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A study of adsorption and absorption mechanisms of copper in palygorskite

¹ State Key Laboratory of Mineral Deposits Research, Nanjing University, Nanjing, 210093, P. R. China, and² Department of Earth Sciences, School of Earth Sciences and Engineering, Nanjing University, Nanjing, 210093, P. R. China

^* E-mail: caiyf{at}nju.edu.cn

(Received 29 January 2007; revised 12 November 2007)

Desorption experiments performed on four Cu-adsorbed palygorskites suggest that the leached Cu²⁺ ion originates at the surface and/or net-like interstice of the palygorskite fibres. The leached fraction, calculated from the quantities of adsorbed Cu²⁺ before and after desorption, is <1%. This may indicate that the majority of Cu is in inaccessible structural sites. X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared (FTIR) spectroscopy and electron spin resonance (ESR) were used to determine the mineralogical character of the Cu-adsorbed palygorskite. Two photoelectron lines at 932.5 and/or 933.7 eV in the narrow scan Cu 2p_3/2 spectra show that Cu adsorbed on the surface of palygorskite is in the Cu⁺ and Cu²⁺ state. The stretching vibrations of the octahedral cation shift ~3–5 cm^–1 towards a greater wavenumber in the FTIR spectra of Cu-adsorbed palygorskite. It can be deduced that the Cu²⁺ is trapped in the channel of the palygorskite structure. The ESR spectra of the palygorskite give g values of 2.34, 2.12, 2.08 and 2.05, suggesting that some Cu ions cannot be reached by H⁺. These results confirm that Cu is adsorbed by palygorskite via three possible mechanisms: (1) the Cu is adsorbed onto the surface or in a net-like interstice, and its oxidation states are +1 and +2; (2) Cu forms a complex ion –[Cu(H₂O)₄]²⁺ or [Cu(H₂O)₆]²⁺, and is trapped in the channel; or (3) Cu enters into the hexagonal channel of the tetrahedral sites or the unoccupied octahedral sites of palygorskite.

KEYWORDS: palygorskite, Cu, adsorbed site, adsorption, absorption, XPS, FTIR, ESR, XRD