Research On In-situ Modification And Enhanced Adsorption Mechanism Of Biochar

Pyrolysis,one of the important ways of biomass thermochemical utilization,can obtain a variety of energy products such as synthesis gas,bio-oil,bio-char,etc.Biochar can effectively respond to the problems of environmental pollution,which plays an important role in the sustainable development of the world in the future.In many existing preparation methods of biochar,the activators used are mostly not environmentally friendly,and the lack of surface groups limits their high-value applications,such as adsorption.Adsorption performance of biochar in the water is affected by two main factors,physical pore structure?specific surface area?and surface chemical properties?functional groups?.In this paper,the low-corrosion green activator was used to adjust the biochar pores by in-situ modification,and the nitrogen doping was used to further regulate the chemical properties of the carbon surface,while the characteristics and adsorption properties of the modified biochar were investigated.Firstly,the preparing method of in-situ modified biochar under a green system was investigated.The activation effects of potassium bicarbonate?KHCO₃?and potassium oxalate?K₂C₂O₄·H₂O?on biochar at different temperatures were studied.The higher the temperature,the better the pore properties of biochar,which means the more micropores.HCO₃^-11 is easier to decompose than C₂O₄^-2 to produce CO₂ and thus has a stronger etching effect on biochar.So the biochar prepared by KHCO₃ at the same temperature has better pore properties.Biochar prepared at 700?with KHCO₃ as an activator exhibited the highest specific surface area and the highest adsorption capacity for phenol,up to 1328 m²·g^-1 and 157.5 mg·g^-1,respectively.Afterwards,the in-situ pyrolysis biochar rich in nitrogen-containing functional groups was prepared by gaseous nitrogen source NH₃.NH₃ is beneficial to the pore structure development of biochar and the increase of nitrogen-containing functional groups.The prepared nitrogen-doped biochar has a maximum specific surface area of1382 m²·g^-1 and a maximum nitrogen content of 10.8%.Micropores can provide more adsorption sites,while nitrogen-containing functional groups can change the electron cloud distribution in the carbon layer and form bonds with adsorbate molecules.The equilibrium adsorption capacity of the nitrogen-doped biochar for phenol and methylene blue were up to 145.4 mg·g^-1 and 484 mg·g^-1,respectively.Moreover,nitrogen-doped biochar with excellent pore structure was prepared by in-situ pyrolysis mixed with activator and different ratios of homogeneous nitrogen-source urea.Low urea admixture ratio was beneficial to the development of pore structures.The optimum specific surface area of N-doped biochar could be up to 1693m²·g^-1.Nitrogen doping can effectively improve the adsorption capacity of biochar to phenol and methylene blue.Nitrogen-containing functional groups promote the formation of?-?bond between carbon layer and pollutant particles,and the Lewis acid-base action can enhance the adsorption of methylene blue.Biochar prepared at 700?with low urea addition ratio exhibited the highest adsorption capacity for phenol and methylene blue,and the equilibrium adsorption capacity was 169.0 mg·g^-1 and 499.3mg·g^-1,respectively.