**4. Phosphorus in sediments**

The role of sediment as a chemical pollutant depends on various factors, such as the size of the particles that make up the sediment, and the amount of organic matter and nutrients present in it. Nutrients can be released through surface runoff, which is influenced by soil type, vegetation, rainfall, and land use practices. The use of phosphate and other fertilizers in agricultural lands, as well as landfills, can contribute to the transfer of persistent pollutants to the soil and, ultimately, to runoff through leaching [79].

Since erosion tends to selectively transport the smallest soil particles, which are often rich in nutrients and organic substances, the high specific surface area of these particles can lead to the absorption of nutrients, toxins, and pests, resulting in their accumulation in sediments. Suspended sediments are the primary vehicle for transporting nutrients, toxic substances, and chemical elements in aquatic environments. Therefore, sediments from eroded soils can have different environmental impacts depending on the materials and elements they contain.

Concerns about the increase in nitrogen, phosphorus, and pesticide compounds in surface and underground waters have been growing in Europe since the 1970s. Agriculture is identified as the main source of phosphorus compounds and sediments in a recent comparison between domestic, industrial, and agricultural sources of pollution in the Mediterranean basin. The European community has responded with the directive EEC 16/6/1991, which aims to protect waters from pollution caused by nitrates from agriculture. In France, this issue has led to the formation of an advisory committee under the supervision of the Ministry of Agriculture and Environment to reduce nitrogen and phosphorus pollution in agriculture.

An analysis by the United States Environmental Protection Agency in 1994 identified agriculture as the primary land use that degrades wetlands and other wetland areas. As nutrients are removed from the soil during the erosion process and transferred to rivers, water reservoirs, and other water sources, eutrophication and oxygen depletion can occur, leading to a decline in water quality [80–82].

The physical-chemical properties of sediments, including amorphous iron and aluminum content, the amount of clay, and other characteristics, play a critical role in the absorption of phosphorus. Once absorbed, phosphorus can be released back into the water due to various biological and chemical activities, leading to water enrichment in certain cases. As stated by Sposito [83], the absorption and removal of phosphorus from surfaces are more complex than for other elements in soil.

#### **4.1 Adsorption of phosphorus on sediments**

Adsorption of phosphorus onto sediments is subject to complex conditions, as the physical, chemical, and biological characteristics of sediments play decisive roles in the absorption and retention of phosphorus. Under different environmental conditions, each of these features plays a different role. Iron and aluminum oxides and hydroxides are among the important chemical properties known to play a role in phosphorus absorption and storage [17]. The high ability of metal oxides and hydroxides to absorb phosphorus is probably due to their high specific levels [84]. Research has shown that the bond between phosphorus and amorphous aluminum is stronger than that with iron, as the bond with aluminum involves both electrostatic and hydrogen interactions, whereas with iron, it is primarily electrostatic [85]. Conducted research on sediment drains and concluded that over 88% of the phosphorus absorption capacity is related to the iron extracted with ammonium oxalate [86].

The amount of silt and clay in sediments is another parameter known to be important in phosphorus absorption. Sanyal and De Datta [87] have highlighted the significance of the relationship between phosphorus absorption and the amount of clay, which may be due to the high specific levels of clay. Some researchers have shown that reducing the particle size exponentially increases the amount of phosphorus absorption [88, 89]. It should also be noted that smaller particles are more easily disturbed by factors such as water waves and become suspended, providing more opportunities to react with dissolved phosphorus in water compared to larger particles [90]. Phosphorus is absorbed in the form of an inner-sphere complex on colloid surfaces, meaning that phosphorus absorption takes place in specific positions on colloid surfaces, and no water molecule is present between the surface and phosphate ion. The phosphate ion is directly absorbed by the functional groups on the surface of colloids. Phosphorus absorption occurs in two stages: surface absorption and sedimentation in a sequential form [85].

*Phosphorus Dynamics in Soil-Water-Sediment Environment DOI: http://dx.doi.org/10.5772/intechopen.113225*

Organic carbon is one of the most important parameters in absorption, and studies have shown that the absorption capacity of phosphorus is dependent on the amount of organic matter in the sediments [91, 92]. There is a significant relationship between the absorption capacity and the amount of organic matter [20]. Ngoyan and Sukias [86] reported a very high correlation between iron and aluminum extracted with oxalate and organic carbon. This study showed that iron and aluminum may bond with organic carbon and play a much more effective role in phosphorus absorption. Under calcareous conditions, simultaneous absorption or precipitation of phosphorus with calcium carbonate has been reported. Solid phase reactions of CaCO3 control phosphorus reactions. The pH level of sediments also plays a key role in phosphorus absorption. With a change in pH, iron and aluminum precipitation occurs. When the pH is higher than 8, phosphorus precipitates by forming a bond with calcium [93].

