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Novel donor-acceptor reagents in annulation sequences : total synthesis of oplopanane-type sesquiterpenoids

University of British Columbia

An investigation of the effects of soil organic matter on boron retention and release in soils was carried out on soils from British Columbia. The effect of soil humic acid on boron adsorption was studied in detail as a function of pH, boron concentration in equilibrium solution, ionic strength and different cations. Boron adsorption by Ca-precipitated humic acid was found highly pH-dependent, with the maximum adsorption at pH around 9.5, and remained little changed below pH around 6.5. Boron adsorption isotherms by various humic acids were studied at two pH levels and it was found that the increase of boron adsorption with an increase of boron in equilibrium solution at a higher pH level (8.5-9.0) was much more pronounced than that at a lower pH level (near neutral). The adsorption conformed quite well to the Freundlich empirical equation and was fairly well described by the Langmuir theory. The b and kb values calculated from the Langmuir equation increased with increasing pH whereas k values decreased with increase of pH with one exception. High ionic strength in solution favored boron adsorption by soil humic acid. By comparing the b values (high pH level), the amount of boron adsorbed by the two humic acids, WH-HA(a) and LA-HA(b), increased 21% and 12 %, respectively, with an increase of solution ionic strength from 0.03 to 0.3. However, at lower pH levels, the effect seemed insignificant. Results indicated that humic acids extracted from different soils or by different extraction methods adsorbed very different amounts of boron. The difference was attributed to the contribution of iron present in the humic acid samples, and it was confirmed experimentally that Al or Fe precipitated humic acid adsorbed much more boron than Ca-precipitated humic acid. No significant amount of boron adsorption by soil polysaccharides was observed at pH near neutral. However, studies on boron adsorption by organo-clay complexes indicated that coating of soil polysaccharides on illite, montmorillonite and kaolinite reduced boron adsorption by 31.6%, 44.5% and 76.7%, respectively, if compared with that by the pure clays. The effect of coating of the fulvic polyphenol on those clays seemed the least, especially on montmorillonite. From the results of these equilibrium studies, it is proposed that soil organic matter may play an important role on boron adsorption in soils with alkaline reaction, high in organic matter content, and high in extractable iron and aluminum contents. However, the effect would be negligible in soils with low organic matter content but high in clay or sesquioxide contents.

Text

2010-08-02

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http://hdl.handle.net/2429/27079

Chemistry

University of British Columbia

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