Obtención de residuos de segunda generación a partir de posos de café mediante carbonización hidrotermal. Posibilidades de uso agrícola y otros avances para su aplicación en una estrategia de economía circular

Abstract

Spent coffee grounds (SCG) are a residue of the agri-food industry generated in large quantities worldwide. Its rich composition in fiber [62%, cellulose, hemicellulose, lignin], fat (13-18%), protein (14-17%) and ash (1.3%) makes this residue a material with chemical and biotechnological potential that can be used within a concept of circular economy. However, its high content of toxic compounds has made its use in the agricultural field difficult. The objective of this work has been to focus on the revaluation of SCG through hydrothermal carbonization (HTC) as the main transformation process and, from the byproducts generated (hydrochar and liquor), explore the possible applications based on their characterization; this with greater emphasis on the hydrochar. The HTC generated hydrochars with different properties than the SCG. Second generation solid products with a higher amount of fixed C, N, H, C/N ratio, specific surface area and total polyphenols were obtained. On the contrary, other properties were reduced: pH, EC25, assimilable P and K, particle size, volatile matter, and ashes. The application of a defatting process combined or not with an ethanol wash prior to HTC did not have an impact on the significant reduction (p <0.05) of the total polyphenol content or on the improvement of the particles structure. but they could be used for other exploitation purposes. On the other hand, the analysis of the HTC liquors showed a majority composition of reducing sugars, short-chain fatty acids (SCFA) and other toxic compounds. The characteristics and properties of both HTC by-products were highly influenced (they are accentuated) by the temperature applied in the process. For the different applications of SCG hydrochars in the agricultural field, a Mediterranean cultivated soil and lettuce plants (Lactuca sativa var. longifolia) were used as reference. The evaluation of two of these bio-products (H175, H185) as organic soil amendment at doses of 1% and 2.5% resulted in a slight improvement in the physicochemical properties of the soil, but it was not significant with respect to the SCG. The contribution of organic carbon and the mobilization of nutrient elements with the increase of the dose stood out, however, a limitation of the biomass was evidenced even at the lowest dose (1%) due to the greater phytotoxic nature of the hydrochars with respect to the SCG. Based on the mobilizing capacity of elements in the soil, a first evaluation of these bioproducts as biofortification agents was carried out through the functionalization of two hydrochars (H160, H200) with Zn and Fe. The results revealed that there is a better balance between the increase of mineral content and plant growth when bio-chelates are applied to the soil in fixed doses (0.5%) compared to varied doses that depend on an established mineral concentration. Although a certain degree of phytotoxicity was still observed, plant Zn and Fe contents increased by up to 90% (H160-Zn) and 288% (H200- Fe), respectively, compared to non-biofortified lettuces. Subsequently, the activation of the hydrochars prior to their functionalization, caused an increase in the adsorption capacity of Zn (758%, H160) and Fe (230%, H160) and at the same time a reduction of the content of total polyphenols (46-73%). This fact made it possible to reduce the dose tested for biofortification effects (0.2%), obtaining as a result a total suppression of phytotoxicity. Both Zn and Fe contents tripled compared to nonbiofortified lettuces. This effect was also proportionally reflected in the percentage of recommended daily intake (RDI) of each micronutrient. Finally, in all the biofortification tests carried out, the creation of a mineral reserve in the soil that could be used in future crop cycles was evidenced. In general terms, SCG hydrochars exhibited great potential to be used in the agricultural field as biofortification agents as long as they are used in subtoxic doses (≤ 0.2%) and go through a prior activation process that guarantees a good contribution of the tested micronutrient. The liquors also showed attractive fermentation potential for the obtention of high biological value compounds. The techniques tested in this doctoral thesis are subject to optimization and/or modification tests in order to obtain the best use and yield.