The adsorption of various pollutants from aqueous solutions on to activated carbon

Water Res. Vol. 19. No. 4. pp. 491~t95. 1985 Printed in Great Britain. All rights reserved

0043-1354 85 53.00 + 0.00 Copyright ~ 1985 Pergamon Press Ltd

THE ADSORPTION OF VARIOUS POLLUTANTS FROM AQUEOUS SOLUTIONS ON TO ACTIVATED CARBON G. MCKAV *, M. J. BINO: and A. R. ALTAMEMI" *Department of Chemical Engineering, Queen's University, Belfast, Northern Ireland and -'Royal Scientific Society, Amman, Jordan (Received July 1984)

Abstract--The ability of activated carbon, Filtrasorb 400, to adsorb various pollutants from aqueous solutions has been studied. The pollutants investigated are phenol, p-chlorophenol, sodium dodecyl sulphate, mercuric ions and chromic(Ill) ions. The saturation adsorption capacity of the activated carbon for the pollutants is 213,434, 361, 35 and 138 mg g- i for phenol, p-chlorophenol, sodium dodecylsulphate, chromium(IlI) and mercuric(ll) respectively. Equilibrium isotherm analyses were undertaken using Langmuir and Freundlich equations. Key words--adsorption, activated carbon, water pollution, equilibrium isotherms, chlorophenol, sodium dodecyl sulphate, mercuric ions, chromium(III) ions

NOMENCLATURE a = Langmuir constant (dm 3mg -L) c = dimensionless liquid phase concentration Co = initial concentration of pollutant in solution (mg dm- ~) C, = Concentration of pollutant in solution at equilibrium (rag din-3) K = Langmuir constant (dm 3 g-~) K r --- Freundlich constant (mg g-t ) m = concentration of carbon in solution (g dm -3) M --- mass of carbon (g) N = Freundlich exponent q = dimensionless solid phase concentration Q,--- concentration of pollutant in the carbon (mgg -~) r = dimensionless separation factor X = weight of pollutant adsorbed (rag).

INTRODUCTION Activated carbon adsorption offers one of the most efficient processes available for removing certain organics and inorganics from wastewater. The capacity of activited carbon for a certain type of pollutant is important for designing adsorption contacting systems. The aim of the present work is to determine the adsorption capacity of activated carbon for a number of substances in aqueous solution, namely, phenol, chlorophenol, dodecyl hydrogen sulphate sodium salt (SDS), Hg 2+ and Cr 3+. Phenolic compounds (Lewis, 1980) are discharged in wastewaters from industries such as refineries, gas undertakings, pharmaceutical and pesticide (Nemerow, 1971) manufacturers etc. SDS is widely used in many applications as in lube oils to stabilize anti: oxidants in photographic developers, tin plating and many others. Certain inorganic pollutants can create severe problems in conventional wastewater treatment 491

phenol, p-

plants. In the trivalent state chromium is not believed to be toxic but it does inhibit the growth of microorganisms in activated sludge processes. Trivalent chromium is discharged f r o m tanneries and certain metal treatment plants. Mercury ions, Hg -'+, in industrial effluents are frequently related to the discharge of effluent from battery cell manufacturers and chloralkali plants (Cowley et aL, 1966; Caban and Chapman, 1972). Mercury is most toxic to man in its methylated form and certain microorganisms in water are capable of the conversion of mercury to methyl mercury. A series of experiments have been performed in this work to assess the capacity of activated carbon, Type Filtrasorb 400, to adsorb several contaminants from aqueous solutions. Adsorption isotherms have been determined and analysed according to the Langmuir and Freundlich equations. EXPERIMENTAL Materials The solutes used were obtained from BDH chemical suppliers. The isotherms were all conducted using distilled water and all the chemicals studied were readily soluble in water. The granular activated carbon was Filtrasorb 400 and it was supplied by Chemviron Ltd. Table 1 shows the physical properties of this carbon. The carbon was milled in a ball mill and then sieved into various particle size ranges as shown in Table 2. The sieved carbon was washed with distilled water to remove fines and dried to constant weight at 105:C. Drying for 24h was usually sufficient to maintain constant weight, after which the carbon was kept in a desiccator. Analysis Phenol and p-chlorophenol absorb ultraviolet radiation at ).,~x = 270 and 280nm respectively. A Perkin-Elmer Model 550-S spectrophotometer was used to determine concentrations of phenol and p-chlorophenol. Dodecylhy-

