Regeneration of used magnetic seeds with ultrasound employed on the treatment of wastewater from semiconductor industry

Separation and Purification Technology 108 (2013) 89–95

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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Regeneration of used magnetic seeds with ultrasound employed on the treatment of wastewater from semiconductor industry Shu-Min Shen a, Terng-Jou Wan b,⇑, Ya-Lin Shu b a b

Department of Earth and Environmental Sciences, National Chung Cheng University, Chiayi 62102, Taiwan, ROC Department of Safety, Health and Environmental Engineering, YunTech Douliou, Yunlin 64002, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 23 August 2012 Received in revised form 28 January 2013 Accepted 5 February 2013 Available online 13 February 2013 Keywords: Backside grinder wastewater Regeneration Magnetic seeds Ultrasound Reuse cycles Polyaluminum chloride (PAC)

a b s t r a c t In the past, several technologies for the removal of silica particles from chemical mechanical polishing (CMP) or backside grinder (BG) wastewater such as chemical coagulation, electrocoagulation, flotation, membrane filtration, and adsorption. Currently, the successful aggregation technologies by employing magnetic seeds have been reported. This paper assesses the potential for regeneration of used magnetic seeds by ultrasound employed on the treatment of wastewater from the semiconductor industry. The results show that the reuse cycles of magnetic seeds could be increased up to five times for seeds regenerated by ultrasound power of 235 W for 1 min which exhibited the best effect on the regeneration. Furthermore, the reuse cycles of magnetic seeds could be increased up to 12 times using pulsed ultrasound. In this study, the regeneration of magnetic seeds enhanced by using ultrasound could not only increase the reuse cycles of magnetic seeds but also lessen the usages of coagulants and reduce the quantity of chemical waste sludge. It also indicated that such regeneration processes of magnetic seeds enhanced by ultrasound has a high potential as an alternative to chemical and thermal regenerations of magnetic seeds for purposes of reclamation, recycling, and reuse of magnetic seeds, wastewater, and silica particles. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Production of semiconductors is one of the most crucial manufacturing industries, and wafer fabrication processes constitute important branches of those industries, especially those chemical mechanical polishing (CMP) and backside grinder (BG) processes which have been widely adopted by producers in the past decades. Real CMP wastewater generally has zeta potential ranging from 10 mV to 78 mV and pH, silica particle size, conductivity and turbidity ranging from 6.0 to 9.7, 50 to 200 nm, 55 to 1960 ls/ cm, and 65 to 400 NTU, respectively, while BG wastewater has zeta potential varying from 10 mV to 40 mV and pH, silica particle size, conductivity and turbidity varying dramatically from 5.0 to 8.1, 60 to 7720 nm, 34 to 56.4 ls/cm, and 110 to 2500 NTU, respectively [1–6]. This study examined real BG wastewater with a comparatively high turbidity, in the range of 2100–2500 NTU. Conventional technologies for the removal of silica particles from CMP or BG wastewater include chemical coagulation, electrocoagulation, flotation, membrane filtration, and adsorption [3,7– 10]. Among the different treatment technologies described above,

⇑ Corresponding author. Tel.: +886 5 5342601x4410; fax: +886 5 5312069. E-mail address: [email protected] (T.-J. Wan). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.02.004

