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Boric and Silicic Acid Adsorption and Desorption by a Humic Acid

Metropolitan Council Environmental Services - EPE, 2400 Childs Rd., St. Paul, MN 55106

Paul R. Bloom

Dep. of Soil Science, 439 Borlaug Hall, Univ. of Minnesota, St. Paul, MN 55108

^* Corresponding author (meyer_m{at}zeus.metc.state.mn.us).

ABSTRACT

Boron is an essential plant nutrient having a narrow range between concentrations that cause deficiency or toxicity symptoms in plants. Boric acid (B[OH]₃) and silicic acid (Si[OH]₄) have many chemical similarities and both are thought to be associated with organic matter in soils. The objectives of this study were to investigate the variation in boric and silicic acid adsorption and desorption in low ash humic acid over a range of B and Si concentrations and pH values. Borate adsorption by humic acid increased significantly from pH 3.67 to 8.70. The data fit the Freundlich equation better than the Langmnir model at high pH; however, both fit well at pH < 7. Adsorption was greatest at pH 8.5 to 10 and decreased at pH >10. The constant capacitance model predicted B adsorption and the pH maximum using catechol as the adsorbing ligand and published stability constants. The predicted adsorption envelope was narrower than experimental data. This could be due to variation in stability constants that result from variation in substitution on other sites on the aromatic ring. Inclusion of glucose and salicylate in the adsorption model would also broaden the adsorption envelope by increasing adsorption at pH > 9 and < 9, respectively. Desorption of B from humic acid appears to show hysteresis effects at pH 7.6 but not at pH 8.15 and 8.7. Generally, B will be easily released from HA as solution concentrations decrease. Silicic acid did not adsorb in humic acid. The stability constant of Si-catechol humate complexes is apparently too low for adsorption of Si to occur at the pH and solution concentrations used in this experiment.

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