Preparation Of Nitrogen-Rich Activated Carbon Derived From Wetland Plant And Removal Of Heavy Metals From Aqueous Solutions

Heavy metal pollution is a great concern environmental contaminants due to its chronicity,accumulation,latency and irreversibility.In February 2011,the State Council approved the "Twelfth Five-Year Plan" for comprehensive prevention and control of heavy metal pollution,which indicated the start of the work on heavy metal pollution prevention and control.Therefore,it is important to explore the economical and efficient methods for heavy metal treatment in wastewater.Constructed wetlands(CWs)is an ecological wastewater treatment technology which has been widely used in the watershed wastewater treatment,water recycle and reuse,and ecological restoration due to its low cost,stable operation,and convenient management.At present,the wetland area in China is approximately 65.94 million hectares(except rivers and ponds,etc.),accounting for 10%of the world's wetlands,ranking first in Asia and fourth in the world.The areas of CWs is more than 40 millions.However,wetland plants have withered in winter in northern China and large numbers of wetland plants was abandoned during the operation of CWs in winter,which lead to resources waste and secondary pollution,and threaten the stable operation of large-scale CWs.It is significant to explore the wetland plants utilization and make waste profitable and harmless.Wetland plants are used in this study as carbon precursors for activated carbon(AC)production to achieve its efficient recycling.New preparation methods and modification methods were developed to improve the adsorption capacity of heavy metals by AC.The heavy metals adsorption efficiency by AC is limited due to the small radius and hydrated ionic radius of the heavy metal ions.In order to improve the adsorption capacity and removal efficiency of heavy metals by AC,most studies focused on the improvement of oxygen-containing functional groups in AC,which can reduce the value of pHPZC and increase the hydrophilia,adsorptive selectivity,capacity of ion exchange and heavy metals binding onto activated carbon.However,the nitrogen atoms have much lower electronegativities compared to the oxygen atoms and provide lone pair electrons for binding heavy metal ions.Therefore,it is a good way to develop a high-nitrogen microporous AC for heavy metals adsorptive removal from aqueous solutions.This study completed the pretreatment technology of chemical activation method for activated carbon preparation on the basis of lignocellulose-based porous vascular bundle of wetland plants.A new ammoniation-activation method was provided to increase the BET surface area and surface functional groups of AC and then improving the adsorption of Ni(II).The new activated agents of urea phosphate and ammonium phosphate were used for activated carbon preparation.This can improve the microporous content and nitrogen-containing functional groups.Accordingly,the adsorption capacity of Cd(?)and Cr(?)by activated carbon was increased.Activated carbon-polypyrrole composite(AC-PPy)was synthesised by electrolytic deposition method which used to heavy metals adsorption/desorption.This can control TcO4-(ReO4-)deposition and release through changing the potential.The main conclusions are follows:(1)The feasibility of new ammoniation-activation method for activated carbon preparation was verified and completed chemical activated methods.The ammoniation of lignocellulose can cause the expansion of the cell wall,resulting in a loose structure which can increase the available attachment sites for the activating agents and promote impregnation and activation.Sequentially,the content of O,N on the PA can be increased as well,which promotes higher surface functional groups of the AC.The influence factors of the ammoniation parameters on the physicochemical properties and adsorption performance of the AC are in the following order:ammoniating agent>ammoniating time>ammoniating dose>ammoniating temperature.The best parameters are as follows:urea as the ammoniating agent,the ammoniating temperature at 30?,ammonia ratio of 0.07:1(mass ratio of ammonifying agent:raw material),and kept ammoniating for 7 days.Compared to AC-PPA,AC-PPy has 20-40%higher BET surface area,45-70%higher total pore volume,and increased total functional group.Particularly,the N element content and basic functional group of AC were improved obviously.The AAC had 30%higher adsorption capacity of Ni(?)compared to AC-PPA.The adsorption behavior ofNi(?)by AAC was more consistent with the Langmuir isotherm adsorption model and the pseudo-second-order kinetics adsorption model.At pH 3-8,the adsorption capacity of AAC improved as the pH value increased.Meanwhile,the ionic strength of the solution had the negative correlation to the adsorption.The XPS analysis proved both of the nitrogen-containing and the oxygen-containing functional groups played an important role in the chemisorption process.(2)The new activating agent urea phosphate(UP)was used to prepare AC has shown a good performance.The urea phosphate could release organic phosphates and N radicals during the activation process.Consequently,urea phosphate has the activation capacity like phosphoric acid,and could also form nitrogen-containing functional groups on the surface of AC.AC-UP shows great yield of 47.7%due to the great flame retardants of UP.The BET surface area and functional groups of AC-UP are higher than that of biochar.The maximum BET surface area of 846.4 m2/g,the micropores proportion of 68%,the total functional group contents of 3.521 mmol/g,and the basic functional group contents of 1.477 mmol/g.The adsorption capacity of AC-UP for Cd(?)was 40.65 mg/g,which was 5 times higher than that of biochar.The adsorption behavior of Cd(?)by AC-UP was consistent with Langmuir adsorption model and pseudo-second kinetics adsorption model.The adsorption capacity of Cd(?)was highest when the pH value of the solution is neutral.Ions in solution plays an negative role in the adsorption.The XPS analysis proved that the nitrogen-containing functional groups play a major role in the chemisorption process.(3)Since the ammonium ions in the ammonium phosphate are more active than the urea components in the UP during the pyrolysis process,it is proved that the nitrogen-containing functional group can be more easily formed on the AC surface by the ammonium phosphate as the activating agent.According to the comprehensive evaluation of physical and chemical properties and yield of AC,diammonium hydrogen phosphate was selected for further optimization studies.The optimum impregnation ratio of diammonium phosphate as activator was 1.5:1 and the activation temperature was at 700?.AC-DAP had a surface area of 1094.27 m2/g,the microporosity of up to 90%,and the total functional group content of 3.84 mmol/g,in which the content of basic functional groups was 1.83 mmol/g,and the adsorption capacity of AC-DAP for heavy metal Cr(?)was 44 mg/g.The adsorption behavior is consistent with the Langmuir adsorption model and pseudo-second kinetic adsorption model.The increase of pH of the solution plays a negative role in the adsorption.XPS technique proves that in the chemisorption of Cr(?)on AC-DAP,an anion of Cr(?)is adsorbed on the surface of AC-DAP by electrostatic adsorption,and part of Cr(?)is reduced to Cr(?)and then provided by the surface functional group of AC which the nitrogen-containing functional group plays a major role.(4)Activated carbon-polypyrrole composite(AC-PPy)was prepared by electrodeposition method.The BET surface area of AC-PPy was 2550.64 m2/g,hence its three-dimensional porous structure for the removal of heavy metals provides a lot of adsorption sites.The adsorption and desorption of TcO4-(ReO4-)was applied to the electrochemically controlled ion exchange technology.Under the action of the electrochemical potential,the adsorption and desorption of ReO4-were completed in 60 seconds,which greatly improved the efficiency of the adsorption method.The bahaviours of adsorption and desorption of TcO4-(ReO4-)on AC-PPy were through XPS and E-QCMD techniques.AC-PPy exhibits good recovery rate through frequency and quality change.In the third cycle,the recycling rate reaches 100%and adsorption and desorption reach equilibrium.The adsorption and desorption ability of the cyclic voltammetry technique is stronger than that of using a specific voltage technique.This method expands new ideas for the modification and application of AC and uses electrochemical technique to increase the efficiency of the adsorption.