Precipitated Green Pigments: Products of Chromate Postgalvanic

Environ. Sci. Technol. 2008, 42, 7482–7488

Precipitated Green Pigments: Products of Chromate Postgalvanic Waste Utilization ANDRZEJ KRYSZTAFKIEWICZ, BEATA KLAPISZEWSKA, AND TEOFIL JESIONOWSKI* Poznan University of Technology, Institute of Chemical Technology and Engineering, M. Skłodowskiej-Curie 2 Sq., 60-965 Poznan, Poland

Received April 11, 2008. Revised manuscript received July 24, 2008. Accepted July 24, 2008.

The paper reports on obtaining highly dispersed chromium(III) silicates and chromium(III) oxides from postchromating waste by reduction of Cr(VI) to Cr(III) with hydrazine as a reducing agent. Chromium(III) salt solution and sodium silicate or hydroxide solutions have been used to precipitate green pigments (silicates and oxides). The effect of the precipitation parameters on the quality of green pigments obtained has been studied, and the optimum parameters ensuring getting the product of well-developed surface area and possibly lowest diameter of primary particles have been established. The precipitated silicates and oxides have been subjected to physicochemical analysis to determine bulk density; capacities to absorb water, dibutyl phthalate, and paraffin oil; particles size; particle size distribution; and surface morphology of chromium pigment surfaces. The adsorptive properties of the oxide and silicate pigments have also been examined. The XRD analysis documented that the chromium(III) silicate pigments obtained are amorphous. In addition, the pigments have been subjected to a colorimetric appraisal using the CIE L*a*b* color space system. The high-quality green pigments obtained have shown high dispersion, small tendency to agglomerate formation, and repeatable green hue. The parameters of the products are promising for their future technological use.

Introduction Composition of wastes from metal surface processing reflects the type of processes and work system of a given factory. In this study, our interest concentrates on the postgalvanic waste solutions from the chromating processes. They are toxic because of the content of heavy metals, increase the salinity of a water reservoir to which they are discharged, and contribute to the amount of suspended substance. These wastes are also very aggressive toward communal sewage systems and equipment of water treatment plants. A particular problem is the presence of chromates(VI), which must be reduced prior to use or storage (1). One of the methods of neutralization of solid waste containing chromium(III) is its introduction to the rotating furnaces in the process of clinker burning at cement-producing plants (2, 3). The stabilization or solidification of the waste, which can be of chemical, physical, or thermal character (4-7), seems an interesting solution to the problem. The matrix for chro* Corresponding author phone: +48 61 6653720; fax: +48 61 6653649; e-mail: [email protected]. 7482

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mium(III) compounds may be portland cement, pozzolanes, lime, or other binding materials. Attempts have also been made to apply heated postneutralization sediments of chromium(III) as fillers in asphalt road covers. In recent years, attempts have been made to add various types of postgalvanic waste to construction of ceramic products. The attempts aim not only at decreasing pressure on the environment but also at optimizing the properties of the ceramic materials produced (8-10). For instance, galvanic sediments can be applied in the production of ceramic pigments (e.g., chromium, aluminum, zinc and ferrous sediments) (11). The role of activated carbons and low-cost adsorbents for remediation of Cr(III) and Cr(VI) from water has been discussed in detail in ref 12. Waste solutions from plating plants have been considered for use in the production of green inorganic pigments, chromium(III) silicates and oxides (13). The pigments play an important role in several branches of industry, mainly in the production of paints and varnishes and in processing of plastic. In an earlier study (13), waste chromate(VI) solutions were reduced using Fe(II) salts, formic aldehyde, or hydrogen peroxide. All the processes were conducted in an acidic environment. From the Cr(III) salts obtained, chromium(III) oxide was precipitated using sodium hydroxide solution or chromium Cr(III) silicate employing sodium silicate solution. In the present work, hydrazine was suggested for reduction of chromates(VI), and the reduction process was carried out in a neutral environment. The systems obtained were dissolved in sulfuric(VI) acid solution, and from the chromium(III) salts, the green pigments of chromium(III) were expected to precipitate as the respective oxides or silicates. The aim of this study was to establish the effect of the precipitation parameters on the quality of green pigments obtained and to determine the optimum parameters, ensuring that the product has a well-developed surface area and possibly the lowest diameter of primary particles.

