MONITORING OF ENVIRONMENTAL PARAMETERS TO EXPLAIN STONE DETERIORATION: CHURCH OF STA. MARIJA TA'CWERRA, MALTA

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1 265 MONITORING OF ENVIRONMENTAL PARAMETERS TO EXPLAIN STONE DETERIORATION: CHURCH OF STA. MARIJA TA'CWERRA, MALTA TORFS, K. Dept. of Chemistry, University of Antwerp (U.l.A.), B Antwerp, Belgium VAN GRIEKEN,R. Dept. of Chemistry, University of Antwerp (U.IA), B Antwerp, Belgium CASSAR,J. Inst. for Masonry and Construction Research, University of Malta, Malta SUMMARY To evaluate the effects of air pollution and the influence of marine aerosols on the weathering of historical monuments, the Church of Sta. Marija Ta'Cwerra in Malta has been selected. The study included the chemical examination of the weathered stone layer at the outside and the inside of the building. Environmental parameters (total deposition, aerosols and meteorological data) have been monitored during more than one year. 1. INTRODUCTION The church of Sta. Marija Ta'Cwerra is located on the southwest of the island, at a distance of 3 km from the sea. It is a building from the 17th century, less than 10 by 10 metres. The church is built entirely of Globigerina limestone, a highly porous limestone. The four external walls of the church show severe decay. The middle courses are deteriorated in the form of alveolar weathering, as well as powdering of several areas. Most of the mortar has been lost from the joints in this area. At the inside of the building, the plaster has fallen away in several areas revealing powdering and flaking stone underneath (Cassar, 1993). 2. SAMPLING The total deposition is sampled on the roof of the church by means of a funnel with a diameter of 20 cm, attached to a 1 L polyethylene bottle. The funnel is rinsed weekly with 50 ml of deionised water to collect also the dry deposition in the funnel and to ensure a new sample every week. The sampling has been carried out from March 1994 till March 1996; 97 samples have been collected. Meteorological data are gathered from the weather station, 10 km away from the church. To collect outdoor aerosols, a filter unit in cascade geometry, provided with a top-hat inlet and connected to a low volume pump, is used. Polycarbonate filters (Nuclepore) with a diameter of 47 mm and a pore-size of 2 mm and 0.4 mm are placed behind each other, in order to collect coarse and fine particles separately. The indoor aerosol sampling needs only one filter with a pore-size of 0.4 mm. The sampling is carried out during one week. The monitoring at the inside has been started in November 1994 and at the outside in March 1995, both lasted till March More than 60 inside aerosol samples have been collected, as well as 46 outside aerosol samples. At the inside of the church, a stone sample has been taken at a height of 2 m on the south wall (MA-1), efflorescence samples have been taken on the south wall, at a height of 1.7 m (MA-2) and on the north wall at a height of 0.3 m (MA-3). on the outside, stone samples have been taken at the south wall only : MA-4 is a sample from deep destruction of the stone, MA-5 is an exfoliation sample, MA-6 is a sample from the depth in the alveolar weathering.

