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Article

Control of Endogenous Phosphorus Release at the Sediment–Water Interface by Lanthanum-Modified Fly Ash

by
Ying Pan
1,2,
Gang Liu
3,
Beibei Chai
2,4,*,
Xiaohui Lei
5,*,
Lixin He
2,4,
Shuailong Cheng
6,
Yijie Wang
6,
Wenlong Chen
6,
Simin Li
1,
Liang Chen
7 and
Bin Chen
8
1
School of Energy and Environmental Engineering, Hebei University of Engineering, Handan 056038, China
2
Hebei Collaborative Innovation Center for the Regulation and Comprehensive Management of Water Resources and Water Environment, Hebei University of Engineering, Handan 056038, China
3
Xi’an Municipal Design Research Institute Co., Ltd., Xi’an 710068, China
4
Hebei Key Laboratory of Intelligent Water Conservancy, School of Water Conservancy and Hydroelectric, Hebei University of Engineering, Handan 056038, China
5
State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, China Institute of Water Resources and Hydropower Research, Beijing 100038, China
6
Pearl River Hydraulic Research Institute, Guangzhou 510611, China
7
China Three Gorges Corporation, Wuhan 430010, China
8
State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Beijing Normal University, Beijing 100875, China
*
Authors to whom correspondence should be addressed.
Coatings 2022, 12(6), 719; https://0-doi-org.brum.beds.ac.uk/10.3390/coatings12060719
Submission received: 25 March 2022 / Revised: 17 May 2022 / Accepted: 18 May 2022 / Published: 24 May 2022
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Abstract

:
This study optimizes the modification and granulation of fly ash to make it more stable at the sediment–water interface. Through laboratory simulations, the modified fly ash pellets were optimally granulated to cover the sediment–water interface, and its control effect and mechanism were evaluated. The results showed that the phosphorus adsorption effect of lanthanum-modified fly ash was 34% and 40% higher compared with those of acid-modified and alkali-modified fly ash, respectively, with the phosphorus adsorption effect reaching 85%. The best dosing ratio was about 0.3 g/L. Adsorption was affected by pH and was more effective under weak alkalinity, close to the Langmuir adsorption model, which was consistent with the unimolecular layer adsorption characteristics and the presence of chemisorption and physical adsorption. The saturation adsorption amount of phosphate by lanthanum-modified fly ash was 8.89 mg/g. The optimized granulation conditions for lanthanum-modified fly ash pellets were a fly ash/montmorillonite ratio of 7:3, a roasting temperature of 900 °C, a roasting time of 4 h, and a particle size of 3 mm. After 20 days, the orthophosphate removal rate was more than 60% higher than that of the control group, with a total phosphorus removal rate of 43%. After covering for 60 days, active phosphorus in the surface layer of the sediment was gradually transformed into a stable phosphorus form, with calcium phosphorus accounting for 70% of the total inorganic phosphorus. The ability of the sediment to release phosphorus to the overlying water body was also significantly weakened. Meanwhile, the total phosphorus removal rate in the overlying water at the sediment–water interface reached more than 40%, and orthophosphate removal reached more than 60%, indicating an obvious phosphorus control effect. Transmission electron microscopy analysis showed that lanthanum was present at locations enriched with elemental phosphorus and was adsorbed onto the material surface. Therefore, lanthanum-modified fly ash pellets are a promising in situ phosphorus control agent with good endogenous phosphorus pollution control abilities in eutrophic water bodies.

