Solidification/stabilization applied to chromium species

Abstract

It is not easy to stabilize Cr(VI) in treatment systems for waste or wastewater unless it is reduced to Cr(III). Ferrous sulfate is a effective reducing reagent, and can produce stable coprecipitate products like (Cr_xFe_1‐x)(OH)₃. In a cement‐based S/S system, various iron salts are effective at reducing Cr leachabilities, as shown by the TCLP leaching procedure after both short and long cure times. For appropriate contents and time, the leachate concentrations can be brought below the EPA characteristic limit of 5 mg/L. The addition of ferrous sulfate has been investigated in most detail, and ²⁹Si MAS NMR shows retarded hydration by Cr salts alone, or by ferrous sulfate alone or in combination with Cr salts. In addition to retarded hydration, the added ferrous sulfate results in enhanced amounts of chain‐lengthening and chain‐branching silicates, and several explanations are possible for these effects. Leaching behavior of cement‐based S/S samples prepared with binary mixtures of Cr salts with lead and arsenic show synergistic effects that may be due to formation of insoluble salts or to redox chemistry. Cr(III) and Cr(VI) both enhance immobilization of lead, but compromise immobilization of sodium arsenite. Cr(VI) leachability is reduced in the presence of either Pb(II) or As(III).