Analytical method, pattern and sources of polycyclic aromatic hydrocarbons (PAHs) in the stone of the Temples of Agrigento (Italy)

Abstract

This study shows the extraction and analytical conditions for the determination of polycyclic aromatic hydrocarbons (PAHs) in the stone of archaeological and historical interest. The aim of this research was to determine the concentrations, distribution and the main sources of PAHs present in the surface layer of stone of the Temples of Agrigento. Together with PAHs, we analyzed sulphates and nitrates since it has been demonstrated that these are very destructive salts that play an important role in the deterioration of stones. Total PAHs at different sites of the Valley of Temples at Agrigento varied from 18 to 84 μg/kg. Analyses were performed by GC–MS. The relative abundance of molecular weight PAHs, together with the PAHs compound ratios and supported by the total index (by us proposed), demonstrate that the most samples in Hellenistic Agrigento historical buildings owe their PAHs to a predominant single mode of origin, i.e. anthropogenic combustion processes. The same origin of PAHs pollution was confirmed by the good correlations existing between total PAHs concentrations and the content of sulphates and nitrates of the analyzed samples.

Introduction

Since the last century, the degradation of natural and artificial stones has been accelerated by increased air pollution, essentially caused by sulphur and nitrogen oxides as well as hydrocarbons which are emitted into the atmosphere by sources related to industry, transportations and heating [1]. Also biological activity in the surface layers of the material contributes to the weathering of stones in monuments [2]. One aspect of the weathering process is the formation of thin surface layers or, in some cases, black coloured gypsum crusts on the stone of buildings [3]. Surface layers are the areas where atmospheric depositions accumulate along with the products of the chemical transformation of materials. Dust and soot have often been blamed for the dark colour of the weathered surfaces. The surface layers not only impair the aesthetic aspect but they also initiate destruction of the stone surface by exfoliation. Their chemical composition varies from one site to another. Physical and chemical weatherings have been largely investigated [4], [5], but, only a limited amount of knowledge is available regarding the organic compounds present on the surface layer [3], [6], [7] material.

It is known that fluidized beds of limestone have been successfully used to remove polycyclic aromatic hydrocarbons (PAHs) and it seems that there is an effect not only of calcium carbonate but also of the porosity of calcite [8].

The principal aim of this study was to determine the concentrations, distribution and the main sources of PAHs present in the surface layers of stone of some historical buildings located in the Valley of the Temples (Agrigento, Italy) dating from 6th B.C. century to 2nd century A.D.

The interest arises from the fact that air pollution is a major concern in Italy along with the industrial wastes and the traffic congestion in cities all causing pollutants that significantly contribute to environmental damage to cultural heritage and to health. Further, often, during the restoration, operators are exposed [9], [10] to dusts of the material that in this case can contain very hazardous substances as PAHs.

Atmosphere often contains many organic pollutants which are related to incomplete fuel combustion in domestic heating, industrial plants and vehicular exhausts, such as alkanes, carboxylic acids and PAHs [11], [12].

The interest in PAHs, both from analytical and environmental point of view, lies with the fact that, although most of them have no practical use, they are widespread in all the environmental compartments. They can be found, to different extents of concentration, in atmosphere [12], in water [13], in soil [14], in sediments [15], in food [16], [17] and in many matrices [18], [19] and show high toxicity levels towards living organisms [11], [15].

PAHs are chemicals containing two or more fused benzene rings in a linear, angular or cluster arrangement. They are usually generated under inefficient combustion conditions, such as insufficient oxygen by primary natural sources which are forest fires and volcanic activity, but most of the PAHs released into the environment arise from anthropogenic sources such as burning of fossil fuels, biomasses (grass, wood, etc.) in petroleum refineries, industrial processes, as a constituent of coal tar and motor vehicle exhaust [20].

