Ind. Eng. Chem. Res. 2003, 42, 5959-5963
5959
Destruction of Cyanide in Water Using N-Chlorinated Secondary Sulfonamide-Substituted Macroporous Poly(styrene-co-divinylbenzene)† Yu Zhang, David W. Emerson,* and Spencer M. Steinberg Department of Chemistry, The University of Nevada, Las Vegas, Box 454003, Las Vegas, Nevada 89154-4003
Cyanide ion in water is converted to cyanate ion by use of macroporous poly(styrene-codivinylbenzene) bearing regenerable N-chlorinated N-alkylsulfonamide functional groups. Cyanide is converted first to cyanogen chloride that is then rapidly hydrolyzed to cyanate ion, which is stable at high pH. Cyanate ion can be further oxidized by active chlorine supplied by the resin to nitrogen and carbonates under weakly basic conditions or more rapidly hydrolyzed to ammonium ion and carbon dioxide by passage through a bed of a strong acid cation-exchange resin in the H+ form without use of additional active chlorine. Much of the active chlorine that escapes the reactor can be recaptured by unchlorinated polymer for subsequent use. Introduction Wastewaters from various industrial operations and ore processing wastewaters may contain CN- (cyanide) in low concentration that must be decomposed before the water is released to the environment. An extensively used technology is a two-stage, liquid-phase process in which oxidation of the CN- by a strongly basic chlorine solution occurs in the first stage. In the second stage, the pH is lowered and more chlorine is added which oxidizes NCO- (cyanate) produced in the first stage to N2 and carbonate.1,2 Use of insoluble, oxidizing polymers whose oxidizing component is easily restored by slightly acidified bleach is being explored3-10 as an alternative to the solution-phase method. Studies by Bogoczek and Kociołek-Balawejder showed that macroporous styrene-divinylbenzene (DVB) polymers bearing N,N-dichlorosulfonamido groups 1 (PolSO2NCl2) remove CN- from water at high pH under both batch and flow-through conditions.6-10 In even moderately basic solutions, 1 hydrolyzes rapidly to form ClO- (hypochlorite) and the anion 2 (Pol-SO2NCl-), which is reasonably resistant to further hydrolysis. Anion 2 is the conjugate base of an unstable acid resembling N-monochloro-p-toluenesulfonamide (MCT), which has a pKa of 4.55.11 The result is waste of some of the active chlorine if the concentration of rapidly released ClO- considerably exceeds the concentration of CN- in the solution being treated. Macroporous styrene-DVB polymers bearing N-chloroN-alkylsulfonamido groups 3 (Pol-SO2NRCl)4 are less extensively hydrolyzed to produce ClO- than 1 because 4 (Pol-SO2NR-) is a much stronger base than 2. Anion 4 is the conjugate base of a very weak acid, 5 (PolSO2NRH), with pKa > 12.12,13 We undertook a study using 3 for decomposing CN-. There is a possibility that cyanide may react directly with 3 without action of ClOin solution. Studies on N-chlorinated small molecules † Portions of this material were excerpted from the Thesis of Yu Zhang, dated Aug 1997, submitted in partial fulfillment of the requirements for the degree of Master of Science in Chemistry. * To whom correspondence should be addressed. Fax: (702) 895-4072. E-mail:
[email protected].
