A Review of Chemical Examples in the Context of Circular Economy

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MSc Chemistry

Track Molecular Sciences

Literature Thesis

A Review of Chemical Examples in the Context of Circular Economy

by

Narcisa-Lidia Girigan

ID 12294764

July 2020

12 ECTS

Supervisor:

Second Reviewer:

Dr. Chris Slootweg

Dr. Ștefania Grecea

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Abstract

The linear economy is a traditional economic model that follows a take-make-waste pattern and heavily relies on fossil fuels. The environmental damage linked to it is best quantified by the planetary boundaries (e.g. ocean acidification, biogeochemical flows, climate change), which are alarmingly evolving towards high-risk zones. Three industries – metals, fertilisers and plastics – are major contributors to the abundance of unnatural substances in Earth’s spheres (i.e. unexploited waste, reactive species from fertiliser application), creating imbalances in the ecosystems (eutrophication, plastic soup) and endangering the biodiversity (marine life menaced by plastic waste, algal bloom and hypoxia). Element scarcity and increasing greenhouse gas emissions are also consequences of the linear economy, exemplified here in the above-mentioned sectors. By contrast, the circular economy follows a take-make-restore trend and relieves the pressure on natural resources by promoting feedstock from secondary sources. The present literature thesis investigates circular economy as the best possible solution to the problems caused by the linear economy and brings chemistry forward as an important asset in building such a system, with innovation and clever waste valorisation as main pillars. Examples from the scientific literature have been highlighted to emphasise the importance of sustainable thinking and chemical tools

at any stage of the product development - from design to waste management. Research efforts are persistently made in an attempt to find bright closed-loop solutions that tackle the environmental challenges brought by the linear economy. However, space for change can only arise from the extensive collaboration of different stakeholders (e.g. scientists, economists, the media, governments, consumers). Once concerted action is taken on several fronts, scale-ups of scientific findings will pick up economic momentum and the good practices of circular economy will go mainstream.

I. Introduction

You have probably often encountered the word ‘microplastics’ in recent news titles. What about eutrophication and resource scarcity? Maybe less. They are however linked by a common factor, namely the linear economy. The current literature thesis investigates circular economy as the best possible solution to these problems and brings chemistry forward as an important asset in building such a system, with innovation and clever waste valorisation as main pillars.

Appearing as a necessary shift from the linear system, which follows a take-make-waste pattern, the circular economy knows no waste and the emphasis lays on services and renting goods, rather than ownership and buying a product. In this ideal scheme, following a take-make-restore trend, waste becomes a valuable resource leading to starting materials after mechanical or chemical recycling. The circular approach is the answer to several important issues, such as element scarcity and the alarming environmental impact of the linear economy. There must be space for change in order to achieve such a dynamic and clever chemistry is key. Thinking sustainably every step of the way means that chemical innovation can exist at any level of the product development: design, manufacturing, waste management etc.

The alarming evolution of the environmental problems arises from the chemical pollution, which is caused by unnatural substances (i.e. unexploited waste) in the biosphere, that create imbalances in the

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ecosystems and endanger the biodiversity (e.g. effects of ocean soup and excess nitrogen from fertilizers on aquatic life). Research efforts are persistently made in an attempt to find bright closed-loop solutions to tackle the planetary boundaries that have reached high-risk zones (e.g. ocean acidification, biogeochemical flows, climate change). Considering the above-mentioned facts, this literature study will give a review of chemical examples in the circular economy of metals, fertilisers and plastics.

1. Linear economy

Linear economy is a traditional economic model in which “the environment is as a simple natural free resource”1_{. In this context, every activity follows a take-make-dispose behaviour and waste is generated} during the production (raw materials as feedstock) and the end-of-life stages (landfill, nature). Furthermore, linear manufacturing heavily relies on the oil and gas industry and is accountable for huge GHG emission levels; in 2013, 67% of the total GHG emissions were due to excessive exploitation of natural resources and material management2_.

Although this model has been employed for many years now, it has many serious ecological and economic disadvantages. The state of the planetary boundaries best describes the impact of linear economy on the environment; this topic will be dealt with in the following section. Additional consequences are littering (e.g. plastic soup) and resource scarcity. The economic disadvantages are closely linked to raw materials and concern their prices, geopolitical dependence and increasing demand3_.

2. Planetary boundaries – quantified environmental damage of anthropogenic activities

In 2009, Rockström et al. introduced the concept of planetary boundaries and described how the environmental damage of anthropogenic activities can be quantified4_{. This framework is, in fact, a} scientific analysis of the consequences that human perturbations can have on the state of the Earth system and provides a risk assessment by identifying the critical issues. Steffen et al.further developed this system and identified two core planetary boundaries, climate change and biosphere integrity, which are co-dependent and are influenced by the levels of the other planetary boundaries5_{(fig. 1a). For the} purpose of this literature study, the most relevant environmental issues are ocean acidification, biogeochemical flows, and climate change. Some of them have already reached high-risk zones (in red) and they are all directly related to the metal, fertilisers and plastics industries (fig. 1b). Although there is a high uncertainty level for planetary boundaries estimations, they offer a broad idea about the impact of human activities on the health of the planet and they should be closely monitored.

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Fig. 1a) Fig. 1b)

Figure 1. a) Interaction of the planetary boundaries. The two core planetary boundaries, climate change and biosphere integrity are co-dependent

(thick arrows). Moreover, they move towards a zone of risk as the other planetary boundaries move away from their safe zone (positive influence). Thin arrows denote a weaker connection, while dashed arrows indicate an effect with high uncertainties. b) Current status of the 9 planetary boundaries. The safe zone is illustrated in green, while yellow denotes the uncertainty zone and red warns against high risk. Some planetary boundaries cannot be quantified yet (functional diversity, novel entities, atmospheric aerosol warning). Credit: Steffen et al. 20155

a) Ocean acidification

This planetary boundary quantifies the effect of the atmospheric CO2 concentration on the pH of oceans. Oceans absorb around one-third of the atmospheric CO2, as one of nature’s ways to control the greenhouse effect. The benefits of this biomechanism are overshadowed by the escalating levels of atmospheric CO2 - a 40% increase in its concentration since 17506.

When it dissolves, CO2 forms carbonic acid (eq. 1), which quickly decomposes into bicarbonate ions (eq. 2). These can further dissociate into carbonate ions (eq. 3). The last two reactions also produce protons (H+_{). Keeping in mind that pH is given by the negative decimal logarithm of the proton concentration in a} solution (eq. 4), it results that ocean acidity is increased by CO2 dissolution. This phenomenon is known as ocean acidification. Eq. (5) comes as a natural buffer against pH changing and explains why marine organisms lose one of their fundamental building blocks, calcium carbonate (CaCO3), in the fight against ocean acidification7_{. The Bjerrum plot in fig. 2a) shows the correlation between pH and the concentrations} of different carbon species [HCO3-], [CO32-] and [CO2].

The H+_{ion concentration in oceans has increased by 30% since the industrial revolution, causing the global} average pH of the surface ocean to drop from 8.2 to 8.18_._{Figure 2b) projects the change of ocean pH} according to the evolution of atmospheric CO2 concentration.

