The migration, transformation and ecological risk of heavy metals (Cr, As, Ni, Cu, Zn, Cd and Pb) in sewage sludge during the microwave-assisted thermal hydrolysis process were investigated under different temperatures (80 °C, 100 °C, 120 °C, 140 °C, and 160 °C). The potential relationship between the bio-availability of heavy metals and the variables of microwave treatment, including pH, ammonium-nitrogen, soluble chemical oxygen demand, pH, soluble protein, soluble polysaccharide and volatile solids, was also explored. The results showed that the migration of heavy metals between solid–liquid phase mainly depended on the temperature. The percentage of all heavy metals (except Cu) in mobile (acid-soluble/exchangeable and reducible) forms decreased after microwave-assisted thermal hydrolysis treatment. The solubilisation of compounds with C = O and O-H accompanied with the generation of organic and inorganic metal halides were also observed in the treated sludge through Fourier transform infrared spectroscopy analysis. NH4+-N showed the highest negative correlation to the bio-availability of most heavy metals (except Cu and Cr) with coefficients (absolute value) over 0.87 (P < 0.05). VS showed a positive correlation to the bio-availability of most heavy metals (except Cu). The total potential ecological risk index (RI) decreased by 46.65% after microwave treatment at 160 °C.

  • Effect of microwave treatment on heavy metals speciation in sludge was reported.

  • The metal halides related to organic and inorganic compounds were observed.

  • NH4+-N showed the greatest correlation to the bio-availability of most heavy metals.

  • Environmental risk of treated sludge after microwave treatment was alleviated.

Graphical Abstract

Graphical Abstract
Graphical Abstract

Large amounts of sewage sludge are generated during the biological treatment process of municipal wastewater, consequently the treatment and disposal of sewage sludge has become a serious problem in many countries and regions (Zheng et al. 2007; Yang et al. 2017). Land application is considered an economical way for sludge disposal due to its high contents in nutrients and organic materials (Laurent et al. 2011; Zhang et al. 2015; Ali et al. 2018). However, sewage sludge can do harm to the environment as it contains undesirable components such as various toxic organic compounds, pathogens and heavy metals (Laurent et al. 2011; Zhang et al. 2014; Ali et al. 2018). Among them, heavy metals are non-degradable, persistent and bio-accumulative, which restricts the use of sewage sludge as a fertilizer or soil regenerator (Dong et al. 2013; Liu et al. 2018).

Heavy metals are transferred into sewage sludge through a series of interactions involving ion exchange, surface complexation, precipitation, physical adsorption and so on during wastewater treatment processes (Laurent et al. 2011). Furthermore, the toxicity, bio-availability and mobility of heavy metals depend more on their chemical speciation than their total concentration in the sludge (Fontmorin & Sillanpaa 2015; Zeng et al. 2015; Lin et al. 2019). The chemical speciation of heavy metals could be classified as acid-soluble/exchangeable, reduced, oxidizable or residual fractions according to the modified Community Bureau of Reference (BCR) sequential extraction procedures (Rauret et al. 1999). The chemical speciation of heavy metals could be altered during the physicochemical and biological treatment process of sewage sludge (Gu & Wong 2004; Yuan et al. 2011; Dong et al. 2013).

Thermal hydrolysis is an effective pretreatment technique that can improve the anaerobic digestion efficiency of sewage sludge (Barber 2016; Kor-Bicakci & Eskicioglu 2019), and also change the chemical speciation distributions of heavy metals (Wu et al. 2016; Zhang et al. 2016). Meanwhile, thermal hydrolysis treatment has also been applied to improve the dewaterability of sewage sludge before transportation and disposal (Fang et al. 2017). As an alternative technology to conventional heating for sewage sludge treatment, microwave radiation has received much attention due to several advantages such as high energy utilization rate, fast heating rate and good selectivity (Eskicioglu et al. 2007; Fang et al. 2017; Alqaralleh et al. 2020). However, previous studies have mainly focused on the change in physicochemical characteristics of the treated sludge, the migration and chemical speciation transformation of heavy metals in the sewage sludge during the microwave-assisted thermal hydrolysis process remained unclear, and could influence the microbial community of the anaerobic digestion system as well as the subsequent treatment and disposal of the treated sludge.

