- Email: [email protected]
Combination of activated carbon adsorption with light-enhanced chemical oxidation via hydrogen peroxide
PII: S0043-1354(00)00194-9
Wat. Res. Vol. 34, No. 17, pp. 4169±4176, 2000 7 2000 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/00/$ - see front matter
www.elsevier.com/locate/watres
COMBINATION OF ACTIVATED CARBON ADSORPTION WITH LIGHT-ENHANCED CHEMICAL OXIDATION VIA HYDROGEN PEROXIDE NILSUN H. INCE*M and IZZET G. APIKYAN BogÆazic° i University, Institute of Environmental Sciences, 80815, Bebek-Istanbul, Turkey (First received 10 June 1999) AbstractÐA tertiary treatment scheme involving simultaneous operation of activated carbon adsorption and advanced oxidation with ultraviolet light and hydrogen peroxide, followed by ``destructive regeneration'' of the spent adsorbent by advanced oxidation was investigated, using phenol as a model compound. Operational parameters in each step were optimized on the basis of phenol and total organic carbon removal during selected contact times. It was found that in the ®rst stage with adsorption/advanced oxidation, phenol was totally eliminated during the ®rst quarter of the contact time, and 87.5% total organic carbon removal was accomplished at the end. It was further found that advanced oxidation was the dominant pathway in this operation for the disappearance of phenol, while that of total organic carbon was carried out by combined eects of adsorption and oxidative degradation. Optimum regenerating frequency for the spent activated carbon was found to be once every four batches, which was four times slower than the required frequency in the absence of advanced oxidation. In the second part of the operation, where the spent carbon was regenerated destructively via advanced oxidation, 92.5% mineralization was accomplished in the regenerating solution at the end of the optimized contact time. The economic assessment of the system considering the operation of both steps revealed that under the initial and optimized conditions, the operating cost is 2.26 USD per cubic meter of wastewater with 40 ppm phenol. 7 2000 Elsevier Science Ltd. All rights reserved Key wordsÐadvanced oxidation (AOP), granular activated carbon (GAC), destructive regeneration, total organic carbon (TOC), mineralization, degradation
INTRODUCTION
Advanced technologies have recently become essential counterparts of euent treatment plants due to increasing public concern for health related environmental problems, and the corresponding outcome as the need for revision of euent discharge standards. Despite the fact that many of the advanced systems are expensive to install and operate, they are nevertheless unavoidable in tertiary treatment of industrial euents to render detoxi®cation and/or exclusion of refractory substances. Today's engineer, therefore, is challenged by the need to develop optimal technologies that allow safe discharge and disposal of industrial contaminants at appropriate costs. The last decade has witnessed advanced oxidation processes (AOP) emerging as promising alternatives to tertiary treatment, owing to their high potency to *Author to whom all correspondence should be addressed. Tel.: +90-212-263-1500; fax: +90-212-257-5033; email: [email protected]
render partial and ultimate destruction of many refractory compounds including dyestu, halogenated and aromatic organics (Bauman and Stenstrom, 1990; Kusakabe et al., 1991; Ince et al., 1997; Ince, 1998; Ince and Tezcanli, 1999). These processes involve the formation of highly reactive free radical species, which are far more powerful as oxidizing agents than commonly known strong oxidants like molecular oxygen and ozone. The chemistry, kinetics and quantum yields in free radical reactions have been widely investigated and reviewed in the past (Glaze et al., 1987; Staehelin and Hoigne, 1985; Guittonneau et al., 1990; GuÈrol and Vatistas, 1987). These processes have two unique advantages over other advanced treatment processes: (i) they are non-selective to a very broad range of chemicals, and (ii) they involve no sludge production due to the character of their removal mechanism, which is based on the oxidative destruction of organic carbon by conversion to higher oxidation states. Nevertheless, depending on the targeted euent quality, treatment by advanced oxidation may impose high operating costs, due to
4169
4170
Nilsun H. Ince and Izzet G. Apikyan
intensive energy and chemical requirements arising from long retention times to complete oxidation reactions and to accomplish an appreciable degree of mineralization. A more commonly employed advanced treatment technique is adsorption on activated carbon; a process based on the concentration and immobilization of a contaminant on the surfaces of granular or powdered activated carbon. The technique is well known as an ``interface phenomenon'', encompassing a state of dynamic equilibrium between the solute in the aqueous phase and that adsorbed on the carbon surface. When equilibrium is reached, adsorption parameters can be simply established by using an appropriate isotherm equation. As a whole, adsorption is a simple-to-operate process and relatively cost-eective, due to low (or no) energy requirements and the possibility of reclaiming and reusing the spent carbon via regeneration. However, surface phenomena involving the accumulation of undesired contaminants on the solid surface end up in large quantities of ``spent adsorbent'' and/or ``regeneration solutions'', both of which have to be handled as ``hazardous wastes'' (Crittenden et al., 1997). Accordingly, tertiary treatment by adsorption alone is not an environmentally complete system, unless the destruction of immobilized contaminants on the spent or wasted carbon surface is heeded as well. Furthermore, the incomplete system is incomparable to treatment by advanced oxidation, where there is neither any accumulation, nor transport of the contaminant from one medium to another. A recently proposed advanced method to cope with the above problem was called ``phase-transfer oxidation'', which involved two consecutive operational steps characterized by ®xed-bed adsorption for euent treatment and advanced oxidation for destructive regeneration of the adsorbent (Mourand et al., 1995; Liu et al., 1996). The strategy was reported to provide a signi®cant advantage as ``onsite regeneration'' to minimize adsorbent inventory and eliminate unloading, transportation and repacking of the adsorbent (Mourand et al., 1995). The purpose of this study was to investigate a modi®ed version of ``phase-transfer oxidation'', in which euent treatment or the ®rst-step operation was carried out by the simultaneous operation of activated carbon adsorption and advanced oxidation in the same reactor. The second consecutive step similarly involved destructive regeneration of the spent adsorbent by advanced oxidation. The advantages expected from this modi®cation were: (i) delayed exhaustion of the adsorbent ± less frequent regeneration/disposal and fresh supply of activated carbon; (ii) increased cost-eectiveness due to lowered energy and oxidant consumption during the destructive regeneration of the spent carbon. This expectancy is based on the fact that the so called ``regeneration solution'' to be treated by advanced
oxidation in the second-stage will be made of partly degraded by-products of the parent compound as a consequence of its oxidative degradation in the ®rst stage. Phenol was used as a model chemical throughout the study to represent toxic contaminants with aromatic structures. The availability of extensive literature on its immobilization by activated carbon adsorption (Diez et al., 1999; Tressmer et al., 1997; Furuya et al., 1996; Nakhla et al., 1994), and destruction by advanced oxidation (Gould and Weber, 1976; Castrantas and Gibilisco, 1990; Scheck and Frimmel, 1995; Fajerwerg et al., 1997; Mokrini et al., 1997) provided not only the information about the degradation products and adsorption parameters of phenol, but also the opportunity to compare the tested system with singly employed advanced oxidation and activated carbon adsorption systems. The study involved: 1. Construction of adsorption isotherms at equilibrium and the investigation of possible cross eects of H2O2 and UV irradiation with activated carbon. Isotherms were constructed by ®tting the test data into the linearized form of Freundlich equation described by: ln qe ln K
1 ln Cf n
1
where qe is the solid-phase loading of the solute, Cf is the equilibrium solute concentration in solution; K is the unit capacity factor, and 1/n is the Freundlich exponent, representing the sorption capacity of the adsorbent, and adsorption intensity of the solute, respectively (Dobbs and Cohen, 1980). 2. Selection and/or optimization of operating parameters such as contact time, pH, GAC dose, UV intensity, and the frequency of spent adsorbent regeneration. 3. Experimentation with single adsorption and advanced oxidation processes to compare their eectiveness with that of the proposed system. Rates of oxidative degradation in this part were estimated by using the integrated form of the standard ®rst-order kinetic equation: dCt ÿk 0 Ct dt
2
where Ct is the concentration of the contaminant at time t (mol lÿ1), and k' is the ®rst-order reaction rate constant (t ÿ1). 4. Investigation of optimum regenerating conditions for the spent activated carbon and estimation of the operating cost of the system on the basis of complete phenol and at least 85% total organic carbon elimination in both stages of the operation.
