Luciano Poppi

Kinetics of heavy-metal removal and recovery in sepiolite

Fixed beds of Mg-enriched sepiolite were percolated through Co2+, Cu2+, Zn2+, Cd2+ and Pb2+ single- and multicomponent heavy-metal solutions to study both the dynamic interactions between mineral and heavy-metal cations and the ion-sorption kinetics. The metal concentrations in the eluates were determined by atomic adsorption and/or inductively-coupled plasma and kinetics by the classical kinetic approach, using isothermal experiments at room temperature. The experimental results suggest that: (i) the amount of heavy metal sorbed by the mineral increases for smaller cations; ii) the sepiolite sorption efficiency sequence is, for single component solution, Pb2+<Cd2+<Co2+<Zn2+<Cu2+, and, for multicomponent solution, Pb2+=Co2+<Cd2+<Zn2+<Cu2+. Therefore, the ability of sepiolite to remove Cu2+, Zn2+, Cd2+ and Pb2+ is virtually independent of the competitive cation interactions, whereas its affinity for Co2+ is lower when other metals coexist in the solution. The cationic sorption-exchange equilibrium constants (k), obtained by fitting the data with Langmuir equations are: kCo2+=4.798×10−3, kCu2+=3.424×10−3, kZn2+=2.907×10−3, kPb2+=1.009×10−2 [meq min]−1, kCd2+=1.187×10−2 [meq min]−1/2 for monocomponent solution experiment. The desorption study concerning the nature of eluting agents shows that Mg2+ is more effective than Na+ in removing heavy metals. Rapid kinetics and equilibrium of exchange of Mg2+ for heavy metals were observed; this is thought to indicate that the main process occurs at easily accessible sites at the outer and channel surface. Moreover, other processes can be assigned to the exchange of Mg2+ in the octahedral sites at the channel edges.

Crystal chemical variations in Li- and Fe-rich micas from Pikes Peak batholith (central Colorado)

Abstract The crystal structure and M-site populations of a series of micas-1M from miarolitic pegmatites that formed within host granitic rocks of the Precambrian, anorogenic Pikes Peak batholith, central Colorado, were determined by single-crystal X-ray diffraction data. Crystals fall in the polylithionitesiderophyllite- annite field, being 0 ≤ Li ≤ 2.82, 0.90 ≤ Fetotal ≤ 5.00, 0.26 ≤ [6]Al ≤ 2.23 apfu. Ordering of trivalent cations (mainly Al3+) is revealed in a cis-octahedral site (M2 or M3), which leads to a lowering of the layer symmetry from C12/m(1) (siderophyllite and annite crystals) to C12(1) diperiodic group (lithian siderophyllite and ferroan polylithionite crystals). On the basis of mean bond length, the ordering scheme of octahedral cations is mostly meso-octahedral, whereas the mean electron count at each M site suggests both meso- and hetero-octahedral ordering, the calculated mean atomic numbers being M1 = M3 ≠ M2, M2 = M3 ≠ M1 and M1 ≠ M2 ≠ M3. As the siderophyllite content increases, so do the a, b, and c unit-cell parameters, as well as the refractive indices, primarily nβ. The tetrahedral rotation angle, α, is generally small (1.51 ≤ α ≤ 5.04°) and roughly increases with polylithionite content, whereas the basal oxygen out-of-plane tilting, Δz, is sensitive both to octahedral composition and degree of order (0.0 ≤ Δz ≤ 0.009 Å for siderophyllite and annite, 0.058 ≤ Δz ≤ 0.144 Å for lithian siderophyllite and ferroan polylithionite crystals).

Interaction between montmorillonite and pollutants from industrial waste-waters: exchange of Zn2+ and Pb2+ from aqueous solutions

Abstract The interactions between montmorillonite (sample SAz-1 from the Source Clay Repository of the Clay Minerals Society) and Zn2+ and Pb2+ solutions of different ionic strengths (from about 10−5 to 1 M) are studied in order to observe changes in the clay-solution system and mineral crystal chemistry. In both types of solution the stationary state of exchange, attained within 20 min, depends on the solutions ionic strength. The uptake can be interpreted by means of a Langmuir-type equation with Zn2+ somewhat more easily exchanged than Pb2+. The shifting of the d(001) spacing from 15.3 A (natural sample) to 12.8 A (Zn2+-exchanged montmorillonite) and 12.4 A (Pb2+-exchanged montmorillonite) suggests that the kind of interlayer cation affects the c dimension of the layer. Upon heating, the exchanged montmorillonite layer collapses (d(001),Zn = 9.5 A; d(001),Pb = 10.0 A) and the closest packing is attained at T = 200°C for Pb2+ - and at 380°C for Zn2+-exchanged samples. In addition to reactions observed for natural and Pb2+-exchanged samples in the temperature ranges 20–250°C and 500–700°C, the thermal analyses of Zn2+-exchanged montmorillonite show a reaction at about 380°C which can be related either to strongly bonded water molecules or to “brucitic-like” interlayers or to Zn2+-exchanged octahedra.

