Two alternative approaches for the magnetic adsorption of the hydrophobic dye Methylene Blue

Many of the most harmful pollutants contained in the wastewaters are hydrophobic compounds. Consequently, the selection of suitable adsorbents to remove Methylene Blue (MB), adopted as model hydrophobic contaminant, was carried out evaluating two alternative approaches. First, two nanocomposites containing the hydrophobic biochar were tested to exploit the hydrophobic interaction between adsorbent and adsorbate. As an alternative approach, a nanocomposite based on chitosan, a material substantially hydrophilic though containing hydrophobic methyl groups, was found to allow adsorption of pollutants by both hydrophobic and hydrogen-bond interactions. Both biochar- and chitosan- based nanocomposites were inserted in magnetic nanoparticles by crosslinking and impregnation to allow a simpler solid/liquid separation. XRD, FTIR and SEM analyses elucidated the properties of the nanocomposites, showing the magnetic fraction to be mainly composed of maghemite, with a crystallite size of about 8 mm. The M(H) curves showed a pure superparamagnetic behaviour of the nanoparticles, with larger magnetic moments of biochar nanocomposites in comparison to chitosan nanocomposites (5.8 × 10−20 Am2 against 3.5 × 10−20 Am2). The mechanism of MB adsorption on the nanocomposites was discussed considering the zero-point charges of nanocomposites and the oxidation–reduction system of MB molecules. The higher adsorption capacity shown by magnetic chitosan in a lower range of MB concentration (1.57 × 105 mg/kg) was explained taking into account the overlapping effects of both the hydrophobic and the polar interactions. Kinetics and isotherms modelling suggested a multistep adsorption process, due to the different properties of phases constituting the nanocomposites. An efficient regeneration procedure was developed for the chitosan-based nanocomposite, based on the incubation with an acetic acid/ethanol mixture.