Environ. Sci. Technol. 1999, 33, 2095-2099
Potassium Persulfate as Oxidant in Pressurized Hot Water JUHANI KRONHOLM* AND MARJA-LIISA RIEKKOLA Laboratory of Analytical Chemistry, Department of Chemistry, P.O. Box 55, University of Helsinki, FIN-00014 Helsinki, Finland
Sub- and supercritical water oxidation are quite new techniques for wastewater treatment. Temperatures must be high enough and reaction times long enough that the organic compounds are completely oxidized into carbon dioxide and water. The critical temperature and pressure of water are 374 °C and 22.1 MPa. The primary aim of the study was to determine whether potassium persulfate is a feasible choice for oxidant in work clearly below the critical temperature of water. Because potassium persulfate is water soluble, it can be added directly to water containing organic compounds. Simple equipment was constructed for oxidation in high-temperature pressurized water, and oxidation of phenol, 2,3-dichlorophenol, and 1-naphthol was carried out in aqueous environment at 75340 °C with potassium persulfate used as oxidant. The pressure was adjusted to 25-45 MPa. The concentration of the organic compounds was 1.0 mM, and that of the oxidant was 1.0-10.0 mM. Study was made of the effects of concentration, temperature, and reaction time on oxidation efficiency. The removal percentages of phenol were good even at 115 °C. Concentration of the oxidant turned out to be a very important parameter. Also temperature and reaction time had an effect on the results. There were some problems arising from the formation of rust and from blockage of the capillaries of the equipment. However, the blockage could be prevented with use of a precipitant collector.
Introduction The theoretical possibilities of hot and pressurized water for oxidation reactions have been known for more than a century, but experimental and practical experiences were not widely reported until the 1960s. The research on sub- and supercritical water oxidation was first focused on temperatures below the critical temperature and only later on the supercritical state. Today, wastewater treatment techniques exploiting the oxidation power of hot pressurized water offer a promising approach for converting organic substances into harmless compounds such as water and carbon dioxide (1). The critical temperature of water is 374.2 °C, and the critical pressure 22.1 MPa. If both or either of these values are smaller, water is in the subcritical state. In the present work, where the temperature range investigated was wide (75-340 °C), the water is simply said to be pressurized and hot. The physicochemical properties of water change in the supercritical region. Above its critical temperature, water is * Corresponding author phone: +358-9-191 40267; fax: +3589-191 40253; e-mail:
[email protected]. 10.1021/es9805977 CCC: $18.00 Published on Web 05/13/1999
1999 American Chemical Society
like a dense gas and behaves like a fluid. Its density and viscosity are reduced, and the Brownian diffusion is increased (2, 3). The dielectric constant and extent of hydrogen bonding are also reduced (4-7), resulting in increased solubility of organic compounds and gases and decreased solubility of electrolytes (8). As the temperature in the subcritical region is raised, the properties of the water approach those in the supercritical region. For treatment purposes, it is not necessary to raise the temperature up to 374 °C or further if complete destruction of the organic compounds can be achieved at temperatures below 374 °C. The main incentive for working below the critical temperature is to save energy. With large-scale systems consuming large amounts of energy, energy costs become of critical importance. In addition, inorganic compounds are precipitated at temperatures higher than the critical temperature of water, so that working below this temperature may prevent the formation of insoluble matter and blockage of the capillaries of the equipment. To ensure the effective oxidation of organic pollutants into water and carbon dioxide, it is usual to add an oxidant to the reaction chamber. Oxygen and hydrogen peroxide have been widely used with excellent results. For example, pulp and paper mill sludges have been successfully purified (9, 10), and wastewaters containing phenol have been effectively treated under supercritical conditions (11-13). Potassium persulfate, which was the oxidant used in this study, is not a common choice but was of interest to us as an effective and exothermic reactant. Besides oxidants, catalysts (for example, transition metal salts such as CuO, ZnO, MnO2, and V2O5) have been used under supercritical conditions to lower the activation energy of the reaction and thus enhance the destruction rate (14-16). Catalysts have also been used at temperatures clearly below the supercritical temperature to destroy phenolic compounds (17-20). The primary aim of our study was to evaluate the feasibility of using potassium persulfate as oxidant at temperatures clearly below the critical temperature of water. Inorganic compounds are a problem in work in supercritical conditions, and at temperatures clearly below the critical temperature of water, it may be possible to prevent the precipitation of inorganic compounds and the blockage of capillaries. A second aim of the study was to construct simple laboratory equipment allowing the destruction of organic compounds reliably and safely at high temperatures and under high pressures. Phenol, 2,3-dichlorophenol, and 1-naphthol were chosen as the organic contaminants because all are common aromatic pollutants and information is available on the destruction efficiencies of closely related compounds in pressurized hot water.
