Determination of ‘labile’ phosphate in lake sediments using anion exchange resins: a critical evaluation

Abstract

Transfer of phosphate ions from sediment particles to an anion exchanger (Amberlite IR-48 or Duolit A378) has been used to determine the 'labile' (or 'available reactive') phosphorus content of several sah-water lake sediments. An evaluation of the procedure identified several sources of error, e.g. (i) incomplete transfer of P to the exchanger [due to competition for sites by other anions present in the dried sediments]; (ii) slow intermediate processes such as dissolution of inorganic phosphate compounds; (iii) sorbed P not fully retrieved from the exchanger resins [due to the limited efficiency (-90%) of the displacement process]; and (iv) absorbance readings in the colorimetric procedure used to determine phosphate can be enhanced by light scattered from fine particles. Accordingly, for the determination of the amount of 'labile' phosphate present in sediments it is recommended that one (a) stirs the exchanger/suspension system for at least 24 hours; (b) analyses both the aqueous phase and the resin phase; (c) extracts the resin phases at least twice with 1M NaCI; and (d) repeats the "transfer to resin" step after isolating the partly-washed sediment and re-suspending it in water. The contributions from steps (d) and the non-sorbed Pin the aqueous phase segment of (b) changed slightly with changes in the ratio of exchanger to sediment mass.

The Chemistry of Phosphate and Nitrogen Compounds in Sediments

“…For example, House et al (1995) used the iron-oxide stripping method, which might be promising as a fractionation method, with the difficulty that several extractions may be necessary, as tailing (i.e., each next fraction still containing a considerable quantity) will occur. (Waller & Pickering, 1992). They loaded a cation-exchange column with Al(OH) 3 , measured the amount of P-adsorbed, and compared this with P aa .…”

Fractionation and bioavailability of phosphates in lacustrine sediments: a review

“…They loaded a cationexchange column with aluminium hydroxide, measured the amount of phosphate adsorbed, and defined this as Pad. When using these techniques several posible sources of errors must be identified first: incomplete transfer of P to the exchanger, slow dissolution of PPdrt, incomplete retrieval from the exchanger, etc (Waller & Pickering, 1992). The conditions of these extractions must, therefore, be carefully checkedwhich is not often done.…”