Reppe is located in the Kolfe Keraniyo Sub-city of Addis Ababa, where an open dumpsite established in 1964 has become surrounded by expanding residential communities. The site now receives ash from the nearby waste-to-energy plant, creating a direct connection between incineration residues and a long-used disposal area that lacks the engineered barriers found in a modern sanitary landfill. In such settings, rainfall can pass through waste and ash, producing leachate that dissolves or carries contaminants into surrounding soils and potentially toward groundwater. To investigate that pathway, the researchers collected three bottom-ash samples and three fly-ash samples from the plant’s ash-handling system, along with six surface-soil samples from a transect extending from the ash deposition zone toward the nearest residential area. The soil was sampled at depths of 0 to 15 centimetres, the biologically active surface layer most relevant to contact, dust and other exposure routes. Sampling took place during February and March 2024, the dry season, when rain-driven dilution was less likely to obscure contamination patterns.

In the laboratory, the team processed half-gram portions using United States Environmental Protection Agency Method 3050B, an acid-digestion procedure designed to release metals from soil and solid material before instrumental measurement. Chromium, copper, lead and zinc were measured with flame atomic absorption spectrophotometry; cadmium was measured with graphite-furnace atomic absorption, which is more sensitive at low concentrations; and mercury was analysed using a cold-vapour or hydride-generation approach. Each sample was analysed in triplicate. Quality-control procedures included certified reference materials, procedural blanks, daily calibration and replicate testing. Recoveries from the reference materials ranged from 92 to 105 per cent, while relative standard deviations were below 5 per cent. These checks indicate that the analytical procedures were precise for the samples tested, although the study’s conclusions remain limited by the small number of samples and the absence of a local uncontaminated reference soil. Instead, background values used in several calculations came from average upper-continental-crust concentrations, which may overestimate or underestimate enrichment at a particular Ethiopian site.

The most striking results came from the ash. The study reports mercury concentrations reaching 2,630 micrograms per litre and cadmium concentrations reaching 1,530 milligrams per litre in ash samples, with values varying substantially between locations and ash types. The reported mercury maximum was thousands of times higher than the comparison soil limit cited in the study, while the cadmium maximum was hundreds of times above the World Health Organization guideline used by the researchers. Because mercury and cadmium are toxic at relatively low concentrations and can be mobile under suitable chemical conditions, their presence in loosely managed ash is especially important. The study’s contamination-factor analysis identified mercury as the dominant contaminant, with ash values ranging from 12.9 to 65.8 times the chosen background. Cadmium showed moderate to severe enrichment, particularly in fly ash, whereas chromium, copper, lead and zinc generally displayed much lower contamination factors relative to the background used. The authors describe the resulting ash profile as mercury-dominated, followed by cadmium, with the other metals contributing less to enrichment.

The distinction between bottom ash and fly ash helps explain why combustion residues can concentrate particular contaminants. Bottom ash is the heavier material that remains in the furnace, while fly ash consists of finer particles carried with combustion gases and captured by pollution-control equipment. During incineration, volatile elements such as mercury and cadmium can enter the gas phase and later condense onto fine particles as the gases cool. Those particles may therefore carry a disproportionate share of certain metals. Statistical results from the Reppe ash samples support this interpretation. Principal-component analysis found that two components explained 94.5 per cent of the total variation. The first, accounting for 70.5 per cent, carried strong loadings for lead, cadmium and copper, a grouping the researchers associate with anthropogenic combustion sources such as mixed municipal waste, batteries and electronic materials. The second explained 24 per cent and was linked mainly to chromium and mercury, suggesting a combination of geochemical background and combustion-related behaviour. A strong positive correlation between lead and cadmium, reported as r = 0.91, further indicates that these elements may share a common source in the ash.

The risk calculations translate those concentrations into a broader ecological warning. The study’s total ecological-risk index for every ash sample fell between 1,086 and 3,000, well above the threshold of 600 classified as very high risk in the assessment framework. Mercury contributed more than 80 per cent of the total toxicity-unit values in the ash samples, reflecting both its measured concentration and its high toxic-response factor. Some bottom-ash samples had summed toxicity units above eight, while fly-ash values ranged from 2.93 to 4.88; the authors interpret these results as evidence that the material should be handled as hazardous waste rather than placed in an uncontained dump. The numbers are screening and assessment metrics, not direct measurements of human illness or exposure. They indicate the potential for ecological harm if contaminants are released, inhaled as dust, ingested or transported into water, but they do not by themselves establish the health risk to a particular resident. That distinction matters, especially because the study did not conduct biomonitoring, groundwater testing or epidemiological assessment.

