Archives of Mining Sciences 50, Issue 1 (2005) 131–142

Archives of Mining Sciences 50, Issue 1 (2005) 131–142
131
EWELINA PIECZABA*, STANISŁAWA SANAK-RYDLEWSKA*, DANUTA ZIĘBA*
REMOVAL OF ARSENIC FROM AQUEOUS SOLUTIONS BY THE METHOD
OF PRECIPITATE FLOTATION
USUWANIE ARSENU Z ROZTWORÓW WODNYCH METODĄ FLOTACJI OSADÓW
This paper presents the results of laboratory studies on elimination of arsenic from aqueous solutions
by flotation with colloidal precipitate of iron hydroxide (also known in the literature as ferrihydrite, goethite, or oxide-hydroxide phases). Sodium dodecyle sulphate and dodecylamine hydrochoride were used
as collectors. The study was performed for various pH values. The results were interpreted in terms of
measured values of electrophoretic mobility for iron hydroxide, and thus the calculated values of the zeta
potential. The paper also gives possible mechanisms of removing arsenic from aqueous solutions by the
adsorbing colloid flotation (ACF) method, using hydrated iron(III) oxides. Under the conditions developed
for co-precipitation and flotation, about 96% of the arsenic could be removed from the studied systems.
Keywords: arsenic, iron hydroxide, zeta potential, precipitate flotation, wastewater treatment
W pracy przedstawiono wyniki badań laboratoryjnych eliminacji arsenu z roztworów wodnych za
pomocą flotacji na koloidalnym osadzie wodorotlenku żelaza (nazywane też w literaturze ferrohydratem,
getytem lub fazami tlenowo-wodorotlenkowymi). Jako kolektory zastosowano dodecylowy siarczan sodu
lub chlorowodorek dodecyloaminy. Badania wykonano w zależności od pH środowiska. Wyniki interpretowano posługując się zmierzonymi ruchliwościami elektroforetycznymi dla osadu wodorotlenku żelaza
i na tej podstawie obliczonymi wartościami potencjału dzeta.
Z naszych badań wynika, że osad wodorotlenku żelaza(III) wykazuje dodatnie wartości potencjału
dzeta w środowisku kwaśnym i zbliżonym do obojętnego. W środowisku alkalicznym wartość potencjału
jest ujemna, ale mniejsza, co do bezwzględnej wartości, niż w środowisku kwaśnym. Punkt izoelektryczny
osadu jest przy pH około 7,1 i jest to wartość zbliżona do wartości jaką otrzymał Parks (Parks, 1964).
Wartość punktu izoelektrycznego (IEP) osadów wodorotlenku żelaza wielokrotnie wyznaczało wielu
autorów. Świadczą o tym wartości zebrane w pracy (Parks, 1964).
W artykule zamieszczono badania flotacji wykonane według dwóch różnych procedur. W pierwszej serii badań do świeżo strąconego osadu wodorotlenku żelaza(III), wprowadzano jony arsenu(V)
oraz odczynnik flotacyjny dodecylowy siarczan sodu lub chlorowodorek dodecyloaminy. Początkowe
*
ZAKŁAD PRZERÓBKI KOPALIN, OCHRONY ŚRODOWISKA I UTYLIZACJI ODPADÓW, AKADEMIA GÓRNICZO-HUTNICZA, AL. MICKIEWICZA 30, 30-059 KRAKÓW, POLAND
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pH wynosiło około 5. Doświadczenia wykonano dla różnych stosunków molowych żelaza do arsenu
wynoszących od 2 do 12. Wyniki flotacji z użyciem tych kolektorów przedstawiono na rys. 3 i 4. Na
ich podstawie można stwierdzić, że niezależnie od czasu flotacji (stosując dodecylowy siarczan) oraz
od stosunku Fe/As, stężenie arsenu w roztworze zawsze wynosiło powyżej 18 mg/dm3. Uzyskano więc
nieznaczne zmniejszenie stężenia kolligenda. Natomiast zaobserwowano niekorzystny wzrost kwasowości
roztworu z wartości pH około 5 do około 2 po flotacji (rys. 3). Jeżeli pH roztworu jest poniżej 3 to jony
arsenu(V) tworzą niezdysocjowane cząsteczki kwasu arsenowego(V). Obojętne cząsteczki H3AsO4 nie
zostały aktywnie związane z strąconym osadem i być może dlatego nie były wyniesione z produktem
pianowym podczas flotacji.
