Simultaneous Removal Of Ammonia-nitrogen And Phosphate By Lanthanum Hydroxide Loaded On Y-type Molecular Sieve Adsorbent:Performance And Mechanism

The input of ammonia-nitrogen(NH4+-N)and phosphate(PO4)can cause overproduction of organic matter in the water,resulting in eutrophication of water.Studies have shown that lanthanum-based materials have an efficient and selective adsorption effect on phosphate in water.Therefore,the 4A-type molecular sieve supported lanthanum hydroxide type adsorbent(LHMSA)and the Y-type molecular sieve supported lanthanum hydroxide type adsorbent(LHMSY)prepared by coprecipitation method were applied to remove low concentration NH4+-N in simulated wastewater.Optimum adsorbent was selected through its investigation on the adsorption performance of NH4+-N in water,and then it was used in the simultaneous removal of NH4+-N and PO4,and the possible adsorption mechanism was proposed through the study of its adsorption behavior.The experimental results showed that:The equilibration time of LHMSA and LHMSY were 120 min and 30 min,respectively.Compared with LHMSA,LHMSY had significantly better resistance to pH change shock and ion interference.Under the optimum dosage and adsorption equilibrium time,the pH value was around 2.5 and the reaction temperature was 25℃,the best removal efficiency of LHMSA and LHMSY for NH4+-N were 70.13%and 76.00%,respectively.After 4 times of desorption,the adsorption capacity of LHMSA was only 11.4%of the initial adsorption capacity;while the adsorption capacity of LHMSY still reached 85.7%of the initial adsorption capacity,and LHMSY had better adsorption regeneration capacity.Therefore,LHMSY was more suitable for simultaneous NH4+-N and PO4 removal than LHMSA.The LHMSY was applied to the simultaneous NH4+-N and PO4 removal experiments,and it was found that the adsorption equilibrium time was 30 min,and the adsorption process was more in line with the pseudo-second-order kinetic model.The adsorption isotherm data of NH4+-N and PO4 on LHMSY fit the Freundlich model and Langmuir model,respectively.Under acidic conditions,the adsorption effect of LHMSY on NH4+-N was better,and the adsorption capacity of LHMSY on PO4 was hardly affected by the change of pH value.The maximum adsorption capacity for NH4+-N and PO4 reached 1.38 mg/g and 0.49 mg/g when pH=3.0 and temperature=25℃,respectively.Coexisting ions(K+,Ca2+,Na+,HCO3-,SO42-and NO3-)had little effect on PO4 removal,however,the presence of K+,Ca2+ and Na+ in the coexisting system would produce adsorption competition,resulting in a significant decrease in the adsorption capacity of LHMSY for NH4+-N.After 4 desorption cycles,the adsorption capacity of LHMSY for NH4+-N still reached 85.0%of the initial adsorption capacity,and the adsorption capacity for PO4 was comparable to the initial adsorption capacity.The results of adsorption performance experiments and characterization showed that the ion exchange reaction was the main reason for the adsorption of NH4+-N by LHMSY.In addition,the LaPO4 generated by the ligand exchange reaction was the main reason for the selective adsorption of La(OH)3 by LHMSY.In order to improve the separation ability of the adsorbent LHMSY in the water,polyvinyl alcohol(PVA)was used as the cross-linking agent,and the spherical adsorbent PVA@LHMSY was prepared by controlling the experimental conditions such as the concentration of the cross-linking agent,the cross-linking time,and the amount of LHMSY embedded.And the optimal preparation conditions were selected by its NH4+-N and PO4 removal behavior.The experimental results showed that the mass fraction of polyvinyl alcohol solution was 10wt.%,the mass fraction of boric acid solution was 5wt.%,the cross-linking time was 30min,and the optimal embedding amount of adsorbent was 3.0gLHMSY/20mLPVA(10wt.%),the spherical adsorbent PVA@LHMSY could be obtained.The NH4+-N and PO4 removal behavior of PVA@LHMSY conformed to the pseudo-second-order kinetic model and the Langmuir model.The pH of the solution had little effect on the simultaneous NH4+-N and PO4 removal process.The coexisting ions(K+,Ca2+,Na+,Cl-,SO42-and HCO3-)had almost no effect on the adsorption process of PO4,and the presence of K+,Ca2+ and SO42-would obviously reduce the removal efficiency of NH4+-N.