in aqueous solutions showed that active halogen is transferred rapidly from one N-chlorinated compound to another nitrogen-containing compound, or to nucleophiles such as phenols, without participation by intermediates such as ClO- or ClOH (hypochlorous acid) that could result from hydrolysis of the N-chloro compounds.14-18 Moreover, these workers showed that the active chlorinating compound involved in the ratedetermining step of chlorination of cresol was dichloramine-T (DCT, N,N-dichloro-p-toluenesulfonamide),16 which bears a close resemblance to 1. It arose from disproportionation of MCT to DCT and p-toluenesulfonamide, which resemble 1, the conjugate acid of 2, and the unchlorinated primary sulfonamide form of the polymer, respectively. Materials and Methods Resins. Primary and secondary sulfonamide-substituted macroporous poly(styrene-co-DVB) resins were prepared from Amberlyst-15 (6) as described elsewhere.3,4 The secondary sulfonamide was Haloscrub A15-2,2 [mostly Pol-SO2NH(CH2)2NHSO2-Pol4,19 (5a), in which 1,2-diaminoethane was the primary amine used in the preparation]. The resins 5a were chlorinated to 3a using commercial laundry bleach, nominally 5.25% sodium hypochlorite with 5 vol % of glacial acetic acid added. Resin in a packed column was regenerated by pumping the acidified bleach into the column containing resin or by removing the resin from the column, chlorinating it externally, and reinstalling it in the column. Analysis of Functional Groups in Resins. These resins have some sulfonic acid groups as well as the sulfonamide groups.3,4 Total exchange capacity for strong acid groups was performed by a standard method;20 the active chlorine content was determined either by the hydrazine method3 or by oxidation of sulfite ion.5 Analysis of Intermediates and Products. The substances were analyzed by well-accepted methods as follows: active halogen in water by the diethyl-p-phenylenediamine (DPD)-iron(II) titrimetric method;21 ammonia,21,22 chloride,22 and cyanide21,22 by electrode methods; cyanogen chloride21,22 and cyanate21 by colorimetric methods.
10.1021/ie030151l CCC: $25.00 © 2003 American Chemical Society Published on Web 10/09/2003
5960 Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003
Apparatus for Flow Experiments. The flow experiments were conducted in an apparatus previously described.19 For two-stage experiments, the effluent from the first reactor was passed into a second reactor that contained 6 (H+). A sampling port was placed between the two reactors. Decomposition of Cyanide, Batch Scale. A 1 g sample of chlorinated resin 3a (2.18 mmol of Cl/g) was added to a 1 L volumetric flask containing a solution of 1 mmol of potassium cyanide and 1 mmol of sodium hydroxide. The mixture was stirred, and samples were withdrawn at fixed time intervals and analyzed for cyanate, cyanide, cyanogen chloride, ammonia, and chloride. At the conclusion, the resin was removed and analyzed for active chlorine. Effect of pH on Cyanide Decomposition, Batch Scale. Sodium carbonate (0.1 M) buffers of pH 13, 12, and 11 were used with 1 mmol of potassium cyanide to make 1 L solutions. A total of 2 g of resin 3a (2 mmol of Cl/g) was added to each of the three solutions, stirring was begun, and samples were withdrawn periodically for nondestructive cyanide analysis by electrode methods, after which the samples were returned to the flask. After each run, the resin was removed for residual active chlorine analysis. Temperature Effect on Cyanide Decomposition, Batch Scale. The rates of cyanide decomposition at 0, 25, and 40 °C were studied using 1 L solutions containing 1 mmol of potassium cyanide in a 0.1 M phosphate buffer of pH 11 and 1 g of 3a (2 mmol of Cl/g). Release of Active Chlorine from 3a at Different Mole Ratios of Active Cl to Cyanide, Batch Scale. Two-, four-, and eightfold excesses of 3a were added to three volumetric flasks each containing 1 L of a solution of 1 mmol of potassium cyanide and a phosphate buffer of pH 10. The flasks were stirred, and samples were withdrawn after fixed time intervals and analyzed nondestructively for free cyanide. After analysis, the samples were returned to the flasks. Oxidation of Cyanate, Batch Scale. A 20 g sample of 3a (2.18 mmol of active Cl/g) was added to a 1 L volumetric flask containing 10 mmol of potassium cyanate and a 0.1 M phosphate buffer of