Reducing the Risks of Hydrogen Peroxide Bleaching in Presence of Iron Ions in Paper

Abstract

Abstract Several reagents were tested as possible future pre-treatments to hydrogen peroxide bleaching of discoloured paper with the goal of minimizing the degrading effect of hydrogen peroxide on cellulose. Two historic rag papers were used as testing substrate, one with high, the other with low levels of iron ion content. The two papers underwent the following test protocol: all samples were rinsed in deionized water for ten minutes. Samples were then divided to undergo seven different bleaching pre-treatments: (1) deacidification with calcium hydroxide solution at pH 9; (2) deacidification with magnesium hydrogen carbonate solution at pH 7.5; (3) hydrochloric acid (HCl, 0.1 M, pH 1) followed by deacidification with calcium hydroxide; (4) calcium phytate (1.75 mmol/L, pH 5.3) followed by deacidification with calcium hydroxide; (5) magnesium phytate (pH 6.5) followed by deacidification with magnesium hydrogen carbonate solution; (6) and (7) the chelating agent diethylene-triamine-pentaacetic acid (DTPA, 0.005 mol/L, pH 3) followed by deacidification either with calcium hydroxide or magnesium hydrogen carbonate. All samples were immersed in 3% hydrogen peroxide baths (pH 9, adjusted with calcium hydroxide). They were dynamically aged in a customized setup in a stack (20°C-80°C, three hour intervals, six weeks). The samples underwent analyses before treatment, after bleaching and after accelerated ageing. The molecular weight and carbonyl group content of the cellulose were determined with fluorescence labelling in combination with gel permeation chromatography (GPC-MALLS). Brightness reversion of the papers was determined by colourimetry. The iron content level was determined by inductively coupled plasma-mass spectrometry (ICP-MS) before and after treatment. The paper with low iron ion content did not benefit measurably from the pre-treatments, whereas the paper with high iron content benefited from some of the pre-treatments, as determined after accelerated aging. The least increase in carbonyl group content after accelerated ageing was achieved with calcium phytate (4); followed by magnesium phytate (5); and DTPA with magnesium hydrogen carbonate deacidification (7). The highest carbonyl group increase was caused by the calcium hydroxide pre-treatment (1). The Mw was best with the phytate-treated samples (4 and 5), followed by DTPA with magnesium hydrogen carbonate deacidification (7) and hydrochloric acid (3); the calcium-hydroxide deacidified sample showed the greatest Mw loss (1). All of tested iron removal and complexation treatments (3-7) diminished brightness reversion. DTPA (6 and 7) and hydrochloric acid (3) diminished the iron ion content of the high-iron-content paper by 30%. In sum, the tests 3-7 showed distinct benefits over deacidification alone (1 and 2) and may become viable as pre-treatment agents before hydrogen peroxide bleaching after further testing on object materials.

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