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Environmental Processes

A comparative analysis of photocatalytic and electrochemical degradation of 4-Ethylphenol in saline conditions Robert Bruninghoff, Alyssa van Duijne, Lucas Braakhuis, Pradip Saha, Adriaan W Jeremiasse, Bastian Mei, and Guido Mul Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b01244 • Publication Date (Web): 08 Jul 2019 Downloaded from pubs.acs.org on July 21, 2019

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Environmental Science & Technology

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A comparative analysis of photocatalytic and

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electrochemical degradation of 4-Ethylphenol in

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saline conditions

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Robert Brüninghoff †, Alyssa K. van Duijne †, Lucas Braakhuis †, Pradip Saha ‡, Adriaan

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W. Jeremiasse †,‡, Bastian Mei †, Guido Mul †,*

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† PhotoCatalytic Synthesis Group, MESA+ Institute for Nanotechnology, Faculty of

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Science and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The

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Netherlands.

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‡ Department of Environmental Technology, Wageningen University and Research, P.O. Box

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17, 6700 AA Wageningen, The Netherlands

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†,‡ MAGNETO special anodes B.V. (an Evoqua brand), Calandstraat 109, 3125 BA Schiedam,

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The Netherlands

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* Corresponding author: Guido Mul

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E-mail: [email protected]

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Phone:+31534893890 1 ACS Paragon Plus Environment


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ABSTRACT

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We evaluated electrochemical degradation (ECD) and photocatalytic degradation (PCD)

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technologies for saline water purification, with focus on rate comparison and formation and

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degradation of chlorinated aromatic intermediates using the same non-chlorinated parent

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compound, 4-ethylphenol (4EP). At 15 mA.cm-2, and in the absence of chloride (0.6 mol.L-1

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NaNO3 was used as supporting electrolyte), ECD resulted in an apparent zero-order rate of

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30 μmol.L-1.h-1, whereas rates of ~300 μmol.L-1.h-1 and ~3750 μmol.L-1.h-1 were computed for

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low (0.03 mol.L-1) and high (0.6 mol.L-1) NaCl concentration, respectively. For PCD, initial

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rates of ~330 μmol.L-1.h-1 and 205 μmol.L-1.h-1 were found for low and high NaCl

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concentrations, at a photocatalyst (TiO2) concentration of 0.5 g.L-1, and illumination at

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λmax ≈ 375 nm, with an intensity ≈ 0.32 mW*cm-2. In the chlorine mediated ECD approach,

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significant quantities of free chlorine (hypochlorite, Cl2) and chlorinated hydrocarbons were

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formed in solution, while photocatalytic degradation did not show the formation of free

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chlorine, nor chlorine-containing intermediates, and resulted in better removal of non-purgeable

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hydrocarbons than ECD. The origin of the minimal formation of free chlorine and chlorinated

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compounds in photocatalytic degradation is discussed based on photoelectrochemical results

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and existing literature, and explained by a chloride-mediated surface-charge recombination

34

mechanism.

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INTRODUCTION

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Water pollution is one of the greatest challenges of modern society.1,2 The treatment of polluted

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water to enable its re-use is essential,3 while efficient removal of (recalcitrant) organic

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pollutants, present in low concentrations, requires development of innovative technology.4–7

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Particularly difficult is treatment of industrial effluents containing high amounts of sodium

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chloride, which impedes biological treatment due to negative effects of the NaCl on the

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microbial flora such as plasmolysis.8,9

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Advanced oxidation processes (AOP) such as photocatalytic degradation (PCD) or

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electrochemical degradation (ECD) are promising methodologies for the removal of

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(recalcitrant) organics in saline conditions. Highly reactive oxygen species (ROS), e.g.

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hydroxyl radicals (●OH) are generated oxidatively (anodically) in both methodologies.10

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Sodium chloride, ubiquitous in many industrial wastewaters,8 is known to further enhance

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degradation rates in ECD, due to (anodic) chloride oxidation and the consecutive formation of

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reactive chlorine species (RCS: e.g. Cl●, Cl2, or HOCl).11–15 However, the major disadvantage

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of ECD in saline media is the formation of chlorinated by-products.12,16–20 These by-products

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typically induce high water toxicity, and thus deteriorate the environment.21–23

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For TiO2-based photocatalytic degradation, chloride is usually considered to be an inhibitor.

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Several mechanisms have been proposed to explain the inhibition: scavenging of holes or OH-

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radicals by chloride ions;24–27 blocking of active surfaces sites by chloride ions;25,28–32,

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formation of an inorganic salt layer;33 or chloride acting as surface-charge-recombination-

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center for photogenerated charge carriers.34 Often, more than one mechanism is used to explain

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the inhibiting effect.35–40 In addition, aggregation of TiO2 particles has been reported to reduce

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the number of absorbed photons influencing degradation rates.41 While the inhibiting effect of

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chloride, using TiO2 as photocatalyst, has been discussed frequently in the literature, detailed

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studies regarding the formation of reactive chlorine species (RCS), such as hypochlorite, and 3 ACS Paragon Plus Environment


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chlorinated intermediates in aqueous solution (starting from a non-chlorinated parent molecule)

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are rare, and obtained results seem to be inconsistent. For example, negligible amounts of

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organochloride compounds have been reported during phenol degradation in high saline media

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[50 g.L-1 NaCl],38,42,43 which appears in disagreement with detection of several chlorinated

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intermediates arising from the photocatalytic degradation of the azo dye Acid Orange 7 at

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moderate salinity [5.8-11.8 g.L-1].39

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The role of chloride ions in TiO2-based PCD of organic pollutants is thus not fully

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understood44,45 and requires further studies.46 In particular, the formation of toxic by-products

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needs attention, to reveal the environmental impact of AOPs.4,20,47–52

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Although some comparative studies between different AOPs, or a combination of technologies

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have been reported,53–62 to the best of our knowledge a comparative study specifically

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addressing the concentration dependent formation of chlorinated intermediates during ECD,

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PCD and photoelectrochemical degradation (PECD), starting from a non-chlorinated parent

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compound in saline solutions, has not yet been performed.

