In-situ Technologies for Controlling Sediment Phosphorus in Eutrophic Shallow Lakes: A Review

0. INTRODUCTION

Eutrophication has become a worldwide challenge for the management of aquatic environments. Water eutrophication is caused by the excessive enrichment of nitrogen (N), phosphorus (P), and other nutrient elements in water, which result in the rapid algae growth, outbreaks of cyanobacteria blooms, severe decline of water quality, and destruction of aquatic ecosystems (Smolders et al., 2001). P is the primary limiting nutrient in eutrophic waters. Therefore, water eutrophication can be effectively limited by controlling the concentration of P in the water (Schindler et al., 2008). The sources of P in water primarily include exogenous inputs, and internal release from sediment, with a small contribution from atmospheric deposition. Primary sources of exogenous P are chemical fertilizers, farmland irrigation, industrial wastewater and domestic sewage discharge. These exogenous sources can be specifically controlled. However, relevant studies have shown that even after effective management, the release of P from the sediment to the overlying water still leads to cyanobacterial blooms, which can last for decades (Wang and Jiang, 2016).

Sediment is the natural intersection of the hydrosphere, lithosphere, soil sphere, and biosphere, and plays a vital role in the material and energy cycle of critical zones and aquatic ecosystem, as well as in the changes in environmental quality. Various forms of P are imported into lakes via sewage discharge, surface runoff, aquatic biological remains, and groundwater input. Some of the P in water gets stored in lake sediments through sediment adsorption, chemical flocculation, and coprecipitation. The gradual enrichment of P fractions in sediments forms the internal loading of eutrophic lakes. The release of P from sediments increased in changing environmental conditions, such as temperature, pH of the overlying water, dissolved oxygen (DO), redox potential (Eh), and microbial activity, eventually resulting in eutrophication (Zhang Y et al., 2016). Research on the endogenous pollution of Taihu Lake in Jiangsu, China, found that the internal P release could reach 2–6 times the exogenous input (Qin et al., 2006). In another study, 50% of the P in Lake Okaro, New Zealand, was from sediment release (Özkundakci et al., 2011). Furthermore, 18%–88% of lake nutrients in Scotland and England were from sediment release (Carvalho et al., 2005). Therefore, controlling the P release from lake sediments is crucial for the rapid restoration of lake water quality.

Currently, sediment P control technologies mainly include ex-situ control and in-situ methods. Sediment ectopic control technology directly removes contaminated sediment as the source of pollution. Sediment dredging is used worldwide to reduce internal pollution loads in sediments (Chen et al., 2018). However, a series of negative effects of sediment dredging, including re-suspension, residuals, and release of contaminants, makes it a controversial technology for the remediation of contaminated sediments. In-situ control and remediation of contaminated sediments refers to the direct on-site treatment of the sediments without moving the contaminated sediments or changing the original state of the sediments. Furthermore, path isolation, structural degradation, and activity passivation of pollutants are treated through physical, chemical, and biological technological means. In comparison to ex-situ control technology, in-situ control technology has several significant advantages: it effectively mitigates the issue of sediment re-suspension that can occur during dredging, reduces the overall quantity of pollutants released during sediment relocation, fundamentally decreases sediment volume, diminishes pollutant toxicity, and helps manage the migration of contaminants. Additionally, this technology is often more cost-effective. Thus, sediment in-situ control technology has attracted extensive attention in recent years. In-situ sediment control techniques include physical and chemical control, ecological remediation, and combined control technologies (Figure 1). Currently, in-situ control technology has been widely used in various polluted lakes in China and abroad.

Figure  1.  In-situ technologies for sediment P control.

DownLoad: Full-Size Img PowerPoint

In-situ sediment control technology is to reduce the concentration of pollutants in sediments through physical adsorption, chemical reaction or biological methods to prevent the release of pollutants up into the overlying water. In-situ control technology can significantly reduce the P content in sediments (Fan et al., 2017). The contaminated sediment can be treated directly in-situ, which prevents the release of P into the water caused by the re-suspension of the sediment (Zhang C et al., 2016). No additional sites are occupied and no disposal facilities or equipment for monitoring dredged sediments are required. The cost of in-situ control technology is lower than ex-situ control technology.

Bibliometric analyses are useful for identifying the key aspects and hotspots of a certain research subject (Deng et al., 2021). The results of bibliometric network mapping using VOSviewer software (version 1.6.13) based on our searched publication data showed that the two themes of "sediment phosphorus remediation and adsorption in eutrophic lake" and "eutrophication control and water quality improvement" are the focus of most research (Figure 2).

Figure  2.  Bibliometric network map generated in VOSviewer using the 256 results extracted from the Web of Science database (until Dec 31, 2022) using the search terms "Sediment phosphorus control AND Remediation". The embedded sub-plot in the top left corner represents the annual number of publications on this topic.

DownLoad: Full-Size Img PowerPoint

In this review, we will re-examine the results of engineering practice and experimental research to assess the advantages and disadvantages of physical, chemical, and biological technologies for control of sediment P and discuss about the promising combined technology that synthesizes characteristics of existing technologies.

1. IN-SITU TECHNOLOGIES FOR CONTROL OF SEDIMENT P

1.1 In-situ Physical Control Technology

1.1.1   Mechanism of in-situ capping technology for control of sediment P

In-situ capping technology is commonly used for physical control of polluted sediment. This technology involves placing one or more layers of capping on the sediment to physically isolate it from the water body and to prevent or control the diffusion or migration of pollutants in the sediment to the water body. The interaction between P_ortho and the adsorbent results in the formation of surface adsorption complexes (Figure 3). The restoration of water bodies and sediments by overburden mainly includes three main functions (Libralato et al., 2018). First, the blocking effect. Material exchange between the overlying water and sediment is blocked using capping materials. This prevents sediment pollutants from migrating to the overlying water. The second function is adsorption. Porous materials with large surface areas and strong adsorption performances, such as zeolites and biochar, are used to adsorb nutrients in sediment. And the third function is degradation. Through biochemical or chemical reactions, most pollutants are quickly and efficiently degraded into harmless and non-toxic substances.

Figure  3.  Schematic diagram of in-situ capping technology.

DownLoad: Full-Size Img PowerPoint

1.1.2   Main capping material and method

The adsorption capacity of the capping material is directly related to capping effects. The most widely used materials are unpolluted lake sediments, riverine stone sand, gravel, clay, and artificial composite materials (Zhang C et al., 2016). The effects of different capping materials are closely related. Three main characteristics for selecting capping materials are: First, progressively smaller particle sizes of the capping material have stronger barrier and lower penetration capacities. Second, characteristics of organic material (OM) content, surface area, and porosity of capping materials are related to the adsorption capacity of pollutants; and Third, specific gravity or density of the capping material is related to its resistance to water disturbance and the stabilization of polluted sediment. In-situ capping layers can be divided into single- and multi-layer overlays. Single-layer capping involves the use of one capping material. Multiple capping involves layers of multiple capping materials, with small material typically in the lower layer (Lampert et al., 2011).

