Photocatalytic Oxidation in Drinking Water Treatment Using Hypochlorite and Titanium Dioxide

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Photocatalytic Oxidation in Drinking Water Treatment

Using Hypochlorite and Titanium Dioxide

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Photocatalytic Oxidation in Drinking Water Treatment

Using Hypochlorite and Titanium Dioxide

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universtiteit Delft,

op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,

in het openbaar te verdedigen op woensdag 25 september 2013 om 10:00 door

Amer EL-KALLINY

Master of Science in Chemistry Ain Shams University, Egypt

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Dit proefschrift is goedgekeurd door de promotor: Prof. dr. ir. L.C. Rietveld

Samenstelling promotiecommissie: Rector Magnificus Voorzitter

Prof. dr. Ir. L.C. Rietveld Technische Universiteit Delft, promotor Dr. H.W. Nugteren Technische Universiteit Delft

Prof. dr. ir. P.W. Appel Technische Universiteit Delft

Prof. dr. R. Abdel Wahaab National Research Center (NRC), Cairo, Egypt Prof. dr. M. Kennedy UNESCO-IHE/Technische Universiteit Delft Prof. dr. D. Wolbert École Nationale Supérieure de Chimie de Rennes Prof. dr. T.C. Schmidt University Duisburg-Essen, Germany

Prof. J.P. van der Hoek Technische Universiteit Delft, reservelid

This research was conducted within the framework of INNOWATOR project and financially supported by Agentschap NL and the Organization for Prohibition of Chemical Weapons (OPCW).

Proefschrift, Technische Universiteit Delft

Met samenvatting in het Nederlands/With summary in Dutch ISBN 978-94-6186-214-3

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IN THE MEMORY OF MY FATHER TO MY FAMILY(MOTHER,WIFE&DAUGHTER)

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Summary

Not only the water shortage is the main issue in the global water crisis, but also the water quality tends to be a central factor. The point and non-point sources of pol-lution, including sewage and industrial discharge, affect water quality. The increase in population and the expansion of urbanized and industrialized, combined with in-tensified agricultural activities, are the main causes for increased water pollution. Of particular concern are the so-called micropollutants (e.g., pesticides, pharmacuticals, etc.), which are not effectively removed by the conventional treatment steps in a wa-ter purification plant. Hence, additional, yet cost-effective treatment steps are being developed and implemented to cope with these issues. In this context, advanced ox-idation processes (AOPs) can play an important part due to their ability to produce highly oxidative hydroxyl radicals (•_{OH), which is capable of destruction emerging}

organic pollutants (e.g., endocrine disrupters, pesticides, pharmaceuticals, etc.). In ad-dition, use of renewable energy resources, as in the case of solar photocatalysis, would reduce the treatment costs and make AOPs more attractive to the water industry. The use of photochemical conversion and photo-oxidation technology to remove not only the aforementioned hazardous chemicals but also disinfection byproducts (DBPs) caused by chlorination has attracted a considerable interest in the last decade. Nowa-days, the use of UV and solar light for not only water disinfection but also for oxida-tion of undesirable chemical contaminants is increasingly being applied in the field of water treatment. Here, the oxidation is achieved by the production of highly oxidative

•_{OH by combining light with a suitable precursor (e.g., H}

2O2, O3, semiconductor).

This thesis focuses on two AOPs, NaOCl/UV and TiO₂/UV solar light, which are representative for homogeneous and heterogeneous photo-oxidation, respectively. Chapter1starts by introducing some important concepts related to the fundamentals of homogeneous and heterogeneous photo-oxidation mechanisms in the water phase. It introduces and discusses the present knowledge in photo-oxidation. Furthermore, a short description of different immobilized photocatalysts and solar reactors is pre-sented.

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xiv Summary

The research questions regarding the potential of the homogeneous photo-oxidation NaOCl/UV process as an AOP for the degradation of humic acids (HAs) and its risk for the formation of disinfection byproducts are addressed in Chapters 2 and3. In Chapter2, The production of •_{OH via the low pressure UV (LPUV)/NaOCl process}

was detected by a photoluminescence (PL) technique using terephthalic acid (TA) as a probe molecule, as the LPUV/NaOCl process has been considered previously as an AOP due to its ability to form•OH radicals. The PL method proves that the Cl• radicals plays an important role in the free radical initiation step and enhances the oxidation process to a comparable extent as •OH. In order to investigate the effect of Cl•radicals separately on the oxidation of HAs, sodium bicarbonate is used as •OH scavenger. It was found that the formation of adsorbable organic halogens (AOX) due to the combined action of •_{OH and Cl}• _{is higher than in the absence of} •_OH,

be-cause •_{OH has the potential to hydroxylate aromatic structures, thereby increasing}

the chlorination potential of HA. Once formed, chloroform can be degraded by •_OH,

but only very slowly: concentrations of up to 130 μg/L are still found. These results draw attention to the risk of producing additional disinfection byproducts when using LPUV with hypochlorite as an AOPs for the oxidation of organic material. Because most industrial AOPs use a medium pressure UV (MPUV) light source, we also in-vestigated the risk for formation of DBPs when using this light source in the presence of hypochlorite Chapter2. In this chapter, the results are discussed of a comparative study between MPUV/NaOCl and LPUV/NaOCl.

AOPs are driven by the rate of free radical formation. However, comparison between MP and LP so far has been based on time or on emitted photons. In our investigation, the comparison between two processes is based on the moles of free radicals (•OH and Cl•) formed per unit reactant volume(Γ) by UV-illumination of NaOCl, as these are the driving force for the reaction. For this purpose, the concentration of NaOCl was maintained constant during the reaction period by compensating its degradation due to photolysis.Γ in mol/L was calculated by multiplying the moles of NaOCl decom-posed by photolysis (qdecin mol/L · s), which equivalent to the moles of NaOCl dosed

(qdosed in mol/L · s) by time of the reaction in seconds. It was shown that by usingΓ

as the parameter, a reasonable agreement was found between the two processes. This proves that using this experimental method, AOPs can be compared and up-scaled in a straightforward manner, without the need to calculate the light distribution in reactors with often complicated geometries. The degradation of HA with MPUV/NaOCl in the presence and in the absence of •OH scavenger had a similar trend as LPUV/NaOCl process. This indicates that Cl• radicals played the same role in HA fragmentation. The combined effect of •OH and Cl• result in higher AOX levels in MPUV/NaOCl process and LPUV/NaOCl, because •OH radicals have the potential to hydroxylate aromatic structures and can increase the chlorination of HA. It was found that at high free-radical dose, the same levels of AOX and CHCl3 formed with LP and with MP

respectively. This may be relevant for process with high free-radical dose, as in swim-ming pools with recirculating water. In addition, risk for CHCl₃and AOX formation is

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Summary xv

similar for LP and MP at same free-radical dose. CHCl₃, once formed, is not degraded with either LP or MP. Moreover, the photo-degradation of HA by LPUV/NaOCl pro-cess with and without•_{OH scavenger are 1.6 times higher than that for MPUV/NaOCl}

process, which result in higher initial rate of AOX and CHCl₃formation. This raised the attention to the risk of using the LPUV/NaOCl process especially at the short reaction times that are relevant for water treatment.