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In this study the influence of NaCl on the degradation rate and mechanism of the model

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compound 4-ethylphenol (4EP) is investigated. Two salt concentrations, corresponding to

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brackish (1.75 g.L-1) and sea water salt concentrations (35 g.L-1), covering a relevant

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concentration range of industrial effluents,8 have been studied. TiO2 particle suspensions or

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TiO2 thin films illuminated by UV light are used for PCD and PECD, respectively. A platinized

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Ti (Ti/Pt) anode was utilized for ECD, and the performance compared to a Boron-Doped-

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Diamond (BDD) electrode.

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The obtained results clearly demonstrate that the rate of degradation of 4EP in ECD & PECD

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is mainly the result of chemical chlorination, since free chlorine and various chlorinated

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hydrocarbons were detected. In contrast, the formation of chlorinated compounds was 4 ACS Paragon Plus Environment

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significantly smaller in PCD, and the overall removal effective, despite an inhibiting effect of

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chloride.

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MATERIALS AND METHODS

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Chemical and Reagents

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The following chemicals were used in this study: titanium dioxide (Hombikat UV 100,

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Sachtleben (Venator)), 4-Ethylphenol (Sigma Aldrich 99%), sodium chloride (Sigma Aldrich

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≥ 99%), sodium nitrate (Sigma Aldrich ≥ 99.0%), demineralised water (Merck MilliQ system,

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resistivity > 18MΩ*cm), water (LC-MS grade, Biosolve BV), acetonitrile (LC-MS grade,

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Biosolve BV), formic acid (98-100%, LC-MS grade, Merck), 1-(4-Hydroxylphenyl)ethanol

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(synthesis

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4-Hydroxybenzaldehyde (Sigma Aldrich 98%), 4-Hydroxyacetophenone (Sigma Aldrich

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99%), 4-Ethylresorcinol (Alfa Aesar 98%), 4-Ethylcatechol (Sigma Aldrich 95%), and

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2-Chloro-4-ethylphenol (AKos GmbH, > 95%), 2,6-Dichloro-4-ethylphenol (AKos GmbH,

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> 90%).

see

SI),

2-(4-Hydroxylphenyl)ethanol

(Sigma

Aldrich

98%),

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Degradation experiments

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If not otherwise stated all experiments were performed in solutions containing NaCl or NaNO3

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at various concentrations (0.03 mol.L-1 or 0.6 mol.L-1). Sodium nitrate (NaNO3) was used for

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comparison, since NaNO3 is supposed to be an “inert” water additive in oxidative processes.

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Photocatalytic degradation

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Pre-treatment of TiO2 photocatalyst, via annealing at 600 °C for 4 h (named in the following

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H600), was performed to achieve an improved photocatalytic performance (optimum between

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adsorbed surface OH groups and available holes at the surface), as reported in earlier work by

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our group.63 The presence of the anatase phase was confirmed by XRD analysis (see XRD

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patterns in Figure S1). 5 ACS Paragon Plus Environment


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4EP belongs to the group of alkylphenols, which have been found in process-water from the

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petrochemical industry65, or as degradation product from alkylphenol ethoxylates (used in the

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chemical industry).66 These compounds are suspected to induce endocrine disruptive effects,

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and thus pose a health risk.67 In this study 4EP was used as model compound offering several

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experimental advantages, for example a moderate toxicity and an adequate water solubility.

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Moreover, the structure consists of three functional groups (benzene ring, alcohol group, alkyl

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chain) representing important functional groups of more complex compounds found in

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wastewater5,6 and is suitable to study the selectivity in reactions with chlorine.68,69

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PCD experiments were performed as described in the following: 4EP stock solution (approx.

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50 mg.L-1) was pre-saturated with air for 20 min. The catalyst H600 [25(+/- 1) mg], the required

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amount of salt and 50 mL stock solution were mixed in a quartz glass beaker, covered with a

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quartz glass lid and stirred (350 rpm) in the dark for 30 min before the first sample was taken

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(0 h measurement). UV-irradiation of the suspension was achieved using a custom-made closed

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box reactor64 equipped with eight Philips UV lamps (TL-D 18W BLB, λmax ≈ 375 nm, intensity

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≈ 0.32 mW.cm-2). Any influence of UV light absorption by the salt solutions on the PCD rate

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was excluded (Figure S2)33. Liquid samples were taken after 0.5 h, 1 h, 2 h, 3 h, 4 h and 5 h of

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continuous illumination. Each time approx. 1.5 mL solution was taken. Afterwards, the samples

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were filtered using a Phenex RC membrane filter (0.2 µm; Phenomenex), and analysed by

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HPLC and LC-MS.

