Mobilization potential of heavy metals: A comparison between river and lake sediments

Toxic metals introduced into aquatic environments by human activities accumulation in sediments. A common notion is that the association of metals with acid volatile sulfides (AVS) affords a mechanism for partitioning metals from water to solid phase, thereby reducing biological availability. However, variation in environmental conditions can mobilize the sediment-bound metal and result in adverse environmental impacts. The AVS levels and the effect of AVS on the fate of Cu, Cd, Zn, Ni in sediments in the the Changjiang River, a suboxic river with sandy bottom sediment and the Donghu Lake, a anoxic lake with muddy sediment in China, were compared through aeration, static adsorption and release experiments in laboratory. Sips isotherm equation, kinetic equation and grade ion exchange theory were used to describe the heavy metal adsorb and release process. The results showed that AVS level in the lake sediment are higher than that of the river. Heavy metals in the overlying water can transfer to sediments incessantly as long as the sediment remains undisturbed. The metal release process is mainly related to AVS oxidation in lake sediment while also related to Org-C and Fe-Mn oxyhydroxide oxidation in river sediment. The effect of sulfides on Zn and Ni is high, followed by Cd, and Cu is easy bound to Org-C. AVS plays a major role in controlling metals activity in lake sediment and its presence increase the adsorption capacity both of the lake and river sediments.

Effects of heavy metals on plants and resistance mechanisms

Goal, Scope and Background. As one of the consequences of heavy metal pollution in soil, water and air, plants are contaminated by heavy metals in some parts of China. To understand the effects of heavy metals upon plants and the resistance mechanisms, would make it possible to use plants for cleaning and remediating heavy metal-polluted sites. Methods. The research results on the effects of heavy metals on plants and resistant mechanisms are compiled from Chinese publications from scientific journals and university journals, mostly published during the last decade. Results and Discussion. Effects of heavy metals on plants result in growth inhibition, structure damage, a decline of physiological and biochemical activities as well as of the function of plants. The effects and bioavailability of heavy metals depend on many factors, such as environmental conditions, pH, species of element, organic substances of the media and fertilization, plant species. But, there are also studies on plant resistance mechanisms to protect plants against the toxic effects of heavy metals such as combining heavy metals by proteins and expressing of detoxifying enzyme and nucleic acid, these mechanisms are integrated to protect the plants against injury by heavy metals. Conclusions. There are two aspects on the interaction of plants and heavy metals. On one hand, heavy metals show negative effects on plants. On the other hand, plants have their own resistance mechanisms against toxic effects and for detoxifying heavy metal pollution. Recommendations and Outlook. To study the effects of heavy metals on plants and mechanisms of resistance, one must select crop cultivars and/or plants for removing heavy metals from soil and water. More highly resistant plants can be selected especially for a remediation of the pollution site. The molecular mechanisms of resistance of plants to heavy metals should be studied further to develop the actual resistance of these plants to heavy metals. Understanding the bioavailability of heavy metals is advantageous for plant cultivation and phytoremediation. Decrease in the bioavailability to farmlands would reduce the accumulation of heavy metals in food. Alternatively, one could increase the bioavailability of plants to extract more heavy metals.

Efficiency of constructed wetlands in decontamination of water polluted by heavy metals

A twin-shaped constructed wetland (CW) comprising a vertical flow (inflow) chamber with Cyperus alternifolius followed by a reverse-vertical flow (outflow) chamber with Villarsia exaltata was assessed for decontamination of artificial wastewater polluted by heavy metals. After application of Cd, Cu, Pb, Zn over 150 days, together with Al and Mn during the final 114 days, no heavy metals with the exception of Mn could be detected in either the drainage zone at the bottom, shared by both chambers, or in the effluent. The inflow chamber was, therefore, seen to be predominantly responsible for the decontamination process of more toxic metal species with final concentrations far below WHO drinking-water standards. About one-third of the applied Cu and Mn was absorbed, predominantly by lateral roots of C. alternifolius. Lower accumulation levels were observed for Zn (5%), Cd (6%), Al (13%). and Pb (14%). Contents of Cd, Cu, Mn, and Zn in soil were highest in top layer, while Al and Pb were evenly distributed through the whole soil column. Metal species accumulating mainly in the top layer can be removed mechanically. A vertical flow CW with C. alternifolius is an effective tool in phytoremediation for treatment of water polluted with heavy metals. (C) 2002 Elsevier Science B.V. All rights reserved.

