Environ. Sci. Technol. 2010, 44, 9157–9162
Use of 4,4′-Dinitrostilbene2,2′-Disulfonic Acid Wastewater As a Raw Material for Paramycin Production WENCHAO PENG, YING CHEN, SHIDONG FAN, FENGBAO ZHANG, GUOLIANG ZHANG, AND XIAOBIN FAN* School of Chemical Engineering and Technology, Tianjin University, Tianjin, People’s Republic of China, 300072
Received June 9, 2010. Revised manuscript received October 11, 2010. Accepted October 18, 2010.
This study uses 4,4′-dinitrostilbene-2,2′-disulfonic acid (DNS) wastewater to produce paramycin (4-amino-2-hydroxybenzoic acid), an antitubercular agent and important pharmaceutical intermediate. The high concentrations of aromatic sulfonic acids contained in the wastewater, derived from a DNS production facility, have been transformed to paramycin in yields of more than 85%. This waste-disposal strategy, which combines oxidation using NaClO, reduction using iron powder, and subsequent alkaline fusion with NaOH, has been proven to be successful in dealing with ton-scale DNS wastewater. Compared with common treatment methods, which usually involve degrading the compounds, this new method recycles most of the aromatic sulfonic acids in the wastewater to produce paramycin. This effectively solves the associated environment problems associated with DNS wastewater and is also potentially profitable. The present approach could also lead to alternative solutions for dealing with other industrial wastewaters generated from oxidative coupling reactions of nitro-substituted toluenes to the corresponding substituted stilbenes.
1. Introduction 4-Amino-2-hydroxybenzoic acid (PAS), which is commonly called paramycin, is an antitubercular agent used with other antituberculosis drugs (most often isoniazid) for the treatment of all forms of active tuberculosis due to susceptible strains of tubercle bacilli. It is also an important intermediate, widely used in the production of medicines, polymers, and pesticides (1). Usually, PAS is produced by carboxylation of 3-aminophenol or reduction of 4-nitro-2-hydroxybenzoic acid (2, 3). This study describes a novel method of producing paramycin from the aromatic sulfonic acids dissolved in the wastewaters resulting from the production of 4,4′-dinitrostilbene-2,2′-disulfonic acid (DNS), an important synthetic intermediate. Aromatic sulfonic acids (ASAs) such as the benzene and stilbene derivatives are widely used in various industrial processes. For example, benzene sulfonic acids are used as intermediates in producing ion-exchange resins, pesticides, and wetting agents, and stilbene sulfonic acids are used in the preparation of optical brighteners and synthetic dyes (4, 5). Because ASAs are produced in millions of tons on an annual basis, are highly soluble in water, and are not easily
degraded in environmental biological systems, their persistence and accumulation in natural ground and surface water has the potential to cause serious environmental problems (6-12). The treatment of industrial ASA wastewaters containing concentrated ASA solutions is therefore an urgent task. Nearly 100 kilotons of DNS are used each year for the production of direct dyes, fluorescent brighteners, and mothproofing agents (5, 6). Oxidation of p-nitrotoluene-osulfonic acid (NTS) (Scheme 1a) is the industrial process for the production of DNS, with a yield of ∼85% (13, 14). More than 15 kilotons of NTS are therefore transformed into various ASAs, which are dissolved in water forming 2 million m3 of DNS wastewater (COD∼2 × 104 mg L-1) (15). There are several methods for treating this wastewater on an industrial scale, including biological degradation, oxidation with Fenton’s reagent, ozonation, resin absorption, and incineration (15-18). However, biological processes cannot degrade the ASAs in wastewater effectively; resin absorption produces mixtures that require further treatments; the Fenton’s reagent and ozone oxidation methods are more effective but are still unable to recycle ASAs (Scheme 2, Route I). The incineration process (Scheme S5) previously developed by our laboratory can deal effectively with tons of DNS wastewater effectively, but this method degrades all of the ASAs into significant amounts of acidic gases (NxOy, SOz, CO2), which need further treatment prior to release into the atmosphere (14). In this study, the ASAs in the DNS wastewater were identified to have the same structural unit (Figure 2) by highperformance liquid chromatography-electrospray ionizationmass spectrometry (HPLC-ESI-MS). Based on this finding, we developed a new strategy for treating DNS wastewater, which combines an oxidation process, a reduction process, and subsequent alkaline fusion, and can transform more than 85% of the ASAs in wastewater into PAS (Scheme 2, Route II). Therefore, the new strategy not only solves the environment problems effectively but also results in substantial economic benefits. Notably, this approach could also lead to alternative solutions for dealing with other industrial wastewaters generated from oxidative coupling reactions of nitro-substituted toluenes to the corresponding substituted stilbenes (Scheme 1b) (19).
