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Stabilization of arsenic sulfide sludge by hydrothermal treatment
Hydrometallurgy 191 (2020) 105229
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Stabilization of arsenic sulfide sludge by hydrothermal treatment a
Hui Xu , Xiaobo Min a b
a,b
a,b,⁎
, Yunyan Wang
, Yong Ke
a,b,⁎
a
a
, Liwei Yao , Degang Liu , Liyuan Chai
T a,b
School of Metallurgy and Environment, Central South University, Changsha 410083, China Chinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, Changsha 410083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Arsenic sulfide sludge Stabilization Hydrothermal treatment Arsenic
Arsenic sulfide sludge (ASS) generated during the treatment of strongly acidic wastewater with sulfide precipitation method has long been a problem for the nonferrous smelting enterprises. In this study, stabilization of ASS from a lead and zinc smelter by hydrothermal treatment was investigated. The ASS was characterized with high contents of arsenic (46.9%) and sulfur (32.6%), and leaching concentration of arsenic was determined to be 702.0 mg/L. The parameters of the hydrothermal treatment were studied in detail. It was found that the leaching concentrations of As, Pb and Cd, dehydration ratio, density and morphology of ASS changed dramatically during the hydrothermal treatment. Comprehensive consideration of these changes, the optimal conditions were determined to be temperature of 200 °C, reaction time of 4 h, liquid-to-solid (L/S) ratio of 1:1 and the initial pH of 2. Under the optimal conditions, the leaching concentration of As for the treated ASS decreased to < 5 mg/L. The yellow muddy ASS changed into a big bulk with metallic luster. The density of the ASS increased from 1.20 g/cm3 to 2.29 g/cm3, the moisture content decreased from 62.59% to 6.50%, and the volume reduction ratio reached 91.67%. After the hydrothermal treatment, the speciation transformation of As, Pb and Cd occurred. Some of the As, Pb and Cd in ASS transformed into a more stable speciation after the hydrothermal treatment. As a result, the stabilization of As, Pb and Cd attributed to both of the microstructure transformation of ASS and the speciation transformation of As, Pb and Cd.
1. Introduction Arsenic is an abundant element in the earth's crust and is found in > 300 minerals (Drahota and Filippi, 2009) generally in combination with sulfur and metals. Arsenic that occurs in base metal ores and/or concentrates enters into the metallurgical process accordingly and then is released to the environments (Ke et al., 2017; Min et al., 2019b; Wang et al., 2019; Yang et al., 2015). Arsenic is highly toxic and adversely affects human health. Therefore, arsenic has become a worldwide environmental concern especially in metal industry (Chai et al., 2018; Fei et al., 2017). Most of the arsenic is volatilized and oxidized to As2O3 and As4O6 during the smelting of non-ferrous metals-bearing concentrates and then emitted with flue gas. Prior to the sulfuric acid production, arsenic compounds in flue gas must be eliminated because the arsenic influences the quality of the produced sulfuric acid. Hence, spray washing with the diluted acid is performed to remove the dust and arsenic from the pre-cleaned gas. Hence, strongly acidic wastewater with a high arsenic concentration is generated. Presently, sulfide precipitation processes with Na2S, NaHS and H2S have been developed and widely adopted to eliminate arsenic from the acidic wastewater due to its high ⁎
efficiency and ease of operation (Kong et al., 2017a; Opio, 2013). Therefore, a large amount of arsenic sulfide sludge (ASS) with great risk is generated. Its safety disposal is of great importance for non-ferrous smelters. To prevent arsenic pollution, numerous methods including vitrification, extraction, and stabilization/solidification (S/S) have been developed to treat arsenic-containing hazardous waste (Chai et al., 2016; Lei et al., 2017; Li et al., 2019; Li et al., 2016; Liang et al., 2017; Min et al., 2019a; Min et al., 2017; Peng et al., 2016; Zhao et al., 2016). Up to now, S/S with binders, such as cement, pozzolanic matter and geopolymer, is the most widely used method for the disposal of arsenic and potentially toxic metals-bearing waste (Bose and Sharma, 2002; Desogus et al., 2013; Jang and Kim, 2000; Singh and Pant, 2006; Van Jaarsveld et al., 1997; Van Jaarsveld et al., 1999). However, it will result in a large compatibilization ratio and high cost when ASS is treated with cement. For example, Long et al. (2014) reported the S/S of ASS with oxidant, polyferric sulfate (PFS), polyacrylamide (PAM), lime and cements. Although this method achieved the desired results, a lot of additives were added, which leads to a higher cost of landfill disposal subsequently. The hydrothermal treatment has now been widely used to prepare
Corresponding authors at: School of Metallurgy and Environment, Central South University, Changsha 410083, China. E-mail addresses: [email protected] (Y. Wang), [email protected] (Y. Ke).
https://doi.org/10.1016/j.hydromet.2019.105229 Received 13 August 2019; Received in revised form 20 November 2019; Accepted 13 December 2019 Available online 17 December 2019 0304-386X/ © 2019 Published by Elsevier B.V.
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Intensity (a.u.)
crystals with special performance (Byrappa and Adschiri, 2007; Byrappa and Yoshimura, 2013). Some studies showed that it can effectively achieve the S/S of hazardous components in hazardous waste. For instance, hydrothermal precipitation of arsenical natroalunite has been proposed to stabilize calcium arsenate waste from a copper smelter, which skipped the production of the intermediate arsenical gypsum because it was transformed into arsenic-free anhydrite (Viñals et al., 2010). Hydrothermal treatment with microwave heating is also a feasible approach for the solidification of potentially toxic metals in fly ash (Qiu et al., 2016). Moreover, most importantly, hydrothermal treatment can simultaneously realize dehydration and volume reduction of waste compared to the above-described S/S method (Nazari et al., 2017). Our previous works preliminary indicate that the hydrothermal treatment had obvious effects on the detoxication, dehydration and volume reduction for several types of ASS (Yao et al., 2019; Yao et al., 2018), but the specific parameters of hydrothermal process and its effects are not studied systematically. The main purpose of this work is to optimize the hydrothermal stabilization process of the ASS generated from the sulfide precipitation of high arsenic wastewater with H2S. To explore the transformation of the ASS during the hydrothermal stabilization process, the changes in morphology, dehydration ratio, density and the leaching concentrations and speciation of As, Pb and Cd had been investigated in detail.
10
20
30
40
50
60
70
80
2 theta (degree) Fig. 1. X-ray diffraction pattern of the ASS.
exposed to the natural environment directly. Combining with the elemental composition, it is concluded that the ASS can be identified as the hazardous waste, which should be solidified or stabilized before landfill.
2. Materials and experimental methods
2.2. Experimental procedure
2.1. Materials
Five grams of the original collected ASS and the deionized water with an appropriate liquid-to-solid (L/S) ratio were loaded into a stainless steel autoclave with a capacity of 25 mL. Then it was placed into the oven at a given temperature in range of 160–210 °C for 0–10 h. Thereafter, the autoclave was cooled to the room temperature naturally. The as-treated ASS was collected and washed with the deionized water to remove the remained ions. Finally, it was dried in a vacuum oven at 60 °C for 12 h.
