Ind. Eng. Chem. Res. 1991,30, 1893-1896
1893
Process Development for Removal and Recovery of Phosphorus from Wastewater by a New Adsorbent. 1. Preparation Method and Adsorption Capability of a New Adsorbent Kohei Urano* and Hirotaka Tachikawa Laboratory of Safety and Environmental Engineering, Yokohama National University, 156 Tokiwadai, Hodogaya-ku, Yokohama, 240 Japan
Alophene and activated alumina can adsorb phosphate ion, and their adsorption capacities have been increased by adding aluminum sulfate or iron sulfate, respectively, to form complex salts on their surfaces. In particular, activated alumina combined with 2.0 X lo4 mol of aluminum sulfate/g has a large adsorption capacity and good characteristics. This new adsorbent adsorbs various inorganic phosphorus species in a pH range from 4 to 7. Coexistence of various inorganic and organic anions does not disturb the adsorption of phosphate ion, but coexistence nitrate and sulfate ions disturb the adsorption slightly. Phosphate is adsorbed in the early stage by the reaction with the complex salt A14(OH)6(S04)3, which was formed by the reaction of aluminum sulfate and aluminum hydroxide on the surface of activated alumina, and in the delay time by the reaction with the aluminum hydroxide.
'
1. Introduction A feasible process for removing phosphorus from water must be developed to alleviate eutrophication of lakes, bays, and inland seas. Recovering phosphorus from wastewater is also required because it is expected that high-quality resources of phosphorus will be lacking in the near future. Several processes by coagulation, crystallization, or biosorption have been developed for removing phosphorus from wastewater (Urano and Tachikawa, 1987). These processes need complex and strict control of the operating conditions, and some of them produce much sludge. Furthermore, it is difficult to recover phosphorus by those processes. On the other hand, an adsorption process can be operated simply, produces little sludge, and enables phosphorus recovery. Clay minerals (Okubo and Matsumoto, 1984, Hashimoto and Ozaki, 1985),zirconia (Suzuki and Fujii, 1987),titania, and activated alumina (Yee, 1966; Ames and Dean, 1970; Brattebo and Odegaard, 1986) were investigated as adsorbents of phosphate in water. These conventional adsorbents, however, may not be feasible in practical wastewater treatment because their adsorption capacities are insufficient and the total processes using these adsorbents have not been developed. The adsorbent for removing phosphorus from wastewater must satisfy the following conditions: (1)high capacity, selectivity, and rate of adsorption; (2) granular type; (3) high chemical and physical strengths; (4) no hazardous pollutants contained within it; (5) easy regeneration of spent adsorbent; (6) low cost and steady supply. In this study, numerous clay minerals and aluminas were tested and improved in consideration of the above conditions, and a new superior adsorbent was developed for removing and recovering phosphorus from wastewater. The studies for plant design, the regeneration conditions for spent adsorbent, and the method of recovering phosphorus will be reported subsequently. 2. Materials and Methods 2.1. Raw Adsorbents. Nine kinds of clay minerals and six kinds of aluminas were examined as raw adsorbents. and SiOzand the micropore volumes The contents of A1203 and surface areas of them are shown in Table I. The contents of A1203 of the clay minerals were high in the following order: kaolinite A > alophene A > kaolinite B
Table I. Properties of Raw Adsorbents components 46 pore sample Al,OI SiO, area"/(m*/d volumeO/(cma/.d alophene A 39.4 38.1 335 0.29 alophene B 0.08 35.5 28.8 93 58.0 0.18 323 montmorillcmite 21.9 bentonite 15.4 69.5 0.23 372 kaolinite 40.5 45.9 0.03 29 alumina A 80.4 11.6 0.40 290 93.7 0.3 alumina B 0.29 246 alumina C 93.7 0.3 0.26 228 0.21 alumina D 93.7 0.3 213 alumina E 93.7 0.3 0.17 123 0.24 alumina F 93.7 0.3 135 alumina G 0.0 1 0.00 98.5 OThe values for pores smaller than 20 nm in diameter.
