Synthesis of Zr metal-organic frameworks (MOFs) to remove Fe3+ from water.

Although a trace amount of Fe3+ is essential for human physiological function, excessive amounts are lethal. Here, we present a protocol for removing Fe3+ from water through highly crystalline and stable thiol-functionalized Zr metal-organic frameworks (Zr-MOFs). We provide details of the MOFs synthesis and post-functionalization procedures, and include key performance data of the Zr-MOFs for removing Fe3+, which were collected from the inductively coupled plasma-optical emission spectrometer (ICP-OES) and inductively coupled plasma mass spectrometer (ICP-MS). For complete details on the use and execution of this protocol, please refer to Yuan et al. (2022).

Before you begin

Iron is one of the essential nutrients which maintains various human physiological functions. However, an excess amount of Fe3+ can induce various biological disorders. Specifically, 20–60 mg L−1 Fe3+ can damage DNA and cellular components, leading to hemochromatosis, liver cirrhosis, diabetes, and heart disease (Hartmann et al., 2013; Khatri et al., 2017). A concentration of Fe3+ > 200 mg L−1 induces severe Fe3+ poisoning, leading to death. Therefore, it is crucial to exploit technology that can purify the Fe3+ contaminated water to a drinkable level (< 0.3 mg L−1) (WHO World Health Organization, 2003). Compared with water purification methods such as ion exchange, precipitation, electrodialysis, coagulation, and flocculation, adsorption is the most promising feature high removal efficiency, improved recyclability, and cost-effectiveness (Duan et al., 2018; Gabarrón et al., 2016; Nemati et al., 2017; Sun et al., 2018). Therefore, plenty of research interests focus on the synthesis of novel adsorbent. MOFs have the merits of high specific surface area and facile tunable structure, ensuring high removal efficiency for toxic heavy metal ions. Hence, MOFs are prominent for water purification compared with traditional activated carbon and zeolites (Hui et al., 2005; Wang et al., 2009). However, the aqueous stability restricts the application of the MOFs. One solution to improve the stability is synthesizing MOFs from a hard Lewis acid, a hard Lewis base pair, or a soft Lewis acid, and a soft Lewis base pair (Feng et al., 2018; Huang et al., 2020; Yuan et al., 2018a, 2018b). The Zr-based UiO-66 type MOFs have demonstrated high stability in various environments but low water purification efficiency. Therefore, in this protocol, we synthesize UiO-66 type MOFs from the Zr cluster and 2,5-dichloroterephthalic acid (H2BDC-Cl2) under solvothermal conditions and then further post-functionalize with 1,2-Ethanedithiol (EDT) (Scheme 1) to introduce the thiol chelation groups to boost the Fe3+ removal efficiency. Further knowledge of solvothermal synthesis and post-functionalization procedures can be found in the literature (Yuan et al., 2022).

Scheme 1

Synthesis of UiO-66 MOF and post functionalization

Oxygen is shown in red; zirconium in light blue; carbon in gray; and chlorine in green. Hydrogen is omitted.

Reproduced with permission (Yuan et al., 2022); Copyright, 2022 Elsevier Inc.

Preparation of stock solution

Timing: 30 min

Slowly add 113.44 mL 65% HNO3 to 2000 mL of deionized water under vigorous stirring to obtain 5% HNO3.

CRITICAL: Nitric acid is a corrosive chemical; thus, it should be handled in the fume hood. Also, the nitric acid solution should be stored in a glass container, tightly closed and well-ventilated place at room temperature.

Weight 101 mg lead (II) nitrate, 163.1 mg iron (III) nitrate nonahydrate, 133.1 mg cadmium nitrate tetrahydrate, 174.5 mg magnesium nitrate hexahydrate, 126.5 mg copper (II) nitrate hemi(pentahydrate), and 160.2 mg zinc nitrate hexahydrate to 100 mL 5% HNO3 separately to obtain 1000 ppm Pb2+, Fe3+, Cd2+, Mg2+, Cu2+, Zn2+ stock solution.

