Highly Dispersed Green Silicate and Oxide ... - ACS Publications

Environ. Sci. Technol. 2003, 37, 4811-4818

Highly Dispersed Green Silicate and Oxide Pigments Precipitated from Model Systems of Postgalvanic Waste BEATA KLAPISZEWSKA, ANDRZEJ KRYSZTAFKIEWICZ, AND TEOFIL JESIONOWSKI* Poznan University of Technology, Institute of Chemical Technology and Engineering, pl. M. Sklodowskiej-Curie 2, 60-965 Poznan, Poland

A procedure was worked out to obtain highly dispersed green silicate and oxide pigments precipitated from postgalvanic waste. The highly dispersed chromium(III) silicates and oxides were produced from the waste, originating from chromium plating, by reduction of Cr(VI) to Cr(III) employing various reducing agents. All the reductions were conducted in an acidic medium. Solutions of Cr(III), obtained in reducing processes, were employed to precipitate silicate pigments (using sodium metasilicate solution and containing mainly chromium(III) silicates) and oxide pigments (using sodium hydroxide and containing chromium(III) oxides). The precipitated silicates and oxides were subjected to a comprehensive physicochemical analysis (estimating bulk density, capacities to absorb water, dibutyl phthalate, paraffin oil, particle size distribution, and morphology of particle surface). Precipitation process (its parameters) and heating of the reactive mixture exerts a significant effect on the principal physicochemical properties of the pigments. The heating significantly affects first of all color shade of the obtained silicate and oxide pigments as well as their dispersion. Coprecipitated chromium(III) and iron(III) silicates exhibit a brownish color and a reasonably uniform character. Apart from primary agglomerates (in the range of 414-717 nm), they contain small amounts of secondary agglomerates (in the range of 4154-6445 nm). Best physicochemical parameters have been demonstrated by chromium pigments which have been precipitated from chromium solutions reduced using hydrogen peroxide. Chromium(III) oxides deserve particular distinction since their structure includes primary particles, primary agglomerates but is completely free of secondary agglomerates. The pigments manifest a brightly green color and a low capacity to absorb water (100 cm3‚100 g-1). Application of hydrophobicity-inducing agents in the course of precipitation has corrected physicochemical parameters of both the oxides and silicates of chromium(III). Their bulk densities have been clearly decreased (to as low as below 250 g‚L-1 in the case of chromium(III) silicate), while capacities to absorb paraffin oil have increased to as much as 750 cm3‚100 g-1 for chromium(III) oxide. The respective particle size distribution has shown a tendency * Corresponding author phone: +48 61 6653720; fax: +48 61 6653649; e-mail: [email protected]. 10.1021/es020973u CCC: $25.00 Published on Web 09/18/2003

 2003 American Chemical Society

for disappearance of large accumulations of secondary agglomerates.

