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Introduction
EHC-M is a specially formulated version of our controlled-release, integrated carbon and zero valent iron (ZVI) technology for in situ chemical reduction (http://www.adventus.us/ehc.htm). EHC-M encourages the precipitation and adsorption of arsenic and other dissolved metals (such as chromium, lead and mercury) to limit their movement downstream of a treatment zone. It can be applied to the subsurface environment in a number of ways to quickly reduce the concentration of arsenic in groundwater in a safe and timely manner.


The problem with Arsenic.

Arsenic in ground water is largely the result of minerals dissolving from weathered rocks and soils (http://water.usgs.gov/nawqa/trace/arsenic). As summarized below, arsenic is naturally occurring in the environment and is present in groundwater at concentrations ranging from 1 to >50 micrograms per liter (ug/L).

However, several types of cancer have been linked to arsenic in water. Therefore, in 2001 the US Environmental Protection Agency lowered the maximum level of arsenic permitted in drinking water from 50 to 10 ug/L. A number of sites exceed this value do to a combination of natural and/or anthropogenic arsenic sources. In turn, an effective, cost-efficient in situ remedial solution is required.

EHC-M for Removal of Arsenic in Groundwater

The primary mechanism of removal entails physical precipitation of arsenic with iron and other inorganic compounds, especially those associated with the reduction of sulfate to form arsenopyrite (EPA, 2000; Craw et al 2003 as shown right). Given that the removal mechanisms are precipitation and adsorption, the arsenic is transferred from the aqueous phase to a solid phase.

EHC-M Treatment Performance

EHC-M has been shown to rapidly reduce the concentration of dissolved arsenic in groundwater from >1,000 to <10 ug/L. Under continuous-flow laboratory conditions, removal efficiencies exceeding 98% have been maintained for over a year.

Arsenic removal using EHC-M technology is NON REVERSIBLE by change in Eh or pH (Figure 2 right) hence rebound should not be observed. EHC-M is designed to create very low redox (Eh) conditions and neutral pH. Once stabilized, arsenic is not liberated upon exposure to oxygenated water. This supports the premise that arsenopyrite is the primary precipitation product (Craw et. al., 2003). Likewise, acidification to pH 4 did not increase the concentration of arsenic in groundwater nor did exposure to simultaneous aerated and acidic water or simultaneous aerated and basic water.

Only 4% of the mass of arsenic that was eluted from the control column was eluted by the EHC- M column during the second (aerated), third (acidified pH 4), fourth (simultaneous aeration and acidification), and fifth (simultaneous aeration and basic pH 9) phases of the study. These data illustrate that EHC-M is capable of creating and maintaining reducing conditions effective for the removal of arsenic from the water phase, despite significant change in physiochemical conditions that in theory could reverse the stabilized condition (Figure 3 right).

The high longevity of this process is demonstrated by the fact that the column has been operating for more than three years at room temperature. It is expected that EHC-M's longevity under field conditions will be longer than that in the laboratory due to lower average temperatures.

The Cost

At $2/lb, EHC-M offers a very cost efficient means of in situ stabilization of dissolved arsenic. Field application methods consist of various injection methods for plume cut-off, plume treatment, and source-zone reduction, or trench-type applications for plume cut-off.

The Installation

The EHC-M is provided in 50-lb bags as a dry powder and mixed with water on site into a slurry. The EHC-M slurry can be injected into the subsurface in a variety of ways including direct injection and hydraulic fracturing or through direct soil mixing. EHC-M has also been applied on top of sediments in combination with Bauxsol for removal of Arsenic from surface water.

References

Craw D., Falconer D., and Youngson J.H. 2003. Environmental arsenopyrite stability and dissolution: theory, experiment, and field observations. Chemical Geology (199) p. 71-82.

EPA (United States Environmental Protection Agency). 2000. Technologies and Costs for Removal of Arsenic from Drinking Water. EPA 815-R-00-028. December 2000.