Removal of the Ammonium Cation From Waste Water Using Ion Selective Battery Electrodes.

[NNadir]

The paper I will discuss in this post is this one: Ammonium Removal from Domestic Wastewater Using Selective Battery Electrodes (Logan et al Environ. Sci. Technol. Lett., 2018, 5 (9), pp 578–583

One of the things that really freaks me out as I contemplate what we have done in my generation to screw over all future generations is the situation with respect to fixed nitrogen.

Specifically in my rants and rages, I am concerned with the accumulation of growing levels nitrous oxide - "laughing gas" that isn't funny - in the atmosphere where it acts as a greenhouse gas but, of more concern, as an ozone depleting agent. Nitrous oxide, N2O, is of course, a part of the normal nitrogen cycle, which has operated for billions of years, but the invention of Haber-Bosch process in the early 20th century severely impacted the established balance while simultaneously preventing world wide starvation and making it possible for people without access to salt peter mines to make gun powder for their much loved wars. (Germany would have lost World War I in 1915 were it not for the Haber Process.)

I suspect that nitrous oxide will be the major ozone depletion agent by the end of the 21st century should there be an end to the 21st century. It is not easy to ban - actually it's impossible to ban - because the world food supply does, and always will, depend on access to nitrate and ammonia fertilizers.

One of the other effects of excess nitrogen is to destroy rivers and lakes and - as we are seeing in Florida now in part because of "Red Tide Rick's" traditional Republican hatred of anything involving saving the environment - large stretches of ocean with eutrophication, a burst of growth of microorganisms which die and consume all the oxygen in the water as they decompose, thus killing any aquatic creature that needs, um, oxygen.

It is therefore with great interest that I read the paper cited at the outset of this post.

The introduction covers the problem pretty well and reviews the proposed approaches and problems with those approaches to dealing with this very serious problem:

The anthropogenic release of nutrients into the environment can drive eutrophication, threatening the health of aquatic ecosystems.(1) Removal of ammonium from wastewater before its discharge to the environment is particularly needed because it is a major component of the nitrogen species in wastewaters, and nitrogen is the critical limiting nutrient for eutrophication of many receiving waters.(2) While biological processes are the most common approach for ammonium removal,(3) other technologies are being developed with the purpose of ammonium separation and recovery from wastewater, not only to avoid its release but also to enable its reuse. For example, several absorbents can be used for ammonium removal by ion exchange, but these approaches require salty brines to regenerate absorbents for further removal of ammonium in subsequent cycles.(4?6) Although the use of an ion exchange membrane can avoid the need for regeneration, the selectivity of the membrane for ammonium versus other cations is often low or unknown.(7?14) Ammonium can be removed from water through its conversion into volatile ammonia by raising the solution pH using chemicals or electrochemical systems and, then, its removal using stripping towers or membrane contactors.(15?21) These approaches can capture ammonium into valuable salts, such as (NH4)2SO4, but raising and lowering solution pH can be expensive. Bioelectrochemical systems have also been proposed for ammonium recovery from wastewaters based on using the electrical power produced by microorganisms degrading organic matter in the wastewater; however, rates of ammonium separation are limited by the low current densities the bacteria produce, and removal is not selective for ammonium.(22?25)

Their approach arises from a consideration of a type of electrode that has been developed specifically for batteries, a "Prussian Blue" electrode comprised of copperhexacyanoferrate, which they noted, involves the capture and release of different cations utilized in electrolytes at different voltages. They make one of these electrodes under fairly mild conditions - it is supported on a carbon cloth - and then run some tests, beginning with some laboratory solutions of common salts found in wastewater in the presence of ammonium salts, and then with "synthetic wastewater" and finally with real wastewater.

Here's a cartoon describing their system along with some bar graphs showing the selectivity of this system toward the ammonium ion as compared to sodium:



The caption:

Figure 1. (a) Schematic of the system for ammonium removal using two copper hexacyanoferrate (CuHCF) battery electrodes in two channels divided by an anion exchange membrane (AEM). (b) Conductivity profiles of treated and concentrated waters (synthetic wastewater containing 20 mM NaCl and 5 mM NH4Cl) recirculated in each channel at the flow rate of 4 mL/min at constant voltages of 0.1, 0.2, and 0.3 V. (c) Concentration and (d) percent removed and selectivity of Na+ and NH4+ as a function of the cell voltage. Error bars show the range from duplicate experiments.

This graphic shows other results for the removal of cations from waste water.



