Ecotoxicology and Environmental Safety 102 (2014) 18–24

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Sequential extraction of anaerobic digestate sludge for the determination of partitioning of heavy metals Neng-min Zhu a,b,n, Qiang-Li a,b, Xu-jing Guo a,b, Hui-Zhang a,b, Yu-Deng a,b a

Biogas Institute of Ministry of Agriculture, 13 4th Section Renmin South Road, Chengdu Sichuan Province 610041, China Key Laboratory of Development and Application of Rural Renewable Energy, Ministry of Agriculture, 13 4th Section Renmin South Road, Chengdu Sichuan Province 610041, China

b

art ic l e i nf o

a b s t r a c t

Article history: Received 30 July 2013 Received in revised form 30 December 2013 Accepted 31 December 2013 Available online 1 February 2014

In China, agricultural use of anaerobic digestate sludge is considered a concern due to high heavy metal content of the sludge. In this study, sequential extraction procedure (SEP) was conducted to determine metal speciation which affects release and mobility of metal significantly. The results of SEP showed that each heavy metal possessed different distribution characteristics. Cu mainly reacted with carboxyl functional group to form the fraction bound to organic matter. Zn and Mn were dominated in the fraction bound to Fe–Mn oxides and carbonates, respectively. Pb, Ni, Cr, Cd and As were present as the residual fraction. Examination of mobility factors (MFs) indicated that Zn, Pb, Ni, Mn and Cd were more mobile whereas Cr and As were immobilized in anaerobic digestate. Based on the results, it can be stated that Cu, Zn, Mn, Ni and Cd may be grouped as toxic and active components in sludge and should be regarded as the priority pollutants for elimination. Pb should be monitored in terms of its high mobility factors (MF). Cr and As, nevertheless, were the most stable components in sludge. & 2014 Elsevier Inc. All rights reserved.

Keywords: Anaerobic digestate Heavy metals Sequential extraction Metal speciation Toxic components

1. Introduction In the past decade, a large number of household digester and largescale biogas plants were constructed across the country in China (Chen et al., 2012). Consequently, a great deal of by-products named as anaerobic digestate was generated. According to traditional farming practice in China, the anaerobic digestate is commonly used as organic fertilizer to amend agricultural soil. Heavy metals are extensively used as feeding additive for stimulating poultry growth in China, however, their contents are found to be increased in poultry slurry which is used as main substrate materials for anaerobic digestion (Jiang et al., 2011). These additive metals mainly consist of macroelements Zn, Cu, Mn, and microelements Ni, Pb, Cr, Cd, As (Gosens et al., 2013). As a consequence, the resulting anaerobic digestate contains large amounts of heavy metals whose concentrations may be several times higher than their background concentrations in arable soil (Zhang et al., 2012). Therefore the use of anaerobic digestate as organic fertilizer has been criticized considerably in recent years. Recently, some studies have been carried out to investigate the phytotoxicity and ecotoxicity of these heavy metals after they were introduced into agricultural soil along with anaerobic digestate. However, their toxic effects on plants and living beings were usually concluded on the basis of their total contents in previous reports

(Auda et al., 2011; Demirel et al., 2013; Mantovi et al., 2003; Singh et al., 2008). According to Tessier et al. (1979), the speciation of particulate heavy metal was divided into five fractions and each fraction possessed distinctive characteristics and migration behavior in environmental matrix. This means that each fraction might play a different role considering its phytotoxicity and toxicity to plants and living beings. In fact, it has been proved that metal only in the form of free ionic or exchangeable fraction could migrate in soil and accumulate in plant tissues and therefore cause direct toxicity to the ecosystem (Li et al., 2010a; Salazar et al., 2012; Walter et al., 2006; Yang et al., 2010). Consequently, it is untenable to assess the toxic effect of heavy metal in anaerobic digestate only in view of its pseudo-total content, which supposes that all fractions of a given metal have an equal impact on ecosystem. To our best knowledge, there are limited reports aimed at the partitioning of heavy metal in anaerobic digestate. In this study, speciation distribution characterization as well as the total content of heavy metals in anaerobic digestate was investigated extensively. FTIR, XRD and ICP–OES were performed to assist in illuminating metals distribution characteristic. Meanwhile, a cost-effective and practical method used for feasibility assessment of anaerobic digestate for land use was also proposed in terms of obtained results. 2. Materials and methods

n Corresponding author at: Biogas Institute of Ministry of Agriculture, 13 4th Section Renmin South Road, Chengdu Sichuan Province 610041, China. Tel./Fax: þ 86 28 85215106. E-mail address: [email protected] (N.-m. Zhu).

