FATE AND CONTROL OF NERVE CHEMICAL WARFARE AGENTS IN THE DESALINATION INDUSTRY OF THE ARABIAN-PERSIAN GULf

摘要

This paper assesses the impact of chemical warfare agents (CWAs) on drinking water. The study is focussed on the Arabian-Persian Gulf (APG) and the desalination process. Contamination of seawater with nerve CWAs can be naturally alleviated by degradation mainly through hydrolysis. Hydrolysis is a relatively fast pathway as compared to other processes such as biodegradation and photooxidation. From the review of the open literature and based on the marine environmental conditions in the A.P.G., the following projections are made: 1) Hydrolysis will affect the soluble fraction of nerve CWAs only; 2) the rate of degradation of insoluble agent is determined by the transport phenomena and the available contact surface between the two phases (water/CWA); 3) higher temperature will promote degradation of nerve CWAs by increasing their solubilities and by accelerating their hydrolysis reactions; and 4) higher pH and divalent metal content (Cu, Ca, and Mg) in the A.P.G. will promote the hydrolysis of nerve CWAs. In general, all the environmental factors involved in the aquatic degradation of nerve CWAs in the A.P.G. marine favor the fast degradation process. In case of massive releases of nerve CWAs near the A.P.G. western shorelines, turbulence resulting from tidal cycles will affect the dissolution process and extend the toxicity of the insoluble agent. However, this phenomenon will lead to a faster elimination of the toxic substance from the aquatic environment. The time needed for the transport of the soluble fraction of the agent from the point of release to the intakes of desalination plants should be calculated to estimate the likely residual levels of toxic non-hydrolyzed fractions of the agent. Post- and pre-chlorination during the course of seawater desalination will catalyze and significantly accelerate the hydrolysis processes of the nerve CWAs. The heat exerted on CWAs during the power generation-desalination processes is not expected to thermally decompose them. However, the steam heat will augment the agents' rate of hydrolysis with subsequent acceleration in their rate of detoxification. Available data indicate that breakthrough of nerve agents into the thermally desalinated water is conceivable and that their remnants can pose a serious life threat to consumers. Conventional pretreatment of feed seawater for RO desalination is theoretically capable of reducing the concentration of nerve CWAs by coprecipitation and adsorption on floes formed during coagulation. Prechlorination and prolonged detention time in pretreatment units will simultaneously promote hydrolysis reactions. However, nerve CWAs can breakthrough RO tight membranes into the permeate at levels higher than the accepted tolerance posing a potential life threat to the public. Suggested control measures include boosting of pre- and post-chlorination doses, discarding the yield collected from the first stage in MSF, applying high doses of activated carbon, extending detention time, and utilizing water treatment devices at point of use or boiling the water by consumers.