There are two methods to control the pollution of heavy metals in sewage sludge: namely, heavy metals removal, and heavy metals immobilization. During the first process, heavy metals are removed from sewage sludge by chemical leaching, bioleaching, electrochemical method, or their combination, thoroughly eliminating the pollution risk of heavy metals. The cost of removing heavy metals from sludge is higher than immobilizing heavy metals in sludge. In order to avoid causing secondary pollution, the heavy metal pollutants separated from sewage sludge must be seriously considered [74
]. During the second process, the heavy metals are transferred into more stable fractions, reducing their mobility and bioavailability. The immobilized heavy metals still exist in sewage sludge. With the extension of time or changes in environmental conditions, they have the possibility of reactivation. The commonly used immobilization methods are sewage sludge composting and chemical immobilization.
3.1. Sewage Sludge Composting
Composting is a complex dynamic digestion process that comprises three major phases: the mesophilic phase, the thermophilic phase, and the cooling phase [75
]. Microorganisms utilize OM for metabolism, and transform the biodegradable fraction into stable humic components in the process [76
]. The volume of solid waste can be reduced by 40–50%, and the metabolic heat generated in the thermophilic phase destroys pathogens [77
]. The end product is rich in humic substance, and can be used as soil conditioner/fertilizer due to the presence of N, P, K, and other nutrients. However, the presence of heavy metals in compost restricts its uses as soil conditioner/fertilizer. The heavy metals may be absorbed by plants and pose an indirect risk on human health due to their bioaccumulation and biomagnification properties [79
Sewage sludge is often co-composted with bulking agents to reduce the content of heavy metals and optimize substrate properties such as air space, moisture content, C/N ratio, and pH. Some lignocellulosic byproducts, such as wood chips and sawdust, are commonly used as bulking agents. The addition of bulking agents not only positively affects the composting rate, but also has a dilution effect on the contents of heavy metals. Amir et al. [49
] reported that the contents of heavy metals in compost increased with the increased proportion of sewage sludge in the composting mixture. Other additives, such as zeolite, manure, and red mud, not only function as bulking agents, but also play the role of passivation agents to immobilize heavy metals. Their passivation mechanism will be discussed in Section 3.2
During the composting process, the stabilization of heavy metals was mainly achieved by organic mineralization, microbial adsorption, and the complexation of humic substances. In order to ensure the safe use of compost, it is necessary to study the speciation transformation of heavy metals and their bioavailability during the process. Many researchers have been dedicated to studying the bioavailability of heavy metals during the composting process [47
If there is no metals loss by leaching during the composting process, a continuous increase of total heavy metals concentration is observed due to the weight loss of OM [81
]. Zheng et al. [82
] found that Ni and Cr concentration increased 30.4% and 36.0%, respectively, due to the volatilization of gases such as H2
O and CO2
, which were produced during the degradation process of OM. The proportions of exchangeable, carbonate-bound, Fe–Mn oxide-bound, and organic matter-bound Ni and Cr decreased, while the proportions of residual Ni and Cr increased substantially, reducing their plant availability and environmental risks. Amir et al. [50
] concluded that after a composting period of 180 days, the largest proportions of metals were in the residual fraction and fractions more resistant to extraction. The amount of potentially bioavailable metals was less than 2%.
The speciation of heavy metals in sewage sludge-based compost depends not only on their initial chemical states, but also on the OM transformation during the composting process. Kulikowska [83
] reported that the mineralization of OM during the composting process occurred most intensively in the first 15 days, whereas humification was most intensive during the next three months. During the later stage of humification, the amount of humic substance didn’t change substantially, while its degree of polymerization increased substantially due to the formation of complex molecules of HA from simpler fulvic acid molecules. The concentration of humic substance and the proportion between HA and fulvic acid were important for the stabilization of heavy metals [84
]. The humic substance would provide numerous non-specific and specific sites for metals adsorption. The formation of insoluble organometallic complexes could decrease the mobility and phytotoxicity of heavy metals [85
He et al. [87
] suggested that the transformation of heavy metals speciation and phytotoxicity of sewage sludge were dependent on multiple components, such as pH, OM, dissolved organic carbon (DOC), and mobile metal fractions, rather than a single element. The decomposition of OM during composting was thought to be the most important accessorial factor to influence the phytotoxicity and speciation of heavy metals. Stable and soluble complexes would be formed between DOC and heavy metals, which helped to weaken the risk of heavy metals. The germination index of Pakchoi was predictable from the overall mobile fractions of Cu. For Zn and Pb, the R-values were significantly increased by utilizing other components, such as pH, OM, and DOC.