Flotacja arsenu na Fe(OH)3 za pomocą dodecylosiarczanu sodu zachodzi najefektywniej w środowisku
kwaśnym od pH około 4 do 5 (największa redukcja jego stężenia), a maleje w środowisku zasadowym.
Jeżeli do flotacji zastosowano kationowy kolektor (chlorowodorek dodecyloaminy) to w środowisku
kwaśnym stężenie arsenu utrzymuje się prawie na stałym poziomie i jego wartość praktycznie jest taka
jak we wprowadzonym roztworze (20 mg/dm3). Po przekroczeniu pH 7 następuje szybki spadek stężenia
arsenu poniżej 1 mg/dm3, a następnie znowu rośnie.
Porównując oddziaływanie tych dwóch kolektorów nasuwa się wniosek, że w pobliżu punktu izoelektrycznego osadu (pH około 7; rys. 2) proces flotacji zachodzi z podobną i niewielką wydajnością
- (stężenie As(V) w roztworze wynosi ok. 18 mg/dm3; rys.4).
W drugiej serii badań wykonano flotację doprowadzając pH mieszaniny roztworów: jonów żelaza(III)
i jonów arsenu(V) do wartości około 5,2-5,5. Tak otrzymany osad poddano flotacji za pomocą dodecylosiarczanu sodu. Warunki flotacji były takie jak w poprzedniej serii doświadczeń. Stosunek stężeń Fe
do As wynosił od 2 do 12.
Zależności podane na rysunku 5 wskazują, że wraz ze wzrostem stosunku molowego żelaza do arsenu
rośnie stopień eliminacji arsenu z roztworu. Początkowe stężenie arsenu zmalało z 20 mg/dm3 do wartości
około 0,8 mg/dm3. Istotną rolę odgrywa tu prawdopodobnie wpływ pH środowiska (około 5) i związana
z nim wartość potencjału dzeta powierzchni osadu oraz rodzaj kolektora.
W pracy podano możliwe i cytowane w literaturze mechanizmy usuwania arsenu z roztworów
wodnych metodą flotacji nośnikowej – na uwodnionych tlenkach żelaża(III) (adsorbing colloid flotation
– ACF). Opracowane warunki procesu współstrącania i flotacji pozwalają na usunięcie około 96% arsenu
z badanych układów.
Słowa kluczowe: arsen, wodorotlenek żelaza, potencjał dzeta, flotacja osadów, oczyszczanie ścieków.
1. Introduction
The increased interest in environmental protection results, among other things, in
stricter requirements concerning handling of materials containing toxic substances. To
this group obviously belong arsenic-bearing wastes, e.g., dust from pyrometallurgical
processes, slimes generated during electrorefining of metals (e.g., copper), flotation waste,
metallurgical effluents, etc. The way these wastes are treated affects directly the quality
of soil and water. The most serious source of arsenic exposure for people and animals is
potable water (Dutrizac, 2001; Kumar, 2004). According to the European Union’s new
directive in force since 2003, the concentration of arsenic in potable water must not
exceed 10 g/dm3. This is also the value set by the Minister of Infrastructure of 20 July
2002 on the highest permissible levels of wastewater pollutants, and the value set by the
WHO (Katsoyiannis, 2004; Zang, 2003; Rozporządzenie Ministra Infrastruktury).
133
Arsenic is not only toxic but also mutagenic and carcinogenic. It results from arsenic’s
high affinity for proteins and lipids. Ions containing five-valent arsenic can substitute
phosphate groups. That is why they act as inhibitors in the synthesis of ATP (adenosine
triphosphate) – an energy carrier in biochemical reactions. Ions containing arsenic with
an oxidation number of +3 enter into reactions with thiol groups in proteins, and reduce
their biological activity (Dutrizac, 2001; Katsoyiannis, 2004; Sracek, 2004).
It is important to stress the difference between As(III) and As(V), the two main
oxidation numbers of this metal, due to their ionic forms present in aqueous solution.
