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1.0 INTRODUCTION <br />' Bench-scale laboratory testing was performed on soil and water collected from the <br /> Diamond Walnut Growers site in Stockton, California Four soils (SB-10-1, SB-10-2, <br /> SB-10-3 and SB-11) and 2 waters (MW-3 and site tap water) were received for testing <br /> MW-3 water contained approximately 4 2 mg/L of hexavalent chromium [Cr(VI)], while <br /> the soils contained 0 006 mg/kg or less of Cr(VI) Bench testing evaluated the <br /> effectiveness of two potential reducing agents—ferrous iron solution and calcium <br />,t polysulfide (Cascade°)—to chemically reduce Cr(VI) to trivalent chromium [Cr(III)] <br /> Tests were conducted in the presence and absence of soil to assess the potential for in Situ <br /> remediation <br /> 1.1 Ferrous Iron Solution <br /> Ferrous iron [Fe(H)] reacts with Cr(VI) to form Cr(IH), which precipitates as an <br /> iron/chromium hydroxide when the pH is greater than about 5 (Eqn 1) <br /> Cr(VI) + 3Fe(II) --> Cr(III)+ 3Fe(III) ---> lron/chrorruum hydroxides Eqn 1 <br /> Based on the stoichiometry of Eqn 1, 3 2 g Fe(II) are needed to reduce 1 g of Cr(VI) <br /> Fe(II) must be added as an acidic solution (pH 2-4) in order to prevent its rapid oxidation <br /> to femc iron [Fe(IH)I, which is ineffective for treating Cr(VI) During in situ <br /> remediation, the natural buffering capacity of site soil and groundwater may allow pH of <br /> the sub-surface to return to pre-treatment values once application is complete As the pH <br /> increases to greater than about pH 5, the Cr(III) will co-precipttate with iron to form <br />' iron/chromium hydroxides, which are typically more stable (less soluble) than chromium <br /> hydroxide Excess iron will also precipitate <br /> 1.2 Calcium Polysulfide (Cascade) <br /> Cascade° is a dark orange liquid with a sulfide smell It reduces Cr(VI) to Cr(HI) by a <br /> variety of chemical reactions that ultimately produce solid chromium hydroxide, sulfate, <br /> and sulfur (Egns 2-3) Intermediate products may include sulfide and thiosulfates <br />' Concentrated Cascade is basic (pH -- 10), but the injection pH will be lower because <br /> Cascade& is normally diluted prior to use In addition, the natural buffering capacity of <br /> site soil and groundwater will often minimize long-term changes in pH <br /> r! 2Cr04 + 3CaS5 + 8H2O -� 2Cr(OH)3 + 3Ca(OH)2 +4(OH)- + 15S Eqn 2 <br /> Cr(VI) Cascade CrY11) <br /> rS52" + 3/20-7 —� S2032_ + 3S Eqn 3a <br /> 2Cr04 + 3 52032- + 5H2O -� 2Cr(OH)3 + 3SO42- + 3S +4(OH)- Eqn 3b <br /> Crf VI) Cr(m) <br /> I <br /> PRIMA Environmental 1 Eval of Reductants <br /> January 19 2005 ATC-Diamond Walnut <br />