Effects of Organic Matter on Acceleration of the Iron Elution Rate from Steelmaking Slag

Abstract

An iron (Fe) fertilization method using a mixture of steelmaking slag and compost for restoring seaweed beds in coastal areas of barren grounds has been developed, and mixing of organic matter other than compost with steelmaking slag has been shown to accelerate the Fe elution rate. In this study, Fe elution tests under conditions close to that of the actual sea environment were conducted in Tsushima, Nagasaki Prefecture, Japan, to elucidate the effects of the addition of powdered bamboo on the promotion of Fe elution. We examined the characteristics of Fe elution for three samples: 1) the mixture of steelmaking slag and compost; 2) the mixture of slag and powdered bamboo; and 3) the mixture of slag, compost, and bamboo. Our results showed that addition of bamboo to the mixture of slag and compost was the most effective for accelerating the Fe elution rate, suggesting that the amount of eluted organic matter was important, as were the structural characteristic of the material. The safety of this method was also evaluated by investigating the accumulation of heavy metals in sea creatures. We confirmed that six heavy metals, i.e., cadmium, arsenic, lead, zinc, selenium, and chromium, were not accumulated in the sea cucumber and sea snails.

1. Introduction

Seaweed depletion, which created the so-called “barren ground”, is a serious problem in coastal areas of Japan and worldwide. The lack of iron (Fe), nitrogen (N), and phosphorus (P) is one of the main causes that could account for the generation of barren ground. In addition to removal of herbivorous sea creatures, production of seed and saplings, installation of adhesive substrates, and nutrient fertilization has been conducted for restoration of seaweed beds in such areas.¹⁾ We have previously developed an Fe fertilization method using a mixture of steelmaking slag, and compost containing humic substances^2,3) (hereafter referred to as slag-compost fertilizer). The effectiveness of the method was confirmed by a field test on the coast of the Sea of Japan in Mashike-cho, Hokkaido, Japan.^2,3) We installed the slag-compost fertilizer on the shoreline and monitored changes in the seaweed bed distribution and water quality, such as dissolved Fe, total N (T-N), and total P (T-P). In addition to the field test, fundamental studies such as analyses of the characteristics of Fe elution from the slag-compost fertilizer,^4,5,6) effects of compost on Fe elution from steelmaking slag and formation of Fe-humate complexes,^7,8,9,10) and safety of Fe fertilization in coastal areas¹¹⁾ have also been conducted.

Regarding the characteristics of Fe elution, the effects of humic substances contained in compost have been investigated in detail.^4,5,6,7) In particular, the effects of mixing compost with steelmaking slag for accelerating Fe elution have been evaluated. The mixture was effective not only for increasing Fe elution but also for extending the lifetime of the elution.⁴⁾ The mechanism of Fe elution from the slag-compost fertilizer has also been investigated and proposed.⁵⁾ Additionally, we previously found that the structural features of humic substances are related to the iron-humate formation capacity,⁷⁾ which is associated with the amount of Fe elution. Furthermore, the effects of seawater temperature and organic matter on the Fe elution from slag-compost fertilizer have also been investigated.⁶⁾ It was supposed that organic matter other than compost was also effective for promoting Fe elution, because the formation of complex, organic iron, is one of the important factors for increasing dissolved Fe. We mixed powdered bamboo with the steelmaking slag and compost and used the mixture for Fe elution tests. Bamboo was selected because there is a large amount of unused bamboo biomass produced by neglected bamboo forests in Japan and the efficient utilization is required. Bamboo was examined as a representative type of organic matter mixed with steelmaking slag. The data confirmed that the Fe elution rate from the mixture of bamboo with steelmaking slag and compost was faster than that from the mixture of steelmaking slag and compost.⁶⁾ However, because this elution test was only conducted using small tanks under batch conditions, an additional Fe elution test using large tanks under conditions similar to those of the actual coastal area should be performed. Moreover, we should evaluate the safety of the Fe utilization method using the slag-compost fertilizer in more detail because the safety had been evaluated only for elution of toxic substances, such as heavy metals.¹¹⁾

The objective of the present study was to evaluate the effects of the addition of organic matter on the promotion of Fe elution from steelmaking slag and the safety of the slag-compost fertilizer for sea creatures under conditions similar to those of the actual sea environment. We performed an Fe elution test using large tanks in Tsushima to investigate the effects of mixing of the powdered bamboo with the slag-compost fertilizer on the acceleration of the Fe elution rate and the safety against heavy metal accumulation in sea creatures.

