Feed Ionic Strength Research Articles

Heavy Metal Removal from Aqueous Solutions Using a Customized Bipolar Membrane Electrodialysis Process.

Lack of safe water availability and access to clean water cause a higher risk of infectious diseases and other diseases as well. Heavy metals (HMs) are inorganic pollutants that cause severe threats to humans, animals, and the environment. Therefore, an effective HM removal technology is urgently needed. In the present study, a customized bipolar membrane electrodialysis process was used to remove HMs from aqueous solutions. The impacts of the feed ionic strength, applied electrical potential, and the type and concentration of HMs (Cd2+, Co2+, Cr3+, Cu2+, and Ni2+) on the process performance were investigated. The results showed that feed solution pH changes occurred in four stages: it first decreased linearly before stabilizing in the acidic pH range, followed by an increase and stabilization in the basic range of the pH scale. HM speciation in the basic pH range revealed the presence of anionic HM species. The presence of HMs on anion exchange membranes confirmed the contribution of these membranes for HM removal in the base channels of the process. While no clear trend was seen in the ionic strength solution, the maximum HM removal was observed when 1.5 g/L NaCl was used. The initial HM concentration showed a linear increase in HMs removal of up to 30 mg/L. A similar trend was seen with an increase in the applied electrical potential of up to 15 V. In general, the amount of HMs removed increased in the following order: Cd2+ ˃ Ni2+ ˃ Co2+ ˃ Cu2+ ˃ Cr3+. Under some operational conditions, however, the removed amount of Cu2+, Co2+, and Ni2+ was similar. The mass balance and SEM-EDX results revealed that the removed HMs were sorbed onto the membranes. In conclusion, this process efficiently separates HMs from aqueous solutions. It showed the features of diluate pH adjustment, reduction in the overall stack electrical resistance, and contribution of anion exchange membranes in multivalent cation removal. The mechanisms involved in HMs removal were diffusion and migration from the bulk solution, followed by their sorption on both cation and anion exchange membranes.

Aminophosphonates in Nanofiltration and Reverse Osmosis Permeates.

Aminophosphonates such as aminotris(methylenephosphonic acid) (ATMP) are common constituents of antiscalants. In nanofiltration (NF) and reverse osmosis (RO) processes, ATMP prevents inorganic scaling leading to more stable membrane performance. So far, little attention has been paid to the possible permeation of aminophosphonates through NF and RO membranes. We have investigated the permeability of these membrane types for ATMP and its potential metabolites iminodi(methylenephosphonic acid) (IDMP) and amino(methylenephosphonic acid) (AMPA) with two different NF membranes (TS40 and TS80) and one RO membrane (ACM2) and three different water compositions (ultra-pure water, synthetic tap water and local tap water). We found traces of phosphonates in all investigated permeates. The highest phosphonate rejection occurred with local tap water for all three membranes investigated. Filtration experiments with a technical antiscalant formulation containing ATMP indicated similar trends of phosphonate permeability through all three membranes. We assume that the separation mechanisms of the membranes are the results of a very complex relationship between physico-chemical properties such as Donnan exclusion, feed pH, feed ionic strength and feed concentration, as well as solute–solute interactions.

Scaling sets the limits of large scale membrane distillation modules for the treatment of high salinity feeds

In this study, the dynamics of scaled-up membrane distillation (MD) modules are tackled for the treatment of highly saline desalination brines. Physical phenomena occurring inside the feed chamber during process scale-up including temperature evolution, species distribution and scaling likeliness were explored using a multicomponent computational fluid dynamics (CFD) model that couples momentum, heat, ions transport and water permeation across the membrane. The model was calibrated with experiments carried out on a lab-scale direct contact MD system fed with concentrated seawater with a salinity of 61 g/L. The complete fall-off of the permeate flux occurred when the salinity reached 170 g/L from 61 g/L, caused by a scaling mostly due to calcium sulfate (gypsum). In order to predict scaling occurrence, an in-house code is embedded in the CFD model to solve Pitzer’s equation at every cell of the domain, enabling the calculation of species activity coefficients, the feed ionic strength, species effective concentration and degree of saturation of the solution with respect to gypsum. Results unveil that during the MD process of brines, the degree of saturation increases considerably in membrane vicinity while the average outlet salinity remains close to that at the inlet due to the relatively high flow rate. Extrapolation to longer modules revealed that an increase in the feed temperature increases the scaling likeliness while flow rates, especially in the high range, did not significantly impact scaling formation. The drop in performance from lab-scale module to a scaled-up size is shown for 1 m long generic modules with and without the use of antiscalants.

Achieving low concentrations of chromium in drinking water by nanofiltration: membrane performance and selection.

