TREATMENT OF AQUEOUS ALCOHOL ETHOXYLATES SOLUTION BY TiO2/UV-A PHOTOCATALYTIC OXIDATION

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1 Proceedings of the 13 th International Conference on Environmental Science and Technology Athens, Greece, 5-7 September 2013 TREATMENT OF AQUEOUS ALCOHOL ETHOXYLATES SOLUTION BY TiO2/UV-A PHOTOCATALYTIC OXIDATION I. KABDASLI 1, C. ECER 1, T. OLMEZ-HANCI 1 and O. TUNAY 1 1 Istanbul Technical University, Civil Engineering Faculty, Environmental Engineering Department, Maslak, Istanbul, Turkey kabdasli@itu.edu.tr ABSTRACT Alcohol ethoxylates (AEs) are a class of non-ionic surfactants widely used as detergents, emulsion stabilizers, foaming agents and wetting enhancers. These properties allow large industrial application as additives in the formulation of pesticides, pharmaceuticals, paints and cosmetics, as well as in mineral separation, biotechnology etc. Although there is no known toxic effect of AEs, their concentrations are often relatively high in aquatic environment. Until now, only few publications have been devoted to the degradation of AE with advanced oxidation processes (AOPs). Using semiconductor, more especially TiO 2- related catalysts, to oxidize organic chemicals is a promising AOP which has received much attention recently. Considering the above mentioned facts, in the present experimental study, the photocatalytic degradation of aqueous poly (oxyethylene) (4) loryl ether (commercial name Brij30 ) being selected as a model AE, using UV-A irradiation in the presence of TiO 2 photocatalyst was investigated. The degradation extent has been evaluated by measurements of Brij30 and total organic carbon (TOC). Photocatalytic oxidation experiments were conducted to treat 20mgL -1 aqueous Brij30 solution at different initial ph values (3.0 and solution s original ph 6.0), different initial TiO 2 dosages (1.0, 1.5 gl -1 ) and different reaction periods varying between 5 to 480 minutes. Effect of ph on the photocatalytic reaction was conducted at initial ph values of 3.0 and 6.0, with the Aeroxide P25 dosages of 1.0 gl -1 and 1.5 gl -1.At initial ph 3.0, for 1.0 gl -1 Aeroxide P25 dosage, ~64% Brij30 and ~68% TOC removals were obtained after 10 and 480 minutes of reaction time, respectively. At original aqueous Brij30 ph (6.0), 83% Brij30 removal was observed after 10 minutes reaction time whereas ~ 65% TOC removal was realized after 480 minutes treatment. The effect of Aeroxide P25 dosage was investigated at ph3.0 and 6.0 at two different TiO 2 loads being 1.0 and 1.5 gl -1. For initial ph value of 3.0, increasing the Aeroxide P25 dosage from 1.0 gl -1 to 1.5 gl -1 increased the Brij30 removal from 64% to 79%, respectively. 480 minutes reaction time provided 68% and 88% TOC removals for 1.0 gl -1 and 1.5 gl -1 TiO 2 dosages, respectively. For initial ph value of 6.0, increasing the Aeroxide P25 dosage to 1.5 gl -1 also increased the TOC removal efficiency up to ~86% for a reaction period of 480 minute. The experimental results showed that by using photocatalytic oxidation it is possible to achieve complete degradation of Brij30 accompanying with high TOC removals under investigated reaction conditions. Even in short reaction periods (15 minutes) high rates of Brij30 removals were achieved however longer oxidation periods ( minutes) were needed in order to obtain significant TOC removals. Keywords: Alcohol ethoxylates, nonionic surfactant, Brij30, TiO 2photocatalysis, Advanced Oxidation Process (AOP). 1. INTRODUCTION Alcohol ethoxylates (AE) of the formula RO(CH 2CH 2O) nh are an increasingly important class of non-ionic surfactants widely used as detergents, emulsion stabilizers, foaming

