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Fig. 4. The remaining manganese concentrations in oxygen bleached hardwood and softwood kraft pulps after chelations with EDTA of different concentrations

Figure 4 The remaining manganese concentrations in oxygen bleached hardwood and softwood kraft pulps after chelations with EDTA of different concentrations

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EDTA, ethylenediaminetetraacetic acid, is a multidentate chelating agent with 6 coordinating sites (Fig. 1), and it forms 1:1 complexes with most metal ions (Ringbom 1963). EDTA contains both -COOH and —NH; as the functional groups and forms a ring structured chelate around the metal ion.
EDTA, ethylenediaminetetraacetic acid, is a multidentate chelating agent with 6 coordinating sites (Fig. 1), and it forms 1:1 complexes with most metal ions (Ringbom 1963). EDTA contains both -COOH and —NH; as the functional groups and forms a ring structured chelate around the metal ion.
In Fig. 2 the ay-coefficients of iron (IID) and magnesium(II) are plotted as function of pH for EDTA, DTPA, and DCTA, ie. the chelating agents that are commonly used for desorption of metal ions in pulping processes. The ay-coefficients presented in Figs. 2 and 3 are based on the equilibrium constants given by Ringbom (1963) at 20 °C or 25 °C. The side reactions of the metal ions with hydroxide ions are included in the given am-coefficients. The ligand concentrations are assumed to be 0.01 M. As the trivalent iron has harmful effects on the pulping process, the Ofe-coefficient should be as high as possible in order to enable an effective desorption of iron(II). All the three chelating agents show quite similar curves (Fig. 2a) but DCTA has, compared with EDTA and DTPA, a slightly better ability to complex with iron(II) at pH below 9. Chelation of iron(III) is very effective with all three chelating agents in the alkaline and neutral pH regions. However, still at pH 1 the of am-coefficients for DCTA and EDTA are as high as ca 10’. At pH values higher than 11 iron(III) starts to precipitate as Fe(OH); even in presence of the chelating agent.
In Fig. 2 the ay-coefficients of iron (IID) and magnesium(II) are plotted as function of pH for EDTA, DTPA, and DCTA, ie. the chelating agents that are commonly used for desorption of metal ions in pulping processes. The ay-coefficients presented in Figs. 2 and 3 are based on the equilibrium constants given by Ringbom (1963) at 20 °C or 25 °C. The side reactions of the metal ions with hydroxide ions are included in the given am-coefficients. The ligand concentrations are assumed to be 0.01 M. As the trivalent iron has harmful effects on the pulping process, the Ofe-coefficient should be as high as possible in order to enable an effective desorption of iron(II). All the three chelating agents show quite similar curves (Fig. 2a) but DCTA has, compared with EDTA and DTPA, a slightly better ability to complex with iron(II) at pH below 9. Chelation of iron(III) is very effective with all three chelating agents in the alkaline and neutral pH regions. However, still at pH 1 the of am-coefficients for DCTA and EDTA are as high as ca 10’. At pH values higher than 11 iron(III) starts to precipitate as Fe(OH); even in presence of the chelating agent.
Calculated am-values for some metal ions with 0.0027 M EDTA are shown in Fig. 3. Alkali metal ions, like Na, K, and Li that form very weak chelates with EDTA ‘Smith and Martell 1989) are not included in the figure, as their a values are close to 1. it can clearly be seen that the metal ions are chelated by EDTA with varying strengths. Fes Al™, and Cu’ form the most stable chelates, but also Zn", Fe*’, and Mn?" form rather strong chelates with EDTA. It is important to note the large difference in the chelating strength of EDTA for iron(II) and iron(II) within the entire pH range. This means that the oxidation state of iron in the chelating step of the pulping process is of sreat importance. A clear pH effect on the chelating power of the EDTA can also be seen in Fig. 3. The lower the pH is the lower is the chelating effect of EDTA. This is due to srotonation of EDTA.
Calculated am-values for some metal ions with 0.0027 M EDTA are shown in Fig. 3. Alkali metal ions, like Na, K, and Li that form very weak chelates with EDTA ‘Smith and Martell 1989) are not included in the figure, as their a values are close to 1. it can clearly be seen that the metal ions are chelated by EDTA with varying strengths. Fes Al™, and Cu’ form the most stable chelates, but also Zn", Fe*’, and Mn?" form rather strong chelates with EDTA. It is important to note the large difference in the chelating strength of EDTA for iron(II) and iron(II) within the entire pH range. This means that the oxidation state of iron in the chelating step of the pulping process is of sreat importance. A clear pH effect on the chelating power of the EDTA can also be seen in Fig. 3. The lower the pH is the lower is the chelating effect of EDTA. This is due to srotonation of EDTA.
