The heavy metals in sludge, especially flue gas desulphurization wastewater sludge, exceed the standard seriously, which belongs to hazardous solid waste. The annual production of sewage sludge in China has exceeded 50 million tons. It is calculated that only the annual production of flue gas desulphurization wastewater sludge in coal-fired power plants reaches 90 million tons. The heavy metals in sludge, if not disposed of properly, will cause very serious secondary pollution to the ecological environment. This paper introduces the present situation of sludge treatment at home and abroad and the determination and estimation methods of heavy metals in sludge. The advantages and disadvantages of chemical process, electrokinetic treatment, bioleaching process, thermal treatment and stabilization are summarized in detail from the aspects of principle, reaction device, research progress, and hot spot. Furthermore, the authors shed light on the existing problems and development prospects of each method. In the end, the paper puts forward the prospect of economical and efficient implementation device and technical scheme with strong application.
Contents
1 Introduction
2 Determination and Estimation methods
2.1 Determination methods
2.2 Estimation methods
3 Advanced technology for disposal of heavy metal in sludge
Guanyi Chen, Kexuan Han, Caixia Liu, Zeng Dan, Duo Bu.
Removing Heavy Metals from Sludge.
Progress in Chemistry, 2021, 33(6): 998-1009 DOI:10.7536/PC200731
The rapid development of China's economy has led to the improvement of living standards and the promotion of industrialization, the discharge of urban industrial wastewater and domestic sewage has increased, and the output of various types of sludge has increased day by day. According to statistics, 40 million tons of municipal sludge (about 80% moisture content) were produced in 2018 alone[1]. The composition of sludge is generally 59% ~ 88% organic matter, 50% ~ 55% C, 25% ~ 30% O, 10% ~ 15% N, 6% ~ 10% H and a small amount of P and S. There are also serious pollutants and potential carcinogens in sludge.Such as heavy metals, dioxins, furans, pathogenic microorganisms, etc. The water content, viscosity, pH value and other properties often vary with the type and source of sludge. For example, municipal sludge generally has a high water content, printing and dyeing wastewater sludge has a low pH, and desulfurization sludge has a high Ca content and a variety of heavy metal contents seriously exceed the standard[2]. The basic physical and chemical properties of different types of sludge are shown in Table 1.
The contents of heavy metals in municipal sludge in China vary widely, and the speciation distribution characteristics vary greatly with the types of metals and the sources of sludge. The contents of heavy metals in sludge in different regions are also different, and the contents of Hg and As in municipal sludge in northern China are higher.The contents of Cu, Pb, Zn, Cd, Cr and Ni are higher in the southern region, but generally speaking, they all exceed the standard to a certain extent and need further treatment and disposal. The contents of heavy metals in different types of sludge are shown in Table 2[8][8].
European and American countries adopt different treatment and disposal methods according to the types and characteristics of sludge, and municipal sludge with high organic matter content often adopts composting + agricultural use.Desulphurization waste sludge with high calcium sulfate content is often used in building materials manufacturing, while industrial wastewater sludge with serious pollution needs to remove heavy metals and toxic substances before landfill or incineration[9,10][11,12][10]. However, in the past, more attention was paid to whether the water quality of sewage treatment plants was up to standard, and less attention was paid to the treatment and disposal of sludge. If sludge was dumped or used as fertilizer without proper treatment, it would pollute the soil and water body, disturb the ecological balance, and long-term exposure to the environment would also threaten human health[13][14]. Therefore, how to make rational use of sludge and effectively deal with heavy metals in sludge is regarded as a problem to be solved urgently at this stage.
The heavy metals in sludge are large in quantity, wide in source, complex in morphology and various in species, and the current treatment methods have the problems of high cost, high risk of secondary pollution, poor removal effect and the like. In view of the above situation, many new methods, such as chemical method, electrokinetic method and microbial method, have been applied in this field. In this paper, five kinds of advanced technologies for heavy metal treatment in sludge were summarized, and their principles and research progress were introduced.The advantages and disadvantages of different methods were summarized, and the application prospects of economical and efficient implementation devices and technical schemes were put forward, which provided reference for the development of heavy metal treatment technology in sludge.