1.1.3   Effect of in-situ capping control technology on sediment P

In-situ capping can physically isolate all forms of P in lake sediments from benthic organisms and lake water bodies, which can effectively prevent the multichannel migration of sediment P or its resuspension in overlying water bodies. In addition, in-situ capping can reduce the diffusion flux of all forms of sediment P in lake water bodies. Various capping materials have been employed to control sediment P, of which, sand is the most straightforward capping material proposed to date for in-situ control of sediment P. Reportedly, sand capping with a thickness of < 50 mm reduced the P concentration by over 85% and was effective in reducing the amount of sediment P released (Kim and Jung, 2010). Common or modified clay materials are typically used to reduce the release of P from the sediment to the overlying water. P released from sediment could be reduced by approximately 90% and 60% by capping natural gypsum and zeolite, respectively (Yun et al., 2007). Bentonite, illite, maifanite, and zeolite as capping materials can stabilize nutrients and interrupt their release from contaminated lake sediment (Ding et al., 2018; Lin et al., 2011; Xiong et al., 2018; Zamparas and Zacharias, 2014). The 700 ºC-heated natural calcium-rich attapulgite (NCAP700) could immobilize sediment P through the transformation of mobile P to stable Ca-P (Yin and Kong, 2015). An active barrier system using calcite/zeolite mixtures can supply Ca²⁺ through Ca²⁺/NH₄⁺ exchange to improve the ability of the capping material to immobilize the P released from sediments (Lin et al., 2011). Many researchers have investigated in-situ capping engineering applications of sediment P at lake restoration sites (Table 1).

Table  1.  Summary of selected projects of in-situ capping control on sediment P

Project location	Capping materials	Treatment effects	References
Lake Epple, Germany	Rohrbach calcite and merck calcite	The P flux from the sediment was reduced 80% for at least 2–3 months. The release of sediment P was prevented for at least 7–10 months.	(Berg et al., 2004)
Lake Okaro, New Zealand	Aluminium modified zeolite (Z2G1)	Z2G1 could completely block P release from the sediment under aerobic and anoxic conditions and remove P from the overlying water.	(Gibbs and Özkundakci, 2011)
Shiba Bay, in the western Meiliang Bay in Taihu Lake, China	Local clean soils	Capping significantly reduced the internal loading of P. Low concentrations of DRP in pore waters after capping.	(Xu et al., 2012)
Hamilton Harbour, Lake Ontario, Canada	Calcareous mud	The physical capping of sediment had a weak effect on the high release of P and could not control the internal P cycle.	(Orihel et al., 2017)
The pond in the Shanghai Jiao Tong University, China	A zeolite/hydrous zirconia composite	Capping resulted in a more efficient, rapid and sustained decrease in P concentration.	(Fan et al., 2017)

 | Show Table

DownLoad: CSV

1.1.4   Advantages and disadvantages of in-situ physical control technology on sediment P

Compared with sediment dredging technology, in-situ capping control has several advantages. First, the sediment dredging technology greatly disturbs sediment and water. This accelerates the sediment release of N and P. By contrast, in-situ capping technology only covers other cleaning materials on the surface of contaminated sediment and causes little disturbance to the sediment (Zhang C et al., 2016). Second, the dredged sediment is transported to other locations for processing. When a large amount of sediment is involved, challenges in dealing with complex pollutant composition and high moisture content increase the processing costs and environmental health risks. However, the engineering cost of the in-situ capping technology is low. Third, sediment dredging during the sediment transport process sometimes results in sludge spilling or evaporation into the atmosphere, causing secondary pollution (Yu et al., 2017). The disadvantage of in-situ capping technology is that the environmental potential is relatively small. The sediment P is only in-situ blocked without removal, and there is a risk of re-release with wind and wave disturbance.

In-situ capping technology has several advantages over other in-situ processing technologies, such as in-situ chemical technology and biotechnology. First, in-situ capping is suitable for various organic and inorganic polluted sediments; thus, it can effectively control the release of N, P, and other nutrients in the sediment, as well as the release of heavy metals and persistent organic compounds that include polychlorinated biphenyls, polycyclic aromatic hydrocarbons, and phenols (Xiong et al., 2018). Second, in-situ chemical control technology requires the addition of chemicals to water, which affect aquatic organisms and the ecological environment of lake. Third, in-situ biotechnology is influenced by season and temperature, where as in-situ capping is unaffected by such changes (Xu et al., 2012).

1.1.5   Application of in-situ physical control technology on sediment P

In-situ capping is influenced by several factors, which must be considered in its application (Lin et al., 2011; Yin and Kong, 2015). First, external sources of contamination in the water body must be controlled to prevent the deposition of new contaminated sediment on the capping. Second, the technology must be considered only when the toxicity and mobility of sediment contaminants are low. Third, readily available materials suitable for capping contaminated sediments in water must be utilized. Fourth, on-field conditions of the water area, such as the possibility of flooding, extreme wind, and waves, must be monitored to ensure that they do not influence the capping effect. Fifth, the sediment should support the capping weight. Sixth, there should be no impact on present and future infrastructure development of buildings and waterways. Additionally, this technology is unsuitable for building piers, laying pipes, and other functions. Finally, when the site conditions are unsuitable for dredging all the sediment due to prerequisite of optimum water depth or hydraulic power, combining with capping should be considered.

1.2 In-situ Chemical Control Technology on Sediment P

In-situ chemical control technology refers to the addition of specific chemical or enzyme agents to contaminated sediment, which undergo a directional complexation reaction with nutrients in the sediment to produce stable binding compounds that can inhibit the release of P. Owing to the different chemical and enzyme preparations, in-situ chemical control mainly includes redox and chemical passivation technologies (Zamparas and Zacharias, 2014). In-situ redox technology involves the addition of chemical agents to eutrophic waterbodies. Chemical agents react with various forms of P in sediments and change the form of P or produce other substances that are difficult to degrade. Some newly generated substances also provide favorable conditions for subsequent microbial degradation. In-situ chemical inactivation of sediments is widely used to control the internal P loading in lakes because it often works by inactivating P. Oxygen nano-bubble modified mineral (ONBMM) technology reduced the total P (TP), NH₃-N, and total nitrogen (TN) loading from sediments in simulation to 96.4%, 51.1%, and 24.9%, respectively (Wang et al., 2020). In-situ chemical inactivation technology is vital for controlling internal P pollution in lake sediments. This technology makes active P in water and sediment inert by adding materials to reduce the amount of P released from sediments to the overlying water (Gibbs et al., 2011).

1.2.1   Mechanism of In-situ Chemical Inactivation Control Technology on Sediment P

There are three dominant mechanisms of P fixation by passivated materials: chemical precipitation, adsorption and coprecipitation (Figure 4). Because the same material often contains different fixing mechanisms when passivating lake sediment, it is difficult to distinguish the differences between the three inactivation mechanisms of P (Figure 5).

Figure  4.  P immobilization by inactivation materials (Kelly Vargas and Qi, 2019).

DownLoad: Full-Size Img PowerPoint

Figure  5.  Schematic diagram of in-situ inactivation control technology.

DownLoad: Full-Size Img PowerPoint

1.2.1.1   Chemical precipitation

Ferric ions (Fe³⁺) are strongly hydrolyzed by dissolution and water absorption to form various complexes (Gong and Zhao, 2014). These complexes polymerize with PO₄^3- in the water to form Fe and P, and precipitate in lake sediment, thus removing PO₄^3- from the water.

