In order to assess the potential use of low-cost materials for dye removal from aqueous solutions, the adsorption of cationic dye methylene blue (MB) onto orange tree sawdust was studied under static mode using raw sawdust (ROS) and chemically modified sawdust (MOS). The effect of several parameters such as contact time, initial dye concentrations, initial pH, adsorbent dose, and temperature were also investigated. Results showed that the adsorption kinetic data of MB onto both materials were well fitted by the second-order model and the equilibrium state was reached after 180 min of contact time. For both ROS and MOS, MB removal efficiency was improved by the increase in the initial aqueous concentrations, adsorbent dose, and aqueous pH. Moreover, MB adsorption data at equilibrium were well fitted by Langmuir model suggesting a probable monolayer adsorption process. The chemical treatment of the orange tree sawdust with sodium hydroxide (1 M) significantly increased the density of sorption sites and lead to the appearance of new functional groups. Therefore, MB removal capacity increased from about 40 mg/g for ROS to 111 mg/g for MOS at an initial pH value 6.0. The thermodynamic study demonstrated that MB adsorption was endothermic for ROS and spontaneous and exothermic for MOS, respectively. Desorption experiments with HNO₃ acidic solutions proved that MB was significantly desorbed from the tested adsorbents, which offers a possible reusability. All these findings indicate that alkaline-treated orange tree sawdust could be employed as an efficient low-cost and eco-friendly adsorbent for cationic dye removal from industrial wastewaters.

Lignite, as an available and low-cost material, was tested for cadmium (Cd) and copper (Cu) removal from aqueous solutions under various static experimental conditions. Experimental results showed that the removal efficiency of both metals was improved by increasing their initial concentrations, adsorbent dosage and aqueous pH values. The adsorption kinetic was very rapid for Cd since about 78% of the totally adsorbed amounts were removed after a contact time of only 1 min. For Cd and Cu, the kinetic and isothermal data were well fitted with pseudo-second order and Freundlich models, respectively, which suggests that Cd/Cu removal by lignite occurs heterogeneously on multilayers surfaces. The maximum Langmuir’s adsorption capacities of Cd and Cu were assessed to 38.0 and 21.4 mg g⁻¹ and are relatively important compared to some other lignites and raw natural materials. Results of proximate, scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), Fourier transform infrared spectroscopy (FTIR) and X-Ray diffraction (XRD) showed that the removal of these metals occurs most likely through a combination of cation exchange and complexation with specific functional groups. The relatively high adsorption capacity of the used lignite promotes its future use as a low cost material for Cd and Cu removal from effluents, and possibly for other heavy metals or groups of pollutants.

In the present study, methylene blue (MB) removal from aqueous solutions via the photocatalytic process using TiO₂ as a catalyst in the presence of external ultra-violet light (UV) was investigated. The results of adsorption in the absence of UV radiation showed that adsorption reached an equilibrium state at 60 min. The experimental kinetic data were found to be well fitted by the pseudo-second-order model. Furthermore, the isotherm study suggested that dye uptake by TiO₂ is a chemisorption process with a maximum retention capacity of 34.0 mg/g. The photodegradation of MB was then assessed under various experimental conditions. The related data showed that dye mineralization decreased when dye concentrations were increased and was favored at high pH values and low salt concentrations. The simultaneous presence of organic and inorganic pollution (Zinc) was also evaluated. The effect of the molar ratio Zn²⁺/MB⁺ in the solution at different pH values and NaCl concentrations was also monitored. The corresponding experimental results showed that at low values of Zn²⁺ in the solution (30 mg/L), the kinetic of the MB removal became faster until reaching an optimum at Zn²⁺/MB⁺ concentrations of 60/60 mg/L; it then slowed down for higher concentrations. The solutions’ carbon contents were measured during the degradation process and showed total mineralization after about 5 h for the optimal Zn²⁺/MB⁺ condition.