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ATR-FTIR spectroscopic studies of boric acid adsorption on hydrous ferric oxide |
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| Authors: | Derek Peak George W Luther III |
| Institution: | 1 Department of Soil Science, University of Saskatchewan, Saskatoon, SK S7N 5A8, Canada 2 College of Marine Studies, University of Delaware, Lewes, DE 19958, USA 3 Department of Plant and Soil Sciences, University of Delaware, Newark, DE 19717, USA |
| Abstract: | Boron is an important micronutrient for plants, but high B levels in soils are often responsible for toxicity effects in plants. It is therefore important to understand reactions that may affect B availability in soils. In this study, Attenuated Total Reflectance Fourier transform Infrared (ATR-FTIR) spectroscopy was employed to investigate mechanisms of boric acid (B(OH)3) and borate (B(OH)4−) adsorption on hydrous ferric oxide (HFO). On the HFO surface, boric acid adsorbs via both physical adsorption (outer-sphere) and ligand exchange (inner-sphere) reactions. Both trigonal (boric acid) and tetrahedral (borate) boron are complexed on the HFO surface, and a mechanism where trigonal boric acid in solution reacts to form either trigonal or tetrahedral surface complexes is proposed based upon the spectroscopic results. The presence of outer-sphere boric acid complexes can be explained based on the Lewis acidity of the B metal center, and this complex has important implications for boron transport and availability. Outer-sphere boric acid is more likely to leach downward in soils in response to water flow. Outer-sphere boron would also be expected to be more available for plant uptake than more strongly bound boron complexes, and may more readily return to the soil solution when solution concentrations decrease. |
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