Selectivities of Potassium-Calcium and Potassium-Lead Exchange in Two Tropical Soils

gram (Adriano, 1986; Davies, 1992; Forstner, 1995; Ma and Rao, 1997). Measurement of cation selectivity in soils provides important infor­ mation about the affinity and binding strength of a particular cation In many cases, much of the Pb in uncontaminated on soil surfaces. Gaines-Thomas (KGT) selectivity coefficients were determined for a variety of K/Ca and K/Pb ratios on an Oxisol and Ultisol soil from Puerto Rico. The calculated KGT values indicated a preference for K over Ca2� or Pb2� . The selectivity for Pb2� was significantly greater than that for Ca2� due to Pb2�’s larger hydrated charge density relative to that of Ca2�. The patterns of selectivity were independent of metal type. The selectivity of the Oxisol for Ca2� or Pb2� exhibited no trend and changed little with changes in divalent metal surface coverage. The Ultisol displayed a decrease in selectivity for Ca2� and Pb2� with increasing surface coverage of these ions. This was attributed to the presence of smectite in the Ultisol, which was able to partially collapse when K saturated. Some of the Pb sorption in the soils was due to chemisorption. The Oxisol chemisorbed ≈3000 mg Pb kg�1 while that value for the Ultisol was ≈1900 mg kg�1. The differences were due to the greater quantities of Fe/Al oxides and organic matter in the Oxisol relative to the Ultisol. Scanning electron microscopy-energy dispersive X-ray (SEM-EDX) spectros­ copy detected discrete Pb-C phase in both soils. The C was from organic matter. Under experimental conditions, any Pb-carbonate phase would not have been stable. It was possible Pb was associated with organic sulfhydral groups. The selectivity exhibited by soil sys­ tems for various nutrient and heavy metals is important in elucidating how available these metals will be for plant/animal uptake as well as their mobility and stability in the soil environment. L is an important heavy metal that may threaten soil quality and human health at elevated levels (Adriano, 1986; Kabata-Pendias and Pendias, 1992). It is one of the most widespread elemental contaminants worldwide (Nriagu, 1998) and has been shown to cause anemia, kidney disease, brain damage, impaired func­ tion of the peripheral nervous system, high blood pres­ sure, reproductive abnormalities, developmental de­ fects, abnormal Vitamin D metabolism, and in some situations death (Hrudey et al., 1995; USEPA, 1992). The average concentration of Pb in the earth’s crust is 13 mg kg�1 (Brown et al., 1999). Release of this metal into soils via a variety of industrial, urban, and agricul­ tural applications has lead to very high soil Pb concen­ trations of up to several thousand milligrams per kiloC. Appel, Earth and Soil Sciences Dep., Cal Poly State Univ., San Luis Obispo, CA 93407; L.Q. Ma, R.D. Rhue, and W. Reve, Dep. of Soil and Water Science, Univ. of Florida, Gainesville, FL 32611-0290. Approved for publication as Florida Agricultural Experiment Station Journal Series No. R-07373. Received 3 July 2002. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 67:1707–1714 (2003).  Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA and contaminated soils is not present in readily available fractions (RAF; we have operationally defined RAFPb to equal the water-soluble and salt exchangeable fractions of Pb found in soils). In several studies looking at the sequential extraction of Pb (and other heavy met­ als) from contaminated soils, researchers have shown the amount of RAF-Pb relative to the total soil Pb is generally �1 to 10% (Boruvka et al., 1997; Kabala and Singh, 2001; Ma and Rao, 1997). This corresponded to RAF-Pb ranging from ≈0.4 to 4000 mg kg�1 (3.9 � 10�4 to 3.9 cmolc kg�1) using the high RAF value of 10%. Thus, depending on a soil’s cation-exchange capacity (CEC) and the types of cations on the cation-exchange sites, Pb potentially has the ability to occupy part or a majority of these sites. The degree to which a cation adsorbs to an exchanger depends largely on its selectivity relative to other cations in the soil solution (McBride, 1994). A cation with high selectivity for a particular soil surface (exchanger) will be preferentially adsorbed relative to one with lower selectivity. Many studies, considering sorption of heavy metals (i.e., Pb, Cu, Zn, Cd, and Ni) added to soils and pure minerals (i.e., goethite, alumina, kaolinite, and zeolite), have inferred higher Pb selectivity relative to other heavy metals based on differences in total amount of metals sorbed or desorbed (Ahmed et al., 1998; Gao et al., 1997; Pardo, 2000; Yong and Phadungchewit, 1993). These studies did not focus on the exchangeable Pb fraction. Ahmed et al. (1998) examined competition for zeolite exchange sites between Pb2� and K, how­ ever, no known studies have determined Pb selectivity nor selectivity coefficients relative to the important cat­ ions commonly found on soil cation-exchange sites (i.e., Ca, Mg, K, and Na). These cations may compete strongly with Pb for adsorption. Understanding how Pb competes for exchange sites in soils, at varying concentrations and saturations on the exchanger relative to a typical cation (i.e., Ca or K), would provide useful information about Pb selectivity and, thus, mobility and diffusion in soils. In the soils used in this study, Ca2� plus K occupied 23 and 31% of Oxisol and Ultisol cation exchange sites, as deterAbbreviations: AAS, atomic adsorption spectroscopy; CEC, cation exchange capacity; EDX, energy dispersive X-ray; I, ionic strength; Ksp, solubility product constant; KGT, Gaines-Thomas selectivity coeffi­ cient; PDI, potential determining ions; pKH, negative log of the hydro­ lysis constant; RAF-Pb, readily available fraction of lead; SEM, scan­ ning electron microscopy; TGA, thermal gravimetric analysis; XRD, X-ray diffraction; �GEX�, standard Gibbs free energy of exchange; �Hhyd, hydration energy; % R, percent recovery.