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Suppression of phosphate liberation from sediment by using iron slag
War. Res. Vol. 21, No. 3, pp. 325-333, 1987 Printed in Great Britain. All rights reserved
0043-1354/87 $3.00+0.00 Copyright © 1987 PergamonJournals Ltd
SUPPRESSION OF PHOSPHATE LIBERATION FROM SEDIMENT BY USING IRON SLAG HISASHI YAMADA,j MITSU KAYAMA,2 KAZUO SAITO3 and MASAKAZUHARA3 tTokai Regional Fisheries Research Laboratory, the Ministry of Agriculture, Forestry and Fisheries, Arasaki, Nagai, Yokosuka 238-03, 2Laboratory of Marine Biochemistry, Faculty of Applied Biological Science, Hiroshima University, Midori-machi, Fukuyama 720 and 3Fukuyama Ironworks, Nippon Kokan Co. Ltd, Kokan-machi, Fukuyama 720, Japan (Received February 1986)
Abstract--In order to suppress the phosphate liberation from sediment to sea water, several suppressing materials such as four kinds of iron slag, fiver and sea sands, and glass bead were tested. The sediment and sea water were sampled from Hiroshima Bay which seemed to be a eutrophicated area. The suppressing materials were covered on the surface of sediment, and sea water was introduced in the aquarium. The concentrations of phosphate, sulfide and dissolved oxygen were measured during the incubation period under the condition of anaerobic state. The suppressing effects of slags, sea and river sands were compared with that of glass bead used as control materials, and the suppression efficiencies were measured. The suppression efficiencyof glass bead was 54.1%, and those of slag-2, -3 and -4 were 97.3, 96.1 and 98.8%, respectively.The suppression efficiencydepended on the amounts of slag used, and about 85% of liberated phosphate were suppressed by slag-3 of 7.5 kg m -2. The sulphide ion generated under anaerobic state reacted with some metals on the surface of slags and sands, and the dissolution of precipitated phosphate (Fe--P, A l P and Ca-P) was suppressed by formation of metal sulfide on the surface of slags and sands. The suppression mechanisms on phosphate liberation, such as covering, chemical and adsorption effects were proposed and discussed. The suppression by slag was mainly carried out by the adsorption effect, in addition to the covering effect. Key words--iron slag, phosphate, liberation, suppression efficiency, adsorption, sediment
INTRODUCTION It is well known that the accumulated nutrients in the shallow and coastal sediment play an important role for the eutrophication in a closed water area. The supply of nutrients from the sediment is one of the main sources, and has an influence on the change of nutrient concentration. The suppression of nutrient liberation from sediment has been recognized as one of the important research to control the eutrophication caused by the excessive nutrients. According to Lee et al. (1976), Lijkleme (1976) and Banoub (1976), orthophosphate has been precipitated on sediment under the aerobic condition, and it has been dissolved under the anaerobic condition. Hosomi et al. (1982) divided phosphorus states in the lake sediment into five forms, such as inorganic phosphate adsorbed on the clay particle, inorganic phosphate precipitated with aluminium (AI-P), with iron (Fe-P), and calcium (Ca-P) and organic phosphorus. Joh (1983) demonstrated that AI-P and Fe-P in marine sediments have changed gradually to a dissolved form in the oxygen-deficient condition. It has been well known that the considerable amounts of phosphate in the sediment were released in the anaerobic state (Fillos and Molof, 1972; Mortimer, 1971; Serruya, 1971). In order to suppress the liberation of phosphate from sediment, many kinds of substances which wa. 2,/r-v
adsorb phosphate or form insoluble compounds with phosphate have been investigated by several researchers. Sodium aluminate (Peterson et al., 1976), lanthanum rare earth chloride (Peterson et al., 1976), zirconium tetrachloride (Peterson et al., 1976; Sanville, 1982), fly ash (Higgins et al., 1976), lime (Higgins et al., 1976; Nishimura and Seki, 1983; Ishio and Kondo, 1980) and gypsum (Higgins et al., 1976) were studied, and these substances have suppressed a considerable amount of phosphate by adsorbing phosphate or producing a unsoluble compound with phosphate. The characteristics of adsorption of phosphate on the slag surface was fundamentally studied in our previous paper (Yamada et al., 1986). It was found that adsorption of phosphate on slag surface and re-dissolution of the adsorbed phosphate from the slag surface depended on pH of solution. Adsorption of phosphate on the slag surface was high at pH 8, and re-dissolution of the adsorbed phosphate was low at same pH of 8. Since pH of natural sea water is around 8, it is considered that slag removes phosphate from natural sea water by adsorbing it on the surface of slag. The adsorption of phosphate on slag surface was influenced by the concentration of co-existing ion, that is, adsorption of phosphate decreased with the increase of the concentration of co-existing ion, but the decrease of phosphate adsorption was not significantly
325
HISASHI YAMADAet al.
326
Gas inlet
influenced by the concentration o f co-existing ion in natural sea water. Accordingly, it is expected that slag adsorbs phosphate dissolving in natural sea water, and that slag suppresses p h o s p h a t e liberation from sediment even under anaerobic conditions. In this paper, the effect o f suppression on p h o s p h a t e liberation from sediment by using slag was investigated by small scale experiment. A system including sediment, sea water and several kinds o f slag or other materials was designed in this experiment to clarify the suppression o f p h o s p h a t e liberation from sediment under the anaerobic condition. The suppression efficiencies o f several kinds o f slag and other materials were also determined, and these effects were discussed.
I
BoLt
The experiment on the liberation of phosphate from sediment were carried out by using a plexiglass container of
~
SampLing tube Rul )r tub
Sintered gLass ball
~
iiiiii;ili iiii!iiii i[iiiiiiiiSediment ?iii?il
20 mm Fig.
1. Schematic presentation of the experimental container.
which the length, width and height were 100, 200 and 276 mm, respectively. The used sediment and sea water were obtained at the pier of the Nansei Regional Fisheries Research Laboratory located in Hiroshima Bay, Hiroshima, Japan. The apparatus presented in Fig. 1 contains sediment, sea water and suppressing materials, such as glass bead, sea and river sands and slags. Incubation was carried out for the period at room temperature (about 20-25°C). In order to study the suppression of phosphate liberation from sediment, two experiments were carded out as follows; a control experiment was carried out by using both sediment and sea water, and only sea water in container. Glass bead, sea sand, river sand and several slags were used as sup-
Table I. Chemical and physical characteristicsof slag and sand Sand Chemical composition (%) SiO2 CaO A1203 FeO MgO S MnO TiO2 P205 Particle size (~) Median diameter 0~) Density (gem -~) p'* Pt Porosity (%)
Slag-1 33.10 41.80 13.90 0.34 6.42 1.05 0.68 1.76 -63-2000 800 2.59 2.72 5.0
Rubber packing
Wa~r
Materials
Experimental procedure
I"
II
I N
MATERIALS AND METHODS In the process of iron and steel production, the blast furnace slag, the converter slag and the electric arc furnace slag are produced independently. Iron ore and caustic lime charged into the furnace are melted, and finally converted into two phases of molten slag and molten iron in this process. Since the former is lower than the latter in density, the former floats on the latter. Slag is molten state at the temperature higher than 1500°C, and the slag is cooled by several methods and the different physical properties are gotten. Slag cooled slowly has ordinarily a hard and dense crystal structure, while slag cooled rapidly contains much more foams in it. The molten slag from the blast furnace is rapidly cooled by the pressurized water, and two types of slag are formed under the conditions of temperature of molten slag, ratio of water to slag, and water pressure. Lightweight granulated slag contains more foams and is called soft granulated slag, and heavier one with less foams is called hard granulated slag. The materials used in this research were soft and hard granulated slag, sand and glass bead. Slag was manufactured and supplied by Nippon Kokan Co. Ltd, Fukuyama, Japan. The chemical composition and physical properties of the materials are summarized in Table 1. The glass bead (GMB-60) was used as a control material, which was supplied by Nippon Rikagakukikai Co., Japan, and the particle size was between 0.5 and 0.7 mm.
