Investigation on the simultaneous removal of fluoride, ammonia nitrogen and phosphate from semiconductor wastewater using chemical precipitation

Investigation on the simultaneous removal of fluoride, ammonia nitrogen and phosphate from semiconductor wastewater using chemical precipitation

Accepted Manuscript Investigation on the simultaneous removal of fluoride, ammonia nitrogen and phosphate from semiconductor wastewater using chemical...

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Accepted Manuscript Investigation on the simultaneous removal of fluoride, ammonia nitrogen and phosphate from semiconductor wastewater using chemical precipitation Haiming Huang, Jiahui Liu, Peng Zhang, Dingding Zhang, Faming Gao PII: DOI: Reference:

S1385-8947(16)31226-8 http://dx.doi.org/10.1016/j.cej.2016.08.134 CEJ 15700

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

20 July 2016 27 August 2016 29 August 2016

Please cite this article as: H. Huang, J. Liu, P. Zhang, D. Zhang, F. Gao, Investigation on the simultaneous removal of fluoride, ammonia nitrogen and phosphate from semiconductor wastewater using chemical precipitation, Chemical Engineering Journal (2016), doi: http://dx.doi.org/10.1016/j.cej.2016.08.134

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Investigation on the simultaneous removal of fluoride, ammonia nitrogen and

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phosphate from semiconductor wastewater using chemical precipitation

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Haiming Huang, Jiahui Liu, Peng Zhang, Dingding Zhang, Faming Gao*

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Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering,

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Yanshan University, Qinhuangdao 066004, PR China

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Abstract: This study investigates the simultaneous removal of the total ammonia nitrogen (TAN),

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phosphate (PO4-P) and fluoride (F–) from semiconductor wastewater by chemical precipitation.

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The lab-scale experiment results revealed that the fluoride removal by using magnesium salts

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produced a good performance. The fluoride present could significantly inhibit the struvite

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crystallization, in this process. The inhibition ratio of the fluoride on struvite crystallization

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remarkably increased with an increase in the fluoride concentration and a drop in the pH value.

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The optimal pH for struvite precipitation in the semiconductor wastewater was taken as 9.5, the

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value at which the fluoride effect significantly decreased. Therefore, to further lower the fluoride

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effect, an overdose of the magnesium source was required in the process of struvite precipitation.

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The experimental results thus indicated that overdosing the bittern was the more effective method

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to treat the semiconductor wastewater compared with a brucite overdose; this was because large

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amounts of un-reacted brucite remained in the solution, causing increased costs and operation

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difficulty when it was employed as magnesium source. The pilot-scale study demonstrated that

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97% of the PO4-P, 58% of the TAN and 91% of the F– could be removed from semiconductor

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wastewater by a two-stage precipitation process. An economic analysis showed that the treatment

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cost of the process proposed was approximately 1.58 $/m3.

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Keywords: Ammonia nitrogen, phosphate, fluoride, struvite, semiconductor wastewater.

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*Corresponding Author: Phone: +86 335 8387 741; Fax: +86 335 8061 569;

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E-mail: [email protected] 1

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1. Introduction

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As the global demand for electronic products has rapidly increased over the past decade,

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the global semiconductor industries have made considerable strides in development [1, 2].

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However, this speedy development of the semiconductor industry also triggers some

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environmental risks including the generation of large amounts of wastewaters and high water

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demand [3]. The wastewater generated from the semiconductor manufacturing process generally

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contains high levels of total ammonia nitrogen (TAN), fluoride (F–) and phosphate (PO4-P) [4].

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The TAN and PO4-P are the well-known significant nutrient substances that induce water

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eutrophication. When they exist in substantial quantities in the water bodies, large amounts of

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algae and microorganisms would breed, resulting in a higher dissolved oxygen depletion and fish

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toxicity. Although fluoride is one of the essential elements of the human body, the excessive

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fluoride intake can result in dental and skeletal fluorosis [5]. The safe prescribed fluoride level in

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drinking water, according to WHO, is less than 1.5 mg/L [6]. Therefore, a good efficient treatment

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of semiconductor wastewater plays a crucial role in the prevention of environment pollution and

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human health risk.

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Commonly, biological treatment is accepted as the economical and feasible process to

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remove nutrients from the wastewaters. However, biological processes may not be very feasible in

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the treatment of semiconductor wastewater because of its high content of toxic substances, which

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can inhibit the microorganism activity in the biological treatment system [7, 8]. Although fluoride

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may be efficiently removed from aqueous solution by the electrodialytic method [9–11], this

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process is difficult to be applied to the treatment of the semiconductor wastewater due to the

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complexity of the wastewater. As an alternative, precipitation using calcium salts is often used to

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treat the semiconductor wastewater [12]. Unfortunately, this process cannot simultaneously

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remove the TAN and PO4-P, because it is quickly interrupted by the presence of PO43–, SO42– and

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NH4+ in the wastewater, resulting in a decrease in the recovery factor of the CaF2 for several

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industrial purposes [13, 14]. Additionally, because the chemical precipitation produces very fine

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CaF2 precipitates, flocculants like polyferric sulfate and polyaluminum chloride need to be added

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to accelerate the solid separation process [15]. Compared with the precipitation using calcium salts,

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struvite crystallization can help remove both the TAN and PO4-P, and has been largely considered

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a promising pretreatment method to remove nutrients from various types of wastewaters [16–20]; 2

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this process also has several advantages including the high reaction rate, simple operation and

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excellent solid-liquid separation performance. Besides, the struvite thus recovered finds use as a

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valuable slow-releasing fertilizer. In Japan, struvite has been commercially recovered by Unitika

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Ltd., and sold to American fertilizer companies [21]. Hence, struvite crystallization appears to be

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an attractive process to pre-treat semiconductor wastewater.

