Environ. Sci. Technol. 2003, 37, 1457-1462
Thiocyanate/Phenol Wet Oxidation Interactions JESU Ä S V I C E N T E A N D M A R I O D IÄ A Z * Departamento de Ingenierı´a Quı´mica y Tecnologı´a del Medio Ambiente, Universidad de Oviedo, E 33071 Oviedo, Spain
Simultaneous phenol and thiocyanate wet oxidation has been studied in a 1-L semi-batch reactor using oxygen as oxidant at 100 atm and 200 °C in absence of any promoting substance. Phenol initial concentration has been kept in 1000 ppm while thiocyanate initial concentration has ranged from 50 to 200 ppm. Reaction kinetics of the process have been analyzed under a kinetically controlled regime paying special attention to the key role of pH and to the interactions between both pollutants. When individually oxidized under alkaline pH conditions, phenol wet oxidation is prejudiced with respect to the conventional process with no base addition. The presence of thiocyanate in the reaction media also inhibits phenol oxidation, and a linear dependence is established between thiocyanate initial concentration and phenol reaction rate. In the case of thiocyanate, its degradation rate is significantly enhanced when simultaneously oxidized together with phenol. A kinetic model for wet oxidation of each pollutant is proposed, with kPh,SCN 0.26 ) 2.33 × 10-7 and kSCN,Ph 3.44 ) 1.21 × 106 (mol/L)1-nT s-1 being the values of the kinetic constants obtained for phenol and thiocyanate, respectively.
Introduction Wet oxidation is a general technique of relevant importance for wastewater treatment that is particularly useful for hazardous and toxic streams, which present concentrations of pollutants too high to be biotreated and too low to be incinerated. It involves the liquid-phase oxidation of organics and oxidizable inorganic components at elevated temperatures (125-320 °C) and pressures (5-200 atm) employing a gaseous source of oxygen. The high temperatures are needed to guarantee that the oxidation takes place at an adequate rate, while the high pressure is necessary to favor high oxygen concentration and to accelerate the oxidation rate. The degree of oxidation is a function of temperature, oxygen partial pressure, residence time, and oxidizability of the pollutants. This technique has been demonstrated for oxidizing organic compounds to partially oxidized organics and CO2 plus other innocuous end products. Biological treatment is an adequate degradation technique for most cases, but when the wastewater contains important amounts of non-easily biodegradable and/or toxic substances, its effectiveness drops. Coke-oven wastewater is a good example of this kind of effluent, containing important concentrations of phenol and thiocyanate as the main pollutants that need to be eliminated by an adequate treatment before the biological one. Phenols are mainly of coal tar origin and hence present in the effluent from coke * Corresponding author telephone: +34-98-5103439; fax: +3498-5103434; e-mail:
[email protected]. 10.1021/es0201045 CCC: $25.00 Published on Web 03/06/2003
2003 American Chemical Society
ovens, blast furnaces, and shale oil processing, but they also are present in the effluents from the chemical process industries that are either manufacturing or using them. The importance of phenol in water pollution stems from its extreme toxicity to the aquatic life and resistance to biodegradation and because it is one of the major constituents of several wastewaters. Thiocyanate is usually present in phenol-containing wastewaters (i.e., coke-oven wastewater) and probably contributes to the development of the oxidation process, interfering in phenol degradation and modifying the yield of the treatment. For this type of wastewaters and when the biological treatment is not the most suitable one, techniques such as wet oxidation have been employed, achieving good mineralization levels (1-3). There is a widespread literature concerning phenol wet oxidation (4) but fundamental information on oxidation kineticssreaction orders and activation energiessand on reaction mechanisms is extremely meager and often contradictory because of several reasons such as phase equilibrium effects, apparent errors in the experimental procedure, the huge variety of contactors and working conditions used (5), the use of different catalysts or co-oxidants as promoters for the oxidation. Several authors have not considered necessary