Trace Metal Enrichment Driven by Phosphate Functional Group Binding Selectivity

Abstract

AbstractTrace metals have been found to be selectively enriched in the sea surface microlayer (SSML) due to association with organics, but the underlying cause of this selectivity has not been as well studied. Therefore, a systematic cation affinity study on surfactant lipids containing various functional headgroups (‐OH, ‐H2PO4, ‐COOH, ‐N(CH3)2) was conducted at pH 3 to further explain cation enrichment selectivity using Brewster angle microscopy, surface tension salt titration, and infrared‐reflection absorption spectroscopy (IRRAS). At pH 3, the headgroups chosen are neutral, and therefore the variability of surface charge on binding is eliminated. The phosphate ester headgroup was observed to exhibit the strongest trace metal binding, followed by the carboxylic acid headgroup. Hydroxy and dimethylamine headgroups did not exhibit significant trace metal binding. By applying the Langmuir‐Szyszkowski model to the surface tension data, the cation surface binding affinities for phosphatidic acid were determined, with Al3+ > Fe3+ > Zn2+> Mg2+ > Ni2+ > Mn2+∼ Ca2+, an order not predicted from bulk properties such as solid formation constants of metal‐phosphonates. Thus cation binding to surface active organic molecules with phosphate ester headgroups is indicated to be a significant source of trace metal enrichment within the SSML.