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Ind. Eng. Chem. Res. 2009, 48, 9902–9909
Wet Oxidation of Thiocyanate under Different pH Conditions: Kinetics and Mechanistic Analysis Sergio Collado, Adriana Laca, and Mario Dı´az* Department of Chemical Engineering and EnVironmental Technology, UniVersity of OViedo, E-33071, OViedo, Spain
The wet oxidation of thiocyanate has been investigated in a 1-L semibatch reactor at temperatures in the range of 438.15-468.15 K and total pressures between 2.0 × 103 kPa and 8.1 × 103 kPa. The initial pH value of the reaction media was varied between pH 2 and pH 12 and was found to be a key parameter in the rate of thiocyanate degradation. The evolution of concentration of substrate and products (sulfate and ammonium) were analyzed. Based on the experimental data and bibliography information, a kinetic model was obtained and, here, possible pathways, depending on the initial pH of the media, are suggested for thiocyanate oxidation. Introduction Thiocyanates (SCN-) are present in several industrial wastewaters such as those from coal gasification factories, metal extraction industries, refineries, or agrochemistry plants.1-3 Although thiocyanate is a less-toxic form than cyanide, many deleterious effects of thiocyanate on human health and the aquatic ecosystem have been documented.4 Toxicity effects include respiration problems and can provoke death in humans, because of the formation of toxic gases from contact with acids.5,6 A biological process is the most commonly used method for removing this pollutant from wastewaters.2 Thiocyanate biological treatment has been studied for decades,3,7 but its application range is limited to low pollutant concentrations, because of inhibitory effects on the activity of the micro-organisms. Moreover, the presence of some compounds, such as cyanide or phenol, delays its degradation and can inhibit it totally when the compounds are present at high concentrations.8 Different oxidizing chemical agents have been also assayed over the past few years, and hydrogen peroxide has stood out as an effective one.9,10 Despite their effectiveness, these treatments are normally expensive and they have the additional inconvenience of introducing nondesired substances into the wastewater. Wet oxidation is an effective alternative treatment for wastewaters containing toxic compounds at levels that make biological treatment impossible or difficult.11 This technique involves the liquid-phase oxidation of organics and oxidizable inorganic compounds at relatively elevated temperatures and pressures using a gaseous source of oxygen. The process has been used to treat wastes that contain free cyanide, metal cyanide, or cyanide derivatives, giving carbon dioxide and ammonia as final products. Mishra and Joshi12 have studied the kinetics of hydrolysis and Wet Air Oxidation (WAO) of aqueous solutions of sodium cyanide. The hydrolysis followed a firstorder reaction, with respect to cyanide in the temperature range of 100-180 °C. These authors suggested that the destruction of NaCN occurred mainly by hydrolysis. The wet oxidation has been also used to treat other cyanide compounds, such as sodium cyanocuprate13 or acrylic acid.14 Despite the good results that have been obtained with other cyanide compounds, to the best of our knowledge, there are * To whom correspondence should be addressed. Tel.: 34 985103439 Fax: 34 98103434. E-mail:
[email protected].