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Electrochemical degradation

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ECD of 4EP was performed in a custom-made single-compartment cell (see scheme in

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Figure S3). A platinized Ti plate electrode or BDD electrode (Magneto special anodes B.V;

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geometric surface area 3.14cm2) was used to study the electrochemical degradation of 4EP. A

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Pt mesh and a Ag/AgCl electrode (3 M NaCl, BASi) were used as a counter electrode and as a

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reference electrode, respectively. The reactor was filled with 70 mL 4EP stock solution (pre6 ACS Paragon Plus Environment

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saturated with air). Degradation experiments were carried out under galvanostatic conditions at

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15 mA.cm-2 (VersaSTAT4 potentiostat, PAR or biologic SP-300/VMP3). The electrolyte was

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constantly stirred during the experiments. Sampling was performed as described for PCD. For

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high NaCl concentration, samples were also taken after 1 min, 2.5 min, 5 min, 10 min, 15 min,

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20 min and 30 min. The cell was not purged with air, and the cathodic reaction is thus likely

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the formation of hydrogen. The surface area of the Pt Gauze was significantly larger than the

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applied anode, and therefore the cathodic reaction was not limiting the degradation of

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hydrocarbons (See Figure S3).

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To evaluate reactivity of 4EP with reactive chlorine species, 9.5 mL of 4EP stock solution

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[50mg/L] was mixed with sodium hypochlorite solution (0.5 mL, equal to approx. 20 mg.L-1

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free chlorine) and treated for 15 min. Filtration was followed by LC MS analysis.

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Photoelectrochemical degradation

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PECD experiments were carried out in a custom-made Teflon based PEC-cell. The reactor was

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equipped with a thin film of TiO2 on a Ti-electrode (for the preparation see SI; geometric surface

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area 3.14 cm2 [TiO2/Ti]) or H600 coated on FTO ([H600/FTO], 2.54 cm2]) and a Pt-mesh

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counter electrode. The reactor was filled with the 4EP stock solution containing 0.03 mol.L-1

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NaCl, and pretreated by purging air. Experiments were carried out under potentiostatic

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conditions at 0.1 V vs. Ag/AgCl reference electrode (3 M NaCl, BASi) using a potentiostat

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(VersaStat3, PAR or biologic SP-300). The electrolyte was constantly stirred and illuminated

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with a 365 nm LED [for TiO2/Ti experiments] or 375 nm LED [for H600/FTO experiments]

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(Roithner Lasertechnik GmbH, APG2C1-365/375-E, 200 mW output power) placed in front of

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the glass window of the PEC-cell.

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Analytical methods

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HPLC analysis was performed by a ThermoFisher Scientific Dionex Ultimate 3000 HPLC

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system equipped with a UV detector and a reversed phase Luna Omega polar C18 column 7 ACS Paragon Plus Environment


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(3 µm, 150 x 2.1 mm, protected by a polar C18 security guard column (both Phenomenex)).

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The sample injection volume was 5 µL. The oven temperature was set to 30 °C. A water ([A];

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0.1% formic acid) - acetonitrile (0.1% formic acid) gradient system was used at a flow rate of

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0.200 mL/min and a total run time of 60 min. (0 min 100% A, 1 min 100% A, 30 min 50% A,

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35 min 50% A, 45 min 5% A, 50 min 5% A, 55 min 100% A, 60 min 100% A). For detection,

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UV absorption was measured at 276 nm. For the quantification, an external standard calibration

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was applied (for more information see SI Figure S4 & Table S1-3).

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For the identification of intermediates, a Bruker amaZon SL ion trap using electrospray

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ionization (conditions: capillary 4500 V, end plate offset 500 V, 15 psi nebulizer gas pressure

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(N2), 8 L/min dry gas flow (N2), 200 °C dry gas temperature, scan between 50 to 1000 m/z) was

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coupled to the HPLC system. A divert-valve was used for salt separation. Ionization was

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performed both in positive and negative mode under alternating conditions using automatic

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fragmentation for data-dependent acquisition of both MS and LC MS data.

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Free chlorine ((FC); total concentration of Cl2 + HOCl + OCl-) measurements were performed

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in adaption of the EPA-DPD method 330.5, using a Hanna Instruments free chlorine test kit

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(HI93701) and photometer (HI83099) for a concentration range of 0.00 to 2.50 mg.L-1

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[accuracy +/- 0.03 mg.L-1]. For higher free chlorine concentrations a Hanna Instruments test kit

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(HI95771) and photometer (HI96771) were used [accuracy +/- 2 mg.L-1].

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Non-purgeable organic carbon (NPOC) content was determined with a SHIMADZU total

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organic carbon analyser, TOC-L CPH/CPN (see SI for more information)

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RESULTS AND DISCUSSION

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Electrochemical degradation (ECD) of 4EP

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Degradation curves of 4EP obtained for ECD at various salt conditions (NaNO3 or NaCl

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containing electrolytes), at constant current density of j = 15 mA.cm-2, are shown in Figure 1.

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A linear degradation behavior was obtained for both NaNO3 concentrations (Figure 1a), and

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zero-order kinetic constants can be estimated to be ~20 μmol.L-1.h-1 and 30 μmol.L-1.h-1 for the

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low or high salt concentrations, respectively (voltage-time curves are shown in Figure S5 and

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quantified numbers are presented in Table S4).

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In contrast to NaNO3, which cannot be oxidized to reactive radicals, the degradation of 4EP in

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chloride-containing media proceeded much faster (Figure 1b). Again assuming linear

191

regression (zero-order), constants of ~300 μmol.L-1.h-1 and ~3750 μmol.L-1.h-1 can be

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computed for low and high NaCl concentration, respectively. The fast 4EP degradation in the

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NaCl experiments can be explained by chlorine mediated electrochemical oxidation, where free

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chlorine (i.e. hypochlorite) and chlorine radicals support the degradation of organic

195

pollutants.11–15 In agreement with this hypothesis, the measured concentrations of FC amounted

196

to 0.95 mg.L-1 (after 5 h at 0.03 mol.L-1 NaCl) and 111 mg.L-1 (after 30 min at 0.6 mol.L-1).

