The context is a region under acute water stress. The Horn of Africa experienced its warmest five-year stretch since 1981 between 2021 and 2025, with severe droughts in 2021–2022 and again in 2024 leaving soil moisture below normal across central and eastern Ethiopia. As climate pressures intensify, smallholder farmers across sub-Saharan Africa have intensified pesticide use to protect yields, introducing a growing class of emerging contaminants into already strained water resources. In eastern Ethiopia specifically, khat cultivation requires intensive management and regular pesticide application, and runoff from treated fields carries these chemicals into soil, rivers, lakes, and groundwater.

The health stakes are considerable. Long-term exposure to organophosphates has been linked to endocrine disruption, confusion, anxiety, memory loss, depression, and personality changes. Organophosphate poisoning accounts for roughly 38 percent of all emergency room poisoning admissions in Ethiopian teaching hospitals, and a retrospective study at the University of Gondar Teaching Hospital found that 90 percent of poisoning cases studied involved ingested organophosphates, with a mean patient age of just 25.5 years. Earlier work in the Ethiopian highlands detected dimethoate in surface waters at average concentrations of 0.38 micrograms per liter and in groundwater at 1.24 micrograms per liter, indicating preferential transport to aquifers during the rainy season.

To establish the scale of contamination, the team collected two-liter samples from each of the four sites, kept them refrigerated at 4 degrees Celsius, and extracted the pesticides using liquid-liquid extraction with n-hexane, achieving a pre-concentration factor of 200:1 before analysis. Khat leaf samples were dried, ground, and extracted with acetone-water solution using a similar approach. Quantification was performed by high-performance liquid chromatography with diode array detection at 210 nm, using both isocratic and gradient elution on a reversed-phase C18 column.

The results were unambiguous. Profenofos concentrations in the Haramaya, Ganda Qallu, and Adele water samples exceeded the World Health Organization’s recommended limit of 0.003 milligrams per liter, with the highest single reading at 0.14 milligrams per liter in Ganda Qallu. Dimethoate residues in Haramaya, Ganda Hayyo, and Adele samples surpassed the WHO limit of 0.02 milligrams per liter. Malathion remained below its 0.05–0.06 milligram per liter limit, but the summed pesticide load in every water sample exceeded the European Union’s guideline range of 0.001–0.003 milligrams per liter. Solubility and adsorption tests showed the pesticides dissolve readily in water and bind strongly to soil colloids, meaning they can move both in surface runoff and by leaching through soil.

For treatment, the researchers turned to the classical Fenton process, in which ferrous iron reacts with hydrogen peroxide under acidic conditions to generate hydroxyl radicals — highly reactive, non-selective oxidants that attack persistent organic molecules through hydroxylation, dealkylation, and ring-opening pathways, ultimately converting them to smaller, less hazardous intermediates and, ideally, to carbon dioxide and water. The team chose the homogeneous process deliberately: it offers direct contact between dissolved iron and peroxide, avoids the synthesis and regeneration burdens of heterogeneous catalysts, and does not depend on the light intensity and reactor geometry that solar and photo-Fenton variants require.

Batch experiments in 600-milliliter reactors explored the effects of pH, ferrous sulfate dose, hydrogen peroxide dose, and reaction time, and a response surface methodology with a central composite design was used to optimize the four variables simultaneously. The statistical model explained 99.17 percent of the variance in profenofos removal, with pH, peroxide dose, and reaction time all highly significant. The optimum emerged at pH 3.4, 0.7 grams per liter of ferrous sulfate, 5 grams per liter of hydrogen peroxide, and 42 minutes of reaction time — conditions predicted to remove 99.83 percent of profenofos. Validation runs achieved 99.21 percent removal on average, within 0.62 percent of the prediction.

The chemistry behind the optimum is well understood. Acidic pH keeps iron soluble and catalytically active, while higher pH triggers iron hydroxide precipitation and peroxide auto-decomposition. Both iron and peroxide show nonlinear behavior: beyond an optimum, excess ferrous ions and peroxide themselves scavenge hydroxyl radicals, reducing the fraction available to attack the pesticide. Strong interactions between pH and reaction time, and between iron and peroxide doses, confirmed that the reagents must be balanced rather than maximized. Thermodynamic analysis showed the process is spontaneous and endothermic, with Gibbs free energy becoming more negative as temperature rose from 298 to 328 kelvin.

After treatment, profenofos in the Haramaya, Ganda Hayyo, and Adele samples fell to 0.00084, 0.0006, and 0.0012 milligrams per liter respectively — all below the WHO limit — corresponding to removal efficiencies of 97.6 to 98.9 percent. Dimethoate removals ranged from roughly 98 to 99 percent, and overall organophosphate removal reached approximately 98.5 to 99 percent across the treated sites. Chemical oxygen demand also dropped substantially, from 128 to 32 milligrams per liter at Haramaya and from 256 to 96 milligrams per liter at Adele, bringing those samples within permissible ranges, while Ganda Qallu and Ganda Hayyo achieved complete COD removal at the optimal pH.

An economic assessment using SuperPro Designer simulation, based on a 10-cubic-meter-per-day treatment capacity operating 300 days per year, estimated a total capital investment of about 78,450 US dollars, an annual operating cost of roughly 42,680 dollars, and a treated-water cost of approximately 0.56 dollars per cubic meter. Chemicals dominated the cost structure at about 42 percent of total expenditure, followed by power at 18 percent. With a payback period of 2.7 years, a net present value of about 135,800 dollars over ten years, and an internal rate of return near 28.4 percent, the authors argue the process is financially viable and well suited to decentralized deployment in regions where advanced infrastructure is limited. They caution that pesticide disappearance does not guarantee complete mineralization, and that further work on transformation products and large-scale operation is needed — but for a region where khat farming, drought, and pesticide-laced runoff converge, a cheap bottle of iron salt and peroxide may prove to be a powerful line of defense.

Subject of Research: Fenton oxidation removal of organophosphate pesticides from contaminated water and wastewater in eastern Ethiopia

Article Title: Organophosphate pesticides in water/wastewaters in eastern Ethiopia: identification and abatement study using fenton oxidation process

Article References: Jabesa, A., Gindaba, G. T., Abo, L. D., Hamda, A. S., Edae, I. T., & Areti, H. A. (2026). Organophosphate pesticides in water/wastewaters in eastern Ethiopia: identification and abatement study using fenton oxidation process. Environmental Challenges, 25, Article 101684. https://doi.org/10.1016/j.envc.2026.101684

Image Credits: AI Generated

DOI: 10.1016/j.envc.2026.101684

Keywords: organophosphate pesticides, Fenton oxidation, water treatment, Ethiopia, profenofos, dimethoate, malathion, advanced oxidation processes, response surface methodology, khat cultivation, wastewater remediation, hydroxyl radicals