A Mechanistic Study on the Electrocoagulation of ...

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Abstract The removal mechanisms involved in the continuous-flow electrocoagulation for the treatment of a polishing wastewater containing negatively charged ...
A Mechanistic Study on the Electrocoagulation of Silica Nanoparticles from Polishing Wastewater

W. Den*, C. Huang**, H.-C. Ke** *Department of Environmental Science and Engineering, Tunghai University, Taichung-Kan Road, Sec. 3, #181, Taichung, Taiwan 407,Republic of China. E-mail: [email protected] ** Institute of Environmental Engineering, National Chiao Tung University, Po-Ai St., #75, Hsinchu, Taiwan 300, Republic of China. E-mail: [email protected] Abstract The removal mechanisms involved in the continuous-flow electrocoagulation for the treatment of a polishing wastewater containing negatively charged silica particles are described in the report. The mechanisms are derived from the experimental measurement of pH, zeta-potential and turbidity as a function of reaction time, as well as from the sludge characterization. Two types of distinct mechanisms are proposed, one involving particle destabilization by oppositely charged ferrous ions, the other involving a complexation or physical bridging by iron hydroxides/hydroxyl complexes. The former mechanism is apparently responsible for the formation of “surface sludge”, whereas the latter mechanism is responsible for the “sediment sludge”. Both mechanisms are important to the silica removal based on the measurement of sludge quantity. Keywords chemical mechanical polishing, corrosion, electrocoagulation, industrial wastewater, silica, zeta potential

Introduction In recent years, electrochemical processes have received renewed interest as an alternative treatment method for industrial wastewater containing dispersed charged particles without chemical additions [Matteson et al., 1995; Holt et al., 2003; Mollah et al., 2004a, b]. One of the most notable sources producing wastewater containing highly charged ultra-fine particles is chemical mechanical planarization (CMP), a process vital in the fabrication line for the multilevel design of integrated circuit. With the continuing pursuit of functionally stronger devices with smaller feature sizes (sub 0.10 µm), the device fabrication will undoubtedly and increasingly rely on the CMP processes. As a consequence, the quantity of CMP wastewater generated is expected to increase proportionally with the growing needs of the CMP processes [Golden et al., 2000; Golden and Carrubba, 2001]. CMP effluents typically contain suspended solids originated from slurry abrasive particles of SiO2, Al2O3, or CeO2, depending on the nature of the CMP applications. Other contaminants, including insoluble metal oxides and nitrides and soluble chemicals, also exist in the wastewater in a much lesser quantity. In principal, CMP wastewater contains very dilute slurry abrasives that are narrowly ranged between 50 nm and 200 nm, and possess highly negative surface charges that repel adjacent particles when they are immersed in base solutions. These wastewater characteristics render conventional coagulation-flocculation technology less ideal to remove such nano-scale particles from the CMP wastewater. Recently, membrane separation process has become a mainstream choice as a single unit or as a post separator for coagulation/flocculation process [Kim et al., 2002; Pan et al., 2005]. The use of membrane filtration has demonstrated success in the removal of suspended solids, and owns the Water Practice & Technology Vol 1 No 3 © IWA Publishing 2006 doi: 10.2166/WPT.2006049

A Mechanistic Study on the Electrocoagulation of Silica Nanoparticles from Polishing Wastewater added value of re-concentrate the valuable slurry for possible reuse. However, the capital and operating costs and maintaining the necessary degree of permeate flux is still of issue of concern. Alternatively, a number of studies have demonstrated that electrodecantation (ED) and electrocoagulation (EC) occurred in a batch and continuous-flow electrochemical reactors [Belongia et al., 1999; Lai and Lin, 2004; Den and Huang, 2005a]. The occurrence of these phenomena strongly depends on the type of slurry and the conductivity of the suspension. For EC, the sacrificial anodic electrodes, such as iron and aluminium, are used to continuously supply metallic ions as the source of coagulants. These electrochemically generated metallic ions can hydrolyze near the anode to form a series of metallic hydroxides capable of destabilizing dispersed particles. The simultaneous electrophoretic migration of the negatively charged particles (e.g., silica particles) toward the anode forces chemical coagulation between particles and metallic hydroxides in the vicinity of the anode, forming flocs that either settles or re-deposits onto the anode. Den and Huang (2005b) have also demonstrated efficient particle separation by EC of dispersed silica particles in a continuous-flow mode, and has addressed the roles of particle transport and the rate of reaction in such reactors. However, these studies were primarily focused on the system performances and parameter optimization, the reaction mechanisms occurring in the process of EC of CMP wastewater has not been discussed in depth. Therefore, the primary objective of the present work is to clarify the possible removal mechanisms involved in the EC with a continuous-flow reactor for treatment of CMP wastewater consisting predominantly of nano-size silica particles.

