Mantle-derived gaseous components in ore-forming fluids of the Xiangshan uranium

The Xiangshan U deposit, the largest hydrothermal U deposit in China, is hosted in late Jurassic felsic volcanic rocks although the U mineralization post dates the volcanics by at least 20 Ma. The mineralization coincides with intrusion of local mantle-derived mafic dykes formed during Cretaceous crustal extension in South China. Ore-forming fluids are rich in CO2, and U in the fluid is thought to have been dissolved in the form of UO2 (CO3)22− and UO2 (CO3) 34− complexes. This paper provides He and Ar isotope data of fluid inclusions in pyrites and C isotope data of calcites associated with U mineralization (pitchblende) in the Xiangshan U deposit. He isotopic compositions range between 0.1 and 2.0Ra (where Ra is the 3He/4He ratio of air=1.39×10−6) and correlates with 40Ar/36Ar; although there is potential for significant 3He production via 6Li(n,α)3H(β)3He reactions in a U deposit (due to abundant neutrons), nucleogenic production cannot account for either the 3He concentration in these fluids, nor the correlations between He and Ar isotopic compositions. It is more likely that the high 3He/4He ratios represent trapped mantle-derived gases. A mantle origin for the volatiles of Xiangshan is consistent with the δ13C values of calcites, which vary from −3.5‰ to −7.7‰, overlapping the range of mantle CO2. The He, Ar and CO2 characteristics of the ore-forming fluids responsible for the deposit are consistent with mixing between 3He- and CO2-rich mantle-derived fluids and CO2-poor meteoric fluids. These fluids were likely produced during Cretaceous extension and dyke intrusion which permitted mantle-derived CO2 to migrate upward and remobilize U from the acid volcanic source rocks, resulting in the formation of the U deposit. Subsequent decay of U within the fluid inclusions has reduced the 3He/4He ratio, and variations in U/3He result in the range in 3He/4He observed with U/3He ratios in the range 5–17×103 likely corresponding to U concentrations in the fluids b0.2 ppm.