0147-6513/$ - see front matter & 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.ecoenv.2013.12.033

2.1. Chemicals and anaerobic digestate Analytical or higher grade chemical reagents involved in this study were used without any pretreatment, including nitric acid, hydrogen peroxide, hydrochloric

N.-m. Zhu et al. / Ecotoxicology and Environmental Safety 102 (2014) 18–24 acid, perchloric acid, hydrofluoric acid, acetic acid, hydroxylamine hydrochloride, anhydrous sodium acetate and ammonium acetate. Milli-Q ultrapure water (18.2 MΩ cm  1) was used for all the experiments. Anaerobic digestate was obtained from a biogas plant with 800 m3 of space volume which is located in a large-scale intensive pig farm. The pig slurry in this farm was introduced into biogas plant as main substrate for anaerobic digestion. After one stabilization period, 50 kg of anaerobic digestate were sampled and stored in refrigerator at 4 1C for subsequent analysis. 2.2. Sequential extraction procedure For sequential extraction, 1 kg of raw anaerobic digestate was centrifuged at 10,000 rpm to achieve liquid–solid separation. The resultant liquid phase was digested with aqua regia for ICP–OES analysis and defined as water-soluble fraction. The solid phase was dried at 105 1C in a forced air oven for 24 h. Dry solid phase was ground in an agate mortar, homogenized and stored at 4 1C for subsequent extraction procedures. A four-step extraction procedure was performed sequentially to determine the metal speciation according to previous literatures (Mahan et al., 1987; Tessier et al., 1979), and the details are listed in Supporting Information (Table S1). In brief, 2 g of dry sample (at 105 1C) was mixed with extractant in 100 ml Teflon bottles and subjected to extraction in sequence. After one extraction step, the extract was filtrated using 0.45 μm pore water-phase filter paper. Subsequently, the solid residue from pervious extraction step was subjected to the next extraction step. Each aqueous extract sample was digested with aqua regia at 120 1C in order to destroy organic materials completely for ICP–OES analysis. Extraction step 1–2 and sub-step 3 in step 4 were conducted with continuous shaking at 163 rpm and the other steps were carried out with occasional shaking. Batch experiments were performed in triplicate to ensure the reproducibility and each data point reported in all figures and tables represents the mean of triplicates with standard deviation. 2.3. Analytical methods X-ray diffraction (XRD) analysis was carried out using PANalytical X0 Pert Powder X-ray diffractometer (Netherlands). Fourier transform infrared spectroscopy (FTIR) was performed using ThermoFisher Nicolet 6700 spectrometer (USA). Inductively coupled plasma–optical emission spectrometry (ICP–OES) was conducted using Perkin Elmer OPTIMA 2000 spectrometer (USA).

3. Results and discussion 3.1. Normal physicochemical properties of anaerobic digestate Table 1 shows the normal physicochemical characteristics of the anaerobic digestate involved in this study. As can be seen, its moisture content is higher than 95% corresponding to its low dry matter content below 5%. In this intensive pig farm, the pig slurry consisting of excrement and urine was not subjected to solid– liquid phase separation but put into the biogas digester directly. Accordingly, the moisture content in digester was much higher and subsequently resulted in the water content in anaerobic digestate being beyond 90% in present study. In this atmosphere, the substrates were present as fluid state and therefore a slight turbulence appeared occasionally during digestion process, which might affect the distribution of metals among different phases (Bollon et al., 2013; Dąbrowska and Rosińska, 2012). In terms of high water content, COD (844 mg/kg DM) and TOC (241 mg/kg DM) shown in Table 1, this type of anaerobic digestate can be regarded as “organic waste water” and should be subjected to Table 1 Normal physical and chemical characteristics of anaerobic digestate. Density (g mL  1) Dry matter (DM %) Moisture content (%) Total N (g kg  1 DM) Total C (g kg  1 DM) Total P (g kg

