Steam- and carbon dioxide-gasification of coal combustion ash for liquid phase cadmium removal by adsorption

Chemical Engineering Journal 207–208 (2012) 66–71

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Steam- and carbon dioxide-gasification of coal combustion ash for liquid phase cadmium removal by adsorption Marco Balsamo a, Francesco Di Natale a, Alessandro Erto a, Amedeo Lancia a, Fabio Montagnaro b,⇑, Luciano Santoro b a b

Department of Chemical Engineering, Università degli Studi di Napoli Federico II, Piazzale Vincenzo Tecchio 80, 80125 Naples, Italy Department of Chemical Sciences, Università degli Studi di Napoli Federico II, Complesso Universitario di Monte Sant’Angelo, 80126 Naples, Italy

h i g h l i g h t s " Coal ash beneficiation by gasification was investigated by tests on Cd adsorption. " The CO2-gasified ash appeared as a better Cd sorbent than the steam-treated ash. " Both treatments were able to improve the sorbent characteristics. " Gasified ashes showed higher removal efficiencies at low pollutant/sorbent ratios. " An analysis concerning the involved processes characteristic times was developed.

a r t i c l e

i n f o

Article history: Available online 8 July 2012 Keywords: Coal ash CO2-gasification Steam-gasification Cadmium Adsorption Industrial waste reuse

a b s t r a c t This paper is collocated after two other works in which sieved or demineralized coal combustion ashes were used as cadmium (Cd2+) sorbent for water treatments, and the changes in raw ash properties were followed upon steam- and CO2-gasification. In the present paper, ash samples activated by steam- or CO2gasification were used to adsorb Cd2+ in model aqueous solutions, to elucidate the relationships among ash properties, gasification treatments, properties of the gasified ash and its sorption behavior. To this end, equilibrium and kinetic cadmium adsorption tests were carried out using the gasified ash (treated with either steam or CO2) as sorbent and under fixed operating conditions (for example, the contact time was 7 days and the initial Cd2+ concentration in the aqueous solution to be treated varied between 5 and 50 mg L1 for equilibrium tests, while kinetic tests were carried out for times ranging from 10 min to 7 days using an initial Cd2+ concentration equal to 50 mg L1). Results were expressed in terms of adsorption isotherms, removal efficiency and kinetic curves. Post-processing of experimental data allowed to provide estimations of relevant parameters such as the equilibrium and the kinetic constants, the separation factor, the initial adsorption rate, the characteristic adsorption time. Besides a direct confrontation between the two gasification treatments, results were also compared with former experiments on the same raw material, and with the main outcomes emerging from the pertinent literature, confirming that the gasified materials resulted as very attractive sorbents for Cd2+ capture from wastewaters. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Coal combustion ash (CCA) is a residue whose disposal raises several environmental criticisms [1]. Different alternatives to CCA landfilling have been explored in the scientific literature. For example, Koukouzas et al. [2] investigated the possibility to produce high-value zeolitic adsorbents via hydrothermal treatment of coal fly ashes derived from fluidized bed combustors. Moreover, highCa ash particles have been employed to produce Al composites ⇑ Corresponding author. Tel.: +39 081 674029; fax: +39 081 674090. E-mail address: [email protected] (F. Montagnaro). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.07.003

with improved mechanical and tribological properties to be applied in different sectors including automotive and aerospace [3]. While in the past this research group has investigated CCA possible reuse in fields concerning the development of construction materials [4,5] or the use as SO2 sorbents [6–8], in this paper gasified CCA application to adsorption for the remediation of heavy metals-polluted water is dealt with, following the path drawn by the authors in two previous published works [9,10]. In fact CCA can be considered, together with other non-conventional materials, as a cheaper alternative to the use of activated carbons [11– 19]. In order to better employ the CCA as sorbent, a gasification treatment (to be carried out either in steam or in carbon dioxide)

