The Influences of Chemical Compositions and Molecular Sizes of Humic Substances on Cr(VI) Photo-Reduction.

Humic substances (HS) strongly affect the fate of trace metals in soils and aquatic environments. Because HS are chemically complex with molecular sizes ranging from several hundred to several hundred thousands, it is very difficult to elucidate which components of HS possess photo-induced properties and are responsible for photo-reduction of toxic Cr(VI) to Cr(III). In this study, an ultrafiltration technique was used to fractionate HS, originated from a Taiwan peat soil, into four fractions of different nominal molecular weight (>100 kD, 50~100 kD, 10~50 kD, and <10 kD). Each fraction of HS was first characterized by elemental analysis and spectroscopic techniques followed by examination the sorption and reduction of Cr(VI) on each fraction of HS under illumination. The results of elemental analysis indicated that the content of C, H, N, and S decreased with decreasing molecular weight of the fraction. C¹³-NMR studies showed that the aromatic C increased with decreasing molecular weight of the fraction. The preliminary study of photo-induced Cr(VI) reduction by each fractionated HS suggested that the Cr(VI) reduction proceeded more rapid with a smaller molecular weight.