> alophene B > montmorillonite > bentonite A > bentonite B > cristobalite > sepiolite. The pore volumes and the surface areas of the clay minerals were large in the following order: alophene A > sepiolite > bentonite A > montmorillonite > cristobalite > bentonite B > alophene B > kaolinite B > kaolinite A. Alumina A-F are commercial activated aluminas, and they have micropore volumes from 0.17 to 0.40 cm3/g and surface areas from 123 to 290 m2/g. Alumina G is a crystal alumina, having a small pore volume and small surface area. 2.2. Addition of Inorganic Salts. Inorganic salts were added to the raw adsorbent for improving the adsorption capability. Since magnesium, calcium, aluminum, and iron form insoluble salts with phosphate ion, their chlorides and sulfates were added to alophene A or activated alumina A. The raw adsorbent of 100 g was put into 200 mL and 0.75 equiv/L of solution of the inorganic salt and mixed for 1 h. The mixture was filtered, and the adsorbent was washed with twice the volume of pure water. The adsorbent was dried at 110 O C for 12 h. The combined ratio of the salt was calculated from the difference between the added amount and the eluted amounts in the filtrate and the washing water. 2.3. Adsorption Test Method. At first, batchwise adsorption testa were carried out at 10 mg of P / L of sodium biphosphate (Na2HP04)at pH 5 for comparison of adsorption capabilities among the adsorbents. An amount of 50 mg of each adsorbent was added to 100-mL solutions and shaken for 7 days at 25 f 0.5 "C.The concentration of phosphorus after the adsorption test was analyzed, and
0888-5885/91/2630-l893$02.50/00 1991 American Chemical Society
1894 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991 Table XI. AdsorDtion Abilities of Raw Adsorbent sample adsorbed amount"/(mg of P/L) alophene A 10.5 alophene B 1.5 montmorillonite 0.0 bentonite 0.0 kaolinite 0.0 alumina A 17.5 alumina B 14.8 alumina C 13.8 alumina D 11.6 alumina E 10.8 alumina F 8.4 1.9 alumina G
" A 50-mg amount was added to 100 mL of 10 mg of P / L solution. Table 111. Combined Ratio a n d Effect of Inorganic Salt adsorbed amount"/ salt combined ratio/% (mn of P / L ) 24.1 0 23.0 24.4 0 23.8 0 25.0 5 24.2 2 33.0 83 33.4 66 34.0 81
" A 50-mg amount was added in 200 mL of 10 mg of P / L solution. Poly(a1uminum chloride).
the adsorbed amount was calculated. Next, batchwise adsorption tests of biphosphate, biphosphite (Na2HP03),hypophosphite (NaH2P02),and tripolyphosphate (Na,P,O,,) were carried out at different concentrations and pH for the best adsorbent, and their adsorption isotherms were obtained. Batchwise adsorption tests were also carried out with the coexistence of sodium chloride, carbonate, nitrate, or sulfate. 2.4. Analytical Method. The concentration of orthophosphate type phosphorus was determined by colorimetry or ion chromatography (Shimazu HIC-6A). Concentrations of phosphite, hypophosphate, and tripolyphosphate types were determined by colorimetry after changing to orthophosphate. The concentrations of chloride ion, nitrate ion, and sulfate ion were determined also by the ion chromatography, and the concentration of carbonate ion was determined by a carbon analyzer (Shimazu TOC 500). 3. Results and Discussion 3.1. Adsorption Capability of Raw Adsorbent. The results of the adsorption tests of biphosphate solution are compared in Table I1 for all the raw adsorbents. Montmorillonite, bentonite A and B, kaolinite A and B, cristobalite, and sepiolite adsorbed little orthophosphate type phosphorus, and alophene B adsorbed a little. On the other hand, alophene A, which contains much alumina and has a large pore volume and surface area, adsorbed phosphate type phosphorus more than the other clay minerals. The activated aluminas adsorbed orthophosphate more than the clay minerals, but crystal alumina adsorbed little. The aluminas having larger surface areas adsorbed more orthophosphate, and alumina A was the best. The adsorbed amounts on alophene A and alumina A, however, were not sufficient for economical use in wastewater treatment. Therefore, addition of the various inorganic salts on alophene A and alumina A was examined
Added
O 0 4 ' n o l / ~ ~
a m o u n t
Figure 1. Relationships between added amounts and combined amounts of sulfates.
-
? 401
1
I P
=
; 20y :-
0
I 2
1 mb
I
1
, r
3 1
ii / Y
Figure 2. Influence of combined amounts of aluminum sulfate on adsorption ability.
