(171e) Formation of Halogenated Organic Compounds in Seawater

The use of seawater in industrial cooling is a common practice in many parts of the world that have limited fresh-water resources. One of the primary operational problems of using seawater in cooling is biofouling. Biofouling can result from growth of microorganisms on surfaces where they form biofilms or from the growth of macro-organism such as clams. Biofilms tend to stick to heat-exchange surfaces, thereby significantly reducing heat-transfer coefficients. In some cases, excessive bio-fouling can lead to plugging of heat exchangers. There are several techniques for preventing biofouling of both types, but application of chlorine-based biocide is most common.

Chlorine is added to the seawater to control biofouling of the cooling system. The added chlorine reacts with bromide and other compounds in the water to produce a wide range of chemical oxidants. These include conversion of bromide ion to hypobromous acid and other reactive forms of bromine. These brominated products are the active forms of biocide in seawater systems and their relative concentrations change on time scales from fractions of seconds to days. Understanding their behavior is critical to insuring effective control of biofouling within the plant and minimizing environmental impacts outside the plant. Additionally, the brominated residual biocide also can react with natural organic matter in the seawater to form a number of halogenated organic compounds that are toxic to aquatic life as well as humans. Brominated forms of biocide are much more effective in producing halogenated organics than chlorine, but have not been studied as extensively.

Understanding the reaction scheme of chlorine, brominated products and natural organic matter is critical to optimizing performance of biofouling control systems while minimizing impacts to the aquatic environment and to human health. Studies by Shams et al. (1991) on Umm Al Nar seawater desalination plant in Abu Dhabi showed that bromoform represented 95% of the trihalomethanes (THM) that were formed. Ali and Riley (1986) reported that THM concentrations as high as 90 µg/L were observed in the vicinity of discharges from combined power/desalination plants in Kuwait. Many other reports are available on the production of halogenated organics in freshwater systems such as water treatment plants and water distribution systems. However, limited data are available on biocides chemistry and their reaction by-products in seawater.

This purpose of this study is to develop the understanding of biocide chemistry in seawater and apply that understanding to develop methods to quantitatively predict changes in concentrations of biocides and reaction products in cooling water during use and after release to the environment. This will provide the basis for developing a full kinetic model to describe the biocide chemistry in seawater and for developing a simulation tool to determine the fate and transport of biocides and their reaction products in the environment.

Kinetic experiments were conducted to evaluate the kinetics of formation of halogenated compounds in seawater. All experiments were conducted using seawater sampled from the Arabian Gulf in Qatar. In order to study the effect of chlorine dose on the kinetics of trihalomethanes formation, various chlorine doses (1, 2, 3, and 4 mg/L) were reacted with the seawater. Samples were taken at times 0.0, 0.5, 1, 6, 18, 24, 48, 72, 96, 120 and 168 hours and were analyzed for trihalomethane compounds using Gas Chromatography (GC) equipped with electron capture detector (ECD). Results of these experiments showed that concentrations of trihalomethanes increased rapidly during the first half hour of the reaction. Bromoform was found to be the dominant halogenated compound formed in seawater.

References:

Ali, M. and Riley, P. (1986) The distribution of halomethanes in the coastal waters of Kuwait, Marine Pollution Bulletin 17, 409?414, 1986.

Batchelor, B., (1989) A Kinetic Model for Formation of Disinfection By-Products, Fellowship Report, American Academy for the Advancement of Science/Environmental Protection Agency Fellowship Program, Washington, DC.

Goodman P.D. (1987) Effect of chlorine on materials for sea water cooling systems: A review of chemical reactions. British Corrosion Journal 22: 56-62.

Johnson, J.D., Inman, G.W., Trofe, T.W., (1982) Cooling Water Chlorination: The Kinetics of Chlorine, Bromine, and Ammonia in Sea Water?, NUREG/CR-1522 RE, Office of Nuclear Regulatory Research, U. S. Nuclear Regulatory Commission, Washington, DC.

Johnson, J.D., Overby, R. (1971) Bromine and Bromamine Disinfection Chemistry, Journal of the Sanitary Engineering Division, ASCE, 97(SA5): 617-628, 1971.

Shams El Din, A. M., Rasheed, A. A. and Hammoud A. A. (1991) A contribution to the problem of Trihalomethane formation from the Arabian Gulf Water, Desalination 85, 13?32.