Fast Diffusion Reaction in the Composition and Morphology of

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Fast Diffusion Reaction in the Composition and Morphology of Coprecipitated Carbonates and Nitrates of Copper(II), Magnesium(II), and Zinc(II) J. Michael Davidson,*,† Khellil Sefiane,†,§ and Tiffany Wood‡ †

School of Engineering, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JL, U.K. International Institute for Carbon Neutral Energy Research (I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 818-0395, Japan ‡ School of Physics and Astronomy, James Clerk Maxwell Building, University of Edinburgh, Edinburgh EH9 3JZ, U.K. §

S Supporting Information *

ABSTRACT: The role of membrane formation, diffusion, and segregated reaction in determining the composition and morphology of copper, magnesium, and zinc carbonate and nitrate precipitates has been investigated. Photography of the coalescence of drops of divalent nitrate solutions into a shallow pool of sodium carbonate, at millisecond intervals, shows the formation of lamellae of fixed diameter comprised of a network of membranes separating the solutions. Segregated reaction within a lamella yields basic nitrates internally and mainly basic carbonates externally. On the macroscopic scale, with stirring, the precipitates are gels and, as precipitated, have been investigated by chemical analysis, IR spectroscopy, and microscopy through narrow pH ranges. Such gels are complexes of water, sodium carbonate, and metal basic carbonates corresponding to typical alkali complex carbonates, NamM(CO3)(n+0.5m)(OH)(2−2n) (m and n are the ratios of sodium and carbonate ions with respect to divalent cations), together with basic nitrates. Hydrolysis during washing changes the composition and yields basic carbonates having low values of n as reported hitherto. solutions can be achieved (∼10−4 s).5,6 Practical laboratory or industrial preparations require much higher concentration, leading to large supersaturation ratios and the formation of amorphous precipitates at rates several orders of magnitude faster still. For the first time, fast photography and microscopy are applied here to examine the formation and physical structure from the earliest stages during precipitation (1 ms resolution), and this shows that interfacial and segregated reactions are important in determining the outcome of these fast reactions. We observe that, upon coalescence of a falling single drop of metal nitrate solution into a pool of sodium carbonate, reaction takes place without mixing and with the formation of interfacial membranes that subsequently control the diffusion and segregated reaction observable on a time scale of milliseconds through to completion within a few seconds. Surprisingly, little attention has been given previously to either the pH history or the composition of the precipitates, which have invariably been washed before characterization. We report that the unwashed precipitates formed by copper(II), magnesium(II), and zinc(II) invariably contain sodium in a stoichiometric amount with respect to the divalent metals; they can be described as gels having as components metal carbonates and hydroxocarbonates, anionic complex carbonates in sodium form, basic nitrates or chlorides, and water. The composition changes upon washing by removal of sodium salts

1. INTRODUCTION The preparation of carbonates by precipitation is of interest widely in chemistry, especially in catalyst preparation, mineralogy, pharmaceuticals, and commerce generally. The basic carbonates of copper and zinc are precursors for the important family of copper/zinc oxide catalysts. In the teaching laboratory, reactions of metal salts with alkali carbonates exemplify “double decomposition” and have been carried out by millions of individuals.1,2 These laboratory experiments are not correctly described because changes in the pH during reaction, subsequent conventional washing, diffusion effects, and the mode of contact all change the chemical and physical structures of the precipitates, which prove to be mixtures. On the simplest view a very dilute, hydrated divalent metal ion reacts in a homogeneous solution according to eq 1 until the solubility limit of the normal carbonate is reached; it then precipitates or crystallizes. M(NO3)2 + Na 2CO3 = MCO3 + 2NaNO3

(1)

For Mg2+, the second-order rate constant for a typical dissociative ligand exchange is3 k2 = 1.5 × 106 L/mol·s, and for Cu2+ and Zn2+, the rates are “immeasureably fast”.4 The ionization of hydrates to form M(OH)+, where the rate constants for Cu2+ and Zn2+ are k1 > 107 s−1, is a necessary step in the formation of hydroxocarbonates of hydrozincite [Zn5(CO3)2(OH)6] or malachite [Cu2(CO3)(OH)2] composition. In general, these rates of homogeneous reaction are comparable with the rates of diffusion of ions in solution, and furthermore, even in dilute solution (∼10−6 M), second-order half-lives are similar to the time in which mixing of the reactant © 2015 American Chemical Society

Received: Revised: Accepted: Published: 1555

October 23, 2014 January 12, 2015 January 14, 2015 January 14, 2015 DOI: 10.1021/ie504192u Ind. Eng. Chem. Res. 2015, 54, 1555−1563

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Industrial & Engineering Chemistry Research

