Abstract--The present work examines the possible use of fly ash, a by-product of coal power stations, as a means of removing arsenic OF) from water, ...
Wat. Res. Vol. 27, No. 12, pp. 1773-1777, 1993 Printed in Great Britain.All rightsreserved
0043-1354/93 $6.00+0.00 Copyright © 1993PergamonPress Ltd
As(V) REMOVAL FROM AQUEOUS SOLUTIONS BY FLY ASH EVAN DIAMADOPOULOS1'2., SOCRATESIOANNIDIS2'3 and GEORGEP. SAKELLAROPOULOS2'3 ~Department of Production Engineering and Management, Technical University of Crete, 73100 Chania, 2Chemical Process Engineering Research Institute, P.O. Box 1520, 54006 Thessaloniki and 3Department of Chemical Engineering, Aristotle University of Thessaloniki, 54006 Thessaloniki, Greece
(First received January 1992; accepted in revised form March 1993) Abstract--The present work examines the possible use of fly ash, a by-product of coal power stations, as a means of removing arsenic OF) from water, or equivalently, of restricting its movement in the solid wastes or the soil. Kinetic and equilibrium experiments were performed in order to evaluate the removal efficiency of lignite-based fly ash. Both adsorption and desorption experiments were done at three pH levels, namely 4,7 and I0. The results indicated that arsenic can be removed from water by fly ash, yet the degree of removaldepended markedly on the pH. Removal at pH 4, as demonstrated by the adsorption isotherms, was significantlyhigher than that at the other two pH values. For 80% removal of arsenic, the solid phase concentration at pH 4 was up to 4 times greater than that at the other two pH levels. During the desorption studies only a small amount of the pre-adsorbed arsenic was released into the water. This amount was practically independent of the initial fly ash loading. This indicates that adsorption of arsenic on fly ash is almost irreversible and, therefore, there are good prospects for arsenic fixation on fly ash in practical applications.
Key words--fly ash, arsenic, adsorption, desorption
INTRODUCTION Arsenic is a toxic element that can be found in natural waters, as well as in various industrial wastes, solid or liquid (Ferguson and Gavis, 1972). Leaching of arsenic into the groundwater may cause significant contamination. Among the methods examined for arsenic removal from water, adsorption on activated carbon has been shown to be very efficient (Huang and Fu, 1984; Huang and Vane, 1989). In these studies, commercially available activated carbons were used. Their results indicated that the higher the ash content of the activated carbons, the higher the removal efficiency. Similar results were obtained with lignite-based laboratory-produced activated carbons (Diamadopoulos et al., 1990). It was found, that the removal of arsenic (V) was not affected by the surface area of the carbon, while its removal from water was enhanced up to 5 times for the high ash activated carbons. These results were explained by a strong interaction between the arsenate ion and the inorganic part (ash) of the activated carbon. Therefore, it was decided to further investigate the adsorption of arsenic (V) solely on fly ash. Fly ash is a solid waste produced in large quantities at coal power stations. Its ability to remove cations from water has been demonstrated in the literature *All correspondence should be addressed to: Dr Evan Diamadopoulos, Department of Production Engineering and Management, Technical University of Crete, 73100 Chania, Greece.
(Bhattacharya et al., 1984; Panday et al., 1985; Yadava et al., 1987). However, limited information exists on the removal efficiency of anions (Gangoli and Thodos, 1974; Tsitouridou and Georgiou, 1988; Alberts et al., 1988). The present work examines the potential use of fly ash as an adsorbent for the removal of arsenate ion from water under kinetic and equilibrium conditions. Furthermore, it looks into the extent of desorption of the pre-adsorbed arsenic (V), so that the possible use of fly ash for arsenic fixation is investigated. MATERIALSAND METHODS Fly ash was obtained from Kardia thermal power plant. Chemical analysis by a Siemens X-ray fluorescence spectrometer (Table 1) indicated that the major constituent of this fly ash was CaO, which characterizes alkaline ashes, i.e. ashes which raise the pH of the water. The fly ash was passed through a stack of sieves and only the size fraction between 106 and 125/~m was used in this study. The BET surface area of the particles was analyzed with a Model QS-7 Quantasorb Surface Area Analyzer (Quantachrome Corp.) and found to be 0.8 m2/g. Sodium hydrogen arsenate (Na2HAsO4 • 7H20) obtained from Merck was used as the arsenic (3/) source. All experiments were conducted in 500 ml polyethylene bottles containing 150 ml of As(V) solution of 50 mg/l (expressed as As). NaHCO3 was also added to a concentration of 0.33 × 10-3 M to provide su~cient buffering capacity. All chemicals used were of analytical grade. The water used was distilled and deionized. The bottles with the arsenic solution and the fly ash were placed in a temperature controlled water bath at 20°C and shaken with uniform speed 005 strokes per min).