the use of additional substances to reach good depuration levels by conventional wet oxidation (6, 7), but some others have introduced acids and bases (8) to modify the pH of the media, interfering in the reaction kinetics of the process. However, many workers have focused their research on the use of different catalysts. Thus, phenol oxidation in aqueous solution has been carried out using sodium sulfite plus oxygen as oxidant (9), by means of heterogeneous catalysts such as zeolites (10) together with hydrogen peroxide or just focusing on the catalytic activity of homogeneous metal salts (i.e., copper, manganese, and cobalt) in both presence and absence of hydrogen peroxide (11). More recently, Centi et al. (12) have worked with heterogeneous and homogeneous Fenton-type catalysts. For the case of thiocyanate, studies dealing with its biological treatment have led to relevant results such as the presence of thiocyanate in effluents from coal-coking processes inhibited phenol biodegradatrion (13). On the other hand, thiocyanate chemical oxidation has been widely studied with different reactants (14-16), but to the best of our knowledge, no data have been reported for its chemical oxidation together with other pollutants. In this way, wet oxidation is here proposed as a suitable degradation technique for streams containing phenol and thiocyanate as the main pollutants. In previous works, phenol (17) and thiocyanate (18) wet oxidation have been individually studied, analyzing the influence of the different working conditions, determining the kinetics of the system, and suggesting possible reaction mechanisms. As both compounds appear as important pollutants in several wastewaters, we consider that it is very interesting to study their simultaneous oxidation paying special attention to the possible synergism or inhibition interactions between them. However, it is not easy to accurately determine the role of each component because of the huge variety of wastestreams and to the even wider range of compositions, but some general clues can be deduced if the simultaneous wet oxidation of both pollutants is undertaken. Initially, the research has been carried out assuming that phenol is the main pollutant and analyzing how thiocyanate presence in the bulk liquid affects its oxidation, but the experiences have also been very useful for the analysis of the VOL. 37, NO. 7, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Simultaneous phenol and thiocyanate wet oxidation in standard conditions: [Ph]o ) 1000 ppm, [SCN-]o ) 100 ppm; (b) phenol, (]) thiocyanate, and (/) pH. Solid lines correspond to theoretical models (eqs 8 and 9). changes provoked on thiocyanate kinetics when oxidized in the presence of phenol. Then, the aim of the present work is to gain experimental information about simultaneous phenol and thiocyanate wet oxidation, focusing the research toward the kinetics of the process for each individual pollutant and analyzing the key role of the pH of the reation media.
Experimental Section Apparatus and Procedure. Experiments were completed by means of the same experimental apparatus employed in phenol and thiocyanate individual wet oxidations (17), using molecular oxygen as oxidant and working in batch mode. Before the equipment was pressurized with oxygen and preheated to the desired working temperature, a precalculated amount of a concentrated aqueous solution of potassium hydroxide was added to the reaction media in order to provide the alkaline conditions required for the process. A sample reservoir connected to the vessel was used to introduce a predetermined amount of a concentrated solution of phenol and thiocyanate (T ) 20 °C) at the end of the heating period by means of the pressure supplied by the bottled compressed oxygen, being the zero time established by this injection. A valve and a coil fitted on the top of the vessel allowed the withdrawal of samples during the reaction. Liquid samples were periodically withdrawn and analyzed until the concentration was less than 1% of its initial value. Analysis. The concentration of phenol in the aqueous phase was measured using the 4-amino antipyrine colorimetric method developed by Ettinger et al. (19), while thiocyanate concentration was determined using the iron(III) complex method (20). A selective electrode Crison micro CM 2001 pH meter has been used for pH determination.
