Fabrication of Biochar-Based Marine Buoy Composites from Sargassum horneri: A Case Study in Korea

Abstract

The recurrent influx of invasive Sargassum horneri along the coasts of South Korea poses significant ecological and economic challenges, including habitat disruption, aquaculture damage, and shoreline pollution. This study investigates a sustainable valorization pathway by converting SH into functional biochar through slow pyrolysis and utilizing the product as a core material for eco-friendly marine buoys. Biochars were produced at pyrolysis temperatures ranging from 300 °C to 700 °C and characterized for elemental composition, FT-IR spectra, leachability (CODcr), and biodegradability. Higher pyrolysis temperatures resulted in lower H/C and O/C molar ratios, indicating enhanced aromaticity and hydrophobicity. The biochar produced at 700 °C (SFBW-700) exhibited the highest structural and environmental stability, with minimal leachability and resistance to microbial degradation. A composite buoy was fabricated by mixing SFBW-700 with natural binders (beeswax and rosin), forming solid specimens without synthetic polymers or foaming agents. The optimized composition (biochar:beeswax:rosin = 85:10:5) showed excellent performance in density, buoyancy, and impact resistance, while fully meeting the Korean eco-friendly buoy certification criteria. This work presents a circular and scalable approach to mitigating marine macroalgal blooms and replacing plastic-based marine infrastructure with biochar-based eco-friendly composite alternatives. The findings suggest strong potential for the deployment of SH-derived biochar in marine engineering applications.

1. Introduction

Massive blooms of floating macroalgae—particularly Sargassum species—have become increasingly frequent and disruptive in coastal regions worldwide. These so-called “golden tides” form extensive rafts that drift across oceans, eventually inundating shorelines, where they foul beaches, damage ecosystems, and hinder economic activities such as tourism, fisheries, and aquaculture. Among the most problematic species is Sargassum horneri (SH), whose proliferation has intensified due to climate change and nutrient enrichment in marine environments [1]. Warmer waters and eutrophication promote rapid algal growth, while ocean currents transport the detached biomass across large distances [2]. In the northwestern Pacific, SH blooms originating near China are carried toward Korea by the Kuroshio and Yellow Sea currents [1].

In South Korea, SH “golden tides” have emerged as a recurring phenomenon since the mid-2010s, especially along Jeju Island and southern coasts [3]. These blooms disrupt marine ecosystems, block sunlight, and deplete oxygen in nearshore waters. As the algae decay, they release nutrients and toxic compounds, degrade water quality, and produce foul odors. The accumulation also clogs fishing gear, damages aquaculture infrastructure, and contributes to substantial economic losses [4]. For instance, a large SH event in the Yellow Sea caused the collapse of seaweed farming structures and losses exceeding $70 million [4].

In Jeju alone, the annual volume of collected SH rose from 860 tons in 2019 to nearly 9800 tons by 2021 [3]. This biomass is often incinerated or landfilled due to limited reuse options. Though occasionally applied as fertilizer, its high salt and fibrous content hinder widespread agricultural use [5]. Incineration, while convenient, is energy-intensive and releases stored carbon as CO₂, counteracting environmental goals [6].

Concurrently, South Korea’s aquaculture sector faces another environmental challenge: the widespread use of expanded polystyrene (EPS) foam buoys. These materials, although lightweight and inexpensive, easily fragment into microplastics, which persist in marine ecosystems [7,8,9]. EPS debris accounts for a significant portion of plastic pollution on Korean beaches, prompting government policies to phase out EPS buoys by 2025 [10].

While “eco-friendly” alternatives are being promoted, many substitutes still contain plastic components or suffer from low durability and excessive weight [4]. As a result, over 95% of newly certified buoys remain plastic-based, and sustainable replacements are still in development [7].

Given these dual environmental concerns—harmful Sargassum blooms and plastic buoy pollution—there is growing interest in circular solutions that address both. Thermochemical conversion of SH into biochar presents a promising strategy [11,12,13]. Pyrolysis converts seaweed biomass into a stable, carbon-rich material with reduced weight and volume [7]. Seaweed-derived biochars exhibit high fixed-carbon content, porosity, and adsorption capacity, making them suitable for environmental applications [14,15,16,17,18].