Introduction A postgalvanic waste is qualified as a hazardous waste and, due to its vast amounts, creates a problem the solution of which would be important for environmental protection and waste management. Therefore, many tests and studies are performed worldwide in multiple centers, aimed at utilization of postgalvanic sediments. With increasing frequency the sediments are processed by hydrometallurgical procedures orsin metallurgysby pyrometallurgical procedures (United States, Germany, and Austria). Mixed with a slag, the waste is used in road construction or is transformed into construction, ceramic, or magnetic materials. The composition of waste from metal surface processing reflects the type of processing and working procedures in a given factory. The wastes are toxic due to their content of cyanides and heavy metals, and they may also include also wastes containing chromium(VI) compounds. Processes for stabilization/solidification of these wastes fail to neutralize the metals and usually do not neutralize organic components. Following solidification and stabilization (S/S), the wastes should be evaluated, i.e., in respect to their leakage of toxic substances (1). Originally, the S/S processes employed Portland cement (2). Addition of chromate in the form of K2CrO4 was found to extend the preliminary and the final setting time of the cement. X-ray diffraction tests demonstrated that Cr(VI) inhibited cement hydration by interaction with Ca2+ in course of the process. Increasing content of Cr(VI) led to the lowering of the crushing strength of the cement. Leaching of Cr(VI) reflected its original concentration and the duration of the test. To fulfill leakage limits (Cr(VI) concentration in the eluate e5 mg‚L-1), the K2CrO4:cement ratio had to be below 0.2% (2). Optimum solidifying mixtures for postgalvanic and tannery slimes continue to be searched for, employing for this purpose cements, flue dusts, cinders, and lime. The tests have demonstrated that Portland cement represents the most effective binding agent for Na2Cr2O7‚2H2O (3). The content of flue dusts and cinders have not favored binding of chromium(VI) (4). On the other hand, Park (5) examined leakage of chromium ions stabilized in ordinary Portland cement or in cement modified with burned clinker dust and found lower leakage of metals from the latter stabilizer. The modified cement demonstrated also higher crushing strength (6). Binding of chromium waste in ceramic materials seems to represent a promising strategy (7-10). As compared to S/S technologies, ceramic technologies are thought to be a more radical way of turning a hazardous waste inert since they destroy organic substances and permanently immobilize metals in matrices, in this way transforming chemical waste into materials which are useful and potentially applicable in building industry. A variable composition of postgalvanic waste narrows potential for its application and for this reason selected sediments are used, which manifest a defined composition. Some types of the postgalvanic waste, which contain nonferrous metals or salts, may be applied for coloring ceramic products, substituting for, e.g., pure chromium, copper, nickel, etc. compounds (11). Ceramic pigments can be produced using chromium, iron, nickel, etc. sediments. VOL. 37, NO. 20, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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The mechanism of toxic action of chromium on human body involves formation of stable bonds with various cell components, i.e., with DNA. This leads to DNA lesions and various type mutations. For an adult man lethal dose (LD) amounts to around 20 g of Cr(VI). Thus, it seems important to handle chromium(VI) in the form reduced to Cr(III). In sewages drained to waters and to the ground the maximum concentration of Cr(III) amounts to 0.5 mg of Cr‚L-1 and that of chromium(VI) to 0.2 mg of Cr‚L-1 (12, 13). In the present study on Cr(VI) waste we obtained green pigments: chromium(III) silicates and oxides. Earlier we succeeded in the synthesis of red, green, and white pigments, taking advantage of industrial wastes (14-16).

Experimental Details Reagents. Highly dispersed chromium(III) silicates were obtained from the waste originating from chromium plating by reduction of Cr(VI) to Cr(III), employing various reducing agents. The latter included iron(II) salts, lead(II) salts, methanal, and hydrogen peroxide. All the reduction had performed in an acidic medium. The Cr(III) solutions obtained in the reduction procedures were employed to precipitate silicate pigments (using sodium metasilicate solution and containing mainly chromium(III) silicates) and oxide pigments (using sodium hydroxide and containing chromium(III) oxides). To obtain highly dispersed pigments, aqueous solution of sodium metasilicate (VITROSILICON SA, Poland) and aqueous solution of sodium hydroxide were used. Surface morphology and dispersion of the green pigments were corrected by conducting the precipitation in the presence of hydrophobicity-inducing agents from the group of oxyethylenated compounds (ROKITA SA, Poland). The agents included the following: (1) Rokanol K-7soxyethylenated nonsaturated fatty alcohol, with the formula RO(CH2CH2O)nH, where R ) C16 - C22, nav ) around 7 and (2) Rokafenol N-9snonylphenylpolioxyethyleneglycol ether, of the following formula

where nav ) around 9.7 Procedures and Methods. The reactive system for precipitation of silicates and oxides using sodium metasilicate solution or sodium hydroxide, respectively, consisted of chromium(III) chloride and chromium(III) sulfate. The solutions were obtained by reduction of chromates(VI) or bichromates(VI). Sodium or potassium chromates(VI) represent very frequently encountered industrial (e.g., postgalvanic) waste components, which contain extremely toxic chromium(VI). The studies were conducted in a model system consisting of chemically pure potassium bichromate(VI) and potassium chromate(VI). The agents used to reduce chromates(VI) and bichromates(VI) included iron(II) sulfate, iron(II) chloride, formaldehyde (methanal), and hydrogen peroxide. The redox reactions could be presented as follows:

K2Cr2O7 + 6FeSO4 + 7H2SO4 f K2SO4 + 3Fe2(SO4)3 + Cr2(SO4)3 + 7H2O (1) K2Cr2O7 + 6FeCl2 + 14HCl f 2KCl + 2CrCl3 + 6FeCl3 + 7H2O (2) 2K2CrO4 + 6FeSO4 + 8H2SO4 f 2K2SO4 + 3Fe2(SO4)3 + Cr2(SO4)3 + 8H2O (3) K2Cr2O7 + 3HCHO + 4H2SO4 f K2SO4 + Cr2(SO4)3 + 3HCOOH + 4H2O (4) 4812

