Environ. Sci. Technol. 2001, 35, 1379-1384
Decomposition of β-Alaninediacetic Acid and Diethylenetriaminepentaacetic Acid by Hydrogen Peroxide in Alkaline Conditions M I K A E . T . S I L L A N P A¨ A¨ * , † , ‡ A N D J A A K K O H . P . R A¨ M O ¨ ‡ Laboratory of Environmental Protection Technology, Helsinki University of Technology, P.O. Box 6400, FIN-02015 Hut, Finland, VTT Chemical Technology, P.O. Box 1401, FIN-02044 Espoo, Finland
FIGURE 1. Pilot-plant scale flow-through system used in the experiments.
The chemical decompositions of β-alaninediacetic acid (ADA) and diethylenetriaminepentaacetic acid (DTPA) were studied in a pilot-plant flow-through system simulating alkaline (pH 10-11) hydrogen peroxide bleaching environments. The amount of hydrogen peroxide decomposition was evaluated, and the distribution calculation was performed. Under the conditions investigated, ADA was more degradable than DTPA (average residual 71% vs 94%). The decomposition of hydrogen peroxide was not dependent on the chelate; the residual percent of hydrogen peroxide was 40 in both cases.
Introduction Complexing agents are extensively used by the pulp and paper industry to form stable water-soluble chelates with transition metal ions and to remove these metals from the pulp before hydrogen peroxide bleaching (1-3). Since complexing agents also prevent the contact of transition metals and hydrogen peroxide, they reduce the catalytic decomposition of the bleach (4). The conventional agents are ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA). In addition to total chlorine-free (TCF) bleaching, peroxide stages are becoming more common in elemental chlorine-free (ECF) sequences, so that the total use of sequestering agents is increasing. There may be environmental impacts related to the use of these compounds. As nitrogen-containing compounds, they increase the total nitrogen content of wastewaters. They also have been shown to remobilize toxic heavy metals from soils and sediments into the overlaying water phase, extending the biological life cycles of heavy metals (5-7). In view of the initial speciation of the complexing agents and their slow cation-exchange kinetics, however, this last effect should not be overestimated (8). As a third effect, ligand removal of iron from precipitated phosphates converts the phosphate to soluble form (9). On the basis of these considerations, it is essential that an effective way is developed for the removal of chelating agents or that more degradable compounds, such as hydroxycarboxylic acids (10), phosphonic acids, and * Corresponding author present address: Laboratory of Water Resources and Environmental Engineering, University of Oulu, P.O. Box 4300, FIN-90014, University of Oulu, Finland; telephone: 3588-5534500; fax: 358-8-5534399; e-mail:
[email protected]. † Helsinki University of Technology. ‡ VTT Chemical Technology. 10.1021/es000167s CCC: $20.00 Published on Web 02/22/2001
2001 American Chemical Society
aminopolycarboxylic acids other than DTPA and EDTA (11) are taken under consideration. Since neither biochemical (12, 13) nor photochemical (14, 15) degradation rates of DTPA and EDTA are rapid enough to exclude concern about their ultimate release to the environment, it would be highly desirable if they or an alternative chelating agents were chemically degraded already in the industrial bleaching stage. We have compared the oxidative decomposition of DTPA and an alternative complexing agent β-alaninediacetic acid (ADA) under simulated hydrogen peroxide bleaching conditions. ADA is readily biodegradable (degradation of 98% has been obtained in laboratory-scale activated sludge simulation) and has exceptionally low toxicity (for example, no inhibition was observed in the Daphnia magna 24-h test up to a concentration of 1 g/L) (16). It also improves the whiteness gains of hydrogen peroxide bleaching processes (17). Since degradation may be significantly dependent on the chemical speciation, the distribution of the different metal chelates of DTPA and ADA was estimated from the assumed prevailing thermodynamical equilibrium. In addition, to compare the technical performance, the ability of the two agents to protect hydrogen peroxide against decomposition was studied by determining the residual percent of the bleach.