In this study, we explore the conversion of SH biomass into biochar and its use as a material for eco-friendly marine buoys. SH samples collected from Korean coasts were pyrolyzed under various conditions to optimize biochar yield and properties. Comprehensive analyses—including elemental composition, porosity, density, and hydrophobicity—were conducted to assess material suitability.

Environmental evaluations, such as heavy metal content and carbon footprint comparison against conventional disposal, were also performed. Using biodegradable binders, biochar was molded into buoy components without synthetic resins. Prototype buoys were then tested for buoyancy, water absorption, and mechanical stability under simulated marine conditions.

By linking Sargassum management with sustainable material development, this research demonstrates a viable strategy to upcycle invasive biomass into a value-added product. The approach not only reduces environmental burdens from golden tides and marine plastics but also aligns with national goals to promote eco-friendly marine infrastructure. Ultimately, this study contributes to a broader framework for climate adaptation, waste valorization, and circular economy in coastal systems.

2. Materials and Methods

2.1. Sample Collection and Preparation

Figure 1 schematically illustrates the pretreatment process of SH. The SH samples used in this study were collected from the coastal area of Jangheung-gun, Jeollanam-do, Republic of Korea, and stored in a dried state prior to experimentation. The collected samples typically comprised approximately 85% seaweed, 10% salt, and 5% moisture. Dried SH generally contains 30–60% carbohydrates (including cellulose, fucoidan, laminaran, and alginate), ~15.8% protein, 27.5% ash, and ~5% lipid content. To improve biochar quality, the SH biomass was washed with tap water to remove surface salts, dried at 80 ± 3 °C for over 48 h, and ground to a particle size of 1.0 ± 0.2 mm.

In addition, the produced biochar was subjected to an additional washing step, followed by drying at 80 ± 3 °C for 48 h and grinding to a particle size of 1.0 ± 0.2 mm. The resulting sample was designated as SFBW.

2.2. Pyrolysis Process

Figure 2 illustrates the pyrolysis process for producing biochar using SH. To valorize SH, slow pyrolysis was conducted using a Lab House electric furnace (2200 W). The temperature ramp rate was set at 8 °C/min, and a nitrogen gas (99.99% purity) flow of 8 L/min was maintained to ensure an inert atmosphere. Target pyrolysis temperatures were set at 300 °C, 400 °C, 500 °C, 600 °C, and 700 °C. Upon reaching the target temperature, samples were held for 30 min. After pyrolysis, the furnace was allowed to cool naturally below 100 °C before samples were retrieved to prevent spontaneous ignition. The resulting biochar samples were denoted SFB-300, SFB-400, SFB-500, SFB-600, and SFB-700, respectively, and were subjected to physicochemical analysis for resource utilization assessment.

2.3. Characterization of SH and Biochar

2.3.1. Elemental Analysis (EA)

Elemental compositions of the biochar (C, H, N, S) were determined using an EA1112 elemental analyzer (Thermo Fisher Scientific, Waltham, MA, USA). Prior to analysis, samples were dried at above 80 °C for over 48 h and ground for homogenization. The atomic ratios of O/C and H/C, calculated from the elemental composition, were used as indicators of biochar stability and hydrophobicity.

2.3.2. FT-IR Spectroscopy

To analyze the functional groups on the biochar surface, Fourier-transform infrared (FT-IR) spectroscopy was performed using a PerkinElmer Spectrum Two instrument (PerkinElmer, Shelton, CT, USA). The analysis was conducted in ATR (attenuated total reflectance) mode over a range of 4000–400 cm⁻¹. Functional groups such as –OH, –COOH, C=O, and C=C were examined to evaluate structural changes and stability under different pyrolysis conditions.

2.3.3. Leachability (CODcr)

To assess the leachability of water-soluble organic compounds, chemical oxygen demand (CODcr, where cr refers to the use of potassium dichromate, Cr₂O₇²⁻, as the oxidizing agent) was measured. Biochar samples (SFBW-300 to SFBW-700) were mixed with deionized water at a 1:100 (w/v) ratio and stirred at 200 rpm for 2 h at 20 °C. The mixture was then filtered through 0.45 µm GF/C filter paper, and the filtrate was analyzed using Hach Vial reagents for CODcr.

