Removal of chlorinated phenols from aqueous solutions by adsorption on alumina pillared clays and mesoporous alumina aluminum phosphates

Pergamon PII: S0043-1354(97)00206-6

War. Res. Vol. 32, No. 2, pp. 295-302, 1998 © 1998 ElsevierScienceLtd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00

REMOVAL OF CHLORINATED PHENOLS FROM AQUEOUS SOLUTIONS BY ADSORPTION ON ALUMINA PILLARED CLAYS A N D MESOPOROUS ALUMINA A L U M I N U M PHOSPHATES THEOPHARIS G. DANIS, T R I A N T A F Y L L O S A. ALBANIS®*, DIMITRIOS E. PETRAKIS and PHILIP J. POMONIS Department of Chemistry, University of Ioannina, Ioannina 45110, Greece (First received March 1996; accepted in revised form June 1997)

Abstract--Alumina pillared montmorillonite (AIPMt) and mesoporous alumina aluminum phosphates (AAPs) were contacted with aqueous, 2,4-dichlorophenol, 2,4,6-trichlorophenol and pentachlorophenol solutions, at a concentration range between 25 and 250 #g/litre, in batch equilibrium experiments, in order to determine their adsorption properties. The removal of chlorophenols by the adsorbents increase with increasing chlorine substitution in their molecules. In the case of pentachlorophenol, the increased affinity allows adsorption to occur much more efficiently than in the case of other compounds. AIPMt material adsorbs 26.3% of 2,4-dichiorophenol, 75.6% of 2,4,6-trichlorophenol and 95.2% of pentachlorophenol at equilibrium. The adsorption of chlorophenols on mesoporous AAPs is much less pronounced as compared to clays but increases with the ratio of P/AI, as the surface acidity of those solids also increases. The AAP mixture with a ratio P/AI = 0.6 adsorbs 14.8% of 2,4-diehlorophenol, 27.1% of 2,4,6-trichlorophenol and 58.3% of pentachlorophenol. The amounts of chlorophenols decomposed during the treatment increase in AAPs and especially in those with a higher P/AI ratio (=0.6). © 1998 Elsevier Science Ltd. All rights reserved Key words--chlorophenols, alumina pillared montmorillonite, alumina aluminum phosphates, removal, adsorption, degradation

INTRODUCTION

Among the different pollutants of aquatic ecosystems, phenols, especially the chlorinated ones, are considered as priority pollutants since they are harmful to organisms even at ppb levels (Chapman et al., 1982). Chlorinated phenols with less than three chlorines are not used extensively, except in the production of higher chlorophenols and chlorophenoxyacetic acid herbicides. They have been used in many applications over the last 50 years, and especially pentachlorophenol (PCP) is used mainly as a fungicide and insecticide for wood preservation. PCP and some tetrachlorophenois are used primarily as wood preservatives or fungicides (Exon, 1984). Residues of chlorophenols have been found worldwide in soil, water and air samples, in food products, in human and animal tissues and body fluids (Crosby, 1981; Paasivirta et al., 1985). Environmental contamination with these chemicals occurs from industrial effluents, decoloration of paper and paper mill effluents, agricultural runoff, breakdown of chlorophenoxyacetic acid herbicides and hexachlorobenzene and from spontaneous for*Author to whom all correspondence should be addressed.

mation following chlorination of water for disinfection and deodorization (Exon, 1984). The removal of such compounds at such low levels constitutes a difficult problem. Among the methods employed are either destructive oxidation with ozone (Hoignt, 1985), hydrogen peroxide (Moza et al., 1988; Kochany and Bolton, 1992), and manganese oxides (Ulrich and Stone, 1989; Ukrainczyk and McBride, 1992), often assisted by light (Moza et al., 1988; Kochany and Bolton, 1992; Lipczynska-Kochany and Bolton, 1992) or adsorption into porous solids such as activated carbon (Speitel et al., 1989; Paprowicz, 1990); fly ash (Bishop et al., 1990) and clays, either natural or pillared (Mortland et al., 1986; Boyd et al., 1988; Ziekle and Pinnavaia, 1988; Srinivasan and Fogler, 1990; Michot and Pinnavaia, 1991; Wang, 1991; Kowalska et al., 1994). Although activated carbons are among the most effective adsorbents with high surface areas and can be regenerated by thermal desorption or combustion of the toxicant in air, a substantial fraction of the carbon is lost with each oxidation cycle. This loss of adsorbent is a major economic consideration in any large-scale remediation application (Guymont, 1984). In the past few years, there has been increasing interest in develop-

