Electrodialysis of Deep Sea Water for the Production of Value Added Vegetables – Engineering Essay
Abstract – The present research was carried out to apply deep sea water to hydroponics in order to improve the yield and nutritional value of vegetables. Deep sea water was electrodialyzed using a selective membrane
cartridge for the exchange of monovalent ions, and then it was used to prepare nutrient solutions for the hydroponical production of spinach.
Experimental results showed that sodium chloride was removed selectively from deep sea water by electrodialysis. The highest yield of spinach with a normal content of nutritional components was obtained when the electrodialyzed deep sea water of 5 dS/m was applied in nutrient solution. Increasing the salinity of electrodialyzed deep sea water caused an increase in the total ascorbic acid content but a decrease in the yield of spinach.
Keywords: deep sea water, electrodialysis, ion concentration, hydroponics, nutrient solution, vegetable, yield, nutritional value
Deep sea water contains more than 80 kinds of inorganic elements (Nozaki, 1997) and some organic substances. It is almost free of harmful bacteria and has a constant temperature of 4-6 ? at a depth of 500 m. These characteristics have brought deep sea water many applications in marine products industry, food industry and health drink manufacturing (Watanabe, 2000; Yamaoka, 2000; Nakagawa, 2000).
Vegetables are produced hydroponically using nutrient solution that usually contains only 17 kinds of elements. Applying deep sea water to nutrient solution is the best way to provide its components to vegetables, which is expected to result in the improvement of nutritional value of vegetables. Some of the components existing in deep sea water might have growth promotion effects on vegetables, however, surplus sodium chloride must be removed. The objectives of the present work were (1) to selectively remove sodium chloride from deep sea water, (2) to prepare nutrient solution with deep sea water for the hydroponical production of spinach, and (3) to investigate the effect of deep sea water on yield and content of nutritional components of spinach.
2-1 Deep sea water
Deep sea water was obtained from a depth of 500 m in the Pacific Ocean at a latitude of 33°56’N and a longitude of 136°21’E, where is near Owase City, Mie Prefecture, Japan. Surface sea water was also obtained at the same place. The sea water was stored at 5 ? before used.
2-2 Electrodialysis of deep sea water
A small-scale electrodialyzer?(Micro Acilyzer S3, Asahi Chemical Industry Co., Ltd.) fit up with a selective membrane cartridge for the exchange of monovalent ions (AC-110-550, Asahi Chemical Industry Co., Ltd.) was used to remove sodium chloride (NaCl) from the sea water. Figure 1 shows its basic principles (Azuma, 1997). When a direct voltage is applied between the anode and the cathode, cation existing in stream moves toward the cathode, while anion in stream moves toward the anode. Cation may be passed through cation-exchange membranes but stopped by anion-exchange membranes, and anion may be passed through anion-exchange membranes but stopped by cation-exchange membranes, which results in moving of monovalent ions from the dilution stream to the concentration stream.
The sea water was electrodialyzed at a voltage of 10 V. Electrical conductivity of the electrodialyzed sea water was measured at 25 ? using an electrical conductivity meter (CM-40S, TOA Co., Ltd.). Potassium (K), sodium (Na), calcium (Ca) and magnesium (Mg) existing in the electrodialyzed sea water were analyzed by atomic absorption spectrophotometry (AA-6200, Shimadzu Co., Ltd.).
2-3 Hydroponics of spinach
A schematic diagram of the experimental apparatus for hydroponics is shown in Fig. 2. The bed was 600 mm long, 420 mm wide and 110 mm high, and it contained 25 L of nutrient solution. Air was supplied into the nutrient solution using an air pump. The deep sea water was electrodialyzed to an electrical conductivity of 5, 9, 16, 25 dS/m (25 ?) respectively, and then diluted with deionized water by 10 times in volume ratio before used to prepare nutrient solutions. Deionized water was used as the control condition, assuming that its electrical conductivity was 0 dS/m. Elements of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) were controlled at the same concentrations for all nutrient solutions as the prescription B of OTSUKA HOUSE (Otsuka Chemical Co., Ltd.). A same amount of iron (Fe), boron (B), copper (Cu), zinc (Zn), and molybdenum (Mo) were added to all nutrient solutions, neglecting the original existence of these elements in the water used. Table 1 summarizes ion concentration, pH and electrical conductivity of the nutrient solutions. The nutrient solutions had an identical pH at 6.1, but different electrical conductivities ranging from 2.4 to 5.1 dS/m due to their different content of inorganic elements.
