Matthew J. Eick

Kinetics of NI(II) sorption and desorption on kaolinite: Residence time effects

Recent studies have shown that aging or increased residence time can reduce the availability of trace element cations sorbed to common soil minerals. Numerous explanations have been given to explain the observed residence time effect. However, most of these studies begin only with sorbed species and not surface precipitates. The formation of Ni2+ surface precipitates on common soil minerals has been observed in the laboratory by a number of researchers. Accordingly, the influence of residence time on the sorption/desorption kinetics of Ni2+ on kaolinite was examined. Nickel sorption kinetics were conducted at three aqueous concentrations (0.10, 0.50, 0.75 mM) of Ni2+ in the presence of 25 g L−1 kaolinite at pH 7.5. More than 99% of the Ni2+ was sorbed to the kaolinite surface at the end of 14 days for all aqueous concentrations of Ni2+. Adsorption was characterized by an initial fast reaction followed by a slower reaction. Both reactions followed first order kinetics. Based on previous spectroscopic studies, the fast reaction was attributed to chemisorption, whereas the slow reaction was attributed to nucleation and surface precipitation of a Ni-Al layered double-hydroxide (LDH). Desorption experiments were conducted on kaolinite samples after 14 days (short-term) and 20 weeks (long-term) in the presence of 1 mM oxalate at pH 6.0. Similar to adsorption kinetics, desorption kinetics were characterized by an initial rapid reaction followed by a slower reaction, both of which followed first order kinetics. For all surface coverages the total quantity of Ni2+ desorbed and the desorption rate coefficients (k1 and k2) were greater for the short-term than for the long-term experiments. It is suggested that the residence time effect observed for the slow desorption/dissolution reaction was caused by an increase in crystallinity of the LDH surface precipitate and, to a lesser extent, phase transformation into a Ni-Al phyllosilciate. In contrast, several processes may be responsible for the residence time effect observed for the fast desorption/dissolution reaction, including movement of weakly bound Ni2+ to a more strongly bound phase (eg, change in the type of surface complex), diffusion into micropores or intraparticle spaces, or an increase in crystallinity (eg, Ostwald ripening) of weakly precipitated Ni2+. The above results demonstrate and suggest potential mechanisms for the long-term natural attenuation of trace metal cations such as Ni2+ adsorbed to mineral surfaces.

EFFECT OF SILICA POLYMERIZATION ON THE OXALATE-PROMOTED DISSOLUTION OF GOETHITE

Numerous studies have investigated the ligand-promoted dissolution of Fe (oxyhydr)oxides. In natural environments, inorganic ligands can compete with organic ligands for surface sites on (oxyhydr)oxides which may influence dissolution rates. Published research of this interaction and its effect on the dissolution of (oxyhydr)oxides is rare. The objective of the present study was to examine the extent to which silica, as a naturally occurring competitive ligand added in the form of silicic acid, impacts the oxalate-promoted dissolution of the common soil Fe (oxyhydr)oxide goethite. Sorbed silica reduced the oxalate-promoted dissolution rate of goethite at all surface coverages investigated. As initial silica solution concentrations increased from 0.50 mM to 5.0 mM, relatively little change in the dissolution rate was observed. Fourier-transform infrared (FTIR) spectra indicated that, as silica-surface coverages increased, the silica underwent polymerization on the goethite surface. Initially, silicate was associated with surface functional groups, but as polymerization occurred some of the silica appeared to desorb from the goethite surface without being released into the bulk solution, suggesting that silica polymers formed discrete islands or surface clusters that grew away from the goethite surface rather than expanding epitaxially across the surface. Minimal changes were observed in the quantity of reactive goethite surface, which is responsible for the observed dissolution rates, as silica-surface coverages increased.

EFFECTS OF OXYANIONS ON THE EDTA-PROMOTED DISSOLUTION OF GOETHITE

Organic ligands, such as EDTA, accelerate the dissolution of silicate and oxide minerals. In natural systems, oxyanions can compete with organic ligands for mineral surface sites thereby affecting ligand-promoted dissolution rates, either enhancing or inhibiting dissolution, depending upon pH. The influence of selenite, molybdate and phosphate on the EDTA-promoted dissolution of goethite has been examined and a mechanism proposed for the observed differences in dissolution rates over a pH range of 4–8. We propose that the surface complex formed by EDTA is the controlling factor for the observed dissolution rate, with mononuclear complexes accelerating dissolution compared to bi- or multinuclear complexes. Dissolution results from our experiments suggest that EDTA forms multinuclear complexes at pH values ⩾6 and mononuclear complexes at pH values <6. Dissolution results show that when the oxyanion and the EDTA are present in the system at concentrations nearly equalling the surface sites available for adsorption, the oxyanion reduces the adsorption of EDTA and inhibits dissolution. However, if the oxyanion is present at lower concentrations at pH values ⩾6, EDTA is adsorbed but the number of carboxylic groups that can bind to the surface is reduced causing the formation of mononuclear complexes. This shift to a weaker surface complex enhances the EDTA-promoted dissolution of goethite in the presence of the oxyanions compared to EDTA-promoted dissolution in their absence.

