Preparation Of Layered Double Metal Hydroxide-based Materials And Their Adsorption Properties For Hexavalent Chromium

Human activities and industrial development bring about many energy and environmental problems.Due to its toxicity and ecological persistence,heavy metal pollution has caused human health harm and ecological environment deterioration.Hexavalent chromium（Cr（Ⅵ））is widely exists in a variety of industrial wastewater,which has carcinogenic and mutagenic to human body.It mainly exists in the form of oxygen anions in water environment,and possesses high solubility and mobility.Traditional adsorbents often face many problems in the removal of Cr（Ⅵ）,such as slow adsorption rate,incomplete removal of pollutants and the fixation of Cr（III）,etc.It is still a focus of people to find new adsorbents with excellent Cr（Ⅵ）removal ability.Layered double hydroxide（LDHs）,as a kind of adsorption material with simple synthesis process,which have abundant surface hydroxyl groups and positive charge in aqueous solution,showing great potential for Cr（Ⅵ）removal.However,the affinity of pure LDHs materials for Cr（Ⅵ）pollutants has some limitations.Therefore,according to the characteristics of hexavalent chromium pollutants,how to target the surface modification of LDHs to obtain the target material with excellent Cr（Ⅵ）removal performance has attracted widespread attention.In view of the above mentioned research background,the surface charge density and affinity for Cr（Ⅵ）of LDHs were controlled by surface modification strategy in this paper,in order to improve the adsorption performance of Cr（Ⅵ）.The mechanism of the adsorption reaction and the reason for the improvement of the adsorption performance were investigated by sequential batch experiments and various characterization techniques.The main research contents and conclusions are as follows:（1）The surface charge density of nickel iron layered double hydroxide（NiFe LDH）was adjusted by changing the valence state of iron ions in the precursor solution,in order to improve the adsorption performance of Cr（Ⅵ）.The oxidation topological transformation from Ni²⁺-Fe²⁺LDH to Ni²⁺-Fe³⁺LDH increases the hydroxyl groups and charge density on the surface of NiFe LDH.The introduction of iron divalent in the precursor solution enhanced the surface positivity and reducibility of the target LDHs(Fe²⁺-LDH),which increased the maximum adsorption capacity of Cr（Ⅵ）by nearly 150%.The Fe²⁺-LDH prepared by topological chemical method has strong p H and ionic strength dependence of Cr（Ⅵ）adsorption.The adsorption process is more consistent with the pseudo-second-order kinetic model,which can better explain that the total reaction rate of the adsorption process is controlled by chemisorption.Langmuir isothermal model indicates that the monolayer adsorption occurs in the adsorption process.The main Cr（Ⅵ）removal mechanisms by Fe²⁺-LDH includes electrostatic attraction,interlayer anion exchange and adsorption-coupled reduction.The results suggest that Fe²⁺-LDH adsorbent can achieve rapid removal of Cr（Ⅵ）pollutants and has excellent regeneration performance.（2）Nickel iron layered double hydroxide（NiFe LDH）was prepared by a simple coprecipitate method,and then polypyrrole coated NiFe LDH organic-inorganic composites（PPy/LDH）were prepared by in situ oxidative polymerization.Compared with other LDHs and polypyrrole based materials,the synthesized PPy/LDH still maintains good adsorption performance under slightly acidic conditions,and the maximum adsorption capacity of Cr（Ⅵ）can reach 240.4 mg/g at p H 5.More significantly,PPy/LDH effectively reduced Cr（Ⅵ）concentration from 10 mg/L to 0.028 mg/L,which is lower than the maximum allowable concentration of Cr（Ⅵ）in drinking water（0.05 mg/L）.In addition,the obtained material has good ionic stability.Under the condition of p H 5,nickel ions and iron ions are undetected in the reaction solution after adsorption,and the trivalent chromium is fixed on the material well,which means the reduction product is no longer released into the solution.By analyzing the adsorption behavior and mechanism of PPy/LDH,it was found that adsorption coupled reduction and surface complexation were the main mechanisms of Cr（Ⅵ）removal.In general,PPy/LDH composite material has the advantages of simple synthesis,strong adsorption capacity and effective removal of low concentration Cr（Ⅵ）in water,thus has a great application prospect in Cr（Ⅵ） efficient removal.