E V A N D~AMADOPOULOSet al.
Three sets of experiments were done: adsorption kinetic and equilibrium experiments, as well as desorption studies. All adsorption kinetic experiments were carried out with 1.0 g fly ash. At the end of predetermined time intervals the bottles were withdrawn one by one from the water bath, their content was filtered through membrane filters and the filtrate was, subsequently, analyzed for its arsenic content. For the adsorption equilibrium experiments, varying amounts of fly ash were used. The contact time was determined from the kinetic experiments. The desorption studies involved the careful filtration of all fly ash particles and their addition, along with the membrane filter, into a bottle containing only distilled and deionized water and buffer. Desorption equilibrium was considered to happen at the same time as adsorption equilibrium. All sets of experiments were performed at three pH levels, namely 4,7 and 10. The pH adjustment was done with 0.1 N HCI or NaOH. During the experiments, the pH of the solution in the bottles was checked and corrected with the addition of a few drops of 0.I N HCI solution. At the end of each experiment, deviations in the final pH up to 0.5 units from the desired values were acceptable. Blank experiments were also performed at all three pH levels. No adsorption of arsenic on the PE bottle walls occurred, while no dssorption of any arsenic (V) or phosphate from the fly ash in the absence of arsenic solution took place.
Chemical analysis of arsenic (V) was done spectrophotometrically using the molybdenum blue method (Johnson and Pilson, 1972). Absorbance measurements were made at 885 nm using a Shimadzu UV-1201 Spectrophotometer. RESULTS AND DISCUSSION
The rate of adsorption of As(V) on fly ash for the three pH levels is presented in Fig. 1. This figure shows the remaining As concentration in solution as a function of time. A rapid adsorption takes place for the first 20 h, while after this very little adsorption occurs. In addition to this, adsorption at p H 4 is twice as effective as at the other two pH levels. As a result of the above observations, it was considered that equilibrium was practically complete after 72 h contact time. This time period was, subsequently, used in the equilibrium experiments, both adsorption and desorption. The results from the equilibrium studies are presented in Figs 2, 3 and 4 which correspond to pH levels 4, 7 and 10, respectively. F o r all three pH values, the adsorption isotherms indicate a favorable type of adsorption. However, significant differences exist in relation to pH. At pH 4, arsenic adsorbs more strongly on the fly ash, followed by adsorption at pH 10, while adsorption at pH 7 is the least effective. The ash loading (solid phase concentration) is greater at pH 4 as opposed to the other two p H levels.
o pH4 pH 7 ¢ pH~O 4O
Time (h) Fig. 1. Effect of contact time on arsenic (V) adsorption on fly ash. Conditions: initial As concentration 50 rag/l; fly ash concentration I g/I; 20~C. Maximum loading at pH 4 reaches 30 mg As/g, while at the other two p H values it does not exceed 10mgAs/g. Equivalently, for an 80% removal of arsenic (i.e. a liquid phase equilibrium concentration of 10 mg/l) the ash loading at pH 4 is 4 times as high as at p H 7 and 3 times as high as at p H 10. This behavior may be explained as the result o f interaction of the arsenate ion with the various oxides which exist in the solid phase. Strong adsorption at pH 4 is due to the ferric and aluminum oxides, while adsorption at pH 10 is due to calcium and magnesium oxides. At pH 7, none of the two classes of oxides is efficient enough to interact with the arsenate ion and, therefore, adsorption at p H 7 is the lowest. These considerations are in good agreement with similar findings in the literature. Alberts e t al. (1988) report that both arsenic (III) and (V) can be effectively removed from water by coal ash at acidic pH. A review of the efficiency of common coagulants to remove arsenic from water has indicated that arsenic can be effectively removed by precipitation with aluminum sulfate at a pH less than 7, ferric chloride at a pH less than 8.5 and lime at a pH higher than 10.5 (Trace Inorganic Substances Research Committee, 1988). The same article reports adsorption of As(V) on 30
Table 1. Chemicalanalysisof the fly ash Constituent Percent by weight C_~O
F~O 3 MgO TiO2
7.6 5.2 0,9
~ Adsorption data o
C~saotption data , !
