Επεξεργασία πόσιμου νερού με χρήση διεργασιών προσρόφησης
Authors: Κεκές Τρύφων 
Published: March 2022

Water is an essential element of human life, with safe and adequate access to clean and potable water being recognized as an inalienable human right. However, water can be contaminated by a variety of contaminants rendering it unsafe for human consumption. Therefore, various methods of water purification have been developed and applied in order to remove contaminants from it.In the present dissertation, three different water contaminants were selected and their removal from water was investigated by means of adsorption techniques. In particular, indigo carmine, hexavalent chromium and paracetamol were selected as target contaminants, as they represent three different categories of contaminants (organic compounds, heavy metals and emerging contaminants, respectively) in order to study in detail the efficacy of adsorption towards the removal of water contaminants. Adsorption is a very attractive method of water purification, as it is environmentally friendly and shows great efficiency in a wide range of contaminants. In addition, it exhibits various other advantages, such as the low operating costs required, its high efficiency even in mild pH and temperature conditions and the ability to regenerate the adsorbing agents in order to reuse them. Therefore, the adsorption processes and the investigation of the possibilities of the adsorbents already used along with the further research and development of innovative adsorbing agents are in the center of interest regarding water purification.Specifically in the present dissertation, four innovative adsorbents based on chitosan and β-cyclodextrin were synthesized (chitosan and chitosan/β-cyclodextrin beads and chitosan/cerium and chitosan/β-cyclodextrin/cerium granules) due to their high efficacy as potential adsorbents. For the synthesis of the adsorbing agents, sodium tripolyphosphate was used as cross-linking agent in the beads and glutaraldehyde in the granules. In addition, manganese dioxide was studied as adsorbent due to its ability to simultaneously adsorb and oxidize water contaminants, as well as a zirconium-based metal organic framework due to the ever-increasing interest in the use of such materials.Initially, the porosity of the chitosan and chitosan/β-cyclodextrin beads was measured equal to 88.3 ± 0.1% and 94.7 ± 0.2%, respectively, while the porosity of the chitosan/cerium and chitosan/β-cyclodextrin/cerium granules was 91.1 ± 0.3% and 95.6 ± 0.3%, respectively. Fourier-Transform Infrared Spectroscopy (FTIR) analyses for all four innovative adsorbents synthesized in the present dissertation confirmed the successful crosslinking of chitosan and β-cyclodextrin with the chemical agents used as ligands. In addition, Scanning Electron Microscope (SEM) analyses indicated that the surface of the chitosan and chitosan/β-cyclodextrin beads consists of a rough and irregular polymer network with irregular pores, while the surface of the chitosan/cerium and chitosan/β-cyclodextrin/cerium granules is also rough and displays various cavities, in which adsorption can occur. Also, X-Ray Diffraction (XRD) analysis of chitosan/cerium and chitosan/β-cyclodextrin/cerium granules confirmed the successful coupling of the materials. All adsorption experiments of the different contaminants were performed by conducting batch experiments and various conditions (initial concentration of contaminant, adsorbent concentration, pH and temperature) were examined to investigate their effect on the process.The experimental study of the adsorption of Indigo Carmine using chitosan and chitosan/β-cyclodextrin beads (1st experimental series) was performed at different Indigo Carmine concentrations (50-200 mg/L), adsorbent concentrations (100-1000 mg/L), pH (3.0-6.0) and temperature values (15-50oC). According to the specific experimental study, chitosan/β-cyclodextrin beads displayed higher adsorption capacity, exhibiting on average in all studied experimental conditions a higher adsorption efficiency (94.5%) than the chitosan beads (90.5%). Based on the results, it emerged that the optimum pH value was 4.0, while a decrease in the efficiency of the process was observed with an increase in the temperature values (from 25 to 50oC). Additionally, an increase in the concentration of the used adsorbent (700 and 1000 mg/L) led to an increase in adsorption efficiency. Adaptation of the experimental data to the adsorption isotherms showed that the Langmuir model better expresses the adsorption of Indigo Carmine onto both beads, therefore the adsorption takes place on a homogeneous surface, leading to the formation of a monolayer of Indigo Carmine on the surface of the beads. In addition, the Langmuir model constants indicated that the adsorption of Indigo Carmine was favorable onto both adsorbing agents, with the maximum