experiences with solar application of photocatalytic …...experiences with solar application of...

Experiences with solar application of photocatalytic oxidation for dye removal from a model textile industry wastewater

H. Gulyas, R. Stiirmer and L. Hintze Institute of Municipal and Industrial Wastewater Management, Technical University of Hamburg-Harbuug, Germany

Abstract

Aqueous solutions of the azo dye "Acid Orange 7" simulating dye-containing textile industry wastewaters have been irradiated by sunlight during circulation through a 2 cm deep channel conducted as a meander in a 2.5 X 1 m double-skin sheet reactor made of plexiglasB in batch experiments after addition of different titanium dioxide concentrations (1 to 10 gll). The experiments have been performed in a region with relatively low solar irradiation (HamburgIGermany). Depending on titanium dioxide concentration, different portions of the dye have been mineralized as recorded by TOC analyses. With the highest investigated titanium dioxide concentration, specific TOC removals (g TOCkWh photons passing the reactor surface) were increasing linearly with increasing TOC concentrations of the aqueous solutions. These data were used for developing a simple design method (concerning demand of irradiation time and area) for photocatalytic oxidation sequencing batch reactors eliminating the investigated azo dye and were compared to results derived from irradiation experiments of aqueous Acid Orange 7 solutions utilizing a UV-A lamp. In some experiments, decolorization of the solutions had been complete. The results have shown that dyes can be appropriately removed from aqueous solutions by this process. No addition of chemicals is required, because the catalyst can easily be removed by sedimentation and reused. The photocatalytic oxidation process can also be applied in a "low-tech" version (even avoiding pumps) and is thus proposed for treating textile dyeing wastewaters in very poor countries.

1 Introduction

Because of a huge variety of textile products, textile industry wastewaters are very complex and exhibit a high variability of constituents [l]. Textile industry

Transactions on Ecology and the Environment vol 49, © 2001 WIT Press, www.witpress.com, ISSN 1743-3541

comprises a couple of processing steps which cause specific partial streams of wastewater. Among textile wastewater partial streams of major ecotoxicological (and aesthetic) concern are those containing dyes. A huge variety of different dyes are applied in textile industry. It is reported that these dyes belong to 19 different classes of chemical compounds [l]. Among these, azo dyes represent the class with the largest number of individual dyes. They are looked at as "ubiquitous commercial chemicals that present unique environmental problems" [2]. When azo compounds are discharged to surface waters, the cleavage of the azo group leads to the formation of anilines - potential carcinogens - in anaerobic compartments, e.g. in sediments [3]. Because of the widespread use of azo dyes and because of their ecological concern, in this study a model wastewater containing an azo dye was investigated. The selected azo dye Acid Orange 7 (Cl6Hl2o4N2S) is an aerobically biodegradable representative [4]. However, most azo dyes are not degraded under aerobic conditions. In an investigation on biodegradability of azo dyes it has been found that of 18 tested azo dyes only three were degraded in an activated sludge system to a significant degree [5]. Acid Orange 7 concentrations in the mg/l range have been identified to inhibit nitrification but not growth of nitrifying bacteria [6]. Thus, dyeing wastewaters containing Acid Orange 7 discharged to sewers potentially disturb the operation of municipal wastewater treatment plants.

Several processes can be applied for treating dyeing wastewaters: oxidation, coagulation, adsorption, ion exchange, membrane processes [l]. Although the activated sludge process is of limited efficiency for removing dyes from wastewaters, other processes utilizing biomass might be promising, e.g. biodegradation of dyes by fungi [7] or adsorption of dyes to activated sludge [g, 91. Moreover, anodic oxidation [l01 and a couple of advanced oxidation processes (AOPs, i.e. chemical oxidation processes which generate OH radicals at normal temperatures, including ozonation [ l 1, 121) have been investigated for dye removal from wastewaters, among them ozonation combined with hydrogen peroxide [ l l], Fenton's reagent [ l 31, photo-assisted Fenton reaction [l 31, UVIozone, UVIhydrogen peroxide, UVIozonelhydrogen peroxide [14], photocatalytic oxidation [15-191, and electrochemically assisted photocatalytic oxidation [2]. Also the influence of photocatalytic oxidation on biodegradability of dyes in aqueous solutions has been determined [20].

