Experimental Study On Preparation Modified Granular Activated Carbon And Adsorption Of Heavy Metal Cr(Ⅵ) From Anaerobic Fermentation Broth Of Swine

The anaerobic fermentation broth of swine wastewater could be an important source of water in agriculture.It contains elements such as N,P,K,which could be absorbed and utilized by plants.Water resources could be save and water shortage could be relieved.However,a amount of heavy metals,such as copper,zinc,iron,arsenic,chromium,lead,are added in the feed during the breeding process,in order to avoid livestock sickness and improve their feed utilization.These elements are difficult to be digested and absorbed by the livestock and poultry,and the most of them are excreted by feces and urine.If the farmland was irrigated by the wastewater containing heavy metals for a long time,it would cause harm to soil,groundwater and crops.In order to utilize nitrogen and phosphorus,removing the heavy metals to avoid the harm of the anaerobic fermentation broth of swine has become a hot research topic.In the study,heavy metals were removed from the anaerobic fermentation broth of swine wastewater by activated carbon adsorption.Activated carbon has the advantages of large specific surface area,well-developed pores and easy availability.In order to improve the adsorption performance of activated carbon for heavy metals,the activated carbon was modified by citric acid.Taking Cr（Ⅵ）as the research object,discuss the surface physical properties of citric acid modified activated carbons and its adsorption effect and adsorption mechanism on Cr（Ⅵ）.The main contents and conclusions in the study are as follows:（1）Four traditional activated carbons with different material（coal,wood,apricot shell,coconut shell）were used in the experiment.The optimal modification conditions were as follows:citric acid concentration 2%of coal and apricot shell,citric acid concentration 4%of wood and coconut shell,reaction time of 6 hours,reaction temperature of 25℃.Through the single factor adsorption test,the adsorption capacity of all kinds of modified activated carbon under the optimal conditions was:coconut shell>wood>apricot shell>coal.（2）The physicochemical properties of modified coconut shell activated carbons were characterized with Boehm,BET,FT-IR,SEM and XRD.The results showed that the citric acid modified caused the activated carbon pore collapse,specific surface area,micropore area and volume decreased.The citric acid modification increased the surface functional groups of activated carbon,increased 0.470mmol/g,especially the carboxyl content,increased 0.434mmol/g,that was consistent with the FT-IR results.（3）The effect of different water conditions on the adsorption of Cr（Ⅵ）by activated carbon was studied by single-factor experiments.The adsorption mechanism was studied through the adsorption isotherm model and adsorption kinetics model.In addition the optimal regeneration conditions of citric acid modified activated carbon was studied.The results showed that pH was the most important factor influenced the adsorption of Cr（Ⅵ）by activated carbon.At pH=4,the removal rate of Cr（Ⅵ）solution at 5 mg/L reached a maximum of 97.73%.With the progress of adsorption,the dynamic equilibrium was reached at 360 min.The anion concentration had a great influence on the activated carbon adsorption of Cr（Ⅵ）.With the anion concentration increased,the removal rate of Cr（Ⅵ）by activated carbon was decreased.The adsorption process of modified carbon followed the Langmuir isothermal adsorption equation,and the adsorption form was mainly monolayer adsorption;it conformed to the pseudo-second-order kinetic model.Regeneration with the NaOH of 0.2 mol/L and the time of 6h had the best effect on the modified carbon.（4）Taking the anaerobic fermentation broth of swine wastewater as the research object,the influence factors in the adsorption process were analyzed and the influence of these effects on the adsorption of nitrogen and phosphorus by activated carbon were studied.With Cu（II）as the representative,the competitive adsorption in the anaerobic fermentation broth of swine wastewater was studied,too.The results showed that the complex water environment of anaerobic fermentation broth of swine wastewater reduced the removal rate of Cr（Ⅵ）under the same conditions.pH was the main factor affected the adsorption of nitrogen and phosphorus by modified carbon,mainly the existence form of NH₄⁺and P were changes with pH.The removal rate of NH₄⁺by the modified carbon increased first and then decreased with the increase of pH.When the pH was increased from 4 to 9,the removal rate of NH₄⁺by the modified carbon increased from 11.07%to27.09%;when the p H continued to increased from 9 to 11,the removal rate of NH₄⁺by the modified carbon decreased from 27.09%to 24.36%.The removal rate of P by the modified carbon decreased with increase of pH,and decreased from 14.23%to 8.50%.The adsorption of Cu（II）by the modified carbon increased from 52.18%to 83.00%with the increase of pH.The removal of Cu（II）by the activated carbon was partly self-adsorption,and the other part was precipitation.Controlling the pH was very important for the removal of heavy metals in anaerobic fermentation broth of swine wastewater by the modified charcoal.The adsorption process of modified charcoal followed the Langmuir isothermal adsorption equation,and the adsorption form was mainly monolayer adsorption;it conformed to the pseudo-second-order kinetic model.