Metales pesados en suelos. Identificación de sorbentes, distribución entre fases amorfas y cristalinas y respuesta fisiológica de especies nativas / heavy metals in soils. Identification of sorbents, distribution among amorphous and crystalline phases and physiological response of native species

Resumo

The contamination of soils by heavy metals is an increasingly worrying issue because many of the contaminants are toxic for organisms and they can enter the trophic chain, thus bringing about harmful effects for humans. These elements are used in a great deal of industrial, urban and agricultural applications and quite often, more than one can be detected, sometimes even a wide range; in the contaminated sites, not only in those affected by the introduction of residues, but above all also in areas dedicated to extraction activities. Therefore, it is important to understand the interaction of these elements with the different components of the soils where they are found, fundamentally to have information about their bioavailability in the short, mid and long term. Understanding the capacity of soils to fix these metals is important because it affects key environmental issues, such as the prediction of risks derived from their release, the recovery of contaminated soils and waste management. The bioavailability and the destination of heavy metals in the environment are both controlled by sorption and desorption processes; in solid-liquid interfaces, these processes play an important role in environmental studies, especially regarding the control of metallic cations transferring to surface and underground waters. As a result, understanding as deeply as possible the bioavailability of contaminants in the short, mid and long term depends on the understanding of the sorption and desorption reactions which take place on the surfaces of soil particles and the distribution of these metals among the geochemical phases of the. Therefore, it is necessary to be aware of their chemical and structural properties, given that the sorption of metals, dissolution/precipitation reactions and pore transferring phenomena take place in interfaces, whose physicochemical properties will be different to those of the individual components of the soil. This work increases, deepens and gives new knowledge to a research line that gave rise to highly relevant results related to, individual and competitive, sorption and desorption of heavy metals by different soils. From this research line they were derived predictive and explanatory models of the sorption and retention capacities, of the hysteresis and of the migration throughout different soil profiles. It is essential to expand the knowledge of the sorption and retention capacity of heavy metals and other potentially toxic elements in all of the horizons from different soils to evaluate the ability to fix them. Based on the results and previous findings, it is considered very necessary to deepen the knowledge of the distribution of retained metals among the different geochemical phases of soils for estimating the possibility of flow throughout the profile because until now the research done on mobility, distribution and competition is not enough. Therefore, different natural uncontaminated soils were selected to know the properties and characteristics of all horizons, especially the chemical, structural and textural ones and to perform experiments of sorption and desorption, individual and competitive, of potentially toxic heavy metals, byall the horizons. After these experiments were performed, they were determined the concentration and distribution of the metal, or heavy metals, in all phases, crystalline or amorphous, by means of chemical sequential extraction. Likewise they were also evaluated the specific organo-mineral associations, the particles present in the soils and which ones retain the metal or metals added. The main constituents were characterized by field emission scanning electron microscopy with energy dispersive X-ray spectroscopy (FE-SEM/EDS). The combination of the previous techniques with the time of flight secondary ion mass spectrometry (TOF-SIMS) was used to know after direct visualization the distribution of these metals in the solid fraction of the soils. Since the use of the chemical sequential extraction gives rise to artifacts that often do not correspond to the real situation, the joint use of all of these techniques allowed to properly assess the competition, selectivity and mobility of heavy metals; providing further information on their availability. The combined use of the methods of speciation by sequential chemical extraction, along with the analysis with FE-SEM/EDS and TOF-SIMS, was also applied to the study of contaminated soils, especially by extractive activities. In the minespoils, settling ponds and mining places of these areas, there are soils with high contents of heavy metals or other toxic elements. The objectives were to assess the total content of heavy metals and also to check the validity of the techniques used in previous studies to understand their distribution among the different geochemical phases of the soils and thus be able to propose non aggressive measures for recovery and to ostensibly improve the soil quality of these areas. In general, the soils developed in areas of extraction of metallic minerals, the vegetation growth is inhibited and not optimal coverage develops. This fact increases erosion and promotes the loss of nutrients and the release of contaminants. However, many of these areas are relatively covered by native plant species that grow spontaneously and which have a high capacity to adapt to high concentrations of metals and other potentially toxic elements. The growth of these species in polluted areas is due not only to its phytostabilising capacity, their tolerance is the result