Figure 1. Carbonate/metal ratios, n, versus pH: (○) unwashed samples; (●) washed samples. (a) Zinc nitrate single run. Horizontal bars represent pH intervals. (b) Zinc composite data: nitrate and chloride. (c) Copper composite data: nitrate only. (d) Magnesium composite data: nitrate and chloride. These data correlate with Tables S1−S3 in the SI.

without chemical analysis or pH measurement, and in the case of the as-formed, i.e., unwashed, precipitates, this neglect appears to be total. In this work, the colloidal carbonates were prepared in narrow pH ranges, centrifuged, and then drained between filter papers under firm manual pressure until a gel that did not wet fresh filter paper was obtained. These gels contained ∼80% w/w water but handled as friable solids. A comparator specimen was washed, using water, and separated four times, yielding different gels. Carbonate analyses were carried out in vacuo by thermal decomposition (temperature: Cu, 623 K; Mg, 973 K; Zn, 573 K) and acid decomposition (5% HNO 3 ) with separation of water by fractional condensation at 195 K and collection of CO2 at 77 K, using sequential traps. In acid decomposition, CO2 was separated by three cycles of freeze and melt. The molar quantity of CO2 was estimated from pressure measurement in a standard volume. Both methods allowed gravimetric estimation of the water in the gels. Acid decomposition gives the total carbonate, whereas thermal decomposition gives only carbonate bound in stoichiometric proportion to the divalent metals and not sodium carbonate. The method suffers from interference by nitrates, evident as brown NO2, which was, however, useful for the qualitative identification of nitrate. The gas was transferred to an infrared cell for identification. C,H,N analyses were carried out mainly to quantify nitrate. If the precipitate contains sodium, upon thermal decomposition this will remain as Na2CO3 mixed with the oxide residue, e.g., for a zinc complex carbonate (eq 2):

and especially because of hydrolysis. Crystalline minerals are presumably formed slowly on a geological time scale from very dilute solutions close to equilibrium.

2. NOMENCLATURE Crystalline compounds having established stoichiometry will be given their mineral names; nonstoichiometric basic carbonates and basic nitrates will be referred to as ZnBC or CuBN, etc. The molar ratios of sodium and carbonate ions to the divalent metal ions in the basic carbonates are m and n, respectively. Tables, figures, and videos in the Supporting Information (SI) are designated by the prefix S. 3. EXPERIMENTAL SECTION Elemental or group analysis remains the most reliable, and perhaps the only, means to establish the composition of colloidal carbonate precipitates, whereas speciation of complex substances within colloids is difficult. X-ray diffraction (XRD) can be applied to the wet precipitates but is of limited value because their amorphous nature;7−9 this limited applicability is exemplified in the SI (section 3). Raman spectra can be obtained from wet samples, but unfortunately there is thermal damage even at low power (Renishaw InVia backscattering microscope with a 514 nm laser). IR spectroscopy is always applicable (PerkinElmer 1420 dispersive and PerkinElmer Frontier FTIR/ATR spectrometers), but in the present case, assignments prove to be difficult because of sensitivity to the method of sample preparation and sample presentation between transmission mode and attenuated total reflection. These problems are also discussed in the SI (section 2). Preparations of metal carbonates have often been reported

Na 2Zn3(CO3)4 = 3ZnO + 3CO2 + Na 2CO3 1556

(2)

DOI: 10.1021/ie504192u Ind. Eng. Chem. Res. 2015, 54, 1555−1563

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Industrial & Engineering Chemistry Research Metals were analyzed by either inductively coupled plasma optical emission spectroscopy, ethylenediaminetetraacetic acid titration, or gravimetry using the metal oxides from the thermal decompositions, giving m and n. Preparations were carried out at ambient temperature in a beaker (90 mm diameter) having a stirrer (300−500 rpm) with a rectangular blade (70 × 35 mm) and using 400−700 mL of a 0.5 M carbonate/bicarbonate solution together with a 0.5 M divalent metal salt solution in a buret (50 drops/min). Additions were mainly restricted to 5− 6% of the initial volume of the carbonate solution, after which the precipitates were collected in a centrifuge, or by filtration, and the clear solution was returned to the reactor for further additions as necessary. The pH was monitored at the beginning and end of each addition and, for example, is recorded as horizontal bars in Figure 1a, in which the mean value is plotted as the abscissa versus the ratio, n. Some reactions were carried out initially free of nitrate or chloride by mixing sodium carbonate and bicarbonate solutions, giving some desired pH. Carbonate phase formation by contacting metal salt solutions with sodium carbonate solutions was amenable to study by photography at moderate magnification (Mikrotron camera, Nikkor 105 mm f 2.8 lens, 1000 frames/s) and by optical microscopy (Zeiss Observer Z1 with LSM 700 software and 63× objective). Coalescence of single drops in shallow pools gave lamellae of volume similar to that of initial particles formed in titrations. Monitored at 1000 frames/s, the formation of membranes and the segregation of reactants can be seen, with the rates differing for copper, magnesium, and zinc. The single-drop reactions were carried out at ambient temperature in a small Petri dish constructed of a glass ring of 30 mm diameter cemented to a microscope slide. The syringe dropper and the dish were mounted on a platform having vernier adjustments that allowed measurement of the position of the capillary orifice and the liquid surface. A number of single-drop reactions were carried out external to the fast photographic apparatus in order to record reflection photographs and to combine multiple samples (usually five, dried) for IR analysis. Optical microscopy revealed the internal structure of the lamellae to below 1 μm, while internal transport of colloids by Brownian motion could be seen. Dried lamellae were examined by scanning electron microscopy (SEM).