In this thesis, it is shown that a fixed-bed photocatalytic reactor based on TiO2coated

stainless steel woven meshes fitted in layers has major advantages over the commonly used flat-plate reactor and the dispersed-phase reactor. This presents a novel reactor in the oxidation of water contaminants based on heterogeneous solar photocatalysis. The challenge of designing an efficient solar photocatalytic reactor is in using a suitable catalyst structure both to optimize the area covered by photocatalytic particles and ef-ficiently distribute the incident light throughout the reactor volume. There have been many attempts to immobilize TiO₂ photocatalyst on different structures of supports to increase the surface/volume ratio and to enhance the photocatalytic oxidation effi-ciency of flat-bed photocatalytic reactors. Chapter 4deals with the immobilization of TiO₂photocatalyst on stainless steel woven mesh structures to be used in a fixed-bed solar reactor for water purification. Immobilization of such a complex shape needs a special coating technique. For this purpose, dip coating and electrophoretic deposition (EPD) techniques were used. The EPD technique using a stable TiO₂suspension in sol gel O500 as electrolyte at 300 celsius dryness temperature gave the TiO2coating

films better homogeneity and adhesion, fewer cracks, and higher •OH formation than the dip coating technique. Coating with EPD is up-scalable for large stainless steel woven meshes. A simple model was sufficient to describe the distribution of light in the reactor in the presence of absorbing medium (e.g., contaminated water with humic acids) in order to determine the optimum number of mesh layers and the separation distance between them. The dimensionless parameter β = ε · C · d can be used to op-timize the fraction of the captured light by meshes in a colored medium in order to be applicable for various light absorbing mediums with different extinction coefficients and for different concentrations.(d) in cm is the separation distance between mesh layers,(ε) in Lmg−1cm−1is the extinction coefficient(ε), and (c) in mgL−1is the concentration of the colored medium. It was found that three to four mesh layers are enough to harvest the light in the presence of colored water with HA. It was also found that the optimum separation distance between mesh layers to have as much as possible illuminated light on top of the last mesh layer in the bottom of the reactor is 0.2 cm. This distance meets the requirement of combining sufficient vertical mixing with a high efficiency of captured of photons per unit reactor volume.

In Chapter 5, the rate of humic acids oxidation by a solar photocatalytic fixed-bed reactor fitted with stainless steel woven meshes coated with TiO2photocatalyst on a

lab scale compared to the flat plate. It is shown that per unit surface area exposed to the sun, a fixed-bed solar reactor fitted with stainless steel meshes degrades HA 3.4

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xvi Summary

times faster than a flat-plate reactor. The comparison of the adsorption behavior of the small coated meshes in the beaker and the large coated meshes in the fixed-bed reactor shows that the catalyst morphology is scale-independent as the adsorption kinetics are scale-independent, and that no other scale effects play a role. On the other hand, the degradation kinetics of HA using small coated meshes in the beaker and large coated meshes in the fixed-bed solar reactor obey Langmuir-Hinshelwood model. It was found that the actual rate constants of the reaction between •OH radicals and HA in the large and small scale mesh structure are close, which also indicates that up-scaling of such reactors to a larger scale is feasible.

In Chapter6, the solar photocatalytic degradation of atrazine was tested using the im-mobilized TiO₂over stainless steel woven meshes fitted in layers and compared with flat plate. In addition, the regeneration efficiency of the immobilized TiO₂ photocat-alyst through illuminating the catphotocat-alyst surface in the presence of deionized water is investigated by measuring the •_{OH formation with a photoluminescence technique.}

It was found that atrazine was degraded photocatalytically by using the coated mesh structure. The solar photocatalytic degradation of atrazine using four coated woven meshes was 2.3 times faster than that of the flat-plate due to the surface area of the mesh structure is higher than that of flat-plate for the same volume of water and the availability of light penetration through them. Moreover, the rate of solar degradation of atrazine in a solar fixed-bed reactor using meshes coated with photocatalytically active TiO2is not determined by the rate of adsorption of atrazine. On the other hand,

the photocatalyst can be reactivated by irradiation the coated meshes for 30 min or longer in the presence of deionized water.

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Samenvatting

Naast het tekort aan water vormt ook de kwaliteit van het beschikbare water een belangrijk aspect van de wereldwijde watercrisis. Lokale en diffuse bronnen, zoals huishoudelijke en industriële afvalwaterlozingen beïnvloeden de waterkwaliteit. Bevolkingstoename, groei van steden en industrie en de intensivering van landbouw en veeteelt zijn de belangrijkste oorzaken van toenemende waterverontreiniging. De huidige technieken voor afvalwaterzuivering zijn niet in staat microverontreinigingen (bijvoorbeeld pesticides en medicijnen, of producten daarvan) op adequate wijze te verwijderen. Extra kosteneffectieve zuiveringsstappen worden momenteel ontwikkeld en uitgetest om aan dit probleem het hoofd te kunnen bieden. Geavanceerde oxi-datieprocessen (AOPs) kunnen hierbij een belangrijke rol spelen omdat daarbij zeer reactieve hydroxylradicalen (•_{OH) worden gevormd, die in staat zijn deze organische}

verontreinigingen (endocriene disruptors (hormoonverstorende stoffen), pesticides, geneesmiddelen, etc.) af te breken. De combinatie met het gebruik van hernieuw-bare energiebronnen, zoals fotokatalyse op basis van zonne-energie, kan de zuiver-ingskosten verlagen en maakt het gebruik van AOPs aantrekkelijk voor de waterindus-trie.

Gedurende de laatste decennia heeft fotochemische omzetting en foto-oxidatie van zowel bovengenoemde verontreinigingen als van bijproducten van desinfectie met chloor, speciale aandacht getrokken. UV-straling en zonlicht worden tegenwoordig steeds vaker toegepast in de waterzuivering voor desinfectie en voor de oxidatie van chemische verontreinigingen. De oxidatie vindt daarbij plaats door de werking van

•_{OH radicalen die gevormd worden door bestraling van een daarvoor geschikte}

pre-cursor (H₂O₂, O₃of een semiconductor). In dit proefschrift komen twee AOPs aan de orde, NaOCl/UV en TiO₂/zonlicht UV, welke representatief zijn voor respectievelijk homogene en heterogene foto-oxidatie.

In hoofdstuk1worden een aantal fundamentele aspecten geïntroduceerd van homo-gene en heterohomo-gene foto-oxidatie in de waterfase. De huidige kennis van foto-oxidatie wordt behandeld en bediscussieerd. Verder wordt er een korte beschrijving gegeven van de verschillende types geïmmobiliseerde fotokatalysatoren en zonnereactoren.