Add 1 mL 1000 ppm Pb2+, Fe3+, Cd2+, Mg2+, Cu2+, Zn2+ As (V) (AsO43−), Cd2+, and Cr (VI) (Cr2O72−) stock solution to 99 mL 5% HNO3 separately to obtain 10 ppm Pb2+, Fe3+, Cd2+, Mg2+, Cu2+, Zn2+ As (V) (AsO43−), Cd2+, and Cr (VI) (Cr2O72−) stock solution. The stock solution should be stored in a glass container, tightly closed, and well ventilated at room temperature.

Key resources table

CRITICAL: 1,2-Ethanedithiol has a strong and unpleasant odor; handle it inside the fume hood.

We use the screw-capped glass jar for MOFs synthesis in this protocol. However, alternative tools such as autoclaves can also be used.

The resources listed above were only based on our experience. The chemicals and resources obtained from reliable commercial sources are feasible.

Step-by-step method details

MOFs synthesis

Timing: 48 h

The initial step for the synthesis of UiO-66-S is the UiO-66-Cl synthesis. We synthesize UiO-66-Cl via a solvothermal method; the details are listed below (Figure 1).

Figure 1

Synthetic procedure for the UiO-66-Cl

Oxygen is shown in red; zirconium in light blue; carbon in gray; and chlorine in green. Hydrogen is omitted.

Solvothermal conditions can yield highly crystalline MOFs in 48 h. Reproduced with permission (Yuan et al., 2022); Copyright, 2022 Elsevier Inc.

Weigh 13.2 mg (0.041 mmol) Zirconyl chloride octahydrate and 9.6 mg (0.041 mmol) 2,5-dichloroterephthalic acid to a screw-capped glass jar.

Add 4.1 mL DMF and 0.82 mL formic acid to the jar.

Sonicate the mixture for 30 min until the solid is fully dissolved.

Put the jar in the oven and heat at 120°C for 48 h.

After cooling down to room temperature, collect the white-colored octahedral crystal, UiO-66-Cl, by centrifugation (Yield: 84%, based on Zr).

Activation of MOFs

Timing: 6 days

After the MOFs synthesis, remove the unreacted starting materials. In addition, replace the high boiling point solvent, DMF, inside the pore of MOFs with a low boiling point solvent that can be easily removed.

Immerse the as-synthesized UiO-66-Cl in 10 mL DMF and stir for three days, during which replace the DMF three times per day by centrifugation.

Immerse the as-synthesized UiO-66-Cl in 10 mL of anhydrous acetone and stir for three days, during which replace the anhydrous acetone three times per day by centrifugation.

Then dry the sample under vacuum at 100°C overnight to obtain the activated UiO-66-Cl.

Post-functionalization of UiO-66-Cl to UiO-66-S

Timing: 48 h

Use deprotonation reaction to post-functionalize the UiO-66-Cl to UiO-66-S, in which nucleophilic 1,2-Ethanedithiol easily replaces the chlorides.

Add 1.175 g UiO-66-Cl, 1.68 mL 1,2-Ethanedithiol, 2.764 g K2CO3, and 58.1 mg potassium iodide (KI) to a 50 mL reaction vial.

Change to the atmosphere in the reaction vial to nitrogen by Schlenk line.

Add 20 mL DMF to the reaction vial under a nitrogen atmosphere.

Heat the mixture to 85°C and keep at different times (24 h, 48 h, 72 h, 96 h, and 120 h).

After cooling down to room temperature, collect the product by centrifugation.

Wash the product with 20 mL H2O six times.

Immerse the as-synthesized UiO-66-S in 10 mL DMF and stir for three days, during which replace the DMF three times per day by centrifugation.

Immerse the as-synthesized UiO-66-S in 10 mL of anhydrous acetone and stir for three days, during which replace the anhydrous acetone three times per day by centrifugation.

Then dry the sample under vacuum at 100°C overnight to obtain the activated UiO-66-S.

Characterizations of MOFs

Timing: 24 h

After the synthesis of UiO-66-Cl and UiO-66-S, use scanning electron microscopy (SEM), powder X-ray diffraction (PXRD), surface area, thermogravimetric analysis (TGA), and Fourier-transform infrared spectroscopy (FTIR) characterizations to characterize the synthesized MOFs (Figure 2). (Area et al., 1996; Mohammadinezhad and Akhlaghinia, 2019; Yuan et al., 2022).

