Characterization of ferricyanide-humate complexes by a voltammetric approach

The industrial sites that deal with production and/or use of cyanide in the ir processes often have contamination problems by cyanide In soils and wate r. The behavior of these hazardous compounds in soil and the interactions w ith soil components are mostly unknown The toxicity of cyanide and Its tate In soil are strongly influenced by the formation of complexes with iron, i .e., ferric - and ferrous cyanides (Fuller, WH, 1994, Theis and West, 1986) These complexes may interest with the soil inorganic fraction containing f errous or ferric iron, with consequent formation of stable precipitates (fe rric ferrocyanide or ferrous ferricyanide, the Prussian Blue and the Turnbu ll Blue, respectively). However, no information is available on the capacit y of humic substances to immobilize the cyano-complexes and thus to reduce their toxicity and the leaching, and the consequent risk of contamination o f watertable. In fact, one of the most significant properties of humic subs tances is their ability to interact with xenobiotics to form complexes of d ifferent solubility and chemical and biochemical stability. In this paper we report the results obtained in a voltammetric investigatio n aimed to study the behavior of the ferricyanide/ferrocyanide redox couple chosen as the indicator of redox system (Helburn and MacCarthy, 1994) In t he absence and in the presence of humic fraction at nominal molecular weigh t lower than 5kDa (HSLMW) extracted from a surface horizon of a Haplumbret soil. The results obtained in these investigations combined with the eviden ce gained by spectrophotometric measurements carried out in parallel, point out that ferrocyanide-HSLMW and ferricyanide-HSLMW complexes are more stab le than the parent FE-CN- ones. These complexes undergo electrochemical pro cesses involving a slow homogeneous chemical reaction preceding the electro n transfer step and causing their kinetic control (CE processes). The volta metric measurements allowed the conditional constants K to be determined fo r ferric and ferrous cyanide-HSLMW complexes, respectively: they turn turn out to be about three orders of magnitude higher than those for the ferricy anide and ferrocyanide complexes alone, respectively. From the data treatme nt we may hypnothize that two humate ligands, on average, are accommodated in the coordination sphere of both iron (III) and iron (II) in the correspo nding complexes.