adsorption technologies are perhaps the most attractive due to their efficiency, economy, and simple operation [3,11–15]. Recently, the successful application of aggregation technologies by employing magnetic seeds has been reported [3,6,16]. Magnetic separation has attracted great attention because the magnetic force is a long-distance attraction, and thus enhances the removal of waste silica particles. Wastewater can be treated by using magnetic separation in a very short period of time, and the quantity of chemical waste sludge can be reduced [3,6]. As a result, it has been widely used in the textile industry, and in the fields of biology (such as DNA and cell extraction) and environmental protection [3,16–20]. Highly turbid river water has been effectively treated using Fe3O4 magnetic seeds, which can reduce turbidity from 9600 to 20 NTU [13]. The coagulant dosage of BG wastewater treated by using coagulant PAC combined with magnetite (Fe3O4) seeds could be reduced by about 80% and reduce the turbidity of BG wastewater from 2500 to 23 NTU and the turbidity of CMP wastewater from 110 to 1 NTU by using magnetite particles [3,6]. Feng et al. reported that magnetic iron can adsorb and remove Cu, Zn, and Cd from sandy soil [21]. The adsorption capacity of Fe3O4 magnetic seeds has been shown to be as high as 35.46 mg/ g metal ions and arsenic removal efficiency increased from 87% to 98.7% upon addition of 108.7 mg L 1 of magnetic seeding by polymetric ferric sulfate [22,23]. Furthermore, Tural investigated and

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showed that glucamine modified magnetic sorbent has the ability to separate and pre-concentrate boron [24]. Common technologies for regeneration of absorbents are chemical extraction, bioregeneration, solvents, high pressure/high temperature and ultrasound, etc. [25–30]. Due to ultrasonic acoustic cavitation and shock waves which can reduce the thickness of liquid films, ultrasound treatment thus can enhance gas transfer by means of reducing bubble coalescence to rise in the interfacial area [31–33]. The bubbles eventually become unstable and implode, producing high-speed micro-jets of liquid, intense localized heating and high-pressure shock energy waves, i.e. called ‘‘hot spot’’ [31]. Furthermore, the enhancement of desorption by ultrasound is explained by the phenomenon of acoustic cavitation [34–36], improving and accelerating the mass transport of both intra- and inter-particles [37,38], increasing in surface diffusivity, and decreasing the activation energy [38]. The intense localized heating and high-pressure shock waves caused by cavitation were found to be the major cause of the decrease of activation energy [38]. The mechanisms explaining ultrasonically enhanced desorption are both the thermal and non-thermal (hydro-dynamical) effects of ultrasound. The non-thermal effect of ultrasound is greater than the thermal effect [30]. Previous reports have indicated the feasibility of using ultrasound for desorption of Cu (II) and Pb (II) from bentonite [39]. Ultrasonic regeneration has shown the possibility to serve as an alternative to chemical and thermal regenerations of GAC [40]. Wang found that magnetic hydroxyapatite can adsorb 90% of the phenol in wastewater and it can be reused six times [41]. To the best of our knowledge, magnetite particles have been successfully regenerated by cetyltrimethyl ammonium bromide (CTAB), acid, base and solvent, and so on [16,42–47]. Recently, one promising method of adsorbent regeneration is the application of ultrasound [38,40,48,49]. According to those reports, the magnetic seeds could reduce time-consumption, chemical wastewater, and lessen the usages of coagulants to reduce the quantity of chemical waste sludge. However, the preparation cost of magnetic seeds is still higher than other methods costs for treating wastewater and sludge. Thus, this study investigated the effects of regeneration by ultrasound on the reuse cycles of magnetic seeds to be a cost-effective alternative. In addition, we studied the effects of solution pH, proper exposure duration time of regeneration by ultrasound, suitable ultrasonic power, and the patterns of ultrasound application (patterns A and B) on reuse cycles of magnetic seeds and the residual turbidity of treated water. 2. Materials and methods 2.1. Samples and water quality of BG wastewater BG wastewater samples were obtained from the silicon wafer backside grinding process used in semiconductor companies in Taiwan. The basic wastewater quality parameters, i.e., pH, suspended solids (SSs), turbidity, and the particle size of silica particles in the wastewater were measured. The pH of the wastewater was generally close to neutrality (pH 5.8–7.4). SS ranged from 120 to 180 mg L 1 and its color was dark brown. The turbidity of the wastewater ranged from 2100 to 2500 NTU from BG wastewater (Table 1) and the silica size was about 138–142 nm (Fig. 1). The particle sizes reported in another research ranged from 60 to 7720 nm [2]. 2.2. Preparation and characterization of magnetic seeds Magnetic seeds were synthesized by the chemical co-precipitation method. FeCl3
6H2O (ferric chloride, Shimakyu, Japan) of

Fig. 1. The average particle sizes of magnetic seeds and silica in wastewater.

