Waste streams as current sources of persistent organic pollutants and organophosphate esters in Africa - a critical review

Concentrations reported in air from dumpsites and landfills

In total, 35 research articles were found to report the selected contaminants for about 50 African landfills or dumpsites [Supplementary Table 1]. The reported concentrations are summarised in Table 1, while the concentrations for the individual compounds are summarised in Supplementary Tables 2-6. In South Africa, Katima et al. reported high concentrations of alternative flame retardants (AFRs) comprising 2-ethyl-hexyl tetrabromobenzoate (EHTBB), bis(2-ethylhexyl) tetrabromophthalate (BEHTBP) and 1,2-bis(2,4,6-tribromophenoxy)ethane (BTBPE) as 4.7 ng/m³ in air at landfill sites^[25]. In the same study, high concentration ranges of 0.050-12 and 0.95-2.8 ng/m³ were reported for hexabromocyclododecane (HBCDD) and ∑₉BDEs, respectively. These concentrations exceeded the concentrations reported in the same studies for the industrial and highly populated locations. DecaBDE (BDE-209), PentaBDE (BDE-99 and BDE-100) and AFRs (EHTBB, BEHTBP and BTBPE,) congeners were dominant compounds reported in all seasons, with a suggestion that these congeners have been extensively used in consumer products. The authors attributed the high concentrations of the contaminants to the lack of treatment (besides compaction) of brominated flame retardant (BFR) wastes at the disposal sites. The concentrations of HBCDD, PBDEs and AFRs in the landfill air were reported to be higher in summer than winter due to warmer temperatures in summer intensifying BFR volatilisation emission rates from landfilled waste.

Hogarh et al. reported the highest concentrations for ∑₁₉₀PCBs 4.6 ng/m³ in air samples collected from Agbogbloshie. Agbogbloshie is a huge scrapyard and dumpsite location in central Accra, Ghana, infamous for informal waste electrical and electronic equipment (WEEE) recycling activities^[26]. In particular, excessive polychlorinated biphenyl (PCB) concentrations (11 ng/m³) were reported from a plume resulting from uncontrolled open burning of WEEE. The tri-PCBs were reported to dominate these PCB concentrations. Using statistical source apportionment, the authors concluded that the WEEE recycling site is a major source of atmospheric PCBs in Ghana. Adesina^[27] similarly reported high concentrations of ∑₁₅PCBs 3.1-5.0 ng/m³ around open burning municipal dumpsites in Nigeria, with PCB-52 (mean = 2.3 ng/g) reported as the dominant congener. In Dar es Salaam, Tanzania, high concentrations of short-chain chlorinated paraffins (SCCPs) (4.0-59 ng/m³) and medium-chain chlorinated paraffins (MCCPs) (1.0-33 ng/m³) were also reported in air from dumpsite sites^[13]. The authors referred to waste handling sites as important emission sources of chlorinated paraffins in the studied urban, suburban and rural locations.

Concentrations reported in soil from landfills and dumpsites

Sixteen studies were obtained capturing landfill/dumpsite soils [Table 1]. In a 2010 study, Fujimori et al. reported concentrations of dioxin-like PCBs (median = 5.5 ng/g) in soil from Ghana’s Agbobgloshie WEEE dumpsite, with sources pointing to the open burning of WEEE^[28]. In 2014, Oteng-Ababio et al. identified several PBDE congeners (BDE 1, 7, 28, 47, and 99/100) in soil collected from Agbobgloshie WEEE informal recycling site^[29]. The highest concentrations of PBDEs were found at burning sites and areas of intense incineration for copper retrieval. The authors raised concern about the possible impacts of emissions of PBDEs from the uncontrolled informal WEEE recycling and dumping activities at Agbobgloshie on local vegetable farm soils. Subsequently, Akortia et al. found high PBDE concentrations (∑₁₅BDEs 16-97 ng/g) from the soil of the Agbobgloshie WEEE dumpsite directly related to the informal WEEE recycling activities present in 2017^[30]. Sources of lower-brominated PBDEs not usually utilised in treating electrical and electronic goods, such as BDE-28, were most likely due to the debromination of more commonly used higher-brominated PBDE congeners formed during high-temperature burning conditions from low-tech recycling operations. Similarly, Tue et al. also reported high concentrations of PBDEs in soils from the Agbobgloshie WEEE Site in 2019, with ∑₈BDEs ranging from 21-6900 ng/g. Concentrations were specifically reported to be higher in dismantling areas when compared to the areas of burning^[31]. However, generally, the concentrations in these two areas were reported to be greater than for soils collected from locations other than directly from the Agbobgloshie WEEE dumpsite.

