Sulfur-nitrogen co-doped porous carbon / geopolymer composite material as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of high polymer materials, and particularly relates to a sulfur-nitrogen co-doped porous carbon / geopolymer composite material as well as a preparation method and application thereof. A geopolymer is combined with a plant-based material, and sulfur, nitrogen and other components are doped in the geopolymer and the plant-based material, so that the sulfur-nitrogen co-doped porous carbon / geopolymer composite material is obtained. The preparation method has the beneficial effects that the geopolymer is modified by adopting components such as a plant-based material and thiourea, so that the compact structure of the geopolymer is effectively loosened; meanwhile, by doping heteroatoms such as nitrogen and sulfur, the effect of Lewis alkaline sites is favorably played, the active adsorption sites of the composite material are increased, the adsorption capacity of the composite material on heavy metals in wastewater is improved, and the adsorption capacity of the composite material obtained after modification on heavy metal ions Cd < 2 + > in water is improved by 317% compared with that of GM before improvement.

Description

Technical Field

[0001] The present invention belongs to the technical field of polymer materials, and in particular relates to a sulfur-nitrogen co-doped porous carbon / geopolymer composite material, a preparation method and applications thereof. Background Art

[0002] Heavy metal adsorption materials are a type of substance that can effectively reduce or remove the concentration of heavy metal ions in the environment. However, at present, heavy metal adsorption materials generally have problems such as high cost, poor regeneration, and limited adsorption capacity.

[0003] Geopolymers, also known as geopolymers, are a new type of green inorganic gel material obtained by reacting silicon-alumina materials containing silicon oxide and aluminum oxide as the main components under the action of an activator through appropriate process reactions.

[0004] Geopolymer materials have an anionic skeleton structure similar to that of zeolite and can be prepared based on industrial waste. They are inexpensive, environmentally friendly, stable, and have good ion exchange capacity, showing competitive advantages in the field of heavy metal adsorption. However, the surface of geopolymer materials lacks functional groups, has fewer active sites, and has a relatively dense structure, resulting in low mass transfer efficiency. As a result, when used as an adsorption material, the adsorption equilibrium time is long and the adsorption capacity is low, making it difficult to meet the needs of rapid processing.

[0005] Porous carbon materials are widely used for the adsorption of heavy metals due to their high specific surface area and rich pore structure. However, defects such as unstable material structure and easy collapse during use limit their wide application.

[0006] Therefore, combining geopolymer and porous carbon to prepare a heavy metal adsorption material with good performance can not only improve the utilization rate of industrial and agricultural waste resources, but also reduce the preparation cost of adsorption fillers, which can be said to kill two birds with one stone. Summary of the Invention

[0007] In order to solve the above technical problems, the present invention provides a sulfur-nitrogen co-doped porous carbon / geopolymer composite material, a preparation method and application thereof.

[0008] The sulfur-nitrogen co-doped porous carbon / geopolymer composite material provided by the present invention is prepared by the following steps: S1: washing, air-drying, crushing, and sieving the plant material to prepare plant material powder; Preparation of S2 geopolymer: Kaolin is calcined in a muffle furnace to obtain metakaolin. Saturated sodium silicate solution is added dropwise to the obtained metakaolin for alkaline excitation. The addition is stopped when the metakaolin becomes viscous. After solidification, the geopolymer is ground to obtain GM. Preparation of S3 sulfur-nitrogen co-doped porous carbon / geopolymer composite material: Weigh the plant material powder prepared in S1 and the geopolymer GM prepared in S2, add deionized water into a reaction vessel, stir continuously for 20-60 min, then add thiourea and continue stirring, react at 100-200 ° C for 2-24 h, after the reaction is completed, centrifuge and discard the supernatant, wash and dry the precipitate, place it in a tube furnace, and heat it under argon protection at 8-12 ° C min -1 The heating rate is increased to 300-900° C. and calcined for 1-6 h to obtain the sulfur-nitrogen co-doped porous carbon / geopolymer composite material, which is named SN-PC / GM.

[0009] In the above preparation method, preferably, the plant material described in S1 is selected from at least one of pine cones, wheat straw, corn straw, and peanut shells.

[0010] More preferably, the plant material described in S1 is pine cone.

