Study On Effect Of Clay Mineral Colloid On The Environmental Behavior Of Pb

Irrigation water and soil contamination is getting worse, in order to reveal the migration machanism and environmental behavior of of heavy metal pollutants in soil with different colloidal systems, we investigated the mechanism and related theory of inorganic colloids and colloids bounded contaminants transport in porous media, which will benefit the understanding of groundwater pollution and improve the research and application on wastewater treatment. Most scitensists have achieved a lot of successfull results on the research of contaminants migration in soil media, but much work need to be studied and further improved. Based on the past literature, most work focused on the trasnport of contaminant with a single heavy metal ion in the soil media, while the research of the transport mechanism of complexes consisted of heavy metals and inorganic colloids, and the complex contaminants which have reactive characters with the migration media is inadequate. Based on the develement of discipline at home and abroad and the crtitical need for objective reality, this study selected kaolinite and montmorillonite as the main experimental material, which are two different structure colloidal clay minerals in common types soil. Using laboratory experiments and mathematical model simulation, based on the study of transport mechanism of the two inorganic colloids and Pb in porous media, and the interaction of inorganic colloid and Pb, this study focused on the differentiation and migration mechanism of complexs transport in porous media, which consisted of inorganic mineral colloid and Pb, and explored the remediation principle and technology for heavy metal pollutants. This thesis achieved the following major conclusions:(1) The characters of media, colloid type and pore water physical and chemical conditions will impact the transport of clay mineral colloids in saturated porous media. The difference surface characteristics of quartz sand (washed sand and baked sand) results the different absorption force to colloidal clay mineral in the transport process, which is the external factors affecting colloid transport. The difference of clay minaral colloidal structure and significant difference of particle size determines the reaction of adsorption and deposition during the colloidal clay mineral migration, which is the internal factor affecting colloid transport. The change of shear stress generated from the varity of pore water velocity (colloid transport velocity) will affect the colloidal mobility, also pH and ionic strength (IS) of solution will strongly affect the surface charge andξpotential of colloidal clay minerals, which is the environmental conditions impacting the colloid transport, and the effect was significantlly difference,(2) All the different types of clay mineral colloids have some adsorption on Pb2+, but the adsorption mechanism and characteristics are significantly different, the adsorption of Pb2+ on kaolinite is mainly saturation adsorption of exchange mechanism, though the montmorillonite is unsaturated adsorption of ion exchange mechanism. pH is an important factor to impact heavy metal ions adsorption and desorption in colloidal suspension, different clay mineral colloids have different reaction with solution pH. The solution pH has greater impact to the adsorption of Pb2+ on kaolinite which mainly has pH variable charge, but which has relatively small impact to the adsorption of Pb2+ on montmorillonite which mainly has permanent negative charge. Ionic strength also is an important affect factor to the adsorption and desorption of colloid and heavy metals, in different ionic strength, the desorption process of adsorbed Pb2+ in different types clay mineral colloids surface was distributed to different stage over time, the first stage is initial fast desorption, followed with a slow desorption process, and as the increasing of ionic strength, the desorption quantity of adsorbed Pb2+on kaolinite gradually increasing. But montmorillonite colloid will inhibit the desorption of adsorbed Pb2+ when the ionic strength reach a certain concentration, and at the same ionic strength concentration, the desorption quantity of Pb2+ is far less than kaolinite, which further show the adsorption mechanism of two different types colloidal clay minerals have different mechanism to adsorpt heavy metals. The characteristics of Pb adsorption-desorption on the clay mineral colloids further showed, when the low concentration of Pb2+ got into the soil, the Pb2+ would immediately be attracted and stabliablized, and the Pb2+ activity decreased. However, when high concentration Pb2+ got into soil, because of soil adsorption, the Pb2+ accumulated in the soil and saturated eventually. The changing of external environment, such as rainfall, irrigation and fertilization, will lead to soil colloids adsorbed Pb2+ desorption, which release a greater ecological risk. Therefore, only the understanding of mechanism of lead migration in soil was improve, that the soil lead pollution could be controlled.(3) Compared with the pure colloid transport, the Pb transport in saturated porous media was more complicated, including static and dynamic reaction, resulting abundant Pb stored in the porous media in the migration process. Such a strong trend of Pb combining with other materials and low movability in porous media is the main reason of Pb contamination in soil.Clay mineral colloids can accelerate the migration of Pb in porous media, resulting the Pb leach out ahead of schedule. Which proved that natural colloid can be a "carrier" to accelerate the migration of heavy metals. Although the input material was colloid adsorbed Pb, but the transport of colloidal clay minerals and lead in porous media was not synchronized absolutely, which showed that colloidal clay minerals and Pb migrated as a two-reaction complex, furthermore Pb would always redistribute between inorganic colloids and the media in the migration process. Thus, when describing the migration characteristics of complex of colloids and heavy metal pollutants in porous media, the two indicators, "migration rate" whose indicator was material detection time, and "migration ability" whose indicator was detection quantity, were indispensable to characterize the migration of heavy metal ions in the case of a transport carrier.(4) As colloid and lead facilitated transport, when clay mineral colloids transported in Pb contaminated porous media, the correlation of colloid and Pb is much higher than the correlation of colloid and complex. The main reason is that in the two different situations, the peocess and direction of adsorption and desorption between Pb and colloid are different, and the ability of Pb carrying and migration is different, thus there are "adsorption carrying" and "desorption retardation" phenomenas. "Adsorption carrying" is the process that the colloid is added to the contaminated media, which erased the pollutants via adsorption carrying. "Desorption retardation" is the process that when the complex of colloid and pollutants was added into the porous media, contaminants desorbed from colloids, which was retarded in porous media, so the porous media was cotaminatinated. Fot the two cases mentioned above, they can be named as "adsorption and desorption process effect of colloids on pollutants".(5) Colloids deposited in porous media can be release and remediation via increasing the pore water velocity, reducing the ionic concentration, leaching with diluted alkali solution and other methods. When the soil has a high ionic strength, no matter it was irrigated with sewage or other reasons, then in the ionic strength decreasing process, the colloidal substances in soil will release produce incremental release, which will inhance the migration of contaminants adsorbed on soil colloids, which may threaten deep soil and groundwater. The harm to the environment can be reduced via artificially adjusting the ionic strength, such as artificial irrigation and other management method.(6) When dealing with the data of Pb adsorption and desorption on colloidal clay mineral experiments and transport of clay minerals and colloids in porous media experiments, the data resulting from Langmuir model and the HYDRUS-1D software has significant correlation.