Study On The Phophate Removal Performance Of Layered Double Hydroxides And The Modified Composite With Amino Acid Intercalation

Phosphorus is an essential element for all life forms,and it is one of the main causes of eutrophication.According to the practical operation of wastewater treatment plants,the traditional biological treatment method cannot maintain a high operation efficiency and guarantee a low phosphate concentration in the effluent,and the traditional chemical precipitation method often encounters with problems like excess dose of chemicals and chemical sludge disposal.In terms of phosphate pollution control,adsorption method can efficiently separate phosphate from wastewater,which makes it promising in application in advanced treatment and guarantee a low P concentration,as well as in the preparatory process of phosphate recovery.However,traditional adsorbents usually have low adsorption capacities,which makes them practical application.Compared to other phosphate adsorbents,as a class of laminar nanomaterials with variable chemical composition,layered double hydroxides?LDHs?have advantages in structural stability and adsorption capacity.At the same time,there still remains problems for the study of LDHs,such as the optimization of basic chemical composition of LDHs as phosphorus removal materials,the further improvement of the adsorption rate and adsorption selectivity of LDHs for phosphorus.In view of the above problems,the chemical composition of LDHs as phosphorus adsorbents was designed as a template based on the analysis of chemical composition characteristics.14 LDHs with different chemical compositions were prepared through the co-precipitation method and their phosphate removal performance were compared.Then,amino acids were intercalated into the LDHs layer to improve the adsorption rate and selectivity due to the fact that the interlayer spacing of LDHs is adjustable,which is one of the characteristic properties of LDHs.The main conclusions of this paper are as follows:?1?Characteristic structures and compositions of LDHs were confirmed by characterizations including scanning electron microscopy equipped with X-ray energy spectrum,X-ray diffraction,Fourier transform infrared spectroscopy,thermogravimetry-differential thermal thermal analysis,Zeta potential analysis,and specific surface area and pore structure analysis.The specific surface area of LDHs composed of different elements ranged from 10 to 50 m²·g^–1.The results of screening tests based on LDHs with different divalent cations,trivalent cations,interlayer anions,and different cation ratios showed that LDHs with trivalent cations of Al³⁺have better removal efficiency than LDHs with trivalent cations of Fe³⁺,and the best n(Mg²⁺/Al³⁺molar ratio)value is 2.The screened out LDHs is Mg₂-Al₁-Cl-LDHs with the divalent metal ion of Mg²⁺,the trivalent metal ion of Fe³⁺,the molar ratio of the divalent and trivalent metal ions of 2,and the interlayer anion of Cl^–.?2?The selected Mg₂-Al₁-Cl-LDHs showed the best adsorption performance when the initial pH was 4-6.For the 20 mg·L^–1 phosphate solution,the optimum dosage of the adsorbent is 0.5 g·L^–1;The order of the influence of four common coexisting anions on the removal of phosphate by Mg₂-Al₁-Cl-LDHs was SO₄^2–>CO₃^2–>Cl^–>NO₃^–;As temperature rose,the adsorption performance decreased.Phosphate adsorption by Mg₂-Al₁-Cl-LDHs was in accordance with the pseudo-second-order kinetics model.At a contact time of 1 min,the phosphorus removal rate was about 50%.At 40 min,the adsorption reached equilibrium.The phosphate adsorption isotherm of Mg₂-Al₁-Cl-LDHs accorded with the Temkin and Langmuir adsorption models.The adsorption capacity was69.4 mg·g^–1.The adsorption reaction was spontaneously exothermic in nature.Based on the material characterization before and after adsorption and the calculation of ion exchange capacity,the mechanism of phosphate adsorption by Mg₂-Al₁-Cl-LDHs was determined.The mechanism included physical adsorption,ion exchange,and coordination exchange,in which ion exchange was the main route.?3?Glycine and alanine intercalated LDHs were synthesized on the basis of Mg₂-Al₁-Cl-LDHs.The morphology and structure of Mg₂-Al₁-Gly-LDHs and Mg₂-Al₁-Ala-LDHs were almost the same as LDHs before intercalation.However,due to the intercalation of amino acids,the interlayer spacing increased and the zero charge point increased.The results of the study on the phosphorus adsorption performance of Mg₂-Al₁-Gly-LDHs and Mg₂-Al₁-Ala-LDHs showed that the optimum pH range for the adsorption of phosphate by amino acid intercalated LDHs adsorbents was 3-6,which is wider than LDHs before intercalation.For 20 mg·L^–1 phosphate solution,the optimum dosage of the two adsorbents was 0.5 g·L^–1,and the remaining phosphate concentration after adsorption was lower than the detection limit of 0.01 mg·L^–1 by the phosphorus molybdenum blue spectrophotometric method,which is lower than that of LDHs before intercalation.The order of the influence of the four common anions on the phosphate adsorption was SO₄^2–>CO₃^2–>Cl^–>NO₃^–.After the amino acid intercalation,the selectivity against SO₄^2–was higher than that before intercalation.Also,the higher the temperature,the worse the adsorption performance.?4?The speed of adsorption of phosphate was significantly increased after amino acid intercalation.At 1 min,the removal rates of phosphorus by Mg₂-Al₁-Gly-LDHs and Mg₂-Al₁-Ala-LDHs reached 57.99%and 81.18%,respectively.The kinetics of phosphate removal by Mg₂-Al₁-Gly-LDHs and Mg₂-Al₁-Ala-LDHs fitted the second-order kinetic model best.The adsorption isotherm model was best described by the Temkin and Langmuir models.The k₂ values of the kinetic parameter were significantly larger than before the intercalation,indicating a significant increase in the adsorption rate.The adsorption capacities of the Mg₂-Al₁-Gly-LDHs and Mg₂-Al₁-Ala-LDHs fitted by the Langmuir model were 55.82 mg·g^–1 and 58.17 mg·g^–1,respectively.?5?Based on the material characterization before and after adsorption and the calculation of ion exchange capacity,the main mechanism of phosphorus adsorption by amino acid intercalated LDHs composites were physisorption,ion exchange,electrostatic adsorption and coordination exchange,in which ion exchange was the main route.The intercalation of amino acid increased the interlayer spacing of LDHs,which improved the contact area of adsorbent and the adsorbate.As the result,the speed of adsorption increased.At the same time,when the pH was<6.0,the amino acid had a positive charge,which facilitated the electrostatic adsorption effect for phosphate.