G. McKAY et al.

492

Table I. Physical properties of Fihrasorb 400 (I.05--1.2) x 106 Total surface area [(N., BET method)m: kg-'] 2100 Solid phase density (kg m -J) 1300-1400 Particle density (wetted in water) (kg m -~) 0.38 Porosity 1000-1100 Iodine number 260-300 Methylene Blue number

Table 2. Carbon particle size range Particle size range

Average particle diameter (dp)

(~m)

(~m)

150-250 250--355 355-500 500-710 710-1000

200 303 428 605 855

Mesh size 65 48 35 28 20

drogen sulphate (SDS) concentrations were measured usmg Beckman Model 915-B Total Organic Carbon Analyser. Hg2~ and Cr 3* were determined using Perkin-Elmer 300 atomic absorption spectrophotometer. Mercury concentrations were measured by the sodium borohydrate flameless technique, while Cr 3" was determined using an air/acetylene flame.

Experimental Stock solutions of the adsorbates were prepared with distilled water in the desired initial concentration and subsequent concentrations were made by dilution. Adsorption isotherms were obtained by shaking a fixed weight of carbon with 0.05 dm ~ of pollutant solution of known initial concentration. Various solute concentrations were used and the system was allowed sufficient time to reach equilibrium. The effect of carbon particle size was also studied. The adsorbed amounts were calculated from:

Q~

x

Co-C

M

rn

(1)

where m

=

g carbon dm -3

RESULTS AND DISCUSSION

Equilibrium isotherms Adsorption at equilibrium conditions was determined for each pollutant separately. Phenol solutions

were observed to become slightly turbid after 4 days of contact with carbon. This was attributed to some sort of bacterial growth in the solution and in particular the liquid-solid interface. Therefore, a procedure whereby a vigorous shaking of adsorbent/solution mixtures of all solutes, except p-chlorophenol, for 2 days was found enough to attain equilibrium, pChlorophenol, on the other hand did not develop such complications and a contact time of up to 4 weeks was found necessary to establish equilibrium using a much lower degree of agitation. Adsorption isotherms of the studied organic compounds were of the Langmuir form and typical of those encountered in the adsorption of most organic compounds from dilute aqueous solutions (Weber and Morris, 1964; Glasstone, 1951; McKay, 1982). Adsorption isotherms of mercuric and chromic ions were of the Freundlich type. Figures 1-5 show the adsorption isotherms of the studied adsorbates; Figs 1, 2, 4 and 5 show isotherms for different particle size ranges and Fig. 3 shows the effect of changing pH on the adsorption of Cr 3+ ions. The effect of pH on the adsorption of Cr 3+ was studied to a limited extent and is shown in Fig. 3. Cr 3÷ tends to form a precipitate at pH higher than pH 5.0 and therefore adsorption by carbon is difficult to quantify at the higher pH value of 5.7 and the " t r u e " adsorption at this pH is masked by precipitation. Consequently only the data at pH 5 are analysed to determine the isotherm constants. The adsorption curves in Figs 1, 2, 4 and 5 indicate a particle size effect. The difference in adsorption capacity with particle size is small but significant, it is not proportional to the external surface area of the

200

///.

00<.m,

100 O 605 T •

I

100

I

I

200 300 C, (mg din-31

18°C

I

I

400

500

Fig. 1. Adsorption isotherms for phenol on carbon.

