## 1 Introduction
However, to prevent desorption under subsequent changes in environmental conditions, and to enhance the total capacity of the process for NO\({}_{x}\) storage beyond that of surface saturation alone, it is beneficial to have a material that promotes a material surface conversion reaction rather than pure adsorption, which enables the ultimate complete transformation of the host sorbent into a stable nitrogen species such as a nitrate salt. In addition, if the nitrate salt itself can contribute to a further sought-after product, then it not only supplies a solution for mitigation but also valorises a pollutant gas by generating an onward use, which results in significant contribution to the circular economy. In light of the stoichiometric reaction for calcium carbonate in the form of ground limestone or marble, such as architectural waste cuttings, with NO\({}_{x}\) gas in the presence of moisture, as shown in Equation (1), it can be hypothesised that, under conditions of sufficient exposure to NO\({}_{x}\), calcium nitrate (Ca(NO\({}_{3}\))\({}_{2}\)) can be generated. Calcium nitrate is considered a premium fertiliser for use in horticulture as a concentrate or hydroponic nutrient and, in slurry dilution, for agricultural crops and sustainable forestry plantations. \[\text{CaCO}_{3}+\text{H}_{2}\text{O}+2\text{NO}_{x}=1,2+\frac{1}{2}\text{O}_{2 }\rightarrow\text{Ca(NO}_{2,3})_{2}+\text{H}_{2}\text{CO}_{3} \tag{1}\]  
Furthermore, where calcium micronutrient is required, such as for repeated cycles of forestry re-planting and fruit production in which mineral depletion can become endemic, Ca(NO\({}_{3}\))\({}_{2}\) can advantageously dissociate readily into Ca\({}^{2+}\) for mineralisation and 2 NO\({}_{3}\)\({}^{-}\) for promoting plant growth. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Burden of NO\({}_{2}\) Exposure** & **2007** & **2008** & **2009** & **2010** & **2011** & **2012** & **2013** & **2014** \\ \hline Attributable fraction 1 (\%) & 2.19 & 2.29 & 2.26 & 2.26 & 1.86 & 1.87 & 1.58 & 1.77 \\ \hline Attributable deaths 2 & 7832 & 8157 & 8035 & 7960 & 6343 & 6531 & 5605 & 5966 \\ \hline Years of life lost 3 (YLL) & 69,244 & 71,396 & 69,526 & 68,428 & 53,489 & 54,536 & 46,795 & 49,726 \\ \hline YLL per 100,000 population & 122.78 & 126.40 & 123.04 & 120.85 & 94.10 & 95.48 & 83.13 & 87.96 \\ \hline \end{tabular} 1
[FOOTNOTE:1]Footnote 1: Percentage of NO\({}_{2}\)-attributable cases of cardiovascular mortality. 2  
\end{table} 
Table 2: Data Translated from [PERSON] et al. [4], Federal Environment Agency, Germany, under the Creative Commons attribution-non-commercial-no processing 4.0 international license ([[https://creativecommons.org/licenses/by-nc-nd/4.0/](https://creativecommons.org/licenses/by-nc-nd/4.0/)]([https://creativecommons.org/licenses/by-nc-nd/4.0/](https://creativecommons.org/licenses/by-nc-nd/4.0/))). In addition, if magnesium deficiency occurs, then dolomitic stone (CaMg(CO\({}_{3}\))\({}_{2}\)) can be used in place of limestone (CaCO\({}_{3}\)) to rectify it. The following report unveils the step-by-step actions taken and solutions that have been found to achieve proof-of-principle in the development of a working model filter-element structure suitable for either a pressure-driven through-flow or, preferentially, a zero-energy-acting surface flow concept for NO\({}_{x}\) capture. The filter element was formed by coating calcium carbonate, either as valorised ground mineral product (tailings or cuttings waste), or, if needed, specially treated for additional functionality on a porous carrier consisting of no-longer-viable recycling-sourced cellulose paper fibre. In this way, the target of NO\({}_{x}\) capture extended further to form a mineralic micronutrient nitrate-based fertiliser, which could be used as an aqueous humus mulch after re-pulping. If needed, the mulch could be dried into a concentrate, or the nitrate could be extracted by washing to form a value-added calcium nitrate concentrate.