Photocatalytic oxidation, a process suitable for mineralization of organics in wastewater, means UV irradiation of wastewater in the presence of suspended photosemiconductor particles. During illumination of the photosemiconductor material electrons of the valence band are energetically elevated to the conductivity band thus becoming mobile and migrating to the particle surface. Also the remaining electron holes h+vB are migrating to the surface unless they recombinate with an excessive electron e-cB. These recombinations increase with increasing UV intensity because this leads to enhanced amounts of electronthole pairs. The wavelength of photons which can produce electronthole pairs in


photosemiconductor materials depends on the energy difference between the valence and the conductivity band: The commonly used titanium dioxide modification anatase (which shows a higher photocatalytic activity than the rutile form) exhibits a bandgap of about 3.2 eV at pH 2 [21]. Thus, only light with h c 400 nm is capable to form e-/h+ pairs in titanium dioxide. Both species, electrons as well as holes, which have migrated to the particle surface undergo several reactions with substances attached to the particle surface, among them water molecules, which are oxidized to OH radicals by electron holes:

Surface-trapped holes can also oxidize adsorbed organic molecules usually leading to organic radical intermediates which are further oxidized e.g. by binding dissolved oxygen. Oxidation of intermediates in several steps finally leads to formation of carbon dioxide. The organic wastewater constituents can also be oxidized by the OH radicals formed on the surface of the semiconductor particles. On the other hand, the excessive electrons may cause reductions of electron acceptors, as e.g. dissolved oxygen, thus generating superoxide anion radicals, so2', and hydrogen peroxide. Superoxide anion radicals capture a proton forming hydroperoxyl radicals, H02-, and hydrogen peroxide again is capable to form OH radicals due to oxidation by electron holes. Some impressions about the complexity of reactions related to illuminated photosemiconductor particles in aqueous suspensions are given in [22].

As the main oxidants in this process, surface-trapped holes and OH radicals generated at the particle surface, are quite short-lived, adsorption of organics to the semiconductor surface is important for their oxidation. That is one reason why photocatalytic oxidation is pH-sensitive, because the charge of surface groups of the Ti02 particles depends on pH: With low pH positively charged =TiOH2+ - groups are formed while at high pH the negatively charged =TiO- group is present. The second factor, which is important for adsorption capacity, is the charge (which may also be affected by pH) or dipole character of the organic molecule to be adsorbed. Because of these both factors, some organics are adsorbed (and degraded) preferably at high pH [21] and others at low pH [15]. According to the importance of adsorption of organics to the semiconductor, the degradation rate r during photocatalytic oxidation can be described by the Langmuir-Hinshelwood relationship:

In this equation, k' is the rate constant of the reaction (in mg/[l.min]), K is the adsorption constant (in Ilmg), and c is the concentration of the organic (in mgll). The (photo) adsorption coefficient K sometimes is higher in illuminated


156 Water Poliurior~ 17

semiconductor suspensions for particular organics than in non-illuminated, but for other organics it may be decreased by illumination. This can be explained by bandgap illumination increasing the number of charge carriers on the semiconductor particle [23] influencing the extent of adsorption of species that carry a charge. However, it is not predictable whether the net effect of illumination will be an adsorption or a desorption [23]. Serpone et al. [22] point out that the Langmuir-Hinshelwood relationship is just manifesting a saturation- type kinetics which are general in chemical kinetics, and that nothing can be concluded about the operational mechanism in a heterogeneous photocatalysis experiment. They gave a more complex model based on this saturation-type kinetics containing additional parameters like irradiated fraction of particle surface area, lifetime of OH radicals bound to the semiconductor surface, number of oxidative active sites, rate constant of electrodhole recombination, concentration of ions (which affect photoadsorption [23]), and oxygen concentration [22]. When the concentration of organics is very high, the degradation rate decreases below the saturation level [24], which is assumed to be a consequence of formation of ionic degradation products (e.g. chloride ions in the case of chlorinated organics) competing for adsorption sites on the semiconductor particles. For low organic concentrations, the Langmuir- Hinshelwood equation can be simplified to give an apparent first order kinetics.

