Study On Adsorption Of Heavy Metal Ions By Tannin-Dialdehyde Carboxymethyl Cellulose

The adsorption methods separate heavy metal ions by functions of the structure and surface properties of adsorbent particles,which has the advantages of less reagent consumption,low cost,no pollutants increase in sewage body,easy recovery of heavy metals,no secondary pollution,and adsorbent reuse.Common heavy metal ion adsorbents are usually used to adsorb one kind of metal ions,cations or anions.The broad-spectrum adsorbents which can adsorb cations and anions efficiently and can be used in various complex sewage systems are industriously searched for by the researchers of water purification technology.Tannin originates from plants and is rich in phenolic hydroxyl groups which can chelate heavy metal ions in wastewater to achieve the effect of removing heavy metal ions.However,tannin is water-soluble,which limits its use as adsorbent.The solution is one of carriers is needed to solidify tannin.In this study,carboxymethyl cellulose（CMC）was oxidized to dialdehyde carboxymethyl cellulose（DCMC）,then DCMC loaded tannin to prepare the tannin-dialdehyde carboxymethyl cellulose（TA/DCMC）.The adsorption properties of TA/DCMC for Cr（Ⅵ）and Ni（Ⅱ）ions under different adsorption conditions were investigated.The TA/DCMCs before and after adsorption were characterized by SEM,EDS,FTIR and XPS.The adsorption data were fitted by kinetic models,isothermal models and thermodynamic models,respectively.And the adsorption mechanism was determined.The main contents and results of this paper were as follows.The experimental results of TA/DCMC for Cr（Ⅵ）adsorption showed that the adsorption capacity of Cr（Ⅵ）can reach 281.06 mg/g when the concentration of Cr（Ⅵ）solution was 300mg/L,the volume was 100 m L,the pH was 1.5,the adsorption temperature was 333.15 K and the dosage of adsorbent was 50 mg.In the selective experiments,the inhibition order of the four anions to Cr（Ⅵ）adsorption was NO₃^->Cl^->H₂PO₄^->HCO₃^-.The adsorption-desorption experiments were repeated with 1 M potassium hydroxide aqueous solution as desorption agent.The fifth adsorption capacity of TA/DCMC was 148.65 mg/g,and 52.89%of that of the first adsorbent.The pseudo-second-order kinetic model and Langmuir model respectively were the best for fitting different adsorption data,so it was speculated that the adsorption process was monomolecular chemisorption.The thermodynamic fitting results showed that the adsorption process was spontaneous and endothermic.The increase of temperature was beneficial to the adsorption process.The characterization of FTIR results showed that the adsorbent was successfully prepared.After Cr（Ⅵ）adsorption on TA/DCMC,the TA/DCMC surface in SEM image had large agglomerates and its structure became loose,and in EDS image Cr（Ⅵ）was evenly distributed on TA/DCMC.XPS analysis showed that electron transfer took place during the adsorption process.Highly toxic Cr（Ⅵ）was changed into low toxic Cr（ⅡI）.And there were interactions between the oxygen of O-H and O-C groups in TA/DCMC and chromium ions.Based on the analysis results described above,it was speculated that TA/DCMC adsorbed chromium ions by ion exchange.The experimental results of TA/DCMC for Ni（Ⅱ）adsorption showed that the adsorption capacity of Ni（Ⅱ）was 43.68 mg/g when the initial concentration of Ni（Ⅱ）solution was 100mg/L,the volume was 100 m L,the pH was 5,the adsorption temperature was 313.15 K,and the dosage of adsorbent was 50 mg.The inhibition of the four coexisting cations to Ni（Ⅱ）adsorption was in the order of Pb（Ⅱ）>Ca（Ⅱ）>Cu（Ⅱ）>Zn（Ⅱ）.The adsorbent after Ni（Ⅱ）adsorption was desorbed by 1M nitric acid.The desorption-adsorption experiments were repeated 4 times.The fifth adsorption capacity of Ni（Ⅱ）on the adsorbent was 30.12 mg/g which was 68.95%of that in the first adsorption,which suggested TA/DCMC had good reusable property.Pseudo-second-order kinetic model and Freundlich isothermal adsorption model can fit the adsorption data well.The former results indicated the adsorption process was chemical adsorption.Thermodynamic model fitting showed the adsorption was spontaneous and endothermic process.The SEM images showed that the surface of TA/DCMC became rough and appeared bulges and agglomerates after Ni（Ⅱ）absorption.The EDS image showed that Ni（Ⅱ）was evenly distributed on TA/DCMC.The main functional groups of adsorbent for Ni（Ⅱ）adsorption were phenolic hydroxyl groups.The adsorption mechanism of nickel ions by TA/DCMC was supposed to be ion exchange.The experimental results of TA/DCMC adsorbed Cr（Ⅵ）and Ni（Ⅱ）at the same time showed that the pH value of solution had a great influence on the adsorption performance.Cr（Ⅵ）was mainly adsorbed in the range of pH 1-3,while high Ni（Ⅱ）adsorption capacity was in the range of pH 4-7.The pseudo-second-order kinetic model and Langmuir isothermal adsorption model separately were the best for different Cr（Ⅵ）adsorption data,and the pseudo-second-order kinetic model and Freundlich isothermal adsorption model separately were the best for different Ni（Ⅱ）adsorption data fitting.Thermodynamic fitting showed that the simultaneous adsorption of Cr（Ⅵ）and Ni（Ⅱ）by TA/DCMC were spontaneous and endothermic.In order to make TA/DCMC can be used in a broader pH range,chitosan（CT）which can protonize in higher pH was used to modify TA/DCMC to obtain the adsorbent of TA/CT/DCMC.Compared with TA/DCMC,the pH value of zero charge point of TA/CT/DCMC increased 0.2.Accordingly,in the same pH condition the potential for TA/CT/DCMC was higher and the Cr（Ⅵ）adsorption capacity of TA/CT/DCMC was larger.From the foregoing,the reducibility of tannin combines the potential change for oxygen of O-H and O-C groups of tannin in different pH conditions,which makes TA/DCMC can remove both the anionic Cr（Ⅵ）and the cationic Ni（Ⅱ）.This study provides a feasible idea that one adsorbent is used to adsorb anions and cations separately under different conditions.