2 Overview of Measurement and Evaluation Methods
Heavy metals in sludge can be enriched in animals and plants through water, soil and even gas, and then enter the human body through the food chain or directly, which will damage environmental safety and human health. At present, there is no unified way to classify the existing forms of heavy metals. In 1992, the Euro-pearl Community Bureau of Reference proposed a three-stage four-step extraction method (BCR method) based on the Tessier method, which divides the existing forms of heavy metals into water-soluble and acetic acid extractable forms, reducible forms, oxidizable forms and residual forms. Different chemical forms of heavy metals have different effects on the environment. The extractable form of heavy metals in acetic acid is easy to migrate and transform, the reducible and oxidizable forms are affected by pH value, and the residual form is usually stable. The extractable and reducible forms of acetic acid have direct biotoxicity and bioavailability, while the oxidizable form has potential biotoxicity and bioavailability, and the residual form has no biotoxicity but low bioavailability.
2.1 Method for measuring heavy metal content
2.1.1 Determination of heavy metal content
The commonly used digestion systems are HNO3, HNO3-HF and HNO3-HF-H2O2 digestion systems. In view of the possible presence of silicates in sludge samples, the determination of heavy metals in sludge is usually carried out by digestion with HNO3-HCl-HF(10, 5, 0.5 mL) acid system. The specific steps are as follows.
(1) Digestion: Accurately weigh (0.1 ± 0.01) G of sludge sample, put it into a polytetrafluoroethylene digestion tank, add various acids in turn (15.5 mL in total), shake slightly to make no sludge sample remain on the tank wall, mix it with the acid evenly, and let it stand at room temperature for 10 min. After the reaction is stable, cover the digestion tank and put it into a graphite digestion instrument.
(2) acid removal: operation steps of the digestion instrument: heating to 120 ℃ and keeping for 10 min, then heating to 160 ℃ and keeping for 40 ~ 60 min to completely dissolve the sample. Observe the digestion tank from time to time during digestion. When the sample is completely dissolved in the acid, open the lid of the digestion tank, raise the temperature to 180 ℃ for acid removal, and observe the sample. When the acid is almost removed (about 1 ~ 2 mL left), take out the digestion tank and cool it.
(3) Constant volume: After the digestion tank is cooled, add 5 mL of deionized water, shake and mix well, filter through a 0.45 μm filter membrane, transfer to a 25 mL colorimetric tube, rinse the digestion tank and the lid with deionized water for three times, and then fix the volume to 25 mL, which is stored in a refrigerator at 4 ℃ for testing.
And (4) teste: determining that concentration of the heavy metal in the solution by adopting ICP-OES for Ca, Mg, Fe, Cr, Cd, Cu, Ni, Pb, Zn, As and Hg.
2.1.2 Leaching toxicity analysis of heavy metals
The three commonly used methods are sulfuric acid-nitric acid method, acetic acid buffer solution method and horizontal oscillation method. Considering the complexity of sludge composition, the sulfuric acid and nitric acid method is often used for leaching, and the specific methods are as follows.
(1) Sample and leaching agent preparation: weigh 5 G of sludge sample and put it into a 2 L leaching bottle. Add 2 drops of mixed solution of concentrated sulfuric acid and concentrated nitric acid (mass ratio: 2:1) into 1 L of water, and adjust the pH value to (3.20 ± 0.05).
(2) Vibrating extraction: add the extraction agent according to the liquid-solid ratio of 10 ∶ 1, oscillate for 8 H at room temperature (25 ℃) with a rotation speed of 200 R/min, and then stand for 16 H. The supernatant is filtered through an aqueous filter membrane (with a pore size of 0. 45 um) and transferred to a centrifuge tube. Add a few drops of nitric acid and store at 4 ℃ for testing.
(3) Test: same as 2.1.1.
2.1.3 Heavy metal speciation
According to the Modified BCR Sequential Extraction Procedure, heavy metal elements are divided into four forms. The acid exchangeable form has strong mobility and high toxicity. The reducible state is not easy to release; The oxidizable state can be degraded under certain conditions and partially converted into the acid exchangeable state for biological utilization; The residue state is relatively stable, and its mobility is very small, so it is generally not used by organisms. Tessier method is also a common method.
