Fate and transport of chromium in industrial sites: Dynamic simulation on soil profile.

Direct discharge of chromium-containing waste water and improper disposal of waste residues in industrial sites may lead to the vertical migration of metals into aquifers, posing serious threat to soil-groundwater system. The heterogeneity in soil profile further aggravates the complexity and unpredictability of this transport process. However, topsoil was the main focus of most studies. Herein, the vertical transport and transformation of Cr in soils at different depths in three industrial sites (i.e., Shijiazhuang, Zhuzhou, and Guangzhou) were investigated to delineate Cr transport and retention characteristics under complex conditions. Regional and vertical differences in soil properties led to the specificity in Cr migration behaviors among these three sites. Correlation analysis showed that soil pH (r = -0.909, p < 0.05) and Fe content (r = 0.949, p < 0.01) were the major controlling factors of Cr(VI) migration and transformation in aquifers. Furthermore, the soil of Zhuzhou site showed the maximum adsorption capacity for Cr(VI) (0.225 mol/kg), and the strongest reduction ability of Cr(VI) was observed in the Guangzhou soil. Results of model-based long-term forecast indicated that the Cr(III) concentration in the liquid phase of Guangzhou subsoil could reach 0.08 mol/m3 within 20 years. Heavier rainfall condition exacerbated the contamination due to an increased pollutant flux and enhanced convection. Specially, Cr was fixed in the topsoil of Zhuzhou site with the formation of PbCrO4 and presented least vertical migration risk. The conclusions above can provide scientific theoretical guidance for heavy metal pollution prevention and control in industrial contaminated regions.

Effect of overlying water pH, temperature, and hydraulic disturbance on heavy metal and nutrient release from drinking water reservoir sediments.

How environmental factors impact the release of pollutants from sediment is critical to ensure the safety of drinking water, especially when the seasons change. Here, we investigated the effect of water pH, temperature, and hydraulic disturbance on the release of heavy metals and nutrients from the sediment of drinking water reservoir. The results show that lower initial water pH promoted the Zn release, while low temperature enhanced the Mn flux after 15 days. Meanwhile, continuous disturbance caused more metals releasing from sediment than intermittent disturbance due to greater shear stress and turbulence effect. However, intermittent high-speed disturbance greatly altered the dynamic release of Zn from L-shaped curve to U-shape in water column. Moreover, lower water pH caused higher ammonium in water but lower nitrate since H+ restrained the nitrification. Yet, higher temperature inhibited the release of ammonium from sediment, which might relate to the accelerated mineralization of organic nitrogen and elevated dissolved oxygen caused by the algae growth. Notably, hydraulic disturbance with various intensity and duration greatly influenced the fluxes of various species of nitrogen and soluble phosphate in water column, because the disturbance facilitated the nitrogen and phosphorus exchanges between sediment-water and water-air interfaces. PRACTITIONER POINTS: Lower water pH induced Zn release, while low temperature gradually enhanced Mn level. More metals were released from sediment under continuous disturbance than intermittent disturbance. Lower water pH caused higher ammonium nitrogen in water but lower nitrate nitrogen. Higher temperature inhibited the release of ammonium nitrogen from sediment. Hydraulic disturbance greatly changed the release of different species of nitrogen and soluble phosphate from sediment.

Release of odorants from sediments of the largest drinking water reservoir in Shanghai: Influence of pH, temperature, and hydraulic disturbance.

