**Yan Liu, Limin Zhang \* and Rajendra Prasad Singh**

School of Civil Engineering, Southeast University, Nanjing 210000, China; liuyian@seu.edu.cn (Y.L.); rajupsc@seu.edu.cn (R.P.S.)

**\*** Correspondence: 220181172@seu.edu.cn; Tel.: +86-151-9590-8761

Received: 21 April 2020; Accepted: 18 May 2020; Published: 24 May 2020

**Abstract:** Fly ash and steel slag both have a good adsorption performance and many researchers have mixed the two to make effective adsorbents. Based on previous knowledge, activated clay is added in this study. In order to deep dephosphorize wastewater, two different industrial wastes (steel slag, fly ash) are blended into activated clay as adsorption substrates, supplemented with a binder and foaming agent to prepare a Residue and Soil Phosphorus Removal Composite (RSPRC). This is prepared to carry out experimental research on the decolorization effect and phosphorus removal characteristics of RSPRC. Meanwhile, a self-developed concentric circular diversion wall adsorption reactor is implemented to study the effect of phosphorus removal. It is found that the addition of activated clay can significantly improve the phosphorus removal performance. The results suggest that the phosphorus concentration in the effluent from the reactor can be stably reduced to below 0.10 mg/L. The concentric circular diversion wall adsorption reactor and RSPRC will have broad application prospects in phosphorus removal.

**Keywords:** phosphorus; adsorption; steel slag; fly ash; activated clay; reactor

## **1. Introduction**

Phosphorus (P) usually originates from human and animal wastes, household detergents, food-processing effluents, commercial fertilizers, and agricultural land runoffs. Phosphorus is a major nutrient for biomass growth [1–3]. However, excessive concentrations of P in water bodies such as lakes, lagoons or rivers cause an abnormal growth of algae and aquatic plants resulting in the degradation of the water quality [4,5]. The State Environmental Protection Administration of China has recommended that total P should not exceed 0.5 mg/L according to the class A demands of discharge standard of pollutants for municipal wastewater treatment plants (GB18918-2002). The European Union (EU) maintains that the cut-off for total P concentration between risk and no risk of eutrophication in lakes is <10 µg/L to >100 µg/L, and for rivers, while total P concentrations below 0.01–0.07 µg/L are considered ideal. Therefore, the enhanced removal of phosphorus is a current trend. At present, removal methods of phosphorus from wastewater can be roughly divided into chemical precipitation [6], advanced biological treatment [7,8] and adsorption [9,10]. The biological phosphorus removal process is complicated and easily affected by the environment, which means that the effluent phosphorus concentration stability is poor [11]. Although the chemical removal method has a good effluent effect, most phosphorus removal agents used are industrial products such as lime, aluminum salt, iron salt, ferrous salt and magnesium salt, which lead to high phosphorus removal costs and a large amount of sludge. Thus, chemical precipitation has not been widely applied in practice [12]. Compared with other kinds of phosphorus removal methods, adsorption has the advantages of easy operation, simple process, reliable operation, high efficiency, low consumption, etc. [13]. Therefore, it has broad application prospects.

Studies over the past two decades have provided significant information on the profound phosphorus removal performance of steel slag and fly ash. However, there is clearly a lack of research related to the phosphorus removal performance of activated clay. Although extensive research has been carried out on steel slag and fly ash, there has been little research on combining active clay, steel slag and fly ash to remove phosphorus. Most studies in the field of phosphorus adsorption have only focused on the adsorbents. So far, very little attention has been paid to the role of reactors used for adsorption reactions. Currently, high efficiency phosphorus removal material with strong adsorption capacity, certain strength and chemical stability, low water flow resistance and low cost should be developed urgently and applied to the phosphorus removal process of various wastewaters to ensure the phosphorus removal effect on the wastewater and recover the phosphorus [14]. In addition, the development of a reactor that is easy to operate, has a large capacity for water treatment and can guarantee an excellent adsorption effect for a long period of time is also one of the hot topics in recent years.

Therefore, the prepared RSPRC was used as the adsorbent and a concentric circular diversion wall adsorption reactor was developed as the adsorption device. The main objectives of the current work are as follows: (1) assess the decolorization and dephosphorization performance of activated clay at different ratios to determine the optimum ratio of raw materials for preparing RSPRC. (2) Investigate the phosphorus removal characteristics of RSPRC. (3) Investigate the effect of RSPRC dosage on the phosphorus removal efficiency of the reactor. (4) Investigate the effect of RSPRC distribution on phosphorus removal efficiency to optimize the placement of RSPRC for adsorption reactors. Moreover, (5) investigate the effect of hydraulic retention time (HRT) in the reactor on the phosphorus removal efficiency.

#### **2. Materials and Methods**

#### *2.1. The Concentric Circular Diversion Wall Adsorption Reactor*

The design scheme of the concentric circular diversion wall adsorption reactor was as follows: comparing the advantages and disadvantages of the existing reactors, combined with the purpose of this research, the preliminary design scheme of the reactor type, namely the fixed bed type, was proposed. In order to realize the functions of multi-stage series adsorption and flexible distribution of adsorbents, taking the oxidation ditch as a reference, the reactor adopted the shape of concentric circles and was equipped with slots and grills. Based on the preliminary design scheme, comprehensively considering the advantages and disadvantages of various types of materials, after repeated research and discussion with the manufacturer on the details of the reactor, stainless steel was chosen as the reactor material and the necessary adjustments of the reactor structure were made. Finally, the reactor dimensions were determined, such as the height-to-diameter ratio of the reactor, the size of the water inlet and outlet, the size of the overflow port and so on.