2.2 Heavy metal pollution level/risk assessment method
Geoaccumulation index and potential ecological risk index were introduced to quantify the pollution degree and risk assessment of heavy metals in sludge, and the pollution degree of heavy metals in sludge was quantitatively assessed from the aspects of heavy metal concentration, human activities and ecological risk, which provided a reference for measuring the hazard degree of sludge.
2.2.1 Geoaccumulation index
Muller, a German scholar, proposed the index of geoaccumulation (Igeo) in 1969, which is to determine the degree of heavy metal pollution by comparing the geochemical background value (mostly determined by referring to the national soil environmental standards) with the content of heavy metals.
In the formula, the Cn is the concentration of the heavy metal element n, the (mg/kg),Bn is the geochemical background value of the heavy metal element, and 1.5 is the geological layer difference fluctuation coefficient. The Igeo index divides the risk of heavy metals into seven levels, and the specific pollution classification standards are shown in Table 3.
2.2.2 Potential ecological risk index
Hakanson, a Swedish geochemist, put forward the potential ecological risk index method in 1980, whose highlight is to distinguish different forms of heavy metals for measurement. The Er value is the ratio of the sum of the concentrations of exchangeable, reducible, and oxidizable forms of heavy metal acids to the concentration of residual forms.
Among them, the represents the pollution factor of a single heavy metal, and the represents the sum of the concentrations of available and potentially available heavy metals (acid exchangeable, reducible and oxidizable).The represents the stable (residual) concentration of heavy metals, the represents the ratio of the sum of the concentrations of exchangeable, reducible and oxidizable heavy metals to the residual concentration, and the represents the toxicity response coefficient of heavy metals. RI is the potential ecological risk index of heavy metals in sludge. See Table 4 for classification standard of potential hazard index.
3 Advanced Technologies for Heavy Metal Disposal of Sludge
3.1 Chemical method
Chemical methods usually refer to changing the pH value of sludge to convert heavy metals into soluble state, or adding complexing agents, chelating agents and biosurfactants to convert heavy metals into complexes, so as to promote the better resolution of heavy metal ions from sludge, so as to achieve the purpose of reducing the total amount of heavy metals in sludge. The chemical extraction method has gone through inorganic acids (HNO<sub>3, HCI, H2SO4), inorganic salts (FeCl3, CaCl2, NaNO3), organic acids (citric acid, oxalic acid, acetic acid),The development of chelating agents (EDTA, EDDS, PESA, PASP, Na2EDTA, IDS, GLDA) and biosurfactants (sodium cholate, rhamnolipid, tea saponin, sophorolipid, α-amylase).
There are various experimental devices and disposal processes for chemical methods, but laboratory tests are generally carried out in beakers or conical flasks. Kou et al. mixed the sample with the chemical agent and put it into a conical flask, shaking at 250 R/min for 24 H at room temperature[15]; Chen Xi et al separated hard plastic tubes (3 cm in diameter and 15 cm in length) with gauze, and then leached with different concentrations of chemicals[16]. Ren et al. mixed CA and EDTA with sludge in a ball mill[17]. Each reaction form has its advantages and disadvantages, and should be selected according to the characteristics and needs of the sample to be treated. The comparison of different additives is shown in Table 5.
At present, the research focus of chemical methods at home and abroad is to explore new biodegradable chelating agents, such as polyepoxysuccinic acid (PESA), aspartic acid (PASP) and glutamic acid N, N-diacetic acid (GLDA), which have strong metal chelating ability, good biodegradability and no risk of secondary pollution. Lu Jianping and Zhang Liying's research proved that the extraction rate of 30 mg/mL PESA for Hg in sludge was 63%, and the extraction rate for Cr in manganese mud residue was 95%[19][20]. Zhang Hua et al. Found that under low pH conditions, the extraction rates of Zn, Ni and Cu by aspartic acid and CA were more than 85%, and the extraction ability of PASP for Ni and Cu was stronger than that of CA[21]. When GLDA was used to extract heavy metals from sludge, the removal efficiency of Cd, Ni, Cu and Zn was high, and the removal efficiency of heavy metals other than residue was good[23]. Wu Qing et al showed that the extraction rate of Cd, Ni, Cu and Zn in sludge by GLDA was more than 80%, but only 32% for Zn[18]. According to Suanon et al., GLDA has better extraction effect than CA in sludge with high water content, and the removal rate is more than 80%[24].