1

DM)

0.96 4.77 95.22 64.43 258 14.01

pH COD (g kg  1 DM) TOC (g kg  1 DM) Electrical conductivity (EC mS cm  1) Total dissolved solids (TDS g kg  1DM) Oxidation–reduction potential (ORP mv)

7.38 844 241 12.32 132  145

19

some pretreatment before use as fertilizer, such as dilution with fresh water. Oxidation–reduction potential (ORP) and pH are considered as important factors which can affect metabolism activity of microorganism remarkably (Tanwar et al., 2008; Watling et al., 2012). Hence, the necessary adjustment of ORP and pH should be conducted in order to alleviate their impact on microbe community in soil. In fact, items in Table 1 can be adjusted easily via simple physical treatment in order to meet the requirement for land application. Therefore, characteristic indexes in Table 1 need not be paid special attention to in terms of agricultural use of anaerobic digestate. As for heavy metal, however, both its concentration and speciation should be given more special attention due to its accumulation in plant tissues, which would pose potential risk to living beings via the food chain. 3.2. FTIR and XRD properties of anaerobic digestate As shown in Fig. S1, the FTIR spectrum reveals a number of absorption peaks indicating the complex nature of the anaerobic digestate. A strong-broad peak at 3307 cm  1 was ascribed to the hydroxyl group break in cellulose to form diols during the fermentation process (Zhao et al., 2012). Both a sharp peak at 2926 cm  1 and a weak intense peak at 2854 cm  1 were the characteristics of C–H asymmetrical and symmetrical stretching vibration of alkyl chains, respectively (Abidi et al., 2014). In some cases, the peak at 1656 cm  1 was regarded as a chelated form of the carbonyl on the carboxyl group or a C¼ O stretching mode of amide I band (Sawalha et al., 2007; Majumdar et al., 2008). The appearance of a weak intensive peak at 1544 cm  1 was ascribed to the combination of N–H bending and C–N stretching vibrations of amide II band (Majumdar et al., 2008). A medium strong peak at 1418 cm  1 showed the presence of the COO– in carboxylate group, implying that these reactive functional groups might react with some metal ions to form chelate (Pagnanelli et al., 2009). Some previous studies indicated that the weak and broad peak at 1240 cm  1 might be ascribed to both the overlapping of symmetric carboxyl stretching vibrations from unionized carboxylates and C–O–C stretching vibrations from esters (Gardea-Torresdey et al., 2002). The intense and broad peak at 1056 cm  1 was attributed to the C–O stretching vibration mode (Majumdar et al., 2008). The peak at 873 cm  1 and 696 cm  1 were assigned to the β-linkage of cellulose and C–H rocking in crystalline cellulose, respectively (Alonso-Simon et al., 2011). The corresponding spectra assignments were presented in Table 2. As mentioned above, the FTIR spectra in Fig. S1 confirms the presence of amide, hydroxyl, and carboxyl groups in this anaerobic digestate. In terms of chemical reactivity to metal ions, carboxyl and NH2 groups were much more reactive than other functional groups such as hydroxyl and carbonyl group (Pagnanelli et al., 2009; Zhao et al., 2012). It is hard to identify the chemical bond between functional groups and metal ions definitely but some changes of the peak position in FTIR spectra could be ascribed to the interaction of metal ions with functional groups. Pagnanelli et al. (2009) reported that the enhancement of peak intensity at 1418 cm  1 was ascribed to the interaction of metals with carboxylic adsorbent sites. Meanwhile, Majumdar et al. (2008) also suggested that the shift of peak position from 1544.9 cm  1 and 1407.9 cm  1 to 1523.2 cm  1 and 1399.8 cm  1, respectively, resulted from the interaction of copper ions with the amine and carboxyl groups. In the present study, the weak peak at 1516 cm  1 in Fig. 1 might be assigned to the vibration of carboxyl groups in chelated form of the (COO–)Cu(COO–) according to the FTIR characteristic described by Majumdar et al. (2008). Sawalha et al. (2007) also pointed out that the weak peak at 1656 cm  1 was mainly ascribed to the binding Cd(II) and Cr(III) with carboxyl functional group, which is consistent with that in Fig. S1. As mentioned above, some metal ions interact with reactive functional group during anaerobic digestion process to form metal-organic