Combination of activated carbon adsorption with light-enhanced chemical oxidation MATERIALS AND METHODS
Materials Phenol was supplied by Riedel-De Haen AG in 99.5% purity. Hydrogen peroxide (30% w/v), NaOH and HCl (used for pH adjustment) were all analytical grade (Merck). Granular activated carbon was prepared in pellets of 20 60 mesh by grinding standard commercial size (2 8 mesh) Aquatech and W1, followed by washing them with deionized water and drying for 12 h at 1058C. Catalase of bovine liver was purchased from Fluka Chemie AG. The light source was made of four Philips 15-W lowpressure mercury UV lamps, emitting monochromatic light at 253.7 nm. Set-up A schematic diagram of the experimental set-up is given in Fig. 1. It consisted of a 5-l Pyrex reactor equipped with four UV lamps situated 6 cm from the surface of the solution. Contents of the reactor were stirred with magnetic stirrers located at its bottom. A 500-mesh ®lter was placed at the sampling port to prevent free GAC particles escaping the solution during the withdrawal of euent samples for chemical analyses. Analytical Concentration of phenol was analyzed by a standard method based on spectrophoto-metric analysis at 510 nm of the developed color resulting from the reaction of phenol with 4-aminoantipyrene (APHA/AWWA, 1992). Hydrogen peroxide was analyzed by the ``HP-02'' triodide method (Klassen et al., 1994) and destroyed by catalase. All spectrophotometry was carried out by a Shimadzu UV-160 Double Beam Spectrophotometer. Mineraliztion or TOC removal from solution was estimated by monitoring the TOC of the euent samples using a Fisions TCM 480 Analyzer. The UV intensity was determined by a chemical actinometer based on the photolysis of an O2saturated potassium peroxy-disulfate tert-butanol solution to form sulfuric acid (Mark et al., 1990). In accordance, acid formation was monitored by the reduction in pH during 25-min irradiation of the solution by four 15 WUV mercury lamps (253.7 nm), and the data were plotted against time to estimate the photon ¯uence rate from the slope of the obtained straight line. Methodology Adsorption isotherms were constructed at equilibrium by contacting 40 mg lÿ1 phenol with 200, 500, 800, 1000, 1200 and 1500 ppm GAC, respectively. The data were ®t into equation (1) to estimate Freundlich parameters. The eect of a strong oxidizing agent on the adsorption of phenol was investigated by reconstructing the isotherms in the presence of H2O2. The operating parameters of the ®rst-stage operation were determined by irradiating a synthetic wastewater (contaminated with 40 mg lÿ1 phenol) in the presence of 5 10ÿ3 M H2O2 and 1000 mg lÿ1 GAC until complete phenol and at least 85% TOC removal was accomplished. The dose of GAC was selected on the basis of increased
Fig. 1. Schematic diagram of the reactor.
4171
adsorption upon increasing doses of GAC addition. The initial H2O2 concentration was adjusted in accordance with the stoichiometric requirements of the chemical oxygen demand reaction of the phenol test solution. The absorption spectrum and pH of euent samples were periodically monitored during contact to verify the occurrence of oxidative degradation. The experiments were repeated with GAC and AOP alone to investigate the eectiveness of running the two processes simultaneously. The second stage of the operation involved two substeps: isolation of spent GAC particles from treated euent for remobilization of the sorbed species from GAC surfaces into solution, and exposure of this contaminated solution to advanced oxidation by UV/H2O2 until mineralization was close to completion. The ®rst sub-step involved contacting the exhausted GAC particles with deionized water at optimum pH and temperature to provide maximum desorption. In the second sub-step, the ``regenerating solution'' (after pH adjustment to a neutral level) containing organic matter desorbed from GAC surface was exposed to destruction by UV/H2O2 for a reasonable contact time, beyond which further reduction in TOC was practically insigni®cant. The dose of H2O2 in this operation was ®xed with respect to the chemical oxygen demand reaction of total organic carbon. RESULTS AND DISCUSSION
First-stage operation Determination of optimum working pH, Freundlich parameters, GAC dose and UV intensity. Previous researchers have reported that for organic compounds with acidic or basic properties, adsorption is strongest in the pH region which yields the highest proportion of undissociated molecules (Martin and Iwugo, 1982). We observed however, that the eect of pH from the acidic to the neutral range was insigni®cant for our test concentration of phenol. Our observation was in agreement with those of Halhoili et al. (1995) and Cooney and Wijaya (1987), who have reported that for phenol concentrations below 70 ppm, the eect of pH up to 10 is negligible. Furthermore, in a pH range between 3 and 11, we found that oxidative degradation of phe-
Fig. 2. Time-dependent degradation of phenol by UV/H2O at various pH levels. Initial conditions were 40.62 mg lÿ1 phenol, 5.04 10ÿ3 M H2O2 and 104.78 W mÿ2 UV intensity. (k ' denotes pH dependent ®rst-order degradation constants.)
4172
Nilsun H. Ince and Izzet G. Apikyan
nol and total organic carbon by UV/H2O2 was most ecient when pH was 7, as shown in Fig. 2. Hence, the working pH was ®xed at 7 for all isotherm tests, single AOP and single adsorption runs, as well as the ®rst- and second-stage operational steps of our system (except for desorption runs, which were employed at a basic pH). Adsorption isotherm parameters, or the coecients of equation (1) (K and 1/n ) were estimated graphically as 23.57 and 0.35, respectively, as shown in Fig. 3. Isotherm tests conducted at the same initial phenol concentration but with H2O2 resulted in a slight alteration in Freundlich parameters K 21:53, 1=n 0:397), implying the likelihood of some H2O2 adsorption on GAC, along with phenol. This was veri®ed by monitoring the concentration of 5 10ÿ3 M H2O2 in a phenol-free solution during 90-min contact with 1 g lÿ1 GAC. The data showed that ®nal H2O2 residual in the euent was 30% lower than that in the control solution (no GAC). However, when H2O2 was irradiated either in the presence or absence of GAC for 90 min, the take-up rate was nearly the same, and much faster than the take-up by adsorption in a light-free solution. Hence, it was concluded that the main tendency of H2O2 in a medium of UV irradiation and activated carbon is to dissociate by photolysis rather than adsorb on the GAC surface. This lack of competition for adsorption and photolysis (regarding H2O2 elimination) indicates that during the simultaneous operation of advanced oxidation and activated carbon adsorption, there will be no interactive eects to inhibit the adsorption of phenol and/or lower free radical yields. The optimal UV power in both operational steps was 60 W, because the rates of phenol and/or total organic carbon degradation were by far the fastest when all four of the mercury cathode lamps were turned on. The corresponding UV intensity, estimated by chemical actinometry (described in the
Fig. 3. Freundlich adsorption isotherm for phenol (initial concentration=40 mg lÿ1). The data labels refer to the applied GAC dose.
Fig. 4. The impact of increased GAC dose on removal of 40 ppm phenol from solution.
previous section) was 2.22 10ÿ4 Einstein/mÿ2 sÿ1 or 104.78 W/mÿ2. The optimum dose of GAC was determined by monitoring the residual phenol in solution (as an indicator of the degree of adsorption) during 90-min contact with increasing levels of GAC in the absence of UV irradiation and hydrogen peroxide. The data are presented in Fig. 4. Since no signi®cant increase in adsorption was recorded at GAC doses larger than 1 g lÿ1, this was selected as the operating dose. We also observed in preliminary tests with the ®rst-stage operation (using 1 g lÿ1 GAC) that the mass of GAC remained constant even after being used in four consecutive batch operations to treat 16 l of phenolic wastewater. This observation justi®ed our expectation that activated carbon was not exposed to any degradation reactions in the combined system by the oxidative eects of H2O2 and/ or UV light. Process analysis and comparison with GAC-absent AOP The impact of increased exposure on the rate of
Fig. 5. The impact of detention time on the rate of phenol and TOC removal by adsorption, advanced oxidation and GAC-added advanced oxidation. Initial conditions were 40.62 mg lÿ1 phenol, 29.89 mg lÿ1 TOC. (Doses of GAC, H2O2 and UV in related processes were 1000 mg lÿ1, 5.04 10ÿ3 M and 104.8 W mÿ2, respectively.)