theoretical adsorption capacity (qm) of chitosan/β-cyclodextrin beads being twice as much as the one of the chitosan beads (1000.0 and 500.0 mg/g, respectively). Also, the pseudo-second order kinetic model better describes the adsorption of Indigo Carmine on the chitosan and chitosan/β-cyclodextrin beads and therefore, the adsorption can be considered to be controlled by a chemical process. According to the thermodynamic analysis, the adsorption is spontaneous at all studied temperatures, and is characterized as exothermic (ΔΗ = -17.38 and -35.06 kJ/mol for chitosan and chitosan/β-cyclodextrin beads, respectively). Furthermore, based on the calculated values in the change of entropy, a reduction in the randomness at the solid/liquid interface is occurring due to the possible formation of an activated complex between Indigo Carmine and the adsorbents. The experiments regarding the regeneration of the adsorbents indicated that their efficiency in the adsorption of Indigo Carmine decreased significantly during the second regeneration cycle, while after the end of the third regeneration cycle the efficiency of the chitosan beads presented a smaller decrease (11.0 versus 15.7% of chitosan/β-cyclodextrin beads). Finally, the adsorption of Indigo Carmine onto the chitosan and chitosan/β-cyclodextrin beads was confirmed by FTIR and SEM analyses. The experimental study of the adsorption of hexavalent chromium using chitosan and chitosan/β-cyclodextrin beads (2nd experimental series) was performed at different hexavalent chromium concentrations (5-100 mg/L), adsorbent concentrations (1-20 g/L), pH (2.0-7.0) and temperature values (15-50oC). The results of this study displayed that chitosan/β-cyclodextrin beads possess a higher adsorption capacity, exhibiting on average a higher adsorption efficiency (84.6%), compared to chitosan beads (82.0%). Based on the results, the optimum pH value was equal to 4.0, while the efficiency of the process increased at a reduced temperature value (15oC) and at increased concentrations of the adsorbent (15-20 g/L). According to the experimental data, the Langmuir model best expresses the adsorption of hexavalent chromium onto both beads, and therefore the adsorption takes place on a homogeneous surface, effectively leading to the formation of a monolayer of hexavalent chromium on the surface of the beads. Moreover, from the calculated constants of the Langmuir model it is concluded that the adsorption of hexavalent chromium onto both beads is favorable , with the maximum theoretical adsorption capacity (qm) equal to 400.0 and 555.6 mg/g for chitosan and chitosan/β-cyclodextrin beads, respectively). Furthermore, the pseudo-second order kinetic model better describes the adsorption of hexavalent chromium onto the chitosan and chitosan/β-cyclodextrin beads and based on this, the adsorption can be considered to be controlled by a chemical process. Thermodynamic analysis of the process indicated that the adsorption becomes favorable at low temperatures (15-25oC) and is characterized as exothermic (ΔH = -2.99 and -3.80 kJ/mol for the chitosan and chitosan/β-cyclodextrin beads, respectively), while based on the calculated values in the change of entropy, there is a decrease in the randomness at the solid/liquid interface due to the possible formation of an activated complex between the hexavalent chromium and the adsorbents. Adsorption regeneration experiments indicated that chitosan beads exhibited a smaller reduction in adsorption efficiency compared to the chitosan/β-cyclodextrin beads, with the decrease in efficiency of the process after the end of the third regeneration cycle being 11.9 and 26.5%, respectively. Finally, the adsorption of hexavalent chromium to the chitosan and chitosan/β-cyclodextrin beads was verified by FTIR and SEM analyses.The experimental study of the adsorption of Indigo Carmine using chitosan/cerium and chitosan/β-cyclodextrin/cerium (3rd experimental series) was performed at different Indigo Carmine concentrations (50-200 mg/L), adsorbent concentrations (50-500 mg/L), pH (3.0-6.0) and temperature values (15-50oC). According to the specific experimental study, chitosan/β-cyclodextrin/cerium granules displayed higher adsorption capacity, exhibiting on average in all studied experimental conditions a higher adsorption efficiency (92.3%) than the chitosan/cerium granules (90.5%). Based on the results, the optimum pH value was 3.0, while an increase in the efficiency of the process was observed at high values of temperature (50oC) and concentration of the adsorbent (500 mg/L). In this case (use of granules), the adaptation of the experimental data to the adsorption isotherms indicated that the Freundlich model, in contrast to the beads, better expressed the adsorption of Indigo Carmine onto the two adsorbents, thus adsorption occurs on a heterogeneous surface, leading to the formation of multiple