During our sulfurization experiments, As was desorbed from As-loaded model peat in all treatments, demonstrated consistently by both total As solid-phase (study 3, Figure 1b) and solution data (study 3, Figure 2a-c). In control experiments without addition of reduced S (as PS or dissolved sulfide), 35%, 32%, and 20% of initial As content remained adsorbed at pH 4.5, 7.0 and 8.5, respectively, after 180 h. Upon sulfurization of model peat, the proportion of As desorption was altered. Under acidic conditions, more than half of the As (56% and 54%) was still found in the solid phase for the sulfide and PS treatment, respectively, compared to only 35% in controls without S addition. This observation matches the findings from study 1 (study 1, Figure 2, Figure SI-18) that arsenite sorption to thiols is increased at slightly acidic pH. In contrast to acidic conditions, reduced S significantly increased As desorption compared to the controls at neutral to alkaline pH. The proportions of solid-phase As in sulfurized treatments were overall
Results and Discussion
between 9% and 14%. No significant differences were observed between pH 7.0 and 8.5, which suggests that a maximum of S-induced As desorption was already reached at neutral pH. The As desorption was slightly but consistently higher in PS compared to sulfide experiments. These differences can be attributed to the additional zero-valent S in the PS treatment, as the sulfide concentrations over time were nearly identical in all sulfurized treatments (study 3, Figure SI-8).
The As K-edge XAS solid-phase speciation of the initial As-loaded peat revealed that 42% of As were bound via thiol groups and 58% via O-containing groups at pH 7.0 (study 3, Figure 3). With sulfide and PS addition, the As(III)-S fraction increased to 55% and 58%, respectively, whereas the As(III)-O fraction decreased correspondingly, indicating stronger relative binding via As(III)-S bonds. In control experiments, the As(III)-S fraction stayed almost constant and As(III)-O appeared to be partially oxidized to As(V)-O after equilibration with S-free solution, indicating that arsenite bound via O-containing groups is less stable than thiol-bound As. The suggested weaker binding of As(III)-O moieties indicates that arsenite was likely the species desorbed from the solid phase rather than As-S species. Furthermore, surface complexation of stable thioarsenates with peat, by reaction of aqueous reduced S species with As(III) bound to NOM, is unlikely on the basis of our shell-fitting results. Fitted As-S bond-lengths of 2.24-2.27 Å (study 3, Table SI-7) were significantly longer than the typical bond distances observed in thioarsenates (2.13-2.18 Å).38 Moreover, no As-S minerals could be fitted to the XAS spectra, showing low probability of precipitation of mineral phases in the incubation experiments.
Our aqueous As speciation showed that in control experiments without addition of reduced S, arsenite was the dominant species at all pH values (study 3, Figure 4). Addition of sulfide or PS yielded similar effects on As desorption and similar As speciation patterns. At all pH values, thioarsenate formation increased significantly upon sulfurization compared to controls. Under acidic conditions, arsenite remained the dominant species. After 12 h of reaction, a maximum of 26% and 38% thioarsenates was observed with PS and sulfide addition, respectively. However, the contribution of thioarsenates decreased again to 4% and 6% of total As after 180 h, confirming their instability at acidic pH.40,168
At neutral and alkaline conditions, thioarsenates dominated As speciation throughout the experiment (study 3, Figure 4). At the start of the experiment (0 h), and for both pH 7.0 as well as pH 8.5, 77 and 74-79% of As (0.72-0.80 and 0.85-87 µM) were present as thioarsenates in sulfide and PS treatments, respectively. A maximum concentration of thioarsenates was reached after 4-12 h, remaining at a constant level of 1.3±0.2 and 1.2±0.1 µM, respectively, until the end of the experiment. Until thioarsenates reached a constant concentration, arsenite was present in low concentrations (until 12 h) and started to increase thereafter, reaching similar concentrations as thioarsenates at pH 7.0 (~1.1 µM), but only less than half the amount at pH 8.5 (~0.5 µM). Since the concentrations of thioarsenates showed no decreasing trend (only minor influence of arsenate), the increasing arsenite concentrations seemed to originate directly from a release from peat surfaces.
