Name: optimized procedure for determining the adsorption of phosphonates onto granular ferric hydroxide using a miniaturized phosphorus determination method
Uploaded: 2018-05-18T14:00:00
Duration: 8 min 21 s
Description: Watch this Scientific Journal Video about Optimized Procedure for Determining the Adsorption of Phosphonates onto Granular Ferric Hydroxide using a Miniaturized Phosphorus Determination Method at JoVE

The authors are grateful for the financial support by the Willy-Hager-Stiftung, Stuttgart. We would also like to thank the employees of Zschimmer &amp; Schwarz Mohsdorf GmbH &amp; Co. KG for providing phosphonate samples.

1. Preparation of All Required Solutions for the Total P Determination
NOTE: The preparation of some of the solutions described below is explained in ISO 687828. These preparation methods have been slightly adapted to the method of this work. The required degree of purity of chemicals can be found in the attached material list.
<ol>
	<li>Preparation of H2SO4 solutions (13.5, 9 and 0.9 M H2SO4) 
	Caution: Work under fume hood.
	<ol>
		<li>Preparation of 13.5 M H2SO4
		<ol>
			<li>Fill a 100 mL graduated cylinder with 25 mL of water and transfer it into a 100 mL glass bottle surrounded by ice cubes placed in a beaker.</li>
			<li>Fill the same graduated cylinder with 75 mL of concentrated sulfuric acid and transfer it under stirring to the water in the bottle. Caution: Heat development.</li>
			<li>Take the bottle carefully out of the beaker as soon as it is sufficiently cooled down (max. 40 &#176;C).</li>
		</ol>
		</li>
		<li>Preparation of 9 M H2SO4 (required for the preparation of molybdate solution)
		<ol>
			<li>Fill a 1 L graduated cylinder with 700 mL of water and transfer it into a 3 L glass beaker surrounded by ice cubes placed in a bucket.</li>
			<li>Fill the same 1 L graduated cylinder with 700 mL of concentrated sulfuric acid and transfer it under stirring to the water in the 3 L beaker. Caution: Heat development.</li>
			<li>Take the 3 L beaker carefully out of the bucket as soon as it is sufficiently cooled down (max. 40 &#176;C) and transfer its content into a 2 L glass bottle.</li>
		</ol>
		</li>
		<li>Preparation of 0.9 M H2SO4
		<ol>
			<li>Fill a 250 mL volumetric flask with about 100 mL of water.</li>
			<li>Transfer 25 mL of 9 M H2SO4 (see 1.1.2) into the 250 mL volumetric flask using a 25 mL volumetric pipette. Caution: Heat development.</li>
			<li>Fill the 250 mL volumetric flask with water up to the 250 mL ring mark.</li>
			<li>Close the volumetric flask with a stopper, shake it several times for homogenization and transfer the content of the volumetric flask into a 250 mL glass bottle.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Preparation of HCl rinsing solution (approx. 2 M) 
	Caution: Work under fume hood.
	<ol>
		<li>Fill a 2 L graduated cylinder with 1 L of water.</li>
		<li>Fill this graduated cylinder with 400 mL of 32% HCl (w/w) solution.</li>
		<li>Now add 600 mL of water to gain a total volume of 2 L in the graduated cylinder.</li>
		<li>Stir the content of the graduated cylinder with a rod (e.g., graduated pipette) and transfer the content of the graduated cylinder into a 2.5 L glass bottle.</li>
		<li>Close the bottle and shake it upside down several times for homogenization.</li>
		<li>Reuse this solution only until a color change becomes apparent. Then discard the rinsing solution and prepare a new one.</li>
	</ol>
	</li>
	<li>Preparation of HCl solutions (10.2 and 2 M) 
	Caution: Work under fume hood.
	<ol>
		<li>Use 32% HCl (w/w) as 10.2 M HCl.</li>
		<li>Preparation of 2 M HCl
		<ol>
			<li>Fill a 100 mL volumetric flask with 15 mL of 32% HCl (10.2 M) using a 15 mL volumetric pipette.</li>
			<li>Add another 4.67 mL of 32% HCl (10.2 M) to the volumetric flask using a micropipette.</li>
			<li>Fill the volumetric flask with water up to the 100 mL ring mark.</li>
			<li>Close the volumetric flask with a stopper and shake it upside down several times for homogenization and transfer the content of the volumetric flask into a 100 mL glass bottle.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Preparation of NaOH solutions (10, 2, 1.5 M NaOH) 
	Caution: Work under fume hood.
	<ol>
		<li>Weigh 100.0 g (for 10 M), 20 g (for 2 M) or 15 g (for 1.5 M) of NaOH into a small beaker and transfer the content of the beaker into a 250 mL volumetric flask.</li>
		<li>Fill the volumetric flask with water up to the 250 mL ring mark. Close the volumetric flask with a stopper and shake it upside down several times for homogenization (Caution: solution can become hot). If the height of the water level no longer corresponds to the ring mark, add more water (the total volume changes as a result of the dissolving process).</li>
		<li>Transfer the content of the volumetric flask into a 250 mL plastic bottle (Caution: Do not use glass bottles for NaOH solutions).</li>
	</ol>
	</li>
	<li>Preparation of K2S2O8 solution/suspension (8.33, 41.67, 50.00, 58.33, 66.66 g/L) 
	NOTE: Differently concentrated peroxodisulfate mixtures are required for phosphorus determination. Since some of them are above the saturation limit of K2S2O8 of approx. 50 g/L at 20 &#176;C, it is advisable to weigh the K2S2O8 directly into a brown glass bottle and pour a corresponding volume of water over it (do not use volumetric flasks for the preparation).