2 SAMPLE PREPARATION AND ANALYSIS TECHNIQUES The volume of the total deposition samples is measured. The sample is filtered over a Whatman 41 filter. Weighing the filter before and after filtration gives the mass of the total suspended particles (TSP). The ph of the filtrate is measured using a ph electrode, which is calibrated with buffer solutions of ph 4 and 7. The HC0 3 - content is determined by titration : HCI is added to 1 O ml of the sample till the colour of the methylorange indicator changes from yellow to orange. The anion and cation determinations are canied out using ion chromatography (IC), atomic absorption and atomic emission spectrometry (AAS and AES). The aerosols are first analyzed by energy dispersive X-ray fluorescence (EDXRF). Afterwards, the filters are leached in exactly 25 ml of deionised water. The suspension is filtered and the soluble ions are determined by IC, AAS and AES. To determine the soluble salts in the stone, the samples are crushed and the powder is dispersed in water. The aqueous suspension is filtered and the filtrate is analyzed for the ionic content using IC, AAS and AES. For EDXRF analysis, about one gram of the crust samples is wet-ground in a McCrone Micronizing Mill. A few millilitres of the suspension is brought on a Mylar foil, to obtain a sample thickness of around 1 mg/cm 2. The foil is dried at a temperature of 60 C. Ion chromatography (IC) is used for er, N0 3 - and sol determinations; a Dionex 4000i instrument, equipped with a AS11 separator column is used; the eluent is 20 mn NaOH. Ca 2 + and Mg 2 + are determined by means of AAS, whereas Na+ and I< are measured by AES, both by means of a Perkin Elmer 3030 spectrometer. A Tracor Spectrace 5000 instrument is used for EDXRF. The instrument is equipped with a Si(Li) detector and a low power X-ray tube with a Rh target. The X-ray spectra are accumulated for 3000 sec and analyzed using the AXIL software (Van Espen et al., 1986). For the aerosol samples, the AXIL program calculates the concentration of the elements on the filters in mg/m 2 Since the surface of the filter and the volume that has been sampled are determined, the concentrations of the elements can be expressed in ng/m 3. The calculation of the concentrations in the stone samples has been checked with soil standards : IAEA Soil 5 and BCR DISCUSSION OF THE RESULTS A. Total deposition and aerosols The ionic balance has been calculated to check whether the major ionic species in the total deposition samples have been determined. The balance, defined as the ratio of the sum of the cations to the sum of the anions, has an average value of 1.29 ± This confirms that the major content of the samples is determined. Table 1 Vol H Weekly average total deposition (meq/m 2 /week) (the average volume is expressed in vm21week). Ca 2 + Mg2+ Na+ K+ HC03- er N03- sol The weekly average total deposition of the ions is presented in Table 1. Ca 2 +, Na+, HC0 3 - and er are the major ions in the total deposition. The Ca 2 + content is quite high and the acidity low, because of the neutralizing action of calcite particles from the monument. Roekens et al. (1988) observed the same phenomenon near the cathedral of Mechelen (Belgium). In the Mediterranean, the high Ca 2 + content may also be explained by the presence of Saharan dust (Laye-Pilot et al., 1986), but only this can not be the cause of the high value, we observe. When comparing our results with results of rainwater, obtained in Sicily (Alaimo et al., 1989), it seems that we determine higher concentrations of Ca 2 + and er, but that the sol content is lower in the samples from Malta, maybe due to less pollution or emissions of Etna in Italy. The seasonal variation in the deposition, has been evaluated by grouping the sampling periods, according to the season during which they were collected (Table 2). The deposition is clearty lowest during summer, due to

3 267 the long dry periods. Especially for er, Na+ and Mg 2 +, the deposition dominates during winter, which may be explained by higher wind velocities, causing more breaking waves. The deposition has the highest acidity during winter, maybe due to less neutralising particles in the atmosphere. Most rainfall occurs during autumn and winter. Table 2 Seasonal variation of the total deposffion (expressed in%). Vol H+ Ca 2 + Mg2+ Na+ K+ HC03. er N03. so/ spring summer autumn winter Enrichment factors, relative to the average composition of seawater, using Na as indicator element, have been determined to evaluate to which extent the sea influences the composition of the total deposition samples. The following equation has been used: F'.reawaler (X) (X /Na )101al dep. (XI Na ) seawater with X and Na the concentration of the investigated element and Na, in the total deposition and seawater, respectively. The total deposition samples are enriched relative to seawater in ea 2 + (EF=40); this may be explained by calcite particles from the monument, while the other elements originate from seawater (EF close to 1). The relationship between two variables is considered by use of the concept of the correlation coefficient. A high correlation is observed between Na+, Mg 2 + and er (r-0.9), elements derived from the sea. The volume is correlated with the ea 2 + and Heo 3 - content (0.8). The acidity (H} is correlated with the Na+, Mg 2 + and er content (r=0.7). ea 2 + is correlated with the Heo 3 - and so/ content (r=0.7). HC03-, N03- and sol are correlated (r-0.7). The temperature is not significantly correlated with any ion, but all correlation coefficients are negative, which means that the lower the temperature the higher the deposition (winter rains). Principal component analysis (PeA) has been applied to the data set. The first component is explained by the volume, e r, Na+, Mg 2 + and ea 2 +. Na+, Mg 2 + and er form one cluster in the loading plot, while HC03-, sol and Ca 2 + form another cluster of variables. This implies that these elements do not reflect the action of the same source: a marine factor is acting together with the influence of calcite particles from the monument. The temperature seems to be negatively related with e r, Na+ and Mg 2 + (the lower the temperature the higher the deposition of these elements, i.e. more breaking waves, generating sea salt in winter). The composition of the total deposition is mainly influenced by the vicinity of the sea (high Na+, Mg 2 + and er concentration). The highest deposition occurs during winter, especially for the elements generated by the sea. Ca 2 + is correlated with Heo 3 - and S , which indicates the action of a second source (anthropogenic influence or calcite particles from the monument). The average inside and outside aerosol concentration is given in Table 3. The concentrations of the 2 and 0.4 mm pore size Nuclepore filter from the outside are given separately, while the sum is also presented to compare with the inside aerosols.

