Removing Metallic Stains from Paper Objects Using Chelating Agent EDTA

Removing Metallic Stains from Paper Objects Using Chelating Agent EDTA by D. G. SURYAWANSHI & S. K. BISARIA

INTRODUCTION Among the many kinds of damage found on old paper-based documents, books, archival materials and art objects, disfiguring stains and spots are something particular and quite common. Metals, in particular iron, copper and their salts, can play a vital role in provoking such disfiguring stains if, under favourable atmospheric conditions they react with non fibrous materials in the paper or with substances in polluted surrounding air. Foxing is a very common example. Such stains are believed to occur due to the presence of compounds such as iron oxide or iron hydroxide combined with fungi. Fungus, too, may be seen as the cause of foxing, but the distinctive colour is believed to be the result of the transformation of a colourless iron compound, evenly distributed throughout the paper. Prolonged development of stains and spots does not only cause disfiguring, but can also accelerate the rate of deterioration of paper objects. Such stains cannot be removed just by using either water or an organic solvent or by oxidative bleaching. Therefore, with a view to solving the problem of removing unwanted stains and spots from paper objects, the use of a chelating agent is discussed. We chose a quite common one for our studies, i.e. ethylenediamine tetra acetic acid (EDTA).

CHELATING AGENTS Chelating agents can be defined as organic compounds which complex or sequester metal ions. The formation of the complex prevents the metal ions from taking part in chemical reactions in which it would normally participate. It is, in effect, eliminated from the system. In paper conservation chelating agents are used for two purposes, namely the removal of iron stains and the stabilisation of bleach solutions, in particular hydrogen peroxide. Here the former function is studied. In order to be called a chelating agent, a molecule must have two appropriate functional groups, each having a donor atom, which is capable of combining with a metal atom by donating a pair of electrons. In effect, a positively charged metal centre combines with the negative centre of the chelating molecule. These centres, which donate electrons, can be grouped into two broad categories: • Acidic Compounds: Acids which can lose a proton to create a negative charge. This anion then complexes with the positively charged metal centre. As the loss of the proton is pH dependent, so is the ability of a particular acid to act as an effective chelating agent. Carboxylic acid- (R-COOH), hydroxyl group (R-OH) and sulphonic acid (H2SO3) are examples for this kind of chelating. • Basic Compounds: The second group, which can donate electrons and complex to a positive metal centre, is that of basic compounds. In this case, the negative centre comes from an available set of electrons and does not require the loss of a proton or the presence of a negative charge. These basic groups are called coordinating groups. Carbonyl (R-HCO), amine (R-N2H) and hydroxyl group (R-OH). Two acidic groups can be present in the same chelating agent. Oxalic acid (HOOC-COOH) is an example for this type of chelating agent. Ethylene-diamine (H2N • CH2 - CH2 • NH2) is an example of a chelating agent that has two basic and one acidic group. One basic group can be present in the same chelating agent. An example for this type is glycine (H2N-CH2-COOH). The second requirement for chelation to take place is that these functional groups must be situated in such a way that they permit the formation of a ring with a metal centre as the closing member. Chelating agents can have several locations which can complex with a metal centre. Ethylenediamine tetra acetic acid (EDTA), a very common chelating agent, has six such sites: sites which can complex with most cations in a one-to-one ratio; e.g. one molecule of EDTA to one ion of iron. Under acid conditions the solubility of EDTA is fairly low. It increases quickly as the pH becomes more alkaline. EXPERIMENTAL Paper samples: Whatman chromatography paper In order to conduct the experiments, Whatman chromatography paper (W), a pure cellulose paper, was chosen and tested for its physical and chemical parameters such as acidity or alkalinity, brightness, copper number and folding endurance

Table 1 : Properties of W Paper before and after accelerated ageing.

Table 2: Properties of W paper after treatment with EDTA solutions.

Table 3 : Properties of W paper samples before and after iron staining.

