effects of the seaweed bed construction using the mixture

1919 © 2011 ISIJ

ISIJ International, Vol. 51 (2011), No. 11, pp. 1919–1928

Effects of the Seaweed Bed Construction Using the Mixture of Steelmaking Slag and Dredged Soil on the Growth of Seaweeds

Akio HAYASHI,1,2) Hirokazu TOZAWA,2) Katsuya SHIMADA,3) Katunori TAKAHASHI,2) Ryoko KANEKO,1)

Fumitaka TSUKIHASHI,4) Ryo INOUE5) and Tatsuro ARIYAMA5)

1) JFE Mineral Company, LTD. E-mail: [email protected], [email protected]) JFE Steel Corporation. E-mail: [email protected], [email protected], [email protected]) IDEA Consultants, Inc. E-mail: [email protected]) Department of Advanced Material Science, The University of Tokyo. E-mail: [email protected]) Institute of Multidisciplinary Research for Advanced Materials, Tohoku University. E-mail: [email protected],[email protected]

(Received on March 7, 2011; accepted on July 28, 2011)

Supply of Fe ions is considered to be effective for the growth of kelp (a kind of seaweed). In the presentresearch, a demonstration experiment in which seaweed beds/shoals were formed using a mixture ofsteelmaking slag and dredged soil was carried out in a marine area of Kawasaki City, Japan. The averagestrength of the mound for seaweed beds, which was made of a mixture of dredged soil and steelmakingslag, was 109.7 kN/m2. The shape of the mound was stable during the experiment period. The Fe contentof the water above the mound made of the mixture was around 5 ppb higher than that above moundsmade of natural sand. The average dry weight of the soft seaweed (Undariapinnatifida:wakame inJapanese) and brown seaweed (Sargassumhorneri) taken from mounds of the mixture including steelmak-ing slag were respectively 1.1 times and 2.1 times as much as that from mounds of natural sand. Theseresults indicate that Fe ions, which dissolved from the steelmaking slag, have a positive effect on thegrowth of brown seaweed.

KEY WORDS: global warming; steelmaking slag; dredged soil; Fe ion; seaweed.

1. Introduction

To cope with global warming, CO2 reduction is veryimportant in all areas. A number of countermeasures havebeen implemented and have been under development sincethe 1990s. The Japanese steel industry is not only devotinggreat effort to reducing CO2 emissions from iron andsteelmaking processes by energy conservation, but is alsocontinuing research and development on CO2 fixation tech-nologies. These technologies include a technology whichutilizes CO2 absorption by seaweeds. It is known that supplyof Fe ions accelerates the growth of seaweeds and increasesfixation of CO2 by this marine vegetation.1–4) Steelmakingslag, which is a by-product of the steel manufacturing pro-cess, is one of the prominent candidates as a source of ironions.

The purpose of the present research was to develop a newtechnology for creating mounds for seaweed beds and seagrass beds in coastal areas using a mixture of steelmakingslag and dredged soil. The aim is to accelerate CO2 absorp-tion by promoting the growth of seaweed and sea grassusing Fe ions from steelmaking slag. This technology hasthe additional benefit of enabling recycling of dredged soil.

Algae in coastal areas form colonies. Marine algae andseaweeds absorbCO2 by photosynthesis, supply oxygen tothe environment, and thereby support the growth of fish and

shellfish. According to a report compiled by the UnitedNations Environment Programme(UNEP) in October2009,5) mangrove forests, seaweed beds, salt marshes, andother coastal ecosystems worldwide absorb between 0.87billion and 1.65 billion tons of CO2 annually, which is com-parable to Japan’s total annual emissions of CO2. The UNEPreport also pointed out that stopping the progressive des-truction of coastal ecosystems caused by reclamation anddevelopment is an effective approach to preventing globalwarming.5)

When seaweeds absorb nutrients such as nitrogen andphosphorus from seawater, enzymes are needed, and microelemental Fe is indispensable for the smooth functioning ofthese enzymes.6) Lack of the proper amount of Fe also hin-ders formation of chlorophyll in seaweeds.

