1. Introduction
With the increases in population, industrialization, urbanization, and agricultural modernization, water pollution is becoming increasingly serious [
1]. A large amount of phosphorus-rich domestic sewage, industrial wastewater, and farmland runoff are discharged into lakes, rivers, oceans, and other water bodies, causing eutrophication of water bodies [
2,
3,
4]. Eutrophication of water bodies poses a huge threat to human health [
5]. Therefore, it is very important to remove phosphorus from water.
The commonly used phosphorus removal methods mainly include chemical and biological methods [
6]. The chemical precipitation method is widely used in engineering, but this method has a high cost and a large amount of precipitated sludge. Coagulant addition can easily cause equipment corrosion, blockages, and water pollution [
7,
8]. Biological dephosphorization methods have lower operating costs, but the effectiveness of dephosphorization generally does not exceed 30% [
9]. Biological dephosphorization has low stability and strict operation requirements. The effectiveness of dephosphorization is greatly affected by wastewater temperature and pH [
9]. Therefore, it is necessary to research and develop new efficient and feasible phosphorus removal technologies. As an alternative to conventional and chemical methods in ecological engineering, the employment of constructed wetlands (CW) to treat phosphorous wastewater is becoming increasingly popular [
10]. The main mechanisms for removing phosphorus from water in constructed wetlands include substrate removal, plant removal, and microbial assimilation. A large number of studies have shown that the substrate in constructed wetlands is the main determinant of phosphorus removal [
11,
12]. The substrate in the constructed wetland provides growth media for plants and microorganisms, and at the same time, can directly remove phosphorus through ion exchange, specific or nonspecific adsorption, complexation, and precipitation. The most important thing is that the constructed wetland substrate contains abundant metal elements, such as iron, aluminum, and calcium. The ions of these metal elements can combine with the phosphate ions in the water to form a precipitate to achieve the purpose of removing phosphorus. Therefore, choosing the appropriate substrate is the key to improving the phosphorus removal capacity of constructed wetlands.
In recent years, iron-carbon (Fe-C) micro-electrolysis technology has been used in many applications for the treatment of phosphorus-containing wastewater [
13]. This method is also called the internal electrolysis method and iron-carbon method [
14]. Iron-carbon micro-electrolysis technology uses wastewater as the electrolyte, iron in the micro-electrolytic substrate as the anode, and activated carbon as the cathode to form a “primary cell.” The discharge is used to form an electric current to electrolytically oxidize and reduce the wastewater discharge. The principle is based on using the combined effects of electrochemistry, oxidation-reduction, and flocculation to achieve the purpose of removing organic pollutants [
15]. Iron-carbon micro-electrolysis has the advantages of a wide application range, low cost of processing, short processing time, convenient operation and maintenance, low power consumption, etc. [
16,
17].
The preparation technology of iron-carbon (Fe-C) substrates has also evolved greatly. In previous studies, iron-carbon micro-electrolytic substrates were mainly synthesized by physically mixing or sintering iron and carbon as raw materials [
18]. In 2014, Zhou et al. used active iron-carbon micro-electrolysis systems to remove organic phosphates from discharged circulating cooling waters [
19]; Shi et al. prepared an iron oxide/activated carbon composite adsorbent (activated carbon loaded with iron oxide) that can effectively treat phosphate-contaminated water [
20]; Chen et al. synthesized an iron-carbon (Fe-C) micro-electrolysis material substrate through a carbothermal reduction process using flotation waste copper slag as a carbon source and anthracite as a carbon source [
21]. Although these substrates have a good removal effect, they have the following problems: (1) The iron scrap landfill treatment time is long, and packing compaction is prone to occur [
22]; (2) iron-carbon stacks act as iron-carbon primary cells to obtain a good phosphorus removal effect, but with increasing processing time, cathode and anode separation and blocking problems will occur [
23]; and (3) through firing and the formation of a regular structure, a substrate can be obtained with strengthened contact between the cathode and anode to avoid separation problems, but the energy consumption is high, and air pollution is caused during firing [
24]. To solve the problems related to the application of traditional iron-carbon micro-electrolysis, improvements must be made. There are two ways to solve the problems of traditional iron-carbon substrates: (1) Combine new technologies, such as oxidation, on the basis of traditional iron-carbon substrates; (2) develop new iron-carbon substrates. However, combining micro-electrolysis with other technologies and developing new reactors will increase the costs of system operation and maintenance to a certain extent and complicate the operation. In this paper, a new type of iron-carbon substrate is prepared, which not only solves the problems of iron powder and carbon powder loss, and cathode and anode separation, but also solves the serious problem of substrate compaction, saves energy, and avoids air pollution.
