Optimized Procedure for Determining the Adsorption of Phosphonates onto Granular Ferric Hydroxide using a Miniaturized Phosphorus Determination Method

The overall goal of this procedure is the reliable investigation of the absorption capacity of the selective iron hydroxide containing materials such as granular ferric hydroxide regarding phosphonates with the low workload in significant cost-savings compared to conventional methods. This method can help answer key questions on how well phosphonates can be technically eliminated from industrial waste water such as membrane concentrates, in which phosphonates are present as antiscalants. The main advantage of this modified phosphorus determination using 10 milliliter vials and a thermostat is that more results can be produced within shorter time compared to the conventional method.

Demonstrating the procedure will be Ellen Raith-Bausch, a technician from our laboratory. To begin the experiment, select one or more buffers with suitable properties for this technique that buffer the pH range or ranges of interest. For each buffer to be prepared, weigh or pipette the needed quantity of solute to prepare a 0.01 mole per liter solution into a volumetric flask.

Add reverse osmosis treated de-ionized water until the flask is about 3/4 full. Add the needed volume of a phosphonate stock solution with the concentration of one gram of phosphorus per liter to achieve the target phosphorus concentration in the buffer solution. Then fill the flask to the ring mark with water.

Stir the mixture until the solute has completely dissolved, or for five minutes for a liquid solute. Then transfer the buffer solution to a labeled glass bottle. Adjust the pH of the solution with either hydrochloric acid or sodium hydroxide to the desired pH value while stirring.

To begin the absorption test, weight the desired quantity of the chosen washed and dried filter material into a 50 milliliter centrifuge tube. Set a rotator to run at 20 revolutions per minute, and start the rotator. Transfer about 250 milliliters of the pH adjusted phosphonate containing buffer solution to a glass beaker.

Working quickly, fill the tube of filter material with the phosphonate containing buffer to the 50 milliliter mark. Close the tube, and clamp it into the running rotator. Rotate the tube at 20 rpm for the desired absorption time.

Then, filter 10 to 20 milliliters of the supernatant through 0.45 micron syringe filter into a glass bottle for analysis. Measure and record the pH of the filtrate. Repeat this process for each phosphonate containing buffer to be tested.

Prior to the analysis, clean and dry a 10 milliliter screwed cap vial and the PTFE-lined cap for each sample to be tested. If the expected phosphorus concentration for a sample is within the calibration curve, pipette four milliliters of the sample into the vial. If the concentration of the sample is expected to exceed the calibration curve, instead dilute a smaller volume of the sample with water to achieve a total volume of four milliliters.

Next, pipette 0.2 milliliters of 0.9 molar sulfuric acid solution into the vial. If the sample has a sodium hydroxide concentration of one molar, instead carefully add 0.2 milliliters of 13.5 molar sulfuric acid to the vial. Then, add to the vial 4.8 milliliters of a stock solution or a suspension of potassium persulfate of the appropriate concentration for the buffer and the sample.

Tightly cap the vial and shake the vial for one second. The potassium persulfate dosage amount needs to be matched to matched to each buffer and the solution individually since each buffer has different COD. Heat the sample at 148 to 150 degrees Celsius for one hour in a digestion thermostat.

Then remove the vial from the thermostat and allow it to cool to room temperature. The ISO 6878 method suggest the use of Erlenmeyer flask for digestion. However, this is very labor intensive.

Further, with fixed NaOH dosages, no manual pH adjustment as stipulated in the conventional ISO method, is required. Next, in sequence, add 0.4 milliliters of a 1.5 molar sodium hydroxide solution, 0.2 milliliters of a 100 gram per liter solution of ascorbic acid, and 0.4 milliliters of molybdate two stock solution. Tightly cap and shake the vial.

Wait between 15 minutes to four hours for color development in the solution. Finally, measure the absorbance of the sample at 880 nanometers with a spectrophotometer. A loading of up to 0.55 milligrams of NTMP phosphorus per gram of GFH was achieved with one hour of contact in acidic acid buffer.

Although loading decreased with increasing pH, absorption was observed in a sodium hydroxide system at pH 12. This indicates that successful desorption requires more concentrated sodium hydroxide solutions. Changes in pH were observed for various buffer concentrations after one hour of contact between NTMP phosphorus and GFH in concentrations of one milligram per liter and 2.5 grams per liter, respectively.

Relatively reliable pH adjustment for the effect of contact with GFH was only achieved at buffer concentrations of 10 millimole per liter. The 10 millimole per liter buffers also provided the most reliable absorption results. Furthermore, it could be shown that phosphonate absorption is not influenced significantly by parallel absorption of the buffer as independent of the buffer concentration predominantly similar phosphonate absorption occurred.

When 10 millimole per liter buffers were used, the color complexes were most stable when the buffers were dosed with 0.6 millimole of sodium hydroxide. Potassium persulfate quantities only needed to exceed the potassium dichromate based chemical oxygen demand, as the nitrogen and some sulfo groups were not expected to decompose completely. I first had the idea for this technique when I've analyzed the total phosphorus content of more than 1, 000 samples according to the conventional methods for my doctoral thesis.

This was very time consuming. The procedure presented here shows that investigations regarding elimination of phosphonates by absorption on polar iron oxide containing materials such as granular ferric hydroxide can be carried out quickly and reliably when in accordance with the given protocol. With this procedure, various influencing factors on the loading of granular ferric hydroxide with phosphonates can be investigated, such as pH, temperature, kinetics, initial phosphonate, and GFH concentration, or the presence of typical waste water components such as calcium.

While attempting this procedure, remember to avoid contamination by cleaning each piece of equipment properly in advance. When appropriately performed, up to around 50 total phosphorus contaminations can be carried out in only a few hours.