Metal contamination issues are becoming increasingly common in India and elsewhere, with many documented cases of metal toxicity in mining industries, foundries, smelters, coal-burning power plants and agriculture. Heavy metals, such as cadmium, copper, lead, chromium and mercury are major environmental pollutants, particularly in areas with high anthropogenic pressure. Heavy metal accumulation in soils is of concern in agricultural production due to the adverse effects on food safety and marketability, crop growth due to phytotoxicity, and environmental health of soil organisms. The influence of plants and their metabolic activities affects the geological and biological redistribution of heavy metals through pollution of the air, water and soil. This article details the range of heavy metals, their occurrence and toxicity for plants. Metal toxicity has high impact and relevance to plants and consequently it affects the ecosystem, where the plants form an integral component. Plants growing in metal-polluted sites exhibit altered metabolism, growth reduction, lower biomass production and metal accumulation. Various physiological and biochemical processes in plants are affected by metals. The contemporary investigations into toxicity and tolerance in metal-stressed plants are prompted by the growing metal pollution in the environment. A few metals, including copper, manganese, cobalt, zinc and chromium are, however, essential to plant metabolism in trace amounts. It is only when metals are present in bioavailable forms and at excessive levels, they have the potential to become toxic to plants. This review focuses mainly on zinc, cadmium, copper, mercury, chromium, lead, arsenic, cobalt, nickel, manganese and iron.

Heavy metals, occurrence and toxicity for plants: a review

http://www.hyperquad.co.uk/documents/hyss.pdf

Hyperquad simulation and speciation (HySS): a utility program for the investigation of equilibria involving soluble and partially soluble species

Introduction Fundamentals of biochemical modeling Balance equations Rate laws Generalized mass-action kinetics Various enzyme kinetic rate laws Thermodynamic flow-force relationships Power-law approximation Steady states of biochemical networks General considerations Stable and unstable steady states Multiple steady states Metabolic oscillations Background Mathematical conditions for oscillations Glycolytic oscillations Models of intracellular calcium oscillations A simple three-variable model with only monomolecular and bimolecular reactions Possible physiological significance of oscillations Stoichiometric analysis Conservation relations Linear dependencies between the rows of the stoichiometry matrix Non-negative flux vectors Elementary flux modes Thermodynamic aspects A generalized Wegscheider condition Strictly detailed balanced subnetworks Onsager's reciprocity reactions for coupled enyme reactions Time hierarchy in metabolism Time constants The quasi-steady-state approximation The Rapid equilibrium approximation Modal analysis Metabolic control analysis Basic definitions A systematic approach Theorems of metabolic control analysis Summation theorems Connectivity theorems Calculation of control coefficients using the theorems Geometrical interpretation Control analysis of various systems General remarks Elasticity coefficients for specific rate laws Control coefficients for simple hypothetical pathways Unbranched chains A branched system Control of erythrocyte energy metabolism The reaction system Basic model Interplay of ATP production and ATP consumption Glycolytic energy metabolism and osmotic states A simple model of oxidative phosphorylation A three-step model of serine biosynthesis Time-dependent control coefficients Are control coefficients always parameter independent? Posing the problem A system without conserved moieties A system with a conserved moiety A system including dynamic channeling Normalized versus non-normalized coefficients Analysis in terms of variables other than steady-state concentrations and fluxes General analysis Concentration ratios and free-energy-differences as state variables Entropy production as response variable Control of transient times Control of oscillations A second-order approach A quantitative approach to metabolic regulations Co-response coefficients Fluctuations of internal variables versus parameter perturbations Internal response coefficients Rephrasing the basic equations of metabolic control analysis in terms of co-response coefficients and internal response coefficients Control within and between subsystems Modular approach Overall elasticities Overall control coefficients Flux control insusceptibility Control exerted by elementary steps in enzyme catalysis Control analysis of metabolic channeling Comparison of metabolic control analysis and power-law formalism Computational aspects Application of optimization methods and the interrelation with evolution Optimization of the catalytic properties of single enzymes Basic assumptions Optimal values of elementary rate constants Optimal Michaelis constants Optimization of multienzyme systems Maximization of steady-state flux Influence of osmotic constraints and minimization of intermediate concentrations Minimization of transient times Optimal stoichiometries.

The Regulation of Cellular Systems

A review on heavy metal pollution, toxicity and remedial measures: Current trends and future perspectives

This paper presents the results from biomass gasification tests in a pilot-scale (6.5-m tall × 0.1-m diameter) air-blown circulating fluidized bed gasifier, and compares them with model predictions. The operating temperature was maintained in the range 700–850°C, while the sawdust feed rate varied from 16 to 45 kg/h . Temperature, air ratio, suspension density, fly ash re-injection and steam injection were found to influence the composition and heating value of the product gas. Tar yield from the biomass gasification decreased exponentially with increasing operating temperature for the range studied. A non-stoichiometric equilibrium model based on direct minimization of Gibbs free energy was developed to predict the performance of the gasifier. Experimental evidence indicated that the pilot gasifier deviated from chemical equilibrium due to kinetic limitations. A phenomenological model adapted from the pure equilibrium model, incorporating experimental results regarding unconverted carbon and methane to account for non-equilibrium factors, predicts product gas compositions, heating value and cold gas efficiency in good agreement with the experimental data.