In fiscal 2004–2007, the Japan Iron and Steel Federation,with financial assistance from Japan’s Ministry of Econo-my, Trade and Industry (METI), conducted an “Environ-mental improvement study and evaluation of the maritimeuse of steelmaking slag”. The following technical knowl-edge was obtained as a result of this study: Mixing of steelslag and dredged soil improves the soil strength and controlsleaching of high pH water from steelmaking slag.7)

Previous research confirmed that a fertilizer unit consist-ing of steelmaking slag and humus soil which was artifi-cially fermented from waste wood chips substantially

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ISIJ International, Vol. 51 (2011), No. 11

increases kelp growth.1–4,8) Some previous research showsthat seaweed utilizes Fe ions dissolved from the mixture ofthe steelmaking slag and artificial humus soil containinghumus acid.8)

The purpose of the experiment was to demonstrate andevaluate the effect of a mixture of steelmaking slag anddredged soil on the growth of seaweeds in a marine envi-ronment at Kawasaki harbor. The concept of the demonstra-tion experiment is shown in Fig. 1. Certain dredged soilscontain organic acids such as fulvic acid and humic acid.These organic acids capture Fe ions eluted from steelmakingslag and form complexes. It is known that seaweed absorbsFe ions from these complexes. One key difference betweenthe existing technologies and this newly developed tech-nology is the use of dredged soil containing naturally occur-ring organic acids. Dredged soil recovered from the seabedis considered to be a lower impact on ecosystems. From theviewpoint of effective use of resources, the development ofa new technique which utilizes dredged soil and by-productslag from the steelmaking process is highly significant forimproving environmental preservation.

2. Experimental

2.1. Laboratory Experiments2.1.1. Solidification by Mixing Dredged Soil and Steel-

making SlagIn order to determine the proper mixing ratio, a mixing

test was carried out using steelmaking slag and dredged soilfrom Ukishima in the Port of Kawasaki, which was per-mitted to use by the port authority. The soil characteristicsof the dredged soil and chemical composition of the steel-making slag are shown in Tables 1 and 2, respectivelySteelmaking slag of 0–5 mm in diameter and two types ofdredged soil in its nature state are mixed with a paddle typemixer with the ratio ranging from 20:80 to 50:50. The blend-ed material was cast into 5 × 10 cm molds and sealed. Aftercuring for 28 days, the compressive strength was measured.

The effects of the free-CaO ratio in the slag and grain sizeof the slag were examined. Steelmaking slags with dia-meters of 0–5 mm, 0–13 mm and 0–30 mm were mixed withUkishima soil at a ratio of 30:70. Compressive strength testswere carried out after curing for 3, 7 and 28 days.

2.1.2. Dissolution of Fe from Mixture of Steelmaking Slagand Dredged Soil

The dissolution of Fe ions from the mixtures of steelmak-ing slag and dredged soils from various locations (Asanocanal, Ukishima, Tama River) was measured to observe thedifferences among the dredged soils from different locationsby ICP measurement. In addition, the dissolution of Fe fromthe mixture of steelmaking slag and the dredged soil ofUkishima, which was planned to be used in the demonstra-tion experiment, was measured by atomic absorption spec-trophotometry.

The oxidation-reduction potential (ORP) and organic car-bon concentration in the dredged soil, which was extractedin 0.1 M NaOH water, were measured.

2.2. Demonstration Experiment2.2.1. Division of Experimental Areas

This experiment was performed in an inlet at EastOgishima Island at the Port of Kawasaki, Kawasaki City,Japan. Figure 2 shows the experiment site, which was

Fig. 1. Image of CO2 fixation processes by seaweed formation of seaweed beds.

Table 1. Characteristics of the dredged soil sampled in Ukishima.

Moisture ratio w % 111.8

Wet density ρt g/cm3 1.408

Dry density ρd g/cm3 0.67

Soil particle density ρs g/cm3 2.64

Liquid limit wL % 109.7

Plastic limit wP % 39

Plasticity index IP % 70.7

Liquidity index (w-wP)/(wL-wP) – 1.07

Proportion of particles under 75 mm % 91.6

Total organic carbon % 1.2

Ignition loss % 8.5

Table 2. Chemical composition of steelmaking slag.