The lighter PAHs (2–3 rings) are generally found in the gas phase while the heavier ones are mainly associated with airborne particulates. PAHs with more than three rings are speedily transferred to particles, by adsorption or condensation upon cooling of fuel gas. PAHs are transported in the atmosphere over short and long distances in both gaseous and particulate forms. These compounds, in the environment (air, water, sediments, soils, etc.) are subject to redistribution and transformation processes [21], [22]. Atmospheric deposition constitutes the main input of PAHs to soil and to monuments surface [3]. Once absorbed in the materials they accumulate [8] and are likely to be retained for long time due to their persistence and hydrophobicity, especially in the presence of organic matter [15], [22], [23]. The environmental occurrence of PAHs has been associated with adverse effects on public health [24], [25], [26], [27]. The effects of airborne PAHs on both human and environmental health have recently forced the European Union (European Community, 2005) to issue strict regulation with regard to these pollutants [25]. This in turn places heavy demand on efficient monitoring of these regulated pollutants. Owing to their persistency and bioaccumulation, often PAHs have been used as biomarkers for purposes of environmental bio monitoring [11].

In order to get a better insight in the environmental levels and distribution of these compounds, reliable analytical methods capable of measuring a large range of PAHs at low concentrations are required. There are some studies on the concentrations of PAHs in different matrices but information about the distribution of PAHs in ancient building materials is rare [3], [7]. Some authors have investigated the factors influencing the accelerated solvent extraction for the analysis of PAHs [28], [29], [30]. Martınez-Arkarazo et al. [7], in an integrated analytical approach, characterized several organic compounds (mainly PAHs) by GC–MS. Concentrations higher than 20 mg/kg of total PAHs were measured in the black-crusts of the rain-protected areas. They concluded that the presence of certain PAHs indicated combustion of fuels as the main source of deterioration for the palace house building materials. Pyrolysis-gas chromatography/mass spectrometry was used to study the organic compounds present in weathered building materials from historic buildings and monuments. Different materials were investigated: a black layer from a terracotta statue from the Pardon Gate, Cathedral of Seville; a green layer found beneath a black sulphated crust from the Cathedral of Seville; and a black coating covering the stones of the Old Church of Delft. N-alkanes and PAHs were the main components of analyzed samples [31].

The organic and elemental carbon content in Saxonean sandstones as well as a few microbiological data of black surfaces on historic buildings in Dresden has been obtained during preliminary studies. Organic compounds in the samples after extraction were analyzed by GC–MS. About 150 compounds were detected. Their origin is discussed and correlated with data from the literature about organics in the atmosphere and with the microbiological results [32].

The difficulties of analysis of the environmental matrices are due to their high variability (qualitative and quantitative) and their complexity. It is necessary to check the validity of the analytical method using the material to be analyzed.

Here we report an analytical method for 16 PAHs which adopts and improves previous knowledge and affords better results on the concentration levels and the distribution of PAHs in surface layer of eight samples of stone of the Temples of Agrigento.

The investigations have been performed on the fifteen PAHs recommended by US-EPA as priority pollutants to be monitored in the framework of the environmental quality control [26]. Several PAHs species have been classified into probable (2A) or possible (2B) human carcinogens by the International Agency for Research on Cancer [27]. Moreover, perylene, non-US-EPA listed PAHs, has been investigated with the aim to obtain additional information on PAHs’ origin.

In this work, together with PAHs, the chemical composition of the leachable stone fraction is measured. We quantified sulphates and nitrates in the solution of soluble salts because it has been demonstrated that sulphates are very destructive salts that play an important role in the deterioration of stone. In coastal areas like Agrigento, marine aerosol could be the source of a fraction of sulphate ions.

Nitrates have been investigated with the aim to obtain additional information on PAHs origin.

The pollution related atmospheric SO₂ is also a very common source of sulphates. Particulate derived from fossil-fuel combustion and humidity has been shown to play an important role in the sulphatation of carbonate stones. Particulate appears to contribute to the catalytic oxidation of SO₂ to form SO₃, which successively forms sulphuric acid in the presence of water. Transformation of calcite and dolomite by H₂SO₄ leads to precipitation of gypsum and epsomite blocking organic pollutants on the stone surface.

Nitrogen oxides are gaseous components which react directly with the stone surface by forming acids in the presence of water and oxidising agents. These acids react with the stone to form salts (nitrates and nitrites) which either crystallise out within the stone resulting in damage or they are washed away resulting in a loss of material. Nitrates are more soluble salts and expand less than sulphates, leading to little, if any, damage by this mechanism. Removal of sulphates and nitrates by appropriate cleaning procedures is one of the goals of the restoration techniques.