Preliminary Analyses of Controlled Release of Potassium Permanganate Encapsulated in Polycaprolactone ()
1. Introduction
Remediation of water and soil systems has long utilized various oxidation methods to treat contaminants in the environment. The aggressiveness of most oxidants to oxidize all organic materials has led to encapsulation of these chemicals in various polymers to cause their slow controlled release in these aqueous and soil systems [1] [2] [3]. In Situ Chemical Oxidation (ISCO) is the process that introduces the selected oxidant into the contaminated environment to reduce the harmful effects on the media [4] [5] [6] [7]. To increase efficiency of targeted oxidant contact in ISCO processes, Controlled Release Material (CRM) delivery systems have been developed for environmental purposes such as environmental protection, decontamination, and remediation, especially during treatment of groundwater, wastewater, and drinking water [8] [9]. Throughout the evolution of CRM, synthetics have been utilized to deliver the chemical oxidants including polyester mesh bags, Polymethyl Methacrylate (PMMA), Piccolyte resin S115, Epolene C-16, and stearic acid [10] [11] [12] [13]. However, often these synthetic polymers did not have a high rate of biodegradability and demonstrated harm to the environment and its organisms [14]. In response to these deficiencies, several studies have reported the safe and successful controlled slow-release of chemical oxidants using clay, waxes, and gels for remediation in various water systems [1] [2] [15]. However, the work to find the optimum oxidant for these controlled slow-release systems continues, as system and contaminant variability is challenging.
Various oxidants have been used for subsurface remediation such as hydrogen peroxide, persulfate, Fenton’s reagent, and zero-valent iron [16] [17] [18] [19]. One highly effective oxidizing chemical agent for the removal of contaminants and the inactivation of microorganisms has been potassium permanganate [10] [20] [21]. In numerous instances, potassium permanganate (KMnO4) has been found to produce environmentally harmless by-products in the degradation of organic and inorganic compounds within soil and groundwater [13] [22] [23] [24]. Other overall advantages of utilizing KMnO4 in remediation include its chemical stability, cost effectiveness, and the less hazardous biodegradable composition of the oxidant [25] [26]. Yet, the level of effectiveness of the KMnO4 for remediation is in direct correlation to the method its delivery. Methods for KMnO4 delivery have included pressurized direct-push, soil mixing, well flushing, treatment walls and oxidant, however these did not offer sufficient delivery control of the chemical oxidant [13]. This uncontrolled flow of the oxidant through these non-restrictive processes resulted in reactions with natural organic matter and inorganic soil constituents, such as metals. This reduced the amount of oxidant attacking the target contaminant and would often require additional measures to further remediate the media [1] [2]. Controlled-release delivery methods have been applied to KMnO4 with some success [7] [27] [28] [29]. Despite these applications, there is still much more information to be gathered surrounding polymers and their potential to increase delivery efficiency while reducing the negative impacts and by-products.
One polymer that has shown great promise regarding oxidant encapsulation for remediation of water sources is Polycaprolactone (PCL). PCL has a relatively
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Figure 1. Structure of Polycaprolactone (PCL).
long half-life with little or no residual side effects or change in the molecular weight of the internal bulk of the polymer [14]. Bulk degradation of the PCL transpires when hydrolysis occurs after the entire polymer has been penetrated with water. In addition, PCL has a low melting point, exceptional blending capabilities, malleability into large ranges of shapes and sizes, and has been FDA approved [14] [30]. The long chain structure of PCL, which includes five non-polar methylene groups and a polar ester group (Figure 1), taken into account with its physiochemical characteristics make PCL an excellent candidate for CRM in water treatment systems [31].
This attractiveness of PCL as a CRM has led to it being central to a patented process (US Patent 8,519,061 B2) for controlled release of KMnO4 [32]. The patent entitled Controlled Released Biodegradable Polymer (CRBP) utilizes PCL for controlled delivery of the chemical oxidant to treat contaminated water systems over a particular span of time which may reduce the number of times the treatment needs to be administered [6] [21] [24]. Though these studies show promise, more information on the specifics of the oxidant release with respect to aqueous systems I s necessary. Once the release of the oxidant from PCL is better understood, the CRBP mechanism would be of more robust use in the oxidation of organic contaminants in soil and water. The main objective of the present study is to evaluate the release characteristics of KMnO4 encapsulated in PCL in an aqueous system.
