The Use of PRBs (Permeable Reactive Barriers) for Attenuation of Hexavalent Chromium and Cadmium from Industrial Contaminated Soil

Permeable reactive barriers are considered among the most promising new technologies to remediate contaminated soil.  Zero-valent iron, hydroxyapatite, and organic compost (with and without sulfate as a terminal electron acceptor) were assessed in column studies for their ability to attenuate soluble hexavalent chromium or cadmium via chemical reduction, precipitation, and/or sorption mechanisms. PVC columns (30 cm length, 5 cm i.d.) were packed with the reactive media, and Cr or Cd solutions were pumped through the columns.  Cr and Cd were applied to columns at three concentrations (5, 50 and 200 mg/L). Collected solutions were measured for Cr and Cd concentration using flame atomic absorption spectrophotometry. Zero-valent iron was significantly superior in its ability to attenuate soluble Cr; therefore it removed 90.4% and 38.8% more than the hydroxyapatite and the organic compost respectively, after 12 pore volumes (i.e., 5 mg/L per volume). At the completion of the solution runs, the treatment materials were dried and subsequently analyzed via x-ray diffraction for Cr and Cd minerals which may have crystallized. In the second part of the study, soil collected from a contaminated site in central Indiana (and enriched in both Cr and Cd) was sieved and packed into PVC columns. The reactive material demonstrating the greatest Cr and Cd removal from the first tests (i.e., zero-valent iron) will be packed up-gradient of the soil. Acidified deionized water will be pumped through the columns and Cr and Cd retention will be evaluated. The Cr and Cd elution data will be input into mathematical equations in attempts to formulate models to accurately explain and predict the behavior of both metal and reactive medium. Experimental data will be treated statistically by one-way analysis of variance (ANOVA) using SPSS®. Those data found to be significantly different at p<0.05 will be subjected to post-hoc Bonferroni t-tests.