Ind. Eng. Chem. Res. 2003, 42, 5751-5761
5751
Solar Photochemical Degradation of Aminosilicones Contained in Liquid Effluents. Process Studies and Neural Network Modeling Antonio Carlos S. C. Teixeira, Roberto Guardani, and Claudio A. O. Nascimento* Chemical Engineering Department, University of Sa˜ o Paulo, Avenida Prof. Luciano Gualberto, Travessa 3, 380-05508-900 Sa˜ o Paulo, SP, Brazil
The effectiveness of the solar-driven photo-Fenton process in treating wastewater contaminated with aminosilicone compounds has been evaluated in a parabolic trough reactor under variable weather conditions. On sunny days, after 3 h of irradiation, more than 80% of the initial COD has been removed. The degradation generates compounds that might be readily biodegradable and/or a solid phase that is easily separated by mechanical means. An important interaction between manipulated [H2O2 and Fe(II) concentrations] and nonmanipulated (direct and diffuse components of solar radiation) variables was detected. Therefore, degradation was possible even on cloudy days, provided that the H2O2 and Fe(II) concentrations are conveniently selected. The neural network technique is an effective, simple approach to successfully modeling the solardriven photo-Fenton degradation. The model might therefore be useful in process optimization, as well as in the design and scale-up of solar reactors for industrial application. 1. Introduction 1.1. Silicones and the Environment. Poly(organosiloxanes) comprise a class of compounds known as silicones. The presence of different monomer units, side chains, and functional groups gives these polymers suitable properties for a wide number of industrial processes. Silicone-in-water emulsions are used as materials for surface impregnation and water repellency treatments, as antifoaming agents, as dielectric and heat-transfer fluids, and as fabric softeners.1 Silicones therefore enter the environment through municipal sewage systems and mainly through industrial wastewater.2,3 Silicones and silicone derivatives are found to be exceptionally resistant to hydrolytic and oxidative breakdown under conventional wastewater treatment.4 Only a slow, acid-catalyzed hydrolysis of some silicone polymers has been observed in dry soils in combination with clay minerals,4,5 resulting in a series of water-soluble silanol compounds. In this case, the main breakdown product, monomeric dimethylsilanediol [(CH3)2Si(OH)2], can be removed from soil by biodegradation to CO2 and inorganic compounds5 and by volatilization to the atmosphere where it is photolytically degraded in the presence of sunlight.5 In contrast, silicone biodegradation is not observed in wastewater treatment facilities because the Si-O bond is resistant to enzymatic attack.3,6 Biodegradation of Si-C bonds also seems to be difficult.7 No degradation of silicone fluids has been observed in sludge,2 and the further fate of such substances depends on disposal conditions. Once carried to the aquatic environment, silicones are tightly adsorbed by and deposited in sediments, where they accumulate. Consequently, silicone polymer fluids are considered as environmentally persistent compounds6,7 that could present a potential threat to the environment. Their increased use and the need for effective treatment processes for silicone-containing wastewater has therefore become a matter of environmental importance. * To whom correspondence should be addressed. Tel.: +5511-3091-2237. Fax: +55-11-3813-2380. E-mail:
[email protected].
