Nature-Based Approaches for Managing Bioavailable Phosphorus in Aquatic Ecosystems

Abstract

High levels of phosphorus cause eutrophication, leading to water blooms and making the water undesirable in aquatic environments. Surface water pollution by phosphorus (P) is caused by both point and diffuse sources. Despite the recent technological advancements in wastewater phosphorus removal, this element persists in aquatic ecosystems, particularly in sediments, often in non-bioavailable forms (in the case of precipitation by aluminum salts) or within biomass associated with high concentrations of heavy metals, rendering it unsuitable for reuse. In this paper, we review the measures and methods commonly used for reducing or removing bioavailable phosphorus, with a focus on the strategies and methods for direct in situ phosphorus removal or reuse, including the use of microbial biofilms and aquatic macrophytes, natural and constructed wetlands, and biotised (biologically enhanced) solid-phase sorbents or woodchip bioreactors. This paper also highlights the significance of bioavailable phosphorus from both the hydrochemical perspectives, examining phosphorus speciation, solubility, and the geochemical interactions influencing mobility in water and sediments, and the biological perspectives, which consider phosphorus uptake, bioaccumulation in aquatic organisms, and the role of microbial and plant communities in modulating phosphorus cycling. This overview presents sustainable phosphorus management approaches that are key to reducing eutrophication and supporting ecosystem health.

1. Introduction

Phosphorus (P) is an essential nutrient for plant growth; however, when it is present in excessive amounts in water, it can lead to significant ecological problems. The over-enrichment of water bodies with phosphorus, primarily due to human activities, results in a process known as anthropogenic or cultural eutrophication [1,2]. This phenomenon triggers the overgrowth of algae and other aquatic plants, disrupting the ecological balance and adversely affecting biodiversity. Phosphorus is the primary nutrient responsible for the eutrophication of surface waters and often serves as the limiting factor in these ecosystems [3,4]. Phosphorus is introduced into streams, rivers, lakes, and ponds via multiple pathways and at varying temporal scales from both natural and anthropogenic sources, including wastewater discharges and runoff from impervious surfaces as well as pervious landscapes such as forests, croplands, and pastures [5].

In Western Europe, approximately 79% of total phosphorus (TP) used is allocated for fertilizer production, while around 11% is used for feed additives and about 7% for detergent manufacturing [6]. Between 2008 and 2012, the average annual use of phosphorus in fertilizers in Western Europe was 5.2 kg per hectare [7]. It is important to emphasize that only a fraction of the phosphorus applied through fertilizers is readily taken up by plants, while the remainder persists in the soil [8], from where it can be washed out or transported by runoff and subsequently affect aquatic ecosystems. Consequently, agricultural runoff represents a significant source of phosphorus in river systems, alongside contributions from wastewater [9,10,11]. The transport of agricultural phosphorus, particularly from upland regions, poses a threat to water quality and aquatic ecosystems downstream. Therefore, implementing strategies to reduce excess phosphorus loading from agricultural catchments prior to its discharge into downstream surface waters [9] is essential, with a particular emphasis on utilizing or remobilizing phosphorus already present.

The European Water Framework Directive 2000/60/EC regulates the use of phosphorus in agriculture to maintain good surface water quality. The critical concentrations for eutrophication control (10–20 μg P/L) are about an order of magnitude lower than the soil P concentrations required for crop growth (200–300 μg P/L), indicating the high sensitivity of surface water to P enrichment. Thus, phosphorus removal methods and techniques have been the focus of extensive research over many years [12,13,14,15,16,17,18,19,20,21].

This paper considers the review of the measures and methods commonly used to remove, mitigate, or retain bioavailable phosphorus from surface water, especially by nature-based approaches. Optimal methods shall combine effectiveness with sustainability, ensuring the future health of aquatic ecosystems and supporting the reuse or recycling of P whenever possible.

2. Bioavailable Forms of Phosphorus

There are two different approaches to evaluating phosphorus in the aquatic environment—the chemical and the biological. From a hydrochemical point of view, phosphorus can be sequentially fractionated, starting with easily extractable forms using weak extractants (such as water and weakly acidic ammonium chloride) and progressing to more potent extractants (including caustic soda, hydrochloric acid, sulfuric acid, and perchloric acid). Each extraction reagent targets different fractions of phosphorus (such as labile phosphorus, apatite phosphorus, non-apatite phosphorus, or metal-bound phosphorus), allowing for a comprehensive analysis of phosphorus availability within water or sediment samples.

Biologically available phosphorus, a key nutrient for organisms, is mainly considered to be labile phosphorus. This form of phosphorus is readily soluble in water, mostly in the form of orthophosphates [22,23]. However, there is also a limnological way of looking at the availability of phosphorus to organisms. Under unfavourable conditions, some organisms, such as cyanobacteria and algae, produce enzymes (e.g., alkaline phosphatase) that promote the bioavailability of phosphorus from various sources, including apatite and organic forms. In both the limnological and hydrochemical contexts, the following forms of phosphorus are recognized as being bioavailable in sediments and water: labile phosphorus, loosely sorbed phosphorus, organic phosphorus, and metal-bound phosphorus [3,22,24,25,26,27,28,29].

In addition, biologically available phosphorus encompasses phosphates that have already been incorporated into organisms (Scheme 1), including phosphorus derived from biogenic metabolites and biomass residues, which are released from hydrolysable fractions and fractions capable of functional group exchange [24]. In contrast, most other forms of phosphorus, particularly those associated with alkaline earth metals, such as aluminum (Al) and iron (Fe), exhibit poor bioavailability. For instance, orthophosphate ions adsorbed to metal oxides/hydroxides are generally inaccessible to organisms unless they undergo desorption, which can be triggered, e.g., by changes in redox conditions. Under reducing conditions, Hupfer et al. [30] found that changes in the iron-bound fraction were significantly correlated with changes in the concentration of iron. The authors suggested that the increase in phosphorus concentration, especially in the spring season, was due to the increased metabolic activity of iron- and manganese-oxidizing bacteria. Phosphorus desorption is particularly critical in shallow lakes, where iron-bound phosphorus may be released under changing conditions, significantly contributing to the occurrence of cyanobacterial blooms.

The bioavailability of phosphorus corresponds to its measurability as molybdate-reactive or dissolved reactive phosphorus [24]. Readily bioavailable phosphorus in water represents two parts: (i) P determined by the molybdenum blue method in a fraction containing orthophosphate ions, which passes through 0.5 µm filters [24,25,26] and represents the available part of P, (ii) the part of P already incorporated in biomass. There are several concepts in the P fractionation nomenclature; for example, Rönspieß et al. [27] examined “unfiltered” and “10 µm”- and “1.2 µm”-filtered water samples for bioavailable phosphorus fractions. Bioavailability (uptake by organisms within a few days) was found to be between 9% and 100% for dissolved phosphorus and between 34% and 100% for particulate phosphorus.

Bioavailable phosphorus retrievable from sediment particles is the sum of the fractions of labile phosphorus, iron-bound phosphorus, and metal oxide-bound phosphorus. This bioavailable fraction, also known as non-apatite inorganic phosphorus, may include a part of the organic phosphorus [28], because organic phosphorus extractable with alkalis also contains readily available organic phosphorus in microbial biomass, labile organic phosphorus, and slowly available organic phosphorus associated with fulvic and humic acids [3]. This bioavailable fraction may contain phosphorus associated with calcium (Ca) and may also be available to microorganisms [28]. Sodium hydroxide (NaOH) can be used to extract phosphorus available for algae and to estimate both short- and long-term available phosphorus in sediments [29], although Dunne et al. [3] reported that it is not readily available to all phytoplankton groups. Distilled water is used for the leaching of loosely sorbed phosphorus [31,32,33] from sediments in addition to ammonium chloride, potassium chloride, and magnesium chloride salts [3].

Bioavailable phosphorus enters aquatic systems primarily as a result of aqueous leaching from the hydrological catchment [24]. While phosphorus exists in numerous chemical forms within aquatic environments, perspectives on its biological availability to organisms such as phytoplankton, microbial biofilms, and plants can differ significantly. The presence of phosphates has been shown to disrupt the ecological balance within water bodies, with the consequent effect of reducing dissolved oxygen levels, primarily due to heterotrophic respiration of bacteria involved in the decomposition of algal biomass [14]. The best way to tackle eutrophication is to prevent phosphorus from entering, but that can sometimes be complicated, inefficient, or impossible.

3. Methods and Technologies for Phosphorus Removal, Reduction, or Mitigation by Precipitation and (Bio)Sorption

Currently, there is an increasing number of methods that can be used to reduce the concentration of phosphorus in water, and thus limit its bioavailability for unwanted phytoplankton, microbial biofilms, or aquatic plants. The removal of phosphorus is dominantly realized via its transformation to a solid fraction. This fraction can be an insoluble salt precipitate, a microbial mass in a reaction sludge, or a plant biomass.

Current phosphorus removal methods include chemical or physicochemical techniques ranging from adsorption, crystallization, electro-coagulation, or precipitation to enhanced biological approaches like constructed wetlands [14,19,34]. Materials used for adsorption and precipitation can be natural or anthropogenic (e.g., metal oxides and hydroxides, furnace slag, fly ash, and chemically modified clays) [31]. They can remove phosphorus by a ligand exchange mechanism at a neutral or acidic pH [34].

