1. Introduction Environmental concerns require treatment of industrial wastes [1]. Economic and ecological advantages make waste glass recycling attractive [2]. Recycling glass saves raw materials, conserves natural resources, and decreases energy consumption so less CO2 is emitted. Landfill volume is also reduced [3].
Fly ash utilization is more recent, as it was formerly released into the atmosphere. Current control measures require that it be captured and disposed in landfills [4,5]. This may result in groundwater contamination so the fly ash must be made non-hazardous [6], usually by stabilization in cement [7,8]. An alternative is vitrification, which reduces the volume, destroys residual organics, and immobilizes heavy metals [9]. Its economic disadvantages – it is energy and capital intensive - may be counterbalanced by conversion of the resulting glasses into marketable products: glass ceramics [10], glazes [11], glass foams [12-14], glass or glass-ceramic matrix composites [15,16], or glass fibers [17].
Fly ash is readily available at low or even negative cost. Its polarity and high specific surface make it a
desirable adsorbent for Cr6+ removal from water, but the chromium must then be immobilized due to its high pollution potential. Vitrification is feasible, but the >1400°C required is economically unacceptable. Using common waste glasses and borax with the fly ash lowers the melting temperature by 200°C, making the process more economic.
This paper presents the vitrification of chromium-adsorbed fly ash in waste glass. Mixtures containing common waste glasses, fly ash, and borax flux were used to prepare glasses of practical and economic interest. Two different glass : borax : fly ash weight ratios were examined to establish the minimum borax amount required to lower the melting temperature. Further decreasing the borax yields incompletely fused glasses without practical interest.
2. Experimental procedure
2.1. Sample preparationThe waste glasses were tableware, bottle glass and window panes. Their compositions, determined by X-ray
Central European Journal of Chemistry
New vitreous matrix for chromium waste immobilization
Politehnica University Timisoara, Faculty of Industrial Chemistry and Environmental Engineering, 300223, Timisoara, ROMANIA
Ioan Lazău, Cosmin Vancea*
RICCCE 18
Common waste glasses (window, bottle glass or tableware) with fly ash form a glass matrix for chromium waste immobilization. Soluble chromium from residual waters was adsorbed on fly ash; the resulting solid contained 23.7% Cr6+. The three glass wastes, chromium-containing fly ash, and borax were used to make glasses in weight ratios waste glass : borax : fly ash of 1 : 1 : 1 and 1.5 : 0.5 : 1. The hydrolytic stability ranged from 18.46 to 28.13 µg g-1 soluble Na2O, qualifying them in the HGB1 class. The chemical stability, characterized by the dissolution rate, was 0.011-0.077 µg cm-2 h-1, depending on the glass composition and the aggressive medium pH. The chromium leachability is influnced by the glass composition and the pH of the leaching solution, ranging between 0-0.015% of the total chromium. Chromium waste vitrification is a viabile solution with multiple economic advantages.
Cent. Eur. J. Chem.DOI: 10.2478/s11532-014-0509-3
New vitreous matrix for chromium waste immobilization
fluorescence using a Niton XL 3 analyzer, are presented in Table 1. The composition of the fly ash from a thermal power plant is shown in Table 2.
The fly ash, having a BET surface of 6.3 m2 g-1, first adsorbed soluble chromium from waste water containing 2.4 g L-1 Cr6+. The adsorption conditions were pH = 5, shaking time = 60 minutes, solid : liquid = 1 : 100. The chromium removal efficiency was > 99% and the resulting solid contained 23.7% Cr6+.
The waste glass powders (< 100 µm), the fly ash containing adsorbed Cr6+, and borax were weighed, mixed and ground in a porcelain mortar. The mixtures were melted in an electric furnace for 90 min at 1200°C. The melt was press-quenched between two stainless steel blocks and annealed to remove stress.
The glass : borax : fly ash weight ratios are in Table 3 and the product glass compositions are in Table 4.
2.2. Characterization methodsThe product glass densities were measured by the pycnometric method using isopropanol.
The hydrolytic stability was determined following ISO 719-1985. Two grams of each glass (< 500 µm) in 50 mL de-ionized water were kept at 98°C for 60 min. 25 mL of the resulting solution was titrated against 0.01 M HCl. Its volume gave the equivalent Na2O extracted.