	<ol>
		<li>Weigh 2.08 g (for 8.33 g/L), 10.42 g (41.67 g/L), 12.50 (50.00 g/L), 14.58 g (58.33 g/L) or 16.67 g (66.66 g/L) of solid K2S2O8 directly into a brown 250 mL glass bottle.</li>
		<li>Fill a graduated cylinder with 250 mL of water and pour this water over the K2S2O8 in the bottle.</li>
		<li>Stir the content of the bottle until all ingredients are dissolved or until there is only a slight turbidity.</li>
		<li>Carry out the extraction of K2S2O8 under high turbulence on the magnetic stirrer to ensure that the undissolved K2S2O8 can also be extracted as homogeneously as possible.</li>
	</ol>
	</li>
	<li>Preparation of 100 g/L ascorbic acid solution
	<ol>
		<li>Weigh 50 g of ascorbic acid into a 500 mL volumetric flask.</li>
		<li>Fill the volumetric flask with water up to the 500 mL ring mark.</li>
		<li>Stir the content of the volumetric flask on the magnetic stirrer until the ascorbic acid is completely dissolved. It may be necessary to correct the level of the water surface to make it congruent with the ring mark by adding a little more water (be careful of the stirring bar giving volume as well). Then transfer the content of the volumetric flask into a brown 500 mL glass bottle.</li>
	</ol>
	</li>
	<li>Preparation of Molybdate I solution (required for phosphate determination)
	<ol>
		<li>Weigh 13.0 g of solid (NH4)6Mo7O24&#8729;4H2O directly into a 100 mL glass bottle. Fill a graduated cylinder with 100 mL of water and pour it into the bottle. Stir the content of the bottle on a magnetic stirrer until it is completely dissolved.</li>
		<li>Weigh 0.35 g of solid K(SbO)C4H4O6&#8729;&#189;H2O directly into a fresh 100 mL glass bottle. Fill a graduated cylinder with 100 mL of water and pour it into the bottle with K(SbO)C4H4O6&#8729;&#189;H2O. Stir the content of the bottle until it is completely dissolved.</li>
		<li>Fill a graduated cylinder with 300 mL of 9 M H2SO4 (see 1.1.2) and pour it into a brown 500 mL glass bottle.</li>
		<li>Add the (NH4)6Mo7O24&#8729;4H2O solution to the 300 mL of 9 M H2SO4. Then add the K(SbO)C4H4O6&#8729;&#189;H2O solution to this mixture. Close the bottle and shake it several times upside down for homogenization.</li>
	</ol>
	</li>
	<li>Preparation of Molybdate II solution (required for total P determination)
	<ol>
		<li>Weigh 13.0 g of solid (NH4)6Mo7O24&#8729;4H2O directly into a 100 mL glass bottle. Fill a graduated cylinder with 100 mL of water and pour it into the bottle. Stir the content of the bottle on a magnetic stirrer until it is completely dissolved.</li>
		<li>Weigh 0.35 g of solid K(SbO)C4H4O6&#8729;&#189;H2O directly into a fresh 100 mL glass bottle. Fill a graduated cylinder with 100 mL of water and pour it into the bottle with K(SbO)C4H4O6&#8729;&#189;H2O. Stir the content of the bottle until it is completely dissolved.</li>
		<li>Fill a graduated cylinder with 70 mL of water. Add 230 mL of 9 M H2SO4 (see 1.1.2) to the water in the graduated cylinder (i.e., fill up to 300 mL). Carefully homogenize the content of the graduated cylinder with a rod (e.g., graduated pipette). Transfer the content of the graduated cylinder into a brown 500 mL glass bottle (current content: 6.9 M H2SO4).</li>
		<li>Add the (NH4)6Mo7O24&#8729;4H2O solution to the 300 mL of 6.9 M H2SO4. Then add the K(SbO)C4H4O6&#8729;&#189;H2O solution to this mixture. Close the bottle and shake it several times upside down for homogenization.</li>
	</ol>
	</li>
	<li>Preparation of internal quality standard (IQS: 1 mg/L KH2PO4-P in 0.9 mM H2SO4)
	<ol>
		<li>Dry a few grams of KH2PO4 in a small glass dish at 105 &#176;C in the drying oven until mass constancy is achieved and then cool the KH2PO4 down to room temperature in a desiccator.</li>
		<li>Weigh 0.2197 g &#177; 0.0002 g of KH2PO4 directly from the desiccator into a 1 L volumetric flask and add approx. 800 mL of water into the volumetric flask.</li>
		<li>Now add 5 mL of 9 M H2SO4 (see 1.1.2) to the flask using a 5 mL volumetric pipette and fill the flask with water up to the 1 L ring mark.</li>
		<li>Stir the content of the volumetric flask on the magnetic stirrer and transfer the content of the volumetric flask into a 1 L glass bottle (current content: 50 mg/L KH2PO4-P in 45 mM H2SO4). This solution can henceforth be used as a stock solution for the preparation of IQS.</li>
		<li>Transfer 10 mL of this solution into a 500 mL volumetric flask using a 10 mL volumetric pipette, fill the volumetric flask with water up to the 500 mL ring mark and stir the content of the volumetric flask on the magnetic stirrer.</li>
		<li>Transfer the content of the volumetric flask into a 500 mL glass bottle (current content: 1 mg/L KH2PO4-P in 0.9 mM H2SO4). This solution is the IQS.</li>
	</ol>
	</li>
</ol>
2. Preparation of Phosphonate-Containing Buffered Solutions
<ol>
	<li>Weigh or pipette the desired buffer into a volumetric flask (at a target concentration of 0.01 M buffer in 1 L, e.g.: 572 &#181;L of 100% AcOH, 2.1014 g of CitOH&#183;H2O, 1.9520 g of MES, 2.0926 g of MOPS, 2.3831 g of HEPES, 2.5233 g of EPPS, 2.3732 g of CAPSO, 2.2132 g of CAPS, 5 mL of 2 M NaOH).</li>
	<li>Fill the volumetric flask to about three quarters with water and add a previously prepared 1 g/L phosphonate-P stock solution (for a target concentration of 1 mg/L P in 1 L, e.g., 1 mL of 1 g/L phosphonate-P).</li>
	<li>Fill the flask with water up to the ring mark, stir the content of the flask on the magnetic stirrer until all ingredients are dissolved and transfer it into a glass bottle.</li>
	<li>While stirring, adjust the desired pH value in the buffer solution (e.g., pH 6 at MES) with HCl (e.g., 2 and 10.2 M) or NaOH (e.g., 2 and 10 M) (the addition of both acidic and basic solution should be avoided to prevent an unnecessary increase of ionic strength).</li>
	<li>To determine the phosphonate-P concentration, proceed according to step 4.</li>
</ol>
3. Procedure of the Adsorption Test
<ol>
	<li>Wash the filter material thoroughly with distilled water (e.g., over a sieve with a mesh size of 0.5 mm) and then dry it at 80 &#176;C. 
