Four fractions(A,B,C,and D)of humic acids(HAs)were separated based on the polarity from weak to strong.UV-vis absorption and Fourier transform infrared spectroscopy(FTIR)analysis show that the fractions C and Dpossessedmore aromatic C5C content.The influences of HAs and their fractions on the photolysis were investigated by the photodegradation of 2,4-D solutions under simulated solar light irradiation.The degradation rate of 2,4-D was found to decrease in the presence of bulk HAs or their fractions especially at high HAs concentration.The fractions of strong polarity C and D retarded the degradation rate more than the fractions of weak polarity A and B.This could be attributed to the different absorption intensity of the four HAs fractions in the order of D≥C>A>B,and the stronger π-π electron donor-acceptor interactions between the strong polar fractions and 2,4-D.
