Adsorption of Alkaline Earth, Transition, and Heavy Metal Cations by Hydrous Oxide Gels of Iron and Aluminum

Abstract

Freshly precipitated Fe and Al gels (points of zero change at pH 8.1 and 9.4, respectively) strongly specifically adsorb divalent cations from 1M NaNO3 solution. Adsorption from a mixed solution of eight divalent cations (each 0.125 × 10‐3M) in suspensions of freshly precipitated Fe and Al gels (0.093M with respect to Fe or Al) was measured as a function of pH in 1M NaNO3. The selectivity sequence (lower pH = greater selectivity) for the retention of the alkaline earth cations by Fe gel was Ba > Ca > Sr > Mg, but for the Al gel was Mg > Ca > Sr > Ba. The selectivity sequence (Figures in parentheses indicate pH ± 0.2 for 50% retention) for the Fe gel was: Pb (3.1) > Cu(4.4) > Zn(5.4) > Ni(5.6) > Cd(5.8) > Co(6.0) > Sr(7.4) > Mg(7.8), whereas the sequence for the Al gel was: Cu(4.8) > Pb(5.2) > Zn(5.6) > Ni(6.3) > Co(6.5) > Cd(6.6) > Mg(8.1) > Sr(9.2). Significant adsorption occurred even when the extent of cation hydrolysis was much < 1%, and invariably occurred at a pH lower than that for hydroxide precipitation. Although the adsorption‐pH sequences are related to cation hydrolysis and hydroxide precipitation pH values, the relationship is far from perfect, as is evidenced by the different sequences for the two gels. On aging of the Al gel in the presence of alkaline earth cations, the retention of Mg increased, while that of Ca, Sr, and Ba decreased. This result was thought to result from the structural incorporation of some Mg and the exclusion of the other cations.

Copper adsorption as a function of solution parameters of variable charge soils

“…Surface functional groups of Fe, Al and Mn oxides are very selective for Cu adsorption, making highly stable inner-sphere complexes. 17,18,60,61 …”

“…12 Organic complexes are particularly important in Cu retention by the soil under a wide variety of conditions, [13][14][15][16] that influence the adsorption 7 and desorption of the metal. 17 Copper adsorption was studied in pure systems, such as Fe and Al (oxy)hydroxides (hematite and goethite) [18][19][20] and clay minerals (kaolinite, illite etc), [21][22][23] and in complex systems such as soil, under natural, agricultural or waste application conditions. 7,[24][25][26] Metal adsorption by soil (oxy) hydroxides has been identified as the main mechanism by which the metals are retained by the inorganic constituents of weathered soils.…”

Adsorption and Precipitation of Aqueous Zn(II) on Alumina Powders

“…The sorption of dissolved Zn(II) onto hydrous metal oxides, clays, and organic matter has been observed to play an important role in controlling the dissolved zinc concentrations in soils and natural waters, but the underlying chemistry is poorly understood (13)(14)(15)(16)(17)(18). A number of macroscopic investigations of Zn(II) sorption behavior on Al and Fe (hydr)oxides have observed strong uptake of Zn(II) well below saturation with respect to homogeneous precipitation, and further suggest that Zn(II) is bound as an inner-sphere complex due to the negligible salt effect and strong uptake at pHs below the point-of-zero charge of the solids (19)(20)(21)(22)(23)(24)(25)(26). Although several adsorption reactions have been postulated for modeling the uptake of Zn(II) to various metal-(hydr)oxides, including both inner-sphere monodentate and bidentate species (21,24,(26)(27)(28)(29)(30)(31), little work has been done on the structural identification of sorption products.…”

Determination of trace elements bound to soil and sediment fractions (IUPAC Technical Report)

“…In addition to other differences, the wide range of cations used suggests that most methods do not address any "specifically sorbed" fraction, and, likely even do not extract the same operationally defined solubility class. To extract specifically sorbed trace metals, Pb(NO 3 ) 2 seems to be most adequate, due to its low pK (7.7) and large atomic radius, and it is being effective in displacing other trace metals, i.e., Cd (pK = 10.1), Ni (pK = 9.9), Co (pK = 9.7), Zn (pK = 9.0) and Cu (pK = 7.7), with smaller atomic radius than Pb [134]. Pb(NO 3 ) 2 was found to extract less metal than HAc, probably indicating that the later was more specific [135].…”