From an earlier 2015 investigation of the Agbobgloshie WEEE site, Möckel et al. reported 6.3-7700 ng/g ∑₂₅BDEs, 150-28,000 ng/g SCCPs, 1300 ng/g MCCPs and 6.5-830 ng/g ∑₇PCBs^[23]. Additionally, concentrations of ∑₂₅BDEs (1.2-100 ng/g), SCCPs (69-1,600 ng/g), MCCPs (nd to 1400 ng/g) and ∑₇PCBs (0.74-43 ng/g) were reported in soils from the Kingtom WEEE dumpsite in Freetown, Sierra Leone. The concentrations of PBDEs in these two locations were reported to exceed the background concentrations from Tanzania and Kenya. Sources were associated with the burning of WEEE and/or the incorporation of waste particles in the soil. Here, BDE-209 was reported as the predominant congener with concentrations ranging from 1.2-100 ng/g^[23].

In Nigeria, Ohajinwa et al. reported concentrations of ∑₁₇BDEs (median = 0.0032-21 ng/g) for informal WEEE recycling locations in Ibadan, Lagos and Aba in 2019^[32]. These concentrations were reported to be higher at the burning sites than at the dismantling sites, and the concentrations from the two sites exceeded those of the other locations studied. BDE-209 was reported as the predominant congener with 18 ng/g (upper bound concentration). Conversely, in a subsequent 2020 study^[33] from Benin City, Nigeria, the authors reported only BDE-47 above the detection limit in the concentration range of nd to 1.90 ng/g from the PBDE congeners investigated (BDEs 47, 79, 99, 153 and 209). The sources were attributed to the burning and leaching of PBDEs from WEEE plastics into the soil.

Olutona et al. reported concentrations of six individual PBDE congeners (BDEs 28, 47, 99, 100, 153, and 154) in soils sampled from Obafemi Awolowo University’s dumpsite, Ile-Ife, Nigeria, with BDE-153 being the dominant BDE congener (14 ng/g)^[34]. Higher concentrations of PBDEs were reportedly found in the top layer of soil than in the layers of soil below, indicating either atmospheric deposition or upward mobility of PBDEs in soils to enrich the top layer. Oloruntoba et al. reported excess concentrations of ∑₇BDEs (110-370 ng/g) in soil samples collected from Karmo and Anjanta dumpsites in Abuja, Nigeria^[35]. These concentrations were reported to significantly exceed the control sites. The authors considered these high concentrations as possible reasons for the high levels of PBDEs reported in eggs and vegetation around the same dumpsites^[36], suggesting the potential transfer of PBDEs from contaminated soil into the food chain. PBDE concentrations were reported to be higher in the wet season (June) compared to the dry season (March), associated with a high “wash out” of atmospheric PBDEs in the wet season.

Oketola and Akpotu^[37] reported ∑₆PCBs (3-410 ng/g) in soils from seven dumpsites located in Lagos and Ibadan, Nigeria. This study indicated higher concentrations of PCBs in samples collected within Lagos, which is a more industrial and populated location. Meanwhile, lower ∑₁₅PCBs concentrations (24-29 ng/g) were reported in soil collected from Afe Babalola University’s dumpsites in Ado-Ekiti, Nigeria, = by Adesina^[27]. Ouabo et al. reported 32-73 ng/g for ∑₃₀PCBs in soil samples from abandoned WEEE sites in Douala, Cameroun^[38]. In this study, PCB-52 was reported as the dominant congener (0.43-7.4 ng/g). Relatively high concentrations for ∑₁₈PCBs (1000-4900 ng/g) were reported by Debela et al. from soil samples collected from an old transformer dumpsite in Addis Ababa, Ethiopia^[39]. The dominant congener was again reported as PCB-52 (mean = 360 ng/g).