They also differ in mobility and toxicity. Arsenic (V) is more mobile and toxic (25-60
times) than As(III) (Kumar, 2004). The oxidation number of arsenic ionic forms depends
on the oxidation-reduction properties of the medium, and its pH (Fig. 1).
2,0
3
1,6
Potential, E[mV]
5
O2
1,2
6
0,8
0,4
4
H3 AsO4
AsH3 (g)
AsO43-
9
HAsO2
-0,4
10
11
H3AsO3 0
-0,8
H2AsO3
As
1
-1,2
HAsO4 2-
8
As2O3
H2
0,0
H2AsO4 -
7
AsO+
-
2
-1,6
-2
-1
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
pH
Fig. 1. E – pH equilibrium diagram for the As – H2O system at 298 K (25°C)
(Dutrizac, 2001; Porubaix, 1963)
Rys. 1. Diagram równowag E – pH układu As – H2O w temp. 298 K (25°C)
In oxidizing aqueous media, at pH values greater than 7, HAsO42– ions predominate,
whereas in slightly acidic media, H2AsO4– ions predominate. These ions come from arsenic (V) oxide, which is hygroscopic and very readily soluble in aqueous solutions – that is
why it is not present in the diagram. In oxidizing media, H2AsO4– and HAsO42– ions can
also come from amphoteric arsenic (III) oxide, which is electrochemically more stable
than As2O5 and is readily soluble in a wide range of pH, from 1 to 8. If the solution has
reduction properties, arsenic occurs in an undissociated form as H3AsO30 at pH below 9.2
(Dutrizac, 2001; Porubaix, 1963; Kumar, 2004; Katsoyiannis, 2004; Ghimire, 2003).
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The ionic form in which arsenic occurs in the studied system determines the methods
and treatment procedures to be used for its removal. Most of the solutions proposed so
far have confirmed, that As+5 can be removed from aqueous solutions easier and more
effectively than As+3 (Dutrizac, 2001; Kumar, 2004; Katsoyiannis, 2004; Katsoyiannis, 2002; Ghimire, 2003; Leist, 2000; Apostoluk, 1996). And this is in spite of the fact
that at ambient temperatures, and under conditions that most technological processes
are run, it is As(III) that is more stable and predominant (Dutrizac, 2001). That is why
it is suggested that As(III) is oxidized with appropriate reagents, e.g., H2O2, KMnO4,
Cl2, O3, O2, SO2+O2 (Dutrizac, 2001; Katsoyiannis, 2002). Most recent technological
solutions suggest the use of bacteria to remove arsenic from aqueous solutions (Katsoyiannis, 2004; Katsoyiannis, 2002), or dead biomass and plant remains, e.g., orange peels
(Ghimire, 2003; Loikidou, 2003).
In natural environment, arsenic ions are very strongly adsorbed and effectively immobilised by mildly acidic FeO(OH)(H2O)1+x (hydrated iron oxides, goethite), manganese and aluminium hydroxides, which are common soil components. A shift to a more
reducing environment and the action of micro-organisms may result in freeing mobile
and toxic arsenic ions (Dutrizac, 2001).
Arsenic ions are most often removed from aqueous solutions by chemical precipitation
or co-precipitation. An often used reagent is lime, producing various forms of calcium
arsenate Ca4(OH)2(AsO4)2  4H2O; Ca5(AsO4)3OH). Unfortunately, if these substances
are stored in the presence of carbon dioxide and water, arsenic is liberated back to the
environment (Dutrizac, 2001; Kumar, 2004; Ghimire, 2003). Studies have been conducted, in which arsenic was precipitated with salts of aluminium, sodium sulphate, hydrates
of bisulphates represented by a general formula MI MIII (SO4)2  12H2O, where MI = Na,
K, Rb, Cs, NH4+, Tl, MIII = Al, Fe, Cr, Co, Mn, Sc, Ga (known widely as alums). But
in most experiments the concentration of arsenic was not sufficiently reduced. There
are also problems with dewatering and storage of thus produced precipitates (Dutrizac,
2001; Ghimire, 2003).
The most widely applied is the method in which arsenic is co-precipitated with
iron(III) hydroxide. The numerous modifications of this method show that it is most
effective from both technological and economic points of view (Dutrizac, 2001; Kumar,
2004; Wang, 2001; Zang, 2003).
