This thesis aimed to characterize the physical and chemical properties of biochar (BC) and a biochar–compost mixture (BCO) using suit of analytical techniques and assess their surface reactivity via flow microcalorimetry (FMC). We hypothesized that compost addition enhances BC’s surface reactivity. Analyses included elemental composition, mineralogical assessment, functional groups analysis, and surface charge reactivity. FMC measured the heats of ion exchange (Qexch) at pH 3.5, 5.8, and 8.0 to estimate surface reactivity and the point of zero net charge (PZNC). BCO showed enriched mineralogical and functional group content, with a PZNC below pH 3.5, compared to BC’s PZNC at 6.1. This shift is attributed to (1) a higher abundance of acidic functional groups in BCO with low pKa values and (2) more diverse oxygen-containing functional groups. The lower pKa enhances BCO’s surface reactivity for immobilizing cationic contaminants, highlighting its implications for the biogeochemical cycling of contaminants in biochar-amended soils.