2. Materials and Methods 2.1. Chemicals. p-Nitrotoluene-o-sulfonic acid (NTS), 4-nitrobenzaldehyde-2-sulfonic acid (NSB), 4-nitro-2-sulfobenzonic acid (NSBA), 4-amino-2-sulfobenzonic acid (ASBA),
SCHEME 1. Oxidative Coupling Reactions of (a) NTS to DNS and (b) Nitro Substituted Toluenes to the Corresponding Stilbenes
* Corresponding author e-mail:
[email protected]. 10.1021/es101950k
2010 American Chemical Society
Published on Web 11/05/2010
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SCHEME 2. Treatment of DNS Wastewatera
a
I, conventional process; II, new route proposed in this study.
FIGURE 1. DNS wastewater (a) before and (b) after redox treatment. 4,4′-dinitrostilbene-2,2′-disulfonic acid (DNS), 4,4′-diaminostilbene-2,2′-disulfonic acid (DSD), 4-amino-4′-nitrostilbene-2,2′-disulfonic acid (ANSD), 6-6′- (oxirane-2,3-diyl) bis(3-nitrobenzenesulfonic acid) (DNS-epoxide), 4,4′-dinitrobibenzyl-2,2′-disulfonic acid (DND), and poly-4,4′-dinitrostilbene-2,2′-disulfonic acid (poly-DNS) were provided by the Huayu Chemical Co. (Cangzhou, China) and were recrystallized from water to obtain purities of more than 99.5%. Other chemical reagents and solvents (Merck, USA) were either analytical or chromatography grade and were used without further purification. 2.2. Analysis. The HPLC system used for identification of the products was a Finnigan Surveyor HPLC containing quaternary gradient pumps, a photodiode array detector, an auto injector, a degasser, a system controller, and an autosampler coupled with an LCQ Advantage Max ion-trap mass spectrometer (all from Thermo Fisher Scientific, San Jose, CA, USA) equipped with electrospray (ESI) and atmospheric pressure chemical ionization (APCI) interfaces. Xcalibur software was used for data acquisition and processing. For both systems, a reversed-phase Hypersil ODS-2 C18 column (250 × 4.6 mm i.d., 5 µm particle size) (Thermo Scientific, US) with a guard column made of the same packing material was used. The mobile phase was acetonitrile-0.01 M ammonium acetate, and the elution profile was as follows: initially 100% 0.01 M ammonium acetate, then 8% acetonitrile after 8 min, 25% acetonitrile after 35 min, and 100% 0.01 M ammonium acetate after 40 min. The analysis was carried out at a flow rate of 0.8 mL min-1 at 25 °C. A detection wavelength of 310 nm was used for quantification of the original compounds. The Microcoulometric Synthesize Analyzer (WK-2D, Jiangsu, China) used for determination of organochlorine 9158
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content contains a temperature and flow controller, a coulomb amplifier, an autosampler, a pyrolysis tube, and a titration cell. The titration cell was filled with 70% acetic acid aqueous solution, and the reference electrode was filled with silver acetate. Its detection range for chlorine is 0.5-5000 ppm. 2.3. Wastewater Sample Preparation. DNS wastewater samples produced on various dates (10 samples) were collected from the Huayu Chemical Co. (Cangzhou, China) and stored at 4 °C. 2.4. Selection of the Oxidant and Reductant. NTS aqueous solution (1%, 100 mL) was adjusted to the desired pH value with dilute NaOH or HCl solution. The solution was then heated to the desired temperature, and the chosen oxidant was added. The temperature was maintained for 5 h. Samples of the products were then removed and diluted 100times with cold water (0-5 °C) to terminate the reaction. The samples were analyzed by HPLC-ESI-MS. 2.5. Treatment of the Wastewater Samples. NaOH (4 g) was added to a mixture of 80 mL of DNS wastewater and 20 mL (0.03 mol) of aqueous NaClO. The resulting solution was heated at reflux for 5 h with vigorous stirring. The pH value of the mixture was then adjusted to 5-6, and 10 g of zerovalent iron powder was added. Refluxing was continued for another 30 min to finish the wastewater treatment process. The pH value of the treated wastewater was then adjusted with NaOH to ∼9 to precipitate the Fe2+ and Fe3+ with NaOH. After removing the iron mud, the treated wastewater was adjusted to pH ∼ 1 to precipitate the ASBA. The conversions from pure standards of the ASAs in the wastewater to ASBA were also conducted by the procedures described above. 2.6. Alkaline Fusion of ASBA to Paramycin. The ASBA obtained as described in section 2.5 was washed with diluted HCl (pH ∼ 1) and then dried under vacuum at 50 °C for 2 h. Then 2 g of dried ASBA and 1.5 g of NaOH were placed in a 25-mL autoclave. After sealing the reactor, its air content was purged by flushing twice with nitrogen at a pressure of 10 bar. Next, the autoclave was heated up to 230 °C, and the temperature was maintained for 2 h. After cooling the autoclave, the products were neutralized with 30% HCl aqueous solution at 25 °C. PAS was obtained by filtration.
