Quantification of the Adsorption Capacity of Fly Ash - Industrial

(13) The fly ash iodine number, however, followed the capacity trend better than the fly ... Adsorption is a combination of many processes where the p...
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Quantification of the Adsorption Capacity of Fly Ash Zeyad T. Ahmed* and David W. Hand Department of Civil & Environmental Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931, United States ABSTRACT: Fly ash adsorption capacity is a critical property for the beneficial utilization of fly ash; although this capacity could be favorable for some applications, it may not favor the use of fly ash for other applications such as cement and concrete production. In both cases the lack of fly ash adsorption capacity quantification tools and procedures limits the beneficial utilization of fly ash. This study compared the results of the currently available fly ash adsorption capacity tests to a newly developed test for the direct measurement of the adsorption capacity of fly ash. The study showed, as previous work did, that the commonly used thermogravimetric method, called loss on ignition (LOI), is not a good measure for quantifying the adsorption capacity of fly ash, the foam index test was more successful as an indicator rather than a measurement of the adsorption capacity, and the fly ash iodine number successfully quantified the adsorption capacity of fly ash and correlated very well with the direct measurement of the air entraining admixture (AEA) adsorption by fly ash. In addition, this study showed that the relationship between the fly ash iodine number and the AEA adsorption capacity measured via the direct adsorption isotherms can be used to determine the quantity of AEA adsorbed by the fly ash using the fly ash iodine number of the fly ash of interest. This AEA quantity represents the AEA dosage adjustment required to compensate for the adsorption of the AEA by fly ash in concrete.

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INTRODUCTION Coal fly ash is a solid waste generated as byproduct from coal burning, more than 70 million tons of fly ash is produced in the United States every year. Only about 40% of this fly ash is beneficially utilized,1 and the remainder is handled as a costly solid waste. The main beneficial use of fly ash is in the cement and concrete industry; fly ash improves many concrete features and reduces the amount of cement required for the concrete mixture, making fly ash a promising sustainable material for infrastructure. Fly ash is also used in numerous applications such as low-cost adsorbents for removal of methylene blue and humic acid from aqueous solutions2 and removal of heavy metal ions from municipal solid waste leachate.3 The utilization of fly ash not only reduces the environmental impact of coal burning but it also reduces the carbon foot print of concrete and provides a low cost treatment option. AEAs are organic surfactants used in concrete to entrain about 4−6% air void content to improve the duribility of the concrete and increase the concrete resistance to the freeze− thaw cycles in climates with wide temperature variations. AEAs interact with cement and fly ash in a complex manner due to the complex composition of AEAs and the presence of various types of minerals in the concrete mixture. Residual carbon in the fly ash adsorbs some of the AEAs reducing their availablitily to function in the mixture, consequently, the hardened concrete fails to meet the air void content requirments. For other fly ash uses where the adsorption capacity is favorable, the lack of standard adsorption capacity testing procedure forces users to deal with the unknown capacity of fly ash and experimentally examine all the possibly available fly ashes to assess their suitability. It is vital to develop a simple and accurate test for the quantification and characterization of fly ashes adsorption capacity, a test similar to the iodine number test commonly used by activated carbon producers to characterize activated carbons. © 2014 American Chemical Society

Fly ash utilization is limited and challenged by the uncertainty of the fly ash quality, especially the adsorption capacity of fly ash and the lack of adequate adsorption capacity quantification tools and procedures. Developing reliable fly ash adsorption capacity quantification methods is crucial to increase the confidence in the fly ash quality and facilitate further utilization of ashes otherwise considered risky for use. Various procedures of loss on ignition (LOI) and foam index tests have been used to assess the adsorption capacity of fly ash. However, both tests do not provide a direct or accurate measurement of the adsorption capacity. The fly ash iodine number test4,5 provides a precise measurement for the adsorption capacity of fly ash, measured as mg of iodine adsorbed by a gram of fly ash. This test is extremely important for the specification and characterization of fly ash and can be used to determine the suitability of fly ash for use in concrete or as a low cost adsorbent. However, it does not directly quantify the adsorption of AEAs by the fly ash. The direct adsorption isotherm test measures the adsorption capacity of fly ash for the specific AEA used in the test.4,6,7This test provides a direct and accurate measurement of the amount of AEA adsorbed by the fly ash in the concrete mixture, and the results of this test can be used to determine the AEA dosage adjustment required to compensate for the AEA volume adsorbed by the fly ash. In this study, all four tests were performed on eight fly ash specimens of varying carbon contents. The LOI, foam index, and fly ash iodine number tests results were compared to the direct AEA adsorption measurements made using the recently developed combined adsorption isotherm test, and conclusions Received: Revised: Accepted: Published: 6985

