Preparation Of Walnut Shell Micro/Mesoporous Biochar And Adsorption Characteristics For Arsenic

Juglans regia is one of the most important economic tree species in the world.With the increase of planting area,the yield of walnut is increasing year by year.and global walnut yield has reached around 5 million tons in 2019.After the walnut kernels have been used to produce various products such as edible oils and walnut lotions,a large amount of walnut shells are left behind.Walnut shells are used as polishing powders,filtering materials and culture media,but the low added value and low utilization rate has led to a large amount of walnut shells being discarded directly.As the degree of industrialization continues to deepen,the problem of heavy metal pollution has become increasingly serious,and biochar can play an important role in the treatment of heavy metal pollution.Therefore,preparing walnut shells into biochar can not only reduce the waste of walnut shell resources,improve the economic value of walnut shells,but also adsorb heavy metals and contribute to the restoration of the ecological environment.In this study,walnut shells were used as the research material and a slow pyrolysis method was used to prepare walnut shell biochar.Walnut shell micro mesoporous biochar was prepared using a potassium hydroxide modification method and the adsorption characteristics of heavy metal arsenic were analyzed.The main results of the research are as follows:1.Preparation and modification of walnut shell biocharAfter passing through an 80 mesh sieve,the walnut shell powder was ball-milled for 1h,3h,6h,9h and 12h with a planetary ball mill（No.WN-1,WN-3,WN-6,WN-9 and WN-12）.The ball-milled samples were slowly pyrolysis in a hot kiln to give walnut shell biochar（No.WN-1B,WN-3B,WN-6B,WN-9B and WN-12B）,which yielded 42.78%,42.26%,40.29%,39.68%and 39.03%.The walnut shell biochar was modified with potassium hydroxide to increase and enlarge the pores of the walnut shell biochar to obtain walnut shell micro mesoporous biochar（No.WN-1BM,WN-3BM,WN-6BM,WN-9BM,WN-12BM）.2.Changes in functional groups of walnut shell biochar before and after modification.The functional group changes of walnut shell microporous biochar were analyzed by Fourier transform infrared spectroscopy using potassium bromide filming method.The results show that the functional groups in the unmodified walnut shell biochar are mainly carboxyl groups,double bonds（carbons and carbon-oxygen）and amino groups.The functional groups of modified walnut shell micro mesoporous biochar are mainly carboxyl groups and double bonds（carbon carbon,carbon and oxygen）,hydroxyl groups.It is presumed that the type of functional group has changed due to the chemical reactions that occur during the high temperature modification process.3.Characteristics of specific surface area and pore size of walnut shell biochar before and after modificationSpecific surface area and pore size distribution of walnut shell micro mesoporous biochar were characterized using a specific surface area and pore size distribution instrument,scanning electron microscopy（SEM）and transmission electron microscopy（TEM）.Based on the principle and method of nitrogen isotherm adsorption,the specific surface area of unmodified walnut shell biochar was found to be between 1.25-2.10m²·g^-1,while the modified specific surface area was found to be between 1077-1261m²·g^-1,an increase of nearly 1000 times.Combining the SEM and TEM results,the pore size of the unmodified walnut shell biochar was found to be non-porous or macroporous（>50nm）,while the modified pore size reached the mesoporous（2nm-50 nm）or even microporous level（within 2nm）,and the pore size of the modified walnut shell biochar was also found to be 0.56-3.23nm by the BJH method.4.Comparative analysis of arsenic adsorption characteristics of walnut shell biochar before and after modificationICP-MS and single-factor experimental methods were used to compare unmodified and modified WN-1BM,WN-3BM and WN-6BM with larger surface areas for adsorption experiments,comparing dynamic adsorption,quasi-first-order kinetics,and quasi-second-order kinetics of arsenic.The results show that the adsorption processes of unmodified WN-1B,WN-3B and modified WN-1BM,WN-3BM and WN-6BM are in accordance with quasi-second-order kinetics,while unmodified WN-6B is in accordance with quasi-first-order kinetics.When the biochar concentration was 0.4m²·g^-1,the unit adsorption amounts of arsenic by modified WN-1BM,WN-3BM and WN-6BM were 9.49m²·g^-1,11.22m²· g^-1 and 7.35m²·g^-1 after 30 minutes of dynamic adsorption with an increase of 48.28%,70.78%and 231.08%per unit,respectively.When the biochar was added at 0.8m²·g^-1,the modified WN-1BM and WN-3BM showed the greatest removal of arsenic,with an increase of 44.39%and 76.52%compared to the unmodified ones.This all indicates that the modified treatment can effectively improve the adsorption capacity of walnut shell microporous biochar.The results of this study show that the use of potassium hydroxide modified walnut shell biochar can effectively increase the specific surface area,increase the number of pores,reduce the pore size and increase the porosity.The adsorption capacity of walnut shell micro mesoporous biochar for arsenic was significantly improved.The results of this study will provide theoretical basis and technical support for the development of high value-added walnut shell products.