This is a review of studies on ion-exchange membranes preferentially permselective for monovalent ions, which are used for electrodialytic concentration of sea water. Divalent ions (Ca2+, Mg2+, and SO42-, etc.) should be rejected at the membrane surface not to permeate through the membranes, in order to inhibit formation of scale consisting of CaCO3 and CaSO4, etc. The preferential permselectivity of the cation-exchange membranes is endowed by forming a very thin layer of polycation on the membrane surface. In the case of the anion-exchange membranes, SO42- is less able to penetrate into the embranes than Cl-, and it is considered that the anion-exchange membranes are modified to make their structure more compact although the details are obscure.
An ion-sieve manganese oxide was prepared by extracting magnesium ions from Mg2MnO4 using (NH4)2S2O8 as an extracting agent. The preparation conditions of the ion-sieve adsorbent were studied. Mg2MnO4 could be obtained by heating a mixture of MnOOH and Mg(OH)2 in a molar ratio of 1:2 at 900°C for 2.5h. The extractability of Mg2+ reached above 96% by the treatment of Mg2MnO4 with 1-M (NH4)2S2O8 solution at 90°C for 2h, while the dissolution of Mn was less than 0.2%. The adsorbent obtained showed the lithium adsorptivity from sea water; the equilibrium adsorptivity of lithium reached 6.4mg/g.
The permselective characteristics of monovalent cation permselective membrane were evaluated, and the following results were obtained. 1) The relative transport number is slightly dependent on the salt concentration. 2) The concentration ratio of monovalent cations to total cations in the brine increases with increasing the current density and lowering the feed seawater temperature. 3) The decrease in ratio of sodium chloride in the brine is 1.0% for three years.
A dried sample of 10-15g was ground by a vibrating mill for 4min, and pressed into an aluminum ring (40mm i. d., 5mm high) under a total pressure of 30 ton to make a sample briquet. Peak intensities for BrKα and ClKα of the briquet were measured by the method of fixed time, 100 and 10s, respectively. Background intensities were also measured and subtracted from the peak intensities to get net intensities. Standard samples for calibration curves were prepared by addhig known amounts of sodium bromide to a high-purity sodium chloride. Linear calibration curves were obtained of both BrKα intensity and BrKα/ClKα intensity ratio vs. bromine concentration of up to 700ppm. Relative standard deviation was about 1.5% for determination of a refined salt containing 65ppm bromine, and the detection limit of bromine was estimated as about 3ppm. The pulverization and mixing of samples were critical. Analytical results by the proposed method agreed well with those by ion chromatography.
Determination of cadmium in environmental water was studied by a coprecipitation method with diethyldithiocarbamate (DDTC) and zinc (II) and the subsequent graphite furnace atomic absorption spectrometry (GFAAS). One milligram of zinc and 5 ml of 10 w/v% ammonium carbamate buffer were added to sample water containing cadmium, the pH of which was adjusted to 7.5-8.0, followed by adding 5 ml of 2 w/v% DDTC solution. The precipitated DDTC complex including cadmium was collected onto a membrane filter (0.45μm), and then dissolved by heating with 25 ml of nitric acid (1+40). The solution was used as the sample solution for GFAAS. The detection limit for cadmium was 0.005μg/l, the lower limit of determination was 0.016μg/l in the case of 500 ml of sample water. This method was not influenced by sample volume, and was satisfactorily applied to the determination of cadmium without any interfering effects of the copresent ions in environmental water.
A dialytic battery of flow type was constructed; 41 compartments were made with 20 sets of cation-exchange membranes and anion-exchange membranes, each having an area of π cm2 and Ag-AgCl electrodes were used for the anode and cathode. Output voltage and current of the battery were measured with three concentration levels of concentrated aqueous NaCl solution, 30, 14, and 3.8 g/l, and several levels of dilute solutions. The analysis of the result obtained shows that internal electric resistance due to the resistance of the dilute solution compartment is very large at low level of concentration and that the output power obtained is about a half of that predicted due to the short circuit which occured in the connecting line. When the battery is scaled up enough to diminish the effect of short circuit, the output power will be obtained as 0.6 W/(m2 of a pair of ion-exchange membranes) in the case of concentration of the concentrated solution 30 g/l and that of the dilute 3g/l.
A 5-MGD MSF seawater desalination plant built in 1970 in Saudi Arabia was rehabilitated in 1989 with the largest reverse osmosis unit in the world. Commercial operation started in May 1989 and at present maintains a constant recovery ratio of 35 percent with 15-MGD production. Performance characteristics of 10-inch hollow fiber membranes are far superior in respect to the quality and quantity of the permeate.