ELECTROCHEMICAL TREATMENT OF AC ID WASTEWATERS CONTAINING [601124]

ELECTROCHEMICAL TREATMENT OF AC ID WASTEWATERS CONTAINING
METHYLORANGE ADRIANA SAMIDE1 , M ĂDĂLINA DUMITRUa , ADINA
CIUCIUa , BOGDAN TUTUNARUa , MIRCEA PREDAa ABSTRACT. The
degradation process of solutions contai ning Methylorange using electrochemical
measurements, on Ti electrode was studied. In order to estim ate the contribution of pure
adsorption, behavior of Methyl orange in open circuit was in vestigated. The color removal
(CR) due to adsorption was estimated at 8.86%. From the anodic polarization, Ti
electrode in the blank solution of 0.1 M HCl containing 0.035 M NaCl (ASB) or dye
solution of 0.1 M HCl containing 0.035 M Na Cl and Methylorange (ASC) a different
behaviour was observed. In cathodic polar isation case, the ASC behavior being
completely different towards ASB. It can be concluded that the polarisation curve for
ASC in the potential range of -426mV to -900mV can be attr ibuted to the dye reduction;
thus the color removal was estimated at value of 23.6%. Galvanostatic method to
evaluate the effect of curre nt density on solution discolor ation process was applied. The
color removal at 250min increases of the valu e of 68.4% calculated at current density of
5mA/cm2 until the value of 73.4% at current density equal to 15 mA/cm2 . Degradation
process of Methylorange follows a first orde r kinetics reaction. Rate constants increase
with the increase of current densities and their values ar e: 0.0047min-1, in the case of
degradation at a current density of 5 mA/cm2 , and 0.006min-1, in the case of
degradation at a current density of 15mA/cm2 . Keywords: Methylorange,
electrochemical degradation, titanium el ectrode, kinetics process INTRODUCTION
Wastewaters contain a range of organic pollutants (about 10.0 00 different dyes) such as
acids, alkalies, solid particles, toxi c compounds and dyes which even in low
concentrations must be removed. Synthetic dye s are used in textile industry (60%), paper
industry (10%), plastic manufact ure (10%) and it is estimated that 10-15% of the dyes is
lost during fabrication processes. It is reported that approximately 5 tonnes of dyes
discharge from coloration industries every year. Furthermore, some azo dyes, their
precursors and a number of their reacti on products are carcinogenic. Due to
environmental requirements in recent years, different techniques have been used for removal of such waste as adsorption, oxida tion, reduction and electrochemical reactions.
To eliminate dyes from aqueous coloured 1 Universitatea din Cr aiova, Facultatea de
Chimie, Str. Calea Bucure ști, Nr. 107I, RO-200285, Craiova, Romania,
[anonimizat] A. SAMI DE, M. DUMITRU, A. CIUCIU, B.
TUTUNARU, M. PREDA 158 effl uents and reduce their ecological impact, several
biological, physical and chemical methods ha ve been proposed: biological treatment [1-
4], physical or chemical flocculation, electrofilte ration, membrane filtration,
electrokinetic coagulation [5-8], adsorption a nd precipitation [9-11] and other oxidative/
reductive chemical and photochemical processe s [12-17].These methods have individual
advantages, but also have some constrai ns when they are applied individually.
