Supercritical Water Oxidation vs Supercritical Water Gasification

Jan 14, 2015 - (21) Before simulation, reaction models are constructed using the Amorphous Cell module in Materials Studio software. One model is show...
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Supercritical Water Oxidation vs Supercritical Water Gasification: Which Process Is Better for Explosive Wastewater Treatment? Jinli Zhang,† Jintao Gu,† You Han,*,† Wei Li,† Zhongxue Gan,‡ and Junjie Gu§ †

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China ENN Group, State Key Laboratory of Low Carbon Energy of Coal, Langfang 065001, Hebei Province, China § Mechanical and Aerospace Engineering, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S5B6, Canada ‡

S Supporting Information *

ABSTRACT: 2,4,6-Trinitrotoluene (TNT), as a representative component of explosive wastewater, is treated in supercritical water gasification (SCWG) and supercritical water oxidation (SCWO) using molecular dynamic simulations based on ReaxFF reactive force field as well as density functional theory (DFT). The detailed reaction processes, important intermediates and products distribution, and kinetic behaviors of SCWG and SCWO systems have been analyzed at the atomistic level. For the SCWG system, TNT is activated by water cluster or H radical and the N atom is mainly converted into NH3 more than N2 through two significant intermediates NOH and C−N fragment. In addition to water cluster and H radical, the TNT is activated by O2 in the SCWO system. Besides, the N atom is transferred into N2 more than other N-containing products after 750 ps simulation. Combined with the calculated cracking energy of the bonds in TNT, SCWG can accelerate its degradation and is easier for C−N bond breaking or changing through other reactions because of its low cracking energy (69.6 kcal/mol in thermal decomposition and 59.0 kcal/mol in SCWG). In addition, a large amount of H2 molecules is produced in SCWG, which is a meaningful way of transforming waste to assets. On TNT degradation, SCWO with inadequate O2 that can be treated as partial oxidation reaction (SCWPO) can combine the advantages of SCWG and SCWO (with enough O2) to convert TNT into CO2, H2O, as well as H2 and NH3 with high economic value. Finally, a kinetic description is performed whose activation energies (17.6 and 18.4 kcal/mol) are theoretically consistent with experimental measurements.

1. INTRODUCTION The activities in modern society involved with mining, construction, explosives, and defense-related operations contribute to the numerous energetic materials,1which caused large quantities of explosive wastewater produced. As a result, explosive wastewater has become a serious problem in current society, which has also caused much harm to our daily life. In China, the amount of produced wastewater in 2012 was nearly 70 billion tons,2 which contains a large amount of nonbiodegradable wastewater such as explosive wastewater. Thus, it is important to treat the wastewater, especially for explosive wastewater, effectively with appropriate methods. In modern industry, there exist several wastewater treatment methods, including physical method, biochemical method, photocatalytic oxidation method, and electrochemical oxidation method, and these methods have their own advantages and disadvantages.3 A physical method such as adsorption is usually used to pretreat wastewater because of its low removal rate of chemical oxygen demand (COD).4−6 The biochemical method is simple and safe, but it takes a lot of area and has a low removal rate of COD as well.7 Photocatalytic oxidation can remove the nitrogen-containing organic components effectively from wastewater.8 However, the reaction system must have a good absorption of light, which is unsatisfactory for explosive wastewater treatment. For the electrochemical oxidation method, hydroxyl radicals produced by oxidation reaction can react with organic components in wastewater and some small molecules including water and carbon dioxide will be generated.9 It certainly can treat explosive wastewater © 2015 American Chemical Society

effectively. However, its shortage is obvious for its short life of electrode and longtime treatment processes. As an innovative technology of treating nonbiodegradable wastewater, supercritical water oxidation (SCWO) has been used more widely than other methods.10−12 The SCWO method can remove the organic components of explosive wastewater with great effectiveness. Chang13 applied the SCWO method to investigate the degradation of 2,4,6trinitrotoluene (TNT). He concluded that at 550 °C, 24 MPa, 120 s, and oxygen excess 300%, the TNT removal rate could be as high as 99.9%, that is, it is quite effective for treating explosive wastewater by the SCWO method. Besides, another method which is similar to SCWO called supercritical water gasification (SCWG) in treating organics has also become popular, and it is mainly applied in biomass treatment.14−16 When the biomass is treated in SCWG, it will be converted into available products such as H2, CH4, and so on. Our group also calculated the gasification of coal based on the wiser model in SCWG using ReaxFF and DFT methods.17 On the basis of this study, we find that the SCWG method in treating organics can produce large amounts of H2, which has significant economic value. Jarana et al.18 studied the supercritical water gasification of industrial wastes on a laboratory-scale continuous-flow system with two different Received: Revised: Accepted: Published: 1251

November 5, 2014 January 14, 2015 January 14, 2015 January 14, 2015 DOI: 10.1021/ie5043903 Ind. Eng. Chem. Res. 2015, 54, 1251−1260