Soils along the leachate-affected transect contained lower metal concentrations than the ash, yet mercury remained the dominant concern. Reported mercury values in the soil samples ranged from 205 to 1,015 micrograms per litre, and contamination factors ranged from 5.1 to 25.4 times the selected background. Three soil samples had ecological-risk indices at or above 600, placing them in the study’s very-high-risk category, while the full range for soils was 255 to 1,065. Cadmium was generally less enriched according to the contamination-factor analysis, and chromium, copper, lead and zinc were often near the chosen background values. However, correlations among chromium, copper and lead suggested that localized waste disposal or metal-bearing refuse had influenced parts of the transect. Mercury behaved differently: its inverse relationship with some of those metals and positive association with zinc were consistent with separate combustion and atmospheric-deposition pathways. The findings therefore do not describe a uniform plume. They show a heterogeneous site where some contaminants remain close to the ash source, while mercury may be redistributed through volatilization, condensation, leachate movement and re-emission.

Source-apportionment results estimated that anthropogenic inputs accounted for 58 per cent of the metal signal in ash and 38 per cent in leachate-impacted soil, with the remainder assigned to geogenic or mixed sources. Those percentages are model-based estimates rather than direct measurements of individual waste streams, and they depend on the background assumptions and the small dataset. Still, they reinforce the central message: the contamination cannot be explained solely by the natural composition of local soil. The authors recommend that ash, particularly fly ash exceeding hazardous-waste thresholds, be stabilized and disposed of in engineered facilities equipped with leachate collection rather than placed in the open dumpsite. They also call for groundwater monitoring, restricted access to high-risk areas, remediation of contamination hotspots and stronger national standards for waste-to-energy residues. Separating batteries, electronic waste and mercury-containing products before combustion could reduce the metals entering the ash. The study sampled a plant operating at less than half capacity, so contamination could differ with full operation or changes in the waste feedstock. Even with those limitations, the results offer a baseline for Addis Ababa and a broader warning: converting waste into energy does not eliminate hazardous elements, and the safety of the system depends on how its ash is contained after combustion.

The study’s dry-season design provides a snapshot of conditions when rainfall was unlikely to dilute surface contamination, but it cannot establish how concentrations change during wet periods. Seasonal sampling would help determine whether intensified runoff mobilizes metals beyond the sampled transect or instead redistributes them within the dump. Likewise, the six soil samples were selected purposively along visible leachate pathways, making them useful for identifying hotspots but unsuitable for estimating contamination across the entire dumpsite. Additional samples from uncontaminated soils, different depths and groundwater would improve comparisons and clarify whether metals are retained near the surface or migrating downward.

The reported measurements also require careful interpretation because the study expresses some ash and soil results in mass-per-volume units, whereas ash and soil are solid matrices. Direct comparison with soil guidelines therefore depends on the extraction and reporting basis used. Risk indices are valuable for prioritizing investigation, but they combine measured concentrations with toxicity factors and reference values; they are not substitutes for exposure measurements. Follow-up work could pair chemical testing with leachability experiments, dust monitoring and bioavailability analyses. Such evidence would show which fraction of the total metal burden can actually move into water, air or living organisms, helping regulators design controls proportionate to the site’s most plausible exposure pathways.