3.1. Precipitation of Phosphorus by Metal Salts

The chemical precipitation of phosphorus (soluble phosphates) usually utilizes the chemical interaction of P with inorganic salts based on iron (II), iron (III), and aluminum [35,36]. Calcium and magnesium salts are used less frequently than Fe and Al, although the final precipitation product is more important from the P recycling point of view, which is a challenge that will become more and more important in the near future due to the consequences of phosphorus scarcity. The use of zero-valent iron (Fe-0)-based materials for phosphorus removal from aquatic environments has been explored as a promising strategy, as Fe-0 generates positively charged iron cations at a pH above 4.5, which strongly bind phosphorus and form stable, less bioavailable compounds [37]. This process is only seldom used in aquatic ecosystems due to the high variability in its efficiency caused by factors like unstable water quality, pH value, or operating conditions. Phosphorus in water or sediments treated by Al or Fe salts becomes biologically unavailable and cannot be recycled. This limitation must be considered in lake restoration and phosphate reuse projects.

3.2. Solid-Phase Sorbents

Solid-phase adsorbents are often used to remove phosphorus in various applications, such as wastewater treatment, potable water supplies maintenance, and constructed wetlands. Commonly investigated and long-term used adsorbents or materials used for phosphorus removal include the following [38,39,40,41]:

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Naturally occurring minerals from soils (e.g., Fe-oxides/oxyhydroxides, allophone)

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Naturally occurring (poly-mineral) soils or sands.

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Derivatives from mineral deposits (e.g., wollastonite) or other natural materials (e.g., shale, serpentinite, marl).

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Synthetic analogues of natural minerals produced on an experimental or industrial scale (e.g., polymeric hydrogels, hydrotalcites).

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Expanded clay aggregates, including extruded clays fortified by nanomaterials, or metals.

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Nanoscale zero-valent iron (nZVI).

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Waste materials from industrial processes that adsorb phosphate or that may also be further modified to enhance their uptake capacity (e.g., electric arc furnace, steel slag, blast furnace slag, red mud).

In most cases, the efficiency of each type of adsorbent, measured by its adsorption capacity and relative cost, varies widely, and there is a significant overlap between the different types. In addition, adsorbents vary in their sensitivity to changes in pH and redox conditions [42].

Another way of categorizing divides the sorbents into four main types, inorganic sorbents, organic sorbents, sorbents made from industrial by-products, and sorbents made from biological waste, as proposed in a comprehensive review by Loganathan et al. [43]. Inorganic sorbents are usually metal oxides and hydroxides, calcium and magnesium carbonates and hydroxides, or layered double hydroxides. Organic sorbents are based on activated carbon or anion exchange resins or involve other organic compounds. Loganathan et al. [43] classified the red mud, slag, fly ash, and similar industrial wastes as industrial by-products. Sorbents of agricultural origin, which improve the chemical, physical, and biological properties of soils, are included in the last category of biological waste. A summary of the sorbents and other methods suitable for phosphorus removal is presented in Table 1, together with the relevant details.

Nanoscale zero-valent iron (nZVI)-based materials including pristine nanoparticles and combinations of nZVI with nanocomposites were tested to achieve low cost, highly porous, and highly specific sorbent surface areas. Suazo-Hernández et al. [41] showed that under optimal conditions, such as an acidic pH, higher oxygen availability, or optimal dosage, nZVI can be highly potent in phosphorus removal. Biochar enriched with nZVI nanoparticles was tested as an environmentally friendly approach to remove phosphorus from wastewater as an alternative to conventional water treatment [44]. The removal efficiency rate was 60% at a phosphate concentration of 25 mg/L, and the adsorption capacity increased with increasing phosphorus concentration (100 mg/L achieved 89% efficiency).

Another interesting type of sorbent is gel sorbent, prepared, e.g., from pectin-containing organic waste by saponification with calcium hydroxide. A gel prepared from orange juice factory waste, immobilized with zirconium (IV), had nearly four times higher adsorption capacity for phosphate, 57 mg P/g, than that of zirconium ferrite [14]. The highest phosphate removal rate was observed at a low pH; however, even up to pH 9, a greater than 85% phosphate removal rate was observed. Another gel sorbent, polyamine polymer hydrogel, was used to remove phosphorus from aquaculture and poultry wastewater effluent [45].

Biochar is a highly porous material with a high content of carbon (over 60%). The composition of biochar depends on the origin of the biomass used and the production conditions. Biochar is often produced from agricultural, aquatic, or woody biomass. It can also be produced from waste such as municipal and paper waste [46]. Biochar is used as an adsorbent for wastewater and soil treatment, and also for the removal of pollutants (nitrogen, pesticides, metals, pathogenic microorganisms) due to its abundant functional groups. Chemical activation, post-pyrolysis, or modification with metals (oxides, salts, transition metals) are commonly used to increase its sorption capacity [47,48]. Producing biochar at high temperatures enhances its surface area and porosity, making it more effective for water treatment [46].

The successful use of biochar in the treatment of wastewater for the removal of phosphorus has been described, e.g., by Kopecky et al. [49] with biochar made from coconut shells with a sorption efficiency of 20–34%, or by Ouyang et al. [50] with a bamboo biochar composite with La₂(CO₃)₃ and MnFe₂O₄-loading. Similarly, Linag et al. [51] reported an adsorption capacity reaching 29.7 mg/g, a level which exceeds the adsorption capacities of other comparable materials, in the case of mulberry rods biochar used for the preparation of composites with a magnesium/iron layered bimetallic oxide. Xiao et al. [52] prepared biochar using canna aquatic plant waste, modifying it with MgO. The results demonstrated the importance of MgO in the modified biochar for phosphorus adsorption.

Certain forms of biochar are endothermic and increasing the temperature can increase the adsorption capacity for phosphorus binding, achieving removal rates as high as 96% [53]. A high efficiency of phosphorus removal (288 mg/g) at pH 5–12 in biochar produced from eggshell and maize straw was published by Xu et al. [54]. Wystalska et al. [55] characterized a biochar prepared from activated sewage sludge in accordance with the principles of the circular economy. When modified by KOH, it demonstrated the most effective phosphorus (86%) and nitrogen (72%) removal compared with a modification with MgCl₂. The enhancement of the nitrogen and phosphorus content of a fecal-derived biochar was investigated by Koulouri et al. [56]. Biochar had limited phosphorus adsorption capacity, but combining magnesium oxide with a lower dose of biochar produced the richest fertilizer in terms of nitrogen and phosphorus. Similarly, the potential of poultry feces for the synthesis of Mg/Al nanoparticles-containing biochar for the recovery of phosphorus was investigated with the highest observed yield of 88% at an initial pH of 9.0 [57]. It should be noted that high initial phosphorus concentrations may lead to misleading conclusions when expressed as percentage removal; therefore, evaluating efficiency in terms of absolute concentrations is more appropriate.

In addition to biochar, other natural solid-phase organic sorbents are also utilized. Li et al. [58] used bamboo powder and rice husk powder, natural organic compounds, to condition the sludge to improve phosphorus removal in wastewater treatment. The dewatered sludge had a higher total phosphorus (TP) content after conditioning with these natural compounds than with conventional chemicals, which makes these agricultural waste products suitable for the reuse of waste resources in land use (an ecological risk assessment was carried out). Similarly, a Ca-biocomposite prepared from a mixture of banana peel and eggshell was found to have a maximum P adsorption capacity of 214 mg/g and strong chemical bonds between the adsorbent and P species [59].

There are many other solid sorbents used for phosphorus removal, such as the well-known activated carbon, which is also used for water treatment (Table 1). Some of these solid sorbents can also be considered filter materials. Activated carbon, sand, clay, resins, or other filters, with the addition of specific materials, are solid filtration materials suitable for removing phosphorus from water. In this context, solid sorbent material works as a filter that mechanically retains particles but also chemically absorbs the ions. Plach et al. [60] installed phosphorus sorbent and woodchip filters in the experimental fields and quantified the filters’ ability to remove soluble reactive phosphorus, total phosphorus, and total suspended solids from the surface runoff. The filters did not remove dissolved phosphorus species effectively and were even a source of phosphorus at one site. They found that dissolved phosphorus was retained as a loosely bound fraction so that it was in a bioavailable form and was able to be remobilized.

Table 1. Overview of the methods of P removal by metal salts and solid-phase sorbents.

