Adsorption Of Cr(Ⅵ) On Modified AttapuLgite Via One-step Hydrothermal Carbonization Of Chitosan

Heavy metal chromium is one of the pollutants under control in China,mainly from the electroplating,metallurgy,leather,printing,dyeing and chemical industry emissions.The main forms of chromium in water are Cr(III)and Cr(VI)water.Compared with Cr(Ⅲ),Cr(Ⅵ)is more toxic,absorbed by the body can cause stomach pain,abuse,diarrhea,vomiting,liver cancer and other diseases.At present,the main treatment methods of chromium containing wastewater include chemical precipitation,ion exchange,membrane separation,photocatalysis,electrochemistry,biochemistry and so on.Compared with the above methods,the adsorption process has been widely used to treatment with chromium containing wastewater because of its simple operation,cheap and good treatment effect.Attapulgite(AT)has the advantages of large specific surface area,good adsorption performance,has been widely used in water treatment field.Due to its own structure has a negative charge and negative surface charge,resulting in poor adsorption of Cr(VI),therefore,it is necessary to modified the attapulgite to improve the adsorption performance of Cr(Ⅵ).Instead Chitosan(CS)is abundant with such active groups as-OH and-NH2 and other active groups.It can help remove heavy metal ions and other pollutants by electrostatics or complexion.However,since chitosan is easily dissolved in acidic aqueous solutions and forms colloids,it is usually not possible to use it directly under acidic conditions.For the removal of Cr(VI),only in acidic conditions will have a good effect.Therefore,this paper proposes ready attapulgite@chitosan nanocomposites(AT@CS)using attapulgite and chitosan via one step hydrothermal carbonized process.And analyze properties of AT@CS through diversified methods such as SEM、Raman、FTIR、XRD、XPS、EA、Zeta and TGA.In addition,this paper studies how factors like pH 、 Na+ 、 initial concentration and temperature would influence Cr(Ⅵ)removal,analyzed the kinetics and thermodynamics of adsorption behavior of AT@CS was studied;the dynamic adsorption process of AT@CS on the total chromium;and to investigate the adsorption mechanism,the main conclusions are as follows:(1)Attapulgite was organically modified through one-step hydrothermal carbonization of chitosan as a carbon source under mild temperature.Characterization was examined by Raman spectroscopy(Raman),Fourier transform infrared spectroscopy(FTIR),Elemental analysis(EA)and X-ray photoelectron spectroscopy(XPS)analysis showed that the surface of the attapulgite was rich in C=O,C-OH,COOR and-NH2.(2)The removal rates of Cr(Ⅵ)and total chromium were 89.5% and 80.2% by AT@CS,higher than attapulgite and AT@C.Although the removal efficiency of Cr(Ⅵ)is similar to activated carbon(AC),but AT@CS is significantly higher than activated carbon(AC)in the removal rate of total chromium,and the cost of activated carbon is high,the preparation temperature is high,will produce toxic and harmful gases and other defects,and AT@CS in the preparation process is not toxic and harmful gas release,the cost is low.It shows that AT@CS is a new,environmentally friendly and high-performance adsorbent.(3)In the range of pH=1~8,with the increase of pH,the removal rate of Cr(VI)increases first and then decreases,while the removal rate of total chromium is gradually decreased.At pH 2,the removal rates of Cr(VI)and total chromium were91.0% and 80.8%,respectively.With the increase of Na+ concentration,the competitive adsorption of Cl-with Cr(VI)and Na+ with Cr(III)increased,which inhibited the adsorptionand.(4)The results of dynamic adsorption experiments show that the dynamic adsorption behavior of attapulgite and AT@CS on total chromium is in accordance with the LDF dynamic model,indicating that the dynamic adsorption behavior of attapulgite and AT@CS on total chromium is mainly controlled by internal diffusion.(5)In the range of 100~200 mg/L,the maximum adsorption capacity of AT@CS to total chromium increased from 35.887 mg/g to 73.836 mg/g,and its kinetic adsorption behavior was accordance with the quasi-second order kinetic equation.The apparent adsorption activation energy is 13.4 kJ·mol-1,indicating that the adsorption process has both physical adsorption and chemical adsorption,and the physical adsorption is the main.(6)In the range of 298~328 K,the maximum adsorption capacity of AT@CS to total chromium increased from 210.939 mg/g to 231.630 mg/g,0(35)H is 19.8 kJ·mol-1,0(35)G is-19.5~-23.7 kJ·mol-1,0(35)S is 131.8~72.1 J·mol-1·K-1,indicating that the adsorption is an endothermic,spontaneous,entropy Increase the process.The thermodynamic adsorption behavior of AT@CS on total chromium is in accordance with Langmuir equation,indicating that the adsorption process has both electrostatic and coordination adsorption,and is mainly electrostatic adsorption.(7)With XPS analysis,it is obvious that,after absorbed,chromium has present as Cr(VI)and reduced Cr(III).Partial Cr(III)involved in the process of ion exchange between Mg 1s and Si 2p in the adsorption process,and partial Cr(III)is released into solution.Cr(VI)is not only adsorbed on the surface of the adsorbent with coordination complexation with COOH,but also reacts with-NH2 to form-NH2 Cr,which indicates that the adsorption process of Cr(Ⅵ)contains both physical absorption and chemisorption.