Boron Adsorption.

BORON ADSORPTION

 Boron (B) is an essential micronutrient for plants. Its deficiency is a widespread problem in relatively humid areas, especially in sandy soils. Otherwise, the toxicity of this nutrient tends to occur in arid zone soils. The soil organic matter (SOM) can be a reservoir through fertilization or in contaminated soils. The SOM is also important because its content can be changed with soil management. In this study, it was evaluated B adsorption onto two Oxisols, from Paraná State-Brazil (Rhodic Eutroperox and Anionic Acroperox) and two Alfisols (Natric Palexeralfs and Aridic Paleustalfs) and an Entisol (Xeric Torrifluvents) from the United States of America. Part of the sample was treated with sodium hypochlorite in order to remove SOM. B adsorption isotherms were measured using different B concentrations (0-4.630mmol L^-1) and NaNO₃ (0.05 M) as background solution at pH 7. B adsorption envelopes were also measured using 0.463 mmol L^-1 B and three ionic strengths (0.05, 0.1 and 1M) and NaNO₃ as background solution at different pH values (3-12). The cation exchange capacity (CEC), specific surface area (SSA), free Al and Fe oxides, organic (OC) and inorganic carbon (IC) content of the soils were also determined. The Langmuir isotherm and the constant capacitance model were adjusted to the B adsorption data and the parameters obtained were related to the chemical properties by multiple linear regression equations using the SAS stepwise routine. B maximum adsorption capacity (MAC_B) showed that the North American soils untreated samples had higher values when compared with the treated ones. Otherwise, treated Brazilian soils had higher MAC_B values when compared to untreated samples. This behavior was confirmed by the adsorption envelopes. Apparently SOM in Brazilian soils does not have a large B adsorption capacity when compared to Fe oxides Al, which are mainly responsible for B adsorption in these soils. Otherwise SOM is the most important component in B adsorption in American soils, which mineralogy is based on aluminosilicate. SOM in American soils has a higher B adsorption capacity when compared to SOM in Brazilian soils and the iron oxides. MAC_B was related to the chemical properties by the equation: MAC_B =1.5063 - 0.0659 OC + 4.0040 Al - (R² = 0.94). The complexation constant for the SH₃BO₄^- complex (LogK_B-) presented the following regression model: LogK_B-= - 3.1751 + 2.13 ln (SSA) - 0.1646 ln(IC) (R² = 0.79).

Key words: B adsorption, SOM, chemical properties.