The use of the electrocoagulation method (with an iron electrode) makes it possible
to produce the adsorbate, that is, Fe(OH)3, in situ. Simultaneously, As(III) is oxidised to
As(V) at the anode. Arsenic ions are removed with an efficiency of 99% (Kumar, 2004).
Interesting results were obtained in studies which took advantage of both the adsorptive properties of iron hydroxides, and the high affinity of rare metal hydroxides, and
Ce(IV) in particular, for arsenic. If the pH was below 5.8, the Ce-Fe adsorbent showed
the highest efficiency of arsenic ion removal, as its surface was positively charged and
the ions were very strongly bound (Zang, 2003).
135
Since the 1980s, the so-called adsorbing colloid flotation (ACF) method has been
used, involving flotation of colloids with adsorbed forms of the ion being removed.
This method has many advantages: the process is relatively fast, the recovery of sublate
is considerable, and its costs are low (Eric Helnen De Carlo, 1981; Shreedhara, 1983;
Feng Xi, 1984).
Studies in this field are performed by the team led by Clarka A.N. of the Vanterbilt University, USA. Their work mainly involved the removal of oxyanions, such as
AsO43– , PO43–, and metal ions (Clarka, 1985). Tomotaka studied the ACF process for
phosphate(V) ions floated with aluminium hydroxide precipitate, with sodium naphtate
as a collector. Tomotaka demonstrated that cyanide ions (CN–), which form complexes
with Cu, Cr, Ni and Co ions, have an advantageous effect on the flotation recovery (Eric
Helnen De Carlo, 1979).
This paper presents the results of experiments which take advantage of the adsorption
properties of iron(III) hydroxide and employ the Adsorbing Colloide Flotation method
(Sebba, 1962).
2. Subject and method of the study
The model solution contained 20 mg/dm3 of arsenic(V). The solution was prepared
from a standard solution (1000 mg/dm3 As2O5 in 0.5 M HNO3). Iron(III) hydroxide
was precipitated from iron(III) sulphate(VI) with a 10–1 molar NaOH solution. After
the reaction the precipitate was washed until negative reaction for sulphate ions was
obtained. Next, zeta potential was measured with the electrophoretic method. Zeta potential measurements were performed at an ionic strength of 210–3 mole/dm3, which
was controlled by NaClO4. The pH of the flotation solutions was controlled by aqueous
solutions of H2SO4 or NaOH at a concentration of 10–1 mole/dm3 each.
Iron(III) hydroxide precipitate was floated in a 0.5 dm3 flotation column which had
a G-4 Schott filter at the bottom. An ionic collector – sodium dodecyle sulphate (C12
H25OSO3Na), and a cationic collector – dodecylamine hydrochloride (C12H25NH2HCl)
were used as flotation reagents. The experimentally determined concentrations of these reagents, which acted as both collectors and frothers, were 40 mg/dm3 for each of them.
The solution was aerated with an inert gas (nitrogen) which was supplied to the
column at a measured rate of 50 cm3/min.
Arsenic was removed from the solution in two ways:
• The first one involved precipitation with iron(III) hydroxide, followed by the
introduction of arsenic (V) and the above specified quantity of collector, and then
flotation. The flotation time was 3 to 4 minutes. The results of the experiments are
shown in Fig. 3.
• The other variant involved flotation of the precipitate obtained by introducing a salt
of arsenic(V) to the iron(III) sulphate(VI) solution, the pH of which was brought
136
to the values between 4.8 and 5.2 (precipitation of the hydroxide). As in previous
experiments, the flotation time was 3 to 4 minutes. The results of the experiments
are shown in Figs. 4 and 5.
The concentration of arsenic was determined by Atomic Absorption Spectrometry.
3. Results and discussion
Precipitates of iron(III) hydroxides (hydrated iron oxides) were used to remove
arsenic(V) ions from the model solutions containing this colligend at a concentration
of 20 mg/dm3. Fig. 2 shows changes in the electrokinetic potential zeta of Fe(OH)3 as
a function of pH at a constant ionic strength.
