Study on immobilization of nitrogen and phosphorus in sediments using modified fly ash composite material

With the continuous acceleration of economic development and urbanization, many rivers, lakes and other water bodies have experienced different degrees of eutrophication and black smell. These will affect people's lives and destroy the ecological environment. Therefore, sediment treatment has become a hot issue studied by domestic and foreign experts and scholars (Yue et al. 2021). River sediment is not only an ‘assembly point’ for containing and decomposing pollutants in the river water body, but also a ‘source’ for releasing nutrients and organic matter to the overlying water, which has a great impact on the water environment of the river. In the case of hydraulic erosion and external disturbance, unstable pollutants are re-released from the sediment and enter the water body again, causing the water body to be polluted again (Zhu et al. 2021). With the effective control of external pollution and the advancement of river water treatment, the problem of internal pollution caused by polluted sediments has become increasingly prominent, and it has become particularly important to repair river sediments and control the release of sediment pollutants (Yixuan et al. 2021).

As a by-product of coal-fired power plants, fly ash is also one of the largest emissions of solid waste (Dongdong et al. 2021). Due to its pozzolanic properties and high content of silicon-aluminum oxide, a large number of studies have confirmed that it has the effect of removing inorganic pollutants such as nitrogen and phosphorus. Many countries recognize it as a ‘green remediation’ material, and it is widely used in the treatment of various types of sewage and the pollution control of sediments (Tomasevic et al. 2013; Yamamoto et al. 2013; Nakamoto et al. 2015).

However, according to the application of fly ash in various studies, it is found that the individual fly ash and modified fly ash materials are both powdery, and the cementation effect achieved by the pozzolanic reaction of fly ash itself is not obvious. There is a risk of secondary pollution to water quality. Therefore, from a large number of literatures, we can find that cement (Woo et al. 2019), water glass (Tian-ling et al. 2021), gravel (Rodrigue Kaze et al. 2021), slag (Yongqi 2017) and other materials with low cost, hydration activity, good gelling effect and certain strength are also widely used in the treatment of sewage and polluted sediments. In the research of this paper, an attempt is made to composite the fly ash with these types of materials with a cementitious effect to improve the integrity and durability of the material. Risks to water bodies from powdered materials are avoided and recycling of materials is facilitated.

The focus of this research is to realize the modification and molding of fly ash, and further improve the material properties of fly ash and determine the best material ratio by adding four auxiliary materials such as cement, water glass, quartz sand and slag. Scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD) and other characterization methods are used to analyze the mechanism of action, and the inhibitory effect of different types and different covering thicknesses of modified fly ash composite material on the release of nitrogen and phosphorus from the sediment is observed through a small-scale test.

Fly ash was provided by a coal-fired power plant in Shandong Province. The cement grade was P.O42.5, provided by a technology company in Shandong. The slag used was commercially available industrial S95 grade alkaline blast furnace slag powder. The water glass was a commercially available industrial-grade white powder with an initial modulus of 3.4(M = n(SiO₂)/n(Na₂O)). The quartz sand was 200 mesh laboratory-specific high-content silica sand. The chemical components of the four materials are shown in Table 1. The sediment used in the experiment was taken from a river in Jinan City, and its basic physical and chemical properties were measured three times as shown in Table 2. The experimental water was ultrapure water filtered by a water purifier, and the water quality indicators are shown in Table 3.

The modified fly ash material was prepared according to the following steps. In the first step, the original fly ash of the coal-fired power plant was calcined in a muffle furnace at 500 °C for 4 hours and then cooled through a 200-mesh screen. In the second step, it continued to mix with 3 mol/L NaOH solution according to the solid-to-liquid ratio of 1:6, then was placed on a magnetic stirrer at 1,500 r/min and stirred for 30 min at room temperature, taken out and washed with deionized water until pH = 7. Then, it was dried in an oven at 80 °C to a constant weight and ground through a 200-mesh screen to obtain alkali-modified fly ash. Finally, it was mixed uniformly with CaO powder at a ratio of 10:1 and calcined at 500 °C for 2 hours to finish the modified fly ash material (Tomasevic et al. 2013; Yamamoto et al. 2013; Nakamoto et al. 2015; Elyamany et al. 2018).

The modified fly ash was mixed with four kinds of auxiliary materials (cement, slag, water glass, quartz sand) at a mass ratio of 9:1 and a water-binder ratio of 0.6. Molds were used to make pellets of about 3 mm, which were cured for 14 days according to the standard curing conditions of cement specimens after drying at room temperature [temperature (20 ± 2) °C, relative humidity ≥95%] (Jipeng 2021; Shuangyue 2021). Finally, four modified fly ash composite materials were obtained, and the physical and mechanical properties, microscopic characterization and application effects of different types of modified fly ash composite materials were compared.

