Boron Adsorption on California Soils

Boron adsorption on 15 arid zone soil samples was investigated both as a function of equilibrium B concentration (0-100 g irr) and as a function of solution pH (5.5-11.5). Boron adsorption as a function of equilibrium B concentration could be described by the Freundlich adsorption isotherm over the entire concentration range studied and by the Langmuir adsorption isotherm over a limited concentration range. Boron adsorption increased from pH 5.5 to 8.5, exhibited a peak in the pH range 8.5 to 10, and decreased from pH 10 to 11.5. The adsorption maximum was highly correlated with both specific surface area and clay content. It was assumed that B adsorbs by ligand exchange with hydroxyl groups at the edges of clay particles and Al and Fe oxide minerals. The constant capacitance model was able to describe B adsorption on the soil samples. While the fit was generally good, the chemical significance of one of the surface complexation constants is uncertain for a few soil samples. An average set of surface complexation constants could be used to predict B adsorption on most of the soil samples studied. Additional Index Words: anion adsorption, ligand exchange, surface chemistry, constant capacitance model, FITEQL. Goldberg, S. , and R.A. Glaubig. 1986. Boron adsorption on California soils. Soil Sci. Soc. Am. J. 50:1173-1176. for the adsorption peaks was shifted upward to pH 8 to 10 (Goldberg and Glaubig, 1986). The constant capacitance model (Stumm et al., 1980) was applied to describe B adsorption on Al and Fe oxides both crystalline and amorphous (Goldberg and Glaubig, 1985), and on the clay minerals: kaolinite, montmorillonite, and illite (Goldberg and Glaubig, 1986). The model is based on a ligand exchange mechanism of adsorption, considers the charge on both the adsorbent and the adsorbate, and is able to represent the experimental adsorption data under changing conditions of solution pH. Little information about B adsorption on soil samples as a function of pH is available. Boron adsorption behavior as a function of pH for two amorphous soils from Mexico (Bingham et al., 1971) and four soils from Israel (Mezuman and Keren, 1981) was found to be similar to that for oxide and clay minerals, exhibiting an adsorption maximum near pH 9. In this paper, the role of the pH variable in B adsorption on fifteen soil samples is investigated. The constant capacitance model is extended for the first time to B adsorption on soil samples. B is an important element in plant nutrition being essential at low concentrations and producing toxicity symptoms at higher concentrations. The concentration range between plant deficiency and toxicity is narrow (Keren and Bingham, 1985). For this reason understanding of reactions affecting B availability in soils is necessary. Boron present in soil solution or applied via irrigation water equilibrates with soil solid surfaces. The extent of this adsorption reaction is affected by clay mineral type and content, Al and Fe oxide content, organic matter content, CaCO3 content, and soil pH (Keren and Bingham, 1985). Boron adsorption data on arid zone soils have been described previously with the Freundlich adsorption isotherm (Elrashidi and O'Connor, 1982). Elrashidi and O'Connor (1982) were able to describe these data over a limited concentration range using the Langmuir adsorption isotherm as well. The adsorption behavior of B on Al and Fe oxide minerals (Sims and Bingham, 1968; McPhail et al., 1972; Choi and Chen, 1979; Goldberg and Glaubig, 1985) as well as on layer silicate clay minerals (Kingston, 1964; Sims and Bingham, 1967; Keren and Mezuman, 1981; Mattigod et al., 1985; Goldberg and Glaubig, 1986) has been investigated previously. Boron adsorption on both types of minerals exhibited strong pH dependence. Adsorption peaks for the oxide minerals occurred in the pH range 7 to 9 (Goldberg and Glaubig, 1985). For the clay minerals the range 1 Contribution from the U.S. Salinity Laboratory, USDA-ARS, 4500 Glenwood Drive, Riverside, CA 92501. Received 27 Jan. 1986. 2 Soil Scientist and Biological Laboratory Technician-Soils, respectively. MATERIALS AND METHODS Boron adsorption was studied using 15 surface and subsurface soil samples from eight different soil series. Soil classifications are given in Table 1. The soils were chosen because of their low organic and inorganic C contents to insure that B adsorption would occur primarily on oxide mineral and clay surfaces. The soil samples used consisted of the <2-mm fraction. Particle size distributions were determined using the hydrometer method described by Day (1965). Specific surface areas were determined using ethylene glycol monoethyl ether (EGME) (Cihacek and Bremner, 1979). Inorganic and organic C analyses were determined using the method described by Nelson and Sommers (1982). Clay minerals present in the <2-^m fractions were determined by x-ray analysis (B.S. Kapoor, 1985, personal communication). Boron adsorption experiments were carried out in batch systems to determine both adsorption isotherms (amount of B adsorbed as a function of equilibrium B concentration) and adsorption envelopes (amount of B adsorbed as a function of solution pH per fixed total B concentration). Boron adsorption envelopes were determined using the identical procedure used previously by Goldberg and Glaubig (1985, 1986). The suspension density of solid was 200 kg m~. For determination of B adsorption isotherms the procedure was modified so that the equilibrating solution contained 0, 1, 2, 5, 10, 15, 25, 50, or 100 g B m~ and no acid or base additions were used. The decantates were analyzed for pH, filtered, and analyzed for B concentration using a Technicon Auto Analyzer II and the Azomethine-H method described by Bingham (1982). The constant capacitance model (Stumm et al., 1980) was used to describe the B adsorption envelopes obtained for the soil samples. The computer program FITEQL (Westall, 1982) was used to fit intrinsic surface complexation constants to the experimental data using the model assumptions and procedure described in detail by Goldberg and Sposito (1984). Surface reactions, intrinsic conditional equilibrium constants, mass balances, and charge balance were analogous to