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In this work, the adsorption behavior of Sr onto a synthetic iron(III) oxide (hematite with traces of goethite) has been studied. This solid, which might be considered a representative of Fe3+ solid phases (iron corrosion products), was characterized by X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS), and its specific surface area was determined. Both XRD and XPS data are consistent with a mixed solid containing more than 90% hematite and 10% goethite. The solid was further characterized by fast acid-base titrations at different NaCl concentrations (from 0.1 to 5 M). Subsequently, for each background NaCl concentration used for the acid-base titrations, Sr-uptake experiments were carried out involving two different levels of Sr concentration (1 × 10−5 and 5 × 10−5 M, respectively) at constant solid concentration (7.3 g/L) as a function of −log([H+]/M). A Surface Complexation Model (SCM) was fitted to the experimental data, following a coupled Pitzer/surface complexation approach. The Pitzer model was applied to aqueous species. A Basic Stern Model was used for interfacial electrostatics of the system, which includes ion-specific effects via ion-specific pair-formation constants, whereas the Pitzer-approach involves ion-interaction parameters that enter the model through activity coefficients for aqueous species. A simple 1-pK model was applied (generic surface species, denoted as >XOH−1/2). Parameter fitting was carried out using the general parameter estimation software UCODE, coupled to a modified version of FITEQL2. The combined approach describes the full set of data reasonably well and involves two Sr-surface complexes, one of them including chloride. Monodentate and bidentate models were tested and were found to perform equally well. The SCM is particularly able to account for the incomplete uptake of Sr at higher salt levels, supporting the idea that adsorption models conventionally used in salt concentrations below 1 M are applicable to high salt concentrations if the correct activity corrections for the aqueous species are applied. This generates a self-consistent model framework involving a practical approach for semi-mechanistic SCMs. The model framework of coupling conventional electrostatic double layer models for the surface with a Pitzer approach for the bulk solution earlier tested with strongly adsorbing solutes is here shown to be successful for more weakly adsorbing solutes. [ABSTRACT FROM AUTHOR]

Magnetite nanoparticles are a promising cost-effective material for the remediation of polluted wastewaters. Due to their magnetic properties and their high adsorption and reduction potential, they are particularly suitable for the decontamination of oxyanion-forming contaminants, including the highly mobile selenium oxyanions selenite and selenate. However, little is known how the remediation efficiency of magnetite nanoparticles in field applications is affected by partial oxidation and the formation of magnetite/maghemite phases. Here we characterize the retention mechanisms and capacity of partially oxidized nanoparticulate magnetite for selenite and selenate in an oxic system at different pH conditions and ionic strengths. Data from adsorption experiments showed that retention of selenate is extremely limited except for acidic conditions and strongly influenced by competing chloride anions, indicating outer-sphere adsorption. By contrast, although selenite adsorption capacity of oxidized magnetite is also adversely affected by increasing pH, considerable selenite quantities are retained even at alkaline conditions. Using spectroscopic analyses (XPS, XAFS), both mononuclear edge-sharing (2E) and binuclear corner-sharing (2C) inner-sphere selenite surface complexes were detected, while reduction to Se(0) or Se(–II) species could be excluded. Under favourable adsorption conditions, up to ~pH 8, the affinity of selenite to form 2C surface complexes is higher, whereas at alkaline pH values and less favourable adsorption conditions 2E complexes become more dominant. Our results demonstrate that magnetite can be used as a suitable reactant for the immobilization of selenite in remediation applications, even under (sub)oxic conditions and without the involvement of reduction processes. [Display omitted] • Oxidized magnetite can effectively remediate selenite contaminated wastewaters. • No reduction processes are required to remove large amounts of Se oxyanions. • Selenite immobilization is driven by formation of inner-sphere adsorption complexes. • Partially oxidized magnetite nanoparticles behave like maghemite phases. [ABSTRACT FROM AUTHOR]