[0011] Preferably, the calcination in S2 is performed at a temperature of 3-8°C / min -1 The temperature is raised to 300-900 °C at a rate of 1-2 hours and the calcination time is 1-5 hours.

[0012] As a further preferred embodiment, the calcination in S2 is performed at a temperature of 3-8°C·min -1 The temperature is raised to 600-800℃ at a rate of 1-3 h and the calcination time is 1-3 h.

[0013] Preferably, the curing temperature in S2 is 60-120° C., and the curing time is 2-12 h.

[0014] As further preferred, the curing temperature in S2 is 60-100° C., and the curing time is 2-8 h.

[0015] Preferably, the mass ratio of the plant material powder to the geopolymer in S3 is 1:0.8-1.5, and the mass ratio of thiourea to the geopolymer is 0.5-2:1.

[0016] As a further preference, the mass ratio of the plant material powder to the geopolymer in S3 is 1:0.8-1.2, and the mass ratio of thiourea to the geopolymer is 1-1.8:1.

[0017] Preferably, in S3, the precipitate is first washed alternately with deionized water and anhydrous ethanol 2-3 times, and then vacuum dried at 50-120° C. for 6-24 h.

[0018] As a further preferred method, thiourea is added to S3 and the mixture is stirred continuously, and the reaction is carried out at 150-180° C. for 5-15 h.

[0019] As a further preferred method, in S3, the temperature is 8-12°C·min -1 The heating rate is increased to 600-900℃ and calcined for 1-3 h.

[0020] In addition, the present invention also provides an adsorbent containing the sulfur-nitrogen co-doped porous carbon / geopolymer composite material.

[0021] Furthermore, the application of the sulfur-nitrogen co-doped porous carbon / geopolymer composite material or the adsorbent in sewage treatment is also the technical content protected by the present invention.

[0022] Preferably, in the above application, the sewage is heavy metal sewage, and the heavy metal ions contained in the heavy metal sewage include but are not limited to Cd 2+ 、Ni 2+ 、Hg 2+ 、Zn 2+ 、Cu 2+ .

[0023] As a further preferred embodiment, the sewage contains Cd 2+ sewage.

[0024] The beneficial effects of the present invention are: The present invention provides a sulfur-nitrogen co-doped porous carbon / geopolymer composite material, namely, the geopolymer is modified by using plant materials, thiourea and other components, which effectively loosens the dense structure of the geopolymer; at the same time, the doping of heteroatoms such as nitrogen and sulfur is conducive to the role of Lewis basic sites, increases the active adsorption sites of the composite material, and improves its adsorption capacity for heavy metals in wastewater.

[0025] The experimental results show that when the adsorption time is 300 min, the adsorption of GM on Cd 2+ The adsorption capacity was only 3.43 mg·g -1 ; PC / GM vs. Cd 2+ The adsorption capacity reached 4.64 mg·g -1 SN-PC / GM obtained after doping S and N has a significant effect on Gd 2+ The adsorption effect of PC / GM was significantly higher than that of GM and PC / GM, reaching 14.30 mg·g -1 , the adsorption capacity increased by 317% and 208% respectively. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The scanning electron microscope morphology characterization images of the three materials of GM, PC / GM and SN-PC / GM of the present invention; Figure 2 FT-IR scanning patterns and XRD patterns of various materials of the present invention; Figure 3 The effects of various factors on the Cd 2+ The influence of adsorption effect; Figure 4 SN-PC / GM to Cd 2+ Cyclic adsorption experimental results (a) and the adsorption of Cd by SN-PC / GM in different water samples 2+ Effect of adsorption (b); Figure 5 The comparison of adsorption effects of composite materials obtained by pine cone powder and geopolymer at different ratios is shown; Figure 6 The adsorption effect of the composite material obtained without adding pine cone powder was compared with that of geopolymer and SN-PC / GM. DETAILED DESCRIPTION

[0027] In order to enable those skilled in the art to better understand the present invention, the present invention will be further described in conjunction with specific embodiments.