#### **4.2 Phosphorus adsorption isotherms**

Previous studies on soil have shown that the process of phosphorus absorption is non-linear due to varying energy levels in different binding sites on particles. The high-energy sites are usually occupied and filled first. The exchange of phosphorus between sediments that are resuspended in water and the surrounding water follows a two-stage dynamic, with rapid exchange occurring at the surface of particles and much slower exchange occurring internally [94]. Short-term equilibrium in the range of several days and long-term equilibrium over several months to several years are assumed to model these exchanges in soils. During periods of rapid transport, the absorption of phosphorus into soil materials decreases due to short contact times, low water-to-soil ratios in the flow path, and possibly facilitated colloidal transport. As a result, heavy rainfall and high concentrations of phosphorus may have strong impacts on the dynamics of phosphorus in agricultural basins with low drainage, and high concentrations of phosphorus may occur during periods of high flow. However, true equilibrium is rarely achieved.

Isotherm equations are used to study the mechanisms of phosphorus absorption in sediments and soil [84]. These equations allow for the determination of important absorption parameters, such as the phosphorus concentration at the equilibrium point (EPC0), the degree of phosphorus saturation, and absorption energy [86, 95, 96]. If the EPC0 and degree of phosphorus saturation in the soil are low and absorption energy is high, the sediments will have a high capacity to absorb phosphorus. Comparing the EPC0 and the concentration of reactive phosphorus in the water solution (DRP) can determine whether the sediments act as a phosphorus reservoir or a source of phosphorus.

In general, there are three main reasons for using absorption isotherms: (1) identification of compounds that play a role in the absorption and release of phosphorus, (2) forecasting the amount of fertilizer needed to maximize production, and (3) studying the nature of absorption to understand the mechanism of these processes [87].

To study phosphorus absorption in sediments, a surface adsorption isotherm is generated by shaking the sediment sample with increasing concentrations of the ion solution. The amount of ion absorbed in each sample is calculated by measuring the difference between the initial concentration and the equilibrium concentration. Laboratory methods, such as those developed by Nelson and Logan in 1983, are commonly used to measure the characteristics of phosphorus absorption in sediments. The results obtained from these methods are then fitted with standard absorption

#### *Phosphorus in Soils and Plants*

isotherm models, such as Langmuir, Freundlich, and Temkin, as described in the studies by Rhue and Harris [97] and Graetz and Nair [98]. These equations are suitable for methods that reach equilibrium in a short time, but their range of success is limited to a certain concentration of phosphorus, as reported by [99].

Various equations have been employed to elucidate the relationship between the amount of phosphorus absorbed per unit weight of the adsorbent and the concentration of phosphorus in the solution.

#### *4.2.1 Langmuir equation*

The Langmuir equation is widely used in soil science. It was first used by Fried and Shapiro [100] and later by Olsen and Watanabe [99] to explain phosphate absorption in soil. The Langmuir equation is based on three main assumptions [101]:


The general form of the Langmuir equation is as follows (Eq. 2):

$$S = \frac{bKC}{1 + KC} \tag{2}$$

The surface adsorption isotherm can be expressed using the Langmuir equation, where S represents the amount of absorption, b (also known as Smax) is the maximum absorption of a single layer that occurs as the equilibrium concentration, C, increases, and K is a parameter that reflects the surface's absorption capacity. The Langmuir equation has one main advantage: it reveals the maximum absorption capacity of a surface (Smax). Although other equations, such as Redlich-Peterson and Fowler-Guggenheim, can predict the maximum absorption, they are more complex and challenging to use. In comparison to other isotherm equations, the Langmuir equation offers more comprehensive information about phosphorus absorption in soil and sediment [102]. Haines et al. [103] has utilized mathematical methods to further explore the features of these equations and showed that the slope of the Langmuir equation at a concentration close to zero is equal to:

$$\lim\_{\mathbf{c}\to 0} \frac{\mathbf{ds}}{\mathbf{dc}} = \mathbf{b}\mathbf{K} \tag{3}$$

She also showed that we will achieve maximum absorption on surfaces when the equilibrium concentration is large enough:

$$\lim\_{\mathbf{c}\to\infty} \mathbf{s} = \lim\_{\mathbf{c}\to\infty} \frac{bKC}{\mathbf{1} + KC} = \frac{bK}{K} = b \tag{4}$$

In this equation, it is assumed that the absorbing sites do not participate in the absorption after absorbing phosphorus. In simpler terms, phosphorus absorption on absorbent surfaces occurs as a single layer [97, 102]. The graphic form of this equation is in **Figure 6**.

