The studies of the kinetics and isotherms adsorption of the Reactive Black 5 (RB5) onto bentonite clay were explored in a batch study in a laboratory. The maximum RB5 adsorption conditions of bentonite clay were optimized such as shaking speed (100 rpm), temperature (323 K), pH (10), contact time (40 min), initial dye concentration (170 mg·L⁻¹), and particle size (177 µm). The adsorbent surface was characterized using Fourier Transform Infrared Spectroscopy spectroscopy. The mechanisms and characteristic parameters of the adsorption process were analyzed using two parameter isotherm models which revealed the following order (based on the coefficient of determination): Harkin-Jura (0.9989) > Freundlich (0.9986) and Halsey (0.9986) > Langmuir (0.9915) > Temkin (0.9818) > Dubinin–Radushkevich (0.9678). This result suggests the heterogeneous nature of bentonite clay. Moreover, the adsorption process was chemisorption in nature because it follows the pseudo-second order reaction model with R² value of 0.9998, 0.9933 and 0.9891 at 25, 75 and 100 mg·L⁻¹ RB5 dye in the solution, respectively. Moreover, based on the values of standard enthalpy, Gibbs free energy change, and entropy, bentonite clay showed dual nature of exothermic and endothermic, spontaneous and non-spontaneous as well as increased and decreased randomness at solid–liquid interface at 303–313 K and 313–323 K temperature, respectively.

Eucalyptus camdulensis biochar (Ec-bio) was used to adsorb crystal violet (CV) and methylene blue (MB) dyes, which was optimized and further evaluated using different isotherm and kinetic models. Microscopy and spectroscopy techniques showed the interactions of the dyes with the surface functional groups of the Ec-bio, resulting in the removal of the dyes from aqueous solution. Both dyes were immediately uptaken, with equilibrium reached in 60 min, with a higher sorption efficiency of CV compared to MB. Thermodynamic parameters showed endothermic adsorption and the nonspontaneous adsorption of both dyes onto the Ec-bio. Both the adsorption capacity and percentage removal increased with the increasing solution pH from 2.0 to 4.0 and to 10 for CV and MB. An increase in adsorption capacity was observed upon increasing the initial concentrations, with a corresponding decrease in the percentage removal. The pseudo-second-order (PSO) and Elovich kinetic models (nonlinear approach) were a good fit to the data of both dyes, confirming a chemisorptive adsorption process. The Langmuir isotherm fitted well to the CV data, supporting its monolayer adsorption onto the Ec-bio, while the Freundlich isotherm was a good fit to the MB dye data, suggesting the surface heterogeneity of the Ec-bio. The Dubinin–Radushkevich isotherm was a good fit to the adsorption CV data compared with the MB dye, suggesting the physisorption of both dyes onto the Ec-bio due to its mean free energy of adsorption of <8 kJ mol⁻¹.

The production of biosorbents by waste biomass has attracted considerable attention due to the low cost and abundance of the raw materials. Here biochar produced from Eucalyptus camdulensis sawdust (EU-biochar) via pyrolysis at 600 °C was used as a potential biosorbent for Ni²⁺ and Pb²⁺ metal ions from wastewater. Characterization experiments indicated the formation of C- and O-bearing functional groups on the EU-biochar surface, while shifts and changes in the shape of C–H bands suggested the adsorption of Ni²⁺ and Pb²⁺ onto EU-biochar by interacting with surface carboxylic groups. Pb²⁺ was adsorbed more quickly than Ni²⁺, indicating a faster and stronger interaction of Pb²⁺ with EU-biochar compared to Ni²⁺. As the initial concentrations of both metal ions increased, the percentage removal decreased, whereas increasing the EU-biochar dose improved the percentage removal but impaired the adsorption capacity for Ni²⁺ and Pb²⁺. The adsorption capacity could only be improved without affecting the percentage removal of both ions by increasing the pH of the metal solutions. The sorption efficiency of EU-biochar and the removal mechanism of Ni²⁺ and Pb²⁺ were further explored using non-linear and linear forms of kinetic and isotherm models.