Abiotic Reduction of Mercury(II) in the Presence of Sulfidic Mineral Suspensions

Monomethylmercury (CH3Hg) is a neurotoxic pollutant that biomagnifies in aquatic food webs. In sediments, the production of CH3Hg depends on the bacterial activity of mercury (Hg) methylating bacteria and the amount of bioavailable inorganic divalent mercury (HgII). Biotic and abiotic reduction of HgII to elemental mercury (Hg0) may limit the pool of HgII available for methylation in sediments, and thus the amount of CH3Hg produced. Knowledge about the transformation of HgII is therefore primordial to the understanding of the production of toxic and bioaccumulative CH3Hg. Here, we examined the reduction of HgII by sulfidic minerals (FeS(s) and CdS(s)) in the presence of dissolved iron and dissolved organic matter (DOM) using low, environmentally relevant concentrations of Hg and ratio of HgII:FeS(s). Our results show that the reduction of HgII by Mackinawite (FeS(s)) was lower (<15% of the HgII was reduced after 24 h) than when HgII was reacted with DOM or dissolved iron. We did not observe any formation of Hg0 when HgII was reacted with CdS(s) (experiments done under both acidic and basic conditions for up to four days). While reactions in solution were favorable under the experimental conditions, Hg was rapidly removed from solution by co-precipitation. Thermodynamic calculations suggest that in the presence of FeS(s), reduction of the precipitated HgII is surface catalyzed and likely involves S−II as the electron donor. The lack of reaction with CdS may be due to its stronger M-S bond relative to FeS, and the lower concentrations of sulfide in solution. We conclude that the reaction of Hg with FeS(s) proceeds via a different mechanism from that of Hg with DOM or dissolved iron, and that it is not a major environmental pathway for the formation of Hg0 in anoxic environments.

Usage of new materials during rehabilitation of road structure using cold recycling technology

Until recently, the most common way for recovering of damaged and worn asphalt pavements on Ukrainian roads remains the provision of additional reinforced layers over the old pavement with patching. However, such measures give only a short-term effect because after one or two years, the existing deformations and fractures beneath reinforced layers occur, especially in conditions of insufficient strength of the foundations. But nowadays, recycling technology of different variations became the main method of existing pavement renovation. The economic attractiveness of cold recycling technology is primarily in the reuse of existing road material for arrangement of new pavement layers, so there is no need to arrange special areas for storage and disposal of old asphalt. In addition, the use of such technology helps to minimize the harmful impacts on the environment during road repair works. The essence of cold recycling technology, which is the most widely used in Ukraine for the arrangement of a road foundation layer, is in the fact that the defective and destroyed pavement layers are strengthened directly by complex admixtures of organic (hot bitumen, bituminous emulsion, foamed bitumen) and mineral suspensions, lime) binders. Cold recycling, according to the complications of the work, is divided into several types, depending of the depth of cutting. The choice of a particular type of recovery depends mainly on the condition of the entire pavement structure, which is determined prior to the start of repair works. Optimal mis design of the organic and mineral mixture for the arrangement of the road foundation layer by cold recycling technology is also executed before the beginning of the works. Actually, the main direction of cold recycling technology research in Ukraine is the usage of new materials such as fiber - basalt or polymer, stabilizing additives (ionic or polymeric), industrial waste - slags of various types of production or other by-products. Performed studies have shown that the use of organic and mineral mixtures of optimal design with the insertion of basalt fiber increases crack resistance and durability of the arranged road foundation layer.

Application of Molecular Weight Regulators for the Synthesis of Sodium Polyacrylate Thinners of Mineral Suspensions

The synthesis of additives for thinning mineral suspensions based on sodium polyacrylate was carried out. The effect of molecular weight regulators on the molecular weight characteristics of the polymer and the effect of such polymers on the rheological properties of suspensions was studied. Sodium acrylate polymers are synthesized by free radical polymerization in aqueous solution using molecular weight regulators. The molecular weight characteristics of the polymeric samples were estimated by viscometry using Mark-Houwink-Kuhn-Sakurada (MHKS) equation. Synthesized polymers were used as thinners ceramic slurries, prepared according to the recipe of the enterprises producing ceramic products. The thinning ability of polymer samples with different molecular weights was estimated using an Engler viscometer from the time of the ceramic slurry flow. The influence of the type and amount molecular weight regulator on polyacryates was revealed. It was found that molecular weight synthesized samples was in the range of 21000 - 91000. It was determined that samples with a molecular weight of 28000 - 35000 synthesized using mercaptoethanol (at a dosage of 0.5-1.5% by weight of the monomer) provide optimal fluidity to the ceramic slurry.