Crystal chemistry of Mg-, Fe-bearing muscovites-2M 1

Abstract Phengitic muscovite-2M1 crystals [[12](K0.88-0.99Na0.01-0.09Ca0.00-0.06Ba0.00-0.01)[6](Al1.64-1.88Fe2+0.06-0.29Fe3+0.01-0.16Mg0.00-0.16Mn0.00-0.07Ti0.00-0.06)[4](Si2.87-3.30Al0.70-1.13)(OH)1.56-2.07F0.00-0.41O9.91-10.25] from pegmatites and peraluminous granites were refined to investigate the influence of phengitic substitution on the mica structure. Single-crystal X-ray diffraction data were collected for eleven crystals in the C2/c space-group (agreement factor 2.1% ≤ Robs ≤ 3.9%). Tetrahedral Si and Al cation disorder was found for each sample, with the mean tetrahedral cation-oxygen distances ranging from 1.639 Å ≤ ≤ 1.647 Å and 1.640 Å ≤ ≤ 1.646 Å. As phengitic substitution increases, the octahedral sheet expands and requires a less distorted (more hexagonal) tetrahedral ring (7.70° ≤ α ≤ 11.38°) and low corrugation of the basal O plane (0.1796 Å ≤ Δz ≤ 0.2296 Å). The electron density at the M2 site is greater than that required for the ideal muscovite-2M1 structure, and a small excess of electron density is found in the M1 site. The inner sixfold coordination of the interlayer (A) cation is elongated along c*, which is consistent with the high α values and the long A-O11 bond length.

Interaction between aqueous chromium solutions and layer silicates

Abstract The interactions between Cr in aqueous solutions and phyllosilicates were studied to determine: (a) the amount of Cr(VI) to Cr(III) reduction in aqueous solutions by Fe(II)-bearing phyllosilicates; (b) the removal of the Cr species from solution by interaction with phyllosilicates, as a function of Cr(III) concentration and anionic environment. Chlorite, corrensite and montmorillonite were reacted with solutions containing Cr(VI) (1.62×10 −3 N, 5.77×10 −3 N and 1.32×10 −1 N, respectively). The sorption/desorption of Cr(III) by saponite was investigated in different anionic environments (Cl − , NO − 3 and CH 3 COO − ) and at different initial Cr(III) concentrations (3.21×10 −3 N, 5.49×10 −3 N and 8.49×10 −3 N). The extent of Cr(VI) reduction and the amount of Cr removed by phyllosilicates were measured by analysis of the liquid portion separated by centrifugation after controled periods of exposure. The minerals were studied by chemical, thermal and X-ray powder diffraction analyses. The results show that: (i) Fe(II)-bearing phyllosilicates sorb Cr and reduce Cr(VI) to Cr(III); (ii) the extent of reduction depends on the solution concentration and on mineral crystal chemistry; (iii) Cr(III) sorption isotherms show that the degree of uptake depends both on the initial concentration of metal in solution and on the anionic environment, the order of effectiveness being Cl − ≅NO − 3 >CH 3 COO − ; (iii) Cr(III) is retained in the mineral substrate and its release is difficult.

Crystal chemistry of Al-rich biotites coexisting with muscovites in peraluminous granites

Abstract A comparison was made between single-crystal structure refinements, electron microprobe analyses and octahedral site populations of seven biotite crystals and data obtained previously for coexisting muscovite in peraluminous granites using the same methods. Both micas, from several plutons of Northern Victoria Land (Antarctica) and Sardinia (Italy), show significant octahedral substitutions: biotite has a composition characterized by relatively high [6]Al content (0.32 ≤ [6]Al ≤ 0.59 apfu), whereas muscovite is characterized by phengite-like substitutions [0.12 ≤ [6](Mg + Fe + Ti + Mn) ≤ 0.35]. Mean bond-lengths and electron count data for six biotites-1M (space group C2/m, agreement factor 2.7% ≤ Robs ≤ 3.6%) and a biotite-2M1 (space group C2/c, Robs = 2.8%) show that Al substitutes for divalent cations in the octahedral M2 site and that the Fe and Mg distribution is disordered. The mean tetrahedral bond lengths determined for biotite-2M1 reflect Al-Si disorder. In coexisting muscovite-2M1 crystals, small positive electron density residuals close to M1 site position as well as the increase in M2 mean atomic number is in agreement with the presence of a significant phengitic component. Reduction in biotite unit-cell dimensions with the increase of Al follows a pattern similar to that of associated muscovites, and the octahedral site volumes of both micas are influenced by the Al saturation index (ASI) of the rock. These results, and the calculated partition coefficients between biotite and muscovite for elements in M sites, are consistent with continuous reaction and re-equilibration of biotite and muscovite during crystallization of peraluminous granitic melts.

