Chemical Relaxation and Double Layer Model Analysis of Boron Adsorption on Alumina

Boron is a plant nutrient essential for adequate plant growth, yet the range between B deficiency and toxicity levels is rather narrow. Boron adsorption reactions with soil components, particularly sesquioxides, most often regulate the amount of B in the soil solution. The reaction mechanisms of B adsorption on oxides have not been fully characterized, however. Pressure-jump relaxation experiments were conducted to measure the rates and determine the reaction mechanism for B adsorption on an alumina (y-Al2O3) surface from B(OH)3-B (OHK solutions. Relaxation times (t) were measured from pH 7.0 to 9.7 in alumina suspensions with 0.012 mol L' total B. A plot of T~ ' vs. B(OH)4 plus surface site concentration obtained from the triple layer model (TLM) assuming inner sphere B(OH)4 adsorption yielded an adsorption rate constant (kf) of 3.3 x 10 L mol~' s~' and a desorption rate constant (*?") of 1.8 x 10~ L mol" s". The ratio kf/k? yielded an equilibrium constant (log /aTM) of 8.26, in agreement with the intrinsic equilibrium constant for B(OH)4" adsorption (log K& = 7.69) obtained from adsorption isotherms. Four additional surface complexation models were tested for their ability to model both the equilibrium and kinetic data simultaneously: the constant capacitance model, the diffuse layer model, a Stern model variant, and the TLM assuming outer sphere B(OH)4 adsorption. Only the TLM, assuming both B(OH)3 and B(OH)4 were adsorbed via ligand exchange on neutral surface sites, was successful. The TLM indicated that B(OH)4~ is the predominant adsorbed species throughout the pH range 7.0 to 10.8. T EQUILIBRIUM between B in the soil solution and adsorbed B plays a pivotal role in determining the amount of B available for plant uptake. Perhaps the most important soil components affecting B adsorption are soil oxides and oxyhydroxides (Keren and Bingham, 1985). The frequent occurrence of these oxides as coatings on mineral surfaces results in a physicochemical influence far in excess of what their contribution to the total soil mass would suggest, due to their highly reactive nature and great surface exposure (Sims and Bingham, 1968b). Various researchers have demonstrated the propensity of soil oxides to adsorb substantial amounts of B from solution (Goldberg and Glaubig, 1985; Rhoades et al., 1970). In a comparison of Al oxides with Fe oxides, Sims and Bingham (1968a) found that Al oxides removed nearly an order of magnitude more B from solution than Fe oxides on a weight basis. Goldberg and Glaubig (1985) studied B adsorption on several Al and Fe oxides and found that B adsorption was comparable when compared on a surface-area basis. Although B fixation by adsorption is most often considered the dominant phenomenon in controlling B availabilDepartment of Plant and Soil Sciences, Univ. of Delaware, Newark, DE 19717-1303. The authors appreciate the partial support of this research from the U.S. Borax Corporation. The senior author also acknowledges the receipt of a graduate research fellowship from the Univ. of Delaware. Received 20 Sept. 1993. "Corresponding author (dlsparks@brahms. udel.edu). Published in Soil Sci. Soc. Am. J. 59:395-404 (1995). ity, the reaction mechanisms of B adsorption have never been clearly established. Adsorption of B is believed to be specific hi nature, occurring by ligand exchange with surface hydroxyl groups of variable-charge oxides and broken edges of clays (Kingston et al., 1972; Sims and Bingham, 1968a). Specific adsorption of B may occur irrespective of the sign of the net surface charge (Kingston et al., 1972). Various models have been employed in describing B adsorption behavior in soils. These include the Langmuir equation (Bingham et al., 1971; Goldberg and Forster, 1991; Goldberg and Glaubig, 1986), as well as a phenomenological equation that assumes that two B solution species are competing for the same surface sites, boric acid [B(OH)3] and borate [B(OH)4-] (Keren et al., 1981; Mezumen and Keren, 1981). Goldberg and Glaubig (1985) have used the CCM to describe B adsorption on several Fe and Al oxides across a wide pH range (4-11). The CCM was also able to simulate B adsorption on 15 arid-zone soils (pH 5.511.5) using a set of surface complexation constants averaged across all 15 soils (Goldberg and Glaubig, 1986). In each application of the model to B adsorption, the researchers assumed that B(OH)3 is the only B solution species adsorbed and only neutral OH sites participate in the ligand exchange, with water as the leaving ligand. Since this proposed reaction mechanism involves no charged species, surface charge has no effect on the adsorption process. Because the CCM does not consider adsorption of the background electrolyte, the B adsorption constant is only valid for a given ionic strength (Hayes et al., 1991). Boron adsorption on kaolinite has been modeled using the TLM (Singh and Mattigod, 1992). Adsorption on kaolinite was well described by the TLM from pH 6.0 to 10.5 at constant ionic strength with either Ca(ClO4)2 or KC1O4 as the background electrolyte. Singh and Mattigod (1992) assumed that B adsorbs as both B(OH)3 and B (OH)4~ in the presence of K and Ca, with B(OH)4 forming both monoand bidentate inner sphere surface complexes. Boron adsorption on kaolinite in the presence of Ca is enhanced by the adsorption of Ca-B (OH)4 ion pairs. The FITEQL program (Westall, 1982) was used to simultaneously optimize multiple equilibrium constants for B adsorption in both systems. The TLM has been adapted by Hayes and Leckie (1987) from its original form to consider the formation of inner and outer sphere coordination complexes with the surface, a more realistic assumption considering that the specifically adsorbed ion is also a component of the EDL. In contrast Abbreviations: BA, boric acid; BT, borate; CCM, constant capacitance model; DLM, diffuse layer model; EDL, electric double layer; EGME, ethylene glycol monoethyl ether; MSM, modified Stern model; TLM, triple layer model; TLM-IS, triple layer model, inner-sphere adsorption; TLM-OS, triple layer model, outer-sphere adsorption.