3. Results and discussion 206
3.1. Electrolysis of soil washing effluents 207
Aliivibrio fischeri. The sample toxicity was evaluated as toxicity units (TU), calculated 199
as 100/IC50, where IC50 represents the percent dilution of the initial solution causing 200
50% reduction of the luminescence (Sanchis et al., 2014). The luminometer employed 201
was a Junior LB 9509 portable tube luminometer from Berthold.
202 203 204 205
3. Results and discussion
interferences caused by partial degradation of the high surfactant concentration in SWE.
224
The oxidation ability of the anodes in the groundwater medium increased in the order 225
MMO-Ru < MMO-Ir < CF < BDD. This trend agrees with the expected oxidative 226
superiority of non-active BDD anodes over the other three active anodes (Boye et al., 227
2002; Marselli et al., 2003; Panizza and Cerisola, 2009; Ridruejo et al., 2017). The good 228
performance of BDD over atrazine removal has been previously reported in some 229
research works (Borràs et al., 2010; dos Santos et al., 2015; Bravo-Yumi et al., 2018; Bu 230
et al., 2018). Garza-Campos et al. (2014) reported an atrazine degradation efficiency 231
higher than 90% under analogues conditions, applying a current density of 13.3 mA cm- 232
2 using BDD and the initial atrazine concentration was 20 mg dm-3. 233
234
235
Figure 1. Evolution of the atrazine relative concentration with the applied electrical 236
charge when treating polluted SWE with different anodic materials.
237 238
According to the background in introduction section, the main oxidant species in the 239
system depend on the anode nature. BDD anode is considered as non-active electrode, 240
where the degradation of the organic molecules is carried out by hydroxyl radicals.
241
0 5 10 15 20 25
0.0 0.5 1.0
BDD CF MMO-Ir MMO-Ru
C/C0
Q (A h dm-3)
K (min-1) r2
BDD 0.10666 0.9798
CF 0.04108 0.9117
MMO-Ir 0.03131 0.9256
MMO-Ru 0.01086 0.991
Conversely, the other anodes (CF and MMO) are active-electrodes and hydroxyl radical 242
can interact with the anode surface in spite of attacking the target pollutant. In the 243
present work, active chlorine species should be considered because the medium contains 244
chloride ions, however the concentration of NaCl is only 84 mg dm-3, and it remains 245
below the optimal concentration found in bibliography to take advantage about active 246
chlorine oxidant species. In addition, in the case of BDD, the subsequent formation of 247
chlorite, chlorate and perchlorate ions is expected to occur.
248
The initial stage of atrazine depletion, from 0 to 5 A h dm-3 of electrical applied charge 249
can be adjusted to a first-order kinetic model. Figure 1 also summarizes the apparent 250
rate constants and the square of correlation coefficient of the linear fittings. Kinetics 251
analyses corroborate the statement of the superior ability of BDD to degrade organic 252
matter.
253
It is known that the active anode MMO-Ru has good performances in chloride medium, 254
because it allows the production of active chlorine species that can attack rapidly the 255
aromatic structures, and they are good catalysts for chlorine evolution (Panizza and 256
Cerisola, 2009). Groundwater medium in the present work contains a concentration of 257
chloride ions about 84 mg dm-3 and therefore, in principle it was expected that MMO- 258
Ru could act as a good anode material for atrazine removal. However, the behaviour of 259
MMO-Ru in atrazine degradation was not good as expected. A reason that could justify 260
the unexpected operation of this anode is that such chloride concentration, as previously 261
indicated, is not high enough. Rajkumar and Palanivelu (2003) optimized a NaCl 262
concentration about 2.5 g dm-3 to degrade cresol using MMO-Ru anodes. On the other 263
hand, Panizza et al. (2007) reported an optimal chloride concentration of 1.2 g dm-3 to 264
degrade methylene blue through anodic oxidation with MMO-Ru anode. As indicated, 265
the chloride concentration in the synthetic groundwater in the present work was only 84 266
mg dm-3, this value is below to the optimum NaCl concentrations observed in 267
bibliography.