Distribution and mobility of metals in agricultural soils near a copper smelter in South China

The distribution and mobility of heavy metals in the paddy soils surrounding a copper smelting plant in south China was investigated. We assessed the degree of metal contamination using an index of geoaccumulation. The metals were divided into two groups: (1) Cu, Zn, Pb and Cd, whose concentrations were heavily affected by anthropogenic inputs, and (2) Ni, Co and Cr, which were mainly of geochemical origin. Concentrations of Cu, Cd, Zn, and Pb in the polluted soils were higher than the Chinese soil quality criteria. The chemical partitioning patterns of Pb, Zn and Cu indicated that Pb was largely associated with the residual and NH2OH HCl extractable fractions. In contrast, Cd was predominantly associated with the MgCl2 extractable fraction. A large proportion of Cu was bound to the acidic H2O2 extractable fractions, while Zn was predominantly found in the residual phase. The fraction of mobile species, which potentially is the most harmful to the environment, was found to be elevated compared to unpolluted soils in which heavy metals are more strongly bound to the matrix. The mobility of the metals was studied by water extraction using a modification of Tessier's procedure, and the order of mobility was Zn > Cd > Cu > Ce > Pb.

A meso elastoplastic constitutive model for polycrystalline metals based on equivalent slip systems with latent hardening

A meso material model for polycrystalline metals is proposed, in which the tiny slip systems distributing randomly between crystal slices in micro-grains or on grain boundaries are replaced by macro equivalent slip systems determined by the work-conjugate principle. The elastoplastic constitutive equation of this model is formulated for the active hardening, latent hardening and Bauschinger effect to predict macro elastoplastic stress-strain responses of polycrystalline metals under complex loading conditions. The influence of the material property parameters on size and shape of the subsequent yield surfaces is numerically investigated to demonstrate the fundamental features of the proposed material model. The derived constitutive equation is proved accurate and efficient in numerical analysis. Compared with the self-consistent theories with crystal grains as their basic components, the present theory is much simpler in mathematical treatment.

Behaviour of Gaseous Chlorine and Alkali Metals During Biomass Thermal Utilisation

The behaviour of gaseous chlorine and alkali metals of three sorts of biomass (Danish straw, Swedish wood, and sewage sludge) in combustion or gasification is investigated by the chemical equilibrium calculating tool. The ranges of temperature, air-to-fuel ratio, and pressure are varied widely in the calculations (T=400-1800 K, gimel=0-1.8, and P=0.1-2.0 MPa). Results show that the air excess coefficient only has less significant influence on the release of gaseous chlorine and potassium or sodium during combustion. However, in biomass gasification, the influence of the air excess coefficient is very significant. Increasing air excess coefficient enhances the release of HCl(g), KOH(g), or NaOH(g) as well as it reduces the formation of KCl(g), NaCl(g), K(g), or Na(g). In biomass combustion or straw and sludge gasification, increasing pressure enhances the release of HCl(g) and reduces the amount of KCI(g), NaCl(g), KCI(g), or NaOH(g) at high temperatures. However, during wood gasification, the pressure enhances the formation of KOH(g) and KCI(g) and reduces the release of K(g) and HCl(g) at high temperatures. During wood and sewage sludge pyrolysis, nitrogen addition enhances the formation of KCN(g) and NaCN(g) and reduces the release of K(g) and Na(g). Kaolin addition in straw combustion may enhance the formation of potassium aluminosilicate in ash and significantly reduces the release of KCl(g) and KOH(g) and increases the formation of HCl(g).