O" Pinchcock
Slag-2
Slag-3
Slag-4
Beach
River
32.30 34.30 33.40 71.73 77.81 41.90 40.50 42.30 0.39 2.10 14.00 14.60 13.70 9.33 9.00 0.37 0.30 0.32 0.91 0.76 6.41 6.88 6.69 0.24 0.27 I. 19 0.87 0.93 0.002 0.02 0.65 0.50 0.60 0.12 0.15 2.17 1.23 I. 13 0.63 0.57 0.02 -0.01 0.004 0.006 63-2000 63-2000 63-2838 63-2000 63-2000 720 860 1550 530 1404 2.47 2.02 1.17 2.59 2.59 2.71 2.63 2.63 2.61 2.62 8.8 23.3 55.6 1.0 I. 1
*p' was determined with a pygnometer. tP was determined with a pycnometexafter the removal of air included in the materials by boiling.
Suppression of phosphate liberation pression materials. After sea water (100 ml) was sampled through sampling tube once a day, hydrogen gas was bubbled through a sintered glass ball to keep an anaerobic state in the container. The used amounts of sediment, sea water and suppressing materials are described in the captions of Figs 2, 3, 4 and 5, respectively. It is considered that the liberation of phosphate from sediment is influencedby sulfide ion generated under anaerobic conditions. In order to make clear the influence of sulfide ion on the phosphate liberation, another experiment was carded out. Sediment of 20 g in wet state was put into a BOD bottle of 100 ml. Sea water was filtered with glass fiber filter OVhatman GF/C) and filtered sea water containing 0.05% sucrose was introduced. Dissolved oxygen in the sea water was excluded by bubblingnitrogen gas previously. A control bottle consists of both sediment and the sea water containing 0.05% sucrose. The experiment bottles consist of each 20 g of glass bead and sea sand, and 15 g of slag-3 with 20 g of sediment and the sea water, respectively. Both control and experimental bottles were incubated at 25°C for 5 days. After incubating the bottles, the concentrations of dissolved oxygen, hydrogen sulfide and orthophosphate in the sea water were determined. In order to study the relationships between suppression efficiencyand amount of slag, six containers were prepared. The slag-3 (0-250 g) was introduced into each container containing 3.1 1. of sea water and 2.5 kg of sediment. In the experimental container containing 250 g of slag-3, a slag layer of ~ 1cm covered the surface of sediment. The containers were kept under an anaerobic State, and the concentrations of dissolved oxygen and the orthophosphate were measured during the 7 days incubation. The suppression efficiencies were calculated from the difference between the amount of phosphate released from sediment in a control container (not containing slag-3) and that in experimental container.
Analytical procedure The densities (p and p') of four kinds of slag and two kinds of sand were determined by using a pycnometer according to Japanese Industrial Standard (JIS A 1202, 1970). The value ofp was determined after removing air in the material by boiling it, and p' was the density of the material containing air. The volume percentage of air (Va) in the materials was calculated by the following equation. Va = (1/p" -- 1/p)/(l/p') x I00. The chemical compositions of slag and sand, such as SiO2, CaO, AI203, FeO, MgO, S, MnO, TiO2and P205 were analyzed by using a X-ray fluorescence analyzer (Shimazu, Model VQX-130A). The concentrations of heavy metals, cadmium, lead, copper, zinc and total chromium, in four kinds of slag were analyzed by using an atomic absorption spectrnphotometer according to Japanese Industrial Standard (JIS M 8135, 1981; JIS M 8123, 1977; JIS M 8121, 1982; JIS M 8124, 1979), respectively. The concentration of ammonium ion was determined by measuring the absorption of blue co[or of indophenol according to Liddicoat et al. (1975) and Sasaki and Sawada (1980). Nitrite and nitrate ions were determined according to Strickland and Parsons (1968) and Wood et al. (1967), respectively. The concentration of orthophosphate in the sea water was determined by ascorbic acid-molybden blue method according to Koroleff (1979). The samples containing excess amounts of orthophosphate were diluted by using artificial sea water (25 g NaC1 and 8 g MgSO4 in 1 liter of redistilled water). The concentration of hydrogen sulfide in sea water was measured by a Gastec detector tube (Kitazawa Sangyo Co.), in which lead acetate coated on silica gel was packed as the reactor. The concentration of hydrogen sulfide was determined by the length of the tube turned to black with hydrogen sulfide (Arakawa, 1980). The
327
concentration of dissolved oxygen was determined by the modified Winkler's method (Strickland and Parsons, 1968). RESULTS AND DISCUSSION
Suppression by covering and mixing o f slag The surface of sediment was covered with slag, and the concentrations of dissolved oxygen, orthophosphate and dissolved inorganic nitrogen (DIN) in sea water were measured. The changes of those concentrations are shown in Fig. 2. The concentration of orthophosphate in sea water of the control experiment increased with the decrease of dissolved oxygen during the anaerobic incubation, on the contrary, the concentration of orthophosphate in sea water of the suppression experiment was low during the anaerobic incubation. Comparing the concentrations of DIN in sea water of both experiments, no considerable difference was observed. Although pH of sea water in both the control and suppression experiments increased slightly from 8 to 9 during the anaerobic incubation, the pH decreased to 8 by bubbling air into the sea water. The pH change might be caused by bicarbonate derived from carbon dioxide in air. The concentration of orthophosphate in sea water decreased gradually in the control experiment during the aerobic incubation after bubbling air. It was considered that the decrease of orthophosphate was caused by the precipitation formed with metal ions, such as Fe 2+ or Al 3+. For the suppression experiment, the concentration of orthophosphate increased at the beginning of the aerobic incubation and then decreased gradually. In the experiment of mixing slag with sediment, the suppression effect of phosphate liberation was studied as shown in Fig. 3. Comparing the concentration of orthophosphate in control with that in suppression experiment, the phosphate concentration increased rapidly in the former and remained at low level in the latter. Accordingly, it was clear that the liberation of phosphate was suppressed by mixing slag with sediment. Ishio and Kondo (1980) demonstrated that phosphate in the presence of calcium forms a unsoluble compound like hydroxyapatite, Cas(OH) (PO4)3, which inhibits the dissolution of precipitated phosphorus in sediment. As slag contains large amounts of calcium, the unsoluble compounds may be formed by the mixture of slag and sediment, and the suppression of the phosphate liberation might be progressed.
Suppression effects by slag, sands and glass bead In order to compare the suppression effects on orthophosphate liberation by various materials, slag1, -2, -3, -4, river sand, sea sand, and glass bead were used, and their effects were investigated. As seen in Figs 4 and 5, the variations of phosphate and dissolved oxygen in sea water were measured in the course of incubation period. The increase of the phosphate concentration in sea water was recognized
HISASrIIYAMADAet al.
328 o pH
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• I
..-'-+ 7
e~ "-°
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8
" 9
i
I 10
11
period ( d )
Fig. 2. The changes of pH and concentrations of dissolved oxygen, ortbophosphate and dissolved inorganic nitrogen (DIN) during anaerobic or aerobic incubation. 2 kg of sediment and 2630 ml of sea water were used for the control experiment and 232 g of slag-3 was used for the suppression experiment
with sediment and sea water. during 10-40days under the conditions of control, and covered with glass bead, sea and river sands and slag-1 on sediment, respectively. But the significant increases of the phosphate concentration were not recognized under the covering conditions of slag-2, -3 and -4 on sediment and only sea water (without sediment in Fig. 5), respectively. Therefore, it was considered that the increase of phosphate ion in sea water was mainly caused by the phosphate liberation from sediment and was not caused by the mineralization of organic phosphorus in sea water.
7
•
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F~4 P I Experiment
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53 C2 8
~
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7
8
9
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period (d)
Fig. 3. The changes of concentrations of dissolved oxygen and orthophosphate during anaerobic incubation. 1.5 kg of sediment and 2300 ml of sea water were used for the control experiment. 500g of slag-3 was mixed with 1.5 kg of sediment for the suppression experiment.
The amounts of orthophosphate released from sediment during 40 days experimental period were calculated by integrating the changes of orthophosphate concentrations shown in Figs 4 and 5. The suppressed amounts of orthophosphate liberation by using the suppressive materials were obtained by subtracting released phosphate during 40 days in the experimental container from that in the control container. The suppression efficiencies (ratio of the suppressed amount of phosphate to the released amount of phosphate in the control experiment) were calculated as shown in Table 2. The suppression efficiencies of soft granulated slags such as slag-2, -3 and -4 were 97.3, 96.1 and 98.80/0, •respectively, and were larger than those of glass bead, sand and the hard granulated slag such as slag-i. It was found that soft granulated slag suppressed effectively the phosphate liberation from sediment. As shown in Table 2, 54.1% of orthophosphate in the control experiment was suppressed by glass bead which did not absorb phosphate. Therefore, it was understood that the orthophosphate liberation was physically suppressed by covering the surface of sediment.