云南兰坪金顶超大型铅锌矿区镉的环境地球化学研究

矿产资源开采利用过程中导致的重金属环境污染问题日益严重。我国铅锌矿资源丰富，其开采利用过程中镉的环境污染也日益突出。本文通过对云南兰坪金顶Pb-Zn矿区矿床开采利用过程中镉等重金属元素的环境地球化学行为及矿区生态环境的研究，得出如下主要结论。 1. 矿石淋滤实验表明矿区部分氧化铅锌矿石可以很快被再次氧化或者被溶解并释放出大量镉等有害元素，滤出元素可以迅速发生沉淀或被沉淀物包裹，其释放能力表现为Zn>Pb>Cd。铅锌氧化矿石中菱锌矿组分含量是影响镉淋失的主要因素。在开放体系的水-岩作用下，矿区岩石、矿物的自然风化极易造成当地水系统中镉污染。 2. 矿区不同岩（矿）石中镉含量分布差异比较大，围岩中镉含量为50-650 ppm，平均310 ppm，原生矿中镉含量为14-2800 ppm，平均767 ppm，氧化矿中镉含量为110-8200 ppm，平均1661 ppm，其平均值最高。Zn、Cd地球化学性质的差异导致了二者在原生矿和氧化矿中的不同地球化学分配特点，原生矿Zn/Cd高于氧化矿Zn/Cd，表明氧化环境中镉更容易在氧化矿中富集，而锌更容易被氧化析出到环境中。氧化矿中Cd与Ca呈负相关，这表明Cd的富集和Ca的氧化淋失是同时进行的，并且还可能有Cd对Ca的类质同像代替存在。 3. 矿区上游对照区土壤中的高含量Cd浓度是因土壤母质层重金属高背景值造成的，而非人为污染。矿区中心区土壤受到严重Cd污染，可能与选厂、采场废石堆、尾矿库和露采矿山大范围暴露有关。矿区沿沘江下游两岸土壤中Cd含量远远超出上游土壤背景值和金顶全区土壤背景值，这可能是与污水灌溉有关。通过加权综合污染指数评价法发现矿区土壤污染的主要因子是Cd，其次是Zn和Pb，矿区土壤重金属污染贡献顺序为：Cd>Zn>Pb。矿区土壤污染主要表现为：矿区土壤污染有从中心区向沘江下游扩散区土壤中蔓延的趋势。 4. 矿区水体中出现较高含量的镉，高出天然河流中镉含量的50-100倍。矿区架崖山、北厂和跑马坪等采矿区水体中镉浓度范围在15-30 µg/L之间。矿区水体中镉含量水平表现为：矿山浅层地下水>矿山溪流水>沘江河水。研究结果表明，矿区沘江下游河段水体明显受镉污染，其中水体中镉的平均含量为15.7 µg/L，悬浮物中镉含量为49.3 mg/kg，沉积物中镉含量为203.7 mg/kg。矿区载镉岩石和矿物的自然风化是造成矿区水环境中镉污染的直接原因。 5. 跑马坪采场的废弃石具有较低的Cd含量，而北厂、架崖山采场的废弃石具有较高的Cd浓度，可能与废弃矿石类型本身差异有关。尾矿剖面中的Cd含量，在表层中随剖面深度呈递减趋势，在中层随剖面深度变化不明显，而在底层中明显富集。尾矿库表层尾矿样品中弱酸提取态和可还原态Cd高于底层尾矿样品，相比之下，表层尾矿中Cd等重金属元素易于释放到环境中，对环境的潜在危害大。老尾矿库尾矿砂中Cd金属总量高于新尾矿库尾矿砂，可能还是因为选矿工艺、技术的差异造成的。 6. 矿区污染段水体中硫同位素值较低，远远低于上游非污染区硫同位素值。矿区水体中δ34S值保持了金顶铅锌矿山源区矿山物质硫同位素的特征，显示了矿山来源物质的影响。根据水体硫酸盐中硫同位素稀释原理，研究发现沘江下游水体SO42-中85 %的硫来源于矿山物质。 7. 从矿区筛选出Cd、Zn、Pb的超富集植物共有4种：其中Cd超富集植物有2种，分别是本地生条裂萎陵菜（Potentilla lancinata Card. In Lecomte）和辣子草（Galinsoga parviflora Cav.）；Zn超富集植物仅发现有1种植物，为节节草（Equisetum ramosissmum Desf.）；Pb超富集植物发现了1种植物，为毛莲菜（Picris hieracioides L.）。这些植物均具备了超富集植物的基本特征，在污染土壤治理与修复方面具有一定的实践意义。 8. 建立了金顶铅锌矿山（床）地质环境模型。Cd的释放、迁移扩散模式为：雨水淋滤时，矿山固体废弃物产生富Cd的酸性或弱酸性矿山排水，通过下渗淋滤发生测向和垂向迁移，进入周边水体和土壤，然后被水系沉积物中针铁矿、方解石等吸附，并在沉降物中沉淀富集，导致矿区主要河流沘江水体的自净能力下降，加速水体的进一步恶化，破坏生物生存环境。矿区受污染水体、土壤和大气中的有害物质通过生物链进入动植物体内，进而危害人类健康。