Sₒ: It is known as the amount of primary phosphorus in the absorption phase. EPCₒ is also known as the concentration of phosphorus at the equilibrium point. At this concentration, the absorption and excretion of phosphorus in sediments reach equilibrium [93, 104]. Between So and EPCₒ, phosphorus is released into the water. The slope of the isotherm line, referred to as K or absorption energy as mentioned before, represents the rate of change. In this graph, it is observed that after the initial rapid absorption, the amount of absorption gradually increases with the equilibrium concentration until reaching the maximum absorption (Smax). After reaching Smax, the slope of the line remains constant.

#### *4.2.2 Freundlich equation*

The Freundlich equation is commonly used for describing phosphorus absorption in soils and has the ability to fit absorption data quite well in most soil types [101]. Freundlich discovered a certain relationship that describes absorption from dilute solutions. This equation is primarily derived from experimental observations, but it can also be obtained theoretically by assuming that the bond energy decreases exponentially with the surface coverage, which is likely closer to the actual absorption conditions [105].

The Freundlich equation is considered superior to the Langmuir equation because it is simple and based on more realistic assumptions. It can account for non-ideal absorption on heterogeneous surfaces and the absorption of multiple layers. Assuming that the reduction in absorption energy with increasing surface coverage is due to the uniformity of the absorbing surfaces justifies the Freundlich equation [101]. The Freundlich equation is commonly defined as Eq. 5:

$$\mathcal{S} = K\_f \mathcal{C}^a \tag{5}$$

In this equation, α (alpha) and Kf are called adjustable positive values, where α is usually between zero and one, but in some cases it is seen that it is more than one. By using logarithms on equal sides, this equation becomes a straight line. Although the

**Figure 6.** *Schematic display of Longmuir isotherm.*

equation itself is sometimes used unchanged for simplicity. This equation with all its ability is not able to predict the maximum amount of absorption, that is, if the equilibrium concentration moves toward its maximum possible value, the amount of absorption also moves toward its maximum value, but it does not reach a constant value. (Opposite to the Langmuir equation), that is:

$$\lim\_{\varepsilon \to \infty} \mathcal{S} = \lim\_{\varepsilon \to \infty} K\_f \mathcal{C} = K\_f(\infty) = \infty \tag{6}$$

Therefore, this equation suggests that as the equilibrium concentration increases, the absorption on the surfaces also increases, and this increase in the amount of absorption is exponential according to the equation. According to some researchers, this equation is better at low equilibrium concentrations [2].

#### *4.2.3 Tamkin's equation*

This equation has found a special place in soil science due to its simpler formula compared to other equations.

$$S = K\_1 \quad \text{In } \ c + K\_2 \tag{7}$$

In this equation, K1 and K2 are constant values. The weak point of this equation is the inability to predict the maximum amount of absorption on the surface [103].

#### *4.2.4 Risk assessment indicators*

In order to identify the points of the sediments that have the greatest potential to release phosphorus into the water, two important parameters, the equilibrium concentration of phosphorus at the zero point (EPCₒ) and the degree of phosphorus saturation (DPS) are used.

#### *4.2.4.1 Phosphorus concentration at the equilibrium point*

The most important result obtained from isotherm studies is the equilibrium phosphorus concentration (EPCₒ), which represents the equilibrium between absorption and desorption processes in pollution studies [36, 95]. By comparing the EPCₒ value to the concentration of dissolved reactive phosphorus (DRP) in the water solution, it is possible to determine whether sediments act as a source or sink of phosphorus. If EPCₒ is higher than the DRP concentration (EPCₒ > DRP), phosphorus will be released from the sediments into the water column. Conversely, if EPCₒ is lower than the DRP concentration (EPCₒ < DRP), sediments are absorbing phosphorus from the water column [84]. In simpler terms, sediments can either release or absorb phosphorus depending on the EPCₒ value, with higher EPCₒ values indicating a higher likelihood of phosphorus being released from sediments into the water column. The EPCₒ value is graphically represented by the point where the isotherm curve intersects the x-axis (equilibrium concentration axis).

