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In Chapter 2, catalytic reduction, using biogenic Pd/Au nanoparticles, has been investigated on a total of 5 TrOCs, being atrazine, bromoxynil, carbamazepine, diatrizoic acid and dinoseb. Previously conducted studies have only been performed on chlorinated solutes, i.e. diclofenac and trichloroethylene [137, 144], reporting hydrodechlorination reactions. In this study, evidence of a wider range of reactions occurring with bio-Pd/Au is provided, as all TrOCs were effectively reduced, yielding hydrogenation, hydrodeoxygenation, and different hydrodehalogenation products.

Of the 5 TrOCs investigated, carbamazepine, bromoxynil and dinoseb were completely degraded within 30 minutes, atrazine within 4 hours and diatrizoic acid within 8 hours. In comparison, complete oxidative removal of the parent TrOCs by ozone was found for carbamazepine within 12.5 minutes, > 88% oxidative removal within 20 minutes for dinoseb and bromoxynil, and 43% and 62% removal for the more ozone-persistent solutes diatrizoic acid and atrazine respectively after 20 minutes. Catalytic reduction, even when using an efficient bimetallic catalyst [137], is thus a considerably slower mechanism than ozonation, being the state-of-the-art oxidative technique. Even though TrOC degradation is occurring, the slow rate would require large contacting reactors in practice, which will hamper practical applicability of catalytic reduction. The reaction rate can be increased by using a higher catalyst dose, yet the applied dose in this study is generally considered to be an overdose of what is deemed as economically viable [147]. Other problems arising with catalytic reactions are catalyst fouling (largely due to inorganic or organic sulfur containing ions

and compounds), which is difficult or sometimes impossible to reverse [145]; and catalyst leaching, especially when using nanoparticles [146]. For this reason, bio-Pd/Au should in fact be considered as a chemical which is “consumed” in the process, rather than a truly reusable catalyst material [147]. Together with the necessity to dose an electron-donor (such as H2 gas), bio-Pd/Au should

not be considered as a chemical-free method.

Considering the drawbacks as they stand at this moment and are likely to remain in the future, catalytic reduction will not be suitable to be generally implemented in (municipal or drinking) water treatment. However, still, the results prove that the reductive degradation concept is interesting in terms of formed by-products, and could thus be useful for TrOC abatement. However, it is not likely that this will occur through catalytic reduction, rather a different (i.e., faster and less expensive) reductive technique.

It is important to keep in mind that neither catalytic reduction and ozonation result in TrOC mineralization, rather only transformation occurs. If transformation products are of concern, both techniques therefore should be followed by a second subsequent treatment step, capable of removing these transformation products. In this study, activated carbon adsorption was used as a treatment step to remove transformation products. Conclusions concerning the complementarity between AC adsorption and the oxidative and reductive pre-treatments will be discussed in paragraph 1.2.1.

1.1.2. Advanced reduction

Advanced reduction results in a faster degradation of TrOCs than catalytic reduction, indicating it may be a more viable reduction method than catalytic reduction. In the UV/sulfite ARP, degradation can occur through both photolysis and reaction with reducing radicals. A mixture of 27 and 29 TrOCs was subjected to UV/sulfite ARP in Chapter 3 and Chapter 4, respectively, whereby similar results were found in terms of major removal mechanism. Except for diglyme, all TrOCs showed both photolytic degradation and reductive degradation. In Chapter 3 and Chapter 4, respectively 17 solutes and 19 solutes were degraded the fastest in presence of reducing radicals compared to photolytic degradation. The other solutes are removed more efficiently by photolysis, i.e. in absence of sulfite.

Degradation proves to be dependent on the applied sulfite dose. Solutes which are removed well by photolysis show decreased reactivity upon addition of sulfite (due to competition for photons), while solutes which are removed well by reducing radicals benefit from a higher sulfite concentration. It is thus possible that by altering the solution chemistry in order to favor

degradation of one group of solutes, the degradation of other solutes can be suppressed. Thus, in practice, an assessment will have to be made about which sulfite dose should be applied, based on the TrOCs present in the feed water.

However, solutes which are removed faster with photolysis than under ARP conditions are not necessarily unreactive towards reducing radicals, as an increase in reduction product formation is still found for those TrOCs which are categorized with photolysis as the major degradation mechanism, in ARP experiments than in photolysis experiments. Reaction with reducing radicals therefore only occurs slower than photolytic degradation for these TrOCs.

Degradation is also found to be dependent on pH. Photolytic TrOC degradation rates are affected by solution pH, however no clear link with different microspecies, present at different pH, is found. This is supported by some previously observed trends in literature as well, where photolytic removal could not be related to (de)protonated microspecies present either. The exact factors governing (the effect of pH on) photolytic removal are thus still to be elucidated.

In the UV/sulfite ARP, different reducing radicals are being produced under different solution pH, and TrOC removal is affected accordingly. At alkaline pH, hydrated electrons are formed, which overall result in a faster TrOC removal compared to hydrogen atoms, formed at acidic and neutral pH.