Experimental Section Reagents. Postgalvanic waste solutions from chromating processes were used to obtain green pigments. The wastes originated from metal surface finishing works in the processing of zinc and aluminum products. The process was electrophoretically implemented in baths containing chromic acid, sodium and potassium bichromate, and small amounts of sulfuric(VI) acid. The study was conducted on waste 1 and waste 2 solutions originating from two distinct chemical charges of two waste solutions. The postgalvanic waste solutions differed slightly in their content of principal components (mainly chromates), and their basic chemical composition is presented in Table 1. The very high toxicity of chromium(VI) compounds (14, 15) prompted us to reduce Cr(VI) to Cr(III) with subsequent formation of green silicates and oxides of Cr(III) (13). Hydrazine was suggested to serve as a reducing agent.

TABLE 1. Content of Sulphates(VI) and of Chromium(VI) in Samples of Waste Chromate Solutions evaluated substances (g · L-1)

waste no.

1 2

pH

SO42-

CrO42-

ZnO

Al2O3

2.1 2.2

26.3 27.1

2.0 2.1

3.2 3.7

10.1021/es800416e CCC: $40.75

4.2 4.4

 2008 American Chemical Society

Published on Web 08/29/2008

TABLE 2. Physicochemical Properties of Silicate Pigments Precipitated from Solutions Formed Following Reduction of Chromate(VI) Using Hydrazine (for Wastes 1 and 2) sample no.

temp (°C)

1A 1B 1C 1D 2A 2B 2C 2D

capacity to absorb water (cm3 · 100 g-1)

20 40 60 80 20 40 60 80

capacity to absorb dibutyl phthalate (cm3 · 100 g-1)

200 150 150 200 150 100 150 250

capacity to absorb paraffin oil (cm3 · 100 g-1)

250 250 300 350 300 200 200 350

250 450 400 450 400 250 300 350

bulk density (g · L-1)

average particle diameter (nm)

229 245 223 217 268 437 363 299

672 500 763 648 462 344 703 662

chemical composition (%) SiO2

Cr2O3

ZnO

Al2O3

H2O

58.5 59.4 58.8 60.1 56.5 57.0 57.8 58.2

19.4 19.0 19.2 19.0 21.0 20.7 21.1 20.1

0.6 0.5 0.6 0.6 0.5 0.5 0.6 0.6

0.9 1.1 0.8 0.9 1.0 1.1 1.0 0.9

20.8 19.7 20.1 19.2 20.4 20.5 19.2 20.0

TABLE 3. Physicochemical Properties of Chromium(III) Oxides Obtained by Precipitation Using Sodium Hydroxide Solution and Solutions Formed Following Reduction of Chromates(VI) Using Hydrazine (Waste 1) sample no.

temp (°C)





mean particle diameter (nm)

1A* 1B* 1C* 1D*

20 40 60 80

150 150 100 100

300 200 250 250

350 300 350 350

490 458 502 444

4966 2809 5078 3970

TABLE 4. Physicochemical Properties of Chromium(III) Silicates Obtained by Precipitation Using Sodium Metasilicate Solution and Solution of KCr(SO4)2 · 12H2O sample no.

temp (°C)





mean particle diameter (nm)