Materials and Methods Autoclave Simulations. The purpose of our experiments was to study the durability of ADA and DTPA in alkaline hydrogen peroxide environments. The experimental results were compared with the calculated concentrations, and the degradation percentages were calculated on this basis. In addition, the degree of decomposition of hydrogen peroxide was evaluated. Pulp was not added to the autoclave because it blocks the pipes and makes stabilization of the hydrogen peroxide level extremely difficult. The achievement of constant hydrogen peroxide concentration is, of course, essential in making direct comparisons between the abilities of chelating agents, which was the object of our study. Since both ADA and DTPA are extremely hydrophilic, no adsorption to pulp is expected to occur, and the absence of pulp should not interfere with the results. The stablization of the experimental system was highly time-consuming and demanded much work before reproducible runs could be performed. The organic load of the actual process makes the chelating agents less vulnerable to chemical oxidation, but the organics are not expected significantly to compete with chelating agents for metal complex formation. Both ADA and DTPA are thermodynamically exceptionally stable compounds. Experiments with ADA and DTPA were executed in the pilot-plant system presented in Figure 1. Two flows were VOL. 35, NO. 7, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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organic phase was dried with anhydrous Na2SO4 and analyzed with a Hewlett-Packard HP5860 series II Plus GC-NPD (gas chromatograph equipped with a nitrogen-phosphorus detector). The reproducibilities of the analytical procedures are at maximum 6%. The procedures are described in detail in refs 18 and 19. Total hydrogen peroxide concentration ([H2O2] + [HO2-]) was determined immediately after the sampling by adding 4 mL of concentrated sulfuric acid, 10 mL of 10% KI, 50 mL of deionized water, and 3 drops of 3% ammonium molybdate to 1 mL of sample. The solution was titrated iodometrically with 2 M sodium thiosulfate solution with use of starch as an indicator (20). Thus, the percentage error is at most 10%. pH was controlled continuously, using a CONTRONIC STE140 glass electrode. Metals strongly affect the behavior of hydrogen peroxide (21-23) as well as the speciation of chelating agents. In alkaline environments, metals exist as oxides, oxyhydroxides, carbonates, colloidal hydroxides, or complex compounds. Since the colloids in some degree may adsorb on the walls of the reservoir and piping, there is a need to monitor the metal concentrations, and this was done after pH and hydrogen peroxide had reached steady-state level. Metal analyses were initiated by adding 50 mL of concentrated nitric acid to 50 mL of effluent of the autoclave. The solutions were analyzed by AAS using a Perkin-Elmer 4100 flame atomic absorption spectrometer. Speciation Simulation. Metal (M) interacts with organic ligand (L) (L ) DTPA, ADA) and proton (H) according to the following equations (the charges are different and are omitted for simplicity) (24):
FIGURE 2. Experiment with DTPA: (a) pH (×), total hydrogen peroxide concentration ((), and HO2- concentration ()); b) total concentrations of DTPA and metals. diverted through a mixing autoclave; one a hydrogen peroxide flow (21 mL/min) and the other (115 mL/min) consisting of chelating agent, hydroxide, and metals present at the typical concentrations found in Finnish bleaching systems (Figure 1). The metal concentrations were chosen on the basis of the results of filtrate metal analysis performed in the actual mill process waters. In actual processes, metal concentrations vary with the wood, raw water, and purity of the chemicals. Chelating agents were used in stoichiometric excess of 1.3. The temperature was set at 60 °C, corresponding to the actual process. The stream from the chelating agent, hydroxide, and metals reservoir was pumped through the autoclave before start up of the hydrogen peroxide pump, which was set as the zero point of time, and samples for the total hydrogen peroxide analyses were taken at 20-30-min intervals. When pH and hydrogen peroxide concentration had reached the target levels (pH 10.5, hydrogen peroxide concentration 1200 mg/L), hydrogen peroxide flow was slowed from 27 to 21 mL/min. As a result, the hydrogen peroxide concentration stopped increasing, and a steady-state condition (pH 10.610.7, hydrogen peroxide concentration 1000 mg/L) was achieved. Chemicals. All chemicals were of analytical grade and were purchased from Merck. Used water was Milli-Q (18 MΩ/cm). Hydrogen peroxide (32%) was obtained from Baker. Analytical Procedures. Samples were analyzed for total chelating agents according to the following procedure. The sample was evaporated to dryness, pH was dropped, and the chelating agent was derivatized to the corresponding ethyl ester. After esterification, the sample was extracted into toluene and neutralized with 1 M KHCO3. The extracted 1380
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aH + L S HaL
(1)
aH + bM + cL S HaMbLc
(2)
bM(aq) S Mb(OH)a + aH
(3)
H2O S H + OH
(4)
Equation 1 describes the ligand protonation, eq 2 describes the metal complex formation, eq 3 describes metal hydrolysis and solubility of metal hydroxides, and eq 4 describes the ion product of water. The term “uncomplexed” refers to the ligands that are not metal-bound but can be protonated depending on pH. The complexation of metals by ligands in a solution of known metal and ADA and DTPA concentrations can be presented as curves of percentage distribution of metals among the complex species as a function of pH. The distributions were calculated for constant conditions of 25 °C and I ) 0.1. The values for hydrolysis of the metals and the constants of protonation and metal complexation are taken from refs 25 and 26. The distribution curves have been drawn for complexing agents and iron at the concentration levels presented in Figures 2 and 3 using the SPE program (27).
