Removal of Arsenic Using a Cationic Polymer Gel Impregnated with Iron Hydroxide

Summary

In this work, we prepared an adsorbent composed of the cationic N,N-dimethylamino propylacrylamide methyl chloride quaternary (DMAPAAQ) polymer gel and iron hydroxide for adsorbing arsenic from groundwater. The gel was prepared via a novel method designed to ensure the maximum content of iron particles in its structure.

Abstract

In this work, we prepared an adsorbent composed of a cationic polymer gel containing iron hydroxide in its structure designed to adsorb arsenic from groundwater. The gel we selected was the N,N-dimethylamino propylacrylamide methyl chloride quaternary (DMAPAAQ) gel. The objective of our preparation method was to ensure the maximum content of iron hydroxide in the structure of the gel. This design approach enabled simultaneous adsorption by both the polymer structure of the gel and the iron hydroxide component, thus, enhancing the adsorption capacity of the material. To examine the performance of the gel, we measured reaction kinetics, carried out pH sensitivity and selectivity analyses, monitored arsenic adsorption performance, and conducted regeneration experiments. We determined that the gel undergoes a chemisorption process and reaches equilibrium at 10 h. Moreover, the gel adsorbed arsenic effectively at neutral pH levels and selectively in complex ion environments, achieving a maximum adsorption volume of 1.63 mM/g. The gel could be regenerated with 87.6% efficiency and NaCl could be used for desorption instead of harmful NaOH. Taken together, the presented gel-based design method is an effective approach for constructing high-performance arsenic adsorbents.

Introduction

Water pollution is a great environmental concern, motivating researchers to develop methods for removing contaminants such as arsenic from wastewaster¹. Among all the reported methods, adsorption processes are a relatively low cost approach for heavy metal removal^2,3,4,5,6,7. Iron oxyhydroxide powders are considered to be one of the most efficient adsorbents for extracting arsenic from aqueous solutions^8,9. Still, these materials suffer from a number of drawbacks, including early saturation times and toxic synthetic precursors. Additionally, there is a severe adverse effect in the water quality when these adsorbents are used for a long period of time¹⁰. An additional separation process, such as sedimentation or filtration, is then needed to purify the contaminated water, which increases the cost of the production further^8,11.

Recently, researchers have developed polymer gels such as cationic hydrogels, microgels, and cryogels that have demonstrated efficient adsorption properties. For example, an arsenic removal rate of 96% was achieved by the cationic cryogel, poly(3-acrylamidopropyl) trimethyl ammonium chloride [p(APTMACl)]¹². Additionally, at pH 9, approximately 99.7% removal efficiency was achieved by this cationic hydrogel¹³. At pH 4, 98.72 mg/g of maximum arsenic adsorption capacity was achieved by the microgel, based on tris(2-aminoethyl) amine (TAEA) and glyceroldiglycidyl ether (GDE), p(TAEA-co-GDE)¹⁴. Although these gels demonstrated good adsorption performances, they failed to effectively remove arsenic from water at neutral pH levels, and their selectivities in all studied environments were not reported¹⁵. A maximum adsorption capacity of 227 μg/g of was measured when Fe(III)-Sn(IV) mixed binary oxide-coated sand was used at a temperature of 313 K and a pH of 7¹⁶. Alternatively, Fe-Zr binary oxide-coated sand (IZBOCS) has also been used to remove arsenic and achieved a maximum adsorption capacity of 84.75 mg/g at 318 K and a pH of 7¹⁷. Other reported adsorbents suffer from low adsorption performances, lack of recyclability, low stability, high operational and maintenance costs, and the use of hazardous chemicals in the synthesis process⁴.

We sought to address the above limitations by developing a material with improved arsenic adsorption performance, high selectivity in complex environments, recycling capability, and efficient activity at neutral pH levels. Therefore, we developed a cationic gel composite of N,N-dimethylamino propylacrylamide methyl chloride quaternary (DMAPAAQ) gel and iron(III) hydroxide (FeOOH) particles as an adsorbent for arsenic removal. We chose to combine FeOOH with our gel because FeOOH increases the adsorption of both forms of arsenic¹⁸. In this study, our gel composite was designed to be non-porous and was impregnated with FeOOH during preparation. In the next section, the details of the gel preparation method, including our strategy for maximizing the content of FeOOH is discussed further.

Protocol

CAUTION: Arsenic is extremely toxic. Please use gloves, long sleeve clothing, and experimental goggles at all times during the experiment to prevent any contact of arsenic solution with the skin and eyes. If arsenic comes into contact with any part of your body, wash it immediately with soap. Additionally, please clean up the experimental surroundings regularly so that you and others do not come into contact with arsenic, even when the experiment is not being performed. The symptoms of arsenic exposure may appear after a long period of time. Prior to cleaning the equipment, first rinse it with clean water and dispose the water separately into an experimental waste container designated for arsenic. Then, clean the equipment well with detergent. To prevent arsenic contamination of the environment, take precautions while disposing of arsenic samples. Dispose of them separately into experimental waste containers designated for arsenic. After the adsorption or desorption experiment is performed, the gels contain a high amount of arsenic. Therefore, dispose of the gels separately to a designated experimental waste bin for only arsenic-containing gels.

