Layered double hydroxide–agarose composites for water treatment: carbonate contamination during the drying process

Abstract

Layered double hydroxides (LDH) are anion exchangers and have been investigated intensively as adsorbents for removal of hazardous and toxic anionic substances in water treatment. However, LDH, which are usually synthesized in the form of a powder, are often difficult to separate from suspensions in water treatment. Incorporating LDH into composite hydrogels is a strategy to avoid this difficulty: in this study, aqueous suspensions of Cl-containing LDH of high and low crystallinity were mixed with agarose solution while heating, and the mixtures were allowed to cool to form gels. However, in wet conditions, LDH would be susceptible to carbonate contamination from atmospheric CO2. In this context, LDH–agarose composite hydrogels were synthesized, and gels dried in three different ways (air drying, oven drying in air at 110°C, and vacuum drying) were evaluated as anion adsorbents by using SO42−. The air-dried gels showed significant reductions in SO42− adsorption abilities compared to those of the original undried gels: 56% reduction for air-dried gels containing the low-crystallinity LDH and 29% reduction for air-dried gels containing the high-crystallinity LDH. This result shows that carbonate contamination was extremely serious for low-crystallinity LDH in the gel. In contrast, the oven- and vacuum-dried gels retained adsorption abilities that were almost the same as those of the original undried gels, regardless of crystallinity of LDH in the gels. Powdery form of LDH showed similar results about drying. These results show how important the drying methods are, and provide fundamental information about carbonate contamination.