(mass%)

SiO2 Al2O3 CaO MgO MnO P2O5 Al2O3 Total-Fe Metal-Fe FeO

10.7 3.70 38.1 6.69 2.43 2.24 3.70 22.6 1.94 9.71

f-CaO : 3.1%


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approximately 75 m east-to-west and approximately 110–150 m north-to-south. The water depth was 3–4 m. Thenorth side of the inlet is a comparatively quiet sea area withlittle navigation adjoining the Keihin Canal.

In the seaweed bed creation experiment, four test areas(Mounds A–D and two control areas (Mounds E–F) wereused. Mound A was assumed to be the standard mixture, andMound D was used to confirm changes in strength associ-ated with differences in the grain size of the steelmakingslag. Mound A was constructed with a trapezoidal shapewithout an embankment to confirm that the material used inthis experiment is capable of maintaining the shape of themound in seawater. In Mounds B–F, the mixed materialswere placed in container boxes. Several natural stones andartificial stones, which were manufactured by adding organ-ic acid to slag, were placed on each mound. The stones were300–400 mm in size and 10–20 kg in weight.

The locations of Mounds A–F and the arrangement of theseaweeds and seaweed bases are shown in Fig. 2, and thesize of each mound and placed material conditions areshown in Table 3.

2.2.2. Mound Construction and MonitoringHeavy metal leaching tests were performed with the steel-

making slag, mixture of steelmaking slag, and artificial

stones before use as materials for mound construction. Alldata were within the levels specified in the environmentalstandards of Japan’s Ministry of the Environment. (In thisproject, verification as an environmental technology wasconducted under the auspices of the Ministry of the Envi-ronment in order to receive a third-party evaluation).

The target strength of the mixture was set at 100 kN/m2.To obtain this strength level, the 0–13 mm size of the steelslag was selected and the mixing ratio was set at soil: steel-making slag=70:30. The dredged soil and the steelmakingslag used for these mounds were same materials as labora-tory mixing test, as shown in Tables 1 and 2. To make themixture, steelmaking slag was introduced into the dredgingsoil on a hopper-barge and was stirred for approximately 90min. Part of the mixture was sampled to monitor the hard-ness of the mound The mixture was cured on the barge for2 days and then placed in the intended sea area by a grabdredger, and was shaped by divers. Construction workstarted on July 31 and ended on August 3, 2009. No rise inpH was observed during this work. Soil-based cloudinesswas observed during placement of the material, but the tur-bidity of the seawater returned to its original level after 90minutes.

Water quality was monitored after the construction,including pH, ORP, DO, COD, T–N, T–P, PO4–P, sulfide

Fig. 2. Location of mounds and arrangement of test seaweeds.

Table 3. Test cases in demonstration tests in inlet of East Ogishima island in sea area of Kawasaki Port Tunnel.

Classification Mound Embankment Seaweed basis Mound size (m) Soil volume (m3) Notes

Study area(slag mixtlulre)

A Mixtlulre of dredged soil and slag(Type I)*

Artificial stoneNatural stone 11 × 5 × 1 32.5 Slop gradient 1:3

B Mixtlulre of dredged soil and slag(Type I) Natural stone 2 × 2 × 1 4 2 large-scale

containers

C Mixtlulre of dredged soil and slag(Type I) Artificial stone 2 × 2 × 1 4 2 large-scale

containers

D Mixtlulre of dredged soil and slag(Type II)* Artificial stone 2 × 2 × 1 4 2 large-scale

containers

Control are E Natural sand Artificial stone 2 × 2 × 1 4 2 large-scalecontainers

F Natural sand Natural stone 2 × 2 × 1 4 2 large-scalecontainers

*Type I: The volumetric mixing ratio is set at dredged soil and slag A=7:3 The grain size of steel slag is 0–13 mm*Type II: The volumetric mixing ratio is set at dredged soil and slag A=7:3 The grain size of steel slag is 0–30 mm

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ion, Fe, Mg, and hazardous substances. The Fe content wasmeasured by atomic absorption spectrophotometry after fil-tration using a 0.45 μm filter. The Fe dissolutions of thematerials sampled from Mound A and Mound B were alsomeasured by atomic absorption spectrophotometry.