1. Synthesis of the DMAPAAQ+FeOOH gel composite

1. Dry two 20 mL measuring flasks and two 20 mL beakers equipped with magnetic stir bars.
2. Transfer 2.07 g of DMAPAAQ (75%), 0.15 g of N,N’-methylene bisacrylamide (MBAA), 0.25 g of sodium sulfite and 1.68 g of NaOH to one 20 mL beaker.
3. Dissolve the solution wholly in distilled water as ‘solvent’ and stirring it for 30 min with a magnetic stir bar.
4. Transfer the mixture from the beaker to one 20 mL measuring flask and add distilled water to generate a 20 mL solution. Label the solution as the “monomer solution”.
5. Similarly, take 0.27 g of ammonium peroxodisulfate (APS) and 3.78 g of FeCl₃ in another 20 mL beaker.
6. Dissolve the solution completely in distilled water and stirring it for 30 min with a magnetic stir bar.
7. Transfer the mixture from the beaker to another 20 mL measuring flask and add distilled water to compose a 20 mL solution. Label the solution as the “initiator solution”.
8. Prepare the experimental setup as shown in Figure 1.
9. Transfer the solutions into the respective 20 mL separating funnels.
10. Purge the solutions with N₂ gas for 10 min.
11. Mix the solutions together, stir them in a 50 mL test tube with an electric stirrer, and then place the mixture into a chiller maintained at 10 °C for 40 min.
12. Take out the gel block from the test tube and place it on a flat cutting board.
13. Cut the gel block into a cubic shape, 5 mm in length.
14. Soak the gel slices with de-ionized water for 24 h to remove the impurities.
15. After 12 h, replace the water and soak the gel slices again.
16. Spread the gel slices onto a Petri dish and dry them at room temperature for 24 h.
17. Place the Petri dish with the gel slices in the oven at 50 °C for 24 h.

2. pH sensitivity analyses

1. Dry nine 40 mL plastic containers.
2. Measure nine 20 mg dried gel pieces and put each of them in a separate 40 mL plastic container.
3. Add 20 mL of a 4 mM disodium hydrogenarsenate heptahydrate (Na₂HAsO₄·7H₂O) solution to each container.
4. To control the pH levels, add 20 mL of NaOH solution or HCL solution with different concentrations (0.1, 0.01, 0.001, 0.0001 M) in the respective containers to maintain pH levels of 2, 6, 8, 10, 12, 13 and label them.
5. Keep the containers in the stirrer at 20 °C and 120 rpm for 24 h.
6. Collect a 5 mL sample from each container and place each sample in a plastic tube using a micropipette.
7. Measure the equilibrium pH for all the samples.
8. Measure the remaining concentration of arsenic in the solution using a high performance liquid chromatography (HPLC). Use an analytical column (4 x 200 mm), a guard column (4 x 50 mm) and a 4 mm suppressor with the following conditions:
   Flowrate: 1.5 mL/min;
   Amount of injected sample: 10 μL;
   Column temperature: 30 °C;
   Eluent solution: 2.7 mM Na₂CO₃ and 0.3 mM NaHCO₃;
   Pump pressure: 2000 psi;
   Electric conductivity detection: Suppressor method.
   NOTE: We procured 1 mL of the sample into a 1 mL single-use syringe. The syringe was coupled with a syringe membrane filter (pore size: 0.22 μm, diameter: 13 mm) to discrete the microscopic fragments of the gel from the sample. About 0.7 mL of sample was instilled into the column. Distilled water was infused before the inception of injecting the samples as blank sample. Peaks denoting the existence of arsenic in the sample was detected at 13 min.
   CAUTION: After injecting the sample, please leave the syringe into the suction head of HPLC for nearly 2 min with roughly 0.2-0.3 mL of sample remaining in it. Because the dust and air could penetrate the column and alter its adeptness, which possibly will result in erroneous outcome.

3. Arsenic adsorption experiment

1. Dry five 40 mL plastic containers.
2. Measure and place 20 mg of dried gel in each 40 mL plastic container.
3. Add 40 mL of disodium hydrogenarsenate heptahydrate (Na₂HAsO₄·7H₂O) solution to each container at the following concentrations: 0.1, 0.2, 0.5, 1, 2 mM.
4. Keep the containers in the stirrer at 20 °C and 120 rpm for 24 h.
5. Collect a 5 mL sample from each container and place in a plastic tube using a micropipette.
6. Follow step 2.8 to assess the equilibrium arsenic levels in the solutions using HPLC.

