Subsurface interfaces are unique boundaries that sustain a multitude of biogeochemical processes. Redox transformations at these interfaces can promote mobilization of organic matter (OM), metal micronutrients, and contaminants, helping to mediate the quality of downstream waters.  Furthermore, changing hydrological conditions further influence the redox gradients.

We commonly detect reduced Fe-rich colloids (20-100 nm) at our Slate River Floodplain Field Site, at depths were anoxic conditions prevail. The abundance and composition of the colloids varies with season, suggesting transformation or transport of the colloids. The composition, architexture and mechanisms responsible for the colloid dynamics are poorly understood. Thus, our overarching goal is to identify the mechanisms controlling colloid fate in heterogeneous and dynamic environments.

Related publications:

Engel M., Noël V., Pierce S., Kovarik L., Kukkadapu R., Lezama Pacheco J., Qafoku O., Runyon R., Chorover J., Zhou W., Cliff J., Boye K. and Bargar J. (2023) Structure and composition of natural ferrihydrite nano-colloids in anoxic groundwater. Water Research. Just Accepted. https://doi.org/10.1016/j.watres.2023.119990 

Alluvial aquifers are one of the world's largest sources of drinking water. They are composed of hydraulically productive sand and gravel, embedded with fine-grained lenses, which are typically rich in reactive materials, such as clay, organic matter and reduced iron and sulfur. The ability of the aquifer to mitigate groundwater quality depends on the dynamic framework of the aquifer network, which largely depends on the composition and spatial distribution of the sediments.

Heavy metal contamination of aquifers poses a major threat to the quality of groundwater and safety of crops and wildlife. The availability of heavy metals depends on their chemical speciation, which is reliant on multiple factors such as soil texture, pH and organic matter. Particularly, redox conditions have a strong influence on heavy metal behaviour; however, the impact of temporally and spatially heterogeneous redox regimes on heavy metal mobility in intricate soil and groundwater systems remains largely unexplored. Therefore, we are predominantly interested in heavy metal and nutrient cycling in these complex and dynamic systems. 

Related publications:

•  Engel M., Noël V., Kukkadapu R., Boye K., Bargar J. and Fendorf S. (2022) Nitrate controls on the extent and type of metal retention in fine-grained sediments of a simulated aquifer. Environmental Science and Technology 56:14452-14461. https://doi.org/10.1021/acs.est.2c03403 

• Babey T., Boye K., Tolar B., Engel M., Noël V., Perzan Z., Francis C., Bargar J. and Maher K. (2022) Simulation of anoxic lenses as exporters of reactivity in alluvial sediments. Geochimica et Cosmochimica Acta 334:119-134. https://doi.org/10.1016/j.gca.2022.07.018 

• Aeppli M., Babey T., Engel M., Fendorf S., Bargar J. and Boye K. (2022) Export of organic carbon from reduced fine-grained zones governs biogeochemical reactivity in simulated aquifer. Environmental Science and Technology 56:2738-2746. https://doi.org/10.1021/acs.est.1c04664 

• Engel M., Boye K., Noël V., Babey T., Bargar J. and Fendorf S. (2021) Simulated aquifer heterogeneity leads to enhanced attenuation and multiple retention mechanisms of Zn. Environmental Science and Technology 55:2932-2948. https://doi.org/10.1021/acs.est.0c06750 

Soil organic compounds are ubiquitous but quite heterogeneous in the environment; they can enhance or retard heavy metal adsorption through direct complexation or by altering mineral surfaces. Thus, it is crucial to examine the effects of natural organic compounds and soil minerals on heavy metal behaviour in soil systems. 

This project focused on 2-line ferrihydrite, a common soil iron mineral, and three heavy metals with various Lewis aid-base properties (Ni, Zn and Cd). Five natural organic compounds were selected to account for the various compounds that contribute to soil organic matter in order to investigate how mineral-organic associations influence the binding rate and capacity of heavy metals. 

Related publication:

Engel M., Lezama Pacheco J., Noël V., Boye K. and Fendorf S. (2020). Organic compounds alter preferences and rates of heavy metal adsorption on ferrihydrite. Science of the Total Environment 750:141485. https://doi.org/10.1016/j.scitotenv.2020.141485 

Carbon nanotubes (CNTs) exhibit toxic properties that may be utilised for water disinfection purposes. To permit reusability of the CNTs, they were decorated with magnetic iron oxide nanoparticles. This work was initiated during my BARD Graduate Fellowship Program, which as hosted by Prof. Menachem Elimelech at at Yale University, and continued at HUJI with the help of Prof. Yitzhak Hadar and Prof. Shimshon Belkin. We evaluated the efficiency of the nanocomposite material against different strains of bacteria (gram + and -), and tested its recycling using various washing techniques. Finally, we gained mechanistic information on the nanocomposite's anti-bactericidal activity using single-gene knockout mutant strains of E.coli. 

Related publications:

• Engel M., Hadar Y., Belkin S., Lu X., Elimelech M. and Chefetz B. (2018). Bacterial inactivation by a carbon nanotube-iron oxide nanocomposite: A mechanistic study using E.coli mutants. Environmental Science: Nano 5:372-380.  https://doi.org/10.1039/C7EN00865A

• Bhaduri B., Engel M., Polubesova T., Wu W., Xing B. and Chefetz B. (2018). Dual functionality of an Ag-Fe3O4-carbon nanotube composite material: Catalytic reduction and antibacterial activity. Journal of Environmental Chemical Engineering 6:4103-4113. https://doi.org/10.1016/j.jece.2018.06.023

Carbon nanotubes (CNTs) are a novel nanomaterial with remarkable features that may be harnessed for their potential use as adsorbents and disinfectants to remove hazardous substances from water. However, in light of their increasing implementation in various products, CNTs are also destined to invade environmental systems where they may interact with organic pollutants and dissolved organic matter (DOM).

DOM is a mixture of chemically complex molecules present in almost all aquatic environments. DOM plays a key role in many biochemical processes in natural water systems, therefore, a fundamental understanding of DOM-CNT interactions is  essential.

In this project we focused on configurational changes and fractionation of natural DOM upon adsorption to CNTs; to the DOM adsorption and desorption kinetics; and to the effects of varying solution conditions on DOM uptake by CNTs. Moreoever, we investigated the effects of DOM on adsorption of organic pollutants by CNTs under different solution conditions and stages of introduction. Our findings contributed to the understanding of the complex DOM-CNT-organic pollutant system.  

Related publications:

• Engel M. and Chefetz B. (2019). The missing link between carbon nanotubes, dissolved organic matter and organic pollutants. Advances in Colloid and Interface Science 271:1-8. https://doi.org/10.1016/j.cis.2019.101993 

• Engel M. and Chefetz B. (2016). Removal of triazine-based pollutants from water by carbon nanotubes: Impact of dissolved organic matter (DOM) and solution chemistry. Water Research 106:146-154.  https://doi.org/10.1016/j.watres.2016.09.051 

• Engel M. and Chefetz B. (2016). Adsorption and desorption of dissolved organic matter by carbon nanotubes: Effects of solution chemistry. Environmental Pollution 213:90-98.  https://doi.org/10.1016/j.envpol.2016.02.009 

• Engel M. and Chefetz B. (2015). Adsorptive fractionation of dissolved organic matter (DOM) by carbon nanotubes. Environmental Pollution 197:287–294.  https://doi.org/10.1016/j.envpol.2014.11.020