4EP [0.03 mol*L-1 NaCl]

400

concentration [mol*L-1]

concentration [mol*L-1]

400

300

200

100

4EP [0.03 mol*L-1 NaNO3]

4EP [0.6 mol*L-1 NaCl]

300

200

100

4EP [0.6 mol*L-1 NaNO3] 0

0 0

1

2

3

4

5

0

1

2

3 time (h)

4

197

a)

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Figure 1. Electrochemical degradation of 4EP at various salt conditions: a) 0.03 mol.L-1

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NaNO3 (light blue), 0.6 mol.L-1 NaNO3 (dark blue); b) 0.03 mol.L-1 NaCl (light green),

time (h)

b)


5


200

0.6 mol.L-1 NaCl (dark green). ECD was performed at a Ti/Pt working electrode at constant

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current density of 15 mA.cm-2.

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Theoretically an electric charge of 110 Coulomb is required for full mineralization of 4EP,

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corresponding to approx. 40 min. of continuous treatment at the applied current density of

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j = 15 mA.cm-2.

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The presence of chlorine radicals was found to induce the formation of undesired chlorinated

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organic intermediates and by-products (RCl), in agreement with literature.12,16–20 LC-MS

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analysis resulted in the identification of 2-chloro-4-ethylphenol (2C4EP) and 2,6-dichloro-4-

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ethylphenol (26DC4EP), see Figure 2. The same intermediates were also formed during

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chemical chlorination of 4EP with sodium hypochlorite solution (20 mg.L-1 FC), showing that

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conversion of 4EP in saline conditions resembles chemical chlorination. The chlorination of the

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aromatic ring at the ortho position is directed by the adjacent OH-group and is a typical reaction

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of phenolic compounds with electrophilic oxidants.70,71 Furthermore, the identified

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intermediates are in good agreement with other literature reports on phenol and bisphenol

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conversion, using degradation at BDD electrodes at low salt concentrations16,18,19 and the

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chemical chlorination of methylphenols.72

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217 218

Figure 2. Reaction path of 4EP with RCS (mainly hypochlorite) during ECD at 15 mA.cm-2

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at a Ti/Pt electrode or BDD electrode in NaCl media (0.03 mol.L-1 NaCl & 0.6 mol.L-1 NaCl). 10 ACS Paragon Plus Environment

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2-chloro-4-ethylphenol (2C4EP) and 2,6-dichloro-4-ethylphenol (26DC4EP) were identified

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as main aromatic intermediates.

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In order to further understand the subsequent degradation process of the two intermediates the

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concentration vs time profiles were determined (Figure 3). Obviously, 4EP is converted into

224

2C4EP almost instantly after the start of the experiment. For the low NaCl concentration

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(Figure 3a) a maximum of 2C4EP formation is obtained after 1 - 1.5 h of treatment time, with

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2C4EP already exceeding the concentration of 4EP. The measured concentration of

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149 μmol.L-1 [23.3 mg.L-1] of 2C4EP corresponds to a molar fraction of 34% relative to the

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initial molar concentration of 4EP (for more information see also Figure S6).



4EP [0.03 mol*L-1 NaCl]_Pt 2C4EP [0.03 mol*L-1 NaCl]_Pt 26DC4EP [0.03 mol*L-1 NaCl]_Pt

300 34 mol% [23.3 mg*L-1]

200

100

4EP [0.6 mol*L-1 NaCl]_Pt 2C4EP [0.6 mol*L-1 NaCl]_Pt 26DC4EP [0.6 mol*L-1 NaCl]_Pt

400 concentration (mol*L-1)

concentration (mol*L-1)

400

10 mol% [8.6 mg*L-1]

300

200 21 mol% [14.1 mg*L-1] 100

7 mol% [5.7 mg*L-1]

0

0 0

1

2

3

4

0

5

time (h)

5

-1

4EP [0.03 mol*L NaCl]_BDD 2C4EP [0.03 mol*L-1 NaCl]_BDD 26DC4EP [0.03 mol*L-1 NaCl]_BDD

300 35 mol% [24.0 mg*L-1]

200

13 mol% [10.5 mg*L-1]

100

15 20 time (min)

25

30

4EP [0.6 mol*L-1 NaCl]_BDD 2C4EP [0.6 mol*L-1 NaCl]_BDD 26DC4EP [0.6 mol*L-1 NaCl]_BDD

400 concentration (mol*L-1)


400

10

b)

a)

300

200

24 mol% [16.4 mg*L1] 8 mol% [7.1 mg*L1]

100

0

0 0

c)

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1

2

3

4

0

5

time (h)

5

10

15 20 time (min)

25

30

d)

229

Figure 3. Formation and degradation of 2C4EP (red) and 26DC4EP (blue) formed during

230

ECD of 4EP (black) in 0.03 mol.L-1 NaCl (a & c) and 0.6 mol.L-1 NaCl solution (b & d)

231

using Ti/Pt (a & b) or BDD electrode (c & d).

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26DC4EP is already formed during accumulation of 2C4EP indicating a continuous

233

consecutive transformation of 4EP into 2C4EP and 26DC4EP (Figure 2). Again these results

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are in agreement with the chemical chlorination of phenol70 and recent studies on phenol

235

degradation at a BDD electrode in low saline solution.16 A concentration maximum of

236

45 μmol.L-1 [8.6 mg.L-1] of 26DC4EP corresponding to a molar fraction of 10% relative to the

237

initial molar concentration of 4EP was obtained after 2 h. It took as long as 4 h of ECD at low

238

NaCl concentration, to remove both intermediates completely from solution (below the limit of

239

detection (LOD), see Table S3 for more information).