Experimental Section Materials. Phenol (E. Merck, Darmstadt), 2,3-dichlorophenol (Fluka AG, Buchs SG), and 1-naphthol (E. Merck, Darmstadt) were chosen as the model contaminants. Concentration of each compound in water (distilled and deionized) was 1.0 mM. The water containing the contaminants is afterward simply referred to as the wastewater. Potassium persulfate (E. Merck, Darmstad) was added to the wastewater as an oxidant (c ) 1.0-10.0 mM). Two internal standards were used in sample analysis: m-cresol (Fluka AG, Buchs SG) and hexachlorobenzene (Sigma-Aldrich). Dichloromethane (LabScan, Analytical Sciences, HPLC grade) was used as the solvent in extraction. Instrumentation. Figure 1 shows a simplified view of the subcritical water oxidation equipment. All the capillaries (1/ VOL. 33, NO. 12, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Equipment constructed for oxidation in pressurized hot water. The precipitant collector is described in detail in the area bounded with a dashed line. 16 in. o.d.), the reaction chamber (Keystone Scientific extraction chamber, V ) 5.0 mL), and the precipitant collector (V ) 2.0 mL) were made of stainless steel. One high-pressure pump (Perkin-Elmer Series 1) and a heating oven (Carlo Erba Fractovap model G1) were used in this study, an ice-bath was added to cool the hot effluent, and a pressure regulator (needle valve) was installed to adjust the pressure. A Hewlett-Packard model 5890 gas chromatograph and a model 5989A mass spectrometer (EI, 70 eV) were employed to analyze the compounds left in the effluent. The analytical column of the gas chromatograph was a polar Nordion NB 351 (20.0 m, 0.2 mm i.d., 0.2 µm). The precolumn (2.5 m, 0.53 mm i.d.) attached in front of the analytical column was deactivated with DPTMDS (1,3-diphenyl-1,1,3,3-trimethylsilazane). The injection mode was on-column. Procedure. Most of the experiments were carried out with phenol as the organic pollutant, and a few additional experiments were carried out with 2,3-dichlorophenol and 1-naphthol. The wastewater, containing the added oxidant, was pumped into the heated reaction chamber by means of a high-pressure pump. In some arrangements, a precipitant collector was installed before the reaction chamber to prevent blockage of the capillaries. The flow rates of the wastewater feed were adjusted to 1.9-5.4 mL/min. In this text, we prefer the expression reaction time to flow rate because flow rate is strictly related to the high-pressure pump, whereas the reaction time expresses the time the aqueous waste is subjected to certain reaction conditions. Pressure and flow rates fluctuated a little during sample collection (by about 5 MPa on average), and the values given are averages of the minimum and maximum values. However, pressure was kept high enough to maintain water in liquid state. Three effluent samples were collected under each set of conditions (experiments with phenol). Each point in the graphical presentations (Figures 2-4) represents an average removal percentage of phenol. The relative standard deviations of the replicates are noted in Table 1. After the samples were collected at the outlet of the capillary, they were extracted with dichloromethane (4 × 2.0 mL) and finally concentrated to 2.0 mL by gentle nitrogen evaporation. Two internal standards were used: one (m-cresol, added before the extraction) to calculate the removal percentages of the organic pollutants and the other (hexachlorobenzene, added after the extraction) to calculate the extraction efficiencies. The removal percentage entered in the tables is the amount of compound that was destroyed under the conditions in question. This was calculated as follows:
[ ]
std1a X1a removal percentage ) 100 × 1 std1b X1b
where std1a is the peak area of the internal standard (mcresol) added to the reference sample before extraction, std1b 2096
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FIGURE 2. Removal percentages of phenol (c ) 1.0 mM) at four temperatures (115, 195, 275, and 340 °C) and three concentrations of potassium persulfate. Average reaction times were 1.4-2.5 min, and the average pressure was 33-38 MPa (exact values in Table 1).
FIGURE 3. Removal percentages of phenol (c ) 1.0 mM) at four temperatures (75, 115, 250, and 340 °C) and three concentrations of potassium persulfate. Average reaction times were 1.2-1.5 min, and the average pressure was 32-43 MPa (exact values in Table 1).