Therefore, the thermal hydrolysis treatment of sewage sludge using microwave irradiation was conducted in this study. The migration and chemical speciation distributions of chromium (Cr), arsenic (As), nickel (Ni), copper (Cu), zinc (Zn), cadmium (Cd) and lead (Pb) in the sewage sludge after microwave treatment under different temperatures were analyzed, as well as the relationship between bio-availability of heavy metals and the indexes of sewage sludge after microwave treatment, including pH, ammonium-nitrogen (NH4+-N), soluble chemical oxygen demand (SCOD), soluble protein, soluble polysaccharide and volatile solids (VS). In addition, the ecological risk of heavy metals in the sewage sludge was also evaluated.

Materials preparation

The sludge used in this study is waste activated sludge collected from the secondary sedimentation tank in a local municipal wastewater treatment plant (WWTP) in Tianjin, China which adopted an anaerobic-anoxic-oxic (A-A-O) process as the biological treatment. Before experiments, the supernatant was removed after gravity settling for 24 h and the moisture content is 98.99%. Then the sludge samples were stored at 4 °C for further use. The main characteristics of the raw sewage sludge are listed in Table 1. All the agents used in the experiment were analytically pure and produced in Tianjin, China.

Table 1

Characteristics of the raw sewage sludge

ItemValue
Total solids (TS, g/L) 10.09 ± 0.12 
Volatile solids (VS, g/L) 4.83 ± 0.16 
pH 7.22 ± 0.24 
Total chemical oxygen demand (TCOD, mg/L) 6001.6 ± 82.04 
Soluble chemical oxygen demand (SCOD, mg/L) 194.83 ± 15.67 
Soluble protein (mg/L) 67.24 ± 9.24 
Soluble polysaccharide (mg/L) 11.13 ± 0.35 
Ammonium-nitrogen (NH4+-N, mg/L) 17.45 ± 1.27 
ItemValue
Total solids (TS, g/L) 10.09 ± 0.12 
Volatile solids (VS, g/L) 4.83 ± 0.16 
pH 7.22 ± 0.24 
Total chemical oxygen demand (TCOD, mg/L) 6001.6 ± 82.04 
Soluble chemical oxygen demand (SCOD, mg/L) 194.83 ± 15.67 
Soluble protein (mg/L) 67.24 ± 9.24 
Soluble polysaccharide (mg/L) 11.13 ± 0.35 
Ammonium-nitrogen (NH4+-N, mg/L) 17.45 ± 1.27 

Experimental apparatus and procedures

A microwave digestion system (Milestone ETHOS A, Italy) was employed to carry out the microwave irradiation treatment for the sewage sludge in batches. Firstly, 50 mL sludge was subjected to 100 mL polytetrafluoroethylene vessels. Then the sludge was heated by microwave irradiation to a fixed temperature (80 °C, 100 °C, 120 °C, 140 °C and 160 °C) at different microwave power levels set automatically by the microwave digestion system with heating holding time of 30 min according to a pre-set procedure. Finally, the treated samples were cooled to room temperature for subsequent analysis. To avoid the contingency of the experiment, the experiments with the same parameters were repeated three times, and then the average data were taken for analysis.

Analytical methods

The total solids (ITS), volatile solids (VS), pH, NH4+-N, soluble chemical oxygen demand (SCOD) and total chemical oxygen demand (TCOD) of the sludge were determined based on standard methods (APHA 2005). The concentration of the soluble protein was determined using the Folin phenol reagent method (Frlund et al. 1995). The principle of this method was that soluble protein and Cu2+ could form a complex under alkaline conditions, then the Folin reagent was reduced by this complex with the formation of a dark blue compound. The color depth was correlated with the soluble protein content and could be measured by a spectrophotometer under a wavelength of 500 nm. The soluble polysaccharide was measured using the phenol–vitriol colorimetry method (Kim et al. 2020). Furfural and hydroxymethyl furfural could be generated by the dehydration of polysaccharide using concentrated sulfuric acid. Red compounds could be formed through the condensation reaction of the generated furfurals and phenol. The color depth was correlated with the soluble polysaccharide content and could be measured by a spectrophotometer under a wavelength of 485 nm. The functional groups in sludge before and after microwave irradiation were analyzed by Fourier transform infrared spectroscopy (FT-IR, TENSOR-27, USA).

