酸性含Fe3+溶液作用下铀的溶解迁移特征

采用不同Fe~(3+)浓度的酸性溶液,对取自新疆伊犁盆地的砂岩铀矿石进行了溶浸对比试验,探讨了铀在酸性含Fe~(3+)溶液作用下的溶解迁移动力学特征及其与Fe~(3+)的关系。结果表明,在Fe~(3+)的氧化作用下,铀从矿石向溶液的迁移于10小时内快速达到平衡,溶解速度衰减迅速;铀的溶解速度与Fe~(3+)向Fe~(2+)转化速度呈正相关的指数函数关系,当Fe~(3+)向Fe~(2+)转化速度趋近零时,铀的氧化溶解基本停止,溶液中的铀达到平衡浓度;Fe~(3+)向Fe~(2+)浓度达到2g/L可使铀强烈溶解迁移,而溶液酸度增高会弱化铀溶解速度与Fe~(3+)转换速度的关系,但酸度在2g/L~4g/L(对应pH值1.65~1.33)之间变化不会对铀的浸出产生显著影响;保持Fe~(3+)浓度为2g/L、酸度为2g/L(pH值1.65)的水化学条件对铀的溶浸是经济且足够有效的。     

碘量法测定锰矿中二氧化锰

本法基于MnO在酸性介质中与KI发生氧化还原反应而定量析出I_2,然后以淀粉为指示剂,用Na_2S_2O_3标准溶液滴定I_2间接测定MnO_2。Cu~(2+)、Fe~(3+)在上述条件下干扰测定。在锰矿中Cu~(2+)含量甚微,不足以影响结果。在H_3PO_4溶矿的测定体系中,H_3PO_4掩蔽了Fe~(3+),同时控制了酸度。本法快速、简便、准确度高,适用于锰矿中MnO_2的测定。     

离子色谱法测定六价铬和三价铬

本文提出了用HPIC-AG4柱和用0.5mmol/L Na_2CO_3为淋洗剂(2.0ml/min),离子色谱法测定土壤中Cr(Ⅵ)。50μl进样体积时的检测限为0.1μg/ml Cr(Ⅵ),同时在0.01mol/L NaOH的介质中,H_2O_2将Cr~(3+)氧化成CrO_4~(2-),然后测定总Cr量,由此计算Cr~(3+)的含量。     

聚噻吩插层有机蒙脱土导电复合材料的制备及其修饰

实验采用Fe~(3+)-H_2O_2催化氧化体系,以水为介质,十二烷基苯磺酸钠(十二烷基苯磺酸钠)为乳化剂,室温下制备聚噻吩(PTP)有机蒙脱土(OMMT)复合材料。当有机蒙脱土/聚噻吩=2g/ml时,反应时间为12 h,制得复合材料电导率最高,其值为3.44×10~(-5)S/cm;在Fe~(3+)-H_2O_2和过硫酸铵(APS)两种氧化体系下,分别采用苯胺(ANI)和吡咯(PY)对聚噻吩-有机蒙脱土进行修饰,制得聚苯胺-(聚噻吩-有机蒙脱土)和聚吡咯-(聚噻吩-有机蒙脱土)复合材料的电导率均达到10~(-2)S/cm~10~(-1)S/cm,较未修饰时提高了10~3倍~10~4倍。傅里叶红外光谱(FTIR)和X射线衍射(XRD)表明噻吩(TP)进入有机蒙脱土层间聚合,形成插层复合,经苯胺和吡咯修饰后,插层结构没有被破坏;重分析(TG)则证实复合后材料热稳定性得到提高,500℃时失重在11.69%左右,但经修饰后材料的热稳定性有所降低500℃时失重达到80%以上。扫描电镜(SEM)表明在Fe~(3+)-H_2O_2氧化体系下,经修饰后材料形貌更规整,且均为球形。     

