Opportunity and challenges in large-scale geothermal energy exploitation in China

Under the significant pressure to diversify energy supply and reduce the dependence on fossil fuels, harnessing the vast geothermal resources has been made a priority in China’s renewable energy development plans. This review provided an overview on the development of geothermal power generation and direct use in China, identified the key barriers for geothermal energy utilization, particularly in power generation, and proposed corresponding measures for boosting the growth of geothermal industry. Little progress was made in geothermal power generation over the past decades, while geothermal direct use, particularly geothermal heat pumps (GHPs) and district heating, has been growing quickly. Development of geothermal power generation was primarily limited by the lack of high temperature hydrothermal resources near the major demand centers. A series of technological challenges associated with drilling under extremely high pressure and temperature conditions, transportation of heavy equipment, long-distance electricity transmission, and hot dry rock (HDR) exploitation, also hindered large-scale development of geothermal resources for power generation and district heating. There were no social barriers for geothermal exploitation except in hot spring tourism areas, while the high cost could limit the consumer acceptance of geothermal heating/cooling solutions. Significant investment in research and development of enhanced geothermal system (EGS) technologies for exploiting HDR resources and establishment of favorable policy instruments are recommended as key technological and policy measures to accelerate the growth of geothermal energy market, while the potential social barriers for the development EGS projects in the densely populated areas should also be recognized and mitigated in the future.</p

Emission of Polycyclic Aromatic Hydrocarbons in China

Emission of 16 polycyclic aromatic hydrocarbons (PAHs) listed as U.S. Environmental Protection Agency (U.S. EPA) priority pollutants from major sources in China were compiled. Geographical distribution and temporal change of the PAH emission, as well as emission profiles, are discussed. It was estimated that the total PAH emission in China was 25 300 tons in 2003. The emission profile featured a relatively higher portion of high molecular weight (HMW) species with carcinogenic potential due to large contributions of domestic coal and coking industry. Among various sources, biomass burning, domestic coal combustion, and coking industry contributed 60%, 20%, and 16% of the total emission, respectively. Total emission, emission density, emission intensity, and emission per capita showed geographical variations. In general, the southeastern provinces were characterized by higher emission density, while those in western and northern China featured higher emission intensity and population-normalized emission. Although energy consumption in China went up continuously during the past two decades, annual emission of PAHs fluctuated depending on the amount of domestic coal consumption, coke production, and the efficiency of energy utilization

Environmental and human health impacts of geothermal exploitation in China and mitigation strategies

Exploitation of the vast reserve of geothermal energy for power generation and direct use has been elevated to the top of China’s energy agenda, while the environmental impacts and human health risk have received little attention. This review systematically summarizes the environmental and human health impacts of geothermal utilization in China, and identifies the key technologies and policy measures that can be implemented to mitigate them. Geothermal fluids are typically enriched with a range of dissolved species and non-condensable gases, which originate from magma degassing and/or water-rock interaction under high temperature and pressure, and often low pH conditions in the reservoirs, and their releases could pollute local surface water, groundwater, soil, and air, and adversely affect biodiversity. The toxic elements (primarily As and F) could negatively impact the health of people living in geothermal areas through contaminated surface water, groundwater, and food crops. Among the gaseous pollutants, hydrogen sulfide often poses the most significant health concern, while radon might pose potentially significant radiological risk to the workers of geothermal spa and hot spring facilities. Improper disposal of the geothermal wastes enriched with natural radionuclides might elevate the radiation exposure of workers at geothermal facilities and people living in their vicinity. To minimize the potential environmental impacts and human health risk from large-scale expansion of geothermal exploitation, heat-extraction-only technologies should be promoted, while geothermal facilities should actively adopt pollution control and prevention measures. Government regulation and supervision can further ensure the negative environmental and human health impacts are adequately mitigated.</p

Sorption and Competition of Aromatic Compounds and Humic Acid on Multiwalled Carbon Nanotubes