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Table 2 Assignments of the main vibrations in FTIR spectra. IR region (cm  1)

Vibrations (cm  1)

Intensity shape

Assignments

3100–3500 2800–3000 1500–1800

3307 2926, 2854 1656 1544, 1516 1418 1240 1058 873 696

Strong-broad Sharp and medium-strong Sharp and medium-strong Weak Weak Weak Strong-broad Sharp Weak

OH-stretching vibrations CH2 asymmetrical and symmetrical stretching C ¼ O stretching in carboxyl N–H bending in amide II and C–N stretching in –CO–NHC ¼ O-stretching in carboxylate C ¼ O stretching C-O stretch β-Linkage of cellulose CH2 rocking

1200–1500 650–1200

additives in feedstuff could be hardly detected via XRD due to its small fraction. And the appearance of Cu1 peak in Fig. S2 might be ascribed to the co-introduction of Cu1 into feedstuff along with metal salts as additives.

100% 90%

80% 70%

3.3. Pseudo-total concentration of metals in anaerobic digestate

60% 50%

40% 30%

20% 10%

Cu

Zn

Pb

water-soluble Fe-Mn oxides

Ni

Cr

exchangeable organic matter

Cd

Mn

As

carbonates residual

Fig. 1. Percentage distribution of different fractions of each heavy metal in anaerobic digestate.

components which was defined as fraction bound to organic matter by Tessier et al. (1979). As can be seen from XRD patterns of the anaerobic digestate in Fig. S2, the main characteristic peaks were associated with crystal SiO2. These SiO2 might come from the crop straws which are mainly used as silage for poultry feeding. SiO2 existed in plant tissues considerably and could not be digested by poultry and anaerobic organism but remained as SiO2 in anaerobic digestate. In spite of its large fraction as inorganic components, these SiO2 could not impose a threat to soil ecosystem from the standpoint of toxicity. In addition, it is interesting that the characteristic peaks of crystal Si1 and Cu1 were also observed in Fig. S2 with weak peak intensity. Since metals used as additives are present as metal salts in feedstuff, such as CuSO4, ZnSO4, the emergency of crystal Cu1 indicated that either metal ion was reduced to zerovalent species during fermentation process or the zerovalent Cu was introduced into feedstuff randomly along with additive. Kieu et al. (2011) reported that Cu2 þ , Zn2 þ and Ni2 þ were precipitated as CuS, ZnS and NiS in anaerobic tank reactors in the presence of consortium of sulfate-reducing bacteria. This suggests that heavy metal ions were hard to reduce to their zerovalent species in anaerobic reducing environment. The ORP and pH shown in Table 1 indicated that the anaerobic digestate was a slight reducing and alkaline matrix. Under this atmosphere, heavy metals could hardly be present as charged free ions but formed the precipitation or organic–metallic compounds. In general, the detection limit of most metals for XRD analysis is beyond 5% (wt%), but the weight percentage for each heavy metal in present anaerobic digestate was below 0.5% (wt%) in Table 4. Accordingly, Cu2 þ introduced as