Fig. 2. Adsorption-desorption isotherms at pH 4. The continuous line represents the Freundlich equation.
As(V) removal by fly ash
Table 2. Adsorption equilibrium parameters estimated by non-linear regression
Freundlich model pH 4
I & Adsorption data 0 Daaorptiondata I
Fig. 3. Adsorption-desorption isotherms at pH 7. The continuous line represents the Freundlich equation.
amorphous Fe(OH)3 , amorphous AI(OH)3, activated alumina and activated carbon. In all cases, the amount of arsenic adsorbed increased for a pH less than 7 and became maximum in the pH region 4-5. The beneficial role of iron in the adsorption of arsenate has also been observed by Huang and Vane (1989). These investigators impregnated ferrous ions onto activated carbon and achieved an increase in the arsenic removal from less than 10% before the impregnation to over 90% after it. Modeling of adsorption equilibrium is usually done by applying either the Langmuir or the Freundlich equation. Both equations relate the equilibrium solid phase concentration, qe, with the liquid phase concentration, Co. The Langmuir equation is expressed as
qeffi l + K--~
The Freundlich equation is expressed as qe=kC n
The adsorption equilibrium parameters for these two models are presented in Table 2, while the Freundlich model simulations of the experimental data are also
~ Adsotptios data Dnotpt|cm data
Fig. 4. A d s o r p t i o n - d e s o r p t i o n isotherms at p H 10. The continuous line represents the Fretmdlich equation.
9.92 i.69 4.06
0.263 0.389 0.206
27.78 10.46 7.71
0.329 0.058 1.036
shown in Figs 2, 3 and 4. Both models demonstrate reasonably good fit and this is in agreement with the adsorption of other inorganic species on fly ash (Yadava et aL, 1987; Gupta et aL, 1990). Furthermore, arsenic (V) adsorbs stronger on fly ash than on activated carbon. Huang and Fu (1984) report values of the Langmuir Qm parameter (an indication of the maximum loading achievable) as being less than 4 mg/g for a variety of activated carbons at pH 4.5. This value is almost 7 times smaller than the same parameter for the fly ash adsorption experiments at pH 4. Contrary to the adsorption results, desorption data follow a totally different pattern. All desorption data appear to be independent of the fly ash loading and lie in a narrow concentration region. The upper limit of this region is 5 mg/l for pH 4, 8 mg/1 for pH 7 and 10 mg/l for pH 10. If the adsorption of arsenic on fly ash was completely reversible, then the adsorption and desorption isotherms would coincide. On the other hand, if no resorption takes place, all desorption data points should lie on the loading axis at zero mg/l equilibrium concentration. Judging from the results, little desorption takes place particularly at pH 4 and 7, which indicates that adsorption is, to a large extent, irreversible. Irreversibility effects have also been demonstrated to occur during the adsorption of pesticides on soil (Swanson and Dutt, 1973; Koskinen et al., 1979) and PCBs on sediments (Horzempa and Di Toro, 1983). Modeling of desorption isotherms is quite complex when irreversibility effects occur. The desorption points depend on both the preceding adsorption, as well as the desorption procedure and, therefore, it cannot be assumed that at equilibrium the same isotherm applies for both adsorption and desorption as is the case of reversible adsorption. Thus, in the case of irreversibility for each adsorption isotherm point, there exists a desorption isotherm which starts from this point and is directed towards the loading axis. In the case of arsenic desorption from fly ash, the desorption isotherms are depicted by the straight lines joining the adsorption and desorption points as shown in Fig. 5. It should be mentioned that desorption isotherms derived after several desorption cycles may also follow curvilinear trends (Di Toro and Horzempa, 1982). A simple linear mathematical model has been employed to describe the desorption data. This model has been successfully used for the irreversible adsorption of PCBs on sediments (Di Toro and Horzcmpa,