2.3.4. Antimicrobial/Leachate Toxicity Screening

The disk diffusion method was used to evaluate the impact of biochar on microbial activity [19]. Mixed microbial cultures were obtained from activated sludge in a wastewater treatment plant and spread onto nutrient agar plates. Biochar extract (from SFBW samples dried at 110 °C for >24 h) was absorbed onto paper disks and placed on the agar surface. The plates were incubated statically at 25 °C for 24 h. Growth inhibition zones were visually assessed, and inhibition diameters were measured using a digital caliper. Each test was performed in triplicate.

This assay was intended as a qualitative, screening-level evaluation of potential leachate toxicity and microbial interaction rather than a direct biodegradability or “microbial stability” test [19]. Its results were interpreted as indirect indicators of the presence or absence of leachable organic compounds, consistent with previous studies reporting that high-temperature biochars have lower labile carbon contents and exhibit greater chemical recalcitrance [20,21].

2.4. Buoy Specimen Fabrication

2.4.1. Binder Selection

To develop eco-friendly buoy composites based on SH biochar, various natural binders were screened. Initial experiments compared a hydrophilic binder (starch) and a hydrophobic binder (beeswax), mixed at an 8:2 biochar-to-binder ratio, and submerged in distilled water to evaluate decomposition. Among hydrophobic candidates, beeswax and carnauba wax were further compared based on density, biodegradability, hydrophobicity, and moldability. Ultimately, beeswax was selected as the primary binder, with gum rosin added as a secondary binder to improve mechanical strength. Both rosin and gum rosin have also been reported to be biodegradable natural resins, which supports their suitability for use in eco-friendly marine applications [22,23].

The binder ratios tested are shown below.

2.4.2. Specimen Preparation

Biochar obtained at 700 °C (SFBW-700) and sieved to 100–200 µm was used. Ethanol (2–3 wt%) was added to the biochar-–binder mixture to enhance adhesiveness and moldability.

The mixture was placed into a cylindrical mold (30 mm diameter × 50 mm height) and compressed at 10 MPa for 3 min using a hydraulic press. Molded specimens were demolded carefully, air-dried for 24 h, and oven-dried at 60 °C for 6 h to ensure binder curing and structural stability. Final specimens were used for evaluating physical properties such as density, buoyancy, impact resistance, and saltwater leachability.

2.5. Evaluation of Eco-Friendly Buoy Performance (Korean Case Standard)

Table 1. Evaluation of Korean standards for eco-friendly buoys.

Parameter	Performance Standard
Dimensions (Weight)	Specified individually (design-specific)
Buoyancy (N)	(Displayed Volume (L) × 0.7) – weight (N)
Impact Resistance	No cracks, fractures, or tears
Internal Density	≥18

presents the evaluation of Korean standards for eco-friendly buoys. To assess the applicability of the SH biochar composite in marine infrastructure, the fabricated specimens were evaluated against Korea’s eco-friendly buoy standards. These standards, set by the Ministry of Environment, aim to prevent marine pollution and require the absence of hazardous leachates and ecological toxicity.

2.5.1. Density Measurement

Specimen density was measured via the Archimedes method, referencing KS F 2409 [24]. The procedure was as follows:

• Measure dry weight (W_dry) using an analytical balance
• Immerse specimen in distilled water and record submerged weight (W_wet)
• Assume water density ($\rho$_water) = 0.998 g/cm³
• Calculate density ($\rho$, g/cm³):

  $$\rho =W_{dry}/(W_{dry}-W_{wet})$$

  (1)

Five specimens were tested per composition, and average values with standard deviations were reported.

2.5.2. Impact Resistance Test

Impact resistance was evaluated via a drop weight test. A free-falling mass was dropped from a known height, and the specimen was examined for cracking or deformation. The impact energy (E, J) was calculated using:

$$\mathrm{E}=\mathrm{m}\times \mathrm{g}\times \mathrm{h}$$

(2)

where m = mass (kg), g = 9.81 m/s², h = drop height (m). Five replicate specimens were tested for statistical robustness. Post-impact integrity (fracture, indentation, cracking) was visually assessed and recorded.

