SIMULATING DISSOLVED INORGANIC CARBON AND STABLE CARBON ISOTOPE EVOLUTION IN AMD USING ACIDIFIED SODIUM BICARBONATE SOLUTION

Dissolved inorganic carbon (DIC) loss and stable carbon isotopes (δ¹³C_DIC) enrichment have been observed in acid mine drainage (AMD) contaminated streams. Varying geochemical, biological and physical processes and their isotopic fractionations affect stream δ¹³C_DIC. While protons produced by AMD dehydrates HCO₃^- and shifts carbonate equilibrium to CO_2(g), low pH, biological activity and metal cycling in AMD-contaminated streams affect the transformation of dissolved organic carbon (DOC) to DIC, impacting the δ¹³C_DIC. It is therefore difficult to determine how progressive acidification of stream water by AMD affects fractionation of carbon during DIC loss.

This study determined the magnitude of δ¹³C enrichment associated with DIC loss due to acidification in the absence of in-stream processes, high metal concentration and DOC typical of AMD-impacted streams. To simulate AMD effects on carbonate equilibrium and δ¹³C fractionation, we progressively acidified NaHCO₃ solutions in 20 L reactors to pH <3 in the laboratory. One set of two samples was acidified in reactors open, and another set closed to the atmosphere. A set of un-acidified samples served as control. Headspace CO₂ from closed reactors and DIC from all reactors were periodically measured and δ¹³C determined. The average initial δ¹³C of the NaHCO₃ used was -3.5 ± 0.1‰. The δ¹³C_DIC of samples from open and close reactors ranged from -3.6 to –2.3‰ and -3.4 to 0.74‰,respectively and headspace CO₂ ranged from -10.8 to +1.3‰. δ¹³C_DIC of un-acidified samples ranged from -3.3 to -2.7‰. The un-acidified sample lost little DIC and had δ¹³C enrichment of 0.6‰ compared to 1.3 and 2.7‰ for the open and closed acidified reactors samples which lost more that 66% and 92% of DIC respectively. The isotope enrichment for the acidified samples was much smaller than the equilibrium enrichment of 10.1‰ for HCO₃^- dehydration and CO₂ diffusion from solution. However, the δ¹³C of the initial CO₂ evolved from the first acid application was 7.3‰ depleted relative to the NaHCO₃ solid. We hypothesize that the lower enrichment factor for the DIC in the acidified samples was due to re-equilibration of CO₂ released from acidification with HCO₃^- in solution. Our results suggest that loss of large amounts of DIC due to AMD impact will be accompanied by relatively small enrichment of δ¹³C_DIC.