Remediation of Contaminated Soils Using Organic Waste and Waste Products in Sub-Saharan Africa: A Review of Technologies, Adoption and Challenges

Abstract

Soil contamination in Sub-Saharan Africa (SSA) is increasingly driven by rapid industrialization, intensive agriculture, mining activities, and urban expansion, posing significant risks to food safety, ecosystem services, and human livelihoods. Despite the growing scale of the problem, low-cost, locally adaptable remediation technologies are widely available and technically feasible within the region. Organic waste and waste-derived products—such as compost, manure, biochar, vermicompost, digestate, and agro-industrial residues—have emerged as sustainable and cost-effective amendments for the remediation of contaminated soils. These materials can immobilize heavy metals, enhance the microbial degradation of organic pollutants, and improve soil health, making them especially suitable for resource-constrained settings. This review synthesizes the current knowledge on the use of organic waste-based remediation approaches in SSA, highlighting technologies already applied at the laboratory, pilot, and field scales, as well as their effectiveness across different contaminant types. However, despite their demonstrated potential, their widespread adoption remains limited. The primary challenge is not the absence of affordable solutions, but rather the systemic constraints characteristic of many SSA countries, including limited technical capacity, weak policy and regulatory frameworks, low stakeholder awareness, and insufficient financial and institutional support for large-scale implementation. To enable broader uptake, there is a need to strengthen waste segregation and treatment systems, standardize composting and pyrolysis processes, and develop robust regulatory guidelines and certification schemes. Investments in monitoring infrastructure, practitioner training, and knowledge transfer mechanisms will also be critical to translating scientific advances into scalable, field-ready solutions for sustainable soil remediation in SSA.

1. Introduction

Soil contamination by toxic elements and organic pollutants is an escalating environmental and public health concern across Sub-Saharan Africa (SSA). The multiple, geographically concentrated soil-contamination hotspots of SSA are a direct result of a multitude of factors which include rapid urban expansion, industrial activities, informal mining, intensive unsafe agrochemical use and unregulated waste disposal systems. Together, these have contributed to elevated levels of heavy metals such as lead (Pb), cadmium (Cd), chromium (Cr), arsenic (As), and nickel (Ni) in urban and agricultural soils, leading to the creation of contamination hotspots across the region [1,2]. Estimates further show that around 25–30% of the total land area in SSA is degraded, including various forms of contamination, and that around 65% of croplands in SSA are degraded to varying degrees [3,4]. Recently SSA’s soil contamination problem has grown to include non-conventional contaminants, namely electronic waste (E-waste) and radioactive materials [5,6]. While much of the peer-reviewed reporting on the E-waste contamination of soils focuses on Ghana, Nigeria and South Africa [7], informal and risky recycling activities have been reported in several other countries in the region [8]. There is only limited information on the magnitude of the problem of soil contamination by radioactive substances in SSA. A few recently reported cases are summarized in Figure 1. Despite the existence of widespread contamination, significant spatial research gaps do exist, with many countries lacking contamination data despite rapid urban growth, agricultural demands and industrial pressures [9].

Conventional high-technology remediation options such as thermal treatment, large-scale soil washing, and engineered pump-and-treat systems have proven effective in industrialized contexts but are often prohibitively expensive, resource-intensive, and technically demanding [10]. Such constraints limit the scalability and adoption of these high tech technologies in SSA, where financial resources, infrastructure, and regulatory capacity are frequently limiting. There remains an urgent need, therefore, for cost-effective, low-technology, and locally appropriate remediation strategies that align with resource limitations and development priorities within the region.

Against this backdrop, the utilization of organic waste and waste-derived products as soil amendments for remediation is attracting increasing scientific and practical interest. SSA generates large volumes of organic residues and by-products such as crop wastes (rice husks, maize cobs, cassava peels), manure, sewage sludge, and agro-industrial materials (bagasse, palm kernel shells). These organic waste and waste-derived products are often underutilized or contribute to additional environmental burdens. Converting them into value-added soil amendments such as compost, biochar, and digestates offers a promising circular economy approach that simultaneously addresses waste management and soil contamination challenges [11].

However, much of the existing evidence remains fragmented. Studies show that while many waste- and waste-product-based remediation technologies have been tested at the laboratory or screen house scale, only a few field-scale application studies have been reported [12,13]. Variations in feedstock quality, pyrolysis parameters, amendment rates, and soil conditions make cross-study comparison challenging. In addition, there is limited understanding of the long-term stability of immobilized contaminants, potential secondary risks (e.g., the introduction of pathogens or undesirable elements from poorly characterized waste inputs), and the fate of organic amendments under diverse climatic and soil regimes typical of SSA. Equally important to technical feasibility are socio-economic and institutional factors that influence adoption. The availability and seasonality of waste feedstock, cost and labor requirements for amendment production, land tenure dynamics, farmer knowledge and perceptions, and policy or regulatory frameworks governing waste reuse and soil quality standards all shape the practicality and scale of organic amendment deployment. In many SSA countries, the absence of clear guidelines for amendment quality and safe application creates uncertainty, reducing uptake among land managers and practitioners [14].

Despite growing global interest in organic waste-based remediation, only a few studies have specifically contextualized these technologies within SSA’s complex environmental, socio-economic, and institutional landscape. To address this gap, we review the available evidence on the use of organic waste and waste products for soil remediation in SSA, integrating technological mechanisms with real-world adoption dynamics and practical challenges. We explore the available evidence on the soil contaminants most commonly found in the region as well as available technologies and the extent of their adoption for the remediation of contaminated soils in SSA. Through the current peer-reviewed and gray literature on organic waste-based remediation technologies, we discuss the available evidence on organic waste availability and generation capacity in the SSA region as well as co-benefits and socio-institutional barriers to adoption. Future research focus areas and the technical and policy directions for the scaling up of safe and effective practices across SSA are also discussed.

2. Soil Contaminants Most Commonly Found in the SSA Region

In SSA, soil contamination is characterized by a complex mixture of organic and inorganic contaminants, microbiological, radioactive materials, and emerging contaminants (

Table 1. Soil contaminants commonly found in various countries in SSA.