Usually, the removal rate of chemical method is high, but its dosage is large and the risk of secondary pollution is high, which is not conducive to the utilization of sludge resources. In the process of chemical extraction, more than 90% of heavy metals can be extracted by inorganic acid, but when the pH value is in the range of 1.5 ~ 2, a large amount of acid and water are needed to wash the sludge, and more lime is needed to neutralize the pH value of the sludge after extraction, which makes the cost of the chemical method higher. In addition, inorganic acids are not biodegradable, which means that the risk of secondary contamination is high. The unique advantage of chemical method lies in the diversity of reaction forms and devices, which can be adjusted according to the characteristics of sludge, and can be repaired in situ or in situ, but it is necessary to prevent secondary pollution and control costs, so the research of chemical method tends to the development of new chemical agents. With the continuous extension of the field of biochemistry and the development of nanotechnology, chemical methods still have good prospects for development.
3.2 Electrokinetic method
Under the action of electric field, heavy metals in sludge are transformed, migrated and separated from sludge under the action of electrokinetic effects such as electroosmosis, electromigration and electrophoresis. Under the action of DC electric field, the value of H+,pH produced by the oxidation reaction of electrolyzed water in the anode area is low, and the value of OH-,pH produced by the reduction reaction in the cathode area is high. Electromigration causes metal cations to migrate toward the cathode, where they precipitate as hydroxides, carbonates, or other compounds under the action of OH- plasma near the cathode[25]. The main factors affecting electrokinetic remediation are sludge pH, electrolyte concentration, voltage gradient, current density and electrode materials.
There are basically two types of electrokinetic reactors: cuboid and cylinder. The cuboid reactor used in the laboratory is generally made of plexiglass. The soil chamber and the electrode chamber are separated by cotton fiber filter paper or cationic membrane. Stainless steel and graphite are commonly used as electrode materials, both of which have good corrosion resistance[26]. A cathode chamber (annular cylinder) of the cylindrical electrokinetic remediation reactor wraps an anode chamber (cylinder) and is separated by a cation exchange membrane, the anode is made of ruthenium platinum titanium alloy, the cathode is made of stainless steel mesh, and a water inlet and a water outlet are arranged in the cathode chamber to realize electrolyte reflux[27]. Li Wanran et al. Added a magnetic field on the basis of the conventional electrokinetic reactor, and the magnetic line of force is perpendicular to the electric field, which can effectively promote the migration of heavy metals and thus improve the removal rate[28]. The two typical reaction devices are shown in Figure 1, and the comparison of electrokinetic method under different experimental conditions is shown in Table 6.
Electrokinetic remediation is an economically feasible and promising technology for heavy metal removal from sludge, which does not cause secondary pollution to the environment, and has the advantages of high removal efficiency and short treatment time.However, the efficiency of single electrokinetic remediation method is limited, and the energy consumption problem restricts its further expansion and industrial application. In addition, when the electrokinetic method is adopted to remove the heavy metal, the sediment near the cathode can affect the dissolution and migration of the heavy metal, and the H+ and the OH- generated by electrolyzing water at the cathode and the anode enter the sludge under the action of an electric field, so that the acidity of the sludge area is gradually reduced from the anode to the cathode,In the middle of the reactor, there is a sudden change in pH value, which leads to the precipitation of metal cations in this area by a large number of OH-, which is one of the key factors causing the accumulation of heavy metals in a certain area after electrokinetic treatment in the rectangular reactor[26]. Therefore, it is necessary to improve the traditional electrokinetic reaction process, break through the obstacles of the electrokinetic process, further improve the reaction efficiency and removal rate, and reduce the cost.
