# Synthesis of Zeolitic Material with High Cation Exchange Capacity from Paper Sludge Ash Using EDTA
## 3 Results and Discussion
A higher number of prismatic crystals can be observed in the product synthesized in a solution with 1 M EDTA than in that synthesized in a solution without 1 M EDTA. Platy shape crystals of tobormite can be observed in the product synthesized at 160 \({}^{\circ}\)C without the addition of 1 M EDTA (Figure 7c). Furthermore, cubic crystals of zeolite-A with a size of 1-2 \(\upmu\)m can be observed in the product synthesized at 80 \({}^{\circ}\)C with EDTA addition (Figure 7d), but these crystals are diminished in the products synthesized at 120 \({}^{\circ}\)C and 160 \({}^{\circ}\)C with EDTA addition (Figure 7e,f). This is because zeolite-A is a metastable phase at 60-100 \({}^{\circ}\)C and is easily transformed into the stable phases hydroxyocalite and hydroxycraninite at high temperatures [1]. These results suggest, to synthesize a product with zeolite-A phases, which have a high CEC, the reaction temperature should be 80 \({}^{\circ}\)C.  
Figure 8 show the XRD patterns of the products synthesized from raw ash during the reaction at 80 \({}^{\circ}\)C with and without 1 M EDTA addition. During synthesis without 1 M EDTA, almost the same patterns were confirmed from 0.5 h to 12 h, and the low peaks of zeolitic product phases (hydroxysodalite and zeolite-P) could be confirmed after the 24 h reaction. In contrast, during the synthesis with 1 M EDTA, almost the same patterns were confirmed from 0.5 h to 4 h, zeolite-A and zeolite-P were confirmed at 6 h, and hydroxysolat  
Figure 7: SEM photos of the synthesized products: (**a**) 80 \({}^{\circ}\)C without 1 M EDTA, (**b**) 120 \({}^{\circ}\)C without 1 M EDTA, (**c**) 160 \({}^{\circ}\)C without 1 M EDTA, (**d**) 80 \({}^{\circ}\)C with 1 M EDTA, (**e**) 120 \({}^{\circ}\)C with 1 M EDTA, and (**f**) 160 \({}^{\circ}\)C with 1 M EDTA (NaOH concentration: 3 M NaOH, Reaction time: 24 h). The concentrations of Si, Al, and Ca during synthesis with and without 1 M EDTA are shown in Figure 9, and the relative crystallinities of the product phases and CECs of the product during the reaction are shown in Figure 1:0 In the case of the reaction without 1 M EDTA, the concentration of soluble Al always exceeded that of Si during the reaction, and the Ca concentration was almost zero. Initially, the concentrations of Si and Al increased upon the addition of PSA; however, as the reaction progressed, the Si concentration gradually decreased owing to the formation of aluminosilicate or calcium aluminosilicate gels [55]. The Al concentration started increasing and remained almost constant after 4 h of the reaction. The relative crystallinities of zeolite-P and hydroxysoalite remained almost constant until 12 h and increased afterwards, while the CEC of the product gradually increased during the reaction because of the formation of silicate gels and zeolite crystals. In the case of the reaction with 1 M EDTA, the concentrations of Si, Al, and Ca initially increased, that of Si gradually decreased, that of Ca increased, and that of Al decreased to zero, reaching a steady-state after 8 h. This residual Al concentration was lower than that in the former case, indicating that a large amount of aluminosilicate precipitated out of the solution. The relative crystallinities of zeolite-A and zeolite-P increased after 6 h, whereas that of hydroxysoalite increased after 8 h. The CEC of the product increased for 8 h and then gradually increased, which is consistent with the relative crystallinities of the zeolite phases. Figure 1:1 XRD patterns of the products synthesized at 80 \({}^{\circ}\)C (**a**) without the addition of 1 M EDTA and (**b**) with 1 M EDTA during the synthesis reaction (NaOH concentration: 3 M NaOH). In summary, we propose the following mechanism for 1 M EDTA addition at 80 \({}^{\circ}\)C, as shown in Figure 1:2 [PERSON] et al. studied the formation of zeolites from coal fly ash and found that zeolite synthesis from coal fly ash consists of three reactions: dissolution, gelation, and crystallization [59].