Development of the appropriate techniques for treatment of dye wastewater is important
for the natural waters protect ion. Other techniques including radiation and discoloration
with ozone in combination with H2O2 are also employed [18]. Electrochemical
treatments have certain advantages by co mparison with other methods, namely: wide
application, simple equipment, easy opera tion, lower temperature requirements and no
sludge formation. It is importa nt to select the proper elect rode material, because the
reaction products strongly depend on those materials as well as the experimental

conditions. Selection of a proper electrode material is vital for an efficient and enduring
operation of an electrode. Several resear chers have tested the feasibility of
electrochemical degradation using various el ectrode materials:glas sy carbon, Pt, Pt+Ir,
Ti, Al, Co+Pd, Fe, IrO2, PbO2, SnO2, di amond paste doped with boron [19-33]. The
objective of this research was to evaluate the electrochemical discoloration process of
solutions with Methylorange content by dire ct electrochemical degradation, with an
electrode made of Ti using a synthetic solu tion with Methylorange disolved in 0.1 M HCl
aditivate with 0.035 M NaCl. RESULTS AND DISCUSSION The Methylorange was the
organic compound studied, whose structure and spectrum are shown in Figure 1. This azo
dye was purchased from Fluka and used as received. All others chemical compounds
used were of analytical type. As it can be seen from Figure 1, the maximum absorption
for dye in the visible region was at 502 nm. Beside the main peak, other two characteristic absorption peaks at the wa velength of 273 and 321 nm in the ultraviolet
region, were identified. Figur e 1. UV-Vis spectrum of initial Methylorange solution and
molecular structure of Methylorange in acid medium aditivate with NaCl. ELECTROCHEMICAL TREATMENT OF AC ID WASTEWATERS CONTAINING
METHYLORANGE 159 Study of Me thylorange adsorption pro cess In order to estimate
the contribution of pure adsorption, behaviour of Methylorange in open circuit was
studied. In the open circuit, in ASB solu tion, the potential is stabilized after
approximately 10 min, being of -306 mV at th e end of the experiment (Figure 2 as an
example). In the dye presence (ASC), within the first 25 minutes, the potential changes
are major and can be attributed to a tende ncy of adsorption-desorbtion of dye on the
titanium surface with the form ation of a non-adherent a nd ruggedness film, which is
interposed at the metal / so lution interface. After 40 min th e potential is stabilized,
reaching the value of -266mV, at the end of the experiment. The potential shift to less
negative values can be associated with the fo rmation of a relative, uniform film, which is
due to Methylorange adsorption on the tita nium surface. This adsorption has a weak
effect on the discoloration and it results from the fact that the titanium surface texture
shows affinity for Methylorange, which can be adsorbed on the substrate through free
electrons from azot atoms. Figure 2. The potenti al dependence of time registered in open
circuit in the absence and in the presence of Methylorange in 0.1 M HCl / 0.035 M NaCl
solution. UV-Vis spectra registered in open circ uit, at the initial time and after 60 minutes
are shown in Figure 3. A decrease of abso rbance value from 2.03 to 1.85 which is
attributed of Methylorange adsorption on the titanium surface was observed. Color
removal (CR) due to the pure adsorption wa s determined using the following equation:
CR = (1 – A / A0 ) · 100 (1) where: A0 a nd A represents the initial absorbance and
absorbance at a moment “t”, 60 min in this case, respectively. Figure 3. UV-Vis spectrum of Methylorange so lution registered in open circuit. The
colour removal was 8.86% at an initial Met hylorange concentration of 0.15 M. Study of
anodic and cathodic processes The polarization curves of ASB and ASC on Ti electrode
are shown in Figures 4 and 5. From the anodi c polarization (Figure 4), it can be observed
that Ti has a different beha viour in the blank solution and dye solution, respectively.
Figure 4. Anodic polarization curves of ASB and ASC on Ti electrode.
ELECTROCHEMICAL TREATMENT OF AC ID WASTEWATERS CONTAINING
METHYLORANGE 161 For the titanium el ectrode ASC has a major effect on the

reaction in the range of positive potential; thus the potentials were shifted to less negative
values, and current densities decreased. Th ese changes may be associated with the
formation of a film on the titanium surface, which modified the electronic change at
metal / electrolyte interface. The anodic peak which appears at 200 mV is not attributed
to an electro-oxidation, but to a tendency of titanium passi vation. In spectrum UV-Vis the
absorbance level was maintained on the in itial value of 2.03. From cathodic polarization
curve, a different behaviour of titanium electrode in ASC solution, by comparison with the ASB solution was observed. In the presen ce of Methylorange th e current densities
increased from a value of potential equal to -430 mV to – 556 mV. This evolution could
be attributed to the fact that the cathodic pr ocess is too slow to be controlled, therefore no
characteristic peaks are obtained. Literature data [33] mentioned on glassy electrode the
existence of two reduction peaks of azo group, first bielectronic at ε = – 425 mV / Ag,
AgCl, with the formation of hydrazoderivate (according to Scheme 1), and the second
tetraelectronic, at more negative potentials , up to -875 mV / Ag, AgCl leading to
adequate amines according to Scheme 2: N N + 2e , + 2H NH NH Scheme 1 +4e , 4H
+4e , 4H R NH2 + H2N R1 R N N R1 Scheme 2 Therefore, the ASC behaviour being
completely different towards ASB, it can be concluded that the polarisation curve
carriage for ASC in the potential range of – 426 mV to -900 mV can be attributed to dye
reduction. A. SAMIDE, M. DUMITRU, A. CIUCIU, B. TUTUNARU, M. PREDA 162
Figure 5. Cathodic polarization curves of ASB and ASC on Ti electrode. UV-Vis
spectrum of Methylorange solution registered at the end of the cathodic process is shown
in Figure 6. A decrease of the absorbance fr om the 2.03 to 1.55 was observed, therefore
the color removal calculated using the equation (1) has been calculated 23.6%.
on discoloration process of dye solution. Electrochemical measurements, for two
different current densities 5 mA/cm2 and 15 mA/cm2 , respectively, were carried out.
