Electrochemical oxidation of chemi-thermo-mechanical pulping wastewater

Chemi-thermo-mechanical pulping (CTMP) wastewater has high chemical oxygen demand (COD), which inhibits the activity of microorganisms during biological oxidations. Conventional wastewater treatment technologies such as dissolved air flotation, coagulation, and biological treatment, are insufficient to treat the CTMP wastewater to meet the environmental requirements. There remains a need for advanced wastewater treatment technologies, which can be integrated into existing wastewater treatment processes to improve the end-of-pipe water quality. In this study, a heterogeneous electrochemical oxidation process was developed to treat chemi-thermo-mechanical pulp mill wastewater. The Ti substrate anodes were prepared by thermal decomposition and electro-deposition, and characterized by SEM and Tafel curve. The impacts of electrode modification, flowrate, current density, and aeration on the process efficiency were investigated. COD and color density were used to evaluate the removal efficiency of organic pollutants. Results show that the Ti/SnO2+Sb2O3+MnO2/PbO2 electrode exhibited the best performance in terms of COD and color removal efficiency. SEM observation revealed that the electrode surface was smooth and compact, with numerous uniform micro pores of which the inner walls were covered with tiny crystals. Under the studied conditions, color removal reached 90%, while COD removal was about 60%.


INTRODUCTION
2][3][4] Pulp and paper mill wastewater is characterized with very high chemical oxygen demand (COD), which inhibits the activity of microorganisms during biological oxidations.The traditional wastewater treatment technologies, such as biological, physical, chemical, and their combinations, cannot substantially eliminate the recalcitrant contaminants in wastewater.In particular, chemi-thermo-mechanical pulp (CTMP) mill wastewater has a very high COD, and its color is also very pronounced.The organics difficult to degrade are of a wide range of resources, and its efficient treatment remains to be a big challenge.Therefore, there remains a need to integrate advanced wastewater treatment technologies into treatment processes to improve effluent quality.
Advanced oxidation processes (AOP) 5,6 such as Fenton reaction, electrochemical oxidation (EO), ozonation, and photocatalysis, frequently referred to as the prototypical "green" technologies of the future, have been used for the treatment of wastewater containing biorefractory organics.EO process has attracted increasing interests because of high oxidation efficiency, mild process conditions, and good environmental compatibility.EO is capable of producing hydroxyl radicals, which are very powerful oxidants capable of oxidizing a wide variety of organics. 7,8 ][11][12] However, the low efficiency of electrochemical oxidation limits its commercial application.The research activities in this area can be divided into two categories.The current efficiency of an electrochemical oxidation process strongly depends on anode material.One is to develop high-performance anodes including IrO2 13 , RuO2 14 , SnO2 15 , PbO2 16 , and boron-doped diamond (BDD) 17 anodes as regards to high catalytic activity, long life-time, etc.9][20] Another approach is to make use of granular activated carbon (GAC) or metal particles, namely three-dimensional (3D) particle electrodes, for enhancing the conductivity and mass transfer or the adsorption of pollutants. 21Indeed, there are more reactive sites than traditional electrodes for pollutants removal or even catalytic reactions, resulting in higher organics removal. 22,23  present study, heterogeneous electrochemical oxidation (HEO) of CTMP pulping wastewater was investigated with 3D particle electrodes in combination with aeration.The modified anodes were examined by SEM, and the effect of liquid flow rate, current density, and aeration on COD and color removal were studied.The effluent samples were also characterized by FTIR before and after the electrochemical oxidation treatment.

Pulp mill wastewater
The wastewater used in this study was obtained from a chemi-thermo-mechanical pulping pilot line (State Key Laboratory of Pulp & Paper, China).The main equipment pertaining to the pulping process was an Andritz Sprout/Bauer 12/ITP laboratory pressure refiner.The conditions of the chemical pretreatment stage of the CTMP process were: wood chips (bone-dry) of 100 kg, NaOH dosage of 4% (based on bone dry wood), Na2SO3 dosage of 4% (based on bone dry wood), liquid-to-wood ratio of 1:4, max temperature of 130 o C, pressure of 0.2MPa, and retention time at the max temperature of 15 min.

Modified electrode
The Ti plates were subjected to surface pretreatment with corundum sand blasting and hot hydrochloric acid picking.Then, the plates were coated with active middle layer through thermal decomposition so as to enhance the interaction between the coating layer and the substrate and increase their life-time.Finally, the active and functional layer was coated onto the plates through electro-deposition.

3D particle electrodes
3D particle electrodes were made from granular activated carbon, metal/metal oxide, and adhesive.The mixture was formed into particles by extrusion and high-temperature sintering.It had an average particle size of about 3 mm.Before the treatment, the particles were pre-saturated by wastewater to eliminate the adsorptive effect on the experimental results.

Experimental setup
The experiments were conducted in an open cell reactor of 5 dm 3 made of organic glass, as shown in Fig. 1.The main anode and cathode situated 12 cm apart from each other.Self-made 3D particle electrodes were fixed into the electrode gap.There was a micro-pore plate at the bottom of the reactor for aeration.The aeration and water flow resulted in the fluidization of the particles.Several anodic materials were used in the experiments for comparison purposes.The D.C. stabilized power was used to monitor the current and voltage.

