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DIVISION S-2—SOIL CHEMISTRY

Boron Release from Weathering of Illites, Serpentine, Shales, and Illitic/Palygorskitic Soils

^a USEPA, Office of Research and Development, National Risk Management Research Laboratory, 919 Kerr Research Drive, Ada, OK 74820
^b USDA-ARS, George E. Brown, Jr. Salinity Lab, 450 West Big Springs Road, Riverside, CA 92507-4617

^* Corresponding author (su.chunming{at}epa.gov).

Despite extensive research on B adsorption and release from soils, mineral sources of B within natively high B soils remain poorly understood. The objectives of this study were to identify source minerals contributing to the continued B release after extraction of soluble B and to estimate B release rate from weathering of B-containing minerals and soils. Two specimen illites (Morris and Fithian), two shales (Salt Creek and Moreno Gulch), a fresh and a weathered serpentine (antigorite) from the Coastal Range of California, a Traver silt loam (coarse-loamy, mixed, superactive thermic Natric Haploxeralfs) and a Twisselman clay loam [fine, mixed (calcareous), superactive thermic Typic Torriorthents] both containing illite and palygorskite were successively extracted 7 to 26 times following each 12-h equilibration in 0.1 and 0.01 M CaCl₂ solution until the supernatant solutions contained less than the detection limit of 0.001 mmol B L^–1. Subsequently, the <2-µm and 2- to 20-µm size fractions were separated and reacted in deionized water at pH 5, 7, and 9 adjusted with HCl and NaOH. The total B of the separated fractions ranged from 5.1 to 28 mmol B kg^–1 and the surface areas from 5.7 to 126 m² g^–1. Boron release rates decreased with time and increasing pH. Average B release rates from 150 to 180 d ranged from 0.005 fmol m^–2 s^–1 for Salt Creek shale (2–20 µm) to 0.342 fmol m^–2 s^–1 for Traver silt (<2 µm) at pH 5, 0.004 fmol m^–2 s^–1 for Salt Creek shale (2–20 µm) to 0.060 fmol m^–2 s^–1 for Traver silt (<2 µm) at pH 7, and 0.002 fmol m^–2 s^–1 for weathered serpentine (2–20 µm) to 0.044 fmol m^–2 s^–1 for Traver silt (<2 µm) at pH 9. Nonstoichiometric dissolution was found for all materials at all pH levels. Illite, chlorite, and palygorskite were identified in the clay and silt fractions of the soils. Boron release from the two soils was accompanied with high Mg release into the solution, suggesting palygorskite as a major source for B.

Abbreviations: CEC, cation-exchange capacity • DI, deionized water • ICP-AES, inductively coupled plasma-atomic emission spectrometry • XRD, X-ray diffraction