Effect of hydrogen sulfide on phosphate liberation It was recognized that the color of slag and sand layers turned to black during the anaerobic incubation, and the color again turned to the original color by changing to the aerobic state. It was under-
Suppression of phosphate liberation
329
Control
2030 --
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1 40
Ineubotion period {d)
40
Incubation period (d) Fig. 4. The changes of concentrations of dissolved oxygen and orthophosphate in sea water during anaerobic incubation for 40 days. 2.5 kg of sediment and 3200 ml of sea water were used for the control experiment (Control) and 250g of materials covering sediment were used for the suppression experiment (Sea sand, River sand, Slag-2, Slag-4). Q--dissolved oxygen; O - - o r t h o p h o s p h a t e .
stood that some metals on the surface of slag and sand reacted with hydrogen sulfide produced by the anaerobic process. In order to investigate the effect of hydrogen sulfide on phosphate liberation from sediment, the relationships between the concentrations of hydrogen sulfide and phosphate in sea water were studied by using control and experimental bottles
Fig. 5. The changes of concentrations of dissolved oxygen and orthophosphate in sea water during anaerobic incubation for 45 days. 2.5 kg of sediment and 3200ml of sea water were used for the control experiment (Control) and 250g of materials covering sediment was used for the suppression experiment (Glass bead, Sea sand, Slag-l, Slag-4). Q~dissolved oxygen; ©---orthophosphate.
under the strong anaerobic state. The results are shown in Table 3. Experiment 1 shown in Table 3 was carried out with control bottle containing only sediment and sea water, and also experiments 2-4 with the experimental bottles containing glass bead, sea sand and slag-3 in addition to sediment and sea water, respectively. Comparing the hydrogen sulfide concentration of the control bottle with that of the experi-
Table 2. Suppression efficiency of phosphate liberation by several materials
Experiment
Materials Control Sea sand River sand Slag-2 Slag-4 Control Glass bead Sea sand Slag-I Slag-3
Released amount of PO3- during 40 days
Suppressed amount of PO43- during 40 days
(~g)
¢~g)
Suppression efficiency'[" (%)
2584.0 957.0 934.7 70.6 30.6 2570.1 I 180.3 1101.6 1055.8 100.4
-1627.0 1649.3 2513.4 2553.4 -1389.8 1468.5 1514.3 2469.7
-63.0 63.8 97.3 98.8 -54.1 57.1 58.9 96. I
?These values were the ratios of suppressed amount of phosphate(*) to the control values (2584.0 in experiment I and 2570.1 in experiment 2). *These values were obtained by subtraction of the phosphate amount released during 40 days from the control value.
330
HISASHI YAMADAet al.
mental bottle, the concentrations of the latter were lower than those of the former and approx, onequarter of the former except glass bead. The hydrogen sulfide concentration of experiment 2 (glass bead) was almost as same as that of experiment 1 (control bottle). As described in a previous paper (Yamada et al., 1986), metals such as magnesium, aluminium, calcium and iron were detected on the surface of slag by the X-ray microanalyzer, and almost the same metals might be naturally presented on the surface of sand. Considering the results of color change on the surface of sea sand and slag and of low concentrations of hydrogen sulfide in experiments 3 and 4, it was found that hydrogen sulfide converted from sulfate by sulfate reducing bacteria reacted with metals on the surface. Therefore, the undissolved metal sulfides were produced on the surface, and the surface color of sea sand and slag changed, and the hydrogen sulfide concentration in sea water became low. The concentrations of phosphate in the sea water of experiments 2-4 were lower than that of experiment I, especially that of experiment 4 was the lowest. Although the concentrations of hydrogen sulfide of experiments 1 and 2 were not so different, the concentration of phosphate of experiment 2 was lower than that of experiment 1 as shown in Table 3. It was considered that dissolution of the precipitated phosphorus was inhibited by covering the surface of sediment with glass bead. On the interaction between hydrogen sulfide and liberation of phosphate from sediment under anaerobic state, Sugawara et al. (1957) demonstrated that precipitated phosphorus (Fe-P, AI-P and Ca-P) is dissolved by the reaction with hydrogen sulfide, and the phosphate is released into overlying water as follows. Fe (PO4)--* Fe3 (PO4)2
(1)
SO~ --,S- 2
(2)
Slag or Sand (Fe, M n ) + S2- ~ F e S , MnS
(3)
Fe3(PO4)2 + S:- ~ 3 F e S + 2PO 3-. (4) That is, the precipitated phosphate of Fe(PO4) in sediment is reduced to Fe3(PO4)2 at anaerobic state
[equation (1)] and also sulfate is reduced to sulfide ion by sulfate reducing bacteria [equation (2)]. The generated sulfide ion reacts with Fe3(PO4)2 and the phosphate ion is formed and liberated into sea water [equation (4)]. In the presence of sea sand and slag covered on the surface of sediment, the generated sulfide ion reacts with the metals on the surface of sea sand and slag, and the metal sulfides precipitates on it [equation (3)], and the reaction of sulfide ion with Fe3(PO4)2 on the surface of sediment does not progress, and consequently the liberation of phosphate is suppressed. A consideration on suppression o f phosphate liberation
The suppression effect on the phosphate liberation from sediment was considered from the results shown in Tables 2 and 3 as follows; (1) Suppression efficiency of glass bead covered on the surface of sediment was 54.1% in spite of no adsorption of phosphate and no reaction of sulfide on the surface of glass bead (in Table 2), that is, this effect was caused by covering the surface of sediment with materials (covering effect); (2) The liberation of phosphate was suppressed by the formation of metal sulfide on the surface of slag (in Table 3), that is, this effect was caused by chemical reaction between hydrogen sulfide and precipitated phosphorus (chemical effect); (3) The suppression efficiencies of slag-2, -3 and -4 were 97.3, 96. l and 98.8%, that is, these effects were caused by not only the above two effects but also adsorption of phosphate liberated from sediment (adsorption effect). A suppression mechanism of phosphate liberation was considered on the basis of three effects described above. The first is covering effect of materials such as glass bead on the surface of sediment. As shown in Table 2, the suppression efficiency of phosphate liberation was 54.1% by glass bead. The liberation of phosphate from sediment was considerably suppressed by just covering materials on the surface of sediment. The second is chemical effect by hydrogen sulfide generated at anaerobic state. From the result of experiment 3 using sea sand as shown in Table 3, the generated sulfide ion reacted with some metals on the surface of sea sand and the liberation of phos-
Table 3. Relationshipsbetween concentrationsof hydrogen sulfide and phosphate at anaerobicstate Sediment weight Materials and weight DO H2S PO~ -P Experiment No. (g) (g) (ml I t) (rag ml ~) (#g-at I ') 0
0.095
0.921
0 0 0 0 0 0
0.075 (0,082)* 0.075 0.065 0.070 (0.072) 0.080 0,020
0,653 (0.758)* 0.701 0.267 -- (0.223) 0.178 0.252
20
0 0 0
0.022 (0.020) 0.019
0.089 (0.173) 0.178
15
0 0
0.018 0,013 (0.019) 0.027
0.148 0,001 (0,094) 0.133
I Controlledbottle
20
--
--
2 Experimentalbottle
20
Glass bead
20
3 Experimental bottle
20
Sea sand
4 Experimentalbottle
20
Slag-3
*The values in parentheses exhibit the mean value of measured three values.
331
Suppression of phosphate liberation I00
15
~
effee!
