Heavy metal removal by the photosynthetic microbial biomat found within shallow unit process open water constructed wetlands

Nature-based solutions offer a sustainable alternative to labor and chemical intensive engineered treatment of metal-impaired waste streams. Shallow, unit process open water (UPOW) constructed wetlands represent a novel design where benthic photosynthetic microbial mats (biomat) coexist with sedimentary organic matter and inorganic (mineral) phases, creating an environment for multiple-phase interactions with soluble metals. To query the interplay of dissolved metals with inorganic and organic fractions, biomat was harvested from two distinct systems: the demonstration-scale UPOW within the Prado constructed wetlands complex (“Prado biomat”, 88 % inorganic) and a smaller pilot-scale system (“Mines Park (MP) biomat”, 48 % inorganic). Both biomats accumulated detectable background concentrations of metals of toxicological concern (Zn, Cu, Pb, and Ni) by assimilation from waters that did not exceed regulatory thresholds for these metals. Augmentation in laboratory microcosms with a mixture of these metals at ecotoxicologically relevant concentrations revealed a further capacity for metal removal (83–100 %). Experimental concentrations encapsulated the upper range of surface waters in the metal-impaired Tambo watershed in Peru, where a passive treatment technology such as this could be applied. Sequential extractions demonstrated that metal removal by mineral fractions is more important in Prado than MP biomat, possibly due to a higher proportion and mass of iron and other minerals from Prado-derived materials. Geochemical modeling using PHREEQC suggests that in addition to sorption/surface complexation of metals to mineral phases (modeled as iron (oxyhydr)oxides), diatom and bacterial functional groups (carboxyl, phosphoryl, and silanol) also play an important role in soluble metal removal. By comparing sequestered metal phases across these biomats with differing inorganic content, we propose that sorption/surface complexation and incorporation/assimilation of both inorganic and organic constituents of the biomat play a dominant role in metal removal potential by UPOW wetlands. This knowledge could be applied to passively treat metal impaired waters in analogous and remote regions.