Phenanthrene Adsorption And Desorption By Melanoidins And Marine Sediment Humic Acids

Abstract

Sediments are major reservoirs of persistent petroleum contamination in marine environments. Petroleum hydrocarbons associate with the sediment organic matter, of which humic acids are an important constituent. This study examined the role that humic acid and its structure plays in the kinetics and mechanisms of polycyclic aromatic hydrocarbon (PAH) interactions with sediments. Natural humic acids, with a wide range of properties, were isolated from Alaska coastal marine sediments. Melanoidins were synthesized and used as humic acid analogs. The humic acids were characterized by elemental and isotopic analyses, Fourier transform infrared spectroscopy, and cross-polarized magic angle spinning 13C nuclear magnetic resonance spectroscopy. The humic acids were coated onto a standard montmorillonite clay, and the adsorption and desorption of phenanthrene was measured using a radiotracer. Adsorption required about one week to reach steady state, indicative of slow diffusion of PAH within the humic acid. The composition of the humic acids had a greater effect on phenanthrene adsorption than their concentrations on the clay. Organic carbon normalized adsorption partition coefficients were closely correlated with the sum of amide and carboxylic carbons, a measure of the polarity of the humic acids, but were independent of initial phenanthrene concentration, indicating that the binding sites were unlimited and uniform in strength. This explains the fact that initial adsorbed concentration of phenanthrene had no effect on subsequent phenanthrene adsorption. Desorption of phenanthrene was not related to any of the humic acid structural characteristics measured. The initial desorption rate was linearly related to the initial adsorbed concentration, as expected for a diffusive process, and was negatively correlated with the carbon content of the humic acid coated clay. Under most conditions, desorption was complete after one to seven days; there was little evidence for irreversible adsorption. Because of the substantial variability of adsorption and desorption behavior with organic matter characteristics, interactions of aromatic hydrocarbons with marine sediments cannot be predicted based on total organic matter concentration alone. Information on aspects of organic matter composition is needed in order to make accurate predictions.