Oxidation mechanisms and chemical bioavailability of chromium in agricultural soil – pH as the master variable

Chromium (Cr) is a heavy metal that exists in soils in two stable oxidation states, +III and +VI. The trivalent species is an essential nutrient, whereas the hexavalent species is highly toxic. This study investigated the environmental impact of Cr^III potentially released into soil from wastes and various materials by determining the risk of oxidation of initially soluble inorganic Cr^III into hazardous Cr^VI. The principal aim was to describe the pH-dependent mechanisms that regulate 1) the formation of Cr^VI from the easily soluble Cr^III and 2) the potential bioavailability of Cr^III and that of Cr^VI species produced in the oxidation of Cr^III in agricultural soil (fine sand, organic carbon 3.2%). The amount of Cr^VI formed in oxic soil conditions was regulated by two counteracting reactions: 1) oxidation of Cr^III into Cr^VI by manganese oxide (Mn^IVO₂) and 2) the subsequent reduction of Cr^VI by organic matter back to Cr^III. The effect of pH on this net-oxidation of Cr^III and on the chemical availability of both Cr^III and Cr^VI species was investigated in soil samples incubated with or without excessive amounts of synthetic MnO₂, over the chemically adjusted pH range of 3.9–6.3 (+22 °C, 47 d). In soil subsamples without added MnO₂, the net-oxidation of Cr^III into Cr^VI (1 mM CrCl₃ in soil suspensions, 1:10 w/V) was negligible. As for the MnO₂-treated soils, at maximum only 4.7% of added Cr^III was oxidized – regardless of the high oxidation potential of these subsamples. The lowest production of Cr^VI was observed under acidic soil conditions at pH ∼4. At low pH, the net-oxidation diminished as result of enhanced reduction of Cr^VI back to Cr^III. At higher pHs, the oxidation was limited by enhanced precipitation (or adsorption) of Cr^III, which lowered the overall amount of Cr^III susceptible for oxidation. Moreover, the oxidation reactions by MnO₂ were inhibited by formation of Cr(OH)₃ coverage on its surface. The pH-dependent chemical bioavailability of added Cr^III differed from that of the Cr^VI formed. At elevated pHs the chemical availability of Cr^III decreased, whereas that of Cr^VI produced increased. However, the risk of Cr^VI formation through oxidation of the easily soluble inorganic Cr^III was considered to be low in agricultural soils high in organic matter and low in innate MnO₂.     

铬(VI)对浮游植物生长率的影响(英文)

本工作使用一次培养法初步研究了Cr(VI)对菱形硅藻和隐藻的毒性,实验结果表明,当Cr(VI)浓度大于16ppb时,则对其生长率产生抑制作用,其72小时EC_(50)值分别为260ppb和230ppb.根据活性点位模式计算了Cr(VI)与藻体的结合常数,约为2×10~5藻体中Cr含量与培养液中Cr浓度有关,两者近似指数关系,根据藻体中Cr含量和溶液中Cr的浓度,估计每个硅藻细胞与Cr最大结合容量约为5×10~(-17)mol/cell.     

Competition between enzymatic and abiotic reduction of uranium(VI) under iron reducing conditions

Reduction of U(VI) under iron reducing conditions was studied in a model system containing the dissimilatory metal-reducing bacterium Shewanella putrefaciens and colloidal hematite. We focused on the competition between direct enzymatic uranium reduction and abiotic reduction of U(VI) by Fe(II), catalyzed by the hematite surface, at relatively low U(VI) concentrations (< 0.5 μM) compared to the concentrations of ferric iron (> 10 mM). Under these conditions surface catalyzed reduction by Fe(II), which was produced by dissimilatory iron reduction, was the dominant pathway for uranium reduction. Reduction kinetics of U(VI) were identical to those in abiotic controls to which soluble Fe(II) was added. Strong adsorption of U(VI) at the hematite surface apparently favored the abiotic pathway by reducing the availability of U(VI) to the bacteria. In control experiments, lacking either hematite or bacteria, the addition of 45 mM dissolved bicarbonate markedly slowed down U(VI) reduction. The inhibition of enzymatic U(VI) reduction and abiotic, surface catalyzed U(VI) reduction by the bicarbonate amendments is consistent with the formation of aqueous uranyl-carbonate complexes. Surprisingly, however, more U(VI) was reduced when dissolved bicarbonate was added to experimental systems containing both bacteria and hematite. The enhanced U(VI) reduction was attributed to the formation of magnetite, which was observed in experiments. Biogenic magnetite produced as a result of dissimilatory iron reduction may be an important agent of uranium immobilization in natural environments.     

New seismic data support Cenozoic rifting in George VI Sound, Antarctic Peninsula

New multifold seismic-reflection and wide-angle reflection/refraction data across George VI Sound, Antarctic Peninsula, show the presence of graben and horst structures indicating an extensional origin. The data suggest that rocks of an accretionary complex and fore-arc basin underlie the Sound and are in faulted contact along its eastern boundary with volcanic and plutonic rocks of the associated Mesozoic arc of western Palmer Land. A cover of possible syn- and postglacial Cenozoic deposits drapes the structures. The combination of new seismic, synthetic-aperture radar and previously acquired data suggests subduction-related rifting in the Sound was segmented, with opening in the south predominately by normal extension whilst in the north, dextral transtension predominated.     

