(234g) Indirect Electrochemical Treatment of Textile Wastewater: Influence of Design and Operational Parameters on Color Removal

Effluents
from textile industry pose threats to the ecosystem due to the presence of
elevated levels of toxic colorants and inorganic salts.  Electrochemical methods have
recently gained attention as promising alternatives to traditional chemical
treatments for textile wastewaters. 
Indirect electrochemical treatment makes use of sodium chloride (NaCl)
present in textile wastewater to produce active chlorine based oxidants at the
anode that decolorize highly-colored azo compounds forming colorless
end-products.  Field scale application of
electrochemical treatment systems are limited by power requirements.  The major goals of this study were to: 1)
develop design strategies providing maximum color removal utilizing minimum
power, and 2) investigate the effect of design and operational parameters on
decolorization rate and power consumption.

Electrochemical
cell configuration, surface area-to-volume (S-V) ratio and applied current were
the critical factors chosen for the investigation.  S-V ratio plays a vital role in reactor
scale-up and most investigators have combined this parameter with applied
current, expressing the combination as current density.  In this study, individual effects of
electrode surface area and applied current on decolorization rate were
quantified.  Fixed variables
included: 1) solution pH (pH = 7), 2) cell re-circulation flow rate (1650
ml/min), 3) bulk solution ionic strength (1 M),    4) contaminant:  Acid Alizarin Violet N (AAVN)
concentration (14.65 ppm), 5) NaCl concentration (20.48 g/L).  Graphite and stainless steel rods were
employed as anodes and cathodes, respectively.  Two electrochemical cell designs, split
and undivided cell were tested in this study.  In the split cell configuration, the
anode and cathode compartment are separated using a cation exchange Nafion
membrane.  In the undivided cell
configuration, both electrodes are installed in the same compartment.  Applied current levels were 72 mA
and 144 mA.  The S-V ratio levels
were 0.0185 m²/m³ and 0.037
m²/m³.  The
treatment runs were terminated after reaction completion, overcoming the
limitations of conventional time-based studies.  The re-circulation loop was placed in the
path of UV-Vis diode array detector, which recorded the in situ solution absorbance every 3
minutes for real-time dye concentration monitoring.

The initial pseudo-first order rate constant, k, was calculated as the slope of the
line fitted to a ln(C/Co) versus time curve up to a cut off point of ln(C/Co) =
0.5.  The computed reaction rate
constant and power consumption values for the treatment runs are summarized in
Table 1.  Decolorization
rates for comparative runs with different cell designs revealed that the split
cell design showed higher removal rates than the undivided cell except for the
72 mA and 0.037 m²/m³ run.  Higher decolorization rates observed in
the split cell were due to the conservation of oxidants produced at the vicinity
within the anode compartment by the Nafion membrane.  In the undivided cell, oxidants produced
at the anode were reduced to other compounds at the cathode, resulting in lower
decolorization rates.  Increasing
the S-V ratio increased the decolorization rate at all tested conditions.  This result might be due to the
occurrence of unwanted side reactions competing for electrode surface along with
the preferred oxidation reaction.  A
linear relationship between applied current and decolorization was observed.
 Influence of design and operational
parameters were also verified using Analysis of Variance.  Cell type, S-V ratio and applied current
had a significant effect on decolorization rate at 95 % confidence limit.

When the
applied current was doubled, power consumed was increased in the undivided cell
as opposed to a decrease recorded in the split cell.  This conflicting trend in power
consumption confirms that adding more electrons to the undivided cell may
increase rates initially, but excess electrons are lost to unwanted side
reactions resulting in a loss of efficiency.  Electrochemical
reactor design strategies to attain maximum color removal and minimum power
consumption developed from this study will be presented.  Significance of considering surface
area-to-volume ratio in electrochemical investigations will also be discussed.

Table 1.  Experimental conditions and
results

^* Power consumption: P = Current (amp) * Voltage (V) *
Time (sec), computed at 50 % color removal