Chelating agents or surfactants, such as ethylenediamine disuccinic acid (EDDS), ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), ethylene glycol bis (2-aminoethyl ether) tetraacetic acid (EGTA), have been the focus of electrokinetic method. Tetrasodium iminodisuccinate (IDS) is an environmentally friendly chelating agent with strong biodegradability, low toxicity and good biodegradability, which has little impact on the environment. The extraction efficiency of IDS for Cd, Ni and Cu can reach 68%, 90.7% and 91.3% respectively[31]. Citric acid can promote the removal of Pb and Cu, but the effect on Cu is not obvious[26]. EDTA as a complexing agent is beneficial to the increase of electroosmosis flow rate and the removal of Cr[32]. Both EDTA and CA have activation effect, which can significantly reduce the power consumption and the amount of activator compared with the traditional electrokinetic repair[33]. When ammonium citrate was used as an auxiliary agent for electrodialysis remediation of MSWI fly ash, the release of heavy metals increased significantly[34]. However, some chelating agents and organic acids have the disadvantages of poor degradability, high cost and high carcinogenic risk, which will cause secondary pollution to the environment[35]. Therefore, the pretreatment of heavy metals in sludge and the removal of heavy metals in sludge by electrokinetic method can effectively reduce the energy consumption of electrokinetic process, reduce the obstacles of electrokinetic process, improve the efficiency of heavy metal removal, and will not cause secondary pollution to the environment, which is a relatively promising heavy metal disposal technology.
3.3 Bioleaching method
Bioremediation is to metabolize, absorb and transform heavy metals in sludge through specific microbial metabolic activities, so as to achieve the goal of heavy metal treatment in sludge. At present, the research focus is bioleaching. In 1993, Sreekrishnan et al. Identified that sludge solid concentration, pH value, acid concentration and other parameters have a certain impact on process performance[36]. Two kinds of bacteria, At. thiooxidans and At. Ferrooxidans, can survive in acidic environment and oxidize insoluble iron and sulfur compounds respectively, and then metabolize, absorb and transform them to repair sludge.
In the process of sulfur-based bioleaching, the dissolution of metal sulfides has two mechanisms: direct and indirect[37]. In the direct leaching process, the bacteria are in direct contact with the metal sulfides in the sludge, and the metal sulfides are directly oxidized by the bacteria. In general, metal sulfides such as NiS, CuS and ZnS in sludge can be dissolved through the above mechanism. In indirect bioleaching, metals are generally in the divalent state, and elemental sulfur or reduced sulfur compounds in the sludge are oxidized to sulfuric acid by sulfur-oxidizing bacteria, which reduces the pH value of the sludge medium and increases the solubility of metals.
There are also direct and indirect mechanisms for the oxidation of reduced iron and sulfur compounds during iron based bioleaching[38]. Non-ferrous metal sulfides are directly oxidized in the bacterial direct leaching process. In the indirect process, the bacteria oxidize the Fe2+ to Fe3+ in the liquid phase, and the Fe3+ is leached out by a chemical reaction. In this mechanism, the bacteria do not need to be in contact with the reactant surface.
The reaction devices of bioleaching are mostly conical flasks or Petri dishes, ranging in size from 10 mL to 1 000 mL. Generally, stirring or shaking is required to promote microbial reaction, and certain environmental conditions (temperature, pH value, etc.) Need to be maintained to improve the survival rate and reproduction rate of microorganisms[37,38]. Different strains have different requirements for reaction equipment and environmental conditions. For example, the strain used by Chen et al. Is Sulfobacillus acidophilus, and the culture dish is placed at 200 R/min and 55 ℃, and good heavy metal removal effect is achieved[39].
The key factor affecting the extraction rate of bioleaching is pH value. pH not only plays an important role in the dissolution of metals, but also affects the growth of microorganisms. In order to provide suitable conditions for the growth of bacteria in the sludge, the pH value of the sludge must be reduced. However, the pH value of the sludge produced by the sewage treatment plant is unstable, and if the extraction rate of the bioleaching process is to be improved, the effectiveness of the leaching microorganism needs to be tested in a wide range of sludge pH values. The research progress of bioleaching with different strains is shown in Table 7.
