Oxidation Efficiencies of Potassium Persulfate and ... - ACS Publications

Jun 10, 2000 - Juhani Kronholm, Pertti Vastamäki, Riikka Räsänen, Annukka Ahonen, Kari Hartonen, and Marja-Liisa Riekkola. Industrial & Engineering...
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Ind. Eng. Chem. Res. 2000, 39, 2207-2213

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Oxidation Efficiencies of Potassium Persulfate and Hydrogen Peroxide in Pressurized Hot Water with and without Preheating Juhani Kronholm,* Pentti Jyske, and Marja-Liisa Riekkola Laboratory of Analytical Chemistry, Department of Chemistry, PO Box 55, University of Helsinki, Helsinki FIN-00014, Finland

Potassium persulfate was compared with hydrogen peroxide as oxidant in pressurized hot water. Oxidant (in excess) and organic model pollutants (phenol, 2,3-dichlorophenol, and m-cresol, c ) 0.3-0.5 mM each) were introduced to the reaction capillary with and without preheating. Reaction time was 4-57 s, temperature 80-400 °C, and pressure 220-310 bar. Without preheating, potassium persulfate was clearly more efficient than hydrogen peroxide in oxidizing the model compounds: for example, with potassium persulfate, removal percentages close to 100 were obtained in 55 s at 110 °C, whereas with hydrogen peroxide the required temperature to obtain similar results in any reaction time was nearly 300 °C. Total organic carbon was removed more effectively by potassium persulfate under mild conditions. Overall, oxidation efficiencies were decreased when capillaries were preheated, especially with potassium persulfate as oxidant. Corrosion, measured as Ni concentration of the effluent, was more severe with potassium persulfate than hydrogen peroxide. Introduction Wastewater purification in pressurized hot water is a well-known technique. Under optimized conditions, organic pollutants such as phenol, substituted phenols, PCBs, and dioxins are efficiently oxidized and destroyed.1-4 The main reaction products are water and carbon dioxide. The critical temperature of water is 374.2 °C, and the critical pressure is 221 bar. When water is employed at higher temperatures and pressures, the technique is referred to as supercritical water oxidation. The physicochemical properties of water change in the supercritical region: density, viscosity, dielectric constant, and the extent of hydrogen bonding are reduced and Brownian diffusion is increased, resulting in increased solubility of organic compounds and gases and decreased solubility of electrolytes.5-14 Below the critical temperature or pressure, water is said to be in the subcritical state. Oxygen, compressed air, and hydrogen peroxide are the most widely used and investigated oxidants in pressurized hot water. Catalysts are sometimes introduced to lower the activation energies of the oxidation reactions and enhance reaction rates. For example, with use of Na2SO3 (3 g/L), Cu2+ (765 mg/L), and 0.25 MPa oxygen partial pressure, phenol (100 mg/L) is completely destroyed in 15-18 min at 110 °C.15 With metal salts (containing Cr(VI), Co, Zn, Pb, Hg, and Mn(VII)) and hydrogen peroxide as oxidant, Brett et al.16 obtained 74.2% (mean result) COD removal from sewage sludge at 170 °C (mean value) and 15 min. In a different study, hydrogen peroxide was used as oxidant together with sodium peroxodisulfite and MnO2.17 Yet another good catalyst is FeSO4.18 Various free radicals, such as SO3•-, SO4•-, SO5•-, O2•-, OH•-, HSO3•-, HSO4•-, and HSO5•-, have been shown to take part in the oxidation process.19-21 In our preliminary study, potassium persulfate was used as single oxidant and was found to be effective at >115 °C.22 Nevertheless, potassium persulfate is * Corresponding author. Phone +358-9-191 40270; fax +3589-191 40253; e-mail [email protected].

rarely used as oxidant in wastewater purification in pressurized hot water: it is an inorganic salt and starts reacting exothermally only at about 100 °C.23,24 When heated in aqueous environment, potassium persulfate is decomposed and oxygen is released:

2K2S2O8 + 2H2O f 4KHSO4 + O2

(1)

Hydrogen peroxide is a widely used oxidant in pressurized hot water. When heated in water, it decomposes as follows:25,26

H2O2 + (X) f 2OH + (X)

(2)

OH + H2O2 f H2O + HO2

(3)

OH + HO2 f H2O + O2

(4)

