The first sustainable material designed for air particulate matter capture: An introduction to Azure Chemistry

Highlights

• A new porous material is designed to capture particulate matter (PM) in urban areas.

• Energy and emission involved in its synthesis are evaluated.

• Two different pollution sources are considered: diesel exhausts and incense smoke.

• Results show a good capacity of the proposed material for PM capture.

• A new chemistry approach (Azure Chemistry) is proposed.

Abstract

This work presents a new porous material (SUNSPACE) designed for air particulate matter (PM) capture. It was developed in answer to the European Commission request of an innovative, affordable, and sustainable solution, based on design-driven material, to reduce the concentration of air particulate matter in urban areas. SUNSPACE material was developed from by-products and low-cost materials, such as silica fume and sodium alginate. Its capability to catch ultrafine PM was evaluated by different ad-hoc tests, considering diesel exhaust fumes and incense smoke PM.

Despite the fact that procedures and materials can be designed for remediation, the high impact on the environment, for example in terms of natural resources consumption and emissions, are not usually considered. Instead, we believe that the technologies must be always evaluated in terms of material embodied energy (EE) and carbon footprint (CF). We define our approach to solve environment problems by a sustainable methodology “Azure Chemistry”. For the SUNSPACE synthesis, the multi-criteria decision analysis was performed to select the best sustainable solution. The emissions and the energies involved in the synthesis of SUNSPACE material were evaluated with the Azure Chemistry approach, showing that this could be the best available technology to face the problem of capturing the PM in urban area.

Introduction

The European Environment Agency (“Air quality in Europe — 2016 report — European Environment Agency,” 2016) estimated that in the 2012–2014 period 50–63% and 85–91% of the urban population in Europe were exposed to levels of PM₁₀ and PM_2.5 (respectively) which exceeded the recommended World Health Organization (WHO) annual limits (PM₁₀: 20 μg/m³ and PM_2.5: 10 μg/m³). In the same report, it was reported that 467,000 premature deaths in Europe could be attributed to PM_2.5 in 2013. PM pollution due to fine particles (2.5 μg or less) is particularly harmful since fine PM can penetrate human bronchi and lungs owing to the small particle size (Harrison and Yin, 2000).

The number of publications per year devoted to “air pollution” problems and solutions is growing from 1960 (Fig. 1) and the trend is still growing, while the number of publications about “filter” and “air” is quite constant in the last ten years (the data were obtained by SCOPUS). This may suggest that the technologies for PM filtration is set and few researches are devoted to find new approaches to face the problem. Indeed, several techniques are already assessed for filtration (Liu et al., 2017a).

Air pollution control systems also concern anthropogenic emissions reduction (such as CO₂, SO₂, NO_x) such as gas capture (Zhang et al., 2018, Yan et al., 2018).

Possible, emerging nano-fibers materials, to reduce PM, may be produced in the future at accessible costs (Liu et al., 2015, Liu et al., 2017c, Singh et al., 2017). Since it is urgent to develop low-cost technologies to promote the air quality in urban areas, the European Commission encourages the research in the field of new materials, devoted to reducing the concentration of particulate matter in urban areas. The aim was to highlight innovative, affordable, and sustainable solutions, based on design-driven materials.

The research presented in this paper started in 2015 by using the multi-criteria decision analysis (Jahan et al., 2016) to support the most suitable synthesis strategy able to produce a new material to be used for the PM reduction in the urban area.

The new proposed material is designed to reduce the PM present in the urban air by using green and sustainable materials, process, and technologies (as formulated in the Green chemistry). We called this chemistry approach that want to link Green Chemistry and Remediation “Azure Chemistry”: the goal is to restore or reconstruct the ecosystems by sustainable solutions in terms of energy, materials and emissions. Azure Chemistry concerns, for example, carbon dioxide sequestration, PM pollution reduction, waste minimisation, and energy neutrality. It requires low-energy paths, manufacturing and technologies reducing the use of non-renewable resources, and in which wastes and by-products are employed. Overall, Azure Chemistry approach must minimize the global impact of the remediation processes. Indeed, literature reports several examples of declared low-cost materials, synthesised for instance starting from wastes and by-products (Nayak and Pal, 2017, Silva et al., 2017, Wong et al., 2016). However, the declared sustainability of these new proposed materials is rarely demonstrated and quantified.

The most promising new materials for air filters, e.g. polyethylene (PE), polypropylene (PP), polyamide (PA), and polystyrene (PS), are petroleum-based materials (Khalid et al., 2017, Liu et al., 2015). This makes the environmental costs in their production high. Indeed, the emissions associated with the production of these polymer materials can range from 30 to 150 kg-CO₂eq.

Even when biomaterials are used to make filters (as for example soy protein isolate and bacterial cellulose) (Liu et al., 2017b), the synthesis requires toxic chemicals, thermal treatments and refrigeration, that makes filters production not suitable to solve the PM problem of the urban areas. Moreover, the filter regeneration is usually not considered, making the material substitution the most common practice to replace exhaust filters, triggering a secondary environment pollution.

Several materials are reported in the literature and databases and they can be grouped in two categories (Jahan et al., 2016): traditional and, more recently developed, engineered materials (see Fig. 2). Engineered materials have been developed to guarantee functional and structural characteristics that cannot be provided by a single component alone. Two main distinct strategies design engineering materials:

• conventional design strategy that aims to obtain materials with a uniform and homogeneous structure.

• newly developed design strategy in which the goal is to disperse the constituents by a controlled and selectively heterogeneous way. By this approach, materials for niche applications have been obtained (Jahan et al., 2016).

By the second approach, it is possible to create porous materials that combine desirable properties of organic polymers (flexibility and elasticity) with those of inorganic solids (rigidity and chemical resistance) (Zou et al., 2008). Porous materials are used for trapping and encapsulation functional biomolecules, enzymes, drugs, and nutrients (Kim et al., 2015). Various methods have been used to obtain pore sizes control, as specific precursors for nano-casting or variable size surfactant templates (Suib, 2017).

The new material for air particulate trapping must have a wide range of pores sizes, from few nanometers (nm) to some microns (μm) to allow the capture of all sizes of PM. In particular, pores in the nanometers range are necessary for the trapping of the most dangerous smallest PM.

Indeed, these particles contain toxic substances such as heavy metals, PAHs, polychlorinated dibenzo-p-dioxins and dibenzofurans and polychlorinated biphenyls, making them more hazardous and carcinogenic (Dzierżanowski et al., 2011, Silverman et al., 2012).

For this reason, the newly developed material performances have been assessed in trapping of diesel exhaust and incense smoke PM. Indeed, diesel exhaust is enriched in ultrafine particles (<0.1 μm in diameter) (Williams, 2011) and incense burning emits particles with the mean particle diameter of about 100 nm (Ji et al., 2010, Tirler and Settimo, 2015).