The main research contents were as follows:

• (1)

  The Zeiss scanning electron microscope (SEM and EDS) was used to observe the morphology and element distribution of the material, and the German Bruker X-ray powder diffractometer (XRD) to analyze the crystal and gel phase changes of different materials.

• (2)

  According to the current national and urban construction industry standards for fly ash ceramsite requirements and testing methods (Development 2008; Administration S. 2010a, 2010b), the density, strength, wear rate, void ratio, hydrochloric acid dissolution rate and other indicators of several modified fly ash composite materials were detected.

• (3)
  As shown in Figure 1, based on the above indicators and the results of characterization analysis, the optimal composite material was selected for the application test of the nitrogen and phosphorus suppression effect of sediment. Use A 250 mL conical flask was used to cover the volume ratio of water: sediment: material = 5:2:1, and the vertical thickness of the material cover layer was 1 cm. The control group (No. ①) did not add covering materials, and three experimental groups (No. ②, ③, ④) were added with original fly ash, modified fly ash and modified fly ash composite materials. Three groups of different coverage thicknesses ④, ⑤, ⑥ were set simultaneously. To better mimic the dark environment at the bottom of the lake, the sediments of the Erlenmeyer flasks were partially wrapped in tinfoil, and experiments were carried out at room temperature in the laboratory for 22 days. According to the ‘Technical Specification Requirements for Surface Water and Waste Monitoring’ (Administration S E P 2002) issued by the State Environmental Protection Administration, the concentration changes of ammonia nitrogen (NH_4þ-N), total nitrogen (TN) and total phosphorus (TP) in the overlying water were detected and recorded. Use inhibition rate calculation to reflect its immobilization effect on pollutants, and use formula (1) to calculate the inhibition rate of each index.

  

  (1)

• C₀: Concentration of pollutants in the overlying water released by the uncovered sediment

• C_t: Concentration of pollutants in the overlying water of the cover group

It can be seen from Figure 2 that the unmodified original fly ash in (a) has a smooth, independent spherical structure inside. In (b), the fly ash undergoes a composite modification process of NaOH and CaO, the spherical structure is destroyed, adheres to each other, and the surface is wrapped by flocs. This was the result of the dual effects of alkali dissolution and quicklime calcination. When immersed in a high concentration NaOH solution, the internal vitreous structure of fly ash was destroyed, the bonding energy of Si-O and Al-O bonds was reduced, and it reacted with free Na+ to produce N-A-S-H gel (Rovnaník 2010). After being calcined at a high temperature of 500 °C, the N-A-S-H gel was transformed into a crystalline zeolite, and the vitreous structure was further melted, causing the surface of the sphere to become rough and blocky. The CaO powder also adhered to the surface of the sphere, showing a cotton-like attachment form, which made the fly ash structure loose and surface pores larger.

In (c), (d), (e), (f), various auxiliary materials were added on the basis of modified fly ash and cured into granular form. It can be clearly seen that the addition of auxiliary materials has completely destroyed the internal structure of fly ash. As shown in (c), the hydration reaction of cement is complementary to the pozzolanic effect of fly ash. A large number of columnar and reticulated aluminosilicate gel phases appeared, the spherical structure almost disappeared, and the inside of the material became uneven and the pores were obvious. In (d), the main components of slag are calcium oxide and silicon aluminum oxide, which have high hydration activity and have good synergistic performance with fly ash. It can be seen from the figure that the hydration products are uniformly attached to the surface of the sphere, which significantly improves the pores of the fly ash surface. In (e), the water glass itself acts as a binder, it has high activity and contains dissolved silicon granules. It was easy to react during the polymerization process to form a silica-aluminum gel product, which enhanced the activity and hardness of the material. In (e), the addition of quartz sand increased the Si-O bond inside the structure, and the free Na⁺ continued to polymerize with it to form a gel phase and a zeolite structure. It can be clearly seen from the figure that the interior presents a uniform macroporous morphology, and the gel product and floc are uniformly attached to the surface of the sphere, which is similar to the effect of the slag in (d).

