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Section 4 <br />Discussion <br />4.1 Arsenic <br />The isotherm data summarized in Table 3-7 suggest that arsenic is controlled by <br />adsorption in samples V4WR4, SJWR3, and TPWR3. However, the adsorption <br />coefficients range from 8 to 16 L/kg which are lower than most literature values, <br />which range from 20 to 30 L/kg (Table 3-8). The low Kd is likely due to the relatively <br />high pH of the solutions (pH = -8 su for all batches except WSWR3, which was 7.2 to <br />7.4 su). Under oxidizing conditions (>400 mv), arsenate (As+5) is the predominant <br />oxidation state of arsenic. At pH 8, the main species of arsenate in solution are HAsO4 <br />2 and H2AsO4- both of which are anions (see Figure 4-1). In addition, the surface <br />charge of most mineral surfaces are negative at pH 8. Even amorphous iron <br />hydroxide is very near the zero point of charge (ZPC) at pH 8. The combination of the <br />negative charge on the arsenic in solution and on the surfaces of the minerals in the <br />soil resulted in a lack of coulombic adsorption, which limited all arsenic adsorption to <br />specific adsorption (surfaces bonding) and resulted in an overall low range of Yd <br />values. <br />1.4 <br />1.2 <br />1.0 <br />0.8 <br />0.6 <br />? 0.4 <br />0.2 <br />0.0 <br />-Os2 <br />-0.4 <br />-0.8 <br />-0.8 <br />-1.0 <br />- • ?? _, @VVSVVP3 <br />LSJWR3 <br />H3A5 04 o TPWR3 <br />(aq) o SVWR2 <br />?_`_` *V4WR4 <br />H2As04 (aq) ' <br />FisO?{aq <br />H3ASO3 {aq) 0 <br />?? <br />- ? 4 <br />•r <br />r? <br />?r <br />r? <br />rr <br />1r <br />H2ASO3 (aq) <br />0 2 4 8 pH 6 10 12 14 <br />Figure 4-1- Eh-pH Diagram for the System As-O-H at 25e C and 1 atm.. <br />• <br /> <br />• <br />CM 4-1 <br />T:N64936-Denison Min"ITask Order 3 - DMO Sampling and Analysis Plan%Task 3.12 - Soil, Ore, Rock Data Assessment Reportt,Sunday Rods ReportAAltechmentskAftenustion Report_working draft 040309.doc