Results and Discussion
Arsenic release from the solid to the aqueous phase may result from arsenite desorption from relatively weak O-containing groups like carboxylic or phenolic groups (see study 2) or desorption of As bound via thiol groups, either by breaking the bond between S and C or between As and S. Due to the higher stability of thiol bound As in comparison with As bound to O-containing groups, as discussed above, arsenite desorption from As-O-Corg is considered the more likely pathway. This assumption is also consistent with the increase in aqueous arsenite concentrations and no observed transformation of thioarsenates over time.
The larger fraction of the released arsenite likely formed thioarsenates spontaneously by reaction with aqueous or surface-associated reduced S species similarly at pH 7.0 and 8.5 shifting the equilibrium toward thioarsenates. After reaching their steady-state concentration, only arsenite concentrations increased thereon over time (study 3, Figure 4). The overall similar concentrations of S(0)-species in both sulfurization treatments may explain the comparable total As desorption and speciation patterns over time for both neutral and alkaline conditions (study 3, Figure 2, Figure 4), additionally suggesting that low amounts of S(0)-species are sufficient for thioarsenate formation and an excess of S(0)-species does not necessarily lead to an increase in thioarsenate concentrations. A dominance of higher thiolated As species in the sulfurization experiments (TriTAs(V) > DTAs(V) > MTAs(V)), and the observations in Gola di Lago (study 1), where MTAs(V) was the most abundant species and dissolved sulfide was below detection limit, could be explained by the high dissolved sulfide concentrations in the present experiments. This observation fits the current theory that high amounts of free dissolved sulfide are necessary to form higher thiolated thioarsenates.39,40
While the results of study 3 showed enhanced mobilization of As upon sulfurization of As-bearing peat organic matter at neutral to alkaline pH, study 4 had a closer look onto potential As mobilization mechanisms from sulfate-rich alluvial aquifer sediments, where high-organic matter reductive zones were embedded in otherwise naturally low arsenic aquifer sand.
Within column experiments fed with artificial groundwater (pH 8), where 1, 2, or 3 organic-rich, sulfidic lenses were embedded in natural aquifer sand, aqueous As, Fe, and sulfide concentrations in aquifer ports (groundwater samples) increased as a function of time until day 40 of the experiment (study 4, Figure 1).
Concentrations were considerably higher in ports immediately downstream from an organic-rich, sulfidic lens (max. ~600 nM As, ~40 µM Fe, ~18 µM sulfide). After day 40, concentrations of As, Fe, and sulfide decreased again, particularly in ports directly downstream of the lenses. Sulfide and As concentrations always increased directly after an organic-rich lens, but then decreased with increasing distance from lenses.
In contrast, aqueous Fe concentrations remained elevated independent of the distance from, or the number of, lenses. Concentrations of As, Fe, and S in control columns only showed little increase over time or remained close to, or below, the detection limit throughout the experiment (study 4, Figure SI-3). In contrast to groundwater samples, pore water from inside the lenses exhibited low aqueous Fe concentrations (<5 µM) and lower As concentrations (~200 nM) but high sulfide (up to 60 µM) concentrations, which were sustained until the end of the experiment (study 4, Figure SI-4). Aqueous As speciation of end-point
Results and Discussion
groundwater samples downstream of the lenses (ports 3, 5, and 7) showed that ~30-40% of total aqueous As were thioarsenates, with MTAs(V) being most abundant (study 4, Figure 2). Arsenite and arsenate, were also present in these samples, with the relative fraction of arsenate generally increasing with distance away from reducing lenses.
