# Deciphering Competitive Interactions of Natural Organic Matter Components at Metal Oxides: Insights from Experiments and Modeling

## TL;DR

This study explores how different components of natural organic matter compete for space on metal oxides, affecting the movement of important environmental compounds like arsenate and phosphate.

## Contribution

The study introduces a mechanistic model to explain competitive adsorption of humic and fulvic acids on metal oxides, revealing a high mass exchange ratio.

## Key findings

- Fulvic acid (FA) particles effectively remove humic acid (HA) particles from metal oxide surfaces due to interfacial space limitations.
- The adsorption of FA reduces HA adsorption, leading to a high HA/FA mass exchange ratio of ∼2.4 ± 0.6.
- This competitive interaction impacts the mobility of oxyanions like arsenate and phosphate in the environment.

## Abstract

Natural organic matter (NOM) is a heterogeneous mixture,
including
humic acid (HA) and fulvic acid (FA), that competitively interacts
with metal (hydr)­oxides. Despite its environmental importance, this
competition has not yet been measured extensively, and mechanistic
modeling is lacking. The present work examined the competitive adsorption
to goethite and the corresponding molecular fractionation of HA and
FA using UV–vis spectroscopy, acid precipitation, and size
exclusion chromatography (SEC). Our findings reveal that on a mass
basis, FA particles effectively remove HA particles from the surface.
This efficiency can be mainly attributed to an interfacial space limitation
in which FA restricts HA adsorption, as evidenced by mechanistic modeling
with the Consistent Competitive Ligand and Charge Distribution (LCDcc) approach for the heterogeneous adsorption of NOM. The adsorbed
FA particles occupying part of the surface prevent HA from accessing
the corresponding double-layer space, disproportionally reducing HA
adsorption. This restriction leads to a high HA/FA mass exchange ratio
(∼2.4 ± 0.6), consequently affecting the mobility and
transport of oxyanions (arsenate and phosphate) in the environment.
The difference in the partitioning of NOM is also relevant for soil
carbon sequestration via the selective preservation of NOM by association
with oxide minerals.