Combination of activated carbon adsorption with light-enhanced chemical oxidation
phenol and TOC removal by the ®rst-stage operation is presented in Fig. 5. The optimum contact time was selected on the basis of maximum TOC removal in time, and because total TOC removal remained almost constant beyond 90-min exposure to the tested system, this was selected as the operating contact time. The eect of combining two processes in the same reactor was assessed by monitoring the individual performances of singly operated adsorption and advanced oxidation systems. The observed data are added onto Fig. 5 for comparison with the combined operation. Although the rate of phenol (or TOC) removal by adsorption is faster than that of TOC removal (upon mineralization) by AOP, the two processes are not environmentally equivalent due to the accumulation of unconverted contaminants on the solid phase by the former. The data also show that the rate of phenol (or TOC) accumulation by adsorption is equal to the rate of TOC elimination (upon oxidation and accumulation) by the combined system. However as discussed in the next section, much larger volumes of TOC-accumulated sludge was generated by the former process, turning it to a notably less ecient system. Comparison of the two oxidation-involving processes showed that running advanced oxidation and absorption simultaneously provided a distinct advantage over running AOP alone, in particular for the elimination of total organic carbon. While 87.4 % TOC was removed by the former in 90 min, 250 min of contact was required by the latter for achieving an equivalent eciency. Furthermore, the combined process, despite the partial accumulation of organic matter on the solid phase (requiring substantial treatment for destructive regeneration) was far more cost-ecient than AOP alone, due to much lower rates of energy and H2O2 consumption for equivalent eciency of total mineralization. This is a consequence of the fact that in the AOP system, mineralization did not start before 50 min of contact, whereas in the combined operation, a ®rst-order TOC removal rate was observed throughout the contact period (upon combined eect of adsorption and degradation of oxidation intermediates). This implied that some early intermediates of oxidation were trapped by the adsorbent before
4173
they had a chance to undergo further oxidation for initiation of the mineralization process. Accordingly, the faster elimination of total organic carbon in the simultaneous operation was due to enhanced mineralization eect, arising from partial adsorption of organic intermediates on the GAC surface, or reduced competition of scavengers for free radicals and UV light. Estimated rate constants for ®rst-order reactions in all three systems are presented in Table 1. Oxidative degradation of phenol in the combined process was veri®ed by the changes observed in the absorption spectrum of the solution during 90-min contact with the GAC-UV/H2O2 system, as presented in Fig. 6. Initially, the solution was characterized by a peak at 270 nm (typical of phenol), that was observed to diminish shortly after contact. The optical density of the solution thereafter was positive but declining with time, indicating and implying respectively, the presence and degradation of organic oxidation byproducts. Furthermore, continuous monitoring of pH during the operation showed that there was an immediate pH drop as the system was turned on, leveling o at a value close to 4 after the ®rst 20 min. The shift of pH towards the acidic range indicates the formation of organic acids, as proposed in the literature for the transformation mechanism of phenol upon exposure to advanced oxidation by UV/H2O2 (Castrantas and Gibilisco, 1990). Second-stage operation Selection of regeneration parameters. The deterioration in GAC upon multiple use was investigated by comparing the total fraction of TOC removal at the end of each consecutive batch with a controlÐ the euent of a UV/H2O2 system, yielding 65% TOC removal in a 90-min batch. The test was repeated in the absence of UV/H2O2 to isolate the eect of advanced oxidation on saturation time or volume (Fig. 7).
Table 1. Comparison of ®rst-order reaction rate constants by adsorption, advanced oxidation and adsorption-enhanced advanced oxidation Processa
Phenol
TOC
ADS AO ADS+AO
k '=0.034 minÿ1 k '=0.16 minÿ1 k '=0.31 minÿ1
k'=0.034 minÿ1 k' not availableb k'=0.035 minÿ1
a
``ADS'' and ``AO'' refer to adsorption on GAC and advanced oxidation by UV/H2O2, respectively. b Rate of TOC removal by AO was not ®rst-order.
Fig. 6. Absorption spectra of the test solutoin at the 200± 330 nm band during exposure to GAC-UV/H2O2 systems. Labels a, b, c, d, e and f refer to exposure duration as 0, 10, 20, 30, 60 and 90 min, respectively. Initial conditions were 40.62 mg lÿ1 phenol, 5.04 10ÿ3 M H2O2 and 1000 mg lÿ1 GAC.
4174
Nilsun H. Ince and Izzet G. Apikyan
Fig. 7. Depreciation of GAC upon use in ®ve consecutive batches. Initial conditions in each batch were 29.89 mg lÿ1 TOC, 5.04 10ÿ3 M H2O2 and 1000 mg lÿ1 GAC.
It was observed that in GAC-added advanced oxidation, 42% excess TOC removal (142% of the control) was possible initially when the GAC was fresh, and the system was still 6% better than the control (106% of the control) after processing 20 l of euent. When GAC was used alone, the depreciation upon increased batch operation was considerably faster, as indicated by the sharp decline in TOC removal eciency from 123% of the control (23% excess) when GAC was fresh (batch 1) to 11% of the control (89% de®ciency) when it was saturated (batch 5). The remarkable delay in the degree of saturation with increased wastewater volume in the combined operation is simply due to reduced organic loading on the carbon surface as a consequence of oxidation reactions. This led to the fact that while in the combined system, regenerating the spent GAC after four consecutive batches was sucient to maintain economic and technical feasibility, the operation of adsorption alone required regeneration after each 90-min batch to accomplish an equivalent TOC removal eciency. The ®rst step of regeneration was the transport of immobilized particles from surfaces of exhausted carbon particles into solution (desorption) by contacting the spent GAC for 90 min with deionized water at constant pH and temperature. These constants were selected on the basis of maximum TOC transport into 4 l of deionized water during contact of 4 g of spent GAC with various levels of pH and temperature. The tests revealed that desorption was very sensitive to pH increases (upon controlled ad-
dition of sodium hydroxide) in the basic range, but less so to temperature eects. The data are presented in Table 2. Large increases in desorption upon pH elevations in the basic range are due to the formation of water soluble sodium salts of phenol and its acidic oxidation intermediates with increased alkalinity. On the other hand, when pH was kept constant at a neutral level and the temperature was raised from 25 to 658C, the degree of desorption increased by only 10%; to be attributed to the exothermic nature of the adsorption process (Castrantas and Gibilisco, 1990). As listed in Table 2, maximum desorption was obtained when the spent GAC was contacted with a regenerating solution at pH=12.5 and T=658C for 90 min. The closest value to this maximum was achieved when the temperature was lowered to 258C, pH being kept constant at 12.5. Reuse of the two sets of GAC retained under these two conditions showed that the reclamation in terms of total organic carbon adsorption was very close (91.1% and 92.8% of fresh GAC upon regeneration at 25 and 658C, respectively). Hence, for the sake of obvious economic advantages, regeneration conditions were ®xed as t = 90 min, pH=12.5 and T = 258C. Treatment of the regeneration solutions. The second-step of regeneration was the destruction of remobilized adsorbates by exposing the regenerating solution (readjusted to neutral pH) to the previously de®ned UV/H2O2 operation for 90 min. The control parameter was TOC during this stage, for the fact that no phenol could be detected in regenerating solutions by chemical analysis. This notably implies that ``oxidative degradation'' is the favorable exposure pathway for phenol in a system where routes to ``adsorption'' and ``advanced oxidation'' are equivalent. Under the experimental conditions de®ned, it was found that ``destructive regeneration'' provided 92.5% TOC removal from the regenerating solutions of the combined system in 90 min. On the
Table 2. Eect of pH and temperature on remobilization of 144.04 mg TOC from 4 g of spent GAC in 90 min pH
Temperature (Celcius)
TOC in solution (mg)
7.0 7.0 9.2 10.8 12.5 12.5
25 65 65 25 25 65
0.72 11.52 20.77 47.48 85.48 110.46
Fig. 8. Degradation of total organic carbon by UV/H2O2 in the regeneration solutions from saturated GAC of GAC-UV/H2O2 process (``REG1''), and saturated GAC of the GAC process (``REG2''). The straight lines represent linear ®ts of the two data sets to y=ax, where y = ln(c/ c0), x = time, and a = ®rst-order reaction rate constant.
Combination of activated carbon adsorption with light-enhanced chemical oxidation
other hand, ``destructive regeneration'' of spent GAC from ``single GAC system'' (as ``spent'' by use in one 90-min batch, or 4 l of phenol solution) showed that the rate of mineralization within the same period of time was signi®cantly lower. The two sets of data are plotted in Fig. 8. The pseudo ®rst-order rate constants kREG1 and kREG2 corresponding to the degradation of total organic carbon in the two referred solutions were estimated by regression analyses of the TOC-time data, using equation (2). Note that the large dierence in the initial TOC concentrations of the two solutions is a consequence of multiple use of GAC in the combined system, and single use in the other. The faster rate of mineralization in the ®rst solution (that from the combined system) despite its higher TOC content (for 4 times longer exposure of GAC to the wastewater) is due to dierences in chemical structure and complexity between the two solutions. The ®rst one is made of a mixture of randomly distributed oxidation by-products, those of phenol and its degraded forms. The second solution, however, contains exclusively the parent compound, requiring a larger number of oxidation steps with longer reaction times to accomplish an equivalent degree of mineralization. As a result, 92.5% of TOC was mineralized in the second solution in 90 min, as opposed to 56% mineralization in the ®rst one, corresponding to ®nal euents of 3.82 and 12.87 mg lÿ1 as TOC, respectively. Hence, the rate of mineralization in the regenerating solutions was inversely proportional to the number and complexity of the compounds present, the latter re¯ecting the activation energy and nature of chemical bonding between organic carbons. Summary of operating parameters and cost evaluation. It was found that the ®rst operational step required 375 h of contact with the system for processing 1 m3 of wastewater with 40 mg lÿ1 phenol to accomplish 87.4 % TOC removal along with complete phenol elimination. The total GAC consumption during this operation was 125 g, which resulted in 125 l of regenerating solution to be processed in the second operational step by advanced Table 3. List of operating parameters and cost estimation for the GAC-added advanced oxidation system Parametera
Contact time GAC (0.68 USD/kg) H2O2 (0.295 USD/l) NaOH (0.268 USD/l) Electrical power (0.055 USD/kWh) UV lamp-life (1.25 USD/1000 h) Cost a
Consumption (per m3 wastewater) First stage
Second stage
375 h 125 g 0.58 l ± 22.5 kWh 37.5% 1.96 USD
46.9 h ± 0.032 l 0.281 l 2.81 kWh 4.69% 0.30 USD
Numbers in parentheses indicate unit prices, except for that of electrical power, which is the local price.