Adsorption of pollutants on to carbon

5oo~

493

75

o•O57 • 50

400

i"

S 3O0

50

dp • 303/t.m

dp(~m)

~A /.~

o /

0 200 A 303 ~ 428 • 605

d

T - 18"C

"~'------"

25

o

/

~

• 855

x x

200

T-20"C

[ 100

I 200

Ce ( mg dm -3 ) 100

Fig. 3. Effect ofpH on the adsorption isotherm for Cr 3~ on carbon.

I

I

100

200

I

400

3OO

Ce(mg dm -3 ) Fig

2 Adsorption

isotherms for pochlorophenol carbon.

on

particles. Consequently, the effect is difficult to explain. It is possible that the adsorbate molecules may be unable to penetrate all the internal pores within the carbon particles. The greater the diffusion path (proportional to particle radius), the more difficult

o

150

[

o

A

100 E:n

d 5O

0

I

I

50

100

C, (mg dm "3)

Fig. 4. Effect of particle size for the adsorption of Hg -'÷ on carbon.

400

300 A-----'---

>/K / 6

dp (,u.m)

oo :

20O • 428 • 605

100

T , 27"C

I

I

I

I

100

200

300

400

I

500

I

I

600

700

Ce (rag dm-3) Fig. 5. Adsorption isotherms for sodium dodecyl sulphate on carbon.

G. McKAY et al.

494

so L

becomes complete saturation of the particle; the greater will be the probability that the solute molecules will come up against pores too small for the solute to penetrate. Alternatively it is possible that the fine particles are derived from the highly activated outer portion of the granules of carbon and the coarser particles are from the less activated inner core. Equations (2) and (3) represent the Langmuir and the Freundlich equations respectively.

X KC,, ;vl 1 + aC~ Z O = 7 ~ = K F C,t.,.v.

Q, = - - = 9 - -

1

u

(2)

0.1

(3)

Figures 6 and 7 show the Langmuir isotherm plots for phenol and chlorophenol. The linear forms of Langmuir and Freundlich isotherms are given by equations (4) and (5) respectively. The Freundlich plots for four of the adsorbates on to carbon, size range 500-710/~m, are shown in Fig. 8. As the isotherms reach saturation and monolayers are formed the Freundlich plots are not applicable but up to monolayer formation the plots in Fig. 8 are linear. C~

0.3

;o I 100

1 50

I 150

Ce ( m g d m -3 )

Fig. 7. Linearized Langmuir plot for p-chlorophenol adsorption. 2.B~

a

X/M = -K + K C¢

(4)

X 1 In ]-~ = In Kr + ~ In C,.

(5)

16

The Langmuir and Freundlich isotherm constants of the relevant adsorbates are listed in Table 3. The theoretical plots shown in Fig. 9 show that both Freundlich and Langmuir equations are in agreement with experimental results for low values of C,. The Langmuir isotherm gives the best agreement over the whole adsorption range since it predicts the

1.2

. ~ ~, Phenol o /P - ChloropheooI • Mercuric ions • Sodium dodecyl sulphate I 1.0

I 2.0

log C,

Fig. 8. Freundlich isotherm plots for the adsorption of phenol, p-chlorophenol, mercuric ions and sodium dodecyl sulphate on to carbon (500-710pm).

0.006

"~ ~ ~- ~'-

•

/ l 0.50

•

"

0.6

-i

^( ." ." / 2' ,'.," / I d / I

I

/

/

/

/ /

/

/ /

/ I

0.5

0.4

025

/

5

(

/ 0

i

i

50

100

Ce (rag drn-3)

Fig. 6. Linearized Langmuir plot for phenol adsorption on carbon.

0

02

I 0 4

I 0.6

I 0.8

1.0

c Fig. 9. Dimensionlessconcentration isotherms on functions of separation factor. C) SDS, r = 0.006: A Hg2", r = 0.17; • Cr 3-, r =0.23.

