Effect of colloidal goethite and kaolinite on colorimetric phosphate analysis

Filtration through 0.45-mu m filters provides a convenient and widely used operational separation between dissolved and particulate P, yet colloidal P is included in the filtrate with true dissolved P. In addition, centrifugation is often used to separate solids from solution during PO4 sorption experiments, but colloids can be present in the supernatant solution. It is not known whether colloid-sorbed PO4 is detected colorimetrically in solutions that have not been pretreated to release colloidal PO4. The objectives of this research were to determine whether colloid-sorbed PO4 is detected colorimetrically and whether the type and size of colloid, the pH during PO4 sorption, or the sorption reaction time affect the extent to which colloid-sorbed PO4 is detected colorimetrically. To accomplish these objectives, 10 mg L(-1) suspensions of 40- to 100-nm and 100- to 450-nm kaolinite and goethite were reacted with sodium phosphate solutions (4, 8, and 16 mu M PO4 at pH 4.5 and 7 in 10 mM NaCl for 1 and 10 d. The concentration of PO4 that was not detected colorimetrically in the presence of colloids (i.e., nonreactive PO4) was calculated as the difference between PO4 in colloid-free blanks and colloid-containing samples, whereas PO4 sorption was measured in samples that had been ultrafiltered through a 4-nm membrane to separate colloids from solution. Nonreactive PO4, which represents the fraction of sorbed PO4 for which desorption kinetics are slow compared with the colorimetric analysis time, ranged from 2% to >35% of total PO4 (P-tot) and from 20 to 100% of sorbed PO4, depending upon P-tot, colloid type, and reaction time. Nonreactive PO4 was about two times greater after 10-d than 1-d reaction and about 1.4 times greater for large than small colloids, even though small colloids sorbed more PO4. Nonreactive PO4 was greater at pH 4.5 than 7 and was greater for goethite than kaolinite. Nonreactive PO4 was independent of P-tot, although sorbed PO4 increased with increasing P-tot. Thus, each type of colloid apparently has a finite capacity to retain sorbed PO4 in nonreactive sites. Because sorbed PO4 is not uniformly detected colorimetrically, accurate quantitation of the colloid-sorbed PO4 will require additional pretreatments such as dissolution of the colloids in strong acid.