Analytical methods
CODCr, BOD5, TSS, and SS were tested by following a Chinese standard method. 24Color was measured by following a CPPA standard method. 25The pH was determined with a pH S-3C acidity meter.

Tafel curve and electrochemical kinetics parameters
The polarization curves of the modified electrodes in CTMP effluent and 0.1 mol/L H2SO4 were monitored through a three-electrode system, and Tafel curves were obtained by steady state polarization curve.The linear relationship between over potential and Tafel was produced from the linear regression of mathematical treatment.The electrochemical kinetics parameters (α, b, β, i 0 et.al.) of the electrode in the solution would be calculated by the experience formula of the electrochemical reaction current.

Characteristics of wastewater
The main water quality parameters of CTMP effluent are listed in Table 1.As shown in Table 1, the CTMP effluent was rich in contaminants.Biodegradability was only 0.22, which was unsuitable for biochemical treatment.The apparent color was dark brown.The conductance rate was high, therefore it was beneficial for the electrochemical treatment, and there was no need to add electrolytes.

Modified electrodes
In this study, nine different modified electrodes such as Ti/RuO2, Ti/NiO, Ti/CoO+ZnO, Ti/SnO2+Sb2O3 and Ti/SnO2+Sb2O3+MnO2/PbO2 were prepared and used as anode materials of the HEO system.SEM images of the modified electrodes are shown in Fig. 2. In order to compare the electro-chemical performances, the exchanging current density (i 0 ) and oxidative reaction transfer coefficient (β) of different electrodes were calculated on the basis of Tafel curve (Table 2).
As shown in Fig. 2, there were many large and uneven turtle cracks on the RuO2 electrode surface, and the NiO electrode showed a convex surface.On the other hand, the CoO+ZnO electrode surface exhibited irregular, concave, and convex structures.The addition of Sb2O3 resulted in a relatively smooth electrode surface.In addition, there were lots of compact, homogeneous granules on the MnO2 electrode surface.
It could be observed from Fig. 2 (i) that the PbO2 electrode surface was smooth, compact, with many uniform micro pores, but without cracks; the inner wall of the micro pores was covered with small, dense crystals.The crystals had large specific surface area (assembling fluffy cotton), which might provide more reactive sites and enhance the adsorption of hydroxyl radical.Meanwhile, it could effectively prevent the spreading of oxygen into the substrate, reduce the formation of TiO2 insulating layer, and improve the life-time of the electrode.Therefore, the Ti/SnO2-Sb2O3-MnO2/PbO2 appeared to be the best composition for the electrodes in terms of surface morphology.
As shown in Table 2, the exchanging current density (i 0 ) and oxidative reaction transfer coefficient (β) of Ti/SnO2+Sb2O3+MnO2/PbO2 electrode were 10 -1.72 and 0.784 respectively, which were the biggest among the electrodes.The results indicated that the electrochemical performance of PbO2 anode was most excellent for the treatment of CTMP wastewater.There were two competitive reactions including organic oxidation decomposition and oxygen evolution at anode.The potential of organic decomposition was mostly lower than the potential of oxygen evolution, so very little of the current would be wasted to split water, which is due to the electrochemical oxidation reaction, proceeding with high current efficiency on the PbO2 electrode.

Effect of liquid flow rate
The liquid flow rate significantly influenced the mass transfer, which in turn affected both electrocatalysis and adsorption.Experiments were carried out at different liquid flow rates.The flow rate could control the retention time indirectly.Ti/SnO2+Sb2O3+MnO2/PbO2 electrode was chosen as the anode material, and stainless steel plate acted as cathode.Other operating parameters were: current consistency of 10 mA .cm -2 , pH 6, and ambient temperature.The results are shown in Fig. 3.
It could be seen from Fig. 3 that the color and CODCr removal decreased with increasing flow rate, and for various flow rates the difference in removal was evident.The curve corresponding to 40-70 ml .min -1 decreased significantly.However, when liquid flowrate increased in the range of 20-40 ml .min -1 , such a reduction became insignificant.This was due to the fact that the color and CODCr removal was no longer a mass-transfer-controlled process when the liquid flow was below 40 ml/min, implying that optimal flow rate for this reactor was 40 ml/min.The reason could be that in the initial stage the density of H + and the pollutants were both higher.They could quickly diffuse onto the electrode surface for the oxidation reactions.