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Similar concentration profiles were obtained for the high salt conditions, however within

241

shorter reaction times (Figure 3b). Already after 5 min., 2C4EP reached its maximum

242

concentration (90 μmol.L-1 [14.1 mg.L-1], corresponding to 21 mol% of the initial 4EP

243

concentration). 26DC4EP reached its maximum concentration after 10 min (30 μmol.L-1

244

[5.7 mg.L-1], corresponding to 7 mol% of the initial 4EP concentration). After 20 min of

245

treatment both intermediates were already removed from solution. For comparison, the

246

performance of a BDD electrode is shown in Figure 3 c & d) for the low and high NaCl

247

concentration. While small changes in the kinetic curves of the degradation of 4EP and

248

formation of chlorinated intermediates are present, generally both electrodes perform similarly

249

under the studied conditions.

250

The results suggest that significant amounts of the same mono- and di-chlorinated intermediates

251

were formed and built-up in the solution for both NaCl concentrations and both electrode types.

252

Although the apparent removal of the starting compound 4EP is fast (compared to NaNO3) a

253

large fraction of 4EP is directly converted into chlorinated compounds in a consecutive reaction

254

path (Figure 2). Interestingly for the higher salt concentration less 2C4EP and 26DC4EP were

255

measured, however the higher FC concentration led to a faster removal rate which most likely

256

suppresses more accumulation of these intermediates.

257

In addition to the two identified intermediates various other chlorinated compounds were

258

detected by MS. While identification of their structure also exceeds the scope of this work, it is

259

interesting to note that the degradation of these intermediates is significantly delayed

260

(Figure S7). The degradation pattern indicates that these compounds are obtained by additional

261

reactions of the two main intermediates, 2C4EP and 26DC4EP. Hydroxylation72,73 and

262

subsequent ring opening73,74 might cause the formation of several other intermediates. In

263

addition, dimerization has been proposed for chemical chlorination of methylphenols leading

264

to polychlorinated methylphenoxymethylphenols72 and the formation of polychlorinated phenol 13 ACS Paragon Plus Environment


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oligomers was observed during ECD of phenol and 2-chlorophenol.74,75 Therefore, it is likely

266

that the detected intermediates are congeners of the reported polychlorinated phenol oligomers.

267

The formation of small chlorinated end products such as chloromethanes17,76 and chlorinated

268

acetic acids17 has been reported for ECD in saline water, which processes likely occur 4-5 hours

269

after start of the experiment. In order to analyze the residual organic content after oxidation of

270

4EP, non purgeable organic carbon (NPOC) measurements were performed. The NPOC values

271

(Figure 4) clearly indicate that large amounts, between 64% to 86% of the initial organic

272

carbon, were still present. Even after extended treatment times of 5 h (high salinity) only 23%

273

of organic carbon was effectively removed. These non-volatile compounds are most probably

274

low molecular weight organic compounds such as organic acids, which have been detected after

275

aromatic ring cleavage during ECD in NaCl solution74 and might also consist of chlorinated

276

derivates such as chlorinated organic acids.17

100

NPOC c/c(0) (%)

80

ECD (0.5 h or 5 h) with Ti/Pt or BDD 86% (5h)

83% (0.5h)

84% (0.5h)

77% (5h)

64% (5h)

60

40

20

0

0.03

0.6 0.03 0.6 salt concentration (mol*L-1)

0.6

277 278

Figure 4. Non purgeable organic carbon (NPOC) measurements of treated waters after ECD

279

of 4EP at constant current conditions (15 mA.cm-2) at a Ti/Pt or BDD electrode in NaCl


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media. NPOC values were measured after 5 h ECD in 0.03 mol.L-1 NaCl media and after

281

30 min and 5 h for 0.6 mol.L-1 NaCl.

282

The experimental observations highlight a major bottleneck of electrochemical AOP

283

techniques. Though ECD in saline conditions appeared to be efficient, especially compared to

284

inert salt conditions, undesirable (poly)chlorinated aromatic intermediates were formed for both

285

electrodes that require long treatment times. Moreover, the degradation of 4EP and

286

(chlorinated) intermediates on a Ti/Pt electrode led only to a breakdown of the aromatic

287

compounds and the total removal rate of organic carbon was quite poor. Only for BDD at low

288

salt concentration slightly higher NPOC removal was achieved, most likely due to an increased

289

contribution of generated OH radicals. Still, 64 % of the NPOC remained after 5 h treatment

290

and significant amounts of RCl were formed (Figure 3), leading to the conclusion that for both

291

electrodes chlorination is the main degradation pathway under the studied conditions, which is

292

in agreement with a recent report of high faradaic efficiency for chloride oxidation on BDD

293

electrodes.77

294 295

Photocatalytic degradation (PCD) of 4EP

296

The photocatalytic degradation of 4EP at different salt conditions is shown in Figure 5. Using

297

a pseudo first order approximation, implying surface mediated conversion is dominating, rate

298

constants can estimated to be 1.13 h-1 in the absence of salt, 0.8 h-1 in the presence of NaNO3

299

(high and low concentration), and 0.8 h-1 and 0.5 h-1 for low and high concentration of NaCl.