CO2(aq) + H2O ↔ H2CO3 (1)

H2CO3 ↔ HCO3- + H+ (2)

HCO3- ↔ CO32- + H+ (3)

pH = -log10[H+] (4)

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Fig. 2a) Fig. 2b)

Figure 2. Ocean acidification is one of the causes of the increasing concentration of atmospheric CO2. a) Bjerrum plot showing the correlation

between pH and the concentration of carbon species [HCO3-], [CO32-] and [CO2]. The decrease in pH caused by ocean acidification translates to a

lower concentration of [CO32-] and a higher concentration of [HCO3-]. Credit: Nature Education, 20127. b) Change of ocean pH according to the

evolution of atmospheric CO2 concentration are projected to drop at least another 0.2 units before 2100 if no change in anthropogenic emissions

occurs. Credit: Turley et al. 20049_{. For estimation of data accuracy, on May 7, 2020, the atmospheric CO}

2 concentration was 18.3 mmol m-310.

The biggest concerns related to this planetary boundary are the weakening of the marine carbon sink and the increase of atmospheric CO2, which amplifies global warming.

b) Biogeochemical flows

The planetary boundary of biogeochemical flows has already been exceeded and is linked to the perturbation of the natural N and P cycles as a result of industrial activity. The main consequence of reaching the high-risk zone for this planetary boundary is eutrophication. Nitrogen oxides (N2O, NOx), ammonia (NH3) and phosphate (PO43−) enter the water system from fertiliser application mostly, enriching

the nutrient levels and destabilising the aquatic ecosystems (e.g. excessive algae growth). The control variables for this planetary boundary are thus the flows of P and N, respectively, from soil to water systems.

Phosphorus

The planetary boundary for phosphorus flow is quantified by looking at mined P that reaches the soils and waters through fertiliser application. Manure is a natural fertiliser and is considered to be internally recycled P. To prevent an excess of P reaching the natural systems, it is important to recognise efficient ways of recycling manure in order to reduce the use of synthetic fertiliser. Nitrogen

The highest share of anthropogenic N input comes from the fertiliser industry via the Haber-Bosch process. Other sources are the emission of nitrogen oxides (NOx) from transport and the biological N fixation. Although eutrophication is the main environmental concern of overflowing N, atmospheric NH3 concentration, climate forcing with N2O and NO3- contamination of drinking water are also important consequences.

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8 c) Climate change

The control variable for climate change is the atmospheric CO2 concentration. The risk of crossing this planetary boundary is increasing and legal frameworks have been implemented to improve the global response face to this menace (e.g. The Paris Agreement). Since climate change is a core planetary boundary, it is directly influenced by the weakening of marine carbon sink due to ocean acidification and by greenhouse gas emissions, climate forcing, and eutrophication caused by the biogeochemical flows.

3. Legal framework

In light of the planetary boundaries moving away from their safe zone, legal framework has been developed and agreed upon by many states to make sure everyone understands and is held responsible for the consequences of their activities, while striving for less negative impact on the environment.

a) Sustainable Development Goals

The Sustainable Development Goals (SDG) is a plan of action formulated for people, planet and prosperity by the United Nations (UN). Although only adopted in September 2015, the SDG agenda is the result of many years of work and partnerships that started at the Earth Summit in 199211_{. Circular economy would} accelerate the achievement of at least four goals on the 2030 agenda: decent work and economic growth (#8), responsible consumption and production (#12), climate action (#13) and partnership for the goals (#17)12_.

b) REACH

Adopted on the 18th_{December 2006 by the EU, this regulation concerns the Registration, Evaluation,} Authorisation and Restriction of Chemicals and is meant to shield the human health and the environment from avoidable danger linked to chemical use, while boosting competitiveness of the chemical industry13_. REACH contributes to achieving the UN SDG concerning chemicals by maintaining an updated log with the properties and risk assessment of substances and tracking their use.

c) Paris Agreement

The Paris Agreement is a framework adopted by the United Nations in 2015. It recognises climate change as a “common concern of humankind” and targets the reduction of GHG emission while encouraging sustainable development14_{. The ultimate aim of this agreement is to limit the increase in the global} average temperature to 1.5°C above pre-industrial levels.

Circular economy is a complex strategy to achieve the UN SDG and the targets of the Paris Agreement, while complying to REACH. Its weapons can tackle the consequences we see today from the linear economy, as a response to climate change, economic vulnerability, biodiversity loss, resource scarcity, waste, and pollution15_.

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4. Circular economy – what goes around must come around

Circular economy is an industrial system intentionally designed to be restorative and regenerative16_{. Being} inspired by nature and its many intertwined circles, it strives for resource preservation, circular flows of materials and decarbonisation of energy, all without tampering with progress. Circular economy is driven by both ecological and economic forces. On one hand, this scenario assures lower GHG emissions, no waste and conservation of natural resources. On the other hand, lower costs for producers and stable prices for consumers will follow.

The concept of circular economy was first described in 1990 by David Pearce and R. Turner17_{and has been} successfully applied on small scale since then18_{. Launched in 2010, the Ellen MacArthur Foundation} thoroughly researches and boosts awareness about circular economy, especially for policy makers. Its founder, Dame Ellen MacArthur, acknowledged the importance of resource management while competing to become the fastest solo sailor to circumnavigate the globe and then concluded that the same approach should be applied to the economy19_{. Although a relatively new and daring concept, China} showed a positive attitude towards circular economy and expressed their intention to vigorously develop it as early as 201120_.

In a context where the dynamics of production-consumption vs resource availability are radically changing, circular economy comes as a saving grace. Ellen MacArthur Foundation (EMF) formulated the principles that should rule such an innovative economic model (fig. 3) and ranked the power of circles within16_{(fig. 4).}

Circular economy comes with new patterns of production, consumption and use. For example, the value of a product is defined by its value retention potential, rather than immediate benefits. As dictated by the cradle-to-cradle concept, at the end of a cycle, all materials will be “nourishment for the next production process”17

. Although this might seem idealistic and unattainable, it is a good reference to look up to21. The

change from a linear economy to a circular one will without doubt be radical; figure 5 summarises the main elements of this much-awaited transition.

Figure 3. Principles of circular economy, introduced by the Ellen MacArthur Foundation16 Principles of circular economy

1. Design out waste: biological nutrients go back in the biosphere and technical nutrients are upcycled

2. Aim for durability and anticipate easy disassembling to facilitate recycling 3. Use renewable energy

4. Lease-and-use instead of buy-and-consume 5. Think in systems

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10 Figure 4. The power of circles. In order to get the best possible outcome, circular economy should aim for: prioritising tighter circles that lead to

higher savings (inner circle), maximising the number of consecutive cycles (circling longer), diversifying reuse across the value chain before recycling (cascaded use) and keeping material streams uncontaminated for increased efficiency (pure circles). These four principles were proposed by the EMF in the “Towards a Circular Economy” report16_{and illustrated by my dear friend Ruxandra Păcurar}22_.

