Superb adsorption of organic dyes from aqueous solution on hierarchically porous composites constructed by ZnAl-LDH/Al(OH)3 nanosheets

Abstract

Hierarchically porous composites constructed by ZnAl-Cl type layered double hydroxides (LDH) nanosheets and amorphous Al(OH)3 nanosheets were synthesized via a facile sol-gel route for the adsorption of two typical anionic dyes Methyl Orange (MO) and Congo Red (CR) from aqueous solutions. The micro-macrostructure and adsorption properties of hierarchically porous ZnAl-LDH/Al(OH)3 composites before and after calcination were investigated in detail. The adsorption mechanism and adsorption–desorption cycles were also discussed. Materials characterization confirm that the resultant hierarchically porous composites possess three-dimensional macrostructure with interconnected macropores and co-continuous skeletons constructed by ZnAl-LDH and Al(OH)3 nanosheets. Calcination at 500 °C allows the transformation from LDH composite to layered double metal oxides (LDO), while the macrostructure is basically maintained with a macropore size of 0.5 μm, a porosity of 53.1% and a BET surface area as high as 122 m2 g−1. Adsorption performances show that hierarchically porous composites exhibit ultrafast adsorption rates and high adsorption capacities in the removal of anionic dyes CR and MO. The maximum adsorption capacities of hierarchically porous LDH-based composites and LDO composites via calcination at 500 °C are 2348.3 and 1574.4 mg g−1 for CR and 1013.5 and 1015.1 mg g−1 for MO, respectively. XRD and FTIR results after the dyes adsorption indicate that the intercalation of MO− and CR− anions into the LDH structure via ion exchange with the pristine chloride ions group and the attachment on the external Al(OH)3 surface determine the adsorption behavior, thus leading to an expansion of the interlayer space. And the calcined products LDO-based composites can be reversibly transformed back into the LDH structure via reconstruction procedure in the presence of dyes aqueous solution. Furthermore, the adsorption–desorption cycles confirm that the as-prepared hierarchically porous composites own excellent recyclable properties and the removal efficiency for CR and MO are 80.2% and 73.7% even after five cycles, respectively.