Method for preparing aluminum-based hydrotalcite by utilizing aluminum-containing solid waste, prepared aluminum-based hydrotalcite and application of prepared aluminum-based hydrotalcite

Abstract

The invention discloses a method for preparing aluminum-based hydrotalcite by utilizing aluminum-containing solid waste, the prepared aluminum-based hydrotalcite and application of the prepared aluminum-based hydrotalcite. The method for preparing the aluminum-based hydrotalcite by using the aluminum-containing solid waste comprises the following steps: (1) calcining the aluminum-containing solid waste generated in the synthesis process of a benzotriazole ultraviolet light absorber to obtain low-grade Al2O3; (2) measuring the content of aluminum in the low-grade Al2O3; and (3) taking low-grade Al2O3 as an aluminum source, adding a proper amount of divalent metal hydroxide, an anion source and deionized water, uniformly stirring, and carrying out hydrothermal reaction to obtain the aluminum-based hydrotalcite. The invention provides application of the aluminum-based hydrotalcite as a heavy metal ion treating agent in water or soil. According to the method, the recycling difficulty and cost of aluminum resources in the aluminum-containing solid waste are reduced, resource utilization of the aluminum-containing solid waste is achieved, the production cost of the aluminum-based hydrotalcite is reduced, and the obtained aluminum-based hydrotalcite has good adsorption performance on heavy metal ions.

Description

Technical Field

[0001] The present invention relates to the field of comprehensive utilization of aluminum resources, and specifically to a method for preparing aluminum-based hydrotalcite by utilizing aluminum-containing solid waste (i.e., aluminum-containing solid waste), the prepared aluminum-based hydrotalcite and its application in adsorbing heavy metal ions. Background Art

[0002]

[0003] In the process of reducing azo intermediates to benzotriazole ultraviolet absorber products, alkaline aluminum-containing waste liquid is generated due to the use of aluminum powder as a reducing agent, the main component of which is NaAlO2. The waste acid generated in the pickling process neutralizes the alkaline aluminum-containing waste liquid to generate aluminum-containing waste slag mainly composed of Al(OH)3, which also contains Cl - 、SO4 2- , nitrogen-containing organic matter and tar and other impurities. Most of the existing treatment methods are to calcine the waste residue at high temperature to burn off the organic impurities and form low-grade Al2O3 for recycling. According to the current production scale of benzotriazole ultraviolet absorber products, tens of thousands of tons of aluminum-containing waste residue will be produced each year, which will cause great environmental pressure and treatment costs to industry enterprises, and low-quality Al2O3 has low added value and low utilization rate. Therefore, it is urgent to develop new treatment technologies for aluminum-containing wastes and effectively utilize aluminum-containing wastes as resources to improve the market competitiveness of enterprises.

[0004] At present, most of the research on scrap aluminum resources focuses on the recycling and reuse of scrap aluminum. The inevitable result of scrap aluminum recycling is that the quality of recycled aluminum will be reduced. Generally, aluminum resource recycling can be divided into three categories, namely downgrade recycling, grade-retaining recycling and leapfrogging recycling (extracting original aluminum from scrap aluminum alloy). At present, my country's scrap aluminum recycling is still mainly based on remelting and smelting downgrade recycling. This recycling method involves sorting technology and selective impurity removal. It has high energy consumption, a lot of waste slag, and a low aluminum recycling rate. It is also difficult to meet the requirements of forged aluminum. Unless innovative aluminum recycling technology is developed, about 3.6 million tons of secondary aluminum will become "dead metal" by 2040. Grade-retaining recycling of scrap aluminum can maintain most of the value of scrap aluminum. At present, grade-retaining recycling is the main way to recycle scrap aluminum. However, the large amount of aluminum produced by grade-retaining recycling cannot be digested, which will also lead to the accumulation of a large amount of aluminum resources. Leapfrogging recycling is the main direction of the future recycled aluminum industry, but leapfrogging recycling requires a high level of technology, which is difficult for small and medium-sized aluminum industries to afford and cannot be promoted on a large scale.

[0005] It can be seen that the current comprehensive utilization of waste aluminum resources is mostly to recover metallic aluminum from waste aluminum resources through various regeneration technologies to achieve the reuse of aluminum resources, and there is little research on the use of waste aluminum resources to prepare high-value-added aluminum-containing materials.