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11 Figure 5. Linear to circular transition. Such a movement can be achieved only by taking a systems approach. Divided industries shall thus become interconnected. The emphasis is laid on stewardship and services, rather than ownership of products. When designing goods, the aim should be durability and easy recyclability; degradability leads to waste and loss of value. Consumers become users and possibly creators if they repurpose

an object. (*) People are central to this model. Credit: Ruxandra Păcurar22

5. Circular chemistry

Circular economy is the result of concerted actions on all fronts and chemistry is key to optimising resource efficiency, maintaining or increasing value across the loops and designing out waste. The concept of “circular chemistry” was coined by Keijer et al. in 2019 and it was introduced as a natural evolving step after green chemistry. A list of 12 principles was formulated to better frame circular processes from a chemical point of view23_:

Principles of circular chemistry

1) Collect and use waste 2) Maximise atom circulation 3) Optimise resource efficiency 4) Strive for energy persistence 5) Enhance process efficiency 6) No out-of-plant toxicity 7) Target optimal design 8) Assess sustainability 9) Apply ladder of circularity 10) Sell service, not product 11) Reject lock-in

12) Unify industry and provide coherent policy framework Figure 6. Principles of circular chemistry as formulated by Keijer et al. 201923

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12 The above list shows that chemistry implications must exist at every step of a process in order to make it circular. In a perfect loop system, waste management is improved to assure complete flow of materials and, therefore, no discharged waste; this step highly benefits from the discovery of novel reactions and maximises atom circulation. In a company that sells a service, rather than a product, waste management will be carried out more effectively because they have the know-how and the means to repurpose their materials.

Reusing waste as feedstock for further production allows to preserve finite resources, while renewable resources are a tolerated alternative. Bio-based and bio-degradable products do not fit perfectly in this circular scenario that promotes durability because of end-of-life and, respectively, feedstock related issues. Value must be conserved or increased along the loops and this involves stocking energy in materials (e.g. CO2-based chemistry24).

Early in the design phase, the most appropriate end-state of the product should be identified and taken into consideration for easy recycling/conversion. To this end, molecular complexity must be kept to a minimum and traceability is recommended25_._{An evaluation using the life cycle assessment (LCA)}26_and the ladder of circularity23_{is imperative at this stage. With regards to end-of-life actions, those in charge} must aim for the top of the ladder shown below.

Figure 7. The 11 Rs. This ‘ladder of circularity’ offers options for end-of-life actions; their desirability decreases from top to bottom. Credit:

adapted from Keijer et al. 201923_{by Ruxandra Păcurar}22

Finally, circular economy requires heavy communication and collaboration from stakeholders (i.e. everyone). Industrial sites benefit from being interconnected and discharge of chemicals in nature will be drastically reduced; as they say, one man’s trash is another man’s treasure. Furthermore, change will be supported by a policy framework that educates on and rewards circularity.

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II. Scarce metals and specialty elements

The fast-moving technological progress that is characteristic to the innovative world we live in often translates to an unsustainable materials infrastructure, the main issue being the current end-of-life management (poor recovery and recycling processes).

Many of the elements required for the manufacturing of novel performant (electronic) products are now critical materials, which means that they have a high economic impact, paired with an increasing risk of supply shortage. In 2017, the EU list of Critical Raw Materials included the platinum group metals (PGMs),

the rare earth elements (REEs) and germanium, among others27_{. Platinum group metals (PGMs) are} iridium, platinum, palladium, rhodium, ruthenium. The rare earth elements (REEs) include the 15 lanthanides, as well as scandium and yttrium. These elements are essential resources for technological development because of their unique chemical profile, showing strong magnetic properties and relative stability at increasing temperatures. Sadly, enhancing performance comes at a price – an increased demand of specialty elements and, consequently, resource scarcity. If we look at the evolution of computers, for instance, not only did the material demand rise with the accelerated production capacity, but so did the number of elements in the chips as the processor speed increased (16 in the 1990s vs. 60 by mid 2000s)28_.

Element scarcity can be easily visualised in an illustration of the periodic table made by the European Chemical Society in 2019 (fig. 8). While all the PGMs are at risk due to increased used, out of the REEs, only scandium and yttrium are associated to rising and serious scarcity, respectively. Besides components of these groups, other elements will be discussed in this chapter, some of them present in smartphones: gold (limited availability and conflict zone origin), silver (serious scarcity threat), lithium (limited availability), germanium (serious scarcity threat). The EuChemS periodic table of element scarcity ensures that everyone who sees it makes the first step towards taking action - understanding element vulnerability. Then, developing new methods for the sustainable separation, recovery and recycling of critical materials must be at the top of scientific research in order to bring element circulation to circular economy levels.

Figure 8. Element Scarcity – EuChemS Periodic Table. The tile size is proportional, on a logarithmic scale, to the amount of the corresponding

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whether its scarcity is due to natural limited availability or increased use and lack of recycling. Information on elements extracted in conflict zones and elements presence in smartphones is also given. This work is licensed under the Creative Commons Attribution NoDerivs CC BY-ND. Credit: EuChemS29_.

1. E-waste

E-waste comprises all the electronic or electrical devices that have been discarded. In 2016, their amount was estimated at 6kg per person30_{. While being a serious threat for the environment because of the} naturally leaching chemicals, E-waste also represents a secondary source of valuable materials and its recycling should be incentivized in order to move away from raw materials.

Mobile phones are a valuable source for recovering metals in higher concentrations than obtained from the natural rock (e.g. in the case of Au, 255g/t from mobile phones vs 4g/t from mining). Other metals that have high recovery/extraction ratios are Co and Cu31_{. Despite the promising outcome of urban} mining, electronics are complex products, and this usually comes with raised levels of difficulty to separate and recycle their components. Product design should therefore aim for resource efficiency, by avoiding the use of incompatible metal mixtures to lessen recycling complications32_{. In addition, recovery} technologies only exist for a limited number of elements and are usually associated with high costs; cheaper alternatives have yet to be developed.

Metal extraction is energy-intensive and generates large amounts of waste. These negative effects are even higher in the overall production process, when the manufacturing stage is considered. Therefore, urban mining is key to reducing the environmental impact of the metals industry and replacing primary sources. One first step towards enhancing recycling is to implement an output-pulled mentality, instead of the current input-pushed one33_{. Umicore is one of the companies that recycle mobile phones in Europe} and they mainly valorise silver, gold, palladium and copper34_.

As countries work on achieving the renewable energy goals, the global demand for metals such as aluminium, cobalt, copper, lithium and zinc is significantly increasing35_{. It is therefore imperative to meet} these requirements by metal recycling. The copper industry shows perfectly how circularity should look like. Copper knows no end-of-life. Once extracted, it can be infinitely recycled with no subsequent value or propriety loss. Hence, there is no surprise that the copper industry has been sustainably relying on circular principles and at least 50% of the material used in Europe comes from secondary copper sources. Besides the economic benefits of value perpetuity, copper recycling requires up to 85% less energy than primary production and saves CO236.

2. Current recycling processes

The traditional recycling protocols of E-waste start with manual disassembly (mechanical processing) to select and separate materials, followed by technologies based on mechanical, chemical and thermal methods 37_{. Smelting (pyrometallurgical processing)}38–40_{is the go-to technique for metal recovery and} consists of melting the waste. In hydrometallurgical processing, the solid material is dissolved with acid or basic attacks41,42_{and the metals are isolated and concentrated by either subjecting the solution to} separation processes (precipitation, filtration, distillation etc.)38_{or to ion exchange (electrometallurgical} processing)38,43_{. Another recovery process is biohydrometallurgy}43,44_{, which makes use of bacteria to}

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15 extract metals from waste. The previously mentioned metal recycling techniques have many disadvantages that stop them from being green. While generating a lot of waste, they are all solvent or energy intensive and the former is especially difficult to scale up33_.

