Chapter 3. Properties of chelating agents
- Table of Contents
- 3.1. Decomposition
- 3.2. Solubility
Complexing agents are extensively used in many applications, the major use being in the detergent component. In the pulp industry, they are utilized to form stable water soluble chelates with transition metal ions, and so remove these metals before hydrogen peroxide bleaching [8, 9]. In addition to assisting the removal of metals, chelating agents may prevent their contact with hydrogen peroxide and so reduce the catalytic decomposition of the bleach [9, 22]. The use of chelators can be expected to increase further if closure of water cycles and TCF bleaching processes will be applied more widely. Chelating agents may also find use in ECF-processes that incorporate hydrogen peroxide stages.
The behaviour of DTPA and EDTA in waste water effluents [23] and a natural aquatic environment [24-27] has received attention. In addition to increasing the total nitrogen contents, DTPA and EDTA remobilize the most toxic heavy metals from solid matter into water solution and thus extend their biological life cycles [28-31]. Moreover, when iron is removed by these ligands from precipitated phosphates, the phosphate is converted to a soluble form [32]. Although successful biodegradation in certain industrial wastewater treatment conditions has been observed [33], and elevated pH has been observed to increase decomposition rates in activated sludge plants [34, 35], biochemical [36, 37] and photochemical [38, 39] degradations are not totally adequate to eliminate the environmental threat. The influence of metal speciation is of the utmost importance to understand the environmental behaviour of these compounds [40].
It may be reasonable and even necessary to find alternative, more biodegradable chelating agents of the future such as phosphonic acids, methylenglycine diacetic acid (MGDA), iminodisuccinate (IDS), Glutamate, N, N-bis(carboxymethyl)(GLUDA) or S, S’-ethylenediamine disuccinic acid (EDDS) [24]. According to a recent investigation, especially S, S’-EDDS is a viable replacement ligand in bleaching applications, and also IDS is comparable to EDTA [25]. β -alaninediacetic acid (ADA) is a potential alternative, too, because it has been shown to improve the whiteness gain [41]. Exceptionally low toxicity and 98% biodegradation have been observed in laboratory scale activated sludge simulation [42]. Hence, ADA was investigated together with DTPA and EDTA in the present study. Chelating of the pulp can also be carried out using a mixture of a nitrogen-containing agent and one or more non-nitrogen containing chelator like lactic, citric, tartaric, gluconic or glucoheptonic acid [43, 44].
In the work for this dissertation, decomposition (I, II) and solubility (III) of DTPA, EDTA and ADA were investigated under simulated bleaching conditions. Since these properties significantly may be dependent on chemical speciation, distribution of the different metal chelates was calculated from the assumed prevailing thermodynamic equilibrium, which is realistic since ligands were added to the experimental system as uncomplexed forms, reactions of which are rapid (except those with trivalent cations). The calculations were based on equilibrium constants of ligand protonation, metal complex formation, metal hydrolysis and solubility of metal hydroxides as well as the ionic product of water [45-47]. The complexation of metals in a known solution may be visualized by drawing curves of their percentage distribution among different complex species, e.g. as a function of pH. As an example, the percent EDTA and Fe(III) distributions in typical concentrations of the alkaline hydrogen peroxide bleaching stage are presented in Fig. 2 [48]. As can be seen, chelation of iron(III) is restricted due to its strong self hydrolysis.
In real bleaching lines, the results of speciation calculations should be considered critically. If the chelating agent is added before the alkaline hydrogen peroxide bleaching stages at pH 4-5, iron(III)chelate, if formed, might play an important role later at the higher pH due to the slow kinetics of trivalent cations. The initial speciation has an outstanding impact on properties of chelating agents also in waste- and receiving waters as well as in natural aquatic environment. While evaluating these properties, it must be noted, too, that the agents can hardly exist in their free or protonated forms in any practical circumstances [49].
Figure 2. Percentage distributions of (a) EDTA and (b) Fe(III) in concentrations typical of the alkaline hydrogen peroxide bleaching stage: EDTA 0.026 mmol/l; Ca 0.40 mmol/l; Mg 0.67 mmol/l; Fe 0.014 mmol/l and Mn 0.0058 mmol/l [48].
3.1. Decomposition
As noted above, biochemical [36, 37] and photochemical [38, 39] degradations do not eliminate the possible environmental impacts of DTPA and EDTA. Therefore, chemical decomposition of the chelating agents already in industrial processes is of interest. The chemical degradation of EDTA in real bleaching lines has been evaluated earlier. According to mass balance calculations, EDTA did not degrade chemically in mechanical pulping when hydrogen peroxide was the bleach. Instead, ozone in combination with hydrogen peroxide decomposed EDTA, and also DTPA to some extent. The percentage of residual of EDTA in a real bleaching process using ozone and peroxide was 60% and that of DTPA 87% [50]. In another study, dealing with TCF effluents [51], the percentage of the residual of 20 % for both the chelating agents after 15 min was achieved, but degradation by the ozone alone was poor. On the other hand, ozone has been shown to degrade EDTA considerably [52, 53], and a mixture of oxidants [54] especially in combination with the ultraviolet light has been effective.
In this study, chemical decompositions of DTPA, EDTA and ADA in hydrogen peroxide bleaching conditions were investigated with a system described in Figure 1 in strictly controlled chemical conditions [I, II].
