| 其他摘要 | In recent decades, the studies of REE distribution and behavior are the necessary parts of trace element geochemistry, because REE constrain the sources of materials and their evolution processes in many fields of earth science, furthermore, REE are sometimes used as chemical hornologues for radionuclide (i.e. Am +, Cm +, and Cr+). Although extensive experimental studies have been carried out on the control of particle/water interaction on the distribution of the REE in the aquatic environment. Several studies now are available concerning the REE adsorption onto colloid or particulate material, moreover, the present studies failed to discuss the: effect of solution chemistry (e. g. pH, ionic strength, ligand species, solid/solution ratio) and natural organic material. It is still short of experimental basis about the variety of REE and other minor elements concentration in aquatic system. We carried out the research on the batch adsorption experiments of REE on kaolin from SuZhou and American Clay Minerals Society(Kga-lb) and montmorillonite from American Clay Minerals Society(Swy-2), considering the effect of solution chemistry(e. g. pH, ionic strength, solid/solution ratio), ligand (Cl- ClO4-, SO42-, HCO3-) and Fluka humic acid, and discussing the interaction of HA and clay and the REE speciation in the presence and absence of humic acid. The important conclusions have been summarized as follows: 1 The reproductive experiments results of REE distribution in the SuZhou kaolin/water interface were: the most value of the relative standard deviations of distribution coefficient (D) was 8.4% for Eu, others were between 5.0 and 6.6. The most value of the relative standard deviations of adsorption ratio (Rd) was 2.2% for Dy, most of others were under 2%. The little error means that we can use this experiment method to study REE distribution and fractionation in the water/clay interface. 2, The kinetic experimental results of REE adsorption on the SuZhou kaolin showed: REE were adsorbed onto the kaolin in short time (several minutes), then the D for REE varied due to the layer structure of kaolin and exchange beiween the metals among the clay layers and REE. The reaction reach a "steady state" after 20h. Therefore, we performed our experiments in a 24-h reaction time. 3 Langmuir isotherm can model the REE adsorption on SuZhou kaolin at pH 4.5 and 6.5. At the same time, MINEQL+ software can describe the Langmuir isotherm. It is consistent with electrostatic adsorption that the amount of adsorbate required to rbrm a monalyer on available surface sites was bigger at pH 6.5 than at pH 4.5. REE fra-ctionation occured at different REE concentration. The trend is REE is insignificant in hi^h REE concentration. 4- REE adsorption onto SuZhou kaolin in the presence of different Ligands at near neutral showed: Rd for REE decreased as ligand concentration (Cl\ C1O4\ SO42') increased, especially the effect of SO42" was the most, which reflect the effect of Na+ mass action and complexation of anion. Fractionation between LREE and HGREE were more and more significant, La/Yfo=0.14-0.96, as ligand concentration increased due to the difference of the complexation of ligand and REE. Generally, more HREE were absorbed with increasing ligand concentration, especially for Cl- and SQ42+. Although HCO3+ can form stable complex with REE., the effect of HCCV concentration on REE distribution was low maybe dwe to the low HCCV concentration (<0.0025mol/L). 5. In the different pH arid the present of different ligand, REE speciation distribution are different, which would influence REE distribution in the water/particulate interface. We researched on the R￡E speciation distribution using MINEQL+ model and considering the complexation of i*EE with hydrate, carbonate and humate. The results showed there were great effect of .HA on REE speciation distribution: at the absence of HA, REECCV' become predominant between pH7 and 8, REE3+ and REE(CO3)2" predominat at lower and higher pH separat Y- However, in the presence of HA, the results are very different, it would be expected tLSat REEHA species will predominate at pH 3-9, with REE3+ and .REE(CO3)2- predominat at + lower and Ugher pH separately. 6- The adsorption of HA onto clays occurred over a wide pH range, which suggests that in most natural aquatic systems containing HA, mineral surfaces will be coated with HA. Increasing the pH resulted in a decrease in the adsorption of HA onto minerals, which reflect the specific binding by ligand exchange or surface cornplexation reactions, there may also be possible contributions from other effects such as hydrophobicity, solubility of humic material and HA conformational changes. HA concentration, mineral surface and ionic strength also have effects on HA adsorption, and these will determine, along with solution chemistry, whether complete surface coverage of the clay by HA is achieved. As a result, the presence of HA would influence on the REE adsorption onto clay. 7 The effects of solution chemistry on REE adsorption onto the clay (Kga-lb and SWy-2) were as follows: REE adsorption onto the bare kaolin was shown to low pH dependent expect for low pH, which are mainly consistent with electrostatic and ionic exchange; the trend of REE adsorption on montmorillonite decrease with increasing pH, which shows the main mechanism is exchange reactiion. REE adsorption on the kaolin and montmorillonite decreased With increasing ionic strength, the Na mass action may be important. 8. HA coatings on the minerals were able to confer a different characteristic on the clay surface. In the presence of HA, REE adsorption reflect the distribution of REEHA and HA in the solid/water interface, the electrostatic and other mechanisms maybe have effects on the adsorption. The presence of HA caused a enhancement of adsorption of REE in kaolin system at low pH (<4) and a decrease in adsorption at intermediate and high pH. But the presence of HA decreased significantly adsorption of REE in montmorillonite system in the whole pH range (3-10). REE adsorption onto kaolin increased and then decreased with increasing HA concentration. However, in the montmorillonite system, REE adsorptin decreased exponently to several percent at low HA concentration (<5mg/L), and have no significant change at high HA'1 concentration, with increasing HA. The results presented here have shown that the presence of organic material generally lower trace metals adsorption onto particulate and increase the solubility of trace metals in the aquatic environment, therefore, improve the migration of them in long distance. 9. The REE fractionation during adsorption/desorption in both the absence and presence of HA was not remarkable as function of pH or ionic strength, but occurs as function of HA concentration due to difference between complexation of humic substance and mineral with REE series. In general, high concentration of HA in the solution increases the adsorption of light REE onto the kaolin/montmoriilonite. The results are experimental verification of a process that may generate the enrichment of ligh REE in the particulate phase in the presence of humic acid, which is consistence with the fractionation of REE in most of natural waters. |
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