	Note: The protocol can be paused here.</li>
	<li>Weigh the filter material (e.g., granular ferric hydroxide) into a 50 mL centrifuge tube. 
	Note: The protocol can be paused here.</li>
	<li>Quickly fill the 50 mL centrifuge tube with the phosphonate-containing solution from step 2 up to the 50 mL mark.</li>
	<li>Quickly close the tube and clamp it into the running rotator (the contact time starts from now on).</li>
	<li>Rotate the tube at 20 revolutions per minute for a specific amount of time (e.g., 1 h).</li>
	<li>Filter approx. 10-20 mL of the supernatant with a syringe filter (0.45 &#181;m pore size) into an empty glass bottle. 
	Note: The protocol can be paused here.</li>
	<li>Determine the pH value of the filtrate and to determine the phosphonate-P concentration proceed with step 4. In the case of investigating phosphate adsorption, proceed with step 5.</li>
</ol>
4. Determination of Total P (Phosphonate-P) According to ISOmini
NOTE: The following procedure is also shown in Figure 3.
<ol>
	<li>Transfer an aliquot of the sample to be analyzed (Vsample, max. 4 mL) using a micropipette into a 10 mL screw cap vial (the vial including the cap should be pre-rinsed with HCl (see 1.2) and H2O and dried at 80-100 &#176;C). 
	Note: The protocol can be paused here.</li>
	<li>Add water with a micropipette to obtain a total volume of 4 mL together with the sample previously added (Vwater = 4 mL-Vsample). 
	Note: The protocol can be paused here.</li>
	<li>Add 0.2 mL of 0.9 M H2SO4 solution (see 1.1.3) using a micropipette. If there is a concentration of 1 M NaOH in the sample, as is often the case with regeneration solutions, add 0.2 mL of 13.5 M H2SO4 solution (see 1.1.1) (Caution: This solution of sulfuric acid is highly concentrated). 
	Note: The protocol can be paused here.</li>
	<li>Add 4.8 mL of a K2S2O8 solution/suspension (see 1.5) the concentration of which depends on the buffer contained in the sample (corresponding to ISO at 0.01-1 M NaOH: 8.33 g/L K2S2O8; 0.01 M CitOH, AcOH, MES: 41.67 g/L; 0.01 M MOPS: 50.00 g/L; 0.01 M HEPES: 58.33 g/L; 0.01 M EPPS, CAPSO, CAPS: 66.66 g/L).</li>
	<li>Close the vial with the cap very tightly and shake it.</li>
	<li>Heat the vial in a thermostat at 148-150 &#176;C for 1 h.</li>
	<li>Take the vial out of the thermostat and let it cool down to room temperature. 
	Note: The protocol can be paused here.</li>
	<li>Open the vial and add 0.4 mL of 1.5 M NaOH solution (see 1.4). 
	Note: The protocol can be paused here.</li>
	<li>Add 0.2 mL of 100 g/L ascorbic acid solution (see 1.6).</li>
	<li>Subsequently, add 0.4 mL molybdate II solution (see 1.8).</li>
	<li>Close the vial and turn it upside down for homogenization.</li>
	<li>Wait minimum 15 minutes to a maximum of 4 h for color formation.</li>
	<li>Measure spectral absorbance (A) at a wavelength of 880 nm using a photometer.</li>
	<li>Carry out the steps 4.1-4.13 regularly for 4 mL of water (for the determination of Ablind) as well as for 4 mL of an IQS (see 1.9).</li>
	<li>Calculate the total P or phosphonate-P concentration of the analysis sample on the basis of the specific absorbance of the analysis sample (A), the absorbance of the blind sample (Ablind) and the sample volume (Vsample) using the following equation (0.287 corresponds to the slope of the calibration line with 1 cm cuvettes and can deviate depending on the photometer): 
	<img alt="Equation 2" src="/files/ftp_upload/57618/57618eq2v2.jpg" /></li>
</ol>
5. Determination of o-PO43--P According to ISOmini
NOTE: This determination method can be used when the adsorption of inorganic ortho-phosphate on granulated filter materials is to be investigated. In this case, the sample to be tested does not have to be digested.
<ol>
	<li>Transfer an aliquot of the sample to be analyzed (Vsample, max. 9.4 mL) by means of a micropipette into a 10 mL screw cap vial (the vial including the cap should be pre-rinsed with HCl (see 1.2) and H2O and dried at 80-100 &#176;C). 
	Note: The protocol can be paused here.</li>
	<li>Add water with a micropipette to obtain a total volume of 9.4 mL together with the sample previously added (Vwater = 9.4 mL-Vsample). 
