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 μmol [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]•9H2O, 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.