The aim of this work is to study the degradation rate of adipic acid in wet FGD plants using forced oxidation. The investigation is experimentally demanding because the degradation rate must be studied under realistic conditions present in pilot plants or industrial plants only. This is the first systematic investigation including both chemical and biological degradation. The influence on the degradation rate of adipic acid was studied: The concentration of adipic acid (0-2100 mg/l), trace-metals, Cl- (0-50 g/l), pH (4.7 and 5.4), and temperature (32, 42 and 50 degrees C). Furthermore. the degradation rate of adipic acid was examined in two types of limestone/gypsum slurry: one based on limestone, distilled water and flue has from natural Gas combustion, the other on slurry liquid taken from the wet FGD plant of a full-scale power plant (coal combustion) with limestone subsequently added. The first order rate constant, using slurry based on natural gas combustion, was estimated to 0.60+/-0.10 day(-1) which is more than twice the value of the rate constant estimated from experiments based on slurry front the full-scale wet FGD plant (0.25+/-0.10 day(-1)). Both types of slurry were examined for biological activity. In the slurry based on natural gas combustion no biological activity was found. Independent laboratory tests showed that biological activity contributed to the degradation rate of adipic acid in the slurry liquid from the full-scale wet FGD plant, though the effect could not be quantified. Analysis of the slurries for selected trace metals showed significantly higher concentrations in the slurry from the full-scale plant. It was found that increasing concentrations of trace metals and chloride inhibits the chemical degradation of adipic acid. (C) 2005 Elsevier Ltd. All rights reserved.