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combined DFT method to investigate the reaction pathways and product distribution in both SCWG and SCWO of TNT reactions in this work. These simulation results will not only help to better understand the essential difference of the reaction process between SCWG and SCWO but also contribute to designing a reasonable reaction plan for explosive wastewater treatment, thus promoting efficient utilization of obtained products.

industrial wastewaters that contain a high concentration of organics. An important conclusion has been made that a maximum of 0.19 mol of H2 per initial CODm (CODm is given as mol of O2 consumed for total oxidation) was obtained in the gas phase under the best conditions by their research group. TNT, as a representative component of explosive, is usually chosen as a model (Figure 1) to be investigated about its

2. COMPUTATIONAL METHODS In this work, we perform molecular dynamics (MD) simulations using ReaxFF21 to investigate the degradation mechanism of TNT in SCWG and SCWO. The force field parameters we used here are the same as the ReaxFF MD study of Goddard’s group.22 The ReaxFF method that can accurately describe the chemical bond breaking and formation has been applied to combustion,22,23 coal pyrolysis,24 and nanotube formation.25 Besides, Goddard’s group compared the ReaxFF results with DFT calculations and found that both of the results from ReaxFF and DFT are consistent with each other.21 Before simulation, reaction models are constructed using the Amorphous Cell module in Materials Studio software. One model is shown in Figure S1, Supporting Information, which shows that the reactants are equally distributed. The simulation time step is set as 0.1 fs, which is consistent with other researchers’ work.26 A multithermostat with a damping constant of 10 ps calculation is employed for the initial temperature from 0 to 300 K with a rising rate of 30 K/ps; then the system is equilibrated at 300 K for 1 ps. Next, the systems are heated up to the reaction temperature with a rising rate of 100 K/ps. The NVT ensemble is applied all over reaction simulations. In order to save simulation time, we select a relatively high temperature of 2500, 3000 and 3500 K for all simulations concerned with subsequent sections 3.1, 3.2, 3.3, and 3.4. Another way to save simulation time is to increase pressure by increasing density. On the basis of our previous work17 and calculation, we set the density of pure SCWG to be 0.20 g/cm3. In fact, researchers always use a relatively high temperature range to do ReaxFF research. For example, Wang et al.27 set the temperature range as 2500−3000 K to study the thermodynamics of spinel surface deposition, and Wei’s group28 studied lignite depolymerization in supercritical methanol with ligniterelated model compounds using the ReaxFF reactive force field for molecular dynamics simulations in a temperature as high as 2200 K. Thus, we also choose a high temperature range from 2500 to 3500 K to perform our work. In fact, we simulated TNT SCWO and SCWG reactions at lower temperatures, such as at 1500 and 2000 K, and the results showed that the temperature had little effect on the degradation mechanism of

Figure 1. Structure of TNT (grey, carbon; red, oxygen; black, hydrogen; blue, nitrogen).

thermal decomposition or degradation because of its wide application. Experiments concerned with explosives such as TNT are usually not safe. Consequently, many researchers begin to investigate the degradation or thermal decomposition mechanism using simulated methods. For example, Cohen et al.19 calculated unimolecular dissociation reactions of TNT using the density functional theory (DFT) method. Three initial reactions of TNT, (1) NO2 → ONO, (2) C−NO2 bond breaking, and (3) C−H alpha breaking, have been concluded. Temperature has a significant effect on the three pathways, of which a detailed illustration was given in ref 19. Rom20 studied the reaction kinetics of the thermal decomposition of hot, dense liquid TNT from a first-principle-based reactive force field (ReaxFF) multiscale reactive dynamics simulation strategy. From the results they pointed out that the TNT molecule was decomposed into radicals, carbon-cluster products, and finally stable gaseous products. Although the thermal decomposition mechanism of TNT was investigated by many researchers, the reaction mechanism of SCWG or SCWO of TNT is lacking, which is important for development and application of the explosive wastewater treatment. Because the SCWG and SCWO reactions are really very fast, it is a great challenge for experimental scientists to disentangle the various fundamental reaction steps from the intermediate and final product distribution. Therefore, we chose to use the ReaxFF molecular dynamic simulations Table 1. Simulation Conditions in SCWG and SCWO no. 1 2 3 4 5

system SCWG SCWO SCWPO SCWG SCWO

TNTa 10 10 10 50 50

O2 0 100 20 0 200

densityb (g/cm3)

H2O 500 500 500 400 400

0.25 0.32 0.27 0.52 0.70

temp. (K)

simulation time (ps) c

2500−3500 2500−3500 3500 1700−2200d 1700−2200

750 750 500 100 100

a

Quantity of TNT molecules in periodic box corresponding to the LAMMPS software. bDensity of reaction system. cTemperature range from 2500 to 3500 K with an interval of 500 K, including 2500, 3000, and 3500 K. dTemperature range from 1700 to 2200 K with an interval of 100 K, including 1700, 1800, 1900, 2000, 2100, and 2200 K. 1252

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Figure 2. Initial pathways of TNT’s degradation in SCWG.