only two works that have studied thiocyanate wet oxidation, individually and together with phenol.15,16 In these works, the wet oxidation of thiocyanate at pH 12 was first analyzed over a wide range of temperatures, pressures, and initial concentrations and simultaneous phenol and thiocyanate wet oxidation was also studied, paying special attention to the interactions between both pollutants. From the point of view of industrial practice, it is interesting to know if a change in the initial pH of the wastewater significantly changes the kinetics of the wet oxidation process. Consequently, the objective of the present work was to investigate the oxidation kinetics of thiocyanate in aqueous solutions, paying special attention to the effect of the pH of the reaction media. Here, the wet oxidation of thiocyanate at different initial pH values is analyzed at a wide range of temperatures, pressures, and initial concentrations, to obtain reliable kinetic data and to make an analysis of the possible reaction mechanisms for the wet oxidation process. Experimental Methods Figure 1 shows a scheme of the equipment used to conduct the catalytic wet oxidation experiments. The reactor (Parr Model T316SS) had a capacity of 1 L and was equipped with two sixbladed magnetically driven turbine agitators (identified as “1” in the figure). The reactor was preceded by a 2-L stainless steel water reservoir (“2” in the figure). The loaded volume in each vessel is ∼70% of the total, to ensure adequate safety conditions. The equipment, which contained water, was pressurized and preheated to the desired operating conditions while the stirrer speed was adjusted to 500 rpm for all the experiments. The operating pressure was provided by bottled compressed oxygen (identified as “3” in the figure) and controlled by an electronic mass flow controller (“4” in the figure) and a backpressure controller located at the end of the gas line (“5” in the figure) (the oxygen flow rate is 2.3 × 10-5 N m3/s). The oxygen was bubbled through the water reservoir to become saturated with water vapor, and then it was sparged /introduced into the reaction vessel. Once the desired conditions were achieved, a predetermined volume of a concentrated thiocyanate solution (potassium thiocyanate) was injected into the reactor (“7” in the figure). The injection time was taken as the zero time for the reaction. A valve and a coil (“8” in the figure) fitted to the top of the reaction vessel allowed the withdrawal of samples during the reaction. The reaction temperature and pressure were maintained constant during the experiments by a proportional integral
10.1021/ie9006485 CCC: $40.75 2009 American Chemical Society Published on Web 09/11/2009
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Figure 1. Experimental equipment.
differential (PID) controller (identified as “6” in the figure). Two bubblers (“9” in the figure) that were filled with a concentrated hydroxide solution were installed at the end of the gas line. Cyanide was not detected in the bubbler liquid at any time. To avoid leaching of the metal, the wall of the reactor and the stirrer were protected from the acidic media using a polytetrafluoroethane (PTFE) spray before each run. A comparison between identical experiments showed that the procedure has good reproducibility, with standard deviations of 4, the thiocyanate ion is the predominant species and the reaction rate is constant. However, the reaction rate increases significantly when the concentration of HSCN begins to be appreciable. Figure 2b suggests that oxygen oxidizes the thiocyanic acid more easily than the thiocyanate ions, with higher degradation rates for lower pH values. As shown in Figure 2, when the concentration of HSCN begins to increase, k also increases.
453.1 8.1 × 103 5.6 8.6 7.2 × 10-2 5.8 × 10-5 0.995
453.1 8.1 × 103 2.2 17 7.2 × 10-2 5.3 × 10-5 0.95
453.1 8.1 × 103 2.2 26 7.2 × 10-2 3.7 × 10-5 0.98
When the concentration of HSCN is negligible, for pH >4, the kinetic constant is less dependent on pH. The dependence of kNC on the initial concentration of protons has been obtained (r2 ) 0.997) as k(min-1) ) 2.97 × 10-pH + 2.59 × 10-3
(1)
In all cases, the concentration of protons increased during the wet oxidation process (denoted by the horizontal lines in Figure 2b); obviously, the decrease in pH was more marked when the initial pH was almost neutral. Effect of the Oxygen Pressure. Figure 3 shows a plot of the evolution of the thiocyanate concentration for runs performed at 453.15 K with an initial thiocyanate concentration of 1.72
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Figure 3. Evolution of thiocyanate concentration during wet oxidation conducted at different pH values and pressures: (4, 2) 2.0 × 103 kPa, (0, 9) 5.1 × 103 kPa, and (O, b) 8.0 × 103 kPa. (Solid symbols represent pH 5.6 data, and hollow symbols represent pH 2.2 data.) Conditions for all cases: initial thiocyanate concentration ) 1.72 mM; T ) 453.15 K. Solid lines denote models according to Table 1.