to improve their adsorption capabilities. 3.2. Addition of Inorganic Salt. The combined ratios of the inorganic salts are shown in Table 111. All the chlorides combined somewhat on the raw adsorbents because the chlorides dissolved in the washing water. On the other hand, the sulfates combined much more on the raw adsorbents. The results of the adsorption tests of biphosphate solution for these adsorbents are shown also in Table 111. The adsorbed amounts were barely increased by adding chlorides. On the other hand, the adsorbed amounts were substantially increased by adding aluminum sulfate or iron(II1) sulfate. However the adsorbed amount for alophene A after adding the sulfates was smaller than that for the raw alumina A. Furthermore, it might be difficult to supply a great deal of the alophene whose properties were constant. Therefore, alumina A was employed as the raw adsorbent. The relationships between the added amounts and the combined amounts of the sulfates were examined and the results are shown in Figure 1. About 80% of the added aluminum sulfate and iron(II1) sulfate combined closely and did not dissolve in the washing water because these sulfate forms complex salts with aluminum hydroxide on the surface of the activated alumina, as mentioned later. The influence of the combined amounts of aluminum sulfate or iron(II1)sulfate on the adsorption of biphosphate solution was examined, and the results are shown in Figure 2. It was found that the adsorbed amounts were increased by combining the sulfates, but the adsorbed amounts decreased in the case of too much combination. Consequently, approximately 2.0 x loa mol of aluminum sulfate or iron(II1) sulfate/(g of alumina A) was the best combining amount, and it corresponded to the added amounts mol/g. of 2.5 X The pore-size distributions of these adsorbents were calculated by the methanol adsorption method (Urano,
Ind. Eng. Chem. Res., Vol. 30,No. 8, 1991 1895
Pore
diameter
(nm)
6
Figure 3. Change in pore-size distribution with combining aluminum sulfate.
0
-2
3
4
5
6
7
8
9
PH
Figure 5. Influence of pH on adsorption of phosphate. $00
Table V. Influence of Coexistent Ion ion none
: I : '0,
0
4
,
, , , , ,,,,
2
5 conc
e n
,
10 t rat
i
on
20
, ,
,
c1-
,,,,I
50
HCOS-
NO;
100
NO;
(m~-P/l)
Figure 4. Adsorption isotherms of biphosphate (pH = 6, 25 "C). Table IV. Characteristics of New Adsorbent particle size packed density packed void 660 kg/m3 0.40 1.7-mm 0.d.
strength 1.4 Kg
1975)and mercury pressing method for pores smaller than 2000 nm in radius. The results are shown in Figure 3. The pore volume decreased with the combined amounts of the sulfate. Therefore, too much combination might disturb diffusion of the biphosphate ion in the pores, and the adsorbed amounts were decreased. From the above results, it was found that the combination of 2.0 x IO4 mol of aluminum sulfate or iron(II1) sulfate/(g of alumina A) could greatly increase the adsorption capability. However, iron(II1) ion colors water and iron phosphate releases phosphate ion in anaerobic atmosphere. Therefore, aluminum sulfate was employed as the adding salt in this study. The properties of this new adsorbent, which was prepared from alumina A by combining aluminum sulfate at 2.0 X lo4 mol/g, are shown in Table IV. Because the shape of this adsorbent was a sphere of 1.7 mm of mean diameter, it might be useful in the fixed-bed adsorber for a long time without clogging with suspended solid. The physical strength was large enough for general use in wastewater treatment, though the total pore volume was large. 3.3. Adsorption Isotherms of Orthophosphate Type Phosphorus. The influence of the concentration of biphosphate solution on adsorbed amounts was examined for the raw alumina A and the new adsorbent at 25 "C and pH 5. The adsorption isotherms are shown in Figure 4. The adsorption isotherms could be represented by the following Freundlich equations for raw alumina A and the new adsorbent, respectively.
Q = 2OC0.'O
for the raw alumina A
(1)
Q = 37PM for the new adsorbent (2) Here, Q is the adsorbed amount (mg of P/g), and C is the equilibrium concentration (mg of P/L).
SO,% SO4ZS0,Z-
concentration/ (mg/L)
710 610 300 600 20 100 200
adsorbed amounta/ (ma of P/L) 33 34 33 29 26 32 28 25
'A 50-mg amount was added to 200 mL of 10 mg of P / L solution.