Figure 3. Detailed process of −NO2 → −ONO for TNT in the SCWG media.

3. RESULTS AND DISCUSSION

TNT in SCWG and SCWO. Goddard’s group also tested the effect of the different temperature scales between ReaxFF MD simulation and experiment on the thermal decomposition of brown coal, and they concluded that the high simulation temperature may certainly affect product distributions, but the reaction processes were not affected.29 A summary of all the input conditions can be seen in Table 1. The conditions of nos. 4 and 5 are used to analyze the kinetic behavior of both SCWG and SCWO for TNT degradation. In addition, cracking energy has been also calculated using the Dmol3 module by Materials Studio software. DFT calculations using Materials Studio software are at the level of the generalized gradient approximation (GGA) with the Becke−Lee−Yan−Parr (BLYP) functional employed.30 The double-numerical plus polarization (DNP) basis set is also employed here, and the energy convergence criteria were set to be 0.00001 Hatree.

3.1. Detailed Analysis of Initial Degradation for TNT in SCWG and SCWO. Although investigation of the degradation or decomposition of TNT has been performed by experiment13 or simulation,20,31,32 researchers have seldom investigated TNT’s degradation in SCWG and SCWO using molecular dynamics simulation, and its initial reaction mechanism is still unclear. In this subsection we analyze the initial degradation of TNT in SCWG and SCWO at the atomistic level, especially focusing on the detailed process of how the TNT molecule changes in SCWG and SCWO. By tracing the atoms in the generated documents using Materials Studio software, we obtain all possible reaction pathways of the SCWG system as shown in Figure 2. According to the simulation results (Figure 2) it can be found that initiation of TNT’s degradation in SCWG is mainly through three pathways. The first way is NO2 → ONO, which has been also observed in its thermal decomposition by ReaxFF.19 The reaction goes through a transition state under 1253

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Figure 4. Detailed process of how O2 imbeds in the aromatic ring of TNT in SCWO.

In general, the initial mechanisms of TNT’s degradation in SCWG and SCWO are similar except the O2 pathway (Figure 4). Having a good understanding of the initial reaction mechanism is very important for researchers; thus, many researchers have focused on this investigation.22,24,29 It is certainly crucial for us to study the initial mechanism because it not only helps us know the detailed reaction process but also makes us learn the crucial step about the reaction and indicates a clear direction for experimental research. 3.2. Chemical Bonds Cleavage of TNT Structure. The degradation of TNT consists of the aromatic ring opening reaction and other chemical bonds cleavage. Among which, the latter part are considered as the primary steps in the reactions. On the basis of the original structure of TNT (Figure 1), the cracking energies of different types of chemical bonds are calculated as listed in Table 2. We compare the calculated

catalysis of SCW cluster as shown in Figure 3. The water cluster consisted of three water molecules that act with each other by hydrogen bond interactions with −NO2, which leads to the breaking behavior of the C−N bond and formation of the C−O bond through a transition state like Figure 3b shows. Thus, rearrangement of −NO2 has finished, and the water cluster then breaks away from −ONO. In addition, the interaction between water cluster and −NO2 can also promote the −NO2 group to leave away from the TNT molecule, and it results in formation of 2, 6-dinitrotoluene radical and NO2 (this reaction is shown in Figure 5). This is another way of TNT’s initial degradation in SCWG, which has been found in the thermal decomposition of TNT as well.19 In SCWG environment, there usually exist quite a few H radicals and OH radicals produced from the water molecule and water cluster.33,34 The H radical is observed to attack the O atom, locating in −NO2 to form into a −NOOH structure. Besides, the H atom coming from −CH3 can also connect with −NO2. This course is the third pathway, and next, two routes will be found: (a) the N−O bond breaks producing a OH radical and (b) the C−N bond breaks generating a HNO2 molecule. Our results have already been proved from the energy change of every structure in other researchers’ work using the DFT method.19 After the initial reaction, the remaining large molecular fragments continue being degraded until they are almost converted into final products like CO2, H2O, and NH3. To date, people usually use SCWO, which has great potential in many fields more to treat wastewater than SCWG. Thus, it is significant for us to understand the mechanism of SCWO. A similar analysis of the initial mechanism of TNT’s degradation in SCWO is performed as well. Most of the pathways in SCWO are the same with that in SCWG for the reason that the SCWO system is the combination of SCWG and O2. Consequently, the pathways in SCWG as shown in Figure 3 are all existent in SCWO. On the other hand, owing to addition of O2, a new pathway is observed (Figure 4). The O2 molecule can move toward to the TNT molecule and then interacts with its aromatic ring, finally connecting with it. As simulation continues, the oxygen atom from O2 then embeds in the aromatic ring as Figure 4d shows. After this initial process, the ring of the embedded structure will be broken up and into small fragments. The O2 molecule can accelerate the aromatic ring opening in this way for the reason that the C−O bond in the produced ring structure is much weaker than the C−C bond. Thus, the ring opening reaction in the generated substance in Figure 4d can happen more easily.

