Five systems consisting of different sorbate-sorbent combinations were studied using experimental volumetric adsorption techniques. Multiple adsorption isotherms were measured at low temperatures and low pressures for all of the systems studied which included CO2 adsorption on single walled carbon nanotubes (CO2 – SWCNT), Ethane adsorption on closed carbon nanohorns (Ethane-cNH), Ar adsorption on open carbon nanohorns (Ar – oNH), CO2 adsorption on zeolitic imidazolate framework-8 (CO2 – ZIF-8), and O2 adsorption on ZIF-8 (O2 – ZIF-8). Each of these systems offers a unique study of the relationship between the physical properties of the adsorbate and substrate and the effects of these properties on the thermodynamics and kinetics of adsorption. In addition to being of fundamental interest, the thermodynamics and kinetics of adsorption are important to understand for practical considerations in research fields such as gas storage and gas separation via adsorption processes, among other applications. CO2 – SWCNT is a system with a small linear molecular adsorbate with a permanent quadrupole moment adsorbing on a substrate with quasi-1D grooves and convex outer adsorption sites. Ethane-cNH is a system with a linear alkane adsorbing on a substrate with conical pores and convex outer adsorption sites. Ar – oNH is a system with a spherical atom sorbing in a substrate with two different groups of conical adsorption sites and both convex and concave surface sites. CO2 – ZIF-8 and O2 – ZIF-8 are both systems with small linear molecules sorbing in a flexible microporous scaffold-like substrate. Adsorption isotherms were analyzed to identify features corresponding to adsorbate-adsorbate and adsorbate-substrate interactions. Namely, the presence of substeps in the semi-logarithmic data were identified and interpreted to correspond to groups of adsorption sites of similar binding energy which likely depend on the morphology and/or structural flexibility of the substrates. All of the systems, with the exception of CO2 - SWCNTs, yielded at least some isotherms with substeps at pressures below that corresponding to saturation. Effective specific surface areas for all adsorbent-substrate combinations were calculated using the BET and Point-B methods for the sake of comparison. These surface area measurements are very dependent on the porosity and morphology of the substrate as well as the size and shape of the adsorbate atoms/molecules and therefore the values vary greatly between the different systems. The isosteric heat of adsorption was calculated using isotherms over the full range of temperatures for each system. A variant of the Clausius-Clapeyron equation was used for this purpose and the results were analyzed based on adsorbate-adsorbate and adsorbate-substrate interactions. Plateaus in the isosteric heat data for Ethane – cNH and Ar – oNH were related to the morphology of the substrates and properties of the adsorbate species. For CO2 – SWCNTs, the isosteric heat at all but the lowest coverages is below the latent heat of deposition. This is due to the quadrupole moment of CO2. For both of the studies of adsorption on ZIF-8, the isosteric heat contains peaks in the data which likely are the result of the flexibility of the ZIF-8 structure. The kinetics of adsorption (or, the rates at which the adsorption systems approach equilibrium) were analyzed as functions of isotherm temperature and coverage, vapor pressure, and fractional uptake point by point along individual isotherms using the linear driving force model. Certain trends in the kinetics of adsorption are consistent for all the systems studied and others vary depending on the specific adsorbate-substrate combination. As with the thermodynamic results, trends in the kinetics of adsorption are discussed in terms of the effects of adsorbate-adsorbate and adsorbate-substrate interactions.