Graphical Abstract

1. Introduction

Phosphorus is a fundamental nutrient that constitutes organisms, playing a significant role in life activities and influencing nitrogen fixation by plants, and thus limiting the carbon cycle and indirectly affecting global climate change. Phosphorus is considered a nutrient-limiting factor in water bodies and is an important element in controlling primary productivity [1]. Phosphorus concentrations above 0.02 mg/L are susceptible to eutrophication [2]. Eutrophication of water bodies is among the water environment pollution problems requiring urgent address, with the United Nations Environment Program (UNEP) survey of water bodies globally showing that about 30%–40% of lakes and reservoirs are affected by eutrophication [3]. Eutrophication of water bodies not only deteriorates water quality, but also breaks the functional balance of water ecosystems and threatens human health [4]. This causes large outbreaks of cyanobacteria and diatoms, increases plankton and suspended matter, increases organic matter in the water column, creates an irritating taste, decreases dissolved oxygen, and leads to the death of aquatic organisms such as fish [5], which adversely affects the aquaculture industry. This, in turn, affects local tourism and causes economic problems. As eutrophic water also affects the uptake of crop nutrients when irrigating agricultural fields [6], control of phosphorus levels is essential.
Phosphorus sources in water are mainly divided into endogenous phosphorus, exogenous phosphorus, and a small amount of atmospheric deposition phosphorus [7]. After exogenous phosphorus input, a series of physical, chemical, and biochemical effects occurs in the water column, and part of the phosphorus is deposited on the bottom. However, this is not stable and will be released into the overlying water with changes in environmental factors, such as dissolved oxygen, temperature, and season, so that eutrophication will still occur in the water column [8]. Therefore, controlling endogenous phosphorus in the substrate becomes key to preventing and controlling eutrophication in the water column. Current measures to control endogenous phosphorus release include substrate dredging [9], active material cover [10], in situ chemical application [11], bioremediation [12], and in situ sorbent amendment [13]. Substrate dredging [14,15] might result in secondary pollution and complicated subsequent treatment. The active material cover method [16,17,18] reduces the risk of phosphorus overlying the sediment by placing blunt phosphorus materials at the sediment–water interface to form an active cover layer, with technical difficulty derived from identifying green, environmentally friendly, and economical active cover materials [19,20]. In situ chemical methods [21,22] can add iron, aluminum, and calcium salts to form precipitates with phosphate, but too high a dosage can cause pollution. Bioremediation methods are combined with processes, hanging membranes, and long operating cycles [23]. As phosphorus is a strategic resource, its recycling should receive more consideration [24,25]. The adsorption method [26] has advantages of high efficiency, simple operation, no secondary pollution, and recyclability, among others, and has been widely favored by domestic and foreign researchers in recent years [27,28,29]. The identification of cost-effective adsorbents is key to phosphorus removal by adsorption, with some industrial waste materials, such as fly ash, applied as phosphorus removal materials [30].
Fly ash is a by-product of thermal power plant emissions and is rich in active vitreous particles which contain large numbers of fine active pore channels, giving fly ash a large specific surface area. However, fly ash does not have a high adsorption capacity and adsorption efficiency, and it should be modified to achieve better adsorption [31,32]. Such modification not only improves the effectiveness of fly ash treatment of wastewater, but also turns fly ash into a valuable resource, solving the fly ash pollution problem while achieving resource utilization [33,34]. Recently, many researchers have studied rare earth salt ions, finding that lanthanum metal [35,36] has a strong affinity for phosphates [37,38,39] even at a trace level. Lanthanum (La) can combine with PO3−4 to form a small amount of redox-stable and soluble precipitates (LaPO4) [35] with a high adsorption capacity over a wide pH range and does not affect water’s oxidation-reduction potential [40,41]. The La-P complex, which is the most insoluble rare earth phosphorus complex, can be formed at low concentration, and the precipitate formed with phosphate is less affected by pH. Lanthanum has a single oxidation state, good chemical stability [42], and is environmentally friendly. Lanthanum is also abundant in nature, and cheap relative to other metals [34,43]. Thus, La-based P adsorbents have drawn significant attention in the last decade [44]. However, incomplete binding of lanthanum with phosphate alone greatly reduces its effective utilization [45,46], making phosphorus removal difficult and causing harm to aquatic organisms. To overcome this challenge, researchers have prepared different lanthanum-doped materials by combining lanthanum with other materials [47]. Lanthanum-modified adsorbents have promising applications as new adsorbent materials. Currently, most studies focus on the desorption and resorption efficiencies of lanthanum-based adsorbents. Only a few studies focus on the influencing factors in the adsorbent regeneration process, and future research can focus more on this aspect, which is beneficial to reduce energy and water consumption and improve adsorption efficiency [48].
In this study, fly ash was subjected to different modifications and the adsorption performance was compared after modification. Lanthanum modification achieved a better adsorption effect and was preferentially selected. To make the modified fly ash more stable at the sediment–water interface and less likely to be disturbed and float, further optimized granulation experiments were conducted to determine optimal process parameters. The optimized granulated lanthanum-modified fly ash pellets were placed at a simulated sediment–water interface in the laboratory to study the effect on in situ endogenous phosphorus pollution at the sediment–water interface of eutrophic water bodies. Thus, the fly ash, which was waste originally, can be used as an endogenous phosphorus controller combined with lanthanum, and the phosphate adsorbed, which was harmful to water environment, can be revised and reused again. The pollution control mechanism was also explored to provide a theoretical and scientific basis for the practical application of lanthanum-modified fly ash adsorption to the release of endogenous phosphorus in water.

2. Materials and Methods

2.1. Materials

Fly ash was purchased from Zhengzhou Yuen Long Water Purification Co., Ltd., (Zhengzhou, China) washed with deionized water, and then dried and set aside. The original components of fly ash are shown in Table 1.
The reagents used for the experiments (lanthanum chloride, potassium dihydrogen phosphate, ascorbic acid, sulfuric acid, hydrochloric acid, sodium hydroxide, ammonium molybdate, sodium potassium tartrate, ammonium chloride) were obtained from Comio (New Delhi, India).

