Characterization of a Sodium Persulfate Sustained Release Rod for in

Apr 19, 2017 - In situ chemical oxidation (ISCO) using sodium persulfate (SPS) is an increasingly implemented process for the .... 2016 55 (8), pp 230...
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Characterization of a Sodium Persulfate Sustained Release Rod for in Situ Chemical Oxidation Groundwater Remediation Chenju Liang* and Cheng-Yu Chen Department of Environmental Engineering, National Chung Hsing University, 250 Kuo-kuang Road, Taichung 402, Taiwan S Supporting Information *

ABSTRACT: In situ chemical oxidation (ISCO) using sodium persulfate (SPS) is an increasingly implemented process for the remediation of subsurface contamination. For this study, a SPS sustained release rod (SRrod) was prepared by mixing SPS particles and melted paraffin wax with a SPS/wax mass ratio of 6/1. Observation of the rod in potassium iodide solution revealed that, as soon as the rod was placed in the solution, the color turned yellow due to the oxidation of I− to form I2. The solution became darker at the bottom and lighter toward the top because the density of dissolved SPS causes it to sink along the surface of the rod. On the basis of the results of the column experiment which creates a steady SPS release from the rod, a matrix boundary diffusion controlled two-film theory model was derived to explain the SPS release behavior. On the basis of the model developed and the effective SPS diffusion coefficient within the rod matrix, the minimum release time for the rod can be determined. The designed sustained release system would be expected to have a SPS duration of at least longer than the determined minimum release time regardless of site conditions. The SPS SR-rod minimum release time as a function of the radius of the rod was established. With a better understanding of the characteristics of the sustained release system, the SPS SR-rod could potentially be developed as a practical approach for in situ remediation of contaminated aquifers. blended in a matrix.4 The preparation of sustained release ISCO oxidant in a mixture is a matrix-type formulation. Instead of a conventional well-based injection, a replaceable SPS SRrod in a groundwater well may be considered as a way to create chemically active ISCO zones and extend the oxidant reactivity lifespan at certain locations in the subsurface. Numerous materials such as wax or polymers have been developed with varying degrees of success for the controlledrelease of potassium permanganate5−7 and SPS.8,9 Ranjha et al.10 indicated wax as a common carrier in the design of a sustained drug delivery technique with the advantages of good stability at varying pH, nonswellable, water insoluble nature, protection of a drug against chemical degradation, and biocompatible safe application in humans. Therefore, wax would be compatible with environmental applications because residual wax in the subsurface would pose no risk to human health or the environment. The oxidant blended into the wax matrix will diffuse from the matrix through interconnected pores in the wax and will continuously dissolve into the aqueous phase once water molecules penetrate into the wax matrix. Kambhu et al.8 prepared a prototype of slow-release persulfate (PS) candles (0.71 and 1.27 in diameter; 2.38 cm in

1. INTRODUCTION In situ chemical oxidation (ISCO) using sodium persulfate (SPS) (Na2S2O8) is an increasingly implemented process for the remediation of groundwater contaminated with a variety of organic compounds.1,2 One of the important factors for a successful ISCO field application is the effective delivery of the oxidant (e.g., SPS) into the contaminated zone. Conventional liquid oxidant delivery approaches may include gravity- or pump-feed vertical/horizontal well injection, pressurized directpush technology, or recirculation systems.1 Persulfate is typically injected as an aqueous solution, and its delivery within the soil medium to the target area is usually challenging. The rate of success can be unpredictable due to the nonuniform distribution of oxidant caused by preferential pathways in the subsurface. One delivery approach for creating an oxidantcontaminant zone can be to employ a sustained oxidant delivery system, which enables oxidant migration by molecular diffusion or advection into the soil matrix (e.g., low permeability media) without disturbing site conditions (e.g., contaminant migration can be caused by conventional liquid injection). A well-based persulfate sustained releasing rod (SRrod) reactive barrier such as a curtain-like setup can be designed as a long-term passive treatment option for contaminant plumes in aquifers.3 The most common sustained releasing mechanism is diffusion, and there are generally two types of diffusion-controlled systems, one where a chemical is surrounded by a film, and a second where the chemical is © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

