Mineral Surface Complexation Research Articles

Mechanisms on the Impacts of Alkalinity, pH, and Chloride on Persulfate-Based Groundwater Remediation.

Persulfate (S2O82-)-based in situ chemical oxidation (ISCO) has gained more attention in recent years due to the generation of highly reactive and selective sulfate radical (SO4•-). This study examined the effects of important groundwater chemical parameters, i.e., alkalinity, pH, and chloride on benzene degradation via heterogeneous persulfate activation by three Fe(III)- and Mn(IV)-containing aquifer minerals: ferrihydrite, goethite, and pyrolusite. A comprehensive kinetic model was established to elucidate the mechanisms of radical generation and mineral surface complexation. Results showed that an increase of alkalinity up to 10 meq/L decreased the rates of persulfate decomposition and benzene degradation, which was associated with the formation of unreactive surface carbonato complexes. An increase in pH generally accelerated persulfate decomposition due to enhanced formation of reactive surface hydroxo complexation. A change in the chloride level up to 5 mM had a negligibly effect on the reaction kinetics. Kinetics modeling also suggested that SO4•- was transformed to hydroxyl radical (HO•) and carbonate radical (CO3•-) at higher pHs. Furthermore, the yields of two major products of benzene oxidation, i.e., phenol and aldehyde, were positively correlated with the branching ratio of SO4•- reacting with benzene, but inversely correlated with that of HO• or CO3•-, indicating that SO4•- preferentially oxidized benzene via pathways involving fewer hydroxylation steps compared to HO• or CO3•-.

Chromium geochemistry and speciation in natural waters, Iceland

Natural waters in Iceland were collected and analyzed for chromium concentration and speciation (CrIII, CrVI and CrTOT). The water sampled included non-thermal surface and spring water, surface geothermal water, and single and two-phase geothermal well discharges with sampling temperatures of 0–178°C, pH of 2.0–9.5, and total dissolved solids (TDS) of 35–4030ppm. The total Cr concentration was between <0.01 and 660ppb with highest concentrations in waters with the lowest pH. At pH>4 the measured CrIII concentration was low, generally <1ppb but with decreasing pH higher CrIII concentrations were observed reaching values of hundreds of ppb. At pH<6 no measurable CrVI was detected whereas in neutral to alkaline waters measured CrVI concentrations were as high as 3ppb, frequently dominating over CrIII. The Cr chemistry in natural waters associated with mafic rocks in Iceland is largely influenced by the water pH. At low pH Cr-containing minerals are unstable and Cr leaches into the waters as CrIII. Possible oxidation of CrIII to CrVI is minimized thermodynamically and the mobility of CrVI is further reduced by surface complexation reactions. At neutral to alkaline pH the primary Cr-containing mineral phases like titanomagnetite and chromite may be stable, limiting Cr-rock leaching. Solubility of secondary CrIII–FeIII-mineral phases may further reduce and/or limit the CrIII availability. In contrast, CrVI mobility is enhanced at a pH>8 associated with decreasing importance of mineral surface complexation. Hence, CrVI becomes an increasingly dominant form of dissolved Cr at pH above 7–8. Many groundwater drinking supplies associated with mafic rocks are characterized by moderately alkaline pH resulting in CrVI concentrations of a few ppb.

Electrochemical properties and relaxation times of the hematite/water interface.

Electric double layer properties and protonation rates at the surface of a mechanically and chemically polished (001) surface of hematite (α-Fe2O3) contacted with aqueous solutions of NaCl were extracted by electrochemical impedance spectroscopy (EIS). Effects of pH (4-12) and ionic strength (10-1000 mM) on the EIS response of the electrode were predicted using an electrical equivalent circuit model accounting for hematite bulk and interfacial processes. These efforts generated diffuse layer as well as compact layer capacitances and resistance values pertaining to interfacial processes. Diffuse layer capacitance values lie in the 0.5-0.6 μF cm(-2) region and are about 1.5 times smaller than those obtained on a roughened hematite surface. Compact layer capacitances are strongly pH dependent as they pertain to the transfer of ions (charge carriers) from the diffuse layer onto surface (hydr)oxo groups. These values, alongside those of resistance adsorption, pointed a 50% decrease in proton adsorption/desorption resistance under acidic and alkaline conditions relative to that of the point of zero charge (pH 8-9). Increasing ionic strength generally induces larger diffuse layer capacitances, larger adsorption capacitances, and lower resistance values. Such a response is in line with the concept for thinner electric double layers and facilitated proton adsorption reactions by solutions of high ionic strengths. Relaxation times pertaining to the transfer of charge carriers across the compact plane induced by the EIS experiments lie in the 0.7-4.2 s range and become larger under acidic conditions. Decreases in site availability and increases in electrostatic repulsion are two possible contributing factors impeding reaction rates below the point of zero charge. Collectively, these finding are underpinning important relationships between classical views on mineral surface complexation reactions and electrochemical views of semiconductor/water interfaces.