	Note: The protocol can be paused here.</li>
	<li>Add 0.2 mL of 100 g/L ascorbic acid solution (see 1.6).</li>
	<li>Subsequently, add 0.4 mL of molybdate I solution (see 1.7).</li>
	<li>Close the vial and turn it upside down for homogenization.</li>
	<li>Wait minimum 15 minutes to a maximum of 4 h for color formation.</li>
	<li>Measure spectral absorbance (A) at a wavelength of 880 nm using a photometer.</li>
	<li>Carry out the steps 5.1-5.7 regularly for 9.4 mL of water (for the determination of Ablind) as well as for 4 mL of an IQS (see 1.9).</li>
	<li>On the basis of the specific absorbance of the analysis sample (A), of the blind sample (Ablind) and the sample volume (Vsample), the ortho-phosphate-P concentration of the analysis sample can be calculated using the equation in 4.15.</li>
</ol>

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K., Raue, B., Brauch, H. J., Sacher, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Trace-level+analysis+of+phosphonates+in+environmental+waters+by+ion+chromatography+and+inductively+coupled+plasma+mass+spectrometry.">Trace-level analysis of phosphonates in environmental waters by ion chromatography and inductively coupled plasma mass spectrometry.</a> International Journal of Environmental Analytical Chemistry. 94 (4), 385-398 (2014).</li><li>Nowack, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determination+of+phosphonates+in+natural+waters+by+ion-pair+high-performance+liquid+chromatography.">Determination of phosphonates in natural waters by ion-pair high-performance liquid chromatography.</a> Journal of Chromatography A. 773 (1-2), 139-146 (1997).</li><li>Murphy, J., Riley, J. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+modified+single+solution+method+for+the+determination+of+phosphate+in+natural+waters.">A modified single solution method for the determination of phosphate in natural waters.</a> Analytica Chimica Acta. 27, 31-36 (1961).</li><li>Worsfold, P., McKelvie, I., Monbet, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determination+of+phosphorus+in+natural+waters:+A+historical+review.">Determination of phosphorus in natural waters: A historical review.</a> Analytica Chimica Acta. 918, 8-20 (2016).</li><li>Going, J. E., Eisenreich, S. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spectrophotometric+studies+of+reduced+molybdoantimonylphosphoric+acid.">Spectrophotometric studies of reduced molybdoantimonylphosphoric acid.</a> Analytica Chimica Acta. 70, 95-106 (1974).</li><li>Pai, S. C., Yang, C. C., Riley, J. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effects+of+acidity+and+molybdate+concentration+on+the+kinetics+of+the+formation+of+the+phosphoantimonylmolybdenum+blue+complex.">Effects of acidity and molybdate concentration on the kinetics of the formation of the phosphoantimonylmolybdenum blue complex.</a> Analytica Chimica Acta. 229, 115-120 (1990).</li><li>Eisenreich, S. J., Bannerman, R. T., Armstrong, D. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+simplified+phosphorus+analysis+technique.">A simplified phosphorus analysis technique.</a> Environmental Letters. 9 (1), 43-53 (1975).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> ISO 6878:2004. Water quality-Determination of phosphorus-Ammonium molybdate spectrometric method. , (2004).</li><li>Freundlich, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=&#220;ber+die+Adsorption+in+L&#246;sungen+(About+the+adsorption+in+solutions).">&#220;ber die Adsorption in L&#246;sungen (About the adsorption in solutions).</a> Z. Physical Chemistry. 57, 385-470 (1907).</li><li>Ho, Y. S., Porter, J. F., McKay, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Equilibrium+isotherm+studies+for+the+sorption+of+divalent+metal+ions+onto+peat:+Copper,+nickel+and+lead+single+component+systems.">Equilibrium isotherm studies for the sorption of divalent metal ions onto peat: Copper, nickel and lead single component systems.</a> Water Air &amp; Soil Pollution. 141 (1-4), 1-33 (2002).</li><li>Li, L., Zhang, S., Li, G., Zhao, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determination+of+chemical+oxygen+demand+of+nitrogenous+organic+compounds+in+wastewater+using+synergetic+photoelectrocatalytic+oxidation+effect+at+TiO2+nanostructured+electrode.">Determination of chemical oxygen demand of nitrogenous organic compounds in wastewater using synergetic photoelectrocatalytic oxidation effect at TiO2 nanostructured electrode.</a> Analytica Chimica Acta. 754, 47-53 (2012).</li></ol>

No conflicts of interest declared.

The increasing significance of phosphonates requires research for reliable methods of removing these compounds from wastewater to protect wastewater treatment plants or receiving water bodies. At present, very few studies have been carried out on the removal of phosphonates from industrial wastewater5,11,12,13,14,16. The procedure presented here shows that investigations regarding the elimination of phosphonates by adsorption on polar iron oxide containing materials, in particular granular ferric hydroxide, can be carried out quickly and reliably when in accordance with the given protocol.
The decisive point in conducting adsorption studies is the maintenance of the pH value. This cannot be done in rotating centrifuge tubes without using a buffer. In this article, it was shown that Good buffers allow an acceptable pH adjustment only at a concentration of 0.01 M and even at this concentration have no significant influence on the adsorption of phosphonates onto GFH. The application of Good buffers is also the reason why the procedure presented here cannot be used for studies on adsorption of phosphonates onto rather non-polar materials such as activated carbon. Good buffers would compete with phosphonates for free adsorption sites.
Since the direct analysis of phosphonates by means of HPLC22 or IC-ICP-MS21 is very complex and expensive, the presented method suggests that the phosphonate after contact with the adsorbent should be measured indirectly via the determination of the total P. A standardized method (ISO 687828) is generally used for the total P determination, in which a digestion is carried out by means of H2SO4 and K2S2O8 on a hotplate, the pH value is then set to 3-10 by means of NaOH and a blue color complex (the color intensity of which is linearly proportional to the phosphate concentration) is formed with the aid of ascorbic acid and molybdate solution. This standardized method is very labor and time consuming, which is why a faster variant of the ISO method (ISOmini) was developed. The ISOmini method reduces the total volume to one-fifth. The digestion takes place comfortably in a thermostat and the NaOH dosage after digestion is fixed. This method enables a large number of phosphorus determinations to be carried out within a very short time and does not compromise accuracy in comparison to the ISO method.
