Desalination Property of Various Calcined Layered Double Hydroxides from Seawater
Abstract
Now, approximately 20% of farmland in the world becomes salt damage soil with unsuitable properties for agriculture. In general, salt was removed from soil by flushing outwith rainwater due to the improvement of soil permeability using gypsum and so on. However, there are arid and semi-arid areas with an insufficient supply amount of rainwater to remove salts from soil. In this study, a novel method to capture salt in soil using various calcined layered double hydroxides (LDHs) as a desalination agent was attempted to estimate desalination property from seawater. 4 kinds of LDH with the different M2+/M3+ ratios are prepared using 2 kinds of Mg2+ and Ca2+ as M2+ and 2 kinds of Al3+ and Fe3+ as M3+. The desalination ability of these calcined LDHs was investigated using seawater. As a result, the decrease of salinity was confirmed using all samples. Among these samples, the calcined Mg-Al LDH with Mg/Al = 3.45 and Ca-Fe LDH with Ca/Fe = 2.35 indicated the highest desalination property, due to the decrease of Cl-and SO 2-from seawater, by 4 reconstruction reaction. Ca-Fe LDH was calcined at various temperatures and the desalination ability at different calcined temperatures was also evaluated. It was found that the desalination ability of calcined LDH depends on the calcination temperatures and Ca-Fe LDH calcined at 500℃ indicated the highest desalination ability.
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Introduction
In the 21st century, global environmental issues are very serious, and effective utilization of energy and mineral resource and the securing of food and water are urgent problems.
Now, there are about 1/5 of salt-damaged soils in the world farmland. The demand for food is increasing due to the global population growth, and production of a stable supply of food is essential to sustain human life. Securing farmland is one of the most important factors in food production. Furthermore, in 2011, great earthquake occurred in Tohoku area of Japan and farmland in Tohoku was damaged by Tsunami. A lot of farmlands are damaged by salt, and it is desired to improve the salt-damaged soil into plantable soil.
There are some desalination methods, such as leaching, elution, disposal of outer layer soil, and salt absorption due to the halotolerant plant. Now, leaching method, which makes farmlands good water permeability and flush out salt with water, is a popular desalination method to improve salt-damaged farmlands. However, the method needs along time to improve the salt damaged soil completely, and depends on weather. There are some studies to improve salt-damaged soil rapidly. For example, electorostatic desalination was studied, but this technology is expensive and needs a large space. In this study, we develop a new salt-damaged soil improving agent prepared from calcined layered double hydroxide (LDH). Chemical formula of LDH expresses as [M2+ M3+ (OH) ][An- •mH O] (M2+: divalent metal ions, M3+: trivalent metal ions, 1-x x 2 x/n 2 An-: anionic species, x = 0-1), and composed of metal complex hydroxide known as inorganic anion exchangers [1]. Hydrotalcite is well known for Mg/Al type LDH, and the uptake of anions onto hydrotalcites from aqueous solution was occurred by two mechanisms: (1) intercalation by anion exchange; (2) intercalation by reformation of calcined samples [2]. Intercalation by anion exchange was used for the removal of the anion or oxi-anions, such asH AsO , from water solution 3 4 [3]. Intercalation by reformation of calcined LDH was also used for anion removal, e.g. adsorption of VO 3-or NO - [4]. In 4 3 our previous study, calcined hydrotalcite could desalinate seawater and the solution could use for plant growth by combination with natural zeolite treatment [5]. However, little information can be available on desalination property of calcined LDH. The objective of this study was to investigate the desalination properties of various calcined LDH from seawater.
Conclusion
In this study, we investigated the desalination properties of calcined LDH. As a result, Mg-Al oxide and Mg-Fe oxide peaks confirmed in calcined Mg-LDH and peak intensity of Mg-Al oxide and Mg-Fe oxide increased with increasing Mg2+/M3+ ratio of the LDH. In calcined Ca-LDH, calcite peaks were mainly confirmed. After desalination test, for all calcined LDHs, the structure of calcined LDH returned to that of LDH by reformation reaction. In calcined Ca-LDH, calcite was confirmed in the LDHs after desalination test. The salinity decreases in all seawater after treatment with calcined LDH. The highest desalination from seawater were treated with the calcined Mg-Al LDH with Mg/Al = 3.45 and calcined Ca-Fe LDH with Ca/Fe = 2.32, because the structure of calcined LDH can remove high amounts of Cl-, Na+ and Mg2+ from seawater. The removal amounts of Cl-, Na+ and Mg2+ in seawater treated with calcined Ca-LDH were more than Mg-LDH. Because Ca2+ and Fe3+ are cheaper than Mg2+ and Al3+, it would be considered that Ca-Fe LDH (Ca/Fe = 2.32) is the best desalination product for desalination in this experiment. For Ca-Fe LDH calcined at various temperatures, salinity decreased in seawater after treatment with all calcined LDHs. The highest removal of salt from seawater is approximately 20% using the Ca-Fe LDH with Ca/Fe = 2.32 calcined at 500oC.