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In the earlier literature, some papers reported that the struvite crystallization process is

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significantly influenced by certain inorganic ions like Ca2+, K+, Fe3+, CO32–, etc. [22–24], which

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could interfere with the nucleation of the struvite crystal or compete with the NH4 + and Mg2+ for

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the HPO42–, inhibiting the struvite formation. Besides, some researchers also reported that some

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organic substances like citric acid [25] and humic substances [26] have an observable inhibitory

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impact on the struvite crystallization. However, some papers are available in the literature which

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studied the effect of the F– ions on the TAN and PO4-P removal by struvite crystallization. The F–

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concentration in the semiconductor wastewater is usually as high as several hundred to several

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thousand mg/L, based on the operational conditions [15]. Although Ryu et al. [27] have confirmed

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that the high concentration of F– may inhibit the removal of TAN and PO4-P by struvite

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precipitation, their investigation did not specifically report the influence mechanism and impact

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strength of the F– on the struvite crystallization under different conditions. Therefore, in this study,

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it is necessary to further investigate the mechanism of influence and strength of the fluoride on the

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struvite crystallization and identify a process to eliminate the effect.

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The objective of this study was to simultaneously remove F–, TAN and PO4-P from the

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semiconductor wastewater. To achieve this, first the fluoride was removed from synthetic

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wastewater by chemical precipitation utilizing magnesium salts. Second, the experiments were

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performed to investigate the influence of fluoride on struvite crystallization at different pHs and

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fluoride concentrations. Third, to eliminate the fluoride effect, overdosing with bittern and brucite

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mineral powder during chemical precipitation was done; besides, the economic feasibility of both

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the magnesium sources was evaluated and compared. Finally, the pilot-scale treatment of

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semiconductor wastewater was conducted.

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2. Materials and methods

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2.1. Experimental materials

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The raw semiconductor wastewater used in the experiments was supplied by a 3

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semiconductor manufacturer from Shenzhen, China. To ensure sample stability, the wastewater

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was stored below 4 oC before use. The main characteristics of the wastewater sample are described

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as follows: pH, 7.5 ± 0.05; F–, 1280 ± 56 mg/L; PO4-P, 201 ± 3.4 mg/L; TAN, 131 ± 5.1 mg/L.

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Besides, in the experiments, natural brucite mineral powder (particle size, 50-80 µm) and bittern

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were used as the magnesium sources of chemical precipitation. The brucite mineral powder was

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purchased from a mineral processing plant in Dandong city and about 94% of it was composed of

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magnesium hydroxide. The bittern was collected from a solar salt field in Shenzhen, and the chief

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constituents were as follows: Mg2+, 45.3 g/L; K+, 10.1 g/L; Ca2+, 142 mg/L. All the other

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chemicals used in the experiments were of analytical grade and purchased from the Tianjin

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Fengchuan Chemical Reagent Plant, China.

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2.2. Experimental methods

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1) Removal of fluoride: As the semiconductor wastewater has high fluoride content, the

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removal of fluoride ions by co-precipitating with Mg2+ was first investigated. In this work,

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synthetic wastewater containing a fluoride concentration of 1280 mg/L was used, which was

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prepared by dissolving sodium fluoride (NaF) in deionized water. The experimental procedures are

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described as follows: 100 ml of the synthetic wastewater was first added to a 200 ml beaker placed

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on a magnetic stirrer, followed by the addition of magnesium chloride (MgCl2· 6H2O) in a

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desirable Mg:F molar ratio (0.5–1), and then the wastewater was stirred at a designed pH (8–10)

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for 30 min. After the stirring was completed, the mixture was left for 30 min to be precipitated,

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and 5 ml of the supernatant was removed and filtered through 0.22-µ m filter membranes for

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composition analysis.

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2) Effect of fluoride on the struvite crystallization: To determine the degree of the influence

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of the fluoride on the struvite crystallization, a series of experiments was performed. The

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wastewater used in the experiments were prepared by dissolving Na2HPO4· 12H2O and NH4Cl in

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deionized water. It was found to contain PO4-P 201 mg/L and TAN 90.7 mg/L. The experiments

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were performed as follows: 100 ml of synthetic wastewater was first taken in a 200 ml beaker and

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placed on a magnetic stirrer. Next, several milliliters of the NaF stock solution was added to the

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wastewater to adjust the concentration of the fluoride ions to be in the range of 0–2100 mg/L.

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Then, magnesium chloride was added to the wastewater in a stoichiometric ratio of Mg:N:P. After

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that, the mixed solution was stirred for 30 min and the solution pH was controlled at a designed 4

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value (8–10) by the addition of 0.1 M NaOH. Finally, the solution at the end of reaction was

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allowed to settle for 30 min and 5 ml of the supernatant was drawn and filtered through a 0.22-µ m

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filter membrane for component analysis.

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3) Simultaneous removal of F–, PO4-P and TAN from real semiconductor wastewater: In

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this work, bittern and natural brucite were used as the magnesium source for the chemical

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precipitation. The experimental procedures using the bittern were similar to those of the

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experiments performed for the effect of fluoride on struvite crystallization. In the experiments, the

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molar ratio of Mg:P ranged from 1 to 3, and the solution pH was maintained at 9.5. Regarding the

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use of natural brucite, the experiments were conducted according to the following procedures. 500

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ml of the semiconductor wastewater was first taken in a 1000 ml beaker, followed by the addition

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of natural brucite in different doses (2.4–4.8 g/L, i.e. Mg:P molar ratio, 6–12). Next, the mixture

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was stirred for 6 h. During the reaction, the solution pH was measured at every 5 minutes and 1 ml

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of each sample was drawn at every 20 minutes and filtered through 0.22-µ m filter membranes for

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composition analysis.

