Co-existence of humic acid enhances reductive removal of diatrizoate via depassivating zero-valent iron under aerobic conditions
Surface passivation is one of most challenging aspects in successful application of zero-valent iron (ZVI) for water decontamination, especially when oxygen is inevitably present. This study found that co-existence of HA ranging from 0 to 40.0 mg-C/L appreciably improved the pseudo-first-order kinetic rate constant of pollutant diatrizoate (DTA) reduction by ZVI from 0.0454 to 0.1365 h-1 under aerobic conditions. For the first time, it revealed that co-existence of HA could accelerate reductive removal of organic contaminant via ZVI surface depassivation rather than previously proposed electron-shuttle mechanism under aerobic conditions. Interestingly, both DTA removal and Fe0 consumption were found to undergo two stages in the presence of HA, with an initially slight enhancement attributed by Fe(III)-HA complexes formation partially suppressing iron precipitation, followed by a remarkable promotion arising from HA-associated iron oxides detachment from Fe0 surface. These findings not only provide a good strategy to enhance ZVI reactivity with minimal extra cost and complexity under aerobic conditions, but also help understand ZVI behavior in practical applications.