Faculty of Science Engineering and Technology

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    Assessment of heavy metal pollution and the phytoremediation potential of nonedible plants in Mwea irrigation scheme, Kirinyaga county, Kenya
    (Chuka University, 2024) Silas Edward Njagi
    A clean environment is critical for existence and development of all living organisms. The pollution of various sectors of environment are difficult to reverse and present a serious threat to humankind. Modern agricultural practices pollute soil to a large extent due to huge quantities of fertilizers, pesticides, herbicides and soil conditioning agents which are employed to increase crop yield. Long-term use of these agrochemicals has resulted to accumulation of heavy metal such as cadmium, lead, chromium, mercury, copper, nickel and arsenic which are toxic to plants and animals. Heavy metals cause deleterious effect on soil fertility because they are sorbed more strongly than alkali and alkaline earth metals making the essential metals leach from top soil which affect cation exchange of the soil and therefore affect crop production. Rice farming involves uses of agrochemicals which may contain heavy metals that can accumulate in various environmental segments and may enter the food chain. In humans, heavy metals toxicity is manifested by growth retardation, reproductive cycle changes, mortality, chronic diseases development and formation of tumours among other health effects. Therefore, there is need for continuous assessment of heavy metals in the environment. This study intended to assess the levels of toxic heavy metals in soil, rice, sediment and water from Mwea irrigation scheme in Kenya; determine the phytoremediation potential of common plants found in the rice farms; and propose a way to enhance phytoremediation potential of the plants. The study adopted descriptive survey design, CRD and RCBD. Questionnaires were used to establish the agrochemicals used in the study area. Soil, rice, water, sediment, fertilizers and plant shoots samples were collected, dried and digested then heavy metals analysed by ICP-MS. Data obtained was analysed using t-tests and ANOVA using SPSS version 26 and means were separated using Tukey’s HSD test at α = 0.05. The survey revealed that fertilizers, pesticides and other chemicals were used during rice farming with fertilizers used during planting containing high amounts of the heavy metals. Sediment from rivers Thiba and Nyamindi contained Cd, Pb, Zn, As and Se amounts below WHO and KEBS/WASREB limits but Cr, Ni and Mn amounts were above the limits; water from the rivers contained Cr and Mn amounts above set limits during rainy season but during dry season, the amounts of all the heavy metals determined were below the limits implying that the water was not polluted with the heavy metals during dry season. Paddy water had higher concentration of all the heavy metals than river water except for Pb and Mn. Paddy soil, rice straw, rice husks and rice grains from the scheme were found not to be contaminated with the heavy metals. The amount of Cd, Cr, Ni, Pb, Zn, and As in rice grains were found to be below the upper limit set by WHO and FAO. BAFs for rice grains were 0.5744, 0.0374, BDL, 0.0148, 0.0403, 0.0150, 0.0254 and 0.0049 for Cd, Cr, Ni, Pb, Zn, As, Mn and Se respectively indicating that little amount of the heavy metals were transferred from soil to rice grains. The EFs for all the heavy metals in Cyperus difformis (rice sedge), Tradescantia fluminensis (wandering jew), Echinochloa crus-galli (cockspur grass), Cyperus rotundus (nut grass), Ludwigia adscendens (water primrose) were below or close to 1.0 except for Ni which ranged from 10.1442 to 25.3863. Wandering jew and cockspur grass were found to be better phytoremediators than the other plant species studied. This study found that agrochemicals were the main source of heavy metal pollution at Mwea irrigation scheme although the pollution had not reached alarming level. Continuous monitoring of heavy metals situation is necessary to ensure that residents of Mwea and consumers of rice from Mwea irrigation scheme are safe from heavy metals.
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    Ab initio defect engineering and green synthesis of ca-mg Codoped titania and zirconia for photocatalytic Applications
    (Chuka University, 2025-10) Mbae, Jane Kathure
    The growing demand for long-term solutions to environmental pollution and energy concerns has sparked interest in photocatalysts that work efficiently in visible light. In particular, TiO2 and ZrO2 photocatalysts have shown great potential in photocatalytic degradation and disinfection applications. However, traditional TiO2 and ZrO2photocatalysts feature metastable phases that are better photocatalysts, large bandgaps, low solar energy utilization, and rapid charge carrier recombination, limiting their usefulness. This study addresses these limitations by investigating Ca-Mg codoping as a defect engineering strategy for improving the photocatalytic performance of TiO2 and ZrO2. The primary objectives were to determine the phase stability of Ca-Mg codoped TiO2 and ZrO2 polymorphs using Density Functional Theory (DFT), to evaluate the effects of oxygen vacancies on their electronic structures, to establish optimal dopant concentrations for visible light activation, to green synthesize and characterize the predicted visible-active materials, to assess photocatalytic activity via Rhodamine B degradation under visible light and to evaluate antibacterial performance against E. coli and S. aureus. In this study, a combined ab initio and experimental approach was undertaken. Density Functional Theory (DFT) method was used to simulate the codoped polymorphs at different doping levels. Optimal codoped systems found suitable for visible light photocatalysis were green synthesized and characterized using XRD, ICP-MS and UV-VIS spectroscopy. The photocatalytic properties of the synthesized materials were evaluated by degradation of Rhodamine B (RhB) under visible light and the disinfection capability was tested against Escherichia coli and Staphylococcus aureus bacteria strains. The results yielded reasonable agreement between experimental and theoretical results, as well as a better understanding of the system. DFT results showed that Ca-Mg codoping stabilizes the anatase phase in TiO2 by increasing the c/a ratio and induces tetragonalization in monoclinic ZrO2 through lattice distortions and oxygen vacancy formation. Defective oxygen vacancy Localized mid-gap states were discovered in Ca-Mg codoped TiO2 and ZrO2 systems. These states served as trapping sites for the photogenerated charge carriers and significantly decreased the energy band, improving visible light absorption. Experimentally, the green synthesized codoped materials revealed smaller crystallite sizes (5.567 nm for TiO2 and 8.647 nm for ZrO2) and narrower bandgaps (1.92 eV and 2.4 eV, for TiO2 and ZrO2 respectively). Codoped TiO2 degraded 99.3% of Rhodamine B in 120 minutes and completely inactivated S. aureus under visible light in 60 minutes. RhB degradation % for 5 cycles of codoped TiO2 and ZrO2 nanoparticles have remarkable stability and reusability, which is the key reason for their widespread use in photocatalytic degradation of dyes. Based on these findings, Ca-Mg codoping is recommended as an economical, non-toxic and effective strategy for tuning the structural and electronic properties of TiO2 and ZrO2 to enhance visible-light photocatalysis. Future work should focus on advancing the green synthesis technique and testing against a larger spectrum of contaminants and microbial strains to further establish its environmental application.