February 2, 2014 April 7, 2014 April 9, 2014 April 9, 2014 dx.doi.org/10.1021/ie500484d | Ind. Eng. Chem. Res. 2014, 53, 6985−6989

Industrial & Engineering Chemistry Research

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Table 1. Properties of the Fly Ash Specimens, wt % fly ash ID

FA1

FA7

FA8

FA10

FA15

FA20

FA39

FA40

SiO2 Al2O3 Fe2O3 Total: SiO2, Al2O3, Fe2O3 CaO SO3 MgO LOI Alkali

60.1 29.9 2.7 92.7 0.9 NA NA 0.87 0.61

53.94 27.66 8.29 89.89 1.45 0.08 1.15 2.25 0.64

60.85 25.7 4.66 91.2 3.46 0.29 1.12 0.17 0.69

45.95 23.61 22.31 91.88 1.28 0.77 0.99 1.26 0.77

58.92 16.17 4.71 79.81 10.24 0.86 3.13 1.5 0.73

44.81 23.08 9.51 77.4 13.58 0.96 2.97 0.39 0.89

39.6 20 12.7 72.3 9.1 1.1 2.28 10.49 NA

53.9 26.3 6.24 86.4 4.0 0.2 0.86 3.35 NA

Figure 1. LOI correlation to fly ash iodine number, foam index, and fly ash capacity measured using the direct adsorption isotherms.

of AEA used is then determined and called the foam index test, expressed commonly as mL AEA/g fly ash. It is very important to emphasize that the foam index test is a dynamic test, and it is not based on equilibrium or near equilibrium conditions. Therefore, every single detail in the procedure especially the total test run time may impact the test results. Accordingly, the procedure reported by Watkins13was used in this study because it accounts for the total test time and it addresses the dynamic nature of the test. The Fly Ash Iodine Number Test. The fly ash iodine number test, as reported by Ahmed,4 accurately measures the adsorption capacity of fly ash using iodine as a standard adsorbate. A similar concept is also widely used to determine the adsorption capacity of activated carbon14and carbon black.15 The fly ash iodine number test requires fly ash treatment before equilibration with iodine. The treatment removes the sulfur, which interferes with the iodine concentration measurement, and acidifies the fly ash to prevent the conversion of iodine to iodide. The fly ash iodine number test does not utilize AEAs in the measurement; therefore, it does not measure the adsorption of AEAs by fly ash. However, the test provides an accurate and extremely useful tool for the specification of ashes based on their adsorption capacities; this is also important for almost all other beneficial uses of fly ash. In addition, based on comparison to LOI and foam index tests, it was determined in a previous work that the fly ash iodine number test is able to represent the fly ash adsorption capacity better than the LOI test, especially for low carbon fly ashes usually used in concrete applications.4,5 The Direct Adsorption Isotherms Test. The direct adsorption isotherm test is a direct measurement of the

were drawn regarding the ability of these tests to represent the adsorption capacity of fly ash.