Modeling the influence of organic acids on soil weathering

Biological inputs and organic matter cycling have long been regarded as important factors in the physical and chemical development of soils. In particular, the extent to which low molecular weight organic acids, such as oxalate, influence geochemical reactions has been widely studied. Although the effects of organic acids are diverse, there is strong evidence that organic acids accelerate the dissolution of some minerals. However, the influence of organic acids at the field-scale and over the timescales of soil development has not been evaluated in detail. In this study, a reactive-transport model of soil chemical weathering and pedogenic development was used to quantify the extent to which organic acid cycling controls mineral dissolution rates and long-term patterns of chemical weathering. Specifically, oxalic acid was added to simulations of soil development to investigate a well-studied chronosequence of soils near Santa Cruz, CA. The model formulation includes organic acid input, transport, decomposition, organic-metal aqueous complexation and mineral surface complexation in various combinations. Results suggest that although organic acid reactions accelerate mineral dissolution rates near the soil surface, the net response is an overall decrease in chemical weathering. Model results demonstrate the importance of organic acid input concentrations, fluid flow, decomposition and secondary mineral precipitation rates on the evolution of mineral weathering fronts. In particular, model soil profile evolution is sensitive to kaolinite precipitation and oxalate decomposition rates. The soil profile-scale modeling presented here provides insights into the influence of organic carbon cycling on soil weathering and pedogenesis and supports the need for further field-scale measurements of the flux and speciation of reactive organic compounds.

Structural environments of carboxyl groups in natural organic molecules from terrestrial systems. Part 1: Infrared spectroscopy

Carboxyls play an important role in the chemistry of natural organic molecules (NOM) in the environment, and their behavior is dependent on local structural environment within the macromolecule. We studied the structural environments of carboxyl groups in dissolved NOM from the Pine Barrens (New Jersey, USA), and IHSS NOM isolates from soils and river waters using attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy. It is well established that the energies of the asymmetric stretching vibrations of the carboxylate anion (COO −) are sensitive to the structural environment of the carboxyl group. These energies were compiled from previous infrared studies on small organic acids for a wide variety of carboxyl structural environments and compared with the carboxyl spectral features of the NOM samples. We found that the asymmetric stretching peaks for all NOM samples occur within a narrow range centered at 1578 cm −1, suggesting that all NOM samples examined primarily contain very similar carboxyl structures, independent of sample source and isolation techniques employed. The small aliphatic acids containing hydroxyl (e.g., d-lactate, gluconate), ether/ester (methoxyacetate, acetoxyacetate), and carboxylate (malonate) substitutions on the α-carbon, and the aromatic acids salicylate ( ortho-OH) and furancarboxylate ( O-heterocycle), exhibit strong overlap with the NOM range, indicating that similar structures may be common in NOM. The width of the asymmetric peak suggests that the structural heterogeneity among the predominant carboxyl configurations in NOM is small. Changes in peak area with pH at energies distant from the peak at 1578 cm −1, however, may be indicative of a small fraction of other aromatic carboxyls and aliphatic structures lacking α-substitution. This information is important in understanding NOM–metal and mineral-surface complexation, and in building appropriate structural and mechanistic models of humic materials.

Experimental study of mineral surface and aqueous aluminum complexation with aniline and 2-chloroaniline

The thermodynamic stabilities of the aqueous and surface Al-aniline and Al-2-chloroaniline complexes were investigated using solubility and adsorption experiments. Aqueous complexation was studied by measuring the solubility of gibbsite (Al(OH) 3) as a function of aniline and chloroaniline concentrations. Experiments measuring aniline and chloroaniline adsorption onto corundum (α-Al 2O 3) were conducted as a function of pH, from pH 1.5 to 9.0, at room temperature in 0.1 molal NaCl solutions, with starting concentrations of 10 −3.0 and 10 −3.3molal aniline, 10 −2.7 molal 2-chloroaniline, and 2.72 × 10 −3 molal total surface sites. The experiments place quantitative constraints on the thermodynamic properties of the aqueous and the surface Al-aniline complexes. The solubility data indicate that if Al-aniline or Al-2-chloroaniline aqueous complexes exist, they are not stable enough to significantly affect the solubility of gibbsite under the experimental conditions. Conversely, the adsorption data provide unequivocal evidence for the presence of two distinct surface Al-aniline and Al-chloroaniline complexes according to the following reactions (1) Aniline 0 + ≡ Al(OH) 0 ⇔ ≡ AlOH(Aniline) 0 (2) Aniline 0 + ≡ Al(O) − ⇔ ≡ AlO(Aniline) − (3) Chloroaniline 0 + ≡ Al(OH) 0 ⇔ ≡ AlOH(Chloroaniline) 0 (4) Chloroaniline 0 + ≡ Al(OH 2 ) + ⇔ ≡ AlOH 2 (Chloroaniline) + We use a constant capacitance model to quantify the stability constants for reactions 1–4, and the results yield log equilibrium constant values of 2.09, 2.67, 2.87, and 2.30, respectively. The experimental results indicate that for most contaminated systems, aqueous Al-aniline, and Al-chloroaniline complexation does not have a significant effect on aniline speciation in groundwater, but that mineral surface complexation can significantly affect total aniline and chloroaniline budgets.