2. ENVIRONMENTAL ChEMISTRy & TEChNOLOGy 2.1. Lectures

Chem. Listy, 102, s265–s1311 (2008) Environmental Chemistry & Technology
effect can be considered as negligible. In a previous study 4
the zeta potential of akaganeite was studied and the point of
zero charge was found at pH 7.5. At pH 3.5, the zeta potential
was around +20 mV and the predominant As (V) species
is H 2 AsO 4 – . So, attractive electrostatic forces favour
sorption. However, at pH 8.0, the zeta potential is negative
and the predominant species is HAsO 4 –2 . electrostatic forces
are repulsive but still sorption is achieved due to the existence
of specific forces. Calculations with Geochemist Workbench
(data not shown here) show that in the presence of
iron mineral As (V) can form scorodite (iron arsenate) on the
mineral surface and the maximum is obtained at pH 2.0, This
coincides very well with the maximum sorption at pH 2.0 in
our experiments (Fig. 1.). The existence of scorodite on akaganeite
surface was also proved by FTIR spectra 6 . The chemical
nature of the sorption can also explain the sorption of
As (V) at pH 8.0. In our results, the effect of ionic strength
was negligible. However, in the literature 6 a significant effect
was observed, but with KnO 3 and in smaller sorbent
concentrations (< 2 g dm –3 ). The effect was decreasing with
increasing sorbent concentrations. This was attributed to
the elimination of the negative charge of akaganeite by the
potassium cations and the easier approach of As (V) species
to the surface of the sorbent. It is well known that the increase
of the ionic strength decreases the effective radius of the
electric double layer. The arsenate species can come closer to
the surface so that the attractive chemical forces overcome
the electrostatic repulsion forces and sorption is achieved.
Fig. 1. Effect of solution ph on As (V) sorption
E f f e c t o f I n i t i a l A r s e n i c
C o n c e n t r a t i o n
The effect of initial arsenic concentration (20–
200 mg dm –3 ) was studied with 2 g dm –3 sorbent concentration,
ambient temperature and pH values 3.5 and 7.0. The
results are given in Fig. 2. (data points: experimental data,
lines: isotherms). Sorption isotherms were evaluated (from
experimental data) using the Freundlich model:
Q KC
= (2)
b
eq
s472
where Q is amount sorbed per unit mass of sorbent, K and b
are constants and C eq is the equilibrium concentration. Freundlich
sorption parameters (K, b, R 2 ) were calculated and are
given in Table I. The maximum sorption capacity was around
53 and 37 mg g –1 for pH values 3.5 and 7.0 respectively. The
R 2 values show a good agreement of the model to the experimental
data. As is expected, the K value (which shows the
affinity of akaganeite to As (V) ) decreases with the increase
of the pH.
Fig. 2. Sorption isotherms
Table I
Freundlich adsorption parameters
pH Freundlich parameters
K b R 2
3.0 32.26 0.10 0.98
7.5 12.84 0.23 0.97
E f f e c t o f t e m p e r a t u r e
The effect of temperature on arsenic sorption was investigated
at initial arsenic concentration 100 mg dm –3 , sorbent
concentration 2 g dm –3 and 4 different temperatures (ambient,
30, 40 and 50 °C). The experimental results are presented
in Table III. The maximum sorption capacity increased with
increasing temperatures indicating an endothermic reaction
6 . The maximum sorption capacity (at 50 °C) was around
43 mg g –1 .
Table II
The effect of temperature
T [°C] Q [mg g –1 ]
ambient temperature 37
30 38
40 40
50 43
E f f e c t o f S o r b e n t D o s e
The effect of sorbent dose was studied at ambient temperature,
initial arsenic concentration 100 mg dm –3 and sorbent