Chemosphere 117 (2014) 374–381

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Pilot-test of the calcium sodium phosphate (CNP) process for the stabilization/solidification of various mercury-contaminated wastes Jae Han Cho, Yujin Eom, Tai Gyu Lee ⇑ Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea

h i g h l i g h t s  The Hg leaching from the field-collected waste samples were investigated.  A calcium sodium phosphate process is proposed to treat Hg-contaminated wastes.  A pilot-scale treatment plant was designed and manufactured using the CNP process.  The waste samples were successfully treated using the pilot-scale treatment plant.

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Article history: Received 29 May 2014 Received in revised form 21 July 2014 Accepted 24 July 2014 Available online 25 August 2014 Handling Editor: O. Hao Keywords: Mercury Waste Leaching Stabilization Solidification CNP

a b s t r a c t A pilot-scale calcium sodium phosphate (CNP) plant was designed and manufactured to examine the performance of recently developed stabilization/solidification (S/S) technology. Hg-contaminated wastes samples generated via various industrial processes in Korea, including municipal, industrial, and medical waste incineration, wastewater treatment, and lime production, were collected and treated using the pilot-scale CNP plant. S/S samples were fabricated according to various operating conditions, including waste type, the dose of the stabilization reagent (Na2S), and the waste loading ratio. Although the performances (Hg leaching value and compressive strength) were reduced as the waste loading ratio increased, most of the S/S samples exhibited Hg leaching values that were below the universal treatment standard limit of 25 lg L1 and compressive strengths that exceeded the criterion of 3.45 MPa. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction The Minamata Convention on Mercury, which is the international binding treaty for Hg, was adopted by representatives from all participating countries on October 10, 2013 in Minamata, Japan (UNEP, 2013a; Mackey et al., 2014). The goal of this treaty is to protect humans and the environment from hazardous Hg and its compounds and to minimize the release of Hg and Hg compounds. Hg mining and Hg emissions will be controlled by this convention, and the production, import and export of Hg-containing products under the certain conditions set by the Minamata Convention, such as batteries, switches, fluorescent lamps, barometers, and thermometers, will be banned in 2020. In addition, Hg wastes must be managed using disposal methods that are appropriate and environmentally sound management according to the guidelines ⇑ Corresponding author. Tel.: +82 2 2123 5751; fax: +82 2 6008 0560. E-mail address: [email protected] (T.G. Lee). http://dx.doi.org/10.1016/j.chemosphere.2014.07.080 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.

developed under the Basel Convention (Kaghazchi and ShamsiJazeyi, 2011; Asasian et al., 2012; Lee and Lee, 2012; Cho et al., 2013a; UNEP, 2013b; Won et al., 2013; Mackey et al., 2014). Article 11 of the Minamata Convention stipulates meanings of Hg wastes under the convention as follows: (1) wastes consisting Hg or Hg compounds; (2) wastes containing Hg or Hg compounds; or (3) wastes contaminated with Hg or Hg compounds (UNEP, 2013b). Industrial process residues (e.g., fly ash, bottom ash and sludge) generated from various Hg emission facilities, including waste incinerator, coal-fired power plant, and cement kiln, are typical Hg-contaminated wastes (Kosson et al., 2014; Xu et al., 2014). These residues must be treated using an appropriate treatment method before being disposed of in a landfill because of their high Hg contents and leaching values (Stark et al., 2006; Cho et al., 2013b). A calcium sodium phosphate (CNP) process was recently developed by our group to treat these different types of Hg-contaminated wastes using stabilization/solidification (S/S) technology