2 agents and wetting enhancers. These properties allow large industrial application as additives in the formulation of pesticides, pharmaceuticals, paints and cosmetics, as well as in mineral separation, biotechnology and other purposes (Brand et al., 2000; Pagano et al., 2008; Enget al., 2010; Pagano et al., 2012). They are not single components but complex mixtures with a distribution of alkyl and ethoxylate groups. The R group given in the above formula is typically an alkyl carbon chain with 12, 13, 14 or 15 carbon atoms where number of ethoxylate groups can vary between 0-30 (Evans et al., 1997). AEs enter the environment mainly through the discharge of industrial wastewaters into sewage treatment works and natural water bodies. Due to the relatively high concentration of AEs in industrial wastewaters advanced remediation techniques have to be developed for the treatment of AEs. Among alternative treatment options, so-called advanced oxidation processes (AOPs) have been applied to eliminate non-ionic surfactants efficiently from water (Pagano et al., 2008, Arslan-Alaton and Olmez-Hanci, 2012; Arslan-Alaton et al., 2012, Pagano et al., 2012). As one of the AOPs, photocatalytic oxidation of a wide variety of organic contaminants by semiconductors has been investigated in more detail for more than 20 years (Olliset al., 1985; Konstantinou and Albanis, 2003). Titanium dioxide (TiO 2) is the most frequently used semiconductor in photocatalysis due to its efficiency, nontoxicity, high activity, photochemical inertness and low cost. Heterogeneous photocatalysis by using TiO 2 involves the irradiation of TiO 2 with UV light ( <400nm) to produce electron/hole pairs (Equation 1). Hydroxyl radicals (HO ) are generated upon the oxidation of OH - or H 2O by these photogenerated holes (Equations 2 and 3). Such holes are principally responsible for the destruction of organic species. Oxygen is mainly used as an efficient electron trap (Equation 4), preventing the recombination of electrons and photogenerated holes. Equations 5-7 represent the other reactions of irradiated TiO 2 (Konstantinou and Albanis, 2004; Paul et al., 2007; Vasconceloset al., 2009; Paul et al., 2010). TiO 2 + h TiO 2 (e CB- + h VB+) (1) TiO 2(h VB+)+ OH - TiO 2+ HO (2) TiO 2 (h VB+) + H 2O TiO 2 + H + + HO (3) TiO 2 (e CB-) + O 2 TiO 2 + O 2 - (4) O H + HO 2 (5) TiO 2 (e CB-) + H + + O 2 - HO 2 - (6) HO H + H 2O 2 (7) In the present experimental study, the photocatalytic degradation of aqueous poly (oxyethylene) (4) loryl ether (commercial name Brij30 ) being selected as a model AE, using UV-A irradiation in the presence of TiO 2 photocatalyst was investigated. The experiments were conducted in aqueous suspensions of TiO 2, varying different parameters such as the amount of catalyst, UV irradiation time and initial ph. The degradation extent has been evaluated by measurements of Brij30 and total organic carbon (TOC). To the best of our knowledge, this is the first study of its kind on the degradation of AE based non-ionic surfactant Brij30 by TiO 2 photocatalysis under UV-A irradiation.