The ASS used in the experiments was sampled from a lead and zinc smelter of Hunan province, China. The original ASS collected was yellow mud with moisture of 62.59% and density of 1.20 g/cm3. In order to determine the elemental composition of ASS, the ASS was dried, ground, sieved and digested. The elemental composition of the ASS determined by ICP-OES is presented in Table 1. The main elements in the ASS were arsenic and sulfur with the content of 46.90% and 32.60%, respectively. The atomic molar ratio of As and S was about 1:1.63, which is close to the molar ratio of As and S in As2S3. The higher S content indicated that the ASS might be composed of As2S3 and elemental S0 which was generated by the oxidation of S(-II) to S(0) during the sulfide precipitation process (Kong et al., 2018; Peng et al., 2018; Rochette et al., 2000). Additionally, the content of Cd was 3.73%, and small amount of Zn, Fe, Pb, Cr and Cu were also found in the ASS. Wherein, elements of As, Cd, Pb and Cr were toxic components which must be prevented from releasing into the environment. The XRD pattern of ASS presented in Fig. 1 indicates that the ASS had a poor crystallinity without any sharp diffraction peaks. According to Kong (Kong et al., 2017b), the broad humps at 2θ around 18o, 32o and 56o were corresponded to the amorphous As2S3. Fig. 2 shows the SEM-EDS images of the ASS. The ASS was composed of the extreme fines of AseS compounds, and the particles were irregular and flocculent with particle size of 2–5 μm. The concentrations of arsenic and potentially toxic metals in the leachate of the ASS after the toxicity characteristic leaching procedure (TCLP) were presented in Table 2. As seen from Table 2, the leaching concentrations of Zn (52.2 mg/L) and Cr (0.3 mg/L) were below the allowable limits set by United States Environmental Protection Agency (USEPA). The leaching concentrations of As, Cd and Pb were 702.0 mg/ L, 15.1 mg/L and 6.5 mg/L, respectively, which exceeded the limits. It implies that the ASS will result in severe environmental pollution if
2.3. Analysis methods 2.3.1. Determination of elemental composition To determine elemental composition content in the ASS and the treated ASS, the samples were dried at 60 °C in a vacuum oven. Then, they were ground and sieved for obtaining the samples with particle size of 100–200 mesh. Therewith, the samples were digested using a mixture of concentrated HCl and HNO3 (3:1, v/v). The solutions were filtrated with filter paper (1 μm pore), and then element concentrations in filtrate were analyzed with inductively coupled plasma optical emission spectrometer (ICP-OES, Agilent 5100). 2.3.2. TCLP test and BCR sequential extraction To evaluate the stability of the ASS and the treated ASS, the TCLP test was performed according to USEPA Method 1311 (USEPA, 1992). A BCR sequential extraction procedure was employed to assess the environmental activity and potential mobility of As, Pb and Cd in the ASS and the treated ASS (Davidson et al., 1998; Kazi et al., 2005). The concentrations of As and potentially toxic metals in the leachate obtained during the TCLP test and BCR sequential extraction were analyzed by ICP-OES.
Elements
As
S
Cd
Zn
Fe
Pb
Cr
Cu
2.3.3. Determination of volume reduction ratio, apparent density and dehydration ratio The volume of samples before and after hydrothermal treatment was measured by the method of water displacement (King II, 1993). The volume reduction ratio can then be calculated according to Eq. (1).
Wt%
46.90
32.60
3.73
0.86
0.50
0.14
0.05
0.01
Volume reduction ratio =
Table 1 The main elemental composition of the ASS.
2
V − V′ × 100% V
(1)
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Fig. 2. SEM-EDS images of the ASS. Table 2 Concentrations of arsenic and potentially toxic metals in leachate of the ASS after TCLP test. Threshold
The ASS
As Cd Pb Cr Zn
≤5 ≤1 ≤5 ≤5
702.0 15.1 6.5 0.3 52.2
Not specified by USEPA.
where V (mL) is the raw volume of the ASS; V′ (mL) is the volume of the treated ASS. Based on the Archimedes law, the apparent density of the ASS and the treated ASS were determined and calculated as:
60 50 40 30 20 10 0
160
170
180
210
200
190
Temperature (°C)
m0 ρw ρ= m2 − m1
(2)
Fig. 3. Effect of the temperature on the leaching concentrations of As, Pb and Cd for the treated ASS (4 h, L/S ratio of 1:1, initial pH of 2).
where m0 (mg) is the weight of sample in air; m1 (mg) is the weight of the beaker with the distilled water; m2 (mg) is the weight of the beaker with distilled water and sample; ρw (mg/cm3) is the density of water. Moisture content was measured by drying the samples at 60 °C under vacuum oven to a constant weight ( ± 0.01 g). Then, the dehydration ratio was calculated according to Eq. (3).
M × φ − M ′ × φ′ × 100% M×φ
3.2
100
Density Dehydration ratio
Density (g/cm3)
Dehydration ratio =
As Pb Cd
(3)
where M (g) is the weight of the ASS; φ (%) is the moisture content of the ASS; M' (g) is the weight of the treated ASS; φ' (%) is the moisture content of the treated ASS. 2.3.4. Others The phases of the ASS was characterized by X-ray diffraction (XRD: D/max 2500 VB+) with a scanning speed of 10° min−1 from 10° to 80°. The microstructural observation and qualitative elements analysis of the ASS and the treated ASS were performed by scanning electron microscopy equipped with energy dispersive X-ray spectrometry (SEMEDX: Nano SEM 230).
2.8
80
2.4
60
2.0
40
1.6
20
1.2
Dehydration ratio (%)
a
a
Leaching concentration (mg/L)
Elements
70
0
160
170
180
190
200
210
Temperature (°C) Fig. 4. Density and dehydration ratio of the treated ASS with different temperatures (4 h, L/S ratio of 1:1, initial pH of 2).
3. Results and discussion only the leaching concentrations of them exceeded the limits (Table 2). As shown in Fig. 3, the temperature affected the leaching concentrations of As, Pb and Cd significantly. As a whole, the leaching concentrations decreased with the improvement of temperature. When the temperature varied from 160 °C to 190 °C, the reduction in leaching concentration of As is remarkable which decreased from 65.19 mg/L to
3.1. Effect of each parameter on the stabilization of ASS 3.1.1. Temperature The leaching concentrations of As, Pb and Cd and the density for the treated ASS obtained at different temperatures are presented in Fig. 3 and Fig. 4, respectively. The As, Pb and Cd should be focused because 3
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Fig. 5. SEM images of the treated ASS at different temperatures: (a) 160 °C, (b) 170 °C, (c) 180 °C, (d) 190 °C, (e) 200 °C, (f) 210 °C (4 h, L/S ratio of 1:1, initial pH of 2).
200
Density (g/cm3)
Leaching concentration (mg/L)
As Pb Cd
100
20 10
100
Density Dehydration ratio
2.8
80
2.4
60
2.0
40
1.6
20
0 0
2
4
6
8
10
1.2
Time (h)
0.5
1
1.5
2
3
4
5
6
8
Dehydration ratio (%)
3.2
300
0 10
Time (h)
Fig. 6. Effect of the reaction time on the leaching concentrations of As, Pb and Cd for the treated ASS (200 °C, L/S ratio of 1:1, initial pH of 2).
Fig. 7. Density and dehydration ratio of the treated ASS with different reaction time (200 °C, L/S ratio of 1:1, initial pH of 2).
5.34 mg/L. With the further increase of temperature from 190 °C to 210 °C, leaching concentration of As can be reduced to 0.67 mg/L. For Pb and Cd, the leaching concentration was below the allowable limits as soon as temperature reached 170 °C. The above phenomena illustrate that temperature plays a vital role in hydrothermal process. The higher the temperature, the better the stabilization effect of ASS. It can be seen from Fig. 4 that the density of the treated ASS and the dehydration ratio increased almost linearly with the temperature rising from 160 °C to 210 °C. These results indicated that densification and dehydration occurred during the hydrothermal treatment of the ASS. Fig. 5 shows the surface characteristics of the treated ASS obtained at different temperatures. Compared to Fig. 2, the microstructure of the ASS was completely different after hydrothermal process. The treated ASS exhibited a loose network-like microstructure after it was hydrothermal treated at 160 °C. Moreover, with the increase of hydrothermal
temperature, the network became dense. The network bonded and fused together when the temperature increased from 160 °C to 190 °C. With the temperature varying from 190 °C to 210 °C, the network almost disappeared (Fig. 5d~f). They converted into a bigger bulk, which explained why the density of ASS increased and which may be the reason for the decrease in leaching concentrations of As, Pb and Cd during the hydrothermal treatment. In summary, increasing the temperature promotes the transformation of the microstructure and the dehydration of the ASS. During the hydrothermal treatment, the water was removed from the ASS, and the microstructure of ASS changed from flocculent to reticulate and finally to bulk, which resulted in the increase in the density of ASS. With the changes in microstructure and density, the leaching concentrations of As, Pb and Cd for the treated ASS also reduced gradually. When the 4
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Fig. 8. SEM images of the treated ASS for different reaction time: (a) 0 h, (b) 1 h, (c) 2 h, (d) 3 h, (e) 4 h, (f) 10 h (200 °C, L/S ratio of 1:1, initial pH of 2).
