Sabine Goldberg

Reactions of boron with soils

Boron is an essential micronutrient for plants, but the range between deficient and toxic B concentration is smaller than for any other nutrient element. Plants respond directly to the activity of B in soil solution and only indirectly to B adsorbed on soil constituents. Soil factors affecting availability of B to plants are: pH, texture, moisture, temperature, organic matter and clay mineralogy. Boron adsorbing surfaces in soils are: aluminium and iron oxides, magnesium hydroxide, clay minerals, calcium carbonate, and organic matter. Boron adsorption reactions can be described empirically using the Langmuir adsorption isotherm equation, the Freundlich adsorption isotherm equation, and the phenomenological Keren model. Chemical models such as the constant capacitance model, the triple layer model, and the Stern VSC-VSP model can describe B adsorption over changing conditions of solution pH and B concentration. Boron desorption reactions often exhibit hysteresis. The rate of B desorption can be described using the first order rate equation, the Elovich reaction rate equation, and the power function equation.

Factors affecting clay dispersion and aggregate stability of arid-zone soils

We investigated the stability of 34 aridzone toil samples from 15 soil series, using clay dispersion and aggregate stability as structural indexes. The study evaluated clay dispersion and aggregate stability as affected by: pH, electrical conductivity, sodium adsorption ratio, soluble silica, cation exchange capacity, exchangeable sodium percentage, inorganic carbon, organic carbon, free iron oxide, free aluminum oxide, clay, surface area, quartz, kaolinite, illite, chlorite, vermiculite, and montmorillonite. The most significant single-variable linear regressions were obtained for percentage of clay dispersed versus log (montmorillonite) (r2 = 0.52**) and for percentage of aggregate stability versus organic carbon (r2 = 0.27**). Significant variables for multiple linear regression for percentage of clay dispersed were montmorillonite, exchangeable sodium percentage, and electrical conductivity (r2 = 0.67**). For percentage of aggregate stability, significant variables in the multiple linear regression were quartz, montmorillonite, and surface area (r2 = 0.49**). Principal factor analysis results indicated that the structural indexes were related most to the soil variables stabilizing structure by physically binding particles. These binding agents are aluminum and iron oxides and organic matter.

Effect of aluminum and iron oxides and organic matter on flocculation and dispersion of arid zone soils

We investigated the structural stabilities of eight arid zone soils using percent optical transmittance as a measure of flocculation-dispersion behavior. The soils were sodium saturated and their stabilities determined in sodium chloride solutions of varying concentrations. We treated the soils with hypochlorite for organic matter removal, with oxalate for removal of amorphous aluminum and iron oxides, and with dithionite for removal of crystalline aluminum and iron oxides. The removal of amorphous and/or crystalline oxides increased the clay dispersivity. This indicated that amorphous and crystalline oxides play important, stabilizing roles in soil structure. The removal of organic matter decreased the clay dispersivity. This indicated that dissolved organic matter enhances clay dispersion. The well-known positive effect of organic matter on soil tructure probably occurs through binding of soil particles by roots and hyphae at the aggregate level, but at the clay-particle level the negative charge of organic anions enhances clay dispersion.

BORON SORPTION ON CALCAREOUS SOILS AND REFERENCE CALCITES

Boron sorption on two calcareous soils, one noncalcareous soil, and two reference calcites was investigated in batch systems both as a function of solution pH (5.5–12) and as a function of initial B concentration (1–250 g B m−3). Boron sorption was investigated on untreated soil samples, and soil samples treated with dilute acid to remove soil calcite. Boron sorption on the soil samples increased from pH 5.5 to 9, exhibited a peak near pH 9.5, and decreased from pH 10 to 11.5. Boron sorption on the two reference calcites exhibited an adsorption envelope with maximum sorption occurring near pH 9.5. The magnitude of the B adsorption maximum was significantly lower for the soil samples treated for calcite removal than for the untreated soil samples. This result indicates that calcite plays an important role in B sorption by calcareous soils. Boron sorption as a function of equilibrium B concentration could be described by both the Freundlich and Langmuir adsorption isotherms over the entire concentration range studied on all materials. Goodness of fit of the Langmuir adsorption isotherm equation was much improved over linear transformations by using the program ISOTHERM with its nonlinear least squares optimization routine.

Sensitivity of surface complexation modeling to the surface site density parameter

Previous research has shown that adsorption of many inorganic anions on soil mineral surfaces can be described equally well by chemical surface complexation models using either inner- or outer-sphere surface complexes. At the same time, goodness of fit of these models to adsorption data has been used to distinguish between inner- and outer-sphere adsorption mechanisms. In this study the ability of chemical surface complexation models to describe anion adsorption on goethite using both inner- and outer-sphere surface complexes was evaluated and found to be sensitively dependent on the value of the surface site density. Application of a goodness of fit criterion leads to the choice of an inner-sphere adsorption mechanism for small surface site densities and an outer-sphere adsorption mechanism for large values of this parameter. Experimentally determined values of surface site density vary by an order of magnitude depending upon the method used. It is suggested that uncertainty in the value of the surface site density parameter currently invalidates use of surface complexation models to predict anion adsorption mechanisms on soil mineral surfaces.