Although recycling is an established practice for costly (PGM) or highly requested metals (e.g. aluminium, nickel, copper) with rates greater than 50%, many RESEs that are key for new technologies do not offer the same economic incentives and have near-zero recovery rates (e.g. germanium, lithium, rare earths)45_. This is mostly because, since these materials are used in small quantities, their recovery is more expensive than extraction and often leads to lower purities. Furthermore, the input rates of the secondary materials are much lower because of the quality criteria they must meet to re-enter the manufacturing process (e.g. 10-20% for aluminium and copper)46_.

3. Chemical innovation for metal recovery

Inorganic chemistry is essential to develop new metal recycling strategies and responds to the challenges of circular economy by tackling issues such as selectivity and recovery of elements in trace amounts47_{. In} the next pages of this chapter, innovations in this field will be discussed for a selection of metals: Au, Ag, Pd, Pt, Li, Ge, REEs.

a) Gold

Gold (Au) is a precious metal with many applications in electronics, catalysts and anti-corrosion materials. Its availability in nature is limited, making the extraction and recovery processes very complicated. On the other hand, the gold demand is firmly increasing and so its recovery from waste solutions is highly incentivised. A variety of techniques have been explored over the years to recover gold from wastewater (chemical precipitation48,49_{, membrane separation}50,51_{, solvent extraction}52_{, and adsorption}53–55_{). It has} been concluded that adsorption is the most eﬀective method, because it is economical and applicable to low concentrations of gold – which is the case for industrial waste.

Cyanide leaching, the most common hydrometallurgical process for extracting gold from ores, is known to have hazardous environmental and health impacts. The alternatives presented below are cyanide-free gold leaching methods and are effective for the selective recovery of gold, while being environmentally friendly.

Doidge et al. reported the successful use of the primary amide 3,5,5-trimethylhexanamide L (H2NC(O)CH2CH(Me)CH2tBu) in the selective extraction of gold in the organic phase (toluene) over other metals from a mobile-phone waste aqueous HCl solution56_{. It was proved that phase transfer occurs by} hydrogen bonding of protonated and neutral amides (HL+_and_{L) with [AuCl}

4]- ions, which results in the immediate formation of supramolecular aggregates in toluene (fig. 9). This method leads to the significantly selective extraction of 82% of the gold present in the initial mixture, compared to 6.4% iron and 2.7% tin. This is one of the first examples of chemical understanding coming to the aid of metal recovery. However, this technique is limited by the amount of wastewater it produces. Moreover, the steps necessary for the metal gold extraction are not accounted for in the abovementioned publication.

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16 Figure 9. Neutral and protonated 3,5,5-trimethylhexanamides form aggregates with [AuCl4]- ions in organic phase through hydrogen bonding.

Credit: Doidge et al. 201656

Zhichang Liu et al. researched the noncovalent bonding interactions between macrocyclic hosts e.g. α-cyclodextrin (α-CD, shown in fig. 10a) and alkali metal halo-aureate salts, also known as second-sphere coordination57_{. The study shows that, in aqueous solution of KAuBr}

4 and CD, co-precipitation of an α-CD·[K(OH2)6]+·[AuBr4]- adduct (fig. 10b) occurs within minutes. This is made possible by the incredible molecular match of α-CD, K+_{and [AuBr}

4]−.

An αCD macrocycle has two faces: one with primary OH groups (1°) and another one with secondary -OH groups (2°). The α-CD tori are stacked in a crystal due to head-to-head and tail-to-tail [O-H···O] hydrogen bonds. Two types of second-sphere cavities are thus created: one described by the interaction of two 1° α-CD faces and another one between two 2° α-CD faces, respectively. In aqueous solution of KAuBr4, the primary cavity of α-CD will host the [AuBr4]− anion by means of [C−H···Br−Au] hydrogen bonds, while the secondary cavity will be filled by the hexaaqua K+_{cation ([K(OH}

2)6]+). The noncovalent interaction of these two ions drives the coprecipitation of the α-CD·[K(OH2)6]+·[AuBr4]- sandwich-type adduct.

Subsequent reduction reactions yield Au(0) and α-CD, which can be reused in the gold separation process. When applied to yellow and rose gold waste containing impurities such as Ag, Cu, and Zn, this method allowed the recovery of high-purity gold metal in 89% and 92% yield, respectively.

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17 Figure 10. a) Molecular structure of α-cyclodextrin. The macrocycle has two faces: one with primary -OH groups and another one with secondary

-OH groups. Source: Wikipedia b) Schematic of the α-CD·[K(OH2)6]+·[AuBr4]- sandwich-type adduct. The α-CD macrocycles are stacked in a crystal

due to head-to-head and tail-to-tail [O-H···O] hydrogen bonds, forming primary and secondary spherical cavities. In aqueous solution of KAuBr4,

the 1° cavity of α-CD hosts the [AuBr4]− anion, while the 2° cavity is filled by the hexaaqua K+ cation ([K(OH2)6]+). Reproduced after Zhichang Liu et

al. 201657

One method described by Korolev et al. uses repetitive Electrochemical Deposition-Redox Replacement (EDRR) cycles to recover gold in limited concentrations from cupric solutions58_{. This is a green process in} the sense that it does not use any additional chemicals. Solutions with different Au:Cu ion concentration ratios were used together with a three electrode platinum cell in the EDRR process (fig. 11).

Figure 11. Mechanism of the electrodeposition-redox replacement in cupric chloride solution. After introducing the working electrode in the

solution (A), Cu(0) accumulates on its surface at constant potential of the cathode (Eset) for a set time (tdep). In the next step, the circuit is left open

(B) until the termination potential is reached (Ecut). Cu(0) is replaced by Au(0) because of the difference in their standard electrode potential (C).

Copper residue might be present in the final layer if the redox reaction does not have enough time to complete (D). Cyclic voltammetry was used to establish the values of Eset and Ecut. Credit: Korolev et al. 201758

The method described above may yield Cl2 as a side-product, which is hazardous and toxic if not addressed properly. EDRR presents however many advantages, such as: reduction of hazardous substances traditionally associated with the extraction of gold, replacement of primary sources with recovery processes and lower resulting costs.

Wenqi Liu et al. reported another macrocyclic system that has the potential to selectively capture halide coordination complexes of gold (III) over palladium (II) and platinum (II) equivalents59_{. Metal gold can then} be isolated through precipitation. Two examples of such molecules are shown in figure 12, together with a conceptual design. The two tetralactam macrocycles M1 and M2 present π-donating aromatic moieties that interact with the electropositive metal centre of the guest and amide protons that participate in hydrogen-bond interactions with its electronegative halide ligands.

M1’s distinctive structural quality is that its amide NH bonds are always directed inward. Thus, when

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18 major conformational change of the receptor happens. In the final adduct, the metal centre Au(III) is sandwiched between two anthracene sidewalls of M1, while the halide ligands engage in hydrogen-bonds with the NH amide residues. M2 was designed to have increased affinity and selectivity for AuX4- guests, by replacing the two anthracene sidewalls with two durene (2,3,5,6-tetramethylbenzene) moieties. Indeed, the AuCl4- and AuBr4- binding constants increased tenfold, due to the stabilising CH···XAu non-covalent interactions of the durene methyl groups with the guest.