2.2. Methods

2.2.1. Preparation of Modified Fly Ash

The purchased fly ash particles were rinsed with tap water, repeatedly rinsed with distilled water to neutral pH, placed in an oven, and baked at 120 °C for 2 h. The different fly ash particle sizes were then screened for use. In acid modification, the original fly ash was placed in aqueous hydrochloric acid solution (2 mol/L), shaken at 200 rpm and 25 °C for 36 h, and left to stand to allow sediment–water separation. The supernatant, which is left to stand until the muddy water is separated, is removed, then washed with deionized water to neutral pH, dried, and set aside. In Alkali modification, the original fly ash was placed in aqueous sodium hydroxide solution (2 mol/L), shaken at 200 rpm and 25 °C for 36 h, and left to stand to allow sediment–water separation. The supernatant was then washed with deionized water to neutral pH, dried, and set aside. In Lanthanum modification, pretreated fly ash or the original fly ash (10 g) was placed put into a 250 mL conical bottle. Deionized water was added and ultrasonication was performed to obtain a fly ash liquid dispersion. Lanthanum chloride particles were then added to obtain a lanthanum chloride solution (1.5 mol/L). The mixture was stirred at 250 rpm at 25 °C for 10 h, and then the conical flask was plastic-wrapped and placed in a constant temperature oscillator at 25 °C for 180 rpm for 24 h. The supernatant was washed with deionized water to neutral pH, and lanthanum-modified fly ash, alkali-modified fly ash, or acid-modified fly ash was obtained by drying [49,50].

2.2.2. Adsorption Experiment

Phosphorus adsorption experiments [51] were conducted using the fly ash reagents obtained above. Phosphate solution with an initial concentration of 5 mg/L was prepared for the adsorption experiments. Fly ash was evenly dispersed in the solution by ultrasonication, and then adsorption was performed in the water bath shock chamber at 30 °C with a shock intensity of 200 rpm for 16 h. After being allowed to stand, the supernatant was removed and filtered through a 45 μm filter membrane, and the phosphate content in the remaining aqueous solution was determined. The modified fly ash that showed the best adsorption effect was identified.

2.2.3. Optimal Dosage Test

The optimal dosage was determined for the lanthanum-modified fly ash showing the best adsorption effect, using the initial phosphate solution (5 mg/L), by testing dosages of 0.05, 0.1, 0.2, 0.3, and 0.5 g/L. Three groups of parallel experiments were conducted, and the optimal dosage was determined according to the adsorption effect.

2.2.4. Optimal pH and Temperature Experiments

With the optimal dosage ratio in hand, different pH ranges and temperatures were used to explore the adsorption effect of lanthanum-modified fly ash.

2.3. Fly Ash Granulation Experiment

2.3.1. Process Flow

After mixing the fly ash and montmorillonite evenly, adding water glass, stirring to make it viscous, semi-mechanized to make it into a ball, the sticky substance with a certain viscosity was made into a bar by a manual pelletizing machine. Then, we placed the strips in the pellet machine model, and pushed them back and forth to make them into balls (Figure 1). The prepared spherical fly ash pellets usually contain a certain degree of moisture, and they were first dried in a constant temperature drying oven. After setting, it was finally placed in a muffle furnace and calcined to obtain spherical fly ash pellets.

2.3.2. Orthogonal Experimental Design

To investigate the influence of preheating temperature, preheating time, sintering temperature, and sintering time on the properties of the prepared fly ash pellets, different raw material ratios were selected for an orthogonal analysis experiment. Orthogonal analysis was conducted on five factors at four levels, as follows: Calcination temperature (600, 700, 800, and 900 °C), calcination time (1, 2, 3, and 4 h), fly ash/montmorillonite ratio (4:6, 5:5, 6:4, and 7:3), and particle size (3, 5, 8, and 11 mm). A blank test was also conducted. The effect of montmorillonite on phosphate removal from water bodies was investigated, and its physical properties were analyzed [52,53,54].

2.4. Study of Lanthanum Modified Fly Ash Pellets to Control Endogenous Phosphorus Release at the Sediment–Water Interface

Laboratory simulation experiments were conducted in two tanks containing mud (15 L) and water (60 L) in a 1:4 ratio, with lanthanum-modified fly ash pellets added to one tank and the other used as a blank control [55,56]. The simulated experimental setup is shown in Figure 2. First, the reactor was released and incubated. After allowing the sediment to slowly sink, the contents of different forms of phosphorus in the sediment were measured after phosphorus release reached equilibrium. Fly ash pellets were then added into the water body at a ratio of 2 g/L and covered the top of the substrate for regular sampling and analysis. The indexes of pH, ORP, DO, and water temperature in the water body were monitored. When these indexes were stabilized, the adsorption effect of fly ash pellets on various forms of phosphorus in the water body, and the in situ control effect of phosphorus release from the sediment, were investigated.

2.5. Raw Water Quality

The sediments in the bottom of the center of Changle Park Lake in Xi’an were collected with a Peterson dredger, and the overlying water was collected with a sampler, and immediately returned to the laboratory for processing. Then, the concentrations of different pollutants were determined, including the related determinations of TP, SRP, and TN. Table 2 shows the contents of related indicators in the overlying water.

