Spectroscopic studies of metal sorption reactions to soil minerals
Combined spectroscopic and macroscopic studies were conducted on Ni and Pb sorption to an array of mineral sorbents. In all systems, the metal speciation was found to vary as a function of parameters such as pH, ionic strength, reaction time, and the presence of competing mineral surfaces and co-adsorbing oxyanions.
An X-ray absorption fine structure spectroscopy (XAFS) study of Ni sorption in a 1:1 pyrophyllite-montmorillonite clay mixture showed that Ni uptake occurred via both adsorption to the montmorillonite phase and Ni-Al hydroxide precipitation induced by the pyrophyllite phase.
In another XAFS study, the Ni speciation in Ni-illite suspensions was found to be strongly dependent on pH, ionic strength and reaction time. At pH > 6.25, Ni-Al hydroxide phases formed, whereas at lower pH values, Ni adsorbed to illite edge and planar sites. Formation of Ni-Al hydroxide precipitates continued over time (3h–1 week), and accelerated with increasing pH. At a given Ni sorption level, the Ni speciation was found to be pH-dependent. This indicated differences in either the composition or the amount of the Ni-Al hydroxide phases formed. Lowering the ionic strength resulted in increased Ni sorption due to the formation of outer-sphere Ni sorption complexes.
An XAFS study of Pb sorption to amorphous silica showed that at low pH values (<4.5), significant outer-sphere Pb sorption occurred, along with inner-sphere adsorption to high affinity sites. As pH increased from 4.5 to 6.3, Pb inner-sphere adsorption and nucleation became increasingly important. Lowering the ionic strength increased the overall Pb sorption, but did not change the average Pb coordination environment. This indicated that the additional Pb sorption occurred via both outer- and inner-sphere Pb complexation, as well as Pb nucleation, dependent on pH.
Infrared and XAFS studies were conducted on the coadsorption of Pb and SO4 to goethite at pH 4.5–6.0. The results pointed to the formation of the Pb-SO4 ternary complexes forming at the goethite surface. In addition to the formation of ternary complexes, Pb adsorption also had an electrostatic effect on SO4 sorption to goethite. This effect was more pronounced at higher pH values and lower Pb additions.
0749: Agricultural chemicals