A New Approach: Azure Chemistry
Low-Cost and Sustainable New Hybrid Material for Reducing Air and Water Pollution
- Fig. 1: A picture of the new hybrid porous material, that shows macro-pores. It could be applied as a plaster to reduce PM pollution, as it occurs in nature with leaves.
- Fig. 2: Embodied energy and carbon footprint associated with the new material production. In comparison also other materials (such as some polymers, and activated carbon) are reported. It is evident that the new proposed material results much more sustainable in respect to polymeric materials used in commercial filters and to activated carbon.
- Fig. 3: The new porous hybrid material may be used, for example, to cover street borders, improving the PM capture by the porous material proximity to the emission sources.
A new class of low-cost and sustainable hybrid materials could possibly displace activated carbon as the preferred choice for reducing wastewater. The studies show that the material, synthesized inexpensively from by-product materials and alginates (an abundant polysaccharide material obtained from algae), can reduce pollutants in wastewater with a material much more sustainable than activated carbon, the current gold standard adsorbent [1,2].
Additionally, this material is the first proposed low-cost material to be used to reduce particulate matter (PM) concentration in air, as shown in a very recent paper . Finally, the use of sustainable materials to reduce pollutants allows to introduce a new chemistry approach: Azure Chemistry.
Air Particulate Matter
The European Environment Agency estimated that 467,000 premature deaths in Europe could be attributed to air particulate matter (PM) pollution, only in 2013. PM is a complex mixture of extremely small particles and liquid droplets that get into the air. It is emitted from different natural and anthropogenic sources such as power plants, industry, automobiles and fires.
In this frame, in 2015 the European Commission’s requested to develop an affordable, sustainable and innovative design-driven material solution that can reduce the concentration of particulate matter in urban areas.
Most materials, today, are made by mining and processing minerals drawn from the planet’s mineral reserves. This requires energy (more than 20% of all the useful energy available is consumed in material production) and produces emissions (CO2). At the end of product life the material may be rejected or it may recirculate through the supply chain via recycling, remanufacturing, or reuse. The other major natural resources (energy, land, water, and air) have a renewable component. Minerals are a large resource, but they are a finite reserve. For this reason, it is fundamental to promote measures to recycle waste and by-products to limit the use of natural resources.
Conservation of Natural Resources
To support natural resources conservation the author proposed a new method to evaluate a raw-material sustainability based only on two parameters: “embodied energy” and “CO2 footprint”.
The materials production from feedstock and ores needs energy. It is named “embodied energy” and includes all energies necessary for the production of 1 kg of a specific material. The CO2 footprint (or carbon footprint) corresponds to the equivalent mass of greenhouse gases (CO2-eq) released into the atmosphere for the production of 1 kg of the material. The embodied energy accounts for the resources and the CO2 footprint for the emissions, involved in a material production .
To respond to European Commission call about a synthesis of a new sustainable material, able to reduce PM pollution, the multi-criteria decision analysis for supporting the material selection was used. The basic idea to develop a new hybrid material was derived from the limitation of the existing air filter technologies, to be applied as PM removal. The already existing materials for PM filters, e.g. polyethylene (PE), polypropylene (PP), polyamide (PA), and polystyrene (PS) are often petroleum based materials. This makes the environmental costs in production of these filters extremely high and not competitive. Another disadvantage of traditional filters is that they are disposable, intended to be used until no longer useful, and then thrown away.
The new material design principles were based on the necessity to reduce environmental impact of the proposed solution designed for the specific need of PM reduction. Then, for the synthesis, it was chosen to use low-cost materials and by-products that require little energy for processing: a naturally abundant raw material, sodium alginate (a polysaccharide that can be extracted from seaweed and algae) was combined with a high-volume industrial by-product, silica fume (a by-product of ferrosilicon or silicon metal alloy processing) to produce a new eco-material. The synthesis method described in the study is very simple and easy to scale up. It only requires mixing. Then, taking advantage of the gelling properties of alginate, it is possible to combine it with silica fume, and to consolidate a new hybrid material, that can be applied by direct foaming, extrusion, 3D printing, or deposited by spray, or by brush. The decomposition of food-grade sodium-bicarbonate (baking soda) at low temperature (70-80°C) allows to produce pores; then only a moderate thermal treatment is necessary to obtain a porous material, able to trap PM.
New Porous Material
Figure 1 shows a picture of new porous material obtained by direct foaming. The new material was studied in detail as reported. Its structure, composition, and morphology are presented and the material chemistry is also suggested . Some tests about the capability to reduce wastewater pollution were performed using methylene blue, a dye, as a model pollutant. The hybrid material adsorbed the dye, even at high concentrations, with 94% efficiency. It also showed good photocatalytic performance when coated with a thin layer (about 100 nm) of titania. The paper published on Frontiers of Chemistry also reports the preliminary results about new porous material capability in trapping air particulate matter.
