Project 4 - Arsenic, Iron, Sulfur and Organic Carbon Speciation

Seeking to determine impact of sediment mineralogy and related transformations on arsenic groundwater concentrations

Our investigators research 1) the role of mineralogy and organic matter source and properties on the expression of dissimilatory iron and sulfur reduction and 2) how such properties affect arsenic (As) solid solution partitioning in a wide array of environments. These processes are intimately linked to the location of groundwater arsenic contamination.  They impact the location and extent of human and environmental risk, yet we do not understand them. Our overall goal is to understand the fundamental connection between sediment properties, biogeochemical processes, and arsenic fate and transport. We make use of samples from field environments at both existing Superfund sites (in collaboration with Project 5) and Bangladesh (in collaboration with Project 6).

This project examines both the connection between (a) sediment mineralogy and (b) organic matter source on biogeochemical processes in arsenic-contaminated aquifers. (a)Many, but not all,  iron minerals retain arsenic strongly, minimizing its solubility and thus limiting its transport, and bioavailability/toxicity. Despite their importance, it is quite difficult to accurately quantify these phases, or to study their properties.

Ferrihydrite has a major role in regulating the fate of arsenic. Ferrhyrite and other reactive Fe(III) oxides are not only much more reactive as scavengers of aqueous arsenic, but their reduction is rapid and linked to the release of adsorbed As into solution under reducing conditions. In 2013, we collected additional samples from a variety of field environments and determined their mineralogy using three methods of characterizing Fe mineralogy. In all, we analyzed the Fe and As speciation in more than 100 new core and sediment samples during the year.

First, we used existing and well developed methods of linear combination analysis to determine the fractional composition of iron minerals in this diverse set of samples. These results suggested that ferrihydrite was an important mineral component in many, but not all, samples.  Second, we developed a standard addition method to quantify ferrihydrite, goethite, and magnetite in these sediments. This application of standard addition is the first of its kind for XAS data, and has improved the detection limits for ferrihydrite and other minerals to a few hundreds of ppm.  Third, we used statistical methods including principal components analysis (PCA) and cluster analysis. These methods make it possible to determine oxidation state and to define a signature of Fe mineralogy that is attributed to the source location (provenance) and potentially the age of sediments that does not change in response to sediment reduction.Remediation strategies based on inducing magnetite formation are currently being developed in collaboration with Project 5 to enhance in situ magnetite formation as a method of immobilization-based remediation strategies.

To properly assess the long term risks of arsenic contamination at contaminated sites and in groundwater, we need to be able to predict arsenic adsorption and transport properties. To this end, we have continued to develop our methodologies for measuring the adsorption and transport properties of arsenic under field conditions. We built upon our previous efforts examining iron mineralogy and As desorption at synchrotrons to examine the advection of As from high-As aquifers to low-As aquifers in model systems.

These data show that groundwater plumes at Superfund sites containing As may not be effectively decreased and, thus, As is susceptible to off-site migration. Enhancing As retention should target either the kinetic limitations to adsorption or the solution composition that inhibits adsorption. In reference to groundwater contamination in Bangladesh (Project 6), these data indicate that sediments containing Fe oxides provide some measure of protection from the horizontal transfer of high-As groundwater. This protection is important in preserving groundwater quality in aquifers currently used for drinking.

In 2013, our researchers used model systems containing real sediments collected from Superfund sites to study the conditions affecting iron mineralogy and to test the potential of in situ methods for the remediation of groundwater As contamination. Such remediation is complex, in that remedial efforts can facilitate the release of natural levels of As in sediments into unsafe levels in groundwater. Under oxidizing conditions, our studies show that the simultaneous addition of ferrous iron and nitrate produced magnetite that was an effective trap for groundwater arsenic. Additional data suggest that magnetite synthesis in contaminated environments could be an effective means of groundwater remediation for many years.

The microbes that reduce reactive iron phases like ferrihydrite require a reactive organic matter source. We have collected DNA from a new series of arsenic-contaminated aquifers in Bangladesh to ascertain the source of organic matter that drives this iron reduction. The data will help reveal the relative role of infiltration rates and groundwater age on carbon substrate utilization. As explained in our Nature publication, such processes can be important in perturbed groundwater systems, contributing to the contamination of previously low-As aquifers. The samples selected for radiocarbon DNA analysis also have been studied to detail the mineralogy and to determine, using genomic approaches, the microorganisms that are present in these environments. They allow us to directly relate the activity of Fe reduction and other biogeochemical processes with sediment properties such as mineralogy and with aquifer properties like dissolved As and Fe concentrations.

The chemical parameters that describe arsenic transport are needed to understand the distribution of arsenic in the environment and to predict how this spatial distribution will evolve over time. The physic-chemical parameters derived from these studies, and the underlying importance of specific mineral phase concentrations on these parameters, should be useful both to characterize contamination and to develop effective remediation strategies for arsenic. In these studies, we have found that reactive phases such as ferrihydrite affect arsenic fate. Since these phases are found in limited concentrations in many natural systems, there is a limited capacity of natural aquifer sediments to retard arsenic. The reduction of these phases can release arsenic into solution, but can in some cases form magnetite, which in turn immobilizes arsenic. Our project continues to focus on how to enhance magnetite formation to improve remediation technologies and how to improve the binding efficiency of As to sediments by tuning mineralogy as part of a comprehensive remediation plan.

Project/Core Scientists

Experimental Officer
Co-Investigator (Co-I)
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