Journal of Hazardous Materials 265 (2014) 96–103
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Transformation of heavy metal speciation during sludge drying: Mechanistic insights Huan-Xin Weng a,∗ , Xue-Wen Ma b , Feng-Xia Fu a , Jin-Jun Zhang a , Zan Liu a , Li-Xun Tian a , Chongxuan Liu c,∗∗ a
Institute of Environment & Biogeochemistry, Zhejiang University, Hangzhou 310027, PR China Research Institute for Environmental Science, Shanxi University, Taiyuan, 030006, PR China c Geochemistry, Fundamental, and Computational Science Directorate, Pacific Northwest National Laboratory, USA b
h i g h l i g h t s • Drying process stabilizes heavy metals in sludge through species transformation. • Metal transformation and stabilization occur regardless of sludge source and type. • Various mechanisms responsible for the speciation changes of heavy metals in sludge.
a r t i c l e
i n f o
Article history: Received 20 August 2013 Received in revised form 22 November 2013 Accepted 27 November 2013 Available online 1 December 2013 Keywords: Sewage sludge drying Heavy metal Transformation Stabilization
a b s t r a c t Speciation can fundamentally affect on the stability and toxicity of heavy metals in sludge from wastewater treatment plants. This research investigated the speciation of heavy metals in sludge from both municipal and industrial sources, and metal speciation change as a result of drying process to reduce sludge volume. The changes in sludge properties including sludge moisture content, temperature, density, and electrical conductivity were also monitored to provide insights into the mechanisms causing the change in heavy metal speciation. The results show that the drying process generally stabilized Cr, Cu, Cd, and Pb in sludge by transforming acid-soluble, reducible, and oxidizable species into structurally stable forms. Such transformation and stabilization occurred regardless of the sludge source and type, and were primarily caused by the changes in sludge properties associated with decomposition of organic matter and sulfide. The results enhanced our understanding of the geochemical behavior of heavy metals in municipal sludge, and are useful for designing a treatment system for environment-friendly disposal of sludge. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Safe and cost-effective disposal of sewage sludge from waste treatment plants is still a worldwide environmental challenge. A common goal in the sludge treatment is to reduce toxicity, decrease volume, and convert sludge into useful resources [1]. The decrease in volume by reducing moisture content in the sludge is one of the key steps in the sludge treatment. It becomes only feasible to convert the sludge into nontoxic resources when the sludge volume is significantly reduced [2]. Sludge moisture content is typically reduced to 75–85% after concentration, digestion and mechanical dewatering to remove free water and capillary water in sludge.
∗ Corresponding author. Tel.: +86 13071807599; fax: +86 0571 87951336. ∗∗ Corresponding author. Tel.: +86 509 371 6350. E-mail addresses: [email protected] (H.-X. Weng), [email protected] (C. Liu). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.11.051
The treated sludge still contains too high water content to convert sludge into useful resources [2]. Additional treatment through lowtemperature drying process has to be used to further reduce sludge volume by evaporating residual capillary water, interstitial water, as well as adsorbed water [3]. Heat is often provided to enhance the evaporation process. Heat drying can reduce sludge moisture content to less than 10% [4]. Sludge from sewage treatment plants is a solid-liquid mixture containing organic fragments, bacteria, inorganic particles and colloids [5]. Heavy metals can associate with all these sludge components with complex speciation that can have different eco-environment effects [6]. The drying process will increase solid/liquid ratio that may lead to the changes in the speciation of metals and their relative abundance in the sludge components. Previous studies indicated that drying affect bioavailability of metals in sludge [7], and sludge drying changed the nature of organic compounds leading to changes in their solubility [8]. The mechanisms for the change in metal speciation during the drying process
H.-X. Weng et al. / Journal of Hazardous Materials 265 (2014) 96–103
97
Table 1 Types and characteristics of sewage sludge. Sludge number
Type
Moisture content (%)
pH
Organic content (%)
Source
S1 S2 S3 S4 S5
Municipal sludge Municipal sludge Municipal sludge Leather-tanningsludge Dyeing sludge
83 77 64 67 78
6.6 7.5 6.9 8.3 8.5
54.45 40.4 30.8 28.9 29.5
Hefei Hangzhou Sibao Shanghai Haining Wujiang
are, however, still poorly understood. To safely and cost-effectively dispose the sludge, it is important to understand how the metal speciation and relative abundance in different sludge components change before and after drying processes [9]. In this paper, we report an investigation on the effect of sludge drying process on the speciation of several environmentally important heavy metals including As, Cd, Cr, Cu, Ni, Pb, and Zn. Sludge collected from both municipal and industrial wastewater treatment plants were dehydrated under different temperatures. The objectives of this study are to determine the effect of temperature and sludge source on the speciation of heavy metals and their evolution; and to reveal mechanisms causing the metal speciation changes. Sequential extraction of heavy metals in the sludge samples was used to determine metal speciation. Physical properties, such as sludge moisture content, temperature, density, and electrical conductivity were also measured and analyzed to provide insights into the mechanisms of metal speciation changes.
