PhD Thesis Defence Presentations - SPYRIDON GIANNAKOPOULOS

The continuous and ever-escalating increase in the number and needs of the world population has multiplied the consumption of goods, sometimes accompanied by the excessive and reckless consumption of pharmaceutical products related to the treatment of health problems, such as antibiotics for the treatment of bacterial infections (Sulfamethoxazole, Ampicillin) and medicines to fight high blood pressure (Losartan). In addition, the modern way of life mainly of Western and economically and technologically developed societies, directly linked to the standard of today's consumer, has strengthened the excessive purchase of products in which chemical substances with a proven negative effect on the human body are detected. A typical example is Bisphenol A, found in a wide range of goods and products such as tableware, plastic bottles and sports equipment, which is responsible for causing toxicity and affecting the human hormonal system. This development triggered the ban on the incorporation of this chemical compound into a series of products (baby utensils) and its subsequent replacement by Bisphenol S. 

Due to the extremely complex chemical structure of the mentioned organic molecules, related to the existence of an aromatic structure, polycarbonate chains, a variety of heteroatoms and unsaturated bonds, their biodegradation in traditional wastewater treatment plants is most of the time insufficient as the microorganisms are unable to <<recognize>> and ultimately break down the given components completely. The almost unaffected concentration of organics at the secondary treatment influent compared to the corresponding one at the effluent contributes to the channeling of pollutants to surface receivers (rivers, sea, aquifer) whose accumulation even in concentrations of the order of μg L^-1 or ng L^-1 is able to affect not only microorganisms but also animals and humans. Alongside the ineffectiveness of existing treatment systems, the aquaculture, animal husbandry and the majority of chemical industries can exacerbate the problem through the excretion of unmetabolized pharmaceuticals by animal organisms and the incomplete degradation of the wastewater organic matter.

The criticality and gradual deterioration have sparked the interest of the scientific community on a global scale in search of alternative practices with promising results limiting the environmental footprint. One such category is the so-called Advanced Oxidation Processes (AOPs) being intensively studied in recent decades mainly on a laboratory and pilot scale, showing encouraging results by oxidizing a wide range of organic compounds detected in the aquatic environment. These practices constitute a set of chemical processes with the common denominator of the strong reactive oxygen species production and the general avoidance of causing secondary pollution. Among the most widespread and used techniques, the heterogeneous activation of mild oxidants to the formation of stronger oxidizing species, such as reactive radicals, has been at the center of attention of the scientific community receiving rave reviews in recent years due to the highly successful removal of complex and bio resistant organic molecules from the liquid phase. Materials that are mostly utilized are catalysts-activators based on metals, such as Manganese (Mn), Cobalt (Co), Nickel (Ni), Iron (Fe) etc., while recently carbonaceous materials such as biochar are in the spotlight because of their desirable and attractive physicochemical characteristics (electrical conductivity, porous structure, variety of surface functional groups, high specific surface area). It is a carbon based, stable material produced through the thermochemical treatment of dry biomass in conditions of high temperatures (350-900 ^oC) and full or limited presence of oxygen. Based on the perspective of the integration and full exploitation of biochar in Advanced Oxidation Processes, the present thesis had as its ultimate goal the synthesis, physicochemical-electrochemical characterization and utilization of biochar derived from various initial biomass in differential chemical processes aiming at the decomposition of organic micropollutants in the liquid phase. 