Akortia et al. reported ∑₇BDEs (7.1-11 ng/g) from landfill soil samples collected from sites in Gauteng Province, South Africa, with BDE-183 the predominant congener^[40]. Sources were related to consumer goods and abrasions of materials containing PBDEs within the landfill. The authors reported higher concentrations of PBDEs within the coarse soil fraction (150-250 μm) compared to a finer fraction (45-150 μm). In Dar es Salaam, Tanzania, Nipen et al. reported concentrations of SCCPs and MCCPs as 11-5300 and 26-5100 ng/g, respectively^[13]. The concentrations from the municipal waste dumpsite and that of the WEEE sites were specifically reported to exceed all other studied locations. Compared to this study and the one by Möckel et al., Nipen et al. attributed higher MCCP/SCCP ratios in their data to a higher volume of WEEE processed in Ghana, cruder processing methods used for WEEE dismantling/recycling, differing socioeconomic development in Tanzania and/or the later year of study, during which time the use of modern electronics could potentially contain greater quantities of MCCPs than SCCPs due to international policy shift from SCCPs to MCCPs use in these products^[13,23].

Concentrations reported in sediment and leachate from landfills and dumpsites

Five studies were identified to target landfill/dumpsite sediments, while nine were found for leachates [Table 1]. All of these documents originated from South Africa. Olukunle and Okonkwo^[41] reported ∑₃AFRs (EHTBB, BEHTBP and BTBPE) (mean = 71 ng/g) and HBCDD (mean = 33 ng/g) in selected landfill sediments from Gauteng Province, South Africa. In leachates, HBCDD and ∑₃AFRs concentrations were reported as 0.14 and 0.072 ng/L, respectively. The authors reported detection of most AFRs in samples of unlined/unprotected landfill, while HBCDD was mostly detected in samples of lined/protected landfills equipped with incinerators. According to the authors, this study was the first to confirm the presence of AFRs and HBCDD in municipal solid waste landfills in South Africa.

Sibiya et al. observed PBDE concentrations (∑₉BDEs 2.5-4.9 ng/g and 0.32-1.4 ng/L in sediment and leachate, respectively) from seven functional landfill sites in Johannesburg and Pretoria, South Africa^[42]. These concentrations were reported to be higher in landfills that were lined/protected with geomembrane liners. One landfill site (Hatherly) without geomembrane liners reported high concentrations of PBDEs (∑₉BDEs 0.045-0.15 ng/L) in groundwater beneath the unlined landfill site, indicating possible migration of PBDEs to the surrounding groundwater. However, higher concentrations of ∑₉BDEs (0.29-0.449 ng/L) were reported when monitoring groundwater around a lined landfill (Marie Louis). The authors attributed these differences in concentrations to the size or incompleteness of the geomembrane liner, age or possible degradation/damage of the liners and/or contamination from improper disposal from nearby industries. BDE-209 was reported as the dominant PBDE congener in both landfill leachates (0.14-0.47 ng/L) and landfill sediments (1.2-3.0 ng/g).

Further South African studies from Johannesburg and Pretoria by Sibiya et al. indicated ∑₁₀OPFRs (120-1660 ng/g), ∑₅BDEs (0.82-1.4 ng/g) and ∑₇PCBs (2.3-6.9 ng/g) from landfill sediment samples and ∑₁₀OPFRs (560-17,000 ng/L) from landfill leachate samples^[43]. The high concentrations of OPFRs compared to the relatively low concentrations of PBDEs and PCBs were attributed to the possible replacement of PBDEs and PCBs by OPFRs in consumer products imported into South Africa. The initial 2010 investigation on Pretoria’s landfill sediment by Odusanya et al. reported ∑₁₅BDEs (33 ng/g). In this report, BDE-209 was reported below the limit of detection, attributed to photolytic degradation of samples^[44]. Daso et al. reported concentrations of nd to 50 ng/L γ-HBCDD, not detected for tetrabromobisphenol A (TBBPA) and 40-480 ng/L for ∑₅BDEs in landfill leachates collected from sites in Johannesburg and Pretoria (Gauteng Province)^[45]. Similar to the study by Odusanya et al., BDE-209 was reported as not detected^[44]. α-HBCDD and β-HBCDD were also reported below the limit of detection. The authors attributed the non-detection of α- and β-HBCDD to the low composition of α-HBCDD (10%-13%) and β-HBCDD(1%-12%) in the HBCDD technical mixture, as well as the extremely low water solubility of γ-HBCDD (2.1 μg/L) together with its affinity for suspended solids in aqueous media. The authors suggested consideration of the distinctive physicochemical parameters of TBBPA when developing the chromatographic methods for its quantification by LC-TOF-MS/MS, a factor they believed could affect the detection of TBBPA.