Contaminant Category	Contaminant Subcategory/Group	Contaminant Type	Location/Context	Notes	Permissible Limits (Means) *	References
Inorganic Contaminants	Heavy metals	Pb,	* Kabwe (town/mining areas), Zambia	Extremely elevated soil Pb and blood lead levels in children of up to 11.9 μg/dL.	<5 μg/dL for Pb,	[15]
		Pb, Ni, Cu, Cr	Katoro, artisanal gold-mining areas in Geita, Tanzania	Pb, Cr, Cu, and Ni had soil concentrations (mg/Kg) of 11.1–14.4, 117–1255, 81–104, and 89–271, respectively, from sampled areas.	50 mg Pb/kg 50 mg Ni/kg 100 mg Cu/kg 100 mg Cr/kg	[16,17,18]
		Mn, Zn Pb, Cd	* Minna, Niger State, Nigeria	Soils were found to have up to (mg/kg) 2.87, 286.21, 15,375.46, 2233.60 and 131.71 for Cd, Zn, Fe, Mn, and Pb, well above FAO/WHO permissible limits.	80 mg Pb/kg 500 mg Mn/kg 1 mg Cd/Kg 200 mg Zn/kg	[17,18,19,20]
	Metaloids	As	Marrakech, Morocco	Urban soils were found to have up to  8.64 mg/kg, mainly attributed vehicle emissions and corrosion of source materials.	20 mg/kg	[17,18,19,20,21]
			Kumasi, Ghana	Soil concentration of As was measured at 54.3 mg/kg, attributed to automotive repairs associated with auto-mechanics workshops.		[22]
		Sb	Thebephatshwa (TAB) shooting range, Botswana	Soil concentrations of Sb were found to be 38.8–283.5 mg/kg, attributed to shooting range activities.	20 mg/Kg	[17,23,24]
Organic Contaminants	Petroleum hydrocarbons	TPH, PAHs	* Niger Delta/Ogoniland (coastal wetlands)	Concentrations above permissible limits, reaching 14,087 to 36,775 ppm of TPH and 3248.75 ppm of PAH.	TPH < 100 μg/kg PAH < 200 μg/kg	[25]
	Persistent organic pesticides (POPs)	DDT, Diedrin,	* Tanzania (agricultural soils)	Soils had concentrations of 2.29 ng/g DDT, 1.57 ng/g Diedrin.	0.1 mg/kg for DDT, 0.05 mg/kg for Diedrin.	[26,27]
Radioactive contaminants	Naturally occurring radioactive materials (NORMs) in soils	238U, 232Th,	Manyoni, Tanzania (uranium deposit)	Levels of each NORM were above global average (in Bq/kg). 238U = 302 232Th = 78. Some areas of Manyoni were classified as high-background-radiation areas (HBRAs).	International limits (Bq/kg) are 238U = 33 232Th = 45	[28]
	A mixture of NORMs in Industrial zone	238U, 232Th, 40K	Johannesburg, South Africa	Levels of soil0related materials around industrial zones were (Bq/kg) 238U = 37.6 40K= 657.	International limits (Bq/kg) are 238U = 33 40K = 400–420	[29]
Microbiological contaminants	Solid Fecal contaminant materials	Ascaris lumbricoides	Addis Ababa, Ethiopia	Near-universal presence of helminth ova in soils.	≤1 viable parasitic helminth egg per liter	[30,31]
	Contaminated wastewater for irrigation	Fecal coliforms, enteric pathogens	Addis Ababa, Ethiopia	Extensive contamination across soil–water–crop continuum.	<10 CFU fecal coliforms/g of soil	[32]
	Contaminated irrigation water	E. coli, Salmonella, helminths	Northern Ghana	Measurable pathogen loads of up to 4.58 log10CFU gDW−1 in soils were detected.	<10 CFU fecal coliforms/g of soil	[33]
	Fecal contaminated wastewater used for irrigation	Escherichia-Shigella, Clostridiaceae, Enterococcaceae	South Africa	Diverse pathogenic microbial communities linked to fecal contamination.	0 CFU of –Escherichia-Shigella	[34]
Emerging Contaminants	Pharmaceuticals and personal care products (PPCPs) and per- and polyfluoroalkyl substances (PFAs) and microplastics		South Africa, Nigeria, Kenya, Ghana, and Tanzania	Wastewater irrigation, sludge use and unregulated waste dumping.	NA *	[35]
			South Africa, Nigeria, and Ghana	Unregulated E-waste-disposal hotspots. These countries showed the largest amounts.	NA *	[7,36,37]

* Original studies used a mixture of national standards, international guidelines provided by the WHO/FAO and the EU directive 86/278/EEC on use of sewage. sludge.

). Accordingly, all such contaminants end up in soils through a combination of anthropogenic pressures, namely, industrial growth, waste mismanagement, agricultural intensification, and informal sector activities on the one hand and inherent geological characteristics on the other hand. Despite the documented presence of these contaminants, regional monitoring remains fragmented, leading to a persistent problem of data gaps, which complicates efforts to assess spatial patterns and to formulate robust regulatory responses.

Toxic trace elements constitute one of the most extensively reported categories of soil contaminants in SSA. This category is made up of toxic inorganic elements, commonly referred to as heavy metals or potentially toxic elements (PTEs), which include lead (Pb), cadmium (Cd), chromium (Cr), arsenic (As), mercury (Hg), nickel (Ni), zinc (Zn), copper (Cu), manganese (Mn), antimony (Sb) and cobalt (Co). PTEs may originate from one or several of the following: mining activities, industrial effluents, municipal dumpsites, vehicle emissions, battery recycling, and poor waste handling practices. Recent studies and reviews underscore the pervasive nature of these pollutants and emphasize the limited monitoring and regulatory enforcement across many SSA countries. While contamination by Pb and Cd are a common occurrence in areas near mining sites and municipal waste dumpsites, high concentrations of Cr are characteristically reported in areas dominated by tannery wastes [38,39]. It is also common, however, to find reports of multiple PTEs in urban soils due to industrial and municipal sources [40].

