Arsenate–Ferrihydrite Systems from Minutes to Months: A Macroscopic and IR Spectroscopic Study of an Elusive Equilibrium

Sorption by ferrihydrite is an important control on As­(V) concentrations in many oxic aquatic systems. There are significant discrepancies in reported sorption constants (log­(<i>K</i>_As)), which presents a problem for quantifying and understanding this important system. A review of reported ferrihydrite–As­(V) sorption studies indicated a positive correlation between reaction time used in the experiments and the log­(<i>K</i>_As) values derived from the data. In this paper, we study the kinetics of As­(V) sorption over ≈3000 h in nine systems with varying pH and As­(V)/Fe. Ferrihydrite was stable in all systems containing As­(V), and the [As­(V)] in solution decreased linearly as a function of log­(<i>t</i>) (termed Elovich kinetics) over the full 3000 h in most systems. A stable [As­(V)] was only observed in systems with low As­(V)/Fe and low pH. Apparent As­(V) sorption constants were derived from the data at specific time intervals using the diffuse layer model and equations describing log­(<i>K</i>_As) values as a function of time provide a way to describe this elusive equilibrium. IR spectra support the hypothesis that slow interparticle diffusion is responsible for the slow approach to equilibrium. This work resolves discrepancies in previous studies of As­(V)–ferrihydrite and provides equations to allow for system appropriate log­(<i>K</i>_As) values to be used

Germination Shifts of C₃ and C₄ Species under Simulated Global Warming Scenario

<div><p>Research efforts around the world have been increasingly devoted to investigating changes in C₃ and C₄ species' abundance or distribution with global warming, as they provide important insight into carbon fluxes and linked biogeochemical cycles. However, changes in the early life stage (e.g. germination) of C₃ and C₄ species in response to global warming, particularly with respect to asymmetric warming, have received less attention. We investigated germination percentage and rate of C₃ and C₄ species under asymmetric (+3/+6°C at day/night) and symmetric warming (+5/+5°C at day/night), simulated by alternating temperatures. A thermal time model was used to calculate germination base temperature and thermal time constant. Two additional alternating temperature regimes were used to test temperature metrics effect. The germination percentage and rate increased continuously for C₄ species, but increased and then decreased with temperature for C₃ species under both symmetric and asymmetric warming. Compared to asymmetric warming, symmetric warming significantly overestimated the speed of germination percentage change with temperature for C₄ species. Among the temperature metrics (minimum, maximum, diurnal temperature range and average temperature), maximum temperature was most correlated with germination of C₄ species. Our results indicate that global warming may favour germination of C₄ species, at least for the C₄ species studied in this work. The divergent effects of asymmetric and symmetric warming on plant germination also deserve more attention in future studies.</p></div

Thermal time model parameter estimates (<i>T_b</i>, minimum temperature; <i>θ</i>₁, thermal time constant) for C₄ and C₃ species under the symmetric warming (SW) and asymmetric warming (AW) alternating temperature regimes.

<p>Thermal time model parameter estimates (<i>T_b</i>, minimum temperature; <i>θ</i>₁, thermal time constant) for C₄ and C₃ species under the symmetric warming (SW) and asymmetric warming (AW) alternating temperature regimes.</p

The Influence of Surface Structure on H₄SiO₄ Oligomerization on Rutile and Amorphous TiO₂ Surfaces: An ATR-IR and Synchrotron XPS Study

Silicic acid (H₄SiO₄) is ubiquitous in natural aquatic systems. Applications of TiO₂ in these systems will be influenced by H₄SiO₄ sorption and oligomerization reactions on the TiO₂ surface, and this can affect many aspects of TiO₂ reactivity. The spatial arrangement of sorption sites on a metal oxide surface can promote specific lateral interactions, such as oligomerization, between sorbed species. In this work we explore the relationship between surface structure and interfacial H₄SiO₄ oligomerization by quantifying the extent of H₄SiO₄ sorption and oligomerization on three TiO₂ phases; a rutile phase having well-developed (110) faces (R180), a rutile phase with poorly developed (110) faces (R60), and an amorphous TiO₂ (TiO_2(am)). The <i>in situ</i> ATR-IR spectra measured over time as 0.2 mM H₄SiO₄ reacted with TiO₂ were quite different on the three TiO₂ phases. The percentage of the surface H₄SiO₄ that was present as oligomers increased over time on all phases, but after 20 h almost all H₄SiO₄ on the R180 surface was oligomeric, while the H₄SiO₄ on TiO_2(am) was predominantly monomeric. The extent of H₄SiO₄ oligomerization on R60 was intermediate. When the TiO₂ phases reacted with 1.5 mM H₄SiO₄ the ATR-IR spectra showed oligomeric silicates dominating the surface of all three TiO₂ phases; however, after 20 h the percentage of the surface H₄SiO₄ present as three-dimensional polymers was ∼30, 10, and 0% on R180, R60, and TiO_2(am) respectively. The Si 2s photoelectron peak binding energy (BE) and the H₄SiO₄ surface coverage (Γ_Si) were measured by XPS over a range of Γ_Si. For any given Γ_Si the Si 2s BE’s were in the order R180 > R60 > TiO_2(am). A higher Si 2s BE indicates a greater degree of silicate polymerization. The ATR-IR and XPS results support the existing model for interfacial H₄SiO₄ oligomerization where linear trimeric silicates are formed by insertion of a solution H₄SiO₄ between suitably orientated adjacent bidentate sorbed monomers. The TiO_2(am) has previously been shown to consist of ∼2 nm diameter particles with a highly disordered surface. When compared to the TiO_2(am) surface, the regular arrangement of TiO₆ octahedra on the rutile (110) face means that sorbed H₄SiO₄ monomers on adjacent rows of singly coordinated oxygen atoms are oriented so as to favor linear trimer formation. Higher silicate polymers can form between adjacent trimers, and this is favored on the rutile (110) surfaces compared to the TiO_2(am). This is also expected on the basis of the arrangement of surface sites on the rutile (110) surface and because the high surface curvature inherent in a ∼2 nm spherical TiO_2(am) particle would increase the spatial separation of adjacent trimers

