Preparation Of Biomass-based Carbon Composites And The Study For Removal Of Heavy Metal Pollutants

With the rapid development of industrialization,electroplating,metallurgy and leather industries have produced large amounts of various heavy metal wastewater containing chromium,copper,cadmium and nickel,which caused serious damage to the water environment.Chromium is one of the most toxic heavy metals that is harmful to humans.Chromium mainly exists in two forms of Cr???)and Cr???in nature.The ecotoxicological effect of Cr???is much greater than that of Cr???,which can cause lung cancer,dermatitis and other diseases.Therefore,taking effective measures to reduce the content of Cr???in wastewater is of great significance for public health and environmental protection.Biochar is often used to remove heavy metal ions in wastewater,but due to its limited adsorption capacity,chemical modification is usually used to enhance its adsorption capacity.In this study,agricultural and forestry waste biomass was selected as the raw material to prepare porous biochar by pyrolysis.The aluminium/manganese oxides were used to modify the porous biochar,and the removal mechanism of Cr???in wastewater by aluminium/manganese modified biochar was investigated.The contents are as follows:?1?The porous biochar was prepared by KOH modification using corn straw as raw material.It had a specific surface area of 2131.18 m² g^-1.The proportions of micropore surface area and micropore volume were 91.93%and 80.43%,respectively.The formation mechanism was the evaporation of CO,and other gases generated in the pyrolysis process created micropores on/in the biochar.Besides the porous biochar contained abundant oxygen functional groups,such as carboxyl group?-COOH?,ether group?C-O-C?and hydroxyl group?-OH?,etc.which contributed to remove Cr???from wastewater through ion exchange,reduction reaction,and complexation reaction.At 25°C,the saturated adsorption capacity of Cr???by porous biochar was 175.44 mg g^-1,and the removal rate of Cr???after recycling four times still reached 52.39%,which indicated this material has an excellent recycling performance.The adsorption process of Cr???by porous carbon conformed to the pseudo-second-order model and Langmuir adsorption isotherm model,which indicate that the removal of Cr???was mainly based on chemical adsorption of a monomolecular layer.The removal rates of Cr???in Cr???-nickel and Cr???-copper coexisting ion wastewater were 99.81%and 99.72%,respectively.Which were higher than in single Cr???wastewater,indicated that porous biochar had high adsorption selectivity for Cr???coexisting ionic wastewater.The removal rate of Cr???in decorative chromium plating wastewater of electroplating workshop reached 97.90%.Porous biochar may have great potential in decorative chromium wastewater treatment.The better recycling performance and higher adsorption selectivity indicate that porous biochar may have a broad application prospect in heavy metal wastewater treatment.?2?Biochar was prepared with Phoenix tree fallen leaves?abbreviated as BC?followed bymodification?KBC?.The KBC was then modified with aluminium/manganese oxides to get Al/Mn-modified biochar composites?AMKBC?.According to the results of X-ray photoelectron spectroscopy?XPS?,scanning electron microscope?SEM?and energy dispersive spectrometers?EDS?,aluminium/manganese oxides have been effectively embedded in the surface or pores of AMKBC.They mainly exist in the form ofand Mn O.Under certain adsorption conditions,when the mass ratio of aluminium/manganese oxides to KBC was 3/4,the adsorption capacity of Al/Mn-modified biochar composites for Cr???reached a maximum of 56.87 mg g^-1.Which was higher than BC(10.76 mg g^-1),KBC(39.82 mg g^-1)or AMKBC composites under other preparation conditions.Nitrogen adsorption and desorption results showed that the total specific surface area and total pore volume ofwere 1173.36 m² g^-1 and 0.54 cm³ g^-1,which were 15.25 and 8.87 times of BC,respectively.The proportions of micropore surface area and micropore volume reached 93.97%and 83.92%,respectively.They were significantly higher than BC.At25°C,the saturated adsorption capacity of Cr???was 152.86 mg g^-1.The adsorption process ofto Cr???conformed to the pseudo-second-order kinetic model and Langmuir adsorption isotherm model,indicating that the removal of Cr???was based on chemical adsorption of a monomolecular layer,accompanied by reduction reaction,channel interception and other mechanisms.The preparation of Al/Mn-modified biochar composites provides a new adsorbent for the treatment of heavy metal wastewater,and may have a great application prospect.