Two observed characteristics of Th isotope and stable metal sorption in natural aquatic systems are seemingly at odds with physico-chemical adsorption theory: (1) characteristic sorption times of days to weeks and (2) Kds which are inversely related in magnitude to particle concentrations. In addition, sorption rate constants are positiveiy correlated with particle concentrations and Kd. This paper presents a conceptual and mathematical model with which it is proposed that these metal sorption characteristics have the same underlying physical process in common: the coagulation of colloidal (nonfilterable) particles onto larger (filterable) particles. “Brownian pumping” (the transfer of truly dissolved metal species to filterable particles through a colloidal intermediate) consists of two rate steps: (1) rapid formation of metal/colloid surface site complexes (adsorption) and (2) slow coagulation of colloids with filterable particles. The Brownian-pumping model is tested against field and laboratory data. The field data, obtained from the literature, covers different regions of the oceans: deep ocean environments, euphotic zone, coastal and estuarine systems. The laboratory data involved 228Th sorption in suspensions of goethite and polystyrene latexes. Although the model has general applicability, results and discussions herein emphasize thorium isotope behavior. The Brownian-pumping model suggests that Th or other strongly sorbing elements may be useful as in situ “coagulometers” either at relatively high (e.g., greater than 5–10 mg/l) particle concentrations or when the mass ratio of colloids (C*p) to filterable particles (Cp) is known. The model also indicates that the ratio of colloids to filterable particles in marine systems, may be, by a first approximation, described by the relationship log C*p = 0.7 log Cp – 2.6 (in units of kg/l).