[0028] Example 1 The sulfur-nitrogen co-doped porous carbon / geopolymer composite material is prepared as follows: S1: Wash the pine cones, air-dry them, crush them, and pass them through a 50-mesh sieve to prepare pine cone powder; Preparation of S2 geopolymer: Kaolin was placed in a muffle furnace at 5 °C·min -1 The temperature was raised to 750°C and calcined for 2 h to obtain metakaolin. Saturated sodium silicate solution was added dropwise to the obtained metakaolin for alkaline activation. The addition was stopped when the metakaolin became viscous. The metakaolin was then placed in an oven and cured at 80°C for 5 h. After curing, it was ground to obtain a geopolymer, named GM. Preparation of S3 sulfur-nitrogen co-doped porous carbon / geopolymer composite material: 0.8 g of pine cone powder prepared in S1 and 0.8 g of geopolymer prepared in S2 were weighed in a reaction container, 60 mL of water was added, and the mixture was stirred continuously for 30 min. Then 1.2 g of thiourea was added and stirred continuously. The mixture was reacted at 160 °C for 12 h. After the reaction was completed, the mixture was centrifuged at 10,000 rpm for 5 min, the supernatant was discarded, and the precipitate was washed alternately with deionized water and anhydrous ethanol three times, and vacuum dried at 60 °C. Finally, the mixture was placed in a tube furnace and annealed at 10 °C min under argon protection. -1 The heating rate was increased to 800° C. and calcined for 2 h to obtain the sulfur-nitrogen co-doped porous carbon / geopolymer composite material, which was named SN-PC / GM.

[0029] A porous carbon / geopolymer composite material without thiourea was prepared by the same steps as above as a control and named PC / GM.

[0030] Example 2 The various materials in Example 1 (geopolymer GM, sulfur-nitrogen co-doped porous carbon / geopolymer composite material SN-PC / GM, porous carbon / geopolymer composite material PC / GM without thiourea) were characterized.

[0031] Attachment Figure 1 (a), (b), and (c) are scanning electron microscope morphology characterization images of the three materials GM, PC / GM, and SN-PC / GM, respectively.

[0032] Attachment Figure 1 It can be seen that the surface structure of the GM material is dense and relatively smooth, and is blocky. After modification with the addition of plant material (pine cone powder here, hereinafter referred to as PC), the dense structure of the GM material surface is destroyed, and the obtained PC / GM composite material presents the characteristics of looseness and porosity. After sulfur and nitrogen modification, the surface structure of the SN-PC / GM composite material is further destroyed, the pore area is increased, and the pores are interconnected.

[0033] The unique morphology and structure of the SN-PC / GM composite material provides more adsorption sites and a larger specific surface area for the adsorption of heavy metals, which is beneficial for the diffusion of metal ions into the pores and reduces the resistance of the mass transfer process.

[0034] Attachment Figure 2 (a) and (b) are the Fourier transform infrared (FT-IR) scanning patterns and X-ray diffraction (XRD) patterns of various materials, respectively.

[0035] Figure 2 (a) can be seen at 3440 cm -1 The peak centered at 712 cm belongs to the -OH stretching vibration of water molecules, while the peak at 712 cm -1 The peak at corresponds to the Al-O-Si bending vibration peak, which indicates that all three materials retain the basic structure of GM.

[0036] In addition, PC / GM and SN-PC / GM have the same wavelength at 1600 cm -1 The characteristic peaks appearing at 1304 cm are attributed to the C=O stretching vibration peaks in the conjugated ketone and quinone in PC, indicating that PC was successfully compounded during the preparation of the two composite materials. -1 The corresponding peak belongs to CN stretching vibration, while 607 cm -1 The corresponding peak belongs to the CS stretching vibration, which proves the successful doping of non-metallic elements S and N.

[0037] Figure 2 (b) shows that the three materials have similar crystal compositions, including Al2O3 and SiO2, which indicates that the characteristic structure of GM is obvious in the three materials. θ = 21.243° and 34.961° correspond to the (10-2) crystal plane and (020) crystal plane of Al2O3 JCPDS No.50-1496, respectively. θ The peaks at = 23.279° and 30.012° are attributed to the (402) and (-512) crystal planes of SiO2, JCPDS No. 18-1170, respectively. Compared with PC / GM, it can be clearly observed that the GM matrix has a higher degree of crystallinity, which also means that the structure of PC / GM gradually transforms into an amorphous structure. After doping with S and N, the peak shape of SN-PC / GM does not change much, indicating that S and N do not react with PC / GM to form new crystalline components.

[0038] The above characterization results show that all three materials were successfully prepared.

