Chlorophenol reactions on fly ash .1. Adsorption desorption equilibria and conversion to polychlorinated dibenzo-p-dioxins

Tetrachlorophenol (T4CP) reactions to PCDD products in the presence of MSWI fly ash were measured as a function of gas-phase precursor concentration, reaction time, and reaction temperature. Tetrachlorophenol concentrations ranged from 150 to 700 ng/ mL in 10% O-2, and temperatures ranged from 250 to 400 degrees C, which resulted in PCDD formation rates from about 0 to 35 mu g of PCDD (g-fly ash)(-1) min(-1). Reaction conditions were found such that the reactor approached differential behavior, which led to the breakthrough of the T4CP and the desorption of adsorbed PCDD products into the gas-phase. At constant temperature, the surface coverage of chlorophenols on fly ash was found to be constant for reaction times between 2 and 120 min, suggesting that the system had quickly reached adsorption/desorption equilibrium. Yields of PCDD were observed to increase with increasing precursor concentration but either peaked, leveled off, or increased as the temperature was raised. Studies of T4CP reactions on different fly ashes showed a correlation between total conversion of T4CP-to unknown products-and total yield of the trace reaction forming PCDD.