1. Introduction
Phosphorus, as an essential nutrient for biological growth, plays an irreplaceable role in modern agriculture and industrial production [
1]. Driven by rainfall and wastewater discharges, excessive phosphorus runoff into nearby waters can lead to non-point source phosphorus pollution, such as eutrophication, algal blooms, reduction of biodiversity, deterioration of water quality, and human health problems [
2]. Therefore, rigorous control of phosphorus load in waters has become a new challenge for water environment management.
Currently, the main methods such as biological methods, chemical precipitation, ion exchange, and adsorption have been widely applied to remove phosphorus from water [
3]. The adsorption method is regarded as a good application prospect due to its simple, low-cost, and high efficiency of phosphorus removal [
4]. In recent years, different types of adsorbents have been developed for phosphate (PO
43−-P) removal, including carbon-based materials [
5], zeolites [
6], silica [
7], diatomaceous earth [
8], and so on. However, the practical application was hindered by the limitations of the high cost and isolation difficulty. To achieve excellent PO
43−-P adsorption performance at an acceptable cost, industrial waste-based adsorbent materials have attracted wide attention [
9]. A lot of work has been reported on the basis of industrial waste-based material to prepare adsorbents for the removal of phosphorus, including municipal sludge [
10], coal fly ash [
11], cement blocks [
12], red mud [
13], and slag [
14]. Lin et al., (2021) reported the high adsorption capacity of red mud-based ceramsite for PO
43−-P [
15]. A study by Gu et al., (2021) showed that coal fly ash achieved phosphorus removal through adsorption and precipitation [
16]. Similarly, Liu et al., (2020) innovatively used waste concrete to enhance the removal of phosphorus, and demonstrated that the phosphorus adsorption capacity of modified waste concrete was up to 100 mg/g [
17]. Industrial waste-based ceramsite can improve the level of resource utilization and address the issues of environmental pollution. This is a necessary condition for sustainable development strategy.
In China, more than 10 million tons of dry sludge was produced annually from wastewater treatment plants [
9]. Sludge disposal by landfills and incinerators could not satisfy the trend of sustainable development in the future. The construction industry is reportedly the largest consumer of natural resources, producing an enormous amount of 2.65 billion tons of waste annually [
18]. Moreover, finding ways of sustainable alternatives to reuse and recycle these industrial wastes is gaining importance due to the amount of wastes generated and concerns about inadequate final disposal. The porous structure and high specific surface area of coal fly ash and cement from industrial wastes make it possible to adsorb and precipitate phosphorus [
19]. However, in order to further improve the adsorption capacity of adsorbents on pollutants, metal ions have been widely used in the surface modification of adsorbents [
20]. Yang et al., (2018) pointed out that the iron-modification waste-activated sludge (WAS)-based biochar contributed to PO
43−-P adsorption, and the maximum PO
43−-P adsorption capacity of 111.0 mg/g was observed using FeCl
3-impregnated WAS-based biochar [
21]. Deng et al., (2021) used Mg-modified-biochar composites for removing phosphate from waste streams and reported the maximum phosphate adsorption capacity of 128.21 mg/g [
22]. Using industrial wastes as raw material for the preparation, ceramsite is a sustainable development strategy which can not only improve industrial waste management, but also reduce environmental effects.
This study synthesized a composite ceramsite composed of sludge, cement blocks, and coal fly ash, which was further doped with aluminum salts to form Al-doped waste ceramsite (Al-ceramsite) to enhance PO43−-P removal from aqueous solutions. The optimal process parameters were determined by a static adsorption experiment, and the desorption behavior of Al-ceramsite were further understood. The dynamic adsorption column was to gain a deeper understanding of the adsorption performance of Al-ceramsite in rainwater under more realistic operating conditions. Therefore, the adsorption mechanism of Al-ceramsite on PO43−-P were analyzed by the X-ray photoelectron spectroscopy (XPS). With high efficiency and low cost, the waste-based ceramsite is of significant importance for phosphorus removal, and provides an economically sustainable way for rainwater purification.
2. Materials and Methods
2.1. Materials
All reagents were of analytical grade. Aluminum nitrate Al(NO3)3, sodium sulphate (Na2SO4), sodium chloride (NaCl), sodium phosphate (Na3PO4), ascorbic acid (C6H8O6), magnesium sulphate (MgSO4), and sodium fluoride (NaF) were bought from Aladdin Reagents Co., Ltd. (Shanghai, China). Potassium dihydrogen phosphate (KH2PO4), hydrochloric acid (HCl), and ammonium molybdate ((NH4)2MoO4) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Sodium hydroxide (NaOH) and antimony potassium tartrate (C4H4KO7Sb·0.5H2O) were from Macklin Biochemical Co., Ltd. (Shanghai, China). Sodium acetate (NaAC) and sodium carbonate (Na2CO3) were from Bide Pharmatech Ltd. (Shanghai, China). Calcium oxide was from Meryer Chemical Technology Co., Ltd. (Shanghai, China). All solutions were prepared with deionized water. Sludge, cement block, and coal fly ash were used to prepare ceramsite. Sludge was collected from a sewage-treatment plant in Nanjing (Jiangsu, China), cement block was produced from a construction site in Nanjing (Jiangsu, China), and coal fly ash was acquired from a power plant in Nanjing (Jiangsu, China).
