~ Pergamon

PH: S0273-1223(97)QO 112-1

Wal. Sci. Tech. Vol. 35. No.7. pp. 39-46. 1997. Ie 1997 IA WO. Published by Elseyier Science Lid

Printed in Great Britain. 0273-1223197 $17'00+ 0-00

REMOVAL OF PHOSPHORUS FROM WASTEWATER BY ACTIVATED ALUMINA ADSORBENT

Tadashi Hano*, Hirokazu Takanashi *, Makoto Hirata*, Kohei Urano** and Shunji Eto***

• Department of Applied Chemistry. Oita University. Dannoharu 700. Oita 870-11. Japan •• Laboratory of Environmental and Safety Engineering. Yokohama National University. 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240, Japan ... Aquatech Ltd. 23-2 Yorozucho, Hachioji 192, Japan

ABSTRACf

The adsorption characteristics of activated alumina treated with aluminum sulfate were studied to develop a new removal process for low concentration phosphorus in the waters of rivers and lakes. The equilibrium study showed that the adsorption capacity was enhanced about 1.7-fold by treating with aluminum sulfate. The effective intraparticle diffusion coefficients. detennined by the Boyd's method based on batch runs, were hardly affected by such a low phosphorus concentration as observed in the water of rivers and lakes under investigation. The temperature dependence of the intraparticle diffusion coefficients based on the concentration in solid showed the activation energy of 29.7kJ·mor- I , which was a little higher than that in usual pore diffusion. The maximum continuous operation term (regeneration cycle) of the present phosphorus adsorption system was estimated. Under the conditions of influent phosphorus concentration of 0.1 g. m-3, removal extent of 90%, particle size of 2X 1Q-3m, temperature of 298K and space velocity of 1.39 X 1O-3s-1 (5h-I), the present removal system remained effective for about 500 days. © 1997 IA WQ. Published by Elsevier Science Ltd

KEYWORDS

Activated alumina, adsorption, closed water area, eutrophication, phosphorus removal

INTRODUCTION

Phosphorus is one of the main causes of eutrophication and several processes have been developed to remo~e it from wastewater. Almost all of them are, however, developed for industrial or domestic wastewater m which phosphorus concentration is rather high. So far, an effective and economical process applicable to low concentration phosphorus in surface water has not been established yet.

The adsorption method is considered to be one of the best treatment processes for waters of rivers and lakes since it is effective especially for phosphorus of low concentration. Many adsorbents for phosphorus have been investigated (Yee. 1966; Ames and Dean, 1970; Okubo and Matsumoto, 1984; Hashimoto and Ozaki, 1985; Brattebo and Odegaard, 1986; Suzuki and Fujii, 1987). Recently, a novel adsorbent prepared by treating activated alumina with aluminum sulfate has been proposed for both industrial and domestic

39

40 T. HANO~' al.

wastewater treatments (Urano and Tachikawa. 1991a,b). This adsorbent can remove any type of inorganic phosphates and various coexisting inorganic anions do not disturb the adsorption of phosphate. except when the concentrations of nitrate and sulfate ions are high. Furthermore. this adsorbent satisfies the following general requirements: (I) high capacity and selectivity; (2) granular type; (3) high chemical and physical strength; (4) no hazardous pollutants contained; (5) simple regeneration; (6) low cost and sustainable supply.

Another advantage of adsorption method is the simplicity of phosphorus recovery. Phosphorus is one of the most important resource. but it will be lacking in the near future. For example. Kotabe (1987) pointed out that it would be exhausted in 2035. Therefore. the establishment of phosphorus recycling process is an urgent subject to be investigated. The waters of eutrophied lakes will become one of the phosphorus resources of the highest quality when such a process is developed. The phosphorus on the adsorbent can be desorbed by alkali and phosphorus desorbed can be coagulated by CaCI2 as Ca(H2PO,v2 which is a useful chemical manure (fertilizer).

The aims of the present study are to analyze the adsorption behavior of new activated alumina developed by authors and to evaluate the adsorbent from the viewpoint of long-term applicability to the removal of low concentration phosphorus.

MATERIALS AND METHODS

Preparation of activated alumina treated by aluminum sulfate

Table I shows the principal properties of the activated alumina particles supplied from Mizusawa Industrial Chemicals. Ltd. and used as the base of the present adsorbent. The particle size of alumina available was 1.41-3.36X JO-3m. It was crushed and the particles of 1.25-1.77 X l0-4m and 4.2-S.0X l0-4m were used to evaluate the characteristics easily and quickly. This activated alumina had a large specific surface area and pore volume.

