Detection and behavior of iron-cyanide complexes in soils of a former Manufactured Gas Plant site
(2014)
In Germany, soil and groundwater atmore than a thousand sites are contaminatedwith ironcyanide complexes. These contaminations originate from the gas purification p process that was conducted inManufactured Gas Plants (MGP).There is a risk of human threat depending on the stability dissolved iron-cyanide complexes due to the potential release of toxic free cyanide, CN(aq) and HCN(g), (aq). The aim of this thesis was to evaluate a new method for the cyanide detection and to investigate the stability and retardation of the iron-cyanide complexes in soils by batch and column experiments in order to improve the knowledge concerning the fate of cyanide compounds in soil and groundwater. This study seeks for enhancing the efficiency of phytoremediation as a proper approach to decontaminate ironcyanides complexes and thus tries to make a contribution to environmental protection.
Soil contamination with iron-cyanide complexes is a common problem at former manufactured gas plant (MGP) sites. Dissolution of the cyanide, from Prussian Blue (ferric ferrocyanide), creates an environmental hazard, whereas the risk of groundwater contamination depends on the stability of dissolved iron–cyanide complexes. Lack of a standard leaching method to determine the water-soluble (plant-available) cyanide fraction generates potential limitations for implementing remediation strategies like phytoremediation. Applicability of neutral solution extraction to determine the water-soluble cyanide fraction and the stability of Prussian Blue in surface and near-surface soils of an MGP site in Cottbus, undersaturated and unsaturated water conditions, was studied in column leaching and batch extraction experiments. MGP soils used in the long-term tests varied according to the pH (5.0–7.7) and the total cyanide content (40–1718 mg kg−1). Column leaching, after four months of percolation, still yielded effluent concentrations exceeding the German drinking water limit (> 50 μg L−1) and the solubility of Prussian Blue reported in the literature (< 1 mg L−1) from both alkaline and acidic soils. Long-term (1344 h) extraction of MGP soils with distilled water was sufficient to dissolve 97% of the total cyanide from the slightly alkaline soils and up to 78% from the acidic soils. Both experiments revealed that dissolution of ferric ferrocyanide under circum-neutral pH and oxic water conditions is a function of time, where the released amount is dependent on the soil pH and total cyanide content. Unexpectedly high and continuous solubility of Prussian Blue, both in acidic and slightly alkaline MGP soils, implies the need to introduce an additional cyanide fraction (“readily soluble fraction”) to improve and specify cyanide leaching methods. Long-term extraction of cyanide-contaminated soil in neutral solution seems to be a promising approach to evaluate the potential hazard of groundwater pollution at the MGP sites.