Key Cr species controlling Cr stability in contaminated soils before and chemical stabilization at a remediation engineering site.

Linking chromium (Cr) speciation with its stability in soils is vital because insoluble Cr(VI) and chemically adsorbed Cr(VI) could hinder the remediation efficiency and release Cr(VI) for a prolonged period of time. In this study, we investigated key Cr species to probe the mechanisms controlling the release of insoluble Cr(VI) at Cr-contaminated sites using synchrotron-based X-ray absorption near-edge structure (XANES) for the first time. Chromite, stichtite and Cr-silicate were predominant forms of Cr(III). Insoluble Cr(VI) was hosted by layered double hydroxides (LDHs) such as brownmilerite and hydrotalcite. Anion competition tests documented a substitution of absorbed Cr(VI) by SO42- and NO3-. Acid extraction released 6.7-25.7% more Cr(VI) than anion extraction, possibly attributing to the erosion of LDH and CaCrO4 in calcite rather than Cr-bearing minerals. Brown and red soils released maximally 62% and 44% of total Cr(VI) by 10 mol/(kg soil) and 2 mol/(kg soil) of H+, respectively. SO42-, H2O and H+ contributed to more release of total Cr(VI) in brown soils (22%, 33% and 7%) than red soils (25%, 17% and 2%). More crystalline Cr structures were found after chemical stabilization, indicating a higher Cr stability in chemically stabilized soils. Cr and Mn exhibited an overlapped distribution pattern in both contaminated and chemically stabilized soils, hinting at the re-oxidation of Cr(III). Insoluble Cr(VI) could be released by acidic rainfalls and soil organic matters, posing potential threats to Cr long-term stability in field-scale remediation.

Controllable Design and Preparation of Hollow Carbon-Based Nanotubes for Asymmetric Supercapacitors and Capacitive Deionization.

Carbon-based materials are important desirable materials in areas such as supercapacitors and capacitive deionization. However, traditional commercial materials are heterogeneous and prone to agglomeration in nanoscale and have structural limitation of electrochemical and desalination performance due to poor transport channels and low capacitance of prepared electrodes. Here, we introduce the facile strategy for controllable preparation of two types of hollow carbon-based nanotubes (HCTs) with amorphous mesoporous structures, which are synthesized by employing a MnO2 linear template method and calcination of polymer precursors. The porous N-doped HCT (NHCT) shows a specific capacitance of 412.6 F g-1 (1 A g-1), with 77.3% rate capability (20 A g-1). The fabricated asymmetric MnO2//NHCT supercapacitor displays the energy density of 55.8 Wh kg-1 at a power density of 803.9 W kg-1. Furthermore, two typical MnO2//HCT and MnO2//NHCT devices both show the selective desalination performance of sulfate, and the MnO2//NHCT device possesses a high deionization value of 11.37 mg g-1 (500 mg L-1 Na2SO4). These fabricated hollow carbon-based architectures with functional characteristics promise potential applications in energy and environmental related fields.

Indirect effect of nutrient accumulation intensified toxicity risk of metals in sediments from urban river network.

The levels of metals in sediments of urban river ecosystems are crucial for aquatic environmental health and pollution assessment. Yet little is known about the interaction of nutrients with metals for environmental risks under contamination accumulation. Here, we combined hierarchical cluster, correlation, and principal component analysis with structural equation model (SEM) to investigate the pollution level, source, toxicity risk, and interaction associated with metals and nutrients in the sediments of a river network in a city area of East China. The results showed that the pollution associated with metals in sediments was rated as moderate degree of contamination load and medium-high toxicity risk in the middle and downstream of urban rivers based on contamination factor, pollution load index, and environmental toxicity quotient. The concentration of mercury (Hg) and zinc (Zn) showed a significant correlation with toxic risks, which had more contribution to toxicity than other metals in the study area. Organic nitrogen and organic pollution index showed heavily polluted sediments in south of the study area. Though correlation analysis indicated that nutrients and metals had different input zones from anthropogenic sources in the urban river network, SEM suggested that nutrient accumulation indirectly intensified toxicity risk of metals by 13.6% in sediments. Therefore, we suggested the combined consideration of metal toxicity risk with nutrient accumulation, which may provide a comprehensive understanding to identify sediment pollution. Graphical abstract Toxicity rate of metals in sediments from urban river network indirectly intensified by nutrients accumulation.

Carbon Black Oxidized by Air Calcination for Enhanced H2O2 Generation and Effective Organics Degradation.

Carbon black (CB) has a high conductivity and a large surface area, which are the basis of an excellent electrocatalyst. However, CB itself is usually less active or even inactive toward two-electron oxygen reduction reaction (2e- ORR) due to the absence of highly active functional groups with low oxygen content. To activate commercial CB for 2e- ORR, oxygen-containing functional groups were introduced onto the CB surface by a simple air calcination method. After the oxidation treatment at 600 °C (CB600), the oxygen content increased from the initial 1.17 ± 0.15 to 4.08 ± 0.60%, leading to a dramatic increase of the cathodic current from only -8.1 mA (CB) to -117.6 mA (CB600). The air cathode made of CB600 achieved the maximum H2O2 production of 517.7 ± 2.4 mg L-1 within 30 min, resulting in the removal of ∼91.1% rhodamine B in 2 min and an effective mineralization of ∼76.3% in an electro-Fenton reactor. This performance was much better than that obtained using the CB catalyst (65.3 ± 5.6 mg L-1 H2O2 production, and ∼20.3% mineralization). This excellent activity of CB600 toward 2e- ORR was greatly improved by the introduction of O═C-OH and C-O-C groups. The successful improvement of the 2e- ORR activity of CB using air calcination enables its practical application in electrochemical advanced oxidation processes.