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Evaluation of Chemical Amendments for pH and Redox Stabilization in Aqueous Suspensions of Three California Soils

^a Dep. of Environmental Sciences, University of California, Riverside, CA 92521
^b USDA-ARS, U.S. Salinity Lab., 450 W. Big Springs Rd., Riverside, CA 92507
^c USDA-ARS, SJVASC-Water Management Research, 9611 S. Riverbend Ave., Parlier, CA 93648

View larger version (28K): [in this window] [in a new window]   Fig. 1. Stability of pH in Lillis soil suspensions after titration with HCl or NaOH (Days –13 to –1.2) and separation and enclosure into serum bottles followed by addition of 20 mM Good Buffers (Day 0). Symbols represent live (solid and hatched), heat-killed (AC, open square), or antibiotic (Chl, open triangle) samples.

View larger version (13K): [in this window] [in a new window]   Fig. 2. The pH and Eh values of Lillis soil suspensions on Day 0 (plus symbols) and Day 8 (small symbols: controls [closed circles and open triangles]; autoclaved [cross-hatched]; chloramphenicol [open squares]). Theoretical EH–pH values for water in equilibrium with H2 gas partial pressure = 2.53 kPa at the initial pH are also indicated (diamonds).

View larger version (22K): [in this window] [in a new window]   Fig. 3. Changes in EH with time for Lillis soil suspensions with 5 mM soluble or solid-phase suspension concentrations of various chemical amendments. Sample labeled "Control" had no amendment, and the concentration for the lactate sample was 1.67 mM. Abbreviations: ferrihydrite (Fh), goethite (Gt), hematite (Hm), magnetite (Mag). Samples labeled +Fe(II)Cl2 were amended with 1 mM FeCl2 in addition to 5 mM solid-phase suspension.

View larger version (35K): [in this window] [in a new window]   Fig. 4. Changes in EH with time for Hanford (HA) and Imperial (IM) soil–water suspensions for samples with varying amounts of gases (O2 or H2) and soluble Fe, Ti, or ascorbate (as ascorbic acid). Concentrations (in mM) are listed following sample label.

View larger version (33K): [in this window] [in a new window]   Fig. 5. Changes in EH with time for Hanford (HA) and Imperial (IM) soil–water suspensions for samples with varying amounts of Fe or Mn (hydr)oxides (as solid–liquid suspensions) and S compounds. Abbreviations for Fe (hydr)oxides are ferrihydrite (Fh), goethite (Gt), hematite (Hm), and magnetite (Mag). Concentrations (in mM) are listed following sample label.

View larger version (34K): [in this window] [in a new window]   Fig. 6. Ratios of 1 M HCl extractable Fe(II)/Fe(III) on Days 1 and 48 for the 5.0 mM soluble or suspended solid-phase chemical amendments. Note missing value for Imperial soil amended with MnO2, Day 48.

View larger version (50K): [in this window] [in a new window]   Fig. 7. Effects of chemical amendments on 1 M HCl extractable Fe(II)/Fe(III) (Day 48) and soluble S(–II)/SO4 ratios (Day 56) for all Hanford and Imperial soil suspensions. Method detection limit (MDL) for S(–II)/SO42– ratio is 0.05. Note missing values for Imperial Valley, MnO2 amendment, Day 48.

View larger version (18K): [in this window] [in a new window]   Fig. 8. Predicted vs. measured (Pt combination redox electrode) Eh based on soluble concentrations of Fe2+ (measured) and Fe3+ (estimated, in equilibrium with amorphous (am-)Fe(OH)3 or -FeOOH solid phase) for Hanford soil–water suspensions. The EH comparisons based on the SO42––HS– (measured) redox couple are also shown. Measured Fe2+ concentrations for highlighted (square-enclosed) samples were less than the detection limit (9.0 x 10–6 M) and were estimated at 1.0 x 10–6 M. Straight line indicates 1:1 correlation between measured and predicted EH values.