SCHOOL OF SCIENCE

Valorization of diatomaceous earth as an alternative activator for pumice–based geopolymer concrete: Experimental and multiple linear regression modeling

2026-01-01T00:00:00Z

Valorization of diatomaceous earth as an alternative activator for pumice–based geopolymer concrete: Experimental and multiple linear regression modeling Wafula, Dorothy; Shikuku, Victor; Saouma, Felix; Tome, Sylvain Geopolymer cement (GC) is gaining attention as a sustainable alternative to Portland cement due to its lower CO₂ emissions. However, GC formations rely on sodium silicate, which is expensive, and its production has a high carbon footprint. The development of alternative silica and aluminosilicate sources is therefore critical for improving the sustainability and economic viability of geopolymer concrete. This study focuses on the use of diatomaceous earth (DE) as an alternative silica source and examines the effect of the activator composition on the compressive strength of pumice-based geopolymer concrete, relative to commercial sodium silicate (CSS) as a control, using a one-step synthesis approach. The study evaluates the effects of curing temperature (30, 50, and 70 ◦C), sodium hydroxide concentration (6, 8, and 10 M), and silica mass in the activator (40, 80, and 100 g) on the compressive strength of the resulting geopolymer concrete. The compressive strengths for CSS and DE samples were comparable (<6%). The Multiple Linear Regression (MLR) model (y = 0.3319[NaOH]-0.0231 [DE]+0.0304[Temp]+0.4508) indicated that NaOH has the highest positive effect on compressive strength, while excessive silica content has a negative effect on the compressive strength of pumice-based geopolymer concrete. Compressive strength increased (~50%) from 2.8 to 4.41 MPa when NaOH concentration increased from 6 M to 10 M for DE samples, respectively. In contrast, an increase in silica mass from 40 g to 100 g reduced the strength by ~28.9% and ~38% for CSS and DE samples, respectively. The results indicate that at high NaOH concentration, low DE content and elevated temperatures, DE-based activator produced an activator with comparable results to CSS. In the absence of established geopolymer concrete standards in Kenya, a laboratory scale mix design was adopted to enable consistent comparison between DE and CSS based systems. Further research should focus on mix optimization and strategies to enhance strength development for broader construction applications and standardization.

A review on the recent advances in the use of hydrochar for adsorption of methylene blue dye from aqueous systems

2026-01-01T00:00:00Z

A review on the recent advances in the use of hydrochar for adsorption of methylene blue dye from aqueous systems Onyango, Collins; Nyairo, Wilfrida; Shikuku, Victor This study reviewed the use of hydrochars in the adsorption of methylene blue, a toxic dye, from aqueous phases. Journal articles published predominantly between 2020 and 2025 were assessed. The articles were mainly identified through keyword searches on ScienceDirect, Scopus and Web of Science. The studies reported a high efficacy of hydrochars against the dye. Organic wastes were primarily used as a feedstock in the hydrothermal carbonization processes, aligning to the waste to resource principle, an efficient and cost effective route of synthesis of adsorbents. The pseudo-first order, pseudo-second order and the Elovich model best described the kinetic data while the Freundlich, Sips and Langmuir isotherms best fitted the experimental data, with model predicted adsorption capacities in the range of 30-1060 mg g− 1. All the reviewed studies reported a spontaneous and feasible adsorption process (negative ΔG), while the entropy (ΔS) and enthalpy (ΔH) values varied with feedstock used, the specific modification routes and modification conditions. Several systems retained between 65 and 95% capacity over five cycles with eluents such as HCl, ethanol and methanol. The mechanism of adsorption was reported by most of the studies to be through electrostatic interactions, hydrogen bonding and π-π interactions.

Adsorption of heavy metals onto food wastes: a review

2025-01-01T00:00:00Z

Adsorption of heavy metals onto food wastes: a review Nyairo, Wilfrida; Njewa, Joel B.; Shikuku, Victor O. here has been an increase in the production of food waste materials worldwide due to rapid population growth. The ineffective and sometimes unscientific and ad hoc disposal of these food waste materials has led to environmental pollution. Studies have reported the occurrence of heavy metals in water resources posesserious health threats to the environment and human health. Heavy metals are documented to be recalcitrant to conventional water treatment facilities since they are non-biodegradable. The use of food waste-based adsorbents provides an alternative solution for the adsorption of heavy metals in water resources, with concomitant benefit of valorization of otherwise waste materials. Therefore, this study examined the applications of food waste-based adsorbents for the removal of heavy metal ions. The study adopted a literature-based approach which involved reviewing published papers from selected science databases. The results indicate that these bioadsorbents have great removal efficiencies for different heavy metals with, rice husks and sugarcane bagasse demonstrating special sorption properties, especially for chromium and lead metal ions, respectively. The adsorption data were mostly best described by the Langmuir and Freundlich isotherm models, suggesting a monolayer coverage with similar sites and a heterogeneous surface, respectively. Further, the kinetic studies indicated that the adsorption processes largely followed a pseudo secondorder model, showing chemisorption-mediated rate-limiting steps. However, regardless of these encouraging results attained, the use of food waste-based adsorbents has limitations such as variation in the composition and the structure. This leads to inconsistencies in adsorption efficiencies, regenerations and reuse, and reduced removal capacities. There is also the possibility of leaching of heavy metals from the adsorbents which may in-turn cause secondary pollution. Sustainability investigations such as life cycle assessment, cost-benefit analysis, pilot-scale studies and optimization studies present areas for future research.

Unary Adsorption of Phthalates from Wastewater onto Water Hyacinth Biochar: Parameters, Drivers and Mechanism

2025-01-01T00:00:00Z

Unary Adsorption of Phthalates from Wastewater onto Water Hyacinth Biochar: Parameters, Drivers and Mechanism Ogora, Elkanah N.; Getenga, Zachary M.; Gichumbi, Joel M.; Shikuku, Victor O. In this study, water hyacinth root-derived biochar (WHB) was prepared as a low-cost adsorbent for the removal of three phthalates, namely, benzyl butyl phthalate (BBP), dimethyl phthalate (DMP) and bis(2-ethylhexyl) phthalate (BEHP) from single solute aqueous solutions. The equilibrium data were best described by the adsorption isotherm models in the order Freundlich>Langmuir>Dubinin-RadushkevichKaganer (D-R-K) isotherms. The maximum monolayer adsorption capacity (Qo) was 1.83, 1.77, and 1.62 mg/g for DMP, BBP, and BEHP, respectively. The adsorption of the phthalates was diminished by increased molecular weight and molar volume of the molecules but compensated by their hydrophobicity. The kinetic data were best described by the pseudo-second order (PSO) model and pore diffusion was not the sole operative rate-determining step. The calculated thermodynamic functions, changes in Gibb’s free energy (ΔG<0), enthalpy (ΔH<0), and entropy (ΔS<0) demonstrate the adsorption of DMP, BBP, and BEHP onto WHB is energetically favorable, exothermic, spontaneous and of a physical type controlled by hydrophobic interactions. The comparative adsorption capacities imply that WHB would sequester phthalates regardless of their physicochemical profiles.