The role of magnesite spatial distribution patterns in determining dissolution rates: When do they matter?

Abstract

We systematically explore the role of magnesite distribution patterns in dictating its dissolution rates under an array of flow velocity and permeability contrast conditions using flow-through column experiments and reactive transport modeling. Columns were packed with magnesite distributed within quartz matrix in different spatial patterns: the Mixed column has uniformly distributed magnesite while the zonation columns contain magnesite in different number of zones parallel to the main flow. Dissolution rates are highest under conditions that maximize water flowing through the magnesite zone. This occurs under fast flow and high-permeability or uniformly distributed magnesite zones. Under high flow and low permeability magnesite conditions, dissolution only occurs at the magnesite–quartz interface, leading to rates an order of magnitude lower in the One-zone columns than those in the Mixed columns. Spatial patterns do not make a difference under low flow conditions when the system approaches equilibrium (v<0.36m/d) or under conditions where magnesite zones have higher permeability than quartz zone. The bulk column-scale rate depends on Ae through RMgCO3,B(mol/s)=10-9.60Ae, where Ae is the surface area that effectively dissolves with IAP/Keq<0.1. The rate constant of 10−9.60 is very close to 10−10.0mol/m2/s under well-mixed conditions, suggesting the potential resolution of laboratory-field rate discrepancy when Ae, instead of the total BET surface area AT, is used. The Ae values are 1–3 orders of magnitude lower than AT. The effectively-dissolving magnesite–quartz interface areas vary between 60% and 100% of Ae, pointing the importance of “reactive interfaces” in heterogeneous porous media. This work quantifies the significance of magnesite spatial distribution patterns. It has important implications in understanding biogeochemical processes in the Critical Zone and in the deep subsurface, where spatial variations in mineral properties prevail.