HO2 + HO2 f H2O2 + O2

(5)

where the collision partner, X, is water. Radicals such as HO2•, and especially OH•, have been found to be highly reactive in the conversion of phenol to phenoxy radical.27 The free-radical chemistry of phenol is essentially the chemistry of the phenoxy radical. It has been suggested that H2O2 is important only at the onset of the reaction. Once the reacting medium becomes homogeneous, oxidation by O2 dominates.28 Preheating of the oxidant and organic waste stream has been studied by Li et al.29 Relative to fast preheating, the oxidation efficiency was decreased when oxygen and the phenolic waste stream were preheated slowly before introduction to the reaction zone. This was explained in terms of the thermal pyrolysis of organic compounds, which becomes more dominant in slow preheating. In the absence of oxidant in the preheater, the free-radical organic pyrolysis intermediates are subject to coking, and polymerization and may form products less easy to oxidize in the reaction zone. Our primary objective in this work was to evaluate the potential of potassium persulfate as an alternative and new oxidant in wastewater treatment at temperatures

10.1021/ie990755i CCC: $19.00 © 2000 American Chemical Society Published on Web 06/10/2000

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Figure 1. Instrumentation with nonpreheated capillaries. Temperature was measured at points 1 and 2, and the average was recorded. A indicates the reaction capillary, and B and C indicate the preheating capillaries. The instrumentation with preheated capillaries was identical except that capillaries B and C were inside the oven.

clearly below the critical temperature of water. For comparison, hydrogen peroxide as oxidant and temperatures above the critical temperature of water were studied as well. In view of the encouraging results of Li et al. and the known decomposition of hydrogen peroxide and potassium persulfate during preheating, we also investigated the effects of preheating of the oxidant and organic model compounds. No attempt was made to analyze reaction intermediates or kinetics. Experimental Section Phenol (E. Merck, Darmstadt, Germany, g99,5%), 2,3-dichlorophenol (Fluka AG, Buchs SG, Switzerland, g97%), and m-cresol (Fluka AG, Buchs SG, Switzerland, >98%) were the organic model contaminants (c ) 0.3 or 0.5 mM each, in distilled deionized water). Either potassium persulfate (E. Merck, Darmstadt, Germany, 99%, c ) 16.1-26.9 mM in distilled deionized water) or hydrogen peroxide (Riedel-deHae¨n, Seelze, Germany, 30% aqueous solution, c ) 129-191 mM in distilled deionized water) was used as an oxidant. Two internal standards were used in sample analysis: pentachlorobenzene (Sigma-Aldrich, Steinhem, Germany, 98%) and hexachlorobenzene (Sigma-Aldrich, Steinhem, Germany, 99%). Dichloromethane (Lab-Scan, Analytical Sciences, Dublin, Ireland, HPLC grade) was used as solvent in the extraction of organic pollutants from the effluent. Figure 1 shows a simplified view of the equipment. When the capillaries delivering oxidant and organic model pollutants were preheated, they were situated inside the oven (Carlo Erba Fractovap model G1). All the capillaries (1/8 in. o.d.) in the oven were made of Inconel 600 (nickel alloy) because Inconel is more resistant to corrosion than ordinary stainless steel.30,31 The volume of the reaction capillary was 1.3 mL (preheating not used) or 2.0 mL (with preheating). The volumes of the preheating capillaries were 1.2 mL (for the organic model compounds) and 1.0 mL (for the oxidant). The capillary (1/16 in. o.d.) leading from the precipitant collector to the pressure regulator was made of ordinary stainless steel, and the capillary (1/16 in. o.d.) leading from the pressure regulator to the sample collection was made of Teflon. Temperature inside the oven was measured at two points, and the average was reported. The temperature range was 80-400 °C. Two high-pressure pumps (an LKB-2150 HPLC pump for oxidant and a Jasco PU-980 HPLC pump for organic model contaminants) were employed to deliver aqueous solutions to the reaction zone. Reaction time (4-57 s) was altered by changing the pumping speed of the high-