CRYSTAL CHEMISTRY OF CORRENSITE: A REVIEW

Statistical analyses of chemical data from the literature of corrensite minerals suggest a large compositional variability, more evident in octahedral than in tetrahedral coordination. Mg occupies 40–80% of the octahedral sites, with Al and Fe2+ making up the remainder. Approximately 15–30% of the tetrahedral sites are filled by Al. Despite this compositional variability, distinct fields for the several types of mixed-layer trioctahedral chlorite/trioctahedral swelling layer are not apparent. Statistical analyses of the composition of corrensite compared with saponite, vermiculite, and chlorite suggest that corrensite is an intermediate between trioctahedral chlorite and trioctahedral smectite. If Fe/(Fe + Mg) > 50%, chlorite alone is favored, but with increasing Mg, chlorite appears to transform into corrensite and then, by iron oxidation, into trioctahedral smectite. Despite the chemical variability between corrensite, chlorite, and saponite, corrensite appears chemically to be a well-defined species. On the other hand, corrensite cannot be characterized chemically on the basis of its swelling component. Thus, the current definition of corrensite as a regular 1:1 interstratification of trioctahedral chlorite and either trioctahedral smectite or vermiculite is appropriate.РезюмеСтатистические анализы литературных химических данных по коррензитовым минералом выказывают большую композиционную разнообразность, которая более очевидна в октаэдрической, чем тетраэдрической координации. Мg занимает 40–80% октаэдрических мест, тогда как остальные места заполняются А1 и Fe2+. Приблизительно 15–30% тетраэдрических мест заполнено Аl. Несмотря на композиционную разнообразность нет очевидных четких областей для нескольких типов смешанно-слойного триоктаэдрического хлорита/триоктаэдрического набухающего слоя. Сравнение статисти¬ческих анализов композиции коррензита с сапонитом, вермикулитом и хлоритом наводит на мысль, что коррензит является промежуточным соединением между триоктаэдрическим хлоритом и три-окраэдрическим смектитом. Если Fе/(Fе + Ме) > 50%, только хлорит является предпочтительным, но с увеличением М§ хлорит, по видимому, преобразовывается в коррензит и при последующем окислении железа—в триоктаэдрический смектит. Несмотря на химическое различие между коррен-зитом, хлоритом и сапонитом, коррензиты, по видимому, являются химически хорошо-определен¬ными видами. С другой стороны, коррензиты не могут быть химически охарактеризованы на основе их набухающего компонента. Таким образом, современное определение коррензита как регулярной прослойки 1:1 триоктаэдрического хлорита и триоктаэдрического смектита или вермикулита яв¬ляется соответствующим. [E.G.]ResümeeStatistische Analysen von chemischen Daten aus der Literatur über Corrensitminerale deuten auf eine große Variabilität der Zusammensetzung hin, die in der oktaedrischen Koordination ausgeprägter ist als in der tetraedrischen Koordination. Mg besetzt 40–80% der oktaedrischen Plätze, während Al und Fe den Rest besetzen. Ungefähr 15–30% der Tetraederplätze werden von Al besetzt. Trotz dieser chemischen Variabilität sind keine getrennten Bereiche für die verschiedenen Arten von Wechsellagerung aus trioktaedrischem Chlorit und trioktaedrischer quellfähiger Schicht zu erkennen. Statistische Analysen der Corrensitzusammensetzung im Vergleich zu Saponit, Vermiculit und Chlorit deuten daraufhin, daß der Corrensit ein Zwischenglied zwischen trioktaedrischem Chlorit und trioktaedrischem Smektit ist. Wenn das Verhältnis Fe/(Fe + Mg) > 50% ist, wird Chlorit allein bevorzugt, aber mit zunehmendem Mg-Gehaltscheint Chlorit in Corrensit umgewandelt zu werden und dann durch Oxidation von Fe in trioktaedrischen Smektit. Trotz der chemischen Variabilität zwischen Corrensit, Chlorit und Saponit scheint der Corrensit eine gut definierte Phase zu sein. Auf der anderen Seite kann der Corrensit chemisch nicht aufgrund seiner quellfähigen Komponente charakterisiert werden. Daher ist die gegenwärtige Definition von Corrensit als eine regelmäßige 1:1 Wechsellagerung von trioktaedrischem Chlorit und entweder trioktaedrischem Smektit oder Vermiculit zutreffend. [U.W.]RésuméDes analyses statistiques de données chimiques de la littérature concernant les minéraux cor-rensites suggèrent une variabilité de composition très grande, plus évidente dans la coordination octaèdrale que tetraèdrale. Mg occupe 40-80% des sites octaèdraux, avec Al et Fe2+ remplissant le reste. Approximativement 15-30% des sites tetraèdraux sont remplis par Al. Malgré cette variabilité de composition, il n’y a pas d’apparence de champs distincts pour les différents types de couches mélangées chlorite trioctaèdrale/couche gonflante trioctaèdrale. Des analyses statistiques de la composition de corrensite comparée à la saponite, la vermiculite et la chlorite suggèrent que la corrensite est un intermédiaire entre la chlorite trioctaèdrale et la smectite trioctaèdrale. Si Fe/(Fe + Mg) > 50%, la chlorite seule est favorisée, mais avec l’augmentation de Mg, la chlorite semble se transformer en corrensite et ensuite, par oxidation de fer, en smectite trioctaèdrale. Malgré la variabilité chimique entre la corrensite, la chlorite et la saponite, la corrensite semble être chimiquement une espèce bien définie. Mais d’autre part, la corrensite ne peut pas être caractérisée chimiquement sur la base de son composé gonflant. Ainsi, la définition actuelle de la corrensite en tant qu’interstratification 1:1 de chlorite trioctaèdrale et soit de la smectite ou de la vermiculite est appropriée. [D.J.]