2. Materials and Methods
2.1. Materials
Potassium permanganate, KMnO4, (ACS reagent grade, 99%) was purchased as dark purple odorless crystals from Sigma Aldrich. The hydrophobic biodegradable polymer, Polycaprolactone, was also purchased from Sigma Aldrich. All solutions used in this study were prepared using ultrapure water from a bench ultrapure water system. As a biocompatible and biodegradable polymer, PCL was used to encapsulate KMnO4 to formulate a CRBP system to observe controlled-release kinetics and characteristics of encapsulated KMnO4 when introduced to aqueous media.
2.2. Encapsulation of KMnO4 CRBP
In this study, the first phase consisted of formulating CRBP pellets using the baking method for encapsulation described by the literature [32]. To produce the CRBP delivery system, KMnO4, as the chemical oxidant, and PCL, as the hydrophobic polymer, were utilized. The encapsulation of this KMnO4 oxidant allowed for stabilizing control of chemical release. The study modeled basic reproducible components of Teasley et al. studies [6] [21] [24] for establishment of standard measurements including a KMnO4 to PCL at a ratio of 1:5 or 20 wt%. The same standard measurements were used for all CRBP pellet formulations. Initially, using a laboratory muffle furnace (Thermo Scientific, US), PCL was heated at 90˚C. Once PCL was molten, KMnO4 particles were added to the soften polymer and stirred thoroughly to achieve a homogeneous mixture using a stainless-steel spatula. After blending for uniform particle dispersion, and while malleable, the molten mixture was then placed in a Parr stainless steel pellet press and pelletized. Solid circular pellet structures were produced with 20 wt% KMnO4-PCL and size of 1.3 cm in length capable of the controlled-release (Figure 2(a)). Pellets were then preserved and stored in a dry cool place until use for batch release experiments. Pellets were designed based on a combined mechanism [32] consisting of controlled dissolution and diffusion of KMnO4 for an extended period of time.
2.3. Release Experiments
The study was conducted in the form of a series of batch release experiments, similar to what is seen in Figure 2(b). Experiments were conducted with 1:5 ratio of CRPB KMnO4/PCL (also referred to as 20 wt%), approximately 0.60 g in mass, pellets submerged in 1 L of ultra pure water and synchronously released over the 96 hour period. Each of 12 Pyrex glass beakers were used as batch reactors for individual pellets. These reactors were tightly capped at room temperature under constant mixing with a mechanical stirrer to ensure a homogenous mixed solution. Aluminum foil was wrapped around the glass reactors for protection from ambient light. Preliminary experiments confirmed the findings of prior studies noted that the water volume did not affect the amount of relative mass lost over a 24 h period [7]. The release of KMnO4 from the PCL was observed periodically by measuring the oxidant concentration in the aqueous solution. Samples of aqueous solution were withdrawn using a sterile serological pipette at various times within the 1-week period. At each time interval after
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(b)
Figure 2. (a) Pellets with KMnO4 encapsulated in PCL; (b) Pellets release KMnO4 in water.
release, pellets were collected from reactors, desiccated for 24 hours and weighed for calculating changes and averages in mass.
2.4. Analysis
Water samples collected from the batch experiments were analyzed for KMnO4 concentrations using a UV-Vis spectrophotometric by measuring the absorption at λ = 525 nm wavelength at each sampling time interval, which is within the maximum absorbance range for KMnO4. UV-Vis spectrophotometric analysis indicated the amount of additive KMnO4 integrated in the aqueous solution. UV-Vis spectrophotometric used the absorbance measurements to determine the levels of KMnO4 concentration in the aqueous solution. The analyzed absorbance and concentration for the samples were taken for each interval of time. The surface morphology of the encapsulated KMnO4 CRBP before and after release experiments were characterized using FEI Helious G4 UC field emission scanning electron microscope (FESEM).
