1.1 Preparation of CROF

The samples of *C. rupestris* was collected from a pond water of Hefei in Anhui Province, China(32°30′ N, 116°17′E), these raw materials were rinsed with running water repeatedly to remove dirt particles and impurities, later by ultrapure water several times, dried in an oven(80℃,12 h) to constant weight, and then the dried *C. rupestris* was crushed into powder to obtain particle size of 100 mesh by screening, CROF was saved in a brown bottle for standby application.

1.2 Batch adsorption experiments and Pb2+ content analysis

1.599 g Pb(NO3)2 dissolve in 1000 mL ultrapure water, stock solution of Pb2+(1000 mg/L)was obtained with 0.01 mol/L NaNO3 as the background electrolyte. 0.100 g of CROF and 25 mL of Pb2+ solutions were shaking at a rate of 180 rpm, then centrifuged at 10000 rpm, the supernatants were filtering by a 0.45 µm filter and analysis Pb2+ concetrations. Pb2+ concentrations were measured by a flame atomic absorption spectrometer(FAAS, ZEEnit 700P,GER Analytic Jena) with the absorbance at a wavelength of 283.3 nm.

1.3 Characterizations and combined forms analysis

The solide mixtures were lyophilized by Freeze Dryer(FD,SIM-4,USA).

1.3.1 SEM–EDS determination

A field emission Scanning Electron Microscope (SEM; S-4800, Hitachi, Japan) was used to look into micromorphology of CROF samples both prior to and after Pb2+ adsorption. Energy Dispersive X-ray Spectroscopy (EDS; X-MAXN 150, OXFORD, UK) equipped with SEM was used simultaneously to quantify the composition of elements on CROF surface(Yang et al. 2022).

1.3.2 PSD and BET analysis

A laser particle size analyzer(LPSA; BT-9300H Danton) was employed to assess the the pore size distribution(PSD) of CROF,the N2 adsorption/desorption isotherms was used to analyze pore structures of CROF with the method of Brunauer-Emmett-Teller(BET; Micromeritics ASAP 2460 USA) ( Chen et al. 2021b;Esakkimuthu et al. 2022).

1.3.3 pHpzc analysis

0.100 g samples were taken into the volume of 25 mL in 0.01 mol/L NaNO3 solutions with starting pH 2–9(Ahmad et al. 2019). Under the condition of 180 rpm, at 25 ℃, the suspensions reached equilibrium for 48 hours, then the final pH was assayed. When the initial pH = final pH(△pH = 0), the pH value is Point of zero charges (pHpzc).

1.3.4 FTIR analysis

0.001 g dried CROF treated with Pb2+ concentrations were 0, 5.0 and 50 mg/L were ground in an agate mortar with 0.100 g KBr and then compressed and molded into a small disk for the Fourier transform infrared spectroscopy analysis (FTIR; Nicolette is 50, Thermo Scientific, USA). Spectra were captured with a resolution of 2 cm− 1 and wavenumbers of 4000 to 400 cm− 1(Chen et al. 2021a).

1.3.5 XPS analysis

The combined form of the elements(C, O and N) of the dried CROF treated with Pb2+ (0, 5.0 mg/L, 50 mg/L) were determined by X-ray photoelectron spectroscopy (XPS; Escalab 250, Thermo-VG Scientific, USA) with a monochromatized Al Ka (hv = 1486.6 eV) excitation source and a pass energy of 30 eV. Binding energies with the C 1s peak of the carbon contaminant at 284.80 eV were calibrated (Chen et al. 2021a; Liang et al. 2020).

1.4 Data Analysis

1.4.1 Determining the adsorption capacity and the removal percentage

The adsorption capacity and the removal percentage of Pb2+ on CROF could be calculated by the method of Chen et al. ( 2021b) and Xu et al. (2021).

\(\) 1.4.2 adsorption kinetic models

The adsorption kinetics was studied by at constant temperature (25℃) and shook at 180 rpm with regular time intervals which were set as 1,3,5, 10 25,20, 25, 30, 35, 40, 45, 50, 55, 60 min .The results for adsorption capacity can be fitted by three adsorption kinetic models: Pseudo-first-order (PFO)(Eq. (1)), Pseudo-second-order(PSO) (Eq. (2)), and Elovich models(Eq. (3))(Xu et al. 2021):

$${q}_{t}={q}_{e}(1-{e}^{-{K}_{1}t})$$

1

$${q}_{t}={K}_{2}{q}_{e}^{2}t/\left(1+{K}_{2}{q}_{e}t\right)$$

2

$${q}_{t}=\left(\frac{1}{\beta }\right)In[1+\left(\alpha \beta t\right)]$$

3

where K1, K2 \(and {\beta }\)are constant \(\)of three adsorption kinetic models.

1.4.3 Isotherm adsorption models

The isotherm adsorption models were examined at a series of initial concentrations including 0, 5, 25, 30, 35, 40, 45, 50 mg/L and was shaken at 180 rpm and 30 ℃ for 24 h. The Langmuir model(Eq. (4)), Freundlich model(Eq. (5)) and Sips model (Eq. (6))were adopted to simulated the capture process of Pb2+(Chen et al. 2021b; Wang et al. 2022c):

$${q}_{e}=\frac{{q}_{max}{K}_{L}{C}_{e}}{1+{K}_{L}{C}_{e}} \left(4\right)$$

$${q}_{e}={K}_{f}C{e}^{\frac{1}{n}} \left(5\right)$$

$${q}_{e}=\frac{{K}_{LF}{C}_{e}^{n}}{1+{\partial }_{LF}{C}_{e}^{n}} \left(6\right)$$

$${R}_{L}=\frac{1}{1+{K}_{L }{C}_{0}} \left(7\right)$$

where \({K}_{L}\) \(and {K}_{f}\) are the Langmuir and Freundlich constant, respectively, n is the Freundlich dimensionless exponent related to how favourable of the adsorption process. And the Langmuir parameter RL(Eq. (7)) is also used to further investigate the viability of the adsorption (Zhang et al. 2022).

1.4.4 Adsorption thermodynamic models

The adsorption thermodynamic models were performed at 293, 298, 303 and 308 K. \({\varDelta H}^{\theta }\), \({\varDelta S}^{\theta }\) and \({\varDelta G}^{\theta }\) were evaluated by the Van’t Hoff(Eq. (8) and(Eq. (9))) and Gibbs free energy(Eq. (10)) formulas(Rahim et al. 2020;Zhang et al. 2022):

$${K}_{T}=\frac{{q}_{e}}{{c}_{e}}$$

8

$$In\left({K}_{T}\right)=-\left(\frac{{\varDelta H}^{\theta }}{RT}\right)+\left(\frac{{\varDelta S}^{\theta }}{R}\right)$$

9

$${\varDelta G}^{\theta }=-\text{R}\text{T}\text{l}\text{n}{K}_{T}$$

10

where, \({K}_{T }\text{a}\text{n}\text{d} \text{R}\) is the constant. The values of \({\varDelta H}^{\theta }\) and \({\varDelta S}^{\theta }\) is calculated from the slope and intercept t by the linear relationship between ln(qe/ce) and 1/T.

1.4.5 Statistic analysis

The relative errors were all less than 5% and every experiment was done in triplicate. OMNIC and Avantage were used for spectral processing. Origin 2018 was used to map the fitted data and facilitate simulations of adsorption kinetic, isotherm adsorption and adsorption thermodynamic models.