Análisis, interpretación y discusión de resultados

DOM is known to absorb light of different wavelengths, leading to the formation of reactive entities such as reactive oxygen species (OH•_, 1_O

2, O2•−), reducing radicals (𝑒𝑎𝑞− ) and triplet states

of DOM (3_DOM*_{) [331, 351–353]. In reducing environment and in the presence of a hydroxyl}

radical scavenger, 𝑒𝑎𝑞− , 1O2 and 3DOM* are expected to be the most important photolytic generated

reactive species from DOM [331, 354]. 3.3.1. Photolysis

In general, photolysis of TrOCs in the presence of all three different DOM types resulted in a lower removal degree (Figure 4.3-A and Table D.3 in Appendix D). Even though UV fluence90 was

already corrected for the light screening effect of DOM, still a general decrease in reactivity is observed. Similar as with the inorganic matrix substances, this may be due to an effect of DOM on the photolysis excited states of the TrOCs. An additional effect (other than light screening) of DOM on photolytic removal of polybrominated diphenyl ether has been observed before, which was also attributed to association of the target compounds with DOM [297]. Additionally, Chen et al. (2013) [355] reported that under UV253.7 nm irradiation, humic substances decreased photolysis of

estrogenic solutes. The general inhibition order of the different DOM substances is NR-NOM > HA > alginate. The inhibiting effect of organic substances thus proves to be dependent on the characteristics of the DOM, as different DOM will interact differently with excited TrOCs, and will form different types of reactive species (such as 3_DOM*_{) upon UV irradiation.}

Interestingly, ibuprofen and gemfibrozil are removed better in the presence of NR-NOM, which was not the case for HA or alginate. It is speculated that this is due to reaction with reactive oxygen

be produced more from NR-NOM than from HA and alginate. Gemfibrozil is a structurally similar solute towards ibuprofen, which could explain its similar behavior. This effect is only observed with these two solutes, which may be because they are relatively inert towards direct photolysis, with high fluence90 of 16670 mJ cm-2 and 20180 mJ cm-2 in milli-Q, therefore amplifying the effect

of the secondary reaction. Similarly, caffeine (also relatively inert towards direct photolysis) is degraded more easily with HA, possibly due to reaction with another reactive species photogenerated from HA. In addition, dimethoate presents a vast decrease in fluence90 during

photolysis when either type of DOM is present, possibly due to easy reaction with singlet oxygen or different types of 3_DOM*_.

3.3.2. ARP

Figure 4.3-B and Table D.4 in Appendix D demonstrate the effect of the three types of DOM on ARP. The influence of NR-NOM and alginate on any ARP was examined here for the first time. Contrary to what is observed with photolysis (paragraph 3.3.1), the presence of NR-NOM results in a lower fluence90, or more efficient removal. Since NR-NOM did not (in general) enhance

removal during photolysis, the higher removal observed during ARP must be because of an interaction between (photolytic excited) NR-NOM and sulfite or sulfite-generated radicals. It is possible that the presence of Na2SO3 enabled a better transfer of reactive entities generated from

NR-NOM photolysis, which manifests itself in a more obvious fluence90 decrease for the solutes

of group B, however this hypothesis cannot be unambiguously confirmed from these results. HA on the other hand has a generally inhibiting effect on TrOC removal by ARP, which is in agreement with previously conducted studies performed on bromate and perfluorooctane sulfonate, which both showed a decreased removal in the presence of HA [328, 329]. On the other hand, Li et al. (2014) [161] found no influence of HA on chloroacetic acid removal. We found that in presence of HA only ketoprofen, dimethoate and naproxen show a better removal in ARP. Addition of alginate also decreases TrOC removal, however less pronounced as HA. The exact phenomena influencing the different removal mechanisms are still unclear, and should be further investigated in detail, however it is obvious that different types of DOM influence the removal of TrOCs in ARP experiments in a different manner, and the influence of DOM is also TrOC-specific.

3.3.3. Screening for reduction products

Very similar trends in terms of formation of reduction products are observed in milli-Q and DOM matrices for the photolysis experiments (Figures D.2-A to D.16-A in Appendix D). Since parent compound degradation was affected by addition of DOM, this indicates that a variety of photolysis products are being formed (especially in milli-Q), part of which remain undetected in our

experiments. These undetected photolysis products may have different production and subsequent degradation trends in different water matrices, however photolytic dehalogenation and hydrogenation product formation is not found to be markedly influenced by DOM.

However, large differences in reduction product trends are observed during ARP experiments with milli-Q and in the presence of DOM (Figures D.2-B to D.16-B) in Appendix D). The presence of NR-NOM considerably decreases production and further degradation of all detected reduction products. The generally lower fluence90 found for parent compound removal in NR-NOM matrix

suggests faster degradation reactions, however the lower formation of dehalogenation and hydrogenation products indicates that different transformation products are formed. These results support the theory that presence of Na2SO3 enables a better transfer of reactive entities generated

from NR-NOM photolysis, which may not be dehalogenating or hydrogenating in nature. In addition, for 7 out of 15 detected reduction products (DH-carb, tric-ClH2, PYAA, sima-H, atra-H,

DABA and fenuron, Figures D.4, D.6 – D.9, D.13 and D.16 in Appendix D), HA shows to (partly) inhibit further degradation of these reduction products, whereas the initial formation of these dehalogenation products was not majorly affected.

Interestingly, regardless of the inhibiting effects of some DOM types on parent TrOC and reduction product removal, the formation and/or further degradation of all reduction products is in general still higher in ARP conditions than in photolysis for all DOM types, indicating that ARP are definitely more effective in TrOC reduction than just photolysis.