2.5.3. Buoyancy in Saltwater

To simulate marine conditions, buoyancy was tested in 3.5 wt% saline water at 25 °C. Each specimen was immersed and stabilized for 2 min before buoyant force (N) was measured using a spring scale. Measurements were repeated on five specimens to obtain average values and standard deviations.

3. Results

3.1. Characteristics of Produced Biochar

3.1.1. Yield Analysis

The pyrolysis temperature plays a critical role in determining the yield of biochar derived from SH. In this study, pyrolysis was conducted at five different temperatures ranging from 300 °C to 700 °C at 100 °C intervals to evaluate the temperature-dependent yield behavior. As shown in Figure 3, the biochar yield decreased with increasing pyrolysis temperature. The yield was highest at 300 °C (~69%), likely due to the limited volatilization of organic matter at this relatively low temperature. A sharp decrease in yield (~45%) was observed at 400 °C, suggesting the onset of significant devolatilization and moisture loss. At 500 °C and 600 °C, the yield plateaued around 47% and 45%, respectively. The lowest yield (~40%) was recorded at 700 °C, attributed to enhanced decomposition and carbon loss at high temperatures [25,26]. It should be noted that all reported yields were calculated after washing (SFBW), and the associated mass loss due to salt and impurity removal during the washing step was included in the calculations. Thus, the yield values reflect the effective recovery of purified biochar rather than the unwashed raw product (SFB). These results are consistent with typical biomass pyrolysis behavior, where higher temperatures promote the removal of volatile organics, thereby reducing the solid product yield. This confirms the technical feasibility of converting marine waste SH into biochar and highlights the importance of optimizing pyrolysis conditions to balance yield and material performance.

3.1.2. Elemental Analysis (EA) Results

Table 2. Biochar EA results.

Biochars	Elemental Composition(wt%)					H/C	O/C
	C	H	O	N	S
SF-RAW	49.02	6.95	39.99	3.14	0.90	0.14	0.82
SFB-300	58.21	5.93	30.70	4.64	0.52	0.10	0.53
SFB-400	60.61	4.58	29.51	4.36	0.94	0.08	0.49
SFB-500 SFB-600	65.03	2.66	28.63	3.57	0.11	0.04	0.44
	68.56	1.99	26.00	3.24	0.21	0.03	0.38
SFB-700 SFBW-300 SFBW-400	67.90	2.22	25.73	3.89	0.26	0.03	0.38
	60.70	6.11	27.59	5.60	0.00	0.10	0.45
	68.36	5.02	21.80	4.82	0.00	0.07	0.32
SFBW-500 SFBW-600	70.34	2.89	22.55	4.21	0.00	0.04	0.32
	72.12	1.77	22.59	3.52	0.00	0.02	0.31
SFBW-700	72.43	1.84	22.14	3.60	0.00	0.03	0.31

provides a comprehensive summary of the EA results. The raw SH material (SF-RAW) exhibited high H/C and O/C molar ratios of 0.96 and 0.82, respectively, indicating a high content of hydrogen and oxygen functional groups. However, biochars produced via pyrolysis showed a consistent decrease in both H/C and O/C ratios as the temperature increased. The H/C ratio dropped sharply from 0.96 (SF-RAW) to 0.62 at 300 °C and gradually declined to 0.13 at 700 °C. A H/C ratio below 0.7 generally implies the development of thermally stable, aromatic carbon structures. These results suggest that high-temperature pyrolysis promotes extensive dehydrogenation and aromatization. The O/C ratio also decreased from 0.82 to 0.36 as temperature increased, indicating a reduction in oxygen-containing hydrophilic functional groups. An O/C ratio between 0.2 and 0.6 is associated with moderate hydrophobicity. Therefore, high-temperature pyrolysis enhances both the thermal stability and hydrophobicity of the biochar, improving its suitability as a buoyant material for marine applications.

Both SFB and SFBW samples showed decreasing trends in H/C and O/C ratios with increasing temperature. The SFBW samples, which were washed to remove salts and impurities, consistently exhibited lower ratios than SFB. In particular, SFBW-700 showed the lowest values (H/C = 0.03, O/C = 0.31), suggesting a more aromatic and hydrophobic structure.