Theoretical investigation of epitaxial deformation and the hcp-bcc transition of alkali metals

The deformation of alkali metals K, Rb, and Cs under epitaxial deformation is studied via the ab initio pseudopotential plane wave method using the local-density approximation. Under loading from the stable fee phase, metastable stares along directions [001], [111], and [201] are identified. One metastable state, presented at direction [201], has a very low symmetry in contrast to the planes [001] and [201]. Our results show that the softening direction and sequences of growth is significantly affected by the existence of the metastable states and magnitude of the energy barrier. The resulting softening sequences from soft to hard are [201], [110], [001], and [111] under biaxial compression and [001], [111], [201], and [110] under biaxial tension. An orthorhombic deformation path is used to investigate the fact, that the structure of the alkali films K and Cs evolve from the quasihexagonal structure into the (110)-oriented bcc structure, observed by experiments.

Function Innovation in Combined Surface Technologies for Metals

表面技术能够显著提高材料功能而成为工程和产品设计的重要组成部分, 但前提是表面技术必须具有可设计性。为此, 需要开拓和发展表面组合加工技术, 进行创新。该技术的内涵体现了新材料与新技术、基础研究与产业化有机结合的特点, 具有重要的价值和广阔的前景。

Deformation Twinning In Bulk Nanocrystalline Metals: Experimental Observations

Deformation twins have been observed in nanocrystalline (nc) fcc metals with medium-to-high stacking fault energies such as aluminum, copper, and nickel. These metals in their coarse-grained states rarely deform by twining at room temperature and low strain rates. Several twinning mechanisms have been reported that are unique to nc metals. This paper reviews experimental evidences on deformation twinning and partial dislocation. emissions from grain boundaries, twinning mechanisms, and twins with zero-macro-strain. Factors that affect the twinning propensity and recent analytical models on the critical grain sizes for twinning are also discussed. The current issues on deformation twinning in nanocrystalline metals are listed.

New Deformation Twinning Mechanism Generates Zero Macroscopic Strain In Nanocrystalline Metals

Macroscopic strain was hitherto considered a necessary corollary of deformation twinning in coarse-grained metals. Recently, twinning has been found to be a preeminent deformation mechanism in nanocrystalline face-centered-cubic (fcc) metals with medium-to-high stacking fault energies. Here we report a surprising discovery that the vast majority of deformation twins in nanocrystalline Al, Ni, and Cu, contrary to popular belief, yield zero net macroscopic strain. We propose a new twinning mechanism, random activation of partials, to explain this unusual phenomenon. The random activation of partials mechanism appears to be the most plausible mechanism and may be unique to nanocrystalline fcc metals with implications for their deformation behavior and mechanical properties.

An anisotropic elastic-plastic constitutive model for single and polycrystalline metals. II—Experiments and predictions concerning thin-walled tubular OFHC copper

Predictions based on an anisotropic elastic-plastic constitutive model proposed in the first part of this paper are compared with the experimental stress and strain data on OHFC copper under first torsion to about 13% and partial unloading, and then tension-torsion to about 10% along eight different loading paths. This paper also describes the deformation and stress of the thin-walled tubular specimen under finite deformation, the numerical implementation of the model, and the detailed procedure for determining the material parameters in the model. Finally, the model is extended to a general representation of the multiple directors, and the elastic-viscoplastic extension of the constitutive model is considered.

A theoretical analysis on stochastic behaviour of short cracks and fatigue damage of metals

In this paper, the closed form of solution to the stochastic differential equation for a fatigue crack evolution system is derived. and the relationship between metal fatigue damage and crack stochastic behaviour is investigated. It is found that the damage extent of metals is independent of crack stochastic behaviour ii the stochastic deviation of the crack growth rate is directly proportional to its mean value. The evolution of stochastic deviation of metal fatigue damage in the stage close to the transition point between short and long crack regimes is also discussed.