Arsenate adsorption on ruthenium oxides: A spectroscopic and kinetic investigation

Arsenate adsorption on amorphous (RuO(2)1.1H(2)O) and crystalline (RuO(2)) ruthenium oxides was evaluated using spectroscopic and kinetic methods to elucidate the adsorption mechanism. Extended X-ray absorption fine structure spectroscopy (EXAFS) was used to determine the local coordination environment of adsorbed arsenate. Additionally, pressure-jump (p-jump) relaxation spectroscopy was used to investigate the kinetics of arsenate adsorption/desorption on ruthenium oxides. Chemical relaxations resulting from the induced pressure change were monitored via electrical conductivity detection. EXAFS data were collected for two initial arsenate solution concentrations, 3 and 33 mM at pH 5. The collected spectra indicated a similar coordination environment for arsenate adsorbed to RuO(2)1.1H(2)O for both arsenate concentrations. In contrast the EXAFS spectra of RuO(2) indicated differences in the local coordination environments for the crystalline material with increasing arsenate concentration. Data analysis indicated that both mono- and bidentate surfaces complexes were present on both RuO(2)1.1H(2)O and RuO(2). Relaxation spectra from the pressure-jump experiments of both ruthenium oxides resulted in a double relaxation event. Based on the relaxation spectra, a two step reaction mechanism for arsenate adsorption is proposed resulting in the formation of a bidentate surface complex. Analysis of the kinetic and spectroscopic data suggested that while there were two relaxation events, arsenate adsorbed to ruthenium oxide surfaces through both mono- and bidentate surface complexes.

Adsorption of Selenite and Selenate on Ferrihydrite in the Presence and Absence of Dissolved Organic Carbon

This study examines selenite [Se(IV)] and selenate [Se(VI)] adsorption on two-line ferrihydrite in the presence and absence of two low-molecular-weight dissolved organic carbon (DOC) species, citric and salicylic acid. Ferrihydrite surface potential measurements were also examined to identify shifts in isoelectric points, which suggest possible adsorption mechanisms. Sorption was completed in batch reactor systems at environmentally relevant pH. Our results indicate citric acid suppressed both Se(IV) and Se(VI) sorption on ferrihydrite, which may be caused by competition. This was especially evident at pH 5 to 7 for Se(IV) and pH 5 to 6 for Se(VI). Little sorption suppression was observed for both Se species in the presence of salicylic acid. In the presence of Se(IV) and Se(VI), citric acid adsorption was reduced (pH 5-8). Salicylic acid sorption was almost completely suppressed in the presence of Se(IV) throughout the entire pH range examined, with minimal sorption occurring at pH 5. In the presence of Se(VI), the largest reduction in salicylic acid sorption occurred at pH 5 to 6. Small shifts in the surface potential of ferrihydrite at higher pH suggest that Se(VI) and salicylic acid form weak, outer-sphere complexes. However, at pH 5 and 6, there is a shift in the surface potential measurements to more negative values, indicating possible formation of stronger, inner-sphere complexes. Larger surface potential shifts for Se(IV) and citric acid suggest the formation of strong, inner-sphere complexes. This work demonstrates the ability of low-molecular-weight DOC species (particularly for citric acid) to increase Se(IV) and Se(VI) solubility through sorption suppression.

Selenium speciation in phosphate mine soils and evaluation of a sequential extraction procedure using XAFS

Selenium is a trace element found in western US soils, where ingestion of Se-accumulating plants has resulted in livestock fatalities. Therefore, a reliable understanding of Se speciation and bioavailability is critical for effective mitigation. Sequential extraction procedures (SEP) are often employed to examine Se phases and speciation in contaminated soils but may be limited by experimental conditions. We examined the validity of a SEP using X-ray absorption spectroscopy (XAS) for both whole and a sequence of extracted soils. The sequence included removal of soluble, PO4-extractable, carbonate, amorphous Fe-oxide, crystalline Fe-oxide, organic, and residual Se forms. For whole soils, XANES analyses indicated Se(0) and Se(-II) predominated, with lower amounts of Se(IV) present, related to carbonates and Fe-oxides. Oxidized Se species were more elevated and residual/elemental Se was lower than previous SEP results from ICP-AES suggested. For soils from the SEP sequence, XANES results indicated only partial recovery of carbonate, Fe-oxide and organic Se. This suggests Se was incompletely removed during designated extractions, possibly due to lack of mineral solubilization or reagent specificity. Selenium fractions associated with Fe-oxides were reduced in amount or removed after using hydroxylamine HCl for most soils examined. XANES results indicate partial dissolution of solid-phases may occur during extraction processes. This study demonstrates why precautions should be taken to improve the validity of SEPs. Mineralogical and chemical characterizations should be completed prior to SEP implementation to identify extractable phases or mineral components that may influence extraction effectiveness. Sequential extraction procedures can be appropriately tailored for reliable quantification of speciation in contaminated soils.