Study of simultaneous adsorption of ammonium and phosphate in waters by La-F4A zeolites prepared from spent FCC catalyst

— The problem of resource disposal of massive spent fluid catalytic catalyst needs to be solved urgently. Thus an environmentally friendly, highly efficient and economical cost adsorbent used for nitrogen and phosphorus removal was prepared from spent fluid catalytic catalyst. A novel 4A zeolite adsorbent loaded with lanthanum hydroxide (La-F4A) was prepared by acid leaching pretreatment, alkali fusion roasting hydrothermal and co-precipitation method. And use La-F4A in the study of simultaneous adsorption of ammonia-nitrogen and phosphorus in water. Adsorption experiments were carried out under the condition that the primordial concentrations of PO 4 and NH 4+ -N were respectively 1 & 10mg/L. La-F4A has an adaptive adsorption ability on NH 4+ -N and PO 4 in the pH range of 4-9. When the temperature is 298K, the dosage is 1.0 g/L, the contact time is 240 min, and test water pH=7, the maximum adsorption capacities for NH 4+ -N and PO 4 are 7.64 mg/g and 0.93 mg/g respectively. The behaviours accord with the pseudo-second-order adsorption kinetic model. The adsorption of PO 4 conforms to the Langmuir isotherm model while NH 4+ -N conforms to the Freundlich isotherm model. Coexisting anions have almost no effect on the adsorption of PO 4 , coexisting cations have different inhibitory effects on the adsorption of NH 4+ -N. After four desorption cycles, the removal efficiency of La-F4A for NH 4+ -N and PO 4 was 77% and 93% of the initial capacity. There are both chemical adsorption and physical adsorption in the process. The ammonium removal mechanism is the ion exchange between NH 4+ and Na + in the framework, and the phosphate removal mechanism is the ligand exchange between PO 4 and hydroxyl to form LaPO 4 . La-F4A is regarded to be an excellent adsorbent for ammonium and phosphate removal.


Introduction
Fluid catalytic cracking (FCC) is a secondary crude oil processing method for modern refineries to transform heavy oil to light oil. The catalyst used in the FCC process is called FCC catalyst.
With the increasing demand for light oil such as gasoline and diesel, the annual growth rate of FCC catalyst usage is about 3.76%, China's demand is growing even faster 1 . This means that more and more deactivated FCC catalysts are being produced. Spent FCC catalysts contain a rich amount of rare earth elements such as cerium and lanthanum (La), as well as heavy metals including iron, vanadium, nickel. Direct discharge not only pollutes the environment but also spends resources 2 . Therefore, it is essential to study the resource utilization of spent FCC catalyst.
In recent years, various methods have been developed to treat and dispose of spent FCC catalyst, including landfilling 3 , chemical regeneration to remove heavy metals 4 , the recovery of rare earth elements and valuable metals 56 . Another promising method is using them as raw materials for the production of new synthetic materials. Scholars have utilized spent FCC catalyst for the preparation of adsorbents used in the removal of CO 2 and other gases, and heavy metals such as Ni, Zn, and Cu 78 . It has become a research hotspot, because it not only reduces the cost of new materials, but also reduces the environmental pollution caused by the disposal.
Eutrophication of water bodies refers to the overproduction of organic matter caused by inputs of nitrogen and phosphorus. Water treatment nitrogen and phosphorus removal technologies include chemical precipitation, biological methods, adsorption, membrane separation, electrochemical methods, etc. Adsorption method is often used for low-concentration nitrogen and phosphorus in wastewater due to the simple operation and high removal efficiency. Among various adsorbents, zeolites are widely used in the adsorption of ammonium due to their low price and excellent cation exchange capacity 910 . Lanthanum-based materials have excellent affinity for phosphate. Lanthanum is relatively cheap and environmentally friendly among the rare earth elements 1112 . At the same time, the harmless waste FCC catalyst can be used as raw material to prepare adsorbent, but there is no research on efficient simultaneous adsorption of nitrogen and phosphorus, which provides a new idea for its resource utilization.
This study will explore the preparation of highefficiency ammonium and phosphate removal adsorbent from spent FCC catalyst. Firstly, the spent FCC catalyst was treated harmlessly by acid leaching, and then various conditions for hydrothermal processes of zeolite 4A were optimized respectively. Finally, the particle adsorbent La-F4A was prepared by loading lanthanum hydroxide on the surface. La-F4A was applied to the batch adsorption experiments of ammonium and phosphate to explore its adsorption performance and mechanism, which provided useful reference for the resource utilization of spent FCC catalyst and advanced treatment of nitrogen and phosphorus in tail water.