4175
oxidation. The contact time for 92.5% TOC degradation in this volume of a solution was 46.9 h. Table 3 summarizes the operational parameters of the system for processing 1 m3 of wastewater with 40 ppm phenol. The value of total GAC consumption was estimated by considering regeneration every four batches, and fresh supplies every eight batches. Caustic consumption is due to pH adjustment of the deionized water to 12.5 to prompt the transfer of the sorbed species back into aqueous medium. The operating cost of the proposed system was estimated on the basis of four 90-min batch operations (16 l) with 4 g of GAC, followed by regeneration of the spent carbon and destruction of the mobilized species in the regenerating solution by exposure to UV irradiation and hydrogen peroxide. In addition to reagents and power, deterioration of UV lamps was also considered a consumption item, the cost of which was estimated by multiplying the total irradiation time as a fraction of the life time of a low-pressure 60-W UV mercury lamp (01000 h) by its unit price. Accordingly, total operating cost of the system per cubic meter was found to be 2.26 USD, as the sum of the estimated costs of ®rst and second-step operations. CONCLUSION
An advanced treatment approach involving simultaneous operation of adsorption and advanced oxidation, followed by ``destructive regeneration'' of the spent adsorbent was investigated for treating wastewaters of industrial origin. It was found that while oxidative degradation was the prevailing elimination route for phenol, the intermediates or byproducts of this route were removed by combined eect of adsorption and mineralization. It was also found that there were no cross eects between reagents of the system to inhibit adsorptive and/or free radical generation processes. Furthermore, as a consequence of enhanced mineralization during the ®rst-stage operation, destructive regeneration of the spent carbon was found to proceed with lower energy and hydrogen peroxide consumption than it would if treatment were carried out by adsorption alone. The system described herein provides apparent advantages over singly operated adsorption and advanced oxidation processes as on-site disposal of the spent adsorbent and lower reagent and power consumption, respectively. Moreover, it is preferrable to the previously reported ``phase transfer oxidation'' approach (where advanced oxidation was used only for destructive regeneration) for the fact that much smaller volumes of regenerating solutions are generated in the proposed combination, due to reduced rate of saturation. In addition to its lower volume advantage, the regenerating solution of the GAC/UV±H2O2 combined system was noted for its
4176
Nilsun H. Ince and Izzet G. Apikyan
simpler chemical structure, which made it more easily and rapidly degradable by advanced oxidation for a more complete mineralization of organic carbon. REFERENCES
APHA/AWWA (1992) Standard Methods for the Examination of Water and Wastewater, 18th ed. American Public Health Association, Washington, DC. Bauman L. C. and Stenstrom M. K. (1990) Removal of organohalogens and organohalogen precursors in reclaimed wastewaterÐI. Wat. Res. 24(8), 949. Castrantas H. M. and Gibilisco R. D. (1990) UV destruction of phenolic compounds under alkaline conditions. ACS Symposium Series 422, 77±99. Cooney D. O. and Wijaya J. (1987) Eect of pH and added salts on the adsorption of ionizable organic species onto activated carbon from aqueous solution. In Proceedings of the 2nd Engineering Foundation Conference on Fundamentals of Adsorption, ed. A. I. Liapis. Engineering Foundation, New York. Crittenden J. C., Suri R. P. S., Perram D. L. and Hand D. W. (1997) Decontamination of water using adsorption and photocatalysis. Wat. Res. 31(3), 411±418. Diez M. C., Mora M. L. and Videla S. (1999) Adsorption of phenolic compounds on bleached kraft mill euent. Wat. Res. 33(1), 125±130. Dobbs R. A. and Cohen J. M. (1980) Carbon Adsorption Isotherms for Toxic Organics, US Environmental Protection Agency, Municipal Environmental Research Laboratory, Oce of Research and Development, Cincinnati. EPA-600/8-80-023. Fajerwerg K., Foussard J. N., Perrad A. and Debellefontaine H. (1997) Wet oxidation of phenol by hydrogen peroxide: the key role of pH on the catalytic behavior of Fe-ZSM-5. Wat. Sci. Tech. 35(4), 103±110. Furuya E. G., Chang H. T., Miura Y., Yakomura H., Tajima S., Yamashita S. and Noll K. E. (1996) Intraparcticle mass transport mechanism in AC adsorption of phenols. J. Env. Eng-ASCE 122(10), 906±916. Gould J. P. and Weber W. J. (1976) Oxidation of Phenols by Ozone. J. WPCF 48(1), 47±54. Glaze W. H., Kang J. W. and Chapin D. H. (1987) The chemistry of water treatment processes involving ozone, hydrogen peroxide and ultraviolet radiation. Ozone Sci. Eng. 9, 335. Guittonneau S., De Laat J., Duguet J. P., Bonnel C. and Dore B. (1990) Oxidation of PCNM in dilute aqueous solutions by O3+UV and H2O2+UV: a comparative study. Ozone Sci. Eng. 12, 73±94. GuÈrol M. D. and Vatistas R. (1987) Oxidation of phenolic compounds by ozone and ozone/uv radiation: a comparative study. Water Res. 21(8), 895.
Halhoili K. A., Darwish N. A. and Al-Dhoon N. M. (1995) Eects of pH and inorganic salts on the adsorption of phenol from aqueous systems on activated decolorizing charcoal. Separation Science and Technol. 30(17), 3313±3325. Ince N. H., Stephen M. I. and Bolton J. R. (1997) UV/ H2O2 degradation and toxicity reduction of textile azo dyes. J. Adv. Oxid. Technol. 2(3), 442±448. Ince N. H. (1998) Light enhanced chemical oxidation for tertiary treatment of municipal land®ll leachate. Water Env. Res. 70(6), 1161±1168. Ince N. H. and Tezcanli G. (1999) Treatability of textile dyebath euents by advanced oxidation: preparation for reuse. Wat. Sci.Tech. 40(1), 183±190. Klassen N. V., Marchington D. and McGrowen C. E. (1994) Method HP-02: Analysis of hydrogen peroxide. Anal. Chem. 66, 2921±2925. Kusakabe K., Aso S., Wada T., Hayashi J., Moroka S. and Isomuta K. (1991) Destruction rate of volatile organochlorine compounds in water by ozonization with ultraviolet radiation. Water Res. 25(10), 1199. Liu J., Crittenden J. C., Hand D. W. and Perram D. L. (1996) Regeneration of adsorbents using heterogeneous photocatalytic oxidation. J. Env. Eng.ÐASCE. 122(8), 707±713. Mark G., Schumann M. N., Schummann H. P. and Sonntag C. (1990) A chemical actinometer for use in connection with UV treatment in drinking water processing. J. Water S.R.T.ÐAqua. 39(5), 309±313. Martin R. J. and Iwugo K. O. (1982) The eects of pH and suspended solids in the removal of organics from waters and wastewaters by the activated carbon adsorption process. Water Res. 19, 73±82. Mokrini A., Ousse D. and Esplugas S. (1997) Oxidation of aromatic compounds with UV radiation/ozone/ hydrogen peroxide. Wat. Sci. Technol. 35(4), 95±102. Mourand J. T., Crittenden J. C., Hand D. W., Perram D. L. and Notthakun S. (1995) Regeneration of spent adsorbents using homogeneous advanced oxidation. Water Env. Res. 67, 355±363. Nakhla G., Abuzaid N. and Farooq S. (1994) Activated carbon adsorption of phenolics in oxic systems: eect of pH and temperature variations. Water Env. Res. 66(6), 842±850. Scheck C. K. and Frimmel F. H. (1995) Degradation of phenol and salicylic acid by ultraviolet radiation/hydrogen peroxide/oxygen. Wat. Res, 29(10), 2346±2352. Staehelin J. and Hoigne J. (1985) Decomposition of ozone in water in the presence of organic solutes acting as promoters and inhibitors of radical chain reactions. Env. Sci. & Technol. 19, 1206. Tressmer C. H., Vidic R. D. and Uranowski L. J. (1997) Impact of oxygen containing surface functional groups on AC adsorption of phenols. Env. Sci. & Technol. 31(7), 1872±1878.