295

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296

Table 1. Physiochemicalproperties of ehlorophenols used 2,4-Dichlorophenol Molecular weight Solubility in water (mg/litre) Log Kow* pKa

163 4500 2.90 8.15

2,4,6-Trichlorophenol

Pentachlorophenol

197.5 800 3.72 5.99

266.4 14 5.25 4.74

*Log Kow, logarithm of compounds partition coefficient between octanol and water ing recyclable inorganic adsorbents, particularly pillared clays for the efficient removal o f organic pollutants from aqueous solutions. Chlorinated phenols c a n b i n d to pillared clay surfaces by a variety o f m e c h a n i s m s depending o n p H conditions. I f the p H o f aqueous solutions is n o t m o r e t h a n one unit above the pKa value o f the c o m p o u n d , the cont r i b u t i o n of p h e n o l a t e sorption m a y be neglected (Schellenberg et al., 1984; Shimizu et al., 1992). The oxidation processes are facilitated by modified clays a n d o t h e r mineral surfaces containing metal ions, such as Cu, Fe, Ti a n d M n (Shindo a n d H u a n g , 1985; Ulrich a n d Stone, 1989; G a g n o n et aL, 1992). Nevertheless, such a d s o r p t i o n studies have often t a k e n place at p p m levels which are three orders o f m a g n i t u d e a b o v e the p p b levels imposed as acceptable limits by the EC. So, in the present study, the a u t h o r s considered work to check the a d s o r p t i o n capacity o f pillared clays at these low levels. Besides, they t h o u g h t t h a t a c o m p a r i s o n o f such m i c r o p o r o u s materials with m e s o p o r o u s a l u m i n a a l u m i n u m p h o s p h a t e ( A A P ) solids (Petrakis et al., 1991) m i g h t be o f interest as indicative o f the mechanisms controlling the process. MATERIALS AND METHODS

Chemicals and materials The tested chlorophenols in this study, 2,4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP) and pentachlorophenol (PCP) were purchsaed from Chem Service (West Chester, PA, U.S.A.) and used without further purification. The physiochemical properties of these chlorophenols are shown in Table I. Aqueous solutions of each chlorophenol were prepared in 0,01 M CaCI2 and the final concentrations were 25, 50, 100, 150, 200 #g/litre. The alumina pillared montmorillonite AIPMt) used was prepared from clay (Greek origin from the Aegean island of Melos) which contains almost 85% montmorillonite. The raw clay was crushed and the fraction of d < +200 gm was separated by precipitation in aqueous solutions. This clay fraction was exchanged by Na ÷ in a dialysis tube and pillaring was achieved by using an Al-hydroxide solution as pillaring agent, according Lahav et al. (1978). The Al-hydroxide solution was prepared by titrating aqueous 0.2 M AICI3 with aqueous 0.2 M NaOH until the OH/AI ratio was equal to 2. After aging for 30 days, the resulting solution was reacted with a proper amount of 1% aqueous suspension of Na-exchanged clay, keeping an Al/clay ratio of 2 nmol/g. The system was left for several hours and the supernatant was then removed after centrifugation. The sediment was filtered, air-dried and heated at 500°C in air before use. The XRD analysis at small angles of the pillared clay spread and dried at 500°C on a small glass plate, showed a rather broad peak corresponding to do01 spacing from 12-18 A. The specific sur-

face of 130+2m2/g was measured in a SORPTY 1750 Carlo Erba (Milano, Italy) single pint system and a SORPTY 1900 Fisous (Milano, Italy) apparatus and found 130 + 2 m2/g. The adsorption desorption loops of N2 at T = 77 K was of the H 3 type according to IUPAC classification. Treatment of these data according to standard software procedure on the SORPTY 1900 system resulted in a sharp peak of p.s.d, around 20-21 A. Nevertheless, such results are usually produced as an artefact steaming from the N2-N2 quadrupole interactions; therefore, the only conclusion that could be actually reached is that the AIPMt material is simply a microporous one. The mesoporous AAPs were prepared as reported by Petrakis et al. (1991) and had a P/AI ratio equal to 0.3, 0.4 and 0.6 with surface areas of 162-130m2/g-1 (BET) and pores with rp 80-100 A (Table 2).