3. Results and Discussion 3.1. Wastewater Analysis. Generally, the DNS yield from the oxidation of NTS is only 85% or less, and about 15% of the reactants are transformed into various byproduct through three main routes (Section 1 in the Supporting Information) (13, 20). Therefore, the concentration of ASAs remains high in the wastewater and, as a result, is extremely toxic to organisms (15, 16). Direct discharge of the wastewater will cause serious environmental problems. Furthermore, conventional treatment methods will destroy the ASA structure, making it impossible to reuse this resource. To study the wastewater components systematically, a typical DNS waste-
FIGURE 2. Detailed structures of compounds in the wastewater.
SCHEME 3. Oxidation Process (a) and the Reduction Process (b) during the Wastewater Treatment
water was analyzed by HPLC-ESI-MS; the results are listed in Figure 1a, Figure S1, and Table S1 (5, 21). Detailed structures of these components and other minor compounds in wastewater were described in Figure 2. Despite the wide array of functional groups found on the ASA that were identified in the DNS wastewaters, it was determined that they all shared the same basic structural unit (Figure 2). Therefore, the reactions illustrated in Scheme 3a are expected based on treatment with an appropriate oxidant. During these reactions, the methyl group, the
aldehyde group, and the CdC double bond can be oxidized into carboxyl groups, and some of the amino groups will be transformed into nitroso, nitro, azo, and azoxy groups (22-24). Subsequently, all the nitroso, nitro, azo, and azoxy groups in the oxidation products can be reduced to amino groups by selected reductants to give 4-amino-2-sulfobenzonic acid (ASBA) (Scheme 3b) (25, 26). 3.2. Selection of the Oxidant and Reductant. A systematic survey of suitable oxidant/reductant combinations was therefore conducted. Since NTS represents a relatively large VOL. 44, NO. 23, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Performances of Three Conventional Oxidants towards NTS
oxidant
temperature
pH
t/h
% yielda
NaClO H2O2 O2
100 °C 100 °C 100 °C
14 2 14
5 5 5
93 5 10
a
% yield based on the formation of NSBA.
