ELECTRODIALYTIC REMOVAL OF HEAVY METALS FROM TREATED TIMBER WASTE
Adélia M. Castro1, Filipa A. Pico2, Alexandra B. Ribeiro2, Eduardo P. Mateus2 & Arménio V. Oliveira1
1
Departamento de Ciência do Solo, Estação Agronómica Nacional, Av. República, 2784-505 Oeiras,Tel. 214403500
fax 214416011, email: [email protected]
2
Departamento de Ciências e Engenharia do Ambiente,Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa,
2829-516 Caparica, Tel. 212948300, email: [email protected]
RESUMO
O problema ambiental associado aos resíduos de madeira tratada com preservantes, nomeadamente com cobre,
crómio e arsénio (CCA) ou creosote, necessita de uma solução a curto prazo. Espera-se que nos próximos anos e
décadas, grandes quantidades de madeira tratada com CCA, existentes, cheguem ao fim do seu ciclo de vida e,
ainda não existe disponível nenhuma técnica de tratamento bem documentada. No entanto, a legislação recente
limita a sua deposição em aterro sanitário. A utilização do processo electrodialítico surge como uma alternativa
interessante, uma vez que permite a reciclagem/reutilização da madeira e possivelmente também dos metais
removidos.
Este trabalho apresenta os principais resultados de dois ensaios electrodialíticos (1 e 2) em amostras de madeira
tratada com CCA, fora de uso, recolhida em Leiria (poste de Pinus pinaster Ait.). Estes ensaios foram realizados
numa célula electrodialítica à escala piloto onde a estilha foi saturada com uma mistura de 1/1 de ácido oxálico
2.5% (p/p) e ácido fórmico 5.0 % (p/p), como agente adjuvante, aplicando-se uma corrente directa de 500 mA,
durante 86 e 30 dias, respectivamente para os ensaios 1 e 2. No ensaio 1 foram obtidos valores de eficiências de
remoção do cobre, crómio e arsénio acima de 98%, enquanto que, no ensaio 2, as eficiências de remoção para o
crómio e arsénio ultrapassaram os 90% e apenas 30% para o cobre. Continua a ser necessário prosseguir a
investigação de forma a optimizar condições.
Palavras-chave: processo electrodialítico; resíduos de madeira tratada com CCA; cobre; crómio; arsénio.
INTRODUCTION
Treated timber with chromated copper arsenate (CCA) is resistant to fungal decay and insect attack, allowing it to
be used in a wide variety of end uses that are not suitable for untreated timber. CCA remains the most widely used
timber preservative in the whole world.
Regulatory controls on chemicals and pesticides have become much more strict on interests of public health and
safety and for protection of the environment. Recently, there has been a concern about the use of preservatives
particularly about the rate of leaching of CCA components from the treated timber. The amounts produced of treated
timber waste have dramatically increased in the last few years, and, since the time of service of treated wood is
between 20 and 50 years, treated wood today will be a waste after the referred period of time.
The electrodialytic process has proved to be a cleaning technology for this type of waste. The electrodialytical
(ED) concept was developed for soil cleaning at the Technical University of Denmark (1). It uses a low-level direct
current as the “cleaning agent”, combining the electrokinetic (EK) movement (mainly due to eletromigration), with
the principle of electrodialysis (1-2).
This work describes the results of Cu, Cr and As removal from chipped CCA-treated timber waste in a small ED
pilot plant.
MATERIALS AND METHODS
Wood waste: Well homogeneized CCA treated wood waste from a Portuguese out of service CCA treated Pinus
pinaster Ait. poles, collected at Leiria, was used. The CCA formulations as well as the treatment schemes used are
unknown.
Pilot scale: The principle of the electrodialytical cell used for the experiments (Fig.1) was developed at the
Technical University of Denmark (1). A full description of the cell is provided elsewhere (1-2).
