Surface Complexation of Calcium Minerals in Aqueous Solution (original) (raw)
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American Journal of Science, 2005
In order to understand the interactions of inorganic and organic species from solution with mineral surfaces, and more specifically, with the growth and dissolution behavior of minerals, we start by reviewing the most basic level of interaction. This is the influence of single monovalent ions on the growth and dissolution rate of minerals consisting of divalent ions. Monovalent ions as background electrolyte can change the morphology of growth features such as growth islands and spirals. These morphology changes can be similar to the ones caused by organic molecules and are, therefore, easily mixed up. Both Na ؉ and Clpromote growth and dissolution of some divalent crystals such as barite and celestite. In addition, morphology changes and the stability of polar steps on sulfates are explained using atomistic principles.
ADSORBABILITY OF ALIZARIN RED S ON Fe(III)- AND Pb(II)-TREATED HYDROXYAPATITES IN WATER
Phosphorus Research Bulletin, 2010
We studied on adsorbability of alizarin red S (ARS) on Fe(III)-and Pb(II)-treated hydroxyapatites in distilled water (H 2 O) and phosphate buffer solution (PBS). The Fe(III)-treated apatites indicated high adsorption capacity from their isothermic curves both in H 2 O and PBS: The capacities in PBS were larger than those in H 2 O. On the other hand, adsorption capacities of Pb(II)-treated apatites in PBS were much lower than those in H 2 O. Surface analysis of those treated apatites by electron probe micro analysis and FT-IR microspectroscopy affords significant information on surface element (P, Ca, Fe, or Pb) distribution and interaction patterns between ARS and the corresponding metal (Fe or Pb) sites. Eventually, predominant adsorption mechanisms were elucidated as follows, chelate form adsorption for Fe(III)-treated apatites and salt form adsorption for Pb(II)-treated apatites.
Fluoride removal from aqueous solutions using a-Al 2 O 3 as an adsorbent was investigated through adsorption and electrokinetic studies, using two different initial fluoride concentrations relevant to fluoride contents in drinking water, namely 3 and 10 mg/L. Maximum fluoride removal was achieved between pH 5 and 6 at 258C. Adsorption at both pH 5 and 9 correlates to the Langmuir isotherm. The point of zero charge (pzc) of a-Al 2 O 3 at 208C was determined using three different concentrations of NaNO 3 as an indifferent electrolyte. It was found to occur at pH 9.2. Through mineral/solution equilibria, this pzc was determined to be close to pH 9.5 of minimum solubility of amorphous aluminum hydroxide, 1973 suggesting the formation of this species onto the adsorbent's surface. Zeta potential of a-Al 2 O 3 in the presence of fluoride reversed sign towards more acidic pH values in comparison to the pzc, indicating specific adsorption of fluoride. Fluoride adsorption onto alumina under mildly acidic conditions seemed to involve mainly a replacement of surface OH 2 by F 2 . Adsorption is small at pH 9 and up to about 10 and it seems to take place through hydrogen bonding.
Adsorption
Research on Ca2+ adsorption onto the mineral surface is of significant importance with regard to geochemical processes. Sverjensky (Geochim Cosmochim Acta 70(10), 2427–2453, 2006) assumed that alkaline earths form two types of surface species on oxides: tetranuclear (> SOH)2(> SO−)2_M(OH)+ and mononuclear > SO−_M(OH)+. To look into the above assumption we investigated calcium adsorption on SiO2 and Al2O3 because they are the most widespread minerals in the environment. We have determined the proton surface charge, electrokinetic potential and metal adsorption as a function of pH. The Ca2+ uptake and concentration in the system were monitored by the calcium ion-selective electrode (Ca-ISE). The Ca-ISE measurements indicated a similar affinity of Ca2+ for both materials despite their differently charged surface, negative for silica and mainly positive for alumina. This may suggest that simple electrostatic interactions are not the primary driving force for calcium adsorption,...