1.2. The Photo-Fenton Reaction. Several photochemically initiated reactions are known that can rather unselectively oxidize a wide range of organic compounds with a diversity of chemical structures and functional groups. These reactions, when applied to the degradation of pollutants, are usually termed advanced oxidation processes (AOPs).8,9 Oxidation of organic pollutants implies in most cases generation and subsequent reactions of hydroxyl radicals (HO•), which are short-lived, powerful oxidizing agents, capable of oxidizing organic compounds by hydrogen abstraction, addition to unsaturated compounds, or electron transfer.10 In favorable cases, these radicals oxidize organic contaminants completely into water, carbon dioxide, and inorganic compounds of all heteroatoms other than oxygen. These reactions generate organic radicals as transient intermediates, which then undergo further reactions. Among AOPs, the Fenton reaction11 [based on hydrogen peroxide and an Fe(II) salt] and especially the photochemically enhanced Fenton (photo-Fenton) reaction12 are considered most promising for the remediation of wastewater containing a variety of toxic organic compounds. In a simplified scheme, the Fenton reaction is a redox process in which Fe(II) is oxidized to Fe(III) and H2O2 is reduced to hydroxide ion and hydroxyl radical
Fe2+ + H2O2 f Fe3+ + HO• + OH-
(1)
Hydroxyl radical is widely accepted in the literature as the main primary oxidizing species of the Fenton reaction.13 In the absence of light, the ferric ion produced in reaction 1 is reduced back to ferrous ion
Fe3+ + H2O2 + H2O f Fe2+ + H3O+ + HO2•- (2) The thermal reduction given by reaction 2 ultimately determines the overall rate at which the process proceeds. A more detailed description of these reactions, considering hydrated iron-H2O2 complexes, including rate constants and redox potentials, is presented elsewhere.10 Although chemically very efficient for the
10.1021/ie0303350 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/18/2003
5752 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003
removal of organic pollutants, the Fenton reaction slows appreciably after the initial conversion of Fe(II) to Fe(III). Exposure of Fenton reaction systems to UV-visible radiation strongly accelerates both Fenton (H2O2/Fe2+) and Fenton-like (H2O2/Fe3+) reactions, improving the degradation rates of a variety of organic pollutants.10,14,15 This enhancement has been explained by Fe3+-photocatalyzed reactions, i.e., photolysis of hydroxide complexes of Fe3+ yielding hydroxyl radicals and regenerating Fe2+ (reaction 3) and photochemical reactions of complexes formed between Fe3+ and the organic substrate or its intermediates of degradation, especially organic acids, which exhibit strong ligand-to-metal charge absorption bands in the near-UV and visible regions of the spectrum (reaction 4)15 hν
Fe(OH)2+ 98 Fe2+ + OH• hν
hν
Fe3+-L 98 [Fe3+-L]* 98 Fe2+ + L•
(3) (4)
The contribution of the production of HO• by H2O2 photolysis is probably of little importance, considering that the molar absorption coefficient of H2O2 above 280 nm is very small (18.7 L mol-1 at 254 nm). It should be noted that both components of the Fenton reagent react with HO•. These species oxidize Fe(II) to Fe(III), and they can be trapped by H2O2 (reaction 5), leading to the formation of hydroperoxyl radicals (HO2•, conjugated acid of the superoxide anion O2-•) that are much less reactive than HO•.16 However, the rate constant of HO• with organic compounds is generally 1-3 orders of magnitude larger than that of reaction 5,17 and the latter reaction can be minimized by continuously feeding H2O2 solution (at an appropriate H2O2 concentration) into the reactor.
H2O2 + HO• f HO2• + H2O
(5)
Many papers have appeared in the literature describing the use of the Fenton and photo-Fenton processes for the degradation of waterborne organic contaminants.10-12,14,15,18 A preliminary process study by the authors reported on the photo-Fenton treatment of wastewater contaminated with the aminosilicone poly[dimethyl, methyl(aminoethyl aminoisobutyl) siloxane], commonly used in the textile industry, with a 450-W medium-pressure mercury lamp.19 The effects of the Fe(II) and H2O2 concentrations, temperature, and UVvisible light irradiation on chemical oxygen demand (COD) removal for aminosilicone-in-water emulsions were examined. On the basis of experimental findings and theoretical considerations, oxidation mechanisms were proposed for highly concentrated and dilute emulsions with COD0 > 80 000 mg L-1 and COD0 < 1000 mg L-1, respectively. 1.3. Solar Technologies. One of the common drawbacks of photochemical processes is the great demand for electrical energy by UV lamps.18 An auspicious possibility is the utilization of solar light, which could provide a major step toward commercial application. The use of solar radiation looks extremely attractive at sites with annual global insolation of 1700 kWh m-2 or higher.20 Also, hybrid UV-visible lamp/solar photochemical reactors could be employed in situations and/ or areas with low solar radiation intensity.