Daso et al. in Cape Town, South Africa, reported a PBDE concentration range for ∑₈BDEs (0.28-2200 ng/g) for Bellville, Coastal Park and Vissershok landfills^[46]. The authors attributed the PBDE concentrations to the huge volume of wastes deposited into the landfills, frequency of precipitation, generally hot climate, degree of waste compaction and age of the landfill sites. PBDE sources at Coastal Park and Vissershok landfills were largely attributed to the nearby wastewater treatment plant (WWTP)-derived sludge. However, multivariate analysis revealed multiple sources for PBDEs. The authors suggested that the observed differences in the PBDE levels between the Vissershok (0.28-21 ng/L) and Coastal Park landfills (1.7-1200 ng/L) could be due to the differences in sampling method employed during leachate collections, with organic matter from bottom sediments influencing the availability of the contaminants.

Odusanya et al. reported ∑₁₃BDEs (8.4-55 ng/L) from leachates collected from Soshanguve, Temba, Garankuwa, Hatherley and Kwaggasrand landfills, South Africa^[47]. The lowest PBDE concentration was reported in Kwaggasrand, which was considered the youngest landfill (seven years old). PBDE congeners BDE 28, 47, 71 and 77 were detected in leachate samples from all landfill sites; all congeners were reported in two of the oldest landfill sites (both ten years old). The authors reported that higher levels of organic materials may have significantly contributed to elevated PBDE concentrations in leachate.

Olukunle et al. reported PBDE concentrations for the ∑₇BDE for sediment (0.8-8.4 ng/g) and leachate (0.13-3.7 ng/L) in samples collected from landfills in Gauteng, South Africa^[48]. BDE-209 (mean = 1.6 ng/g) was reported as the dominant congener in the majority of sediment samples. The highest concentrations of ∑₇BDEs were reported in geomembrane-lined landfills. The authors observed a possible positive relationship between trace metal concentrations and PBDE concentrations in leachate samples. Ololade et al. reported concentrations of ∑₁₄PCBs (3.0-41 ng/L) in leachate samples collected from Lagos (Olusosun) and two other dumpsites at Ondo (dumpsite of Ondo State Waste Management Authority) and Ikare-Akoko^[49]. These dumpsites were reported not to have any protective liner. Higher concentrations were reported in Olusosun compared to the other landfills studied, with factors of population density, age of dumpsite, quantity and nature of deposited wastes and higher market density attributed to these differences. Concentrations of ∑₁₄PCBs (nd to 67 ng/L) [Table 2] were reported in the groundwater samples around these dumpsites, suggesting percolation of PCBs into local groundwater. Ibor et al. reported ∑₈PFASs in stimulated leachate from Lenna solid waste dumpsite, Calabar, Nigeria^[50]. The highest PFAS detected was reported as perfluorodecanoic acid (PFDA) (5.0 ng/g), while the lowest detected PFAS was perfluorobutane sulfonic acid (PFBS) (0.05 ng/g).

Concentrations reported in human/biological samples, indoor dust and groundwater from landfills and dumpsites

A summary of the concentrations reported on human/biological samples is presented in Table 2, while the details of individual compounds are shown in Supplementary Tables 2-6. In two studies conducted in 2019 and 2021, high levels of PBDEs were reported from selected plants (∑₇BDEs 8.5-61 ng/g dw)^[35], free-range eggs (∑₇BDEs 190-370 ng/g lipid wt.)^[50] and cow milk samples (∑₇BDEs 33-100 ng/g lipid wt.)^[50] collected from Karmo and Anjanta dumpsites, Abuja, Nigeria. Oloruntoba et al. reported that these concentrations significantly exceeded those of the control samples^[35,36]. ∑₇BDEs were reported to vary between plant roots (25-61 ng/g dw) and shoots (8.5-32.2 ng/g dw)^[35]. In the egg samples, BDEs 47, 99, 100 and 153 were reported as the predominant congeners in egg samples, while BDEs 47 and 99 were the dominant ones in milk samples. The authors attributed the difference to metabolism and debromination of higher-brominated PBDEs during transfer into milk. Oloruntoba et al. provided evidence of dietary transfer of contaminants from dumpsites to humans through contaminated vegetable, egg and dairy consumption^[35,36]. While contamination of dumpsite soil and plants was directly associated with waste disposal and open burning of deposited wastes, the authors associated the relatively lower contamination of the control soil and plants with air pollution and atmospheric deposition.