3A 3B 3C 3D

20 40 60 80

150 150 150 150

200 250 300 250

400 350 350 300

400 350 350 300

640 720 685 805

To obtain highly dispersed pigments, an aqueous solution of sodium silicate (Na2O · mSiO2 · nH2O; SiO2/Na2O molar ratio MS ) 3.3; density equal 1.39 g · cm-3), made by Vitrosilicon S.A., or an aqueous solution of sodium hydroxide (purchased form POCh S.A.) was used. Procedures and Methods. The reactive system in which the silicates and oxides were precipitated using sodium silicate of sodium hydroxide solutions was made of Cr(III) salts-containing solutions. The solutions were obtained by dissolving the sediments, formed following the reduction of postgalvanic chromates(VI)containing waste, in sulfuric(VI) acid. Effective reduction of the hexavalent to the trivalent chromium was preconditioned by selective collection of the waste streams. The two collected streams were marked as waste 1 and waste 2. Hydrazine was employed to reduce chromates(VI). The implemented redox reaction can be presented as follows: 2H2Cr2O7 + 3N2H4 f 4Cr(OH)3 + 3N2 + H2O

(1)

From the solutions obtained, the silicate and oxide pigments were precipitated using sodium silicate or sodium hydroxide. 2Cr(OH)3 + 3H2SO4 f Cr2(SO4)3 + 6H2O

(2)

Cr2(SO4)3 + 3Na2O · 9.9SiO2·nH2O f 2Cr(OH)3 · 9.9SiO2·nH2O + 3Na2SO4 (3) Cr2(SO4)3 + 6NaOH f 2Cr(OH)3 + 3Na2SO4

(4)

From the solutions, a precipitate of gray-blue chromium(III) hydroxide or a coprecipitate chromium(III) hy-

droxide with silica were obtained. The chromium(III) hydroxide composition was variable CrOx(OH)y · zH2O. The asprecipitated hydroxide is a bridge layer polymer, with the OH- and H2O groups acting as ligands and the -OH acting as bridges. On heating, Cr(OH)3 loses activity and is transformed into a dark green powder. 600 ° C

2Cr(OH)3 98 Cr2O3 + 3H2O

(5)

For comparative reasons, a solution of hydrated (pure) salt of chromium(III)-potassium chromate was used, from which silicate pigments were directly precipitated using sodium silicate solutions. The conditions of the process were selected so that the reduction of Cr6+ to Cr3+ would proceed with an efficiency of 100% (pH ) 7.3; 10% excess of hydrazine). Precipitation of silicates and oxides was performed in a reactor of 0.5 dm3 capacity and placed in a thermostat to allow temperature control. The system was subjected to intense mixing (1900 rpm). The solution of sodium silicate or sodium hydroxide was dosed to the reactive system at a constant rate using a peristaltic pump. Five-percent solutions of sodium silicate, sodium hydroxide, and solutions of chromium(III) salts were used. The reaction yielded colorful sediments of silicates or oxides. The samples obtained were dried for 48 h in a stationary drier at 105 °C. The main physicochemical properties of the pigments studied were examined, including bulk density and capacities to absorb water, dibutyl phthalate, and paraffin oil. Measurements of these parameters were repeated three times for each sample studied, and the mean values rounded to VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Multimodal particle size distribution of chromium(III) oxide particles; sample 1D*.

FIGURE 1. SEM micrograph (a) and multimodal particle size distribution (b) of chromium(III) silicate; sample 1D.

FIGURE 2. Multimodal particle size distribution of chromium(III) silicate particles; sample 2D. whole numbers are given in the tables. The contents of the metals (expressed in the content of the oxides) were determined by the atomic absorption spectroscopy (AAS Perkin-Elmer 700); the amount of SiO2 was weighted. A series of precipitated pigments of variable physicochemical and morphological properties were evaluated. Particle shape and morphology of the silicates and oxides were examined using a scanning electron microscope, Philips SEM 515. Polydispersity and particle size distribution (PSD) were assessed on a ZetaPlus apparatus (Brookhaven Instruments Co.) using the technique of dynamic light scattering (DLS) (16, 17). The resolution of determination of the mean particle diameter was 0.1 nm. The measurements of particle 7484

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FIGURE 4. SEM micrograph (a) and multimodal particle size distribution (b) of chromium(III) silicate; sample 3A. size and polydispersity were repeated 10 times for each sample of the green pigment studied. Specific surface areas of the obtained products were determined by N2 adsorption (BET method) using an ASAP 2010 instrument (Micromeritics Instrument Co.). Moreover, the volume and size of the pores of the precipitated materials were examined. For adsorptive characterization of each sample tested, three measurements were made. Because of the high accuracy of the instrument applied ((0.01 m2 · g-1), the surface area values were rounded up to whole numbers. The structure of selected samples was studied by XRD using a SIMENS D-500 X-ray diffractometer with Cu KR radiation.