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2.3. Inhibition model of fluoride on struvite crystallization

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To determine the degree of the effect of F– on the removal of TAN and PO4-P by struvite

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crystallization, an inhibition model was introduced into this study [26]. The inhibition ratios (IR)

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of fluoride on the TAN and PO4-P removal efficiency are described as follows.

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IR N =

NRE 0 − NRE i × 100% NRE 0

(1)

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IR P =

PRE0 − PREi × 100% PRE 0

(2)

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where NRE0 and NREi are represented as the TAN removal efficiency (%) in the absence and

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presence of fluoride, respectively. Similarly, the PRE0 and PREi are the PO4 -P removal efficiency

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(%) in the absence and presence of fluoride, respectively.

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2.4. Pilot-scale treatment of semiconductor wastewater

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The pilot-scale treatment of semiconductor wastewater was performed at a semiconductor

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plant located in a suburban area of Shenzhen. In this study, the pilot-scale test was operated in a

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continuous-flow mode and lasted for 100 h without interruption. The specific schematic diagram

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is shown in Fig. 1. As observed in Fig.1, the semiconductor wastewater treatment was divided into 5

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two phases, namely, the first and second stages of precipitation. 1) First stage precipitation: the

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semiconductor wastewater was first continuously pumped into the Reaction Tank (a), which is a

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barrel having a 70 L reacting zone. In this stage, the influent flux of the semiconductor wastewater

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was maintained at 70 L/h, which implied a hydraulic retention time (HRT) of 1 h. To

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simultaneously remove the phosphate, fluoride and ammonia nitrogen, the bittern stored in the

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liquid storage tank was continuously added to the wastewater in a 2:1 molar ratio of Mg:P, and the

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solution pH was constantly maintained at 9.5. The mixture in the Reaction Tank (a) flowed

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through the metering valve to the Sedimentation Tank (a) (HRT, 2 h). After precipitation, the

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precipitates collected at the bottom of the sedimentation tank were discharged at 3 L/h. 2) Second

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stage precipitation: the supernatant in the Sedimentation Tank (a) was pumped into the Reaction

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Tank (b) (a barrel with a reacting zone of 67 L; HRT, 1 h) for the further removal of phosphate,

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fluoride and ammonia nitrogen. In this process, the bittern which served as the magnesium source

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was continuously added to the supernatant, and the pH and the Mg:F molar ratio of the solution

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were controlled at 9.5 and 1:1, respectively. Through the second precipitation reaction, the mixing

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solution was continuously discharged into the Sedimentation Tank (b) (HRT, 2 h). The precipitates

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collected at the bottom of the sedimentation tank were drained into a subsequent sludge

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concentration tank through the hydrostatic pressure, and the resulting supernatant was discharged

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into a subsequent treatment process for advanced treatment. In the tests, all the pumps and pH

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meter were controlled by a Programmable Logic Controller and a computer. The stirring rate in

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the Reaction Tanks (a) and (b) was kept at 200 rpm. To maintain the stable removal of the

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pollutants from the semiconductor wastewater, the main parameters in the pilot-scale treatment

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such as the solution pH, the Mg:P molar ratio and Mg:F molar ratio in the Reaction Tanks (a) and

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(b) need to be accurately controlled. The deviations of the Mg:P and Mg:F molar ratios should be

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controlled within 10%, and the deviation of the solution pH was maintained at ± 0.2. During the

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test, at every 2 h, 10 ml of the supernatant in both the sedimentation tanks were drawn and filtered

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through 0.22 µ m filter membranes for composition analysis.

Fig. 1 here

173 174

2.5. Chemical analysis

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Various parameters of semiconductor wastewater were analyzed according to the American

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Public Health Association (APHA) standard methods [28]. The concentrations of TAN and PO4-P 6

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of wastewater samples were colorimetrically determined using a 752N spectrophotometer (China).

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The F– concentration in the sample was determined using an ion selective electrode (Orion 720

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A+ Ion analyzer). During experiments, the solution pH was recorded using a pH meter (pHS-3C;

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China). The precipitates formed in the experiments were collected and washed thrice with

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deionized water, and oven dried at 40°C for 24 h. The morphology of the dried precipitates was

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observed using a scanning electron microscope-energy dispersive spectrometer (SEM-EDS; FEI

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Nova NanoSEM 450; American). In this study, all the tests were performed in triplicate, and their

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average values were reported.

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3. Results and discussion

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3.1. Removal of fluoride by the co-precipitation of Mg2+

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To determine the performance of the Mg2+ on the fluoride removal, a series of experiments

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was performed. The removal efficiency of F– by Mg2+ is shown in Fig. 2. As observed in Fig. 2,

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the F– removal efficiency increased as the solution pH and the Mg:F molar ratio increased. When

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the Mg:F molar ratio was 1 and the pH of solution was 10, the removal efficiency of F– by Mg2+

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reached > 90%. When the Mg2+ was added to the wastewater rich in fluoride, if the ionic product

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of Mg2+ and F– was greater than the solubility product of the product (Ksp), F– could be removed

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as insoluble fluoride precipitates, according to the following reaction equation [29].

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2F – + Mg 2 + → MgF2

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Furthermore, during the process of the experiments, the settleability of the fluoride precipitates

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formed by Mg2+ was observed. The fluoride precipitates formed by the Mg2+ were observed to

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show good settleability, which formed an obvious thin sedimentation layer after 90 min of free

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sedimentation. However, in this study, we have performed control experiments using calcium salts

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to precipitate the fluoride, and found that although the use of calcium salts as the precipitator of

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fluoride can achieve fluoride removal efficiency comparable to that by the Mg2+, the settleability

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of the formed precipitates was very poor. It was observed that the resulting solution system was

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still turbid after 90 min of free sedimentation, without any obvious solid-liquid separation

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interface. The CaF2 precipitate was very fine with a particle size of around 1µ m, which could have

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been responsible for the difficulty in settling [15, 29]. Therefore, in the CaF2 precipitating process,

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polyaluminum chloride and polyelectrolyte are normally required to be added for attaining a good

(3)

7

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solid separation from water [30]. Hence, from the viewpoint of the settleability, it was considered

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that the fluoride removal with the Mg2+ appeared to be more feasible than it did with the Ca2+.