In this study, it has been attempted to utilize a simple first order kinetic equation analogue (where the irradiation time is substituted by the accumulated radiant energy transferred to the Ti02 suspension) with an empirically determined constant for designing a sequencing batch photocatalytic oxidation process for removal of the model dye "Acid Orange 7" from aqueous solutions in a plexiglasB double-skin sheet reactor [25] utilizing sunlight.

2 Methods

2.1 Model wastewaters and photocatalyst

Model wastewaters have been prepared by dissolving the dye "Acid Orange 7" (purchased from Aldrich, Taufkirchen, FRG) in tap water to give concentrations of 25 to 100 mgA without any pH adjustment (initial pH was between 7.7 and 8.1). The Ti02 photocatalyst "P25" (Degussa-Hiils, Hanau, FRG) has been suspended in the model wastewaters in concentrations between 1 and 10 g/l.

2.2 Photocatalytic oxidation utilizing sunlight

The experimental set-up for solar irradiation of the Ti02 suspensions in model wastewaters is given in fig. 1. The ~ l e x i ~ l a s @ double-skin sheet reactor (A in fig. 1; length: 2.5 m; breadth: 1.0 m; depth: 0.02 m) consisted of 30 channels which were connected resulting in a meandric flow of the photocatalyst suspension and


has been purchased from Rijhm GmbH, Chernische Fabrik, Darmstadt (FRG). Because of the Plexiglas@ walls between the channels, the effective illuminated area of the reactor was 2.29 m2.

Figure 1: Scheme of the experimental arrangement with Plexiglas@ double-skin sheet reactor for photocatalytic oxidation of model wastewaters using sunlight; for details see text.

Liquid volume in the double-skin sheet reactor itself was 28.6 1, connecting tubes exhibited a volume of 7.5 1. Before each experiment, the surface of the reactor has been cleaned. The slope of the reactor was 40". Its cardinal orientation was adjusted manually following the sun during an experiment. A total volume of 50 1 of the photocatalyst suspension in model wastewater was recirculated through the reactor with a centrifugal pump (C in fig. 1). The flow had been regulated to 600 l/h by means of a bypass (not shown in fig. 1). Mean hydraulic retention time as determined in tracer experiments (injection of 10 % sodium chloride solutions and recording of conductivity of the reactor effluent) was 179 sec (compared to a mean theoretical hydraulic retention time of 172 sec) under these conditions. The tracer signals indicated that there were no stagnant volumes or short circuits. At the start of each experiment, the reactor was filled with tap water, and the dye Acid Orange 7 was added to vessel B. Then the water volume was accomplished to give a total of 50 1. Complete mixing of the dye solution was achieved by recirculating the solution through the reactor for 15 min. Then an amount of the photocatalyst "P25" was added to vessel B which resulted in the desired Ti02 concentration. In order to avoid time-consuming total emptying of the entire reactor system between two experiments, the settled photocatalyst was kept in vessel B and reused for the following experiment. The subsequent experiment was performed with a higher Ti02 concentration and the difference of photocatalyst mass was added after addition of water and dye and a mixing period of 15 min. By this procedure a residue of organic material had been retained in the system which resulted in an additional TOC at the start of most of the experiments (unless the whole reactor had been emptied before). After addition of the photocatalyst, an initial sample was taken at the sampling port D directly at the effluent of the double-skin sheet reactor. Further sampling was executed each hour. During solar photocatalytic oxidation experiments, solar and


