Governing Constraints of Chromium(VI) Formation from Chromium(III)-Bearing Minerals in Soils and Sediments

Debra Hausladen,Sarah Fakhreddine,Scott Fendorf

doi:10.3390/soilsystems3040074

Debra Hausladen, Sarah Fakhreddine + Show 1 more

Open Access

https://doi.org/10.3390/soilsystems3040074

Copy DOI

Journal: Soil Systems	Publication Date: Nov 13, 2019
Citations: 11	License type: CC BY 4.0

Affiliation: Stanford University, Université de Sherbrooke

Abstract

The potential for geogenic Cr(VI) contamination is vast, yet it is difficult to predict susceptible environments as groundwater Cr(VI) concentrations vary significantly, even within a given aquifer, due to physical and hydrologic heterogeneity. The physical structure of soils and sediments exerts a dominant control on Cr(VI) production by dictating the separation distance of reactive phases, the diffusion distance from Cr(VI) generation sites to advecting groundwater, and by influencing infiltration rates and porewater velocity. Here, we used a dual-pore domain model to investigate the relative control of these parameters on Cr(VI) production. The reaction distance between Cr(III)-bearing minerals and Mn oxides predominantly controls Cr(VI) export to advecting groundwater, while changes in diffusion distance between sites of Cr(VI) generation and advective flow channels generally have little impact on steady-state Cr(VI) concentrations. Changes in Cr(VI) diffusion distance can, however, increase the time required for groundwater Cr(VI) concentrations to reach a steady-state; thus, under fluctuating hydrologic and biogeochemical conditions, long diffusion distances still have the potential to suppress Cr(VI) supply to advecting water. Furthermore, we show that high porewater flow velocities effectively dilute Cr(VI) diffusing from soil/sediment aggregates, thus minimizing Cr(VI) concentrations relative to lower porewater velocities. The strong control that the physical/hydrologic parameters exert on Cr(VI) production appears to overwhelm the impact of Cr(III)-mineral solubility within soils and sediments.

Highlights

Hexavalent chromium concentrations exceeding recommended drinking water limits have increasingly been found in aquifers devoid of industrial or other anthropogenic sources [1,2,3,4,5,6,7,8,9]
Given the wide-distribution of Cr(III)-bearing mineral phases that reside in ultramafic igneous rocks such as peridotites and dunites, their metamorphic derivatives such as serpentinites, and associated soils [10,11], the growing prevalence of Cr(VI) generated from geogenic sources is of concern, predominantly in regions throughout the circum-Pacific and Mediterranean regions [10,11]
We evaluated the constraints of each of five parameters (Cr(III) mineral solubility (Ksp ), pH, separation distance between reactive phases, distance from Cr(VI) production sites to advective flow channels, and porewater velocity (v)) on Cr(VI) concentrations in advecting water

Summary

Introduction

Hexavalent chromium concentrations exceeding recommended drinking water limits have increasingly been found in aquifers devoid of industrial or other anthropogenic sources [1,2,3,4,5,6,7,8,9]. Given the wide-distribution of Cr(III)-bearing mineral phases that reside in ultramafic igneous rocks such as peridotites and dunites, their metamorphic derivatives such as serpentinites, and associated soils [10,11], the growing prevalence of Cr(VI) generated from geogenic sources is of concern, predominantly in regions throughout the circum-Pacific and Mediterranean regions [10,11]. In these ultramafic igneous rocks and their metamorphic derivatives, primarily found along convergent margins, Cr resides in low solubility minerals

Methods

Results

Conclusion