# Removal of Contaminants in Water with Scallop Shell Waste
## 3 Results  
### Characterization of Scallop Shell Waste  
Results of the X-ray fluorescence spectroscopy (XRF) analysis show that all samples are mainly composed of calcium in the form of calcium oxide (CaO), which represents more than 97% of the shell composition (Table 2). Therefore, the scallop shell waste are an attractive bio-resource of high-purity calcium carbonate, as shown in various studies of other bivalve shells worldwide [35]. The crystal structure of the analyzed scallop shell powders (Figure 3) confirms the composition obtained by the XRF technique. These powders have a single phase and correspond to a rhombohedral calcite crystal structure calcite because the diffraction peaks coincide satisfactorily with the JCPDS 05-586 card. The most representative peaks are located at angles (20) 23.06\({}^{\circ}\), 29.41\({}^{\circ}\), 31.42\({}^{\circ}\), 35.97\({}^{\circ}\), 39.4\({}^{\circ}\), 43.17\({}^{\circ}\), and 47.48\({}^{\circ}\), corresponding to planes (112), (104), (006), (110), (113), (201), and (018), respectively. It has to be noted that the scallop shells possess a pure calcite CaCO\({}_{3}\), which differs from most other shells that present an aragonitic phase, and a large part of the species at a global level present both polymorphs, but with an ergonomic predominance [36]. The scallop shell from northern Chile has the same calcite crystalline phase present in commercial CaCO\({}_{3}\) coming from limestone rocks. This scallop shell waste can be used for practically the same industrial applications that use commercial CaCO\({}_{3}\)[37]. In order to determine the thermal behavior of the scallop shell waste powder, it was subjected to differential scanning calorimetry (DSC) with a dynamic variation of temperature at a scanning speed of 10 \({}^{\circ}\)C/min. Figure 4 shows that the shell waste powder accumulates heat steadily as the temperature it is subjected to increases. This endothermic behavior can be attributed to the moisture of the organic matter present on the sample. \begin{table}
\begin{tabular}{c c} \hline \hline
**Oxide** & **(\%)** \\ \hline CaO & 97.68 \\ SiO\({}_{2}\) & 0.08 \\ Na\({}_{2}\)O & 0.81 \\ Fe\({}_{2}\)O\({}_{3}\) & 0.10 \\ SO\({}_{3}\) & 0.35 \\ SrO & 0.33 \\ MgO & 0.21 \\ Al\({}_{2}\)O\({}_{3}\) & 0.03 \\ P\({}_{2}\)O\({}_{5}\) & 0.40 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Chemical composition of scallop shell waste powder obtained with X-ray fluorescence (XRF). Figure 2: System used for colorimetric determination of arsenic(III). Thermogravimetric analysis (TGA) shows the decomposition periods of scallop shell waste powders (Figure 5). In the beginning, the thermogram exhibits a loss of 2.13% in weight coming from the organic matter that seashells have within them, which remains in the crushed samples despite the shells having been washed previously [38]. After 600 \({}^{\circ}\)C, a strong reduction in weight can be seen, corresponding to the thermal decomposition of calcium carbonate up to 688.35 \({}^{\circ}\)C. Due to the fact that scallop shell waste has very few impurities, it can be assumed that the resulting sample is calcium oxide (CaO), and 42.23% by weight is released as carbon dioxide (CO\({}_{2}\)) into the environment, as shown in Equation (1) [39]. Figure 4: Differential scanning calorimetry (DSC) of scallop shell waste. Figure 3: X-ray diffractogram (XRD) of scallop shell waste powder. Therefore, through this analysis, it is possible to observe the thermal behavior of calcitic CaCO\({}_{3}\) from scallop shells for future applications. Also, a promising bioresource of calcium oxide can be obtained through calcination, a material used for desulfurization, cement manufacturing, as a catalyst to produce biofuels, and as an antimicrobial agent, among others [40; 41; 42; 43]. \[\text{CaCO}_{3}\rightarrow\text{CaO}+\text{CO}_{2} \tag{1}\]  
Figure 6 shows the images of scanning electron microscopy (SEM).