Table 2. Concentrations of Different lons in Softwood Kraft Pulps Before and After Chelation with 0.2% EDTA at pH 5, 7, and 10 Determined by the Semiquantitative ICP-MS Technique As can be seen in Table 1, the concentrations of several metal ions, especially sodium, potassium, and manganese, were much higher in the oxygen bleached hardwood kraft pulp than in the unbleached hardwood kraft pulp. One reason for this may be that he oxygen bleaching step increases the concentration of metal binding anionic groups in he pulp. Su et al. (2004) showed by potentiometric acid-base titrations that the concen- ration of carboxylic groups increased during oxygen delignification of kraft pulps. Table 2, however, indicates that in the softwood kraft pulp the concentration of the metal ions both in unbleached and oxygen bleached pulps were rather similar. Although there were big differences in the original metal ion concentrations between unbleached and oxygen bleached hardwood pulps, the concentrations of magnesium, manganese and iron in the unbleached and oxygen bleached pulps were rather similar after the chelations, indicating a rather effective chelation power of EDTA under the used experimental conditions. Hence, the chelation can successfully be done either before or after the oxygen delignification.
Table 2. Concentrations of Different lons in Softwood Kraft Pulps Before and After Chelation with 0.2% EDTA at pH 5, 7, and 10 Determined by the Semiquantitative ICP-MS Technique As can be seen in Table 1, the concentrations of several metal ions, especially sodium, potassium, and manganese, were much higher in the oxygen bleached hardwood kraft pulp than in the unbleached hardwood kraft pulp. One reason for this may be that he oxygen bleaching step increases the concentration of metal binding anionic groups in he pulp. Su et al. (2004) showed by potentiometric acid-base titrations that the concen- ration of carboxylic groups increased during oxygen delignification of kraft pulps. Table 2, however, indicates that in the softwood kraft pulp the concentration of the metal ions both in unbleached and oxygen bleached pulps were rather similar. Although there were big differences in the original metal ion concentrations between unbleached and oxygen bleached hardwood pulps, the concentrations of magnesium, manganese and iron in the unbleached and oxygen bleached pulps were rather similar after the chelations, indicating a rather effective chelation power of EDTA under the used experimental conditions. Hence, the chelation can successfully be done either before or after the oxygen delignification.
Fig. 6. The remaining manganese and magnesium concentrations vs. time during chelation of oxygen bleached softwood kraft pulp (0.2% EDTA, pH 5) . In order to study the time required for the chelation reaction, pulp samples were taken from the pulp slurry every 5" minute during the chelation experiment, followed by an immediate thorough washing. The chelation was done with 0.2% EDTA at pH 5 on oxygen-bleached softwood pulp. The manganese and magnesium concentrations during this chelation experiment are shown in Fig. 6.
Fig. 6. The remaining manganese and magnesium concentrations vs. time during chelation of oxygen bleached softwood kraft pulp (0.2% EDTA, pH 5) . In order to study the time required for the chelation reaction, pulp samples were taken from the pulp slurry every 5" minute during the chelation experiment, followed by an immediate thorough washing. The chelation was done with 0.2% EDTA at pH 5 on oxygen-bleached softwood pulp. The manganese and magnesium concentrations during this chelation experiment are shown in Fig. 6.