Table 3 presents the pseudo-total content of heavy metals in this anaerobic digestate. The concentration of heavy metal is expressed with per unit/dry matter of anaerobic digestate (mg kg  1 DM). As shown in Table 3, each metal fraction in solid phase accounted for approximately 90% (Csolid phase/Csum) of its total amount except for Ni whose content in solid phase was 83.85% of its total content. This implies that all given heavy metals possessed great affinity to solid particle and could be prone to form precipitation in this anaerobic digestate. The total concentrations of Cu, Zn, Mn were beyond 1000 mg/kg whereas that for other metals is below 250 mg/kg, indicating that Cu, Zn and Mn were macroheavy metals in this anaerobic digestate, which agrees with previous reports (Govasmark et al., 2011; Tani et al., 2006). As well known, Cu, Zn and Mn were used as macronutrient elements in foodstuff additives to enhance growth rate, which thereby resulted in their accumulation in excreta at high levels (Xiong et al., 2010). In contrast, As, Cd, Ni and Cr were used as microfunction elements in additives to enhance muscle productivity and immunity to disease (Li et al., 2010b), which thus caused their accumulation in excreta at low levels. Especially in the case of Hg, it was not detected both in aqueous and solid phase, indicating that either its concentration was below detection limit of ICP-OES or it was absent in this anaerobic digestate. Due to its more serious toxicity to mammals and human beings as compared with other heavy metals, Hg has been strictly forbidden to use as additive in China. Consequently, Hg was hardly detected both in feedstuff and poultry manure in recent years, which resulted in its absence in this anaerobic digestate. 3.4. Speciation of heavy metals in anaerobic digestate According to Tessier et al. (1979), the chemical speciation of heavy metals in solid environmental matrix consisted of five fraction. In present study, water-soluble fraction from the liquid phase of anaerobic digestate referred to in Section 2.2 was defined as another metal fraction besides fractions described by Tessier et al. (1979). This classification is performed because the mobility, bioavailability and eco-toxicity of heavy metal depend on its speciation more closely than its total content. Accordingly, accurate quantification of each metal speciation in anaerobic digestate is necessary considering its land application. The SEP results are summarized in Table 3 and Fig. 1. For all metals, the sum of six fractions is in good agreement with the pseudo-total concentration with satisfactory recoveries of 91–109%, which confirms the validity and reliability of the obtained results.

N.-m. Zhu et al. / Ecotoxicology and Environmental Safety 102 (2014) 18–24

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Table 3 Chemical fractionation (mg kg  1 DM) and corresponding MF of heavy metals in anaerobic digestate.

Water-soluble Exchangeable Carbonates Fe–Mn oxides Organic Residual Solid phaseb Sumc Pseudo-totald Recovery (%)e MF

Cu

Zn

Pb

Ni

Cr

Cd

Mn

As

101.117 2.24 42.337 2.15 21.81 7 1.78 77.65 7 4.27 804.99 720.23 83.75 7 3.54 1030.53 1131.64 1158.28 7 32.63 98 14.6

294.81 7 3.85 38.29 7 0.86 315.137 6.75 1744.69 7 19.86 240.277 7.34 108.047 1.59 2446.42 2741.23 2743.497 56.87 100 23.6

7.86 70.19 22.577 1.07 26.49 71.94 31.75 7 1.73 17.7071.02 118.78 7 5.39 217.29 225.15 217.51 710.23 103 25.3

12.017 0.28 7.667 0.32 6.34 7 0.42 14.39 7 1.07 6.96 7 0.36 26.95 7 0.86 62.30 74.31 72.727 5.67 102 35

1.96 70.17 0.53 7 0.05 1.55 70.21 2.05 7 0.29 3.11 7 0.12 54.337 4.52 61.57 63.53 60.93 7 6.16 104 6.3

0.447 0.05 1.47 70.16 1.08 70.08 2.317 0.17 1.677 0.08 3.25 7 0.53 9.78 10.22 10.487 0.54 97 29.3

61.58 7 1.79 28.86 7 3.06 960.447 14.36 552.01 77.68 23.89 7 2.54 112.107 5.76 1677.30 1738.88 1900.99 738.1 91 60.4

nda nda 0.447 0.07 8.127 0.36 5.017 0.37 12.22 70.76 25.79 25.79 23.58 72.13 109 1.7

a

“nd” means “not detected”. Sum of fraction in form of “exchangeable, carbonates, Fe–Mn oxides, organic and residual” defined as Tessier et al. (1979). c Sum of all six fractions. d Total metal concentration in anaerobic digestate from aqua regia digestion procedure. e Recovery rate of heavy metals (sum/pseudo-total). b

