An uncomplexed Cu pool was the smallest among observed TEs and almost completely depleted (down to ~1%) even at slightly basic conditions (pH 7.5). Over the most tested pHs organo-com-plexation of Cu dominated, with exception at pH 9.0 (Fig. 22.6a). In non-contaminated conditions at pHs <5.0 the LMW-Cu pool dominated, and thereafter chemosorption with humic substances (55-99%) prevailed. Compared to non-contaminated conditions, in the contaminated model the Cu2+ pool increased only by several %s, but Cu was markedly redistributed in all other pools (Fig. 22.6a). Under contaminated conditions, Cu complexation in the LMW pool occurred over a wider range of pHs (vs. non-contaminated conditions) and dominated at pH <6.5, whereas compl-exation with inorganic ligands (mostly with CO3; data not shown) started to increase from the neutral pH, and dominated (63%) in the basic (9.0) pH (Fig. 22.6a).

As confirmed by the model, Cu possesses the highest affinity to OM among observed TEs. In the model were included organics differing in functional, mostly acidic radicals, with strong potential to complex Cu. Soluble metallo-organo complexes undergo microbial degradation and thereafter (1) a portion of metal is able to enter

Fig. 22.6 Distribution of Cu species (a) among four pools [non-contaminated conditions = L (left bars); contaminated conditions = R (right bars with bold %)] and activities of some of the most abundant Cu species (b) inside each pool (non-contaminated conditions only)

Fig. 22.6 Distribution of Cu species (a) among four pools [non-contaminated conditions = L (left bars); contaminated conditions = R (right bars with bold %)] and activities of some of the most abundant Cu species (b) inside each pool (non-contaminated conditions only)

plants roots as complex fragments (Evangelou et al. 2004), whereas the remaining portion may be (2) leached to deeper soil profiles/groundwa-ter and/or (3) re-adsorbed to soil matrix (e.g. Gondar and Bernal 2009) ; To what extent a certain process would prevail depends on many physical (soil temperature, moisture), chemical (thermodynamic stability of particular metallo-complexes, salinity, DOC) and biological (micro-bial activity) conditions in the rhizosphere. Recently, Ondrasek et al. (submitted) working with salty (0-60 mM NaCl) and Cd-contaminated (4.9-39 mg/L) rhizosphere solution have confirmed a significant decrease in DOC (probably under diminished photosynthetical and microbial activity) and an increase in Cu (Zn, Cd) concentrations, either in the rhizosphere or in leaf/fruit tissues of radish. In the same study, under slightly acidic rhizosphere (pH ~6) organically com-plexed Cu predominated (>99%) in all tested conditions. Evangelou et al. (2006) observed improved Cu uptake and accumulation (up to

2.3-fold) in tobacco shoots with the addition of LMW-OAs to soil. Although LMW and HMW organics have significantly different properties, particular components such as acidic radicals are inherent to both of them (Sect. 3).

Investigating Cu binding to OM fractions in soil amended with olive mill residue, Gondar and Bernal (2009) found that at low metal concentrations (at pH 5.3), the amount of Cu bound by FAs and HAs was much higher than the water soluble (WS) fraction, whereas with metal concentration increasing, WS bound Cu to the same extent as FA, but the HA binding capacity was significantly lower. Such behaviour authors explained by qualitative/quantitative differences among OM fractions, i.e. lower content of carboxylic and higher content of phenolic groups in HAs (0.96 and 2.06 me/g, respectively) compared to FAs (5.32 and 1.26 me/g, respectively), and also by greater than twofold lower metal binding capacity of HAs (vs. FAs). Also, FAs are known to form more soluble and mobile metallo-complexes than HAs due to their higher acidic functional group content, smaller molecular weight and solubility over a wider range of pHs (2-12) (e.g. Smith and March 2007).

Phenolic functional groups have weaker tendency for deprotonation than carboxylic groups under acid conditions, and acidity of the hydroxyl groups in phenols (pKa 10-12) is lower than in carboxylic groups (e.g. pKa of acetic, formic and oxalic acids are 4.76, 3.77 and 1.27, respectively) (e.g. Smith and March 2007). Therefore, it appears that carboxylic groups are more important for metals binding under lower pHs, whereas in higher pHs other functional groups such as phenolic could be more involved in metallo-complexation. This hypothesis has been confirmed by the proposed model (Fig. 22.6b) where organo-complexation of Cu with carboxylate anions from LMW (e.g. oxalate and malate) and HMW [i.e. FA1-Cu(6)] dominated in acidic pHs (<6.0), whereas a continuous increase in organo-complexation with an increase in pH was due to enhanced deprotonation of LMW phenolic groups and Cu chemosorption with phenolate anions [i.e. FA2-Cu(6)].

With their huge reactive interfaces, predominantly polycarboxylic (FAs) and polyphenolic (HAs) (e.g. Gondar and Bernal 2009), HMW ensure a plenty of sorption sites for Cu and other TEs. Such sorption is mostly with a higher metal affinity than for mono/di carboxylic LMW-OAs, and therefore indirectly decreases TE bioavailability, although underlying mechanisms remain unclear.

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