Unlike more frequently monitored heavy metals and/or metalloids, such as Pb, Cd, Zn, Ni, As and Hg, research on some potentially toxic elements, such as antimony (Sb) and cobalt (co), remains limited in SSA, with most available data tied only to localized environmental studies and mining-related assessments rather than broad regional surveys. Antimony, for example, has been reported to contaminate soils near mining dumpsites in Nigeria [41] and Botswana [25]. Similarly, a couple of recent studies have documented cobalt contamination in SSA soils, particularly in mining regions and urban and peri-urban agricultural areas of the region [42,43,44]. The studies highlighted above underscore the environmental persistence of both antimony and cobalt in soils of SSA, which requires a collective soil monitoring and risk assessment plan of action in regions with intensive mining, industrial, or urban activity.

Africa, and particularly SSA, has an endemic problem of the contamination of its soils by persistent organic pollutants (POPs). Recent reviews have shown, for example, that perfluorooctane sulfonate, a synthetic perfluoroalkyl substance listed as a POP under the Stockholm Convention in 2009, has contaminated soils across the region, reaching recorded levels of up to 275.3 ng/g with potential ecological and human health implications [45]. POPs of an organochlorine nature such as lindane and DDT have a long history of use in agriculture, and thus contamination in agricultural soils above permissible limits has been reported years after they were banned from use [46]. Polychlorinated biphenyls, a classic POP group with strong persistent bioaccumulative behavior, have also been reported in soils near urban centers of SSA [47]. An extensive review of the data from previous years has documented a persistent and measurable trend of polycyclic aromatic hydrocarbon (PAH) contamination throughout SSA environments, with ranges of up to 435.7 ng/g in surface soils in Ethiopia and Sierra Leone [48,49,50]. There is also documented widespread environmental exposure linked to combustion processes, petrochemical effluents, and biomass fuel use, with implications for soil and the broader environmental contamination of PAH in the region [51]. The evidence presented here from both targeted studies and regional reviews underscores that soil contamination by organic pollutants in SSA is not isolated but broadly distributed, with varying concentrations tied to urbanization, industrial activities, agricultural history, and waste management practices.

Naturally occurring radioactive materials (NORMs) like uranium series (²³⁸U and ²²⁶Ra), thorium series (²³²Th), and potassium-40 (⁴⁰K) are intrinsic constituents of the Earth’s crust. Their concentrations in soil vary with local geology, mineralization, and land-disturbing activities such as mining or quarrying. Recent studies confirm that NORMs are widely present in SSA soils, with measurable concentrations of ⁴⁰K, ²³⁸U, ²²⁶Ra, and ²³²Th documented in Tanzania, Nigeria, South Africa, and Ethiopia. Uranium-rich geological formations such as those found in Manyoni, Tanzania, for example, can produce elevated levels of ²³⁸U and ²³²Th, with significant exposure risks for humans and animals surrounding the deposits [28]. Similarly, mining and quarrying operations often expose deeper rocks, increasing measurable levels of the natural radioactivity of ⁴⁰K, ²³⁸U, and ²³²Th at the soil surface in Nigeria [52]. In Ethiopia, baseline soils in non-industrial regions still contain measurable NORM concentrations reflective of natural geological variability [53]. A separate study found that most radionuclide concentrations in South African soil samples taken near industrialization activities are above the permissible limits [54].

Soil contamination from electronic waste (e-waste) has emerged as a significant environmental issue in Sub-Saharan Africa, particularly around informal recycling and disposal sites where crude recovery practices prevail. The informal processing of e-waste involves open burning, manual dismantling, and acid leaching to recover valuable metals, releasing hazardous substances such as lead (Pb), cadmium (Cd), mercury (Hg), brominated flame-retardants, and other persistent organic pollutants directly into surrounding soils [1]. Elevated concentrations of these toxic elements have been documented in soils at major e-waste hubs, exceeding international safety limits and contributing to widespread contamination that extends into residential, agricultural, and recreational areas. This contamination threatens soil fertility, undermines food safety through crop uptake of heavy metals, and exacerbates environmental health disparities across the region.

Soils in sub-Saharan Africa are increasingly impacted by contaminants of emerging concern (CECs), including pharmaceuticals and personal care products (PPCPs), per- and polyfluoroalkyl substances (PFAS), endocrine-disrupting compounds, and microplastics [55,56]. Soil acts as a major sink for these contaminants, primarily through wastewater irrigation, sludge application, landfill leachate, and urban runoff [56]. Evidence from continental reviews and case studies shows that PFAS are now detectable in soils, crops, and sediments, with documented ecological risks and bioaccumulation potential in soil–plant systems [45,57,58]. Similarly, PPCPs—particularly antibiotics and analgesics—are widely reported at environmentally relevant concentrations, often exceeding ecological risk thresholds [58]. Despite increasing detection, data remain spatially uneven across SSA, with Southern and West Africa dominating the literature [58]. Overall, the growing occurrence and persistence of these contaminants highlight an urgent need for soil-focused monitoring frameworks and regulatory strategies in the region [58].

3. Available Technologies and Extent of Their Adoption for Remediation of Contaminated Soils in SSA

There exists a range of technologies that can be adapted to local conditions for more effective soil contaminant removal in various countries of SSA. The technologies span a wide spectrum from cost-intensive engineered methods to low-cost biological and plant-based strategies suitable for many SSA contexts. Based on the overall outcome, the remediation technologies fall into two broad categories: (i) those that immobilize contaminants in situ, reducing their mobility and bioavailability without outright removal from the contaminated medium and (ii) technologies that achieve either the removal or destruction of contaminants—physically extracting or chemically and/or biologically degrading them. The literature further classifies remediation technologies into physical, chemical, biological, and thermal approaches, based on the dominant mechanisms governing contaminant removal, transformation, or stabilization [59,60,61,62]. This distinction is critical for selecting cost-effective and context-appropriate solutions, especially in soil and groundwater systems.

