PO43 1

1
The P Biogeochemical Cycle
units - 1012 g or 1012 g/yr
5 oxidation state. Essentially no atmos.
component (land dust and sea spray). Oceans
huge reservoir (mainly dissolved) due to large
volume of oceans (1370 x 106 km3) Essential
nutrient PO43-(oceans) 8x1016/31/1370x1018M
2?M (cf lt 0.1?M for surface tropical
waters)
P in marine waters
PO43- - 1
2
Sediment huge reservoir Ca5(PO4)X (X OH, F)
hydroxy- fluoro-apatite Mineable reserves
0.002 of sediment. Mining flux 10x the
natural flux to oceans (use dissolved flux as
natural flux ??) Anthropogenic flux from land
5x natural weathering. Mining flux 3x
anthropogenic flux ?? Particulate load to
oceans gtgt dissolved load adsorption to
particles.
Residence Times i) TR (atm. over land)
0.025/4.2 years 2 days ii) TR (atm.
Over sea) 0.003/1.3 years 1
day iii) Biota P (2600 land 100 sea) 90 on
land. iv) TR (land) 13 years. v) TR (sea)
1 month.
3
The P Biogeochemical Cycle
The Biological Functions
60 bones, 70 teeth (enamel 100 hydroxy-
apatite - F- strengthens teeth). 3 - 5 kg
apatite in adults. Urinate 3 to 4 gP/day - P
first isolated (H. Brandt 1669) from urine
(Greenwood and Earnshaw)
ATP
essential to biochemical processes - ATP, DNA,
RNA. ATP 2H2O ? ADP HPO42- H3O ?Go
-41 kJ/mol K 1.3 x 107. ATP 2H2O ? AMP
HP2O73- H3O ?Go -44 kJ/mol K 4.2 x
107. Energy releasing reactions (Greenwood and
Earnshaw, Chemistry of the Elements p 611)
Nucleic acids phosphate diester polymers with
phosphate-sugar backbone and attached bases.
The Sources and Uses of P
3. 95
1. 90
H3PO4.
Fertilizers
Ca5(PO4)3(F,OH)
5
2. 10
Food (sequesting agent, sour taste in soft
drinks, acid source in baking powder),
Detergents, Water softening.
80
80
P
H3PO4. pure
20
20
Other chemicals
Metal Processing
PO43- - 2
4
The P Biogeochemical Cycle
The Sources and Uses of P cont. 1. Ca5(PO4)3(OH)
5H2SO4 ? 5CaSO4 3H3PO4 H2O 2.
4Ca5(PO4)3(OH) 20SiO2 30C ? 20CaSiO3 3P4
2H2O. 1500C - expensive 3. 2Ca5(PO4)3(OH/F)
7H2SO4 ? (3Ca(H2PO4)2 7CaSO4) 2H2O/2HF -
superphosphate 2Ca5(PO4)3(OH/F) 14H3PO4 ?
10Ca(H2PO4)2 2H2O/2HF - triple-superphosphate
The use of phosphates in detergents utilizes
their complexing (sequestering) properties to tie
up metals ions and hence soften the water. Used
when water will not be frequently released to the
environment - boiler waters. Na5P3O10 -
polyphosphate chains or Na-hexametaphosphate - a
mixture of cyclic polymers Calgon - not
structurally defined.
PO43- when present as the limiting reagent (ie
there is plenty of C and N) for photosynthesis
50 will taken up by phytoplankton within 1
minute, 80 within 3 minutes.
Reactions in the Environment Limited to the
aquatic and soil spheres. Concentrations in the
aquifers controlled by precipitation reactions.
H3PO4 is a tribasic acid pK1 2.15 pK2 7.2 pK3
12.37
PO43- - 3
5
The P Biogeochemical Cycle
Reactions in the Environment cont. 1. Under
basic conditions Ca5(PO4)3(OH) ? 5Ca2 3PO43-
OH-. Ksp 10-57 Ca2 5PO43- 3OH- At
pH 7 Phosphate present as 50/50 H2PO4-/HPO42-
- See speciation diagram HPO42- H2O ? PO43-
H3O. K3 PO43-H3O/HPO42-
PO43- K3 HPO42- /H3O ? Ksp 10-57
Ca2 5(K3 HPO42- /H3O)3OH- using
Ca2 as 1mM (typical of fresh waters) gives
Ksp 10-57 (10-3)5 x (10-12.37)3 x
(10-7)/(10-7)3 x HPO42- 3 and
HPO42- 10-57 x 10-14/10-37 x 10-151/3
4.6 x 10-7 mol dm-3 Given the 50/50 mix of
H2PO4-and HPO42-. Then PT 2 x 4.6 x 10-7 mol
dm-3 ? 1?M At pH 8 only the OH- and H3O
change in the above expressions and the PT will
drop by about an order of magnitude to 0.1
?M. Thus the concentrations of phosphate in soil
solutions and in aquifers will be very low if
calcium ions are present (I.e. normally).