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MATERIALS AND METHODS The Loss on Ignition (LOI) Test. The loss on ignition test (LOI) is used to estimate the carbon content of fly ash; the carbon content is proportional to the adsorption capacity because the unburned carbon is responsible for the AEA adsorption capacity of fly ash. In order to determine LOI according to ASTM C311,8 a gram of fly ash is burned in a muffle furnace for 15 min at 750 ± 50 °C. Several reported modifications of the test include increasing the time to 2 h,9 3 h,10 preheating at 140 °C, and burning for several hours.11 Many factors can contribute to the weight loss of fly ash after burning. These include decomposition of carbonate (CaCO3) and portlandite Ca(OH)2, removal of water bound in clay minerals, and combustion of carbon. Depending on the composition of fly ash, the error between carbon content and LOI can be as low as 1%12 or as high as 75%.11In this study, the LOI tests were performed according to ASTM C3118with the burning time extended to 5 h. The Foam Index Test. The foam index test is a dynamic test used to determine the relative amount of AEA required in concrete containing fly ash. The foam index test is a simple titration procedure in which a solution of a fly ash and water or a fly ash, water, and cement is titrated incrementally in a specified dose with diluted AEA. After each addition the container is closed, agitated for certain time, and opened, and the foam formation at the surface of the solution is observed. The procedure is repeated until the foam covers the container surface and remains stable for a prespecified time. The volume 6986

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Figure 2. The relationship between the fly ash iodine number, the foam index, and the capacity of fly ash at 0.4% vol. MB VR concentration measured using direct adsorption isotherms.

This agrees with the findings of Brown and Dykstra11 who reported that depending on the composition of fly ash, use of LOI to measure the carbon content can be as accurate as 99% or can have up to 75% error between carbon content and LOI. Based on these results of LOI correlation to adsorption and the correlation of LOI to iodine number and foam index presented in previous work,5 the researchers suggest that LOI is not a reliable test for low carbon fly ash because the noncarbon mass loss might be significant. However, LOI is a good indicator of relative adsorption capacity for high carbon fly ashes because the loss in mass due to burning carbon is significantly higher than the mass loss by the other mechanisms. Powdered activated carbon (PAC) is increasingly being used to treat the stack emissions in power plants for pollution control. This causes a small increase in the LOI of the fly ash, as only small amounts of PAC are needed to achieve the treatment goals, and a high increase in the adsorption capacity, as the PAC has a very high adsorption capacity. In this case, although the LOI test may be able to represent the carbon content, it should not be used as an indication of the fly ash adsorption capacity, because the LOI test measures the mass of carbon, and a gram of nonactivated, low adsorption capacity fly ash carbon is the same as a gram of high adsorption capacity PAC. However, the difference in adsorption capacity between the two carbon types can be reflected in the foam index test and can be accurately measured by using the iodine number test and the direct adsorption isotherms. Based on these findings and facts the LOI test should not be used for the quantification of the adsorption capacity of fly ash, especially for PAC containing fly ash. The Correlation among the Adsorption Capacity Tests. The relationships between the direct adsorption isotherms, the fly ash iodine numbers, and the foam index results of the eight fly ash specimens is illustrated in Figure 2. The results showed that although it is conceptually different, foam index test results showed a clear trend, following the fly ash iodine number, and correlating to the AEA adsorption capacity measured by the direct adsorption isotherms. In Figure 2, the capacity measured by direct adsorption isotherms was used as the base of comparison and correlated to both the fly ash iodine number and the foam index tests. The direct adsorption isotherm test is the only direct method for the measurement of AEA adsorption by fly ash, as compared to the indirect measurement of both fly ash iodine number and foam

adsorption capacity of AEA materials by fly ash. The test provides an accurate measurement of the amount of AEA adsorbed by fly ash on a volume to mass basis. AEAs interact with fly ash and cement in a complex manner; a separate cement and fly ash isotherms test was developed and tested4,6 to determine the amount of a given AEA chemisorbed by cement and physically adsorbed by fly ash. The combined adsorption isotherms test, a simpler and more accurate procedure, was later developed by combining the cement and fly ash into one combined adsorption isotherm test.4,7 Since AEA materials are composed of more than one organic compound, the chemical oxygen demand (COD) test is used for the measurement of the AEA concentrations. A blank of an AEA solution and cement determines the AEA concentration available for adsorption by fly ash, and then various masses of fly ash are utilized to determine the isotherm points and the adsorption capacity of the fly ash for the specific AEA utilized. All direct adsorption isotherm tests used in this study were performed according to Ahmed.4,7 Fly Ash Specimens. All tests described in the previous section were performed on eight specimens of coal fly ash from several sources; Table 1 illustrates the properties of these fly ash specimens. MB VR, a Vinsol resin admixture manufactured by BASF, was utilized for the foam index test and for the direct adsorption isotherms.

