Inorganic anion sorption and interactions with phosphate sorption by hydrous ferric oxide gel

Abstract

SUMMARY The sorption of inorganic anions by hydrous ferric oxide gel (Fe gel) from 10 −1 M NaClO 4 at pH 6.5 decreased in the order: orthophosphate (H 2 PO 4 )>arsenate (H 2 AsO 4 ) = selenite (HseO3) > silicate (H 4 SiO 4 ) > molybdate (MoO 2 4 − ) > sulphate (SO 2 4 − ) > selenate(SeO 2 4 − )>chloride (Cl − ) = nitrate (NO − 3 ). When each anion was added to Fe gel with an equimolar quantity of H 2 PO − 4 , there was no detectable effect of SO 2 4 − , SeO 2 4 − , Cl − , and NO − 3 on the amount of H 2 PO − 4 sorbed. Other anions depressed H 2 PO 4 sorption in the order H 2 AsO 4 >HseO 3 > H 4 SiO 4 > MoO 2 4 . At the lowest level of anion addition (500 mmol kg −1 ), H 2 PO 4 sorption was depressed by no more than 6% of the sorption level in the absence of a competing anion. In contrast, at the highest level of anion addition (1700 mmol kg‐ 1 of each), H 2 AsO 4 decreased H 2 PO 4 sorption by 44%. The sorption of SO 4 − was completely eliminated when this anion was added with equimolar amounts of H 2 PO 4 . The ability of anions to compete with H 2 PO 4 for sorption sites could not be explained solely by the results obtained for the sorption of each anion alone. Thus, H 2 AsO 4 was more competitive than H 2 PO 4 when added together, even though more H 2 PO 4 than H 2 AsO 4 was sorbed when each anion was added alone. Although H 2 PO 4 was sorbed in larger amounts, there is no evidence to suggest that H 2 PO 4 . H 2 AsO 4 , and HseO 3 were sorbed on different sites.