*2.3. Characterization of SF@Cu-HNFs*

The morphologies of the prepared SF@Cu-HNFs products were characterized by a scanning electron microscope (SEM, EV0 MA15, Carl Zeiss, Germany) at an acceleration voltage of 20 kV. All samples were sputter-coated with gold using an E1045 Pt-coater (Carl Zeiss, Germany) before SEM observation. Elemental analysis was conducted with an energy dispersive X-ray spectrometer (EDS) equipped in the SEM.

The crystal structures of the nanoflower products were characterized by X-ray di ffraction (XRD) analysis (X Pert PRO MPD, PANalytical, Holland). Radial scans using Cu K α radiation source at 20 mA and 40 kV were recorded in the reflection scanning mode from 2θ = 10 to 80◦ at a scanning rate of 1◦ min−1.

The chemical structures of the nanoflower products were measured by Fourier transform infrared spectroscopy (Nicolet 6700 FTIR, Thermo Fisher Scientific Corp., USA) in the range of 400−4000 cm<sup>−</sup><sup>1</sup> with KBr pellets. The thermogravimetric analysis (TGA) of nanoflowers was measured with a thermogravimetric analyzer (STA449F3, Netzsch, Germany) in a dynamic atmosphere of dinitrogen with 20 cm<sup>3</sup> min−<sup>1</sup> flow rate. The TGA measurements were performed with a temperature ranging from 40 to 700 ◦C in an alumina crucible at a rate of 5 ◦C min−1.

To evaluate the surface adsorption and interaction process of Pb(II) on nanoflowers adsorbent, the surface characteristics of SF@Cu-HNFs were furthermore investigated with the addition of di fferent Pb(II) concentrations. The surface zeta potential was determined by dynamic light scattering (DLS) measurements (NANO ZS, Malvern Instruments Ltd., UK) equipped with the DTS Ver. 4.10 software package.