The effects of adding aluminum salt includes strong electric neutralization of Al(OH)₃ colloidal substances in water, bridging adsorption of Al³⁺ hydrolysates to suspended solids in water; Al(OH)₃ colloids adsorption and precipitation of dissolved active P and suspended matter in overlying water to reduce P content and increase water transparency. Furthermore, it forms a masking layer on the sediment surface to reduce the release of active P from the sediment to overlying water by physical barriers and chemisorption. The precipitation reaction is performed by adding potash-alum (KAl(SO₄)₂·12H₂O) to water (Gong and Zhao, 2014). The formation of aluminum phosphate is superior to that of Al(OH)₃ in terms of thermodynamics and kinetics (Gong and Zhao, 2014).

Coprecipitation and adsorption are the two main mechanisms of calcium salt fixation of PO₄^3-. Calcium and P mainly exist in the extractable form of hydrochloric acid, which is inert, stable, and not easily released from the sediment. Thermally modified calcium-rich, attapulgite-based, thin-layer cap can reduce internal sediment P loading from eutrophic lakes (Gong and Zhao, 2014). When a calcium-containing passivation material is added to the sediment, excess calcium ions (Ca²⁺) adsorb PO₄^3- from the sediment and form a hydroxyapatite (Ca₅(PO₄)₃(OH)) precipitate (the most stable form of calcium phosphate) (Gong and Zhao, 2014).

In addition, lanthanum (La) supported on lanthanum modified bentonite can chemically precipitate with PO₄^3- in water to form insoluble and stable compounds, such as LaPO₄·nH₂O, to achieve dephosphorization.

1.2.1.2   Adsorption

Adsorption refers to the preferential and selective adsorption of passivated materials to polar substances. Unlike chemical precipitation, adsorption does not require formation of new substances (Kelly Vargas and Qi, 2019). The central atom receives a lone pair from the ligand as a Lewis acid. La³⁺, Ca²⁺, Al³⁺ and other metal cations on the surface of clay minerals have Lewis acid properties. These metal cations exhibit strong ligand adsorption to HPO₄^2- and H₂PO₄^- in water through a Lewis acid-base interaction. The metal cation is a Lewis acid, which is an electron pair acceptor that selectively binds to a Lewis base. PO₄^3- is a Lewis base that provides electron pairs and forms the inner layer complex by binding to the central atom of the metal oxide. Bentonite modified with lanthanum adsorbs PO₄^3- in water by anion coordination exchange, and hydroxyl (OH^-) and hydration functional groups are present on its surface (Acelas et al., 2015). La³⁺ and Ca²⁺ ions on the bentonite surface have a strong adsorption capacity for HPO₄^2- in water and can be combined with an oxygen bridge (La/Ca-O-P). HPO₄^2- can exchange ions with OH^- to remove P from water. Hydrated iron oxide (HFeO) and hydrated zirconia (HZrO) are highly selective for ligands. Since these hydration oxides can only form outer complexes with other anions, such as SO₄^2-, Cl^-, NO₃^-, and others, through coulomb interaction, HFeO and HZrO selectively adsorb PO₄^3- without interference in water bodies where many anions exist (Smolders et al., 2001).

1.2.1.3   Coprecipitation

Coprecipitation is a process associated with chemical precipitation in which the precipitate can capture ions existing in the solution, rather than the supersaturated ions being precipitated. When precipitates exit a solution, some soluble substances adsorbed on the precipitate surface are unable to partition back into solution and are covered by more precipitate. When Ca²⁺ is added to a solution containing carbonate, coprecipitation of phosphate occurs, and PO₄^3- is captured by the resulting CaCO₃ precipitation (Dittrich et al., 2011).

1.2.2   Evaluation of inactivation materials on sediment P

The development and application of inactivation materials is critical for in-situ chemical inactivation technology. Commonly used passivation materials include metal salts (e.g., Fe, Al, and Ca) and modified clays. The choice of passivation material is complicated as it must consider the economic benefits, service life, and ecological security of the passivation agent itself, as well as the appropriate lake type (Smolders et al., 2001).

1.2.2.1   Metal salts

Iron salts are a critical group of chemicals in the control of internal P. Fe³⁺ in iron salts hydrolyzes to form Fe(OH)₃ in water and sediments. Fe(OH)₃ reduces dissolved P in water via surface adsorption and forms a small amount of FePO₄ with PO₄^3- (Ding et al., 2018). Simultaneously, a micro-oxide layer formed on the sediment surface to prevent the release of P from the sediment. Common iron salts include ferric chloride (FeCl₃) and ferric sulfate (Fe₂(SO₄)₃). Ferric salts are more suitable for acidic water with a pH of 4.5–5.0. An appropriate dosage (≥ 200 g/m²) of iron salt (Fe(OH)₃ or FeCl₃) applied to eutrophic water can effectively control P loading in lakes for a long time (Smolders et al., 2001). Fe³⁺ are strongly hydrolyzed through dissolution and water absorption to form various complexes. The release of phosphate appears to increase strongly when Fe : PO₃^4- ratio is less than unity. This indicates that lake aquatic plant communities can recover under iron salt treatment (Smolders et al., 2001). However, this favorable treatment effect is often affected by the water redox potential. One of the main disadvantages of using iron salt to treat lake eutrophication is the redox sensitivity of iron (Smolders et al., 2001). Under anoxic conditions resulting from the bacterial decomposition of organic matter (OM), the dissolution of ferric ions diminishes, facilitating the secondary release of iron-bound phosphorus (Fe-P) from the sediment (Wang et al., 2016). This process underscores the intricate interplay between biochemical transformations and sediment phosphorus dynamics (Kleeberg et al., 2013). Therefore, a low redox potential substantially reduces the treatment effects of Fe³⁺ salts. Fe salt also has some toxic effects on the water ecological environment. A high dose of FeCl₃ may reduce the alkalinity of water, increase the chloride concentration in water, and harm aquatic ecosystems. However, the toxic effect of FeCl₃ is not as pronounced as that of aluminum chloride (AlCl₃).

Al salt treatment was one of the earliest methods used to reduce the internal P loading in lakes. Common Al salts, including aluminum sulfate (Al₂(SO₄)₃), alum (XAl(SO₄)₂∙12H₂O), and polyaluminum chloride, have been used to deactivate sediment P, thus reducing the P loading in lakes (Huser et al., 2016a; Lu et al., 2013). The Al salt passivating agent exhibit the best passivating effect when the pH is 6–8. Al₂(SO₄)₃ converts weakly bound P and Fe-P to Al-P, resulting in a 53% reduction in the removable P in the sediment (Lin et al., 2017). Proper XAl(SO₄)₂∙12H₂O treatment can reduce sediment P loading by 70%–85%, and reduce the frequency and density of algal blooms (Brattebo et al., 2017). The application of aluminum-modified clay can effectively inhibit the release of P from the sediments and remarkably improve the water qualiy. High doses of Al salt inhibit the growth of plankton and submerged plants in water, and have toxic effects on aquatic ecosystems (Huser et al., 2016a; Reitzel et al., 2005). Therefore, the dosage of Al salt in water treatment is a crucial factor. In most cases, the calculated dosage of Al salt needs to be added multiple times, based on the fluctuation of the lake morphology, to ensure maximum passivation efficiency. Batch dosing can change the dosing strategy by transforming P in water and avoiding the ecological risk caused by excessive dosage of a single dose (Kuster et al., 2020).