Fig. 2 here

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3.2. Effect of fluoride on the removal of TAN and PO4-P by struvite crystallization

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The experiments done to investigate the effect of fluoride on the struvite crystallization were

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performed at the Mg:N:P molar ratio of 1:1:1. Figs. 3a and 3b show the changes in the TAN and

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PO4-P removal efficiency with solution pH and fluoride concentration, respectively. The results

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demonstrated that the fluoride present in solution could significantly influence struvite formation.

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As observed in Fig. 3a, at a given fluoride concentration, the TAN removal efficiency increased

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with increasing the solution pH from 8 to 9.5, and reached a maximum at pH 9.5; however, it

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progressively decreased with a further increase in the pH in the range of 9.5–10. This finding was

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consistent with reports in published investigations [31]. Solution pH is an important factor that

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influences struvite crystallization [25]. It could critically influence the existing form and activity

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of the NH4+, Mg2+ and HPO4 2– species of the struvite crystal [32]. Struvite formation proceeds by

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the following reaction equations:

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Mg 2+ + NH 4 + + HPO 4 2 – + 6H 2 O → MgNH 4 PO 4 ⋅ 6H 2 O ↓ + H +

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Within the pH range of 8–9.5, the H+ concentration in the solution gradually decreased with the

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increase in the pH value, which is beneficial to the crystallization reaction from left to the right,

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thus resulting in an obvious increase in the efficiency of TAN removal. However, at pH > 9.5, as a

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large quantity of the NH4+ in the solution was converted to NH3 which cannot be precipitated by

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struvite crystallization, and the phosphate began to react with Mg2+ to form Mg3(PO4)2 [32, 33],

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these would markedly obstruct the reaction process of Eq. (4), causing a drop in efficiency of the

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TAN removal in the pH range of 9.5–10. In addition, the decrease of Mg2+ for the formation of

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Mg(OH)2 at pH > 9.5 may be another reason resulting in a drop in efficiency of the TAN removal.

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Further, it is evident from Fig. 3a that at a given pH, the TAN removal efficiency rapidly

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decreased with the increase in the fluoride concentration. When the solution pH was 9.5, the TAN

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removal efficiency decreased from 85.6% to 56.3% with the rise in the fluoride concentration

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from 0 mg/L to 1200 mg/L.

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(4)

Fig. 3b shows the effects of the pH of the solution and fluoride concentration on the PO4-P 8

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removal, which reveals that at a given fluoride concentration, the PO4-P removal efficiency

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increases with an increase in the pH, whereas at a given pH it decreased with the increase in the

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fluoride concentration. In this study, quite different from the removal of TAN, which was only

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through struvite formation (the NH3 volatilization can be ignored), the PO4-P removal could

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proceed through the formations of struvite and magnesium phosphate. At pH >9.5, although the

240

struvite crystallization was inhibited, magnesium phosphate formation was accelerated. This may

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be responsible for the increase in the PO4-P removal efficiency with a rise in the pH value.

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Furthermore, combining Figs. 3a and Fig. 3b, it became evident that at a given pH, the TAN and

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PO4-P removal efficiency simultaneously decreased with the increase in the fluoride concentration.

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Based on the results in Section 3.1, it can be confirmed that the F– can compete with NH4+ and

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HPO42– for Mg2+ to form MgF2, which resulted in a decrease in the quantity of Mg2+ used for the

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struvite crystallization. Besides this, F– as the foreign ions in the crystallization reaction can be

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easily adsorbed onto the struvite crystal surface, which can induce the slowing down of the crystal

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growth [7]. In the earlier literature, Ryu et al. [27] reported that as the fluoride concentration was

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above 600 mg/L, the removal of TAN and PO4-P by forming struvite were obviously inhibited,

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which concurs with our findings.

251

To further determine the effect mechanism of the fluoride on the struvite crystallization, the

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struvite precipitates obtained at pH 9.5 and different fluoride concentrations were characterized by

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SEM-EDS (see Fig. 4). The results indicated that the shape of the struvite crystal produced was

254

markedly affected by the fluoride ions. Fig. 4 reveals that the morphology of the struvite crystals

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obtained in the absence of the fluoride ions was columnar with smooth surfaces, whereas the

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shape of the crystals formed at fluoride concentrations of 600 mg/L and 1500 mg/L gradually

257

became irregular blocks with fine amorphous particles on their surfaces. This suggested that the

258

presence of fluoride may influence the nucleation and growth of the struvite crystals, producing

259

different shaped products. Besides, the EDS patterns clearly demonstrated that the fluoride

260

element was present in the precipitates formed, and its peaks were intensified with the increase in

261

the fluoride concentration.

