Immobilization of chromate from coal fly ash leachate using an attenuatingbarrier containing zero-valent iron

The purpose of this investigation was (i) to test the effectiveness of a ba rrier engineered to remove Cr(VI) from leachates of higher pH and salinity typical of coal burning ashes and iii) to determine which geochemical proce sses control Cr immobilization. Laboratory column and batch desorption expe riments show that a barrier composed of sand, Fe(0), and bentonite irrevers ibly immobilizes Cr. Concentrations fall from 25 mg Cr L-1 in the leachate to below detection limits (0.0025 mg Cr L-1) and solution pH increases by a bout two units. Solid-phase analytical techniques such as SEM, EDS, XPS, an d TOF-SIMS were used to characterize the barrier material prior to and afte r exposure to the Cr leachate. In the barrier material, Cr(III) was found a ssociated with Fe(III)-oxides, as separate Cr oxides and as a Ca,Cr phase, probably Cachromite, CaCr2O4. The attenuating barrier can be an alternative to traditional liners and leachate collection systems at coal ash storage and disposal sites.