Endogenous pollution from sediments is gradually becoming a critical pollution source of the drinking water reservoir. Odorants can be released from sediments into the overlying water which further deteriorate the water quality of the drinking water reservoir. In this work, we set the sediment-overlying water systems under various water pH (6.5, 8 and 9), temperature (4, 20 and 30 °C) during 30 days and intermittent or continuous hydraulic disturbances (at 100 r/min or 200 r/min) in 5 days, and investigated the dynamic release of odorants from the drinking water reservoir sediments via using headspace solid-phase microextraction (HSPME) and gas chromatography-mass spectrometry (GC-MS). The result shows that weakly alkaline environment slightly but not significantly increased the concentration of dimethyl disulfide (DMDS) in the overlying water. Furthermore, low temperature promoted the release of bis(2-chloroisopropyl) ether (BCIE) and geosmin to 108.36 and 18.98 ng/L, respectively, while high temperature facilitated the DMDS release to 20.33 ng/L. Notably, hydraulic disturbances drastically elevated the level of seven odorants released from the sediments. Specially, benzaldehyde exhibited highest concentration at 260.50 ng/L. The continuous disturbance greatly enhanced the release of benzaldehyde, DMDS, dimethyl trisulfide (DMTS), BCIE and 1,4-dichloro-benzene (1,4-DCB) from sediments with a positive disturbance speed-dependence. However, the intermittent disturbance promoted higher level of geosmin in the overlying water compared to the continuous disturbance. Only continuous hydraulic disturbance at high speed could lead to the release of ethylbenzene from sediments, which was up to 4.89 ng/L in 12 h.

AMn2O4 (A=Cu, Ni and Zn) sorbents coupling high adsorption and regeneration performance for elemental mercury removal from syngas.

AMn2O4 (A= Cu, Ni and Zn) spinel sorbents synthesized by a low-temperature sol-gel auto-combustion method were for the first time used to eliminate elemental mercury (Hg0) from syngas. CuMn2O4 sorbent exhibits the highest Hg0 adsorption performance under simulated syngas, higher than 95 % Hg0 capture efficiency is obtained at 200 °C. Adsorption-regeneration experiments demonstrate that the regenerability of CuMn2O4 is excellent. The influences of syngas compositions on Hg0 elimination over CuMn2O4 were examined. H2S plays the most important role in Hg0 removal from syngas, which can be adsorbed on CuMn2O4 surface and transformed into active sulfur species to react with Hg0 to form surface-bonded HgS. X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) experiments prove that HgS is formed on the spent sorbent. The surface Cu2+ cations and chemisorbed/lattice oxygens participate in Hg0 adsorption and transformation. HCl promotes Hg0 removal by forming surface active chlorine species. H2, CO, and H2O inhibit Hg0 removal under N2 atmosphere. However, they exhibit no obvious effect on Hg0 removal with the assistance of H2S. The excellent Hg0 capture performance, good regenerability and H2O resistance of CuMn2O4 make it to be a very promising sorbent for Hg0 removal from syngas at higher temperature.

Insights into the catalytic behavior of LaMnO3 perovskite for Hg0 oxidation by HCl.

LaMnO3-based catalysts with perovskite structure have gained increasing interest for Hg0 oxidation owing to their excellent catalytic activity, high thermal stability and unique redox behavior. Understanding the Hg0 oxidation behavior on LaMnO3 will broaden the application of LaMnO3-based perovskites in Hg0 removal field. Density functional theory (DFT) calculations were conducted to examine the catalytic mechanism of Hg0 oxidation by HCl on LaMnO3 surface. The results indicate that Mn-terminated LaMnO3(010) surface is more active and stable than La-terminated surface. Hg0 and HgCl2 are chemisorbed on LaMnO3(010) surface. HgCl can be molecularly chemisorbed on LaMnO3(010) and serve as an intermediate in Hg0 oxidation reaction. HCl dissociatively adsorbs on LaMnO3(010) and generates surface active chlorine complexes. Langmuir-Hinshelwood mechanism, where the chemisorbed Hg0 reacts with the dissociatively adsorbed HCl, is responsible for Hg0 oxidation by HCl on LaMnO3(010). Catalytic Hg0 oxidation over the surface contains four-steps: Hg0 → Hg(ads) → HgCl(ads) → HgCl2(ads) → HgCl2, and the second step (Hg(ads) → HgCl(ads)) is the rate-determining step because of its relatively larger energy barrier (0.74 eV).