Each experiment ran for a time of 250 mi n, samples being analysed at certain time
intervals, namely: 20 min, 40 min, 60 min, 80 min, 100 min, 150 min, 200 min, 250min.
At specified time intervals absorbances values were evaluated. The degradation process of dye was followed by means of color rem oval (CR) values, which were calculated
using equation (1). UV-Vis spectra of discol orated Methylorange at different current
densities, 5 mA/cm2 and 15 mA/cm2 re spectively, applied for 250 min, at room
temperature are shown in Figures 7 and 8. Figure 7. UV-Vis spectru m of Methylorange
solution registered for current density of 5 mA/cm2 , for 250 min. Figures 7 and 8 show
that absorbance decreases in time, on the same current density, which demonstrates Methylorange degradation and discolorati on. Absorbance values, at the same reaction
time, present a significant decr ease, hence a greater rate of degradation, with the current
density increase. This may be shown by dete rmining the colour re moval (CR) using the
equation 1, at time values when the spectra l recordings were pe rformed. Dependence of
colour removal of time, at different current densities, is shown in Figure 9. Thus, the
color removal at 250 min increases from 68.4% calculated at current density of 5 mA/cm2 to the value of 73.4% at a current density equal to 15 mA/cm2 . A. SAMIDE,
M. DUMITRU, A. CIUCIU, B. TUTUNA RU, M. PREDA 164 0 0.5 1 1.5 2 2.5 200 250
300 350 400 450 500 550 600 650 Wavelength (nm) Absorbance 0 min 20 min 40 min 60 min 80 min 100 min 150 min 200 min 250 min Figure 8. UV-Vis spectrum of
Methylorange solution register ed for current density of 15 mA/cm2 , for 250 min. Figure

9. Dependence of color removal on time at different current densities.
ELECTROCHEMICAL TREATMENT OF AC ID WASTEWATERS CONTAINING
METHYLORANGE 165 Kinetics of Methylorange degrada tion 1). Determination of
apparent rate constant In general, degradati on curves of dyes follow a first order kinetics
reaction. The first order kinetic can written as follows: -d[dye ] / dt = kap[dye] (2) where:
kap is apparent rate constant . Integrating the equation (2), a first order kinetics equation
was obtained: ln [dye]0 / [dye]t = kap × t (3) Straight line plots ln [dye]0 / [dye]t = f(t)
allow us to obtain the apparent rate constants for Methylora nge, at both current densities.
The absorbance is directly proportional w ith the concentration of Methylorange
degraded; equation (3) can be written as: ln A0 / At = kap × t (4) where: A0 and A
represent the initial absorbance and absorb ance at a given “t” time, respectively. 2)
Verification of first order ki netics a) Absorbance variati on in time The variation of
absorbances resulted in the given experimental conditions were be evaluated, in time and
as a function of the current densities whereat the degrada tion process of Methylorange
had been studied. Absorbance versus time e volutions are shown in Figure 10. Figure 10.
Absorbances variation during Methylorange de gradation at different current densities
applied. A. SAMIDE, M. DUMITRU, A. CI UCIU, B. TUTUNARU, M. PREDA 166 It
was noticed that, for the same current dens ity, absorbance decreases exponentially with
time, according to a relation A = A0 exp(-kt), which corresponds to a first order kinetics,
related to Methylorange degrad ation. From Figure 10, it can be seen that apparent rate
constants values are: 0.0048 mi n-1, in the case of degradatio n at a current density of 5
mA/cm2 , and 0.0062 min-1, in the case of degradation at a current density of 15
mA/cm2 . b) Integrated equation constant ra te verification. Rate constants determination
The curves ln A0 / A = f (time) obtained fr om experimental data during the degradation
process of Methylorange are shown in Figur e 11. From Figure 11, it can be noticed that
straight lines were obtained, passing thr ough the origin and whos e slope is dy / dx =
kap;it can be observed that the value of R2 is approximately equal 1;this proves that the
degradation of ASC respects the first order kinetic. Rate consta nts (kap) increase with the
increasing of current densities and have values ranging of 0.0047min-1, in case of
degradation at a current dens ity of 5 mA/cm2 and 0.006 min-1, in case of degradation at a
current density of 15 mA/cm2 , respectivel y. Figure 11. Diagrame corresponding with the
first order kinetics in case of Methylorange degradation at different current densities.