In this study, a new type of iron-carbon-loaded substrate (NICLS) was prepared for the first time. Using water-based polyurethane as the binder, hydroxyethyl cellulose as the thickener, iron and carbon as the coating, and zeolite as the aggregate, single-factor experiments were used to optimize the NICLS to determine the optimal ratio of the substrate raw materials. A static test was used to study the effects of different factors (pH, initial concentration, reaction time, and temperature) on phosphorus removal by the NICLS. A specific surface area analyzer (BET), scanning electron microscope (SEM), X-ray diffractometer (XRD), and energy-dispersive spectrometer (EDS) were used to determine the specific surface area of the NICLS, analyze its composition, examine its surface structure, analyze the type and content of elements in the substrate, and explore its mechanism of phosphorus removal, laying the foundation for the practical application of this substrate. The innovation of this research lies in the fact that the NICLS was first made by loading iron carbon on zeolite, which not only achieves zeolite recycling but also improves the removal of phosphorus, promotes the development of phosphorus removal substrate materials, improves water quality, and has good environmental and social benefits.
2. Materials and Methods
2.1. Materials
Zeolite particles, iron powder, and activated carbon (coal activated carbon, wood activated carbon, tar activated carbon, and coconut shell activated carbon) were all obtained from the Zhengzhou Building Materials Market (Zhengzhou, China). The particle size of the zeolite was 10–16 mm, and the surface was smooth. The zeolite was cleaned and dried before use.
2.2. Chemicals
All the chemicals used in this study were of analytical reagent grade. Phosphorous solutions with different concentrations were prepared from KH
2PO
4, and the pH value of the solutions was adjusted with 0.1 mol/L hydrochloric acid and 0.1 mol/L sodium hydroxide [
25]. Water-based polyurethane, hydroxyethyl cellulose, potassium dihydrogen phosphate (KH
2PO
4), ammonium molybdate [(NH
4)
6Mo
7O
24·4H
2O], concentrated sulfuric acid, ascorbic acid (C
6H
8O
6), potassium persulfate, and potassium antimony tartrate (C
4H
4KO
7Sb·1/2H
2O) [
26] were obtained from Zhengzhou Heshun Technology Co., Ltd. (Zhengzhou, China), and all solutions were prepared using deionized water (the conductivity of deionized water is 1.0 µs/cm).
2.3. Preparation of NICLS
The NICLS is composed of basic ordinary zeolite and a cladding layer, which is composed of components (iron and carbon) capable of removing a large amount of phosphorus. The iron-carbon, ordinary zeolite, and binder are mixed to adhere the iron carbon to the ordinary zeolite.
The specific procedure was as follows: First, the zeolite was washed with water. Then, 3.0 g of hydroxyethyl cellulose was added (to increase the viscosity of the water-based polyurethane) to 30 mL of water-based polyurethane, and the mixture was stirred evenly. Then, 20 g of clean zeolite was added, and the mixture was stirred continuously to coat the zeolite with a layer of binder. This mixture was then added to 10 g of iron-carbon polymer, stirred well, and shaken to wrap the iron-carbon on the zeolite. Finally, the material was dried naturally to obtain the NICLS, as shown in
Figure 1.
2.4. Optimization of NICLS Preparation Conditions
To optimize the NICLS preparation conditions, the removal capacity of phosphorus was taken as the detection target, and a single-factor method was used to study different iron-carbon mass ratios (Fe/C = 4:1, 3:1, 2:1, 1:1, 1:2, 1:3, and 1:4), thickener contents (0.01, 0.1, 0.5, 1.0, and 1.5 g), carbon types (coal, wood, tar, and coconut shell), and carbon particle sizes (>60 mesh, 200-60 mesh, and <200 mesh). The influence of these four factors on the phosphorus removal performance of the NICLS was experimentally determined to obtain the best preparation conditions for the NICLS, and the significant difference analysis of the experimental data.
2.5. Comparison with Other Constructed Wetland Substrates
In this experiment, five types of artificial wetland substrates were selected for comparison, namely ceramsite, gravel, volcanic rock, steel slag, quartz sand, etc., all of which come from the building materials market.