"~ ~. o 50
Chemicot
effect
kg m - Z i .o o
Covering o "6
Fig. 6. Suppression efficiency by various suppressing materials phate was suppressed by the formation of metal sulfide as described in the section above. In our previous paper (Yamada et al., 1986), authors concluded that sand did not absorb orthophosphate because adsorption of phosphate on the surface of sand could not apply the Freundlich adsorption isotherm. Therefore, it was considered that the suppression of orthophosphate liberation by sand was caused by the covering effect and the chemical effect. Considering that the covering effect by glass bead was identical with that by sand, the chemical effect was estimated by the difference between the suppression efficiency of sand and that of glass bead, and it could be determined from the present result that the chemical effect was 7.2%. The third is adsorption effect by slag self. As shown in Table 2, the suppression efficiencies to the orthophosphate liberation of soft granulated slags such as slag-2, -3 and -4 were high. It was found in our previous paper (Yamada et al., 1986) that the soft granulated slag adsorbed much amount of orthophosphate, and adsorption of orthophosphate was related to the surface characteristics of slags such as porosity. Therefore, it was considered that these high suppression efficiencies of the soft granulated slags were due to the covering effect, the chemical effect and the adsorption effect. The adsorption effect of slag-2, -3 and -4 was estimated by subtracting the covering effect (54.1%) and the chemical effect (7.2%) from the suppression effect of slags. The adsorption effect of slag-2, -3 and -4 were 36.0, 34.8 and 37.5%, respectively. The adsorption effect of hard granulated slag such as slag-I is unclear at present, however, this fact may depend on the low adsorption coefficient as shown in our previous paper (Yamada et al., 1986). As described above, three effects were concerned with the suppression of phosphate liberation from sediment. This consideration was presented schematically in Fig. 6. The soft granulated slags such as slag-2, -3 and -4 had an effective suppression on the orthophosphate liberation, and the suppression was mainly carried out by both covering effect and adsorption effect. Suppression efficiency depending on the amount of slag It is important to estimate how much slag is
1
2
3
4
5
6
7
Experimental period ( d )
Fig. 7. The changes of concentration of orthophosphate in five containers containing different amount of slag-3. necessary for the suppression of phosphate liberation from sediment. A series of containers containing 50, 100, 150, 200 and 250 g of slag-3 with sediment and sea water, and a container including only sediment and sea water as control were prepared. The concentrations of dissolved oxygen and orthophosphate in sea water were determined over 7 days. The concentration of dissolved oxygen decreased gradually, and dissolved oxygen was not detected in the sample solution of third day of the experimental period. The changes of orthophosphate concentration are shown in Fig. 7. When the amount of slag was expressed with a unit of kg m -2, slag of 50 g corresponded to 2.5 kg m-:, because the area of sediment surface in the container was 200 cm 2. As shown in Fig. 7, the increase of phosphate concentration was recognized, and the increase rate of phosphate concentration decreased with increasing the amount of slag in the container. The increase of phosphate concentration was not recognized in a container containing slag of 250 g. The suppression efficiencies were obtained from the changes of phosphate con-
100
u
._~ == 03
~ I 5
e / ! e
I 10
Amount of stag (kg rn-2)
Fig. 8. Suppression efficiencydepending on the amount of slag-3.
332
HISASHIYAMADAet al.
centration as described in above section. The relationships between the suppression efficiency and the amount of slag are shown in Fig. 8. The suppression efficiency depended on the amount of slag, and increased significantly with the increase of slag from 2.5 to 7.5 kg m -2, but the changes of suppression efficiency were small at much more amount of slag from 7.5 to 12.5 kg m -2. From the present results, it was found that about 85% of orthophosphate liberation were suppressed by using 7.5 kg m -2 of slag. Influence o f slag on ecosystem
In the case of using slag for the suppression of orthophosphate liberation in water region, the influence of slag on the ecosystem must be researched. F o r this purpose, it is necessary to study the influences of slag on the mortality of organisms such as zooplankton and juvenile fish, on the growth rate of fish, on the biomass and fauna of benthic organisms, and on the concentration of toxic substances such as heavy metals etc. in the slag. The concentrations of several heavy metals, cadmium, lead, copper, zinc and chromium, in four kinds of slag were determined by the atomic absorption spectrophotometer, and the results were shown in Table 4. The concentration of cadmium was low and almost the same as that in the earth's crust. The concentrations of lead, copper, zinc and chromium were lower than that in the earth's crust reported by Yamagata (1977). The concentrations of cadmium, lead, copper and zinc in the sediment in the coastal area like Tokyo Bay, Japan were 1.2-3.0, 26.6-66.7, 17.5-86.5 and 130.3-436.8 ppm, respectively (Yamada, unpublished data). Therefore, the concentrations of heavy metals in slag were lower than those in the environmental materials. Hereupon, it was considered that the concentrations of heavy metals in slag were too low to influence the ecosystem in natural water region. The influence of slag on the mortality of marine organisms such as rotifers and juvenile of Oplegnathus fasciatus, and the influence of slag on the growth rate of juvenile of O. fasciatus were studied by Iidaka et al. (personal communication). It was found from their results that slag had not wrong effect on these organisms. In the case of covering the surface of sediment by slag, the life of the benthic organisms may be affected. Hence, the influence of covering slag on biomass and fauna of the bottom dwelling organisms should be studied further. Table 4. Concentrationsof severalheavy metalssuch as cadmium, lead, copper, zinc and chromium in slag Ca
Cd Slag-I <0.05 Slag-2 <0.05 Slag-3 <0.05 Slag-4 <0.05 Earth's crust* 0.02 *Yamagata (1977).
Pb 0.3 0.4 0.9 0.3 12.5
(ppm) 1.0 0.9 0.6 0.8 55.0
Zn 6.5 1.7 1.9 2.2 70.0
Total-Cr 29.0 35.0 21.0 17.0 100.0
CONCLUSIONS (I) The suppression effect on the phosphate liberation was recognized by covering the sediment surface with iron slag or by mixing it with sediment. Comparing by four kinds of slag, glass bead and two kinds of sand, the suppression efficiencies of slag-2, -3 and -4 were larger than those of glass bead and sands. The suppression efficiency of glass bead was 54. 1%, but those of slag-2, -3 and -4 were 97.3, 96.1 and 98.8%, respectively. The suppression efficiency depended on the amount of slag used for the suppression experiment, and about 85% of phosphate liberation was suppressed by using 7 . 5 k g m -2 of slag-3. (2) The concentration of sulfide ion in the sea water decreased in the presence of slag and sand. Sulfide ion generated under the strong anaerobic condition reacted with some metals on the surface of slag and sand to produce metal sulfides. As the reaction of sulfide ion with precipitated phosphate was inhibited, the liberation of phosphate was suppressed. (3) As for the suppression mechanisms of phosphate liberation by suppressing materials, it was considered that three effects of suppressing materials were concerned with the phosphate liberation from sediment. The first was covering effect of materials such as glass bead on the surface of sediment. The second was chemical effect by hydrogen sulfide generated at anaerobic state. The third was adsorption effect. Since slag has all of three effects, it was effective suppressing materials for the phosphate liberation from sediment. The suppression of phosphate liberation by slag was mainly carried out by the covering and adsorption effects. (4) The concentrations of several heavy metals in slag, such as cadmium, lead, copper, zinc and chromium, were low compared with the concentrations of those in the coastal sediment. Acknowledgements--The authors are grateful to Professor Hiroshi Sunahara (Faculty of Engineering, Hiroshima University) and Dr Yoshihiro Satomi (Water Pollution Division, Tokai Regional Fisheries Research Laboratory) for their helpful advice and critical reading of this manuscript.
REFERENCES
Arakawa K. (1980) Determination of sulfide in bottom sediments. In Suishitsu Odaku Chosa Shishin (Edited by Nippon Suisan Shigon Hogo Kyokai), pp. 254-257. Koseisha Koseikaku, Tokyo. Bonoub M. W. (1976) Experimental investigation on the release of phosphorus in relation to iron in freshwater mud system. Proceedings of International Symposium on Interaction between Sediments and Freshwater, Amsterdam, pp. 324-330. Fillos J. and Motof A. H. (1972) Effect of benthal deposits on oxygen and nutrient economy of flowing water. J. Wat. Pollut. Control Fed. 44, 644--662. Higgins B. P. J., Mohleji S. C. and Irovine R. L. (1976) Lake treatment of fly ash, lime and gypsum. J. Wat. Pollut. Control Fed. 45, 2153--2164.
Suppression of phosphate liberation Hosomi M., Okada M. and Sudo R. (1982) Release of phosphorus from lake sediments. Envir. int. 7, 93-98. Ishio S. and Kondo K. (1980) Studies on the scarcity of Red Tide in the eutrophicated waters of Ariake Bay. I. Dissolution of phosphate ion from bottom mud by hydrogen sulfide. Bull. Jap. Soc. Sci. Fish. 46, 977-989. Japanese Industrial Standard (1970) Method of test of specific gravity of soil, JIS A 1202-1970. Japanese Industrial Standard (1977) Methods for determination of lead in ores, JIS M 8123-1977. Japanese Industrial Standard (1979) Methods for determination of zinc in ores, JIS M 8124-1979. Japanese Industrial Standard (1981) Methods for determination of cadmium in ores, JIS M 8135-1981. Japanese Industrial Standard (1982) Methods for determination of copper in ores, JIS M 8121-1982. Joh H. (1983) Fractionation of phosphorus and releasable fraction in sediment mud of Osaka Bay. Bull. Jap. Soc. Sci. Fish. 49, 447-454. Koroleff F. (1979) Determination of dissolved inorganic phosphate. In Methods o f Sea Water Analysis (Edited by Grasshoff K.), pp. 117-122. Verlag Chemie, New York. Lee G. F., Sonzogni W. C. and Spear R. D. (1976) Significance of oxic vs anoxic condition for Lake Mendota sediment phosphorus release. Proceedings o f International Symposium on Interaction between Sediments and Freshwater, Amsterdam, pp. 294-306. Liddicoat M. I., Tibbitts S. and Butler E. I. (1975) The determination of ammonia in sea water. Limnol. Oceanogr. 20, 131-132. Lijkleme L. (1976) The role of iron in exchange of phosphate between warter and sediments. Proceedings of International Symposium on Interaction between Sediments and Freshwater, Amsterdam, pp. 313-317.