262

The IR values of fluoride on the TAN and PO4-P removal efficiencies are shown in Figs. 3c

263

and 3d, respectively. As evident, the IR values were closely related to the solution pH and fluoride

264

concentration. At a given fluoride concentration, the IR values of the fluoride on the TAN and 9

265

PO4-P removal efficiencies decreased rapidly with the increasing pH in the range of 8–9.5, and

266

plateaued in the pH range of 9.5–10. Otherwise stated, the lower the pH, the greater the inhibition

267

level of the fluoride ions on the struvite crystallization. For example, when the fluoride

268

concentration was 300 mg/L, the IR value of the fluoride on the TAN removal efficiency increased

269

from 5% at pH 9.5 to 31% at pH 8. Any rise in the fluoride concentration could rapidly increase

270

the IR values. When the fluoride concentration increased from 300 mg/L to 2100 mg/L at pH 9,

271

the IR values of the fluoride for the removal efficiencies of TAN and PO4-P increased from around

272

5% to around 70%. Therefore, it can be confirmed that maintaining a high pH level and lowering

273

the fluoride ion concentration could contribute to the struvite formation in the semiconductor

274

wastewater.

275

Fig. 3 here

276

Fig. 4 here

277

3.3. Simultaneous removal of F–, PO4-P and TAN from real semiconductor wastewater

278

Based on the results mentioned above, it can be confirmed that an excessive magnesium dose

279

is required to achieve the efficient removal of PO4-P and TAN from real semiconductor

280

wastewater by struvite crystallization. Therefore, in the subsequent experiments, bittern and

281

brucite mineral powder were used as the magnesium sources of chemical precipitation to

282

simultaneously remove F–, TAN and PO4-P from the semiconductor wastewater.

283

3.3.1. The use of bittern

284

In the investigation mentioned above, the inhibition of fluoride on struvite crystallization was

285

understood to be closely related to the solution pH and fluoride concentration. Therefore, it was

286

assumed that if the fluoride concentration in the semiconductor wastewater was decreased at a

287

given pH, the inhibition of fluoride on the struvite crystallization would correspondingly decrease,

288

resulting in the improvement of the removal efficiencies of the TAN and PO4-P. Hence, in this

289

work, the bittern was overdosed to eliminate the fluoride influence on the struvite crystallization.

290

The results of the removal efficiencies of F–, PO4 -P and TAN from real semiconductor wastewater

291

by chemical precipitation are shown in Fig. 5. As observed from Fig. 5a, the removal efficiency of

292

the F– rapidly escalated with the increase in the magnesium dose, implying that the fluoride

293

inhibition on the removal of PO4-P and TAN would be decreased. In fact, from Fig. 5b, it is noted

294

that with the increase in the molar ratio of Mg:P in the range of 1–1.75, the TAN removal 10

295

efficiency rose from 39.9% to 53.2%. This may be attributed to the lowered inhibition of the

296

fluoride and the increase in the quantity of magnesium utilized in the struvite crystallization.

297

Certain publications have reported that the correct increase of the magnesium dose could promote

298

the removal of TAN by struvite crystallization [34, 35]. However, a further increase in the molar

299

ratio of Mg:P in the range of 1.75–3 produced a drop in the removal of TAN. This result concurred

300

with the finding in an earlier publication in the literature [14]. This may have occurred due to the

301

fact that an excessive magnesium dose may have induced the formation of magnesium phosphate,

302

which in turn could have competed with the NH4+ for HPO42– and Mg2+. Although the magnesium

303

phosphate formed could result in a decrease in the removal of TAN, it was advantageous in the

304

removal of phosphate. This could be corroborated by the results shown in Fig. 5c. Fig. 5c reveals

305

that the PO4-P removal efficiency rose as the Mg:P molar ratio increased. The PO4-P removal

306

efficiency achieved 90.1% when the molar ratio of Mg:P was 2:1, and it further increased to

307

95.5% when the ratio was increased to 3:1. This finding was consistent with the result reported by

308

Warmadewanthi and Liu [14], who used magnesium chloride to treat semiconductor wastewater

309

and achieved a PO4-P removal efficiency of 92.1% at a molar ratio of Mg:P of 3:1. Fig. 5 here

310 311

3.3.2. The use of brucite mineral powder

312

In order to compare the performance of bittern, the utilization of brucite mineral powder for

313

treating semiconductor wastewater was further investigated. In this investigation, the brucite

314

dosage ranged from 2.4 g/L to 4.8 g/L, implying that the Mg:P molar ratio is in the range of 6–12

315

or the Mg:F molar ratio is between 0.58 and 1.16. The results explaining the effects of the brucite

316

dose and reaction time on the pH value and the removal efficiencies of PO4-P, TAN and F– are

317

shown in Fig. 6(a–d). In Fig. 6a, the solution pH values for all the brucite doses tested it can be

318

observed to rapidly increase with the escalations of the brucite dose and reaction time in the first

319

240 min, and then gradually reach a plateau between 240 and 360 min. This plateau may be

320

attributed to the achievement of the dissolution equilibrium of Mg(OH)2. Fig. 6b shows that when

321

the brucite dose was 2.4 g/L, the TAN removal efficiency progressively increased within the

322

reaction time tested. However, when the brucite dose was greater than 2.4 g/L, the TAN removal

323

efficiency rapidly increased during the initial 160 min, followed by a slow drop in the reaction

324

time range of 160–360 min. Besides, from Fig. 6b it is evident that the TAN removal efficiency 11

325

decreased with an increase in the brucite dose between 240 min and 360 min. This event may have

326

resulted from the high pH, which was higher than 9.5 for the brucite dose, greater than 2.4 g/L in

327

the reaction time range of 160–360 min.