Results and Discussion The theoretical concentrations of chelating agents and hydrogen peroxide were calculated using the differential equation for an ideal blender (5) in which the outcoming stream mirrors the situation inside the blender. The values obtained describe the theoretical concentrations when no degradation occurs. The residual percentages of complexing agents were calculated by dividing the experimental result by this theoretical value:
FIGURE 3. Experiment with ADA: (a) pH (×), total hydrogen peroxide concentration ((), and HO2-concentration ()); (b) total concentrations of ADA and metals.
d(Cout(t)V) ) Cin(t)Q - Cout(t)Q dt
(5)
where V is the volume of the blender, constant, 13.3 L; Q is the stream going through blender, constant, 0.1365 L/min; Cin is the ingoing concentration, 0.034279 mmol/L for chelating agent, 7502 mg/L for hydrogen peroxide; and Cout is the outcoming concentration and concentration in the blender. Since V and Q remained constant, we obtain
∫
Cout(t)
Cout(0)
d(Cout(t)) (Cout(t) - Cin(t))
)-
Q V
∫ dt t
0
(6)
Integration and rearrangement give the concentration of the outcoming stream, which is equal to the concentration inside the blender:
Cout(t) ) e-Qt/V (Cout(0) - Cin) + Cin
(7)
Adsorption on the walls of the reservoir and piping (28) would cause the concentration of complexing agents entering the reactor to be slightly less than the concentration added to the reservoir. The residual percentages of the agents would then be greater than the calculated values. To check for possible adsorption, we determined the concentrations of ADA and DTPA before the peroxide addition. No adsorption was found, i.e., the calculated and measured concentrations of the liquid phase were the same. Evidently adsorption was insignificant in the present study because ADA and DTPA are highly hydrophilic. Since EDTA and DTPA are not expected to adsorb onto the pulp in actual bleaching
FIGURE 4. Percentage distribution of DTPA (a) and iron(III) (b) in the steady-state situation of the experiment with DTPA (43). processes (29), we furthermore assume that the absence of pulp did not significantly not affect peroxide bleaching simulation. As indicated in the Experimental Section and in Figures 2 and 3, conditions in the autoclave vary in the course of the experiment. At the beginning of both experiments, the pH value decreased after startup of the peroxide pump. This is because hydrogen peroxide functions as an acid, but the dilution of hydroxide may also play a role. The steady-state situation, in which conditions in the autoclave were constant, commenced when the peroxide pump was slowed from 27 to 21 mL/min at time 100 min. The amount of the hydrogen peroxide feed and decomposing hydrogen peroxide were then equal, and the hydrogen peroxide formed a buffer with sodium hydroxide. The pH during this steady-state situation was 10.6 for DTPA and 10.7 for ADA, and the total hydrogen peroxide concentration was 1000 mg/L. These conditions correspond well with industrial pulp bleaching processes. The measured metal concentrations in the autoclave were approximately 1.1 mg/L for Fe, 0.4 mg/L for Mn, 11 mg/L for Ca, and 16.3 mg/L for Mg. Dilution caused by the hydrogen peroxide stream partially explains the differences in metal concentrations in the reservoir and autoclave. In addition to the dilution, some precipitation of the metals was observed. The concentrations measured in the autoclave were used for speciation calculations (Figures 4 and 5). Speciations of the ligands and iron(III) are presented in Figures 4 and 5. The chelation of iron appears to be limited, evidently because of its self-hydrolysis and precipitation as colloidal hydroxides. As can be seen in Figure 4, with increase in pH the proportion of Fe-DTPA decreases dramatically. In the steady-state situation, no Fe is chelated by DTPA. VOL. 35, NO. 7, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Percentage distribution of ADA (a) and iron(III) (b) in the steady-state situation of the experiment with ADA (43).
