Exploring the Phosphorus Biogeochemical Cycle with Analytical Chemistry

Topics phosphorus biogeochemical cycle, eutrophication, HPLC, mass spectrometry

Phosphorus (P) is an essential element for life. It is a key component of the phospholipid membranes that hold our cells together, and it forms the phosphate-sugar backbone of deoxyribose nucleic acid (DNA) and in the energy currency adenosine triphosphate (ATP). It cycles through the biosphere (living things) and the geosphere (non-living things like soil, water, rock) in the phosphorus biogeochemical cycle (see Figure 1). P is mined from phosphate containing minerals for chemical phosphate fertiliser, providing increased agricultural yields. However, the replacement of that rock from geological processes takes many thousands or millions of years, meaning phosphate rock is essentially a non-renewable resource in our lifetimes.

Figure 1: The phosphorus biogeochemical cycle. Blue arrows indicate transportation processes and green arrows indicate transformation processes.

While addition of P to farmland is important for crop productivity, however, leaching of P to watercourses, such as rivers and lakes, causes excess algal growth (eutrophication) which can lead to the killing of fish through oxygen depletion when the algae die and decay. Sources of polluting P in the environment are agricultural water run-off from fertiliser and animal waste, and wastewater effluent discharge. P in watercourses eventually makes its way to the ocean and cannot be economically recovered. Between mining, agricultural use, and discharge of P to freshwater and the ocean, anthropogenic (human) influence on the P biogeochemical cycle has been excessive and we have now breached the threshold for the “safe operating space for humanity” as defined by Johann Rockstrom et al., in their 2009 paper (see reference) i.e. the identification and quantification of planetary boundaries that must not be crossed, helping to prevent anthropogenic activities from causing unacceptable environmental change.

Figure 2: Compound classes and examples of P compounds.

P in the environment is found in many thousands of different compounds – both organic and inorganic. We know more about the inorganic salt forms of P (for example the apatite minerals found in clay soils) than we do about the organic forms produced by living organisms. Examples of P compounds from each of the P class are given in Figure 2.

The determination of molecular forms of P in the environment has been challenging because they occur in very low concentrations relative to all the other compounds and materials that make up soil, wastewater effluent, animal manures, and water. Current research into the molecular character of P in the environment has advanced due to recent developments in the sensitivity of analytical technology. We use high performance liquid chromatography (HPLC) to separate P compounds from their matrix (i.e. the soil, manure, effluent environment) and quantify them. High-resolution mass spectrometry (HRMS) can accurately detect low concentration analytes in complex mixtures and enable identification of individual compounds based on of their mass-to-charge ratio (m/z). Using tandem mass spectrometry (MS/MS) experiments, compounds are bombarded by high energy gases causing them to fragment. P compounds can be identified in the fragmentation mass spectrum by the presence of the characteristic PO3− ion at m/z 78.9585.

Using these analytical methods, we can determine which P compounds occur in environmental matrices, how much of the compounds are present, and how they behave. Answering these questions about P compounds allows us to:

· identify how they cause pollution,

· devise better ways to retain P in agricultural soils,

· develop new technology to recover P from wastewater streams,

and, therefore, manage anthropogenic (human) influence on the P biogeochemical cycle.

Reference

Rockström, J.; Steffen, W.; Noone, K.; Persson, Å.; Chapin, F. S.; Lambin, E. F.; Lenton, T. M.; Scheffer, M.; Folke, C.; Schellnhuber, H. J.; et al. A Safe Operating Space for Humanity. Nature 2009, 461 (7263), 472–475.

Dr Catherine McIntyre is currently a Research and Development Analytical Scientist at the global taste and nutrition company, Kerry Group, Ireland. Catherine has recently completed her PhD at the Organic Geochemistry Unit in the University of Bristol, where she has been developing a novel method for the characterisation of organic phosphorus compounds in environmental matrices using ion chromatography and high-resolution mass spectrometry. She gained her first degree in chemistry at the National University of Ireland, Galway.

Exploring the Phosphorus Biogeochemical Cycle with Analytical Chemistry

Questions

1. What is the molecular mass of the phosphate ion? [1 mark]

2. What does ATP stand for? [1 mark]

3. What are ‘analytes’? [1 mark]

4. How does a phosphate mono ester resemble a carbon ester such as ethyl ethanoate? [1 mark]

5. Explain the eutrophication process. [3 marks]

6. According to Wikipedia ‘calcium pyrophosphate dihydrate crystal deposition disease, also known as pseudogout and pyrophosphate arthropathy is a rheumatologic disease which is thought to be secondary to abnormal accumulation of calcium pyrophosphate dihydrate crystals within joint soft tissues.’ What is the formula of calcium pyrophosphate dihydrate. [2 marks]

Extension work

Explain how high performance liquid chromatography (HPLC) works. [4 marks]

Exploring the Phosphorus Biogeochemical Cycle with Analytical Chemistry

Questions

1. What is the molecular mass of the phosphate ion? [1 mark]

2. What does ATP stand for? [1 mark]

3. What are ‘analytes’? [1 mark]

4. How does a phosphate mono ester resemble a carbon ester such as ethyl ethanoate? [1 mark]

5. Explain the eutrophication process. [3 marks]

6. According to Wikipedia ‘calcium pyrophosphate dihydrate crystal deposition disease, also known as pseudogout and pyrophosphate arthropathy is a rheumatologic disease which is thought to be secondary to abnormal accumulation of calcium pyrophosphate dihydrate crystals within joint soft tissues.’ What is the formula of calcium pyrophosphate dihydrate. [2 marks]

Extension work

Explain how high performance liquid chromatography (HPLC) works. [4 marks]