[0039] Example 3 The various composite materials prepared in Example 1 were used as adsorbents to remove Cd from wastewater. 2+ The adsorption effect was verified, and the specific experimental operation was as follows: Prepare 5 mg·L -1 Cd 2+ The solution was placed in a conical flask, and 25 mg of the composite material was added as an adsorbent for adsorption experiments. Samples were taken at time points of 0, 30, 60, 90, 120, 180, 240, and 300 min, and the residual Cd was determined by atomic absorption spectrophotometry after passing through a 0.22 μm water filter membrane. 2+ concentration.

[0040] 3.1 Effect of adsorbent type on Cd adsorption 2+ Effect: Prepare 5 mg·L -1 Cd 2+ The solution was placed in a conical flask, and 25 mg of GM, PC / GM and SN-PC / GM were added respectively. The pH value was adjusted to 6.0 before adsorption experiments.

[0041] 3.2 Effect of pH on Cd 2+ Influence of adsorption effect: Prepare 5 mg·L -1 Cd 2+ The solution was placed in a conical flask, 25 mg of SN-PC / GM was added, and the pH value was adjusted to 3.0, 4.0, 5.0, 6.0, and 7.0 before adsorption experiments were performed.

[0042] 3.3 Cd 2+ Effect of concentration on adsorption effect: 2 mg·L -1 , 5 mg·L -1 、10 mg·L -1, 20mg·L -1 CD 2+ The solution was placed in a conical flask, 25 mg of SN-PC / GM was added, and the pH value was adjusted to 6.0 before the adsorption experiment.

[0043] 3.4 Effect of adsorbent dosage on Cd adsorption 2+ Effect: Prepare 5 mg·L -1 Cd 2+ The solution was placed in a conical flask, and 5 mg, 10 mg, 25 mg, and 30 mg of SN-PC / GM were added, respectively. The pH value was adjusted to 6.0 and the adsorption experiment was carried out.

[0044] Effects of various factors on Cd 2+ Adsorption effect see attached Figure 3 shown.

[0045] Attachment Figure 3 In (a), the effect of adsorbent type on the adsorption of Cd 2+ (b) The effect of pH on Cd 2+ The influence of adsorption effect; (c) is the Zeta potential diagram of SN-PC / GM; (d) is the Cd 2+ Effect of concentration on adsorption effect; (e) Effect of the amount of adsorbent SN-PC / GM on the adsorption of Cd 2+ The impact of the effect.

[0046] Figure 3 Figure (a) shows that GM has a strong effect on Cd 2+ The adsorption effect of α-glutamyl is the worst, and the maximum adsorption capacity is only 3.43 mg·g -1 After adding PC, the PC / GM ratio of Cd 2+ The adsorption performance was improved to 4.64 mg·g -1 This is because compared with GM, the pore structure of PC / GM changes, the specific surface area increases, and the mass transfer efficiency increases, so it can adsorb more Cd 2+ SN-PC / GM composite materials obtained by doping S and N on the basis of PC / GM have a great influence on the performance of Gd 2+ The adsorption effect of PC / GM was significantly increased compared with that of GM and PC / GM, reaching 14.30 mg·g -1 .

[0047] Figure 3 Figure (b) shows that when pH < 7.0, as the pH of the solution increases, SN-PC / GM has a 2+ The adsorption capacity of SN-PC / GM composite material increased when pH = 6.0. 2+The adsorption amount reaches the maximum and the adsorption performance is the best. The reason for the above phenomenon may be that the surface charge of the solution changes under different pH conditions and affects the state of heavy metal ions in the solution.

[0048] Figure 3 Figure (c) shows the Zeta potential changes of SN-PC / GM at different pH values. It can be seen from the figure that within the pH range of 1.0-4.0, the surface of the SN-PC / GM composite material carries a positive charge, and the protonation effect at low pH increases the positive charge density on the surface, thereby causing Cd 2+ electrostatic repulsion; in addition, the positively charged H + Also works with Cd 2+ The adsorption sites on the surface of the competing materials are not conducive to adsorption. Therefore, the Cd 2+ When the pH is 5.0-6.0, the surface of the composite material has a negative charge and adsorbs heavy metal Cd 2+ It will produce electrostatic attraction, which is beneficial for the composite material to Cd 2+ After pH>7.0, although the surface of the composite material is still negatively charged, Cd 2+ Easily generate Cd(OH)2 or Cd(OH)3 - It precipitates from the aqueous solution, thereby hindering the adsorption of heavy metal Cd by the SN-PC / GM composite material.