268
Regarding carbon felt electrode, it offered similar behaviour to MMO-Ir. Generally, the 269
use of carbon-based material as anode generates low degradation efficiencies because 270
they can also be electrochemically incinerated (transformed into carbon dioxide) during 271
the electrochemical process when using the high voltages required to oxide organic 272
pollutants (Sopaj et al., 2015). Additionally, it should be considered the great porosity 273
of the carbon felt. Due to this fact, the electrode surface is much larger than that of the 274
other electrodes, which helps to explain the better performance in atrazine degradation 275
when carbon felt electrodes are used instead of the MMO anodes.
276
At this point, it is known that the atrazine molecule is degraded during the electrolysis 277
but, both atrazine and surfactant could be converted into intermediate compounds or 278
they could be completely mineralized to CO2.Partial mineralization is usually necessary 279
to generate products with good biodegradability, but strong mineralization is not a 280
desired result in the present work. Note that strong mineralization would produce high 281
operation cost as it would reduce the fraction of organic pollution to be oxidized by a 282
subsequent biological step. The mineralization rate of the solution is directly related to 283
the degradation of the surfactant, SDS. It is important to remark that the SWE has an 284
important amount of TOC due to the use of SDS during the SW process. For this 285
reason, TOC is a significant parameter to consider. Relative TOC depletions 286
(TOC/TOC0) appear in Figure 2.
287
Figure 2 shows that the BDD anode is the most powerful material that mineralizes most 288
of the organic matter in solution, it means that BDD was powerful enough not only to 289
remove atrazine (Figure 1), but also to mineralize a large amount of its by-products and 290
the surfactant. On the contrary, MMO-Ru was the most inefficient anode to transform 291
the intermediates into CO2. This is not surprising based on the very slow degradation 292
rate observed for atrazine. Regarding the other two materials, MMO-Ir caused an 293
important TOC removal, reaching a 50% of TOC removal. Finally, CF only achieved a 294
20% of mineralization, which in principle may be a good result (atrazine is removed 295
while TOC is not strongly reduced) if subsequent tests would show a biodegradability 296
increase. The mineralization results obtained with active anodes agree to the expected 297
because these kinds of materials allow the partial organic degradation, along the 298
formation of many refractory species as final products (Moreira et al., 2017). The 299
greater mineralization achieved with BDD could then be accounted for two factors: the 300
low formation of active chloride species, with subsequent accumulation of a lower 301
amount of chloroderivatives, and the larger ability of BDD to mineralize the 302
intermediates formed (Ridruejo et al., 2017).
303
304
Figure 2. TOC removal over the electrical applied charged for the electrochemical 305
oxidation of polluted SWE.
306 307
TOC depletion is also related to the generation of sulphate when SDS is oxidized, 308
because the sulphate group contained in the tail of SDS molecule is broken and 309
0 5 10 15 20 25
0.0 0.5 1.0
BDD CF MMO-Ir MMO-Ru
TOC/TOC0
Q (A h dm-3)
generates sulphate (dos Santos et al., 2016). The generation of sulphate ions is plotted in 310
Figure 3.
311
The results shown in Figure 3 agree to the ones in Figure 2 (TOC depletion). The 312
highest sulphate concentration is achieved working with BDD anode, which 313
corroborates the good ability of BDD to eliminate TOC. In the experiment carried out 314
with CF, the concentration of sulphate remains constant during electrolysis, indicating 315
again that this anodic material does not degrade the surfactant. Using MMO anodes 316
cause a slight increase of the sulphate ion concentration.Despite MMO-Ir causes higher 317
TOC removal than MMO-Ru (as shown in Figure 2), no clear differences have been 318
observed between both MMO anodes regarding sulphate ions produced during 319
electrolysis (Figure 3). Using BDD anodes, hydroxyl radicals are available to be 320
combined with sulphate, forming sulphate radical and mono and diperoxosulphate, a 321
powerful oxidant, which may extend the oxidation from the nearness of the electrode 322
surface to the bulk (Barrera-Díaz et al., 2014). On the other hand, with MMO and CF 323
anodes, these radicals are produced but are consumed in other processes (reversible 324
oxidation of the electrode components) before they can oxidize sulphate anions to 325
sulphate radicals. They are transformed into oxygen, which help to explain the lower 326
efficiency observed since active electrodes have low O2 overpotentials. It means that 327
hydroxyl radicals are quickly converted into oxygen, in contrast to BDD electrode, 328
where these radicals could de accumulated and used to degrade organics (Sopaj et al., 329
2015).