of the activation of mechanisms that act against oxidative stress caused by pollutants. Therefore, the presence of these species favours the development of vegetation cover, increases the organic matter content, improves the texture and structure of the soils and thus reduces the exposure to erosion and leaching of toxic elements contents in soils of mining areas. Therefore these plant species can be used as a strategy for remediation of contaminated soils. Therefore the objectives of this study were: i) to increase the knowledge of the sorption capacity and retention capacities of heavy metals in all the different soil horizons for evaluating the ability to fix them, deepening in determining the distribution of retained metals between the different geochemical phases of the soils through the combined use of chemical sequential extraction, FE-SEM/EDS and TOF-SIMS; ii) to study soils contaminated by heavy metals coming from areas dedicated to extractive activities like metal mines and quarries of ultrabasic rocks; analysing not only the total content but also the risk of mobility of these metals, by studying their distribution and interaction with the geochemical soil phases; validating the combined use of the methods of speciation by sequential chemical extraction, along with the analysis with FE-SEM/EDS and TOF-SIMS for identifying sorbents that fix the heavy metals in the soils of these areas and for assessing the distribution and association with the different geochemical phases of the soil and iii) to investigate the physiological response of Cistus monspeliensis L. growing spontaneously in mining areas of the Iberian Pyrite Belt (Portugal) to estimate whether it can be used as a phytoremediator species. According to the proposed objectives the following overall conclusions can be drawn: 1. In natural soils, the fixation of heavy metals added individually or in competition, it is higher in surface than in subsurface horizons and it mainly occurs in exchangeable sites of organic matter, oxides, especially amorphous Fe oxides, and clays. The crystalline Fe oxides and the residual fraction barely influence their retention. It was demonstrated that the addition in an acidic medium produced alteration reactions in several primary minerals, increasing the amorphous content and forming new crystalline phases. 2. The elemental maps obtained by means of TOF-SIMS showed the overlapping between the soil components and the added metals. The combined use of this technique with FE-SEM/EDS demonstrated that, generally, they are associations between soil components those which jointly retain the metals, rather than individually. 3. It was verified that the combination of FE-SEM/EDS and TOF-SIMS together with X ray diffraction and sequential chemical extraction is an effective tool for identifying the sorbents of the soil and for determining their affinity for the fixation of heavy metals as well as for assessing their competition. 4. In the soils from extractive areas the total heavy metal contents are higher than the contamination levels indicated in different international guidelines. They also have limiting organic matter content, low cohesion and unfavourable texture and structure. Although a high proportion of the heavy metals is in the residual fraction, the Fe and Mn oxides play a fundamental role because they fix important quantities. The ones associated to the organic matter and to the exchangeable fraction are lower than 10% of the total content. 5. In the mine and quarry soils, the combined use of FE-SEM/EDS, TOF-SIMS, sequential chemical extraction and statistical analysis was also validated as a suitable tool for identifying the sorbents of the soil, determining their affinity for the cations of the heavy metals and for probing their distribution between the amorphous and crystalline phases. 6. The risk of mobilization and lixiviation of significant quantities of contaminants in the soils from the tailings is very high. Together with the acid drainage produced by the oxidation of the metallic sulphides, there are unfavourable soil conditions that inhibit the growth of the vegetation and hamper the development of an optimal cover. 7. Cistus monspeliensis L. is a plant from the Cistaceae family that spontaneously grows in soils from the Iberian Pyrite Belt providing them a relative cover. Its adaptation to high concentration of metals was proved after the artificial contamination of soils with Zn, finding that the contents retained in the roots were higher than in aerial part, thus avoiding toxicity problems and minimizing the oxidative stress. Moreover, if there are high concentrations of Zn in the aerial part, the defence mechanisms are also activated by increasing the activity of antioxidative enzymes and the glutathione-ascorbate cycle. 8. In soils with high concentrations of Zn, the contents of chlorophyll, anthocyanin and glutathione in the plants decrease and the ones of ascorbate and also of H2O2 increase, therefore the oxidative stress also increases. 9. The comparison between the C. monspeliensis growth in mining areas with high concentrations of As, Cd, Pb and Zn and the ones from non contaminated areas showed that it is adapted to unfavourable environments by showing higher enzymatic activity and contents of ascorbate and glutathione as a response to the oxidative stress. 10. The phytostabilising potential of C. monspeliensis and its tolerance to high concentrations of heavy metals and other potentially toxic elements allow suggesting its use as phytostabilising species when climatic conditions are favourable. It is expected that the lixiviation and mobility of the toxic elements present in the soils decrease because the vegetal cover will increase the organic matter content and will improve the texture and structure of the soils, thus reducing the risks of erosion.