Modeling boron adsorption on five soils before and after removal of organic matter

Boron- B concentrations that cause deficiency and those that cause toxicity appear to be very similar, compared to other nutrients, which can complicate successful management of this element in soils. In this study, B adsorption onto two Oxisols from Brazil (Rhodic Eutroperox and Anionic Acroperox), two Alfisols (Natric Palexeralf and Aridic Paleustalf) and an Entisol (Xeric Torrifluvent) from the United States of America were evaluated. The samples were treated with sodium hypochlorite in order to remove soil organic matter. Both treated and untreated samples were used to determine B adsorption isotherms using different B concentrations (0-4.630 mmol L–1) and NaNO3 (0.05 M) as background electrolyte solution at pH 7. Boron adsorption envelopes were also measured using 0.463 mmol L–1 B at three ionic strengths (0.05, 0.1 and 1M) and NaNO3 as background electrolyte solutions at different pH values (3-12). The cation exchange capacity, specific surface area, free Al and Fe oxides, organic and inorganic carbon content, mineralogy and particle size distribution of the soils were also determined. The Langmuir isotherm and the constant capacitance model were fit to the B adsorption data and the parameters obtained were related to the chemical attributes by multiple linear regression equations. Boron maximum adsorption capacity (BMAC) and the complexation constant for the SH3BO4– inner-sphere complex (LogKB–) could be predicted under all experimental conditions. The Alc content was the main soil chemical attribute associated with the BMAC under the conditions evaluated and the LogKB–(int) in untreated and treated samples.

Modeling boron adsorption on five soils before and after removal of organic matter

Boron- B concentrations that cause deficiency and those that cause toxicity appear to be very similar, compared to other nutrients, which can complicate successful management of this element in soils. In this study, B adsorption onto two Oxisols from Brazil (Rhodic Eutroperox and Anionic Acroperox), two Alfisols (Natric Palexeralf and Aridic Paleustalf) and an Entisol (Xeric Torrifluvent) from the United States of America were evaluated. The samples were treated with sodium hypochlorite in order to remove soil organic matter. Both treated and untreated samples were used to determine B adsorption isotherms using different B concentrations (0-4.630 mmol L–1) and NaNO3 (0.05 M) as background electrolyte solution at pH 7. Boron adsorption envelopes were also measured using 0.463 mmol L–1 B at three ionic strengths (0.05, 0.1 and 1M) and NaNO3 as background electrolyte solutions at different pH values (3-12). The cation exchange capacity, specific surface area, free Al and Fe oxides, organic and inorganic carbon content, mineralogy and particle size distribution of the soils were also determined. The Langmuir isotherm and the constant capacitance model were fit to the B adsorption data and the parameters obtained were related to the chemical attributes by multiple linear regression equations. Boron maximum adsorption capacity (BMAC) and the complexation constant for the SH3BO4inner-sphere complex (LogKB–) could be predicted under all experimental conditions. The Alc content was the main soil chemical attribute associated with the BMAC under the conditions evaluated and the LogKB–(int) in untreated and treated samples.(AU)

Modeling selenium (IV and VI) adsorption envelopes in selected tropical soils using the constant capacitance model.

The adsorption of selenium (Se) on soil is important because of the relevance of Se to environmental and health issues. The adsorption of Se(IV) and Se(VI) was evaluated on soil samples from São Paulo State, Brazil, as a function of varying pH, and the experimental data were fitted to the constant capacitance model. Adsorption experiments were conducted for 15 soil samples, after the addition of 20 µmol L(-1) of either Se(IV) or Se(VI), and the adjusted pH ranged between 2.5 and 10. Selenite adsorption was high for all soils, decreased with increasing pH, and was strongly correlated with Fe and Al oxide content. In contrast, Se(VI) adsorption was very low at pH values commonly found in agricultural soils, except for the highly weathered Rhodic Acrudox. The constant capacitance model fitted the Se(IV) and Se(VI) adsorption data well. Optimizations of mono- and bidentate complexation and surface protonation constants were used for the Se(IV) adsorption data. For Se(VI), optimizations for the 2 monodentate species were employed.

Studies of the surface charge of amorphous aluminosilicates using surface complexation models.

Synthetic noncrystalline aluminosilicates with variable charge, similar to allophanes present naturally in volcanic soils, were studied. The surface charge behavior was determined by zero point charge (ZPC) measured by electrophoretic mobility (isoelectric points, IEP) and determined by potentiometric titration (point of zero salt effect, PZSE). The ZPC calculated by Parks model (ZPC(c)), compared with IEP values, showed that the aluminosilicate (AlSi) surface was slightly enriched by AlOH (34% Al(2)O(3) and 66% SiO(2)) compared with the bulk composition (29% Al(2)O(3) and 71% SiO(2)). For aluminosilicate coated with iron oxide (AlSiFe) the ZPC(c) (4.4) was lower than the IEP (8.46), showing that the surface composition is formed mainly from iron oxide. The PZSE values for AlSi and AlSiFe were 6.2 and 4.8, respectively. The differences between the IEP and PZSE are attributed to the formation of Si-O-Fe or Si-O-Al bonds; therefore, the reactivity of Fe and Al atoms was modified on the surface. Two mechanistic models, the constant capacitance model (CCM) and the triple layer model (TLM), using the program FITEQL 3.2 were able to describe the surface behavior of both synthetic aluminosilicates. The acidity constants determined using both models for the aluminosilicates showed differences with respect to pure oxide, mainly attributed to the presence of SiOH sites on the internal surfaces. The ionic strength showed a good relation with the parameters obtained using the CCM (pK(int)(a1), pK(int)(a2) and capacitance values) and the TLM (pK(int)(a1), pK(int)(a2), pK(int)(Cl-), pK(int)(K+), and inner capacitance) for both aluminosilicates. However, the TLM was able to describe the acidity and complexation constants better since it considered the formation of the outer sphere complex between the background electrolyte and the surface. Then, the TLM makes it possible to describe real systems.