High concentrations of PBDEs (770-1300 ng/g lipid wt.), TBBPA (< 4.2-150 ng/g lipid wt.), SCCPs (310-2100 ng/g lipid wt.), ICES indicator ∑₇PCBs (290-620 ng/g lipid wt.) and ∑₃AFRs (41-57 ng/g lipid wt.) were reported in free-range eggs collected around Agbogbloshie WEEE dumpsite, Ghana^[51]. These concentrations were related to high informal activities and processes in the WEEE scrapyard. The concentrations of PCBs, PBDEs and SCCPs reported in Ghanaian free-range eggs^[51] generally exceeded similar reports on free-range eggs collected from Yaoundé, Cameroon’s dumpsite (PCBs: 28-36 ng/g lipid wt., SCCPs 150 ng/g lipid wt. and BDEs 0.2-2.8 ng/g lipid wt.). A similar investigation from Mbeubeuss dumpsite in Dakar, Senegal, by IPEN^[52] indicated contamination by ∑₇PCBs (29 ng/g lipid wt.) in free-range eggs.

Asamoah et al. reported mean concentrations of 4.4 ng/g lipid wt. for ∑₆PCBs from the milk of volunteer mothers in the hotspot of Agbogbloshie WEEE dumpsite, Ghana^[53]. This concentration was reported to significantly exceed the concentration (0.03 ng/g lipid wt) of samples from “non-hotspot” areas, with the predominant PCB congener reported as PCB-28 [mean = 1.3 ng/g (lipid weight)]. From the same Agbogbloshie WEEE dumpsite, Wittsiepe et al. reported a concentration of 82 ng/L for PCB congeners 138, 153 and 180 in workers from the WEEE scrapyard and the control sites^[54]. Interestingly, and perhaps counterintuitively, the concentrations reported from the control group were significantly higher than those of the Agbogbloshie scrapyard workers. The authors reported that WEEE-related activities had no influence on internal exposure and called for further investigation of their observation of higher PCB exposure for people living in areas not associated with WEEE activities.

Kaifie et al. reported PCB concentrations of 340 ng/L (mean) for ∑₆PCBs in human blood plasma from WEEE recycling workers at Agbogbloshie, Ghana^[55]. All target PCB congeners were reported in the plasma of the WEEE site workers in this study. In contrast to Wittsiepe et al., the authors reported a significant difference for lower chlorinated PCBs (PCBs 28, 52 and 101) when compared with the control group^[54]. According to Kaifie et al., the PCB congeners 138, 153 and 180 monitored by Wittsiepe et al. are associated with food ingestion, which is age or time related, in addition to occupational exposure^[54,55]. Lower PCB congeners were regarded as accurate markers to differentiate between environmental exposure and occupational exposure as a result of their shorter half-lives since they are not confounded by age or dietary habits. Kaifie et al. reported dismantlers and burners to have the highest value of occupational related PCBs 28 and 52^[55].

Abafe and Martincigh^[56] reported high concentrations of ∑₈BDEs (2600-44,000 ng/g) and ∑₃PCBs (54-490 ng/g) in the indoor dust samples collected around WEEE recycling locations in South Africa. Higher PBDE concentrations were reported around sampling points characterised by WEEE polymers compared to locations characterised by internal components of personal computers, mobile phones and fridges. BDE-99 and BDE-209 were reported as the most prevalent congeners, while PCB-180 was reported as the dominant PCB congener. Concentrations of both PBDEs and PCBs in dust were reported to reduce after the clean-up of the WEEE recycling site.

Concentrations of target contaminants reported in African wastewater and wastewater treatment sludge

In total, eight documents were found on African wastewater and wastewater treatment sludge. The concentrations obtained are summarised in Table 3, while the details of the individual compounds are presented in Supplementary Tables 2-6. Sindiku et al. reported low ∑₁₀PFASs (0.10-0.54) ng/g in sludge samples collected from hospital, industrial and domestic WWTPs, in Lagos, Oyo and Ogun, Nigeria^[57]. Perfluoroalkyl carboxylates having carbon chain with ≥ 8 fluorinated carbons were reported at higher levels than those with < 8 fluorinated carbons. The authors reported that PFAS concentrations were lower compared to other regions in the world. No point sources were attributed by the authors. The low concentration of PFASs from municipal sewage plants was related to low PFAS uses in Nigerian residential settings. The highest concentration of PFAS detected, perfluorooctane sulfonate (PFOS) (0.54 ng/g) reported in hospital sewage sludge, was attributed to minor releases from medical equipment.