January 7, 2017 April 2, 2017 April 19, 2017 April 19, 2017 DOI: 10.1021/acs.iecr.7b00082 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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distribution within the SR-rod and the presence of continuous connected pores (high porosity) allow for water penetration, SPS dissolution, and steady persulfate release. Several preliminary tests were conducted to evaluate the optimum mixtures of wax and SPS. The interaction between SPS (1 and 100 g L−1) and wax (14 g) in a 200 L beaker was evaluated by comparing SPS concentration variations in aqueous solution. 2.3. Batch Release Experiment. The SPS SR-rod was placed in a 1 L flask with KI solution (10 g L−1), and the color change from transparent to dark yellow due to the SPS oxidation of iodine (I−) to iodine (I2) was recorded (at 20 °C). This color change is a visual indication of the release of SPS from the SR-rod. 2.4. Column Release Experiment. A SPS SR-rod (length: 13 cm; diameter: 2.25 cm) was placed in a one-dimensional vertical glass column (15 cm in length and I.D. of 2.5 cm, ACE Glass, Inc.), which was equipped with a threaded PTFE top cover and a PTFE stopcock plug (both ends), and glass beads (2 mm) were placed on the bottom of the column to support the rod. A peristaltic pump (Cole Parmer Master Flex L/S) was used to deliver RO water from the bottom to the top, and effluent was collected in a flask. The released SPS was continuously flushed by the water flow to maintain the lowest SPS concentration outside the rod and create a maximum diffusive concentration gradient within the rod. The schematic diagram showing the column SPS SR-rod release experiment is presented in Figure S2. Different flow rates (0.2, 1, and 5 mL min−1) were utilized, and the matrix boundary diffusion model was developed to estimate the matrix effective diffusion coefficient and the longevity (minimum release time) of the SPS SR-rod. 2.5. Analysis. A desktop meter was used to monitor pH, temperature, and conductivity (CyberScan PC5000). The persulfate anion concentration was determined by a spectrophotometer method (Hach DR/2400) at 400 nm.14

length) as a way to treat groundwater contaminated with organic compounds such as benzene, toluene, ethyl-benzene, and xylenes (BTEX). As reported by several studies demonstrating the successful remediation of BTEX contamination with PS,11−13 a PS candle with a persulfate release rate of 117−188 mg d−1 was able to degrade BTEX8 in aqueous solution. For field application of the sustained release oxidant matrix, in which oxidant particles are uniformly distributed and the sustained oxidant release mechanism is diffusion through the oxidant-wax system, the oxidant release rate is dependent on the diffusion rate within the specific type of sustained release matrix. This rate is governed by the concentration gradient between the boundary layer of the matrix and the adjacent aqueous phase. The radius of influence of the oxidant, from the release point in the subsurface, would be dependent on the concentration used to treat a specific contaminant and the rate of reaction with nontarget contaminant reactants in the subsurface. For any sustained release oxidant matrix used in an aqueous system, the maximum release (diffusion) rate will occur when the sustained release system is working at its highest concentration gradient. At this diffusion rate, the longevity of the system can be calculated and is regarded as a “minimum release time”. Therefore, a designed sustained release system would be expected to have a duration at least as long as the estimated minimum release time, regardless of site conditions. When selecting wax for large scales uses in environmental remediation, one should consider the cost for wax and the amount of energy used. Table S1 classifies paraffin wax based on their melting points and associated prices. It can be seen that the price is lowest for wax with melting points between 136 and 142 °F, and therefore, this wax was selected for mixing melted paraffin wax and SPS particles. Characterization of the SPS SR-rod was conducted by (1) visual observation of SPS release, examining the interaction between wax and SPS, and (2) developing a matrix boundary diffusion model to predict the effective diffusion coefficient within the release matrix and thereby determining the minimum release time for the rod.

3. RESULTS AND DISCUSSION 3.1. Characterization of the SPS SR-Rod. By well shaking melted wax and SPS particles, it was observed that a SPS/wax mass ratio of 6/1 resulted in SPS completely mixed within the wax. This ratio allows melted wax to fill within the SPS particle stacking pores and support the formation of the SR-rod. At lower wax/SPS ratios, excessive wax would block water penetration and reduce SPS dissolution; at higher ratios, some SPS particles would not be surrounded by wax, and the rod becomes easily breakable (see Figure S1 for a schematic illustration of variations in the uniformity of particle spatial distribution within the mixture of SPS and wax). A picture of the prepared SPS SR-rod and a tabulation of its associated properties are provided in Table 1. The average density of the SPS SR-rod is 1.94 g cm−3, which is heavier than water (note: SPS and wax specific gravity are 2.59 and 0.774 g cm−3, respectively), and the porosity (without SPS particles and assuming SPS is completely released) is 64%. Therefore, when the SPS in the SR-rod is exhausted (i.e., SG = 0.28 g cm−3), the rod density becomes lighter than water. In order to further explore the interaction between wax and SPS, paraffin wax was submerged in the SPS solution for 30 d and SPS concentration was monitored with time (data presented in Figure S3). When comparing the SPS selfdecomposition rate in the absence of wax to those obtained in the presence of wax, it was seen that differences are minor and overall SPS decompositions are within 5% variations. Also, the