Chromium immobilization was evaluated from its leaching in the US EPA Extraction Procedure Toxicity Test [18]. Three extraction media were prepared by adding concentrated aqueous ammonia to 2.5% v/v glacial acetic acid in water until the pH reached 5.5, 7.0 or 8.5. The pH was measured using an E-500 digital pH meter. 2 g of each product was shaken with 250 mL buffer for 1, 14 or 28 days at 20±2°C, then filtered. The chromium in the filtrate was measured using a Varian SpectrAA 110 spectrometer with Mark 7 flame atomization system.
The chemical stability was determined under the same conditions as the chromium leachabilty [19]. It is expressed by the dissolution rate:
[µg cm-2 h-1] (1)
where Dm is the weight loss at time t and S the sample surface area.
3. Results and discussion
3.1. Glass densitiesProduct density (reflected in the molar volume) varies with composition and coordination geometry. The composition parameter R and the molar volume VM were calculated from
Oxide Composition (weight %)SiO2 Na2O K2O MgO CaO Al2O3 Fe2O3
Fly ash 46.20 6.23 4.17 3.30 8.60 23.20 8.10
Table 3. Batch composition (weight ratios).
Sample Waste glass Borax Fly ash Tableware Bottle
glassWindow
pane
P1 1 - - 1 1
P2 - 1 - 1 1
P3 - - 1 1 1
P4 1.5 - - 0.5 1
P5 - 1.5 - 0.5 1
P6 - - 1.5 0.5 1
I. Lazău, C. Vancea
x is the mole fraction of alkali, alkaline-earth, or other modifying oxide and ( 1 - x) the mole fraction of glass former.
ρMVM = (3)
M represents the molecular weight of the glass composition and ρ the density [20].
The product densities, molar volumes and composition parameters are summarized in Table 5.
The higher proportion of waste glass in P4-P6 (giving smaller R) forms more compact glass networks with smaller molar volumes and higher densities.
3.2. Product hydrolytic stability The hydrolytic stability of the products is given in Fig. 1.
All fit ISO 719 hydrolytic class HGB1, having very good water stability. For P1-P3, 25.97< Na+<28.13 µg g-1 was leached and between 18.31-23.91 µg g-1 for P4-P6. The alkali leachability depends on the Na2O content; the lower borax content in P4-P6 improves hydrolytic stability. 3.3. Product chemical stabilityFig. 2 shows the chemical stability, given by the dissolution rate in aggressive solutions.
The dissolution rates range between 0.011-0.077 µg cm-2 h-1, depending on the product glass composition and pH. The products are most sensitive to alkaline attack, with dissolution rates between 0.062-0.077 µg cm-2 h-1 after 28 days. Acid has a moderate effect, giving rates between 0.017-0.026 µg cm-2 h-1. The neutral medium has a very low effect (Dr < 0.019 µg cm-2 h-1). Regardless of pH and attack time products P4-P6 showed higher resistances, confirming the negative effect of B2O3.
There is good correlation between density and product chemical stability. Products P4-P6, containing
Figure 1. Product Na2O content and hydrolytic stability.
New vitreous matrix for chromium waste immobilization
more SiO2, have more compact networks and higher densities. These resist acid and neutral medium attack better than more open networks.
3.4. Chromium immobilization in the product glassThe chromium leached from the products is shown in Fig. 3
The extracted chromium was between 0 and 0.015% for all products. More chromium was extracted by the alkaline medium at all times. The acid and neutral media leach very little chromium (< 0.002%) after 14 days, and under 0.004% after 28 days. The P4-P6 products have superior chromium immobilization as the more open
networks of P1-P3 allow it to be extracted more easily. The lower chromium extraction, regardless of pH and attack time, for products containing more waste glass and less borax suggests that decreasing borax improves chromium retention.
4. ConclusionsThe chromium in fly ash used to remove Cr+6 from wastewater may be immobilized by vitrification using waste glass and borax. This provides multiple economic advantages.
(a)
(b)
(c)
Figure 2. Chemical stability of the products.