Methods CMP wastewater preparation The CMP wastewater used throughout this study was obtained from a full-scale semiconductor manufacturing plant in the Science Industrial Park in Hsinchu, Taiwan. The characteristics of the “oxide” CMP wastewater, which was segregated from other sources of wastewater, typically contained turbidity in the range between 150 and 450 NTU, conductivity between 50 and 150 µS/cm, and pH between 6.5 and 9.0. The size and distribution of the particles in the CMP wastewater were measured by a Malvern particle analyzer (Mastersizer 2000, Malvern Instruments, UK). The electrophoretic mobility of the particles was measured by a zeta (ζ) potential analyzer (ZetaPlus, Brookhaven Instruments, USA). A representative set of particle characteristics with respect to ζ-potential and particle size as a function of wastewater pH is shown in Figure 1. The isoelectric point of the wastewater was approximately 2.7, resulting a mean particle size larger than 2 mm. Furthermore, the particle size appears to stabilize (~150 nm) at pH greater than 4, suggesting that particle collision energy was sufficient to overcome the electrical repulsive force for ζ-potential less than -20 mV, leading to particle agglomeration and rapid size growth. Experimental Methods An 8-liter continuous-flow reactor channelized by the electrode plates (20 cm × 14 cm) was designed and fabricated with clear acrylic to perform the experiments. The anodic plates (ASTM A36-97a iron or AA-JIS1050 aluminium) were completely submerged, and the cathodic plates (JIS SUS304 stainless steel) were partially submerged in the suspension. The electrode plates were interposed equidistantly to create vertical flow channels with uniform electrical field strength under monopolar electrical arrangement supplied by a manually controllable DC power

A Mechanistic Study on the Electrocoagulation of Silica Nanoparticles from Polishing Wastewater supply (Model GPS, Goodwill Instruments, Taiwan) operating in constant-current mode (0-3 A range). The voltage response (0-200 V range) was monitored and recorded by a data-acquisition system. During experiment, the reactor was initially filled with the CMP suspension such that the liquid level was approximately 1 cm above the anodic plates and below the top of the cathodic plates. The wastewater was continuously pumped into the reactor using a microcomputercontrolled peristaltic pump (Masterflex L/S, Cole Parmer, IL) at various flow rates. Samples from the influent and effluent ports were regularly taken and measured for turbidity, conductivity, and pH by a Hach ratioTM/XR turbidity meter (Hach Co., Loveland, CO). Also, the particle size and ζ-potential were analyzed as the wastewater flows through each channel. The sludge was scraped off of the anodic surfaces and reactor bottom, and its wet and dry weights were measured. In addition, the crystalline composition of the sludge was analyzed by an X-ray diffractometer (Shimadzu XRD-600). 20

3000

Zeta potential (mV)

2000 -20 1500 -40 1000 -60

Mean particle size (nm)

2500

0

500

-80

0 0

2

4

6

8

10

12

pH

Figure 1. Variation of ζ-potential (!) and mean particle size (!) as a function of initial CMP wastewater pH.

Results and discussion The effects of surface state of anodes on EC efficiency Table 1 summarizes the operating variables using the new iron plates as the anodes (hereinafter referred to as “new”) and those having been previously used for three months (“used”). Under the same operating current density (5.71 A/m2), the steady-state effluent pH, conductivity, as well as the corresponding monopolar potential were nearly identical between the new and used anodes, however the turbidity removal efficiency with the “used” anodes was much superior. These results indicate that the reaction (coagulation) mechanisms involved in the EC were similar, but the surface state of the anodes caused very different types of floc characteristics and sinks. When the “used” plates were employed during EC, vast majority of the flocs were attached to the anodic surfaces, with very little remaining in the suspended state. In contrary, when “new” plates were employed, the flocs became visibly loose in structure, and nearly all flocs floated on top of the liquid surface, which partly contributed to the higher effluent turbidity.