[0006] Aluminum-based hydrotalcite, as a type of layered double hydroxides (LDHs), has a two-dimensional layered structure, and its chemical composition can be expressed as:

[0007]

[0008] Among them, M 2+ Mg 2+ , Ni 2+ , Co 2+ , Zn 2+ ,Cu 2+ Equivalent metal cations; M 3+ For Al 3+ ; A n- Anions, such as CO3 2- , NO3 - , Cl - , OH - ,SO4 2- , PO4 3- , C6H4(COO)2 2- The interlayer distances of LDHs are different for inorganic and organic ions and complex ions and different interlayer anions.

[0009] Aluminum-based hydrotalcite has shown significant application potential in the adsorption of heavy metal ions due to its special layered structure and physical and chemical properties. Aluminum-based hydrotalcite can be used to treat wastewater containing heavy metal ions, and remove heavy metal ions from wastewater through adsorption to achieve the purpose of purifying water. For soil contaminated by heavy metals, aluminum-based hydrotalcite can be used as a soil remediation agent to reduce the bioavailability of heavy metal ions in the soil by adsorbing and fixing them, thereby reducing the harm to the environment and human body. In industrial fields such as electroplating, metallurgy, and chemical industry, the wastewater generated often contains high concentrations of heavy metal ions. Aluminum-based hydrotalcite can be used as a treatment agent for these industrial wastewaters, effectively removing heavy metal ions in the wastewater and achieving standard discharge of wastewater.

[0010] The invention uses aluminum-containing solid waste generated in the synthesis process of benzotriazole ultraviolet absorbers as raw materials, prepares aluminum-based hydrotalcite after calcination and washing, and applies the aluminum-based hydrotalcite to the field of heavy metal pollution control, thereby achieving effective utilization of aluminum resources. It is unexpectedly found that the treatment effect of the aluminum-based hydrotalcite on heavy metal cadmium ions is even better than that of the aluminum-based hydrotalcite prepared from pure alumina. Summary of the invention

[0011] In view of the insufficiency of the existing waste aluminum resource utilization, the purpose of the present invention is to provide a method for preparing aluminum-based hydrotalcite using aluminum-containing solid waste generated in the synthesis process of benzotriazole ultraviolet absorbers as aluminum source raw material, the aluminum-based hydrotalcite prepared according to the method, and the use of the aluminum-based hydrotalcite in heavy metal ion treatment. The present invention not only reduces the difficulty and cost of recycling aluminum resources in aluminum-containing solid waste, but also realizes the resource utilization of aluminum-containing solid waste, and the prepared aluminum-based hydrotalcite has a good adsorption effect on heavy metal ions.

[0012] In order to achieve the above-mentioned object of the invention, the technical solution of the present invention is:

[0013] In a first aspect, the present invention provides a method for preparing aluminum-based hydrotalcite using aluminum-containing solid waste, comprising the following steps:

[0014] (1) Calcining aluminum-containing solid waste generated during the synthesis of benzotriazole ultraviolet absorbers to burn out organic impurities and obtain low-grade Al2O3;

[0015] (2) determining the aluminum content in the low-grade Al2O3 obtained in step (1);

[0016] (3) Weigh a certain amount of low-grade Al2O3 obtained in step (1) as an aluminum source and place it in a hydrothermal reactor. According to the aluminum content measured in step (2), add an appropriate amount of divalent metal hydroxide and an anion source into the hydrothermal reactor, wherein the divalent metal hydroxide is calcium hydroxide and the anion source is a chlorine source, so that the molar ratio of calcium to aluminum is 1:1 to 5:1, and add deionized water. After stirring evenly, carry out a hydrothermal reaction in an oven. The hydrothermal reaction temperature is 60 to 150°C and the hydrothermal reaction time is 8 to 36 hours. After sufficient reaction, take out the reaction mixture from the hydrothermal reactor, cool it, filter it, wash the precipitate until the filtrate is neutral, and then dry and grind it to obtain aluminum-based hydrotalcite.

[0017] Preferably, in step (1), the calcination temperature is 550-750°C, more preferably 600-700°C, and further preferably 600°C; the calcination time is 2-6h, more preferably 3-4h, and further preferably 3h.