天然有机质与多环芳烃相互作用的荧光光谱学研究—以苝、菲和蒽为例

自然界水体（如地下水、湖泊、河流及海洋）中的天然有机质（NOM）因具有显著的生态及环境功能而受到人们的广泛关注。NOM作为全球碳循环的主要组成部分，是水体中异养型微生物生长所需碳及能量的重要来源。此外，NOM对湖泊生态系统中的多种物理、化学过程具有不同程度的影响：NOM中的有机酸对淡水体系的酸度具有控制作用而且对酸沉降有一定的缓冲作用；NOM通过减弱可见光及紫外线照射从而影响淡水系统的光化学环境，这一方面降低了水体中自养生物的繁衍速度，同时保护水生生物免受有害的紫外线辐射。更为重要的是，NOM能够影响环境中污染物的命运，比如重金属离子和疏水性有机污染物与天然有机质结合后，其迁移途径、溶解度、生物可利用性及毒性受到明显的控制，这样其对环境中生物的危害性减弱。多环芳烃（PAHs）是一类对环境危害极大的疏水性有机污染物，多数PAHs对有机体具有毒性和致癌性而被美国环保署（EPA）列为优先处理的污染物。因此，确定NOM-PAHs反应的强弱，即PAHs在NOM中的分配系数（Kdoc）以及NOM的物化特性对Kdoc的影响对于预测PAHs毒性和生物可利用性大小、了解NOM-PAHs相互作用的机理及确立污染预测模型具有十分重大的意义。前人的研究结果表明，多环芳烃与天然有机质相互作用的分配系数Kdoc的与PAHs的疏水度（即Kow大小）、NOM的物化特性（如芳香度、脂肪碳含量、分子量及极性等）和水化学参数（pH、离子强度等）有很大的相关性。然而，许多学者在NOM的物化特性对分配系数的影响方面存有较大分歧。前人在研究天然有机质与多环芳烃之间的相互作用时所用的NOM多为腐殖质，即水体NOM中的疏水性组分，而其中的亲水性组分与PAHs的结合作用文献中却鲜见提及，因此人们对其结合作用的特性如反应机理和结合作用强弱等的了解十分有限。 本论文利用XAD树脂分离技术把红枫湖水体中天然有机质分成了疏水性酸、碱、中性物质和亲水性酸、碱、中性物质等六种有机组分。同时运用元素分析、有机碳分析仪、紫外－可见分光光度，高效液相体积排阻色谱、三维荧光光谱和荧光偏振技术等现代分析方法，对各有机组分的物理、化学特性进行了表征。荧光光谱技术被用于研究水化学参数对腐殖质荧光特性和分子构型的影响以及测定Amherst腐殖酸（Amherst HA）、垃圾渗滤液富里酸（LF FA）和红枫湖NOM有机组分与多环芳烃苝、菲和蒽的结合系数（分配系数、条件稳定常数）并探讨了影响分配系数的各种因素和NOM与多环芳烃相互作用的机理。本论文的获得的主要结果如下： 1. 在水化学参数对腐殖质荧光特性及分子构型的影响方面： 在NOM与PAHs相互作用过程中，腐殖质的分子构型起着关键性的作用，而其分子构型又受到其本身的浓度、溶液的pH和离子强度等水化学参数的影响。本论文运用三维荧光光谱和荧光偏振技术研究了水化学参数对腐殖质荧光特性的影响，并由此推断其分子构型变化情况。实验结果表明随着腐殖质浓度、溶液的pH和离子强度的改变，腐殖质荧光特性（如荧光强度、荧光峰位和荧光偏振值等）出现不同程度的变化，揭示了腐殖质分子构型的改变。因此，本实验结果有利于深入了解腐殖质的分子构型及其对吸附PAHs的重要影响，对理解NOM的环境行为具有一定的理论价值。 2. 运用荧光猝灭滴定法研究Amherst HA和红枫湖NOM有机组分对多环芳烃苝、菲和蒽的吸附作用，结果表明NOM对多环芳烃的吸附能力与其中芳香结构单元有很强的相关性，logKdoc值与NOM在280 nm处的摩尔吸收（ε280）和分子量有线性度较高的正比关系，而C/H原子比值对不饱和碳含量（芳香度）并没有很好的指示作用。同时，脂肪碳（0－50 ppm）中的聚亚甲基碳对吸附PAHs也有很大的贡献，而有机组分的极性与分配系数有明显的负相关性。不同种类的多环芳烃与NOM有机组分的结合能力有很大的差异，这取决于不同PAHs的疏水度大小。本研究结果有利于弥合前人在NOM物化特性对PAHs吸附能力的影响方面的分歧，揭示了NOM的组成和结构在与PAHs等疏水性有机污染物相互作用方面的重要影响。 3. 在结合机理方面，苝与NOM有机组分结合的Stern－Volmer图具有较高的线性度，这表明其结合机理以疏水反应（或分配）为主，而菲和蒽与有机组分相互作用的吸附等温线显示较多的非线性特征表明极性反应为主要反应机理，结合过程以吸附为主。在红枫湖NOM有机组分中，疏水性组分显示对PAHs较高的结合能力，同时疏水性组分与PAHs相互作用的吸附等温线具有较高的线性度。这一结果揭示了不同种类的PAHs和各NOM有机组分之间结合机理的差异，加深了人们对NOM与PAHs相互作用机理的认识。 4. 前人在研究pH、离子强度等水化学参数对分配系数的影响方面得到的结果多有矛盾之处，同时，对实验结果的解释也不尽相同。本实验结果表明分配系数随pH增加而下降；离子强度对分配系数的影响比较复杂，就总体趋势而言，增加离子强度有利于对PAHs吸附能力的提高，这一结果初步揭示了水化学参数对PAHs分配系数的影响，丰富了人们对其影响机理的认识。 5. 运用荧光偏振技术测定了蒽与Amherst HA、LF FA和红枫湖NOM有机组分的条件稳定常数，结果如下：随着NOM浓度增加，蒽的荧光偏振值不断下降，这表明蒽的分子构型由平面线性向圆柱状转变。在所有NOM样品中，土壤源的Amherst HA与蒽结合的条件稳定常数最大（pH 4时logK=5.6，pH 6时 logK=5.4）；就NOM有机组分而言，疏水性组分的logK变化范围在pH 6时为（4.4－5.2），而亲水性组分为（4.3－4.8），这表明疏水性组分具有相对较高的蒽吸附能力。实验结果有利于了解PAHs与NOM相互作用前后的分子构型变化情况，揭示了不同的NOM有机组分在吸附PAHs过程中的差异。

