34    COMPLEXING OF HEAVY METALS WITH PHOSPHATES
Kitt Farrell-Poe, Assistant Professor
Environmental Engineering Extension Specialist
Utah State University
Logan, Utah 84322
James E. Etzel
Vice-President, Heritage Environmental Services
Indianapolis, Indiana 46251
INTRODUCTION
Heavy metals in the environment are of concern because microorganisms, fish, and plants in the
human food chain can bioaccumulate them, particularly mercury, cadmium, and lead. The metals can
accumulate in stream sediments which may be translocated during high flows, and some metals, like
mercury, can be transformed into more hazardous metalorganic forms by microorganisms.'
Industrial sources of heavy metal bearing effluents are diverse and typically include metal processing and refining, metal plating, chloralkali production, storage battery manufacturing, pigment
manufacturing, tanning, anodizing, photographic film manufacturing, ore benefication, and automotive production. These effluents may contain cadmium, nickel, copper, lead, zinc, and chromium
at concentrations ranging up to 50,000 mg/L.2,3 The volume of heavy metal bearing wastewater
produced annually in the United States by these industries amounts to billions of gallons.
The Resource Conservation and Recovery Act of 1976 (RCRA) created regulations for hazardous
waste management. A waste is classified as hazardous if it is a "listed" substance (40 CFR 261.30), or
if it exhibits any of four characteristics which classify it as a hazardous waste (40 CFR 261.20):
corrosivity, reactivity, ignitability, or toxicity (as measured by the Extraction Procedure (EP) Toxicity
or the Toxic Characteristic Leaching Procedure (TCLP). (Foundry waste is toxic because of the
presence of heavy metals. Therefore either procedure, EP toxicity or TC toxicity, would be correct.) A
waste is considered EP toxic if it produces over 100 times the National Interim Primary Drinking
Water Standards for arsenic, barium, cadmium, chromium, lead, mercury, selenium, or silver in the
Extraction Procedure (EP) test or the TCLP test.4
There are numerous methods for treating heavy metal bearing wastewater—ion exchange, adsorption, and chemical precipitation to name a few. Ion exchange and adsorption are basically volume
reduction processes. The ions are removed from the wastewater and concentrated on the exchange
resin. During regeneration, the ions are released from the resin and are solubilized in the regeneration
wastewater. Therefore, the volume of contaminated wastewater is reduced but it still needs ultimate
treatment. Hydroxide precipitation is a popular method of removing soluble heavy metals. However,
upon neutralization, the heavy metals go back into solution. Therefore, there is a need to find a
process which removes heavy metals from wastewater and produces a non-hazardous sludge at near
neutral pH values. Ideally, the process should also be able to convert a hazardous sludge from
conventional heavy metal precipitation to non-hazardous with as much facility as it does when used as
the primary heavy metal removal process.
Triple superphosphate (TSP) is a phosphate fertilizer which has been used with success in complex-
ing heavy metals and producing a sludge which is non-EP toxic.3 Etzel has shown that phosphate
fertilizer can be used both as a wastewater and sludge treatment method.
Triple superphosphate, Ca(H2P04)2, is used mainly as a commercially available fertilizer, but it is
also used as an acidulant in baking powders and wheat flours, mineral supplement for foods and
feeds, and in enameling.6 It is made by the phosphoric acidulation of phosphoric rock or monobasic
calcium phosphate.
Etzel used phosphate fertilizer in the treatment of ion exchange regeneration wastewaters. He used
0.5% to 1.0% by weight based on the total weight of the regeneration wastewater volume being
treated. The mixture was allowed to settle and the supernatant decanted. The settled volume of the
46th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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