Application of Iron-Modified Activated Carbon for Phosphate Removal in Aqueous Systems

Abstract

Phosphate pollution in aquatic environments leads to eutrophication and harmful algal blooms, significantly impacting ecosystems and water quality. The current study evaluates the effectiveness of surface-modified activated carbon (SMAC) in suppressing phosphate release from sediments. Using soil samples from Daecheong Lake, the performance of SMAC adsorption for phosphate was analyzed under various SMAC modification scenarios. Experiments showed that SMAC achieved approximately twice the phosphate removal efficiency compared to conventional activated carbon, with increasing effectiveness under higher flow velocities. Additionally, SMAC significantly reduced phosphate concentrations within the sediment layers, proving its effectiveness in the soil remediation process as well. The results highlight SMAC as a promising solution for mitigating pollutant release in rivers, lakes, and coastal areas, offering both short-term and cumulative long-term benefits for water quality improvement and ecosystem protection.

1. Introduction

With the industrial and agricultural intensification in contemporary society, there is a growing trend of increased inflow of organic matter and nutrients into aquatic systems [1]. As a result, significant algal blooms occur from late spring to autumn [2], with algae triggering the release of toxins and odorous compounds [3]. Microcystin, toxins produced by certain strains of cyanobacteria [4], pose a significant threat to the supply of drinking water and irrigation water [5]. In 1991, along the Darling River in New South Wales, Australia, a stretch of 1200 km was affected by microcystin produced by cyanobacteria, resulting in the deaths of 1600 livestock [6]. In 1996, at a blood dialysis center (dialysis center A) in Caruara, Brazil, 116 out of 130 patients died due to the influx of microcystin [7], and the high incidence of liver cancer in Southeast Asia (Qidong Contry and Jiangsu Province in China) and Sub-Saharan Africa (Inhambane Province in Mozambique) has been reported to be associated with contaminated drinking water due to microcystin [8].

The issues stemming from algal blooms and microcystin are worsening annually in watersheds across South Korea, particularly experiencing a rapid increase since the implementation of the Four Major Rivers Project from 2008 to 2012 [9]. In the lower reaches of the Han River passing through Seoul, from July to September 2015, concentrations of chlorophyll-a and toxic cyanobacteria exceeded 25 mg/and 5000 cells/mL, respectively, triggering a second-level warning in the South Korean algal bloom alert system [10]. In the case of the Nakdong River (Gyeongsang-do, South Korea) and Yeongsan River (Jeolla-do, South Korea), the median concentrations of chlorophyll-a increased from 12.8 µg/L to 17.6 µg/L between 2005 and 2012, and from 13.9 µg/L to 20.0 µg/L between 2013 and 2016. Particularly, the Geum River exhibited the highest increase, rising from 9.3 µg/L to 23.0 µg/L, indicating the most significant elevation [11].

The Daecheong Lake (Chungcheong-do, South Korea), formed by a composite dam completed in 1981, is a reservoir situated in the upper reaches of the Geum River [12], providing water resources for a population of two million in the Daejeon and Chungcheong regions, supplying 922,000 of water daily [13]. Since the implementation of an algal bloom alert system in 1997 [14], except for the years 1998 and 1999, Daecheong Lake has been under algal bloom advisories and warnings for as few as 14 days to as many as 90 days annually [15].

Various technologies have been developed to prevent algal blooms and control the amount of nutrients in aquatic ecosystems [16]. Physical–chemical treatment methods include coagulation–precipitation treatment using aluminum-based coagulants [17] and adsorption methods using ion exchange resins [18], while biological treatment methods involve the utilization of polyphosphate accumulating organisms (PAOs) [19]. In the case of Daecheong Lake, various facilities such as floating aerators, plasma generation devices, algal bloom barriers, and submerged plant cultivation islands have been installed for algal bloom control [20]. However, coagulant chemicals and biological treatment methods pose issues such as sludge generation and accumulation of chemicals [21], making them less suitable for application in natural environments. Additionally, the effectiveness of algal bloom prevention facilities in Daecheong Lake has been reported to be limited [20].