Six beds with different nutrient solutions were placed in an artificial weather room. Twenty plants of spinach were grown in each bed for 21 days after transplantation. As shown in Fig.3, the artificial weather room had a light intensity of 175 ?mol/m2s, temperature of 23 ?, relative humidity of 70 % at light period from 6:00 to 18:00, and a temperature of 18 ?, relative humidity of 85 % at dark period from 18:00 to the next 6:00.
All the nutrient solutions were kept at the same level in the beds. Changes in pH and electrical conductivity of the nutrient solutions were measured. Fresh matter of spinach was investigated by weighing the leaf and stem of each plant immediately after the harvest. Moisture (60?-48 h), crude ash (600?) and total ascorbic acid (Sakaki, 1983; The Vitamin Society of Japan, 1990) contained in the leaf of spinach were analyzed.
3. Results and Discussion
3-1 Variation of ion concentration during the electrodialysis of deep sea water
Deep sea water and surface sea water had an electrical conductivity of 45, 50 dS/m, respectively. As shown in Fig.4, the electrical conductivity of deep sea water was reduced from 45 dS/m to 10 dS/m by electrodialysis. K concentration decreased from 619 ppm to 21 ppm, and Na concentration decreased from 11,212 ppm to 999 ppm in correspondence with the decrease of electrical conductivity. For both deep sea water and surface sea water, K oncentration (Ck, ppm) and Na concentration (Cna, ppm) could be expressed as a linear function of electrical conductivity (Ec, dS/m) respectively, in the range of 10 to 50 dS/m as follows:
The concentrations of Ca and Mg did not change in the electrical conductivity range of 50 to 25 dS/m, and they decreased slightly when the electrical conductivity decreased from 25 to 10 dS/m as shown in Fig. 5. The decreases in the concentrations of Ca and Mg were much smaller than those of K and Na. It is clear that monovalent ions were selectively removed, while most of non-monovalent ions were left in the electrodialysis of deep sea water.
3-2 Growth of spinach in hydroponics
The changes in pH and electrical conductivity of the nutrient solutions during growing of spinach are shown in Fig. 6. All the nutrient solutions were exchanged on the 14th day because of the increases in both pH and electrical conductivity of the nutrient solutions. pH increased identically from 6.1 to 7.0, but the electrical conductivity varied differently. The electrical conductivity of nutrient solution DSW0 was almost constant at 2.7 dS/m, while the electrical conductivity of nutrient solution DSW25 increased from 5.1 to 5.8 dS/m. The increase of electrical conductivity related to the accumulation of inorganic elements in the nutrient solution.
The effect of salinity of electrodialzed deep sea water on spinach yield is shown in Fig.7. The mean fresh matter of spinach was 32.7 g/plant at 0 dS/m. It had the highest value of 33.5 g/plant at 5 dS/m, and then decreased linearly with the salinity of electrodialyzed deep sea water in the range of 9 to 25 dS/m. In the t test (Abacus Concepts, Inc., 1996; Morita, 1973), however, the differences in mean fresh matter, however, were not significant expect that at 25 dS/m.
The moisture content and crude ash content of spinach were approximately constant at 92 %,w.b. and 2.0 %,w.b. respectively, in the salinity range of 0 to 25 dS/m as shown in Fig. 8. The effect of salinity of electrodialzed deep sea water on total ascorbic acid content of spinach is shown in Fig.9. Total ascorbic acid content was almost constant at 28.5 mg/100g in the range of 0 to 9 dS/m, and increased with the salinity of electrodialyzed deep sea water ranging from 9 to 25 dS/m.
1) Sodium chloride was removed from deep sea water without a big loss of non-monovalent ions by electrodialysis using a selective membrane cartridge for the exchange of monovalent ions. K concentration and Na concentration of the electrodialyzed deep sea water could be expressed as a linear function of its electrical conductivity, respectively.
2) After electrodialyzed to an electrical conductivity of 0 to 25 dS/m, and then diluted with deionized water by 10 times in volume ratio, deep sea water could be used to prepare nutrient solution for hydroponics of spinach.
3) Spinach had the highest yield and almost the same content of moisture, crude ash and total ascorbic acid when the electrodialyzed deep sea water of 5 dS/m was applied. Increasing the salinity of electrodialyzed deep sea water caused an increase in the total ascorbic acid but a decrease in the yield of spinach.
This work was financially supported by Mie Prefecture and Owase City, Mie Prefecture through a grant for feasibility study on the utilization of deep sea water. The authors are grateful to Mr. I. Azuma, manager of Industrial Membranes Division, Asahi Chemical Industry Co., Ltd. for kindly offering the electrodialyzer, Micro Acilyzer S3 and valuable advice.
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