Ceramsite is made of natural clay minerals or solid waste as the main raw material, supplemented by a small amount of additives, and mixed and granulated by a sintering process. The ceramsite has the characteristics of large specific surface area and developed porosity, which can realize the removal of phosphate ions in water [
27].
Gravel is a natural pellet made of exposed rocks through weathering or long-term transport by water. Gravel has a wide range of sources and low cost. It is one of the most widely used substrates in experiments and engineering at home and abroad [
28].
The main components of volcanic rock are SiO
2, Al
2O
3, Fe
2O
3, and CaO, and it has good surface activity and pore structure [
29].
Steel slag is mainly composed of calcium, iron, silicon, magnesium, and a small amount of oxides such as aluminum, manganese, and phosphorus. It has a pore structure and a large specific surface area. Steel slag has stable performance, low price, and good adsorption capacity, which is used to treat wastewater [
30].
Quartz sand is one of the most widely used and most used water purification materials. Quartz sand has stable chemical properties, strong dirt interception ability, and has the advantages of long service life, wide application range, and low processing cost [
31].
2.6. NICLS Removal Experiment of Phosphorus in Simulated Wastewater
The NICLS was prepared under the optimal conditions (the optimal synthesis conditions of the NICLS are 1 g hydroxycellulose, wood activated carbon as the cathode, an activated carbon particle size of 200-60 mesh, and an Fe/C ratio of 1:1), and experimentally prepared simulated phosphorus solution (0.2197 g KH2PO4 dissolved in 100 mL water) was used as the treatment object. The influence of different pH values, initial phosphorus concentrations, and reaction times on the treatment effect of the NICLS on wastewater was investigated using a single-factor analysis method, and blank and parallel experiments were carried out throughout the whole process.
The specific procedure was as follows: A total of 0.5 g of the NICLS was weighed into a 250 mL Erlenmeyer flask, 100 mL of a set concentration of phosphorus solution was added, and the pH of the solution was adjusted to 6 with 0.1 mol/L hydrochloric acid or 0.1 mol/L sodium hydroxide solution. The solution was shaken in a constant-temperature shaker at 150 r·min
−1 and 25 °C for 24 h. A syringe was used to take out 5 mL of the supernatant from the Erlenmeyer flask and filter it with a 0.45 µm filter membrane; the phosphorus concentration in the filtrate was determined by ammonium molybdate spectrophotometry (Multi-parameter water quality analyzer (5B-3B(V8)) of Lianhua Technology, Zhengzhou, China), and the removal capacity of phosphorus on the NICLS was calculated according to Equation (1) [
32].
where
C0 is the initial phosphorus concentration of the solution, mg/L;
Ae is the removal capacity at equilibrium, mg/g;
Ce is the phosphorus concentration at reaction balance, mg/L;
m is the mass of the NICLS, g; and
V is the volume of the phosphorus solution, L.
2.6.1. Effect of Reaction Time and Initial Phosphorus Concentration
Five series of 250 mL Erlenmeyer flasks were prepared: 0.5 g of the NICLS and the phosphorus solution was added to reach the final phosphorus concentrations of 10, 30, 50, 100, or 150 mg/L, and the pH of the solution was adjusted to 6 with 0.1 mol/L hydrochloric acid or 0.1 mol/L sodium hydroxide solution [
33]. The flasks were incubated at 25 °C and 150 r·min
−1, and samples were collected at different times (1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, and 24 h). After the water sample was left and centrifuged, the phosphorus content in the solutions was determined by ammonium molybdate spectrophotometry, and the corresponding removal capacity was calculated [
34]. The calculation formula used to determine the removal capacity is formula (1).
2.6.2. Effect of pH
Using the same test method of phosphorus removal by the NICLS, we set different pH values (3, 5, 7, 9, and 11) to determine the effect of pH on the NICLS removal of phosphate ions [
35]. The calculation formula used to determine the removal capacity is formula (1).
2.7. Characterization of the NICLS
A surface area analyzer was used to measure the BET surface area (SBET) and pore volume of the NICLS from the 77 K nitrogen adsorption/desorption isotherm [
36]. The pore size was derived using the BET and DFT methods [
37]. Scanning electron microscopy (SEM) was used to characterize the surface morphology of the NICLS before and after the reaction, and energy-dispersive X-ray spectroscopy (EDS) was used to determine the composition change in the NICLS before and after the reaction [
38,
39]. X-ray diffraction (XRD) patterns of the NICLS were recorded with an X-ray diffractometer (China DX-2700), (Cu Kα, 30 mA, 40 kV) [
40].