The oxidation of dissolved Fe(ii) in near-neutral natural waters leads to the formation of Fe(iii)-precipitates. Organic ligands, PO₄, calcium (Ca) and other solutes affect the composition, bulk and nanoscale structure and colloidal properties of Fe(iii)-precipitates and their impacts on co-precipitated compounds in interdependent ways. In this study, we quantified the effects of four low molecular weight organic acids (LMWOAs) with different Fe(iii) complexation strengths (2,4-dihydroxybenzoic acid (2,4-DHB) ∼ galacturonic acid (Galact) ≪ 3,4-dihydroxybenzoic acid (3,4-DHB) < citric acid (Citr)) and of leonardite humic acid (LH) in combination with PO₄ and Ca on Fe(iii)-precipitate structure and composition in a multifactorial experiment. Precipitates were synthesized by oxidation of 0.5 mM Fe(ii) in bicarbonate-buffered solutions at pH ∼ 7, at molar (OC/Fe) ratios of 0.1 to 9.6, at molar (PO₄/Fe) ratios of 0.05 and 0.25, and without/with 4 mM Ca. OC-free controls consisted of amorphous (Ca)Fe(iii)-phosphate and ferrihydrite (Fh) aggregated in core-shell nanoparticles with lepidocrocite (Lp) crystallites attached to their surface, reflecting their formation sequence. The LMWOAs promoted the formation of OC-loaded Fh at the expense of Lp and thereby led to more effective PO₄ retention and lower residual dissolved PO₄. This effect increased with ligand concentration and Fe(iii) complexation strength. At a higher PO₄ level, the co-precipitation of all LMWOAs was reduced, reflecting the stronger affinity of PO₄ for Fe(iii). At higher (OC/Fe), the strong ligands 3,4-DHB and Citr increasingly co-precipitated with PO₄ into (Ca)-Fe(iii)-PO₄-OC nanoparticles and led to the formation of Fe(iii)-colloids and Fe(iii)-organic complexes that passed 0.2 μm filter membranes at the highest tested (OC/Fe) levels. Macromolecular LH (humic acid) had similar effects on Fe coordination and PO₄ uptake to 2,4-DHB and Galact, but distinct effects on the nanoscale precipitate structure and colloidal properties. Ca enhanced the co-precipitation of PO₄ and OC with Fe(iii), the aggregation of Fe(iii)-precipitate particles, and LH coagulation. Collectively, the insights presented in this study highlight the need to take the type of OC, its concentration relative to Fe, and interdependent effects of OC, PO₄, SiO₄, and Ca into account when assessing the formation of Fe(iii)-precipitates by Fe(ii) oxidation in near-neutral natural waters and their impacts on the cycling of PO₄, OC and other co-precipitating compounds.

