Preparation of Triiodide Resin Using Potassium Iodide and Peracetic Acid: Application to Wastewater Treatment

Abstract

Triiodide resin has an antimicrobial effect on bacteria in water. In the traditional TR manufacturing method, potassium iodide (KI) and crystalline I₂ are reacted to form triiodide ion (I₃⁻). However, I₂ is difficult to use and store because it is vaporizable and poorly soluble in water. This study was conducted to develop a method of producing triiodide resin (TR) without using crystalline I₂. A chemical radical reaction between a commercially available peracetic acid (PAA) solution and a potassium iodide (KI) solution was used to produce I₂ and I₃⁻ ions, which combined with a strong basic anion exchange resin to produce TR. The disinfection of pathogenic microorganisms (e.g., Escherichia coli, Salmonella spp.) present in anaerobically digested livestock wastewater is essential prior to its discharge into public water systems or marine environments, in order to safeguard environmental integrity and public health. Anaerobically treated contaminated livestock wastewater was sterilized through three rounds of treatment with a TR column and prepared by the oxidation of a 100 mM KI solution.

1. Introduction

As the population increases rapidly, water pollution is also increasing. Relatively few methods are available to chemically treat water to destroy microorganisms without leaving behind residual disinfectants [1,2,3,4]. Halogen compounds (strong oxidizing agents) such as bromine, fluorine, chlorine, and iodine are used to chemically purify water [5,6]. Among these, iodine is known to exhibit lower reactivity toward organic nitrogen pollutants than other halogen compounds and to have a long residence time in aqueous solution [7,8,9]. In aqueous solution, iodine exists in the forms of hypoiodous acid (HIO), iodide ions (I⁻), hypoiodite ions (IO⁻), triiodide ions (I₃⁻), iodic acid (HIO₃), and iodate ions (IO₃⁻) [5,10]. Among these, the I₃⁻ ion is known to exhibit the strongest sterilizing power [6,11,12]. It is used in the form of triiodide resin (TR), which is prepared by combining an I₃⁻ ion with a strong anion-exchange resin [13,14,15,16,17,18]. The mechanism of sterilization by TR involves the inactivation of the cell membrane via electrostatic interaction, and the killed bacteria do not remain attached to the resin but will readily pass on through a bed of the resin [19,20]. Therefore, TR can be used to continuously sterilize large amounts of contaminated wastewater without releasing an unpleasant amount of iodine into the water [21].

A common method of producing TR is to dissolve crystalline solid I₂ in an aqueous solution of sodium iodide (NaI) or KI and then combine it with a resin in a glass column [22,23]. In this case, because crystalline I₂ exhibits low solubility in water, this process requires dissolving solid I₂ in a high-temperature (>80 °C) KI aqueous solution, and repeated washing with a large amount of distilled water is required to obtain uniformly loaded TR [24]. In addition, because the high volatility of crystalline I₂ makes its storage and use difficult, the TR manufacturing method using crystalline solid I₂ has limitations in economically mass-producing TR [25,26].

In the present study, an I₃⁻ ion was prepared using only a room-temperature chemical radical reaction between a commercially available peracetic acid (PAA) solution and a KI solution, without using crystalline I₂, and the generated I₃⁻ ion was subsequently combined with a strong basic anion-exchange resin to produce TR.

CH₃COOOH + KI → K⁺CH₃COOO⁻ + H⁺ + I⁻

[I] total = [I₂] + [I⁻] + [I₃⁻]

PAA and KI react to form a complex of K⁺CH₃COOO⁻, and the iodide ion dissociates in the form of I₃⁻, I₂, and I⁻. TR was prepared by adding the I₃⁻ to a strong basic anion-exchange resin. In the present study, we verified the sterilizing power of the TR manufactured by the proposed method by passing livestock wastewater containing various microorganisms through the resin column.