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xx Samenvatting

In de hoofdstukken 2 en 3 wordt aandacht besteed aan de mogelijkheden om NaOCl/UV in te zetten als AOP voor de afbraak van humuszuren en aan de risico’s van de vorming van schadelijke bijproducten die daarbij optreden. UV-straling van een lage druk kwiklamp in combinatie met NaOCl (LPUV/NaOCl) kan gezien worden als een AOP. De geproduceerde•_{OH werd gedetecteerd en gekwantificeerd door middel}

van een fotoluminescentietechniek waarbij tereftaalzuur als gevoelig meetmolecuul werd gebruikt. Deze methode laat zien dat Cl• radicalen tijdens de initiële fase van het oxidatieproces een even belangrijke rol spelen als de•OH radicalen. Om te onder-zoeken wat het effect van Cl• op het oxidatieproces van humuszuren is, werd natri-umbicarbonaat toegevoegd om de werking van •OH uit te schakelen. Er werd gevon-den dat de vorming van adsorbeerbare organische halogeenverbindingen (AOX) hoger was bij de combinatie van•_{OH en Cl}•_{dan bij afwezigheid van}•_{OH. Dit wordt}

verk-laard doordat•_{OH in staat is aromatische structuren te hydroxyleren waarbij de kans}

op chlorinatie van humuszuren toeneemt. Wanneer chloroform gevormd is kan dit door

•_{OH weer worden afgebroken, maar dit gebeurt zeer traag. Concentraties tot 130 μg/L}

werden nog steeds aangetroffen. Deze resultaten laten de risico’s van de vorming van desinfectiebijproducten zien wanneer LPUV/NaOCl als een AOP voor de oxidatie van organisch materiaal wordt gebruikt. Omdat de meeste industriële geavanceerde oxidatieprocessen de middendruk kwiklamp als lichtbron gebruiken (MPUV), hebben we ook de risico’s van de vorming van desinfectiebijproducten onderzocht met deze lichtbron in combinatie met hypochloriet (NaOCl). In hoofdstuk 3 worden de resul-taten van de LPUV/NaOCl en MPUV/NaOCl systemen met elkaar vergeleken. De effectiviteit van AOPs wordt bepaald door de mate waarin radicalen worden gevormd. Tot nog toe werd de vergelijking tussen de LP en MP systemen gebaseerd op tijdseenheden of op de emissie van fotonen. Omdat vrije radicalen de drijvende kracht voor de oxidatiereacties zijn, is in deze studie echter de vergelijking gedaan op basis van de hoeveelheden geproduceerde vrije radicalen •_{OH en Cl}• _per

reac-torvolume(Γ), die gevormd werden door UV-straling op NaOCl. Om dit te kunnen uitvoeren werd de NaOCl concentratie tijdens de reactie constant gehouden door te compenseren voor de NaOCl die door fotolyse werd afgebroken. De hoeveelheid vrije radicalenΓ (in mol/L) werd berekend door de hoeveelheid NaOCl die werd afgebroken door fotolyse, qdec(deze is gelijk aan de hoeveelheid gedoseerde NaOCl,

qdosed) (beiden in mol/L · s) te vermenigvuldigen met de reactietijd (in s). DoorΓ als

parameter te kiezen werd een aanvaardbare overeenkomst gevonden tussen de twee processen. Dit toont aan dat het mogelijk is door deze methode te gebruiken, om zonder berekeningen van lichtverdeling in reactoren met vaak complexe geometrieën, deze twee systemen te vergelijken en op te schalen. De afbraak van humuszuren in het MPUV/NaOCl systeem met en zonder •OH onderdrukking vertoont een zelfde trend als in het LPUV/NaOCl systeem. Dit betekent dat Cl• radicalen in beide systemen dezelfde rol spelen in de fragmentatie humuszuren. In beide systemen wordt ook meer AOX gevormd door de gecombineerde actie van Cl•en•_{OH dan met}

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Samenvatting xxi

daardoor chlorinatie van humuszuur te verhogen. Bij een hoge dosis vrije radicalen werden voor beide systemen dezelfde hoeveelheden AOX en chloroform gevormd. Dit kan van belang zijn in processen met hoge dosering vrije radicalen zoals in zwembaden met continu rondgepompt water. De risico’s voor vorming van AOX en chloroform zijn voor de LP en MP systemen vergelijkbaar bij dezelfde dosering van vrije radicalen. Wanneer chloroform eenmaal is gevormd wordt het niet meer afgebroken door bestraling met de LP noch met de MP lampen. Verder blijkt de foto-oxidatie van humuszuren in het LPUV/NaOCl systeem met of zonder •OH onderdrukker 1,6× hoger dan in het MPUV/NaOCl systeem. Dit resulteert in een hogere initiële snelheid van vorming van AOX en chloroform en verhoogt dus het risico op desinfectiebijproducten voor het LPUV/NaOCl systeem, vooral gezien de korte reactietijden die in de waterzuivering gebruikelijk zijn.

In dit proefschrift is aangetoond dat een gepakt bed fotokatalytische reactor, bestaande uit lagen van geweven roestvrijstalen zeefstructuren die gecoat zijn met TiO₂, grote voordelen heeft ten opzichte van de vaak gebruikte vlakke plaat reactor en de TiO₂ dispersie reactor. De gekozen reactor is nieuw als toepassing voor de oxidatie van verontreinigingen in water gebaseerd op heterogene fotokatalyse met zonlicht. De uitdaging een efficiënte fotokatalytische reactor te ontwikkelen is erin gelegen dat er een geschikte katalysatorstructuur moet worden gevonden die gelijktijdig het beschik-bare oppervlak aan katalytisch actieve deeltjes en de verstrooiing van licht binnen het reactorvolume optimaliseert. Er zijn verschillende manieren onderzocht om fotokat-alytisch actieve TiO2 deeltjes te immobiliseren op diverse structuren en dragers met

de bedoeling de oppervlakte/volume verhouding te vergroten en de oxidatie efficiëntie van de reactor te verhogen. Hoofdstuk4behandelt de immobilisatie van TiO2deeltjes

op roestvrijstalen geweven zeefstructuren. Vanwege de complexe vorm van de struc-tuur is er een speciale coating techniek vereist. Dip coating en afzetting door middel van elektroforese werden hiervoor onderzocht. Gebruikmakend van een stabiele TiO₂ suspensie in sol/gel O500 als elektrolyt, resulteerde bij 300 celsius droging een TiO₂ gecoate film met een betere homogeniteit en hechting, minder scheurvorming en een hogere •_{OH vorming dan met de dip coating methode werd bereikt. Deze methode}

bleek ook op te schalen voor grotere oppervlakken. De lichtverdeling in de reactor in de aanwezigheid van de zeefstructuur en vervuild water kon met een eenvoudig model adequaat worden beschreven. Door de dimensieloze parameter β te gebruiken (β = εCd, waarbij ε de extinctiecoëfficiënt van het medium is (in L/mg · cm), C de concentratie van de absorberende stof in het medium (in mg/L) en d de afstand tussen de zeeflagen (in cm)) kon het optimale aantal zeeflagen en de afstand tussen de zeven worden geoptimaliseerd. Drie tot vier zeeflagen bleken voldoende om vrijwel alle licht te benutten in een door humuszuren gekleurd water. Verder werd gevonden dat de op-timale afstand tussen de lagen, waarbij de onderste laag nog substantieel bijdraagt aan de reactie, ongeveer 0,2 cm bedraagt. Bij deze afstand is er voldoende verticale meng-ing in de reactor mogelijk in combinatie met hoge efficiëntie van meng-ingevangen fotonen per reactor volume eenheid.

