Sorption—desorption of trace elements

FACTORS AFFECTING THE SORPTION-DESORPTION OF TRACE ELEMENTS IN SOIL ENVIRONMENTS... 

In contrast to sorption studies, relatively little information is available on the desorption of trace elements from soils or soil components as affected by organic and inorganic ligands. Desorption studies have showed biphasic reaction processes for sorption and desorption of trace elements (Sparks, 1990) a fast reaction followed by a slow reaction. 

The main soil parameters governing processes of sorption and desorption of trace elements can be presented as follows (i) pH and Eh values (ii) fine granulometric fraction <0.02 mm (iii) organic matter (iv) oxides and hydroxides of Fe, Mn and Al and (v) microorganisms. 

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. 

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. 

Ainsworth et al. (1994) observed that oxide aging did not cause hysteresis of trace element cation sorption-desorption. Aging the hydrous ferric oxide with trace elements cations resulted in hysteresis with Cd and Cu, but little hysteresis was observed with Pb. The extent of reversibility with aging for Co, Cd, and Pb was inversely proportional to the ionic radius of the ions (i.e., Co < Cd < Pb). The authors attibuted the hysteresis to Co and Cd incorporation into a recrystallizing solid (probably goethite) via isomorphic substitution, not to micropore diffusion. 

I.I. Effect of Residence Time on Desorption Some researchers found that trace elements [Ni, Pb, As(V)] reacted with metal oxides and pyrophyllite over longer times resulted in either irreversible or reversible sorption mechanisms. Violante et al. (2003) studied the effect of residence time on the sorption of Zn onto ferrihydrite in the presence of Cu. As Cu has a greater affinity than Zn for tire surfaces of ferrihydrite, Cu was added from 1 to 336 hours after Zn at a Zn/Cu molar ratio of 2. Zinc sorption increased, particularly when Cu was added 6 to 336 hours after Zn. A possible explanation of these findings is that trace elements initially sorbed on the surfaces of variable-charge minerals slowly form precipitates with time. As discussed before, sorption is considered to be the predominant sorption mechanism responsible for trace element uptake on mineral surfaces within the first few hours, while surface precipitation is considered to be a much slower process, occurring on a time scale of hours to days (McBride, 1994 Scheidegger et al., 1997 Sparks, 1999 Borda and Sparks, Chapter 3, this volume). Clearly, Cu added many hours or days after Zn addition cannot replace Zn ions that have formed precipitates on the surfaces of the ferrihydrite. 

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