| Better understanding radionuclides migration is of significance to both the treatment of radioactive pollution and the disposal of radioactive waste.In recent years,many field studies revealed that the contribution of ubiquitous environmental colloids to the migration of radionuclides is important.However,investigation on the migration of radionuclides in the presence of colloids is still insufficient.Few studies have focused on the migration of radionuclides in the presence of combined colloids composed of difference kinds of colloids.In this context,the transport of U(VI)in saturated porous media(quartz sand and Beishan particulate granite)were comparatively studied using gibbsite colloids and humic acid as two kinds of model colloids.The effects of velocity,pH,ionic strength,initial U(VI)concentration and humic acid concentration were studied,respectively.The transport experimental data were fitted to one-dimensional advecation-dispersion equation and the transport mechanisms were discussed.The dissertation includes the following aspacts:(1)Gibbsite colloid was prepared and characterized,and the effects of gibbsite colloid on U(VI)transport through saturated quartz column as a function of flow rates,U(VI)concentration,pH and ionic strength were studied,respectively.For individual U(VI)transport,an obvious retardation was observed and the retardation increased with U(VI)concentration decreased and pH increased.Ionic strength and flow rates had no significant effects on U(VI)retention.For individual gibbsite colloid transport,gibbsite colloid transport was seriously blocked by quartz sand and the retention of gibbsite colloid increased with increasing pH and ionic strength.For gibbsite-U(VI)system,it was found that the presence of gibbsite colloid facilitated U(VI)transport only at relatively low U(VI)concentrations,whereas,at high U(VI)concentration,U(VI)transport was inhibited.At high U(VI)concentration,the inhibiting effects of gibbsite colloid on U(VI)transport were enhanced with pH and ionic strength.(2)A negelible retention of HA was occurred due to the repulsion force between HA and quartz sand surface.As a result,HA can significantly facilitate U(VI)transport and the facilitating effects increased slightly with pH and ionic strength increased.(3)When both of gibbsite colloid and HA were present,it was found that,owing to electrostatic attraction,HA and gibbsite colloids associated with each other and transported synchronously regardless of U(VI)presence.Besides the impact of pH and ionic strength,whether gibbsite colloids facilitated U(VI)transport depended on HA concentration.Gibbsite colloids impeded U(VI)transport at relatively low HA concentration(≤ 5mg/L),indicating low concentration HA could not alter the inhibiting effects of gibbsite on high concentration U(VI)transport.At relatively high HA concentration(20 mg/L),however,the associated colloids showed negative zeta potential which facilitated U(VI)transport because of repulsion between simultaneously negatively charged colloids and quartz sand.Meanwhile,high concentration of HA dramatically accelerated the transport of gibbsite colloids.(4)Laboratory column experiments were carried out to study the transport and cotransport behavior of U(VI),gibbsite colloid and HA through saturated granite particle column at varying conditions.The results showed that U(VI)and colloids transport and co-transport behaviors were similar to results in saturated quartz sand column,but the retention effects of U(VI)and colloids were more obvious due to the stronger interaction between granite,U(VI)and colloid than quartz.(5)The mechanisms controlling the binary and ternary co-transport processes were confirmed by analyzing the zeta potentials and size distributions of colloids in influent and representative effluents.The hydrodynamic diameters in the effluents increased dramatically in the initial several PVs,and then decreased gradually until to reach their initial size.Correspondingly,the absolute magnitude of the zeta potentials displayed an opposite trend.These evolution trends of effluent particle size can be explained by the colloid size exclusion effect,that is,the contact efficiency decreased with increasing particle size.Consequently,colloids with bigger particles were expected to have a slower deposition rate than the smaller ones and prior passed through the column.Thus,the average sizes of effluent colloids are expected to increase in the initial several PVs then decrease gradually with the depletion of deposition sites in the porous media,which is likely induced by granite surface charge heterogeneity and/or surface roughness.(6)In this work,the one-dimensional form of the convection-dispersion equation modified by the two-site non-equilibrium sorption model and two kinetic retention sites model were used to simulate U(VI)and colloid transport and retention using STANMOD and HYDRUS-1D,respectively.The parameters showed that two-site nonequilibrium sorption model and two kinetic retention sites model can well described U(VI)and gibbsite transport through saturated porous media.For U(VI)associated colloid system,to date,there is no generally accepted model to predict the cotransport behavior of both U(VI)and colloid accurately.Thus,in this work,we did not attempt to develop a model to identify the specific retention mechanisms of HA colloids on U(VI)transport without sufficient experimental evidence.More attention will to be paid into this area in our future research. |