FIGURE 5. SEM micrograph (a) and multimodal particle size distribution (b) of chromium(III) silicate; sample 3C. The colorimetric data of the pigments obtained were collected on an instrumented colorimeter (Specbos 4000, JETI Technische Instrumente GmbH) based on the CIE L*a*b* color space system.

Results and Discussion The previous paper reported on the properties of highly dispersed green silicate and oxide pigments precipitated from postgalvanic wastes in which chromates(VI) were reduced to chromium(III) compounds using salts of iron(II), methanal, or hydrogen peroxide (13). The processes were conducted in an acidic environment at room temperature. Application of iron(II) salts failed to yield satisfactory results (reduction of Cr6+ to Cr3+ was complete), since apart from the green chromium(III) sulfate, the solution contained brown iron(III) sulfate. Its presence significantly deteriorated the quality of the green color of the pigment obtained. Following the reduction with methanal, formic acid remained in the solution, which negatively affected the dispersion of the produced silicate and oxide pigments. On the other hand, application of hydrogen peroxide also resulted in a complete reduction of chromates(VI) to salts of chromium(III), but with progress of oxygen release, a slow reciprocal reaction could be noted; that is, oxidation of Cr3+ again to Cr6+. For the above reasons, hydrazine was selected as a reducer of chromates(VI). Following the reduction, hydrazine becomes oxidized to nitrogen so that no potential would exist for reverse oxygenation of Cr3+ to Cr6+.