The physical technologies category includes technologies such as soil washing and physical separation methods, soil vapor extraction, excavation and off-site disposal, and containment and physical separation techniques such as capping (engineered surface covers), encapsulation, physical separation bars and/or containment cells and thermal destruction and volatilization techniques [10]. Under this category, technologies are usually designed to achieve the mechanical removal and transportation of the contaminant for off-site treatment or secure containment. Soil washing, for example, uses water, surfactants or other agents to detach contaminants from soil particles, effectively reducing contaminant mass and enabling reuse of the cleaner soil fraction. These processes leverage differences in physical properties—mainly density, particle size, and solubility—to extract pollutants into liquid or vapor phases for collection and treatment. Similarly, the ex situ physical removal of contaminated topsoil, also known as soil excavation, typically involves digging out the contaminated soil for off-site treatment or secure disposal. It is recognized as a conventional method for heavily polluted urban, industrial, or mining-impacted lands. A classic example of physical removal is the work in Zamfara State, Nigeria, where more than 27,000 m³ of lead-contaminated soil and mining waste were excavated from severely contaminated soils around residences in the early 2010s, leading to an approximate 89% reduction in soil lead concentrations and dramatic health improvements in children [63].

Thermal technologies are a variant of physical remediation technologies, and they include technologies such as thermal desorption, incineration, and other heat-based treatments. These use heat to volatilize, destroy, or mobilize contaminants, effectively reducing the organic pollutant loads of a contaminated soil, while electrochemical and electrical technologies such as electrokinetic remediation and electrical resistance heating involve the use of electric currents or fields to mobilize ions and contaminants, which enhances the transport and extraction of pollutants [64]. Thermal technologies have been deployed to remediate contaminated soils in parts of SSA, including Cape Town, South Africa, where in situ thermal remediation was carried out to address subsurface petroleum hydrocarbon contamination at a former service station [65], and in Nigeria, where indirect thermal desorption units were used treat oily sludge, drill cuttings, and hydrocarbon-impacted soils [66].

Common chemical technologies include in situ chemical oxidation, nanotechnology and advanced materials-based technologies, which use engineered nanomaterials such as nanoscale zero-valent iron and/or carbon nanotubes to enhance contaminant removal through high-reactivity, adsorption, and redox processes. A feasibility analysis specifically for Kenya lists chemical oxidation and reduction, soil washing, and soil flushing among effective physical/chemical remediation options that could be implemented locally, particularly where the rapid treatment of organic and inorganic contaminants is required [67]. Soil washing and chemical extraction were reported to successfully help clean-up efforts in Nigeria [68]. In Nigeria, for example, soil washing has been applied mainly at laboratory and pilot scales to remediate soils contaminated with both heavy metals and petroleum-related organic pollutants, often occurring together in mixed-contamination environments. Studies from auto-mechanic and urban waste sites demonstrate the removal of toxic metals such as cadmium (Cd), lead (Pb), copper (Cu), nickel (Ni), and zinc (Zn) alongside total petroleum hydrocarbons (TPHs) using surfactant-assisted washing techniques, with removal efficiency influenced by operating conditions such as pH, surfactant concentration, and washing time [69]. Similarly, chemical extraction methods for soil remediation have been primarily applied to heavy metal-contaminated soils, particularly in Nigeria and South Africa, using chelating agents such as EDTA, EDDS, and organic acids. These approaches have demonstrated the effective removal of metals including Pb, Cd, Zn, Cu, and Ni, with additional applications in co-contaminated soils containing petroleum hydrocarbons through combined surfactant–chelant systems [68]. While most applications remain at the laboratory or pilot scale, the results highlight the strong potential of chemical extraction technologies for treating mining, industrial, and oil-impacted soils in SSA, particularly when coupled with metal recovery or integrated remediation strategies.

Biological methods such as bioremediation and phytoremediation deploy living organisms or their enzymes to degrade, transform, or accumulate contaminants [10,70]. Biological remediation methods, especially phytoremediation and bioremediation, are among the most widely studied technologies for managing contaminated soils and water in the Sub-Saharan African (SSA) context due to their relative cost-effectiveness, environmental compatibility, and suitability to resource-limited settings. A recent regional review explicitly identifies bioremediation and phytoremediation as feasible strategies in SSA, noting their use of indigenous microorganisms and plants to transform or remove contaminants [10]. Similarly, continent-wide analyses of potential phytoremediation species have highlighted hundreds of native plants with remediation potential across African mining-impacted landscapes, underscoring the extensive research focus on plant-based cleanup strategies in response to heavy metal pollution [71]. Recent research shows further that the application of microbial communities naturally adapted to contaminated mine soils across SSA can help to metabolize, transform, and stabilize toxic compounds in metal-laden soils. For example, studies show that proteobacteria and firmicutes with metabolic traits that support natural attenuation have proved useful in the stabilization of contaminants in heavy metal-contaminated environments [72]. Overall, across Sub-Saharan Africa (SSA), biological remediation using microbial consortia has demonstrated high degradation efficiencies for organophosphates, while fungal and bacterial strains can degrade persistent organochlorines. In addition, inoculated systems enhance simazine degradation and reduce mobility, and tropical soil microorganisms have shown the ability to degrade pyrethroids [73,74,75]. Studies from oil-impacted regions such as Nigeria show that microorganisms can degrade total petroleum hydrocarbons (TPHs) and polycyclic aromatic hydrocarbons (PAHs) into less harmful products [76,77]. Biological processes have also been applied to heavy metal-contaminated soils in mining regions (e.g., Zambia and South Africa), where mechanisms such as biosorption and bioaccumulation contribute to the attenuation of metals including Cu, Zn, Pb, Cd, and Ni [10].

Despite the availability of diverse soil remediation technologies, as discussed above, most of them are rarely implemented at scale in SSA due to a combination of economic, technical, institutional, and contextual constraints. Many physical, thermal, and advanced chemical technologies are capital-intensive, requiring sophisticated equipment, a continuous energy supply, and highly skilled personnel, all of which are unavailable in many resource-constrained countries of SSA [10,78,79]. Since most countries in SSA are resource-constrained, low-cost, nature-based approaches to remediation, including bioremediation and phytoremediation, or hybrid, low-cost remediation options such as compost or the biochar-assisted immobilization of contaminants are the most widely tested technologies, although these are usually applied at the local scale.