Figure 2. Formation of macroscopic membranes by copper basic carbonate. A 0.5 M copper nitrate solution flows from a buret tip into 0.5 M sodium carbonate. The coherent membrane and internal fluid are blue/green, the color of which diffuses into the external fluid only very slowly. Ruptures in the membrane lead to outgrowths that are rapidly bound by fresh membrane.

using color tracers, with the dispersal as lamellae being unconstrained in time and space. In the case of a metal nitrate drop and a sodium carbonate pool, fast interfacial reaction at contact leads to the formation of membranes on and within the lamella by precipitation on a time scale evident within a few milliseconds and commensurate with the hydrodynamic events. In monochrome transmission images, the development of these membranes is clearly seen as a darkening as the transparency is reduced. Segregated reaction then proceeds on a longer time scale by diffusion reaction at low pH and high nitrate concentration within a lamella of fixed diameter (average 9.6 mm) and at high pH and high carbonate concentration on its surface. Figure 3a shows a reflected light photograph, taken against a dark background, of a lamella derived from magnesium nitrate at final conversion; this shows the complexity of these segregated reactions, “frozen” compared with the nonreactive case: concentric toroidal vortex rings, an unreacted core residual from an impact crater, azimuthal development of cells within the vortices, and capillary waves or fingers. Vortex formation can be driven by surface tension effects, whereas azimuthal effects are of random origin (such as entrained bubble formation or prolate/oblate oscillation within the falling drop), vitiating the prospect of replication. Videos of two such reactive events are presented as SI: normal addition of magnesium14 and inverse addition to zinc.15 Parts b−k of Figure 3 show selected transmission images of these developments for a magnesium nitrate drop. Figure 3b is the drop at impact, and Figure 3c is an exposure 3 ms later. An impact wave appears with a shadow, where the lamella has started to form; the radial dimension is now fixed. The central, more transparent core is unreacted solution under an impact crater. After 22 ms (Figure 3d), a ring structure of membranes becomes evident and the impact wave has subsided. At 39 ms, fluid in the core from the drop has shrunk to a small remnant

4. RESULTS AND DISCUSSION Segregated Reaction. The role of membrane formation in the contacting of divalent metal salt and alkali carbonate solutions (0.5M) is readily demonstrated at the macroscopic level by introducing a slow bleed of a copper nitrate solution (0.5 mL/min) from a buret tip below the surface of a pool of sodium carbonate. A membrane forms at the buret tip and expands into a bubble of diameter ∼1 cm (Figure 2). Where the bubble appears translucent, the color is blue, whereas the internal precipitate is green. It is about 2 h before color develops external to the bubble because of diffusion of a blue, soluble copper complex. Segregated precipitation was principally monitored by singledrop reactions of nitrate solutions (0.5 M) in a shallow pool of sodium carbonate (0.5 M; pH adjusted to 10.8; depth of 0.87 mm) illuminated from above and observed from below by digital photography. The average drop size was 11 mg (∼3 mm diameter) released from a height of 3 mm diameter, with the low Weber number, around unity, reducing hydrodynamic disturbance. There are extensive reports10−13 of the impact and coalescence of nonreactive drops in shallow, stagnant liquids 1557

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Figure 3. Lamellae from single-drop reactions: (a) Reflective image from a magnesium drop. Complete reaction. (b−i) Sequential transmission images in sequence from part b. Millisecond times from part b. (j) Finger growth in a zinc nitrate lamella. (k and l) Dispersal of a preformed magnesium carbonate colloid.

