Phosphate trap

As phosphate case is referred to in the Limnologie under aerobic occurring conditions constant deposition of phosphate in the form of Fe (III) PO ₄ (iron phosphate) into the sediment of a lake .

In the near-surface water layers, phosphate is bound by the primary production of algae and thus brought into the form of biomass particles , also via the food chain . Biomass particles that sink into the deeper water layers due to cell death (apoptosis) release the phosphate there again when they decay. Under aerobic conditions (with oxygen), Fe (II) ions can be oxidized to Fe (III) ions and the precipitation reaction of phosphate with Fe (III) ions to form iron phosphate (Fe (III) PO ₄ ) can take place. This sinks to the bottom and remains with other sedimenting substances ( e.g. calcium carbonate and organic detritus ).

Oxidation of Fe (II) to Fe (III) :

Fe ²⁺ → Fe ³⁺ + 1e ^-

Phosphate trap :

Fe ³⁺ + PO ₄ ³⁻ → FePO ₄

Over time, more and more phosphate is stored at the bottom of the lake. Only when there is no more oxygen above the lake floor is the iron reduced; the phosphate that has been “trapped” up to then is released again and returned to the lake's mineral cycle; this can lead to an algal bloom if the released phosphate gets back into the epilimnion (surface water) in the course of the autumn or spring circulation . As a result, so much oxygen can be consumed by the sinking biomass in the deep water ( hypolimnion ) that the phosphate trap is permanently disabled. The transition into this state is called tipping over .

Reduction of Fe (III) to Fe (II) :

Fe ³⁺ + 1e ^- → Fe ²⁺

Release of the phosphate :

FePO ₄ + 1e ^- → Fe ²⁺ + PO ₄ ³⁻