Study of the wavelength effect in the photolysis and heterogeneous photocatalysis

Abstract

In this work, photocatalytic and photolytic degradation of two model pollutants has been done. The chosen substances were 2,4-dichlorophenol (DCP) and sulfamethoxazole (SMOX), both with different absorptive properties in the UV range. The experiments were carried out under UV-A and concomitant UV-ABC radiation, studying the effect of each type of radiation on each type of degradation. Different kinetic orders were applied to the experimental series, results were fitted to the time and incident radiation. Also, the photon flow absorbed by the suspension and the quantum yield was measured for each condition tested. It was found that UV-ABC radiation can be more efficient than UV-A, although it depends also on the properties of the pollutant. It was demonstrated that the plotting of compounds degradation versus the incident UV radiation offers more accurate perspectives in establishing comparisons between different radiation sources. In this way, the conclusions are very similar to those obtained calculating the quantum yield, which is probably one of the best options to analyze a process.

Introduction

Heterogeneous photocatalysis has been one of the most studied oxidation processes over the last decades, because of its efficiency in the treatment of a wide variety of pollutants in wastewaters [1], [2], [3]. In these processes, TiO₂ is the most employed catalyst. As known, electron-hole pairs can be generated in titania when it absorbs radiation with an energy level higher than the one corresponding to the band gap (3.2 eV for titania), that is, radiation with a wavelength (λ) lower than 387 nm, corresponding to the UV region of the electromagnetic spectrum (200–380 nm). On the other hand, photolysis is other phenomenon that can take place, when a solution is irradiated. The mechanism of the photolysis is based on the fact that the chemical species undergo photochemical reactions, by which molecules are broken down into smaller molecules, merely through the absorption of light.

The photon flux necessary to initiate these processes can be supplied whether by the sunlight, or by artificial lamps. There is a broad variety of artificial radiation sources when working at the laboratory: black, germicide, solar simulating lamps, etc. The most commonly used are usually high-, medium- and low-pressure mercury lamps, and xenon lamps, to generate UV radiation [4].

Photocatalytic studies are normally carried out under UV-A (320–380 nm) and UV-B (280–320 nm) radiation range [5], [6], since this is the spectrum range of the solar radiation, in the UV range, arriving at the surface of the earth and photocatalysis is commonly understood as a process to be used with solar energy. In fact, the UV-C radiation emitted by the lamps mentioned above is normally cut off by filters or by the material of the photoreactor. On the contrary, very little work has been made under UV-C radiation to study the efficiency or parallel effects of photochemical reactions occurring. Chun et al. [7] studied the photocatalytic oxidation of phenol under UV light at λ > 330 nm and λ > 200 nm, and they obtained a slightly better response in the last case, both with and without catalyst. Matthews and McEvoy [8] obtained similar results, when working on the degradation of phenol and salicylic acid employing radiation of 254 and 350 nm. The considerably more effectiveness of 254 nm radiation was attributed to the shorter penetration capability of the higher energy photons, so there were more electron-hole pairs available for the target compounds. Most recently, Puma and Yue [9] followed this trend and compared the effectiveness of UV-A and concomitant UV-ABC in the photocatalytic and photolytic degradation of 2-chlorophenol, obtaining a slight higher yield for the UV-ABC radiation. A similar study and results were obtained by Lee et al. [10] but using TDAB instead of TiO₂. Cao et al. [11] used different glass filters to work at 300 or 350 nm and degraded chlorfenapyr using photocatalytic and photolytic processes. Again, the 300 nm radiation demonstrated to be more efficient that the longer one. Some different results were obtained by Wong and Chu [12], who photodegraded alachlor by means of 254, 300 and 350 nm radiation, finding that the best quantum yield was obtained at 300 nm, followed by the one at 254 nm and finally 350 nm. In summary, it seems that the lower the wavelength of the radiation, the better efficiency is obtained, although some authors disagree about this point. Unfortunately, some of the studies do not consider the incident photonflow as an important parameter. When a filter is placed between the lamp and the reactor, part of the available radiation is lost, and therefore a lower degradation must be expected for the same time of irradiation. The same may occur when the lamp is changed. It is also important to control the lamp aging, since spectral energy distributions and intensities can change along the time [13]. Actually, only Puma and Yue [9] and Wong and Chu [12] take some of these facts into account to some extent, and they do not come exactly to the same conclusions. Therefore, it is not totally clear from the previous literature, if the most energetic radiation is more efficient than the less energetic one or if the results are disguised by some other considerations.

When the effectiveness of photocatalysis and photolysis are compared in the degradation of wastewaters, the former usually yields to a higher degradation rate than the latter, due to the enhancing effect of the catalyst on the use of the light. It is a subject of discussion how much the photolysis interferes in the photocatalysis process, since it is important to know the extension of the photocatalytic process to understand it [12]. A first clue about the importance of the photolysis can be obtained comparing the absorbance spectrum of the target compounds with the emission spectrum of the lamp [14]. Obviously, if the compounds can absorb light in the region of the lamp emission, both the catalyst and the molecules will compete for the photons. Nevertheless, one of the two mechanisms, either photolysis or photocatalysis, may predominate in the process. How important is the photolysis on the photocatalytic process might be deduced from the study of the reaction pathways.

The aim of the present work is to determine the importance of the photolysis in a photocatalytic process, by studying the degradation of two organic compounds, under UV radiation of λ > 235 nm (UV-ABC) and λ > 300 nm (mainly UV-A). It is also focused on the effectiveness of each type of radiation in a photocatalytic process, paying special attention to the incoming and absorbed radiation in the system. To reach these goals, photooxidation of two model compounds (2,4-diclhorophenol (DCP) and sulfamethoxazole (SMOX)) has been done. These substances can absorb light until a wavelength of 315 nm. Although both compounds absorb approximately in the same range of the UV spectrum, SMOX retains quite higher amount of radiation than DCP. Thus, it is also possible to study the effect when working in a medium more or less avid for the entering radiation.

These chemicals are also chosen because both are typical and toxic pollutants, and SMOX is in addition an emerging pollutant. DCP is a key intermediate in the synthesis of the herbicide 2,2-DT, and it is classified as a toxic, non-biodegradable compound [15] and it is usually chosen as a representative of chlorophenol's family. SMOX is an antibiotic commonly used in the treatment of urinary tract infections [16].