A sustainable replacement for TiO₂ in photocatalyst construction materials: Hydroxyapatite-based photocatalytic additives, made from the valorisation of food wastes of marine origin

Highlights

• Sustainable photocatalyst mortar made with wastes using 100 times less TiO₂.

• First ever use of HAp or fishery/marine waste as additive in construction materials.

• This wastes-derived photocatalyst additive has a very low TiO₂ level of only 1 wt%.

• Only 1–5 wt% added to mortar, so it contains only 0.01–0.05 wt% TiO₂ (100–500 ppm).

• Excellent photocatalyst, degrading 23.8% NOx after 45 min under solar light.

Abstract

The use of waste materials and by-products in building materials is of increasing importance to improve sustainability in construction, as is the incorporation of photocatalytic materials to both combat atmospheric pollution and protect the structures and façades. This work reports the innovative use of photocatalytic hydroxyapatite (HAp) based powders, derived from Atlantic codfish bone wastes, as an additive to natural hydraulic lime mortars. HAp is the main component of bone, and hence is non-toxic and biocompatible. This is the first time that such a calcium phosphate-based photocatalyst, or indeed any fish/marine derived wastes, have been added to building materials. A key factor is that this HAp-based photocatalyst contains only 1 wt% TiO₂, the material usually used as a photocatalyst in construction materials. As we only add 1–5 wt% of our total HAp-based material to the mortar, this means our photocatalytic mortars only contain 0.01–0.05 wt% titania (100–500 ppm), two orders of magnitude less than the quantities of 2–10 wt% TiO₂ which are usually needed. Our photocatalyst is made from a sustainable waste stream by simple solution and thermal processing, and thus with a much smaller impact on the environment. Specimens were made by either traditional intermixing techniques, or by a post-curing coating procedure. All showed gas-phase photocatalytic activity for abatement of NO_x pollutants under solar light. With intermixing, NO_x abatement of 6.3–8.3% was observed. However, for coated mortars, superior NO_x conversion rates were achieved of 7.1% and 23.8%, with 1 and 5 wt% additions, respectively. These results show the potential of this naturally-derived photocatalyst for applications in the construction industry, leading to lower atmospheric pollution and the creation of more durable/lower maintenance building façades, and environmentally sustainable materials for the preservation of cultural heritage.

Introduction

Construction has been a major human activity for millennia, representing one of the most important production sectors. However, developments in the manufacture of building materials, and the construction industry in general, have a significant impact on the environment in terms of energy consumption, use of natural resources and environmental pollution. Indeed, 24% of the raw materials extracted from the lithosphere are used to fabricate building materials (Zabalza Bribián et al., 2011), and this figure will only grow in the future, due to increases in both world population, and construction taking place in developing countries. Several studies show that the current state of the construction industry is not sustainable in the long term (Shafigh et al., 2014). One way to address this problem is the use of partly recycled materials, i.e. valorised wastes, by-products and end-of-life products, both from the construction industry and from other sectors. These are commonly referred to as “secondary raw materials” enabling a “production-use-reuse” cycle (Cumo et al., 2015; Morgan, 2009; Letcher and Daniel, 2011; Wyatt and Kibert, 1994).

Use of wastes and by-products are attracting increasing attention because, beyond decreasing the amount of raw materials employed, it also addresses the ever-increasing problem of the amount of wastes generated by modern society – data from 2010 showed that waste production in the European Union was 2.5 billion tonnes, a value ∼25% higher than that of 2006 (European Commission, 2015). The re-use/valorisation of these wastes could, therefore, be a profitable way to solve this problem. Wastes from the food industry sector, in particular, make up about 30% of the total, and have great potential in many different applications. Literature reports exist of construction materials made with the addition of wastes, particularly from the food industry, as whole or partial replacement of conventional aggregates (Shafigh et al., 2014; Aprianti et al., 2015; Barbieri et al., 2013; Basri et al., 1999; Muños Velasco et al., 2014; Pavŝiĉ et al., 2014).