70
Leaching concentration (mg/L)
Leaching concentration (mg/L)
5
As Pb Cd
4 3 2 1 0
0:1 0.5:1 1:1 1.5:1 2:1 2.5:1 3:1
As Pb Cd
60 50 40 30 20 10 0 0
4:1 5:1
1
2
3
4
5
6
7
8
Initial pH
L/S ratio (mL/g)
Fig. 10. Effect of the initial pH on the leaching concentration of As, Pb and Cd for the treated ASS (200 °C, 4 h, L/S ratio of 1:1).
Fig. 9. Effect of the L/S ratio on the leaching concentrations of As, Pb and Cd for the treated ASS (200 °C, 4 h, initial pH of 2).
2.5 h. Further extension of the reaction time, however, the leaching concentration of As hardly changed. Besides, when the reaction time reached 2 h, leaching concentrations of Pb and Cd decreased to 0.02 mg/L and 0.19 mg/L, respectively, which were much lower than the allowable limits set by USEPA. Thereafter, the leaching concentrations of Pb and Cd remained stable regardless of the change of reaction time. Fig. 7 shows the density and dehydration ratio of the treated ASS with different reaction time. It was evident that reaction time had dramatically impact on the density of ASS and dehydration ratio. According to our tests, the original ASS had a low density of 1.2 g/cm3 and a high moisture content of 62.59%. After 4 h of hydrothermal treatment, the density of the ASS increased to 2.29 g/cm3, which was almost twice of the density of the original ASS. As the reaction time continued to increase, the density of ASS kept constant. Correspondingly, when the reaction time was < 4 h, the dehydration ratio also increased
temperature varied from 200 °C to 210 °C, the decrease of the leaching concentrations of As, Pb and Cd was not obvious (Fig. 3), and the microstructure and density changed slightly. Therefore, 200 °C was selected as the optimum temperature. 3.1.2. Reaction time The effect of reaction time on leaching concentrations of As, Pb and Cd for the treated ASS is investigated under the conditions of temperature of 200 °C, reaction time of 0-10 h, L/S ratio of 1:1 and initial pH of 2. Fig. 6 presents the variation of leaching concentrations for As, Pb and Cd with the reaction time. The curves showed a fast decrease on leaching concentration of As in initial 2 h. When the reaction time was longer than 2 h, the leaching concentration of As decreased slowly until it was stable. The leaching concentration of As decreased from 702.0 mg/L to 1.41 mg/L with the reaction time varying from 0 to 5
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Fig. 11. SEM images of the treated ASS for different initial pH: (a) pH = 1, (b) pH = 2, (c) pH = 3.5, (d) pH = 4.5, (e) pH = 6.5, (f) pH = 8 (200 °C, 4 h, L/S ratio of 1:1).
Fig. 12. The comparison of the ASS before and after the hydrothermal treatment.
with a smooth surface at 4 h. The treated ASS in Fig. 8e is denser than that in Fig. 8d, which would improve the structural stability of ASS and inhibit the release of As, Cd and Pb. With the reaction time prolonging to 10 h, the microstructure changes of the treated ASS were not significant (Fig. 8f). In addition, there were some spherular pits on the fracture surface of the bulk, which might be caused by the gas of H2S generated by sulfur decomposition. Because the amorphous and
gradually, whereas the dehydration ratio remained unchanged when the reaction time exceeded 4 h. Fig. 8 shows the SEM images of the treated ASS obtained at different reaction time. It was clearly seen that there were tremendous differences in the morphology of the ASS with different reaction time. After being hydrothermal treated for 2 h, the flocculent particles in ASS began to bond together. Then they gradually fused into a bigger bulk 6
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The ASS As
95.93% Oxidizable fraction
The treated ASS 76.40% Oxidizable fraction
1.52% Residul fraction
0.03% Acid soluble fraction
2.49% Acid soluble fraction
0.04% Reducible fraction
0.06% Reducible fraction
Pb
18.31% Oxidizable fraction
75.63% Residual fraction
8.35% Oxidizable fraction
1.93% Reducible fraction
4.48% Reducible fraction
25.88% Oxidizable fraction
0.37% Residual fraction
89.72% Residual fraction 0% Acid soluble fraction
1.58% Acid soluble fraction
Cd
23.53% Residual fraction
87.07% Oxidizable fraction
12.86% Residual fraction 0.01% Acid soluble fraction
71.44% Acid soluble fraction 2.31% Reducible fraction
0.06% Reducible fraction
Fig. 13. The speciation of As, Pb and Cd for the ASS and the treated ASS.
flocculent ASS was converted into a big bulk, the water content and volume of ASS reduced dramatically. This phenomenon was also found in the hydrothermal treatment of coal (Zhang et al., 2016). In summary, the hydrothermal treatment is a novel and effective technique for densification, volume reduction, dehydration of the ASS, and especially for the stabilization of As, Pb and Cr. According to the results above, the optimum reaction time was considered to be 4 h.
the treated ASS increased slightly with the pH rising from 0.5 to 3, and then increased rapidly to 64.79 mg/L when the pH increased to 8. The higher pH value, the higher leaching concentration of As for the treated ASS, which agrees with the fact that arsenic sulfide is soluble in alkali solution (Darban et al., 2011). The lowest leaching concentration of As is 0.54 mg/L at the initial pH of 0.5, implying that strong acidic conditions facilitates the hydrothermal stabilization process. The leaching concentrations of Pb and Cd kept below the allowable limits in the entire pH range studied. Because the pH of the original ASS was about 2, it does not need to add any chemical agent to adjust the pH. In order to explain the reason of the increase in leaching concentration of As when the initial pH exceeded 3, the surface characteristics of the treated ASS obtained at different initial pH were characterized by SEM. It can be seen from Fig. 11 that the treated ASS existed as bigger bulk with a smooth surface compared to the original ASS of flocculent particles at the initial pH of 1 and 2. With the increase of initial pH to 3.5, the treated ASS seems to dissolve, the surface of ASS became rough. A large number of fine particles are clearly visible on the surface of bulk particles when the initial pH prolonged to 4.5, 6.5 and 8 (Fig. 11d~f). The results explained why the leaching concentration of As increased as shown in Fig. 10 after initial pH > 3.
3.1.3. Liquid-to-solid ratio The liquid-to-solid (L/S) ratio affects the hydrothermal reaction via influencing the degree of liquid-solid mixing and pressure. Thus, the effect of the L/S ratio adjusted by changing the dosage of the deionized water on the stabilization effect of ASS was investigated. Fig. 9 shows that the leaching concentrations of As, Pb and Cd decreased with the increase of L/S ratio. For leaching concentration of As, it decreased rapidly from 4.28 mg/L to 1.70 mg/L when the L/S ratio changed from 0:1 to 1:1.Then it decreased further to 0.83 mg/L when the L/S ratio increased to 2.5:1. As the L/S ratio varying from 2.5:1 to 5:1, leaching concentration of As fluctuated slightly. As for Pb and Cd, the leaching concentrations were less affected by L/S ratio, which always remained lower than the allowable limits. In this study, L/S ratio of 1:1 was chosen for the subsequent experiments.
