Adsorption behavior and mechanism of aqueous Cr(III) and Cr(III)-EDTA chelates on DTPA-chitosan modified Fe3O4@SiO2

Abstract

The presence of widely used chelating agents such as ethylenediaminetetraacetic acid (EDTA) in aqueous solution increases the difficulty of heavy metal removal by the formation of heavy metal chelates. In this study, diethylenetriaminepentaacetic acid (DTPA)-chitosan functionalized magnetic silica (Fe3O4@SiO2/Cs-DTPA) was developed for the removal of Cr(III) and Cr(III)-EDTA. The obtained results from characterization confirmed that Fe3O4@SiO2/Cs-DTPA with the core-shell structure and superparamagnetic properties was synthesized. The maximum theoretical adsorption capacity of Cr(III) and Cr(III)-EDTA chelates on the adsorbents were 39.27 and 22.24 mg/g at 25 °C and pH 4.0, respectively. Adsorption equilibrium of Cr(III) and Cr(III)-EDTA on Fe3O4@SiO2/Cs-DTPA was achieved within 15 min, and the adsorption data fit the pseudo second-order model. The co-existing complex agents (EDTA) and cations (Na+, Ca2+, and Mg2+) in the aqueous solution reduced Cr(III) adsorption by the competition with Cr(III) and Cr(III)-EDTA chelate for active groups of the adsorbent. The affinity of the Fe3O4@SiO2/Cs-DTPA for Cr(III) and Cr(III)-EDTA was excellent even at high salt and organic chelates concentration. The Cr(III) adsorption was attributed to surface complexation between Cr(III) and DTPA moieties of the adsorbent, while Cr(III)-EDTA as a whole was adsorbed on the adsorbent by the complexation and electrostatic interaction. Cr(III) saturated Fe3O4@SiO2/Cs-DTPA can be readily regenerated in acid solution and Cr(III) or Cr(III)-EDTA adsorption of regenerated adsorbent exhibits no obvious reduce.