The degradation of aromatic amino acids, including L-tryptophan, L-tyrosine, and L-phenylalanine, by δ-MnO2 was investigated. L-tryptophan and L-tyrosine underwent rapid oxidative degradation in the δ-MnO2 suspension solution. In contrast, L-phenylalanine was not oxidized by δ-MnO2. The degradation of L-tryptophan was faster than that of L-tyrosine. The oxidation rates of amino acids increased with increasing concentration of δ-MnO2. The increase of concentrations of substrates and pH disfavored the degradation. The kinetics modeling showed that the reaction orders of amino acids, MnO2, and H+ were 0.67, 1.24, 0.72 and 0.32, 1.20, 1.05 for L-tryptophan and L-tyrosine, respectively. The coexistence of metal ions markedly inhibited the degradation in the order of Al3+ > Mn2+ > Zn2+ > Ca2+ > Mg2+. The inhibition effect of metal ions was related to the stability constant and hydration radius of metal ions. The addition of fulvic acid to the reaction solutions did not affect the degradation of substrate. This finding suggests that δ-MnO2 selectively oxidize aromatic amino acids and it can be used as an efficient oxidant for the removal of certain aromatic acid pollutants.
Keywords: Aromatic amino acids; fulvic acid; kinetics modeling; metal ions; δ-MnO2.