Effect of current density
In the present study various voltages were examined to investigate the effect of current density on the treating efficiency.Other operating parameters were fixed at: liquid flow rate of 40 ml .min -1 , pH=6 and ambient temperature.
It was apparent that the higher applied current density resulted in the higher efficiency of color and CODCr removal (Fig. 4).With respect to the current density from 2 to 6 mA .cm -2 , an increase in the applied voltage led to an enhancement of color removal efficiency from 50.9% to 83.6% and CODCr from 24.6% to 60.0%.These enhancements were attributed to the strengthening of the electrolysis driving force.The reason was that the potential was the major driving force for the respective phenomena of interest in electrochemical reactors.However, the enhancement of the removal efficiency of color and CODCr was much less than that of the applied current density.This result implied that when the current was too big the side-reaction of water electrolysis increased rapidly, and the

Effect of aeration
The color removal efficiency of the electrochemical treatment was also investigated with and without aeration.The operating parameters were fixed at: current density of 8 mA .cm -2 , liquid flowrate of 40 ml .min -1 , pH=6, and ambient temperature.As could be seen in Fig. 5, the color removal was faster with aeration, and a higher maximum color removal efficiency could be reached.In the initial reaction (<40min), the color removal increased rapidly.In 30 min of treatment, the decoloration reached 75%.And then it started t slow down.The decoloration rate reached a plateau of 95% at about 60 min.The concentration of the colored organics in initial stage was high and could be degraded easily.The electrochemical oxidation reactions generated reactive species in the system, including [H], • OH, • HO 2-, H2O2, etc., which can readily react with the colored organics in the wastewater, as shown in Figure 1.The rapid redox reactions with these species led to modification of the chromophores and degradation the chemical structure of the organics, and consequently eliminated the color from the wastewater.From the above analyses, aeration played a significant role in the electrochemical reaction.It could be explained in two ways.On one hand, the gas was pump into the reactor from bottom and constructed the reaction system in a fluidized state.Friction occurred between the particle electrodes, reducing the concentration polarization on the electrode surface and improving the mass transfer rate of the electrode surface, thereby enhancing the reaction efficiency.On the other hand, aeration changed the corrosion potential of the systems.Corrosion potential changed under environmental conditions, and the fundamentals could be illustrated in Fig. 6.Anodic polarization and cathodic polarization curves crossed at the point of S1, and the corresponding corrosion potential and current were E1 and I1 respectively.If there was a material with high potential such as O2 could be restored in the solution, and the reduction of cathodic polarization curves and anodic polarization curves crossed at the S2, the corresponding corrosion potential E2 was greater than E1, and the corrosion current I2 was greater than I1.

FTIR analysis
The wastewater samples before and after electrochemical oxidation were characterized by FTIR, and the FTIR spectra were shown in Fig. 7.The change of the FTIR spectra indicated that the pollutants' chemical structure was significantly altered in the electrochemical process.In the case of the FTIR spectrum pertaining to electrochemical oxidation as shown in Fig. 7 (a) and Fig. 7 (b), the absorption near 3440 cm -1 assigned to the O-H stretching became narrow and weak.It indicated that the number of lignin-derived phenol hydroxyls was significantly reduced due to the electrochemical effect.The absorption near 1655 cm -1 assigned to the non-conjugated C=O stretching almost disappeared.The absorption pertaining to the conjugated C=C stretching shifted to higher wave numbers near 1601 cm -1 and 1422 cm -1 .These bands-shift might be due to the breakage of the conjugated p-π or π-π.The absorption intensity near 1343 cm -1 assigned to O-H in-of-plane deformation of the polysaccharides was greatly reduced, which indicated that the degradation products of cellulose and hemicellulose were significantly removed after treatment.The absorption near 1215 cm -1 assigned to the conjugated C=O stretching disappeared completely, which proved that the conjugated chain was broken as a result of electrochemical oxidation.The absorption near 1121 cm -1 assigned to the asymmetry C -O -C stretching was remarkably enhanced due to the increase of the relative contents of aliphatic hydrocarbons.The peak intensity of 930 cm -1 was greatly reduced, which was related to the conjugated C=C out-of-plane flexural vibration.

CONCLUSIONS
Electrochemical oxidation treatment can effectively destroy the colored pollutant in the CTMP pulping effluent.The removal efficiency was up to 95% for color and 60% for COD.It was found that the electrode composition had a critical effect on the efficiency of the electrochemical treatment.The best electrochemical performance was obtained with the electrode composition of Ti/SnO2+Sb2O3+MnO2/PbO2. SEM observation revealed that the electrodes of this chemical composition displayed a smooth and compact surface with many uniform micro pores of which inner wall was covered with dense tiny crystals.This unique surface structure probably hindered the diffusion of oxygen into the substrate, and thus reducing the formation of TiO2 insulation layer.This hypothesis was supported by observed the higher exchanging current density (i 0 ) and oxidative reaction transfer coefficient (β).FTIR spectra indicated that the organic pollutants in CTMP wastewater were degraded effectively in the electrochemical oxidation treatment.

Fig. 1
Fig. 1 Schematic diagram of electrochemical reactor (top) and mechanistic aspect of the process (bottom)

Fig. 3
Fig. 3 Effect of liquid flow rate on CODCr and color removal

Fig. 4
Fig. 4 Effect of current density on CODCr and color removal

Fig. 7 FTIR
Fig. 7 FTIR Spectra of wastewater samples before (a) and after (b) treatment.

Table 1
Main water quality parameters of CTMP wastewater

Table 2
Electrochemical parameters from steady state polarization curves of electrodes . cm -2 was the proper current density.