2.2.3. Seaweed TransplantationSmall stocks of the soft seaweed called “wakame” in

Japanese (Undariapinnatifida), kelp (genus Laminaria), andbrown seaweed (Sargassumhorneri) were bonded to theartificial stones and natural stones (in the following, theseseaweeds are referred to as wakame, kelp, and brownseaweed, respectively). The stocks of brown seaweed weretransplanted on November 6, and the stock of wakame andkelp, on November 22. Wakame and brown seaweed weretransplanted again on December 25, as desirable growth wasnot obtained due to the high temperature of the water aftertransplantation.

The growth conditions of the seaweed and benthos wereinvestigated at set periods. The number of growing stocksand the length of the seaweeds were measured by a diver.At the end of the experiment the seaweeds were sampledand their wet and dry weights were measured.

3. Results

3.1. Laboratory Experiment3.1.1. Solidification by Mixing Dredged Soil and Steel-

making SlagThe results of the uniaxial compressive strength test are

shown in Fig. 3. Basically, compressive strength increaseswith the compounding ratio of steelmaking slag, however,the strength levels varied depending on the type of dredgedsoil used in the mixture. In the case of Kawasaki area soil,Ukishima-type soil is suitable for stable mound construc-tion. Figure 4 shows the relationship between the strengthof the mixed materials and the amount of free-CaO in theslag with a diameter of 0–5 mm. The strength of the mixedproducts depended on the particle size of the slag, as shownin Fig. 5. Mixed products using smaller sizes of slag havehigher strength.

These results suggested that the mound strength is con-trollable by changing both the chemical composition andsize of the steelmaking slag. For this mound construction,the target strength of the mixture at the seabed was set at100 kN/m2. In the case of cement-treated soil, the typicalstrength ratio between experiments and real construction isabout 0.5, referring to which, the mixing condition wasselected against an experimental value of 200 kN/m2. Steel-making slag with a diameter of 0–13 mm was selected, anda 70:30 mixing ratio of dredged soil and steelmaking slagwas applied.

3.1.2. Dissolution of Fe from Mixture oF SteelmakingSlag and Dredged Soil

Among the mixtures of steelmaking slag and dredgedsoils from various locations, dissolution of Fe was observedwith the mixture of steelmaking slag and dredged soil fromthe Asano Canal (Table 4). In measurements of the oxida-tion-reduction potential (ORP) of the dredged soil, thedredged soil from the Asano Canal showed a low level of

ORP. The concentrations of organic carbon of the dredgedsoil from the Asano Canal showed higher levels comparedto those from Ukishima and the Tama River (Table 5). Theamounts of Fe dissolved from the mixture of steelmakingslag and the dredged soil of Ukishima were <0.01 and 0.16(Table 6).

3.2. Demonstration Experiment3.2.1. Shape and Hardness of Mound A

Figure 6 shows the relationship between wet density andstrength of the mixture of steelmaking slag and dredged soil

Fig. 3. Relationship between actual slag volumetric ratio andstrength measured in laboratory strength test.

Fig. 4. Relationship between the strength of the mixed materialsand the amount of free-CaO in steelmaking slag with diam-eter of 0–5 mm.

Fig. 5. The strength of the mixed materials with various particlesize of steelmaking slag.


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sampled during construction of Mound A. The averagestrength of the mixture of Mound A was 109.7 kN/m2,which satisfied the target strength of 100 kN/m2. The shapeof the mound observed by a diver was also stable during theexperiment period.

3.2.2. Fe Dissolution from Mixture and Water QualityThe amount of Fe ions dissolved from the mixture at

Mound A and Mound B was 0.03 mg/kg-mixture in bothcases, as shown in Table 6.

