Hematite decorated multi-walled carbon nanotubes (α-Fe2O3/MWCNTs) as sorbents for Cu(ii) and Cr(vi): comparison of hybrid sorbent performance to its nanomaterial building blocks

Abstract

Major hurdles in the application of engineered nanomaterials for water treatment include loss of reactive surface area arising from particle aggregation and the development of application platforms that limit their potential release into the treated water supply. Here, we develop hybrid nanostructures through the growth of hematite (α-Fe₂O₃) nanoparticles, which are recognized sorbents for various heavy metals, on multi-walled carbon nanotubes (MWCNTs). The hybrid nanostructures were synthesized via hydrolysis of ferric nitrate in the presence of carboxylated MWCNTs, and their activity as sorbents toward Cu(II) and chromate (CrO₄²⁻) was examined as a function of pH (i.e., pH-edge experiments) and initial metal concentration (i.e., adsorption isotherms). Characterization of α-Fe₂O₃/MWCNT nanostructures via Raman spectroscopy and transmission electron microscopy (TEM) with selected area electron diffraction (SAED) confirmed the deposited iron phase as α-Fe₂O₃. Further, complementary acid digestions and TEM imaging revealed that the amount (0.1 and 0.5 g g⁻¹ α-Fe₂O₃/MWCNT) and size [5.9 (±1.1) and 8.9 (±1.5) nm, respectively] of α-Fe₂O₃ nanoparticles immobilized on MWCNTs were tunable during synthesis. Generally, mass-normalized concentrations of adsorbed Cu(II) and CrO₄²⁻ were greatest for α-Fe₂O₃/MWCNT hybrids relative to adsorption on either carboxylated MWCNTs or freely suspended α-Fe₂O₃ nanoparticles, with evidence implicating α-Fe₂O₃ as the active sorbent phase in hybrid materials. Indeed, per unit mass of available α-Fe₂O₃, hybrid sorbents exhibited capacities comparable to or exceeding most other iron-based sorbents for Cu(II) and CrO₄²⁻ (from 220 to 470 mg Cu(II) per g α-Fe₂O₃ and 60 mg CrO₄²⁻ per g α-Fe₂O₃, respectively, at pH 6 and 20 °C). The enhanced sorption capacity of the hybrid nanostructures is due, at least in part, to the greater available surface area of α-Fe₂O₃ nanoparticles immobilized on MWCNTs when compared to their more extensively aggregated state in suspension. Notable differences in the pH-dependent trends of Cu(II) and CrO₄²⁻ uptake on α-Fe₂O₃/MWCNT hybrids, along with differences in zeta potential measurements across pH, also suggest that the immobilized α-Fe₂O₃ nanoparticles may exhibit unique surface reactivity relative to their freely suspended analogs as a result of their association with the negatively charged MWCNT surface.