proportion of the ASAs in the wastewater and is difficult to oxidize to the corresponding carboxylic acid, it was used as a standard in selecting the most suitable oxidant. Three conventional oxidants were chosen, and their performances at optimized pH values were investigated. As can be seen in Table 1, NaClO performs best, and it was selected for the oxidation process. Zero-valent iron powder was chosen as the reductant because it is the most widely used reductant for reducing nitro, azo, and azoxy groups with high efficiency under relatively mild condition (27, 28). NaClO and iron powder were therefore selected as the redox pair. The amount of wastewater is very large, so the oxidants and reductants were all used at atmospheric pressure in open systems; even the pressurized oxygen was bubbled into the system at a steady rate of 0.5 L min-1, so scale-up of the process will be convenient. 3.3. Treatment of the Pure Standards and Wastewater Samples. The performance of the NaClO and iron powder redox pair with respect to all of the components (pure standards) in the wastewater are listed in Table 2. Some of the amino groups could be oxidized into phenolic derivatives (Scheme S3), leading to degradation of the common structural unit, and subsequently low yields of ASBA (29-31). Although the amino compounds account for only a small proportion in the wastewater, they are relatively difficult to transform; most of the components can be transformed into the desired product (4-amino-2-sulfobenzonic acid, ASBA) in yields of more than 90% (Table 2). The NaClO/Fe redox pair was then used to treat the wastewater samples. A typical HPLC chromatogram illustrating the results of the treatment process is shown in Figure 1b. The detailed quantitative methods used to determine the proportion of each ASA are recorded in the Supporting Information (Section 4). It was found that the ASAs in the DNS wastewater had been transformed into ASBA;
FIGURE 3. Solubility curve of ASBA as a function of solution pH (the intrinsic pH of the ASBA solution is ∼1, and the pH of the solution was adjusted by addition of 2 N NaOH aqueous solution). the percentage conversion from the raw material (NTS) could be as high as 90% (i.e., ∼5.3 kg of ASBA of purity >98% can be obtained from every cubic meter of DNS wastewater). 3.4. Separation and Purification of ASBA. After the redox treatment, the wastewater contains a mixture of ASBA and NaCl. ASBA is an ampholyte; it contains both -NH2 and -SO3H groups. The solubilities of ampholytes should follow the Henderson-Hasselbalch (HH) equation (32) log S ) log S0 + log(1 + 10pH-pKa(acid) + 10pKa(base) - pH) (1) where S is the solubility at a given pH value, S0 is the intrinsic solubility, and pKa is the dissociation coefficient of the acid or the base. The pKa values of ASBA predicted by Marvin software were pKa (acid) ) -3.64 and pKa (base) ) 1.88. According to the above equation and predicted pKa values, the ASBA solubility should decrease with decreasing pH under acidic conditions. Consequently, the acidification precipitation method should be effective for the separation of ASBA. The optimum pH value for the ASBA precipitation was determined by measuring the solubility of ASBA by the static analytical method (Section 3 in the Supporting Information); its pH-solubility curve is shown in Figure 3. It was found that the solubility of ASBA decreases rapidly with decreasing pH, which is in agreement with eq 1. A pH value of ∼1 was therefore used in the precipitation to obtain ASBA with a
TABLE 2. Average Proportion of Each ASA in the Wastewater and Yields of the Pure Standards Transformed to ASBA by Redox Treatment
abbreviation
yielda (%)
proportionb (mol %)
abbreviation
yielda (%)
proportionb (mol %)
ASBA NSBA Trans-DSD Trans-ANSD NTS DNS-epoxide
50 98 40 82 92 97
1.9 2.8 0.9 2.2 3.2 10.1
Trans-DNS Cis-DNS NSB ASB DND Poly-DNS
95 95 98 48 93 90
65.2 9.2 98%. After precipitation of ASBA with 30% HCl, the wastewater was evaporated to obtain the byproduct (NaCl). 3.5. Alkaline Fusion of ASBA to PAS. The resulting ASBA can then be treated with NaOH by alkaline fusion at 230 °C to give PAS with a yield of >95% (Scheme 4). Due to the presence of the -NH2 group on ASBA, it is necessary to exclude the oxygen in the autoclave before alkaline fusion to prevent the oxidation of the amino group. 3.6. Environmental Impact of the New Recycling Method. E-factors, which are a measure of the mass of waste produced by a method, compared to the mass of useful material obtained, were calculated to quantify the cost savings based on the use of DNS wastewater for the production of paramycin, as compared to the conventional methods for the preparation of paramycin (Table 3). Some types of waste cannot be reused in the corresponding production, but they are useful in other fields and were defined as recoverable wastes in the calculation of the corresponding E′-factors ((mass of total waste minus the mass of recoverable wastes) per mass of product). For example, iron mud can be recycled to produce iron oxide red, sponge iron, and FeSO4 · H2O; Na2SO3 can be recycled as a mild reductant (33, 34). Detailed information on the three paramycin-production methods are listed in Scheme S4 in the Supporting Information. It can be seen from Table 3 that the E′-factor for this new paramycin-production method (method I) is smaller than those for the two conventional methods (method II and III). This new method also significantly decreases the E′-factor for the oxidation process from NTS to DNS (from 0.2 to 0.017). The nitro groups of the wastes remaining after the recycling process were reduced to amino groups, which increases the degradability of the remaining organic wastes (35). Furthermore, the -SO3H groups of some of the remaining organic wastes were transformed into -OH groups, which will decrease the solubilities of the organic wastes. Both of these transformations will be convenient for treating the remaining wastes (36, 37). In addition, the organochlorine content (wt %) of the remaining organic wastes was determined to be