As shown in Table 3, each heavy metal possessed its own distinctive distribution characteristics, suggesting that they were subjected to different physical and chemical transformations during the same anaerobic fermentation process. Meanwhile, there was significant difference in the dominant fraction distribution among these heavy metals. With regard to main element Cu, Zn and Mn, their dominant fractions were organic, Fe–Mn oxide and carbonate, respectively. Other microelements Pb, Ni, Cr, As, Cd were mainly present as residual fraction. As for most reactive fraction, water-soluble fraction should be paid more attention to due to its direct and acute toxicity to ecosystem. Among these heavy metals, the percentages of Cu, Ni and Zn in water-soluble fraction were almost beyond 9% of their total concentration especially in the case of 16.2% for Ni in Fig. 1. From the standpoint of toxicity of element Ni, its fraction in liquid phase of this anaerobic digestate should be considered to be removed. As for Cu element, its organic fraction accounted for 71.13% of its total content shown in Fig. 1, which was mainly ascribed to the reaction of Cu with carboxyl group to form chelate shown in Fig. S1 (Majumdar et al., 2008). Generally speaking, Cu in the organic fraction was relatively stable but could be released into aqueous environment once the organic matter was degraded via biochemical process under oxidizing conditions (Tessier et al., 1979). With prolonged retention time in soil, these organic matters from anaerobic digestate could decompose into small water-soluble fraction gradually via microbe metabolism following the release of the attached heavy metals. Consequently, physicochemical properties of agriculture soil, such as pH, ORP and water content, should be determined extensively to assist to evaluate their effects on the release risk of different metal fractions. In the case of Zn, nearly 63.64% of its total content was present as Fe–Mn oxides fraction as compared with lowest 1.40% in exchangeable fraction. Some previous reports (Kirkelund et al., 2010; Tu et al., 2012) have pointed out that Zn was bound to Fe–Mn oxides strongly and caused its dominant Fe–Mn oxides fraction in the sludge. Fe–Mn oxides fraction was also defined as “reducible fraction” (Álvarez-Valero et al., 2009), which means that Zn bound to Fe–Mn oxides can be easily reduced and transformed into exchangeable fraction especially under the surface layer of agriculture soil which usually maintains anaerobic atmosphere. This fraction is identified as potential effective fraction with high bioavailability and potential eco-toxicity and therefore special attention should be given. Nevertheless, some other reports showed that Zn was mainly present as organic and residual fraction in different solid matrix. Álvarez-Valero et al. (2009) found that Zn existed as nearly 60% residual fraction in sulphide

tailing and Dong et al. (2013) reported that Zn bound to organic fraction was increased from 78.12% to 85.73% during sewage sludge anaerobic digestion. This indicates that the speciation of a heavy metal in different matrix is changeable and affected significantly by many factors, such as weather conditions, exposure duration and geochemical properties of matrix. As a consequence, any organic matrix should be subjected to SEP to determine each heavy metal fraction accurately before it is used as fertilizer or soil amendment. Meanwhile, the results in previous reports should be cited and used cautiously when first-hand information about heavy metal fraction in matrix is scarce. As for element Mn, sum of fraction bound to carbonates and Fe–Mn oxides accounted for almost 86.97% of its total content. This agrees with findings reported by Tessier et al. (1979) and Mahan et al. (1987). Metals bound to carbonates and Fe–Mn oxides are considered as reactive and potential reactive fraction respectively from standpoint of leaching toxicity. In spite of its exemption from the list of priority pollutants regulated by China, element Mn also should be given suitable notice taking into account its neurotoxicity to human brain and considerable total content in present anaerobic digestate. In the case of higher toxic elements like Pb, Ni, Cr, Cd and As, their residual fractions were present as dominant species and accounted for 52.76%, 36.27%, 85.52%, 31.80%, 47.38% of their total content, respectively. As described by Tessier et al. (1979), metals in residual fraction were bonded to the crystalline structure of the minerals tightly and hardly escaped from the restriction of crystal lattice. This implies that these higher toxic heavy metals possessed stronger affinity to mineral particles as compared with Cu, Zn and Mn, which is in good agreement with previous reports (GarcíaDelgado et al., 2007; Qiao et al., 2013). As described in the case of Zn, some reports also showed that the residual fraction of Pb, Ni, Cr, Cd and As in different matrix was not their dominant species. Anju and Banerjee (2010) found that the Cr and Pb were mainly present as fraction bound to carbonates and Fe–Mn oxides in old tailing dam. Dong et al. (2013) reported that Ni existed in Fe–Mn oxides fraction dominantly and Kaasalainen and Yli-Halla (2003) confirmed that Cd was dominant in exchangeable fraction in surface soil. This difference in distribution characteristic of heavy metals in different solid matrix demonstrated that not only the chemical properties of metal element but also geochemical properties of environment matrix could affect the formation of metal species significantly. The formation of metal species in solid matrix was a complicated and long-term process which was sensitive to many environmental factors. From the standpoint of toxicity caused by heavy metals both the speciation of heavy metals and

