Molybdenum Adsorption on Oxides, Clay Minerals, and Soils

Molybdenum adsorption behavior was investigated on various crystalline and x-ray amorphous Al and Fe oxide minerals, clay minerals, CaC@, and arid-zone calcareous and noncalcareous s soils. Molybdenum adsorption on botb Al and Fe oxides exhibited a maximum at low pH extending to about pH 4 to 5. Above pH 5 adsorption decreased rapidly, with little adsorption occurring above pH 8. Molybdenum adsorption was higher for the oxide minerals having higher specific surface area and lower crystallinity. Molybdenum adsorption on the clay minerals exhibited a peak near pH 3 and then decreased rapidly with increasing pH until adsorption was virtually zero near pH 7. The magnitude of MO adsorption on clays increased in the order: kaolinite < illite < montmorillonite. Shifts in point of zero charge were observed on Al and Fe oxides and kaolinite following M O adsorption, indicating an inner-sphere adsorption mechanism for MO on these surfaces. Molybdenum adsorption behavior on three arid-zone noncalcareous soils resembled that on clays, exhibiting a peak near pH 3 to 4 and decreasing with increasing pH up to pH 7. This behavior is expected since the oxide content of these soils is low. Molybdenum adsorption on calcite and two calcareous arid-zone soils was low, indicating that CaCOJ is not a significant sink for MO in soils. M OLYBDENUM IS A TRACE ELEMENT required by both plants and animals. Molybdenum is readily taken up by plants and can accumulate to levels detrimental to grazing ruminants (Reisenauer et al., 1962). Especially in contrast to phosphate, studies of MO adsorption are limited. Molybdenum adsorption by Al and Fe oxides has been investigated (Jones, 1957; Reisenauer et al., 1962; Reyes and Jurinak, 1967; Kyriacou, 1967; McKenzie, 1983; Ferreiro et al., 1985; Zhang and Sparks, 1989; Vordonis et al., 1990; Spanos et al., 1990a,b; Bibak and Borggaard, 1994). Molybdenum adsorption was studied on halloysite, nontronite, and kaolinite (Jones, 1957), kaolinite, montmorillonite, and illite (Motta and Miranda, 1989), and kaolinite (Phelan and Mattigod, 1984; Mikkonen and Tummavuori, 1993). Molybdenum adsorption by soils has been investigated previously (Jones, 1957; Reisenauer et al., 1962; Barrow, 1970; Theng, 1971; Gonzalez et al., 1974; Jarrell and Dawson, 1978; Karimian and Cox, 1978; Roy et al., 1986, 1989; Xie and MacKenzie, 1991; Xie et al., 1993; Mikkonen and Tummavuori, 1993). Iron oxide adsorbed more MO than did Al oxide (Jones, 1957). Molybdenum adsorption on oxides increased from pH 2 to 4, exhibited a peak near pH 4, and decreased with increasing pH above 4 (Jones, 1957; Kyriacou, USDA-ARS, U.S. Salinity Lab., 450 Big Springs Rd., Riverside, CA 92507. Contribution from the. U.S. Salinity Lab. Received 18 Jan. 1995. *Corresponding author. Published in Soil Sci. Soc. Am. J. 60:425-432 (1996). 1967; McKenzie, 1983; Ferreiro et al., 1985). The decrease in adsorption with increasing pH above 4 occurred more rapidly for Al oxide than for Fe oxide (Ferreiro et al., 1985). The mechanism of MO adsorption on Al and Fe oxides is suggested to be ligand exchange with surface hydroxyl ions (Jones, 1957; Ferreiro et al, 1985). Ligand exchange is a mechanism by which ions become specifically adsorbed as inner-sphere surface complexes. Inner-sphere surface complexes contain no water molecules between the adsorbing ion and the surface functional group (Sposito, 1984). The point of zero charge (PZC) of variablecharge minerals is shifted to a more acid pH value following specific adsorption of anions. Molybdenum adsorption lowered the PZC of goethite, indicating specific adsorption (McKenzie, 1983). By studying the effects of ionic strength on anion adsorption, Hayes et al. (1988) were able to distinguish between innerand outer-sphere Se surface complexes. Outer-sphere surface complexes contain at least one water molecule between the adsorbing ion and the surface functional group (Sposito, 1984). Hayes et al. (1988) suggested that since selenite showed little ionic strength dependence in its adsorption behavior, it was specifically adsorbed on goethite in an inner-sphere surface complex. Zhang and Sparks (1989) found little ionic strength dependence for MO adsorption on goethite and interpreted this result as supporting evidence for inner-sphere surface complexation. Molybdenum adsorption on the clay minerals metahalloysite, nontronite, and kaolinite showed pH-dependent adsorption behavior similar to that on oxides with an adsorption peak near pH 4 (Jones, 1957; Mikkonen and Tummavuori, 1993). The relative adsorption on the clay minerals was less than on Al and Fe oxide and decreased in the order: metahalloysite > nontronite > kaolinite (Jones, 1957) and montmorillonite > kaolinite > illite (Motta and Miranda, 1989). Similar behavior was observed for a soil clay dominant in kaolinite and illite (Theng, 1971). Adsorption on soil clays was decreased by removal of amorphous Fe and Al oxides (Theng, 1971). Molybdenum adsorption by soils has been investigated (Jones, 1957; Reisenauer et al., 1962; Barrow, 1970; Roy et al., 1986, 1989). Adsorption of M O was highly correlated with extractable Al (Barrow, 1970) and Fe (Gonzalez et al., 1974; Jarrell and Dawson, 1978; Karimian and Cox, 1978). Removal of amorphous Fe oxides drastically reduced MO adsorption by an Australian soil (Jones, 1957). The pH dependence of MO adsorption on soil resembled that on oxides and clay minerals. Adsorption increased with increasing pH up to a peak near pH 4 and then decreased with increasing pH above 426 SOIL SCI. SOC. AM. I., VOL. 60, MARCH-APRIL 1996 Table 1. Characterization of oxides and clay minerals used in this MATERIALS AND METHODS study. Solid Surface Point of Trace area zero charge impurities