The Congo Red dye was removed from a simulated textile wastewater solution using fly ash from a local power plant. The characterisation of fly ash was studied in detail by SEM, EDX, XRD, FTIR, BET surface area and TGA techniques. The influence of four parameters (contact time, initial concentration, adsorbent dose, and temperature) was analysed, the results showing that the adsorption capacity depends on these parameters. Thermodynamic and regeneration investigations as well are presented. The fit to pseudo-second-order kinetics models suggests that the removal process is a chemical adsorption. The Langmuir model fitted the experimental data, with a maximum adsorption capacity of 22.12 mg/g. The research is a preliminary case study that highlights that fly ash posed a very good potential as a material for Congo Red dye removal.

The amendments may be natural or synthetic and by their origin organic or inorganic. Among amendments, the inorganic ones are recommended, such as gypsum, lime, zeolites and altered rocks, including sand, ceramic, perlite, ash, slag, zeolite, pyrites, dolomite, calcined clay, etc. Fly ash-based zeolites can be used as amendments to improve the use of the soil for agricultural purposes. Generally, there are different methods for the synthesis of new materials using solid waste -fly ashes. Methods are known to be composed of a single stage or two stages. The first method -the conventional -is direct hydrothermal conversion of the mixture of ash and alkaline solution (NaOH or KOH), but only 50% of ashes can be converted into zeolite. The second method consists in mixing the ash with KOH and fusion at elevated temperature. The method leads to an advanced conversion -the type of zeolite depending on treatment conditions. Other studies recommend the conversion of ash using microwave ovens or ultrasound bath; in this case, the conversion time is reduced to 1-2 hours. From these methods, the direct method was proposed to be used in synthesizing zeolites for agriculture, with a part in controlling release of fertilizers and as a soil amendment. The synthesized materials were characterized with respect to microstructure (electronic microscopy SEM) and chemical and mineralogical composition (EDAX, X-ray diffraction). By analyzing the obtained data, one may observe the destruction of the ash network and crystallization of the zeolitic phase, especially in the case of treatment by diffusion. The XRD analysis confirms the presence of zeolite in materials obtained. Good results were also obtained in the case of using ultrasound treatment for zeolite synthesis; also, the time of treatment significantly decreased in this case.

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The initial characteristics of Romanian fly ash from the CET II Holboca power plant show the feasibility of its application for the production of a new material with applicability in environmental decontamination. The material obtained was characterized using standard techniques: scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), instrumental neutron activation analysis (INAA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), the Brunauer–Emmett–Teller (BET) surface area, and thermogravimetric differential thermal analysis (TG-DTA). The adsorption capacity of the obtained material was evaluated in batch systems with different values of the initial Cu(II) ion concentration, pH, adsorbent dose, and contact time in order to optimize the adsorption process. According to the experimental data presented in this study, the adsorbent synthesized has a high adsorption capacity for copper ions (qmax = 27.32–58.48 mg/g). The alkali treatment of fly ash with NaOH improved the adsorption capacity of the obtained material compared to that of the untreated fly ash. Based on the kinetics results, the adsorption of copper ions onto synthesized material indicated the chemisorption mechanism. Notably, fly ash can be considered an important beginning in obtaining new materials with applicability to wastewater treatment.

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