Oil–Water Separation using Synthetic Trees

Abstract

Existing oil−water filtration techniques require gravity or a pump as the driving force for separation. Here, we demonstrate transpiration-powered oil−water filtration using a synthetic tree, which operates pumplessly and against gravity. From top to bottom, our synthetic tree was composed of: a nanoporous "leaf" to generate suction via evaporation, a vertical array of glass tubes serving as the tree's xylem conduits, and filters attached to the tube inlets to act as the oil-excluding roots. When placing the tree in an oil emulsion bath, filtrate samples were measured to be 97−98% pure water using gravimetry and refractometry. The spontaneous oil−water separation offered by synthetic trees could be useful for applications such as oil spill cleanup, wastewater purification, and oil extraction.

Oil Sequestration in Synthetic Trees with Nanoporous Leaves and Oleophilic Roots

“…After each sequestration experiment, 50 μL samples were extracted from three randomly selected tubes and analyzed by gravimetry and refractometry. The volume fraction of oil in the permeate (η o,p ) was calculated using a linear interpolation of gravimetric values: 20…”

“…14−18 An alternative strategy is to selectively evaporate the water phase, leaving concentrated oil in the reservoir. 19,20 Synthetic trees, which are designed to mimic the transpiration cycle of natural plants, 21−27 can offer a renewable approach to pumping. 28−31 For both natural and synthetic trees, as water evaporates from a saturated leaf into the subsaturated ambient environment, a negative water potential is generated that enables spontaneous liquid ascent across the tree.…”

“…A theoretical analysis revealed that this concept could be extended to the portable desalination of 1 L/day of seawater in a water bottle by using solar heat to boost the transpiration rate . Very recently, we demonstrated the selective uptake of water from an oil–water emulsion bath by bonding oleophobic membranes to the entrances of the tree’s vertical tube array . These collective works have demonstrated the use of synthetic trees for preferential water uptake (excluding salt or oil); to date, the extraction of oil from an oil–water mixture has not been demonstrated with synthetic trees.…”

“…Differentiation from Previous Synthetic Tree Studies. While our recent work also used synthetic trees for oil−water separation, 20 the present use of sequestering oil (rather than excluding oil) has important fundamental and practical distinctions. Our current setup profoundly alters the transpiration behavior.…”

Highly Permeable Sulfonated Polydopamine Integrated MXene Membranes for Efficient Surfactant-Stabilized Oil-in-Water Separation

“…Conventional approaches can be expensive, difficult to handle, and might even lead to higher levels of pollution . Therefore, new methods and technologies are required to deal with oil/water emulsions. – …”