OVERVIEW

In the 20th century, the invention of the Haber-Bosch process for converting atmospheric nitrogen into ammonia, together with the Green Revolution that dramatically improved agricultural yields, eliminated natural limits on bioavailable nitrogen and enabled an expansion of world population from 1.6 billion to 7 billion. Globally, the amounts of chemically and biologically fixed nitrogen in our food supply are now comparable, meaning that chemically-fixed nitrogen feeds about half of the world’s population. However, when one limiting element becomes abundant, another becomes limiting: that element is now phosphorus. It is extracted from phosphate rock (phosphorite), a sedimentary deposit. The world’s largest producers, China and the US, are expected deplete their reserves in the next 50 years, assuming continued extraction at current rates (we note that this is an optimistic scenario, and does not account for further population expansion, increased dietary protein, or the demand for phosphate-based fertilizers from the emerging biofuels industry). The most productive of the US mines, located in Florida, will be depleted within 20 years, and the US recently became a phosphate importer. China now imposes seasonal tariffs exceeding 100 % on exported phosphate, in order to conserve its resources for domestic use. Globally, phosphate appears to be more geographically concentrated than crude oil; if industry estimates are accurate, a single country (Morocco) controls a higher fraction (70 %) of global reserves than the crude oil in all 12 member states of OPEC combined. This represents an important supply interruption risk, and a potential threat to global food security. We urgently need to develop sustainable chemistry to enable the recovery and reuse of phosphorus. Human consumption and excretion of phosphorus amounts to 3 million tons annually, turning our cities into valuable sources of this essential and completely non-substitutable element in human nutrition, via recovery during wastewater treatment. New phosphate recovery technologies, possibly including adsorption, ion exchange, and nano-filtration, followed by precipitation or crystallization, are urgently needed.