The Adsorption Of Organic Pollutants Affected By The Interaction Between Adsorbents And Solvents

The specific surface area(SSA)refers to the total surface area per unit mass of materials.It is a basic property and closely associated with other physical and chemical properties,including the cation exchange capacity,organic matter content,porosity and geotechnical characteristics.In addition,many applications of particular materials are dependent on their SSA,such as paint covering power,the activity of catalysts,food taste and texture,bioavailability and drug toxicity.Thus,the SSA has been measured frequently to determine some properties of the substance.Gas adsorption method is currently the most widely used for finely divided and porous solids and the most common form of this technique is the BET method.But this method is only used for the determination of dry powders and requests samples to be initially degassed to remove material adsorbed on the particle surface,and the key is that nitrogen(N2)gas is adsorbed on a sample kept at liquid N2 temperature at a series of different pressures,the entire process takes several hours.In fact,both in the natural environment and in laboratory-scaled simulation,a vast majority of samples systems make initially in aqueous phase.Many substances on dry condition may be completely different from that in liquid state.Hence,the SSA of materials in the dry state which obtained via conventional BET method to explain and predict the behavior of environmental contaminants in aqueous phase must exist deviation,also,it cannot provide effective SSA information in aqueous phase,making the dry powder area measurement meaningless,especially for those who are created directly by a condensation method.Nuclear magnetic resonance(NMR)solvent relaxation measurement breaks some limitations of current traditional techniques who can reflect the available wetted particle SSA through the change of relaxation rate of solvent molecules in aqueous environment.This article mainly conducted a series of batch adsorption experiments and NMR solvent relaxation determination in 5 kinds of solvents respectively whose polarity get weaker successively,by using different sized hydroxylated and dehydroxylated nanosilica.Compared with the traditional BET method with the batch adsorption experiments,then having a better understanding of the difference between the gas adsorption method and aqueous adsorption measurement.Further to put forward that the NMR solvent relaxation technique has an obvious advantage in the measurement of the SSA in aqueous phase.In this thesis,the major findings and conclusion of the study were as follows:(1)Removal of nanosilica hydroxyl groups is comparable,feasible and effective;(2)After dehydroxylation,the primary particle size barely changes,but the clusters of primary particles aggregate seriously after annealing;(3)Hydroxylated particles possess a larger BET surface area than the same dehydroxylated ones.The hydroxylated BET surface area increases with the particle enlarges whilest the dehydroxylated BET surface area increases with the particle lessens;(4)For dehydroxylated silica surface,only partial rehydroxylation takes place in polar solvents,that can not reach the degree before dehydroxylation;(5)The number of surface OH groups(i.e.,the interaction between OH groups and solvent molecules)appears to dominate the relaxation rate in polar media and particle size(surface area)becomes the secondary element.However,in nonpolar organic solvents who possessed a stronger nonpolarity and a suitable conformation can easily wedge into the groove and interstitial areas,say n-hexane,the relative exposed surface area can be acquired accurately.(6)The batch adsorption of phenol and β-naphthol experiments,to some extent,are valid for the determination of NMR solvent relaxation methods in reliability and accuracy of the surface area.In summary,the determination of surface area via NMR solvent relaxation technique in polar solvent is remarkably affected by the presence of surface OH groups.A better understanding of surface area in gas or aqueous environment should be obtained unless a complete removal of OH groups by making use of effective method.In polar solvents,even if the elimination of hydroxyl groups,partial rehydroxylation still take place.As a result,the measurement results are not necessarily reliable whether dehydroxylation or not.Nevertheless,in nonpolar solvents,especially in a certain solvent,it is possible to obtain an accurate result of the relative surface area in comparison with hydroxylation and dihydroxylation,A combination of different appropriate techniques to measure the specific surface area enables us to get more accurate and useful information about the specific surface area.