Adsorption of boric acid on pure and humic acid coated amorphous-aluminum hydroxide : a Raman and XANES spectroscopy study

Abstract

The fate and mobility of boric acid in the environment is largely controlled by adsorption reactions with soil organic matter and soil minerals to form surface complexes (Marzadori et al., 1991; Su and Suarez, 1995; Yermiyahu et al., 1995; Peak et al., 2002). The effects of humic acid (HA) and dissolved CO2 on boric acid adsorption on amorphous (am)-Al(OH)3 were investigated as their influence on sorption is potentially important. Although a model system was used in the studies, the findings should be generally useful to better understand the mobility and bioavailability of boric acid in the soil ecosystems.In this dissertation, boric acid adsorption on pure am-Al(OH)3 and 5% w/w HA coated am-Al(OH)3 were investigated both as a function of pH (4.5 – 11) and initial boric acid concentration (0 – 4.5 mmol L-1). Batch adsorption isotherm experiments were also conducted with samples exposed to atmospheric CO2 and anaerobic (N2) conditions to examine the effects of dissolved CO2 on boric acid adsorption. Both the pH envelope and the adsorption isotherm experiments showed that the HA coating on am-Al(OH)3 and the presence of dissolved CO2 decreased boric acid adsorption. Raman spectroscopy and boron (B) K-edge X-ray Absorption Near Edge Structure (XANES) spectroscopy were used to investigate the coordination of boric acid adsorbed at mineral/water interfaces. The Raman spectroscopy was less successful than expected as there were difficulties in identifying B bands in the 5% w/w HA coated am-Al(OH)3 samples.The B K-edge XANES spectroscopy yielded better results. The XANES spectra of boric acid adsorption samples showed that both trigonal BO3 and tetrahedral BO4 coordinated complexes are present on the pure and HA coated mineral surfaces. At pH 7.0 and 9.2, the adsorption of boric acid on am-Al(OH)3 is predominantly inner-sphere trigonal complexes; with minor amounts of inner-sphere tetrahedral complexes. Both macroscopic and spectroscopic experiments revealed that the combination of HA coating on am-Al(OH)3 and dissolved CO2 decreased boric acid adsorption compared to adsorption on pure am-Al(OH)3.The discoveries in this dissertation contribute significantly in understanding the effects of HA and dissolved CO2 has on boric acid adsorption in the environment. Since B speciation and the stability of am-Al(OH)3 mineral and HA changes with pH, the bioavailability of B is expected to change as well with pH. The adsorption on boric acid on am-Al(OH)3 and/or HA coated am-Al(OH)3 is expected to decrease the amount of boric acid available to plants. Therefore the nutrient management regimen will have to be modified for soils that are high in natural organic matter, carbonate and/or aluminum hydroxyl species.