2.6. Analytical and Characterization Methods

Emission scanning electron microscopy (FESEM), (SU 8020, Hitachi, Japan) was used to obtain the apparent morphology, pore structure, and cross-sectional element distribution of materials, X-ray diffractometry (XRD)( was used to analyze the surface crystal-phase changes of materials before and after loading HLO, pattern of samples was determined by D/max-2500 X-ray diffractometer (X’Pert PRO, PANalytical, Almelo, Netherlands) in the 2θ range of 10–80. Information about the functional groups and specific chemical bonds on the surface of materials was obtained with FTIR (Nicolet 8700, Thermo Fisher Scientific, New York, NY, USA) [57] (The IR spectra were recorded at room temperature with a Nicolet Magna 550 FTIR spectrometer (resolution 4 cm−1) after quenching the samples), and a specific surface (the total pore volume, and the porosity of the adsorbents before and after adsorption were determined using an ASAP2020M porosimeter analyzer (Micromeritics, Thermo Fisher, New York, NY, USA). The samples were degassed at 80 °C for 24 h before BET analysis) area test was used to analyze the modified fly ash. Transmission electron microscopy (TEM; FEI Tecnai G2 F20, DeFelsko, New York, NY, USA), X-ray fluorescence spectrometry, XRD, and other techniques were used to characterize the blank fly ash pellets and fly ash pellets from the experimental water body and to study their adsorption principle. The different phosphorus forms in sediment were determined by the SMT [58,59] (The SMT method was used to determine the content of different forms of phosphorus in the sediment, mainly including TP (total phosphorus), IP (inorganic phosphorus), OP (organic phosphorus), NAIP (non-apatite inorganic phosphorus), and AP (calcium phosphorus)) method. Each experiment was set up to repeat the experiment, and three parallel experiments were carried out.

3. Results and Discussion

3.1. Study of Adsorption Efficiency of Modified Fly Ash

3.1.1. Adsorption Effect and Characterization of Different Modified Fly Ash Materials

The lanthanum-modified fly ash showed the best removal effect for orthophosphate. At the same initial concentration, the lanthanum-modified, alkali-modified, and acid-modified fly ash materials showed approximately 85%, 56%, and 46% removal of phosphate, respectively, after adsorption equilibrium, which was reached after approximately 120 min.
It can be seen from Table 3 that compared with the original fly ash, the specific surface area of the lanthanum-modified fly ash adsorbent material was increased three-fold and the total pore volume was increased seven-fold, resulting in significantly improved adsorption performance.
Table 4 shows that the SiO2 contents of differently modified fly ashes were reduced to varying degrees compared with the original fly ash. The exchange of inherent elements is caused by the iron, aluminum, and calcium compounds in fly ash having the ability to form precipitates with phosphates in aqueous solution, while the iron, aluminum, and calcium compounds in acid- or alkali-modified fly ash have lower contents than the original fly ash.
As shown in Figure 3, the analysis in Figure 3 observes and analyzes the material morphology and size of the four different modified fly ash by SEM (Electron microscope magnified 400 times). The lanthanum-modified fly ash had a rougher surface, resulting in an increased specific surface area, which can also be confirmed by the BET analysis (as shown in Table 3) with nitrogen as the adsorbate and helium or hydrogen as the carrier gas, mixed in a certain proportion. The sample will adsorb nitrogen, and an adsorption peak will appear; after nitrogen is desorbed, a desorption peak will appear. A calibrated peak was obtained by injecting a known volume of pure nitrogen into the mixture. According to the peak area of the calibration peak and the area of the desorption peak, the adsorption amount of the sample under relative pressure can be obtained, and then the specific surface area can be calculated according to the BET formula. The physical appearance of fly ash has not changed greatly compared with the original form, but it can be seen from the figure that many denser dots have been added to the outer surface morphology of fly ash. It is speculated that the dots may be modified by lanthanum. However, the acid-modified and alkali-modified fly ashes were smoother than the original sample, which might be due to acid and alkali elution of the surface impurities, respectively. The micro-molecular structure of acid-modified fly ash showed more obvious molecular structure agglomeration or collapse and a smaller specific surface area, resulting in a poor adsorption effect. The alkali-modified fly ash did not show an obvious surface microscopic phase change, and the molecular structure had sparing open internal pore channels, resulting in an increased specific surface area, which makes the lanthanum-modified fly ash have a sufficient specific surface area and distributes a large number of effective phosphorus binding sites.
As shown in Figure 4, according to infrared spectra analysis of four modified fly ashes, the peak observed at about 1100 cm−1 was more prominent for lanthanum-modified fly ash compared with the other three types of fly ash. This peak corresponded to the quantity of C=O bonds generated which had an adsorption effect on lanthanum ions. Therefore, the lanthanum-modified fly ash had more lanthanum ion binding sites, enhancing the capture and adsorption of phosphate in aqueous solution. This generates a relatively stable lanthanum phosphate compound which can be statically adsorbed on the fly ash surface to achieve phosphorus removal. Meanwhile, lanthanum-modified fly ash showed a weak peak at 2500 cm−1, which might correspond to the CO2 level, indicating that lanthanum-modified fly ash contained less impurities. At the same time, it can be seen from the XRF diagram that the surface loaded lanthanum in the lanthanum-modified fly ash particles is more, accounting for about 4.70%, while the loading of lanthanum in the fly ash after acid–base modification is 0.27% and 0.50%, respectively. The main reason for the better adsorption effect of lanthanum-modified fly ash particles is that lanthanum plays a decisive role in the adsorption of phosphorus in water.

