Electrokinetic behavior of artificial and natural calcites: A review of experimental measurements and surface complexation models

Effect of calcium and phosphorus on ammonium and nitrate nitrogen adsorption onto iron (hydr)oxides surfaces: CD-MUSIC model and DFT computation

Calcium (Ca2+) and phosphorous (PO43-) significantly influence the form and effectiveness of nitrogen (N), however, the precise mechanisms governing the adsorption of ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3--N) are still lacking. This study employed batch adsorption experiments, charge distribution and multi-site complexation (CD-MUSIC) models and density functional theory (DFT) calculations to elucidate the mechanism by which Ca2+ and PO43- affect the adsorption of NH4+-N and NO3--N on the goethite (GT) surface. The results showed that the adsorption of NH4+-N on the GT exhibited an initial increase followed by a decrease as pH increased, peaking at a pH of 8.5. Conversely, the adsorption of NO3--N decreased with rising pH. According to the CD-MUSIC model, Ca2+ minimally affected the NH4+-N adsorption on the GT but enhanced NO3--N adsorption via electrostatic interaction, promoting the adsorption of ≡FeOH-NO3- and ≡Fe3O-NO3- species. Similarly, PO43- inhibited the adsorption of ≡FeOH-NO3- and ≡Fe3O-NO3- species. However, PO43- boosted NH4+-N adsorption by facilitating the formation of ≡Fe3O-NH4+ via electrostatic interaction and site competition. DFT calculations indicates that although bidentate phosphate (BP) was beneficial to stabilize NH4+-N than monodentate phosphate (SP), SP-NH4+ was the main adsorption configuration at pH 5.5-9.5 owing the prevalence of SP on the GT surface under site competition of NH4+-N. The results of CD-MUSIC model and DFT calculation were verified mutually, and provide novel insights into the mechanisms underlying N fixation and migration in soil.

Carbonation inhibitor by polyethylene glycol encapsulation of calcium hydroxide fine particles to improve antimicrobial and root canal penetration properties

The carbonation of calcium hydroxide (Ca(OH)₂) is affected by humidity and a saturated atmosphere. Ca(OH)₂ from nature is easily carbonation and self-aggregates into calcium carbonate (CaCO₃), resulting in larger particle size impairing the antimicrobial properties due to lack of penetration into the dentinal tubules and lower ion dissociation. To reduce the particle size, the wet beads milling process with distilled water as the medium is commonly used, but often results in great carbonation of the final product. Polyethylene Glycol (PEG) may inhibit the carbonation process as well as re-agglomeration. However, it requires intensive drying of the fine Ca(OH)₂ particles. As an alternative, we used ethanol as a medium in the milling process, which is easily dried and compatible with PEG as a surfactant. This study aimed to evaluate PEG 400 as a dispersing agent in ethanol medium in the beads milling process to prevent carbonation of the fine Ca(OH)₂ particles. The following groups were analysed CaP-PEG (Ca(OH)₂-PEG) with ethanol as a medium, CaP-Eth (Ca(OH)₂ with ethanol as a medium), CaP-DW (Ca(OH)₂ with distilled water as a medium), CaPC (Ca(OH)₂-carbonated) as the negative control and CaC (Ca(OH)₂ analytical grade) as the positive control The final particle results were characterized to evaluate the crystal structure, functional groups, and particle size. The corresponding pH and antimicrobial activity against Enterococcus faecalis were assessed at 1, 3, 7, and 14 days. The penetration ability was evaluated by Scanning Electron Microscope. The data obtained were analysed by ANOVA with a significance level of 5%. PEG was able to inhibit carbonation and stabilize pH for up to 14 days, providing increased antimicrobial activity against E. faecalis. PEG also facilitates the ability of fine Ca(OH)₂ particles to penetrate deeper into the dentine tubules by reducing particle size.

Prediction of electrostatic properties of reservoir rock in low salinity water injection into carbonate reservoirs

Geochemical modeling along with chemical reactions is one of the challenges in modeling of low salinity water injection. The most important issue in the geochemical model is to determine the correct electrical charge distribution model and its tuning parameters. The composition of the rock as well as the candidate water used is effective in determining the type of model and its parameters, so that the tuning parameters are determined based on the history of zeta potential experiments. In this study, in order to determine the correct model of electrical charge distribution and its tuning parameters in carbonate rock samples, first, equilibrium samples of Candidate water with crushed rock are subjected to static zeta potential tests. Then, the diffuse electrical double layer model is used to determine the electrical charge of the rock/water and water/oil surfaces and to predict the zeta potential. In the following, by adjusting the tuning parameters of the model to match the prediction results of the model with the history of the laboratory data, the density of the carbonate rock surface, the equilibrium constants and the kinetics of the governing reactions are determined. The obtained results show that the range of error in zeta potential prediction by the model compared to the laboratory data is from 2 to 20%, which is within the acceptable range of the performance of electrical charge distribution models. Moreover, it could be observed that the error of prediction using DLM model is significantly less than the conventional models (CD-MUSIC and BSM) for different candidate water. Finally, the effect of calculated zeta potential changes is used to calculate the contact angle changes of low salinity water injection based on the coupling of DLVO theory and geochemical model. The results of the study prove that the prediction error is less than 5% compared to the results of the static wettability tests. Based on this, according to the good match between the model and the laboratory results, it is possible to determine the properties of surface sites in surface complexation models of carbonate samples using the proposed approach and the subsequent tuning data of the geochemical model.

Probing interaction forces associated with calcite scaling in aqueous solutions by atomic force microscopy

The prevention of calcite aggregation and scaling remains a challenging problem in aqueous based systems and environmental science. Decades of research studies have proposed microscopic mechanisms of aggregation control, but experiments at the nanoscale and molecular level are rarely conducted. Here we show that the nanoscale topographic features of calcite during its aggregation depend significantly on the intermolecular and surface forces involved in this process. By measuring the forces between a calcite or silica particle and a calcite surface in aqueous solutions using atomic force microscopy, we found that higher solution pH and inhibitor concentration and lower salinity resulted in a system of stronger repulsion and weaker adhesion, which is favorable for reducing the possibility of calcite aggregation and surface deposition. Conflicting roles of Mg²⁺ in calcite aggregation prevention, being positive in acidic pH and negative in alkaline pH, were also observed. The nanoscale structural changes of calcite, visualized by atomic force microscopy or scanning electron microscopy, indicated a size dependence of aggregated and deposited calcite crystals on the calcite-calcite and calcite-silica interactions, respectively. The generalized framework of the calcite aggregation mechanism achieved in this work can be extended to other types of systems and provides a basis for investigating the anti-aggregation strategy of calcite from industrial and environmental perspectives.

Theoretical determination of zeta potential for the variable charge soil considering the pH variation based on the Stern-Gouy double-layer model

Electrokinetic remediation (EKR) is a promising alternative for the contaminated soil with low hydraulic permeability. The nonlinearity of the electroosmotic flow (EOF) is mainly induced by the nonuniform variation of the pH and thus the zeta potential of the soil during the EKR process. The empirical relation between the zeta potential and the pH for kaolinite is currently applied to analyze the nonlinearity of the EOF. A new perspective for theoretical determination of the zeta potential for the variable charge soil is proposed in this study. The prediction model incorporates the pH, the valence and concentration of the electrolyte, and the temperature and permittivity of the solvent surrounding the clay particles. Satisfying agreement between the calculated and measured curves of zeta potential versus pH for three types of variable charge soil was achieved. This model would act as a useful tool to simulate the nonlinearity of the electroosmosis of the variable charge soil and provide guidance and precise control mechanism for maximizing the efficiency of the EOF.