Abstract

Artisanal mining of cassiterite, coltan, and wolframite in Rwanda has left extensive areas of strongly acidic soil contaminated with toxic heavy metals. However, cost-effective remediation technologies adapted to local conditions are limited. This study aimed at assessing the synergistic effect of coffee pulp compost (CPC), a major agro-industrial by product in Rwanda, and indigenous phosphate solubilising bacteria (PSB) consortium on immobilization of lead (Pb), cadmium (Cd) and zinc (Zn) in contaminated orthic Acrisol from the mining area of Rutongo. A 90-day pot experiment was conducted using a completely randomized design with seven treatments and four replicates, including an unamended control, autoclaved PSB control, CPC at 5% and 10% (w/w), a live PSB consortium (Bacillus megaterium BM-PSB1 and Pseudomonas fluorescens PF-PSB2), and two CPC + PSB combinations. Diethylenetriaminepentaacetic acid (DTPA)-extractable metals, sequential extraction, soil physicochemical properties, microbial biomass carbon (MBC), dehydrogenase activity (DHA), and X-ray diffraction (XRD) mineralogy were assessed on days 30, 60, and 90. The combined application of 10% CPC with live PSB was the most effective, increasing the soil pH from 3.82 to 6.47 and reducing the DTPA-extractable Pb, Cd, and Zn concentrations by 72.4%, 64.8%, and 58.6%, respectively.  No significant metal reduction was observed for autoclaved PSB, which was used to confirm that living bacteria activity was responsible for any effect observed. Sequential extraction revealed a pronounced shift of metals from exchangeable to residual and organic-bound fractions, while MBC and DHA increased by 3.8- and 4.2-fold, respectively. XRD analysis confirmed the formation of new pyromorphite and hopeite, indicating phosphate-mediated metal precipitation. The integrated use of CPC and indigenous PSB is a low cost and locally adaptable method to immobilize heavy metals and restore functions of biological systems in contaminated acid mining soil in addition to the valorisation of coffee waste.

Keywords: bioremediation, coffee pulp compost, phosphate-solubilizing bacteria, heavy metal immobilization, acidic soil, Rwanda.

References

Ahmed H, Abolore RS, Jaiswal S, Jaiswal AK. 2024. Toward Circular Economy: Potentials of Spent Coffee Grounds in Bioproducts and Chemical Production. Biomass. 4(2):286–312. doi:10.3390/biomass4020014.

Beiyuan J, Tsang DCW, Bolan NS, Baek K, Ok YS, Li X-D. 2018. Interactions of food waste compost with metals and metal-chelant complexes during soil remediation. J Clean Prod. 192:199–206. doi:10.1016/j.jclepro.2018.04.239.

Bertozzi J, Andrade DS, Oliveira CC, Bala A, Caviglione JH. 2020. Microwave assisted biocidal extraction is an alternative method to measure microbial biomass of carbon from cultivated and non-cultivated soils. Brazilian J Microbiol. 51(1):255–263. doi:10.1007/s42770-019-00186-z.

Dusengemungu D-R, Zhou Z, Liu J. 2023. Overview of mineral reserves availability in Rwanda: opportunities and challenges. Environ Dev Sustain. 25(10):10599–10626. doi:10.1007/s10668-022-02548-4.

Kazindu H, Mupenzi C, Maniragaba A. 2020. Environmental Impacts Analysis of Mining on the Surrounding Communities: A Case Study of Rutongo Mines Ltd in Rulindo District, Northern Province of Rwanda. 9(2):56–69. doi:10.11648/j.jenr.20200902.12.

Mo J, Xin L, Zhao C, Qin Y, Nan Q, Mei Q, Wu W. 2023. Reducing nitrogen loss during kitchen waste composting using a bioaugmented mechanical process with low pH and enhanced ammonia assimilation. Bioresour Technol. 372:128664. doi:10.1016/j.biortech.2023.128664.

Muhirwa F, Shen L, Elshkaki A, Hirwa H, Umuziranenge G, Velempini K. 2023. Linking large extractive industries to sustainable development of rural communities at mining sites in Africa: Challenges and pathways. Resour Policy. 81:103322. doi:10.1016/j.resourpol.2023.103322.

Nayak A, Rühl M, Klüber P. 2024. Hermetia illucens (Diptera: Stratiomyidae): Need, Potentiality, and Performance Measures.

Ngango J, Kim SG. 2019. Assessment of Technical Efficiency and Its Potential Determinants among Small-Scale Coffee Farmers in Rwanda. Agriculture. 9(7):161. doi:10.3390/agriculture9070161.

Noller C, Friesl-Hanl W, Hood-Nowotny R, Puschenreiter M, Watzinger A. 2021. Effect of Chelant-Based Soil Washing and Post-Treatment on Pb, Cd, and Zn Bioavailability and Plant Uptake. Water, Air, Soil Pollut. 232(10):405. doi:10.1007/s11270-021-05356-0.

