Equilibrium Solution Concentration Research Articles

Polyacrylonitrile‐based porous polymer spheres and their conversion to N‐doped carbon materials for adsorption and electrocatalysis

AbstractBecause of their tunable porosity and specific surface area, porous materials are of high interest for the purification of wastewater by adsorption as well as in electrocatalysis, where, in particular, developing metal‐free carbon‐based catalysts for the oxygen reduction reaction (ORR) is researched. The carbon spheres presented here can meet these two requirements simultaneously. Porous polyacrylonitrile (PAN) spheres were firstly prepared by droplet shaping cum nonsolvent induced phase separation. Then, they were converted to nitrogen‐doped carbon spheres by two‐step carbonization via preoxidation and pyrolysis. During the pyrolysis step, carbon dioxide was used to reopen the pores of the materials that had been blocked after the preoxidation step. By this way, specific surface area values of carbon spheres of more than 1000 m2/g could be obtained. Best performing carbon materials (C#3) showed high adsorption capacity (e.g., 296 mg/g for methyl orange at solute equilibrium concentration of 24 mg/L in water). The XPS analysis revealed that only quaternary (N‐Q) and pyridinic (N‐6) nitrogen sites were found in these carbon spheres. The best ORR performance was found also for C#3 carbon spheres, with a potential of 0.81 V versus reversible hydrogen electrode (RHE) at −1 mA/cm2 and electron transfer number of 3.5 at 0.6–0.8 V versus RHE, as determined by rotating disk or rotating ring disk electrode tests. In conclusion, all results confirm that these PAN‐derived carbon spheres are potentially valuable materials for both wastewater treatment by adsorption and electrocatalytic oxygen reduction.

Regulating microstructure and improving precipitation hardening response of fine-grained Mg-RE-Ag hot-extruded alloy by extreme short-time heat treatment

The difficulty in exerting beneficial effects of fine grains, high number density nano-precipitates as well as weakened texture simultaneously is currently the bottleneck limiting the development of high performance (high strength, high plasticity, and isotropic) age-hardenable Mg-RE alloys. The key difficulty is that the generation of excessive dynamic precipitates is always accompanied by the formation of fully dynamic recrystallized ultrafine grains during thermal deformation, causing a significant depletion of solutes within the entire matrix and finally seriously weakening the subsequent aging nano-precipitation. In this work, the process of extreme short-time (2 min) heat treatment (ESHT) followed by peak aging treatment is applied to an ultrafine-grained weak-textured Mg-RE-Ag as-extruded alloy with numerous dynamic precipitates. The precipitation hardening response is dramatically increased (ΔHV = 37.2 HV) and the yield strength is increased by nearly 50 MPa. This is attributed to the fact that concentration of main solutes in matrix is greatly increased within only 2 min of heat treatment promoting the precipitation of nano-precipitates (prismatic β′ and basal γ′′) in the absence of excessive grain growth. The large difference between the initial and equilibrium solute concentrations, the extremely short distance required for solute diffusion within sub-microscale grains, and the effective pinning of grain boundaries by excess dynamic precipitates are responsible for the unusual microevolution during the ESHT. In addition, the formation of precipitation-free zones (PFZs) is attributed to ‘uphill’ diffusion during aging, and novel γ′′-PFZs and β′-PRZs are found. The ESHT followed by peak aging treatment provides a new way to simultaneously exert the beneficial effects of fine grains and high number density nano-precipitates.

Effect of solute concentration on microstructure evolution during static recrystallization in Mg-0.2%Ce alloy using cellular automata

A new cellular automata model has been developed to evaluate the effect of solute drag on the microstructure and texture evolution during static recrystallization of Mg and its alloys in the temperature range of 200 – 400 °C. In the present model, the site saturation nucleation was assumed and all the nuclei are present at the grain boundaries. The growth of each nucleus depends on the solute concentration and dislocation density difference between neighboring grains. The microstructural analysis of the newly developed cellular automata model showed the evolution of grain size at different temperatures compared to the equilibrium microstructure containing the equilibrium concentration of solute atoms at the grain boundaries. Further, the texture evolution showed that there is an increase in the texture strength with an increase in the annealing temperature when the solute concentration was lower than the equilibrium concentration at the boundaries, whereas the texture strength decreased with increasing the annealing temperature when the solute concentration was higher than the equilibrium solute concentration at the boundaries. The present model shows that solute drag pressure increased continuously with increasing the annealing temperature when the solute concentration was lower than the equilibrium concentration at the boundaries. However, the solute drag pressure decreased with an increase in the annealing temperature when the solute concentration was higher than the equilibrium solute concentration at the boundaries.