Method	Principles and Additional Information	Reference
Precipitation and sorption by metals or metal salts	Iron(II), iron(III), aluminum	[35,36]
	Polyaluminium chloride (liquid)	[39]
Solid-phase sorbents INORGANIC	Naturally occurring minerals from soils (Fe-oxides/hydroxides, allophone)	[39]
	Sand filter amended with biochar coating with iron hydroxides	[38]
	Synthetic analogues of natural minerals produced on an industrial scale (polymeric hydrogels, hydrotalcites)	[39]
	Expanded clay aggregates, extruded clays fortified by nanomaterials or metals	[39]
	Waste materials from industrial processes (electric arc furnace, steel slag, blast furnace slag, red mud) and basic oxygen furnace slag and converter slag	[39]
	Other by-products, iron oxide tailing material; an industrial waste derived from a mineral processing industry; wollastonite tailing (calcium meta-silicate), iron-containing humic materials as iron humate by-product obtained from alkali humates wastewaters from manufacturing industry with iron salts (critical review)	[43]
	Metals, metal oxides, and hydroxides; Mn, Ti, Zr, lanthanum, iron oxide, nano-sized magnetite, Fe-Zr binary oxides, Ca-based layered double hydroxides, Mg-based layered double hydroxides, Mg-Ca-based layered double hydroxides, Zr-modified Mg-Fe-based layered double hydroxides	[43]
	Mesoporous silica doped with La, Zr, Ti, Fe, and Al oxides (critical review)	[43]
	Minerals and composite sorbents; goethite, akaganeite, lepidocrocite, hematite, magnetite, vesuvianite, limestone, dolomite, calcium and magnesium carbonates; bentonite, coal, and derived from mineral deposits (wollastonite) or other natural materials (shale, serpentinite, marl) (critical review)	[43]
	Clay with positive charges; layered double hydroxides or hydrotalcite adsorb oxyanions, e.g., phosphate	[43]
	Activated carbon prepared from shells, hulls, sobs, of plants, coal, lignite, wood (critical review)	[43]
Solid-phase sorbents ORGANIC	Cellulose and its derivatives; grafted iron-coordinated amino-functionalized polymers on the cellulose (critical review)	[43]
	Allepo pine saw dust containing lignocellulosic materials by acid prehydrolysis and urea addition (critical review)	[43]
	Mesostructural materials; ammonium functionalized mesoporous silica; Cu and Fe ethylene diamine complexes anchored inside mesoporous silica compound (critical review)	[43]
	Carboxylic acid-coated alumina and surfactant-coated nano-crystalline iron oxide, with a high surface area of the nano-sized material (critical review)	[43]
	Bamboo powder and rice husk powder; enrichment of P and N in sludge cake	[58]
	Mining, mineral processing by-products	[39]
	Orange juice plants, immobilized with zirconium	[14]
Biochar and sorbents of biological origin	Biochar production methods; application, development (review)	[46]
	Biochar modified by FeCl3 holds one-third of the agricultural superphosphate amount	[49]
	Overview of the preparation of biochar; adsorption of N, P by biochar	[53]
	Overview of the preparation and modification of biochar and adsorption of N, P	[54]
	Comparison of uncoated and Fe-coated biochars in sand filters in relation to the bacteria removal	[38]
	Sewage slug biochar modified by metals (lanthanum nitrate)	[47]
	Global meta-analysis; the most effective methods for activating the biochar to maximize heavy metal removal	[48]
	Biochar composites loaded with metals (La, Mn, Fe); recyclability by magnetisation	[50]
	Mulberry rods biochar composites with metal (Mg, Fe) layers	[49]
	Biochar from activated sewage sludge modified by MgCl2, KOH, or activated by CO2	[55]
	Fecal-derived biochar (addition of undiluted human urine), MgO addition	[56]
	Biochar from canna aquatic plant waste, modified by MgO	[52]
	Wheat straw anion exchanger, reed, soybean hulls, saw dust from Allepo pine (critical review)	[43]
	Adsorbents produced from waste and biosorbents (sugarcane bagasse, rice hull, coconut shells, wheat straw charcoal, almond shell, sugar beet pulp, mustard straw charcoal) for nitrate removal (review)	[61]

Table 1. Overview of the methods of P removal by metal salts and solid-phase sorbents.

Method	Principles and Additional Information	Reference
Precipitation and sorption by metals or metal salts	Iron(II), iron(III), aluminum	[35,36]
	Polyaluminium chloride (liquid)	[39]
Solid-phase sorbents INORGANIC	Naturally occurring minerals from soils (Fe-oxides/hydroxides, allophone)	[39]
	Sand filter amended with biochar coating with iron hydroxides	[38]
	Synthetic analogues of natural minerals produced on an industrial scale (polymeric hydrogels, hydrotalcites)	[39]
	Expanded clay aggregates, extruded clays fortified by nanomaterials or metals	[39]
	Waste materials from industrial processes (electric arc furnace, steel slag, blast furnace slag, red mud) and basic oxygen furnace slag and converter slag	[39]
	Other by-products, iron oxide tailing material; an industrial waste derived from a mineral processing industry; wollastonite tailing (calcium meta-silicate), iron-containing humic materials as iron humate by-product obtained from alkali humates wastewaters from manufacturing industry with iron salts (critical review)	[43]
	Metals, metal oxides, and hydroxides; Mn, Ti, Zr, lanthanum, iron oxide, nano-sized magnetite, Fe-Zr binary oxides, Ca-based layered double hydroxides, Mg-based layered double hydroxides, Mg-Ca-based layered double hydroxides, Zr-modified Mg-Fe-based layered double hydroxides	[43]
	Mesoporous silica doped with La, Zr, Ti, Fe, and Al oxides (critical review)	[43]
	Minerals and composite sorbents; goethite, akaganeite, lepidocrocite, hematite, magnetite, vesuvianite, limestone, dolomite, calcium and magnesium carbonates; bentonite, coal, and derived from mineral deposits (wollastonite) or other natural materials (shale, serpentinite, marl) (critical review)	[43]
	Clay with positive charges; layered double hydroxides or hydrotalcite adsorb oxyanions, e.g., phosphate	[43]
	Activated carbon prepared from shells, hulls, sobs, of plants, coal, lignite, wood (critical review)	[43]
Solid-phase sorbents ORGANIC	Cellulose and its derivatives; grafted iron-coordinated amino-functionalized polymers on the cellulose (critical review)	[43]
	Allepo pine saw dust containing lignocellulosic materials by acid prehydrolysis and urea addition (critical review)	[43]
	Mesostructural materials; ammonium functionalized mesoporous silica; Cu and Fe ethylene diamine complexes anchored inside mesoporous silica compound (critical review)	[43]
	Carboxylic acid-coated alumina and surfactant-coated nano-crystalline iron oxide, with a high surface area of the nano-sized material (critical review)	[43]
	Bamboo powder and rice husk powder; enrichment of P and N in sludge cake	[58]
	Mining, mineral processing by-products	[39]
	Orange juice plants, immobilized with zirconium	[14]
Biochar and sorbents of biological origin	Biochar production methods; application, development (review)	[46]
	Biochar modified by FeCl3 holds one-third of the agricultural superphosphate amount	[49]
	Overview of the preparation of biochar; adsorption of N, P by biochar	[53]
	Overview of the preparation and modification of biochar and adsorption of N, P	[54]
	Comparison of uncoated and Fe-coated biochars in sand filters in relation to the bacteria removal	[38]
	Sewage slug biochar modified by metals (lanthanum nitrate)	[47]
	Global meta-analysis; the most effective methods for activating the biochar to maximize heavy metal removal	[48]
	Biochar composites loaded with metals (La, Mn, Fe); recyclability by magnetisation	[50]
	Mulberry rods biochar composites with metal (Mg, Fe) layers	[49]
	Biochar from activated sewage sludge modified by MgCl2, KOH, or activated by CO2	[55]
	Fecal-derived biochar (addition of undiluted human urine), MgO addition	[56]
	Biochar from canna aquatic plant waste, modified by MgO	[52]
	Wheat straw anion exchanger, reed, soybean hulls, saw dust from Allepo pine (critical review)	[43]
	Adsorbents produced from waste and biosorbents (sugarcane bagasse, rice hull, coconut shells, wheat straw charcoal, almond shell, sugar beet pulp, mustard straw charcoal) for nitrate removal (review)	[61]

3.3. Biosorption by Microbial Assemblages Including Bacteria, Cyanobacteria, Microalgae, Fungi, or Filamentous Algae

Microorganisms can be another valuable tool for phosphorus removal (see Table 2). Shao et al. [62] studied nanospheres containing microorganisms (photosynthetic bacteria, lactic acid bacteria, yeast, and actinomycetes) fixed in charcoal nanoparticle carriers resulting from the pyrolytic processing of bamboo wood, zeolite, and aquaculture pond sediment fermented with the microorganisms in active calcium solution. The results showed that the maximum total phosphorus removal ranged from 50 to 84%. It is important to note that nano-bamboo microbial material must be aerated [62].

Bacteria and cyanobacteria are also used to remove primarily bioavailable phosphorus. Besides others, filamentous cyanobacterium Phormidium bohneri was investigated to remove dissolved inorganic nutrients from fish farm effluents. The average efficiencies of ammonia nitrogen (NH₄-N) removal from rainbow trout (Oncorhynchus mykiss) farm effluent were 82% and 85% for soluble orthophosphate, over a period of one month. These results showcase the potential use of P. bohneri as an alternative for the tertiary treatment of fish farm effluents [63]. Similarly, a photobioreactor containing cells of the purple non-sulphur bacterium Rhodobacter capsulatus immobilized on cellulose beads removed organic carbon, ammonium ions, and phosphate ions from a diluted growth medium over a period of 19-22 d with a residence time of 20.6 or 10.3 h at 35 (±1) °C and continuous light of 60 μE/m²/s [64].