The toxic metals distribution and bioavailability in the final compost depend on the speciation of metals themselves, sludge characteristics, composting process, and physicochemical properties of the final compost, such as amount of organic carbon, humic matter content, and pH, etc. [87
]. As an effective method to reduce the mobility of heavy metals in sewage sludge, composting has the shortcomings of a large one-time investment, a wide area, and a long period of composting. New composting technology with low investment and high efficiency is needed to improve the quality of compost and reduce its investment [83
3.2. Chemical Immobilization
The migration of heavy metals can be reduced by reacting with chemical passivating agents through precipitation, chelation, adsorption, and ion exchange. During the process, the volume of sludge does not increase or increases only a little, thus reducing the subsequent cost for sludge transportation and storage. Chemical immobilization will be an attractive technology to immobilize heavy metals in sewage sludge if the chemical passivating agent is cheap and ease of application. The commonly used additives include basic compounds, aluminosilicate, phosphorus-bearing materials, and sulfides.
3.2.1. Basic Compounds
Basic compounds are used for an increase of pH in sludge. Heavy metals are precipitated in metal hydroxide form with the increased pH. The increased pH also enhances the precipitation of metal carbonates, thus reducing the exchangeable metal concentration. What’s more, the variable charges of the sludge surface are increased, which reduces the specific ability to adsorb heavy metals. Lime is the most commonly used basic compound to stabilize heavy metals in sludge. Jim et al. [89
] found Cu, Zn, and Ni contents in Altari radish
leaves in the industrial sewage sludge-treated soil were reduced by the addition of lime, and were negatively correlated with soil pH. Li et al. [90
] added 5%, 7%, 10%, 12%, and 15% of lime into dewatered sludge with 86.0% moisture content, and found that the contents of Cd, Cu, and Zn in acid-extractable form were significantly decreased, while those in iron-manganese oxide form, organic form, and residual form were increased. The inorganic form of Pb was slightly increased, and the residual form remained unchanged. The highest passivation efficiency was achieved by adding 7% of lime. If excess lime is added, the pH would exceed the appropriate scope of land use, and thus limit the subsequent land use of sludge. Once the environmental conditions change or the pH value is reduced, the precipitation of heavy metal oxides will be partially dissolved, once again harming the environment [91
3.2.2. Aluminosilicate Materials
Zolite, fly ash, and bentonite are all aluminosilicate materials that are usually used as chemical passivating agents to stabilize heavy metals in sludge. Their mineral phase composition is similar to that of soil, so that they would not have a significant influence on the phase composition of soil minerals even if they are used in soil for a long time.
Fly ash (FA) is generated in huge quantities from coal-fired power plants, and mainly consists of silica (SiO2
), alumina (Al2
), calcium oxide (CaO), iron oxide (Fe2
), magnesium oxide (MgO), sodium oxide (Na2
O), potassium oxide (K2
O), unburned carbon, and sulfate (SO42−
]. It is a heterogeneous mixture of both amorphous and crystalline phase, with a high specific surface area. The predominantly amorphous aluminosilicate glassy spheres have a strong capacity for ion exchange, thus producing isomorphous replacement with heavy metals in sludge. The pH of FAs is linearly associated with the content of CaO, or the CaO/SO42−
ratio. It varies from 4.5 to 12.0. The majority of FAs are alkaline. The high pH value is beneficial for the oxidizable fraction to transfer into the residual fraction [93
]. The combined use of FA and sewage sludge has been proposed to reduce the bioavailability of heavy metals [27
Wang et al. [95
] found that with the increasing addition of FA, the exchangeable, carbonate, and organic matter contents of Cr, Cu, Mn, Pb, and Zn in sludge decreased, while the iron and manganese oxidation state and residual state contents increased. In stabilized sewage sludge with a volumetric ratio to soil of either 1:1 or 1:5, the contents of Cu, Cr, Mn, Pb, and Zn in soil-percolating water were much lower than the sludge treatment without the addition of FA. Bian et al. [96
] used the microwave/alkali method to modify FA, and found that the modified FA had the larger specific surface area, and a large number of active groups were produced on the surface, which helped to form covalent linkage with heavy metals ions. Hence, the modified FA was conducive for the stable transition of heavy metals, and its passivation performances on the exchangeable fraction, reducible fraction, oxidizable fraction, and residual fraction of Cu were 18.1%, 24.48%, 33.25%, and 249.19% higher than the unmodified FA, respectively.
Zeolites are microporous aluminosilicate minerals that consist of three-dimensional frameworks of [SiO4]4− and [AlO4]5− tetrahedra. The replacement of Si4+ by Al3+ produces a net negative charge, which gives rise to a high cation exchange capacity (CEC). Zeolites are not only naturally occurring, they are also produced industrially on a large scale. Both kinds of bentonite have a high specific area and CEC, which help their ability to immobilize heavy metals.
