Biofilms coat the exterior of most water-exposed interfaces, from the surfaces of sediments and rocks to the interior walls of fluid transport systems and even medical and dental apparatus. Composed of a diverse assemblage of microbial species growing in a matrix of extracellular polymeric substances (EPS), biofilms are well-known for their ability to sorb metals and nucleate mineral phases. In this study, purified alginate, a major polysaccharide component of some algal and bacterial EPS, was studied to ascertain its chemical reactivity towards dissolved cadmium and protons, and thus better constrain its role in overall EPS reactivity. FTIR analysis and compositional constraints based on known molecular structure indicate that alginate’s geochemical behaviour is dominated by a single carboxyl functional group. Correspondingly, potentiometric titration data were best fit using a single functional group acidity constant (pKa
) and site concentration of 3.98 ± 0.01 and 1.728 ± 0.02 mol/kg, respectively, which are in agreement with typical carboxyl acidity (pKa
3–6) and carboxyl functional group concentration based on alginate polymer composition. The logarithm of the Cd-carboxyl complexation constant (log K) was determined to be −0.52 ± 0.22, lower than carboxyl-Cd stability constants reported from independent studies of isolated microbes. Together, these results place important constraints on organic matrix contributions to overall biofilm reactivity.