Treatment and stabilization of a hexavalent chromium containing waste material

Abstract

AbstractIn order to stabilize such wastes, it is necessary to convert the Cr+6 to the trivalent (Cr+3) form, before solidifying the waste through the use of cementitious materials such as portland cement or pozzolime (cement kiln dust). A treatment scheme was developed using ferrous ammonium sulfate (FAS) as reducing agent. With FAS the reaction kinetics are sufficiently rapid at neutral or alkaline pH ranges. With most other reducing agents, such as sodium metabisulfite or ferrous sulfate, it would first be necessary to adjust the pH to about 2 S.U. in order for the reduction reaction to proceed in reasonable time. Bench scale tests were conducted in which the waste material containing about 6,000 mg/kg of Cr+6 and with pH approximately II S.U., was reacted with varying doses of FAS at pH levels of 4.5, 7, and as received. The reaction was found to be essentially complete after 3 days and selective for Cr+6. Further tests were conducted in which pozzolime was added to the mixture after the reduction. With an approximately 1:1 ratio of FAS: waste and pozzolime ratios of between 0.5:1 and 1:1, residual hexavalent chromium and total chromium concentrations were well below 100 mg/kg of waste, which corresponds to the EP Toxicity limit of 5 mg/L. The costs for full‐scale FAS/pozzolime stabilization were estimated. Testing of alternative reducing agents, which would require first acidifying with sulfuric acid, was deemed not warranted. The acid quantities required would be costly, and such operations in field conditions were felt to be not prudent.