Binding mechanism of Cu(II) at the clay–water interface by powder and polarized EXAFS spectroscopy

The sorption mechanism of Cu(II) on the 2:1 magnesian clay hectorite, the 2:1 aluminous clay montmorillonite, and the 1:1 aluminous clay kaolinite was investigated at pH 5.4–6.0 and 0.5 M NaCl ionic strength by solution chemistry and powder and polarized XANES and EXAFS spectroscopy. Divalent copper was not photoreduced under the intense synchrotron X-ray beam and retained its usual square (bi)pyramidal coordination with four equatorial oxygens (Oeq) at 1.93–1.96 Å and one to two axial oxygens (Oax) at 2.56–2.58 Å. The angular dependence of XANES spectra, the measured angle of 57–58° between the Cu–Oeq atomic pair and the direction perpendicular to the clay layer plane ([0 0 1]∗), and the detection and polarization dependence of neighboring Al/Mg and Si cationic shells, together indicate that Cu(II) is bonded to the edges of the clay layers as an inner-sphere complex. Based on the number of Mg/Al and Si nearest neighbors, the Cu(Oeq)4(Oax)1–2 polyhedron is attached to the clay surface by sharing one to three edges with the structural Al/Mg octahedra and zero to three corners with the Si/Al tetrahedra. Copper has an unusually high coordination on the two dioctahedral aluminous clays, explained by the presence at their surface of distorted empty cavities which can accommodate irregular coordination polyhedra. The steric match between the distorted empty octahedral cavities and the Jahn–Teller distorted Cu polyhedra provides a rationale to explain the higher affinity of Cu(II) for Al octahedral sheets, including the hydrargillite sheet of lithiophorite ([Al,Li][MnO2]O2[OH]2) as observed in nature.