Results of the tests on principal physicochemical properties of precipitated silicates and oxides, obtained by precipitation using sodium silicate or sodium hydroxide solutions and solutions originating from hydrazine-induced reduction of chromates(VI), are presented in Tables 2 and 3. Results of the tests on physicochemical properties of silicates precipitated using sodium silicate solution and a solution of the pure chromium(III) salt, KCr(SO4)2 · 12H2O, are presented in Table 4. As evident from the data of Tables 2 and 3, green pigments of high dispersion were obtained when either waste solution 1 or 2 was used for precipitation. The lowest bulk densities and the highest capacities to absorb paraffin oil were obtained at 80 °C (Table 2). Chromium(III) oxide of the lowest bulk density and the highest capacity to absorb paraffin oil was obtained from the chromium waste 1 solution, but on treatment at a lower temperature (i.e., 60 °C; see Table 3). The silicate obtained by precipitation from a pure solution of KCr(SO4)2 · 12H2O demonstrated parameters similar to those of chromium(III) silicate precipitated from the waste solution of chromate 2 (Table 4). Sample 1D belonged to the best quality green pigments precipitated. It was obtained by precipitation using the reduced chromate waste (waste 1) and sodium silicate solution at 80 °C. The sample demonstrated very low bulk density (217 g · L-1) and relatively high activity, since its capacity to absorb paraffin oil amounted to 450 cm3 · 100 g-1. The SEM microphotograph and particle size distribution for this chromium(III) silicate are shown in Figure 1. The particle size distribution of the chromium(III) silicate sample showed two bands of similar intensity (Figure 1b). The presence of primary particles was reflected by the band within the range of 252-320 nm (the maximum intensity of 98 corresponded to primary particles of 283.7 nm in diameter). Thus, the particles were the smallest from among all the silicate samples obtained. Unfortunately, the pattern was deteriorated by the presence of another band of a maximum intensity in the range of 648-1132 nm, which could be ascribed to the presence of primary agglomerates (aggregates); their maximum intensity of 100 corresponded to the aggregate diameter of 952.7 nm. The SEM microphotograph (Figure 1a) confirmed the character of the sample; that is, the presence of primary particles and their aggregates. The particle size distribution of chromium(III) silicate precipitated at 80 °C but from the chromate waste 2 solution, enriched in sulfates and chromates (Table 1), is presented in Figure 2. The distribution manifests the presence of three bands assigned to particles of medium size, which pointed to a deteriorated homogeneity of the silicate. The appearance of small particles of very low diameters, in the range of 91-160 nm (maximum intensity of 30 corresponded to the primary particles of 120.9 nm in diameter), was most advantageous. Primary particles of higher diameters were responsible for a very intense band in the range of 277-482 nm (maximum intensity of 100 corresponded to the primary particles of 365.6 nm in diameter). Unfortunately, secondary agglomerates were also present, in the range of 1674-2911 nm (maximum intensity of 48 corresponded to secondary agglomerates of 2207.8 nm in diameter). Thus, the presence of secondary agglomerates was the principal factor defining the very low uniformity of the silicate. Even if manifesting no particularly unfavorable physicochemical properties, chromium(III) oxide precipitated from sodium hydroxide solution, and the reduced chromate waste solution (waste 1) at 80 °C showed very low uniformity. It was confirmed by the respective particle size distribution (Figure 3) showing three intense bands. The band corresponding to the primary particles was present in the range of 448-780 nm (the maximum intensity of 90 corresponded VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Nitrogen adsorption-desorption isotherms of chromium(III) oxide and silicate: (a) samples 1D* and 1D and (b) samples 3A, 3B, and 3D. to the primary particles of 624.8 nm in diameter). The aggregates or primary agglomerates corresponded to the band in the range 1358-1895 nm (the maximum intensity of 79 corresponded to the aggregates of 1695.4 nm in diameter). The highly intense band extending above 8000 nm attested to the presence of secondary agglomerates (the maximum intensity of 100 corresponded to the secondary agglomerates of 8950 nm in diameter). Oxide pigments, even if ensuring higher color intensity, manifested less favorable physicochemical parameters as compared to silicate pigments. High bulk density of the oxide pigments frequently restricted their application range, since they easily underwent sedimentation in dispersion media. In this respect, silicate pigments proved to be much better. SEM and particle size distribution of chromium(III) silicate precipitated from chemically pure solution of KCr(SO4)2 · 12H2O at 20 °C are shown in Figure 4. As shown by the particle size distribution for the silicate (sample 3A), the sample was not fully uniform (Figure 4b) because it showed two bands of different intensities. The 7486

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more intense band in the range 275-373 nm could be assigned to primary particles of chromium(III) silicate (the maximum intensity of 100 corresponded to the primary particles of 330 nm in diameter). The other band of the particle size distribution could be ascribed to primary agglomerates (aggregates). This band spanned the range of 928-1260 nm (the maximum intensity of 69 corresponded to the aggregates of 1049 nm in diameter). The presence of the two forms of chromium(III) silicate particles, primary particles and large aggregates, was confirmed by Figure 4a. Silicate sample 3C, precipitated at 60 °C using the solution of chemically pure KCr(SO4)2 · 12H2O, manifested a relatively highly uniform character. In the particle size distribution (Figure 5b), two bands were present, differing from each other only slightly in their intensities. The band reflecting the presence of primary particles spanned the range of 288-362 nm (the maximum intensity of 95 corresponded to the primary particles of 323.3 nm in diameter). On the other hand, primary agglomerates corresponded to the band in the range of 893-1184 nm (the maximum intensity of 100

FIGURE 7. XRD patterns for selected silicate pigments: (a) sample 1D and (b) sample 3D.

