Hydrogen peroxide- metals- chelating agents; interactions and analytical techniques

Jaakko Rämö

Department of Process and Environmental Engineering, University of Oulu

Abstract

Information about interactions among metals, hydrogen peroxide and chelating agents is needed to develop environmental technology and the operating efficiency of modern elemental chlorine free and total chlorine free bleaching processes. The work presented here focused on the properties of metal chelates and corrosion of titanium in an alkaline hydrogen peroxide solution. A comparative study between three rapid analysis methods, ICP-AES, XRF and ISE, was performed in pulp matrix and error sources of ISE were investigated in detail. Sensitive and selective GC methods for chelating agents ADA and NTA in water matrices were developed.

Decomposition of ADA (percentage of residual 71) was observed already at the hydrogen peroxide anion level of 400 mg/l in which DTPA was more persistent (percentage of residual 94). EDTA was stable even in the hydrogen peroxide anion level of 1200 mg/l, in which its percentage of residual was 94. DTPA, EDTA and ADA were more soluble in the presence of iron and manganese than in the absence of these metals. The chelation of iron appeared to be thermodynamically limited in hydrogen peroxide bleaching conditions.

Unalloyed (Grade 2) and alloyed (Grade 5) titanium corroded at the hydrogen peroxide anion level of 200 mg/l. The presence of calcium and silica inhibitors and further iron and manganese enhanced the critical hydrogen peroxide anion levels. Grade 5 was inferior to Grade 2. During rapid uniform corrosion, the potential of unalloyed titanium was under 200 mV (SHE) and lower than that of platinum.

Over 90% of manganese and many other metals could be leached into aqueous phase for ICP-AES analysis using chelation or acid hydrolysis. An XRF method for manganese, iron and copper in pulp including little or no sample treatment was developed. Measuring temperature differences and atmospheric carbon dioxide were observed to be notable error sources of the ISE technique.

Table of Contents
Preface
Glossary
List of original papers
1. Introduction
2. Hydrogen peroxide in alkaline solutions
2.1. Dissociation
2.2. Decomposition
2.3. Experimental investigation of alkaline hydrogen peroxide solutions
3. Properties of chelating agents
3.1. Decomposition
3.2. Solubility
4. Uniform corrosion of titanium in alkaline hydrogen peroxide environments
4.1. Critical hydrogen peroxide anion levels and the influence of inhibitors and transition metals
4.2. The relationship between the corrosion rate and potential
5. Analytical techniques in pulp and water matrices
5.1. Metals
5.1.1. ICP-AES
5.1.2. X-ray fluorescence
5.1.3. Ion selective electrodes
5.1.4. Comparison of metal analysis methods
5.2. pH and hydrogen peroxide
5.3. Chelating agents ADA and NTA
6. Summary and conclusions
References
List of Tables
1. Conditions and results of the decomposition experiments [I, II].
2. Rate of corrosion of titanium under different experimental conditions [IV, V].
3. Comparison of results of manganese analyses with and without pretreatment, mg/kg [VII].
4. Measurements of calcium in a TCF bleaching line, using an ion selective electrode with and without an ionic strength adjustor, and by AAS (x = average, s = standard deviation, n = sample amount).
5. Measurements of sodium in a TCF bleaching line, with an ion selective electrode with and without an ionic strength adjustor, and by AAS (x = average, s = standard deviation, n = sample amount).
6. Comparison of metal analysis methods.
7. Optimized sample pretreating conditions and analytical parameters [X, XI].
List of Figures
1. The pilot plant scale system used in investigation of alkaline hydrogen peroxide solutions.
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. Molecular structures of MeEDTA2- and MeADA- complexes.
4. Results for the experiment where EDTA, DTPA and ADA were used simultaneously: A) pH and total hydrogen peroxide concentrations as a function of time; B) total concentrations of metals and concentrations of soluble EDTA, DTPA and ADA [III]. Reproduced by permission of PAPTAC.
5. Corrosion rates of the unalloyed titanium (Grade 2) as a function of hydrogen peroxide anion (HOO-) concentration.
6. Corrosion rates of the alloyed titanium (Grade 5) as a function of hydrogen peroxide anion (HOO-) concentration.
7. Corrosion potential vs. corrosion rate of titanium Grade 2; a) 0-4 mm/y, b) 0-90 mm/y.
8. pH values measured using an ion selective electrode on-line in the process as a function of values measured later at room temperature. Reproduced by permission of APPITA.
9. Formation of a carbonate buffer shown as inaccuracy of the pH values measured in solutions after alkalization with NaOH and neutralization with HCl. Reproduced by permission of APPITA.
10. Chromatogram of ethyl esters of DTPA, EDTA ADA and NTA. Reproduced by permission of Taylor & Francis Ltd.
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