Ca salts are essential passivating materials for sediments. Ca salts that are commonly used to control internal P loading in lakes include calcium chloride (CaCl₂), calcium nitrate (Ca(NO₃)₂), calcium peroxide (CaO₂), and lime (CaCO₃/Ca(OH)₂. Ca and Ca salts in water can form hydroxyapatite (Ca₅(PO₄)₃(OH)) and calcium carbonate (CaCO₃). The solubility of hydroxyapatite (Ca₅(PO₄)₃(OH)) and CaCO₃ precipitates is small at pH ≥ 9.5. Thus, Ca and Ca salts are suitable for alkaline hard-water lakes. However, when the carbon dioxide (CO₂) content in the aqueous solution increases or the pH value decreases, the dissolution of CaCO₃ and Ca₅(PO₄)₃(OH) increase, and the adsorbed P will be released into the water. Ca salts have a strong inhibitory effect on the release of active P from sediment. Under natural conditions, calcium and P are difficult to release into water, and can effectively inhibit the release of other forms of P under reducing conditions, such as hypoxia (Zhou et al., 2019; Gong and Zhao, 2014).

1.2.2.2   Modified clay

Modified clay, as a P inactivation material, can safely and effectively solve the problem of P loading in lakes (Kelly Vargas and Qi, 2019; Wang and Jiang, 2016). Common modified clay materials, including attapulgite, bentonite, and zeolite, have been used to passivate sediment P (Liu et al., 2022a, 2017; Yin et al., 2018, 2017). The application of modified clay minerals in P passivation technology is mainly based on the minerals' surface adsorption, ion exchange, and interlayer adsorption properties. These clay minerals can reduce the P content in water, settle on the surface of sediments to form a capping layer, and prevent the release of P from the sediments. The Phoslock^® form of La-modified bentonite is based on the modification of Ca-based bentonite materials to acquire P products (Lin et al., 2016). The maximum P adsorption capacity of Phoslock^® is 9.06 mg/g (Li et al., 2020). The passivation efficiency of Phoslock^® is the highest when the pH value is 5–7 and substantially decreases at pH ≥ 9. In shallow lakes with high temperatures and oxygen deficiencies in summer, algal blooms often result in a higher pH in the water. Therefore, Phoslock^® alone cannot be used in such an environment. The most suitable time for the passivation of Phoslock^® is winter in temperate regions, with minimal side effects on the aquatic biota (Li et al., 2020). Free La³⁺ may have toxic effects on aquatic ecosystems. As La is easily dissolved under acidic conditions, Phoslock^® is unsuitable for water bodies with low pH values.

1.2.3   Effect of in-situ chemical control technology on sediment P

In-situ chemical control technologies have been widely used for endogenous sediment remediation. The optimal dosage of Ca(OH)₂ as a P passivator in lakes is approximately 10 mg Ca/L and that in ponds is between 27 and 135 mg Ca/L. An increase in Ca(OH)₂ dosage can lead to increased pH of the water body. For most lakes, increased pH causes the release of Fe, manganese (Mn), and Al combined with P, which harms aquatic organisms. Therefore, Ca(OH)₂ can only be used at low doses. An Al modified zeolite (Z2G1) coating with a thickness of 2 mm has been used to effectively control the release of internal P under different oxygen content conditions and effectively reduce the P content in water. Z2G1 exhibits a synergistic purification effect, removing NH₃-N and heavy metals. 110 t (350 g/m²) of Z2G1 was applied to the entire Lake Okaro, New Zealand, using a lower water detention belt and passivation agent. This in-situ control technology reduced the release of P (38 mg/(m² d)) and NH₃-N (212 mg/(m²d)) (Gibbs et al., 2011). Zirconium modified zeolites also exhibit good passivation. The maximum single-layer P adsorption capacity of zirconium modified zeolite is 10.2 mg/g at 25 ºC and a water pH of 7 (Yang et al., 2015). Zirconium modified zeolites can reduce the concentration of soluble reactive P in pore water, the concentrations of reactive and bioavailable P in sediments, and the release of reactive P under hypoxic conditions. P-inactivated solid-phase materials (PISMs) are effective at controlling the release of endogenous P into overlying water (Xia et al., 2023; Zhang et al., 2023). Iron-based PISMs (Fe-PISMs) have also attracted considerable attention due to their high phosphate adsorption ability, environmentally friendly nature, and inexpensive cost (Lin et al., 2023a; Xia et al., 2023; Zhang et al., 2023). Many researchers have investigated in-situ chemical engineering applications for control of sediment P at lake restoration sites (Table 2).

Table  2.  Summary of selected projects of in-situ chemical control on sediment P

Project location	Materials	Treatment effects	References
Lake Long, New Brighton	Liquid calcium nitrate (LCN)	LCN could eliminate virtually all P release from the sediments, and result in the sediments becoming a sink for P in the water column.	(Willenbring et al., 1984)
Frisken Lake, British Columbia	Calcium hydroxide	Adding CH precipitated more than 89%–96% of the SRP.	(Murphy et al., 1988)
Lake Xiapu, Japan	Calcium nitrate	Sediment P was reduced by 79%.	(Mikuniya Corporation, 1984)
Foxcote Reservoir, England	Ferric sulphate	Dissolved phosphorus values declined after ferric sulphate dosing	(Daldorph, 1999)
Bautzen Reservoir, Germany	Fe salts	The SRP contents in the whole water body dropped by 72% and 54%, respectively, while the TP contents dropped by 45% at each period.	(Deppe and Benndorf, 2002)
Lake Dagowsee, Germany	Nitrate storage compound	The release of P from the anoxic sediments was completely suppressed even one year.	(Wauer et al., 2005)
Lake Terra Nov, Netherlands	FeCl3	The majority of the added Fe is still undergoing redox cycling within the top 10 cm of sediment accounting for the binding of up to 70% of sedimentary P.	(Münch et al., 2024)

 | Show Table

DownLoad: CSV

1.2.4   Advantages and disadvantages of in-situ chemical control technology on sediment P

Given the importance of controlling external pollution of lakes, treatment of the internal pollution of lake sediments has become crucial for improving water quality in lakes. In-situ chemical control technology has played an essential role in treating the internal pollution of eutrophic lake sediments (Huser et al., 2016b). Currently, commonly used in-situ chemical passivating agents include Al, Fe, and Ca salt, and clay minerals. Various passivating agents have advantages, disadvantages, and suitable application conditions.

The use of Al salt (Al₂ (SO₄)₃) as a passivating agent has the following four advantages. First, the inactivation effect of Al₂(SO₄)₃ on P is stable and not affected by the redox potential. Second, the inactivation effect on sediment P is good. Third, the effective time is long. The adequate period can reach 10–15 years. Fourth, the control cost is low.

The disadvantage of using Al³⁺ salt as a passivating agent is that it could increase the content of Al³⁺ in the water body, which can cause chronic Al poisoning, reduce the diversity of organisms, and cause gill deformity and even death of fish. Since most passivated materials contain metallic substances, the harm caused by sediment resuspension after passivation is unpredictable. When applying Al salts to shallow lakes, the risk of secondary suspension must be low to allow 2–4 months for floc stabilization. Resuspension could lead to an increase in pH and Al concentrations in water, which is toxic to aquatic life. Excessive aluminum hydroxide (Al(OH)₃) flocs suspended in water cover the surface of animal cells for a long time, resulting in the distortion and death of aquatic animals, and eventually resulting in the imbalance and deterioration of the aquatic ecosystem (Egemose et al., 2009). Fe³⁺ and Ca²⁺ salts are nontoxic passivating agents. However, Fe³⁺ salts are easily affected by redox conditions and pH values, whereas Ca²⁺ salts increase the pH of water, limiting their applications.

