| Excessive phosphate discharging in industrial,agricultural and domestic sewage has caused serious eutrophication of water.At present,adsorption is one of the most effective methods to remove phosphate in water among many treatment methods.The current adsorption research mainly focus on the preparation of adsorbents for highly efficient removal of phosphate.Metal oxides with abundant phosphate adsorption sites have been widely applied by researchers in recent years as potential high-efficiency phosphate adsorbents.Zirconia and lanthanum oxides have high affinity for phosphate binding and can maintain physicochemical stability in a wide pH range.However,due to their small specific surface areas and less exposed adsorption sites,their practical applications in phosphate removal are limited.Therefore,we propose to load zirconia and lanthanum oxides onto inert carriers with large specific surface areas to fully expose the active sites.Among many carriers,ordered mesoporous materials(such as mesoporous carbon)can provide restrictive space to,disperse metal oxides,inhibit the agglomeration of metal oxides in the mesoporous pore channels,enable metal oxides to form nano-sized particles in the mesoporous pore channels,and improve the adsorption capacity of phosphate.At the same time,the limited space can inhibit the leaching of nano-sized metal oxides at low pH value and improve the application range of adsorbents.In this study,hard template method was used to synthesize mesoporous carbon CMK-3 and nitrogen-doped mesoporous carbon MCN.Zirconium oxychloride and lanthanum nitrate were impregnated on CMK-3 and MCN by the wet impregnation method,and ZrO2@CMK,3,ZrO2@MCN,La203@CMK-3 and La2O3@MCN composite adsorbents were obtained by the vapor deposition method.In the preparation process,by regulating the dosage of zirconium chloride and lanthanum nitrate,the composite adsorbents with low,medium and high(5%,10%and 20%)loadings were prepared.For comparison,ZrO2/La2O 3@AC composite adsorbent with a corresponding proportion of ZrO2/La2O3 loading on activated carbon(AC)was also prepared.The structure and surface characteristics of zirconium-/lanthanum-modified porous carbon matrix composites were studied by means of XRD,TEM,N2 adsorpt-desorption and Zeta potential determination,etc.The adsorption properties of phosphate in water before and after zirconium/lanthanum loading in the three carbon materials were systematically investigated.The characterization results showed that MCN and CMK-3 synthesized with SB A-15 as the hard template successfully replicated the two-dimensional hexagon mesoporous structure of SBA-15,and the carbon matrix composites after zirconium/lanthanum addition basically maintained such structural characteristics.The specific surface area and pore volume decreased after ZrO2/La2O3 loading on CMK-3/MCN,which may be related to the limitation of ZrO2/La2O3 in the CMK-3/MCN mesoporous pore channel.The adsorption mechanism of zirconium/lanthanium-modified porous carbon-based materials on phosphate was discussed,and the effects of pH and ionic strength on phosphate adsorption were studied.CMK-3/MCN presented a low phosphate adsorption,while the adsorption capacity greatly increased after the loading of ZrO2/La2O3,which was positively correlated with the loading amounts.La2O3(20wt.%)@MCN showed the highest adsorption capacity with 41.8mg/g.The significant increase of phosphate adsorption capacity by loading ZrO2/La2O3 on mesoporous CMK-3 and MCN was related to the increase of isoelectric point,indicating that the adsorption mechanism was mainly electrostatic attraction and coordination exchange.The adsorption equilibrium time of phosphate on ZrO2/La2O3(10wt.%)@MCN was about 110 min,which was faster than that of ZrO2/La2O3(10wt.%)@CMK-3 and ZrO2/La2O3(10wt.%)@AC.When the loading ratio of ZrO2/La2O3 increased from 5 to 20 wt%,the normalized adsorption amounts decreased significantly,indicating that the active species were effectively dispersed at low loadings.The fitting results show that the adsorption kinetics followed the pseudo-second-order model,and the Freundlich and Langmuir models can describe the adsorption isotherms of phosphate adsorption on zirconia and lanthanum oxide-modified porous carbon-based adsorbents respectively.The adsorption of phosphate by porous carbon based materials after zirconium/lanthanum loading was mainly controlled by pH.When pH was below the isoelectric point of the materials,the adsorption performance increased with the decrease of pH,and the adsorption capacity significantly decreased under strong alkaline conditions.Except that the adsorption effect of lanthanum on phosphate was significantly inhibited by SO42-,other materials were less affected by Cl-,NO3-and SO42-.In general,the porous carbon matrixs modified by lanthanum oxide have higher adsorption capacity under same conditions,maintain the adsorption stability in a wider pH range,shorten the adsorption equilibrium time and have better quasi-secondary kinetic fitting results.The results further indicats that the porous carbon materials modified with lanthanum oxide are more ideal phosphate adsorbents.In addition,carbon based materials modified by lanthanum with low loading capacities have higher adsorption efficiency and can be used as an efficient adsorbent for phosphate removal. |
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