Maja Čolnik

The degradation of bisphenol A (BPA), the main monomer of polycarbonate, was investigated under subcritical water conditions to better understand its decomposition as a function of process conditions and to provide useful data for designing a recycling process to convert polycarbonate into valuable products. Hydrothermal experiments were conducted in a batch reactor at temperatures ranging from 250 to 350 °C, with reaction times from 5 to 30 min and water-to-material ratios of 5, 10, and 15 (mL/g), following a Box–Behnken design with response surface methodology (RSM). The influence of process parameters on phase distribution, total carbon content, and product composition was evaluated. The results showed that temperature and reaction time were the most significant factors affecting BPA decomposition, while the water-to-material ratio had a minor effect. The recovery of the DEE (diethyl ether)-soluble phase decreased with increasing temperature and time, accompanied by a corresponding increase in the water-soluble phase yield and total carbon content. Analysis of the DEE-soluble fraction revealed the sequential transformation of BPA into 4-isopropenylphenol, 4-isopropylphenol, and phenol, with phenol becoming the dominant degradation product at higher temperatures. These findings provide new insights into the hydrothermal decomposition mechanism of BPA and form a basis for understanding polycarbonate degradation and developing sustainable subcritical water recycling processes for polymeric materials.

Corncobs are a widespread and renewable by-product of corn cultivation that are typically considered waste or low-value material. Corncobs contain hemicellulose, cellulose, and lignin, which can be converted into valuable products using suitable techniques. Subcritical water is increasingly used as a green medium for the extraction of valuable components from biomass, as it has many advantageous properties (high yield, pure extracts, shorter times) compared to other organic solvents. For this reason, subcritical water was used in this study to extract valuable components from corncobs at different temperatures (150–250 °C) and reaction times (10–60 min). During the decomposition of corncobs, numerous valuable products are formed in the aqueous phase depending on the temperature and reaction time. In addition to sugars and their derivatives, phenolic compounds were also formed, which are of great importance in numerous applications. It was found that at low temperatures (150–170 °C) the hemicellulose in the corncobs begins to decompose and, in particular, the sugars (glucose, xylose, arabinose, and galactose) are initially formed in the aqueous phase. Higher temperatures (200 and 250 °C) are more favorable for the decomposition of corncobs into valuable components. The yield of sugars increases with temperature due to the degradation of the cellulose content of the lignocellulosic biomass. At the same time, several new valuable products (furfural, 5-hydroxymethylfurfural (5-HMF), 1,3-dihydroxyacetone, levulinic acid, and formic acid as well as phenolic components) are formed through the degradation of lignin and the further degradation of sugars. The most important products are certainly the furfurals, which are central platform compounds. The highest furfural content was reached at 200 °C and 60 min and accounted for almost half of all components in the aqueous phase (472.01 ± 5.64 mg/g dry extract). These biomass-derived sugars and derivatives can be used in the production of fuels, pharmaceuticals, biodegradable polymers, and surfactants.

Tetra pak packaging is one of the most frequently used types of packaging in the food industry. The recycling of the tetra pak packaging waste presents a difficult task because of its multi-layered, multi-component structure. In this study, the degradation of tetra pak packaging in subcritical (SubCW) and supercritical (SCW) water was investigated. The experiments were carried out in one (SCW) or two stages (SubCW and SCW), whereby the influence of the reaction temperature and time on the yield and composition of the products obtained was investigated. The maximum oil phase yield achieved in a one-stage and a two-stage degradation process was 60.7% and 65.5%, respectively. The oil and gas phases were composed of different types of hydrocarbons. Higher temperature and longer time led to higher amounts of saturated aliphatic hydrocarbons in both the oil and gas phases. The aqueous phase contained sugars (glucose, fructose) and sugar derivatives (levulinic acid, glyceraldehyde, furfurals). Based on these results, the degradation pathway of waste tetra pak packaging in SubCW and SCW was proposed. The results of the study show that the degradation of waste tetra pak packaging with SubCW and SCW is a promising recycling process.