3. Results and Discussion
The KMnO4 oxidant encapsulated in PCL polymer was released in reagent grade water to measure release characteristics of the system. Throughout the experimentation, the fraction of the oxidant released was seen to continually increase over the 96 hour sampling period for the present study (Figure 3). As with similar experiments, the greatest percentage of oxidant release was within the first 24 hours, reaching the maximum rate of release between the 12 hour and 24 hour time interval. This correlates to the prior findings related to this rapid release biphasic phase in the CR process [2] [33].
3.1. Surface Morphology of the CRBP
The release of the KMnO4 particles from the PCL matrix can readily be seen in the SEM images (Figure 3). Increasing oxidant removal from the PCL surface was evidenced by the increase in size of cavities in the waxy surface of the polymer as time from release increased. These crevices during oxidant release also mirror what was seen from other CR systems utilizing KMnO4 release [1] [27]. KMnO4 progresses initially from the preliminary contact between KMnO4 particles on the surface of the matrix and water (Figure 3(a)) to 24 hours (Figure 3(b)). The dissolution of the polymer bearing KMnO4 particles occurred as the surfaces were freely accessible to water in the batch reactors. The PCL polymer degraded slowly in the presence of aqueous solution, similar to what has been stated in the literature in that the embedded KMnO4 was released by dissolution-diffusion which resulted in the formation of KMnO4 solution that would be available to oxidize contaminants in the aqueous phase [1] [27] [29].
At the 24 hour mark, the SEM image indicates a maximum imbibition of the shell matrix (Figure 3(b)). It is at this point that the amount of hydrolysis in the PCL shell peaks, KMnO4 release decreased as the system transitioned from the dissolution phase to the diffusion phase followed by a controlled and slower release of KMnO4. Though beyond the sampling intervals for this study, a 1-week SEM picture was also taken of the system to show long-term effects of release on the polymer surface (Figure 3(c)). After a week of uninterrupted contact with water, KMnO4 dissolution left empty pores and larger crevices allowing the embedded KMnO4 to diffuse in aqueous solution, clearly seen in the SEM images and in agreement to what has been noted in the literature [29]. These results support the conjecture that the combined control CRBP delivery mechanism for PCL is more effective for long term release of the oxidant.
3.2. Release Data
The experimental release data for the KMnO4 from the PCL matrix (Figure 4) was found to best fit the following equation (R2 = 0.93):
(1)
whereQ represents the mass fraction of KMnO4 released over time t, k is a constant
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Figure 3. SEM images of the surface of encapsulated 20 wt% KMnO4 CRBP in (a) before release experiment, (b) after 1 day of release, (c) after 1 week of release. Magnification = 50 X, kV = 5.00 Scale bar = 500 λm.
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Figure 4. Experimental and calculated values (Equation (1)) for KMnO4 released over time from PCL in reagent grade water.
incorporating polymer and oxidant characteristics, and n is the diffusional exponent. The fit of the data has been used to describe the release process of other oxidants from wax matrices as non-Fickian diffusion with constant pseudo-convection due to stress within the wax or polymer matrix itself [1] [27] [34]. These values of n and k can be gathered once Equation (1) has been linearized in logarithmic form (Equation (2)) and by plotting the experimental data to determine the slope and the intercept on the log Qt axis, respectively.
(2)
Even with the linearized model, it has been shown that the estimated values might not necessarily simulate the nonlinear release kinetics [1] [34]. Therefore, estimation of parameters can be done with an optimization method that minimizes the sum of the square of errors (SSE) between the experimental and calculated data (Equation (3)).
(3)
3.3. Comparison Release Studies
The present study measured release of KMnO4 from PCL at a ratio of 1:5, otherwise known as 20 wt%. Results from this present study were compared to parallel release studies with the same oxidant, KMnO4, encapsulated in different wax matrices at similar ratios. Kang et al. [1] examined the use of paraffin wax (Figure 5) to encapsulate the KMnO4 at a ratio of 1:5, similar to that of the present work.
