Crystallization of Layered Double Hydroxides by Ultrasound and the Effect of Crystal Quality on Their Surface Properties

Abstract

The ultrasound technique was applied to the synthesis of layered double hydroxides. The use of ultrasound in the synthesis of these compounds was studied in terms of its effect on their crystal qualities and surface properties. The crystal qualities of the compounds synthesized with ultrasound were compared with those of compounds synthesized without ultrasound to elucidate the effect of the ultrasound on the synthesis. The effect of crystal quality (crystallite size) on the adsorption behavior of humic substances was examined. The compounds synthesized under ultrasonic conditions showed a larger crystallite size and a larger adsorption capacity for humic substances than those synthesized without ultrasonic treatment. The degree of adsorption correlated well with crystal quality. INTRODUCTI ON Layered double hydroxides (LDHs) such as hydrot a l c i t e a n d h y d r o t a l c i t e -l i k e c o m p o u n d s M 2+x M 3+ y (OH) 2(x+y) A n¡ y/n ·mH 2 O (where M 2+ is a divalent metal ion, M 3+ is a trivalent metal ion, and A n¡ is an intercalated anion) are well known as inorganic anion exchangers. Because of their large ion-exchange capacity and structural thermal stability, which are difficult to achieve simultaneously in conventional anionexchange resins, these compounds are used in many industrial processes, e.g. for polymer synthesis, and as coatings that are halogen scavengers and neutralizers (Cavani et al., 1991).These compounds are commonly synthesized by precipitation from a parent salt solution of divalent and trivalent metal ions, followed by ageing at an elevated temperature (hydrothermal treatment) in the laboratory. The conditions of synthesis, such as solution pH, temperature and metal composition, along with physicochemical structure of the compounds have been studied extensively by many researchers (Allmann et al., 1970;

Increasing the basicity and catalytic activity of hydrotalcites by different synthesis procedures

“…Kapustin [19] examined 30 different samples and observed that insonating the solution during crystallization alters the grain size of crystals and produces finestructured crystals which have smaller grain sizes than the crystals prepared without insonation. Interestingly, we have not found any report on the application of the ultrasound irradiation during the synthesis of hydrotalcites; however, Seida et al [20] applied sonication after the synthesis of the hydrotalcite. The authors found in this case that the LDHs present larger crystallites and BET surface areas than conventionally prepared samples.…”

Combined SANS and SAXS study of the action of ultrasound on the structure of amorphous zirconia gels

“…For example, nickel hydroxides obtained by ultrasonic-assisted techniques [20][21][22][23] possessed higher electrochemical performance. Similarly, layered double hydroxides obtained by an ultrasound-enhanced technique showed larger adsorption capacity for humic substances [24]. Recently, formation of high specific surface area porous adsorbents with the use of ultrasound was also reported [25,26].…”

Facile synthesis of Zn-Al layered double hydroxide from aqueous suspension of zinc oxide and aluminum hydroxide

“…36) The preparation of LDH has actively been investigated from both fundamental and practical viewpoints and several new or modified synthetic methods, which utilize sonication, 38) microwave irradiation, 39),40) colloid mill, 41) and an in line dispersion-precipitation method, 42) have been reported recently. Homogeneous precipitation using urea hydrolysis has been applied for the synthesis of carbonate LDHs with large and controlled particle size.…”