pressure pumps, and the reaction time was measured by dividing the volume of the reaction capillary by the volumetric flow rate. Water expands when heated, and the density decreases. This leads to decreased reaction times in the reaction capillary though the pumping speed of the influent is constant. This was taken into account when reaction times were calculated. A Software program NIST/ASME Steam Properties Standard Reference Database 10 Version 2.01 was used to calculate the density of water at various temperatures and pressures. For example, at 80 °C and 220 bar, the density of water is 981 kg/m3 but at 340 °C and 220 bar only 645 kg/m3. If the reaction time is 60 s at room temperature (P ) 220 bar), it is 58 s at 80 °C (P ) 220 bar) and only 38 s at 340 °C (P ) 220 bar). An ice bath was used to cool the stream before it entered the precipitant collector (added to collect the insoluble particles in the effluent and prevent blockage of the pressure regulator) and stainless steel pressure regulator (Jasco). The preheating time for the oxidant was the same as the reaction time and for the organic model pollutants 1.2 times bigger than the reaction time. Percentage values were obtained from the ratios of the volumes of the preheating and reaction capillaries. (The flow velocity in the preheating capillary is half of that in the reaction capillary.) The pressure range in the experiments was 220-310 bar. Pressure has no essential effect (reaction time kept constant) on the oxidation efficiency when water is in the liquid state and prevented from vaporizing by adjustment of the pressure to a high enough level.32 For GC-MS analysis, three effluent samples (10 mL each) were collected under each set of conditions. Reference samples were collected at 25 °C. The samples were extracted with dichloromethane (4 × 2,5 mL) and finally concentrated to 1.5 mL by gentle nitrogen evaporation using a Pierce Reacti-Therm heating module (T ) 30 °C). A Hewlett-Packard model 5890 gas chromatograph and a model 5989A mass spectrometer were employed to analyze the compounds left in the effluent. The software used in the computer connected to the GC-MS was a Hewlett-Packard ChemStation B.02.05. All MS analyses were carried out in SCAN mode with electron impact ionization (EI, 70 V). The analytical column of the gas chromatograph was a 20.0 m NB 351 (HNU Nordion, Espoo, Finland) with 0.2 mm i.d. and 0.2 µm film thickness or a 20.0 m BGB-5 (BGB Analytik AG, Rothenfluh, Switzerland) with 0.2 mm i.d. and 0.15 µm film thickness. A retention gap (1.0-2.0 m, 0.25 mm i.d.), deactivated with DPTMDS (1,2diphenyl-1,1,3,3-tetramethyldisilazane), was used in front of the analytical columns. Samples were injected in the splitless mode (Vinj ) 2.0 µL). For total organic carbon analysis (TOC) three effluent samples (10 mL each) were collected under each set of conditions, and a Shimadzu TOC 5000 analyzer was employed to analyze total organic carbon left in the effluent. TOC content was measured in the samples at various reaction times and three temperatures (150, 250, and 390 °C). For corrosion and pH measurements, one effluent sample was collected under each set of conditions. Corrosion was measured as nickel concentration in the effluent with a Perkin-Elmer 272 atomic absorption spectrophotometer (AAS). The calibration curve for Ni was prepared with nickel(II) sulfate (E. Merck, Darmstadt, Germany, >99%). The wavelength used in the

Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2209 Table 1. Removal Percentages of the Model Pollutants (cinit ) 0.5 mM Each) under Various Conditions with Capillaries Not Preheated and Potassium Persulfate (cinit ) 26.9 mM) as Oxidanta no.

t (s)

T (°C)

P (bar)

removal % phenol

RSD (%)

removal % m-cresol

RSD (%)

removal % 2,3-dichlorophenol

RSD (%)

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 28 29 31 32 33 34 35

6 6 7 8 9 9 9 9 9 9 9 12 16 18 19 19 19 19 24 32 36 37 38 38 38 38 46 53 54 55 55 55 55

359 386 401 255 123 153 158 168 178 198 399 356 253 152 110 121 130 140 356 254 346 152 102 110 122 130 248 148 119 80 90 99 110

260 250 260 240 220 240 250 250 250 240 240 230 230 240 240 240 240 240 240 230 240 220 240 250 240 250 230 230 230 220 230 240 250

>99.9 99.4 98.7 >99.9 5.8 23.3 25.7 74.1 >99.9 >99.9 97.7 99.3 >99.9 99.7 8.3 46.4 83.7 >99.9 98.5 99.7 97.0 >99.9 44.5 60.8 98.1 >99.9 99.8 >99.9 >99.9 18.8 53.6 71.2 >99.9

