ADSORPTION OF (POLY)MALEIC ACID AND AN AQUATIC FULVIC-ACID BY GOETHITE

The adsorption of Suwannee River fulvic acid (SRFA) and a synthetic or ganic polymer, polymaleic acid (PMA) by goethite was studied. The adso rption density of the sorbates decreased with increasing pH, and the d ata could best be described by the Langmuir adsorption equation. The n umber- and weight-averaged molecular weights, M-w and M-n, of the sorb ates in the solution phase before and after adsorption by goethite wer e measured by high-pressure size exclusion chromatography. The observe d decreases in the M-n and M-w of SRFA in the solution phase after ads orption demonstrated that fractionation of SRFA on the basis of molecu lar size occured. UV molar absorptivities measured at 280 nm also decr eased, which suggests that the more aromatic moieties are preferential ly adsorbed. Potentiometric titrations conducted on SRFA revealed chan ges in the conditional acidity constants (expressed as the negative lo garithm, pK(a)) of the fulvic acid after adsorption. The increased pK( a) value of the humic material remaining in the solution after adsorpt ion implies that the strongly sorbing compounds possessed stronger aci dic functional groups. Fractionation of PMA after adsorption, however, did not occur to the same extent compared to SRFA because PMA is rela tively homogeneous. The experimental results confirm that molecular we ight, aromaticity, and organic acidity are major factors controlling t he fractionation of natural organic matter (NOM) by adsorption onto hy drous mineral oxides. PMA, a relatively uniform and less polydisperse organic polymer, may serve as an endmember material characteristic of NOM that exhibits the least amount of fractionation upon adsorption. C opyright (C) 1997 Elsevier Science Ltd.