3.2. Solubility
Metal complexes are generally hydrophilic compounds, but this does not exclude adsorption and different adsorption properties between chelate species. In natural aquatic conditions, adsorption is considered to be an important pathway of removal. EDTA has been found in lake sediments [60] and differences of the extent of adsorption between metal complexes of EDTA have been observed [61]. Adsorption mechanisms of EDTA on iron and aluminium oxides have been investigated in detail [62, 63].
In addition to natural conditions, adsorption of metal complexes may have a role during the bleaching process itself. An earlier investigation carried out in real bleaching lines nevertheless suggests that pulp has no significant ability to bind DTPA and EDTA [64]. In this study, solubility of DTPA, EDTA and ADA under alkaline conditions was investigated [III]. The purpose was to compare the solubility of the different complex species. Chelating agents in the aqueous phase were analysed by GC from simulated hydrogen peroxide bleaching solutions. The conditions in all systems were as follows: pH 10-12, hydrogen peroxide concentration during simulation 3000-5000 mg/l, concentration of Mg 0.5-0.8 mmol/l. Otherwise, the samples could be divided into two groups; those with 0.015 – 0.025 mmol/l of iron and manganese, and those with absence of these metals. An exact experimental description is presented in article III.
In the absence of the transition metals, soluble chelates were detected by GC to a limited extent. It could be shown by the species distribution calculations that ligands were totally present as magnesium complexes. Over 90% of magnesium existed as hydroxide, Mg(OH)2, the rest being complexed by ligand. It is quite possible that the magnesium complex adsorbed onto the magnesium hydroxide and hence was not found in the solution.
When the iron and manganese were introduced to the systems, solubilities were observed to be enhanced considerably. Soluble ligand content increased up to 20-25%, 80% and 30% of the added amount for DTPA, EDTA and ADA, respectively. This supports the conclusion on high chemical durability of EDTA presented in section 3.1. With regards to the species distribution calculation, for DTPA 45-60% of the added manganese was in the form of Mn-DTPA. The corresponding percentages for EDTA and ADA were 100 and 40, respectively. Evidently, manganese formed a stable complex transporting ligands into solution. Iron was calculated to exist totally as hydroxy species Fe(OH)4-.
In the experiments described above, EDTA was more soluble than the other two agents. To confirm this conclusion, and to achieve better comparison between the three complexing agents, an experiment in which all the ligands were added was performed. The results of this experiment are presented in Fig. 4. in which chemical environment and the soluble chelating agent concentrations are presented as a function of experimental time. At first, when the transition metals were present, no DTPA, but almost all the EDTA, was in the solution. This may indicate that complexation of Mn(II) was more rapid with EDTA than with DTPA. In the course of time, part of the DTPA was transported into the solution, probably as a manganese complex. During the whole experiment, however, more EDTA was found in the aqueous phase as compared to the other two agents.
In summary, the solubility of the three chelating agents in an alkaline hydrogen peroxide bleaching environment is limited, but increases in the presence of iron and manganese. A possible explanation for the limited solubility is that the complexes, particularly the magnesium complex, are adsorbed onto magnesium hydroxide precipitate. Under the conditions investigated, manganese is well chelated especially by EDTA. Chelation of iron is thermodynamically difficult due to its strong self-hydrolysis.
These experiments also support the conclusion of high chemical durability of EDTA presented in section 3.1.
