Adsorption And Reduction Of Chromium (Ⅵ) From Electroplating Wastewater By Nano-zero-valent Iron Modified Biochar

Chromium is widely used in electronic and electrical manufacturing,wood anticorrosion,leather tanning,metallurgy,alloy preparation and electroplating industries,especially there are a lot of hexavalent chromium in electroplating wastewater.Cr(Ⅵ)is extremely toxic,100 times more toxic than Cr(Ⅲ).It is carcinogenic and genotoxic,easy to be absorbed by the human body,but not easy to be metabolized by the human body,causing a serious threat to human health.Effective treatment of Cr(Ⅵ)in electroplating wastewater is one of the environmental problems to be solved urgently.Biochar,as a heavy metal adsorbent,has the characteristics of economic efficiency and environmental friendliness,but it cannot effectively reduce chromium and reduce the toxicity of Cr(Ⅵ).Nanosized zero-valent iron modified biochar can effectively solve the above problems and achieve Cr(Ⅵ)adsorption and reduction at the same time.In this study,black agaric spent mushroom substrate was used as raw material to prepare biochar(BBC),and zero-valent iron nanoparticles were loaded onto biochar by liquid phase reduction method to prepare zero-valent iron nanoparticles modified biochar(BBC-n ZVI).The adsorption and reduction capacities of Cr(Ⅵ)were compared by batch adsorption tests,and the optimum conditions of adsorption reaction were determined.The adsorption mechanism was analyzed by thermodynamic and kinetic models.Using sodium alginate as a gel to make BBC-n ZVI into a spherical adsorbent.The continuous flow column bed technology was used to treat actual electroplating wastewater,and the best column bed condition was determined,and the feasibility of the column bed in actual electroplating wastewater treatment was determined by model fitting.This study provides a theoretical basis for the application of zero-valent iron modified biochar adsorbent in the treatment of heavy metal wastewater.The main results obtained in this study are as follows:(1)Compared with biochar,the prepared nanometer zero-valent iron modified biochar has larger specific surface area and higher load of nanometer zero-valent iron particles.The specific surface area of the modified BBC-n ZVI is about three times that of the unmodified BBC,which expands from 5.742 m~2/g to 18.592 m~2/g.Elemental analysis of biochar before and after modification showed that the mass percentage(Wt%)of Fe in BBC-n ZVI increased from 0.39%to 38.49%,indicating that zero-valent iron nanoparticles were successfully loaded on biochar.By comparing the morphology of biochar before and after modification by scanning electron microscopy,it can be found that the specific surface area of BBC-NZVI and the abundant pores of biochar complement each other,so that the nanosized zero-valent iron particles are evenly distributed on its surface.(2)Nano-zero-valent iron modification can improve the adsorption and reduction capacity of biochar and shorten the reaction time.Batch adsorption test results show that:p H has a great influence on the adsorption process.When p H<5,the adsorption capacity of Cr(Ⅵ)increases with the decrease of p H.Under the optimal adsorption conditions of p H=1,Dose=2 g/L,t=120min,the adsorption capacity of unmodified biochar was 19.59 mg/g,and the reduction rate was65.31%;Under the optimal adsorption conditions of p H=2,Dose=1 g/L,t=10 min,the adsorption capacity of biochar modified with nano-zero-valent iron was 59.63 mg/g,and the reduction rate was 99.39%.Elemental analysis of BBC-NZVI before and after adsorption shows that the mass fraction of Fe on the surface of BBC-n ZVI after adsorption changes from 38.49%to12.64%.This indicates that zero-valent iron nanoparticles provide the main reaction binding site,indicating the feasibility of the removal and reduction of Cr(Ⅵ)by BBC-NZVI composite adsorbent.(3)The multilayer adsorption and chemisorption of Cr(Ⅵ)on biochar can be improved by nano zero-valent iron modification.Isothermal adsorption model analysis results show that:The Langmuir and Freundlich models have a good fit for the adsorption process of Cr(Ⅵ)by BBC-NZVI at different temperatures.This indicates that the BBC-n ZVI adsorbent has both monolayers and multilayers of adsorption and has good adsorption capacity.The maximum adsorption capacity is 123.58 mg/g according to Langmuir model.The results of kinetic model analysis show that pseudo-second-order kinetic model can better describe the reduction kinetic behavior of BBC-n ZVI.This indicates that the adsorption process is dominated by chemical adsorption while physical adsorption exists.Thermodynamic model analysis shows thatΔG°<0、ΔH°>0、ΔS°>0.This indicates that the adsorption process is spontaneous and endothermic with increasing entropy,which is beneficial to the adsorption process.(4)Nano-zero-valent iron modification can improve the ability of biochar to effectively adsorb and reduce Cr(Ⅵ)in actual electroplating wastewater.Continuous flow column bed adsorption test results show that:When the initial concentration is 60mg/L,the flow rate is3m L/min,and the column height is 15cm(optimum technology parameter),the maximum adsorption capacity of Cr(Ⅵ)in electroplating wastewater by BBC-NZVI is 42.04mg/g,and the reduction rate is 81%.In conclusion,nano-zero-valent iron modification can significantly improve the adsorption and reduction capacity of biochar for Cr(Ⅵ).The BBC-n ZVI adsorbent can be used in the treatment of electroplating wastewater by batch experiment and continuous flow column bed experiment.This research provides a theoretical basis for the application of BBC-n ZVI in semi-industrial electroplating wastewater treatment.