Enhanced Fluoride Uptake by Layered Double Hydroxides under Alkaline Conditions: Solid-State NMR Evidence of the Role of Surface >MgOH Sites.

Layered double hydroxides (LDHs) are potential low-cost filter materials for use in fluoride removal from drinking water, but molecular-scale defluoridation mechanisms are lacking. In this research, we employed 19F solid-state NMR spectroscopy to identify fluoride sorption products on 2:1 MgAl LDH and to reveal the relationship between fluoride sorption and the LDH structure. A set of six 19F NMR peaks centered at -140, -148, -156, -163, -176, and -183 ppm was resolved. Combining quantum chemical calculations based on density function theory (DFT) and 19F{27Al} transfer of populations in double resonance (TRAPDOR) analysis, we could assign the peaks at -140, -148, -156, and -163 ppm to Al-F (F coordinated to surface Al) and those at -176 and -183 ppm to Mg-F (F coordinated to surface Mg only). Interestingly, the spectroscopic data reveal that the formation of Al-F is the predominant mode of F- sorption at low pH, whereas the formation of Mg-F is predominant at high pH (or a higher Mg/Al ratio). This finding supports the fact that the F- uptake of 2:1 MgAl LDH was nearly six times that of activated alumina at pH 9. Overall, we explicitly revealed the different roles of the surface >MgOH and >AlOH sites of LDHs in defluoridation, which explained why the use of classic activated alumina for defluoridation is limited at high pH. The findings from this research may also provide new insights into material screening for potential filters for F- removal under alkaline conditions.

Application of ecogeochemical prediction model to safely exploit seleniferous soil.

Seleniferous soil and crops have recently attracted attention worldwide. Cultivating seleniferous crops in the absence of heavy metals is greatly challenging. This study aimed to develop approaches for the safe exploitation of seleniferous soil. We collected 246 pairs of rice grain samples and their corresponding rhizosphere soil samples and 8542 topsoil samples from Binyang and Xingbin in Guangxi. The Cd, Cu, Hg, Pb, Zn, and Se contents of soil and rice grain samples were tested. Several soil properties, including CaO, Mn, Mo, and S contents; total organic carbon content; and pH were also measured. Soil and rice grain samples that were classified as seleniferous accounted for 85.77% and 88% of all samples, respectively. More than 30% of soil and rice grain samples were potentially polluted by Cd. The percentage of seleniferous rice grain samples increased as soil Se concentration increased. Notably, however, the percentage of Cd-polluted rice grain samples decreased with the increase in soil Cd concentration. Models for the prediction of BAFs of heavy metal and Se were accurately established on the basis of significant partial correlations between log10 (BAFs) and log10 (soil properties). Farmlands with seleniferous soil under preferential protection and with safely exploited seleniferous soil accounted for 82.61% of the total study area. Sites that require remediation or land-use changes accounted for only 14.7% of the total study area and were mainly distributed in the center of the study area.