Bacillus subtilis, Pseudomonas sp., Achromobacter sp., Spirulina platensis, and Chlorella vulgaris are also reported to remove phosphates and nitrates from water [61]. The removal of antibiotics in wastewater has been the subject of research by Zhou et al. [65] who prepared gel particles containing Chlorella vulgaris and Bacillus subtilis to monitor nitrogen/phosphorus removal and feedback during tetracycline loading. The removal rates of NH₄-N, TP, chemical oxygen demand (COD), and tetracycline were 96%, 95%, 81%, and 74%, respectively.

Microbial inoculants containing Bacillus subtilis, Paracoccus pantotrophus, and Pseudomonas putida were investigated in a study by Simon et al. [66]. After 72 h of treatment, Bacillus subtilis achieved a maximum removal rate of organic load (chemical and biological oxygen demand) of 86% and 90%, respectively. Paracoccus pantotrophus and Pseudomonas putida microbial inoculants showed the highest total nitrogen (TN) removal (66%) after 72 h of treatment. These inoculants could be used as a tool for the remediation of polluted surface water and also for the treatment of wastewater through bioaugmentation. A mixture of Bacillus strains (Bacillus megaterium, Bacillus subtilis, and Bacillus coagulans) has also been observed to improve water quality and maintain fish health in aquaculture, as reported by Li et al. [67]. An analysis of the microbial community showed an increase in the relative abundance of bacteria involved in nitrogen (N) and phosphorus removal (Proteobacteria, Actinobacteria, Comamonas, and Stenotrophomonas) and a decrease in the relative abundance of pathogenic bacteria and fungi.

Spirulina platensis was used for nutrient removal from mariculture effluent, desalination effluent, or wastewater treatment (cultivation in synthetic human urine) with biomass production [65,66] because of its high biomass yield and excellent adaptability to saline wastewater. The removal rates were about 86% of total nitrogen and 98% of TP in mariculture wastewaters [68,69]. The efficiency of nitrogen and phosphorus removal by Spirulina platensis was 82% and 81% in synthetic municipal wastewater [69], and the addition of glucose to the mixotrophic culture resulted in enhanced biomass growth. A high efficiency of phosphate removal (79%) was observed in diluted wastewater effluent [70].

The use of biosorbents in wastewater treatment is on the rise. Macrophytes and microalgae (as a part of both phytoplankton and phytobenthos) are commonly found in freshwater systems but are also being studied in the context of wastewater treatment. This is a promising approach to natural technologies, and given the rise in publications in this field, it highlights a significant potential for natural water treatment. However, technologies in this area vary widely in terms of economy and efficiency.

The use of phytoplankton species for P removal is severely limited by the difficulty of harvesting the enormous population of microalgae that develop in the water after treatment [16]. For easy removal by sedimentation, e.g., spherical gel capsules containing microalgae were developed. For example, Chlorella vulgaris, immobilized in two natural polysaccharide gels (carrageenan and alginate), was used to treat primary domestic wastewater. Over 95% of ammonium and 99% of phosphates were removed from the wastewater in 3 days of treatment [16]. In a study by Tarabukin et al. [71], a hydrogel complex containing Chlorella vulgaris was also used to investigate the potential of microalgae (Tetradesmus obliquus and Chlorella vulgaris) to remove nutrients from domestic wastewater. A hydrogel complex containing microalgal cells was formed, with cell viability higher than 95%. Nitrogen was primarily taken up by living microalgae cells, while phosphorus removal exceeded 95%, mainly through the polymeric component of the hydrogel complex. Due to its potential in wastewater treatment, Chlorella sp. was also tested in combination with other hydrogels (methylcellulose, alginate) as part of a 3D-printed microbial biocarrier. The soluble nitrogen and phosphorus removal efficiencies were 52% and 88%, respectively [72]. The removal of nutrients from water by Chlorella sp. has also been studied by Asaad et al. [73], who have shown that the rate of adsorption via C=C and N-H amide bonds is much higher than the rate of desorption. Chlorella vulgaris and Scenedesmus bijugatus immobilized in calcium alginate have been studied by Megharaj et al. [74], showing that Chlorella was more efficient in the removal of phosphorus and nitrogen. Also, Scenedesmus sp. showed a high efficiency of phosphate removal even in non-diluted wastewater effluent [70].

In addition to Chlorella sp. and Scenedesmus sp., other microalgae have been tested in the microalgae utilization system, including Chlamydomonas sp. and Nannochloropsis sp. [11,75]. These organisms have been shown to almost completely or completely remove phosphorus from wastewater. In addition, the coccolithophore Emiliania huxleyi was included in the testing process. The effectiveness of these microorganisms in phosphorus removal is influenced by several factors, including light intensity, photoperiod and wavelength, pH, and temperature. Further research of the recovery and reuse of phosphorus from water bodies has involved gene manipulations in Chlamydomonas. These genetically engineered algae were able to recover phosphorus from wastewater three times faster than wild-type Chlamydomonas and had a three times larger phosphorus storage capacity [11].

In terms of industrial water, Chlorella vulgaris has also been studied [76]. The combination of microalgae (Chlorella vulgaris) and macrophytes (Lemna minuscula) was used for the biological treatment of industrial effluent. This setup prevented the growth of macrophytes. Firstly, Chlorella vulgaris reduced ammonium ions (72%), phosphorus (28%), and COD (61%). Consequently, Lemna minuscula was able to grow in the treated wastewater, precipitate the microalgal cells by shading the culture, and reduce organic matter and colour. However, Lemna minuscula did not significantly improve further nutrient removal [76]. In accordance with these results, C. demersum and C. vulgaris were used for the effective treatment of agro-industrial wastewater [77]. The growth and metabolic activity of the algae and plants also improved significantly after treatment with wastewater.

Qv et al. [78] investigated a symbiotic system between microalgae and bacteria, specifically Chlorella sorokiniana and activated sludge including Brevundimonas, Dokdonella, and Thermomonas. The results confirm the system’s ability to remove ammonia nitrogen and effectively eliminate 65% of total nitrogen along with 43% of phosphorus. The study revealed that Brevundimonas facilitated the growth of microalgae, which in turn promoted the development of Dokdonella and Thermomonas, leading to an increase in nitrogen removal.

The interaction between microalgae and the bacterium Azospirillum is recommended as a biological application for wastewater treatment. These organisms contribute to the removal of nutrients and improve water quality [79].

The microalgae consortium, which includes microalgae, bacteria, and other microscopic organisms that have not been fully characterized, transfers nutrients from wastewater to biomass. The removal activity of the microalgae consortium was found to be influenced by temperature and correlated with photosynthetic activity [80].

The removal of nutrients by Cladophora glomerata and Elodea canadensis was observed by Gumbricht et al. [81]. The mean weekly reduction in nutrients was found to be 32% of nitrogen and 62% of phosphorus. The harvested biomass analysis revealed that 10% of nitrogen and 20% of phosphorus were removed. The high potential of filamentous algae for nutrient removal and their bioremediation was discovered [82]. Cladophora sp., Oedogonium sp., Rhizoclonium sp., and Spirogyra sp. were observed under summer and winter conditions. Nutrient removal rates of 0.17 g PO₄/m²/d were observed in summer and 0.085 g PO₄/m²/d in winter. According to the authors, the most suitable filamentous algae seem to be Oedogonium sp. or Cladophora.

The filamentous algae Cladophora sp. was studied in constructed wetlands [83]. The results demonstrated a fast and stable operational efficiency for these algae, with a total nitrogen consumption rate of 2.65 mg/g/d. Additionally, a symbiotic relationship between the filamentous algae and bacteria was observed, which contributed to nutrient removal.

Table 2. Overview of the methods of P removal by biosorption.

Method	Principles and Additional Information	Reference
Biosorption by bacteria and cyanobacteria	Phormidium bohneri; removal of dissolved inorganic nutrients from fish farm effluents	[63]
	Rhodobacter cupsulatus; on cellulose beds for removal of N, P, and organic carbon from wastewater	[64]
	Bacillus subtilis, Pseudomonas, Achromobacter, Spirulina platensis, and Chlorella vulgaris for phosphates removal (review)	[61]
	Chlorella vulgaris, Bacillus subtillis in symbiotic particles using sol–gel method for removal of antibiotics from wastewater	[65]
	Bacillus subtillis, Paracoccus pantotrophus, and Pseudomonas putida for organic load and total N removal from surface water (poor removal of phosphates)	[66]
	Bacillus megaterium, Bacillus subtilis, and Bacillus coagulans for P, N and pathogen removal from aquaculture	[67]
	Proteobacteria, Actinobacteria, Comamonas, and Stenotrophomona for P and N removal to improve water quality in aquaculture	[67]
	Nano-bamboo charcoal powder, zeolite powder, and aquaculture pond sediment formed into nanospheres to observe the purification effect	[62]
	Spirulina platensis cultivated in human urine for wastewater treatment and biomass production	[68]
Microalgae and filamentous algae	Scenedesmus sp., Arthrospira platensis for nutrients, metals, and organic materials removal from desalination systems wastewater	[70]
	Chlorella vulgaris, Scenedesmus dimorphus for P removal from wastewaters, Spirulina platensis for nitrates, ammonia, and phosphates (review)	[16]
	Chlorella vulgaris for reducing colour, organic matter, ammonium, P, and chemical oxygen demand	[76]
	Tetradesmus obliquus and Chlorella vulgaris in hydrogel complex for N and phosphate removal from domestic wastewater	[71]
	Chlorella sp. in 3D-printed biocarriers for wastewater treatment, P and N removal	[72]
	Chlorella vulgaris ability to eliminate N and phosphates as biosorbent	[73]
	Chlorella vulgaris and Scenedesmus bijugatus in calcium alginate beads, removal of N and P	[74]
	Chlorella sorokiniana and activated sludge (Brevundimonas, Dokdonella, and Thermomonas); efficient removal of nutrients in wastewater	[78]
	Azospirillum spp. with microalgae in alginate; wastewater treatment	[79]
	Cladophora glomerata, Elodea Canadensis for P and N removal	[81]
	Cladophora sp., Oedogonium sp., Rhizoclonium sp., and Spirogyra sp.; P and N removal. Oedogonium sp. is recommended for all-year cultivation, as it is a good species for biomass production and bioremediation	[82]
	Cladophora sp., bacteria; algal–bacterial symbiosis improves N removal; wetland with filamentous algae; good ability for N removal; Cladophora resistant to ammonia N	[83]

Table 2. Overview of the methods of P removal by biosorption.

