## 2 Materials and Methods
### Characterization of Materials  
The JJ sample, the product of its alkaline decomposition, and the final product of the As(V) and Cr(III) adsorption stage (liquid and solid), were characterized using the following analytical techniques.  
To disclose the mineral species contained in the samples, X-ray diffraction using the INEL Diffractometer Equinox 2000 model located at the Autonomous University of the State of Hidalgo, Mexico (AUSH) and manufactured by INEL at Artenary, Centre Val de Loire, France. For this analysis, the sweep time was 0.5 min for each sample, and the indexing for the obtained diffractograms was carried out with the MATCH version 1.1 software (developed by Crystal Impact, Bonn, Germany).  
Similarly, the morphological and punctual composition analysis was performed using the JSM-IT300 JEOL Scanning Electron Microscope, manufactured by JEOL Tokyo, Japan (and located at the AUSH), using an operating voltage of 30 keV and equipped with an Oxford energy dispersive spectrometer (EDS).  
In addition to determining the presence both of As(V) and Cr(III) in the resulting solids after adsorption experiments, a total reflectance Fourier-transform infrared (ATR-FTIR) analysis was carried out. This examination was performed using a Perkin Elmer Frontier FTIR spectrometer manufactured in Waltham, MA, USA (and located at the AUSH), which is a high-sensitivity and high-resolution instrument capable of analyzing the infrared spectral region [56]. A 10 mg sample of powdered material was carefully placed on the crystal surface of the equipment, and each obtained spectrum was recorded as the absorbance under 75%. Each spectrum was scanned between the 4000 and 400 cm\({}^{-1}\) wavelengths, covering the mid-infrared and near-infrared spectral regions [57]. FTIR spectroscopy is a widely used technique for analyzing the chemical composition of samples, providing information on the presence of functional groups and chemical bonds [58]. In this case, FTIR was used to detect the presence of As(V) and Cr(III) in the resultant solids after adsorption. The ATR-FTIR technique was employed to analyze the sample in its solid state, eliminating the need for sample preparation in a liquid or gaseous form [59].  
On the other hand, the X-ray photoelectron spectroscopy (XPS) analysis was executed in a K ALPHA Surface Analysis Machine (Thermo Fisher Scientific) manufactured in Waltham, MA, USA (and located at the laboratory of Nano and Biomaterials, CINVESTAV-IPN, Merida Yucatan, Mexico). This instrument has a hemispherical (180\({}^{\circ}\)) analyzer of double approach and a 128-channel detector with a base pressure of 2 \(\times\) 10\({}^{-9}\) mbar. The XPS spectra were first obtained in wide sweep (0-1350 V) at 1 eV/step, and then in the small-window mode at 0.1 eV/step, with a step energy of 50 eV.  
Finally, the analysis performed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was carried out with the purpose of determining the As(V) and Cr(III) contents in solution before and after conducting the adsorption experiments with the JJ samples. For this analysis, the ICP-MS equipment model 2100, manufactured by Perkin Elmer, Waltham MA, USA (and located at the AUSH), was used.