Trace elements in anionic form

In contrast to the trace elements in cationic form, the adsorption of trace elements in anionic form (e.g. As, selenite, molybdate, chromate) usually decreases with increase in pH owing to a decrease in the positive charge of the sorbent at higher pH values (Fig. 3). However, some ligands (e.g. arsenite and selenite) may be adsorbed more easily at high pH values because they form weak acids at low pH values and may consequently only be dissociated in alkaline environments (Sparks, 1995 Goldberg etal, 1996). 

The presence of organic and inorganic ligands that strongly interact with variable-charge minerals and soils also affects the adsorption of trace elements in anionic form through competition for available binding sites. The competition... 

Sorption of trace elements onto soil components is greatly affected by pH, ionic factors, nature of the sorbents, redox reactions, and so on, but the sorption of elements in cationic form differs greatly from that of elements in anionic form. The presence of organic and inorganic ligands (including nutrients) in soil environments has a very important role in the sorption-desorption processes of trace elements. 

Anion exchange chromatography coupled with ICP MS was used in the simultaneous speciation analysis of As, Se, Sb and Te compounds in extracts of fish [230]. Size exclusion chromatography (SEC) coupled with specific detectors is frequently used to analyse species of trace elements in protein-rich materials, such as extracts of meat and plant tissues. For instance, SEC hyphenated with ICP MS was used for the speciation analysis of Cu and Zn in samples of leguminous plants [191]. The same technique was applied to the speciation analysis of Cu, Cd, Zn, Se, As and Ca in fish [220] and Fe, Zn, Cu, Ag, Cd, Sn and Pb in mussels [189]. SEC HPLC coupled with GF AAS mmed out to be very useful for determining levels of Fe species in baby food [312]. With gel permeation chromatography (GPC) GF AAS, the speciation forms of Cd were determined in two kinds of vegetables contaminated with this element [216]. 

Trace elements may be present in solution with positive or negative charges and in different redox states. They occur predominantly in cationic form [Pb, Cu, Zn, Ni, Cd, Hg, Cr(III), and Co], but some trace elements are present in anionic form [As, Se, Cr(Vl), Mo, and B]. Redox reactions, both biotic and abiotic, are of paramount importance in controlling the oxidation state, and thus mobility, phytoavailability, and toxicity of many trace elements, including Cr, Se, Co, Pb, As, Ni, and Cu (Huang and Germida, 2002 Sparks, 2003). 

The aim of this chapter is to provide the current state of knowledge on the factors that affect the mobility of trace elements in soil environments. Special attention is given to the influence of inorganic and organic ligands, including nutrients and root exudates, on the sorption—desorption processes of trace elements in cationic and anionic forms on/from soil components and soils. 

Most competitive sorption studies have been carried out adding the ions contemporaneously. In natural environments, however, it is more likely that the ions will come in contact with a sorbent sequentially (i.e., the solid is exposed to one ion first, with the second ion coming in contact with a solid at a later time). The sorption of trace elements in cationic or anionic form is strongly influenced by the order of addition of organic and inorganic ligands and trace elements on the sorbents. 

The chemical composition of soils is diverse and governed by many different factors, of which parent materials and climatic factors usually predominate. Although trace elements (both cationic and anionic forms) are minor components of the soil, they play an important role in soil bioactivity and fertility. Behavior of trace elements in soils depends upon complex reactions between their ionic forms and various components of the various soil phases solid, aqueous, and gaseous. This relationship is closely related to the main features of the soil biogeochemical system, which are (i) seasonal and spatial alteration of major soil variables (ii) heterogeneous distribution of compounds and components (iii) transformation of element species (iv) complexa-... 

Trace elements in cationic and anionic forms show a different adsorption capacity on metal oxides, organic matter, and organo mineral complexes. The isotherms of Cu, Zn, and As absorbed at pH 4.0 onto POL (extracted from ohve oil mill waste waters), a Fe (OH) -POL complex, and ferrihydrite are shown in Fig. 4. 

The inspection of Table 6 shows that Na", K, NH/ and a trace of Rb" are present as cations in the aerosol. Rubidium is detected as a natural trace element in potassium salts. The densitometer readings provide approximate information on the relative abundances, although ammonium compounds may be underestimated due to some evaporation of their neutral components, especially at higher temperatures. The only anions detected in the aerosol are NOj", SO , HSO and Cl . The predominance of the nitrate ion is noteworthy, stressing the importance of aerosols as a sink for NO2 in the atmosphere. NO2 has been shown to react rapidly with NaCl to form NaNOj and NOCl as an intermediate of presumably short lifetime in the atmosphere. The product NaNOj is most abundantly observed in the aerosol, besides some unreacted NaCl. 

The Fermentation Process The process by which this antifungal substance is produced is an aerobic fermentation of an aquaous nutrient medium inoculated with a pimaricin-producing strain of Streptomycesgihrosporeus. The nutrient medium contains an assimilable source of carbon such as starch, molasses, or glycerol, an assimilable source of nitrogen such as corn steep liquor and Inorganic cations such as potassium, sodium or calcium, and anions such as sulfate, phosphate or chloride. Trace elements such as boron, molybdenum or copper are supplied as needed in the form of impurities by the other constituents of the medium. 

D. Logie (83) described a new analytical separation technique by applying ion-exchange membranes, which can be used for the determination of boron in sodium metal. By treatment with water, the Na is converted to NaOH, borate being formed from the boron. When the solution is introduced in the anode chamber of a two-cell apparatus fitted with a negative membrane, the Na+ ion is transported to the cathode chamber, whereas the borate anion remains in the anode chamber. In general this method can be applied, if the trace element yields an ion with a charge which opposite to that of the main component. 

Laboratory studies of metal ion-ligand or metal ion-organism interactions are carried out under controlled conditions. In the marine evironment, however, organisms encounter metal ions under a variety of conditions. Most trace elements rarely occur in sea water as simple hydrated ions, in complexes with the major marine anions (Cl", HCO5, SO2"), or in complexes with dissolved organic compounds. Although these forms can serve as metal ion sources to organisms, as in the case of vanadium and tunicates, other sources are more abundant. 

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