东川铜矿与大姚铜矿的实验地球化学研究及其成因探讨

本文通过对东川铜矿紫色层和大姚铜矿红层的浸出实验和构造模拟实验研究后认为，东川因民紫色泥砂质白云岩在NaCl, NaHS, NaHCO_3, HAc, NaAc溶液的作用下，铜的浸出量很低，铜很难被热卤水活化、迁移；而大姚铜矿的红色砂岩中的铜在NaCl等水溶液的作用下，铜的浸出量达一、二十个ppm，可以被热卤水活化、迁移。岩石中非成矿元素的赋存状态对铜的活化、迁移影响很大。根据两矿闲的地质地球化学特征，进一步探讨了两矿床的矿床成因。两者虽然在地质特征上有许多相似的地方，但它们的成矿物质来源不同，东川铜矿的铜来源于火山期后气液喷发；大姚铜矿的铜则来源于陆源风化。

The effect of chromium on sulfur resistance of Pd/HY-Al2O3 catalysts for aromatic hydrogenation

Sulfur is a major poison to noble metal catalysts for deep aromatic hydrogenation in the petroleum refining industry. In order to study the sulfur resistance of Pd-based catalysts, a series of Pd, Cr, and PdCr catalysts supported on HY-Al2O3 were studied by NH3-TPD, pyridine-adsorption IR, TPR, IR spectra of adsorbed CO, and toluene hydrogenation in the presence of 3000 ppm sulfur as thiophene under the following conditions: 533-573 K, 4.2 MPa, and WHSV 4.0 h(-1). Cr has no influence on the acidity of the catalysts. TPR patterns and in situ IR spectra of adsorbed CO revealed a strong interaction between Cr and Pd, and the frequency shift of linear bonded CO on Pd indicates that the electron density of Pd decreases with the increase of the Cr/Pd atomic ratio. The catalytic performance of Pd, Cr, and PdCr catalysts shows that the sulfur resistance of Pd is strongly enhanced by Cr, and the activity reaches its maximum when the Cr/Pd atomic ratio equals 8. The active phase model "Pd particles decorated by Cr2O3" is postulated to explain the behavior of PdCr catalysts. (C) 2001 Academic Press.