2. Experimental Methods

An Fe elution test was conducted using 300 L tanks located at the Tsushima Aquaculture Center in Nagasaki Prefecture, Japan. The amount of seawater in each tank was maintained at 150 L; 0.6 L/min of actual seawater collected from the area around the center of the east side of Tsushima Island was exchanged continuously. The following three samples, which were prepared by mixing steelmaking slag (hereafter referred to as slag), compost, and powdered bamboo (hereafter referred to as bamboo), were used in this study:

Sample 1: Mixture of slag and compost (volume ratio of 1:1)

Sample 2: Mixture of slag and bamboo (volume ratio of 1:1)

Sample 3: Mixture of slag, compost, and bamboo (volume ratio of 1:0.5:0.5)

The amount of slag used in all three samples was constant (i.e., 800 g). Compost and bamboo were added in volumes equal to that of the slag (i.e., 800 g) to prepare samples 1 and 2, whereas compost and bamboo were added in volumes equal but half of that of the slag (i.e., 800 g) to prepare sample 3. Each sample was placed at the bottom of the tank. We prepared eight tanks in a single room: two tanks were used for each of samples 1–3, and two blank tanks were prepared for control conditions. We used the same kinds of slag having a composition listed in Table 1. The surface of the slag was covered with CaCO₃ by carbonation arrangement to reduce the alkaline elution.^4,7) The compost used in this study was produced from thinnings, as described in a previously described method.⁷⁾ The bamboo was made from waste bamboo that was dried naturally and contained some moisture.⁶⁾ We used the same kinds of compost and bamboo in the previous study.⁶⁾ The compositions of the compost and bamboo were as described in Tables 2 and 3.⁶⁾ The particle sizes of the slag and compost were less than about 5 mm, and these materials were prepared with a 3.5-mesh sieve; the particle size of the bamboo was less than 2 mm. Fifteen snails and three sea cucumbers were placed into each tank to investigate the safety for using slag, compost, and bamboo. Since the main purpose was to investigate the characteristics of Fe elution from the samples, we did not feed either the sea cucumbers or sea snails during the experiment to avoid alterations in Fe concentrations in the tanks. We assumed that the small populations of algae that grew on the walls of the tanks spontaneously after starting the elution test served as feed for the sea cucumbers and snails.

Table 1. Composition of steelmaking slag [wt%].

T–Fe	M–Fe	FeO	Fe2O3	CaO	SiO2	MgO	MnO	Al2O3	P2O5	C	Na	f-CaO
26.16	13.85	8.05	8.65	35.2	14.7	2.07	2.45	1.86	1.94	1.2	0.2	0.68

Table 2. Composition of compost [wt%, dry-basis].

Component	C	N	Humic acid	Fulvic acid	C/N
Content	48.1	0.47	7.56	7.92	102

Table 3. Composition of bamboo [wt%, dry-basis].

Component	C	H	N	O	Cl	F(μg/g)	Others
Content	50.0	6.4	0.3	43.0	0.2	260	0.1

The Fe elution test was initiated on January 2013 and continued for 5 months. The seawater in each tank was sampled every 2 weeks for water analysis. Iron concentrations, defined as dissolved Fe in this study, were analyzed by spectrophotometry,⁴⁾ as were the concentrations of total organic carbon (TOC), T-N, and T-P. We also monitored pH and concentrations of dissolved oxygen (DO). TOC, T-N, and T-P were analyzed by a total organic carbon analyzer (TOC-V_CSN, SHIMADZU), ultraviolet spectrophotometry and molybdenum blue assays, respectively. pH and DO were measured using pH and DO meters, respectively (AS-600, AS ONE Corporation; DO-24p, DKK-TOA, respectively). The accumulation of six heavy metals in the bodies of the sea cucumbers and snails, including cadmium (Cd), arsenic (As), lead (Pb), zinc (Zn), selenium (Se), and chromium (Cr), was analyzed with inductively coupled plasma mass spectrometry (ICP-MS) after finishing the elution test.