A Mechanistic Study on the Electrocoagulation of Silica Nanoparticles from Polishing Wastewater Table 1. Comparison of parametric values and removal efficiency with “new” and “used” iron anodic plates. Parameter

“New” anodes

“Used” anodes

Current density (A/m2)

5.71

5.71

Initial turbidity (NTU)

268

243

40.4

44.9

9.5

10.0

32

33

68.8

98.1

Reduction of effluent conductivity (%) Final pH Cell potential (V) Turbidity removal efficiency (%)

Figure 2 shows the images of the original “new” (2a) and “used” (2b) anodic plates, as well as the plates with floc deposition (2c). When an electrical current passes through a new plate, the native metal oxide passivating the anodic surface is stripped, thereby corroding the surface and liberating metal ions into the liquid. The corroded surface (Fig. 2b) provides pits and crevices that favour deposition of flocs. Subsequently, it is thought that the depositing flocs form a gelatinous layer that causes increase in electrical potential due to the added resistivity of the layer, eventually leading to “pitting” corrosion. (a)

(b)

(c)

Figure 2. The corrosion process of iron anodes: (a) new plate, (b) old plate, (c) plate with deposits showing the oxidized and unoxidized flocs. Due to the nature of low conductivity of the CMP wastewater, the process leading to pitting corrosion was relatively long. Hence, to accelerate the occurrence of pitting corrosion, we intentionally scratched the surfaces of new aluminium anodes, knowing that corrosion normally initiates from the defects of a surface. Using the identical current density, we again observed much better turbidity removal efficiency with the scratched anodes as compared to the untreated ones. Also, as shown in Fig. 3b, the EC with the scratched anodes was essentially free of flocs in suspended state, as opposed to EC with the untreated anodes (Fig. 3a), demonstrating the importance of the surface state (for both iron and aluminium anodes) in operating and shortening the start-up period of an EC system.

(a)

(b)

A Mechanistic Study on the Electrocoagulation of Silica Nanoparticles from Polishing Wastewater

Figure 3. Aluminium anodes: (a) EC with unscratched Al anodes, (b) EC with scratched anodes. The variation of pH and ζ-potential during EC with iron anodes Figure 4 shows the variation of pH, effluent turbidity removal efficiency, and the particle’s ζpotential with the treatment time of EC. As the effluent turbidity removal efficiency gradually reached a steady-state, the wastewater pH and ζ-potential gradually approached 9.5 and –35 mV, respectively. Based on the Pourbiax (pE-pH) diagram of iron, the hydroxyl species under basic and well-aerated conditions are predominantly Fe(OH)3 and Fe(OH)4-. It was interesting because these hydroxyl species carrying neutral or negative charges cannot proceed with charge neutralization with the negatively-charged silica particles in the CMP wastewater, and thus coagulation was unlikely to occur. To clarify this point, we collected the solids from the effluent by filtering with 0.45 µm membranes, and found that the ζ-potential of the filtered solids was clearly greater than that of the unfiltered effluent (Fig. 4), indicating that the filtered solids were the main contributors of the highly negatively ζ-potential of the effluent. To further investigate this question, these iron hydroxyl species were separately produced by EC using RO water in place of the CMP wastewater. The pH and ζ-potential of the steady-state effluent were 9.2 and –25 mV, respectively. Furthermore, the ζ-potential measurement showed that the IEP of the EC effluent using RO water was at pH ~ 6, which further demonstrates the coagulation via charge neutralization with silica particles (IEP ~ 2) was improbable because both were highly negatively-charged at pH above 9. Jar-tests between the EC effluent and the CMP wastewater (rapid mixing at 200 rpm for 1 min; slow mixing at 30 rpm for 20 min; settling for 30 min) also showed no evidence of coagulation as well as no significant change in ζ-potential. Although the possibility of particle removal by means other than charge neutralization, such as the Fe(OH)3 forming suspended gel capable of binding colloids or particles (Mollah et al., 2004a), cannot be ruled out, a more plausible explanation would be that the ferrous ion (Fe2+) liberating from the anodes immediately reacted (charge neutralized) with the silica particles which migrated to the vicinity of the anodic surface by electrophoresis. The flocs then redeposited onto the pits of the surfaces, forming greenish deposits characteristic of the Fe2+ or its hydroxyl species. These greenish deposits would eventually turned yellowish due to oxidation of the ferrous species.

