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

Abstract

<p>Under anoxic conditions, clay minerals sorb dissolved Mn(<sc>ii</sc>) predominantly by forming surface precipitates, which increase Cd sorption.</p>

The Effect of Aeration on Mn(II) Sorbed to Clay Minerals and Its Impact on Cd Retention

“…It was demonstrated by Van Groeningen et al that the addition of Mn 2+ increased Cd 2+ sorption in anoxic suspensions of both clays, KGa-1b and Syn-1. As shown in Figure , the aeration of the anoxic samples led to the release of Cd to solution, which increased with oxic equilibration time.…”

“…To understand the effect of aeration on the speciation of Mn, it is important to know how Mn is sorbed under anoxic conditions prior to O 2 exposure. In our previous work, we showed that under anoxic conditions, Mn 2+ sorbed to the clay minerals predominantly through the formation of Mn­(II)-bearing solid phases, although bulk solutions were undersaturated with respect to known Mn phases. LCF of Mn K-edge XANES spectra revealed that in the presence of KGa-1b, Mn was removed from solution by forming mainly MnAl-LDH and pyrochroite (Mn­(OH) 2 ) and in the presence of Syn-1, by forming MnAl-LDH and rhodonite (MnSiO 3 ) (Figure ).…”

“…The precipitation of Mn­(III,IV)-(oxyhydr)­oxides as expected by thermodynamic calculations (Figure S3) is paired with a release of protons and explains the observed decrease in suspension pH upon aeration (Table S3). The desorption of adsorbed Mn 2+ from clay surfaces as a result of the decrease in suspension pH is expected to be negligible for our samples, as sorption of Mn 2+ in the anoxic clay suspension was predominantly through the formation of Mn­(II) phases …”

“…In water-saturated soils, Mn 2+ can become a major cation with total dissolved Mn 2+ concentrations in the lower millimolar range and may represent an important competitor for adsorption of trace element cations (e.g., Zn 2+ , Cd 2+ ) to mineral surfaces. Yet, Van Groeningen et al demonstrated that in clay suspensions, elevated Mn 2+ concentrations did not cause a net release of Cd 2+ due to competition but instead increased Cd 2+ sorption due to the formation of Mn­(II) surface precipitates, which adsorbed or incorporated Cd . Similarly, several studies have shown that during sorption of metals onto Al-containing solid phases, Me­(II)­Al-layered double hydroxides (LDH; Me = first-row transition metal) formed in pH-neutral to alkaline conditions and sorbed or incorporated contaminants. − As soil pH develops toward circumneutral values during anoxia, precipitation of Me­(II)-incorporating phases may significantly alter the sorption of trace metals in anoxic systems.…”

“…Similarly, fast oxidation of the anoxically formed Mn­(II)-containing LDH may occur. The oxidative transformation of Mn­(II)-containing LDH may also influence the retention of trace metals as Mn­(II)-bearing solid phases formed in anoxic clay suspension were reported to be sorbents for trace elements such as Cd 2+ . However, direct experimental evidence on the impact of oxidation is lacking.…”