The results of the measurements of water quality, nutrientsalts, and Fe content are presented in Figs. 7–10. Changesin the levels of pH, ORP, DO, COD, T–N, and T–P of theseawater above the mounds showed basically the same ten-dencies at all mounds. The Fe content of the water aboveMound A and Mound B was around 5 ppb higher than at theother mounds in November and December, and the Fe con-tent of the water above the study area(Mounds A–D) wasalso around 5 ppb higher than that above the control area(Mounds E–F) in February.

Table 4. Fe content dissolved from the mixture of dredged soil andsteelmaking slag.

Solid/Solutionratio

(g/mL)

Dredged soil Slag/dredged

soil

FepH ORP

(mV)Classification μg/L mg/kg-Slag

1/10

Asano canal 2.33 400 6 11.4 65

1.00 90 2 10.4 113

0.43 35 1 9.9 136

0.10 10 1 9.5 171

Ukishima 0.43 <10 <1 10.2 168

0.10 <10 <1 9.6 154

Tama River 0.43 <10 <1 10.2 137

0.10 <10 <1 9.7 135

1/3

Asano canal 0.03 1 150 115 9.1 –376

Ukishima 0.03 <10 <1 9.4 143

Tama River 0.03 <10 <1 9.4 136

Eluted time: 24 hr, 200 rpm shaking

Table 5. Organic carbon content in dredged soils.

SampleControlled area Dredged soil and bed material sample

Humus soil Asano canal Ukishima Tama River

TOC (mg/L) 2 900 650 97 280

Table 6. Fe content dissolved from mound A and B varied withtime.

Sample Mound Slag Grain size ofsteel slag

Fe [kmg/Kg-mixture]

Prepared inlaboratory

A 30 vol% –0.6 mm <0.01

B 30 vol% –0.6 mm 0.16

Immediately afterexecution

A 30 vol% 0–13 mm 0.03

B 30 vol% 0–13 mm 0.03

180 days afterexecution

A 30 vol% 0–13 mm <0.01

B 30 vol% 0–13 mm 0.03

Solid/Solution ratio(g-mixture/ml-artificial seawater): 1/10, Eluted time: 24hr, 200 rpm shaking

Fig. 6. Comparison of compressive strength of mixture made bylab-mixer and backhoe mixing.

Fig. 7. Monthly changes of (a) water acidity of seawater and (b)dissolved oxygen in seawater.

Fig. 8. Monthly changes of (a) Chemical Oxygen Demand and (b)Oxidation-Reduction Potential of seawater.

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There was no substantial difference of Fe content of thewater above between Mound E and Mound F, although arti-ficial stones were placed on Mound E and natural stoneswere placed on Mound F.

A leaching test was also performed with the mixture actu-ally used in the seawater over a period of a few months afterthe construction of Mound A. All data were within the levelsset in Japanese environmental standards (Table 7).

3.2.3. Seaweed GrowthAlthough a simple comparison is not possible because of

individual variability, the average dry weight of the wakameseaweed taken from each study area was 1.1 times higherthan that of the same seaweed taken from the control areaas shown in Fig. 11.

Table 7. Contents of various hazardous heavy metals dissolvedfrom mound A after 180 days (mass mg/L).

(mg/l)

Element Mound A Bottom sedimentstandards

Hg or Hg compound <0.0005 0.005

Cd or Cd compound <0.01 0.1

Pb or Pb compound <0.01 0.1

Cr(VI) or Cr(VI) compound <0.05 0.5

Cu or Cu compound <0.1 3

Zn or Zn compound <0.1 2

Be or Be compound <0.1 2.5

Cr or Cr compound <0.1 2

Ni or Ni compound <0.1 1.2

V or V compound <0.1 1.5

Fig. 9. Contents of total nitrogen (a) and total phosphorus (b) inseawater.

Fig. 10. Content of Fe ion (a) and magnesium ion (b) in seawater.

