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Article

Dissolved Iron from Steel Slag with Its Chelating Agent Promotes Seaweed Growth

by
Paul Kwangho Kwon
,
Hyung-Suek Kim
and
Sung Woo Jeong
*
Environment & Energy Research Division, Research Institute of Industrial Science and Technology, Pohang 37673, Korea
*
Author to whom correspondence should be addressed.
Sustainability 2022, 14(9), 5498; https://doi.org/10.3390/su14095498
Submission received: 7 March 2022 / Revised: 20 April 2022 / Accepted: 29 April 2022 / Published: 3 May 2022
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Abstract

:
Blue carbon ecosystems are crucial for carbon sequestration on a global scale. However, it is unclear how we could promote and maximize carbon sequestration. Here, we demonstrate that providing an iron source to seaweed fostered its growth through increased photosynthetic efficiency and transformed the carbon into a biomass. Firstly, we revealed that the mixture of the steel slag and DTPA eluted iron dramatically in seawater. Next, we applied the eluate of the slag-DTPA pellet to the seaweed. The results for the eluate treatment showed a 25.8% increase in the photosynthetic pigment level and a 44.9% increase in the seaweed weight. Furthermore, we confirmed no elution of potential toxic substances from the steel slag and DTPA pellet. Finally, we applied the pellet at a depth of 15 m near seaweeds and observed a 52.0% increase of carbon weight in the pellet treated group, while the non-treated group showed only a 10.3% increase for five months. This study indicated that steel slag-DTPA pellet treatment induced seaweed growth and efficiently transformed its carbon into a seaweed biomass. Thus, steel slag and its chelating agent may contribute to the promotion of sea forestation and a subsequent increase in carbon sequestration known as blue carbon.

1. Introduction

The carbon sequestered in coastal and marine ecosystems is known as blue carbon. Among marine ecosystems, seaweeds possess a highly efficient mechanism of sequestering significant amounts of carbon along with seagrasses as carbon sources [1,2,3]. Moreover, seaweeds have been reported to serve a critical role in sea forestation’s carbon sequestration thanks to the installation of artificial reefs [4,5]. However, marine deforestation has been observed in the world’s large coastal areas [6,7]. Specifically, it has been reported that ocean desertification is underway and possibly even intensified in 18,759 ha out of 40,868 ha surveyed along the coastal area of Korea [8]. The causes of this deforestation are related to complex factors, such as rising sea temperatures, decrease in salinity, and decline in nutrients required for proper seaweed growth and development [9,10].
Generally, a seaweed absorbs the nutrients via their blades and uses it for photosynthesis and metabolism. Among the nutrients, iron is essential for chlorophyll synthesis in phototrophs including seaweeds [11,12,13]. However, it has been reported that dissolved iron concentrations were extremely low or under the limit of quantification in the specific areas where sea desertification has been observed [9,13,14]. In the case of the macroalgae Gracilaria tenuistipitata, chlorosis and inhibited growth have been observed in iron-deficient conditions [11]. Furthermore, iron availability limits populations of phytoplankton, specifically in shallow marine systems [15].
Steel slag is a by-product of iron production and has been applied in constructing artificial reefs to enhance sea forestation because of its high specific gravity and excellent stability against wave forces [5,16,17,18]. Steel slag also contains solubilized iron that is easily absorbed by seaweeds, but it is immediately oxidized by dissolved oxygen in seawater due to its pH and precipitated in the form of colloidal hydroxide to become insolubilized [13]. In order to solve this issue, a few trials were conducted by exposing steelmaking slag to chelating agents, such as EDTA, humic acid or fulvic acid, as a way to prevent iron oxidation [9,14].
In the agricultural field, it has been reported that the iron chelating ability varies depending on the soil pH [19]. In particular, it has been confirmed that DTPA effectively increases the solubility of iron compared to EDTA in alkaline soil. On the other hand, there is no comparable information on the soluble iron availability for the growth of seaweeds by the addition of chelating agents such as EDTA and DTPA in seawater conditions. Moreover, the correlation data between the growth effect of seaweeds and their carbon weight may contribute to the carbon sequestration quantification and further blue carbon strategy. DTPA is also one of the well-known iron chelating agents, which is inexpensive and easily obtained on the market. Thus, DTPA could hold the key to advancing the commercialization of seaweed farming.
In this study, an effective soluble iron supply method was proposed under seawater conditions. Restoration of the sea forest was also demonstrated by promoting the growth and the photosynthetic capacity of seaweed, which would lower the cost of carbon sequestration and boost the income from seaweed farming, if the product of this study was applied.