[0049] Figure 3 (d) shows that as Cd 2+ With the increase of Cd concentration, the adsorbent 2+ The adsorption capacity of Cd 2+ The concentration is 20 mg·L -1 When -1 This is because Cd 2+ The higher the concentration, the greater the driving force for adsorption on the surface of SN-PC / GM composite material, and the adsorption amount also increases. 2+ The concentration reaches 5 mg·L -1 After that, the adsorption amount increased with the increase of its concentration, but the rate of increase slowed down, indicating that the adsorption sites on the surface of SN-PC / GM have been gradually occupied by Cd 2+ The saturation of adsorption sites makes the adsorption process reach the surface of the composite material and the Cd 2+ The state of near equilibrium between the concentrations. Considering the adsorption efficiency and the 2+ The concentration of 5 mg·L -1 Cd 2+ Conduct pollutant adsorption experiments.

[0050] Figure 3 Figure (e) shows that under the same adsorption conditions, the adsorption effect of SN-PC / GM composite material is the worst when the dosage is 5 mg, and the adsorption amount is only 3.31 mg·g -1 When the amount of composite material was 10 mg, the adsorption capacity increased to 4.16 mg·g -1 When the amount of the composite material increased to 25 mg, the adsorption effect was significantly improved, and the adsorption capacity could reach 14.30 mg·g -1 This is because as the amount of composite adsorbent increases, the adsorption sites and adsorption area of SN-PC / GM increase, while Cd 2+ The interaction between the composite material and the adsorbent was more effective. Then, as the amount of adsorbent continued to increase (30 mg), the adsorption amount remained almost unchanged. This may be due to the excessive amount of adsorbent material, which caused some pore structures to stack and block each other, thus limiting the adsorption of heavy metal Cd by the composite material. 2+ adsorption effect.

[0051] Example 4 Verification of adsorption stability and reusability of SN-PC / GM composite materials.

[0052] Attachment Figure 4 (a) shows the effect of SN-PC / GM composite material on Cd 2+ (b) is the cyclic adsorption experimental results of SN-PC / GM composite materials on Cd in different water samples. 2+ The effect of adsorption.

[0053] Figure (a) shows that in the first round of adsorption experiments, the SN-PC / GM composite material has a strong affinity for Cd 2+ The adsorption capacity can reach 14.30 mg·g -1 The composite material was washed alternately with deionized water and ethanol, filtered, dried, and recovered for the second round of adsorption experiments. The results showed that the adsorption capacity of the recovered SN-PC / GM composite material could reach 9.38 mg·g -1 Although the adsorption amount decreased, it still 2+ It has a certain adsorption effect.

[0054] Attachment Figure 4 Figure (b) shows the use of SN-PC / GM composite materials to analyze the Cd content in different water samples. 2+ As can be seen from the figure, the adsorption effect of SN-PC / GM composite material on Cd in the water system of the Jihe River 2+ The adsorption effect of α-glutamyl is the worst, with an adsorption capacity of only 4.21 mg·g -1 This may be because the actual water sample contains a variety of metal ions, which will react with Cd2+ Compete for adsorption sites. 2+ Compared with the adsorption process, the SN-PC / GM composite material has a better effect on the adsorption of Cd in tap water. 2+ The adsorption capacity of Cd in tap water was relatively low in the initial 120 min, but as the adsorption time increased, its adsorption capacity for Cd in tap water increased. 2+ The adsorption effect of the deionized water system was significantly higher than that of the deionized water system, with an adsorption capacity of up to 19.12 mg·g -1 This is mainly because tap water contains a large amount of chloride ions and organic matter. The coexisting chloride ions and organic matter can promote the absorption of Cd by enhancing electrostatic attraction and complexation. 2+ In addition, Cl in tap water - It can also form Cd-Cl complex with Cd, which is beneficial to the adsorption of Cd. In the first 120 min of adsorption, due to the presence of other metal ions and Cd in the system, 2+ competitive adsorption of Cd 2+ The adsorption capacity is small.

[0055] The above results show that SN-PC / GM composite materials are expected to be used in the analysis of Cd in actual water samples. 2+ adsorption.

