Cristina Lugli

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).

REDUCTION AND SORPTION OF CHROMIUM BY Fe(II)-BEARING PHYLLOSILICATES: CHEMICAL TREATMENTS AND X-RAY ABSORPTION SPECTROSCOPY (XAS) STUDIES

The reduction of hexavalent chromium species in aqueous solutions by interaction with Fe(II)-bearing solid surfaces was studied using a 0.96 × l0−3 M Cr(VI) solution and iron-rich clays with different Fe(II)/Fe(III) ratios, layer charge, and exchange properties, i.e., chlorite, corrensite, and montmorillonite. Experimental studies demonstrated that Fe(II)-bearing phyllosilicates reduce aqueous Cr(VI) ions at acidic pH. Chlorite and corrensite, owing to the high Fe(II)/Fe(III) ratio, are electrochemically reactive, as rapid Cr(VI) reduction indicated. In contrast, montmorillonite showed minimum to nil reactivity towards Cr(VI). Furthermore, corrensite, which is high in both Fe(II)/Fe(III) ratio and exchange capacity, adsorbs the greatest amount of chromium.X-ray absorption spectroscopy at Al, Mg, Fe, and Cr K-edges was used to investigate the adsorbed chromium species. The montmorillonite sample, unaffected by treatment with Cr(VI) solution, displays no change at any investigated edge. Edge shape and energy also do not change for the Mg and Al spectra in corrensite, and changes are minor in chlorite. By contrast, the Fe K-edge changes both in chlorite and corrensite, and indicates an increase of Fe(III) in treated samples at the expense of pre-existing Fe(II). Cr K-edge spectra show that chlorite and corrensite sorb Cr(III), which implies its reduction from Cr(VI) in the interacting solution.

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.

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.

Interaction between glycine and Na-, Ca-and Cu-rich smectites

Abstract The interactions between glycine and two Na-, Ca- and Cu-exchanged smectites with different layer-charge location were studied. The sorption of glycine depends on the nature of the interlayer cations (Ca < Na < Cu), and on the type of smectite. Sequential extraction procedures were carried out in order to test the possibility of removing metals and/or glycine from the smectite interlayer. By the end of the treatments, the release of the amino acid from the substrates, with the exception of Cu-rich smectite, was virtually complete. The thermal curves of glycine-smectites confirm the stronger bonding of the amino acid with Cu-exchanged samples, and FTIR spectra indicate that glycine is mainly sorbed in the zwitterionic form. The data obtained suggest that in investigations into mechanisms of the binding of metals by minerals and their subsequent mobilization, amino acids merit close attention.

Iron-rich saponite; dissolution reactions and Cr uptake

Abstract To gain a deeper understanding of the geochemical processes involved in the interactions between ionic solutions and clay minerals in natural systems, the dissolution rate of an Fe-rich saponite from Mt. Prinzera (Taro Valley, Italy) was measured as a function of pH and time at 25°C. Also, its ability to adsorb Cr was studied at varying metal concentrations (3.22, 5.50, 8.50 mEq l-1) and with different competing anions (CH3COO-, Cl-, NO3- ). It was found that there is a correlation between the experimentally determined release rate constant (k), as defined by the relase of Si, and pH. In acidic solutions, the k values correlate negatively with pH, whereas the opposite occurs for basic solutions. The extent of Cr uptake was measured by analysis of the liquid portion separated by centrifugation after a controlled period of exposure. The Cr adsorbed for different concentrations and anionic environments shows the dependence of Cr uptake on the initial metal concentration and on accompanying anions in the order CH3COO- < Cl- < NO3-.