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Trapping phosphogypsum radiation

A method for immobilizing phosphogypsum radiation has been developed. Phosphoghypsum is a radioactive waste generated during the fabrication of phosphoric acid. The new method combines phosphogypsum with a sulfuric polymer, obtaining as a result a new polymeric cement with radiation levels below the legal limits, that could be safely applied for construction.

Phosphogypsum stack located near Fort Meade, Florida. Author: Harvey Henkelmann. Wikimedia CommonsPhosphogypsum is a gypsum sulfate that contains several radionuclides. It is generated in the production of phosphoric acid, used to obtain catalysts or phosphates that are applied as fertilizers or water softeners in soaps. Also, phosphoric acid is used to produce the additive E-338 and adhesives for removable dentures.

The wide use of phosphoric acid and the fact that the most efficient method to obtain it is from naturally radioactive phosphoric rocks, has caused an environmental problem difficult to solve. In spite of being a low-level, natural radiation, its effect is magnified by the concentration of large amounts of waste (millions of tons) in very few places.

A question of accumulation

The production of phosphoric acid is located in very few places in the world, where phosphoric rocks collected worldwide are kept and vast quantities of phosphogypsum are stored as waste.

One of these places is in the province of Huelva (Spain). According to figures by CEDEX up to 120 million tons of phosphogypsum have been disposed during the last 40 years in the Rio Tinto marshes, in Huelva. Another of these production places is in Central Florida (EUA), where more than a billion tons of this material can be found.

Producing a single ton of phosphoric acid generates between 4.5 and 5.5 tons of phosphogypsum as waste. It is estimated that every year between 100 and 250 million tons are produced worldwide.

Scientists at CSIC, the centre CIEMAT and the Universidad de Huelva, led by Felix A. López, research professor at CSIC, have developed a method to transform and stabilize this waste material.

The emissions from phosphogypsum radionuclides are reduced to levels below the limits accepted

Immobilization of phosphogypsum is achieved by adding it to a thermoplastic material formed by a mixture of least gravel, sand, elemental sulfur and a catalyzer (sulfuric polymer or modified sulfur). As a result, the emissions from phosphogypsum radionuclides are reduced to levels below the limits accepted by international regulations, which implies that the material material can be employed for building purposes with no restrictions.

Safe for construction

The material obtained can be used safely since there is no risk of lixiviation. Lopez, at the CSIC’s center for research on metals CENIM, explains that “using this material and with normal ventilation, the radon concentration indoors is lower than the recommended concentration in international regulations”. Prof. López keeps explaining: “Besides, the accelerated aging tests have shown that there is no mobilization of Ra-226 (radium, which generates radon gas)”. As a matter of fact, this is the most worrying pollutant substance from phosphogypsum.

The method has been patented worldwide by CSIC, CIEMAT and the Universidad de Huelva. Now it is ready to be transferred to the industry as a safe solution to phosphogypsum wastes.

Phosphoric rocks from which phosphogypsum is obtained have different radionuclides, uranides and lanthanides, depending on where the rock comes from, explains Felix A. López.

Some of them, as most of the uranium (U-238) and thorium (Th-230), are diluted during the process and accompany the final phosphoric acid. Up to 95% or more of radon, 80% of lead (Pb-210) and 90% of polonium (Po-210) remain bound to phosphogypsum.

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