4. Selectivity analyses of the DMAPAAQ+FeOOH gel

1. Dry five 40 mL plastic containers.
2. Place 20 mg of dried gel in each of the five 40 mL plastic containers.
3. Add 20 mL of a 0.4 mM disodium hydrogenarsenate heptahydrate (Na₂HAsO₄·7H₂O) solution to each container.
4. Add 20 mL at concentrations of 0.5, 1, 2, 5, 10 mM Na₂SO₄ to the five containers.
5. Keep the containers in the stirrer at 20 °C and 120 rpm for 24 h.
6. Collect a 5 mL sample from each container and place into separate plastic tubes using micropipettes.
7. Follow step 2.8 to quantify the remaining concentration of arsenic in the solution using HPLC.

5. Equilibrium rate analyses

1. Dry seven 40 mL plastic containers.
2. Place 20 mg of dried gel in each of the 40 mL plastic containers.
3. Add 40 mL of a 0.2 mM disodium hydrogenarsenate heptahydrate (Na₂HAsO₄·7H₂O) solution to each of the containers.
4. Keep the containers in the stirrer at 20 °C at 120 rpm for the duration of their designated times.
5. Collect 5 mL samples in plastic tubes using micropipettes after 0.5, 1, 3, 7, 11, 24, and 48 h.
6. Follow step 2.8 to determine the equilibrium arsenic level in each solution using HPLC.

6. Regeneration analysis

1. Adsorption analysis
   1. Dry a 40 mL plastic container.
   2. Take 20 mg of dried gel and place it in the 40 mL plastic container.
   3. Add 40 mL of a 0.2 mM disodium hydrogenarsenate heptahydrate (Na₂HAsO₄·7H₂O) solution to the container.
   4. Keep the container in the stirrer at 20 °C and 120 rpm for 24 h.
   5. Collect a 5 mL sample in a plastic tube using a micropipette.
   6. Refer to step 2.8 to evaluate the equilibrium arsenic level in the solution using HPLC.
2. Cleaning the gel
   1. Obtain a mesh sieve.
   2. Carefully collect the gel pieces one at a time so that they do not break and place them in the mesh sieve.
   3. Wash the gel several times (minimum five times) using de-ionized water so that any remaining arsenic on the surface of the gel is washed away.
      CAUTION: The gel pieces are fragile. Handle them with care while washing and transferring them from the arsenic solution to the NaCl solution.
3. Desorption analyses
   1. Dry a 40 mL plastic container.
   2. Put the gel pieces from step 6.2 into a 40 mL plastic container.
   3. Add 40 mL of a 0.5 M NaCl solution to the container.
   4. Keep the container in the stirrer at 20 °C and 120 rpm for 24 h.
   5. Collect a 5 mL sample in a plastic tube using a micropipette.
   6. Follow the step 2.8 to evaluate the equilibrium arsenic level in the solution using HPLC.
4. Repetition of the process
   1. After collecting the gel from step 6.3, repeat the process in the following sequence for eight complete cycles: 6.2 > 6.1 > 6.2 > 6.3 > 6.2 > 6.1 > 6.2 > 6.3.

Results

Figure 1 describes the experimental setup for the preparation of the DMAPAAQ+FeOOH gel. Table 1 illustrates the compositions of the materials involved in the preparation of the gel.

Figure 2 shows the relation of contact time with the adsorption of arsenic by the DMAPAAQ+FeOOH gel. In the figure, the amount of adsorption of arsenic was examined at 0.5, ...

Discussion

The main advancement of our developed method is the unique design strategy of the gel composite. The purpose of our gel preparation method was to maximize the amount of iron content in the gel. During the preparation, we added FeCl₃ and NaOH to the “initiator solution” and the “monomer solution,” respectively. Once the monomer solution was mixed with the initiator solution, there was a reaction between FeCl₃ and NaOH, producing FeOOH inside the gel. This phenomenon ensured ma...

Materials

Name	Company	Catalog Number	Comments
N,N’-dimethylamino propylacrylamide, methyl chloride quaternary (DMAPAAQ) (75% in H2O)	KJ Chemicals Corporation, Japan	150707
N,N’-Methylene bisacrylamide (MBAA)	Sigma-Aldrich, USA	1002040622
Sodium sulfite (Na2SO3)	Nacalai Tesque, Inc., Japan	31922-25
Sodium sulfate (Na2SO4)	Nacalai Tesque, Inc., Japan	31916-15
Di-sodium hydrogenarsenate heptahydrate(Na2HAsO4.7H20)	Nacalai Tesque, Inc., Japan	10048-95-0
Ferric chloride(FeCl3)	Nacalai Tesque, Inc., Japan	19432-25
Sodium hydroxide(NaOH)	Kishida Chemicals Corporation, Japan	000-75165
Ammonium peroxodisulfate (APS)	Kanto Chemical Co. Inc., Japan	907W2052
Hydrochloric acid (HCl)	Kanto Chemical Co. Inc., Japan	18078-01
Sodium Chloride (NaCl)	Nacalai Tesque, Inc., Japan	31320-05