Strong effect of transitional metals on the sulfur resistance of Pd/HY-Al2O3 catalysts for aromatic hydrogenation

In order to improve the sulfur resistance of noble metal catalysts in the aromatic hydrogenation of diesel fuel, the alloying effect of non-noble metals with Pd was studied. Toluene hydrogenation over Pd and Pd-M bimetallic catalysts (M = Cr, W,La, Mn, Mo, Ag) on a mixed HY-Al2O3 support was investigated in the presence of 3000 ppm sulfur as thiophene in the feedstock. The results showed that the addition of the second metals strongly affected the activity of toluene hydrogenation, which suggests that the sulfur resistibility of Pd-M bimetallic catalysts is much different from single Pd. La, Mn, Mo and Ag decreased the sulfur resistance of the palladium catalysts. For example, the toluene conversion at 553 K was observed to decrease sharply from 39.4 wt.% on Pd to 1.6 wt.% on Pd-Ag, which is by a factor of 25. One of the important findings in this article is that Cr and W increase hydrogenation activity of Pd catalysts. The reactions occurring on these catalysts include hydrogenation, isomerization and hydrocracking, The addition of the second metals has no noticeable effects on the hydrogenation and isomerization selectivity, but it slightly suppresses hydrocracking reactions. The four typical catalysts, Pd-Cr, Pd-W, Pd-Ag and Pd were characterized by infrared (IR) spectroscopy of pyridine and CO. LR spectra of CO revealed the strong interaction between Pd and the second metal as Cr, W and Ag (or their oxide), indicating that the improvement in sulfur resistance originates from electron-deficient Pd with the addition of second metals. (C) 2001 Elsevier Science B.V. All rights reserved.

Preparation and characterization of multi-walled carbon nanotubes supported PtRu catalysts for proton exchange membrane fuel cells

A series of PtRu nanocomposites supported on H2O2-oxidized multi-walled carbon nanotubes (MWCNTs) were synthesized via two chemical reduction methods - one used aqueous formaldehyde (HCHO method) and the other used ethylene glycol (EG method) as the reducing agents. The effects of the solvents (water and ethylene glycol) and the surface composition of the MWCNTs on the deposition and the dispersion of the metal particles were investigated using N-2 adsorption. TEM. ICP-AES. FTIR and TPD. The wetting heats of the MWCNTs in corresponding solvents were also measured. The characterizations suggest that combination of the surface chemistry of the MWCNTs with the solvents decides the deposition and the dispersion of the metal nanoparticles. These nanocomposites were evaluated as proton exchange membrane fuel cell anode catalyts for oxidation of 50 ppm CO contaminated hydrogen and compared with a commercial PtRu/C catalyst. The data reveal superior performances for the nanocomposites prepared by the EG method to those by the HCHO method and even to that for tile Commercial analogue. Structure performance relationship of the nanocomposites was also studied. (C) 2005 Elsevier Ltd. All rights reserved.

Assimilacao do nitrogenio pelas plantas de panicum maximum, cv. Vencedor, submetidas a diferentes proporcoes NH4+/NO3-.

Visando a elucidar alguns aspectos do metabolismo do nitrogenio por plantas de Panicum maximum, estudaram-se os efeitos da proporcao NH +4/NO-3 na variacao do pH das solucoes nutritivas de tratamento, no acumulo de materia seca e na distribuicaoo de nitrogenio na planta. Para tal, as plantas foram cultivadas em camara de crescimento na solucao nutritiva (pH 5,5) completa, contendo nitrogenio somente na forma nitrica. Aos 25 dias depois da germinacao, as plantas foram submetidas a 100 ppm de nitrogenio fornecido nas formas nitrica e amoniacal ou amonical, nas seguintes proporcoes 0/100,25/75,50/50,75/25 e 100/0, durante quatro dias. O pH das solucoes nutritivas foi medido e corrigido diariamente. Os resultados demostraram que o pH aumentou na presenca exclusiva de nitrato (pH 6,5) e decresceu na presenca exclusiva de amonio (pH 3,5). Na presenca de ambas as formas de nitrogenio, o pH permaneceu em torno de 5,0 nos dois primeiros dias entao diminui para cerca de 4,5 nos dias subsequentes. Esses resultados indicam que as plantas de Panicum maximum absorveram o nitrato, inicialmente, porem com o tempo adaptaram-se a presenca de amonio e tenderam a absorver ambas as formas. As plantas que receberam nitrato, combinados ou não com amonio, apresentaram o peso da materia seca, N-total e N-soluvel total superiores as que receberam somente amonio como fonte de N, havenso pequena tendencia de maior acumulo na presenca de ambas as fontes. No tratamento 100% de amonio, pN-total e o N-soluvel total diminuiram apreciavelmente, indicando baixa capacidade de essa especie utilizar amonio como fonte única de nitrogenio. Os resultados indicam que, embora as plantas possa absorver as duas formas de nitrogenio, a combinacao entre ambas parece ser a forma ideal para o crescimento das mesmas.