It can be seen from Figure 3 and Table 4 that the peaks and proportions of C, Si, Al and Fe elements in the modified fly ash all show a downward trend, while the peaks of Na, Ca and O elements rise. This phenomenon is due to the dissociation of the internal chemical bonds of the original ash during the modification process, the breakage of the glass body and the generation of new phases so that the excess impurities in the original ash were dissolved. And metal oxides react with Na⁺, Ca²⁺ and hydroxyl radicals to form new chemical products and gel products. The large amount of CaO in the cement made the peak of Ca in (c) rise significantly. CaO and silicon aluminum oxide in the slag increased the peaks of Ca, Al and Si in (d). Water glass is used as a binder and activator, and the gel produced during the curing process increased the proportion of metal elements in the material. Quartz sand and fly ash are also rich in SiO₂, which has little effect on the overall element distribution of the material. It is mainly used as a framework to enhance the strength of the material.

The selection of auxiliary materials mainly relies on its material composition and its own properties. After the modified fly ash was soaked in strong alkali and calcined at high temperature, the element composition of the internal structure changed to a certain extent. As auxiliary materials, cement, slag, water glass and quartz sand, the main material composition provided are the corresponding oxides of Ca, Al, Si, Na and other elements. It can make up for the large number of Si-O and Al-O bonds that are broken after the glass body structure of fly ash is damaged. The increase in CaO also enhances the alkalinity during the water curing process. As a result, the newly added Si-O and Al-O bonds take the opportunity to continue to react with the hydroxyl group, the fly ash activity is continuously activated, and the aluminosilicate gel product is also continuously produced. The increase of the gel product will inevitably increase the overall strength of the material. This series of reactions is the main significance of the water curing process.

In Table 2, the silicon-to-aluminum ratio of various materials is calculated based on the element specific gravity. Experimental studies have found that the silicon-to-aluminum ratio has a greater impact on the strength of the material. Materials greater than 1.97 will experience strength shrinkage in the later period of curing (Huimei et al. 2021). The specimens with low silicon-aluminum content can generate more aluminum-rich gel phases and have a denser microstructure, resulting in higher strength in the curing stage.

As shown in Figure 4, from the XRD spectrum (a) of the original fly ash, it is found that the main diffraction peaks appear at 2θ = 21° and 2θ = 26°. At this time, the main mineral components in the fly ash are quartz and mullite. Compared with the original fly ash, the diffraction peaks of quartz and mullite of the modified fly ash (b) are significantly weaker. In addition, a new diffraction peak appeared near 2θ = 29°, indicating that Na-P1 zeolite and Na-A zeolite were partially formed in the modified fly ash (Li et al. 2003). It can be seen from the figure (c) that after the modified fly ash and cement are mixed and cured, the diffraction peaks of quartz and mullite in the XRD spectrum are further weakened. It shows that during the curing process, the hydration of cement stimulated the active reaction of silica aluminum oxide in fly ash. The Si-O and Al-O bonds in the modified fly ash and cement, as well as the free Na⁺ and Ca²⁺, broke and recombined in an alkaline environment. The specific manifestation is that the diffraction peaks of the zeolite phase around 2θ = 29° are enhanced. In the range of 2θ = 30–50° and 2θ = 10–30°, new diffraction peaks appear obviously. It proves that the reorganization of chemical bonds not only increased the content of zeolite, but also produced cohesive gel products (C-S-H, C-A-S-H and N-A-S-H) (LaRosa et al. 1992; Qing et al. 2020).

In figure (d), the modified fly ash is mixed with slag for curing, and the slag contains more active silicon-aluminum-calcium oxides. Specifically, the diffraction peak at 2θ = 26° is stronger than (b) and (c), while the diffraction peak near 2θ = 29° is significantly stronger than (b) and weaker than (c). According to the literature (Fernández-Jiménez et al. 2005; Yin et al. 2018; Da et al. 2021), slag and fly ash have good synergistic performance. However, when the fly ash content is relatively large, it will reduce the production of torbe mullite, thereby inhibiting the crystallization of calcium silicate hydrate, and will also cause the mechanical properties of modified fly ash + slag granules to decrease. In Figure 4(e), it can be seen that after the modified fly ash is cured and molded with water glass, the characteristic peaks are similar to those in (d), but they tend to weaken compared to (c) and (d). In Figure 4(f), since the main component of quartz sand is SiO2, the diffraction peak of quartz at 2θ = 21° is sharper than that of other types of granules, but the intensity of the diffraction peak near 2θ = 29° becomes weaker. It can be seen that the combination of sodium silicate and quartz sand with the modified fly ash has little effect on the internal mineral composition of the modified fly ash. As the hydration reaction cannot be carried out, the output of the gel product is reduced.

