Calcium carbonate minerals as scavengers of metals and radionuclides: Their role in natural attenuation and remediation

Calcium carbonates (CaCO3) are ubiquitous naturally occurring minerals in soils and subsurface sediments. Calcium carbonate coatings are common in many terrestrial environments and are known to coat a variety of reactive mineral surfaces in soils and sediments, altering their reactivity and chemical behavior. For these reasons, contaminant interactions with CaCO3 minerals are relevant and important to evaluation of the overall contaminant fate and transport in different surface and subsurface environments. Significant advancements have been made recently toward identifying relevant contaminant (e.g., uranium (U), iodine (I), and chromium (Cr), but also others, such as arsenic (As)) attenuation mechanisms, which may also serve as a basis for developing in situ remediation strategies. Hence, the objective of this review paper is to (1) describe the state-of-knowledge of calcium carbonate minerals as universal scavengers of metals (Cr and As) and radionuclides (U and I); (2) provide a state of the science of how calcium carbonate interactions affect contaminant fate and transport in the subsurface environmental systems; and (3) provide insights on the use of calcium carbonate natural attenuation mechanisms and their application to the remediation of complex sites. Knowledge gaps are also identified that, when closed, can advance the use of carbonate minerals for in situ remedy applications.