300

To compare the rates to ECD, initial rates can be calculated (using the initial concentration of

301

410 μmol.L-1 [50 mg.L-1]) to be ~330 μmol.L-1.h-1 or 205 μmol.L-1.h-1 for the low or high NaCl

302

concentration respectively. At low NaCl concentration, the rates are comparable, but in contrast

303

to ECD, addition of salts, and in particular NaCl at high concentration, had a negative effect on 15 ACS Paragon Plus Environment


304

the degradation rates of the parent compound 4EP, leading to an order of magnitude difference

305

under these conditions (~3750 μmol.L-1.h-1 vs 205 μmol.L-1.h-1). The inhibiting effect of NaCl

306

is in agreement with observations reported for PCD of phenol.38 Since the degradation at

307

0.6 mol.L-1 NaCl is clearly more effected than for 0.6 mol.L-1 NaNO3, the inhibition is mainly

308

attributed to the presence of Cl-.24

309

4EP [no salt] 4EP [0.03 mol*L-1 NaCl] 4EP [0.6 mol*L-1 NaCl] 4EP [0.03 mol*L-1 NaNO3]


400

300

4EP [0.6 mol*L-1 NaNO3]

200

100

0

310

0

1

2

3 time (h)

4

5

311

Figure 5. Photocatalytic degradation of 4EP at various salt conditions: no salt (black curve),

312

0.03 mol.L-1 NaNO3 (light blue), 0.6 mol.L-1 NaNO3 (dark blue), 0.03 mol.L-1 NaCl (light

313

green), 0.6 mol.L-1 NaCl (dark green). PCD was performed under 375 nm illumination in a

314

TiO2 (H600) slurry [0.5 g.L-1].

315 316

While the exact reason for the negative effect of anions is still under debate, blocking of active

317

surface sites25,28–32 appears unlikely as the main explanation. Considering a point of zero charge

318

(PZC) for TiO2 between 5 to 678, the TiO2 particle surface charge in this study is neutral as

319

measurements were performed at circumneutral conditions. Moreover, under illumination the

320

build-up of a negative surface charge has been observed for TiO2 particles (pH > isoelectric

321

point)79,80 hindering a competitive adsorption or blocking of active surface sites by chloride 16 ACS Paragon Plus Environment

Page 16 of 38

Page 17 of 38


322

anions. In the case of chloride absorption on the TiO2 particles (pH < PZC, for example possible

323

due to a pH drop during PCD because of dissolved CO2 produced during degradation; see also

324

Figure S8) the oxidation of chloride ions, by either photogenerated valence band holes and/or

325

hydroxyl radicals becomes likely and is proposed frequently in literature.24–27,35–37,39,40,57 As a

326

consequence RCS formation would occur supporting the 4EP removal as observed in ECD.

327

However, our results indicate that the degradation pathways of 4EP during PCD and ECD are

328

different and RCS likely does not contribute to PCD. Thus, just blocking of active sites by Cl-

329

cannot fully explain the observations. In order to verify this conclusion we analyzed the

330

intermediates formed during (PCD).

331

Unlike in the case of ECD, chlorinated intermediates were identified only in extremely low

332

quantities, and LC-MS showed the following aromatic intermediates in significantly larger

333

quantities:

334

Hydroxyacetophenone, (4) 4-Ethylresorcinol and (5) 4-Ethylcatechol (Figure 6a; more

335

information SI Table S1). Thus, the model compound 4EP is oxidized at least at three carbon

336

atoms during PCD, which is typical for non-selective radical reactions (i.e. by •OH). The

337

formation of the isomer 2-(4-Hydroxyphenyl)ethanol could not be revealed due to overlapping

338

retention times with (1) (SI Table S1). However, the increased stability of secondary alkyl

339

radicals favors the formation of intermediate (1) and its subsequent oxidation product (3).

340

Therefore, the formation of 2-(4-Hydroxyphenyl)ethanol is of minor contribution during PCD

341

of 4EP. The same intermediates were detected for all PCD conditions. The non-selective radical

342

reaction pathway is also in good agreement with other reports, e.g. for PCD of ethylbenzene,81

343

phenol37,42,82 or p-cresol,83 confirming that RCS are barely involved in the degradation path.

(1)

1-(4-Hydroxyphenyl)ethanol,

(2)

4-Hydroxybenzaldehyde,

344


(3)

4-



12

Page 18 of 38

4HAP [no salt] 4HAP [0.03 mol*L-1 NaCl] 4HAP [0.6 mol*L-1 NaCl] 4HAP [0.03 mol*L-1 NaNO3]

10

4HAP [0.6 mol*L-1 NaNO3]

8

1.7 mol% [0.90 mg*L-1]

6 4 2 0 0

1

2

3

4

5

345

a)

346

Figure 6. a) Chemical structures of the identified aromatic intermediates during PCD of 4EP

347

observed for all studied salt conditions (no salt, 0.03 mol.L-1 NaNO3, 0.6 mol.L-1 NaNO3,

348

0.03 mol.L-1 NaCl, 0.6 mol.L-1 NaCl: (1) 1-(4-Hydroxyphenyl)ethanol, (2) 4-

349

Hydroxybenzaldehyde, (3) 4-Hydroxyacetophenone, (4) 4-Ethylresorcinol, (5) 4-

350

Ethylcatechol. b) Formation and degradation of intermediate (3) 4-Hydroxyacetophenone

351

during PCD of 4EP at various salt conditions: no salt (black curve), 0.03 mol.L-1 NaNO3

352

(light blue), 0.6 mol.L-1 NaNO3 (dark blue), 0.03 mol.L-1 NaCl (light green), 0.6 mol.L-1 NaCl

353

(dark green).

b)

time (h)

354 355

For the most abundant intermediate (3), its formation and degradation at different salt

356

conditions is shown in Figure 6b. At high NaCl concentration a pronounced inhibition of

357

formation and subsequent degradation was observed, whereas the influence of NaNO3 or low

358

NaCl concentration is less. The observations are related to the 4EP removal, where the strongly

359

inhibited 4EP degradation explains the delayed formation of (3) at high NaCl concentration.