Table 1. Principal properties of activated alumina

Particle size Specific surface area Pore volume Average pore diameter Chemical composition Si02 Ai2O:3

(m) (m2'g- l) (m3'g-') (m) (%)

1.25-1.77 X 10-4 or 200 5.0X 10-7 I.OX 10-8 9.0 80.6

4.2-S.0X 10-4

The treatment with aluminum sulfate was performed as follows. The activated alumina was put into 6mol· m-3

of aqueous aluminum sulfate solution at the prescribed solidlliquid ratio. Shaken for 24h at 298K. it was dried at 378K until its weight reached constant. Then. the alumina was washed by pure water sufficiently to remove the residual aluminum sulfate. 1be ratio of aluminum sulfate combined with activated alumina was calculated from the decrease of the aluminum sulfate concentration which was detennined by modified 8-<juinolinol method. This modified analytical method was similar to Japan Industrial Standard KOI02 except that toluene was used instead of chloroform.

Phosphorus removal by activated alumina adsorbent 41

Measurement of adsorption isotherm and evaluation of diffusion coefficient

The activated alumina was added into aqueous sodium biphosphate solution of 30g-P.m-3 to measure the adsorption isotherm at pH5. The concentrations of phosphorus before and after adsorption were measured by molybdenum blue method with 0.1 m cells according to Japan Industrial Standard KO I 02.

The intrapartic1e diffusion coefficients based on the concentration in solid. Dj '. were evaluated at various temperatures by the Boyd's method according to Eqs. (I) and (2).

F=Q,IQ"" -log(l-F2) = 41t2D;'t 12.3d2

(I)

(2)

where F is the adsorption coverage. Q, the amount adsorbed at time t, Q"" the equilibrium amount adsorbed, d the diameter of the adsorbent and t the adsorption time. The values of Dj ' were obtained from the slope of the plots of -Iog( I-F2) versus t.

The continuous adsorption runs were performed with columns of 4X IO-3m inner diameter and 0.070m length in which the activated alumina of 3.9X 1O-7m3 or 5.9X to-7m3 was packed. The particle size was 1.25-1.77 X 104 m. The aqueous solution of sodium biphosphate, 0.22g-P·m-3 or 0.68g-P.m-3, was fed to the column at the space velocity, S V. of 0.1 Os·1 or 0.2Is-1 at 298K and pH5.

RESULTS AND DISCUSSION

Optimum amount of aluminum sulfate to be combined with activated alumina

The relation between the amount of aluminum sulfate combined and adsorption capacity was investigated to find the optimum conditions. Figure I shows the effect of the combined amount on adsorption capacity under the same experimental conditions.

22r---------------------~

";'0) 20 ",0 b 18

-g 16 -e o -jg1 III If)

~ 12 .r: Co ~ 10 .r: a.

298K,168h dp=4.5-5.0 Xl 0-4m Adsorbent=O.l 9 Co=30 9 of P.m-3

l/q. volume=1.00X10-4m3

8~~-*----~-*----~-*----~ o 0.2 0.4 0.6 0.8 1.0

Aluminum sulfate combined [10-3mol.g-1)

Fig.1 Effect of aluminum sulfate combination on adsorption capacity

42 T. HANO et al.

It was found that the adsorbed amount of orthophosphate reached the maximum in the range of 2.0-6.0X lO-4mol aluminum sulfate per one gram of activated alumina and then dropped appreciably It I.OX 1O-3mol.g-l . We confinned 3.0X lO-4mol· g-1 as the best combined amount from the viewpoint of saving aluminum sulfate to be consumed. Hereafter. the adsorbent was prepared by combining 3.0X 10-4 mol.g- I .

TIle behavior observed in Figure I can be explained as follows. The combination of aluminum sulfate fonns a complex effective for adsorption as will be discussed at the next section. but simultaneously the pore volume decreases with increasing the amount of aluminum sulfate combined (Urano and Tachikawa. 1991a). Therefore. excessive combination decreases the surface area which plays an important role in adsorption capacity and the reSUlting optimum amount of combination appears. In addition. the decrease of average pore diameter may disturb the diffusion of biphosphate ion through the pores. which is likely to lower the adsorption rate.

Effect or combination or aluminum sulrate on adsorption isotherm

The adsorption isothenns at 298K were shown in Figure 2. The Freundlich adsorption isotherms for the combined activated alumina and the uncombined raw alumina could be represented by Eqs. (3) and (4). respectively.

For uncombined alumina Q= 8.7CIl4.7

For combined alumina Q = IS.OCI/5.S (3) (4)

where Q is the amount adsorbed and C is the eqUilibrium concentration. By combining aluminum sulfate. about 1.7-fold enhancement of the adsorption capacity was attained in low concentration of phosphorus (O.lg.m-3). In addition. a lower the value of exponent of Eq. (4) than that of Eq. (3) indicated that the combination of aluminum sulfate was effective to increase the adsorption power at low concentration range of orthophosphate which is common to the waters of rivers and lakes under investigation.