Sludge characterization Sludge was collected from the anodic surfaces and from the bottom of each cell after 2 hr of continuous-flow operation (9.6 L of CMP wastewater treated). Sludge volume measurements showed that approximately 60% of the sludge volume was generated from the surface-bound deposits (hereinafter referred to as the “surface sludge”), and the remaining 40% from the sediments (“sediment sludge”). Also, the dry mass of the surface sludge nearly doubled that of the sediment sludge.

A Mechanistic Study on the Electrocoagulation of Silica Nanoparticles from Polishing Wastewater XRD analysis of the sludge exhibits a wide variety of crystalline, including iron oxides

120

-25

100

-30

80

-35

60

Zeta potential Zeta potential (filtered) Residial turbidity pH

-40

-45

40

20

-50 0

20

40

60

80

100

120

140

0 160

10.0

9.5

9.0

8.5

pH

-20

Turbidity removal (%)

Zeta potential (mV)

(Fe2O3, Fe21.3O32, etc.), goethite (α -FeOOH), FeO(OH), among other minor iron- and siliconcontaining crystalline. None of the crystalline, however, showed a Fe-Si composite, thereby confirming that silica particles are removed strictly by coagulation (i.e., no chemical reaction forming Fe-Si bonding).

8.0

7.5

7.0

6.5

Time (min)

Figure 4. The ζ-potential, pH, and turbidity as a function of EC time. EC mechanisms Based on the experimental results presented above, two possible reaction mechanisms may be operative in the EC process: (i). When the applied overpotential is sufficiently high such that the kinetic and activation overpotentials can be compensated to liberate Fe2+ from the iron anodes, these ferrous ions immediately participate in charge neutralization with the silica particles already migrated to the vicinity of the anodic surfaces (Eq. 1). This directly leads to particle destabilization and subsequently coagulation, forming flocs that mostly redeposit onto the anodic plates through electrostatic attraction (Eq. 2). This mechanism is primarily responsible for the “surface sludge”. Fe(s) → Fe2+(aq) + 2eFe

2+

+ SiO (silica) → floc (↓ ) -

(aq)

(1) (2)

(ii) The Fe2+ that do not participate in direct charge neutralization with the silica particles slowly diffuse outwards into the bulk liquid and form various iron hydroxyl complexes. Additionally, in liquid with sufficiently high dissolved oxygen concentration, the ferrous ions are oxidized to ferric ions (Eq. 5b), and thereby the formation of both ferrous and ferric hydroxyl species are possible. If these hydroxyl complexes are not consumed in charge neutralization, then the stable hydroxides will form as the end-products (Eq. 5a and 5c). Fe(s) → Fe2+(aq) + 2e-

(3)

A Mechanistic Study on the Electrocoagulation of Silica Nanoparticles from Polishing Wastewater 2H2O + 2e- → 2OH- + H2(g) Fe

2+

Fe

2+

Fe

3+

(4)

(aq)

+ 2 OH → Fe(OH)2(s)

(5a)

(aq)

+ 4H + O2 → Fe + 2H2O

(5b)

(aq)

+ 3 OH → Fe(OH)3(s)

(5c)

-

+

3+

-

Under high pH and oxygenated conditions, the ferrous ions and its hydroxyl complexes can also easily form Fe(OH)3 andα -FeOOH (goethite) as co-products (eq. 6). They may further react to form hydrogenated iron hydroxyl species, which can be bound to the silica particles to become flocs that precipitate to the bottom of each cell (Eq. 7 and 8). It is noted that both α -FeOOH and FeO(OH) have been identified by XRD analysis. As mentioned earlier, these iron hydroxide/hydroxyl species are not coagulated with the silica particles through charge neutralization as identified in mechanism (i), but they can capture or bind to the particles by complexation or bridging, and eventually co-precipitate. This mechanism is mainly responsible for the “sediment sludge”. Fe(OH)2(s) + H2O + O2 High pH Oxygenate

Fe(OH)3(S) +α -FeOOH(s)

(6)

Fe(OH)3(s) +α -FeOOH(s) → H(aq)(OH)OFe(s)

(7)

H(aq)(OH)OFe(s) + SiO (silica) → floc (↓ ) + H2O

(8)

-

Conclusions ! !

