Size and shape of lead-organic associations

The relevance of mobile organic matter for the transport of Pb through soils has been shown through several studies. Up until this study, no investigation has been published on whether the adsorption of Pb alters the size and/or shape, and consequently the mobility, of organic substances. We hypothesize that the addition of Pb induces the formation of organic precipitates by intermolecular cross-linking. We extracted organic matter from forest floor samples and determined the size and shape of the organic colloids via small angle X-ray scattering (SAXS) and their zeta potential. Measurements were conducted at three Pb concentrations (0, 0.05 and 0.10 mmol l−1) and at varying pH values (2.5, 4, 7, and 8). Under neutral and alkaline conditions, we observed geometrically weakly defined colloids possessing a maximal dimension of 33 nm. Either decreasing the pH value or increasing the Pb concentration induced the formation of organic matter rods with a corresponding decrease in the maximal dimension to 24 nm. Zeta potential measurements indicate that this decrease is due to a reduction of the repulsion between functional groups of the natural polyelectrolytes. SAXS curves indicate the formation of polymolecular Pb-organic agglomerates at pH 4 and 0.1 mM Pb, which exhibit a high colloidal stability. We conclude from our results that Pb is not merely passively co-transported by organic matter in soils but that it may also accelerate its mobile sorbents through the compaction of organic molecules or by the formation of highly mobile organic colloids.