100 ml and FeCl2
4H2O (ferrous chloride tetrahydrate, Hanawa, Japan) of 100 ml were added to a 50 ml NaOH solution (sodium hydroxide, Merck, Germany) by mixing at 300 rpm, at room temperature [3,6]. The optimum molar ratio of Fe2+–Fe3+ was taken as 2:3, as this ratio has been shown to produce a synergistic effect [45,50]. The prepared mixture was black in appearance. Subsequently, the magnetic seeds (Fe3O4) were washed with de-ionized water. There were still impurities remaining in the colloidal water suspension (pH = 5.5 ± 0.5) when the aggregation reaction was completed. In this study, the particle sizes of the magnetic seeds in the slurry was about 25.7 nm (pH = 5) as illustrated in Fig. 1 and the magnetic seeds’ images before and after regeneration by ultrasound were examined by SEM, and are shown in Fig. 2a and b, respectively. 2.3. Aggregation of magnetic seeds and silica and sedimentation experiments Magnetic seeds with dosages from 2.49 to 3.74 g were mixed with one liter of BG wastewater in a beaker. PAC (Nippon Shinyaku Co., Japan) of 0.01 g L 1 was added to improve removal efficiency of the wastewater so that it could reduce the usage of the magnetic seeds to the most cost-effectiveness dosage. Sodium hydroxide (NaOH, Merck, Germany) and sulfuric acid (H2SO4, Merck, Germany) of 0.1 M or 1 M were used to adjust the pH of the wastewater and magnetic seeds separately with pHs ranging from 6 to 10. The mixture was rapidly mixed at 100 rpm for 3 min and then slowly mixed at 30 rpm for 20 min. The modified jar test (Cherng Huei, CG-8277) was used in this study. After the jar test, a magnetic field of 1000g was applied for 30 min during aggregates sedimentation [6]. The magnetic field was applied by magnet. The residual turbidities of the supernatants were measured during the initial 30 min once every 5 min. 2.4. The power and duration time of ultrasound applied for regeneration and reuse of magnetic seeds In this study, to determine the proper power and exposure duration time of ultrasound applied for regeneration and reuse of magnetic seeds, first of all, magnetic seeds were regenerated by ultrasonic powers of 145 W, 195 W, and 235 W for exposure duration times of 1–5 min at 25 °C. Next, the regenerated magnetic seeds were filtered and recycled for reuse. Subsequently, a new

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Fig. 2. (a) SEM image analysis of magnetic seeds before used; (b) SEM image analysis of magnetic seeds after five times regenerated by ultrasound.

wastewater sample and 0.01 g L 1 PAC were added into the beaker for the next reuse cycle of magnetic seeds for the aggregation with silica particles from BG wastewater treatment.