2. MATERIALS AND METHODS 2.1. Materials. The water used was purified by a reverse osmosis (RO) purification system (Sky Water XL-300A). Sodium persulfate (Na2S2O8, >99.0%) was purchased from Merck. The paraffin wax used as the blending material was purchased from Taiwan Wax Company Ltd., Taiwan, with the following characteristics: melting point of 140 °F (60 °C), specific gravity (SG) of 0.774, and molecular mass of 740 g mol−1. Potassium iodide (KI, >99.5%, Union Chemical Works Ltd., Taiwan), sodium bicarbonate (NaHCO3, >99.7%, SigmaAldrich), acetic acid (CH3COOH, ≥80%, Taiwan Maxwave Company Ltd.), and sodium thiosulfate (Na2S2O3·5H2O, ≥99.5%, Sigma-Aldrich) were used for persulfate analysis by the iodometric titration method. 2.2. SPS SR-Rod Preparation. The SPS SR-rod was prepared by the hot melt encapsulation method. The solid paraffin wax was melted in an oven at 70 °C, adding SPS particles to the melted wax and stirring well using an overhead stirrer. The wax-SPS mixture was then poured into a preheated cylindrical stainless steel mold (2.25 cm in diameter and 13 cm in length) with a stainless steel cap. The mold was gently tamped to eliminate any trapped air bubbles and set aside to cool down at room temperature. The SPS SR-rod was pushed from the mold after cooling. Uniformity of SPS particle B

DOI: 10.1021/acs.iecr.7b00082 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. Properties of the SPS SR-Roda

that of water in accordance with eq 1.15 Therefore, the higher density of dissolved SPS would sink along the surface of the rod to the bottom of the flask under static experimental conditions (see Figure 1b for a schematic illustration of SPS release). SPS density(g mL−1) = density H2O + 6.709 × 10−4X − 1.4934 × 10−6X1.5

(1)

where X = solution concentration (g L−1) and density H2O = 0.99707 (g mL−1) at 25 °C. 3.2. SPS SR-Rod Release in Column Experiments. A one-dimensional column SPS SR-rod release experiment was conducted to simulate a maximum concentration gradient within the rod matrix, where fresh water flushed out the released SPS and a zero SPS concentration can be maintained at a certain distance away from the rod. Therefore, a steady SPS release from the rod matrix can be maintained. The results of the temporal SPS release under different rates of water flushing are presented in Figure 2. SPS concentrations were initially high at approximately 50 000−250 000 mg L−1 at 1 h and then rapidly decreased over 1 d of testing and reached complete release SPS on day 8 based on mass balance calculations (see Figure 2, inset). The faster water flushing rate resulted in lower aqueous SPS concentrations than those observed with lower flushing rates, due to dilution of the released SPS. Although the surrounding dissolved aqueous SPS concentration varied with different water flushing rates, the total release times were similar and were not affected by the water flushing rate along the outside of the rod. Therefore, the column data may suggest that the primary SR-rod SPS release mechanism is dissolution− diffusion within the rod matrix. This matrix-type formulation

Note: SPS specific gravity (SG) = 2.59 g cm−3; wax SG = 0.774 g cm−3; calculation: SPS/wax weight ratio = Total SPS/Total wax; SPSrod SG = (Total SPS + Total wax)/Total volume; SPS-rod porosity = (Total SPS/SPS SG)/Total volume; Pore volume = SPS SR-rod porosity × Total volume. a

pH, conductivity, and oxidation−reduction potentials in solutions showed nearly no differences. Figure 1 displays the picture of observations of the rod in KI solution. It can be seen that, as soon as the SPS SR-rod was placed in solution, the color turned yellow (i.e., I2 color, due to oxidation of I− to I2 by persulfate) (at 10 s for the reaction time in Figure 1) and became darker from the bottom moving up over time (Figure 1a). Also, it was observed that many tiny dark brown spots appeared on the outside of the rod (Figure 1b), which may indicate the pores where the interpenetration of SPS and water occurred through the rod. The density of SPS solution increases when the concentration increases; e.g., when the concentration is at the SPS solubility limit (550 g L−1 at 20 °C), the density could be approximately 1.5 times higher than

Figure 1. (a) Photographs of observed color variations due to SPS release from the SPS SR-rod in KI solution and (b) postulated SPS releasing behavior. C

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Higuchi16 to describe the mechanism of drug release from a silicone rubber matrix. The rate of diffusion under steady-state condition across a plane of unit area is given by Fick’s First Law,

Fx = −D

dC dx

(2)

where Fx is the flux of diffusion across the plane (M L−2T−1); D is the diffusion coefficient (L2 T−1); dC/dx is the concentration gradient (M L−3 L−1). Assuming a steady state release of SPS occurred within the rod, eq 2 is modified to represent SPS release across a cylinder surface area: dM dC = 2πrhDe dt dr Figure 2. SPS release from the SPS SR-rod in the column with different water flushing rates. Inset shows cumulative mass of SPS released as a function of time.