Results and Discussion Simultaneous wet oxidation of phenol and thiocyanate has been carried out under standard working conditions fixed in 100 atm, 200 °C, 2.33 m3 s-1 for the oxygen flow, and 500 rpm for the speed of stirring. The initial concentrations were 1000 ppm for phenol and 100 ppm for thiocyanate, according to the conditions previously employed in the wet oxidation of each pollutant individually. Kinetic control conditions have been guaranteed for all the runs in terms of the Hatta number and in the same way as discussed in previous research concerning thiocyanate wet oxidation (18). Figure 1 shows the experimental points obtained for each pollutant when simultaneously oxidized under the abovementioned standard conditions. After a first qualitative analysis based on the shape of the degradation curves of phenol in the presence of thiocyanate as compared with the results in the absence of thiocyanate (17), it can be said that phenol wet oxidation is modified, taking place in a single stage without the existence of an induction period. It seems that its degradation is significantly prejudiced by the presence 1458
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of thiocyanate because more than 4 h are needed to reach a 99% conversion when normally this conversion is reached in hardly 20 min under the same working conditions when oxidized alone. On the other hand, wet oxidation of thiocyanate has been accelerated by the presence of phenol, achieving longer half-reaction times to reach the same conversion. In a preliminary kinetic discussion, it was assumed that each pollutant oxidation could be independently fitted to a pseudo-first-order behavior despite being oxidized at the same time. So, phenol and thiocyanate individually will react with oxygen to form the different oxidation products according to eqs 1 and 2: k′Ph,simul
Ph + bO2 98 phenol oxidation products
(1)
k′SCN,simul
SCN- + b′O2 98 thiocyanate oxidation products (2) The proposed pseudo-first-order kinetic expressions for each compound individually considered are rPh ) -(d[Ph]/dt) ) k′Ph,simul[Ph] and rSCN- ) -(d[SCN-]/dt) ) k′SCN,simul[SCN-]; the slopes of the linear plots of ln(Ci/Ci,o) versus t give the values of the apparent kinetic constants. In the case of phenol, k′Ph,simul ) 2.50 × 10-4 s-1 (r 2 ) 0.997) is considerably lower than the one obtained when phenol is individually oxidized (17) (k′Ph,ind ) 8.35 × 10-3 s-1). This phenomenon was already reported by Neufeld and Valiknac (13) in their works concerning the biological treatment of phenol, where thiocyanate concentrations ranging from 0 to 250 ppm inhibited phenol biodegradation. However, not only is phenol behavior affected, but also thiocyanate wet oxidation is modified according to the reaction rate obtained for its oxidation in the presence of phenol (k′SCN,simul ) 1.83 × 10-3 s-1, r 2 ) 0.991), which is substantially higher than the one obtained when individually oxidized (18) (k′Ph,ind ) 1.83 × 10-4 s-1). The effect in this case is the opposite, the presence of phenol makes it possible to reach 99% conversion, reducing reaction time from 4 to 2 h. Although the shape of the curve and the mechanism do not look to change, thiocyanate wet oxidation is clearly enhanced. Special precautions have been taken to avoid the eventual release of toxic gases (HCN) because of the possible cyanides formation from thiocyanate oxidation. In such a way, an alkaline pH has been established at the beginning of each run by the addition of an adequate amount of KOH during the reactor loading. This is rather a preventive safety measure since the results of previous work concerning the thiocyanate wet oxidation mechanism (18) indicate that they are immediately oxidized because of the excess of oxygen employed. Anyway, this measure makes necessary the analysis of the pH influence on the kinetics of the process, and pH evolution with reaction time has been followed for every run. Initial pH values from 8 to 9 (pHo) after the pollutants were injected into the bulk liquid (pH around 11) at zero reaction time gradually decreased during the oxidation process until reaching a value close to 6. This is mostly due to the carboxylic acids formation during phenol oxidation, and it has been stated that phenol reaction rate was seriously prejudiced by the initial alkaline pH of the media. As mentioned before, phenol wet oxidation has already been isolately studied with no KOH addition to the reaction media, with the initial pH of the bulk liquid being the one given by an aqueous solution of 1000 ppm phenol concentration (around 6.5). Under these pH conditions, good oxidation yields in short residence times were achieved, but when the process is carried out under alkaline conditions together with thiocyanate, phenol oxidation rate is considerably slowed.
