Sustainable bioremediation of HCHs-contaminated soils using organic amendments: A circular economy approach

This study investigates the potential of two organic waste-derived amendments, horse manure compost (E H ) and sewage sludge (E S ), as cost-effective, scalable solutions for bioremediating hexachlorocyclohexanes (HCHs)-polluted soils. Laboratory experiments used field soils artificially spiked with HCHs particles (≈165 mg/kg soil ) to simulate representative conditions at the Bailín landfill site (Sabiñánigo, Spain). Amendments were applied at 5% (E H ) and 2.5% (E S ) dry weight, and degradation was monitored over 90 days under control conditions. E S stimulated short-term microbial and enzymatic activity, achieving rapid removal of labile isomers (α-HCH and γ-HCH), but its efficacy declined due to nutrient depletion. In contrast, E H induced sustained enzymatic activation, leading to progressive degradation of all HCH isomers, including recalcitrant β-HCH, with >95% total HCH removal by day 90. Enzymatic profiling indicated the involvement of multiple biogeochemical cycles (carbon, nitrogen, phosphorus, and sulphur). While E S generated early enzymatic peaks (e.g., α-GLU, URE, PHOS), E H promoted long-term enzymatic functionality, supporting persistent microbial activity. This study emphasizes the practical relevance of organic amendments for field-scale remediation of persistent organic pollutants, highlighting their engineering feasibility, low cost, and alignment with circular economy principles. The amendments did not introduce secondary pollutants, avoiding cross-media contamination and supporting safe application in integrated soil recovery strategies. Increased dehydrogenase and phenoloxidase activities paralleled HCH degradation, highlighting their role as functional indicators of the bioremediation process.