TABLE 1. Speciation of DTPA and ADA in the Steady-State Situation Fe-L Mn-L Ca-L Mg-L
DTPA
ADA
0 25 70 5
0 5 20 75
Correspondingly, Figure 5 shows that Fe-ADA speciation plays no role in the pH 8-11 range. This is an obvious drawback under process conditions since the main function of complexing agents in non-chlorine bleaching processes is to inactivate detrimental transition metals, especially Fe and Mn. Actually, several reports show that, in contrast to manganese, the residual iron content in pulp after the chelation step is not significantly reduced: Brown and Abbot (30) report a drop in the Mn content in pulp from 40-60 to 3-6 mg/kg and a drop in Fe from 12-17 to just 10-13 mg/kg. Similarly, Lapierre et al. (1) report a drop in the Mn level from 47 to below 5 mg/kg and a drop in Fe from 11 to just 6-8 mg/kg. Iron may be more strongly adsorbed to the pulp than manganese, but our study also shows that the chelation conditions are unfavorable to iron complexation. The dominating iron species in the steadystate situation is Fe(OH)4-. In addition to soluble forms, also insoluble iron, copper, and manganese have been shown to catalyze hydrogen peroxide decomposition (21). The mechanisms of the hydrogen peroxide degradation reactions are difficult to predict because the insoluble chemical forms are unknown. Table 1 shows the ligand speciations of ADA and DTPA with Fe, Mn, Ca, and Mg at thermodynamic equilibrium. 1382
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The properties of DTPA and EDTA, including ecotoxicity and biochemical and photochemical degradation, are strongly dependent on the metal speciation (12-15, 31, 32). No corresponding information is available for ADA. Because of the structural similarity, however, it is reasonable to assume that the metal speciation also has an impact on the fate of ADA. Probably some metal complexes of ADA are more chemically degradable than others. The analytical procedure applied here for chelate determination did not allow direct measurement of the remaining species, because esters are formed from the target compounds in the derivatization step, and only total analysis of the complexing agents was possible. It has been suggested that cation-exchange reactions are so slow that the initial speciation of EDTA plays a significant role in the chelate speciation in waste and natural waters. Thus, theoretical calculations assuming thermodynamical equilibrium are of limited use under waste and natural water conditions when the exact speciation is to be predicted (8, 33-36). Given their structural similarity, the same argument can be expected to apply to DTPA and ADA. In the present study, however, the ligands were in non-metal-bound form when added to the reservoir, which contained known amounts of alkaline earth and transition metals. The reactions of free anionic and protonated ligands with metals are intrinsically fast (8, 33). Therefore, a speciation simulation based on the assumption of thermodynamic equilibrium is more realistic for the conditions applied here. Sometimes the chelation step is performed before the bleaching stage, at pH 4-5 range, and under acidic conditions, Fe(III)-chelate is thermodynamically stable. In that case, if the hydrolysis kinetics are slow, Fe-chelate might play some role later on at the higher pH of the hydrogen peroxide bleaching stages. Hydrogen peroxide anion, HO2-, is often considered a bleaching as well as a corroding species. Its function is to degrade chromophores of lignin, but it may also nonselectively break any organic bonds, including those of chelating agents. Thus, in addition to the total hydrogen peroxide concentration, it is useful to know the concentration of HO2anion. The HO2- concentration was calculated with eq 8, which was derived from the acidity constant of hydrogen peroxide [pKa value of 11.0 at 60 °C (37)]. The activity coefficient, f1 value of 0.8 for univalent ions calculated by the Davies equation, was used for the calculation (38). The HO2concentrations are presented in Figures 2 and 3:
C(HO2-) (mg/L) )
10pH-pKaC(H2O2 + HO2-) (mg/L) f1 + 10pH-pKa
(8)
It is reported that nucleophilic addition reactions of HO2constitute the main part of the hydrogen peroxide bleaching process (39). Electrophilic reactions arise by metal-catalyzed reductive cleavages of the bleaching reagent. The occurrence of the latter reactions depens strongly on conditions. In pulp bleaching by hydrogen peroxide, nucleophilic reactions are lignin-retaining, whereas radical reactions are lignin-degrading. Only a small extent of lignin degradation can be observed in hydrogen peroxide bleaching (39). The generation of hydroxyl and superoxide radicals in the presence of iron and manganese is inhibited (1, 40). It has been suggested that the main transition metals, namely, Mn and Fe, do not decompose peroxide via a free radical chain mechanism. The opposite was observed for copper however, which induced the formation of a large number of free radicals during hydrogen peroxide decomposition (1). Copper was not included in the present study because it usually appears in very low concentrations and does not affect the bleaching. Thus, we can rule out the possibility that radical mechanisms play a significant role. The degradation results (the residual percentages of the chelating agents) are presented together with the percentages