This study evaluates nanofiltration as a feasible process to reach low concentrations of chromium in drinking water and provides means for the selection of the most suitable membrane based on the specific treatment needs. Chromium removal is concerning since new stringent limits (10μg/L) for hexavalent Cr concentration in potable water were recently adopted in various countries. Three commercial nanofiltration membranes were tested against this threshold value: two membranes made of semi-aromatic polyamide and the third having a sulfonated polyethersulfone asymmetric film as the selective layer. The rejection observed as a function of chemical composition in the feed solution suggests that electrostatic effect is an important mechanism of chromium(VI) removal for the membranes with higher surface charge and lower film density. The performance of such membranes is strongly affected by the presence of salts, especially divalent cations, which reduce both Cr(VI) rejection and the permeate flux. The removal of Cr(VI) by denser membranes is dominated by solution-diffusion and is not influenced by feed ionic strength. The exposure of membranes to high chromium concentrations and to hypochlorite, typically employed as an oxidizing agent in water treatment plants, was also investigated. An analysis of the operational membrane life is thus discussed, based on the loss in performance due to active film degradation. All three membranes showed adequate rejection of chromium from tap and well water of diverse chemical composition, suggesting that nanofiltration is an effective process to remove chromium for the production of safe drinking water. However, membranes with different properties should be adopted depending on specific feed water composition and on the productivity required from the system. A final analysis is presented to help with the choice of the most suitable nanofiltration membrane based on initial and target Cr(VI) concentration in feed and product water, respectively.

Influence of Feed-Electrode Concentration Differences in Flow-Electrode Systems for Capacitive Deionization

The common performance metrics ascribed to flow electrode capacitive deionization systems can vary significantly depending on the mode of operation and initial system conditions. Through varying the flow electrolyte ionic strength, performance values such as average salt adsorption rate and energy consumption can vary by as much as 51 and 55%. This variability from cycle to cycle is in part due to changes in the electrical conductivity (ohmic) but is also due to the introduction of competing transport processes. Diffusive transport can enhance or diminish the rate of desalination and energy recovery, creating a larger degree of error in reported values. Here, we propose a dimensionless ratio comparing diffusion and electromigration-based transport to measure performance stability. Unstable system performance is most prominent when the feed ionic strength does not match the flow electrode ionic strength. In this scenario, the ratio of the diffusion to electromigration transport reached a maximum. Conversel...

Impact of feed ionic concentration on colloidal and organic fouling of osmotically driven membrane process

The impact of feed ionic concentration in the presence of organic and colloidal foulant on osmotically driven membrane processes (ODMPs) was demonstrated. The normalized flux of ODMPs was significantly decresed in both DI and 10 mM NaCl feed solution when the feed solution contained colloidal particles as foulants due to the deposition of colloidal particles on the membrane surface. The deposited colloidal particles strongly promoted cake-enhanced osmotic pressure near the surface of the membrane. In contrast, the normalized flux of ODMPs was slightly increased when humic acids (HA) were added as the organic foulant into the feed solution. The attachment of HA molecules changed the membrane surface conducting it more hydrophilic, and induced a stronger diffusion of water molecules into the membrane active layer. As a result, both the increased feed ionic strength and deposited colloidal fouling did not significantly alter the reversal salt selectivity of the ODMPs, while the reversal salt selectivity was significantly incresed after the addition of HA molecules to the feed solution because the HA molecules changed the properties of the membrane surface.

Nitrate removal from aqueous solution by direct contact membrane distillation using two different commercial membranes

In the present work, the removal of nitrate from aqueous solution by direct contact membrane distillation (DCMD) using flat sheets polypropylene (PP) and polyvinylidene fluoride (PVDF) membranes was studied. Effect of operating parameters, such as feed temperature, feed flow rate, initial nitrate concentration, feed ionic strength, and competing co-existing anions on permeate flux and nitrate rejection, was investigated. In all DCMD experimental runs, an almost complete nitrate rejection was achieved (higher than 99.90%) and the permeate nitrate concentration was largely below the maximum permissible level in potable water. Under the same operating conditions, PVDF hydrophobic membrane showed a higher permeate flux of 37.21 L/m2h than PP membrane with a permeate flux of 4.12 L/m2h. For both the membranes, feed temperature is the important operating parameter which enhanced exponentially the permeate flux. Likewise, a positive effect on permeate flux was found when feed flow rate was increased. However, no significant effect was found by varying initial nitrate concentration, feed ionic strength, and the presence of co-existing anions on DCMD permeate flux or nitrate rejection efficiency.

