Plant metal inhibition

The physiology of metal toxicity in plants has been reviewed by Foy et al. (1978). The most widely described effects of metal toxicity in plants are inhibited root growth, depressed shoot and leaf growth, and general chlorosis of the younger leaves (Bradshaw and McNeilly, 1981 Baker and Walker, 1989). Literature on the relative toxicities of different metals to different species is limited to a few, mostly species-specific studies. 

Similar to an earlier discussion of F effects on plants, F inhibits a large number of enzymes in animals and humans. The general mode of F inhibition includes direct interaction with enzymes and formation of metal-F complexes. A brief description follows ... 

The increase in the content of metal in the solution was resulted in a decrease of the growth rate of plants. At the concentration of 10 mg/L the growth of shoots of control plants was inhibited by 28%. Roots are noticeably responsive to the presence of cadmium and its growth was inhibited by 39%. 

Precipitation is often applied to the removal of most metals from wastewater including zinc, cadmium, chromium, copper, fluoride, lead, manganese, and mercury. Also, certain anionic species can be removed by precipitation, such as phosphate, sulfate, and fluoride. Note that in some cases, organic compounds may form organometallic complexes with metals, which could inhibit precipitation. Cyanide and other ions in the wastewater may also complex with metals, making treatment by precipitation less efficient. A cutaway view of a rapid sand filter that is most often used in a municipal treatment plant is illustrated in Figure 4. The design features of this filter have been relied upon for more than 60 years in municipal applications. 

The other advantages which sulfuric acid has as an inert electrolyte are (i) it increases the conductance of the bath (ii) it is inexpensive (iii) it strongly inhibits the hydrolysis of cuprous sulfate (iv) it is nonvolatile and may be used at high concentrations and temperatures and (v) it does not attack lead, so that it is possible to use this metal for plant construction. The only inconvenience of sulfuric acid is that copper dissolves in it essentially as the divalent ion this means that the current consumption is double of that which would be consumed if the electrolysis were to be carried out in an electrolyte solution containing Cu+ ions. Attempts to implement this alternative have not been very successful so that the use of sulfuric acid is yet to be challenged. 

More than 3000 different enzymes have been extracted from animals, plants and microorganisms. Traditionally, they have been used in impure form since purification is expensive and pure enzymes may be difficult to store and use. There is usually an optimum temperature and pH for maximum activity of an enzyme. Outside these optimum conditions, activity may simply be held in check or the enzyme may become denatured , i.e. altered in such a way that activity is lost permanently, although some forms of denaturing are reversible. Many enzymes are also sensitive to transition-metal ions, the effect being specific to particular metal ions and enzymes. In some cases, certain metal ions are essential for the stability and/or activity of an enzyme. In other cases, metal ions may inhibit the activity of an enzyme. Similarly, certain organic compounds can act as enzyme inhibitors or activators. 

However, if the raw materials are contaminated or the composting process is incomplete, unfavorable effects must be expected. Heavy metals may be introduced into the compost with communal waste. To ensure that these do not enter the food chain, authorised limit values must be strictly adhered to. The same is true of organic contaminants (particularly polyaromatic or chlorinated hydrocarbons), the effect of which is extremely complex. If the fermentation process is not satisfactory, putrefaction will occur, the by-products of which (S02, NH3, N02, organic acids, cadaveric alkaloids, etc.) inhibit plant growth and attract pests. 

As a consequence of the detection of catalytic pathways for formation of PCDD/F, special inhibition methods have been developed for PCDD/F. By this approach the catalytic reactions are blocked by adding special inhibitors as poisoning compounds for copper and other metal species in the fly ash. Special aliphatic amines (triethylamine) and alkanolamines (triethanolamine) have been found to be very efficient as inhibitors for PCDD/F, and have been used in pilot plants. The effect can be seen in Figure 8.6. The inhibitors have been introduced into the incinerator by spraying them into the postcombustion zone of the incinerator at about... 

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