#### *4.2.4.2 Degree of phosphorus saturation in sediments*

The investigation of the degree of phosphorus saturation can be traced back to the research of Breeuwsma and Silva [43], who aimed to establish a correlation

*Phosphorus Dynamics in Soil-Water-Sediment Environment DOI: http://dx.doi.org/10.5772/intechopen.113225*

between soil phosphorus content and its transfer to groundwater in Dutch soil. Since then, many researchers have used and modified their approach [86, 106, 107] to determine the critical level of phosphorus saturation. Points with a degree of saturation higher than this critical limit are more likely to release phosphorus into water compared to those with lower saturation levels. Breeuwsma and Silva [43] determined the critical level of phosphorus saturation in Dutch soils to be 25%. Sallade and Sims [40] raised this limit to 40% for sediments in American drains. Ngoyan and Sukias [86] found that although the degree of phosphorus saturation in some sediments located in drains was between 64 and 68%, the dissolved phosphorus concentration in the water of these drains was still low, indicating that the level was not sufficient to cause enrichment. On average, the degree of phosphorus saturation increased to more than 65%. The degree of phosphorus saturation reflects the actual state of phosphorus in the soil [108]. It indicates the amount of phosphorus accumulation in sediments relative to the maximum absorption capacity of phosphorus [86], as expressed in Eq. 8:

$$DPS(\%) = \frac{TP}{PSC} \times 100\tag{8}$$

That DPS is the degree of phosphorus saturation in percentage and PSC indicates the maximum absorption capacity of phosphorus at the desired depth from the soil [17]. TP is the amount of total phosphorus (which is obtained by digesting the sediment sample). The higher the degree of saturation in sediments, the greater the risk of releasing phosphorus into water [86]. Various methods have been used to measure the degree of saturation. Sometimes formula 9 is used in acid soils:

$$DPS(\%) = \frac{P\_{ox}}{Fe\_{ox} + Al\_{ox}} \times 100\tag{9}$$

Pox in this formula is phosphorus extractable by ammonium oxalate and Feox + Alox is the sum of iron and aluminum extractable by ammonium oxalate [109, 110]. Sometimes Olsen or Mehlich phosphorus together with single point absorption index is used to determine the degree of saturation [102, 111]. These methods are referred to as indirect methods. From the direct method to measure the degree of phosphorus saturation, we can refer to the isotherm equations. Which is obtained by determining the amount of absorbed phosphorus and total phosphorus in sediments.

Methods such as the first two mentioned can be used to determine the degree of phosphorus saturation in specific soils, but the isotherm method is also time-consuming [108]. To address these challenges, Pöthig et al. [108] proposed a simpler method. They demonstrated that this method is applicable to all soil types and is independent of land use.

Water-soluble phosphorus (WSP) is determined based on the ratio of phosphorus absorption to total phosphorus (SP/TP). Each soil has its own level of phosphorus absorption and total phosphorus, but dividing these values cancels out their mutual effects:

$$DPS(\%) = \frac{TP}{PSC} \times 100\tag{10}$$

$$P\mathbb{S}\mathbb{C} = TP + \mathbb{S}P \tag{11}$$

where SP is known as absorbed phosphorus and is calculated by isotherm experiments [108]. From the combination of the above two equations, the following relationship is obtained:

$$DPS(\text{@}) = \frac{1}{1 + \frac{SP}{TP}} \times 100\tag{12}$$

Therefore, WSP can be used instead of SP/TP ratio:

$$DPS(\%) = \frac{1}{1 + f(WSP)} \times 100\tag{13}$$

After examining more than 400 different soil samples (sandy, loamy, peaty and calcareous soils), the following relationship was proposed:

$$DPS(\text{@}) = \frac{1}{1 + \mathbf{1.25} \times WSP^{-0.75}} \times 100 \tag{14}$$

In this method, when the concentration of phosphorus soluble in water exceeds 5 mg/kg, the degree of phosphorus saturation approaches 70–80%, which indicates a high risk of phosphorus transfer from soil to water. Due to the ease of determining the degree of phosphorus saturation, this method can determine the dangerous points easily and in the shortest time [108].
