2. ENVIRONMENTAL ChEMISTRy & TEChNOLOGy 2.1. Lectures

Chem. Listy, 102, s265–s1311 (2008) Environmental Chemistry & Technology
P68 ARSENIC REMOVAL FROM wATER by
SyNThETIC AKAGANEITE
MIROSLAVA VáCLAVíKOVá a,c , KATARínA
ŠTEFUSOVá a , GEORGE P. GALLIOS b and ŠTEFAn
JAKABSKý a
a Institute of Geotechnics, Slovak Academy of Sciences, Watsonova
45, 043 53 Košice, Slovakia,
b Laboratory of General. & Inorganic Chemical Technology,
School of Chemistry, Aristotle University, 540 06 Thessaloniki,
Greece,
c School of Chemistry, Royal Military Academy, Renaissancelaan
30, 1000 Brussels, Belgium,
vaclavik@saske.sk
Introduction
Arsenic is considered one of the most toxic contaminants
found in water-streams. It poses serious health risks
to humans (e.g. cancer, cardiovascular and neurological effects).
There are two main groups of arsenic pollution sources;
natural (dissolution of As containing mineral ores) and
anthropogenic sources (e.g. arsenic-based insecticides and
pesticides, fertilizers, coal combustion, mining, semiconductor
industies). The maximum contaminant level of arsenic in
drinking water (both in EU and USA) is 10 μg dm –3 .
Arsenic has several oxidation states (–3, 0, + 3, + 5) but
the most common forms in natural waters are trivalent arsenite
[As (III)] and pentavalent arsenate [As (V)]. The pH and
the redox potential (Eh) of the aqueous system determine the
predominant form of As; As (III) is dominant in reducing
conditions while As (V) in oxidizing conditions. Generally,
As (III) is considered to be more toxic than As (V) 1 .
There are various methods for arsenic removal from
water streams, such as sorption, ion-exchange, precipitation,
coagulation and flocculation, reverse osmosis, membrane
technologies, electrodialysis, biological processes as well as
lime softening. A good overview 2 and a critical review 3 of the
available methods are given recently. An effective and commonly
used method for water treatment is sorption of arsenic
on natural or synthetic sorbents. The most commonly used
sorbents can be classified in two main groups: (i) sorbents
based on iron compounds, which are the most frequently used
(e.g. several iron (III) oxides/hydroxides, materials based on
iron oxides/hydroxides, natural iron ores and waste materials
containing iron particles) and (ii) sorbents based on aluminium
compounds (e.g. activated alumina or gibbsite). Several
other sorbents (clays, manganese dioxide, activated carbon,
ion-exchange resins, biosorbents, etc.) have also been studied
for As removal 2 .
Experimental
R e a g e n t s
Analytical grade chemicals were used in all experiments.
Model solutions were prepared by dissolving
AsHna 2 O 4 . 7H2 O in deionized water, 0.01 M nanO 3 and
s471
0.1 M nanO 3 . The pH of the solutions was adjusted with suitable
concentrations of naOH and HnO 3 .
P r e p a r a t i o n a n d C h a r a c t e r i z a t i o n
o f S o r b e n t
Synthetic akaganeite was prepared by hydrolysis of partially
neutralized FeCl 3 by addition of naOH. The precipitate
thus obtained was centrifuged and it was subsequently submitted
to dialysis to remove Cl – ions. The material was dried and
used for arsenic sorption. XRD analysis 4 confirmed that the
material produced was akaganeite. The specific surface area
(measured by BET) was 151.3 m 2 g –1 .
S o r p t i o n E x p e r i m e n t s
The effect of pH, initial arsenic concentration, sorbent
dose and temperature as well as ionic strength effect at arsenic
sorption were studied in batch type experiments. The
experiments were performed in a rotary shaker set at 30 rpm
and equilibrium time 24 hours. Preliminary experiments have
shown that equilibrium was established. The arsenic quantity
in solutions was determined by AAS and spectrophotometry
before and after the sorption experiments. The sorption capacity
(Q) of akaganeite was calculated using the equation:
C − C
Q =
C
0 eq
s
where C 0 and C eq are the initial and equilibrium arsenic concentration,
respectively and C s is the sorbent concentration
in solution.
Results
E f f e c t o f S o l u t i o n p H
It is well known that the solution pH plays an important
role in all sorption experiments in aqueous systems. It determines
the aquatic chemistry of the system under study (in this
case As speciation) and also the charge density of the solid
surface (sorbent). It is related to the sorption mechanisms and
reflects the nature of the physicochemical interactions of the
species in solution and the active sites on the sorbent 5 .
The effect of solution pH on As (V) sorption on akaganeite
was examinated at pH values 2.0–9.0 at ambient temperature.
Sorbent concentration in solution was 2 g dm –3 and
initial arsenic concentration was 100 mg dm –3 . The results are
given in Fig. 1. for various electrolyte (nanO 3 ) concentrations.
The best sorption is observed at pH 2.0. However, in
this case, akaganeite is dissolved and iron was determined in
filtrates. For this reason, all sorption experiments were carried
out at pH 3.5, where no iron was found in solution. At
this pH, the sorption capacity was a bit smaller than at pH 2.0
and around 40 mg As (V) g –1 of sorbent. This is considered
quite good compared to the average value reported in the literature
3 . The effect of the ionic strength was also studied by
adding 0.01 and 0.1 M nanO 3 . It is shown that (Fig. 1.) the
(1)