2. Materials and Methods

2.1. Seaweed Collection and Analysis

Gametophytes of Saccharina japonica (S. japonica) were obtained from the National Fisheries Research and Development Institution [20] (NFRDI) (Haenam-gun, Korea) and sporophytes of S. japonica were purchased from Woori-poja (Wando-gun, Korea) in February 2021. Ecklonia cava (E. cava) was obtained from the selected site (37°28.186′ N, 130°49.650′ E) near Ulleung Island in Korea. Gametophytes and sporophytes of S. japonica were maintained in 500 mL flasks (Pyrex) under light irradiance of 40 μmol m−2 s−1 and 12:12-h light/dark photoperiod at 15 °C. The eluate or seawater was exchanged every 3 days. Incubators (Daewon, Bucheon, Korea) and air pumps (Dongyang Co., Busan, Korea) were also used to culture the seaweed. Provasoli-enriched seawater (PES) without the iron sources was also used [21]. To measure the weight of S. japonica and E. cava, the samples were washed with water before incubating the samples in a drying oven (N-BIOTEK, Bucheon, Korea) at 60 °C for 72 h. Next, measurement was conducted by using a weighing scale (DAIHAN Scientific, Wonju, Korea) and total carbon of the E. cava was measured by using an elemental analyzer (LECO, St. Joseph, MI, USA).

2.2. Element Elution and Analysis

Seawater was obtained from Mopo-ri, which is near Pohang-si. Firstly, we filtered the seawater with a membrane filter (ADVANTEC, Tokyo, Japan) and sterilized it in an autoclave (CRYSTE, Gwangmyeong, Korea). A tester (DAIHAN Scientific) was used to mix 3 g of steel slag (POSCO, Pohang, Korea), EDTA (Sigma, St. Louis, MO, USA) and DTPA (Sigma) in 30 mL of seawater at 60 rpm for 48 h. In the case of the mixture of the 3 g of the slag and 0.03/0.3/3 g of DTPA in 30 mL of seawater, the treatment of the mixture was conducted for 48 h to elute the iron with the jar tester. Ten grams of the pelletized slag-DTPA in 1 L of seawater (1% of seawater weight) was also treated for 48 h with the jar test. After the treatment, filtration was performed by using a membrane filter (Sartorius, Göttingen, Germany) to analyze the elements. Inductively Coupled Plasma Mass Spectrometer (ICP-MS) (iCAP TQ™, Thermo Scientific, Waltham, MA, USA) was performed to measure the concentration of iron. The instrument condition was described as follows: RF plasma power: 1500 W, Argon gas flow rate of coolant flow: 13 L/min, Argon gas flow rate of auxiliary flow: 0.9 L/min, Argon gas flow rate of nebulizer flow: 1.1 L/min, Sampler cone: Ni, Skimmer cone: Ni. Furthermore, Al, Cr, Ni, Zn, As, Cd, Hg, and Pb concentrations were also measured after the treatment of the pelletized slag-DTPA for 30 days by using the ICP-MS. We used CRM-TMDW (trace metals in drinking water, High-Purity Standards, Charleston, SC, USA). The recoveries of CRM-TMDW are also described as follows: (Fe: 103%, Al: 102%, Cr: 99.6%, Mn: 96.5%, Ni: 94.6%, Zn: 102%, As: 92.5%, Cd: 96.9%, Pb: 92.7%). The limits of detection (LOD) were as follows: (Fe: 0.01, Al: 0.05, Cr: 0.01, Ni: 0.01, Zn: 0.01, As: 0.05, Cd: 0.01, Hg: 0.01, Pb: 0.01, unit: ppb). The limits of quantification (LOQ) were as follows (Fe: 0.05, Al: 0.26, Cr: 0.04, Ni: 0.05, Zn: 0.06, As: 0.24, Cd: 0.05, Hg: 0.05, Pb: 0.07, unit: ppb). The mode of each heavy metal was provided as follows (Fe, Al, Cr, Mn, Zn, As were formed by KED mode. Cd, Hg, Pb were formed by STD mode). R2 values for all elements of the calibration curve exceeded 0.999.

