Sedimentary humic substances isolated from a coastal lagoon of the Nile Delta: Physical and chemical characteristics

Humic substances extracted from the sediments of Lake Edku (Egypt) were fractionated to their humic and fulvic acid constituents; their physical characteristics [ratio of optical density at 465 and 665 nm (E4/E6), infra-red spectra (IR)] and chemical composition (elemental composition, functional groups) were measured. Fulvic acids were characterized by their lower E4/E6, lower carbon content and higher oxygen content. C/H and C/O ratios were lower in fulvic acids, but the C/N ratio was comparable in both components. Fulvic acids were also more acidic in nature, due to a higher carboxyl group content which constituted the major part of the total acidity. Carboxyl group concentrations averaged 10.6 mu eq mg(-1) C and 6.1 mu eq mg(-1) C in fulvic and humic acids, respectively. Infra-red spectra revealed a similar structural configuration. The geographical distribution of the physical and chemical characteristics did not reveal distinct and systematic features that could be related to the origin of the material. This was mainly attributed to the predominance of the internally produced humic substances over the land-derived material, and the relative distribution of humic and fulvic acids in both of them. Observed geographical variations appeared to be the product of varying conditions in the local depositional environment. (C) 1996 Academic Press Limited