nitrogen atom, making it less vulnerable to nucleophilic attack. In addition, when ADA exists as a cyclic chelate, the attractive effect of the metal cation is concentrated on one nitrogen rather than three as in DTPA, increasing the electron deficit. On this basis in addition to its almost total degradation in activated sludge treatment (16), ADA might be oxidized already in the bleaching processes and thus be more favorable than DTPA from the environmental point of view. In contrast to the biochemical and photochemical degradations, the mode of chemical decomposition of DTPA and ADA has received little attention. According to a recent investigation, the degradation of DTPA results from cleavage of the C-N bond and substitution of one acetic acid group for hydrogen. The main product of this breakdown has been identified as glyoxalic acid, which oxidizes to oxalic acid (29). The reaction of peroxide with tertiary amines is described in the literature (21) as
FIGURE 6. Minimum residual percentages of chelating agents (9) and residual percentages of hydrogen peroxide ((): (a) experiment with DTPA; (b) experiment with ADA. of hydrogen peroxide in Figure 6. In the steady-state situation, the average minimum residual percentage of ADA was 71% and that of DTPA was 94%. Thus, ADA seems to be considerably more degradable under these conditions than DTPA. It can be inferred that all DTPA species present are recalcitrant to oxidation, but Mn-ADA and Ca-ADA may be more readily degradable chemically. In fact, the DTPA degradation (6%) is almost within the analytical reproducibility (18). Moreover, in the actual process, other organic compounds would effectively compete with chelating agents for the reactive hydrogen peroxide species. On this basis, we conclude that DTPA is chemically nondegradable under oxidizing conditions simulating peroxide bleaching and that ADA is considerably more degradable. The residual percentage of hydrogen peroxide is 40 for both compounds, indicating no significant dependence on the chelating agent. In terms of technical performance in the actual process, this shows that ADA is comparable with DTPA in inactivating transition metals, and its advantage is higher chemical degradability. The bond with lowest bond energy in the ADA and DTPA molecules is C-N (41). In all likelihood, the species making nucleophilic attack on the C-N bond is the HO2- anion. Theoretically, ADA should be the better substrate since there is a greater deficit of electron density on the nitrogen as compared with DTPA. In ADA, relative to DTPA, the three carboxylic groups cause the single nitrogen to be more positively charged and consequently more favorable for the attack of the HO2- anion. In DTPA, only one (at the center of the molecule) or two (at the remaining two nitrogens) carboxylic groups are attracting the electron density of the
R3N + HOOH f R3N+OH + HO-
(9)
R3N+OH f R3N+O- + H+
(10)
According to the mechanisms presented by eqs 9 and 10, chelating agents are not directly degraded by the reaction with hydrogen peroxide, which may partly explain the relatively high residual percentages observed in this work. The decomposition of DTPA in real bleaching processes including ozone and hydrogen peroxide stages was recently evaluated in mass balance calculations (42). According to these calculations, the residual percentage of DTPA was 87, indicating poor chemical degradability. This is in excellent agreement with the results presented here. To sum up, ADA holds good promise as a sequestering agent that could substitute for DTPA in pulp bleaching. In addition to its low toxicity and high biochemical degradation, we have shown that it is noticeably more degradable by chemical oxidation than DTPA while still maintaining good technical performance in the process. In contrast to DTPA, degradation of ADA might already occur to some extent in the industrial process before the stage of wastewater treatment.
Acknowledgments Financial support was receivedfrom the Academy of Finland, the Maj and Tor Nessling Foundation, the Jenny and Antti Vihuri Foundation, the Foundation of Technology, and the Kemira Foundation. We thank Dr. Marjatta Orama for speciation calculations and Ms. Salla Tuulos-Tikka for technical assistance in chelate analysis. Dr. Kathleen Ahonen is thanked for improving the language of the manuscript.
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Received for review July 27, 2000. Revised manuscript received December 15, 2000. Accepted January 5, 2001. ES000167S