3.1.5. Comparison of the ASS before and after the hydrothermal treatment According to the results above, the optimum conditions of hydrothermal treatment were determined to be temperature of 200 °C, reaction time of 4 h, L/S ratio of 1:1 and initial pH of 2. Fig. 12 shows the comparison of the ASS before and after the hydrothermal treatment. It
3.1.4. Initial pH The pH is one of the key parameters for the hydrothermal treatment. In this study, it was adjusted from 0.5 to 8 with the dilute HCl and NaOH solution. Fig. 10 shows that the leaching concentration of As for 7
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H. Xu, et al.
China (2018YFC1903301, 2018YFC1900301), National Science Fund for Distinguished Young Scholars (51825403), the National Natural Science Foundation of China (51634010 and 51904354).
could be seen that there were obvious differences of the morphology and physicochemical properties for the ASS before and after the hydrothermal treatment. The leaching concentrations of As, Pb and Cd for the treated ASS reduced to 0.67 mg/L, 0.04 mg/L and 0.10 mg/L, respectively, all of which were far below the allowable limits. From the appearance, the yellow muddy ASS changed into a big bulk with metallic luster. Under the SEM view, the flocculent amorphous ASS was seen to fuse together and solidified into bulk substance with smooth surface after the hydrothermal treatment. Therefore, it is assumed that the low leaching concentrations of As, Pb and Cd may be attributed to its specific morphological and microstructural features. Besides, the density of the ASS increased from 1.20 g/cm3 to 2.29 g/cm3, the moisture content decreased from 62.59% to 6.50%, and the volume reduction ratio reached 91.67%, indicating that the hydrothermal treatment also shows excellent ability in dehydration and volume reduction.
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Liang, Y., Min, X., Chai, L., Wang, M., Liyang, W., Pan, Q., Okido, M., 2017. Stabilization of arsenic sludge with mechanochemically modified zero valent iron. Chemosphere 168, 1142–1151. Long, D., Jia, J., He, T., Li, Y., 2014. Stabilization /solidification of arsenic sulfide residue and effect evaluation. Environ. Protection & Technol. 20 (3), 7–11. Min, X., Li, Y., Ke, Y., Shi, M., Chai, L., Xue, K., 2017. Fe-FeS2 adsorbent prepared with iron powder and pyrite by facile ball milling and its application for arsenic removal. Water Sci. Technol. 76 (1), 192–200. Min, X.-B., Liu, D.-G., Chai, L.-Y., Ke, Y., Liang, Y.-J., Shi, M.-Q., Li, Y.-C., Tang, C.-J., Wang, Y.-Y., Wang, Z.-B., 2019a. Comparison of arsenic immobilization properties among calcium silicate hydrate, ettringite, and friedel’s salt in a slag-based binder. Environ. Prog. Sustain. Energy 38 (s1), S422–S488. Min, X.-b., Peng, T.-y., Li, Y.-w.-j., Ke, Y., Liang, Y.-j., He, X.-y., 2019b. Stabilization of ferric arsenate sludge with mechanochemically prepared FeS2/Fe composites. T Nonferr Metal Soc 29 (9), 1983–1992. Nazari, A.M., Radzinski, R., Ghahreman, A., 2017. Review of arsenic metallurgy:
3.2. Speciation variation of As, Pb and Cd In order to further analyze the reasons for the stability of As, Pb and Cd in ASS, we used BCR method to analyze the speciation of As, Pb and Cd. Based on BCR method, the speciation of As, Pb and Cd are divided into the acid soluble fraction, reducible fraction, oxidizable fraction and residual fraction (Xie et al., 2013). Fig. 13 shows that As in original ASS existed mainly in the speciation of oxidizable state with a percentage of 95.93%. After hydrothermal treatment, although some of As was converted into residual fraction, the oxidizable fraction still occupied 76.40%. Combined with the microstructure transformation of ASS, it can be inferred that the microstructure transformation of ASS is the main reason for the stabilization of As, while the change in As speciation is the minor reason. As for Pb, it mainly existed as the residual state in both ASS and the treated ASS, which explains why the leaching concentration of Pb was low as shown in Fig. 3, Fig. 6, Fig. 9 and Fig. 10. Besides, the Pb in residual state increased somewhat, suggesting that the speciation transformation also contributes to the stabilization of Pb. For Cd, acid soluble Cd was the main speciation with percentage of 71.44% for ASS. After hydrothermal treatment, the main speciation of Cd changed into oxidizable state (that is, CdS) with a percentage of 87.07%, indicating that both of speciation changes and microstructure transformation of ASS are the reasons for the stabilization of Cd. 4. Conclusions Hydrothermal treatment can achieve the stabilization of the arsenic sulfide sludge effectively and economically. Of all factors, temperature had the greatest effect on the stabilization of the ASS. The optimal conditions were temperature of 200 °C, reaction time of 4 h, liquid-solid ratio of 1:1, the initial pH value of 2. Under these conditions, arsenic leaching concentration decreased from 702.0 mg/L to 0.67 mg/L after hydrothermal treatment, and the concentrations of Pb and Cd in leachate were 0.04 and 0.1 mg/L, all of which were less than the allowable limits set by USEPA. In hydrothermal treatment process, the flocculent amorphous ASS fused together and solidified into bulk with smooth surface, which may result in the decrease of arsenic leaching concentration. Besides, the density increased from 1.20 g/cm3 to 2.29 g/ cm3, the moisture decreased from 62.59% to 6.50%, and the volume reduction ratio was up to 91.67%. These results indicate that the dehydration and volume reduction of the ASS had realized. In addition, the speciation transformation also contributed to the stabilization of the As, Pb and Cd in ASS. After hydrothermal treatment, As, Pb and Cd in residual state increased. Acknowledgements This research was supported by the National Key R&D Program of 8
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characteristics. Miner. Eng. 12 (1), 75–91. Viñals, J., Sunyer, A., Molera, P., Cruells, M., Llorca, N., 2010. Arsenic stabilization of calcium arsenate waste by hydrothermal precipitation of arsenical natroalunite. Hydrometallurgy 104 (2), 247–259. Wang, Z., Zhao, Z., Zhang, L., Liu, F., Peng, B., Chai, L., Liu, D., Liu, D., Wang, T., Liu, H., Liang, Y., 2019. Formation mechanism of zinc-doped fayalite (Fe2-x,ZnxSiO4) slag during check copper smelting. J. Hazard. Mater. 364, 488–498. Xie, X.D., Min, X.B., Chai, L.Y., Tang, C.J., Liang, Y.J., Li, M., Ke, Y., Chen, J., Wang, Y., 2013. Quantitative evaluation of environmental risks of flotation tailings from hydrothermal sulfidation-flotation process. Environ. Sci. Pollut. Res. 20 (9), 6050–6058. Yang, Z., Liu, L., Chai, L., Liao, Y., Yao, W., Xiao, R., 2015. Arsenic immobilization in the contaminated soil using poorly crystalline Fe-oxyhydroxy sulfate. Environ. Sci. Pollut. R. 22 (16), 12624–12632. Yao, L., Min, X., Ke, Y., Wang, Y., Liang, Y., Yan, X., Xu, H., Fei, J., Li, Y., Liu, D., Yang, K., 2019. Release behaviors of arsenic and heavy metals from arsenic sulfide sludge during simulated storage. Minerals 9 (2), 130–145. Yao, L., Min, X., Xu, H., Ke, Y., Liang, Y., Yang, K., 2018. Hydrothermal treatment of arsenic sulfide residues from arsenic-bearing acid wastewater. Int. J. Env. Res. Pub. He. 15 (9), 1863–1877. Zhang, Y., Wu, J., Wang, Y., Miao, Z., Si, C., Shang, X., Zhang, N., 2016. Effect of hydrothermal dewatering on the physico-chemical structure and surface properties of Shengli lignite. Fuel 164, 128–133. Zhao, Z., Song, Y., Min, X., Liang, Y., Chai, L., Shi, M., 2016. XPS and FTIR studies of sodium arsenate vitrification by cullet. J. Non-Cryst. Solids 452, 238–244.
