# Removal of Contaminants in Water with Scallop Shell Waste
## 3 Results
### Remotion of Water Waste Contaminants
It is important to note that the adsorption process slows down at pH 6.5, meaning that this pH value would be most useful in a situation where contaminants need to be removed in a controlled manner, as shown in Figure 10. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample Concentration** & **Adsorption (\%)** & **Adsorption (\%)** & **Starting pH** & **Final pH** \\ (ppm) & **20 min** & **90 min** & **Starting pH** & **Final pH** \\ \hline
200 & 54.2 \(\pm\) 0.4 & 85.4 \(\pm\) 0.3 & 4.6 & 7.0 \\
500 & 74.4 \(\pm\) 0.5 & 98.5 \(\pm\) 0.2 & 4.5 & 6.8 \\
1000 & 84.2 \(\pm\) 0.4 & 98.6 \(\pm\) 0.2 & 4.6 & 9.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Adsorption percentage and pH variation after 90 min using different northern scallop shell concentrations, \(p<0.0001\). Figure 10: Removal percentage of caffeic acid at different initial pH values; pH and time effect are statistically significant (\(p<0.0001\)). The initial pH value selected for this study was of 4.6, as it allows for a faster caffeic acid adsorption rate while also needing no previous adjustments to sample pH. The adsorption behaviors seen in Figure 10 can be explained by calcium carbonate's point of zero charge value of 8.3 [58]. Below this point, the carbonate compound's surface charge remains positive, while at pH values above 8.3, surface charge becomes negative. Furthermore, a pH 3.0 caffeic acid solution has a neutral charge, which makes its adsorption onto the shell residue difficult. Increasing pH to 5.0 negatively polarizes caffeic acid by about 70%, and further increasing pH to 7.0, caffeic acid polarization increases to about 100%, making it easier for it to be adsorbed onto the shell residue's surface. On the other hand, at a pH value of 8.0, calcium carbonate's surface charge is almost neutral; therefore, the seashell residue's capacity to adsorb contaminants begins to decrease, as the primary intermolecular forces acting at this point are Van der Waals forces. In addition, high adsorption percentages were observed at pH values of 4.6 and 6.5. This can be explained by the seashell residue's surface being positively charged while the caffeic acid contaminant is negatively charged at this pH range, as shown in Figure 11, enabling the adsorption process through an electrostatic interaction between shell residue and contaminant. Table 5 shows the variation in contaminant concentration during the adsorption tests over a timespan of 90 min. The removal percentage of a starting caffeic acid concentration of 10 ppm is 69.2%, while doubling that initial concentration to 20 ppm yields a removal percentage of 34.9%, an efficiency reduction of almost 50%, which suggests that a 500 ppm sample of shell residue may only remove up to 10 ppm of caffeic acid at a time, leaving the rest in solution due to the saturation of the material's surface. The final pH values in Table 5 show that increasing the caffeic acid contaminant's concentration also increases pH. All conditions analyzed in this work determine that 500 ppm of shell residue are needed in order to remove 10 ppm of caffeic acid out of a water sample, as well as a pH value of 4.6 and a timespan of 90 min, allowing for high adsorption percentages of the contaminant on the seashell powder. Conversely, arsenic removal testing did not include setting new operational definitions, as the studies on this subject were directly carried out on water meant for human consumption, and thus working conditions were defined by their characteristics. The adsorption mechanism for this system consists of arsenic ions gathering on the shell residue's surface. Initially, these ions interact with the material's surface through electrostatic forces, subsequently diffusing through its pores until finally settling down on the capillary spaces of the material used as an adsorbent [59; 60; 61]. Table 6 shows a comparison between the removal percentages obtained in this work and those of materials commonly used in wastewater treatment for these specific contaminants.