Boletín de la RSEHN. Sección Geológica

En este trabajo se estudia la cristalización de algunos fosfatos de cobre mediante la técnica del gel de sílice, a partir del sistema CuCl2-Na3PO4-H2O. Los geles de pH inicial ≥ 7 actúan como “gel activo” frente a la atacamita, Cu2Cl(OH)3, que se forma gracias a su bajo producto de solubilidad y a la aportación de grupos OH- por parte de dichos geles, además de la contribución, en mayor o menor medida, de iones Cl-, ya que la mezcla gelificante utilizada fue acidificada con HCl. Cuando los reactivos que contradifunden, alcanzan la sobresaturación y demás condiciones necesarias, se forma una segunda zona de reacción (Zri), constituida por esferulitos de cornetita, Cu3PO4(OH)3, que empobrecen el medio en [Cu2+] y [OH-], dando paso a otra fase, identificada como a-Cu2P2O7. La posición inicial de Zri se ve afectada por la formación previa de atacamita en los pH neutros o alcalinos. La relación inicial [CuCl2]/[Na3PO4] determina la evolución temporal de Zri: Cuando es 1/1 domina la cornetita y avanza hacia el reservorio del CuCl2, lo cual se justifica por la elevadísima actividad iónica del Cu2+, necesaria para la cristalización de cornetita. Cuando es 2/1 avanza hacia el reservorio del Na3PO4 y está integrada por dos fases diferentes, a-Cu2P2O7 y CuH2P2O7, que requieren una actividad iónica de Cu2+ menor, más ajustada al gradiente de [Cu2+] generado al difundirse éste, alejándose de su reservorio, y que son estables en condiciones de mayor acidez.

The immobilization of heavy metals as sparingly soluble salts constitutes one of the most powerful and cost-effective measures against pollution of soil and water. Among the number of anion which forms this kind of salts, phosphate is of great practical importance: it is low-priced, non-toxic, stable in oxidation or biodegradation conditions and turns out to be effective in a wide range of pH. Besides, copper compounds have received considerable attention in research and applications because of their ionic conductivity and superconductivity. In this sense, knowledge of mechanisms and conditions of crystallization from the involved systems (“phosphate-heavy metals-...” and “copper-anions-...”) is essential to develop the most appropriated precipitation and crystallization methods. However, in the copper-phosphate system, characteristics of the processes mentioned are not known with accuracy. This ignorance is due to the peculiar crystal chemistry of copper (II), coming from the variable geometries of Cu (II) environment; to the great variety of phosphates which can be obtained from this element and last to the lack of thermodynamics data available for most of these compounds. In this work, crystallization of some copper phosphates in silica gel from the system “CuCl2-Na3PO4-H2O” is studied.

The technique of crystal growth in gels requires some simple and economical experimental devices, is particularly suitable for obtaining crystals of slightly soluble compounds and allows to observe directly and anytime the process development that takes place in the gel’s inside. Silica gel was prepared by the acidification of a Na2SiO3 solution (density 0,79 g/cm3 and pH ≈ 11) with HCl (1M) solution until the desired initial pH (pH0) level was obtained. This gelling mixture was introduced into some glass U-tubes and, once the gel was formed in the horizontal branch of the tubes, reagents were introduced in the vertical branches. Combining gels’ pH0 (5,5; 7,0 and 8,5) and reagents’ (CuCl2and Na3PO4) concentrations (0,5 and 0,25 M), a set of 6 experiments was prepared (Table I). All of them were carried at room temperature (25 ºC). The growth evolution was monitored by binocular lens and the solid phases were identified by X-ray powder diffraction and scanning electronic microscopy.

In the gels of pH0 ≥ 7, the first precipitate is produced immediately in the interface CuCl2-gel. This is constituted by a continuous band of atacamite [Cu2Cl(OH)3] microspherolites (Plate I, figs. 1 a - b), which moves forward to the phosphate reservoir and ends in a series of Liesegang rings. That is, pH0 ≥ 7 gels act as “active gel” for atacamite, Cu2Cl(OH)3. Its low solubility product and the supply of ions OH- , which can be found in great quantity in gels of these pH ranges, make possible the formation of atacamite in conditions of high metastability. It has to be considered too the plausible adding of Cl- by the gel, as this has been acidified with HCl.

After some time, a second reaction zone is formed and immediately evolves until it disappears, leaving instead a thick ring constituted by two different kind of spherolites, much bigger than the previous ones. One kind is intense blue colour and perfectly spherical (Plate I, figs. 2 a - b) identified as cornetite, Cu2Cl(OH)3. The other kind, identified as a-Cu2P2O7, has an accentuated radial morphology (Plate. I, fig. 3). Gels of pH0 5,5 behave in a similar way (apart from the fact that, in them, atacamite is not formed initially). The location of this second reaction zone (Fig. 1) results affected by the formation of atacamite in the alkaline pH0, which consumes part of the Cu2+, delaying its diffusion. This way, PO43+ must overtake the middle of the gel column and approach the CuCl2 reservoir to reach the supersaturation and “equality range” necessary for the precipitation of copper phosphates. The evolution of this second reaction zone is very similar in all the experiments: cornetite, Cu3PO4(OH)3, crystallizes, followed by the phase a-Cu2P2O7, which requires less Cu2+ concentration and is stable in a more acid environment, poor in OH-, which have been consumed by cornetite (Fig. 2).

In six months time, it can be observed a clear influence of the initial ratio [CuCl2] / [Na3PO4] in the characteristics of the reaction zone (Zrf). When that ratio is 1/1, the Zrf is mainly constituted by cornetite, Cu3PO4(OH)3, and moves forward to the CuCl2 reservoir (Figs. 1 and 2), which is justified by the high activity of Cu2+, necessary for the crystallization of cornetite. When the ratio is 2/1, the Zrf moves forward to the Na3PO4 reservoir (Fig. 1) and it’s formed by two different phases, a-Cu2P2O7 and CuH2P2O7 (Fig. 2). They require an ionic activity of Cu2+ much lower than cornetite and more adjusted to the concentration gradient generated by the diffusion of Cu2+ while it moves away from its reservoir. Besides, both phases are stable in more acid conditions, like the ones likely to be generated with the diffusion of CuCl2.

Finally, the framboidal and botryoidal morphologies (Plate. I, fig. 4) shown in the cornetite aggregates at the most alkaline pH0, compared to the large size and crystallinity of the aggregated developed during the experiments at pH0 5,5 (Plate I, figs. 2 a - b), seem to indicate that this is a very dependable-on-pH compound.