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xxii Samenvatting

In hoofdstuk5werden op laboratoriumschaal de met zeefstructuren gepakte bed re-actor en de vlakke plaat rere-actor, beide gecoat met TiO₂, met elkaar vergeleken door voor beide de kinetiek van de oxidatie van humuszuren te bestuderen. Er is gebleken dat per eenheid door de zon belicht oppervlak de gepakte bed reactor humuszuren 3,4× sneller afbreekt dan de vlakke plaat reactor. Aannemende dat adsorptiekinetiek schaalonafhankelijk is, kan uit een vergelijkend onderzoek van gecoate zeefstructuren in een (klein) bekerglas en in de gepakte bed reactor worden geconcludeerd dat de morfologie van de katalysator schaalonafhankelijk is en dat er geen andere schaal-effecten een rol spelen. De afbraak van humuszuren met zowel de kleine structuren in een bekerglas als de grotere structuren in de gepakte bed reactor volgen beide het Langmuir-Hinshelwood kinetische model. Uit de experimenten bleek dat de reacties-nelheidsconstanten voor de afbraakreactie van humuszuren met •_{OH voor de kleine}

en de grotere zeefstructuren vrijwel gelijk waren, hetgeen er op duidt dat opschaling van dergelijke reactoren tot de mogelijkheden behoort.

In hoofdstuk 6 werd de fotokatalytische afbraak van atrazine bestudeerd voor de gepakte bed reactor en de vlakke plaat reactor. Verder werd ook de mogelijkheid van regeneratie van het TiO₂gecoate katalysatoroppervlak onderzocht door het meten van

•_{OH radicalen met behulp van de fotoluminescentietechniek. Atrazine werd}

inder-daad in de fotokatalysatorreactor afgebroken en met vier gecoate zeefstructuren in de gepakte bed reactor was de afbraak 2,3× sneller dan in de vlakke plaat reactor met het-zelfde watervolume. De afbraaksnelheid van atrazine in de gepakte bed reactor blijkt niet beperkt door de adsorptiesnelheid van atrazine aan het katalytisch actieve opper-vlak. Het fotokatalytische oppervlak kan worden geregenereerd en gereactiveerd door het tenminste 30 min te belichten in demiwater.

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1. Introduction

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2 Chapter 1

1.1 Introduction

More than 3.4 million people die each year from water pollution, lack of sanitation, and hygiene-related causes [1]. Together, pneumonia and diarrhoea, due to lack of access to clean water and sanitation, are responsible for an estimated 40% of all child deaths around the world year [2]. The problem of water pollution has increased in both developed and developing countries. It negatively affects economic growth as well as the physical and environmental health and quality of life for billions of people. All these facts should be sufficient to mobilize international action about water quality. Water quality is highly variable from one country to another, as it depends on social, economic, and geographical factors. As a result, there is no “one-size-fits-all” solu-tion [3]. In addisolu-tion, a major challenge facing the developing countries is to close the rapidly increasing gap between water demand and supply and between consumption and potentially available safe water resources. They need to expand water supply ser-vices to meet the ever increasing needs of industry and to support growing population. The increase in population and the expansion of urbanized and industrialized areas indicate that water pollution problems are on the rise. On the other hand, due to the polar nature of some of these pollutants (such as pesticides, hormones, pharmaceuti-cals, and industrial chemicals), they are not completely removed by traditional water treatment barriers. At the same time, the conventional water treatment methods form carcinogenic and/or mutagenic compounds during the disinfection process [4]. Con-sequently, there is a need to develop water treatment methods that are able to treat a large variety of water contaminants, in particular the above-mentioned contaminants in groundwater and in surface water. In this context, advanced oxidation processes (AOPs) can play an important role in improving the drinking water quality as they are capable of reducing the concentration of many emerging organic pollutants (e.g., estradiol, acetominophen, diclofenac, etc.) [5–7].

1.2 Advanced oxidation processes

AOPs were defined by Glaze et al. as near ambient temperature and pressure wa-ter treatment processes which involve the generation of highly reactive •OH radicals in sufficient quantity to affect water purification [8]. These treatment processes are considered as very efficient methods for the remediation of contaminated ground, sur-face, and wastewaters containing non-biodegradable organic pollutants. The oxidation potential values of different oxidants are listed in Table 1.1and show that the •_OH

radicals are extremely powerful oxidants. The •_{OH radicals can subsequently, when}

sufficient contact time is available, oxidize organic species into CO₂ and H₂O [9]. These radicals can oxidize natural organic matter (NOM) and disinfection byproducts

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Introduction 3

Table 1.1 Standard electrochemical reduction potentials of common oxidants [9].

Oxidant Half-cell reaction Oxidation

potential (V) •_{OH (hydroxyl radical)} •_{OH + H}+_{+ e}−_{→ H} 2O 2.80 O₃(ozone) O₃(g) + H+_{+ 2e}−_{→ O} 2(g) + H2O 2.07 H₂O₂(hydrogen peroxide) H₂O₂+ 2H+_{+ 2e}−_{→ 2H} 2O 1.77

HOCl (hypochlorous acid) 2HOCl + 2H++ 2e−→ Cl₂+ 2H₂O 1.49

Cl₂(chlorine) Cl₂(g) + 2e−→ 2Cl− 1.36

(DBPs) precursors by removing hydrogen atoms or adding electrophiles to their dou-ble bonds [10]. There are many types of treatment combinations that can produce •OH, such as photo-Fenton or Fenton reagent, O₃/H₂O₂, O₃/UV, H₂O₂/UV, HOCl/UV, and TiO₂/UV [11]. Among these types, the UV-based AOP offers a great potential because of the already wide acceptance and application of UV light as an effective disinfection method [10].

Photo-oxidation reactions are defined here as oxidation reactions of dissolved sub-stances with•_{OH radicals or Cl}•_{radicals that are generated by the combination of UV}

light and a free-radical precursor. In homogeneous photo-oxidation, the precursor is dissolved in water, and the generated free radicals react with the dissolved substances in the aqueous phase. In heterogeneous photo-oxidation, the precursor is a solid; the free-radicals are generated at the solid-water interface, and the oxidation reaction takes place at the solid-water interface with the adsorbed substance.