Sorption of humic acid (HA) and aromatic compounds by multiwalled carbon nanotubes (MWCNTs), and their competition on MWCNTs were examined. HA sorption by MWCNTs was regulated by their surface area (SA). Hydrophobic and π−π attractions of HA with MWCNTs were main driving forces for their interactions. Kd/Kow values of phenanthrene (Phen), naphthalene (Naph), and 1-naphthol (1-Naph) by individual MWCNTs were positively correlated with their molecular size, suggesting that micropore-filling could not be a predominant mechanism. HA had the lowest competition with Phen and 1-Naph on MWCNT20, due to its greatest abundance of sorption sites. Competition between HA and 1-Naph followed an order MWCNT40 < MWCNT60 < MWCNT100, due to their reduction in SA and porosity. Micropore blockage and direct competition by HA increased with deceasing SA and porosity of MWCNTs. MWCNT20 had much more sorption sites than other MWCNTs, leading to insignificant difference in competition between 1-Naph and Phen with HA. Also, HA had higher competition with Phen on MWCNT40, MWCNT60, and MWCNT100 than 1-Naph. Our results highlight the significance of MWCNT SA for HA sorption and the associated influence on sorption of aromatic compounds. Further, molecular size and hydrophobicity of aromatic compounds strongly affected their competition with HA on MWCNTs

New Mechanism via Dichlorocarbene Intermediate for Activated Carbon-Mediated Reductive Dechlorination of Carbon Tetrachloride by Sulfide in Aqueous Solutions

Although activated carbon (AC) is widely used as an adsorbent and barrier for contaminated sediment remediation, little attention has been paid to its mediation effects on reductive dechlorination of chlorinated solvents by commonly presenting sulfide. Here, we reported that highly porous, graphitized AC (250 mg L–1) suspended in deoxygenated aqueous solutions could increase the pseudo-first-order rate constant of sulfide-induced dechlorination of carbon tetrachloride (CCl4) by more than 1 order of magnitude. Carbon disulfide (CS2) was the only main product, with no production of chloroform or dichloromethane. The minimum promotion of CCl4 reduction observed with electro-conductive but nonporous graphite and a microporous but electro-insulative resin (XAD-4) indicates that graphitic carbons and micropores both play key roles in AC-mediated dechlorination of CCl4 by sulfide. The detection of dichlorocarbene (:CCl2) by free radical trapping experiments combined with the high suitability of the Langmuir–Hinshelwood model led us to propose a new mediation mechanism: CCl4 molecules adsorbed within the deep regions of AC micropores formed by graphitic carbons accept two electrons transferred from sulfide to form :CCl2, which is impeded from hydrolysis and hydrogenolysis by the hydrophobic micropore and further reacts with sulfide to generate CS2. Consistently, the production of :CCl2 was very low when AC was replaced with graphite or XAD-4. The proposed mechanism was further validated by the enhanced mediation effects of another two carbonaceous materials (template-synthesized mesoporous carbon and covalent triazine-based framework) that are electro-conductive and have well-developed micropore structures. These findings highlight the importance of pore properties of carbonaceous materials as mediators or catalysts for reductive dechlorination reactions and shed light on the development of coupled adsorption-reaction systems for remediation

Impact of De-Ashing Humic Acid and Humin on Organic Matter Structural Properties and Sorption Mechanisms of Phenanthrene

Organic matter−mineral interactions greatly affect the fate of hydrophobic organic compounds (HOCs) in the environment. In the present study, the impact of organic matter−mineral interaction on sorption of phenanthrene (PHE) by the original and de-ashed humic acids (HAs) and humin (HM) was examined. After de-ashing treatment, the overall polarity of organic matter in HAs and HM consistently decreased. Differently, the surface polarity of HAs increased but that of HM decreased. No correlation between Koc values of PHE by all tested sorbents and their bulk polarity was observed due to inaccessibility of a portion of interior sorption domains. The inaccessibility of interior sorption domains in HAs and HM was partly due to the crystalline structure in organic matter as indicated by differential scanning calorimetric (DSC) and 13C NMR data and the interference from minerals. A good correlation between surface polarity of the original and de-ashed HAs and HMs and their Koc values for PHE indicated its importance in HOC sorption. Dissimilar changes in surface polarity of HAs and HM after de-ashing treatment can be ascribed to the distinct interactions between organic matter and minerals. The solid-state 13C NMR, XPS, and elemental composition data of all tested sorbents revealed that a larger fraction of O atoms in HAs were involved in organic matter−mineral interaction as compared to HM. Results of this work highlight the importance of soil organic matter (SOM)-mineral interactions, surface polarity, and microscaled domain arrangement of SOM in HOC sorption