Each buffer has a different COD. In addition, the relatively high necessary buffer concentration of 0.01 M means that, in order to ensure sufficient digestion of the sample constituents, considerably higher amounts of oxidizing agent have to be dosed than it is stipulated in the ISO method. If the K2S2O8 dosage is too low or too high, incorrect measurement results do occur. In the ISOmini method, this K2S2O8 dosage is thus matched to each buffer individually. Another critical point is the dosage of NaOH. As a rule, regeneration solutions have NaOH concentrations of &#62; 0.1 M. In order to avoid that the [H+]:[Mo] ratio required for the formation of the color complex25,26 is not adhered to, a proper adjustment of the H2SO4 quantity prior to digestion is therefore necessary. The problem arises when the regeneration solution is reused several times, thereby changing its pH value and COD. Since a reliable and simple pH measurement is not possible in screw cap vials and an appropriate pH adjustment is not provided, the ISOmini method presented here, thus, reaches its limits for samples with very high pH values. For regeneration solutions it is therefore recommended to use the ISO method.

Motivation
The efforts to reduce nutrient inputs into surface waters, which are necessary, inter alia, in the context of the implementation of the European Water Framework Directive1, require a more detailed examination of phosphorus emissions. The substance group of phosphonates (Figure 1), which are used as bleach stabilizers in textile and paper industries, as antiscalants in drinking water treatment, as hardness stabilizers of cooling water and in detergents and cleaning agents, is particularly relevant in terms of quantity and environmental relevance2. Phosphonates are suspected of contributing to long-term eutrophication of water bodies2,3,4. For example, due to UV radiation of sunlight or in the presence of MnII and dissolved oxygen, phosphonates can be degraded into microbiologically available phosphates5,6. The oversupply of phosphate is an essential characteristic of ecologically unbalanced water bodies, which makes phosphorus an important target substance for the sustainable improvement of the ecological status of water bodies.
Phosphonates can be removed from wastewater by precipitation/flocculation when using iron or aluminum salts7,8,9,10. In this process, metals are transformed into hardly soluble metal hydroxides. These polar flocks with a relatively large specific surface serve as adsorbents for the negatively charged phosphonates. However, the flocculation process can have two main disadvantages. Depending on the wastewater, sludge volumes of up to 30% of the sample volume can occur11. This sludge has to be separated, treated and disposed of in a further sedimentation or filter stage. Furthermore, phosphonates can complex the added flocculants and thus prevent the formation of flocks, especially in wastewater with low water hardness. This effect can be compensated by increased quantities of flocculant. However, this leads to increased &#946; values (&#946; = molar ratio of flocculant to phosphorus in wastewater)11,12. A complex wastewater matrix, therefore, can complicate the control of an optimum flocculant dosage.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/57618/57618fig1.jpg" /> 
Figure 1: Structural formulae of important phosphonates11. <a href="/files/ftp_upload/57618/57618fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
A possible alternative that exploits the high adsorption affinity of phosphonates to metal-containing surfaces and that does not have the above-mentioned disadvantages are filter materials based on iron (hydr)oxides. For such filter materials, the literature mainly presents investigations into the elimination of phosphate13,14,15,16. This paper introduces a procedure which allows the investigation of the adsorption capacity of selective granulated filter materials, in this work in particular with granular ferric hydroxide (GFH), regarding phosphonates with little workload and significant cost saving. The study of the adsorption capacity can be divided into the following steps: preparation of the phosphonate solution, adsorption test (contact of the phosphonate solution with the granulate) and phosphonate analysis. All steps must be perfectly coordinated.
Concept for adsorption test and the use of suitable buffers 
For the study of adsorption capacity, batch or column tests can be carried out. In order to determine adsorption isotherms or pH-dependencies of the adsorbent, the batch approach is preferred since many results can be obtained within a short period of time by the possibility of varying several parameters. The pH value is one of the most important factors influencing adsorption. Compliance with or adjustment of the pH value is a great challenge for the laboratory technician, as the simple adjustment of the pH value in the sample solution previously to the contact with the adsorbent is usually not sufficient. Each adsorbent material is usually striving to approximate the pH around its point of zero charge (PZC). Accordingly, it is possible that an aqueous solution, e.g., adjusted to pH 3, changes to a pH value of 8 when in immediate contact with the adsorbent. Wastewater mostly has a natural buffering capacity, which attenuates this effect. If, however, only the removal of a particular target substance is to be investigated with a particular adsorbent, synthetic wastewater must be used, i.e., pure water, which is specifically spiked with the target substance or, e.g., competitive anions. In contrast to powdered adsorbents, where the pH value can be easily maintained in the desired range by adding acids and bases in the open stirring vessel, no pH adjustment in this form can be done in a batch approach with granulates. In order to keep granules homogeneously suspended, very high stirring speeds are required, which would result in very rapid abrasion of the material. If such abrasion is unintended, the gentlest method is to rotate closed centrifuge tubes to keep the granules mixed continuously in the solution. The only way to keep the pH value constant in this case is to use buffers.
The following requirements for buffers must be met in order to be able to investigate the adsorption of phosphate and phosphonates on iron-containing filter materials: free of phosphorus; colorless; soluble; at best, no complexing agents; no competition with phosphonates regarding adsorption onto polar filter materials; similar structure of the different buffers used; and buffers or their degradation products must not have a negative effect on the spectral absorbance of the color complex after digestion for total P determination. For the biochemical research field, so-called Good buffers were developed17,18,19, which have exactly these properties. Thus, for the investigations of this work, the buffers in Table 1 were selected. The pKa value of each buffer indicates the range that can be kept constant by the buffer. For the pH range &#60; 5, however, organic acids such as citric acid (CitOH) and acetic acid (AcOH) must be used. Citric acid is a complexing agent, but it buffers in a pH range where most iron-containing filter materials become unstable anyway. Acetic acid and MOPS were already used by Nowack and Stone7 to investigate the adsorption of NTMP on slurry goethite (&#945;-FeOOH) at pH 4.6 and 7.2. However, their experiments on the pH-dependency of adsorption took place without buffering.
<img alt="Table 1" src="/files/ftp_upload/57618/57618table1.jpg" /> 
Table 1: pKa values20, theoretical oxygen demand (ThOD) and analyzed actual chemical oxygen demand (COD) of buffers used in this study.
