Studies On The Adsorption Behavior Of Main Heavy Metal Ions In Environmental Wastewater By Modified Polyvinyl Alcohol

At present,the problem of heavy metal pollution seriously affects the ecological balance and human health.In addition,with flowing characteristics,if malignant heavy metal pollution accidents can not be treated timely,that will threaten the downstream in the direction of flow and then eventually trigger a regional water supply crisis.There are many methods to deal with heavy metal polluted water;including adsorption method as a kind of traditional classic methods.Activated carbon and iron oxide are classical adsorption materials,with raw material sources,low cost,easy to use,but small adsorption capacities,not forming material,not favor in fluidization process,etc...Based on the accidental heavy metal water pollution,mainly on cadmium,lead,nickel,we prepared a modified polyvinyl alcohol;studied the thermodynamics and kinetics of its adsorption on cadmium,lead,nickel;evaluated its coordination in materials and environment,with the balance of functionality,environment,and pursuit of economy;investigated its applicability in removing the heavy metals in the flow of water simulation experiment.The main conclusions of this paper are as the follows.?1?A modified polyvinyl alcohol was synthesized after cross-linking with maleic anhydride polymer and dehydrated and carbided with concentrated sulfuric acid.By optimizing esterification conditions,the mass ratio of polyvinyl alcohol and maleic anhydride was 1:2.the reaction temperature was 90?,reaction time was 3 h,the grafting ratio of maleic anhydride on polyvinyl alcohol was 25.89%.Modification results show that the BET surface area of modified polyvinyl alcohol is 5 times of that before modified,and the numbers of pore are increased.The results of FTIR spectrum analysis found that the group structure of modified PVA surface had been changed and successfully introduced a large number of carboxyl groups.And the mechanism of adsorption heavy metals was mainly ion exchange,coordination and electrostatic force on weakly acidic function groups with metal ion.?2?Batch static adsorption results show that the adsorption capacities of modified PVA are Cd 1.16 mmol/L,Pb 1.39 mmol/L and Ni 1.30 mmol/L,respectively.The batch equilibrium data fit well into the Langmuir and Dubinin-Radushkevich isotherms model,which can better describe the modified polyvinyl alcohol on the adsorption behavior of heavy metal ions.The best fitting model is Langmuir adsorption model.The calculated adsorption capacities are well as to the experimental adsorption capacities,which are Cd 1.19 mmol/L,Pb 1.48 mmol/L and Ni 1.35mmol/L,respectively.The value of E calculated by Dubinin-Radushkevich model is between 17.96 k J/mol and 28.87 k J/mol,means that the series of adsorption process is not only physical adsorption,but also chemical adsorption.The thermodynamic parameters showed that the adsorption of Cd and Pb onto modified PVA are spontaneous and endothermic process,whereas for Ni is an endothermic reaction,and the rising of temperature is advantageous to the adsorption of Ni.Kinetic data were best fitted with the pseudo-second-order kinetic model.The adsorption process was controlled by the intra-particle diffusion and membrane diffusion process.?3?Continuous fixed-bed column studies showed that the exhaustion time decreased with the decrease of bed depth,increase of flow rate and influent concentration.Thomas,Yoon-Nelson and BDST models were good applied to the adsorption of Cd²⁺,the fitting degree were more than 0.9.Under the experimental condition(C_0,Cd=1.08 mg/L,V=50 m L/min,Z=7 cm),when the adsorption time was no more than 118 h,total of 0.35 tons of wastewater,the concentration of Cd²⁺in the outlet solution was less than the detection limit of ICP-MS(C_Cd<0.002 ug/L),the removal rate of Cd²⁺in wastewater was close to 100%;when the adsorption time was no more than 227 h,total of 0.68 tons of wastewater,the concentration of Cd²⁺in the outlet solution was less than 5 ug/L,the concentration of Cd²⁺in the water reaches the national drinking water standard.These showed that the modified polyvinyl alcohol had great application prospect in the development of low concentration cadmium contaminated water.The recycle of modified polyvinyl alcohol has a good performance in the adsorption of heavy metals.After 38 consecutive adsorption-desorption experiments for 200 mg/L cadmium test aqueous solution,the removal rate of modified polyvinyl alcohol onto Cd²⁺remained stable,more than 97%,and the water extract of modified polyvinyl alcohol has no significant toxicity to the cell with no more than 8%concentration;the acute toxicity reaction of mice showed the water extract of modified polyvinyl alcohol toxicity of the samples for class 1,the actual non-toxic role.?4?The study on the emergency simulation treatment of heavy metal wastewater by modified polyvinyl alcohol was carried out.In the dynamic simulation experiment of acyclic system,considering the factors of water self-purification,the removal rate of cadmium ion in“mobile corps”?5 L,concentration of 1.15 mg/L cadmium pollution simulated wastewater?by modified polyvinyl alcohol was 40.9%;in the dynamic simulation experiment of circulation system,after the sixed time processing the modified PVA,the concentration of Cd²⁺in pollution simulated wastewater,total of 80 L,reduced from 1.20 mg/L to 2.7 ug/L,below to the Standards for Drinking Water Quality?GB/T 5749-2006?.Results showed that the modified polyvinyl alcohol had a great application prospect in the purification of heavy metals pollution water on the emergency treatment.The modified polyvinyl alcohol block emergency treatment process can solve the problem of flocculation precipitation secondary pollution,and the problem of the suspended adsorption tank on the water resistance,low water flux,inducing difficult to widely use in emergency.The industrial wastewater of an automobile fittings factory in Guangzhou development zone was used as the research object.Under the static adsorption conditions,the concentration of zinc decreased from 28.8 mg/L to 0.32 mg/L,chromium decreased from 1.1 mg/L to 0.01 mg/L.The experimental results showed that the modified polyvinyl alcohol had a good effect on the industrial wastewater,and the content of the metal ions after the modified PVA treatment were lower than that of the current discharge water.Under the studies of continuous fixed-bed column of the industrial wastewater?V=50 m L/min,Z=7 cm?,we found that the modified polyvinyl alcohol caculated saturated adsorption capacity of Zn²⁺was 53.6 mg/g,and 2.10 cm.This series of data for the modified polyvinyl alcohol in the practical application of industrial wastewater has a good guiding significance;in the dynamic simulation experiment of circulation system,after the fourth time processing the modified PVA,the concentration of Zn²⁺reduced from 28.8 mg/L to 1.18 mg/L in 80L industrial wastewater,below the Emission Standard of Pollutants for Electroplating?GB/T 21900-2008?.Experimental data showed that in the pollution wastewater of high concentration of heavy metals,high concentration of salt,with organic pollution,the modified polyvinyl alcohol block treatment process still have good adsorption and strong selectivity to heavy metal ions.Modified polyvinyl alcohol in the control of heavy metal pollution of urban river water has a strong feasibility.