Effect of texture on the dynamics of a water saturated biochar

Biochar is defined as charred organic matter applied to soil in a deliberate manner, with the intent to improve soil properties (Lehmann et al., 2009). Biomass-derived char can be used as energy carrier, as adsorber and as material for the improvement of soil properties. Carbonized organic matter can have different physical and chemical properties depending on the technology used for its production. Moreover char texture influences deeply physical and chemical properties. In this work effect of texture on the dynamics of a water saturated biochar was analyzed by FFC NMR. An industrial biochar derived from gasification of poplar wood was sieved to determine influence of texture on water dynamic on particles surface. Sieves with 2, 1 and 0.3 mm have been used to separate three different texture fractions. Each sample has been saturated with water and then analysed by FFC NMR relaxometry by applying a range of magnetic field from 40MHz to 0.01MHz. Three profiles with different longitudinal relaxation rate have been carried out. Sample with texture 1-2mm has a profile with the lowest longitudinal relaxation rate (R1). Profile of sample with textures <0.3mm has the profile with highest R1. Behavior of water can be explained associating an increment of longitudinal relaxation rate with an increment of porosity. Surface area reduction is achieved when the sizes of the pores increase. As water molecules flow through larger sized pores, their motion occurs at a frequency that is broader than that of water molecules constrained in smaller sized pores. For this reason quickly moving water can not efficaciously interact with either neighboring molecules or with the molecular sites on the surface at the liquid-solid interface. As a consequence intermolecular dipolar interactions are weakened and a reduction of the proton longitudinal relaxation rate (shorter R1 values) can be observed compared with the R1 values for slowly moving or immobilized water systems.