Construction Of Three-dimensional Porous Structure Of Biochar And Its Application In Cr(Ⅵ) Adsorption

Biochar materials have complex pore structure,large specific surface area and special functional groups,and thus exhibit strong adsorption properties,which can be used to adsorb heavy metal ions in wastewater.The heavy metal Cr(Ⅵ)is easily absorbed by human body and causes irreversible damage to multiple organs,and long-term exposure to Cr(Ⅵ)also has the risk of cancer.Biochar exhibits efficient adsorption performance for Cr(Ⅵ)due to its large specific surface area,but its adsorption performance depends on the structure of biochar.And the carbon framework structure of biochar is determined by the biological structure of biomass materials,which makes it difficult to regulate the adsorption performance of biochar effectively.Therefore,this study proposed to realize the controlled construction of three-dimensional porous carbon framework structure of biochar with soy protein gel as the substrate,and investigated the effects of soy protein concentration,KOH thermal etching temperature and holding time on the construction of three-dimensional porous structure of biochar,as well as the relationship between three-dimensional porous structure of biochar and Cr(Ⅵ)adsorption performance,and explored the mechanism of Cr(Ⅵ)adsorption.The following research conclusions were obtained:(1)The three-dimensional porous structure of soy protein biochar can be autonomously constructed through the controllable gel structure of soy protein,and its structure depends on the concentration of soy protein,KOH etching temperature,and holding time.The optimum three-dimensional porous structure of the biochar ASPB-2-75(4h)is prepared under conditions of soy protein concentration of 8.863%and thermal etching with KOH at 750°C for 4 h.The biochar ASPB-2-75(4h)obtained a three-dimensional porous structure with interpenetrating pores and an open"hemispherical"micro-mesoporous structure,with a maximum specific surface area of2945 m~2/g and a pore volume of 1.484 cm~3/g,and an average pore size of 2.502 nm.The three-dimensional porous soy protein biochar surface is rich in both N-based and O-based functional groups.(2)The soy protein biochar ASPB-2-75(4h)with the best three-dimensional porous structure obtained a maximum adsorption capacity of 392.81 mg/g for the adsorption of250 mg/L Cr(Ⅵ)solution.The special three-dimensional porous structure enabled it to show an excellent Cr(Ⅵ)selective adsorption capacity in the Cl~--Cr(Ⅵ)competitive adsorption,with the Cr(Ⅵ)adsorption capacity reaching 97.65%of that for the adsorption of pure Cr(Ⅵ)solution.(3)The three-dimensional porous structure of the connected main framework and the"hemispherical"micro-pores produced by KOH thermal etching provide effective transport channels and storage space for Cr(Ⅵ)adsorption,respectively,so that pore filling,electrostatic attraction,redox and complexation combine to enhance the adsorption of Cr(Ⅵ)and form multilayer adsorption in the micro-mesopores.The Cr(Ⅵ)adsorption reaction is a spontaneous and heat-absorbing process controlled by chemisorption,and the adsorption behaviour is in accordance with the Freundlich adsorption isotherm and the Pseudo-first-order kinetics.