## 1 Introduction  
Fossil fuels have always been the primary source of the global energy supply. Algeria has an important energy potential: It holds the tenth largest gas reserve in the world and the third largest oil reserves in Africa [1]. Among all fossil fuel energy sources, natural gas (NG) is the cleanest and is a safe source when transported, stored, and used [2]. The chemical composition of the crude NG found in oil and gas reservoirs is composed of methane (as a major component), and in smaller quantities, ethane, propane, and butane, among other gaseous hydrocarbons. It also contains undesirable compounds such as water vapor, carbon dioxide, nitrogen, and hydrogen sulfide [3; 4]. In industrial use, natural gas is subjected to different cleaning processes to meet product quality standards. Two main standards can be distinguished: Liquid natural gas (LNG) and pipeline gas [5]. The water vapor contained in NG must be removed from the gas stream for the following reasons: Reducing the heating value of the produced gas, condensation of the water in the pipelines, and the risk of corrosion [2]. Extensive literature is available on common gas dehydration systems including glycol absorption, zeolite adsorption, and refrigeration-based systems [6]. Among these processes, adsorption on zeolite, especially the Temperature Swing Adsorption (TSA) system, is the most frequently used industrially for technical and energetic considerations [7]. The simplest configuration for the application of a TSA system to obtain a continuous flow of produced gas is the fixed double-bed system, with one operating in adsorption mode and the second in desorption mode (regeneration) [5]. Due to the higher capacity of water adsorption and selectivity, as well as its regeneration potential for several cycles of adsorption, the 4A zeolite adsorbent is widely used for the fine drying of NG in the TSA system [5; 7]. Zeolites [8] are crystalline microporous aluminosilicates with a three-dimensional framework structure bearing AlO\({}_{4}\) and SiO\({}_{4}\) tetrahedra. These are linked to each other by sharing all of the oxygen to form interconnected cages and channels [9] with the characteristics of high ion exchange capacity and excellent selectivity (size, charge, and shape) and porosity on the surface [10]. Zeolites present a high potential for the purification of gases, especially those containing moisture [11]. 4A zeolite belongs to the LTA-type, with a pore size of 0.4 nm and a Si/Al ratio of approximately 1. This low Si/Al ratio, owing to the high aluminum content, makes the material highly hydrophilic [12]. The dehydration unit in the gas plant TFT of the Sonatrach company in Algeria generates important amounts of 4A zeolites after using the material in several adsorption and regeneration cycles, which reduces its adsorptive capacity with respect to the desired product quality during the dehydration operation. Under such a situation, the adsorbent is treated as solid waste to be disposed of in the technical landfill center near the plant site. The reuse of this adsorbent in wastewater treatment can be a possible way to valorize it by increasing its lifetime before it is considered waste. Several studies have applied natural and synthetic zeolites [13; 14; 15; 16; 17; 18], as well as other eco-friendly materials [19], in wastewater treatment. In the present paper, batch experiments were carried out to separately remove methylene blue (MB) or malachite green (MG), two very popular and toxic cationic dyes [20; 21], by the adsorption technique using 4A zeolite solid waste from the gas dehydration unit of TFT plant (Sonatrach company). This adsorbent material is named '4A zeolite waste' (4 AZW) here. The characterization of the 4 AZW sample was performed by BET surface area analysis, X-ray diffraction, and FT-IR analysis. The influence of temperature, pH, amount of adsorbent, contact time, and initial MB or MG concentrations on the removal rate of those dyes were studied under shaken conditions.