Did Salts in Seawater Play an Important Role in the Adsorption of Molecules on Minerals in the Prebiotic Earth? The Case of the Adsorption of Thiocyanate onto Forsterite-91.

This study aims to provide a better basis for application of adsorption models for metal (hydr)oxides to natural multicomponent systems. Adsorption of any ion in the environment will be potentially influenced by the effect of other ions present like calcium, phosphate, carbonate etc. The study starts with a detailed study of the binding of ions as outersphere complexes. The CD model has been extended to use the charge distribution for ions that bind as outersphere surface complex. This indicates that neither innersphere nor outersphere surface complexes are treated as point charges anymore. The new approach was applied to describe the adsorption of various electrolyte ions, phosphate, and carbonate. Batch experiments were performed using goethite as an adsorbent to determine the adsorption behavior of electrolyte ions (Li +1 , Na +1 , K +1 , Cs +1 , Ca +2 , Mg +2 , Cl -1 , NO 3-1 ), phosphate, and carbonate. The adsorption of phosphate and carbonate ions was studied in a 'single ion' system and their interaction in a competition system. The charge distribution value of innersphere surface complexes of phosphate and carbonate was calculated using the new approach. New is also the use of quantum chemical calculations to derive the CD value based on a calculated geometry of the surface complexes. The calculated geometries were interpreted with the Brown bond-valence model, resulting in a calculated CD of the surface complex. The calculated CD values were used as a constraint in the surface complexation modeling. The CD model for inner- and outersphere surface complexation successfully described the adsorption data of electrolyte ions, phosphate, and carbonate. For accommodation of adsorbed ions within the Stern layer, a Three Plane (TP) model was used as a framework. For outersphere surface complexes, it was shown that the minimum distance of approach of adsorbed ions depends on the finite size of ions and their degree of hydration, which determine their relative distances to the surface of minerals. It has been shown that the capacitance of the inner Stern layer is determined by the minimum distance of approach of the ion closest to the surface, while the capacitance of the outer layer is determined by the minimum distance of approach of the ion furthest away from the surface. Modeling of phosphate adsorption data revealed that phosphate adsorbed mainly as a bidentate surface complex. At low pH, protonated species of phosphate are a combination of monodentate and bidentate surface complexes. The new CD approach shows that phosphate interacts with sodium at the mineral surface, which could not be detected using previous approaches. Carbonate adsorption data were successfully described using a bidentate surface complex. This complex interacts with sodium at high pH and high salt level. The new approach not only predicts the shift in the isoelectric point as a function of phosphate loading, but also the measured zeta potential is in quite good agreement with predictions based on the assumption that the zeta potential coincides with the potential of the head end of the DDL. Furthermore, it has been shown that the parameterized CD model can be used to determine the effective reactive surface area of metal (hydr)oxides and the total reversibly adsorbed phosphate fraction in soils.

Adsorption mechanisms of selenium oxyanions at the aluminum oxide/water interface

Microscopic level investigation of Ni(II) sorption on Na-rectorite by EXAFS technique combined with statistical F-tests

EXAFS study of the interfacial interaction of nickel(II) on titanate nanotubes: Role of contact time, pH and humic substances

RETRACTED: Determination of Ni(II) uptake mechanisms on mordenite surfaces: A combined macroscopic and microscopic approach

An electron paramagnetic resonance study of Cu(II) sorbed on quartz

The most crucial role played by minerals was in the preconcentration of biomolecules or precursors of biomolecules in prebiotic seas. If this step had not occurred, molecular evolution would not have occurred. Thiocyanate is an important molecule in the formation of biomolecules as well as a catalyst for prebiotic reactions. The adsorption of thiocyanate onto ferrihydrite was carried out under pH and ion composition conditions in seawater that resembled those of prebiotic Earth. The seawater used in this work had high Mg2+, Ca2+ and SO42- concentrations. The most important result of this work was that ferrihydrite adsorbed thiocyanateata pH value (7.2 ± 0.2) that usually does not adsorb thiocyanate. The high adsorptivity of Mg2+, Ca2+ and SO42-onto ferrihydrite showed that seawater ions can act as carriers of thiocyanate to the ferrihydrite surface, creating a huge outer-sphere complex. Kinetic adsorption and isotherm experiments showed the best fit for the pseudo-second-order model and an activation energy of 23.8kJmol-1forthe Langmuir-Freundlich model, respectively. Thermodynamic data showed positive ΔG values, which apparently contradict the adsorption isotherm data and kinetic data that was obtained. The adsorption of thiocyanate onto ferrihydrite could be explained by coupling with the exergonic SO42- adsorption onto ferrihydrite. The FTIR spectra showed no difference between the C≡N stretching peaks of adsorbed thiocyanate and free thiocyanate, corroborating the formation of an outer-sphere complex. All the results demonstrated the importance of the artificial seawater composition for the adsorption of thiocyanate and for understanding prebiotic chemistry.