J.H. Cho et al. / Chemosphere 117 (2014) 374–381

(Cho, 2014). This approach is a novel immobilization method to transform potential hazardous waste into less hazardous materials (Chen et al., 2009; Zheng et al., 2010; Song et al., 2013). Hazardous Hg-contaminated ash treated using the CNP process had a Hg leaching value below the universal treatment standard (UTS) limit of 25 lg L1 and superior mechanical compressive strength (Cho, 2014). Therefore, Hg-contaminated ash was successfully stabilized and solidified using the CNP process at the laboratory scale. The CNP process is based on chemically bonded phosphate ceramic (CBPC) technology. More specifically, insoluble ceramic material is formed through the chemical reaction of calcium oxide (CaO) and sodium phosphate (Na2HPO4) (Cho, 2014). A coordinated network ceramic is fabricated via an acid–base reaction and sol–gel process between the metal oxide and the acid–phosphate solution. Moreover, the ceramic immobilizes and transforms the Hg-contaminated waste into thermodynamically stable material (Durate and Brandao, 2008; Donahue and Aro, 2010; Formosa et al., 2012; Manso et al., 2014; Viani and Gualtieri, 2014). In this study, a pilot-scale CNP S/S plant was manufactured, permitting larger scale experiments based on the lab-scale design. Hgcontaminated wastes generated from various industrial processes, including municipal, industrial, and medical waste incineration, wastewater treatment, and lime production, were stabilized and solidified using the pilot-scale CNP S/S plant. S/S samples were fabricated according to various operating conditions, such as the Hgcontaminated waste type, the stabilization reagent (Na2S) dose, and the waste loading ratio. Moreover, the Hg leaching value and compressive strength of each S/S sample were analyzed, and the differences in microstructure, mineralogy, and crystallinity were compared. 2. Materials and methods 2.1. Hg-contaminated wastes and preparation A total of 15 Hg-contaminated wastes, including fly ash and sludge, were collected from various industrial facilities in Korea. These facilities included three municipal waste incinerators (MWIs), three industrial waste incinerators (IWIs), three medical waste incinerators (MDWIs), three wastewater treatment facilities (WTFs), one steel production facility (SPF), one aluminum production facility (APF), and one lime production facility (LPF). Fly ash was obtained from dust-collecting devices that were installed in each facility, while dewatered sludge was collected from the wastewater treatment facilities. The Hg content and the toxic characteristic leaching procedure (TCLP) values of the Hg-contaminated wastes were determined according to the USEPA methods 7471B and 1311 using a cold vapor atomic absorption (CVAA)-type Hg analyzer (RA-915+/RP-91, Lumex, Russia) (US EPA, 1992, 1994, 2007). Based on the TCLP value of each waste sample, Hg-contaminated wastes with TCLP values exceeding 25 lg L1 were selected for further S/S investigation. In addition, chemical composition tests were performed on the selected Hg-contaminated waste samples using X-ray fluorescence spectrometry (XRF; S4 PIONEER, Bruker, Germany).

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the mixing reactor using ball valves and a screw conveyer. The desired amounts of reagents and waste were packed into each storage tank according to specific operating conditions, such as the Na2S dosage, the total volume of solidification binder, and the waste loading ratio. Additionally, each reagent and waste was sequentially injected according to the procedure determined in this study. The mixing unit consisted of a propeller-type agitator (5.5 kW), a cone-type stainless reactor (400 L), and a water injection system. Reagents and waste from the injection unit were mixed with a predefined amount of water. Thereafter, sol-type mixtures were combined to facilitate acid–base and hydration reactions. The mixing unit was designed to minimize dead volume within the reactor, to homogeneously blend the entire sol-type mixture and to form a coordinated network and monolithic ceramic. Because the workability time for mixing during the CNP process was determined to be approximately 30 min in a previous lab-scale study, the total mixing time for each mixture in all of the S/S experiments was adjusted to 30 min (Cho, 2014). In the extraction unit, mixed CNP binder and waste were discharged into the outlet of the reactor, and the final product was poured into cylinder molds with volumes of 50, 100, or 200 L. When the mixing time exceeded the maximum workability time for mixing, the viscosity of the mixture increased. Therefore, a considerable amount of the mixture could not be contained in the mold. While the gel-type mixture was solidified in the reactor, it was not easily separated from the reactor. Thus, the optimum coordinated ceramic could not be formed. A total of five Hg-contaminated wastes (four fly ash and one sludge) were treated using the pilot-scale CNP S/S plant. CaO (Samchun Pure Chemical, Korea) and Na2HPO4 (Samchun Pure Chemical, Korea) were used as a solidification binder, and Na2S (0.5 wt% of the solidification binder dosage) was introduced as a stabilization reagent to test the effects of sulfide on the TCLP values and compressive strengths of the S/S samples. In addition, S/S samples were fabricated according to the operating conditions (including the waste loading ratio (25, 50, or 100 wt% of solidification binder dosage) and total volume (3, 10, 50, or 100 L) of the mixture). Moreover, the performances (e.g., TCLP value and compressive strength) of each S/S sample were compared. 2.3. Hg content analysis The Hg content of the Hg-contaminated wastes was analyzed using a CVAA-type Hg analyzer, in accordance with USEPA method 7471B; three samples were tested in Hg content analysis of each waste (US EPA, 2007). The detection limit was 0.5 ng L1, and the measurement range was 100–10 000 ng L1. A calibration curve was constructed using Hg standard solutions with concentrations of 100–10 000 ng L1, which were prepared from a Hg standard reference material (3133, NIST, Gaithersburg, USA). The analysis of each Hg-contaminated waste was conducted with a certified reference material (industrial fly ash, No. 882-1, Swerea KIMAB AB, Stockholm, Sweden) to confirm the adequacy and the reliability of the measurements and the procedure. The Hg content of CRM tested in this study was always within the range of certified Hg concentrations (750 ± 140 lg kg1).

