Preparation Of Lignite-based Adsorbent And Study On Adsorption Of Cd²⁺, Zn²⁺ In Water And Removal Of Leaching Humic Acid

With the continuous acceleration of the urbanization process and the rapid development of industry,the pollution of heavy metal ions such as Cd²⁺and Zn²⁺in the water environment is becoming more and more serious.The adsorption method is widely used in the treatment of colored,odorous,organic and inorganic wastewater due to its many advantages such as low cost and convenient subsequent treatment.The core of this method is to select a suitable and efficient adsorbent to remove pollutant ions as far as possible.Lignite,as a kind of mineral with huge reserves,rich oxygen-containing functional groups and humic acid macromolecular substances and other advantages,has been widely used in environmental protection and other fields in recent years,but in the process of utilization,there are also disadvantages such as low mechanical strength,high impurity content,prone to shrinkage or expansion effect.Therefore,in this paper,a brown coal-based adsorbent with a variety of excellent properties was successfully prepared by optimizing the nitric acid modification conditions using brown coal from a coal mine in Mile,Yunnan,the performance characterization and adsorption capacity test were performed,the dissolution of humic acid in the adsorption process was also concerned.After the adsorption process,free humic acid,heavy metal ions,and humic acid-heavy metal complex substances are removed simultaneously by adding an aluminum salt flocculant,and finally the discharge safety of the aqueous solution is guaranteed.The main research content of this paper is the following four parts:Preparation of nitric acid modified lignite based adsorbent:Taking the lignite from a coal mine in Mile,Yunnan as the raw material,the reaction time,nitric acid concentration and acid coal ratio are roughly determined by single factor method,aiming at the humic acid content of lignite and the removal effect of Cd²⁺and Zn²⁺;Then,response surface method was used to optimize the modification conditions,and a preparation condition was obtained,which had a better removal effect on the solution containing Cd²⁺and Zn²⁺pollutants,and effectively improved the humic acid content of lignite.The nitrated lignite used in the subsequent experiments was prepared under this condition.Characterization of adsorbent and adsorption batch test:BET,SEM-EDS,FTIR,XRD,zeta potential and other characterization methods were used to analyze the original lignite and nitric acid modified lignite.The results showed that the porosity of lignite based adsorbent increased after the modification with nitric acid of appropriate concentration;more adsorption sites were exposed on the surface of the material,and Cd²⁺and Zn²⁺were successfully loaded on the lignite based adsorbent;the microcrystalline structure of lignite was not destroyed;a large number of oxygen-containing functional groups were introduced on the surface,resulting in the negative zeta potential in the studied p H range,improving the stability of the reaction system.The adsorption performance of Cd²⁺and Zn²⁺on lignite and nitric acid modified lignite and the dissolution of humic acid organic matter under various single factors were systematically studied.For Cd²⁺and Zn²⁺,the removal efficiencies of both pristine lignite and nitric acid modified lignite increased with the increase of p H value,increased with the decrease of particle size of adsorbent,increased firstly and then balanced with the increase of adsorbent dosage,and the fitting results of kinetic experimental data were more in line with the Pseudo-second-order kinetic model,indicating that the adsorption behavior was mainly chemical adsorption.According to the fitting results of isotherm experiment data,the Langmuir isotherm model shows that the adsorption is mainly a single layer adsorption on the surface.The separation factor R_L also shows that the adsorption of Cd²⁺and Zn²⁺by the two adsorbents is easy.The thermodynamic calculation parameters show that the adsorption of Cd²⁺and Zn²⁺by the two adsorbents is spontaneous endothermic reaction.As for the dissolution of humic acid,with the increase of p H value,the dissolution amount of humic acid of the two adsorbents increased slowly at first,when the solution was weak alkali or alkaline,humic acid began to dissolve in large quantities;with the decrease of the pristine lignite particle size,the dissolution amount of humic acid gradually increased;with the increase of the amount of two adsorbents and the extension of reaction time,the content of humic acid in the solution increased firstly and then balanced.With the increase of temperature,the amount of humic acid dissolved in water by the two adsorbents increased to a certain extent.Regeneration and adsorption mechanism of lignite based adsorbent:The adsorbents which have adsorbed the heavy metal ions were desorbed by several salt solutions often contained in industrial wastewater.It was found that the combination of two heavy metal ions with lignite-based adsorbent was very stable.In the whole adsorption process,the adsorption effect is the result of the interaction of lignite and humic acid organic matter on heavy metal ions.Lignite molecule contains a large number of phenolic hydroxyl,carboxyl,carbonyl and other oxygen-containing functional groups,humic acid organic matter molecule contains a large number of carbonyl,quinone,carboxyl,hydroxyl,methoxy and other oxygen-containing functional groups.These functional groups determine that lignite and humic acid organic matter have a certain cation exchange and chelating ability to heavy metal ions and the possibility of a series of other complex chemical reactions occurred,finally show that both of them work together to remove Cd²⁺and Zn²⁺.The reaction between nitric acid and lignite promotes the formation of porous morphology and the increase of the content of nitro-humic acid,which further improves the removal efficiency of nitric acid modified lignite on heavy metal ions.The results show that the lignite based adsorbent modified by nitric acid has good performance in the treatment of solutions containing Cd²⁺and Zn²⁺,and has potential application.Advanced treatment of solution after adsorption:The effluent after adsorbing still contains some unremoved Cd²⁺and Zn²⁺ions,free humic acid molecules and their combined substances,which could not meet the discharge standard.The two pollutants can be removed simultaneously by adding aluminium flocculant(Al Cl₃·6H₂O)and using hydrolysis conditions of aluminium salt.The effects of solution p H,dosage of aluminium salt flocculant and initial concentration of humic acid organic matter on effluent treatment were investigated,and the optimal treatment conditions were obtained,which ensured the safety of effluent discharge after final treatment.