Figure 2

Characterization of UiO-66 MOFs

SEM of UiO-66-Cl and UiO-66-S before and after adsorption.

(A and B) Pristine crystal of UiO-66-Cl and crystal of UiO-66-Cl after complete Fe3+ adsorption.

(C and D) Pristine crystal of UiO-66-S and crystal of UiO-66-S after complete Fe3+ adsorption.

(E) Powder X-ray diffraction patterns of UiO-66-Cl and UiO-66-S.

(F) Nitrogen adsorption and isothermal desorption curves of the MOFs.

(G) FTIR spectra of the MOFs.

(H) TGA of the MOFs. Reproduced with permission (Yuan et al., 2022); Copyright, 2022 Elsevier Inc.

Stability of MOFs

Timing: 35 days

Immerse the UiO-66-S and UiO-66-Cl in water (pH = 6.8), 0.1 mol L−1 HCl, and 0.01 mol L−1 NaOH aqueous solutions at room temperature for different times to investigate the stability of the MOFs (Figure 3), which demonstrate their high chemical stability (Yuan et al., 2022).

Figure 3

Stability of UiO-66 MOFs

(A and B) PXRD patterns of UiO-66-Cl and UiO-66-S after being immersed in water (A and B).

(C and D) Chemical stability of UiO-66-Cl and UiO-66-S (C and D), testing in 0.1 mol L–1 HCl and 0.01 mol L–1 NaOH. Reproduced with permission (Yuan et al., 2022); Copyright, 2022 Elsevier Inc.

Optimization of post-functionalization

Timing: 120 h

Optimize the sulfur content in the UiO-66-S by changing the reaction time of the post-functionalization. The crystallinity is maintained during the post-functionalization process (Figure 4) (Yuan et al., 2022).

Figure 4

Optimization of UiO-66-S MOFs

(A) The sulfur content of UiO-66-S under different post-functionalization times was measured by X-ray fluorescence (XRF).

(B) PXRD pattern of the UiO-66-S with different post-functionalization times. Reproduced with permission (Yuan et al., 2022); Copyright, 2022 Elsevier Inc.

Post-functionalization – digested NMR

Timing: 2 h

Analyze the sulfur content in the UiO-66-S by the Nuclear Magnetic Resonance (NMR) spectrum of digested UiO-66-S (Figure 5). Comparing with the 1H NMR spectrum of digested UiO-66-Cl, UiO-66-S shows three new peaks at δ 0.98–1.02 ppm, 3.4–3.44 ppm, and 7.09 ppm. After post-functionalization, dithiol replaces part of the Cl, resulting in the upshift of aromatic proton resonance, making a new resonance peak at 7.09 ppm. The peaks at 0.98–1.02 ppm correspond to the proton of methyl in CH3CH2SH, and the peaks at 3.40–3.44 ppm are the proton of -CH2CH2- and methylene of CH3CH2SH. The conversion efficiency of post-functionalization is about 43.67%. The gas chromatography-mass spectrometry results show that CH3CH2SH exists in the digested UiO-66-S, which results from the cleavaged C-S bond (Yuan et al., 2022).

Figure 5

Digested NMR

(A) 1H NMR spectra of the digested UiO-66-S.

(B) Stacked 1H NMR spectra of the digested UiO-66-S, digested UiO-66-Cl, and 1,2-Ethanedithiol.

(C) Gas chromatography-mass spectrometry (GC-MS) of digested UiO-66-S.

(D) Proposed UiO-66-S digested in 0.4 mL DMSO and 0.2 mL NH4F in H2O stock solution (4.14 M). Reproduced with permission (Yuan et al., 2022); Copyright, 2022 Elsevier Inc.

Add 30.64 mg NH4F to the mixture of 0.4 mL DMSO-d6 and 0.2 mL D2O to obtain the deuterated stock solution.