1.3 Homogeneous photo-oxidation with hypochlorite

Chlorination is the disinfection process mostly used in water treatment plants (WTPs), because it is cost-effective for the deactivation of pathogenic microorganisms. More-over, chlorine can be easily dosed, measured, and controlled. However, protozoa that can be present in surface waters (e.g., Cryptosporidium parvum and Giardia lamblia) cannot be deactivated by chlorine [12–14]. Moreover, the reaction between humic acids (HAs), which are the major organic contaminants in surface water, and chlorine leads to the formation of chlorination disinfection byproducts (CDBPs), that are considered potentially carcinogenic [15]. As a result, the introduction of new technologies in water treatment plants (WTPs) has become a requirement to improve the drinking water quality. For this purpose, UV irradiation has been applied as it is also highly effective against chlorine-resistant protozoa (e.g., Cryptosporidium parvum and Giardia lamblia) [12–14]. In addition, coupling UV with oxidant such as

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4 Chapter 1

H₂O₂ is gaining interest due to its ability for degradation of organic micropollutants by both UV photolysis and UV/H₂O₂oxidation [7]. However, the presence of residual hydrogen peroxide in water may require a separate removal step downstream of the process. From this point of view, the presence of the chlorination agent NaOCl can be an alternative for H₂O₂ where UV irradiation of NaOCl produces •_OH,

which turns disinfection and photolysis processes into an AOP. The homogeneous photo-oxidation using UV/NaOCl has been the subject of some research related to wastewater treatment [16–18] and drinking water treatment applications [19, 20]. The free chlorine exists as hypochlorous acid (HOCl) and hypochlorite (OCl–). Only the neutral form is responsible for disinfection. HOCl is formed when chlorine reacts with water as follows [19]:

Cl2+ H2O → HOCl + HCl. (1.1)

The concentration of HOCl and OCl–species is strongly pH-dependent and follows the equilibrium [19–21]

HOCl OCl−_{+ H}+_(pK

a= 7.5 at 25◦C). (1.2)

On the other hand, the UV/NaOCl process is capable of producing the hydroxyl and chlorine radicals simultaneously through the following reactions [19]:

HOCl + hν →•_{OH + Cl}•

, (1.3)

OCl−_{+ hν → O}•−_{+ Cl}•_, _(1.4)

O•−_{+ H}

2O →•OH + OH−. (1.5)

However, so far, the risk of formation of CDBPs, when applying UV treatment in chlorinated surface water, has attracted little attention, despite the well-known fact that under these conditions the formed chlorine free radicals may lead to an increased risk for the formation of unwanted CDBPs. Similar questions arise when using UV in chlorinated swimming pools [22].

1.4 Heterogeneous photo-oxidation

In general, AOPs are expensive to operate, due to the usage of costly chemicals and the consumption of energy [6]. Use of renewable energy resources, as in the case of solar photocatalysis, would reduce the treatment costs and make AOPs more attractive to the water industry [6]. Future applications of AOPs could be developed by using immobilized reusable catalysts and solar energy. Therefore, research is increasingly focusing [23] on the heterogeneous photocatalysis with TiO₂/UV solar light.

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Introduction 5

Unlike the homogeneous photo-oxidation, in heterogeneous photo-oxidation, the pre-cursor of the hydroxyl radicals and the reaction medium are in different phases, with the advantage that the catalyst can be separated from the treated water effluent. The overall process in the aqueous phase can be described with the following five steps [24]:

(1) transfer of the reactants from the bulk to the catalyst interface, (2) adsorption of the reactant on the surface of the catalyst, (3) reaction on the surface of the catalyst in the adsorbed state, (4) desorption of the products from the surface of the catalyst, (5) products removal from the catalyst interface region.

The photocatalyst is a semiconductor. It is activated by photons (step 3), where pho-tons are consumed to form electron-hole pairs that initiate the oxidation reduction re-actions. The advantages of using this process for the treatment of contaminated water were summarized by Malato et al. [23] as follows:

(1) the process takes place at normal pressure and at ambient temperature; (2) the oxygen demand for the electron capture reaction can be directly obtained

from atmosphere;

(3) the catalyst is inexpensive, safe, and, in principle, can be reused; (4) the catalyst can be immobilized on different types of substrates; (5) solar light can activate and excite the catalyst.

Among different semiconductors, TiO₂ is the most used photocatalyst since it is af-fordable, environmentally friendly, non-toxic, chemically resistant, and reusable [25].

1.4.1 Mechanism of heterogeneous photocatalysis (UV/TiO

₂

)

The electronic structure of the TiO₂semiconductor plays a key role in photocatalysis. It consists of a valence band and a conduction band. The energy difference between these two levels represents the band gap energy(Eg). When the TiO2semiconductor is

excited by photons with an energy amount equal to or higher than its band gap energy, electrons receive energy from the photons and are thus transferred from the valence band to the conduction band (see Figure 1.1). In the case of anatase TiO₂, the band

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6 Chapter 1

Figure 1.1 Schematic reaction mechanism of TiO2 photocatalyst with UV light and the

pro-duction of holes in the VB by the excitation of electrons to the CB. Holes and electrons are responsible of oxidation and reduction reactions, respectively [9].

gap is 3.2 eV, therefore UV light(λ ≤ 387 nm) is required [9] and the UV part of the solar light can activate it. The reaction is expressed as follows [25]:

TiO₂_{−→ e}hν ₋_(TiO

2) + h+(TiO2). (1.6)

The photo-generated electrons and holes can recombine within a very short time, re-leasing energy in the form of heat. Therefore, oxygen is essential for capturing the electrons and for preventing hole-electron recombination. Electrons and holes that migrate to the surface of the TiO2semiconductor without recombination can,

respec-tively, reduce and oxidize the reactants adsorbed by the semiconductor. The oxidation reaction is the basic mechanism of photo-catalytic water purification. Both surface adsorption as well as photo-catalytic reactions can be enhanced by nano-sized semi-conductors as more reactive surface area is available.