Such low H/C and O/C ratios indicate improved water resistance and structural durability—critical properties for marine buoy applications. Therefore, SFBW-700 was identified as the most suitable biochar for buoy fabrication in terms of stability and hydrophobicity.

3.1.3. FT-IR Analysis

Fourier-transform infrared (FT-IR) spectroscopy revealed a clear reduction or disappearance of characteristic absorption peaks as the pyrolysis temperature increased. As shown in Figure 4, the intensity of bands corresponding to –OH and –NH stretching (~3300 cm⁻¹), aliphatic C–H (~2800 cm⁻¹), and carbonyl and imine groups (C=O, C=N at ~ 1600 cm⁻¹) significantly decreased at higher pyrolysis temperatures [27,28]. These changes are consistent with the elemental analysis results, particularly the decrease in H/C and O/C ratios. The diminishing absorption at 3300 cm⁻¹ and 1600 cm⁻¹ suggests dehydration and decarboxylation reactions leading to the loss of oxygen-containing functional groups. The weakening of the ~2800 cm⁻¹ band implies the decomposition of aliphatic C–H bonds and the formation of more aromatic structures. Notably, the aromatic C–H out-of-plane deformation bands in the 680–880 cm⁻¹ region were retained or even enhanced at higher temperatures, supporting the development of aromatic carbon domains. For this analysis, washed biochar samples (SFBW) were used to minimize the effect of residual surface salts on FT-IR spectra. All spectra were presented in terms of Absorbance (a.u.), and all wavenumbers are reported in cm⁻¹, which is the standard convention for solid-phase FT-IR biochar analysis.

In summary, the FT-IR and elemental analysis collectively indicate that higher pyrolysis temperatures result in biochar with fewer hydrophilic functional groups and enhanced aromaticity. This transformation enhances both the water resistance and structural integrity of the biochar, making SFBW-700 the most promising candidate for marine buoy applications.

3.1.4. Leachability Evaluation (CODcr)

Figure 5 visualizes the temperature-dependent COD concentration of SFBW. The leachability of water-soluble organic compounds from SFBW biochars was assessed using chemical oxygen demand (CODcr) analysis. Biochars produced at lower pyrolysis temperatures (300–500 °C) exhibited a measurable time-dependent increase in COD concentration during stirring. At 300 °C, COD increased from 0 mg/L at 0 min to 1447 mg/L at 120 min, while at 400 °C, COD increased from 0 to 1142 mg/L over the same period. In contrast, the increase at 500 °C was minor, ranging only from 0 to 83 mg/L after 120 min. No measurable COD release was detected at 600 °C and 700 °C. These results confirm that incomplete decomposition at lower temperatures leads to higher leaching of organic matter, whereas biochars produced at ≥600 °C showed negligible leachability.

In contrast, biochars produced at 600 °C and 700 °C showed minimal changes in COD levels throughout the test duration. This stability is attributed to more complete thermal decomposition of organics at higher temperatures, resulting in structurally stabilized biochar. Therefore, biochars generated at temperatures ≥600 °C demonstrate superior environmental stability and are more appropriate for use in aquatic applications such as marine buoys.

3.1.5. Antimicrobial/Leachate Toxicity Screening Results

Antimicrobial/Leachate Toxicity Screening was conducted using the disk diffusion method with microbial cultures. Washed biochar samples (SFBW) were used in the experiments to minimize the influence of residual salts. Biochars produced at 300 °C and 400 °C formed distinct microbial inhibition zones on nutrient agar plates, which can be interpreted as the release of residual soluble organic compounds exhibiting antimicrobial effects. Conversely, no inhibition zones were observed for samples produced at ≥600 °C, indicating that high-temperature biochars showed little interaction with microorganisms and maintained higher stability in microbial environments.

The CODcr analysis results were consistent with these observations. The COD concentrations of the 300 °C and 400 °C SFBW samples were 1447 mg/L and 1142 mg/L, respectively, while those of the 600 °C and 700 °C samples were nearly undetectable. These findings suggest that the inhibitory effects observed in low-temperature biochars are attributable to leachable organics, whereas high-temperature biochars exhibit greater resistance to microbial decomposition and leachate toxicity, making them more suitable for long-term marine infrastructure applications.

