Solar efficiency of a new deposited titania photocatalyst: chlorophenol, pesticide and dye removal applications

Abstract

A specially designed titania photocatalyst was prepared by coating Ahlstrom non-woven paper, used as a flexible photocatalytic support, with Millennium PC500 anatase. At the same time, a new solar photoreactor (STEP) was designed based on the multi-step cascade falling-film principle to ensure good exposure to sunlight and good oxygenation of the effluent to be treated. Several types of reactants were treated: 4-chlorophenol as a model organic pollutant; formetanate, a widely used pesticide in horticulture; a mixture of pesticides used in vineyards; and indigo carmine (IC) and Congo red (CR), which are complex multifunctional dye molecules. Each reaction was performed simultaneously in a solar CPC slurry photoreactor and in the STEP photoreactor under identical solar exposure to better evaluate and validate the results obtained. The STEP solar reactor was found to be as efficient as the CPC for 4-chlorophenol and formetanate total degradation. In contrast, both dyes required longer treatment in STEP experiments. This new system, in which the final tedious filtration can actually be avoided, constitutes a good alternative to slurries.

Introduction

Heterogeneous photocatalysis is considered one of the new “Advanced Oxidation Technologies” (AOT) for air and water purification treatment. Several books and reviews have recently been devoted to this problem [1], [2], [3], [4], [5], [6], [7], [8]. A recent review reported more than 1500 references on the subject [9].

The Plataforma Solar de Almerı́a (PSA) photocatalytic pilot plant is unique in Europe [10]. Removal of several organic pollutants has been successfully tested at the PSA: phenol [11], 2,4-dichlorophenol [12], pentachlorophenol [13], [14], atrazine [15], dichloracetic acid [16] and other commercial pesticides [17], [18], [19], [20], [21]. In a previous study [21], it was demonstrated that the kinetic results obtained with micro-photoreactors at laboratory scale could be extrapolated to pilot photoreactors having capacities of several hundred litres by using a circulating suspension of titania. The variation in photo-efficiency under high solar flux, as well as possible thermal effects of solar light, has also been studied. In two other studies performed at the PSA, photoactivity of several new industrial anatase titania samples (Millennium Inorganic Chemicals) have been tested [21], [22]. However, in all the previous studies performed at PSA, the suspension of photocatalyst employed has required a final tedious filtration. Therefore, in an attempt to eliminate this drawback, many studies of titania coatings on supports, such as stainless steel [23], quartz [24], Pyrex [25], paper [26], fibreglass [27], fabric [28], [29] and monoliths [30], [31] have been carried out.

In the present study, an estimation of the performance of titania coated Ahlstrom non-woven paper is compared to a suspension of PC-500 in the degradation of three types of pollutants: (i) 4-chlorophenol (4-CP) a model pollutant which has been the subject of much recent research [32], [33] and references therein; (ii) several pesticides, such as formetanate, intensively tested, as reported in the literature and also previously tested at laboratory scale by some of the authors [34], a mixture of different pesticides used in vineyards (Burgundi, France); and (iii) two dyes: indigo carmine (IC) and Congo red (CR), also studied at laboratory scale by the authors [35], [36]. Large amounts of pesticides are frequently found in water [37], [38] and 15% of world dye production is lost during the textile dyeing process and released into effluents [39]. Therefore, their removal from water is a real ecological problem. Unlike the low-level contamination involved in drinking water, agricultural wastewater may be highly contaminated. The major sources of pollution by pesticides are wastewater from agricultural industries, formulation of pesticides and manufacturing plants. Wastewater from those sources may contain pesticides at levels as high as several hundreds of mg l⁻¹. The main characteristics of such wastewater are its extreme toxicity, low volume and well-defined location. A suitable treatment is therefore needed for its detoxification rather than its propagation in the environment. Such localized sources of pollution may be ideally treated in small-scale treatment units. Furthermore, formulation additives may affect the degradation process and, unfortunately, very little information is available about the effects of these additives on photocatalytic degradation. Therefore, all the tests were carried out with commercial products, since a treatment plant must destroy not only the active substance, but also any other organic compound contained in the formulation.

This work was divided into two parts:

• Determination of optimum working conditions (weight of TiO₂ used in suspension, irradiated surface and reproducibility).

• Efficiency of TiO₂ coated on Ahlstrom non-woven paper in the degradation of the three different families of molecules indicated above (phenolic compounds, pesticides and dyes).