Using waste as resource to realize a circular economy: Circular use of C, N and P

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Introduction
The year 2019 was the International Year of the Periodic Table (#IYPT2019), where chemists celebrated the 150th birthday of the Periodic Table of Chemical Elements in the format proposed by Mendeleev in 1869 [1e7]. The European Chemical Society (EuChemS) created for the occasion a special representation of the periodic table that highlights element scarcity ( Figure 1) [8]. EuChemS hereby emphasizes that 'we have to use our precious resources with much more care in the years to come, and we need to carefully look at our tendencies to waste and improperly recycle such items. Unless solutions are provided, we risk see many of the natural elements that make up the world around us run out d whether because of limited supplies, their location in conflict areas, or our incapacity to fully recycle them' [9*]. This means that from now on, we have to fully grasp the issue of element scarcity and take action to realize element circulation. Conservation of our elementary building blocks can only be carried out by recovery and recycling them after their use [10e13]. The development of chemistry that enables the circular use of our elements, molecules and materials is therefore key [14**,15**], next to preventing chemicals from entering the environment [16,17], and thus avoiding them to cause pollution [18]. Safe and circular by design of molecules and materials for a sustainable future is thus of utmost importance [19*,20]. Inducing such change from the current linear 'take-make-dispose' model to a more circular one requires a holistic approach [21**e23] to design a new system of using and reusing our precious elements.

Waste as resource
There is obviously plenty of waste available for use as resource materials but where to start? The most stringent waste problems are addressed by the nine planetary boundaries of Steffen, Rockström et al. [25**,26], which are: climate change, loss of biodiversity, ozone depletion, ocean acidification, biogeochemical flows (the flow of nitrogen and phosphorus), land-system change (deforestation), fresh water use, atmospheric aerosol loading and chemical pollution ( Figure 2). Shockingly, society's activities have pushed four of these sustainability targets beyond the boundaries into unprecedented territory, namely: extinction rate (one of two indicators for biosphere integrity), atmospheric carbon dioxide (an indicator for climate change), and the biogeochemical flow of nitrogen and phosphorus, of which the latter three can be solely ascribed to the chemistry of three elements: Carbon, nitrogen and phosphorus. Urgent action therefore needs to be taken to return to safe operating space in these processes.
The focus of this short review is, therefore, on these three elements (C, N, and P) that are key players in the suite of major biogenic elements, often termed 'CHONPS', needed in large quantities to make living organisms, as well as contribute heavily to three of the most stringent environmental concerns ( Figure 2). Interestingly, the EuChemS periodic table (Figure 1) highlights that element scarcity is far from alarming for C, N, and perhaps to a lesser extent for P [27]. The planetary boundaries concept teaches us clearly that next to advancing resource management, sustainable chemistry is also urgently required to tackle environmental waste issues. So what are the waste products that enter the environment? For carbon, this mainly concerns the greenhouse gases carbon dioxide and methane that are expelled to the atmosphere. The nitrogen waste issue is caused by nitrogen oxides (N 2 O, NO x ) and predominately ammonia (NH 3 ) that are discharged into the aquatic environment and/or atmosphere. For phosphorus, it concerns phosphate, which is next to ammonia essential for plant growth, yet this building block of life also ends up in aquatic systems causing eutrophication [28].