2.3. Analysis of the Photosynthetic Pigment

Each sample of 100 mg of gametophytes [20] and 300 mg of sporophytes of S. japonica were collected into each 50 mL tube before centrifugation at 4000 rpm for 5 min. Next, the seaweed was treated with 80% acetone and the flask covered with aluminum foil overnight to extract the photosynthetic pigment [22,23]. On the following day, centrifugation at 12,000 rpm for 5 min was conducted and the supernatant was measured at 440 nm by using a microplate reader (Thermo Scientific). Four replicates were performed.

2.4. Pelletization of the Slag and DTPA

Pelletized slag and DTPA were prepared using steel slag (POSCO), DTPA (Shijiazhuang Jackchem Co., Shijiazhuang, China) and molasses (Evermaricle, Jeonju, Korea) as a binder. Firstly, the steel slag was dried at 105 °C for 24 h before crushing it evenly to make the 500 µm radius powder. Finally, the ratio of steel slag, DTPA and molasses (79:13.5:7.5) was mixed and the particles were pelletized. The ratio was determined by the solidity of the pellet.

2.5. Growth Measurement in the Sea

On the selected site (37°28.186′ N, 130°49.650′ E) near Ulleung Island in Korea, the 400 kg of the pelletized slag-DTPA was applied on the 3 sites and 12 plots at a depth of 15 m in January 2021. E. cava was labeled with a designated number (Originally, we labeled 40 tags, however, 24 tags were lost due to ocean current) and the thallus heights of the labeled E. cava were measured by using a ruler at a depth of 15 m in January 2021 and in June 2021. The biometric parameters of E. cava were previously mentioned [24].

2.6. Statistical Analysis

The quantitative data are shown as means ± SEM, which were statistically analyzed by unpaired Student’s t tests in the case of the two groups. One-way or two-way analysis of variance (ANOVA) was conducted by using PRISM software (GraphPad Software), which was previously described [25,26,27,28]. A result of p < 0.05 was considered to represent significance. p-values greater than 0.05 were considered not significant.

