Kela P. Weber

Planted and unplanted recirculating constructed wetland mesocosms maintained under non-aerated (reducing) and aerated (oxidizing) conditions were compared in their ability to remove 1 mg/L As(V) from simulated domestic wastewater over a 33-week period. Each week, one day prior to the addition of arsenic, non-aerated mesocosms removed 70% of the nitrate and 95% of the organic carbon likely through denitrification while aerated mesocosms removed organic carbon only (97%). Upon arsenic addition and treatment for the remainder of the week, non-aerated mesocosms removed more arsenic (72%) than aerated mesocosms (43%). For the first time in the literature, we estimated the proportion of arsenic found in biofilms detached from gravel and incorporated this aspect into mass balance calculations. We discovered that biofilms from non-aerated mesocosms contained a higher proportion of arsenic (28%) than biofilms from aerated mesocosms (5%). Plants from both types of mesocosms removed a negligible amount of arsenic (<0.41%). Through X-ray fluorescence (XRF) analysis, it was discovered that biofilms from non-aerated mesocosms had the highest iron concentrations (35,000 mg/kg), and this iron was identified as pyrite (2–15 wt%) through X-ray diffraction (XRD) analysis. The iron concentration of aerated biofilms was no different from that found in clean gravels (17,500 mg/kg), and no iron minerals were detectable in these samples using XRD analysis. Although some of the arsenic removal in both types of wetlands was thought to be attributable to adsorption onto substrates and biofilms, as well as uptake into biofilms, removal of arsenic in the non-aerated mesocosms was likely enhanced by the presence of pyrite because pyrite can adsorb arsenic directly or indirectly through the formation of iron oxides that can adsorb or coprecipitate with the iron oxides.

Planted and unplanted recirculating constructed wetland mesocosms maintained under non-aerated (reducing) and aerated (oxidizing) conditions were compared in their ability to remove 1 mg/L As(V) from simulated domestic wastewater over a 33-week period. Our results show that non-aerated mesocosms removed more arsenic than aerated mesocosms from the wastewater. For the first time in the literature, we estimated what proportion of arsenic was found in gravel detached biofilms and incorporated this aspect into mass balance calculations. We discovered biofilms from both types of mesocosms contained a higher proportion of arsenic than plants, and that the proportion of arsenic found in non-aerated biofilms was higher than for aerated biofilms. We discovered that non-aerated biofilms contained a higher concentration of iron than aerated mesocosms and this was attributable to the formation of biogenic pyrite. There was no formation of arsenic adsorbing biogenic minerals found in biofilms from aerated mesocosms. Although some of the arsenic removal in both types of wetlands was attributable to adsorption onto substrates and biofilms, removal of arsenic in the non-aerated mesocosms was likely enhanced by the presence of pyrite in the non-aerated mesocosms where arsenic adsorbed to the pyrite and/or co-precipitation of arsenic occurred with iron oxides that formed on the surface of the pyrite. The removal of arsenic through the formation of orpiment and arsenian pyrite was also possible in the non-aerated mesocosms but these minerals were not directly detected.

The chemical properties of perfluoroalkyl and polyfluoroalkyl substances (PFAS) pose a significant remediation challenge. This study investigated smoldering combustion to destroy PFAS while scaling up from the lab to field implementation. The first phase consisted of bench-scale tests using a model soil system. Calcium oxide (CaO) was used as a soil amendment in some of the test cases. For all test conditions, greater than 99.9% removal of PFAS was achieved. Post-treatment soils without CaO amendments were found to have a significant reduction in total fluorine concentrations, while the fluorine concentrations in soils with CaO amendments were similar following treatment. This suggests that fluorine emissions from smoldering treatment of PFAS are captured by the presence of the calcium ion (Ca2+). Using new analytical methods, we better characterized the mass balance of the system. These lab results were carried out in a pilot study using soils from a PFAS-impacted site. Two large-scale tests treating 10 m3 of soil were completed. Results of the large-scale smoldering tests agreed with the results from the lab phase. The results from both phases provide a greater understanding of the fate of PFAS when it is treated by smoldering and detail the first large-scale demonstration of smoldering treatment for PFAS-impacted soils.

The transport of per- and polyfluoroalkyl substances (PFAS) in soil and groundwater is important for site investigation, risk characterization, and remediation planning. The adsorption of PFAS at air-water interfaces has been shown to significantly contribute to PFAS retention, with subsequent effects on concentrations and the time scales of transport. In this study, column experiments were conducted to investigate the transport of perfluorooctanesulfonic acid (PFOS), perfluorooctanoic acid (PFOA), and 6:2 fluorotelomer sulfonate (6:2 FTS) individually and in binary mixtures in the presence of a trapped gas phase, using clean sands to isolate adsorption to air-water interfaces. Consistent with previous studies, the transport of PFOS, PFOA, and 6:2 FTS was retarded by adsorption at the air-water interface, with greater retention of PFOS due to its higher affinity for the air-water interface. Chromatographic separation occurred in the experiments using binary mixtures of PFOS and PFOA, with greater retention at lower influent concentrations. The mixture experiments also showed enhanced breakthrough of PFOA in the presence of PFOS, where effluent concentrations of PFOA were temporarily greater than the influent concentration prior to the breakthrough of PFOS. This enhanced breakthrough was attributed to competition between PFOS and PFOA for adsorption to the air-water interface.