http://reposit.library.du.ac.bd:8080/xmlui/xmlui/handle/123456789/64 2026-11-11T02:48:05Z 2026-11-11T02:48:05Z Ahmmed, Hd. Razu http://reposit.library.du.ac.bd:8080/xmlui/xmlui/handle/123456789/4918 2026-10-19T06:31:19Z 2026-10-19T00:00:00Z

Citric Acid and Gypsum Production from Wastes Biomass and Industrial By-products Using Microbial Fermentation A Ahmmed, Hd. Razu The economic potential of microbial citric acid (CA) production is substantial due to its wide-ranging applications in the beverage, food, detergent, cosmetic, and pharmaceutical industries. In Bangladesh, the majority of CA demand is met through imports, while annual consumption is increasing steadily at a rate of 3.5–4%, underscoring the need for sustainable domestic manufacturing alternatives. Large-scale production must therefore adopt eco-friendly processes that utilize readily available and inexpensive agro-industrial byproducts, while ensuring high production yields. CA is predominantly produced via microbial fermentation using Aspergillus niger. The present study aimed to optimize citric acid production from sugarcane molasses employing A. niger (F-81) strain under submerged fermentation, with gypsum as a byproduct. The concentration of CA in the fermentation broth was determined using the Marier–Boulet method. Maximum production was observed in the pretreated molasses sample (12.49 mg/ml), compared to the non-pretreated counterpart (7.3 mg/ml), under identical fermentation conditions (10 days, 2% inoculum size, and an initial pH of 5.6). These findings demonstrate the significance of substrate pretreatment in enhancing yield. The production rate of CA is influenced by multiple factors, including regulatory genes of A. niger, pH, temperature, inoculum size, substrate concentration, and the presence of heavy metals, which warrant further investigation. In addition, citric acid biosynthesis was evaluated using six indigenous fungal strains (FS1–FS6) isolated from soil, cultured under standardized fermentation conditions with 5% molasses as the carbon source, at an initial pH of 6.0 and ambient temperature (30ºC). The produced citric acid was isolated through precipitation method using calcium hydroxide and sulfuric acid, in which gypsum was found as a by-product. Citric acid and gypsum production was confirmed via FTIR analysis. Among the isolates, FS2 exhibited the highest and most consistent productivity, yielding 0.93 mg/ml 1.09 mg/ml and 1.11 mg/ml after 5 days, 7 days and 10 days of incubation, respectively. Moderate production was observed in FS5 (0.55–0.61 mg/ml) and FS3 (0.51–0.61 mg/ml), whereas FS1 (0.41–0.60 mg/ml) and FS4 (0.32–0.40 mg/ml) displayed comparatively lower yields. FS6 was the least productive, reaching only 0.14 mg/ml by day 10. Overall, the results identify A. niger (F-81) strain as the most promising candidate for industrial-scale citric acid production under the tested conditions in Bangladesh. This thesis is submitted for the degree of Master of Philosophy.

2026-10-19T00:00:00Z Hoque, Mohammad Amirul http://reposit.library.du.ac.bd:8080/xmlui/xmlui/handle/123456789/4867 2026-08-04T05:42:53Z 2026-08-04T00:00:00Z