Compared with chemical methods, bioleaching requires less chemical reagents, has the advantages of low cost, high efficiency and significant improvement of sludge dewatering performance, but it also has shortcomings.For example, the remediation cycle is long, the culture time of bacteria is long, and it is difficult to treat the sludge with high concentration of heavy metals, and the initial reaction conditions, inoculum size, bacteria and other factors seriously affect the treatment effect of heavy metals. The practical application of continuous treatment of large amounts of sludge is still difficult, and the bioleaching and sludge digestion under continuous operation mode need to be further studied[47]. In recent years, some scholars have tried to use transgenic technology to improve the strain, and improve the efficiency of bioleaching by improving the tolerance of the strain to the living environment. The continuous improvement of transgenic technology has also opened up a new path for the development of biological methods, and it can be predicted that biological methods will be further improved in the near future[48].
3.4 Heat treatment method
Thermal treatment is one of the most common physical methods for the treatment of metal-contaminated materials, which mainly heats the materials to 150 ~ 400 ℃ to promote the migration and transformation of many inorganic elements in the sludge, so as to remove heavy metals. Some metals are exposed during high temperature evaporation, or condense in ash or evaporated water, and are eventually captured. In general, the higher the temperature, the better the metal treatment effect.
The reaction equipment of heat treatment method varies greatly due to different reaction conditions, and it is often necessary to design the equipment to meet the requirements of temperature, pressure and reaction chamber[49]. In the early 1980s, researchers began to apply hydrothermal treatment technology to the disposal of heavy metals in sludge, but the mechanism of pollutant migration and removal was not studied in depth. Therefore, there are few engineering applications of thermal treatment methods in the world. In 1997, a sludge thermal treatment plant with 5 tons of dry sludge per day was built in California. In 2008, American Enertech Company completed the Slurrycard Sludge Cryogenic Cracking Treatment Plant with a capacity of 675 tons/day[50]. However, China began to pay attention to sludge disposal after 2005, and sludge hydrothermal treatment technology was applied in sludge heavy metal disposal. A demonstration project of 100 tons/day sludge hydrothermal treatment technology has been built in Jinzhong City. Fig. 2 shows a typical heat treatment reaction device. See Table 8 for comparison of heat treatment methods at different reaction temperatures.
In recent years, the research focus of thermal treatment methods is the migration and removal mechanism of heavy metals. The experimental results of Sun Xueping et al show that most of the heavy metals exist in the solid phase products as residues after the hydrothermal treatment of sludge[53]. Wang Yuexiang et al studied the effect of pH value on the distribution forms of Cr and Cd, and proved that Cr was mainly in residual form, the proportion of oxidized form and reduced form changed with pH value, and the content of exchangeable form was low[57]. The oxidation state of Cd increased with the increase of pH value, while the exchangeable state decreased with the increase of pH value. Zhao et al. Used XPS and FTIR to study the release of nitrogen-containing functional groups during hydrothermal treatment of sludge. The results showed that the structure of protein in sludge changed after hydrothermal treatment, the binding energy of N — H and C — N decreased, the volatilization characteristics of organic matter improved, and the emission of NO decreased[58]. He et al. Proved that after hydrothermal treatment of sludge, the amount of sulfur emission from combustion was reduced by half, and 40% of nitrogen was retained in the solid phase products[59]. Therefore, it can be concluded that N and S are transferred and transformed by hydrothermal treatment.
Heat treatment is fast and removes the risk of heavy metals while eliminating potential organic contaminants. There are many influencing factors, such as reaction temperature, additives (catalyst and other biomass), solvent type and reaction time[60]. There are two main disadvantages of thermal treatment. On the one hand, researchers pay more attention to the calorific value of sludge and the conversion of biomass such as liquefaction or gasification of organic matter, but less to heavy metals. On the other hand, the thermal treatment technology at high temperature (≥ 300 ℃) not only consumes a lot of energy, but also leads to a large number of decomposed organic matter and reduces the fertility of sludge, so the thermal treatment products are not suitable for land use resources. The balance between energy consumption and removal efficiency is the key issue in the application of thermal treatment, and the selection of appropriate thermal treatment scheme according to the characteristics of pollutants and the subsequent treatment needs is the key to consider the feasibility of thermal treatment.