The concentric circular diversion wall adsorption reactor was composed of an outlet pipe, an inlet pipe, an overflow port, a slot, removable grilles, diversion walls, diversion tubes, a constant flow pump and ball valves (Figure 1a,b). The total height of the reactor was 80 cm, the effective height was 50 cm and the support foot height was 30 cm. The diameter was 120 cm and the effective diameter was 100 cm. The inflow height was 78 cm and the effluent height was 28 cm. The total volume was about 400 L and the effective volume was approximately 375 L. The diameters of the diversion walls from the outside to the inside were 50 cm, 40 cm, 30 cm, 20 cm and 10 cm, respectively. The total heights of the diversion walls were 50 cm and the effective height was 48 cm, 46 cm, 44 cm, 42 cm and 40 cm from the outside to inside. The distances from the outside to the inside of the diversion wall were 2 cm, 4 cm, 6 cm, 8 cm and 10 cm. The dimensions of the removable grilles were: length × width × thickness <sup>=</sup> 50 cm <sup>×</sup> 10 cm <sup>×</sup> 1 cm; and the measured mesh area was about 0.04 cm<sup>2</sup> . The outer and inner diameters of the diversion tube used in current work were 20 mm and 19 mm respectively.

**Figure 1.** (**a**) The concentric circular diversion wall adsorption reactor schematic; (**b**) the concentric circular diversion wall adsorption reactor. **Figure 1.** (**a**) The concentric circular diversion wall adsorption reactor schematic; (**b**) the concentric circular diversion wall adsorption reactor.

#### *2.2. RSPRC Preparation 2.2. RSPRC Preparation*

**Composition of steel** 

The raw materials of RSPRC were divided into two types: substrate and auxiliary material. The substrate consisted of steel slag, fly ash and activated clay. The auxiliary material was cement as binder and the foaming agent to increase the specific surface area [15]. The steel slag and fly ash used in this experiment were taken from a steel plant and a power plant in Nanjing. The chemical composition of the steel slag and fly ash was analyzed by wavelength dispersive X-ray fluorescence analyzer (XRF-1800) produced by Shimadzu, Kyoto, Japan. Table 1 shows the chemical composition and content of the waste residue substrate and Table 2 presents the particle size distribution of the waste residue substrate. The raw materials of RSPRC were divided into two types: substrate and auxiliary material. The substrate consisted of steel slag, fly ash and activated clay. The auxiliary material was cement as binder and the foaming agent to increase the specific surface area [15]. The steel slag and fly ash used in this experiment were taken from a steel plant and a power plant in Nanjing. The chemical composition of the steel slag and fly ash was analyzed by wavelength dispersive X-ray fluorescence analyzer (XRF-1800) produced by Shimadzu, Kyoto, Japan. Table 1 shows the chemical composition and content of the waste residue substrate and Table 2 presents the particle size distribution of the waste residue substrate.

**Table 1.** Percentage of oxides in waste residue substrate. **Table 1.** Percentage of oxides in waste residue substrate.



**Particle size of steel slag/mesh** <12 12~80 80~320 >320 -

**slag (%)** 55.0 21.5 1.51 13.4 3.65 1.75 0.512 0.417 0.296 0.077 - - **Table 2.** Size distribution of the waste residue substrate.

**proportion/%** 2.49 7.68 32.40 57.36 - The preparation of activated clay is divided into original soil treatment and modification treatment [16]. This experiment adopted acid modification treatment [17]. The bentonite was taken from Tangshan, Jiangning District, Nanjing, China. For the acid modification treatment, the bentonite was immersed in a 3 mol/L sulfuric acid solution, activated for 30 min, and stirred at a speed of 1000 r/min in a constant temperature environment of 100 °C. After the activation was completed, it was washed with distilled water to neutrality, dried, ground, and passed through a 100-mesh sieve to obtain experimental activated clay. Respectively, the principal oxides SiO2, Al2O3, MgO, Fe2O3, and CaO in the activated clay account for 64.23%, 16.88%, 0.76%, 2.05% and 2.45%. The The preparation of activated clay is divided into original soil treatment and modification treatment [16]. This experiment adopted acid modification treatment [17]. The bentonite was taken from Tangshan, Jiangning District, Nanjing, China. For the acid modification treatment, the bentonite was immersed in a 3 mol/L sulfuric acid solution, activated for 30 min, and stirred at a speed of 1000 r/min in a constant temperature environment of 100 ◦C. After the activation was completed, it was washed with distilled water to neutrality, dried, ground, and passed through a 100-mesh sieve to obtain experimental activated clay. Respectively, the principal oxides SiO2, Al2O3, MgO, Fe2O3, and CaO in the activated clay account for 64.23%, 16.88%, 0.76%, 2.05% and 2.45%. The preparation of the adsorbent was carried out according to the different proportion of various substrates. The specific percentages of substrate and binder are presented in Table 3.

preparation of the adsorbent was carried out according to the different proportion of various

substrates. The specific percentages of substrate and binder are presented in Table 3.