4 268 Table 3 Average concentration of the aerosol samples (concentrations are expressed in ng!m 3 ). N03. Na+ Mg2+ Al Si so/ er K+ Ca IC AES AAS XRF XRF IC IC AES XRF in out out out Ti v Cr Mn Fe Ni Cu Zn Br Sr Pb XRF XRF XRF XRF XRF XRF XRF XRF XRF XRF XRF in out out out Ca, Na+, er and sol are the most abundant elements in the aerosols. The inside concentration is more or less the same as (or a bit higher than) the outside concentration; Ca, Fe, Al, Ti, Mn, Ni and Sr are more abundant at the outside. The concentration of the fine fraction is lower than of the coarse fraction; only I( is more abundant in the small particle fraction. In comparison with the aerosol composition around the cathedral in Mechelen (Belgium) (Leysen et al., 1989), it seems that the er and Ca concentration is higher in Malta, due to the vicinity of the sea and the higher Ca content is possibly influenced by the composition of the soil in Malta. The concentration of sol, Pb, Fe, V and Ni is higher in Mechelen, caused by more anthropogenic activities. The course of the aerosol concentration throughout the seasons is given in Table 4. Differences between the seasons are not as explicit as for the total deposition. The concentration of er, Na+, Mg 2 + and Br in the aerosols is highest during winter, maybe caused by more breaking waves, while for t< and sol, highest atmospheric concentrations occur during summer. For metals (Fe, Ti, Mn, Cr), only small variations are observed throughout the year; the concentration reaches its minimum during autumn, which can be explained by the start of rain events after the dry summer. Table 4 Seasonal variation of the aerosols at the outside (expressed in%). N03- Na+ Mg2+ Al Si so/ er K+ Ca spring summer autumn winter Ti v Cr Mn Fe Ni Cu Zn Br Sr Pb spring summer autumn winter The correlation coefficient has been calculated between the inside and the outside concentration for each element. Only for V, S and Ni, some correlation is observed between the inside and the outside. It seems that the concentration inside is almost independent of the outside concentration, for short time intervals like one week. This may be explained by the fact that the church is rarely used nowadays (Cassar, 1993). The contribution of seawater and mineral dust to each element in the aerosol is assessed by means of enrichment factors (Table 5). Al is used as indicator element for soil dust; the average soil composition is

5 269 taken from Mason (1966). Mg, Cl, S, Br and Sr originate mainly from the sea. Si, Mg, Fe and Ti behave as non-enriched elements to crustal concentrations, while the other elements are enriched. For most elements, the enrichment factors are larger for the small particles than for the large particle fraction, indicating that soil dust mainly generates large particles. Mg, K and Sr seem to originate from soil dust in the coarse particle fraction, while these elements are enriched to soil dust for the fine fraction. Table 5 Aerosol enrichment factors. Na Mg Al Si s Cl K Ca Ti Fe Br Sr vs_ seawater (and Na as indicator element) in out out out vs. soil dust (and Al as indicator element) in out out out The correlation matrices have been calculated for the inside and outside aerosols. At the inside, a high correlation is observed between Na, Mg, Cl, Kand Ca (r=0.7). Fe is correlated with Si and K (r=0.9), indicating the origin of the soil. At the outside, a correlation is observed between Na, Mg and Cl (r-0.9). Al, Si, K, Ca, Ti, Mn, Fe and Sr are all correlated (r-0.8). Pb is correlated with some metals 01. Cr, Mn, Ni, Zn) (r-0.8). Cl is negatively correlated with the temperature and positively with the wind speed. Na, Mg and Cl, elements originated from the sea, are correlated in the coarse and the fine fraction, while Al, Si, Fe and Ti, elements derived from soil dust, are only correlated in the coarse fraction and not in the fine one. PCA indicates that the first component of the inside aerosols is composed of Fe, Si, Kand Ca and explains 32 % of the variance. For the outside aerosol