Through the comparison of six sets of XRD, it is not difficult to see that the effect of the modification is to destroy the glass body in the original fly ash, and the result is to produce a Na-type zeolite phase and a small amount of gel product. The purpose of curing the modified fly ash with cement, slag, water glass and quartz sand is to use the respective components and properties of these four materials to further activate the modified fly ash and increase the integrity and intensity of the material. The XRD spectra of the four composite materials all show diffraction peaks of different intensities for the zeolite phase and the gel phase, and the effect shown in the spectrum of modified fly ash + cement (c) is better.

It can be seen from Table 5 that combining the three indexes of actual density, bulk density and particle strength, the composite material made of modified fly ash mixed cement has better mechanical properties. The bulk density is 805 kg/m³, the sum of the wear rate and the damage rate is 5.3% < 6%, which has reached the requirements for medium and high-strength fly ash ceramsite in 《GB/T 17431.1-2010 Lightweight aggregate and its test method》 (Administration S. 2010b) actual application conditions. The hydrochloric acid dissolution rates of the four modified fly ash composite materials are all less than 2%, and they have good corrosion resistance. However, the particle strength and wear rate of the three composite materials mixed with slag, water glass and quartz sand can not meet the basic requirements of the index. This is related to the hydration reaction in the curing process. Both cement and slag have certain hydration properties. The silica-alumina gel product produced during the hydration process is the main source of material strength.

From the above XRD analysis, it is mentioned that too much fly ash will reduce the formation of tobe mullite during the slag hydration process, thereby inhibiting the crystallization of calcium silicate hydrate, which will lead to a decrease in the strength of the material. However, water glass and quartz sand are materials with high strength and are widely used in the production of ceramsite and geopolymers (Criado et al. 2005; Elyamany et al. 2018). In this experiment, considering the control of a single variable and ensuring that fly ash is the main raw material, the mixing amount of water glass and quartz sand is limited. It is speculated that continuing to increase the ratio of the two materials will further enhance the strength of the granules and ensure their integrity.

Based on the above characterization and mechanical performance analysis, cement was finally used as an auxiliary material to produce a large number of modified fly ash composite materials and carry out the experiment of the inhibition effect of nitrogen and phosphorus in the sediment of the water body as shown in Figure 1, and carry out data detection and analysis.

In Figure 5, the difference in the concentration of pollutants released from the sediment covered by different materials is more obvious. From the NH₄⁺-N concentration graph, it can be seen that the original fly ash-covered device will release more NH₄⁺-N to the overlying water in the early stage than the control group. This is because the untreated fly ash is attached to the residual denitrification agent and its by-products (NH₄HCO₃ and NH₄HSO₄) from the denitration process of coal-fired power plants (Hongcai 2013; Yuejin & Nianxin 2019). The calcination and strong alkali soaking in the above-mentioned fly ash modification process can effectively solve the drawbacks caused by the denitration process. And high-temperature calcination and soaking in alkaline solution also promote the decomposition of these residues into gas volatilization (Yu et al. 2015).

Therefore, it can be seen from Figure 5 that the modified fly ash has a better inhibitory effect on the NH₄⁺-N in the sediment. However, the granular material after cement composite shows a more continuous and stable effect. The reason for this partly comes from the removal of excess impurities in the original fly ash by the preliminary calcination, and partly comes from the changes in the internal structure and surface pores of the fly ash during the modification process. In particular, the Na-type zeolite produced during the soaking process of the alkaline solution has a better adsorption effect on NH₄⁺-N through ion exchange (Yu et al. 2015; He et al. 2017). After 22 days, the overlying water NH₄⁺-N concentration of the uncovered control group was 10.444 mg/L, and the TN concentration was 22.743 mg/L. The concentration of NH₄⁺-N and TN in the overlying water ammonia and TN in the three overburden devices of original fly ash, modified fly ash, and modified fly ash composite materials are basically stable. The NH₄⁺-N concentration was 7.836 mg/L, 2.046 mg/L and 1.413 mg/L, and the TN concentration was 16.143 mg/L, 8.072 mg/L and 6.972 mg/L. According to the inhibition rate change curve shown in Figure 6, after 22 days, the inhibition rates of the three overburdens on the NH₄⁺-N of the water body sediment were 24.97%, 80.41% and 86.47%, respectively, and the inhibition rates on the TN were 29.02%, 64.51% and 69.34%, respectively.

