Mineralogy of Pb-P grains in the roots of Agrostis capillaris L-by ATEM and EXAFS

Analytical transmission electron microscopy (ATEM) and X-ray absorption spe ctroscopy (XAS) have been used to determine the mineralogy of Pb-P deposits in the roots of the heavy metal tolerant grass cultivar Agrostis capillari s L. cv. Parys Mountain. The deposits have a pyromorphite (Pb-5(PO4)(3)Cl)t ype structure and composition although some of the Cl may be substituted by OH. Energy-dispersive mapping under the scanning electron microscope demon strated that the majority of these deposits are present in the outer cell w all of the epidermis (the outermost layer of root cells). The phosphate com position of these grains contrasts with the phytate (C6H18O24P612-) composi tion of Zn-P deposits observed in similar electron microscopy studies. The physiological role of heavy metal P deposits is unclear. Heavy metal P prec ipitates may form actively as a tolerance mechanism to heavy metals or pass ively, sequestering P in a metabolically inactive form.