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RESULTS AND DISCUSSIONS LOI Correlation to the Adsorption Tests. The relationship between the fly ash iodine number test, the foam index test, the fly ash capacity measured by the direct adsorption isotherms, and LOI for 8 fly ashes are shown in Figure 1. The fly ash capacities measured using the direct adsorption isotherms were taken at an AEA concentration of 0.4% vol. For all three adsorption testing methods, Figure 1 shows that adsorption capacity somewhat correlates to LOI primarily at higher LOI values. However, the figure also shows that LOI does not provide a good correlation to adsorption capacity of fly ash at low LOI values. This is obvious in the case of FA15 and FA7. All three adsorption tests showed that FA7 has less adsorption capacity than FA15 in contrary to what LOI suggests since FA7 has LOI of 2.25 and FA15 has LOI of 1.5%. The same is true for FA8 and FA20 where FA8 has more capacity than FA20, as foam index, fly ash iodine number, and adsorption isotherms showed, although FA20 has higher LOI. 6987

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Figure 3. The correlation between the fly ash iodine number and various concentrations of the Vinsol resin admixture MB VR.

possible under any circumstances that two very different solutes exhibit the same adsorption behavior unless they are chemically and physically identical. Iodine is a single solute and has a much smaller molecular size than the complex organics that AEA solutions are composed of; therefore, they have different adsorbabilities. Adsorption is a combination of many processes where the pore size (Micro versus Macro) governs the adsorption capacity. Therefore, the capacity determined via the direct adsorption isotherms will always be different, even trend-wise from the fly ash iodine number. This may explain why in some instances (as seen in Figure 2) the fly ash iodine number does not respond proportionally to the AEA adsorption capacity increase. Inconsistencies may also result from the experimental error, as well as the power regression used to represent the isotherm in both tests (Freundlich equation). Despite all of the above, there is a very good correlation between both tests, and the adsorption capacity of the AEA can be determined form the correlation using the fly ash iodine number. The AEA dosage adjustment predictions determined in this manner can be fairly accurate, especially when considering how industry currently applies the foam index test results to obtain AEA dosage adjustments. In summary, the relationship between the fly ash iodine number and the adsorption capacity of the AEA presented in Figure 2, describes the relative adsorbability of an AEA to iodine number. Consequently, if the iodine number is known for a given fly ash, an AEA dosage can be determined. This relationship can be used by the concrete industry to specify AEA dosages for all AEAs if correlations like the one in Figure 2 are developed for all the known AEAs. More research is needed to discern the region where the iodine number is very low and the fly ash still shows some AEA background capacity, it is expected that various AEAs respond differently to the presence of fly ash, even at very low carbon fly ash. This may be attributed to the composition of the AEA and the presence of highly adsorbable compounds in the AEA. Performing these tests on a larger set of fly ash specimens is also needed to increase the certainty and the confidence level of the correlation. Also inconsistencies in AEA composition may occur for the same AEA in various production batches, and this has to be considered and studied although it is not expected to affect the correlation significantly.

