静态法和EXAFS技术研究Eu(III)在钛酸纳米管上的吸附行为和微观机制

Adsorption of Eu(III) on titanate nanotubes studied by a combination of batch and EXAFS technique

结合静态实验和X射线吸收精细结构谱学(EXAFS)技术研究了pH、时间、有机配体等环境因素对放射性核素Eu(III)在钛酸纳米管上的吸附行为和微观机制的影响.宏观实验结果表明:Eu(III)在钛酸纳米管上的吸附在pH<6.0条件下受离子强度影响,而在pH>6.0条件下不受离子强度影响;腐殖酸HA/FA在低pH条件下可以促进Eu(III)在钛酸纳米管上的吸附,而在高pH条件下抑制Eu(III)在钛酸纳米管上的吸附.EXAFS微观分析结果表明:在pH<6.0条件下,吸附属于外层吸附机理;在pH>6.0条件下,吸附属于内层吸附机理.pH<6.0时,中心原子Eu周围只有Eu-O一个配位层,其平均键长为2.40,配位数在9左右;随着pH逐渐升高,第一配位层的配位数下降,表明吸附Eu原子配位的对称性下降.当吸附时间延长或pH升高,吸附原子Eu周围出现了Eu-Eu和Eu-Ti第二配位层,其平均键长分别为3.60和4.40,配位数分别在2或1左右,表明形成了内层吸附产物或表面沉淀或表面多聚体.腐殖酸HA/FA的存在,可以改变Eu(III)在钛酸纳米管表面的吸附形态和微观原子结构,Eu(III)不仅可以与钛酸纳米管的表面羟基直接键合形成二元表面复合物(Eu-TNTs),还可以通过HA/FA的桥连作用形成三元表面复合物(HA/FA-Eu-TNTs).这些研究结果对于评估放射性核素Eu(III)与纳米材料在分子水平上的作用机理及分析Eu(III)在环境中的物理化学行为具有重要的意义.

The effects of pH,contact time and natural organic ligands on radionuclide Eu（III） adsorption and mechanism on titanate nanotubes（TNTs） are studied by a combination of batch and extended X-ray absorption fine structure（EXAFS） techniques.Macroscopic measurements show that the adsorption is ionic strength dependent at pH 6.0,but ionic strength independent at pH 6.0.The presence of humic acid（HA）/fulvic acid（FA） increases Eu（III） adsorption on TNTs at low pH,but reduces Eu（III） adsorption at high pH.The results of EXAFS analysis indicate that Eu（III） adsorption on TNTs is dominated by outer-sphere surface complexation at pH 6.0,whereas by inner-sphere surface complexation at pH 6.0.At pH 6.0,Eu（III） consists of？？ 9 O atoms at REu-O ？？2.40？the first coordination sphere,and a decrease in NEu-O with increasing pH indicates the introduction of more asymmetry in the first sphere of adsorbed Eu（III）.At long contact time or high pH values,the Eu（III） consists of？？2 Eu at REu-Eu ？？3.60？？？ 1 Ti at REu-Ti ？？4.40？icating the formation of inner-sphere surface complexation,surface precipitation or surface polymers.Surface adsorbed HA/FA on TNTs modifies the species of adsorbed Eu（III） as well as the local atomic structures of adsorbed Eu（III） on HA/FA-TNT hybrids.Adsorbed Eu（III） on HA/FA-TNT hybrids forms both ligand-bridging ternary surface complexes（Eu-HA/FA-TNTs） as well as surface complexes in which Eu（III） remains directly bound to TNT surface hydroxyl groups（i.e.,binary Eu-TNTs or Eu-bridging ternary surface complexes（HA/FA-Eu-TNTs））.The findings in this work are important to describe Eu（III） interaction with nanomaterials at molecular level and will help to improve the understanding of Eu（III） physicochemical behavior in the natural environment.