40    THE EFFECT OF ADDITIVES ON THE OXIDATION OF
DIMETHYL METHYLPHOSPHONATE IN
SUPERCRITICAL WATER
Jacqueline K. McKendry, Graduate Research Assistant
Lixiong Li, Research Scientist
Earnest F. Gloyna, Bettie Margaret Smith Chair,
Environmental Health Engineering
The University of Texas
Austin, Texas 78758
'     INTRODUCTION
Supercritical water oxidation (SCWO) has been demonstrated to be effective in the destruction of
hazardous and toxic wastes. This chemical oxidation process takes place at temperatures above
374.15°C and 22.1 MPa. High destruction efficiencies (> 99.99%) over relatively short residence
times (on the order of seconds to minutes) are achieved due to the high solubility of organic compounds and oxygen, alleviating mass transfer hindrances. Residual organic compounds, such as acetic
acid, are relatively harmless and may be treated readily by conventional treatment techniques.
SCWO is one technology currently being investigated to address the disposal of chemical warfare
agents and munitions which are stored at several sites within and outside the United States. The
stockpile of chemical agents and munitions consists of several nerve and blistering agents as well as
explosives or energetic materials. Dimethyl methylphosphonate, DMMP (C3H9O3P), is the simulant
selected by the Department of the Army to study the behavior of the nerve agent GB, or Sarin
(C4H|0FO2P). These two compounds are similar in structure and properties; DMMP can be used to
study the potential fate of GB during SCWO, without producing harmful by-products, in particular,
hydrogen fluoride gas.
Recent research activities involving SCWO have investigated the advantages of producing byproducts rather than forcing the complete oxidation of larger, complex and hazardous organic compounds. For example, in a limited oxygen environment, methane, a useful, high-energy content fuel,
can be produced. This production may be enhanced using additives. The term "additives" refers to a
substance added to a reaction medium which participates in or affects the reaction. Additives may
affect reactions either by homogeneous catalysis or by altering the properties of the supercritical water
medium.
The objective of this research was to study the effect of additives on the destruction of dimethyl
methylphosphonate (DMMP) at supercritical conditions. The additives chosen for this study were
selected primarily to observe their effect on by-product formation upon the destruction of DMMP in
supercritical water. Additives which could possibly create a buffering environment were selected as
potentially having the ability to neutralize the acidic effluents generated during the SCWO process. By
neutralizing the acidic effluent resulting from certain waste streams, corrosion may be inhibited, and
reactor lifetimes prolonged.
This research was limited to laboratory bench-scale tests using a batch reactor. Fixed conditions
were initial concentration of DMMP (10,000 mg/L), residence time (five minutes), temperature
(450°C), and pressure (27.6 MPa). Variables included the use of three additives (NaOH, Na2B407,
Na2C03), and three oxygen concentrations (in excess of 100%, 17% and 0% of stoichiometric
demand). Oxidation and hydrolysis by-products were identified and quantified as a function of
oxygen concentration. By-products included methyl phosphonic acid, phosphates, and methanol in
the liquid phase, and methane, carbon dioxide, carbon monoxide, and hydrogen in the gas phase. The
final pH was measured. Corrosion products, chromium and iron, were quantified as well.
49th Purdue Industrial Waste Conference Proceedings, 1994 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
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