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Mortimer C. H. (1971) Chemical exchange between sediments and water in the Great Lake--speculation of probable regulatory mechanisms. Limnol. Oceanogr. 16, 387-404. Nishimura A. and Seki M. (1983) Effect of lime for the improvement of mariculture grounds. Bull. Jap. Soc. Sci. Fish. 49, 353-358. Peterson S. A., Sanville W. D., Stay F. S. and Powers C. F. (1976) Laboratory evaluation of nutrient inactivation compounds for lake restoration. J. Wat. Pollut. Control Fed. 48, 817-831. Sanville W. D., Powers C. F., Schuytema G. S., Stay F. S. and Lauer W. L. (1982) Phosphorus inactivation by zirconium in a eutrophic pond. J. Wat. Pollut. Control Fed. 54, 434-443. Sasaki K. and Sawada Y. (1980) Determination of ammonia in estuary. Bull. Jap. Soc. Sci. Fish. 46, 319-321. Serruya C. (1971) Lake Kinnert: the nutrient chemistry of the sediments. Limnol. Oceanogr. 16, 501-521. Strickland J. D. H. and Parsons T. D. (1968) A Practical Handbook of Seawater Analysis. Fisheries Research Board Canada, Bulletin No. 167. Sugawara K., Koyama T. and Kamata E. (1957) Recovery of precipitated phosphate from lake mud related to sulfate reduction. J. Earth Sci. 5, 6(L67. Wood E. D., Armstrong F. A. J. and Richard F. A. (1967) Determination of nitrate in seawater by cadmiumcopper reduction onto nitrite. J. mar. biol. Ass. U.K. 47, 23-31. Yamada H., Kayama M., Saito K. and Hara M. (1986) A fundamental research on phosphate removal by using slag. Wat. Res. 20, 547-557. Yamagata N. (1977) Biryogenso. Kankyo Kagaku Tokuron, Sangyotosho, Tokyo.
0043-1354/87 $3.00+0.00 Copyright © 1987 PergamonJournals Ltd
SUPPRESSION OF PHOSPHATE LIBERATION FROM SEDIMENT BY USING IRON SLAG HISASHI YAMADA,j MITSU KAYAMA,2 KAZUO SAITO3 and MASAKAZUHARA3 tTokai Regional Fisheries Research Laboratory, the Ministry of Agriculture, Forestry and Fisheries, Arasaki, Nagai, Yokosuka 238-03, 2Laboratory of Marine Biochemistry, Faculty of Applied Biological Science, Hiroshima University, Midori-machi, Fukuyama 720 and 3Fukuyama Ironworks, Nippon Kokan Co. Ltd, Kokan-machi, Fukuyama 720, Japan (Received February 1986)
Abstract--In order to suppress the phosphate liberation from sediment to sea water, several suppressing materials such as four kinds of iron slag, fiver and sea sands, and glass bead were tested. The sediment and sea water were sampled from Hiroshima Bay which seemed to be a eutrophicated area. The suppressing materials were covered on the surface of sediment, and sea water was introduced in the aquarium. The concentrations of phosphate, sulfide and dissolved oxygen were measured during the incubation period under the condition of anaerobic state. The suppressing effects of slags, sea and river sands were compared with that of glass bead used as control materials, and the suppression efficiencies were measured. The suppression efficiencyof glass bead was 54.1%, and those of slag-2, -3 and -4 were 97.3, 96.1 and 98.8%, respectively.The suppression efficiencydepended on the amounts of slag used, and about 85% of liberated phosphate were suppressed by slag-3 of 7.5 kg m -2. The sulphide ion generated under anaerobic state reacted with some metals on the surface of slags and sands, and the dissolution of precipitated phosphate (Fe--P, A l P and Ca-P) was suppressed by formation of metal sulfide on the surface of slags and sands. The suppression mechanisms on phosphate liberation, such as covering, chemical and adsorption effects were proposed and discussed. The suppression by slag was mainly carried out by the adsorption effect, in addition to the covering effect. Key words--iron slag, phosphate, liberation, suppression efficiency, adsorption, sediment
INTRODUCTION It is well known that the accumulated nutrients in the shallow and coastal sediment play an important role for the eutrophication in a closed water area. The supply of nutrients from the sediment is one of the main sources, and has an influence on the change of nutrient concentration. The suppression of nutrient liberation from sediment has been recognized as one of the important research to control the eutrophication caused by the excessive nutrients. According to Lee et al. (1976), Lijkleme (1976) and Banoub (1976), orthophosphate has been precipitated on sediment under the aerobic condition, and it has been dissolved under the anaerobic condition. Hosomi et al. (1982) divided phosphorus states in the lake sediment into five forms, such as inorganic phosphate adsorbed on the clay particle, inorganic phosphate precipitated with aluminium (AI-P), with iron (Fe-P), and calcium (Ca-P) and organic phosphorus. Joh (1983) demonstrated that AI-P and Fe-P in marine sediments have changed gradually to a dissolved form in the oxygen-deficient condition. It has been well known that the considerable amounts of phosphate in the sediment were released in the anaerobic state (Fillos and Molof, 1972; Mortimer, 1971; Serruya, 1971). In order to suppress the liberation of phosphate from sediment, many kinds of substances which wa. 2,/r-v
adsorb phosphate or form insoluble compounds with phosphate have been investigated by several researchers. Sodium aluminate (Peterson et al., 1976), lanthanum rare earth chloride (Peterson et al., 1976), zirconium tetrachloride (Peterson et al., 1976; Sanville, 1982), fly ash (Higgins et al., 1976), lime (Higgins et al., 1976; Nishimura and Seki, 1983; Ishio and Kondo, 1980) and gypsum (Higgins et al., 1976) were studied, and these substances have suppressed a considerable amount of phosphate by adsorbing phosphate or producing a unsoluble compound with phosphate. The characteristics of adsorption of phosphate on the slag surface was fundamentally studied in our previous paper (Yamada et al., 1986). It was found that adsorption of phosphate on slag surface and re-dissolution of the adsorbed phosphate from the slag surface depended on pH of solution. Adsorption of phosphate on the slag surface was high at pH 8, and re-dissolution of the adsorbed phosphate was low at same pH of 8. Since pH of natural sea water is around 8, it is considered that slag removes phosphate from natural sea water by adsorbing it on the surface of slag. The adsorption of phosphate on slag surface was influenced by the concentration of co-existing ion, that is, adsorption of phosphate decreased with the increase of the concentration of co-existing ion, but the decrease of phosphate adsorption was not significantly
325
HISASHI YAMADAet al.
326
Gas inlet
influenced by the concentration o f co-existing ion in natural sea water. Accordingly, it is expected that slag adsorbs phosphate dissolving in natural sea water, and that slag suppresses p h o s p h a t e liberation from sediment even under anaerobic conditions. In this paper, the effect o f suppression on p h o s p h a t e liberation from sediment by using slag was investigated by small scale experiment. A system including sediment, sea water and several kinds o f slag or other materials was designed in this experiment to clarify the suppression o f p h o s p h a t e liberation from sediment under the anaerobic condition. The suppression efficiencies o f several kinds o f slag and other materials were also determined, and these effects were discussed.