Metals retention was thought to be an ion exchange process. Zeolite acts as a binder during the process. Metal ions move not only through the pores of zeolite mass, they also move through the channels of the lattice replacing exchangeable sodium cation and calcium cation, and then are fixed in the zeolite matrix [97
]. Ashmawy et al. [36
] showed that the incorporation of Pb2+
, and Zn2+
into the zeolite frameworks changed the lattice parameters slightly through XRD analysis. It was concluded that CEC mainly affected the immobilization of heavy metals, rather than the pH value. The retention percent of heavy metals at the optimum zeolite/sludge ratio (10%) was more than 96% for Cd, Cu, Pb, and Ni, and about 79% for Zn.
With the addition of zeolite, environmental alkalinity increases, which helps promote the metal adsorption via surface complexation. Antoniadis et al. [99
] studied the combined effect of liming and zeolite on metal availability, and found that the plant availability of Cu and Zn in limed soil decreased significantly 50 days after sowing. Meanwhile, the availability of heavy metals after 100 days would increase if excess zeolite was added, due to the metals that were initially sorbed onto zeolite perhaps desorbing back into the soil solution. Hamidpour et al. [100
] studied the desorption in a batch test, and found that up to 40% of Cd was desorbed from zeolite. More measurements must be considered to reduce the desorption of heavy metals from zeolite if zeolite is to be used as the additive to immobilize heavy metals in sludge.
The fixation ability of zeolite for heavy metals depends on its structure and physicochemical properties. Kosobucki et al. [101
] applied ultrasonic energy to remove adsorbed water in the skeleton of zeolite, and found that more active centers were made for the metal ions by the ultrasonic treatment. The adsorption of heavy metals by the modified zeolite increased 3–7% in comparison with the unmodified zeolite.
Bentonite is rich in montmorillonite from the smectite group and also contains a variety of accessory minerals, such as quartz, feldspar, calcite, illite, and mica, depending on the nature of their genesis. Montmorillonite is an aluminosilicate layer formed from sandwiching a single (Al, Mg, Fe) octahedral sheet between two sheets (Al, Si) of tetrahedra (referred to as a 2:1 layer). Isomorphous substitution of cations in the 2:1 layers creates surfaces with a permanent negative charge. Its surface area is 700–800 m2
/g. These features, together with its environmental compatibility and ready availability, make bentonite a cost-effective amendment option for immobilizing heavy metals [102
]. Usman et al. [103
] found that bentonite achieved the highest decrease in heavy metals availability to wheat among the three clay minerals, iron oxides, and phosphate fertilizers.
The structural properties of bentonite, such as its specific surface area and surface electrostatic charge, could be substantially changed by various modifications. Kumararaja et al. [104
] used the aluminum-pillared bentonite to enhance the efficiency of metal immobilization, and the pillared bentonite at a 2.5% application rate demonstrated the best immobilization effectiveness to the heavy metals (Cu, Zn, and Ni), significantly reducing their bioavailability. Yang et al. [105
] studied the characteristics of organ-montmorillonite modified by tetramethylammonium (T-Monts) and hexadecyltrimethylammonium (H-Monts). It has been suggested that both T-Monts and H-Monts have a stronger ability to immobilize chromium ions than the unmodified montmorillonite. The enhanced chromium stabilization capacity of T-Monts and H-Monts was ascribed to the enlarged surface area and positively charged surface, respectively. Yu et al. [106
] investigated the interaction between organ-bentonite and the metals. It was found that the main interactive mechanisms for Cu, Zn, and Cd proceeded via cation exchange, Hg proceeded via physical adsorption and partitioning, and Cr and As proceeded via specific adsorption and electrostatic attraction, respectively.
Further investigation is needed to evaluate the critical factors that enhance the immobilization capacity of heavy metals and control their stability in various environmental conditions.
3.2.3. Phosphorus-Bearing Materials
Phosphorus-bearing materials have been widely used as chemical passivating agents to control heavy metal pollution in soil. Good results have been achieved in the control of single metal and multi-metal pollution [107
]. The heavy metals were mainly fixed by three kinds of mechanisms: adsorption, complexation reaction, and the formation of insoluble phosphates with heavy metals. The stabilization process was not the result of a single mechanism, but rather the combination of multiple mechanisms [112
]. The heavy metals in sludge could also be immobilized by the same mechanism, transforming the unstable fraction into a more stable fraction to reduce their mobility and bioavailability.