[0018] Preferably, step (1) is specifically implemented as follows: the aluminum-containing waste is placed in a crucible, which is then moved into a muffle furnace, heated to a calcination temperature, kept at the temperature for a certain period of time, then cooled and taken out, washed, dried, and ground to obtain low-grade Al2O3.

[0019] Preferably, step (2) is to dissolve the obtained low-grade Al2O3 in acid at high temperature, and after it is dissolved, determine the aluminum content thereof by EDTA titration method. More preferably, the acid is sulfuric acid, and the concentration is 0.1-1 mol / L, preferably 0.2-0.5 mol / L. The high-temperature acid dissolution has a reaction temperature of 60-100°C, preferably 80-100°C.

[0020] Preferably, in step (3), the chlorine source is sodium chloride or calcium chloride, and calcium chloride is most preferred. The amount of the anion source added is determined according to the elemental stoichiometric ratio of the aluminum-based hydrotalcite and the amount of aluminum and divalent metal in the system.

[0021] As a further preference, in step (3), the molar ratio of calcium to aluminum is 2:1 to 4:1, and more preferably 2:1.

[0022] Preferably, in step (3), the feed ratio of the low-grade alumina to deionized water is 1-2 g: 40-60 mL.

[0023] Preferably, in step (3), the hydrothermal reaction temperature is 120-150° C., more preferably 150° C.; the hydrothermal reaction time is 16-24 h, more preferably 24 h.

[0024] Preferably, in step (3), the drying temperature is 60 to 100° C., preferably 80 to 90° C.; and the drying time is 4 to 8 hours.

[0025] The present invention particularly prefers the following technical solution: in step (3), the divalent metal hydroxide is calcium hydroxide, the anion source is calcium chloride, the molar ratio of calcium to aluminum is controlled to be 2:1, the hydrothermal reaction temperature is 150° C., and the hydrothermal reaction time is 24 h.

[0026] In a second aspect, the present invention provides an aluminum-based hydrotalcite prepared according to the method described in the first aspect.

[0027] In a third aspect, the present invention provides use of the aluminum-based hydrotalcite described in the second aspect as a heavy metal ion treatment agent in water or soil.

[0028] Preferably, the heavy metal ions are cadmium ions.

[0029] Compared with the existing aluminum-containing solid waste utilization technology, the present invention has the following advantages:

[0030] (1) Starting from aluminum-containing solid waste, low-grade alumina is obtained by calcining at a suitable temperature, and then aluminum-based hydrotalcite is obtained through a simple hydrothermal reaction, which can be used for the effective treatment of heavy metals in water and soil, and has good application prospects. The treatment technology involved in the present invention reduces the difficulty and cost of recycling aluminum resources in aluminum-containing solid waste, realizes the resource utilization of aluminum-containing solid waste, and has good application and promotion value.

[0031] (2) When the aluminum-based hydrotalcite prepared from aluminum-containing solid waste is used to treat heavy metals in water and soil, its optimal treatment effect exceeds that of the aluminum-based hydrotalcite prepared from pure alumina. Therefore, the present invention can not only turn aluminum-containing solid waste into treasure and significantly reduce the production cost of aluminum-based hydrotalcite, but also improve its treatment effect on heavy metal ions. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 is the XRD pattern of solid aluminum waste;

[0033] Figure 2 The XRD diagram of the aluminum source after calcination in Example 1-4;

[0034] Figure 3 This is the XRD pattern of the aluminum-based hydrotalcite obtained in Example 5;

[0035] Figure 4 This is the XRD pattern of the aluminum-based hydrotalcite obtained in Example 6;

[0036] Figure 5 This is the XRD pattern of the aluminum-based hydrotalcite obtained in Example 7;

[0037] Figure 6 This is the XRD pattern of the aluminum-based hydrotalcite obtained in Example 8;

[0038] Figure 7 This is the XRD pattern of the aluminum-based hydrotalcite obtained in Example 9;

[0039] Figure 8 This is the infrared spectrum of the aluminum-based hydrotalcite prepared in Example 6;

[0040] Fig. 9 It is shown that the initial pH of cadmium nitrate aqueous solution has an effect on the adsorption of Cd on aluminum-based hydrotalcite 2+ The impact of

[0041] Fig.10 It shows that the amount of aluminum-based hydrotalcite has an effect on the removal of Cd 2+ The impact of

[0042] Fig.11 The effect of oscillation time on the removal of Cd from Al-based hydrotalcite is shown 2+ impact. DETAILED DESCRIPTION

[0043] The present invention is further described below in conjunction with the embodiments and drawings. However, the embodiments of the present invention are not limited thereto, and there may be many variations. If there are any details not particularly described below, they can be understood by those skilled in the art according to conventional expressions or implemented according to the prior art.