Hybrid remediation technologies integrate biological processes with chemical or physical enhancements, which offer a particularly useful pathway for decontaminating soils in Sub-Saharan Africa (SSA). Organic amendment-assisted phytoremediation is a form of hybrid remediation technology which takes the form of compost- or biochar-supported systems by combining microbial stimulation, sorption, pH adjustment, and plant uptake or stabilization mechanisms to reduce metal bioavailability while restoring soil fertility. In one study, for example, researchers demonstrated that biochar application can help enhance plant growth and influence heavy metal bioavailability to Tithonia diversifolia, leading to their phytoextraction and phytostabilization [80]. Work on species like Acacia karroo in South Africa and Cyperus rotundus on Kenyan artisanal gold mine soils provides evidence that native or regionally adapted plants possess the ability to uptake or stabilize trace elements, supporting the case for plant-based remediation in SSA settings [81,82].

Hybrid systems that use waste and waste products being prepared for compost, biochar or other organic waste amendments to directly extract contaminants from the soil or form part of organic amendment-assisted phytoremediation have a key advantage of enhancing the degradation of hydrocarbons or immobilization of trace metals. This is achieved through complementary bio-physico-chemical processes, thereby addressing a key limitation of standalone bioremediation, that is, slow kinetics or limited contaminant accessibility [61,83]. Beyond single amendment effects, combined strategies—such as biochar with compost or organic fertilizer—have shown enhanced soil health benefits and reduced contaminant transport, while altering the microbial community dynamics in contaminated soils. Biochar also interacts with phytoremediation processes to influence the plant uptake of contaminants, with recent studies showing how amendment type and rate can shift plant species from accumulating to stabilizing specific elements. These multifaceted mechanisms suggest that organic waste-based remediation can offer gentle, sustainable, and multifunctional remediation options that improve soil quality alongside contaminant attenuation [57].

4. Organic Waste and Waste Products: Availability and Use in Remediation of Contaminated Soils of SSA

Apart from crop residues [84] and animal manure [85], the main source of organic waste in Sub-Saharan Africa, like anywhere else in the world, is the municipal solid waste (MSW) generated in various cities of the region. MSW generation is rapidly growing in the region, with the current total of 244 Mg y⁻¹ projected to reach over 269 million Mg y⁻¹ by 2030 [86]. Organic biodegradable waste constitutes up to 63% of all MSW produced in SSA [61] and is projected to exceed 100 million Mg annually by 2030 [87]. This trajectory is set to grow in SSA, where roughly 37% of all the food produced is either lost or wasted, with an average per capita food wastage of 170 kg per annum. The organic fraction of municipal solid waste (OFMSW) currently produced in various cities of SSA ranges from 40 to 65% [88,89,90] which is equivalent to about 70–99 M Mg/yr OFMSW regionally [91]. Most of the organic waste generated in the region is either landfilled, openly dumped or burned due to the rudimental or complete absence of waste collection and treatment infrastructure.

The biggest challenge to governments and other private actors in SSA is the ability to divert waste from crop residues and the OFMSW into alternate uses, such as recycling for nutrients or development of waste products for use in climate mitigation and the remediation of contaminated soils. Accordingly, where they have been deployed, organic waste or waste products have achieved both a rapid increase in soil organic carbon (SOC) and the removal of contaminants and phytopathogenic agents from the soils in various countries of SSA [92,93,94,95]. A selection of successful projects on the use of biodegradable organic waste in the remediation of contaminated soils from the region include the following:

(a)

The Lead (pb) Legacy of Kabwe, Zambia: Kabwe carries a canonical example of intensive Pb contamination from historic lead–zinc mining in Zambia, which led to over 3000 mg·kg⁻¹ of lead at some of its contamination hotspots. However, remediation research has shown that biochar and phosphate amendments have helped to reduce bioavailable Pb and lower the plant uptake of the contaminant [95]. Biochar immobilizes Pb in contaminated soils through multiple mechanisms that reduce its solubility, mobility, and plant uptake. This is because Pb²⁺ ions bind to oxygen-containing functional groups (–COOH, –OH, carbonyl) on biochar surfaces via adsorption and complexation [96]. Its alkaline nature raises soil pH, promoting the precipitation of Pb(OH)₂ and PbCO₃, while mineral constituents like carbonates and phosphates facilitate the formation of stable, insoluble Pb minerals such as pyromorphite [96]. Biochar also enhances the soil cation exchange capacity, shifting Pb from bioavailable to stable fractions, thereby lowering plant uptake and reducing ecological and human health risks [97]. Phosphate, on the other hand, immobilizes Pb by forming stable Pb–phosphate minerals, enhancing adsorption by soil particles, and reducing soluble Pb in soil solutions, which together decrease Pb bioavailability and plant uptake [97]. It is clear, therefore, that biochar helps to immobilize the contaminant Pb, averting its uptake and translocation to edible parts of crop plants, which in turn prevents its entry into the food chain.

(b)

Niger Delta Oil Contamination Problem in Nigeria: The oil contamination crisis in the Niger Delta, Nigeria, is among the most severe environmental challenges in Sub-Saharan Africa. Decades of oil exploration have led to thousands of spills, contaminating soils, mangroves, and water bodies, particularly in Ogoniland. A landmark assessment by the United Nations Environment Programme [98] found widespread hydrocarbon pollution and warned that full restoration could take 25–30 years [98,99]. Studies show, however, that compost, poultry manure, chicken manure digestate (CMD) and pig dung were successfully deployed in the Niger Delta to provide essential nutrients (N, P, K) to indigenous microbes, which in turn, helped in the degradation of hydrocarbons in contaminated soils. These organic amendments were shown to stimulate bacterial, especially Pseudomonas aeruginosa and fungal growth, and thus the biodegradation of petroleum hydrocarbons, achieving, in some cases, between 70 and 95% petroleum hydrocarbon reduction in contaminated soils [100,101,102].

(c)

Smallholder/Municipal Composting Pilot Projects for Remediation of Low-Level Heavy Metal-Contaminated Urban Areas in Ghana and Tanzania: Smallholder and municipal composting pilot projects in Ghana are emerging as a sustainable, low-cost strategy to remediate urban soils lightly contaminated with heavy metals including Pb, Cd, Zn, and Cu. These initiatives focus on using organic waste to rehabilitate soil, enhancing nutrient levels while reducing the mobility and bioavailability of contaminants in urban agriculture [103,104,105]. Similarly, in Tanzania, pilot projects and initiatives for composting to address heavy metal-contaminated soils are primarily concentrated in Dar es Salaam, where key strategies include converting municipal organic waste into soil conditioners, implementing small-scale community composting, and employing phytoremediation to stabilize heavy metals in urban areas [106]. Further studies have shown that the community composting of market/yard waste to produce compost for urban vacant lots and peri-urban farms with low-level mixed contamination of heavy metals and hydrocarbons were proven to successfully reduce PCB/TPH fractions in the soils to which they were applied [107,108].