2. Materials and methods The sludge samples used in this study were taken from municipal, and dyeing and leather-tanning industrial wastewater treatment plants (Table 1). Total 5 sludge samples were collected: 3 municipal sludge samples and 2 industrial samples, from east China cities. Each sample with 4 replicates was dried under different temperatures (25, 105, 150, and 300 ◦ C). After drying, the sludge samples were ground with agate mortars, sieved through 120 mesh (pore diameter 125 m) and stored in polyethylene bags. The dry and ground samples were used for sequential extraction of heavy metals using the BCR method as described elsewhere [10,11]. The total concentrations of heavy metals in the dried sludge samples were determined by first digesting the samples using HCl HNO3 HClO4 [11]. The acid digestion suspensions were then filtered (0.45 um) and the filtrates were measured to determine total metal concentrations using inductively coupled plasma atomic emission spectroscopy (ICP-AES). The organic matter in the sludge was determined using the carbon analyzer (SXL-1304 Programmable Box-type Furnace, Hangzhou Zhuochi Instruments Co., Ltd.). In all the analysis, the variation between the replicates for each sample including blank control was below 5%. The effect of drying process on the sludge physical properties including sludge moisture content, density, temperature, and electrical conductivity were determined as a function of drying time at a drying temperature of 150 ◦ C. Sludge sample from the municipal wastewater treatment plant at Hangzhou Sibao (S1 in Table 1) was used for detail study with measurement procedures provided elsewhere [12]. Briefly, fresh sludge was homogenized and separated into 14 subsamples, which were then weighted (40.0 g each) into 500 ml beakers labeled from 1 to 14. Each sludge subsample was dried in an oven for a specified time (0, 15, 30, 45, 60, 75, 90, 105, 150, 180, 240, 270, 300 or 600 min). After drying, the subsamples were weighted again. Each dried sample was then mixed with 80 ml of deionized water in 200 ml volumetric vessel to determine the total volume (VT ) of the suspension. From VT , the volume (V) and density () of the dried sludge samples can be calculated using: V = VT – 80; = W/V, where W is the weight of the
dried sludge subsample. The conductivity of each subsample was determined after the suspension was settled for 30 min and the supernatant was taken for conductivity measurement using a conductivity meter. The moisture content in the dried subsamples was calculated using the equation: P = (m − M)/m × 100%, where M is the mass of completely dried sludge, m is mass of sludge dried at a specific temperature. The dry-based moisture content was calculated by equation X = P/(1 − P) (kg H2 O/kg DS). In the drying process, sludge temperature will rise and gas will release in concomitant with water vaporization. The sludge temperature was measured using an electrothermal prober (TC15) inserted into the sludge. The composition of the gas released from the sludge samples during the heat drying was also analyzed using the Sibao municipal sludge as an example. Fresh sludge samples (50 g) were placed in flasks heated at 100, 150, or 300 ◦ C for 3 h. The released gas was collected by condensation, filtered, and extracted by petroleum ether. The extraction and fractionation process was conducted at 55 ◦ C. The extracted air samples were analyzed using gas chromatography and mass spectroscopy (GC/MS, HP-150 m × 0.2 mm × 0.33 m, high-purity helium as a carrier gas, initial head pressure:1.6 psi, carrier gas flow rate of 1.02 ml/min, and final head pressure: 30.0 psi) following the method described elsewhere (USEPA,97001). 3. Results 3.1. Physical properties of sludge The physical properties of the sludge vary with sludge source (Table 1). The moisture and organic contents are generally higher in the municipal sludge than that in dyeing and leather-tanning industrial sludge. Among the municipal sludge, the Hefei sludge has the higher organic content (54.45%). The pH values in the municipal sludge is circumneutral, while those in the dyeing and leathertanning sludge are mild alkaline. 3.2. The speciation of heavy metals in sludge Heavy metal concentrations in different types of the sludge are listed in Table 2. There was no significant difference in As concentration in all the studied sludge samples, but the concentrations of other heavy metals varied widely. Zn has the highest concentration among all the measured metals except Cr in the leather-tanning sludge because of the widespread adoption of galvanized pipes in water delivery system. Among the municipal sludge, the Zn content is the highest in the sludge from Shanghai and lowest from the Table 2 Total concentration of heavy metals in sludge (mg kg−1 ). Sludge number
S1 S2 S3 S4 S5
Heavy metals (mg kg−1 ) As
Cd
Cr
Cu
Ni
Pb
Zn
34.7 33.0 33.3 35.2 25.9
1.4 2.9 2.2 – 3.1
129.9 151.1 293.7 17399 294.8
172.5 164.6 517.4 31.3 9.1
45.4 35.6 159.25 6.8 13.7
32.1 32.7 141.1 64.2 15.5
846.5 1343 1731 553.3 277.5
98
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Hefei. The Ni concentrations followed the same trend as Zn among different sources of the sludge. The Ni in the sludge from Shanghai is 3.51 and 4.47 times, respectively, as those from the Hefei and Hangzhou Sibao. The Shanghai sludge also has a higher Pb content, 4.39 and 4.32 times as that in Hefei and Hangzhou sludge. The Cr content in the leather-tanning sludge is the highest, as expected, nearly 17399.75 mg kg−1 because of the use of Cr-containing chemical compounds during the leather-tanning process [13]. The Cr was enriched through sewage treatment process, leading to a higher concentration in the leather-tanning sludge. The metal contents in the dyeing sludge are relatively lower than those in the other sludge except Cr and Cd, which are the same as or higher than those in the municipal sludge. The speciation of heavy metals in the sludge can be operationally divided into acid soluble, reducible, oxidizable and residual fractions based on the concentrations of the metals extracted using the BCR method [10,11]. The acid soluble fraction consisting of adsorbed ions on ion exchangeable phases, and those associated with carbonate minerals and poorly crystalline minerals. This part of the metals is considered to be mobile, and bioavailable to