+
-
+
-
Wood chips contaminatedNO3-
+
Na
Legend
OH-
+
Anode (+)
H
-
H2AsO4-
AN
Cathode (-)
AN – Anion exchange membrane
CAT – Cation exchange membrane
Cu2+
CAT
Fig. 1. Schematic representation of the eletrodialytic cell used in experiments
(Adapted of RIBEIRO, 1998)
The “total” Cu, Cr and As contents in the wood waste was determined according to BS 5666 (3). Before put in
the small ED pilot plant the wood chips were saturated [Fig. 2a)] with an assisting agent: mixture of 1/1 with oxalic
acid 2.5 % (w/w) and formic acid 5.0 % (w/w). The experiments were carried out in a small ED pilot plant [Fig. 2b)],
which consisted of 3 compartments, two electrode compartments and a central one (45 x 30 x 35 cm) for the wood
chips. Each electrode compartment contained 10-2 M NaNO3 as electrolyte solution [Fig. 2c)] and it was equiped
with a circulation system. A power supply (Hewlett Packard E3612A) was used to maintain a constant current and
the voltage drop was monitored by means of a Multimeter (Kiotto KT1000H). The electrodes were platinized
titanium bars with a lenght of 29 cm and a diameter of 3 mm (Bergøe Anti Corrosion A/S, Denmark). The
compartments and the wood chips were separated by ion exchange membranes [Fig. 2c)] (anion exchange
membrane: IA1-204SXZL386; cation exchange membrane: CR67HMR-P1220B, both from Ionics Inc.
Massachusetts, USA). The experiments where carried out for 30 and 86 days [Fig. 2d)] and 500 mA were applied as
initial current. During the experiments and at their end, samples of the electrolyte solutions (catholyte and anolyte)
were collected and analyzed for Cu, Cr and As. At the end of each experiment, the “total” Cu and Cr content in the
wood chips was also determined in accordance with BS 5666. Copper and chromium were determined by Atomic
Absorption Spectrophotometry (Perkin Elmer 300 AAS) and arsenic by Inductively Coupled Plasma (ISA Jobin-Yvon 24).
a) Wood chips saturated with
the desorbing agent
b) Small ED pilot plant
c) Ion exchange membranes
fixing set + electrolyte
compartment
d) Wood chips in ED pilot
plant
Fig. 2. Assemblage sequence used for the experiments 1 and 2 [a) - d)].
RESULTS AND DISCUSSION
Table 1 presents Cu, Cr and As removal efficiencies obtained for experiment 1 (86 days) and experiment 2 (after
30 days). The results obtained for experiment 2 were 33 % of Cu, 92 % Cr and 92% for As, after an ED treatment
which represents a great difference particularly comparing to the Cu, removal efficiency after experiment 1: 99,5%
and the above 98% removal efficiencies obtained for Cr and As.
Removal efficiency (%)
Experiments
Cu
Cr
As
Exp. 1 – 500 mA, 86 days
99.5
98.3
98.3
Exp. 2 – 500 mA, 30 days
33.4
92.0
92.3
Table 1. Cu, Cr and As removal efficiencies obtained at the end of experiments 1 and 2.
Figure 3 presents the voltage drop between working electrodes during the experiments 1 and 2.
Experiment 1
Experiment 2
40
Voltage betw een w orking
electrodes (V)
Voltage betw een w orking
electrodes (V)
40
30
20
10
0
30
20
10
0
0
10
20
30
40
50
60
70
80
90
0
10
Time (days)
20
30
Time (days)
Fig. 3. Voltage drop between working electrodes during the experiments 1 and 2.
Both experiments show low values for the electrical resistance measured. Lower cell voltage means a lower
energy consumption. For experiment 1, the voltage was higher than experiment 2, but it wasn’t significant.
Table 2 shows Cu, Cr and As amounts found in the electrode cell compartment sections (anode compartment:
anolyte and anion exchange membrane; cathode compartment: catholyte, cathode and cation exchange membrane), at
the end of the experiments 1 e 2.
Cell section
Cu (mg)
Cr (mg)
As (mg)
Anolyte
AN
Catholyte
Cathode
CAT
Exp. 1
166.6
6.52
32.1
21.3
0.390
Exp. 2
562.9
2.36
8.62
14.4
0.491
Exp. 1
95.6
95.2
15.1
n.d.
0.044
Exp. 2
1323
107.6
36.0
n.d.
2.056
Exp. 1
266.3
0.597
99.0
0.810
0.101
Exp. 2
346.2
0.772
269.9
1.243
0.829
n.d. – not detected
Table 2. Cu, Cr and As amounts found in the electrode cell compartment sections at the end of
experiments 1 and 2.