TABLE 1. Kinetic Data for Individual and Simultaneous Phenol and Thiocyanate Wet Oxidation ki ((mol/L)1-nT s-1) phenol SCN-
pH influence pH influence
phenol
only pH influence pH and SCN- influence only pH influence pH and phenol influence
SCN-
Simultaneous Oxidation kPh,OH sim 0.87 ) 2.15 × 10-5 kPh,SCN 0.26 ) 2.33 × 10-7 kSCN,OH sim 1.74 ) 2.42 × 102 kSCN,Ph 3.44 ) 1.21 × 106
FIGURE 2. Individual phenol wet oxidation at standard conditions: phenol (O) and pH (/) evolution with no base addition; phenol (b) and pH (+) evolution with base addition. The data above presented for the simultaneous oxidation of both pollutants do not provide enough information to discern if phenol oxidation rate inhibition is exclusively due to the thiocyanate presence, to the alkaline pH of the media, or to both factors. For this reason, further experiences for individual phenol wet oxidation have been carried out under the same working conditions but adding KOH to the reaction media to obtain identical alkaline conditions as in the simultaneous process. Influence of pH. Initially, the individual oxidation processes will be considered previously to the discussion of the simultaneous one. Figure 2 shows the phenol degradation and the pH evolution curves obtained when working under initially alkaline/neutral conditions and the curves obtained when no alkali is added to the reaction bulk, neutral/acidic conditions. Differences between both curves are significant. When no base is added to the solution, phenol is demoted through a rapid stage preceded by an induction period according to a radical chain mechanism. On the contrary, the induction period disappears and phenol oxidation elapses in a single but longer and slower degradation stage. In this last case, the samples withdrawn from zero reaction time present a brown-yellowish coloration because of the presence of quinones in the reaction media that also makes a radical mechanism for phenol oxidation, so the reaction mechanism does not seem to change with pH. When working under alkaline conditions, a 99% conversion for phenol is reached in approximately 2 reaction hours. If there is no base addition, around 20 min is needed for a similar conversion rate, so it is evident that pH has a relevant influence on the process. Equation 3 is a proposed kinetic expression for phenol reaction rate where pH influence is taken into account in the hydroxide ion concentration term. First order with respect to phenol has been assumed as discussed above for the determination of the apparent reaction rate constants:
rPh ) -
nPh
Individual Oxidation with Base Addition kPh,OH ind 0.87 ) 5.17 × 10-5 1 kSCN,OH ind 1.74 ) 1.07 × 10-2 0
d[Ph] ) {kPh,OHnT[OH-]nOH}[Ph]1 dt
(3)
nSCN 0 1
1 1 0 1.70
0 -0.61 1 1
nOH
nT
-0.13 0.74
0.87 1.74
-0.13 -0.13 0.74 0.74
0.87 0.26 1.74 3.44
FIGURE 3. pH influence on individual phenol and thiocyanate wet oxidations. If this expression is rearranged and the logarithm of each side is further taken, eq 4 is obtained:
ln
( ) rPh
[Ph]1
) ln(kPh,OHnT ) + nOH ln[OH-]
(4)
From the experimental data, both sides can be evaluated, and the slope of the linear plot shown in Figure 3 provides the reaction order with respect OH- concentration while the intercept provides the rate constant of the process kPh,OH ind 0.87 ) 5.17 × 10-5 (mol/L)0.13 s-1 (Table 1). The negative reaction order obtained (nOH ) -0.13, r 2 ) 0.97) indicates that alkaline pH inhibits phenol wet oxidation as we already have indicated. If the process is simplified to pseudo-first-order kinetics, the apparent constant can be evaluated (k′ ) 5.17 × 10-4 s-1, r 2 ) 0.996) and compared to the value obtained for phenol when simultaneously oxidized with thiocyanate (k′Ph,simul ) 2.50 × 10-4 s-1) and when individually oxidized with no base addition (k′Ph,ind ) 8.35 × 10-3 s-1). The alkaline media clearly prejudices the phenol oxidation process with respect to the conventional one, but it is even more