Trace Metal Enrichment Driven by Phosphate Functional Group Binding Selectivity

Abstract

Trace 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.

Zinc–Carboxylate Binding in Mixed Octadecanoic Acid and Octadecanol Monolayers on Proxy Seawater Solution Surfaces

“…Contrary to monovalent Na + , multivalent metals have the ability to form strong ionic and, in some cases, covalent bonds with surfactant headgroups. Trace metals such as Zn 2+ have previously been identified in the SSML at concentrations reaching 100 times above those found in the subsurface bulk seawater, indicating significant enrichment factors. , When wave breaking or bubble bursting events occur, surfactant-bound trace metals can be transferred into sea spray aerosol.…”

“…With an average concentration of 468 mM, Na + is the most concentrated metal ion in the ocean . Zn 2+ is known to have an average oceanic concentration of 6 nM and a high SSML enrichment factor relative to other oceanic metal ions despite its low concentration. , For scale, the total concentration of multivalent trace metals in the ocean is estimated to be about 164 nM . Trace metal ions like Zn 2+ have seawater concentrations many orders of magnitude lower than major metal ionic components like Na + and Mg 2+ and yet have been found to be enriched in sea spray aerosol at significantly higher relative proportions. , Ion–surfactant interactions are hypothesized to cause this disparity in sea spray aerosol enrichment.…”

“…Aspects of surfactant film structure including surface molecule orientation and packing can be altered by interactions with ions. ,, Previous studies have shown considerable ordering effects when divalent cations such as Zn 2+ interact with a fatty acid film composed of OA, indicating strong binding and even metal-induced deprotonation of the −COOH group. ,, Monovalent Na + , on the other hand, is not expected to interact as strongly with the carboxylate headgroup due to its smaller charge and larger ionic radius. , When compared to the metal complexation strength of OA, OL’s binding affinity is assumed to be negligible due to its hydroxy headgroup …”

“…The primary goal of this study is to probe the effect of mixed monolayer composition on trace metal binding with monolayers composed of fatty acid and fatty alcohol molecules, typical components in SSML and sea spray aerosol. Mixed monolayers of OA and OL are thus systematically evaluated at four different ratios, and each ratio is studied on eight aqueous solutions of differing concentrations of ZnCl 2 while maintaining a constant and large NaCl ionic strength background equivalent to seawater NaCl concentrations . Here, Zn 2+ is chosen as the model trace metal due to its previously reported interaction strength with OA and its large enrichment factors in sea spray aerosol. , Infrared reflection–absorption spectroscopy (IRRAS) is used to probe these surface films and to quantify apparent surface binding constants using the integrated areas of carboxylate asymmetric stretch peaks as reporters.…”

Insights into the behavior of nonanoic acid and its conjugate base at the air/water interface through a combined experimental and theoretical approach

“…Surface pressure titration experiments were performed by a computer-controlled film balance (KSV NIMA LB, S/N AAA100505) with a slightly modified side hole Petri dish, designed by Allen and co-workers. 37 Nonanoic acid solutions at different concentrations were made in water or 0.5 M NaCl with an initial pH around 11.5. A 40 mL solution was used for each experiment and was titrated with 1 N HCl via a syringe through the side hole of the Petri dish.…”