Table 2. Thermodynamic Analysis via DFT Calculations for TNT’s Degradation in SCWG cracking bond C(1)− C(7) C(2)− N(1) C(4)− N(2) C(7)−H C(3)−H

Ecrack‑SCWG (kcal/mol)

ΔEcracka (kcal/mol)

refs (kcal/mol)

94.8

17.3

101.1−103.135

69.6

58.9

10.7

69.7−73.536

75.6

65.3

10.3

96.4 146.9

78.8 134.6

17.6 12.3

Ecrack (kcal/mol) 112.1

b

89.8−92.237 112.4−113.538

ΔEcrack = Ecrack− Ecrack‑SCWG (Ecrack, the cracking energies without solvent effect; Ecrack‑SCWG:, the cracking energies with SCWG solvent effect). bCalculated values of cracking energy at 298 K. a

cracking energies with relative reports,35−38 finding that the calculated values are consistent with experimental results except C(3)−H. This may be because the cracking energy value in ref 38 is measured using benzene, but the present value is calculated based on the TNT molecule in which −NO2 and −CH3 groups may enhance the C(3)−H bond energy. The thermodynamic analysis using DFT calculations shows that the first bond cracking will happen at the C(2)−N(1) bond with a cracking energy of 69.6 kcal/mol and followed by the C(4)− N(2) bond with the cracking energy of 75.6 kcal/mol. During the simulation processes we also find that the C(2)−N(1) bond breaks earlier than the C(4)−N(2) bond. It indicates that the −NO2 group is easier to break off the aromatic ring than the −CH3 group and H atom. The cracking energies of another 1254

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Figure 5. Effect of SCWG on C−N bond breaking (Angstroms).

two bonds, C(7)−H and C(3)−H, are 96.4 and 146.9 kcal/ mol, respectively. It can be seen that C(7)−H breaks up more easily than C(3)−H, which is why we observe the transformation of the H atom from −CH3 to −NO2 but not from C(3)−H. Because there is no solvent effect in the thermal decomposition of TNT its cracking energies of different bonds are corresponding to Ecrack. Thus, our calculated cracking energies also prove the reaction behavior including abstraction of NO2 from the TNT molecule and transformation of aromatic H from −CH3 to −NO2 of TNT’s thermal decomposition by DFT study.19 Because organics are soluble in SCWG, the SCWG will have an effect in the degradation of TNT. The cracking energies of the TNT bonds in SCWG are also calculated as listed in Table 2. Compared with the thermal decomposition of TNT, the cracking energies of those bonds are weaker in SCWG, differing from 10.3 to 17.6 kcal/mol. Thus, the SCWG can factually accelerate the degradation of TNT. Similarly, it is also easiest for the C−N bond to break up among all bonds with cracking energies of 58.9 and 65.3 kcal/mol. We take the dissociation of NO2 from aromatic ring for example to illustrate the detailed process of SCWG effect on TNT degradation shown in Figure 5. The −NO2 group in the ortho position breaks off the aromatic ring under the catalysis of water cluster with a changing energy of 58.9 kcal/mol. After dissociation, the interaction between the water cluster and NO2 disappears. The distance of the C−N bond goes through 1.492, 2.135, and 4.085 Å. Thus, we suppose that the cracking energy of the C− N bond gets lower as a result of the water cluster. Similarly, other bond breaking reactions happen under the SCWG effects as well, resulting in a reduction of the reaction activation energy. 3.3. Detailed Analysis of Reaction Pathways for the N Element of TNT in SCWG and SCWO. Migration of nitrogen is a key problem because of its huge harm to the environment. Previous studies have reported the routes of nitrogen in SCWO,39,40 but the conclusions were variable. In this subsection, we analyze how the N element transfers in the TNT’s degradation in SCWG and SCWO in detail. All N elements are traced during the whole simulation time, and the pathways are concluded in the following discussions. In SCWG, N element routes have been summarized in Figure 6 and Table 3 as supplement. Four initial pathways for −NO2 are observed in all SCWG simulations. The first pathway is C−N bond breaking with a NO2 molecule generated. Then an H radical may attack NO2 to form HNO2, which will be divided into an OH radical and a NO molecule. Most of the NO molecules interacte with H radical to transfer into NOH, which plays an important role in the N-related routes. Besides, the NO2 molecule can also react with reactive OH radical to produce HNO3, but this situation is quite secondary (not shown in Figure 6). Combination of H radical and R−NO2 are the second and third pathways, resulting in formation of R− NOOH or R−ONOH. Next, for R−NOOH, the bond between

Figure 6. Routes of the N element in SCWG. Detailed routes of 1, 2, 3, and 4 are separately listed in Table 3. NOH is the abbreviation of the structure ON−H. R in R−NO2 represents the group of TNT except −NO2.