**Table 3.** Percentage composition of substrate and binder in Residue and Soil Phosphorus Removal Composite (RSPRC) in four groups. **Table 3.** Percentage composition of substrate and binder in Residue and Soil Phosphorus Removal Composite (RSPRC) in four groups.

A schematic flow chart of the RSPRC preparation process is provided in Figure 2. At first, the selected substrate (fly ash and steel slag) was mechanically pulverized. After that, an appropriate amount of activated clay, binder and foaming agent were blended and thoroughly mixed. Later, an appropriate amount of tap water was added to the materials and these materials were stirred evenly. Then, all the materials were poured into the drum granulator. After the granulation was completed, the water was sprinkled after 6~8 h. Finally, RSPRC was maintained for several days under normal temperature and pressure to keep certain humidity on the surface of the particles. A schematic flow chart of the RSPRC preparation process is provided in Figure 2. At first, the selected substrate (fly ash and steel slag) was mechanically pulverized. After that, an appropriate amount of activated clay, binder and foaming agent were blended and thoroughly mixed. Later, an appropriate amount of tap water was added to the materials and these materials were stirred evenly. Then, all the materials were poured into the drum granulator. After the granulation was completed, the water was sprinkled after 6~8 h. Finally, RSPRC was maintained for several days under normal temperature and pressure to keep certain humidity on the surface of the particles.

**Figure 2.** RSPRC preparation process. **Figure 2.** RSPRC preparation process.

According to the experimental conclusion of Wu et al. [18], the composite adsorbent with a particle size of 5 mm had a more efficient phosphorus removal effect than that with particle sizes of 10 and 20 mm. Therefore, the particle size selected for this experiment was 5 mm. Zhou [19] pointed out in their research on Efficient Phosphorus Removal Composite (EPRC)that the use of a plant-type foaming agent in the preparation process made it more difficult to cause secondary pollution, and that the optimal dosage was 8 mL/kg. RSPRC and EPRC are basically the same in terms of material physical properties. Consequently, the preparation of the adsorbent was carried out with an 8 mL/kg foaming agent. The plant-type foaming agent was bought from Shanghai Fangbao Building Material Technology Co., Ltd., Shanghai, China. The plant-type foaming agent is prepared from rosin and sodium hydroxide through a saponification reaction and had the advantages of its low cost and large foam production. According to the experimental conclusion of Wu et al. [18], the composite adsorbent with a particle size of 5 mm had a more efficient phosphorus removal effect than that with particle sizes of 10 and 20 mm. Therefore, the particle size selected for this experiment was 5 mm. Zhou [19] pointed out in their research on Efficient Phosphorus Removal Composite (EPRC)that the use of a plant-type foaming agent in the preparation process made it more difficult to cause secondary pollution, and that the optimal dosage was 8 mL/kg. RSPRC and EPRC are basically the same in terms of material physical properties. Consequently, the preparation of the adsorbent was carried out with an 8 mL/kg foaming agent. The plant-type foaming agent was bought from Shanghai Fangbao Building Material Technology Co., Ltd., Shanghai, China. The plant-type foaming agent is prepared from rosin and sodium hydroxide through a saponification reaction and had the advantages of its low cost and large foam production.

#### *2.3. Study on Decolorization and P Removal of RSPRC with Activated Clay 2.3. Study on Decolorization and P Removal of RSPRC with Activated Clay*

following formula:

The colored wastewater used in the experiment was derived from a river in the Jiulonghu Campus of Southeast University. A total of 250 mL wastewater was taken and stored in conical flasks. As shown in Table 3, 5 g adsorbents of different groups were respectively added. The colored wastewater was slowly stirred for 20 min and stood for 2 h. After that, the wastewater was filtered and the filtrate was taken to measure absorbance. The absorbance was measured by a spectrophotometer at a wavelength of 750 nm. The decolorization rate was calculated by the The colored wastewater used in the experiment was derived from a river in the Jiulonghu Campus of Southeast University. A total of 250 mL wastewater was taken and stored in conical flasks. As shown in Table 3, 5 g adsorbents of different groups were respectively added. The colored wastewater was slowly stirred for 20 min and stood for 2 h. After that, the wastewater was filtered and the filtrate was taken to measure absorbance. The absorbance was measured by a spectrophotometer at a wavelength of 750 nm. The decolorization rate was calculated by the following formula:

$$\mathbf{Q} = \frac{(\mathbf{A}\_0 - \mathbf{A}\_\mathbf{e})}{\mathbf{A}\_0} \times 100\% \tag{1}$$

A

where Q is the decolorization rate, A<sup>0</sup> is the initial absorbance of the colored wastewater and A<sup>e</sup> is the absorbance of the colored wastewater after adsorption.

A total of 10 g of different groups of adsorbents were placed into 250 mL Erlenmeyer flasks. Then, the simulated solution, with a phosphorus concentration of 2 mg/L, prepared with deionized water and KH2PO4, was separately added to the Erlenmeyer flasks up to the mark. The pH of the simulated solution was adjusted to 7 by adding HCL and NaOH solution. Finally, they were oscillated in a constant temperature oscillator. When the adsorption duration was 60 min, 120 min and 180 min, the supernatant was taken, and the P concentrations were determined using the molybdenum blue spectrophotometric method stated in American Public Health Association (APHA) [20] in order to investigate the effect of the ratio of activated clay in the adsorbent on phosphorus removal.