sky radiation was measured with a type of photoelectric-cell which is used for powering satellites and which had been calibrated. The cells were oriented parallely to the double-skin sheet reactor. The intensities of the whole spectrum (visible plus UV light) were recorded by a computer and hourly integrals of irradiance were calculated in W/m2. The solar experiments were performed in summertime (July and August) each lasting for 4 to 5 hours. Temperatures of model wastewaters at the beginning of the experiments were between 20 and 30°C maximum temperatures between 33 and 43°C were measured during these oxidation experiments. An additional experiment was executed in October with wastewater temperatures between 12 and 19°C.

2.3 Photocatalytic oxidation with UV-A lamp

Photocatalytic oxidation experiments in lab scale were performed with a 75 W facial tanning unit (HD 172, Philips GmbH, FRG) with known UV-A intensity. A volume of one 1 of the suspension of TiOz in model wastewater was stirred in a porcelain bowl by means of a magnetic stirrer. Distance between lamp and liquid surface was 10 cm. The UV-A irradiance (visible light was negligible) was about 70 W/m2 at this distance, the liquid surface area was 295 cm2. Thus, the radiant flux through the liquid surface was calculated to be about 2 W. Samples for TOC and dye analysis were taken hourly.

2.4 Analyses

Samples were centrifuged for TiOl removal and the supernatants were analyzed for TOC with a TOC analyzer TOCOR 2 (Maihak, Hamburg, FRG). Dye concentrations were determined photometrically by measuring the absorbance at 485 nm (difference between maximum and valley-to-valley baseline in order to avoid errors due to scattering at residual T i02 particles).

3 Results and Discussion

In fig. 2, dye removal in the solar experiments is presented as a function of light energy (UV + visible), E, which has been transferred to the liquid in the reactor. These graphs are analogous to kinetics: the time has been substituted by the accumulated radiant energy as solar radiation was not constant during the experiments. Although in all experiments (except in 3 experiments with 10 g Ti02/l and in one experiment with 3 g TiOdl) the initial Acid Orange 7 concentration was 5 0 mgll, some differences in initial dye concentrations occurred after TiOz addition which can be explained by different extents of adsorption of the dye to the catalyst (probably due to different temperatures). Highest dye removal rates were observed with a catalyst concentration of 5 gll. A slower removal of Acid Orange 7 with 10 g Ti02/l can be explained by shading of the dye solution by additional catalyst particles. The data in fig. 2 a and b


indicate a lag phase for dye removal with low concentrations of the photocatalyst. This is frequently observed with low light intensities [15, 201.

-. - -- ,~mean solar' 4-

C q, *and sky b rn;ps;,"Iar radiatron IW/rn2j t, L, 3 a Ti02/1 Iradratron [w/rn2]

accum. radiant energy IkWhl accurn. radiant energy [kWhj

Figure 2: Decrease of Acid Orange 7 concentrations during solar photocatalytic oxidation in the PlexiglasB double-skin sheet reactor with different concentrations of Ti02.

The pH of the model wastewaters was not markedly changed during the experiments. Values between 7 and 8.2 had been determined, most of them between 7.5 and 8.0 (data not shown).

In the same experiments, also the TOC data exhibited a lag phase with low photocatalyst concentrations (fig. 3). The lag phase decreased with increasing TiOz concentrations and was no longer observed at a TiOz concentration of 10 gll. It can be stated that specific TOC removal AmToc/AE (similar to specific colour removal) was not affected by the intensity of solar and sky radiation. With the highest applied photocatalyst concentration (fig. 3 d) an analogy to an apparent first order kinetics can be assumed with the radiant energy E substituting the time:

When equation 3 is multiplied with the liquid volume VL of the reactor, a constant k'VL is obtained which should no longer depend on the volume of the wastewater processed in the reactor in one batch operation. For finite portions of light energy E transferred to the reactor content, equation 4 is obtained:


0 ' ~ ' , , 0 2 4 6 8 1 0 1 2

accurn. radiant energy [kWh]

10 g Ti02/1 + 320 c , - -

0 2 4 6 8 1 0 7 2 accurn. radiant energy IkWhl

Figure 3: Decrease of TOC concentrations during solar photocatalytic oxidation of model wastewaters containing Acid Orange 7 in the Plexiglas@ double-skin sheet reactor with different concentrations of TiO,.