X-rays and other very short-wave radiation of the solar spectrum are absorbed high in the ionosphere by nitrogen, oxygen, and other atmospheric components. Most of the ultraviolet radiation is absorbed by ozone. At wavelengths longer than 2500 nm, a combination of low extraterrestrial radiation and strong absorption by CO2 means that very little energy reaches the ground. Thus, from the viewpoint of terrestrial applications of solar energy, only radiation of wavelengths between 290 and 2500 nm need be considered.21 The amount and spectral distribution of the solar energy reaching the surface of the earth change remarkably over the course of a day, as well as during the year. Research efforts have attempted to understand the influence of several interrelated factors such as altitude, latitude, local weather patterns, topographical and geographical features, climate, and atmospheric conditions (e.g., haze, clouds, airborne particulate matter).20 Generally speaking, the solar radiation incident on the earth’s surface is split into direct-beam radiation and diffuse radiation. Direct radiation is the solar radiation that reaches the surface without being scattered or absorbed in the atmosphere. Diffuse radiation refers to scattered radiation. The sum of the two types is known as global solar radiation. A comprehensive article was published in 2000 concerning ongoing R&D directed toward solar photocatalytic reactors.22 The simplest and most developed are parabolic trough reactors (PTRs), in which parabolic collectors concentrate sunlight onto fluid-filled transparent receiver tubes positioned along the line of focus in the troughs.20,22 1.4. Model of the Degradation Process. Owing to the complexity of the photo-Fenton process, phenomenological modeling of the various steps involved is a difficult task. Moreover, well-known difficulties exist in modeling of the radiation field, even in systems of simple geometry with lamps of constant radiant power.22 Modeling becomes increasingly complex if the position and spectral characteristics of the light source change while the system is being irradiated. Neural network models have been attracting great interest within chemical engineering. They are able to extract information from experimental data and to handle the nonlinearities and complex behavior of a given process in an efficient manner. Neural networks have been successfully used in modeling and optimizing complex industrial process.23,24 This approach has also been applied to the TiO2 photocatalytic treatment of wastewater,25 as well as to the photo-Fenton degradation of pollutants in artificially irradiated photoreactors.26 One of the advantages of empirical models based on artificial neural networks is that a mathematical description of the phenomena involved is not required. However, the choice of the variables affecting the process and a representative set of experimental data (i.e., processing conditions and corresponding responses) are essential requirements for a reliable neural-network-based model to be obtained. Accordingly, the quality of information, particularly in terms of the data distribution in the experimental region of interest, is of primary importance. The experimental design adopted in this work satisfies these requirements. Table 1 shows the conditions adopted in generating the experimental data used here. 1.5. Objectives. In this work, the effectiveness of the solar-driven photo-Fenton process in treating synthetic wastewater contaminated with aminosilicones in a
Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 5753 Table 1. Experimental Conditions According to the Doehlert Design experimenta
[H2O2]b (mmol L-1)
[Fe(II)]c (mmol L-1)
COD0d (mg L-1)
Ediff/Edire
t30f (min)
t50g (min)
t80h (min)
COD removal after 300 min (%)
1Ai 2A 3A 4A 5A 6A 7A 1Bi 1Ci 1Di 2C 2D 3B 4B 5B 6B 7B
500 387.5 50 162.5 387.5 275 162.5 500 500 500 387.5 387.5 50 162.5 387.5 275 162.5
2.8 0.5 2.8 0.5 5.0 2.8 5.0 2.8 2.8 2.8 0.5 0.5 2.8 0.5 5.0 2.8 5.0
629 671 694 692 636 680 662 653 585 665 630 625 621 692 600 669 699
0.30 0.22 0.15 0.15 0.19 0.09 0.11 1.90 8.09 6.37 0.35 0.41 1.21 0.82 1.01
15 17 30 49 11k 37 44 38 46 34 95 225 59 55 19 34 46
29 43 49 89 20k 66 88 91 81 76 163k 296 114 99 42 77 88
89 110 128 n.a.j 71 155 180 n.a. n.a. n.a. 294 n.a. 227 n.a. 92 259 181
88 89 89 79 88 85 89 63 72 77 84 50 87 66 82 86 88
a T ) 40 °C and pH ) 2.8-3.0 in all experiments. The Doehlert design is complete only for irradiation conditions A, except for replications at the center. b H2O2 concentration of the feed solution. c Initial Fe(II) concentration of the aqueous emulsion. d Initial COD of the aqueous emulsion. e Ratio between the accumulated diffuse radiant energy (Ediff) and the accumulated direct radiant energy (Edir) at time t (time averaged values). f -hIrradiation times required to achieve COD removals of 30, 50, and 80%, respectively. i Irradiation conditions: set A, clear, sunny days with the sky free or almost free from moving clouds; set B, sky moderately covered with clouds; set C, heavily clouded days; set D, darkness. j 80% COD removal not achieved after 300 min of irradiation. k Values estimated from the COD-time curve.