For determination of the concentrations and chemical speciation of heavy metals, sludge samples were centrifuged at 5,000 r/min for 10 min and freeze dried to constant weight. A freeze-dried sludge sample of 0.25 g was digested with 2.0 mL H2O2 (30%) and 6.0 mL HNO3 at 160 °C for 15 min and 200 °C for 20 min using a microwave digestion system (Milestone ETHOS A, Italy). The digestion solution was filtered through a 0.45 μm microporous membrane after cooling to room temperature, and the concentrations of heavy metals were determined using inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7700, USA). The chemical speciation distributions of heavy metals were determined using the improved BCR method (Rauret et al. 1999). The procedures of the improved BCR method were as follows:

Step 1: Acid-soluble/exchangeable fraction (ionic and carbonate-associated metals). A sludge sample of 0.5 g and 20 mL acetic acid (0.1 mol/L) was put in a 50 mL polypropylene centrifuge tube and shaken at 20 °C for 16 h. The solid and liquid phases were separated by centrifugation at 4,000 r/min for 20 min. Next, the liquid phase was filtered through a 0.45 μm membrane filter and the solid phase was used in the following experiment.

Step 2: Reducible fraction (metals associated with Fe/Mn oxides). The solid phase from Step 1 was immersed in 20 mL of hydroxyammonium chloride (0.1 mol/L, adjusted to pH = 2 with nitric acid) and shaken for 16 h. The liquid phase was separated according to Step 1.

Step 3: Oxidizable fraction (metals bound to organic compounds). The solid phase was contacted with 5 mL hydrogen peroxide (30%) and reacted at room temperature (25 ± 3 °C) for 1 h, with intermittent oscillation during the reaction. The solid phase was digested with a second 5 ml hydrogen peroxide (30%) for 1 h at 85 °C (water bath). After the liquid phase was evaporated to 1–2 mL volume, 25 mL ammonium acetate (1.0 mol/L, adjusted to pH = 2 with nitric acid) was introduced, and then shaken and centrifuged. The liquid phase was separated according to Step 1.

Step 4: Residual fraction. The residues from Step 3 were extracted in the same way as the total concentrations of heavy metals.

Ecological risk calculation

Based on the BCR extraction method, the acid-soluble/exchangeable and reducible fractions of heavy metals were classified as mobile forms, which represent direct toxicity and bio-availability, while heavy metals present in oxidizable and residual fractions have low toxicity and bio-availability (Liu et al. 2008; Jin et al. 2016). The potential ecological risk factor (Er) proposed by Hakanson (1980) was used to quantitatively evaluate the contamination degree and ecological risk of heavy metals in the sludge. The formulas are as follows:
formula
(1)
formula
(2)
formula
(3)
where Cf is the contamination factor of individual heavy metal; Ci is the sum of the concentrations of the acid-soluble/exchangeable fraction, reducible fraction and oxidizable fraction, Cn is the concentrations of the residual fraction, Er is the potential ecological risk factor, and Tr is the toxic response factor for the heavy metal. Tr values of the heavy metals in the sludge were as follows: Cr (2), As (10), Mn (1), Ni (5), Cu (5), Zn (1), Cd (30), and Pb (5) (Hakanson 1980; Huang et al. 2011). RI is the total potential ecological risk index caused by all the heavy metals. The classification criteria of potential ecological risk based on Cf, Er and RI are shown in Table 2.
Table 2

Classification criteria of potential ecological risk (Jin et al. 2016)

CfMetal contaminationErPotential ecological riskRISludge contamination
Cf < 1 Clean Er < 40 Low RI < 150 Low 
1 < Cf < 3 Low 40 < Er < 80 Moderate 150 < RI < 300 Moderate 
3 < Cf < 6 Moderate 80 < Er < 160 Considerate 300 < RI < 600 Considerate 
6 < Cf < 9 Considerate 160 < Er < 320 High RI > 600 High 
Cf > 9 High Er > 320 Very high   
CfMetal contaminationErPotential ecological riskRISludge contamination
Cf < 1 Clean Er < 40 Low RI < 150 Low 
1 < Cf < 3 Low 40 < Er < 80 Moderate 150 < RI < 300 Moderate 
3 < Cf < 6 Moderate 80 < Er < 160 Considerate 300 < RI < 600 Considerate 
6 < Cf < 9 Considerate 160 < Er < 320 High RI > 600 High 
Cf > 9 High Er > 320 Very high   