328

Figs. 6c and 6d show the changes in the PO4-P removal efficiency and the F– removal

329

efficiency with the brucite dose and reaction time, respectively. As observed in Fig. 6c, although

330

the PO4-P removal efficiency progressively increased with the increases in the brucite dose and

331

reaction time, it never exceeded 80% in the batch experiments. This finding does not concur with

332

the publications reported in the literature, in which the phosphate in wastewater can be almost

333

completely removed by overdosing brucite or Mg(OH)2 as the magnesium source [16, 36]. In this

334

study, the competition between F– and PO43– for Mg2+ may be the main reason for the occurrence,

335

which may induce a lack of Mg2+ for the phosphate removal. As seen in Fig. 6d, when the brucite

336

dose increased from 2.4 g/L to 4 g/L, the F– removal efficiency also increased; however, further

337

increases in the brucite dose caused only a very small rise in the fluoride removal. Furthermore,

338

for all the brucite doses tested, the F– removal efficiency gradually plateaued when the reaction

339

time crossed 200 min. This suggested that the reaction between F– and Mg2+ was almost

340

completed. However, in this study, the brucite dosage far exceeded that required for the removal of

341

phosphate and fluoride. This suggested that large quantities of brucite were not involved in the

342

removal reaction.

343

From the results mentioned above and the published literatures [37, 38], we proposed a

344

reaction mechanism for the simultaneous removal of phosphate, ammonia nitrogen and fluoride by

345

using brucite as a magnesium source, the sketch map of which is described in Fig. 7. As observed

346

in Fig. 7, the reaction process was distinguishable into three stages: 1) in the first stage, the Mg2+

347

and OH– were rapidly released from the brucite and diffused into the bulk solution, while the F–,

348

PO43– and NH4+ diffused into the brucite surface at the same time, followed by the formation of

349

struvite and magnesium fluoride; 2) in the second stage, the struvite and magnesium fluoride grew

350

on the brucite surface and the pH of the bulk solution rapidly increased, which blocked any further

351

brucite dissolution; 3) in the third stage, the brucite dissolution achieved an equilibrium state and

352

the removal reactions of phosphate and fluoride culminated. In this reaction mechanism proposed,

353

only the formation of struvite and magnesium fluoride was considered. However, some other

354

compounds such as magnesium phosphate may also be formed. 12

355

Fig. 6 here

356

Fig. 7 here

357

3.3.3. Comparison analysis of both magnesium sources

358

In this investigation, a comparative analysis for the simultaneous removal of phosphate,

359

ammonia nitrogen and fluoride employing bittern and brucite as the magnesium sources was

360

performed. The analysis mainly related to the economic evaluation and the difficulty level of the

361

operation. In this comparative analysis, the reaction conditions were set as follows: bittern, at a

362

dose of 7g/L (i.e. Mg:P molar ratio, 2:1) and a pH of 9.5 for 60 min; brucite, at a dose of 4 g/L for

363

240 min. In the economic evaluation, to simplify the calculation, only the costs of the chemicals

364

utilized and the energy consumed were considered; the manpower and equipment maintenance

365

costs were disregarded. The results of the economic evaluation are shown in Table 1. The cost for

366

treating semiconductor wastewater using bittern was found to be 0.44 $/m3 , which has a better

367

economic advantage compared to that by using brucite (0.76 $/m3). From the operation aspect,

368

although the removal efficiencies of phosphate, ammonia nitrogen and fluoride employing brucite

369

were comparable to those by using bittern, a big difference was observed in the level of difficulty

370

of their operation. On the one hand, because the use of brucite as the magnesium source required a

371

longer stirring reaction time compared with the use of bittern, this implied that a bigger reactor

372

was needed, especially constructed for the semiconductor wastewater treatment. This would

373

greatly increase the investment cost for the equipment. On the other hand, when brucite was

374

employed as the magnesium source, more precipitates were produced when compared with the

375

utilization of bittern because a large amount of un-reacted brucite remained in the solution. This

376

increased the difficulty in the subsequent disposal of solid waste. Therefore, from the viewpoints

377

of economy and operability, it can be confirmed that the treatment of semiconductor wastewater

378

employing bittern as the magnesium source was optimal. Hence, in the subsequent pilot-scale

379

treatment, bittern alone was used as the magnesium source for the removal of phosphate, ammonia

380

nitrogen and fluoride from semiconductor wastewater.

381

Table 1 here

382

3.4. Pilot-scale treatment of semiconductor wastewater

383

To further determine the feasibility of the simultaneous removal of phosphate, ammonia

384

nitrogen and fluoride from semiconductor wastewater by chemical precipitation, a pilot-scale test 13

385

was performed based on the lab-scale experiment results. In this test, to maintain the stability of

386

water quality, all the wastewater required during test was stored in advance, in a 10 m3 storage

387

pool. The characteristics of the semiconductor wastewater used in the pilot-scale study were as

388

follows: pH, 7.55; F–, 1162 mg/L; PO4-P, 205 mg/L; TAN, 135 mg/L. The results of the pilot-scale

389

test are shown in Fig. 8, and the concentration changes of the pollutants in the remained solution

390

at different precipitation stages are given in Table 2. As observed in Fig. 8 and Table 2, the

391

removal performance of the phosphate, ammonia nitrogen and fluoride by the pilot-scale process

392

were found to be consistent with those in the lab-scale experiments. In the first precipitation stage,

393

the average removal efficiencies of the phosphate, ammonia nitrogen and fluoride reached 90 ±

394

3%, 50 ± 4% and 20 ± 2%, respectively, and the average remaining concentrations of the

395

phosphate, ammonia nitrogen and fluoride decreased to 21, 68 and 930 mg/L, respectively;

396

subsequently, in the second precipitation, the average removal efficiencies of the three pollutants

397

further rose to 97 ± 1.5%, 58 ± 2% and 91± 2.4%, respectively, and their average remaining

398

concentrations were further decreased to 6.2, 57 and 105 mg/L, respectively. In the published

399

literature, some papers are available on the treatment of semiconductor wastewater by struvite

400

crystallization. Ryu et al. [27] considered that struvite precipitation was an effective process for

401

the treatment of semiconductor wastewater, and can achieve 89% ammonia nitrogen removal

402

efficiency. Kim et al. [7] in their investigation on the effects of the mixing intensity and mixing

