Funding 

My research activities at UCR have been supported by more than 30 grants and contracts from federal, state, foundational, industrial and international agencies totaling $11.5 million, of which $5.3 million are for my research lab. We thank the support from different funding agencies and our partners for the generous support. Our current and ongoing research thrusts are described below:

Advanced oxidation processes for water reuse and treatment

Water shortage has become a global crisis. Restoration of contaminated source water and reuse of treated wastewater effluent are the two promising solutions to address the water crisis. Advanced oxidative treatment for groundwater remediation and wastewater treatment has gained increasing attentions in recent years due to its capacity to remove contaminants of emerging concerns such as flame retardants and personal care products. However, little is known about the nature of oxidation by-products and their toxicological effects. Furthermore, traditional oxidants are usually non-selective and have low treatment efficiencies. The applications of photochemistry principles to our benefits in water purification and wastewater reclamation is an important path to water sustainability. My research team is among the first to discover the unique chloramine photochemistry to generate reactive radical species that is beneficial to potable reuse treatment (Environ. Sci. Technol. Lett. 2017, 4, 26-30). Combining bench-scale experiments with kinetic modeling and computational calculations, we investigated the photolysis of chloramines and its applications to the removal of neutral and small-molecule-size contaminants that are present in recycled water (Environ. Sci. Water Res. Technol. 2017, 3, 128-138 and Environ. Sci. Technol. 2018, 52, 11720-11727). To further commercialize the chloramine-based water reuse technology, we collaborated with Orange County Water District, which operates the world’s largest potable reuse facility, to investigate the chloramine at a pilot-scale and developed guidelines to optimize chloramine-based advanced oxidation process (Environ. Sci. Technol. 2019, 53, 13323-13331). We revealed the balance between photon energy input and the control of residual hydrogen peroxide was critical to its full-scale implementation.More recently, we investigated chloramine redox chemistry and hydrolysis in the absence of UV irradiation. We discovered a pathway to promote the activation of chloramines into reactive hydroxyl radicals via an intra-conversion to peroxynitrite (J. Hazard. Mater. Lett. 2022, 3, 100054 and J. Hazard. Mater. 2022, 440, 129760). Out studies revealed the unique pathway by which chloramines and ammonia-chlorine mixture can be tuned into a strong oxidative system for the degradation of trace organic contaminants in recycled water. Our ongoing research on oxidative treatment also found that sulfate radical is a promising oxidant. My group investigated the application of sulfate radical in water reuse systems and revealed sulfate radical and chloramine photolysis can function synergistically to enhance the oxidative capacity (Environ. Sci. Technol. 2018, 52, 6417-6425). Production of sulfate radical is also more efficient that hydroxyl radical when generated by persulfate reacting with iron oxides or clay minerals (Environ. Sci. Technol. 2017, 51, 3948-3959). We applied advanced computational tools to understand the electrophilic attack of sulfate radical with aromatic ring structures (Environ. Sci. Processes Impacts. 2017, 19, 395-404). 
 