Hydrolysis efficiency of the sewage sludge

Figure 1 shows the hydrolysis efficiencies of the sewage sludge after microwave treatment under various temperatures. The reductions in the concentrations of TS and VS were proportional to the temperature as shown in Figure 1(a). The concentration of TS in the raw sludge decreased by 16.15% under 160 °C because of hydrolysis of organic matter, gasification of volatile substances and mineralization (Li et al. 2013). Furthermore, amorphous hydroxides could be converted to oxides after microwave irradiation (Obrador et al. 2001), also resulting in the reduction of TS. In addition, under the treatment temperatures of 80 °C, 100 °C, 120 °C, 140 °C and 160 °C, VS content of the treated sludge decreased by 2.69%, 6.83%, 14.49%, 17.18% and 19.88%, respectively. The release of intracellular and extracellular substances after microwave radiation could be represented by SCOD in the supernatant (Alqaralleh et al. 2020). As shown in Figure 1(b), the SCOD concentration reached the highest value of 1,645.25 mg/L at 160 °C. The concentrations of polysaccharide and protein in the liquid phase increased along with the increasing temperature, and reached 421.27 mg/L and 876.25 mg/L (160 °C), respectively. The fluctuating pH values of the treated sludge were mainly due to the generation of NH4+-N and volatile fatty acids (VFAs). The NH4+-N concentration increased with the increasing temperature due to the decomposition of nitrogenous organic compounds. A similar phenomenon was also found in a previous study using conventional thermal hydrolysis (Jeong et al. 2019).

Figure 1

Hydrolysis effects of sludge by microwave treatment at different temperatures. (a) The concentration of VS and TS with their ratio. (b) The concentration of soluble polysaccharide, protein, SCOD, NH4+-N and pH.

Figure 1

Hydrolysis effects of sludge by microwave treatment at different temperatures. (a) The concentration of VS and TS with their ratio. (b) The concentration of soluble polysaccharide, protein, SCOD, NH4+-N and pH.

Close modal

FT-IR spectroscopy of the sewage sludge

The FT-IR spectroscopy of the treated sewage sludge is shown in Figure 2. Similar to previous studies, the main characteristics bands of the FT-IR spectra obtained in this study were associated with chemical bonds belonging to functional groups existed in polysaccharides and proteins, such as -COOH, -NH2 and O-H (Laurent et al. 2011). The peak at a wavenumber of about 3,430 cm−1 was usually caused by the stretching vibration of O-H in water molecules or metal hydroxide, or by being -NH or -NH2. The protein content in sludge decreased after microwave treatment, but the peak around 3,430 cm−1 was enhanced, which may be due to the formation of metal hydroxides. The peaks at 2,920 cm−1, 2,850 cm−1 and 1,650 cm−1 were due to the anti-symmetric and symmetric absorption peak of =CH2 and the stretching of CO-NH, respectively (Jin et al. 2016). The peaks at a wavenumber of 1,540 cm−1 could be attributed to the bending vibration in the N-H plane (Wu et al. 2016). The intense peak at about 1,240 cm−1 was linked to the stretching and deformation vibrations of C = O and O-H. The intensity of this peak decreased along with increasing temperature, which indicated the interactions of carboxyl and hydroxyl groups and the solubilization of compounds with these functional groups (Laurent et al. 2011). In addition, heavy metals complexed with C = O and O-H could also be transformed into other forms. The peak at 600–800 cm−1 indicated the existence of aromatic and hetero-aromatic compounds. The aromatic structure could provide π-electrons, which were able to bond with heavy metal cations (Jin et al. 2016). The peak at 669 cm−1 appeared after microwave irradiation treatment might be the metal halides related to organic and inorganic compounds (Hossain et al. 2011; Jin et al. 2016; Udayanga et al. 2019). On the whole, the change of group was not obvious after microwave irradiation treatment due to the relatively mild conditions in this study, which was different from the results of pyrolysis of sludge at high temperature (Jin et al. 2016; Udayanga et al. 2019), but a similar phenomenon was also found in previous studies using conventional thermal hydrolysis (Laurent et al. 2011; Wu et al. 2016).

