Research

In 2050, 6.4 billion people will suffer from water stress, and renewable energy needs to meet 49% of total energy production in 2030 according to the sustainable development scenario. Crisis in the water-energy nexus is one of the main challenges in the 21st century.

My research is focused on advancing separation technologies for sustainable production of water and energy, and resource recovery. Past and ongoing projects are i) designing and evaluating selective separations for lithium-ion battery recycling, ii) advancing transport theory for membranes used in water-energy applications, iii) treatment and management of high-salinity brines, and iv) energy conversion of low-temperature heat resources. 

Current demand of lithium is expected to exceed global lithium reserves by 2040. While a vast amount of lithium exists in used lithium-ion batteries (LIBs), brines, and wastewater, current technologies for mining and recycling lithium from the aqueous phase suffer from high energy intensities and inadequate selectivity of lithium from co-existing ions. Advancing solute-specific selectivities has also become the new frontier of separation sciences. Collaborating with an company that recycles used LIBs, Redwood Materials, I lead a life cycle comparison of industrial-scale LIB recycling and mining supply chains (manuscript under review in Nature Communications), and I am working on developing selective membrane materials that enable efficient separation of lithium from coexisting aqueous ions.

Lithium recycling is one example of resource recovery using precision separations. Improving solute-specific selectivites for separation processes is key in resource recovery, and my review papers in ESE and I&EC Research provide considerations of various separation technologies and materials.

Membrane-based reverse osmosis (RO) technology is the most energy-efficient method today for desalination of seawater and brackishwater, and the thin-film composite polyamide (TFC-PA) membranes are widely appied in RO. Understaning of the first principles governing the permeability-selectivity peformance of the TFC-PA membrane is still imcomplete, which significantly frustrates the informed design of next-generation desalination membranes. My work advances a cohesive framework to better understand the intrinsic transport mechanism of RO, focusing particularly on the tradeoff phenomenon between the water permeability and solute selectivity (see the left figure). More details are in my papers published in Water Research and ACS ES&T Engineering.

Birnes generated from inland desalination and oil and gas industries have exceedingly high salinities (2 × that in seawater), and treatment of these hypersaline brines has rapidly become an important water-environment global challenge. Conventional evaporative methods are intrinsically energy-intensive, whereas the energy-efficient reverse osmosis today is unsuitable for dealing with high salinities. I developed a novel cascading osmotically mediated reverse osmosis (COMRO) technology for high-salinity desalination (see the left upper figure), which can achieve sustantial energy savings while using only moderate hydraulic pressures. Read more in my publications in ES&T and Desalination.

In addtion to membrane, I also participated in developing a temperature swing solvent extraction (TSSE) technology achieve zero liquid discharge treatment of ultrahigh-salinity brines (in ES&T), and a critical review (in Desalination) providing high-level comparisons of existing and emerging technologies for treatment and management of hypersaline brines.

A vast amount of low-temperature heat (< ≈100 ºC) exists in industrial waste heats and geothermal energy housed in the Earth's crust. Current technologies cannot directly utilize low-temperature heat, and they often requires expensive exotic materials or suffer from poor energy efficiencies. The emerging vapor pressure-driven osmosis (VPDO) technology enables direct conversion of low-temperature heat to electricity/mechanical work (see the left schematic), and my work provides fundamental understanding on the mass and heat transfer processes in VPDO. Read more in my JMS paper.