Exfiltration of anoxic phosphate-rich groundwater into surface water leads to the oxidation of dissolved Fe(II) and the formation of Fe(III)-precipitates that can retain phosphate (PO₄) and thereby attenuate eutrophication. Fresh Fe(III)-precipitates transform into more stable phases over time, and retained PO₄ may be released again. In parallel, CO₂ outgassing can promote the formation of Ca-phosphates or -carbonates that also sequester PO₄. In laboratory experiments, we studied the effects of Mg, Ca, silicate (SiO₄) and PO₄ on these processes. Fresh Fe(III)-precipitates were formed in bicarbonate-buffered aqueous solutions at pH ∼ 7.0 via the oxidation of 0.5 mM Fe(II) in the presence of 0.15 or 0.025 mM PO₄, at Mg or Ca concentrations of 0, 0.4, 1.2 or 4 mM and in the absence or presence of 0.5 mM SiO₄. After CO₂ outgassing, the suspensions were allowed to age for 100 d at pH ∼ 8. Changes in the composition and structure of Fe(III)- and Ca-precipitates over time were probed with spectroscopic and microscopic techniques and were linked to variations in the retention of PO₄. The oxidation of Fe(II) led to effective PO₄ removal via the formation of Fe(III)-precipitates that consisted of amorphous (Ca-)Fe(III)-phosphate ((Ca)FeP), ferrihydrite (Fh) and, in SiO₄-free treatments, lepidocrocite (Lp). During aging, FeP and Fh that had formed in the absence of Mg, Ca and SiO₄ rapidly and nearly completely transformed into Lp. Via effects on molecular- and nanoscale precipitate structure, Mg slowed down FeP transformation into Fh, stabilized Fh, and decreased the crystallinity of Lp (in SiO₄-free suspensions), Ca stabilized CaFeP against transformation into Fh, and SiO₄ stabilized Fh and (Ca)FeP. Core/shell CaFeP/Fh particles formed in electrolytes that contained Ca and SiO₄ hardly transformed within 100 d. Calcite only formed at low dissolved PO₄ concentrations and, by incorporation of PO₄, contributed to PO₄ retention. Higher levels of dissolved PO₄ inhibited calcite formation but could induce Ca-phosphate precipitation. Differences in precipitate formation and transformation pathways and kinetics were reflected in the extents of PO₄ release over the 100-d aging period, ranging from rapid release of 77% of the total PO₄ in the treatment without Mg, Ca and SiO₄ at 0.15 mM total PO₄ to slow release of only 0.1% of the total PO₄ at initial concentrations of 4 mM Ca, 0.5 mM SiO₄, and 0.025 mM PO₄. In summary, this study reveals the conditions and the extents and timescales over which Fe(III)- and Ca-precipitates form and transform and how these processes affect PO₄ immobilization in near-neutral natural waters. The detailed new insights into the coupling between Fe(III)- and Ca-precipitate formation and into the interdependent effects of Mg, Ca, SiO₄ and PO₄ are not only relevant with respect to PO₄ but also with respect to the cycling of trace elements in natural and engineered systems.

The adsorption of thallium (Tl) onto phyllosilicate minerals plays a critical role in the retention of Tl in soils and sediments and the potential transfer of Tl into plants and groundwater. Especially micaceous minerals are thought to strongly bind monovalent Tl(_I), in analogy to their strong binding of Cs. To advance the understanding of Tl(_I) adsorption onto phyllosilicate minerals, we studied the adsorption of Tl(_I) onto Na- and K-saturated illite and Na-saturated smectite, two muscovites, two vermiculites and a naturally Tl-enriched soil clay mineral fraction. Macroscopic adsorption isotherms were combined with the characterization of the adsorbed Tl by X-ray absorption spectroscopy (XAS). In combination, the results suggest that the adsorption of Tl(_I) onto phyllosilicate minerals can be interpreted in terms of three major uptake paths: (i) highest-affinity inner-sphere adsorption of dehydrated Tl⁺ on a very low number of adsorption sites at the wedge of frayed particle edges of illite and around collapsed zones in vermiculite interlayers through complexation between two siloxane cavities, (ii) intermediate-affinity inner-sphere adsorption of partially dehydrated Tl⁺ on the planar surfaces of illite and muscovite through complexation onto siloxane cavities, (iii) low-affinity adsorption of hydrated Tl⁺, especially in the hydrated interlayers of smectite and expanded vermiculite. At the frayed edges of illite particles and in the vermiculite interlayer, Tl uptake can lead to the formation of new wedge sites that enable further adsorption of dehydrated Tl⁺. On the soil clay fraction, a shift in Tl(_I) uptake from frayed edge sites (on illite) to planar sites (on illite and muscovite) was observed with increasing Tl(_I) loading. The results from this study show that the adsorption of Tl(_I) onto phyllosilicate minerals follows the same trends as reported for Cs and Rb and thus suggests that concepts to describe the retention of (radio)cesium by different types of phyllosilicate minerals in soils, sediments and rocks are also applicable to Tl(_I).

