Fractionation of Dissolved Organic Matter by (Oxy)Hydroxide‐Coated Sands: Competitive Sorbate Displacement during Reactive Transport

Abstract

Sorptive retention of dissolved organic matter (DOM) at soil particle surfaces controls C flux through the critical zone. Prior studies have shown that pristine Al‐ and Fe‐(oxy)hydroxide surfaces are especially reactive toward DOM sorptive stabilization. However, the impact of progressive and/or preexisting organic surface coatings on further surficial uptake and exchange during repeated DOM infusion episodes remains unclear. In this study, DOM solutions were extracted from organic horizons in grassland (G) and mixed conifer forest (F) vegetation types in the Jemez River Basin Critical Zone Observatory. Extracted DOM solutions were used to sequentially irrigate columns packed with either quartz sand (Qtz), Al‐hydroxide‐coated quartz sand (Al‐Qtz), or Fe‐hydroxide‐coated quartz sand (Fe‐Qtz). Use of distinct DOM sources enabled investigation of how sorption, fractionation, and exchange ensued during reactive transport through mineral media progressively coated with sorbate organic matter (SOM). During initial irrigation of fresh mineral media with G‐DOM, the magnitude of DOM sorption (per unit sorbent mass) followed the trend: Al‐Qtz ≥ Fe‐Qtz > Qtz. Effluent solutions showed diminished molar absorptivity and humification index (HIX) values, indicating preferential uptake of high‐molar‐mass aromatic constituents. Introduction of F‐DOM to G‐SOM‐coated surfaces revealed competitive desorption of G‐SOM from the organo–mineral interface. During F‐DOM irrigation, high HIX values were observed in effluent solutions, indicating remobilization of G‐SOM by displacement. According to spectroscopic analyses, the displaced G‐SOM consisted of aromatic phenolic acids with high excitation–emission “fingerprints” characteristic of fulvic‐ and humic‐acid‐like compounds, providing evidence for kinetic DOM exchange reactions.