Figure 2

FT-IR spectroscopy of sludge treated by microwave irradiation at different temperatures.

Figure 2

FT-IR spectroscopy of sludge treated by microwave irradiation at different temperatures.

Close modal

Migration and transformation of heavy metals in the sewage sludge

Heavy metals in the sewage sludge are present inside microbial cells, or adsorbed by EPS (Dewil et al. 2006, 2007). Microwave treatment could destroy the structure of cells and EPS of the sludge (Wang et al. 2015; Liu et al. 2016), which may also lead to the release of metals from solid phase into liquid phase. Thus, the release of metals could result in the decrease in total concentrations (mg/kg-TS) of metals in the solid phase, with the reduction of heavy metals present in unstable forms. Furthermore, thermal hydrolysis could also cause the loss of solid mass of sludge. For these metals which were not easily released into the liquid phase, their total concentrations might increase due to the decrease in TS (Wu et al. 2016). In addition, the released metals could also combine with inorganic or organic ligands instead of being dissociated as free ions (Zhang et al. 2016). It is easier for heavy metals to enter into the solid phase where they could be absorbed by hydrolyzed metal oxides and form polyphosphates by combining with uncompleted hydrolyzed amino compounds or hydrolyzed phosphorus compounds (Zhang et al. 2016). These reactions could also cause an increase in the concentrations of metals in the sludge and change their chemical forms. The total concentrations and chemical speciation of heavy metals in the sludge after microwave treatment at different temperatures are shown in Figure 3.

Figure 3

Chemical speciation and total concentration changes of heavy metals at different temperatures.

Figure 3

Chemical speciation and total concentration changes of heavy metals at different temperatures.

Close modal

The total concentrations of Cr and As in the sludge showed a downward trend after microwave treatment as shown in Figure 3. A similar phenomenon was found in a previous study using traditional thermal treatment (Obrador et al. 2001). As shown in Figure 3, the concentrations of Cr and As in mobile forms (acid-soluble/exchangeable and reduced fractions) also decreased after microwave treatment, which are consistent with the results found by Zhang et al. (2016) carrying out traditional thermal hydrolysis treatment of sewage sludge. The concentrations of Cr present in acid-soluble/exchangeable fractions decreased by 51.06%, 51.79%, 39.79%, 52.19% and 61.03% at 80 °C, 100 °C, 120 °C, 140 °C and 160 °C, respectively. First, the VFAs produced during sludge treatment process could dissolve Cr in acid-soluble/exchangeable fraction due to its instability under weak acid conditions (Karwowska et al. 2015; Zou et al. 2019). Furthermore, partial complex organic matters were hydrolyzed into small molecules and lost the binding sites of heavy metals (Wei et al. 2019), resulting in the release of Cr in an acid-soluble/exchangeable form. The concentrations of Cr in the reducible fraction (mainly oxide and hydroxide) decreased by 45.16%, 59.31%, 34.78%, 47.51% and 54.36% at 80 °C, 100 °C, 120 °C, 140 °C and 160 °C, respectively. The main reason might be that H+ accumulated on the surface of Fe/Mn oxide, and the Cr lost the chance to be adsorbed on the surface of Fe/Mn oxide (Wu et al. 2016). In addition, the proportion of As in acid-soluble/exchangeable and reducible forms decreased gradually with the increase in treatment temperature. Cr and As mainly existed in residual fractions in the raw sludge with the proportions of 51.25% and 48.00%, respectively, which also decreased in the treated sludge. The concentrations of Cr and As in residual fractions decreased by 30.28% and 13.32% at 160 °C, respectively, probably due to the destruction of the crystal structure by high temperature (Wu et al. 2016).

