Desalination of brackish water using activated carbon - the solution to the water crisis?

How a method that was developed in the seventies and shelved due to various technical reasons, was pulled out of the drawer and after forty years could possibly alleviate the current water crisis *

We all know activated carbon from household water filters (Brita filter, Tammy 4, etc.). Another application of them is in the gas masks that were distributed in the Gulf War and are now being distributed again. It turns out that the activated carbon can be used not only to filter gases or organic substances but also salts, and this if in addition to the large surface area the good electrical conductivity property of the coal is also used.

Dr. Yaniv Bohdana, recently completed his doctorate in the laboratory of Prof. Doron Orbach in the chemistry department at Bar Ilan University and under the direct guidance of Dr. Avraham Sofer. Hella first developed the method in the seventies as part of his doctoral thesis at the Technion. This work was abandoned due to various technical reasons. About six years ago, in light of the ongoing water crisis, it was decided to pull it out of the drawer, and it was Bohdana who continued the research.

Dr. Sofer focused on the theoretical side and he even guided several students in their doctoral theses - one of them is Prof. Yoram Oren, currently head of the Department of Water and Soil at the Desert Research Institute at Ben Gurion University. The subject was studied again in the nineties in the laboratory of Prof. Joseph Farmer from the Lawrence Livermore National Laboratories in the USA.

"One of the reasons for neglecting the field was technological. The coal they used was of the Carbon Aerogel type. It is a commercial product whose surface area can reach hundreds and thousands of square meters per gram, but its synthesis is expensive. In our research, we decided to go for another type of carbon - Carbon Cloth - which is produced from synthetic fabrics. Because of this, its mechanical strength is high and its surface area ranges from one thousand to two thousand square meters per gram of material, and of course it is very cheap to produce. Unlike the aerogel, which is built from solid blocks that the water flows next to, the carbon cloth allows water to pass through it. This feature increases by orders of magnitude the kinetics of the process in the adsorption and release of the salt because the flowing solution comes in maximum contact with the surface of the coal surface with minimum reliance on comparing concentrations in diffusion between the flowing solution and the stationary solution on the surface of the coal.

"Even before we performed a single experiment, we spent about two years building the system, which will allow us to conduct experiments automatically to test both the stability and the long-term efficiency of the process, something that experiments of one or two days do not allow," notes Dr. Bohdana.

As mentioned, salts do not naturally adsorb to coal as organic substances do, for example, and therefore it is necessary to invest electrical energy.

"We put salt water into the chamber (known as the CDI chamber). A voltage is applied between two electrodes - one positive and one negative. The positive ions of the salt are attracted to the negative electrode and the negative ions are attracted to the positive electrode. The salt remains attached to the electrodes and the desalinated water exits the cell. In the next step, the cell is again filled with salty water, this time shorting the electrodes and all the salt caught on them is released into the water and makes it even more salty - this is the waste water of the process. And so it is possible to route desalinated water to one pipe and concentrated water to another pipe, and of course to transfer the desalinated water for domestic or agricultural use," Bohdana explains. "We tested various aspects of the process, especially the stability of the process over time. If we want to make the process commercial, the electrodes need to last thousands of cycles without having to replace them. The problem is that coal has the property that it oxidizes in the presence of water under an electric charge. This problem was investigated in the current project and a number of effective techniques were proposed in order to deal with it where the combination of the techniques constitutes the solution to the stability problem. In addition, we examined the issue of energy and electrical efficiency, because in practice, for every ten electrons that pass between the electrodes, 5-7 molecules of salt are adsorbed. We want to increase efficiency and maintain this increase over time. The subject of efficiency has been studied and will be studied a lot because the factors that affect efficiency are many and varied and many techniques have been proposed in order to improve efficiency in a good way.

A third area that was tested was the replacement of the conventional flow regime of constant flow in a uniform direction to operation in opposite flow directions with the voltage applied in only one direction. This operating regime leads to the creation of a concentration amplitude (the difference in values ​​from peak to peak) along the cell, similar to the reality in a distillation column where in each layer there is a different ratio between the various components in the column. The same is true in the CDI chamber where each layer will have water of a different level of salinity, and thus it is possible to remove only the waste from one side or only the desalinated water from the other side. This engineering improvement, predicted mainly theoretically by Prof. Oren, will save a lot of energy and allow great flexibility in the cell's operating capabilities for a large range of feed concentrations and different product concentrations. In other words - it will be possible to desalinize water at any level of salinity, at any level of desalination that we want. Further studies will be carried out in order to realize this method, which from an engineering point of view is the most effective.

In conclusion, the research is in the advanced stages of a laboratory prototype and will move in the coming years to commercial realization and a combination of forces with parties in the international water industry.

* The article was published in 2011 on the joint news site of Bar-Ilan University and Haaretz newspaper

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