I. Lazău, C. Vancea
The product glasses range between 2.527-2.555 g cm-3, comparable to industrial glasses. There was good correlation between the glass composition, molar volume, and glass density. Increasing the proportion of waste glass increases the SiO2 content, the vitreous network compactness, and the density.
The hydrolytic stability of the products qualifies them as HGB1 glasses, having very good resistance to water. The correlation between product glass density and hydrolytic stability was good.
All the glasses produced had very good chemical stability. Their dissolution rates were less than 0.077 µg cm-2 h-1, making them suitable for industrial use.
The more compact networks containing more waste glass and less borax are more resistant to acid and base attack.
The products immobilized chromium well; less than 0.08% was leached.
The most stable products were P4-P6, produced with less borax. These gave the least alkaline extraction (<24.00 µg g-1 after 28 days), the best chromium immobilization (leachate < 0.012% total Cr) and the highest chemical stability (dissolution < 0.065 µg cm-2 h-1).
The best waste glass was tableware, products P1 and P4 having slightly higher density, chemical stability and chromium retention, but the differences among the waste glasses are not significant.
(a)
(b)
(c)
Figure 3. Total chromium ions leached from the products.
New vitreous matrix for chromium waste immobilization
T. Basegio, A.P. Beck Leão, A.M. Bernardes, C.P. Bergmann, J. Hazard. Mater. 165, 604 (2009)P. Colombo, G. Brusatin, E. Bernardo, G. Scarinci, Curr. Opin. Solid State Mater. Sci. 7(3), 225 (2003)R. Gutman, Glastech. Ber. Glass Sci. Technol. 69(9), 223 (1996)K. Al-Zboona, M.S. Al-Harahshehb, F.B. Hania, J. Hazard. Mater. 188, 414 (2011)L. Barbieri, A.C. Bonamartini, I. Lancellotti, J. Eur. Ceram. Soc. 20, 2477 (2000)P. Kavouras, G. Kaimakamis, Th.A. Ioannidis, Th. Kehagias, Ph. Komninou,S. Kokkou, E. Pavlidou, I. Antonopoulos, M. Sofoniou, A. Zouboulis, C.P. Hadjiantoniou, G. Nouet, A. Prakouras, Th. Karakostas, Waste Manage 23, 361 (2003)M.J. McCarthy, R.K. Dhir, Fuel 84, 1423 (2005)A. Duran-Herrera, C.A. Juarez, P. Valdez, D.P. Bentz, Cem. Concr. Compos. 33(1), 39 (2011)Vitrification technologies for treatment of Hazardous and radioactive wastes, Handbook, EPA/625/R-92/002 (US EPA, Cincinnati, Ohio, 1992)E. Bernardo, M. Varrasso, F. Cadamuro, S. Hreglich, J. Non-Cryst. Solids 352, 4017 (2006)V. Dima, M. Eftimie, A. Volceanov, A. Melinescu, A. Petrescu, M. Ionescu, N. Argintaru, N. Ziman, D. Tita, Rom. J. Mater. 4, 321 (2006)
V. Ducman, M. Kovacevic, Key Eng. Mater. 132–136, 2264 (1997)I. Lazău, C. Vancea, Rom. J. Mater. 42(3), 270 (2012)C. Vancea, G. Moşoarcă, Proc. of The Fourth Edition of the Symposium with International Participation “New trends and strategies in the chemistry of advanced materials” (Timisoara, Romania, 2010) ISSN 2065-0760 P.A. Trusty, A.R. Boccaccini, Appl. Composite Mater. 5(4), 207 (1998)A.R. Boccaccini, M. Bucker, J. Bossert, K. Marszalek, Waste Manage 17, 39 (1997)G. Scarinci, G. Brusatin, L. Barbieri, A. Corradi, I. Lancellotti, P.Colombo, S. Hreglich, R. Dall’Igna, J. Eur. Ceram. Soc. 20, 2485 (2000)US EPA, Extraction procedure toxicity test, in: Stabilization/Solidification of CERCLA and RCRAWastes. US EPA625/6-89/022 (US EPA, Cincinnati, Ohio, 1986)I. Lazău, C. Vancea, G. Moşoarcă, Rom. J. Mater. 43(1), 68 (2013)M. Altaf, M.A. Chaudhry, J. Mod. Phys. 1, 201 (2010)
