Effect of Humic Substance Photoalteration on Lead Bioavailability to Freshwater Microalgae

The present study provides results on the influence of humic substance (HS) photoalteration on lead availability to the freshwater microalga Chlorella kesslerii. The evolution of the free lead-ion concentrations measured by the ion exchange technique Pb and intracellular lead contents was explored in the presence of Suwannee River humic: (SRHA) and fulvic (SRFA) acids, as well as Aldrich humic acid (AHA) exposed at increasing radiance doses under a solar simulator. Modifications of HS characteristics highly relevant to Pb complexation and accumulation of HS to algal surfaces, including Fourier transform infrared spectroscopy, were followed. It was demonstrated that simulated sunlight exposure of HS increased Pb, in the medium for SRFA and SRHA, but had no effect for AHA. No clear relationship was observed between the changes in free lead-ion concentrations and intracellular content in alga for all studied HS, suggesting that HS photodegradation products also exhibit Pb complexation properties, and that direct interactions between HS and alga are affected. Indeed, photoalteration of humic substances reduced the adsorption of HS to the algal surface; the effect was more pronounced for SRFA and AHA and less significant for SRHA. The bioavailability results were consistent with the characterization of the phototransformation of humic substances: Pb speciation changes followed the modification of the relative abundance of the carboxylic groups and their molecular environment, while the reduced HS adsorption to the alga correlated with losses of the double bond abundance and aromaticity.