3.5 Stabilizing method
The stabilization method of heavy metals in sludge is mainly to add stabilizers to promote the transformation of heavy metals from unstable forms to more stable forms, so as to reduce the unstable heavy metals, reduce the ecological hazard index of heavy metals in sludge, and then reduce the direct risk of sludge. The key of stabilization method is to change its existing form in sludge. The method can effectively reduce the mobility and the bioavailability of the heavy metal, and simultaneously reduce the pollution of the heavy metal to the underground water body[61]. Commonly used stabilizers are mainly alkaline substances (such as lime, fly ash), sulfides (such as Na2S, sulfur, FeS, ammonium sulfate, ferric sulfate), salts (such as phosphate, silicate, humus, etc.)[62,63]. See Table 9 for the comparison of the increase of heavy metals in the stable state of the stabilization method under the addition of different drugs.
The reaction device of the stabilization method is relatively simple, and often only a container of appropriate size is required to allow the sludge to fully react with the stabilizer[64]. In practical applications, appropriate containers are often selected according to the subsequent disposal methods. If landfill or incineration is needed, it can be transported to the treatment plant after reaction in the cement pool. If dehydration is needed, it can be put into geotextile bags for simultaneous dehydration and stabilization reaction[65,66].
Sludge stabilization research mainly focuses on the stabilization and remediation of one or two heavy metals, such as the use of cement synergistic stabilization, brick making, iron salt and so on to reduce the heavy metal leaching toxicity of municipal sludge, various printing and dyeing sludge and leather sludge[70][71][67]. The functions of the alkaline agent added in the stabilization method are as follows: firstly, the pH value of the sludge is adjusted, so that heavy metals are converted into hydroxides with low solubility and precipitated; Secondly, it reacts with hydroxyl, carboxyl and other functional groups in sludge to produce carbonate radicals, and then to produce metal carbonate precipitates. Thirdly, it can increase the variable charge on the surface of sludge particles and reduce their specific adsorption capacity, which is conducive to the desorption of heavy metals in sludge. Stabilizer Na2S can effectively solidify Cd, Cu and Pb in the sludge, but it will cause a decrease in pH, thus affecting its reuse as a fertilizer.CaO stabilizer treatment will increase the pH value of the sludge, affecting the solubility of heavy metals in the soil[72]. At present, some researchers are trying to use the mixture of CaO, Na2S and FeCl3 to stabilize the desulfurization wastewater sludge, hoping to get better stabilization effect without changing the pH value of the sludge. The stabilization method has the advantages of low cost and high short-term solidification efficiency, but the long-term stability of heavy metals in the stabilization method is poor, resulting in the sludge can not be used as a resource, thus limiting the development of the stabilization method. It is undeniable that as one of the methods with low economic cost, quick effect and strong operability, the stabilization method is widely used in practice, but considering its instability, it may cause serious secondary pollution, so it is necessary to be cautious to apply the stabilization method to the disposal of heavy metals in sludge[73].
4 Conclusion and prospect
The species and forms of heavy metals in sludge are quite different, and the water-soluble, exchangeable and organically bound heavy metals may enter the natural environment without proper disposal, and then endanger environmental safety and human health through the water or food chain. In general, the removal of heavy metals in sludge is to separate them from sludge or change their existing form, so as to reduce or even eliminate their potential harm to the environment. There are great differences in the removal efficiency of heavy metals in sludge, which is mainly attributed to the differences in sludge composition, sludge pretreatment methods, concentration and form of metals in sludge, and extraction conditions. See Table 10 for comparison of sludge heavy metal treatment methods.
Although the chemical method has the advantages of short time consumption and high efficiency, it requires a large amount of acid (0. 5-0. 8 G per gram of dry sludge), is difficult to operate, requires acid-resistant corrosion devices, and is difficult to regenerate, which limits the promotion and application of the chemical method. In contrast, organic acids and biosurfactants have better application prospects in chemical extractants. Organic acids and biosurfactants have high extraction efficiency for heavy metals in weak acidic environment (pH 3 ~ 4). In addition, the organic acid and the surfactant are easy to be degraded, which is beneficial to the composting of the disposed sludge. Therefore, the use of organic acids, biosurfactants and other new agents as extractants to remove heavy metals from sludge is emerging.
