Study On Removal Of Nitrate And Phosphate From Water By Modified Corn Straw Adsorbent

Nitrogen and phosphorus pollution are two important factors leading to eutrophication of rivers and lakes in China;the random disposal of agricultural and forestry straws may cause environmental hazards.In order to realize the comprehensive utilization of agricultural straws and resource treatment of nitrogen and phosphorus wastewater,this paper selected corn stalk?CS?as representative product of agricultural and forestry straws.Epichlorohydrin,N,N-dimethylformamide and triethylene tetra added into CS in sequence,the modified corn straw?MCS?was obtained by grafting quaternary ammonium functional groups with amine and triethylamine.Based on the CS and MCS characterization,the adsorption properties,adsorption mechanism and dynamic adsorption,desorption and regeneration of nitrate and phosphate by MCS were studied.The ability of MCS to adsorb nitrate and phosphate is evaluated in all aspects.In order to compare the changes of corn straw properties before and after modification,the surficial and internal structural changes of CS and MCS were characterized by elemental analysis?EA?,Zeta potential,electron microscopy?SEM?and Fourier transform infrared spectroscopy?FTIR?.EA analysis found that C/N decreased from 31.9179 to 3.7913,indicating an increase in nitrogen content in MCS;at pH 7.0 and 25°C,the zeta potential increased from-23.5 mV to 18.1 mV,indicating a positively charged group grafted on CS sucessfully;SEM analysis found that MCS was rougher,porous and cracked,had larger surface area,and may have stronger adsorption capacity;FTIR found that the characteristic peak of quaternary ammonium salt appeared in 1330 cm^-11 in MCS,1053cm^-11659cm^-1specific band enhancement indicating adsorption of phosphate and a characteristic peak of1384 cm^-1 indicating that MCS can adsorb nitrate.In order to investigate the adsorption performance of MCS,the dosage,pH,initial concentration,coexisting ions(Cl-,SO₄^2-)were the main factors,and the adsorption amount of nitrate and phosphate was used as the response index to optimize the adsorption conditions.The removal rate of phosphate was 10.97%higher than that of nitrate,indicating that MCS had a stronger adsorption capacity for phosphate.Nitrate and phosphate adsorption had a wide range of pH adaptation?4-10?,but under extreme pH conditions,both exhibited opposite characteristics,which may be related to ion exchange and valence;SO₄^2-and Cl^-to nitrate and phosphate adsorption had a negative effect,and the former had a greater impact.In order to explore the possibility of MCS application,this paper dynamically investigated the adsorption capacity of MCS for nitrate and phosphate by column adsorption,column desorption and column regeneration,and used various kinetic models to fit the dynamic adsorption process.The column adsorption test showed that the flow rate,column height and initial concentration had no significant effect on the amount of MCS unit adsorption.At the condition of pH 7.0,flow rate 10 mL/min,column height 2.0 cm,and initial concentration 50 mg NO₃-N/L,the adsorption amount of MCS on nitrate was 34.76 mg/g.At the condition of pH 6.0,flow rate 10 mL/min,column height 2.0 cm,and initial concentration50 mg PO₄^3--P/L,the adsorption amount of MCS on phosphate was 66.94 mg/g,whose adsorption capaticy was about 2 times compared with nitrate.According to the R² and standard deviation,the degree of fitting of the nitrate and phosphate adsorption models was Yoon-Nelson>Yan>Thomas model from the largest to the smallest,and the model parameter analysis found lower k_YN and higher₍?8?67??may be more conducive to increasing the adsorption capacity of the adsorbent.Nitrate and phosphate were desorbed with different concentrations of NaOH and HCl,and it was found that the desorption capacity of HCl was higher than that of NaOH,but the concentration was higher than the latter.Column regeneration showed that HCl had a significant increase in the regenerating ability of nitrate and phosphate.After three regeneration cycles,the regeneration rates were 129.32%and 128.43%,respectively,which may mean that the adsorption performance of MCS can be improved by HCl activation.Through the research in this paper,the mechanism of different modified straws on different anions adsorption performance can be clarified from the microscopic point of view,enriching theoretical research,and providing corresponding method guidance for nitrogen and phosphorus removal.