Risk assessment of the utilization of basic oxygen furnace slag (BOFS) as soil liming material: Oxidation risk and the chemical bioavailability of chromium species

Basic oxygen furnace slag (BOFS), a by-product of steel manufacturing, is efficient as liming material. It contains, however, trace amounts of heavy metals such as chromium (Cr). Under environmental conditions Cr exists in two stable oxidation states: +III and +VI. From those, Cr-III is an essential nutrient, whereas Cr-VI is highly toxic. In soils, soluble Cr-III is potentially oxidized to Cr-VI by manganese (hydr)oxides (MnO2). We investigated the release of bioaccessible Cr-III or Cr-VI from BOFS to field soil. Prior to characterization of the bioaccessible, as well as less bioavailable Cr fractions, field soil was incubated with BOFS in quantities equivalent to 10 and 100 t ha(-1) (7 weeks at 22 degrees C, moisture content at 50% of water holding capacity). Liming with BOFS elevated the acid soluble (known as semi-total) Cr but not the bioaccessible Cr-III or Cr-VI. In addition, potential oxidation of initially soluble Cr-III (mimicking leachates of BOFS) was determined by a short-term net-oxidation test in soil suspension with 1 mM CrCl3 (1:10 soil to solution). In field soil incubated with BOFS, the net-oxidation (oxidized Cr-III-Cr-VI reduced back to Cr-III) of added Cr-III into Cr-VI without MnO2 was negligible. To produce highly oxidizing conditions, the test was further modified with excessive addition of synthetic (MnO2)-O-IV. Liming with BOFS diminished the production of Cr-VI, whereas added (MnO2)-O-IV notably promoted oxidation of Cr-III. Overall, the oxidation risk of Cr-III in BOFS to Cr-VI by innate soil MnO2 can be expected to be minimal due to the low solubility of Cr-III in field soil. (C) 2018 Published by Elsevier B.V.