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In the present study, the reduced adsorption of propanil on black carbon (BC) influenced by dissolved organic matter (DOM) was verified to be closely related to DOM molecule size and loading mode. Two congenetic carbons, a rice-residue-derived BC and the reduced product (RC), were characterized by similar specific surface area and different surface properties. Reduced product exhibits higher adsorption of propanil and DOM than BC. A series of model DOMs, including tannic acid (TA), pentagalloylglucose (PA), 3-O-galloylmucic acid (OA), and gallic acid (GA), characterized by different molecule sizes and molecular weights, were used to evaluate the different inhibitory effects. The DOM adsorption (mmol/g) on BC and RC follows the order of GA > PA > OA > TA, whereas the reduction of propanil adsorption influenced by the model DOM follows the order of PA ≈ TA > OA ≈ GA. The suppressive degree is connected to their molecule sizes rather than to molecular weights. Tannic acid and PA weakened propanil adsorption more effectively than OA and GA because the large DOM molecules may hinder propanil molecules into the micropore regions. Because of the similar molecule size, TA and PA present a similar suppressive effect on propanil adsorption. The influence of the DOMs was greater when preloaded than when in competition with propanil. The preloading of macromolecules (TA and PA) and OA on the carbons may lead to secondary and primary micropore blocks, respectively. The preloading of GA may cause partial GA molecule sequestration in the primary micropore, thus leading to strong attenuation of propanil adsorption on the carbons. Environ. Toxicol. Chem. 2012;31:1187-1193. © 2012 SETAC [ABSTRACT FROM AUTHOR]