However, in aquatic environments such as lakes, rivers, and seas, activated carbon can be an effective method for pollution control, particularly targeting sediment layers. Activated carbon, characterized by its porous structure and large surface area [22], possesses high adsorption capacity [11]. Activated carbon is widely used across various fields for pollution remediation purposes [10]. Preceding research includes controlling various hydrophobic organic compound (HOC) substances contained in lakes and marine sediments [23] as well as controlling polycyclic aromatic hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs) in marine sediments [24]. Furthermore, activated carbon treatment has been reported to be environmentally safe [25].

Meanwhile, activated carbon has been shown to exhibit particularly strong adsorption capabilities for organic compounds compared to metals or inorganic substances [26]. To leverage the advantages of activated carbon effectively, methods such as surface modification using acids, alkalis, microwaves, etc. are employed [9]. Among these, surface-modified activated carbon using cationic metals has demonstrated enhanced adsorption capabilities for inorganic substances [27]. For example, nitrate and fluoride showing improved performance due to electrostatic interactions between positive charge of metal sites, and negative charged pollutants [28,29,30]. In the case of phosphate, several studies have reported that activated carbons impregnated with metals such as lanthanum, zirconium and iron exhibit high sorption efficiency through mechanisms including electrostatic attraction and surface complexation [31,32,33].

In this study, we conducted a case study with surface-modified activated carbon to prevent phosphorus release from river sediments. The study evaluated the adsorption capacity of surface-modified activated carbon for phosphate, and examined the differences in adsorption capacity according to the modification conditions. To assess the field applicability to Daecheong Lake soil, the study investigated the release of phosphate under hydraulic stress conditions simulated by a stirred batch simulators. Additionally, the study determined the impact of modified activated carbon on phosphate removal from the soil, leading to a possibility for river bed remediation.

2. Materials and Methods

The In this study, the activated carbon modification method using ferric chloride was primarily based on the procedure outlined by Lee et al. [21].

2.1. Materials

Palm-based Activated Carbon (BSC-001, Bscarbon Co., Seoul, Republic of Korea, size range of 0.5 mm to 4.0 mm), iron(iii) chloride hexahydrate (97%, SAMCHUN Extra pure chemicals, Daejeon, Republic of Korea), potassium phosphate monobasic (99.0%, SAMCHUN Extra pure chemicals, Daejoen, Republic of Korea), Phosver3 phosphate reagent (PERMACHEM REAGENT of Hach Korea, LLC., Seoul, Republic of Korea), and a pore water sampler (Rhizon SMS Sampler, Rhizonsphere Research Product Co., Wageningen, Gelderland province, The Netherland) were purchased.

2.2. SMAC Modifying Process

AC was prepared by washing with water, until there was no dust or another particle. Initially, 5 g of AC and solutions of was added and shaken for 8 h on a shaking plate at 150 rpm.

Iron-adsorbed AC was collected on a drying plate and dried in a 150 °C oven (DO-81, Han Yang Scientific Equipment Co., Ltd., Seoul, Republic of Korea). After one hour of the drying process, the modified AC was washed until wash water became clear, indicating complete removal of unbound ferric chloride. After washing, the modified AC was dried at 50 °C.

A SEM-EDS (SU8230, HITACHI Korea, Ltd., Seoul, Republic of Korea) was used to observe the surface structure and composition of the material of AC and SMAC modified with 0.2 M and 0.5 M solutions. For sample preparation, AC and SMAC were blended using a mortar and pestle to particles under 263 μm diameter while minimizing friction during the blending process. Before analysis, the samples were coated with platinum layer (~5 nm) and examined in a high-vacuum chamber.

SEM-EDS analysis was performed using an accelerating voltage of 20 kV and a magnification of 5000×, with a working distance of 15 mm. Elemental mapping was conducted over a selected surface region of 20 ${\mathsf{\mu }\mathrm{m}}^2$. The EDS resolution was 125.5 eV, and data were collected with a live time of 307.2 s. The temperature and relative humidity were maintained at (24 ± 2) °C and (45 ± 2) °C R.H

2.3. Phosphate Adsorption to SMAC

The phosphate solutions were prepared, resulting in concentrations ranging from 10 mg/L to 250 mg/L. Initially, 40 mL of phosphate solution of varying concentrations was aliquoted into plastic tubes, and 5 g of AC and SMAC was each added to separate tubes. The tubes were then shaken at 90 rpm on the shaking plate for 24 h. After the process, 10 mL of the supernatant was sampled and analyzed with a UV–Vis spectrophotometer (UV-1900i, SHIMADZU Korea Corp., Daejoen, Republic of Korea) at 880 nm wavelength.