403

duration on the struvite precipitation in semiconductor wastewater, reported that under certain

404

conditions, the removal efficiencies of phosphate, ammonia nitrogen and fluoride can reach

405

approximately 70%, 80% and 20%, respectively. Warmadewanthi and Liu [14] reported that

406

84.2% of the PO4-P and 33.5% of the TAN were removed from semiconductor wastewater when

407

the ratio of Mg:P was 2.5:1 at pH 9. On comparison with these literatures, we can state that the

408

process proposed in this study can simultaneously achieve higher removal efficiencies of

409

phosphate and fluoride. Besides, based on the economic evaluation method in Section 3.3.3, the

410

cost incurred for treating 1 m3 semiconductor wastewater by the pilot-scale process involving two

411

stages of precipitations was calculated to be approximately $1.58. Overall, this cost is acceptable

412

by most semiconductor manufacturing enterprises in China. As for the final disposal of the

413

precipitates recovered by the proposed process, since the precipitates contained large amounts of

414

fluoride, it cannot be used as the agricultural slow release fertilizer; however, it can be served as a 14

415

phosphate rock to be reused in industrial production. Furthermore, through the pretreatment of the

416

proposed process, the amount of the harmful substances in the semiconductor wastewater will be

417

greatly reduced, which would be beneficial to the stable operation in the following biological

418

treatment system.

419

Fig. 8 here

420

Table 2 here

421

4. Conclusions

422

In the present study, chemical precipitation was practiced to simultaneously remove the

423

phosphate, ammonia nitrogen and fluoride from semiconductor wastewater. The lab-scale

424

conditional experiments were initially performed to investigate the effects of different parameters

425

on chemical precipitation. Subsequently, a pilot-scale study was performed to determine the

426

feasibility of treating semiconductor wastewater with a two-stage precipitation process. The main

427

conclusions drawn from the study are given below:

428 429 430 431

1) Magnesium salts as the precipitator exhibited a superior settleability with respect to fluoride removal when compared with the calcium salts. 2) Fluoride significantly inhibited struvite crystallization and its inhibition ratio increased with the rise in fluoride concentration and the lowering pH value.

432

3) Despite the usage of bittern and brucite as the sources of magnesium to attain comparable

433

phosphate, ammonia nitrogen and fluoride removal from semiconductor wastewater, the

434

utilization of bittern was more advantageous in terms of treatment cost and the operability

435

compared with the use of brucite.

436

4) The pilot-scale process proposed, involving a two-stage precipitation, was economical and

437

feasible for the treatment of semiconductor wastewater, as it could achieve a high degree of

438

success in the removal of PO4-P and F– (97% and 91%, respectively).

439

Acknowledgments

440

This work was financially supported by the National Natural Science Foundation of China

441

(Grant No. 51408529), the Natural Science Foundation of Hebei Province (Grant No.

442

E2014203080), the Outstanding Young Scholars Project of Colleges and Universities of Hebei

443

province (Grant No. BJ2014059), and China Postdoctoral Science Foundation Funded Project

444

(Grant Nos. 2015M580215, 2015M581319 and 2016T90215). 15

445

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446

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swine wastewater using microbial fuel cell, Bioresour. Technol. 114 (2012) 303–307.

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ammonium nitrogen from semiconductor wastewater, J. Hazard. Mater. 156 (2008)

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163–169.

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fluorine-containing wastewater treatment, Appl. Mech. Mater. 361–363 (2013) 755–759.

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[31] T. Zhang, L. Ding, H. Ren, X. Xiong, Ammonium nitrogen removal from coking

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wastewater by chemical precipitation recycle technology, Water Res. 43(2009)

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[33] I. Stratful, M.D. Scrimshaw, J.N. Lester, Conditions influencing the precipitation of

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magnesium ammonium phosphate, Water Res. 35 (2001) 4191–4199.

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[34] T. Zhang, L. Ding, H. Ren, Pretreatment of ammonium removal from landfill leachate

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by chemical precipitation, J. Hazard. Mater. 166 (2009) 911–915.

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[35] S. Uludag-Demirer, M. Othman, Removal of ammonium and phosphate from the supernatant 18

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100 (2009) 3236–3244.

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[36] H. M. Huang, X. M. Xiao, L. P. Yang, Removal of ammonium from rare-earth wastewater

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using natural brucite as a magnesium source ofstruvite precipitation, Water Sci. Technol. 63 (2011)

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[37] J.M. Chimenos, A.I. Fernández, G. Villalba, M. Segarra, A. Urruticoechea, B. Artaza, F.

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Espiell, Removal of ammonium and phosphates from wastewater resulting from the process of

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cochineal extraction using MgO-containing byproduct, Water Res. 37 (2003) 1601–1607.

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[38] T. Chen, X. Huang, M. Pan, S. Jin, S. Peng, P. H. Fallgren, Treatment of coking wastewater

540

by using manganese and magnesium ores, J. Hazard. Mater. 168 (2009) 843–847.

541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 19

561 562 563

Table captions

564

Table 1. The market prices of the chemicals used and energy consumed and economic evaluation

565

of using bittern and brucite as magnesium sources.

566

Table 2. The changes of the remaining concentrations of the phosphate, ammonia nitrogen and

567

fluoride in the remained solution at different precipitation stages.

568 569

Figure captions

570

Fig. 1. The flow diagram of the pilot-scale process for treating the semiconductor wastewater by

571

chemical precipitation.

572

Fig. 2. The removal efficiency of the fluoride by Mg2+ at different solution pHs and Mg:F molar

573

ratios.