3. Results and Discussion

3.1. Characteristics of Fe Elution

Figure 1 shows changes in Fe concentrations in the seawater of the tanks for samples 1, 2, and 3, as measured using Fe elution tests. The Fe concentration for each sample and under control conditions is presented as the average value obtained from two tanks. Our data confirmed that Fe concentrations in samples 1, 2, and 3 were higher than those under control conditions, indicating that Fe elution could be continued over 150 d using samples consisting of slag, compost, and bamboo. Additionally, Fe concentrations in samples 1 and 3 were higher than those in sample 2. However, it was difficult to determine the qualitative differences in Fe concentrations between samples 1 and 3. We estimated the Fe elution rates in each sample as listed in Table 4. The Fe elution rates were calculated as follows. The amount of Fe elution was estimated by using Fe concentrations analyzed every two weeks. Average Fe elution rates were derived from dividing the amount of Fe elution by the duration of the elution test (154 d). Blank values of Fe concentrations were removed. The Fe elution rate in sample 3 was the highest among all three samples. The amount of Fe eluted under each condition was in the order of sample 3 > sample 1 > sample 2. We found that addition of the compost-bamboo mixture to slag, namely, the addition of bamboo to the slag-compost fertilizer, was effective for accelerating the Fe elution rate, although the addition of bamboo to slag was less effective than the addition of compost to slag. Figure 2 shows the amount of Fe eluted from each sample and indicates half of the totals of samples 1 and 2 (hereafter referred to as condition A). The composition of condition A was considered the same as that of sample 3. We found that the amount of Fe elution in sample 3 was greater than that in sample 1, whereas the amount of Fe elution in sample 3 was greater than that in condition A. These results corresponded to the result of a previous study that was conducted under the same conditions in a batch-type elution test with 11 L seawater.⁶⁾

Fig. 1.

Characteristics of Fe elution from three samples: the mixture of steelmaking slag and compost (sample 1); the mixture of slag and powdered bamboo (sample 2); and the mixture of slag, compost, and bamboo (sample 3).

Table 4. Differences in average Fe elution rates from the three samples.

	Sample 1	Sample 2	Sample 3
Iron elution rate [mg/d]	7.43	4.39	8.32

Fig. 2.

Differences in the amounts of eluted Fe from three samples during the elution test.

Figure 3 shows the amount of organic carbon (TOC) eluted from three samples in the experimental tanks. Although we predicted that the Fe elution rates were related to the amount of organic matter eluted from the samples, the amount of TOC eluted from sample 1 was lower than that from sample 2. This did not correspond to the results of Fe elution. We predicted that not only the amount of organic matter but also the characteristics of this material were related to the acceleration of Fe elution from the slag. In a previous study, the capacity for complex formation between Fe and humic substances, including the binding capacity and the conditional complexation constant, was shown to depend on the structural characteristics of humic acids and fulvic acids.⁷⁾ Acid functionalities, such as carboxyl groups, are related to binding capacity, whereas N and sulfur are related to the conditional complexation constant. For sample 2, we assumed that acid functionalities and elements, which strongly affect the complexation ability of the material, were not as abundant as those in compost. Moreover, the amount of TOC elution from sample 3 was over two times higher than that in condition A, indicating that the mixing of compost with bamboo was effective for accelerating the Fe elution from slag compared with that of condition A. Because the amount of TOC eluted from sample 3 appeared to be strongly related to the amount of Fe elution from sample 3, bamboo may have a role in accelerating the Fe elution rate when mixed with slag. There is a possibility that organic matter other than bamboo also has a role for acceleration of the Fe elution, if the organic matter is easy to be eluted in seawater. The increase of organic matter in seawater is related to the increase of acid functionalities and elements which combine with Fe. The type of organic matter suitable for promoting Fe elution from slag should be investigated in detail in future works.

Fig. 3.

Differences in the amounts of eluted organic carbon (TOC) from three samples during the elution test.