Total P determination (ISOmini) adapted to the buffer solution 
Following each adsorption test, each solution must be analyzed for the residual phosphonate concentration. Only recently, a method for the determination of phosphonates in environmental samples with limits of quantification in the range of 0.1 &#181;g/L was introduced. It is based on the IC-ICP-MS method and the use of cation exchangers (for the conversion of phosphonates into &#34;free&#34; phosphonic acids) and anion exchangers (for the pre-concentration of phosphonates)21. Furthermore, already in 1997 a method from Nowack22 was introduced with higher limits of detection of 15-100 &#181;g/L, which is based on the pre-complexation of phosphonates with FeIII, retention using HPLC and the photometric detection of these complexes. However, these methods are very time-consuming and expensive. In studies with synthetic wastewater in which the only phosphorus-containing compound is a phosphonate, it is sufficient to determine the phosphonate concentration by determining the total P concentration. The determination of inorganic phosphate presents the experimenter with far fewer problems than the determination of total P, as the latter requires previous digestion. The amount of chemicals that have to be added priorly must be matched precisely to the compounds present in the sample.
The determination of phosphate is currently carried out mainly using the method introduced by Murphy and Riley23. This method is based on the spectrophotometric detection of an intensely colored phosphomolybdenum blue complex ([PSb2Mo12O40]&#8722; with &#955;max at 880 nm) which is formed in the presence of phosphate and acidified molybdate using ascorbic acid and antimony(III) as reducing agents24. In other studies, the optimal ratio of [H+]:[Mo] was determined to be 60-8025,26. In order to determine total P, digestion, i.e., the breaking of P-O-P, C-O-P and C-P bonds in phosphorus-containing compounds and the oxidation of phosphorus to phosphate must be carried out prior to the phosphomolybdenum blue formation24. Eisenreich et al.27 presented a simplified method based on the use of the oxidizing agent peroxodisulfate (K2S2O8) in the acidic milieu. Many of these findings have been incorporated into the development of ISO 687828, which systematically explains the procedure for the determination of phosphate-P and total P concentrations in water samples (wastewater and seawater).
The total P determination according to ISO 6878 (Figure 2) requires the sample to be digested in an Erlenmeyer flask by K2S2O8 at an acidic pH (use of sulfuric acid) for at least 30 min. After digestion, the pH value is set to 3-10 using NaOH and the content of the Erlenmeyer flask is transferred to a 50 mL volumetric flask. In this flask, ascorbic acid and an acidic solution containing molybdate and antimony are added to the sample and then filled with water. After 10-30 minutes, the intensity of this blue coloration is measured at a wavelength of 880 nm. In the case of phosphate determination, the digestion is omitted. This means, the sample is mixed in a 50 mL volumetric flask with ascorbic acid and a solution containing molybdate as well as antimony, and the intensity of the blue coloration is measured in the photometer.
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/57618/57618fig2.jpg" /> 
Figure 2: Procedure of total P determination according to ISO 6878 applying digestion using sulfuric acid and potassium peroxodisulfate, a subsequent pH adjustment with NaOH and coloration using ascorbic acid and molybdate-containing solutions. <a href="/files/ftp_upload/57618/57618fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
The procedure of total P determination is very complex since during digestion it must always be taken care of that the sample does not boil over and the adjustment of the sample to pH 3-10 takes a long time. In order to be able to analyze as many samples as possible in a very short time, a miniaturized form of the total P and ortho-phosphate determination was developed based on this ISO method. Figure 3 summarizes the individual steps of this method. In this miniaturized determination method (ISOmini), the final volume of the color solution is 10 mL (in the ISO method, this is 50 mL). Accordingly, the ISOmini method reduces the amount of the solutions to be used to one-fifth. In the ISOmini method, the digestion is carried out in a thermostat (in contrast to the ISO method, where digestion is proposed in an Erlenmeyer flask on a hotplate) at 148-150 &#176;C to obtain the highest possible oxidation. NaOH is added after digestion together with the ascorbic acid and acidic molybdate solution.
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/57618/57618fig3.jpg" /> 
Figure 3: Procedure of total P determination according to a modified and miniaturized form of ISO 6878 (ISOmini) using 10 mL screw cap vials, buffer-dependent potassium peroxodisulfate concentrations, heating in a thermostat and addition of color reagents directly to the digested sample without transferring it previously. <a href="/files/ftp_upload/57618/57618fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
The organic buffers contained in the samples must be present in relatively high concentrations (10 mM) in comparison to the phosphonate (5-30 &#181;M) in order to maintain the pH value effectively. These buffers must be digested for the analysis of the total P after the adsorption test. Accordingly, the dosed amount of oxidizing agent must be matched to each buffer, taking into account that too much oxidizing agent should not interfere with the formation of the color complex formed after digestion. In order to be able to estimate the K2S2O8 quantity required for the digestion of each buffer in the total P determination based on the analyzed chemical oxygen demand (COD), a comparison of how many electrons can be converted during the reduction of O2 and K2S2O8 is necessary:
O2 + 4 H+&#160;+ 4 e-&#160;&#8594; 2 H2O
S2O82-&#160;+ 2 e-&#160;&#8594; 2 SO42-
Thus, the oxidation of a particular molecule requires twice as many peroxodisulfate molecules as O2 molecules. Accordingly, in the case of a sample volume of 20 mL, the COD of the sample must not exceed 500 mg/L when using the ISO method. However, even in the case of MES, the Good buffer with the smallest molar mass from Table 1, already a COD of 2.4 g/L is present at a concentration of 10 mM. In addition to the step-by-step protocol of the adsorption test and ISOmini method, this paper, therefore, investigates the required buffer concentration, the influence of the buffers on phosphonate adsorption and the K2S2O8 quantity and NaOH dosage required for their digestion in the ISOmini method.
Freundlich model of adsorption 
Adsorption isotherms, i.e., loading q (e.g., in mg P/g adsorbent) applied over the dissolved concentration c (in mg/L P) of adsorptive after a specific contact time, can be modeled using the equation proposed by Freundlich29:
<img alt="Equation 1" src="/files/ftp_upload/57618/57618eq1.jpg" />
If the experimentally obtained values q and c are plotted in the form of a function ln(q) over ln(c), the slope of this function determined by linear regression corresponds to 1/n and the y-axis intercept to the KF value30.