Add 10 mg UiO-66-S to the deuterated stock solution and sonicate until fully dissolved.

Measure the NMR spectrum by the Bruker 400 MHz NMR.

After the NMR measurement, remove the solvent of the mixture, add the chloroform to the residual, and then filtration. The gas chromatography-mass spectrometry was conducted for the filtrate.

Ion adsorption – Uptake tests

Timing: 24 h

Use the stock solution with different concentrations (100 ppm, 200 ppm, 400 ppm, 500 ppm, 600 ppm, 800 ppm, 1000 ppm) to determine the adsorption capacity of the synthesized MOFs; details are listed below.

Add 10 mL Fe3+ stock solution with different concentrations (100 ppm, 200 ppm, 400 ppm, 500 ppm, 600 ppm, 800 ppm, 1000 ppm) to 7 batches of 20 mL glass vial.

Add 10 mg of as-synthesized MOFs (UiO-66-Cl or UiO-66-S) to every stock solution vial.

Stir the mixture for 24 h.

Filter the solution through a syringe filter and use the inductively coupled plasma-optical emission spectroscopy (ICP-OES) to measure the Fe3+ concentration in the filtrate.

Before running the ICP-OES test, prepare three standard stock solutions, 5% HNO3, 500 ppm, and 1000 ppm Fe3+ stock solution, for calibration.

If the correlation coefficients (R2) values of the calibration curve are larger than 0.9999, measure the Fe3+ concentration of the filtrate by ICP-OES (Figure 6A).

Figure 6

Adsorption performance of UiO-66 MOFs

(A) Uptake.

(B) Kinetics.

(C) Selectivity.

(D) Recyclability. Reproduced with permission (Yuan et al., 2022); Copyright, 2022 Elsevier Inc.

Ion adsorption – Kinetic tests

Timing: 24 h

In this protocol, we show the synthesized MOFs for heavy metal ion removal and investigate their adsorption performance, including uptake, kinetics, selectivity, and recyclability. We use the same amount of MOFs to remove the metal ions at different times to investigate the adsorption kinetics. Details are listed below.

Add 10 ppm Fe3+ stock solution to 10 batches of 20 mL glass vial, 10 mL for each vial.

Add 10 mg of as-synthesized MOFs (UiO-66-Cl or UiO-66-S) to every stock solution vial.

Stir the mixture for 20 min, 40 min, 60 min, 90 min, 2 h, 2.5 h, 3 h, 4 h, 6 h, and 8 h, respectively.

Filter the solution through a syringe filter and use the ICP-MS to measure the Fe3+ concentration in the filtrate (see Figure 7 for Fe3+ adsorption kinetics measurements of UiO-66-S, Methods Video S1 for UiO-66-S removing 500 ppm Fe3+ stock solution from 0 to 240 min, related to step 30).

Figure 7

Procedure of Fe3+ adsorption kinetics measurement

500 ppm Fe3+ stock solution was used as such a high concentration of Fe3+ solution had a yellow color, and purification progress was visible to the eyes. Reproduced with permission (Yuan et al., 2022); Copyright, 2022 Elsevier Inc.

Before running the ICP-MS test, prepare the standard stock solution, 5% HNO3, 2.5 ppm, 5 ppm, 7.5 ppm, and 10 ppm Fe3+ stock solution, for calibration.

If the R2 values of the calibration curve are larger than 0.9999, measure the Fe3+ concentration of the filtrate by ICP-MS (Figure 6B).

Methods Video S1. UiO-66-S removes 500 ppm Fe3+ stock solution from 0 to 240 min

This movie is related to steps 30 and 31. Reproduced with permission (Yuan et al., 2022) Copyright, 2022 Elsevier Inc.

Ion adsorption – Selectivity tests

Timing: 24 h

We use the stock solution containing multiple metal ions to test the MOF’s selectivity toward the ions.

Add 10 mg of as-synthesized MOFs (UiO-66-Cl or UiO-66-S) to a 20 mL glass vial.

Add 10 mL aqueous mixed solution (1.25 mL each 1000 ppm Fe3+, As (V) (AsO43−), Cd2+, Cr (VI) (Cr2O72−), Pb2+, Mg2+, Cu2+, Zn2+ stock solution) to the vial.