Wat. Res. Vol. 34, No. 17, pp. 4169±4176, 2000 7 2000 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/00/$ - see front matter
www.elsevier.com/locate/watres
COMBINATION OF ACTIVATED CARBON ADSORPTION WITH LIGHT-ENHANCED CHEMICAL OXIDATION VIA HYDROGEN PEROXIDE NILSUN H. INCE*M and IZZET G. APIKYAN BogÆazic° i University, Institute of Environmental Sciences, 80815, Bebek-Istanbul, Turkey (First received 10 June 1999) AbstractÐA tertiary treatment scheme involving simultaneous operation of activated carbon adsorption and advanced oxidation with ultraviolet light and hydrogen peroxide, followed by ``destructive regeneration'' of the spent adsorbent by advanced oxidation was investigated, using phenol as a model compound. Operational parameters in each step were optimized on the basis of phenol and total organic carbon removal during selected contact times. It was found that in the ®rst stage with adsorption/advanced oxidation, phenol was totally eliminated during the ®rst quarter of the contact time, and 87.5% total organic carbon removal was accomplished at the end. It was further found that advanced oxidation was the dominant pathway in this operation for the disappearance of phenol, while that of total organic carbon was carried out by combined eects of adsorption and oxidative degradation. Optimum regenerating frequency for the spent activated carbon was found to be once every four batches, which was four times slower than the required frequency in the absence of advanced oxidation. In the second part of the operation, where the spent carbon was regenerated destructively via advanced oxidation, 92.5% mineralization was accomplished in the regenerating solution at the end of the optimized contact time. The economic assessment of the system considering the operation of both steps revealed that under the initial and optimized conditions, the operating cost is 2.26 USD per cubic meter of wastewater with 40 ppm phenol. 7 2000 Elsevier Science Ltd. All rights reserved Key wordsÐadvanced oxidation (AOP), granular activated carbon (GAC), destructive regeneration, total organic carbon (TOC), mineralization, degradation
INTRODUCTION
Advanced technologies have recently become essential counterparts of euent treatment plants due to increasing public concern for health related environmental problems, and the corresponding outcome as the need for revision of euent discharge standards. Despite the fact that many of the advanced systems are expensive to install and operate, they are nevertheless unavoidable in tertiary treatment of industrial euents to render detoxi®cation and/or exclusion of refractory substances. Today's engineer, therefore, is challenged by the need to develop optimal technologies that allow safe discharge and disposal of industrial contaminants at appropriate costs. The last decade has witnessed advanced oxidation processes (AOP) emerging as promising alternatives to tertiary treatment, owing to their high potency to *Author to whom all correspondence should be addressed. Tel.: +90-212-263-1500; fax: +90-212-257-5033; email: [email protected]
render partial and ultimate destruction of many refractory compounds including dyestu, halogenated and aromatic organics (Bauman and Stenstrom, 1990; Kusakabe et al., 1991; Ince et al., 1997; Ince, 1998; Ince and Tezcanli, 1999). These processes involve the formation of highly reactive free radical species, which are far more powerful as oxidizing agents than commonly known strong oxidants like molecular oxygen and ozone. The chemistry, kinetics and quantum yields in free radical reactions have been widely investigated and reviewed in the past (Glaze et al., 1987; Staehelin and Hoigne, 1985; Guittonneau et al., 1990; GuÈrol and Vatistas, 1987). These processes have two unique advantages over other advanced treatment processes: (i) they are non-selective to a very broad range of chemicals, and (ii) they involve no sludge production due to the character of their removal mechanism, which is based on the oxidative destruction of organic carbon by conversion to higher oxidation states. Nevertheless, depending on the targeted euent quality, treatment by advanced oxidation may impose high operating costs, due to
4169
4170
Nilsun H. Ince and Izzet G. Apikyan
intensive energy and chemical requirements arising from long retention times to complete oxidation reactions and to accomplish an appreciable degree of mineralization. A more commonly employed advanced treatment technique is adsorption on activated carbon; a process based on the concentration and immobilization of a contaminant on the surfaces of granular or powdered activated carbon. The technique is well known as an ``interface phenomenon'', encompassing a state of dynamic equilibrium between the solute in the aqueous phase and that adsorbed on the carbon surface. When equilibrium is reached, adsorption parameters can be simply established by using an appropriate isotherm equation. As a whole, adsorption is a simple-to-operate process and relatively cost-eective, due to low (or no) energy requirements and the possibility of reclaiming and reusing the spent carbon via regeneration. However, surface phenomena involving the accumulation of undesired contaminants on the solid surface end up in large quantities of ``spent adsorbent'' and/or ``regeneration solutions'', both of which have to be handled as ``hazardous wastes'' (Crittenden et al., 1997). Accordingly, tertiary treatment by adsorption alone is not an environmentally complete system, unless the destruction of immobilized contaminants on the spent or wasted carbon surface is heeded as well. Furthermore, the incomplete system is incomparable to treatment by advanced oxidation, where there is neither any accumulation, nor transport of the contaminant from one medium to another. A recently proposed advanced method to cope with the above problem was called ``phase-transfer oxidation'', which involved two consecutive operational steps characterized by ®xed-bed adsorption for euent treatment and advanced oxidation for destructive regeneration of the adsorbent (Mourand et al., 1995; Liu et al., 1996). The strategy was reported to provide a signi®cant advantage as ``onsite regeneration'' to minimize adsorbent inventory and eliminate unloading, transportation and repacking of the adsorbent (Mourand et al., 1995). The purpose of this study was to investigate a modi®ed version of ``phase-transfer oxidation'', in which euent treatment or the ®rst-step operation was carried out by the simultaneous operation of activated carbon adsorption and advanced oxidation in the same reactor. The second consecutive step similarly involved destructive regeneration of the spent adsorbent by advanced oxidation. The advantages expected from this modi®cation were: (i) delayed exhaustion of the adsorbent ± less frequent regeneration/disposal and fresh supply of activated carbon; (ii) increased cost-eectiveness due to lowered energy and oxidant consumption during the destructive regeneration of the spent carbon. This expectancy is based on the fact that the so called ``regeneration solution'' to be treated by advanced
oxidation in the second-stage will be made of partly degraded by-products of the parent compound as a consequence of its oxidative degradation in the ®rst stage. Phenol was used as a model chemical throughout the study to represent toxic contaminants with aromatic structures. The availability of extensive literature on its immobilization by activated carbon adsorption (Diez et al., 1999; Tressmer et al., 1997; Furuya et al., 1996; Nakhla et al., 1994), and destruction by advanced oxidation (Gould and Weber, 1976; Castrantas and Gibilisco, 1990; Scheck and Frimmel, 1995; Fajerwerg et al., 1997; Mokrini et al., 1997) provided not only the information about the degradation products and adsorption parameters of phenol, but also the opportunity to compare the tested system with singly employed advanced oxidation and activated carbon adsorption systems. The study involved: 1. Construction of adsorption isotherms at equilibrium and the investigation of possible cross eects of H2O2 and UV irradiation with activated carbon. Isotherms were constructed by ®tting the test data into the linearized form of Freundlich equation described by: ln qe ln K
1 ln Cf n
1
where qe is the solid-phase loading of the solute, Cf is the equilibrium solute concentration in solution; K is the unit capacity factor, and 1/n is the Freundlich exponent, representing the sorption capacity of the adsorbent, and adsorption intensity of the solute, respectively (Dobbs and Cohen, 1980). 2. Selection and/or optimization of operating parameters such as contact time, pH, GAC dose, UV intensity, and the frequency of spent adsorbent regeneration. 3. Experimentation with single adsorption and advanced oxidation processes to compare their eectiveness with that of the proposed system. Rates of oxidative degradation in this part were estimated by using the integrated form of the standard ®rst-order kinetic equation: dCt ÿk 0 Ct dt
2
where Ct is the concentration of the contaminant at time t (mol lÿ1), and k' is the ®rst-order reaction rate constant (t ÿ1). 4. Investigation of optimum regenerating conditions for the spent activated carbon and estimation of the operating cost of the system on the basis of complete phenol and at least 85% total organic carbon elimination in both stages of the operation.
