Trace Metal Removal by Benthic Microbial Biomats as Compared to an Iron (Oxyhydr)oxide Sorbent

In regions of past and current mining activities, elevated levels of potentially toxic metals are an important environmental problem to both local communities and surrounding aquatic ecosystems. Various materials have been studied for metal removal, with increasing attention given to diatom-related materials in recent years. Unit process open water engineered wetlands select for benthic microbial mats (biomat) that contain a diatom-bacteria/archaea consortium, organic matter, and mineral phases such as iron (oxyhydr)oxides. We conducted laboratory batch experiments to compare the adsorption behavior of a mixture of metals of concern (Zn, Cu, Pb, Cd, and Ni). We used lab-synthesized iron (oxyhydr)oxides and two kinds of biomat (a) from the open water cells situated within the Prado Engineered Wetlands (altitude 206m )that receive Santa Ana River water (Prado biomat, 2.3 wt% Fe) and (b) from a pilot-scale wetland (altitude 1768m) receiving on-site sequencing batch membrane bioreactor permeate that was seeded at start up with Prado biomat (MP biomat, 0.24 wt% Fe). All three materials showed high removals of all investigated metals (>50% removal with iron (oxyhydr)oxides and >83% removal with two biomats). Based on Fe content, iron (oxyhydr)oxides accounted for 5-9% of the metal removal in MP biomat and 36-61% of the metal removal in Prado biomat, indicating that other phases such as labile organics might also play an important role. Ethylenediaminetetraacetic acid (EDTA) extraction and acid digestion were used to evaluate the mobility of the adsorbed metals from these three materials. Metals were easier to extract (i.e., only required EDTA) from iron (oxyhydr)oxides (88-96%) than from MP biomat (56-91%) than from Prado biomat (50-77%). Sequential extraction results demonstrated that 902% of Zn and 734% of Pb were associated with adsorbed/carbonate/exchangeable and labile organics in MP biomat, whereas 332% of Zn and 513% of Pb were associated with those two phases in Prado biomat. By comparing the metal mobility in iron (oxyhydr)oxides with two biomats, we hope to improve our understanding of the removal processes and subsequent stability of metals in open water wetlands, which is critical for exploring their feasibility to treat and sequester heavy metals in mining and other impacted waters.