Release and mobility of polycyclic aromatic hydrocarbons and iron-cyanide complexes in contaminated soil

Export of organic and inorganic pollutants from the unsaturated zone of manufactured gas plant (MGP) sites may endanger groundwater quality. The release and mobility of polycyclic aromatic hydrocarbons (PAH) and iron-cyanide complexes (FeCN) in soils from a former MGP site were studied in column and batch experiments. The unsaturated column experiment involved two parts. (1) The development of steady-state effluent concentrations was studied under constant irrigation. (2) Rate-limited contaminant release was studied by the system's response to flow interruption. Transport data were supplemented by a sequential batch-desorption experiment. During constant irrigation the initial concentrations of total PAH and FeCN, but also the concentration of dissolved organic carbon (DOC) were higher than the steady state level, indicating that a specific fraction of the DOC and the contaminants was readily mobilized. Cyanide reached a constant effluent concentration after seven pore volumes (pvs), whereas equilibration of effluent PAH took 20 pvs. After flow interruption, rate-limited release of DOC, PAH, and FeCN was observed by increased effluent concentrations. Comparison of FeCN levels with theoretical equilibrium concentrations indicated that in addition to the dissolution of iron(II)ferrocyanide (Berlin Blue), Fe₄[Fe(CN)₆]₃, other cyanide-species may control the release and such the mobility of FeCN. This was confirmed by decreasing FeCN concentrations during sequential batch extractions. Due to rate-limited release, seepage water concentrations of DOC, PAH, and FeCN in alkaline MGP soils are influenced by the flow regime. Experiments performed at continuous irrigation may therefore severely underestimate the release of PAH and FeCN under field conditions. The effect of DOC on the release and mobility of PAH and FeCN under transient conditions has to be further investigated.