2.4. Soil Phosphate Release

The soil was sampled at Chudong area, Sediment Monitoring Site 1 in Daecheong Lake. Soil samples were washed to remove debris. To analyze the particle size distribution of the soil, a sieve analysis was conducted using a mechanical shaker under dry conditions. Stainless steel sieves (ChungGye Sieve Co., Ltd., Gyeonggi-do, Republic of Korea) with mesh sizes of #4 (4.75 mm), #2 (2.0 mm), #16 (1.18 mm), #40 (0.425 mm), #60 (0.250 mm), #100 (0.150 mm), and #200 (0.075 mm) were used. The sieves were stacked in descending order from the smallest mesh size at the bottom to the top, and 0.5 kg of washed and dried soil was poured onto the top sieve. The sample was then shaken for 5 min using a mechanical sieve shaker (CG-211-8, ChungGye Sieve Co., Ltd., Gyeonggi-do, Republic of Korea) at a consistent amplitude, following standard soil testing procedures. After this, 0.5 kg soil was stored with the 0.5 L phosphate solution on a large stainless steel plate for phosphate adsorbed onto the soil. After 48 h, the supernatant was decanted and the wet soil was dried. This will be referred to as “P-Soil”.

The control group samples (P-Soil only) were prepared by filling each beaker with 0.6 kg of P-Soil (approximately 300 mm in depth). The first experimental group (P-Soil + AC) and second experimental group (P-Soil + SMAC) were prepared by adding 30 g of SMAC (forming a 30 mm surface layer) on top of each control sample. Each beaker was filled with 0.5 L of distilled water.

To simulate water flow, a jar tester was used, where the range of rpm for stirring was from 0 to 80. The stirring time was set to 6 h considering the time required to reach concentration equilibrium. After mixing, the supernatant was collected to measure the phosphate concentration.

2.5. In Situ SMAC Uptake

The batch was a cylindrical container made of acryl plastic with a volume of 10 L and an inner height of 25 cm. Holes were drilled at 2.5 cm intervals along the container for connecting a pore water sampler to the batch. The pore water sampler is composed of a membrane filter, a silicon line, and 40 mL syringe, which can be attached and detached as needed. The samplers were held to the holes using waterproof tape and hot-melt adhesive, ensuring that only the membrane filter section was inserted. After inserting the sampler, the batch was filled with 4 kg P-Soil (10 cm depth) and 1.5 L of distilled water (preparation method of P-Soil was the same as in 2.4). The phosphorus concentration at various soil depths was monitored until reaching equilibrium. Sampling was conducted by fully inserting the piston of the syringe, connecting it to the line, and ejecting 3 mL of pore water. After equilibrium was reached, 400 g of SMAC was placed on top of the P-Soil, and 0.5 L of water was added. Phosphate concentrations at different depths were measured over a period of 3 weeks.

3. Results and Discussion

3.1. Soil Sample Acquisition

The soil used in the study was collected at Sediment Measurement Network Site 1, located near Chudong Intake Tower within the Chudong Water area, Daecheong Lake, as marked in Figure 1a. To analyze the particle size distribution of the soil used in the experiment, the collected soil samples were washed and subjected to a sieve test. The results are presented in (b). Based on the analysis, $D_{10}$ (the effective diameter) was 0.150 mm and $D_{60}$ was 0.250 mm, yielding a uniformity coefficient ($D_{60}$/$D_{10}$) of approximately 1.67. This can be considered relatively uniform in particle size distribution. The Chudong water area experiences consistent foot traffic and hosts various commercial and recreational facilities. The waterfront includes walking trails and an array of trees, contributing to the influx of organic materials, such as leaves and branches, as well as other pollutants. Juwon Stream, which flows into the Chudong water area, has a relatively small watershed, and features a topography that widens significantly near the inflow zone. Due to these characteristics, the Chudong water area is less influenced by fluctuations in inflow volume and water quality caused by rainfall compared to other regions of Daecheong Lake [34]. The sediment in the Chudong water area consists of clay, silt, and sand particles, with its soil texture classified as silt, clay. Particles such as clay and fine silt, which have small sizes and high specific surface areas, are known to effectively adsorb nitrogen and phosphorus [35]. These soil characteristics can lead to internal loading.