(Figure 3e). The shadows intensify within the multiple ring structure, having also a network of internal membranes (56 ms, 2f). Parts g−i of Figure 3 show the progressive development of azimuthal periodicity within the vortex rings, out of phase with shallow fingers; the membrane appears thicker (darker), appearing as a cell pinched by surface tension. Figure 3h shows the fully developed membrane system at 435 ms, and in Figure 3i (2964 ms), the optical density has increased within the confines of this established pattern, consistent with mainly diffusion of reactants onto the surface and penetration into the lamella. Parts h and i of Figure 3 also show a fixed frontal development, although less pronounced than that in Figure 3a. The latter is similar to the more marked ephemeral temporal development of fingers shown in the dispersal of a soluble tracer,10 which gives rise to the appearance of local symmetry because of the merging of fingers also seen in Figure 3a. The single-drop reactions of copper and zinc nitrates were similar to those of magnesium but slower and with less clearly defined successive vortex rings. Figure 3j (221 ms) was obtained by inverse addition using a low-pH Zn(NO3)2 pool and displays a remarkable development of broad fingers, as shown in the video.15 In a blank experiment, the dispersal of a drop of preformed colloidal magnesium carbonate into a sodium carbonate solution at pH 11.55 develops a transient ring system but with unstructured distribution of the particulates, which continue to disperse in the absence of the membrane present in a reactive system (Figure 3k,l). Coherent membrane formation allows advection over the initial vortex ring with the formation of a series of further vortex rings (Figure 3i), reminiscent of the Liesegang effect,16,17 which has been ascribed to autocatalytic reaction restricted by diffusion. In the present case, the reaction rate may also initially accelerate because of nucleation, followed by reaction on the increasing surface area of the particles. Precipitate from the cores of six copper-containing lamellae was combined and identified (infrared spectrum) as mainly basic nitrate, gerhardtite [Cu2(NO3)(OH)3], which, unlike the basic carbonates, was strongly crystalline, as shown in the SEM images (Figure 4a,b). Carbonate frequencies were barely discernible in the IR

Figure 4. SEM images from lamellae with normal addition: (a) unwashed copper nitrate; (b) washed copper nitrate; (c) unwashed zinc nitrate.

spectrum, indicating a high nitrate, low-pH reaction. Likewise, SEM reveals bladed crystals formed in a single-drop reaction of zinc nitrate (Figure 4c) of appearance very similar to those of Zn5(OH)8(NO3)2·2H2O, prepared in bulk by Li et al.18 If a single droplet of nitrate solution were dispersed into the bulk carbonate, whether stagnant or stirred, then the nitrate concentration would be very low and unable to sustain the relatively high selectivity for precipitated basic nitrates, which 1558

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Figure 5. Optical micrographs of lamellae with normal addition of copper(II) and zinc(II): (a) center of a toroidal ring almost devoid of colloids; (b and d) concentrated copper(II) colloidal precipitate with donut morphology at the inner part of the toroidal ring; (c and e) fine membranes at the outer edge of the lamellae; (f) colloids, mainly gerhardtite, formed from copper nitrate. Scale bars = 20 μm.

Na2Zn3(CO3)4·3H2O (A),21,22, were crystalline and readily identified by their sharp IR spectrum and XRD pattern (compare parts a and b of Figure S6 in the SI). The stoichiometry of precipitation at high pH in a carbonate medium, exemplified by zinc(II), is given by eq 4, which correlates decreasing pH in the titration with an increase in the bicarbonate/carbonate ratio. With respect to the equilibrium equation (3), the concentrations of carbonate and bicarbonate are equal at about pH = 10. In a titration carried out in a closed, evacuated system to quantify gaseous CO2 , evolution commenced at pH = 9.2, simultaneously with a change in the morphology, to accord with eq 5. At low pH, normal carbonates are formed from bicarbonate with CO2 evolution and A can be described as a complex carbonate derived from 3 mol of ZnCO3.

are therefore necessarily products of segregated reaction. Throughout the present investigations, the formation of basic nitrates of low solubility has been ubiquitous, indicating that segregation is a significant factor even in vigorously stirred reactions (see nitrogen analyses in Tables S1−S3 in the SI), and, further, microscopy reveals differences between the precipitated phases. Lamellae were also prepared in situ under a microscope. Parts a−f of Figure 5 show an outer membrane structure with a mobile, colloidal core (Brownian motion). The center of the core within the toroidal ring is initially free from precipitate. The inner part of the toroidal ring is composed of a concentrated colloid with donut-ring-shaped particles approximately 1 μm in diameter, as shown in Figure 5b,d. These colloidal particles slowly diffuse inward toward the center. The colloidal structure of gerhardite in the core (Figure 5f) is comparable with the SEM crystalline structure (dry) of Figure 4b. External to the colloidal region, parts c and e of Figure 5 show a fine membrane structure composed of submicron material, which prevents outward diffusion of the colloidal precipitate such that no precipitate was found outside of the rings on the time scale of the experiment. Divalent cations diffusing outward through the membranes would react at the surface. The addition of NaHCO3 into copper nitrate (low pH; 4.27 → 7.42) gives mainly gerhardtite. Stirred Batch Reactions. Precipitations were carried out by semibatch addition of the metal salt solution, most often nitrate, into stirred sodium carbonate (“normal mode”). Asformed precipitates were recovered as gels and then drained under compression. Large numbers of analyses gave good replication, revealing consistent trends in n, determinate water content and often without detectable nitrate, indicating that entrainment of the medium was minimal. The washed products are those reported hitherto7−9,19,20 to be amorphous forms of hydrozincite, malachite, etc. In this work, the main components of the washed product are either nonstoichiometric hydroxocarbonates, M(CO3)n(OH)2−2n, mixtures of stoichiometric ones, or hydroxocarbonates diluted by metal oxide or hydroxide. Even fractions collected in narrow pH ranges do not correspond to hydroxocarbonates of rational stoichiometry. Four substances, chalconatronite [Na2Cu(CO3)2], gerhardtite [Cu2(NO3)(OH)3], nesquehonite [Mg(CO3)·3H2O], and