I
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The experiment on the liberation of phosphate from sediment were carried out by using a plexiglass container of
~
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~
iiiiii;ili iiii!iiii i[iiiiiiiiSediment ?iii?il
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which the length, width and height were 100, 200 and 276 mm, respectively. The used sediment and sea water were obtained at the pier of the Nansei Regional Fisheries Research Laboratory located in Hiroshima Bay, Hiroshima, Japan. The apparatus presented in Fig. 1 contains sediment, sea water and suppressing materials, such as glass bead, sea and river sands and slags. Incubation was carried out for the period at room temperature (about 20-25°C). In order to study the suppression of phosphate liberation from sediment, two experiments were carded out as follows; a control experiment was carried out by using both sediment and sea water, and only sea water in container. Glass bead, sea sand, river sand and several slags were used as sup-
Table I. Chemical and physical characteristicsof slag and sand Sand Chemical composition (%) SiO2 CaO A1203 FeO MgO S MnO TiO2 P205 Particle size (~) Median diameter 0~) Density (gem -~) p'* Pt Porosity (%)
Slag-1 33.10 41.80 13.90 0.34 6.42 1.05 0.68 1.76 -63-2000 800 2.59 2.72 5.0
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Experimental procedure
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MATERIALS AND METHODS In the process of iron and steel production, the blast furnace slag, the converter slag and the electric arc furnace slag are produced independently. Iron ore and caustic lime charged into the furnace are melted, and finally converted into two phases of molten slag and molten iron in this process. Since the former is lower than the latter in density, the former floats on the latter. Slag is molten state at the temperature higher than 1500°C, and the slag is cooled by several methods and the different physical properties are gotten. Slag cooled slowly has ordinarily a hard and dense crystal structure, while slag cooled rapidly contains much more foams in it. The molten slag from the blast furnace is rapidly cooled by the pressurized water, and two types of slag are formed under the conditions of temperature of molten slag, ratio of water to slag, and water pressure. Lightweight granulated slag contains more foams and is called soft granulated slag, and heavier one with less foams is called hard granulated slag. The materials used in this research were soft and hard granulated slag, sand and glass bead. Slag was manufactured and supplied by Nippon Kokan Co. Ltd, Fukuyama, Japan. The chemical composition and physical properties of the materials are summarized in Table 1. The glass bead (GMB-60) was used as a control material, which was supplied by Nippon Rikagakukikai Co., Japan, and the particle size was between 0.5 and 0.7 mm.
O" Pinchcock
Slag-2
Slag-3
Slag-4
Beach
River
32.30 34.30 33.40 71.73 77.81 41.90 40.50 42.30 0.39 2.10 14.00 14.60 13.70 9.33 9.00 0.37 0.30 0.32 0.91 0.76 6.41 6.88 6.69 0.24 0.27 I. 19 0.87 0.93 0.002 0.02 0.65 0.50 0.60 0.12 0.15 2.17 1.23 I. 13 0.63 0.57 0.02 -0.01 0.004 0.006 63-2000 63-2000 63-2838 63-2000 63-2000 720 860 1550 530 1404 2.47 2.02 1.17 2.59 2.59 2.71 2.63 2.63 2.61 2.62 8.8 23.3 55.6 1.0 I. 1
*p' was determined with a pygnometer. tP was determined with a pycnometexafter the removal of air included in the materials by boiling.
Suppression of phosphate liberation pression materials. After sea water (100 ml) was sampled through sampling tube once a day, hydrogen gas was bubbled through a sintered glass ball to keep an anaerobic state in the container. The used amounts of sediment, sea water and suppressing materials are described in the captions of Figs 2, 3, 4 and 5, respectively. It is considered that the liberation of phosphate from sediment is influencedby sulfide ion generated under anaerobic conditions. In order to make clear the influence of sulfide ion on the phosphate liberation, another experiment was carded out. Sediment of 20 g in wet state was put into a BOD bottle of 100 ml. Sea water was filtered with glass fiber filter OVhatman GF/C) and filtered sea water containing 0.05% sucrose was introduced. Dissolved oxygen in the sea water was excluded by bubblingnitrogen gas previously. A control bottle consists of both sediment and the sea water containing 0.05% sucrose. The experiment bottles consist of each 20 g of glass bead and sea sand, and 15 g of slag-3 with 20 g of sediment and the sea water, respectively. Both control and experimental bottles were incubated at 25°C for 5 days. After incubating the bottles, the concentrations of dissolved oxygen, hydrogen sulfide and orthophosphate in the sea water were determined. In order to study the relationships between suppression efficiencyand amount of slag, six containers were prepared. The slag-3 (0-250 g) was introduced into each container containing 3.1 1. of sea water and 2.5 kg of sediment. In the experimental container containing 250 g of slag-3, a slag layer of ~ 1cm covered the surface of sediment. The containers were kept under an anaerobic State, and the concentrations of dissolved oxygen and the orthophosphate were measured during the 7 days incubation. The suppression efficiencies were calculated from the difference between the amount of phosphate released from sediment in a control container (not containing slag-3) and that in experimental container.
Analytical procedure The densities (p and p') of four kinds of slag and two kinds of sand were determined by using a pycnometer according to Japanese Industrial Standard (JIS A 1202, 1970). The value ofp was determined after removing air in the material by boiling it, and p' was the density of the material containing air. The volume percentage of air (Va) in the materials was calculated by the following equation. Va = (1/p" -- 1/p)/(l/p') x I00. The chemical compositions of slag and sand, such as SiO2, CaO, AI203, FeO, MgO, S, MnO, TiO2and P205 were analyzed by using a X-ray fluorescence analyzer (Shimazu, Model VQX-130A). The concentrations of heavy metals, cadmium, lead, copper, zinc and total chromium, in four kinds of slag were analyzed by using an atomic absorption spectrnphotometer according to Japanese Industrial Standard (JIS M 8135, 1981; JIS M 8123, 1977; JIS M 8121, 1982; JIS M 8124, 1979), respectively. The concentration of ammonium ion was determined by measuring the absorption of blue co[or of indophenol according to Liddicoat et al. (1975) and Sasaki and Sawada (1980). Nitrite and nitrate ions were determined according to Strickland and Parsons (1968) and Wood et al. (1967), respectively. The concentration of orthophosphate in the sea water was determined by ascorbic acid-molybden blue method according to Koroleff (1979). The samples containing excess amounts of orthophosphate were diluted by using artificial sea water (25 g NaC1 and 8 g MgSO4 in 1 liter of redistilled water). The concentration of hydrogen sulfide in sea water was measured by a Gastec detector tube (Kitazawa Sangyo Co.), in which lead acetate coated on silica gel was packed as the reactor. The concentration of hydrogen sulfide was determined by the length of the tube turned to black with hydrogen sulfide (Arakawa, 1980). The
327
concentration of dissolved oxygen was determined by the modified Winkler's method (Strickland and Parsons, 1968). RESULTS AND DISCUSSION
Suppression by covering and mixing o f slag The surface of sediment was covered with slag, and the concentrations of dissolved oxygen, orthophosphate and dissolved inorganic nitrogen (DIN) in sea water were measured. The changes of those concentrations are shown in Fig. 2. The concentration of orthophosphate in sea water of the control experiment increased with the decrease of dissolved oxygen during the anaerobic incubation, on the contrary, the concentration of orthophosphate in sea water of the suppression experiment was low during the anaerobic incubation. Comparing the concentrations of DIN in sea water of both experiments, no considerable difference was observed. Although pH of sea water in both the control and suppression experiments increased slightly from 8 to 9 during the anaerobic incubation, the pH decreased to 8 by bubbling air into the sea water. The pH change might be caused by bicarbonate derived from carbon dioxide in air. The concentration of orthophosphate in sea water decreased gradually in the control experiment during the aerobic incubation after bubbling air. It was considered that the decrease of orthophosphate was caused by the precipitation formed with metal ions, such as Fe 2+ or Al 3+. For the suppression experiment, the concentration of orthophosphate increased at the beginning of the aerobic incubation and then decreased gradually. In the experiment of mixing slag with sediment, the suppression effect of phosphate liberation was studied as shown in Fig. 3. Comparing the concentration of orthophosphate in control with that in suppression experiment, the phosphate concentration increased rapidly in the former and remained at low level in the latter. Accordingly, it was clear that the liberation of phosphate was suppressed by mixing slag with sediment. Ishio and Kondo (1980) demonstrated that phosphate in the presence of calcium forms a unsoluble compound like hydroxyapatite, Cas(OH) (PO4)3, which inhibits the dissolution of precipitated phosphorus in sediment. As slag contains large amounts of calcium, the unsoluble compounds may be formed by the mixture of slag and sediment, and the suppression of the phosphate liberation might be progressed.
