Study On Nitrogen And Phosphorus Adsorption Mechanism Of Modified Corn Straw Hydrothermal Carbon

Seeking environmentally friendly adsorption materials for ammonium nitrogen and phosphate in polluted water to achieve high-value utilization of agricultural waste corn straw under mild hydrothermal conditions.Using corn straw as raw material,the effect of KOH and Fe Cl₃ modification methods on the hydrothermal carbonization of corn straw was investigated.The surface functional groups,structure and morphology of the modified hydrothermal carbon were characterized by Fourier transform infrared spectroscopy and scanning electron microscopy.The adsorption mechanism of ammonium nitrogen and phosphate in water was investigated through adsorption kinetics and thermodynamics experiments.The main research conclusions are as follows:（1）Modified hydrothermal carbon had a well-developed carbon microsphere structure and more pores.Compared with unmodified hydrothermal carbon,C2（iron chloride modified hydrothermal carbon）and C3（potassium hydroxide modified hydrothermal carbon）have significantly increased specific surface areas by 14.81%and 42.79%.The surface of the modified hydrothermal carbon was rich in oxygen containing functional groups,mainly including（-C-O,-OH,-O-）.（2）The adsorption of ammonium nitrogen on modified hydrothermal carbon conformed to a quasi second order kinetic equation（R²>0.95,P<0.05）,and the adsorption thermodynamics most conformed to the Langmuir equation（R²≥0.96,P<0.05）,ΔG^θLess than 0,ΔH^θGreater than 0 indicated that the adsorption of ammonium nitrogen by different modified hydrothermal carbon was a spontaneous endothermic reaction;With the increase of adsorption temperature and ammonium nitrogen concentration,the adsorption amount of ammonium nitrogen on hydrothermal carbon gradually increased.Adsorption was a single molecular layer adsorption controlled by a rapid reaction.Both KOH modified and Fe Cl₃ modified hydrothermal carbons could significantly improve the ability to adsorb ammonium nitrogen.The adsorption of ammonium nitrogen after modification with Fe Cl₃ mainly depended on the pore filling capacity,and the specific surface area of the modified hydrothermal carbon increases;The adsorption of ammonium nitrogen by KOH modification mainly depended on pore filling and electrostatic attraction.The modified hydrothermal carbon had a significant increase in specific surface area and a richer oxygen containing functional group.The adsorption effect of ammonium nitrogen by KOH modification was better than that of Fe Cl₃modified hydrothermal carbon.The maximum adsorption amount was 22.61 mg/g at 45℃.（3）The adsorption kinetics of phosphate on modified hydrothermal carbon conformed to a quasi second order kinetic equation（R²>0.95,P<0.05）,and the adsorption thermodynamic characteristics conformed to the Langmuir equation（R²≥0.94,P<0.05）;This adsorption process was a spontaneous endothermic reaction（ΔG^θLess than 0,ΔH^θGreater than 0）,and was adsorbed by a single molecular layer and controlled by a rapid reaction.By modifying Fe Cl₃ and KOH,hydrothermal carbon could significantly improve the ability to adsorb phosphate radicals.The main reason for the adsorption of phosphate radical after Fe Cl₃ modification was that they have good electrostatic attraction ability;The adsorption of phosphate radical by KOH modification mainly depended on a large specific surface area and ion exchange.The corn straw hydrothermal carbon modified with Fe Cl₃ has a large adsorption capacity,with a maximum adsorption capacity of 2.25 mg/g at 45℃.