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Background:

In order to separate the different phosphorus (P) species in lake or marine sediments, one can use sequential P fractionation method. Its main idea is to use extractants with weak or strong affinity to P to extract the sediment sample step by step. By doing this, different operationally defined P fractions are extracted sequentially.

There are many methods available, and my question is about the method proposed by Hieltjes & Lijklema (1980). Its procedure is shown in the first screenshot below.

Question:

I'm wondering why Fe-P is in the "NaOH extraction" part, which is "17 hours with 50 ml 0.1N NaOH".

In the Table 2 (see second screenshot below) of the original paper (Hieltjes & Lijklema, 1980), we can see that 98.9% of Fe-P is extracted from the 0.1 N NaOH solution. The authors mentioned that this synthetic Fe&P compounds is "... a suspension of freshly precipitated ironhydroxy-phosphate, aged by standing or moderate heating." However, they didn't explain why this synthetic Fe hydroxides can be dissolved in the NaOH solution.

I understand that Al oxides can be dissolved by NaOH, so Al-P is in the "NaOH extraction" part. But Fe oxides can not be dissolved by NaOH, for example in this posted question. Then why Fe-P is in this extration?

Fig.1

Fig.3

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  • $\begingroup$ Why don't you give us more information about the problem ? You say that you will extract phosphate from "sediments". Well ! Which sort of sediment ? Organic sediments ? Silicate sediments ? Both ? Anything in Waste Water ? $\endgroup$
    – Maurice
    Dec 4, 2022 at 10:21
  • $\begingroup$ @Maurice Thanks Maurice! The question has been updated now, adding the Background part and one table from the orignal paper for better explanation. $\endgroup$
    – T X
    Dec 4, 2022 at 11:32
  • $\begingroup$ @ TX. You promise to show the process in Figure 1 and 2 below. And there is no figures below !! Apart from this, you gave us one Table $1$, and two Table $2$. Yes two ! Then, the table $1$ is no use for the present problem. The second table $2$ says that $98.9$% of the phosphate can be recovered form $\ce{FePO4}$ by $\ce{NaOH}$ treatment. The authors do not say that the iron is also dissolved. In my opinion, the iron is transformed into insoluble $\ce{Fe(OH)3}$ which remains in suspension in the liquid. $\endgroup$
    – Maurice
    Dec 4, 2022 at 15:09
  • $\begingroup$ @Maurice Thanks for your proposal about the Fe-P extraction mechanism! It reminds me that maybe PO4 ion on the solid Fe is replaced by OH ion, thus NaOH procedure can "dissolve" Fe-bound P when solid Fe itself is not dissolved? (though I'm not sure since I'm not major in chemistry) And I have edited my question to remove one repetitve table, and use the word "screenshot" instead of "figure" to avoid misunderstanding. $\endgroup$
    – T X
    Dec 5, 2022 at 4:47
  • $\begingroup$ @ TX. Your text about dissolution of hydrated ferric phosphate can be considered as a repetition of my last comment 18 hours ago $\endgroup$
    – Maurice
    Dec 5, 2022 at 9:25

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