Modified clay minerals as passivating agents can reduce the cost of sediment restoration and effectively control the release of sediment N, P, and some heavy metals. These minerals do not cause secondary pollution in the low-dose range and are the first choice as P-passivating agents. However, clay minerals are rarely used in endogenous lake pollution control, and their passivation effects and ecological and environmental impacts need to be further studied and verified.

1.2.5   Comparison of materials and feasibility for improvement

Overall, new and improved materials can be developed to control sediment P loading. From an economic and performance point of view, there are currently no materials that can be applied for the wide range of complex circumstances requiring a solution for internal loading. While metal salts seem to be more limited to a narrower range of physic-chemical conditions, modified clays could benefit from a more economical option. The cost of chemical treatments was presented in Figure 6. The application of 17.5 tons of Phoslock^® (a La-modified bentonite) over a 6-hectare lake (a relatively low dose), places the chemical cost at nearly 6 000 USD/h (Epe et al., 2017). A dose of 278 g/m² would be required to completely prevent P_ortho release from the sediment (Gibbs et al., 2011). At this dose, the cost of Phoslock^® treatment per hectare would increase up to $9 000 USD. According to Blazquez-Pall, to remove 35% of the TP in lake sediments, one kilogram of P would require roughly $1 400 USD of Phoslock^® material. This is 14 times higher than what was estimated for aluminum sulfate, and more than 10 times what has been established as a sustainable target by the George Barley Water Prize Competition.

Figure  6.  Cost analysis for the practice of chemical addition including a bar chart comparing total costs and a pie chart of the total cost breakdown (Kelly Vargas and Qi, 2019). 950 km was considered as the distance for transporting the chemical from the industrial source to the treatment sit (Prepas et al., 2001). A chemical transportation cost of 6 cents per ton per mile was chosen as per a 2018 report from the Association of American Railroads (Association of American Railroads, 2018). The total cost of the chemical was taken from average USD/ha values applied to a lake of 70 ha. The application costs also considered a 70 ha lake and the average cost required to treat this area with either injection systems or barge/weed harvesters (Moore et al., 2009).

DownLoad: Full-Size Img PowerPoint

1.2.6   Application of in-situ chemical control technology on sediment P

Deepwater lakes have a high pollution load capacity and strong stability. The water body volume and the restoration environmental ability of shallow lakes are small. Thus, cyanobacterial blooms can easily occur under conditions of high temperature and anoxia in summer. In shallow lakes, surface sediments are often disturbed by wind. Therefore, disturbance is a key factor limiting the application of in-situ P control technology in sediments. The effect of both moderate (5.1 m/s) and strong winds (8.7 m/s) on the stability of sediment treated by two geoengineering materials, Phoslock^® and thermally treated Ca-rich attapulgite, in a sediment resuspension generating system was investigated. The passivation efficiencies of the two modified clay materials decreased with an increase in sediment resuspension time. Both modified clay materials reduced the mobile P concentration in surface sediments (0–4 cm), and reduced the depth of sediment erosion by wind, and the P flux at the cement interface during sediment resuspension (Yin et al., 2016). Frequent wind disturbances gradually erode the surface sediments, making the sediments of shallow lakes very active. In Lake Taihu in eastern China, the summer monsoon suspended surface sediments back into the light zone with a high pH value (9–10), resulting in drastic changes in the concentration of suspended matter in the overlying water and secondary release of P (Deng et al., 2018). In recent decades, the annual average wind speed in Lake Taihu has substantially declined. The decrease in high and medium wind speeds was conducive to maintaining the steady state of the cement interface. Long-term low wind speeds may cause a reduction in the sediment surface dissolved oxygen concentration, promote the release of sediment P, and improve the difficulty of passivating sediment. Thus, attention should be paid to the number of passivation materials and their applications in passivation projects.

The application of in-situ chemical inactivation P control technology has obvious regional limitations due to the many factors affecting sediment P control via in-situ passivation. For example, large shallow eutrophic, deepwater, and urban lakes have notable differences in many limnological characteristics, such as water surface area, water depth, disturbance intensity, and exogenous input intensity. Therefore, applying in-situ chemical inactivation P control technology to sediments will completely achieve other internal P control effects in the different aforementioned types of lakes (Yuan et al., 2023; Deng et al., 2018). Generally, in-situ chemical inactivation technology for sediment P suits lakes with reasonable external control, weak hydrodynamic disturbances, and small water areas.

1.3 In-situ Ecological Remediation Control Technology on Sediment P

The ecological remediation of contaminated sediments is mainly based on the oxidative metabolism, absorption, and enrichment of pollutants by microorganisms and phytoremediation. Microbial remediation is usually performed by injecting specific microbial agents into contaminated sediments to promote the degradation or morphological transformation of contaminants (Horppila and Nurminen, 2003). As a common microbial in-situ remediation technology, the selection, delivery, long-term performance, and high efficiency of functional bacteria are often the core economic indicators. Phytoremediation is the most commonly used technology for sediment bioremediation (Xu et al., 2019). Using the implantation of large aquatic plants (mainly submerged plants) on sediments, utilization of N and P, and the enrichment of heavy metals in sediments are realized through the absorption of pollutants by the roots, stems, and leaves of plants.

1.3.1   Mechanism of phytoremediation control on sediment P

Sediment nutrients can promote plant growth and diversity of sediment microorganisms. Harvesting submerged plants can remove nutrients from aquatic environments. The mechanism of submerged phytoremediation is illustrated in Figure 7. The N and P required for the growth of aquatic plants are mainly absorbed by the roots of sediments, which leads to a reduction in the nutrient content of rhizosphere sediments (Han et al., 2018; Wu et al., 2018). Submerged plants can enhance microbial activity and promote nutrient salt mineralization (Wang et al., 2017). The roots of submerged plants provide an attachment environment for the mass reproduction of microorganisms. Rhizosphere oxygen can change the local redox state of sediments. Root secretions provide rich nutrients for rhizosphere microorganisms, affecting their metabolic activities and composition in sediments (Wang L Z et al., 2012). Some studies have found that submerged plants can increase alkaline phosphatase activity (APA) and decrease NaOH-P and organic P (OP) contents. These results indicated the relative importance of submerged plants in the release and transformation of sediment P in eutrophic lakes (Tang et al., 2015). Submerged plants can improve the aquatic environment and deactivate nutrients. Sufficient DO in submerged plant roots and other oxidizing substances secreted by roots, such as oxidase, make the redox potential of the rhizosphere microenvironment relatively high, which can promote metal ions from a reduced to an oxidizing state, and substantially increase the adsorption of P by sediments (Miretzky et al., 2004). pH is also an essential factor affecting sediment P release. Under neutral conditions, P bonds easily with metals in sediments and is adsorbed by the sediments. A pH value that is too high or too low can promote P release from sediments. During growth, submerged plants can promote the mineralization cycle and change the microbial community structure through oxygen secretion in the roots. This can buffer the change in sediment pH and maintain the sediment pH within a neutral range, which can reduce the release of sediment P (Zhu et al., 2015). The shoots and rhizomes of submerged plants release oxygen that oxidizes surface sediments and reduces the release rate of sediment P (Horppila and Nurminen, 2003). Submerged plants can slow water flow and wind, and use developed root systems to form a barrier at the sediment-water interface, inhibiting sediment suspension and nutrient release (Grisé et al., 1986). In addition, plant growth can substantially improve sediment density, reduce sediment moisture content, effectively improve the flow state of surface sediments, and inhibit sediment suspension (Horppila and Nurminen, 2005).