7Be Recovery from Seawater by Sorbents of Various Types.

For the first time, a comprehensive study of sorbents based on manganese dioxide was carried out for beryllium sorption from seawater in laboratory and expeditionary conditions. The possibility of using several commercially available sorbents based on manganese dioxide (Modix, MDM, DMM, PAN-MnO2) and phosphorus(V) oxide (PD) for 7Be recovery from seawater for solving oceanological problems was evaluated. Beryllium sorption under static and dynamic conditions was studied. The distribution coefficients and dynamic and total dynamic exchange capacities were determined. Sorbents Modix (Kd = (2.2 ± 0.1) × 103 mL/g) and MDM (Kd = (2.4 ± 0.2) × 103 mL/g) showed high efficiency. The dependences of the degree of recovery on time (kinetics) and the capacity of the sorbent on the beryllium equilibrium concentration in solution (isotherm) were established. The data obtained were processed using kinetic models (intraparticle diffusion, pseudo-first and pseudo-second orders, Elovich model) and sorption isotherm equations (Langmuir, Freindlich, Dubinin-Radushkevich). The paper contains results of expeditionary studies to evaluate the sorption efficiency of 7Be from large volumes of the Black Sea water by various sorbents. We also compared the sorption efficiency of 7Be for the considered sorbents with aluminum oxide and previously obtained sorbents based on iron(III) hydroxide.

Removal of amoxicillin and ampicillin using manganese dioxide/montmorillonite composite

AbstractBACKGROUNDClay‐based materials represent great potential for the development of efficient and environmentally friendly sorbents. The study focuses on a laboratory‐obtained manganese dioxide/montmorillonite (MnO2/MMT) composite for removal of two types of antibiotics – amoxicillin (AMX) and ampicillin (AMP) – from aqueous solution.RESULTSThe composite was successfully prepared using a reduction procedure involving the reaction between potassium permanganate (KMnO4) and hydrochloric acid (HCl) to form MnO2 followed by the addition of MMT. X‐ray analysis, scanning electron microscopy, X‐ray fluorescence and Fourier transform infrared spectroscopy were performed for characterization of physicochemical and structural properties, simultaneous thermogravimetry and differential scanning calorimetry for estimation of thermal stability and high‐performance liquid chromatography for determination of antibiotic equilibrium concentrations in aqueous solution. The precipitated MnO2 component, manifested by long fibers, corresponded to the tunnel structure of cryptomelane. In the case of MnO2/MMT it is evident that MnO2 developed short fibers with the participation of the MMT matrix. The thermal data suggested that the MnO2 phase upon contact with the clay support showed better thermal stability. The final decomposition of Mn2O3 was shifted to higher temperature of 985 °C. Adsorption procedure in a batch regimen showed sufficient sorption ability for both antibiotics with over 90% efficiency. For AMP the value of qmax was about 45 mg g−1 and for AMX it was only 21 mg g−1.CONCLUSIONSThe results provided valuable information for the design of a potentially inexpensive clay‐based adsorbent and demonstrated the removal of two types of broad‐spectrum β‐lactam antibiotics from aqueous solution to a sufficient degree. © 2022 Society of Chemical Industry (SCI).

Influence of ionic strength and temperature on adsorption of tetrakis-N-methylpyridyl porphyrin onto mesoporous silica

In the present study, we examined the effects of ionic strength and temperature for adsorption of tetrakis-N-methylpyridyl porphyrin (TMPyP) on mesoporous silica with 7.0 nm in pore diameter. The adsorption of TMPyP in 10 mM phosphate buffer solutions containing 0–150 mM NaCl showed linear adsorption isotherm plotted as inverse of the adsorption amount of TMPyP versus inverse of the equilibrium solution concentration of TMPyP. From the analysis of the adsorption isotherm by electrostatically modified Langmuir model, decrease in apparent adsorption constant with increasing the ionic strength was ascribed to the surface charge screening by electrolyte ions. In the temperature dependence on the TMPyP adsorption, non-linear van’t Hoff plots were analyzed by including heat capacity change during adsorption. The results of the van’t Hoff analysis indicated that the TMPyP adsorption at low temperature was exothermic and enthalpy-driven reaction, whereas it became endothermic and entropy-driven reaction with temperature elevation. In addition, the surface diffusivity was suggested as a primary factor to govern the maximum adsorption amount of TMPyP on mesoporous silica.

