Production of a generic magnetic Fe₃O₄ nanoparticles decorated tea waste composites for highly efficient sorption of Cu(II) and Zn(II)

In this paper, we reported an environmental friendly and cost-effective method to prepare magnetic nanoparticles decorated tea waste (Fe₃O₄-tea) composites by chemical co-precipitation of Fe³⁺/Fe²⁺ on waste tea leaves. Various physical and chemical characterizations indicated that the synthetic Fe₃O₄ nanoparticles were combined to the tea powder surface by chemical bonds. When tested as an adsorbent for Cu(II) and Zn(II) removal, the Fe₃O₄-tea composites showed an unexpected high sorption capacity and satisfactory regeneration performance, better than those of naked Fe₃O₄ and other reported materials. The results showed that the maximum sorption capacities of Fe₃O₄-tea calculated from Langmuir isotherm model were 95.44 mg g^-1 for Cu(II) and 68.78 mg g^-1 for Zn(II) at pH 6.0. Interestingly, it was found that much higher adsorption capacity was achieved for Cu(II) on Fe₃O₄-tea rather than that for Zn(II) adsorption, which was attributed to the higher ionic potential of Cu(II), preferably penetrating into smaller pores. Furthermore, in the binary-solute system, Cu(II) exhibited greater inhibition for Zn(II) sorption, suggesting stronger affinity of Fe₃O₄-tea for Cu(II). Relatively, pseudo-second-order equation was the optimal model to describe the kinetics of Cu(II) and Zn(II) sorption, suggesting a chemisorption with the adsorption mechanism via M-O or M-N bonding. X-ray photoelectron spectroscopy (XPS) analysis showed that the enrichment of Cu(II) and Zn(II) was attributed to the abundant binding sites (e.g., -OH, -COOH and -NH groups) of Fe₃O₄-tea, which made stable complex compound with Cu²⁺ or Zn²⁺. This work provides a facile and general approach to synthesize promising magnetic biochar materials for wastewater treatment.