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the soil characteristics should be investigated extensively when the anaerobic digestate is used as organic fertilizer or soil amendment.

Table 4 Comparison of metal concentration in anaerobic digestate with that regulated by national control standards. Metal Concentration of monitored metals

3.5. Mobility of heavy metals in anaerobic digestate Mobility of element is defined by its capacity to pass into matrix compartments where it is less energetically retained (Achiba et al., 2010; Juste, 1988), which is always used to reveal its migration possibility as runoff in environment. Mobility factor (MF) is commonly used to determine element mobility, which is calculated using species fraction of element by the following equation (Achiba et al., 2010; Kabala and Singh, 2001) MF ¼

F1 þ F2 þ F3  100 F1 þ F2 þ F3 þ F4 þ F5 þ F6

ð1Þ

Anaerobic digestate (mg kg  1 DM)

Cu Zn Pb Ni Cr Cd As Hg

1158.28 2743.49 217.51 72.72 60.93 10.48 23.58 ndd

Suma (mg kg  1 DM)

1047.89 2633.19 106.37 47.36 9.2 6.97 13.57 ndd

Sumb (mg kg  1 DM)

165.25 648.23 56.92 26.01 4.04 2.99 0.44 ndd

GB4284-84c (mg kg  1) c1

c2

r 250 r 500 r 300 r 100 r 600 r5 r 75 r5

r 500 r 1000 r 1000 r 200 r 1000 r 20 r 75 r 15

where F1, F2, F3, F4, F5, F6 are water-soluble fraction, exchangeable fraction, fraction bound to carbonates, fraction bound to Fe–Mn oxides, fraction bound to organic matter and residual fraction, respectively. The calculated MFs for heavy metals in present study are also listed in Table 3. As can be seen, the MFs for all heavy metals with the exception of Cr and As exceed 20 especially in the case of highest 60.4 for Mn. Whereas the MFs for Cr and As were 6.3 and 1.7, respectively, which is nearly 10 and 35 times lower than that for Mn. This indicates that Cu, Zn, Mn, Ni, Pb and Cd possessed more migration possibility than Cr and As and were likely to be taken up by plant and living beings. These heavy metals with higher MF may create a serious risk to ecosystem via increasing accumulation in agriculture soil after they were introduced into soil along with the anaerobic digestate periodically. However, Cr and As can be regarded as the most stable heavy metals due to their low MFs and dominant residual fraction in this anaerobic digestate, which implied they need not be given more attention in view of their toxicity to environment. In addition, it should be noted that the main fraction contributed to high MFs for Cu, Zn, Mn, Ni, Pb and Cd is different. For Cu, Zn, Ni, Pb and Cd, combined water-soluble and exchangeable fractions were dominantly associated with their MFs, whereas fraction bound to carbonates was the main contributor to MFs of Mn. Heavy metals in water-soluble and exchangeable fraction were found to be the most mobile and active species in environment (Álvarez-Valero et al., 2009; Sánchez-Martín et al., 2007), which will create the most serious damage to ecosystem. Metal species in exchangeable fraction were susceptible to the flush flow containing cation and readily exchanged by other cations. So, irrigation with mixture of chemical fertilizer and water should be carried out carefully to prevent these exchangeable species from being released especially in the case of flood irrigation. Accordingly, Cu, Zn, Ni, Pb and Cd need to be given more intensive surveillance. Combined their speciation and corresponding MFs, these heavy metals in this anaerobic digestate were divided into three groups in terms of their eco-toxicity and bioavailability. Cu, Zn, Mn Ni and Cd were identified as direct toxic and active components. Pb was grouped into potential toxic and effective species. Cr and As were labeled as inert substances which are stable in natural environmental conditions.

