Study On Adsorption Properties Of Corn Stalk For Heavy Metal Ions

The rapid development of China’s industry has led the increase of water pollution to become more and more serious,especially industrial wastewater containing various types of heavy metals.As a by-product of agriculture,corn stalk is rich in cellulose,hemicellulose and lignin,and holds high utilization value.The preparation of adsorbent from corn stalk for the use of adsorption of heavy metals can promote the comprehensive utilization of straw.This is of great significance for the stabilization of agricultural ecological balance,the reduction of environmental pressure,and the realization of the goal of"treating waste with waste".There are some common problems when considering corn stalk,such as poor selectivity and small adsorption capacity when the stalk is used for adsorption.Thus,the study on the adsorption performance of modified corn stalk can provide more theoretical basis for its practical application.In this paper,corn stalk was treated with nitric acid,sodium hydroxide and citric acid respectively to prepare three adsorbents:nitric acid modified corn stalk（SCS）,sodium hydroxide modified corn stalk（JCS）and citric acid modified corn stalk（CACS）.At the same time,the properties and adsorption performance of various adsorbents are explored,and their modification and adsorption mechanisms are elaborated.This thesis mainly includes the following:that is the modification of corn stalk,the adsorption of modified stalk to copper ion,lead ion and chromium ion.The research results are as follows:1.Treatment of corn stalk with nitric acid,sodium hydroxide and citric acid,using the adsorption amount of three heavy metal ions of copper,lead and chromium as the index,optimizing the modification conditions,and obtaining the best modification conditions for adsorption performance of each ion.The results show that the ability of the modified stalks to adsorb three heavy metal ions was improved to some extent.SEM,EDS,FTIR,XRD were used to characterize the performance of each adsorbent and explore the modification mechanism.It was found that nitric acid could oxidize the glycosidic bonds and eventually form carboxyl groups;sodium hydroxide could dissolve lignin,weaken the hydrogen bond and ester bond,exposing more-OH;and citric acid could react with free hydroxyl group in the cellulose sugar unit to introduce carboxyl group at high temperature.2.The adsorption performance of adsorbent on copper ion was studied.The results showed that the adsorption amount of corn stalk adsorbent for copper ion increased with the increase of pH,and the adsorption amount was the highest at pH 5.The adsorption amount of per unit mass of adsorbents for copper ion decreased as a result of increasing the adsorbents amount.The adsorption rate of copper ion was very fast in the first 20 minutes,and the adsorption equilibrium was reached at 100min.The adsorption amount increased with the increase of temperature and initial concentration.The adsorption behavior of the adsorbents for copper ion matches with the Freundlich isotherm adsorption model and Pseudo second-order dynamics model.FTIR and EDS analysis of the adsorbent changes after adsorption,showed the result that-C=O and-OH absorption peaks changed significantly;There were characteristic peaks of Cu element at 0.9-1.0 keV and 8.0-8.1 keV.It can then be inferred that the copper ion adsorption mechanism of corn stalk was the combination of Cu（Ⅱ）and functional groups on the surface of the stalk to form a complex.Ion exchange occurred in the adsorption process of SCS and CACS.3.The adsorption performance of adsorbent on lead ion was then studied.The results showed that the adsorption amount of corn stalk adsorbents for lead ion increased with the increase of pH.When the pH level increased from 2 to 3,CACS adsorption amount for lead ion increased significantly.At a pH of 5,the adsorption amount was the largest.The adsorption amount of per unit mass of adsorbent for lead ion decreased with increases of the adsorbent amount,and the removal rate of lead ion increased.The higher the initial concentration of lead ion was,the higher the initial adsorption rate and the larger the adsorption amount at equilibrium.The adsorption equilibrium was reached after 1 hour.The adsorption amount decreased with the increase of temperature.The adsorption behavior of adsorbents for lead ion is in agreement with Freundlich isotherm’s model and the pseudo second order dynamics model.FTIR and EDS analysis of CACS changes after adsorption is as follows.It showed the absorption peak of-C=O was significantly weakened,and there was a characteristic peak of Pb²⁺at 2.3-2.4 keV.It could then be inferred that the lead ion adsorption mechanism of CACS was a chemisorption process in which CACS surface functional groups combined with Pb²⁺to form a complex,and there was also ion exchange between H⁺on CACS surface and Pb²⁺in the solution.4.The adsorption performance of the adsorbent on chromium ion was then studied.The results showed that the adsorption amount of corn stalk adsorbents decreased with the increase of pH.With the increase of CACS usage,the removal rate of chromium ion increased,and the adsorption amount of per unit mass of stalk decreased.The adsorption process of chromium ion reached equilibrium slowly,and the adsorption reached equilibrium at 24h.The adsorption amount increased significantly with the increase of temperature and the initial concentration.The adsorption behavior of the adsorbent on chromium ion is in accordance with the Freundlich isotherm adsorption model and the pseudo second-order dynamic model.FTIR and EDS analysis of adsorbent changes after adsorption is as follows.It showed that-C=O,-CH,-OH,-C-O-C are related to the adsorption of chromium ion.There are characteristic peaks of chromium at 5.3-5.5keV and 5.9-6.0keV.It can be inferred that Cr（Ⅵ）combined with the surface functional groups of stalk to form a complex under electrostatic attraction,or Cr（Ⅵ）was reduced by contact with the electron donor groups on the surface of corn stalk.