SORPTION AND TRANSPORT OF IRON-CYANIDE COMPLEXES IN UNCONTAMINATED SOIL INVESTIGATED IN COLUMN EXPERIMENTS

Abstract

The occurrence of the iron-cyanide complexes ferrocyanide, [FeII(CN)6]4−, and ferricyanide, [FeIII(CN)6]3− in soil and groundwater is caused by disposal of industrial wastes and by the use of road salt containing ferrocyanide. We investigated the sorption and transport of iron-cyanide complexes in sandy uncontaminated soil (Ap horizon, Dystric Gleysol; 857 g sand kg−1, organic C 18 g kg−1, pH 5.8) in column experiments at saturated conditions and two concentrations, 0.03 and 10 m M [Fe(CN)6]. When using 10 m M, flow was interrupted for 36 h. Batch desorption experiments using phosphate and chloride were conducted to evaluate fractions of sorbed iron-cyanide complexes differing in their affinity for the soil matrix. Tracer breakthrough experiments using chloride indicated strongly advective flow in all experiments. The transport of both iron-cyanide complexes was characterized by rate-limited and nonlinear sorption affected by hysteresis. Rate-limited sorption was shown by the decrease of effluent concentrations after flow interruptions and by tailing. Nonlinear sorption became evident by the retardation of the breakthrough curves, depending on influent concentrations. Breakthrough of ferrocyanide took longer than that of ferricyanide, indicating stronger sorption of ferrocyanide. The extent of iron-cyanide complex desorption depended on (i) the amount of iron-cyanide complexes sorbed previously, (ii) the type of complex sorbed, and (iii) the solution used to desorb. For the experiment, higher amounts of iron-cyanide complexes sorbed resulted in lower desorption. More ferricyanide than ferrocyanide was desorbed, and phosphate desorbed more iron-cyanide complexes than chloride. Precipitation of iron-cyanide complexes following desorption experiments was indicated by the presence of blue precipitates in the soil matrix. Consequently, the sorption behavior of iron-cyanide complexes in uncontaminated soil was complex due to rate-limited and nonlinear sorption and precipitation. The experiments revealed that soils may act as a sink for iron-cyanide complexes at low concentrations, e.g., those originating from the use of road salt or occurring in contaminated seepage and groundwater. However, the long-term behavior of iron-cyanide complexes in this soil is characterized by continuous desorption.