Removal of Cadmium and Cyanide from Aqueous Solutions through Electrodialysis

A descarga de efluentes contendo metais pesados e cianeto pelas indústrias de galvanoplastia é uma das maiores fontes de poluição dos recursos hídricos. O emprego da técnica de eletrodiálise para o tratamento de um efluente sintético contendo aproximadamente 0,0089 mol L de cádmio e 0,081 mol L de cianeto foi estudado utilizando uma célula de eletrodiálise de cinco compartimentos. Os resultados demonstram que a remoção de cádmio e cianeto depende da densidade de corrente aplicada e é limitada pela precipitação de cádmio sobre a membrana catiônica.


Introduction
The release of industrial wastewaters containing cadmium and cyanide to the environment is strictly controlled due to the toxic nature of these substances.To reduce the environmental impacts it is necessary to remove these substances from wastewater before its discharge in the environment. 1One of the largest cadmium and cyanide sources, which contributes largely to water pollution, are the electroplating industries that use cadmium cyanide baths.The main problems associated to this process, from the environmental point of view, are the deposition baths and rinsewaters discharges.The alkaline-chlorination process for cyanide destruction with further cadmium hydroxide precipitation is a common method employed for the treatment of the wastewaters characterized by the presence of these substances.However, chlorination of cyanides can result in highly toxic intermediates (cyanogen chlorides) and other toxic organo-chlorines that can be formed.These compounds together with residual chlorine create additional environmental problems.][12][13] Electrodialysis is a membrane separation process in which ionic species in solution are transported through ionexchange membranes, under the influence of an electric field from one solution to another.In an electrodialysis stack, as shown in Figure 1, cation-exchange and anionexchange membranes are alternately arranged in a filter press-like system to form diluted and concentrated flows between the membranes.The electrodes, disposed at the end of the stack, are in contact with an electrode rinse solution.The applied direct current electric field produces the transport of cations towards the cathode and anions towards the anode.
During this transport anions are able to permeate through the anion-exchange membrane, but are blocked by the cation-exchange membrane.The opposite occurs with cations.As a result, two different solutions are produced inside the stack: one more diluted (the diluate) and another more concentrated (the concentrate) than the feeding solution. 14,15Considering that this membrane separation technique is able to transform a concentrated solution in two other solutions, one more concentrated and another more diluted than the original solution (feeding solution), electrodialysis presents large application in what concerns the galvanic industries wastewater treatment, because it makes it possible to reuse the more diluted solution as rinsewater and the more concentrated solution in the deposition bath. 16,17he objective of this work was to verify the possibility of use of the electrodialysis technique to remove cadmium and cyanide from a synthetic wastewater containing approximately 0.0089 mol L -1 cadmium and 0.081 mol L -1 cyanide applying different current densities.

Electrodialysis cell and solutions
The electrodialysis cell used in this work and represented in Figure 2 was composed of five compartments separated alternately by cation-exchange and anionexchange membranes.The cell was manufactured in transparent acrylic.The cation-exchange and anionexchange membranes used were NAFION 450 and SELEMION AMV, respectively.The effective membrane area was 16 cm 2 .The two outer compartments contain the working electrodes (platinum sheets) both with 16 cm 2 effective area.The solutions in the three central compartments circulate independently while in the electrode compartments the same solution is circulated in closed circuit to avoid pH variations due to electrodes reactions.In each cell compartment 500 mL of solution circulate.
The solution to be treated (0.0089 mol L -1 CdO, 0.081 mol L -1 NaCN and 0.018 mol L -1 NaOH) circulates in the diluate central cell compartment.A 0.054 mol L -1 NaOH solution circulates in the anodic and cathodic concentrated cell compartments and a Na 2 SO 4 5% solution circulates in the electrodes compartments.The solutions were prepared with analytic grade reagents with distilled and deionized water.The current was applied through a 2.5A -80V power supply.The experiments were accomplished at least in duplicate at 25 °C.Cadmium concentration in the solutions was measured by Inductively Coupled Plasma Spectroscopy.Cyanide was measured by direct titration with an AgNO 3 solution. 18The electrodialysis performance was evaluated in terms of percent extraction (pe) and current efficiency (ce), defined respectively by equations 1 and 2. 19,20 (1) where C i is the initial concentration of the ion and C f is the final concentration of the ion. ( where z is the valency of the ion, F is the Faraday constant (96500 C mol -1 ), m i is the initial number of moles of the ion (moles), m f is the final number of moles of the ion (moles), I is the electric current applied (A) and ∆t is the time interval (s) and n is the number of membrane pairs.
At the experiments done at this work, the number of membrane pairs was considered equal to one, since the work solution is in contact just with one membrane pair (diluate central cell compartment).