Experimental procedure The batch adsorption experiments were carried out in 40-ml glass centrifuge tubes where 0.3 g of AIPMt or AAPs and 25 ml of the above chiorohphenol solutions (25-200 #g/litre) were added. The amounts of the sorbent as well as that of the aqueous phase were determined by weight. The centrifuge tubes were subsequently capped and shaken in a wrist-action shaker for 24 h at 20°C. Preliminary kinetic experiments demonstrate that adsorption equilibrium was reached within 24 h. At the end of the equilibrium period the samples were centrifuged at 6000g for 15 rain, the supernatant was obtained carefully for determination of chlorophenol equilibrium concentration and the pH was measured immediately. To the remaining soil, 25 ml of CaCI2 0.01 M were added. The centrifuge tube was capped and shaken for 6h. Preliminary kinetic experiments demonstrate that desorption equilibrium was reached within 6 h. After centrifugation the supernatant was taken for determination of chlorophenol desorption equilibrium concentration. A second desorption step was carried out using 15 mi of acetone and shaking for 30 min. Triplicate samples were run.

Analytical procedure All the aqueous phases were acidified with sulphuric acid 1:! (v/v) until pH 2 had been reached, and they were extracted three times with 5 ml dichloromethane. The combined extracts were filtered through 5 cm of anhydrous Na2SO4 and collected into a test-tube. Then, 2 ml of acetone and 250 #1 of 10% K2CO 3 solution were added to organic extract to ca 5 ml under reduced pressure (waterbath temperature 35°C). The concentrated extract was Table 2. Surface areas of adsorbent solids and their pH values in experimental aqueous suspensions (solid/water = 1.2 g per 100 ml) Adsorbent solids 1. Alumina pillared montmorillonite 2. Alumina aluminum phosphate (P/AI = 0.3) 3. Alumina aluminum phosphate

Surface area (mZ/g) 129

pH 4.9

162

5.3

139

5.1

130

4.8

(P/AI = 0.4)

4. Alumina aluminum phosphate (P/AI ~ 0.6)

297

Removal of chlorinated phenols by adsorption transferred quantitatively to a graduated centrifuge tube and immediately 2ml acetone were added to original flask, the container rinsed and set aside. The extract in the centrifuge tube was evaporated to 0.8 ml in a bath at 50°C under gentle stream of dry nitrogen. Then, 3 ml of 0.55% K2 CO3 solution were added to the acetone extract and the mixture was shaken. Next, 2 ml of n-hexane and 50/~1 of acetic anhydride were added to the sample and the mixture was stirred slowly for I min (Lee et al., 1984). The collected n-hexane phase was kept for GC analysis. The chlorophenols were determined by GC using a Vanan-3300 equipped with an EC detector, Ni-63. The column was 5% OV-17 + 4% QF-1 on chromosorb W, 80-100 mesh. Nitrogen was used as carrier gas (25 ml/ min) and the temperatures of the column, the injector and the detector were 180, 220 and 300°C, respectively.