In another study, Ighere and Chawla [27] used of Poly-methyl-methacrylate (PMMA) as polymer at a ratio of 1:8 KMnO4 to PMMA. PMMA is a hydrophobic synthetic resin and an ester of methacrylic acid generally produced from propylene (Figure 5). These comparable polymers had different chemical structures (Figure 5, Figure 6) than PCL which makes the information from their release compared to its release of KMnO4 valuable.
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Figure 5. General structure of paraffin waxes.
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Figure 6. Structure of poly methyl methacrylate.
Figure 7 shows the data in this present study with 1:5 ratio of KMnO4 to PCL agrees favorably with data from the compared studies which included Kang et al. [1] with 1:5 ratio KMnO4 to paraffin wax and that of Ighere and Chawla [27] with 1:8 ratio of KMnO4 to PMMA. The data from our present study was normalized by a factor of 20, since its initial mass of oxidant in the PCL was 1/20th of that utilized by the other comparable studies.
Furthermore, the PCL release data modeled also compare favorably with the model parameters from the comparative studies (Table 1). The values of n and k for all the studies were calculated from Equation 2 based on established research on this model by Sinclair and Peppas [34]. Evaluating these model parameters further indicated that the PCL release of KMnO4 of the present study aligned with this oxidant’s release from the comparable studied polymers. The slight differences can be attributed to both the different chemical structures and physical characteristics of the different polymers used, as well as the higher mass amounts utilized in the data from the previous literature.
Though the present study paralleled the release from the compared studies, it most agreed with the Kang et al. [1] study. This was expected since that study and the present research utilized the same ratio of 1:5 or 20wt%. Further agreement
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Figure 7. Theoretical data showing the rate of release of KMnO4 for this study alongside two other works utilizing KMnO4 as a controlled release oxidant in batch release experiments.
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Table 1. Comparison of model parameters from experimental batch release.
between the present study and Kang et al. [1] study. This was noted in the calculation of SSE from Equation 3. The data of the present study was calculated to have a SSE value of 1.9 × 10−5, while Kang et al. was 1.7 × 10−4. This indicated that not only was the data relationships between these studies agreed, but also, that the data from the present data better fit the model as it was lower than that of the comparable study. Overall, the similarity of release data between the diversity of polymers shows that the CRBP with PCL provides effective release of the KMnO4 with the added benefit biodegradable nature of PCL, as mentioned previously [6] [21] [32].
4. Conclusion
Successful implementation of controlled release materials relies upon increased understanding of release patterns for CRBP systems. One such system includes Polycaprolactone polymer to control the slow release of KMnO4 oxidant as part of a patented process to remove contaminants from aqueous systems. Experiments were conducted to analyze the release data from this CRBP system into water at a ratio of 1:5 KMnO4 to PCL. Based on the experimental data, the fraction of the KMnO4 released from PCL into water was seen to continually increase over the 96 hour sampling period and resulted in the increase in size of cavities in the waxy surface of the polymer as seen in SEM images. Data from release of KMnO4 from PCL was found to fit non-Fickian diffusion after dissolution (R2 = 0.93) similar to other systems that describe the dispersal of other oxidants from wax matrices. Release data and model parameters for data of this present study were also found to be comparable to previous release studies with the same oxidant, KMnO4, encapsulated in different wax matrices at similar ratios. Overall, the similarity of release data between the diversity of polymers shows that the CRBP with PCL provides effective release of the KMnO4 with the added benefit biodegradable nature of PCL.
Acknowledgements
The authors of this paper wish to thank the following for funding support: National Science Foundation (Award #: 1740463); U.S. Department of Education GAANN Program (Award #: P200A180074); U.S. Department of Education MSEIP Program: Program of Excellence in STEM (Award #: P120A160115); Florida Agricultural & Mechanical University, including the Title III Graduate Engineering Program for funding support; and North Carolina Agricultural & Technical State University. Thanks are also given to the Civil and Environmental Engineering Department of the FAMU-FSU College of Engineering. The authors would like to thank, above all, their Lord and Savior Jesus the Christ for guidance during this endeavor.