| The high specific surface area and excellent adsorption capacity make biochar widely used in the removal of organic pollutants.Based on the introduction of the structural characteristics of biochar,this study proposed a new activation method-two-stage activation method involving acid pickling and reheating,by summarizing the related scholars’research on the biochar activation method and the effect of adsorbing pollutants.The two-stage activated biochar was prepared by using alfalfa as biomass raw material,and four other styles of biochar,all prepared through precarbonization at the same temperature,then were treated with acid pickling or ultrapure water washing without reheating,to be regarded as controls.The elemental analysis,specific surface area analysis,particle size analysis,scanning electron microscopy,X-ray photoelectron spectroscopy,Fourier-transform infrared spectroscopy and X-ray diffraction were used to systematically analyze the structural composition and functional groups change of biochar.Three kinds of phenolic pollutants such as phenol,p-nitrophenol and o-nitrophenol were selected as adsorbates to investigate the adsorption effects of different activation stages biochar.The adsorption behaviors of the two-stage activated biochar in single-and ternary-pollutant systems were further studied,and the effects of pollutant concentration,pH and temperature on the adsorption capacity were explored,then the adsorption mechanism was discussed.It provided a new way to improve the adsorption performance of biochar.The main conclusions of this study are as follows:(1)The surface morphology,pore structure and functional group composition of biochar at different activation stages were compared.The results showed that the specific surface area and pore volume of biochar increased significantly after activation by two-stage method.Pickling and reheating could effectively remove tar and other mineral impurities,clear the channels,and expose more active sites.After pickling,the hydrophilicity,polarity and the number of oxygen-containing functional groups increased.The biochar formed a denser aromatic ring structure after reheating.(2)The adsorption properties of biochar on three phenolic pollutants were investigated.The reason for the improvement of adsorption efficiency and the adsorption mechanism were analyzed in combination with the physical and chemical properties of biochar.When the initial concentration of pollutants was 100 mg/L,the pH value was 5,and the temperature was 25°C,the adsorption capacities of phenol,p-nitrophenol and o-nitrophenol on biochar prepared by two-stage activation method was 39.73 mg/g,49.03 mg/g and 48.54 mg/g,respectively,and the removal rates reached 77.99%,97.43%and 93.33%.Under the same reaction conditions,the adsorption capacities of other biochar for the three pollutants were only in the range of 2.2-5.26 mg/g within 12 hours.The adsorption data conformed to the pseudo-second order model and the adsorption rate depended on the available adsorption active sites on the surface of the biochar.(3)In the ternary pollutant system,the adsorption capacity of two-stage activated alfalfa biochar for three pollutants decreased significantly.When the pH value was 5,the temperature was 25°C,and the initial concentration of phenol,p-nitrophenol and o-nitrophenol was 33.3 mg/L,the adsorption capacities of two-stage activated alfalfa biochar for phenol,p-nitrophenol and o-nitrophenol in 12 hours were 10.14 mg/g,16.51 mg/g and 15.15 mg/g,respectively,indicating that the three phenolic contaminants inhibited each other.The kinetic data of the three pollutants had the best fitting effect with the pseudo-second-order model.The R~2 values were closer to 1,and theχ~2 values were the smallest.The isothermal adsorption data had the highest goodness of fit with the Langmuir model,the adsorption of three pollutants on biochar was single layer covering on the outer surface.And the presence of phenol enhanced the individual adsorption of p-nitrophenol and o-nitrophenol on two-stage activated biochar.From the thermodynamic point of view,the adsorption process was spontaneous and endothermic.(4)The removal of three phenolic contaminants by two-stage activated biochar relied on surface adsorption.Activated biochar had a tighter aromatic structure,higher specific surface area and porosity.The forces involved mainly included electrostatic attraction,π-πEDA interactions,and hydrogen bonding between the functional groups present on the biochar surface(carboxylic acids,lactones and phenolic hydroxyls)and in the phenol,p-nitrophenol and o-nitrophenol molecules. |
PDF Full Text Request