The phosphorus removal effect of RSPRC in the advanced treatment of wastewater was mainly reflected in the phosphorus removal rate and the amount of phosphorus adsorbed by RSPRC. The adsorption capacity (Qe, mg/g) or amount of phosphorus adsorbed by RSPRC and removal rate (R) of phosphorus were calculated from the following equations:

$$\mathbf{Q\_e} = \frac{(\mathbf{C\_0} - \mathbf{C\_e})\mathbf{V}}{\mathbf{m}} \tag{2}$$

$$\text{TR } (\%) = 100 \times \frac{\text{C}\_0 - \text{C}\_{\text{e}}}{\text{C}\_0} \tag{3}$$

where Co is the initial concentration of the P (mg/L), Ce is the equilibrium or residual P concentration (mg/L), V is the volume of the solution (L) and m is the mass of adsorbent (g).

#### *2.4. Study on P Removal Characteristics of RSPRC*

At first, for the purpose of exploring the effect of dosage on the phosphorus removal of RSPRC, 2, 3, 4, 5, and 6 g RSPRC samples with a particle size of 5 mm were placed in five conical flasks, which contained 100 mL of KH2PO<sup>4</sup> standard solution with a P concentration of 0.5 mg/L and pH = 7. The five conical flasks were shaken well in a 150 r/min shaker. After 24 h, the supernatant was taken and the P concentration was measured. Secondly, in order to study the effect of contact time, when the static adsorption times were 4, 8, 24, 72 and 144 h, the supernatant was taken and the P concentration was determined. Thirdly, 5 g of RSPRC was separately put into 1 L of simulated wastewater with pH = 7 and P concentrations of 0.3, 0.5 and 1.0 mg/L. The P concentration was detected at different times to investigate the effect of initial P concentration on the phosphorus removal of RSPRC.

## *2.5. Study on the Influence of RSPRC Distribution in the Reactor on Phosphorus Removal E*ff*ect*

In an attempt to study the influence of different distributions of RSPRC in the reactor on the phosphorus removal effect, four different distributions were designed and a certain amount of adsorbent was placed in the reactor in the specified way. Four distributions of RSPRC are presented in Figure 3. The initial P concentration of the experimental influent was set to 0.5 mg/L, the pH was adjusted to 7, and the total amount of adsorbent was 10 kg, so as to better observe the phosphorus removal effect of the reactor under different distributions. Additionally, the hydraulic retention time was 3 h and the water flow rate was 125 L/h.

#### *2.6. Study on the Influence of RSPRC Dosage in the Reactor on Phosphorus Removal E*ff*ect*

In this experiment, the initial phosphorus concentrations of the wastewater were set to three gradients: 0.3, 0.4 and 0.5 mg/L. The pH of wastewater was adjusted to 7. At present, the reaction time required for the wastewater advanced treatment process is 1~6 h, and hence the adsorption reaction time set in this experiment was 2~4 h, which is far less than the previously set reaction balance time. Therefore, for the purpose of reaching the set target effluent P concentration (<0.1 mg/L), the experiment needed to increase the dosage of the adsorbent and shorten the time required for the

static adsorption to reach equilibrium. The initial doses of the adsorbent for the entire reactor were 4, 6, 8, and 10 kg, which were numbered as group 1, group 2, group 3, and group 4. The distribution of the adsorbents was set between the diversion walls at equal intervals, as shown in experimental condition d in Figure 3, and the hydraulic retention time was set to 3 h. Finally, the effluent P concentration of each group was determined. were 4, 6, 8, and 10 kg, which were numbered as group 1, group 2, group 3, and group 4. The distribution of the adsorbents was set between the diversion walls at equal intervals, as shown in experimental condition d in Figure 3, and the hydraulic retention time was set to 3 h. Finally, the effluent P concentration of each group was determined.

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**Figure 3.** Adsorbent distribution of four experimental conditions (**a**)–(**d**). **Figure 3.** Adsorbent distribution of four experimental conditions (**a**–**d**).

## *2.7. Study on the Influence of HRT in the Reactor on Phosphorus Removal E*ff*ect*

*2.7. Study on the Influence of HRT in the Reactor on Phosphorus Removal Effect*  Keeping in mind the time required for the wastewater treatment process in the self-developed reactor, the initial phosphorus concentration of the statically adsorbed wastewater was set to 0.5 mg/L, and the hydraulic retention times were set to 2, 3 and 4 h, respectively. The pH of the wastewater was adjusted to 7. The P concentration in the effluent of each group was detected at the Keeping in mind the time required for the wastewater treatment process in the self-developed reactor, the initial phosphorus concentration of the statically adsorbed wastewater was set to 0.5 mg/L, and the hydraulic retention times were set to 2, 3 and 4 h, respectively. The pH of the wastewater was adjusted to 7. The P concentration in the effluent of each group was detected at the corresponding moment.