pH 10. The mixture was stirred, and samples were withdrawn at intervals and analyzed for cyanate. The resin was separated and analyzed for active Cl. Hydrolysis of Cyanate Using 6 (H+), Batch Scale. A 10.99 g sample of 6 (4.37 mequiv of H+/g of dry resin) was placed in a 1 L volumetric flask containing 10 mmol of potassium cyanate. The mixture was stirred, and samples were withdrawn at timed intervals and analyzed for cyanate. Release of Active Chlorine from 3a in a Flow System at Different Flow Rates. A reactor tube, 2.2 cm i.d. × 31.0 cm height, was equipped with a foundation of glass wool and a mixture of 3 and 4 mm glass beads. When resin 3a was swollen (20 g, 40 mL), 1.5 mmol of Cl/g was introduced, making a 10.5 cm column of resin. Sodium hydroxide solutions, either 2.5 × 10-2 or 1.0 × 10-2 M, were pumped through the reactor at 5 mL/min (7.5 bed volumes/h), 10 mL/min (15 bed volumes/ h), or 15 mL/min (22.5 bed volumes/h). Consecutive 100 mL samples were analyzed for active chlorine. Hydrolysis of Cyanate in a Flow System. A reactor was loaded with 20 g of 6 (4.9 mmol of H+/g), with a bed volume of 55 mL after swelling with water and a column height of 15 cm. A solution of 1 × 10-2 M
potassium cyanate was pumped through the reactor at 5 mL/min (5.5 bed volumes/h), 10 mL/min (11 bed volumes/h), 15 mL/min (16.5 bed volumes/h), and finally 5 mL/min again. Consecutive 100 mL samples were analyzed for ammonia and cyanate until breakthrough of cyanate occurred. Cyanide Oxidation in the Flow System. With the column, resin, and methodology described above, solutions of 10 mM potassium cyanide and either 25 or 10 mM sodium hydroxide were pumped through the reactor at 5, 7.5, 10, and 15 mL/min (7.5, 15, and 22.5 bed volumes/h, respectively). Consecutive 100 mL samples were analyzed for pH, cyanide, cyanogen chloride, cyanate, ammonia, chloride, and active chlorine until breakthrough of cyanide occurred. Cyanide Decomposition with Concomitant Hydrolysis of Cyanate in the Second Stage. After appropriate regeneration of the two resins (3a in the first column and 6 in the second), a solution of 10 mM potassium cyanide and 25 mM sodium hydroxide was pumped through the system at 5 mL/min (7.5 bed volumes/h in the first column and 5.5 bed volumes/h in the second). Consecutive 100 mL samples were analyzed for pH, cyanide, cyanogen chloride, cyanate, ammonia, and chloride until the breakthrough point of cyanide from the first reactor occurred. Scavenging and Recovery of Active Cl from a Solution at pH 12.3. A 30 mM active chlorine solution was prepared from bleach and a phosphate buffer solution. This solution was pumped at 0.3 L/h (6.5 bed volumes/h) through a bed of 20 g of 5a (dry sulfonic acid groups in the Na+ form) that had a chlorine-holding capacity of 1.6 mmol/g. A total of 100 mL of the buffer was pumped through the system and discarded, after which samples of 150, 307, 250, 256, and 260 mL were withdrawn and analyzed for active Cl by the DPDiron(II) titrimetric method.21 The percentages of removal of active Cl for these samples were 99.6, 97.7, 86.1, 74.5, and 63.6%. The percentages of the Cl-holding capacity of the resin (32 mmol) used up were 18, 53, 73, 85, and 93% for these samples. Results and Discussion In batch and flow-through systems, some of the active chlorine of 3a resides on the insoluble polymer and some is in solution as ClO- when the pH is high enough (g10) to maintain the cyanide mainly as CN- rather than the very weak acid HCN. Cyanide is known to react with ClO- to form ClCN (cyanogen chloride), which is subsequently hydrolyzed to NCO- under basic conditions [reactions (1) and (3)].2 However, cyanide ion is a small, reasonably aggressive nucleophile that might compete directly for the active chlorine of 3a, without hydrolysis of the resin, to form ClCN, a known intermediate in the chain of reactions occurring during the decomposition of cyanide by active chlorine. Thus, we propose the sequence of reactions (1)-(5) that incorporate two pathways, reactions (1) and (3), both of which lead to the formation of the unstable intermediate ClCN. Reaction (2) produces ClO-. Reaction (4) is hydrolysis of ClCN to NCO-. Reaction (5) is an alternate way to remove NCO- and produces an effluent possibly suitable for irrigation or disposal through a wastewater treatment facility that employs denitrification.
Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003 5961
Figure 1. Batch-scale reactants, and products during CN- contact with 3a (1 g of resin, 2.18 mmol of SO2NClR/g, 1 mM KCN, 1 mM NaOH): (b) Cl-; (9) CN-; (2) CNCl; (1) CNO-; ([) NH3; (`) SO2NClR (lost).
Pol-SO2NRCl + CN- + H2O f Pol-SO2NRH + ClCN + HO- (1) Pol-SO2NRCl + HO- f Pol-SO2NRH + ClO- (2) ClO- + CN- + H+ f ClCN + HO-
(3)
ClCN + 2HO- f NCO- + H2O + Cl-
(4)
NCO- + H2O + 2H+ f NH4+ + CO2
(5)
Batch Experiments. The oxidation/hydrolysis system was tested in batch mode, and the concentrations of several components were measured. The CN- concentration declined steadily while NCO- and Cl- rose steadily, and ammonia and ClCN were almost imperceptible (Figure 1). The rate of disappearance of CNincreased as a function of pH, which may reflect the increase of active Cl in solution with increasing pH (Figure 2). Similar rate increases were seen with increases in the mole ratio of active Cl on the resin to CN- (Figure 3). The rates of disappearance of CN-, measured at 0, 25, and 40 °C, showed a typical temperature dependence (Figure 4). Hydrolysis of NCO- to NH4+ and CO2 in the presence of an acid ion-exchange resin is much faster than oxidation of NCO- in basic solution to nitrogen and CO32-, as shown in Figure 5. Flow-through Reactor Experiments. Pumping water of differing hydroxide concentrations and different flow rates revealed that the concentration of active Cl in the effluent from a reactor containing 3a is greater at 5 mL/min than at 15 mL/min at the same [OH-] and less at lower [OH-] than at higher [OH-] at the same flow rate (Figure 6). When CN- and OH-, each at a concentration of 10 mM, were used as feed and the feed rate was varied, the results shown in Figure 7 were obtained. It is evident that rate-controlled processes, some combination of reactions (1)-(3), are involved and that maximum use of the active chlorine occurs at a flow rate of 7.5 bed volumes/h or fewer at ambient temperatures. The resin loading was 30 mmol of active Cl on 20 g of resin 3a. Breakthrough of unreacted CNoccurred at 0.90, 1.5, and 1.8 L at feed rates of 15, 10, and 5 mL/min, respectively. Active Cl consumed in converting CN- to ClCN, assuming a 1:1 stoichiometry,
Figure 2. Batch-scale concentration of CN- during contact with 3a at a pH range from 11 to 13 (2 g of resin, 2 mmol of SO2NClR/g, 1 mM KCN, 0.1 mM carbonate buffer).
Figure 3. Batch-scale effect of the mole ratio of KCN/active Cl in 3a (1 g of SO2NClR/g, pH 11 phosphate buffer).
would thus be 9, 15, and 18 mmol, respectively. There was a substantial amount of active chlorine in the effluent of this reactor. In another experiment conducted with the same active chlorine loading as above with concentrations of CN- and OH- at 10 and 25 mM, respectively, and a flow rate of 10 mL/min, the pH and active Cl content of the effluent were measured, as was the CN- breakthrough volume of 75 mL/g (1.5 L total) (Figure 8). It is possible to estimate a material balance using data from Figure 8. Through 1.5 L of feed (75 mL/g of resin), 15.2 mmol of CN- was removed, requiring 15.2 mmol of active Cl (eqs 1 and 3). There was little ammonia produced, and 10.5 mmol of NCO- was produced. We infer that 4.9 mmol of NCO- was further oxidized to carbonate and N2, requiring 7.3 mmol of
5962 Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003
Figure 4. Batch-scale effect of the temperature on the decomposition of CN- at pH 11 (1 g of 3a, 2 mmol of SO2NClR/g, 1 mM KCN).
Figure 6. Active Cl in the effluent at different NaOH concentrations and flow rates (20 g of 3a, 1.5 mmol of SO2NClR/g): (b) 2.5 mM NaOH at 15 mL/min; (9) 2.5 mM NaOH at 5 mL/min; (2) 1.0 mM NaOH at 15 mL/min.