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Nomenclature C (mg L1) Cd2+ concentration C0 (mg L1) Cd2+ initial concentration Ce (mg L1) Cd2+ equilibrium concentration D (m2 s1) Cd2+ effective diffusivity ds (m) mean Sauter particle diameter K (L mg1) adsorption equilibrium constant kkin (g mg1 min1) specific kinetic constant m (g) sorbent mass P/S (mg g1) pollutant-to-sorbent mass ratio q (mg g1) specific adsorption capacity qe (mg g1) equilibrium specific adsorption capacity qm (mg g1) maximum equilibrium specific adsorption capacity qmax (mg g1) maximum specific adsorption capacity

R2 () t (min) V (L)

determination coefficient adsorption time aqueous solution volume

Greek symbols g () Cd2+ removal efficiency 1 h (mg g min1) initial adsorption rate r () separation factor sext (min) characteristic time for external mass transfer smacro (min) characteristic time for macropore diffusion s(smicro) (min) characteristic adsorption time (under micropore diffusion control)

has been proposed [20–23]. CCA gasification involves the conversion of the unburnt carbon into gaseous products, and it can be very effective in the development of ash porosity. In particular, this paper intends to complete a trilogy of works also consisting in a first paper [9] in which CCA beneficiation treatments other than gasification (e.g. mechanical sieving and demineralization) were investigated and the treated ash was employed as cadmium sorbent, and in a second paper [10] in which the changes in CCA physico-chemico-microstructural properties were followed upon steam- and CO2-gasification. Starting from the results reported in this second work, samples activated by either steam- or CO2-gasification under the most interesting operating conditions are, in this third paper, used as sorbents for cadmium removal from model aqueous solutions, to better elucidate the relationships among ash properties, gasification treatments, properties of the gasified ash and its sorption behavior (through equilibrium and kinetic adsorption tests). Cadmium was chosen as representative of a highly toxic and soluble heavy metal pollutant present in several industrial wastewaters [11,24–28]. Results obtained in this paper are also critically discussed with reference to both data obtained in the first work for the untreated ash and main outcomes emerging from the pertinent literature.

bent amount of m = 1 g was contacted in a stirred glass beaker with an aqueous solution having volume of V = 0.1 L and Cd2+ initial concentration (C0) ranging from 5 to 50 mg L1. The contact time was 7 days, chosen on the basis of preliminary kinetic tests as largely sufficient to achieve equilibrium conditions. The initial cadmium solution pH was adjusted with HNO3 (1 M) solution, to obtain an equilibrium pH around 7.5. In fact, preliminary tests indicated that, in the presence of SG10 or DG10 sample, the equilibrium pH of cadmium solutions equaled to 12. This strongly alkaline character is likely to be related to calcium ions leaching from CCA, as reported in Balsamo et al. [9]. These operating conditions affect cadmium adsorption, due to Cd(OH)2 precipitation which hinders the assessment of reliable adsorption data. After each test, liquid solutions were filtered and analyzed by atomic absorption spectrophotometry (using a Varian SpectrAA 220 apparatus), to determine the equilibrium Cd2+ concentration Ce and, therefore, the equilibrium specific adsorption capacity qe:

2. Experimental

Ce Ce 1 ¼ þ qe qm Kqm

2.1. Sorbent materials The coal combustion fly ash was generated in a 240 MWe Italian pulverized coal power plant operated by ENEL, and its characterization was reported in Balsamo et al. [10], while in Balsamo et al. [17] information concerning its leaching characteristics and its sorbent behavior toward arsenate ions can be found. Chemical characterization, together with porosimetric, diffractometric and granulometric analyses allowed to individuate, as best candidates for further adsorption tests, the materials called SG10 (CCA steam-gasified at 850 °C for 10 min) and DG10 (CCA CO2-‘dry’-gasified, same conditions). The burn-off degree upon gasification was 48.1% and 44.6% and the increase in porosity with respect to the untreated ash was 110% and 63% for SG10 and DG10, respectively. The decrease in the mean particle diameter with respect to CCA was 40% for both SG10 and DG10.