电子探针定量分析直接测定含铁矿物中二价和三价铁

研究表明,含铁矿物中铁的平均价态(反映Fe~(2+)和Fe~(3+)含量)与Fe Lβ/Fe Lα值呈正比关系曲线。未知样品只要测得TFeO含量和Fe Lβ、Fe Lα的计数值,就可以分别获得FeO和Fe_2O_3的定量分析结果。经几种含铁矿物测定实例验证,说明方法可行。     

电气石/H_2O_2体系降解Cu(Ⅱ)-EDTA废水的研究

Cu(Ⅱ)-EDTA废水由于其螯合性难以处理.采用电气石/H_2O_2体系进行降解,结果显示废水处理达到较好的效果.COD和Cu~(2+)的去除率与电气石投加量、H_2O_2用量和温度呈正相关性.溶液pH=3时,两者去除率最大.紫外-可见吸收光谱显示,处理后的Cu(Ⅱ)-EDTA被降解为小分子有机物.通过对比电气石反应前后的XRD图谱和红外光谱发现,电气石与EDTA降解中间产物发生络合.反应动力学研究结果表明,电气石/H_2O_2体系降解废水的反应为拟一级反应.     

蒙脱石向海绿石转变的“海绿石化”模拟研究

海绿石是一种浅海相沉积指示矿物,形成成熟的海绿石需1Ma。本文以含铁量很低的蒙脱石和0.1mol/LFe~(3+)、0.2mol/LK~+的离子溶液分别为初始物质在50℃恒温环境和不同的pH、Eh条件下,模拟海绿石化过程,并通过XRD、FTIR、SEM、ESR等检测反应产物。研究发现,在SEM下观察到球形纳米颗粒的海绿石出现;同时XRD谱图上除出现云母类矿物的001衍射峰外,还出现d值为0.1520nm和0.1498nm的代表海绿石和铁伊利石的060特征衍射峰。pH和氧化还原条件对产物的影响比较显著,pH为酸性时出现海绿石和铁伊利石的组合,其红外吸收谱中出现AlFe~(2+)OH或Mg~(2+)Fe~(3+)OH(3547~3562cm~(-1))、Fe~(3+)Fe~(3+)OH的伸缩振动峰,与之相应的ESR中出现g=1.978的八面体位Fe~(3+)的共振峰;当pH介于中性的时候,红外光谱谱图上出现明显的AlFe~(2+)OH、Mg~(2+)Fe~(3+)OH(3550~3562cm~(-1))和Al~(3+)Fe~(3+)OH(870cm~(-1))振动;pH为碱性时,出现Fe~(3+)Mg~(2+)OH(3560cm~(-1))振动;上述红外吸收振动的出现表明Fe~(3+)已经进入了蒙脱石结构的八面体位。蒙脱石在上述条件下发生明显的改变,发生海绿石化或铁伊利石化,pH和氧化还原条件对结果的影响比较显著,酸性的环境中海绿石化显著,而中性至碱性的环境则发生铁伊利石化。     

Cd~(2+)胁迫下小球藻(Chlorella vulgaris)对岩溶水HCO_3~-、Ca~(2+)利用研究——以桂林寨底地下河为例

初级生产者藻类对维持生态系统稳定具有重要的意义。2012年底广西龙江重金属Cd~(2+)污染对其下游水体中水生生物造成了严重的彩响,为了解Cd~(2+)对岩溶水体中藻类碳汇效应的影响,针对广西龙江重金属Cd~(2+)污染,文章通过室内封闭培养体系研究了在0、10、20、40μmol/L不同Cd~(2+)浓度胁迫下,小球藻对岩溶水中游离CO_2、HCO_3和Ca~(2+)的利用情况以及体系中pH和生物量的相应变化。结果表明:当Cd~(2+)浓度在0~10μmol/L时,小球藻对岩溶水中Ca~(2+)和HCO_3~-的利用基本上没有受到影响;当Cd~(2+)浓度在10~40μmol/L时,对小球藻利用Ca~(2+)和HCO_3~-具有一定的抑制作用;当Cd~(2+)浓度高于40μmol/L时,小球藻将不能利用岩溶水中Ca~(2+)和HCO_3~-同时pH漂移实验表明:当Cd~(2+)浓度在0~20μmol/L时,小球藻能同时利用岩溶水中游离CO_2和HCO_3~-进行光合作用;Cd~(2+)浓度为10μmol/L时,体系中藻细胞生物量与空白对照组基本相同;当Cd~(2+)浓度在20~40μmol/L时,小球藻只能利用岩溶水中游离CO_2进行光合作用;当Cd~(2+)浓度为20μmol/L时,藻细胞生物量为空白对照组的一半;当Cd~(2+)浓度为40μmol/L时,小球藻生物量仅为20μmol/L时的一半。     