360

This is in agreement with reports for PCD of phenol.38


Page 19 of 38


361

As 2C4EP and 26DC4EP were detected as main chlorinated intermediates in ECD the obtained

362

chromatograms were thoroughly analyzed in PCD. Only for high salt concentration small

363

amounts of 2C4EP could be measured and quantified (Figure 7). Similarly to ECD

364

measurements, formation of 2C4EP occurs immediately, slowly accumulates in the reactor, and

365

its degradation is strongly retarded in agreement with intermediate (3) monitored during PCD

366

at high saline conditions (see Figure 6b). If at all formed, the concentration of 26DC4EP

367

remained below the LOD.

368

2C4EP [0.6 mol*L-1 NaCl]

1.4


1.2 1.0 0.3 mol%

0.8

[0.20 mg*L-1]

0.6 0.4 0.2 0.0

369

0

1

2

3 time (h)

4

5

370

Figure 7. Formation of the monochlorinated intermediate 2C4EP during PCD of 4EP in

371

0.6 mol.L-1 NaCl solution.

372 373

Again FC concentrations were determined. For low NaCl concentrations the measured values

374

are below the limit of accuracy; for high NaCl concentrations only small amounts of FC could

375

be detected [0.15 mg.L-1]. This shows PCD is hardly mediated by FC species, in agreement

376

with the low quantities of chlorinated intermediates.



377

In general the oxidation of chloride by scavenging OH radicals is not favored at the conditions

378

of this study.45,52,84 Zhang et al. concluded chloride being a poor hydroxyl radical scavenger at

379

circumneutral to alkaline conditions, because the equilibrium of the reaction is favored for the

380

reverse reaction to Cl- and •OH (> 99.98%).52 The absence of FC for smaller Cl- concentrations

381

is also in agreement with findings reported by Krivec et al.,32 who did not detect Cl• radicals.

382

For higher NaCl concentrations the results match the observations by Azevedo et al.38 who

383

reported small amounts of 4-Chlorophenol (not quantified) formed during phenol PCD at high

384

salt concentrations. Anyway, the results indicate that chloride is barely oxidized during PCD,

385

and formation of RCS is negligible.

386

As for the ECD experiments, the overall removal of organic carbon during the PCD treatment

387

was estimated by determination of the NPOC content. Results obtained after 5 h of PCD in

388

saline water are shown in Figure 8. In the absence of any salt NPOC was below the LOD. This

389

is in agreement with the non-affected 4EP and intermediate removal (compared to salt

390

containing solutions) and indicates a complete removal of the organic carbon within the

391

duration of the PCD experiments. For the low and high NaCl concentration, residual NPOC

392

remained in the solution in agreement with the delay in 4EP degradation. For example, although

393

4EP and the main intermediate (3) were removed after 5 h treatment in 0.03 mol.L-1 NaCl

394

solutions, NPOC of 13% was measured. Since the UV chromatogram did not show any

395

significant peaks after 5 h of PCD, the residual organic carbon most likely exists in compounds

396

arising from ring-opening reactions, such as short chain aliphatic compounds and carboxylic

397

acids.82,85,86 At high NaCl concentration, a residual NPOC of 46% remained. Calculated

398

photonic efficiencies of PCD experiments for the various salt conditions are shown in Table S5

399

and an estimation of the energy consumption compared to ECD experiments is shown in

400

Table S6. For the applied conditions in this study the energy consumption of the PCD treatment

401

was approx. between one and two orders of magnitude higher compared to the ECD; however, 20 ACS Paragon Plus Environment

Page 20 of 38

Page 21 of 38


402

it is important to note that the applied reactors were not optimized for energy consumption, and

403

the applied light sources have low efficiency for illumination of the photocatalytic slurry (less

404

than 1%). Although ECD thus seems to appear beneficial in terms of consumed energy per kg

405

removed non purgeable organic carbon (NPOC), degradation of 4EP by PCD leads to

406

significantly smaller NPOC than ECD at comparable rate of 4EP conversion (low NaCl

407

concentration). Moreover, the concentration of chlorinated intermediates formed during PCD

408

is significantly lower (for low & high NaCl concentration) compared to ECD, thus

409

photocatalysis appears the preferred method for application, in particular if illumination

410

efficiency and reactor design are optimized.

100

PCD in NaCl (5h)

NPOC c/c(0) (%)

80

60

46% 40

20

0

13% 0.03

0.6

salt concentration (mol*L-1)

411 412

Figure 8. Non purgeable organic carbon (NPOC) measurements of treated waters after 5 h

413

PCD of 4EP in 0.03 mol.L-1 NaCl and 0.6 mol.L-1 NaCl.