50
40
Zeta potential [mV]
30
20
10
7,1
0
-10
-20
-30
-40
-50
2
3
4
5
H2SO4
6
7
pH
8
9
10
11
12
series 1
series 2
series 3
NaOH
Fig. 2. Dependence of zeta potential (ξ) of Fe(OH)3 precipitate on the pH of the medium,
at a constant ionic strength I = 2  10–3 mole/dm3
Rys. 2. Wykres zależności potencjału dzeta (ξ) osadu Fe(OH)3 od pH środowiska,
przy stałej sile jonowej I = 2  10–3 mol/dm3
It results from the shown dependence that the iron(III) hydroxide precipitate has
positive values of the zeta potential in acidic and close to neutral media. The isoelectric
point of the precipitate lies near pH 7.2. In an alkaline medium the potential assumes
negative values, but lower (in terms of the absolute value) than in an acidic medium.
The value of isoelectric point (IEPS) of iron hydroxide precipitates was determined many
137
times for various sets of experimental conditions. For iron(III) hydroxide precipitated
from Fe2(SO4)3 solution with NaOH (i.e., for such conditions as those adopted for this
study) the isoelectric point was determined at pH 7.0 (Parks, 1964).
In the first stage of the study, arsenic(V) ions and the flotation reagent – sodium
dodecyle sulphate – were introduced to freshly precipitated iron(III) hydroxide. The
initial pH was about 5. The experiments were conducted for various iron to arsenic ratios –
from 2 to 12. The results show that the concentration of arsenic was always greater than
18 mg/dm3, regardless of the flotation time and the Fe/As ratio. Hence, only a slight
decrease in the colligend concentration was achieved. It was noted, however, that an
increase in the acidity from the initial pH 5 to about pH 2 after flotation had an adverse
effect (Fig. 3). If the pH of the solution is about 3, the arsenic(V) ions form undissociated
molecules of arsenic(V) acid (Fig. 1). Neutral H3AsO4 were not strongly bound with the
precipitate and that could be why they did not float together with the foam product.
21,0
As concentration [mg/dm3 ]
20,5
20,0
19,5
pH0/pHk
19,0
pH0/pHk
18,5
pH0/pHk
17,5
17,0
0
30
60
90
120
150
180
210
240
270
300
Fe/As = 4
4,94/2,10
Fe/As = 6
4,92/2,14
pH0/pHk
Fe/As = 8
4,91/2,05
Fe/As = 10
4,94/2,03
pH0/pHk
Fe/As = 12
4,89/2,04
pH0/pHk
18,0
Fe/As = 2
4,88/2,14
330
t [s]
Fig. 3. Dependence of the concentration of arsenic(V) remaining in the pulp after flotation
with the Fe(OH)3 precipitate on the flotation time (collector – sodium dodecyle sulphate
(C12H25OSO3 Na))
Rys. 3. Wykres zależności stężenia arsenu(V) pozostałego po flotacji na osadzie Fe(OH)3 od czasu
flotacji (kolektor – dodecylosiarczan sodu (C12H25OSO3 Na))
The next step of experiments involved flotation of arsenic on colloidal ion(III) hydroxide precipitate, in order to determine the process efficiency on the medium’s pH.
In both experiments the Fr to As ratio was maintained at a level of 12. The obtained
dependencies are shown in Fig. 4.
138
22
20
As concentration [mg/dm3 ]
18
16
Fe / As = 12
C0 [As] = 20 mg/dm 3
14
Codczynnika = 40 mg/dm 3
t flotacji = 3-4 min.
12
Vazotu = 50 cm 3/min
10
8
6
4
dodecylamine
hydrochloride
2
0
sodium
dodecyle sulphate
3
4
5
6
7
8
9
10
11
12
pH
Fig. 4. Dependence of the concentration of arsenic(V) remaining in the pulp after flotation
with the Fe(OH)3 precipitate on the flotation time (collector – sodium dodecyle sulphate
and dodecylamine hydrochloride)
Rys. 4. Wykres zależności stężenia arsenu(V) pozostałego po flotacji na osadzie Fe(OH)3
od pH środowiska (kolektory – dodecylosiarczan sodu i chlorowodorek dodecyloaminy)
When a cationic collector (dodecylamine hydrochloride) was used in flotation, the concentration of arsenic in an acid medium was almost constant and its value was practically
the same as in the solution (20 mg/dm3). For pH values greater than 7, the concentration
of arsenic rapidly decreased to values below 1 mg/dm3, and then rose again.