From the TP change curve in Figure 5, it can be seen that the three types of fly ash materials all show good suppression effects. This is mainly due to the precipitation of metal ions and phosphate radicals from the dissociation of calcium and iron oxides in fly ash to achieve the purpose of fixing phosphorus (Ragheb 2019). The modified and compounded fly ash, due to the addition of quicklime and cement, increases the content of CaO, which is more conducive to the stable removal of phosphate (Kim et al. 2018). Combining the effect curves of TP in Figures 5 and 6, after 22 days of experiment, the TP concentration of the overlying water in the control group without covering layer was 0.192 mg/L. The TP concentrations of the overlying water in the original fly ash, modified fly ash and modified fly ash composite materials are 0.041 mg/L, 0.024 mg/L, and 0.021 mg/L, respectively; the inhibition rates are 78.65%, 87.50% and 89.06%, respectively.

According to the capacity of the experimental device, the three coverage thicknesses shown in Figure 1 are set. It can be seen from Figures 7 and 8 that increasing the coverage thickness will enhance the inhibitory effect of the modified fly ash composite material on nitrogen and phosphorus in the sediment. It can be compared from the changes in the concentration of NH₄⁺-N and TN that the suppression effect of 2 and 3 cm coatings is more stable than that of 1 cm. After 22 days of the experiment, the NH₄⁺-N concentration of the overlying water in the 1 cm, 2 cm and 3 cm cover layer devices were 1.413 mg/L, 0.949 mg/L, 0.971 mg/L, and the inhibition rates were 86.47%, 90.91% and 90.70%, respectively. The TN concentration was 6.972 mg/L, 5.672 mg/L and 5.200 mg/L, and the inhibition rates were 69.34%, 75.06% and 77.14%, respectively. The TP concentrations were 0.021 mg/L, 0.018 mg/L and 0.012 mg/L, and the inhibition rates were 89.06%, 90.63% and 93.75%, respectively.

It can be seen from the data that increasing the thickness of the covering layer has little effect on the nitrogen and phosphorus concentration of the overlying water in the later period. Combining the stability of the experiment and the utilization of materials, it is believed that the 2 cm modified fly ash composite material cover layer has the best inhibitory effect on the nitrogen and phosphorus of the sediment.

In the research scope of this article, fly ash is used as raw material to make modified fly ash composite materials by means of modification and compounding. The following conclusions have been obtained during structural characterization, performance testing and application tests:

• (1)

  The microscopic characterization and internal mineral composition of fly ash modified by NaOH and CaO changed significantly. The alkaline environment accelerates the rupture of Si-O bonds and Al-O bonds in fly ash, and reacts with free Na+ to produce N-A-S-H gel, which is further transformed into crystalline zeolite. The CaO powder is in the form of a cotton-like attachment, which makes the fly ash loose in structure and has large surface pores. Excess impurities in the original ash are also dissolved during the modification process, and the content of unburned carbon and heavy metal ions attached during the coal combustion process is reduced, avoiding the risk of fly ash releasing pollutants into the water.

• (2)

  Four kinds of modified fly ash composite materials were made by compounding modified fly ash with cement, slag, water glass and quartz sand. From the microscopic characterization and mineral composition, a silica-alumina gel product that can improve the overall strength of the material was produced. The cohesive gel products (C-S-H, C-A-S-H and N-A-S-H) produced by the cement hydration reaction have a significant effect on the strength of the composite material. Therefore, the modified fly ash composite material mixed with cement had stronger disturbance resistance and durability than other materials.

• (3)

  The bulk density, wear rate and breakage rate of the composite material prepared by modified fly ash + cement reached the requirements for medium- and high-strength fly ash ceramsite in the standard. The soluble rate of hydrochloric acid is less than 2%, and the water absorption rate of 1 h is less than 20%, indicating good corrosion resistance and stability. It can be determined that the composite material meets the conditions for practical application.

• (4)

  Through the small-scale test, when the covering thickness was 2 cm, the inhibition rates of the modified fly ash composite material on NH_4þ-N, TN and TP in the sediment reached 90.91%, 75.06% and 90.63%, respectively. The immobilization effects of this composite material on pollutants in water sediment could be well confirmed.

Through experimental research in this paper, the modified fly ash composite material obtained is a granular material with small particle size, high strength and good adsorption performance. It can realize the stable immobilization of sediment pollutants and maintain the water quality of the overlying water body, providing more possibilities for the actual engineering utilization of fly ash. However, the subsequent reuse rate and specific use cycle of this composite material still need further discussion and research. It can be confirmed that the modified fly ash composite material prepared in this experiment can become a water body sediment restoration material with experimental research value.

No funds.

No conflict of interest.

All relevant data are included in the paper or its Supplementary Information.