EVAN DIAMAVOPOULOSet 30
• J_ !0
Adsorption data Desorption data
Fig. 5. True and model regressed desorption isotherms for pH4. 1982), as well as river water organics on activated carbon (Narbaitz, 1985). The model considers the adsorption isotherm to be a straight line, so that q ~ = q + a (C,~ - C) where q,~ ffi equilibrium solid phase concentration after the desorption cycle (mg/g) q •equilibrium solid phase concentration before desorption (rag/g) C ~ -- equilibrium liquid phase concentration after the desorption cycle (mg]l) C ffi liquid phase concentration at the end of the adsorption cycle (C is in equilibrium with q) (rag/l) a = slope 0/g). It is further considered that the slope, a, can be expressed as an exponential function of q, i.e. a f
g l q s2
where g, and g2 are constants whose values can be estimated by statistical regression analysis. By doing so, a single equation can be used to describe all desorption isotherms. The parameter estimates are presented in Table 3 for all pH levels studied, while the true and model-regressed desorption isotherms for pH 4 are illustrated in Fig. 5. From this figure it can be seen that the model describes the desorption data satisfactorily. Similar results are obtained with the model at the other two pH levels. The results presented above strongly suggest that the adsorption of arsenic ('V) on fly ash is, to a large extent, irreversible and pH dependent. In order to explain the observed effects, one has to consider the Table 3. Parameter estimates of the desorption model pH
4 7 10
0.19 x 10 -5 0.13 × 10 -I 0.1 ! x 10 -5
2.18 0.78 5.85
possible mechanisms of adsorption. These are electrostatic attraction (which gives rise to ion exchange with the counter-ions in the diffuse layer) and specific adsorption (surface complexation). The possible sites on the fly ash surface for specific adsorption at low pH include hydrous oxides of aluminum and iron. Surface complexation may occur when a proton from an undissociated arsenate ion forms a molecule of water with the hydroxyl group of the hydrous oxide followed by its displacement by the arsenate ion. The potential for surface complexation will depend on the protonation state of the arsenate ion and will be more favorable at lower pH. As pH increases, the ability of displacing the hydroxyl groups from the hydrous oxides is reduced, since less arsenate ions are protonated. Removal of arsenic at high pH is primarily due to adsorption on calcium and magnesium oxides. This is the reason why adsorption at pH 10 was more effective than at pH 7. The considerations given above were based on the interactions among the arsenate species and the individual hydrous oxides assumed to exist on the surface. It is likely, however, that the adsorptive capacity of fly ash cannot be solely explained as in the case of a single oxide or hydroxide. Recent evidence on the combined use of lime, ferric and aluminum coagulants has shown that these substances are more effective in combination than individually (Harper and Kingham, 1992). For these reasons, the modeling approach followed was empirical, as opposed to equilibrium reaction modeling between the adsorbing ion and the specific groups on the surface. The effect of particle surface area has not been studied, since fly ash is an industrial by-product and its properties cannot be tailored. From a theoretical standpoint, higher surface area will make more active sites available for the adsorption interactions, yet the measured surface area of the fly ash particles is the combined surface area of both the strictly inorganic matter, which participates in the arsenate adsorption process, and the remaining unburnt carbon. Further chemical analysis and characterization of the surface of the fly ash particles is required. Yet, the results presented above indicate that there are good prospects for practical applications of fly ash for arsenic removal from wastewater, as well as arsenic fixation during the co-disposal of fly ash with arsenic-laden solid wastes. CONCLUSIONS