3.1.2. Experimental Investigation of the Adsorption Efficiency Resulting from Lanthanum Modification

Figure 5 shows that phosphate removal from water increased with an increasing fly ash dosage, with the removal effect basically remaining stable after the dosage ratio reached 0.3 g/L. At a dosage of 0.3 g/L, lanthanum and phosphate binding sites in the fly ash essentially reached saturation, and the exchange of inherent fly ash elements and phosphate ions reached equilibrium.
As shown in Figure 6, as the pH increased, phosphate removal first decreased and then increased. In a neutral or weakly alkaline water body, lanthanum-modified fly ash removed about 78% of phosphate from the water body. In an acidic solution, fly ash components might preferentially undergo ion exchange with H+ in the water column over phosphate ions, thus affecting the adsorption performance. Meanwhile, the effect of temperature on the removal rate was not significant, with the highest difference among the four temperatures tested being 7%. An increase in temperature will be accompanied by an increase in the free energy between molecules and an increase in contact opportunities between molecules. However, too high a temperature will destroy the structure of the molecular layer, causing desorption and affecting the equilibrium. A removal effect up to 70% was achieved at a temperature of 30 °C.
The lanthanum-modified fly ash fitted better with the Langmuir model, and the saturation adsorption of phosphate was 8.89 mg/g. Phosphate adsorption by the modified fly ash was close to the unimolecular layer adsorption theory and was a synergistic process of physical and chemical adsorption.

3.2. Orthogonal Analysis of Modified Fly Ash Granulation

From the variance ratio, the magnitude of extreme differences in fly ash/montmorillonite ratio, roasting temperature, roasting time, and particle size were 0.381, 0.050, 0.037, and 0.198, respectively. Therefore, the effect on phosphorus adsorption in water by fly ash pellets was in the following order of fly ash/montmorillonite ratio > particle size > roasting temperature > roasting time. The ratio of raw materials had the most significant effect on phosphate adsorption by the modified fly ash pellets. The calcination temperature affected the evaporation of internal water form fly ash pellets and the size of internal pores formed. The particle size of the fly ash pellets affected the size of the exchange area with the aqueous solution. At the same dosage, pellets with a smaller particle size will have a larger effective area to contact water in the water body, resulting in a better phosphorus adsorption effect. The roasting time had the smallest effect on particle formation and adsorption because the roasting time mainly affected the water vapor emission time from fly ash pellets and the speed of liquid-phase formation by hot-melt substances, and this effect was weak because the two materials react quickly.
In summary, the optimal synthesis conditions for fly ash pellets were fly ash/montmorillonite ratio of 7:3, a roasting temperature of 900 °C, a roasting time of 4 h, and a particle size of 3 mm.

