3.1. UV and UV/H2O2
The efficacy of direct UV photolysis at pH 6 and 10.5, and UV with added 60 ppm H
2O
2 at pH 6, for two different UV fluence values, on degradation of the selected TOrCs is presented in
Figure 2. The degradation percentage in the presence of H
2O
2 was favorable for all compounds (compared to UV alone) at pH 6. Sulfamethoxazole (SMX) and particularly diclofenac (DCF) were susceptible to photolysis, with a minor improvement when H
2O
2 was added. Carbamazepine (CBZ), bezafibrate (BZF), and venlafaxine (VLX) were only marginally degraded by direct UV photolysis, but their degradation increased significantly (by 15–30%) in the presence of H
2O
2. IHX and lamotrigine (LMG) also showed low- to moderate degradation by UV photolysis, but there was no substantial improvement with the addition of H
2O
2. Except for DCF, all compounds demonstrated better degradation at pH 6 than at pH 10.5.
The superior efficiency of UV/H
2O
2 vs. UV alone at the same pH has already been demonstrated for several types of TOrCs [
29,
38,
39]. In our case, the indirect mechanism of radical degradation seems to be more effective than photodegradation for all of the studied TOrCs. CBZ, BZF, and VLX have higher K
OH values than SMX, IHX, and LMG (
Table 1), and the relative improvement in degradation is therefore higher. Nevertheless, the overall degradation of the studied TOrCs, even at a higher UV fluence and with the addition of H
2O
2, was lower than 50%, except for DCF and SMX which are known to have a high reaction rate under direct photolysis [
40].
Other studies on TOrC removal from WWROC by UV/H
2O
2 [
13,
40] have also demonstrated sufficient degradation efficiency, even with lower H
2O
2/DOC ratios. However, the deviations in WWROC parameters from individual production processes differ significantly between sites [
17], in addition to the impact of influent parameters, season and the RO setup [
9], affecting the UV/H
2O
2 efficiency (for example, Justo et al. [
13] demonstrated over 70% degradation for all of their studied TOrCs from ratios of 0.54–0.72 ppm H
2O
2/total organic carbon—TOC). Another major factor regarding UV radiation is the variety of possible setups. Every setup will result in different degradation efficiency (e.g., UV lamp locate above the sample surface, as on this study, versus UV lamp immersed inside the sample [
26]). Therefore, an accurate comparison between treatments studied on WWROC from different sites is difficult to achieve. However, in comparison to the one-stage RO process in Justo et al. [
13], this study was done with a two-stage RO process, characterized by higher values of DOC, conductivity, UVA
254 and metal ions, indicating the presence of high concentrations of organic and inorganic compounds that may interfere with the degradation process (scavenging rate).
Even though a relatively high ratio of 1 ppm H
2O
2/1 ppm DOC was applied, H
2O
2 also has HO· scavenging characteristics [
41]. At high H
2O
2 concentration, a competitive reaction between H
2O
2 and HO· becomes significant [
31]:
Therefore, the possibility that the H
2O
2 concentration used here (60 ppm) increased the scavenging rate over TOrCs degradation cannot be ruled out. In addition, the UV light absorbance of H
2O
2 at high concentrations may screen the TOrCs and reduce the rate of their direct photolysis [
31].
3.2. OH· Scavengers
Except for H
2O
2, some other compounds found in wastewater are generally known as OH· scavengers. Those could be organic and inorganic molecules that also react with the OH·, since it is not a selective radical which react strongly with almost any compound. Such known scavengers are the unspecific effluents organic matter (EfOM) [
40,
42], usually expressed by TOC or DOC, inorganic anions as halides [
40,
43], carbonates and bicarbonates [
44], and suspended particles [
45]. Based on the work of Rosenfeldt and Linden [
41], Sharpless and Linden [
44], and Lester et al. [
31] it was possible to calculate the OH· scavenging rate by the matrix (calculations not shown, see
Supplementary Data). Briefly, para-chlorobenzoic acid (pCBA) was added into the WWROC matrix with different concentrations of H
2O
2 and UV radiation. The p-CBA degradation rate was compared to the total OH· formation by each concentration to calculate the scavenging rate. The obtained value of scavenging rate was 1.29 × 10
4 s
−1. This value is relatively low, similar to the values found in lakes [
46] rather than wastewater, which is at least one-fold higher [
46,
47].
Low scavenging rate obtained here might be a result in the acidification stage before RO filtration, which eliminate carbonates/bicarbonates due to the relatively low pH (pH~6 or lower transform bicarbonates to carbonic acid). In addition, the Ultra-filtration (UF) step before the RO process (
Figure 1) significantly reduces suspended particles, which also contributes to OH· scavenging. Finally, the quality of the effluent and its source (e.g., municipal wastewater from rural settlements and small towns), efficiency of the secondary biological treatment and seasonality can also have a major effect on the scavenging rate.
Overall, it seems that no major OH· scavenging is arisen by the studied WWROC matrix. However, this might be a result of the specific WWROC production process used here and cannot be deduced for every WWROC.
3.3. Ozone and O3/H2O2
The efficacy of direct ozonation at pH 6 and 10.5, and ozone with added
60 ppm H
2O
2 at pH 6, for two different ozonation times, for degradation of the selected TOrCs, is presented in
Figure 3. Samples undergoing shorter ozonation time (5 min) demonstrated a lower degree of degradation for the slow and moderately reactive compounds. At the longer ozonation time (12 min), all of the fast and moderate reaction rate compounds demonstrated over 90% degradation for all tested conditions. The slow-reacting IHX and LMG also demonstrated much better degradation efficiency for all tested conditions for this time interval (a longer time interval results in a higher ozone dose).
