Electrodialytic properties of aromatic and aliphatic type hydrocarbon-based anion-exchange membranes with various anion-exchange groups

Anion-exchange membranes (AEMs) with various membrane structures were prepared by introducing various amines: trimethylamine (TMA), triethylamine (TEA), tri-n-propylamine (TPA) and tri-n-butylamine (TBA) into precursor membranes prepared from chloromethylstyrene (CMS)–divinylbenzene (DVB) and glycidyl methacrylate (GMA) – DVB. Their properties for ionic transport and anti-organic fouling were examined. Almost all of the prepared AEMs have excellent ionic transport properties: transport number of anions >0.94 and membrane resistance <4 Ωcm². The voltage change through the AEMs during electrodialysis operation using solutions containing sodium dodecylbenzene-sulfonate as a foulant indicated that there are three cases of fouling mechanism being related to membrane structure: (a) aliphatic AEMs show lower fouling than aromatic ones; (b) the lower the water content of an AEM, the more remarkable the fouling; (c) the longer the chain length of the alkyl groups of the anion-exchange groups of an AEM, the more remarkable the fouling.     

Separation of Calcium and Cadmium by Electrodialysis in the Presence of Ethylenediaminetetraacetic Acid

ABSTRACT

The separation of calcium and cadmium ions in a system containing ethylenedi-aminetetraacetic (EDTA) as the complexing agent has been studied using a laboratory-scale batch electrodialyzer. The theoretical distribution diagram constructed for the Ca—Cd—EDTA system suggested that cadmium preferably formed negatively charged complexes while calcium remained in the positively charged uncomplexed form. The experimental results confirmed that under the influence of an electric field, calcium was exclusively transported to the cathode while more than 90% of cadmium totally removed from the middle compartment of the batch electrodialyzer migrated toward the anode. The separation effect resulting from EDTA complexation was studied within the 1·l5–4·0 pH range.     

Evaluation of treatment and recovery of leachate by bipolar membrane electrodialysis process

Recovery studies are frequently carried out for electrodialysis (ED) processes. In this study, beyond examining the recovery of leachate components in an electrodialysis process, the use of that process to treat leachate-containing wastewater was simultaneously tested. Leachates were initially pre-treated (ultra filtration + cation exchange) to prevent clogging and harmful effect to the bipolar electrodialysis membranes. Optimum operating conditions were determined at the end of the experimental studies. Online observations during the electrodialysis process included the temperature-dependent reaction time, conductivity and changes of molar concentrations of H⁺ and OH⁻ ions in both the anolyte and catholyte compartments in which removed ions were collected. The most important contaminants in leachates are organic substances and nitrogen compounds. For this reason, representations of organic substances, such as the chemical oxygen demand (COD), and nitrogenous compounds, such as total Kheldahl nitrogen (TKN) and ammonia-nitrogen (NH₃-N), were also monitored in the electrodialysis effluent. Under the optimum operating conditions, removal of NH₃-N, TKN and COD were determined in the effluent at 96.2%, 92.8% and 86.7%, respectively. The conductivity value was determined to be 1.97 mS/cm at the end of the study.     

ELECTRO-DEIONIZATION OF Cr (VI)-CONTAINING SOLUTION. PART II: CHROMIUM TRANSPORT THROUGH INORGANIC ION-EXCHANGER AND COMPOSITE CERAMIC MEMBRANE

Cr (VI) transport through a composite ceramic membrane containing an ion-exchange component, namely xerogel of hydrated zirconium dioxide, was investigated. The diffusion coefficient of Cr (VI) species through the membrane, which has been determined under open circuit conditions, is 1.80 × 10^?10 m² s^?1. The transport number of Cr (VI) species through the ceramic membrane was found to rise with increasing voltage and reached 0.17 under “over-limiting current” conditions. On the other hand, the transport of chromate ions through hydrogel of hydrated zirconium dioxide becomes more intensive with a decrease in potential drop through the system involving ion-exchanger bed and ceramic membrane due to decrease in the membrane resistance. The diffusion coefficient of Cr (VI) ions in hydrogel of the inorganic ion exchanger was estimated as 4.36 × 10^?12 m² s^?1. A possibility of Cr (VI) removal from a weakly acidic diluted solution using an electro-deionization method was shown: the degree of solution purification was found to reach 50%. The transport of species is realized through both the solution and the ion exchanger.     

