NOx is unavoidably emitted during combustion in air at high temperature and/or pressure, which, if exceeding recommended levels, has a negative impact on the population.
The authors found that when moist, limestone (CaCO3) readily sorbs NO2 to form calcium nitrate, which provides the basis for developing a surface flow filter.
The substrate was made from “over-recycled” cellulose fibres such as newsprint, magazines, or packaging fibre, which are too weak to be used in further recycling.
The substrate was specially-coated with fine-ground calcium carbonate and micro-nano-fibrillated cellulose, which was used as a binder and essential humectant to avoid formation of a stagnant air layer.
Pre-oxidation countered the action of denitrification bacteria colonising the cellulose substrate.
The by-product CO2 produced in situ during carbonate to nitrate conversion was adsorbed by perlite, which is an inert high surface-area additive.
After use, the nitrate-rich CaCO3-cellulose-based filter was proposed to be mulched into a run-off resistant soil fertiliser and micronutrient suitable, e.g., for renewable forestry within the circular economy.
Belgrade, Serbia, which is a highly polluted city, was used as a laboratory test bed, and NO2 was successfully removed from an inlet of city air. A construct of street-side self-draughting or municipal/commercial transport vehicle-exterior motion-draught filter boxes is discussed.
The following introduction aims to explain the claim that NOx released by any form of combustion in air at high temperature is a pollutant that will remain as transport, industry, and energy generation evolve to adopt sustainable materials and processes.
It is this continuing NOx release that motivates the sustainable bio-resourced filter development presented in this paper.
Sustainable production methods and processes are either based on the best available technologies (BAT) or contribute to them. Such processes within the European Union (EU) context are certified via the controlling issuance of permits based on provided reference documents (BREFs) outlining the BATs employed.
The procedures for assessing the sustainable verification of production processes in this way stems from the Industrial Emissions Directive 2010/75/EU (IED) .
Although the evolution of regulatory has been primarily focused on industry, the support infrastructure for industry itself has become increasingly relevant, which includes emissions contributors such as energy generation, raw material extraction, and synthesis, and required naturally resourced services such as water and air.
A prime challenge to assessing sustainability is not only defining the production process itself, but also the product arising from it with increasing scrutiny being given to ensuring product longevity, repairability, and recyclability.
In defining recyclability as an aim, benefits arise only after the product lifecycle has been considerably extended. In this context, the ‘design for excellence’ (DFX) concept provides a practical tool for product engineers in which the basis has been focused on a small domestic appliance and enables a systematic assessment of products even in the concept phase to recycle after use, more specifically after ‘shredding’ .
Small domestic appliances are an example of the growth in the demand for sustainability with ultimate recyclability. In the context of this paper, the principles encapsulated have a far wider context when considering the design of technologies targeting the mitigation of gaseous and particulate pollutants, i.e., they are as such appliances, and should not, therefore, fail to meet the criteria of sustainability, repairability, and ultimate recyclability.
Many materials, even those sustainably sourced, ultimately fail to meet the recyclability criterion. Their properties deteriorate so drastically during recycling that they no longer meet the requirements of the product from which they are derived, and, by definition, the same product they re-use to produce.
An example of this arises from the recycling of printing and writing paper and board packaging to such an extent that the cellulosic fibrous material is mechanically degraded and stripped of the necessary surface chemical bonding groups (OH), which are both needed to re-constitute a uniformly strong matrix able to meet the demands of the end-application, be it printing, binding, folding, or containment of goods during transportation and storage.
Other criteria also come into play when considering the recyclability of paper and board. For example, cross-product contamination during recycling, e.g., de-inked fibre, being regarded as unsuitable for food packaging due to mineral oil residue, and derivatives arising from retained printing ink, glues, polymer binders, and more.
Additionally, recycling chemical aids themselves are considered as undesirable residues, as outlined in “Guidelines on the safe use of paper and board made from recycled fibres for food contact use” (FoodDrinkEurope) .
This incorporates the EU Packaging and Packaging Waste Directive, and, in turn, embodies the German National Institute for Risk Evaluation (Bundesinstitut für Risikobewertung (BfR)) Recommendation XXXVI  or the Council of Europe Resolution RESAP (2002).
$The lifecycle of such materials becomes strongly limited due to their deteriorating properties and applicable regulatory requirements, and, thus, drop out of the ultimate definition of recyclability. Such materials are then mainly considered waste, and fail in terms of the standard recyclable target despite the green labelling they receive.
Transport is an integral component of local, national, and international commerce and the global economy. As a major CO2 emitter, efforts move apace to consider transport energy sources that are non-carbon based.
However, frequently, the larger picture of total carbon emissions for a given transport vehicle manufacture, its energy transforming mechanism, and lifecycle use is either not considered at all or misrepresented as a side effect of political and regulatory haste in meeting the popular demand of providing a demonstrable reaction to the threat of global climate change.
Recent studies have shown that misappropriation of an ill-studied or superficially evaluated assessment criterion, usually due to component isolation from the total lifecycle with respect to carbon emissions, is rife.
For example, recent literature shows that the large increase in the carbon footprint arising from the duplication of motive power in hybrid vehicles, the incorporation of complex battery technology, and the accompanying demand for rapidly diminishing rare elements, including cobalt and lithium, with internal combustion engine (ICE) support, exceeds significantly the overall lifecycle vehicle carbon emissions of a solely ICE-powered unit (Buchal et al.) .
