Biologically Mediated Transformations - Contaminant Geochemistry - page 384

Table 15.4 Bacteria capable of denitrification (Firestone 1982 )

Genus Interesting characteristics of some species

Alcaligenes Commonly isolated from soils

Agrobacterium Some species plant pathogens

Azospirillum Capable of N 2 fixation, commonly associated with grasses

Bacillus Thermophilic denitrifiers reported

Flavobacterium Denitrifying species isolated

Halobacterium Requires high salt concentrations for growth

Hyphomicrobium Grows on one-carbon substrates

Paracoccus Capable of both lithotrophic and heterotrophic growth

Propionibacterium Fermentors capable of denitrification

Pseudomonas Commonly isolated from soils

Rhizobium Capable of N 2 fixation in symbiosis with legumes

Rhodopseudomonas Photosynthetic

Thiobacillus Generally grow as chemoautotrophs

products, sludge, and effluents). This element may be found in inorganic or organic
forms, which are in a dynamic equilibrium with dissolved P in the subsurface
liquid phase.
Phosphorus transformations in the subsurface environment, known as the P
cycle, depend on their original state. Subsurface transformation of phosphorus is
due to fundamental processes such as mineral equilibrium (dissolution-precipi-
tation) and interactions between P in solution and the solid phase (adsorption-
desorption). Alternatively, biologically mediated conversion of P between inor-
ganic and organic forms (mineralization-immovability) and bonding of inorganic P
by organic ligands (complexation) may affect P transformation in the subsurface.
Figure 15.9 shows the phosphorus forms found in the upper layer of the subsur-
face, as result of various transformations occurring during the P cycle.
In subsurface aqueous solutions, P may be found primarily as PO 4 3- and to
lesser extent as HPO 4 2- and H 2 PO 4 - . In general, orthophosphate species in
solution vary in relative concentration as a function of pH. An increase in pH
causes larger concentrations in the secondary species and changes the ratio of
primary to secondary orthophosphates. For example, at a pH of 4.0-5.5, the pre-
dominant P form is H 2 PO 4 - , while at pH [ 8.0, HPO 4 2- species predominate.
Phosphorus species concentrations in subsurface aqueous solutions are affected by
anthropogenic factors, either directly by the P species disposed of on a particular
site or indirectly via pH changes resulting from municipal, agricultural, or
industrial composition of the disposed materials. Ambient temperature and the
water-air ratio in the subsurface also control rate factors of biologically mediated
P species transformation.
Here, we do not discuss P transformation resulting from chemical equilibrium
and adsorption-desorption processes; the reader is directed to the comprehensive
review of Sims and Pierzjinski ( 2005 ). Instead, we focus on biologically mediated
transformation of organic P in the subsurface. The ultimate and ''natural'' sources
of organic P in the subsurface environment are animal and plant residues. These