Comparing operational conditions, the high-pH sample demonstrated the highest degradation efficiency, 70% for both IHX and LMG (under the longer ozonation time). Encouraging, however, was that the sample at pH 6 with the addition of H2O2 (1 ppm for every ppm DOC) demonstrated good TOrC degradation results, only 10% lower than the high-pH condition for LMG and IHX. Compared to samples at pH 6 without H2O2, the degradation efficiency of the former was 15–24% higher.
Ozone reactions can degrade TOrCs via two pathways: direct (O
3) and indirect (HO·) [
48]. While the direct mechanism is specific for each compound and depends on its reaction rate (K
O3), the indirect mechanism is relatively non-selective and usually much more reactive, since it involves radical reactions with the HO· generated during ozonation [
49]. The reaction rate of a TOrC with HO· (K
OH), as already discussed, is usually several orders of magnitude higher than that compound’s K
O3 (
Table 1).
In this study, four out of the seven compounds had relatively high K
O3 values (CBZ, VLX, DCF, and SMX), hence their degradation by ozone was very rapid and did not allow distinguishing the effects of the different tested conditions. Although VLX is considered a fast ozone reactant, its K
O3 is at the lower limit of this definition (
Table 1) and at the lower ozone dose tested, it did not fully degrade. Hence VLX, BZF, LMG, and IHX are the compounds that should be examined for degradation efficacy under the different conditions, with a better distinction between compounds observed at the lower ozone dose interval (5 min of ozonation).
As implied by the results, there is an increment of 6–14% degradation (for each individual TOrC) by the addition of H
2O
2 to the WWROC ozonation at the 5-min interval, and 15–24% at the 12-min interval, when the pH is held constant at 6. The addition of H
2O
2 to the ozonation process enhances HO· formation and the radical reactions in the matrix [
49]. Here, the positive correlation between percent degradation and H
2O
2 addition indicates a preferred indirect oxidation mechanism.
Ozonation at basic pH also accelerates the formation of HO· because the presence of hydroxide ions can initiate ozone decomposition to form HO· [
49]:
Therefore, the effect of ozonation with high-pH WWROC was expected to produce better TOrC degradation than at lower pH. Indeed, the result for ozonation at pH 10.5 demonstrated higher percent degradation for all of the TOrCs, under both ozonation intervals, than at pH 6.0, with and without H2O2 (with one exception—CBZ at the 5 min interval). These results highlight the selected TOrCs preference for the indirect mechanism.
Another important observation is the high efficiency of the indirect mechanism, where a lower ozone dose is required to achieve better degradation, in comparison to the direct mechanism.
Figure 4 shows the dynamic ozone consumption by the samples, during 12 min ozonation for the three different conditions. The accumulated transferred ozone dose (TOD) during the first 2 min is about 50% higher for pH = 6, then pH = 10.5 and pH = 6 with H
2O
2, and the total slope from 2–12 min is 15% higher. This indicating that more ozone was required for potential oxidative reactions in the sample at pH = 6 only, while the two other conditions (addition of H
2O
2 and pH = 10.5) demonstrated lower ozone requirement, since the formed OH· also react with the compounds in the samples, lowering their total direct ozone oxidation potential. Calculated TOD/DOC ratios of ozonation with H
2O
2 (1.24 and 1.72 for the 5- and 12-min intervals, respectively) are smaller than the ratios of the other conditions (1.42 and 1.52 for 5 min; 1.72 and 2.23 for 12 min). Overall, this emphasizes the advantage of the ozone/H
2O
2 process over direct ozonation, as well over the high-pH adjustment of the matrix, since it is actually not practical to increase the WWROC pH to high values on a large scale.
The obtained results are consistent with other studies on wastewater and WWROC ozonation. Lakretz et al. [
25] demonstrated, in their pilot system at SHAFDAN municipal WWTP (Israel), high degradation efficiency by O
3/H
2O
2 (at a continuous flow rate, set to achieve a TOD/DOC ratio of 1.0–1.2) of pretreated secondary effluent, where the fast and moderate ozone-reacting compounds (same as in this study—SMX, CBZ, VLX, DCF, and BZF) were 96–100% degraded. The slow ozone-reacting compounds in their study (IHX as here, iopromide and iopamidol) were 41–81% degraded. However, in their case, the H
2O
2/DOC ratio was about 2.7, much higher than in the current study. Justo et al. [
13] also applied ozonation on a one-stage WWROC matrix (with lower concentrations of TOC, COD, conductivity, and UVA
254) at pH 8.3 (favoring indirect mechanisms), and demonstrated high degradation efficiency (~100%) for several fast-ozone-reacting TOrCs (including DCF, CBZ, and SMX) at a TOD/TOC value of 1.38. Degradation of the slow to moderate-reacting compound atenolol (K
O3 ~ 1.7 × 10
3 M
−1 s
−1 [
50]) was about 80%. For lower ozone doses (TOD/TOC ratios of 0.82 and less), the percent degradation of the tested TOrCs decreased significantly (DCF ~65%, CBZ ~60%, and SMX ~80%).
Overall, the trends from these two examples (and more) are similar to the trends in this study, demonstrating the efficiency of ozone (mainly by indirect mechanism) at degrading TOrCs in solid wastewater matrices.
An additional comparison between the untreated WWROC sample and O
3/H
2O
2-treated WWROC is presented in
Table 6. Several quality parameters were analyzed, and the results indicated the effectiveness of the O
3/H
2O
2 treatment, mostly for reduction of organic parameters (color, UVA
254 and DOC). However, this treatment was insufficient for inorganic pollutants, such as the toxic metals copper, manganese, and nickel, which were not removed at all, and the concentrations of which remained above the recommended and acceptable limits for reclaimed wastewater for irrigation in some countries, including Israel and the United States [
51,
52].