2HCO3− = CO32 − + CO2 (g)

(3)

Zn(NO3)2 + (2 − n + 0.5m)Na 2(CO3) + (2 − 2n)H 2O = Na mZn(CO3)n + 0.5m (OH)2 − 2n + (4 − 2n)Na + + (2 − 2n)HCO3− + 2NO3−

(4)

3Zn 2 + + 8NaHCO3 = Na 2Zn3(CO3)4 + 6Na + + 2H 2O + 4CO2

(5)

7,8,18,19,22−25

Numerous papers have described the batch (integral in pH) precipitation of copper(II) and zinc(II) salts to form colloidal precipitates, which, together with those of magnesium(II), are our present concern. These are largely amorphous, and upon aging, especially at higher temperature (Ostwald ripening), broadened XRD lines appear7,8 that have been assigned to the patterns of hydrozincite, malachite, etc. In this paper, we show that incremental precipitation of divalent ions by titration with a differential decrease in the pH (mainly 0.2−0.5 units in the range of 12 to 5) yields a series of changing mixtures. The titration media were centrifuged after small pH intervals, and the supernatant returned to the beaker for immediate continuation. Both drained and washed samples were then analyzed. Strictly, the above procedure is a series of batch microchanges in the pH rather than a continuous differential change because the rheology is perturbed after each 1559

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0.021/pH unit, which is within the standard deviation of the mean values from a large body of analyses (20 samples analyzed by thermal decomposition, naverage = 0.316, and σ = 0.028; 20 different samples analyzed by acid decomposition, naverage = 0.304, and σ = 0.025). A value of n near 0.30 can be taken as representative of washed ZnBC; even two outliers (n = 0.370 and 0.355) do not approach the hydrozincite value. Complex B thus suffers hydrolysis as well as loss of Na2CO3 in the course of washing. IR spectroscopy offers some insight into the composition of the unwashed gel, B, further exemplified in the SI (section 2). The amorphous hydrozincite precursor is characterized by bands close to those of the mineral26 (1507 and 1390 cm−1); these frequencies may not be present at all at the highest pH, whereas at pH = 9.5, they predominate strongly in both unwashed and washed precipitates. A further band at 1350 cm−1, sometimes the strongest in the spectrum, can be assigned to another basic carbonate. It is removed sequentially by washing but remains as a shoulder in all spectra. A band at 1460 cm−1 in the spectra of high-sodium unwashed materials could be assigned to free Na2CO3, but it is absent from many such samples. Bridged carbonates such as A also have a band at 1460 cm−1. There are several ways in which the stoichiometry of B could arise: in a covalent polyzinc complex anion, by intercalation of sodium ions in a layered structure, or through bonding by weak forces. One water-stable complex hydroxocarbonate of zinc has been reported (n = 0.84 and m = 0.48).27 The presence of basic nitrate is evident from nitrogen analysis and qualitatively from the formation of NO2 by thermal decomposition; it is a main component formed by initial segregated reaction or by inverse addition. The SEM image of a dried single-drop lamella (Figure 4c) reveals a crystalline, bladed habit similar to that of Zn5(OH)8(NO3)2·2H2O reported by Li et al.18 and which has a strong peak at 1380 cm−1, unfortunately overlapping the doublet of hydrozincite. Within hours, their substance decomposes sequentially to εZn(OH)2 and ZnO,18 both of which would be difficult to identify admixed with ZnBC. Thus, the lowered values of n at high pH might be attributed to molar amounts up to ∼10% of Zn5(OH)8(NO3)2·2H2O and its decomposition products in a mixture with a hydroxocarbonate precursor. These reactions are one possible cause of low values of n. However, there are also several reports of low-n zinc compounds, Zn(CO3)n(OH)2−2n; e.g., carbonation of ZnO in water at subambient temperature with reaction times extending to 100 days27 yields three crystalline phases, one of which is tentatively identified as Zn3(CO3)(OH)4·2H2O. The mineral sclarite, Zn7(CO3)2(OH)10, has an n value of 0.28. Such substances would also be in accordance with the low values of n recorded for our washed products especially if they are structurally related to hydrozincite. At about pH = 9 in the titration of zinc(II) into sodium bicarbonate/carbonate solution, there is a strongly marked change in the morphology as a dense precipitate replaces the colloidal product with a step change in composition, unchanged by washing, with n above 1.0 and up to 1.22 (Figure 1a,b and Table S2 in the SI) and m up to 0.44. The dried powder was identified by IR and XRD as mainly A, also prepared by normal addition of Zn(NO3)2 into sodium bicarbonate at pH = 8.0− 7.5. Reactions of Copper(II). The unwashed light-blue precipitate obtained from copper nitrate is described as a form of