Arsenic (As) is a toxic element, and elevated levels of geogenic As in drinking water pose a threat to the health of several hundred million people worldwide. In this study, we used microfluidics in combination with optical microscopy and X-ray spectroscopy to investigate zerovalent iron (ZVI) corrosion, secondary iron (Fe) phase formation, and As retention processes at the pore scale in ZVI-based water treatment filters. Two 250 μm thick microchannels filled with single ZVI and quartz grain layers were operated intermittently (12 h flow/12 h no-flow) with synthetic groundwater (pH 7.5; 570 μg/L As(III)) over 13 and 49 days. Initially, lepidocrocite (Lp) and carbonate green rust (GRC) were the dominant secondary Fe-phases and underwent cyclic transformation. During no-flow, lepidocrocite partially transformed into GRC and small fractions of magnetite, kinetically limited by Fe(II) diffusion or by decreasing corrosion rates. When flow resumed, GRC rapidly and nearly completely transformed back into lepidocrocite. Longer filter operation combined with a prolonged no-flow period accelerated magnetite formation. Phosphate adsorption onto Fe-phases allowed for downstream calcium carbonate precipitation and, consequently, accelerated anoxic ZVI corrosion. Arsenic was retained on Fe-coated quartz grains and in zones of cyclic Lp-GRC transformation. Our results suggest that intermittent filter operation leads to denser secondary Fe-solids and thereby ensures prolonged filter performance.

The sorption of thallium (Tl) onto manganese (Mn) oxides critically influences its environmental fate and geochemical cycling and is also of interest in water treatment. Combined quantitative and mechanistic understanding of Tl sorption onto Mn oxides, however, is limited. We investigated the uptake of dissolved Tl(I) by environmentally relevant phyllo- and tectomanganates and used X-ray absorption spectroscopy to determine the oxidation state and local coordination of sorbed Tl. We show that extremely strong sorption of Tl onto vacancy-containing layered δ-MnO₂ at low dissolved Tl(I) concentrations (log K_d ≥ 7.4 for ≤10^-8 M Tl(I); K_d in (L/kg)) is due to oxidative uptake of Tl and that less specific nonoxidative Tl uptake only becomes dominant at very high Tl(I) concentrations (>10^-6 M). Partial reduction of δ-MnO₂ induces phase changes that result in inhibited oxidative Tl uptake and lower Tl sorption affinity (log K_d 6.2-6.4 at 10^-8 M Tl(I)) and capacity. Triclinic birnessite, which features no vacancy sites, and todorokite, a 3 × 3 tectomanganate, bind Tl with lower sorption affinity than δ-MnO₂, mainly as hydrated Tl⁺ in interlayers (triclinic birnessite; log K_d 5.5 at 10^-8 M Tl(I)) or tunnels (todorokite). In cryptomelane, a 2 × 2 tectomanganate, dehydrated Tl⁺ replaces structural K⁺. The new quantitative and mechanistic insights from this study contribute to an improved understanding of the uptake of Tl by key Mn oxides and its relevance in natural and engineered systems.

Iron(III)-precipitates formed by the oxidation of dissolved Fe(II) are important sorbents for major and trace elements in aquatic and terrestrial systems. Their reductive dissolution in turn may result in the release of associated elements. We examined the reductive dissolution kinetics of an environmentally relevant set of Fe(II)-derived arsenate-containing Fe(III)-precipitates whose structure as function of phosphate (P) and silicate (Si) content varied between poorly-crystalline lepidocrocite, amorphous Fe(III)-phosphate, and Si-containing ferrihydrite. The experiments were performed with 0.2–0.5 mM precipitate-Fe(III) using 10 mM Na-ascorbate as reductant, 5 mM bipyridine as Fe(II)-complexing ligand, and 10 mM MOPS/5 mM NaOH as pH 7.0 buffer. Times required for the dissolution of half of the precipitate (t_50%) ranged from 1.5 to 39 h; spanning a factor 25 range. At loadings up to ~ 0.2 P/Fe (molar ratio), phosphate decreased the t_50% of Si-free precipitates, probably by reducing the crystallinity of lepidocrocite. The reductive dissolution of Fe(III)-phosphates formed at higher P/Fe ratios was again slower, possibly due to P-inhibited ascorbate binding to precipitate-Fe(III). The slowest reductive dissolution was observed for P-free Si-ferrihydrite with ~ 0.1 Si/Fe, suggesting that silicate binding and polymerization may reduce surface accessibility. The inhibiting effect of Si was reduced by phosphate. Dried-resuspended precipitates dissolved 1.0 to 1.8-times more slowly than precipitates that were kept wet after synthesis, most probably because drying enhanced nanoparticle aggregation. Variations in the reductive dissolution kinetics of Fe(II) oxidation products as reported from this study should be taken into account when addressing the impact of such precipitates on the environmental cycling of co-transformed nutrients and contaminants.