ND 0.1 0.1 ND AV 26.9 36.8 2.8 ND ND 0.1 0.2 ND 0.1 56.4 19.6 AV ND 0.1 0.1 0.7 ND AV 22.7 0.7 ND 0.0 ND ND 32.3 23.4 9.5 ND

>99.9 >99.9 >99.9 >99.9 6.1 22.4 33.6 78.4 >99.9 >99.9 >99.9 >99.9 >99.9 >99.9 9.9 45.1 86.6 >99.9 >99.9 >99.9 >99.9 >99.9 46.3 58.6 98.9 >99.9 >99.9 >99.9 >99.9 18.7 53.1 72.9 >99.9

ND ND ND ND AV 52.5 35.9 2.4 ND ND ND ND ND ND 53.5 20.8 AV ND ND ND ND ND AV 21.5 0.6 ND ND ND ND 21.8 18.6 9.2 ND

>99.8 >99.8 99.2 >99.8 2.4 16.3 30.7 56.0 >99.8 >99.8 99.3 99.7 >99.8 >99.8 4.6 29.9 65.7 >99.8 98.6 99.7 99.7 >99.8 11.0 33.6 97.4 >99.8 >99.8 >99.8 >99.8 8.0 41.3 60.1 >99.8

ND ND 0.1 ND AV 38.9 47.2 6.4 ND ND 0.2 0.1 ND ND 81.9 36.3 AV ND 0.2 0.1 0.0 ND AV 60.4 1.0 ND ND ND ND 107 7.6 15.0 ND

a AV means that only two replicates were performed under each set of conditions and that no RSD is included. ND means that the compound was not detected in analysis.

Figure 2. Removal percentages of phenol (cinit ) 0.5 mM) at various temperatures with potassium persulfate as oxidant (cinit ) 26.9 mM) and capillaries not preheated. For complete set of values see Table 1.

Figure 3. Removal percentages of m-cresol (cinit ) 0.5 mM) at various temperatures with potassium persulfate as oxidant (cinit ) 26.9 mM) and capillaries not preheated. For complete set of values see Table 1.

measurements was 232.0 nm. Chromium concentration of the effluent was measured by AAS at wavelength 357.9 nm. The calibration curve for Cr was obtained with potassium dichromate (E. Merck, Darmstadt, Germany, analytical grade). Acidity of the effluent was measured with a Jenway 3030 pH meter. Results and Discussion Figures 2-6 show the removal percentages of the three organic model pollutants at various reaction times and temperatures. Each point represents an average removal percentage (n ) 3) of the compound. All the results are not presented graphically; the total removal percentages for all experiments, and the relative standard deviations for the removal percentages, are presented in Tables 1-4. The limits of detection for the model pollutants were 0.56 ng for phenol, 0.65 ng for m-cresol, and 1.65 ng for 2,3-dichlorophenol. If the compound was not detected in the analysis, >99.9% (for

Figure 4. Removal percentages of 2,3-dichlorophenol (cinit ) 0.5 mM) at various temperatures with potassium persulfate as oxidant (cinit ) 26.9 mM) and capillaries not preheated. For complete set of values see Table 1.

phenol and m-cresol) or >99.8% (for 2,3-dichlorophenol) was assumed for the removal percentage. The concentration of the oxidant was assumed to be stoichiometrically sufficient: both oxidants were used

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Table 2. Removal Percentages of the Model Pollutants (cinit ) 0.5 mM Each) under Various Conditions with Capillaries Not Preheated and Hydrogen Peroxide (cinit ) 191 mM) as Oxidanta no.

t (s)

T (°C)

P (bar)

removal % phenol

RSD (%)

removal % m-cresol

RSD (%)

removal % 2,3-dichlorophenol

RSD (%)

1 2 3 4 5 6 7 8 9 10 11

5 6 7 8 8 13 16 36 42 46 53

375 348 295 248 400 348 248 345 298 249 149

260 240 250 230 240 250 250 240 240 240 240

>99.9 >99.9 99.3 27.5 >99.9 99.9 87.2 >99.9 99.9 85.7 21

ND ND 0.1 55.6 ND 0.0 1.9 ND 0.0 1.9 116

>99.9 99.7 99.9 36.5 >99.9 97.4 92.2 >99.9 >99.9 94.2 26.5

ND 0.3 0.0 33.4 ND 1.3 1.1 ND ND 0.7 88.8

>99.8 >99.8 95.6 11.7 >99.8 >99.8 38.8 >99.8 99.6 67.6 25.0

ND ND 0.9 122 ND ND 13.2 ND 0.3 5.2 77.9

a

ND means that the compound was not detected in analysis.