separation. All of the fractions yielded changing mixtures of products comprised of basic carbonates, M(CO3)n(OH)2−2n not of rational stoichiometry, basic nitrates, sodium complex salts, and possibly hydroxides or oxides. All noncarbonate coproducts introduce a systematic reduction in the observed values of n. Comprehensive data from all titrations using chlorides, nitrates, and sulfates of copper(II), magnesium(II), and zinc(II) are given in Tables S1−S3 in the SI. The values of n versus pH derived from analyses by acid decomposition in all titrations are combined in Figure 1b−d. Changes in the morphology of the precipitates are seen that cannot be observed in continuous titrations. Initially, there are highly dispersed aggregates of size up to 1−2 mm, associated with segregated reaction. Soon the medium is transformed into a colloidal solution, and microscopic examination in the case of copper(II) then showed that the particles had been reduced below 1 μm. Upon loading onto a rheometer (TA Instruments, AR2000 with 40 mm, 1° cone), the storage modulus of the dispersed colloid was higher than the loss modulus, thus indicating that the agitated precipitate forms a weak colloidal gel, as shown in Figure 6. In low-pH ranges, magnesium(II) and zinc(II) yield noncolloidal precipitates.

Figure 6. Storage modulus, G′ (solid circles), and loss modulus, G″ (open circles), versus strain. 0.5 M sodium carbonate, 25% converted to copper basic carbonate emulsion.

Reactions of Zinc(II). In Figure 1a relating to one complete titration using Zn(NO3)2, values of n are plotted versus pH showing the pH intervals and analytical data (n, m, and C, H, N) tabulated for both unwashed and washed gel samples. Analysis of the as-formed zinc(II) precipitates reveals mixtures always containing basic carbonate plus sodium and basic nitrate (up to ∼10% ZnBN molar basis) at high initial bulk pH. Collected data of n versus pH for numerous runs are given in Figure 1b. Neglecting the presence of nitrate, at high pH, there is a group of both m and n values close to 0.8 [with the average for eight runs using 0.5 M carbonate at pH = 11.21−10.60 being n = 0.780 (σ = 0.039) and m = 0.808 (σ = 0.115)] that combine to suggest a 2:1 complex, B, between sodium carbonate and a hydrozincite precursor: 2Na 2 CO 3 · Zn5(CO3)2(OH)6. The gel, B, was formed with the same composition using both 0.5 and 0.25 M reagents (Table S2 in the SI, experiments 263 and 264), and, remarkably, both were in a group of several samples having nearly constant 84% w/w water. Below pH = 11, the trend in n values is downward to meet those from washed samples near pH = 9.2. The observed stoichiometry of B appears to preclude the precipitation of significant amounts of zinc hydroxide as a free phase at the initial high pH. In the pH range 11.2 → 9, washing removes 2 mol of Na2CO3. For washed samples precipitated in this range, n increases slightly on a linear trend with a slope of Δn = 1560