Figure 12. (top) Conceptual design of a macrocycle that could host an AuX4- metal halide. The sidewalls should be π-electron donor moieties to

stabilise the electropositive metal centre, while the halide ligands should interact with H-bond donors that are oriented inward. (bottom) Two tetralactam macrocycles used in this publication. M1 has anthracene sidewalls and M2 has durene sidewalls which increase the affinity and selectivity for AuX4- guests. Credit: Wenqi Liu et al. 201859

The effective adsorption and recovery of Au3+_{ions were obtained by Zhang et al. using terminal} triethylenetetramine hyperbranched dendrimer-like polymer modified silica-gel (SG-TETA and SG-TETA2) in water60_{. The complexation mechanism in this process reveals an adsorptive reduction (fig. 13). It was} shown that Au(III) ions are present in the form of Au(0) and Au3+_{when adsorbed on the surface of}

SG-TETA and SG-SG-TETA2. Thus, the amine moieties have a double role: to coordinate the Au3+ _{ions and to}

reduce them to metal gold.

To prove that the adsorption of gold on the surface of SG-TETA and SG-TETA2 is reversible, adsorption-desorption cycles were repeated 3 times by use of a 4% thiourea/0.1molL-1_{HCl solution. The Au(III)} recovery was maintained above 91%, indicating a relatively stable adsorption capacity of the reused polymers. SG-TETA2 has a slightly higher adsorption capacity than SG-TETA due to long and flexible triethylenetetramine chains with more N sites.

Both SG-TETA and SG-TETA2 showed infinite adsorption selectivity towards Au(III) in binary mixtures with Cu(II), Pb(II) and Ni(II), respectively. The stable regeneration of polymers, their good adsorption capacity and their selectivity for gold guarantee for the great potential of this method.

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19 Figure 13. Complexation mechanism of SG-TETA and SG-TETA2 of silica gel surface with Au(III) ions. The Au(III) ions may chelate with two nitrogen

atoms via their lone electron pairs or participate in electrostatic interactions with a protonated nitrogen atom. After adsorption, Au(III) ions are reduced to Au(0). Credit: Zhang et al. 201960

b) Silver

Halli et al. managed to selectively recover silver present in trace amounts from hydrometallurgical solutions with high concentration of zinc sulfate (60g/L) using EDRR (fig. 14)61_{. As previously mentioned,} EDRR relies on the difference in potential for two metal-ion pairs; in this case, 1.6V between Zn/Zn2+_and Ag/Ag+_{. The replacement of metallic Zn, initially deposited on the electrode, with the more noble metallic} Ag thus happens through spontaneous redox reactions. After the EDRR procedure, the electrode is removed from the sulfate solution to recover the deposited Ag(0).

Zinc ores or concentrates are not normally considered viable sources for silver recovery because of sulfate contamination during processing. However, silver has limited solubility in sulfate media and can be successfully recovered from zinc waste. EDRR is an energy-efficient and chemical-free method that creates new secondary sources by making possible the recovery of many critical metals present in trace amounts in industrial hydrometallurgical solutions.

Figure 14. EDRR in ZnSO4 solution with trace amounts of silver. Zn2+ ions (empty grey circles) are reduced and Zn(0) (filled grey circles) is deposited

on a platinum electrode (electrodeposition step). Then, due to the favourable change in potential, Ag+_{ions (empty black circles) oxidise the}

metallic Zn to ions that return in solution and metallic Ag (filled black circles) is deposited on the electrode surface (redox replacement step). Credit: Halli et al. 201761

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20 c) Palladium

The largest application of palladium is for catalytic converters. Together with platinum and rhodium, it is used, for example, in three-way catalysts (TWC) to transform major exhaust gas pollutants into harmless compounds62_.

Serpe et al. studied the dissolution of Pd(0) using N,N’-dimethylperhydrodiazepine-2,3-dithione diiodine adduct (Me2dazdt2·I2, 1 in fig. 15)63. This reagent is water and oxygen-stable and had been previously shown to be successful in Au recovery under mild conditions64_{. The same reaction profile was applied to} Pd(0) and lead to the formation of the square-planar complex 2 (fig. 15), which then undergoes reduction to yield metallic palladium. By repeating the same methodology with platinum and rhodium, Me2dazdt2·I2 showed no reactivity, even in refluxing solvents. This method is therefore ideal for recovering palladium from TWC catalysts and was shown to leach >90% palladium from mixtures with typical TWC metal loadings.

Jantan et al. investigated several palladium dithiooxamide complexes (2 included) obtained through sustainable Pd(0) leaching and established that they can be directly valorised for catalysing selective C-H bond functionalization reactions, leading to tighter loops in a circular economy65_.

Figure 15. Dissolution of Pd(0) with Me2dazdt2·I2 (1). The reaction was carried in THF (i) at room temperature and without protection from air and

moisture. Credit: Serpe et al. 200563

d) Platinum

The production of catalytic converters accounts for more than half of the platinum demand and has little to no chances to decrease since it is essential for improving air quality by reducing pollutants from exhaust systems. Besides being prepared to satisfy the rising demand and to lower the GHG emissions and energy consumption associated to platinum extraction, developing sustainable recovery systems would also make the supply chain more secure; most of the platinum is imported in the EU from two countries, Russia and South Africa66_{. The main bottleneck of platinum recycling is its presence in negligible amounts in} industrial waste solutions.

Halli et al. reported the recovery of trace amounts of platinum from hydrometallurgical solutions with high nickel content (> 140 g/L) by EDRR on a pyrolyzed carbon (PyC) electrode67_{. This procedure follows} the principles described in figure 14 and allows to selectively recover 90% of the platinum Pt contained in complex nickel solutions, creating the opportunity for new secondary sources in the attempt of reaching circularity.

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21 Starbon® is a carbon-rich mesoporous material derived from waste polysaccharides (starch)68_{. Muñoz} Garcia et al. investigated its application for the selective adsorption and separation of PGM ions (Au3+_, Pd2+_{, Pt}2+_{) from a mixture containing earth abundant elements, with the recovery of their respective metal} nanoparticles69_{. The mechanism of this method comprises the reduction of the metal ions by oxidation of} the carbonaceous structure of Starbon®, which yields metal nanoparticles. While the material shows a significantly higher adsorption preference for gold (>99%), palladium (>90%) and platinum (>80%) compared to the other cations present in solution (Ni2+_{, Zn}2+_{, Cu}2+_{), it only partially adsorbs Ir}3+_{, with a} capacity of 31%. These values are independent of the starting metal concentrations. Therefore, this application has potential for the recovery of PGM from low-concentration waste or catalysts.

e) Lithium

The demand for lithium has significantly increased in the past years due to its application in electric vehicles. Therefore, methods for lithium recovery from spent batteries are highly needed in order to create secondary sources that could reinforce the supply chain and stabilize the metal price.

