## 2 Materials and Methods
### Analytical Methods  
Powder X-ray diffraction (XRD) was performed using a Seifert XRD 3000P diffractometer (Seifert, Ahrensburg, Germany) with CoK\(\alpha\) radiation (\(\lambda\) = 0.179026 nm, graphite monochromator and goniometer with Bragg-Brentano geometry) in a 20 range of 5-60\({}^{\circ}\) with a step size of 0.05\({}^{\circ}\).  
X-ray fluorescence analyses (XRFs) of the solid phase were performed using an ARL 9400 XP+ spectrometer (ARL, Ecublens, Switzerland) at a voltage of 20-60 kV and a probe current of 40-80 mA, with an effective area of 490.6 mm\({}^{2}\). UniQuant v.4 software was used to evaluate the data (Thermo ARL, Ecublens, Switzerland).  
The specific surface area (S\({}_{\mathrm{BET}}\)) was measured on a Micromeritics ASAP 2020 (accelerated surface area and porosimetry) analyser (Micromeritics, Norcross, GA, USA) using gas sorption. The ASAP 2020 device assesses single- and multipoint BET surface areas and the Langmuir surface area, performs Temkin and Freundlich isotherm analyses, and determines pore volume and pore area distributions in the micro- and macropore ranges. Macro- and micropore samples were analysed using the Horvath-Kavazoe method and Barret-Joyner-Halenda method (BJH), respectively. In the BHJ method, N\({}_{2}\) was used as the analysis adsorbent, and the analysis bath temperature was \(-\)195.8 \({}^{\circ}\)C. The samples were degassed at 313 K for 1000 min.  
The point of zero charge (pH\({}_{\mathrm{ZPC}}\)), which generally allows the estimation of the preferential targeting of adsorption in aqueous systems [30], was determined using zeta potential analyser Stabino, version 2.0 (Particle Metrix GmbH, Meerbusch, Germany) equipped with a cylindrical polytetrafluoroethylene-measuring beaker and piston. The pH\({}_{\mathrm{ZPC}}\) was measured using the pH titration method to assess stable pH regions and determine the isolelectric point (IEP) with an extremely high resolution (0.1 pH). Three suspensions of the same solid sample in a KCl solution with an identical solid-to-liquid ratio (1:100) but different ionic strengths of KCl (0.1 M, 0.01 M and 0.001 M) were titrated to the IEP by 0.01 M HCl or NaOH in a dynamic regime. The pH\({}_{\text{ZPC}}\) was calculated as the average of three pH values corresponding to the zero potential with a standard deviation range of 0.05-0.7 pH units.  
The structure of samples was investigated by scanning electron microscopy (SEM) on a Tescan Vega 3, Czech Republic, and energy dispersion spectrometry on an Inca 350 spectrometer, Oxford Instruments, Abingdon, UK.  
The concentrations of Pb in liquid samples were measured by performing atomic absorption spectrometry (AAS) using a SpectrAA-880 VGA 77 unit (Varian, Palo Alto, CA, USA) in flame mode. The accuracy of the AAS analyses was guaranteed by the Laboratory of Atomic Absorption Spectrometry of the University of Chemistry and Technology, Prague, Czech Republic, with a detection limit of 0.5 \(\mu\)g L\({}^{-1}\) and a standard deviation ranging from 5% to 10% of the mean.  
The concentration of total As as AsO\({}_{4}\)\({}^{3-}\) in the liquid samples was measured by performing hydride generation atomic fluorescence spectrometry (HG-AFS) using a PSA 10.055 Millennium Excalibur system (PS Analytical, Kent, UK). The samples were pretreated with a solution of HCl (36% \(w/v\)) and KI (50%) with ascorbic acid (10%). The measurement was performed in milligram per litre and microgram per litre modes using HCl (12%), with a KI + ascorbic acid solution as a reagent blank and 7% NaBH\({}_{4}\) in 0.1 mol L\({}^{-1}\) NaOH as a reductant. The detection limit was 5 \(\mu\)g L\({}^{-1}\), and the standard deviation was experimentally determined to be 2.5%.