prejudiced by the presence of thiocyanate. As the wet oxidation of phenol produces organic acids, the pH of the media decreases during the course of the reaction, with this effect being added to the presence of phenol, radicals, and oxidation intermediates formed during the reaction. It follows that any variation of reaction rate with pH is an important parameter in the process, so pH influence should be considered in all the kinetic discussions together with the prevailing working conditions and the thiocyanate presence. In accordance with this discussion, Kolaczkowski et al. (8) reported the complexity of pH influence on phenol wet oxidation with significant phenol decomposition when no acid or base were added to the bulk liquid (conventional process), but when the pH of the solution was below 2 or between the neutrality and 10, phenol degradation was considerably slowed. If working under strong alkaline conditions (pH set above the dissociation constant of phenolspK ) 9.95spH > 12), the reaction rate was enhanced significantly. The reason suggested for this different behavior at pH values from 2 to 12 was the different reactivities with VOL. 37, NO. 7, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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oxygen of three different forms of phenol found in the liquid, namely protonated, nonprotonated, and phenolate ion. As we will consider simultaneous phenol and thiocyanate oxidation, it is also necessary to know the influence of pH on individual thiocyanate wet oxidation before undertaking the discussion of the simultaneous process. This can be done in an identical way than the one previously used for phenol. In this case, using eq 5 as the final kinetic expression from the correspondent linear plot, kSCN,OH ind 1.74 ) 1.07 × 10-2 (mol/L)-0.74 s-1 and nOH ) 0.74 (r 2 ) 0.973) have been obtained:
rSCN- ) -
d[SCN-] ) {kSCN,OHnT[OH-]nOH}[SCN-]1 dt
(5)
The reaction order of 0.74 obtained for OH- concentration indicates that alkaline pH has a considerably enhancing effect over thiocyanate wet oxidation. If the simplification to a pseudo-first-order process is considered, the apparent constant obtained is significantly lower than the one obtained when thiocyanate is oxidized in the presence of phenol as previously described (k′SCN,ind ) 1.83 × 10-4 s-1 vs k′SCN,simul ) 1.83 × 10-3 s-1). In some cases, when the process is brought about in sufficiently basic solution, a dependence on pH in the basic region can be established for the rate constant, with the reaction being catalyzed by the hydroxide ions present in the alkaline media (21). This phenomenon seems to occur here for thiocyanate wet oxidation, the alkaline media provides a high enough hydroxide ions concentration so that the oxidation process is significantly enhanced, and a specifictype basic catalysis takes place. Interactions of the Oxidation. Traslating these results into the simultaneous phenol and thiocyanate wet oxidation, the influence of the alkaline media should be evaluated in order to quantify the effect of the pH on the process and later on determine the overall kinetic expressions for each pollutant. The procedure is similar to the one described above for the individual processes, but it must be noted that, in all these calculations, average reaction orders have been considered for hydroxide ion concentration. These average values are the reaction orders that better fit all the runs simultaneously. In the case of phenol, the nOH values obtained, nOH ) -0.10 (r 2 ) 0.973) for isolated phenol oxidation and nOH ) -0.15 (r 2 ) 0.991) in the simultaneous process, give an average value of nOH ) -0.13 that was employed in the kinetic expressions. Accordingly, in the isolated thiocyanate wet oxidation, the reaction order was nOH ) 0.57 (r 2 ) 0.985) whereas in the simultaneous oxidation, nOH ) 0.91 (r 2 ) 0.993); then, nOH ) 0.74 has been taken as the average reaction order for pH