3. Results

3.1. Fe-DTPA Elutes the Soluble Iron in the Range of Oceanic pH

Steel slag is a by-product of the steel making process, which contains about 35.7 wt% of iron complex (Figure 1a). Here, we measured the soluble iron elution levels among steel slag, slag plus EDTA, and slag plus DTPA mixture (Figure 1b). We used seawater as a control group for the iron elution test. Seawater, slag, and slag plus EDTA groups eluted a tiny amount of iron, while the slag plus DTPA group eluted a much higher iron concentration than other groups.
Fe3+ is able to be chelated with oxygen atoms of DTPA. Thus, we tested the iron elution for the steel slag and DTPA combination. The mixture of a minor portion of DTPA with steel slag did not show any differences in soluble iron concentration. Strikingly, when the ratio of DTPA weight was 100% of total the input slag weight, it was highly effective in eluting soluble iron (Figure 1c). The data indicated that the mixture of the same weight portion of DTPA with steel slag eluted soluble iron effectively.
Sustainability 14 05498 g001 550
Figure 1. Fe-DTPA elutes the soluble iron in the range of oceanic pH. (a) The composition for steel slag was analyzed through X-ray Fluorescence (XRF) (RIGAKU, Primus II), which showed that steel slag contains an iron complex. The total weight percent of the iron complex was about 35.7% of the total steel slag weight; (b) An iron elution test for slag, slag + EDTA, and slag + DTPA was performed. Iron concentrations were measured by using ICP-MS. The results show that only the Fe + DTPA mixture dramatically eluted a soluble iron. Seawater was used as a negative control. (ns, not significant, * p < 0.05; by ordinary one-way ANOVA followed by Dunnett’s multiple comparisons test; error bars indicate SEMs); (c) An assessment of the iron elution capacity for the combination of steel slag and DTPA was conducted. Seawater was used as a negative control group. Soluble iron concentrations were measured through ICP-MS. (DTPA) −: negative control, +: 1% of slag weight, ++: 10% of slag weight, +++: 100% of slag weight (ns, not significant, * p < 0.05; by ordinary one-way ANOVA followed by Dunnett’s multiple comparisons test; error bars indicate SEMs).
Figure 1. Fe-DTPA elutes the soluble iron in the range of oceanic pH. (a) The composition for steel slag was analyzed through X-ray Fluorescence (XRF) (RIGAKU, Primus II), which showed that steel slag contains an iron complex. The total weight percent of the iron complex was about 35.7% of the total steel slag weight; (b) An iron elution test for slag, slag + EDTA, and slag + DTPA was performed. Iron concentrations were measured by using ICP-MS. The results show that only the Fe + DTPA mixture dramatically eluted a soluble iron. Seawater was used as a negative control. (ns, not significant, * p < 0.05; by ordinary one-way ANOVA followed by Dunnett’s multiple comparisons test; error bars indicate SEMs); (c) An assessment of the iron elution capacity for the combination of steel slag and DTPA was conducted. Seawater was used as a negative control group. Soluble iron concentrations were measured through ICP-MS. (DTPA) −: negative control, +: 1% of slag weight, ++: 10% of slag weight, +++: 100% of slag weight (ns, not significant, * p < 0.05; by ordinary one-way ANOVA followed by Dunnett’s multiple comparisons test; error bars indicate SEMs).
Sustainability 14 05498 g001

3.2. Slag-DTPA Promotes Growth of Seaweed

Firstly, we demonstrated the effect of Fe-DTPA (Sigma). We used the gametophyte which is the microscopic phase or the sporophyte which is the dominant phase of S. japonica. Next, we used the gametophytes of S. japonica as a model for measuring the photosynthetic pigment. The absorbance spectrum of the brown seaweed was measured after its extraction with acetone. The results showed that the F-D (Fe-DTPA) treated group increased the absorbance level to around 440 nm, which showed the photosynthetic pigment increase of the gametophytes (Figure 2a). Next, we confirmed statistical significance of the photosynthetic pigments at the wavelength of 440 nm. The data showed a 25.8% increase for the F-D treated group compared to the non-treated group (PES media only) (Figure 2b). Next, we pelletized the steel slag and DTPA to control the iron release effectively (Figure 2c). The iron elution test for the pellet was executed. The iron elution test for the pelletized slag and DTPA showed a dramatic increase in iron concentration, while seawater showed only an insignificant concentration of iron. The result showed that the pellet eluted a dramatic iron concentration (Figure 2d). We monitored the eluent elements for potential harmful effects on the seaweed and humans for 30 days (Figure 2e). The data showed that only the iron concentration was increased more than twice after 30 days. We also observed that other potential harmful elements were not highly increased due to the slag-DTPA pellet treatment. Next, we elucidated the biological effect of the S. japonica sporophytes. Firstly, we measured the weight of the sporophytes after treatment with the pellet eluate. The weight levels of sporophytes were especially significantly increased when 1% pellet eluate was applied, while applications of 0.01% and 0.1% pellet eluate treated groups resulted in only slight increases. The data showed that the weight level of the pelletized slag-DTPA eluate treated group was increased by 44.9% in comparison to the seawater group (Figure 2f). Moreover, we measured the absorbance of the non-treated (seawater only) group and the 1% pellet eluate treated group and found that the photosynthetic pigment level of the pellet eluate treated group was higher than the non-treated group (Figure 2g). These data suggested that the pellet elute treated group supported the photosynthetic pigments increase and the growth of S. japonica.