FIGURE 8. Spectrum of color shades of precipitated chromium(III) silicates: (a) from solution formed following reduction of chromate(VI) using hydrazine (for waste 1); (b) from solutions of Na2SiO3 and KCr(SO4)2 · 12H2O. corresponded to the agglomerates of around 1000 nm in diameter). No signs of the presence of secondary agglomerates were noted. Thus, the precipitation temperature of 40-60 °C seemed to be preferable for precipitation of chromium(III) silicates.

The adsorptive properties of the silicate and oxide pigments obtained were also evaluated. The specific surface area (ABET), total volume of pores (Vp), mean pore diameter (Sp), and isotherms of nitrogen adsorption-desorption recorded for the samples of chromium(III) oxide and for the silicate obtained from chromium waste solutions are presented in Figure 6a. Chromium(III) oxide was characterized with a definitely low specific surface area, 60 m2 · g-1, and the course of respective adsorption-desorption isotherm was typical of samples of low total pore volume (0.14 cm3 · g-1). The isotherm began to rise as late as at the relative pressure of 0.8. At the maximum value of the relative pressure (p/p0 ) 1), the volume of nitrogen adsorbed at the chromium oxide surface manifested a relatively low value (∼140 cm3 · g-1). Definitely more advantageous structural and adsorptive parameters were demonstrated by the sample of precipitated chromium(III) oxide (Figure 6). Chromium(III) silicate obtained from waste chromium compounds was much more active: its specific surface area amounted to as much as 313 m2 · g-1, while the total volume of its pores increased to 0.75 cm3 · g-1. The adsorption-desorption isotherm recorded for this sample was different, with its rising part beginning at a relative pressure of p/p0 ) 0.3. The sample reached a high nitrogen adsorption capacity of 550 cm3 · g-1 range. Preliminary analysis of the shape of the hysteresis loop allowed an evaluation of pore sizes. If the nitrogen adsorption rapidly advanced at low values of p/p0 and a hysteresis loop started also at low relative pressures, we would deal with a microporous adsorbate. When the adsorption is insignificant at high values of p/p0 and a hysteresis loop starts at p/p0 values close to value of 1, the adsorbent is microporous. Thus, in view of the results obtained, chromium(III) oxide evidently is a microporous adsorbent. On the other hand, chromium(III) silicate is mesoporous, as shown by the shape of the hysteresis loop, which starts at much lower values of p/p0 as compared to chromium(III) oxide. They manifested a similar mean pore diameter, ranging from 9.0 to 9.6 nm. Structural and porous characteristics of chromium(III) silicates, obtained from pure chromium(III) salt at various temperatures, is shown in Figure 6. Chromium(III) silicates precipitated at lower temperatures (20 or 40 °C) exhibited similar structural and adsorptive properties: slightly lower specific surface area and total pore volume, with the mean pore diameter from 9.8 to 11.0 nm. Chromium(III) silicate precipitated from solution of a pure salt at 80 °C had a slightly altered porous structure, which was best evidenced by the course of the isotherm of nitrogen adsorption-desorption on its surface. The isotherm showed a typical course, and its rising tendency could be noted when the relative pressure p/p0 exceeded 0.5, reaching over 500 cm3 · g-1 at p/p0 ) 1. the specific surface area of the silicate increased to 323 m2 · g-1; the mean pore diameter decreased to 7.5 nm. The silicate pigments obtained are amorphous, irrespective of whether the substrate was the postgalvanic waste or KCr(SO4)2 · 12H2O, which has been confirmed by the diffractograms shown in Figure 7. Colorimetric studies, performed in the CIE L*a*b* system, permitted evaluation of the changes in individual colors following precipitation of appropriate pigments (Figure 8). Figure 8 presents an evident decrease in the lightness (L*) and augmented share of green color (negative values of the coordinate a* were high) while the share of yellow color (positive values of coordinate b*) remained at a constant level.

Acknowledgments This work was supported by the Poznan University of Technology Research Grant No. 32-117/08-BW. VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Precipitated Green Pigments: Products of Chromate Postgalvanic

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