## 3 Experimental
### Meeting Regulatory Requirements: Heavy Element (HE) Analysis  
We do not intend to list exhaustive regulatory demands for various end-uses for the circular economy concept of the proposed filter. However, to meet standards for use in agriculture for food production, compliance is required, which differs regionally but is stringent in most cases, with respect to heavy elements. Application in horticulture for garden plants or forestry is less restrictive.  
Figure 31: The release of selectively intercalated NO from sepiolite upon exposure to CO\({}_{2}\).  
Figure 32: Non-selective pore sorption of NOx components resulted in their release on exchange with CO\({}_{2}\).  
However, using an actively reactive component as only a fraction of the total filter can eliminate the role of any contaminants occurring naturally in the minerals making up that fraction, such as ancillary components, when meeting suitability demands for paper production. However, contamination during application may be more serious depending on the environment. In this context, results from exposure to Belgrade ambient air over the time periods used in these experiments followed a methodology defined by the European Community Bureau of Reference ([PERSON] et al.) [36].  
The behaviour of chemical elements in the environment depends on the form in which they occur. The way that an element is bound to a solid phase in the environment affects its mobility, and, therefore, its potential bioavailability and toxicity to living organisms. The result of this is a serious interest in improving the understanding of the system-element interaction, i.e., elemental interaction with the solid phase.  
Heavy Element Analysis  
The combination of multiple extraction agents, i.e., their successive application on the sample, was first applied by [PERSON] [37] to investigate suspended river sediment. This method and its modifications were applied by a number of authors. The most notable was [PERSON] et al. [38]. Many sequential extraction procedures (SEP) in the literature represent variants and modifications of the same sequential extraction developed by [PERSON] et al. ([PERSON] et al.) [38; 39].  
Generally, the most widely used SEP is the Community Bureau of Reference (BCR) three-step procedure modified by the European Community Bureau of Reference (now Standards, Measurement, and Testing ([PERSON] et al.)) [36].  
The adapted SEP method used here, which focused on the likely prevalent heavy elements (HEs) at a roadside site is shown in Figure 33. Using this kind of methodology could provide some valuable information on heavy element mobility with changing environmental conditions, such as leaching due to rainfall (step I) or acid rainfall (step II).  
Heavy elements (HE) analysis  
Figure 33: Adapted methodology to focus on the elected heavy elements (HEs) Pb, Cd, Hg, As, Sb, and Cu.  
Analytical measurements were taken in triplicate for all samples. Inductively coupled plasma-mass spectrometry (ICP-MS) (iCAP(tm) Qc ICP-MS, Thermo Scientific, Waltham, MA, USA) was applied to analyse for HE in the water soluble and acid soluble extract fractions. Results are shown in Table 13. Results for elected HEs all show either very low levels or are below the detection limits (\(<DL\)). It is again stressed that these findings are not intended to suggest regulatory compliance and are indicative only.