Method	Principles and Additional Information	Reference
Biosorption by bacteria and cyanobacteria	Phormidium bohneri; removal of dissolved inorganic nutrients from fish farm effluents	[63]
	Rhodobacter cupsulatus; on cellulose beds for removal of N, P, and organic carbon from wastewater	[64]
	Bacillus subtilis, Pseudomonas, Achromobacter, Spirulina platensis, and Chlorella vulgaris for phosphates removal (review)	[61]
	Chlorella vulgaris, Bacillus subtillis in symbiotic particles using sol–gel method for removal of antibiotics from wastewater	[65]
	Bacillus subtillis, Paracoccus pantotrophus, and Pseudomonas putida for organic load and total N removal from surface water (poor removal of phosphates)	[66]
	Bacillus megaterium, Bacillus subtilis, and Bacillus coagulans for P, N and pathogen removal from aquaculture	[67]
	Proteobacteria, Actinobacteria, Comamonas, and Stenotrophomona for P and N removal to improve water quality in aquaculture	[67]
	Nano-bamboo charcoal powder, zeolite powder, and aquaculture pond sediment formed into nanospheres to observe the purification effect	[62]
	Spirulina platensis cultivated in human urine for wastewater treatment and biomass production	[68]
Microalgae and filamentous algae	Scenedesmus sp., Arthrospira platensis for nutrients, metals, and organic materials removal from desalination systems wastewater	[70]
	Chlorella vulgaris, Scenedesmus dimorphus for P removal from wastewaters, Spirulina platensis for nitrates, ammonia, and phosphates (review)	[16]
	Chlorella vulgaris for reducing colour, organic matter, ammonium, P, and chemical oxygen demand	[76]
	Tetradesmus obliquus and Chlorella vulgaris in hydrogel complex for N and phosphate removal from domestic wastewater	[71]
	Chlorella sp. in 3D-printed biocarriers for wastewater treatment, P and N removal	[72]
	Chlorella vulgaris ability to eliminate N and phosphates as biosorbent	[73]
	Chlorella vulgaris and Scenedesmus bijugatus in calcium alginate beads, removal of N and P	[74]
	Chlorella sorokiniana and activated sludge (Brevundimonas, Dokdonella, and Thermomonas); efficient removal of nutrients in wastewater	[78]
	Azospirillum spp. with microalgae in alginate; wastewater treatment	[79]
	Cladophora glomerata, Elodea Canadensis for P and N removal	[81]
	Cladophora sp., Oedogonium sp., Rhizoclonium sp., and Spirogyra sp.; P and N removal. Oedogonium sp. is recommended for all-year cultivation, as it is a good species for biomass production and bioremediation	[82]
	Cladophora sp., bacteria; algal–bacterial symbiosis improves N removal; wetland with filamentous algae; good ability for N removal; Cladophora resistant to ammonia N	[83]

3.4. Woodchip Bioreactors for Nutrients Removal

Woodchip denitrifying bioreactor technology is commonly used to remove nitrogen, mainly in agricultural wastewater treatment, and is also being tested for phosphorus removal. In many cases, woodchip bioreactors are combined with filtering materials to improve phosphorus removal (see Table 3). For example, optimized woodchip bioreactors in combination with two types of phosphorus filters (acid mine drainage treatment residues and steel slag) were investigated for phosphorus removal [84]. Similarly, woodchip bioreactors (N removal) and aluminized zeolite filters (phosphate P removal) combined with constructed wetlands were used to remove nitrogen and phosphorus [85], with a 15% reduction in the total phosphorus load in the catchment.

Published experiments with woodchips bring inconsistent results and readers should follow experimental conditions carefully, as there are several papers presenting the misuse of woodchips, with contradictory results. A special consideration in experimental and technological design is the redox potential and oxygen conditions, as a successful process, in this case, runs under aerobic conditions. For example, in experiments in a real ecosystem with a woodchip wall (Pinus radiata and gravel) to remove nitrates from groundwater, the water quality of the influent, the woodchip wall, and the treated water was monitored [86]. They found that after burying the woodchips, phosphorus, ammonium, and organic carbon were released into the groundwater. This was due to the low redox state caused by the released dissolved organic carbon. Woodchip reactors were also used in mesocosms to monitor the reduction of nitrate and soluble reactive phosphorus [87]. The reactors were successful in reducing nitrate. However, soluble reactive phosphorus was sometimes higher in treated variants than in the control. Their results show that rotating wetting and drying cycles can prevent phosphorus release. We suggest that the mechanism of phosphorus retention may be related to the cell wall polymers of the wood vessels. The annual nitrogen removal efficiencies ranged from 6 to 55%, and bioreactors retained from 12 to 77 mg P/m³/d. The further investigation of the use of oak woodchips in denitrifying bioreactors is recommended as the results showed good potential [88].

In the United States, woodchips from high-tannin tree species (oak) are no longer recommended by the federal standard to be used in bioreactors due to the concerns about potential phosphorus release [87,88] because tannins can negatively affect denitrifying bacteria or aquatic organisms downstream. However, iron (Fe) redox cycling is known to be a key factor in the phosphorus dynamics in woodchip bioreactors [89]. Fe-oxide-based media should be placed under oxic conditions near the inlet of the denitrifying bioreactor. Microbial activity is also a key factor in phosphorus removal in nutrient recycling bioreactors, depending on P availability and the hydraulic retention efficiency, and phosphorus removal rates can be similar to those of nitrate.

Table 3. Woodchip bioreactors used for P removal.

Method	Principles and Additional Information	Reference
Woodchip bioreactors	Combination of bioreactors with filters; dissolved P removal, pollution swapping by-products based on flow conditions	[84]
	Study of N and P reduction efficiency in edge-of-field mitigations; constructed wetlands, aluminised zeolite filters; the highest reduction made using the catchment collective approach	[85]
	Pinus radiata woodchips and gravel; evaluation of pollution swapping	[86]
	Reduction of N in mesocosms by woodchip bioreactors, but soluble reactive P was higher	[87]
	Autochthonous P generated by reduction of iron; microbiological removal of P depends on P availability, and hydraulic retention efficiency may be comparable to nitrate removal	[89]
	Woodchip filters with P sorbent; ability to remove total suspended sediments, but the ability to remove P is minimal	[60]

Table 3. Woodchip bioreactors used for P removal.

Method	Principles and Additional Information	Reference
Woodchip bioreactors	Combination of bioreactors with filters; dissolved P removal, pollution swapping by-products based on flow conditions	[84]
	Study of N and P reduction efficiency in edge-of-field mitigations; constructed wetlands, aluminised zeolite filters; the highest reduction made using the catchment collective approach	[85]
	Pinus radiata woodchips and gravel; evaluation of pollution swapping	[86]
	Reduction of N in mesocosms by woodchip bioreactors, but soluble reactive P was higher	[87]
	Autochthonous P generated by reduction of iron; microbiological removal of P depends on P availability, and hydraulic retention efficiency may be comparable to nitrate removal	[89]
	Woodchip filters with P sorbent; ability to remove total suspended sediments, but the ability to remove P is minimal	[60]

4. Wetlands and Macrophytes

Wetlands (constructed or natural) can effectively remove phosphorus and nutrients from various sources (Table 4), including domestic and industrial wastewater, as well as surface runoff from agricultural ecosystems. Surface runoff significantly contributes to phosphorus contamination, particularly during high-volume, intense rainfall events. According to Heathwaite et al. [18], high phosphorus concentrations were recorded in the dissolved fraction of surface runoff, with storm runoff exhibiting phosphorus levels six times higher than those during baseflow conditions. Wetlands used to remove phosphorus from agricultural runoff [20] are discussed as a sink of phosphorus because of the sedimentation of suspended particles and organic material [9]. Wetlands retain phosphorus through a combination of short-term and longer-term mechanisms. Uptake by macrophytes and riparian vegetation represents temporary storage, as nutrients are re-released upon plant senescence and decomposition. In contrast, sediment can act as either a temporary reservoir or a more stable sink, depending on the strength of binding and redox conditions. This distinction underlines the role of wetlands as both dynamic regulators and potential long-term sinks of phosphorus.

