The oxidation of
herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) by different
iron-mediated processes, with or without the presence of ultraviolet (at 253.7 nm) and
oxalate, was investigated and compared. The initial decay rate and the overall removal percentage were used as the performance indexes. To extensively explore the associated processes, the following combinations or blank systems were investigated: UV radiation only, Fe(2+)/H(2)O(2), Fe(2+)/H(2)O(2)/UV, ferrous
oxalate/H(2)O(2), ferrous
oxalate/H(2)O(2)/UV, Fe(3+)/H(2)O(2), Fe(3+)/H(2)O(2)/UV,
ferrioxalate/H(2)O(2), and
ferrioxalate/H(2)O(2)/UV. This study showed that the degradation of
2,4-D by sole UV or dark processes (without UV) is generally slow, except by the conventional Fenton's process (Fe(2+)/H(2)O(2)). However, these slow reactions can be accelerated by exposure to UV irradiation, which can increase the initial
2,4-D decay rate from ten to more than one hundred times. Furthermore, if the reaction is initiated by ferrous
oxalate or
ferrioxalate instead of Fe(2+) or Fe(3+)
ions, the rates can be further improved, because of the higher
light sensitivity of the organometallic complexes. These reactions were also found sensitive to the initial
hydrogen peroxide concentration. The competition of
hydroxyl free radicals by the primary intermediate,
2,4-dichlorophenol, was also observed.