Dissociation of Orthophosphoric Acid

Transcription

1 { Phosphorus

2 General Essential for all living things Component of DNA, RNA, ADP, ATP, and bone Usually the most limiting nutrient in phytoplankton productivity Many forms Soluble inorganic, soluble organic, particulate organic, and particulate inorganic Occurs naturally in most soils Not toxic but can lead to eutrophication

3 Phosphorus in Plants Phytic acid, C 6 H 18 O 24 P 6, is the storage form in plant tissues Not readily digestible by animals unless the enzyme phytase is present (mainly microorganisms) Required in large amounts, but there s only low concentrations available Thus, high rates of plant growth (in aquatic ecosystems) require continuous input of phosphorus

4 P-Cycle Not well defined cycle P has several valence states, -3 to +5, with the most common of +5 in nature Reactions are mediated by chemotropic bacteria mainly by a chemical cycle, not biological.

5 Dissociation of Orthophosphoric Acid H 3 PO 4 = H + + H 2 PO 4 - K1= H 2 PO 4 - = H + + HPO 4 2- K2= HPO 4 2- = H + + PO 4 3- K3= Polyphosphates such as H 6 P 4 O 13, has a greater proportion of P than does orthophosphate: PO 4 is 25% P which P 4 O 13 is 30.8% P ph 6-9, H 2 PO 4 - and HPO 4 2- dominates in natural waters Example: Calculate the percentages of H 2 PO 4 - and HPO 4 2- at a ph of 6

6 Fig 12.2

7 Phosphorus-Sediment Reactions Inorganic P reacts with iron and aluminum in acidic sediments: AlPO 4 2H 2 O + 2H + = Al 3+ + H 2 PO H 2 O K= FePO 4 2H 2 O + 2H + = Fe 3+ + H 2 PO H 2 O K= Thus, decreasing ph favors solubility of Fe and Al phosphates Example: Estimate the solubility of P from variscite (AlPO 4 2H 2 O) for ph 5 and ph 6 Example: Estimate the solubility of P from strengite FePO 4 2H 2 O) for ph 5 and ph 6

8 More Dissolution P solubility in aerobic water is controlled by iron and aluminum minerals. Most common are gibbsite, Al(OH) 3, and iron (III) hydroxide, Fe(OH) 3. Al(OH) 3 + 3H + = Al H 2 O K=10 9 Fe(OH) 3 + 3H + = Fe H 2 O K= Solubilities increase while ph decreases Al compounds are more soluble than iron compounds Example: Estimate the solubilities of gibbsite and iron (III) hydroxide at ph 5 and ph 6

9 Availability of P from sediment tends to decrease with decreasing ph Iron and aluminum oxides and hydroxides tend to be more abundant in sediment than aluminum and iron phosphates When a highly soluble source of P is added to a sediment at ph of 7 or below, the P will react with aluminum and iron and precipitate.

10 P absorption by iron and aluminum oxides H 2 PO Al(OH) 3 = Al(OH) 2 H 2 PO 4 + OH - H 2 PO FeOOH = FeOH 2 PO 4 + OH - In soil and sediment, especially the tropics, much of the clay fraction is in these forms. Clays are colloidal and have a large surface area; thus, they can bind to large amounts of P Silicate clays also can fix P

11 Calcium Phosphates Primary P compounds in neutral and basic sediments Most soluble compound is monocalcium phosphate, Ca(H 2 PO 4 ) 2 : Ca 5 (PO 4 ) 3 OH + 7H + = 5Ca 2+ +3H 2 PO 4 - +H 2 O K= Apatite is not soluble at ph 7 or above High Ca 2+ and high ph favors formation of hydroxyapatite Example: Estimate the solubility of P in waters of ph 7 and ph 8 with 15 mg/l calcium ( M)

12 Anaerobic Sediment Max. availability of P in aerobic soil occurs between ph 6-7 Here, there is less Al 3+ and Fe 3+ to react with Iron P becomes more soluble when the redox potential drops, thus ferric iron reduces to ferrous iron. P is unavailable to the water column until thermal stratification, but then, it s only brief

13 Organic Phosphorus Dry matter of plants Vertebrate animals (fish) Invertebrates Crustaceans % P 2-3+% P % P 1% P P in organic matter is mineralized in the same manner as nitrogen The Nitrogen:Phosphorus ratio in living organisms and in decaying organic residues varies from 5:1 to 20:1

14 Total Phosphorus (TP) vs Soluble Reactive Phosphorus (SRP) TP P concentration from raw water SRP P concentration from filtrated water, amount available to plants Surface waters <0.5 mg/l TP (Mostly Particulate) (<0.05 SRP) Sediment 10-3,000 mg/kg TP (Mostly bound, 85.6% is not removable) Typically, 10% or less of TP is SRP and readily available for plants

15 Phytoplankton absorbs P very quickly mg/l can be removed within a few hours Macrophytes also remove P quickly, but they can also remove P from anaerobic zones in the sediment Most TP is contained in phytoplankton cells Plants can take up extra P and store it, or they have a luxury consumption Plant Uptake

16 Fig 12.4

17 Water Sediment Interface Sediment is a Phosphorus sink 2 ways that P gets into water: 1) Bioturbidity 2) Diffusion **Difficult to get into water column

18 Fig 12.7

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20 Eutrophication mg/l TP Oligotrophic Mesotrophic Eutrophic Hyper-eutrophic Not an exact relationship due to availability of essential nutrients, degrees of turbidity and source of the turbidity. Hydraulic flushing rate the percentage of the P input that is retained increases as the hydraulic retention time increases A lake with high ph and high Ca 2+ (high TA and high TH) would require a greater P load to cause eutrophication

21 Redfield Ratio Phosphorus and Nitrogen have a linear relationship: Typical ratio (5:1 to 20:1): 7:1 N:P Marine plankton: 106:15: 16:1 C:Si:N:P P concentration will cause a greater response in plant growth than will an increase in N concentration Nitrogen is readily recycled in the water system while Phosphorus (limiting recycling) is bound in sediment or plant growth.

22 Significance Phosphorus is needed for all living organisms Main source in agriculture and industry is mineral apatite, or calcium/rock phosphate Municipal and agricultural pollution is a major source of Phosphorus in many water bodies Forms of Phosphorus in water are dependent on the ph Sediment is a sink for Phosphorus due to the low solubility of Al, Fe, and Ca phosphates (P is tied up in Al and Fe in acidic soils, and CaPO4 in alkaline soils)

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