organisms. The residual fraction of the heavy metals is considered to be associated with stable minerals such as silicates and crystallized oxides of magnesium and aluminum. This fraction of metals has the lowest mobility. Fig. 1 is a triangle phase diagram showing the distribution of each heavy metal in different operational fractions after the drying process at 25 ◦ C. In the diagram, the metal concentrations in the acid soluble (F1) and reducible fractions (F2) were combined to form one end member because the metal in both fractions are likely highly mobile when environmental conditions such as pH and Eh changes. The other two end members are oxidizable (F3) and residual fractions (F4). Fig. 1 indicates that Zn and Ni have the similar species distribution in different types of sludge. They are low in the stable fraction (F4) and high in acid soluble (F1) and reducible state fractions (F2), suggesting that these two metals have a high migration potential. The results are consistent with previous reports [14–16]. The Cd content in all the sludge is low (Table 2). The Cd, however, concentrates at the F1 + F2 phase angle (Fig. 1), indicating that Cd has a high potential of migration despite its low content in the sludge. The As speciation is more diverse as it shows in all the fractions. The Cu speciation is dominated by the oxidizable form regardless of sludge source. It is most likely that Cu was associated with organic matters, forming stable chelated substances in the sludge [15,17]. Cr mainly existed in the oxidizable and residual fractions with a relatively higher percentage in the oxidizable fraction. The higher percentage of Cr in the oxidizable fraction was attributed to its association with sulfur to form stable sulfide minerals [14,15,18]. The sulfur content was high in the sludge as discussed further in the section 4.2. The distribution of Pb is similar as Cr. The difference is that it has a higher percentage in the residual fraction. Overall, these results indicated that sludge from different locations contains variable contents and speciation of heavy metals, reflecting the nature and source of sewages collected at different municipal places and industries. Among all metals analyzed, Zn, Ni, and Cd had the most potential for migration, while Cr, Pb, and Cu were mostly associated with the oxidizable fraction, which may become unstable and release to environment in oxidizing environments. As was partially associated with the mobile and oxidizable fractions except for the sludge from Shanghai, in which most As was associated with stable form.
the drying at 25, 105, 150, and 300 ◦ C. For example, using the speciation at 105 ◦ C as the reference, the relative percentage of Cd in the acid soluble fraction in the Hefei sludge decreased from 21 to 12%, in the reducible fraction decreased from 51 to 37%, while in the oxidizable fraction remained relatively stable, and generally increased in the residual fraction (Fig. 2S1). After drying of the Hefei sludge at 300 ◦ C, the relative percentage of Cr in the oxidizable fraction decreased respectively from 96.89% at 105 ◦ C and 85.42% at 150 ◦ C to 53%, while in the residual fraction it increased from 8 to 42%. The increase in the relative percentage in the residual fraction also observed for As, but not for Zn. The result indicated that the drying process stabilized As, Cd, and Cr, but had no effect on Zn in the Hefei municipal sludge. The similar transformation of As, Cd, and Cr from other fractions to the residual fraction was also observed for Hangzhou sludge (Fig. 2S2). In addition, the relative percentages of heavy metals in the sludge are nearly identical after drying at 105 or 150 ◦ C, implying that the temperature effect became negligible in the range of 105–150 ◦ C. However, after drying at 300 ◦ C, part of oxidizable Cr and Cu transformed to the residual fraction, Ni shifted from the acid soluble to residual fraction, and Zn slightly from the reducible to residual fraction. The results indicated that the stability of all studied heavy metals in this sludge increased as their relative percentages in the residual fraction increased, especially after high temperature (300 ◦ C) treatment. In the Shanghai sludge after drying at 105 and 150 ◦ C (Fig. 2S3), the combined acid soluble and reducible Cr was the one tenth of its total. The oxidizable Cr decreased to 60%, while residual fraction increased to 10%. Most Cu (80%) in the dried sludge was in the oxidizable fraction. The relative percentage of combined acid soluble and reducible Ni was in the range of 43–48%, and oxidizable fraction was 24–29%. Most Zn (70%) was in the reducible fraction. The result suggested that metals in the Shanghai sludge have a high mobility potential even after treatment at 105 and 150 ◦ C, consistent with previous observations [16]. On the other hand, after drying at 300 ◦ C, Cr, Cu, and Pb partially shifted from the oxidizable to residual fraction. Their percentages in the residual fraction, respectively, increased to 38, 46, and 54%. In leather-tanning sludge, the percentages of Cr, Cu, Ni, and Pb changed little after drying at 105 and 150 ◦ C relative to those after drying at 25 ◦ C (Fig. 2S4). The acid solute Zn, however, increased to 57% after drying at 105 ◦ C. The relative percentage of Cr and Ni, however, changed significantly after drying at 300 ◦ C. The oxidizable Cr declined from 81 to 25%, while the corresponding residual fraction increased from 19 to 75%. The oxidizable Ni became negligible, while reducible Ni increased a little after 300 ◦ C treatment. Compared with other types of the sludge, the dyeing sludge contained the highest percentage of reducible metals that were likely in the association with microganisms. Most Cr and Pb were transformed to the residual fraction after drying (Fig. 2S5). The behavior of Zn in dyeing sludge was different from other types of sludge. The dominated Zn speciation in fresh dyeing sludge shifted from oxidizable to residual fraction after drying at 105 and 150 ◦ C. However, after drying at 300 ◦ C, oxidizable fraction dominated Zn speciation again. The reason for the Zn increase in the oxidizable fraction from 150 to 300 ◦ C was unclear. The Zn increase from 0% at 150 ◦ C to about 85% at 300 ◦ C in the oxidizable fraction suggested that a significant amount of Zn sulfide minerals was formed for unexplained reasons.
