A research group that included Yurina Sekine, a researcher the Japan Atomic Energy Agency (JAEA)’s Materials Sciences Research Center, Takuya Nankawa, a researcher at the JAEA’s Advanced Science Research Center, as well as the University of Tokyo and National Institute for Materials Science(NIMS) has announced the development of an absorbent that possesses high absorption performance for hazardous metals such as strontium (Sr) and cadmium (Cd), and uses waste pork bones as its raw material. The agent was realized by analyzing the high absorption performance boasted by bones. It can be manufactured using a simple procedure, and has been confirmed to have 20 times the absorption performance of the natural zeolite currently used as an absorbent. The results are expected to prove useful for contaminated water purification, environmental purification and hazardous metals collection. The results were published in the January 21 online edition of the “Journal of Environmental Chemical Engineering,” an international scientific journal.
Natural and human-derived hazardous metals exist in soil, underground water, seawater, marine sediment and elsewhere. Up to now also, pollution by these hazardous metals has resulted in large-scale contamination events worldwide. In Japan also, “itai itai disease” caused by Cd occurred from the 1910s, and other hazardous metals are also commonly and widely contained in soil. It is also known that radioactive Sr occurs as a result of the nuclear fission of uranium when nuclear power is produced, and that one of its properties is that it is readily captured by bones. There is a need to prevent the diffusion or discharge of hazardous metals such as these, but absorbents such as synthetic and natural zeolites or oxidized titanium have not been put into practical use due to their high cost as well as issues on the performance front. Attempts had been made to absorb uranium and Sr in the soil by burying waste beef bones, but the performance that was obtained was not sufficient enough to warrant the technique being put into practical use.
With that in mind, this research group clarified the mechanism by which bones absorb metals such as Sr and Cd, with the aim of developing an absorbent that would utilize the same mechanism.
Animal bones are composed of inorganic matter known as apatite, and organic tissue such as collagen. Apatite is an inorganic matter with a hexagonal crystalline structure constructed of calcium, phosphate and other minerals. Unlike synthetic (man-made) apatite, the apatite derived from living things contains carbonic acid. Additionally, synthetic apatite had been known to be less capable of absorbing metals than apatite derived from living organisms.
In light of that, the research group studied the effect that controlling the amount of carbonic acid in the bones of living things has on the metals absorption performance that the bones possess. At the same time, the research group explored the development of an absorbent that uses a cheap ingredient—waste pork bones—as its base material. Worldwide, 7.5 billion tons of waste pork and beef bones are generated every year, and disposing of this waste poses a challenge.
To begin with, in order to remove the organic tissue from the pork bones they were heated at high temperature and high pressure, and were subsequently immersed in an aqueous solution of sodium hydrogen carbonate (baking soda) in order to add carbonic acid. This process removed the organic tissue. Examination by transmission electron microscope also confirmed that the organic tissue had been removed and that apatite crystals had been obtained.
By measuring the introduction of carbonic acid using Fourier transform infrared spectroscopy (FTIR), it was also confirmed that it was possible to increase the volume of carbonic acid introduced by increasing the concentration of the baking soda introduced.
The absorption performance was evaluated by immersing the pork bone-derived material that was produced in an Sr aqueous solution (0.1 millimole/liter), stirring it for 24 hours, and then measuring the Sr concentration of the aqueous solution once the material alone had been removed. It was also confirmed that by altering the percentage of carbonic acid content, the absorption performance for Sr rose dramatically accompanying the increased quantity of carbonic acid.
Of the material that was produced, when the partition coefficient (the ratio of whether the Sr is more likely to migrate to the water or the material) was studied for the material made with the highest volume of carbonic acid—an aqueous solution of 20% baking soda—it showed that the material had a performance of around 250 times that of untreated bones, and around 20 times that of clinoptilolite (a natural zeolite), an absorbent for Sr. Furthermore, when the amount of Sr absorbed by one gram of the material was studied, it was found to be 125 milligrams/gram, or a performance of approximately five times that of untreated bones and around five times that of clinoptilolite. The material’s performance was also studied for Cd and lead, which are also known as hazardous metals, and it showed a similarly high absorption performance. For Cd it showed a performance of 23 times that of untreated bones and 370 times that of clinoptilolite, and for lead it showed a performance of 14 times that of untreated bones and 1.5 times that of clinoptilolite.
When the research group studied the electrical charge on the material’s surface in order to study the absorption mechanism, it found that accompanying an increase in the volume of carbonic acid introduced, the negative charge increased, and that the charge became positive following Sr absorption. It is believed that absorption sites for the positive Sr and other metals to bond to are being formed on the negatively-charged surface of the material produced.
The high-performance absorbent for hazardous metals that was developed can be produced in large quantities and at low cost, and there is a possibility it will be able to be broadly utilized as an environmental purification material and for recovering useful metals. The research group reports that trials conducted in seawater and other environments have also yielded favorable results. Going forward the group will verify whether the material can be preserved stably following absorption, for example.
Yurina Sekine said, “We believe that in the future it will be possible to develop this as a low-cost, eco-friendly environmental purifier by combining it with a waste disposal system.”