influence on thiocyanate oxidation. The reaction rate constants obtained in this way for all cases are collected in Table 1 together with the corresponding average reaction orders. Once the pH influence on the simultaneous process has been evaluated, the kinetic analysis can be fulfilled with the study of the interactions between phenol and thiocyanate. Despite both pollutants being oxidized in the same reaction media, each one of them independently reacts with molecular oxygen, but there is not a direct reaction between the two compounds. However, there exist interactions between phenol and thiocyanate wet oxidations because some of the radicals involved are common to both processes and are therefore consumed in both oxidations, leading to significant changes on the kinetics of the overall process. Each compound has been treated separately for the kinetic analysis, a first order with respect to the main considered pollutant has been assumed for each case, with the influence of pH being included in the resulting expressions. The corresponding reaction order for the respective co-pollutant has been determined from the experimental data analysis. Equations 1460
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FIGURE 4. Determination of the reaction order for phenol and thiocyanate when respectively considered as a co-pollutant: (b) order of thiocyanate and (]) order of phenol. 6 and 7 show the general kinetic expressions proposed for phenol and thiocyanate, respectively:
rPh ) rSCN- ) -
d[Ph] ) {kPh,SCNnT[OH-]nOH[SCN-]p}[Ph]1 dt
(6)
d[SCN-] ) {kSCN,PhnT[OH-]nOH[Ph]q}[SCN-]1 (7) dt
Operating on these expresions and taking logarithms of each side, the linear plots shown in Figure 4 are obtained. The slopes provide the reaction orders of each co-pollutant, and the intercepts give the value of the kinetic constants. The final kinetic expressions for phenol and thiocyanate when oxidized simultaneously are presented in eqs 8 (r 2 ) 0.996) and 9 (r 2 ) 0.991), respectively. A good agreement between the proposed models and the experimental results has been found as stated in the profiles shown in Figure 1:
rPh ) -
d[Ph] ) {2.33 × 10-7[OH-]-0.13[SCN-]-0.61}[Ph]1 dt (8)
rSCN- ) -
d[SCN-] ) {1.21 × 106[OH-]0.74[Ph]1.70}[SCN-]1 dt (9)
From eq 8, it is deduced that the thiocyanate presence clearly inhibits phenol wet oxidation as indicated by the negative reaction order obtained (nSCN ) -0.61), leading to the same result previously pointed out by the apparent constant values obtained from the pseudo-first analysis simplification. For phenol oxidation in thiocyanate presence, k′Ph,simul ) 2.33 × 10-7[OH-]-0.13[SCN-]-0.61 ) 2.50 × 10-4 s-1, which is approximately half of the value obtained when phenol is isolated and oxidized under alkaline pH (5.17 × 10-4 s-1) and more than 30 times smaller than when it is oxidized with no base addition to the bulk liquid (8.35 × 10-3 s-1). As phenol and thiocyanate wet oxidation processes involve free radicals, both pollutants consume radicals during their respective oxidations. When individually oxidized, phenol alone generates and consumes the radicals, but the presence of other pollutants like thiocyanate could act as a “radical scavenger”, consuming radicals for its degradation and slowing down the oxidation of phenol because of a lack of radicals in the reaction media to keep an adequate oxidation rate. For thiocyanate the effect is the opposite. A reaction order of 1.70 is obtained for phenol, which means that the presence
initial concentrations. A linear dependence with respect to thiocyanate initial concentration has been found for each pollutant reaction rate accordingly to eq 10, where i and j refer to the main pollutant and the co-pollutant, respectively: m ki,j,nT{CSCNo-} ) ko,iC SCN o
FIGURE 5. Initial thiocyanate concentration influence on simultaneous phenol and thiocyanate wet oxidation: [Ph]o ) 1000 ppm, [SCN-]o ) 200 and 50 ppm; / and ) denote pH.