qe ¼

ðC 0  C e ÞV m

ð1Þ

The adsorption isotherms were then described by the linearized form of the Langmuir equation, which best fitted equilibrium adsorption data:

ð2Þ

where K is the adsorption equilibrium constant and qm is the maximum equilibrium adsorption capacity (a more complete discussion concerning the various models employable to represent the isotherms has been recently published on this Journal by Foo and Hameed [29]). The Langmuir equation can be used to obtain the value of K and the separation factor r (as defined by Webi and Chakravort [30]) known to characterize a favorable adsorption if smaller than 1:

r¼

1 1 þ KC 0

ð3Þ

Finally, the efficiency curves were evaluated, and expressed as the Cd2+ removal efficiency g vs. the pollutant-to-sorbent mass ratio P/S:

g¼

C0  Ce P C0V vs: ¼ S m C0

ð4Þ

2.2. Equilibrium adsorption tests 2.3. Kinetic adsorption tests Cadmium adsorption thermodynamic tests, aimed at evaluating the adsorption isotherms, were conducted at room temperature using, alternatively, SG10 and DG10 as sorbents. To this end, a sor-

Cadmium kinetic adsorption tests were conducted under the same operating conditions as above, with the following exceptions:

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(i) C0 = 50 mg L1 for each test; (ii) adsorption time t ranged from 10 min to 7 d. After each test, the Cd2+ concentration C(t) was determined to give the specific adsorption capacity:

y=1.292+0.3981x R2=0.9930

8

ðC 0  CðtÞÞV m

ð5Þ

t t 1 ¼ þ qðtÞ qmax kkin q2max

7 −1

The adsorption kinetic curves were then described by the linearized form of the pseudo-second order Ho and McKay [31] equation (for a detailed discussion on the different kinetic models, see the work by Rudzinski and Plazinski [32]), which resulted to fit the experimental data better than other laws:

6 5 4

y=0.2705+0.2951x R2=0.9985

3 2

ð6Þ

1

where kkin is the specific kinetic constant and qmax the maximum q(t)-value. The value of kkin and the initial adsorption rate h:

dqðtÞ h¼ ¼ kkin q2max dt t¼0

0 0

2

4

6

8

10

12

14

16

18

20

−1

Ce, mg L

ð7Þ

were correspondingly determined. Moreover, the experimental results allowed the estimation of a characteristic adsorption time, defined as:

s¼

SG10 DG10

9

Ce/qe, g L

qðtÞ ¼

10

Fig. 2. Equilibrium Cd2+ adsorption data on the coal combustion ash steam (SG10)and CO2 (DG10)-gasified fitted by the Langmuir Eq. (2).

1.0

ðds =4Þ2 D

ð8Þ

0.9

where ds is the mean Sauter particle diameter and D is an effective Cd2+ diffusivity estimated following the path described in Balsamo et al. [9].

0.8 0.7

η, −

0.6

3. Results

0.5 0.4

3.1. Equilibrium adsorption tests

0.3

Fig. 1 reports the adsorption isotherms obtained using either SG10 or DG10 as sorbents. These isotherms show saturation conditions around Ce comprised between 10 and 20 mg L1, with corresponding qe-values equal to 3.2 and 2.2 mg g1 for the CO2 and the steam-gasified sample, respectively. Manipulation of equilibrium data according to Eq. (2) is reported in Fig. 2. In this Ce/qe vs. Ce plot, it can be seen that the data quite satisfactorily lay on a straight line (R2 = 0.9985 and 0.9930 for DG10 and SG10, respectively), whose y-axis intercept gives (known qm) the value of 1.09 (DG10) and 0.31 (SG10) L mg1 for K. Applying Eq. (3), and by letting C0 = 5 mg L1 so to obtain the maximum value for the separation

SG10 DG10

qe, mg g

−1

2.5 2.0 1.5 1.0 0.5 0.0 0

2

4

SG10 DG10

0.1 0.0 0

1

2

3

4

5

6

−1

P/S, mg g

Fig. 3. Removal efficiency vs. pollutant-to-sorbent ratio for Cd2+ adsorption under equilibrium conditions on the coal combustion ash steam (SG10)- and CO2 (DG10)gasified.

