The effect of EDTA-2Na on the photocatalytic performance of bismuth-based compounds varied with their chemical compositions and EDTA dosage. For Bi2O3 and Bi5O7I with high Bi/anions ratios, coordinating with EDTA-2Na at molar ratios of 1.5:1 for Bi2O3 and 5:1 for Bi5O7I significantly improved photocatalytic activity of methyl violet (MV, 50 mg.L-1) degradation under visible irradiation, with apparent rate constants (k) of 0.0715 min-1 and 0.0261 min-1, which were 102 and 44 times of those for pure Bi2O3 and Bi5O7I, respectively. Besides, these samples also exhibited enhanced adsorption performance to cation dye MV due to decreased zero zeta potential. However, If the molar ratio of EDTA to Bi2O3 exceeded 4:1 and the molar ratio of EDTA to Bi5O7I exceeded 5:1, the samples possessed the poor photocatalytic performance for the degradation of MV, even inferior to pure Bi2O3 and Bi5O7I. Conversely, BiOX (X = Br, I) and Bi2O2CO3 with low Bi/anion ratios didn't show significant improvement in photocatalytic activity, and BiVO4 and Bi2MoO6 with lower Bi/anion ratios exhibited even worse photocatalytic performance after coordinating with EDTA-2Na. Photocatalytic mechanism suggests that EDTA-2Na's electron transfer capabilities to the conduction band of high Bi/anion ratio compounds promote reactive oxygen species generation, enhancing their photocatalytic performance. For the low Bi/anion ratio compounds, the electrons of EDTA would transit to VB and trap the holes, hindering the hole-dominant photo catalysis process.
