A negligible depletion-solid phase microextraction (nd-SPME) method is developed to measure free concentrations of two alkyl phenols (4-t-octylphenol, nonylphenol) and bisphenol A in activated sludge, with which aerobic biodegradation kinetics of three pollutants was determined. For the degradation of octylphenol, nonylphenol and bisphenol A, the apparent rate constant based on total concentration is 0.02, 0.01 and 0.03 h-1, respectively; with the half life being 34.9, 54.3 and 22.3 h correspondingly. Meanwhile, real rate constants based on free concentration are 0.74, 0.60 and 1.03 h-1, respectively; with the half life being 0.9, 1.2 and 0.7 h accordingly. Differences between two constants revealed that desorption of targeted pollutants from activated sludge should be the rate-limiting step. Compared to the rapid biodegradation of alkyl phenols and bisphenol A in free state, the desorption of pollutants from the sludge is relatively slow, which made the apparent biodegradation rate constants much lower.