Reducing radicals (𝑒𝑎𝑞− and 𝐻•) are found to be more effective for dehalogenation than UV253.7 nm

irradiation, and the hydrated electrons prove to be the most efficient dehalogenating entities (more than 𝐻•). However, since 𝑒𝑎𝑞− is a negatively charged species, electrostatic repulsion can occur at

negatively charged reducible moieties in the target molecules, resulting in a reduced reactivity. This phenomenon is observed for ibuprofen and gemfibrozil, containing negatively charged carboxylate functional groups. However, other mechanisms should also be involved, since for other solutes also containing anionic carboxylate moieties (such as clofibric acid, diatrizoic acid and diclofenac), no higher 𝐻•-induced removal is observed compared to photolysis.

The presence of inorganic matter (NaCl and Na2CO3) results, in general, in a decreased photolytic

TrOC degradation, due to a deactivation of photolytically generated excited states of TrOCs and/or by quenching TrOC based radicals formed upon UV253.7 nm irradiation. An organic matter

matrix also generally decreases photolytic TrOC degradation, in the order of Nordic Reservoir NOM (NR-NOM) > humic acid > alginate. Some exceptions exist towards this photolysis inhibition by these (in)organic matrix substances, indicating component dependent behavior. Dimethoate, paracetamol, naproxen, dinoseb and diatrizoic acid show a faster degradation in NaCl

or Na2CO3 water matrices. Also, ibuprofen and gemfibrozil are removed better when NR-NOM is

present, caffeine removal is faster with addition of humic acid, and all three DOM types result in a faster removal of dimethoate. As a result, photolytic degradation of these TrOCs shows to be influenced by reactive species produced from DOM, such as 1_O

2 and 3DOM*.

TrOC degradation in the UV/sulfite ARP was not largely affected by NaCl, however bicarbonate – surprisingly – enhanced degradation of halogenated solutes. Interestingly, no increased formation of dehalogenation products could however be observed in this case, with the exception of triazine herbicides. This shows that other transformation products than halogenated ones must be formed in parallel. As for dissolved organic matter, both humic acid and alginate decrease TrOC degradation in the UV/sulfite process, while NR-NOM speeds up TrOC degradation. It is thus clear that different types of DOM alter micropollutant removal in a different manner, likely due to different 3_DOM* _{species being generated by different DOM types. Since this effect is not observed}

during photolysis, as a result, Na2SO3 must be responsible for the increased micropollutant

degradation, possibly by enhancing transfer of reactive species from 3_DOM* _{to the target TrOCs.}

As indicated by a lower formation and slower subsequent degradation of reduction products, the reactive 3_DOM*_{-generated species have a lower dehalogenating nature than 𝑒}

𝑎𝑞− . Degradation is

thus very dependent on the solution chemistry.

Interestingly, a more extensive degradation (i.e., further transformation of reduced reaction products) is found with advanced reduction compared to catalytic reduction. This is shown by atra- H, DABA and DH-carb being end products in catalytic reduction, but degrading further in advanced reduction. In this study, it was not possible to determine whether photolysis or reducing radicals are responsible for this further degradation. However, this clearly demonstrates that the UV/sulfite ARP is a more powerful technology than biogenic Pd/Au nanocatalysts for TrOC degradation.

Also the UV/sulfite ARP is not a chemical-free method, due to the necessity to dose Na2SO3 as

electron-donating agent. It must be noted that sulfite is a chemical allowed to be used in drinking water treatment according to the Flemish legislation [32]. In addition, the final end product of sulfite in ARP is sulfate [171], which is a harmless macro-element, naturally present in drinking water. Still, because of the sulfite dose, the sulfate concentration will increase, and thus care must be taken not to exceed the sulfate norm for drinking water of 250 mg/L. Note that the experiments in this study were not conducted at environmentally relevant concentrations. Therefore it is hard to make a statement about what the required sulfite dose would be to achieve adequate TrOC

Compared to the UV/H2O2 AOP, the UV/sulfite ARP requires higher energy inputs to achieve

the same TrOC removal degree, because the effective generation of reducing radicals is lower than the generation of oxidative radicals at 253.7 nm irradiation. The required UV fluences for obtaining 90% TrOC removal in ARP are in the order of several hundred to several thousand mJ cm-2_,

whereas for AOP the energy inputs are usually below 1000 mJ cm-2_{. As such, UV}

253.7 nm/sulfite

cannot compete with UV253.7 nm/H2O2 from an economical point of view.

Since the UV/sulfite ARP is a UV based process, with germicidal 253.7 nm irradiation, it can be expected that ARP will simultaneously act as a disinfection step in water treatment. The necessary UV fluence for reductive TrOC degradation (several hundred to several thousand mJ cm-2_{) is much}

higher than the UV fluence required for disinfection (around 60 mJ cm-2_{) [392]. This is a practical}

advantage in drinking water treatment, where disinfection is a very important aspect in the treatment process. A thorough UV-disinfection during ARP will significantly decrease the number of alive micro-organisms, which may result in a lower required NaOCl dose in post-chlorination, necessary to prevent biological contamination of the drinking water network.