Mechanochemically sulfidated zero-valent iron as persulfate activation catalyst in permeable reactive barriers for groundwater remediation - A feasibility study

The technology of permeable reactive barriers is reliable and economically effective to prevent the spread of pollutants in groundwaters. Yet, it is efficacious only with easily reducible chemicals such as heavy metals and halogenated organics. In the present study, sulfidated zero-valent iron solventless synthesized by ball-milling is proposed as a possible barrier filling for activation of persulfate to achieve sound removal of reduction-resistant organic pollutants (the herbicide atrazine was used as a model pollutant). Preliminary batch experiments demonstrated rapid degradation of atrazine. Continuous experiments executed in columns proved the superior efficiency of sulfidated iron as a persulfate activator, compared to zero-valent iron, in terms of removal of both atrazine and byproducts. Optimal atrazine removal in the column was achieved with 10% sulfidated iron packing, and 9 mM persulfate at a hydraulic residence time of 6.02 h. Under such conditions, the estimated bed length of the reactive barrier for 99% atrazine removal was 8.69 cm. The morphology and surface-active species in the column demonstrated that activation of persulfate mainly occurred at the inlet of the column until the complete usage of the active species. Batch experiments with coexisting ions suggested that they have a minor influence on atrazine removal percentage, while Mg2+, Ca2+, CO2-and HCO- had a significant impact on the kinetics of the process. However, analogous column experiments demonstrated that the coexisting ions have a negative influence on both atrazine and its byproducts. The results obtained in this study corroborate the potential application of persulfate-enhanced permeable reactive barriers for in situ removal of atrazine from underground water.