#### corresponding moment. **3. Results and Discussion**

#### **3. Results and Discussion**  *3.1. E*ff*ect of Proportion of Activated Clay in RSPRC on Decolorization and Phosphorus Removal*

*3.1. Effect of Proportion of Activated Clay in RSPRC on Decolorization and Phosphorus Removal*  The experimental results reveal that the decolorization rate was only 46.7% without the addition of activated clay in RSPRC; following the addition of activated clay, a significant increase in the decolorization rate of the wastewater was recorded. When the proportion of activated clay was 15%, the decolorization rate was as high as 70.8%. This indicates that the adsorbent RSPRC is equipped with a better decolorization performance because of the addition of activated clay. The experimental results of exploring the phosphorus removal performance of activated clay are given in Figure 4. The phosphorus removal rate was 75.20% when the proportion of activated clay in group 3 was 15%. On the other hand, the phosphorus removal rate of the adsorbent containing no activated clay in group 4 was merely 68.74% at 180 min. The results show that there is a clear trend of an increasing phosphorus removal rate with the addition of activated clay. For instance, the phosphorus removal rate of the adsorbent with 15% increased by 6.46%. It is apparent that the addition of activated clay can effectively improve the phosphorus removal performance of the adsorbent. This result may be explained by the fact that the activated clay was obtained by rinsing and drying bentonite after it was activated by inorganic acid, and that its main composition is montmorillonite, which has a high specific surface area [21]. During the acid activation treatment, The experimental results reveal that the decolorization rate was only 46.7% without the addition of activated clay in RSPRC; following the addition of activated clay, a significant increase in the decolorization rate of the wastewater was recorded. When the proportion of activated clay was 15%, the decolorization rate was as high as 70.8%. This indicates that the adsorbent RSPRC is equipped with a better decolorization performance because of the addition of activated clay. The experimental results of exploring the phosphorus removal performance of activated clay are given in Figure 4. The phosphorus removal rate was 75.20% when the proportion of activated clay in group 3 was 15%. On the other hand, the phosphorus removal rate of the adsorbent containing no activated clay in group 4 was merely 68.74% at 180 min. The results show that there is a clear trend of an increasing phosphorus removal rate with the addition of activated clay. For instance, the phosphorus removal rate of the adsorbent with 15% increased by 6.46%. It is apparent that the addition of activated clay can effectively improve the phosphorus removal performance of the adsorbent. This result may be explained by the fact that the activated clay was obtained by rinsing and drying bentonite after it was activated by inorganic acid, and that its main composition is montmorillonite, which has a high specific surface area [21]. During the acid activation treatment, Wang et al. found that Na+, Mg2+, K+, Ca2<sup>+</sup> and other cations between the bentonite layers can be converted into soluble salts and dissolved

out, increasing the layer spacing, and forming a porous active substance with a microporous mesh structure and a large specific surface area. Additionally, the impurities distributed in the bentonite structure channels can also be removed. The pore volume was increased, which is beneficial to the diffusion of adsorbate molecules [17]. converted into soluble salts and dissolved out, increasing the layer spacing, and forming a porous active substance with a microporous mesh structure and a large specific surface area. Additionally, the impurities distributed in the bentonite structure channels can also be removed. The pore volume was increased, which is beneficial to the diffusion of adsorbate molecules.[17].

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**Figure 4.** Effect of proportion of activated clay on Phosphorus removal efficiency. **Figure 4.** Effect of proportion of activated clay on Phosphorus removal efficiency.

Although the phosphorus removal performance of the adsorbent was improved with the increase in the proportion of activated clay in the substrate, the improvement was not obvious enough. Taking into account economic factors, the basic ratio of adsorbent materials was fly ash:steel Although the phosphorus removal performance of the adsorbent was improved with the increase in the proportion of activated clay in the substrate, the improvement was not obvious enough. Taking into account economic factors, the basic ratio of adsorbent materials was fly ash:steel slag:activated clay:cement = 14:2:3:1 in the subsequent experiments.

slag:activated clay:cement = 14:2:3:1 in the subsequent experiments.

**(mm)** 

#### *3.2. Characterization of RSPRC*

**Parameter** 

*3.2. Characterization of RSPRC*  The microstructure of RSPRC was observed by a Nova scanning electron microscope (SEM) The microstructure of RSPRC was observed by a Nova scanning electron microscope (SEM) using Nano SEM 450. Table 4 presents the physical properties of RSPRC used in experiments.


**(%)** 

**(m2/g)** 

**(g/cm3)** 

using Nano SEM 450. Table 4 presents the physical properties of RSPRC used in experiments. **Table 4.** Physical properties of RSPRC used in experiments.