FIGURE 4. Removal percentages of phenol (c ) 1.0 mM) as a function of potassium persulfate concentration (1.0, 2.0, 5.0, and 10.0 mM) at four temperatures. Average reaction times were 1.2-1.5 min, and the pressure was 32-43 MPa (exact values in Table 1). is the peak area of the internal standard (m-cresol) added to the collected sample before extraction, X1a is the peak area of the analyte in the reference sample, and X1b is the peak area of the analyte in the collected sample. The reference sample was a sample treated at room temperature.
TABLE 1. Oxidation Efficiencies for Phenol (c ) 1.0 mM) at Various Temperatures and Oxidant (Potassium Persulfate) Concentrationsa exp
temp (°C)
oxidant concn (mM)
reaction time (min)
pressure (MPa)
removal (%)
RSD (%)
precipitant collector in use
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
75 75 75 75 115 115 115 115 115 115 115 195 195 195 250 250 250 250 275 275 275 340 340 340 340 340 340 340
1.0 2.0 5.0 10.0 no oxidant 1.0 1.0 2.0 5.0 10.0 10.0 no oxidant 1.0 10.0 1.0 2.0 5.0 10.0 no oxidant 1.0 10.0 no oxidant 1.0 1.0 2.0 5.0 10.0 10.0
1.4 1.3 1.4 1.5 1.4 2.3 1.4 1.2 1.4 2.3 1.5 2.4 2.3 2.3 1.5 1.3 1.5 1.4 2.4 2.4 2.4 2.3 2.2 1.5 1.2 1.5 2.5 1.5
35 32 37 43 33 35 37 35 40 37 40 34 38 35 40 40 38 40 35 33 37 37 33 35 40 38 37 42
0 10.4 3.3 29.1 12.5 23.9 7.3 7.4 99.6 99.7 99.8 8.1 59.6 99.9 41.2 95.5 98.9 99.5 0 62.9 99.3 0 65.4 40.9 88.7 97.0 99.3 97.4
X 141.4 173.2 83.6 86.9 89.7 105.9 87.2 0.1 0.1 0.2 125.9 22.0 0.0 6.6 0.2 0.1 0.1 X 10.1 0.1 X 2.8 9.7 3.4 0.3 0.0 0.3
yes yes yes yes no no yes yes yes no yes no no no yes yes yes yes no no no no no yes yes yes no yes
a Reaction times, pressures, and removal percentages are averages (n ) 3). The reaction volume was 7.0 mL when the precipatant collector was in use and 5.0 mL when it was not. An X means that only two replicates were performed under each set of conditions and that no RSD is included.
FIGURE 5. Some oxidation reactions of phenol, 2,3-dichlorophenol, and 1-naphthol. In practice, inorganic compounds are present as salts or ions in aqueous environment. The reactions in the figure are highly simplified, and no intermediate reaction products are presented.
Results and Discussion Figures 2-4 present the oxidation efficiencies for phenol at different temperatures and different concentrations of oxidant. Table 1 gives the numerical and exact information on which the figures are based. The theoretical ratio of oxidant to phenol needed to convert all the phenol into carbon dioxide and water is 7/4 (see Figure 5). Note: The reactions presented in Figure 5 are theoretical because no intermediate products are included, and only the initial and final products are shown; the main reason for presenting the figure is to indicate the stoichiometry of the reactions and thus the equivalent amounts of oxidant needed for the oxidation process. The ratio 7/4 means that a potassium persulfate concentration of 2.0 mM should be stoichiometrically high enough to oxidize all the phenol (c ) 1.0 mM). A good removal percentage was obtained when the concentration of potassium persulfate was 2.0 mM and the temperature at least 250 °C (see Figures 3 and 4). When the oxidant concentration was increased to 5.0 mM, results were excellent even at temperatures as low as 115 °C. A temperature of 75 °C was clearly too low, and 1.0 mM was too low a
concentration of the oxidant to obtain good results (see Figures 2-4). It was evident that by increasing the concentration of potassium persulfate from 2.0 to 5.0 mM and further up to 10.0 mM the oxidation efficiency was improved. Potassium persulfate is an inorganic salt, soluble in water (5.3 g/100 mL, 20 °C) (21) that begins to react strongly and release oxygen when the temperature is raised to 100 °C. The reaction is strongly exothermic. Possibly not all the oxidant present is available for oxidizing phenol since there will always be small amounts of impurities in the water even though the water is distilled and deionized and all the reagents are of high purity. There may also be a number of side reactions occurring simultaneously before the final products appear, and this could lead to increased oxidant consumption. When the concentration of potassium persulfate was below the stoichiometric requirement (0-1.0 mM), the results were poor even when the temperature was raised to 340 °C. The results demonstrate the importance of the stoichiometric requirement of the oxidant. The temperature, reaction time, and pressure employed are of great importance. The temperature must be high enough to increase the exothermic energy to values able to break down the chemical bonds and to produce