| The high concentration of inorganic oxygen anions such as nitrate and hexavalent chromium in water will not only cause some environmental problems,but also do harm to human health.Biochar is a kind of carbon material produced by biomass pyrolysis at high temperature.Because of its rich pore structure and high specific surface area,it has been used to adsorb different pollutants.It can be modified to improve the adsorption of pollutants.In this paper,we synthesized Fe/Mn bimetallic oxide coated biochar sorbents(BC,BCFe,BCMn,BCFe0.5Mn1,BCFe1Mn1 and BCFe1.5Mn1)by pyrolysis of wheat straw impregnated with ferric chloride and potassium permanganate,and investigated their potential to adsorb nitrate and hexavalent chromium in water.The adsorption properties of biochar on nitrate and hexavalent chromium were investigated by batch adsorption experiments and dynamic adsorption experiments,and the removal mechanism was also discussed.The main results are as follows:(1)X-ray photoelectron spectroscopy and scanning electron microscopy analysis suggests that Fe(?)/Mn(?)bimetallic oxide particles emerge on the sorbents.The optimized sorbent could achieve a specific surface area of 153.116 m2·g-1and a point of zero charge of 9.76.(2)BCFe1Mn1 has the highest removal rate of NO3-,mainly due to its higher zero charge point and specific surface area.Batch nitrate adsorption experiments were carried out to investigate the influence of various factors such as sorbent dosage,initial solution p H and co-existing anions.Results show that the sorbent maintained high adsorption capacity of75.40%-78.70% over a wide range of p H from 1.00 to 9.05,and the sorption mechanism was interpreted as ligand exchange.The effects of co-existing anions on the nitrate sorption followed the decreasing order of Cl-?SO42-?PO43-.Furthermore,the adsorption isotherms were well described by the Langmuir model,and the sorbent could exhibit a quite competitive high capacity of 37.3613 mg·g-1 for nitrate removal.Meanwhile,the accordance of sorption kinetic with the pseudo-second order model implied that the sorption could be a multi-stage controlled chemical process.In addition,the thermodynamic parameters suggested that the sorption reaction could be a spontaneous and endothermic process.The results demonstrated that the Fe/Mn bimetallic oxide coated biochar could serve as a promising agent for nitrate removal from water.In addition,after three rounds of regeneration,the adsorption capacity of biochar to NO3-is still high,up to32.8673 mg·g-1.The results of dynamic adsorption experiments show that the smaller the bed height,the larger the flow rate,the faster the initial concentration,the faster the penetration time is,but the adsorption efficiency is reduced.(3)BCFe1Mn1 removal Cr(?)maintained high adsorption capacity at acidic or alkaline conditions.At temperatures of 298 K,p H=3.00,BCFe1Mn1 removal Cr(?)reached a maximum value of 73.50%;the adsorption obeys the Langmuir isotherm equation,at a temperature of 308 K,the maximum adsorption amount of BC,BCFe,BCMn and BCFe1Mn1 is 51.5732 mg·g-1,71.2529 mg·g-1,58.5684 mg·g-1 and 85.3913 mg·g-1respectively.The adsorptive capacity was: BCFe1Mn1> BCFe> BCMn> BC;the adsorption process followed pseudo two order kinetics equation,mainly controlled by chemical adsorption;the adsorption process was spontaneous endothermic adsorption process.In addition,the adsorption capacity of biological carbon to Cr(?)was still high after three rounds of regeneration,which was 87.32% before parsing.The smaller the height of the adsorption bed,the larger the flow rate,the more the initial concentration is,the faster the penetration time is,but the adsorption efficiency is reduced. |
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