Machine learning method for simulation of adsorption separation: Comparisons of model’s performance in predicting equilibrium concentrations

In this work, we implemented different models for predicting adsorption separation of a dye from aqueous solution using porous materials. The equilibrium data of solute concentrations were collected from resources and used in the models for training and verification purposes to develop the models. For prediction of the equilibrium solute concentrations (Ce), we used tree models: Multi-layer Perceptron (MLP), Passive aggressive regression, and Decision Tree (DT) Regressor. In the modeling, we considered the adsorbent dosage as well as solution pH as the input parameters to the model, and the model was able to generate the output values, i.e., equilibrium concentrations based on the input variables. The evaluation of the models’ performances revelated that the final R 2 scores are 0.99, 0.98, 0.93 for DT, MLP and Passive-Aggressive, respectively and a very low RMSE of 0.055 for decision tree that shows this model is the best among models used in this study. Indeed, decision tree model is recommended among the other three models to be employed for correlation of adsorption equilibrium data.

Artificial intelligence simulation of water treatment using a novel bimodal micromesoporous nanocomposite

Molecular separation using nanostructured materials have attracted much attention recently for various applications. Different classes of materials have been used among which metal organic framework (MOF) and layered double hydroxide (LDH) have been recently developed due to their superior structure in separation, specifically adsorption process. In this work, we have studied removal of dye from water using a nanocomposite of MOF/LDH. The method of investigation is development of an artificial intelligence-based model for prediction of the adsorption process. The adsorption data have been obtained for removal of a dye (orange II reactive dye) from water at different conditions. The model was proposed using artificial neural network for simulation of the adsorption output which was considered to be equilibrium concentration in the solution (Ce). Indeed, the equilibrium concentration of solute was assumed as the main output in developing the model, while two inputs were postulated including the pH and adsorbent dosage. The model was built considering two hidden layers in the neural network. The validation and training steps were carried out and statistical analysis indicated excellent agreement between the simulated and measured data possessing high coefficient of determination (R2 greater than 0.999). The model revealed to be a high-performance model for simulation of dye removal using adsorption process with high accuracy.

Stability of Darcy thermosolutal convection in bidispersive porous medium with reaction

AbstractAnalysis of fluid flow in a bidispersive medium with reactions is explored. The Darcy model is used in the momentum equation with the density being a linear function in temperature and salt concentration. Two cases are regarded which are heated below and salted above system and heated and salted below system. It is presumed that the equilibrium solute concentration follows a linear relationship to temperature. A linear instability and nonlinear stability theories are conducted, and from the outputs of the analysis, the Chebyshev collocation approach is used to solve the resulting eigenvalue systems. The effects of the chemical reaction and other parameters on temperature Rayleigh number are described graphically.

