1513
Znd. Eng. Chem. Res. 1992,31,1513-1515
each regenerated cycle was compared with that of the virgin adsorbent in Figure 8. It was confirmed that the adsorption property of the regenerated adsorbent for biphosphate ion was maintained for over 80% of the virgin adsorbent during 20 reuse cycles. Consequently, these reuse cycles enable the adsorbent to be used for a long time to remove phosphorus from wastewater. Registry No. P, 7723-14-0; A1203,1344-28-1; Na(OH), 131073-2;HzS04,7664-93-9; A12(S04)3,10043-01-3.
Literature Cited Urano, K.; Tachikawa, H. Process Development for Removal and Recovery of Phosphorus from Wastewater by a New Adsorbent. 1. Preparation Method and Adsorption Capability of a New Adsorbent. Znd. Eng. Chem. Res. 1991a,30, 1893-1896. Urano, K.; Tachikawa, H. Process Development for Removal and Recovery of Phosphorus from Wastewater by a New Adsorbent. 2. Adsorption Rates and Breakthrough Curves. Ind. Eng. Chem. Res. 1991b, 30, 1897-1899.
Received for review March 6, 1992 Accepted March 23, 1992
Process Development for Removal and Recovery of Phosphorus from Wastewater by a New Adsorbent. 4. Recovery of Phosphate and Aluminum from Desorbing Solution Kohei Urano,* Hirotaka Tachikawa, and Masataka Kitajima Laboratory of Safety and Environmental Engineering, Yokohama National University, 156 Tokiwadai, Hodogaya-ku, Yokohama, 240 Japan
Processes were developed for recovering phosphate and reusing aluminum from alkaline solutions that have been used for desorbing phosphate from the spent adsorbent. The phosphate was completely recovered as the precipitate of calcium hydroxyphosphate, which can be used as fertilizer, by adding calcium chloride. The aluminum was also completely recovered as the dense precipitate of aluminum hydroxide, which can be reused for regenerating the adsorbent, by slowly adding sulfuric acid in the pH range from 11to 10. A total system for removing and recovering phosphorus from wastewater by using the new adsorbent was established, and only sodium sulfate and sodium chloride were discharged from this process. 1. Introduction A feasible method for removing and recovering phosphorus from wastewater is required to improve eutrophication of lakes, bays, and inland seas and to make up for the lack of phosphorus resources. A new superior adsorbent was developed, and its adsorption capacities, adsorption rates, breakthrough curves, desorption method, and regeneration method for removing various inorganic phosphorus from wastewater have been represented in previous papers (Urano and Tachikawa, 1991a,b, 1992). Thus, it has been shown that the newly developed adsorbent from an activated alumina by combining aluminum sulfate has high adsorption properties for inorganic phosphorus. The adsorbed phosphate on this new adsorbent was desorbed sufficiently by sodium hydroxide solution, and the adsorbent was reusable over 20 times by regeneration with acidic aluminum sulfate solution. However, phosphate in the alkaline desorbing solution could not be reused as it was, and this desorbing solution could not be thrown away. In this study, a feasible process for recovering phosphate and reusing aluminum from this alkaline desorbing solution was developed.
2. Materials and Methods 2.1. Materials. The desorbing solutions passed through the fixed bed column, which was used for the adsorption of orthophosphate from wastewater, contained the ions of phosphate, sulfate, hydroxide, carbonate, aluminum, and sodium. The samples of desorbing solution used in this study were obtained by the method described in the third paper in this series (Urano and Tachikawa, 1992). That is, 1.0 mol/L sodium hydroxide solution of 3 bed volumes
Table I. Compositions of Desorbing Solutions Used (mmol/L) ion solution A solution B Na+ 1000 lo00 P04380 20 so:4.2 4.3 Al(OH)(60 60 OH690 870 (PH 13.84 13.94)
was circulated in the column at a space velocity of 30/h after the adsorption test for sodium biphosphate solution of 21 mg of P/L at a space velocity of 10/h. The components of the desorbing solutions used are shown in Table I. 2.2. Recovery of Phosphate. Phosphate in the desorbing solution was precipitated by neutralization with acid, but the obtained precipitate was a mixture of aluminum hydroxide and aluminum phosphate, which was difficult to reuse. Therefore, calcium chloride was added to the desorbing solution for recovering phosphate as calcium salt, which could be reused easily as fertilizer. Various amounts of a solution of 2 mol/L calcium chloride were added into the desorbing solution, which contained 20 or 80 mmol/L phosphate. After 30 min, the concentration of phosphate in the supernatant was determined by colorimetry, and the recovering efficiency was calculated. 2.3. Recovery of Aluminum. Aluminum in the desorbing solution was precipitated by neutralization under general conditions with concentrated sulfuric acid after recovering phosphate by addition of calcium chloride, but the obtained precipitate was loose flock of aluminum hydroxide and contained large amounts of water. Therefore,
0888-5885/92/2631-1513$03.00/00 1992 American Chemical Society
1514 Ind. Eng. Chem. Res., Vol. 31, No. 6, 1992
8
O7 Molar
a d d i n g
r a t i o
Figure 1. Change in recovery of phosphate with molar adding ratio of calcium to phosphate.