He et al. explored the use of ion-pair receptors to selectively recover lithium salts from organic solvents (CHCl3/CH3CN/CH3OH)70. The hemispherand-strapped calix[4]pyrrole (fig. 16a) has appropriate binding sites to coordinate both the cation and the anion of a metal salt (fig. 16b). For instance, in the case of lithium chloride, the Li+_{cation is adsorbed in the pocket created by the amide group, nitrogen atom of} pyridine and the neighbouring methoxy group of the receptor. The Cl-_{anion, on the other hand, is} adsorbed with the help of the NH groups in the calix[4]pyrrole subunit through hydrogen bonds. The metal ion remains bound to one water and one methanol molecule from the solvent, with the water molecule acting as a bridge between the cation and the anion in the receptor.

Figure 16. a) Molecular structure of the hemispherand-strapped calix[4]pyrrole. b) Different binding sites of the ion-pair receptor. For the

adsorption of lithium chloride, the Li+_{cation is found in the cavity described by the amide group, nitrogen atom of pyridine and the neighbouring}

methoxy group of the receptor. The Cl-_{anion is coordinated by the NH groups in the calix[4]pyrrole subunit through hydrogen bonds. The metal}

ion remains bound to solvent molecules, with the water molecule acting as a bridge between the cation and the anion. Credit: He et al. 201670

The previously described method is selective for the recovery of Li+_{salts in solution because of its lower} binding energy in comparison to Na+_{, K}+_{and Rb}+_{salts. Although this approach would be beneficial for} curbing the lithium industry, the low yield for the synthesis of the receptor (19%)and the use of toxic solvents e.g. chloroform represent serious bottlenecks that need to be addressed before scale-up. However, the compound in figure 16a) is the first ion-pair receptor to successfully extract lithium nitrite

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22 under both solid−liquid and liquid−liquid conditions; this is of great importance considering the harmful presence of the nitrite ion in the environment.

Bae et al. developed a waste-to-lithium cell (fig. 17) that extracts more than 75% of high-purity Li metal from waste when charged71_{. Since Li metal is unstable and very reactive, commercial precursors LiOH and} Li2CO3 are formed upon discharge via electrochemical water reactions.

The system is composed of three compartments separated by two Li+_{conducting membranes (LATP): a} first cathode for Li-containing waste, an anode for harvesting Li metal and a second cathode for recycling it into air-stable Li compounds. The first step is delithiation of Li solids when the cell is charged, by conversion of the Li+_{ions resting on the current collector (carbon paper) into Li metal. The anode harvests} Li metal on a Ni mesh current collector that converts it back to ions upon discharge of the cell. The ions pass through the ceramic membrane and react with water in the recycling cathode, forming LiOH. However, because of the reaction of LiOH with CO2 dissolved in water, Li2CO3 is the end-product. The yield of this process was found to be low at first, ~25% for obtained Li2CO3. This issue was overcome by replacing the Ni mesh in the anode with a graphite electrode, which stores Li+_{ions instead of forming Li metal and} directly participates to water reactions to produce air-stable lithium precursors.

The previously described waste-to-lithium cell operates at room temperature and does not make use of harmful chemicals, hence becoming a great alternative to the chemical- and heat-dependent lithium metal extraction from ores.

Figure 17. Waste-to-lithium multi-layer electrolyte system containing a waste cathode (top), a harvesting anode (middle) and a recycling

cathode (bottom). Credit: Bae et al. 201671

Guan et al. investigated the mechanochemical extraction of lithium and cobalt from spent lithium-ion batteries (LIB)72_{. Pure lithium cobalt oxide (LiCoO}

2) powder, which is a LIB cathode material, was grinded with iron powders (1:1 ratio) before leaching with dilute nitric acid solution at room temperature. The first process is an activation step, which leads to the cleavage of bonds within LiCoO2. While the structure

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23 of the LiCoO2 crystals is gradually destroyed during grinding, reaching an amorphous state, the iron powder acts as a reducing agent and changes the oxidation state of cobalt (eq. 6).

3Co3+_{+ Fe}0_{→ 3Co}2+_{+ Fe}3+ ₍₆₎

The process was ultimately tested on waste LIB cathode materials and lead to the low-cost extraction of 77% of Li, 91% of Co, 100% of Mn and 99% of Ni, which can be directly used to source the production of new lithium-ion batteries.

Swain points out that hydrometallurgical processes for LIB recycling mainly focus on the recovery of Co, Mn, Al and Ni, while Li remains in the wastewaters. Hence, he proposes a cost-effective method to recover Li from LIB industrial recycling waste using reverse osmosis (RO) and subsequent precipitation73_{. Firstly,} lithium enrichment is achieved by reverse osmosis (fig. 18). In this process, wastewater is pumped into a container equipped with a semi-permeable RO membrane that purifies the water by collecting the salts in the concentrate stream. This rejected stream is enriched with lithium but may contain residual metal ions. These can be precipitated as hydroxides at pH 12 and filtered out. Then, by increasing the pH≥14 of the purified solution and by adding a saturated solution of sodium carbonate, lithium can be finally precipitated as lithium carbonate.

Figure 18. Lithium enrichment through reverse osmosis. Credit: Swain 201873

Li et al. developed a green and simple mechanochemical method to selectively recover lithium from spent LiFePO4 (LFP) cathode materials, which are difficult to recycle through hydrometallurgical processes because of the stable olivine structure of the crystals (fig. 19)74_{. This process uses low-cost mild chemicals,} while consuming less energy than other LFP treatments, and was shown to have a 99.35% efficiency for Li+_recovery.

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24 Figure 19. Mechanochemical recycling process for LFP cathode materials in lithium-ion batteries. Credit: Li et al. 201974

LFP materials are grinded with citric acid (H3Cit) powder and hydrogen peroxide (H2O2) using a ball mill at room temperature (eq. 7). The mixture is then leached with deionised water and filtered. While Li+_ions remain in solution as Li3Cit, Fe2+ ions are oxidised by H2O2 to Fe3+ and filtered out as FePO4 precipitate. However, some Fe3+_{ions remain as impurities in the solution. After stirring the solution at 60°C to} evaporate the water, the impurities are precipitated as Fe(OH)3 with the addition of NaOH and removed by filtration. The remaining solution is then heated up to 95°C and saturated Na2CO3 is added to precipitate Li+_{ions as Li}

2CO3.

2H3Cit + 6LiFePO4 + 3H2O2 → 2Li3Cit + 6FePO4 + 6H2O (7) f) Germanium

Germanium is an important element for electronics and optical devices. Because of its reactivity, it is rarely present in nature in its pure form and can be found as an oxide in minerals. Its extraction is energy-intensive and requires the use of toxic chemicals (e.g. hydrochloric acid)75_{. Thus, recovering purified} germanium from industrial waste using low-energy and benign chemicals is essential.

Glavinović et al. developed a recyclable quinone/catechol redox system in the presence of pyridine (L) to replace the use of toxic chemicals Cl2 and HCl in traditional germanium processing (fig. 20a)76. Germanium is commonly found as a dioxide (GeO2) in mineral deposits of zinc oxide (ZnO). After industrial purification by chlorination and distillation, germanium is extracted in the end as germanium tetrachloride (GeCl4). Converting germanium metal or dioxide to germanium tetrachloride is also the first step in postconsumer recycling. GeCl4 is sensitive to moisture and easily hydrolyses back into GeO2 and HCl; the germanium oxide may be further reduced to obtain germanium metal.GeCl4 is a key compound in the lifecycle of germanium, taking part in ligand substitution reactions that yield organogermanes. However, it participates in energy-intensive low-yield reactions and gives toxic side-products because of its sensitivity to moisture.

