Climate change impact on the photodegradation of polycyclic aromatic hydrocarbons in soils
- Autores: Montse Marquès Bueno
- Directores de la Tesis: Martí Nadal Lomas (dir. tes.), José Luis Domingo Roig (codir. tes.), Montserrat Mari Marcos (codir. tes.)
- Lectura: En la Universitat Rovira i Virgili ( España ) en 2017
- Idioma: español
- Tribunal Calificador de la Tesis: Nuno Miguel Ratola Neto (presid.), Eva Pocurull Aixalà (secret.), Esther Martí Verge (voc.)
- Materias:
- Enlaces
- Tesis en acceso abierto en: hdl.handle.net TDX
- Resumen
The assessments of the Intergovernmental Panel on Climate Change (IPCC) have evidenced that, due to increasing greenhouse gases, the Earth’s climate is substantially changing faster. Several studies have confirmed that the global mean temperature increased by 0.6±0.2ºC during the 20th century, while it is projected to increase up to 1.8-4.0ºC by the end of the 21st century, under a range of probable greenhouse gas emission scenarios. Moreover, the increase in the surface UV‐B radiation induced by ozone depletion has received wide attention as an environmental issue of great concern. High latitude regions are those more severely affected. In addition, the Mediterranean region is pointed out as a vulnerable zone because of lying in the transition between high and low latitude processes. Therefore, the impact of climate change on the environment has become a topic of notable concern.
The expected increase in temperature and UV-B radiation are key parameters that will alter the fate and behavior of a wide range of pollutants, such as persistent organic pollutants (POPs). Persistent organic pollutants (POPs) are chemicals of concern because of their toxicity, resistance to degradation, potential to be bioaccumulated and ability to be transported over long distances from sources by air and ocean currents. Although not all PAHs are considered POPs, they have a photosensitive nature, and therefore, are potentially vulnerable to climate change.
The present thesis was aimed at reporting the impact of the rising temperature and light intensity on the fate of PAHs in two typical Mediterranean soils. The monitoring of PAHs concentrations and ecotoxicity, as well as the identification of PAHs photodegradation by-products were carried out at laboratory scale by the simulation of the current climate and the extreme climate change scenario (RCP 8.5) for the Mediterranean region according to IPCC. In addition, a field experiment was performed in order to assess the PAHs degradation under real Mediterranean conditions. Finally, PAHs levels were determined in soils from another potential vulnerable region to climate change: the Arctic.
In Chapter I, the state of the art of the impact of climate change on environmental concentrations of POPs, as well as on human health risks, was reviewed. Climate change and POPs are a hot issue, for which further attention should be paid not only by scientists, but also and mainly by policy makers in order to adapt outdated regulations. Most of the studies performed so far were found to be focused on legacy POPs, such as polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), polychlorinated biphenyls (PCBs) and pesticides. However, the number of investigations assessing the impact of climate change on the environmental levels of polycyclic aromatic hydrocarbons (PAHs) was limited. Some studies pointed out that as a result of the photosensitivity of PAHs, more toxic photodegradation byproducts may be formed and result in adverse health effects, becoming an unquestionable gap which should be evaluated.
Chapter II presents the photodegradation rates of PAHs in soils under the current Mediterranean climate scenario. Arenosol and fine-textured Regosol soils, representative of the typical Mediterranean soils, were spiked with PAHs and exposed to controlled conditions of temperature (20 °C) and low light intensity (9.6 W m-2) for up to 28 days. Concentrations of PAHs were further monitored and supported with Microtox® ecotoxicity assessment. In addition, H isotopes of benzo(a)pyrene were analyzed to confirm its degradation. Photodegradation was found to be dependent on exposure time, specific physicochemical properties of each hydrocarbon, and soil texture. Sorption and photodegradation processes were enhanced in fine-textured soil in comparison to Arenosol soil. Significant photodegradation rates were detected for a number of PAHs, namely phenanthrene, anthracene, benzo(a)pyrene, and indeno(123-cd)pyrene. Benzo(a)pyrene, commonly used as an indicator for PAH pollution, was below the limit of detection after 7 days of light exposure. Ecotoxicity assessment, performed by means of Microtox®, showed a higher detoxification trend in fine-textured soil than in Arenosol soil, agreeing with its higher photodegradation rates. Reported differences between both soils were mostly attributed to the higher content of metal oxides in fine-textured Regosol soil, behaving as potential PAHs photocatalysts. Finally, the strong isotopic effect observed in benzo(a)pyrene suggested, on one hand, that compound-specific isotope analysis (CSIA) may be a powerful tool to monitor in situ degradation of PAHs, and on the other hand, evidenced an unknown degradation process of benzo(a)pyrene simultaneously occurring in the darkness.
Chapter III was aimed at explaining the differences in PAHs photodedradation rates found between Arenosol and fine-textured soil in chapter II. It was hypnotized that the main difference could be related to the photocatalysis caused by the joint impact of light exposure and iron (III) oxide presence in soils. The catalysis contribution was mostly attributed to iron (III) oxide because it is the most abundant metal oxide in both tested soils, being higher in Regosoil than in Arenosoil. Reported results showed that iron (III) oxide had a slight potential role in the photocatalysis of fluorene, phenanthrene, anthracene, fluoranthene and pyrene, while it was very significant on benzo(a)pyrene. Therefore, iron (III) oxide would not be the only responsible for the higher degree of PAHs photodegradation in fine-textured Regosol soil than in Arenosol soil. Indeed, soil is a complex matrix with several elements, such as a wide range of metal oxides, in addition to humic acids, and a specific texture, having each one of them a potential specific role on PAHs fate.