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Industrial & Engineering Chemistry Research georgite,7 itself a form of malachite, which is green. The initial high-pH precipitate, on vacuum drying, gives a sharp IR spectrum (Figure S2a in the SI) indicative of a mixture of at least three components, two of which are crystalline [gerhardtite (ger; ν 1420 s, 1346 vs cm−1); chalconatronite (chal; ν 1605 s, 1530 vs, 1385 s, 1355 w, 1330 s cm−1] and a copper basic carbonate of variable composition (CuBC; ν 1470 vs, 1400−1385 vs cm−1). Analytical data (Table S1 in the SI; n, m, and C, H, N) are in accordance with mole ratios ger/chal/ CuBC of approximately 0.35:0.35:0.3. Gerhardtite is initially the predominant product, and this outcome is similar to that of the segregated, low-pH region in a lamella. Chalconatronite forms in small amounts from the supernatant liquids from the centrifuge and hence may also have resulted from slow crystallization during titration. Sodium salts including chalconatronite are removed from high-pH precipitates by washing or hydrolysis. Evidence of the changing ratio CuCO3/Cu(OH)2 by hydrolysis comes from the observation of gas evolution from hydrolysis of the gel during washing. Gerhardtite is difficult to remove completely by washing and crystallizes partially to give an powder XRD pattern in an otherwise amorphous material. The IR spectra of washed precipitates show that the malachite precursor is a component despite disparate values of n. At pH = 11.0−9.2, the low values of n indicate the presence of nitrates and hydrolysis products; at low pH = 8.5−6.5, n is raised by small amounts of chalconatronite. Reactions of Magnesium(II). The circumstances of the precipitation of magnesium carbonates differ from those of copper and zinc (full analytical data are given in Table S3 in the SI). The solubility is greater and increases as the pH decreases below 11. Copper and zinc have concentration minima at pH = 8.9 and 9.2, respectively, of around 10−6 M as hydroxides. At pH = 10.5, the solubility of magnesium is ∼10−4 M. Over a wide pH range, the relatively soluble nesquehonite crystallizes slowly in competition with fast precipitation of amorphous materials and in preparations continues to crystallize from supernatants over 1−2 days. A normal batch titration using 0.5 M Na2CO3 terminates at about pH = 9.6 because of increasing solubility but can be continued using NaHCO3 presumably driven by the irreversible formation of gaseous CO2 from decomposition of magnesium bicarbonate and precipitation of magnesium carbonate. Below pH = 11, the trend in n for unwashed precipitates is a constant value, approaching 1.0. Analyses of precipitates from normal titrations in the range pH = 10.99−9.65 using MgCl2 or Mg(NO3)2 were as follows: 13 samples collected in the range pH = 10.99−10.35 had naverage = 0.961 with σ = 0.036; for 8 samples, maverage = 0.420 with σ = 0.046. These data correspond approximately t o Na2Mg5(CO3)5(OH)2 (C) in some colloidal form having ∼77% w/w water; this, upon washing, hydrolyzes either to Mg5(CO3)4(OH)2, stoichiometrically related to either hydromagnesite or dypingite (n = 0.8) or else to readily identified nesquehonite (n = 1.0). Above pH = 11, some lower values of n are indicative of noncarbonate products. There is analytical evidence of the formation of nitrate compounds of magnesium, unusual for a group IIA element, but none forming a phase suitable for identification. Initial precipitates above pH = 11 are very coarse and redissolve with stirring; additionally, the collected precipitate redissolves in water. These indicators suggest that the particles contain nitrate segregated by a carbonate membrane. A typical such sample has a hard morphology upon drying and a high sodium content and

yields NO2 and a volatile solid nitrate upon thermal decomposition in vacuo (875 K). Analytical data for the washed gels are quite highly scattered possibly because of slow crystallization processes. Nevertheless, a significant number of data points are close to n = 0.8 within a generally increasing trend; this corresponds to removal of Na2CO3 by washing without further hydrolysis of the carbonate function. At about pH = 10 in the titration, there is a change in the morphology, and the colloid is replaced by a dense solid that sediments rapidly. The n value indicates a normal carbonate, but its IR spectrum is not that of nesquehonite. The IR spectra of magnesium(II) precipitates gave little useful information, although nesquehonite can be distinguished from MgBC by the sharp peak at 3563 cm−1. Ill-defined basic carbonates of magnesium have also been prepared free of sodium by a carbonation method, and the possibility of a layered structure was discussed.28,29 The as-formed carbonate precipitates from magnesium(II) and zinc(II) contain respectively ∼77% and ∼84% w/w water in combination with stoichiometric amounts of Na2CO3 and hydromagnesite or zinc basic carbonate precursors. These original gels may be similar to the prenucleation clusters that have been observed in the precipitation of calcium carbonate.30−32 Washing removes Na2CO3 and is accompanied by hydrolysis, lowering n. Crystallization of Malachite Close to Equilibrium. Crystalline chalconatronite and nesquehonite continue to form slowly in reactions of copper(II) and magnesium(II) subsequent to fast precipitation reactions. It seems likely that such processes in very dilute aqueous media mirror mineral formation in nature, which is often difficult to replicate in the laboratory. We have tested for such equilibrium reactions using bicarbonate solutions free of sodium carbonates and sodium nitrate. A sample of blue, five-times-washed CuBC (0.012 g prepared at pH 10.85−10.62) was carbonated in water (100 mL, HPLC grade) at 1 atm of CO2. CuBC dissolved in the course of 2 h to give a clear solution having pH = 4.62. The resultant solution was opened to air and allowed to desorb CO2 until green malachite started to crystallize at pH = 5.35, as identified by XRD (Figure S6a in the SI) and IR spectroscopy. Treatment with CO2 caused the malachite to redissolve, confirming the reversibility of the reaction. Carbonation of Cu2O in the presence of a little air or of CuO, which was slower, similarly yielded malachite. Crystallization of malachite around pH = 5 and of chalconatronite between pH = 9 and 10 in a carbonate medium should be compared with the thermodynamic data presented by Stumm and Morgan.33 Plausible precursors in solution are described in the appropriate pH ranges. Thus, Cu2(OH)22+ occurs at pH = 4−5, and Cu(CO)22− occurs at pH 9−10. These results suggest that quite stringent conditions of the pH and concentration of carbonate must be met for crystalline basic carbonates to form. Similar treatment of magnesium(II) and zinc(II) oxides yielded only amorphous materials.

