Enfoque multianalítico para evaluar la contaminación por plomo en suelos / multianalytical approach to assess the soil contamination by lead

Resumo

Soil is the unconsolidated top layer of the earth crust that develops on the surface of the rocks due to their physical alteration, chemistry and influence climate, topography, organisms and time. It is a living system and its existence and conservation are due to the dynamics and ecological balances of the large number of living beings that inhabit it; influencing plant growth and development. This is because many processes of food energy transfer (through soil organisms) take place. Each one feeds on the foregoing and, in turn, feeds the next. Soils are exposed to a natural and/or anthropogenic set of pressures and degradation processes that affect their physical, chemical and biological properties. The soil is a finite resource, which implies that its loss and degradation are not reversible, even in the long term. It is under increasing pressure due to intensified agricultural, forestry, pastoral and urban development purposes, as well as satisfying food, energy and raw material production demands, and because of its use as a support for certain activities that may contaminate it, such as military or sport shooting. A soil can be degraded by accumulating potentially hazardous elements (PHEs) to levels that adversely affect its behaviour. Such elements, at these concentration levels, are toxic to soil organisms. It is a chemical degradation that causes partial or total loss of soil productivity. Soil contamination with PHEs is alarming because many of them are toxic to organisms and they can get into the food web producing harmful effects on humans. They come from different industrial, urban and agricultural sources, and are often found together in contaminated sites. In contamination studies, it is not sufficient to detect the presence of contaminants, but it is necessary to define the maximum levels and also analyse the possible factors that can influence the soil response to pollutants. Lead is an element that can produce very harmful effects if it reaches the food web. It is a heavy metal that forms many salts, oxides and organometallic compounds. Its most important minerals are galena (PbS), cerussite (PbCO3), crocoite (PbCrO4) and pyromorphite (Pb5 (PO4)3Cl), although it is also found in various uranium and thorium minerals as they come directly from radioactive disintegration. It is used in the production of accumulators, batteries, pigments, pesticides, explosives, chemical reagents, solders, anti-knock additives for petrol (now banned in many countries), pottery, covers for protection against X-rays, pipes, projectiles, etc. It is a serious pollutant of the atmosphere, hydrosphere and edaphosphere. The addition of Pb to petrol causes an unnatural cycle of this metal. It is burned in car engines, thus generating Pb salts (chlorides, bromides, oxides) that enter the environment through the exhaust pipes. The large particles fall into soil or water, and the small ones travel long distances through the air and remain in the atmosphere. Some of these then fall with rain. Therefore, the Pb cycle, due to anthropic activity, is much more widespread than the natural cycle, causing pollution. Lead reaches soil from industrial and mining waste, atmospheric deposition, military activities and from bedrock, if it is rich in minerals containing this metal. Although it can be immobilized in organic matter and clay soil, if it is acid, or is acidified, Pb is mobile and can be absorbed by plants and through them it can reach animals and man. The low mobility causes the permanence of Pb in the surface horizons and therefore, it is not easily assimilated by plants in the same degree as other potentially toxic elements. Thus, soils are an important sink for lead compounds. It is necessary to consider that as Pb is more mobile in acid mediums, common in mining waste and landfill leachates, it can be released and get into the food web from these sources. In addition to acidity, mineralogy, texture and organic matter content, Pb availability also depends on the nature of the Pb compounds that reach the soil. If Pb is present in bioavailable form, it can be absorbed by the different components of the food web: modifying microbial activity; accumulating in invertebrates and the plant root system, and generating adverse effects on primary producers. Consequently, Pb and its compounds tend to be accumulated in soils and sediments, and part of this metal is in bioavailable form for a long period of time, so there is a risk of entry into the food web. In recent years, many efforts have been made to reduce the danger associated with Pb-contaminated soils, recognise contaminated sites and locate contamination sources, since they often come from diverse and highly dispersed sources. It is, therefore, necessary to deepen the knowledge of analytical techniques which determine the origin of Pb and