Synthesis, Characterization and Application of Graphene Based Materials Hoque, Mohammad Amirul Graphene is being exponentially utilized across diverse research fields due to its extraordinary ability to enhance material properties with its miraculous attributes. Still, the material is expensive due to difficulties in the production of high-quality graphene. In this study, a few initiatives were approached to reduce the production costs of graphene with improved properties. At first, Tour's modified Hummer’s method was adopted to synthesize improved graphene with the feed acid liquor (FAL) recycling technique. About 90% of the FAL were recycled and reused five times as feed for successive production batches. Another focus was on the oxidation reaction to achieve graphene oxides (GOs) with higher oxygen to carbon ratios. The changes in recycled FAL and synthesized GOs properties due to repeated recycling were evaluated. The synthesized GOs were utilized for the removal of arsenic (As3+) ions from water, showing a qmax of 343.14 mg/g with 98.4% removal efficiency from 300 ppm to 22 ppm. The GOs were then reduced thermally at various temperatures in water, kerosene, and kerosene followed by ascorbic acid methods and times. The major objective of the reduction was to achieve maximum reduction of GO. The synthesized reduced graphene oxides (rGO) were evaluated for the antimicrobial properties. Although the Hummer’s method produces GO and rGO with improved properties, the method is difficult to carry out, very slow, and expensive. To obtain an easy and faster method for the production of graphene in bulk, a microwave assisted rapid exfoliation process of graphene exfoliation was examined along with the effect of variation of the intercalating agent ratio. All the products of FAL, GOs, rGOs, and microwave graphene (MG) were characterized using a Karl-Fischer moisture analyzer, IC, AAS, FT-IR (ATR), FT-Raman, UV-Visible Spectroscopy, PS-Zeta potential, XPS, XRD, STA, TGA, FE-SEM, and TEM analyzer. The FAL had increased moisture content with the successive recycling, but had no significant changes in properties and composition of GO. However, GO properties greatly changed with temperature and time of reaction, and the amount of oxidizing agent. The degree of reduction of GOs in N2 atmosphere at 193.4 oC was 95% of its initial mass, even at a slow rate of heating, but at this temperature, GO explosively degraded. In water medium as well as in kerosene, the explosive degradation can be omitted, and even at 220 oC, affording 40% reduction, which was mainly due to the removal of oxygen atoms as evident from the XPS analysis. The extremely reduced rGO contained an oxygen-to-carbon ratio of only 2.49% based on its initial oxygen content. In this study, XRD data also supported the composition of rGO and showed that it contained a homogeneous amorphous or viii nanocrystalline structure, as demonstrated by the 2θ = 24.91° peak in the (002) plane, which was shifted from the GO peak at 2θ = 12.86° in the (001) plane. The SEM image measured by ImageJ software showed that the obtained grain sizes were between 100 and 200 nm for both the GO and rGO. These rGO also showed strong activities against gram-positive and gram-negative bacteria, such as B. subtilis, S. aureus, E. coli, and S. typhi. In the case of MG, the degree of exfoliation was directly proportional to the intercalating agent ratios, and this method directly produces pristine graphene. This MG was highly thermo-stable up to 700-800 oC with high crystallinity, having 2θ = 26.56° at the (002) plane. The crystalline graphene showed excellent adsorption of acid blue-25 dyes with qmax 472.8 mg/g and 94.56% removal efficiency in the case of a 300-ppm dye solution. This thesis is submitted for the degree of Doctor of Philosophy

2026-08-04T00:00:00Z Akter, Nasrin http://reposit.library.du.ac.bd:8080/xmlui/xmlui/handle/123456789/4854 2026-08-03T05:15:07Z 2026-08-03T00:00:00Z

