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Effect of Colloidal Goethite and Kaolinite on Colorimetric Phosphate Analysis
Author(s) -
Anderson Sharon J.,
Sanders Keith E.,
Steyer Karyn J.
Publication year - 1996
Publication title -
journal of environmental quality
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.888
H-Index - 171
eISSN - 1537-2537
pISSN - 0047-2425
DOI - 10.2134/jeq1996.00472425002500060023x
Subject(s) - colloid , sorption , goethite , chemistry , phosphate , desorption , filtration (mathematics) , kaolinite , chromatography , centrifugation , adsorption , nuclear chemistry , inorganic chemistry , mineralogy , organic chemistry , statistics , mathematics
Filtration through 0.45‐µm 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 PO 4 sorption experiments, but colloids can be present in the supernatant solution. It is not known whether colloid‐sorbed PO 4 is detected colorimetrically in solutions that have not been pretreated to release colloidal PO 4 . The objectives of this research were to determine whether colloid‐sorbed PO 4 is detected colorimetrically and whether the type and size of colloid, the pH during PO 4 sorption, or the sorption reaction time affect the extent to which colloid‐sorbed PO 4 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 µ M PO 4 ) at pH 4.5 and 7 in 10 m M NaCl for 1 and 10 d. The concentration of PO 4 that was not detected colorimetrically in the presence of colloids (i.e., nonreactive PO 4 ) was calculated as the difference between PO 4 in colloid‐free blanks and colloid‐containing samples, whereas PO 4 sorption was measured in samples that had been ultrafiltered through a 4‐nm membrane to separate colloids from solution. Nonreactive PO 4 , which represents the fraction of sorbed PO 4 for which desorption kinetics are slow compared with the colorimetric analysis time, ranged from 2% to >35% of total PO 4 (P tot ) and from 20 to 100% of sorbed PO 4 , depending upon P tot , colloid type, and reaction time. Nonreactive PO 4 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 PO 4 . Nonreactive PO 4 was greater at pH 4.5 than 7 and was greater for goethite than kaolinite. Nonreactive PO 4 was independent of P tot , although sorbed PO 4 increased with increasing P tot . Thus, each type of colloid apparently has a finite capacity to retain sorbed PO 4 in nonreactive sites. Because sorbed PO 4 is not uniformly detected colorimetrically, accurate quantitation of the colloid‐sorbed PO 4 will require additional pretreatmeats such as dissolution of the colloids in strong acid.