3.3. In Situ Control of Endogenous Phosphorus at the Sediment–Water Interface by Modified Fly Ash Pellets

The pH of the experimental group and control group did not change significantly, with stability observed under neutral and weak alkaline conditions and suitable phosphorus release conditions in the sediment. At different pH values, phosphate salts exist in different forms. Under acidic conditions, dihydrogen phosphate is the dominant form, while hydrogen phosphate is dominant under alkaline conditions. As the reactor was mainly alkaline, hydrogen phosphate was mainly adsorbed. A higher ORP value results in a higher dissolved oxygen content in the water, which has a positive impact on the ecological balance of the water environment [60]. At low ORP values, anaerobic microorganisms play a major role, consuming a large amount of oxygen to reduce the oxygen content in the lower water body, which is positively correlated with the release of pollutant elements from the sediments. Therefore, ORP can reflect changes in the dissolved oxygen content and the water quality and can be used as an indicator for the restoration of eutrophic water bodies. This experiment was conducted in autumn and winter, and the ORP of the water body was decreasing. However, compared with the control group, the amount of floating was not large, because the dissolved oxygen content in the experimental group was similar to that in the control group. Therefore, adding pulverized coal pellets will not have a large effect on the ORP of the water column, making it of practical value in engineering.
During the substrate release experiment, no obvious changes in the phosphate concentration were observed in the first 3 days, as the sediment did not release phosphorus. From 2 to 7 days, the phosphate concentration increased rapidly, because the substrate released phosphorus. After a further 7 days, the phosphate concentration remained basically unchanged, as phosphorus release by the substrate tended to stabilize. At this time, fly ash pellets were added to the water body for monitoring and analysis of the adsorption effect.
In eutrophic water bodies, aquatic plants and animals generally mainly use orthophosphate. Through long-term monitoring and analysis, the main adsorption in the water body before the adsorbent material was added was SRP, with captured PO4−3 in the overlying water. When the material settles on the substrate surface, the orthophosphate released from the substrate is gradually controlled. The adsorption of orthophosphate in the water body is mainly due to the adsorption effect lanthanum loaded in the fly ash pellets. When the fly ash pellets enter the water column, the numerous phosphorus binding sites on their surfaces adsorb a large amount of SRP in the water column. When the fly ash pellets settle, SRP in the water column is fixed more permanently to the sediment’s surface. Afterwards, orthophosphate release from the substrate stabilized and reached equilibrium. As shown in Figure 7, the orthophosphate in the injection group basically reached adsorption equilibrium after 20 days of experimental observation and testing, with an orthophosphate removal rate of more than 60% higher than that of the control group achieved. Furthermore, the SRP dropped to about 0.1 mg/L, showing potential for the treatment of eutrophic water bodies.
As shown in Figure 8, after adding fly ash pellets, the total phosphorus in the water column of the injection group showed an overall decreasing trend, which was due to the adsorption of orthophosphate by the fly ash pellets. The total phosphorus in the control group also decreased to some extent, but some increases and decreases were observed, which might be due to phosphorus release from the sediment to the overlying water column. The phosphorus content in the sediment had an important influence on the total phosphorus content in the overlying water column. The connection between these two parameters mainly depended on the concentration gradient gap. When the phosphorus concentration in the sediment far exceeded the phosphorus concentration in the overlying water, the sediment phosphorus release potential was highest, and the phosphorus release rate was fastest. However, a dynamic equilibrium was present between these two parameters, when phosphorus release from the sediment was persistent and slow. This also confirmed that the fly ash pellets had a certain adsorption capacity for phosphorus release from the sediment. Modified fly ash pellets showed a positive effect on total phosphorus control. When the total phosphorus concentration basically reached adsorption equilibrium, the total phosphorus concentration in the control group was more than 1.5 times that in the experimental group, indicating that the modified fly ash pellets achieved the expected control effect on total phosphorus in the water body.
As shown in Figure 9, inorganic phosphorus accounted for a relatively large proportion of phosphorus in the sediment. Inorganic phosphorus was the main source of phosphorus released to the sediment studied, accounting for about 65% of the total phosphorus content, with a content range of approximately 480–510 mg/kg. After observation for 60 days, the change in total phosphorus content in the sediment was not obvious, showing a slight downward trend. Experimental results were obtained over a long observation period after injecting modified fly ash pellets (2 g/L) into the experimental water body. A comparison of the changes in phosphorus content in the sediment before and after the experiment showed that the total phosphorus content in the substrate changed less, while changes in percentage contents of different forms of phosphorus were more obvious. The percentages contents of inorganic and organic phosphorus did not change significantly, but the different metal-bound forms of phosphorus in the organic phosphorus content showed a more obvious change trend in the sediment at 60 d.
As shown in Figure 10, the difference between HCl–P and NaOH–P in the original sediment sample was large, with the HCl–P content in a more stable state accounting for only approximately 26% of inorganic phosphorus. By overlaying the cover material, the ratio of HCl–P to NaOH–P changes greatly, with the more active NaOH–P continuing to release phosphorus into the overlying water. This phosphorus is effectively intercepted by modified fly ash pellets on the sediment surface, forming relatively stable phosphorus compounds, while the content of inert phosphorus in the substrate continued to increase as the experiment proceeded. After 20 days, the HCl–P content accounted for about 40% of inorganic phosphorus, while after 60 days, it accounted for about 70% of total inorganic phosphorus. At this time, activated phosphorus in the sediment was greatly decreased, and the ability of the sediment to release phosphorus to the overlying water body was weakened. During detection of the relevant phosphorus content in the overlying water body, all forms of phosphorus in the overlying water body were found to decreased by different magnitudes, indicating that the fly ash pellets had a certain remediation effect on endogenous pollution in eutrophic water bodies.

3.4. Study of Adsorption Mechanism

A schematic diagram of the P-binding mechanism of different La species and the structures of different types of La-based adsorbents has been studied by others, as shown in Figure 11 and Figure 12 [61]. In this study, Figure 13 shows that the particle surface structure did not change significantly after fly ash pellets were adsorbed by the experimental water, but an irregular combination of adsorbed substances was observed on the surface of particles. As shown by TEM at 2 μm (Figure 13), a large number of point fast phosphorus binding compounds were produced on the surface of fly ash by the adsorption of phosphorus, nitrogen, and other substances in experimental water. These point fast phosphorus binding compounds might be the binding state compounds generated by lanthanum and phosphate in the water. Elemental scan analysis showed that phosphorus was uniformly adsorbed on the surface of fly ash spheres, demonstrating that lanthanum was the key element for phosphorus binding.
Figure 14 shows the results of XRF analysis before and after adsorption of modified fly ash pellets. TEM elemental scan analysis showed that more phosphorus elements were enriched at positions where lanthanum was present, which might correspond with lanthanum phosphate generated by the combination of lanthanum and phosphorus in the experimental water. This was consistent with the experimental concept and demonstrated the strong adsorption ability of lanthanum for phosphorus in water, confirming that lanthanum is a rare earth element with an important adsorption effect on phosphorus in water.
Figure 15 and Figure 16 show the semi-quantitative XRF analysis of modified fly ash pellets before and after adsorption. The phosphorus content of fly ash before and after casting of the water body was significantly different. The percentage of elemental phosphorus before adsorption by modified fly ash pellets was about 1.13%, while the characteristic profile after adsorption showed that the percentage of elemental phosphorus had increased by 2% to 3.10%.