(d)

Use of Organic Waste to Promote Remediation of Contaminated Soils in Witwatersrand, South Africa: Soil contamination in this area is a severe, long-standing environmental problem primarily driven by gold mining activities, leading to high levels of heavy metals like arsenic (As), lead (Pb), copper (Cu), zinc (Zn), cadmium (Cd), and chromium (Cr) from mine tailings and acid mine drainage (AMD). Organic amendments like compost derived from organic waste enhance soil fertility, increase organic matter and nutrient availability, and create favorable conditions for plant establishment and phytoremediation on degraded mine tailings, which are otherwise hostile to vegetation due to heavy metal contamination and poor structure [109]. Studies have reported the efficacy of compost and Moringa leaf extract (MLE) in enhancing the phytoremediation potential of vetiver grass on the gold mine tailings dam in the Witwatersrand goldfields [109]. A similar report in South Africa showed that organic waste-based compost enhances the capacity of vetiver grass to accumulate and stabilize multiple heavy metals in metal-contaminated soils and waters, which indicates the synergistic benefits of combining organic waste-derived amendments with plant-based remediation [110]. Generally, however, most studies in the region have focused on the immobilization of potentially toxic elements such as lead, cadmium, zinc, copper, arsenic and mercury rather than complete contaminant removal. Among organic waste types, compost—derived from OFMSW and animal manure—and biochar—derived from agricultural residues such as rice husks, maize cobs or coffee husks—are the most frequently investigated [111,112,113]. Compost and manure amendments are primarily used to enhance soil organic matter, buffer pH, and reduce metal bioavailability, while biochar is increasingly favored for its strong sorption capacity, persistence, and multifunctional benefits. Integrated approaches—combining compost, biochar, and phytoremediation—represent an emerging research frontier and are widely regarded as the best practice for SSA conditions.

5. Effects of Organic Amendments on Accessibility and Bioavailability of Soil Contaminants

The interactions between organic amendments and pollutants in soil are complex and can significantly change the behavior, mobility, and ecological impact of both natural and harmful compounds, which in turn influences the accessibility and/or bioavailability of soil contaminants [114]. Organic amendments such as biochar, compost, farmyard manure, and residue-derived organic materials play a critical role in modifying the accessibility and bioavailability of soil contaminants, particularly heavy metals and metalloids, by altering soil chemical, physical, and biological properties [115]. Primarily, organic amendments tend to increase soil organic carbon content, which may lead to alterations in the pH and cation exchange capacity of the soil. Alterations in the pH of the soil will in turn lead to alterations in contaminant solubility, speciation and mobility [116], while the additional OC content of the soil also increases sorption sites for both organic and inorganic contaminants, thus potentially reducing their mobility and bioavailability to plants and soil microbes [115,117]. For example, biochar’s high porosity, large surface area, and functional groups strongly adsorb heavy metals, decreasing their leachability and bioavailability to plants and soil biota [118]. When combined with biological amendments such as arbuscular mycorrhizal fungi, the reduction in heavy metal uptake can be even greater, with studies showing reductions in lead (Pb), cadmium (Cd), and nickel (Ni) in grain by up to 93%, 76%, and 83%, respectively, while also enhancing plant growth and soil enzymatic activity [118,119]. A summary of the main courses of action when organic amendments are introduced into contaminated soil is shown in Figure 2.

The specific properties of organic amendment including their pH, nature and quantity of surface functional groups, as well as their structural stability, significantly influence the mechanisms by which they remediate contaminated soils. Materials like biochar and compost, for example, can alter soil pH, which affects contaminant speciation and sorption. This is because a higher pH often increases negative surface charge and enhances the electrostatic adsorption of metal cations, promoting the precipitation of metal hydroxides/carbonates and reducing their bioavailability. These pH-mediated shifts also influence the dissociation of functional groups such as carboxyl and hydroxyl groups, altering metal complexation and mobility in the soil system [120].

The surface functional groups such as –COOH and –OH on residues provide active binding sites for contaminants. As a result, oxygen-containing moieties on biochar and organic matter can form stable complexes with heavy metals and facilitate hydrogen bonding and other interactions with organic pollutants, thereby enhancing sorptive immobilization. These groups also act as nucleation points for the ion exchange and surface complexation reactions that sequester metal ions away from soil pore water [121].

The structural stability of the organic amendment, which is influenced by properties like porosity, aromaticity, and resistance to decomposition, determines how long residues persist in soil and how effectively they influence contaminants’ fate. More stable amendments, such as high-temperature biochars with developed pore networks and aromatic structures, can offer greater specific surface areas and sustained adsorption capacity, while less stable materials may contribute to increased labile organic matter that more rapidly alters microbial activity and contaminant transformation pathways. Mechanistically, therefore, increased soil organic matter due to use of organic amendments does provide binding sites and promote formation of metal–organic complexes, effectively reducing contaminant solubility in the soil solution, while enhanced aggregation and microbial processes can further stabilize pollutants or facilitate their degradation under certain conditions.

In regions such as Sub-Saharan Africa, where soils are increasingly affected by contamination from mining, waste disposal, wastewater irrigation, and agrochemical residues, organic amendments offer accessible and sustainable remediation options that complement local agricultural practices. Research indicates that locally available organic residues such as crop residues can reduce heavy metal mobility while improving soil fertility, water-holding capacity, and nutrient availability, which is vital for the smallholder systems common in SSA [10,122]. Although region-specific field studies on contaminant bioavailability remain limited in the literature, broader reviews emphasize the need to integrate locally produced organic amendments into soil restoration strategies to mitigate contamination risks, improve food safety, and enhance ecosystem function across diverse SSA landscapes.