mM and oxygen pressures between 2.0 × 103 and 8.1 × 103 kPa (corresponding to oxygen concentrations between 10-2 and 7.2 × 10-2 M). The solid symbols denote the degradation of the thiocyanate when the initial pH was slightly acid (5.6), and the hollow symbols correspond to the results obtained for a more acidic initial pH (2.2). Considering that the oxygen consumption was very slow in comparison with the supply, the concentration of oxygen can be considered constant for a given run but varied from run to run as the pressure changes. The increase in operating pressure leads to higher reaction rates and reduces the reaction time needed to oxidize the thiocyanate for both assayed pH values. In the range of pressures considered, the behavior of the system could be properly fitted to a pseudo-first-order model. Solid lines in Figure 5 (shown later in this paper) denote the model curve according to the calculated pseudo-first-order constants (see Table 1). The value of the initial pH had a greater influence on reaction rate than the effect of the operating pressure. So, as can be observed in Figure 3, the degradation of thiocyanate at an acidic pH and 2.0 × 103 kPa was faster than at a slightly acidic pH in all the cases, including when the pressure was four times higher. The equilibrium concentration of oxygen was calculated employing Henry’s law and empirical correlations.20 The reaction order of oxygen (R) was determined by correlating the oxygen concentration and the reaction rate constant at different working pressures (k ) k′CO2R), yielding a value of R ) 0.61 for pH 5.6 (r2 ) 0.98) and R ) 0.32 (r2 ) 0.997) for pH 2.2. A value of R ) 0.86 was reported14 for the kinetic order of the oxygen during the wet oxidation of the thiocyanate at an initial pH value of 12. Dependence of the reaction rate on operating pressure is higher for more-basic media. The reaction order was pseudo-first-order, with respect to oxygen, has been reported in nitrile21 and sulfite22 oxidation, whereas the order obtained for wet oxidation of cyanides was 0.4.11 Merchant et al.23 proposed reaction orders of 0.14 and 0.74 for the oxidation of acetonitrile and acrylonitrile, respectively. Thus, it can be stated that the range of oxygen reaction orders is usually between 0.14 and 1 for the oxidation of inorganic compounds, in agreement with the values found in this work. Effect of Temperature. Figure 4 shows the results of two sets of runs performed with an oxygen pressure of 8.1 × 103 kPa and an initial thiocyanate concentration of 1.72 mM. Again, two initial pH values were tested: pH 5.6 and pH 2.2. The concentration of oxygen was constant for a given run but varied from run to run as the temperature changed.
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Figure 4. Evolution of thiocyanate concentration during wet oxidation conducted at different temperatures and pH values: (4, 2) 438.15 K, (0, 9) 453.15 K, and (O, b) 468.15 K; pH 5.6. (Solid symbols, pH 5.6; hollow symbols, pH 2.2). Conditions for all cases: initial thiocyanate concentration ) 1.72 mM; P ) 8.1 × 103 kPa. Solid lines denote model data according to Table 1.
Figure 5. Evolution of thiocyanate concentration during wet oxidation conducted at different initial concentrations and pH: (2) 0.86 mM and pH 5.6, (9) 1.72 mM and pH 5.6, (b) 3.45 mM and pH 5.6, ([) 8.62 mM and pH 5.6, (0) 1.72 mM and pH 2.2, (O) 5.17 mM and pH 2.2, (]) 17.24 mM and pH 2.2, and (4) 25.86 mM and pH 2.2. Conditions for all cases: T ) 453.15 K and P ) 8.1 × 103 kPa. Solid lines denote model data according to Table 1.