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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 20, 2003

K2Cr2O7 + 3H2O2 + 4H2SO4 f K2SO4 + 3O2 + Cr2(SO4)3 + 7H2O (5) Conditions in which the reactions took place were selected so that reduction of Cr(VI) to Cr(III) showed efficiency of 100%. The procedure of silicate precipitation was conducted to select its parameters which warranted a product of optimum physicochemical and morphological properties. The precipitation was conducted in a reactor of 0.5 L capacity, placed in a thermostate bath to control the temperature of the procedure. The reactor was equipped with a top stirrer (1900 rpm). Sodium metasilicate solution or sodium hydroxide solution was dosed to the reactive system at a constant rate using a peristaltic pump. The reactions yielded colorful sediments of silicates or oxides, which then were subjected to filtration under lowered pressure. Samples obtained in this way were dried for 48 h in a stationary drier, at 105 °C. The resulting silicates and oxides were subjected to several tests which aimed at determining all the physicochemical parameters necessary for determining the quality of the products. These parameters included bulk density, capacities to absorb water, dibutyl phthalate, paraffin oil, grain size, particle size distribution, and morphology of particle surface. The bulk densities were determined using a WE-5 electromagnetic volumeter (Poland). The end point of water absorption capacity was noted when an excess of a single drop induced an evident liquefaction of the formed paste. The end point of dibutyl phtahalate or paraffin oil absorption capacities was registered when an excess of a single phthalate or oil drop altered abruptly the consistency of the paste which adhered to a glass plate. Testing of particle size distribution included weighing out an appropriate amount of the tested sample (0.2 ( 0.001 g) and suspending it in a small amount of double distilled water (50 cm3), and then, the examination was performed by DLS technique, using ZetaPlus apparatus (Brookhaven Instruments Co., U.S.A.). For evaluation of morphology of the pigment surface, selected samples were examined in a scanning electron microscope (SEM) to observe the rough surface of the solids, such as fracture planes, surface structure, and pigment agglomerates. In the studies the scanning electron microscope Philips SEM 515 was used. Specific surface areas of the silica powders were determined by N2 adsorption (BET method) using Micrometrics ASAP 2010 instrument. Samples were outgassed at 120 °C for 2 h prior to measurements.

Results and Discussion Principal physicochemical properties of pigments obtained by precipitation from sodium metasilicate solution and solutions originating from reduction of chromates(VI) using iron(II) salts are presented in Table 1. Pigments characterized in the table represented coprecipitated chromium(III) and iron(III) silicates. Silicates precipitated from the system of a bichromate reduced with iron(II) chloride in the medium of H2SO4 exhibited high values of bulk density (even above 400 g‚L-1)ssamples I A and I B. The definitely more favorable parameters were demonstrated by coprecipitated chromium(III) and iron(III) silicates obtained in the system of potassium bichromate reduced by iron(II) chloride in the medium of hydrochloric acid (samples II A and II B of bulk densities slightly above 300 g‚L-1 and the capacity to absorb paraffin oil of 350 cm3‚100 g-1, i.e., higher capacity than those presented by samples I A and I B). The particle size distribution and a SEM microphotograph of chromium(III) silicate coprecipitated with iron(III) silicate

TABLE 1. Physicochemical Properties of Silicate Pigments Precipitated from Postgalvanic Waste after Reduction with Iron(II) Salts

sample no.

temp (°C)

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

IA IB II A II B

80 25 80 25

50 100 300 150

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

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

bulk density (g‚L-1)

average particle diameter (nm)

250 200 350 350

200 200 250 200

403 483 345 313

663 883 519 800

TABLE 2. Physicochemical Properties of Silicate Pigments Precipitated from Postgalvanic Waste after Reduction with Methanal

sample no.

temp (°C)