[0044] Examples 1-4

[0045] Take a certain amount of aluminum-containing solid waste (the measured COD value is 591 mg / L, and the XRD pattern is shown in Figure 1 ) was placed in a crucible, and then moved into a muffle furnace, and the temperature was raised to 550°C (Example 1), 600°C (Example 2), 650°C (Example 3), and 700°C (Example 4) at a rate of 5°C / min, and calcined at this temperature for 3 hours. After cooling, it was taken out, washed, dried, and ground to obtain low-grade alumina. The XRD pattern of the low-grade alumina is shown in Figure 2 .

[0046] A certain amount of low-grade alumina was dissolved in 0.5 mol / L dilute sulfuric acid at 100°C for 2 h. After it was dissolved, the aluminum content was determined by EDTA titration method. The aluminum content was converted into Al2O3 content. The results are shown in Table 1.

[0047] Table 1 Al2O3 content in low-grade alumina obtained in Examples 1-4

[0048]

[0049] Example 5

[0050] 2.368g Ca(OH)2, 0.932g NaCl and 1.432g low-grade alumina obtained by calcining at 650°C for 3h in Example 3 were weighed and placed in a 100mL hydrothermal reactor, 50mL deionized water was added and stirred for 15min, and then heated at 150°C for 24h. After the reaction, the reactor was cooled, filtered, washed and precipitated until the pH of the filtrate was neutral, and then dried at 80°C, ground and sieved to obtain aluminum-based hydrotalcite CaAl-LDHs with a calcium-aluminum molar ratio of 2:1. Its XRD pattern is shown in Figure 3 .

[0051] Example 6

[0052] 1.776g Ca(OH)2, 0.888g CaCl2 and 1.432g low-grade alumina obtained by calcining at 600°C for 3h in Example 2 were weighed and placed in a 100mL hydrothermal reactor, 50mL deionized water was added and stirred for 15min, and then heated at 150°C for 24h. After the reaction, the reactor was cooled, filtered, washed and precipitated until the pH of the filtrate was neutral, and then dried at 80°C, ground and sieved to obtain aluminum-based hydrotalcite CaAl-LDHs with a calcium-aluminum molar ratio of 2:1. Its XRD pattern is shown in Figure 4 , infrared spectrum see Figure 8 .

[0053] Example 7

[0054] 1.776g Ca(OH)2, 0.888g CaCl2 and 1.432g low-grade alumina obtained by calcining at 700°C for 3h in Example 4 were weighed and placed in a 100mL hydrothermal reactor, 50mL deionized water was added and stirred for 15min, and then heated at 120°C for 16h. After the reaction, the reactor was cooled, filtered, washed and precipitated until the pH of the filtrate was neutral, and then dried at 80°C, ground and sieved to obtain aluminum-based hydrotalcite CaAl-LDHs with a calcium-aluminum molar ratio of 2:1. Its XRD pattern is shown in Figure 5 .

[0055] Example 8

[0056] 2.96g Ca(OH)2, 0.888g CaCl2 and 1.432g low-grade alumina obtained by calcining at 600°C for 3h in Example 2 were weighed and placed in a 100mL hydrothermal reactor, 50mL deionized water was added and stirred for 15min, and then heated at 90°C for 16h. After the reaction, the reactor was cooled, filtered, washed and precipitated until the pH of the filtrate was neutral, and then dried at 80°C, ground and sieved to obtain aluminum-based hydrotalcite CaAl-LDHs with a calcium-aluminum molar ratio of 3:1. Its XRD pattern is shown in Figure 6 .

[0057] Example 9

[0058] 4.144g Ca(OH)2, 0.888g CaCl2 and 1.432g low-grade alumina obtained by calcining at 600°C for 3h in Example 2 were weighed and placed in a 100mL hydrothermal reactor, 50mL deionized water was added and stirred for 15min, and then heated at 60°C for 8h. After the reaction, the reactor was cooled, filtered, washed and precipitated until the pH of the filtrate was neutral, and then dried at 80°C, ground and sieved to obtain aluminum-based hydrotalcite CaAl-LDHs with a calcium-aluminum molar ratio of 4:1. Its XRD pattern is shown in Figure 7 .