Subject of Research: Heavy-metal contamination in waste-to-energy ash and leachate-impacted soils

Article Title: Heavy metal contamination in thermal power plant ashes and leachate-impacted soils at Reppe dumpsite in Addis Ababa, Ethiopia

Article References: Tamene, M., Admassu, T., Alemayehu, T., & Mekonen, S. (2026). Heavy metal contamination in thermal power plant ashes and leachate-impacted soils at Reppe dumpsite in Addis Ababa, Ethiopia. Discover Soil, 3(1), Article 145. https://doi.org/10.1007/s44378-026-00291-0

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00291-0

Keywords: heavy metals, mercury, cadmium, waste-to-energy, fly ash, leachate, soil contamination, ecological risk, Addis Ababa, Heavy, metal, contamination

The study draws attention to the increasing amount of municipal solid waste (MSW) generated globally and the environmentally hazardous byproducts resulting from its improper treatment, particularly heavy metals. MSWI is a widely adopted method for solid waste management, significantly reducing waste volume while generating energy. However, during the incineration process, heavy metals such as lead, cadmium, and mercury can be released, posing severe risks to ecosystems and human health. Understanding how these metals behave during the incineration and subsequent vitrification can inform better practices in waste management and pollution control.

Through a series of meticulously designed experiments, the researchers examined how the physical and chemical properties of fly ash influence the mobility of heavy metals during the melting process. The study reveals that the transformation of solid waste into a glassy material through vitrification can effectively immobilize heavy metals, thus reducing their potential leachability into the environment. This transformation not only enhances the stability of heavy metals but also provides a viable pathway for recovering valuable materials from waste.

The researchers utilized temperature-controlled melting processes, which allowed them to assess the degree of vitrification achieved and the resultant heavy metal content within the new material. By varying the temperature and the composition of additives, they were able to optimize conditions that maximized the immobilization of heavy metals while minimizing the emission of harmful gases. The results indicated a clear correlation between the melting temperature and the effectiveness of heavy metal containment, leading to recommendations for optimal operational practices in waste management facilities.

Additionally, the study highlights the importance of understanding the chemical interactions between different compounds present in fly ash during the melting process. These interactions can either facilitate or hinder the vitrification process, affecting the final properties of the vitrified product. The research team employed advanced analytical techniques, including scanning electron microscopy and X-ray diffraction, to acquire detailed insights into the microstructure of the vitrified materials, further providing a clear characterization that could guide future engineering applications.

The implications of this research extend beyond academic interest; they hold significant relevance for policymakers and industrial practitioners aiming to enhance current waste management practices. The findings advocate for the adoption of vitrification as a reaffirmed strategy in the sustainable management of municipal waste, promoting a circular economy where resources are reused and environmental impacts minimized.

With the global push toward stricter environmental regulations and increased pressure to develop innovative waste management solutions, the research conducted by Li et al. represents a timely contribution to the discourse. It underscores the necessity for a paradigm shift in how we view and treat waste, recognizing its potential as a resource rather than merely a byproduct to be disposed of.

Furthermore, this study highlights the urgent need for multidisciplinary approaches when addressing environmental challenges. Collaboration between engineers, environmental scientists, and waste management professionals will be vital in developing robust systems that can effectively manage the complex interplay of waste disposal, resource recovery, and environmental protection.

As cities around the world continue to grapple with rising waste generation, studies like this pave the way for future innovations in waste treatment technologies. By understanding the behaviors and properties of materials such as fly ash, we can develop more effective systems to mitigate pollution and recover energy and materials from waste streams.

In conclusion, the research by Li and colleagues shines a light on an often-overlooked aspect of municipal solid waste management while providing critical insights for future developments in the field. The ability to understand and control the migration of heavy metals during the melting and vitrification of fly ash not only enhances current waste treatment practices but also holds promise for advancing sustainable waste management strategies globally.

This research represents a crucial step toward a future where waste is effectively transformed into valuable resources, demonstrating that with innovative thinking and scientific inquiry, we can tackle the pressing environmental challenges of our time.

Subject of Research: Migration behaviour of heavy metals and vitrification characteristics in melting of municipal solid waste incineration fly ash.

Article Title: Migration behaviour of heavy metals and vitrification characteristics in melting of municipal solid waste incineration fly ash.

Article References:

Li, Q., Gao, Y., Geng, C. et al. Migration behaviour of heavy metals and vitrification characteristics in melting of municipal solid waste incineration fly ash.
ENG. Environ. 20, 52 (2026). https://doi.org/10.1007/s11783-026-2152-6

Image Credits: AI Generated

DOI: 20 January 2026

Keywords: Heavy metals, vitrification, municipal solid waste incineration, fly ash, waste management, environmental impact.