layers of Indigo Carmine onto the surface of the adsorbents. Additionally, based on the calculated constants of the Freundlich model, adsorption of Indigo Carmine onto both adsorbing agents is favorable, with the chitosan/β-cyclodextrin/cerium granules having a higher value of the KF constant (228.9 versus 196.5 (mg/g)*(l/mg)1/n of chitosan/cerium granules), thus indicating that they are more effective than the chitosan/cerium granules. Also, the pseudo-second order kinetic model better describes the adsorption of Indigo Carmine, therefore adsorption can be considered to be controlled by a chemical process. According to the thermodynamic analysis, the adsorption is spontaneous at all studied temperature values and is characterized as exothermic (ΔΗ = -30.70 and -51.33 kJ/mol for the chitosan/cerium and chitosan/β-cyclodextrin/cerium granules, respectively), while the calculated values in the change of entropy indicate an increase in the randomness of the system. Adsorbent regeneration experiments indicated that their efficacy towards the adsorption of Indigo Carmine decreased significantly during the second regeneration cycle, with the efficiency of chitosan/cerium granules presenting a smaller decrease after the end of the third regeneration cycle (6.8 versus 8.0% of chitosan/β-cyclodextrin/cerium granules). Finally, the adsorption of Indigo Carmine on chitosan/cerium and chitosan/β-cyclodextrin/cerium granules was confirmed by FTIR and SEM analyses. The experimental study of the adsorption of hexavalent chromium using chitosan/cerium and chitosan/β-cyclodextrin/cerium (4th experimental series) was performed at different hexavalent chromium concentrations (5-100 mg/L), adsorbent concentrations (0.10-2 g/L), pH (2.0-11.0) and temperature values (15-50oC). According to the specific experimental study, chitosan/β-cyclodextrin/cerium granules displayed higher adsorption capacity, exhibiting on average in all the studied experimental conditions a higher adsorption efficiency (82.1%) than the chitosan/cerium granules (77.1%). Based on the results it emerged that the optimum pH value was equal to 3.0, while the efficiency of the process increased at high temperature (50oC) and adsorbent concentration (1.50-2.00 g/L). Adaptation of the experimental data to the adsorption isotherms indicated that the adsorption of hexavalent chromium onto both adsorbents is better expressed by the Freundlich model, thus adsorption takes place on a heterogeneous surface, leading to the formation of multiple layers of hexavalent chromium onto the surface of the adsorbents. In addition, according to the calculated values of the Freundlich model constants, the adsorption of hexavalent chromium is favorable, with the chitosan/β-cyclodextrin/cerium granules exhibiting a higher value of the KF constant (20.8 versus 9.3 (mg/g)*(l/mg)1/n of chitosan/cerium granules), thus indicating that they are more effective than chitosan/cerium granules. Moreover, the pseudo-second order kinetic model better describes the adsorption process and based on this, it can be noted that adsorption is controlled by a chemical process. The thermodynamic analysis indicated that adsorption is spontaneous at all studied temperatures and is characterized as exothermic (ΔΗ = -17.91 and -41.19 kJ/mol for the chitosan/cerium and chitosan/β-cyclodextrin/cerium granules, respectively), while the calculated values in the change of entropy indicate an increase in the randomness of the system. According to the regeneration experiments of the adsorbents, the efficiency of chitosan/cerium granules presented a smaller decrease than that of chitosan/β-cyclodextrin/cerium granules, with a decrease in adsorption efficiency at the end of the third regeneration cycle equal to 4.9 and 13.5% for chitosan/cerium and chitosan/β-cyclodextrin/cerium granules, respectively.. Finally, the adsorption of hexavalent chromium on chitosan/cerium and chitosan/β-cyclodextrin/cerium granules was confirmed by FTIR and SEM analyses.The experimental study of the oxidation-adsorption of paracetamol (C8H9NO2) (5th experimental series) using manganese dioxide was performed at different paracetamol concentrations (25-100 mg/L), manganese dioxide concentrations (100-800 mg/L), pH (3.0-6.0) and temperature values (15-50oC). The results indicated that the removal of paracetamol from water occurs more rapidly at high concentrations of manganese dioxide (400-800 mg/L), high temperatures (35-50oC) and low pH values (3.0-3.5). According to the kinetic analysis of the process, the reaction between paracetamol and manganese dioxide has as rate-limiting step the electron transfer in strong acidic conditions (pH < 4.0), while at pH ≥ 4.0 the rate-limiting step of the process is the complex formation. Therefore, under strong acidic conditions the adsorption of paracetamol to the surface of manganese dioxide occurs at a much greater extent