Effects of humic substances on Fe(II) sorption onto aluminum oxide and clay

Abstract

We studied the effects of humic substances (HS) on the sorption of Fe(II) onto Al-oxide and clay sorbents at pH 7.5 with a combination of batch kinetic experiments and synchrotron Fe K-edge EXAFS analyses. Fe(II) sorption was monitored over the course of 4 months in anoxic clay and Al-oxide suspensions amended with variable HS types (humic acid, HA; or fulvic acid, FA) and levels (0, 1, and 4 wt%), and with differing Fe(II) and HS addition sequences (co-sorption and pre-coated experiments, where Fe(II) sorbate was added alongside and after HS addition, respectively). In the Al-oxide suspensions, the presence of HS slowed down the kinetics of Fe(II) sorption, but had limited, if any, effect on the equilibrium aqueous Fe(II) concentrations. EXAFS analyses revealed precipitation of Fe(II)–Al(III)-layered double hydroxide (LDH) phases as the main mode of Fe(II) sorption in both the HA-containing and HA-free systems. These results demonstrate that HS slow down Fe(II) precipitation in the Al-oxide suspensions, but do not affect the composition or stability of the secondary Fe(II)–Al(III)-LDH phases formed. Interference of HS with the precipitation of Fe(II)–Al(III)-LDH was attributed to the formation organo-Al complexes HS limiting the availability of Al for incorporation into secondary layered Fe(II)-hydroxides. In the clay systems, the presence of HA caused a change in the main Fe(II) sorption product from Fe(II)–Al(III)-LDH to a Fe(II)-phyllosilicate containing little structural Al. This was attributed to complexation of Al by HA, in combination with the presence of dissolved Si in the clay suspension enabling phyllosilicate precipitation. The change in Fe(II) precipitation mechanism did not affect the rate of Fe(II) sorption at the lower HA level, suggesting that the inhibition of Fe(II)–Al(III)-LDH formation in this system was countered by enhanced Fe(II)-phyllosilicate precipitation. Reduced rates of Fe(II) sorption at the higher HA level were attributed to surface masking or poisoning by HA of secondary Fe(II) mineral growth at or near the clay surface. Our results suggest that HS play an important role in controlling the kinetics and products of Fe(II) precipitation in reducing soils, with effects modulated by soil mineralogy, HS content, and HS properties. Further work is needed to assess the importance of layered Fe(II) hydroxides in natural reducing environments.

Interactions of ferrous iron with clay mineral surfaces during sorption and subsequent oxidation

“…S3 †), e.g., by 0.02 mol Fe per kg clay at pH 7. This observation is consistent with ndings by Zhu et al, 34 who observed an increase in Fe 2+ sorption to Syn-1 during 5 days at pH 7.5. Such increases of Fe 2+ sorption suggest that slow sorption processes were taking place.…”

Surface precipitation of Mn2+ on clay minerals enhances Cd2+ sorption under anoxic conditions

“…At elevated concentrations, many divalent metal cations (Me 2+ , rst-row transition series) have been shown to precipitate as Me 2+ Al 3+ -layered double hydroxides (LDH) under alkaline conditions in presence of Al-containing minerals. [19][20][21][22][23][24][25] Starcher et al demonstrated that the FeAl-LDH phase formed during Fe 2+ sorption to an Al-bearing sorbent can sorb or incorporate Zn 2+ when both metals were co-sorbed to the Albearing mineral. 26 However, Bhattacharya and Elzinga demonstrated that dissolved Mn 2+ concentrations signicantly higher than normally encountered in anoxic soils (i.e., [Mn 2+ ] > 10 À2 M) are required to precipitate Mn 2+ Al 3+ -LDH, making the precipitation of this phase unlikely.…”

A Comparison of the Solubility Products of Layered Me(II)–Al(III) Hydroxides Based on Sorption Studies with Ni(II), Zn(II), Co(II), Fe(II), and Mn(II)

“…Since these compounds are common in soils and sediments, they may hinder the formation of MeCO 3(s) in these systems even when substantial supersaturation occurs [64,65]. In contrast, LDH phases have been shown to form in whole soils [36][37][38][39][40][41][42][43][44][45] as well in model systems containing humic substances and arsenate (the chemical analogue of phosphate) [28,[72][73][74][75]. This suggests that LDH formation is not particularly susceptible to interference by foreign solutes.…”