Figure  7.  Schematic diagram of submerged phytoremediation.

DownLoad: Full-Size Img PowerPoint

1.3.2   Effect of phytoremediation control on sediment P

Phytoremediation is widely used to control sediment P. Many types of plants can be used for phytoremediation. The most common plant is Vallisneria natans (V. natans) that can effectively capture pollutants and reproduce in many types of water environments owing to its strong survival ability. Therefore, V. natans has been cultivated in many locations in China for water environment restoration.For shallow lakes, sediment resuspension caused by wind and waves is the main factor leading to increased P release fluxes in sediments. Field investigations showed that the change of TP from the sediments in the nonvegetative area could reach 15–1 200 mg·m^-2·d^-1 under wind and wave conditions (Wang et al., 2018; Horppila and Nurminen, 2003). Because of the large amounts of granular nutrients entering the water, the TP fluxes are two to four orders of magnitude higher than those under static conditions. Submerged plants can effectively control the resuspension of sediments and reduce the TP flux to 5.0–140.9 mg·m^-2·d^-1 under wind and wave conditions, and the control rate of P flux under wind and wave disturbance can at least reach more than 50%. Submerged plant biomass has a significant impact on sediment resuspension. When the plant volume inhabited exceeds 30%, sediment resuspension can be significantly reduced (Wang et al., 2018). Submerged plants can remove a certain content of sediment P and inhibit the release of sediment P (Zhou et al., 2000). Removing P from sediments by submerged plants is a relatively slow process that occurs along with the plant growth process. This process influences the physical and chemical environment of the sediments. Many researchers have investigated in-situ ecological control engineering applications of sediment P in laboratory settings or lake restoration sites (Table 3).

Table  3.  Summary of selected studies and projects of in-situ ecological control on sediment P

Project Location	Submerged plants	Treatment effects	References
Lake Donghu, China	V. nantans and Potamogeton crispus	The macrophytes decreased the concentration of orthophosphate, coupled with the decreasing function of OP hydrolysis.	(Zhou et al., 2000)
Lake Hiidenvesi, Finland	Ranunculus circinatus, Ceratophyllum demersum and Potamogeton obtusifolius	Submerged macrophytes reduced sediment resuspension, water turbidity and sediment internal P loading.	(Horppila and Nurminen, 2005, 2003)
Lake Memphremagog, Canada	Submerged macrophyte beds: Myriophyllum spicatum, Potamogeton spp., and Elodea canadensis.	Macrophyte beds served as important sediment traps, and but that a large portion of initially sediment-associated P is subsequently lost to the open water.	(Rooney et al., 2003)
River Spree, Germany	Sagittaria sagittifolia, Nuphar lutea and Potamogeton pectinatus	N and P were retained by deposition by 2.5% and 12.2%, respectively.	(Schulz et al., 2003)
Wuliangsuhai Lake, China	Potamogeton pectinatus, and Phragmites australis	Although rooted macrophytes could directly uptake P from sediments, it is responsible for increasing the internal P loading especially OP by reducing current velocities and generating organic residue.	(Liu et al., 2007)
Lake Schlachtensee and Lake Tegel, Germany	Najas marina subsp. intermedia, Myriophyllum spicatum, Chara contraria, Potamogeton pectinatus, and Potamogeton berchtoldii	Submerged macrophyte could reduce TP concentrations in lakes, eventually resulting in increased water transparencies.	(Hilt et al., 2010)
Lake Gucheng, China	E P. crispus, P. maackianus, and V. natans.	Submerged macrophytes could decrease the concentrations of all P fractions. The decreasing order of P fractions was IP > HCl-P > NaOH-P > OP > BD-P > NH4Cl-P.	(Wang et al., 2012)
River Vasse, Australia	V. natans	High plant density may have reduced oxygenation by restricting mixing and benthic primary production, causing P release.	(Paice et al., 2016)
Lake Dongpo, China	V. natans	Water quality was substantially improved, and nutrient concentrations, particularly TP, TN and Chl-a were significantly reduced.	(Chen et al., 2020)
Lake Datong, China	Vallisneria denseserrulata Makino and Hydrilla verticillata	Half of the P content in overlying water and sediments, particularly dissolved P in overlying water and Ca-P in sediments, was removed after restoration.	(Li et al., 2021)
Lake Datong, China	V. denseserrulata, M. spicatum, H. verticillate, and V. spinulosa	Recovery of plants reduced the P content of the primary pollutant in sediment and water.	(Chao et al., 2021)
Lake Qin, China	V. natans	V. natans growth reduced the TP and OM contents of the sediments and increased the bioavailable iron (Fe) and Fe-bound P contents.	(Wang et al., 2021)

 | Show Table

DownLoad: CSV

1.3.3   Advantages and disadvantages of in-situ ecological remediation technology

The use of in-situ ecological remediation technology to control internal P loading has several advantages. First is the low cost of ecological restoration projects. Biological organisms have a strong reproductive ability and can grow in large quantities when the environment is suitable, thus reducing the cost of control. Second is the low risks of secondary pollution and ecological risks associated with ecological remediation. The natural purification of organisms does not damage ecosystems. Although the decomposition of plant residues will release a certain amount of C, N, and P into the water body short-term, the biomass and community structure of submerged plants can be effectively regulated by adjusting the water level change, releasing herbivorous fish, artificial harvesting, and other measures, which ultimately leads to the formation of a stable and benign ecosystem. Dredging can lead to secondary pollution and damage to sediment habitats (Yin et al., 2021). Chemical passivation may lead to the abnormal release of secondary pollution and other pollutants, whereas sediment mulching has problems that include failure to remove pollutants and destruction of sediment capping over time. The third advantage is the substantial persisting effect of ecological restoration is substantial, which helps in restoring aquatic ecosystems. Submerged plant restoration can control internal P loading, improve water quality, and provide a site for other aquatic organisms to reproduce and grow, thereby directly promoting the restoration of the water ecosystem. Fourth, microbial remediation has a certain degree of persistence compared with other remediation technologies. Due to the characteristics of bacteria themselves, specific bacteria can only act on specific substances. Therefore, different colonies can come together to address pollutants. Finally, many types of microorganisms in water may promote or inhibit the degradation of complex pollutants.

However, ecological remediation to control internal P loading also has certain limitations. First, submerged plants can effectively reduce and control sediment internal P loading, but excessive nutrients stress the growth of submerged plants (Zhang et al., 2013). Thus, submerged plant restoration technology is unsuitable for the restoration and treatment of heavily polluted sediments. Second, in addition to sediment conditions, restoration of submerged plants is also affected by various environmental factors, such as light, hydrological conditions, water temperature and aquatic organisms, which substantially limit the application range of submerged plant restoration technology. This technology has broad application prospects in closed and semi-closed water bodies that include shallow lakes, urban river channels, and landscape water bodies. Third, owing to slow plant growth and microbial reproduction, ecological control and repair still have the disadvantages of long cycles and slow effects. Submerged plant restoration technology mainly removes nutrients or inhibits their release through the growth, propagation, and absorption of submerged plants. The treatment period is longer than that of dredging, sediment passivation, or sediment mulching techniques.