The percentages of residuals of the chelating agents were based on division of the analysed concentration by the theoretical concentration calculated by equation (3.1).
Cout concentration in the out coming stream = concentration inside the blender
Cin concentration in the incoming stream
V volume of the blender
Q stream flowing through the blender
Equation (3.1) is derived from the general equation of an ideal blender [I, II], and is thus based on the assumption that the concentration in the outcoming stream is the same as that inside the blender. Also percentages of residuals of hydrogen peroxide were determined with equation (3.1). The experiments were started by running solution from the reservoir through the autoclave (Fig. 1). This was followed by starting the hydrogen peroxide pump to enhance the hydrogen peroxide level to the desired steady-state, in which the amount of decomposing hydrogen peroxide and the feed of hydrogen peroxide were equal. Decomposition determinations were made when the system was in the steady state to get parallel results. Table 1 presents the chemical conditions and the results of the experiments.
Table 1. Conditions and results of the decomposition experiments [I, II].
| Experiments with EDTA | Experiments with DTPA | Experiments with ADA | |
|---|---|---|---|
| pH | 10.5 | 10.6 | 10.7 |
| Concentrations (mg/l) | |||
| HOOH+HOO- | 5000 | 1100 | 1000 |
| HOO- | 1200 | 400 | 400 |
| Mn | 0.4 | 0.4 | 0.4 |
| Fe | 0.8 | 1 | 1 |
| Ca | 16 | 11 | 11 |
| Mg | 16 | 16 | 16 |
| Ligand | 10 | 11 | 3 |
| Ligand speciation (%) | |||
| Ligand as Mn(II) complex | 20 | 25 | 15 |
| Ligand as Fe(III) complex | 0 | 0 | 0 |
| Ligand as Ca(II) complex | 80 | 70 | 80 |
| Ligand as Mg(II) complex | 0 | 5 | 5 |
| Results (%) | |||
| Residual of ligand | 94 | 94 | 71 |
| Residual of hydrogen peroxide | 74 | 40 | 40 |
The results in Table 1 reveal that EDTA is a persistent compound in a solution of high total hydrogen peroxide and hydrogen peroxide anion concentrations, 5000 and 1200 mg/l, respectively. This is in accordance with the documented reaction of undissociated hydrogen peroxide with tertiary amine [2].
According to the mechanisms of equations (3.2) and (3.3), chelating agents are not directly degraded through the reaction with undissociated hydrogen peroxide, which partly explains the high percentages of EDTA residual observed in this work and suggests that the undissociated form is not responsible for the decomposition. It has also been observed earlier that evidently no reaction occurs between the Cu(II)-EDTA complex and hydrogen peroxide in the absence of biological reductants [55].
Table 1 reveals that ADA decomposed clearly already at the HOO- level of 400 mg/l, in which DTPA was persistent. In conclusion, ADA is more degradable than the other two agents.
As mentioned earlier, hydrogen peroxide anion is often considered to be an outstanding bleaching [1] as well as corroding [14] species. It may break organic bonds other than chromophores of lignin including those of the chelates, in which the chemical bonds are C-C, C=O, C-O, C-H, O-H, and C-N with bond energies of 339, 724, 331, 410, 456, and 276 kJ/mol, respectively, for hemolytic bond dissociation [15]. In all likelihood, nucleophilic bond breaking by HOO- occurs at the weakest bond, C-N. Figure 3 reveals that ADA should be a better substrate than EDTA. In ADA, three carboxylics cause the single nitrogen to be more positive charged and consequently more favorable for the attack of the HOO- anion. In EDTA, only two carboxylic groups are attracting the electron density of the nitrogen atom making it less vulnerable to the nucleophilic attack. Also in DTPA, two carboxylic groups are attracting the electron density of nitrogen atoms, or even one in the nitrogen in the center of the molecule. In an earlier investigation, it has been stated that the degradation of DTPA and EDTA indeed results from cleavage of the C-N bond. In this mechanism, one acetic acid is substituted by hydrogen. The main product of this breakdown has been identified as glyoxylic acid, which further oxidizes to oxalic acid [56].
It can be concluded from the species distribution calculations that under 10% of manganese remained unchelated in the experiments with DTPA and EDTA. In the case of ADA, 70% existed in unchelated form, due to the observed decomposition and due to the lower stability constant of Mn(II)-ADA chelate as compared to DTPA and EDTA. This gives rise to the formation of manganese (III, IV) oxides, which are facile oxidants for organic compounds, for example EDTA [57]. This may also explain the better decomposition of ADA as compared to the other two agents.
To summarize, EDTA appears to be a durable compound in alkaline hydrogen peroxide bleaching conditions [I]. The poor biodegradation has been reported earlier [23, 24, 36, 37]. It is recommended, therefore, to minimize the use of EDTA, until an effective means, such as chemical oxidation with an effective catalyst [58, 59], or improved biodegradation conditions [33-35] for its removal have been developed. An approach worth considering would also be the use of less recalcitrant ligands. ADA degrades chemically better than DTPA and EDTA [I, II], is biodegradable [42], has good technical performance in bleaching [41] and is thus a potential replacement ligand. The technical performance of biodegradable S, S’-EDDS [25] and mixtures of chelating agents [43, 44] should also be kept in mind.