[0059] Comparative Example 1

[0060] 3.552g Ca(OH)2, 0.888g CaCl2 and 0.816g pure γ-Al2O3 (200-300 mesh, Shanghai MacLean Biochemical Technology Co., Ltd.) were weighed and placed in a 100ml hydrothermal reactor, 50ml deionized water was added and stirred for 15min, and then heated at 150℃ for 24h. After the reaction, the reactor was cooled, filtered, washed and precipitated until the pH of the filtrate was neutral, and then dried at 80℃, ground and sieved to obtain aluminum-based hydrotalcite CaAl-LDHs with a calcium-aluminum molar ratio of 2:1.

[0061] Example 10

[0062] 50 ml of cadmium nitrate aqueous solution with a cadmium nitrate concentration of 100 ppm and adjusted to pH 2, 3, 4, 5, 6 and 7 with 0.1 mol / L nitric acid was prepared respectively and poured into a 100 ml stoppered conical flask, 0.05 g of the aluminum-based hydrotalcite prepared in Example 6 was added, and then the conical flask was placed in a constant temperature oscillator and oscillated at 25° C. and 200 rpm for 2 h, then taken out and filtered, and the filtrate was measured by atomic absorption spectrophotometer for Cd 2+ Concentration, calculate Cd 2+ Removal rate, the results are as follows Fig. 9 The diagram shows that the aluminum-based hydrotalcite has a good removal effect on 100 ppm cadmium solution in a wide pH range (pH 3 to 7).

[0063] Embodiment 11

[0064] Prepare 50 ml of 100 ppm cadmium nitrate aqueous solution and pour it into a 100 ml stoppered conical flask, add 0.01 g, 0.02 g, 0.035 g, 0.05 g, 0.075 g, 0.1 g, 0.15 g and 0.2 g of the aluminum-based hydrotalcite prepared in Example 6, respectively, and then place the conical flask in a constant temperature oscillator, oscillate and react at 25 ° C and 200 rpm for 2 h, then take out and filter, and the filtrate is used to determine Cd by atomic absorption spectrophotometer. 2+ Concentration, calculate Cd 2+ Removal rate, the results are as follows Fig.10 The figure shows that when the dosage of the aluminum-based hydrotalcite is 0.01g, the removal rate of cadmium ions can reach 90%; when the dosage is 0.05g, the cadmium ion removal rate remains basically unchanged, reaching 98~99%, showing a good removal efficiency of cadmium ions.

[0065] Example 12

[0066] 50 ml of 100 ppm cadmium nitrate aqueous solution was prepared and poured into a 100 ml stoppered conical flask, 0.05 g of the aluminum-based hydrotalcite prepared in Example 6 was added, and then the conical flask was placed in a constant temperature oscillator and oscillated at 25° C. and 200 rpm for reaction. Samples were taken and filtered after 2 min, 5 min, 10 min, 20 min, 30 min, 60 min, 90 min and 2 h of reaction, and the filtrate was measured by atomic absorption spectrophotometer for Cd 2+ Concentration, calculate Cd 2+ Removal rate, the results are as follows Fig.11 The figure shows that when the oscillation time is 5 minutes, the removal rate of cadmium ions by aluminum-based hydrotalcite can reach more than 97%, showing an excellent removal rate in a short time.

[0067] Example 13

[0068] The aluminum-based hydrotalcite prepared in Examples 5-9 was used to simulate the treatment of cadmium-containing wastewater. 50 mL of 100 ppm was measured and poured into a 100 mL stoppered conical flask, and 0.05 g of the aluminum-based hydrotalcite prepared in Examples 5-9 was added. The conical flask was then placed in a constant temperature oscillator and oscillated at 25°C and 200 rpm for 2 h. Afterwards, it was taken out and filtered, and the filtrate was used to determine the Cd content using an atomic absorption spectrophotometer. 2+ Concentration, calculate Cd 2+ The removal rate is shown in Table 2. In addition, 50 mL of cadmium-containing solutions of different concentrations were treated with 0.05 g of the aluminum-based hydrotalcite prepared in Example 6 and Comparative Example 1, and the reaction was oscillated at 25 ° C and 200 rpm for 2 hours. The results are shown in Tables 3 and 4.