Table 3. Flow Chart for Generation of N2 and NH3 from NOH and C−N Fragment in SCWG products N2

no. 1a 3 1 and 3

NH3

2 4

reactions 2NOH → 2HON• → HON−NOH → N2 + 2OH• C1b−N• + C2−N• → C1−N−N−C2 → N2 + C1 + C2 HON• + C3−N• → HON−N−C3 → N2 + OH• + C3 NOH + H• → HO−NH•; HO−NH• + 3H• → NH3 + H2O C4−N• + H• → C4−NH; C4−NH + H• → C4−NH2 → C4 + NH2•; NH2• + H• → NH3

a

Reaction number in Figure 6. bC-containing structure; the same with C2, C3, and C4.

R and NOOH will break, which leads to formation of HNO2, while for R−ONOH the R−ONOH bond or RO−NOH bond will break in the subsequent stage; the following reaction is shown in Figure 6. In the fourth pathway, sometimes the N element, does not leave the C-containing structure, so the C−N fragment occurred with reaction proceeding. In SCWO environment, the main pathways about the N element are similar to that in SCWG, so Figure 6 can also describe the relationship among N-containing molecules in SCWO. Additionally, except the processes in SCWG, the NO2 molecule can react with reactive O to generate the NO3 group in SCWO. The N-containing products observed in our simulation are consistent with the experimental results.41 The structure of the C−N fragment is quite complex containing C, N, and O, so its degradation is as important as TNT. From the statistical results (Figure 7) we find that the temperature has an important effects on intermediate or product distribution. When it varies from 2500 to 3500 K through a middle value of 3000 K, more and more C−N 1255

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Figure 7. Distribution of intermediates and products about N element.

generated NH3 and N2 in SCWG are 22 and 4, respectively, differing from the values of 5 and 10 in SCWO. In SCWG, the reaction system is a H-rich environment and H radicals are more likely to react with a N-containing group than other radicals or molecules, resulting in formation of NH3. In contrast, most H radicals react with O2 to produce H2O; then the interaction among N-containing groups take place, mainly forming N2. Three routes about N2 generation and two NH3generated ways are summarized and listed in Table 3. NOH and C−N fragment are both important resources for production of NH3; meanwhile, the H radical is also necessary. Another important phenomenon is discovered that the C−N fragment is more easily degraded in SCWO in the same condition resulting from the fact that the number of C−N fragments in SCWO at every temperature is smaller than that in SCWG before absolutely being degraded. The existence of O2 accelerates C−N degradation and makes the N element in the C−N fragment change into simple molecules following a pathway as follows. The O2 molecule can react with C atoms in the C−N fragment and further produce CO2. Then the left N atom is converted into inorganics. 3.4. Detailed Analysis of Simple Molecules’ Generation in SCWG and SCWO. In SCWG or SCWO, the goals of

fragments are degraded into small molecular inorganics such as NO2, NOH, NH3, and so on in the same simulation time. At one temperature, 3000 K of SCWG, for example, when the time is 250 ps, the number of C−N fragments is 7, but when it comes to 500 ps, the quantity of C−N fragments turns into 3 and decreases to 0 at 750 ps. Similarly, the same situation appears at 2500 K in SCWG by a changing quantity of C−N fragments through 10, 9, and 5 at 250, 500 and 750 ps, respectively. As a consequence, extending simulation time can make C−N fragments degraded more, which are also applied to SCWO systems. Combined the the influences of temperature and simulation time, we are sure that increasing temperature and extending simulation time have the same effects on TNT degradation. Those three species including NH3, C−N fragment, and N2 are emphasized because NH3 and N2 are the main products of the reaction and the C−N fragment is the structure we attempt to degrade. While other intermediates, NO, NO2, and NOH, for example, exist for a while and then disappear, involving in further reaction, we just count their quantity and summarize their routes (shown in Figure 6). It is observed that NH3 is the main product in SCWG, while N2 is the main product in SCWO. For instance, at 3500 K within 750 ps, the quantity of 1256