2.2. Preparation of Ceramsite
Al-ceramsite was prepared based on a previous study [
23]. Preparation conditions were as following: firstly, sludge, cement blocks, and coal fly ash were crushed and ground by a predetermined mass ratio (3:2:6) and then fed into the granulator to form granules. Secondly, the particles of 4–6 mm were screened and dried at 105 °C for 2 h. Finally, undoped-ceramsite (un-ceramsite) was obtained by preheating at 350 °C for 10 min and calcining at 1150 °C. Al-ceramsite was prepared via doping Al(NO
3)
3 solution with ceramsite under the optimum conditions (Al(NO
3)
3 concentration of 0.75 mol/L, the ratio of ceramsite mass to Al(NO
3)
3 solution of 1:3, doping time of 3 h, and temperature of 25 °C). The Al-ceramsite was washed with distilled water to neutral and calcined at 600 °C for 4 h. After cooling, Al-ceramsite was collected and reserved. Of these, the physicochemical properties of Un-ceramsite and Al-ceramsite are shown in
Table 1.
2.3. Experimental Setup
The static adsorption performance of ceramsite was tested in a beaker. A certain mass of ceramsite was added to the PO
43−-P solution and mixed in a constant temperature shaker (THZ-C, Taicang Qiangle Experimental Equipment Co., Ltd., Jiangsu, China) at 120 r/min. Dynamic adsorption experiments were tested in polyvinylchloride (PVC) tubes with an inner diameter of 3.5 cm and a height of 40 cm.
Figure 1 reveals the setup of the static adsorption and the continuous flow dynamic adsorption column experiment. Experiments were performed at room temperature. Natural rainfall (rainwater was collected from natural rainfall in Nanjing, China, in the summer of July–August 2019, with a PO
43−-P content of 0.58 mg/L and a rainwater pH of 7.2–8.7) was selected as the influent water for the column experiment. Flow rate maintained by a peristaltic pump (BT100-2J, Longer Precision Pump Co., Ltd., Hebei, China). Samples were taken at scheduled times for determination of PO
43−-P concentration.
2.4. Experimental Procedure
2.4.1. Static Adsorption of PO43−-P by Al-Ceramsite
To investigate the effect of Al-ceramsite on PO43−-P adsorption, Un-ceramsite and Al-ceramsite were added to 10 mg/L of PO43−-P solutions (pH = 7 and temperature of 25 °C) for 20 h, respectively. The effect of Al-ceramsite on PO43−-P removal was further studied in different environmental factors (dosage of Al-ceramsite, initial PO43-P concentration, and initial pH). Effect of coexisting ions were evaluated by using different coexisting ions (Ca2+, Mg2+, SO42−, CO32−, F−, and Cl−), and the influence of varying coexisting ion concentrations (2–10 mmol/L) on PO43−-P absorption was investigated.
2.4.2. Effect of Phosphorus Desorption
The adsorption-saturated Al-ceramsite was obtained at about 25 °C, pH = 6.5, and 120 r/min for 48 h. Firstly, the desorption performance of Al-ceramsite was examined at pH = 4–10. Secondly, the effect of desorbent type (NaAC, Na2CO3, and NaOH) on the desorption of Al-ceramsite was investigated. Finally, the efficiency of desorbent concentration (0.25–2 mol/L) on the desorption of Al-ceramsite was analyzed.
2.4.3. PO43−-P Removal by Dynamic Adsorption Column
To understand the effect of Al-ceramsite on the adsorption of PO43−-P from rainwater by column experiments, different submerged zone depths (SZD) and filter media heights (FMH) was investigated. Then, samples were collected at intervals for PO43−-P concentration determination. Subsequently, the effect of stormwater biofilter on PO43−-P removal in practical application was simulated, and the columns were filled with Al-ceramsite and gravel as fillers, respectively. The effect of columns on the removal of PO43−-P from rainwater was analyzed in different influent flows (0.04–0.12 mL/s), and antecedent dry day (ADD) of 3–21 days, where SZD was 20 cm and FMH was 15 cm.
2.5. Analytical Methods
A pH meter (PHS-3C, Shanghai INESA Scientific Instrument Co, Ltd., Shanghai, China) was used to monitor the solution pH. The concentration of PO
43−-P was determined by ultraviolet spectrophotometer (GEN10S UV-VIS, Thermo Fisher Scientific Inc. Shanghai, China) and based on standard methods [
24] as follows: PO
43−-P (molybdate colorimetric method). X-ray photoelectron spectroscopy (XPS) was obtained using an ESCALAB 250, Thermo instrument. Each batch was repeated three times using the mean value for analysis. Origin 2018 software was used to analyze the data.
The PO
43−-P adsorption capacity (Q
e, mg/g) on ceramsite is calculated by Equation (1). The PO
43−-P removal rate (R, %) is calculated by Equation (2). The PO
43−-P desorption efficiency (D, %) is calculated by Equation (3).
where Q
e is the adsorption capacity (mg/g); V is the solution volume (L); C
0 and C
e are the initial and equilibrium concentrations of PO
43−-P (mg/L), respectively; m is the mass of ceramsite (g); R is the PO
43−-P removal efficiency (%); Q
ed is the PO
43−-P desorption capacity; and Q
ts the theoretical saturation adsorption capacity.