Figure 5. Comparison of the rates of decomposition of NCO- by acid hydrolysis or oxidation by 3a.
active Cl (eqs 6 and 7). The amounts of carbonate and nitrogen were not measured. The active Cl escaping was
3HOCl + 2NCO- f 3Cl- + 2CO2 + N2 + H+ + H2O (6) 3ClO- + 2NCO- + H2O f 3Cl- + 2HCO3- + N2 (7) 1.6 mmol, and measurement of active Cl remaining on the resin later in a run was 0.11 mmol/g or 2.2 mmol. The total active Cl accounted for is thus 26.3 mmol. Considering the possible errors in the many measurements, this is in fair agreement with the 30 mmol of active Cl loading of the resin. The pH of the effluent was surprisingly low for the first two-thirds of the run. This may indicate that some active Cl is lost by reaction with components of the resin, such as oxidation and/or substitution. Decomposition of NCO- by hydrolysis under acidic conditions is rapid in a flow system. When the effluent from the flow-through reactor containing 3a is subsequently passed into a flow-through reactor containing the poly(styrene-DVB) sulfonic acid cation exchanger, 6 (H+ form), the resulting products are 6 (NH4+ form)
Figure 7. Breakthrough volume of CN- at various flow rates (20 g of 3a, 1.5 mmol of SO2NClR/g, 0.01 M NaOH).
and CO2 [reaction (5)]. Negligible quantities of CN-, ClCN-, NCO-, and NH4+ were found in the effluent. This reaction converts NCO- to NH4+ without consuming any active chlorine. The oxidation pathways [reactions (6) or (7)], however, require 1.5 mol of active chlorine/mol of NCO-. Under some circumstances, operating the system to produce ammonium salts could be an advantage depending on the disposition of the effluent water. The ammonia could be used to fertilize a tailings pile if restoration of plant life was desired and consumption of chlorine would be minimized. A previous study demonstrated the effectiveness of the Haloscrub family of resins for scavenging active Cl, Br, and chloramines at a pH between 7 and 8.7.19 However, it was not known whether such resins would scavenge active Cl from a solution at high pH. An experiment showed that 5 removed up to 86% or more of the active Cl from a solution at pH 12.3 until 73% of the chlorine-holding capacity of the resin was reached.
Ind. Eng. Chem. Res., Vol. 42, No. 24, 2003 5963
is available free of charge via the Internet at http:// pubs.acs.org. Literature Cited
Figure 8. Composition of the effluent with CN- and OH- at 1 × 10-2 and 2.5 × 10-2, respectively (Cl load on 10 g of 3a, 1.5 mmol of SO2NClR/g, flow rate of 10 mL/min): (9) CN-; (2) CNCl; ([) NH3; (`) pH; (1) active Cl (mM).
This shows that much of the active Cl in the effluent of the first reactor should be recoverable by interposing a reactor containing 5 between the first and second reactors. Thus, less fresh hypochlorite would be required to regenerate the resin than if active Cl were allowed to escape. Conclusions These results show that having active chlorine partly fixed in secondary sulfonamide groups on a matrix of insoluble polymer and partly in the surrounding solution is highly effective at decomposing cyanide in a synthetic mixture. Wastewater samples from “real sources” were not available to us. The question of whether cyanide complexes would be degraded by this method was not addressed. Most of the active chlorine that escapes the reactor can be trapped on a second column containing unchlorinated, sulfonamide-containing polymer, 5, thus reducing the amount of active chlorine needed to regenerate the N-chlorinated form of the polymer, 3a. Further reduction in chlorine demand can be attained by hydrolysis under acidic conditions of the