574

Fig. 3. Struvite crystallization at different pH and fluoride concentrations (a) effect of fluoride on

575

TAN removal, (b) effect of fluoride on PO4-P removal, (c) IR value of fluoride on TAN removal

576

efficiency and (d) IR value of the fluoride on the PO4-P removal efficiency.

577

Fig. 4. SEM-EDS characterization results of the struvite crystals obtained at pH 9.5 and different

578

fluoride concentrations: (a) and (b) are the SEM micrograph and the EDS pattern at a fluoride

579

concentration of 0 mg/L, respectively; (c) and (d) are the SEM micrograph and the EDS pattern at

580

a fluoride concentration of 600 mg/L, respectively; (e) and (f) are the SEM micrograph and the

581

EDS pattern at a fluoride concentration of 1500 mg/L, respectively.

582

Fig. 5. The changes in the removal efficiency of fluoride, TAN and PO4 -P from semiconductor

583

wastewater by chemical precipitation with the Mg:P molar ratio (a) fluoride removal efficiency, (b)

584

TAN removal efficiency and (c) PO4-P removal efficiency.

585

Fig. 6. The changes in the solution pH (a), TAN removal efficiency (b), PO4-P removal efficiency

586

(c) and fluoride removal efficiency (d) with the brucite dose and reaction time.

587

Fig. 7. Diagrammatic sketch of simultaneously removing the phosphate, ammonia nitrogen and

588

fluoride with brucite.

589

Fig. 8. Pilot-scale treatment of semiconductor wastewater by chemical precipitation involving two

590

stages of precipitations: (a) the removal efficiency of PO4 -P, TAN and F– in the first stage of 20

591

precipitation, (b) the removal efficiency of PO4-P, TAN and F– in the second stage of precipitation.

592 593

Table 1

594 595

Table 1. The market prices of the chemicals used and energy consumed and economic evaluation of using bittern and brucite as magnesium sources. Market price ($/kg)

Cost of using bittern ($/m3)

Cost of using brucite ($/m3)

Bittern

0.04

0.28

NaOH

0.31

0.11

– –

Brucite

0.14



0.56

0.1$/kW·h

0.05

0.2



0.44

0.76

Chemicals/energy

Energy consumed Total

596 597 598 599 600 601 602

Table 2

603

Table 2. The changes of the remaining concentrations of the phosphate, ammonia nitrogen and

604

fluoride in the remained solution at different precipitation stages. Initial concentration

Average remaining concentration

Average remaining concentration in

of the influent

in the first stage precipitation

the second stage precipitation

(mg/L)

(mg/L)

(mg/L)

F

1162

930

105

PO4-P

205

21

6.2

TAN

135

68

57

Pollutant

605 606 607 608 609 610 611 21

612 613 614

Figure 1

615 616 617

9.55

9.55

618 619 620

Fig. 1. The flow diagram of the pilot-scale process for treating the semiconductor wastewater by chemical precipitation.

621 622 623 624 625 626 627 628 629 630 22

631 632 633

Figure 2

634 635 636

637 638 639

Fig. 2. The removal efficiency of the fluoride by Mg2+ at different solution pHs and Mg:F molar ratios.

640 641 642 643 644 645 646 647 648 649 650 23

651 652 653

Figure 3

654 655 656 657

658 659 660 661

Fig. 3. Struvite crystallization at different pH and fluoride concentrations (a) effect of fluoride on TAN removal, (b) effect of fluoride on PO4-P removal, (c) IR value of fluoride on TAN removal efficiency and (d) IR value of the fluoride on the PO4-P removal efficiency.

662 663 664 665 24

666 667 668

Figure 4

669 670 671

672 673 674 675 676

Fig. 4. SEM-EDS characterization results of the struvite crystals obtained at pH 9.5 and different fluoride concentrations: (a) and (b) are the SEM micrograph and the EDS pattern at a fluoride concentration of 0 mg/L, respectively; (c) and (d) are the SEM micrograph and the EDS pattern at a fluoride concentration of 600 mg/L, respectively; (e) and (f) are the SEM micrograph and the 25

677

EDS pattern at a fluoride concentration of 1500 mg/L, respectively.

678 679 680

Figure 5

681 682 683 684 685

686 687 688 689

Fig. 5. The changes in the removal efficiency of fluoride, TAN and PO4 -P from semiconductor wastewater by chemical precipitation with the Mg:P molar ratio (a) fluoride removal efficiency, (b) TAN removal efficiency and (c) PO4-P removal efficiency.

690 691 692 693 26

694 695 696 697

Figure 6

698 699 700 701 702

703 704 705

Fig. 6. The changes in the solution pH (a), TAN removal efficiency (b), PO4-P removal efficiency (c) and fluoride removal efficiency (d) with the brucite dose and reaction time.

706 707 708 709 710 27

711 712 713 714

Figure 7

715 716 717 718

719 720 721

Fig. 7. Diagrammatic sketch of simultaneously removing the phosphate, ammonia nitrogen and fluoride with brucite.

722 723 724 725 726 727 728 729 730 731 28

732 733 734 735

Figure 8

736 737 738

739 740 741 742

Fig. 8. Pilot-scale treatment of semiconductor wastewater by chemical precipitation involving two stages of precipitations: (a) the removal efficiency of PO4 -P, TAN and F– in the first stage of precipitation, (b) the removal efficiency of PO4-P, TAN and F– in the second stage of precipitation.

743 744 745 746 29

747 748 749

Graphical Abstract

750 751

752 753

30

Highlights

754 755 756 757 758 759 760

    

Treatment of semiconductor wastewater by chemical precipitation was investigated. Removal of fluoride by Mg2+ showed good performance. Fluoride ions exert a significant inhibitory effect on struvite crystallization. Bittern is suitable to treat semiconductor wastewater. PO4-P and F– can be efficiently removed using the two-stage precipitation process.

761 762

31