In the investigated PlexiglasB double-skin sheet reactor, k was (0.138 .t 0.042) kWh-' under the given experimental conditions with a TiOz concentration of 10 g11 resulting in a constant k,VL of 6.9 UkWh. In the laboratory scale experiments a constant k.VL of (36.8 * 7.3) l/kWh had been determined. By means of these constants, lines were drawn in fig. 4 d for both types giving a dependence of the specific TOC removal AmToc/AE on TOC concentration. Values for AmToc/AE were calculated from individual data of the graphs cToc =

f(E) obtained from solar experiments by determining the slope between two points of the graph. The abszissa related to the h o c l A E data points in fig. 4 was the mean TOC between these two points.

Specific TOC removal AmToc/AE was higher in experiments with the face- tanning unit than in solar experiments for all investigated TiOz concentrations (fig. 4 a-d). This is according to the higher portion of W - A of the light emitted by the lamp (nearly 100 %) compared to the sun (only 5 % of the spectrum of solar and sky radiation is in the UV range), because only light with wavelengths below about 400 nm is able to produce holelelectron pairs in the TiOz particles. However, the slope of the graph AmToc/AE = f(cToc) derived from kinetic data of lamp experiments with a catalyst concentration of 10 g11 was only about five


times higher (instead of expected 20 times) than the slope for solar experiments conducted with the same photocatalyst concentration. There can be several explanations for the lower specific TOC removal by the face tanning unit than expected: analytical errors (especially for the points in fig. 4 d high scattering must be taken into consideration because they were derived from subtracting one large number from another), ageing of the lamp (performance at time of experiments might have been lower than at time when the intensity had been measured), different depths of liquid phase in both reactors, different hydraulic conditions. Moreover, dissolved oxygen concentrations which had not been measured during the experiments might have been different in both reactors. However, as high oxygen concentrations will accelerate the process, in the lamp experiments higher specific TOC removals are expected (because the specific gaslliquid interface was higher in that experimental set-up than in the solar reactor), which should result in a ratio of (~mToc/AE)l,,/(AmToc/AE)Sun > 20. Therefore, the other mentioned causes declining the performance of the lamp- powered process might have been of larger influence than dissolved oxygen concentration.

31 lamp

TOC lmg/ll 0 20 40 60 80

TOC [mg/ll

Figure 4: Specific TOC removal in solar experiments and lamp experiments (face-tanning unit) as a function of TOC concentration at different TiOa concentrations.

In contrast to results obtained with a catalyst concentration of 10 gll, the specific TOC removal did not show a steady increase with TOC concentration in the solar experiments applying lower TiOz concentrations (fig. 4 a-c). These results are reflecting the lag phases shown in fig. 3 a-c. On the opposite, specific TOC


162 Water Pollurion L7

removal was decreasing with increasing TOC concentrations, when certain concentrations were exceeded. This has also been observed in photocatalytic oxidation experiments with aqueous solutions of dichloroacetic acid [24] and is explained by the formation of degradation products which compete for adsorption sites on the photocatalyst particles with other organic solutes.

