Submit your papersSubmit Now
For Enquiries: [email protected]
IIARD LogoIIARD

Assessment of Heavy-Metal Partitioning and Speciation in Digestate from Municipal–Agro Residue Co-Digestion: Implications for Risk Beyond Total Concentrations

Nkechi Blessing Chinedu, Gospel Effiong Isangadighi, Jessica Gospel Isangadighi, Goodness Effiong Isangadighi

Abstract

Increasingly, anaerobic co-digestion of municipal solid waste and agro-residues is considered in the context of renewable energy recovery and nutrient recycling. Still, the safety of digestate reuse remains unclear when assessed solely by total heavy-metal concentration. The study aimed to compare heavy-metal partitioning and chemical speciation in co-digested and digestate to reframe environmental risk beyond concentration-based metrics. A mesophilic anaerobic co-digestate was analysed for total concentrations of Cd, Pb, Cr, Cu, Zn, and Ni, followed by sequential extraction- based speciation, Risk Assessment Code analysis, multivariate statistics, and a recently developed speciation-weighted heavy-metal risk index. Although no cases of exceeding international legislative thresholds for total metal concentrations were observed, speciation analysis revealed significant metal-specific risk patterns, with 34.2% and 22.6% of cadmium and zinc, respectively, in the exchangeable fraction (high and medium ecological risk). By contrast, chromium and nickel were characterised by residual fractions exceeding 50% and 40%, respectively, suggesting high geochemical stability and minimal short-term risk. Principal component analysis accounted for 91.8% of the total variance, with the first component loading primarily on mobile-risk metals (Cd, Zn) and structurally bound metals (Cr, Ni). Among the risk drivers, although regulatory requirements appear to have been met, Cd (27.6) and Zn (22.4) were the most important. The present research contributes to knowledge by translating heavy-metal speciation theory into a quantitative, decision-relevant risk framework that shows that anaerobic co-digestion of heavy metals does not remove risk but redistributes it. It is also determined that total concentrations cannot be relied upon to infer the safety of digestates, and that specification-based regulations and site-specific digestate management practices are highly advisable.

Keywords

Heavy-metal speciationDigestate risk assessmentAnaerobic co-digestionMunicipal solid wasteAgro-residuesMetal partitioningSpeciation-weighted risk indexEnvironmental sustainability