The same study further ranks electric vehicles, operated under today’s prevalent electricity generating plant mix in Germany, as the second largest carbon emitting option, after hybrid, available for personal automobiles.
The decreasing lifecycle carbon footprint is followed by petrol, and then diesel. Such commentary is made under the caveat that eventual transformation of the electricity generation plant to fully sustainable sourcing would bring about a more level playing field for electric vehicles but does not obviate the question of sourcing increasingly rare materials often under questionable working and social conditions.
The work of Buchal et al.  is challenged by a contemporary report issued by the Fraunhofer ISI (Wietschel et al.)  in which the authors claim that “electric cars purchased and used in Germany today have a better carbon footprint than diesel or gasoline-powered cars” as quoted by Wietschel and Jung  who suggest that these emission savings amount to up to 28% fewer greenhouse gas emissions than a top-range diesel and up to 42% fewer than a small gasoline-powered car over a 13-year vehicle lifespan.
However, to support the claim, the report specifies certain actions needed to achieve this if the market for electric vehicles grows.
Namely, (i) “charging at home” using self-produced solar power, (ii) using “green electricity” from additional renewable sources, (iii) using “renewable energies” when producing batteries, and (iv) employing “smart load management” by charging on cheap tariff, which they claim coincides with a higher proportion of renewably generated electricity.
Though the survey suggests that half of the owners of electric cars in Germany already draw charge from their own photovoltaic system, this shows how the owners of such cars invest personally in additional renewable generating facilities.
This is partly historical due to high levels of government subsidies for solar generation installations in Germany, which is a strategy that is certainly not politically global.
Though the Fraunhofer report  promotes these clearly beneficial developments, it effectively presents a rose-tinted view of electric vehicles that is dependent on rapid societal change, which is only to be encouraged, but it should be driven by the overall sustainability need rather than to make a choice of electric vehicle sustainable for the individual.
Additionally, if the car user is dependent on the power grid and adopts “smart load management,” this will inevitably be a short-term policy.
Increased vehicle charging will rapidly offset any current lower charge period, as the demand will grow per vehicle sold, which will rapidly exceed sustainable power generation.
Ensuring that batteries are made using renewable energy is also a challenge, as rare earth metals are extracted in countries not readily amenable to moving from fossil fuel energy production to sustainable solutions.
In short, the Fraunhofer report  can support longer-term change to electric vehicles, which contributes to the needed balance between pro and con arguments and the adoption of electric vehicles only where optimal.
The question of variable production from solar and wind power also remains, which urgently demands advances in electricity storage and an understanding of the implications for deciding the future transport power source.
The cracking of water to release hydrogen is one of the most efficient options to provide such storage.
Thus, it may be simpler to adopt hydrogen alone as the prime distributed fuel for industrial and transport needs. In such a case, the need for fuel cell technology can be removed.
Ozcanli et al. describe how the rotary Wankel engine, with its extended combustion volume and ignition timing and its separation of cooler and high temperature regions to prevent premature ignition, is extremely suitable for direct hydrogen combustion .
Given the desire to opt for the minimal carbon emissions pathway to navigate the evolving regulatory environment aimed at promoting sustainably favourable energy sourcing and motive power, these findings indicate that ICE transition from petrol to diesel and then to hydrogen provides the greatest relief to the climate change and global resourcing threat.
Even if these scientific findings are ignored and political expediency alone prevails in the field of transport, the use of hydrogen for energy storage and for industrial energy processes cannot be ignored on economic grounds.
In 2019, the technology for water separation into hydrogen and oxygen took such a great a leap forward that hydrogen production efficiencies of up to 98.7% may be achievable, which could lead to a cost reduction of 50%.
The technology is based on decoupling the electrolysis into two reaction steps in which the first is an electrochemical step that reduces water at the cathode while oxidising the anode.
The process is then followed by a chemical step driven spontaneously at raised temperature, which reduces the anode back to its initial state by re-oxidizing water.
The authors, Dotan et al., claim to be able to split water at average cell voltages of as little as 1.44 V to 1.60 V using current densities of as low as 10,200 mA·cm−2 in a two-electrode cell without needing a membrane .
This means that point-of-use combustion will remain and move from carbon burning to hydrogen burning. Burning, by definition, means oxidation, which is most efficient at high temperature and pressure.
These are the conditions under which oxidation derived from air, which is a mix of oxygen and nitrogen, inevitably leads to the formation of oxides of nitrogen (NOx).
The unavoidable consequence is that NOx pollution at ground level will remain a hazard to human health even if society becomes carbon neutral or abandons carbon altogether.
It is imperative to find a way to mitigate this threat without creating further negative effects, such as ammonia release.
Ammonia release can lead to particulate formation in the air when using NOx inhibitors as additives directly in the combustion environment.
Therefore, capture of released NOx is a viable and sustainable approach for relieving the burden of pollution in the habitable surrounding atmosphere, i.e., a problem that can be solved at ground level while preventing the unnecessary release of excess greenhouse gases into the upper atmosphere by forcing the immediate change to electric vehicles rather than transitioning smoothly through low carbon emitting diesel and methane (natural gas) ICEs directly to zero-carbon-emitting hydrogen-powered ICEs.
Current EU regulatory demands for NO2 are shown in Table 1, and exemplify the global trend for NOx reduction.