Bulk As K-edge XANES spectra taken from samples of the glycerol-preserved column with 3 lenses (study 4, Figure 4) showed that thiol-bound As(III) was the only species exhibiting a clear change in relative abundance within lenses compared to the initial material. Both As(III) and As(V) were further adsorbed to FeS in the reducing lenses. Interestingly, the As speciation in the aquifer material shifted dramatically along the flow path and compared to the initial aquifer material, where As(V) adsorbed to Fh was dominating in addition to a small pool of As(III) adsorbed to Fh (Figure 4). Before the first fine-grained reducing lens, mineral As(III)S (realgar) dominated, thereafter As(III) adsorbed to FeS and As(V)-Fh were the dominant species with a small (but increasing along the flow path) pool of S-complexed As (represented by MTA) adsorbed to FeS. Notably, the decreasing pool of As(V) remained adsorbed to Fh, whereas the increasing pool of As(III) was adsorbed to FeS in the ports upstream from the reduced lenses.
Also S exhibited major shifts in solid-phase speciation (study 4, Figure 5). Before the first lens (port 1), some S(0) was detected, but no sulfide minerals were present. After the first lens, no S(0) was detected anymore; instead, the relative abundance of FeS increased and that of sulfate decreased after each lens (ports 3 and 5). The lenses themselves also exhibited a shift in inorganic S species. The original material contained S(0)-species together with FeS and sulfate, while sulfate dominated along the flow path, probably translocated from the upstream aquifer material. However, sulfate decreased again from lens 1 to 3, while FeS and small portions of thiol-S increased.
The combined results of aqueous and solid phase speciation of As, Fe, and S reveal that the fate of As was governed by redox processes. It appears that high sulfate concentrations in the groundwater, together with the abundance of microbially available organic carbon within reducing lenses, stimulated sulfate reduction within and along the edges of lenses to produce dissolved sulfide that dispersed within the groundwater (study 4, Figure 1). The sustained sulfide supply drove reductive dissolution of Fe(III)-oxides to aqueous Fe(II), producing S(0)-species, as shown before,78,79 and releasing adsorbed As. On the influent (upstream) side of the first lens, however, aqueous Fe(II)-concentrations were too low to precipitate FeS (study 4, Figure 1). Instead, the relative lack of aqueous Fe(II) (sulfide:Fe ratios >1) induced precipitation of realgar (study 4, Figure 4), thus keeping the aqueous As concentrations relatively low.
Similarly, within the organic-rich lenses, sulfide concentrations were consistently higher than the aqueous Fe concentrations (study 4, Figure SI-4). However, reaction of dissolved sulfide with NOM leads to formation of S(0)-species and thiol groups121, as discussed before, and consequently surface As(III)-S-Corg
complexes could form, in contrast to precipitation of realgar. Probably dissolved sulfide and the slightly alkaline pH 8 induced the formation of thioarsenates and their mobilization downstream of the reduced
Results and Discussion
lenses (study 4, Figure 2). These observations are consistent with recent findings of mobile thioarsenates in the vicinity of organic-rich reduced zones in lake sediments,174 in a Bangladeshi groundwater aquifer,153 and probably also with observed As mobilization from an As-bearing alluvial aquifer peat-layer.137 The formation of FeS precipitates downstream the organic-rich lenses (study 4, Figures 1, 5 and Figure SI-4), where aqueous Fe concentrations were constantly higher than the aqueous sulfide concentrations (Fe:sulfide ratios >1), did not hinder thioarsenate mobility, as observed before.73
Our results reveal that organic-rich phases not only lead to the retention of As, but reduced sulfur species can also mobilize As through formation of thioarsenates under certain conditions in a broader range of solid NOM-containing systems then previously known. At slightly acidic to circumneutral pH and in presence of reduced S, solid NOM acts as a sink for As via formation of stable As-S-Corg bonds. Increases in pH in the presence of reduced S, from near-neutral to slightly alkaline pH, can turn NOM from an As sink into a source, and demonstrate the pH as a sensitive variable for As retention in sulfidic, organic-rich systems.
3.3 Binding mechanisms of antimonite to organic functional groups of model peat and