Concentration, pH, and Surface Charge Effects on Cadmium and Lead Sorption in Three Tropical Soils

blood pressure, reproductive abnormalities, developmental defects, abnormal vitamin D metabolism, and in Reactions of heavy metals with soil are important in determining some situations death (Hrudey et al., 1995; USEPA, metal fates in the environment. Sorption characteristics of two heavy metals, Cd and Pb, in three tropical soils (Mollisol, Oxisol, and Ultisol) 1992). from Puerto Rico were assessed at varying metal concentrations (0 Many studies have evaluated heavy metal concentrato 1.2 mM ) and pH values (approximately 2 to 7). All soils sorbed tions, mobilities, and partitioning in temperate soils more Pb than Cd. Sorption maxima were obtained for each metal for (Barbarick et al., 1998; Chang et al., 1984; Gong and the Oxisol and Ultisol soils, but not the Mollisol. Sorption appeared to Donahoe, 1997; Jang et al., 1998; Johnson and Petras, depend more on soil mineralogy than organic matter content. Sorption 1998; Jordan and LeChevalier, 1975) as well as pure isotherms were linear within the sorption envelope with similar slopes mineral specimens and synthetic analogs (i.e., kaolinite, for each soil–metal curve, when plotting metal sorption as a function gibbsite, goethite; Balistrieri and Murray, 1982; Bibak, of pH. Cadmium and Pb isotherms yielded average slopes of approxi1994; Dalang et al., 1984; Eick et al., 1999; Forbes et mately 36 1 and 28 1 units (percent increase in metal sorption al., 1976; Haas and Horowitz, 1986; Jenne, 1968; Puls et per 1-unit increase in pH), respectively. Metal sorption depended more on metal type than soil composition. Cadmium sorption displayed al., 1991; Rose and Bianchi-Mosquera, 1993); however, a greater pH dependence than Pb. Cadmium sorption was less than relatively few experiments have been conducted on or equal to the amount of negative surface charge except at pH values tropical soils (Hanafi and Maria, 1998; Hanafi and Sjigreater than the point of zero net charge (PZNC). This suggests that ola, 1998; Hue and Ranjith, 1993; Naidu et al., 1997; Cd was probably sorbed via electrostatic surface reactions and/or Wilcke et al., 1998). This has resulted in a large disparity possible inner-sphere complexation at pH 3.7. However, the amount between what is known about heavy metal contaminaof Pb sorbed by the Oxisol was greater than the amount of negative tion in temperate region soils compared with their tropisurface charge, suggesting that Pb participates in inner-sphere surface cal counterparts. reactions. Lead was sorbed more strongly than Cd in our soils and The properties of many tropical soils differ signifiposes less of a threat to underlying ground water systems due to its cantly from properties of soils in temperate regions. lower mobility and availability. Whereas temperate region soils are composed of clays with mostly permanent negative surface charge, highly weathered tropical soils (Oxisols, Ultisols, Andisols, and T increasing consumption, production, and exacid Alfisols) generally have low surface charge density ploitation of the earth’s raw materials (fossil fuels and consist, predominantly, of materials with variable and minerals), coupled with the exponential growth of charge or pH-dependent charge (McBride, 1994). Surthe world’s population over the past 200 years, have face charge in these systems depends on activities of resulted in environmental buildup of waste products, of potential-determining ions (H and OH ) and electrowhich heavy metals are of particular concern (Adriano, lyte concentrations (Barrow, 1987; van Olphen, 1977). 1986; Purves, 1977). Soils are an important sink for these Depending on soil pH, these surfaces can be negatively metals due to soils’ high metal retention capacities. or positively charged or exhibit a point where the net Important heavy metals posing threats to soil quality total charge on the particle is zero (PZC). and human health include Cd and Pb. They are used Due to their low negative surface charge densities at for a wide variety of industrial, urban, and agricultural common pH values (4–5), highly weathered tropical applications and can be toxic to humans (Adriano, 1986; soils may exhibit relatively low affinities for heavy metAngelone and Bini, 1992; Forstner, 1995; Kabata-Penals (McBride, 1994). The persistence and mobility of dias and Pendias, 1992). People exposed to low levels Cd and Pb in these soils are dictated by the extent to of Cd over time may incur kidney damage as well as which the metals sorb to solid phases, which is a function lung, bone, cardiovascular system, liver, and reproducof reactions affecting surface charge (i.e., pH and ionic tive system damage (Hrudey et al., 1995; USEPA, 1992). strength; Naidu et al., 1997). In children, Pb has been known to cause decreases in Increasing ionic strength (I) and the pH to greater IQ scores, retardation of physical growth, hearing probthan the PZC in tropical soil systems increases negative lems, impaired learning, as well as decreased attention surface charge (Marcano-Martinez and McBride, 1989; and classroom performance. In individuals of all ages, van Olphen, 1977; van Raij and Peech, 1972). Many Pb may cause anemia, kidney disease, brain damage, researchers have shown increased Cd and/or Pb sorption impaired function of the peripheral nervous system, high in tropical soils and/or in pure oxidic mineral systems (i.e., goethite) with increasing pH (Bruemmer et al., Department of Soil and Water Science, Univ. of Florida, Gainesville, FL 32611-0290. Approved for publication as Florida Agricultural Experiment Station Journal Series no. R-08001. Received 28 Mar. 2001. Abbreviations: BET, Brunauer–Emmett–Teller; CEC, cation ex*Corresponding author ([email protected]). change capacity; I, ionic strength; pH50, pH of 50% sorption; PZC, point of zero charge; PZNC, point of zero net charge. Published in J. Environ. Qual. 31:581–589 (2002).