100 0 Combined alumina ... ,

CI • C> Uncombined alumina C')

b .-"C

~ 0 If)

~ If)

2 0 .c Q.

298K.168h If) 0 .c

dp:1.25- .77X10-4m c. 1 .01 .1 1 10

Equilibrium concentration [g-P.m-3j

Fig.2 Adsorption Isotherms of combined and uncombined activated alumina

Phosphorus removal by activated alumma adsorbent 43

The mechanism of phosphorus adsorption on the combined alumina is considered as follows (Vrano and Tachikawa, 199Ia). Aluminum hydroxide is fonned on the surface of activated alumina in water and then it combines with aluminum sulfate to fonn a complex salt as expressed in Eqs. (5) and (6), respectively. Finally, aluminum hydroxide and the complex may react with orthophosphate according to EQs. (7) and (8), respectively.

A1P3 + 3H20 -+ 2AI(OH)3 2AI(OH)3 + A1 2(S04)3 -+ A14(OH)6(S04)3

AI(OH)3 + H2P04 - -+ AlP04 + OH- + 2H20 A14(OH)6(S04)3 + 4H2P04 - -+ 4A1P04 + 3S0i- + 2H+ + 6Hp

(5)

(6) (7) (8)

The combination ratio of alumina in Fig.2 was 68%. The relationship between the combination ratio and the amount of aluminum sulfate added is under investigation.

Effective intraparticle diffusion coefficient

Figure 3 shows the breakthrough curves of orthophosphate at different S V. The curves of similar shape If different SV suggest that the rate detennining step of adsorption is the intraparticle diffusion. Hence, the effective intraparticle diffusion coefficient, De. was calculated from the breakthrough curves. Almost the same value of De' 9.0X 1O- 15m2·s- l , was obtained at the influent phosphorus concentrations, CO' of 0.22 and 0.68g.m-3. This is a little lower than the usual values reported in water treatment processes. Probably, the combined alumina do not have many pores. The same values of De at different Co suggest that the influence of phosphorus concentration on the intraparticle diffusion coefficient is negligibly small and the same De can be used for the waters of rivers and lakes under investigation.

1.0r------------.

o ~ 0.8 u

~ ,g 0.6 a. en o a 0.4 '0 CD g>

.:.! 0.2 rd

~

o

298K

• Co=0.22g-P.m-3

- Calculated curve

20 40 60 80 Feed time [h]

Fig.3 Breakthrough curves of orthophosphate at different SV

Temperature dependence of Intrapartlcle diffusion coefficients based on the concentration

in solid

Temperature dependence of D is important infonnation in the design of treatment plants. It is troublesome. however. to measure the br:akthrough curves at all temperatures. Therefore. the intraparticle diffusion

44 T. HANO et al.

coefficients based on the concentration in solid, D;", was evaluated instead of De at 218K, 288K and 298K by batch runs according to the Boyd's method. In calculating Dj ', it was assumed that the equilibrium amount of phosphorus adsOlbed at 218K and 288K is equal to that at 298K, since the temperature dependence of adsorbed amount was negligibly small. The Boyd's plots and the D;' obtained at various temperatures are shown in Figure 4.

It was confirmed that the Boyd's method could be applicable for the present use since the relationships between t and -log(l-F2) were linear. The value of D.' at 298K was calculated as 2.2 X lo-I4m2· s-I, but the value of 9.0X 1O-15m2's-1 was obtained as De from the breakthrough curves at 298K. This disagreement is considered to be caused by great difference of phosphorus concentration in bulk solution (Urano and Tachikawa, 1991b).

As the temperature dependence of D;' was not negligibly small, the activation energy, E, was evaluated from the plot of logarithmic Dj ' and the reciprocal of T as shown in Figure 5.

0.3,.....-------------.

0.2

dp: 1.25-1. nx 1 0-4m

C0=30g of P·m-3

~ o 298K 6. 288K o 278K .....

~

~ , 0.1

o 2 3 4 5 Adsorption time [h)

Fig.4 Boyd's plots at various temperatures

-22.2,.....-------------.

-22.4

-22.6 21-s

-22.8

-23.0

E.29.7kJ·mor1

-23.2 L-___ .J....---'----........ 3.3 3.4 3.5 3.6

103IT[,(1)

Fig.5 Relationship between the reciprocal of Tand logarithmic Di' of combined alumina

Phosphorus removal by activated alumina adsorbent 45

1be activation energy was obtained as 29.7kJ·mol- l , which was a little higher than usual in pore diffusion.