Effect of Schwertmannite Surface Modification by Surfactants on Adhesion of Acidophilic Bacteria

Bacterial cell adhesion onto mineral surfaces is important in a broad spectrum of processes, including bioweathering, bioleaching, and bacterial cell transport in the soil. Despite many research efforts, a detailed explanation is still lacking. This work investigates the role of surface-active compounds, cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and pure rhamnolipid (RH), in the process of bacteria attachment on the schwertmannite surface. The surface energy was calculated based on the wettability of the tested systems, and for bacteria it was 54.8 mJ/m2, schwertmannite-SDS 54.4 mJ/m2, schwertmannite-CTAB 55.4 mJ/m2, and schwertmannite-RH 39.7 mJ/m2. The total energy of adhesion estimated based on thermodynamic data was found to be negative, suggesting favorable conditions for adhesion for all examined suspensions. However, including electrostatic interactions allowed for a more precise description of bacterial adhesion under the tested conditions. The theoretical analysis using the extended Derjaguin-Landau-Verwey-Overbeek (DLVO) approach showed a negative value of total adsorption energy only in bacteria-mineral suspensions, where SDS and rhamnolipid were added. The calculated data were in good agreement with experimental results indicating the significance of electrostatic forces in adsorption.

High-Quality Cement-Mineral Suspensions Using Stone Crushing Waste

The quality of concrete is determined by the physico-mechanical and rheological properties of the original components. Rheological properties depend on the amount and concentration of the rheological matrix. Cement-mineral matrices of modern high-quality concrete are rheologically active suspensions which allow to provide the necessary fluidity and mobility of the concrete mix, as well as the required strength of concrete. The multicomponent composition of the water-dispersed phase is represented by cement, stone flour and hyperplasticizer. The article presents the results of studies confirming the possibility of replacing finely ground quartz with other highly dispersed rocks, including stone-crushing waste. It was revealed that increasing in the concentration of the water-dispersed phase by 10% made it possible to reduce the water-solid ratio while maintaining the required mobility of the mixture, and in some cases improved the spreadability of the suspension by 3-13%. It was determined that the physicomechanical properties of reaction-powder concretes depend on the density and porosity of finely dispersed components to a greater degree.

Sharp changes in properties of aqueous sodium chloride solution at “critical” concentrations of the salt

It was shown that some physicochemical properties of aqueous solutions of sodium chloride (contact angle, surface tension, pH, and others) demonstrate a nonlinear dependence on the salt concentration. The maxima or minima in the graphs are most frequently observed near the concentrations of NaCl we call "critical": 3, 5, 7, 12, 17, and 21 g/L. Similar nonlinear peak-shape dependence of the property on salinity was observed in more complex systems containing NaCl (sedimentation rate of mineral suspensions, viscosity of clay pastes, rate of a redox reaction of periodate with amine). We hypothesized that the said critical points of salt concentration may exist when the clusters of water molecules rearrange themselves upon the addition of a definite quantity of salt. The obtained results were used in the discussion of avalanche sedimentation of suspended particulate matter and colloids transported by rivers through the river mixing zones (estuary, delta) and the critical salinity under which the river biota gives place to marine biota.