Influence Of Anion Competition On Boron Adsorption By Clays And Soils

Boron adsorption on the clay minerals, kaolinite and montmorillonite, and two arid zone soils was investigated as a function of solution pH (3-12) and presence of competing anions (nitrate, sulfate, molybdate, and phosphate) after 2 h of reaction time. Boron adsorption on all materials increased from pH 3 to 8, exhibited a peak at pH 8 to 10, and decreased from pH 10 to 12. Boron adsorption was greatest using a NaNOJ background electrolyte. The competitive anion effects on B adsorption increased in the order sulfate < molybdate < phosphate. The competitive effect on B adsorption was small even for the strongly adsorbing anion, phosphate. Our results suggest that B-adsorbing sites are, generally, specific to B and act independently of competing anions. This result will simplify the description of B transport in that changes in solution concentration of competing anions may not have to be considered.

Temperature effects on boron adsorption by reference minerals and soils

Information on the effect of temperature on B adsorption by soils and soil minerals is scant. These data are needed to understand B availability. Boron adsorption on goethite, gibbsite, kaolinite, montmorillonite, calcite, and two arid zone soils was investigated as a function of solution pH (3–12) and reaction temperature (10, 25, and 40|MoC) after 2 h of reaction time. Boron adsorption on all materials increased from pH 3 to 7, exhibited a peak at pH 7.5 to 10, and decreased from pH 10.5 to 12. Temperature dependence measured as the increase of the B adsorption maximum at 10|MoC compared with 40|MoC on reference minerals increased in the order: calcite (3|X%) < goethite (7|X%) < gibbsite (18|X%) < montmorillonite (-20|X%) < kaolinite (26|X%). The kaolinitic soil exhibited greater temperature dependence than the smectitic soil. The B adsorption reaction was exothermic since B adsorption decreased with increasing temperature for all materials, except for montmorillonite at high pH. Highly specific ion adsorption is expected to be exothermic, suggesting an inner-sphere adsorption mechanism for B on all reference minerals except montmorillonite.

Application of surface complexation models to anion adsorption by natural materials

Various chemical models of ion adsorption are presented and discussed. Chemical models, such as surface complexation models, provide a molecular description of anion adsorption reactions using an equilibrium approach. Two such models, the constant capacitance model and the triple layer model, are described in the present study. Characteristics common to all the surface complexation models are equilibrium constant expressions, mass and charge balances, and surface activity coefficient electrostatic potential terms. Methods for determining parameter values for surface site density, capacitances, and surface complexation constants also are discussed. Spectroscopic experimental methods of establishing ion adsorption mechanisms include vibrational spectroscopy, nuclear magnetic resonance spectroscopy, electron spin resonance spectroscopy, X-ray absorption spectroscopy, and X-ray reflectivity. Experimental determinations of point of zero charge shifts and ionic strength dependence of adsorption results and molecular modeling calculations also can be used to deduce adsorption mechanisms. Applications of the surface complexation models to heterogeneous natural materials, such as soils, using the component additivity and the generalized composite approaches are described. Emphasis is on the generalized composite approach for predicting anion adsorption by soils. Continuing research is needed to develop consistent and realistic protocols for describing ion adsorption reactions on soil minerals and soils. The availability of standardized model parameter databases for use in chemical speciation-transport models is critical.

Soil boron extractions as indicators of boron content of field-grown crops

Determining the relationship between soil B and crop B content can help predict when crops will respond to B fertilizer and when B toxicity may be expected. Such a relationship can then be used to make fertilizer recommendations or to flag conditions of potential B toxicity. Soil samples were obtained from 65 sites located in the Broadview Water District in the San Joaquin Valley of California. A diverse set of extractants was evaluated including: hot water-soluble, 1:1 soil:distilled water and 1:2 soil:distilled water, ammonium acetate, calcium chloride-mannitol, and DTPA-sorbitol extracts. Soil extract B values were correlated significantly with various B reactive soil constituents, including aluminum and iron oxide, clay, organic matter, and calcium carbonate content. The 1:1 water extract B was highly significantly correlated (99% level) with other measures of extractable B used in the study. Extractants were compared on soil samples collected from six depths at 65 field sites in the San Joaquin Valley of California that were cropped to alfalfa, melons, and cotton. Boron concentrations of whole plants and composites of 10 leaves were determined. Plant sampling occurred at the time of soil sampling for the alfalfa. Cotton and melons were sampled at flowering and prior to fruit set, the recommended growth stages, respectively, for tissue sampling, and 6 weeks thereafter. Five weeks later the cotton was sampled a third time. Significant correlations (95% level) between extractable soil B and plant B were found for melons and cotton but not for alfalfa. Correlation coefficients for the ammonium acetate, DTPA-sorbitol, and 1:1 water extract were not statistically significantly different (95% level). Although significant correlations (95% level) were obtained, the equations provided relatively poor predictive capability. These results illustrate the difficulty of predicting plant B content based on soil B analyses from a single soil sampling.

Molybdenum Adsorption by Volcanic Italian Soils

Abstract Molybdenum (Mo) adsorption was investigated on five soils from the Gauro and Roccamonfina volcanoes of Campania, a southern Italian volcanic region. The soils were characterized by different degrees of development. Molybdenum adsorption exhibited a maximum near pH 4–5 and decreased with increasing pH above 5. The constant capacitance model was able to fit Mo adsorption on the soils as a function of pH. A general regression model was used to predict a Mo surface complexation constant from routinely measured soil chemical parameters. The predicted Mo constant was used to predict adsorption on the soils, thereby providing a completely independent evaluation of the ability of the model to describe Mo adsorption. The prediction equation, developed from a set of soils primarily from California, was able to predict Mo adsorption on four out of five soils from Italy, suggesting wide applicability for describing Mo adsorption by soils of diverse mineralogy and parent material.