Adsorption of pollutants on to carbon

495

Table 3. Langrnuir and Freundlich constants for activated carbon. de 500-710~m Langmuir Freundlich K a K "a Kr 1 Pollutant (din3m8-' ) (dm~g-K) (mgg-L) (mgg-t ) =

Phenol p-Chlorophenol SDS Cr3" Hg"*

0.10 0.1-14 0.180 0.015 0.034

21.3 62.50 65.06 0.52 4.68

experimentally observed monolayer coverage better than the exponentially increasing Freundlich isotherm. The theoretically predicted Langmuir monolayer coverages, defined by the K/a ratio in Table 3, are 213, 434, 361, 35 and 138mgg -t carbon for phenol, p-chlorophenol, sodium dodecyl sulphate, chromic and mercuric ions. There is a large difference in the monolayer adsorption capacities between phenol and chlorophenol. Although chlorophenol is a larger organic molecule than phenol, its adsorption capacity is around 400 mg g-' carbon, having a small particle size dependence, whereas the adsorption capacity of phenol is around 210-230mgg -~ carbon. T h e difference in adsorption capacities must therefore be linked to the solute adsorbent bonding. The presence of the chlorine atom produces a more polar charge over the molecule and enhances its affinity for adsorption at the carbon surface.

213 434 361 35 138

50 126 72 2.3 20

0.26 0.25 0.27 0.'4Z 0.46

Values of r < 1.0 represent favourable adsorption and values greater than 1.0 represent unfavourable adsorption. The results for three of the adsorption systems, namely, sodium dodecyl sulphate, Hg -'+ and Cr ~+ are shown in Fig. 9 in the form of the dimensionless isotherm. The r values for all three systems are favourable. Theoretical isotherms

The theoretical isotherms can be obtained from equations (2) and (3) for Langmuir and Freundlich analysis. The theoretical results are compared with the experimental results in Fig. 10. The agreement between experimental values and theoretical data is good over most of the isotherm except at high Ce values where the Langmuir gives the best fit.

CONCLUSION

Effect of isotherm shape

The influence of isotherm shape on whether adsorption is "favourable" or "unfavourable" has been considered Weber and Chakravorti (1974). For the Langmuir-type adsorption process the isotherm shape can be classified by a term "r" a dimensionless constant separation factor. 1

r = - 1 + aC0"

(6)

The adsorption capacity of five pollutants in ous solutions on to activated carbon has been studied. The Langmuir and Freundlich constants have been determined. The Langmuir monolayer adsorption capacities for phenol, p-chlorophenol, sodium dodecyl sulphate, mercuric ions and chromium ions on to carbon are around 213, 434, 361, 138 and 35 mg g - ' respectively. The capacities show a small particle size dependence possibly due to the fact that the solute molecules cannot completely penetrate all the pores within the carbon particles.

REFERENCES

L_____

I

100

I

'

200 3(30 Ce (mq dm -3)

1

400

Fig. 10. Comparison of theoretical isotherms for phenol. A Experimental; © Langmuir; • Freundlich.

Caban R. and Chapman T. W. (1972) A.I.Ch.E.J! 18, 892. Cowley W. E. et al. (1966) Chem. Engng 345. Glasstone S. (1951) Textbook of Physical Chemistry, 2nd Edition. Macmillan, London. Lewis W. M. (1980) Developments in Water Treatment. Applied Science, London. McKay G. (1982) J. Chem. Technol. Biotechnol. 32, 759. Nemerow N. L. (1971) Liquid Wastes from Industry-Theories, Practices and Treatment. Addison-Wesley, Philippines. Weber T. W. and Chakravorti R. K. (1974) A.LCh.E.JI 20, 228. Weber W. J. Jr and Morris J. C. (1964) J. sanit. Engng Div. Am. Soc. cir. Engrs 90, 79.