Pistachio and walnut shells accumulate in large quantities as waste during food processing and represent a promising lignocellulosic biomass for the extraction of valuable components. Subcritical water technology was used as an environmentally friendly technique to study the extraction of active ingredients and other valuable degradation products from walnut and pistachio waste. Subcritical water extraction (SWE) was carried out under different process conditions (temperature (150–300 °C) and short reaction times (15–60 min)) and compared with conventional extraction using different organic solvents (acetone, 50% acetone and ethanol). The extracts obtained from pistachio and walnut shell waste are rich in various bioactive and valuable components. The highest contents of total phenols (127.08 mg GA/g extract at 300 °C for 15 min, from walnut shells), total flavonoids (10.18 mg QU/g extract at 200 °C for 60 min, from pistachio shells), total carbohydrates (602.14 mg TCH/g extract at 200 °C for 60 min, from walnut shells) and antioxidant activity (91% at 300 °C, for 60 min, from pistachio shells) were determined when the extracts were obtained via subcritical water. High contents of total phenols (up to 86.17 mg GA/g extract) were also determined in the conventional extracts obtained with ethanol. Using the HPLC method, sugars and their valuable derivatives were determined in the extracts, with glucose, fructose, furfurals (5-hydroxymethylfurfural (5-HMF) and furfural) and levulinic acid being the most abundant in the extracts obtained by subcritical water. The results show that subcritical water technology enables better exploitation of biowaste materials than conventional extraction methods with organic solvents, as it provides a higher yields of bioactive components such as phenolic compounds and thus extracts with high antioxidant activity, while at the same time producing degradation products that are valuable secondary raw materials.

The isolation of keratin from poultry feathers using subcritical water was studied in a batch reactor at temperatures (120–250 °C) and reaction times (5–75 min). The hydrolyzed product was characterized by FTIR and elemental analysis, while the molecular weight of the isolated product was determined by SDS-PAGE electrophoresis. To determine whether disulfide bond cleavage was followed by depolymerization of protein molecules to amino acids, the concentration of 27 amino acids in the hydrolysate was analyzed by GC/MS. The optimal operating parameters for obtaining a high molecular weight protein hydrolysate from poultry feathers were 180 °C and 60 min. The molecular weight of the protein hydrolysate obtained under optimal conditions ranged from 4.5 to 12 kDa, and the content of amino acids in the dried product was low (2.53% w/w). Elemental and FTIR analyses of unprocessed feathers and dried hydrolysate obtained under optimal conditions showed no significant differences in protein content and structure. Obtained hydrolysate is a colloidal solution with a tendency for particle agglomeration. Finally, a positive influence on skin fibroblast viability was observed for the hydrolysate obtained under optimal processing conditions for concentrations below 6.25 mg/mL, which makes the product interesting for various biomedical applications.

The particles from the gas-saturated solutions (PGSS) process was employed to micronize brown algae pigments separated by different extraction techniques. The particle formation of pigments with a coating material, polyethylene glycol (PEG), was carried out by the PGSS process using supercritical CO₂. Environmental scanning electron microscopy (ESEM) and Fourier-transform infrared spectroscopy (FTIR) were performed to characterize the produced particles, while encapsulation efficiency was determined using spectrophotometric methods. The physical properties of obtained microparticles were also determined. The PGSS process enabled a high encapsulation yield in the range from 61.60 to 73.73%, and high encapsulation efficiency in terms of chlorophyll a, chlorophyll b, and carotenoid content. The release of CO₂ during the PGSS process gave the microparticles their characteristic open and porous form, and enhanced the solubility and flow properties at the same time.

In the following work, the hydrothermal degradation of polypropylene waste (PP) using supercritical water (SCW) has been studied. The procedure was carried out in a high-pressure, high-temperature batch reactor at 425 °C and 450 °C from 15 to 240 min. The results show a high yield of the oil (up to 95%) and gas (up to 20%) phases. The gained oil phase was composed of alkanes, alkenes, cycloalkanes, aromatic hydrocarbons, and alcohols. Alkanes and alcohols predominated at 425 °C and shorter reaction times, while the content of aromatic hydrocarbons sharply increased at higher temperatures and times. The higher heating values (HHVs) of oil phases were in the range of liquid fuel (diesel, gasoline, crude and fuel oil), and they were between 48 and 42 MJ/kg. The gas phase contained light hydrocarbons (C₁–C₆), where propane was the most represented component. The results for PP degradation obtained in the present work were compared to the results of SCW degradation of colored PE waste, and the potential degradation mechanism of polyolefins waste in SCW is proposed. The results allowed to conclude that SCW processing technology represents a promising and eco-friendly tool for the liquefaction of polyolefin (PE and PP) waste into oil with a high conversion rate.

The chemical degradation of PVC waste in SCW between 400 and 425 °C and reaction times from 30 to 60 min was studied. The PVC waste in SCW decomposed into the gas, oil, water soluble, and solid phases. The highest yield of the gas and oil phases was achieved at the temperature of 425 °C after 60 min. By increasing the reaction time at 400 °C, the yield of chloride ions in the aqueous phase increased and reached the maximum at 60 min. The gas and oil phases contained many valuable compounds similar to crude oil. Alkanes and chloroalkanes; alkenes, alicyclic, and aromatic hydrocarbons; as well as alcohols were the main groups of hydrocarbons in the oil phase, while the gas phase contained only light hydrocarbons (C₁–C₆), CO_2, and small amounts of H₂. This confirmed that the largest chlorine content remains in the aqueous phase and does not pass into the gas phase. It can be concluded that SCW presents effective decomposition media for plastic waste.