АДСОРБЦІЯ ФЕНОЛЬНИХ СПОЛУК АКТИВОВАНИМ ВУГІЛЛЯМ, УТВОРЕНИМ ПРИ КАРБОНІЗАЦІЇ БУРОГО ВУГІЛЛЯ З ГІДРОКСИДОМ КАЛІЮ

The aim of the work is to evaluate the adsorption capacity of activated carbons (ACs) from brown coal in relation to phenol (Ph) and 4-chlorophenol (CPh) and the influence of the AСs formation temperature under carbonization with potassium hydroxide on capacities. The samples of ACs were prepared by heating with KOH (1 g/g, 1 h) at a given temperature in the range of t=400-800°C and marked as AC(t). The ACs porosity characteristics were determined by low-temperature (77 K) adsorption – desorption nitrogen isotherms (Micromeritics ASAP 2020) calculated by the 2D-NLDFT method. They are as follows: total pore volume Vt (cm3 /g), specific surface area S (m2 /g), volume (Vmi) and surface (Smi) of micropores, volume (V1nm) and surface (S1nm) of subnanopores, the total surface of meso- and macropores Sme+ma. The adsorption of phenol and 4-chlorophenol was determined at equilibrium concentrations in aqueous solutions ≤5 mmol/l (25 °C). The alkaline carbonization temperature of brown coal was found to be a key factor in the formation of micro- and subnanopores, the growth of the AC specific surface area (from 12.8 m2 /g to 1142 m2 /g) and adsorption activity against phenolic compounds. Its increase to 800°C causes an exponential increase in the AC adsorption capacity in 8.7 times (Ph) and 6.7 times (CPh), which is proportional to the concentration of surface adsorption centers (AdCs). The values of the effective activation energy of forming AdCs being active in relation to adsorbates were determined as 29.5 kJ/mol (Ph) and 31.5 kJ/mol (CPh). The kinetics of Ph and CPh absorption was found to obey the pseudosecond-order model, and the adsorption rate is limited by the interaction of the adsorbate molecules with the AdCs. Adsorption isotherms at equilibrium concentrations ≤ 5 mmol/l are approximated by the Langmuir model (R2 ≥ 0.994). Compared with Ph, the degree of CPh extraction is much higher, which is a consequence of its stronger connection with the AC surface. The specific adsorption capacity for Ph and CPh shows a sharp decrease (10-16 times) with increasing carbonization temperature from 400° C to 550 °C and a weak temperature dependence at 550-800 °C. In this range, ACs are formed with similar concentrations of AdCs, but different for various phenolic compounds. Adsorption on brown coal ACs was postulated to include π-π interaction, formation of electron-donor-acceptor complexes and formation of hydrogen bonds, but their contributions depend on adsorbate nature and they change while increasing alkaline carbonization temperature. Keywords: brown coal, alkaline carbonization, activated carbon, porosity, adsorption, phenol, 4-chlorophenol. Corresponding author Таmarkina Ju.V., e-mail: Tamarkina@nas.gov.ua

Synthesis of poly (hydroxamic acid) ligand for removal of Cu (II) and Fe (II) ions in a single component aqueous solution

Biosorbents are the alternatives for the removal of heavy metals because they are cheaper and more effective. This is attributed to the metallic elements interactions with the functional groups on the surface of the adsorbent. Additionally, the surface of biosorbents can further be improved by pretreatment and modification leading to the formation of functional derivatives onto the cellulose backbone. In this study pine cone powder was pretreated using Fenton reagent and modified by grafting with acrylic acid in order to enhance structural characteristics of the material. Grafting of acrylic acid onto Fenton treated pine cone powder was achieved by a free radical procedure in which ceric ammonium nitrate (CAN) was used as an initiator. Furthermore, a new poly (hydroxamic acid) ligand was synthesized from the poly(acrylic acid) grafted pine cone powder for heavy metal ions removal from aqueous media. The observed FTIR bands that shifted from 1727 cm−1 shifted to 1616 cm−1 confirmed that ester functional groups were converted to hydroxamic acid groups, which was further confirmed by elemental analysis results. The poly (hydroxamic acid) ligand was further characterized by TGA/DTA, XRD, SEM-EDS techniques. The TGA/DTA results revealed that the poly(acrylic acid) grafted pine cone powder had better thermal stability. The XRD spectra revealed that the structure of pine cone powder is composed mainly of crystalline cellulose. The SEM results indicated that the grafted pine cone powder showed a smooth surface, which is an indication that the acrylic acid was coated on the surface of the Fenton treated pine cone powder. It further revealed a coarse surface the poly (hydroxamic acid) ligand confirming the conversion of the carboxyl group to hydroxamic acid group. The linear Langmuir isotherm plot gave an R2 values of 0.9997 for Cu (II), 0.9961 Fe (II) biosorption while Freundlich isotherm plot R2 values were 0.9581 for Cu(II) and 0.9671 Fe (II) biosorption. These results demonstrate that the Freundlich isotherm was not a good fit to describe the process of adsorbed metal ions and their equilibrium concentration in solution. The theoretical values qe (mg g−1) of the pseudo second order model compared with those for the pseudo first order kinetic model agree with the experimental value qe (mg g−1). Therefore, these results confirm the chemisorption of Cu (II) and Fe (II) onto poly (hydroxamic acid) ligand.