An increase in operating temperature results in an increase in the thiocyanate degradation rate for the two initial pH values used (see Figure 4). When the working temperature was 468.15 K, a conversion of 50% was reached within 1 h at pH 5.6, whereas, within the same time, only 10% conversion was obtained when working at 438.15 K. In the former case, only 150 min are needed to reach a conversion of 90%. In all cases, a pseudo-first-order model could be assumed (see Table 1). These k constants are dependent on the oxygen concentration with kinetic orders of 0.86 (pH 5.6) and 0.61 (pH 2.2) (see eqs 2 and 3, presented later in this paper). The oxygen concentrations for the operating conditions were calculated in each case and the new k′ constants were obtained. These k′ constants followed an Arrhenius-type behavior, and the activation energies were determined to be 141 and 116 kJ/mol for pH 2.2 and 5.6, respectively; these values are very different from the value obtained for pH 12 (84.4 kJ/mol).14 Effect of Initial Thiocyanate Concentration. Figure 5 shows the effect of the initial thiocyanate concentration on the reaction rate. Concentrations of 0.86-8.62 mM were studied at pH 5.6, and similar reaction times were required to obtain similar concentrations. The reaction did not show the usual pattern for radical systems; that is to say, when the initial concentration of substrate is lower, fewer free radicals are generated and the
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reaction time is longer.10 To verify that the initial concentration exerted no effect on the rate of reaction, a Student’s t-test was applied.24 Working with repeated experiments of wet oxidation of 1.72 mM of thiocyanate, it was shown that the initial concentration of thiocyanate did not affect the reaction rate, with 99% confidence. The behavior observed when the pH was slightly acidic can only be explained by considering that, in this case, wet oxidation of thiocyanate occurs through a nonradical mechanism. In contrast, when the initial pH was 2.2, a different type of behavior was observed: an analysis of data reveals that the higher the initial concentration, the lower the conversion at any given reaction time over the entire range of concentrations studied (1.7-25.9 mM). It can be observed that, after 2 h of reaction in acidic media, the highest thiocyanate removal (90%) corresponded to the lowest initial concentration and the highest initial concentration produced the lowest degradation (20%). Once more, the pH value is determined to be a key variable in the wet oxidation of thiocyanate. According to the pseudokinetic constants shown in Table 1, a value of -0.86 (r2 ) 0.998) was calculated for the exponential fitting of the initial concentration during the wet oxidation of thiocyanate at acidic pH (2.2). Kinetic Model. Taking into account the experimental results previously referenced, the following kinetic models can be proposed for the wet oxidation of thiocyanate in slightly acidic (eq 2) and acidic media (eq 3) (time, temperature, and concentrations are given in terms of seconds, Kelvin, and molarity, respectively): C ( -14561 T )
-rSCN- ) 3.25 × 1010 exp
0.61 SCN-CO2
(2)
C ( -16968 T )
-rSCN- ) 5.13 × 1010 exp
0.32 CSCN0--0.86 SCN-CO2
(3) All the experiments presented in this work were simulated using eq 2 or eq 3 with a good degree of accordance (r2 > 0.98) in every case). Evolution of Products and Mass Balances. To elucidate the oxidation route of thiocyanate, the evolution of the reaction products during the oxidation of thiocyanate at different initial pH values was studied. Figure 6 shows the evolution of concentrations of thiocyanate and the main products during oxidation at pH 5.6. Thiocyanate degradation occurred in a single stage and the formation of sulfate and ammonium occurred during the reaction. The presence of cyanides was tested for at different reaction times and was not detected at any moment. The evolution of ammonium and sulfate concentrations were modeled according to eqs 4 and 5. dCSO 42dt dCNH 4+ dt
dCSCN) -YS/TkCSCNdt
(4)
dCSCN) -YA/TkCSCNdt
(5)
) -YS/T
) -YA/T
where k is the pseudo-first-order kinetic constant for the degradation of thiocyanate, YS/T the ratio between sulfate and thiocyanate molecular masses (YS/T ) 1.65), and YA/T the ratio between ammonium and thiocyanate molecular masses (YA/T ) 0.31). There is a good agreement between experimental and model data for NH4+ and SO42- (see Figure 6) when the initial
Figure 6. - Evolution of the SCN-, SO42-, and NH4+ concentrations during thiocyanate wet oxidation: initial pH ) 5.6; initial concentration ) 1.03 mM; T ) 453.15 K; P ) 8.1 × 103 kPa. Lines show model data (from eqs 4 and 5).