samples, the first component is explained by Mn, Fe, K, Ca, Ti, Sr, Si and Al. The second component is explained by Cl, N0 3 -, Na, Mg and Cl and the wind speed and negatively by the temperature. The first component contains soil derived elements, while the second factor, which explains less of the variance, is composed of marine elements. The aerosol concentration is influenced by the sea and the vicinity of the monument. The high Ca concentration is mainly due to the loss of calcite particles from the church. Sea derived elements (Na and Cl) are very abundant in the aerosols. Soil derived elements (Fe, Ti, Si, Sr and AQ determine to a higher extent the aerosol composition of the coarse fraction than of the fine fraction. B. Stone samples Table 6 presents the results of the leaching experiment. The concentrations of the ions are expressed in weight percent to the dissolved mass of the sample. Small enrichments of Cr, sol. ca 2 and Na in sample MA-5 and of er, N03-, ca 2 and Na are observed in samples MA- Table 6 Results of IC, AAS and AES on leachate samples (weight %). er sol N03- Ca 2 Mg2 Na K MA MA MA MA MA MA

6 270 4 and MA-6. At the inside of the church, a higher concentration of Na and er is found (MA-1) than at the outside, while the concentration of N03- and sol is almost the same as at the outside. The salt efflorescence from the south wall exists mostly of NaCl, while the efflorescence from the north side consists mainly of Nai$0 4. Fassina (1993) also found that the efflorescences contain mainly chloride at the south wall and mainly sulphate at the north wall. The height of the sampling influences the distribution of the efflorescence : in the lower zone, the less soluble and less hygroscopic sulphates are present, while in the upper zone, the chlorides accumulate (Arnold and Zehnder, 1989). This is in agreement with our results: the sample taken at 0.3 m contains mainly so/, while the sample from 1.7 m height contains er. Table 7 shows the EDXRF results of some samples. The samples from the outside contain quite high amounts of Si; S is almost not present in the outside and inside samples. The composition of the inside sample is comparable with the outside stone samples, although the concentration of Pb is higher in the stone samples from the outside than the inside. The inside efflorescence sample (MA-2) contains a high amount of Cl and S, as is observed with the leaching experiment, while the Ca content is lower (because this sample does not contain limestone). The efflorescence contains lower concentrations of Ti, Mn and Fe than the stone samples. The enrichment factors with respect to average carbonate rock have been calculated to identify the component due to the deposition of atmospherical gases and aerosols on the stones. The elemental composition of carbonate rock, reported by Mason (1966). is used; Ti is used as indicator element (Sabbioni, 1995). To estimate the importance of sea derived elements and soil dust on the stones, enrichment factors relative to seawater and the average soil dust composition have been determined as well. Results are presented in Table 8. Enrichments to carbonate rock have been found for Cl, S and Na. Cl, Mg, S and K seem to originate from the sea. Ca in the crust is enriched with respect to seawater, because of the under1ying limestone; the Ca content of the efflorescence seems to be influenced by seawater. For Mg, K, Si, Mn and Fe, enrichment factors to soil dust are close to 1, indicating that soil dust may be their major source. The weathering layer on the building seems to be mostly affected by the action of seawater. The efflorescence sample from the inside contains mostly sulphates and chlorides, depending on the height. Enrichment factors indicate that the weathered layers are enriched mainly in er and Na. which originate from the sea, and that soil dust could be the source of Si and Fe, but these elements are not enriched relative to the average carbonate rock composition. 5. CONCLUSION It is striking that in the crust and deposition samples in Malta, the NaCl concentrations are much higher than in Eleusis, Greece and Bari, Italy (Van Grieken and Torts, 1996) Table 1 Si(%) P (ppm) S(%) Cl(%) K(ppm) Ca(%) Ti (ppm) Cr (ppm) Mn (ppm) Fe (ppm) Ni (ppm) Cu (ppm) Zn (ppm) As (ppm) Br (ppm) Rb (ppm) Sr (ppm) Y (ppm) Zr (ppm) Pb (ppm) Results of EDXRF on the stone samples. MA-1 MA-2 MA-4 MA N.D. N.D N.D N.D N.D N.D N.D. N.D. N.D N.D. N.D

