Synthesis of iron phosphate-SAPO-34 composite and its application as effective absorbent for wastewater treatment

. High-purity FePO 4 was purified from iron-based phosphating slag as raw material, and FePO 4 @SAPO-34 was synthesized by hydrothermal crystallization method under the action of templating agent-diethylamine. The synth esized FePO 4 @SAPO-34 samples were characterized by x-ray diffraction (XRD), scanning electron microscopy (SE M) and Fourier transform infrared spectroscopy (FT-IR). The effects of different crystallization time on the morpholo gy and crystallization of FePO 4 @SAPO-34 crystals were investigated. The removal of heavy metal ion wastewater by low-cost FePO 4 @SAPO-34 was investigated. The experimental results show that when the reaction time is 180 °C an d the reaction time is 72h, the crystallization of FePO 4 @SAPO-34 is the best. When the dosage is 0.6g, the removal e fficiency of heavy metal ions is the highest.


Introduction
Phosphating is a commonly used surface treatment technology. Phosphating slag is a precipitated substance that is produced on the surface of a metal phosphating process. Its main components are metal ions such as phosphate, iron and zinc [1]. In order to effectively control the pollution of phosphating slag, researchers have carried out a lot of research work and made some progress. In this study, the iron phosphate is purified by phosphating slag and has various crystal structures. It is provided with unique catalytic properties, ion exchange properties, etc. Research reports show that iron phosphate is negatively charged (zeta = -30 ~ -40mV) in wastewater, and Its electronegative properties produce electrostatic adsorption on positively charged ions [2].
The molecular sieve has a uniform pore structure. According to the molecular size, it can selectively adsorb. Thus, it is mostly used for adsorbents and catalysts [3]. In 1982, SAPO-34 was first synthesized as a SAPOS series molecular sieve by Wilson et al [4]. It has CHA structure and medium strength acid center. SAPO-34 exhibits excellent performance in adsorption and catalysis [5]. Because SAPO-34 has a neutral framework structure, it is difficult to provide the active center required for many reactions [6]. Therefore, it is very important to modify the material. In summary, this study prepared a new catalytic adsorption material of FePO4@SAPO-34. New materials were applied to remove heavy metal ions from wastewater.

Adsorbent preparation
A certain amount of phosphating slag, water and phosphoric acid were uniformly mixed in an appropriate ratio, and placed in an oven at 80℃ for 4 h, and the sample was suction filtered and dried overnight; after three times of reaction, a phosphating slag was obtained to purify the sample iron phosphate. The experiment used hydrothermal synthesis of FePO4@SAPO-34. Firstly, the silicon source, the phosphorus source, the aluminum source, the FePO4 and the appropriate amount of water are uniformly mixed according to a certain ratio, and then the templating agent diethylamine is added, and after pretreatment, it is transferred to a polytetrafluoroethylene reactor, and heated to 180 °C, and is self-generated [5]. Crystallize under pressure for different times, after suction filtration, washing and drying. Then, it was baked in a muffle furnace at 550 °C for 6 h to obtain a modified new material.

Heavy metal ion removal experiment
The experiment of removal of heavy metal ions in simulated wastewater was carried out with a material dosage of 0.6 g, lead-containing (Pb(NO3)2) wastewater concentration of 100 mg/L. Adesignated intervals, samples of the solution were collected and immediately filter to monitor the adsorption efficiency by inductively coupled plasma-atomic emission spectrometry (ICP-AES) (ICAP 6000, Thermo Corporation, USA). All adsorption experiments were stirred at 350 r/min. After adsorption experiment, the FePO4@SAPO-34 adsorbent was collected with strong acid and then reused.

Characterization methods
The morphology of the samples was characterized by sca-nning electron microscopy (SEM) (S-4800, Hitachi Corp-oration,Japan) with the accelerating voltage at 10 kV and current at 10 mA. X-ray diffraction (XRD) (D8-Advance, Bruker Corporation, Germany) measure ments were un-dertaken to record the phase composition and crystalline structure of the powder samples, operating at 40 kV and 40 mA with a monochromatized Cu Kα radiation (λ = 0.15418 nm) source. The chemical bonds on the surface of the samples were examined by Fourier transform infrared spectroscopy (FTIR) (Vertex70, Bruker Corporation, Germany). KBr was chosen as a diluent. The surface area was measured based on the Brunauer Emmett Teller (BET) (ASAP-2020M, Micromeritics Corporation, USA) model over a relative pressure range of 0.05-1. (JCPDS), respectively [6][7]. With the extension of time, the peak intensity gradually increased. When t=72 h, the peak intensity was the strongest, but when the time was extended to 84 h, the peak intensity weakened and the impurity peak began to appear. The crystallinity of SAPO-34 was set to 100%. The relative crystallinity of FePO4@SAPO-34 is calculated as follows [7][8]:

