# Synthesis of Zeolitic Material with High Cation Exchange Capacity from Paper Sludge Ash Using EDTA
## 1 Introduction  
Zeolites are a group of microporous aluminosilicate minerals that are tectisolicates with three-dimensional aluminosilicate structures containing water molecules, alkali, and alkaline earth metals in their structural framework [1, 2]. Zeolites have unique properties owing to their specific structure, i.e., their aluminosilicate skeleton structure with a hierarchical character. The center of the tetrahedron is an atom \"T\" (Si or Al), which is linked to four other T-atoms by an oxygen bond. The tetrahedrons AlO\({}_{4}\) and SiO\({}_{4}\) are known as the primary building units (PBUs). These tetrahedrons can be arranged into larger structures called secondary building units (SBUs) consisting of up to 16 T atoms, and it is assumed that the entire framework is made up of only one type of SBU. Presently, 23 different types of SBUs are known to exist, and approximately 250 zeolite framework types have been defined in the \"Atlas of zeolite framework types\" [3]. Based on the Si/Al ratio, zeolites can be divided into: (i) low (1-1.5), (ii) intermediate (2-5), (iii) high silica zeolites (10-100), and (iv) silica molecular sieves (>100) [4]. With decreasing Si/Al ratios, the zeolites are more hydrophilic and have a greater affinity for polar adsorbates. Generally, according to Loewenstein's role, the amount of tetrahedral AlO\({}_{4}\) should not be higher than SiO\({}_{4}\) in the zeolite structure, and the lowest Si/Al ratio has the highest negative charge in the skeleton structure [5]. Zeolites occur naturally in the environment, but they can also be synthesized in a laboratory or industry [6]. The outstanding physicochemical properties of zeolites, such as their cation exchange, catalysis, and sorption, make them very useful in a variety of industrial [7, 8], environmental [9, 10, 11], biotechnological [12, 13, 14], biomedical [15, 16], and medical applications [17, 18]. Numerous researchers have reported the synthesis of various types of zeolites (such as zeolite-A, P, and X, and ZSM-5) from industrial waste (for example, coal fly ash, paper sludge ash, municipal incinated ash, and waste porcelain or perlite), which are mainly composed of SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\) in amorphous phases [19, 20, 21, 22, 23, 24, 25, 26, 27], via differenthydrothermal activation methods, such as classic alkali activation [28], alkali fusion [29], two-stage synthesis [30], acid leaching [31], and addition of silica sources [32; 33]. In the manufacture of recycled paper, paper sludge is discharged as industrial waste, and its amounts have continued to increase annually worldwide, for example, 11 million tons were discharged in Europe [34], 5.1 million tons were generated in Japan [35], and 2.6 million tons were generated in Korea [36]. Paper sludge ash (PSA) is produced by the fluidized bed combustion of paper sludge for the volume reduction of waste sludge for landfills (80-90% reduction) and partial recovery of energy from paper sludge combustion. Although a part of PSA is used for some applications such as soil improvement, cement additives, and water treatment [37; 38], a large part of the ash is landfilled to cause environmental problems, incurring high costs for the paper industry due to the shortage of landfill sites. Therefore, alternative routes to the landfilling and valorization of PSA are desired. In the past few decades, PSA has been used for the synthesis of functional minerals, such as zeolites and tobermorites [39; 40]. Owing to their ion-exchange properties and unique structures, they are used in various applications such as gas removal and water purification [41; 42; 43]. Some methods for the chemical conversion of PSA into functional materials exist, such as the conventional hydrothermal reaction [44], alkali fusion [45], and the use of silica additives [46]. Calcite, which is used in this study as a filler, has been recently used to make ash with high calcium content in the form of anorthite, gehlenite, and amorphous phases.