VOC and SVOC Remediation Using Thermally Enhanced Hydrolysis

Hydrolysis is a water substitution reaction that aids in the breakdown of many organic molecules. While it has been long studied and used by chemists, hydrolysis has been underutilized by the environmental remediation community. Hydrolysis has been overlooked because its reaction rates, at ambient conditions, are in most cases too slow for efficient degradation of harmful organic compounds. In the last decade new subsurface thermal remediation technologies have been used to treat sites containing organic contaminants. These thermal treatment methods have opened up the possibility of utilizing the hydrolysis reactions as part of site treatment. Applicability of thermally enhanced hydrolysis for various contaminants, remedial optimization techniques, cost savings, and a remediation case study are discussed. INTRODUCTION During hydrolysis, an organic compound chemically reacts with water. The reaction takes place without regard to redox conditions, dissolved minerals, or the presence of soil microbes. The organic compound needs to be dissolved into water for the reaction to take place. The general hydrolysis reaction, XH ROH O H RX + = + 2 (1) involves the exchange of some functional group X, on an organic molecule R, with the hydroxide group from water (Washington, JW. 1995). Through the hydrolysis reaction, the organic compound either mineralizes or converts into another organic compound that is less toxic or less recalcitrant than its parent. The authors know of no common environmental contaminant for which hydrolysis is considered to be an unfavorable reaction. Hydrolysis tends to be important for chlorinated and other halogenated compounds, especially halogenated alkanes (Jeffers et. al. 1986). Most hydrolysis reactions are considered pseudo first order reactions (Weintraub, 1986). Pseudo first order reactions are defined as systems where all but one of the reactants are in a supply so large that their concentrations can be considered constant. For compounds of environmental interest, only the concentration of the contaminant affects the reaction rate and therefore the reaction can be considered to be first order. EFFECT OF PH Three principal hydrolysis mechanisms for halogenated compounds have been identified (Morrison and Boyd, 1960): elimination by dehydrohalogenation, unimolecular nucleophilic substitution (SN1), and bimolecular nucleophilic substitution (SN2). Dehydrohalogenation and SN2 are favored by higher pH; SN1 is not pH-dependent. Therefore, the hydrolysis rates of some compounds increase with pH. Each unit increase of pH will increase the concentration of the hydroxyl ion by a factor of ten and the hydrolysis rate will increase by a factor of ten if the hydrolysis reaction is pH-dependent. The overall rate constant, k, for most hydrolysis reactions is described by: [ ] N B k OH k k + = − (2) where kB is the rate constant for the base reaction, OH is the hydroxyl ion concentration, and kN is the rate constant for the neutral pH reaction (Washington, JW. 1995). Equation 2 shows the dependence of the overall rate constant on pH. Depending on the pH of the system, one of the rate constants will become the dominant reaction. In basic solutions, the kB rate constant will be multiplied by a high hydroxide ion concentration, thus increasing its effect on the overall hydrolysis rate. In neutral solutions the kN reaction is usually dominant. While pH does have an effect on the rate of hydrolysis reactions, there are few environmental situations where it can be taken advantage of. Because most environmental conditions are near a neutral pH the easiest and most effective control over hydrolysis comes from adjusting the temperature. EFFECT OF TEMPERATURE The pseudo first order hydrolysis reactions have a simple and constant relationship for the half-life of the reactant. A half-life is described as the time it takes for 50% of a reactant to be consumed by a reaction. In the case of a pseudo first order reaction, the half-life is described by: k t 2 ln 2 / 1 = (3) where k is the reaction rate constant. The reaction rate constant describes how long it will take for a given amount of reactant to be consumed by the reaction. The k value is constant at any given temperature, but changes exponentially in relation to a change in temperature. This relationship is described by the Arrhenius Equation, T R Ea e A k ⋅ − ⋅ = (4) where A is a constant specific to each reaction, Ea is the activation energy for the reaction, R is the universal gas constant, and T is the temperature of the system where the reaction is taking place. Exponents raised to a negative power return positive answers with values less than one. The larger the negative power, the smaller the positive result from the exponent. In order to increase the reaction rate constant, k, one must only reduce the size of the negative exponent. Of all of the numbers in the equation, only the temperature is variable. By increasing the value of temperature, the negative value in the exponent term will become smaller. This will in turn increase the value of the reaction rate constant. The rate of hydrolysis increases markedly with temperature as described by the Arrhenius reaction. Heating the subsurface from ambient temperatures to 80°C will increase the hydrolysis rates of most organic compounds by a factor of over 1000. Heating to 100°C will increase hydrolysis by a factor of over 10,000. Heating can lead to dramatically fast and effective remediation of compounds that are subject to hydrolysis. The relationship between hydrolysis half-life and temperature is represented in Figure 1. The hydrolysis half-life is represented on a log scale. 0.01 0.10 1.00 10.00 100.00 40 50 60 70 80 90 100 110 120 hy dr ol ys is h al f-l ife a t p H 7 (d ay s) Temperature (°C) Hydrolysis Rates of Selected Halogenated Alkanes