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Pharmaceutical compounds (PCs) and dissolved organic matter (DOM) are co-introduced into soils by irrigation with reclaimed wastewater. We targeted carbamazepine (CBZ) as a model compound to study the tertiary interactions between relatively polar PCs, DOM, and soil. Sorption-desorption behavior of CBZ was studied with bulk clay soil and the corresponding clay size fraction in the following systems: (i) without DOM, (ii) co-introduced with DOM, and (iii) pre-adsorption of DOM before CBZ introduction. Sorption of the DOM to both sorbents was irreversible and exhibited pronounced sorption-desorption hysteresis. Carbamazepine exhibited higher sorption affinity and nonlinearity, and a higher degree of desorption hysteresis with the bulk soil than the corresponding clay size fraction. This was probably due to specific interactions with polar soil organic matter fractions that are more common in the bulk soil. Co-introduction of CBZ and DOM to the soil did not significantly affect the sorption behavior of CBZ; however, following pre-adsorption of DOM by the bulk soil, an increase in sorption affinity and decrease in sorption linearity were observed. In this latter treatment, desorption hysteresis of CBZ was significantly increased for both sorbents. We hypothesize that this was due to either strong chemical interactions of CBZ with the adsorbed DOM or physical encapsulation of CBZ in DOM-clay complexes. Based on this study, we suggest that DOM facilitates stronger interactions of polar PCs with the solid surface. This mechanism can reduce PC desorption ability in soils.

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.

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