35    ARSENIC REMOVAL USING ELECTROCHEMICALLY
GENERATED IRON IN CONJUNCTION WITH
HYDROGEN PEROXIDE ADDITION
Michael D. Brewster, Research Chemist
Michael N. Laschinger, Process Engineer
Andco Environmental Processes, Inc.
Amherst, New York 14228-2380
INTRODUCTION
Arsenic is a common contaminant that exhibits varying degrees of toxicity depending on its chemical form. Its potential to form highly toxic compounds, coupled with the fact that arsenic chemistry
allows it to form soluble compounds has made arsenic one of the inorganic contaminants regulated in
the National Primary Drinking Water Regulations. As of May, 1990, the Environmental Protection
Agency (EPA) Maximum Contaminant Level (MCL) allowed for point source discharges is 0.05
milligrams per liter.' Many sources of arsenic contaminated water are required to meet even stricter
limits due to state and local regulations. A factor in determining these point source limits is where the
discharge is being sent. Surface water discharges regularly have lower limits than discharges to a
municipality. It is common to have limits of 0.025 ppm arsenic.
There are many sources of arsenic. Naturally occurring arsenide is commonly present in many
sulfide ores such as cobalt, copper, lead, silver, tin, and zinc. Arsenic is found in the minerals realgar
(AsS) and orpiment (As2S3). These mineral forms are highly insoluble and pose little threat to the
environment. The chemistry of arsenic and its presence at trace levels in soils have resulted in arsenic
being detected in waters, plant tissues, and animals throughout the world. Levels of natural occurring
arsenic seldom pose health risks.
Arsenic does have industrial uses. It is commercially produced as a by-product from ores of metals.
Arsenic trioxide (As203) is obtained during the smelting process. From this oxide, many other arsenic
compounds are produced, notably those used for pesticide and herbicide formulations. Refined
arsenic trioxide is also used to decolorize glass and as an ingredient of some pyrotechnics. It is present
in some wastewaters from chemical production plants, tannery operations, wood preserving and
timber products processing facilities, paint/pigment formulation companies and semiconductor producers. While arsenic occurs naturally in coal, it becomes an industrial source when coal is converted
to energy. As a result, power plant fly ash ponds typically contain soluble arsenic compounds. Soluble
forms of arsenic are of much greater concern than insoluble arsenic compounds. Their ability to leach
through soils and contaminate groundwater is much greater. Nearly all of the hazard associated with
arsenic compounds is the result of industrial products or byproducts. This paper addresses and
compares the treatment of a range of arsenic contaminated water sources.
THE CHEMISTRY OF ARSENIC
Arsenic, atomic number 33, is one of the elements located in group VA of the periodic table.2
Within this group, metallic characteristics increase as atomic number increases. Thus, arsenic will be
more metallic than phosphorous, which has atomic number 15. A characteristic of metals, in aqueous
solutions, is the loss of electrons and resulting formation of cations. This is only observed for the
heavier members of this group, which are antimony and bismuth. These two elements can readily lose
three electrons to form simple cations. Arsenic will loose electrons to be in the + 3 or +5 states
though in aqueous solutions it exists tightly bound to oxygen which results in an anionic form.
Arsenic's anionic form is important when discussing the chemistry of arsenic and how it relates to
whether or not successful treatment is achieved.
Four stable states are known to occur in nature, they are +5, +3, -3, and 0. The + 5 or pentavalem
forms are As04'3, HAs04"2 and H2As04"'.3 They will predominate and be stable in oxygen rich water
where mild oxidizing conditions are present. These forms of arsenic in its pentavalent state are
46th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
339