Based on the results presented above, the following conclusions can be drawn: • Arsenic (V) adsorbs strongly on fly ash. Equilibrium is practically achieved in less than 72 h, while most of adsorption takes place in less than 24h. • Equilibrium studies of As(V) on fly ash show that arsenic adsorbs more strongly at pH 4 as opposed
As(V) removal by fly ash
Gupta G. S., Prasad G. and Singh V. N. (1990) Removal of chrome dye from aqueous solutions by mixed adsorbents: fly ash and coal. Wat. Res. 24, 45-50. Harper T. R. and Kingham N. W. (1992) Removal of arsenic from wastewater using chemical precipitation methods. Wat. Erwir. Res. 64, 200-203. Hozempa L. M. and Di Toro D. M. (1983) The extent of reversibilityof polychlorinated biphenyl adsorption. Wat. Res. 17, 851-859. Huang C. P. and Fu P. L. K. (1984) Treatment of arsenic (V)-containing water by the activated carbon process. J. Wat. Poilut. Control Fed. 56, 233-241. Huang C. P. and Vane L. M. (1989) Enhancing As removal by a Fe-treated activated carbon. J. Wat. Pollut. Control Fed. 61, 1596-1603. Johnson D. L. and Pilson M. E. Q. (1972) Spectrophotometric determination of arsenite, arsenate, and phosphate in Acknowledgement--This studywas financially supported by natural waters. Analyt. chim. Acta 58, 289-299. the General Secreteriat of Research and Technology of Koskinen W. D., O'Connor G. A. and Cheng H. H. (1979) Greece. Characterization of hysteresisin the desorption of 2,4,5-T from soils. Soil ScL Soc. Am. Proc. 43, 871-874. REFERENCF~ Narhaitz R. M. (1985) Modeling the competitive adsorption of l,l,2-trichloroethane with naturally occurring backAlberts J. J., Weber M. E. and Evans D. W. (1988) The ground organics onto activated carbon. Ph.D. thesis, effect of pH and contact time on the concentration of McMaster University. As(IIl) and As(V) in coal ash systems. Envir. Technol. Panday K. K., Prasad G. and Singh V. N. (1985) Copper Lett. 9, 63-70. (II) removal from aqueous solutions by the fly ash. Wat. Bhattacharya A. K. and Venkobachar C. (1984) Removal of Res. 19, 869-873. cadmium (II) by low cost adsorbents. J. envir. Engng II0, Swanson R. A. and Dutt G. R. (1973) Chemical and 110-122. physical processes that affect atrazine distribution in soil Diamadopoulos E., Samaras P. and Sakellaropoulos G. P. systems. Soil Sci. Soc. Am. Proc. 37, 872-876. (1992) The effect of activated carbon properties on the adsorption of toxic substances. War. Sci. Teclmol. 25, Trace Inorganic Substances Research Committee (1988) A review of solid-solution interactions and implications for 153-160. the control of trace inorganic materials in water treatDi Toro D. M. and Horzempa L. M. (1982) Reversible and ment. J. Am. War. Wks Ass. 80, 56-64. resistant components of PCB adsorption-desorption Tsitouridou R. and Georgiou J. (1988) A contributionto the isotherms. Envir. Sci. Technol. 16, 594--602. study of phosphate sorption by three Greek fly ashes. Ferguson J. F. and Gavis J. (1972) A review of the arsenic Toxic. envir. Chem. 17, 129-138. cycle in natural waters. Wat. Res. 6, 1259-1274. Yadava K. P., Tyagi B. S., Panday K. K. and Singh V. N. Gangoli N. and Thodos G. (1974) Kinetics of phosphate (1987) Fly ash for the treatment of Cd (II) rich effluents. adsorption on alumina and fly ash. J. Wat. Pollut. Control Envir. Technol. Lett. 8, 225-234. Fed. 46, 2035-2042. to pH 7 and 10. Complete removal of arsenic is possible at pH 4 at high fly ash concentrations. • The maximum ash loading at pH 4 is 4 times as high as at pH 7, and 3 times as high as at pH 10. • Very little arsenic desorbs from fly ash at the same pH as the adsorption studies. This is more prominent at pH 4. This observation indicates that a large part of As(V) adsorbs irreversibly on fly ash. • A linear desorption model describes the desorption isotherms satisfactorily at the three pH levels.