factor, it is r < 0.16 (dry-gasified sample) and r < 0.39 (steam-gasified sample), so witnessing a favorable cadmium adsorption. Fig. 3 shows the removal efficiency curve: g decreases as P/S increases, with a maximum equal to 95.9% (DG10) and 86.3% (SG10) at P/ S = 0.5 mg g1. In particular for the dry-gasified sample, the efficiency is still larger than 90% when the pollutant-to-sorbent ratio is 2.6 mg g1, and it is anyway larger than 50% throughout the investigated range of P/S. It is recalled here that low Ce values correspond to conditions far from sorbent saturation and to low values of the mass ratio cadmium/ash, and vice versa. Jointly, it is observed that practical systems characterized by a low P/S value are of high interest because they refer to the purification of wastewaters with low pollutant concentration. Therefore, the high efficiency values observed for both sorbents (and, in particular, for the CO2-gasified one) are promising when the ash gasification process aimed at improving its adsorptive properties is considered.

3.5 3.0

0.2

6

8

10

12

14

16

18

20

−1

Ce, mg L

3.2. Kinetic adsorption tests

2+

Fig. 1. Cd adsorption isotherms on the coal combustion ash steam (SG10)- and CO2 (DG10)-gasified. Adsorption time of 7 days, initial Cd2+ concentration ranging from 5 to 50 mg L1, room temperature, equilibrium pH = 7.5.

Fig. 4 depicts the results of the Cd2+ adsorption kinetic tests, when either SG10 or DG10 were used as sorbent materials. Adsorp-

M. Balsamo et al. / Chemical Engineering Journal 207–208 (2012) 66–71

[10]. From Eq. (8), for both SG10 and DG10 samples the characteristic adsorption time results s = 4 days, in very good agreement with the experimentally observed times required for q(t) to closely approach qmax (cf. Fig. 4). This highlights the building-up of a micropore diffusion-controlled regime for the system cadmium/ ash. In fact, external mass transfer and macropore Cd2+ diffusion were ruled out, as detailed in the Discussion section.

3.5

SG10 DG10

3.0

−1

2.5

q, mg g

69

2.0 1.5

4. Discussion 1.0 0.5 0.0 10

100

1000

10000

t, min Fig. 4. Specific adsorption capacity vs. time for Cd2+ adsorption on the coal combustion ash steam (SG10)- and CO2 (DG10)-gasified. Initial Cd2+ concentration equal to 50 mg L1.

tion time is reported on logarithmic scale so to better visualize the whole set of data. The q(t)-values increase with time reaching a maximum equal to 3.2 and 2.4 mg g1 after about 4 days for DG10 and SG10, respectively. It is highlighted that these qmax-values are very close to the equilibrium values for the specific adsorption capacity as observed in the thermodynamic tests for C0 = 50 mg L1, i.e. the value constantly used for carrying out the kinetic tests (see Fig. 1). The kinetic data were arranged according to Eq. (6) (Ho and McKay model), and the results are shown in Fig. 5. Also in this t/q(t) vs. t plot, the values of the R2-coefficient are very high (0.9902 and 0.9892 for the steam- and CO2-gasified material, respectively). The intercepts of the fitting lines with the y-axis give (known qmax) the values of 1.5  103 and 1.8  103 g mg1 min1 for the kinetic constant of the cadmium adsorption process on DG10 and SG10, respectively. The initial adsorption rate is known from Eq. (7) and equal to h = 1.5  102 and 1.0  102 mg g1 min1 for DG10 and SG10, respectively. Moreover, the application of the Reichenberg equation to the kinetic data following the procedure detailed in a former work [9] led to the determination of the effective Cd2+ diffusivity in the ash sorbent particle (D = 5.8  1018 m2 s1 for both materials). A characterization of gasified sorbents reported in a previous work showed a Sauter particle diameter ds = 6 lm for both materials 3000

t/q, min g mg

−1

SG10 DG10

y=97.94+0.4154x R2=0.9902

2000

y=66.94+0.3233x 2 R =0.9892

1000

0 0

1000

2000

3000

4000

5000

6000

t, min Fig. 5. Kinetic Cd2+ adsorption data on the coal combustion ash steam (SG10)- and CO2 (DG10)-gasified fitted by the Ho and McKay Eq. (6).