Reagents and instruments
The sample of the spent FCC catalyst was supplied by a chemical plant in East China.All the chemical reagents used in the experiment are analytically pure. The experimental water is deionized water.

Preparation of adsorbents
For the preparation method of F4A, see the reference13: Screen the spent FCC catalyst sample with a 100-mesh sieve for later use. Weigh 8.0000 g of spent FCC catalyst sample into a 250 mL conical flask, add 100 mL HCl solution with a concentration of 1 mol/L. Shake it in water bath at 75℃ and 120 rpm for 8 h, and then regulate pH value to 7.0±0.1. Filter and wash with deionized water for several times, and dry at 105℃ for 2 h. Put the dried sample with solid NaOH into agate mortar according to the mass ratio of 1:1.4, ground them evenly and then transfer to crucibles. Roast in muffle furnace at 700℃ for 2 h and cool. Transfer to a 150 mL PTFE lined bottle after grinding, and add deionized water at solid-liquid ratio of 1:6. Stir by magnetic force for 1 h to make it mix evenly, and then let it stand at room temperature for 20 h. Transfer to reaction kettles and keep hydrothermal reaction for 10 h at 90℃. Adjust the pH value to 6.5±0.1 after cooling, filter and wash the sample. Finally, dry at 105℃ overnight to obtain zeolite 4A, which is named as F4A.
Grind through a 100-mesh sieve to obtain new adsorbents with different mass fractions of La, which are named as x wt.% La-F4A.

Batch adsorption experiments
According to GB 18918-2002, aqueous solution was prepared with ammonium initial concentration of 10 mg/L and phosphate of 1 mg/L as test water, using ammonium chloride (NH4Cl) and dipotassium phosphate (KH 2 PO 4 ) respectively.
Add 100 mL of test water into a 250 mL erlenmeyer flask, weighing 0.1000 g x wt. % La-F4A and add into test water. Adjust the pH value to 7.0±0.1, oscillate and absorb at 25℃ and 200 rpm for 4 h. Filter with 0.45μm after standing, examine contents of NH 4 + -N and PO 4 in the supernatant respectively. Determine the optimal loading fraction of LaCl 3 according to the test results. Using the same adsorption test method, control test conditions to explore influence of dosage, pH value, coexisting ions on the removal of NH 4 + -N and PO 4 . And fit the adsorption isotherm and kinetic models of La-F4A. After adsorption of NH 4 + -N and PO 4 in the test water for 4 h, La-F4A was desorbed with 1 mol/L NaOH solution for 10 h. After desorption, La-F4A is subjected to a new adsorption experiment, which is a cycle of regeneration.
The concentration of NH 4 + -N in the solution was determined according to HJ 535-2009. The concentration of PO 4 was determined according to GB 11893-89.

Effect of LaCl3 mass fraction on adsorption of ammonium and phosphate
LaCl 3 was used to modify the prepared F4A. In alkaline environment, La(OH) 3 was loaded on the surface of F4A by coprecipitation. Materials with different LaCl 3 fractions (x wt.% La-F4A) are used to adsorb test waters, where x=0.04%, 0.08%, 0.12%, 0.16%, 0.20%, 0.40%, 0.60%, 0.80%. Their adsorption effect on NH 4 + -N and PO 4 is shown in Fig.1 (a). When the mass fraction of LaCl 3 is 0.00-0.08%, the removal efficiency of PO 4 rises sharply to over 99.5%, and the removal efficiency of NH 4 + -N decreases slightly to 80.2%. With the continuing increase of LaCl 3 mass fraction, the removal of PO 4 remained basically stable, while the that of NH 4 + -N decreased obviously. To sum up, the optimal loading fraction of LaCl 3 should be 0.08%, and it should be used as the adsorbent for the following batch adsorption experiments. For convenience, 0.08 wt.% La-F4A would be simplified as La-F4A.

Effect of dosage on adsorption of ammonium and phosphate
The curve of removal efficiency with dosage of La-F4A is shown in Fig.1 (b). When the dosage range of La-F4A is 0.20-1.00g/L, the removal rate of PO 4 rapidly rise to above 99.0%, and that of NH 4 + -N significantly rise to about 77.9%. The removal efficiency of PO 4 and NH 4 + -N can reach 100.0% and 95.0% by increasing the dosage of La-F4A, but the adsorption capacity will be greatly reduced, which weakens the practical performance of the adsorbent. According to the actual emission standards and the economic costs, the dosage of La-F4A is 1.00 g/L.