Thermodynamic Analysis of Zinc Sulfide Dissolution Stoichiometry in Sulfuric Acid Solution with Oxygen Participation

In this work, a thermodynamic study of zinc sulfide high temperature oxidative leaching was carried out. At the metals sulfides dissolution under oxidants action in acidic solutions several simultaneously proceeding reactions are possible. With the aim finding the proportion of potential reactions, a thermodynamic calculation was carried out using stoichiometric equations with equal oxidant consumption. Moreover, stoichiometric coefficients were chosen in such a way that reagents could exchange 1 mole of electric charge. Such an approach allows one to ensure a comparison of different oxidants’ usage effectiveness for sulfide leaching. Thermodynamic analysis results are in accordance with experimental data, which confirm that reactions with the formation of sulfur and sulfate-ions are uppermost at zinc sulfide dissolution in sulfuric acid solutions under oxygen action. Oxygen consumption and initial sulfuric acid concentration influence on proportion of this reactions and zinc equilibrium concentration in solution were investigated. Thermodynamic analysis showed that at insufficient acid concentration, which limits the maximum degree of zinc sulfide oxidation reaction advancements with sulfur formation, oxygen is consumed on oxidation reaction with the formation of sulfate-ions, which spend oxygen less effectively, due to four times smaller zinc dissolution. Thermodynamic calculations carried out allowed us to find out optimal proportions of oxygen consumption and initial sulfuric acid concentration, which provide achievement of a maximal zinc equilibrium concentration in solution at a higher effectiveness of oxidant consumption without labor-intensive experiments. At optimal acid consumption, a zinc equilibrium concentration is directly proportional to initial acid concentration. Directly proportional dependence of zinc cations’ formation on acid consumption was also detected at acid concentration optimal values.

Interface kinetics of rapid solidification of binary alloys by atomistic simulations: Application to Ti-Ni alloys

Using molecular dynamics and Monte Carlo simulations we investigate the non-equilibrium interfacial kinetics during rapid solidification of Ti-Ni alloys. According to the existing theories, the kinetic coefficient is related, via an analytical expression, to the equilibrium and non-equilibrium solute concentration profiles across the crystal-melt interface, the interface temperature and velocity, and the drag coefficient. The kinetic coefficient was obtained by deriving these properties from specifically designed atomistic simulations and then fitting using the analytical expression. The results show that the kinetic coefficient is only weakly anisotropic and increases with increasing temperature. The velocity-dependent partition coefficient, as described by two solute trapping models, the continuous growth and the local non-equilibrium models, were fitted to the molecular dynamics simulation results. In addition, molecular dynamics and semi-grand canonical Monte Carlo simulations suggest that complete solute trapping might occur only under certain conditions. This can be explained by the fact that the maximum achievable effective free energy driving force for solidification, which defines the complete solute trapping, is limited by the chemical potential and free energy profiles for the Ti-Ni alloy. The investigations, using molecular dynamics simulations, of the dependence of crystal-melt interface width on solidification velocity show, at high velocities, similar trend to that predicted by the hyperbolic phase field model which is suitable for studies of rapid solidification of alloys.

Synthesis and Characterization of Al-Pillared Bentonite for Remediation of Chlorinated Pesticide-Contaminated Water

AbstractThe removal of pesticide contaminants from water is a key priority in environmental remediation, and requires intensive effort; this necessitates modification of the properties of pillared clays (PILCs) such as porosity, pore-volume, surface area, and synthesis methods. The purpose of the present study was to test the ability of Al-pillared bentonite (Al-PILB), using [Al13O4(OH)24(H2O)12]7+ and [Al30O8(OH)56(H2O)24]18+ (keggin cations, Al13 and Al30) as pillars, to adsorb chlorinated pesticides from contaminated water. In order to maximize intercalation and uniformity of layer stacking, various ratios of the nitrate forms of the synthesized keggin cations were intercalated into the natural bentonite (BT). The synthesized materials (Al-PILBs) were characterized by various techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), Fourier-transform infrared (FTIR) spectroscopy, UV-Vis spectroscopy, and N2 adsorption-desorption measurements. Increases in basal spacing, surface area, and pore volume were observed. The adsorption capacity of the Al-PILBs for 17 types of chlorinated pesticides from contaminated water was better than using the BT alone, e.g. for heptachlor epoxide, dieldrin, and endrin at natural pH, the maximum adsorptions obtained at equilibrium solution concentrations of 16, 20, and 20 μg/L, respectively, were 59.2, 59.15, and 60 μg/g, whereas corresponding values using pristine BT were 34.68, 39.45, and 38.9, respectively. The data were best described by the Freundlich adsorption model.

