Study On Adsorption Of Copper And Lead Ions In Wastewater By Layered Double Hydroxides

Toxic heavy metals copper（Cu（II））and lead（Pb（II））in wastewater are non-degradable and can enter the food chain from aquatic systems,posing a serious threat to the health of humans and other organisms.Adsorption method is widely used in the treatment of heavy metals because of its low cost,simplicity and rapidity.Layered Double Hydroxides（LDHs）are anionic clay materials with unique physical and chemical properties,which make them develop rapidly in the field of adsorption.In order to obtain adsorption materials with excellent performance,LDHs were modified by ion exchangeability between layers.Since the sulfur-containing functional groups have high affinity for most heavy metals and have good effects in water treatment,they are combined with LDHs to improve the adsorption performance.Therefore,three kinds of hydrotalcite composite adsorbents were prepared in this paper,and their adsorption effects on heavy metals Cu（II）and Pb（II）were studied.In this paper,three kinds of hydrotalcite composite adsorbents were prepared.The effects of heavy metal concentration,experimental temperature,adsorption time,coexisting ions and p H value on the adsorption properties of Cu（II）and Pb（II）in water were studied.In the influence experiment of coexisting ions(Na⁺、K⁺、NH₄⁺、Ca²⁺、Mg²⁺),the three adsorbents are stable for the adsorption of Cu（II）and Pb（II）.In the range of p H=3-7,the adsorption capacity of Cu（II）and Pb（II）by the three adsorbents increased.The samples were characterized by XRD,FT-IR,SEM and XPS,and the adsorption mechanism was analyzed by calculating the adsorption binding energy with Materials Studio molecular simulation software.The main results are as follow:（1）Nitrate hydrotalcite（NO₃-LDH）adsorbent was prepared.The adsorption thermodynamics,adsorption kinetics and isothermal adsorption of Cu（II）and Pb（II）by NO₃-LDH showed that the adsorption was an endothermic reaction,which conformed to the pseudo-second-order kinetics and Intra-particle diffusion model,and the adsorption is controlled by the number of adsorbent sites.In line with the Freundlich isotherm model,It shows that the adsorption is heterogeneous.The adsorption mechanism is that Cu（II）or Pb（II）continuously reacts with OH^-in the NO₃-LDH laminate,resulting in the collapse of its structure and the formation of Pb（OH）₂ and Jahn-Teller distorted Cu₅（SO₄）₂（OH）_6.（2）Molybdenum tetrasulfide intercalated hydrotalcite（Mo S₄-LDH）adsorbent was prepared by co-precipitation method.The adsorption experiments of Cu（II）and Pb（II）on Mo S₄-LDH were divided into two parts:single system and mixed system.The adsorption of a single system is an endothermic reaction;it conforms to the quasi-second-order kinetics and internal diffusion model and Freundlich isothermal model.Compared with NO₃-LDH,the adsorption amount of Mo S₄-LDH for Cu（II）increased from 6.48 mmol/g to10.46 mmol/g,and the adsorption capacity of Pb（II）increased from 3.44 mmol/g to 5.71mmol/g.The kinetic adsorption of Pb（II）in the mixed system reached the adsorption equilibrium faster,and the adsorption amount of Pb（II）in the isothermal adsorption was dominant.The adsorption binding energy calculated by Materials Studio molecular simulation software reveals that Mo S₄-LDH has a preferential adsorption effect on Pb（II）in the mixed system.The adsorption mechanism of the single system and the mixed system is the same.The adsorption of Cu（II）or Pb（II）by the sulfur group in the Mo S₄^2-interlayer of Mo S₄-LDH,increases the interlayer adsorption amount,and then combines with the hydroxyl group to form Pb（OH）₂ and Cu₄SO₄（OH）₆.（3）Molybdenum polysulfide intercalated hydrotalcite(Mo₃S₁₃-LDH)adsorbent was prepared by Ion exchange method.The adsorption experiments of Cu（II）and Pb（II）by Mo₃S₁₃-LDH were divided into two parts:single system and mixed system.The adsorption of a single system is an endothermic reaction;it conforms to the quasi-second-order kinetics and internal diffusion model and Freundlich isothermal model.Compared with Mo S₄-LDH,the adsorption amount of Mo₃S₁₃-LDH for Cu（II）increased from 10.46 mmol/g to 11.16 mmol/g,and the adsorption capacity of Pb（II）increased from5.71 mmol/g to 6.72 mmol/g.The kinetic adsorption of Pb（II）in the mixed system reached the adsorption equilibrium faster,and the adsorption amount of Pb（II）in the isothermal adsorption was dominant.The adsorption binding energy calculated by Materials Studio molecular simulation software reveals that Mo₃S₁₃-LDH has a preferential adsorption effect on Pb（II）in the mixed system.Pb₄（OH）NO₃ and Cu₃（SO₄）₂（OH）₂ precipitates were formed after adsorption in both single and mixed systems,and the formation of Pb Mo O₄ in the mixed system further increased the adsorption capacity.The adsorption mechanism is to use various types of sulfur ligands in Mo₃S₁₃-LDH interlayer Mo₃S₁₃^2-to fix Cu（II）or Pb（II）.When the LDHs layer collapses,the interlayer Mo₃S₁₃^2-also precipitates with Pb（II）to increase its adsorption amount.Therefore,the adsorption effect of Mo₃S₁₃-LDH on Cu（II）or Pb（II）is more obvious.