The central focus of Dr. Xu’s research program is to understand the various ways that nature detoxifies contaminants and use this knowledge to design engineered systems to better retain and/or degrade contaminants. Through our group's research activities, we have advanced the understanding of the adsorptive properties and reactivity of pyrogenic carbonaceous matter (PCM), which includes black carbon (i.e., fossil fuel soot and chars), biochar, and activated carbon. Contrary to the conventional wisdom, recent studies by my group and others have suggested that PCM is reactive and can promote chemical and microbial synergies, affecting global biogeochemical processes of redox-active elements and mitigating climate change. However, due to PCM’s inherent heterogeneity, it is difficult to predict the degree of reactions occurring, limiting its potential engineering applications.

We have engaged in two areas of endeavors to address this very issue (Figure 1). Firstly, we pioneered in employing a tunable PCM-like polymer network that resembles PCM’s key attributes to delineate the contribution of individual properties (e.g., functional groups, porosity) to the reactivity of PCM. By selecting different monomers, we can control the functionality of PCM-like polymers via a bottom-up synthesis. The porosity of PCM-polymers is controlled by rigid node-strut topology, while carbonization confers the conductivity of PCM. This approach is well-poised to elucidate the currently poorly understood mechanisms affecting PCM reactivity in abiotic and biological systems (see projects 1 & 2). Leveraging the knowledge gained from these interfacial processes, we can tailor PCM as a soil amendment for simultaneous adsorption and destruction of contaminants of concern (e.g., insensitive high explosives, per- and polyfluoroalkyl substances; see project 3 & 4). PCM can also be engineered and incorporated into reactive membranes to promote disinfection by-product removal in drinking water (see project 5). Details of these projects are provided below:

Figure 1. Xu's research program at Villanova.

Project 1 (NSF CAREER; midway): Harnessing the synergy between naturally existing PCM and sulfide for detoxifying halogenated contaminants.

Many halogenated pollutants are toxic and enter the environment as pesticides, surfactants, and industrial chemicals. These toxic pollutants are often found in sediment where PCM and sulfide naturally co-exist. My group has demonstrated that PCM and sulfide together promote the abiotic degradation of several halogenated pollutants, releasing lower toxicity products. This dehalogenation reaction represents a vast untapped resource for detoxifying pollutants as both PCM and sulfides are naturally occurring in environmental sediments. Yet, due to the complexity that arises from the heterogeneity of the PCM system in the environment, the required conditions for dehalogenation are not well understood. My group aims to employ polymer chemistry and surface characterization techniques (e.g., XANES) to understand how the interaction between PCM and sulfide degrades these halogenated pollutants. The goal is to apply these naturally occurring reactions to produce solutions that effectively detoxify pollutants.

Project 2 (NIH R01 Program; new start): Elucidating mechanisms for enhanced microbial activities by PCM using an integrated material science and molecular microbial ecology approach.

A common bioremediation strategy for halogenated pollutants in groundwater and sediments is anaerobic reductive dehalogenation by organohalide-respiring bacteria (OHRB). Although effective, OHRB-driven bioremediation strategies are often incomplete in field applications. Recent research highlights PCM’s potential to promote synergistic interactions among OHRB and the auxiliary microbial community and subsequently improve OHRB-driven bioremediation efficacy. However, the underlying mechanisms of how PCM properties best support microbial network interactions, enhance OHRB performance, and promote contaminant biodegradation remain unknown. This project aims at closing the knowledge gap concerning specific surface effects of PCM on the performance of pollutant-degrading microorganisms, especially OHRB. The central hypothesis is that key PCM properties will shape microbial community structure and drive the expression of metabolic functions associated with reductive dehalogenation processes. Elucidating positive impacts between PCM and OHRB will allow for the development of tailored PCM that foster synergistic microbial network interactions and facilitate more effective and sustainable bioremediation. I am collaborating with Dr. Tim Mattes (University of Iowa) on this project through the NIH support. 

Project 3 (DoD; midway): Optimizing carbon amendments that simultaneously adsorb and transform insensitive high explosives (IHEs)

High concentrations of IHEs are commonly found in soil at DoD testing and training ranges, posing a significant safety threat to military personnel and surrounding communities. Many IHEs are highly water-soluble and can easily migrate from the soil and thereby contaminate groundwater. In this project, we tailor PCM to effectively sequester IHEs and hydroxide ions on PCM surfaces, promoting a reaction that leads to IHE degradation. For IHEs that are not prone to hydrolysis, PCM is modified to better retain IHEs. Because coupled interfacial processes are likely to be involved, computational modeling is used to aid the design of experiments and interpret results. I am the lead Principal Investigator on this DoD-funded project. I have assembled and managed a team of collaborators, including Dr. Joseph Pignatello (Connecticut Agricultural Experiment Station), Dr. Paul Tratnyek (Oregon Health & Science University), Dr. Eric Bylaska (Pacific Northwest National Laboratory), and Dr. Samuel Beal (U.S. Army Corps of Engineers). I plan to transition this project to the next stage through field demonstration.

Project 4 (EPA; new start): Enhancing the anion-exchange capacity of biochar for per- and polyfluoroalkyl (PFAS) stabilization in contaminated soils.

Per- and polyfluoroalkyl substances (PFAS) are a suite of ionizable synthetic organofluorine chemicals that have been frequently detected in various environmental compartments, posing global threats to public health. One underexplored source of PFAS in the environment stems from the land-application of biosolids generated at municipal wastewater treatment plants. No suitable remedial technology is currently in place due to the pervasiveness nature of this issue. In this project, we propose to tailor biochar from two waste sources to promote their anion-exchange capacities (AEC) for PFAS sequestration during the land application of biosolids: (i) coagulant-rich water treatment residuals and (ii) agriculture waste. Subsequently, these biochar materials will be incorporated into municipal biosolids to understand their efficacy in sequestering PFAS in agricultural soils. This project aims to create an educational ecosystem, where students will work collaboratively across disciplines (engineering, science, and business) and interact with advisory committee members from the academic, government, industry, and agricultural sectors to design a strategy that is economical, scalable, and feasible for PFAS remediation and ultimately protect public health.

Project 5 (EPA; completed): Develop efficient yet cost-effective strategies to mitigate the formation of disinfection byproducts (DBPs) during water treatment.

Recent studies suggest that a suite of toxic chemicals are produced during water disinfection processes. These chemicals, collectively referred to as disinfection byproducts (DBPs), pose human health concerns due to their potential link with increased bladder cancer rates and negative impact on human reproduction and development. My research group has been focusing on developing strategies to mitigate the formation of DBPs. Specifically, we have demonstrated that the formation of DBPs, such as N-nitrosodimethylamine (NDMA), can be effectively reduced by chemically altering their precursors in drinking water. Moreover, we have shown that the formed DBPs, namely NDMA and haloacetic acids, can be effectively removed by a PCM modified reactive electrochemical membrane system with a highly efficient single-pass flow-through configuration. This technology could be beneficial to small drinking water systems that require low-cost and easily implementable technologies. The work conducted here is in collaboration with Dr. Brian Chaplin in Chemical Engineering at the University of Illinois of Chicago through the EPA Drinking Water Research Center’s support. We now have a pending proposal for the Water Research Foundation to further develop this approach into a point of entry technology for water distribution systems.