Humic acid forms different soluble and insoluble complexes with Fe(III)/Fe(II). Therefore you can't have the hydroxyl radical generating Fenton system in the presence of humic acid. Peroxynitrite, O=N-O-OH, quickly decomposes to HO(.) and NO2(.). Peroxynitrite could be an alternative to the Fenton reagent. Google for "peroxynitrite"