Production of activated carbon from sugar beet pulp and its application in silver based biocomposite polylactic acid films

In this thesis study, activated carbon was produced from the sugar beet (Beta vulgaris L.) pulp, a solid waste from the sugar industry, through chemical activation. Silver ions were added to the obtained activated carbon to impart antimicrobial properties. Subsequently, the activated carbon-silver composite structure was incorporated into biodegradable polylactic acid (PLA) films, and the improvements in the mechanical, barrier, morphological, and antimicrobial properties of the film were investigated. The production process of activated carbon was optimized using the Central Composite Design (CCD) of Response Surface Methodology (RSM). Activated carbon was produced from sugar beet pulp through chemical activation with zinc chloride (ZnCl2). In the production process, carbonization temperature (500-800 °C) and ZnCl2 impregnation ratio (1-4) were determined as variables, and the effects of these variables on activated carbon yield (%) and methylene blue removal (%MBR) responses were investigated. The activation of samples impregnated with different ratios of ZnCl2 was carried out at 500-800 °C with a heating rate of 5 °C/min, under a nitrogen gas flow of 200-300 cm3/min, and held at the final temperature for 2 hours. According to the optimization analysis, the ZnCl2 impregnation ratio of 1 and the activation temperature of 500 °C were determined to be the best conditions for the highest activated carbon yield and methylene blue removal. Under the determined parameters, the activated carbon yield from sugar beet pulp was found to be 20.88±0.822%, and the methylene blue removal was 86.80±0.213%. As a result of the optimization study, the activated carbon (AC) obtained under suitable parameters was analyzed for its elemental (C, H, N, S, O), moisture, ash, volatile matter, fixed carbon, BET, SEM, FTIR, XRD, and TG properties. According to the elemental analysis, after the activation process, the carbon content increased while the hydrogen and oxygen content decreased. It was observed that the volatile matter and ash content decreased, whereas the fixed carbon content increased in the obtained activated carbon. The BET surface area of the activated carbon obtained under optimum conditions was determined to be 1045 m2/g, and the micropore volume was 0.395 m3/g. Under these conditions, it was determined that approximately 74% of the total pore volume consisted of micropores, with an average pore diameter of 2.04 nm. In SEM images, it was observed that the activated carbon had a porous structure. FTIR spectra showed that the activation process significantly altered the chemical composition and structural properties of the sugar beet pulp. According to the XRD analysis, the activated carbon was determined to be amorphous. Based on the TG curve of sugar beet pulp, thermal degradation occurred in three stages, with a mass loss of approximately 93.11%. The activated carbon was found to be less affected by temperature changes, with a total mass loss of 29.04%. Silver ions were added to the obtained activated carbon through a chemical reduction method. In SEM images, the presence of silver particles on the surface of the activated carbon was clearly observed. XRD results showed that a crystalline structure formed when silver was added to the activated carbon. The TG curve of the activated carbon-silver (AC-Ag) composite structure showed a total weight loss of 22.98%. The antimicrobial effect of the obtained AC-Ag composite was investigated using the disk diffusion method, and antimicrobial activity was detected against E. coli and S. aureus. The produced activated carbon (AC) and AC-Ag samples were incorporated into PLA films at different ratios (1%, 3%, 5%, 7%, 10%). The films were prepared using the solvent casting method. Chloroform was used as the solvent to obtain a 5% (w/w) PLA solution. To this solution, 10% polyethylene glycol (PEG 400) plasticizer was added. Subsequently, different ratios of AC (PLA/10%PEG/AC) and AC-Ag (PLA/10%PEG/AC-Ag) particles (<50 µm) were added to the solution, and the mixture was stirred for 12 hours at 60°C. The prepared solutions were poured into 9 cm diameter petri dishes at 7.5 mL each and left to dry in an oven at 75°C for approximately 35 minutes. Additionally, a pure PLA film without PEG 400 and AC or AC-Ag, as well as a PLA/10%PEG film containing only PEG 400, were prepared. The obtained films were analyzed for their physical (color and thickness measurement), mechanical (tensile strength, elongation at break, and thermal adhesion strength), barrier (water vapor transmission rate, WVTR), antimicrobial activity (Escherichia coli and Staphylococcus aureus), structural (SEM, XRD), thermal (TGA/DSC), and surface (FTIR) properties. The produced pure PLA film exhibited a brittle and transparent structure, whereas the PLA films with 10% PEG 400 added were more flexible and opaque in appearance. SEM images showed that pure PLA had a smooth surface, while the film containing 10% PEG 400 exhibited roughness. In SEM images, it was observed that PLA had a smooth surface, while the film containing 10% PEG 400 showed roughness. As the AC content increased from 1% to 10%, it was observed that the AC particles tended to agglomerate rather than distribute homogeneously throughout the PLA matrix. In the SEM images of films containing AC-Ag, a better distribution was observed compared to the AC-incorporated films. The FTIR spectrum showed that the PLA and PLA/10% PEG films had similar chemical compositions and that the PLA/10% PEG film contained the characteristic functional groups of PEG. The composite films with AC and AC-Ag additives also exhibited the same characteristic peaks as PLA/10% PEG. According to the XRD analysis, the addition of 10% PEG did not significantly alter the crystalline structure of PLA. Thermal analysis results indicated that the plasticizing effect of PEG disrupted the thermal stability of PLA, causing the material to degrade at a lower temperature. It was observed that the thermal stability did not change for the films with AC and AC-Ag additives compared to PLA/10% PEG. As the ratio of AC and AC-Ag increased, an increase in the total color change (ΔE) of the films was observed, while a decrease in brightness (L*) value occurred. The AC-Ag containing films showed higher yellow-red color values due to the color of the composite structure. When AC and AC-Ag particles were added to the PLA matrix, a decrease in tensile strength values was observed. Compared to the PLA/10% PEG film, films containing 1-10% AC showed a decrease in tensile strength values from 14.63% to 33%, while films containing AC-Ag showed a decrease from 2.32% to 22.69%. The highest tensile strength values were obtained in films with 1% AC-Ag at 31.47 MPa and 3% AC-Ag at 29.45 MPa. Although the tensile strength values of the PLA films decreased after the addition of AC and AC-Ag, they were found to be comparable to petroleum-based commercial plastic films. The highest elongation at break and thermal adhesion strength values were also recorded in the 1% and 3% AC-Ag films. The highest elongation at break and thermal adhesion strength values were also recorded in the 1% and 3% AC-Ag films. The water vapor transmission rate increased by approximately 34% in the films containing 1% AC and AC-Ag compared to PLA/10% PEG. The antimicrobial activities of the films were investigated using the disk diffusion method. Antimicrobial properties were detected only in the films containing 3%, 5%, and 7% AC-Ag. In conclusion, sugar beet pulp has been determined to be a good raw material for the production of activated carbon. Impregnating the obtained activated carbon with silver ions imparted antimicrobial properties to the AC-Ag composite material. The biocomposite films obtained by incorporating the activated carbon-silver composite structure into the PLA matrix have the potential to be used as sustainable active food packaging, replacing petroleum-based commercial plastic films.