RESULTS AND DISCUSSION

Adsorption of chlorophenols by alumina pillared montmorillonite ( AIPMt) Under the experimental pH conditions (4.6-5.3) chlorinated phenols will be present as the neutral

phenol and not as the phenolate form since their pK, values are 4.7-8.2 (Table 1). Figure 1 displays the adsorption isotherms at 20°C, of 2,4-DCP, 2,4,6-TCP and PCP from water onto AIPMt. The isotherms in Fig. I(A) exhibit marked differences for the chlorophenols. As revealed by the water solubility and partition coefficient between octanoi and water data (Table 1), the polarity of these compounds and, thus, their affinity towards the aqueous phase decrease with increasing chlorine substitution. As expected, the uptake of chlorophenols increases with the number of chlorines present. PCP, the most hydrophobic of these molecules, was adsorpbed at higher rates. In order to compare the intrinsic affinity of the adsorbent towards the adsorbate, one needs to replot the isotherms with regard to the degree of adsorption under saturation conditions, i.e. ln(Ce/Cs) (Cases, 1979), where Ce is the equilibrium concentration and Cs the solubility limit of the adsorbate. Figure

20(A)

m DCP • TCP m PCP

=k

o ee

10

o

0 0

200

100 Equilibrium

concentration

(~tg/L)

20

i

"

Dcr,

,

TCP

•

PCP

(B)

lO,

0 -13

- 12

-lI

-I0 -9 Ln (Ce/Cs)

-8

-7

Fig. i. Adsorption of chlorophenols on alumina pillared montmorillonite.

-6

Theopharis G. Danis et al.

298

(B) presents the results of such a treatment, and it is possible to draw the following conclusions from the results. The affinity of the surface towards the pollutant increases in the order PCP > 2,4,6TCP>2,4-DCP as the isotherms shift towards lower ln(Ce/Cs) values, in that order. It is interesting to note that the three isotherms are similar in shape. This suggests that the mechanism of adsorption is the same for these three compounds (Michot and Linnavaia, 1991). The isotherms of the three chlorinated phenols show the type S characteristics, which implies that adsorption becomes easier as the concentration in the liquid phase increases (Calvet, 1989). Acccording to Giles et al. (1960), this is observed when the solute molecule is monofactional, has a fairly large hydrophobic part (> C5), has a moderate intermolecular attraction leading to a vertical packing in the adsorbed layer, and meets a strong competition for sites from molecules of the solvent.

Such isotherms are characteristics of low weak adsorbate-adsorbent interactions, which at low concentrations are small (Mortland et al., 1986).

Adsorption of chlorophenols by alumina aluminum phosphates (AAPs) Figures 2-4 display adsorption isotherms at 20°C of 2,4-DCP, 2,4,6-TCP and PCP from water onto AAP mixtures with P/AI ratios equal to 0.3, 0.4 and 0.6, as well as in comparison with the adsorption onto AIPMt. The isotherms in Fig. 2 exhibit marked differences for the chlorophenols. The adsorption of the three compounds increases as the stoicbiometric ratio of P/AI increases from 3:7 to 6:4 in AAP mixtures. This increase of A/AI ratio provokes a significant decrease in specific surfaces of adsorbents but a substantial increase in their surface acidity (Table 2). As expected, the chlorophenols uptake by each AAP mixture increases with the number of chlor-

8"

(A)

•

DCP TCP

2

,I

0

T 100

0

Equilibrium concentration

•

200

(~g/L)

(B)

DCP

• TCP • PCP

i

4'

"o io

/

2 I

,¢ 0

•

-14

-12

,

-10

-8 Ln (Ce/Cs)

!

-6

-4

Fig. 2. Adsorption of chlorophenols on alumina aluminum phosphate with a P/A1 ratio of 0.6

Removal of chlorinated phenols by adsorption

299

/

20 A

i

m AAP-0,3

# AAP-0A • AAP-O,6 A AI-Mont

10 u O ,¢ •

[]

• 0T

FI $

~,

I

0

200

I00

Equilibrium

concentration

(Izg/L)

Fig. 3. Adsorption of pentachlorophenol on alumina pillared montmorillonite (AIPMt) and alumina aluminum phosphates (AAPs).

ines present. PCP, the most hydrophobic of these molecules, was adsorbed at higher amounts on the three AAP sorbents. The replotting of isotherms with regard to the degree of saturation conditions, ln(CJCs) (Cases, 1979) show that it is possible to draw the following conclusions from the results. The affinity of the surface towards the pollutant increases in the order PCP > 2,4,6-TCP > 2,4-DCP as the isotherms shift towards lower ln(CJCs) values. The observed isotherms are also similar in shape, and this suggests that the mechanism of adsorption is the same for these three compounds (Michot and Pinnavaia, 1991).