3 2. MATERIALS AND METHODS 2.1. Materials Model AE was poly(oxyethylene)(4)loryl ether [C 12H 25(OCH 2CH 2) 4OH], commercially known as Brij30 was provided from Sigma Aldrich (USA). It is a colourless-light yellow, tasteless, odorless chemical which is in liquid form at room temperature. As catalyst Aeroxide P25 (formerly known as Degussa P25) was kindly supplied from Evonik Industries AG (Germany) and used without any pre-treatment. The Aeroxide P25 consisting of 80% anatase and 20% rutile had a specific BET-surface area of 50 m 2 g -1 and primary particle size of 20 nm (Bickleyet al., 1991). Aeroxide P25 is the best one which gives an optimal efficiency of catalysis, furthermore, good interparticle contacts are formed between anatase and rutile particles in water (Ohnoet al., 2001). Iodine and potassiumiodade were purchased from Riedel-de Haen (USA) and Merck (Germany), respectively. The ph of the reaction solutions was adjusted using concentrated (6 N) or diluted (1 N) H 2SO 4 and NaOH solutions. Aqueous Brij30 solutions (20 mgl -1 ) were prepared with distilled water. All other chemicals required for analytical and experimental procedures were at least of analytical grade and purchased from Merck (Germany) or Sigma-Aldrich (USA) The photoreactor and experimental procedures Heterogeneous photocatalytic experiments were run in a 500 ml-capacity three-neck quartz flask that was continuously stirred at a constant rate of 100 rpm by a magnetic stir bar from the reactor bottom to ensure sufficient oxygen supply and mixing. A LZC-ORG model (Luzchem Research Inc., Canada) photochemical reaction chamber (dimensions: cm), which consists of side irradiation from total ten lamps (8W each and five lamps on each side) was used for the photocatalytic oxidation experiments. The spectral distribution of UV-A lamps had a Gaussian shape with a central wavelength at 350 nm. The lamps were turned on at least 20 min prior to the reaction to obtain a constant light output in the photoreactor. The total light intensity striking the reaction vessel, placed centrally and equidistant to all the lamps was measured daily by using a powermeter, provided along with the photoreactor assembly. The light intensity was measured as 6.9 W/m 2 when all lamps were turned on. The Brij30 concentration was 20 mgl -1 in each experimental run. For photocatalysis experiments, 400 ml of aqueous Brij30 solution with the specified compositions and conditions to be studied was poured into the reaction vessel. Prior to UV-A illumination the TiO 2 suspension containing 20mgL -1 of Brij30 was exposed to ultrasound for 10 minutes and equilibrated in the dark (no UV radiation) for 20 minutes. Thereafter a sample at t = 0 was taken to monitor the adsorption, desorption and oxidation steps. In the subsequent experiments, the photodegradation system containing TiO 2 was further used to study the extent of degradation efficiency of Brij30 at initial ph of 3.0 and 6.0 (original ph of 20 mgl -1 aqueous Brij30 solution) and TiO 2 dosagesof 1.0 and 1.5 gl -1. Samples were taken from the photoreactor at different reaction periods varying between 5 to 480 minutes, and then filtered through 0,45 µm pore size syringe filters and thereafter analyzed for Brij30, TOC and ph. To avoid any possible organic matter interference, hydrophilic polyvinylidene fluoride filters were supplied by Milipore (USA). In order to avoid changes in irradiated volume in the reactor limited amounts (maximum 25 ml) and numbers (maximum five) of samples were taken during each run.

4 2.3. Analytical procedures Brij30 was determined by the iodine/iodide (I 2/I - ) spectrophotometric method (Baleux, 1972; Brown and Jaffe, 2001). In this method, 0.25 ml of I 2/I - reagent (prepared by mixing 2 g of potassium iodide and 1 g of iodine in 100 ml of deionized water) were added to 10 ml of aqueous non-ionic surfactant solution and allowed to equilibrate for 30 min. The complex formed by I 2/I - and the non-ionic surfactant was quantified spectrophotometrically by using Pharmacia LKB-Novaspek II model spectrophotometer at 500 nm with 1 cm cell path. A surfactant-free solution containing the I 2/I - reagent was used as blank. TOC was measured on a Shimadzu V PCN analyzer (Japan) equipped with an autosampler by catalytic oxidative combustion at 680 C, using an infrared detector. During the experiments ph changes in the solution was followed by Thermo Orion 720A+ model ph meter. 3. RESULTS AND DISCUSSION 3.1. Effect of the TiO 2 dosage The effect of Aeroxide P25 dosage was investigated at ph 3.0 and 6.0 at two different TiO 2 loads being 1.0 and 1.5 gl -1. The effect of the TiO 2 Aeroxide P25 dosage on the removal of Brij30 at initial ph 3.0 was shown in Figure 1, confirming the positive influence of the increased number of TiO 2 active sites on the process performance in terms of both Brij30 (a) and TOC (b) removals. Experimental results conducted by direct UV-A photolysis in absence of the TiO 2 resulted in insignificant Brij30 and TOC removals confirming the HO based oxidation of Brij30. From Figure 1 it is apparent that mineralization of Brij30 was a longer process than its disappearance. For initial ph value of 3.0, increasing the Aeroxide P25 dosage from 1.0 gl -1 to 1.5 gl -1 increased the Brij30 removal from 64% to 79%, respectively. 480 minutes reaction time provided 68% and 88% TOC removals for 1.0 gl -1 and 1.5 gl -1 TiO 2 dosages, respectively. For initial ph value of 6.0, increasing the Aeroxide P25 dosage to 1.5 gl -1 also increased the TOC removal efficiency up to ~86% for a reaction period of 480 minute. These results showed that the Brij30 degradation efficiency increased with the increasing the amount of TiO 2. Since irradiating TiO 2 particles with UV- A light produces electrons and holes, then the HO were formed and their yield increased with TiO 2 dosage. Therefore, the proportion of HO that attacked Brij30 and the reaction intermediates increased with TiO 2 dosage. As already well explained in the literature, the increase in photocatalytic activity might increase with catalyst loading, since the proportion of incident light that was absorbed by the catalyst increased with the amount of TiO 2 in suspension (Kritikoset al., 2007; Kuo et al., 2010). This result also suggested that higher initial Brij30 concentrations should be tested in conjunction with appropriate dosage in order to obtain the optimal TiO 2 dosage Effect of initial ph on the degradation of Brij30 The effect of ph on the degradation efficiency of heterogeneous photocatalysis is one of the major factors influencing the rate of degradation of organic compounds (Wang et al., 2000; Pera-Titus et al., 2004; Laoufiet al., 2008). The surface charges of most semiconductor oxides are dependent on the extent of hydrogen ion (H + ) or hydroxide ion (OH - ) in the water environment, resulting in the available surface sites for the adsorption potential and photocatalytic reactivity.