Effect of pH value on adsorption of ammonium and phosphate
The removal efficiency and adsorption capacity of La-F4A for NH4 + -N and PO 4 are affected by pH value as shown in Fig.1 (c). When the solution pH=3, La-F4A has a maximum adsorption capacity of 0.93 mg/g for PO 4 . The adsorption capacity of La-F4A for PO 4 remains above 0.90 mg/g at the pH of 4-9. When pH > 10, the removal efficiency began to decline faster and faster. The removal effect of La-F4A on NH 4 + -N has a relatively obvious relationship with pH value. Under neutral and weakly acidic conditions, the adsorption capacity is higher, and maximum adsorption capacity of 7.83 mg/g occurs when pH=6. The adsorption capacities of NH 4 + -N in the pH value range of 4-8 are all higher than 7.60 mg/g. When pH>8, the adsorption capacity of NH 4 + -N decreased and very quickly. And the adsorption effect of La-F4A on NH 4 + -N is poor when pH=3.

Adsorption kinetics
The time curve and adsorption kinetics of La-F4A adsorbing NH 4 + -N and PO 4 are shown in Fig.2.
The adsorption of NH 4 + -N reached 96.8% of the maximum after 10 min. For PO 4 , it reached 79.9% of the maximum after 60 min. Their initial adsorption speeds are both fast, and the whole process reaches equilibrium after about 240 min. The equilibrium adsorption capacity of La-F4A for N and P can reach 7.56mg/g and 0.91mg/g.
The fitting parameters of experimental data are shown in Table 1. The data show pseudo-second-order model has the highest R 2 , and the theoretical number is close to the experimental number, which is more congenial to describe the adsorption behavior.

Adsorption isotherms
The adsorption isotherm results of La-F4A on NH 4 + -N and PO 4 were shown in Fig.3. Langmuir and Freundlich isotherm models were used to simulate the data, and model constants and correlationco efficients are listing in Table2.
Temperature has little effect on the removal of NH 4 + -N. The adsorption of PO 4 has a high correlation with temperature. The adsorption of PO 4 on La-F4A conformed to the Langmuir isotherm, indicating its single-layer chemical adsorption process. The Freundlich model fitted the adsorption of NH 4 + -N. The inflection points of the isothermal line indicated that the adsorption of NH 4 + -N was a multi-layer process, and it was deduced that chemical adsorption and physical adsorption existed simultaneously in the adsorption process.

Effect of coexisting ions on adsorption of ammonium and phosphate
The effects of common cations on NH 4 + -N adsorption and common anions on PO 4 adsorption were studied. The concentration of ions was set at 1 mmol/L and 10 mmol/L. The effect of coexisting ions is shown in Fig.4  (a)&(b). The existence of anions has almost no interference on the adsorption of PO 4 , while SO 4 2promotes the removal of PO 4 to some extent, which indicates the high selectivity of lanthanum-loaded adaasorbent for PO 4 . The existence of cations inhibited the adsorption of NH 4 + -N, with the effect of Ca 2+ > K + > Na + > Mg 2+ . These cations exchanged with some sites of La-F4A and formed the competition with NH 4 + -N. The ion concentration in the practical secondary effluent can't meet the test concentration, so La-F4A can be applied to the advanced treatment of ammonia nitrogen.

Desorption and regeneration of La-F4A
In theory, phosphate can be dissolved from the LaPO 4 solid phase under strongly basic conditions (pH>13) 14 . Four cycles of adsorption/desorption experiments were carried out on La-F4A and its reusability is shown in Fig.4 (c).
After four cycles of regeneration, the removal efficiency of PO 4 and NH 4 + -N are 93.8% and 77.0% of the initial adsorption capacity. It shows that most adsorption sites can be regenerated, and adsorption and desorption are mutually reversible processes. La-F4A is a promising adsorbent.