* Gloss Reflectance Meter (AIM-611) manufactured by M/s, AIMIL, New Delhi, India ** TAPPI Standard Method T430: Copper number of pulp, paper and paperboard. *** Model L & W, manufactured by M/s, Switzerland before and after accelerated ageing at 105°C for 72 hours. The results are given in Table 1. EDTA aqueous solutions were prepared in six different concentrations: 0.5%, 1.0%, 1.5%, 2.0%, 2.5% and 3.0% w/v. After measuring the pH of each solution the W paper samples were immersed in a bath for 90 min. The samples were dried at room temperature for about 48 hours and again the physical and chemical properties were tested in order to find out if there was any change that could be identified as being provoked by the EDTA treatment. The results are given in Table 2. To attain iron stains in the W paper samples, pieces of iron were put onto wet blotting paper and kept at room temperature for about 72 hours. The iron pieces corroded slowly and emitted their yellowish orange colour throughout the paper fibres. The stained blotting paper was then used to produce iron stains in the W paper by contact and absorption. The W paper sheets were kept in between two sheets of iron stained blotting paper and maintained under wet conditions at room temperature for about 72 hours. The iron stained W paper samples were dried for a week, conditioned and finally retested for the same properties as before. The results are given in Table 3. EDTA solutions The pH of a chelating agent EDTA is in the acidic range, and it becomes more so as the concentration increases. Because it might be considered dangerous to treat paper in an acidic solution, we modified it by adding alkali. Each EDTA solution was made alkaline by adding 2% potassium hydroxide (KOH) solution, in a comparative study for possibly better results, the procedure was repeated using 2% sodium hydroxide (NaOH).

Sodium dithionite (SDT) A reducing agent, i.e. sodium dithionite (NA2S2O4) (SDT) was tested as well, first alone in two concentrations, i.e. 2% and 4%, and then, as in the case of KOH and NaOH as an additive at both 2% and 4% to each concentration of EDTA.

Treatment The stained W paper samples were kept in the EDTA and SDT solutions described above for 24 hours in order to remove the iron stains. After this treatment they were thoroughly washed with distilled water, dried and again measured for the named qualities. The results of all treatments are given in Tables 4-9, and the results of some are visualized in Fig. 1.

RESULTS AND DISCUSSION The pH of pure cellulose paper was found to be almost neutral; no appreciable change after accelerated ageing at 105°C for 72 hour was found, and also no change even after iron staining. The original brightness of the paper was found to Fig. 1 : Visualization of the paper property numbers given in Tables 4, 5, 6 and 8.

Table 4: Properties of W paper samples after removal of iron stains with EDTA solutions.

be very high; it was reduced by 3.3% during accelerated ageing and by 8.7% after iron staining. The copper number of the untreated paper was very low; it did not increase during accelerated ageing, but it clearly increased considerably after being stained. The original folding endurance was reduced by about 8.1% during accelerated ageing and increased by 11.9% after staining. The effect of EDTA, the EDTA solutions mixed with the two alkalies and the sodium dithionite on the removal of metallic stains and their effect on the properties of the paper were studied as follows.

Effect of EDTA (Table 4) At a lower concentration of EDTA solution, removal of iron stains was found to be very small or even equal to nil during 24 hours, but the effect gradually improved at higher concentrations i.e. from 2.0% to 3.0% over the same period. For complete removal, however, more time was needed. The paper became a little brighter after treatment with higher EDTA concentrations, but also a little more susceptible to oxidisation, i.e. the copper number was increased. Its folding endurance was reduced significantly step by step with increasing EDTA concentration. It is clear from these results that a more concentrated and thus more acidic solution, though helpful in removing the stains, would also cause adverse effects on the properties of paper.

Effect of KOH and NaOH with EDTA (Tables 5 and 6) Each concentration of EDTA solution was made alkaline by adding KOH and NaOH in a minimum concentration i.e. 2% each. Due to the addition of alkali the pH of the treated samples shifted into the alkaline range and did not drop af-

Table 5: Properties of W Paper samples after removal of iron stains with EDTA and KOH (2%) solution. Table 6: Properties of W Paper samples after removal of iron stains with EDTA and NaOH (2%) solution.

ter removal of the stains followed by washing. Brightness increased in the alkaline treatment as compared to the acidic treatment with pure EDTA. The values of copper number considerably increased and folds were decreased. Both alkaline solutions required more time to remove the iron stains from the paper in comparison with the pure EDTA solutions.

Effect of SDT (Table 7) The pH of SDT solutions was very close to pH 7.0 i.e. nearly neutral, as was the paper treated with the solution for stain removal followed by washing with distilled water. The iron stains were removed completely during the stipulated time period, together with an increase in brightness of the paper sample. The copper number also increased considerably and folding endurance decreased. There was only a slight difference in these numbers between the lower (2%) and the higher (4%) concentration of the SDT solution.

Table 7: Properties of W Paper samples after removal of iron stains with sodium dithionite (SDT) solution.

Table 8: Properties of W Paper samples after removal of iron stains with EDTA and SDT (2%) solution. Table 9: Properties of W Paper samples after removal of iron stains with EDTA and SDT (4 % solution.