Figure 4 shows the characteristics of changes in T-N concentrations (a) and T-P concentrations (b) during the elution test for the three samples. The patterns in the changes in T-N concentrations in sample 3 were the same as those in samples 1 and 2. We expected that the amount of eluted N and P also increased by mixing compost with bamboo because N and P have also been eluted from the slag-compost mixture.^11,12) However, the result indicates that N eluted from the three samples was almost the same. Additionally, T-N concentrations under control (blank) conditions were almost the same as those in the three samples and increased gradually after starting the elution test. We assumed that the effects of excrement from the sea cucumbers and snails might have been relatively large, although seawater was supplied at a rate of 0.6 L/min, with appropriate water exchange in the tanks. Furthermore, some of the snails died during the test. These factors were the main reasons for the gradual increases in T-N concentrations during the test. As for T-P concentrations in Fig. 4(b), the patterns of the changes in the concentrations were the same as those for the T-N concentrations, indicating that the excrement from sea creatures may also affect the T-P concentration. However, the T-P concentrations in sample 3 from 100 to 130 d after starting the test were higher than those in other conditions. Although it was difficult to determine the reason for this difference, one possibility was that the analyzed seawater was sampled from near the excrement on day 126 after initiation of the experiment. We expected that the excrement could have affected the DO values. Figure 5 shows the characteristics of the DO. DO values decreased gradually under all the conditions, and there were no differences among conditions during the elution test. There is a possibility that the increase of seawater temperature during the elution test around March affect the decrease of DO values. However, this result was consistent with the results presented in Fig. 4. Our findings suggested that the increase in T-N and T-P concentrations were caused by the excrement from the sea creatures under our experimental conditions. It was also assumed that N and P eluted from samples 1, 2 and 3 were not effective for increasing their concentrations in the tanks.

Fig. 4.

Characteristics of changes in T-N concentrations (a) and T-P concentrations (b) for the three samples.

Fig. 5.

Characteristics of changes in DO concentrations for the three samples.

3.2. Effects of Heavy Metals on Sea Creatures

The contents of six heavy metals, including Cd, As, Pb, Zn, Se, and Cr, in the internal organs and muscles, which are the edible portions, of sea cucumbers are listed in Tables 5 and 6. Table 7 indicates the analytical results of contents of the six heavy metals in the muscles of the snails. Standard values for the concentrations of these heavy metals in foods, such as grains, fish, and shellfish, have been established, and many studies have investigated this topic.^13,14) The standard value of Cd concentration in rice was defined as under 0.4 mg/kg.¹³⁾ The standard values of As and Pb concentrations in agricultural products, such as the Japanese pear, apple, and shells of summer oranges have been reported to be 3.5 and 5.0 mg/kg, respectively.¹⁴⁾ Analyzed Zn, Se, and Cr concentrations in abalone are 7 mg/kg, 70 μg/kg, and 50 μg/kg, respectively.¹⁵⁾ Additionally, Zn, Se, and Cr concentrations in short-nicked clams are 10.0 mg/kg, 380 μg/kg, and 40 μg/kg, respectively.¹⁵⁾

Table 5. Contents of heavy metals in the internal organs of sea cucumbers (μg/g).

	Cd	As	Pb	Zn	Se	Cr
Sample 1	0.01±0.002	1.51±0.60	0.005±0.004	1.35±0.57	0.06±0.01	0.02±0.01
Sample 2	0.01±0.002	2.06±1.39	0.01±0.01	0.79±0.40	0.22±0.10	0.03±0.01
Sample 3	0.01±0.002	2.38±0.89	0.003±0.001	1.92±0.74	0.26±0.10	0.03±0.01
Blank	0.01±0.004	3.57±1.58	0.03±0.04	0.35±0.14	0.06±0.02	0.06±0.01

Table 6. Contents of heavy metals in the muscles of sea cucumbers (μg/g).

	Cd	As	Pb	Zn	Se	Cr
Sample 1	0.01±0.01	3.89±1.51	0.03±0.02	2.75±0.76	0.55±0.18	0.01±0.004
Sample 2	0.01±0.01	1.31±1.30	0.03±0.01	3.66±1.27	0.67±0.35	0.02±0.01
Sample 3	0.01±0.01	3.40±0.69	0.02±0.01	3.94±1.25	0.52±0.21	0.01±0.005
Blank	0.01±0.003	5.49±2.97	0.02±0.01	3.35±0.92	0.85±0.10	0.03±0.01

Table 7. Contents of heavy metals in the muscles of snails (μg/g).