Combination of activated carbon adsorption with light-enhanced chemical oxidation MATERIALS AND METHODS
Materials Phenol was supplied by Riedel-De Haen AG in 99.5% purity. Hydrogen peroxide (30% w/v), NaOH and HCl (used for pH adjustment) were all analytical grade (Merck). Granular activated carbon was prepared in pellets of 20 60 mesh by grinding standard commercial size (2 8 mesh) Aquatech and W1, followed by washing them with deionized water and drying for 12 h at 1058C. Catalase of bovine liver was purchased from Fluka Chemie AG. The light source was made of four Philips 15-W lowpressure mercury UV lamps, emitting monochromatic light at 253.7 nm. Set-up A schematic diagram of the experimental set-up is given in Fig. 1. It consisted of a 5-l Pyrex reactor equipped with four UV lamps situated 6 cm from the surface of the solution. Contents of the reactor were stirred with magnetic stirrers located at its bottom. A 500-mesh ®lter was placed at the sampling port to prevent free GAC particles escaping the solution during the withdrawal of euent samples for chemical analyses. Analytical Concentration of phenol was analyzed by a standard method based on spectrophoto-metric analysis at 510 nm of the developed color resulting from the reaction of phenol with 4-aminoantipyrene (APHA/AWWA, 1992). Hydrogen peroxide was analyzed by the ``HP-02'' triodide method (Klassen et al., 1994) and destroyed by catalase. All spectrophotometry was carried out by a Shimadzu UV-160 Double Beam Spectrophotometer. Mineraliztion or TOC removal from solution was estimated by monitoring the TOC of the euent samples using a Fisions TCM 480 Analyzer. The UV intensity was determined by a chemical actinometer based on the photolysis of an O2saturated potassium peroxy-disulfate tert-butanol solution to form sulfuric acid (Mark et al., 1990). In accordance, acid formation was monitored by the reduction in pH during 25-min irradiation of the solution by four 15 WUV mercury lamps (253.7 nm), and the data were plotted against time to estimate the photon ¯uence rate from the slope of the obtained straight line. Methodology Adsorption isotherms were constructed at equilibrium by contacting 40 mg lÿ1 phenol with 200, 500, 800, 1000, 1200 and 1500 ppm GAC, respectively. The data were ®t into equation (1) to estimate Freundlich parameters. The eect of a strong oxidizing agent on the adsorption of phenol was investigated by reconstructing the isotherms in the presence of H2O2. The operating parameters of the ®rst-stage operation were determined by irradiating a synthetic wastewater (contaminated with 40 mg lÿ1 phenol) in the presence of 5 10ÿ3 M H2O2 and 1000 mg lÿ1 GAC until complete phenol and at least 85% TOC removal was accomplished. The dose of GAC was selected on the basis of increased
Fig. 1. Schematic diagram of the reactor.
4171
adsorption upon increasing doses of GAC addition. The initial H2O2 concentration was adjusted in accordance with the stoichiometric requirements of the chemical oxygen demand reaction of the phenol test solution. The absorption spectrum and pH of euent samples were periodically monitored during contact to verify the occurrence of oxidative degradation. The experiments were repeated with GAC and AOP alone to investigate the eectiveness of running the two processes simultaneously. The second stage of the operation involved two substeps: isolation of spent GAC particles from treated euent for remobilization of the sorbed species from GAC surfaces into solution, and exposure of this contaminated solution to advanced oxidation by UV/H2O2 until mineralization was close to completion. The ®rst sub-step involved contacting the exhausted GAC particles with deionized water at optimum pH and temperature to provide maximum desorption. In the second sub-step, the ``regenerating solution'' (after pH adjustment to a neutral level) containing organic matter desorbed from GAC surface was exposed to destruction by UV/H2O2 for a reasonable contact time, beyond which further reduction in TOC was practically insigni®cant. The dose of H2O2 in this operation was ®xed with respect to the chemical oxygen demand reaction of total organic carbon. RESULTS AND DISCUSSION
First-stage operation Determination of optimum working pH, Freundlich parameters, GAC dose and UV intensity. Previous researchers have reported that for organic compounds with acidic or basic properties, adsorption is strongest in the pH region which yields the highest proportion of undissociated molecules (Martin and Iwugo, 1982). We observed however, that the eect of pH from the acidic to the neutral range was insigni®cant for our test concentration of phenol. Our observation was in agreement with those of Halhoili et al. (1995) and Cooney and Wijaya (1987), who have reported that for phenol concentrations below 70 ppm, the eect of pH up to 10 is negligible. Furthermore, in a pH range between 3 and 11, we found that oxidative degradation of phe-
Fig. 2. Time-dependent degradation of phenol by UV/H2O at various pH levels. Initial conditions were 40.62 mg lÿ1 phenol, 5.04 10ÿ3 M H2O2 and 104.78 W mÿ2 UV intensity. (k ' denotes pH dependent ®rst-order degradation constants.)
4172
Nilsun H. Ince and Izzet G. Apikyan
nol and total organic carbon by UV/H2O2 was most ecient when pH was 7, as shown in Fig. 2. Hence, the working pH was ®xed at 7 for all isotherm tests, single AOP and single adsorption runs, as well as the ®rst- and second-stage operational steps of our system (except for desorption runs, which were employed at a basic pH). Adsorption isotherm parameters, or the coecients of equation (1) (K and 1/n ) were estimated graphically as 23.57 and 0.35, respectively, as shown in Fig. 3. Isotherm tests conducted at the same initial phenol concentration but with H2O2 resulted in a slight alteration in Freundlich parameters K 21:53, 1=n 0:397), implying the likelihood of some H2O2 adsorption on GAC, along with phenol. This was veri®ed by monitoring the concentration of 5 10ÿ3 M H2O2 in a phenol-free solution during 90-min contact with 1 g lÿ1 GAC. The data showed that ®nal H2O2 residual in the euent was 30% lower than that in the control solution (no GAC). However, when H2O2 was irradiated either in the presence or absence of GAC for 90 min, the take-up rate was nearly the same, and much faster than the take-up by adsorption in a light-free solution. Hence, it was concluded that the main tendency of H2O2 in a medium of UV irradiation and activated carbon is to dissociate by photolysis rather than adsorb on the GAC surface. This lack of competition for adsorption and photolysis (regarding H2O2 elimination) indicates that during the simultaneous operation of advanced oxidation and activated carbon adsorption, there will be no interactive eects to inhibit the adsorption of phenol and/or lower free radical yields. The optimal UV power in both operational steps was 60 W, because the rates of phenol and/or total organic carbon degradation were by far the fastest when all four of the mercury cathode lamps were turned on. The corresponding UV intensity, estimated by chemical actinometry (described in the
Fig. 3. Freundlich adsorption isotherm for phenol (initial concentration=40 mg lÿ1). The data labels refer to the applied GAC dose.
Fig. 4. The impact of increased GAC dose on removal of 40 ppm phenol from solution.
previous section) was 2.22 10ÿ4 Einstein/mÿ2 sÿ1 or 104.78 W/mÿ2. The optimum dose of GAC was determined by monitoring the residual phenol in solution (as an indicator of the degree of adsorption) during 90-min contact with increasing levels of GAC in the absence of UV irradiation and hydrogen peroxide. The data are presented in Fig. 4. Since no signi®cant increase in adsorption was recorded at GAC doses larger than 1 g lÿ1, this was selected as the operating dose. We also observed in preliminary tests with the ®rst-stage operation (using 1 g lÿ1 GAC) that the mass of GAC remained constant even after being used in four consecutive batch operations to treat 16 l of phenolic wastewater. This observation justi®ed our expectation that activated carbon was not exposed to any degradation reactions in the combined system by the oxidative eects of H2O2 and/ or UV light. Process analysis and comparison with GAC-absent AOP The impact of increased exposure on the rate of
Fig. 5. The impact of detention time on the rate of phenol and TOC removal by adsorption, advanced oxidation and GAC-added advanced oxidation. Initial conditions were 40.62 mg lÿ1 phenol, 29.89 mg lÿ1 TOC. (Doses of GAC, H2O2 and UV in related processes were 1000 mg lÿ1, 5.04 10ÿ3 M and 104.8 W mÿ2, respectively.)