Hydroxyl radicals (HO•), produced through reactions between H2O2 and iron oxides, drive biogeochemical transformations, mediate organism toxicity, and facilitate advanced oxidation processes. The effectiveness of these processes depends on the spatial proximity between HO• generation and target substrates. Consequently, the oxidation mechanisms should be governed by interfacial interactions among H2O2, substrates, and iron oxide surfaces. Substrate oxidation by iron oxide/H2O2 systems were studied using two probes simultaneously: terephthalate (TPA), forming outer-sphere surface complexes, and coumarin, exhibiting no surface interactions. The reactions were followed as a function of pH, time and H2O2 concentration. Complementary experiments were performed with oxalate inner-sphere surface complexes. Both ferrihydrite and goethite were studied, representing differences in reduction potential. Solution analyses were combined with in-situ infrared spectroscopy probing the interfacial reactions. Both probes were oxidized by ferrihydrite/H2O2. Between pH5.5-6.5, substantial amounts of TPA outer-sphere surface complexes were oxidized, while coumarin was mainly oxidized at pH≤4.5, coinciding with a decrease in TPA oxidation. At all investigated pH values, H2O2 reduced ferrihydrite, and the partitioning of Fe(II) controlled the location of HO• generation. At low pH, Fe(II) diffused into solution triggering homogeneous Fenton reactions, while adsorption and re-oxidation at higher pH confined radical generation to the near-surface region. Oxalate inner-sphere complexes resisted oxidation. Oxidation by the goethite/H2O2 system was low compared to ferrihydrite, consistent with the lower reduction potential of goethite. This work demonstrates that H2O2-promoted reduction of iron oxides is a key reaction leading to HO• oxidation of organic outer-sphere surface complexes.

The formation and deposition of mineral scales impose significant challenges to the safety and integrity of field operations across various sectors such as water treatment, desalination, and oil and gas. To mitigate the mineral deposition issues, scale inhibitors (SIs) with phosphonate and carboxylate-based structures are widely employed. Understanding of how these inhibitors interact with minerals and ions is essential for creating more effective, environmentally friendly solutions that meet operational demands. In this study, we employed molecular dynamics (MD) simulations to explore how widely used phosphonate-based SIs adsorb onto calcite surfaces, examining their behavior under different temperatures and salinity levels, both in bulk solutions and within calcite nanopores. Results reveal that the SIs exhibit high compatibility with salty environments, showing low aggregation tendencies. Interestingly, SIs do not form large aggregates in high-salinity brine (213,000 ppm of total dissolved salts), indicating their compatibility with such conditions. MD simulations reveal that phosphonate-based SIs predominantly adsorbed as hydrated outer-sphere surface complexes (OSSC) within the Stern double layer. While the carboxylate-based SIs formed both inner-sphere surface complexes (ISSC) and OSSC with higher adsorption than phosphonate-based SIs. The observed superior performance of phosphonate-based SIs over carboxylate-based SIs can be attributed to two primary factors: reduced loss into reservoir formations due to weaker adsorption and superior thermal stability. Phosphonate-based SIs were found to be significantly more thermally stable than their carboxylate counterparts, which is crucial for high-temperature applications. Diffusion coefficient calculations further indicated that brine salinity has minimal impact on the mobility of SIs, highlighting their robust performance under varying salinity conditions. This study provides detailed atomistic insights into the adsorption mechanisms of SIs on scale-forming mineral surfaces. These findings offer valuable guidance for the development of more efficient and environmentally sustainable scale inhibitors.

An in Situ ATR-FTIR Investigation of Sulfate Bonding Mechanisms on Goethite

Chapter 14 - Surface and Solution Speciation of AgI in a Heterogeneous Ferrihydrite-Solution System with Thiosulfate

Strontium adsorption was studied by acid–base potentiometric titration at various pH, ionic strength, the sorbate/sorbent ratio, and temperatures (at 25, 50, and 75°C). The experimental data were interpreted using two models of surface complexation with two different electrostatic models of the interface: the constant capacitance (CCM) and triple-layer (TLM) models. Although both models are able to take into account acid–base reactions and surface complexation of Sr on birnessite, we believe that TLM is more suitable for the description of the H+–>MnOH–Sr2+ heterogeneous system. At a low ionic strength and negatively charged surface, Sr2+ ions compete with electrolyte ions and form both inner- and outer-sphere complexes. Although the application of CCM in describing Sr adsorption may be mathematically satisfactory, it has little physical sense. We suggest a model that involves both inner-sphere (>MnOHSr2+, >MnOSr+, >MnOSrOH0) and outer-sphere ([>MnO–Sr2+]+) surface complexes. The corresponding constants of formation of these surface complexes were calculated at 25, 50, and 75°C.

SummaryThe surface sorption process of Eu(III) onto smectite and kaolinite was investigated by time-resolved laser fluorescence spectroscopy (TRLFS) in the trace concentration range. The experiments were performed in 0.025 M and 0.45 M NaClO4. The sorption process of Eu(III) onto smectite was obtained by TRLFS under atmospheric conditions and in absence of CO2. The pH was varied between 3.5 and 9 at a fixed metal ion concentration of 3.3 × 10−6mol/L Eu(III). At low pH (< 4) the metal ion keeps its complete hydration sphere indicating outer-sphere complexation. With increasing pH the formation of an inner-sphere Eu(III) surface complex was observed. The differences in the spectra and the fluorescence emission lifetimes of the surface sorbed Eu(III) in presence and absence of carbonate indicate the formation of ternary clay/Eu(III)/carbonate complexes. The different europium/clay surface complexes were characterized by their fluorescence emission spectra (5D0→7F1/5D0→7F2intensity ratio) and their fluorescence emission lifetime.

Sorption characteristics and mechanisms of oxyanions and oxyhalides having different molecular properties on Mg/Al layered double hydroxide nanoparticles

Macroscopic and spectroscopic exploration on the removal performance of titanate nanotubes towards Zn(II)