Electrokinetic method has the advantages of low current density, short time consumption and high efficiency.It has a good effect on the treatment of heavy metals in sludge with poor permeability and more ionic heavy metals.Its treatment mechanism is mainly that the dissolution and desorption near the anode promote the migration of heavy metals, while the precipitation reaction near the cathode hinders the migration of heavy metals. At present, the electrokinetic remediation method is difficult to balance economic value, time cost and treatment efficiency, and there are often problems such as acid band migration, cathodic precipitation, electrode plate corrosion and passivation, so the electrokinetic remediation method is mostly concentrated in the laboratory.
The efficiency of heavy metal removal by bioleaching is higher than that of conventional chemical treatment methods, the requirement for acid is lower, the cost is generally lower than that of chemical methods, and the bioleaching process does not affect the soil conditioning and fertilization value of sludge, and the concentration of volatile solids and nitrogen, phosphorus and potassium is basically unchanged. Sludge composition, microbial species, reaction cycle and other factors affect the specific treatment effect of different substrates, so it is necessary to investigate and optimize different factors in application. In addition, the growth of microorganisms requires strict control of environmental conditions such as temperature, oxygen content and substrate concentration. At present, transgenic technology can effectively improve the performance of microorganisms, which opens a new path for the further improvement of biological methods.
Sludge thermal treatment not only changes the properties of sludge and realizes the stabilization and recycling of sludge, but also changes the occurrence of heavy metals in sludge, promotes the transformation of heavy metals from unstable state to stable state, and reduces the risk of environmental pollution. Hydrothermal treatment technology is environmentally friendly and has broad prospects for development, which is more efficient than dry pyrolysis technology and has a higher degree of resource utilization. However, the disadvantages of the thermal treatment method lie in the poor treatment effect of heavy metals and the low content of organic matter. Heat treatment is now often combined with other methods to remove heavy metals while achieving proper use of energy and resources.
Stabilization method is generally to transform heavy metals into a stable state, so that they will not migrate and transform in a certain period of time, but it still can not rule out the risk of long-term secondary pollution. Conventional stabilization technology is generally closely related to the pH value, and the stabilized heavy metals may become pollution sources again with the change of environment, so the breakthrough of stabilization method needs to start with new stabilizers, and adopt appropriate stabilizers and subsequent disposal schemes according to the properties of specific sludge.
Chemical method, electrokinetic method, bioleaching method, thermal treatment method and stabilization method have their own advantages and disadvantages, but a single method is often difficult to achieve good results in the treatment of sludge with complex components, so on the basis of in-depth study of the existing methods, it is necessary to continue to study the combined technology of multiple methods. At present, there are three main combined methods being explored by researchers, one is chemical method and electrokinetic method, which use chemical agents (surfactants, chelating agents, etc.) To improve the sludge state.Is beneficial to the desorption and dissolution of heavy metals in the sludge, improves the effect of the electrokinetic method, and has certain treatment effect on pathogens and organic matters in the generated strong acid and strong oxidation environment[74⇓~76]; The second is chemical method and stabilization method, which can improve the effect of stabilization method by means of chemical agents. Chemical method can effectively remove active heavy metals in sludge, and then reduce the migration and bioavailability of metals in sludge by stabilization method, so that sludge no longer has potential ecological hazards[77⇓~79]. The third is the bioleaching method and the stabilization method. The sludge treated by the bioleaching method has uniform and stable properties, which can eliminate the risk of long-term secondary pollution existing in the single stabilization method[80,81].
In different treatment methods, the differences of sludge composition, pretreatment methods, extraction conditions, metal species and content have a great impact on the removal efficiency of heavy metals. Therefore, the appropriate heavy metal removal methods should be selected according to the characteristics of sludge, treatment objectives and subsequent disposal methods. In view of the actual application scenario, the mechanism of each method should be explored in depth, the process of combined technology should be optimized, and more economical and efficient reaction devices should be developed.
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