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their increasing rates are quite different. H2O and H2 are the most abundant products followed by CO2 in SCWG. Also, CO only takes a small proportion as well as NH3. The CO2 molecule is mainly produced from C−O fragments and interaction of CO and OH radical. Thus, it is reasonable for the trend of the CO plot. Within 200 ps, the producing rate of CO is faster than the consumption rate. However, from 200 ps to the end, most of the C atoms have been converted into CO2 and CO, resulting in CO becoming the main resource of CO2, that is, the changing trend of CO is directly correlative with the generation of CO2 in the final stage of reaction. At the end of this simulation the final quantities of CO2 and CO are 65 and 4, respectively, and the only C atom exists in the C−N fragment. We also count the generated water distribution that contains H or O element coming from TNT molecule. The results show that within about 150 ps the water quantity increases quickly, and from 150 to 500 ps its number stays nearly at an unchanged level. Besides, the generated water can also be divided into H and OH radical, so the trend of H2O decreases occasionally. Two meaningful products including NH3 and H2 are observed which may provide important information for industrial application about transforming waste into assets because both of them are widely used in industrial fields. This observation as well as the detailed pathways in formation of H2 is also demonstrated in our calculation on coal pyrolysis in SCWG.17 In order to clarify the simple molecular product distribution in detail in SCWO and SCWPO, Figure 8b and 8c are obtained. In SCWO, the trends of the products (CO2, H2O, H2, CO, and NH3) are quite different from those in SCWG. The most abundant products are H2O and CO2, and three other species (H2, CO, and NH3) are not too many. Notably, the quantity of H2 between Figure 8b and 8c is quite different. This is maybe because O2 can accelerate to convert H2 into H2O, and the number of O2 in system 3 (20 per cell) is much less than system 2. Thus, generation of H2 is not the same. As for CO, no matter OH radical (similar to SCWG) or O2, they both can help transform CO into CO2. Compared with SCWG, it is more difficult for a SCWO or SCWPO environment to produce NH3, which may be due to addition of O2, and it prevents the transformation of the N element to NH3. In general, it takes less time for SCWO to reach the equilibrium state than SCWG. This conclusion is made by the CO2 distribution as a function of time because the C content is in the majority of TNT molecules; the production of CO2 quantity can be seen as the main symbol of the equilibrium state of the degradation reactions. In addition, more H2O molecules have been produced in SCWO, to which we suppose it is also for the reason for O2 addition. On the whole, both SCWO and SCWG are good methods on treating TNT wastewater, but they have advantages and disadvantages, respectively. SCWO (with enough O2) has great potential to treat TNT with high speed but cannot produce the products that can be recycled, while SCWG can help convert TNT into harmless products such as H2 and NH3 that can be even consumed again in industry. Thus, we combine the advantages of SCWO and SCWG by conducting a simulation called partial oxidation in SCW named SCWPO whose results are shown in Figure 8b. When the O2 is not enough, the reaction still reaches the equilibrium state as quickly as SCWO (with enough O2), and it also produces larger amounts of NH3 and H2 the same as SCWG. Thus, this conclusion can lead to

treating wastewater are to remove organics and produce environmentally friendly products such as CO2, H2O, and other simple molecules. In order to study the detailed reaction process about TNT treatment, the multimolecular ReaxFF calculations are performed in the temperature range 2500− 3500 K. The main simple molecules distribution including CO2, H2O, H2, CO, and NH3 as a function of time are given in Figure 8a−c, which are obtained at 3500 K. In Figure 8a, it can be seen that as the simulation proceeds at initial stages the amount of five simple molecules (CO2, H2O, H2, CO, and NH3) are all getting larger and larger. However,

Figure 8. Evolution of simple molecules containing CO2, CO, and NH3 as a function of time. Simulation conditions temper ature of 3500 K in (a) SCWG, (b) (TNT(10):O2(20):H2O(500)), and (c) (TNT(10):O2(100):H2O(500)).

H2O, H2, are at the S CWPO SCWO 1257

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Industrial & Engineering Chemistry Research an important direction for the future study about SCWO and SCWG on treating wastewater. 3.5. Kinetic Analysis of TNT Degradation under the Effects of SCWG and SCWO. For the unimolecular reactions, the pressure affects the reaction order and the reaction rate constant. In our simulation system, the pressure is as high as more than 200 MPa when the simulated temperature is above 2500 K, which is much higher than the experimental pressure.11 Most simulated unimolecular reactions would be at their highpressure limit at such high simulation pressure. In order to avoid the influences of the high-pressure limit we chose lower temperatures (ranging from 1700 to 2200 K) to investigate the kinetic behavior of TNT degradation in SCWO and SCWG. The concentration of TNT is replaced by its molecular number. For each temperature, a total of 14 ps of data is collected because most of the TNT molecules have been consumed for all simulations within the first 14 ps. Many researchers have investigated the TNT degradation, in which the reaction is supposed to be first order for TNT.42−44 The rate constant is determined by the following equation ln N0 − ln Nt = kt

(1)

where N0 and Nt are the molecular number of initial stage and the stage at time t, respectively. Figure S2, Supporting Information, displays the relationship of ln Nt and t in the condition of 2000 K for the SCWG system, in which the slope of the plot is equal to the rate constant k, and the k values in other conditions are calculated by the same way, whose results are listed in Table S1, Supporting Information. From the Arrhenius equation k = A exp( −Ea /RT )

Figure 9. Napierian logarithm of the TNT consumption rate (k): (a) SCWG system and (b) SCWO system.