alternative standards to evaluate the feasibility of this anaerobic digestate for land application in view of heavy metals due to the similar moisture content and physicochemical properties between this anaerobic digestate and sludge. Element Mn in this anaerobic digestate is in default in GB4284-84 because it is not regulated as China0 s priority pollutants. As shown in Table 4, the total concentrations for Pb, Ni, Cr and As were within the permissible concentration in GB4284-84 whereas Cu and Zn exceeded their regulated concentrations considerably. As mentioned above, however, metals only in the form of mobile and active state can migrate in soil environment and be taken up by plant (Adams et al., 2013; Arkoun et al., 2013), which thereby pose risk to aquatic environment and human health. The results in Table 4 showed that Cu, Zn and Cd still exceeded the threshold concentration in GB4284-84 considerably even if the residual fractions were excluded for calculation. Pb, Ni, Cr, and As, however, were within the permissible values. Taking into account three fractions defined as F1, F2 and F3 in Eq. (1), moreover, the calculated sum of concentrations for Zn was still beyond the control value for agricultural use in acid soil. This suggested that Cu, Zn and Cd were indeed the most toxic and active heavy metals in this anaerobic digestate and should be regarded as the priority pollutants for elimination. However, the periodic use of this anaerobic digestate as fertilizer will result in accumulation of heavy metals in soil, irrespective of reactive or potential reactive metal fraction. The potential reactive species except for residual fraction might be transformed into watersoluble or exchangeable fraction and then released into surface and ground water as times goes on. Hence, the sum of all fractions except for residual fraction should be defined as “effective concentration” and used as substitute for “total concentration” to perform risk assessment of the anaerobic digestate as organic fertilizer.

3.6. Comparison of concentrations of heavy metals in anaerobic digestate with that regulated by national control standard

4. Conclusion

Table 4 displays the total concentration, sum of different fractions in Table 3 and the corresponding limited threshold concentration of each heavy metal regulated by national control standards of China. GB4284-84 aiming at concentration control of pollutants for sludge use in agriculture was used as temporary

c1¼ maximum concentration in acid soil (pH o6.5). c2¼ maximum concentration in alkali soil (pH Z 6.5). a

Sum of all fractions except for the residual fraction. Sum of fractions in form of water-soluble, exchangeable and bound to carbonates. c GB4284-84 refers to the “Control standards for pollutants in sludge from agricultural use”. d “nd” means “not detected”. b

The primary findings and conclusions are summarized as follows: (1) Metal species with considerable total contents or large proportion of effective fractions (F1 þF2 þF3) should be regarded

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as priority pollutants and have to be removed from anaerobic digestates such as Cu, Zn Mn Ni and Cd in this study. Metals with dominant residual fraction (F6) and low mobility factors might be exempt from immediate removal, such as Cr and As in this work. (2) Metal elements mainly associated with sum of potential effective and residual fractions (F4 þ F5þ F6) should be paid special attention to due to their accumulation and species transformation in soil as times goes by, such as Pb in present anaerobic digestate. (3) Except for residual fraction, the sum of other five fractions (F1 þF2 þF3 þF4 þ F5) can be used as “effective concentration” to substitute “pseudo-total concentration” for risk assessment of anaerobic digestate for agricultural application. (4) This work can also provide a cost-effective method to evaluate the feasibility of anaerobic digestate for land use from the standpoint of toxic heavy metals since the investigation of phytotoxicity caused by heavy metals is expensive and timeconsuming.

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