Dodecyl sulfate–hydrotalcite nanocomposites for trapping chlorinated organic pollutants in water

Abstract

A series of hybrid organic–inorganic nanocomposite materials was synthesized by three different procedures using sodium dodecyl sulfate (DDS) and magnesium–aluminum layered double hydroxide (Mg/Al LDH with a Mg/Al molar ratio of 2 to 5). Both the pH of the exchange medium (6.5 to 10) and the Mg/Al molar ratio of the LDH affected the basal spacing, the content of DDS retained and the orientation of the DDS chains within the interlamellar space. For LDH with higher charge density (Mg/Al=2 and 3), DDS molecules likely formed a perpendicular monolayer within the LDH interlayer and the solution pH had little effect on the basal spacing, with a mean and standard deviation of 25.5±0.4 Å. However, for LDH with lower charge density (Mg/Al=4 and 5), DDS molecules more likely formed an interpenetrating bilayer, and the basal spacing significantly increased with increasing pH, with a mean and standard deviation of 32.7±5.2 Å. Sorption of trichloroethylene and tetrachloroethylene by DDS-LDH varied with synthesis conditions, LDH type and DDS configuration in the interlayer. DDS-Mg 3Al-LDH had the highest affinity for both trichloroethylene and tetrachloroethylene in water, either comparable to or as much as four times higher than other clay-derived sorbents, followed by DDS-Mg 4Al-LDH and DDS-Mg 5Al-LDH. DDS-Mg 2Al-LDH had the lowest sorption affinity although the highest amount of DDS. The pH of the exchange solution also affected the amount of DDS retained by the LDH as well as the sorption efficiency. Mg 3Al-LDH has a charge equivalent area of 32.2 Å 2/charge, which allows the formation of optimal DDS configuration within its interlayer, thus resulting in the highest affinity for the chlorinated compounds. The DDS-Mg/Al-LDHs can be easily synthesized either ex situ or in situ at low temperature, indicating the feasibility of practical applications. The results obtained by controlling the synthesis procedure suggest that different arrangements of DDS molecules in the LDH interlayers can be obtained and optimized for the sorption of specific sorbates.