Molecular weight-dependent adsorption fractionation of natural organic matter on ferrihydrite colloids in aquatic environment

Abstract

Natural organic matter (NOM), a heterogeneous mixture with various organic components and continuous molecular weight (MW) distribution, is omnipresent in natural waters. To date, understanding of the MW-dependent adsorption fractionation of NOM samples on colloids is limited due to the lack of NOM fractionation and characterization methods. In this study, the ultrafiltration technique was applied to fractionate the bulk NOM samples into low MW (LMW-, <1 kDa) and high MW (HMW-, 1 kDa–0.45 μm) fractions, whose adsorption behaviors on ferrihydrite colloids were comparatively examined via using total organic carbon, chromophoric, fluorescent emission-excitation matrix-parallel factor (EEM–PARAFAC), and two dimension FTIR correlation spectroscopy (2D–FITR–COS). The adsorption behaviors of all samples can be reasonably explained by Langmuir and Frendlich isotherm models, and the HMW-NOM exhibited higher adsorption capacities and affinities (4.71 mg-C/g) than the bulk (3.88 mg-C/g) and LMW- (2.44 mg-C/g) counterparts. Irrespective of MWs, the non-aromatic moieties were preferentially adsorbed by ferrihydrites, causing an enhanced aromaticity for the residual solutions. Two humic-like and one protein-like component were identified by EEM–PARAFAC, and a increase and decrease trend of Ka (adsorption affinity) were observed for humic- and protein-like components, respectively, in the order of LMW-, bulk, and HMW-NOM, showing obvious MW- and component-dependent adsorption fractionation of NOM on ferrihydrite colloids. 2D–FTIR–COS provided further information that, with increasing site limitation, the C–O in carbohydrate was adsorbed prior to the amide I in proteins, which testified the MW-dependent adsorption fractionation of NOM samples and accounted for the enhanced aromaticity in the residual solutions. This study highlighted the importance of MWs in NOM adsorption, which can enhance our understanding of the biogeochemical cycles of colloids and contaminants in aquatic environments.