The caption:

Figure 3. (a) The concentration of removed and effluent cations measured after applying a constant voltage of 0.2 V for 200 s. Error bars show the range from duplicate experiments. (b) Cyclic voltammetry profiles (scan rate = 1 mV/s) of electrolytes containing cations present in wastewater.

They offer their conclusions:

Not only is the use of CuHCF electrodes highly selective for ammonium compared to other systems (ion-selective membranes or nonselective electrodes),(8,9,21,37) but also it required less energy than electrochemical systems combined with ammonia stripping.(18,21) The energy needed using actual domestic wastewater at a constant voltage of 0.2 V was 1.5 kWh/kg-N with 85% nitrogen recovery, which was approximately 7% of the energy needed in a flow-electrode capacitive deionization system using dilute wastewater (21.7 kWh/kg-N, 55.1% nitrogen recovery)(21) and 17% of that needed in electrodialysis using real urine (8.5 kWh/kg-N, 92.7% nitrogen recovery).(18) The low energy consumption using the CuHCF electrodes was due in part to the use of cell voltage (<0.3 V) that was much lower than those required to increase pH by electrochemical reactions (>1.2 V)...

...and then - as honest people do - some caveats and limitations...

Although we demonstrated that the CuHCF electrodes can be used to selectively remove ammonium from wastewater, there are remaining challenges to improve performance. The selectivity toward NH4+ decreased against all cations compared to only Na+, which was due to the presence of Ca2+ and K+ in the domestic wastewater. While the effect of these competing ions on ammonium removal was minimal because of low concentrations (<1 mM) compared to that of ammonium (>3 mM), the use of CuHCF electrodes will be best suited for wastewaters with relatively low Ca2+ and K+ contents. We also observed that treated wastewater had a light yellowish color after completing a cycle, which was likely due to dissolution of CuHCF under mildly basic condition of domestic wastewater.(47) This dissolution was minimal if the pH was reduced below 7, indicating that pH adjustments could be needed for practical applications of alkaline wastewaters. In addition, a concentrated wastewater generated in the other channel will require additional separation steps to further concentrate and remove ammonium...

With... ...technological advancements, our approach could represent an effective method for the selective removal of ammonium from various waters, including domestic wastewater, with low energy consumption.

Nice, I think.

I trust that you are having a pleasant Sunday afternoon.

[Midnightwalk]

1. Thanks

Interesting. Chemistry never has been my thing but I have some questions.


What is the importance of the selectivity over Na ions? Is it to make reclamation of ammonium easier? Or is there a reason to not want to remove the sodium?

Do the last two graphs say you could potentially tune this approach and voltages to select for different cations?

Thanks again.

[NNadir]

2. Thanks for your questions.

To answer the first question, the answer is potentially multifaceted. The authors note in the introduction that there is value in recovering ammonium ions, since they are an important raw material, particularly for fertilizer. Although it is not explicitly stated, water containing only ammonium but not sodium, would be very useful as a fertilizer material. Waste water recovered for irrigation is often problematic owing to the long term accumulation of sodium over periods of evaporation, which in high concentrations, kills plants. Another reason is to save energy. Separations always require energy. Sodium is not problematic when waste water is added to water, ammonium is. It takes more energy to separate ammonium and sodium than ammonium alone. A third possibility is that under the right conditions, one could imagine a similar system being used to oxidize ammonium to nitrogen gas, thus removing its environmental risks. Sodium ions might saturate the system, leaving less capacity for ammonia.

I do not know a definitive answer from a known process where such a consideration applies, but these answers all seem reasonable from my experience. It may be that the practice would depend on this process going commercial, which is a considerable way off I'd guess.

The last two graphs refer to the removal of ammonium in the presence of other ions. Small amounts of the other ions are indeed removed, but the important point is the enrichment of ammonium from the initial ratio in the waste water to a much lower level.

There are in fact many kinds of ion selective membranes and possibly even electrodes. These are often used in analytical chemistry. Many people are familiar with pH which involves a proton selective membrane for detection; there are also pF electrodes for the detection of fluoride, and pCl electrodes to detect chloride, etc, although they are far more esoteric.

The difference with this system opposed to alternatives is the claim that it has lower energy requirements, which is, of course, a good thing.

Thanks for your interest and for asking.

[Midnightwalk]

3. Interesting

Thanks for the detailed response. I hadn't even thought of having to add energy let alone that only separating the ions you want being more efficient. Thanks for some insight and some good things to read more on and think about.