the precipitate was difficult to separate from water and dehydrate. Then, the optimum pH and the additional rate of sulfuric acid were examined to obtain large and dense precipitate of aluminum hydroxide. After the phosphate was recovered, concentrated sulfuric acid was added to the desorbing solution within a short time interval until a certain pH level was attained. The concentration of aluminum in the supernatant was determined, and the interface height of the precipitate was measured after 24 h at each pH level. In a new set of experiments, 1.0 mol/L sulfuric acid was added over different time intervals in the range from 5 min to 13.5 h for the pH range from 11to 10 after adding concentrated sulfuric acid to the pH level of 11 at a stirring rate of 50 rpm. Further, 1.0 mol/L sulfuric acid was added to the solution from pH 11to pH 10 for 5 h with different stirring rates, and the interface heights were also measured after 24 h.
3. Results and Discussion 3.1. Recovery of Phosphate. The phosphate in the desorbing solution was precipitated quickly by adding calcium chloride at a high pH level. The relationship between the added molar ratios of calcium to phosphate and the recovering efficiencies of phosphate precipitate are shown in Figure 1. The phosphate was precipitated completely by adding calcium of ca. 2.4 times from the desorbing solution A containing 80 mmol/L phosphate, but calcium of ca. 3.0 times was necessary for the complete precipitation from the desorbing solution B containing 20 mmol/L phosphate. Calcium carbonate was determined in the precipitate by X-ray diffraction analysis. Therefore, the difference of the necessary amounts of calcium chloride between the desorbing Solutions depended on the amounts of the carbonate ion present. Further, the decrease in concentration of hydroxide ion estimated from the pH change was nearly equal in moles to the decrease of phosphate ion in the solution. From these results, the reaction at the addition of calcium chloride could be shown in eq 1. That is, the necessary
Po,"
9
10
-+
+ OH- + aC032-+ (2+ a)Ca2+
Ca2(0H)P04 aCaC03 (1)
amounts of calcium were the sum of twice the moles of phosphate and equal moles of carbonate. In the cases shown in Figure 1,the concentrations of carbonate in both desorbing solutions were estimated to be ca. 20 mmol/L from eq 1; that is, ca. 89% and ca. 67% of the added calcium were reacted with phosphate in the case of the desorbing solutions containing 80 and 20 mmol/L phos-
12
11
P ii
CaZt/P03
Figure 2. Relationship between pH and residual aluminum in SUpernatant (added concentrated H 8 O I within a short time).
t
o.2
1 1 1 1 1 '7 8 9 10 11 P I3
Figure 3. Change of interface height ratio of aluminum hydroxide after 24 h with pH levels (added concentrated H#O, within a short time).
-R I/ ?. 0.6 Io
0.4
I
.
0
(
,
1
4
2
,
-
I
6 Adding
1
8 t i m e
,
I
I
I
'
I
1 0 1 2 1 4 (h)
Figure 4. Change of interface height ratio of aluminum hydroxide after 24 h with adding 1mol/L H#04 at time intervals for pH range from 11 to 10 at a stirring rate of 50 rpm. 0.5
b 0.4 -
-
=:
0.3-
c
0.2 n
0.1 -