(2)

the activity energy (Ea) and pre-exponential factor (A) are calculated by a linear fitting. The plots in Figure 9 show the relationship of ln k and 1/T, from which we calculated the activation energy as 17.6 kcal/mol in SCWG and 18.4 kcal/mol in SCWO, respectively. The activation energies of the two systems are quite close, which may be because the changing trend of ln k as a functional of 1/T is similar. However, O2 addition contributes to increasing the k value more when temperature rises as the reason for EaSCWO > EaSCWG. Actually, from Table S1, Supporting Information, we can see that the increment of k in SCWO is larger than that in SCWG (3.18− 10.67 × 1010 for SCWG and 4.23−14.58 × 1010 for SCWO) in the same temperature range. The calculated rate constant values are compared with related experimental results in Table 4. The oxidation of TNT in the presence of O2 or just in SCWG is not found in other literature, so there are no existing kinetic experimental results under our simulation conditions. The references in Table 4 are about the thermal decomposition of TNT. Our calculated values of Ea are lower than TNT’s thermal decomposition, which may be explained by the fact that the SCWG and SCWO reduce the activation energy of the reactions for TNT degradation. Considering this reason, the ReaxFF simulations are in good agreement with the experimental measurements, although no experimental results are found under the same conditions. Besides, our kinetic results can also demonstrate that the initial reactions about TNT in SCWO proceed faster than that in SCWG, judging from the calculated k values shown in Figure 9. Combined with the water and carbon dioxide trends in section 3.4, we are sure that O2 addition can accelerate the whole degradation of TNT in SCW environment.

Table 4. Activation Energies and Prefactors of TNT Degradation Reactions from Experimental Data and Present Work 45

Zinn et al. Beckmann et al.46 Furman et al.31 Chang et al.13 This work (SCWG) This work (SCWO)

Ea (kcal/mol)

ln[A (s−1)]

41.4 29.4 20.7−27.8 4.63 17.6 18.4

30.4 22.8 29.3−30.0 29.4 29.9

4. CONCLUSIONS In this article, TNT is chosen as a model to investigate its degradation in SCWG and SCWO, respectively, using ReaxFF and DFT. The simulation results show that both SCWG and SCWO can remove TNT effectually. However, the mechanisms of the two systems are not the same as a result of the addition of O2 in SCWO. For the initial reaction, four pathways are obtained in SCWO including (1) NO2 → ONO, (2) R−NO2 bond breaking to produce a NO2 molecule, (3) R−NO2 turning into R−NOOH under the action of H radical, and (4) O2 interacting with TNT to embed into the aromatic ring, the first three pathways of which are also found in SCWG. The DFT calculations demonstrate that the cracking energy of C−N bonds (69.6 and 75.6 kcal/mol) are much weaker compared with other kinds of bonds (>95 kcal/mol), so it is reasonable that most of the initial reactions are relevant to the −NO2 group. The effects of SCWG on TNT degradation are 1258

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Industrial & Engineering Chemistry Research