compared to its oxidation by it. Thermodynamic analysis of the reaction by utilizing the Arrhenius model indicated that the reaction between acetaminophen and manganese dioxide is endothermic (Ea = 37.35 kJ/mol) and an increase in temperature will result in an increase of the rate of the reaction. The analysis of acetaminophen’s transformation products formed by its reaction with manganese dioxide revealed the formation of acetaminophen’s dimer (C16H16N2O4) and trimer (C24H23N3O6) in the early stages of the reaction due to the oxidation of acetaminophen by manganese dioxide, followed by the coupling of acetaminophen’s molecules. Furthermore, following the complete removal of acetaminophen both acetaminophen’s dimer and trimer were also totally removed due to their subsequent reaction with δ-manganese dioxide. Moreover, four additional transformation products were detected, with their molecular formulas being C14H11NO4 (three isomeric compounds), attributed to aggregates of 1,4-benzoquinone and N-Acetyl-p-benzoquinone Imine, generated by the reaction of acetaminophen and δ-manganese dioxide, and C17H8N3O5, respectively.The experimental study of the oxidation-adsorption of Indigo Carmine (6th experimental series) using manganese dioxide was performed at different Indigo Carmine concentrations (50-200 mg/L), manganese dioxide concentrations (100-800 mg/L), pH (3.0-6.0) and temperature values (15-50oC). The results indicated that the removal of Indigo Carmine from water occurs more rapidly at high concentrations of manganese dioxide (400-800 mg/L), high temperature (50oC) and low pH values (3.0-3.5). According to the kinetic analysis of the process, the reaction between Indigo Carmine and manganese dioxide has as rate-limiting step the electron transfer in strong acidic conditions (pH < 3.5), while at pH ≥ 3.5 the rate-limiting step of the process is the complex formation. Thermodynamic analysis of the reaction by utilizing the Arrhenius model indicated that the reaction between indigo carmine and manganese dioxide is endothermic (Ea = 52.77 kJ/mol) and an increase in temperature will result in an increase of the rate of the reaction.The experimental study of the adsorption of Indigo Carmine using a zirconium-based metal organic framework (7th experimental series) was performed at different Indigo Carmine concentrations (10-50 mg/L), adsorbent concentrations (40-160 mg/L), pH (2.0-10.0) and temperature values (15-50oC). Based on the results, the optimum pH value was equal to 2.0, while the efficiency of the process increased at a reduced temperature value (15oC) and at an increased concentration of the adsorbent (120-160 mg/L). Adaptation of the experimental data to the adsorption isotherms indicated that the Langmuir model better expresses the adsorption process, thus adsorption takes place on a homogeneous surface, leading to the formation of a monolayer of Indigo Carmine onto the surface of the metal organic framework. Additionally, according to the calculated constants of the Langmuir model, the adsorption of Indigo Carmine is favorable and the maximum theoretical adsorption capacity (qm) of the adsorbent was 333.3 mg/g. Moreover, the pseudo-second order kinetic model better describes the adsorption process, thus it can be considered that adsorption is controlled by a chemical process. Finally, according to the thermodynamic analysis, the adsorption is spontaneous at all studied temperatures, is characterized as exothermic (ΔΗ = -52.77 kJ/mol), and based on the calculated value in the change of entropy an increase in the randomness of the system is occurring.Additionally, a study was performed based on literature review for the introduction of adsorption processes, as they were investigated in the present dissertation, during the application of HACCP (Hazard Analysis Critical Control Points) with the simultaneous adaptation of FSOs (Food Safety Objectives) in drinking water treatment plants. The analysis and evaluation of the microbiological, chemical and physical hazards of water at all stages of its treatment from raw to drinking water indicated that critical control points of the water purification process are the water source (initial level of microbiological, chemical and physical hazards) and the disinfection process (microbiological hazards elimination and non-development of chemical hazards such as disinfectant residues or disinfectant by-products), while adsorption processes are also a critical control point of the process as they are specially designed processes to remove chemical contaminants (such as: heavy metals, emerging contaminants). To conclude with, the introduction of adsorption processes as an additional step of conventional drinking water production as well as the adaptation of FSOs in the HACCP will enhance the efficiency of the water purification process and ensure the quality and safety of the final drinking water.