Stir the mixture for 24 h.

Filter the solution through a syringe filter and use the ICP-OES to measure the ion concentration in the filtrate.

Before running the ICP-OES test, prepare three standard stock solutions, 5% HNO3, 60 ppm, 125 ppm mix ion stock solution (Fe3+, As (V) (AsO43−), Cd2+, Cr (VI) (Cr2O72−), Pb2+, Mg2+, Cu2+, Zn2+) for calibration.

If the R2 values of all the calibration curves are larger than 0.9999, measure the Fe3+ concentration of the filtrate by ICP-OES (Figure 6C).

Ion adsorption – Recyclability tests

Timing: 198 h

After ion adsorption, we collect the MOFs powder to desorb the ion, which is used for another round of ion adsorption—repeating the adsorption/desorption cycles to analyze the recyclability of the MOFs.

Add 10 mg as-synthesized UiO-66-S to a 20 mL glass vial.

Mix 10 mL Fe3+ stock solution (10 ppm) with the MOFs.

Stir the mixture for 24 h.

Collect the filtrate by centrifuge, and measure the Fe3+ concentration by the ICP-MS.

Immerse the residual UiO-66-S in 10 mL citric acid (10 mM) for 9 h to desorption.

Wash the residual UiO-66-S with distilled water and anhydrous acetone and dry it in a vacuum oven overnight.

Use the regenerated UiO-66-S to measure the recyclability by repeating the adsorption/desorption process.

Before running the ICP-MS test, prepare the standard stock solution, 5% HNO3, 2.5 ppm, 5 ppm, 7.5 ppm, and 10 ppm Fe3+ stock solution for calibration.

If the R2 value of the calibration curve is larger than 0.9999, measure the Fe3+ concentration of the filtrate by ICP-MS (Figure 6D).

Adsorption process analysis

where qe (in mg g−1) is the amount of Fe3+ adsorption at equilibrium, C0 (in mg L−1) is the initial concentration of the used stock solution, and Ce (in mg L−1) is the measured concentration of the filtrate by the ICP-OES or ICP-MS. V (in L) is the volume of the solution used for measurement, and m (in mg) is the corresponding weight of the MOFs.where qmax is the maximum adsorption capacity. b and Kf are Langmuir constant and Freundlich constant, respectively. n is dimensionless exponent of Freundlich equation. Setting Ce as x-axis, Ce/qe as y-axis, using excel to linear fit the data collected from the uptake tests. According to Equation 2, the slope and intercept of the fitted curve correspond to the 1/qmax and 1/(qmaxb), respectively. Then the maximum adsorption capacity was obtained.where qt and qe (in mg g−1) are the amounts of Fe3+ adsorbed at time t and at equilibrium, respectively. k1 (in min−1) and k2 (in g mg−1 h−1) are the rate constant of pseudo-first-order and pseudo-second-order adsorption, respectively. Setting t as x-axis, log(qe-qt) as y-axis, using excel to linear fit the data collected from the kinetics tests. According to Equation 4, the slope and intercept of the fitted curve correspond to the −k1/2.303 and log(qe), respectively. Then the rate constant k1 was obtained.

Timing: 24 h

Calculate the adsorption amount of Fe3+ using Equation 1,

Use the Langmuir isotherm model and Freundlich isotherm model to analyze the adsorption process according to Equations 2 and 3. Treating the data collected from the uptake tests by the below two equations (Yao et al., 2022).

Set ln(Ce) as an x-axis, ln(qe) as a y-axis, using excel to linear fit the data collected from the uptake tests. According to Equation 3, the slope and intercept of the fitted curve correspond to the 1/n and ln(Kf), respectively.

Compare the R2 values obtained from the fitted results of Equations 2 and 3. The Langmuir model describes the adsorption process of Fe3+ better (Figures 8A and 8B), indicating that the Fe3+ is adsorbed on the monolayer surface of MOFs and the adsorption process is uniform chemical adsorption.