treatment of arsenical minerals and the immobilization of arsenic. Hydrometallurgy 174, 258–281. Opio, F., 2013. Investigation of Fe(III)-As(III) Bearing Phases and their Potential for Arsenic Disposal. ProQuest Dissertations Publishing, Queen’s University (Canada). Peng, B., Song, T., Wang, T., Chai, L., Yang, W., Li, X., Li, C., Wang, H., 2016. Facile synthesis of Fe3O4@Cu(OH)2 composites and their arsenic adsorption application. Chem. Eng. J. 299, 15–22. Peng, X., Chen, J., Kong, L., Hu, X., 2018. Removal of arsenic from strongly acidic wastewater using phosphorus Pentasulfide as precipitant: UV-light promoted Sulfuration reaction and particle aggregation. Environ. Sci. Technol. 52 (8), 4794–4801. Qiu, Q., Jiang, X., Lv, G., Lu, S., Ni, M., 2016. Stabilization of heavy metals in municipal solid waste incineration Fly ash in circulating fluidized bed by microwave-assisted hydrothermal treatment with additives. Energy Fuels 30 (9), 7588–7595. Rochette, E.A., Bostick, B.C., Li, G., Fendorf, S., 2000. Kinetics of arsenate reduction by dissolved sulfide. Environ. Sci. Technol. 34 (22), 4714–4720. Singh, T.S., Pant, K.K., 2006. Solidification/stabilization of arsenic containing solid wastes using Portland cement, fly ash and polymeric materials. J. Hazard. Mater. 131 (1–3), 29–36. USEPA, 1992. Toxicity Characteristics Leaching Procedure, Method 1311. (Test Methods for the Evaluation of Solid Waste). Van Jaarsveld, J.G.S., Van Deventer, J.S.J., Lorenzen, L., 1997. The potential use of geopolymeric materials to immobilise toxic metals: part I. theory and applications. Miner. Eng. 10 (7), 659–669. Van Jaarsveld, J.G.S., Van Deventer, J.S.J., Schwartzman, A., 1999. The potential use of geopolymeric materials to immobilise toxic metals: part II. Material and leaching
9
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Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
Stabilization of arsenic sulfide sludge by hydrothermal treatment a
Hui Xu , Xiaobo Min a b
a,b
a,b,⁎
, Yunyan Wang
, Yong Ke
a,b,⁎
a
a
, Liwei Yao , Degang Liu , Liyuan Chai
T a,b
School of Metallurgy and Environment, Central South University, Changsha 410083, China Chinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, Changsha 410083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Arsenic sulfide sludge Stabilization Hydrothermal treatment Arsenic
Arsenic sulfide sludge (ASS) generated during the treatment of strongly acidic wastewater with sulfide precipitation method has long been a problem for the nonferrous smelting enterprises. In this study, stabilization of ASS from a lead and zinc smelter by hydrothermal treatment was investigated. The ASS was characterized with high contents of arsenic (46.9%) and sulfur (32.6%), and leaching concentration of arsenic was determined to be 702.0 mg/L. The parameters of the hydrothermal treatment were studied in detail. It was found that the leaching concentrations of As, Pb and Cd, dehydration ratio, density and morphology of ASS changed dramatically during the hydrothermal treatment. Comprehensive consideration of these changes, the optimal conditions were determined to be temperature of 200 °C, reaction time of 4 h, liquid-to-solid (L/S) ratio of 1:1 and the initial pH of 2. Under the optimal conditions, the leaching concentration of As for the treated ASS decreased to < 5 mg/L. The yellow muddy ASS changed into a big bulk with metallic luster. The density of the ASS increased from 1.20 g/cm3 to 2.29 g/cm3, the moisture content decreased from 62.59% to 6.50%, and the volume reduction ratio reached 91.67%. After the hydrothermal treatment, the speciation transformation of As, Pb and Cd occurred. Some of the As, Pb and Cd in ASS transformed into a more stable speciation after the hydrothermal treatment. As a result, the stabilization of As, Pb and Cd attributed to both of the microstructure transformation of ASS and the speciation transformation of As, Pb and Cd.
1. Introduction Arsenic is an abundant element in the earth's crust and is found in > 300 minerals (Drahota and Filippi, 2009) generally in combination with sulfur and metals. Arsenic that occurs in base metal ores and/or concentrates enters into the metallurgical process accordingly and then is released to the environments (Ke et al., 2017; Min et al., 2019b; Wang et al., 2019; Yang et al., 2015). Arsenic is highly toxic and adversely affects human health. Therefore, arsenic has become a worldwide environmental concern especially in metal industry (Chai et al., 2018; Fei et al., 2017). Most of the arsenic is volatilized and oxidized to As2O3 and As4O6 during the smelting of non-ferrous metals-bearing concentrates and then emitted with flue gas. Prior to the sulfuric acid production, arsenic compounds in flue gas must be eliminated because the arsenic influences the quality of the produced sulfuric acid. Hence, spray washing with the diluted acid is performed to remove the dust and arsenic from the pre-cleaned gas. Hence, strongly acidic wastewater with a high arsenic concentration is generated. Presently, sulfide precipitation processes with Na2S, NaHS and H2S have been developed and widely adopted to eliminate arsenic from the acidic wastewater due to its high ⁎
efficiency and ease of operation (Kong et al., 2017a; Opio, 2013). Therefore, a large amount of arsenic sulfide sludge (ASS) with great risk is generated. Its safety disposal is of great importance for non-ferrous smelters. To prevent arsenic pollution, numerous methods including vitrification, extraction, and stabilization/solidification (S/S) have been developed to treat arsenic-containing hazardous waste (Chai et al., 2016; Lei et al., 2017; Li et al., 2019; Li et al., 2016; Liang et al., 2017; Min et al., 2019a; Min et al., 2017; Peng et al., 2016; Zhao et al., 2016). Up to now, S/S with binders, such as cement, pozzolanic matter and geopolymer, is the most widely used method for the disposal of arsenic and potentially toxic metals-bearing waste (Bose and Sharma, 2002; Desogus et al., 2013; Jang and Kim, 2000; Singh and Pant, 2006; Van Jaarsveld et al., 1997; Van Jaarsveld et al., 1999). However, it will result in a large compatibilization ratio and high cost when ASS is treated with cement. For example, Long et al. (2014) reported the S/S of ASS with oxidant, polyferric sulfate (PFS), polyacrylamide (PAM), lime and cements. Although this method achieved the desired results, a lot of additives were added, which leads to a higher cost of landfill disposal subsequently. The hydrothermal treatment has now been widely used to prepare
Corresponding authors at: School of Metallurgy and Environment, Central South University, Changsha 410083, China. E-mail addresses: [email protected] (Y. Wang), [email protected] (Y. Ke).
https://doi.org/10.1016/j.hydromet.2019.105229 Received 13 August 2019; Received in revised form 20 November 2019; Accepted 13 December 2019 Available online 17 December 2019 0304-386X/ © 2019 Published by Elsevier B.V.
