and reported in Table 1 in order to show the viability of this process as a green alternative for the treatment of urban wastewater.This price was also converted to dollar and reported in the same table.

Conclusions
On the basis of the results obtained for direct and mediated oxidation of a real wastewater effluent, the electrochemical technology is efficient with a low operation cost when the addition of a small amount of NaCl is performed.The results point out the high performance Pt/Ti anodic oxidation for treating wastewater effluent compared to BDD anode.Although, the use of NaCl favors the production of strong oxidant species that together to ROS (such as, hydroxyl radicals) oxidize dissolved organic matter in real effluent (close to anode surface and in the reaction cage) [28][29][30]; particular attention and experimental observations must be taken into consideration before or during its use due to the production of organochloride compounds (pollutants were not detected after electrolysis).Finally, the results reported in the present work have recently allowed to start the design and implementation of a pilot electrochemical cell in the treatment station at the UFRN.These experiments are in progress and their results will be reported in detail in a separate paper in a near future.

Fig. 2 .
Fig. 2. Current density-potential curves for the Pt electrode in the presence of different amounts of NaCl on (a) Pt/Ti and (b) BDD anodes.Na2SO4 as supporting electrolyte; scan rate: 5 mV s −1 and 25 • C.

Fig. 3 .
Fig. 3. (a) % of COD removal and (b) %TCE, as a function of applied current density (2.5 and 5.0 mA cm −2 ), during direct electrochemical oxidation of real effluent by using Pt/Ti and BDD anodes.

Fig. 4 .
Fig. 4. (a) % of COD removal and (b) %TCE, as a function of applied current density (2.5 and 5.0 mA cm −2 ), during Cl-mediated oxidation of real effluent by using Pt/Ti and BDD anodes.