A study on cashew bagasse hydrochars and pyrochars and their interaction with water and soil

Abstract

The Northeast of Brazil has been facing throughout its history problems related to severe droughts. Different projects should be suggested in order to minimize this problem. Among them are new soil additives as biochars, materials made from biomass that, among many possibilities, have the potential to increase the water retention capacity of the soil to which it is added. In order to elucidate the possibility of the preparation of biochar with biomass from the Northeast region, this work was developed with cashew bagasse as the precursor. The objective was to study the preparation, characterization and thermochemical activation of these biochars and clarify the interactions between them and water. So, in Stage I cashew bagasse was first dried and grounded, and then charred by two methodologies: pyrolysis and hydrothermal carbonization. The biochars were then characterized by CHNS Elemental Analysis, Fourier Transform Infrared Spectroscopy (FT-IR), Raman Spectroscopy, X-Ray Diffractometry (XRD), Scanning Electron Microscopy (SEM) and N2 Adsorption-Desorption Isotherms. For Stage II, a new batch of cashew bagasse was retrieved to produce hydrochars and pyrochars in a greater scale for activation. Stage II samples were activated and characterized by the same techniques used before. It was observed that even though precursor biochars presented very different profiles, activated samples were very similar to each other in structure, composition and texture. The thermochemical activation process promoted a decrease in the contact angles with distilled water for all samples, suggesting enhancing in the hydrophilicity, and a greater water soil Plant Available Water Capacity (PAWC) when samples were added to soil. Differential Scanning Calorimetry (DSC) results for water-biochar interactions did not present any easily distinguishable pattern, although for the best PAWC sample, bulk water supercooling was softened and micropore lodged water appeared after activation. The results of cashew bagasse experiments present improvement when compared to experiments with different raw materials described in the literature, meaning that it would probably be suitable to be used as soil conditioner.