3.2. Evaluation of Biochar-Based Buoys

3.2.1. Binder Selection Results

In this study, binder selection was primarily based on underwater stability and structural integrity. Figure 6 visualizes the buoyancy–density relationship by mixing ratio. The black circles represent the buoyancy corresponding to each mixing ratio, while the black squares indicate the selected mixing ratios. Error bars represent the deviations observed at each mixing ratio. Initial experiments with hydrophilic binders such as starch demonstrated poor performance, as the specimens disintegrated upon immersion due to binder solubility. This highlighted the unsuitability of hydrophilic binders for marine environments, and these results are therefore described textually rather than graphically.

Consequently, attention was directed to hydrophobic binders with low water absorption and high structural retention. Beeswax and gum rosin were ultimately selected for their complementary properties. Beeswax, a natural wax with low density (0.96 g/cm³) and hydrophobic character, contributed flexibility and buoyancy, whereas rosin, a natural resin, provided tackiness and mechanical strength. The buoyancy–density relationship of the resulting composites across different binder mixing ratios is shown in Figure 6. In this study, “mixing ratio” refers to the weight ratio of biochar, primary binder (beeswax), and secondary binder (gum rosin) used in composite fabrication (e.g., 85:10:5, 80:10:10, 75:15:10, etc.). All tested compositions are listed in the figure caption for clarity. The marker legend has also been clarified: circular markers indicate biochar–binder mixtures, whereas the square marker represents the selected optimal formulation. Error bars indicate the standard deviation (n = 3), reflecting the variability of replicate measurements.

3.2.2. Optimization of Mixing Ratios

Various binder ratios were tested while maintaining a constant SFBW content to evaluate their influence on physical properties. SFBW content was fixed at 85%, and increasing binder content resulted in a gradual increase in density and a decrease in buoyancy.

Excessively high biochar content reduced moldability, while excessive binder led to higher density, compromising buoyancy. The optimal composition was determined to be 85% SFBW, 10% beeswax, and 5% rosin. This formulation exhibited the best balance between physical integrity and buoyant performance, with a low density of 0.214 g/cm³ and high buoyancy of 0.787 N.

3.2.3. Characterization of Biochar Composite Specimens

Final composite buoy specimens were fabricated by mixing SFBW-700 biochar with natural hydrophobic binders (beeswax and rosin) at a weight ratio of 85:10:5, without foaming agents. This optimized formulation ensured a balance between solidification and mechanical performance.

The specimens exhibited excellent density and durability. In saltwater simulations (3.5% salinity, 25 °C), the buoys maintained higher buoyant force than their own weight and showed no structural degradation or delamination over time.The hydrophobic nature of beeswax, combined with the adhesive properties of rosin, effectively prevented water ingress and surface cracking during immersion.Overall, the SFBW-based buoy composite demonstrated excellent buoyancy, water resistance, and structural cohesion, indicating its high potential for practical deployment in marine environments.

3.3. Comparison with Domestic Standards and Environmental Conditions

Compliance with Korean Buoy Standards

Table 3. Comparison of Korean Standards for Eco-friendly Buoys and Test Specimens.

Parameter	Performance Standard	Test Specimen
Dimensions (Weight)	Specified individually (design-specific)	1000 mm3
Buoyancy (N)	(Displayed Volume (L) × 0.7) – weight (N)	0.19
Impact Resistance	No cracks, fractures, or tears	X
Internal Density	≥18	O

presents a comparison of Korean standards for eco-friendly buoys and the test specimens. The fabricated biochar-based buoy specimens successfully met the performance requirements of Korea’s eco-friendly buoy certification standards. As shown in

Table 4. Density Comparison of Buoy Types.

Parameter	Glass Microspheres	SFBW Composite Result
Bulk Density (g/cm3)	0.62	0.36
True Density (g/cm3)	0.33	0.24

, the specimen’s density was approximately 0.24 g/cm³, providing sufficient buoyant force to float stably in water. The internal density was measured at ~240 kg/m³, well above the minimum standard of 18 kg/m³, which is attributed to the solid-state molding process without foaming agents.