TABLE 2. Rate Constants for Simultaneous Phenol and Thiocyanate Wet Oxidation at Different [SCN-]o CSCNo (ppm)
phenol kPh,SCN 0.26 ((mol/L)1-nT s-1 × 108)
thiocyanate kSCN,Ph 3.44 ((mol/L)1-nT s-1 × 10-5)
50 100 200
7.12 23.3 96.8
6.63 12.1 34.8
of this pollutant in the reaction media strongly enhances thiocyanate wet oxidation. If the apparent constants are again considered, it results that thiocyanate oxidation in the simultaneous process (k′SCN,simul ) 1.21 × 106[OH-]0.74[Ph]1.70 ) 1.83 × 10-3 s-1) is significantly quicker than the individual thiocyanate wet oxidation (1.83 × 10-4 s-1). This could be treated as a synergistic phenomenon where two species are involved, one easily oxidizable and other more refractory to oxidation (5). In this case, thiocyanate would be the refractory one whose oxidation rate would be increased with respect to the isolated process by the presence of phenol. Influence of the Initial Concentration of Thiocyanate. Several runs at different initial concentrations of thiocyanate ranging from 50 to 200 ppm have been carried out. Phenol initial concentration was always kept at 1000 ppm and 100 atm; 200 °C and 500 rpm were fixed as standard working conditions. Figure 5 shows that, independent of the initial concentration of thiocyanate, both pollutants are oxidized in a single stage while the pH of the solution gradually decreases with reaction time until reaching a value around 5.5 at the end of each run. The values of the reaction orders for phenol, thiocyanate, and hydroxide concentration shown in Table 1 have been here used to determine the kinetic constants collected in Table 2 for each pollutant in the runs at different thiocyanate
(10)
Globally, the effect of thiocyanate has shown to slow the kinetics of phenol oxidation as thiocyanate also consumes radicals. The results derived from eq 10 when considering phenol oxidation, m ) 1.88 and ko,Ph) 4.05 × 10-2 (mol/ L)1-(nT+m) s-1 (r 2 ) 0.974), show the contrary: the higher the initial concentration of thiocyanate is, the higher the reaction rate obtained for phenol degradation is. Then there are two opposite effects; thiocyanate presence in the bulk liquid inhibits phenol oxidation (eq 10) but higher thiocyanate concentrations favor the process. The reason can be the increase of radicals in the media when thiocyanate concentration increases. Because the oxidation of each pollutant involves free radicals, it is quite sensible to think about a possible synergistic interaction between the intermediate compounds and the radicals formed during the oxidation of both pollutants simultaneously. In this way, a greater amount of radicals would be generated for higher thiocyanate initial concentrations, enhancing phenol wet oxidation reaction rate. This effect compensates the inhibition caused by thiocyanate, leading to very similar values of the apparent kinetic constants independent of the thiocyanate initial concentration if the overall process is considered (Table 3). The case of thiocyanate is the opposite, the amount of intermediate compounds and free radicals generated during its oxidation is increased with initial thiocyanate concentration, with this accumulation effect being added to the enhancing effect already caused by the radicals formed in the phenol co-oxidation, eq 9. In this way, thiocyanate wet oxidation is more effective when its initial concentration is higher according to the parameters obtained from eq 10, m ) 1.2 and ko,SCN ) 2.85 × 109 (mol/L)1-(nT+m) s-1 (r 2 ) 0.965). To directly compare all the different situations, the pseudo-first-order analysis with respect to the main pollutant has been again considered. The values of the apparent constants collected in Table 3 show that globally the best option for phenol wet oxidation is its isolated oxidation without base addition, with the process being prejudiced if carried out on the basic side of neutrality and inhibited by the presence of thiocyanate. As discussed before, the enhancing effect caused by radical accumulation at greater thiocyanate concentrations compensates its inhibitory effect due to radicals consumption, leading to very similar values of k′Ph,sim. On the other hand, the enhancing effect that phenol cooxidation has in thiocyanate wet oxidation is also remarkable. Although thiocyanate oxidation rate is faster when higher is its initial concentration, its simultaneous wet oxidation together with phenol always leads to considerably shorter reaction times and better reaction rate results. The experimental data here reported indicate the effect of interactions between the free radicals generated during thiocyanate wet oxidation and the radical nature species who
TABLE 3. Pseudo-First-Order Kinetic Parameters in Simultaneous and Individual Phenol and Thiocyanate Wet Oxidation simultaneous with base addition
T (°C)/P (atm)
[Ph]o (ppm)
200 °C/100 atm
1000
individual with base addition
individual with no base addition
[SCN-]o (ppm)
k′Ph,sim (s-1) × 104
k′SCN,sim (s-1) × 103
k′Ph,ind (s-1) × 104
k′SCN,ind (s-1) × 104
k′phenol (s-1) × 103
50 100 200
2.42 2.53 3.18
0.553 1.83 4.07
5.20
1.50 1.83 2.50
8.35
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participate in phenol degradation, suggesting other different possibilities. This phenomenon is particularly emphasized in the study of the simultaneous process with different initial concentrations, and in this way, the results obtained are widely subject to nonmechanistic considerations.