4. Conclusions

In this study, fly ash powder was modified using acid modification, alkali modification, and lanthanum modification methods. These modified fly ash powders were pelletized, and the effect of different ratios on fly ash pelletization was studied. The effect of fly ash pellets on the in situ control of endogenous pollution in eutrophic water bodies was observed by long-term monitoring. The main research findings were as follows:
(i) Compared with acid-modified and alkali-modified fly ash, the adsorption efficiency of lanthanum-modified fly ash was 34% and 40% higher, respectively. The optimal lanthanum dosage was determined to be 0.3 g/L by dosage tests. The adsorption of phosphorus in water by lanthanum-modified fly ash was affected by pH, with better phosphorus removal observed under weakly alkaline conditions. The adsorption isotherm model of modified fly ash was closer to the Langmuir adsorption model, which indicated that monomolecular layer adsorption occurred, with chemisorption and physical adsorption occurring simultaneously. The saturation adsorption amount of phosphate by lanthanum-modified fly ash was 8.89 mg/g.
(ii) In the granulation study of modified fly ash particles, the optimum synthesis conditions were a fly ash/montmorillonite ratio of 7:3, a roasting temperature of 900 °C, a roasting time of 4 h, and a particle size of 3 mm.
(iii) In the laboratory simulation of application to eutrophic water bodies, water quality analysis showed a relatively stable phosphorus content. After experimental monitoring for 20 days, orthophosphate in the injection group basically reached adsorption equilibrium, and the orthophosphate removal rate was more than 60% higher than that of the control group, with a total phosphorus removal rate of 43%.
(iv) The experimental water was generally alkaline, and HCL–P in the sediments was relatively stable. Phosphorus release from the sediment to the overlying water was mainly affected by the iron, aluminum, and phosphorus contents in the sediment. The difference between HCl–P and NaOH–P in the original sediment sample was large, while the HCl–P content in the more stable state at this time was small, accounting for about 26% of inorganic phosphorus. The HCl-P content accounted for about 40% of inorganic phosphorus after 20 days of the experiment, and about 70% of total inorganic phosphorus after 60 days. At this time, the active phosphorus content in the sediment was greatly reduced, and the ability of the substrate to release phosphorus into the overlying water body was weakened.

Author Contributions

Conceptualization, Y.P.; methodology, G.L.; software B.C. (Beibei Chai) and Y.P.; validation, Y.P., G.L. and B.C. (Beibei Chai); formal analysis, Y.P., S.L.; investigation, S.C.; resources, Y.W.; data curation, Y.P.; writing—original draft preparation, Y.P.; writing—review and editing, Y.P.; visualization, B.C. (Beibei Chai), X.L.; supervision, B.C. (Beibei Chai) and L.H.; project administration, B.C. (Beibei Chai) and B.C. (Bin Chen); funding acquisition, W.C. and L.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Grant No. 52070065, and No. 72091510), the Foundation of China Three Gorges Corporation (No. 202003136) and the Natural Science Foundation of Hebei Province (Grant No. E2020402074 and No. E2020402044).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 52070065, and No. 72091510), the Natural Science Foundation of Hebei Province (Grant No. E2020402074 and No. E2020402044), and the Foundation of China Three Gorges Corporation (No. 202003136).

Conflicts of Interest

The authors declare no conflict of interest.

Nomenclature

ORP: oxidation-reduction potential; DO: dissolved oxygen; SMT method: SMT method was applied to analyze phosphorus formations in the original sludge and the hydrochars. SRP: soluble orthophosphate/active phosphorus; BET: The Brunauer–Emmett–Teller theory (BET) is a common approach to estimate SSA that extends the Langmuir monolayer molecular adsorption model to multilayer layers.