The particle sizes of the samples were measured by a Malvern Instruments Zetasizer (Nano-ZS, 3000HSA). Analytical procedures from the standard methods were employed for the measurement of water quality. Suspended solids (SSs) were measured by weighing the dried filter membranes after the filtration of the sample (Method 2540, B and D) [51]. The surface morphology of the magnetic seeds was examined by SEM (JEOL 5410LV). Turbidity was measured by a Model 2100P Hitach meter. Magnetic flux intensity was measured by a Kanetrc Tesla meter TM-701. The trace metals were measured by ICP-AES (OPTIMA 5100DV, Perkin–Elmer). 3. Results and discussion 3.1. Effects of solution pH of slurry on turbidity removal The effects of the solution pH of the slurry which contained magnetic seeds and raw wastewater with silica particles on magnetic seeding aggregations of BG wastewater and the residual turbidity are given in Fig. 3. The residual turbidity was 46, 96, and 524 NTU at pH of 4, 5 and 6, respectively. Wan et al. [6] reported that the zeta potential of magnetic seeds were positive within a pH range of 3–7, but in contrast became negative when pH was greater than 9. Moreover, the magnetic seeding aggregation efficiency was poor at high pH (9–11) because the zeta potential of the magnetic seeds and particles were both negative [6]. Thus, the successful magnetic seeding aggregation could be found when the solution had a pH of 4 or 5. The magnetic seeds which were positively charged successfully captured the waste silica particles which were negatively charged due to their highly opposite zeta potentials having electrostatic attraction, thus, obviously increasing the likelihood of successful aggregation, under silica particle sizes of several hundred nanometers, at pH 4 or 5. This phenomenon is consistent with other reports previously published in the literature [3,6,51–54]. However, the previous reports indicated that it was also reported that magnetite (Fe3O4) can transform into maghemite (Fe2O3) and release ferrous ions (Fe2+) in acidic pH, e.g. especially with pH values lower than 4.5 [55–57]. In the presence of excess cation such as Fe2+ and Fe3+ specific adsorption oc-

Residual turbidity (NTU)

2.5. Analytical methods

1500

1200

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0 4

5

6

7

8

9

Solution pH Fig. 3. Residual turbidity of backside grinder wastewater treated at various solution pH of slurry under 3.74 g of magnetic seeds applied without PAC added and magnetic field of 1000g adopted for liquid–solid (the aggregates of silica particles and magnetic seeds) separation.

curred on the surface of magnetite, which dramatically affects the value of zeta potential [57]. Hence, the appropriate solution pH was all adjusted at 5 in the rest of the experiments in this study. In this study, the surface charge and particle size of magnetic seeds could be easily controlled by pH adjusting and regenerated by ultrasound applying. The smaller the particle size of magnetic seeds, the greater total surface area is which will enhance the effects of adsorption magnetic seeds and silica particles, showed in Fig. 1.

3.2. Effects of ultrasound on reuse cycles of magnetic seeds The use of magnetic seeds might have multiple advantages because it decreases the required dosages of PAC, i.e. a large amount of waste chemical sludge could be decreased, and the time could be saved processing wastewater treatment and liquid–solid separation, thus the cost of wastewater treatment and waste sludge processing could be reduced. Fig. 4 illustrates the comparison of reuse cycles and residual turbidity of BG wastewater treatment between the using of magnetic seeds of 2.49 g L 1 coupled with PAC 0.01 g L 1 and magnetic seeds with 3.74 g L 1 added only under magnetic seeds without ultrasound application for regeneration for each reuse cycle under a magnetic field of 1000g applied for li-

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500

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Without ultrasound applied 145 W 195 W 235 W

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Fig. 4. Comparison of reused cycles of magnetic seeds and residual turbidity of BG wastewater treatment between using of magnetic seeds of 2.49 g L 1 coupled with PAC 0.01 g L 1 and magnetic seeds of 3.74 g L 1 without PAC added. Each reuse cycle of magnetic seeds without regenerated by ultrasound under magnetic field applied of 1000g for liquid–solid (the aggregates of silica particles and magnetic seeds) separation and pH = 5.