2.2. Pilot-scale CNP S/S plant 2.4. TCLP test A schematic diagram of the pilot-scale CNP S/S plant prepared specifically for this study is shown in Fig. 1. The plant consisted of three parts: an injection unit, a mixing unit, and an extraction unit. In the injection unit, the solidification binder (CaO and Na2 HPO4), the stabilization reagent (Na2S), and the waste, which were stored in 100 L cone-type stainless drums, were introduced into

The Hg leaching values of the Hg-contaminated wastes and S/S samples fabricated under various operating conditions were examined via the TCLP (US EPA method 1311) test using a CVAA-type Hg analyzer (US EPA, 1992; Song et al., 2013). First, preliminary tests were performed to determine the appropriate extraction fluid (fluid #1 or fluid #2) for each Hg-contaminated waste and S/S

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J.H. Cho et al. / Chemosphere 117 (2014) 374–381

Fig. 1. A schematic diagram of the pilot-scale CNP S/S plant.

sample. Extraction fluid #1 was prepared by adding 5.7 mL of glacial acetic acid and 64.3 mL of 1 N sodium hydroxide to 1 L of DI water. Extraction fluid #2 was composed of 5.7 mL of glacial acetic acid in 1 L of DI water. Then, 2 L of the selected extraction fluid and 100 g of the homogenized waste or the S/S sample were added to a high-density polyethylene leaching vessel. Each sample was agitated using a rotator for 18 h at 30 rpm and filtered using 0.7-lm certified glass fiber filters (acid-washed TCLP filters, Environmental Express, Charleston, USA) and a zero head-space extractor (ZHE+, Environmental Express, Charleston, USA). The TCLP value was determined by analyzing the Hg concentration in the filtrate using a CVAA-type Hg analyzer according to USEPA method 7470A (US EPA, 1994). The leaching ratio (RL) was calculated according to the determined Hg content and the TCLP value of each waste sample using the following equation (Cho et al., 2013b):

 RL ¼

   C TCLP  2 L W TCLP  100 ¼  100ð%Þ C sample  100 g W sample

25 ± 3 °C with a relative humidity of 50 ± 5%. All compressive strength tests were conducted after curing for 28 d.

2.6. Physical and chemical characterization The physical and chemical characteristics of the S/S samples were analyzed to determine the effect of different operating conditions on the S/S performance (i.e., TCLP value and compressive strength). Differences in the microstructure of each S/S sample were examined using a scanning electron microscope (SEM; JSM7001, JEOL, Japan). The X-ray patterns of the S/S samples were compared to identify the formation of CNP (CaNaPO4) in the final product. Moreover, the relationship between crystallinity and compressive strength was examined using an X-ray diffractometer (XRD; Miniflex, Rigaku, Japan).

ð1Þ

where RL is the ratio of the mass of Hg extracted during the TCLP test to the mass of Hg included in the waste, CTCLP is the TCLP value of the sample (lg L1), Csample is the Hg content of the sample (lg kg1), WTCLP is the mass of Hg in 2 L of leachate (lg), and Wsample is the mass of Hg in 100 g of the sample (lg). 2.5. Compressive strength Various S/S samples were fabricated under various operating conditions (Hg-contaminated wastes type, Na2S dosage, and waste loading ratio) using the pilot-scale CNP S/S plant. The compressive strength of each S/S sample was measured using a universal testing machine (WJ-100S, Woojin, Korea) according to ISO 679. Specimens were prepared using prism molds of 40 mm  40 mm  160 mm, and each S/S sample was cured at a temperature of

