1.14 Study of the Effects of Sulfur Dioxide and Nitrogen Dioxide on Deacidified Papers - Part 2

Daniel Grosjean and Associates
The Getty Conservation Institute
Los Angeles County Museum of Art

Daniel Grosjean
Antoinette Van Neste
Edwin Williams II
James Druzik
Eric Hansen
Victoria Blyth-Hill
Period of Activity: 6/87 to 4/89

Project Abstract
One test paper, new 100% chemical pulp, from Flieder's group and a naturally aged paper were exposed in a Teflon exposure chamber with continuously monitored ambient levels of SO2 and NO2 (less than 0.5 ppm). Subsequent analysis included the measurement of changes in pH, tensile strength, sulfur and nitrate buildup. Analysis techniques supplement the French team as well as provide an independent statistical validation for the observed exposure effects. In addition, French and American exposed samples may have been exchanged.

Major Findings
HPLC was used for bulk extraction analysis of the anionic content of all paper samples. Both deacidified and untreated paper samples absorbed SO2 and NO2 throughout the exposures. Most of the absorbed SO2 was accounted for as sulfate (65 -30%). No sulfite or bisulfite was detected. Absorbed NO2 was accounted for as nitrite and nitrate within experimental error. Paper samples absorbed more SO2 than NO2. Deacidified paper absorbed more SO2 than untreated paper. Paper absorbed less SO2 and NO2 over time whether exposed singlely or together. After 40 days of exposure to NO2 or NO2 SO2 the paper samples were removing only 10 -10% of the inlet NO2 concentration. Whereas it took nearly 29 weeks of exposure to SO2 for the removal of the inlet SO2 concentration to reduce to 10 -10%. All observations made in this study are for the specific type of newspaper and white wove paper we used and caution must be used in extrapolating our results to other types of newsprint and white wove paper. Electron spectroscopy for chemical analysis, ESCA, Electron spectroscopy for chemical analysis, ESCA,; was used to analyze the surface of several unextracted paper samples for sulfate, bisulfate, sulfur, nitrate, nitrite, nitrogen, carbonate, carbon oxygen, oxygen, and metals. No detectable nitrogen or carbonate was found.

Primary Publications
Williams II, E. L., and D. Grosjean, "Exposure of Deacidified Paper to Ambient Levels of NO2 and SO2," 1989 Pacific Conference on Chemistry and Spectroscopy, Division of Atmospheric Chemistry, Pasadena, California, October 18-21.

ABSTRACT-Paper deacidification is a widely practiced protection measure in the conservation community. We have investigated the effectiveness of deacidification for paper exposed to ambient levels of two major air pollutants, SO2 and NO2. Four sets of paper exposure experiments were conducted, each of 13 weeks' duration, to purified air (control) and to purified air containing SO2 SO2 ;(104 ppb), NO2 (92 ppb), and a mixture of NO2 (98 ppb) and SO2 (108 ppb). Sulfur dioxide and nitrogen dioxide concentrations were monitored at the exposure chamber inlet and exit with pulsed fluorescence and chemiluminescence techniques, respectively. Paper samples showed a larger capacity for uptake of SO2 than for NO2. Periodically, paper samples were withdrawn from the chambers and analyzed for pH, nitrate, nitrite, sulfate, sulfite, and bisulfite.The results are discussed in terms of pollutant uptake and retention for both deacidified and untreated paper.

Williams II, E. L., and D. Grosjean, "Exposure of Deacidified Paper to Ambient Levels of SO2 and NO2," Final Report to the (Conservation at the Getty) Institute, February 1990, Daniel Grosjean and Associates, Inc., [4526 Telephone Road, Suite 205, Ventura, California 93003].

EXECUTIVE SUMMARY-Damage to works of art and historical records on paper medium is currently a major focus of conservation efforts. Paper degradation is mostly due to the acidic hydrolysis of the glycosidic linkages in cellulose. Paper may become acidic from the manufacturing process and/or by the absorption of atmospheric pollutants, e.g., sulfur dioxide, sulfur dioxide, ;SO2 and nitrogen dioxide, NO2. In an effort to preserve works of art and historical records, paper deacidification has become a widely practiced protection measure in the conservation community. Paper deacidification neutralizes the acidic content of paper and leaves an alkaline reserve for the uptake of acidic pollutants.

The degradation of paper due to uptake of airborne SO2 has been the object of many studies. However, all previous studies have been carried out at SO2 concentration of 1-10 parts per million (ppm), i.e., 20 to 2,000 times higher than those found in ambient air (typically from less than 5 ppb to about 50 ppb). A potential problem with using such high SO2 concentrations, at which SO2 would readily form sulfuric acid aerosol in air at ambient humidity, is that the observed damage may be due to uptake of sulfuric acid, H2SO4, rather than SO2. At the lower and more realistic SO2 concentrations used in this study, ~100 ppb, no sulfuric acid aerosol is produced and therefore chemical and physical changes can be unambiguously attributed to uptake of SO2. In addition, no one has directly measured the expected products of SO2 absorption on paper, i.e., sulfite, bisulfite, and sulfate. To our knowledge, exposure of paper to NO2 has not been investigated.