Flouroapatite is even less soluble than apatite
and then phosphate concentrations drop to
micromolar levels at pHs as low as 6 - 7. 2.
Under acidic conditions The sparingly soluble
variscite, AlPO4.2H2O, and/or strengite,
FePO4.2H2O, control the aqueous
concentrations. Consider variscite AlPO4.2H2O ?
Al3 PO43- 2H2O Ksp 10-21
Al3PO43- The Al in solution will be
controlled by the an oxyhydroxide of Al (assume
gibbsite) 3H3O Al(OH)3 ? Al3 6H2O
logK 8.5 K Al3/H3O3 108.5 and at
pH 5 Al3 10-6.5. ?PO43-
10-21/10-6.5 10-14.5. But at pH 5 the
dominating P species will be H2PO4-. And
therefore we need to calculate the HPO42- from
the PO43-.
PO43- - 4
6
The P Biogeochemical Cycle
Reactions in the Environment cont. H2PO4- H2O ?
HPO42- H3O. K2 H3OHPO42-/H2PO4-
10-7.2. HPO42- H2O ? PO43- H3O. K3
H3OPO43-/HPO42- 10-12.37. ? HPO42-
H3OPO43-/ K3 10-5 x 10-14.5/10-12.37
10-7.13 M. H2PO4- H3OHPO42-/ K2 10-5
x 10-7.13/10-7.2 10-4.97 M. At lower pHs the
Al3 increases decreasing further the PO43-
and therefore the other P species. At pH 4
H2PO4- is still the dominant species. Al
10-3.5 PO43- 10-17.5 HPO42- 10-9.13
H2PO4- 10-5.97. A similar calculation can be
done for strengite. The total phosphate
concentrations in fresh waters and pore waters ?M
or less outside of the pH 5 - 7 range (see
speciation diagram). Analytical Methods for P. a)
in soils present as available P (see lab for
extraction method) and total P (HF/HNO3 or S2O82-
digestion to convert P to orthophosphate). Thus
analyse for orthophosphate after extractions in
both cases. b) in water present as
orthophosphates, polyphosphates
(hexametaphosphates) organic P. filter at 0.45
?m (dissolved vs suspended) and then i) for the
orthophosphate adjust to pH 7 and analyse. ii)
for hydrolysable phosphates (polyphosphates) add
0.1M H2SO4 and boil to convert all P to
orthophosphate, adjust to pH 7 and analyse for
orthophosphate. iii) for total phosphorus boil
or autoclave with S2O82- in 0.2M H2SO4, adjust to
pH 7 and analyse for orthophosphate. Sampling
and Storage (consult the standard method
recommendations) avoid contamination (clean
bottles with 0.1M HCl, rinse thoroughly with
distilled water water to be sampled). analyse
within a few hours if speciation is to be
determined. Up to 20 can adsorb to glass walls
within 6 hours (use plastic bottles). if
storage is necessary add 2mL conc.H2SO4/L but
will lose speciation.
PO43- - 5
7
Analytical Methods for P cont.
The analytical reactions H2PO42- 14H2O
12HMoO3 ? PMo12O403- 14H3O. pH 7.2, ?
420nm The actual phosphomolybdate formed depends
critically on the conditions PMo11O397- ?
PMo12O403- ? P2Mo18O626- altering the pH alters
the species analytical procedures must be
rigorously followed.
- O2- 4 octahedra per
tetrahdron O4s falls on the centres
of the tetrahedral faces Mo(VI) in (12/40) Oh
holes
4a
1a
4a
1a
1
1
1
1
4a
4a
2
2
4
4
2
2
4a
3
3
3
3
4a
P in the Td hole defined by the tetrahedron to
the left. O4 are P nearest neighbors.
By adding a reducing agent (ascorbic acid or
stannous chloride) some of the Mo(VI) are reduced
to Mo(V) and the charge transfer complex that
forms has an intense blue colour. For analysing
for low concentrations of PO43- use the Mo-blue
method ? 880nm. Any other species that can
fit into the Td hole can interfere with the
colour development. - Si(IV) at ? 430 (or
800-820) nm - As(V) ? 440 (or 840) nm Not only
do these ions fit into the holes but the produced
polymolybdates have similar colours - must
establish that ions that can interfere are absent
or present in only low concentrations.
PO43- - 6