2. Materials and Methods

2.1. Chemicals and Apparatus

KI (DAEJUNG Chemical, Siheung, Republic of Korea), PAA (Dong Myung ONC, Republic of Korea), standard hydrogen peroxide (H₂O₂) solution (Sigma-Aldrich, St. Louis, MO, USA, contained inhibitor, 30 wt% in H₂O, ACS reagent), and acetic acid (AA) (DAEJUNG Chemical, Republic of Korea, 99.7 wt%) were used as chemical reagents. All solutions were prepared using distilled water.

Absorbance values at wavelengths of 226 nm (I⁻), 460 nm (I₂), 290, and 350 nm (I₃⁻), which reflect the volumetric iodine spectrum [27,28], were measured using a UV–Vis spectrophotometer (JASCO V-730, Tokyo, Japan) with a 10 mm cell. The concentration of each iodine compound was calculated using molar absorptivity values (12,600 L·mol⁻¹·cm⁻¹ at 226 nm for I⁻, 728 L·mol⁻¹·cm⁻¹ at 460 nm for I₂; 25,000 L·mol⁻¹·cm⁻¹ at 350 nm for I₃⁻ and 26,000 L·mol⁻¹·cm⁻¹ at 290 nm for I₃⁻) [27,29,30].

2.2. Formation of I₂ and I₃⁻ Ion in KI Solution by Reaction with PAA

To induce the formation of the I₂ and I₃⁻ ion in KI solution, a reaction was carried out by adding 1.9 mL/L of 20% peracetic acid (PAA) to a 10 mM KI solution. The changes in concentration of iodine compounds were observed at determined time intervals during the experiments (5, 10, 15, 30, 60, 120, and 1440 min).

2.3. Reaction of AA and H₂O₂ with KI

The PAA solution consisted of peracetic acid, acetate acid, and H₂O₂. To determine which components of the peracetic solution are involved in the formation of I₂ and I₃⁻ ions from KI solution, AA and H₂O₂ solutions with concentrations equivalent to that of the PAA solution were reacted with 10 mM KI solution. The changes in concentration of iodine compounds were measured at fixed time intervals during the experiments (5 min intervals for either 30 or 60 min).

2.4. Manufacture of TR

To observe the change in concentration of iodine ions that occurred after a strongly basic anion-exchange resin (TRILITE^® MA-12, Samyanglite, Cheongju, Republic of Korea) was added to a solution in which I₂ and I₃⁻ ions were formed by the reaction between 0.5 L of 10 mM KI solution and 0.95 mL of 20% PAA;10 min later, 20 g of beads was added, and the solution was stirred for 30 min. Afterwards, the changes in concentration of iodine compounds were measured at fixed time intervals in the experiments (5 min intervals for 35 min). TR samples loaded with different concentrations of I₃⁻ ions were prepared using 50 mM and 100 mM KI solutions.

2.5. Treatment of Anaerobically Treated Livestock Wastewater by TR

Anaerobically treated livestock wastewater was obtained from Anaerobic Digestion Facility (Yangsan, Republic of Korea). The sample was filtered using fabric, and the upper layer of the sample was used to verify the disinfection effect by TR prepared by the chemical reaction of a KI solution with PAA. The formation of colony-forming units (CFUs) was used to quantify and identify viable bacteria. The bacteria-contaminated wastewater sample (50 mL) was then passed through the TR column (23 mm diameter, 80 mm height) by gravity flow. After the contaminated water was treated with TR once, twice, and three times, each sample for the CFU test was plated onto potato dextrose agar and incubated at 25 °C for 48 h. After incubation, the number of bacterial colonies per sample was counted. Contaminated wastewater not treated using the TR column was used as a control.

2.6. Statistical Analysis

Data were analyzed using Statistical Analysis System software SAS 9.4 (version 9.4 for Windows, SAS Institute, Inc., Cary, NC, USA). Statistical analyses were performed by one-way ANOVA Duncan’s test (p < 0.05).

3. Results and Discussion

3.1. Formation of I₂ and I₃⁻ Ion by Chemical Reaction of KI and PAA

In the present study, PAA was reacted with a KI solution to produce I₂ and I₃⁻ ions in an aqueous solution through the dissociation of the iodide ion bound to the potassium iodine (Figure 1).