一个标准大气压下含铁和铝的钙硅酸盐体的粘度和结构

CaO-SiO_2-Fe-O体系熔馆体的温度-粘度关系已经确定,其数值可与那些组成中代替Si~(4十)的是Al~(3+)而不是Fe~(3+)的类似熔体对比.温度在液相线之上时,含铁熔体粘度的变化范围在5和15泊之间,作为Ca/Si、铁含量和温度(1400—1600℃)的函数,这些熔体粘滞流的活化能在8—50千卡/摩尔之间.与无铁熔体相比,当加入5%(重量)Fe_2O_3时,含铁熔体的粘度值明显增加,如继续加入三价铁就会引起这一趋势的逆转.恒温时,粘滞流的活化能随Fe~(3+)含量的增加而减小,当铁含量固定时,活化能则随温度的增加而减小.而已发表的CaO-Al_2O_3-SiO_2体系熔体的粘度数据表明,随Al_2O_3含量的增加,其粘度值持续增加.这些     

济南市某废弃化工厂区域土壤地球化学特征研究

随着我国城市产业布局升级,高污染、高能耗的化工企业被关停或搬离,研究这些企业遗留地块土壤的地球化学特征对污染场地风险管控具有重要的现实意义。本文在济南市某废弃化工厂原厂区采集了100件土壤样品,测试SiO_2、Al_2O_3、CaO、Fe_2O_3、K_2O、MgO、Na_2O、As、Ba、Br、Ce、Cl、Co、Cu、Ga、Hf、La、Mn、Nb、Nd、Ni、P、Pb、Rb、S、Sc、Sr、Th、Ti、V、Y、Zn、Zr、Cr、Cr(Ⅵ)、铬形态、pH值、有机质等42项指标,研究厂区土壤中元素的地球化学特征。结果表明:研究区土壤中的总铬量在1025~2450 mg/kg,Cr(Ⅵ)含量在557.0~996.5 mg/kg,Cr(Ⅵ)含量占总铬量的46.77%。在0~80 cm深度内,总铬、Cr(Ⅵ)含量随深度的变化并不明显。随着深度增加,离子交换态铬占总铬的比例有逐渐降低的趋势,而残渣态铬占总铬的比例增大。土壤的总铬含量与pH值呈线性负相关,相关系数为-0.8470,而Cr(Ⅵ)与有机质、pH值相关性均不显著。聚类分析发现Cr元素的来源单独成为一类,主要为化工厂长期堆放大量铬渣引起的人为污染所致,其他组分与全国土壤、黄淮海平原土壤则有着相似的自然源。     

Laser-induced time-resolved luminescence of tugtupite,sodalite and hackmanite

We have interpreted a number of luminescence centers in natural tugtupite Na₈Al₂Be₂Si₈O₂₄Cl₂, sodalite Na₈Al₆Si₆O₂₄C₂ and hackmanite Na₈Al₆Si₆O₂₄(Cl₂,S) by use of laser-induced time-resolved luminescence spectroscopy. The main new results are the following: Fe³⁺, Mn²⁺, Eu²⁺, Ce³⁺, mercury type (potentially Pb²⁺, Tl⁺, Sn²⁺ and/or Sb³⁺), radiation induced luminescence centers; several types of S₂ ⁻ centers. Spectral shift connected with the presence of luminescence centers, which are detected together with S₂ ⁻ centers and impossible to resolve with continuous wave luminescence spectroscopy, is the possible reason for spectral diversity of S₂ ⁻ luminescence centers presented in different publications.     