Combination of activated carbon adsorption with light-enhanced chemical oxidation
phenol and TOC removal by the ®rst-stage operation is presented in Fig. 5. The optimum contact time was selected on the basis of maximum TOC removal in time, and because total TOC removal remained almost constant beyond 90-min exposure to the tested system, this was selected as the operating contact time. The eect of combining two processes in the same reactor was assessed by monitoring the individual performances of singly operated adsorption and advanced oxidation systems. The observed data are added onto Fig. 5 for comparison with the combined operation. Although the rate of phenol (or TOC) removal by adsorption is faster than that of TOC removal (upon mineralization) by AOP, the two processes are not environmentally equivalent due to the accumulation of unconverted contaminants on the solid phase by the former. The data also show that the rate of phenol (or TOC) accumulation by adsorption is equal to the rate of TOC elimination (upon oxidation and accumulation) by the combined system. However as discussed in the next section, much larger volumes of TOC-accumulated sludge was generated by the former process, turning it to a notably less ecient system. Comparison of the two oxidation-involving processes showed that running advanced oxidation and absorption simultaneously provided a distinct advantage over running AOP alone, in particular for the elimination of total organic carbon. While 87.4 % TOC was removed by the former in 90 min, 250 min of contact was required by the latter for achieving an equivalent eciency. Furthermore, the combined process, despite the partial accumulation of organic matter on the solid phase (requiring substantial treatment for destructive regeneration) was far more cost-ecient than AOP alone, due to much lower rates of energy and H2O2 consumption for equivalent eciency of total mineralization. This is a consequence of the fact that in the AOP system, mineralization did not start before 50 min of contact, whereas in the combined operation, a ®rst-order TOC removal rate was observed throughout the contact period (upon combined eect of adsorption and degradation of oxidation intermediates). This implied that some early intermediates of oxidation were trapped by the adsorbent before
4173
they had a chance to undergo further oxidation for initiation of the mineralization process. Accordingly, the faster elimination of total organic carbon in the simultaneous operation was due to enhanced mineralization eect, arising from partial adsorption of organic intermediates on the GAC surface, or reduced competition of scavengers for free radicals and UV light. Estimated rate constants for ®rst-order reactions in all three systems are presented in Table 1. Oxidative degradation of phenol in the combined process was veri®ed by the changes observed in the absorption spectrum of the solution during 90-min contact with the GAC-UV/H2O2 system, as presented in Fig. 6. Initially, the solution was characterized by a peak at 270 nm (typical of phenol), that was observed to diminish shortly after contact. The optical density of the solution thereafter was positive but declining with time, indicating and implying respectively, the presence and degradation of organic oxidation byproducts. Furthermore, continuous monitoring of pH during the operation showed that there was an immediate pH drop as the system was turned on, leveling o at a value close to 4 after the ®rst 20 min. The shift of pH towards the acidic range indicates the formation of organic acids, as proposed in the literature for the transformation mechanism of phenol upon exposure to advanced oxidation by UV/H2O2 (Castrantas and Gibilisco, 1990). Second-stage operation Selection of regeneration parameters. The deterioration in GAC upon multiple use was investigated by comparing the total fraction of TOC removal at the end of each consecutive batch with a controlÐ the euent of a UV/H2O2 system, yielding 65% TOC removal in a 90-min batch. The test was repeated in the absence of UV/H2O2 to isolate the eect of advanced oxidation on saturation time or volume (Fig. 7).
Table 1. Comparison of ®rst-order reaction rate constants by adsorption, advanced oxidation and adsorption-enhanced advanced oxidation Processa
Phenol
TOC
ADS AO ADS+AO
k '=0.034 minÿ1 k '=0.16 minÿ1 k '=0.31 minÿ1
k'=0.034 minÿ1 k' not availableb k'=0.035 minÿ1
a
``ADS'' and ``AO'' refer to adsorption on GAC and advanced oxidation by UV/H2O2, respectively. b Rate of TOC removal by AO was not ®rst-order.
Fig. 6. Absorption spectra of the test solutoin at the 200± 330 nm band during exposure to GAC-UV/H2O2 systems. Labels a, b, c, d, e and f refer to exposure duration as 0, 10, 20, 30, 60 and 90 min, respectively. Initial conditions were 40.62 mg lÿ1 phenol, 5.04 10ÿ3 M H2O2 and 1000 mg lÿ1 GAC.
4174
Nilsun H. Ince and Izzet G. Apikyan
Fig. 7. Depreciation of GAC upon use in ®ve consecutive batches. Initial conditions in each batch were 29.89 mg lÿ1 TOC, 5.04 10ÿ3 M H2O2 and 1000 mg lÿ1 GAC.
It was observed that in GAC-added advanced oxidation, 42% excess TOC removal (142% of the control) was possible initially when the GAC was fresh, and the system was still 6% better than the control (106% of the control) after processing 20 l of euent. When GAC was used alone, the depreciation upon increased batch operation was considerably faster, as indicated by the sharp decline in TOC removal eciency from 123% of the control (23% excess) when GAC was fresh (batch 1) to 11% of the control (89% de®ciency) when it was saturated (batch 5). The remarkable delay in the degree of saturation with increased wastewater volume in the combined operation is simply due to reduced organic loading on the carbon surface as a consequence of oxidation reactions. This led to the fact that while in the combined system, regenerating the spent GAC after four consecutive batches was sucient to maintain economic and technical feasibility, the operation of adsorption alone required regeneration after each 90-min batch to accomplish an equivalent TOC removal eciency. The ®rst step of regeneration was the transport of immobilized particles from surfaces of exhausted carbon particles into solution (desorption) by contacting the spent GAC for 90 min with deionized water at constant pH and temperature. These constants were selected on the basis of maximum TOC transport into 4 l of deionized water during contact of 4 g of spent GAC with various levels of pH and temperature. The tests revealed that desorption was very sensitive to pH increases (upon controlled ad-
dition of sodium hydroxide) in the basic range, but less so to temperature eects. The data are presented in Table 2. Large increases in desorption upon pH elevations in the basic range are due to the formation of water soluble sodium salts of phenol and its acidic oxidation intermediates with increased alkalinity. On the other hand, when pH was kept constant at a neutral level and the temperature was raised from 25 to 658C, the degree of desorption increased by only 10%; to be attributed to the exothermic nature of the adsorption process (Castrantas and Gibilisco, 1990). As listed in Table 2, maximum desorption was obtained when the spent GAC was contacted with a regenerating solution at pH=12.5 and T=658C for 90 min. The closest value to this maximum was achieved when the temperature was lowered to 258C, pH being kept constant at 12.5. Reuse of the two sets of GAC retained under these two conditions showed that the reclamation in terms of total organic carbon adsorption was very close (91.1% and 92.8% of fresh GAC upon regeneration at 25 and 658C, respectively). Hence, for the sake of obvious economic advantages, regeneration conditions were ®xed as t = 90 min, pH=12.5 and T = 258C. Treatment of the regeneration solutions. The second-step of regeneration was the destruction of remobilized adsorbates by exposing the regenerating solution (readjusted to neutral pH) to the previously de®ned UV/H2O2 operation for 90 min. The control parameter was TOC during this stage, for the fact that no phenol could be detected in regenerating solutions by chemical analysis. This notably implies that ``oxidative degradation'' is the favorable exposure pathway for phenol in a system where routes to ``adsorption'' and ``advanced oxidation'' are equivalent. Under the experimental conditions de®ned, it was found that ``destructive regeneration'' provided 92.5% TOC removal from the regenerating solutions of the combined system in 90 min. On the
Table 2. Eect of pH and temperature on remobilization of 144.04 mg TOC from 4 g of spent GAC in 90 min pH
Temperature (Celcius)
TOC in solution (mg)
7.0 7.0 9.2 10.8 12.5 12.5
25 65 65 25 25 65
0.72 11.52 20.77 47.48 85.48 110.46
Fig. 8. Degradation of total organic carbon by UV/H2O2 in the regeneration solutions from saturated GAC of GAC-UV/H2O2 process (``REG1''), and saturated GAC of the GAC process (``REG2''). The straight lines represent linear ®ts of the two data sets to y=ax, where y = ln(c/ c0), x = time, and a = ®rst-order reaction rate constant.
Combination of activated carbon adsorption with light-enhanced chemical oxidation
other hand, ``destructive regeneration'' of spent GAC from ``single GAC system'' (as ``spent'' by use in one 90-min batch, or 4 l of phenol solution) showed that the rate of mineralization within the same period of time was signi®cantly lower. The two sets of data are plotted in Fig. 8. The pseudo ®rst-order rate constants kREG1 and kREG2 corresponding to the degradation of total organic carbon in the two referred solutions were estimated by regression analyses of the TOC-time data, using equation (2). Note that the large dierence in the initial TOC concentrations of the two solutions is a consequence of multiple use of GAC in the combined system, and single use in the other. The faster rate of mineralization in the ®rst solution (that from the combined system) despite its higher TOC content (for 4 times longer exposure of GAC to the wastewater) is due to dierences in chemical structure and complexity between the two solutions. The ®rst one is made of a mixture of randomly distributed oxidation by-products, those of phenol and its degraded forms. The second solution, however, contains exclusively the parent compound, requiring a larger number of oxidation steps with longer reaction times to accomplish an equivalent degree of mineralization. As a result, 92.5% of TOC was mineralized in the second solution in 90 min, as opposed to 56% mineralization in the ®rst one, corresponding to ®nal euents of 3.82 and 12.87 mg lÿ1 as TOC, respectively. Hence, the rate of mineralization in the regenerating solutions was inversely proportional to the number and complexity of the compounds present, the latter re¯ecting the activation energy and nature of chemical bonding between organic carbons. Summary of operating parameters and cost evaluation. It was found that the ®rst operational step required 375 h of contact with the system for processing 1 m3 of wastewater with 40 mg lÿ1 phenol to accomplish 87.4 % TOC removal along with complete phenol elimination. The total GAC consumption during this operation was 125 g, which resulted in 125 l of regenerating solution to be processed in the second operational step by advanced Table 3. List of operating parameters and cost estimation for the GAC-added advanced oxidation system Parametera
Contact time GAC (0.68 USD/kg) H2O2 (0.295 USD/l) NaOH (0.268 USD/l) Electrical power (0.055 USD/kWh) UV lamp-life (1.25 USD/1000 h) Cost a
Consumption (per m3 wastewater) First stage
Second stage
375 h 125 g 0.58 l ± 22.5 kWh 37.5% 1.96 USD
46.9 h ± 0.032 l 0.281 l 2.81 kWh 4.69% 0.30 USD
Numbers in parentheses indicate unit prices, except for that of electrical power, which is the local price.