3.3. The effect of drying process on heavy metal speciation
4.1. Sludge physical properties and heavy metal speciation
Fig. 2 shows that the percentage of the heavy metals in different operational speciation fractions changes complexly after
Metal speciation in sludge depends on sludge geochemical properties such as redox potential, surface molecular structure, charge
4. Discussion
H.-X. Weng et al. / Journal of Hazardous Materials 265 (2014) 96–103
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Fig. 1. Triangular diagrams showing heavy metal speciation in different sludge. F1-acid soluble, F2-reducible, F3-oxidizable, F4-residual.
density that can affect metal association with sludge [19–21]. The drying process will change sludge geochemical properties that can affect metal speciation and metal association with the sludge. The sludge visually changed from pasty to granular after drying with concomitant volume decreasing to one third as water evaporated (Fig. 3). Fig. 3 also shows the change in sludge temperature in corresponding to the sludge volume change as a function of drying time. The result in Fig. 3 revealed a preheat process upto 20 min when the sludge temperature gradually increased despite a constant ambient temperature at 300 ◦ C. After 20 min of preheat process, the sludge temperature reached a steady state that is near 300 ◦ C. The temperature change reflected the effect of combined processes of sludge water evaporation and organic matter pyrolyzation that absorbed heat. The initial increase in sludge temperature resulted from water evaporation from surface toward interior. The higher temperature of the sludge than the drying temperature (300 ◦ C) after 20 min of drying indicated the pyrolyzation process. Fig. 3 shows an excellent inverse correlation between the changes in sludge temperature and volume, reflecting that the sludge lost weight as moisture evaporated with increasing temperature. When the temperature reached the steady state, the sludge weight approached a constant value. The lower is the moisture content, the shorter the time of preheating. For example, the moisture content in the Shanghai sludge and leather-tanning sludge are 64
and 67%, respectively (Table 1), the sludge temperature started to increase rapidly after 10 min, and corresponding sludge volume decreased to a steady state within 25 min. In contrast, the moisture contents in Hangzhou, Hefei, and dyeing sludge are 77, 83 and 78%, respectively, the sludge temperature increased rapidly after 25 min, and sludge volume approached a steady stable after 45 min. Overall, the moisture evaporation and organic matter pyrolyzation are apparently the two processes controlling the sludge temperature and volume changes. Metal ions can exist in interstitial, capillary and absorbed water in the wet sludge (Wang et al., 2007) [22]. The drying process will completely eliminate interstitial water, and partially remove capillary and absorbed water depending on temperature. When most capillary and absorbed water vaporizes, the number of aqueous ions will decline significantly in the sludge. Fig. 4 shows that when the moisture content was above 1.25 kg(H2 O)/kg (DS, Sample No1-9), average electrical conductivity in the sludge leachate was 2226 s/cm. When the moisture content was below 1.25 kg(H2 O)/kg (DS, Sample No10-14), average electrical conductivity decreased to 1588 s/cm, reflecting that the content of total aqueous ions in the dried sludge sample decreased [23]. The decrease in total aqueous ion was likely caused by the precipitation of minerals. As sludge desiccated, aqueous ion concentrations and ion activities would increase. Minerals precipitated when ion activities reached their solubility. The precipitation would decrease
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Fig. 2. Heavy metal speciation in sludge after drying at different temperatures.
the total ions in the aqueous phase in the dried sludge samples. When sludge sample was suspended in 80 ml DI water for electrical conductivity measurements, the measured electrical conductivity decreased. The volatization of aqueous sulfide may have also contributed to the decrease in total ions in aqueous phase as temperature increased. The precipitation would also decrease total heavy metals in aqueous phase, which would drive their desorption from the exchanger phases, and decrease total heavy metal in the acid soluble fraction. In addition, the moisture decrease in the sludge would affect the reducible fraction as the amorphous oxides such as iron and manganese oxyhydroxides dehydrated [13]. The dehydration
process may convert these amorphous minerals to crystalline and stable forms. Through this process, the metals originally associated with the poorly crystalline and reducible fractions can then become the residual fraction. 4.2. The organic matter volatilization and heavy metal speciation The oxidizable fraction is an important part in the sludge. The metals in this fraction primarily existed in the form of sulfide- and organic matter- bound species [13]. Both sulfide and organic matter can become unstable at high temperature. In addition, some organic matters in sludge may be volatile [24].
H.-X. Weng et al. / Journal of Hazardous Materials 265 (2014) 96–103
101
Fig. 3. Sludge volume (top plot) and corresponding sludge temperature (bottom plot) as a function of drying time. The ambient (heating) temperature was maintained constant at 300 ◦ C.