intermediate cyanate to ammonium ion and carbon dioxide. This reaction is much faster than oxidation of cyanate to nitrogen and carbonate ion by active Cl under basic conditions. Declines in the pH of solutions passed through the oxidizing reactor indicate that acid is being produced by some components of the system and hydroxide is being consumed. We intend to investigate possible sources of this acidity and whether the resins are being degraded under these severe conditions. Supporting Information Available: Tables 1-8 from which Figures 1-8 were prepared. This material
(1) Palmer, S. A. K.; Breton, M. A.; Nunno, T. J.; Sullivan, D. M.; Surprenant, N. F. Metal/Cyanide Containing Wastes: Treatment Technologies; Noyes Data Corp.: Park Ridge, NJ, 1988; pp 612-624. (2) Lanouette, K. H. Chem. Eng. 1977, 84 (22), 73. (3) Emerson, D. W.; Shea, D. T.; Sorensen, E. M. Ind. Eng Chem. Prod. Res. Dev. 1978, 17, 269. (4) Emerson, D. W.; Gaj, D.; Grigorian, C.; Turek, J. E. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1982, 23, 289. (5) Emerson, D. W.; Grigorian, C.; Hess, J. W.; Zhang, Y. React. Funct. Polym. 1997, 33, 91. (6) Bogoczek, R.; Kociołek-Balawejder, E. Polym. Commun. 1986, 27, 286. (7) Bogoczek, R.; Kociołek-Balawejder, E. Angew. Makromol. Chem. 1989, 169, 119. (8) Bogoczek, R.; Kociołek-Balawejder, E. Pol. PL 151267 1990; Chem. Abstr. 1991, 114, 128453. (9) Kociołek-Balawejder, E. React. Funct. Polym. 1997, 33, 159. (10) Kociołek-Balawejder, E. Eur. Polym. J. 2000, 36, 295. (11) Morris, J. C.; Saltar, J. A.; Wineman, M. A. J. Am. Chem. Soc. 1948, 70, 2026. It should be noted here that, in another context, cross-linking was shown to increase the pKa’s of carboxylic acid polymers in proportion to the extent of cross-linking. Fisher, S. A.; Kunin, R. J. Phys. Chem. 1956, 60, 1030. (12) Min, K.-E.; Lee, H.-K.; Klee, D.-H. Polymer (Korea) 1985, 9, 68. (13) Steinberg, S. M.; Emerson, D. W. The University of Nevada, Las Vegas, unpublished results, 2001. (14) Higuchi, T.; Hasegawa, J. J. Phys. Chem. 1965, 69, 796. (15) Higuchi, T.; Ikeda, K.; Hussain, A. J. Chem. Soc. B 1967, 546. (16) Higuchi, T.; Hussain, A. J. Chem. Soc. B 1967, 549. (17) Higuchi, T.; Hussain, A.; Pitman, I. H. J. Chem. Soc. B 1969, 626. (18) Hussain, A.; Higuchi, T.; Hurwitz, A.; Ptitman, I. H. J. Pharm. Sci. 1972, 61 (3), 371. (19) Emerson, D. W. Ind. Eng. Chem. Res. 1988, 27, 1797. The “A15-2,2” part of the name indicates that the material was prepared from Amberlyst-15, that there are two carbons and two nitrogens in the functional group, and that both nitrogens are bonded to sulfonyl groups on the polymer. Resins 5 have been prepared from various primary alkylamines and alkylenediamines. They display similar chemical behavior.4,5,19 (20) Kunin, R. Ion Exchange Resins, 2nd ed.; Wiley: New York, 1958; p 341 ff. (21) Standard Methods for the Examination of Water and Wastewater, 19th ed.; American Public Health Association, American Water Works Association, Water Environment Association: Washington, DC, 1995. (a) Active chlorine: 4500-Cl F, DPD Ferrous Titrimetric Method. (b) Cyanide, titrimetric method: 4500-CN- D; cyanide-selective electrode method: 4500-CN- F; cyanogen chloride: 4500-CN- J; cyanate: 4500-CN- L. (c) Ammonia: 4500-NH3 D. (22) ASTM Research Report; American Society for Testing and Materials: Philadelphia, PA, 1996. (a) Ammonia nitrogen in water: D 1426-93. (b) Chloride ion in water: D 512-89. (c) Cyanides in water: D 2036-91. (d) Cyanogen chloride in water: D 4165-95.
Received for review February 18, 2003 Revised manuscript received May 21, 2003 Accepted July 27, 2003 IE030151L
