This Step of the virtual experiment will now be conducted through an Interactive Video Experiment. Don't worry! You will still have the opportunity to make decisions that will guide the experiment's progress through a series of questions

Only a slight spontaneous precipitation of ferric phosphate could be observed in the ferric solution at room temperature. This is due to the fact that phosphoric acid is a weak acid that is present in solution as different species, according to this dissociation scheme:

H₃PO₄ → H⁺ + H₂PO₄^-

H₂PO₄^- → H⁺ + HPO₄^2-

HPO₄^2- → H⁺ + PO₄^3-

In a phosphoric acid solution at 40% w/w, not enough PO₄^3- anions are present to lead to the ferric phosphate precipitation. Since phosphate precipitation is controlled by pH value in the solution, it is possible to exploit the electrolysis of water to induce a pH increase in the ferric solution. Indeed, the phosphate salt is soluble at pH below 1 and becomes insoluble at pH around 2. In acid aqueous solutions that undergo electrolysis the following reactions occur:

2H⁺ + 2e → H₂ at the cathode

H₂O → ½0₂ + 2H⁺ + 2e at the anode

According to the current density applied, a concentration gradient is created in the cell, so that H⁺ concentration is maximum at the anode and it is the lowest at the cathode. Therefore, locally at the cathode pH increases and leads to the optimal conditions for the ferric phosphate precipitation along with the contaminants co-precipitation.

The electrochemical precipitation is performed by immersing the electrodes in the ferric solution and applying a voltage by a suitable power supply. The proper voltage and current values to be adopted depend on the cell geometrical configuration and the electrical conductivity of the solution: they need to be optimised case by case. As the current flows, there is gas release at the electrodes and the formation of sludge in the proximity of the cathode. The cell voltage increases with time due to the changes in the solution’s conductivity during phosphate’s precipitation.

At the end of the electrochemical precipitation process, a white pinkish sludge is formed in the cell. The liquid still present is called supernatant. They are separated by centrifugation and it is possible to evaluate the precipitation yield, R, defined as the ratio between the weight of the wet sludge after centrifugation and the initial amount of ferric solution. Typical values are around 40-50%. Centrifugation has to be performed under proper conditions in order to recover a wet sludge suitable for conditioning: optimal conditions could be a centrifugal acceleration of about 3000g for 15 minutes. The so-obtained sludge is made of iron phosphates and mixed iron-contaminants phosphate: it contains the most of the contamination removed from the metallic pipes and is set aside for the next conditioning step. The supernatant could contain some remaining contamination: it is measured by gamma spectrometry in order to quantify the contaminants activity still present and calculate the contaminant percentage recovered in the sludge. For each contaminant it is calculated as follow:

\[ \text{Contaminant %} = \frac{{A_{\text{sludge}}}}{{A_i}} = \frac{{A_i - A_{\text{supernatant}}}}{{A_i}} \]

where A_i is the total initial activity of each contaminant in the ferric solution and A_supernatant is the total activity of each contaminant measured in the supernatant.

Please note that all data collected during this experiment will be automatically recorded in your Personal Journal, so there's no need to take any manual notes for the calculations.

Now it’s time to separate the contaminants from the ferric solution by electrochemical precipitation. In this case, you will help the technician by guiding his work.

Remember that it is important to weigh the different parts of the set up, to obtain all the necessary data to calculate the effectiveness of the process. The electrochemical cell should be weighed when empty and then filled with the ferric solution. The electrodes, made of highly corrosion resistant platinum-niobium, should be immersed in the ferric solution, and the power supply should be turned on once connected. Once electrochemical precipitation begins, you will see bubble formation and white pinkish sludge formation. The lab power supply has a maximum voltage of 12V and a maximum current of 3A. It is important to limit the current to prevent an increase in temperature, but a sufficient voltage must be maintained to ensure complete precipitation of contaminants. Using a voltage close to 8V with a current limit of 3A enables complete precipitation in 5-6 hours.

After completing the process, it is important to weigh the cell also before removing the electrodes to evaluate evaporation. A slight evaporation around 5-10% is always observed. Then, the electrodes should be removed, and the sludge allowed to settle in the cell. The cell should then be closed with the proper cap, and the resulting mixture and transferred into the centrifuge. The tubes should be centrifuged at standardised conditions (4500 RPM for 10 minutes), and the weight of the tubes should be balanced to prevent e equipment damage. The supernatant should be separated by pouring it slowly in a new container for gamma measurement, to evaluate the remaining contamination. The sludge should be set aside for the next conditioning step. The final weight of the sludge and the supernatant will be needed for calculating the precipitation yield.

Pay attention, we are working with radioactive materials!