Around the Chudong Intake Tower, the total phosphorus (TP) concentration in the sediment varies between 1000 mg/kg, annually. This level exceeds the sediment dredging threshold of 800 mg/kg set for the Paldang Dam and the 1000 mg/kg criterion for sediment in the Han River [35]. Meanwhile, the concentration of total nitrogen (TN) in the water remains consistent throughout the year; however, the concentration of TP increases between August and October each year, reaching as high as 0.05 mg/L. As a result, harmful algae thrive from September to October, with densities recorded between 2000~5000 cells/mL.

3.2. Sorption Isotherm of Iron-Functionalized Activated Carbon

To investigate the phosphate adsorption characteristics of SMAC, we conducted batch adsorption experiments using various modification concentrations, and the results are presented in

Table 1. Phosphate Adsorption Capacities of SMAC at Various Modification Concentrations.

${\mathbf{F}\mathbf{e}\mathbf{C}\mathbf{l}}_3\left(\mathbf{M}\right)$
0.00	$C_0$	10	25	50	100	200	250
	$C$	1.16	1.80	7.70	14.20	34.00	128.00
	$X/m$	0.02	0.04	0.08	0.17	0.29	0.37
0.05	$C_0$	10	25	50	100	200	250
	$C$	1.87	7.52	15.66	40.58	116.90	157.10
	$X/m$	0.07	0.14	0.27	0.48	0.66	0.74
0.1	$C_0$	10	25	50	100	200	250
	$C$	1.61	4.95	10.23	29.19	96.90	138.40
	$X/m$	0.07	0.16	0.32	0.57	0.82	0.89
0.2	$C_0$	10	25	50	100	200	250
	$C$	2.13	1.20	34.83	17.60	68.20	114.00
	$X/m$	0.06	0.19	0.36	0.66	1.05	1.09
0.3	$C_0$	10	25	50	100	200	250
	$C$	0.65	2.48	3.15	12.59	58.60	99.30
	$X/m$	0.08	0.19	0.38	0.71	1.14	1.23
0.4	$C_0$	10	25	50	100	200	250
	$C$	0.64	2.21	2.93	10.65	57.00	95.70
	$X/m$	0.07	0.18	0.38	0.71	1.14	1.23
0.5	$C_0$	10	25	50	100	200	250
	$C$	0.61	2.13	2.67	9.81	50.00	88.40
	$X/m$	0.08	0.18	0.38	0.72	1.20	1.29

and Figure 2. While the adsorption pattern resembles the Langmuir isotherm, the Freundlich isotherm provides a better mathematical representation. The phosphate removal was 0.64 mg/g for AC and 1.29 mg/g for SMAC, indicating over twice the adsorption capacity for SMAC. This outcome is due to an increased binding of iron on the AC surface. Under neutral pH, the AC surface exhibits a negative zeta potential, which restricts its capacity to effectively adsorb anionic and inorganic substances [36]. However, when iron oxide particles bind to the AC surface, they create new functional groups that enhance bonding with anionic substances [37], resulting in a positively charged surface. In this study, it is assumed that anionic phosphate is adsorbed and removed through binding with iron, which acts as a cationic functional group.

When the ferric chloride modification concentration increases beyond 0.2 M, the modification efficiency decreases, showing no significant increase in phosphate removal. SMAC sorption capacity of phosphate is shown by its adsorption coefficient. The results of the calculated Freundlich constants are summarized in

Table 2. Freundlich Constants.