From Figures 10 and 11, it can be observed that are obtained values of apparent rate
constants approximately equal, for the same values of current densities; so it can be
concluded that in the case of ASC degrada tion on Ti, it is respected the first order
kinetics CONCLUSIONS Electrochemical degradation process of a synthetic solution with
Methylorange disolved in 0.1 M HCl soluti on aditivate with 0.035 M NaCl was studied.
In order to estimate the cont ribution of pure adsorption, beha viour of Methylorange in
open circuit was studied. UV-Vis sp ectra registered in open circuit, at the initial moment
and after 60 min had shown a decrease of the absorbance from t2.03 to 1.55, which was
atributed to Methylorange ad sorption on the titanium surface. The color removal (CR)
due to adsorption was calculated to be 8.86% for a concentration of 0.08 mM Methylorange in ASB. For the anodic polariz ation, observed that Ti in the blank and dye
solution has a different behaviour. The anodi c peak that appeared at 200 mV was not

attributed to an electro-oxidation, but to a tendency passivation of titanium. In spectrum
UV-Vis the absorbance level was maintained at the initial value of 2.03. In cathodic
polarisation case, the ASC be havior being totaly differe nt towards ASB, it can be
concluded that the polarisation curve carriag e for ASC in the potential range of -426mV
to -900mV can be atributted to the dye reduction. UV-Vis spectrum of Methylorange
solution registered at the end of cathodic process had shown a decrease of the absorbance
from 2.03 to 1.85; thus, the color removal wa s calculated to be 23.6%. Galvanostatic
technique was applied to evaluate the current density effect on discoloration process of
dye solution. Thus, the color removal at 250 min increases from 68.4% calculated at current density of 5 mA/cm2 to the value of 73.4% at a current density equal to 15
mA/cm2 . The degradation process of Methylor ange in ASB at studied current densities,
5 mA/cm2 and 15 mA/cm2 , respectively, follo ws the first order ki netic reaction. Rate
constants (kap) increase with th e increase of current densitie s and their values are: 0.0047
min-1, in the case of degradation at a curr ent density of 5 mA/cm2 , and 0.006 min-1, in
the case of degradation at a current density of 15 mA/cm2 . EXPERIMENTAL
SECTION Electrochemical measurements were carried out using a standard cell, with a
working electrode made of Ti (the surface area of 2 cm2 ), an auxiliary electrode in the
form of glossy platinum plat e (the surface area of 1 cm2 ) and an Ag/AgCl electrode was
used as a reference electrode. The electrode made of titanium was polished with very fine metallographic paper, washed with distilled water, degreased with acetone and dried. By
electrochemical measurements azo dye Meth ylorange was degraded using a titanium
electrode. The electrochemical behaviour of Methylorange was A. SAMIDE, M.
DUMITRU, A. CIUCIU, B. TUTUNARU, M. PREDA 168 evaluated by potentiostatic
method using computerized electrochemical equipment VoltaLab 40 with software. The
electrochemical degradation process was ex amined at positive and negative polarization,
respectively, including open circuit condition, in a range of potential between -200mV
and 800mV for the anodic process, and betw een -200 mV and -1000mV for the cathodic
process, applying a scan rate of 10 mV/s. Galv anostatic technique was applied to evaluate
the effect of current density on dyeing solu tion discoloration process. A SourceMeter
2420 3A potentiostat / galvanostat was used to controll the current density. The
experiments were performed for two differe nt current densities, 5 mA/cm2 and 15
mA/cm2 , respectively. Each experiment ra n for 250 min, samples being collected at
certain time intervals. At specified time in tervals absorbance values were assessed using
an UV-Vis spectrophotometer, Varian Ca ry 50 with software.Temperature was
maintained at 20șC. Electrochemical soluti ons tested contained: acid blank solution
composed of 0.035M NaCl, 0.1M HCl (ASB) and dye solution, which had the following components: 0.08mM Methylorange, 0.035M NaCl, 0.1M HCl (ASC). REFERENCES 1.
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