Formation of calcium oxalate concentric precipitate rings in two-dimensional agar gel systems containing Ca2+–RE3+ (RE = Er, Gd and La)–C2O42−

The concentric precipitate rings are found in urinary stone section and may relate to Liesegang pattern and formation mechanism of urinary stone. Urinary stone is mainly composed of calcium oxalate, so it is significant to study the concentric precipitate rings. The influence of RE3+ on the formation of calcium oxalate concentric precipitate rings is studied for the first time in two-dimensional agar gel systems. Precipitate rings are characterized by means of Fourier transform infrared spectrometry (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and biomicroscope with digital camera. The results show concentric precipitate rings are made of calcium oxalate and RE3+ promote the present of rings. The number of concentric rings are influenced by different RE3+ and the reaction time. The agar gel systems are helpful for the ions to diffuse uniformly, and as a medium where calcium oxalate rings are formed. In comparison with Gd3+ and La3+ ions, the content and size of calcium oxalate monohydrate (COM) crystals are highest and biggest after the addition of Er3+ ions, indicating that Er3+ ions promote the formation of concentric rings effectively. However, in the similar systems containing Cu2+, Fe2+, Fe3+and Co2+, we found the ring only appeared in the system containing Fe3+ ions. And the formation mechanism of the calcium oxalate concentric precipitate rings in two-dimensional agar gel systems containing rare earth ions is also discussed in the paper.