The isotherms of the three chlorinated phenols gave the type S characteristics with the three AAP adsorbents.

Removal of chlorophenols by adsorption and degradation The calculation of mass balances of chlorophenols was based on the following equations: [Initial amount] = [Free amount] + [Adsorbed amounq The adsorbed amount could be analysed as follows:

""

¢~ AAP- 0.3

::t.

• AAP- 0.4 • AAP- 0.6

S~4

# AI-Mont.

4

,,

0

100 Equilibrium

concentration

200 (ttg/L)

Fig. 4, Adsorption of 2,4,6-trichlorophenol on alumina pillared montmorillonite (AIPMt) and alumina aluminum phosphates (AAPs).

300

T h e o p h a r i s G. D a n i s et al.

Table 3. Amounts of chlorophenols in equilibrium concentration, adsorbed, desorbed with water and acetone and remain adsorbed or degradated in aqueous suspensions of alumina pillared montmorillonite (n = 3) a Compounds

Equilibrium conc. (%)

Adsorption (%)

73.7 24.4 4.8

26.3 75.6 95.2

2,4-DCP 2,4,6-TCP PCP

Desorption in water Desorption in acetone Decomposition (%) (%) (%) (or bound residues 15.2 (57.6)** 13.1 (17.3) 7.4 (7.8)

8.5 (32.4) 55.9 (73.9) 79.8 (83.9)

2.6 (9.9) 6.6 (8.7) 7.9 (8.3)

aTriplicate experiments. Listed are the amounts with regard to the initial concentration and, inside of the parentheses, the amounts with regard to the adsorbed amount

Table 4. Amounts of chlorophenols in equilibrium concentration, adsorbed, desorbed with water and acetone and remain adsorbed or degradated in aqueous suspensions of alumina aluminum phosphate, P/AI = 0.6 (N = 3)a Compounds

Equilibrium conc. (%)

Adsorption(%)

85.2 72.9 41.6

14.8 27.1 58.4

DCP TCP PCP

Desorption in water Desorption in acetone Decomposition (%) (%) (%) (or bound residues) 6.3 (42.3)** 14.4 (52.9) 30.2 (51.9)

2.0 (13.4) 4.6 (17.1) 12.9 (22.1)

6.7 (44.4) 8.1 (29.9) 15.2 (26.0)

aTriplicate experiments. Listed are the amounts with regard to the initial concentration and, inside of the parantheses, the amounts with regard to the adsorbed amount

[Adsorbed amount] = [Desorbed amount with water] + [Desorbed amount with acetone] + [Decomposed or bounded residues] The decomposed or permanent adsorbed amount is calculated from the difference between adsorbed amount and the sum of the desorbed with water and acetone amounts. The mass balances for the removal by adsorption as well as the desorption of chlorophenols from the AIPMt and AAP aluminum phosphate with a P/AI ratio of 0.6 are summarized in Tables 3 and 4 and are illustrated in Figs 5 and 6, respectively. Tables 3 and 4 list the corresponding amounts with regard to the initial concentration and, inside the parantheses, the amounts with regard to the adsorbed amount. From the results depicted in Figs 5 and 6, it can be easily seen that the AIPMt is a very good adsorbent of much superior capacity than AAP mixtures.

•

It should be noted that desorption of PCP and TCP from the studied adsorbent materials (AIPMt) was very limited into water solutions and showed a significant hysteresis compared to adsorption, but these compounds were desorbed in high level with acetone. In A A P materials, it seems that the adsorption capacity increases with the addition of phosphorus, which increases the surface acidity, but never reaches the capacity of AIPMt. In these materials the adsorptive capacity increased as the ratio of P/A1 increases. A change of the ratio of P/AI from 3:7 to 6:4 increases the adsorption amount of PCP from 2.8 to 67.9%, for the range of concentrations 50-20 #g/litre. The elimination of chlorophenols in AAP suspensions is increased as compared to the AIPMt suspensions (Table 4 and Figs 5 and 6). It is considered that this fact indicates a kind of catalytic degra-

A d s o r p t i o n (%) Degradation*(%)

""

80

Desorption (%) in water Desorption (%) in acetone

40 .

