Boron Sorption and Release by Allophane
1997
Su, Chunming | Suarez, Donald L.
To predict B behavior in soils and its influence on plants, it is crucial to understand the mechanisms of B sorption by mineral phases. Because B reacts with clay minerals and Al and Fe oxides, it should also sorb to poorly crystalline aluminosilicates such as allophane. Boron sorption, with and following allophane precipitation, was studied in batch experiments. Solutions containing AlCl₃ and Na₂SiO₃ (concentrations <47 mM with Al/Si molar ratio of 1:1) and B(OH)₃ (0, 4.4, and 32 mM) were mixed and titrated to pH 8.0 with 1 M NaOH at two rates (1 and 5 mL min⁻¹) at 23°C, then incubated at 23 and 90°C for 5 d. More B was removed from solution when B was present during allophane precipitation than when B of an equal concentration was reacted with freshly precipitated allophane. In addition, more B was removed when allophane was formed by slower mixing at a slower titration rate, with higher initial B concentration, and incubated at lower temperature. More B was released by repeated washing with dionized water from allophane precipitated in the presence of B(OH)₃ than from allophane reacted with B(OH)₃. Boron sorption was investigated with diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Adsorbed B was both trigonally and tetrahedrally coordinated, with the former being predominant. Boron reacted with allophane precipitate was surface sorbed. Corprecipitated B occurred only in tetrahedrally coordinated positions, presumably by substituting for Si in the tetrahedral layer. The structurally coprecipitated B is expected to be more resistant to release than the adsorbed B.
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