Composting as a Sustainable Solution for the Management of Plant Biomass Contaminated with Hg and As from Puddles Generated by Small-Scale Gold Mining in the Municipality of Unión Panamericana, Colombian Pacific

ASGM in the Chocó region (Colombian Pacific) has generated abandoned puddles with concentrations of Hg; however, these sites can generate ecosystem services from contaminated biomass with a circular economy approach. (1) Background: The plant biomass contaminated with Hg from these puddles could be used to produce compost as a bioremediation strategy and as an alternative to generate ecosystem resources (organic fertilizer) with nature-based solutions, representing a circular economy approach and sustainable management of contaminated biomass. (2) Methods: Six treatments were established to produce compost; closed-system plastic composters were used, with a capacity of 15 kg of biomass and organic waste, consisting of contaminated and uncontaminated biomass of macrophytes with Hg and fruit and vegetable waste (fruit and vegetable waste, pruning waste, and poultry manure). The concentrations of As and total and bioavailable Hg were monitored during the composting process by atomic absorption spectroscopy and so were properties such as pH; electrical conductivity; temperature; CO, TN, S, and SiO 2 contents; and C/N ratio. (3) Conclusions: The concentrations of bioavailable Hg during the composting process were between 658.1 and 1.2 µg/kg, decreasing in the following order: T-2 > T-3 > T-1 > C-3 > C-2 > C-1. Composting in the treatments was shown to be an efficient bioremediation technique, as all of the treatments reduced the bioavailability of Hg to below 1.5%, and the physicochemical properties of pH (r: −0.3675; p < 0.007) and %S content (r: −0.6303; p < 0.0025) showed a significant moderate negative correlation with bioavailable Hg concentrations, playing a crucial role in reducing the bioavailability of Hg during the purification process of composting. The results show that all treatments significantly reduced Hg bioavailability below 1.5%; however, T-2 proportionally showed the highest reductions during the process. Compost production was proven to be a sustainable strategy for the management of biomass contaminated with mercury, which is very important for recovering ecosystem services in communities.