Inhibitory effect of NOM in photocatalysis process: Explanation and resolution

•Photocatalytic degradation of CA was inhibited by the presence of NOM.•Inhibition of NOM was more dependent on the molecular weight.•Higher molecular weight fractions were prone to be adsorbed onto the catalyst.•pH adjustment did not eliminate the inhibition of NOM.•Oxygen addition can enhance the degradation rate of high molecular size fractions.