XRD characterization
(1) Where, Xi is relative crystallinity of the sample; XS is equivalent to 100%; ∑Ii is the sum of the five characteristic peak intensities of the sample; ∑Is is the sum of the five characteristic peak intensities of the standard sample. From Fig. 3, the relative crystallinity of FePO4@SAPO-34 increased from 40% to 98%, indicating that the crystallinity of the sample increased with the crystallization time. But when the time is too long, it could lead to the formation of other crystals. Too short or too long crystallization time was not conducive to the synthesis of FePO4@SAPO-34. The crystallization time was the most favourable for the synthesized FePO4@SAPO-34 at 72 h.

SEM characterization
The scanning electron microscopy (SEM) images showed, as the time increased from 48 h to 72 h, the shape of the material became more and more complete. At 180 ℃, t=72 h, FePO4@SAPO-34 had a typical cubic small square structure of chabazite-based molecular sieves, indicating that the crystal growth was perfect [9][10]. When the time was gradually increased to 84-96 h, the morphology of FePO4@SAPO-34 particles was destroyed, and some amorphous substances appeared in the synthesized product.

FT-IR and BET characterization
FT-IR analysis was further carried out to verify FePO4@SAPO-34 in the reaction process. FT-IR spectrum was sensitive to the skeletal structure of the molecular sieve, and could better detect the molecular sieve crystallite structure. SAPO-34 has a periodic arrangement of PO 2+ , AlO4 and SiO4 tetrahedral framework structures. As shown in Fig 4, the spectrum of FePO4@SAPO-34 was basically consistent with the spectrum of SAPO-34. Peak at 2320 cm -1 could be ascribed to the bending vibration of CO2, while those at 730cm -1 , 640cm -1 , 575cm -1 and 530cm -1 could be assigned to the stretching vibration of OPO (O-Al-O), the double six-membered ring vibration peak and PO4. It may be due to moisture in the air during the test. When FePO4 was doped, since the electron energy of iron was smaller than the electron energy of silicon, the bonding strength of the sample was enhanced, and the corresponding frequency was increased [11][12][13].

Adsorption of pb 2+ experiments
The removal effect of Pb 2+ by SAPO-34 and FePO4@SAPO-34 materials has been shown in Fig.5. It can be seen that SAPO-34 has better absorbing capacity than FePO4@SAPO-34 in the first 45min, which was ascribed to the large specific surface area for pure SAPO-34 with a uniform pore structure [13][14][15]. However, the removal efficiency of two materials has been changed. The removal efficiency of FePO4@SAPO-34 exceeded to that of SAPO-34 due to the electrostatic effect of FePO4 in aqueous solution, further attracting cations which has been reported in other papers. Thus, the FePO4@SAPO-34 showed excellent adsorption performance in pollution removal.
On this basis, this experiment studied the effect of FePO4@SAPO-34 dosage on Pb 2+ adsorption. As shown in Figure 6, as the amount of material increases, it can be seen that, the amount of Pb 2+ adsorption increases. The increase was larger and then tended to be flat. The adsorption efficiency is calculated as follows: (2) Where, C0 is original concentration; Ci is concentration during dynamic adsorption equilibrium.
is adsorption efficiency. When the addition amount was 0.2 g, the adsorption rate was only 67.3%. When the amount added was increased to 0.6 g, the removal rate of Pb 2+ was 89%. When the amount added was 1 g, the adsorption rate was 92%. As the number of exchanged active particles increases, the amount of adsorption of metal ions becomes large [14][15][16]. Therefore, the amount of adsorbent has a great influence on the adsorption effect.

Conclusions
In this study, an effective phosphate residue recovery method was proposed and be used to fabricate a FePO4@SAPO-34 adsorbent. Experiments were conducted to investigate the effect of different crystallization times on the crystal form of the adsorbent. The results showed that the synthesized sample crystal form was more complete at 72 h. Experiments have found that new materials can effectively remove heavy metal ions from wastewater. Compared with SAPO-34, the new synthetic material passes through the electro-negative properties of FePO4, so it electrostatically adsorbs positively charged ions. And FePO4 provides a metal active center for SAPO-34. The synergy between SAPO-34 and FePO4@SAPO-34 improves the adsorption performance of new materials.