Carbon
The negative impact of carbon dioxide on climate change [29] can be reduced by decreasing carbon emissions caused by the burning of gas, oil, coal and biomass for energy production and advancing cement production [30,31], but also by removing CO 2 from the atmosphere by carbon capture [32,33] or even direct air capture [34]. Subsequently, CO 2 is available for recycling, which is established for the synthesis of high-value speciality chemicals [35e37], yet only a few industrial processes use CO 2 as resource [38]. The challenge, though, is to develop novel methods for CO 2 recycling that aim to compete with petrochemistry for the synthesis of bulk chemicals [39, 40*]. In this respect, converting CO 2 into CO is considered as an attractive approach because CO can be readily utilized as a feedstock for value-added chemicals and fuels through the existing downstream thermochemical reactions [41,42]. In addition, methane also contributes significantly to climate change, as it is a roughly 30 times more potent greenhouse gas than CO 2 . Therefore, the use of methane as resource for the production of valueadded products is also of great interest [43*,44]. Particularly appealing is methane dry reforming with CO 2 , thus using both greenhouse gases, to produce In this representation of the periodic table prepared by the European Chemical Society (EuChemS) for the International year of the periodic table, naturally occurring elements (except from some of the rarest ones beyond uranium) are depicted through tiles the size of which gives an indicationusing a logarithmic scaleof how much of the element is present in the Earth's crust and atmosphere. The areas are approximate for all elements, and exaggerated in the case of the least abundant ones shown here (technetium, promethium, polonium, astatine, radon, francium, radium, actinium and protactinium) so that they are noticeable. This version of the table is drawn with no gap between beryllium and magnesium in the s block, as well as boron and aluminium in the p block. The colour code highlights how quickly elemental supplies are being used up. Technetium and promethium, shown here in white and marked as synthetic elements, do also occur naturally on Earth although in very small amounts. Some transuranium elements that are not shown here (such as neptunium, plutonium, curium) also occur naturally in minute amounts [24]. This illustration also draws attention to those elements that are risk of being depleted unless recycling routes are developed, and those that come from countries in which wars are fought over the ownership of the relevant mineral rights. It mentions 31 elements (though other sources list other numbers, up to around 70) that are used in smart phones, which are typically replaced more rapidly than necessary. Credit: © EuChemS, reproduced from https://www.euchems.eu/euchemsperiodic-

Nitrogen
Managing the nitrogen issue to sustain food production and the environment [21**,48*] is effective by advancing ammonia synthesis [49e54] and to reduce nitrogen discharge from livestock, domestic and industrial sources [55] and to restore water quality [56]. Current efforts to reduce pollution through wastewater treatment [57,58] and by improving cropland nitrogen management can remedy this situation. Interestingly, anaerobic ammonium oxidizing (anammox) bacteria own a central position in the global N-cycle, as they have the ability to oxidize ammonium (NH 4 þ ) to N 2 under anoxic conditions using nitrite (NO 2 À ) [59]. Next to being indispensable in marine ecosystems [60], the anammox process is also a sustainable way of removing ammonium from effluents and ammonia from waste gas. Because ammonia is a key fertilizer component, recovery and recycling, instead of decomposition into N 2 , can greatly improve the sustainable use of nitrogen as it alleviates the environmental burden of ammonia as well as creates locally new (renewable) Nresources that can be reused efficiently [61,62].  ]. Next to reducing the environmental impact of eutrophication, this will also provide a local source of renewable phosphates that reduces dependencies on import from elsewhere. To realize a circular phosphorus economy, the biggest challenge is to steer the development and implementation of phosphorus recovery and recycling techniques in such a way that the recovered phosphate waste is always suited for use as resource enabling its recycling into marketable products. Struvite (MgNH 4 PO 4 $6H 2 O) is an interesting candidate in this respect, as it can be produced in good purity and it recovers both phosphorus and nitrogen from the environment [28, 69,70]. Recently, also other promising means of capturing ammonium and phosphate ions together were developed [71,72], which bodes well for the future. These emerging technologies showcase that reuse of water pollutants by extracting carbon, nitrogen and phosphorus from wastewater is feasible and, at the same time, generates renewable resources and saves energy [73*], underlining the potential of improved nutrient recovery and recycling [74].

Conclusions and outlook
It is clear that chemistry needs to adjust its focus on prime resources and also incorporate waste as valuable starting material. By optimizing the use and reuse of our all precious elements, so not just carbon, nitrogen and phosphorus, by applying chemistry as enabling tool we can realize a circular economy. This requires circular thinking and systems thinking in the education of current and future leaders [22,75]. All in all, sustainable chemistry is key in the development of a sustainable future and is of immense importance to realize the United Nations Sustainable Development Goals [76*e ,79].

Conflict of interest statement
J.C.S. is the shareholder and serves as the scientific advisor of SusPhos BV.  [190][191][192][193][194][195]. This commentary coins the term Circular Chemistry and introduces its twelve principles. The concept of circular chemistry aims to replace today's linear 'take-make-dispose' approach with circular processes, which will optimize resource efficiency across chemical value chains and enable a closed-loop, waste-free chemical industry.