Figure 8

Adsorption performance and reaction model simulation of UiO-66 MOFs

(A) Langmuir model fitting according to Figure 6A.

(B) Freundlich model fitting according to Figure 6A.

(C) Pseudo-second-order model fitting according to Figure 6B.

(D) Pseudo-second-order model fitting according to Figure 6B. Reproduced with permission (Yuan et al., 2022); Copyright, 2022 Elsevier Inc.

Use the pseudo-first-order kinetic model and pseudo-second-order kinetic model to analyze the adsorption kinetics according to Equations 4 and 5, respectively.

Set t as an x-axis, t/qt as a y-axis, using excel to linear fit the data collected from the kinetics tests. According to the Equation 5, the slope and intercept of the fitted curve correspond to the 1/qe and 1/(k2 ), respectively. Then the rate constant k2 was obtained.

Compare the R2 values obtained from the fitted results of Equations 4 and 5. The Fe3+ adsorption matches well with the pseudo-second-order reaction model, which indicates the rate-limiting factor in the Fe3+ adsorption process depends on chemisorption. The rate constant k for UiO-66-S and UiO-66-Cl was calculated to be 1.07 and 0.124 g mg−1 h−1, respectively (Figures 8C and 8D).

Expected outcomes

This protocol provides the procedure for MOFs synthesis and post-functionalization. The synthesized MOFs show high crystallinity and stability, which are stable in water (pH = 6.8) at room temperature for 35 days, demonstrating their feasibility for water purification application. The synthesized MOFs show Brunauer–Emmett–Teller (BET) surface area of 446 and 275 m2 g−1 for UiO-66-Cl and UiO-66-S, respectively. Under practical operating temperatures, i.e., < 200°C, UiO-66-S shows improved thermal stability. In addition, the synthesized MOFs have been stable for more than a week under acidic conditions (0.1 mol L−1 HCl). Optimizing the reaction condition of post-functionalization, the maximum 52.13% of Cl was replaced by 1,2-Ethanedithiol after 48 h of reaction time, which shows the highest adsorption capacity.

UiO-66-S can remove Fe3+ with the highest adsorption capacity (481 mg g−1) and fastest kinetics (1.07 g mg−1 h−1) for water purification application. In addition, the UiO-66-S shows high selectivity toward Fe3+. The removal efficiency of UiO-66-S remained at over 89% after six cycles demonstrating its high recyclability.

Limitations

Various conditions, including acidic, basic, and neutral, are common for the practical application of water purification. The synthesized MOFs in this protocol are stable in acidic and neutral conditions. However, it can only be stable in the basic condition (0.01 mol L−1 NaOH) for two days. Therefore, the synthesized MOFs are limited to extremely basic conditions. The reason is the coordination of the hydroxides with Zr4+. This limitation can be improved by hydrophobic surface treatment, insertion of stabilizing pillars, and composite construction. See the reference for more detailed information (Ding et al., 2019).

Troubleshooting

Problem 1

The synthesized MOFs show low crystallinity (steps 1–4 in “MOFs synthesis”).

Potential solution

Add modulators to the reaction solution, such as formic acid, acetic acid, and trifluoroacetic acid. The modulator could coordinate with the metal cluster and slow down the reaction leading to improved crystallinity.

Problem 2

The stock solution is used in the adsorption process (step 18 in “ion adsorption – uptake tests”).

Iron can be easily precipitated by a hydrolytic mechanism. Therefore, the matrix used to prepare the stock solution must be acidic to ensure the existing iron is in an ion state. The potential solution for the stock solution preparation should be 5% HNO3, which is clarified in the “before you begin” section.

Problem 3

The results obtained from ICP-MS were not good enough (step 31 in “ion adsorption – kinetic tests”).

Prepare the stock solution more accurately. The 10 ppm stock solution should be diluted from the 1000 ppm stock solution. In addition, the R2 values of the calibration curve should be larger than 0.9999, and then conduct the ICP-MS measurement.

Problem 4

Incomplete removal of unreacted starting material and DMF molecules (steps 6–8 in “activation of MOFs”).

