Theoretical and Experimental Insights into the Electrochemical Mineralization Mechanism of Perfluorooctanoic Acid

The electrochemical mineralization mechanism of environmentally persistent perfluorooctanoic acid (PFOA) at a Ce-doped modified porous nanocrystalline PbO₂ film anode was investigated using density functional theory (DFT) simulation and further validated experimentally. The potential energy surface was mapped out for all possible reactions during electrochemical mineralization reaction of PFOA. The hydroxyl radical (·OH), O₂ and H₂O took part in the mineralization process and played different roles. The ·OH-initiated process was found to be the main degradation pathway, and the existence of O₂ obviously accelerated the degradation process of PFOA in aqueous solution. On the basis of the DFT calculations, an optimal electrochemical mineralization mechanism of PFOA was proposed, which involved the electronic migration, decarboxylation, radical reaction, hydrogen abstraction reaction, and radical fragmentation reaction. The proposed mechanism was verified by the dynamics and intermediate determination experiments. Furthermore, the observed ·OH concentration showed that the electrolysis system could produce enough ·OH for PFOA mineralization process, indicating that the proposed ·OH-initiated process derived from DFT calculations was feasible. These insightful findings are instrumental for a comprehensive understanding of the mineralization of PFOA in the electrolysis system

Impact of the effective precipitation frequency on soil water content and annual Bowen ratio (β).

<p>The effective precipitation frequency was measured as the number of days with a precipitation amount ≥ 1.5 mm during the dry season, and soil water content was measured at 5 cm (Sw_5 cm) during the dry season.</p

Seasonal and interannual variation of environmental and physiological factors.

<p>Factors include (A) monthly mean air temperature (T_a), (B) monthly mean soil water content at 5, 20, and 50 cm (Sw_5 cm, Sw_20 cm, and Sw_50 cm) and monthly precipitation amount (P), and (C) monthly mean canopy conductance (g_c) and 8-day timescale enhanced vegetation index (EVI).</p

Long-term trends of the Bowen ratio (β), soil water content, and effective precipitation frequency.

<p>Long-term trends were marginally significant for (A) the annual Bowen ratio (p = 0.06), and significant for (B) the soil water content at 5 cm (Sw_5 cm; dashed line, p<0.001) and the effective precipitation frequency (solid line, p<0.001) during the dry season.</p

Interannual variation of annual net radiation (Rn, MJ m⁻² yr⁻¹), air temperature (T_a, °C), vapor pressure deficit (VPD, kPa), soil water content at 5 cm, 20 cm, and 50 cm (Sw_5 cm, Sw_20 cm, and Sw_50 cm (m³ m⁻³)), enhanced vegetation index (EVI), total precipitation amount (P, mm), latent heat flux (LE, MJ m⁻² yr⁻¹), sensible heat flux (H, MJ m⁻² yr⁻¹), and the Bowen ratio (β) during 2003–2012 at the Qianyanzhou site.

<p>SD is standard deviation. CV is the coefficient of variation (ratio of standard deviation to mean value).</p

Relationships between the annual Bowen ratio (β) and dry season soil water content.

<p>Dry season (July-October) soil water content was measured at (A) 5 cm (Sw_5 cm), (B) 20 cm (Sw_20 cm), and (C) 50 cm (Sw_50 cm).</p

The seasonal and interannual variation in the cumulative daily fluxes.

<p>Fluxes include net ecosystem production (NEP) (gray dots), the cumulative daily ecosystem respiration (RE) (green cross), and the cumulative daily gross ecosystem production (GEP) (blue open circle). The 10-day running mean of the NEP (red line) for Yucheng (YC) (a) and Luancheng (LC) (b) on the North China Plain is also presented.</p

The interannual variations in annual and seasonal NEP and NBP.

<p>Interannual variations in annual NEP, annual NBP, seasonal accumulative NEP and NBP during winter wheat growing season, seasonal accumulative NEP and NBP during summer maize growing season in Yucheng (YC) (a) and Luancheng (LC) (b) on the North China Plain are presented.</p

The relationship between the mean annual air temperature (MAT) and the net ecosystem production (NEP).

<p>The data was collected from literature and the dots represent annual NEP at the different sites. The solid line is the regression line, and the dotted line is the 95% band.</p

Comparisons of climate condition, carbon fluxes and net biome productions among different farmlands.

<p>Climate condition includes annual mean air temperature (MAT, °C), annual precipitation (P, mm). Carbon fluxes include gross ecosystem production (GEP, gCm⁻²y⁻¹), ecosystem respiration (RE, gCm⁻²y⁻¹) and net ecosystem production (NEP, gCm⁻²y⁻¹).</p><p>Comparisons of climate condition, carbon fluxes and net biome productions among different farmlands.</p