As shown in Figure 3, the concentration of Ni increased after microwave treatment. This phenomenon was also observed in the conventional thermal hydrolysis treatment process of sewage sludge (Zhang et al. 2016; Wang et al. 2019). Ni mainly existed in acid-soluble/exchangeable fraction in the raw sludge (Figure 3). The concentration of Ni in acid-soluble/exchangeable fraction decreased with the increasing temperature, which was similar to the results of the sludge pyrolysis process (Jin et al. 2016; Liu et al. 2016). The concentrations of Ni in mobile forms decreased by 11.86%, 32.07%, 44.74%, 41.33% and 38.79%, respectively, while the concentration of Ni in oxidizable fraction increased significantly after microwave treatment. Over 65% (more than twice of the initial content) of Ni was presented in the oxidizable fraction (metal sulfide and organic bound form) in the treated sludge (160 °C), which was the main reason for the increase in the total Ni concentration. A previous study had concluded that the enrichment of Ni might be due to its higher thermal stability compared with organic materials (Wang et al. 2019).

Cu mainly existed in the oxidizable fraction (88.12%) in the raw sludge, while Cu in mobile fraction was almost undetectable in this study. A low amount of Cu was released after microwave treatment, and its content was concentrated in the treated sludge due to the loss of TS. Cu had been proven to have the strongest complexing ability with organic materials in sludge (Rudd et al. 1984). The concentration of Cu in the oxidizable fraction increased obviously after microwave treatment, which might be due to the combination of Cu with intracellular organic materials produced by the breakdown of cells (Zhang et al. 2015; Wu et al. 2016). Furthermore, the concentration of Cu in residual fraction also increased after microwave treatment.

The concentration of Zn in the raw sludge was the highest among the heavy metals detected, probably due to the widespread use of zinc-plating pipes in wastewater treatment plants in China (Zhang et al. 2018). The total concentration of Zn decreased slightly after microwave treatment at 80–140 °C. However, the concentration of Zn increased by 5.40% after microwave treatment under 160 °C (Figure 3) due to the obvious loss of sludge TS as shown in Figure 1. The concentrations of Zn in the acid-soluble/exchangeable fraction decreased by 30.36%, 36.39%, 50.83%, 51.03% and 49.63% at 80 °C, 100 °C, 120 °C, 140 °C and 160 °C, respectively. The concentration of Zn in the oxidizable fraction increased after microwave treatment, probably due to the transformation of Zn in the mobile fractions (Shao et al. 2015). In addition, the concentration of Zn in the residual fraction increased markedly at 160 °C, possibly because partial Zn2+ reacted with PO43− with the formation of the insoluble phosphate or entered the crystal lattice at high temperature and pressure (Wu et al. 2016).

The total concentrations of Cd and Pb decreased after microwave treatment (Figure 3). As shown in Figure 3, the concentrations of Cd present in acid-soluble/exchangeable and reducible fractions were reduced by 38.74%–55.17% and 35.91%–52.72% in the treated sludge at different temperatures, respectively. In addition, the concentration of Pb in the mobile forms also decreased by 32.92%–67.52% after microwave treatment. The concentrations of Ni and Pb in the oxidizable fraction also decreased due to the dissolution of organic materials (Yuan et al. 2011). Pb mainly existed in residual form in the raw sludge and microwave treatment further increased its concentration. Similar findings were reported when treating sewage sludge by thermal treatment (Naoum et al. 1998) and pyrolysis (Jin et al. 2016) due to the formation of insoluble compounds.

On the whole, the total concentrations of Cr, As, Cd and Pb in the sewage sludge decreased after microwave irradiation treatment mainly due to the release of these metals in mobile form from solid phase into liquid phase. However, the concentrations of other metals increased due to the mass loss of the treated sludge. In addition, the concentrations of most heavy metals (Cr, As, Ni, Zn, Cd and Pb) in mobile form decreased after microwave treatment, which could reduce their bio-availability in the environment.

The reduction of Cr, As and Zn contents in mobile form after microwave irradiation treatment at 160 °C in this study was more efficient than for those treated by traditional thermal hydrolysis at 170 °C for 30 min (Wu et al. 2016). Ozone treatment increased the bio-availability of heavy metals due to the acid-soluble/exchangeable fraction (Zhang et al. 2017). Dong et al. (2013) also found that anaerobic digestion increased the bio-availability of some heavy metals. Therefore, sludge treated by microwave irradiation posed less risk to the environment compared with ozone and anaerobic treatment sludge.