Figure 1. Schematic view of the solar parabolic trough reactor (PTR). M, rotating impeller connected to an agitator device; T, temperature sensor connected to a digital indicator; pH, electrode connected to a digital pH meter.
parabolic trough reactor is evaluated at the bench-scale level. The effects of the Fe(II) and H2O2 concentrations on COD removal were systematically investigated by means of optimally designed experiments under different weather and atmospheric conditions. Use is made of neural network techniques to develop a model for the solar-driven degradation process that could handle variable solar irradiation patterns and controllable process variables. 2. Experimental Section 2.1. Reagents. Experiments were carried out using an aminosilicone containing hydroxyl end groups, poly[dimethyl, methyl(aminoethyl aminoisobutyl) siloxane] (Dow Corning Co.). Hydrogen peroxide (H2O2, analytical grade, 30% w/w in water, Merck) and ferrous sulfate heptahydrate (FeSO4‚7H2O, >98%, Sigma Aldrich) were used. Reactant solutions were prepared with distilled water. 2.2. Equipment and Experimental Procedures. A schematic view of the concentrating parabolic trough reactor (PTR) used in the experiments is shown in Figure 1. The apparatus consisted of a flow-through borosilicate glass tube reactor (internal diameter of 11 mm and length of 1.2 m) connected to a 1-L jacketed glass reservoir. The liquid in the tank is well-mixed, and
its temperature is controlled by means of a thermostatic bath. The tube was positioned along the length of the solar collector at the line-focus of the parabolic trough. The collector, with a 0.26-m2 reflecting surface, was made of polished aluminum as the reflective material. A centrifugal pump provided liquid circulation (1.5 L min-1) for reactor operation in recirculating batch mode. A total of 2 L of aqueous emulsion was treated in each experiment. The most favorable orientation of the PTR depends on the latitude of the specific site. In the present field test study, the module was mounted on a fixed platform inclined 23° (from ground) and facing north to maximize its performance, according to the geographical localization of Sa˜o Paulo City (latitude, 23° 32.0′ S; longitude, 46° 37.0′ W). The Laboratory of Micrometeorology (Astronomical and Geophysical Institute, University of Sa˜o Paulo, Sa˜o Paulo, Brazil) provided radiation data. Global solar radiation was measured by means of a pyranometer (Eppley, model PBW) in the range of 2852800 nm. The diffuse component of the global solar radiation was measured by means of a pyranometer (Eppley, model PSP) with a shading ring to eliminate direct radiation. A correction for this shading was estimated and applied to the diffuse radiation.21 Data were recorded every 5 min and stored prior to the data analysis. The experimental procedure was the same in all runs. First, the silicone-in-water emulsion at an initial concentration of 1290 mg L-1 (average COD0 ≈ 653 mg L-1) was added to the system. This concentration level is typical of end-processing textile wastewater. The pH was adjusted to 2.8-3.0, optimal for the Fenton process,27 by the addition of H2SO4. A weighed amount of solid FeSO4‚7H2O was added. The PTR was exposed to solar light, and the H2O2 solution was then introduced continuously throughout the whole reaction time (300 min) at a controlled flow rate (4 × 10-4 L min-1) by means of a peristaltic pump. The temperature of the emulsion was maintained at 40 °C. This temperature was selected on the basis of previous tests to account for the observed average temperature rise under variable insolation conditions.