**RSPRC** 5~20 1.16 30.98 2.62~8.56 An SEM image of RSPRC is shown in Figure 5. The SEM image shows that RSPRC is composed of particles with different properties, of which spherical particles account for more than 60% (Figure 5a). After analysis, we established that these are the glass bodies in the fly ash, which have stored a high chemical internal energy after high-temperature calcination and are the source of the fly ash activity. Figure 5b shows that the glass body is a hollow sphere. When the sphere breaks, the Al2O3 and SiO2 inside are released, the broken bond increases, the specific surface area increases and the reaction contact area increases, which increases the number of activated molecules and effectively improves the early chemical activity of the RSPRC. Figure 5c presents the fracture surface of the glass body. RSPRC contains a lot of minerals. Steel slag and activated clay are its main sources. An SEM image of RSPRC is shown in Figure 5. The SEM image shows that RSPRC is composed of particles with different properties, of which spherical particles account for more than 60% (Figure 5a). After analysis, we established that these are the glass bodies in the fly ash, which have stored a high chemical internal energy after high-temperature calcination and are the source of the fly ash activity. Figure 5b shows that the glass body is a hollow sphere. When the sphere breaks, the Al2O<sup>3</sup> and SiO<sup>2</sup> inside are released, the broken bond increases, the specific surface area increases and the reaction contact area increases, which increases the number of activated molecules and effectively improves the early chemical activity of the RSPRC. Figure 5c presents the fracture surface of the glass body. RSPRC contains a lot of minerals. Steel slag and activated clay are its main sources. They are 2CaO·SiO2, 3CaO·SiO2, 2FeO·SiO2, 2CaO·Fe2O<sup>3</sup> and free calcium oxide (f-CaO), etc., many of which possess a phosphorus removal ability (Figure 5d) [22].

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**Figure 5.** SEM images of RSPRC. The magnifications from (**a**–**d**) are 5080, 2230, 4070, 26260.

#### **Figure 5.** SEM images of RSPRC. The magnifications from (a) to (d) are 5080, 2230, 4070, 26260. *3.3. Phosphorus Removal Characteristics of RSPRC*

*3.3. Phosphorus Removal Characteristics of RSPRC*  As can be seen from Figure 6, there has been a gradual rise in the phosphorus removal rate with the increasing adsorbent dosage. The phosphorus removal rate of RSPRC reached a peak when the adsorbent dosage was 6 g. However, the adsorption capacity (Qe, mg/g) or amount of phosphorus adsorbed by RSPRC had a downward trend when the dosage of adsorbent increased. Similarly, Ashekuzzaman et al. (2014) discovered that the phosphate sorption capacity was decreased with increasing dose of layered double hydroxides, because with the increasing dose, the adsorbent mass increased in the same volume of adsorbate solution, while the mass of the adsorbate to be sorbed remained the same [22]. As the dosage increased, although the available active sites were increasing, the increase in the density of the adsorbent caused the adsorption active sites to be stacked, so the adsorption efficiency was lowered, resulting in a slow increase in the removal rate of phosphorus As can be seen from Figure 6, there has been a gradual rise in the phosphorus removal rate with the increasing adsorbent dosage. The phosphorus removal rate of RSPRC reached a peak when the adsorbent dosage was 6 g. However, the adsorption capacity (Qe, mg/g) or amount of phosphorus adsorbed by RSPRC had a downward trend when the dosage of adsorbent increased. Similarly, Ashekuzzaman et al. (2014) discovered that the phosphate sorption capacity was decreased with increasing dose of layered double hydroxides, because with the increasing dose, the adsorbent mass increased in the same volume of adsorbate solution, while the mass of the adsorbate to be sorbed remained the same [22]. As the dosage increased, although the available active sites were increasing, the increase in the density of the adsorbent caused the adsorption active sites to be stacked, so the adsorption efficiency was lowered, resulting in a slow increase in the removal rate of phosphorus and the decline in the amount of phosphorus adsorbed by RSPRC [23,24].

and the decline in the amount of phosphorus adsorbed by RSPRC [23,24]. Figure 7 provides the results obtained from the experiment of studying the effect of contact time on the phosphorus removal of RSPRC. As the contact time increases, the effluent P concentration decreases and the phosphorus removal rate increases. With the increase in contact time, different dosages of adsorbent have similar changes in phosphorus removal rate. In the interval of 0~8 h, the effluent P concentration decreased slowly. In the case of the 8~24 h interval, there was a sharp drop in the effluent P concentration. However, in the 24~72 h interval, the effluent P concentration declined gradually, and after 72 h, the effluent P concentration tended to be stable and basically reached the adsorption equilibrium. The observed correlation between contact time and phosphorus removal effect might be explained as follows: there are more active sites on the surface and pores of the RSPRC, where the initial adsorption exists for a short time, so the phosphate ions rapidly occupy the active site, and the adsorption rate is faster. With the extension of time, RSPRC adsorbs more and more phosphorus from the wastewater and the active sites are fewer, which can result in a decline in Figure 7 provides the results obtained from the experiment of studying the effect of contact time on the phosphorus removal of RSPRC. As the contact time increases, the effluent P concentration decreases and the phosphorus removal rate increases. With the increase in contact time, different dosages of adsorbent have similar changes in phosphorus removal rate. In the interval of 0~8 h, the effluent P concentration decreased slowly. In the case of the 8~24 h interval, there was a sharp drop in the effluent P concentration. However, in the 24~72 h interval, the effluent P concentration declined gradually, and after 72 h, the effluent P concentration tended to be stable and basically reached the adsorption equilibrium. The observed correlation between contact time and phosphorus removal effect might be explained as follows: there are more active sites on the surface and pores of the RSPRC, where the initial adsorption exists for a short time, so the phosphate ions rapidly occupy the active site, and the adsorption rate is faster. With the extension of time, RSPRC adsorbs more and more phosphorus from the wastewater and the active sites are fewer, which can result in a decline in the removal rate and the effluent P concentration [25]. Shan et al. (2009) also found that an almost 70% removal of

65

phosphate by fly ash was reached in a short time and progressively increased with the contact time. However, there were no further increases in the percentage of phosphate removal after 20 h [26]. the contact time. However, there were no further increases in the percentage of phosphate removal after 20 h [26]. the contact time. However, there were no further increases in the percentage of phosphate removal after 20 h [26].