1.3.4   Application of in-situ ecological remediation control technology on sediment P

When using submerged plants to control the internal P loading in lakes, plant combinations should be rationally matched on spatial and temporal scales to exert the best control effect. Mosaic planting of different growth types of submerged plants has been used to form an up-middle-down vertical hierarchy in space (Li et al., 2021). This can improve the biomass per unit area and ecosystem diversity. Over time, a reasonable allocation of various submerged plants in different growing seasons can ensure the continuous purification effect of these plants. The submerged plants are harvested in a planned and reasonable way to transfer N and P nutrients and to strengthen the plants purification effect. Simultaneously, submerged plant recovery technology can be integrated with other technologies to achieve better long-term control. In-situ inactivation, capping, and dredging technologies can be combined with submerged plant restoration techniques. Combined technologies can improve the effect of N and P release control on sediments and promote the ecological restoration of water bodies (Fang et al., 2016).

1.4 In-situ Combined Technology of Physicochemical and Ecological Restoration Technique on Sediment P

1.4.1   Mechanism of combined remediation technologies on sediment P

In-situ combined remediation technology of contaminated sediment refers to technology that combines in-situ physical, chemical, and biological remediation technologies. This approach mainly involves the use of natural clay minerals and modified materials that cover heavily polluted lake sediments, controls the release of endogenous pollutants, and promote colonization and growth of submerged plants (Wang et al., 2018). The mechanism of the combined technology is illustrated in Figure 8. Clay mineral materials can absorb nutrients, such as N and P, in sediment and water, and provide nutrients for submerged plants at the early stage of growth while repairing sediment in-situ. Moreover, clay mineral materials can slowly release many major and trace elements required by plants and microorganisms; thus, substantially promoting the colonization, growth, and propagation of submerged plants. This can effectively solve the difficulty of submerged plants to take roots and float during the early stages of lake ecological restoration (Liu et al., 2022b). The application of clay mineral materials has shown that biofilm and rhizosphere microorganisms of submerged plants can degrade mineralized OM, change the redox potential of the sediment habitat, promote the metabolism of anaerobic bacteria in sediment, improve the lake sediment environment, and accelerate the recovery and construction of a healthy ecosystem (Liu et al., 2020). During the process of the combined removal of P from sediments by clay mineral materials and submerged plants, there is an interaction that is beneficial to the removal of P from sediments. Clay mineral materials can provide excellent propagation conditions for plant rhizosphere microorganisms and improve the sediment microenvironment.

Figure  8.  Schematic diagram of combined technology approach (Liu et al., 2022b).

DownLoad: Full-Size Img PowerPoint

1.4.2   Effect of combined control remediation technology on sediment P

There are certain limitations to the use of submerged plants or capping materials alone. The growth of submerged plants is highly dependent on environmental conditions. A polluted water body or sediment may lead to stunted growth and even damage of submerged plants. Capping materials have a limited adsorption capacity for pollutants in the environment, and it is difficult to achieve in-situ regeneration or artificial replacement when adsorption is saturated. Therefore, technologies combining material capping and submerged plants have received increasing attention. The combined use of technologies for sediment P remediation overcomes the technical bottlenecks of a single technology and dramatically improves the removal efficiency of P. Several studies have demonstrated the greater synergistic removal capacity of sediment P using capping materials coupled with submerged macrophytes than either technology alone (Wei et al., 2023; Wang et al., 2022; Yin et al., 2021). Combined technologies can enhance the microbial diversity and abundance of sediments (Wang S R et al., 2012), promote growth of submerged macrophytes (Zhang et al., 2015), and enhance adsorption of sediment P (Li et al., 2017). Calcium silicate hydrate (CSH) coupled with aquatic plants can inhibit the release of sediment P (Li et al., 2017). Submerged plants can take up bioavailable P as a nutrient, thereby reducing P in sediment (Yang et al., 2018). V. natans combined with iron-oxidizing bacteria (IOB) can decrease sediment TP and Fe-P concentrations (Wang et al., 2022). In-situ combined remediation engineering applications for control of sediment P have been robustly investigated in laboratory studies and lake restoration sites (Table 4).

Table  4.  Summary of selected studies and projects in-situ combined control on sediment P

Sediment location	Combined technology	Treatment effects	References
Lake Yuyuantan, China	OM and submerged macrophytes	Sediment TP and P fraction decreased with increasing incubation time, but the decrease of sediment TP was mainly from NaOH-P and OP, but not from HCl-P and IP.	(Wang S R et al., 2012)
Lake Donghu, Wuhan, China	The porous ceramic filter media (PCFM), V. natans, and Potamogeton crispus	The combined technology could achieve a synergetic sediment P removal. The removal rates of the combinations were higher than the sum of that of PCFM and macrophytes used separately.	(Zhang et al., 2015)
Lake Hanze, China	Calcium silicate hydrates (CSH) coupled with Myriophyllum spicatum	The amount of P released to the overlying water was inhibited by 90%. The amount of P in plants with CSH was 1.76 times higher than in plants grown in the sediment without CSH.	(Li et al., 2017)
Lake Nanhu, China	Periphyton biofilm and V. natans	The average daily TN and TP removal rates were 32.6% and 35.4%, respectively, by the periphyton biofilm and V. natans.	(Yang et al., 2018)
Lake Taihu, China	Phosphate-solubilizing bacterium (PSB) strain XMT-5 (Rhizobium sp.) and C. demersum	The sediment TP concentrations in the combined treatments were significantly lower, and the TP contents of the C. demersum were all significantly higher than in other treatments.	(Li H F et al., 2018)
West Lake, China	Sediment microbial fuel cells (SMFCs) and V. natans	The combined treatment decreased the sediment P level. The electrogenesis bacteria achieved stronger P adsorption.	(Xu et al., 2018)
West Lake, China	Clay, PCFM, modified bentonite granular, V. natans, and C. demersum	The ecological restoration project decreased sediment TP and OM, increased submerged macrophyte biomass and sediment microbial diversity in the restored area.	(Liu et al., 2022b)
Lake Taihu, China	V. natans and a lanthanum-modified bentonite (Phoslock®)	The combined treatment led to stronger improvement in water quality and a more pronounced reduction of porewater SRP than each of the two measures.	(Zhang et al., 2021)
Lake Qinhu, China	V. natans and Iron-Oxidizing Bacteria (IOB)	V. natans had significantly decreased OM contents and sediment TP and Fe-P concentrations. Synergistic interactions between V. natans and IOB could enhance Fe-P formation and reduce TP concentrations in sediments.	(Wang et al., 2022)
Lake Rauwbraken, Netherlands	Polyaluminium chloride combined with lanthanum-modified bentonite (LMB)	The treatment reduced the PO4-P release from sediment under anoxic conditions.	(van Oosterhout et al., 2022)
Lake Taihu, China	Combining LMB and V. natans	The N and P release from the sediment in the combined LMB + V. natans treatments had decreased substantially by 97.4% and 94.3%, respectively.	(Han et al., 2022)
River Jialing, China	Combining oxygen-loading zeolite and V. natans	The release of P and N from the sediment was reduced by 86% and 92%, respectively.	(Wei et al., 2023)

