Thilo Rennert

Sorption and desorption of iron-cyanide complexes in deposited blast furnace sludge

Blast furnace sludge is a waste originating from pig iron production and contains small amounts of iron-cyanide complexes. Leaching of iron-cyanide complexes from deposited blast furnace sludge into the ground water seems to be possible in principle. We investigated the sorption of the iron-cyanide complexes ferrocyanide, [FeII(CN)6](4-), and ferricyanide, [FeIII(CN)6](3-), in 22 samples of deposited blast furnace sludge in batch experiments. Subsequently, desorption of iron-cyanide complexes was investigated using 1 M NaCl. Sorption in five samples was evaluated with Langmuir isotherms. The blast furnace sludge samples were neutral to slightly alkaline (pH 7.6-9) and consisted of X-ray amorphous compounds and crystalline Fe oxides primarily. X-ray amorphous compounds are assumed to comprise coke-bound C and amorphous Fe, Zn, and Al oxides. The experiments that were evaluated with Langmuir isotherms indicated that the extent of ferricyanide sorption was higher than that of ferrocyanide sorption. Saturation of blast furnace sludge with iron-cyanide complexes was achieved. Sorption of iron-cyanide complexes in 22 blast furnace sludge samples at one initial concentration showed that 12 samples sorbed more ferrocyanide than ferricyanide. The extent of sorption largely differed between 0.07 and 2.76 Micromol [Fe(CN)6] m(-2) and was governed by coke-bound C. Ferricyanide sorption was negatively influenced by crystalline Fe oxides additionally. Only small amounts of iron-cyanide complexes sorbed in blast furnace sludge were desorbed by 1 M NaCl (ferrocyanide, 3.2%; ferricyanide, 1.1%, given as median). This indicated strong interactions of iron-cyanide complexes in blast furnace sludge. The mobility of iron-cyanide complexes in deposited blast furnace sludge and consequently contamination of the seepage and ground water was designated as low, because (i) deposited blast furnace sludge is able to sorb iron-cyanide complexes strongly, (ii) the solubility of the iron-cyanide-containing phase, K2Zn3[FeII(CN)6] . 9H20, is known to be low, and (iii) a worst case scenario of the transport of iron-cyanide complexes within the blast furnace sludge deposit indicated strong retardation of the complexes within the next 100 years.

Simple modelling of the sorption of iron-cyanide complexes on ferrihydrite

The sorption of the iron-cyanide complexes ferricyanide, [Fe(CN) 6 ] 3- , and ferrocyanide, [Fe(CN) 6 ] 4- , on ferrihydrite was investigated in batch experiments including the effects of pH (pH 3.5 to 8) and ionic strength (0.001 to 0.1 M). The pH-dependent sorption data were evaluated with a model approach by Barrow (1999): c = a exp(bS)S/(S max -S), where c is the solution concentration; S is the sorbed amount; S max is maximum sorption; b is a parameter; and a is a parameter at constant pH. Ferricyanide sorption was negatively affected by increasing ionic strength, ferrocyanide sorption not at all. More ferricyanide than ferrocyanide was sorbed in the acidic range. In the neutral range the opposite was true. Fitting the pH-dependent sorption to the model resulted in a strong correlation for both iron-cyanide complexes with a common sorption maximum of 1.6 μmol m -2 . Only little negative charge was conveyed to the ferrihydrite surface by sorption of iron-cyanide complexes. The sorption of iron-cyanide complexes on ferrihydrite is weaker than that on goethite, as a comparison of the model calculations shows. This may be caused by the lower relative amount of high-affinity sites present on the ferrihydrite surface.

Mobility of Iron-Cyanide Complexes in a Humic Topsoil under Varying Redox Conditions

The potentially toxic Fe-CN complexes ferricyanide, , and ferrocyanide, , undergo a variety of redox processes in soil, which affect their mobility. We carried out microcosm experiments with suspensions of a humic topsoil (pH 5.3; 107 g ) to which we added ferricyanide (20 mg ). We varied the redox potential () from −280 to 580 mV by using , and glucose. The decrease of led to decreasing concentrations of Fe-CN complexes and partial reductive dissolution of (hydrous) Fe and Mn oxides. The dynamics of aqueous Fe-CN concentrations was characterized by decreasing concentrations when the pH rose and the dropped. We attribute these dependencies to adsorption on organic surfaces, for which such a pH/ behavior has been shown previously. Adsorption was reversible, because when the pH and changed into the opposite direction, desorption occurred. This study demonstrates the possible impact of soil organic matter on the fate of Fe-CN complexes in soil.