Fig. 5 shows that the sludge (S1) density increased with decreasing moisture content, reflecting the effect of organic matter decomposition and sulfide conversion to gas phase. In Fig. 5, there are apparently two regions in sludge density changes. When moisture content is >1.0 kg (H2 O)/kg (DS), the sludge density is close to 1.1 within a narrow range between 1.1–1.2 g cm−3 . When the moisture content is
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Transformation of heavy metal speciation during sludge drying: Mechanistic insights Huan-Xin Weng a,∗ , Xue-Wen Ma b , Feng-Xia Fu a , Jin-Jun Zhang a , Zan Liu a , Li-Xun Tian a , Chongxuan Liu c,∗∗ a
Institute of Environment & Biogeochemistry, Zhejiang University, Hangzhou 310027, PR China Research Institute for Environmental Science, Shanxi University, Taiyuan, 030006, PR China c Geochemistry, Fundamental, and Computational Science Directorate, Pacific Northwest National Laboratory, USA b
h i g h l i g h t s • Drying process stabilizes heavy metals in sludge through species transformation. • Metal transformation and stabilization occur regardless of sludge source and type. • Various mechanisms responsible for the speciation changes of heavy metals in sludge.
a r t i c l e
i n f o
Article history: Received 20 August 2013 Received in revised form 22 November 2013 Accepted 27 November 2013 Available online 1 December 2013 Keywords: Sewage sludge drying Heavy metal Transformation Stabilization
a b s t r a c t Speciation can fundamentally affect on the stability and toxicity of heavy metals in sludge from wastewater treatment plants. This research investigated the speciation of heavy metals in sludge from both municipal and industrial sources, and metal speciation change as a result of drying process to reduce sludge volume. The changes in sludge properties including sludge moisture content, temperature, density, and electrical conductivity were also monitored to provide insights into the mechanisms causing the change in heavy metal speciation. The results show that the drying process generally stabilized Cr, Cu, Cd, and Pb in sludge by transforming acid-soluble, reducible, and oxidizable species into structurally stable forms. Such transformation and stabilization occurred regardless of the sludge source and type, and were primarily caused by the changes in sludge properties associated with decomposition of organic matter and sulfide. The results enhanced our understanding of the geochemical behavior of heavy metals in municipal sludge, and are useful for designing a treatment system for environment-friendly disposal of sludge. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Safe and cost-effective disposal of sewage sludge from waste treatment plants is still a worldwide environmental challenge. A common goal in the sludge treatment is to reduce toxicity, decrease volume, and convert sludge into useful resources [1]. The decrease in volume by reducing moisture content in the sludge is one of the key steps in the sludge treatment. It becomes only feasible to convert the sludge into nontoxic resources when the sludge volume is significantly reduced [2]. Sludge moisture content is typically reduced to 75–85% after concentration, digestion and mechanical dewatering to remove free water and capillary water in sludge.
∗ Corresponding author. Tel.: +86 13071807599; fax: +86 0571 87951336. ∗∗ Corresponding author. Tel.: +86 509 371 6350. E-mail addresses: [email protected] (H.-X. Weng), [email protected] (C. Liu). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.11.051
The treated sludge still contains too high water content to convert sludge into useful resources [2]. Additional treatment through lowtemperature drying process has to be used to further reduce sludge volume by evaporating residual capillary water, interstitial water, as well as adsorbed water [3]. Heat is often provided to enhance the evaporation process. Heat drying can reduce sludge moisture content to less than 10% [4]. Sludge from sewage treatment plants is a solid-liquid mixture containing organic fragments, bacteria, inorganic particles and colloids [5]. Heavy metals can associate with all these sludge components with complex speciation that can have different eco-environment effects [6]. The drying process will increase solid/liquid ratio that may lead to the changes in the speciation of metals and their relative abundance in the sludge components. Previous studies indicated that drying affect bioavailability of metals in sludge [7], and sludge drying changed the nature of organic compounds leading to changes in their solubility [8]. The mechanisms for the change in metal speciation during the drying process
H.-X. Weng et al. / Journal of Hazardous Materials 265 (2014) 96–103
97
Table 1 Types and characteristics of sewage sludge. Sludge number
Type
Moisture content (%)
pH
Organic content (%)
Source
S1 S2 S3 S4 S5
Municipal sludge Municipal sludge Municipal sludge Leather-tanningsludge Dyeing sludge
83 77 64 67 78
6.6 7.5 6.9 8.3 8.5
54.45 40.4 30.8 28.9 29.5
Hefei Hangzhou Sibao Shanghai Haining Wujiang
are, however, still poorly understood. To safely and cost-effectively dispose the sludge, it is important to understand how the metal speciation and relative abundance in different sludge components change before and after drying processes [9]. In this paper, we report an investigation on the effect of sludge drying process on the speciation of several environmentally important heavy metals including As, Cd, Cr, Cu, Ni, Pb, and Zn. Sludge collected from both municipal and industrial wastewater treatment plants were dehydrated under different temperatures. The objectives of this study are to determine the effect of temperature and sludge source on the speciation of heavy metals and their evolution; and to reveal mechanisms causing the metal speciation changes. Sequential extraction of heavy metals in the sludge samples was used to determine metal speciation. Physical properties, such as sludge moisture content, temperature, density, and electrical conductivity were also measured and analyzed to provide insights into the mechanisms of metal speciation changes.