镍钛改性膨润土对铬的吸附性能研究

以钠基膨润土为原料,制备镍钛交联改性膨润土、镍钛有机复合改性膨润土,并应用于含铬模拟废水的处理。探讨了改性膨润土的用量、pH值、吸附时间等最佳使用条件,比较了原土、交联改性土、有机复合改性土对铬的吸附效果。结果表明改性土的吸附效果明显优于原土,在最佳实验条件下,交联改性土、有机复合改性土对Cr(Ⅵ)的去除率分别达到了87%和96%。两种改性土对铬吸附行为均符合Langmuir吸附等温方程,饱和吸附量和Lang-muir常数分别为3.1827mg/g、8.5543mg/g和3.5007、1.2738。     

单斜与六方磁黄铁矿处理含Cr(VI)废水过程中pH值变化规律

与六方磁黄铁矿相比,单斜磁黄铁矿Fe缺位较普遍。在初始pH值分别为3.40～9.66和3.47～9.66较宽范围内,利用单斜磁黄铁矿和六方磁黄铁矿处理含Cr(VI) 废水,当反应达到充分平衡时,废水的pH值分别变化在3.61～4.47和5.39～6.57范围内。六方磁黄铁矿除Cr(VI)效果明显不及单斜磁黄铁矿,但被氧化的六方磁黄铁矿除Cr(VI)效率有所 提高。电化学分析表明,在酸性介质中处理的反应过程为H+的消耗过程,而在碱性介质中 则为OH 的消耗过程。根据pH值的这一变化规律,可自行调节处理过程中水质的酸性变化 ,能节省传统工艺中需要加碱以中和处理后酸性水的环节,具有一定的实际应用价值。     

干旱地区植被指数(VI)的适宜性研究

以位于典型干旱区的甘肃河西地区石羊河下游的民勤绿洲为例,对NDVI、SAVI、MSAVI和GEMI等4种VI(植被指数)受土壤背景光谱影响的程度和探测低盖度植被的能力进行了对比研究。通过分析VI提取植被信息时植被土壤噪音比的变化发现,植被覆盖较低条件下VI提取植被信息总体受土壤背景光谱影响程度较低,相比而言,GEMI提取植被信息中受土壤背景影响最小,其他3种VI的区别不太明显。通过分析不同VI随植被覆盖度增加的反映速率变化及不同覆盖条件下不同VI的取值范围的变化发现,NDVI探测低盖度植被的能力最强,GEMI次之。GEMI可能是干旱地区植被探测较适宜的植被指数。     

Removal and Recovery of Molybdenum from Aqueous Solutions by Adsorption onto Surfactant‐Modified Coir Pith,a Lignocellulosic Polymer

Coconut coir pith, a lignocellulosic polymer, is an unwanted by‐product of the coir fiber industry. The pith was used as a biosorbent for the removal of Molybdenum(VI) after modification with a cationic surfactant, hexadecyltrimethylammonium bromide. The optimum pH for maximum adsorption of Mo(VI) was found to be 3.0. Langmuir, Freundlich and Dubinin Radushkevich isotherms were used to model the adsorption equilibrium data and the system was seen to follow all three isotherms. The Langmuir adsorption capacity of the biosorbent was found to be 57.5 mg g^–1. Kinetic studies showed that the adsorption generally obeyed a second‐order kinetic model. Desorption studies showed that the recovery of Mo(VI) from the spent adsorbent was feasible. The effect of foreign anions on the adsorption of Mo(VI) was also examined.     

Proposed Adsorption–Diffusion Model for Characterizing Chromium(VI) Removal Using Dried Water Hyacinth Roots

Experiments have been carried out to characterize the adsorption of chromium(VI) in the aqueous phase onto dried roots of water hyacinth. Results revealed a very high degree of removal efficiency (∼100%). Theoretical analyzes are also made for describing the sorption and diffusion processes. The effective pore diffusivity of chromium(VI) in the water hyacinth roots is determined by a suitable global optimization technique. The depth of penetration, on the other hand, has been estimated for various initial concentrations of chromium(VI). Theoretically predicted concentration profiles are in excellent agreement with the experimental values.     

Removal of Cr(VI) by Zero‐valent,Iron‐encapsulated Alginate Beads

Zero‐valent, iron‐encapsulated alginate beads were synthesized and were applied for the removal of Cr(VI) from aqueous solutions. The effects of several important parameters including solution pH, contact time, initial concentration and reaction temperature on Cr(VI) removal levels were investigated in batch studies. An initial solution pH of 1.0 was seen to be most favorable for Cr(VI) removal. The removal process was quick and almost 80% of the removal was attained within 60 min. The kinetic data followed the second‐order equation well. The Cr(VI) removal was almost reaction temperature‐independent and decreased with an increase in Cr(VI) initial concentration. The removal of Cr(VI) by iron‐encapsulated alginate beads was found to be significantly higher than that of non‐encapsulated alginate beads.