Fig. 11. Change of length (a) and dry weight (b) of seaweeds (wakame) in March with various mounds (L: distancebetween mound surface and fixing point of seaweed).


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The average dry weight of the brown seaweed taken fromeach study area was 2.1 time as much as that of brown sea-weed from the control area, as shown in Fig. 12, althoughagain, a simple comparison is not possible. The data for theprominent stock were not included when calculating theaverage dry weight of the brown seaweed from each studyarea. If these data are included, the average weight of thebrown seaweed from each study area is 5.4 times larger thanthat from the control area. Furthermore, the amount ofmetals contained in the brown seaweed in the study area wasalmost the same as that in the control area, with the excep-tion of iron. The amount of Fe in the brown seaweed in the

study area was approximately double of that in the brownseaweed in the control area, as listed in Table 8. Growth ofkelp could not be confirmed in either the test areas or thecontrol areas. It is assumed that the condition of the seawa-ter, and especially the water temperature, was not suitablefor kelp growth.

3.2.4. Condition of PeriphytonSpecies such as plume worms (serpulids), oysters, and sea

cucumbers, which are frequently found in Tokyo Bay, werealso observed in this experiment. Both the number of spe-cies and the number of individual periphyton showed a ten-dency to be larger in the study area than in the control area,confirming that the mixture used in the mounds did not pre-vent creatures from entering the study area, as seen in Table9.

4. Discussion

4.1. Strength and Mechanism of Solidification of MixedSteelmaking Slag and Dredged Soil

There are several possibilities with regard to solidifica-tion of the mixture, such as lowering of the relative watercontent, agglomeration of soil particles, and pozzolanicreaction. The results of the experiments shown in Figs. 4and 5 suggest that the free-CaO ratio and surface area of thesteelmaking slag strongly affect solidification of mixturescontaining slag. The solidification of the soil and steelmak-ing slag mixture suppose to occur mainly through a hydra-tion reaction between Ca ions in the steelmaking slag andsilicates in the dredged soil and produced pozzolan (C–S–H).7) Our results indicate that pozzolanic reaction increasesthe strength of the mixture.

Table 8. Contents of metals in seaweed (wakame) and brown seaweed.

ElementSeaweed (wakame) Brown seaweed

Mound A B D E A B D E F

Mg (mg/kg) 756 515 361 475 5 370 4 670 6 350 5 170 5 630

P (mg/kg) 553 770 673 552 218 220 221 241 224

Ca (mg/kg) 321 289 215 335 1 144 1 177 1 097 997 1 069

Fe (mg/kg) 9 12 8 6 190 274 113 104 97

Table 9. Monthly change of presence of organisms on various mounds.

Mound A A A B C D E F

Naturalstone

Artificialstone

Side surfaces,foot of structure

Artificialstone

Artificialstone

Artificialstone

Artificialstone

Naturalstone

Number ofanimal species

September 10 12 3 8 10 9 7 9

November 14 10 3 10 10 11 8 6

January 14 14 12 6 7 7 6 9

February 16 15 12 14 14 11 9 9

March 17 16 13 13 14 11 9 11

Number ofplant species

September 1 1 0 1 1 1 2 2

November 1 1 0 1 1 1 1 1

January 1 1 0 1 1 1 1 1

February 1 1 1 1 1 1 1 1

March 2 2 1 2 2 2 1 1

Fig. 12. Change of length (a) and dry weight (b) of brown sea-weeds (Sargassumhorneri) in March with variousmounds.

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The strength of the mixture of steelmaking slag anddredged soil sampled at mound A construction was 109.7kN/m2, which satisfied the target strength of 100 kN/m2. Asthe strength of a mixed product prepared in the laboratoryusing the same materials as in mound construction was198.9 kN/m2, the strength ratio of the field and laboratoryproducts was 0.55. The coefficient of variation of strengthwas 24%. This value is relatively large, which is attributedto heterogeneity due to the mixing method and the low flu-idity of the soil compared with prior sampled soil. However,

35% is used as the coefficient of variation and 0.5 as thefield/laboratory strength ratio. Thus, the design characteris-tics of the mixture of steelmaking slag and dredged soil aresimilar to those of cement-soil mixtures.

