Study On Preparation Of Layered Double Hydroxides Intercalated By Organic Acid Anion And Its Adsorption Of Boron

Magnesium resources of Salt Lake in China are abundant,but their utilization rate have been low.Conducting research on the application development of new magnesium-based functional materials can not only consolidate the theoretical and experimental basis of magnesium-based materials science but also provide a new driving force for the comprehensive utilization of magnesium resources in Qinghai Salt Lake.As a multifunctional material with simple synthesis and diverse properties,magnesium-based layered double hydroxides(LDHs)have interlayer anion exchangeability.The conventional inorganic acidic magnesium-based hydroxides have low boron adsorption capacity,and poor selectivity,and the adsorbed hydroxides cannot be eluted and regenerated.By inserting organic acidic roots with multi-hydroxyl structure into the interlayer of hydroxides,boron adsorbents with high selectivity and theoretical elution and regeneration can be obtained.In this paper,gallic acid(GA)and tartaric acid(TA)as anion sources,and magnesium and aluminum as cation sources were used to prepare gallic acid intercalated layered double hydroxides(GA-LDH)and tartaric acid intercalated layered double hydroxides(TA-LDH)for the adsorption of boron in solution by direct precipitation method.The X-ray diffractometer(XRD),Fourier transforms infrared spectrometer(FT-IR),elemental analyzer,projection electron microscope(TEM),thermogravimetric analyzer(TG/DTA),and contact angle measurement instruments were used to characterize the LDHs.The LDHs synthesized under different conditions were investigated by XRD and adsorption capacity of boron,their adsorption process was optimized,the adsorption kinetics and thermodynamics were investigated,the effect of coexisting ions on the adsorption capacity was explored,the adsorption mechanism was investigated and elution experiments were designed.The main results were as follows.(1)GA-LDH and TA-LDH were prepared by direct precipitation method,and there are characterized by XRD and FT-IR that GA and TA had successfully entered the interlayer of hydrotalcite,and the intercalation content was calculated semiquantitatively by the results of elemental analysis.The optimal reaction conditions were determined by single-factor experiments using XRD and adsorption capacity as evaluation indexes,respectively.For GA-LDH: aging temperature was 20 ℃,the Mg:Al molar ratio was 2:1,and the Al: GA molar ratio was 1:2.For TA-LDH: aging temperature was 60 ℃,the Mg: Al molar ratio was 2:1,the Al: GA molar ratio was 1:2,and the initial p H was 11.The adsorption process conditions for LDHs were optimized:the initial p H of the solution was 8-9,and the reaction time was 3 h.The adsorption capacity of GA-LDH on this basis could reach 10.8 mg·g-1 and that of TA-LDH was 9mg·g-1.(2)The adsorption kinetics and thermodynamics of GA-LDH and TA-LDH were investigated.The adsorption kinetics showed that the reaction rates of both adsorbed B were mainly controlled by the chemisorption mechanism.The thermodynamics revealed that the relationship between the equilibrium concentration of B in solution and the adsorption capacity of TA-LDH on B was more consistent with the Freundlich model,while the relationship between the equilibrium concentration of B in solution and the adsorption capacity of GA-LDH on B was more consistent with the Langmuir model.(3)The interference of coexisting ions was investigated.Compared with NO3-LDH,the coexistence of Cl-and Li+ in solution had little effect on the adsorption of B by GA-LDH and TA-LDH,while the presence of SO42-had a greater effect on their adsorption capacities.(4)The adsorption mechanisms of NO3-LDH,GA-LDH,and TA-LDH on boron were investigated by FT-IR and elemental analysis.The results showed that the adsorption mechanism of NO3-LDH on boron was that NO3-LDH adsorbed boron by ion exchange with B(OH)4-in solution through the interlayer anion,while GA-LDH and TA-LDH adsorbed boron by the interaction of the o-hydroxyl group of the interlayer GA anion and TA anion with the hydroxyl group between B(OH)3 and B(OH)4-.(5)The elution experiments of GA-LDH and TA-LDH were investigated based on their adsorption mechanisms.The elution rates of both GA-LDH and TA-LDH were greater than 70% when ammonium chloride was used as the eluent,but the eluted GALDH lost its adsorption capacity for boron.While TA-LDH still maintained a good adsorption capacity after 5 cycles,and the single adsorption solubility loss rate of Mg2+was 1.5% and the single elution solubility loss rate was 5.4%.Overall,the optimization of the synthesis conditions of two organic acids intercalated hydrotalcite and the study of the adsorption conditions,adsorption mechanism,and elution performance of boron in solution not only lay the theoretical foundation for the synthesis of LDHs boron adsorbents and the efficient separation and extraction of boron in solution but also provide new ideas and methods to broaden the application scope of magnesium-based materials.