Table 3. Removal Percentages of the Model Pollutants (cinit ) 0.3 mM Each) under Various Conditions with Preheated Capillaries and Potassium Persulfate (cinit ) 16.1 mM) as Oxidanta no.

t (s)

T (°C)

P (bar)

removal % phenol

RSD (%)

removal % m-cresol

RSD (%)

removal % 2,3-dichlorophenol

RSD (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

10 12 13 14 14 19 22 25 28 29 38 48 55 57

340 247 388 99 148 347 396 248 150 99 347 248 149 99

220 230 280 230 230 290 290 260 250 250 290 290 310 300

24.7 24.8 40.8 87.2 >99.9 50.5 6.4 27.8 >99.9 87.3 81.8 6.6 7.0 >99.9

37.6 13.8 3.6 1.0 ND 12.7 AV 16.9 ND 4.8 AV 14.0 AV ND

52.6 25.0 50.2 86.1 >99.9 78.8 31.9 21.3 >99.9 82.4 95.4 20.1 0.1 >99.9

11.1 20.6 2.1 1.2 ND 1.6 35.0 43.1 ND 9.3 AV 9.8 AV ND

9.0 13.1 36.7 58.3 >99.8 47.6 2.4 18.5 >99.8 65.2 70.9 0.1 0.1 99.6

AV 18.6 2.2 3.4 ND 3.1 636 57.6 ND 7.9 AV AV AV 0.5

a The preheating time for the oxidant was the same as the reaction time and for the organic model pollutants 1.2 times bigger than the reaction time. AV means that only two replicates were performed under each set of conditions and that no RSD is included. ND means that the compound was not detected in analysis.

Figure 5. Removal percentages of the model pollutants with hydrogen peroxide as oxidant at 248-249 °C. With preheated capillaries the initial concentrations of the model pollutants were 0.3 mM each, and that of the oxidant was 129 mM. With capillaries not preheated the initial concentrations of the model pollutants were 0.5 mM each, and that of the oxidant was 191 mm. For a complete set of values see Tables 2 and 4.

in 10 times excess compared to the amount of organic compounds. The theoretical model oxidation reactions for potassium persulfate as oxidant were presented in our preliminary study.22 When the capillaries were not preheated, the oxidation efficiency, for both potassium persulfate and hydrogen peroxide, increased with temperature and reaction time. Potassium persulfate oxidized the organic model compounds considerably more effectively at lower temperatures than did hydrogen peroxide. For example, with potassium persulfate as oxidant (Figures 2-4, Table 1), oxidation of almost 100% was achieved at 110-

Figure 6. Removal percentages of the model pollutants (cinit ) 0.3 mM each) with potassium persulfate (cinit ) 16.1 mM) as oxidant and preheated capillaries. With 12 s reaction time the reaction temperature for the model pollutants was 247 °C in spite of 248 °C. For a complete set of values see Table 3.

178 °C with reaction times of 55 to 9 s, whereas with hydrogen peroxide as oxidant, the oxidation efficiency was clearly below 100% at 248 °C with reaction times of 46 to 8 s (Figure 5, Table 2). At short reaction times, the required temperatures to achieve very good oxidation efficiencies were clearly lower with potassium persulfate than with hydrogen peroxide (Tables 1 and 2). With potassium persulfate as oxidant, removal percentages of >99.8-99.9% were achieved for all three model pollutants after 9 s at 178 °C, while with hydrogen peroxide as oxidant, similar results were obtained at short reaction times only at ∼300 °C. Under almost all conditions, 2,3-dichlorophenol was the most

Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2211 Table 4. Removal Percentages of the Model Pollutants (cinit ) 0.3 mM Each) under Various Conditions with Preheated Capillaries and Hydrogen Peroxide (cinit ) 129 mM) as Oxidanta no.

t (s)

T (°C)

P (bar)

removal % phenol

RSD (%)

removal % m-cresol

RSD (%)

removal % 2,3-dichlorophenol

RSD (%)