Στην εργασία αυτή μελετήθηκε η ρόφηση και η εκρόφηση του παρασιτοκτόνου ατραζίνη (atrazine) σε πρότυπες εδαφικές ουσίες καθώς και σε ένα τυπικό δείγμα εδάφους, τόσο σε αντιδραστήρες διαλείποντος έργου (batch) όσο και σε πληρωμένες κλίνες (bed). Ως πρότυπες ουσίες επιλέχθηκαν η πυριτία (silica-gel, SiO2), η αλούμινα (γ-alumina, Al2O3), το ανθρακικό ασβέστιο (calcite, CaCO3) και το χουμικό οξύ (humic acid). Η ατραζίνη ροφείται σημαντικά στο χουμικό οξύ στην πυριτία και στο έδαφος, όμως βρέθηκε ότι έχει πολύ μεγαλύτερη συγγένεια για το χουμικό οξύ σε σύγκριση με την πυριτία. Η ατραζίνη δεν έδειξε να ροφείται στη γ-αλούμινα και το CaCO3, ανεξαρτήτως από τις πειραματικές συνθήκες που χρησιμοποιήθηκαν. Σε όλες τις περιπτώσεις, η κινητική μελέτη έδειξε δύο διακριτά στάδια: ένα πρώτο γρήγορο στάδιο ρόφησης της ατραζίνης, διαδεχόμενο από ένα δεύτερο πιο αργό στάδιο. Η κινητική της ρόφησης υπακούει ικανοποιητικά στο μοντέλο Elovich. Στα χρονικά πλαίσια που μελετήθηκε η ρόφηση και η εκρόφηση, η διεργασία είναι αντιστρεπτή και η ατραζίνη εκροφείται ποσοτικά. Οι ισόθερμοι ρόφησης της ατραζίνης λήφθηκαν σε διαφορετικές τιμές ιοντικής ισχύος, pH και θερμοκρασίας και υπακούουν στο μοντέλο Freundlich. Σε όλες τις περιπτώσεις που μελετήθηκαν, η αύξηση της ιοντικής ισχύος του διαλύματος προκάλεσε αύξηση της ροφημένης ποσότητας της ατραζίνης. Η ρόφηση της ατραζίνης μειώνεται καθώς το pH του διαλύματος αυξάνεται. Όσον αφορά στην πυριτία, η ρόφηση της ατραζίνης φαίνεται ότι γίνεται κυρίως μέσω δεσμών υδρογόνου με τις υδροξυλομάδες της επιφάνειας. Όσον αφορά στο χουμικό οξύ, το σημαντικότερο ρόλο στη ρόφηση παίζει η διάχυση της ατραζίνης προς το εσωτερικό του στερεού όπου η ρόφηση επιτυγχάνεται κυρίως μέσω υδρόφοβων αλληλεπιδράσεων. Η προσρόφηση της ατραζίνης στην πυριτία αυξάνεται σημαντικά καθώς αυξάνεται η θερμοκρασία του διαλύματος, μια τάση που δεν παρατηρείται κατά τη ρόφηση της ατραζίνης στο χουμικό οξύ. Η θερμοδυναμική ανάλυση έδειξε ότι η ρόφηση στα μοντέλα εδάφους είναι φυσική αφού παρατηρήθηκαν τιμές της ενθαλπίας ρόφησης στην περιοχή των 10 kJ mol-1. Οι λαμβανόμενες ισόθερμοι ρόφησης από τα πειράματα σε πληρωμένες κλίνες ήταν σε καλή συμφωνία με αυτές που λήφθηκαν από τα πειράματα στους αντιδραστήρες διαλείποντος έργου. Η παρουσία χουμικού οξέος στις κλίνες προκαλεί με το χρόνο δραστική μείωση της διαπερατότητας. / Adsorption and desorption of the herbicide atrazine was investigated on the principal inorganic constituents of soil, as well as on a typical Greek soil sample. The studies were conducted both in batch, stirred reactors and in packed beds. Silica-gel (SiO2), γ-alumina (Al2O3) and calcite (CaCO3), were selected as model inorganic substances. Humic acid was selected as a model substance representative of the organic part of soil. Significant adsorption of atrazine was measured on the humic acid, silica and on the soil suspensions in electrolyte solutions. Atrazine exhibited higher affinity for humic acid rather than for silica. Atrazine did not adsorb on γ-alumina and on CaCO3 regardless the experimental conditions (pH range or total solid available for adsorption). In all cases, kinetic results have shown two distinct features: a first, fast sorption step, followed by a second, slow step. The kinetics data of atrazine uptake on both substrates yielded satisfactory fit to the Elovich model. Atrazine was found to be completely desorbed from both the humic acid and the silica substrates. Adsorption isotherms for atrazine were obtained at different values of ionic strength, pH and temperature. The adsorption data gave the best fit to the Freundlich model. In all cases investigated, the amount of adsorbed atrazine increased upon increasing the ionic strength of the solution. The adsorption of atrazine decreased with increasing solution pH. The adsorption of atrazine on silica was primarily dominated by the formation of hydrogen bonds with the surface hydroxyl groups. In the case of humic acid, the diffusion of atrazine to the interior of the solid seemed to play the most significant role. Inside the organic substance particles, sorption took place mainly through hydrophobic interactions. The sorption of atrazine on silica surface increased considerably with increasing temperature, a trend not found in the case of humic acid. The thermodynamic analysis yielded adsorption energy values of the order of 10 kJ mol-1 suggesting physical adsorption. The isotherms obtained from the packed bed experiments were in a good agreement with those obtained from batch experiments. Finally, humic acid grains, mixed with silica in packed beds, were found to change morphology upon hydration which resulted to swelling. The humic substances clogged a large portion of the pores of the packed beds, decreasing drastically their permeability.