Correlation analysis of the indexes of sewage sludge and the bio-availability of heavy metals

The correlation analysis between the bio-available fractions (acid-soluble/exchangeable and reducible fractions) of heavy metals and the indexes of sewage sludge (pH, NH4+-N, SCOD, soluble protein, soluble polysaccharide and VS) was carried out to explore their potential connections. The correlation coefficients are shown in Figure 4, where −1 indicates a completely negative linear correlation and 1 indicates a completely positive linear correlation between the bio-availability of heavy metals and the variables.

Figure 4

Correlation heat map between the variables (polysaccharide, protein, SCOD, NH4+-N and pH) during microwave treatment and the bio-availability of heavy metals (*:P < 0.05; **:P < 0.01).

Figure 4

Correlation heat map between the variables (polysaccharide, protein, SCOD, NH4+-N and pH) during microwave treatment and the bio-availability of heavy metals (*:P < 0.05; **:P < 0.01).

Close modal

VS showed a good positive correlation to the bio-availability of Ni, Zn and Pb with coefficients of 0.898 (P < 0.05), 0.965 (P < 0.01), and 0.918 (P < 0.01), respectively. Probably because the break of EPS decreased both the VS and concentrations of Ni, Zn and Pb in the acid-soluble/exchangeable fraction with the increasing treatment temperature (Figure 3). NH4+-N showed the greatest correlation to the bio-availability of most heavy metals (except Cu and Cr) with the coefficients (absolute value) over 0.87 (P < 0.05), because NH4+-N could combine with heavy metals or compete for active sites of the organic particle surface against heavy metals (Zheng et al. 2020). Soluble polysaccharide was also well correlated with the bio-availability of Zn (−0.874, P < 0.05) and Pb (−0.831, P < 0.05). The soluble protein and SCOD showed moderate correlation to the bio-availability of most heavy metals (except Cu) with the coefficients ranged from 0.50 to 0.75. It has been reported that the bonds of polysaccharides and proteins (-COO, -OH, C = O, and so on) with heavy metals could be broken down during hydrolysis (Zhang et al. 2016), which is one of the reasons for the correlation between the soluble organic matters in the sludge and the bio-availability of heavy metals. The bio-availability of Cr presented lower correlation to VS and soluble polysaccharide with the coefficients of 0.598 and −0.605, respectively, which might be due to the high stability of Cr in the raw sludge as illustrated in Figure 3. The coefficients between pH and the bio-availability of heavy metals (except Cr) were lower than 0.32. It might be that carbonates or sulfides could not be affected obviously by the change of pH (7.02–8.73) recorded in this study. However, the capture/release of cations caused by pH changes might alleviate/intensify the competition of heavy metal ions for adsorption positions on the surface of organic matter (Zheng et al. 2020), thus affecting the bio-availability of Cr (with the coefficient of −0.502). Cu in bio-available fraction was barely detected in the sewage sludge, resulting in poor correlation between its bio-availability with the variables.

The stepwise linear regression model was established to quantitatively describe the correlation between the variables (pH, NH4+-N, SCOD, soluble protein, soluble polysaccharide and VS) and the bio-availability of heavy metals (except Cu and Cr), as shown in Table 3. Table 3 illustrates that the bio-availability content of As and Cd could be predicted solely based on the concentration of NH4+-N, which further proved the important influence of NH4+-N. The bio-availability content of Ni and Zn could be estimated solely by the concentration of VS in sludge based on their strong negative correlations. The content of Pb in bio-available fraction could be predicted using the concentration of NH4+-N and SCOD in the sludge with a high R2 value of 0.995. The stepwise linear regression model for Cu and Cr cannot be built due to the poor correlation between their bio-availability contents and the variables of microwave treatment.

Table 3

Stepwise linear regression model for predicting the bio-availability of heavy metals based on the variables (VS, NH4+-N and SCOD) during microwave treatment

EquationR2
Bio-As = 11.439 − 0.135b 0.833 
Bio-Ni = −20.35 + 7.839a 0.807 
Bio-Zn = −824.405 + 736.25a 0.932 
Bio-Cd = 2.846 − 0.038b 0.765 
Bio-Pb = 15.862 − 0.359b + 0.003c 0.995 
EquationR2
Bio-As = 11.439 − 0.135b 0.833 
Bio-Ni = −20.35 + 7.839a 0.807 
Bio-Zn = −824.405 + 736.25a 0.932 
Bio-Cd = 2.846 − 0.038b 0.765 
Bio-Pb = 15.862 − 0.359b + 0.003c 0.995 

a: VS b: NH4+-N c: SCOD.

