| | | 13.3 Binding of phosphorus in sediments |
The phosphate ion (PO43-) is a highly particle-reactive anion and thus the sorption properties of sediment are crucial for P-retention capacity. Generally, aerobic conditions are considered to promote P sorption and anoxic conditions to favour P release. After the dissolution of particulate P into inorganic form in sediments, PO43- ion is easily sorbed into inorganic particulate form. The storage of surplus P by microorganisms can, however, compete with the chemical immobilization of P [239]. Normal bacteria have a low P content in living cells but some bacteria accumulate polyphosphate (poly-P) intracellularly in very large amounts, up to ca. 20% of their dry weight. These bacteria store P under aerobic conditions and release intracellular P via enzymatic hydrolysis when conditions turn anaerobic [125]. In organic-rich lake sediments, microorganisms are able to take up and release P, depending on redox conditions, and sterilization of oxic sediments can reduce the microbial take up of P [167]. Both nonbiological and biological binding mechanisms of P are able to keep the pore water P concentration low and reduce the release of P from sediment to water.
Owing to surface area effects, the sorption reactions of P caused by inorganic particles are enhanced as a function of decreasing sediment grain size. The main fine-grained (<2 µm) constituents of sediments are silicates, i.e. clays, carbonates, Fe and Al oxides, and humic acids. The surfaces of fine-grained minerals such as Fe and Al oxides, clay minerals with surficial Fe and Al (hydr)oxides, and possibly also Mn oxides adsorb P efficiently. Humic compounds containing Fe and Al can also sorb P. Silicates and carbonates in larger particles such as sand adsorb P poorly.
It is widely accepted that sedimentary P cycling is linked mainly to the Fe cycle in lake (e.g. [146][323][324]) and marine systems [250][448]. In marine sediments, however, most of the total Fe (60-80%) is bound to sheet silicates (e.g. chlorite). Adsorption of P by silicate-bound Fe is insignificant compared to that by Fe(III) oxides, which are considered to be the main Fe compounds in P binding. Sediments contain a multitude of Fe(III) oxides, often in complexed mixtures, that range widely in degree of crystallinity, particle size, available surface area, reactivity and oxidation state [292]. The P sorption capacity of amorphous, i.e. poorly crystallized, Fe(OH)3 and lepidocrocite (-FeOOH), is about 20 times that of crystalline Fe(III) oxides such as goethite (%-FeOOH) and hematite (%-Fe2O3).
Other negatively charged anions can also compete with the PO43- of adsorption sites in sediments. For example, the effect of pH can be related to competition for adsorption sites, because desorption of P from clay minerals and Fe and Al (hydr)oxides is based on ligand exchange, in which P is substituted for by OH-. An increase in pH can also increase the negative charge of the sorbing oxides [215]. A significant decrease in P sorption to Fe(III) oxides occurs at pH >6.5. Silicon (Si) may also compete with P for adsorption sites [452]. Silicate (SiO4) and P are sorbed onto the surfaces of hydrated Al and Fe oxides by the same specific mechanism and, thus, SiO4 may chemically compete with PO43- for adsorption sites.
| | | 13.3 Binding of phosphorus in sediments |