Characterization of defects and the local structure in natural and synthetic alunite (K, Na, H3O)Al3(SO4)2(OH)6 by multi-nuclear solid-state NMR spectroscopy

Abstract

The local structural environments in a series of natural and synthetic alunite samples [ideally AAl 3 (SO 4 ) 2 (OH) 6 , A = H 3 O + , D 3 O + , Na + , and K + ] have been probed by solid-state 1 H, 2 H, 23 Na, 27 Al, and 39 K NMR spectroscopy. The natural alunite [KAl 3 (SO 4 ) 2 (OH) 6 ] and synthetic hydronium alunite samples contain few structural defects, whereas the synthetic natroalunite and alunite samples have ca. 10% Al vacancies based on 27 Al NMR. A new 27 Al local environment (Al D ) was observed and assigned to Al with one Al vacancy in the fi rst cation sphere. Three different proton environments, Al 2 -OH, Al-OH 2 , and H 3 O + are detected by 1 H and 2 H MAS NMR. The hydronium ion (H 3 O + ) is only observed in hydronium alunite, and is associated with the stoichiometric regions of the sample. It was not detected in 1 H and 2 H NMR spectra of alunite and natroalunite despite K (Na) occupancies of signifi cantly less than 100%, as determined from elemental analysis. Thus, our NMR results suggest that the common assumption, namely that an A vacancy and an Al 3+ vacancy are compensated by adding an H 3 O + and 3 H + (creating 3 Al-OH 2 groups), respectively, is too simplistic. Instead, a signifi cant fraction of the Al 3+ vacancies are compensated for by 4 H + ions, resulting in 4 Al-OH 2 groups per vacancy. This substitution is accompanied by the simultaneous deprotonation of a H 3 O + ion present on the A site. The resultant H 2 O molecule is unnecessary for charge balance, accounting for the A-site defi ciency often observed. The presence of Al 3+ and A + vacancies appears closely correlated based on NMR.

Crystal chemistry of the natrojarosite-jarosite and natrojarosite-hydronium jarosite solid-solution series: A synthetic study with full Fe site occupancy

“…The most recent study done by Nielson et al (2007) found that H 3 O + is not found in alunite or natroalunite in solid state NMR studies that have both A and B site deficiencies. They did see evidence for H 3 O + in hydronium alunite.…”

“…They did see evidence for H 3 O + in hydronium alunite. Nielson et al (2007) postulated that vacancies in the B site are compensated by the addition of 4 H + ions, resulting in 4 Al-OH 2 bonds per vacancy. This substitution is accompanied by deprotonation of H 3 O + in the A site, forming H 2 O, which is then unnecessary for charge balance resulting in an A site vacancy.…”

“…Jarosite with fully occupied B sites have been synthesized by Grohol and Nocera (2002), Grohol et al (2003), and Basciano and Peterson (2007). The presence or absence of H 3 O + in alunite and jarosite group structures has been extensively discussed in the literature (Nielson et al 2007;Drouet and Navrotsky 2003;Kubisz 1970;Dutrizac and Kaiman 1976;Ripmeester et al 1986). The orientation of the hydronium group has not been determined reliably in either X-ray diffraction or neutron diffraction studies (Lager et al 2001;Majzlan et al 2004;Nielson et al 2007) as the molecule is dynamically disordered.…”

“…The presence or absence of H 3 O + in alunite and jarosite group structures has been extensively discussed in the literature (Nielson et al 2007;Drouet and Navrotsky 2003;Kubisz 1970;Dutrizac and Kaiman 1976;Ripmeester et al 1986). The orientation of the hydronium group has not been determined reliably in either X-ray diffraction or neutron diffraction studies (Lager et al 2001;Majzlan et al 2004;Nielson et al 2007) as the molecule is dynamically disordered. The commonly noted cation deficiency in the A site has been assumed in the majority of studies to be the result of hydronium substitution.…”

“…In these cases, it is probable that charge neutrality is maintained by protonation of OH -to form OH 2 (Majzlan et al 2004). Recently, Nielson et al (2007) reinvestigated structural defects in the structure of natural and synthetic alunites with multi-nuclear solid-state NMR spectroscopy, including B site deficiencies and hydronium substitution in the A site. They found that the hydronium ion (H 3 O + ) was observed in hydronium alunite but was not detected in the spectra of alunite and natroalunite that had less than 100% A site occupancy of K or Na.…”