Determination of characteristic properties of mineral suspension for calculating the particle velocity

Calculations of hydraulic devices for separating mineral suspensions and extracting valuable components and waste are based on determining the speed of constrained deposition and surfacing of particles. When determining the speed, it is necessary to know such characteristics of the suspension as viscosity, porosity, density. The article offers a method for analytical calculation of these indicators. Its peculiarity is that the definition of these characteristics is associated with only one parameter – the volume density of the medium (substance). It is easily determined in practice by weighing a sample of suspension in a container of a known volume. This method is illustrated by the characteristics of two water-based mineral suspensions: with a solid particle density of 2.55 g / cm3 (ash and slag waste from a thermal power plant) and a density of 2.0 g/cm3 (crushed tuff). For these suspensions, the approximate dependences of these characteristics on the density of the medium (substance) are established. It is established that the application of the most common suspension model in the form of a discrete cellular structure provides acceptable limitations of the studied characteristics. The obtained values are within the scope of typical indicators of mineral pulps in the working zone of hydro-classifiers and hydro-separators. However, a number of well-known formulas for the speed of restricted movement that use this model are unacceptable for calculating hydraulic devices since they take into account inflated values of characteristics. The described method can be used for dispersed systems of two or more phases, for example, using weighted average values of components. The proposed method for calculating the characteristics of suspensions is suitable for practical use, given that the hydraulic apparatus receives already classified raw materials and the particles have a small variation in shape and size. The obtained analytical dependences of suspension characteristics on density are used for calculating speed of both constrained and free movement of particles, which is the basis for determining technological and design parameters in the design of hydraulic devices and apparatuses.

EFFECT OF ORGANIC AND FINE DISPERSER ADDITIONS ON RHEOLOGICAL PROPERTIES OF MINERAL SUSPENSIONS

In this study, the effect of mineral and organic additives on the properties of mineral suspensions was studied. Disperse mineral additives are used in cement systems to increase the amount of rheological matrix and ensure high fluidity. Due to the high dispersion of mineral additives, high-efficiency plasticizers were used to regulate the rheotechnological properties of cement-based mixtures. The effect of sulfonaphthalene-formaldehyde oligomer and polycarboxylate-based hyperplasticizers on the rheological properties of cement systems as a plasticizer was evaluated according to the methodology proposed by prof. V.I.Kalashnikov. Based on the results obtained, it was determined that limestone powder is more effective than other stone powders. As a result of the research, the optimal amounts of mineral additives and plasticizers were determined and the possibility of obtaining efficient, high-strength cement stone on their basis confirmed.

Effect of Electrolyte Concentration in Process Water on Flocculation

The effect of electrolyte concentration and potential determining ions on the coagulation and flocculation of illite, dolomite, and illite-dolomite mixture suspensions was investigated. Electrokinetic measurements, settling rate tests, and viscosity measurements were performed to examine the stability of these mineral suspensions and to characterize flocculants under various physico-chemical conditions. Two flocculants: A-100 anionic polyacrylamide (PAM) and polyethylene oxide (PEO) were employed. The tests revealed that polyethylene oxide does not flocculate dolomite under any tested conditions. Viscosity results corroborated that the conformation of PAM macro- molecules in water is very sensitive to electrolyte concentration; on the other hand, the conformational state of PEO macromolecules is not affected by ionic strength. The intrinsic viscosity measurements suggest that the unattainable flocculation of dolomite suspensions with PEO must result from poor adsorption of this flocculant onto dolomite particles. In both tested cases, with PAM and PEO, the relationship between coagulation and flocculation was not confirmed. Resumen. Se investigó el efecto de la concentración electrolítica y iones determinantes de potencial en la coagulación y floculación de suspensiones de ilita, dolomita y mezcla de ilita-dolomita. Mediciones electrocinéticas, ensayos de velocidad de sedimentación, y mediciones de viscosidad fueron realizadas para examinar la estabilidad de estas suspensiones minerales y caracterizar floculantes bajo varias condiciones fisicoquímicas. Se empleó dos floculantes: A-100 poliacrilamida aniónica (PAM) y óxido de polietileno (PEO). Los experimentos revelaron que el óxido de polietileno no flocula la dolomita bajo ninguna condición empleada. Los resultados de viscosidad corroboraron que la conformación de las macromoléculas PAM en agua es sensible a la concentración electrolítica; por otro lado, el estado conformacional de las macromoléculas de PEO no se ve afectado por la fuerza iónica. Las mediciones de viscosidad intrínseca sugieren que la floculación inalcanzable de las suspensiones de dolomita con PEO debe ser resultado de una mala adsorción de este floculante en las partículas de dolomita. En ambos casos probados, con PAM y PEO, no se confirmó la relación entre coagulación y floculación.