Wetlands of natural origin are waterlogged ecosystems, usually with seasonal fluctuations in water levels and typically with the growth of hydrophytic plants. These ecosystems have the capacity to capture phosphorus and particulate matter from upland water flow by retaining some of the particles and absorbing bioavailable phosphorus through the roots of macrophytes and riparian vegetation [9]. Constructed and maintained natural wetlands represent a low-cost, low-tech process to control environmental pollution incoming from domestic, industrial, or agricultural effluents to watercourses or water bodies [12]. These systems are designed to enhance water quality through the retention of phosphorus-utilizing physico-chemical and biological processes [9,90]. Furthermore, constructed wetlands have demonstrated the ability to address a variety of additional environmental issues, including high levels of nitrogen, greenhouse gases, organic matter [90], suspended and dissolved solids, toxic compounds, and harmful microorganisms [12]. There are numerous types of constructed wetlands; the main categories include free water surface flow constructed wetlands, subsurface flow constructed wetlands, floating constructed wetlands, and hybrid constructed wetlands [12].

Constructed wetlands (CWs) in the agricultural landscape help to clean polluted water from both point sources like farms as well as diffuse sources such as tile drainage [91,92], especially during the vegetation season [9]. They are popular for their long-term operation and especially for their low maintenance requirements and economic and aesthetic advantages [12]. Constructed wetlands face numerous challenges, including limited oxygen availability, electron limitation, the potential for clogging, and diminished efficiency during low winter temperatures. These challenges can be effectively addressed through several strategies, such as the implementation of artificial aeration, the selection of cold-tolerant plant species, the addition of electron donors, and the establishment of a comprehensive maintenance plan [90]. In fact, constructed wetlands can also just be a container (as small as a bucket or as large as a very large pond) planted with mainly aquatic (submerged or emergent) plants, but sometimes also with terrestrial plants. Inflow wastewater current slowly flows either horizontally or vertically from one part to the other end—outflow. During this process, changes in oxygen levels create an environment suitable for different groups of microorganisms, which remove organic pollutants and nutrients.

Important construction parameters include the type of substrate in which the plants grow and the container material. Both usually have a certain cleaning capacity by themselves. The roots of the plants, especially aquatic macrophytes, both emergent and submerged, act as a giant biological filter, removing or bioaccumulating organic matter of a different composition. At the same time, microorganisms residing in the submerged roots degrade other pollutants that are later absorbed by the plants [16]. After the treatment, water is often discharged into natural water bodies or used to irrigate non-edible plants without further treatment. In some of the constructed wetlands, plant maintenance or replacement is required regularly.

The Sequential Sedimentation–Biofiltration System (SSBS) is a multi-zone constructed wetland composed of three distinct compartments: sedimentation, geochemical, and biofiltration zones [93,94]. The SSBS operates based on the complex relationships among nutrient input, environmental hydrological parameters, the operation of the biological structure, and the associated biogeochemical processes.

Although natural wetlands are not specifically designed to remove nutrients, such as phosphorus, they indirectly achieve this through the plants’ use of phosphorus compounds, as noted by de-Bashan et al. [16]. Typically, dissolved inorganic phosphorus, dissolved organic phosphorus, particulate inorganic phosphorus, and particulate organic phosphorus are the forms of phosphorus that can be intercepted in a wetland. However, organic and particulate forms must be converted to inorganic forms by microbial activity before they are bioavailable to macrophytes [3]. Long-term monitoring in the Norwegian agricultural catchment showed an annual phosphorus removal efficiency of 22%, with a summer phosphorus removal efficiency of 29% in constructed wetlands [91].

Wetlands are often rich in organic matter, and a substantial proportion of phosphorus may be stored in organic forms. The principal sources of organic phosphorus in these ecosystems typically include macrophytes and their decomposed biomass, together with contributions from algae, periphyton, microbes, and organic matter transported by inflowing water. It should be noted, however, that some wetlands, particularly those developed on mineral soils, may contain lower amounts of organic matter and a higher share of dissolved inorganic phosphorus [3]. Organic phosphorus in wetlands in agricultural catchments may include organic forms of phosphorus associated with pesticides (herbicides, fungicides). Phosphorus solubility is influenced by pH and redox potential, being highest at a low pH and low redox potential, where phosphorus may be released from metal-bound forms such as iron and aluminum complexes [3]. Organic phosphorus that can be extracted by alkalis includes the available forms of organic phosphorus, whereas calcium-bound phosphorus (apatite) is usually not available to macrophytes [3]. Calcium regulates the availability of inorganic phosphorus in alkaline wetland sediments. In acidic sediments, the solubility of phosphorus is governed by the presence of iron and aluminum. The inorganic forms of phosphorus dominate the bioavailable fractions, while the organic fractions of phosphorus dominate the total phosphorus content of the wetland. The potassium chloride extracted inorganic phosphorus fraction is considered bioavailable for uptake by vegetation and/or microbes. The overlying water column does not readily absorb the inorganic fraction of the phosphorus extracted by sodium hydroxide [3].

An important component of wetlands is macrophytes and microbial consortia (Table 4). Macrophytes are represented by submerged (e.g., Myriophyllum aquaticum), emergent (e.g., Typha spp. and Phragmites), and natant (e.g., Lemna spp., Pistia stratiotes, Azolla pinnata, and Pontederia crassipes) aquatic plants. The selection of suitable macrophytes is crucial and is guided by the environmental conditions, which may sometimes be specific. For example, in the study by Karstens et al. [95], plants were selected for their ability to withstand salinity fluctuations. In eutrophic shallow lakes, the stability and metabolic activity of submerged and free-floating macrophytes are greatly enhanced by a diverse community characterized by both spatial and temporal variability. This diverse population includes species such as Lemna minor, Hydrocharis dubia, Trapa maximowiczii, Nymphoides peltata, Potamogeton spp., Myriophyllum spicatum, Najas marina, Elodea nuttallii, etc. Macrophytes can rapidly improve the transparency of water, and the mosaic community of macrophytes can maintain clear water for a long period of time. At a retention time of 7 days, a mosaic community of macrophytes removed 58% of algae biomass, 66% of NH₄⁺–N, 60% of TN, 72% of TP, and 80% of PO₄³⁻–P from eutrophic water [96]. During the growing season, wetland macrophytes can translocate between 25% and 75% of phosphorus input into their root tissues. Wetland macrophytes also help to regulate the redox conditions and phosphorus retention and improve oxygen levels in the water column in a similar way to the rhizosphere through the root system [9]. Nie et al. [97] demonstrated that ponds with submerged macrophytes had significantly reduced concentrations of total phosphorus, total nitrogen, and chemical oxygen demand, but increased dissolved oxygen concentrations. However, the effective management of wetlands must consider that the biomass of autotrophic organisms generates oxygen through the process of photosynthesis during daylight hours, while consuming it at night. Therefore, inadequate management may lead to a risk of oxygen depletion in the aquatic environment.

Submerged macrophytes can reduce the concentration of various phosphorus species in the overlying water. This is achieved through the uptake of phosphorus from the water, the inactivation of alkaline phosphatase activity in the sediment and overlying water, a reduction in sediment resuspension, and the control of the release of internal phosphorus loading. The relative growth rate and phosphorus removal capacity of five submerged macrophytes were evaluated in a greenhouse through hydrotropic experiments conducted over two seasons (spring and autumn). The selected macrophytes were Ceratophyllum demersum, Elodea canadensis, Potamogeton crispus, Myriophyllum spicatum, and Vallisneria spiralis. During both seasons, Ceratophyllum demersum had the highest total phosphorus removal rates (92% in spring and autumn) [17]. Typically, species such as Typha latifolia, Phragmites australis, and Juncus effusus are recommended to remove high phosphorus loads [9].

The removal capacity of phosphorus by a natural wetland can be substantial. An assessment of the contribution of duckweed Lemna gibba, a macrophyte, and its associated microorganisms (algae and bacteria forming an attached biofilm) to remove nutrients showed that the biological floating mat complex (plants and microbes) is responsible for removing up to 75% of the nutrients in the wastewater [98]. Drawing inspiration from this natural process, the use of floating wetlands has been shown to enhance water quality in reservoirs and watercourses. They also increase the oxygen levels in the water column and provide an ideal habitat and shade for birds and other aquatic animals.

Floating wetlands are constructed using lightweight, often synthetic materials [95], such as polyethylene terephthalate, polystyrene, polyurethane, polypropylene, and polyvinyl alcohol. However, natural alternatives are becoming more common. The floating part contains a grid—a net of synthetic or natural origin in which aquatic plants are placed. Although floating technological wetlands may serve as an aesthetic element, it is important to remember their primary purpose, i.e., to restore, improve, and manage water quality. The root system of macrophytes used in floating wetlands, along with the bacterial biofilm attached to it [95,99,100], helps to reduce the nutrient load in reservoirs. Microorganisms such as bacteria and algae can also be attached to the macrophytes and consume phosphorus directly from the water column. Winston et al. [101] studied floating treatment wetlands with plants in stormwater ponds. They observed that the concentrations of N, P, and K in plant biomass were higher beneath the floating island mat compared with the levels above it. Additionally, it was found that a minimum percentage (18%) of the pond water surface covered by floating wetlands was sufficient to improve total phosphorus and total dissolved solids, especially if the active management of the floating wetlands position is realized.

