Predicting Boron Adsorption by Soils Using Soil Chemical Parameters in the Constant Capacitance Model

The constant capacitance model, a chemical surface complexation model, was applied to B adsorption on 17 soils selected for variation in soil properties. A general regression model was developed for predicting soil B surface complexation constants from easily measured soil chemical characteristics. These chemical properties were cationexchange capacity (CEC), surface area, organic carbon content (OC), and inorganic carbon content (IOC). The. prediction equations were used to obtain values for B surface complexation constants for 15 additional soils, thereby providing a completely independent evaluation of the ability of the constant capacitance model to fit B adsorption. The model was well able to predict B adsorption on the 15 soils. Incorporation of these prediction equations into chemical speciationtransport models will allow simulation of soil solution B concentrations under diverse environmental and agricultural conditions without the requirement of soil specific adsorption data and subsequent parameter optimization. is both a micronutrient essential for plant growth and a potentially toxic trace element. The range between B deficiency and toxicity symptoms in plants is narrow typically in the range of 0.028 to 0.093 mmol L" for sensitive crops and 0.37 to 1.39 mmol L" for tolerant crops (Keren and Bingham, 1985). Excess soil solution B concentrations can lead to marked yield decrement in crop plants resulting in economic losses. For this reason, careful quantification of soil solution B concentrations is needed, especially in regions such as the southwestern USA where B toxicity often limits full use of water resources. Adsorption of B on soil mineral surfaces is important to managing B toxicity or deficiency since adsorbed B is not perceived as toxic by plants (Keren et al., 1985). Availability of B to plants is affected by a variety of factors including soil solution pH, soil texture, soil moisture, temperature, oxide content, carbonate content, organic matter content, and clay mineralogy (Keren and Bingham, 1985). Boron becomes less available with increasing solution pH. Boron deficiency often occurs on sandy soils and during hot, dry soil conditions. The dominant B adsorbing surfaces in soil are oxides, clay minerals, calcite, and organic matter (Goldberg, 1993). Boron adsorption on soils and soil minerals has been described using various modeling approaches. Such models include the empirical Freundlich and Langmuir adsorption isotherms (Elrashidi and O'Connor, 1982; Goldberg and Forster, 1991) and surface complexation models: constant capacitance model (Goldberg and USDA-ARS, George E. Brown, Jr., Salinity Laboratory, 450 W. Big Springs Road, Riverside, CA 92507. Contribution from the George E. Brown, Jr., Salinity Lab. Received 6 Sept. 1999. * Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 64:1356-1363 (2000). Glaubig, 1985, 1986a, 1986b, 1988; Goldberg et al., 1993), triple layer model (Singh and Mattigod, 1992; Toner and Sparks, 1995), and Stern variable surface charge-variable surface potential model (Bloesch et al., 1987). Parameters from empirical models are only valid for the particular conditions of the experiment. Surface complexation models are chemical models that use defined surface species, chemical reactions, mass balances, and charge balance. They contain molecular features that can be given thermodynamic significance (Sposito, 1983). Chemical modeling of B adsorption at the mineralsolution interface has been successful using the constant capacitance model for oxides, clay minerals, and soils (Goldberg and Glaubig, 1985,1986a, 1986b, 1988, Goldberg et al., 1993). In these studies B adsorption was described as forming a trigonal inner-sphere surface complex. Such a configuration is chemically reasonable since boric acid is a very weak monobasic acid with a pKa of 9.2 and thus trigonal over most of the pH range. Although it is reasonable for the dominant solution species to be dominant on the exchange complex, there is no 1:1 correspondence between solution and surface species (Sposito, 1983). Boric acid acts as a Lewis acid by accepting a hydroxyl ion to form the tetrahedral borate anion: H3BO3 + H2O ̂ B(OH)4[1] Recent research using Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy has provided direct evidence for the presence of tetrahedral B as well as trigonal B adsorbed on the surface of amorphous Al hydroxide (Su and Suarez, 1995). Their study was carried out under aqueous conditions so the results are directly applicable to natural soil systems. Spectroscopic investigations of B adsorbed to organic matter and carbonates have not yet been carried out. Boron adsorption on arid zone soils was successfully described with the constant capacitance model using both trigonal and tetrahedral B surface configurations consistent with microscopic experimental results (Goldberg, 1999). The study of Goldberg (1999) fit the constant capacitance model to each of 14 soil samples and tested the ability of an average set of B surface complexation constants to predict B adsorption. The objectives of the present study are (i) to apply the constant capacitance model to B adsorption on an expanded set of 32 soil samples using both trigonal and tetrahedral surface configurations for adsorbed B; (ii) to relate B adsorption characteristics and model surface complexation constants to easily measured chemical paAbbreviations: CEC, cation-exchange capacity; IOC, inorganic carbon content; OC, organic carbon content; SA, surface area; %A1, mass percent aluminum oxide; %Fe, mass percent iron oxide content.