The commonly used phosphorus-bearing materials can be divided into four categories: (1) fertilizer, such as calcium superphosphate phosphate fertilizer; (2) mineral materials, such as hydroxyapatite and phosphate rocks; (3) biological materials, such as crushed bones, and (4) chemical materials, such as H3PO4, (NH4)2HPO4, and KH2PO4. That phorphorus-bearing materials are used as chemical passivating agents of heavy metals has little influence on the pH value of sludge. The treated sludges were in weak acid or a weak alkaline or neutral state, thus presenting their environmental friendliness. At the same time, phosphorus-bearing materials can supply phosphorus nutrients for plant growth and act as an alternative to phosphate fertilizer, thus reducing the environmental pollution during the production process of phosphatic fertilizer.
Tang et al. [113
] investigated the potential of pretreatment with phosphoric acid (PA) and monobasic calcium phosphate (MCP) for the stabilization of heavy metals in tannery sludge. It was concluded that the extractable concentrations of Pb and Cd in the PA-treated sludge decreased by about 32.6% and 44.7%, respectively. In the MCP pre-treated sludge, the extractable Pb and Cd decreased 26.05% and 30.3%, respectively. The leachability of extractable Cu in the Toxicity Characteristic Leaching Procedure (TCLP) test decreased from 0.228 mg/L to 0.181 mg/L and 0.196 mg/L in the PA- and MCP-treated sludge, respectively. However, the leachability of Zn and Cr enhanced after PA and MCP treatment. The different immobilizing effect on heavy metals was due to the different fixing mechanisms. The main mechanism of Pb fixation by phosphorus-bearing materials was attributed to the thermodynamically favorable reaction for dissolved Pb to react with P to form insoluble Pb phosphate [114
]. Cao et al. [115
] pointed out that Pb reacting with apatite and formed fluorpyromorphite (Pb5
F), whose solubility is very small, was the main mechanism for Pb fixation. The contribution from surface adsorption or complexation only accounted for 21.7%. Compared with Pb, less dissolved Cu and Zn reacted with phosphate to form insoluble Cu- and Zn- phosphates. Meanwhile, the contributions from the surface adsorption or complexation during the process of solid–liquid interface reaction for Cu and Zn were 74.5% and 95.7%, respectively [116
]. The increase of Zn solubility after the addition of PA and MCP may be attribute to the weak bonds of the surface complex mechanism between Zn and phosphate, and bond cleavage, because of the increasing sludge acidity. Cd is fixed mainly by surface complexation and co-precipitation. Raicevic et al. [117
] observed through X-ray emission and the Rutherford backscattering spectrum that Cd entered the hydroxyapatite interior by diffusion and ion exchange.
3.2.4. Red Mud
Red mud is the waste residue produced during the process of alumina production when bauxite is leached by strong alkali. It contains TiO2, a certain amount of dicalcium phosphate, and amorphous aluminosilicate materials. Due to its relatively large surface area and high content of iron, aluminum, calcium oxides, and hydroxides, red mud possesses strong adsorption ability and reactivity to heavy metals. It is considered to be a cost-effective material for phosphorus and heavy metals adsorption and immobilization. The full utilization of red mud is beneficial to reduce its environmental pollution and disposal cost.
Liang et al. [118
] found that the immobilization efficiency of Cd, Zn, and Pb was 100%, 85.7% and 62.2%, respectively, when the sewage sludge was mixed with 5% red mud. XRD and SEM analysis showed that there was no detectable changes in the mineralogy of sewage sludge after chemical amendment by 10% red mud. Liang et al. [119
] selected 10% red mud to mix with sewage sludge, and found that the immobilization efficiency of Cd, Zn, and Pb was 100%, 92%, and 82%, and their plant available was reduced from 18.13%, 17.22%, and 13.64% to 6.98%, 11.36%, and 7.12%, respectively.
Soluble sulfides have been usually used as passivating agents to react with heavy metals in sludge and form insoluble sulfides. Sun et al. [120
] found that sodium sulfide had good stabilization properties on heavy metals such as Zn, Cu, and Cd. When 10% sodium sulfide was used, leaching concentrations of Zn, Cu, and Cd were 4.96 mg/L, 9.65 mg/L, and 0.11 mg/L, respectively. The leaching concentration of Ni did not decrease, obviously. The content of Zn in an unstable fraction was decreased by 50–60% after the treatment. Chen et al. [121
] investigated the effect of Na2
S and (NH4
S on the speciation distribution and bioavailability of heavy metals in the sewage sludge. It was shown that the sulfide amendment had a significant retardation effect on Cu, Zn, Cd, and As. The exchangeable contents of Zn and Cd in the sulfide treatment had maximally been reduced to 98.8% and 98.6%, respectively. The tests for the toxicity of heavy metals to Photobacterium phosphoreum
T3 suggested that the sulfide amendments could help alleviate the toxicity of heavy metals in the sludge.