Mechanism of ammonium and phosphate adsorption
Various characterization methods were used to analyse the adsorbents. The N 2 adsorption-desorption experiments were carried out on commercial zeolite 4A (Myriel, China), F4A and La-F4A synthesized in the experiment. The BET surface areas (S BET ) the total pore volume (V tot ) of commercial zeolite 4A were 51.64 m 2 /g and 0.0264 cm 3 /g. The S BET of F4A and La-F4A were 528.12 m 2 /g and 751.80 m 2 /g respectively, while the V tot were 0.5074 cm 3 /g and 0.7028 cm 3 /g respectively. The S BET and V tot of F4A and La-F4A are increased by dozens of times compared with commercial zeolite 4A, which is beneficial for the target pollutants to adhere or enter, indicating the excellent adsorption performance. The S BET and V tot of La-F4A are higher than F4A, proving that the introduction of La can effectively improve the physical and chemical properties of the adsorbents.
The XRD patterns of spent FCC catalyst, F4A and La-F4A were was shown in Fig.5 to study their crystal structures. The metals in catalyst mainly exist in form of metal oxides, and most of them exist in the amorphous phase. The main mineral components are SiO2 and Al 2 O 3 , with a small amount of La 2 O 3 and Ce 2 O 3 . F4A and La-F4A both showed obvious characteristic peaks of zeolite 4A, which indicated that the loading modification process of LaCl 3 basically did not affect the structure. F4A did not show the characteristic peaks of lanthanumrelated compounds, which proves that La had been leached. La-F4A did not show either, which may be due to uneven distribution on the surface, or may exist in the amorphous state. The FT-IR spectra of La-F4A with and without adsorbing NH 4 + -N and PO 4 was shown in Fig.6 (c). The peaks at 3440cm -1 and 1654cm -1 respectively represent the stretching vibration and bending vibration of O-H bond, showing that there are a lot of hydroxyl groups on the surface of La-F4A. 1002cm -1 is the antisymmetric stretching vibration of Si-O-Al. 667cm -1 is the symmetric stretching vibration of T-O (Si or Al). 465cm -1 stands for the bending vibration of Si-O. 560cm -1 corresponds to the double-ring vibration of molecular sieve. All above shows that the material has the structure of A-type zeolite. After adsorption, peak of N-H bending vibration emerged at 1402cm -1 . It shows that NH 4 + -N is mainly transferred to La-F4A by chemical adsorption. There was no new peak of phosphorus-bond after adsorption, which indicated that PO 4 only adheres to the surface of La-F4A, and did not enter the pores.
Energy-dispersive X-ray spectroscopy (EDS) showed the elements of the adsorbents in Fig.7. There was no La in F4A, while the peak of La was obviously observed in La-F4A, which proved that La was successfully loaded on the surface of F4A. After adsorption, a small amount of N and P elements exist in La-F4A. The content of Na in La-F4A before adsorption is equivalent to the total content of Na and N after adsorption, indicating that the mechanism of ammonium adsorption may be the exchange between Na + and NH 4 + . The content of La decreased obviously after adsorption, which might be due to the reaction with PO 4 to generate LaPO 4 precipitate, which fell off the surface of La-F4A. ZO -Na + + NH4 + → ZO -NH4 + + Na + (1) X-La-OH + H2PO4 -→ L-La-H2PO4 -+ OH -(2) X-La-OH2 + + H2PO4 -→ L-La-HPO4 2-+ H2O (3)

Conclusions
To sum up, the conclusions of this study are as follows: (1) In this study, F4A was prepared from spent FCC catalyst by alkali melting-hydrothermal method. La-F4A was prepared by coprecipitation with La(OH) 3 , which showed a high adsorption capacity for ammonium and phosphate.
(2) Adsorption was rapid within 1h and met equilibrium in 4h. The maximum adsorption capacity of La-F4A were 7.64 mg/g for NH 4 + -N and 0.93 mg/g for PO 4 at 25℃. La-F4A had a wide application range (pH=4-9) in the field of nitrogen and phosphorus removal.
(3) The adsorption behavior of NH 4 + -N and PO 4 conformed to the pseudo-second order kinetic model. The adsorption of PO 4 was fitted with Langmuir isotherm model while NH 4 + -N matched Freundlich isotherm model.
(4) Coexisting anions had little effect on the adsorption of PO 4 . Coexisting cations had competitive effects on the adsorption of NH 4 + -N. After four cycles of adsorption/desorption, LA-F4A still retained high adsorption capacity for NH 4 + -N and PO 4 .
(5) There are both chemical adsorption and physical adsorption in the process. The removal mechanism of PO 4 is the ligands exchange with La(OH) 3 to generate LaPO 4 . The removal mechanism of NH 4 + is ion exchange with Na + in La-F4A skeleton.