Structure and Bonding of Orthophosphate Ions at the Iron Oxide–Aqueous Interface
Journal of Colloid and Interface Science, 1996
namely, thermodynamic modeling of potentiometric and ad-The surface speciation of orthophosphate ions on goethite has sorption data (5, and IR spectroscopic investigations been studied as a function of pH, time, total phosphate concentra-(10-16). Regardless of the technique, the results are usually tion, and ionic medium by means of diffuse reflectance FTIR specinterpreted in terms of surface complexation theory (17). troscopy. The samples were prepared in accordance with a distri-The main objective of the thermodynamic work is to formubution diagram of surface species as calculated from thermodylate a set of equilibria that explains the observed changes namic data. In agreement with the thermodynamic model three in the aqueous phase. This includes the reaction between dominating surface complexes could be distinguished with IR phosphate ions and surface metal ions, and commonly also spectroscopy, and the relative distribution of the species was shown protonation reactions of the adsorbed phosphate. There is to be primarily a function of pH. The IR characteristics of the individual surface complexes were indicative of molecular sym-good agreement between the models, based on thermodymetries of the PO 4 unit of C 3v , C 2v , and C 3v , respectively. This namic data, presented by various authors . The was concluded to be incompatible with the bidentate, bridging dominating surface complexes derived are three species mostructural model previously suggested. Instead, the IR data are in nodentately coordinated to the surfaces, namely, GMe{Ogood agreement with a monodentate coordination of phosphate to PO 3 H 2 , GMe{OPO 3 H 0 , and GMe{OPO 20 3 . In Ref. (5) the surfaces, where the three surface complexes only differ in two minor bridging complexes have also been included in the the degree of protonation. A comparison between the adsorption model. Since protonated complexes are involved the surface behavior of phosphate on goethite and hematite was also made. speciation is strongly pH dependent. The direct bond be-Here the importance of the aqueous stability of the adsorbent on tween the surface metal ions and phosphate implies that the adsorption mechanism was shown. ᭧ 1996 Academic Press, Inc. inner-sphere surface complexes are formed. This description Key Words: iron oxide-aqueous interface; orthophosphate ions; adsorption; diffuse reflectance Fourier transform infrared spec-is supported by practically all experimental observations, troscopy.
Environmental Science and Pollution Research, 2018
Calcium ion-incorporated hydrous iron(III) oxide (CIHIO) samples have been prepared aiming investigation of efficiency enhancement on arsenic and fluoride adsorption of hydrous iron(III) oxide (HIO). Characterization of the optimized product with various analytical tools confirms that CIHIO is microcrystalline and mesoporous (pore width, 26.97 Å; pore diameter, 27.742 Å with pore volume 0.18 cm 3 g −1) material. Increase of the BET surface area (> 60%) of CIHIO (269.61 m 2 g −1) relative to HIO (165.6 m 2 g −1) is noticeable. CIHIO particles are estimated to be~50 nm from AFM and TEM analyses. Although the pH optimized for arsenite and fluoride adsorptions are different, the efficiencies of CIHIO towards their adsorption are very good at pH 6.5 (pH zpc). The adsorption kinetics and equilibrium data of either tested species agree well, respectively, with pseudo-second order model and Langmuir monolayer adsorption phenomenon. Langmuir capacities (mg g −1 at 303 K) estimated are 29.07 and 25.57, respectively, for arsenite and fluoride. The spontaneity of adsorption reactions (ΔG 0 = − 18.02 to − 20.12 kJ mol −1 for arsenite; − 0.2523 to − 3.352 kJ mol −1 for fluoride) are the consequence of entropy parameter. The phosphate ion (1 mM) compared to others influenced adversely the arsenite and/or fluoride adsorption reactions. CIHIO (2.0 g L −1) is capable to abstract arsenite or fluoride above 90% from their solution (0 to 5.0 mg L −1). Mechanism assessment revealed that the adsorption of arsenite occurs via chelation, while of fluoride occurs with ion-exchange.
