Replacement of Heavy Metals

Environmental concern has promoted the development of systems to replace lead (especially from cables and cellular products). Cadmium-based stabilizers are also being phased out. To achieve the same effect, however, the replacements have to be complicated mixtures of salts. Organotin and calcium/zinc systems are favoured at present, but much will depend on the shape of any legislation in the future. In general, alternatives based on calcium and zinc are less effective, but are cheaper than those based on aluminium or magnesium. Water absorption can be a problem with systems not using heavy metals. 

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Colorants Pigments, liquid colours, colour pastes, dyestuffs, special effects Virtually unlimited, added as powders or liquids easier mixing, replacement of heavy metals... 

Replacement of heavy metal formulations is influencing all development, and the introduction of cadmium/zinc systems in a more effective form is significant. Classically, stabilizers have used lead compounds and, on environmental grounds, manufacturers have volunteered to reduce levels to about 60% of today s usage by 2010. Calcium/zinc systems are now not included in the European Commission list of heavy-metal stabilizers, and these are seen as a key material for the future. 

Disadvantages Can contaminate other materials/equipment unless kept separate or used in dust-frec/non-pollutlngform migration if not correctly compounded thermal stability in processing at high temperatures replacement of heavy metals with pigments of comparable performance... 

New developments Kasier forms for use/incorporation In compounds colour concentrates, liquid colours improved dispersibilty. better thermal stabilily replacement of heavy metals inlroduction of new chemisiry... 

New developments Multi-functional stabilizer systems, development of synergistic reactions, improved retention of colour and transparency, more convenient forms for handling replacement of heavy metal compounds. 

FiALS systems mulli-functionul/synergistic effects replacement of heavy metals... 

J. Graff, D. Osmer, Replacement of Heavy Metal Containing Pigments in PVC , J. Vinyl Tech., 1979, 1(1), 20-33. 

Since the introduction of this pigment class, they have successfully competed in the replacement of heavy metal-containing inorganic pigments in paints and plastics, and now form a most important group of organic yellows, oranges and reds. 

Slimicide and biocide toxic pollutants containing pentachlorophenol are used at mills in the pulp, paper, and paperboard industry. Initially, pentachlorophenol was used as a replacement for heavy metal salts, particularly mercuric types. Trichlorophenols are also used because of their availability as a byproduct from the manufacture of certain herbicides. Formulations containing organo-bromides and organo-sulfur compounds are also being used. Substitution of alternative slimicide and biocide formulations can lead to the virtual elimination of pentachlorophenol and trichlorophenol from these sources. 

Once a suitable crystal is obtained and the X-ray diffraction data are collected, the calculation of the electron density map from the data has to overcome a hurdle inherent to X-ray analysis. The X-rays scattered by the electrons in the protein crystal are defined by their amplitudes and phases, but only the amplitude can be calculated from the intensity of the diffraction spot. Different methods have been developed in order to obtain the phase information. Two approaches, commonly applied in protein crystallography, should be mentioned here. In case the structure of a homologous protein or of a major component in a protein complex is already known, the phases can be obtained by molecular replacement. The other possibility requires further experimentation, since crystals and diffraction data of heavy atom derivatives of the native crystals are also needed. Heavy atoms may be introduced by covalent attachment to cystein residues of the protein prior to crystallization, by soaking of heavy metal salts into the crystal, or by incorporation of heavy atoms in amino acids (e.g., Se-methionine) prior to bacterial synthesis of the recombinant protein. Determination of the phases corresponding to the strongly scattering heavy atoms allows successive determination of all phases. This method is called isomorphous replacement. 

The Unipnre Environmental, Unipnre process technology is a unique iron co-precipitation method for removal of heavy metals from waste streams or groundwater. It can act as a primary metal-removal system or as a polishing step to an existing treatment system. The reactor mod-nle replaces the nentrahzation tank in a conventional wastewater treatment system. The process prodnces solids that are extremely insolnble in water and mild acid solutions. 

Mercury fulminate is easy to produce, has been known since earliest times and is still widely used. The scarcity of mercury has however led to many attempts to replace this substance by something else, in particular by substances containing a different metal. Some success was achieved as a result of work of Will and Lenze [5] in 1892 on the application of heavy-metal azides as initiating agents. 

Considerable DNase but no RNase activity results if Ca-+ is replaced by Sr-+, while Fe-+ and Cu J+ cause minimal activation (3, 40). A number of heavy metal cations inhibit DNase and RNase activities competitively with Ca-+ Hg-+, Zn2+, and Cd-+ are the most potent of these (3). Studies with synthetic substrates, to be discussed below, indicate that Ca2+ is not only required for the proper binding of substrates but also that it is required for the subsequent independent hydrolytic process. Although several divalent cations can substitute for Ca2t in the binding function, as evidenced by their competitive inhibition of enzymic activity (3) and their ability to promote nucleotide binding (62), the catalytic role of Ca2+ appears to be unique. 

X-ray and neutron diffraction patterns can be detected when a wave is scattered by a periodic structure of atoms in an ordered array such as a crystal or a fiber. The diffraction patterns can be interpreted directly to give information about the size of the unit cell, information about the symmetry of the molecule, and, in the case of fibers, information about periodicity. The determination of the complete structure of a molecule requires the phase information as well as the intensity and frequency information. The phase can be determined using the method of multiple isomor-phous replacement where heavy metals or groups containing heavy element are incorporated into the diffracting crystals. The final coordinates of biomacromolecules are then deduced using knowledge about the primary structure and are refined by processes that include comparisons of calculated and observed diffraction patterns. Three-dimensional structures of proteins and their complexes (Blundell and Johnson, 1976), nucleic acids, and viruses have been determined by X-ray and neutron diffractions. 

Biocidal applications. The use of quaternary ammonium salts in disinfecting systems for household and industrial cleaners has been known for many years [95, 96]. Alkyl-benzyldimethyl quaternaries, alkyltrimethyl quaternaries, and dialkyldimethyl quaternaries are the more commonly used biocidal quaternary ammonium salts [16]. Recently, dialkyldimethyl quaternary ammonium salts have received renewed attention as potential wood preservatives to replace the heavy metal types [97]. Metal-free wood preservative formulations containing dialkyldimethyl ammonium salts with non-halide anions, such as carboxylates, borates, and carbonates, have been developed [98, 99]. 

Complex polyphosphites such as DP12 and DP675 were shown to outperform simple phosphite esters as replacements for heavy metal components of mixed metal stabilisers for PVC. It was found that the zinc level was critical for optimisation of the performance of the phosphite blends. Synergistic performances were observed with combinations of several phosphite esters, with the additional benefit of cost effectiveness. Other properties of PVC compounds, such as clarity, light stability and plate-out resistance, also improved markedly when heavy metal components of mixed metal stabilisers were replaced with polyphosphites. 4 refs. 

There are several factors however which are related to water acidity (low Ca" ", high content of heavy metals and aluminium) and other abiotic factors (temperature, transparency) which mask or enhance the pH effect. It now seems proven that aluminium is a real toxic agent in lakewater in acidified catchments, this metal being leached in high amounts from soils under acidification. Aluminium buffer system replaces the normal bicarbonate buffer system when lakes are acidified and A1 concentrations... 

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