Utilization of Non-Living Microalgae Biomass from Two Different Strains for the Adsorptive Removal of Diclofenac from Water

In the present work, the adsorptive removal of diclofenac from water by biosorption onto non-living microalgae biomass was assessed. Kinetic and equilibrium experiments were carried out using biomass of two different microalgae strains, namely Synechocystis sp. and Scenedesmus sp. Also, for comparison purposes, a commercial activated carbon was used under identical experimental conditions. The kinetics of the diclofenac adsorption fitted the pseudo-second order equation, and the corresponding kinetic constants indicating that adsorption was faster onto microalgae biomass than onto the activated carbon. Regarding the equilibrium results, which mostly fitted the Langmuir isotherm model, these pointed to significant differences between the adsorbent materials. The Langmuir maximum capacity (Qmax) of the activated carbon (232 mg∙g−1) was higher than that of Scenedesmus sp. (28 mg∙g−1) and of Synechocystis sp. (20 mg∙g−1). In any case, the Qmax values determined here were within the values published in the recent scientific literature on the utilization of different adsorbents for the removal of diclofenac from water. Still, Synechocystis sp. showed the largest KL fitted values, which points to the affinity of this strain for diclofenac at relative low equilibrium concentrations in solution. Overall, the results obtained point to the possible utilization of microalgae biomass waste in the treatment of water, namely for the adsorption of pharmaceuticals.

Quantum chemical calculations and molecular modeling for methylene blue removal from water by a lignin-chitosan blend

A comprehensive analysis of the molecular dynamics and quantum chemical calculation methods for methylene blue removal by adsorption were carried out. The experimentally measured adsorption data of methylene blue from water effluent on a novel lignin and chitosan blend were used to validate the modeling results. The method and required steps for application of quantum chemical calculations and molecular modeling techniques as the state of the art approach for predictive modeling and investigation of adsorption were provided, and the model findings were validated by comparing with the experimental data; close agreement between the results was observed. The conductor-like screening model (COSMO) approach was used for calculation of equilibrium concentration of solute at the adsorbent-solute interface. The results revealed that the approach is highly recommended as it provides an alternative assessment for feasibility analysis before any experiments to be undertaken, resulting in time and cost saving.

Electrokinetic Characteristics of Porous Glasses in Solutions of Sodium and Iron(III) Chlorides

The structural (structural resistance coefficient, bulk porosity, average pore radius, and specific surface area) and electrokinetic (surface conductivity and electrokinetic potential) characteristics of high-silica micro- and macroporous glasses produced from two-phase sodium borosilicate glass have been compared in solutions of an indifferent electrolyte (sodium chloride) and iron(III) chloride containing iron ions, which have a high specificity to silica surfaces. It has been shown that, in the presence of iron ions, the electrokinetic behavior of porous glasses is governed by two factors. First, the superequivalent adsorption of these ions in the Stern layer leads to positive values of the electrokinetic potential, and, second, their mobility in the pore space decreases, thereby resulting in the appearance of equilibrium solution concentration ranges, in which the specific conductivity of a pore solution becomes lower than that of a free solution.

Peculiarities of transport-structural parameters of ion-exchange membranes in solutions containing anions of carboxylic acids

The experimental results of the study of the concentration dependence of the electrical conductivity and diffusion permeability of ion-exchange membranes which are in contact with solutions of weak electrolytes (carboxylic acids) are provided for the first time. An attempt was made to correlate the observed properties of the membrane to their structure, based on the microheterogeneous model. Some features have been revealed that differentiate the behavior of membranes in solutions of strong and weak electrolytes. For example, for anion-exchange membranes, the dependence of the electrical conductivity on the equilibrium solution concentration disappears in the concentration above 0.05 M, and there is an insignificant increase in the electrical conductivity in lower concentrations. An explanation for the observed effects is suggested which takes into account that the electrical conductivity of the gel phase is much higher than the electrical conductivity of an electrically neutral solution. An equation is proposed which connects the electrical conductivity of a membrane with its ion-exchange capacity and mobility of the counterion in the gel phase.Recommendations on the practical aspect of electrodialysis technology for the treatment of weak electrolytes are given. From a fundamental point of view, it is concluded that for a full description of the transport-structural organization of ion-exchange membranes which are in a solution of a weak electrolyte, the knowledge of the concentration dependence of the electrical conductivity and diffusion permeability only is not enough, and other experimental methods are required.

