Clean fresh water is a scarce resource. Areas of the world suffering from drought have to filter the salt out of seawater to make it drinkable. In other areas, the water may instead have a high content of toxic compounds, such as arsenic.
If you think about a water filter as a kind of strainer with tiny holes through it, you would assume that since it does a pretty good job of filtering out the small ions of normal table salt, sodium, and chloride, from seawater it would work even better for the larger arsenic compounds. This is however not the case when it comes to desalination – the technology for producing fresh water from seawater; quite the opposite actually. While sodium and chloride are removed effectively, other, much larger contaminants pass through the filtration materials that are typically used. That indicates there must be another mechanism at work here.
Reverse osmosis or desalination membranes, as the filters are called, have been used for a long time and are mass-produced from common polymers. The surface properties of the polymer dictate to a large degree which contaminant can diffuse through and which one is rejected, either through chemical or electrostatic interactions.
Researchers from Lawrence Berkeley National Laboratory in Berkeley, California and the Fritz-Haber-Institute of the Max-Planck-Society in Berlin recently visited the HIPPIE beamline to better understand the inner workings of desalination membranes. With the methods available at the beamline, it is possible to follow what happens to different types of salts as they are diffusing into the membrane. X-ray photoelectron spectroscopy can show the chemical state of atoms or molecules in a sample. Small shifts in the binding energy of electrons in an atom directly indicate changes in the chemical environment. The beamline HIPPIE is equipped to deliver high electron intensities even at elevated pressures, allowing to probe the sample surface in contact with water using Ambient Pressure X-ray Photoelectron Spectroscopy. The team from Berkeley is combining the strengths of the beamline, which is the chemical analysis of the material at gas pressures close to the intended operating conditions, with a clever method for preparing model membrane samples for the experiment.
Before the experiment, the researchers are growing model membranes in a controlled layer-by-layer fashion on top of an ultra-flat substrate surface. This method was first developed by a group from the National Institute for Standards and Technology , which demonstrated that the membranes could be grown on silicon wafers or gold substrates. In the industrial applications the membranes are more disordered and would be much more challenging to study, so these model membranes are critical for reducing the complexity in the experiment and for focusing on a few key properties that are being investigated here.
The Berkeley team is specifically growing the membranes on multilayer X-ray mirrors to be able to use an experimental method called standing wave photoelectron spectroscopy. In these experiments, the incoming and reflected X-rays interfere to form a standing wave on the surface. By modulating the electrical field at different depths of the membrane one can then probe the chemical state of the membrane and ions at a particular depth.
During this beamtime, the research team managed to study the interactions of the membrane with sodium iodide, and now it is time to go back home and analyze the collected data as well as performing more experiments at the Advanced Light Source in Berkeley. The team plans to build on the MAX-IV experiments and investigate arsenic-containing solutions and their interaction with desalination membranes next.
This work was supported by the Alexander von Humboldt Foundation through a Feodor Lynen Research Fellowship, as well as by the Director, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences of the US Department of Energy under Contract No. DE-AC02-05CH11231.
 Johnson et al., DOI 10.1002/polb.23002