Al citrate complex

As indicated above, citrate influences both metal and ligand chemistry in the rhizosphere via aqueous complexation reactions and reactions at the solid-solution interface. Citrate is an important biomolecule in both plant and animal systems, particularly with respect to Fe(III) bioavailability and internal transport and in the amelioration of Al toxicity (Jones, 1998). Thus, this ligand has received considerable study. Yet despite this importance and relationship to citrate chelation capabilities, the aqueous Fe(III)-and Al-citrate complexation chemistry remains ambiguous. Consequently, in this chapter we focus on an examination of citrate aqueous metal complexation, with specific emphasis on the complexation of Fe(III) and Al. 

Ohman Model Perhaps the first critical examinations of Al-citrate complexation were performed by Ohman and Sjoberg (1983). They conducted potentiometric titrations in the pH range 2.7 to 5 in 0.6 mol L NaCl (25°C) solutions containing Aljs and citjs concentrations that ranged between 0.25 and 8 mmol (cit s/AI s ratios of 1 1, 2 1, 4 1, 8 1, 16 1, and 32 1). In addition to predicting the log values for citric acid... 

Greenaway were attributed to slow exchange of Al between several species, most notably Al(Hcit)+(a ), Alcit (a6y), and Al(H icit) (o(/) (although an additional resonance was observed, but remained unidentified). The predicted distribution of Al-citrate complexes, according to tlie Motekaitis and Martell model, shows that the Al(H icit) (ag) species predominates throughout a wide range of pH values (Figure 10.4Z ). 

Although generally restricted to solutions with cIIts/AIts ratios less than 4 1, numerous studies have documented the slow oligomerization of Al-citrate complexes to form Al3(OH)(H icit)3 (areaction scheme for the time-dependent formation of the Al3(OH)(H icit)3 (fl(y) oligomer in alkaline solution. The dissolution of (NH4)3Al(H icit)2-2H2O(s)... 

Aluminium toxicity is a major stress factor in many acidic soils. At soil pH levels below 5.0, intense solubilization of mononuclear A1 species strongly limits root growth by multiple cytotoxic effects mainly on root meristems (240,241). There is increasing evidence that A1 complexation with carboxylates released in apical root zones in response to elevated external Al concentration is a widespread mechanism for Al exclusion in many plant species (Fig. 10). Formation of stable Al complexes occurs with citrate, oxalate, tartarate, and—to a lesser extent— also with malate (86,242,243). The Al carboxylate complexes are less toxic than free ionic Al species (244) and are not taken up by plant roots (240). This explains the well-documented alleviatory effects on root growth in many plant species by carboxylate applications (citric, oxalic, and tartaric acids) to the culture media in presence of toxic Al concentrations (8,244,245) Citrate, malate and oxalate are the carboxylate anions reported so far to be released from Al-stressed plant roots (Fig. 10), and Al resistance of species and cultivars seems to be related to the amount of exuded carboxylates (246,247) but also to the ability to maintain the release of carboxylates over extended periods (248). In contrast to P deficiency-induced carboxylate exudation, which usually increases after several days or weeks of the stress treatment (72,113), exudation of carboxylates in response to Al toxicity is a fast reaction occurring within minutes to several hours... 

Absorption of intramuscularly injected lanthanides (Durbin et al., 1956a) is greatly accelerated if they are administered as citrate complexes. However, the fractional absorption rate is still a function of the amount injected. One day after injection of < 1 jug of the lanthanide citrates, 0.95 had been absorbed 0.75 was absorbed in the first day if the amount of stable lanthanide administered was 1 to 5 jug. After 256 days, only 0.05 of the injected 152,154Eu plus 5 jug Eu remained unab-... 

The second example is seen in the study of PbSe deposition by Kainthla et al. from selenosnlphate solution [41]. In most examples of CD from alkaline so-Intion, the deposition rate increases with increase in pH. This is due to both the greater rate of decomposition of the chalcogenide precursor at higher pH (this decomposition nsnally involves hydroxide ions) and, in many cases, the greater probability of solid hydroxide formation (as long as this is not excessive). However, for PbSe deposition nsing citrate as complex for the Pb and selenosnlphate as Se precursor, the opposite occurs The deposition rate decreases with increase in pH. This is dne to the specific hydroxy-citrate complex formed ... 

Kainthla et al. carried out an investigation on the parameters that affected deposition rate [62]. The rate increased, as expected, with increase in temperature and selenosulphate concentration. However, it decreased with increase in pH. This was explained on the basis of the expected dominant hydroxy-citrate complex of Pb, [Pb(0H)C6Hs07]. The equilibrium involving this complex is... 

In the case of dissolved metal as major additive compounds, a combination of precipitation and redissolution can be applied for recovery from spent solutions. Gyliene et al. [94] found, for recovery of the main additive in nickel electroless plating, that the Ni(II)-citrate complex could be precipitated with alkali followed by redissolution in citric acid for reuse in electroless nickel plating after separation of the precipitate. Additionally, for decontamination of spent electroless nickel plating solutions Fe(III) can be used to precipitate the pollutant. 

Krishnamoorthy et al. developed a citrate-complex method to prepare single-phase Lai j.Cej Mn03 x = 0.15, 0.2, 0.3) nanopowders... 

Yi et al. reported LaCoOa perovskite NPs confined in SBA-15 silica by a microwave-assissted process with a La-Co citrate complex precursor (Yi et al., 2005). This method allows the incorporation of up to 60 wt% LaCoOa perovskites into the mesopores. The obtained material exhibited high catalytic activity in the complete methane oxidation. 

Worldwide, given adequate nutrients, the presence of AP+ is the main limiting factor in plant productivity in acidic soils. Plants such as tea that accumulate AP+ are rare and do so in acidic soils and evidently detoxify the AP+ by storing a chelated version in cell vacuoles of older leaves separate from the more metabolically active parts of the plant. Tea plants have been found with as much as 3% AP+ in older leaves and only 0.01% in younger ones, a 300-fold difference. Typical tea infusions contain about 50 times as much AP+ as do infusions from coffee. Adding milk to tea should immobilize Al + as an insoluble phosphate, while lemon will strongly complex the Al + in deleterious soluble citrate complexes described below. 

One can envision that pH can impact several aspects of this reaction, including the affinity of citrate for both Fe + and Fe + (Table 10). Abrahamson et al. studied the quantum yield for citrate complexes of Fe + and Fe + as a function of pH and reported that the formation of Fe + is more efficient at pH 4.0 than at pH 2.7, which would predict the opposite trend observed in Figure 7 (36). 

Fk 8. The 104-MHz "Al NMR spectra of 1 1 mixtures of AF and citrate. The sharp peak at -80 ppm in some spectra comes from I All OH I. The remaining signals are from AP -citrate complexes at least three peak.s overlap in the region 0-35 ppm... 

Lothenbach B, Furrer G, Schulin R (1997) Immobilization of heavy metals by polynuclear aluminum and montmorillonite compounds. Environ Sci Technol 31 1452-1462 Marklund E, Ohman L-O (1990) Equilibrium and structural studies of silicon(IV) and aluminum(III) in aqueous solution. 24 A potentiometric and Al NMR study of polynuclear aluminum(III) hydroxo complexes with lactic acid. Acta Chem Scand 44 228-234 Matzapetakis M, Raptopoulou CP, Terzis A, Lakatos A, Kiss T, Salifoglou A (1999) Synthesis, structural characterization, and solution behavior of the first mononuclear, aqueous aluminum citrate complex. Inorg Chem 38 618-619... 

---