1 2 3 4 5 6 7 8

9 10 12 12 19 24 37 48

395 340 248 390 348 249 348 249

240 290 290 280 290 220 240 250

82.7 18.2 0.1 83.5 34.0 14.5 >99.9 37.1

10.1 98.9 AV 7.4 19.6 26.2 ND AV

91.3 50.2 0.1 95.2 64.8 19.1 >99.9 43.4

5.8 23.6 AV 2.2 10.5 35.1 ND AV

45.9 0.0 0.1 57.2 23.7 3.6 >99.8 10.8

10.1 AV AV 13.1 54.9 229 ND AV

a The preheating time for the oxidant was the same as the reaction time and for the organic model pollutants 1.2 times bigger than the reaction time. AV means that only two replicates were performed under each set of conditions and that no RSD is included. ND means that the compound was not detected in analysis.

stable compound, and its removal percentages were the lowest. Usually the removal percentages for m-cresol were the best. Only trace amounts of side oxidation products were found, and no attempt was made to analyze them. The preheated capillaries were situated in the same oven as the reaction capillary, so that their temperatures were the same as that of the reaction capillary. Oxidation efficiencies were lower overall with the preheated than the unheated capillaries, for both potassium persulfate and hydrogen peroxide. With potassium persulfate as oxidant and temperature of 248 °C, oxidation efficiencies did not improve with reaction time (Figure 6, Table 3). We suggest that potassium persulfate is decomposed more effectively in the preheating capillary at higher temperatures (see eq 1), so that oxidation with released oxygen becomes dominant and efficiency is decreased. Clearly, potassium persulfate is a more effective oxidant than oxygen and needs to be decomposed not in the preheating capillary but in the reaction mixture, in the presence of the organic compounds. Nevertheless, moderate preheating had the advantage of reducing energy consumption in the reaction capillary: with potassium persulfate as oxidant at 99 °C, good oxidation efficiencies were obtained for the model pollutants, and the oxidation efficiency was increased with the reaction time (Figure 6, Table 3). At this temperature, the removal percentages for the model pollutants were better than in the system where capillaries were not preheated. This result can be explained by the temperature being low enough so that potassium persulfate was not fully decomposed in the preheating capillary; thus, the energy required to decompose potassium persulfate in the reaction capillary and oxidize the organic model pollutants is lower than in the system where capillaries are not preheated. The optimum preheating temperature depends on the preheating time. It can be estimated, for example, that about 100 °C is a suitable preheating temperature for the whole range of preheating and reaction times studied, and about 150 °C is a suitable temperature up to at least 28 s reaction time and preheating time for the oxidant. The oxidation efficiency drops nearly to zero at 149 °C and 55 s (preheating time for the oxidant), at which point the oxidant is evidently almost totally decomposed. (Table 3). At higher temperatures, with potassium persulfate as oxidant, the oxidation efficiencies of the model pollutants are not good at any preheating time. Oxidation efficiencies overall also decreased with hydrogen peroxide as oxidant when the capillaries were preheated (Figure 5, Table 4). Hydrogen peroxide is decomposed in the preheating capillary, and again oxygen becomes dominant in the oxidation process (see

Figure 7. Effect of the oxidation of model pollutants (cinit ) 0.5 mM each) on TOC level at different temperatures and reaction times with potassium persulfate (cinit ) 26.9 mM) as oxidant, pressure 220-260 bar, capillaries not preheated. Relative standard deviations are marked with error bars. Only two replicates were performed at “a” and “b”, so only mean values were calculated.

Figure 8. Effect of the oxidation of model pollutants (cinit ) 0.5 mM each) on TOC level at different temperatures and reaction times with hydrogen peroxide (cinit ) 191 mM) as oxidant, pressure 240-250 bar, capillaries not preheated. Relative standard deviations are marked with error bars. Only two replicates were performed at “a” and “b”, so only mean values were calculated.

eqs 2-5). For example, over the whole reaction time range oxidation efficiency was clearly lower with capillaries preheated at 248-249 °C than with the unheated capillaries. The drop in oxidation efficiency of the model pollutants was not as great with hydrogen peroxide as oxidant as with potassium persulfate. The total organic carbon (TOC) of the water was measured in experiments with capillaries that were not preheated in order to compare the differences in the oxidation results with potassium persulfate and hydrogen peroxide as oxidants. TOC content of the water was reduced with both oxidants (Figures 7 and 8). Each bar in the figures represents an average TOC removal percentage (n ) 3). Relative standard deviations are marked as error bars. Good removal efficiencies were achieved at clearly lower temperatures with potassium persulfate as oxidant. For example, with potassium persulfate as oxidant (Figure 7) TOC was removed with >94% efficiency at 250 °C and 16 s reaction time, whereas with hydrogen peroxide (Figure 8) similar results were obtained only at 390 °C and 13 s reaction time. The TOC results, together with those obtained by