5. CONCLUSIONS The reactions of nitrates and other salts of magnesium(II), copper(II), and zinc(II) on contacting with sodium carbonate in aqueous solution are very fast in relation to the mixing time of the solutions. As a result, precipitation occurs first at the interface with the formation of a coherent carbonate membrane on a millisecond time scale rather than by homogeneous reaction in solution. In the case of the dropwise addition of 1561

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metal nitrate into a pool of sodium carbonate, the discontinuous phase becomes encased in the membrane as a spreading lamella; segregated reaction then proceeds internally at high nitrate concentration and low pH, yielding colloidal basic nitrates, and externally at the surface and at high pH, yielding basic carbonates by diffusion reaction. The internal structure of the membranes is complex and reminiscent of Liesegang rings. Care has been taken to distinguish the composition of as-formed precipitates from those obtained after washing and/or aging; both are aqueous gels of multicomponent composition characterized by a mole ratio of carbonate to metal ions, n, and a ratio of sodium to divalent ions, m, that change as a function of the pH in the course of reaction and by washing. In stirred semibatch mode, and for zinc(II) as an example, the initial precipitated gel contains little hydrozincite precursor and has n ≈m ≈ 0.8 indicative of sodium carbonate complexed to a basic carbonate in some manner. Chemical analysis also indicates the presence of ∼10% basic nitrate. Very little hydrozincite precursor is present in this early stage. As the semibatch reaction proceeds, n decreases incrementally and the amount of sodium in the gel decreases; IR spectroscopy indicates increasing amounts of amorphous hydrozincite precursor. Upon repeated washing of all precipitates, sodium carbonate is removed and a new gel is formed having n ∼ 0.30. Thus, amorphous hydrozincite contains amounts of either another basic carbonate of low n value or zinc oxide. At about pH = 9.5 in the semibatch operation, the incremental precipitate is mainly hydrozincite precursor before and after washing, and therefore this pH offers optimal conditions for the steady-state production of hydrozincite in a continuous stirred tank reactor. The pH should not drop below 9.5. In the semibatch operation, the initial high-pH precipitate is of large particles containing segregated nitrate; as the reaction proceeds, the medium becomes colloidal, but at pH ∼ 9.0, there is a step change in the precipitate composition to dense noncolloidal Na2Zn4(CO3)4. The precipitation reactions of copper(II) are similar to those of zinc(II) but yield greater amounts of the basic nitrate, gerhardtite, which is stable, crystalline, and difficult to remove by washing. Likewise, magnesium(II) yields insoluble nitrates that have not been observed previously. In general, the fast reactions of concentrated solutions described here yield amorphous products. The equilibria in solution between carbonate, hydroxide, and copper ions and various complexes, including multinuclear ones, are a complicated function of the pH. The obvious precursor to malachite is the dicopper complex cation Cu2(OH)22+, which is dominant at about pH = 5 and 10−6 M copper(II). In practice, crystalline malachite forms reversibly from dilute solution at pH = 5.3 under 1 atm of CO2, an unusual example of such a synthesis of a mineral substance.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +44 (0) 131 650 4853. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.M.D. thanks the University of Edinburgh for the award of an Honorary Fellowship.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

Tables S1−S3 giving full analytical data for precipitation reactions, IR spectroscopy of colloidal carbonate precipitates including Figures S1−S5, patterns of XRD of powders in Figure S6, and videos http://www.see.ed.ac.uk/~ksefiane/JMDmovies/Magnesium.avi 13 and http://www.see.ed.ac.uk/ ~ksefiane/JMD-movies/Zinc.avi.14 This material is available free of charge via the Internet at http://pubs.acs.org. 1562

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