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]