Figure 4 presents pH, Cu, Cr and As concentration profiles as a function of the distance to the anode obtained for
experiments 1 and 2.
Experiment 1
3
Cu - Initial
pH
600
2
0
-
3
Slice
2100
mg Cr/kg
pH
Cr - Initial
pH
2
1
2
3
Slice
3
Cr - Inicial
pH
700
1
1
2
3
Slice
-
2
3500
pH
700
As - Final
2100
pH
1400
As - Initial
mg As/kg
As - Final
pH
mg As/kg
Cr - Final
1400
2800
2100
1400
As - Initial
pH
700
0
+
2
+
2800
pH
-
3
0
-
3500
2
2100
Cr - Final
700
+
1
Slice
1400
Cu - Inicial
1
+
3
0
600
pH
1
Cu - Final
1200
2
+
2
pH
Cu - Final
mg Cu/kg
1200
0
mg Cr/kg
1800
pH
mg Cu/kg
1800
Experiment 2
2
1
2
Slice
3
-
0
1
+
1
2
Slice
3
-
Fig. 4. pH, Cu, Cr and As concentration profiles as a function of the distance to the anode obtained for experiments 1 and 2.
The pH of the anolyte is low (around pH=2) while the catholyte solution was kept acid (pH≈3) by adding
concentrated HNO3.
Copper moved in the electrodialytical cell and, until a certain time, mainly towards the anode compartment
(Fig. 4 and Table 2), most likely in the form of anionic complex CuOx22-, but if more time is given for the
experiments, Cu is expected to move towards the cathode compartment [results based on previous studies, e.g.(4)].
When analysing the Cr data presented in Table 2, it is clear that most of Cr moved preferentially in the
electrodialytical cell mainly towards the anode compartment. This is in accordance to our previous studies, where
there was an exponential removal of Cr as an anion, until a certain time, whereas for longer times of experiments,
cationic Cr species were available for migration towards the cathode compartment (4). The presence of oxalic acid
incubation of the wood chips could cause a reduction of chromate by oxalate, possible in acid conditions. The Cr(III)
ions formed in this way may react to form the oxalate complex. However, at the longer times they both increasingly
moved towards the cathode compartment. On the other side, because of Cr(III) is known to form complexes with
oxalate, the rather stable CrOx22- will, as an anionic complex, move in the direction of the anode.
Concerning the arsenic, it is expected to move mainly towards the anode compartment, as anions (Table 2).
However, AsO+ or As(OH)+, and even in more acid solutions, As3+ ions may exist, capable of moving in the
direction of the cathode compartment (Table 2).
CONCLUSIONS
The application of the electrodialytic process for heavy metal removal from CCA treated timber waste proved to
be an efficient solution and offers new possibilities for its remediation, increasing reuse/recycling opportunities for
this type of waste. Other experimental conditions might possibly optimize the removal rates.
ACKNOWLEDGEMENT
The authors which to thank Dr. Dario Reimão, Estação Agronómica Nacional, for supplying the CCA-treated timber chips
and Instituto de Tecnologia Química e Biológica for ICP facility. This research was funded by the Project
POCTI/32927/AGR/2000, approved by Fundação para a Ciência e a Tecnologia, and POCTI with FEDER funds.
REFERENCES
(1) OTTOSEN, LM, HANSEN HK (1992). Electrokinetic cleaning of heavy metals polluted soil. Internal Report, Fysisk-Kemisk
Institut for Geologi og Geoteknik, Technical University of Denmark, Denmark, 9pp.
(2) RIBEIRO, A. (1998). Use of electrodialytic remediation technique for removal of select heavy metals and metalloids from
soils. Ph.D. Dissertation Technical University of Denmark (in english).
(3) BS 5666 (1991) British Standard Methods of analysis of wood preservatives and treated timber. Part 3. British Standrad
Institution, London, 7 pp.
(4) RIBEIRO, A.B.; MATEUS, E.P.; OTTOSEN, L.M.; BECH-NIELSEN, G. (2000). Electrodialytical removal of Cu, Cr and As
from chromated copper arsenate-treated timber waste. Environmental Science & Technology, 34(5): 784-788.