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DL Sparks

Publications and source records attributed to DL Sparks.

5 recordsLinked to original sources
PubMed1999

The Use of XAFS to Distinguish between Inner- and Outer-Sphere Lead Adsorption Complexes on Montmorillonite.

Adsorption mechanisms of Pb on montmorillonite were investigated by conducting equilibrium and X-ray absorption fine structure (XAFS) spectroscopy studies. Data from the batch equilibrium studies indicate that Pb could be adsorbing via two mechanisms, depending on ionic strength. At low ionic strength (I = 0.006 M) Pb adsorption is pH-independent: 97% of the available Pb was removed from solution at pH 4.42 and 100% at pH 8.0. This behavior is consistent with an outer-sphere complexation mechanism. At high ionic strength (I = 0.1 M) Pb adsorption is pH-dependent, suggesting inner-sphere complexation as the adsorption mechanism: 43% of the available Pb was removed from the solution at pH 4.11 and 98.9% at pH 7.83. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy results reveal that in the sample equilibrated at I = 0.006 M, pH 4.48-6.40 the local atomic structure (LAS) surrounding the adsorbed Pb is similar to the LAS surrounding Pb(2+) (aq), confirming that the adsorption mechanism is outer-sphere complexation. In the system equilibrated at I = 0.1 M, pH 6.77 the XANES and EXAFS results show that the LAS surrounding the adsorbed Pb atom is similar to the LAS surrounding reference compounds in which Pb is forming covalent bonds (Pb(4)(OH)(4+)(4) (aq) and a sample of gamma-Al(2)O(3) with Pb adsorbed via inner-sphere complexation). These similarities indicate that Pb is forming inner-sphere complexes on the montmorillonite at this ionic strength and pH. In samples equilibrated at I = 0.006 M, pH 6.77 and I = 0.1 M, pH 6.31 the XAFS results suggest that Pb is forming both inner- and outer-sphere adsorption complexes. This observation could not be distinguished by making macroscopic observations only. Thus, the results of this study reveal important information on Pb sorption behavior on clays and also provides insights into the use of XAFS to determine sorption mechanisms. Copyright 1999 Academic Press.

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PubMed1999

Nickel Sorption Mechanisms in a Pyrophyllite-Montmorillonite Mixture.

Nickel sorption on pyrophyllite, montmorillonite and a 1:1 pyrophyllite-montmorillonite mixture was studied at pH 7.5 and a reaction time of 40 min. The main modes of Ni uptake under these reaction conditions are adsorption on montmorillonite and surface precipitation on pyrophyllite. For the clay mixture, where adsorption on the montmorillonite component and surface precipitation on the pyrophyllite component compete for Ni uptake, X-ray absorption fine structure spectroscopy (XAFS) was used to estimate the distribution of Ni over the mixture components. This was done by comparison to pyrophyllite-montmorillonite mixtures with known Ni distributions over the mixture components. Nickel uptake on singly reacted pyrophyllite was slightly higher than on singly reacted montmorillonite. This was consistent with the XAFS results for the clay mixture, which suggested that the pyrophyllite component sorbed slightly more Ni than the montmorillonite component. Our findings suggested that both adsorption and surface precipitation were important mechanisms in the overall Ni uptake in the clay mixture, and that neither sorption mechanism truly out-competed the other in the reaction time of 40 min employed. Therefore, both mechanisms should be considered when modeling Ni sorption in similar systems. Copyright 1999 Academic Press.

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PubMed1997

Motor systems.

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PubMed1997

Spectroscopic Evidence for the Formation of Mixed-Cation Hydroxide Phases upon Metal Sorption on Clays and Aluminum Oxides

Retention of heavy metal ions on soil mineral surfaces is an important process for maintaining environmental quality. A thorough understanding of the kinetics and mechanisms of heavy metal sorption on soil mineral surfaces is therefore of fundamental importance. The present study examines the kinetics and mechanisms of Ni(II) sorption onto pyrophyllite, kaolinite, gibbsite, and montmorillonite. Ni sorption reactions were initially fast (15-40% of the initial Ni was removed within the first hour). Thereafter, the rate of sorption decreased significantly. X-ray absorption fine structure (XAFS) spectroscopy was used to determine the local structural environment of Ni(II). Data analysis reveals the presence of polynuclear Ni surface complexes. Ni-Ni bond distances (3.00-3.03 A) were distinctly shorter than in Ni(OH)2(s) (3.09 A). We propose that the reduction of the Ni-Ni distances is caused by the formation of mixed Ni/Al hydroxide phases. The XAFS spectra and derived structural parameters are similar to those in takovite (Ni6Al2(OH)16CO3.H2O), thus suggesting the presence of a Ni phase of similar structure. Even though dissolved Al could not be detected in our samples, Al could have been released into solution and incorporated into mixed Ni/Al hydroxide-like phases. The formation of such phases can explain the finding that the dissolution rates (Si-release) are strongly enhanced (relative to the dissolution rates of the clays alone) as long as Ni sorption is pronounced. We suspect that the release of Al into solution is the rate-determining step for the formation of mixed Ni/Al hydroxide-like phases in our study. Our study demonstrates that mixed Ni/Al hydroxide-like compounds can form when Ni is introduced into a suitable environment in which there is a source of hydrolyzed species of Al. One can speculate that the formation of mixed-cation hydroxide compounds also represents a plausible "sorption mode" for other divalent metal ions when silicates or oxides are present. It has been shown that similar mixed-cation hydroxide compounds can be synthesized when Mg(II), Ni(II), Co(II), Zn(II), or Mn(II) is added to suspensions containing Al(III), Fe(III), and Cr(III). Thus, the formation of mixed-cation hydroxide compounds should be considered when conducting metal sorption experiments, modeling metal surface complexation, determining speciation, and assessing the risk of the migration of contaminants in polluted sites.

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PubMed1995

Neural control.

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