Competitive Sorption and Transport - Transport and Fate of Chemicals in Soils: Principles and Applications - page 232

of addition might significantly affect the competition between As(V) and P
(Liu, De Cristofaro, and Violante, 2001; Zhao and Stanforth, 2001). Liu, De
Cristofaro, and Violante (2001) found that when added sequentially [As(V)
before P versus P before As(V)], more P was replaced by As(V) on goethite
than vice versa. Due to the similar dissociation constants of phosphate
(pK a 1 = 2.23, pK a 2 = 7. 2 , p K a 3 = 12.3) and arsenate (pK a 1 = 2.20, pK a 2 = 6 . 9 7,
pK a 3 = 11.53), adsorption of both anions decreases with increasing pH (Jain
and Loeppert, 2000). When added simultaneously, Jain and Loeppert (2000)
reported that the effect of P on As(V) adsorption on ferrihydrite was greater
at high pH than at low pH.
In most studies dealing with competitive adsorption, equilibrium con-
ditions are often assumed. However, several experiments demonstrated
adsorption of As(V) (Fuller, Davis, and Waychunas, 1993; Grossl et al., 1997;
Raven, Jain, and Loeppert, 1998; Waltham and Eick, 2002; Zhang and Selim,
2008) and P (Barrow, 1992; Wilson, Rhoton, and Selim, 2004) on minerals and
soils are both time dependent.
In the case study presented here, As adsorption as well as desorption
results are presented in the presence of different P concentration (see Zhang
and Selim, 2008). Furthermore, transport results from soil columns that
received As alone and columns that received a pulse having As and P (see
Zhang and Selim, 2007). In the subsequent sections, these two treatments are
referred to as single pulse and mixed pulse, respectively.
Amount of As(V) sorption in the presence of P is depicted in Figure 7.28,
which indicates that As(V) adsorption decreased substantially with increas-
ing P concentrations. Competition for specific adsorption sites is likely the
major cause for the observed competitive effect between As(V) and P shown

1.4

Olivier
Sharkey
Windsor

1.2

1.0

0.8

0.6

0.4

0.2

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Initital Phosphate Concentration (m M )

FIGURE 7.28
Competitive sorption between arsenate and phosphate at 24 h of reactions for Olivier, Sharkey,
and Windsor soils. The initial concentration of arsenate was 0.13 mM.