Zeolites for the Rapid and Selective Uptake of Cesium.

[NNadir]

If you were born after the late 1940's or early 1950's you have always been "contaminated" with the radionuclide cesium-137.

This isotope was released in large quantities during the era of open air nuclear weapons testing, and a clearly detectable amount of it usually leaks out after underground nuclear weapons testing. The isotope is so ubiquitous that it is often utilized as a tracer to understand soil erosion.

I explored this issue of nuclear testing and its relationship to modern soil testing elsewhere: Every Cloud Has A Silver Lining, Even Mushroom Clouds: Cs-137 and Watching the Soil Die.

Of course, this isotope was also released by the nuclear screw up at Chernobyl as well as the failure of the Fukushima reactors in a natural disaster.

As it happens, the amount of radioactivity cesium-137 leaching into the oceans from the Fukushima event are trivial when compared with the natural radioactivity of the ocean, which is largely connected with the huge amounts of naturally occurring (and impossible to avoid) potassium-40 and the polonium-210 which is a product of the decay series. This point was clearly made in the famous "Fukushima Tuna" paper, which was immediately and grotesquely misinterpreted by journalists around the world, causing a fair amount of hysterical gas and coal burning to power computers for people who can't read a scientific paper (or anything else) very well leading them to freak out and announce we're all going to die because of Fukushima.

We didn't.

Here is a link to the famous Fukushima Tuna paper: Pacific bluefin tuna transport Fukushima-derived radionuclides from Japan to California


Here is the text comparing the Fukushima radioactivity with the natural radioactivity that has always been in the ocean and always will be in the ocean, no matter how much we do - and we're doing a lot of it with dangerous fossil fuels - to destroy the ocean:

Inferences about the safety of consuming radioactivity-contaminated seafood can be complicated due to complexities in translating food concentration to actual dose to humans (12), but it is important to put the anthropogenic radioactivity levels in the context of naturally occurring radioactivity. Total radiocesium concentrations of post-Fukushima PBFT were approximately thirty times less than concentrations of naturally occurring 40K in post-Fukushima PBFT and YFT and pre-Fukushima PBFT (Table 1). Furthermore, before the Fukushima release the dose to human consumers of fish from 137Cs was estimated to be 0.5% of that from the α-emitting 210Po (derived from the decay of 238U, naturally occurring, ubiquitous and relatively nonvarying in the oceans and its biota (13); not measured here) in those same fish (12). Thus, even though 2011 PBFT showed a 10-fold increase in radiocesium concentrations, 134Cs and 137Cs would still likely provide low doses of radioactivity relative to naturally occurring radionuclides, particularly 210Po and 40K.

Here is another paper in the same journal by the same authors (with a few added to boot) complaining about the stupidity, fear, and ignorance with which their original paper, which was about the migration of Tuna and not about risk to human health, was handled by the media and the general public:

Evaluation of radiation doses and associated risk from the Fukushima nuclear accident to marine biota and human consumers of seafood (Madigan et al, PNAS, 110, 26, 10670-10675.)

Recent reports describing the presence of radionuclides released from the damaged Fukushima Daiichi nuclear power plant in Pacific biota (1, 2) have aroused worldwide attention and concern. For example, the discovery of 134Cs and 137Cs in Pacific bluefin tuna (Thunnus orientalis; PBFT) that migrated from Japan to California waters (2) was covered by >1,100 newspapers worldwide and numerous internet, television, and radio outlets. Such widespread coverage reflects the public’s concern and general fear of radiation. Concerns are particularly acute if the artificial radionuclides are in human food items such as seafood. Although statements were released by government authorities, and indeed by the authors of these papers, indicating that radionuclide concentrations were well below all national safety food limits, the media and public failed to respond in measure. The mismatch between actual risk and the public’s perception of risk may be in part because these studies reported radionuclide activity concentrations in tissues of marine biota but did not report dose estimates and predicted health risks for the biota or for human consumers of contaminated seafood. We have therefore calculated the radiation doses absorbed by diverse marine biota in which radioactivity was quantified (1, 2) and humans that potentially consume contaminated PBFT

Never underestimate the power of stupidity.

The ocean contains about 530 billion curies of potassium-40, which corresponds to about 2 times 10 to the 22nd power nuclear decays per second, or since, there are 31,556,736 seconds in a sidereal year, 6.2 times 10 to 29th power per year. The specific activity of cesium is 3.12 times 10 to the 12th power bequerels per gram, meaning that to match the natural radioactivity associated with radiopotassium in the ocean, we would need to deliberately and directly dump 6100 tons of cesium-137 directly into the ocean. However, this is more radiocesium, by orders of magnitude, than has ever been produced by all the world's nuclear reactors operating - and all the nuclear weapons detonations - for more than half a century.

In high concentrations, in any case, Cs-137 can and does represent a real risk, not quite the same risk as dangerous fossil fuel waste which kills half of the seven million people who die each year from air pollution, the other half being killed by the combustion products of "renewable" biomass.

