# Synthesis of Zeolitic Material with High Cation Exchange Capacity from Paper Sludge Ash Using EDTA
## 3 Results and Discussion  
The chemical and mineralogical compositions of the PSA used in this study are shown in Table 1 and Figure 1, respectively. Ash is mainly composed of CaO (42.6%), SiO\({}_{2}\) (28.1%), and Al\({}_{2}\)O\({}_{3}\) (18.5%) in the form of crystalline phases, gehlenite (Ca\({}_{2}\)Al\({}_{2}\)SiO\({}_{7}\)), anorthite (CaAl\({}_{2}\)Si\({}_{2}\)O\({}_{8}\)), and amorphous phases [54]. Figure 1 also show the XRD patterns of the products as a function of EDTA addition. PSA contains two mineral phases: gehlenite and anorthite. When 1 M EDTA was added, the product phases changed from a mixture of hydroxyodalite (Na\({}_{6}\)Al\({}_{6}\)Si\({}_{6}\)O\({}_{24}\)-8H\({}_{2}\)O) and zeolite-P (Na\({}_{6}\)Si\({}_{10}\)Al\({}_{6}\)O\({}_{32}\)-12H\({}_{2}\)O) to a mixture of hydroxyodalite, zeolite-P, and zeolite-A (Na\({}_{12}\)Si\({}_{12}\)Al\({}_{12}\)O\({}_{48}\)-27H\({}_{2}\)O), with high peaks for hydroxyodalite and zeolite-A. For concentrations of EDTA above 1 M, the product had almost the same XRD patterns as raw PSA, indicating that the zeolite synthesis reaction did not occur. Hydroxysodalite is a member of the sodalite group, and its structure comprises a framework of four--and six-membered rings of SiO\({}_{4}\) and AlO\({}_{4}\) tetrahedra. Zeolite-P is a member of the gismondine group, and its structural framework is composed of four- and eight-membered rings. Zeolite-A crystallizes in a typical cubic form, and its structural framework consists of four--and eight-membered rings. Notably, the eight-membered rings are larger than the six-membered rings of hydroxysodalite, while the Si/Al ratio is the same (Si:Al = 1:1). The \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Oxide** **(wt.\%)**} & \multirow{2}{*}{**PSA**} & \multicolumn{4}{c}{**Without 1 M EDTA**} & \multicolumn{4}{c}{**With 1 M EDTA**} \\ \cline{3-8}  & & **80 °C** & **120 °C** & **160 °C** & **80 °C** & **120 °C** & **160 °C** \\ \hline SiO\({}_{2}\) & 28.1 & 31.2 & 29.0 & 28.7 & 41.3 & 46.2 & 45.1 \\ Al\({}_{2}\)O\({}_{3}\) & 18.5 & 12.1 & 13.0 & 13.1 & 24.2 & 25.8 & 24.7 \\ CaO & 42.6 & 46.4 & 47.3 & 47.2 & 19.6 & 11.7 & 13.3 \\ MgO & 4.0 & 3.5 & 3.5 & 3.9 & 5.1 & 5.3 & 5.1 \\ Fe\({}_{2}\)O\({}_{3}\) & 1.6 & 1.9 & 1.9 & 1.9 & 2.8 & 3.0 & 3.0 \\ TiO\({}_{2}\) & 2.7 & 3.0 & 3.1 & 3.1 & 4.5 & 4.8 & 4.8 \\ Others & 2.5 & 1.9 & 2.2 & 2.1 & 2.5 & 3.2 & 4.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical compositions of the PSA. Figure 1: XRD patterns of the products obtained with the addition of EDTA (NaOH concentration: 3 M NaOH, Temperature: 80 °C, Reaction time: 24 h). order of the Si/Al ratios in the zeolite structure is hydroxysodalite (1:1) = zeolite-A (1:1) < zeolite-P (5:3), and the CEC of zeolite-A is generally higher than those zeolite-P and hydroxysodalite because zeolite-A has a lower Si/Al ratio and ring size. Figure 2 show the concentration of Si, Al, and Ca in the solution after synthesis as a function of EDTA addition. As the concentration of the added EDTA increased to 1 M, the Ca concentration increased due to Ca-masking with EDTA, and above an EDTA concentration of 1 M, the Ca concentration drastically decreased to zero. The Si concentration increased in the solution with 0-0.3 M EDTA and gradually decreased in the solution with an EDTA concentration higher than 0.3 M. The Al concentration drastically decreased in the solution with 0-0.3 M EDTA and gradually decreased in the solution with an EDTA concentration higher than 0.3 M. Without EDTA addition, the Si, Al, and Ca concentrations in the solution were 227, 2225, and 0 mg/L, respectively, while those in the solution containing 1 M EDTA were 728, 228, and 21,434 mg/L, respectively. Although the reaction between Ca and Si is stronger than that between Si and Al, the Si-Al reaction is promoted by Ca-masking with EDTA [47]. Figure 3 show the relative crystallinities of the zeolite crystals in the product and the CECs of the product as a function of the concentration of the added EDTA.