5 (a) (b) Figure 1.Brij30 degradation (a) and mineralization (b) under 1.0 gl -1 ( ) and 1.5 gl -1 ( ) TiO 2 dosages (Initial conditions: Brij30 = 20 mgl -1, TOC = 13 mgl -1, ph=3.0) In the present experimental study the effect of the initial ph (3.0 and 6.0) on the photodegradation efficiency of Brij30 in water was investigated at the initial Brij30 concentration 20mgL -1 and TiO 2 dosages of 1.0 and 1.5 gl -1. The results of experiment conducted at TiO 2 dosage of 1.5 gl -1 is shown in Figure2. As can be seen from Figure 2 (a), increasing the initial ph from 3.0 to 6.0 enhanced Brij30 degradation. 91% Brij30 removal was observed when the reaction was carried out at ph 6.0, whereas 79% Brij30 was removed at ph 3.0 after 10 min reaction time. Parallel to Brij30 degradation, TOC removals also increased with increasing the initial ph from 3.0 (70%) to 6.0 (82%) for 240 min reaction time as evident in Figure 2 (b). However after this reaction period, the same level of TOC removals were obtained at the investigated ph values, resulting in ultimate TOC removal efficiencies of 88% after 480 min. At initial ph 3.0, for 1.0 gl -1 Aeroxide P25 dosage, ~64% Brij30 and ~68% TOC removals were obtained after 10 and 480 minutes of reaction time, respectively. At original aqueous Brij30 ph (6.0), 83% Brij30 removal was observed after 10 minutes reaction time whereas ~ 65% TOC removal was realized after 480 minutes treatment.

6 At the investigated reaction conditions it was not possible to achieve complete mineralization. In addition to Brij30 and TOC measurements changes in ph values were also followed during the photocatalysis experiments (data not shown). Photocatalytic oxidation of Brij30 at initial ph values of 6.0 the ph gradually decreased to 4.8 at the end of the 480 min reaction time. However, when the process was run at an initial ph of 3.0, the ph remained constant during the whole reaction period. The ph profiles obtained during the photocatalysis experiments indicated that carboxylic acids would be considered as the final oxidation products which are the most difficult compounds to be mineralized (Mantzavinos et al., 1996). (a) (b) Figure 2.Brij30 degradation (a) and mineralization (b) at initial ph of 6.0 ( ) and 3.0 ( ) (Initial conditions: TiO 2 dosage =1.5 gl -1, Brij30 = 20 mgl -1, TOC = 13 mgl -1 )

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