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**Figure 6.** Effect of RSPRC dosage on phosphorus removal efficiency. **Figure 6.** Effect of RSPRC dosage on phosphorus removal efficiency. **Figure 6.** Effect of RSPRC dosage on phosphorus removal efficiency.

**Figure 7.** Effect of contact time on phosphorus removal efficiency. **Figure 7.** Effect of contact time on phosphorus removal efficiency. **Figure 7.** Effect of contact time on phosphorus removal efficiency.

The experimental results on the effect of initial phosphorus concentration on the phosphorus removal of RSPRC are presented in Figure 8. The adsorption capacity (Qe, mg/g) or amount of phosphorus adsorbed by RSPRC significantly increased when the initial P concentration increased. As the contact time is increased, the higher the initial P concentration is, the faster the amount of phosphorus adsorbed by RSPRC is increased. This result indicates that, under the reaction conditions with a high P concentration, the amount of phosphorus adsorbed by RSPRC increased and the efficiency of phosphorus removal was significantly improved [27]. These relationships may partly be explained by the fact that there are sufficient adsorption sites on the surface of the The experimental results on the effect of initial phosphorus concentration on the phosphorus removal of RSPRC are presented in Figure 8. The adsorption capacity (Qe, mg/g) or amount of phosphorus adsorbed by RSPRC significantly increased when the initial P concentration increased. As the contact time is increased, the higher the initial P concentration is, the faster the amount of phosphorus adsorbed by RSPRC is increased. This result indicates that, under the reaction conditions with a high P concentration, the amount of phosphorus adsorbed by RSPRC increased and the efficiency of phosphorus removal was significantly improved [27]. These relationships may partly be explained by the fact that there are sufficient adsorption sites on the surface of the The experimental results on the effect of initial phosphorus concentration on the phosphorus removal of RSPRC are presented in Figure 8. The adsorption capacity (Qe, mg/g) or amount of phosphorus adsorbed by RSPRC significantly increased when the initial P concentration increased. As the contact time is increased, the higher the initial P concentration is, the faster the amount of phosphorus adsorbed by RSPRC is increased. This result indicates that, under the reaction conditions with a high P concentration, the amount of phosphorus adsorbed by RSPRC increased and the efficiency of phosphorus removal was significantly improved [27]. These relationships may partly be explained by the fact that there are sufficient adsorption sites on the surface of the adsorbent. At the same

adsorbent. At the same time, the initial phosphate concentration in the solution is high, so a

time, the initial phosphate concentration in the solution is high, so a sufficient driving force for mass transport promotes the adsorption of phosphate by the RSPRC [28,29]. Meyer et al. obtained the result that the adsorption capacity of electric arc furnace steel slags ranged from 0.09 to 0.28 mg/g and did not increase according to initial P concentrations above 10 mg /L, suggesting that a limit in P removal was reached [30]. On the contrary, when the initial P concentration was from 0.3 to 1 mg/L, the adsorption capacity has been increasing, indicating that the initial P concentration in this experiment was not high enough to make the adsorption capacity reach the limit and that the driving force had not yet achieved its maximum. sufficient driving force for mass transport promotes the adsorption of phosphate by the RSPRC [28,29]. Meyer et al. obtained the result that the adsorption capacity of electric arc furnace steel slags ranged from 0.09 to 0.28 mg/g and did not increase according to initial P concentrations above 10 mg /L, suggesting that a limit in P removal was reached [30]. On the contrary, when the initial P concentration was from 0.3 to 1 mg/L, the adsorption capacity has been increasing, indicating that the initial P concentration in this experiment was not high enough to make the adsorption capacity reach the limit and that the driving force had not yet achieved its maximum.

*Appl. Sci.* **2020**, *10*, x FOR PEER REVIEW 10 of 16

**Figure 8.** Effect of initial concentrations on the absorption of phosphorus. **Figure 8.** Effect of initial concentrations on the absorption of phosphorus.

#### *3.4. RSPRC Adsorption Kinetics 3.4. RSPRC Adsorption Kinetics*

When the adsorbent adsorbs the wastewater, its adsorption function is mainly realized by the chemical bond force, electrostatic attraction and Van der Waals force between the adsorbent surface molecules and the adsorbate molecules in the wastewater. Currently, pseudo-first order kinetic models and pseudo-second order kinetic models are often used to describe the dynamic behavior of solid–liquid static adsorption. When the adsorbent adsorbs the wastewater, its adsorption function is mainly realized by the chemical bond force, electrostatic attraction and Van der Waals force between the adsorbent surface molecules and the adsorbate molecules in the wastewater. Currently, pseudo-first order kinetic models and pseudo-second order kinetic models are often used to describe the dynamic behavior of solid–liquid static adsorption.