330 331
332
Figure 3. Sulphate ions produced during the electrolysis of polluted SWE.
333 334
The changes in pH are shown in Figure 4. In the experiment completed with BDD, the 335
value of pH dropped significantly to a final value close to 3.This behaviour is due to the 336
fact that atrazine can generate many acidic compounds of low molecular weight, which 337
decreases the pH (Martínez-Huitle and Brillas, 2009). This drop in the pH during 338
electrolysis of atrazine with BDD has been previously detected (Bravo-Yumi et al., 339
2018). Moreover, the depletion in pH can be also associated to the release of H+ to the 340
liquid medium during the mineralization or the progressive oxidation of chloride ions 341
which generates H+ (Dominguez et al., 2018).
342
On the other hand, CF produces also a decrease in the pH. This decrease is not so high 343
as the produced with BDD, and thus it would allow a subsequent biological treatment.
344
MMO anodes do not decreased the pH, but just a slight increase, keeping the pH near to 345
9.0. Regarding electrical conductivity, not important changes were appreciated during 346
the different electrolyses, and the values (not shown) were kept constants over electrical 347
charge with the four anodes checked.
348
0 10 20
0 100 200 300 400 500 600
BDD CF MMO-Ir MMO-Ru
Sulphate (mg dm-3 )
Q (A h dm-3)
349
Figure 4. pH values during anodic oxidation of polluted SWE.
350 351
Before finishing the discussion of the effect of the different anodes, the changes in the 352
concentrations of inorganic ions were studied. Table 1 shows the ion concentrations for 353
the initial SWE and after electrolyses using the four anodes tested.
354 355
Table 1. Inorganic ions concentrations before and after electrolysis with different anodic 356
materials.
357
SWE BDD (end) CF (end) MMO-Ir (end) MMO-Ru (end) (mg dm-3) 134.71 152.60 109.06 150.96 151.48 (mg dm-3) 83.98 1.52 46.78 11.38 19.15
(mg dm-3) - 220.23 14.88 124.11 120.99
(mg dm-3) - 179.63 2.53 0 0
358
The concentration increases in the case of using BDD and MMO anodes. This 359
trend is indicative of the presence of much lower amounts of N-derivate in the final 360
solutions using these three anodes. It could be due to the preferential destruction of the 361
by-products by active chlorine. On the contrary, the electrolysis carried out with CF 362
0 5 10 15 20 25
0 2 4 6 8 10
BDD CF MMO-Ir MMO-Ru
pH
Q (A h dm-3)
tried out a decrease in concentration (Ridruejo et al., 2017). Moreover, 363
deamination of the atrazine molecule leads to the release of (Borràs et al., 2010).
364
The nitrogen of atrazine is converted progressively into , as well as to a 365
much lesser extent, but only in the case of BDD, in a concentration of 3.21 mg dm-3. 366
Similar concentrations of and were detected by Borrás et al. (2010) in the 367
degradation of 30 mg dm-3 of atrazine using BDD.
368
The dechlorination process of atrazine is not detected by IC due to that this process 369
competes with the chlorine evolution. ion is unstable and disappears from the 370
solution in all electrolyses. Table 1 shows that initial was more rapidly removed 371
using BDD than using MMO and CF anodes. The larger destruction of with BDD 372
can be ascribed to the quicker consecutive oxidation of active chlorine to and 373
(Ridruejo et al., 2017).
374
3.2. Changes in the biodegradability and toxicity