The hole produced by irradiation reacts with water or surface-bound hydroxyl ion producing hydroxyl radical [26, 27] as follows:

h+_(TiO

2) + H2O →•OH + H+, (1.7)

h+_(TiO

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Introduction 7

The holes can also react directly with adsorbed substrate [28] or reductant (e.g., car-boxylic acids) generating CO₂as follows [29]:

h+_(TiO

2) + RXads→ RX+ads, (1.9)

h+_(TiO

2) + RCOO−→ R•+ CO2. (1.10)

There are two reaction pathways for the electrons released by irradiation of photo-catalyst with dissolved molecular oxygen. Both of them are characterized by the pro-duction of superoxide radical anion, O•₂–. The first reaction pathway is as follows [26, 27]:

e−_(TiO

2) + O2→ O•2−, (1.11)

O•−

2 + e−(TiO2) + 2H+→ H2O2. (1.12) The second reaction pathway is as follows [28]:

2e−_(TiO 2) + 2O2→ 2O•2−, (1.13) O•− 2 + H+→ HO•2, (1.14) O•− 2 + H++ HO•2→ H2O2+ O2. (1.15) The following equation is a summation of (1.11) and (1.12) and also a summation of (1.13)–(1.15):

2e−_(TiO

2) + O2+ 2H+→ H2O2. (1.16) Therefore, one dissolved oxygen molecule can consume two electrons released by irradiation of the photocatalyst. Oxygen acts as an electron scavenger and inhibits the electron hole recombination; on the other hand, hydroxyl radical can be produced due to the formation of hydrogen peroxide as shown in the following reactions [26, 28]:

H₂O₂+ e−_(TiO

2) →•OH + OH−, (1.17)

H2O2 hν

−→ 2•_OH. _(1.18)

Finally, the•_{OH radicals oxidize organic adsorbed pollutants}_(RX

ads) onto the surface of the titanium dioxide particles as follows [28]:

•_{OH + RX}

ads → Intermediate. (1.19)

1.4.2 Immobilization of the photocatalyst

The application of TiO₂in suspension (e.g., TiO₂-P25 Degussa) is, in water purifica-tion, effective in capturing sun light, because suspended TiO₂ powders have a high

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8 Chapter 1

specific surface area in a range from 50 m2_{/g to higher than 300 m}2_{/g [30–33], which,}

in turn, helps in avoiding mass transfer limitations, resulting in a high photocatalytic activity [34]. However, light transport limitation appears with a high catalyst loading. Besides, it is difficult to separate the small TiO₂ particles from water after treatment [33, 35, 36], where the particle size of fine P25 Degussa TiO₂powders is about 21 nm and where, in the aqueous phase, the particles form aggregates within the micron range [33]. To overcome this, the catalyst particles can be immobilized on a surface. This, however, may lower the oxidation potential per volume of water compared to the dispersed-phase system, due to mass transfer limitation and light transport limitation caused by (i) a lower catalyst surface-to-volume ratio, (ii) the presence of substrate that absorbs light and worsens its distribution, and (iii) lack of movement of the cata-lyst particles [36, 37].

There have been many attempts to immobilize TiO₂photocatalyst over different struc-tures of supports whilst increasing the surface/volume ratio at the same time, to en-hance the photocatalytic oxidation efficiency of the reactor. However, the reactor can only be efficient if the total surface area of the catalyst absorbs sufficient light. There are different kinds of materials that have been used as a support for TiO₂, such as glass and borosilicate glass [38–42], cellulose fibres [43], and stainless steel [44– 47]. Among the different supports, stainless steel is an excellent substrate material for many reasons. First, it keeps its structural integrity under the high temperature re-quired for calcination of the TiO2films whereas quartz glass, for example, softens and

deforms. Second, it is not susceptible to attack during the coating process (e.g., sodium ions diffused from the soda-lime glass into the TiO2film and decreased the

photocat-alytic activity [48]). Third, stainless steel can be used in the electrochemical process whereas quartz and ceramics cannot be used because of their dielectric properties. At last, it can be easily used in complex shapes and has excellent mechanical properties [49]. The structures of substrates, so far used, do not allow an even light distribution in a fixed-bed reactor and therefore the photocatalytic efficiency is much lower compared to that of a dispersed-phase reactor. Therefore, the challenge of designing an efficient photocatalytic reactor is in using a suitable catalyst structure to optimize both the area covered by photocatalytic particles and the light distribution. The design criteria of such a reactor should be (i) good vertical mixing, (ii) high surface area per unit reactor volume, and (iii) no shadowing effect.

1.4.3 Solar reactors

Utilization of solar light for activation of the TiO₂photocatalyst and production of the highly oxidative•_{OH radicals can be cost-effective for small-scale applications}

espe-cially in rural areas in developing countries where solar energy is abundant. There are two main systems of solar reactors: light concentrating and non-light-concentrating. The light concentrating systems, such as parabolic-trough reactors and compound

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Introduction 9

parabolic collectors, do not necessarily have advantages over non-light-concentrating systems [50]. The wavelength ranges of the solar light spectrum that can excite TiO₂in the diffuse portion, which is the reflected portion of solar light from a surface broken up and scattered into different directions, are almost equal to those in direct portion of the solar radiation reaching the surface of the earth [51]. This means that a light concentrating system utilizes half of the solar radiation available for photocatalyst activation which is the direct portion of the solar light.

The main disadvantages of light concentrating systems are (i) using only direct radi-ation, (ii) being expensive, and (iii) having low optical and quantum efficiencies as the light concentrating reactor is small, while receiving a large amount of energy per volume unit. On the other hand, non-light-concentrating systems are based on two fac-tors: the high percentage of UV photons in the diffuse component of solar radiation and the low-order dependence of rates on light intensity. Besides, they do not have moving parts or solar tracking devices, which makes its manufacturing and mainte-nance simpler and therefore less expensive. As a result, recently, there is a common trend for constructing installations mainly based on non-concentrating collectors since parabolic-trough concentrators are not the best option [23].

1.5 Thesis aims and outline

The research in this thesis is focusing on two different AOPs which are representa-tive for two different classes of photo-oxidation (homogeneous and heterogeneous) and which are important for improving the drinking water quality: the UV/NaOCl ho-mogeneous photo-oxidation system for large-scale WTP and TiO2/UV solar light for

small-scale water treatment that can serve a small community.

A schematic outline of this thesis is shown in Figure1.2. After this general introduc-tion (Chapter1) in which the most important research topics about AOPs, UV/chlorine process, and solar/TiO₂ process are introduced, there are two chapters for homoge-neous photo-oxidation using the UV/NaOCl process and three chapters for heteroge-neous photo-oxidation using solar/TiO₂process.

The aim of Chapter 2 is to investigate the potential of the low pressure UV (LPUV)/NaOCl process on HA degradation and the formation of CDBPs, because HAs are considered to be the main precursor for the CDBPs in chlorination pro-cesses. In Chapter3, the aim is to make a comparative study based on moles of free radicals formed by NaOCl per unit reactant volume between medium pressure UV (MPUV)/NaOCl and LPUV/NaOCl.