Furthermore, floating wetland macrophytes have been found to mitigate the pollution caused by metals [102], pesticides [103], and toxic substances present in industrial effluents. For example, Cule et al. [104] conducted a study on the phytoremediation potential of various plant species (Phragmites australis, Canna indica, Alissmaplantago aquatica, Menianthes trifoliata, and Iris sibirica) in water contaminated with low concentrations of chromium and nickel. Their results showed that these species were able to bioaccumulate both metals mainly in their roots and rhizomes but did not translocate either metal to their shoots. Notably, P. australis exhibited a great capacity for the bioaccumulation of both metals. The study highlighted the significance of species diversity in the selection of plants for floating treatment wetlands, as certain species were able to accumulate only one of the metals. The study by Pavlidis et al. [103] investigated the effectiveness of floating wetlands in removing agrochemicals. The results for the monitored plants, Lemna minor, Azola Pinnata, and Eichhornia crassipes (water hyacinth), showed that these wetlands reduced pesticide levels by between 12% and 42%, while nutrient reductions ranged from 27% to 83%. In addition, the removal of pollutants was found to be more effective under the conditions of a high temperature and sunlight.

Combining phytoremediation with bioaugmentation, which involves adding bacteria or fungi, significantly enhances the effectiveness of the treatment compared with using either method alone. Added microorganisms can boost plant performance by improving nutrient availability, reducing stress from contaminants, or serving as biocontrol agents against pathogens [105]. A recent study investigated the use of floating wetlands in combination with specific bacteria to mitigate harmful azo dye metabolites. In the presence of Acinetobacter junii, Pseudomonas indoloxydans, and Rhodococcus sp., the plant Phragmites australis was able to degrade dyes into at least 20 different metabolites and remove 96% of the dye from the water [106].

Special attention should be paid to the use of globally invasive plants in treatment wetlands, such as water hyacinth (Pontederia crassipes). In fact, it is used around the world, with positive results in the treatment of water from nutrients and organic pollutants to the reduction in coliforms and Escherichia coli and lower concentrations of microcystins [107]. Further research into the role of native and non-native macrophytes is essential to assess their potential applications in this context. This research will determine whether the negative environmental impacts associated with the use of invasive species, such as water hyacinth, are justified, or whether the use of native plants is more beneficial.

The concept of floating treatment wetlands can be upgraded, and their economic value increased by directing the cleaning process and discarded effluent according to specific requirements, more wisely than as is often performed randomly in natural wetlands. This approach, known as an “engineered floating wetland” or “treatment floating wetlands”, may provide a better solution for operating wetlands in cold climates during winter.

In the context of biomass utilization, as macrophytes senesce and decompose, nutrients can be released back into the water very rapidly, leading to the mineralisation of organic particles and the release of dissolved reactive and organic forms of phosphorus [3]. Consequently, it is essential to implement effective management and treatment strategies for wetlands to maintain their functionality and ecological integrity.

5. Vegetated Buffer Zone

Vegetated buffer strips or buffer zones (VBZs) are a type of constructed ecosystem used to reduce nutrient loading on agricultural land without tiled drainage systems [9] or between agricultural lands and rivers to protect water quality [108]. These designated areas serve as infiltration and percolation systems, cultivated with various woody species, including willow, poplar, and alder, as well as macrophytes, such as reed, water mint, and water iris. They are also used to restore the balance of aquatic ecosystems along watercourses [109]. Additionally, vegetated buffer zones can also be installed between a wastewater treatment plant and a watercourse to further clear the water from nutrients [110].

Pollutant retention processes in VBZs include primarily microbial degradation, infiltration, deposition, filtration, adsorption, and assimilation. Their effectiveness depends on buffer zone width, runoff intensity, slope, soil texture, temperature, vegetation type, and other factors [111]. Furthermore, the efficiency of riparian buffers and wetlands in retaining P is significantly influenced by the specific form of phosphorus entering the wetland. Research indicates that riparian buffers are more effective at retaining particulate phosphorus than dissolved phosphorus. Generally, the retention of dissolved phosphorus is often less than 0.5 kg P per hectare per year; however, this amount can be increased through the harvesting of plant biomass. On the other hand, some studies have indicated that higher concentrations of dissolved phosphorus can be released (mainly due to reductive dissolution of Fe-hydroxides), with rates reaching as high as 8 kg P per hectare per year [92].

In addition to P form, several critical factors strongly influence phosphorus removal efficiency in VBZs: (i) hydrology and hydraulics (residence time, hydraulic loading, flow regime), (ii) substrate properties and sorption capacity (Fe/Al/Ca content, pH, redox, ageing/saturation), (iii) temperature and seasonality, (iv) vegetation composition, biomass and harvesting, (v) the share of particulate/colloidal fraction and its settleability, including the effects of maintenance and sediment resuspension.

Vegetated buffer zones contribute to nitrogen removal through a number of microbial processes, including ammonification, (de)nitrification and ammonia volatilisation, and to a lesser extent by biomass uptake [110]. Phosphorus is absorbed by macrophytes, and the most important process for removing phosphorus is retention by the roots and soil. Riparian vegetation in combination with microbial activity is important for sorption processes (phosphorus and nitrogen removal) through roots, stems, and leaves [112] and it also improves the sorption capacity of sediments. The form of phosphorus further influences the dominant removal mechanism: particulate P is primarily removed by sedimentation, dissolved forms by sorption/precipitation, and bioavailable fractions by plant uptake, although the overall performance is typically governed by loading.

Plant species also influence nutrient remediation. To optimize remediation efforts, it is crucial to select plant species that are specifically suited to the local geography, climate, and environmental conditions [112]. The study conducted by Hu et al. [113] examined the effects of herbaceous vegetation in buffer strips using a simulation device. Total suspended solids, nitrogen, phosphorus, ammonia nitrogen, and chemical oxygen demand were monitored. The findings revealed that variations in the biological structure and growth characteristics of vegetation had an impact on the reduction capacity of pollutants. The authors identified physical adsorption as the predominant mechanism for load reduction, which is closely associated with the morphological characteristics of vegetation. Their findings indicate that the effectiveness of various plant species in load reduction is ranked as follows: Dichondra repens Forst > Cynodon dactylon (Linn.) Pers > Zoysia matrella > Festuca elata Keng ex E. Alexeev > Lolium perenne. Cynodon dactylon and Dichondra repens decrease ammoniacal nitrogen content and TP, whereas Lolium perenne was not able to reduce the observed pollutants. The authors of the study noted that it is important to design the buffer strips for both individual and simultaneous pollutant reduction, because multiple pollutants co-exist in runoff.

Vegetated buffers for phosphorus removal were also studied by Duan et al. [109]. The study used a selection of common plants (Setaria viridis (L.) Beauv., Humulus scandens) in the study area to test their ability to remove pollutants. The width of the VBZ was estimated using a time model and a hydraulic model, resulting in optimal widths of 60 m for the main stream and 40 m for the tributaries. The findings revealed that the nutrient uptake capacity within the 40 to 60 m wide vegetated buffer zones ranked as follows: NH₄⁺-N > TN > TP. Overall, these VBZs demonstrated the ability to remove 20–25% of nutrients from rainfall runoff. This research demonstrates that the careful selection and arrangement of plant materials can significantly enhance water quality by effectively removing pollutants. The optimization of grass buffer strip width was also addressed in the study by Miao et al. [114]. The highest level of removal (total phosphorus, nitrate nitrogen N-NO₃, total nitrogen, NH₄⁺-N) was achieved with buffer strip widths around 25 m, while the maximum cumulative removal rate stabilized at widths of 35–45 m.

Vegetated buffer zones as a complementary treatment of eutrophic substances from wastewater treatment plants were studied by Koenig and Trémolières [110]. They compared two VBZs with different characteristics, such as differences in size, effluent discharge, soil texture, and type of vegetation planted. In particular, soil texture and hydraulic load/surface ratio were the most important parameters in the studied system, with clayey soils being more favourable than sandy soils. Microbial processes of water quality improvement were limited by a low hydraulic retention time and high loading [110]. The study showed that an impressive range of 3000 to 5500 kg P/ha/year were removed. The improved phosphorus removal was explained by better water infiltration and the renewal of the willows.

Vegetated buffer zones together with all types of wetlands for phosphorus removal seem to be a strong method for near-nature measures and should receive greater attention from both scientists and projects developers; when established in the proper way and structure, they can remove phosphorus, which can be used for P recycling, an important part of sustainable P use.

Table 4. Overview of the methods of P removal by wetlands and macrophytes.