Interlayer water and swelling properties of monoionic montmorillonites

Abstract The dehydration of two montmorillonites saturated by 10 different cations is investigated by X-ray spectroscopy, in different conditions of temperature and environmental relative humidity. We consider, in particular, the cation “solvatation” water and propose a model, on the basis of which we show that, in the case of the 15-A structure the stability of the H20-cation-silicate complex depends essentially on the electrostatic energy of the water molecule dipole in the cation field. In the 12-A case, on the other hand, sometimes the energy change associated with the redistribution of the electrical charges of the cation and of the silicate seems to prevail, depending on the interlayer spacing. We also show a correlation between cationic radius and closest interlayer spacing in the conditions of extreme dehydration.

EFFECTS OF EXCHANGE CATIONS AND LAYER-CHARGE LOCATION ON CYSTEINE RETENTION BY SMECTITES

This study investigates the complexes formed between amino acids, which are the natural degradation products of organic matter, and smectites. Thus, the adsorption and desorption behavior of cysteine and Na-, Ca-, Cu-homoionic smectites with different layer-charge location, a montmorillonite, and a beidellite, were studied. The clay samples were treated with Na, Ca, and Cu 1 N solutions and then with a 0.2 M cysteine solution. To test smectite-cysteine stability at acidic pH, the solids obtained were repeatedly treated with distilled water acidified to pH = 5. All treated samples were characterized by thermal, X-ray diffraction, chemical, and infrared analyses. The results showed that: 1) Na- and Ca-rich smectites adsorbed and retained small amounts of cysteine, and did not show interlayer cation-cysteine complexes, whereas the amino acid was strongly retained in the interlayer by Cu-rich smectites; 2) d(001)-values for Na- and Ca-rich smectites showed little or no expansion, whereas for the Cu-rich smectites the intercalation of the organic molecule produced a swelling of the structure; 3) the interaction mechanism of homoionic smectites with cysteine in an aqueous medium occurs by weak interactions, (e.g., van der Waals interactions, hydrogen bonding, dipole-dipole interactions, and other electrostatic forces such as entropy-driven hydrophobic bonding), and/or by complexes involving interlayer cations and organic ligands. The formation of a stable chelate complex with the saturating ion permits cysteine to be adsorbed by Cu(II)-rich smectites and to be resistant to migration in soils and groundwaters.

Chromium-containing muscovite crystal chemistry and XANES spectroscopy

To verify chromium enrichment of the muscovite layer, a crystal chemical and XANES study on chromium-containing muscovite crystals from the South Island of New Zealand was carried out. The crystals studied differ from those of end-member muscovite in that they display variable levels of octahedral substitutions and homovalent and heterovalent substitution of K in interlayer sites. Single-crystal X-ray diffraction data were collected for three crystals in the space group C 2/ c to an agreement factor (R obs ) from 0.025 to 0.033. Tetrahedral cation disorder was found for each sample and the values of mean bond length for both tetrahedra do not depart significantly from that of the end-member muscovite-2 M 1 . Electron density at the M2 site is greater than that required for the ideal muscovite-2 M 1 structure, and a small excess of electron density is found for two crystals in M1. As the octahedral substitution of larger cations for Al increases in the octahedral sites, the match between tetrahedral and octahedral sheets improves and tetrahedral rotation angle, α, decreases. XANES spectra at the Cr K-edge in these chromium-containing muscovite samples exhibit octahedral symmetry. Moreover, a careful analysis of the pre-edge region shows at least two features. A qualitative fitting procedure of the pre-edge region indicates that no more than 0.5% of total Cr(III), if any, may occupy the tetrahedral site.