The data of the solar experiments in fig. 4 d enable the estimation of the area of a double-skin sheet reactor and of duration of solar irradiation required for reducing the TOC concentration of a given Acid Orange 7 solution to a desired level in the presence of 10 g Ti02/l. By means of equation 4, the TOC mass eliminated by introduction of a particular amount of light energy (e.g. 1 kWh) into the TiOz suspension in (model) wastewater at a given TOC concentration can be derived utilizing the determined value of 6.9 YkWh for k.VL. In the next step the expected resulted TOC concentration, CTOC~, is calculated:

This resulting TOC concentration is used for a next calculation step using again equation 4. The obtained AmToccalc. is applied again in equation 5, etc. This method results in calculated TOC concentrations as a function of the accumulated solar and sky radiation energy introduced into the double-skin sheet reactor. From such a function, the light energy E,,, required for reduction of the initial TOC concentration of the (model) wastewater to a desired concentration (e.g. given by a guideline) can be derived. For many regions around the world, data for mean solar and sky radiation, [E/(a.t)]sol+,ky [in ~ / ( r n ~ . d ) ] are available. Presuming that the photocatalytic oxidation of a given volume of the wastewater should be completed (i.e. to result in the desired TOC concentration) in a sequencing batch reactor within one day, the area of the reactor to be designed, a,,,, can be calculated as follows:

As the available [E/(a.t)],ol+sk, data usually are means, one should select a higher reactor area than calculated in equation 6 with E,,, for safety. It has to be noted that the [E/(a.t)]sol+,ky data are not only depending on the region, but also on the season of the year. The [E/(a4)]sol+,ky for the season with the lowest intensity of solar and sky radiation must be applied, when the process should be under operation the whole year. It has also to be noted, that for design of reactors, k.VL data must be experimentally determined for each particular wastewater.


If further studies will reveal that photocatalytic oxidation is a suitable process for the removal of dyes or - more generally spoken - of organics from textile dyeing wastewaters, this process is assumed to be a good option for the reuse of reclaimed dye baths according to investigations of Wenzel et al. [26] who suggested activated carbon adsorption for organic removal from spent dye baths and thus reclamation of the baths. After reclamation, a desired amount of dye can be added again to the aqueous phase, which can now be reused for dyeing. Thus, the spent dyeing bath will not generate any wastewater to be discharged.

Photocatalytic oxidation can also be used for removal of dyes from textile wastewaters in a low-tech version: In a low-tech version, no double-slun sheet reactor is applied, because it requires pumps. In such a simple process, the wastewater containing the dyes is given to a pond with low depth (however, depth should be sufficient in order to prevent total evaporation of the water during the process). For the first operation cycle, the appropriate amount of Ti02 powder is added, and the suspension can be agitated by manpower or by animal power during exposure to sunlight. After required TOC (or colour) removal has been accomplished, Ti02 is allowed to settle over night and the supernatant is discharged e.g. to a surface water. Then the settled TiOz is resuspended into the next charge of wastewater to be treated. However, at this time there is lack of experience concerning technical processes for solar photocatalytic oxidation without pumps or electrically powered stirring devices.

4 Conclusions

The data presented here enable design of double-skin sheet reactors for removing dyes from aqueous solutions by solar photocatalytic oxidation. The design is also possible with data basing on UV-A lamp experiments. It has to be clarified in further investigations whether the suggested simple design method is also applicaple for real textile wastewaters containing dyes. Photocatalytic oxidation is a potential tool for reclaiming dye-baths and can thus contribute to wastewater minimization in textile industry. Solar photocatalytic oxidation for dye removal might also be applied in low-tech mode without pumps or electric stirring devices. So, in regions with high solar and sky radiation it can be economically beneficially implemented even in textile companies which lack of financial power.

Acknowledgements

The companies Rohm GmbH Chernische Fabrik, Darmstadt (FRG), and Degussa- Huls, Hanau (FRG) are greatly acknowledged for the donations of a plexiglasm double-skin sheet reactor and of titanium dioxide "P25", resp. Thanks are due to S. Eggers for TOC analyses, to Dr. U. Nemsmann (System-Analyse-Entwicklung, Prisdorf, FRG) for determination of solar and sky radiation and to J. Jahnke


(Heraeus Noblelight, Hanau, FRG) for intensity measurements of the UV-A lamp.

References

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