establish its contents in different soils, as well as its availability and toxicity as accurately as possible, in order to propose measures to reduce its availability. Hence, this PhD Thesis was planned. In order to do so, affected areas were selected from various human activities that provide high levels of Pb from different origins to the environment. To evaluate its contents and availability in the soils from these areas, different analytical techniques were used. Those chosen were considered adequate for each zone, some of them relatively new, whose validity has been proven in previous studies. Therefore, different soils were selected from disturbed areas by anthropic activities, which introduce Pb and other potentially toxic elements and compounds into the soil. These soils were: i) those from an old Zn/Pb mine, located in Rubiáis (Lugo, Spain); ii) the urban soils from parks and gardens located in the municipality of Vigo (Spain); iii) those from a small arms firing range and a trap shooting range (Monforte, Lugo, Spain) and iv) those from a military manoeuvers and shooting range (El Teleno, Astorga, Spain). All of this allowed us to determine, as accurately as possible, the Pb content and availability, and those of other potentially hazardous elements (PHEs) and compounds from each of these activities. Toxicity and ecological risks in the affected areas were also evaluated, and measures to reduce its content and availability were proposed. They were tested and applied different analytical techniques and the more appropriated were selected to evaluate with the highest accuracy and according to the area and the source of pollution: the contamination level of the soils of each one of the selected areas; the availability, the toxicity and the ecological risks. Finally, it was probed in selected soils the effectiveness of applying Ca3(PO4)2 nanoparticles to decrease the availability of the contaminants. The results obtained allowed to achieve the following conclusions: 1. In all of the selected areas, the different anthropic activities incorporate to the soils high levels of Pb and of other potentially hazardous elements and compounds that cause different degrees of contamination and toxicity. 2. The mine soils have high physical and chemical limitations for plant growth standing out the high contents of Pb and Zn that exceed the official thresholds for ecosystems and agricultural, urban and industrial uses. 3. The urban soils from the city of Vigo are moderately contaminated by Ba, Pb and Cu due to the influence of industrial areas and the main transport routes. 4. The use of the 206Pb/207Pb ratio have allowed to know that the both industrial and fuel emissions are the main sources of Pb in the urban soils from the city of Vigo. 5. The contamination of the soils from the small arm firing range comes from both the Pb ammunition and the steel one that releases Cr, Fe and Ni. 6. Lead is the main pollutant in the soils from the military shooting area followed by Sb, Cu, As, Ni and Cd. 7. The contents of Pb and polycyclic aromatic hydrocarbons in the soils from the trap shooting range exceed the generic levels of reference. 8. CaCl2 is the most efficient reactant for extracting potentially toxic elements and the one that better reflects the available content. 9. Regardless, the source of the contamination, the soils with highest contents of potentially toxic elements lack vegetation. The presence of Agrostis capillaris in the soils from the trap shooting range and of Cytisus scoparius and Betula celtiberica in the mine soils favors the phytostabilization. 10. The combined use of MC-ICP-MS and TOF-SIMS allows studying the interaction between the soil components and the Pb isotopes and identifying the anthropic of geogenic origin of this element. 11. The chemical analysis of the total and available content together with the use of TEM-EDS and TOF-SIMS is the methodology that provides the best information about the interactions between the potentially toxic elements and the soil components and about their possible toxicity, therefore about their availability in a short, medium or long term. 12. The adverse effects identified in Eisenia andrei individuals after the toxicological experiments indicate that the habitat soil function is affected mainly due to the Pb contents. 13. The application to the soils of Ca3(PO4)2 nanoparticles reduces up to 90% the available contents of Pb and Cu due to adsorption processes. 14. In order to select the suitable treatment and dosages for the soils recovery, it is compulsory to carry out more and deeper studies to understand the different nanomaterials capacities for immobilizing the contaminants and the possibility that they cause toxic effects in the soils. 15. More research is needed to be able to determine the available content of each element in each soil. This information together with the one obtained in this work will favor the evaluation and the improvement of the quality of contaminated soils.