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Coatings 12 00719 g001 550
Figure 1. Process flow diagram.
Figure 1. Process flow diagram.
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Figure 2. Schematic diagram of the simulated experimental setup.
Figure 2. Schematic diagram of the simulated experimental setup.
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Figure 3. SEM characterization of four types of fly ash.
Figure 3. SEM characterization of four types of fly ash.
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Figure 4. (A) Infrared spectra of four modified fly ashes; (B) XRF curves of acid modification; (C) XRF curves of alkali modification; (D) XRF curves of lanthanum modification.
Figure 4. (A) Infrared spectra of four modified fly ashes; (B) XRF curves of acid modification; (C) XRF curves of alkali modification; (D) XRF curves of lanthanum modification.
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Figure 5. Optimal dosage of lanthanum modified fly ash.
Figure 5. Optimal dosage of lanthanum modified fly ash.
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Figure 6. (A) phosphate removal at different pH; (B) phosphate removal at different temperature.
Figure 6. (A) phosphate removal at different pH; (B) phosphate removal at different temperature.
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Figure 7. Comparison of orthophosphate control effect.
Figure 7. Comparison of orthophosphate control effect.
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Figure 8. Total phosphorus effect control comparison chart.
Figure 8. Total phosphorus effect control comparison chart.
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Figure 9. (A) The content of different phosphorus forms in the sediment; (B) the change of phosphorus forms.
Figure 9. (A) The content of different phosphorus forms in the sediment; (B) the change of phosphorus forms.
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Figure 10. (A) 60d HCl-P and NaOH-P content change graph; (B) 60d HCl-P and NaOH-P content change percentage graph.
Figure 10. (A) 60d HCl-P and NaOH-P content change graph; (B) 60d HCl-P and NaOH-P content change percentage graph.
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Figure 11. A schematic diagram for the P binding mechanisms of different La species [61].
Figure 11. A schematic diagram for the P binding mechanisms of different La species [61].
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Figure 12. Schematic diagram of the structures of different types of La-based adsorbents: (A) single La compounds; (B) La-metal composites; (C) “La-carrier” absorbents [61].
Figure 12. Schematic diagram of the structures of different types of La-based adsorbents: (A) single La compounds; (B) La-metal composites; (C) “La-carrier” absorbents [61].
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Figure 13. Shape band scan of fly ash pellets before (A) and after (B) adsorption.
Figure 13. Shape band scan of fly ash pellets before (A) and after (B) adsorption.
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Figure 14. Phosphorus spot map (A) and lanthanum spot map (B) after adsorption of fly ash pellets.
Figure 14. Phosphorus spot map (A) and lanthanum spot map (B) after adsorption of fly ash pellets.
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Figure 15. XRF analysis of fly ash pellets before adsorption.
Figure 15. XRF analysis of fly ash pellets before adsorption.
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Figure 16. XRF analysis of fly ash pellets after adsorption.
Figure 16. XRF analysis of fly ash pellets after adsorption.
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Table
Table 1. Main Chemical Components of Fly Ash.
Table 1. Main Chemical Components of Fly Ash.
CompoundSiO2Al2O3Cr2O3Fe2O3MnOCaOTiO2K2OFeSSO3Na2O
Original fly ash36.3%13.0%7.0%3.6%2.3%2.0%4.9%5.0%3.5%15.4%3.9%
Table
Table 2. Raw Nitrogen and Phosphorus Content of the Overlying Water Bodies.
Table 2. Raw Nitrogen and Phosphorus Content of the Overlying Water Bodies.
TP (mg/L)SRP (mg/L)TN (mg/L)
Overlying water0.690.392.53
Table
Table 3. Lanthanum modified fly ash specific surface area pore volume multiplier.
Table 3. Lanthanum modified fly ash specific surface area pore volume multiplier.
Original Fly AshLanthanum-Modified Fly AshMagnification (Lanthanum/Original)
Specific surface area m2/g2.216.213
Pores nm */2~25/
Average pore size nm **/8.5/
Total pore volume%12.8089.637
* Pores are loose rock formations composed of particles of different sizes, voids between particles and particle aggregates, and pores also are the voids between solid mineral particles in rock and soil; ** The average pore size is equal to the corresponding pore volume divided by the corresponding specific surface area and the diameter of the pores on the surface of the object, and refers to the shape and size of the pores in the porous solid.
Table
Table 4. Analysis of elemental composition of four types of fly ash (%).
Table 4. Analysis of elemental composition of four types of fly ash (%).
CompoundOriginal Fly AshAcid-ModifiedAlkali-ModifiedLanthanum-Modified
SiO23627.62921.3
Al2O31715.115.716.5
Cr2O3788.18.5
Fe2O35.64.64.95.2
MnO2.33.63.44.2
CaO5.02.93.52.7
MgO3.15.75.86.1
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Pan, Y.; Liu, G.; Chai, B.; Lei, X.; He, L.; Cheng, S.; Wang, Y.; Chen, W.; Li, S.; Chen, L.; et al. Control of Endogenous Phosphorus Release at the Sediment–Water Interface by Lanthanum-Modified Fly Ash. Coatings 2022, 12, 719. https://0-doi-org.brum.beds.ac.uk/10.3390/coatings12060719

AMA Style

Pan Y, Liu G, Chai B, Lei X, He L, Cheng S, Wang Y, Chen W, Li S, Chen L, et al. Control of Endogenous Phosphorus Release at the Sediment–Water Interface by Lanthanum-Modified Fly Ash. Coatings. 2022; 12(6):719. https://0-doi-org.brum.beds.ac.uk/10.3390/coatings12060719

Chicago/Turabian Style

Pan, Ying, Gang Liu, Beibei Chai, Xiaohui Lei, Lixin He, Shuailong Cheng, Yijie Wang, Wenlong Chen, Simin Li, Liang Chen, and et al. 2022. "Control of Endogenous Phosphorus Release at the Sediment–Water Interface by Lanthanum-Modified Fly Ash" Coatings 12, no. 6: 719. https://0-doi-org.brum.beds.ac.uk/10.3390/coatings12060719

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