Zero-valent iron/persulfate(Fe0/PS) oxidation acetaminophen in water

Zero-valent iron (Fe⁰), as an alternative iron source, was evaluated to activate persulfate (PS) to degrade acetaminophen (APAP), a representative pharmaceutically active compound in water. Effects of key factors in the so-called Fe⁰/PS process, including Fe⁰ dosage, initial pH, temperatures and chelating agents, were studied. Under all the conditions tested, the APAP degradation followed a pseudo-first-order kinetics pattern. The degradation efficiency of APAP was highest when the Fe⁰ to PS molar ratio increased to 1:1, and the degradation rate constant and removal were 23.19 × 10^?3 min^?1 and 93.19 %, respectively. Comparing with Fe²⁺, Fe⁰ served as an alternative iron source that can gradually release Fe²⁺ into water, thereby consistently activating PS to produce sulfate radicals. The Fe⁰/PS system was effective in a broader pH range from 3 to 8.5. Heat could facilitate production of sulfate radicals and enhance the APAP degradation in the Fe⁰/PS system. High reaction temperature also improved the Fe²⁺/PS oxidation of APAP. Finally, sodium citrate (a chelating agent) at an appropriate concentration could improve the APAP degradation rate in the Fe²⁺/PS and Fe⁰/PS system. The optimal molar ratio of Fe⁰ to citrate depended on solution pH. Our results demonstrated that Fe⁰ was an alternative iron source to activate PS to degrade APAP in water.     

Degradation of polyvinyl alcohol in aqueous solutions using UV-365 nm/S2O8 2− process

This investigation evaluates the effectiveness of UV-365 nm/S₂O₈ ^2? process in degrading polyvinyl alcohol in aqueous solutions. The effects of pH, Na₂S₂O₈ dosage, and temperature on the degradation efficiency of polyvinyl alcohol were studied. Under acidic conditions, the degradation efficiency of polyvinyl alcohol exceeded that under alkaline conditions. Additionally, a higher Na₂S₂O₈ dosage and a higher temperature were associated with a higher degradation efficiency of polyvinyl alcohol. The degradation rates of polyvinyl alcohol followed a pseudo-first-order kinetic model. Moreover, the observed degradation rate coefficient increased from 0.0078 to 0.4081 min^?1 when the temperature was increased from 10 to 55 °C. Also, the activation energy estimated using the observed degradation rate coefficients and the Arrhenius equation was 64 kJ/mol. At UV-365 nm, pH 3, an Na₂S₂O₈ dosage of 0.06 g/L, a temperature of 55 °C, and an initial polyvinyl alcohol concentration of 20 mg/L, around 100 % of polyvinyl alcohol was degraded, indicating that UV-365 nm/S₂O₈ ^2? process has great potential in degrading polyvinyl alcohol in aqueous solutions.     

Entropy and structure of oxidized and reduced iron-bearing silicate glasses

The influence of ferrous and ferric iron on the low-temperature heat capacity and vibrational entropy of silicate glasses has been determined by adiabatic calorimetry. Two pairs of samples based on sodium disilicate and calcium Tschermak molecule compositions have been studied. Along with previous data for another Fe-bearing glass, these results have been used to complement the available set of composition independent partial molar relative entropies of oxides in silicate glasses with S₂₉₈ − S₀ values of 56.7 and 116 J/mol for FeO and Fe₂O₃, respectively. The calorimetric data indicate that the fraction of fivefold coordinated Al is significant in the CaO-“FeO”-Al₂O₃-SiO₂ system and that association of Ca²⁺ and Na⁺ with Fe³⁺ in tetrahedral coordination for charge compensation does not entail significant changes in coordination for these two cations. At very low temperatures, however, the heat capacity is no longer an additive function of composition because of unexpectedly high positive deviations from Debye laws. These anomalies are stronger for the reduced than the oxidized glasses and considerably larger than for iron-free glasses, but their origin cannot be established from the present measurements.     