4.2. Fe Dissolution from Mixture of Steelmaking Slagand Dredged Soil

Among the mixtures of steelmaking slag and dredgedsoils from various locations, the dissolution of Fe wasobserved with the mixture of steelmaking slag and dredged

(a) Immediately after transplant action

(b) After 120 daysPhoto 1 Growth of seaweeds (wakame).

(a) Immediately after transplant action

(b) After 120 days

Photo 2 Growth of brown seaweeds.


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soil from the Asano Canal (see Table 4). The reasons whydissolution of Fe from the mixture using dredged soil fromthe Asano Canal was significantly higher than that of mix-tures using dredged soil from Ukishima and the Tama Riveris presumed to be due to the low oxidation-reduction poten-tial (ORP) of the dredged soil from the Asano Canal accel-erates Fe dissolution. It is also supposed that the abundanceof organic acids in the dredged soil from the Asano Canalincreases the dissolution of Fe ions. The remarkably highorganic carbon concentration of the dredged soil from theAsano Canal implies an abundance of organic acids in thissoil (see Table 5).9)

The dissolution mechanism of Fe from steelmaking slagis thought as follows. A high content of organic carbon inthe dredged soil creates a reducing environment, whichaccelerates the dissolution of Fe from the steelmaking slag.Furthermore, the organic acids such as fulvic acid andhumic acid in the dredged soil readily form complexes of Feions, which remain soluble even at pH 8.2.

In normal seawater which contains oxygen, Fe(III) ismore stable than Fe(II).

The solubility of Fe(III) is extremely low, resulting in avery small amount of 10 nML–1=0.6 ppb in water of pH8.2.10) On the other hand in the experiment area in Kawasakiharbor, the measured results were 5–20 ppb in the controlledarea and 5–28 ppb in the study area. The reason for the highdensity of dissolved Fe ions in the seawater is presumed tobe because Fe ions form a complex compound that remainsdissolved in seawater.

Under the reducing environment caused by photochemis-try and presence of enzymes derided from creatures, Fe(III)changes to Fe(II), which readily dissolves in seawater.Fe(II) binds to chelate materials in the periphery, whichremain dissolved in seawater.6) According to the experi-ments in which slag and humic substances from wood wastecompost were mixed in water, chelate of Fe(II) originatesfrom the mixture.11,12) We think that a similar reactionoccurs between the Fe(II) which is eluted from slag andhumic substances originating from dredged soil. And com-plexes of Fe ions, which form from the mixture of slag anddredged soil (see Table 6), might cause the higher concen-tration of Fe observed in the study area than that in the con-trolled area, as seen in Fig. 10.

It is estimated that the Fe ion supply process involving themixture of slag and a dredged soil is similar to the supplymechanism of Fe ions in natural environments. Therefore,the supply method of the Fe ions from the mixture of slagand dredged soil is presumed to have a lower impact on thenatural ecosystem.

Concerning the artificial stones, no substantial differencebetween Fe content of the seawater above Mound E andMound F was observed. It was suggested that Fe ion sup-plied from the only several artificial stones did not influencethe Fe content of the seawater above the mounds signi-ficantly.

4.3. Effect of Fe Ions in Promoting Algae GrowthThe average dry weight of the seaweed (wakame) taken

from each study area was 1.1 times as much as that from thecontrol area, and 2.1 times in case of the brown seaweed, asmentioned in Section 3.2.3. At the time of transplantation,