Pearson correlation analysis of germination percentage (GP) and germination rate (GR) with temperature metrics (TM, minimum, maximum, average, diurnal temperature range (DTR)) from the four alternating temperature regimes for C₄ and C₃ species (<i>r</i>, correlation coefficient; <i>P</i>, probability for the correlation).

<p>Pearson correlation analysis of germination percentage (GP) and germination rate (GR) with temperature metrics (TM, minimum, maximum, average, diurnal temperature range (DTR)) from the four alternating temperature regimes for C₄ and C₃ species (<i>r</i>, correlation coefficient; <i>P</i>, probability for the correlation).</p

The two-way ANOVA analysis of the effects of plant photosynthetic type (PPT) and temperature treatments (T) in four temperature regimes on germination percentage and germination rate.

<p>ns, <i>P</i>>0.05;</p><p>*, <i>P</i><0.05;</p><p>**, <i>P</i><0.01;</p><p>***, <i>P</i><0.001</p>†<p>The degree of freedom for TmaxC temperature regime is 10 and df of other temperature regimes is 5.</p

Germination rates of C₄ and C₃ species under different alternating temperature regimes.

<p>See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0105139#pone-0105139-g001" target="_blank">Fig. 1</a> for symbols.</p

The Influence of Surface Structure on H₄SiO₄ Oligomerization on Rutile and Amorphous TiO₂ Surfaces: An ATR-IR and Synchrotron XPS Study

Silicic acid (H₄SiO₄) is ubiquitous in natural aquatic systems. Applications of TiO₂ in these systems will be influenced by H₄SiO₄ sorption and oligomerization reactions on the TiO₂ surface, and this can affect many aspects of TiO₂ reactivity. The spatial arrangement of sorption sites on a metal oxide surface can promote specific lateral interactions, such as oligomerization, between sorbed species. In this work we explore the relationship between surface structure and interfacial H₄SiO₄ oligomerization by quantifying the extent of H₄SiO₄ sorption and oligomerization on three TiO₂ phases; a rutile phase having well-developed (110) faces (R180), a rutile phase with poorly developed (110) faces (R60), and an amorphous TiO₂ (TiO_2(am)). The <i>in situ</i> ATR-IR spectra measured over time as 0.2 mM H₄SiO₄ reacted with TiO₂ were quite different on the three TiO₂ phases. The percentage of the surface H₄SiO₄ that was present as oligomers increased over time on all phases, but after 20 h almost all H₄SiO₄ on the R180 surface was oligomeric, while the H₄SiO₄ on TiO_2(am) was predominantly monomeric. The extent of H₄SiO₄ oligomerization on R60 was intermediate. When the TiO₂ phases reacted with 1.5 mM H₄SiO₄ the ATR-IR spectra showed oligomeric silicates dominating the surface of all three TiO₂ phases; however, after 20 h the percentage of the surface H₄SiO₄ present as three-dimensional polymers was ∼30, 10, and 0% on R180, R60, and TiO_2(am) respectively. The Si 2s photoelectron peak binding energy (BE) and the H₄SiO₄ surface coverage (Γ_Si) were measured by XPS over a range of Γ_Si. For any given Γ_Si the Si 2s BE’s were in the order R180 > R60 > TiO_2(am). A higher Si 2s BE indicates a greater degree of silicate polymerization. The ATR-IR and XPS results support the existing model for interfacial H₄SiO₄ oligomerization where linear trimeric silicates are formed by insertion of a solution H₄SiO₄ between suitably orientated adjacent bidentate sorbed monomers. The TiO_2(am) has previously been shown to consist of ∼2 nm diameter particles with a highly disordered surface. When compared to the TiO_2(am) surface, the regular arrangement of TiO₆ octahedra on the rutile (110) face means that sorbed H₄SiO₄ monomers on adjacent rows of singly coordinated oxygen atoms are oriented so as to favor linear trimer formation. Higher silicate polymers can form between adjacent trimers, and this is favored on the rutile (110) surfaces compared to the TiO_2(am). This is also expected on the basis of the arrangement of surface sites on the rutile (110) surface and because the high surface curvature inherent in a ∼2 nm spherical TiO_2(am) particle would increase the spatial separation of adjacent trimers