2.3. Biochar Characterizations

Element compositions of pristine cauliflower leaves (PCL), CL and CLB were measured with an Elemental Analyzer (Vario EL cube, Elementar, Hamburg, Germany). CHNS and O modes of operation were selected respectively to determine the corresponding element content. Using the modified dry-ashing method [23], the content of major inorganic elements was identified by an inductively coupled plasma optical emission spectroscopy (ICP-OES) (Agilent 730, Palo Alto, CA, USA). After mixing the biochar sample with DW (1:10 w:v) and shaking them for 1 h in a mechanical shaker, the pH value was measured using a pH meter (Lei-ci PHS-3C, Shanghai, China). To measure specific surface areas (SSA), the surface area analyzer (ASAP2020C, Norcross, GA, USA) was used by the tests of N₂ adsorption-desorption isotherm experiments using BET (Brunauer–Emmet–Teller) method. Based on the method introduced by Johnson et al. (1996) [24], the zeta potential of biochar samples was measured by zeta potentiometric analyzer (HOBRIBA Sz-100, Kyoto, Japan) after they were sonicated in DW for 10 min. The surface morphology of CL, CLB, and CLB loaded with Cu(II)/Pb(II) was investigated at 15 kV voltage by using scanning electron microscopy (SEM, JSM-IT200, Tokyo, Japan) equipped with an energy dispersive X-ray spectroscopy (EDS). EDS was used to identify the composition of the elements of biochars’ surfaces. After shaking the mixture of biochar and 0.3 mM Cu(II)/Pb(II) (0.25:100 w:v) for 24 h at 25 ± 1 °C, post-sorption samples were filtered by membrane filters (pore size = 0.22 μm), washed several times with DW and dried at 80 °C in an oven for overnight.XRD patterns of pre- and post-samples were checked by X-ray diffractometer (XRD, ULTIMA IV, Tokyo, Japan) with Cu Kα radiation (λ = 1.5418 Å), 2θ range from 5° to 60° at the rate of 8°/min, 40 mA electrical current, 40 kV voltage. After pressing the mixture of sample and spectroscopic grade KBr (1:100 w:w) into a flake, the change of surface functional group of the samples before and after metal adsorptionwas determined by Fourier transform infrared spectrometer (FTIR, TENSOR27, Karlsruhe, Germany). The FTIR spectra of sample were recorded in the spectrum ranged from 400 to 4000 cm⁻¹. The surface chemical compositions of sample were measured by an X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi, New York, NY, USA).