Adsorption Behavior Of Uranium (Ⅵ) By Synthetic And Natural Colloids

The removal of uranium(Ⅵ)in liquid phase by adsorption is a simple,efficient and less secondary pollution method.In this paper,the sorption properties of synthetic iron oxide nanomaterials and natural clay Gaomiaozi bentonite colloids(GMZ)are described.The static adsorption experiments of uranium(Ⅵ)were carried out under acidic and neutral pH conditions and the adsorption mechanisms were analyzed at macro-and micro-scales by various characterization methods.The behavioral characteristics of uranium(Ⅵ)and adsorbent were estimated by analyzing the adsorption mechanism(s).The behavioral characteristics of uranium(Ⅵ)and adsorbent materials were studied,with the aim to provide scientific database and guide for removing uranium(Ⅵ)from aqueous media.The research content and conclusions of the work are as follows:(1)The size and zeta potential of the synthesized nanoparticles before and after adsorption and of the extracted Gaomiaozi colloidal particles were characterized by dynamic light scattering.Before adsorption,the bare nanoparticle dynamic diameter was about 14.4 nm and the Zeta potential+27 mV.After modification with polyacrylic acid,the diameter was found to be 23.06 nm and the potential-12.95 mV.The size of GMZ bentonite colloidal particles were 300-500 nm,and the potential-31.84 mV.All of the particles formed stable colloids.After adsorption,the absolute values of the Zeta potentials decreased to some extent,an indicating that the electrostatic equilibrium on the surface of the particles was destroyed by the adsorption process(es).(2)Field emission scanning electron microscopy(FE-SEM)and transmission electron microscopy(TEM)were used to characterize the morphology changes of nanoparticles before and after adsorption.Under acidic pH conditions,agglomeration occured after adsorption of bare particles.Under neutral conditions,uranyl ions were hydrolyzed to form a plate-like hydroxide with the size about 20-50nm,which formed a larger aggregate with the encapsulated nanoparticles.This result was mutually confirmed by the decrease in the absolute value of the Zeta potential.(3)The microstructure and aggregation morphology of the nanoparticles and GMZ were studied by small angle X-ray scattering(SAXS).The nanoparticles after adsorption of uranium(Ⅵ)showed obvious increase in scattering intensities at the low q,an indication that the particles agglomerated.The same behavior also appeared in the scattering measurements obtained from a suspension of the GMZ colloids.When combined with the results of the Zeta potential and electron microscopy studies,it can be inferred that uranium(Ⅵ)destroyed the surface electrostatic equilibrium of the material,and caused the agglomeration of the particles.The in-situ small-angle scattering measurements recorded for GMZ colloidal suspensions at 25℃,55℃ and 85℃ showed that(i)the temperature was not responsible for the changes to the scattering curve of GMZ colloidal particles,and(ii)the scattering intensities after adsorption of uranium(Ⅵ)increased with temperature.The main changes were:(i)the absolute value of the power rate in the low q region become larger,which means that the colloidal particles agglomerated;(ii)the lower scattering intensities indicates that precipitation occurred in the colloidal system;(iii)and the characteristic peak at q=3.15 nm~-11 disappeared.The changes suggest that the addition of uranium(Ⅵ)affected the stability of the GMZ colloidal.(4)The adsorption capacity was directly reflected by the adsorption partition coefficient K_d.The maximum K_d value of the ploymer-coated nanoparticles was3.2×10~3 mL/g,significantly larger than that of the bare particles under acidic conditions,1.3×10~3 mL/g.For GMZ colloidal particles suspended in neutral pH conditions,the maximum K_d value was found to be 1.7×10~3 mL/g,i.e.,only slightly larger than 1.5×10~3 mL/g under acidic conditions,suggesting that the surface-modified iron oxide nanoparticles had selective adsorption to U(Ⅵ).The characteristics and innovations of this paper are as follows:(1)The sorption properties of colloids of(a)ultra-paramagnetic iron oxide nanoparticles and(b)Gaomiaozi bentonite,the recommended buffer/backfill material for an underground repository for high-level radioactive waste in Inner Mongolia,China were selected for uranium(Ⅵ)in water environment were studied.The research results have an intended purpose and directly related to the specific application.(2)For the first time,the X-ray small-angle scattering technique was combined with other methods to systematically investigate the colloidal behavior and interaction mechanism(s)of natural and artificial colloids and U(Ⅵ)colloids in water environment at nanometric resolution.