Preparation Of Modified Bentonite-based Adsorbents And Study On Its Adsorption Performance For Wastewater Containing Pb²⁺

With the rapid development of industries such as batteries,electrical and steel,large amounts of wastewater containing excess Pb²⁺have been generated.The Pb²⁺-containing wastewater can do harm to the human kidneys,nervous system.Traditional technologies for the treatment of Pb²⁺-containing wastewater include membrane separation,coagulation/flocculation,chemical precipitation,ion exchange,electrochemical treatment,and adsorption.Among them,adsorption is regarded as an effective method of eliminating wastewater that contains heavy metals because of high efficiency and ease of operation.Adsorbents often used in industry include activated carbon,zeolites,clays and resins.Bentonite is widely used in the field of wastewater treatment due to its advantages of low cost,wide distribution and large specific surface area.However,bentonite is facing serious challenges due to their limited adsorption capacity and contains impurities such as cristobalite and clinoptilolite.Therefore,bentonite needs to be purified and modified to obtain adsorbents with good Pb²⁺adsorption capacity.Based on the above objectives,this study will be based on low-quality calcium-based bentonite as raw material to modify from the perspectives of interlayer,surface and structure of bentonite.The modified bentonite will be analysed in depth from several aspects such as morphology,structure,properties and adsorption mechanism.The main conclusions obtained are as follows:（1）Low-quality natural bentonites are purified,modified by sodium modification and they are used for the adsorption of Pb²⁺in the aqueous phase.The effects of initial pH of solution,adsorption time,initial Pb²⁺concentration,adsorbent dosage and type on the performance of sodium bentonite for Pb²⁺adsorption were systematically investigated.Under optimal adsorption circumstances,the maximum Pb²⁺adsorption capacity of sodium bentonite up to 165.34 mg/g.The Freundlich model and the pseudo-second-order dynamic model were used to describe the adsorption process.The adsorption process is feasible as a heat absorption process.The adsorption pathways of sodium bentonite for Pb²⁺mainly include ion exchange,surface hydroxyl functional group trapping,electrostatic attraction and chemical precipitation of CO₃^2-in the pore channel to form Pb CO₃.After four regeneration cycles,the sodium bentonite still had good Pb²⁺adsorption performance.This study provides technical support for the promotion of bentonite as an adsorbent material for a wide range of applications.（2）ZIF-8/bentonite composites were synthesized by in situ growth of ZIF-8 on the bentonite surface through surface modified bentonite for efficient adsorption of Pb²⁺from wastewater.ZIF-8 has been successfully grown on the bentonite surface as demonstrated by XRD,TEM,SEM and EDS characterization.The ZIF-8/bentonite adsorption of Pb²⁺was in accordance with the Langmuir model and pseudo-second-order dynamic model.Under the optimal adsorption conditions,the maximum adsorption of ZIF-8/bentonite can reach 336.89 mg/g,which was much higher than that of ZIF-8（117.78 mg/g）and bentonite（85.20 mg/g）.This was attributed to the synergistic effect generated by the combination of ZIF-8 and bentonite.The adsorption mechanism showed that the main pathways of Pb²⁺removal by the ZIF-8/bentonite composite were attributed to ion exchange,electrostatic attraction and hydroxyl complexation.In addition,ZIF-8/bentonite still had good Pb²⁺adsorption performance after three regeneration cycles.Therefore,the synthesized ZIF-8/bentonite composites are a promising candidate for the removal of Pb²⁺from wastewater.（3）Na-P zeolite is synthesised by hydrothermal alkai method by using low grade bentonite and a small aluminium source as raw materials.After hydrothermal treatment,bentonites containing cristobalite and clinoptilolite impurities have been completely converted to Na-P zeolites at a Na OH concentration of 2 mol/L,a hydrothermal time of 6 h and a hydrothermal temperature of 150°C,and used for the removal of Pb²⁺from wastewater.The maximum Pb²⁺adsorption capacity by Na-P zeolite was 371.07 mg/g under the optimal adsorption conditions.The adsorption process follows the Langmuir model and the pseudo-second-order-dynamic model.The thermodynamic parameters indicate a heat absorption reaction and spontaneity.The adsorption mechanism showed that the adsorption of Pb²⁺by Na-P zeolite was mainly through ion exchange,surface hydroxyl complexation and electrostatic attraction.Na-P zeolite has good reusability and stability as an efficient adsorbent.This study suggests that Na-P zeolite is a promising candidate for the adsorption of Pb²⁺in industrial wastewater.