Redox and Electrochemistry of drinking water distribution systems 

Water distribution infrastructure has historically been considered to be a simple means of transmitting water to the public, but it is a complex system with many interactions among reactive components, including pipe materials, accumulated chemicals, and residual disinfectants. Over the next decades, there will be tremendous economic and societal pressure to manage distribution infrastructure effectively. My research was among the first to elucidate the mechanisms of lead release processes associated with the reactions of lead solids with disinfectants in drinking water. These studies combine principles of surface chemistry and electrochemistry to fundamentally understand metal(loid)s mobility and control strategies in drinking water distribution systems. Our recent work elucidated the kinetics of reductive conversion of lead solid species in different oxidation states (Chem. Commun. 2017, 53, 8695-8698 and Environ. Sci. Water Res. Technol. 2021, 7, 357-366), and strategies to maintain the chemical stability of lead corrosion products in the distribution system (Environ. Sci. Water Res. Technol. 2019, 5, 1262-1269). Outcome of these findings will help prevent another Flint, Michigan crisis. My group also studied the accumulated elements (e.g., chromium and vanadium) in water distribution systems, because of emerging concerns on their abundance, toxicity and new regulatory perspectives. We concluded that water chemistry during distribution can be optimized to immobilize contaminants and improve drinking water quality (Environ. Sci. Water Res. Technol. 2016, 2, 906-914). In particular, the release of chromium is closely associated with the redox conditions in water distribution systems, especially reactions mediated by residual disinfectants and the level of halides in source waters. We discovered the catalytic pathway via which trivalent chromium is converted to hexavalent chromium by chlorine as a residual disinfectant (Environ. Sci. Technol. 2016, 50, 701-710 and Environ. Sci. Technol. 2018, 52, 7663-7670). In addition, our recent work using corrosion scales from aging pipelines revelated that the existence of zerovalent chromium is surprisingly important to control chromium release (Environ. Sci. Technol. 2020, 54, 13036-13045). Novel findings from these research efforts received national media coverage, including C&EN. Meanwhile, we developed fundamental electrochemical techniques to investigate the redox chemistry of vanadium , another important element in water distribution system (Environ. Sci. Technol. 2017, 51, 11643-11651). Findings from these studies are not only of interest to the field of environmental chemistry, but also broadly important to the development of effective water infrastructure management strategies.  Based on the newly discovered redox chemistry, my group further developed novel hexavalent chromium removal technologies using stannous chloride (AWWA Water Sci. 2019, e1136) and photocatalyst titanium oxide (Water Res. 2017, 108, 383-390). These new approaches generate non-toxic trivalent chromium as the end product with fast reaction kinetics. We are the first group to develop new chromium removal technologies based on the unique reductive process involving Cr and electrons (Front. Environ. Sci. Eng. 2020, 14, 88).
 

Desalination concentrate treatment

My group develops new brine treatment approaches for inland and brackish water desalination, a topic critical to global water security. We conducted a first of its kind study to evaluate the baseline conditions of inland desalination brine management in California, with emphases on the unique brine chemistry in inland areas (ACS ES&T Engr. 2022, 2, 456-464). We further discovered the importance of phosphonate-based antiscalant to mineral recovery in the desalination brine (Environ. Sci. Water Res. Technol. 2019, 5, 1285-1294). The investigation on antiscalant chemistry led to the development of a sulfate radical-based approach to remove inhibitive effects of antiscalant and promote additional mineral/water recovery from the brine (Water Res. 2019, 159, 30-37). Several manuscripts on this topic are also under review. In addition to mineral and water recovery from the brine, my group developed different photochemical technologies to degrade contaminants of concerns from the brine, including nitrate, residual recalcitrant organic contaminants, and metals (Chem. Engr. J. 2020, 396, 125136). We also evaluated the oxidation of dissolved organic matter in municipal water reuse concentrate and developed a Parallel-Factor (PARAFAC) analysis approach to predict the degradation of pharmaceuticals and personal care products from the brine (Water Res. 2021, 204, 117585). Aligned with these research topics, my group has recently pioneered a photochemical technology to treat poly- and per-fluorinated chemicals (PFAS) from the brine - a group of emerging and persistent organic contaminants that are widely found in water. My group is utilizing a new type of deep UV light source to create an extremely reactive water ionization system and destruct PFAS. 

Water for agriculture and public health implications

One promising strategy to sustain agricultural production and urban landscaping in the face of water scarcity is to increase the reuse of treated municipal wastewater. My group has been working on water reuse technologies for agriculture irrigation and food safety. Supported by multiple current grants from US Department of Agriculture (USDA), my group is developing water reuse technologies that recycle municipal wastewater effluent and use it for edible crop irrigation. We also demonstrated a new disinfection process to inactivate food-borne pathogens (e.g., E. coli. and coliphage) during food production using sulfate radical, which exhibited a much faster initiation of pathogen kill-off than hydroxyl radical (Environ. Sci. Technol. Lett. 2017, 4, 154-160). The progress in disinfection techniques is beneficial to water treatment during the COVID pandemic (Environ. Sci. Water Res. Technol. 2020, 6, 1213-1216). Aligned with this research direction, we evaluated the toxicity of treated water using different oxidative treatment, and established a bioassay approach to evaluate the efficiency of the water treatment based on human toxicity of the byproducts (Environ. Sci. Water Res. Technol. 2018, 4, 1213-1218). We also investigated nutrient management in agricultural drainage water, and developed a carbon-centered radical approach to removal nitrate from the drainage water (Environ. Sci. Technol. 2019, 53, 316-324).