In the marine continuum, adsorption at the solid-liquid interface governs the transport, partitioning, and fate of contaminants. The relationship between adsorption and desorption is described by kinetic equations, among which the Langmuir model remains one of the most widely applied. However, its core assumption is physically untenable at the solid-liquid interface, compromising the accuracy of parameter estimation, the interpretation of key physical processes, and the reliability of subsequent numerical modeling. Building on a solvent–solute exchange framework, we derive a complete kinetic equation, analyze its form and the physical meaning of key parameters, and propose a concentration-weighted strategy for parameter estimation. This work provides a theoretical foundation and a practical methodology for understanding adsorption kinetics in the marine continuum.