Biochars derived from crop residues and green waste as soil amendment for the recovery of the functionality of trace-elements polluted soils

276 páginas.According to estimates, 2.5 million sites in Europe are potentially contaminated. Contaminants, such as trace elements, can be introduced into the environment by human activities. Trace elements are persistent and not biodegradable, consequently they accumulate and threaten soil health and the food chain, in particular if they are bioavailable and occur at elevated concentrations. Common approaches for remediation of such soils include the removal of the polluted soil regions, in the most cases the topsoil, and the off-site clean-up by washing or incineration. However, those strategies are costly and result in a considerable loss of soil. Alternative on-site techniques imply immobilization by precipitation as indissoluble salts or adsorption, for example to organic residues such as compost as well as phytoremediation. Whereas the latter is limited by the tolerance of the plants to the trace element concentration and the respective conditions of the polluted soils (i.e. acidity), the first bears the danger of quick remobilization as a consequence of biodegradation of the amendment. In order to circumvent this problem, several recent studies propose the application of biochar, the solid product of the pyrolysis of biomass, for the remediation of soils polluted by heavy metals, since due to its high aromaticity is expected to resist quick mineralization. A further advantage of this material lays in its high porosity which is likely to support trace metal adsorption, thus their sequestration. It was further hypothized that their potential of trace element sequestration could be high enough to decrease their bioavailable fraction to concentration below the tolerance threshold of accumulating plants. In order to test the feasibility of such a strategy, in the present PhD thesis the remediation potential of biochars derived from agricultural residues was investigated in an area polluted with trace elements due to a mine spill in 1998 (The Guadiamar Green Corridor). The studied soils consisted of Fluvisols with two contrasting levels of acidity and trace elements contents: acidic polluted soil (APS) and moderately acidic polluted soil (MAPS). Considering that biochar amendment may induce changes in soil properties, physical, chemical and biological VI parameters were selected to integrate the three types of soil quality indicators, which allow assessing the capability of the soil to perform its ecological functions. Therefore, the first objectives of this Doctoral Thesis were i) to evaluate how the kind of feedstock [agricultural residues: rice husk (RH), olive pit (OP) and pruning remains of olive trees (OB)] and pyrolysis conditions affect biochar properties and ii) to select those biochars with adequate characteristics for trace element immobilization potential (Chapter 3). Slow pyrolysis was performed in a fixed bed reactor with pyrolysis temperature from 350 to 600 ºC and residence times in the reactor from 0.5 and 4 h. A detailed characterization of physical, chemical and surface properties including techniques such as solid-state 13C nuclear magnetic resonance (NMR) spectroscopy was conducted. Pyrolysis process increased pH, C content and proportion of aromatic C, whereas it reduced the atomic ratio H/C. At least, 500 ºC and 1 h are needed to achieve a high degree of aromatization of the biochars, alkalinity and water holding capacity (WHC). Among the studied feedstock, RH showed the most promising properties (pH >10, WHC >400 %, Aryl C >80 %) for trace elements polluted soil remediation. The next task of this PhD thesis was to study the adsorption of cationic metals in aqueous solution of RH and OP biochars (RHB and OPB) produced in a continuous reactor in comparison with a certified wood biochar (Chapter 4). Biochar chemical properties clearly affected the efficiency of Cu2+ and Pb2+ adsorption, being the acidic sites on biochar surface the key structural feature. RHB showed greater adsorption capacity than OPB. Subsequently, the remediation potential of heavy metal polluted soils by biochars was assessed in greenhouse experiments under controlled conditions. For these incubation experiments, polluted soils from the Guadiamar Green Corridor were collected. RH and OP biochars produced with the continuous reactor were applied at doses of 2 and 5 % (Chapter 4). RHB showed greater improvements in soil properties (pH and WHC) than OPB, enhancing Brassica rapa L. spp pekinensis survival and growth. The application of 5 % of RHB was more efficient than the application of 2 %. VII Afterwards, the remediation potential of 2, 5 and 10 % of RH and OP biochars produced at 400 and 500 ºC using the batch reactor were assessed (Chapter 5). High doses (5 and 10 %) of RH and OP biochars significantly increased soil pH, TC and water holding capacity. Biochar application reduced the bioavailable contents of trace elements (CaCl2-extractable) indicating their immobilization. It was demonstrated that biochars produced at 500 ºC were more efficient in metal immobilization. Particularly, 10 % of RHB produced at 500 ºC obtained the greatest reductions in the bioavailability of trace elements and the restoration of the degraded soils. A 20-months field experiment was conducted for evaluating the effects on soil microbial activity and plant biodiversity by the application of 8 t ha-1 of RHB and OPB produced at a continuous reactor (Chapter 6). Biochar application increased TC content in both soils, whereas soil respiration and enzymatic activities were not modified, indicating the C sequestration potential in soils by biochars. Both biochars improved soil physical properties and vegetation cover in MAPS and microbial community diversity in APS. For a more detailed study of the trace elements immobilization potential of biochars in multicontaminated soils, selected biochars were placed into nylon nets and buried in the studied soils for 2 years. This study showed that biochars had a greater total content of trace elements in those samples that were buried and recovered 2 years later in comparison to the original biochars, indicating metal adsorption. Nevertheless, the application of 8 t ha-1 of biochars to soils did not succeed in reducing the concentration of available metals (CaCl2-extractable fraction) in the soil. This result suggests that in such polluted systems there is an equilibrium in which trace elements are constantly moved from the immobile to the mobile pool, thus the stabilized amount of these elements by biochars is rapidly compensated. In order to study the biochemical degradability of biochars produced at 500 ºC, the CO2 production was measured using a respirometer apparatus and the mineralization constant, k, was determined. The latter allowed the calculation of their mean residence time (MRT) under controlled conditions, which ranged between 82 and 293 years. All studied biochars increased the MRT of the slow C pool of the amended APS and MAPS by 2 to 13 times if compared to the un-amended soils. VIII Moreover, the application of 10 % of RHB500_2, OPB500_2 and ASB500_2 to APS exceeded the threshold of 100 years, which indicates effective C sequestration. Summarizing the results of the present study, we can conclude that the incorporation of biochars from crop residues and green waste to trace element polluted soils participated in soil conditioning, that increase bacterial diversity, promotes plant development and increase soil carbon stabilization. This PhD thesis provides knowledge for the recovery of soil functions that could be applied in future. Lastly, we would recommend the use of RHB produced at 500 ºC for the remediation of multicontaminated soils with trace elements. Reapplications of RHB or the simultaneous use of other remediation technique, such as phytoremediation, may be advantageous if biochar is applied to soil at low dose (8 t ha-1).Se agradece a la Fundación Tatiana Pérez de Guzmán el Bueno la financiación de la beca predoctoral en Medioambiente (2017) para la realización de la tesis doctoral de Paloma Campos Díaz de Mayorga. El presente trabajo se ha realizado en el marco de los proyectos CGL2016-76498-R y CGL-2015-64811-P financiados por el Ministerio de Economía, Industria y Competitividad (MINEICO) y el Fondo Europeo de Desarrollo Regional (FEDER).Peer reviewe