Ecological risk of heavy metals

The changes of Cf, Er and RI of heavy metals in the sewage sludge before and after microwave treatment are shown in Figure 5. Among all the heavy metals, Zn in the raw sludge caused the highest contamination risk with the Cf values of 54.02, as can be seen in Figure 5(a), which could be reduced significantly by microwave treatment (Cf = 14.31, 160 °C). Ni in the raw sludge was also at a high degree of contamination risk, which decreased from 20.10 (high risk) to 6.39 (considerate risk) after microwave treatment at 160 °C. Cu in the raw sludge had a considerable contamination risk with the Cf value of 7.42, which decreased to 3.20 at 140 °C. The Cf values of Cr and Pb in the raw sludge were less than 1.00, indicating little contamination risk. However, the Cf value of Cr slightly increased after microwave treatment under 120–160 °C due to the increase in the content of Cr in oxidizable fractions. As was also at a low risk level with the Cf value of 1.08, which further decreased after microwave treatment. There was no significant change in the Cf values of Cd before and after microwave irradiation, which distributed in the range 3.26–4.41.

Figure 5

Ecological risks of heavy metals in sludge before and after microwave irradiation.

Figure 5

Ecological risks of heavy metals in sludge before and after microwave irradiation.

Close modal

As shown in Figure 5(b), the RI value of heavy metals in the raw sludge was 322.49, indicating that it would cause a considerable ecological risk if the raw sludge was exposed directly to the environment, among which Cd and Ni contributed most of the total potential ecological risk. This was different from the results of Cf value because the Er value takes into account the toxicity of heavy metals. Microwave treatment has a great influence on the Er value of Ni and Zn, which could be decreased by 68.19% and 73.51% at 160 °C, respectively. While the Er value of Cd had a limited decrease from 115.77 to 97.87 after microwave treatment under 160 °C, the Er values of Cr, As, Cu and Pb in raw sludge and treated sludge all showed low potential ecological risks. Correspondingly, the RI value of heavy metals in the treated sludge was significantly decreased to 172.05 by microwave treatment at 160 °C.

Microwave-assisted thermal hydrolysis treatment had a significant effect on the migration and chemical speciation transformation of heavy metals in sewage sludge. Microwave treatment decreased the total concentrations of Cr, As, Cd and Pb in sewage sludge due to the release of metals in mobile forms. However, the contents of Ni, Cu and Zn were concentrated because of the loss of organic matter. In addition, the content of Cr and Zn in the acid-soluble/exchangeable fraction decreased by 61.03% and 49.63% at 160 °C, respectively. Ni in the mobile fractions decreased by 38.79%. However, the content of Cu in the oxidizable fraction increased by 20.63% (160 °C). The content of Cd present in the acid-soluble/exchangeable fraction decreased by 38.74%–55.17%, and the content of Cu in the mobile fractions decreased by 32.92%–67.52% after microwave-assisted thermal hydrolysis treatment. NH4+-N showed the highest negative correlation to the bio-availability of most heavy metals (except Cu and Cr) with the coefficients (absolute value) over 0.87 (P < 0.05), followed by VS with a positive correlation (except Cu) with the coefficients (absolute value) over 0.60. However, soluble polysaccharide, protein and SCOD presented moderate correlations. The ecological risk (reflected by Er and RI values) of all the heavy metals (except Cu) in the treated sludge was reduced significantly. The RI value of heavy metals decreased by 46.65% after microwave treatment at 160 °C. The microwave-assisted thermal hydrolysis treatment can effectively stabilize heavy metals in sludge and reduce their ecological risk.

This work was supported by the research fund of the National Natural Science Foundation of China (No. 51908398), the Tianjin Municipal Education Commission (No. 2016CJ07), Tianjin Science and Technology Committee (No. 14ZCDGSF00032, No. 19JCQNJC08200) and College Student Innovation and Entrepreneurship Training Program (No. 202010792030).

The authors declare no conflict of interest.

All relevant data are included in the paper or its Supplementary Information.

Ali
M.
Huang
Q.
Lin
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