2. Materials and methods The sludge samples used in this study were taken from municipal, and dyeing and leather-tanning industrial wastewater treatment plants (Table 1). Total 5 sludge samples were collected: 3 municipal sludge samples and 2 industrial samples, from east China cities. Each sample with 4 replicates was dried under different temperatures (25, 105, 150, and 300 ◦ C). After drying, the sludge samples were ground with agate mortars, sieved through 120 mesh (pore diameter 125 m) and stored in polyethylene bags. The dry and ground samples were used for sequential extraction of heavy metals using the BCR method as described elsewhere [10,11]. The total concentrations of heavy metals in the dried sludge samples were determined by first digesting the samples using HCl HNO3 HClO4 [11]. The acid digestion suspensions were then filtered (0.45 um) and the filtrates were measured to determine total metal concentrations using inductively coupled plasma atomic emission spectroscopy (ICP-AES). The organic matter in the sludge was determined using the carbon analyzer (SXL-1304 Programmable Box-type Furnace, Hangzhou Zhuochi Instruments Co., Ltd.). In all the analysis, the variation between the replicates for each sample including blank control was below 5%. The effect of drying process on the sludge physical properties including sludge moisture content, density, temperature, and electrical conductivity were determined as a function of drying time at a drying temperature of 150 ◦ C. Sludge sample from the municipal wastewater treatment plant at Hangzhou Sibao (S1 in Table 1) was used for detail study with measurement procedures provided elsewhere [12]. Briefly, fresh sludge was homogenized and separated into 14 subsamples, which were then weighted (40.0 g each) into 500 ml beakers labeled from 1 to 14. Each sludge subsample was dried in an oven for a specified time (0, 15, 30, 45, 60, 75, 90, 105, 150, 180, 240, 270, 300 or 600 min). After drying, the subsamples were weighted again. Each dried sample was then mixed with 80 ml of deionized water in 200 ml volumetric vessel to determine the total volume (VT ) of the suspension. From VT , the volume (V) and density () of the dried sludge samples can be calculated using: V = VT – 80; = W/V, where W is the weight of the
dried sludge subsample. The conductivity of each subsample was determined after the suspension was settled for 30 min and the supernatant was taken for conductivity measurement using a conductivity meter. The moisture content in the dried subsamples was calculated using the equation: P = (m − M)/m × 100%, where M is the mass of completely dried sludge, m is mass of sludge dried at a specific temperature. The dry-based moisture content was calculated by equation X = P/(1 − P) (kg H2 O/kg DS). In the drying process, sludge temperature will rise and gas will release in concomitant with water vaporization. The sludge temperature was measured using an electrothermal prober (TC15) inserted into the sludge. The composition of the gas released from the sludge samples during the heat drying was also analyzed using the Sibao municipal sludge as an example. Fresh sludge samples (50 g) were placed in flasks heated at 100, 150, or 300 ◦ C for 3 h. The released gas was collected by condensation, filtered, and extracted by petroleum ether. The extraction and fractionation process was conducted at 55 ◦ C. The extracted air samples were analyzed using gas chromatography and mass spectroscopy (GC/MS, HP-150 m × 0.2 mm × 0.33 m, high-purity helium as a carrier gas, initial head pressure:1.6 psi, carrier gas flow rate of 1.02 ml/min, and final head pressure: 30.0 psi) following the method described elsewhere (USEPA,97001). 3. Results 3.1. Physical properties of sludge The physical properties of the sludge vary with sludge source (Table 1). The moisture and organic contents are generally higher in the municipal sludge than that in dyeing and leather-tanning industrial sludge. Among the municipal sludge, the Hefei sludge has the higher organic content (54.45%). The pH values in the municipal sludge is circumneutral, while those in the dyeing and leathertanning sludge are mild alkaline. 3.2. The speciation of heavy metals in sludge Heavy metal concentrations in different types of the sludge are listed in Table 2. There was no significant difference in As concentration in all the studied sludge samples, but the concentrations of other heavy metals varied widely. Zn has the highest concentration among all the measured metals except Cr in the leather-tanning sludge because of the widespread adoption of galvanized pipes in water delivery system. Among the municipal sludge, the Zn content is the highest in the sludge from Shanghai and lowest from the Table 2 Total concentration of heavy metals in sludge (mg kg−1 ). Sludge number
S1 S2 S3 S4 S5
Heavy metals (mg kg−1 ) As
Cd
Cr
Cu
Ni
Pb
Zn
34.7 33.0 33.3 35.2 25.9
1.4 2.9 2.2 – 3.1
129.9 151.1 293.7 17399 294.8
172.5 164.6 517.4 31.3 9.1
45.4 35.6 159.25 6.8 13.7
32.1 32.7 141.1 64.2 15.5
846.5 1343 1731 553.3 277.5
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Hefei. The Ni concentrations followed the same trend as Zn among different sources of the sludge. The Ni in the sludge from Shanghai is 3.51 and 4.47 times, respectively, as those from the Hefei and Hangzhou Sibao. The Shanghai sludge also has a higher Pb content, 4.39 and 4.32 times as that in Hefei and Hangzhou sludge. The Cr content in the leather-tanning sludge is the highest, as expected, nearly 17399.75 mg kg−1 because of the use of Cr-containing chemical compounds during the leather-tanning process [13]. The Cr was enriched through sewage treatment process, leading to a higher concentration in the leather-tanning sludge. The metal contents in the dyeing sludge are relatively lower than those in the other sludge except Cr and Cd, which are the same as or higher than those in the municipal sludge. The speciation of heavy metals in the sludge can be operationally divided into acid soluble, reducible, oxidizable and residual fractions based on the concentrations of the metals extracted using the BCR method [10,11]. The acid soluble fraction consisting of adsorbed ions on ion exchangeable phases, and those associated with carbonate minerals and poorly crystalline minerals. This part of the metals is considered to be mobile, and bioavailable to