We investigated the adsorption of Tl⁺ onto purified Illite du Puy (IdP). Distribution coefficients (K_d) for trace Tl adsorption indicated a moderate pH-dependence from pH 2.5 to 11. Adsorption isotherms measured at Tl⁺ concentrations from 10^–9 to 10^–2 M at near-neutral pH on illite saturated with Na⁺ (100 mM), K⁺ (1 and 10 mM), NH₄⁺ (10 mM) or Ca²⁺ (5 mM) revealed a high adsorption affinity of Tl⁺ in Na⁺- and Ca²⁺-electrolytes and strong competition with K⁺ and NH₄⁺. Cation exchange selectivity coefficients for Tl⁺ with respect to Na⁺, K⁺, NH₄⁺, and Ca²⁺ were derived using a 3-site sorption model. They confirmed the strong adsorption of Tl⁺ at the frayed edges of illite, with Tl selectivity coefficients between those reported for Rb⁺ and Cs⁺. X-ray absorption spectra of Tl adsorbed onto Na-exchanged IdP indicated a shift from adsorption of (dehydrated) Tl⁺ at the frayed edges at low loadings to adsorption of (hydrated) Tl⁺ on planar sites at the highest loadings. Our results suggest that illite is an important adsorbent for Tl in soils and sediments, considering its often high abundance and its stability relative to other potential adsorbents and the selective nature of Tl⁺ uptake by illite.

The co-precipitation of arsenate (As(V)) with Fe(III)-precipitates is of great importance in water treatment and critically affects the fate of As in environmental systems. We studied the effects of dissolved phosphate (P; 0-1 mM), silicate (Si; 0 or 0.5 mM) and Ca (0, 0.5 and 4 mM) on the sequestration of 7 μM As(V) by Fe(III)-precipitates formed by the oxidation of 0.5 mM Fe(II) in aerated bicarbonate-buffered solutions with an initial pH of 7.0 as well as the retention or release of As(V) after precipitate aging for 30 d at 40 °C. Dissolved As(V) concentrations in fresh precipitate suspensions greatly varied as a function of the initial dissolved P/Fe ratio ((P/Fe)_init) and the concentrations of Ca and Si. Limited As(V) removal was observed at (P/Fe)_init that exceeded the critical ratio (P/Fe)_crit above which exclusively (Ca-)Fe(III)-phosphate forms. Effective As(V) removal was observed at (P/Fe)_init < (P/Fe)_crit, where initial formation of (Ca-)Fe(III)-phosphate is followed by the formation of Si-ferrihydrite in Si-containing electrolytes and of poorly-crystalline lepidocrocite and hydrous ferric oxide in the Si-free electrolytes. The retention of As(V) and P by fresh Fe(III)-precipitates was most effective in systems containing both Ca and Si. In the Si- and Ca-free electrolytes at (P/Fe)_init of ~0.2–0.6, the rapid onset of precipitate aging with conversion of Fe(III)-phosphate to ferrihydrite resulted in a substantial remobilization of As(V) (up to 55% of initially precipitated As(V)). Ca reduced As remobilization during aging by stabilizing Ca-Fe(III)-phosphate and promoting Ca-phosphate formation, and Si by stabilizing Si-ferrihydrite against transformation. Consequently, also after aging, the lowest dissolved As(V) and P fractions were observed in precipitate suspensions containing both Ca and Si.