quid–solid (the aggregates of silica particles and magnetic seeds) separation and at pH 5. Fig. 4 illustrates that the residual turbidity of both dosage experiments mentioned above were decreased and remained around 10 NTU, when using fresh magnetic seeds. Magnetic seeds of 2.49 g L 1 coupled with PAC 0.01 g L 1 could be reused only one time and the residual turbidity was about 17 NTU. Magnetic seeds of 3.74 g L 1 without PAC added were also reused only one time and the residual turbidity was about 128 NTU (greater than 100 NTU). In other words, without ultrasound application for regeneration of magnetic seeds, it is apparent that the reuse cycles of magnetic seeds was only one time for both dosage experiments of magnetic seeds. Fig. 5 illustrates the effects of ultrasonic power on the reuse cycles of magnetic seeds and residual turbidity of BG wastewater. The experiment was performed with the 2.49 g L 1 magnetic seeds coupled with 0.01 g L 1 PAC under a magnetic field strength of 1000g applied for liquid–solid (the aggregates of silica particles and magnetic seeds) separation and at pH 5. The obtained results in Fig. 4 show that both the residual turbidity of BG wastewater and the reuse cycles of magnetic seeds were significantly improved and enhanced, respectively in the presence of ultrasound for all the studied powers. Ultrasound can increase the reactivity of metal powders by more than 100,000 times [58]. The major phenomenon responsible for this activation is cavitation [59]. Through a series of expansion and compression cycles generated by acoustic streaming, gas bubbles in the liquid may grow larger than 100 lm [31– 33]. In accordance with those results, waste silica particles can be influenced by high power ultrasound. Magnetic seeds with waste silica particles can be separated easily even if waste particles might be adsorbed on high-energy sites of magnetic seeds. Because ultrasonic cavitation can provide more energy ‘‘i.e. spot energy effect, acoustic vortex micro-streaming’’ than required by desorption [60,61]. Fig. 5 also illustrates that the ultrasound power of 235 W has exhibited the best effects on the regeneration of magnetic seeds Table 1 Quality of raw wastewater from backside grinder processes. Parameter

Value

pH Size of silica particles (nm) Turbidity (NTU) SS (mg L 1)

5.8–7.4 138–142 2100–2500 120–180

1st

2nd

3rd

4th

5th

6th

Reuse cylces Fig. 5. Effects of ultrasonic power on the reuse cycles of magnetic seed and residual turbidity of BG wastewater. The experiment was performed with the 2.49 g L 1 magnetic seeds coupled with 0.01 g L 1 PAC under a magnetic field strength of 1000g applied for liquid–solid (the aggregates of silica particles and magnetic seeds) separation and at pH 5.

among the three ultrasound powers of 145 W, 195 W, and 235 W and its reuse cycles were increased by around 20% more than that of the other two experiments of ultrasound powers of 145 W and 195 W. Magnetic seeds were able to be reused for five cycles for the experiment of ultrasonic power of 235 W, and a residual turbidity of about 42 NTU was obtained. Comparatively, the magnetic seeds could be regenerated and reused for around four cycles in the range between 145 W and 195 W, while magnetic seeds could be reused for only one reuse cycle without ultrasound application. Therefore, it is clear that the desorption rates for regeneration were favored by increased ultrasound intensity, and this can also be proven according to the results obtained from Hamdaoui’s report [62]. From the obtained results in this study, ultrasound application was found to significantly enhance the reusability of magnetic seeds. These results could be explained by ultrasound’s ability to cause acoustic cavitation phenomena which produces high-speed micro-jets of liquid, acoustic vortex micro-streaming, intense localized heating, and high-pressure shock waves, i.e. called ‘‘hot spots’’, and then reducing activation energy of desorption from the surface of magnetic seeds, enhancing mass transfer and surface diffusivity of silica particles and magnetic seeds, and all those phenomena were beneficial to desorption. Similar results also have been obtained from several other previous reports in the literature [31–39,63]. The mechanisms of ultrasonically enhanced desorption is due both to the thermal and non-thermal (hydrodynamical) effects of ultrasound. The non-thermal effect of ultrasound is greater than the thermal effect [30].

