Cyanide Speciation in Soil and Groundwater at Manufactured Gas Plant (MGP) Sites

Cyanide species have been observed in soil and groundwater at manufactured gas plant (MGP) sites throughout the United States. Groundwater impacts at these sites can occur as a result of leaching (dissolution) from spent oxide box residuals, which were sometimes used onsite as fill material. The predominant forms of cyanide compounds in these residuals are iron–cyanide solids, the most prominent among them being Prussian Blue or ferric-ferrocyanide (FFC). The dissolution behavior of these iron–cyanide solids is a strong function of porewater pH and pE. Upon dissolution, FFC primarily releases dissolved-phase iron–cyanide complexes. Dissolved phase cyanide complexes and species can exist in a number of different chemical forms, ranging from free cyanide (HCN or CN^–) to weak-acid dissociable complexes (e.g., cyanide complexes with copper, zinc, and nickel) to available cyanides (e.g., weak-acid dissociable complexes plus mercury cyanide complex) to strong-acid dissociable complexes (complexes of cyanide with iron and cobalt). This paper presents the result of a detailed characterization of cyanide species in soil and groundwater samples collected from 10 MGP sites in the State of New York. Also presented in this paper is the confirmation that two different soluble iron–cyanide complexes dominate the site groundwaters and soil leachates at these MGP sites. Finally, site-specific geochemical information, like pH and pE, were used as the master variables to develop semiquantitative predictions of cyanide concentrations in groundwater within or near source materials.