Sorption of metal ions onto PET-derived microplastic fibres†

Abstract

This study investigated microplastic polyester fibres representative of those shed during laundering as sorbents for metal ions. During sewage distribution and treatment, microplastics are exposed to elevated concentrations of metal ions, typically for several days. Cryogenic milling was used to generate polyethylene terephthalate (PET) fibres. Characterisation using optical microscopy and Raman spectroscopy revealed that milling did not cause significant chemical alteration to the fibres. Milled fibres were subsequently assessed in screening tests for their capacity to retain 12 metal ions—Sb(III), As(III), Cd(II), Cr(VI), Cu(II), Co(II), Pb(II), Hg(II), Mo(VI), Ni(II), V(V) and Zn(II)—at pH 8. All metal ions were sorbed onto PET fibres. The highest distribution coefficient (K_d) was observed for Pb²⁺ (939 mL g⁻¹), followed by Cd²⁺ (898 mL g⁻¹), Cu²⁺ (507 mL g⁻¹), Hg²⁺ (403 mL g⁻¹), and Zn²⁺ (235 mL g⁻¹). The extent of sorption is largely explicable by electrostatic interactions between the PET surface (1.95 point of zero net charge) and the predicted metal ion species. The sorption behaviour of Cd²⁺ and Hg²⁺ was examined in more detail since both showed high sorption capacity and are highly toxic. Kinetic experiments revealed that the sorption of both elements was relatively fast, with a steady state reached within six hours. Experimental data from isotherm tests fitted well to the Langmuir sorption model and demonstrated that PET fibres had a much greater sorption capacity for Hg²⁺ (17.3–23.1 μg g⁻¹) than for Cd²⁺ (4.3–5.3 μg g⁻¹). Overall, the results indicate that retention of metal ions onto PET fibres originating from laundry is expected during full-scale sewage treatment, which facilitates the subsequent transfer of metals into the terrestrial environment, given that sewage sludge is commonly applied to agricultural land.