Identifying sources of acid mine drainage and major hydrogeochemical processes in abandoned mine adits (Southeast Shaanxi, China).

Acid mine drainage (AMD) has resulted in significant risks to both human health and the environment of the Han River watershed. In this study, water and sediment samples from typical mine adits were selected to investigate the hydrogeochemical characteristics and assess the environmental impacts of AMD. The interactions between coexisting chemical factors, geochemical processes in the mine adit, and the causes of AMD formation are discussed based on statistical analysis, mineralogical analysis, and geochemical modeling. The results showed that the hydrochemical types of AMD consisted of SO4-Ca-Mg, SO4-Ca, and SO4-Mg, with low pH and extremely high concentrations of Fe and SO42-. The release behaviors of most heavy metals are controlled by the oxidation of sulfide minerals (mainly pyrite) and the dissolution/precipitation of secondary minerals. Along the AMD pathway in the adit, the species of Fe-hydroxy secondary minerals tend to initially increase and later decrease. The inverse model results indicated that (1) oxidative dissolution of sulfide minerals, (2) interconversion of Fe-hydroxy secondary minerals, (3) precipitation of gypsum, and (4) neutralization by calcite are the main geochemical reactions in the adit, and chlorite might be the major neutralizing mineral of AMD with calcite. Furthermore, there were two sources of AMD in abandoned mine adits: oxidation of pyrite within the adits and infiltration of AMD from the overlying waste rock dumps. The findings can provide deeper insight into hydrogeochemical processes and the formation of AMD contamination produced in abandoned mine adits under similar mining and hydrogeological conditions.

Hydrogeochemical characteristics and processes of thermokarst lake and groundwater during the melting of the active layer in a permafrost region of the Qinghai-Tibet Plateau, China.

Permafrost degradation and the development of thermokarst lakes are important factors driving the variability of regional hydrologic processes. Hydrogeochemical and isotopic analyses are important methods for investigating the hydrologic processes of thermokarst lakes. This study focused on comparing the chemical and hydrogeochemical characteristics between lake water and groundwater during the melting of the active layer in a typical thermokarst lake region on the Qinghai-Tibet Plateau (QTP). Ninety-five samples were collected during different periods of active layer melting and analyzed using statistical, isotope, hydrogeochemical, and modeling methods. Statistical results showed that the average concentrations of almost all ions were lower in lake water than in groundwater, with wider spatial variability in groundwater. The lake water is of the ClNa and HCO3-Ca type with low TDS (total dissolved solids), whereas groundwater is of the HCO3-Ca and mixed type (or transition type) with high TDS. The chemical types of the lake water and groundwater are mainly driven by rock weathering. In terms of the saturation index (SI), halite and gypsum are unsaturated dissolved, whereas dolomite and calcite are generally saturated. Evaporation significantly affects the chemical composition of groundwater, while the hydrochemical compositions of lake water are relatively stable under the joint control of evaporation, precipitation, surface runoff, and groundwater. The isotopic analysis results showed that the contribution of permafrost meltwater and precipitation to groundwater and lake water varied during different stages of active layer melting. According to hydrogeochemical modeling, the main chemical reactions in groundwater are the precipitation of calcite and the dissolution of halite, dolomite, and gypsum. The intensity of groundwater flow determines the degree of chemical reactions along the flow path at different stages of active layer melting. The findings can provide deeper insight into hydrogeochemical processes in thermokarst lake regions under the background of permafrost degradation.

Globalization-Driven Industry Relocation Significantly Reduces Arctic PAH Contamination.

Industry relocation under globalization has altered the origins and strength of emission sources of many air pollutants. We develop global emission inventories of polycyclic aromatic hydrocarbons (PAHs) embodied in the production and consumption of goods and services. We implement these inventories within a global atmospheric transport model and simulate spatial-temporal changes in atmospheric concentrations of benzo[α]pyrene (BaP), the most toxic congener in unsubstituted PAHs, and depositions across the Arctic subject to global trade and industry relocation. We show that interregional trade and industry relocation dramatically reduce the atmospheric levels and deposition of BaP in the Arctic. The most significant BaP decline occurs in the European and North American Arctic regions due to attenuated sources in the two well-developed continents proximate to the polar region induced by the relocation of high-PAH pollution industries to many developing countries far from the Arctic. Although BaP emissions embodied in industry relocations in China, India, and South and Southeast Asia resulted in increased BaP contamination in the Asian Arctic, such increases in pollution are minor compared to significant BaP reductions occurring in the European and North American Arctic regions. We find that "North-to-South" industry transfer could reduce trade-related BaP contamination by 60% in the Arctic.

Broadband wave plates made by plasmonic metamaterials.

Although metamaterials wave-plates have been demonstrated previously, many of them suffer from the issue of narrow bandwidth since they typically rely on resonance principles and thus exhibit inevitable frequency dispersions. Here, we show that the dispersion of spoof surface plasmon (SSP) mode supported by a fishbone structure can be freely modulated by varying the structural parameters. This motivates us to establish a general strategy of building broadband wave-plates by cascading two fishbone structures with different propagation constants of SSP modes. We derive a criterion under which the cross-polarization phase-difference across the whole device can maintain at a nearly constant value over a wide frequency band, with frequency dispersions in the two fishbone structures cancelled out. As an illustration, we design and fabricate an efficient microwave quarter-wave plate and experimentally characterize its excellent polarization-control performances over a broad frequency band (7-9.2 GHz). Our findings can stimulate making dispersion-controlled high-performance optical functional devices in different frequency domains.

Graphene-assisted ultra-compact polarization splitter and rotator with an extended bandwidth.

The high refraction-index contrast between silicon and the surrounding cladding makes silicon-on-insulator devices highly polarization-dependent. However, it is greatly desirable for many applications to address the issue of polarization dependence in silicon photonics. Here, a novel ultra-compact polarization splitter and rotator (PSR), constructed with an asymmetrical directional coupler consisting of a rib silicon waveguide and a graphene-embedded rib silicon waveguide (GERSW), on a silicon-on-insulator platform is proposed and investigated. By taking advantage of the large modulation of the effective refractive index of the TE mode for the GERSW by tuning the chemical potential of graphene, the phase matching condition can be well satisfied over a wide spectral band. The presented result demonstrates that for a 7-layer-graphene-embedded PSR with a coupling length of 11.1 µm, a high TM-to-TE conversion efficiency (>-0.5 dB) can be achieved over a broad bandwidth from 1516 to 1602 nm.

Double-stacked hyperbolic metamaterial waveguide arrays for efficient and broadband terahertz quarter-wave plates.

We demonstrate how it is possible to achieve weak dispersion in the phase delay between two orthogonal polarization states by using double-stacked hyperbolic metamaterial (HMM) waveguide arrays. The weak dispersion in the phase delay originates from the different signs of phase delay from the two different HMM waveguide arrays. The condition of dispersion-free phase delay for the transmitted waves has been theoretically derived from the transmission matrix as the propagation characteristic of the HMM waveguide is involved. We further reveal that the designed double-stacked HMM waveguide array can function as an efficient quarter-wave plate that enables the conversion of linearly polarized light to circularly polarized light within a broad frequency band. In addition, the bandwidth over which the degree of linear polarization is nearly unity and over which the angle of linear polarization is kept at approximately 45° is basically consistent with the phase bandwidth. This offers a promising approach for developing a practical polarization converter in the terahertz domain.