Studies on transport properties of short chain aliphatic carboxylicacids in electrodialytic separation

An electrodialysis process was employed to investigate the concentration of formic, acetic and propionic acid solutions under different experimental conditions. In this process interpolymer-type ion-exchange membranes were used due their high chemical stability and durability. The effects of concentration, electric current and time on the electrodialysis process were studied. A mathematical equation representing the water transport behavior of the electrodialysis process for concentrating acids under the influence of different current densities was developed. This equation is valid for any electrodialysis process with ion-exchange membranes used under similar operating conditions. The reliability of the water transport equation was tested through comparison with the experimental data. Results indicate that electrodialysis is an effective method for concentrating shorter chain aliphatic carboxylic acid solutions. Ionic transportation, power consumption and current efficiency data reveal the following trend of the electrodialysis process's efficiency during concentration of carboxylic acid solutions: formic acid > acetic acid > propionic acid.     

Exergy analysis of a combined RO, NF, andEDR desalination plant

A brackish water desalination plant in California that incorporates RO, NF, and EDR units was analyzedthermodynamically using actual plant operation data. Exergy flow rates were evaluated throughout the plant, and the exergy flow diagrams were prepared. The rates of exergy destruction and their percentage are indicated on the diagram so that the locations of highest exergy destruction can easily be identified. The analysis shows that most exergy destruction occurs in the pump/motor and the separation units. The fraction of exergy destruction in the pump/motor units is 39.7% for the RO unit, 23.6% for the NF unit, and 54.1 % for the EDR unit. Therefore, using high-efficiency pumps and motors equipped with VFD drives can reduce the cost of desalination significantly. The plant was determined to have a Second Law efficiency of 8.0% for the RO unit, 9.7% for the NF unit, and 6.3% for the EDR unit, which are very low. This indicates that there are major opportunities in the plant to improve thermodynamic: performance by reducing exergy destruction and thus the amount of electrical energy supplied, making the operation of the plant more cost effective.     

Effect of Operating Parameters on the Condensation of Ammonium Sulfate by Electrodialysis

The separation of ammonium sulfate from dilute solution by electrodialysis was investigated. From the results obtained, it is suggested that it is feasible to separate ammonium sulfate from aqueous solution by electrodialysis. The removal performance of ammonium sulfate obtained was very satisfactory. The optimal cation and anion membranes were found. The removal efficiencies were influenced by voltage, initial concentration, flow rate and temperature. High voltage and high temperature are beneficial to this separation process, but high fluid flow rate and concentrated initial concentration prolong the operation time required to achieve the target value. It was found that the optimal outlet concentration is 1 g/L, since the operation time is prolonged almost two‐fold if the outlet concentration is decreased from 1 g/L to 0.5 g/L.     

Feasibility studies of discontinuous electro-regeneration processes in environmentally-friendly plating for chromate separation from a binary system

Project work was carried out for feasibility and possible design of semi-industrial electro-regeneration units for diluted waters typical for rinse water systems in electroplating industry. Firstly, one subject of interest was the removal of CrO₄²⁻ ions in the concentration range between 0.06 mol m⁻³ and 2.3 mol m⁻³. Ion exchange capacity of several resins under room temperature conditions was tested. A 2-compartment cell with diaphragm was found to be a suitable construction for electrochemical resin regeneration after previous discontinuous loading. The determination of resin conductivities and transport numbers was also part of the studies. Results showed sufficient high current efficiencies up to 70%. Nevertheless, low ion mobility in the resin may limit the removal rate for continuous electrodeionization. In this case, stepwise operation mode in loading and regenerating the resin bed is the only solution. Energy consumption can be lowered by using catholytes of previous electro-regeneration steps. Specific energy consumption until 0.3 kWh per treated cubic meter is to be expected.     

高分子分离膜及其在水处理技术中的应用

介绍了常见的几种高分子分离膜的特点以及在水处理领域的应用情况.     

Candidate membranes for the electrochemical salt-splitting of Sodium Sulfate

This work examines the properties of selected cation and anion exchange membranes for use in the electrochemical separation of sodium sulfate into sodium hydroxide and sulfuric acid. The effect of membrane type, electrolyte composition and ion exchange membrane thickness on the current efficiency was investigated. A decrease in current efficiency was observed due to ion migration under the influence of the electric field, and, to a lesser extent, diffusional losses. Increasing the membrane thickness improves the current efficiency at the cost of higher power due to the higher cell potential.