This project was aimed at investigating the effect of ambient levels of sulfur dioxide and nitrogen dioxide on deacidified and untreated paper. Two types of paper were studied, newsprint and white wove. Four exposures were carried out, one to purified air (control, duration 11 weeks), one to purified air containing SO2 (87 ppb for 29 weeks), one to purified air containing NO2 (92 ppb for 13 weeks), and one to purified air containing a mixture of SO2 and NO2 (108 and 98 ppb, respectively, for 13 weeks). Daytime temperatures were 19 2, 21 2, 19 2 and 19 2 C, respectively, and the corresponding relative humidites were 63 11, 57 8, 62 7 and 62 7%, respectively (within that range, RH fluctuations have no effect on chemical reactions between paper, SO2, and/or NO2). Paper samples were withdrawn from the exposure chamber weekly or biweekly and were analyzed for sulfate, bisulfate, sulfite, nitrate, and nitrite.

Both SO2 and NO2 were continuously absorbed throughout the 13 to 29-week exposure periods. Both SO2 and NO2 were removed at the same rate whether present alone or as a mixture. The paper had a much larger capacity for uptake of SO2 than for uptake of NO2, e.g., 29 weeks (SO2) vs 6.7 weeks (NO2) to reduce the uptake to 10% of the inlet SO2 or NO2 concentration. The paper's ample capacity to remove more NO2 and SO2 after 13 weeks of exposure to 100 ppb of each pollutant was estimated on the basis of bicarbonate content calculations, and was verified experimentally by reexposing the same paper samples to much higher levels of pollutants, e.g., 1,000-1,200 ppb for 4-7 days.

Chemical analysis of the exposed samples led to the following findings:

a.) Sulfate was the only sulfur-containing product observed upon exposure to SO2 and SO2 NO2, irrespective of paper type, deacidified or not. Sulfite and bisulfite were not detected, an observation confirmed by additional experiments involving the treatment of SO2-exposed paper to aqueous and gas phase formaldehyde. Oxidation of SO2-exposed samples by hydrogen peroxide did not reveal the presence of other oxidizable sulfur-containing products.

b.) Nitrite and nitrate were the two nitrogen-containing products observed upon exposure to NO2 and NO2 SO2. Deacidified paper yielded both nitrite and nitrate; untreated paper yielded only nitrate.

c.) Newsprint yielded higher concentrations of sulfate than did white wove paper.

d.) For newsprint, sulfate concentrations were in the order: untreated < aqueous deacidified < nonaqueous deacidified. For white wove, the sequence was: untreated < nonaqueous < aqueous. Thus, untreated paper showed the lowest accumulation of sulfate.

e.) The ratio of nitrate nitrite to sulfate was substantially lower in deacidified paper (0.2-0.5%) than in untreated paper (0.6-2.0%).

f.) The above trends appear to be independent of calculated bicarbonate loading or measured magnesium content.

g.) Nitrite accounted for 41% of the total nitrogen-containing products (nitrate nitrite) upon exposure to NO2 alone but for only 9% upon exposure to NO2 in the presence of SO2.

h.) Mass balance calculations indicated that all absorbed NO2 could be accounted for by the sum of nitrite and nitrate, and that sulfate accounted for 65 - 30% of the absorbed SO2. Two factors suggest that the absorption of NO2 and SO2 involve a diffusion process: newsprint absorbed more SO2 and NO2 than white wove, and ESCA analysis revealed no nitrite, nitrate or carbonate on the surface of samples exposed to SO2 NO2, even though the paper contained significant nitrite and nitrate and had ample capacity for absorption of more NO2 and SO2.

Much work still remains to be carried out to investigate the chemical and physical processes taking place as SO2 and NO2 are absorbed by paper. Future work should include studies of the effectiveness of the deacidification process, and both active and passive mode exposures to NO2 and SO2 should be carried out. In the active mode, purified air containing realistic (i.e., low) levels of SO2, NO2 or both could be "pumped" through paper samples, thereby accelerating pollutant uptake and absorption. These "active mode" experiments would provide a fast screening method to examine the effectiveness of various deacidification processes, and could be used as a guide to select which deacidification process should be studied in more detail in longer-term passive exposures. For passive mode experiments, it would be better to expose only one type of paper at the time so that damage and chemical content could be directly related to dose. Some exposures could be carried out at higher (but still realistic) pollutant concentrations, e.g. 200 ppb, to facilitate observations of physical damage. Consideration should also be given to designing exposure protocols that minimize fluctuations in pollutant absorption.