Due to their cost-effectiveness and high surface area, activated carbons are commonly used for the adsorption of dyes from aqueous solutions. In this study, activated carbon was synthesized from walnut shell waste via KOH activation (1:3 ratio), yielding a surface area of 2347.4 m²/g. Reactive Blue 19 and Reactive Red 195 adsorption behavior were studied under varying experimental conditions. These included natural pH values (6.8–7.2), dye concentrations between 50 and 1250 mg L⁻¹, and adsorbent dosages ranging from 0.1 to 1.0 g. Adsorption equilibrium was achieved within 150 min. The maximum adsorption capacities were found to be 1227.17 mg g⁻¹ for RB 19 and 235.74 mg g⁻¹ for RR 195. Isotherm modeling was conducted using Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich models, with Freundlich providing the best fit for both dyes, indicating multilayer adsorption on heterogeneous surfaces. Thermodynamic analysis revealed that the adsorption processes were spontaneous and endothermic, with negative Gibbs free energy (ΔG°), positive enthalpy (ΔH°), and positive entropy (ΔS°) values. These results highlight the high adsorption performance and practical potential of walnut shell-derived activated carbon for dye removal from wastewater.

Activated carbon is widely recognized as an effective material for removing pollutants, especially pharmaceutical residues, from water. In this study, high-surface-area activated carbon derived from rice husks (RHAC) was synthesized via KOH activation and used for the adsorption of ciprofloxacin, a widely used fluoroquinolone antibiotic. Its adsorption behavior was systematically investigated through batch experiments varying the pH, adsorbent dosage, contact time, initial concentration, and temperature. The RHAC exhibited a high surface area of 1539.7 m²/g and achieved a maximum adsorption capacity of 398.4 mg·g⁻¹. The Freundlich isotherm best describes its adsorption equilibrium, suggesting multilayer adsorption on a heterogeneous surface. Kinetic modeling revealed that the adsorption process followed a pseudo second-order model (R² = 0.9981), indicating chemisorption as the rate-limiting mechanism. Thermodynamic parameters (ΔH° = 6.61 kJ/mol, ΔG° < 0) confirmed that the process was endothermic and spontaneous. These findings demonstrate that RHAC is a highly efficient, low-cost, and sustainable adsorbent for removing ciprofloxacin from aqueous environments.

In this study, the removal of Reactive Blue 19 dyestuffs in aqueous systems was investigated by adsorption method using activated carbon obtained from the pumpkin seed waste. Activated carbon obtained from pumpkin seed waste functionalized with ZnCl2 was used as an absorbent. Pumpkin seed hydrochar was characterized by FT-IR, SEM, TGA-DTA, BET, and XPS. In the experimental stages, the adsorption equilibrium time was determined as 45 minutes, the adsorbent dosage was 0.8 g and the optimum pH was 6.0. After this step, the adsorption parameters of Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherms were investigated. It has been pointed out that the adsorption process fits better with the Freundlich isotherm model, and the adhesion occurs in a multilayered manner and on a heterogeneous surface. Freundlich and Dubinin-Radushkevich isotherms support that the bonding mechanism is realized by physical interactions. When the kinetic data were evaluated, it was found to be compatible with the pseudo-second-order kinetic model. The thermodynamic parameters of adsorption indicate that the system is endothermic, and the adsorption of Reactive Blue 19 on activated carbon is a spontaneous process.

Valorization of agricultural residues into high-performance porous carbons is an effective route to sustainable adsorbents. Here, walnut green outer shell and pistachio pink outer shell were converted into activated carbons via chemical activation using KOH (1:1, 1:2, 1:3) and ZnCl₂ (1:1). Without any pretreatment, precursors were carbonized at 500 °C (1 h, nitrogen atmosphere), followed by activation at 800 °C (KOH, 1 h) or 500 °C (ZnCl₂, 1 h). Textural analyses revealed a strong dependence on both precursor type and activating agent. Walnut-shell carbons reached exceptionally high surface areas of 1028–2347 m² g⁻¹, with the maximum obtained for KOH (1:3), whereas pistachio-shell carbons achieved 788–1324 m² g⁻¹ under the same KOH series. In contrast, ZnCl₂ activation produced markedly lower areas (445–750 m² g⁻¹) and grinding caused only minor changes. FTIR/XRD/SEM/Elemental analyses collectively supported the formation of defect-rich, turbostratic carbon frameworks with well-developed porous morphologies, highlighting walnut shell as a particularly promising precursor for sustainable, high-surface-area activated carbons suitable for adsorption-driven environmental applications.