Effects of strong network modifiers on Fe<Superscript>3+</Superscript>/Fe<Superscript>2+</Superscript> in silicate melts: an experimental study

The effect of CaO, Na₂O, and K₂O on ferric/ferrous ratio in model multicomponent silicate melts was investigated in the temperature range 1450–1550?°C at 1-atm total pressure in air. It is demonstrated that the addition of these network modifier cations results in an increase of Fe³⁺/Fe²⁺ ratio. The influence of network modifier cations on the ferric/ferrous ratio increases in the order Ca?<?Na?<?K. Some old controversial conceptions concerning the effect of potassium on Fe³⁺/Fe²⁺ ratio in simple model liquids are critically evaluated. An empirical equation is proposed to predict the ferric/ferrous ratio in SiO₂–TiO₂–Al₂O₃–FeO–Fe₂O₃–MgO–CaO–Na₂O–K₂O–P₂O₅ melts at air conditions.     

Triple-oxygen-isotope determination of molecular oxygen incorporation in sulfate produced during abiotic pyrite oxidation (pH = 2-11)

Aqueous oxidation of sulfide minerals to sulfate is an integral part of the global sulfur and oxygen cycles. The current model for pyrite oxidation emphasizes the role of Fe²⁺-Fe³⁺ electron shuttling and repeated nucleophilic attack by water molecules on sulfur. Previous δ¹⁸O-labeled experiments show that a variable fraction (0-60%) of the oxygen in product sulfate is derived from dissolved O₂, the other potential oxidant. This indicates that nucleophilic attack cannot continue all the way to sulfate and that a sulfoxyanion of intermediate oxidation state is released into solution. The observed variability in O₂% may be due to the presence of competing oxidation pathways, variable experimental conditions (e.g. abiotic, biotic, or changing pH value), or uncertainties related to the multiple experiments needed to effectively use the δ¹⁸O label to differentiate sulfate-oxygen sources. To examine the role of O₂ and Fe³⁺ in determining the final incorporation of O₂ oxygen in sulfate produced during pyrite oxidation, we designed a set of aerated, abiotic, pH-buffered (pH = 2, 7, 9, 10, and 11), and triple-oxygen-isotope labeled solutions with and without Fe³⁺ addition. While abiotic and pH-buffered conditions help to eliminate variables, triple oxygen isotope labeling and Fe³⁺ addition help to determine the oxygen sources in sulfate and examine the role of Fe²⁺-Fe³⁺ electron shuttling during sulfide oxidation, respectively.Our results show that sulfate concentration increased linearly with time and the maximum concentration was achieved at pH 11. At pH 2, 7, and 9, sulfate production was slow but increased by 4× with the addition of Fe³⁺. Significant amounts of sulfite and thiosulfate were detected in pH ? 9 reactors, while concentrations were low or undetectable at pH 2 and 7. The triple oxygen isotope data show that at pH ? 9, product sulfate contained 21-24% air O₂ signal, similar to pH 2 with Fe³⁺ addition. Sulfate from the pH 2 reactor without Fe³⁺ addition and the pH 7 reactors all showed 28-29% O₂ signal. While the O₂% in final sulfate apparently clusters around 25%, the measurable deviations (>experimental error) from the 25% in many reaction conditions suggest that (1) O₂ does get incorporated into intermediate sulfoxyanions (thiosulfate and sulfite) and a fraction survives sulfite-water exchange (e.g. the pH 2 with no Fe³⁺ addition and both pH 7 reactors); and (2) direct O₂ oxidation dominates while Fe³⁺ shuttling is still competitive in the sulfite-sulfate step (e.g. the pH 9, 10, and 11 and the pH 2 reactor with Fe³⁺ addition). Overall, the final sulfate-oxygen source ratio is determined by (1) rate competitions between direct O₂ incorporation and Fe³⁺ shuttling during both the formation of sulfite from pyrite and from sulfite to final sulfate, and (2) rate competitions between sulfite and water oxygen exchange and the oxidation of sulfite to sulfate. Our results indicate that thiosulfate or sulfite is the intermediate species released into solution at all investigated pH and point to a set of dynamic and competing fractionation factors and rates, which control the oxygen isotope composition of sulfate derived from pyrite oxidation.     

