Hydrolysis

Hydrolysis is easily predicted, provided that the rate constants for a compound are known. The rate of abiotic hydrolysis is given by

where R = the rate of hydrolysis (mol/L/s or ^g/L/s), kH = specific hydrolysis rate constant (L/s), CT = the dissolved plus adsorbed phase concentration of compound C (mol/L or ^g/L).

The hydrolysis rate constant kH is actually the sum of three rate constants:

where kn = the natural hydrolysis rate constant for the pH independent reactions of a chemical with water (L/s), ka = the acid-catalyzed hydrolysis rate constant (L/mol/s), [H+] = the concentration of hydrogen ion (mol/L), kb = the base-catalyzed hydrolysis rate constant (L/mol/s), [OH] = the concentration of hydroxide ion (mol/L).

Note that in an acid solution, kb = 0, and in an alkaline solution, ka = 0. kH can be adjusted to include the effects of adsorption by multiplying (ka [H+] + kb [OH]) times the decimal fraction of the total amount of a dissolved compound, C.58 At any fixed pH, the half-life of a substance is independent of concentration and can be calculated with the equation t1/2 = 0.693/kh (20.16)

Hydrolysis is strongly pH-dependent, with ka dominant at low pH and kb dominant at high pH; at pH 7, kn can often be most important. However, the detailed relationship of pH and rate depends on the specific values of kn, ka, and kb. If these rate constants are known, then the hydrolysis rate at any pH can be readily calculated. Mabey and Mill158 provide these data for a large number of organic compounds, and Ellington159-161 provides data on about 70 regulated hazardous pollutants.

Mills58 describes step-by-step procedures for calculating kH, and Scrivner and colleagues39 describe in detail the modeling of cyanide and nitrite hydrolysis in the deep-well environment.

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