Plant uptake of cyanide

Cyanide (CN) was a by-product of gas production at manufacturing gas plants (MGP). CN was precipitated from the gas using iron oxides to form solid iron CN complexes. The waste products were often used as fill on the MGP site. The speciation of CN at MGP sites is dominated by the two solid complexes Prussian blue and Turnbull’s blue. The complexes are stable at the low pH most often found in CN polluted soil at MGP sites, but the dissolution increases with increasing pH. By dissolution Prussian blue and Turnbull’s blue release the two water soluble iron CN complexes ferro- and ferricyanide. According to equilibrium calculations ferro- and ferricyanide complexes are only stable at pH > 7.

Only at high pH levels in the soil are ferro- and ferricyanide complexes considered to constitute a human health risk. An increase of pH will increase leaching of the complexes to the groundwater and if the groundwater is exposed to sunlight before consumption, free CN can be formed. The risk of ingestion of CN polluted soil is difficult to predict since it will change from site to site depending on concentration, geology, climate etc. Both low and high risk evaluations can be found in the literature but in general it must be concluded to be site specific. Volatilization of free CN is only considered to constitute a risk in closed environments where the soil can be exposed to sunlight, e.g. greenhouses. Plant uptake of CN is not considered as an important exposure route for eatable crops growing above the soil surface, while CN concentrations in root vegetables growing in the soil, like carrots and radish, should be further investigated.

All vascular plants have an enzymatic system to metabolize CN formed during ethylene synthesis. Metabolism of artificially supplied CN was investigated with a phytotoxicity test and a metabolism test. Five different woody plants (willow, poplar, birch, elder and rose) were all able to remove free CN (HCN and CN^-) from a solution. The fastest removal was seen for willow trees (Salix viminalis) where removal rates for leaves and roots were around 9.5 and 7 mg/kg/h, respectively. Willow trees grown in a hydroponic solution containing free CN started to show phytotoxic effects at concentrations of 2 mg/L. When grown in sand the phytotoxicity was approximate a factor 10 lower. More than 90% of the free CN supplied to the willow trees was removed from the closed system (plant + flask) and no accumulation of CN was found in healthy plants. A nonlinear mathematical model was used to balance estimates of uptake and metabolism. The model predicted that at low doses (< 10 mg/L when grown in sand), the CN would be rapidly metabolized. At higher doses, uptake would be faster than metabolism and consequently CN would accumulate in the plant tissue and toxic effects would occur.

When grown in hydroponic solutions containing ferro- or ferricyanide at concentrations up to 10 mg/L as CN, willow trees showed no phytotoxic effects. The speciation in the solutions became dominated by ferrocyanide during the experimental period, when an electron donor was present. It appeared that the removal of CN from the solution decreased as the concentration of ferrocyanide increased, suggesting that uptake of ferricyanide is preferred over uptake of ferrocyanide. Increased concentrations of CN were found in all plant compartments (roots, stem and leaves) and between 10 and 50% of the CN were lost from the system. The highest percentage removal was found in the solutions with ferricyanide and with no additional source of nitrogen which implies that lack of nitrogen increases the uptake and decomposition of iron CN complexes.
The limiting factor for phytoremediation of CN polluted soil was concluded to be the dissolution of the solid iron CN complexes. An increase of soil pH, eventually by spreading of lime, is very likely to increase dissolution and plant uptake of iron CN complexes. In addition, an increase of soil pH can be necessary for the plants to survive in the typically very acidic soil. An increasing solubility of iron CN complexes also increases the possibility of leaching of CN to the groundwater; hence, the transpiration of the plants should be sufficient to prevent migration of water from the unsaturated zone to the groundwater.