	Cd	As	Pb	Zn	Se	Cr
Sample 1	0.16±0.04	20.80±8.74	0.09±0.01	28.00±8.68	0.84±0.19	0.52±0.14
Sample 2	0.16±0.08	21.18±7.32	0.02±0.02	38.66±8.91	0.85±0.34	0.62±0.36
Sample 3	0.17±0.08	22.74±4.94	0.08±0.05	22.98±5.95	0.90±0.26	0.61±0.19
Blank	0.19±0.09	24.39±4.80	0.12±0.04	31.77±7.27	1.05±0.36	0.59±0.20

The concentrations of Cd and Pb shown in Tables 5, 6, and 7 were all lower than the standard values, and there were no differences between the three samples and the blank control. As concentrations in the internal organs and muscles of sea cucumbers were lower than or similar to the standard, except for the concentration in the muscles of sea cucumbers in the blank control. However, As concentrations in the muscles of snails were higher than the standard value. Notably, there were no differences between the three samples and blank sample, indicating that As was not accumulated during the elution test and that As in snails was relatively higher than that in the agricultural products. In food products from the marine environment, As concentrations depend on the type of food. For example, average total As concentrations of Sargassum fusiforme (Harvey) Setchell and kelp in dry condition have been reported as 93 and 54 mg/kg, respectively.¹⁵⁾ For Zn, the concentration in the muscles of snails was relatively large, although there were no differences in Zn concentrations between the three samples and blank control. However, Zn concentrations in scallops and turban shells in raw conditions were reported to be 27.0 and 22.0 mg/kg, respectively.¹⁵⁾ Thus, the content of Zn also depends on the type of sea creatures and algae. Se concentrations in samples 2 and 3 in the internal organs of sea cucumbers were higher than those in the blank control. However, standard deviations in samples 2 and 3 were higher than those in sample 1 and the blank control. Se concentrations in the muscles of sea cucumbers and muscles of snails for the three samples were not higher than those in the blank control. Therefore, we concluded that Se was not accumulated in sea cucumbers and snails. Additionally, Cr was not accumulated in these sea creatures, as listed in Tables 5, 6, and 7. In this case, the average Cr concentration in sample 3 in the muscles of snails (Table 7) was calculated except for an analytical value (1.77 μg/g) that is an outlier based on Dixon test.

We confirmed that the six heavy metals were not accumulated in the sea cucumber and snails. However, it is possible that the concentrations of heavy metals may have increased if the amount of each sample installed in the tank increased. Notably, the conditions of the elution in this study were similar to the conditions in field tests. The difference in Fe concentrations between in the experimental site at 3 m from the shoreline and that at the control site at 50 m from the shoreline was around 10 μg/L, as estimated in a previous study.³⁾ Similarly, as shown in Fig. 1, we found that the difference in the Fe concentration between sample 1 and the blank control was around 10–15 μg/L. Therefore, we concluded that the six heavy metals tested in this study were not accumulated under actual experimental conditions in the field test,³⁾ although the effects of heavy metals should be investigated in more detail in future works.

4. Conclusions

Fe elution tests using actual seawater from Tsushima in 300 L tanks were conducted to elucidate the effects of the addition of organic matter to steelmaking slag and slag-compost fertilizer and to evaluate the safety of the fertilizer in terms of heavy metal accumulation in sea creatures. Our results confirmed that addition of powdered bamboo to the slag-compost fertilizer was most effective for accelerating the Fe elution rate, although the Fe elution rate was lower with the mixture of slag and bamboo than that with the mixture of slag and compost. We assumed that the amount of eluted organic matter affected the elution rate separately from the characteristics of its structure. Moreover, our results confirmed that the six heavy metals Cd, As, Pb, As, Se, and Cr were not accumulated in the sea cucumber or snails in the experimental conditions used in this study. These results are important for understanding the effects of installing the slag-compost fertilizer in the coastal areas of barren grounds.

Acknowledgements

This study was supported by a grant from the Steel Foundation for Environmental Protection Technology. We thank the Tsushima Aquaculture Center and Ohkawa Construction Co., Ltd. for supporting the iron elution tests in Tsushima. We are also grateful to Nippon Steel & Sumitomo Metal Corporation for providing steelmaking slag and compost.

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