Research Synthesis and Properties of Zeolites from Coal Fly Ash C H R I S T O P H E R A M R H E I N , * ,† GHOLAM H. HAGHNIA,‡ TAI SOON KIM,§ PAUL A. MOSHER,† RYAN C. GAGAJENA,† TEDROS AMANIOS,| AND LAURA DE LA TORRE⊥ Department of Soil and Environmental Sciences, University of California, Riverside, California 92521, College of Agriculture, Ferdowsi University, Mashhad, Iran, College of Agriculture, Kon-Kuk University, Seoul, Korea, Eisenhower High School, Rialto, California 92376, and Rialto High School, Rialto, California 92376
Previous attempts to use fly ash as a soil amendment have had limited success because of its low nutrient value, low cation exchange capacity (CEC), and elevated levels of toxic trace elements. However, treating fly ash with NaOH or KOH at an elevated temperature converts the ash into zeolite minerals and solubilizes the toxic trace elements, which are removed in the base solution. The CEC of the untreated fly ash was 1.0 M NaOH solutions at elevated temperatures and the large (>20 µm), cubic trapezohedron crystals of analcime that Ming and Lofgren reported (5). Experiment 3: Cation Selectivity. Figure 7 shows the amount of Na extracted from the treated fly ash with the various solutions (experiment 3). Potassium was the most efficient cation at extracting Na from the zeolite. Cesium, Ca, Mg, Sr, and Ba solutions extracted significantly less Na than the NH4, Li, and K solutions. It is likely that the difference among the various extracting cations can be attributed to differences in cationic size, exclusion from the zeolite channels, and location of the zeolitic charge. There was no significant difference between NH4Cl and ammonium acetate in the amount of Na extracted (1664 ( 32 vs 1686 ( 35 mmolC kg-1). Unexpectedly, the average CEC of the ammonium acetate extracts was lower than previously measured on treated ash that had not been rinsed with 50 mM NaCl. In this study, the average CEC was 1675 mmolC kg-1, and in experiments 1 and 2 the CEC (as measured by ammonium acetate-extractable Na) ranged from 2400 to 3100 mmolC kg-1. This observation suggested incomplete removal of the NaOH with the DIW washings or partial dissolution of the high-charge minerals due to the repeated washing. Experiment 4 (see below) was designed to measure the kinetics of mineral dissolution and its effect on CEC. Experiments on the relative cation affinity in the two ion systems confirmed that the treated ash had a high affinity for K, Ca, and NH4 (data not shown). The highest CEC measured was 4130 ( 20 mmolC kg-1 on the NH4-
VOL. 30, NO. 3, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
739
FIGURE 8. Relative dissolution rates of minerals at pH 4 and pH 5 compared to the fly ash treated in 3.5 M NaOH at 100 °C for 3 days and 1 atm pressure. The references cited are refs 17-23.
saturated ash, extracted with KCl. The fly ash zeolite material that had been repeatedly washed with 1.0 molC L-1 CaCl2, followed by DIW, and equilibrated with 50 mmolC L-1 solutions of NH4Cl and CaCl2 exhibited a strong preference for NH4+ over Ca2+ at low solution concentrations of NH4. This suggested that the treated ash might be useful to remove ammonium from wastewater (see experiment 5). Experiment 4: Mineral Stability Studies. When the treated ash (3.5 M NaOH, 100 °C, 3 days, rinsed four times with DIW, and dried) was added to deionized water, the pH of the water was often higher than 10, depending upon the suspension density. In one study, the treated ash was washed a total of seven times in DIW (0.5 g in 30 mL for 30 min), and the final rinse water pH was 10.6 ( 0.2 with an electrical conductivity (EC) of 0.22 ( 0.04 dS m-1. This washed ash was extracted with 1.0 M ammonium acetate, and the amount of exchangeable Na was 1470 ( 70 mmolC kg-1. Extending each washing to 24 h did not decrease the amount of extractable Na, possibly because of slow mineral dissolution once a high pH was reached. When this same material was saturated with 50 mmolC L-1 CaCl2 and extracted with 1.0 M KCl, the CEC was 2560 ( 50 mmolC kg-1. If the treated ash was shaken in DIW for 13 days, the CEC, as measured by Ca saturation and KCl extraction, was still 2580 ( 80 mmolC kg-1. The high Si and Na concentrations in these DIW batch studies, which averaged 9.8 ( 0.4 mM Si and 8.7 ( 0.8 mM Na, confirm that an initial rapid mineral dissolution had occurred. X-ray diffractograms of the well-rinsed material showed decreased peak intensities for all of the zeolite peaks. In one instance, for the treated ash titrated to pH 4 (see below), two new peaks appeared at 1.47 and 0.72 nm. These were tentatively identified as 2:1 phyllosilicate peaks. If the treated ash was titrated to pH 4, the CEC decreased to 670 ( 30 mmolC kg-1 within 1 h. After reaction at pH 4 for 24 days, the CEC of the treated samples averaged 320 ( 10 mmolC kg-1. At pH 5, the CEC decreased to 1190 mmolC kg-1 after 6 days, to 1030 mmolC kg-1 after 8 days, and to 850 ( 40 mmolC kg-1 after 21 days. These observations led to further short-term studies on the stability and weathering kinetics of the treated fly ash. Based on the rate of acid consumption in a pH-stat titrator, we calculated dissolution rates for the treated ash and compared these rates to other minerals. Figure 8 summarizes the relative rates of mineral weathering at pH 4 and pH 5
740
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 3, 1996
FIGURE 9. (A) Removal of NH4 from synthetic wastewaters using treated fly ash (experiment 5A). (B) pH of the wastewater after the addition of the treated fly ash.
and shows that the treated fly ash dissolved approximately 1000 times slower than calcite and 1000 times faster than heulandite (a natural zeolite from Iceland). Based on these findings, we are suggesting that the fly ash zeolites might be used to treat low pH, low CEC soils by providing both a liming agent and by increasing the cation exchange capacity. Experiment 5: Reactions with Wastewaters Containing NH4 and Metals. Because the treated-ash showed a high preference for NH4 (Figure 7 and other cation selectivity studies), the following experiment was designed to evaluate the potential for using the fly ash zeolites to remove NH4 and heavy metals from wastewater and electroplating wastes. Figure 9A shows very little removal of NH4 from the synthetic wastewaters, which is attributed to the high pH of the solutions at high suspension densities (Figure 9B). As the pH increased with increasing amounts of treated ash, NH4+ was converted to NH30, which did not adsorb. In the earlier study that showed a high affinity for NH4, the treated ash had been extensively washed with high electrolyte solutions prior to use. This removed much of the
FIGURE 10. Effect of treated fly ash on the pH of electroplating waste (experiment 5B).
reactive material, and the suspension pH values averaged 7.5, which favored NH4 adsorption. This study confirms the findings in experiment 4 that the treated ash is quite reactive in water, producing a high pH, even after repeated rinses with DIW. After reaction with the fly ash zeolites, the Cd and Pb concentrations in the synthetic wastewaters dropped from 25 and 20 µg L-1, respectively, to