Chirikona et al. reported concentrations of PFOS (0.9-9.8 ng/L) and PFOA (1.3-28 ng/L) for wastewater samples and PFOS (0.10-0.68 ng/g) and PFOA (0.12-0.67 ng/g) for sludge samples collected from hospital, domestic and industrial WWTPs in Kenya^[58]. Similar to the study by Sindiku et al. all samples indicated PFASs, but higher concentrations were reported in domestic WWTPs^[57].

Daso et al. reported ∑₈BDEs in sewage sludge (13-650 ng/g), raw water (370-4400 ng/L), secondary effluent (19-2600 ng/L) and final effluent samples (90-15,000 ng/L) collected from WWTPs in Cape Town, South Africa^[59]. The PBDE sources were attributed to general wear and tear of contaminated home products such as furniture and other textile materials that could be transferred to the sewer system through washing and floor mopping. The authors highlighted the reuse of treated effluents as a source of PBDEs in the South African environment, particularly when used for agricultural purposes, where transfer of contaminants into the food chain may occur.

Another study from Cape Town (Potsdam, Cape Flats and Bellvill), South Africa, by Fatoki et al. revealed ∑₈BDEs (0.18-4300 ng/g) in sewage sludge^[60]. The variation in concentrations was associated with the time period sampled, the composition of wastewater reaching the WWTPs and changes in lifestyles of the local residents. Similar to the study by Daso et al., the authors attributed the sources of the PBDEs to home products - particularly of second-hand value, such as furniture, electrical and electronic equipment, carpets and upholstery where the contaminants have the possibility of entering the sewer system through floor cleaning and subsequent leaching^[59]. Other attributed sources were associated with industrial-related sources, such as heavy-duty machines, vehicular activities and mismanagement of municipal solid wastes. Most of the congeners were reported to be found to accumulate in the secondary sludge; for this reason, the authors believed that finely suspended particles could be an important mechanism for the removal of PBDEs from aqueous media.

Barakat et al. reported excessive concentrations of ICES ∑₇PCBs (5600-11,000 ng/g) in sewage sludge samples collected from Alexandria, Egypt^[61]. The highest concentration was reported in the anaerobically digested sample (ADS). These high concentrations were associated with the concentration effect of the dewatering process of WWTP and the persistence of PCBs. For wastewater, Dalahmeh et al. reported concentrations of ∑₁₀PFAS in influent (3.4-5.1 ng/L) and effluent (5.6-9.1 ng/L) from Bugolobi WWTP, Uganda^[62]. The higher concentrations of PFAS in the effluent were associated with inefficient removal of PFAS by the treatment processes of sedimentation (primary) and a biological treatment using trickling filters (secondary) within the WWTP. Much higher PFAS concentrations were reported in influents ∑₁₆PFAS (130-630 ng/L-range of means) and effluents ∑₇PFAS (77-220 ng/L-range of means) by Kibambe et al. from wastewater samples collected from Daspoort, Phola and Zeekoegat WWTPs, South Africa^[63]. Similar to the report by Dalahmeh et al., incomplete removal of PFAS was reported in all three WWTPs^[62]. The removal efficiency was reported to vary among the three WWTPs; however, activated sludge recirculation was reported to perform better than biological filtration systems. According to the authors, the removal of PFAS in the WWTPs could be related to type and size of the three WWTPs; load of the PFAS discharged into the WWTPs; distinctive removal efficiencies of the WWTPs, which may depend on the design and operating factors such as temperature, mixed liquor suspended solids, sludge, hydraulic retention times; and feed flow rates in the biological tank.

For PCBs, Samia et al. observed excessive ICES indicator ∑₇PCBs (280,000-1,200,000 ng/L) in untreated textile wastewater samples collected at Oued El bey, Tunisia^[64]. This discharge was reported to contaminate surface and groundwater, reaching concentrations of 90,000-470,000 and 5200-196,000 ng/L, respectively [Table 4]. Badawy et al. reported relatively lower concentrations for ∑₁₂PCBs (mean = 27,000 ng/L) from wastewater collected in 10th of Ramadan industrial city, Egypt^[65]. PCB removal efficiency from the wastewater was reported as 74%, with a residual concentration of 8200 ng/L. The removal rate was reported as 11%-53% in the primary treatment but increased to 33%-74% in secondary treatment due to the degradation of PCBs by biological treatment. The most frequent and abundant PCB congeners were reported as PCBs 18 and 52.