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25 Figure 20. a) Traditional germanes synthesis vs. route described by Glavinović et al. Credit: Glavinović et al. 201776

Figure 20. b) Synthesis of germanium bis(catecholate) complex (3) and formation of organogermanes and germane.Credit: Nelson et al. 201947

The new method involves a redox cycle leads to a stable germanium bis(catecholate) complex (3) that mimics the central importance of GeCl4. By further reacting this complex with various Grignard reagents under mild conditions, organogermanes are prepared in high yields and purity.

There are two ways of making germanium complexes of catechol: by oxidation with ortho-quinone of germanium metal or by ligand substitution between catechol and GeO2 (fig. 20b). Oxidation of Ge(0) to Ge(IV) is made possible by the two-electron reduction of each ortho-quinone unit forming the final metal complex, with pyridine as an auxiliary ligand. The same conditions can be applied to the dehydration of GeO2 with catechol to yield the germanium bis(catecholate) complex. Activation of germanium oxide with catecholate would provide a chlorine-free extraction from ZnO ores at room temperature, by selective complexation and separation of the germanium compound.

The obtained germanium bis(catecholate) complex (3), although bench-stable, acts as a pseudohalide and takes part in ligand exchange reactions with nucleophiles to yield organometallic compounds with the retention of the oxidised metal centre. The higher selectivity and product yields of these syntheses, in comparison to the analogous GeCl4 reactions, are due to the steric strain reduction associated with the conversion of a germanium bis(catecholate) complex to organogermanes. Intermediate (3) was also studied in the synthesis of GeH4, a valuable compound for vapor deposition in electronics. The reaction took place at ambient temperature in dibutyl ether and in the presence of argon, using LiAlH4 as a hydride donor, and resulted in high-quality GeH4. This process could be improved by using a milder hydride donor e.g. NaBH4.

3

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26 Since catechol recovery was proved to be excellent in this process and ortho-quinone is obtained from the catalytic oxidation of catechol by air, the previously described redox system has the potential to revolutionise the germanium processing technologies by using molecular oxygen as oxidant instead of chlorine and having water as side-product.

g) Rare earth elements

The rare earth elements (REEs) contain the lanthanides, scandium and yttrium. These metals are usually found in small concentrations and mixed together with other elements, which makes their extraction energy-intensive and generator of great amounts toxic waste. Moreover, at least half of the rare earth market today is controlled by one major political power – China. REEs are of central importance in the production of high-tech devices, defence systems and clean-energy technologies77_{. Developing viable} methods for the recovery of REEs and securing secondary sources are thus crucial to satisfy the ever-growing demand and to close the loop of their existence.

In 2014, Solvay announced the recycling of phosphorescent powders from fluorescent lamps to extract lanthanum, cerium, europium, terbium, gadolinium, yttrium in the form of oxides78_{. The process has been} tested at industrial scale since 2012 and relies on the chemical attack of phosphors, with subsequent recovery of REEs by precipitation or solvent extraction. The method was proved to yield 1350 t/y of rare earth oxides, which are used as feedstock for the production of low energy consumption lamps. Although this project shows great results, it is difficult to say if it adheres to the principles of circular chemistry because details about the involved chemical substances and processes are not publicly available. For instance, the use of nitric acid, a corrosive chemical, is required during the procedure, but its recycling is not discussed.

Binnemans et al. achieved the photochemical separation of europium from red lamp phosphor waste79_. In aqueous media, EuSO4 is less soluble than Eu(III) salts. Keeping this separation principle in mind, the Eu(II) sulfate salts were precipitated after the photochemical reduction of Eu (III) to Eu (II) in the presence of isopropanol. The process was applied on Eu/Y mixtures and showed >90% efficiency in separating high purity EuSO4 precipitate, depending on the metal ratios in the starting solutions.

Bandara et al. reported the scalable recovery of REEs from end-of-life engines using green chemistry principles80_{. Starting with neodymium (NdFeB) magnets, REE dissolution is achieved using HCl 4M. Then,} oxalic acid is added to precipitate the REEs as oxalate complexes, which can be isolated by filtration. The waste levels are low, with HCl being recycled in the end and the Fe/B oxides recovered. This process could be improved by replacing the use of toxic chemicals and by further developing methods to individually separate the REEs, as well as to valorise the resulting Fe/B oxides. The scale-up was achieved by the Institute of Energy Critical Materials in the U.S. with aqueous Cu(II) salts instead of mineral acids to dissolve REEs.

Each of the previously described systems is proof that clever use of the principles of electrochemistry and inorganic chemistry leads to element separation and recovery processes from electronic metal waste, which build the foundation of a circular metals industry by reducing the environmental costs associated

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27 to it and by securing secondary sources that can cover the demand for specialty elements necessary in high-end technologies.

III. Nitrogen and phosphorus

The world population increased by 5 billion people in the past 70 years and is expected to reach 9.7 billion by 205081_{. The growth rate picked up during the industrial revolution and spiked in the 1970s when} mortality drastically declined thanks to medical advancements. The population growth came with many side-effects, the most relevant for this chapter being that food demand also rose steeply and continues to do so. In order to cover the world’s needs, the agricultural sector saw two possibilities: to expand the cultivated areas and to accelerate crop cultivation. While the first option clearly had limits, the second one seemed to be the ideal choice and brought in the spotlight the use of fertilisers.

Nitrogen (N), phosphorus (P) and potassium (K) are the main macronutrients provided through fertiliser application to plants in order to speed up their growth82_{. Inorganic (i.e. synthetic) fertilisers are primarily} used because they hold higher nutrient concentrations than organic fertilisers (e.g. animal manure) and are readily available for the farmers to use. Although the latter improve soil fertility and have little negative impact, biomass containing nutrients generally needs to undergo purification before being applied as fertiliser.

Chemicals are quickly lost to the biosphere from inorganic fertilisers: nitrogen oxides (N2O, NOx), ammonia/ammonium (NH3/NH4+), phosphate (PO43-). The short-term result is that regular fertiliser application is needed to provide enough nutrients. In the long-term, these molecules alter the natural cycles of N and P, having serious environmental consequences (eutrophication, soil deterioration, GHG emissions). Furthermore, the mineral nutrients contained by inorganic fertilisers are obtained in the linear economy either by extraction (phosphate rock mining for P-fertilisers) or by chemical industrial processes (Haber-Bosch process for N-fertilisers) which are responsible for fossil energy depletion. The anthropogenic inputs that result from fertiliser applications continuously perturb the natural N and P element cycles, pushing the biogeochemical flows towards the high-risk zone and interfering with the core planetary boundaries i.e. climate change and biosphere integrity5_.

The many negative impacts related to the linear fertiliser industry have been long-time overlooked because of convenience, little environmental awareness and lack of suitable alternatives. Closing the loop on nutrients is an essential step in achieving circular economy and will strip the fertilisers industry of its troubles. To this end, primary (fossil-based) sources for fertiliser production will be replaced by secondary (bio-based or waste-based) sources assured by nutrient recovery processes from waste valorisation. Since the consequences mentioned above are mostly due to linear production, the overuse and the waste created by N- and P-containing fertilisers, these elements will be the only ones discussed further in the context of a circular approach for nutrients.