Replace the DMF and acetone in steps 6–8 more frequently. Alternatively, wash the MOFs by Soxhlet extraction with methanol and acetone as a solution, each solution for three days.

Problem 5

Low conversion efficiency of the post-functionalization (steps 9–17 in “post-functionalization of UiO-66-Cl to UiO-66-S”).

Use anhydrous DMF as a solvent and set up the reaction under the inert gas atmosphere. The 1,2-Ethanedithiol (EDT) is air-sensitive and should be stored under inert gas.

Resource availability

Lead contact

Further information and requests for resources and reagents should be directed to and will be fulfilled by the lead contact, Yoonseob Kim (yoonseobkim@ust.hk).

Materials availability

This study did not generate any new unique reagents.

REAGENT or RESOURCE	SOURCE	IDENTIFIER
Chemicals, peptides, and recombinant proteins
Zirconyl chloride octahydrate (98%)	Sigma-Aldrich	CAS: 13520-92-8
2,5-dichloroterephthalic acid (H2DCTP, 98%)	Energy Chemical	CAS: 13799-90-1
N,N-dimethylformamide (DMF, 99.8%, min)	RCI Labscan	CAS: 68-12-2
Formic acid (98%–100%)	Scharlau	CAS: 64-18-6
1,2-Ethanedithiol (EDT, ≥98%)	Sigma-Aldrich	CAS: 540-63-6
Potassium carbonate (≥99.0%)	Sigma-Aldrich	CAS: 584-08-7
Lead (II) nitrate (≥99.0%)	Sigma-Aldrich	CAS: 10099-74-8
Iron (III) nitrate nonahydrate (≥98%)	Sigma-Aldrich	CAS: 7782-61-8
Cadmium nitrate tetrahydrate (98%)	Sigma-Aldrich	CAS: 10022-68-1
Magnesium nitrate hexahydrate (99%)	Sigma-Aldrich	CAS: 13446-18-9
Copper (II) nitrate hemi(pentahydrate) (98%)	Sigma-Aldrich	CAS: 19004-19-4
Zinc nitrate hexahydrate (98%)	Sigma-Aldrich	CAS: 10196-18-6
Potassium iodide (≥99.0%)	Sigma-Aldrich	CAS: 7681-11-0
1000 ppm chromium stock solution	High-Purity Standards Inc.	SKU: 100012-1
1000 ppm arsenic stock solution	High-Purity Standards Inc.	SKU: 10003-1
Acetone (min. 99.5%)	RCI Labscan	CAS: 67-64-1
Nitric Acid (≥65%)	Honeywell/Fluka	CAS: 7697-37-2
Other
Screw-capped glass jar	Synthware Co., Ltd	N/A
Sonicator	Branson Co., Ltd	N/A
Oven	Shanghai Yiheng Co., Ltd (Shanghai, China)	N/A
Stir plate	Heidolph Co., Ltd	P/N: 505-30000-00
Centrifuge	Centurion Scientific Co., Ltd	K3
Hot plate	Heidolph Co., Ltd	P/N: 505-30000-00
Schlenk line	Synthware Co., Ltd	N/A
Inductively coupled plasma mass spectrometer (ICP-MS)	Agilent Co., Ltd	7800
Thermo gravimetric analyzer	PerkinElmer Co., Ltd	UNIX/TGA7
Inductively coupled plasma–optical emission spectrometer (ICP-OES)	PerkinElmer Co., Ltd	7300DV
Fourier-transform infrared spectrometer (FTIR)	Thermo Fisher Scientific Co., Ltd	Nicolet IS50
JSM-7100F field emission scanning electron microscope	JEOL (Europe) BV Co., Ltd	JSM-7100F
Analytical x-ray diffractometer	Malvern Panalytical Co., Ltd	X'pert Pro
JSX-3201Z (JEOL) X-ray fluorescence spectrometer	JEOL Co., Ltd	JSX-3201Z
400 MHz Bruker NMR spectrometer	Bruker	N/A
Gas chromatography-mass spectrometer	Thermo Fisher Scientific Co., Ltd	N/A
Surface area and pore size distribution analyzer	Microtrac Co., Ltd	BELSORP MINI X