The pseudo-first order kinetic equation can express the relationship between the rate of adsorption reaction and the environmental conditions (such as concentration) involved in the adsorption reaction or related to the adsorption reaction. This kinetic model is usually based on the solid adsorption equilibrium capacity. The differential is as follows: The pseudo-first order kinetic equation can express the relationship between the rate of adsorption reaction and the environmental conditions (such as concentration) involved in the adsorption reaction or related to the adsorption reaction. This kinetic model is usually based on the solid adsorption equilibrium capacity. The differential is as follows:

$$\frac{\mathbf{d}\_{\rm qt}}{\mathbf{dt}} = \mathbf{k}\_{\rm l} (\mathbf{q}\_{\rm e} - \mathbf{q}\_{\rm t}) \tag{4}$$

After integrating the differential equation, the pseudo-first order kinetic equation can be expressed as follows: After integrating the differential equation, the pseudo-first order kinetic equation can be expressed as follows:

$$\ln(\mathbf{q}\_{\mathbf{e}} - \mathbf{q}\_{\mathbf{t}}) = \ln \mathbf{q}\_{\mathbf{e}} - \mathbf{k}\_{\mathbf{l}} \mathbf{t} \tag{5}$$

where qe is the adsorption amount of the adsorbent surface to the solute under adsorption equilibrium (mg/g), qt is the adsorption amount of the adsorbent surface to the solute at the specified time (t) during the adsorption process (mg/g), and k1 is the pseudo-first order kinetic equation adsorption rate constant. where q<sup>e</sup> is the adsorption amount of the adsorbent surface to the solute under adsorption equilibrium (mg/g), q<sup>t</sup> is the adsorption amount of the adsorbent surface to the solute at the specified time (t) during the adsorption process (mg/g), and k<sup>1</sup> is the pseudo-first order kinetic equation adsorption rate constant.

The pseudo-second order kinetic model is applicable to the solid–liquid static adsorption reaction in the presence of a saturated site, which can effectively represent the composite effect under the action of multiple adsorption mechanisms. Generally, the kinetic model is based on the adsorption equilibrium capacity. The differential is as follows:

$$\frac{\mathbf{d}\_{\rm qt}}{\mathbf{d}t} = \mathbf{k}\_2 (\mathbf{q}\_e - \mathbf{q}\_t) \tag{6}$$

After integrating the differential equation, the pseudo-second order kinetic equation can be expressed as follows:

$$\frac{\mathbf{t}}{\mathbf{q}\_{\rm t}} = \frac{1}{\mathbf{k}\_2 \mathbf{q}\_{\rm e}} + \frac{1}{\mathbf{q}\_{\rm e}} \mathbf{t} \tag{7}$$

where q<sup>e</sup> is the adsorption amount of the adsorbent surface to the solute under adsorption equilibrium (mg/g), q<sup>t</sup> is the adsorption amount of the adsorbent surface to the solute at the specified time (t) during the adsorption process (mg/g), and k<sup>2</sup> is the pseudo-second order kinetic equation adsorption rate constant.

In general, there is a slight deviation when the single kinetic equation is used to reflect the actual reaction. Therefore, it is necessary to conduct a comparative study after fitting the pseudo-first order kinetic equation and the pseudo-second order kinetic equation. The relevant parameters of the pseudo-first order and pseudo-second order kinetic equations are presented in Table 5.

**Table 5.** Kinetics of phosphorus removal by RSPRC at different initial concentrations.


The pseudo-first order kinetic model and the pseudo-second order kinetic model were utilized to analyze the adsorption kinetics of the adsorbent at different initial phosphorus concentrations. The results illustrate that the correlation coefficients of the pseudo-first order kinetic equation and the pseudo-second order kinetic equation are more than 0.9 and can better reflect the phosphorus adsorption process in different initial P concentrations. The fitting results of the pseudo-second order kinetic model are more accurate than the pseudo-first order kinetic model, because the correlation coefficients exceeded 0.98, while the correlation coefficient of the pseudo-first order kinetic model was only 0.9246 at an initial P concentration of 0.3 mg/L. The adsorption amount of the adsorbent surface to the solute under the adsorption equilibrium (qe) obtained by the pseudo-second order kinetic model was larger than that of the sample measured result (qexp). The reason for the difference is that when the adsorption process continues, the P concentration of the wastewater continues to decrease, the conditions required for each adsorption mechanism change, the molecular force between the surface of the adsorbent and phosphorus is reduced, and the concentration difference required for the chemical adsorption decreases. These all lead to a decrease in the adsorption capacity.

In the second-order kinetic model, k<sup>2</sup> represents the adsorption rate constant of the pseudo-second order kinetic equation, which describes how fast the adsorption is carried out under the corresponding conditions. Table 5 suggests that when the initial phosphorus concentration was 1.0 mg/L, the k<sup>2</sup> was 0.742, which was the maximum value. The k<sup>2</sup> decreased with the increase in the initial P concentration. The k<sup>2</sup> was only 0.301 under the condition that the initial P concentration was 0.3 mg/L. These results further demonstrate that under reaction conditions with a high P concentration, the adsorption capacity of the adsorbent increases, and the efficiency of phosphorus removal also increases remarkably.