The oxidation state of sulfur in synthetic and natural glasses determined by X-ray absorption spectroscopy

Sulfur K-edge X-ray absorption near edge structure (XANES) spectra were recorded for experimental glasses of various compositions prepared at different oxygen fugacities (fO₂) in one-atmosphere gas-mixing experiments at 1400 °C. This sample preparation method only results in measurable S concentrations under either relatively reduced (log fO₂ < −9) or oxidised (log fO₂ > −2) conditions. The XANES spectra of the reduced samples are characterised by an absorption edge crest at 2476.4 eV, typical of S²⁻. In addition, spectra of Fe-bearing compositions exhibit a pronounced absorption edge shoulder. Spectra for all the Fe-free samples are essentially identical, as are the spectra for the Fe-bearing compositions, despite significant compositional variability within each group. The presence of a sulfide phase, such as might exsolve on cooling, can be inferred from a pre-edge feature at 2470.5 eV.The XANES spectra of the oxidised samples are characterised by an intense transition at 2482.1 eV, typical of the sulfate anion SO₄²⁻. Sulfite (SO₃²⁻) has negligible solubility in silicate melts at low pressures. The previous identification of sulfite species in natural glass samples is attributed to an artefact of the analysis (photoreduction of S⁶⁺). S⁴⁺ does, however, occur unambiguously with S⁶⁺ in Fe-free and Fe-poor compositions prepared in equilibrium with CaSO₄ at 4-16 kbar, and when buffered with Re/ReO₂ at 10 kbar. Solubility of S⁴⁺ thus requires partial pressures of SO₂ considerably in excess of 1 bar. A number of experiments were undertaken in an attempt to access intermediate fO₂s more applicable to terrestrial volcanism. Although these were largely unsuccessful, S²⁻ and S⁶⁺ were found to coexist in some samples that were not in equilibrium with the imposed fO₂.The XANES spectra of natural olivine-hosted melt inclusions and submarine glasses representative of basalts at, or close to, sulfide saturation show mainly dissolved S²⁻, but with minor sulfate, and additionally a peak at 2469.5 eV, which, although presumably due to immiscible sulfide, is 1 eV lower than that typical of FeS. These sulfate and sulfide-related peaks disappear with homogenisation of the inclusions by heating to 1200 °C followed by rapid quenching, suggesting that both these features are a result of cooling under natural conditions. The presence of small amounts of sulfate in otherwise reduced basaltic magmas may be explained by the electron exchange reaction: S²⁻ + 8Fe³⁺ = S⁶⁺ + 8Fe²⁺, which is expected to proceed strongly to the right with decreasing temperature. This reaction would explain why S²⁻ and S⁶⁺ are frequently found together despite the very limited fO₂ range over which they are thermodynamically predicted to coexist. The S XANES spectra of water-rich, highly oxidised, basaltic inclusions hosted in olivine from Etna and Stromboli confirm that nearly all S is dissolved as sulfate, explaining their relatively high S contents.     