Further, it turns out that as a strong gamma radiation emitter, cesium-137 should be regarded as an extremely valuable material for mineralizing the vast amounts of halocarbons that have been dumped into the environment, where they represent a huge risk. These are the fluorocarbons like PFOS, chlorocarbons like the famous CFC's, the extremely toxic PCBs, certain insecticides like DDT, and the awful business about brominated flame retardants like the PBDE's and their related compounds.

Thus it is a pretty bad idea to throw this stuff away, either in the idiotic notion of waste dumps, or by either deliberate or unintentional release. Thus we need to recover this stuff in order to utilize it.

Thus it is with interest I came across this paper for highly selective removal of cesium from dilute solutions:

Highly Selective and Rapid Uptake of Radionuclide Cesium Based on Robust Zeolitic Chalcogenide via Stepwise Ion-Exchange Strategy (Feng et al Chem. Mater. 2016, 28, 8774−8780)

From the introduction:

As an efficient and low-carbon power generation method, nuclear power plays a critical role in meeting the increasing energy needs. However, nuclear wastes and reactor accidents could result in the leak of radionuclides into environments, which is a key reason limiting more widespread use of nuclear energy.1,2 Among various radioactive nuclides, 137Cs+ is the most hazardous due to its high fission yield (6.09%), long halflife (∼30 years), and high solubility.3−5 When accidentally released to the sea or ground, it must be decontaminated immediately for public safety. The 137Cs+ ions also need to be recycled effectively from nuclear waste solutions in the reprocessing plants. Therefore, for 137Cs+ cleanup, high selectivity for Cs+ in the presence of relatively high concentration of competing cations (Na+, K+, Ca2+, Mg2+), fast kinetics, and commercial availability are desired in largescale application.6

The authors then describe some problems systems similar to the one they synthesize here were, and claim to address them:

For ion-exchange applications, maximizing the concentration of exchangeable cations is of critical importance for the process efficiency. The concentration of cations in traditional oxidebased zeolites is determined by the framework Al3+/Si4+ molar ratio which has a maximum value of 1 due to the Löwenstein’s rule (as in NaAlSiO4). Surprisingly, the Löwenstein’s rule is not obeyed in zeolite-type metal chalcogenides so that M3+/M4+ ratio can be significantly great than 1.31 This motivates us to initiate investigating the ion-exchange applications of such materials, because we expect that the large negative charge of framework and the associated high concentration of exchangeable cations will lead to a record-high cation exchange capacity. In addition, unlike low-dimensional materials, these zeolite-type chalocgenides have 3-D multidimensional, and mutually intersecting channels that could greatly facilitate ion diffusion and ion exchange kinetics. At the early stage of this study, we encountered a major obstacle to unlock the aforementioned intrinsic advantages of zeolite-type chalcogenides. Specifically, the as-synthesized materials typically contain bulky protonated amines in the channels and their ion exchange process is quite sluggish. 29,32−34

Herein we designed a two-step ion-exchange strategy to address this issue (Scheme 1). In this work, we selected a highly stable and porous amine-directed zeolitic chalcogenide framework, namely UCR-20 (zeolite type code: RWY).34,35 We demonstrated that the protonated amines located in its channels can be exchanged completely into “hard” alkali ions through the stepwise ion-exchange strategy. Interestingly, the K+-exchanged RWY (K@RWY) can rapidly capture Cs+ with high selectivity. This material also shows an excellent ability for Cs+ capture from real water samples including potable water and even seawater.

Here's a cute cartoon of their approach:



Here's some graphic data:



The caption:

Figure 2. (a) Equilibrium curve for cesium uptake fitted by Langmuir model with Ci = 1−500 ppm (RT, V:m = 1000 mL/g). (b) Distribution coefficient and removal efficiency for cesium uptake under different initial concentrations. (c) Adsorption kinetics of K@ RWY and Pristine RWY for Cs+ uptake with initial concentration around 50 ppm at room temperature (V:m = 1000 mL/g). (d) pH dependent cesium uptake. The initial concentrations were set to 10 ppm.

Their conclusion:

In conclusion, we designed and successfully realized a stepwise ion-exchange strategy based on zeolitic chalcogenide (RWY) to replace the organic amines in the channels with “hard” K+. The K@RWY could rapidly capture Cs+ with excellent selectivity, high capacity, good resistance against acid and alkali, and excellent resistance to γ- and β irradiation. High selectivity of Cs+ uptake against Na+, K+, Ca2+, and Mg2+ has been confirmed by further competitive ion exchange experiments. It should also be noted that K@RWY could capture Cs+ efficiently in real water samples including seawater with trace levels. The results indicated that K@RWY is a very promising ion exchanger for the removal of radioactive 137Cs+. Because amine-directed chalcogenide frameworks are a large family of compounds with various compositions and topologies, this strategy reported here could greatly extend the applications of this family of materials to nuclear waste remediation and toxic metal sequestration.

Nice work I think.

Enjoy the coming workweek.