Contribution of dissolved organic carbon to total alkalinity in Enhanced Weathering experiments

Abstract

Total Alkalinity (TA) is widely used as a proxy for captured CO2 in enhanced weathering (EW) applications. However, organic anions can also contribute to TA. To improve carbon accounting in EW, which is often simplified to that TA equals carbonate alkalinity, their contribution should be taken into account. In this study, we tested how dissolved organic carbon (DOC) contributes to non-carbonate alkalinity (ANC) using microcosm experiments with artificial organo-mineral mixtures. We used different combinations of rock powder with straw, microbes and earthworm additions, under ambient air conditions. The microcosms were flow-through columns placed in a climate chamber at 25 °C, which were irrigated with groundwater at rates between 1200 and 3600 mm/yr. The concentrations of several low-molecular-weight organic acids (oxalate, citrate, acetate, gluconate) were quantified to assess which conjugate base anions impact the measured TA. Results revealed a ratio of 3.5 mol DOC per ANC equivalent. In the overall experiment the median contribution of ANC to TA was around 5.5 %. A positive correlation between DOC and charge-balance error suggests that some organic acid anions remained deprotonated during TA titration. Acetate anions found in DOC-rich water samples further support a substantial contribution of organic anions to TA. To investigate the relevance of ANC for natural EW systems, we also quantified ANC contributions in natural waters and leachates from soil EW experiment mesocosms. Because DOC levels were lower, ANC contributions were smaller, ranging from a median of 4.1 % in soil mesocosm leachates down to 0.9 % in Elbe estuary water samples. This ANC contribution, despite seeming small, is relevant for carbon accounting in terrestrial EW practices, where TA is often assumed to be solely carbonate alkalinity.