Inhibition of Mn(II) oxidation performance in the presence of high residual Mn(II) concentrations

Abstract

Bioreactors rich in Mn-oxidizing bacteria hold the potential for removing toxic Mn(II) and generating valuable bio-manganese oxides. Various environmental factors, such as pH, temperature, initial Mn(II) concentration, and dissolved oxygen, could affect microbial Mn(II) oxidation activity. However, limited information is available concerning the effect of residual Mn(II) concentrations on the optimization of bioreactor operations for efficient Mn(II) oxidation. This study aims to explore the influence of residual Mn(II) on the biological Mn(II) oxidation performance in the reactors, employing two identical reactors (R-1 and R-2) operating at varying Mn(II) loading rates. Interestingly, during Phase 2, despite both reactors receiving an equivalent Mn(II) loading rate, R-1 exhibited no residual Mn(II) and demonstrated a higher Mn(II) oxidation rate than R-2, which had a high residual Mn(II) concentration. The maximum Mn(II) oxidation rate in R-1 and R-2 was 0.91 and 0.61 kg m-3 d-1, respectively. The results unequivocally reveal that high concentrations of residual Mn(II) exert a substantial inhibitory effect on Mn(II) oxidation activity. This inhibition can be attributed to the diminished tolerance of specific Mn-oxidizing bacteria, such as Hyphomicrobium, to elevated Mn(II) levels. These findings underscore the critical importance of maintaining low residual Mn(II) concentrations within the reactors. This study carries practical implications for the effective operation of Mn(II) removal systems using bioreactors, highlighting the need to mitigate high residual Mn(II) concentrations for optimized performance.