As can be seen in Fig. 7a the chelation of manganese from oxygen bleached hardwood pulp showed very clear differences in the desorption curves with and without EDTA. Without EDTA the removal is due to ion exchange with hydrogen ions. With EDTA a clear chelation effect can be observed in the whole pH range studied. Removal of iron is more problematic. Even with 0.4% EDTA only slightly more than 50% of iron was chelated (Fig. 7b). In removal of magnesium from oxygen bleached hardwood pulp only a slight chelating effect with EDTA was obtained in the pH region 5 to 10 (Fig. 7c). At pH <5 the main reaction is the ion exchange between magnesium and hydrogen ions. In chelation of manganese from oxygen bleached softwood pulps 0.4% EDTA was very effective (Fig. 8a). At pH < 7 the removal of manganese with EDTA was almost quantitative and more effective than from hardwood pulp. On the other hand, it was more
As can be seen in Fig. 7a the chelation of manganese from oxygen bleached hardwood pulp showed very clear differences in the desorption curves with and without EDTA. Without EDTA the removal is due to ion exchange with hydrogen ions. With EDTA a clear chelation effect can be observed in the whole pH range studied. Removal of iron is more problematic. Even with 0.4% EDTA only slightly more than 50% of iron was chelated (Fig. 7b). In removal of magnesium from oxygen bleached hardwood pulp only a slight chelating effect with EDTA was obtained in the pH region 5 to 10 (Fig. 7c). At pH <5 the main reaction is the ion exchange between magnesium and hydrogen ions. In chelation of manganese from oxygen bleached softwood pulps 0.4% EDTA was very effective (Fig. 8a). At pH < 7 the removal of manganese with EDTA was almost quantitative and more effective than from hardwood pulp. On the other hand, it was more
Table 3. Mg/Mn Ratios in Hardwood Kraft Pulps Chelated with 0.4% EDTA at Different pH Values — Chelation had only a very weak effect on the removal of magnesium from oxygen bleached hardwood pulp (Fig. 7c). Magnesium ions are also only weakly bound to oxygen bleached softwood pulps and are removed mainly by ion exchange with hydrogen ions. The small differences between the curves in Fig. 8c are obviously due to experimental errors and have no practical relevance. The a-coefficients given in Fig. 2b, calculated on theoretical basis, show that magnesium ions form a weak complex with EDTA at pH below ca 8, and in this pH range no chelation effect can hardly be obtained as was experimentally approved in Figs 7c and 8c. Lapierre et al. (1997) have shown that an acid wash (pH=1.5) removed efficiently manganese and magnesium. As we already have shown in Fig. 3, the a-Mn(EDTA) at pH | and a-Mg(EDTA) at pH 4 are ca. 1 and there is no chelation of these ions and the removal of manganese and magnesium at pH- values below 1 and 4. respectively. are entirely due to ion exchange with hydrogen ions.
Table 3. Mg/Mn Ratios in Hardwood Kraft Pulps Chelated with 0.4% EDTA at Different pH Values — Chelation had only a very weak effect on the removal of magnesium from oxygen bleached hardwood pulp (Fig. 7c). Magnesium ions are also only weakly bound to oxygen bleached softwood pulps and are removed mainly by ion exchange with hydrogen ions. The small differences between the curves in Fig. 8c are obviously due to experimental errors and have no practical relevance. The a-coefficients given in Fig. 2b, calculated on theoretical basis, show that magnesium ions form a weak complex with EDTA at pH below ca 8, and in this pH range no chelation effect can hardly be obtained as was experimentally approved in Figs 7c and 8c. Lapierre et al. (1997) have shown that an acid wash (pH=1.5) removed efficiently manganese and magnesium. As we already have shown in Fig. 3, the a-Mn(EDTA) at pH | and a-Mg(EDTA) at pH 4 are ca. 1 and there is no chelation of these ions and the removal of manganese and magnesium at pH- values below 1 and 4. respectively. are entirely due to ion exchange with hydrogen ions.
Table 4. Mg/Mn Ratios in Softwood Kraft Pulps Chelated with 0.4% EDTA at Different oH Values The Mg/Mn ratio for the hardwood pulp, when no EDTA was used, was about 2, and for the softwood pulp 6-9 within the whole pH range, i.e. the pH has only a very small effect on the Mg/Mn concentration ratio in these cases. By using EDTA, manganese was removed, but EDTA had only a weak or even no effect on the removal of magnesium. EDTA improved the removal of manganese at all the tested pH values, but he best improvement was obtained at the neutral pH values (pH 5-8). The highest Mg/Mn ratio, after chelation with EDTA was 38, obtained at pH ca. 4 for the oxygen bleached hardwood kraft pulp (Table 3), and for the oxygen bleached softwood kraft pulp he highest Mg/Mn ratio was as high as 7300 at pH ca. 5 (Table 4). An obvious explanation for the much lower Mg/Mn concentration ratio obtained for the hardwood pulp is that manganese is much more strongly bound to the functional groups in this pulp han to the softwood pulp.
Table 4. Mg/Mn Ratios in Softwood Kraft Pulps Chelated with 0.4% EDTA at Different oH Values The Mg/Mn ratio for the hardwood pulp, when no EDTA was used, was about 2, and for the softwood pulp 6-9 within the whole pH range, i.e. the pH has only a very small effect on the Mg/Mn concentration ratio in these cases. By using EDTA, manganese was removed, but EDTA had only a weak or even no effect on the removal of magnesium. EDTA improved the removal of manganese at all the tested pH values, but he best improvement was obtained at the neutral pH values (pH 5-8). The highest Mg/Mn ratio, after chelation with EDTA was 38, obtained at pH ca. 4 for the oxygen bleached hardwood kraft pulp (Table 3), and for the oxygen bleached softwood kraft pulp he highest Mg/Mn ratio was as high as 7300 at pH ca. 5 (Table 4). An obvious explanation for the much lower Mg/Mn concentration ratio obtained for the hardwood pulp is that manganese is much more strongly bound to the functional groups in this pulp han to the softwood pulp.
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