Overview of the procedure 
The entire process for determining the adsorption capacity of granular ferric hydroxide with regard to phosphonates is divided into several steps and is described in the protocol section. For the analysis, it is necessary to prepare a sufficient amount of reagent solutions (Section 1 in the protocol). These are durable for several weeks. The phosphonate-containing solution is then prepared (Section 2), followed by the adsorption test (contact of the phosphonate solution with the granular material) (Section 3) and the analysis of the total P according to the miniaturized ISO method (Section 4).

<table border="0" cellpadding="0" cellspacing="0" style="width:1359px;" width="1359">
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:465px;">Sulfuric acid (H2SO4)</td>
			<td style="width:352px;">Merck (Darmstadt, Germany)</td>
			<td style="width:119px;">1120802510</td>
			<td style="width:423px;">98% (p.a.)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Hydrochloric acid (HCl)</td>
			<td>VWR Chemicals (Fontenay-sous-Bois, France)</td>
			<td style="width:119px;">20254.401</td>
			<td>32% (AnalaR NORMAPUR, p.a.)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Sodium hydroxide (NaOH)</td>
			<td>Merck (Darmstadt, Germany)</td>
			<td style="width:119px;">1064981000</td>
			<td>&#8805;99% (p.a.)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Citric acid monohydrate (CitOH&#8729;OH)</td>
			<td>VWR Chemicals (Fontenay-sous-Bois, France)</td>
			<td style="width:119px;">20276.292</td>
			<td>99.9% (AnalaR NORMAPUR, p.a.)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Acetic acid (AcOH)</td>
			<td>VWR Chemicals (Fontenay-sous-Bois, France)</td>
			<td style="width:119px;">20104.334</td>
			<td>100% (p.a.)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">2-(N-morpholino)ethanesulfonic acid (MES)</td>
			<td>SigmaAldrich (St. Louis, MO, USA)</td>
			<td style="width:119px;">M3671-250G</td>
			<td>&#8805;99%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">3-(N-morpholino)propanesulfonic acid (MOPS)</td>
			<td>SigmaAldrich (St. Louis, MO, USA)</td>
			<td style="width:119px;">M1254-250G</td>
			<td>&#8805;99.5%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">4-(2-Hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES)</td>
			<td>SigmaAldrich (St. Louis, MO, USA)</td>
			<td style="width:119px;">H3375-250G</td>
			<td>&#8805;99.5%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">4-(2-Hydroxyethyl)-1-piperazinepropanesulfonic acid (EPPS)</td>
			<td>SigmaAldrich (St. Louis, MO, USA)</td>
			<td style="width:119px;">E9502-250G</td>
			<td>&#8805;99.5%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">N-cyclohexyl-2-hydroxyl-3-aminopropanesulfonic acid (CAPSO)</td>
			<td>SigmaAldrich (St. Louis, MO, USA)</td>
			<td style="width:119px;">C2278-100G</td>
			<td>&#8805;99%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">N-cyclohexyl-3-aminopropanesulfonic acid (CAPS)</td>
			<td>SigmaAldrich (St. Louis, MO, USA)</td>
			<td style="width:119px;">C2632-250G</td>
			<td>&#8805;98%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">2-Phosphonobutane-1,2,4-tricarboxylic acid (PBTC)</td>
			<td>Zschimmer &#38; Schwarz (Mohsdorf, Germany)</td>
			<td style="width:119px;">CUBLEN P 50</td>
			<td>50 % technical</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">1-Hydroxyethane 1,1-diphosphonic acid monohydrate (HEDP&#183;H2O)</td>
			<td>SigmaAldrich (St. Louis, MO, USA)</td>
			<td style="width:119px;">54342-50G</td>
			<td>&#8805;95,0 %</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Nitrilotris(methylene phosphonic acid) (NTMP)</td>
			<td>SigmaAldrich (St. Louis, MO, USA)</td>
			<td style="width:119px;">72568-50G</td>
			<td>&#8805;97,0 %</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Ethylenediamine tetra(methylene phosphonic acid) (EDTMP&#183;1.4H2O)</td>
			<td>Zschimmer &#38; Schwarz (Mohsdorf, Germany)</td>
			<td style="width:119px;">-</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Diethylenetriamine penta(methylene phosphonic acid) (DTPMP&#183;6H2O)</td>
			<td>Zschimmer &#38; Schwarz (Mohsdorf, Germany)</td>
			<td style="width:119px;">-</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Potassium dihydrogen phosphate (KH2PO4)</td>
			<td>Merck (Darmstadt, Germany)</td>
			<td style="width:119px;">1048731000</td>
			<td>&#8805;99.5% (p.a.)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Potassium peroxodisulfate (K2S2O8)</td>
			<td>Merck (Darmstadt, Germany)</td>
			<td style="width:119px;">1050920250</td>
			<td>&#8805;99.0% (p.a.)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">L(+)-Ascorbic acid (C6H8O6)</td>
			<td>Merck (Darmstadt, Germany)</td>
			<td style="width:119px;">1004680500</td>
			<td>&#8805;99.7% (p.a.)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24&#183;4H2O)</td>
			<td>Merck (Darmstadt, Germany)</td>
			<td style="width:119px;">1011800250</td>
			<td>&#8805;99.0% (p.a.)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Potassium antimony-(III) oxide tartrate hemihydrate (K(SbO)C4H4O6&#8729;&#189;H2O)</td>
			<td>Merck (Darmstadt, Germany)</td>
			<td style="width:119px;">1080920250</td>
			<td>&#8805;99.5% (p.a.)</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Granular ferric hydroxide (GFH)</td>
			<td>Hego BioTec (Berlin, Germany)</td>
			<td style="width:119px;">-</td>
			<td>FerroSorp RW</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Syringe membrane filters</td>
			<td>Sartorius Stedim Biotech GmbH (G&#246;ttingen, Germany)</td>
			<td style="width:119px;">17765----------Q</td>
			<td>Minisart RC Hydrophilic 25 mm 0.45 &#956;m pore size</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Single-use syringes for membrane filtration</td>
			<td>Henke Sass Wolf (Tuttlingen, Germany)</td>
			<td style="width:119px;">5200.X00V0</td>
