The surface chemistry and morphology of biochars produced by pyrolysis of walnut shells affects their utility for adsorption applications. Yet, little is known about surface interactions at a pore-scale level in these materials, mostly due to the challenges in accessing information at this length scale, and in a non-destructive manner. Here, for the first time, adsorptions strengths of solvents comprising different functional groups over internal (pore) surfaces of walnut shells and derived biochars were investigated using nuclear magnetic resonance relaxation time measurements to non-destructively probe interactions of fluids over pore surfaces. Carbon bonding state compositions of these materials with respect to distance from the particle surface were determined using X-ray photoelectron spectroscopy coupled with ion beam etching. Alkaline pretreatment was found to increase the hydrophilicity of both walnut shells and derived biochars as well as their surface interactions with hydroxyl groups, and to decrease those with methyl groups. Results were contextualised by thermogravimetric analysis, scanning electron microscopy, and in-situ X-ray imaging results. Taken together, the results showed that alkaline pretreatment may be used to modulate responses to pyrolysis temperature of several factors that affect adsorption properties including surface hydrophilicity, particle size, porosity and pore accessibility, and surface texture.