Effect of iron ion on doxycycline photocatalytic and Fenton-based autocatatalytic decomposition

• Doxycycline forms complexes with ferric iron at pH 3.0.
• Iron redox cycling supports the doxycycline autocatalytic decomposition by H2O2.
• Doxycycline oxidation by H2O2/Fe(III) proceeds via hydroxyl radicals.
• Ultraviolet/H2O2 showed the highest doxycycline degradation rate and mineralization.
• The efficacy was: UV/H2O2 ≈ UV/H2O2/Fe(III)>H2O2/Fe(II)>H2O2/Fe(III)≈UV/Fe(III)>UV.

Doxycycline plays a key role in Fe(III)-to-Fe(II) redox cycling and therefore in controlling the overall reaction rate of the Fenton-based process (H2O2/Fe(III)). This highlights the autocatalytic profile of doxycycline degradation. Ferric iron reduction in the presence of doxycycline relied on doxycycline-to-Fe(III) complex formation with an ensuing reductive release of Fe(II). The lower ratio of OH-to-contaminant in an initial H2O2/Fe(III) oxidation step than in that of classical Fenton (H2O2/Fe(II)) decreased the doxycycline degradation rate. The quantum yield of doxycycline in direct UV-C photolysis was 3.1 × 10−3 M E−1. In spite of doxycycline-Fe(III) complexes could produce the adverse effect on the doxycycline degradation in the UV/Fe(III) system some acceleration of the rate was observed upon irradiation of the Fe(III)–hydroxy complex. Acidic reaction media (pH 3.0) and the molar ratio of DC/Fe(III) = 2/1 favored the complex formation. Doxycycline close degradation rates and complete mineralization achieved for 120 min (Table 1) with both UV/H2O2 and UV/H2O2/Fe(III) indicated the unsubstantial role of the reduction of Fe(III) to Fe(II) in UV/H2O2/Fe(III) system efficacy. Thus, factors such as doxycycline's ability to form complexes with ferric iron and the ability of complexes to participate in a reductive pathway should be considered at a technological level in process optimization, with chemistry based on iron ion catalysis to enhance the doxycycline oxidative pathway.

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