Cadmium and cyanide transport through the ion-exchange membranes
The experiments accomplished in the electrodialysis cell demonstrate that cadmium and cyanide are predominantly in anionic form, since they are transported only through the anion-exchange membrane.This behavior can be explained by the formation of negatively charged coordination complexes between cadmium and cyanide.
Cadmium and cyanide, in different proportions, lead up to the formation of different cadmium cyanide complexes in alkaline media.According to Prytz and Osterud, 21 in the interval Cd:CN 1:4 and Cd:CN 1:20, the Cd(CN) 4 2-complex should prevail.Flengas 22 suggests that with excess of cyanide all the cadmium is complexed in the Cd(CN) 4 2-form.Ortega and collaborators 23 suggest that Cd(CN) 4 2-is formed in great excess of cyanide to a solution pH higher than 10.Koivula and co-workers 24 demonstrates that for a Cd:CN 1:10 system, 95% of cadmium are under the Cd(CN) 4 2-form.In this work, the cadmium cyanide complexes speciation was obtained using the program HYDRA MEDUSA. 25Figure 3 shows that at the initial pH and cadmium and cyanide solution concentrations used in this work the predominant cadmium cyanide complex formed is Cd(CN) 4  2-, just like in cadmium-plating baths.The other ionic species present in the solutions are free cyanide (CN -), hydroxide (OH -) and sodium (Na + ) ions. 17 In the electrodialysis cell, only sodium ions can pass through the cation-exchange membrane, whereas metal complexes, free cyanide and hydroxide ions are transported through the anion-exchange membrane, as it can be observed in Figure 2.

Influence of the current density on the Cd(CN) 4 2-and CN - ions removal
Experiments to establish the current density influence on the Cd(CN) 4  2-and CN -ions removal were performed.Table 1 shows results obtained at 10, 15 and 20 mA cm -2 current densities.
The results suggest that the percent extraction of the ionic species is higher when higher current densities are applied, even though the current efficiency is lower.It is clear that the lower current density resulted in better current utilization.The CN -ion removal is higher than the Cd(CN) 4 2-ion.The cadmium cyanide complex is more voluminous than cyanide ion, what indicates that it is more difficult to transport larger ions through the anionexchange membrane.

Cadmium compound precipitation on cation-exchange membrane
Experiments accomplished with 20 mA cm -2 and during 240 minutes of electrodialysis result in 86% Cd(CN) 4 2and 95% CN -extraction, as it can be observed in Figure 4.
However a cadmium compound precipitation on the cation-exchange membrane disposed between the cathodic concentrate and the diluate central cell compartments was observed.Such precipitation is not observed up to 90 min of electrodialysis, where 23% Cd(CN) 4  2-and 43% CN - extraction occur.Figures 5a and 6a show micrographs obtained by SEM (Scanning Electron Microscopy) for a cation-exchange membrane before its use (new membrane) and for the cation-exchange membrane used up to 240 min of electrodialysis, respectively.Figures 5b and 6b show the corresponding membranes EDS (Energy Dispersion Spectroscopy) spectra, respectively.
From Figure 6b it is clear that the precipitate corresponds to a compound containing cadmium.However, a more accurate analysis should be conducted to determine which cadmium compound is formed.This precipitation represents a drawback for the electrodialysis process since the precipitate formation on ion-exchange membranes can reduce the effective membrane area and consequently the process efficiency.Besides, it can cause irreversible membrane damages reducing its life cycle and increasing operational costs.
It is well know in electrodialysis that the precipitation on ion-exchange membranes can be a consequence of operating above the limiting current density.In this case water dissociation in H + and OH -ions can occur, what might cause changes in the pH of the boundary layer formed in the membrane/solution interface, leading to a local change in the characteristics of the ionic species present in the solution.Insoluble compounds can be formed and then precipitate on the ion-exchange membrane.
Previous results have shown that with these membranes and these solutions there is a limiting current density at the cation-exchange membrane at approximately 25 mA cm -2 . 13,26Since in this work 20 mA cm -2 was applied during 240 min for the ions removal, it can be estimated that during this prolonged experimental interval, the system could reach the limiting current density.
Figure 7 demonstrates that if pH increases and cadmium and cyanide concentration decrease, insoluble Cd(OH) 2 can be formed.Cd(OH) 2 might then precipitate on the cation-exchange membrane.
By these considerations it can be estimated that the cadmium compound formed and precipitated on the membrane is Cd(OH) 2 .It is clear that accurate pH measurements should be available near the membrane/ solution interface to confirm these hypotheses.Figure 8 shows that there is a decrease on the bulk pH at the diluate central cell compartment.This decrease could be associated to the OH -consumption that occurs during the formation and precipitation of Cd(OH) 2 .
The conductivity decrease in the diluate central cell compartment indicates the Cd(CN) 4 2-, CN -, Na + and OH -    ions removal (Figure 9).The conductivity increase in the anodic concentrate cell compartment can be attributed to the Na + transport from the electrodes compartment and to the Cd(CN) 4 2-, CN -and OH -transport from the diluate central cell compartment.The conductivity increase in the cathodic concentrate cell compartment can be attributed to the SO 4 2-transport from the electrodes compartment and to the Na + transport from the diluate central compartment.
The cell voltage increase can be attributed to the decrease in the conductivity of the diluate central cell compartment solution as a consequence of the ionic species removal increasing the solution electrical resistance (Figure 10).Besides, when Figures 8 and 10 are compared, it can be seen that when bulk pH decreases in the diluate central cell compartment the cell voltage increases.This behavior could be all related to a limiting current density at the cation-exchange membrane.

Figure 2 .
Figure 2. Five-compartment electrodialysis cell and the expected transport of the ionic species.

Figure 3 .
Figure 3. Speciation of cadmium and cyanide species as a function of the cyanide concentration and the solution pH (0.0089 mol L -1 cadmium).

Figure 7 .
Figure 7. Speciation of cadmium and cyanide ionic species as a function of cadmium and cyanide concentrations and the solution pH.

Figure 10 .
Figure 10.Behavior of the cell voltage during 240 minutes of electrodialysis with 20 mA cm -2 current density.