BayCEER Workshop 2024

The influence of the oxidation state of humic acids on interactions with lepidocrocite

P 6.22 in Posters

The biogeochemical cycling of iron (Fe) and carbon (C) is coupled through diverse interactions, such as adsorption, co-precipitation, redox reactions and complexation. Such interactions between Fe ox(yhydrox)ides and organic matter (OM) are usually investigated individually, such as with batch experiments. In contrast, this project aims to understand the complex interactions of the Fe mineral lepidocrocite with humic acids (HAs) quantitatively, in real time, and simultaneously.

Lepidocrocite was electrochemically deposited on a Quartz Crystal Microbalance (QCM) crystal, and the electron donating and accepting capacities (EDC and EAC) of different standard HAs were measured using mediated chronoamperometry (MCA) and direct oxidation or reduction (DO/R). Humic acids were pumped through a flowthrough system where they are first oxidised or reduced in an electrochemical cell, then passed through the QCM to interact with the deposited lepidocrocite, and finally flow through a HPLC-UV/Vis detector. The QCM was used for quantifying adsorption and mass changes, while dissolved Fe²⁺ concentrations were measured via a modified phenanthroline method in the HPLC-UV/Vis detector.

Results show that lepidocrocite can be electrochemically deposited on a QCM crystal, obtaining a 200 nm thin highly pure layer, analysed by Raman spectroscopy and SEM. DO/R of HAs led to much lower EDCs and EACs compared to MCA. Initial results for the flowthrough method show that adsorption of HAs and mass changes of ng levels could be quantified with the QCM, and thus the result of mass changes can be related to the concentration of Fe²⁺ in the outflow.

This novel approach can be used to investigate multiple processes of the highly environmentally relevant Fe-C interaction. Further studies can be done using other Fe ox(yhydrox)ides and natural HAs.

The research work disclosed in this publication is partially funded by the Endeavour II Scholarships Scheme. The project is co-funded by the ESF+ 2021-2027.

Keywords: Lepidocrocite, Humic Acids, Adsorption, Redox, Quartz Crystal Microbalance