Boron Sorption on Composted Organic Matter

Boron (B) sorption on composted organic matter at various pH levels and ionic strengths was determined. The compost contained 158 g kg −1 of humic acid and 56 g kg −1 of fulvic acid fractions, totaling 21.4% of the organic matter. At pH 8, the amount of sorbed B increased by about 57% with an ionic strength increase from 0 to 0.15 M . For solutions of CaCl 2 at concentrations lower than 5 × 10 −3 M , the effect of CaCl 2 on B sorption was negligible. Sorption isotherms of B on organic matter were determined at pH levels of 7.0, 7.9, and 8.9. The sorption increased with increasing B concentration in solution and pH. A comparison with literature data for clays shows that sorption of B on organic matter at similar pH levels and total B concentration is significantly higher than on clays. A modified Langmuir equation was fitted to the measured data. Based on the isotherms, sorption coefficients were derived and used to calculate the predicted dependence on pH of B sorption on organic matter at various total B levels. Desorption studies of B from organic matter at pH 8 and increasing levels of sorbed B exhibited strong hysteresis effects and the sorbed B was removed to a partial extent only. The formation of dihydroxy and/or hydroxy‐carboxy binding of borate to humic substances and/or carbohydrates is proposed to be the sorption mechanism. These bonds may explain the limited irreversibility of the process.