References

Adelekan, B. A., & Alawode, A. O. (2011). Contributions of municipal refuse dumps to heavy metals concentrations in soil profile and groundwater in Ibadan Nigeria. Journal of Applied Biosciences, 40, 2727-2737. Adeyemi, O., & Isangadighi, G. E. (2025). Diurnal variations and occupational exposure risks of gaseous and particulate pollutants in petrol filling stations of Uvwie, Delta State, Nigeria. Asian Journal of Applied Science and Technology , 9(3), 171-188. Ahanor, E., Obahor, G., & Isangadighi, G. E. (2025). Informal sector dynamics in Nigeria’s e- waste chain: Legal recognition and environmental risk. Journal of Built Environment and Geological Research. Ahmadu, J., Galadima, D. H., Chinedu, N. B., & Eze, O. B. (2019). Effect of erosion on agricultural land in Agyana Community in Abaji, Abuja. Int J Appl Agric Sci, 5, 120-128. Alburquerque, J. A., De la Fuente, C., Campoy, M., Carrasco, L., Nájera, I., Baixauli, C., ... & Bernal, M. P. (2012). Agricultural use of digestate for horticultural crop production and improvement of soil properties. European Journal of Agronomy, 43, 119-128. Alloway, B. J. (1995). Soil processes and the behaviour of metals. Heavy metals in soils, 13, 3488. Angelidaki, I., & Ellegaard, L. (2003). Codigestion of manure and organic wastes in centralized biogas plants: status and future trends. Applied biochemistry and biotechnology, 109(1), 95-105. Brown, K. W., & Ryan, R. M. (2003). The benefits of being present: mindfulness and its role in psychological well-being. Journal of personality and social psychology, 84(4), 822. Chen, S., Rotaru, A. E., Liu, F., Philips, J., Woodard, T. L., Nevin, K. P., & Lovley, D. R. (2014). Carbon cloth stimulates direct interspecies electron transfer in syntrophic co- cultures. Bioresource technology, 173, 82-86. Chinedu, N. B., Nkwoada, A. U., & Opara, A. I. (2024). Boosting biodiesel production from waste vegetable oil water using Acid-Base Sequential Catalysis. GLOBAL NEST JOURNAL, 26(6). Chinedu, N.B., Isangadighi, G.E., Isangadighi, G.E., Jibril, R., Ofoegbu, C.J., Mathew, P., Udeh, J.A., Michael, J.A., & Amba, C.O. (2026). Engineered Nano-Adsorbents for Concurrent Heavy Metal Detoxification and Feedstock Upgrading of Waste Cooking Oils in Delta State, Nigeria. Scientia. Technology, Science and Society, 3(1), 1-23. DOI: 10.59324/stss.2026.3(1).01 Chinedu, P. U., Isah, Y., & Chinedu, N. B. (2022). An Internet of Things Based Smart Agriculture Monitoring System for Enhanced Productivity in a Controlled Farm Environment. ATBU Journal of Science, Technology and Education, 10(3), 125-138. Essien, U. B., Ezejiofor, A. N., Daniel, E. N., Chioma, U. T., Ibanga, F. I., Udoh, U. I., ... & Akpan, L. E. (2025). Environmental and Toxicological Perspectives of Soil Microplastics in Northern Nigeria. Journal of African Innovation and Advanced Studies. Fermoso, F. G., Bartacek, J., Jansen, S., & Lens, P. N. (2009). Metal supplementation to UASB bioreactors: from cell-metal interactions to full-scale application. Science of the total environment, 407(12), 3652-3667. Giacomini, R. (2013). The relationship between DSGE and VAR models. VAR models in macroeconomics–new developments and applications: Essays in honor of Christopher A. Sims, 1-25. Giacomini, R. (2013). The relationship between DSGE and VAR models. VAR models in macroeconomics–new developments and applications: Essays in honor of Christopher A. Sims, 1-25. Hooda, P. S., Edwards, A. C., Anderson, H. A., & Miller, A. (2000). A review of water quality concerns in livestock farming areas. Science of the total environment, 250(1-3), 143-167. Ibe, F. C., Chukwu, F. O., Onyekachi Ibe, B., Shehu Ahmad, A., & Blessing Chinedu, N. (2025). Comprehensive Analysis of the effect of pH on Heavy Metal in Groundwater within Owerri Municipal Area in Imo State, Nigeria. Chemical Research and Technology, 2(2), 75-89. Isangadighi, G. E., & Udeh, J. A. (2023). Emergencies, preparedness, and management: a case study of Nigeria. World Safety Organization, 32(2), 38. Kabata-Pendias, A., & Szteke, B. (2015). Trace elements in abiotic and biotic environments (p. 468). Taylor & Francis. Kumpiene, J., Lagerkvist, A., & Maurice, C. (2008). Stabilization of As, Cr, Cu, Pb and Zn in soil using amendments–a review. Waste management, 28(1), 215-225. Luo, C., Liu, C., Wang, Y., Liu, X., Li, F., Zhang, G., & Li, X. (2011). Heavy metal contamination in soils and vegetables near an e-waste processing site, south China. Journal of hazardous materials, 186(1), 481-490. Mapanda, F., Mangwayana, E. N., Nyamangara, J., & Giller, K. E. (2005). The effect of long-term irrigation using wastewater on heavy metal contents of soils under vegetables in Harare, Zimbabwe. Agriculture, Ecosystems & Environment, 107(2-3), 151-165. Mata-Alvarez, J., Macé, S., & Llabrés, P. (2000). Anaerobic digestion of organic solid wastes. An overview of research achievements and perspectives. Bioresource technology, 74(1), 3-16. McBride, M. B. (1995). Toxic metal accumulation from agricultural use of sludge: are USEPA regulations protective?. Journal of Environmental Quality, 24(1), 5-18. McLaughlin, M. J., Hamon, R. E., McLaren, R. G., Speir, T. W., & Rogers, S. L. (2000). A bioavailability-based rationale for controlling metal and metalloid contamination of agricultural land in Australia and New Zealand. Australian journal of soil research, 38(6), 1037-1086. Nicholson, F. A., Smith, S. R., Alloway, B. J., Carlton-Smith, C., & Chambers, B. J. (2003). An inventory of heavy metals inputs to agricultural soils in England and Wales. Science of the total environment, 311(1-3), 205-219. Nkoa, R. (2014). Agricultural benefits and environmental risks of soil fertilization with anaerobic digestates: a review. Agronomy for Sustainable Development, 34(2), 473-492. Orhuebor, E. N., Chinedu, N. B., Isangadighi, G. E., Ofoegbu, C. J., Essien, U. B., & Damilola, T. E. (2026a). Machine Learning-Driven Predictive Modelling of Toxic Hydrocarbon Emissions and Public Health Risks in The Port Harcourt Refining Corridor: Integrating Data Governance, Ethical AI, and Intellectual Property Protection Frameworks. European Journal of Applied Science, Engineering and Technology, 4(1), 60-72. Orhuebor, E. N., Chinedu, N. B., Isangadighi, G. E., Udeh, J. A., Uchenna, O. A., Isangadighi, G. E., & John, A. M. (2026b). Occupational Health Risk Mapping in the Niger Delta Oil and Gas Fields using Machine Learning and Environmental Exposure Indices. European Journal of Innovative Studies and Sustainability, 2(1), 54-71. Orhuebor, E. N., Isangadighi, G. E., Akpololohor, C. A., & Akinboboye, R. (2025). Data-driven techno-economic and environmental assessment of compressed natural gas vehicles as a source of transportation in Nigeria. European Journal of Applied Science, Engineering and Technology, 3(5), 142-152. Park, J. H., Wacholder, S., Gail, M. H., Peters, U., Jacobs, K. B., Chanock, S. J., & Chatterjee, N. (2010). Estimation of effect size distribution from genome-wide association studies and implications for future discoveries. Nature genetics, 42(7), 570-575. Rauret, G., López-Sánchez, J. F., Sahuquillo, A., Rubio, R., Davidson, C., Ure, A., & Quevauviller, P. (1999). Improvement of the BCR three step sequential extraction procedure prior to the certification of new sediment and soil reference materials. Journal of environmental monitoring, 1(1), 57-61. Sahuquillo, A., Rigol, A., & Rauret, G. (2003). Overview of the use of leaching/extraction tests for risk assessment of trace metals in contaminated soils and sediments. TrAC Trends in Analytical Chemistry, 22(3), 152-159. Singh, R. P., & Agrawal, M. (2008). Potential benefits and risks of land application of sewage sludge. Waste management, 28(2), 347-358. Smith, S. R. (2009). A critical review of the bioavailability and impacts of heavy metals in municipal solid waste composts compared to sewage sludge. Environment international, 35(1), 142-156. Tessier, A. P. G. C., Campbell, P. G., &

More Articles from WORLD JOURNAL OF INNOVATION AND MODERN TECHNOLOGY