/pdf/biomass-gasification-in-a-circulating-fluidized-bed-120fspk3e8.pdf

Biomass gasification in a circulating fluidized bed

The Closed-System Constraint and Chemical Stoichiometry. Chemical Thermodynamics and Equilibrium Conditions. Computation of Chemical Equilibrium for Relatively Simple Systems. Survey of Numerical Methods. Chemical Equilibrium Algorithms for Ideal Systems. Chemical Equilibrium Algorithms for Non-Ideal Systems. The Effects of Problem Parameter Changes on Chemical Equilibria (Sensitivity Analysis). Practical Considerations and Special Topics. Appendixes. List of Symbols. References. Answers to Problems. Author and Subject Indexes.

Chemical Reaction Equilibrium Analysis: Theory and Algorithms

Non-filmwise condensation of binary vapors of miscible liquids

Examination of available experimental information, including that on condensation of binary vapors, indicates that the Marangoni effect provides a consistent interpretation of falling film behavior, when surface tension disturbances are important. Criteria provided by the Marangoni effect are: d/db 0 for stability, where is surface tension and b film thickness, and d/db > 0 for instability; the latter is evidently a necessary but not sufficient condition. Methods are given for evaluating the sign of d/db in terms of the product of two factors, one characteristic of the system and one of the operation.



L'examen des renseignements experimentaux disponibles (y compris ceux qui ont trait a la condensation des vapeurs binaires) indique que l'effet de Marangoni fournit une interpretation logique du comportement d'une pellicule qui tombe, lorsque les perturbations dans la tension superficielle sont importantes. Les criteres fournis par l'effet de Marangoni sont respectivement d/db 0 et d/db > 0, dans le cas de stabilite et d'instabilite ( designant la tension superficielle et b l'epaisseur de la pelli-cule); la seconde relation est evidemment une condition necessaire, mais insuffisante. On indique des methodes pour evaluer le signe de d/db d'apres le produit de deux facteurs, a savoir un qui caracterise le systeme et un autre l'operation.

On the conditions for stability of falling films subject to surface tension disturbances; the condensation of binary vapors

General strategies for solving the problem of computing the equilibrium composition of a multiphase, multispecies, chemically reacting system are discussed. An efficient new algorithm is presented based on these strategies, together with a microcomputer-based implementation. Numerical examples show the algorithm's utility, and its advantages over other recently proposed methods in terns of a number of criteria, including speed and certainty of convergence. The range of problem types within the capability of the new algorithm, and its limitations are discussed.



Nous avons etudie les strategies generales de resolution du probleme de calcul par otdinateur de la composition a l'equilibre d'un systeme de reaction chimique multiphasique mettant en jeu plusieurs especes. Nous presentons ici un nouvel algorithme efficace qui s'appuie sur ces strategies, ainsi que sa mise en oeuvre sur micro-ordinateur. Des exemples numeriques montrent l'utilite de l'algorithme et ses avantages sur d'autres methodes recemment proposees en ce qui a trait au nombre de criteres, y compris le critiere de vitesse et de certitude de convergence. On decrit par ailleurs les problemes rencontres sur les capacites de l'algorithme et ses limites.

Strategies for solving the chemical equilibrium problem and an efficient microcomputer‐based algorithm

Liquid holdup and wetting efficiency were measured by means of stimulus-response (pulse-tracer) experiments in a trickle-bed reactor, packed with a commercial hydrodemetallization catalyst, and operated at 10 MPa and 330–370°C, with a superficial liquid velocity of 1.4 to 8.3 × 10−4 m/s. The total liquid holdup was obtained from the first moments, and the external wetting efficiency from the second moments, of the response curves. The dynamic holdup obtained from the total holdup lies within the range described by existing correlations, but wetting efficiency is significantly lower than literature data obtained at ambient conditions.



On a mesure la retention du liquide et l'efficacite du mouillage par des experiences stimulus-reponse (traceur-impulsion) dans un reacteur a lit ruisselant, garni avec un catalyseur commercial d'hydrodemetalisation, et fonctionnant a 10 MPa et a 330–370°C, avec une vitesse liquide superficielle de 1,4 a 8,3 × 10−4 m/s. On a obtenu la retention liquide totale des premiers moments de la courbe des reponses et l'efficacite du mouillage externe des deuxiemes moments. La retention dynamique obtenue a partir de la retention totale se situe dans la gamme couverte par les correlations existantes, mais l'efficacite du mouillage est nettement inferieure aux donnees publiees aux conditions ambiantes.

R. W. Missen

Papers

Chemical Reaction Equilibrium Analysis: Theory and Algorithms

Non-filmwise condensation of binary vapors of miscible liquids

On the conditions for stability of falling films subject to surface tension disturbances; the condensation of binary vapors

Strategies for solving the chemical equilibrium problem and an efficient microcomputer‐based algorithm

Trickle‐bed reactors: Tracer study of liquid holdup and wetting efficiency at high temperature and pressure