If flotation with Fe(OH)3 was performed with sodium dodecyle sulphate (anionic
collector), the process was most efficient at pH values from about 4 to 5 (the largest
decrease in concentration), whereas in the alkaline medium the efficiency decreased.
Comparison of the two plots leads to the conclusion that in the vicinity of the isoelectric point (about pH 7; Fig. 2) flotation with both collectors has the same, low efficiency
(the concentration of As(V) in the solution is about 18 mg/dm3; Fig.4).
In an acidic medium the surface of the precipitate has a positive value of the zeta
potential (Fig. 2), which favours flotation with an anionic collector. At pH values of
about 4.2, the concentration of arsenic decreased to about 0.5 mg/dm3, whereas in an
alkaline medium the surface of the precipitate was negatively charged, which resulted
in high effectiveness of cationic collectors. At pH of about 9.4, the concentration of arsenic lowered to about mg/dm3. It is also worth noting the different shape of the curves
(Fig. 4). For an ionic collector in a strongly acidic medium from pH 3.5 to about 5, the
139
curve is rather steep. This might be because in this pH range the surface of the precipitate is positively charged, and this is possibly because the predominant form of arsenic
in that solution consisted in the negatively charged monovalent H2AsO4– ions (Fig. 1),
which are probably chemisorbed on the hydroxide surface. Also, the anionic collector
shows affinity for adsorption on the hydroxide surface with a positive value of zeta potential. That is why the effectiveness of ionic collectors in this pH range is high (Fig. 4).
Conversely, in the alkaline medium, at pH greater than 9, arsenic forms bivalent negative ions, HAsO42–, and the precipitate surface is also negatively charged. That is why
a cationic collector, dodecylamine hydrochloride, introduced to the solution not only
adsorbs on the surface but also enters into reaction with the oppositely charged colligend.
Hence, the mechanisms ultimately responsible for flotation performance in the studied
pH range are different.
The last stage of the study involved flotation of the precipitate according to the second variant of the experiment. The precipitate was prepared by bringing the pH of the
mixture of iron(III) and arsenic(V) solutions to values ranging from 5.2 to 5.5. Next,
the solution was floated with sodium dodecyle sulphate. The flotation conditions were
the same as in the previous series of experiments. The Fe to As ratio was variable. The
results are shown in Fig. 5.
22
20
As concentration [mg/dm3 ]
18
16
14
12
pH0/pHk
10
pH0/pHk
8
pH0/pHk
6
pH0/pHk
4
2
0
0
30
60
90
120
150
180
210
Fe/As = 2
5,38/5,19
Fe/As = 4
4,80/4,80
Fe/As = 6
4,95/4,75
pH0/pHk
Fe/As = 8
5,15/4,86
Fe/As = 10
5,49/4,88
pH0/pHk
Fe/As = 12
5,21/4,69
240
t [s]
Fig. 5. Dependence of the concentration of arsenic remaining in the pulp after flotation of iron(III)
arsenate(V) with sodium dodecyle sulphate (C12H25OSO3 Na) on the flotation time
Rys. 5. Wykres zależności stężenia arsenu pozostałego po flotacji arsenianu(V) żelaza(III)
za pomocą dodecylosiarczanu sodu (C12H25OSO3 Na) od czasu flotacji
140
The dependencies presented in Fig. 5 show that the degree of arsenic removal increases
with an increase in the iron to arsenic molar ratio. Hence, the more iron ions, the higher
the yield of arsenic in the froth product. The initial arsenic concentration decreased from
20 mg/dm3 to about 0.8 mg/dm3 (Fig. 5). Obviously, the effect of pH of the medium (of
about 5) and the resulting value of the surface zeta potential, as well as the type of collector, are significant. The pH of the floatation pulp remained almost unchanged.