	0.00	0.05	0.1	0.2	0.3	0.4	0.5
K	0.1060	0.0496	0.0643	0.1096	0.1293	0.1365	0.1407
n	1.6598	1.7876	1.7425	1.9001	1.8488	1.8522	1.8517
${\mathrm{R}}^2$	0.9053	0.9783	0.9565	0.7778	0.9194	0.9100	0.9121

. “K” is related to adsorption capacity (L/mg), and a large value indicates better adsorption ability. Meanwhile, “n” is associated with adsorption intensity, and a higher value of n signifies stronger adsorption strength [38]. The value of K shows a continuous increase with rising modification concentrations. Likewise, n rises to 1.90 at a concentration of 0.2 M but declines to approximately 1.85 at higher modification levels. Although AC exhibits good adsorption capacity for phosphate, its adsorption strength is weak. In SMAC modified at low concentrations, the number of available sites for phosphate adsorption decreases, which reduces adsorption capacity. However, the adsorption strength appears to increase due to the influence of cationic metals. When the modification concentration surpasses a certain level, the increase in adsorption sites causes the K value to continue rising. However, this increase becomes slower as the limited surface of the activated carbon reaches saturation with iron. Simultaneously, the adsorption capacity for phosphate declines and stabilizes at a consistent level.

The effect of changes in the n and K values can be visually confirmed in Figure 3. It shows the distribution of iron on the surface of activated carbon, as analyzed through SEM-EDS for AC and SMAC samples modified with 0.2 M and 0.5 M concentrations. The iron detected using the EDS scan mode is represented by orange dots, which are overlapped on the captured SEM image. Figure 3a represents the image of AC, while Figure 3b and Figure 3c correspond to the 0.2 M and 0.5 M SMAC images, respectively. The absence of iron in Figure 3a confirms that no iron was originally present to influence adsorption, supporting the conclusion that the increase in adsorption capacity is solely due to the cationic iron introduced during the modification process. In both Figure 3b,c, iron is found to be evenly distributed throughout the surface. As seen in elemental mapping images, the overall amount of iron is slightly higher in Figure 3c. This explains the K value of 0.5 M SMAC compared to 0.2 M SMAC in Table 2, as it results from the greater overall Fe attachment to the AC surface. While the 0.5 M SMAC contains more iron overall, large iron aggregates were observed in Figure 3b, which may explain why the n value of 0.2 M SMAC is the highest among all the samples.

Similar trends of decreased modification efficiency in metal-modified activated carbon have been reported in studies involving copper-modified carbon [39]. It has been shown that the modification efficiency of activated carbon using metals can be improved through methods such as pH adjustment and physical surface treatments [40]. Considering these findings, the optimal modification concentration in this study was confirmed to be 0.2 M, and SMAC was manufactured at this concentration for subsequent experiments.

3.3. Prevention of Phosphate Release by Soil

We evaluated the effectiveness of SMAC in preventing the leaching of phosphates from sediments saturated with phosphorous, and the results are shown in Figure 4. For this purpose, we compared three samples: the natural state of P-Soil (soil only) and the treatments where AC and SMAC were applied to the surface. This experiment confirmed the effectiveness of SMAC in preventing the leaching of phosphates through adsorption.

To prepare P-Soil for the experiment, the soil from the Chudong water area was immersed in a solution of concentration for 72 h until it reached equilibrium. We measured the concentration using absorbance every 24 h and found that equilibrium was reached around the 24 h mark. To ensure stability, we set the immersion duration to 48 h. As a result, the phosphate concentrations in the samples were 2100 mg/kg, 1933 mg/kg, and 2033 mg/kg for soil only, AC, and SMAC, in that order. The sediment measurement results from Site 1 of the sediment measurement network in the Chudong area of Daecheong Lake indicated phosphate concentrations ranging from 1000 to 1000~1500 mg/kg throughout the year, which is relatively high. In addition, the leaching of phosphates during the experiment was conducted over a period of 72 h.

The results indicate minimal difference in phosphate leaching between the soil only and AC samples, with a small margin of 2.52%. Meanwhile, the SMAC sample exhibited a significantly reduced leaching amount, 4 to 5 times lower than the other samples. Phosphate leaching levels were 121 mg and 126 mg for soil only and AC, with leaching rates of 19.21% and 21.73%, respectively. In contrast, the SMAC sample recorded a leaching amount of 29 mg and a leaching rate of 4.75%. These results suggest that the SMAC layer effectively captures phosphate during its leaching process from soil to water, thereby reducing phosphate concentration in the water. This experiment demonstrates the significant effectiveness of SMAC in preventing internal phosphate loading from sediment in lake environments. Considering the typical phosphorus concentration in lake water, SMAC may also help prevent external phosphorus loading from accumulating in the sediment.