SILICON BEHAVIOR IN SOILS CONTAINED DIFFERENT SILICON AND PHOSPHORUS CONCENTRATIONS USING ADSORPTION MODELS [

Silicon (Si) is an element, not essential, but is beneficial for some plants. The Freundlich model was used to describe Si adsorption on soil samples incubated with different Si and phosphorus (P) concentrations. Soil incubated samples were: T1 soil had no Si or P (control); T2, T3 and T4 soils contained 50, 100, and 200 mg Si L-1, respectively. T5 and T6 soils contained 50 mg Si L-1 in combined with either 7 or 10 mg P L-1 along with T7 and T8 soil contained 100 mg Si L-1 in combined with either 7 or 10 mg P L-1. In final, T9 and T10 soil contained 200 mg Si L-1 in combined with either 7 or 10 mg P L-1. A series of adsorption experiments were performed using sodium metasilicate pentahydrate (Na2O3Si.5H2O) solution prepared to have concentrations representing 0, 14, 28, 42 and 56 mg Si L-1. The supernatant of Si concentration was determined. The amount of element adsorbed was calculated as the difference between applied element concentrations and that remaining in solution after equilibration. Adsorption isotherms were determined at room temperature (25 oC±1). Results revealed that a positive trend was generally found; increases in amount of adsorption onto soil with increasing Si concentration and equilibrium concentration in concerned solution either applied separately or applied + initial available Si concentration in soil. The Freundlich equation provides a good fit to the sorption data for all incubated soil samples and R2 values were ranged from 0.82 to 0.97. Present study indicated that adsorption capacity value (Kf) decreased from T1 to T3 soil samples then increased at T4 sample, and intensity adsorption values (1/n) gave almost an opposite trend to that of capacity adsorption (Kf) values. Moreover, adsorption of Si onto soil incubated with different Si concentrations in combined with P2 decreased as compared to P1. Opposite trend was obtained with equilibrium Si concentration in concerned solution. Also, present study showed that the higher values of Kf obtained in P1 soils (T5, T7 and T9), compared to Kf values of P2 soils (T6, T8 and T10), and intensity adsorption values (1/n) gave almost an opposite trend to that of capacity adsorption (Kf) in both P1 and P2 soils. Finally, large Si sorption capacity and low Si affinity for the surface sites were observed in soil incubated with high Si concentration compared to soil incubated with low ones.

Relationships between CO2, thermodynamic limits on silicate weathering, and the strength of the silicate weathering feedback

Recent studies have suggested that thermodynamic limitations on chemical weathering rates exert a first-order control on riverine solute fluxes and by extension, global chemical weathering rates. As such, these limitations may play a prominent role in the regulation of carbon dioxide levels (pCO2) over geologic timescales by constraining the maximum global weathering flux. In this study, we develop a theoretical scaling relationship between equilibrium solute concentrations and pCO2 based on equilibrium constants and reaction stoichiometry relating primary mineral dissolution and secondary mineral precipitation. We test this theoretical scaling relationship against reactive transport simulations of chemical weathering profiles under open- and closed-system conditions, representing partially and fully water-saturated regolith, respectively. Under open-system conditions, equilibrium bicarbonate concentrations vary as a power-law function of pCO2(y=kxn) where n is dependent on reaction stoichiometry and k is dependent on both reaction stoichiometry and the equilibrium constant. Under closed-system conditions, bicarbonate concentrations vary linearly with pCO2 at low values and approach open-system scaling at high pCO2. To describe the potential role of thermodynamic limitations in the global silicate weathering feedback, we develop a new mathematical framework to assess weathering feedback strength in terms of both (1) steady-state atmospheric pCO2 concentrations, and (2) susceptibility to secular changes in degassing rates and transient carbon cycle perturbations, which we term 1st and 2nd order feedback strength, respectively. Finally, we discuss the implications of these results for the effects of vascular land plant evolution on feedback strength, the potential role of vegetation in controlling modern solute fluxes, and the application of these frameworks to a more complete functional description of the silicate weathering feedback. Most notably, the dependence of equilibrium solute concentrations on pCO2 may represent a direct weathering feedback largely independent of climate and modulated by belowground organic carbon respiration.