(3)

where De is the effective SPS diffusion coefficient within the rod matrix (cm2 s−1); M is the mass released in the aqueous phase (g); C is the aqueous SPS concentration (g L−1); h is the height of the rod (cm); t is the release time (d); r is the radius of the SPS saturated zone (cm). The effective SPS diffusion coefficient within the rod is given by

would deliver SPS into flowing water by the diffusion of SPS inside the rod matrix. Figure 3 shows a hypothetical diagram of the matrix boundary diffusion controlled two-film theory model for the

De =

nDPS τ

(4)

where DPS is the S2O82− aqueous phase diffusion coefficient (cm2 s−1); n is the porosity within the rod; τ is the tortuosity of the matrix. DPS is calculated in accordance with the WilkeChang equation17,18 as follows: DPS =

T × 7.4 × 10−8 × (φ × MW)0.5 = 8.2 × 10−6cm 2s−1 μ W × VPS0.6 (5)

where T is the absolute temperature (K); μw is the water viscosity (1.002 cP at 20 °C);19 VPS is the S2O82− molecular volume (110.4 cm3 g-mol−1);18 φ is the water association parameter (2.26);18 MW is the water molecular weight (18 g mol−1). Integration of eq 3 utilizing boundary conditions, C = Cs (SPS maximum aqueous solubility 550 g L−1 at 20 °C) at r = r (between r = 0 and r = r0) and C = C0 (SPS concentration g L−1) at r = r0, gives the following equation describing SPS mass released within the rod matrix as a function of time: Figure 3. Schematic diagram of the diffusion controlled two-film theory for the release of SPS from the SPS SR-rod.

2πhDe(C0 − Cs) dM = r dt ln r0

(6)

Additionally, on the basis of eq 3, as soon as SPS is released from the rod matrix-boundary into the aqueous phase, SPS transport from r = r0 to r = ra according to Figure 3 is given by

SPS SR-rod, where the shaded zone represents the SPS saturated zone and the empty zone represents the SPS unsaturated zone inside the rod matrix. Along with SPS release with time, the SPS saturated zone (r0, the radius of SPS SRrod) shrinks toward the center of the rod, and when r = 0, SPS is completely released. As soon as SPS is released into the aqueous phase, flushing water removes dissolved SPS from the surface of the SR-rod. This maintains a maximum SPS concentration gradient between r = r0 (at the boundary of the rod) and r = ra (the radius of column apparatus). In considering diffusive SPS transport within the rod matrix and mass balance, a modified Fick’s First Law was derived in accordance with a model system developed by Roseman and

2πhDPSr0(Ca − C b) dM = dt ra

(7)

where Ca is the concentration in water at r = r0. Assuming Cb = 0 due to water flushing, which brings the released SPS out of the column and results in zero concentration (Cb = 0) at r = ra, eq 7 becomes 2πhDPSr0 dM = Ca dt ra D

(8) DOI: 10.1021/acs.iecr.7b00082 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Under steady state SPS release conditions, the SPS distribution between the matrix and aqueous phases is described as K=

Ca C0

(9)

When substituting Ca in eq 8 into eq 9, C0 at the boundary of the rod matrix in the aqueous phase can be expressed as C0 =

ra dM dt 2πhDPSr0

(10)

K

Further, when substituting eq 10 into eq 6, eq 6 is rewritten as ⎛ ⎞ DeCs dM ⎟ = 2πhKDPSr0⎜⎜ r0 ⎟ dt ⎝ KDPSr0 ln r + Dera ⎠

(11)

Figure 4. Effective diffusion coefficients in the rod matrix as a function of SPS mass released from the SPS SR-rod.