Here it is also worth noting several mechanisms of co-precipitation and adsorption
proposed for the system under study. Obviously they significantly influence the efficiency
of adsorbing colloid flotation (ACF) with the use of hydrated iron(III) oxides. These mechanisms are an attempt to explain the role of iron ions in removing arsenic ions from the
solution. According to Dutrizac J.E., when a solution containing Fe3+ ions is neutralised,
hydrated ion oxides (also termed ferrihydrite, goethite or oxygen-hydroxide phases) are
formed according to this reaction (Dutrizac, 2001; Malinowski, 2004):
Fe3+ + (3 + x) H2O = FeO(OH)(H2O)1+x + 3H+
(goethite)
(1)
Goethite has the same adsorption properties for ions present in a slightly acidic solution. The proposed mechanism is represented by the following reaction:
FeO(OH)(H2O)1+x + AsO43– = AsO43– ⋅ FeO(OH)(H2O)1+x
(2)
This process of co-precipitation and adsorption is most effective at pH of 4-7. In the
presence of such cations as Cd2+, Pb2+, Ca2+, the upper pH limit shifts to about 9. In the
presence of sulphate ions (SO42– ), the pH optimal for precipitation shifts to about 3,
which can indicate that arsenic ions and sulphuric ions compete for their position in the
lattice of the precipitated goethite (Dutrizac, 2001).
In their paper, Reardon E.J. and Wang Y. pointed to the somewhat different character
of the phenomena that occur on the surface of iron hydroxide (goethite), calling this
process an exchange of ligands with the precipitate surface. Products of the reaction
given below can be classified as arsenates (Wang, 2001):
FeOH2+ (= FeO(OH)) + H2AsO4– = FeH2 AsO4 (s) +H2O
(3)
In a different mechanism of adsorption, in which the role of the positively charged
surface of Fe(OH)2+ is clear, products of this process also include double salts (Katsoyiannis, 2002):
M – FeOH + H3AsO4 = M ⋅ Fe ⋅ H2AsO4 + H2O
(4)
Taking into account the proposed mechanisms of the reaction, and above all the conditions selected for the experiments, one can state that in the studied system a precipitate
is formed, consisting of a mixture of iron(III) hydroxide and a double salt whose likely
formula is Na3 Fex H y (AsO4 )3 z H2 O, where: x = 1 – 2, y = 1 – 0,5, z = 0,5 – 1.
141
However, in order to give a precise composition of this precipitate, X-ray diffraction
studies would be needed (Kucharski, 2002).
Adsorbing Colloid Flotation (ACF) on precipitated iron hydroxides can be an effective
method for removing arsenic from aqueous solutions.
4. Conclusions
1. Most of the solutions proposed so far have confirmed that As+5 ions can be removed
from aqueous solutions easier and more effectively than As+3 ions.
2. The most widely applied methods of removing arsenic from aqueous solutions are
chemical precipitation and co-precipitation.
3. The most widely applied is the method involving co-precipitation of arsenic with
iron(III) hydroxide.
4. The isoelectric point of the iron(III) hydroxide deposit lies near pH 7.1 (at a constant ionic strength; Fig. 2).
5. The introduction of arsenic(V) ions to a freshly precipitated iron(III) hydroxide
allowed a slight decrease in the colligend concentration, regardless of the flotation
time and the Fe/As ratio (the concentration of arsenic was above 18 mg/dm3; Fig.3).
Also noted was an unfavourable effect of the solution’s pH from about 5 to about
2 after flotation.
6. At a pH of about 4.2 the use of an anionic collector (sodium dodecyle sulphate)
made it possible to reduce the arsenic concentration to about 0.5 mg/dm3. In an
alkaline medium the surface of the precipitate is negatively charged, which results
in high effectiveness if the cationic collector (dodecylamine hydrochloride).
7. The degree of removal of arsenic from the solution in which a deposit was precipitated from Fe3+ and As5+.solutions increases with an increase in the molar ratio
of iron to arsenic (Fig. 5).
8. Co-precipitation and adsorption significantly affect the efficiency of adsorbing
colloid flotation (ACF) performed on hydrated iron(III) oxides.
9. The adsorbing colloid flotation (ACF) can be an effective method of removing
arsenic from aqueous solutions.
Acknowledgements
The work was realized under a financial support of AGH No. 10.10.100.863
142
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REVIEW BY: PROF. DR HAB. BRONISŁAW JAŃCZUK, LUBLIN
Received: 29 May 2004