(306c) Hydrolysis Kinetics for 2-Chloropyridine in Supercritical Water

Pesticides (especially herbicides, fungicides and insecticides) are substances or products used to treat crops, by preventing, destroying, repelling, or mitigating weeds, insects, rodents, fungi, bacteria or other organisms. They include a large variety of chemicals that owe their effect to one or more specific active groups. The extensive use of pesticides in agriculture generates a large amount of waste products such as plastic containers and wastewaters. Plastic containers can be washed and recycled, but the generated wastewaters contain organic pollutants resistant to conventional biological treatment. Thus, an alternative treatment for the destruction of these low concentration chemicals in residual waters is necessary. Previous studies have demonstrated that Supercritical water oxidation (SCWO) is appropriate to treat water contaminated with non-biodegradable organic compounds at low concentrations (<10% w/w). Supercritical water is an excellent reaction medium for the oxidation of aqueous waste streams, especially at low concentrations. Properties of water above the critical point (T = 374ºC and P = 22.1 MPa) change to those of a non-polar solvent. Many organic compounds and light gases (e.g., oxygen) are highly soluble in supercritical water. Its low viscosity and high diffusivity of reactants yield rapid and complete hydrolysis and/or oxidation reactions. At 450 to 700ºC, roughly half the temperature of conventional incineration, organic compounds can be quickly (0.1 to 100 s) and efficiently (99.9 to at least 99.99%) oxidized, mainly into carbon dioxide, water and nitrogen. These attributes make SCWO attractive for treating organic aqueous wastes, in such as those obtained from the recycling of pesticide containers. In addition, it has also been found that hydrolysis is significant during feed preheating, before supercritical conditions are reached in the SCWO reactor. Therefore, it is necessary to better understand the hydrolysis kinetics since less oxidant or shorter residence times could be needed to assure high degradation conversions in SCWO processes. Hydrolysis is a chemical reaction in which a bond cleavage is affected by water or by acid or base produced from a reaction of water with salts. Individual water molecules may participate in elementary reaction steps as reactants or products. Examples reported in the literature include hydrolysis, hydration, hydrogen exchange, and free-radical oxidation reactions. Hydrolysis of organic compounds in high temperature water (HTW) has been studied in the past for different purposes, including the destruction of organic wastes. Experimental studies have demonstrated that many organic compounds, both small and polymeric, are hydrolyzed to various degrees in HTW, depending on their reactivity. Fully hydrocarbon compounds are generally resistant to hydrolysis in ?pure? HTW or HTW without catalysts added. Compounds that are particularly susceptible to hydrolysis in HTW are those containing a saturated carbon atom attached to a heteroatom-containing functional group. There is a lack of information, however, about the reactivity of many chemicals under such conditions. 2-Chloropyridine (2-CP) is a common degradation product from a wide range of pesticides, and its presence in wastewaters makes it a good candidate to be studied, as a model compound, for treatment by hydrothermal processes of a broader class of nitrogen containing organic compounds. The objective of the current work is to characterize the hydrolysis kinetics of 2-CP in isothermal batch experiments over the temperatures range 400-600ºC. First, some preliminary experiments at temperatures between 400 and 550°C at 280 bar have been carried out in a batch reactor to determine the reactivity of 2-CP in pure water and to establish suitable conditions for the kinetic study experiments. Second,, reaction rate constants for the acid hydrolysis of 2-CP have been determined in the range 400-600°C at 280 bar in the same batch system used in the preliminary experiments. Preliminary oxidation experiments have been also carried to evaluate the effectiveness of SCWO in wastewater treatment process for this and similar pesticides.