Interactions between organic waste soil amendments and soil pollutants are, therefore, context-dependent. This means that the net effect of organic amendment use in contaminated soils is dependent on the amendment type, application rate, pollutant characteristics, soil properties (pH, texture, redox status), and time.

6. Challenges Impeding the Wide-Spread Use of Organic Wastes and Waste Products in the Remediation of Contaminated Soils in SSA

The use of organic waste-based amendments in the remediation of contaminated soils must be implemented with strict regulatory and quality control protocols [64]. This is because while organic waste-based amendments offer a nature-based alternative to expensive engineering techniques by enhancing sorption capacity, stimulating biodegradation, and improving soil structure, some of them may contain many impurities such as heavy metals, pharmaceuticals, pathogens, and microplastics [123]. To ensure that the organic waste amendments used in remediation efforts are free of any hazardous impurities, three main principles must be adhered to, namely, (i) the need to conduct a comprehensive characterization of organic wastes, (ii) the need to match organic waste-based amendment type to contaminant type and (iii) the need to ensure application only occurs at environmentally and scientifically supported rates.

Countries in SSA are increasingly recognizing these waste management challenges and creating general environmental/waste legislation. However, such efforts remain uncoordinated at the regional level, such that specific regulatory and quality control mechanisms for waste-based technologies are highly underdeveloped, and often missing the product standards, clear quality criteria, or enforcement systems needed for their safe large-scale adoption. SSA, therefore, lacks a comprehensive set of formal national regulatory standards for waste-based soil amendments. However, some countries are beginning to build regulatory or quality-control mechanisms through environmental agencies, climate and agricultural frameworks, or project validations. Kenya, Ghana, Nigeria, and South Africa, for example, are among the SSA countries where regulatory bodies or policy frameworks are beginning to provide some structure for managing and monitoring waste-derived soil amendments, but national standards for the quality control of these technologies are generally still developing and are not yet widely implemented or enforced at a region-wide level. A majority of countries have environmental agencies or municipal policies addressing waste and composting, but formal quality-control standards for waste-based soil amendments are often limited, fragmented, or emerging rather than fully codified [124].

The most common bottlenecks to the widespread adoption and use of organic waste-based technologies are (a) a lack of comprehensive and thorough characterization of source wastes, (b) variability in the quality and quantity of organic wastes available for use, (c) increasing contaminant complexity due to the growing magnitude of emerging pollutants that are not routinely monitored in SSA, (d) limited laboratory, monitoring and technical capacity, and (e) barriers related to the economic and financial difficulties of financing even the low-cost technologies [10,124]. Other barriers to the adoption of organic waste-based technologies in the remediation of contaminated soils include the risk of remobilization and uncertain long-term stability of contaminants [125], and poor social acceptance, knowledge gaps and behavioral barriers for organic waste-based remediation technologies [125].

A comprehensive characterization will help determine the presence and level, if any, of heavy metals such as Pb, Cd, Cr, Ni, and Zn, and the presence of organic pollutants such as polycyclic aromatic hydrocarbons and pathogens. Other parameters of importance in this characterization include the pH, electrical conductivity and overall nutrient content of the wastes. This attempt to match amendment to contaminant type is in response to the fact that different waste products have different remediation mechanisms. While the effective immobilization of trace elements in the soil, such as Pb, Cd, Ni, and certain pesticides, may best be achieved using biochar [126] due to its high surface area and multiple functional groups, the enhanced microbial degradation of contaminants such as hydrocarbons, pesticides or certain metal complexes can be achieved through the application of amendments rich in humic substances. In addition to matching contaminant type to amendment type, the application of excessive doses of any one type of amendment should be avoided, as that may increase salinity or result in nutrient imbalances. This should be followed with the regular monitoring of metal bioavailability, enzyme activities, microbial biomass and plant uptake of any suspected contaminants to prevent unnoticed secondary contamination [127]. Accordingly, the most recommended practices in the use of organic waste-based amendments include (a) ensuring full stabilization of the organic wastes through composting to reduce pathogens and enhance remediation efficiency, (b) carrying out analyses of both the intended recipient soil and the amendment before application as this provides information about the appropriate dosing and remediation pathways and (c) deploying only organic wastes-based amendments developed from source-segregated wastes to avoid contamination with plastics, metals or industrial wastes [123]. A summary of the barriers and the associated enablers to overcome or address such bottlenecks is presented in Figure 3.

The variability in the quality of organic wastes in SSA is a growing challenge hindering the widespread use of organic waste amendments in the remediation of contaminated soils. Some of the contributing factors include inconsistent waste handling, limited waste separation at source, inadequate composting infrastructure, and weak regulatory oversight [128]. Consequently, countries in SSA end up using poor-quality materials, raising the risks of introducing pathogens, heavy metals, plastics, persistent organic pollutants (POPs), or suboptimal nutrient contents to the recipient soils alongside the organic waste amendments. This in turn poses a health risk to soils, crops, and the public.

Standardized waste processing is achievable through waste separation-at-source, which is the most critical step in effective recycling, organic waste recovery, and the safe reuse of biosolids and compost. Although there has been growing activity in SSA towards waste separation at source, most of this activity has remained at the pilot stage [128], presented in research studies [129], or only implemented via city-level initiatives [87,130], without system-wide adoption. A selection of projects focused on waste separation at source across the SSA region is presented in

Table 2. Representative projects on waste separation at source in various countries of SSA.

