Role of phosphorus in (im)mobilization and bioavailability of heavy metals in the soil-plant system

A large number of studies have provided conclusive evidence for the potential value of both water-soluble (e.g., DAP) and water-insoluble (e.g., apatite, also known as PRs) P compounds to immobilize metals in soils, thereby reducing their bioavailability for plant uptake. It is, however, important to recognize that, depending on the nature of P compounds and the heavy metal species, application of these materials can cause either mobilization or immobilization of the metals. Furthermore, some of these materials contain high levels of metals and can act as an agent of metal introduction to soils. Accordingly, these materials should be scrutinized before their large-scale use as immobilizing agent in contaminated sites. Although mobilization by certain P compounds enhances the bioavailability of metals, immobilization inhibits their plant uptake and reduces their transport in soils and subsequent groundwater contamination. Whenever phytoremediation of contaminated sites is practicable, appropriate P compounds can be used to enhance the bioavailability of metals for plant uptake. Removal of metals through phytoremediation techniques and the subsequent recovery of the metals or their safe disposal are attracting research and commercial interests. Phosphate compounds can be used to enhance the solubilization of metals, leading to their increased uptake by plants. However, when it is not possible to remove the metals from the contaminated sites by phytoremediation, other viable options such as in situ immobilization should be considered as an integral part of risk management. One way to facilitate such immobilization is by altering the physicochemical properties of the metal-soil complex by introducing a multipurpose anion, such as phosphate, that enhances metal adsorption via anion-induced negative charge (i.e., CEC) and metal precipitation. It is important to recognize that large-scale use of P compounds can lead to surface and groundwater contamination of this element. It is therefore, important that future research should aim to focus on the role of P compounds on in situ remediation and natural attenuation in metal-contaminated sites, with minimum impact of P on quality of water sources.