## 3 Results and Discussion
### Characteristics of Filter Residues
#### 3.6.2 Visual Appearance Analysis  
Figure 6 demonstrates the appearance changes of samples in their original state, after drying at 100 \({}^{\circ}\)C, and after calcination at 650 \({}^{\circ}\)C. In the original solution state, the four sample groups exhibited distinct color characteristics: A1 appeared light yellowish-brown, B1 showed brown, A2 displayed dark brown, and B2 presented a distinctive black color. These color variations likely reflect differences in oxidation states of ferrite treatment  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample Code** & **Treatment Method** & \begin{tabular}{c} **Magnetic Properties in** \\ **Solution** \\ \end{tabular} & \begin{tabular}{c} **100 \({}^{\circ}\)C Thermal** \\ **Treatment** \\ \end{tabular} & **650 \({}^{\circ}\)C Thermal Treatment** \\ \hline A1 & Single Ferrite Addition & No magnetic response & No magnetic response & Complete magnetic attraction \\ \hline B1 & \begin{tabular}{c} H\({}_{2}\)O\({}_{2}\) Pretreatment/ \\ Ferrite Addition \\ \end{tabular} & \begin{tabular}{c} Slight magnetic \\ response \\ \end{tabular} & \begin{tabular}{c} No magnetic \\ response \\ \end{tabular} & \begin{tabular}{c} Nearly complete magnetic \\ attraction \\ \end{tabular} \\ \hline A2 & \begin{tabular}{c} H\({}_{2}\)O\({}_{2}\) Pretreatment/ \\ Sequential Ferrite \\ Addition \\ \end{tabular} & \begin{tabular}{c} Partial precipitate \\ magnetic separation \\ \end{tabular} & \begin{tabular}{c} Nearly complete \\ magnetic attraction \\ \end{tabular} & \begin{tabular}{c} Complete magnetic \\ attraction \\ \end{tabular} \\ \hline B2 & \begin{tabular}{c} Sequential Ferrite \\ Addition \\ \end{tabular} & \begin{tabular}{c} Complete magnetic \\ control of precipitates \\ \end{tabular} & \
\begin{tabular}{c} Complete magnetic \\ attraction \\ \end{tabular} &
\begin{tabular}{c} No magnetic \\ response \\ \end{tabular} \\ \hline \hline \end{tabular} Note: Magnetic properties were assessed by external magnetic field response; “Complete magnetic attraction” indicates that the sample can be fully attracted by a magnet; “No magnetic response” indicates no observable magnetic interaction.  
\end{table}
Table 1: Magnetic characteristics of precipitates under different treatment conditions and temperatures.  
and the degree of interaction with PVA molecules under different processing conditions. Samples with H\({}_{2}\)O\({}_{2}\) pretreatment (B1 and B2) showed notably darker colors, suggesting that pre-oxidation might promote ferrite oxidation and complex formation. After drying at 100 \({}^{\circ}\)C, all samples transformed into powdery substances while maintaining similar color characteristics, indicating that low-temperature drying primarily removed water content without significantly altering their chemical composition. Notably, at this stage, samples A1 and B1 exhibited weak magnetic properties, while A2 and B2 showed stronger magnetic responses, potentially due to the influence of different ferrite treatment strategies on magnetic oxide formation. Upon calcination at 650 \({}^{\circ}\)C, significant color changes occurred. Samples A1, B1, and A2 transformed to deep brownish-red, characteristic of the \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) phase, while maintaining good magnetic properties. However, sample B2 not only became lighter in color but also completely lost its magnetic properties after high-temperature calcination. This phenomenon suggests different phase transformation processes of iron oxides under high temperatures depending on the treatment conditions. These observations reveal the critical influence of H\({}_{2}\)O\({}_{2}\) pretreatment and stepwise ferrite addition strategies on the final product properties.