Browsing by Autor "Stahl, Ullrich"

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Adsorption dynamics of Cd2+(aq) on microwave-synthetized pristine biochar from cocoa pod husk: Green, experimental, and DFT approaches.
(2024) Correa-Abril, Jhonny; Stahl, Ullrich; Cabrera, Elvia V; Parra, Yonathan J; Vega, Michael A; Taamalli, Sonia; Louis, Florent; Rodríguez-Díaz, Joan Manuel
Biochar obtained via microwave-assisted pyrolysis (MAP) at 720 W and 15 min from cocoa pod husk (CPH) is an efficient adsorbent of Cd2+(aq). Biochar of residual biomass of CPH (BCCPH) possesses favorable physicochemical and morphological properties, featuring a modest surface area yet a suitable porous structure. Adsorption, predominantly governed by physisorption, is influenced by the oxygen-containing active sites (-COOR, -C(R)O, and -CH2OR; R = H, alkyl). CdCO3 formation occurs during adsorption. Experimental data were well-fitted into various kinetic models for a broad understanding of the sorption process. Langmuir model indicates a maximum adsorption capacity of 14.694 mg/g. The thermodynamic study confirms the spontaneous and endothermic sorption. Studies at the molecular level have revealed that the Cd2+ ion tends to bind to surface aromatic carbon atoms. This sustainable approach produces BCCPH via MAP as a solution for waste transformation into water-cleaning materials.
Kinetic, equilibrium, and thermodynamic study of Methylene Blue adsorption on orange peel biochar prepared by microwave-assisted pyrolysis.
(2026) Correa-Abril, Jhonny; Cabrera, Elvia V; Robles, Nilo; López Terán, J L; Stahl, Ullrich
UNLABELLED: This study presents a sustainable approach for Methylene Blue (MB) dye removal using pristine, non-activated biochar derived from orange peel waste via Microwave-Assisted Pyrolysis (MAP). The key novelty lies in the systematic comparison of the biochar's adsorption performance under both pH-controlled (constant pH 4) and unregulated pH conditions, demonstrating that pH regulation is essential for optimizing adsorption efficiency. The resulting biochar exhibited a high fixed carbon content (60.89%), an alkaline surface (Point of zero charge (pHpzc) = 11.20, ZPotential = 0.1 mV @ pH 9), and oxygenated functional groups. Best MB removal of 83% was achieved at pH 4, despite the expected electrostatic repulsion. Kinetic studies showed the best fit with the Elovich model, indicating a heterogeneous surface. The Langmuir isotherm accurately described the equilibrium data, revealing a maximum adsorption capacity (qmax) of 20.57 mg g⁻1 under pH-controlled conditions, representing an 83% increase over the 11.24 mg g⁻1 obtained in the unregulated scenario. Thermodynamic analysis confirmed the process is spontaneous (ΔG° < 0), endothermic (ΔH° = + 4.88 kJ mol⁻1 at constant pH), and governed by physisorption mechanisms, including hydrogen bonding and π-π interactions. This work demonstrates that pristine orange peel biochar generated via MAP is a highly effective adsorbent and highlights the critical impact of pH control on improving adsorption capacity and elucidating the dominant physisorption mechanisms. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1038/s41598-026-36741-6.
Long-Term Stability of Polymer-Coated Surface Transverse Wave Sensors for the Detection of Organic Solvent Vapors.
(2017) Stahl, Ullrich; Voigt, Achim; Dirschka, Marian; Barié, Nicole; Richter, Christiane; Waldbaur, Ansgar; Gruhl, Friederike J; Rapp, Bastian E; Rapp, Michael; Länge, Kerstin
Arrays with polymer-coated acoustic sensors, such as surface acoustic wave (SAW) and surface transverse wave (STW) sensors, have successfully been applied for a variety of gas sensing applications. However, the stability of the sensors' polymer coatings over a longer period of use has hardly been investigated. We used an array of eight STW resonator sensors coated with different polymers. This sensor array was used at semi-annual intervals for a three-year period to detect organic solvent vapors of three different chemical classes: a halogenated hydrocarbon (chloroform), an aliphatic hydrocarbon (octane), and an aromatic hydrocarbon (xylene). The sensor signals were evaluated with regard to absolute signal shifts and normalized signal shifts leading to signal patterns characteristic of the respective solvent vapors. No significant time-related changes of sensor signals or signal patterns were observed, i.e., the polymer coatings kept their performance during the course of the study. Therefore, the polymer-coated STW sensors proved to be robust devices which can be used for detecting organic solvent vapors both qualitatively and quantitatively for several years.