Concentrations of target contaminants reported on contaminations from industrial/sewage waste

Five different studies were obtained that targeted our selected contaminants in industrially/sewage-polluted water. The concentrations from the reported studies are summarised in Table 4. On sites along the Vaal River, South Africa, Chokwe et al. reported concentration ranges for ∑₅BDE for water (90-260 ng/L), fish (4.63-33 ng/g lipid wt.) and sediment samples (10-24 ng/g ww); the HBCDD concentrations for water, fish and sediment samples were 510-1770 ng/L, 10-13 ng/g lipid wt. and 15-52 ng/g ww, respectively^[66]. The highest of these concentrations were reported to be found in samples collected from effluents from the Rietspruit WWTP. In South Africa, a study by Groffen et al. on the Vaal River indicated ∑₁₅PFAS concentrations of nd to 39 ng/L from polluted water, nd to 209 ng/g lipid wt for fish and not detected for sediment, with the exception of PFOS [2.36 ng/g dry weight (dw)] detected at Thabela Thabeng^[67].

Olutona et al. reported concentrations of ∑₆BDEs (30-450 ng/L) in water samples collected from Asunle River, an adjoining stream of the Obafemi Awolowo University dumpsite, Nigeria^[68]. These concentrations were reported to decrease downstream with a possible dilution effect resulting from increasing water volumes downriver. Subsequent reports on the river sediment^[69] revealed concentrations of ∑₆BDEs (0.73-10 ng/g). BDE-153 was reported as having the highest concentration among all congeners in both water and sediment samples. The authors observed high concentrations of the BDEs in June (wet season), attributed to the discharge of pollutants from the dumpsite during precipitation.

Comparison of intra- and intercontinental data on African waste streams

The lack of similarity of the target compounds in most of the studies affects the ability to compare them. Comparisons made here are based on articles reporting similar chemical compounds. The range of ICES indicator PCBs (6.5-830 ng/g) reported by Moeckel et al. on Agbobgloshie’s WEEE sites, Accra, Ghana, is higher than the similar reports (0.74-43 ng/g) on Kingtom e-waste samples (Freetown, Sierra Leone)^[23]. These two concentrations far exceed the concentrations reported by Sibiya et al. (2.3-6.9 ng/g) on South Africa landfill sediment^[43].

The upper limit of the concentration range (620 ng/g lipid wt.) of ICES indicator PCBs reported by Petrlik et al. for free-range eggs in Accra, Ghana’s WEEE dumpsite exceeds the values (36 ng/g lipid wt.) reported for Yaoundé, Cameroon and Dakar, Senegal (29 ng/g lipid wt.)^[51,52]. The total range of ICES PCBs indicator reported from three reports on dumpsite/WEEE recycling sites are within 0.74-83 ng/g (lipid wt.), while that of the free-range eggs sampled along dumpsite/WEEE sites are within 28-620 ng/g lipid wt.

The concentration range of HBCDD reported for South Africa’s landfill leachate (mean = 0.024 ng/L) by Olukunle et al. is lower than the by Daso et al. (nd to 50 ng/L)^[41,45]. The upper mean concentration of ∑₈BDEs reported by Daso et al.^[46] (2200 ng/L) is over four times greater than the concentrations of the subsequent report of 480 ng/L^[45].

On intercontinental comparison, the range of 50-120 pg/m³ reported by Katima et al. [Table 1] for HBCDD in South Africa landfill air substantially exceeds the reported concentrations (< 0.05-6.1 pg/m³) of Harrad et al. in Irish landfill air [Table 5]^[39,70]. The mean concentration of the individual BDEs 28, 47, 100, 99, 154, 153, 183 and 209 observed in Ghana’s WEEE soil^[23] [Supplementary Table 4] exceed the reports on Ireland, and particularly BDE-209 is 16 times greater than (mean = 1600 ng/g) the concentration reported for Ireland (upper limit = 100 ng/g).

The ICES PCB concentrations of 1300-3700 ng/g (range of means) reported by Arp et al. for Norway [Table 5] are considerably greater than those reported from Kingtom WEEE sites (mean = 4.7 ng/g) and Agbobgloshie’s WEEE sites (mean = 92 ng/g)^[23,71]. The upper concentrations of PCBs 28, 118, 138 and 153 in human milk samples collected from residents close to Vietnamese WEEE sites^[72] [Table 5] exceed the concentrations reported for human milk samples collected from around Ghana’s WEEE recycling sites [Supplementary Table 6], while concentrations of PCB-180 were similar for Vietnam (0.50-5.5 ng/g lipid wt.) and Ghana (nd to 6.0 ng/g lipid wt.)^[53].