activation energies high enough for the desired reactions. In some cases, the removal percentages of phenol were a little lower when the temperature was raised. The real reason for this is unclear, but these observations are statistically nonsignificant. Water has itself some ability to work as an oxidant (22), but it is not clear why the oxidation efficiency decreased with increasing temperature when no oxidant was used (Figure 2). In theory, pressure may influence the results through altering the density of water, so that an increase in pressure (at constant temperature) increases the density and the number of molecules colliding. It has been observed in another study VOL. 33, NO. 12, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Removal Percentages of Phenol, 2,3-Dichlorophenol, and 1-Naphthola exp
compound
temp (°C)
reaction time (min)
removal %
29 30 31 32 33 34
phenol phenol 2,3-dichlorophenol 2,3-dichlorophenol 1-naphthol 1-naphthol
250 340 250 340 250 340
3.3 5.0 5.0 5.0 5.0 5.0
99.5 97.6 98.4 99.6 94.4 96.3
a Concentration of each compound was 1.0 mM, and concentration of the oxidant (potassium persulfate) was 10.0 mM. The pressure was ≈45 MPa. Relative standard deviations are not included because only one effluent sample was collected under each set of conditions.
(23), where results under pressures of 188 and 278 atm were compared, that pressure influences the destruction efficiencies of phenol clearly at 360-440 °C, but the influence was almost negligible at the temperatures used in our study (at temperatures below 340 °C and where water was still in liquid state). This is why we believe that the differences in pressure in our study do not have a remarkable effect on destruction efficiencies. The pressure effects on rate for elementary reaction steps are pressure dependent according to transition state theory. The pressure effects on rate constant are governed by the relationship
[
]
∂ ln kx ∂P
T
)
-∆V+ RT
where ∆V+ is the activation volume and kx is the rate constant. Some additional studies were carried out with 2,3-dichlorophenol and 1-naphthol (see Table 2) to determine whether these compounds could be removed as effectively as phenol. Only one effluent sample was collected under each set of conditions, so relative standard deviations are not included in the table. Concentrations of these compounds were 1.0 mM, while the concentration of potassium persulfate was kept constant at 10.0 mM. The removal percentages of these compounds were good too, although the reaction time was relatively long (3.3-5.0 min) because the precipitant collector was not used and there were problems in increasing the flow rate due to blockage of the capillaries. Phenol, 2,3-dichlorophenol, and naphthol were collected quantitatively; in other words, no compounds were found in the capillaries or in the reaction chamber when they were eluted with dichloromethane after the oxidation process. It can be concluded that the oxidation process and not the loss of these compounds was responsible for the high removal percentages. We conclude that the removal percentages of phenol, 2,3-dichlorophenol, and 1-naphthol are very good when the parameters (oxidant concentration, temperature, and reaction time) are correctly adjusted. Raising the temperature up to 340 °C is not necessary to achieve good results if the concentration of oxidant is 5.0 mM or higher and the reaction time at least 1.5 min. Catalytic oxidation of phenol has been studied on many occasions. Temperatures between 100 and 200 °C have been widely used, and the reaction times required to oxidize phenol effectively have usually ranged between 5 min and 1 h (17, 18, 20, 24). Faster reaction times have been achieved at temperatures lower than 100 °C, but the catalysts were heterogeneous (19). We found no need to employ a separate catalyst because potassium persulfate worked effectively as oxidant. Water is highly corrosive at high temperatures (25), and at 115-340 °C, the samples collected at the outlet of the equipment were dirty (the effluent was reddish brown). Another source of color and also of inorganic matter is that 2098
