Cyanobacterial extracellular polysaccharides (EPS) produced at high CO₂ enhance sequestration of heavy metals into mineral precipitate

Abstract

Heavy metals are extensively used in industry, and the associated environmental pollution raises risks for human health. Immobilization and long-term sequestration of these metals would reduce such risks. Cyanobacterial extracellular polysaccharides (EPS) have a proven capacity for effectively sorbing cations including heavy metals. Unfortunately, natural degradation of EPS over time can release the sorbed metals back into the environment. In addition to sorption, EPS enriched in negatively charged, acidic residues can induce precipitation of carbonates. We hypothesized that strongly acidic EPS could effectively sorb heavy metals from a solution and facilitate their long-term sequestration into carbonate minerals. To obtain large amounts of strongly acidic EPS, Leptolyngbya boryana strain N1, isolated from the Negev desert soil, was cultivated under high-CO₂ conditions. Chemical and spectroscopic analyses confirmed a high density of negatively charged moieties in the EPS. The strongly acidic EPS sorbed metals better than less acidic EPS of cyanobacteria cultivated under ambient air, low-CO₂ conditions. Birefringent crystals formed in the presence of the strongly acidic EPS. The precipitated carbonates, chlorides and sulfates incorporated Cu²⁺, Zn²⁺, and Li⁺ heavy metals. Therefore, our proof-of-concept study presents a novel approach for long-term sequestration of heavy metals using sustainably produced biopolymers.