To assess the effectiveness of SMAC across various soil environments, we conducted experiments simulating not only stagnant conditions but also environments with hydraulic flow. The results are shown in Figure 5. For this experiment, we used a blade stirrer to measure the leached phosphate over a 6 h period, preparing the P-Soil at a concentration of $2000 \mathrm{m}\mathrm{g}{\mathrm{P}\mathrm{O}}_4/\mathrm{k}\mathrm{g}$. Experiments in Figure 5a were conducted with P-Soil alone, whereas samples in Figure 5b were conducted with SMAC as a separating layer between P-Soil and the bulk water.

The experimental results show that the phosphate leaching rates and concentration of the water increase with rpm, but rise more gradually in the SMAC-applied P-Soil with SMAC. In Figure 5a, phosphate leaching rates were 8.90%, 19.27%, and 39.56% at 0, 40, and 80 rpm, respectively. The increase in leaching rate at 80 rpm reached 444.50% compared to the rate at 0 rpm, where there is no hydraulic flow. This is due to the fact that the hydraulic stress and turbulent flow conditions induce the agitation of the P-Soil, resulting in the higher degree of detachment of phosphorous from the soil. This can also be explained as turbulence-driven.

In contrast, Figure 5b showed phosphate leaching rates of 4.08%, 4.90%, 6.44%, 6.88%, and 8.73% at 0, 20, 40, 60, and 80 rpm, respectively, maintaining leaching rates below 10% across the entire range tested. At 80 rpm, SMAC-applied soil showed 22% phosphate leaching compared to the untreated sample, similar to the leaching rate observed in a stagnant environment under natural conditions. These results indicate that SMAC application effectively suppresses phosphate leaching even in environments with hydraulic flow, with its effectiveness further amplified in stronger flow conditions. The reduction in phosphate leaching with SMAC is due to the sorption of phosphate, and the fact that the additional carbon layer reduces the turbulent conditions the soil is exposed to, leading to lower phosphate leaching. This suggests that SMAC can be effective not only in stagnant water bodies but also in environments with weather-induced inflows or mixing, such as during rainfall or wind-induced waves, as well as in flowing rivers and coastal areas. Furthermore, it indicates that SMAC may be even more effective in these conditions than in situations with no hydraulic flow.

3.4. Effect of SMAC on the Sediment Layer

Nutrient pollutants, which are the causes of algal blooms, require long-term management for improvement. The application of SMAC not only prevents the leaching of phosphates from the soil but also controls the phosphates present within the soil. This contribution is evidenced by the experiments shown in Figure 6. For the experiment, we filled a batch with P-Soil at concentration to a depth of 10 cm and added 1.5 L of distilled water to submerge the soil. Setting the surface at 0 cm, we collected samples of soil pore water at intervals of 2.5 cm to monitor phosphate concentrations. As a result, the phosphate concentrations at different depths reached equilibrium after 6 h. The measured concentration was approximate, which was established as the initial concentration. Subsequently, we covered the soil with 400 g of SMAC and added an additional 0.5 L of distilled water to initiate leaching immediately. Figure 6a,b represents conditions without and with hydraulic flow, respectively, where the results of (b) underwent stirring at the water surface at 100 rpm using a blade stirrer.

Based on the results in Figure 6a,b, the reduction in phosphate through hydraulic stress showed an exponential decrease with soil depth. This phenomenon is known as hyporheic exchange, demonstrated in the experimental study by Chandler et al. [41]. Hyporheic exchange refers to the circulation of water and solutes between surface water and sediment pore water. When surface water flows over the sediment bed, it creates pressure gradients and turbulence at the waterbed. These forces push water into the sediment’s pore spaces where the infiltrated water later returns to the water column, forming a continuous circulation loop known as hyporheic exchange. Its intensity and depth are largely governed by flow conditions and sediment characteristics. Chandler et al. demonstrated that stronger hydraulic flow increases the bed shear velocity, enhancing advective mixing and deepening the active exchange layer within the sediment. The results from the experiments also showed that the pore water concentration would decrease exponentially with the soil depth. Additionally, it was proven that sediments with larger particle diameters exhibited higher permeability, which facilitated faster and deeper solute transport. In our experiments, P-Soil had a $D_{60}$ of 0.250 mm, exhibiting active hyporheic exchange under bed shear velocities ranging from approximately 0.01 to 0.04 m/s, which were well within the experimental parameters conducted in the research of Chandler et al., showing similar results. Additionally, since the SMAC particles applied to the surface had a much larger grain size (0.5–4 mm), the SMAC are unlikely to have affected the pore-scale water flow within the finer underlying soil. Thus, the presence of hydraulic flow appears to have played the dominant role in enhancing solute transport within the soil matrix. This provides experimental support for Figure 6, where the conditions of Figure 6b, with hydraulic flow, showed a significantly faster decrease in phosphate concentration than the results in Figure 6a.