IV A IV B IV C

80 60 25

150 200 150





350 250 250

250 250 250

301 322 349

1114 988 932

of 414-717 nm, reflecting the presence of primary agglomerates and primary particles (maximum intensity of 100 corresponded to particles of 575 nm in diameter). In the distribution, a band of low intensity could also be noted in the range of 4154-6445 nm which reflected the presence of secondary agglomerates (maximum intensity of 18 corresponded to agglomerates of 5174 nm in diameter). Results of tests characterizing principal physicochemical parameters of chromium(III) silicates obtained by precipitation from solutions of sodium metasilicate and solutions formed following reduction of chromate(VI) by methanal (in the medium of sulfuric acid and at various temperatures) are presented in Table 2. Chromium(III) silicates of a clear green shade were obtained by precipitation from solutions obtained by reduction of potassium bichromate with methanal. The silicates demonstrated mediocre physicochemical parameters: bulk densities in the range of 300-350 g‚L-1 and capacities to absorb paraffin oil of 250-350 cm3‚100 g-1. They exhibited higher capacities to absorb water (150-250 cm3‚100 g-1) as compared to chromium(III) silicates obtained following bichromate reduction with iron(II) salts (showing water absorbing capacity of 50-150 cm3‚100 g-1). Particle size distribution and SEM microphotograph of sample IV A (pure chromium silicate precipitated following chromate reduction with methanal) are presented in Figure 2. The sample contained both primary and secondary agglomerates. As demonstrated by Figure 2b, manifestation intensity of the structures was similar: primary agglomerates fitted the range of 632-795 nm (maximum intensity of 100 corresponded to particles of 709 nm in diameter), while secondary agglomerates formed a band in the range of 19882648 nm (maximum intensity of 99 corresponded to particles of 2229 nm in diameter). The distribution of agglomerate structures was confirmed by the respective SEM microphotograph (Figure 2a).

FIGURE 1. SEM microphotograph (a) and multimodal particle size distribution (b) of chromium(III) silicate (sample I A). (sample I A) are presented in Figure 1. Directly after precipitation the sample demonstrated a relatively uniform character, as proved by the SEM microphotograph (Figure 1a) and by particle size distribution (Figure 1b). In the latter distribution the intense band could be observed in the range

Particle size distribution and SEM microphotograph of chromium silicate precipitated from the silicate system reduced by excess methanal at 25 °C (sample IV C) are presented in Figure 3. As shown, the silicate manifested a very strong tendency to form agglomerates and, first of all, secondary agglomerates of high diameters. The particle size distribution showed that very large secondary agglomerates in the range of 4884-10 000 nm (maximum intensity of 86 corresponded to agglomerates of 7508 nm in diameter) were accompanied by primary agglomerates in the range of 427758 nm (maximum intensity of 100 corresponded to primary agglomerates of 569 nm in diameter). In the distribution, the VOL. 37, NO. 20, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3. Physicochemical Properties of Silicate Pigments Precipitated from Postgalvanic Waste after Reduction with Hydrogen Peroxide

sample no.

temp (°C)


VII A VII B VII C VII D

25 40 60 80

150 150 200 200





250 300 200 300

200 200 200 200

264 379 321 328

702 600 9683 3983

FIGURE 2. SEM microphotograph (a) and multimodal particle size distribution (b) of chromium(III) silicate (sample IV A). band of a very low intensity, typical for primary agglomerates, was also observed in the range of 136-241 nm. Results of studies on principal physicochemical properties of chromium(III) silicates obtained by precipitation from solutions of sodium metasilicate and solutions originating from chromate(VI) reduction using hydrogen peroxide (in the medium of sulfuric acid) are shown in Table 3. Results of studies on principal physicochemical properties of chromium(III) oxides obtained by precipitation from 5 wt 4814

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FIGURE 3. SEM microphotograph (a) and multimodal particle size distribution (b) of chromium(III) silicate (sample IV C). % solution of sodium hydroxide and solutions originating from chromate(VI) reduction using hydrogen peroxide (in the medium of sulfuric acid) are listed in Table 4. Physicochemical parameters of chromium(III) silicates and oxides precipitated from solutions following reduction of potassium bichromate using hydrogen peroxide in an acidic medium are compared in Tables 3 and 4. The parameters were more advantageous for the precipitated chromium oxides (as compared to chromium silicates obtained in similar conditions they exhibited lower bulk density and evidently highest capacity to absorb paraffin

TABLE 4. Physicochemical Properties of Oxide Pigments Precipitated from Postgalvanic Waste after Reduction with Hydrogen Peroxide

sample no.