More in detail, the first part of the results concerns the presence of biochar in heterogeneous oxidant activation systems such as persulfate (PS) and hydrogen peroxide (H₂O₂). In the first phase, biochars were synthesized through a pyrolytic process under a limited oxygen atmosphere from spent coffee grounds under different firing temperature varying between 300 and 850 ^oC, thus investigating the possible correlation between the firing temperature effect on the catalytic activity of the given biochar. The reaction temperature, as one of the most crucial parameters of pyrolysis, contributes to the differentiation of the physicochemical properties of the sample which undoubtedly affect the activity of the solid and therefore the reaction kinetics. Capturing this change is achieved by performing characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), calculation of specific surface area and pore size distribution and estimation of catalyst point of zero charge (PZC) through potentiometric titration curves, which revealed that the scaling of the pyrolytic temperature mainly induced the stimulation of the porous structure, graphitic carbon formation, sp² hybridization and specific surface area of the material something that was also reflected in the improvement of the catalytic activity aiming at the decomposition of Sulfamethoxazole through the activation of sodium persulfate. Elevation of temperature is associated with the facilitation of contaminant and oxidant binding onto the catalytic surface and enhancement of electron transfer. A parametric analysis was then carried out examining the effect of firing temperature on the activity of the carbonaceous material under different pH conditions, in the presence of inorganic ions and organic matter found in real environmental samples and in different aqueous matrices. In all cases the higher pyrolysis temperature conduced to the faster final antibiotic conversion. The mechanism of the reaction was also investigated with reference to the sample calcined at 850 ^oC with the aim of ascertaining the produced oxidizing species and the eventual existence of the nonradical activation of persulfate. During the next stage of the study, the samples fired at the lowest and highest temperature (300 ^oC and 850 ^oC) were used in the simultaneous presence of UV-A/ simulated solar radiation in order to activate sodium persulfate to break down Sulfamethoxazole. The combined action of catalyst and light radiation proved to be quite beneficial since a synergistic effect of the individual oxidant activation processes (catalyst, radiation) was found, ensuring a rapid annihilation of the pharmaceutical from the solution. Having determined at this point the favorable thermal conversion of biomass at high temperatures for the reaction kinetics, the biochar synthesis from lemon and orange prunings was exhibited at 850 ^oC which, as in the case of coffee residues, is an abundant unwanted biomass which is utilized for environmental purposes and indeed an unrecommended threat to the ecosystem. The pruning biochars, after initially being characterized physicochemically with the aim of extracting valuable information about their composition, structure, surface morphology and surface functional groups, contributed to the heterogeneous activation of sodium persulfate and hydrogen peroxide for the successful oxidation of Sulfamethoxazole and Ampicillin, respectively. Parameters such as pollutant, catalyst, and oxidant concentration, solution pH, presence of inorganic ions and organic matter in solution, and increasing complexity of the aqueous matrix were thoroughly examined alongside oxidant consumption, reaction mechanism, and catalyst reuse.

In the second part, an attempt was made to utilize biochar in different electrochemical processes. Going deeper, bearing in mind the high catalytic activity of the biochar from lemon stalks, an attempt was made to immobilize the material on a conductive substrate with the aim of placing the carbon electrode on the anode of an electrochemical cell seeking the electrochemical oxidation of Losartan. Initially, the physicochemical characterization of the material and the electrochemical characterization took place accompanied by the analysis of parameters such as the anode material, current density intensity, initial solution pH and electrolyte type. Subsequently, the identification of the intermediate by-products of Losartan and the examination of their computational toxicity in microorganisms were completed. Moreover, wishing to correlate the electrocatalytic activity of different carbonaceous electrodes for the electrochemical oxidation of Bisphenol S with the initial biomass of the biochar (citrus fruit prunings, rice husk, spent coffee grounds), physicochemical and electrochemical characterization of the individual materials was established followed by parametric analysis, identification of intermediate products and examination of computational toxicity based on the most electrochemically active electrode. In addition, the electrochemical activation of persulfate was investigated in the presence of the same electrode placed at the cathode of an electrochemical reactor, decomposing the same pollutant, and finally, the ways of exploiting the same electrode in a photoelectrochemical cell were thoroughly studied, focusing more on the photoelectrochemical decomposition of micropollutants. 

The last two parts of the work concerned the utilization of biochar to improve the photocatalytic activity of semiconductors, mainly connected to the reduction of the charge carriers recombination rate, and to stimulate the acoustic cavitation due to the channeling of solid particles into the solution to be treated. Both processes (photocatalysis and sonocatalysis) involved the decomposition of Bisphenol S. In each case the beneficial action of the biochar, reflected in the reaction rate escalation, emerged through physicochemical and (photo)electrochemical characterization of the respective solids examined.