The ∑₈BDEs concentrations reported in wastewater from Cape Town, South Africa^[59] [Supplementary Table 5] are 18 times higher than those reported in Harbin, China^[73] [Table 5]. While BDE-209 is the common dominant congener, the reported concentrations by Cincinelli et al. (130-9000) from Italian sludge samples substantially exceed those reported by Daso et al. from South Africa (4.9-2300 ng/g)^[59,74]. BDE-153, which is the dominant BDE congener (upper concentration = 97 ng/g lipid wt.) in egg samples collected from Nigerian landfill sites^[36] [Supplementary Table 5], is ten times lower than reported concentrations from Chinese samples (1000 ng/g)^[75] [Table 5].

Factors influencing releases of contaminants from African waste streams

Concentrations of the selected contaminant in African waste streams were reported to vary across locations, sampling periods^{[13,35,46,60]}, activities on dumpsites/landfill^[32], waste treatment methods^[25,61,62] and sometimes study methods including chromatographic methods^[45] and sampling procedures^[59]. Sources were specifically associated with the waste contents^[13,43], which is directly related to the population’s lifestyle^[60]. Possibilities of escape of contaminants such as BDEs into subsoil were reported by Olutona et al. and Oloruntoba et al., particularly for lower brominated congeners^[35,36]. Contaminants of PBDEs and PCBs were reported in groundwater sources around landfills, heavily implying emissions from these sites^[42,49].

From the reported studies, factors influencing releases or increasing levels of contaminants in African waste streams can be highlighted as the volume of waste relative to the capacity of landfill and/or WWTPs^[60]; waste sources or types^[63]; crude processing methods^[13,32]; age of landfill, particularly as related to the effective phasing-out period of regulated contaminants^[35]; potential degradation of heavier brominated congeners leading to volatilisation of lower brominated congeners^[30]; volume, size or type of waste water plant treatment^[63]; weak and/or damaged landfill liners or absence of landfill liners^[42]; no or inadequate treatment procedure for collected landfill leachate; and lack of waste sorting or proper recycling processes^[32,55].

Sibiya et al. and Olukunle et al. reported high contamination of PBDEs in landfill sediment samples. The elevated concentrations were attributed to landfill liners preventing percolation into groundwater systems^[42,48]. Sibiya et al. reported comparatively lower BDE concentrations in older landfills (28 years old), with a suggestion that older landfills contain microorganisms that support the breakdown of contaminants and therefore limit concentrations^[43]. Conversely, Oloruntoba et al. reported higher concentrations of ∑₇BDEs in a slightly older landfill (eight years old) compared to the youngest landfill site^[35].

Ohajinwa et al. reported that concentrations of targeted chemicals followed the order: burning sites > dismantling sites > repair sites > control sites^[32]. High concentrations of BDEs found by Ohajinwa et al. were also attributed to the burning of WEEE^[32]. Similarly, Tue et al. cited thermal debromination as a factor contributing to higher concentrations of lower brominated BDEs at open burning sites^[31]. Debromination was suggested for higher BDE-28 concentration in WEEE recycling soil by Akortia et al.^[30]. Odusanya et al. suggested photolytic properties for less than detectable observation of BDE-209^[44]. However, Moeckel et al. found no major association between burning and volatile PCBs^[23].

Key possible factors influencing the concentration of organic contaminants in landfill were attributed to organic carbon contents. Akortia et al. and Sibiya et al. reported a significant correlation between PBDE concentrations and organic carbon^[30,41]. However, only a weak correlation was observed between lower brominated BDE congeners and organic carbon by Daso et al.^[45].

For seasonal sources, Katima et al. reported higher concentrations of HBCDD, PBDEs and AFRs in landfill air during summer months when compared to winter months^[25]. This was associated with high temperatures in summer, which favour volatilisation emission rates of BFRs from wastes. Higher concentrations of PBDEs were reported in dumpsite soils during the June wet period by Oloruntoba et al. on dumpsite soil^[35], suggesting a possible washout of atmospheric PBDEs^[35]. Similar high June wet period concentrations were reported by Daso et al. for landfill leachate, and flooding effects were suggested as the possible influence^[46]. However, Sibiya et al.^[42] reported slightly higher PBDE concentrations in the winter (3.00-4.91 ng/g) compared to the summer (2.50-3.71 ng/g). The lower concentrations in the summer were attributed to an increase in precipitation rates leading to diluting effects of both leachate and sediments. Meanwhile, Barakat et al. observed no correlation between PCB concentrations and season due to the diffuse origins of PCB sources^[60].