7 271 although the latter are located closer to the sea. This is undoubtly related to the higher average wind speed and the geomorphology of the coast of Malta, causing more sea spray formation, particular1y in winter. The environment around the Church of Sta. Marija Ta'Cwerra is mainly influenced by the sea. The total deposition contains almost only sea derived elements, only the ca 2 deposition is very high, due to erosion of the monument and the limestone soil. The aerosols around the monument seems to be influenced mainly by sea salt particles and soil derived elements. Soil derived elements are abundant in the stone samples from the outside of the building, while also high concentrations of Na and er are observed. On the inside, the efflorescence contains er and sot. depending on the height. It can concluded that the chemical deterioration of the church is mainly caused by the sea and that almost no effects of anthropogenic emissions are found. Table 8 Damage layer enrichment factors Cl s Ca Mg Na K Si Mn Fe Sr Pb vs. carbonate rock (and Ti as indicator element) MA MA MA vs. sea water (and Na as indicator element) MA MA MA vs. soil dust (and Al as indicator element) MA MA MA ACKNOWLEDGEMENT This study was financed by the European Community under contract No EV5V-CT We thank Dr. Fassina from the Soprintendenza ai Beni Aristici e Storici di Venezia (Italy) for helping us with some of the anion analyses of the deposition samples. REFERENCES Alaimo R., Deganello S., Di Franci L., Montana G. (1989) Caratteristiche composizionali del particolata solido atmosferico e chimismo delle acque di precipitazione nell'area urbana di Palermo. Proc. 1st Intern. Symp. on The Conservation of Monuments in the Mediterranean Basin, Ed. Zezza F., Grafo, Bari, Arnold A. and Zehnder K. (1989) Salt weathering on monuments. Proc. 1st Intern. Symp. on The Conservation of Monuments in the Mediterranean Basin, Ed. Zezza F., Grafo, Bari, Cassar J. (1993) The Church of Santa Marija Ta'Cwerra at Siggiewi, 1st Semestrial Report : "Marine spray and polluted atmosphere as factors of damage to monuments in the Mediterranean coastal environment", Ed. Zezza F., R & D Programme in the Field of Environment, Contract No EV5V-CT , Fassina V. (1993) Church of Sta. Marija Ta'Cwerra (Malta) : salt efflorescence analysis by ionic chromatography. 2"d Semestrial Report : "Marine spray and polluted atmosphere as factors of damage to monuments in the Mediterranean coastal environment", Ed. Zezza F., R & D Programme in the Field of Environment, Contract No EV5V-CT , Leysen L., Roekens E. and Van Grieken R. (1989) A ir-pollution induced chemical decay of a sandy-limestone cathedral in Belgium, Sci. Tot. Env. 78, Laye-Pilot M.D., Martin J.M. and Morelli J. (1986) Influence of Saharan dust on the acidity and atmospheric input to the Mediterranean, Nature 321, Mason B. (1966) Principles of Geochemistry, 3'd Ed., J. Wiley & Sons, New York, p Roekens E., Komy z., Leysen L., Veny P. and Van Grieken R. (1988) Chemistry of precipitation near a limestone building, Water, Air and Soil Pollution 38, Sabbioni C.(1995)Contribution of atmospheric deposition to the formation of damage layers, Sci. Tot.env.167, Van Grieken R. and Torts K. (1996) Atmospheric aerosols and deposition near historic buildings: chemistry, sources, interrelations and relevance, in EC Workshop Origin, Mechanisms and Effects of Salts on Degradation of Monuments in Marine and Continental Environment, Ed. Zezza F., Bari. Van Espen P., Janssens K. and Nobels J. (1986) AXIL-PC software for the analysis of complex X-ray spectra, Chemometrics and Intelligent Laboratory Systems 1,