A direct comparison between Cd2+ adsorption data for DG10 and SG10 highlights a somewhat better cadmium uptake ability shown by the CO2-gasified sample. In fact, the specific capacity, the equilibrium constant, the removal efficiency and the initial adsorption rate were all higher for DG10, and the separation factor was favorably smaller for this sample. The circumstance that the CO2-gasified sample showed a better sorption behavior than the more porous steam-gasified material (cf. Section 2.1) could rely on the development, upon the CO2 treatment, of a chemically more kindred sorbent active surface toward cadmium. It has been reported in the literature [33] that the reaction with CO2 can lead to the formation, on the surface of a carbonaceous porous solid, of a significant number of oxygen-containing functional groups able to favorably interact with cationic species, and this could help explaining the higher adsorption capacity shown by DG10. When the data obtained in this work are compared with those valid for the untreated CCA under comparable operating conditions [9], it appears that (Table 1): (i) The steam gasification treatment resulted into a slight increase in the maximum qe-value (from 2.0 to 2.2 mg g1), reflected by comparable results for the equilibrium constant (for CCA, it was K = 0.41 L mg1), while a marked beneficiation was observed for the CO2-gasified ash (cf. data in Section 3.1). (ii) Similar considerations can be developed on the basis of the results coming from kinetic adsorption tests, in particular by observing the values of the initial adsorption rate (Section 3.2). (iii) The decrease in particle size and the increase in porosity driven by both gasification treatments ended up into shorter characteristic adsorption times (for the system cadmium/ raw CCA, it was s = 6 days). (iv) On the basis of the infrared spectroscopy results, it was observed that the gasification processes did not alter the inorganic imprint region of the parent ash, characterized by peaks related to carbonate structures, asymmetric stretching of Al, Si–O groups, presence of Al in aluminosilicates such as mullite, Si–O–Si bending, even if an increase in the peak intensity (promoted by gasification) was observed. On the other hand, the gasification processes were able to induce significant modifications in the ash organic matrix (e.g. opening of the graphitic aromatic rings, presence of carbonylic groups), all chemical alterations that were apparently reflected in the improved Cd2+ sorbent behavior observed in this work. As a consequence of the aforementioned observations, it can be inferred that the gasification treatments resulted in positive effects related to both a faster Cd2+ uptake (due to the ash porosity development and decrease of the mean particle size) and an increase of the metal uptake under equilibrium conditions (as a result of surface chemical modifications). The results heretofore shown are now discussed with reference to some other studies reported in the literature (see also Table 1).

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Table 1 Main parameters for Cd2+ adsorption onto the raw ash (CCA), the steam-gasified (SG10) and the CO2-gasified (DG10) ash, also in comparison with selected literature data. CCA 1

K (L mg ) qmax (mg g1) h (mg g1 min1) s (days)

