Seth B Darling/Argonne National Laboratory
Researchers at Argonne National Laboratory (ANL), Lemont, Illinois, have achieved ultra-high-resolution solute separation using custom-fabricated, defect-free silicon nitride isoporous membranes.
Commercial membranes mostly have pore sizes that aren’t consistent, and so are known as polydisperse. With such membranes, it is nearly impossible to do a sharp separation of materials as different sizes of solutes can fit through different pores.
"Essentially all commercial membranes, all membranes that are actually used for anything, have a wide range of pore sizes — little pores, medium pores and big pores," said Seth Darling, head of the Advanced Materials for Energy Water Systems (AMEWS) Center at ANL.
Separation by membranes where all the pores are the same size, in ANL’s case 17.8 ± 1.3 nm, traditionally is described by hindered transport theory. Here, convective and diffusive hindrance prevents solutes smaller than the pore from passing through.
The ANL researchers have developed a 12-step fabrication process which starts with a silicon wafer coated on both sides with 100 nm of silicon nitride. They combine the finished membrane with a recirculating feed to increase the opportunity for interactions between solutes and the isoporous array.
Results described in a recent issue of Nature Water (DOI: 10.1038/s44221-024-00252-3) show the membrane completely rejects solutes with greater size than the pore size while effectively allowing smaller solutes to permeate through it.
With this traditional hurdle overcome and a steeper size-selective rejection curve achieved, the ANL researchers believe there’s new promise for unprecedented membrane separations through judicious process design and extremely tight pore-size distributions (Figure 1).
“Improving pore size distribution in commercial membranes is a challenge to do at scale and at low cost, but there are potential technologies in research labs that could deliver this. Process design to exploit the concepts we have presented is even more early-stage, and we are hoping this study will inspire folks to pick up that torch and take the ideas further,” explained Darling.
"If these fundamental studies can be successfully transferred to industrial membrane separations, it could have tremendous impact across numerous sectors of our economy," he added.
For now, Darling and his co-workers are following several research directions spawned by the study. One involves new approaches to encourage more solute/membrane interactions to further improve the selectivity of the separation. Another is a systematic investigation of the role that membrane defects play in these processes.