index tests. The foam index test results shows more data scatter than the fly ash iodine number test results when compared to the coefficient of determination (R2) (e.g., 0.98 for the fly ash iodine number vs 0.93 for the foam index). The Foam Index and the Fly Ash Capacity. The foam index test is fundamentally different from the isotherm methodology used in the fly ash iodine number and the direct adsorption isotherm tests. In isotherm experiments, fly ash is equilibrated with a high iodine or AEA initial solute concentrations, and the solute concentration decreases with time due to adsorption until equilibrium is established between the solid phase and liquid phase solute concentration. In the foam index test the system begins with no AEA, and AEA is introduced in an incremental manner, consequently the AEA is adsorbed at very low concentrations in the beginning, then AEA concentration increases dropwise with each cycle until metastable foam is formed. In addition, the foam index test is dynamic, and if the system was left to equilibrate the foam may disappear because adsorption is still taking place and equilibrium is not achieved. On the other hand, isotherms are based on equilibrium or near equilibrium state where no significant change in the solute concentration takes place with time. The foam index test provides a better representation for the adsorption capacity of fly ash than the LOI test, especially if the improved test procedure is used where the total time of the test is considered.13 The fly ash iodine number, however, followed the capacity trend better than the fly ash foam index. This can be seen in the case of FA10 and FA40, where unlike the fly ash iodine number the foam index decreased although the capacity increased. In addition, the grouping of fly ash iodine number test seems to better correlate to adsorption capacity than the foam index test, which can be attributed to the low accuracy, low resolution, and the subjective nature of the foam index test. The foam index test provides a good indication of the adsorption capacity of fly ash; however, it should not be used to measure the adsorption capacity because it does not measure the adsorption capacity. The Fly Ash Iodine Number and the Fly Ash Capacity. Although both are isotherms, and both measure the adsorption capacity, the direct adsorption isotherm is different from the fly ash iodine number because the former utilizes AEA as the adsorbate and the later utilizes iodine as adsorbate. It is not 6988

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The capacity of fly ash for any specific AEA is a function of the concentration of the AEA (4). The correlation between the fly ash iodine number and the AEA presented in Figure 2 was developed using the capacity of MB VR at a concentration of 0.4% volume. Therefore, it is only applicable if this AEA is used at this specific concentration. However, AEAs may be used in various concentrations, and in order to develop a useful graph, it is important to develop a correlation of fly ash iodine number to the various concentrations of AEA used in practice. Figure 3 displays the relationship between iodine number versus AEA concentration for various concentrations of Vinsol resin admixture MB VR. Similar relationships can be developed for other AEAs to be able to predict the AEA capacities for all the different AEAs used in industry. The fly ash capacity for the specific AEA concentration can be determined form the intersection of the Y-axis (the iodine number) with the AEA graph that represents the desired AEA concentration in the concrete mixture (the curves); the value at the x-axis of the intersection point represents the capacity of this fly ash for the AEA at the specified concentration. For example, if a fly ash with iodine number of 1 mg/g is to be used with MB VR at a concentration of 0.6% volume, the y-axis intersects with the curve in the middle (the 0.6% MB VR) at a capacity of 0.01 mL/g. This capacity number multiplied by the mass of fly ash per unit volume of concrete gives the total amount of MB VR required per unit volume of concrete to compensate for the AEA adsorption.

ACKNOWLEDGMENTS Significant part of this study was done under research project 18-13 funded by the National Cooperative Highway Research Program (NCHRP), the Transportation Research Board of The National Academies. The authors would like to thank NCHRP for their cooperation and support. The researchers would also like to express their thanks to the Department of Civil and Environmental Engineering at Michigan Technological University for hosting the research, BASF’s Construction Chemicals for their support and providing the AEA used in this study, and to Dr. Melanie Watkins and Mrs. Angela Keranen for their help and continuous support.

