Tinega, N. Joseph2024-10-012024-10-012024-01https://ir-library.ku.ac.ke/handle/123456789/29002A Thesis Submitted in Fulfilment of the Requirement for the Award of the Degree of Doctor of Philosophy in Environmental Planning and Management in the School of Engineering and Architecture of Kenyatta University, January 2024. Supervisors 1. Sammy Letema 2. saiah Bosire OmosaIn recent years, increased steel production has resulted into increased release of Cr (VI) effluent raising global concern. Due to this a number of conventional Cr (VI) removal technologies that is membrane filtration, nanofiltration, ion exchange, electrochemical and chemical precipitation have been developed. Chemical precipitation is the commonly applied low-cost technology but has proven to be unsustainable due to sludge which creates secondary pollution. This research study established chemical precipitation as the main Cr (VI) treatment technology at the Devki Steel Factory, Ruiru municipality, Kiambu County. The Cr (VI) effluent discharge at the factory was not compliant with WHO, World Bank, EU, USEPA, China and Kenya effluent discharge standard limits. The data obtained necessitated the need to evaluate the potential of macadamia nutshell in Cr (VI) removal from steel industry wastewater as an alternative and eco-friendly technology. Raw macadamia nutshell (RMN) and carbonized macadamia nutshell (CMN) were applied for potential Cr (VI) adsorption. BET, elemental, FTIR and SEM analysis results reveal that carbonization process modified the adsorbent surface properties by increasing the pore volume, surface area, pore size, carbon content and Cr (VI) removal rate from 40.21-380.64 m2 /g, 0.01-0.09 cc/g, 1.63- 8.78 Å, 47.01-57.2% and 71-97% respectively. The optimum Cr (VI) adsorption by the adsorbent was achieved at pH 4, contact time 2hrs, adsorbent mass 0.2g, metal concentration 50mg/l, and temperature 30℃. The adsorption process was exothermic. The equilibrium data was best described by Langmuir and Freundlich isotherms. Pseudo second order kinetics was in agreement with the experimental data. The maximum adsorption capacity and removal rate of 24.3 mg/g and 97.2% were obtained by CMN compared to RMN 17.95 mg/g and 71.6% respectively. The desorption experiment revealed that the spent adsorbent could be reused for a maximum of three adsorption cycles and metal recovery at maximum desorption efficiency of 98.8 % for CMN and 83.43% for RMN making the process eco-friendly. The two-tailed t test and paired t test reveal that successful Cr (VI) adsorption is obtained at p<0.05 in the order CMN>RMN. The study concludes that carbonized macadamia nutshell could potentially be applied as an alternative and eco-friendly technology in Cr (VI) removal from steel industry wastewater and compliant with Cr (VI) effluent discharge standard limits (0.05mg/l) at the local and international level. Thus, reducing human health and ecosystem impacts. The study recommends further study on improvement of metal adsorption efficiency of biomass-based adsorbents with diverse complex wastewater systems to increase their applicability.enThesis