Iodine takes three forms in aqueous solution: the I⁻ ion, I₂, and I₃⁻ ion. The concentrations of these three iodine species were quantified on the basis of the intensity of distinct absorbance peaks at 226 nm (I⁻), 350 nm (I₃⁻), 290 nm (I₃⁻), and 460 nm (I₂) [27,31]. Changes in the concentrations of I₂ and I₃⁻ ions produced by the chemical reaction between KI and PAA were observed through changes in the absorbance value at each wavelength (Figure 2). After a 10 mM KI solution was reacted with a PAA solution, the absorbance at each wavelength was measured. As a result, the I⁻ concentration, which had sharply decreased from 10.71 to 3.13 mM after 5 min of reaction, increased slightly after 30 min, but decreased to zero after 24 h. The I₂ concentration increased 16-fold (from 0.20 to 3.23 mM), and the I₃⁻ concentration increased 44-fold (from 0.01 to 0.46 mM) after 5 min of reaction (Figure 2).

However, the concentrations of generated I₂ and I₃⁻ ions gradually decreased over time and became zero after 24 h. This behavior is attributed to a decrease in the total iodine concentration as a result of the vaporization of I₂ molecules [5].

The PAA solution contains peracetic acid, acetic acid (AA), and H₂O₂ [32]. To determine which of the substances constituting the PAA solution affects the formation of I₂ and I₃⁻ ions, we reacted AA and H₂O₂ with the KI solution individually. First, we confirmed the possibility of producing I₂ and I₃⁻ ions by a chemical reaction between AA and KI. An amount of AA equivalent to the concentration of the PAA solution was reacted with 10 mM KI solution, and the absorbance at the corresponding wavelength was measured for 30 min at 5 min intervals. The I⁻ concentration showed little change: from 10.87 mM before the reaction to 10.98 after 30 min. The I₂ and I₃⁻ concentration showed no change (Figure 3). We therefore concluded that AA alone does not affect the production of I₂ or I₃⁻ ions.

We subsequently evaluated whether the chemical reaction between H₂O₂ and KI affected the production of I₂ and I₃⁻ ions. An amount of H₂O₂ equivalent to the concentration of the PAA solution was reacted with 10 mM KI solution, and the absorbance at each wavelength was measured at 5 min intervals for 30 min. No change was observed in the concentration of I⁻, I₃⁻, and I₂, respectively (Figure 4).

In addition, after KI was reacted with H₂O₂, AA was also reacted (Figure 4). As a result, surprisingly, the I⁻ concentration decreased from 10.97 to 8.40 mM and the I₂ concentration increased from 0.01 to 1.40 mM. The I₃⁻ concentration increased from 0.02 to 0.63 mM. Peracetic acid is generated through the reaction between AA and H₂O₂ [33]. In conclusion, the peracetate ion (CH₃COOO⁻) in the PAA solution dissociates KI to form a complex of K⁺ and PAA and the dissociated iodine finally forms polyvalent iodine (I₂ and I₃⁻).

3.2. Triiodide Resin (TR) Manufacture

The TR was produced by combining polyvalent iodine (i.e., I₂ and I₃⁻) compounds produced by the czzhemical reaction of KI solution and PAA solution with a strong basic anion-exchange resin. To confirm that I⁻ ion and I₂ generated by the reaction with 10 mM KI and 5 mM PAA were removed from the solution by binding with the anion-exchange resin, 20 g of anion-exchange resin was added to the reacted solution at 20 min (Figure 5). The change in absorbance at 226 nm, 290 nm, and 460 nm during the entire reaction process was observed at 5 min intervals for 35 min. The I⁻ concentration decreased from 10.95 (5 min) to 3.08 (20 min) to 0.07 (35 min), the I₂ concentration increased from 0.01 (5 min) to 2.26 (20 min) and then decreased to 0.01 (35 min), and the I₃⁻ concentration increased from 0.01 (5 min) to 0.37 (20 min) and then decreased to 0.01 (35 min) (Figure 5). The results confirmed that the concentrations of the generated I₂ and I₃⁻ ions (including the I⁻ ion) in the solution rapidly decreased as soon as the anion-exchange resin was added at 20 min.