pH value was 5.6. This shows that, in slightly acidic media, these final products are formed more or less simultaneously with substrate consumption. However, eq 5 is not valid for wet oxidation at acidic and basic pH values, which suggests the formation of ammonium and other nitrogen products. The evolution of the products obtained at different pH values was analyzed by employing elemental mass balances. In Figure 7a, the total initial concentration of nitrogen (dotted line) is compared with the sum of nitrogen as SCN- and nitrogen as NH4+ for different initial pH values. In a slightly acidic medium (pH 5.6), the only nitrogen compounds found in the reaction media at any time were thiocyanate and ammonium. The elemental mass balance does not involve the formation of any nitrate, nitrite, or molecular nitrogen as final products and the existence of no appreciable amount of any intermediate nitrogen compound. Figure 7a also shows the evolution of the nitrogen (as thiocyanate and ammonium) with reaction time for a basic pH (12.3). In this case, the nitrogen mass balance did not close correctly; this is a finding that contrasted with wet oxidation at pH 5.6. Therefore, in this case, the ammonium is not the only nitrogen product of the reaction. A possible explanation would be the formation of ammonia at basic pH, which can be stripped by the gas flow or degraded to molecular nitrogen. These explanations are considered to be adequate, because ammonia is less refractory to oxidation than ammonium.25,26 Finally, it can be observed that, during the wet oxidation of thiocyanate at acidic pH, the nitrogen mass balance did not close adequately (see Figure 7a). It is possible that the formation of HSCN gives place to a different reaction pathway, which explains the presence of another nitrogen species as final products. On the other hand, the elemental mass balances for sulfur (Figure 7b) suggest that, whatever the initial pH, sulfate is the only product with sulfur in its composition. Mechanistic Pathways for the Wet Oxidation of Thiocyanate at Different pH Values. According to the experimental data, it seems clear that reaction mechanisms for the wet oxidation of thiocyanate are different according to the initial pH of the system (differences in calculated activation energies, final products and effect of initial concentration were observed for different pH values. Previously, Vicente and Dı´az14 suggested a three-step reaction pathway for the wet oxidation of thiocyanate at basic pH values based on the formation of a series of radicals during the first step and with cyanide and cyanate as the main intermediates. Here, general pathways are proposed
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Figure 7. Elemental mass balances for the wet oxidation at different initial pH values ((2) 2.7, ([) 5.6, and (9) 12.3): (a) sum of N(SCN-) and N(NH+ 4) and (b) sum of S(SCN-) and S(SO24 ). Dotted lines indicate the total initial concentrations of nitrogen and sulfur. Conditions for all cases: initial thiocyanate concentration ) 1.72 mM; T ) 453.15 K, and P ) 8.1 × 103.
for slightly acidic and acidic pH, taking into account experimental results and bibliographic information for chemical oxidation of thiocyanate and cyanide with chemicals.19,27-30 Slightly Acid Media (pH 5.6). In this case, the possibility that the wet oxidation of thiocyanate occurs via a reaction mechanism that involves free radicals, which is what occurs at pH 12,14 was rejected for two reasons: an initial induction period was not observed and the kinetic constant was not a function of the initial concentration. Taking into account these considerations and according to the experimental results, an initial oxidation of the thiocyanate with the dissolved oxygen, yielding a thiocation, seems to be the first step. The thiocation (SCN+) has been reported as an intermediate in many thiocyanate chemical oxidation studies.30 Thiocation formation (eq 6) is considered to be the ratedetermining step, which explains the influence of the oxygen concentration on the reaction rate. The thiocation may then react with SCN- to form thiocyanogen (eq 7).30 At the elevated operational temperatures, the thiocyanogen is very quickly hydrolyzed (eq 8), yielding CN- as an intermediate and SO42as the final product.31,32 The presence of thiocyanogen could not been experimentally proven as a consequence of its extremely short residence time in the reaction bulk. 2SCN- + O2 + 4H+ f 2SCN+ + 2H2O
(6)
SCN+ + SCN- f (SCN)2
(7)
4 5 1 1 8 + CN- + H+ (SCN)2 + H2O f SCN- + SO23 3 3 4 3 3 (8) In our experimental work, no traces of cyanides were detected at any moment of the reaction, which can be explained considering the high working temperatures and the great excess of oxygen used, which leads to a fast oxidation process. Equation 9 is suggested as the fourth step of the proposed mechanism.28 Finally, at acidic pH values, cyanate is hydrolyzed to give ammonium during the last step of the proposed mechanism (eq 10).11 1 1 1 CN + O2 f CNO3 6 3
(9)
1 1 2 1 1 CNO- + H+ + H2O f HCO3- + NH+ 4 3 6 3 3 3
(10)
This pathway is consistent with other reaction pathways that have been proposed for chemical/biological oxidation of thio-
cyanate and with the cyanide wet oxidation mechanism proposed in the bibliography,27-30 and it explains the following experimental observations: (1) Sulfate and ammonium are the final products. (2) The increase in the global rate when the oxygen concentration increases. (3) No intermediates were detected. Acidic Media (pH 2.2). For the wet oxidation of thiocyanate in acidic media (pH 2.2), some experimental observations indicate that a more-complicated mechanism governs the process. The oxidation proceeds significantly faster for pH