(1)

Density Comparison

In general, hollow glass microspheres produced by Camp Shinning (China) and commonly used as additives in conventional polystyrene-based buoyant materials exhibit a true density of 0.29–0.33 g/cm³ and an apparent density of 0.58–0.62 g/cm³ [29]. In contrast, the SFBW composite specimens fabricated in this study demonstrated a true density of 0.24 g/cm³ and an apparent density of 0.36 g/cm³, highlighting their potential as a lightweight and environmentally friendly alternative without the need for foaming processes.

(2)

Impact Resistance Evaluation

Table 5. Impact Resistance Evaluation Metrics.

Parameter	Test Result
Visual Damage	No significant breakage
Max Deformation Depth (mm)	1.8 ± 0.3
Repetitive Impact Durability (cycles)	5 ± 2
Residual Strain Recovery (%)	85 ± 5

summarizes the impact resistance evaluation metrics. Impact resistance was assessed through repeated drop-weight impact testing on the 85:10:5 (biochar:beeswax:rosin) specimens. The maximum deformation depth was 1.8 ± 0.3 mm, attributed to the structural stiffness of the composite and the impact-absorbing nature of the binder.

Minor surface cracks or delamination appeared after 3–7 drops in most specimens, with an average endurance of 5 ± 2 cycles. The residual strain recovery rate was 85 ± 5%, demonstrating the combined rigidity of the biochar and viscoelastic recovery of the natural binders.

(3)

Performance in Saline Conditions

Table 6. Characteristics in Saline Environment.

Parameter	5 °C	25 °C	40 °C
Seawater density (g/cm3)	1027.6	1025.0	1200
Leachability	X	X	X
Buoyancy (N)	185.9	184.9	183.4

summarizes the characteristics in the saline environment. The marine applicability of the SFBW-based composite was evaluated under simulated seawater conditions (3.5% salinity) at temperatures of 5 °C, 25 °C, and 40 °C.

No leachates were detected in any condition, and the specimens maintained structural integrity and buoyancy. Even under elevated temperatures, no deformation or loss in buoyancy was observed, indicating high resistance to saltwater exposure.

4. Conclusions

This study demonstrated the feasibility of converting the invasive brown seaweed SH into biochar through slow pyrolysis, and utilizing the biochar as a core material for eco-friendly marine buoys. Pyrolysis temperature significantly affected both the yield and physicochemical properties of the resulting biochar. Higher temperatures led to lower biochar yields but enhanced thermal stability, aromaticity, and hydrophobicity, as evidenced by decreased H/C and O/C ratios and the disappearance of oxygenated functional groups in FT-IR spectra. Among the tested conditions, biochar produced at 700 °C (SFBW-700) showed the most desirable properties for marine application, including high carbon content, low leachability, and resistance to microbial degradation.

A composite buoy material was successfully fabricated by combining SFBW-700 biochar with natural hydrophobic binders (beeswax and rosin) without the need for synthetic polymers or foaming agents. The optimized composition (biochar:beeswax:rosin = 85:10:5) achieved favorable performance in terms of density, buoyancy, impact resistance, and saltwater durability. The resulting prototype buoy met and exceeded key criteria outlined in Korea’s eco-friendly buoy standards.

This study demonstrated the feasibility of converting the invasive brown seaweed SH into biochar through slow pyrolysis and utilizing it as a core material for eco-friendly marine buoys. The proposed biochar-based composite not only provides a viable alternative to petroleum-derived buoy materials but also contributes to reducing marine plastic pollution and mitigating the impacts of golden tide events in coastal regions.

However, this study was limited to short-term, laboratory-scale feasibility assessments, and the evaluations of biodegradability and saline durability were conducted under restricted conditions. Therefore, the results should be regarded as an initial proof-of-concept rather than full validation for marine infrastructure applications. In addition, detailed characterization of the physical structure of biochar (e.g., pore structure, surface morphology) and the mechanical properties of the composites (compressive strength, force–displacement curves) was not included. This is because the study focused on meeting Korea’s eco-friendly buoy certification standards (density, buoyancy, and impact resistance). These limitations are acknowledged and should be addressed in future work.