!

!

Pre-treatment of the anodic plates is necessary to remove the passivated oxide layer and to generate greater surface defects for deposition. Particle destabilization via charge neutralization by iron hydroxide or hydroxyl complexes is improbable under the experimental conditions, although silica particle removal by these iron species can still occur by complexation and bridging. The sludge was characterized into “surface sludge” and “sediment sludge”, which were caused by different particle removal mechanisms. The surface sludge was likely a result of particle destabilization by charge neutralization between SiO- and Fe2+, which eventually redeposited onto the anodic surfaces. The sediment sludge, on the other hand, was a result of complexation/bridging by iron hydroxides/hydroxyl species that precipitated. The dry mass of the surface sludge nearly doubled that of the sediment sludge. XRD analysis of the sludge samples showed the presence of various iron hydroxides, supporting the occurrence of co-precipitation between silica particles and the iron hydroxides.

A Mechanistic Study on the Electrocoagulation of Silica Nanoparticles from Polishing Wastewater

References Belongia B. M., Haworth P. H., Baygents J. C. and Raghavan S. (1999) Treatment of alumina and silica chemical mechanical polishing waste by electrodecantation and electrocoagulation. J. Electrochem. Soc., 146, 4124-4130. Den W. and Huang C. P. (2005a) Electrocoagulation for removal of silica nano-particle from chemicalmechanical-planarization wastewater, Colloids and Surfaces A: Physicochem. Eng. Aspects, 254, 8189. Den W and Huang C. P. (2005b) Parameter optimization and design Aspect for electrocoagulation of silica nano-particles in wafer polishing wastewater, in Proceedings of IWA-ASPIRE 2005, Paper 11D-2, Singapore, July 11-15. Golden J. H., Small R., Pagan L., Shang, C. and Rghavan, S. (2000). Evaluating and treating CMP wastewater, Semicond. International., 23 (6), 92-103. Golden J. H. and Carrubba J. E. (2001) Chemistry of CMP wastewater. Semicond. Fabtech, 13th ed., Henley Publishing, London, pp. 123-126. Holt P. K., Barton G. W., Wark M., Mitchell C. A. (2002) A quantitative comparison between chemical dosing and electrocoagulation. Colloids and Surfaces A: Physicochem. Eng. Aspects, 211, 233-248. Kim M. S., Woo S. W. and Park J. G. (2002) Point of use regeneration of oxide chemical mechanical planarization slurry by filtrations, Jap. J. Appl. Phy., 41 (11A), 6342-6346. Lai C. L. and Lin S. H. (2004) Treatment of chemical mechanical polishing wastewater by electrocoagulation: system performances and sludge settling characteristics. Chemosphere, 54, 235242. Matteson M. J., Dobson R. L., Glenn Jr. R. W., Kukunoor N. S., Waits III W. H. and Clayfield E. R. (1995) Electrocoagulation and separation of aqueous suspensions of ultrafine particles, Colloids and Surfaces A: Physicochem. Eng. Aspects, 104, 101-109. Mollah M. Y. A., Pathak S. R., Patil P. K., Vayuvegula M., Agrawal T. S., Gomes J. A. G., Kesmez M. and Cocke, D. L. (2004a). Treatment of orange azo-dye by electrocoagulation (EC) technique in a continuous flow cell using sacrificial iron electrodes, Journal of Hazardous Materials, B109, 165-171. Mollah M. Y. A., Morkovsky P., Gomes J. A. G., Kesmez M., Parga J. R. and Cocke, D. L. (2004b). Fundamentals, present and future perspectives of electrocoagulation, Journal of Hazardous Materials, B114, 199-210. Pan J. R., Huang C. P., Jiang W., Chen C. S. (2005) Treatment of wastewater containing nano-scale silica particles by dead-end microfiltration: evaluation of pretreatment methods, Desalination, 179 (1-3), 3140.