In chapter IV, the potential impact of the temperature and light intensity increase on the fate of PAHs in soils surface was assessed. The environmental temperature was increased up to 4 °C, according to RCP 8.5 scenario from IPCC 2013 in the Mediterranean region, while a high light intensity was set (24 W m-2), being samples exposed during 28 days as well. As expected, low molecular weight PAHs were rapidly volatilized when increasing both temperature and light intensity. However, photodegradation of medium and high molecular weight PAHs increased in the coarse-textured Arenosol soil under the climate change scenario, while those rates did not show any significant variation in fine-textured Regosol soil, regardless of the climate scenario. Indeed, the lower content of metal oxides in Arenosol soil might probably need a higher temperature and light intensity to achieve a full photodegradation of PAHs reaction, pointing out a potential impact of climate change. In turn, hydrogen isotopes confirmed that benzo(a)pyrene was degraded in the climate change scenario, not only under light but also in the darkness, as previously occurred in the current climate scenario. Finally, the number of by-products and required time to be formed was enhanced by the increase of temperature and light intensity. Consequently, in an expected climate change scenario, the human exposure to PAHs might decrease while that to PAHs degradation products, which might be more toxic than native compounds, might increase.
Chapter V provides the evaluation of PAHs photodegradation in soil surface caused by solar radiation exposure. Solar intensity is up to 20-times higher than that emitted by common light lamps used for photodegradation studies at lab scale, therefore this field study was fundamental to evaluate the fate of PAHs in the environment. Indeed, PAHs photodegradation caused by sunlight has never been assessed in any matrix (air, water, soil), being this evaluation filling an identified gap. Soil samples spiked with PAHs were deployed in a methacrylate box, and exposed to solar radiation for 7 days, which in turn meant a solar energy of 102.6 MJ m-2. As hypothesized, the individual PAHs were volatilized, sorbed and/or photodegraded, depending on their physicochemical properties, as well as the soil characteristics. Low and medium molecular weight PAHs were more sorbed and photodegraded in fine-textured Regosol soil, while a higher volatilization was observed in the coarse-textured Arenosol soil. In contrast, high molecular weight PAHs were more photodegraded in Arenosol soil, most probably because of an enhanced light penetration in a coarse-textured soil. Specially high photodegradation rates and low half-lives were noted for anthracene, pyrene and benzo(a)pyrene, which had already been found to be the most sensitive to light exposure at laboratory scale. In addition to oxidation products of PAHs previously found at laboratory scale, new oxy-, as well as nitro- and hydro- PAHs were also identified in the field study. The toxic and mutagenic potential of these PAHs byproducts is usually higher than that of the 16 priority PAHs commonly monitored.
Finally, chapter VI shows the levels of PAHs in Arctic soils. Pyramiden (Central Spitsbergen, Svalbard Archipelago) was selected to be sampled because it is a potential contaminated site due to: i) the Long Range Atmospheric Transport (LRAT), ii) the previous coal-mining extraction, and iii) the currently working power plants in this settlement. Furthermore, trace elements were analyzed for further confirmation of anthropogenic pollution. PAHs profiles and molecular diagnostic ratios (MDRs) indicated pyrogenic sources in most samples, being combustion from local power plants the most plausible source. The highest levels of PAHs and trace elements were found in soils close to power plants and those exposed to prevailing winds. Although PAHs levels were higher than ever expected, they only exceeded reference values in three sampling sites. Trace elements were generally lower than reference values, being some of them (Be, Co, Hg, Mn, Ni and Zn) punctually above such references. All those most polluted sites showed a higher organic matter content, pointing out somewhat its role on retaining pollutants in soils. These unexpected high PAH levels found in Pyramiden demonstrated how important environmental monitoring programs in such remote areas are. In turn, in those northern regions there is a potential seasonal effect since the photodegradation of PAHs and the formation of oxy- and nitro- PAHs might be enhanced during the continuous light exposure of midnight sun season. The remobilization and formation of PAHs photodegradation products might be progressively enhanced in a climate change context.
Photodegradation is here reported as an important degradation pathway for PAHs in soil surface in highly irradiated areas, such as the Mediterranean regions, and under a climate change context. Further attention needs to be paid on changes on human health risk mainly in terms of more toxic degradation products. Although PAHs photodegradation will drive to a faster concentration decrease in soils, the oxidation and nitrification reactions causing the formation of oxy- and nitro- PAHs. These chemicals have lower lipophilicity, and therefore, higher mobility, bioavailability, toxicity, and even mutagenicity and carcinogenicity nature than their parent PAHs. Furthermore, there is no regulation regarding and consequently, they are not usually monitored in environmental surveillances. In addition, there is a lack of standardized analytical methods for most PAHs by-products which also difficults its vigilance. Present findings highlight the need for a new regulation of PAHs including their derivatives.
