Mixing of zeolite facilitates re-sequestration of heavy metals via generated Fe-(oxyhydr)oxides in sediments in the presence of NO3- compared to capping

Under nitrate-rich conditions, microbially-driven nitrate reduction (as electron acceptor) mediates the transformation of oxidizable heavy metal (HM) species into acid-soluble and reducible fractions, concomitant with enhanced metal mobilization into overlying water. This study investigates the impact of zeolite application modes—capping and mixing—on controlling HM release with NO₃^- addition. Compared to capping, mixing provides a more sustained inhibition of upward HMs release from sediment. Furthermore, a significant decrease in HM concentrations in overlying water is observed when experiments ended, indicating that sediments transition from being a source to a sink of HMs. Additionally, the study reveals a two-stage Fe(II) oxidation process in sediment, characterized by an initial slow oxidation phase followed by a rapid reaction phase. The distribution of HM fractions suggests that Fe-(oxyhydr)oxides generation is the primary reason for HMs re-sequestration. Therefore, the two-stage Fe(II) oxidation process coupled with zeolite regulates HM mobilization behavior. During the sediment's transition to a pollution source phase (characterized by limited Fe-(oxyhydr)oxides formation), zeolites adsorb dissolved metals from interstitial water. Conversely, upon progression to the pollution sink phase (marked by extensive Fe-(oxyhydr)oxides generation), adsorbed metals by zeolite redistribute into newly-formed Fe-(oxyhydr)oxides, thereby regenerating zeolites' adsorption capacity for HMs.