Suppression effects by slag, sands and glass bead In order to compare the suppression effects on orthophosphate liberation by various materials, slag1, -2, -3, -4, river sand, sea sand, and glass bead were used, and their effects were investigated. As seen in Figs 4 and 5, the variations of phosphate and dissolved oxygen in sea water were measured in the course of incubation period. The increase of the phosphate concentration in sea water was recognized
HISASrIIYAMADAet al.
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with sediment and sea water. during 10-40days under the conditions of control, and covered with glass bead, sea and river sands and slag-1 on sediment, respectively. But the significant increases of the phosphate concentration were not recognized under the covering conditions of slag-2, -3 and -4 on sediment and only sea water (without sediment in Fig. 5), respectively. Therefore, it was considered that the increase of phosphate ion in sea water was mainly caused by the phosphate liberation from sediment and was not caused by the mineralization of organic phosphorus in sea water.
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Fig. 3. The changes of concentrations of dissolved oxygen and orthophosphate during anaerobic incubation. 1.5 kg of sediment and 2300 ml of sea water were used for the control experiment. 500g of slag-3 was mixed with 1.5 kg of sediment for the suppression experiment.
The amounts of orthophosphate released from sediment during 40 days experimental period were calculated by integrating the changes of orthophosphate concentrations shown in Figs 4 and 5. The suppressed amounts of orthophosphate liberation by using the suppressive materials were obtained by subtracting released phosphate during 40 days in the experimental container from that in the control container. The suppression efficiencies (ratio of the suppressed amount of phosphate to the released amount of phosphate in the control experiment) were calculated as shown in Table 2. The suppression efficiencies of soft granulated slags such as slag-2, -3 and -4 were 97.3, 96.1 and 98.80/0, •respectively, and were larger than those of glass bead, sand and the hard granulated slag such as slag-i. It was found that soft granulated slag suppressed effectively the phosphate liberation from sediment. As shown in Table 2, 54.1% of orthophosphate in the control experiment was suppressed by glass bead which did not absorb phosphate. Therefore, it was understood that the orthophosphate liberation was physically suppressed by covering the surface of sediment.
Effect of hydrogen sulfide on phosphate liberation It was recognized that the color of slag and sand layers turned to black during the anaerobic incubation, and the color again turned to the original color by changing to the aerobic state. It was under-
Suppression of phosphate liberation
329
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Incubation period (d) Fig. 4. The changes of concentrations of dissolved oxygen and orthophosphate in sea water during anaerobic incubation for 40 days. 2.5 kg of sediment and 3200 ml of sea water were used for the control experiment (Control) and 250g of materials covering sediment were used for the suppression experiment (Sea sand, River sand, Slag-2, Slag-4). Q--dissolved oxygen; O - - o r t h o p h o s p h a t e .
stood that some metals on the surface of slag and sand reacted with hydrogen sulfide produced by the anaerobic process. In order to investigate the effect of hydrogen sulfide on phosphate liberation from sediment, the relationships between the concentrations of hydrogen sulfide and phosphate in sea water were studied by using control and experimental bottles
Fig. 5. The changes of concentrations of dissolved oxygen and orthophosphate in sea water during anaerobic incubation for 45 days. 2.5 kg of sediment and 3200ml of sea water were used for the control experiment (Control) and 250g of materials covering sediment was used for the suppression experiment (Glass bead, Sea sand, Slag-l, Slag-4). Q~dissolved oxygen; ©---orthophosphate.
under the strong anaerobic state. The results are shown in Table 3. Experiment 1 shown in Table 3 was carried out with control bottle containing only sediment and sea water, and also experiments 2-4 with the experimental bottles containing glass bead, sea sand and slag-3 in addition to sediment and sea water, respectively. Comparing the hydrogen sulfide concentration of the control bottle with that of the experi-
Table 2. Suppression efficiency of phosphate liberation by several materials
Experiment
Materials Control Sea sand River sand Slag-2 Slag-4 Control Glass bead Sea sand Slag-I Slag-3
Released amount of PO3- during 40 days
Suppressed amount of PO43- during 40 days
(~g)
¢~g)
Suppression efficiency'[" (%)
2584.0 957.0 934.7 70.6 30.6 2570.1 I 180.3 1101.6 1055.8 100.4
-1627.0 1649.3 2513.4 2553.4 -1389.8 1468.5 1514.3 2469.7
-63.0 63.8 97.3 98.8 -54.1 57.1 58.9 96. I
?These values were the ratios of suppressed amount of phosphate(*) to the control values (2584.0 in experiment I and 2570.1 in experiment 2). *These values were obtained by subtraction of the phosphate amount released during 40 days from the control value.
330
HISASHI YAMADAet al.
mental bottle, the concentrations of the latter were lower than those of the former and approx, onequarter of the former except glass bead. The hydrogen sulfide concentration of experiment 2 (glass bead) was almost as same as that of experiment 1 (control bottle). As described in a previous paper (Yamada et al., 1986), metals such as magnesium, aluminium, calcium and iron were detected on the surface of slag by the X-ray microanalyzer, and almost the same metals might be naturally presented on the surface of sand. Considering the results of color change on the surface of sea sand and slag and of low concentrations of hydrogen sulfide in experiments 3 and 4, it was found that hydrogen sulfide converted from sulfate by sulfate reducing bacteria reacted with metals on the surface. Therefore, the undissolved metal sulfides were produced on the surface, and the surface color of sea sand and slag changed, and the hydrogen sulfide concentration in sea water became low. The concentrations of phosphate in the sea water of experiments 2-4 were lower than that of experiment I, especially that of experiment 4 was the lowest. Although the concentrations of hydrogen sulfide of experiments 1 and 2 were not so different, the concentration of phosphate of experiment 2 was lower than that of experiment 1 as shown in Table 3. It was considered that dissolution of the precipitated phosphorus was inhibited by covering the surface of sediment with glass bead. On the interaction between hydrogen sulfide and liberation of phosphate from sediment under anaerobic state, Sugawara et al. (1957) demonstrated that precipitated phosphorus (Fe-P, AI-P and Ca-P) is dissolved by the reaction with hydrogen sulfide, and the phosphate is released into overlying water as follows. Fe (PO4)--* Fe3 (PO4)2
(1)
SO~ --,S- 2
(2)
Slag or Sand (Fe, M n ) + S2- ~ F e S , MnS
(3)
Fe3(PO4)2 + S:- ~ 3 F e S + 2PO 3-. (4) That is, the precipitated phosphate of Fe(PO4) in sediment is reduced to Fe3(PO4)2 at anaerobic state
[equation (1)] and also sulfate is reduced to sulfide ion by sulfate reducing bacteria [equation (2)]. The generated sulfide ion reacts with Fe3(PO4)2 and the phosphate ion is formed and liberated into sea water [equation (4)]. In the presence of sea sand and slag covered on the surface of sediment, the generated sulfide ion reacts with the metals on the surface of sea sand and slag, and the metal sulfides precipitates on it [equation (3)], and the reaction of sulfide ion with Fe3(PO4)2 on the surface of sediment does not progress, and consequently the liberation of phosphate is suppressed. A consideration on suppression o f phosphate liberation
The suppression effect on the phosphate liberation from sediment was considered from the results shown in Tables 2 and 3 as follows; (1) Suppression efficiency of glass bead covered on the surface of sediment was 54.1% in spite of no adsorption of phosphate and no reaction of sulfide on the surface of glass bead (in Table 2), that is, this effect was caused by covering the surface of sediment with materials (covering effect); (2) The liberation of phosphate was suppressed by the formation of metal sulfide on the surface of slag (in Table 3), that is, this effect was caused by chemical reaction between hydrogen sulfide and precipitated phosphorus (chemical effect); (3) The suppression efficiencies of slag-2, -3 and -4 were 97.3, 96. l and 98.8%, that is, these effects were caused by not only the above two effects but also adsorption of phosphate liberated from sediment (adsorption effect). A suppression mechanism of phosphate liberation was considered on the basis of three effects described above. The first is covering effect of materials such as glass bead on the surface of sediment. As shown in Table 2, the suppression efficiency of phosphate liberation was 54.1% by glass bead. The liberation of phosphate from sediment was considerably suppressed by just covering materials on the surface of sediment. The second is chemical effect by hydrogen sulfide generated at anaerobic state. From the result of experiment 3 using sea sand as shown in Table 3, the generated sulfide ion reacted with some metals on the surface of sea sand and the liberation of phos-
Table 3. Relationshipsbetween concentrationsof hydrogen sulfide and phosphate at anaerobicstate Sediment weight Materials and weight DO H2S PO~ -P Experiment No. (g) (g) (ml I t) (rag ml ~) (#g-at I ') 0
0.095
0.921
0 0 0 0 0 0
0.075 (0,082)* 0.075 0.065 0.070 (0.072) 0.080 0,020
0,653 (0.758)* 0.701 0.267 -- (0.223) 0.178 0.252
20
0 0 0
0.022 (0.020) 0.019
0.089 (0.173) 0.178
15
0 0
0.018 0,013 (0.019) 0.027
0.148 0,001 (0,094) 0.133
I Controlledbottle
20
--
--
2 Experimentalbottle
20
Glass bead
20
3 Experimental bottle
20
Sea sand
4 Experimentalbottle
20
Slag-3
*The values in parentheses exhibit the mean value of measured three values.