US Foreign Sales Corporations, Export Tax Credits and the WTO

Rhodes, Mark. 'US Foreign Sales Corporations, Export Tax Credits and the WTO', in: 'The WTO and the Regulation of International Trade: Recent Trade Disputes between the European Union and the United States', (Cheltenham: Edward Elgar Publishing, 2005), pp.177-189 RAE2008

An unrecognized source of PCB contamination in schools and other buildings

An investigation of 24 buildings in the Greater Boston Area revealed that one-third (8 of 24) contained caulking materials with polychlorinated biphenyl (PCB) content exceeding 50 ppm by weight, which is the U.S. Environmental Protection Agency (U.S. EPA) specified limit above which this material is considered to be PCB bulk product waste. These buildings included schools and other public buildings. In a university building where similar levels of PCB were found in caulking material, PCB levels in indoor air ranged from 111 to 393 ng/m3; and in dust taken from the building ventilation system, < 1 ppm to 81 ppm. In this building, the U.S. EPA mandated requirements for the removal and disposal of the PCB bulk product waste as well as for confirmatory sampling to ensure that the interior and exterior of the building were decontaminated. Although U.S. EPA regulations under the Toxic Substances Control Act stipulate procedures by which PCB-contaminated materials must be handled and disposed, the regulations apparently do not require that materials such as caulking be tested to determine its PCB content. This limited investigation strongly suggests that were this testing done, many buildings would be found to contain high levels of PCBs in the building materials and potentially in the building environment. The presence of PCBs in schools is of particular concern given evidence suggesting that PCBs are developmental toxins.

Silica polymer bonding of stressed silica grains: An early growth of intergranular tensile strength

© 2015 Elsevier Ltd. All rights reserved.Laboratory tests on microscale are reported in which millimeter-sized amorphous silica cubes were kept highly compressed in a liquid environment of de-ionized water solutions with different silica ion concentrations for up to four weeks. Such an arrangement simulates an early evolution of bonds between two sand grains stressed in situ. In-house designed Grain Indenter-Puller apparatus allowed measuring strength of such contacts after 3-4 weeks. Observations reported for the first time confirm a long-existing hypothesis that a stressed contact with microcracks generates silica polymers, forming a bonding structure between the grains on a timescale in the order of a few weeks. Such structure exhibits intergranular tensile force at failure of 1-1.5 mN when aged in solutions containing silica ion concentrations of 200-to 500-ppm. The magnitude of such intergranular force is 2-3 times greater than that of water capillary force between the same grains.

Menthol attenuates respiratory irritation and elevates blood cotinine in cigarette smoke exposed mice.

Addition of menthol to cigarettes may be associated with increased initiation of smoking. The potential mechanisms underlying this association are not known. Menthol, likely due to its effects on cold-sensing peripheral sensory neurons, is known to inhibit the sensation of irritation elicited by respiratory irritants. However, it remains unclear whether menthol modulates cigarette smoke irritancy and nicotine absorption during initial exposures to cigarettes, thereby facilitating smoking initiation. Using plethysmography in a C57Bl/6J mouse model, we examined the effects of L-menthol, the menthol isomer added to cigarettes, on the respiratory sensory irritation response to primary smoke irritants (acrolein and cyclohexanone) and smoke of Kentucky reference 2R4 cigarettes. We also studied L-menthol's effect on blood levels of the nicotine metabolite, cotinine, immediately after exposure to cigarette smoke. L-menthol suppressed the irritation response to acrolein with an apparent IC₅₀ of 4 ppm. Suppression was observed even at acrolein levels well above those necessary to produce a maximal response. Cigarette smoke, at exposure levels of 10 mg/m³ or higher, caused an immediate and marked sensory irritation response in mice. This response was significantly suppressed by L-menthol even at smoke concentrations as high as 300 mg/m³. Counterirritation by L-menthol was abolished by treatment with a selective inhibitor of Transient Receptor Potential Melastatin 8 (TRPM8), the neuronal cold/menthol receptor. Inclusion of menthol in the cigarette smoke resulted in roughly a 1.5-fold increase in plasma cotinine levels over those observed in mice exposed to smoke without added menthol. These findings document that, L-menthol, through TRPM8, is a strong suppressor of respiratory irritation responses, even during highly noxious exposures to cigarette smoke or smoke irritants, and increases blood cotinine. Therefore, L-menthol, as a cigarette additive, may promote smoking initiation and nicotine addiction.