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Intensity (a.u.)
crystals with special performance (Byrappa and Adschiri, 2007; Byrappa and Yoshimura, 2013). Some studies showed that it can effectively achieve the S/S of hazardous components in hazardous waste. For instance, hydrothermal precipitation of arsenical natroalunite has been proposed to stabilize calcium arsenate waste from a copper smelter, which skipped the production of the intermediate arsenical gypsum because it was transformed into arsenic-free anhydrite (Viñals et al., 2010). Hydrothermal treatment with microwave heating is also a feasible approach for the solidification of potentially toxic metals in fly ash (Qiu et al., 2016). Moreover, most importantly, hydrothermal treatment can simultaneously realize dehydration and volume reduction of waste compared to the above-described S/S method (Nazari et al., 2017). Our previous works preliminary indicate that the hydrothermal treatment had obvious effects on the detoxication, dehydration and volume reduction for several types of ASS (Yao et al., 2019; Yao et al., 2018), but the specific parameters of hydrothermal process and its effects are not studied systematically. The main purpose of this work is to optimize the hydrothermal stabilization process of the ASS generated from the sulfide precipitation of high arsenic wastewater with H2S. To explore the transformation of the ASS during the hydrothermal stabilization process, the changes in morphology, dehydration ratio, density and the leaching concentrations and speciation of As, Pb and Cd had been investigated in detail.
10
20
30
40
50
60
70
80
2 theta (degree) Fig. 1. X-ray diffraction pattern of the ASS.
exposed to the natural environment directly. Combining with the elemental composition, it is concluded that the ASS can be identified as the hazardous waste, which should be solidified or stabilized before landfill.
2. Materials and experimental methods
2.2. Experimental procedure
2.1. Materials
Five grams of the original collected ASS and the deionized water with an appropriate liquid-to-solid (L/S) ratio were loaded into a stainless steel autoclave with a capacity of 25 mL. Then it was placed into the oven at a given temperature in range of 160–210 °C for 0–10 h. Thereafter, the autoclave was cooled to the room temperature naturally. The as-treated ASS was collected and washed with the deionized water to remove the remained ions. Finally, it was dried in a vacuum oven at 60 °C for 12 h.
The ASS used in the experiments was sampled from a lead and zinc smelter of Hunan province, China. The original ASS collected was yellow mud with moisture of 62.59% and density of 1.20 g/cm3. In order to determine the elemental composition of ASS, the ASS was dried, ground, sieved and digested. The elemental composition of the ASS determined by ICP-OES is presented in Table 1. The main elements in the ASS were arsenic and sulfur with the content of 46.90% and 32.60%, respectively. The atomic molar ratio of As and S was about 1:1.63, which is close to the molar ratio of As and S in As2S3. The higher S content indicated that the ASS might be composed of As2S3 and elemental S0 which was generated by the oxidation of S(-II) to S(0) during the sulfide precipitation process (Kong et al., 2018; Peng et al., 2018; Rochette et al., 2000). Additionally, the content of Cd was 3.73%, and small amount of Zn, Fe, Pb, Cr and Cu were also found in the ASS. Wherein, elements of As, Cd, Pb and Cr were toxic components which must be prevented from releasing into the environment. The XRD pattern of ASS presented in Fig. 1 indicates that the ASS had a poor crystallinity without any sharp diffraction peaks. According to Kong (Kong et al., 2017b), the broad humps at 2θ around 18o, 32o and 56o were corresponded to the amorphous As2S3. Fig. 2 shows the SEM-EDS images of the ASS. The ASS was composed of the extreme fines of AseS compounds, and the particles were irregular and flocculent with particle size of 2–5 μm. The concentrations of arsenic and potentially toxic metals in the leachate of the ASS after the toxicity characteristic leaching procedure (TCLP) were presented in Table 2. As seen from Table 2, the leaching concentrations of Zn (52.2 mg/L) and Cr (0.3 mg/L) were below the allowable limits set by United States Environmental Protection Agency (USEPA). The leaching concentrations of As, Cd and Pb were 702.0 mg/ L, 15.1 mg/L and 6.5 mg/L, respectively, which exceeded the limits. It implies that the ASS will result in severe environmental pollution if
2.3. Analysis methods 2.3.1. Determination of elemental composition To determine elemental composition content in the ASS and the treated ASS, the samples were dried at 60 °C in a vacuum oven. Then, they were ground and sieved for obtaining the samples with particle size of 100–200 mesh. Therewith, the samples were digested using a mixture of concentrated HCl and HNO3 (3:1, v/v). The solutions were filtrated with filter paper (1 μm pore), and then element concentrations in filtrate were analyzed with inductively coupled plasma optical emission spectrometer (ICP-OES, Agilent 5100). 2.3.2. TCLP test and BCR sequential extraction To evaluate the stability of the ASS and the treated ASS, the TCLP test was performed according to USEPA Method 1311 (USEPA, 1992). A BCR sequential extraction procedure was employed to assess the environmental activity and potential mobility of As, Pb and Cd in the ASS and the treated ASS (Davidson et al., 1998; Kazi et al., 2005). The concentrations of As and potentially toxic metals in the leachate obtained during the TCLP test and BCR sequential extraction were analyzed by ICP-OES.
Elements
As
S
Cd
Zn
Fe
Pb
Cr
Cu
2.3.3. Determination of volume reduction ratio, apparent density and dehydration ratio The volume of samples before and after hydrothermal treatment was measured by the method of water displacement (King II, 1993). The volume reduction ratio can then be calculated according to Eq. (1).
Wt%
46.90
32.60
3.73
0.86
0.50
0.14
0.05
0.01
Volume reduction ratio =
Table 1 The main elemental composition of the ASS.
2
V − V′ × 100% V
(1)
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Fig. 2. SEM-EDS images of the ASS. Table 2 Concentrations of arsenic and potentially toxic metals in leachate of the ASS after TCLP test. Threshold
The ASS
As Cd Pb Cr Zn
≤5 ≤1 ≤5 ≤5
702.0 15.1 6.5 0.3 52.2
Not specified by USEPA.
where V (mL) is the raw volume of the ASS; V′ (mL) is the volume of the treated ASS. Based on the Archimedes law, the apparent density of the ASS and the treated ASS were determined and calculated as:
60 50 40 30 20 10 0
160
170
180
210
200
190
Temperature (°C)
m0 ρw ρ= m2 − m1
(2)
Fig. 3. Effect of the temperature on the leaching concentrations of As, Pb and Cd for the treated ASS (4 h, L/S ratio of 1:1, initial pH of 2).
where m0 (mg) is the weight of sample in air; m1 (mg) is the weight of the beaker with the distilled water; m2 (mg) is the weight of the beaker with distilled water and sample; ρw (mg/cm3) is the density of water. Moisture content was measured by drying the samples at 60 °C under vacuum oven to a constant weight ( ± 0.01 g). Then, the dehydration ratio was calculated according to Eq. (3).
M × φ − M ′ × φ′ × 100% M×φ
3.2
100
Density Dehydration ratio
Density (g/cm3)
Dehydration ratio =
As Pb Cd
(3)
where M (g) is the weight of the ASS; φ (%) is the moisture content of the ASS; M' (g) is the weight of the treated ASS; φ' (%) is the moisture content of the treated ASS. 2.3.4. Others The phases of the ASS was characterized by X-ray diffraction (XRD: D/max 2500 VB+) with a scanning speed of 10° min−1 from 10° to 80°. The microstructural observation and qualitative elements analysis of the ASS and the treated ASS were performed by scanning electron microscopy equipped with energy dispersive X-ray spectrometry (SEMEDX: Nano SEM 230).
2.8
80
2.4
60
2.0
40
1.6
20
1.2
Dehydration ratio (%)
a
a
Leaching concentration (mg/L)
Elements
70
0
160
170
180
190
200
210
Temperature (°C) Fig. 4. Density and dehydration ratio of the treated ASS with different temperatures (4 h, L/S ratio of 1:1, initial pH of 2).