Different isotopic evolutionary trends of δ34S and δ18O compositions of dissolved sulfate in an anaerobic deltaic aquifer system

Concentration and isotope ratios (δ³⁴S_SO4 and δ¹⁸O_SO4) of dissolved sulfate of groundwater were analyzed in a very large anaerobic aquifer system under the Lower Central Plain (LCP) (25,000 km²) in Thailand. Groundwater samples were collected in two different kinds of aquifers; type 1 with a saline water contribution and type 2 lateritic aquifers with no saline water contribution. Two different isotopic compositional trends were observed: in type 1 aquifers sulfate isotope ratios range from low values (+2.2‰ for δ³⁴S_SO4 and +8.0‰ for δ¹⁸O_SO4) to high values (+49.9‰ for δ³⁴S_SO4 and +17.9‰ for δ¹⁸O_SO4); in type 2 aquifers sulfate isotope ratios range from low values (−0.1‰ for δ³⁴S_SO4 and +12.2‰ for δ¹⁸O_SO4) to high δ¹⁸O_SO4 ratios (+18.4‰) but with low δ³⁴S_SO4 ratios (<+12.9‰). Isotopic comparison with possible source materials and theoretical geochemical models suggests that the sulfate isotope variation for type 1 aquifer groundwater can be explained by two main processes. One is the contribution of remnant seawater, which has experienced dissimilatory sulfate reduction in the marine clay, into recharge water of freshwater origin. This process accounts for the high salinity groundwater. The other process, explaining for the modest salinity groundwater, is the bacterial sulfate reduction of the mixture water between high salinity water and fresh groundwater. Isotopic variation of type 2 aquifer groundwater may also be explained by bacterial sulfate reduction, with slower reduction rate than that of the groundwater with saline water effect. The origin of groundwater sulfate with low δ³⁴S_SO4 but high δ¹⁸O_SO4 is recognized as an important topic to be examined in a future investigation.     

Effect of Fe2+ amendment on photodegradation kinetics of imidacloprid in moist soil

The present study deals with the effect of Fe²⁺ on degradation kinetics of imidacloprid in moist soil under UV system. The moist soil samples were spiked with imidacloprid and irradiated in specially designed UV-photoreactor. The analysis of imidacloprid was carried out by using HPLC–DAD system. UV irradiation caused about ten fold increase in photodegradation rate of the pesticide. Amendment of soil with Fe²⁺ at concentrations of 30 mg/kg led to a further increase in the rate of photodegradation, i.e., a 98 % degradation of imidacloprid was observed in the presence of iron after 32 days of irradiation. Moreover, the half-life of imidacloprid in Fe²⁺ -amended soil was observed to be reduced to 7 days that in the absence of Fe²⁺ was recorded to be 21 days. Iron was also observed to affect the half-life of imidacloprid in dark. When compared with unsterilized Fe²⁺-amended batch treatments, the t _1/2 in sterilized Fe²⁺-amended batch treatments increased from 58 to 96 days. Imidacloprid-urea was detected by HPLC as the only stable photodegradation byproduct of imidacloprid in the soil.     

Effects of grazing and livestock exclusion on soil physical and chemical properties in desertified sandy grassland,Inner Mongolia,northern China

Heavy grazing is recognized as one of the main causes of vegetation and soil degradation and desertification in the semiarid Horqin sandy grassland of northern China. Soil physical and chemical properties were examined under continuous grazing and exclusion of livestock for 8 years in a representative desertified sandy grassland. Exclosure increased the mean soil organic C, total N, fine sand and silt + clay contents, inorganic C (CaCO₃), electrical conductivity, and mineral contents (including Al₂O₃, K₂O, Na₂O, Fe₂O₃, CaO, MgO, TiO₂, MnO), microelements (Fe, Mn, Zn, B, Cu, Mo), and heavy metals (Pb, Cr, Ni, As, Hg, Cd, Se), and decreased the coarse sand content, bulk density, and SiO₂ in the top 100 cm of the soil. Livestock exclusion also improved available N, P, K, Fe, Mn, and Cu, exchangeable K⁺, and the cation exchange capacity, but decreased pH, exchangeable Na⁺, and available S, Zn, and Mo in the top 20 cm of the soil. The greatest change in soil properties was observed in the topsoil. The results confirm that the desertified grassland is recovering after removal of the livestock disturbance, but that recovery is a slow process.