# Adsorption of Methyl Orange on a Novel
## 1 Introduction  
Natural environments, and especially water bodies, receive high amounts of xenobiotics due to human activities [1]. Synthetic dyes represent a class of organic pollutants that end up in wastewater in high amounts, having detrimental effects on the natural environment [2]. They are commonly used in different production lines such as the paper, printing, textile, food, and pharmaceutical industries. The annual discharge of dyes and wastes containing high amounts of dyes into water streams worldwide by textile industries is estimated to be around 5000-8000 tons [3, 4, 5], and many of the dyes found in industrial wastes are harmful for living organisms [6]. Different technologies have been developed and applied for dye removal from effluents, including biological treatment, advanced oxidation, membrane separation, and adsorption [7, 8, 9]. However, some of these methods are insufficient, and they may require the use of specialized set-ups or the introduction of chemicals that end up in the environment [10, 11]. Among the different processes suggested, adsorption is probably the most effective method for contaminated water treatment for the removal of a variety of pollutants, both organic and inorganic. This is mostly due to operational advantages, simplicity, high efficiency, and cost effectiveness [12, 13, 14, 15]. In absorption processes, efficiency is solely defined by the absorbent. Therefore, given their vital role, many different types of adsorbents, such as natural media and bioproducts of another process [16, 17], activated carbon [18], and synthetic porous materials [19], have been applied in wastewater treatment. Among the different types of absorbents, natural clay minerals have been identified as decisive media for dye adsorption due to their inherentproperties such as nonoxicity, swelling, cation exchange, significant surface capacity, and their wide availability [20; 21]. However, although untreated natural clays have high cation exchange capacity due to their negatively charged layers, they present significant limitations in anion adsorption [22; 23]. Palygorskite is characterized by micro-fibrous morphology with a Si\({}_{\text{S}}\)O\({}_{20}\)Al\({}_{2}\)Mg\({}_{2}\)(OH)\({}_{2}\)(OH)\({}_{4}\)\(\cdot\)4H\({}_{2}\)O structural formula [24]. It possesses excellent physical and chemical properties, showing high potential for industrial, catalytic, and environmental cleaning-up applications [23; 25; 26; 27]. Usually, chemical or thermal surface modification is a prerequisite to increase the adsorption ability against anionic chemical moieties [28; 29]. This adds an extra step to the development of efficient absorbents. Metal-organic frameworks (MOFs) are a new class of synthetic materials that possess highly crystalline and porous structures. MOFs have been effectively applied as solid absorbing media in the gas and the liquid phases, including dye removal [3; 11; 30; 31]. They are made of metal clusters or metal ions that are connected together with organic linkers, forming a highly porous structure. Different MOF structures have been studied for the removal of organic dyes, such as UiO-66, MIL-family, ZIF-8, and their derivatives [3; 11; 32]. Due to the vast range of ligands and metal nodes, their properties can be fine-tuned toward a specific application, including dye adsorption [3; 33]. Among the different MOF structures, UiO-66 and its derivatives have received great attention due to their great stability and exceptional tunability and functionality [34]. As a general rule, dye adsorption is mostly driven through electrostatic interactions, although other mechanisms may occur, i.e., via \(\pi\)-\(\pi\) interactions [3; 11]. [PERSON] showed the great efficiency of UiO-66 and modified versions with immobilized phosphate over a range of dyes including anionic dyes [35]. However, other studies showed low removal efficiency via adsorption of methyl orange (MO) using UiO-66 and its NH\({}_{2}\)- derivative [36].