Chromium isotopes are promising for monitoring and quantifying the extent of microbial Cr(VI) reduction. However, the mechanisms and factors that influence Cr isotope fractionation are not well understood. In the present study, we investigated the effects of different metabolic pathways and various environmental parameters on Cr isotope fractionation during Cr(VI) reduction by the extremely thermophilic bacterium Caldicellulosiruptor saccharolyticus . The Cr isotopic compositions were measured on unreacted Cr(VI) in the remaining solution. The reaction product is isotopically lighter than the reactant. The Cr isotope composition for remaining Cr(VI) in the solution followed a Rayleigh fractionation trend and thus became increasingly isotopically heavier as the reduction of Cr(VI) proceeded. Cr(VI) reduction mainly occurs on the cell surface by Ni–Fe hydrogenase; however, in the presence of CuCl 2 , the activity of Ni–Fe hydrogenase was inhibited, and Cr(VI) was reduced inside the cells. We found that despite different Cr(VI) reduction locations, Cr isotope enrichment factors ( ε = ( α − 1) × 1000, where α is the fractionation factor) were identical within uncertainty for experiments with and without CuCl 2 (−2.72 ± 0.67‰ and −2.92 ± 0.14‰, respectively). When added to the cell cultures, neutral red (NR) functioned as an electron carrier that enabled bacteria to drive the electron transfer processes. The ε values were also similar for experiments with and without NR (−2.53 ± 0.40‰ and −2.92 ± 0.14‰, respectively). We compared these ε values to investigate the effects of different electron transport pathways and cellular Cr(VI) transfer processes, including Cr(VI) transport across the cell envelope and binding to reacting enzymes, on Cr isotope fractionation. Furthermore, the effects of various environmental parameters on Cr isotope fractionation were tested, including temperature (50 °C and 70 °C), hydrogen concentration, and different electron donors (glucose and hydrogen). The ε values were similar at both tested temperatures and stayed within the previously observed range for mesophiles ; changes in hydrogen concentration also did not affect the ε values. However, the ε values were slightly higher with hydrogen as the electron donor (−3.64 ± 0.24‰) than those with glucose as the electron donor (−2.92 ± 0.14‰). In short, the Cr enrichment factors remained unchanged despite of changing microbial Cr (VI) reduction pathways and changing temperature conditions, but may be sensitive to the electron donor type.
Yanting Wang,Youmin Sun,Shengying Gong,Zhengqing Cai,Jie Fu
Silver nanoparticles (AgNPs) are one of the main nano-contaminants and their entry into the environment may pose a threat to the aquatic ecosystems . Meanwhile, AgNPs have an important impact on geological processes . In this paper, we studied the influence of AgNPs on the sedimentation of suspended sediments . Results showed the settling rate of sediments increased from 13.06 to 20.08 h −1 in the presence of 0.2 mg/L AgNPs. The morphology characterization indicated the average particle size of sediments increased from 34.74 to 42.87 μm after interacting with AgNPs, resulting in the acceleration of settlement on the basis of Stokes' law. The chemical analysis detected 43–57% of AgNPs were incorporated onto sediments, which was the dominant reason for the increased sediments size and settling rate. The influences of various water quality parameters, e.g., pH, ionic strength , temperature, humic acid and surfactant , on the sediments settlement and interaction with AgNPs were comprehensively studied. Elevating the solution pH, ionic strength, temperature, contents of humic acid and anionic/nonionic surfactants would suppress the settling of suspended sediments , whereas cationic surfactant showed a promoting effect on sediments settlement. In addition, the settling experiments were conducted in collected lake water and seawater , and results showed the real water matrices hindered the sediments settlement and interaction with AgNPs.
Fan Dong,Fan Ge,Xinting Zhao,Daming Sun,Suyu Ren,Yan Wang,Feng Tan
Extensive use of per- and polyfluoroalkyl substances (PFASs) has resulted in their widespread presence in natural waters. Concern for public health requires reliable measurement methods for determining their distribution and risks. Here, a sampling method based on diffusive gradients in thin films (DGT) was developed for measuring PFASs in drinking water sources. Fluorinated graphite (FG) particles were used to prepare the DGT binding gel for selective enrichment of trace PFASs in an aqueous environment. The FG-DGT method did not show sensitivity to relevant environmental parameters including pH (5.0–9.0), ionic strength (0.001–0.5 M), or DOM concentration (0–30 mg/L). The FG-DGT had enough capacity for deployment of up to four months. Six traditional and emerging PFASs including PFOS, PFOA, PFHpA, PFHxS , PFNA, and 6:2 FTSA at the ng/L level were detected in two major reservoirs serving as public drinking water sources by FG-DGT method coupled with liquid chromatography tandem mass spectrometry (LC-MS/MS). PFOA appeared at the highest observed concentrations in the drinking water sources. The research demonstrates that FG-DGT is an effective and efficient tool for monitoring PFASs in drinking water.
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