Adsorbed Phosphorus

This unit appears in the water quality processes section of the 1D Water Quality Solver datafile. The unit models the adsorption of phosphorus on to sediment particles.

Description

Phosphorus is present in aquatic systems in several forms. The most readily available form for uptake by plants and algae is dissolved inorganic phosphorus in the form of orthophosphate. In addition, phosphorus is found adsorbed on to the surface of sediment particles. Algae and plants cannot easily access this form of phosphorus.

The adsorption process is governed by a number of factors including the binding capacity of the sediment, the redox potential and the pH. The concentration of dissolved phosphate in a water body can be reduced by adsorption on to suspended solids. This material can then be deposited on to the bed where conditions may become unfavourable for adsorption, causing phosphorus to be released into the pore water. The phosphate is then available for uptake through the roots of plants which exude nutrients into the water column. Phosphorus may also be returned to the water column by the re-suspension of the bed sediments.

Within 1D Water Quality Solver, sediment is assigned a maximum mass of phosphorus that can be adsorbed per unit mass of solid. The amount of phosphorus adsorbed on to the surface of the sediment is described by a Langmuir adsorption isotherm (see Equation below). This assumes that a relationship exists between the concentration of dissolved phosphate and the amount adsorbed.

Desorption only occurs when the surrounding water effectively becomes anoxic. In 1D Water Quality Solver, this is assumed to occur when dissolved oxygen concentrations fall below 5% of the saturation value. Under these conditions it is assumed that all of the adsorbed phosphorus is instantaneously released from the sediment as dissolved phosphate.

Equations

The Langmuir adsorption isotherm has a similar form to the Michaelis-Menten equation for nutrient uptake by algae:

                    (1)

Where:
S_e = equilibrium mass of adsorbed phosphorus per unit mass of solid
S_max = maximum amount of phosphorus per unit mass of solid that can be adsorbed onto the sediment under any conditions (of the order of 0.05kg of phosphorus per kg of sediment)
k_L = Langmuir half saturation constant (of the order of 0.5mg/l)
P_d = concentration of dissolved phosphate in the surrounding water (mg/l)
S_e represents the amount of phosphorus which would be held on the sediment due to the concentration of dissolved phosphate, if the sediment had remained in contact with the solution for a sufficiently long period of time. If the amount of adsorbed phosphorus is less than Sthen phosphate is adsorbed from the surrounding solution according to a first order process:

                    (2)

Where:

S = actual amount of adsorbed phosphorus per unit mass of solid
K_p= constant (typically 25-50 day)

All of the parameters can be derived experimentally, by exposing sediment samples to solutions containing different phosphate concentrations.

General

The ADSORBED Phosphorus module must be run in conjunction with the PHYTOPLANKTON module.

The ADSORBED Phosphorus module simulates the transported variable name:

• Adsorbed phosphorus (mg/l)

The ADSORBED Phosphorus module simulates the following variable names on the bed:

• Fluffy adsorbed phosphorus (g/m)

• Bed adsorbed phosphorus (g/m)