Sorption of Aromatic Organic Contaminants by Biopolymers: Effects of pH, Copper (II) Complexation, and Cellulose Coating

Sorption of hydrophobic organic compounds (HOCs) (i.e., pyrene, phenanthrene, naphthalene, and 1-naphthol) by original and coated biopolymers was examined. Lignin yielded nonlinear isotherms due to its glassy character. Except for pyrene, cellulose showed linear isotherms for other compounds, indicating a partitioning dominant mechanism. Sorption of 1-naphthol by lignin decreased with increasing pH, attributed to both the increased πeϑ−πeϑ repulsion and weakened hydrogen bonds, while the affinity reduction of cellulose for 1-naphthol with increasing pH resulted from only the decrease in H-bonding due to its absence of benzene ring. Complexation of lignin with Cu2+ increased the sorption affinity for phenanthrene (2.6 times) and slightly enhanced its isotherm nonlinearity. For 1-naphthol, lignin−Cu2+ complex had a much higher sorption capacity (7 times) than the original lignin, accompanied by the increased isotherm nonlinearity. Cellulose-coated lignin showed increased sorption affinity and more pronounced nonlinearity for 1-naphthol than the lignin-Cu2+ complex. In comparison, cellulose coating exhibited little effect on sorption affinity for phenanthrene relative to the lignin−Cu2+ complex. Isotherm nonlinearity of coated lignins increased with increasing cellulose coating, indicating more condensed domains produced, supported by an increase (from 68.9 °C for the original lignin to 82.4 °C for the highest cellulose coating level) in glass transition temperature (Tg). Results of this study highlight the importance of structure, polarity, surface O-containing functional groups, and surface charges of biopolymers in controlling HOC sorption

Emission of Polycyclic Aromatic Hydrocarbons in China by County

Quantitative relationships among social, economic, and climate parameters, and energy consumption for Chinese provinces, provide data for regression models' estimated rates of energy consumption and emission of polycyclic aromatic hydrocarbons (PAHs) by county. A nonlinear model was used for domestic coal combustion with total population and annual mean temperature as independent variables. Linear regression models were utilized for all other types of fuel consumption. Monte Carlo simulation demonstrated that emission factors, rather than the regression modeling, constitute the main source of uncertainty in prediction. Models were validated using available energy data of several northern and southern counties of China from the literature. The total PAHs produced by each county is approximately equivalent to the sum of the total emission from energy, coke, and aluminum production

Public Health Risk of Arsenic Species in Chicken Tissues from Live Poultry Markets of Guangdong Province, China

Arsenic-based feed additives, such as roxarsone (ROX), are still legally and widely used in food animal production in many countries. This study was conducted to systematically characterize the content and speciation of arsenic in chicken tissues from live poultry markets and in commercial chicken feeds in Guangdong, a major poultry production and consumption province in China, and to assess the corresponding public health risk. The total arsenic contents in the commercial feeds could be modeled as a mixture of two log-normal distributions (geometric means: 0.66 and 17.5 mg/kg), and inorganic arsenic occurred at high levels (0.19–9.7 mg/kg) in those with ROX detected. In general, chicken livers had much higher contents of total arsenic compared to the muscle tissues (breast and drumstick), and chicken muscle from the urban markets contained arsenic at much higher levels than that from the rural markets. The incremental lifetime cancer risk (bladder and lung cancer) from dietary exposure to arsenic contained in chicken meat products on local markets was above the serious or priority level (10^–4) for 70% and 30% of the adult populations in Guangzhou and Lianzhou, respectively. These findings indicate the significant need to phase out the use of arsenic-based feed additives in China