Contribution of dissolved organic carbon to total alkalinity

Multiple measurements can be used to quantify the carbon dioxide (CO2) captured during enhanced rock weathering (ERW) applications. In most terrestrial applications, CO2 is dominantly stored as carbonate alkalinity inside the water. Total alkalinity (TA) is often taken as a measure of the CO2 stored in the water. However, the contribution of acids other than carbonic acid to mineral dissolution and, thus, to the generated alkalinity must be studied closely to reliably quantify CO2 capture.
 
In this study, we test how dissolved organic carbon (DOC) impacts non-carbonate alkalinity and the charge balance error of leached waters from a microcosm experiment using organo-mineral mixtures under ambient conditions. Furthermore, we quantified the concentrations of several low-molecular-weight organic acids to assess which conjugate base anions impact TA.
 
Our results reveal a substantial contribution of DOC to non-carbonate alkalinity, yielding a ratio of 3.5 mol DOC per eq of non-carbonate alkalinity. Moreover, we found a positive correlation between DOC levels and charge balance error, indicating that some of the conjugate base anions of the organic acids remained deprotonated in the titration procedure. Acetate anions found in the DOC-rich water samples further support the notion that organic acids have impacted mineral dissolution. The microcosm experimental data show parallels to natural ERW processes in organic-rich soils, demonstrating that organic acid contributions are relevant in mineral dissolution dynamics. These insights are relevant for carbon accounting in terrestrial ERW practices, where TA is often assumed to be solely carbonate alkalinity despite varying environmental conditions.