(3) Forgacs, E.; Cserhati, T.; Oros, G. Removal of synthetic dyes from wastewaters: a review. Environ. Int. 2004, 30, 953−971. (4) Gupta, G. S.; Prasad, G.; Singh, V. H. Removal of chrome dye from aqueous solutions by mixed adsorbents: fly ash and coal. Water Res. 1990, 24, 45−50. (5) Slokar, Y. M.; Majcen Le Marechal, A. Methods of decolouration of textile wastewaters. Dyes Pigm. 1997, 37, 335−356. (6) El-Geundi, M. S. Colour removal from textile effluents by adsorption techniques. Water Res. 1991, 25, 271−273. (7) Oehmen, A.; Lemos, P. C.; Carvalho, G.; Yuan, Z. G.; Keller, J.; Blackall, L. L.; Reis, M. A. M. Advances in enhanced biological phosphorus removal: From micro to macro scale. Water Res. 2007, 41, 2271−2300. (8) Chong, M. N.; Jin, B.; Chow, C. W. K.; Saint, C. Recent developments in photocatalytic water treatment technology: A review. Water Res. 2010, 44, 2997−3027. (9) Chen, G. H. Electrochemical technologies in wastewater treatment. Sep. Purif. Technol. 2004, 38, 11−41. (10) Gong, Y. M.; Wang, S. Z.; Tang, X. Y.; Xu, D. H.; Ma, H. H. Supercritical water oxidation of acrylic acid production wastewater. Environ. Technol. 2014, 35, 907−916. (11) Marrone, P. A. Supercritical water oxidation-Current status of full-scale commercial activity for waste destruction. J. Supercrit. Fluids 2013, 79, 283−288. (12) Shin, Y. H.; Lee, H. S.; Veriansyah, B.; Kim, J.; Kim, D. S.; Lee, H. W.; Youn, Y. S.; Lee, Y. W. Simultaneous carbon capture and nitrogen removal during supercritical water oxidation. J. Supercrit. Fluids 2012, 72, 120−124. (13) Chang, S. J.; Liu, Y. C. Degradation mechanism of 2,4,6trinitrotoluene in supercritical water oxidation. J. Environ. Sci.-China 2007, 19, 1430−1435. (14) de Vlieger, D. J. M.; Thakur, D. B.; Lefferts, L.; Seshan, K. Carbon Nanotubes: A Promising Catalyst Support Material for Supercritical Water Gasification of Biomass Waste. ChemCatChem 2012, 4, 2068−2074. (15) Liao, B.; Guo, L. J.; Lu, Y. J.; Zhang, X. M. Solar receiver/reactor for hydrogen production with biomass gasification in supercritical water. Int. J. Hydrog. Energy 2013, 38, 13038−13044. (16) Lu, Y. J.; Guo, L. J.; Zhang, X. M.; Ji, C. M. Hydrogen production by supercritical water gasification of biomass: Explore the way to maximum hydrogen yield and high carbon gasification efficiency. Int. J. Hydrogen Energy 2012, 37, 3177−3185. (17) Zhang, J. L.; Weng, X. X.; Han, Y.; Li, W.; Cheng, J. Y.; Gan, Z. X.; Gu, J. J. The effect of supercritical water on coal pyrolysis and hydrogen production: A combined ReaxFF and DFT study. Fuel 2013, 108, 682−690. (18) Jarana, M. B. G.; Saanchez-Oneto, J.; Portela, J. R.; Sanz, E. N.; de la Ossa, E. J. M. Supercritical water gasification of industrial organic wastes. J. Supercrit. Fluids 2008, 46, 329−334. (19) Cohen, R.; Zeiri, Y.; Wurzberg, E.; Kosloff, R. Mechanism of thermal unimolecular decomposition of TNT (2,4,6-trinitrotoluene): A DFT study. J. Phys. Chem. A 2007, 111, 11074−11083. (20) Rom, N.; Hirshberg, B.; Zeiri, Y.; Furman, D.; Zybin, S. V.; Goddard, W. A.; Kosloff, R. First-Principles-Based Reaction Kinetics for Decomposition of Hot, Dense Liquid TNT from ReaxFF Multiscale Reactive Dynamics Simulations. J. Phys. Chem. C 2013, 117, 21043−21054. (21) van Duin, A. C. T.; Dasgupta, S.; Lorant, F.; Goddard, W. A. ReaxFF: A reactive force field for hydrocarbons. J. Phys. Chem. A 2001, 105, 9396−9409. (22) Chenoweth, K.; van Duin, A. C. T.; Dasgupta, S.; Goddard, W. A. Initiation Mechanisms and Kinetics of Pyrolysis and Combustion of JP-10 Hydrocarbon Jet Fuel. J. Phys. Chem. A 2009, 113, 1740−1746. (23) Chenoweth, K.; van Duin, A. C. T.; Goddard, W. A. ReaxFF reactive force field for molecular dynamics simulations of hydrocarbon oxidation. J. Phys. Chem. A 2008, 112, 1040−1053. (24) Salmon, E.; Behar, F.; Lorant, F.; Hatcher, P. G.; Marquaire, P. M. Early maturation processes in coal. Part 1: Pyrolysis mass balance

discussed, indicating that SCWG solvent lowers the cracking energy of the chemical bonds in TNT by the affection from water cluster or free radicals. The routes of the N element in the whole reactions are described. The results show that the N transformation pathways are similar for SCWG and SCWO, but the final product distributions are different. Both in SCWG and in SCWO the NOH structure is a key component combining the initial form of the N element and the final product. After enough simulation time at 3500 K, for the SCWG system (750 ps), the main product is NH3 (22/30), while N2 (20/30) is predominant in SCWO (750 ps). The effects of temperature on the final intermediates and products distribution are obvious but influence the pathways little. From the evolution of CO2 as a function of time, a conclusion is made that SCWO can convert the C atoms into CO2 with more efficiency than SCWG, which is also reflected by the quantity of the C−N fragment at one temperature meanwhile. However, large amounts of H2 are produced in SCWG, which provides a huge green energy resource and makes the transformation from waste to assets realized. Therefore, both SCWG and SCWO have great potential on treating nonbiodegradable wastewater. Furthermore, the reaction system SCWPO with an inadequate number of O2 (system 3) can combine the advantages of SCWG and SCWO (with enough O2, system 2, for example) to show a good result. It offers an important direction for future study about SCWO and SCWG on treating wastewater. Finally, the activity energies (17.6 and 18.4 kcal/mol) of TNT degradation in SCWG and SCWO are obtained based on the Arrhenius equation, which is reasonably in accordance with the experimental and simulation results.

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ASSOCIATED CONTENT

S Supporting Information *

Constructed simulation cell obtained by Amorphous module using Materials Studio; Napierian logarithm of TNT’s molecular number within the initial 14 ps of the whole simulation at 2000 K of SCWG is exampled for calculation of constant rate k; results of all constant rates. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the National High-Tech Research and Development Program of China (2011AA05A201), National Natural Science Foundation of China (21106094, 21206021), International Science & Technology Cooperation Program of China (2013DFG42680), and the Program for Changjiang Scholars, Innovative Research Team in University (IRT1161),Tianjin Science Foundation for Youths, China (12JCQNJC03100).

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