Hierarchical layered double hydroxide for the removal of charged dyes: the role of an anionic surfactant

Hierarchical layered double hydroxide (HLDH) was synthesized by using sodium dodecyl sulfate (SDS) as a soft-template agent for the removal of two charged organic dyes (i.e. methylene blue (MB; cationic dye) and methyl orange (MO; anionic dye)). The experimental results based on response surface methodology (RSM) demonstrated distinct removal behaviours of HLDH towards these two dyes: (1) the maximum capacity was 416.7 mg g–1 for MO and 58.7 mg g–1 for MB at 25°C; (2) the increase in temperature could enhance MO removal significantly, whereas it had a negligible effect on the MB treatment process; and (3) rapid removal of MB (5 min) compared to MO (480 min) was observed. In addition, the removal process for both dyes was pH-independent. Multiple characterization techniques further revealed the removal mechanisms, demonstrating that SDS played a significant role in the removal of both dyes; that is, MO replaced SDS to be intercalated into the HLDH interlayer via anion exchange. MB could influence the –SO3 group of SDS, resulting in it modifying the electrodensity of SDS. It could then be further combined with an SDS anion (DS–) via hydrophobic and electrostatic interactions to form DS-MB monolayers. This work not only provides an efficient capture agent for charged dyes, but also offers a deep insight into the underlying removal mechanism.