The aim of Chapter4is to prepare TiO2photocatalyst with a high specific surface area

by immobilizing it on stainless steel woven meshes in order to be used fitted in lay-ers for water purification. In this chapter, immobilization of TiO2photocatalyst over

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10 Chapter 1

Figure 1.2 Schematic reaction mechanism of TA with•OH and Cl•free radicals.

woven meshes was done by the sol-gel dip coating and EPD methods. Also, the aim of this chapter is to determine the optimum number of mesh layers and the separa-tion distance between them in the presence of colored water contaminated with HA, based on the design criteria of a fixed-bed solar reactor. For that purpose a study on the light distribution through the coated catalyst over woven mesh was conducted. In Chapter5, the aim is to investigate the photocatalytic efficiency for the degradation of HA through the TiO2 photocatalyst coated on stainless steel woven meshes in the

presence of UV solar light and to evaluate the performance of the catalyst structure in different scales and compare it to a flat-plate reactor. In this chapter, the adsorption behavior of HA on the coated TiO₂films and the solar photo-degradation of HA using mesh structure in fixed-bed reactor is discussed and compared to a flat-plate reactor. In Chapter6, the solar photocatalytic degradation of atrazine is reported, using immobi-lized TiO₂over stainless steel woven meshes. In addition, the regeneration efficiency of the immobilized TiO₂ photocatalyst through illuminating the catalyst surface in the presence of deionized water is investigated by measuring the •_{OH formation with}

a photoluminescence technique. Finally, conclusions and recommendations are pre-sented in Chapter7.

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Introduction 11

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12 Chapter 1

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Introduction 13

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[26] Pirkanniemi, K., Sillanpaa, M., 2002. Heterogeneous water phase catalysis as an environmental application: a review. Chemosphere 48, 1047–1060.

[27] Baird, N.C., 1997. Free radical reactions in aqueous solutions: examples from advanced oxidation processes for wastewater and from the chemistry in airborne water droplets. The Journal of Chemical Education 74(7), 817–819.

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[29] Houas, A., Lachheb, H., Ksibi, M., Elaloui, E., Guillard, C., Herrmann, J.M., 2001. Photocatalytic degradation pathway of methylene blue in water. Applied Catalysis B: Environmental 31(2), 145–157.

[30] Malato, S., Blanco, J., Vidal, A., Richter, C., 2002. Review: photocatalysis with solar energy at a pilot-plant scale: an overview. Applied Catalysis B: Environ-mental 37, 1–15.

[31] Sunada, K., Watanabe, T., Hashimoto, K., 2003. Studies on photokilling bacteria on TiO₂ thin film. Journal of Photochemistry and Photobiology A: Chemistry 156, 227–233.

[32] Gumy, D., Rincon, A.G., Hajdu, R., Pulgarin, C., 2006. Solar photocatalysis for detoxification and disinfection of water: Different types of suspended and fixed TiO2catalysts study. Solar Energy 80, 1376–1381.

[33] Thiruvenkatachari, R., Vigneswaran, S., Moon, I.S., 2008. A review on UV/TiO₂ photocatalytic oxidation process. The Korean Journal of Chemical Engineering 25(1), 64–72.

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14 Chapter 1

[34] Mehrotra, K., Yablonsky, G.S., Ray, A.K., 2003. Kinetic studies of photocat-alytic degradation in a TiO₂slurry system: distinguishing working regimes and determining rate dependences. Industrial & Engineering Chemistry Research 42, 2273–2281.

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[38] Fujishima, A., Zhang, X., Tryk, D.A., 2008. TiO₂ photocatalysis and related surface phenomena. Surface Science Reports 63(12), 515–582.

[39] Yang, H., Zhu, S., Pan, N., 2004. Studying the mechanisms of titanium dioxide as ultraviolet-blocking additive for films and fabrics by an improved scheme. Journal of Applied Polymer Science 92, 3201–3210.

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[42] Ziegmann, M., Doll, T., Frimmel, F.H., 2006. Matrix effects on the photocatalyt-ical degradation of dichloroacetic acid and atrazine in water. Acta Hydrochimica et Hydrobiologica 34, 146–154.

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Introduction 15

[45] Chen, Y., Dionysiou, D.D., 2006. TiO₂photocatalytic films on stainless steel: The role of Degussa P-25 in modified sol-gel methods. Applied Catalysis B: Environmental 62, 255–264.

[46] Chen, Y., Dionysiou, D.D., 2006. Effect of calcination temperature on the pho-tocatalytic activity and adhesion of TiO₂ films prepared by the P-25 powder-modified sol-gel method. Journal of Molecular Catalysis A: Chemical 244, 73– 82.

[47] Chen, Y., Dionysiou, D.D., 2007. A comparative study on physicochemical properties and photocatalytic behavior of macroporous TiO₂-P25 composite films and macroporous TiO₂films coated on stainless steel substrate. Applied Catalysis A: General 317, 129–137.

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2. UV-induced formation and

breakdown

of

chlorination

disinfection byproducts in

hu-mic acid rich waters

*

The potential of the low pressure UV (LPUV)/NaOCl process as an advanced oxidation process (AOP) for the degradation of humic acids (HAs), and its risk for the formation of disinfec-tion byproducts is investigated. The LPUV/NaOCl process has been considered previously as an AOP due to its ability to form both hypochlorite (OCl•) and hydroxyl radicals (•OH). The production of•OH is detected by a photoluminescence technique using terephthalic acid as a probe molecule. Sodium bicarbonate is used as•OH scavenger in order to investigate the effect of Cl• radicals separately. Adsorbable organic halogens (AOX), and in particular chlo-roform, were formed during the LPUV/NaOCl process. It is found that AOX formation due to the combined action of•OH and Cl• is higher than in the absence of•OH, because•OH has the potential to hydroxylate aromatic structures, thereby increasing the chlorination potential of HA. Once formed, chloroform is can be degraded by•OH, but only very slowly: concentrations of up to 130μg/L are still found. These observations draw the attention to the risk of producing additional disinfection byproducts when using LPUV with organic material in the presence of hypochlorite.

*_{Submitted to: Water Research by Amer S. El-Kalliny, María L. Dell’Arciprete, Henk W. Nugteren, Luuk C.} Rietveld, and Peter W. Appel.

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18 Chapter 2

2.1 Introduction

Ensuring safe drinking water at the tap and minimizing the formation of chlorina-tion disinfecchlorina-tion byproducts (CDBPs) poses a challenge for drinking water plants that use chlorine as a disinfectant, particularly, the case when surface water with high concentrations of humic acids is used. Humic acids (HAs) constitute 40–60% of the naturally-occurring dissolved organic matter (DOM) in aqueous systems that can pass easily through the filtration systems [1]. The reaction between HAs and chlorine leads to the formation of CDBPs, that are considered potentially carcinogenic [2].

One of the major groups of CDBPs is the trihalomethanes (THMs) out of which chlo-roform represents the major part. Due to its potential link to human health effects, the maximum acceptable concentration (MAC) of chloroform in drinking water is, ac-cording to the World Health Organization, 200 μg/L [3]. However, the MAC for total trihalomethanes (TTHM) has been reduced to 100 μg/L and 80 μg/L by the European Community [4] and the US Environmental Protection Agency [5], respectively. Con-sequently, the introduction of new technologies in water treatment plants (WTPs) has become a requirement to improve the drinking water quality. One of these techniques includes the use of UV light. Coupling UV with chlorination disinfection is gaining interest due to the fact that UV irradiation is highly effective against chlorine-resistant protozoa (e.g., Cryptosporidium parvum and Giardia lamblia) [6, 7]. Moreover, irra-diation of the chlorination agent NaOCl produces•_{OH, which turns disinfection and}

photolysis processes into an advanced oxidation process (AOP) [8]. The AOP is capa-ble of reducing the concentration of many emerging organic pollutants (e.g., estradiol, acetominophen, diclofenac, etc.) [9–11].