Pan Y, Yu S-S, Xiao Z-C, Min Y, Tian T, Zheng Y-M, Zhao Q-B, Yuan Z-H, Yu H-Q. 2023. Re-evaluation and modification of dehydrogenase activity tests in assessing microbial activity for wastewater treatment plant operation. Water Res. 246:120737. doi:10.1016/j.watres.2023.120737.

Pande V, Pandey SC, Sati D, Pande V, Samant M. 2020. Bioremediation: an emerging effective approach towards environment restoration. Environ Sustain. 3(1):91–103. doi:10.1007/s42398-020-00099-w.

Patra A, Sharma VK, Nath DJ, Ghosh A, Purakayastha TJ, Barman M, Kumar S, Chobhe KA, Anil AS, Rekwar RK. 2021. Impact of Soil Acidity Influenced by Long-term Integrated Use of Enriched Compost, Biofertilizers, and Fertilizer on Soil Microbial Activity and Biomass in Rice Under Acidic Soil. J Soil Sci Plant Nutr. 21(1):756–767. doi:10.1007/s42729-020-00398-5.

Poria V, De K, Singh S, Pranaw K. 2022. Plant Growth-Promoting Bacteria (PGPB) integrated phytotechnology: A sustainable approach for remediation of marginal lands. October:1–20. doi:10.3389/fpls.2022.999866.

Rashmi I, Kala S, Sharma GK, Kumar A, Ali S, Kumar K, Kumawat A, Meena GL, Meena HR, Pal R, et al. 2024. Impact of Post-mining Restoration Techniques on Soil Health BT - Ecological Impacts of Stone Mining: Assessment and Restoration of Soil, Water, Air and Flora. Di dalam: Sharma GK, Rashmi I, Ali S, Kala S, Kumar A, Madhu M, editor. Singapore: Springer Nature Singapore. hlm 267–284.

Rawat P, Das S, Shankhdhar D, Shankhdhar SC. 2021. Phosphate-Solubilizing Microorganisms: Mechanism and Their Role in Phosphate Solubilization and Uptake. J Soil Sci Plant Nutr. 21(1):49–68. doi:10.1007/s42729-020-00342-7.

Shang Z, Wang T, Ye Q, Wu P, Wu J, Sun L, Zhu N. 2023. Immobilization of Pb2+ and Cd2+ on the novel calcium/magnesium silicate and their transformation in the presence of phosphate. Desalination. 568:117007. doi:10.1016/j.desal.2023.117007.

Tindwa HJ, Singh BR. 2023. Soil pollution and agriculture in sub-Saharan Africa: State of the knowledge and remediation technologies. Front Soil Sci. 2:1101944. doi:10.3389/fsoil.2022.1101944.

Tytła M, Widziewicz-Rzońca K, Bernaś Z. 2022. A Comparison of Conventional and Ultrasound-Assisted BCR Sequential Extraction Methods for the Fractionation of Heavy Metals in Sewage Sludge of Different Characteristics. Molecules. 27(15):4947. doi:10.3390/molecules27154947.

Wang L. 2021. A review of green remediation strategies for heavy metal contaminated soil. March:936–963. doi:10.1111/sum.12717.

Widjaya D, Yanuarti R. 2024. Mapping Coffee Base Countries in the World: Case Studies from Three Continents (Africa-America-Asia). 40(1):61–75. doi:10.22302/iccri.jur.pelitaperkebunan.v40i1.593.

Wróbel M, Śliwakowski W, Kowalczyk P, Kramkowski K, Dobrzyński J. 2023. Bioremediation of Heavy Metals by the Genus Bacillus. Int J Environ Res Public Health. 20(6):4964. doi:10.3390/ijerph20064964.

Yang F, Tang C, Antonietti M. 2021. Natural and artificial humic substances to manage minerals, ions, water, and soil microorganisms. Chem Soc Rev. 50(10):6221–6239. doi:10.1039/d0cs01363c.

Yang Z, Yu L, Yan X, Li P, Si M, Liao Q, Zhao F, Lin Z, Sheng A, Yang W. 2025. Mechanistic and kinetic understanding of Pb-phosphate biomineralization from humic acid-bound Pb under active growth of phosphate solubilizing Enterobacter aerogenes W6. J Environ Manage. 379:124894. doi:10.1016/j.jenvman.2025.124894.

Zhao X, Teng Z, Wang G, Luo W, Guo Y, Ji X, Hu W, Li M. 2023. Anaerobic syntrophic system composed of phosphate solubilizing bacteria and dissimilatory iron reducing bacteria induces cadmium immobilization via secondary mineralization. J Hazard Mater. 446:130702. doi:10.1016/j.jhazmat.2022.130702.