Since the values of the exponent of the Freundlich equations were very small, the concentration was little influenced by the adsorbed amounts for both the adsorbents, especially the new adsorbent. Namely, the values of k and n of the Freundlich equation increased by the addition of aluminum sulfate, and the new adsorbent could adsorb much orthophosphate type phosphorus from the low concentration solution. This adsorption capability of the new adsorbent was far superior than those of conventional adsorbents. 3.4. Influence of pH and Coexisting Ions. The adsorption isotherms of biphosphate for the new adsorbent were obtained at various pH levels. It was found that the values of the exponent of the Freundlich equation were equally small over a wide range of pH levels. However, the values of preexponent factors were changed with pH, as shown in Figure 5. The pH range from 5 to 6 was the best, and the range from 4 to 7 was probable for adsorption of phosphate type phosphorus. In this pH range, most phosphate type phosphorus is in the form of biphosphate ion. Therefore, the difference with pH might not be caused by a difference of ion types, such as H2P04-,HPOZ-, and Po43-,but be related to the solubility of aluminum phosphate. The influence of the coexistence of chloride ion, nitrate ion, carbonate ion, sulfate ion, and various organic anions, such as dodecylbenzenesulfonate, acetic acid, and humic acid, were examined, and the results are shown in Table V. Chloride ion, carbonate ion, and the organic anions did not disturb the adsorption of biphosphate ion. Nitrate ion and sulfate ion somewhat disturbed the adsorption of biphosphate ion, but the decrease of the adsorbed amounts of biphosphate was less than 30% for the coexistence of these ions of 500 mg/L. 3.5. Adsorption Isotherms of Various Types of Phosphorus. Adsorption isotherms for various ions of
1896 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991
aluminum hydroxide is formed on the surface of activated aluminum in water, as shown by eq 3. Therefore, the A1,0, + 3H,O 2Al(OH), (3) adsorption reaction might be expressed by eqs 4 and 5 for the early stage and delay stage, respectively. Namely, the A14(OH)&S04)3+ 4H2P044A1P04 + 3S042-+ 2H+ + 6H20 (4)
-
I ,
5
10
20
50
I
1
100
Figure 6. Adsorption isotherms of various types of phosphorus. I
13
1.5 r
-
Al(OH1, + H2P04- AlP04 + OH- + 2H20 (5) adsorption of biphosphate ion was caused by the quick reaction of eq 4 and the slow reaction of eq 5. And the combined aluminum sulfate is expected to form a complex salt by eqs 3 and 6 on the surface of activated alumina. 2A1(OH)3 + A12(SO4)3 A14(OH),(SO& (6) Registry No. &go3,1344-28-1; AlZ(SO&,, 10043-01-3; FeS04,
-
7720-787; CaCl,, 10043-52-4; MgCl,, 7786-30-3; AlC13, 7446-70-0; FeCl,, 7705-08-0; Fe2C13,7705-08-0; Na2HP0,, 7558-79-4; Na2HPO3, 13708-85-5; NaH2P02,7681-53-0; N%P3OIo,7758-29-4; NO,-, 14797-55-8; SO?-, 14808-79-8; allophane, 12172-71-3; montmorillonite, 131893-0; cristobalite, 14464-46-1; sepiolite, 63800-37-3. A i s o r p t ~ o n clmt.
'11
Figure 7. Change in sulfate ion with adsorption of biphosphate.
phosphite, hypophosphite, and tripolyphosphate are shown in Figure 6. It is confirmed that all isotherms are represented by the Freundlich equation with the small values of exponent. Nearly equal amounts of phosphorus of phosphite and hypophosphite types to that of orthophosphate type could be adsorbed by this adsorbent, but tripolyphosphate type phosphorus could adsorb approximately two thirds of orthophosphate type phosphorus. The effects of pH on their adsorption isotherms were similar to the case of biphosphate, and the suitable pH for the adsorption of these ions was in the range from 4 to 7 . 3.6. Reaction Mechanisms of Combining Sulfate and Adsorption of Biphosphate. Change of pH and the concentration of released sulfate ion with adsorption of biphosphate ion were examined by the batchwise tests, and the results are shown in Figure 7. In the early stage, pH was decreased and 3.0 mol of sulfate ion was released with the adsorption of 4.0 mol of biphosphate ion. The adsorption continued for a long time, and sulfate ion was not released and pH was increased with the adsorption of biphosphate ion in the delay stage. It is well-known that
Literature Cited Ames, L. L.; Dean, R. B. Phosphorus Removal from Effluents in Alumina Columns. J . Water Pollut. Control Fed. 1970, 42, R161-Rl72. Brattebo, H.; Odegaard, H. Phosphorus Removal by Granular Activated Alumina. Water Res. 1986, 20, 977-986. Gangoli, N.; Thodos, G. Phosphate Adsorption Studies. J. Water Pollut. Control Fed. 1973, 45, 842-849. Hashimoto, A.; Ozaki, Y. Phosphorus Removal by Allophane and Its Removal Kinetics. J . Jpn. Sewage Works Assoc. 1985,22 (257), 18-24. Okubo, T.; Matumoto, J. Adsorption of Nutrients on Soil. J . Ind. Water Assoc. Jpn. 1981, 20, 89-95. Suzuki, M.; Fujii, T. Removal of Phosphate from Wastewater by Adsorption of Zirconium Oxide. Proceedings of IVth APCChE Meeting; APCChE: 1987; p 675. Urano, K. Theories, Apparatuses and Problems of Measuring Methods of Pore-distributions and Surface Areas of Porous Materials. Surface (Tokyo) 1975, 13, 738-745. Urano, K.; Tachikawa, H. Techniques and Cost Analysis for Phosporus Removal from Wastewater. J. Water Waste Jpn. 1987,29, 425-434. Yee, C. W. Selective Removal of Mixed Phosphates by Activated Alumina. J . Am. Water Works Assoc. 1966,58, 239-247.
Received for review October 12, 1990 Revised manuscript received March 11, 1991 Accepted March 25, 1991