3.3. Pelletized Slag-DTPA Promotes a Sea Forest and Enhances Carbon Sequestration

Here, we strived to determine the blue carbon effect in the marine forest by using the slag-DTPA pellet. At first, we applied the pellets into the ocean forest near Ulleung Island in Korea (Figure 3a). We measured the thallus height of E. cava [24] in January 2021 and again in June 2021. Next, we compared the thallus heights of E. cava between the non-treated site and the pellet treated site. The results showed that the pellet treated site showed a significant thallus height increase of E. cava in comparison to those in the non-treated site (Figure 3b). These data suggested that the pellet supported the thallus height growth of E. cava. We then derived the relationship between E. cava thallus height and its total carbon weight. To elicit the relationship, we measured both of the thallus heights of E. cava and their total carbon weights step by step. The result showed that the total carbon weights of E. cava were proportional to their thallus heights (Figure 3c). Based on the linear relationship between the thallus heights and the total carbon weights, we could estimate the transformational level which converted the carbon into the biomass. Our data revealed a 52.0% carbon weight increase in the pellet treated group, while the non-treated group had only a 10.3% increase (Figure 3d). These data indicated that applications of pelletized slag-DTPA could foster carbon sequestration and maximize the blue carbon effect.

4. Discussion

Although there is an urgency for reducing carbon dioxide levels on Earth, a proactive trial to sequester carbon by growing the seaweed in the ocean was rarely found. Here, we demonstrated that a seaweed growth-promoting material, which also enhanced the photosynthetic pigment level, significantly increased carbon sequestration.
In fact, iron could be insolubilized by chemical properties such as pH, and in particular, in alkaline conditions, which causes a deficiency problem such as the iron not being absorbed within phototrophic organisms [19,29,30]. Generally, the pH of seawater is affected by precipitation and water temperature and has a weak alkalinity of around 8.1 [31]. Under ocean alkalinity, even if adequate iron is supplied to the seaweed, it becomes insolubilized and may not be used for growth and development of seaweed [9,13,14].
Interestingly, it has been suggested that tomato plants grown at alkaline pH can absorb more soluble iron from Fe-DTPA or Fe-EDDHA than from Fe-EDTA, and the use of Fe-DTPA is preferable due to the high price of Fe-EDDHA [19]. Furthermore, there were some trials where steel slag was combined with chelate elements like EDTA, humic acid, or fulvic acid in steelmaking slag to prevent iron oxidation [9,14]. However, dramatic iron elution has not been observed previously. Thus, we hypothesized that it would be important to supply iron in a stable form at the ambient pH level in seawater. Here, the feasibility of recovering seaweed was confirmed by stimulating the growth and photosynthetic pigments of seaweed, as well as proving the efficacy of iron delivery in real sea conditions.
Firstly, we revealed that the DTPA was a suitable chelating agent for iron elution in seawater (Figure 1b). In the soil, DTPA showed an effective iron availability that exceeded EDTA at a pH of 8.0, as iron insolubilization occurred at a pH of 7.0 [19]. Here, we suggest that the application of the chelating agent would be desirable in consideration of pH to solve an iron deficiency problem in the soil and ocean.
Steel slag, which was obtained from iron production, contains a significant amount of iron (Figure 1a). Thus, steel slag has also been used to produce marine structures such as artificial reefs for sea forests due to its high specific gravity and outstanding resistance to erosion from wave forces [5,16,17,18]. Additionally, the price of steel slag is inexpensive and supplied continually by steel producing companies.
Photosynthesis is the crucial process in the growth and development of seaweed. Furthermore, iron is known as an important factor for photosynthesis by supporting the biosynthesis of photosynthetic pigments [11,12,13]. Here, we showed that steel slag eluted iron effectively and significantly increased the photosynthetic pigments of S. japonica when mixed with DTPA (Figure 1c and Figure 2b). Pelletized slag-DTPA promoted iron elution and photosynthesis without any other toxic heavy metal elution (Figure 2c,e). Steel slag has been used as an additive for soil improvement, and it has been reported that it is safe below the non-hazardous limit in the evaluation of the harmfulness of heavy metals through the elution test [32]. As such, steel slag is a highly desirable source of iron for sea forestation as well as a soil improvement agent.
Finally, we applied the pelletized slag-DTPA onto the artificial reef to confirm the seaweed growth (Figure 3a). The result showed that the thallus height of E. cava and their total carbon weights were meaningfully increased for 5 months when we treated the seaweed growth promoting pellet (Figure 3d). In particular, the heights of E. cava and the total carbon weight showed a linear correlation, which suggests that an increase in the seaweed height could promote additional carbon sequestration.
Though we need further investigation about more numbers of seaweed growth measurements in the sea and various species of seaweed in addition to E. cava and S. japonica, this study proposed the possibility of kelp forest restoration and carbon dioxide reduction through the slag and DTPA treatment. Further study of the seaweed metabolism and photosynthetic gene expression with the use of slag and DTPA is also required. Here, an effective supply method of soluble iron was suggested under ambient seawater pH conditions, and the possibility of restoring seaweed was confirmed by promoting the growth and photosynthetic pigments of seaweed by using steel slag and DTPA, and verifying the efficacy of iron supply in real sea conditions.
Currently, blue carbon is confined to coastal vegetation ecosystems including the tidal marshes, mangrove forests and seagrass beds due to their high carbon burial and accumulation in their sediments. However, seaweeds were not yet considered a blue carbon despite the substantial allochthonous donor of their biomass estimated by 153 Tg C/year−1 in the deep sea (up to 4000 m depth) [1,3,33,34,35]. Though there has been a limitation for the consideration of seaweeds as a blue carbon until now, accumulating evidence indicates that seaweeds are major components of organic carbon standing stocks [1,3,34]. Furthermore, restoration of sea forest ecosystems is considered a way to promote the mitigation of CO2 emissions [33,36]. Thus, this study suggests that marine restoration and forestation by promoting seaweed growth would result in a significant increase in carbon sequestration as blue carbon.