3. Results and discussion only the leaching concentrations of them exceeded the limits (Table 2). As shown in Fig. 3, the temperature affected the leaching concentrations of As, Pb and Cd significantly. As a whole, the leaching concentrations decreased with the improvement of temperature. When the temperature varied from 160 °C to 190 °C, the reduction in leaching concentration of As is remarkable which decreased from 65.19 mg/L to
3.1. Effect of each parameter on the stabilization of ASS 3.1.1. Temperature The leaching concentrations of As, Pb and Cd and the density for the treated ASS obtained at different temperatures are presented in Fig. 3 and Fig. 4, respectively. The As, Pb and Cd should be focused because 3
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Fig. 5. SEM images of the treated ASS at different temperatures: (a) 160 °C, (b) 170 °C, (c) 180 °C, (d) 190 °C, (e) 200 °C, (f) 210 °C (4 h, L/S ratio of 1:1, initial pH of 2).
200
Density (g/cm3)
Leaching concentration (mg/L)
As Pb Cd
100
20 10
100
Density Dehydration ratio
2.8
80
2.4
60
2.0
40
1.6
20
0 0
2
4
6
8
10
1.2
Time (h)
0.5
1
1.5
2
3
4
5
6
8
Dehydration ratio (%)
3.2
300
0 10
Time (h)
Fig. 6. Effect of the reaction time on the leaching concentrations of As, Pb and Cd for the treated ASS (200 °C, L/S ratio of 1:1, initial pH of 2).
Fig. 7. Density and dehydration ratio of the treated ASS with different reaction time (200 °C, L/S ratio of 1:1, initial pH of 2).
5.34 mg/L. With the further increase of temperature from 190 °C to 210 °C, leaching concentration of As can be reduced to 0.67 mg/L. For Pb and Cd, the leaching concentration was below the allowable limits as soon as temperature reached 170 °C. The above phenomena illustrate that temperature plays a vital role in hydrothermal process. The higher the temperature, the better the stabilization effect of ASS. It can be seen from Fig. 4 that the density of the treated ASS and the dehydration ratio increased almost linearly with the temperature rising from 160 °C to 210 °C. These results indicated that densification and dehydration occurred during the hydrothermal treatment of the ASS. Fig. 5 shows the surface characteristics of the treated ASS obtained at different temperatures. Compared to Fig. 2, the microstructure of the ASS was completely different after hydrothermal process. The treated ASS exhibited a loose network-like microstructure after it was hydrothermal treated at 160 °C. Moreover, with the increase of hydrothermal
temperature, the network became dense. The network bonded and fused together when the temperature increased from 160 °C to 190 °C. With the temperature varying from 190 °C to 210 °C, the network almost disappeared (Fig. 5d~f). They converted into a bigger bulk, which explained why the density of ASS increased and which may be the reason for the decrease in leaching concentrations of As, Pb and Cd during the hydrothermal treatment. In summary, increasing the temperature promotes the transformation of the microstructure and the dehydration of the ASS. During the hydrothermal treatment, the water was removed from the ASS, and the microstructure of ASS changed from flocculent to reticulate and finally to bulk, which resulted in the increase in the density of ASS. With the changes in microstructure and density, the leaching concentrations of As, Pb and Cd for the treated ASS also reduced gradually. When the 4
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Fig. 8. SEM images of the treated ASS for different reaction time: (a) 0 h, (b) 1 h, (c) 2 h, (d) 3 h, (e) 4 h, (f) 10 h (200 °C, L/S ratio of 1:1, initial pH of 2).
70
Leaching concentration (mg/L)
Leaching concentration (mg/L)
5
As Pb Cd
4 3 2 1 0
0:1 0.5:1 1:1 1.5:1 2:1 2.5:1 3:1
As Pb Cd
60 50 40 30 20 10 0 0
4:1 5:1
1
2
3
4
5
6
7
8
Initial pH
L/S ratio (mL/g)
Fig. 10. Effect of the initial pH on the leaching concentration of As, Pb and Cd for the treated ASS (200 °C, 4 h, L/S ratio of 1:1).
Fig. 9. Effect of the L/S ratio on the leaching concentrations of As, Pb and Cd for the treated ASS (200 °C, 4 h, initial pH of 2).
2.5 h. Further extension of the reaction time, however, the leaching concentration of As hardly changed. Besides, when the reaction time reached 2 h, leaching concentrations of Pb and Cd decreased to 0.02 mg/L and 0.19 mg/L, respectively, which were much lower than the allowable limits set by USEPA. Thereafter, the leaching concentrations of Pb and Cd remained stable regardless of the change of reaction time. Fig. 7 shows the density and dehydration ratio of the treated ASS with different reaction time. It was evident that reaction time had dramatically impact on the density of ASS and dehydration ratio. According to our tests, the original ASS had a low density of 1.2 g/cm3 and a high moisture content of 62.59%. After 4 h of hydrothermal treatment, the density of the ASS increased to 2.29 g/cm3, which was almost twice of the density of the original ASS. As the reaction time continued to increase, the density of ASS kept constant. Correspondingly, when the reaction time was < 4 h, the dehydration ratio also increased
temperature varied from 200 °C to 210 °C, the decrease of the leaching concentrations of As, Pb and Cd was not obvious (Fig. 3), and the microstructure and density changed slightly. Therefore, 200 °C was selected as the optimum temperature. 3.1.2. Reaction time The effect of reaction time on leaching concentrations of As, Pb and Cd for the treated ASS is investigated under the conditions of temperature of 200 °C, reaction time of 0-10 h, L/S ratio of 1:1 and initial pH of 2. Fig. 6 presents the variation of leaching concentrations for As, Pb and Cd with the reaction time. The curves showed a fast decrease on leaching concentration of As in initial 2 h. When the reaction time was longer than 2 h, the leaching concentration of As decreased slowly until it was stable. The leaching concentration of As decreased from 702.0 mg/L to 1.41 mg/L with the reaction time varying from 0 to 5
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Fig. 11. SEM images of the treated ASS for different initial pH: (a) pH = 1, (b) pH = 2, (c) pH = 3.5, (d) pH = 4.5, (e) pH = 6.5, (f) pH = 8 (200 °C, 4 h, L/S ratio of 1:1).
Fig. 12. The comparison of the ASS before and after the hydrothermal treatment.
with a smooth surface at 4 h. The treated ASS in Fig. 8e is denser than that in Fig. 8d, which would improve the structural stability of ASS and inhibit the release of As, Cd and Pb. With the reaction time prolonging to 10 h, the microstructure changes of the treated ASS were not significant (Fig. 8f). In addition, there were some spherular pits on the fracture surface of the bulk, which might be caused by the gas of H2S generated by sulfur decomposition. Because the amorphous and
gradually, whereas the dehydration ratio remained unchanged when the reaction time exceeded 4 h. Fig. 8 shows the SEM images of the treated ASS obtained at different reaction time. It was clearly seen that there were tremendous differences in the morphology of the ASS with different reaction time. After being hydrothermal treated for 2 h, the flocculent particles in ASS began to bond together. Then they gradually fused into a bigger bulk 6
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The ASS As
95.93% Oxidizable fraction
The treated ASS 76.40% Oxidizable fraction
1.52% Residul fraction
0.03% Acid soluble fraction
2.49% Acid soluble fraction
0.04% Reducible fraction
0.06% Reducible fraction
Pb
18.31% Oxidizable fraction
75.63% Residual fraction
8.35% Oxidizable fraction
1.93% Reducible fraction
4.48% Reducible fraction
25.88% Oxidizable fraction
0.37% Residual fraction
89.72% Residual fraction 0% Acid soluble fraction
1.58% Acid soluble fraction
Cd
23.53% Residual fraction
87.07% Oxidizable fraction
12.86% Residual fraction 0.01% Acid soluble fraction
71.44% Acid soluble fraction 2.31% Reducible fraction
0.06% Reducible fraction
Fig. 13. The speciation of As, Pb and Cd for the ASS and the treated ASS.
flocculent ASS was converted into a big bulk, the water content and volume of ASS reduced dramatically. This phenomenon was also found in the hydrothermal treatment of coal (Zhang et al., 2016). In summary, the hydrothermal treatment is a novel and effective technique for densification, volume reduction, dehydration of the ASS, and especially for the stabilization of As, Pb and Cr. According to the results above, the optimum reaction time was considered to be 4 h.