Method	Principles and Additional Information	Reference
Wetlands	Biological uptake of bioavailable P forms; can remove 15 g/m2/year of TP, but can be a source of P	[3,9,20]
	Constructed wetlands; (sub)surface flow systems; sink or source of P; removal rate of TP depends on loading rate; median of TP was 1.2 g/m2/year	[9,16,92]
	Floating wetlands; Lemna gibba and associated microorganisms; root system of macrophytes with the bacterial biofilm	[98]
	Constructed floating wetland for sewage treatment; P uptake rate by B. articulata was 12.9 g/m2 L. salicaria has the highest phosphorus removal compared with the other tested plants Agrostis alba, Canna spp., Iris hexagona, Juncus effusus, and Sagittaria lancifolia, P uptake rate was comparable for all plant species	[95,99,100]
	Vegetated buffer zones (type of constructed wetland); Setaria viridis (L.) Beauv., Humulus scandens (Lour.) Merr.; important to select species that are common to the local geography, climate, and conditions; Dichondra repens Forst > Cynodon dactylon (Linn.) Pers > Zoysia matrella > Festuca elata Keng ex E. Alexeev > Lolium perenne	[110]
	P in particulate form on the soil surface, plant stems, and roots intercepts soil particles during rapid runoff; buffer zone construction framework (small watershed)	[109]
	Buffer zones larger than 10 m show a total phosphorus intercept of 50%; large amounts of harmful substances are sorbed by plants (roots, stems, leaves) with microorganisms	[112]
	The reduction capacity of buffer strips depends on the differentiation of the biological structures and growth characteristics; high biomass was produced by Dichondra repens Forst and Cynodon dactylon (Linn.) Pers.	[113]
	Sequential Sedimentation–Biofiltration System; multi-zone constructed wetland with average removal efficiencies of 37% (TP), 46% (TN), and 45% (NO3−)	[93,94]
Macrophytes	Typha latifolia, Phragmites australis, and Juncus effusus	[9]
	Ceratophyllum demersum, Elodea canadensis, Potamogeton crispus, Myriophyllum spicatum, and Vallisneria spiralis	[17]
	C. demersum and C. vulgaris	[77]
	Lemna minuscula had no effect on nutrient removal from wastewater after wastewater treatment by Chlorella vulgaris	[76]
	Eichhornia crassipes, Lemna minor, Hydrocharis dubia, Trapa maximowiczii, Nymphoides peltata, three types of Potamogeton, Myriophyllum spicatum, Najas marina, and Elodea nuttallii	[96]
	Local wetland vegetation; Phalaris arundinacea L., Typha latifolia L., Iris pseudacorus L., Glyceria fluitans L., Sparganium erectum L.	[91]
	Water hyacinth	[107]

Table 4. Overview of the methods of P removal by wetlands and macrophytes.

Method	Principles and Additional Information	Reference
Wetlands	Biological uptake of bioavailable P forms; can remove 15 g/m2/year of TP, but can be a source of P	[3,9,20]
	Constructed wetlands; (sub)surface flow systems; sink or source of P; removal rate of TP depends on loading rate; median of TP was 1.2 g/m2/year	[9,16,92]
	Floating wetlands; Lemna gibba and associated microorganisms; root system of macrophytes with the bacterial biofilm	[98]
	Constructed floating wetland for sewage treatment; P uptake rate by B. articulata was 12.9 g/m2 L. salicaria has the highest phosphorus removal compared with the other tested plants Agrostis alba, Canna spp., Iris hexagona, Juncus effusus, and Sagittaria lancifolia, P uptake rate was comparable for all plant species	[95,99,100]
	Vegetated buffer zones (type of constructed wetland); Setaria viridis (L.) Beauv., Humulus scandens (Lour.) Merr.; important to select species that are common to the local geography, climate, and conditions; Dichondra repens Forst > Cynodon dactylon (Linn.) Pers > Zoysia matrella > Festuca elata Keng ex E. Alexeev > Lolium perenne	[110]
	P in particulate form on the soil surface, plant stems, and roots intercepts soil particles during rapid runoff; buffer zone construction framework (small watershed)	[109]
	Buffer zones larger than 10 m show a total phosphorus intercept of 50%; large amounts of harmful substances are sorbed by plants (roots, stems, leaves) with microorganisms	[112]
	The reduction capacity of buffer strips depends on the differentiation of the biological structures and growth characteristics; high biomass was produced by Dichondra repens Forst and Cynodon dactylon (Linn.) Pers.	[113]
	Sequential Sedimentation–Biofiltration System; multi-zone constructed wetland with average removal efficiencies of 37% (TP), 46% (TN), and 45% (NO3−)	[93,94]
Macrophytes	Typha latifolia, Phragmites australis, and Juncus effusus	[9]
	Ceratophyllum demersum, Elodea canadensis, Potamogeton crispus, Myriophyllum spicatum, and Vallisneria spiralis	[17]
	C. demersum and C. vulgaris	[77]
	Lemna minuscula had no effect on nutrient removal from wastewater after wastewater treatment by Chlorella vulgaris	[76]
	Eichhornia crassipes, Lemna minor, Hydrocharis dubia, Trapa maximowiczii, Nymphoides peltata, three types of Potamogeton, Myriophyllum spicatum, Najas marina, and Elodea nuttallii	[96]
	Local wetland vegetation; Phalaris arundinacea L., Typha latifolia L., Iris pseudacorus L., Glyceria fluitans L., Sparganium erectum L.	[91]
	Water hyacinth	[107]

6. Summarisation of P Removal Methods in Point Sources, Diffuse Sources, and Surface Waters

To facilitate the decision-making regarding the choice of phosphorus removal methods, Scheme 2 displays these methods along with their primary applications, key advantages, and disadvantages.

7. Conclusions, Key Findings, Determination of Research Gaps, and Recommendations for Future Research

The management of the phosphorus level in aquatic ecosystems has become an urgent topic, since increasing phosphorus concentrations in surface waters and sediments cause serious health, hygienic, and technological problems for water use. As shown by the current literature search, it is challenging to find all the important data needed to compare different technologies and information from an economic or technological point of view. There are hundreds of published papers using rare or expensive materials, which are far from any consideration for practical use.

Sophisticated technology like struvite production or the pyrolytic recycling of agricultural waste needs a high energy input, and the costs of recycled phosphorus are about 650–700 EUR per ton. Economically and ecologically sustainable technologies, which may be used in realistic practice, should be around 300–330 EUR [115].

That is why grey and green technologies for P recycling have come to be the focus of technology development, as they are simple and cheap to use. However, as is evident from this review, the correct use of some technologies is not trivial and obviously has certain limitations.

Studying and understanding the natural pathways for the removal of anthropogenic phosphorus not only enables the selection of an appropriate and effective process (or a combination of processes) to achieve maximum phosphorus removal but also provides the information for the potential development of ecological management.

To facilitate the easier comparison of different methods and to support decision-making regarding their practical application in specific locations and water bodies, we consider it essential that data on the cost per unit of phosphorus removed or reused become a standard part of publications. Their frequent absence, particularly in the case of nature-based approaches, hinders the comparison of methods and thereby limits both the choice of possible techniques and the recognition of their favourable aspects.

While traditional phosphorus removal methods, such as filtration systems requiring energy and chemicals, remain important at pollution sources like wastewater treatment plants, nature-based solutions present a sustainable alternative. These approaches, which replicate natural ecosystem functions, are cost-effective, manageable, and highly efficient. Their application within natural landscapes—such as wetlands, riparian zones, and fishpond ecosystems—enables broader implementation and integration with ecosystem services, making them suitable for diverse climatic and economic contexts worldwide.

Nonetheless, the limitations of biologically based methods, including decreased efficacy under unfavourable climate conditions and challenges such as costly microalgae harvesting, must be acknowledged. The careful selection of non-invasive native species is crucial to prevent ecological disruption. Filamentous algae and macrophytes that are beneficial for water treatment and are non-invasive within Europe include Cladophora glomerata, Typha latifolia, and Phragmites australis (excluding invasive genotypes, especially from Eurasia). These species are native and widespread across European freshwater and wetland habitats, where they play a natural role in local ecosystems. Their use in water treatment systems is therefore less likely to cause ecological disruption. In contrast, the use of invasive species such as Elodea nuttallii, which is native to North America, and Eichhornia crassipes, native to South America, should be avoided, despite global practical experiences with their high biomass production, as these species pose significant risks to native biodiversity and ecosystem stability.

New scientific approaches for P recycling are abundant in the literature; however, some of the expensive and rare materials are far from up-to-date and therefore are far from technological use. Promising methods include the biotisation of macrophytes, woodchips inoculated with probiotic bacteria, or floating wetlands. It is possible to biotise aquatic plants and transplant them.

Moreover, nitrogen and phosphorus can be easily recycled after removal from water/wastewater as harvested macrophytes, microalgae, algae, or sorbents of biological origin. As is evident from this review, the best solution seems to be a combination of natural methods of nutrient removal, for example, macrophytes can be combined with bacteria in the form of floating wetlands. It is possible to use buffer zones in inflows and biosorbents in point sources of pollution. Bacterial biotisation also helps to treat sediment, which can fix nutrients, and to reduce its amount released into water.

We would also like to emphasize that serious consideration should be given to the use of Fe, Al metal sorbents. Given the limited P bioavailability and reuse, woodchips or biochar technologies appear to be a more appropriate sorption-based and sustainable approach.