1. When nutrients become pollutants

As a result of human activities involving large quantities of nutrients, it is estimated that since the discovery of synthetic fertilisers, the amount of nitrogen in circulation has doubled and that of phosphorus

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28 has tripled83_{. Because of the fertiliser waste remaining unused in the soil and then reaching the aquatic} ecosystems, a more aggressive plant and algal growth is observed, phenomenon known as

eutrophication. Although, this process occurs naturally over centuries, it has become a very expensive

problem; it brings annual costs of $2.2 billion in U.S. alone84_{. Other consequences include hypoxia and} drinking water contamination. The EU Nitrates Directive was adopted in 1991 to protect waters from nitrates pollution originating in agricultural activities across Europe85_{. Although its implementation has} educated in sustainable nutrient management and resulted in lower concentrations of nitrates in surface and groundwater, worrying hotspots remain and eutrophication is still an important environmental consequence of improper fertiliser use. Along the same lines, the European Sustainable Phosphorus Platform monitors the phosphorus challenge and its opportunities86_.

Due to fertiliser application, the N and P levels go beyond the environmental limits and so the planetary

boundary of biogeochemical flows is now in a high-risk zone (fig. 1b). Although the N and P utilization is

intensive in only a few agricultural regions (western Europe, China, North America), it is high enough to take this boundary into the critical zone. The effects further spread to the two core boundaries - climate change (N2O as a GHG, NH3/NH4+ and NOx resulting in aerosol formation and water contamination) and biosphere integrity (reduced biodiversity due to eutrophication, altered N and P levels and ratio)5_. Anthropogenic input of nitrogen is mainly caused by N-fixation via the Haber-Bosch process, while for P it results from phosphate rock mining. Both industrial activities rely on the depletion of non-renewable resources (fossil fuels for hydrogen gas production, phosphate rock) and have high environmental costs (GHG emissions, water contamination), which will be discussed more thoroughly in the next parts of this chapter. Nitrogen and phosphorus are constantly lost to the environment through food waste, wastewater, sewage sludge, manure etc. It is estimated that only 30% of fertiliser P obtained from phosphate rock and only 14% of fertiliser N reach the plants and the consumer87_{. The rest is lost to water} and to the air. Therefore, linear practices are unideal and the nutrient cycles needs to be optimised.

These surroundings force humanity to turn to more sustainable approaches and this is where circular economy comes into play. Nitrogen and phosphorus need to be recovered in order to put an end to them becoming pollutants. The goal is to create a nutrient loop by turning agricultural waste and by-products into fertilisers, while reducing the production and use of the conventional materials. Nitrogen and phosphorus recovery technologies, as well as waste-to-fertiliser plants, are steadily emerging, being key to improving the current nutrient management. They rely on specific waste sources for starting material and make use of different processing techniques, which will be discussed in the following sections.

2. Closing the nitrogen loop

People had noticed that crops grow better when using manure long before the discovery of nitrogen in 1772 by Daniel Rutherford88_{. Moving into the age of N-knowledge, some key conclusions were drawn:} nitrogen is one of the main nutrients needed by plants, it is naturally fixed by microorganisms, it can be industrially fixed through the Haber-Bosch process from atmospheric dinitrogen gas (N2) and hydrogen gas (H2) obtained from fossil fuels, and enough is enough when it comes to anthropogenic inputs.

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29 a) The nitrogen cycle

Although N makes up 78% of Earth’s atmosphere, it is mainly found in nature as the relatively inert dinitrogen gas and cannot be directly assimilated by plants, animals or humans. In the biogeochemical cycle of nitrogen89_{, fixation of atmospheric N}

2 is carried out by nitrogen fixing bacteria through nitrogenases that bring it into the soil as ammonia (NH3). Nitrifying bacteria then oxidise ammonia to nitrites and nitrates (NO2-, NO3-) – N species that can be easily assimilated by plants. Nitrogen is a key element for amino acids, which are, in turn, essential building blocks for cell development (DNA, RNA, proteins etc). Organic N enters the food chains through plants and is eventually consumed by humans according to their dietary choices. N is lost to the environment when an organism produces waste or dies and can be brought back in the soil by decomposers. Then, denitrifying bacteria are responsible of turning NO2- and NO3- back to atmospheric N2 through the activity of nitrate reductases. Intermediates of this process are the greenhouse gases nitric oxide (NO) and nitrous oxide (N2O).

b) The Haber-Bosch process

In 1908, Fritz Haber patented the synthetic fixation of atmospheric N2 gas and 5 years later the industrial synthesis of ammonia was known as the Haber-Bosch process88_{. This discovery marked the beginning of} the synthetic N-fertilisers production and was awarded the Nobel Prize in Chemistry in 1918. The Haber-Bosch synthesis of ammonia is an energy-intensive process having as starting materials N2 gas from air and H2 gas, which is most of the times obtained by steam reforming of natural gas. The ammonia thus obtained is feedstock for the production of mineral N-fertilisers, but also explosives. Urea (CO(NH₂)₂) has the highest N-content among all fertilisers (46%) and is the most used fertiliser around the globe, being synthesised from Haber-Bosch ammonia (NH3) and carbon dioxide (CO2). Other common N-fertilisers are ammonium nitrate, urea ammonium nitrate, calcium ammonium nitrate, calcium nitrate90_{. In the context} of the huge industrial advancement brought by the Haber-Bosch process more than a century ago, manure and other organic fertilisers started to be perceived as agricultural waste.

c) Why is there a nitrogen challenge?

Nitrogen clearly has an important role in the fertiliser industry, maintaining the crop production rates in line with humanity’s needs. The natural nitrogen cycle assures the permanence of this resource. However, great consequences come with the nitrogen excess that has been perturbing it since the discovery of the synthetic fertilisers. 60% of the nitrogen from fertiliser applications escapes the denitrification process which would convert it back to unreactive atmospheric N291. Instead, it infiltrates the soil and the water system as anthropogenic reactive nitrogen species (Nr), altering the natural element ratios and becoming a pollutant. The umbrella term Nr includes inorganic and organic forms of N that are reactive as compared with N2 gas92. Once these species are lost to the environment, disturbances are starting to show at the level of soil (acidification), air (increase of GHG emissions, smog, stratospheric ozone depletion, accelerated climate change), water (eutrophication, hypoxia, water impairment, biodiversity loss), human health (respiratory diseases, cancer)93_{. The circulation of Nr in nature and its effects is referred to as the} N cascade by Galloway et al.92_.

The nitrogen challenge we are facing is based on a series of inequalities. Firstly, the production of ammonia through the Haber-Bosch process is dependent on fossil fuels and is responsible for high levels

A Review of Chemical Examples in the Context of Circular Economy

MSc Chemistry

Track Molecular Sciences

Literature Thesis

A Review of Chemical Examples in the Context of Circular Economy

by

Narcisa-Lidia Girigan

ID 12294764

July 2020

12 ECTS

Supervisor:

Second Reviewer:

Dr. Chris Slootweg

Dr. Ștefania Grecea

Table of Contents

Abstract

I.

Introduction

II. Scarce metals and specialty elements

III. Nitrogen and phosphorus