3.3. Effects of exposure duration time of ultrasound applied on reuse cycles of magnetic seeds Fig. 6 shows the effects of ultrasound exposure duration times on reuse cycles and residual turbidity. By applying ultrasound for 1–5 min, magnetic seeds could be recycled up to five times (each ‘‘time’’ hereafter referred to as ‘‘one reuse cycle’’) and the residual turbidity of treated water was below 100 NTU, containing 3.74 g magnetic seeds without PAC added, under an ultrasound power of 235 W applied, a magnetic field strength of 1000g, and at pH 5. The obtained results shown in Fig. 6 illustrate that both the residual turbidity of BG wastewater and the reuse cycles of magnetic seeds were significantly improved and increased, respec-

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1400

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800 Magnetic seeds of 2.49 g/L + PAC 0.01 g/L

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tively, in the presence of ultrasound for all the studied exposure duration times. The exposure duration time of ultrasound application for 1 min might be enough for the separation of magnetic seeds and silica particles. Moreover, the longer duration times of ultrasound applications as well as the more times the magnetic seeds were regenerated by ultrasound would not be helpful to the reuse cycles of magnetic seeds and would cause the particles or clump size of the magnetic seeds to be smaller (shown as Fig. 2) and probably alter the characteristics of the magnetic seeds. Based on these results, the exposure duration time of ultrasound application for regeneration of magnetic seeds was set exactly for 1 min. 3.4. Effects of the application pattern of ultrasound on reuse cycles of magnetic seeds There are two application patterns of ultrasound application employed, pattern A is that magnetic seeds were regenerated by ultrasound treatment for regeneration only when the residual turbidity rose near or above 100 NTU (shown as Fig. 7a), while pattern B is that magnetic seeds underwent ultrasound pre-treatment for regeneration before each reuse cycle (shown as Fig. 7b). Fig. 7a reveals that the reuse cycles of magnetic seeds could be significantly affected by the application of ultrasound treatment for regeneration. Once residual turbidity was more than 30% of the influent C0, the regeneration process was stopped. For example, as the removal efficiency of magnetic seeds decreased (the residual turbidity of 277 NTU when magnetic seeds of 1.24 g L 1 contained 0.01 g L 1 PAC), magnetic seeds regenerated by ultrasound before the next experiment and the residual turbidity dropped to 61 NTU for the next experiment. These results further provide evidence that ultrasound application can increase reuse cycles of magnetic seeds effectively in BG wastewater treatment. Fig. 7b shows magnetic seeds that underwent ultrasound pretreatment for regeneration each time before the reuse cycle under ultrasonic power of 235 W (pattern B), a magnetic field of 1000g and at pH 5. The reuse cycles went from 1 to 5 times after applying ultrasonic treatment for regeneration of magnetic seeds for each time reuse when using magnetic seeds of 2.49 g L 1 coupled with 0.01 g L 1 PAC added. The reuse cycles were also raised from 2 to 5 times after applying ultrasonic treatment for regeneration of magnetic seeds for each time reuse when only using magnetic seeds of 3.74 g L 1 without PAC added.

(b) Residual turbidity (NTU)

Fig. 6. Effects of exposure duration time of ultrasound application for regeneration of magnetic seeds on residual turbidity of backside grinder wastewater treatment and reuse cycles of magnetic seeds when 3.74 g of magnetic seeds used only without PAC added, under ultrasound power of 235 W, magnetic field of 1000g applied and at pH 5.

800 700

Magnetic seeds of 2.49 g/L + PAC 0.01 g/L

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Magnetic seeds of 3.74 g/L added only

500 400 300 200 100 0 0

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2nd

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Reuse cycles Fig. 7. Effects of application pattern of ultrasound on reuse cycles of magnetic seeds: (a) Application pattern A: ultrasonic treatment for regeneration only when the residual turbidity rose near or above 100 NTU, and (b) application pattern B: each time before the reuse cycle underwent ultrasound pre-treatment for regeneration under ultrasonic power of 235 W, magnetic field of 1000g and pH = 5.

