Phosphates and Global Sustainability

Asignificant effort has been made by the scientific community to evaluate the potential of phosphate minerals and glasses as nuclear waste storage hosts. Radioactive waste–bearing phosphates, including monazites, apatites, and glasses, can be readily synthesized in the laboratory. Because of their low solubilities and slow dissolution rates, these phosphates are more resistant to corrosion by geological fluids than many other potential nuclear waste storage hosts, including borosilicate glass. Phosphates are, however, not currently being used for nuclear waste storage, in part because their synthesis at the industrial scale is relatively labor intensive, often requiring the separation of the waste into distinct fractions of elements. Such limitations may be overcome by adding phosphate amendments to backfill material, which could provoke the precipitation of stable radiactive waste–bearing phosphate minerals in situ.

Phosphorus is a key pollutant in municipal wastewater. To minimise eutrophication, treatment facilities must often reduce phosphorus levels to less than 1 mg L-1. Two main approaches to achieving this are chemical precipitation and enhanced biological uptake. Chemical precipitation is widely used and relatively simple; biological phosphorus removal is more complex but relies less on the addition of chemicals and also offers the opportunity to reuse the phosphorus. Phosphorus can be released from cells and converted to calcium phosphate or the mineral struvite. While the products have been shown to be excellent fertilisers, the economic drivers for recovery are still not clear.

The availability of phosphorus in soils is controlled by the ability of plants to dissolve phosphate-bearing minerals, including apatite and feldspars. To satisfy the requirement of plants for phosphate, mineral dissolution competes with precipitation such as, for example, reactions involving lead or other heavy metals. Plants exude organic acid anions that very effectively enhance mineral dissolution but that may also liberate harmful solutes, such as aluminium. To make readily soluble chemical fertilisers, apatite in igneous and sedimentary rocks is mined and processed; in organic farming, phosphate-rich rocks are crushed and applied directly to the soil, relying on compounds produced by plant roots (exudates) to extract the phosphorus that plants need.

Through evolution, vertebrates have “chosen” the calcium phosphate mineral apatite to mineralize their teeth and bones. This article describes the key characteristics of apatite in biological mineralization and explores how the apatite structure allows biology to control mineral composition and functionality. Through the synthesis and testing of calcium phosphates for biomaterials applications, we have gained further understanding of how sensitive the chemical and physical properties of apatite are to its growth conditions.

The cycling of phosphorus, a biocritical element in short supply in nature, is an important Earth system process. Variations in the phosphorus cycle have occurred in the past. For example, the rapid uplift of the HimalayanTibet Plateau increased chemical weathering, which led to enhanced input of phosphorus to the oceans. This drove the late Miocene “biogenic bloom.” Additionally, phosphorus is redistributed on glacial timescales, resulting from the loss of the substantial continental margin sink for reactive P during glacial sea-level lowstands. The modern terrestrial phosphorus cycle is dominated by agriculture and human activity. The natural riverine load of phosphorus has doubled due to increased use of fertilizers, deforestation and soil loss, and sewage sources. This has led to eutrophication of lakes and coastal areas, and will continue to have an impact for several thousand years based on forward modeling of human activities.

Phosphorus is a unique element. It is the limiting nutrient controlling biological productivity in many terrestrial and marine environments. When in excess, however, dissolved phosphate leads to uncontrollable biological growth and water-quality problems through a process called eutrophication. The use of phosphate minerals and their products as fertilizers has increased tremendously global food production; it would not be possible to feed the current world population without phosphate fertilizers. Yet phosphate is a limited global resource; current estimates suggest economic phosphorus supply may be severely depleted over the next 100 years. Never – theless, mineralogists and geochemists have invested little time investigating phosphate mineral stability, reactivity, and transformations. This issue attempts to bring phosphates to the forefront of our scientific endeavours.