Evaluation of pollution potential and bioremediation of tannery-based chromium wastes in Sub-Saharan Africa: the case of dump sites in Beit Ore Tannery in South Africa and Dogbone Tannery in Kenya

Abstract

The leather tanning technology is one of the oldest industry in humankind’s civilisation history. It converts raw hides and skins into non-putrescible products used in making various leather materials. Globally, Africa accounts for 4% of the world leather production and 3.3% of value addition in leather industry. This value addition, has been accompanied with varied nature and large magnitude of tannery chromium effluent and solid wastes discharge from leather manufacturing. Leather processing wastes are now becoming of great environmental and public health concern in the world but more seriously in Sub Saharan Africa. This study aimed first at investigating chromium status, trace metals contamination and ecological risk assessment of tannery waste disposal of selected tannery waste dumpsites in Kenya and South Africa. Then, it assessed the potential health risk associated with the consumption of edible vegetables grown on Cr (VI) polluted soils. It further investigated the anaerobic bacterial profile of donkey dung assisted anaerobic bioreactor remediation of tannery based chromium wastes. Preliminary study of the use of donkey dung as a sorbent for Cr (VI) sequestration was also performed. This study was carried out in Dogbone (DB) and Beit Ore (BO) tanneries in Kenya and South Africa, respectively. Sampling for soils was done twice to accommodate two major seasons. The dumpsites soil was sampled at a depth of 10 - 20 cm for physico-chemical and heavy metals content analyses in both dumpsites in February, March, July and December of 2018. Wastewater samples were collected between February and March of 2018 at both tanneries. The following plants were sampled at BO dumpsites; Spinacea oleracea, Dactyloctenium aegyptenium, Cynodon dactylon, Alternanthera caracasana, and Corchorus tridens. In DB dumpsites; Amaranthus dubius Thell, Cynodon nemfluensis vanderst and actyloctenium aegyptenium were sampled. The donkey dung used in this study was obtained from Vuwani village of Vhembe district, Limpopo province, South Africa. To achieve the specific objectives of this study, the presence of selected heavy metals (copper, iron, nickel, zinc, cadmium, arsenic, lead, silver, cobalt, manganese) and chromium oxidation status (total chromium and hexavalent chromium) in effluents, soils and plants were investigated.The reduction of total chromium and Cr (VI) by bacteria were assessed using Inductively coupled plasma-optical emission spectrophotometer and Ultra violet visible spectrophotometer, respectively. The occurrence of bacterial community profile in bioreactor and presence of methane gas generated were analysed with Illumina MiSeq System and gas chromatography machine, respectively. The donkey dung was characterised using Fourier transform infrared spectrometer, Thermogravimetric analysis, Scanning electron microscope, Fluorescence excitation emission matrix and Brunau-Emmett-Teller. The results of the first objective investigated chromium status, heavy metals contamination and ecological risk assessment of tannery waste disposal in Kenya and South Africa, and showed that the physico-chemical parameters in sampled soils may have favoured the transformation of Cr (III) to Cr (VI) at BO and DB dumpsites. The levels of Cr (VI) in sampled soils were found to be 0.31 and 0.4 mg/kg in BO and DB, respectively, and exceeded the WHO guidelines of 0.05 mg/kg for Cr (VI) in soils. The results from various pollution indices tools (like geo-accumulation index, contamination degree, the pollution load index, the ecological risk assessment and the potential ecological risk) showed high contamination of soils by tannery waste in relation to the control sites. The chromium contents in edible vegetables like Amaranthus dubius Thell from DB was 12.97 mg/kg Cr while Spinacea oleracea from BO was 12.08 mg/kg Cr above expected level of 1 mg/kg Cr. The study indicated that the sites were ecologically contaminated and the edible vegetables wildly growing were contaminated with Cr, thus posing a health risk to tannery workers and nearby populations who consume such vegetables. The second objective of the study was, the potential health risk associated with edible vegetables grown on Cr (VI) polluted soils, which showed that Vigna angularis was the only edible vegetable that could germinate in highly polluted soil contaminated with Cr (VI) level at 456 mg/kg. The highest total chromium (ChT) factor, bioaccumulated in the roots was found in Phaseolus vulgaris at 0.8. The highest ChT translocation factor in the stem was that of Cicer arietinum and Vigna angularis at 0.30. The same plants translocated the highest ChT factor to the leaf at 0.7. A child or an adult consuming such contaminated Cicer arietinum vegetables is likely to take in between 508 and 785 mg/day of ChT. This is above the World Health Organisation (WHO) guidelines of 220 and 340 mg/day, respectively. The highest Hazard Quotient was found in Cicer arietinum at 8.7 and 13.4 for adults and children, respectively. The same species of plants also recorded the highest Hazard index (17.4 and 27.2) for adults and children, respectively. This imply that consumers of the edible vegetables grown in Cr (VI) rich soils may be exposed to health risks, and the children are more vulnerable to these adverse effects. The third objective of the study was to investigate the anaerobic bacterial profile of donkey dung assisted by anaerobic bioreactor and remediation of tannery chromium wastes. There was 99% removal efficiency of total chromium concentrations. This was observed in solutions mixed with donkey dung in the bioreactors within 30 days of the experiments as compared to 60% in the control. Methane gas was generated in DB bioreactors under alkaline pH but not in acidic pH in BO bioreactors. Abundant bacteria phyla in BO samples were Firmicutes (48.28%) while Proteobacteria (51.70%) were prevalent in DB. The Bacilli (40.29%) class dominated BO when Gammaproteobacteria (20.44%) class was abundant in DB. The genus of Delftia, Streptococcus, Staplococcus and Cutibacterium dominated BO while the genus of Enterobacter, Selenviibrio, Pseudomonas and Alcaligen dominated DB. These microbes may have removed total Cr by either chromosol, plasmid, enzymatic, dissimilatory, persister cell and small colony variant through diverse metabolic mechanisms. The last part of this study (not included in the body of the thesis but attached as Appendix A 1.0), characterised and preliminarily tested the performance of Equus africanus asinus dung on the sorption of synthetic Cr (VI) and real tannery effluents from DB and BO tanneries. The results of the adsorption study showed that when potassium dichromate solution was used as synthetic Cr (VI) wastewater and treated with donkey dung powder, there was a removal efficiency of 93.3% for Cr (VI). However, when the dung was applied to real tannery chromium effluents from BO and DB, the removal efficiency was found to be 83.6% and 85.5%, respectively. The adsorption data fitted better to the pseudo-second order kinetic model (R2 = 1). Thus, the interaction of the cationic species with the donkey dung powder was predominantly via chemisorption. In conclusion, this study has extensively elucidated on pollution potential and bioremediation of tannery-based chromium effluent wastes in BO tannery in South Africa and DB tannery in Kenya. The study has contributed comprehensively on how to assess ecological and health potential risk due to ineffective treatment of chromium effluent and indiscriminate dumping of their solid wastes within tanneries. Due to that, this study has for the first time advance and enhance existing knowledge on the eco-friendly bioremediation technique of tannery chromium effluent using raw organic donkey dung as a bio-stimulant source of naturally occurring microbes and an adsorbent to reduce high concentrations before discharge into the ecosystems.