5754 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003
curves by interpolation. The total extent of the degradation was quantified by the COD percent removal after 300 min of irradiation, that is
total % removal of COD )
(COD0 - COD300) (6) COD0
where COD0 is the initial COD of the silicone-in-water emulsion and COD300 is its final value after 300 min of irradiation. 3. Experimental Results and Discussion
Figure 2. Graphical representation of the Doehlert uniform array. Variables X1 and X2 corresponding to each natural variable are normalized (see section 2.4). The experiment numbers are indicated near the apexes and the center of the hexagon, and the corresponding conditions are listed in Table 1. The Fe(II) and H2O2 concentrations in each run are the same for sets A-D.
2.3. Analyses. Ten-milliliter samples were taken from the tank and vacuum filtered through a 2-µm filter to remove suspended solids. The COD of each sample was measured by the closed reflux colorimetric method.28 The analyses were performed immediately after samples had been collected. 2.4. Experimental Design. The experimental design methodology enables the preformance of a minimum set of well-chosen experiments adequately distributed along the experimental domain. It is especially useful when interacting effects among variables occur. In this work, a Doehlert uniform array design29,30 was used, in which the effects of the hydrogen peroxide concentration of the feed solution ([H2O2], U1) and the initial Fe(II) concentration of the aqueous emulsion ([Fe(II)], U2) were studied in detail by means of seven experiments under variable insolation conditions. The ranges of variation of the H2O2 and Fe(II) doses were 50-500 and 0.5-5 mmol L-1, respectively. A reduced and centered variable Xi is associated with each natural variable Ui (U1 ) [H2O2], U2 ) [Fe(II)]) as
Xi )
(Ui - Ui0) ∆Ui
where
Ui0 )
(Ui,max + Ui,min ) 2
is the value of Ui at the center of the experimental region and
∆Ui )
(Ui,max - Ui,min ) 2
is the step size. The experimental points are uniformly distributed and can be represented as the apexes and center of a hexagon (Figure 2). The concentrations of H2O2 and Fe(II) used in each runs are given in Table 1. The time evolution of the degradation of the aminosilicone-in-water emulsion for each set of experimental conditions was estimated by the irradiation times, in minutes, required to achieve COD removals of 30, 50, and 80% relative to the initial COD. The values of irradiation time for each percent removal (t30, t50, and t80, respectively) were determined from COD-time
Previous studies on the photochemical degradation of aminosilicone-in-water emulsions at different initial COD0 values showed that neither photolysis under solar light nor thermal degradation in the range 30-50 °C occurs.19 Also, no change in the COD of the aminosilicone polymer emulsion was observed when only Fe(II) and solar light, or only H2O2 and solar light, were used.19 The experiments in this work can be analyzed according to four different sets of irradiation conditions: set A, clear, sunny days with the sky free or almost free from moving clouds; set B, sky moderately covered with clouds; set C, heavily clouded days; set D, darkness (solar collector covered). Figure 3 shows representative incident solar radiation profiles (irradiance, in watts per square meter) for sets A-C and the corresponding accumulated radiant energy (direct and diffuse) (in kilojoules). The accumulated radiant energy was obtained by integrating the irradiance versus time curves and then multiplying by the area of the collector. All experiments were carried out within a few weeks, so that seasonal effects should not be important. In Table 1, the values of t30, t50, and t80 are presented, along with the set of irradiation conditions corresponding to each experiment. Unlike other process controllable variables [Fe(II) and H2O2 concentrations, temperature, etc.], in solar photochemical reactors, the incident radiation is not a manipulated variable, but it can be treated as a measured disturbance in the process. Direct radiation entering the collector normal (or almost normal) to its aperture can be focused onto the receiver tube, transferring radiant energy into the reaction medium. However, concentrating solar systems are not suited to capture diffuse sunlight as efficiently as in the case of direct solar beams. The radiation measurements performed in this work indicate that the diffuse component can represent an important amount of the total available radiation. The irradiance plots in Figure 3b show that passing clouds can give rise to operating transients. Under cloudy conditions, mostly the diffuse solar light is available (Figure 3c). An analysis of the irradiation data led to the characterization of the above sets of conditions according to the average ratio between the accumulated diffuse solar energy (Ediff) and the accumulated direct solar energy (Edir) at time t. The three sets were arbitrarily classified as follows
set A
diffuse radiation e 0.3 direct radiation
set B
0.3
4 direct radiation
diffuse radiation
