**1. Introduction**

Wastewater polluted with large amounts of dyes is usually dumped into aqueous effluents from the food and plastic, leather, paper, printing, and textile industries [1]. Dyes have been widely used in many industries for coloration purposes thanks to their good features, such as easy application, low production cost, bright colour, and water-fastness [2]. Nowadays, more than 100,000 commercial dyes exist with over 7 <sup>×</sup> 105 tons of dyes produced per year, and approximately 10–15% are discharged from textile industries [3]. Dyes are organic compounds that are classified according to the chemical composition and type of application. Commercial dyes are categorized into three classes: cationic, anionic, and nonionic, according to the charge after its dissolution in water. Azo compounds are the most typical dyes used for industrial purposes [4–6].

Dyes are non-biodegradable, stable, oxidizing agents and are highly toxic and mutagenic to aquatic life and humans [7]. Their discharge may produce severe hazard to aquatic living organisms, affecting different processes of aquatic vegetation, decreasing the oxygen levels in water and resulting in the choking of aquatic fauna and flora [8]. Thus, it is essential to reduce and remove organics pollutants from wastewater before discharging them [9]. Various methods have been applied for the removal of harmful contaminants from water and wastewater, involving adsorption on activated carbons, reverse osmosis, chemical oxidation, membrane filtration, bacterial action, coagulation and flocculation, activated sludge, ozonation, precipitation, electro-dialysis, ion exchange, and electrochemical techniques [10,11]. However, many of these methods are expensive and exhibit other drawbacks such as poor effectiveness and formation of sludge excess, and are thus unsuitable to be applied by small-scale industries [1]. Between them, adsorption is considered the most feasible method and has become one of the effective and easiest physico-chemical treatment procedures for the decolourization of textile wastewater. It offers several advantages such as low initial costs, high efficiency, producing nontoxic intermediates or by-products, high removal capability, versatility, easy handling, fast adsorption rate, and facile separation [2,12].

The adsorption based on activated carbon is widely employed to remove dyes, however, it still remains an expensive process owing to its high initial cost and the need for a regeneration system of the adsorbent that make it less economically viable [8]. Recently, more effective and cheaper adsorbents based on by-products from poultry waste, agricultural waste, and other natural waste have been developed as an alternative to conventional wastewater treatment processes [13].

In the last decades, researchers have been paying attention to the adsorption properties of agricultural wastes to develop new biosorbents in accordance with circular economy principles. Some alternatives include the orange peel, chitosan, eggshell, grape pomace, coffee residues, rice straw, olive stones, banana peel, artichoke agrowaste, sugarcane bagasse, and bamboo shell, among others [14–24]. This waste, which is eco-friendly, economic renewable, and available in abundance, is a candidate for the treatment of polluted water and wastewater [11]. Among the agricultural poultry by-products, the waste generated can be a promising biosorbent that has slightly increased in recent years. Moreover, the use of by-products from the agro-food sectors in the adsorption process helps to reduce the waste, as well as the low cost biomasses acquisition [17]. Waste materials obtained from different sources have been used as potential adsorbents for the removal of inorganic and organic pollutants. Eggshell and eggshell membranes are waste materials produced in large amounts in the poultry and farms industries as well as restaurants, bakeries, or homes [25].

Approximately 10% of the total mass of hen egg corresponds to eggshell by-product, with an average weigh of 60 g, and it is usually discarded in landfills without any pretreatment. In addition, this waste is commonly reused as soil conditioner, fertilizer, or additive for animal feed. The eggshell consists of three layers: the cuticle on the outer surface (mucin protein), the calcified eggshell (calcite or calcium carbonate crystals), and the eggshell membrane (protein fibers). Numerous pore channels are distributed on the surface of eggshell (between 7000 and 17,000 pores), thus eliciting water transpiration and gaseous exchanges. The porous nature of eggshell makes it a desirable material to be used as an adsorbent [11,13].

Retention of contaminants by adsorbents is not 100% efficient; therefore, additional methods are required to avoid non-retained dyes being disposed of into the environment. The degradation of dyes by an advanced oxidation process (AOP), using pulsed light technology as a photolyzer of hydrogen peroxide, is a novel, fast, and efficient version of AOPs that uses mercury-free lamps to generate energy-dense light [26].

Thus, the main objective of this paper was to evaluate the potential use of eggshell to remove Direct Blue 78 (DB78) dye in aqueous solution by adsorption, and to determine the effect of different parameters such as contact time, initial dye load, pH, and adsorbent concentration. The isotherms of adsorption, as well as the kinetics of dye adsorption on the eggshell, were evaluated by fitting the experimental data to different kinetics and isotherms models. Further, the efficiency of degradation of DB78 by a novel AOP in order to minimize dye discharge to the environment was measured.