In Situ Infrared Speciation of Adsorbed Carbonate on Aluminum and Iron Oxides
Clays and Clay Minerals, 1997
Surface adsorption mechanisms of dissolved inorganic carbon species on soil minerals are not well understood. Traditional infrared (IR) study of adsorbed species of inorganic carbon using air-dried samples may not reveal true species in the solid/water interface in suspension. The ptu'pose of this study was to obtain information on interracial carbonate speciation between solid and aqueous phases. The interaction of bicarbonate and carbonate ions with X-ray amorphous (am) A1 and Fe oxides, gibbsite (~/-AI(OH)3 ) and goethite (a-FeOOH) was examined by electrophoresis and in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The presence of carbonate lowered the electrophoretic mobility and decreased the point of zero charge (PZC) of all minerals, implying specific adsorption. Inner-sphere complexation of bicarbonate and carbonate was supported by a lowering in the anion symmetry due to the interaction with A1 and Fe oxide surfaces. Only complexed monodentate carbonate was identified in am-Al(OH)3/aqueous solution at pH 4.1-7.8 when the solid was reacted with either NanCO 3 or Na2CO 3 solutions. Am-AI(OH)3 was transformed to a crystalline sodium aluminum hydroxy carbonate, dawsonite [NaAl(CO3)(OH)2], and bayerite (ct-AI(OH)3) after reacting with 1.0 M Na2CO3 for 24 h. Gibbsite adsorbed much less carbonate than am-AI(OH)3 such that adsorbed carbonate on gibbsite gave weak IR absorption. It is probable that monodentate carbonate is also the complexed species on gibbsite. Evidence suggesting the presence of both surface complexed bicarbonate and carbonate species in the interfacial region of am-Fe(OH)3 in suspension and the dependence of their relative distribution on solution pH is shown. Only monodentate carbonate was found in the interracial region of goethite in 1.0 M NaHCO3. A ligand exchange reaction was proposed to describe the interaction of bicarbonate and carbonate with the surface functional groups of A1 and Fe oxides.
American Journal of Science, 2008
The Charge Distribution MUltiSite Ion Complexation or CD-MUSIC modeling approach is used to describe the chemical structure of carbonate mineralaqueous solution interfaces. The new model extends existing surface complexation models of carbonate minerals, by including atomic scale information on the surface lattice and the adsorbed water layer. In principle, the model can account for variable proportions of face, edge and kink sites exposed at the mineral surface, and for the formation of inner-and outer-sphere surface complexes. The model is used to simulate the development of surface charges and surface potentials on divalent carbonate minerals as a function of the aqueous solution composition. A comparison of experimental data and model output indicates that the large variability in the observed pH trends of the surface potential for calcite may in part reflect variable degrees of thermodynamic disequilibrium between mineral, solution and, when present, gas phase during the experiments. Sample preparation and non-stoichiometric surfaces may introduce further artifacts that complicate the interpretation of electrokinetic and surface titration measurements carried out with carbonate mineral suspensions. The experimental artifacts, together with the high sensitivity of the model toward parameters describing hydrogen bridging and bond lengths at the mineralwater interface, currently limit the predictive application of the proposed CD-MUSIC model. The results of this study emphasize the need for internally consistent experimental data sets obtained with well-characterized mineral surfaces and in situ aqueous solution compositions (that is, determined during the charge or potential measurements), as well as for further molecular dynamic simulations of the carbonate mineral-water interface to better constrain the bond lengths and the number plus valence contribution of hydrogen bridges associated with different structural surface sites.
Environmental science & technology, 2017
Reactive mineral-water interfaces exert control on the bioavailability of contaminant arsenic species in natural aqueous systems. However, the ability to accurately predict As surface complexation is limited by the lack of molecular-level understanding of As-water-mineral interactions. In the present study, we report the structures and properties of the adsorption complexes of arsenous acid (As(OH)3) on hydrated mackinawite (FeS) surfaces, obtained from density functional theory (DFT) calculations. The fundamental aspects of the adsorption, including the registries of the adsorption complexes, adsorption energies, and structural parameters are presented. The FeS surfaces are shown to be stabilized by hydration, as is perhaps to be expected because the adsorbed water molecules stabilize the low-coordinated surface atoms. As(OH)3 adsorbs weakly at the water-FeS(001) interface through a network of hydrogen-bonded interactions with water molecules on the surface, with the lowest-energy ...