414 415

Finally, photoelectrochemical degradation (PECD), a hybrid technology, was studied. The

416

experiments were performed with a TiO2/Ti electrode at an applied anodic bias of 0.1 V vs

417

Ag/AgCl irradiated with UVA light (see Figure S9 for more information). In recent reports



418

Zanoni et al. showed already that oxidation of chloride to FC at TiO2 photoelectrodes occurs87

419

and the generation of RCS has been proposed to assist the PECD of organic compounds.46,57,61,88

420

Still, reports discussing the formation of intermediates are scarce.50,89 After 15 min of

421

irradiation at low NaCl concentration, 92% of 4EP was removed and an intensive formation of

422

RCS was observed (as detected by FC [27 mg.L-1]). The HPLC analysis (Figure 9) revealed

423

significant amounts of the same intermediates, as observed during ECD (57 μmol.L-1 [9 mg.L-

424

1]

425

(resembling PCD conditions) intermediate (3) was observed as main intermediate and the

426

formation of chlorinated compounds was excluded (Figure S10). Thus, the 4EP reaction

427

pathway can be easily modified by the applied bias. The blank experiment (PECD in dark at

428

0.1 V vs. Ag/AgCl) did not lead to any photocurrent, RCS formation, or 4EP degradation

429

(Figure S11), because the applied potential was below any relevant standard redox potential

430

(e.g 1.36 V vs. SHE for chlorine evolution or 2.8 V vs. SHE for ●OH generation). To verify the

431

observations and exclude a particular effect of the applied TiO2, PECD experiments were

432

conducted with the H600 photocatalyst coated on FTO glass, showing similar trends (see

433

Table S7).

2C4EP and 89 μmol.L-1 [17 mg.L-1] 26DC4EP). Interestingly, without additional bias


Page 22 of 38

Page 23 of 38


100 4EP

dark UVA

Intensity (a.u.)

80

60

40 2C4EP 20

26DC4EP

RCl

0 28

30

32

34

36

38

40

time (min)

434 435

Figure 9. HPLC chromatogram obtained after 15 min PECD of 4EP in 0.03 mol.L-1 NaCl

436

solution using a TiO2/Ti electrode with 0.1 V[vs. Ag/AgCl] bias and 365 nm UV light. The

437

majority of 4EP was removed and transformed into significant amounts of the

438

monochlorinated 2C4EP and dichlorinated 26DC4EP, similar as observed during ECD

439

experiments.

440

The effect of an applied anodic bias can be explained by the following: In a photocatalytic

441

approach charge carrier (electrons and holes) are usually generated close to the particle surface

442

and physical separation of reduction and oxidation sites is hardly achieved on the nanoscale.

443

Thus, oxidation and reduction occur in close proximity and the recombination of oxidized

444

chloride with photogenerated CB electrons is likely. This surface-charge recombination

445

mechanism (quenching of photogenerated charge carriers by chloride ions)34 is effectively

446

suppressed when an anodic bias is applied, e.g. in PECD experiments. Upon biasing the

447

electrode photogenerated conduction band (CB) electrons from the TiO2 surface are pulled

448

towards the back contact and in contrast to PCD, reduction of Cl-radicals by available CB

449

electrons is unfeasible. Therefore, pronounced RCS generation could be observed during

450

PECD, leading to a distinctive formation of RCl. In addition, the coulombic repulsion between

451

the TiO2 surface and chloride ions90 is lowered upon polarization of the TiO2 surface.50 This 23 ACS Paragon Plus Environment


452

facilitates the adsorption of Cl- on the positively charged TiO2 surface allowing for chloride

453

oxidation (RCS formation) and subsequent formation of RCl.

454

Combining all information, it is evident that although not all of the organic carbon could be

455

removed during PCD, the removal of organic carbon was more effective in the purely light-

456

driven approach compared to ECD (under the conditions applied in this study). Especially, the

457

significantly smaller formation of RCS and RCl intermediates during PCD is of importance.

458

During ECD in saline media 4EP was mainly removed by conversion into (polychlorinated)

459

compounds and only small amounts of 4EP could be completely mineralized. In contrast, during

460

PCD only for high NaCl concentrations minor formation of monochlorinated 2C4EP could be

461

detected. The measured 2C4EP concentration during ECD at 0.6 mol.L-1 was nearly 2 orders

462

of magnitude higher than that detected after 5 h of PCD. The difference in the AOP techniques

463

is likely governed by a surface-charge recombination mechanism, which suppresses FC and

464

RCl formation. Overall, the formation of chlorinated intermediates in both AOP technologies

465

is important for consideration of practical wastewater treatment applications.

466


Page 24 of 38

Page 25 of 38

467


ASSOCIATED CONTENT

468 469

Supporting Information. Synthesis of 1-(4-Hydroxyphenyl)ethanol; NPOC measurements;

470

synthesis TiO2 photoelectrodes; 4EP and identified intermediates; coefficient of determination

471

(calibration); limit of detection; iR drop corrected potentials; XRD of pre-treated photocatalyst;

472

UV transmission of salt solutions; scheme of electrochemical cell; calibration curves; potential-

473

time curves; ECD intermediates (molar fraction); ECD additional intermediate; PCD pH

474

development; PCD photonic efficiencies; energy consumption; PECD linear sweep

475

voltammograms; PECD without bias (chromatogram); PECD in dark (photocurrent); PECD

476

intermediates with H600/FTO electrode

477

478

AUTHOR INFORMATION

479

Corresponding Author

480

* E-mail: [email protected]

481

Phone: +31534893890

482

ACKNOWLEDGEMENTS

483 484

This research is financed by the Netherlands Organisation for Scientific Research (NWO),

485

which is partly funded by the Ministry of Economic Affairs and Climate Policy, and co-

486

financed by the Netherlands Ministry of Infrastructure and Water Management and partners of

487

the Dutch Water Nexus consortium (project number 14301). Dyllian Huijgens is acknowledged

488

for PCD experiments. Erna Fräntzel-Luiten and Robert Meijer are acknowledged for technical

489

support. 25 ACS Paragon Plus Environment


490

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491

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