Country/Area in SSA	Evidence/Status of Waste Separation at Source (2020–2025)	Notable Pilot/Success Story	Key References
Kenya	Separation of wet and dry waste at source in the Mvita and Likoni areas in Mombasa.	Pilot led to the growth and expansion of household collection and private BSF (black soldier fly) organic processing.	[128]
Tiassalé, Côte d’Ivoire	Pilot/source-segregation of wastes.	Pilot projects proved the effectiveness of the biowaste source-segregation system in the production of high-quality compost. Up to 75% of the participants accepted the source-segregation system and were ready to pay for the service.	[131]
Ghana (Accra/Kumasi)	MRFs and composting facilities with some elements of source separation.	Large composting/processing plants experienced operational problems, largely due to insufficiently source-separated feedstock.	[132]
Tanzania (Dar es Salaam)	Evidence from household studies and pilots showing low routine city-wide separation—but city systems still mix streams.	Community pilots show feasibility and potential for separation; however, citywide collection still mixes fractions.	[129]
Uganda (Kampala/GKMA)	Evidence of waste segregation at source exists; implementation has remained at the research stage.	Community pilot models and studies show physical space and social networks driving segregation; GIZ and municipal pilots test community collection linked to informal collectors.	[133]
Ethiopia (Addis Ababa)	Sub-city composting (Nifasilk). Some pilot market composting and municipal organizations continue to support the collection of organic wastes from source points in the city.	Composting programs collected organic waste from markets and cooperatives (2020–2021), producing compost for local use and showing that the market–operator separation approach works well in Ethiopia.	[134,135]
Nigeria, Lagos	Lagos policy calls for sustainable waste separation.	Market pilots/scoping studies recommend piloting source separation at large markets (food/vegetable) and show potential for composting if separation is implemented in Lagos’ policy.	[136]

.

Pilots in Kenya and South Africa show that households can and will separate waste when systems (awareness, bags/bins, regular collection) are provided, while meta-analyses and country studies show that low collection coverage, weak policy enforcement, and contamination/infra gaps are limiting the scaling process. Key enablers are clear regulation, financing for collection infrastructure, community engagement, incentives, and integration of the informal sector, as shown in Figure 2.

The main policy and regulatory challenges surrounding the use of organic waste as an oil treatment in the SSA region include policy fragmentation—waste management, sanitation, agriculture and public health commonly sit in different ministries with limited coordination, resulting in serious regulatory gaps [136]. In most African countries, formal, comprehensive national soil quality standards for toxic trace elements are often absent or in development, leading researchers and regulators to reference international benchmarks in environmental assessments. In a few cases, such as that of Tanzania, for example, maximum allowable soil concentrations for several heavy metals are published. These allowable thresholds are used in agricultural soil risk evaluations in the country and are broadly referenced in regional soil studies [137]. Similarly, in South Africa, soil guideline values established by the Department of Environmental Affairs are comparatively stringent (e.g., Pb 20 mg/kg, Cd 8 mg/kg, Zn 240 mg/kg), and these are often applied in contaminated land assessments [138]. Where national soil limits do not exist or are incomplete, scientists in countries such as Kenya, Nigeria, Ghana, and others commonly compare measured soil concentrations to the FAO/WHO recommended limits to assess potential human and ecological risks [139]. The reliance on international guidelines reflects the gap in harmonized soil quality standards within SSA, and the variability that exists among the few national frameworks underscores the importance of context-specific risk assessment. In urban and peri-urban soils across several SSA countries, measured concentrations of metals such as lead and zinc frequently exceed FAO/WHO thresholds, highlighting widespread contamination concerns and the need for national regulatory development and monitoring programs tailored to local land use patterns and exposure scenarios to eventually aid in the national or local, context-specific remediation efforts.

7. Increasing Meaningful Adoption of Organic Waste-Based Remediation Technologies in SSA

We have indicated that most organic waste-based remediation technologies currently implemented in SSA are either at the pilot or large research levels, and that adoption remains desperately low due to socio-economic, technical, institutional, and policy-related constraints. We propose integrated strategies to enhance the meaningful uptake of organic waste-based soil remediation technologies in SSA. Strengthened regulatory frameworks, improved waste quality management, investment in circular economy markets, technology localization, and broader awareness campaigns are critical accelerators for adoption [140,141,142].

Establishing and enforcing quality standards for organic remediation materials is the foremost and most critical step towards the meaningful adoption of organic waste-based remediation technologies. In this, countries need to develop national standards for compost maturity, biochar quality, pathogen limits, and allowable heavy metal concentrations—all of which are either unavailable or, where they exist, not being enforced. This should go hand in hand with both the creation of standardized certification labels that are visible to users and support for the accreditation of regional testing laboratories. Strengthening policy and institutional frameworks is another important step to the effective adoption of technologies. This will require the integration of organic amendments in national soil health strategies, establishing clear regulations governing the reuse of biosolids, digestate, compost, and biochar in the remediation and creation of incentives for municipalities adopting source separation and organic waste valorization.

Promoting waste separation at source through localized, decentralized waste treatment facilities will eventually lead to enhanced feedstock purity, and thus technology adoption across SSA. Countries and municipal authorities in different countries will move forward faster if they invest in supporting small-scale composting hubs, community biochar units, and decentralized bio-waste treatment systems in peri-urban areas. In addition, countries need to promote household, municipal, and commercial source separation systems and support market-based sorting for food waste, which will lead to a cleaner supply of organic residues for composting or biochar production. Incorporating and promoting investment in circular economy markets and principles can also be a cornerstone to the adoption of technologies of interest in SSA. This should include supporting private sector involvement in composting, pyrolysis, and providing performance-based grants, soft loans, or carbon-credit-linked financing for organic waste processors.

8. Conclusions and Future Directions

Across Sub-Saharan Africa (SSA), soils are widely contaminated by heavy metals (Pb, Cd, Zn, Cu, Hg, and As), petroleum hydrocarbons, agrochemical residues, and mining-related pollutants, particularly in urban and extractive regions [143]. Although advanced physical and chemical remediation technologies exist, their adoption remains limited due to high costs and technical demands. Organic waste-based amendments—such as compost, manure, biochar, digestate, biosolids, and agro-industrial residues—offer cost-effective, locally available alternatives. Evidence shows these materials immobilize heavy metals, reduce contaminant bioavailability through adsorption and complexation, stimulate the microbial degradation of hydrocarbons, and improve soil structure and fertility while diverting waste from landfills [144,145,146,147,148]. Their effectiveness is further enhanced when integrated with bioremediation or phytoremediation strategies.

Large-scale adoption, however, remains constrained by inconsistent feedstock quality, pathogen and contaminant risks, weak regulatory frameworks, limited analytical capacity, socio-cultural perceptions, and land-tenure complexities. Scaling up, therefore, requires improved waste segregation and processing standards, strengthened guidelines and certification systems, investment in monitoring infrastructure and practitioner training, participatory community engagement, and supportive policies, as well as financial incentives to achieve mainstream organic waste-based soil restoration across SSA.