Whatever the position of the UV disinfection system, before or after post-chlorination, there will always be free chlorine present in the water when subjected to irradiation. Pre-chlorination is used to control the biological growth in settling basins and filters [12]. This is especially done at high temperatures to control algal growth and filter fouling [13]. Besides, chlorine is an oxidant used in transmission lines prior to en-tering a treatment facility for odor control [14]. On the other hand, post-chlorination is the final necessary step of water purification for protecting drinking water against external contamination and regrowth of bacteria in the distribution system [15]. Shah et al. [16] reported that treatment of nitrate-containing drinking waters with medium pressure UV (MPUV) lamps followed by chlorination could promote chloropicrin for-mation. Besides, halonitromethane formation was not enhanced with LPUV treatment up to 1500 mJ/cm2_{, while MPUV/postchloramination enhanced dichloroacetonitrile}

formation with commercial humic acid.

Jin et al. [8] evaluated the potential of the UV/chlorine process as an AOP on two aspects: the •_{OH production yield factor and the photo-oxidation rate constants of}

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LPUV/NaOCl process based on fluence 19

process would appear to be lower than that of the UV/H₂O₂process. Sichel et al. [17] assessed UV/chlorine process followed by quenching of chlorine in a post treatment at technical scale and at process energies for the removal of emerging contaminants in a WTP. They found that this oxidation process achieved the degradation of de-sethylatrazine, sulfamethoxazole, carbamazepine, diclofenac, benzotriazole, tolyltria-zole, iopamidole, and 17∝-ethinylestradiol with energy reductions of 30–75% com-pared to the UV/H₂O₂process. Besides, they found very low concentration of THMs (3.5 ± 0.4 μg/L) and N-Nitrosodimethylamine (NDMA) (below detection limit) due to quenching of chlorine in the post treatment. Thus, without quenching, much higher THM concentrations could have been measured if the chlorine was not removed. How-ever, so far, the risk of formation of CDBPs when applying UV treatment in chlori-nated surface water has attracted little attention, despite the well-known fact that under these conditions chlorine-based free radicals are being formed. Similar questions arise when using UV in chlorinated swimming pools [18].

Therefore, in this paper, we investigated the potential of the LPUV/NaOCl process on HA degradation and the formation of CDBPs. Because HAs are considered the main precursor for the CDBPs in chlorination processes, they were selected as a model com-pound in this research. Chloroform and AOX were analyzed to monitor the occurring reactions.

2.2 Basic concepts

The concentration of free chlorine species, HOCl (hypochlorous acid) and OCl– (hypochlorite), in aqueous solution is strongly pH-dependent and follows the equi-librium [19]

HOCl OCl−_{+ H}+_(pK

a= 7.5). (2.1)

All the experiments were performed at pH 6.5–7.5. Under such circumstances, both HOCl and OCl– species are present. Feng et al. [19] measured the overall quantum yields of free chlorine (HOCl and OCl–) with 254 nm UV light for different pH (5– 10) and different concentrations (3.5 mg Cl/L and 70 mg Cl/L). At low concentrations, such as those used in this research, the quantum yields of HOCl and OCl–are approx-imately constant at1.0 ± 0.1 and 0.9 ± 0.1, respectively. This is an indication of equal contributions of free chlorine species HOCl and OCl–for hydroxyl and chlorine radicals’ production.

Hydroxyl and chlorine radicals are formed simultaneously in the UV/chlorine process through the following reactions [8]:

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20 Chapter 2

OCl−_{+ hυ → O}•−_{+ Cl}•

, (2.3)

O•−_{+ H}

2O →•OH + OH−. (2.4)

The irradiation of HA with UV light can result in photolysis, whereas the presence of NaOCl can additionally result in the formation of oxidizing species. The direct photolysis is dependent on the ability of the HAs to absorb the emitted light described by the molar absorption coefficient ε.

To model the oxidation of HA using UV radiation, the direct photolysis as well as the oxidation process based on hydroxyl and/or chlorine radicals were taken into account. The following rate equation was used to describe the degradation of HA using the LPUV/H₂O₂process [20]:

−d[HA]

dt = (kp,HA+ k•OH,HA[

•_OH])[HA], _(2.5)

and for the LPUV/NaOCl process in the presence of an•_{OH scavenger (NaHCO}

3):

−d[HA]

dt = (kp,HA+ kCl•,HA[Cl

•_])[HA], _(2.6)

where k_p,HAis the first-order direct photolysis rate constant, k•_OH,HAis the rate con-stant for the oxidation by hydroxyl radicals, k_Cl•_,HA is the rate constant for the oxi-dation by chlorine radicals, [HA] is the concentration of the humic acid, [•OH] is the concentration of hydroxyl radicals, and [Cl•] is the concentration of chlorine radicals. Under the condition that the•OH or Cl•concentration is constant (steady state) due to maintaining the concentration of the H₂O₂or NaOCl constant, respectively, equations (2.5) and (2.6) can be reduced to

−d[HA]

dt = (kp,HA+ k•OH,app)[HA], (2.7)

−d[HA]

dt = (kp,HA+ kCl•,app)[HA], (2.8)

where k•_OH,app and k_Cl•_,app are the apparent reaction rate constants for the ox-idation by hydroxyl radicals in LPUV/H₂O₂ process and by chlorine radicals in LPUV/NaOCl/NaHCO₃ process, respectively. These apparent rate constants can be determined experimentally.

In order to make the irradiation processes comparable to the results in literature, it is essential to convert the time-based rate constants into UV dose or fluence-based constants [21]. The fluence(H) or UV dose (mJ/cm2_{) is calculated by multiplying the}

average fluence rate in the reactor (E in mW/cm2_{) by the exposure time as follows}

[20]:

Photocatalytic Oxidation in Drinking Water Treatment Using Hypochlorite and Titanium Dioxide

Photocatalytic Oxidation in Drinking Water Treatment

Using Hypochlorite and Titanium Dioxide

Photocatalytic Oxidation in Drinking Water Treatment

Using Hypochlorite and Titanium Dioxide

Amer EL-KALLINY

Contents

Summary

Samenvatting

1. Introduction

1.1

Introduction

1.2

Advanced oxidation processes

1.3

Homogeneous photo-oxidation with hypochlorite

1.4

Heterogeneous photo-oxidation

1.4.1

Mechanism of heterogeneous photocatalysis (UV/TiO

)

1.4.2

Immobilization of the photocatalyst

1.4.3

Solar reactors

1.5

Thesis aims and outline

References

2. UV-induced formation and

breakdown

of

chlorination

disinfection byproducts in

hu-mic acid rich waters

*

2.1

Introduction

2.2

Basic concepts