Gem

GEM is a Netherlands Organisation for Scientific Research, Innovational Research Incentives Scheme Veni grant awarded to Case M. van Genuchten, which is running from 15 Feb 2016 to 15 Feb 2019. GEM is a collaboration among several institutions in Europe, North America and Asia.

Title
Geochemical and electrochemical controls on mixed valent Fe(II,III) (hydr)oxide formation by Fe(0) electrocoagulation

Synopsis
This project will investigate the formation, stability, and arsenic removal efficiency of Fe(II,III) minerals generated by Fe(0) electrocoagulation, which is a novel, low-cost arsenic treatment technology for rural communities. This research can lead to technical breakthroughs that increase dramatically the likelihood of sustained EC system operations in poor arsenic-affected rural communities.

Background
Approximately 100 million people worldwide use groundwater contaminated by arsenic (As) as their primary drinking water source. Nearly half live in rural villages of South Asia, too poor to afford conventional As treatment methods. Chronic arsenic exposure leads to painful skin lesions, limb amputation, cancer, and death.  Due to the devastating scope of this problem, it is often referred to as “the largest mass-poisoning in human history”. Despite numerous efforts over the last decade, arsenic continues to be a major global public health concern in rural communities. Electrocoagulation (EC) using Fe(0) electrodes is a promising As removal strategy for resource-scarce areas because it is effective and low-cost, produces minimal waste, and is easy to operate and maintain with locally available materials. In EC, Fe(II) ions are generated by applying a small electric current to a sacrificial Fe(0) anode in contact with an electrolyte (Figure 1).

Figure 1: Fe(0) EC schematic

Although Fe(0) EC systems in the field, such as that pictured at the bottom of the page, are able to remove As below the World Health Organization (WHO) standard of 10 ppb, the full potential of EC systems as a water treatment technology has thus far not been realized. The generation of mixed valent Fe(II,III) oxides (Figure 2) such as magnetite (Fe3O4) or green rust (GR) can lead to breakthroughs in EC treatment efficiency and could dramatically reduce the cost and complexity of EC field operations.  For example, purely Fe(III) phases are generated to remove arsenic in current EC systems in arsenic-affected regions of West Bengal, India, but these precipitates have a nanoscale particle size and thus pose a major challenge for low-cost separation from treated water. The larger particle sizes and settling velocities of GR, or the potential of Fe3O4 suspensions to be separated under low strength magnetic fields, could eliminate the extra costs and complexity of incorporating coagulants and/or filters into EC system design. Additionally, arsenic trapping modes unique to in-situ generated Fe(II,III) oxides, such as multidentate adsorption geometries, may be more stable than the typical binuclear complexes formed on Fe(III) minerals. For these reasons, the formation of Fe(II,III) minerals in EC systems can enhance As removal performance considerably. However, the formation and reactivity of Fe(II,III) (hydr)oxides must be rigorously understood before modifying EC system design toproduce these minerals.

Figure 2: Suspensions of green rust, magnetite, and lepidocrocite generated by Fe(0) EC and some possible As(V) coordination geometries on each mineral

Objectives
This project brings together researchers across several institutions worldwide (Utrecht University, NL; University of Copenhagen, DK; Jadavpur University, IN; University of California, Berkeley, USA) to investigate the formation, stability, and arsenic uptake mechanisms of EC-generated Fe3O4 and GR.

Four main questions will be addressed:
1) What controls the formation of Fe(II,III) (hydr)oxides by Fe(0) EC?
2) How does arsenic interact with each Fe(II,III) mineral?
3) What are the effects of common groundwater ions (e.g. Si, P, Ca, Mg) on the structure and reactivity of Fe(II,III) (hydr)oxides?
4) What happens to the precipitated arsenic with time?
To answer these questions, synchrotron-based X-ray characterization techniques will be employed to characterize the structure of the Fe phase and speciation of surface-sorbed As with molecular-scale levels of detail. This type of information is essential to improve EC system design and to better understand the fate of arsenic at natural redox boundaries in soils, sediments, and groundwater.

Finally, this project also consists of parallel experiments to improve the efficiency of existing Fe(0) EC plants, such as the one shown above, that are currently in operation in arsenic-affected communities in South Asia.

Figure 3: Electrolysis tank of a plant-scale Fe(0) EC system operated in West Bengal, India. Sheets of Fe(0) metal are visible.