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Table 5. Effect of the Oxidation of the Model Pollutants (cinit ) 0.5 mM Each) on Corrosion (Ni Concentration) and pH; Potassium Persulfate (cinit ) 26.9 mM) or Hydrogen Peroxide (cinit ) 191 mM) as Oxidant, Capillaries Not Preheated K2S2O8 as Oxidant

H2O2 as Oxidant

T (°C)

t (s)

P (bar)

Ni (mg/L)

pH

T (°C)

t (s)

P (bar)

Ni (mg/L)

pH

80 80 100 100 120 120 150 150 180 180 200 200 250 250 300 300 350 350 390

20 39 19 38 19 38 18 37 18 36 17 35 16 33 15 30 13 25 9

280 220 240 230 250 240 220 250 220 250 220 290 240 270 240 270 270 250 250

1.9 4.2 2.8 5.0 3.5 7.7 5.3 6.6 5.0 10.4 4.4 10.7 5.7 18.8 9.0 32.5 11.7 16.4 15.3

3.8 3.5 3.6 3.1 3.4 2.4 3.2 2.1 2.6 1.8 2.2 1.9 1.8 1.9 1.9 1.9 1.9 1.9 1.9

80 80 100 100 120 120 150 150 180 180 200 200 250 250 300 300 350 350 390

19 39 19 38 19 38 18 37 18 36 17 35 16 33 15 29 12 25 11

220 240 240 240 240 260 220 260 230 260 230 250 230 250 250 250 250 250 260

0.1 0.1 0.1 0.2 0.2 0.7 0.2 2.9 0.4 8.2 0.4 7.9 1.4 5.9 1.5 5.9 1.5 7.3 5.5

6.9 6.6 6.8 6.2 6.6 5.1 6.3 4.7 5.6 4.5 5.2 4.5 4.4 4.4 4.2 4.1 4.0 3.9 3.8

GC-MS, indicate that, under optimized conditions, only minor amounts of side products were formed; i.e., the oxidation was almost complete. Nickel content of the effluent was taken as the main measure of corrosion because the capillary (Inconel 600) used inside the heated oven was a nickel alloy (Ni content >72%, Cr ∼15.5). The nickel contents measured in the effluent are presented in Table 5. In addition, small amounts of chromium were measured in the effluent. Corrosion tests were performed with capillaries that were not preheated. With potassium persulfate as oxidant, the amount of nickel in the effluent increased with temperature but peaked at 300 °C. The location of the highest corrosion rate below the critical point of water may be explained by the large amount of dissociated ions in solution.33,34 Surprisingly, with hydrogen peroxide as oxidant, the nickel concentration was maximum at 180 °C, but overall the concentrations were clearly lower than with potassium persulfate. The experiments with hydrogen peroxide were performed after the experiments with potassium persulfate as oxidant, but between the two sets of experiments the capillaries were eluted with methanol and deionized water. We think that most of the nickel in the effluent is due to the corrosive effect of the oxidant, and corrosion hysteresis is for the most part irrelevant. The pH of the effluent was measured in capillaries that were not preheated (Table 5). With potassium persulfate as oxidant, the pH maximum of the effluent was 3.9 (at room temperature) and with hydrogen peroxide 6.9. The lowest pH value with potassium persulfate (1.8) was clearly lower than that with hydrogen peroxide (3.8). With both oxidants the nickel content was higher at lower pH. The lower pH values obtained with potassium persulfate than with hydrogen peroxide as oxidant may thus explain the higher nickel content in the effluent where potassium persulfate was oxidant. At pH values 1.8-2.0, KHSO4 occurs mainly as K+ and HSO4- (pKa ) 1.96) ions.35 This means that, theoretically, HSO4- ions may be present and enhance the oxidation process when potassium persulfate is used as oxidant. To summarize, potassium persulfate was very efficient in oxidizing phenol, 2,3-dichlorophenol, and mcresol when capillaries were not preheated. The com-

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Received for review October 18, 1999 Revised manuscript received March 21, 2000 Accepted April 24, 2000 IE990755I