This study investigates the adsorption of americium and its chemical analogue europium on magnetite, which is expected to form as a major long-term steel canister corrosion product under anoxic and highly saline conditions. The sorption of europium on magnetite (solid/liquid ratio = 0.5 g/L) was investigated batch wise in NaCl brines with ionic strength I = 1 m , 3.5 m , and 6.67 m , as a function of pH m for two europium concentrations (6 × 10−10 m , 1.2 × 10−5 m). Information on the chemical nature of the surface species was obtained by X-ray absorption spectroscopy (XAS) at the americium L 3 -edge. Retention of europium by magnetite of >99.5% was found above pH m 6.4 for all ionic strengths for europium concentration of 6 × 10−10 m. No ionic strength effect was observed in this pH m range. At 1.2 × 10−5 m europium concentration, 95 ± 4% sorption was found above pH m 7.5 for I = 1 m and above pH m 8.0 for I = 3.5 m and 6.67 m. A small ionic strength effect was observed in this case. X-ray absorption spectroscopy (XAS) results are consistent with the batch sorption experiment outcomes, showing an insignificant effect of ionic strength on the pH m dependent sorption. Results from potentiometric titrations of the solid phase, batch sorption experiments and spectroscopy were interpreted consistently with a charge distribution multi-site (CD-MUSIC) triple layer surface complexation model assuming surface coordination of the metal ion via a tridentate binding mode. [ABSTRACT FROM AUTHOR]

The solubility of Be(II) is investigated in carbonate-free dilute to concentrated HCl–NaCl–NaOH, KCl–KOH, NaOH and KOH solutions. Undersaturation experiments were performed under Ar atmosphere at T = (22 ± 2) °C. XRD, XPS, SEM and quantitative chemical analysis confirm that α-Be(OH) 2 (cr) is the solid phase controlling the solubility in all evaluated systems. No transformation of α-Be(OH) 2 (cr) to β-Be(OH) 2 (cr) or ternary solid phases Na/K–Be(II)–OH(s) is observed in the investigated systems within the timeframe of this study (t ≤ 600 days). An amphoteric solubility behaviour of Be(II) is observed with a solubility minimum at pH m ≈ 9.5 (with [Be(II)] ≈ 10−6.8 M), regardless of the ionic strength. The combination of solubility data determined in acidic pH m conditions and the hydrolysis scheme reported in the literature for cationic hydrolysis species of Be(II) is used for the determination of the solubility constant of α-Be(OH) 2 (cr), log * K ° s,0 = (6.9 ± 0.4). Slope analysis of the solubility data in alkaline to hyperalkaline conditions and 9Be NMR support the predominance of the monomeric hydrolysis species Be(OH) 2 (aq), Be(OH) 3 – and Be(OH) 4 2− within the strongly alkaline pH m -conditions relevant in cementitious systems. The comprehensive solubility dataset collected within this study in combination with extensive solid and aqueous phase characterization allow the development of a complete chemical, thermodynamic and (SIT) activity model for the system Be2+–Na+–K+–H+–Cl––OH––H 2 O(l). This model provides an accurate and robust tool for the evaluation of Be(II) solubility and speciation in a diversity of geochemical conditions, including source term calculations of beryllium in the context of the nuclear waste disposal Safety Case. • Systematic solubility and spectroscopic study with Be(II) in NaCl and KCl systems. • α-Be(OH) 2 (cr) controls the solubility of Be(II) in the investigated systems. • Comprehensive thermodynamic, chemical and SIT activity models derived. • Accurate description of the system Be2+–Na+–K+–H+–Cl––OH––H 2 O(l) provided. • Be(OH) 3 – and Be(OH) 4 2− confirmed as relevant species in cementitious systems. [ABSTRACT FROM AUTHOR]

Sorption of Eu(III) onto quartz from highly saline solutions (up to 5 M NaCl) for pH c 2–9 ha s been studied by measuring sorption edges. The acid-base titrations of the silica surface suggest the rather unusual presence of two different sites that has been the object of recent discussions in the literature. Europium uptake results show the usual behaviour with a steep pH-edge and nearly complete removal at sufficiently high pH. Previous spectroscopic data on this system suggest the presence of two bidentate surface complexes with different proton stoichiometry. Based on this, a self-consistent Surface Complexation Model (SCM) was fitted to the full set of experimental data, from 0.1 to 5 M NaCl, using a coupled Pitzer/surface complexation approach. The Pitzer model was applied to aqueous species. A Basic Stern Model was used for interfacial electrostatics of the system, which includes ion-specific effects via ion-pair formation. Parameter fitting was done using the general parameter estimation software UCODE coupled to a modified version of FITEQL2 involving separate calculations of the respective ionic strength corrections. At high ionic strength (>1 M), the surface potential is strongly screened by ion-pair formation and the diffuse layer potential is negligibly low, which justifies the extension of the standard electrostatic model to these harsh conditions. Overall, our model is able to describe the full set of analysed data. To the knowledge of the authors this is the first comprehensive work studying solid surface titrations and cation uptake up to high salt levels and applying electrostatics models to describe the solid interface at these particular conditions. It is expected that these first systematic data acquisition along with the detailed modelling can serve as a benchmark for the modelling of future studies on sorption in highly saline systems. [ABSTRACT FROM AUTHOR]