Trace elements, toxicity burdens and industrial water recirculation in molasses-based bioethanol production: A planetary boundary-based life cycle assessment

Background Conventional life cycle assessment (LCA) reports the climate and fossil-fuel benefits of molasses-based bioethanol; however, it does not assess whether trace-element-driven impacts remain within planetary safe operating limits. This study elucidates the role of trace elements (Ni, Cr, Zn, Cu, Pb, Cd, Fe, Co) as dominant contributors to planetary boundary transgression in a molasses-based bioethanol system with multiple-effect evaporator (MEE) based ZLD, including specific emphasis on water-cycle interactions and toxicity impacts. This study is novel in terms of systematic quantification of element-level toxicity contributions, integration of PB-LCA with industrial trace-element flows and identification of critical process-level drivers across coal, molasses and bagasse-based pathways. Method IMPACT World+ and ReCiPe 2016 methods were used for 1 kg of bioethanol as the functional unit. Trace-element flows were sourced from established life cycle inventory databases (Ecoinvent, Agri-Footprint), whereas process data were collected directly from the operational plant. Sixteen midpoint categories were assessed and aggregated to endpoint damage categories (human health, ecosystems and resources). Trace-element-related impacts are primarily reflected in human toxicity (cancer and non-cancer) and freshwater ecotoxicity. Planetary boundaries were downscaled from global to national (egalitarian), sectoral (FCE-based utilitarian) and per-kg levels to calculate Environmental Boundary Ratios (EBR) and Transgression Index (TI). It enables the evaluation of trace metal-driven pressure relative to allocated safe operating space Results Trace element-driven impact shows a significant increase in human toxicity and freshwater ecotoxicity with EBR 6.8 and 3357, respectively. Freshwater ecotoxicity reaches 2.307E+04 CTUe/kg and is dominated by Al at about 80% originated from coal combustion and molasses. Human toxicity (cancer effects) is predominantly driven by chromium (Cr), contributing approximately 85% of the total impact. In the non-cancer human toxicity category, arsenic (As) emerges as the dominant contributor, accounting for approximately 32% of the total impact, followed by lead (Pb) and cadmium (Cd), each contributing around 16%. The primary sources of these toxic emissions identified are coal combustion and molasses-based processing pathways. Although the MEE system achieves up to 98% water recovery, it primarily concentrates trace elements in residual streams rather than eliminating them. Consequently, significant human health damage is observed (199.96 mPt), largely driven by emissions of Al, Cr, Cd, Ni and Pb. Conclusion and recommendation Results show exceedance of the allocated safe operating space, with severe impacts on toxicity indicators, driven mainly by trace metals from coal, bagasse, and molasses. Improving environmental sustainability requires reducing coal use, controlling trace metal inputs and integrating toxicity indicators with PB-LCA thresholds into regulatory frameworks.