Although magnetic seeds regenerated by ultrasound before each reuse cycle (application pattern B) might increase energy consumption compared with regeneration by ultrasound only when the residual turbidity is near or above 100 NTU (application pattern A), the reuse cycles of magnetic seeds by pattern B could be substantially increased by about 100% more than that by pattern A. Magnetic seeds can be reused up to five cycles for the two dosage experiments (2.49 g L 1 of magnetic seeds coupled with 0.01 g L 1 PAC added and 3.74 g L 1 of magnetic seeds without addition of PAC). The turbidity removal efficiency was gradually decreased with the increase of reuse cycles of the magnetic seeds. The adsorption occurred at ‘‘high-energy sites’’ first, and then moved to other sites. Hence, the reason that turbidity removal efficiency was gradually decreased, we suppose, is that the available ‘‘high-energy sites’’ on the surfaces of magnetic seeds were occupied after repeated reuses and the size of particles or the clumps of magnetic seeds decreased which is shown in Fig. 2a and b. These smaller particles of magnetic seeds might increase the residual turbidity or result in the regeneration efficiency being limited due to the magnetic properties of magnetic seeds and possibly the available ‘‘high-energy sites’’ on the surfaces of magnetic seeds might have been changed. 3.5. Effects of PAC addition Conventional chemical coagulation is the most commonly used method to remove waste particles from CMP or BG wastewater,

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which took 0.06 g L 1 PAC to bring the residual turbidity under 100 NTU without the addition of magnetic seeds [6]. However, this method will produce a lot of waste sludge and increase the treatment cost. In this study, the residual turbidity of one experiment (2.49 g L 1 magnetic seeds coupled with 0.01 g L 1 PAC), and another (3.74 g L 1 magnetic seeds without PAC added) are 42 NTU and 43 NTU, respectively, and in both the above experiments the magnetic seeds could be recycled five times. Thus, by adding the appropriate amount of coagulant combined with magnetic separation technology it could reduce the required amount of magnetic seeds, which can significantly lessen the cost of wastewater treatment and the amount of waste sludge produced. 3.6. Effects of the application method of ultrasound on reuse cycles of magnetic seeds There are two application methods of ultrasound application employed, one is that magnetic seeds were regenerated by continuously ultrasound, and the other regenerated method is using pulsed ultrasound. Fig. 8 reveals that the reuse cycles of magnetic seeds could be raised up to 12 times when the experiment was performed with the 2.49 g L 1 magnetic seeds coupled with 0.01 g L 1 PAC. And the reuse cycles of magnetic seeds could be raised up to 8

times when the experiment was performed with the 3.74 g L magnetic seeds coupled with 0.01 g L 1 PAC (Fig. 9).

1

4. Conclusions The feasibility of regeneration of magnetic seeds adsorbed with waste silica particles by ultrasound was evaluated in this work. From the obtained results in this study, it was found that ultrasound application could significantly enhance the regeneration and the reusability of magnetic seeds. The duration time of ultrasound application for 1 min might be enough for the separation or desorption of magnetic seeds and silica particles. Without ultrasound applied for regeneration of magnetic seeds, the reuse cycle of magnetic seeds was only one time. The reuse cycles of magnetic seeds could be raised up to five times for seeds regenerated by ultrasound power of 235 W which exhibited the best effect on the regeneration of magnetic seeds and its reuse cycles was increased by around 20% more than that in the other two experiments of 145 W and 195 W. Magnetic seeds regenerated by ultrasound application pattern B might increase energy consumption compared with application pattern A, and the reuse cycles of magnetic seeds by application pattern B could be substantially increased by about 100% more than that by pattern A. Furthermore, the reuse cycles of magnetic seeds could be increased up to 12 times using pulsed ultrasound. The results show that regeneration of magnetic seeds enhanced by using ultrasound could not only increase the reuse cycles of magnetic seeds, but also lessen the usages of coagulants and reduce the quantity of chemical waste sludge. References

Fig. 8. Effects of application methods of ultrasound on reuse cycles of magnetic seeds. (2.49 g L 1 magnetic seeds coupled with 0.01 g L 1 PAC).

Fig. 9. Effects of application methods of ultrasound on reuse cycles of magnetic seeds. (3.74 g L 1 magnetic seeds coupled with 0.01 g L 1 PAC).

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