4175
oxidation. The contact time for 92.5% TOC degradation in this volume of a solution was 46.9 h. Table 3 summarizes the operational parameters of the system for processing 1 m3 of wastewater with 40 ppm phenol. The value of total GAC consumption was estimated by considering regeneration every four batches, and fresh supplies every eight batches. Caustic consumption is due to pH adjustment of the deionized water to 12.5 to prompt the transfer of the sorbed species back into aqueous medium. The operating cost of the proposed system was estimated on the basis of four 90-min batch operations (16 l) with 4 g of GAC, followed by regeneration of the spent carbon and destruction of the mobilized species in the regenerating solution by exposure to UV irradiation and hydrogen peroxide. In addition to reagents and power, deterioration of UV lamps was also considered a consumption item, the cost of which was estimated by multiplying the total irradiation time as a fraction of the life time of a low-pressure 60-W UV mercury lamp (01000 h) by its unit price. Accordingly, total operating cost of the system per cubic meter was found to be 2.26 USD, as the sum of the estimated costs of ®rst and second-step operations. CONCLUSION
An advanced treatment approach involving simultaneous operation of adsorption and advanced oxidation, followed by ``destructive regeneration'' of the spent adsorbent was investigated for treating wastewaters of industrial origin. It was found that while oxidative degradation was the prevailing elimination route for phenol, the intermediates or byproducts of this route were removed by combined eect of adsorption and mineralization. It was also found that there were no cross eects between reagents of the system to inhibit adsorptive and/or free radical generation processes. Furthermore, as a consequence of enhanced mineralization during the ®rst-stage operation, destructive regeneration of the spent carbon was found to proceed with lower energy and hydrogen peroxide consumption than it would if treatment were carried out by adsorption alone. The system described herein provides apparent advantages over singly operated adsorption and advanced oxidation processes as on-site disposal of the spent adsorbent and lower reagent and power consumption, respectively. Moreover, it is preferrable to the previously reported ``phase transfer oxidation'' approach (where advanced oxidation was used only for destructive regeneration) for the fact that much smaller volumes of regenerating solutions are generated in the proposed combination, due to reduced rate of saturation. In addition to its lower volume advantage, the regenerating solution of the GAC/UV±H2O2 combined system was noted for its
4176
Nilsun H. Ince and Izzet G. Apikyan
simpler chemical structure, which made it more easily and rapidly degradable by advanced oxidation for a more complete mineralization of organic carbon. REFERENCES
APHA/AWWA (1992) Standard Methods for the Examination of Water and Wastewater, 18th ed. American Public Health Association, Washington, DC. Bauman L. C. and Stenstrom M. K. (1990) Removal of organohalogens and organohalogen precursors in reclaimed wastewaterÐI. Wat. Res. 24(8), 949. Castrantas H. M. and Gibilisco R. D. (1990) UV destruction of phenolic compounds under alkaline conditions. ACS Symposium Series 422, 77±99. Cooney D. O. and Wijaya J. (1987) Eect of pH and added salts on the adsorption of ionizable organic species onto activated carbon from aqueous solution. In Proceedings of the 2nd Engineering Foundation Conference on Fundamentals of Adsorption, ed. A. I. Liapis. Engineering Foundation, New York. Crittenden J. C., Suri R. P. S., Perram D. L. and Hand D. W. (1997) Decontamination of water using adsorption and photocatalysis. Wat. Res. 31(3), 411±418. Diez M. C., Mora M. L. and Videla S. (1999) Adsorption of phenolic compounds on bleached kraft mill euent. Wat. Res. 33(1), 125±130. Dobbs R. A. and Cohen J. M. (1980) Carbon Adsorption Isotherms for Toxic Organics, US Environmental Protection Agency, Municipal Environmental Research Laboratory, Oce of Research and Development, Cincinnati. EPA-600/8-80-023. Fajerwerg K., Foussard J. N., Perrad A. and Debellefontaine H. (1997) Wet oxidation of phenol by hydrogen peroxide: the key role of pH on the catalytic behavior of Fe-ZSM-5. Wat. Sci. Tech. 35(4), 103±110. Furuya E. G., Chang H. T., Miura Y., Yakomura H., Tajima S., Yamashita S. and Noll K. E. (1996) Intraparcticle mass transport mechanism in AC adsorption of phenols. J. Env. Eng-ASCE 122(10), 906±916. Gould J. P. and Weber W. J. (1976) Oxidation of Phenols by Ozone. J. WPCF 48(1), 47±54. Glaze W. H., Kang J. W. and Chapin D. H. (1987) The chemistry of water treatment processes involving ozone, hydrogen peroxide and ultraviolet radiation. Ozone Sci. Eng. 9, 335. Guittonneau S., De Laat J., Duguet J. P., Bonnel C. and Dore B. (1990) Oxidation of PCNM in dilute aqueous solutions by O3+UV and H2O2+UV: a comparative study. Ozone Sci. Eng. 12, 73±94. GuÈrol M. D. and Vatistas R. (1987) Oxidation of phenolic compounds by ozone and ozone/uv radiation: a comparative study. Water Res. 21(8), 895.
Halhoili K. A., Darwish N. A. and Al-Dhoon N. M. (1995) Eects of pH and inorganic salts on the adsorption of phenol from aqueous systems on activated decolorizing charcoal. Separation Science and Technol. 30(17), 3313±3325. Ince N. H., Stephen M. I. and Bolton J. R. (1997) UV/ H2O2 degradation and toxicity reduction of textile azo dyes. J. Adv. Oxid. Technol. 2(3), 442±448. Ince N. H. (1998) Light enhanced chemical oxidation for tertiary treatment of municipal land®ll leachate. Water Env. Res. 70(6), 1161±1168. Ince N. H. and Tezcanli G. (1999) Treatability of textile dyebath euents by advanced oxidation: preparation for reuse. Wat. Sci.Tech. 40(1), 183±190. Klassen N. V., Marchington D. and McGrowen C. E. (1994) Method HP-02: Analysis of hydrogen peroxide. Anal. Chem. 66, 2921±2925. Kusakabe K., Aso S., Wada T., Hayashi J., Moroka S. and Isomuta K. (1991) Destruction rate of volatile organochlorine compounds in water by ozonization with ultraviolet radiation. Water Res. 25(10), 1199. Liu J., Crittenden J. C., Hand D. W. and Perram D. L. (1996) Regeneration of adsorbents using heterogeneous photocatalytic oxidation. J. Env. Eng.ÐASCE. 122(8), 707±713. Mark G., Schumann M. N., Schummann H. P. and Sonntag C. (1990) A chemical actinometer for use in connection with UV treatment in drinking water processing. J. Water S.R.T.ÐAqua. 39(5), 309±313. Martin R. J. and Iwugo K. O. (1982) The eects of pH and suspended solids in the removal of organics from waters and wastewaters by the activated carbon adsorption process. Water Res. 19, 73±82. Mokrini A., Ousse D. and Esplugas S. (1997) Oxidation of aromatic compounds with UV radiation/ozone/ hydrogen peroxide. Wat. Sci. Technol. 35(4), 95±102. Mourand J. T., Crittenden J. C., Hand D. W., Perram D. L. and Notthakun S. (1995) Regeneration of spent adsorbents using homogeneous advanced oxidation. Water Env. Res. 67, 355±363. Nakhla G., Abuzaid N. and Farooq S. (1994) Activated carbon adsorption of phenolics in oxic systems: eect of pH and temperature variations. Water Env. Res. 66(6), 842±850. Scheck C. K. and Frimmel F. H. (1995) Degradation of phenol and salicylic acid by ultraviolet radiation/hydrogen peroxide/oxygen. Wat. Res, 29(10), 2346±2352. Staehelin J. and Hoigne J. (1985) Decomposition of ozone in water in the presence of organic solutes acting as promoters and inhibitors of radical chain reactions. Env. Sci. & Technol. 19, 1206. Tressmer C. H., Vidic R. D. and Uranowski L. J. (1997) Impact of oxygen containing surface functional groups on AC adsorption of phenols. Env. Sci. & Technol. 31(7), 1872±1878.