Figure 6c shows the phosphate concentration in the water body during the experiments conducted in Figure 6a,b. The results indicate a minimal amount of phosphate leaching from the P-Soil regardless of the hydraulic stress. This implies that the sorption kinetics of SMAC are sufficient in capturing phosphate before it escapes into the water column. The findings indicate that SMAC is effective not only in reducing phosphate at the soil layers, but also in maintaining low phosphate levels in the water body itself. Therefore, SMAC can be considered a promising material for applications in dynamic environments such as river beds and coastal areas influenced by waves and tides. Furthermore, if SMAC maintains phosphate concentrations within aquatic systems at stable levels, then it can contribute to the suppression of algal blooms and the improvement of water quality. Additionally, it may play a significant role in protecting aquatic ecosystems.

4. Conclusions

The current research has explored the applicability of surface-modified activated carbon to prevent phosphorus leaching from river beds as well as testing for the potential for river bed remediation. Novel approaches were taken to simulate the leaching characteristics under stirred batch simulations. Additionally, the removal of phosphate from sediment layers was also observed under hydraulic stress. The results showed high degrees of phosphorous sorption under various hydraulic stresses, indicating a promising method for algae prevention and river bed remediation. The individual results derived from the study are summarized below:

• Adsorption experiments comparing activated carbon (AC) and ferric chloride-modified activated carbon (SMAC) were conducted to evaluate phosphate removal efficiency. SMAC exhibited significantly enhanced anionic removal capacity due to surface modification, and its adsorption behavior followed the Freundlich isotherm model. The phosphate removal efficiency was positively correlated with the amount of cationic iron bound to the AC surface; however, it was found that metal-modified activated carbon exhibits an optimal modification point for maximizing adsorption efficiency, which in this study was achieved with a ferric chloride concentration of 0.2 M. The adsorption behavior of SMAC followed the Freundlich isotherm model, indicating a heterogeneous surface and multilayer adsorption mechanism.
• Applications of SMAC to river bed conditions were tested to prevent phosphate leaching. For this, field conditions were simulated to verify the effects of hydraulic flow at the soil of Chudong water area. In this study, we used stirred batch simulations to represent the turbulent flow conditions during applications. Under static conditions without hydraulic flow, SMAC exhibited approximately twice the phosphate removal efficiency compared to normal AC. Moreover, as hydraulic flow intensity increased, the effectiveness of SMAC in preventing phosphate leaching became even more pronounced, indicating its superior performance under dynamic water conditions.
• Long-term experiments were conducted to assess phosphate removal within the soil layer under hydraulic stress conditions. Phosphate concentrations in pore water were found to decrease exponentially with depth, consistent with hyporheic exchange processes enhanced by flow conditions. Additionally, the application of SMAC consistently reduced phosphate leaching over time and contributed to soil remediation, regardless of hydraulic stress. Due to its stable performance under both static and dynamic hydraulic conditions, along with its ease of synthesis and field application, SMAC shows strong potential as a versatile material for sediment contamination control in diverse aquatic environments such as river beds, lake beds, and coastal areas.
• To advance the applicability of SMAC in environmental remediation, further research is required to address several limitations of the current study. The present work did not assess competitive adsorption effects in the presence of coexisting anions such as sulfate (SO₄²⁻), nitrate (NO₃₋), and bicarbonate (HCO₃₋) under realistic water chemistry conditions. Additionally, long-term stability, reusability, and regeneration potential remain unexplored, which are essential for field deployment. Moreover, large-scale validation under diverse environmental conditions—such as variations in salinity, pH, and dissolved oxygen—is necessary to optimize SMAC’s performance for practical applications. Addressing these research needs will help establish SMAC as a reliable and robust material for sediment contamination control and water quality management.