331
Suppression of phosphate liberation I00
15
~
effee!
"~ ~. o 50
Chemicot
effect
kg m - Z i .o o
Covering o "6
Fig. 6. Suppression efficiency by various suppressing materials phate was suppressed by the formation of metal sulfide as described in the section above. In our previous paper (Yamada et al., 1986), authors concluded that sand did not absorb orthophosphate because adsorption of phosphate on the surface of sand could not apply the Freundlich adsorption isotherm. Therefore, it was considered that the suppression of orthophosphate liberation by sand was caused by the covering effect and the chemical effect. Considering that the covering effect by glass bead was identical with that by sand, the chemical effect was estimated by the difference between the suppression efficiency of sand and that of glass bead, and it could be determined from the present result that the chemical effect was 7.2%. The third is adsorption effect by slag self. As shown in Table 2, the suppression efficiencies to the orthophosphate liberation of soft granulated slags such as slag-2, -3 and -4 were high. It was found in our previous paper (Yamada et al., 1986) that the soft granulated slag adsorbed much amount of orthophosphate, and adsorption of orthophosphate was related to the surface characteristics of slags such as porosity. Therefore, it was considered that these high suppression efficiencies of the soft granulated slags were due to the covering effect, the chemical effect and the adsorption effect. The adsorption effect of slag-2, -3 and -4 was estimated by subtracting the covering effect (54.1%) and the chemical effect (7.2%) from the suppression effect of slags. The adsorption effect of slag-2, -3 and -4 were 36.0, 34.8 and 37.5%, respectively. The adsorption effect of hard granulated slag such as slag-I is unclear at present, however, this fact may depend on the low adsorption coefficient as shown in our previous paper (Yamada et al., 1986). As described above, three effects were concerned with the suppression of phosphate liberation from sediment. This consideration was presented schematically in Fig. 6. The soft granulated slags such as slag-2, -3 and -4 had an effective suppression on the orthophosphate liberation, and the suppression was mainly carried out by both covering effect and adsorption effect. Suppression efficiency depending on the amount of slag It is important to estimate how much slag is
1
2
3
4
5
6
7
Experimental period ( d )
Fig. 7. The changes of concentration of orthophosphate in five containers containing different amount of slag-3. necessary for the suppression of phosphate liberation from sediment. A series of containers containing 50, 100, 150, 200 and 250 g of slag-3 with sediment and sea water, and a container including only sediment and sea water as control were prepared. The concentrations of dissolved oxygen and orthophosphate in sea water were determined over 7 days. The concentration of dissolved oxygen decreased gradually, and dissolved oxygen was not detected in the sample solution of third day of the experimental period. The changes of orthophosphate concentration are shown in Fig. 7. When the amount of slag was expressed with a unit of kg m -2, slag of 50 g corresponded to 2.5 kg m-:, because the area of sediment surface in the container was 200 cm 2. As shown in Fig. 7, the increase of phosphate concentration was recognized, and the increase rate of phosphate concentration decreased with increasing the amount of slag in the container. The increase of phosphate concentration was not recognized in a container containing slag of 250 g. The suppression efficiencies were obtained from the changes of phosphate con-
100
u
._~ == 03
~ I 5
e / ! e
I 10
Amount of stag (kg rn-2)
Fig. 8. Suppression efficiencydepending on the amount of slag-3.
332
HISASHIYAMADAet al.
centration as described in above section. The relationships between the suppression efficiency and the amount of slag are shown in Fig. 8. The suppression efficiency depended on the amount of slag, and increased significantly with the increase of slag from 2.5 to 7.5 kg m -2, but the changes of suppression efficiency were small at much more amount of slag from 7.5 to 12.5 kg m -2. From the present results, it was found that about 85% of orthophosphate liberation were suppressed by using 7.5 kg m -2 of slag. Influence o f slag on ecosystem
In the case of using slag for the suppression of orthophosphate liberation in water region, the influence of slag on the ecosystem must be researched. F o r this purpose, it is necessary to study the influences of slag on the mortality of organisms such as zooplankton and juvenile fish, on the growth rate of fish, on the biomass and fauna of benthic organisms, and on the concentration of toxic substances such as heavy metals etc. in the slag. The concentrations of several heavy metals, cadmium, lead, copper, zinc and chromium, in four kinds of slag were determined by the atomic absorption spectrophotometer, and the results were shown in Table 4. The concentration of cadmium was low and almost the same as that in the earth's crust. The concentrations of lead, copper, zinc and chromium were lower than that in the earth's crust reported by Yamagata (1977). The concentrations of cadmium, lead, copper and zinc in the sediment in the coastal area like Tokyo Bay, Japan were 1.2-3.0, 26.6-66.7, 17.5-86.5 and 130.3-436.8 ppm, respectively (Yamada, unpublished data). Therefore, the concentrations of heavy metals in slag were lower than those in the environmental materials. Hereupon, it was considered that the concentrations of heavy metals in slag were too low to influence the ecosystem in natural water region. The influence of slag on the mortality of marine organisms such as rotifers and juvenile of Oplegnathus fasciatus, and the influence of slag on the growth rate of juvenile of O. fasciatus were studied by Iidaka et al. (personal communication). It was found from their results that slag had not wrong effect on these organisms. In the case of covering the surface of sediment by slag, the life of the benthic organisms may be affected. Hence, the influence of covering slag on biomass and fauna of the bottom dwelling organisms should be studied further. Table 4. Concentrationsof severalheavy metalssuch as cadmium, lead, copper, zinc and chromium in slag Ca
Cd Slag-I <0.05 Slag-2 <0.05 Slag-3 <0.05 Slag-4 <0.05 Earth's crust* 0.02 *Yamagata (1977).
Pb 0.3 0.4 0.9 0.3 12.5
(ppm) 1.0 0.9 0.6 0.8 55.0
Zn 6.5 1.7 1.9 2.2 70.0
Total-Cr 29.0 35.0 21.0 17.0 100.0
CONCLUSIONS (I) The suppression effect on the phosphate liberation was recognized by covering the sediment surface with iron slag or by mixing it with sediment. Comparing by four kinds of slag, glass bead and two kinds of sand, the suppression efficiencies of slag-2, -3 and -4 were larger than those of glass bead and sands. The suppression efficiency of glass bead was 54. 1%, but those of slag-2, -3 and -4 were 97.3, 96.1 and 98.8%, respectively. The suppression efficiency depended on the amount of slag used for the suppression experiment, and about 85% of phosphate liberation was suppressed by using 7 . 5 k g m -2 of slag-3. (2) The concentration of sulfide ion in the sea water decreased in the presence of slag and sand. Sulfide ion generated under the strong anaerobic condition reacted with some metals on the surface of slag and sand to produce metal sulfides. As the reaction of sulfide ion with precipitated phosphate was inhibited, the liberation of phosphate was suppressed. (3) As for the suppression mechanisms of phosphate liberation by suppressing materials, it was considered that three effects of suppressing materials were concerned with the phosphate liberation from sediment. The first was covering effect of materials such as glass bead on the surface of sediment. The second was chemical effect by hydrogen sulfide generated at anaerobic state. The third was adsorption effect. Since slag has all of three effects, it was effective suppressing materials for the phosphate liberation from sediment. The suppression of phosphate liberation by slag was mainly carried out by the covering and adsorption effects. (4) The concentrations of several heavy metals in slag, such as cadmium, lead, copper, zinc and chromium, were low compared with the concentrations of those in the coastal sediment. Acknowledgements--The authors are grateful to Professor Hiroshi Sunahara (Faculty of Engineering, Hiroshima University) and Dr Yoshihiro Satomi (Water Pollution Division, Tokai Regional Fisheries Research Laboratory) for their helpful advice and critical reading of this manuscript.
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