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REFERENCES

(1) American Coal Ash Association. Coal Combustion Products Production & Use Statistics. http://acaa.affiniscape.com/associations/ 8003/files/1966-2010_FlyAsh_Prod_and_Use_Charts.pdf (accessed April 7, 2014). (2) Wang, S.; Ma, Q.; Zhu, Z. H. Characteristics of Coal Fly Ash and Adsorption Application. Fuel 2008, 87, 3469−3473. (3) Mohan, S.; Gandhimathi, R. Removal of Heavy Metal Ions from Municipal Solid Waste Leachate Using Coal Fly Ash. J. Hazard. Mater. 2009, 169, 351−359. (4) Ahmed, Z. T. The Quantification of the Fly Ash Adsorption Capacity for the Purpose of Charecterization and Use in Concrete. PhD Thesis, Michigan Technological University: Houghton, MI, 2012. (5) Ahmed, Z. T.; Hand, D. W.; Sutter, L. L.; Watkins, M. K. Fly Ash Iodine Number for Measuring Adsorption Capacity of Coal Fly Ash. ACI Mater. J. 2014. URI: http://www.concrete.org/Publications/ ACIMaterialsJournal/ACIJournalSearch.aspx?m=details&ID= 51686582. (6) Ahmed, Z. T.; Hand, D. W.; Watkins, M. K.; Sutter, L. L. Air Entraining Admixtures Partitioning and Adsorption by Fly Ash in Concrete. Ind. Eng. Chem. Res. 2014, DOI: 10.1021/ie4018594. (7) Ahmed, Z. T.; Hand, D. W.; Watkins, M. K.; Sutter, L. L. The Combined Adsorption Isotherms For Measuring Adsorption Capacity of Fly Ash in Concrete. ACS Sustainable Chem. Eng. 2014, DOI: 10.1021/sc500043s. (8) Standard Test Methods for Sampling and Testing Fly Ash or Natural Pozzolans for Use in Portland-Cement Concrete; ASTM C311-04; ASTM International: West Conshohocken, PA, 2004. (9) Fan, M.; Brown, R. C. Comparison of the Loss-on-Ignition and Thermogravimetric Analysis Techniques in Measuring Unburned Carbon in Coal Fly Ash. Energy Fuels 2001, 15, 1414−1417. (10) Zhang, Y.; Lu, Z.; Maroto-Valer, M. M.; Andrésen, J. M.; Schobert, H. H. Comparison of High-Unburned-Carbon Fly Ashes from Different Combustor Types and their Steam Activated Products. Energy Fuels 2003, 17, 369−377. (11) Brown, R. C.; Dykstra, J. Systematic Errors in the Use of Losson-Ignition to Measure Unburned Carbon in Fly Ash. Fuel 1995, 74, 570−574. (12) Dodson, V. H. Concrete admixtures; Van Nostrand Reinhold: New York, 1990. (13) Watkins, M. K. Characterization of a Coal Fly Ash-Cement Slurry by the Absolute Foam Index. Ph.D. Thesis, Michigan Technological University: Houghton, MI, 2013. (14) Standard Test Method for Determination of Iodine Number of Activated Carbon; ASTM D4607.94; ASTM International: West Conshohocken, PA, 1994. (15) Murphy, L. J., Jr. Gravimetric Determination of the Iodine Number of Carbon Black. US Patent 5,002,892.

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CONCLUSIONS The combined adsorption isotherm methodology provides a direct and accurate measurement of the adsorption capacity of AEAs onto fly ash. This method can be performed on the specific concrete mixture materials to determine the exact amount of AEA that will be adsorbed by the fly ash. The adsorbed amount of AEA is the quantity required to compensate for the AEA adsorbed portion by the fly ash. Adding this quantity to the initial AEA concentration will bring the AEA liquid phase concentration of the fly ash containing mixture to the same level of the base fly ash free concrete mixture. The direct adsorption isotherm is the only direct measurement for the adsorption of AEAs by fly ash, and the only method to precisely determine the AEA dosage adjustment required to compensate for the adsorbed AEA. However, this test is technically complicated compared to the fly ash iodine number, which can be performed in a simple field laboratory. The fly ash iodine number also provides a great tool for the quantification of the adsorption capacity of fly ash based on iodine adsorption. This test can be used for the characterization of fly ash and determining whether the fly ash is suitable for the use in concrete and other applications. Furthermore, the fly ash iodine number can be correlated to the adsorption capacities of various AEAs and used directly to determine the AEA adsorption capacity and the AEA dosage adjustment required.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 6989

dx.doi.org/10.1021/ie500484d | Ind. Eng. Chem. Res. 2014, 53, 6985−6989