Beyond sustainability, another challenge facing the construction industry today is the development of functional building materials with enhanced properties and performance; examples include materials with increased insulation, energy saving and/or durability characteristics (Ricciardi et al., 2014; Cuce and Riffat, 2015; Matias et al., 2014). In recent years, there has been increasing interest in creating building materials with photocatalytic properties, especially to combat airborne pollutants such as sulphur dioxide, nitrogen dioxide and oxides of nitrogen, and usually using TiO₂. Such materials offer advantages, both in terms of maintenance/durability, and for the environment. Photocatalytic surfaces can also be self-cleaning, as the photocatalytic effect can combat visible stains on building façades caused by the accumulation of organic or inorganic materials; such stains not only affect the aesthetics of the structure, but in conjunction with water can actually cause physical degradation (Graziani et al., 2014a), leading to higher maintenance costs. A recent study in Portugal, a country where ∼60% of buildings are rendered, found that 36% of buildings suffered from significant exterior stains (Flores-Colen et al., 2008). Preservation of cultural heritage, in which lime-based renders (“nanolimes”) are frequently used, is also an issue of growing importance, and imbuing the surface with added photocatalytic protection could be an important feature of such measures in the future (Nuño et al., 2015). The other major benefit of photocatalytic surfaces in is combating atmospheric pollution, both indoors from volatile organic compounds (VOCs) released from modern furnishings and composite materials (Jiang et al., 2017; Fu et al., 2011), and outdoors from vehicle exhausts such as NO_x (a mixture of NO and NO₂), levels of which can become especially high between tall buildings in “urban canyons”.

In photocatalysis, free radicals or reactive oxygen species are generated under light irradiation (i.e., sun light). Such species can then react with molecules they are in contact with, causing their degradation and/or transformation (Fujishima et al., 2008). They can degrade organic pollutants such as dyes in the liquid phase, or they can react with toxic/harmful gases such as VOCs and NO_x, degrading them and/or converting them into different species. Because of this, photocatalysts are used for environment remediation. Moreover, many photocatalytic compounds also have bactericidal, antimicrobial and antifungal effects, and can be used for treatment of contaminated waters or as self-sterilising surfaces (Kumar et al., 2014). Therefore, the use of photocatalytic construction materials could lead to lower maintenance costs, cultural heritage protection and pollution remediation, with great environmental and health benefits.

Nearly all photocatalytic construction and preservation materials are based on additions of titanium dioxide (TiO₂) to the basic mixture (Chen and Poon, 2009a, 2009b; Folli et al., 2012; Guo et al., 2013; Hüsken et al., 2009; Quagliarini et al., 2013). TiO₂ is chosen as it is the most commonly used photocatalytic compound, being cheap, non-toxic and stable. For example, already by 2003, photocatalytic building materials containing TiO₂ accounted form 60% of sales (¥30 bn) in Japan (Chen and Poon, 2009b; Fujishima and Zhang, 2006), and that value will only continue to increase globally. The use of TiO₂, however, can have some drawbacks; considering its preparation, for instance, the industrial processes employed are environmentally unfriendly, as they generate large amounts of liquid sulphate and chloride wastes and SO_x gas (Franzoni et al., 2014). Life cycle assessment studies have shown that its use as photocatalyst can have an overall benefit for the environment, as the beneficial effects outweigh the negative impact ones (pollution from acidification) from its production; however, clearly minimising the amount used to make a photocatalytic material would enhance the beneficial effects on the environment even more (Babaizadeh and Hassan, 2013). Moreover, the current demand for TiO₂ has increased greatly, reaching 3.7 Mt per year, partly also due to its other uses (i.e. pigments, sunscreen filters). Therefore, it would be useful to find alternative solutions for photocatalytic materials, which would require the minimum possible quantity of titania. The amounts of TiO₂ added to photocatalytic mortars and cements vary, but are quite high: typical reported values are 2% (Jimenez-Relinque et al., 2015) to 10% (Chen and Poon, 2009a, 2009b) when mixed in mortar, 3.5 wt% in cement (Folli et al., 2012), 7.4 wt% when mixed with nanolime (Nuño et al., 2015), and 3 wt% (Chen and Poon, 2009b) and 3.4–5.0 wt% (Franzoni et al., 2014) when coated on the surface. The only other photocatalyst investigated for construction applications is zinc oxide (ZnO) (Senff et al., 2014), but it its performance as a photocatalyst is worse than TiO₂.

Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HAp) is a compound widely used in biomedicine, due to its high biocompatibility (Dorozhkin, 2010), as it is one of the main components of bone, so it is clearly non-toxic. Its structure and properties, however, make it suitable for environmental remediation as well – HAp can in fact be employed for the removal of heavy metals from both contaminated waters and soils (Nzihou and Sharrock, 2010). Moreover, some forms of single-phase HAp also show photocatalytic activity (Nishikawa, 2003). Such property has been attributed to the formation of oxygen vacancies in the HAp lattice (Bystrov et al., 2016). HAp also shows very good photocatalytic behaviour when doped with titanium or in a TiO₂-containing multiphasic material (Anmin et al., 2007; Giannakopoulou et al., 2012a). However, large amounts of TiO₂ were added to all of these previously reported multiphasic materials, typically between 16.7 and 55 wt% (Anmin et al., 2006, 2007; Giannakopoulou et al., 2012a; Nathanael et al., 2010).

Although most commercial HAp is prepared synthetically, the compound can also be extracted from natural sources, such as animal or fish bones (Krishnamurithy et al., 2014; Piccirillo et al., 2013a; Ferraro et al., 2013). Previous work done by the authors showed that single-phase HAp can be obtained from the valorisation of Atlantic cod (Gadus morhua) fish bones, a common waste in Portugal, where 60,000 T of cod is consumed annually (Piccirillo et al., 2013b). Pure HAp, or biphasic material containing HAp and beta tricalcium phosphate (β-Ca₃(PO₄)₂, β-TCP), can be obtained via a simple and economic aqueous treatment and thermal annealing process (Piccirillo et al., 2013b). Moreover, the authors demonstrated that by treating the bones in a Ti(SO₄)₂ containing solution, a triphasic material HAp/β-TCP/TiO₂ was obtained with excellent photocatalytic properties, despite only having a very low TiO₂ content of 1 wt% (Piccirillo et al., 2013c) in the form of anatase. The TiO₂ was added as a simple salt solution, and did not require prior processing or treatment to form titania nanoparticles, further simplifying the process. Only one treatment (soaking for 24 h) and one annealing step (800 °C/1 h) were needed to produce the photocatalytic material from the waste bones. Not only was this an excellent photocatalyst and moderate antibacterial material under UV light, it was also effective under visible artificial white (indoor) light (Piccirillo et al., 2013c). The TiO₂ level of only 1 wt% in this material was much lower than any previously reported HAp-TiO₂ materials.

As discussed above, lime- or cement-based materials are still widely used in construction, with applications including plastering, finishing, masonry binding and preservation. In the work presented here, we use this triphasic photocatalytic material of marine origin, made from valorised wastes, as a photocatalytic additive in a natural hydraulic lime (NHL). Our photocatalyst is made from a sustainable waste stream by simple solution and thermal processing and with a much smaller impact on the environment. NHL was chosen for these preliminary studies as it is one of the most used construction materials, and also due to its relatively simple composition, without too many additives which could interfere with the photocatalyst. The mortars were characterised with several techniques to assess how the additive presence affected both the curing process and the features of the mortars, but it must be stressed that this is only an initial work to prove the photocatalytic functionality of these mortars. Their photocatalytic activity was also tested, by determining NO_x abatement rate under irradiation from a solar lamp (full solar spectrum, replicating sunlight).

To our knowledge, this is the first time that a HAp-based photocatalyst has been used in a construction material. It is also the first time that a product derived from fish bones or fisheries/marine wastes and by-products has been employed as an additive in the formulation of NHL mortars. Of key importance is the fact that we have added between only 1–5 wt% of the HAp-based photocatalyst to the mortars, and this in turn contained only 1 wt% of added TiO₂. This means that we have a high-quality photocatalytic mortar which contains only 0.01–0.05 wt% TiO₂, or 100–500 ppm, compared to the 2–10 wt% usually added to photocatalytic mortars – a very large reduction of two orders of magnitude in the quantity of TiO₂ required.