Effect of SDT with EDTA (Tables 8 and 9) Using mixtures of EDTA solutions with SDT solutions 2.0% and 4.0% the stains were removed very easily in a prescribed time period as compared to the effect of EDTA and SDT solutions used separately. There were also no appreciable changes in the paper properties.

CONCLUSIONS • The higher the concentration of EDTA, the easier and more complete will be the complexing of metal ions. A concentration of 3.0% can be accepted as sufficient to remove iron stains in a short time at pH 4.6. Making the EDTA solution alkaline by adding NaOH or KOH results in a lengthening of time required for stain removal, but also in better stain removal. • Paper strength measured as folding endurance is affected by a high concentration of EDTA and also by making the EDTA solution alkaline. Using alkaline EDTA solutions the higher concentration is of less influence. • EDTA treatment alone, though able to remove iron stains, cannot be suggested for this purpose because of its acidid nature. It is advisable to use it together with alkalies to protect the physical strength properties of the treated paper. • Sodium dithionite, being a reducing agent, helps to remove the stains already in a minimum concentration in a short period but it is reported that the effect is not stable. The reduced ferrous iron can re-oxide and the iron stain can reappear. This is why a chelating agent like EDTA used together with SDT can be very helpful in removing the reduced iron ions from paper. • SDT in the minimum concentration of 2% along with EDTA has been found to be the most effective of the tested procedures to remove iron stains from paper in a comparatively short time.

SUMMARIES Removing metallic stains from paper objects using chelating agent EDTA Paper samples (pure cellulose: Whatman chromatography paper) were stained with iron and then submitted to a treatment with the chelating agent EDTA in different concentrations in order to remove the stains during a stipulated time period (24 hours). As EDTA is acidic, which means that it can harm the paper, the solutions were modified with alkali (NaOH and KOH, 2%). Another variant was to add a reducing agent, i.e. sodium dithionite, to the chelating solution. This last variant gave the best results together with the least reduced fold number and highest brightness. Elimination des taches de rouille des documents en papier à l'aide de l'agent chélateur EDTA Des échantillons de papier en cellulose pure (papier de Whatman) ont été tachés avec de la rouille et soumis à un traitement comportant l'agent chélateur EDTA à des concentrations différentes afin d'éliminer les taches dans une période donnée (24 h). Etant donné que l'EDTA a un pH acide, ce qui signifie qu'il peut causer un dommage au papier, les solutions d'EDTA ont été modifiées avec des alcalins (NaOH et KOH, 2%). Une autre variante consistait à ajouter un agent réducteur, le dithionite de sodium, dans la solution. Cette dernière variante a fourni le meilleur résultat en ce qui concerne la résistance au pliage et le degré de blancheur du papier.

Das Entfernen von Rostflecken aus Papier mit Hilfe des Komplexbildners EDTA Proben aus reinem Cellulosepapier (Whatman Chromatographiepapier) wurden mit Rostflecken versehen und einer Behandlung mit dem Komplexbildner EDTA in verschiedenen Konzentrationen unterworfen, um die Flecken in einer festgesetzten Zeit (24 h) zu entfernen. Da EDTA ein saures pH aufweist, was bedeutet, daß von ihm Schaden für das Papier ausgehen kann, wurden die EDTA- Lösungen mit KOH und NAOH-Lösungen 2% modifiziert. Eine weitere Variante war die Zugabe eines Reduktionsmittels, Natriumdithionit, zu den Lösungen des Komplexbildners. Diese letztere Variante brachte das beste Ergebnis zusammen mit dem geringsten Rückgang der Falzahl und der höchsten Helligkeit des Papiers.

BIBLIOGRAPY 1. Shahini, C.J., & F. H. Hengemihle: The influence of copper and iron on the permanence ofpapef. Preservation of Historic Textile and Paper Materials. Advances in Chemistry Series 212. Washington: American Chemical Society 1986: 387-410. 2. Helen Burgess: The use of chelating agents in conservation treatments. The Paper Conservator, 15 (1991): 36-44. 3. Burgess, H. D.: Practical considerations for conservation bleaching. Journal of the International Institute for Conservation, Canadian Group 13 (1988): 11-26. 4. Burgess, H. D.: The elimination of chloramine-T residues through the use of reducing agent anti-chlors. 6th Triennial Meeting of the ICOM Committee for Conservation. Ottawa 1981. Preprint 14/12. Paris: ICOM 1981: 1-15.

Dr. D. G. Suryawanshi, Sr. Scientific Officer, S. K. Bisaría, Sr. Scientific Assistant NRLC, E/3, Aliganj, Lucknow-226024. India E-mail: [email protected]