20'

0~

2.4-DCP

2,4.6~TCP

PCP

Fig. 5. R e m o v a l o f c h l o r o p h e n o l s by a d s o r p t i o n a n d d e g r a d a t i o n on a l u m i n a pillared m o n t m o r i l l o n i t e a n d d e s o r p t i o n w i t h w a t e r a n d acetone.

Removal of chlorinated phenols by adsorption

5o

4O

•

Adsorption (%)

•

Degradation*(%)

•

Desorption (%) in water Desorption (%) in a c e t o n e

g~l

301

20

0

~ 2,4-DCP

2,4,6-TCP

PCP

Fig. 6. Removal of chlorophenols by adsorption and degradation on alumina aluminum phosphate P/ Al 0.6 and desorption with water and acetone.

dation or transformation of chlorophenols by AAP materials and that this degradation is significant in case of PCP (15.2%). It seems that the acidic surface of AAP has the ability to degradate the adsorbed chlorophenol molecules. As already concluded from the isotherms, adsorption of chlorophenols increases as the number of chlorines in the phenolic ring increases too. The removed amounts of chlorophenols by adsorption on AIPMt reach 26.3% for 2,4-DCP, 75.6% for 2,4,6-TCP and 95.2% for PCP. The removed amounts of these compounds by using the AAP with a P/AL ratio of 0.6 (the best adsorbent among the tested AAPs) is lower as compared with AIPMt and reaches 14.8% for 2,4-DCP, 27.1% for 2,4,6TCP and 58.4% for PCP. The amounts of chlorophenol desorption in water from AIPMt are between 15.2 and 7.4% with regard to the initial experimental concentrations (57.6-7.8% with regard to adsorbed amounts, Table 3). The desorption with acetone, from the AIPMt, increases with the number of chlorines in the phenolic ring, 8.5% for 2,4-DCP, 55.9% for 2,4,6-TCP and 79.8% for PCP. The amounts of chlorophenol desorption in water, from AAP-0.6, are between 6.3 and 30.3% (Table 4). The desorption with acetone increases with the number of chlorines in the phenolic ring, 2,0% for 2,4-DCP, 4.6% for 2,4,6-TCP and 12.9% for PCP. The amounts defined as decomposed or permanently adsorbed increase in AAPs and especially in those with a higher P/A1 ratio (=0.6) which have the more acidic surface. The degradated and/or bound residues of chlorophenols by this material reach 6.5% for 2,4-DCP, 8.1% for 2,4,6-TCP and 15.2% for PCP with regard to initial amount, and 44.4, 29.9 and 25.9%, respectively, with regard to adsorbed amounts.

CONCLUSIONS

It has been demonstrated here that AIPMt and AAP exhibit interesting variations in their adsorption properties with regard to removal of chlorinated phenols from aqueous solutions. Although the two kinds of materials indicate similar pH values in their water suspensions, as well as similar specific surface areas, their adsorption behaviour is quite different and AIPMt is a very good adsorbent of much superior capacity than AAP mixtures. This is in spite of the large pores of the phosphates and the small pores of the clay, a property which does not seem to affect the final outcome. As expected, the uptake of chlorophenols by adsorbents increases with increasing chlorine substitution in their molecules. In the case of PCP adsorption on AIPMt, an improved affinity allows adsorption to occur much more efficiently than in the case of other chlorinated phenols and on AAP. TCP and PCP were desorbed from AIPMt in acetone over 73.9% with regard to the adsorbed amount, The amounts defined as decomposed are increased in AAPs and especially in those with a higher P/A1 ratio (=0.6) which have the more acidic surface. These surface chemical effects may be very useful in designing a variable adsorbent for groundwater treatment applications. The authors acknowledge the support of the General Secretariat of Research and Technology under PENED Grants, Athens. Greece, 91-EA-601. Acknowledgements

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