temp (°C)


VII E VII F VII G VII H

25 80 60 40

100 100 100 100





500 400 350 400

350 300 350 300

265 262 220 244

161 164 451 1058

FIGURE 4. SEM microphotograph (a) and multimodal particle size distribution (b) of chromium(III) silicate (sample VII D). oil). A particularly low capacity to absorb water was shown by all precipitated chromium oxides (samples VII E-VII H - 100 cm3‚100 g-1). Particle size distribution and SEM microphotograph of chromium(III) silicate of an intensely green color, obtained by precipitation from solutions of hydrogen peroxide-reduced chromate (sample VII D), are presented in Figure 4. The particle size distribution (Figure 4b) presented a very intense band of primary agglomerates in the range of 136-737 nm (maximum intensity of 100 corresponded to aggregates of 542 nm in diameter). Unfortunately, the presence of secondary agglomerates of exceptionally high diameters (in the range of 7000-10 000 nm) negatively affected structure and quality of the pigment.

FIGURE 5. SEM microphotograph (a) and multimodal particle size distribution (b) of chromium(III) silicate (sample VII D, heated at 900 °C). The particle size distribution and SEM microphotograph of the same sample VII D heated to 900 °C are presented in Figure 5. Evidently heating of the silicate positively affected morphology, particle size (Figure 5a), and particle size distribution of the pigment (Figure 5b). The particle size distribution documented the presence of primary particles grouped in aggregates of 435-552 nm in size (maximum intensity of 100 corresponded to aggregates of 490 nm in diameter). Very few secondary agglomerates fitted the range of 1430-1815 nm in diameter (maximum intensity of 23 corresponded to secondary agglomerates of 1611 nm in diameter). VOL. 37, NO. 20, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 5. Physicochemical Properties of Silicate and Oxide Pigments Precipitated from Postgalvanic Waste after Reduction with Hydrogen Peroxide (in the Presence of Hydrophobizing Compounds)

sample no.

amount of hydrophobizing agents (wt per wt)

temp (°C)


VII D1 VII G1

1 1

80 60

50 50

VII D2

1

80

50





Rokanol K-7 400 750

300 400

268 250

738 498

Rokafenol N-9 500

600

232

1220

FIGURE 7. Multimodal particle size distribution of chromium(III) silicate (sample VII D1).

FIGURE 6. SEM microphotograph (a) and multimodal particle size distribution (b) of chromium(III) oxide (sample VII F). Particle size distribution and SEM microphotograph of chromium(III) oxide precipitated by reaction of excess of hydrogen peroxide with a potassium bichromate solution are presented in Figure 6. In the particle size distribution (Figure 6b) secondary agglomerates were practically absent. Out of three bands, two bands could be ascribed to the presence of primary particles including the highly intense band in the range of 33-144 nm (maximum intensity of 100 corresponded to primary particles of 115 nm in diameter), 4816

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while the band of poor intensity represented the nanometric size particles of 24-30 nm in diameter. The band ascribed to the presence of primary agglomerates of very small diameters fitted the diameter range of 284-446 nm (maximum intensity of 87 corresponded to agglomerate diameter of 356 nm). No secondary agglomerates were present. Results of studies on principal physicochemical properties of chromium(III) oxides and silicates obtained by precipitation in the presence of hydrophobicity-inducing agents from solutions resulting from chromate(VI) reduction using hydrogen peroxide are listed in Table 5. Application of the hydrophobicity-inducing agents in the course of precipitation improved physicochemical properties of both chromium(III) oxides and chromium(III) silicates. Their presence resulted in clearly lowered bulk densities (even below 250 g‚L-1 in the case of chromium(III) silicate - sample VII D2) and augmented capacities to absorb paraffin oil (to as much as 750 cm3‚100 g-1 for the oxide sample VII G1) and dibutyl phthalate. Particle size distribution of chromium(III) silicate precipitated in the presence of Rokanol K-7 (sample VII D1) is presented in Figure 7. The silicate manifested good morphological characteristics, which was confirmed by particle size distribution of the so obtained chromium silicate. The particle size distribution (Figure 7) demonstrated two bands of closely similar intensities. The more intense band corresponded to primary particles and primary agglomerates, and it fitted the diameter range of 314-455 nm (maximum intensity of 100 corresponded to particles of 364 nm in diameter). The band reflecting the presence of secondary agglomerates of very low diameters occupied the range of 1378-1995 nm (maximum intensity of 87 corresponded to particles of 1598 nm in diameter). Particle size distribution and SEM microphotograph of chromium oxide (sample VII G1) precipitated in the same conditions to those applied for chromium silicate (sample VII D1) are shown in Figure 8. The oxide exhibited very similar morphological and agglomerate characteristics. The

