# Application of Iron-Modified Activated Carbon for Phosphate Removal in Aqueous Systems
## 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]. Microcystn, 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 microcystn 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 microcystn [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 microcystn [8]. The issues stemming from algal blooms and microcystn 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 \(\upmu\)g/L to 17.6 \(\upmu\)g/L between 2005 and 2012, and from 13.9 \(\upmu\)g/L to 20.0 \(\upmu\)g/L between 2013 and 2016. Particularly, the Geum River exhibited the highest increase, rising from 9.3 \(\upmu\)g/L to 23.0 \(\upmu\)g/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].