  Calcium oxalate crystals are the most common biominerals found in plants.
They also make their presence known as painful kidney stones in humans and
animals. Their function in plants is extraordinarily versatile and encompasses
calcium storage and defense mechanisms against herbivores and detoxification
processes. Since plants containing calcium oxalate crystals are often exposed
to metallic environments, we studied the interaction of such environmental
metals with calcium oxalate in vitro. We showed that selected metals are indeed
incorporated into synthetic calcium oxalate, and in a manner that depends on
their ionic radius when precipitated in solution. One such mechanism of
incorporation is based on the replacement of calcium ions by other metal
cations within the host unit cell. The unit cell of calcium oxalate expands
when incorporating elements with larger atomic radii and shrinks when doped
with elements possessing ionic radii smaller than that of calcium. In this
systematic study, metal-doped calcium oxalate crystals were characterized by
means of high-resolution synchrotron X-ray powder diffraction,
energy-dispersive X-ray spectroscopy, inductively coupled plasma atomic
emission spectroscopy, and transmission electron microscopy. Better
understanding of metal incorporation into mineral hosts might lead to ways of
developing new and more efficient sorbent materials for detoxification
processes.