			<td>3-part Soft-Ject Luer 20 mL</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Rotator</td>
			<td>LLG Labware (Meckenheim, Germany)</td>
			<td style="width:119px;">6.263 660</td>
			<td>uniROTATOR2</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Clamp for rotator</td>
			<td>LLG Labware (Meckenheim, Germany)</td>
			<td style="width:119px;">6.263 664</td>
			<td>Clamp for uniROTATOR2</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Screw cap vial</td>
			<td>Glasger&#228;tebau Ochs (Bovenden, Germany)</td>
			<td style="width:119px;">135215</td>
			<td>Pr&#228;paratenglas Duran, 16x100 mm, thread GL18, cap with PTFE seal</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Micropipette</td>
			<td>Eppendorf (Hamburg, Germany)</td>
			<td style="width:119px;">3123000047</td>
			<td>eppendorf Research plus 10&#8211;100 &#181;L</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Micropipette</td>
			<td>Eppendorf (Hamburg, Germany)</td>
			<td style="width:119px;">3123000063</td>
			<td>eppendorf Research plus 100&#8211;1000 &#181;L</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Micropipette</td>
			<td>Eppendorf (Hamburg, Germany)</td>
			<td style="width:119px;">3123000071</td>
			<td>eppendorf Research plus 0.5&#8211;5 mL</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Precision balance</td>
			<td>Precisa Gravimetrics (Dietikon, Switzerland)</td>
			<td style="width:119px;">-</td>
			<td>Precisa LX 220 A SCS</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Thermostat</td>
			<td>Hach (Berlin, Germany)</td>
			<td style="width:119px;">LTV077</td>
			<td>HT200S High Temperature Thermostat</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Thermostat</td>
			<td>Merck (Darmstadt, Germany)</td>
			<td style="width:119px;">1712000001</td>
			<td>Spectroquant TR 320</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Spectrophotometer</td>
			<td>Jasco Labor- u. Datentechnik (Gro&#223;-Umstadt, Germany)</td>
			<td style="width:119px;">-</td>
			<td>UV/VIS Spectrophotometer Jasco V-550</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Centrifuge tube</td>
			<td>Sarstedt (N&#252;mbrecht, Germany)</td>
			<td style="width:119px;">62.559.001</td>
			<td>Tube 50 mL, 115x28 mm, flat/conical base PP, assembled cap</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">pH probe</td>
			<td>WTW (Weilheim, Germany)</td>
			<td style="width:119px;">103635</td>
			<td>WTW pH-Electrode SenTix 41</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">pH device</td>
			<td>WTW (Weilheim, Germany)</td>
			<td style="width:119px;">-</td>
			<td>WTW Multi 350i</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">COD determination</td>
			<td>Hach (Berlin, Germany)</td>
			<td style="width:119px;">LCK514</td>
			<td>100&#8211;2000 mg/L O2</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Sieve</td>
			<td>Retsch (Haan, Germany)</td>
			<td style="width:119px;">60.131.000500</td>
			<td>Test sieve 0.5 mm mesh (ISO 3310/1) stainless steel</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Drying cabinet</td>
			<td>Memmert (Schwabach, Germany)</td>
			<td style="width:119px;">-</td>
			<td>Modell 600</td>
		</tr>
	</tbody>
</table>

optimized procedure for determining the adsorption of phosphonates onto granular ferric hydroxide using a miniaturized phosphorus determination method

This paper introduces a procedure to investigate the adsorption of phosphonates onto iron-containing filter materials, particularly granular ferric hydroxide (GFH), with little effort and high reliability. The phosphonate, e.g., nitrilotrimethylphosphonic acid (NTMP), is brought into contact with the GFH in a rotator in a solution buffered by an organic acid (e.g., acetic acid) or Good buffer (e.g., 2-(N-morpholino)ethanesulfonic acid) [MES] and N-cyclohexyl-2-hydroxyl-3-aminopropanesulfonic acid [CAPSO]) in a concentration of 10 mM for a specific time in 50 mL centrifuge tubes. Subsequently, after membrane filtration (0.45 &#181;m pore size), the total phosphorus (total P) concentration is measured using a specifically developed determination method (ISOmini). This method is a modification and simplification of the ISO 6878 method: a 4 mL sample is mixed with H2SO4 and K2S2O8 in a screw cap vial, heated to 148-150 &#176;C for 1 h and then mixed with NaOH, ascorbic acid and acidified molybdate with antimony(III) (final volume of 10 mL) to produce a blue complex. The color intensity, which is linearly proportional to the phosphorus concentration, is measured spectrophotometrically (880 nm). It is demonstrated that the buffer concentration used has no significant effect on the adsorption of phosphonate between pH 4 and 12. The buffers, therefore, do not compete with the phosphonate for adsorption sites. Furthermore, the relatively high concentration of the buffer requires a higher dosage concentration of oxidizing agent (K2S2O8) for digestion than that specified in ISO 6878, which, together with the NaOH dosage, is matched to each buffer. Despite the simplification, the ISOmini method does not lose any of its accuracy compared to the standardized method.

Watch this Scientific Journal Video about Optimized Procedure for Determining the Adsorption of Phosphonates onto Granular Ferric Hydroxide using a Miniaturized Phosphorus Determination Method at JoVE

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

This paper introduces a procedure to investigate the adsorption of phosphonates onto iron-containing filter materials, particularly granular ferric hydroxide, with little effort and high reliability. In a buffered solution, the phosphonate is brought into contact with the adsorbent using a rotator and then analyzed via a miniaturized phosphorus determination method.

This paper introduces a procedure to investigate the adsorption of phosphonates onto iron-containing filter materials, particularly granular ferric ...

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