the average lengths of wakame and brown seaweed wereabout 10–15 cm. Three months after transplantation thelargest size was 200 cm for the wakame and 600 cm for thebrown seaweed. Except Fe, the properties of the seawater(pH, COD, DO, nutrient salts, dissolved metals) were sub-stantially the same between the study areas and controlareas. Remarkably there was no difference in the concentra-tion of nutrients (nitrogen, phosphorus, etc.) between them.Only the levels of dissolved Fe ions in the study area werehigher than those in the control area (see Fig. 6). Thisimplies that the Fe concentration in the seawater affected thegrowth of the seaweeds. Furthermore, the amount of metalscontained in the brown seaweed in the study area wasalmost the same as that in the control area, with the excep-tion of iron. The amount of Fe in the brown seaweed in thestudy area was approximately twice as much as that in thebrown seaweed in the control area (see Table 8). This sug-gests that the difference in the amount of absorbed Fe wasa factor in accelerating growth of the brown seaweed. Thesetwo results indicate that Fe ions dissolved from the mixtureof the steelmaking slag and the dredged soil had a positiveeffect on the growth of the brown seaweed.

For algae, trace minerals such as Fe ions are indispens-able. It has been reported that decreases in chlorophyll αand the phycobiliproteins are associated with Fe shortage,and it is known that Fe deficiency causes discoloration ofsea laver (nori).13,14) A decrease in chlorophyll reduces thephotosynthesis capacity. In Japan’s Inland Sea, there is anarea of extremely low Fe where the solubility of iron is <1ppb. When discoloration occurs due to iron deficiency, Feions supplied from the seaweed beds of the mixture of steel-making slag and dredged soil may contribute to recoveryand prevention of the discoloration of sea lavers.15)

Concerning the effect of Fe ions on kelp growth, it hasbeen reported that Fe ions play important roles in everystage of growth, including gametophyte maturity, fertiliza-tion, and sporophore development.8) Supplying Fe ions tocoastal waters with low Fe concentrations is reported to bean effective means of reversing ecosystem damage andimproving conditions in coastal areas.2,3)

The mechanism through which seaweed absorbs Fe ionsis still unclear. The uptake mechanism of iron by plants iscommonly classified into two strategies.16) In the first,Fe(III) is changed to Fe(II) by inducible reductases of aplasma membrane and reduced Fe(I) is carried into the cyto-plasm by a transporter. In the second, inducible ligands,which form highly stable complexes with Fe(III), are trans-ferred into the cytoplasm by specific translocator. If theinducible protein with sea laver under iron deficiency can beidentified, it will be possible to determine the uptake mech-anism in this case.

Various findings regarding the dredged soil and steelmak-ing slag were obtained. However, the mechanism respon-sible for the dissolution of Fe ions has not yet been clarified.Influence of dredged soil on Fe dissolution from steelmak-ing slag has neither yet been clarified. How seaweeds utilizeFe ions also remains to be explained. Further research onthese issues is expected.

© 2011 ISIJ 1928


5. Summary

A new technique for forming seaweed beds and shoalsusing a mixture of steelmaking slag and dredged soil hasbeen developed. The effect of a mixture of steelmaking slagand dredged soil on the growth of seaweeds in a marineenvironment was demonstrated in experiments in a marinearea of Kawasaki City, Japan. The growth of seaweedsincreases CO2 fixation. In addition to providing a new CO2

fixation technology, this technique is also environment-friendly from the viewpoint that recycled materials (slag,dredged soil) are used. The following results were obtained:

(1) The average strength of the mound for seaweedbeds, which was constructed using a mixture of dredged soiland steelmaking slag, was 109.7 kN/m2. The shape of themound was stable during the experiment period.

(2) It is considered that organic carbon in the dredgedsoil reduces ORP, and the resulting reducing environmentaccelerates the dissolution of Fe ions from the steelmakingslag, while organic acids in the dredged soil form complexesof Fe ions, even at pH 8.2.

(3) The increase of the soft seaweed and the brown sea-weed compared with the control area implies that Fe ions,which dissolved from the steelmaking slag, had a positiveeffect on the growth of seaweed.

AcknowledgementsMany people and organizations made important contribu-

tions in carrying out these marine experiments. The authorswish to express their great gratitude to Kawasaki City,Kawasaki Port Authority, which provided the experimental

area, the Ministry of Economy, Trade and Industry, whichsupported the experiments financially, and the Ministry ofthe Environment and POs, which gave advice in connectionwith the experiments.

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