Electrodialytic Removal of Cadmium from Brackish Water: Effects of Operating Parameters

The continuous increase of environmental regulations make interesting to find effective and efficient methods for processing effluents containing metal ions. This research focuses on cadmium removal from brackish water by an electro-membrane process: The electrodialysis. Experiments were carried out on synthetic brackish water solutions and using a laboratory scale electrodialysis system. The influence of several parameters on process efficiency was investigated. The efficiency of this process was assessed by the determination of five parameters: The demineralization rate, the removal rate and the transport flux of cadmium, the current efficiency and the specific power consumption. The applied voltage, the feed flow rate, the pH and cadmium initial concentration of the feed solution have a significant effect on the process efficiency and mainly on the cadmium transfer from dilute to concentrate compartment. In contrast, feed ionic strength seems to affect only the SPC and not the R(Cd).

Application of response surface methodology for modeling and optimization of membrane distillation desalination process

In this work, response surface methodology (RSM) was applied for modeling and optimization of operating parameters for water desalination by direct contact membrane distillation (DCMD) process using polypropylene membrane (PP) with low pore size. Operating parameters including vapor pressure difference, feed flow rate, permeate flow rate and feed ionic strength were selected and the optimum parameters were determined for DCMD permeate flux. The developed model for permeate flux response was statistically validated by analysis of variance (ANOVA) which showed a high value coefficient of determination value (R2=0.989). The obtained optimum operating parameters were found to be 0.355×105Pa of vapor pressure difference, feed flow rate of 73.6L/h, and permeate flow rate of 17.1L/h and feed ionic strength of 309mM. Under these conditions, the permeate flux was 4.191L/(m2h). Compared to a predicted value, the deviation was 3.9%, which confirms the validity of the model for the DCMD process desalination optimization. In terms of product water quality, the DCMD process using hydrophobic PP membrane can produce high quality of water with low electrical conductivity for all experimental runs.

Iron removal from brackish water by electrodialysis

The aim of this work is to study the removal of iron from brackish water using electrodialysis (ED). Experiments were carried out on synthetic brackish water solutions using a laboratory-scale ED cell. The influence of several parameters on process efficiency was studied. This efficiency is expressed by the removal rate, transport flux, current efficiency and power consumption. The applied voltage, the feed flow rate, the pH and iron initial concentration of the feed solution have a significant effect on the process efficiency and mainly on the iron transfer from dilute to concentrate compartment. Nevertheless, feed ionic strength does not have an effect on the iron removal. However, the effect is only noted on the specific power consumption.

The effect of feed ionic strength on salt passage through reverse osmosis membranes

Several parameters are known to influence the passage of salts through a reverse osmosis membrane. These parameters include characteristics of both the membrane and the feed water. Of these parameters, the least understood is the effect feed water concentration has on salt passage. At high and very low feed salinities, salt passage can increase by a factor of two or more. As an increasing number of RO systems are designed to treat water at these salinity extremes, a better understanding of this salinity effect is necessary to accurately predict the permeate quality of these systems. This study seeks to demonstrate and characterize the salinity effect on different RO elements treating different feed waters. The magnitude of the salinity effect at any given feed salinity is shown to be influenced by membrane charge and feed water composition. The results of the study on individual elements are used to accurately predict the salt passage in an existing full scale RO system.

Phenomenological theory of pressure-driven transport of ternary electrolyte solutions with a common coin and its specification for capillary space—charge model

Pressure-driven transport of ternary electrolyte solutions have been considered within the scope of irreversible thermodynamics. At small thermodynamic forces, a general solution for an n-ionic system has been obtained and, further, specified and examined for a ternary system with a common coion. With the help of the continuous version of irreversible thermodynamics, two further limiting cases have been considered, namely, one of large Peclet numbers and one of a small addition of one of the electrolytes. Thermodynamic results have been exemplified within the scope of the capillary space—charge model. Parameters which govern the separation of counterions have been examined as functions of fixed charge density, pore size, feed ionic strength, the ratio of counterion mobilities and charges. A significant separation of counterions with different mobilities and/or charges was predicted.

Pressure-driven transport of ternary electrolyte solutions with a common coion through charged membranes. Numerical analysis

Pressure-driven transport of ternary electrolyte solutions with a common coion through charged membranes has been studied numerically for the homogeneous membrane model. Considerable changes in mixture composition have been predicted. The principal features of separation phenomenon are an increase in selectivity with decreasing feed ionic strength and mole fraction of more mobile counterions, though in the case of multiple-charged counterions with a lower mobility, opposite trends and dependencies with extrema also occur. It has been shown that adding electrolytes with less mobile counterions may improve the rejection of a starting electrolyte. The numerical analysis is in agreement with available analytical solutions and experimental data.