3. Results and discussion 3.1. Characterization of Hg-contaminated wastes The Hg contents, TCLP values, and RL values of the Hg-contaminated wastes samples collected from various industrial processes are listed in Table 1. The Hg contents of all of the analyzed wastes were below the USEPA Hg criterion of 260 mg kg1, which is outlined in the USEPA Land Disposal Restrictions. However, the Hg contents exhibited large variation according to the waste and facility types (Randall and Chattopadhyay, 2010). The fly ash collected from MWI #3, MDWI #2, MDWI #3, and LPF #1 exhibited significantly higher Hg contents than the fly ash from MWI #2, IWI #1, and IWI #3. However, the measurements listed in Table 1 do not reflect the full variability of all of the waste generated from each facility due to variations in their characteristics (e.g., Hg content, TCLP value and Hg species) that result from changes in the operat-

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J.H. Cho et al. / Chemosphere 117 (2014) 374–381 Table 1 Hg content, TCLP value, and leaching ratio (RL) of Hg-contaminated wastes. Process

No.

Waste type

Hg content (lg kg1)a

Municipal waste incinerator

1 2 3

Fly ash Fly ash Fly ash

2053 ± 292 585 ± 41 12 792 ± 371

63.1 ± 0.3 6.9 ± 0.2 0.5 ± 0.1

61.4 23.4 0.1

Industrial waste incinerator

1 2 3

Fly ash Fly ash Fly ash

238 ± 14 6134 ± 355 369 ± 29

1.5 ± 0.1 316.0 ± 2.6 3.7 ± 0.1

12.9 103.1 19.9

Medical waste incinerator

1 2 3

Fly ash Fly ash Fly ash

4382 ± 324 32 216 ± 1664 11 765 ± 388

221.0 ± 8.1 5.7 ± 0.5 1.5 ± 0.1

100.8 0.4 0.3

Wastewater treatment facility

1 2 3 1 1 1

Sludge Sludge Sludge Fly ash Sludge Sludge

645 ± 27 481 ± 33 2451 ± 124 68 682 ± 2554 1874 ± 69 58 ± 5

0.2 ± 0.1 1.4 ± 0.1 58.7 ± 0.6 206.6 ± 5.3 1.4 ± 0.2 1.2 ± 0.1

0.4 5.9 47.9 6.0 1.5 41.2

Lime production facility Steel production facility Aluminum production facility a

TCLP value (lg L1)a

RL (%)

Mean ± SD.

ing conditions for each process and the time at which the waste was collected (Tunsu et al., 2014). Among the samples that were analyzed, only three (the fly ash from IWI #2, MDWI #1, and LPF #1) exhibited TCLP values higher than the USEPA limit of 200 lg L1 (Randall and Chattopadhyay, 2004). The TCLP values of two wastes (i.e., fly ash from MWI #1 and sludge from WTF #3) exhibited values that exceeded the UTS limit of 25 lg L1 (Liu et al., 2008). The RL value for each waste sample was calculated based on its Hg content and its TCLP value; four wastes samples (i.e., fly ash from MWI #1, IWI #2, and MDWI #1 and sludge from WTF #3) had RL values higher that exceeded 40%. In this study, 5 waste samples were selected from the Hg-contaminated wastes based on high Hg contents, TCLP values, and RL values as targets for S/S experiments using the pilot-scale CNP plant. These selected samples included four fly ashes (from MWI #1, IWI #2, MDWI #1, and LPF #1) and one sludge (from WTF #3); the performance of each S/S sample was compared. The chemical compositions of the selected waste samples were determined using XRF; the results are listed in Table 2. Na2O, Cl, and CaO are compounds that are commonly found in fly ash collected from waste incinerators; however, the major components of fly ash from LPF #1 were CaO and SiO2. Meanwhile, sludge generated from WTF #3 mainly consisted of Fe2O3, P2O5, and ZnO. Moreover, the percentage of hematite (Fe2O3) in the sludge (47.4%) was much higher than in the other waste samples (

solidification of various mercury-contaminated wastes.

A pilot-scale calcium sodium phosphate (CNP) plant was designed and manufactured to examine the performance of recently developed stabilization/solidi...
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