Rapid mercury removal using living indigenous microalgal communities for water treatment applications

Abstract

This study investigates the remarkable potential of living Mixed Indigenous Microalgae (MIMA) for mercury bioremediation in aquatic environments at environmentally relevant concentrations (10–100 μg/L). Our research demonstrates high mercury removal efficiency (89–94 %) across all tested concentrations, with rapid kinetics achieving equilibrium within just 2 min of contact time. The maximum biosorption capacity reached 0.10 mg/g at 100 μg/L initial concentration. Notably, MIMA maintained both viability and removal efficiency when exposed to Hg for 3 days without any addition of nutrients suggesting resilience under nutrient-limited conditions. Comprehensive isotherm analysis revealed the Dubinin-Radushkevich model provided the best fit (R² = 0.998), indicating physical adsorption as the predominant mechanism, as the calculated mean free energy (E = 5.00 kJ/mol) falls within the 1–8 kJ/mol range characteristic of physical adsorption. Kinetic studies showed superior correlation with the pseudo-second-order model (R² > 0.996), with rate constants decreasing systematically from 767.5 to 216.0 g/mg·min as concentration increased, suggesting secondary chemical interactions may also contribute to the overall mechanism. Advanced characterization revealed significant surface modifications, with Scanning Electron Microscopy (SEM) showing increased surface roughness, Fourier Transform Infrared Spectroscopy (FTIR) indicating involvement of hydroxyl, protein, and carbohydrate functional groups, and X-ray Photoelectron Spectroscopy (XPS) confirming Hg(II) binding to oxygen-containing moieties with distinctive Hg 4f peaks at 101.78 and 105.8 eV. Optical microscopy revealed the formation of sudden spherical-shell boundaries around individual cells providing visual evidence of an immediate physico-chemical response at the cell-mercury interface, correlating with the observed rapid kinetics. This research addresses critical knowledge gaps regarding living microalgae-mediated mercury removal and demonstrates MIMA's potential as a sustainable, efficient solution for mercury contamination in aquatic ecosystems, maintaining viability even under nutrient-limited conditions while effectively reducing mercury concentrations to near guideline values at the lowest initial level (10 μg/L).