 | Show Table

DownLoad: CSV

1.4.3   Advantages and disadvantages of combined control remediation technology on sediment P

In a combined technology, modified materials were artificially capped on the sediment surface to create a physical isolation layer. This action aimed to stabilize the surface sediment and inhibit internal sediment P release. Simultaneously, modified clay materials can absorb P from the overlying water and sediment through physical and chemical adsorption. The introduction of modified clay materials can change the physical and chemical properties of surface sediments and promote conversion of unstable P into more stable P hydroxides (Han et al., 2020). Materials for controlling internal sediment P loading have gradually developed from a single to a multifunctional interfacial P loading control material that can actively adsorb mobile phosphoric acid and regulate the sediment-water interface environmental conditions. In addition, submerged plants develop rhizomes, and the woven root network on the sediment surface can effectively prevent the occurrence of resuspension and improve the transparency of water bodies (Liu et al., 2020). An appropriate dosage of clay mineral material can effectively promote the growth of submerged plants and change the form of residual P through root oxidation and nutrient allocation, allowing easier adsorption by clay materials. Studies have demonstrated that the effect of the combined technology is much higher than that of the clay minerals and submerged plants (Li C J et al., 2018). However, currently submerged plants can effectively reduce the P concentration in water and sediment during the growth process, and P in plant residues is released into the water for a second time during plant death, thus affecting the P removal efficiency of the combined technology. Current in-situ combined control technologies for sediment P need certain upgrades to enhance P removal effectiveness in the sediments.

1.4.4   Application of combined control remediation technology on sediment P

Using combined control remediation technologies in applications that include plant harvesting and herbivorous fish breeding require the effective removal of submerged plant residues. In addition, it is necessary to determine the planting density of submerged plants, added method, and amounts of materials added (Han et al., 2022). Many recent researches have shown that a single clay material is often unable to adapt to complex field conditions, thus making it difficult to achieve the desired water quality and ecological restoration effect (Pan et al., 2011). Therefore, mixing many clay materials is probably more efficient than dealing with a single material. All the foregoing need to be further considered with the current situation of lake pollution and the actual application of clay materials through well-designed laboratory simulation experiments to optimize the application of the combined technology.

2. CONCLUSIONS AND PERSPECTIVES

2.1 Conclusions

With increasingly effective control of external pollution in lakes, the treatment of internal sediment pollution has become the key to improving the water quality of lakes. Current in-situ control techniques for sediment P, including physical capping, chemical inactivation, phytoremediation, and combined control technology, differ in their effectiveness and technical mechanisms of removing P from sediments. In-situ combined control technology, which is the desired technology, can enhance the interaction between materials and plant communities and promote the inactivation of sediment P.

2.2 Perspectives

Current in-situ control technologies for sediment P need to be improved to enhance P removal effectiveness. Cost-effective P control techniques with minimum environmental concerns are essential for eutrophication control. The control materials for internal sediment P loading are mainly divided into chemical passivating agents, clay minerals, industrial by-products, and modified (synthetic) materials, primarily using heat treatment, acid modification, and other physical or chemical modification methods. Materials for controlling internal sediment P loading have gradually developed from single to multifunctional interfacial P loading control material that can effectively adsorb mobile phosphoric acid and regulate the sediment-water interface environmental conditions, thus inhibiting the migration and transformation of P (Zamparas et al., 2013). Currently, most of these materials are still at the experimental research stage and have few practical engineering applications. Sediment internal P loading control materials need to be further studied in terms of their functional properties, pre-evaluation of the P control effect, and engineering applications.

2.2.1   Functional properties of materials

Currently, most of the P adsorption batch tests reported in the literature are aimed at high concentrations of phosphate. The modified materials usually change the pH of the environment, while the actual phosphate concentration at the sediment-water interface is generally < 2 mg/L. To ensure the safety of the lake ecosystem, the materials should not appreciably influence the environmental pH (Fang et al., 2014). However, some materials contain specific P and may exhibit "negative adsorption" when the phosphate concentration is low. Therefore, developing a material with low adsorption and desorption equilibrium concentrations, along with high removal efficiency for low-concentration phosphate that does not cause drastic fluctuations in water chemical conditions, is the preferred choice for controlling P loading at the lake sediment-water interface.

In recent years, modified P adsorption materials based on natural minerals have received increasing attention. These relatively safe modifiers have a strong adaptive value for relatively closed small water bodies with severe pollution. Researchers have focused on the regulatory effects of such materials on the sediment-water interface microenvironment and movable P in sediments (Himmelheber et al., 2008). New materials with slow-release functions and the ability to regulate DO and Eh potential at the sediment-water interface will be an area of priority for material modification research in the future.

As for the present situation of lake pollution, it is essential to control the internal nutrient loading at the sediment-water interface, as well as address persistent organic pollutants and heavy metal pollution. Therefore, capping materials that can simultaneously remove a variety of pollutants and have a lasting effect have a broad application space to strengthen the research, development, and preparation of new passivated P control materials.

2.2.2   Pre-evaluation of P control effect

Different materials have unique and suitable application conditions owing to their various properties. For example, Ca salts are more suitable for alkaline water bodies, whereas La-modified bentonite tends not to be used alone in alkaline lakes during summer. Different lakes exhibit different limnological characteristics. All these limitations limit the range and conditions of in-situ sediment P control technology. Considering the different degrees of eutrophication and types of lakes, and the functional characteristics of in-situ control materials is essential for successful application of in-situ control materials. As in-situ control materials may produce certain toxic effects if they exceed a certain dosage range, in addition to evaluating their P control effects, real-time monitoring of water quality and aquatic organisms should be strengthened during the dosing process to avoid harm to aquatic ecosystems. In addition, for shallow lakes, the bottom sediment is more likely to be affected by wind and wave disturbances. Therefore, prevention of secondary suspension of in-situ control materials should also be the focus of attention.

2.2.3   Engineering applications

Currently, the comprehensive control of polluted lakes is in the critical stage. The establishment and introduction of quality inspection standards for capping materials needs to be done as soon as possible. In addition to analyzing the leaching characteristics of heavy metals and other pollutants in materials, ecotoxicological tests should be performed to avoid potential environmental risks in engineering applications. Furthermore, long-term monitoring of water quality improvements in lakes mulched for restoration is needed. A model of pollutant migration and transformation in the overburden layer has been established through laboratory simulation tests. However, the guiding significance of this model for engineering applications needs to be confirmed and strengthened.

A single in-situ control material is often unable to adapt to complex field conditions, and it is difficult to achieve the desired water quality and water ecological restoration effect. A combination of different materials is probably more efficient than trying to deal with a single material (Lin et al., 2023b). The sediments of eutrophic lakes are soft and cannot support the roots of large plants. Thus, compactness (anti-mobility) of the overburden is very important for the restoration of aquatic plants. Treatment with compactness improves the repair of large plants with roots (Egemose et al., 2010). Research and development of new sediment control materials that meet the needs of lake ecosystem restoration and complete integration of in-situ physicochemical technology with lake ecosystem restoration will be the focus of future research. For example, internal control rotary, multifunctional capping blankets, and other layout forms are used in application processes. Compared with conventional mulching techniques, these new techniques can improve the compaction degree of sediment, slow sediment fluidity, effectively inhibit the destruction of the mulching layer, and provide suitable environmental conditions for the restoration of aquatic vegetation. Large aquatic plants can better anchor sediments and improve water quality (Qin et al., 2023). In-situ combined control technology can enhance the interaction between materials and plant communities and improve the adsorption and fixation of active P in sediments. However, most of these new technologies are still in the conceptual design stage and their specific application effects need to be verified.