Author Contributions

Conceptualization, P.K.K., H.-S.K. and S.W.J.; methodology, P.K.K., H.-S.K. and S.W.J.; investigation, P.K.K. and S.W.J.; writing—original draft preparation, P.K.K., H.-S.K. and S.W.J.; writing—review and editing, S.W.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by POSCO (2019A075).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data that support the findings of this study are available from the first (P.K.K) and corresponding (S.W.J.) authors, upon reasonable request.

Acknowledgments

We thank the National Fisheries Research and Development Institution (NFRDI) (Haenam-gun, Korea) for providing Gametophytes of S. japonica.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 2. Slag-DTPA promotes seaweed growth. (a) Gametophytes of S. japonica were cultured in the Provasoli-enriched seawater (PES) media. The contents of photosynthetic pigments were measured by using a spectrophotometer. The non-treated group and F-D (Fe-DTPA) treated group absorbance levels were shown and compared. PES media was used as a negative control; (b) The absorbance levels of the non-treated group and the F-D (Fe-DTPA) treated group were measured and compared. (n = 4, * p < 0.05; unpaired Student’s t test; error bars indicate SEMs); (c) A picture of pelletized steel slag and DTPA is shown above; (d) The measurement was conducted with an ICP-MS (n = 3, * p < 0.05; unpaired Student’s t test; error bars indicate SEMs); (e) Element analyses for any potential toxic effects were performed for 30 days. The slag-DTPA pellets were applied in seawater for 30 days. The seawater was fully exchanged daily considering the case of the East Sea. The measurement was conducted by using ICP-MS; (f) The sporophytes of S. japonica were cultured and weights measured. The pellet was eluted in seawater for 48 h. −: negative control, + (Pelletized slag-DTPA): 0.01% of seawater weight, ++: 0.1% of seawater weight, +++: 1% of seawater weight (n = 6, ns, not significant, * p < 0.05; by ordinary one-way ANOVA followed by Dunnett’s multiple comparisons test; error bars indicate SEMs); (g) The photosynthetic absorption spectrum of the pelletized slag-DTPA eluate treated group and the non-treated (seawater only) group are shown above.
Figure 2. Slag-DTPA promotes seaweed growth. (a) Gametophytes of S. japonica were cultured in the Provasoli-enriched seawater (PES) media. The contents of photosynthetic pigments were measured by using a spectrophotometer. The non-treated group and F-D (Fe-DTPA) treated group absorbance levels were shown and compared. PES media was used as a negative control; (b) The absorbance levels of the non-treated group and the F-D (Fe-DTPA) treated group were measured and compared. (n = 4, * p < 0.05; unpaired Student’s t test; error bars indicate SEMs); (c) A picture of pelletized steel slag and DTPA is shown above; (d) The measurement was conducted with an ICP-MS (n = 3, * p < 0.05; unpaired Student’s t test; error bars indicate SEMs); (e) Element analyses for any potential toxic effects were performed for 30 days. The slag-DTPA pellets were applied in seawater for 30 days. The seawater was fully exchanged daily considering the case of the East Sea. The measurement was conducted by using ICP-MS; (f) The sporophytes of S. japonica were cultured and weights measured. The pellet was eluted in seawater for 48 h. −: negative control, + (Pelletized slag-DTPA): 0.01% of seawater weight, ++: 0.1% of seawater weight, +++: 1% of seawater weight (n = 6, ns, not significant, * p < 0.05; by ordinary one-way ANOVA followed by Dunnett’s multiple comparisons test; error bars indicate SEMs); (g) The photosynthetic absorption spectrum of the pelletized slag-DTPA eluate treated group and the non-treated (seawater only) group are shown above.