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potassium persulfate is reduced in the oxidation process, and some potassium and sulfur compounds are released. The color was strongest at temperatures of 195-275 °C and got lighter again at 340 °C. Investigations by others suggest that corrosion in oxidation processes in water at high temperatures may be strongest below the critical temperature (26, 27). In addition to this, we observed that the reddish brown color intensified when the concentration of the oxidant was increased. Corrosion and color formation may be heavily dependent on the oxidant, and our observations do not apply beyond systems in which potassium persulfate is the oxidant. Several investigations have shown that ordinary stainless steel is not the best choice for work with water at high temperatures. Better choices may be Inconel 600, Inconel 625, and Hastelloy C-276 (28, 29). In preliminary investigations that we carried out with Inconel 600, rust formation was clearly less, and the samples were cleaner. Rust formation and the precipitation of inorganic compounds caused some problems when the precipitant collector was not connected. To avoid blockage of the capillaries, we occasionally adjusted the pressure regulator so that the flow rate was momentarily increased and the pressure decreased. After this, the pressure regulator was readjusted to return to the original pressure. In some of the experiments the precipitant collector was installed in front of the reaction chamber (see Table 1) allowing the inorganic matter to be collected at the bottom of the collector. This arrangement prevented the capillaries from getting blocked, and there was no need for pressure adjustments. We would conclude that, even when the reaction temperature is clearly below the critical temperature of water, a precipitant collector needs to be used to ensure constant flow. The diameter of our capillaries was small (1/16 in. o.d.) and use of capillaries of larger diameter, 1/8 in. o.d., for example, might have reduced the problems with inorganic compounds. Outside of the problems noted, the organic pollutants were effectively destroyed and no hazards arose. Despite rapid drops in pressure, there were no leaks in the joints of the capillaries. Further investigations will be made with materials more resistant to corrosion than stainless steel and with a two- or three-pump system delivering the organic matter and oxidant to the reaction chamber separately. Also, capillaries with larger diameters will be used to prevent blockage. Further investigations with potassium persulfate as oxidant should be made to determine the effects of temperatures between 340 and 400 °C, an increase in the potassium persulfate concentration beyond 10.0 mM, and the use of a wider range of reaction times. It would also be worth trying to improve the oxidation efficiency by adding potassium persulfate in hydrogen peroxide.
Acknowledgments Financial support for this study was provided by the Maj & Tor Nessling Foundation and the Jenny & Antti Wihuri Foundation.
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(11) Gopalan, S.; Savage. P. E. Innovations in Supercritical Fluids: Science and Technology; ACS Symposium Series 608; American Chemical Society: Washington, DC, 1995; p 338. (12) Shanableh, A.; Gloyna, E. F. Water Sci. Technol. 1991, 23, 389. (13) Koo, M.; Lee, W. K.; Lee, C. H. Chem. Eng. Sci. 1997, 52, 1201. (14) Ding, Z. Y.; Aki, S. N. V. K.; Abraham, M. A. Environ. Sci. Technol. 1995, 29, 2748. (15) Krajnc, M.; Levec, J. Appl. Catal. B: Environ. 1994, 3, L101. (16) Ding, Z. Y.; Aki, S. N. V. K. L.; Abraham, M. A. Innovations in Supercritical Fluids: Science and Technology; ACS Symposium Series 608; American Chemical Society: Washington, DC, 1995; p 232. (17) Pintar, A.; Levec, J. J. Catal. 1992, 135, 345. (18) Fortuny, A.; Ferrer, C.; Bengoa, C.; Font, J.; Fabregat, A. Catal. Today 1995, 24, 79. (19) Atwater, J. E.; Akse, J. R.; McKinnis, J. A.; Thompson, J. O. Chemosphere 1997, 34, 203. (20) Maugans, C. B.; Akgerman, A. Water Res. 1997, 31, 3116. (21) Karama¨ki, E. M. Epa¨orgaaniset kemikaalit; K. J. Gummerus Oy: Jyva¨skyla¨, 1983; p 320. (22) Yang, H. H.; Eckert, C. A. Ind. Eng. Chem. Res. 1988, 27, 2009.
(23) Thornton, T. D.; Savage, P. E. AIChE 1992, 38, 321. (24) Lin, S. H.; Wu, Y. F., Environ. Technol. 1996, 17, 175. (25) Mitton, D. B.; Orzalli, J. C.; Latanision, R. M. Innovations in Supercritical Fluids: Science and Technology; ACS Symposium Series 608; American Chemical Society: Washington, DC, 1995; p 328. (26) Takahashi, Y.; Wydeven, T.; Koo, C. Adv. Space Res. 1989, 9 (8), 99. (27) Gloyna, E. F.; Li, L. Waste Manage. 1993, 13, 379. (28) Downey. K. W.; Snow, R. H.; Hazlebeck, D. A.; Roberts, A. J. Innovations in Supercritical Fluids: Science and Technology; ACS Symposium Series 608; American Chemical Society: Washington, DC, 1995; p 313. (29) Alekseev, A. B.; Averin, S. A.; Geferova, M. N.; Kondrat’ev, V. P.; Shikhalev, V. S. J. Nuclear Mater. 1996, 233, 1367.
Received for review June 12, 1998. Revised manuscript received January 22, 1999. Accepted March 26, 1999. ES9805977
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