Description
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.
Companies On This Project
ADVENTUS GROUP
