Photocatalytic ozonation of 4-chloro-2-methylphenoxyacetic acid and its reaction intermediate 4-chloro-2-methyl phenol R.R. Solı ´s a , F.J. Rivas a, *, J.L. Pe ´ rez-Bote b , O. Gimeno a a Departamento de Ingenierı´a Quı´mica y Quı´mica Fı´sica, Universidad de Extremadura Edificio Jose Luis Sotelo, Av. Elvas S/N, 06006 Badajoz, Spain b Departamento de Anatomı´a, Biologı´a Celular y Zoologı´a, Universidad de Extremadura, Av. Elvas S/N, 06006 Badajoz, Spain 1. Introduction 4-Chloro-2-methylphenoxyacetic acid (MCPA) is a widely used herbicide of the chlorophenoxy type family. Phenoxy herbicides act by simulating the action of natural hormones to produce uncoordinated plant growth. Their action is selective as they are toxic to dicotyledonous plants. The acid is the parent compound, but a number of formulations in use contain the more water soluble amine salts or the ester derivatives, which are readily dissolved in an organic solvent. Regulatory agencies in their evaluations of these two constituents have found MCPA unlikely to be human carcinogen; however, a number of epidemiologic studies have found positive associations between exposure to chlorophe- noxy compounds and an increased risk of some lymphohemato- poietic cancers, primarily non-Hodgkins lymphoma, but also Hodgkin’s disease, soft-tissue sarcoma, and to a lesser extent, leukemia [1]. MCPA can be easily degraded in aqueous solution by means of TiO 2 mediated photocatalysis in the presence of UVA or visible light [2,3]. MCPA photocatalysis leads to the complete removal of the parent compound in a relatively short period of time; however, the herbicide is not completely mineralized and some inter- mediates accumulate in the reaction media [4]. 4-Chloro-2-methyl phenol (CMP) is the main intermediate that accumulates in the MCPA photocatalytic process. CMP is generated from MCPA by direct photolysis and OH/h + reaction (higher CMP yield). CMP is more toxic than the parent compound [5]. Accordingly, water treatment technologies dealing with MCPA removal should take into consideration the evolution of this byproduct. The accumula- tion of CMP in the Degussa P-25 mediated photocatalysis has been previously experienced [6]. In the present study, the photocatalytic ozonation of MCPA is investigated and compared to other systems such as single ozonation, photolytic ozonation, catalytic ozonation and photo- tcatalysis. Use of ozone in photocatalytic processes shows some advantages if compared to the system in the absence of ozone, namely the presence of an additional oxidant (O 3 ) reacting with organic molecules by direct or indirect (OH) pathways, additional generation of free radicals by ozone photolytic scission, potential catalytic ozone decomposition by the TiO 2 surface, etc. [7]. Photocatalytic ozonation has normally been applied by using UV-C radiation, few works have applied cheap black light lamps (UVA radiation), [8,9]. Additionally, the number of studies focused on the photocatalytic ozonation of pesticides is also limited. Some Journal of the Taiwan Institute of Chemical Engineers 46 (2015) 125–131 A R T I C L E I N F O Article history: Received 18 March 2014 Received in revised form 28 August 2014 Accepted 5 September 2014 Available online 28 September 2014 Keywords: MCPA Photocatalytic ozonation 4-Chloro-2-methyl phenol Kinetics Herbicides A B S T R A C T Aqueous 4-chloro-2-methylphenoxyacetic acid (MCPA) has been treated by the systems UVA/TiO 2 /N 2 , O 3 , TiO 2 /O 3 , UVA/O 3 , UVA/TiO 2 /O 2 , and UVA/TiO 2 /O 3 . Under the conditions investigated (T = 20 8C, pH = 4.5, Q gas = 30 L/h, V = 1 L, C O 3 ¼ 5 ppm, C MCPA = 5 ppm, C TiO 2 ¼ 0:5 g=L), MCPA is removed in less than 30 min. Photocatalytic ozonation is the most efficient process both in terms of MCPA removal rate (100% conversion in less than 15 min) and mineralization extent (60% after 3 h and 25 8C). 4-Chloro-2- methyl phenol (CMP) is detected in those systems combining TiO 2 and UVA radiation. The presence of ozone involves the complete depletion of CMP following its generation. The direct rate constant between CMP and ozone corroborates the high reactivity observed (7.2  0.3  10 4 (M s) 1 , 4.4  0.2  10 5 (M s) 1 , and 2.9  0.7  10 6 (M s) 1 at pHs 4, 7 and 10, respectively). Identified intermediates detected in the UVA/ TiO 2 /O 3 applied to MCPA correspond to oxygenated species derived from the parent compound after loss of some substitution groups. No significant toxicity of intermediates is observed in BOD 5 , Daphnia parvula, and Culex pipiens larvae tests. ß 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +34 924289300; fax: +34 924289385. E-mail addresses: fjrivas@unex.es, peloche68@gmail.com (F.J. Rivas). Contents lists available at ScienceDirect Journal of the Taiwan Institute of Chemical Engineers jou r nal h o mep age: w ww.els evier .co m/lo c ate/jtic e http://dx.doi.org/10.1016/j.jtice.2014.09.010 1876-1070/ß 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.