Contributed by Paul Withers, Professor of Geography, Bangor University
Society needs an external source of phosphorus (P) for food, bioenergy and industrial production, but this essential resource is finite, increasingly costly and running out. There is no substitute for P other than the rock P (RP) that is mined out of the ground, and current RP consumption rates are very high (over 20 million tonnes per year driven largely by demand for fertilizers), and forecast to increase. To make matters worse the major stocks of RP are situated in only a few countries raising fears that supplies of affordable fertilizers will become regulated by political forces. Current inefficiencies in the way we use P in society are also leading to widespread P losses to water leading to the eutrophication (algal blooms and hypoxia) of our rivers, lakes, reservoirs and coastal waters. If society does not start to use this rock P more sustainably, its future food and water security in particular will be threatened.
The solution to this issue is in principle quite simple: to develop a circular economy for P based on the green chemistry principles of (a) the development of benign products and processes, (b) the elimination of waste, (c) the use of renewable (secondary) resources, and (d) the design of output-driven production systems with minimum requirements and maximum efficiency. In essence this means we have to use less, recover and recycle more, cut losses and redesign production systems to be more sustainable. Of course these goals are much more difficult to achieve in practice due to technological and economic constraints, but the task is not insurmountable (http://pubs.rsc.org/en/content/articlepdf/2015/gc/c4gc02445a).
The sectors of the P cycle where progress can be achieved are highlighted by the bubbles in Figure 1. This P cycle is inherently inefficient because the vast majority of the P that is mined each year becomes stored in the soil, or is dissipated in low-grade RP ores, manures, by-products, and residues that are not fully re-used or re-used uniformly. Losses and wastage occur in all parts of the cycle (dotted lines in the diagram) and once this P finds its way to the oceans it is impossible to recover. A large proportion of this dissipated P could be re-used and help close the P cycle. Particular green chemistry and/or green engineering challenges are to:
- Recover harmful metals and rare earths from lower grade RP ores to make P products and residues safer to use and re-use and for added value: e.g. co-extraction technologies
- Partially remove P from livestock manures to lower their N:P ratio for more efficient recycling to land and to re-use the recovered P as fertilizer or feed: e.g through biorefinery or membrane filter technologies
- Transform wet bulky manures into transportable products that are directly re-useable or can be further treated to recover P more cost-effectively: e.g. super-critical combustion
- Recover P (and other essential elements or compounds) from the vast range of food, human and industry waste and wastewater streams for re-use; e.g. mining residues, slaughterhouse waste, sewage, food processing waste, steel slags
- Develop novel bioengineering techniques to enable greater soil P acquisition in situ: e.g. plant and microbial engineering
- Improve product formulation design to improve their P utilization efficiency: e.g. refine livestock feeds to make them more digestible, develop novel biofertilizers
- Reduce system P losses through greater P capture, recovery and re-use: e.g.
- Re-use renewable secondary P sources rather than primary RP for production systems
Phosphorus is an element that can be continually re-used, and is thus a prime example of a critical resource that could be utilized more efficiently in a circular economy to support sustainable growth with less pollution. Green chemists and engineers have an essential role in helping to deliver this vision.
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