SG10

0.41 2.0 1.4  103 6

DG10

0.31 2.4 1.0  102 4

1.09 3.2 1.5  102 4

For example, cadmium adsorption was investigated on industrial residues such as coal ash or other materials [2,11,12,24,34–36], on natural sorbents such as clay, bentonite and montmorillonite [37–40], and on agricultural by-products/biomasses such as bagasse, almond/hazelnut shell, bone char, egg shell, orange waste and rice husk [25,26,41–44]. Maximum values for the specific adsorption capacity illustrated in the present work are often in agreement (on an order-of-magnitude basis) with those reported in the quoted papers, even if not always it is possible to compare results as obtained under different operating conditions. Very high cadmium removal efficiencies were also obtained by Koukouzas et al. [2]. The results for the equilibrium constant calculated in this investigation (K = 101–100 L mg1) are in line with those reported in other works: for example, K = 102 L mg1 was reported by Bulut and Tez [42], El-Shafey [43] and Apiratikul and Pavasant [12], K = 101 L mg1 by Cheung et al. [41] and Srivastava et al. [25], K up to 1 L mg1 by Bayat [24]. Thus, the value obtained in this work in particular for the CO2-gasified sample should be regarded as very positive for the Cd2+ adsorption onto a waste-derived sorbent. This consideration is reinforced by considering some values reported for the separation factor (system coal ash/ cadmium, Bayat [24]), on the same order of those obtained in this work. Estimations for the specific kinetic constant drawn applying the pseudo-second order model to Cd2+ adsorption data were reported (data expressed in g mg1 min1) on the order of 6  105–2  104 [11], 2  104–2  103 [35,43], 102– 2  101 [12], 101–3  101 [36,42], and correspondingly initial adsorption rates were reported on the order of 6  103– 2  102 mg g1 min1 by Papandreou et al. [11], and 1 mg g1 min1 by El-Shafey [43], again placing the CCA employed in this study (in particular, when gasified) in a good ranking with respect to the other materials. Finally, a discussion concerning the most plausible controlling mechanisms now follows (Fig. 6), relying on the background developed in Balsamo et al. [9]. The

τmacro & τmicro

τext & τmacro

τext

τmicro

10-1

100

101

102

103

104

105

106

2

0

10 –10 0.7–6.2 6  103–2  102

Refs. [12,24,25,41–43] [24,25,42] [11]

characteristic time for external fluid-to-particle mass transfer (sext) can be estimated by knowing the values for the mean particle diameter (generally falling in the range 10–1000 lm) and for the Cd2+ diffusivity in water (109 m2 s1), and it results in the range 102–102 s. The characteristic time for macropore diffusion (smacro) can be known if, in particular, the values for the particle porosity (0.2–0.6) and the mean pore radius (1–10 nm) are fixed: it results in the range 10–105 s, thus showing a certain overlap with sext. The characteristic time for micropore diffusion (smicro, having the same meaning as s in Eq. (8)) was estimated in this work on the order of 105 s, and rearrangement of data in the literature gave for this parameter values on the order of 102–107 s, thus showing a certain overlap with smacro. In Fig. 6 the typical observed characteristic adsorption times (104–106 s) are reported, too. It can be observed a certain tendency of the systems under investigation to be controlled by metal micropore diffusion, as it was also highlighted for the results obtained in this research.

5. Conclusions The beneficiation of a coal combustion ash by either steam- or CO2-gasification was investigated by means of tests on cadmium adsorption. The CO2-gasified ash appeared as a better Cd2+ sorbent than the steam-treated material. The plausible development, upon CO2 gasification, of a chemically more kindred surface with respect to Cd2+ could justify the higher values observed with reference to this sorbent for the specific adsorption capacity (maximum value of 3.2 vs. 2.4 mg g1), the equilibrium constant (1.1 vs. 0.3 L mg1), the removal efficiency (maximum value of 96% vs. 86%) and the initial adsorption rate (1.5  102 vs. 1.0  102 mg g1 min1). Both gasification treatments were indeed able to generate a material (through a decrease in particle size, an increase in porosity and chemical modifications in the ash organic matrix) with improved sorbent characteristics, and showing higher removal efficiencies at low pollutant/sorbent ratios, an interesting feature when the purification of wastewaters with low pollutant concentration must be dealt with. An analysis concerning the characteristic times of the processes involved in adsorption was reported, too, highlighting the tendency of the investigated system cadmium/ash (and of similar systems reported in the literature) to be controlled by metal micropore diffusion. Acknowledgments Authors wish to thank Enel Produzione Ricerca (Italy) for the supply of the raw ash, and Mr. Fabio Magri for his experimental support.

observed characteristic adsorption times 10-2

In selected literature

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