Superiority Enhancement of Geo-Jute by Chemical Modification Through Gamma Radiation Akter, Nasrin Natural fiber jute inherently possesses greater tensile strength (TS) than other natural fibers and offers numerous applications in geotechnical applications as a replacement for synthetic fibers. This research aims to enhance the physio-mechanical properties of jute fabric by improving its environmental durability. The study improves durability by chemically modifying materials using gamma radiation. Bitumen emulsion and unsaturated polyester resin were used for chemical modification. First, the jute fabric is treated with a chemical (polymer mixture), and a suitable formulation (30% BE + 10% UPR) is chosen based on durability tests. Then, the chemically treated jute fabric is irradiated with gamma radiation, selecting an appropriate dose (5 kGy) through durability testing. Finally, the pre-treated jute fabric is subjected to chemical treatment and gamma radiation, and its durability is assessed using an accelerated weathering tester. Pre-treatment was performed in two ways: (i) HEMA pre-treated, and (ii) benzoyl peroxide pre-treated. The HEMAtreated jute fabric showed better durability than the other treated sample. The tensile strength of HEMA-pre-treated samples was 50% higher than that of the raw jute fabric. At each step, samples were characterized by FT-IR, XRD, and TGA analyses, and their physico-mechanical properties were measured through tensile strength and water absorption tests. After chemical modification, the OH groups in the jute fabrics are reduced, decreasing hydrophilicity and increasing hydrophobicity, thereby enhancing the physico-mechanical properties. This research aims to strengthen these properties without making the fabric entirely plastic by breaking additional OH bonds. Pre-treatment generates free radicals from jute cellulose, and gamma radiation promotes their formation and cross-linking within the cellulose-polymer matrix. Cross-linking improves the fabric's physico-mechanical properties. Durability and physical tests show improvements in these properties. FTIR, XRD, and TGA analyses confirm the formation of free radicals and the crosslinking of jute cellulose with the polymer mixture. This research partially enhances the hydrophobicity of jute fabric. Gamma radiation not only boosts cross-linking but also reduces excess chemicals on the fabric surface, increasing porosity between yarns and making the fabric more biodegradable and eco-friendlier. An accelerated weathering test compares durability under artificial and outdoor conditions. In the lab, an accelerated weathering tester provides quicker results than outdoor exposure by controlling temperature, light, and water spray. Overall, durability testing using this method yields faster results than natural outdoor testing. Significant improvements in TS and durability testing were observed in the treated jute fabric compared to the 4 raw jute fabric. Whereas raw jute fabric deteriorates after 45 days, treated jute fabric showed enough tensile strength after 90 days. On the other hand, in the accelerated weathering durability test, after 28 days, the treated jute fabric showed significantly higher tensile strength and weight than the raw jute fabric. This thesis is submitted for the degree of Doctor of Philosophy.

2026-08-03T00:00:00Z Hassan, Md. Mehedi http://reposit.library.du.ac.bd:8080/xmlui/xmlui/handle/123456789/4796 2026-03-03T07:49:55Z 2026-03-03T00:00:00Z

Non-metal Doped TiO2 Nanocomposites for the Removal of Nuclear Waste from Water Hassan, Md. Mehedi This study focuses on the synthesis and evaluation of non-metal doped titanium dioxide (TiO2) nanoparticles for the potential remediation of radionuclide-contaminated water. Undoped TiO2 and doped TiO2, e.g., B-doped TiO2 (B–TiO2), C-doped TiO2 (C–TiO2), N-doped TiO2 (N–TiO2) were successfully prepared by the sol-gel method. The resultant nanoparticles were thoroughly characterized by XRD, FESEM, EDX, TEM, FTIR, UV–Vis spectroscopy, DLS, zeta potential measurement, and AAS to study their structural, morphological, optical, and adsorption properties. Furthermore, batch adsorption and photocatalytic experiments using adsorbents were conducted to assess their efficiency in radionuclide removal. While attempting to extract radioactive isotopes from water, real radionuclides were not implemented due to safety, cost, and facility constraints. Instead, the nonradioactive analogs—cobalt, iodine, manganese, and zinc (which have similar radioisotopes, such as 60Co2+, 131I-, 54Mn2+, and 65Zn2+)—were employed, due to their similarity with the radioactive elements. XRD results revealed that both the doped and undoped TiO2 were crystalline anatase, and B-doping led to the formation of a small amount of the rutile phase. Both SEM and TEM images showed that the morphology and particle dimension were influenced by doping, with the smallest average particle size being reached for C–TiO2. The adsorption capacity was evaluated using iodine adsorption and the removal of metal ions. The C–TiO2 and N–TiO2 nanocomposites were found to show excellent adsorption properties among the doped samples, which may be due to a higher surface area and improved surface chemistry. The findings indicate that non-metal doping can enhance the photocatalytic and adsorption activity of TiO2. Thus, these materials are promising candidates for treating radioactive wastewater. This thesis is submitted for the degree of Master of Philosophy.

2026-03-03T00:00:00Z