El mercado de carbono y el mecanismo de desarrollo limpio : caso Granja Tres Arroyos S.A. : un análisis desde la Nueva Economía Institucional

El calentamiento global consiste en el aumento de la temperatura de la tierra debido a la acumulación de los gases de efecto invernadero (GEI) en la atmósfera. Estos gases son producidos por actividades de generación de energía, el transporte, el uso del suelo, la industria y el manejo de los residuos. El aumento de GEI en la atmósfera provoca cambios climáticos e impactos en un sinfín de actividades humanas, en la productividad de la agricultura y ganadería, en la infraestructura y turismo, y también daños en la salud. La comunidad científica considera que el aumento de la temperatura para el fin del siglo debería ubicarse en los 2° C, para de esta forma poder limitar los impactos del cambio climático. Ello implicaría restringir las concentraciones de los GEI en valores cercanos a los 450 ppm (partes por millón). El problema económico del cambio climático subyace en que las emisiones de GEI constituyen una externalidad global. Una externalidad ocurre cuando la producción o consumo de un bien afecta a terceros que no participan directamente en su producción, venta o compra. Cuando hay presencia de externalidades, los precios de mercado no reflejan todos los costos ni beneficios sociales asociados a la producción de un bien. En el caso puntual del cambio climático, los emisores de GEI no asumen el costo de emitir gases a la atmósfera. Existen diferentes instrumentos de política ambiental que influyen en la percepción del recurso ambiental por parte del agente económico y que por ende, se reflejan en las decisiones económicas que ellos toman. Todos ellos tienen por objetivo asignar un precio/costo al recurso ambiental. El objetivo de los mercados de emisiones es asignar un precio al carbono. En ellos, se intercambian derechos a emitir cierta cantidad de GEI. El mecanismo de desarrollo limpio (MDL) es un instrumento de mercado definido en el Acuerdo de Marrakech bajo el marco institucional del Protocolo de Kyoto (PK). El MDL establece que un país Anexo I (país desarrollado), con compromisos de reducción de emisiones, invierta en proyectos de reducción o captación de emisiones en un país No Anexo I (país en desarrollo sin compromisos de reducción), mediante la compra de reducciones certificadas de emisiones (RCEs) generados a partir de la implementación de los proyectos. Argentina ratificó el PK a través de la ley nacional 25.438 en el 2001. Como el país se encuentra comprendido en las Partes No Anexo I, sólo puede participar como país anfitrión de un proyecto MDL y ser oferente de RCEs. Hasta la fecha, Argentina desarrolló 65 proyectos que se encuentran en distinta etapa de aprobación nacional o registro internacional. La mayoría corresponden a proyectos vinculados con rellenos sanitarios y energías. Bajo este marco de desarrollo en el país, resulta valiosa la exploración de un caso implementado en la Argentina. El objetivo de la investigación consiste en analizar el mercado de carbono a través del mecanismo de desarrollo limpio y su implementación en una empresa agroindustrial argentina, Granja Tres Arroyos S.A., bajo el enfoque teórico de la Nueva Economía y los Negocios Agroalimentarios.

Fertilización y estres salino en plantas ornamentales anuales

p.93-100

Curvas de tuberización : IV influencia de algunos factores en la pérdida de peso de papa almacenada

p.145-158