Malic acid-enhanced chitosan hydrogel beads (mCHBs) for the removal of Cr(VI) and Cu(II) from aqueous solution

Chitosan hydrogel beads (CHBs) have displayed prominent advantages as adsorbents, including easy separation after adsorption. To further increase their adsorption capacity, CHBs were modified with malic acid and further freeze-dried. The structural characteristics of malic acid modified CHBs (mCHBs) were explored along with the factors primarily influencing their effectiveness. The mechanisms of Cr(VI) and Cu(II) ion removal were re-evaluated in detail. The results show that (1) porous mCHBs were clearly observed by scanning electron microscopy, accompanying with the observation of particles of elemental Cu or Cr about 300 nm in diameter within the pores of the mCHBs; (2) the maximum equilibrium adsorption capacity of mCHBs for Cr(VI) was 383.2 mg g(-1), reached within 2 h, and the removal of Cr(VI) was an efficient adsorption-reduction process. For comparison, the chemisorption of Cu(II) onto mCHBs formed a monolayer with a maximum equilibrium adsorption capacity of 183.8 mg g(-1), reached within 24 h, and the adsorption of Cu(II) ions was spontaneous and endothermic. Moreover, CHBs modified with a series of carboxylic acids also showed enhanced removal of heavy metals. In addition, the presence of Cu(II) ions in the matrix of mCHBs could function as an effective adsorbent for removal of phosphate ions.