To confirm the sterilizing power of the TR produced using this approach, 50 mM KI and 100 mM KI solutions were reacted with appropriate amounts of PAA and then combined with 20 g of strongly basic anion-exchange resin. The TR loaded at different concentrations was filled into a plastic tube (23 mm diameter, 80 mm height) to be used in a contact sterilization experiment.

3.3. Antibacterial Effect of TR

Sterilization using TR is achieved through the process where I₃⁻ present in the TR kills bacteria by inactivating cell wall proteins and the dead bacteria pass through the column without attaching to the resin [34,35]. In this study, 50 mL of bacteria-contaminated wastewater sample was passed three times through a TR column prepared using 50 or 100 mM KI solution and its sterilizing power was subsequently verified through a CFU test (Figure 6).

The contaminated wastewater sample was repeatedly treated as many as three times; however, it could not be completely sterilized at 4.1 × 10³ cells/mL when the TR column prepared with 50 mM KI was used. In contrast, when the TR column prepared with 100 mM KI was used, the contaminated water was completely sterilized after three treatments. For reference, 6.7 × 10⁴ cells/mL were found in the contaminated water that had not been treated with the TR column. The results indicate that the effectiveness of the TR increased in proportion to the amount of I₃⁻ ion loaded, with a comparable sterilization efficacy to that of the I₂-based A605 Iodinated Resin (Purolite Co., King of Prussia, PA, USA) [23].

In the study, the theoretical maximum loading capacity of I₃⁻ ions is calculated to be 39 mM. When using a 100 mM KI solution, the resin was able to accommodate up to 84.6% of its theoretical maximum exchange capacity. To evaluate the potential leaching of iodine species during water treatment, no significant leaching of iodine species was observed during the treatment of up to 3 L of water. Therefore, disregarding the potential loss of I₃⁻ ions upon contact between bacteria and TR, and the possible physicochemical reactions during water treatment, TR is expected to retain its sterilizing efficacy over a prolonged period.

TR exhibits strong potential for field applications as an effective and continuous sterilization agent in various wastewater treatment settings. Given its ability to inactivate a wide range of microorganisms without releasing harmful levels of residual disinfectants, TR is especially suitable for use in livestock wastewater treatment, decentralized rural sanitation systems, and emergency water purification units. The simplified synthesis method proposed in this study using only PAA and KI under ambient conditions enhances the feasibility of on-site TR production without the need for high-temperature processing or volatile crystalline iodine, which is often a limitation in conventional methods. Its ease of integration into column-type systems makes it adaptable for modular treatment plants or mobile disinfection setups in resource-limited environments. Future research should focus on optimizing the loading efficiency and stability of I₃⁻ within the resin under varying environmental and operational conditions. Long term performance assessments, including reusability, breakthrough capacity, and potential for regeneration, are essential for scaling up.

4. Conclusions

We developed a simple method to produce TR by chemically reacting a commercially available PAA solution with a KI solution without using crystalline I₂. In the study, we revealed that only the peracetate ion in the PAA solution reacted with the KI solution, with the effect of forming I₂ and I₃⁻ ions. When 100 mM KI was used for making TR (based on the use of 20 g of resin), all bacteria (6.7 × 10⁴ cells/mL) in the anaerobically digested livestock wastewater were killed when the sample was passed through the prepared TR column three times. This TR synthesis method eliminates the need for high-temperature processing, improves safety by avoiding volatile I₂, and simplifies the overall procedure by removing the need for repeated washing steps. Additionally, the method is cost-effective, scalable, and well suited for large-scale TR production and application, particularly in microbial control in a wide range of wastewater contexts.