Preparation Of Metakaolin Based Geopolymers And Their Adsorption Properties For Metal Ions

With the rapid development of China's industrialization,environmental issues are increasingly becoming one of the key issues about human survival and development,especially heavy metal pollution.Batteries,organic fertilizers,pesticides,heavy metals is widely used in many industries,which brought serious pollution.Wastewater containing heavy metals which are usually directly discharged into the water environment.Due to their inorganic structure,they are not biochemical-degradable in the environment,which is enriched and transmitted through the food chain,endangers the life and health of animals and humans in the ecological environment.The heavy metal wastewater treatment technology mainly includes precipitation method,ion exchange method and adsorption method.Adsorption is widely studied due to its low cost,wide adsorption materials and simple operation.At present,most of the adsorbents are made of carbon materials and molecular sieves as raw materials.High energy consumption and easy to cause secondary pollution.In this paper,metakaolin was used as a synthetic raw material,and a certain proportion of sodium hydroxide,sodium silicate and sodium meta-aluminate were added as activators to synthesize the polymer material by hydrothermal method.The effects of different activator concentrations,ratios and solid-liquid ratio on the adsorption properties of the polymer were investigated,and the optimal conditions were selected.The synthesized geopolymers were used to conduct copper ions at different pH,adsorption dose and reaction time.And adsorption experiments of lead ions;finally,the adsorption isotherms,kinetics and adsorption thermodynamics of the synthesized polymers were studied,and the adsorption mechanism was preliminarily elaborated.The conclusions obtained are as follows:?1?Using metakaolin as raw material,sodium hydroxide,sodium silicate and sodium meta-aluminate as activators for the preparation of geopolymer.The optimal concentration is 0.8 mol/L,0.9 mol/L and 0.6 mol/L;The ratio is also optimized,and the best ratio of activator is n?NaOH?:n?Na₂SiO₃?:n?NaAlO₂?=72:200:90.Among the activators,sodium hydroxide and sodium silicate have a great influence on the adsorption amount of Pb???,while the effect of sodium meta-aluminate is relatively small;?2?Geopolymer was activated using a mixture of three activators under the reaction conditions of an initiator and metakaolin at 180?for 24 hours at 30?,and then heating for 1 hour at a temperature of 60?.Where n?NaOH?:n?Na₂SiO₃?:n?NaAlO₂?equals 72:200:90,the adsorption property of the geopolymer formed after activation is about 50 times higher than that of the original metakaolin,when the solid-liquid ratio is 0.03 g/mL,the adsorption amount of Pb???can reach 228.5 mg/g.?3?The static adsorption test of two metal ions?Cu???and Pb????by geopolymers was investigated.The static adsorption results showed that the removal rate of copper ions reached 94.6%when the dosage of the adsorbent was 1.4 g/L,and the removal rate of lead ions reached 96.6%.The excessive amount of adsorbent will lead to the complete adsorption of metal ions,resulting in the excessive part of the adsorption sites being idle,thus reducing the adsorption capacity of geopolymer per unit mass.When the pH value of the solution is low,H⁺in the solution has competitive adsorption with lead or copper ions.With the increase of pH,the adsorption capacity of geopolymer for two metal ions increased gradually.In the experimental range,the maximum adsorption capacity for Cu???and Pb???was 1.17mmol/g and 1.26 mmol/g respectively.?4?The results of adsorption kinetics show that the adsorption process of geopolymer for Cu???and Pb???ions follows a pseudo-second-order kinetic equation.The equilibrium adsorption capacity fitted by the quasi-second-order kinetic model is closer to the experimental results,in which the actual adsorption capacity of Cu???is 75.61 mg/g and that of lead ion is 262.20 mg/g.The experiment also shows that the adsorption process of geopolymer for Cu???and Pb???is mainly chemical adsorption.The pseudo-second-order kinetic rate constant K₂ of geopolymer adsorbing Pb???is higher than that of Cu???,which indicates that the adsorption rate of geopolymer adsorbing Pb???is faster.Thermodynamic results show that the adsorption of Cu???and Pb???is more in line with Langmuir model,and the maximum adsorption capacity is 178.6 mg/g and 312.5 mg/g,respectively.The adsorption of Cu???and Pb???by geopolymers is monolayer.The results of XRD,FT-IR,SEM and BET analysis showed that metakaolin dissolved in alkaline conditions and formed a three-dimensional network gel structure with a smooth and orderly surface.The specific surface area of geopolymer is nearly 10 times larger than that of metakaolin,reaching 32.05m²/g,and the pore volume Vt reached 0.074,which indicated that the geopolymer is a mesoporous material.Cu???or Pb???as Physical and chemical adsorption are existed in the adsorption of Cu???or Pb???on geopolymer.In the process of adsorption,Cu???or Pb???exchanges with Na⁺in tetrahedral structure of geopolymer,and then the Na⁺are released into the solution.