Acknowledgments The work upon which this paper is based on was financed by the European Union Contract 7220-EB/004.
Nomenclature Ci, [i]
concentration of component i in the reaction mixture (ppm or mol L-1)
Ci,o, [i]o
initial concentration of component i in the reaction mixture (ppm or mol L-1)
k ′i
apparent reaction rate constant (s-1)
ki,j,nT
kinetic constant, units depending on overall reaction order ((mol/L)1-nT s-1)
ko,i
rate coefficient defined by eq 10 ((mol/L)1-(nT+m) s-1)
ri
reaction rate for the component i (mol L-1 s-1)
m, n, p, q partial orders of reaction nT
total order of reaction
P
pressure (atm)
T
temperature (°C)
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(2) Imamura, S. Ind. Eng. Chem. Res. 1999, 38, 1743-1753. (3) Kolaczkowski, S. T.; Plucinski, P.; Beltran, F. J.; Rivas, F. J.; McLurgh, D. B. Chem. Eng. J. 1999, 13, 143-160. (4) Mishra, V. S.; Mahajani, V. V.; Joshi, J. B. Ind. Eng. Chem. Res. 1995, 34, 2-48. (5) Willms, R. S.; Balinsky, A. M.; Reible, D. D.; Wetzel, D. M.; Harrison, D. P. Ind. Eng. Chem. Res. 1987, 26, 148-154. (6) Joglekar, H. S.; Samant, S. D.; Joshi, J. B. Water Res. 1991, 25, 135-145. (7) Portela, J. R.; Lo´pez, J.; Nebot, E.; Martinez, E. Chem. Eng. J. 1997, 67, 115-121. (8) Kolaczkowski, S. T.; Beltran, F. J.; McLurgh, D. B.; Rivas, J. Process Saf. Environ. Prot. 1997, 75, 257-265. (9) Kulkarni, U. S.; Dixit, S. G. Ind. Eng. Chem. Res. 1991, 30, 19161920. (10) Larachi, F.; Levesque, S.; Sayari, A. J. Chem. Technol. Biotechnol. 1998, 73, 127-130. (11) Rivas, F. J.; Kolaczkowski, S. T.; Beltran, F. J.; McLurgh, D. B. J. Chem. Technol. Biotechnol. 1999, 74, 390-398. (12) Centi, G.; Perathoner, S.; Torre, T.; Verduna, M. G. Catal. Today 2000, 55, 61-69. (13) Neufeld, R. D.; Valiknac, T. J. Water Pollut. Control Fed. 1979, 51, 2283-2291. (14) Mishra, D. K.; Dhas, T. P. A.; Bhatnayar, P.; Gupta, Y. K. Indian J. Chem. 1992, 31, 91-96. (15) Christy, A. A.; Egeberg, P. K. Talanta 2000, 51, 1049-1058. (16) Figlar, J. N.; Stanbury, D. M. Inorg. Chem. 2000, 39, 5089-5094. (17) Vicente, J.; Rosal, R.; Dı´az, M. Ind. Eng. Chem. Res. 2002, 41, 46-51. (18) Vicente, J.; Dı´az, M. Environ. Sci. Technol. 2003, 37, 1452-1456. (19) Ettinger, M. B.; Ruchhoft, C. C.; Lishka, R. J. Anal. Chem. 1951, 23, 1783-7188. (20) APHA, AWWA, WEF. Standard Methods for Examination of Water and Wastewater, 20th ed.; APHA: Washington, DC, 1998. (21) Laidler, K. J. Reaction Kinetics, Volume II. Reactions in Solution; Pergamon Press Ltd.: Oxford, 1963; pp 54-71.
Received for review May 27, 2002. Revised manuscript received September 30, 2002. Accepted January 20, 2003. ES0201045