Figure 2 reports embodied energy and carbon footprint associated with the new porous material production. In comparison also other materials (such as some polymers, and activated carbon) are reported. Activated carbon is the most common adsorbent used in wastewater pollution reduction. However, as shown in figure 2, it is expensive to produce it (it requires high energies) and its regeneration (generally made by thermal treatment) need additional energy. Concerning air pollution, no other material exists that can be systematically used in cities to decrease PM. The materials currently used as air filter, i.e. PE, PP, PA, and PS, are also reported in figure 2 as comparison. The emission associated with the production of these polymer materials can be very high ranging from 30 to 150 kg-CO2-eq. It is evident that the new proposed porous material results much more sustainable in respect to polymeric materials used in commercial filters and to activated carbon.
Literature reports that the most sustainable, low-cost, and effective way to entrap air particulate matter (PM) is given by leaves: studies found that PM concentrations in the air decrease by about 9% at distances of 50–100 m into a forest, and that vegetation cover plays a key role in air quality. For this reason we decided to develop a low-cost material, that should be diffused to increase its efficacy (as it is for vegetation); the idea is to use it as a coating, on urban surfaces, like a plaster (as exemplified in fig. 3). In particular, it contains ink-bottle shaped pores, that result very suitable for ultra-fine and fine particles capture, since when the PM enters in the pores it is trapped, due to pore shape and dimensions (the pores dimensions are in the order of 100 nm). Quantitative results about the capability of PM trapping of the new material have been recently published . They show that the new porous material can remove till to 2407 (± 581) μg/cm2 of air particulate matter particles, with dimensions lower than 1 μm. This value corresponds to about 24 (± 6) g/m2, i.e. about two orders of magnitude more than the amount of PM that can be trapped by leaves! The new material regeneration can be obtained by rainfall, as it happens in nature: water is able to wash the pollutants trapped on building external surfaces. After material “washing”, discharge water can be collected in sewage system, then discarded PM is transported to wastewater treatment plants with the wastewater deriving from urban washing (as for example streets washing), with no additional impacts.
Particulate matter is ubiquitous in cities. Moreover, millions of tons of industrial effluents are released into the world’s waters every year. Both particulate matter and organic contaminants are highly toxic to ecosystems and to humanity. PM impacts also on visibility in cities. The challenge has been to find an economical way to remove these pollutants from the environment. 
The new proposed material is designed to reduce pollution (in water and in air) by using green and sustainable materials, process, and technologies (as formulated in the Green chemistry). As a consequence, this allows to introduce a new chemistry approach: the aim is to link Green Chemistry and remediation. This can be resumed as “Azure Chemistry”: the goal is to restore or reconstruct the ecosystems by sustainable solutions in terms of materials, energy, and emissions. Azure Chemistry concerns, for example, wastewater treatment, carbon dioxide sequestration, PM pollution reduction, waste minimization, 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 as remediation agent. As a consequence, Azure Chemistry approach is defined to minimize the global impact of the remediation processes.
In conclusion, analyses revealed that the hybrid material consume less energy to be produced (“embodied energy”) while leaving a similar carbon footprint compared with activated carbon. Upon testing the material demonstrates high PM-trapping capabilities, and also shows good photocatalytic performance when coated with titania.
This new porous material can be applied as a coating, used for spraying or brushing, or for 3D-printed, meaning it could be used to cover external building surfaces to remove PM as well as to design water filtration units.
This versatility is an exciting new addition to the humanity’s toolkit to meet our collective need to reduce air and water pollution.
This work was carried out in the framework of the project: New material based on alginates from airborne particulates, “Basalto,” supported by INSTM (http://www.instm.it/en/instm.aspx) and Regione Lombardia.
INSTM and Chemistry for Technologies Laboratory
Università degli Studi di Brescia
 A. Zanoletti, F. Bilo, L.E. Depero, D. Zappa, E. Bontempi: The first sustainable material designed for air particulate matter capture: An introduction to Azure Chemistry, Journal of Environmental Management, 218, 355–362 (15 July 2018) doi: 10.1016/j.jenvman.2018.04.081.
 Alessandra Zanoletti, Ivano Vassura, Elisa Venturini, Matteo Monai, Tiziano Montini, Stefania Federici, Annalisa Zacco, Laura Treccani and Elza Bontempi: A new porous hybrid material derived from silica fume and alginate for sustainable pollutants reduction, Front. Chem., (19 March 2018) doi: 10.3389/fchem.2018.00060.
 Elza Bontempi: A new approach for evaluating the sustainability of raw materials substitution based on embodied energy and the CO2 footprint, Journal of Cleaner Production, 162, 162-169 (20 September 2017) doi: 10.1016/j.jclepro.2017.06.028.