## 3 Experimental
### Filter Development, Test Methods, and Results
#### 3.1.3 Pluses and Minuses of Adding Inert (Non-Reactive) Surface Area  
Traditional wisdom suggests that increased surface area leads to enhanced absorption efficiency. However, as shown in Equation (1), the task in this study is to promote salt conversion, and, therefore, consumption of the sacrificial calcium carbonate (CaCO\({}_{3}\)). This leads to two important conclusions. First, if the mechanism is one reaction and not simply surface adsorption, then using another material of higher surface area will reduce the end capacity of the filter since capacity is ultimately defined by the salt conversion. Therefore, the sacrificial carbonate is present on the surface area. Second, increasing surface area should only speed the absorption at the outset of exposure. However, since the carbonate is being sacrificed, it is etched during the reaction such that the surface area of the reactive surface automatically increases. The corollary of the second conclusion is that particle size, and, hence, starting surface area within a reasonable range is not of paramount importance. To test these assumptions, the CaCO\({}_{3}\) powder material was modified by employing CaCO\({}_{3}\) in the synthesis of hydroxyapatite (HAP). The formation of high surface area plate-like nanometre-thick flakes of HAP on the surface of the calcium carbonate particles provides an easy way to dramatically increase the overall surface area. The synthesis of HAP was performed by adding NaH\({}_{2}\)PO\({}_{4}\) (Riedel-de Haen(tm), Honeywell, Charlotte, NC, USA) into the CaCO\({}_{3}\) suspension and by leaving the mixture for 1 h to interact and form HAP particles spontaneously (500 cm\({}^{3}\) deionised water + 5 g CaCO\({}_{3}\) (pH 9.20) + 3 g NaH\({}_{2}\)PO\({}_{4}\) (total solution pH 5.88), which resulted in a pH of 7.01 after 1 h and indicated reaction completion). Synthesis was performed at 30 \({}^{\circ}\)C under constant stirring. All reagents were pro analyst (P.A.) grade. The precipitated HAP on carbonate samples was filtered from the mother liquors and washed with deionised water. The samples were dried at 50 \({}^{\circ}\)C overnight and manually ground to powders. Furthermore, SEM, EDS, and BET analyses were performed to characterise the products before NO\({}_{2}\) exposure. The SEM images in Figure 14 display the successful surface transformation of calcium carbonate to hydroxyapatite. Comparative surface area data are shown in Table 7 in which a nearly 20-fold increase in specific surface area was achieved by conversion to hydroxyapatite (HAP). The comparative samples of GCC and HAP were exposed to NO\({}_{x}\) gas using a wash bottle system connected with an Erlenmeyer flask (Figure 15). Five grams of the powder material was first placed in the flask. The NO\({}_{x}\) gas at 10 ppm flowed through the system over the sample from the wash bottle for 1 min. Next, 30 g of water was added, and the mixture was shaken. The solution was then extracted by filtration and analysed again by ion chromatography in accordance with the applicable standard ([PERSON] et al.) [27]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Specific Surface** & **Total Pore Volume,** & **Mesopore** & **Micropore Volume,** & **Pore Diameter from** \\  & **Area, SSA** & **\(V_{\text{nat}}\)** & **Volume, \(V_{\text{nase}}\)** & **\(V_{\text{micro}}\)** & **Alpha S-Plot, \(d_{\text{ar}}\)** \\  & **(m\({}^{2}\)-g\({}^{-1}\))** & **(cm\({}^{-1}\)-g\({}^{-1}\))** & **(cm\({}^{-1}\)-g\({}^{-1}\))** & **(cm\({}^{-1}\)-g\({}^{-1}\))** & **(nm)** \\ \hline CaCO\({}_{3}\) (GCC) & 5.05 & 0.0192 & 0.0184 & 0.0023 & 20.9 \\ \hline HAP & 98.90 & 0.5738 & 0.5743 & 0.0281 & 13.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Specific surface area and surface pore volume characteristics by BET nitrogen adsorption (hydroxyapatite (HAP) phosphate modifications of CaCO\({}_{3}\)).