[0069] Table 2. Treatment results of aluminum-based hydrotalcite prepared in Examples 5-9 on simulated cadmium-containing wastewater

[0070]

[0071] Table 3. Treatment results of aluminum-based hydrotalcite prepared in Example 6 on simulated cadmium-containing wastewater of different concentrations

[0072]

[0073] Table 4 Treatment results of aluminum-based hydrotalcite prepared in Comparative Example 1 on simulated cadmium-containing wastewater of different concentrations

[0074]

[0075] According to Table 2, the aluminum-based hydrotalcites prepared in Examples 5-9 all have good adsorption effects on cadmium ions. The data in the examples show that the calcium-aluminum molar ratio and the hydrothermal reaction conditions in the aluminum-based hydrotalcite have a great influence on the adsorption effect; and it is preferred to introduce the intercalation Cl in the form of calcium chloride.- . Comparison of the experimental data in Table 3 and Table 4 shows that the aluminum-based hydrotalcite prepared by the present invention using low-grade alumina has better adsorption performance for cadmium ions than the aluminum-based hydrotalcite prepared by pure γ-Al2O3, which may be related to the impurities contained in the low-grade alumina. The present invention not only reduces the difficulty and cost of recycling aluminum resources in aluminum-containing solid waste, realizes the resource utilization of aluminum-containing solid waste, and significantly reduces the production cost of aluminum-based hydrotalcite, but also the obtained aluminum-based hydrotalcite has good adsorption performance for heavy metal ions, which is even better than the aluminum-based hydrotalcite prepared by pure γ-Al2O3.

Claims

1. A method for preparing aluminum-based hydrotalcite using aluminum-containing solid waste, characterized in that: The method comprises the following steps: (1) Calcining aluminum-containing solid waste generated during the synthesis of benzotriazole ultraviolet absorbers to burn out organic impurities and obtain low-grade Al2O3; (2) determining the aluminum content in the low-grade Al2O3 obtained in step (1); (3) Weigh a certain amount of low-grade Al2O3 obtained in step (1) as an aluminum source and place it in a hydrothermal reactor. According to the aluminum content measured in step (2), add an appropriate amount of divalent metal hydroxide and an anion source into the hydrothermal reactor, wherein the divalent metal hydroxide is calcium hydroxide and the anion source is a chlorine source, so that the molar ratio of calcium to aluminum is 1:1 to 5:1, and add deionized water. After stirring evenly, carry out a hydrothermal reaction in an oven. The hydrothermal reaction temperature is 60 to 150°C and the hydrothermal reaction time is 8 to 36 hours. After sufficient reaction, take out the reaction mixture from the hydrothermal reactor, cool it, filter it, wash the precipitate until the filtrate is neutral, and then dry and grind it to obtain aluminum-based hydrotalcite.

2. The method according to claim 1, characterized in that: In step (1), the calcination temperature is 550-750°C and the calcination time is 2-6h.

3. The method according to claim 1, characterized in that: In step (3), the chlorine source is sodium chloride or calcium chloride.

4. The method according to any one of claims 1 to 3, characterized in that: In step (3), the molar ratio of calcium to aluminum is set at 2:1 to 4:

1.

5. The method according to claim 4, characterized in that: In step (3), the molar ratio of calcium to aluminum is 2:

1.

6. The method according to any one of claims 1 to 3, characterized in that: In step (3), the hydrothermal reaction temperature is 120 to 150° C. and the hydrothermal reaction time is 16 to 24 hours.

7. The method according to claim 1, characterized in that: In step (3), the divalent metal hydroxide is calcium hydroxide, the anion source is calcium chloride, the molar ratio of calcium to aluminum is controlled to be 2:1, the hydrothermal reaction temperature is 150° C., and the hydrothermal reaction time is 24 h.

8. Aluminum-based hydrotalcite prepared according to the method of any one of claims 1 to 7.

9. Use of the aluminum-based hydrotalcite as claimed in claim 8 as a heavy metal ion treatment agent in water or soil.

10. The use according to claim 9, characterized in that: The heavy metal ions are cadmium ions.

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