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Figure 3. Pelletized slag-DTPA promotes sea forest growth and enhances carbon sequestration. (a) Pictures of the pellet treated site and the non-treated site were presented. The pellets were applied on the artificial reefs where pictures were taken at a depth of 15 m; (b) Box and whisker plots are shown. The thallus heights of E. cava were measured in January 2021 and in June 2021. (n = 8; ns, not significant; * p  <  0.05 by two-way ANOVA followed by Tukey’s multiple-comparison test). Whiskers indicated minimum and maximum values. The boxes showed the 25th, 50th (mean), and 75th percentiles of the distribution; (c) The relationship between the E. cava thallus height and total carbon weight is represented above. The thallus height of E. cava was directly measured in the lab and their total carbon analyses were analyzed. The linear regression equation was y = 0.2942x − 6.7304, R2: 0.9071. The samples were obtained from Ulleung Island, Korea (n = 11); (d) The carbon weight changes (%) of E. cava were estimated based on the linear equation (n = 8; ns, not significant; * p  <  0.05 by two-way ANOVA followed by Tukey’s multiple-comparison test).
Figure 3. Pelletized slag-DTPA promotes sea forest growth and enhances carbon sequestration. (a) Pictures of the pellet treated site and the non-treated site were presented. The pellets were applied on the artificial reefs where pictures were taken at a depth of 15 m; (b) Box and whisker plots are shown. The thallus heights of E. cava were measured in January 2021 and in June 2021. (n = 8; ns, not significant; * p  <  0.05 by two-way ANOVA followed by Tukey’s multiple-comparison test). Whiskers indicated minimum and maximum values. The boxes showed the 25th, 50th (mean), and 75th percentiles of the distribution; (c) The relationship between the E. cava thallus height and total carbon weight is represented above. The thallus height of E. cava was directly measured in the lab and their total carbon analyses were analyzed. The linear regression equation was y = 0.2942x − 6.7304, R2: 0.9071. The samples were obtained from Ulleung Island, Korea (n = 11); (d) The carbon weight changes (%) of E. cava were estimated based on the linear equation (n = 8; ns, not significant; * p  <  0.05 by two-way ANOVA followed by Tukey’s multiple-comparison test).
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Kwon, P.K.; Kim, H.-S.; Jeong, S.W. Dissolved Iron from Steel Slag with Its Chelating Agent Promotes Seaweed Growth. Sustainability 2022, 14, 5498. https://doi.org/10.3390/su14095498

AMA Style

Kwon PK, Kim H-S, Jeong SW. Dissolved Iron from Steel Slag with Its Chelating Agent Promotes Seaweed Growth. Sustainability. 2022; 14(9):5498. https://doi.org/10.3390/su14095498

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Kwon, Paul Kwangho, Hyung-Suek Kim, and Sung Woo Jeong. 2022. "Dissolved Iron from Steel Slag with Its Chelating Agent Promotes Seaweed Growth" Sustainability 14, no. 9: 5498. https://doi.org/10.3390/su14095498

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