Degradability of Iron(III)-aminopolycarboxylate Complexes in Alkaline Media: Statistical Design and X-ray Photoelectron Spectroscopy Studies

Use of ferric-aminopolycarboxylate complexes for odor control via the oxidative scrubbing of H2S and CH3SH contained in pulp and paper noncondensable gas emissions is evoked as potentially beneficial from the standpoint of iron-sequestration and protection against precipitation in the alkaline environments characteristic of the Kraft mill sulfate-pulp processes. In this study, the degradability of two ferric-aminopolycarboxylate complexes in alkaline solutions was investigated by means of replicated four-factor three-level fixed-effects completely randomized factorial (2 * 34) designs. Expressed in terms of ferricethylenediaminotetraacetate (Fe3+EDTA4-) and ferric-trans-1,2-diaminocyclohexanetetraacetate (Fe3+CDTA4-) daily degradation rates, the degradability response was monitored via UV-vis spectrophotometry as a function of temperature (T = 25, 40, 55 (degree)C), alkalinity (pH = 8, 9, 10), ionic strength (I = 0.025, 0.1, 0.5 M) and ferric concentration (CFe = 175, 280, 450 (mu)M). Analysis-of-variance (ANOVA) of the factorial design suggests that pH and temperature are the main factors increasing Fe3+EDTA4- and Fe3+CDTA4- degradation rates. To a lower extent, ionic strength and ferric chelate concentration also promote degradation. At the most severe factor-level combinations (T = 55 (degree)C, pH = 10, and I = 0.5 M), up to 40% of Fe3+CDTA4- and 54% of Fe3+EDTA4- degraded after 1 day, confirming that CDTA is a superior chelating agent against iron precipitation in alkaline solutions. The brownish fresh-state Fe3+EDTA4- or Fe3+CDTA4- solutions evolved with degradation into turbid solutions whereof the precipitated solid was recovered and its surface probed through X-ray photoelectron spectroscopy (XPS). XPS revealed that the solid degradation product was inorganic and mostly contributed by Fe(OH)3. It was, however, not possible to identify which one of the organometallic complex degradation or the ferric dechelation was responsible for iron(III) hydroxide formation since both routes can contribute to its formation.