organisms. The residual fraction of the heavy metals is considered to be associated with stable minerals such as silicates and crystallized oxides of magnesium and aluminum. This fraction of metals has the lowest mobility. Fig. 1 is a triangle phase diagram showing the distribution of each heavy metal in different operational fractions after the drying process at 25 ◦ C. In the diagram, the metal concentrations in the acid soluble (F1) and reducible fractions (F2) were combined to form one end member because the metal in both fractions are likely highly mobile when environmental conditions such as pH and Eh changes. The other two end members are oxidizable (F3) and residual fractions (F4). Fig. 1 indicates that Zn and Ni have the similar species distribution in different types of sludge. They are low in the stable fraction (F4) and high in acid soluble (F1) and reducible state fractions (F2), suggesting that these two metals have a high migration potential. The results are consistent with previous reports [14–16]. The Cd content in all the sludge is low (Table 2). The Cd, however, concentrates at the F1 + F2 phase angle (Fig. 1), indicating that Cd has a high potential of migration despite its low content in the sludge. The As speciation is more diverse as it shows in all the fractions. The Cu speciation is dominated by the oxidizable form regardless of sludge source. It is most likely that Cu was associated with organic matters, forming stable chelated substances in the sludge [15,17]. Cr mainly existed in the oxidizable and residual fractions with a relatively higher percentage in the oxidizable fraction. The higher percentage of Cr in the oxidizable fraction was attributed to its association with sulfur to form stable sulfide minerals [14,15,18]. The sulfur content was high in the sludge as discussed further in the section 4.2. The distribution of Pb is similar as Cr. The difference is that it has a higher percentage in the residual fraction. Overall, these results indicated that sludge from different locations contains variable contents and speciation of heavy metals, reflecting the nature and source of sewages collected at different municipal places and industries. Among all metals analyzed, Zn, Ni, and Cd had the most potential for migration, while Cr, Pb, and Cu were mostly associated with the oxidizable fraction, which may become unstable and release to environment in oxidizing environments. As was partially associated with the mobile and oxidizable fractions except for the sludge from Shanghai, in which most As was associated with stable form.
the drying at 25, 105, 150, and 300 ◦ C. For example, using the speciation at 105 ◦ C as the reference, the relative percentage of Cd in the acid soluble fraction in the Hefei sludge decreased from 21 to 12%, in the reducible fraction decreased from 51 to 37%, while in the oxidizable fraction remained relatively stable, and generally increased in the residual fraction (Fig. 2S1). After drying of the Hefei sludge at 300 ◦ C, the relative percentage of Cr in the oxidizable fraction decreased respectively from 96.89% at 105 ◦ C and 85.42% at 150 ◦ C to 53%, while in the residual fraction it increased from 8 to 42%. The increase in the relative percentage in the residual fraction also observed for As, but not for Zn. The result indicated that the drying process stabilized As, Cd, and Cr, but had no effect on Zn in the Hefei municipal sludge. The similar transformation of As, Cd, and Cr from other fractions to the residual fraction was also observed for Hangzhou sludge (Fig. 2S2). In addition, the relative percentages of heavy metals in the sludge are nearly identical after drying at 105 or 150 ◦ C, implying that the temperature effect became negligible in the range of 105–150 ◦ C. However, after drying at 300 ◦ C, part of oxidizable Cr and Cu transformed to the residual fraction, Ni shifted from the acid soluble to residual fraction, and Zn slightly from the reducible to residual fraction. The results indicated that the stability of all studied heavy metals in this sludge increased as their relative percentages in the residual fraction increased, especially after high temperature (300 ◦ C) treatment. In the Shanghai sludge after drying at 105 and 150 ◦ C (Fig. 2S3), the combined acid soluble and reducible Cr was the one tenth of its total. The oxidizable Cr decreased to 60%, while residual fraction increased to 10%. Most Cu (80%) in the dried sludge was in the oxidizable fraction. The relative percentage of combined acid soluble and reducible Ni was in the range of 43–48%, and oxidizable fraction was 24–29%. Most Zn (70%) was in the reducible fraction. The result suggested that metals in the Shanghai sludge have a high mobility potential even after treatment at 105 and 150 ◦ C, consistent with previous observations [16]. On the other hand, after drying at 300 ◦ C, Cr, Cu, and Pb partially shifted from the oxidizable to residual fraction. Their percentages in the residual fraction, respectively, increased to 38, 46, and 54%. In leather-tanning sludge, the percentages of Cr, Cu, Ni, and Pb changed little after drying at 105 and 150 ◦ C relative to those after drying at 25 ◦ C (Fig. 2S4). The acid solute Zn, however, increased to 57% after drying at 105 ◦ C. The relative percentage of Cr and Ni, however, changed significantly after drying at 300 ◦ C. The oxidizable Cr declined from 81 to 25%, while the corresponding residual fraction increased from 19 to 75%. The oxidizable Ni became negligible, while reducible Ni increased a little after 300 ◦ C treatment. Compared with other types of the sludge, the dyeing sludge contained the highest percentage of reducible metals that were likely in the association with microganisms. Most Cr and Pb were transformed to the residual fraction after drying (Fig. 2S5). The behavior of Zn in dyeing sludge was different from other types of sludge. The dominated Zn speciation in fresh dyeing sludge shifted from oxidizable to residual fraction after drying at 105 and 150 ◦ C. However, after drying at 300 ◦ C, oxidizable fraction dominated Zn speciation again. The reason for the Zn increase in the oxidizable fraction from 150 to 300 ◦ C was unclear. The Zn increase from 0% at 150 ◦ C to about 85% at 300 ◦ C in the oxidizable fraction suggested that a significant amount of Zn sulfide minerals was formed for unexplained reasons.