Estimation of maximum continuous operation term (regeneration cycle)

The breakthrough curves under various operational conditions can be predicted with the Freundlich's equation and De of the combined alumina. From these breakthrough curves, the maximum continuous opemtion terms, i.e., regenemtion cycle, ofthe present phosphorus adsorption system were estimated for discussion while the influent phosphorus concentmtion was kept at O.lg· m-3 and the removal extent at 90%. Figure 6 shows the relationships between SV and the maximum continuous operation tenns at 278K, 288K and 298K for combined alumina of various particle sizes. 1be optimum combination of particle size and S V can be decided from these figures. Based on these calculations, it may be concluded that the present phosphorus adsorption system can be feasible to actual waters of rivers and lakes satisfactorily due to sufficient adsorption capacity. For examfle, under the conditions of particle size of 2 X 1O-3m, temperature of 298K and S V of 1.39 X IO- sol (5h-'), the present removal system will function effectively for about 500 days.

~

o 60 0

40 0

~ 20 E

0

.sl r:: 60 o

0

~ ~ 400 o II) ::l g 200 r:: "" r:: 8 600 E ::l

.~ 400

::E 200

o

SVpO-3s-1j

20 40 60 80

298K ~9.0X'O.15m2~.'

4.0 3.5'3.0'2.5:-"-2.0(X10-3m)

288K ~ D(.5.9~lO.15m2.S.'

3.5\3.02.5 2.0(X10- m)

278K Di~3.8X10-15m2.s-1

~ 3.0 2.5 (2.0X10-3m)

10 20 30

Fig.6 Calculated maximum continuous operation term (Co=0.lg of P.mo3, removal extent=90%, The parameter In each figure Is the diameter of particles)

CONCLUSIONS

The adsorption chamcteristics of activated alumina treated with aluminum sulfate were studied to develop a new removal process for low concentmtion phosphorus in the waters of rivers and lakes. The adsorption

46 T. HAND ", al.

capacity of activated alumina was enhanced 1.7-fold by combining aluminum sulfate. 1be optimum amount to be combined was 3.0X lO-4mol. g-l. The adsorption isothenn could be expressed as Q=15.OCI/5.6 for the combined alumina. The effective intraparticle diffusion coefficient at 298K was evaluated as 9.0X 1O-15m2· s-t

from the breakthrough curves at the influent phosphorus concentration of 0.22 and O.68g·m-3. 'The temperature dependence of the intraparticle diffusion coefficients based on the concentration in solid was determined by the Boyd's method and the activation energy of 29.7kJ·mol-1 was obtained. Under the usual operational conditions, the present phosphorus treatment system was applicable for a long time to actual waters of rivers and lakes where the phosphorus concentration is fairly low.

ACKNOWLEDGEMENT

Authors wish to express their thanks to Mizusawa Industrial Chemicals. Ud. for kindly supplying the activated alumina particles.

NOMENCLATURE

C = equilibrium concentration of orthophosphate in bulk solution Co = initial concentration of orthophosphate in bulk solution d = diameter of adsorbent De = effective intraparticle diffusion coefficient D,' = intraparticle diffusion coefficient based on the concentration in solid E = activation energy F = adsorption coverage Q = amount of orthophosphate adsorbed Q, = amount of orthophosphate adsorbed at t sec. Qoo = equilibrium amount of orthophosphate adsorbed S V = space velocity t = adsorption time T = temperature

REfERENCES

[g_P·m-3] [g_P·m-3]

[m] [m2.s-'] [m2·s-I]

[kJ·mot t ]

[-] [g-p.g- I ] [g_p.g- t ]

[g-p.g- I ] [s-I]

[s] [K]

Ames. L.L. and Dean, R. B. (1970). Phosphorus removal from effluents in alumina columns. J. Water Pollut. Control Fed .• 42, RI61-RI72.

Brattebo, H. and Odegaard, H. (1986). Phosphorus removal by granular activated alumina. Water Res., 20, 977-986.

Hashimoto, A. and Ozaki, Y (1985). Phosphorus removal by allophane and its removal kinetics. J. Jpn. Sewage Worb Assoc., 22(257), 18-24.

Kotabe. H. (1987). Present situation and future of phosphate resources. Gypsum & Ume, 210. 307-316. Okubo, T and Matsumoto, J. (1984). Adsorption of nutrients on soil. J. Ind. Water Ass(lc. Jpn., 20, 89-95. Suzuki, M and Fujii, T. (1987). Removal of phosphate from wastewater by adsorption of zirconium oxide.

Proc. Nth APCChE Meeting, 675. Urano, K. and Tachikawa, H. (1991 a). Process development for removal and recovery of phosphorus from

wastewater by a new adsorbent. 1. Preparation method and adsorption capability of a new adsorbent. Ind. Eng. Chern. Ru., 30, 1893-1896.

Urano, K. and Tachikawa, H. (199Ib). Process development for removal and recovery of phosphorus from wastewater by a new adsorbent. 2. Adsorption rate and breakthrough curves. Ind. Eng. Chem. Res., 30, 1897-1899.

Vee, C. W. (1966). Selective removal of mixed phosphates by activated alumina. J. Am. Water Works As.wc., .5 8,239-247.