Competitive Sorption and Transport - Transport and Fate of Chemicals in Soils: Principles and Applications - page 236

precipitate on ferrihydrite under high As/Fe molar ratio, low pH, and long
reaction time.
Diffusion of As(V) and P to reaction sites within the soil matrix was also
proposed as the explanation of the time-dependent adsorption (Fuller,
Davis, and Waychunas, 1993; Raven, Jain, and Loeppert, 1998). A two-phase
process was generally assumed for diffusion-controlled adsorption, with
the reaction occurring instantly on liquid-mineral interfaces during the first
phase, whereas slow penetration or intraparticle diffusion is responsible for
the second phase. The pore space diffusion model has been employed by
Fuller, Davis, and Waychunas (1993) and Raven, Jain, and Loeppert (1998) to
describe the slow sorption of As(V) on ferrihydrite. For heterogeneous soil
systems, the complex network of macro- and micro-pores may further limit
the access of solute to the adsorption sites and cause the time-dependent
adsorption.
Transport results of As(V) are presented by the BTCs in Figures 7.31 and
7.32. Each soil column received two consecutive As pulses. The BTCs indicate
extensive As(V) retention during transport in both soils. After two As(V)
pulse applications and subsequent leaching by arsenic-free solution for more
than 20 pore volumes, As(V) mass recoveries in the effluent were 82.1% and
72.5% of that applied for Olivier and Windsor soil, respectively. The BTCs
were asymmetric, showing excessive tailing during leaching.

1.0

As(V) Transport
Olivier soil

0.8

0.6

0.4

As(V)
Column

0.2

0.0

0 20 40 60 80

Pore Volumes (V/V o )

FIGURE 7.31
Experimental As(V) breakthrough curves in Olivier soil without addition of P. Solid curves
are single-component multireaction model (MRM) predictions using batch kinetic parame-
ters. The dashed curves depict MRM results based on nonlinear optimization. Arrows indicate
pore volumes when flow interruptions occurred.