3.3. The effect of drying process on heavy metal speciation
4.1. Sludge physical properties and heavy metal speciation
Fig. 2 shows that the percentage of the heavy metals in different operational speciation fractions changes complexly after
Metal speciation in sludge depends on sludge geochemical properties such as redox potential, surface molecular structure, charge
4. Discussion
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Fig. 1. Triangular diagrams showing heavy metal speciation in different sludge. F1-acid soluble, F2-reducible, F3-oxidizable, F4-residual.
density that can affect metal association with sludge [19–21]. The drying process will change sludge geochemical properties that can affect metal speciation and metal association with the sludge. The sludge visually changed from pasty to granular after drying with concomitant volume decreasing to one third as water evaporated (Fig. 3). Fig. 3 also shows the change in sludge temperature in corresponding to the sludge volume change as a function of drying time. The result in Fig. 3 revealed a preheat process upto 20 min when the sludge temperature gradually increased despite a constant ambient temperature at 300 ◦ C. After 20 min of preheat process, the sludge temperature reached a steady state that is near 300 ◦ C. The temperature change reflected the effect of combined processes of sludge water evaporation and organic matter pyrolyzation that absorbed heat. The initial increase in sludge temperature resulted from water evaporation from surface toward interior. The higher temperature of the sludge than the drying temperature (300 ◦ C) after 20 min of drying indicated the pyrolyzation process. Fig. 3 shows an excellent inverse correlation between the changes in sludge temperature and volume, reflecting that the sludge lost weight as moisture evaporated with increasing temperature. When the temperature reached the steady state, the sludge weight approached a constant value. The lower is the moisture content, the shorter the time of preheating. For example, the moisture content in the Shanghai sludge and leather-tanning sludge are 64
and 67%, respectively (Table 1), the sludge temperature started to increase rapidly after 10 min, and corresponding sludge volume decreased to a steady state within 25 min. In contrast, the moisture contents in Hangzhou, Hefei, and dyeing sludge are 77, 83 and 78%, respectively, the sludge temperature increased rapidly after 25 min, and sludge volume approached a steady stable after 45 min. Overall, the moisture evaporation and organic matter pyrolyzation are apparently the two processes controlling the sludge temperature and volume changes. Metal ions can exist in interstitial, capillary and absorbed water in the wet sludge (Wang et al., 2007) [22]. The drying process will completely eliminate interstitial water, and partially remove capillary and absorbed water depending on temperature. When most capillary and absorbed water vaporizes, the number of aqueous ions will decline significantly in the sludge. Fig. 4 shows that when the moisture content was above 1.25 kg(H2 O)/kg (DS, Sample No1-9), average electrical conductivity in the sludge leachate was 2226 s/cm. When the moisture content was below 1.25 kg(H2 O)/kg (DS, Sample No10-14), average electrical conductivity decreased to 1588 s/cm, reflecting that the content of total aqueous ions in the dried sludge sample decreased [23]. The decrease in total aqueous ion was likely caused by the precipitation of minerals. As sludge desiccated, aqueous ion concentrations and ion activities would increase. Minerals precipitated when ion activities reached their solubility. The precipitation would decrease
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Fig. 2. Heavy metal speciation in sludge after drying at different temperatures.
the total ions in the aqueous phase in the dried sludge samples. When sludge sample was suspended in 80 ml DI water for electrical conductivity measurements, the measured electrical conductivity decreased. The volatization of aqueous sulfide may have also contributed to the decrease in total ions in aqueous phase as temperature increased. The precipitation would also decrease total heavy metals in aqueous phase, which would drive their desorption from the exchanger phases, and decrease total heavy metal in the acid soluble fraction. In addition, the moisture decrease in the sludge would affect the reducible fraction as the amorphous oxides such as iron and manganese oxyhydroxides dehydrated [13]. The dehydration
process may convert these amorphous minerals to crystalline and stable forms. Through this process, the metals originally associated with the poorly crystalline and reducible fractions can then become the residual fraction. 4.2. The organic matter volatilization and heavy metal speciation The oxidizable fraction is an important part in the sludge. The metals in this fraction primarily existed in the form of sulfide- and organic matter- bound species [13]. Both sulfide and organic matter can become unstable at high temperature. In addition, some organic matters in sludge may be volatile [24].
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Fig. 3. Sludge volume (top plot) and corresponding sludge temperature (bottom plot) as a function of drying time. The ambient (heating) temperature was maintained constant at 300 ◦ C.
Fig. 5 shows that the sludge (S1) density increased with decreasing moisture content, reflecting the effect of organic matter decomposition and sulfide conversion to gas phase. In Fig. 5, there are apparently two regions in sludge density changes. When moisture content is >1.0 kg (H2 O)/kg (DS), the sludge density is close to 1.1 within a narrow range between 1.1–1.2 g cm−3 . When the moisture content is
