rXXXX American Chemical Society A dx.doi.org/10.1021/ie200412p | Ind. Eng. Chem. Res. XXXX, XXX, 000–000 ARTICLE pubs.acs.org/IECR 4-Chlorophenol Oxidation Photocatalyzed by a Calcined MgÀAlÀZn Layered Double Hydroxide in a Co-current Downflow Bubble Column Eduardo Martín del Campo, † Jaime Sanchez Valente, ‡ Thelma Pav on, § Rubí Romero, †  Angeles Mantilla, r and Reyna Natividad †, * ,^ † Centro Conjunto de Investigaci on en Química Sustentable, Universidad Aut onoma del Estado de M exico, Km 14.5 Carretera TolucaÀAtlacomulco, M exico ‡ Instituto Mexicano del Petr oleo, Eje Central #152, M exico D. F., 07730 § Facultad de Química, Universidad Aut onoma del Estado de M exico, Paseo Colon esq. Paseo Tollocan, Toluca, Estado de M exico, 50120 r CICATA-LEGARIA, Instituto Polit  ecnico Nacional, Av. Legaria #694, M exico D. F., 11500 ABSTRACT: The objective of this work is to study, for the first time, the photodegradation of 4-chlorophenol (4CP) catalyzed by a calcined MgÀZnÀAl layered double hydroxides (MgAlZn LDHs) in a co-current downflow bubble column (CDBC) photoreactor at pilot scale. The effect of initial organic compound concentration (C 4CP0 ), temperature (T), and mass catalyst over reaction rate (Àr 4CP ) was elucidated. An intrinsic kinetic regime was established, and a single-site LangmuirÀHinshelwood mechanism was determined to occur during the organic compound oxidation. The catalyst was characterized by X-ray diffraction (XRD), inductively coupled plasma atomic emission spectrometry (ICP-AES), and ultravioletÀvisible light (UV/vis) spectrophotometry. The reaction progress was verified by UV/vis spectrophotometry and total organic carbon (TOC) content. Degradation and mineralization rate were found to be dependent on T and 4CP concentration. In the range of studied operating conditions, a maximum of 94% 4CP was degraded, while 70% total organic carbon removal was achieved. 1. INTRODUCTION Although environmental risks have always been a constant in chemical processes, it was not until the last decades of the 20th century that efforts to protect the environment were strength- ened. Two general approaches may be identified. The first one is known as being corrective and implies the mineralization of industrial effluents to fulfill regulations, and the second one implies the development of sustainable processes (green chemistry). For both of them, photocatalysis has proven to play a major role since it allows one to carry out complete miner- alization of organic compounds and selective oxidations under mild conditions (low temperature, atmospheric pressure, neutral pH). In this context, the photocatalyzed degradation of phenolic compounds has had priority and therefore has been widely studied, mainly with TiO 2 . Among such organic compounds, chlorophenols (CPs) constitute a particular group of priority pollutants, because most of them are toxic, hardly biodegradable, and difficult to remove from the environment. The half-life for pentachlorophenols (PCPs) in water can reach 3.5 months in aerobic waters and some years in organic sediments. Because of their numerous origins (pesticides, insecticides, paper, and wood preservatives industry), CPs can be found in ground water, wastewater, and soil. They might produce a disagreeable taste and odor to drinking water at concentrations of <0.1 μg/L. The limiting permissible concentration of CPs in drinking water should not exceed 10 μg/L. 1 In particular, 4-chlorophenol (4CP) has been recognized as a precursor of highly toxic compounds. The photodegradation of such a compound has been reported mainly with TiO 2 and in stirred tank reactors at laboratory scale. In related literature, 2 a main concern is the low quantum efficiency attained as a consequence of a fast recombi- nation of the generated holeÀelectron (h + VB /e À CB ) pairs in the catalytic surface. A general accepted solution to this issue has been the addition of an electron acceptor such as oxygen. Then, a substantial responsibility for the success of a photo-oxidation process relies on the design of a reactor that allows the optimization of mass-transfer from gas to liquid. At this point, bubble columns, and specially the co-current downflow bubble column (CDBC), emerge as a promising technology to carry out the catalyzed photo-oxidation of effluents at the industrial scale. To accomplish such a purpose, it is of paramount importance to assess the performance of the CDBC at pilot scale first. The co-current downflow contactor (CDC) or CDBC is a highly efficient mass-transfer device. 3 It consists of a bubble column, where the gas (dispersed phase) and liquid (continuous phase) are introduced co-currently through an orifice at the top of a fully flooded column. The hydrodynamic action of a high inlet liquid velocity, initiates and maintains a turbulent bubble matrix or gasÀliquid dispersion, containing densely packed bubbles (gas holdup given as ε G = 0.4À0.6) that undergo constant coalescence and breakup with a particular bubble diameter related to the system under consideration. 4 When a stable gasÀliquid dispersion is accomplished, the turbulence, the mixing and shear give rise to a large gasÀliquid interfacial area (a = 1000À6000 m 2 /m 3 , depending on bubble size) and, therefore, Received: March 14, 2011 Accepted: August 21, 2011 Revised: July 29, 2011