Analysis of anorthite dissolution at the microscopic scale

Abstract

Constraining the processes behind feldspar dissolution is imperative in understanding the how these reactions facilitate carbon dioxide (CO2) sequestration in ongoing efforts to mitigate climate change. Yet attempts to quantify the kinetics behind these processes typically result in major discrepancies between the field and laboratory observations used to inform models that predict this dissolution. Many of these discrepancies are the result of not properly accounting for the distributions of flow rate, flow path, and solution chemistry from field to laboratory scale of observations. Here we are able to control solution flow rate, flow path and chemistry with a new degree of accuracy at the pore scale using microfluidic devices that are both free of solute build-up and contain an entire flow network comprised of the reactive mineral of interest. In this study, we have analyzed anorthite dissolution using pH 3, 4, and 5 influent solutions at flow rates of 0.56, 1.13, and 2.25 μL min-1, and measured Ca2+ fluxes ranging from 5.53×10-8 to 6.44×10-7 mg min-1 and reaction rates ranging from 7.47×10-10 to 8.83×10-9 mol m-2 sec-1. Our results are among previously measured plagioclase dissolution rates measured at similar pH’s. Furthermore, we have extended the correlation between dissolution rates and residence times (τ), in that our reaction rates are orders of magnitude greater than other rates from the literature while our τ are orders of magnitude lower. This relationship between τ and plagioclase dissolution rates is maintained, in some cases, despite differences in temperature and influent composition from study to study. These observations lead us to consider that residence time, as impacted by flow rate and flow geometry, is a strong control on plagioclase dissolution rates across observation scales. Understanding influences on residence time and its role in mineral dissolution is key as we address climate change with carbon sequestration development in geologic reservoirs.