the treated ASS increased slightly with the pH rising from 0.5 to 3, and then increased rapidly to 64.79 mg/L when the pH increased to 8. The higher pH value, the higher leaching concentration of As for the treated ASS, which agrees with the fact that arsenic sulfide is soluble in alkali solution (Darban et al., 2011). The lowest leaching concentration of As is 0.54 mg/L at the initial pH of 0.5, implying that strong acidic conditions facilitates the hydrothermal stabilization process. The leaching concentrations of Pb and Cd kept below the allowable limits in the entire pH range studied. Because the pH of the original ASS was about 2, it does not need to add any chemical agent to adjust the pH. In order to explain the reason of the increase in leaching concentration of As when the initial pH exceeded 3, the surface characteristics of the treated ASS obtained at different initial pH were characterized by SEM. It can be seen from Fig. 11 that the treated ASS existed as bigger bulk with a smooth surface compared to the original ASS of flocculent particles at the initial pH of 1 and 2. With the increase of initial pH to 3.5, the treated ASS seems to dissolve, the surface of ASS became rough. A large number of fine particles are clearly visible on the surface of bulk particles when the initial pH prolonged to 4.5, 6.5 and 8 (Fig. 11d~f). The results explained why the leaching concentration of As increased as shown in Fig. 10 after initial pH > 3.
3.1.3. Liquid-to-solid ratio The liquid-to-solid (L/S) ratio affects the hydrothermal reaction via influencing the degree of liquid-solid mixing and pressure. Thus, the effect of the L/S ratio adjusted by changing the dosage of the deionized water on the stabilization effect of ASS was investigated. Fig. 9 shows that the leaching concentrations of As, Pb and Cd decreased with the increase of L/S ratio. For leaching concentration of As, it decreased rapidly from 4.28 mg/L to 1.70 mg/L when the L/S ratio changed from 0:1 to 1:1.Then it decreased further to 0.83 mg/L when the L/S ratio increased to 2.5:1. As the L/S ratio varying from 2.5:1 to 5:1, leaching concentration of As fluctuated slightly. As for Pb and Cd, the leaching concentrations were less affected by L/S ratio, which always remained lower than the allowable limits. In this study, L/S ratio of 1:1 was chosen for the subsequent experiments.
3.1.5. Comparison of the ASS before and after the hydrothermal treatment According to the results above, the optimum conditions of hydrothermal treatment were determined to be temperature of 200 °C, reaction time of 4 h, L/S ratio of 1:1 and initial pH of 2. Fig. 12 shows the comparison of the ASS before and after the hydrothermal treatment. It
3.1.4. Initial pH The pH is one of the key parameters for the hydrothermal treatment. In this study, it was adjusted from 0.5 to 8 with the dilute HCl and NaOH solution. Fig. 10 shows that the leaching concentration of As for 7
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China (2018YFC1903301, 2018YFC1900301), National Science Fund for Distinguished Young Scholars (51825403), the National Natural Science Foundation of China (51634010 and 51904354).
could be seen that there were obvious differences of the morphology and physicochemical properties for the ASS before and after the hydrothermal treatment. The leaching concentrations of As, Pb and Cd for the treated ASS reduced to 0.67 mg/L, 0.04 mg/L and 0.10 mg/L, respectively, all of which were far below the allowable limits. From the appearance, the yellow muddy ASS changed into a big bulk with metallic luster. Under the SEM view, the flocculent amorphous ASS was seen to fuse together and solidified into bulk substance with smooth surface after the hydrothermal treatment. Therefore, it is assumed that the low leaching concentrations of As, Pb and Cd may be attributed to its specific morphological and microstructural features. Besides, the density of the ASS increased from 1.20 g/cm3 to 2.29 g/cm3, the moisture content decreased from 62.59% to 6.50%, and the volume reduction ratio reached 91.67%, indicating that the hydrothermal treatment also shows excellent ability in dehydration and volume reduction.
Declration of Competing Interest None. References Bose, P., Sharma, A., 2002. Role of iron in controlling speciation and mobilization of arsenic in subsurface environment. Water Res. 36 (19), 4916–4926. Byrappa, K., Adschiri, T., 2007. Hydrothermal technology for nanotechnology. Prog. Cryst. Growth Charact. Mater. 53 (2), 117–166. Chai, L., Yue, M., Yang, J., Wang, Q., Li, Q., Liu, H., 2016. Formation of tooeleite and the role of direct removal of As(III) from high-arsenic acid wastewater. J. Hazard. Mater. 320, 620–627. Byrappa, K., Yoshimura, M., 2013. Handbook of Hydrothermal Technology. William Andrew. Chai, L., Yue, M., Li, Q., Zhang, G., Zhang, M., Wang, Q., Liu, H., Liu, Q., 2018. Enhanced stability of tooeleite by hydrothermal method for the fixation of arsenite. Hydrometallurgy 175, 93–101. Darban, A.K., Aazami, M., Meléndez, A.M., Abdollahy, M., Gonzalez, I., 2011. Electrochemical study of orpiment (As2S3) dissolution in a NaOH solution. Hydrometallurgy 105 (3–4), 296–303. 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3.2. Speciation variation of As, Pb and Cd In order to further analyze the reasons for the stability of As, Pb and Cd in ASS, we used BCR method to analyze the speciation of As, Pb and Cd. Based on BCR method, the speciation of As, Pb and Cd are divided into the acid soluble fraction, reducible fraction, oxidizable fraction and residual fraction (Xie et al., 2013). Fig. 13 shows that As in original ASS existed mainly in the speciation of oxidizable state with a percentage of 95.93%. After hydrothermal treatment, although some of As was converted into residual fraction, the oxidizable fraction still occupied 76.40%. Combined with the microstructure transformation of ASS, it can be inferred that the microstructure transformation of ASS is the main reason for the stabilization of As, while the change in As speciation is the minor reason. As for Pb, it mainly existed as the residual state in both ASS and the treated ASS, which explains why the leaching concentration of Pb was low as shown in Fig. 3, Fig. 6, Fig. 9 and Fig. 10. Besides, the Pb in residual state increased somewhat, suggesting that the speciation transformation also contributes to the stabilization of Pb. For Cd, acid soluble Cd was the main speciation with percentage of 71.44% for ASS. After hydrothermal treatment, the main speciation of Cd changed into oxidizable state (that is, CdS) with a percentage of 87.07%, indicating that both of speciation changes and microstructure transformation of ASS are the reasons for the stabilization of Cd. 4. Conclusions Hydrothermal treatment can achieve the stabilization of the arsenic sulfide sludge effectively and economically. Of all factors, temperature had the greatest effect on the stabilization of the ASS. The optimal conditions were temperature of 200 °C, reaction time of 4 h, liquid-solid ratio of 1:1, the initial pH value of 2. Under these conditions, arsenic leaching concentration decreased from 702.0 mg/L to 0.67 mg/L after hydrothermal treatment, and the concentrations of Pb and Cd in leachate were 0.04 and 0.1 mg/L, all of which were less than the allowable limits set by USEPA. In hydrothermal treatment process, the flocculent amorphous ASS fused together and solidified into bulk with smooth surface, which may result in the decrease of arsenic leaching concentration. Besides, the density increased from 1.20 g/cm3 to 2.29 g/ cm3, the moisture decreased from 62.59% to 6.50%, and the volume reduction ratio was up to 91.67%. These results indicate that the dehydration and volume reduction of the ASS had realized. In addition, the speciation transformation also contributed to the stabilization of the As, Pb and Cd in ASS. After hydrothermal treatment, As, Pb and Cd in residual state increased. Acknowledgements This research was supported by the National Key R&D Program of 8
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