FIGURE 8. SEM microphotograph (a) and multimodal particle size distribution (b) of chromium(III) oxide (sample VII G1). particle size distribution (Figure 8b) provided evidence for the presence of both aggregates and agglomerates. Primary particles and aggregates were present within the diameter range of 376-459 nm (maximum intensity of 63 corresponded to agglomerates of 437 nm in diameter). Secondary agglomerates of very low diameters formed the more intense band within the range of 1071-1374 nm (maximum intensity of 100 corresponded to secondary agglomerates of 1244 nm in diameter). Particle size distribution and SEM microphotograph of chromium silicate precipitated in the presence of the other hydrophobicity-inducing agent (Rokafenol N-9) are presented in Figure 9. Chromium silicate obtained in the presence of the agent (sample VII D2) also demonstrated positive physicochemical properties. The particle size distribution (Figure 9b) presented two bands of different intensities. The intense band in the diameter range of 283-382 nm (maximum intensity of 100 corresponded to particles of 319 nm in diameter) could be ascribed to both primary particles and primary agglomerates. On the other hand, secondary agglomerates formed the less intense band representing diameters at the transition between primary and secondary agglomerates, in the range of 939-1268 nm (maximum

FIGURE 9. SEM microphotograph (a) and multimodal particle size distribution (b) of chromium(III) silicate (sample VII D2).

SCHEME 1

intensity of 78 corresponded to agglomerates of 1124 nm in diameter). The advantageous effect of oxyethylenated nonsaturated fatty alcohols, involving the decreased tendency of silicate and oxide pigment particles to form large secondary agglomerates, could be explained as follows. The agents become adsorbed on the formed pigment particles due to chemical reaction between hydroxyl groups (tSi-OH, dCr-OH) present at the surface of chromium(III) silicate and alkoxy groups of the alcohol molecule. VOL. 37, NO. 20, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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SCHEME 2

Mechanism of the reaction could be formulated as shown in Scheme 1. A decreased number of hydroxyl groups at the chromium(III) silicate surface following modification with oxyethylenated alcohols results in decreased interactions between primary particles of the silicate and, thus, inhibits binding of the particles into larger agglomerate structures. The modification blocks interactions between hydroxyl groups of the neighboring pigment particles due to formation of hydrogen bonds. Hydroxyl groups (tSi-OH and dCr-OH) are responsible for formation of primary and secondary agglomerates, the presence of which could have been noted in particle size distributions. As indicated by analysis of particle size distributions, destruction of agglomerate structures in pigments results both from modification of their surface with oxyethylenated alcohols and from their heating. The latter in particular (at 900 °C) has resulted in an almost complete disappearance of surface hydroxyl groups and in complete absence of secondary agglomerates. On the other hand, the modification has only decreased the tendency to form secondary structures but has failed to eliminate them completely. The mechanism could be illustrated as shown in Scheme 2. Specific surface areas of chromium(III) silicates are relatively high, fitting the range of 180-350 m2‚g-1. On the other hand, precipitated chromium(III) oxides exhibit definitely lower specific surface areas (in the range of 64-76 m2‚g-1). Chromium(III) silicate, sample I A precipitated from postgalvanic waste following reduction with iron salt, exhibits a gel structure which confirms the exceptionally high specific surface area (354 m2‚g-1) and its high bulk density (above 400 g‚L-1). The silicate precipitated in the presence of hydrophobicity-inducing agent, a nonionic surfactant, exhibits specific surface area similar to highly dispersed pigments (210 m2‚g-1) and a relatively low bulk density (232 g‚L-1).

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Acknowledgments The work was supported by the Poznan University of Technology Research Grant BW 32/001/2002.

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Received for review October 8, 2002. Revised manuscript received July 25, 2003. Accepted July 25, 2003. ES020973U

Highly Dispersed Green Silicate and Oxide ... - ACS Publications

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