[0056] Example 5 The difference from Example 1 is that the mass ratios of pine cone powder to geopolymer are 2:1, 1:1, 1:2, and 1:4 respectively; the rest are the same as Example 1.

[0057] Effect of SN-PC / GM composite materials prepared with different ratios of pine cone powder and geopolymer on Cd in water samples 2+ The adsorption effect of Figure 5 shown. Figure 5 It can be seen that when the mass ratio of the two is 1:1, the obtained composite material has the best adsorption effect.

[0058] Example 6 In this embodiment, the geopolymer is directly reacted with thiourea without adding pine cone powder. That is, based on Example 1, step S1 is omitted and a composite material is prepared using the same method as in Example 1. The composite material obtained by modifying the geopolymer with thiourea without adding pine cone powder is named SN-GM.

[0059] GM, SN-GM, SN-PC / GM and other materials for Cd in water samples 2+ The adsorption effect of Figure 6 shown. Figure 6It can be seen that the adsorption effect of the SN-GM composite material obtained by modifying the SN-GM geopolymer with thiourea is indeed improved compared with GM, but the adsorption effect is far inferior to the SN-PC / GM composite material obtained by adding pine cone powder.

Claims

1. Sulfur-nitrogen co-doped porous carbon / geopolymer composite material, characterized in that: The preparation steps are as follows: S1: washing, air-drying, crushing, and sieving the plant material to prepare plant material powder; Preparation of S2 geopolymer: Kaolin is calcined in a muffle furnace to obtain metakaolin. Saturated sodium silicate solution is added dropwise to the obtained metakaolin for alkaline excitation. The addition is stopped when the metakaolin becomes viscous. After solidification, the geopolymer is ground to obtain GM. Preparation of S3 sulfur-nitrogen co-doped porous carbon / geopolymer composite materials: Weigh the plant material powder prepared in S1 and the geopolymer prepared in S2, add deionized water into a reaction vessel, stir continuously for 20-60 min, then add thiourea and continue stirring, react at 100-200 ° C for 2-24 h, after the reaction is completed, centrifuge and discard the supernatant, wash and dry the precipitate, place it in a tube furnace, and heat it under argon protection at 8-12 ° C min -1 The heating rate is increased to 300-900° C. and calcined for 1-6 h to obtain the sulfur-nitrogen co-doped porous carbon / geopolymer composite material, which is named SN-PC / GM.

2. The sulfur-nitrogen co-doped porous carbon / geopolymer composite material according to claim 1, characterized in that: The plant material described in S1 is selected from at least one of pine cones, wheat straw, corn straw, and peanut shells.

3. The sulfur-nitrogen co-doped porous carbon / geopolymer composite material according to claim 2, characterized in that: The plant material described in S1 is pine cones.

4. The sulfur-nitrogen co-doped porous carbon / geopolymer composite material according to claim 1, wherein: The calcination process in S2 is as follows: 3-8°C·min -1 The temperature is raised to 300-900 °C at a rate of 1-2 hours and the calcination time is 1-5 hours.

5. The sulfur-nitrogen co-doped porous carbon / geopolymer composite material according to claim 1, wherein: The curing temperature described in S2 is 60-120°C, and the curing time is 2-12 h.

6. The sulfur-nitrogen co-doped porous carbon / geopolymer composite material according to claim 1, characterized in that: The mass ratio of plant material powder to geopolymer in S3 is 1:0.8-1.5, and the mass ratio of thiourea to geopolymer GM is 0.5-2:

1.

7. The sulfur-nitrogen co-doped porous carbon / geopolymer composite material according to claim 1, characterized in that: In S3, the precipitate is first washed alternately with deionized water and anhydrous ethanol 2-3 times, and then vacuum dried at 50-120°C for 6-24 h.

8. An adsorbent comprising the sulfur-nitrogen co-doped porous carbon / geopolymer composite material according to claim 1.

9. Use of the sulfur-nitrogen co-doped porous carbon / geopolymer composite material according to claim 1 or the adsorbent according to claim 8 in sewage treatment.

10. The use according to claim 9, characterized in that The sewage is heavy metal sewage, and the heavy metal ions contained in the heavy metal sewage include but are not limited to Cd 2+ 、Ni 2+ 、Hg 2+ 、Zn 2+ 、Cu 2+ .

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