For the condition A ≫ Cs, where A is the oversaturated concentration of SPS within pores of the rod (g SPS porevolume−1 = g mL−1), the rate of SPS mass release from the SPS saturated zone toward the unsaturated zone is expressed as

estimated to be 2.60 ± 0.25 × 10−6 cm2 s−1 (roughly in the range of 2−3 × 10−6 cm2 s−1 as shown in Figure 4).

dM dr = −2πhAr dt dt

M = πhA(r0 2 − r 2)n

It should be noted that the determined model was developed on the basis of an assumption that there would be continuous removal of released SPS from the rod-water boundary and that a maximum SPS diffusion rate within the rod would be maintained. However, the persistence of released SPS outside the rod would affect the release rate of SPS from the rod. A lower diffusion rate would result due to a reduced SPS concentration gradient between the interior and the exterior of the rod. This would result in a longer duration of SPS release from the rod. In the field application of the SPS SR-rod in the subsurface, the SPS release rate would be affected by the surrounding SPS aqueous concentration, for example, consumption (e.g., reactions with soil media) or movement (e.g., groundwater flow) of SPS released in the aqueous phase. These influences would result in different concentration gradients (Ca to Cb) and then affect the lifespan of the rod. As assumed in the column release experiments, when water flushing removes released SPS from the aqueous phase adjacent to the rod, the concentration gradient and the SPS release rate will be greatest. Therefore, on the basis of the estimated De value at the greatest concentration gradient, the minimum release time for the SPS SR-rod can be determined. In other words, the duration of the rod would not be less than the minimum release time. The De range of 2−3 × 10−6 cm2 s−1 is substituted into eq 16, and the SPS SR-rod minimum release time as a function of the radius of the rod is presented in Figure 5. It can be seen that, for example, when the radius is 2 cm, the SPS SR-rod could be used at least 21 to 31 days. In order to verify this speculation, an additional rod with a radius of 2.1 cm was prepared and tested in the column with a water flushing rate of 1 mL min−1. The results are presented in Figure 5, inset. The SPS release pattern in the aqueous phase was similar to those obtained in the previous column release experiments. It was seen that 97% of SPS was released by day 23, which is comparable to the estimated SPS minimum release time. It should be noted that the duration of the rod is likely to be much longer than the estimated minimum release time because the low SPS concentration gradient under slow field groundwater flow conditions would result in slow SPS release from the rod matrix.

(12)

The rate of SPS release from the saturated zone along the reduction of radius of the saturated zone is equal to the rate of SPS dissolution into the aqueous phase, as shown in eq 13. ⎛ ⎞ DeCs ⎟ = −2πhAr dr 2πhKDPSr0⎜⎜ r0 ⎟ dt KD r D r ln + e a⎠ ⎝ PS 0 r

(13)

Integration of eq 13 yields eq 14, for which the limits of integration are specified to yield eq 15, which is the matrix boundary diffusion equation. KDPSr0DeCs A

∫0

t

dt = −

∫r

0

r

r0 ⎛ ⎞ ⎜KD r ln + Dera⎟r dr ⎝ PS 0 r ⎠ (14)

2

DeCst Dera r r 1 = ln + (r0 2 − r 2) − (r − r0) A 2 r0 4 KDPSr0

(15)

For the rod matrix-controlled system, the last part of eq 15 (aqueous controlled release system outside the rod) can be eliminated16 and the matrix boundary diffusion equation reduces to eq 16. DeCst r2 r 1 = ln + (r0 2 − r 2) A 2 r0 4

(17)

(16)

Cumulative SPS mass (M) presented in Figure 2 can be incorporated into eq 17 to calculate the radius (r) of the SPS saturated zone in the rod. The obtained r and corresponding release time (t) are then incorporated into eq 16, and the effective diffusion coefficient (De) within the rod matrix can be obtained. The De values calculated were plotted against the SPS released (M/M0) from the rod (Figure 4). It was seen that the De values in the early stage were scattered, possibly due to the loose structure of the rod where SPS release was disturbed directly by water flow. After approximately 40% of the SPS was released, the scattering of De values were steadily reduced and the SPS release was dominated by diffusion within the rod matrix. Therefore, the De values of the SPS SR-rod were E

DOI: 10.1021/acs.iecr.7b00082 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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4. CONCLUSIONS In this study, the SPS SR-rod was constructed using wax as matrix to blend with SPS particles at a SPS/wax mass ratio of 6/1. Visual observations of SPS release from the rod indicate that SPS released from the rod would initially sink along the surface of the rod to the bottom of the rod. The matrix boundary diffusion model was established, and the minimum release time for the SPS SR-rod is a function of dimensions of the rod (i.e., the radius of the rod). The SPS SR-rod could potentially be developed as an approach to deliver SPS for in situ remediation of contaminated aquifers. Perhaps, the SPS released could reside at a low permeability soil layer and serve as a long-term source of oxidant to facilitate diffusive transport of oxidant through soil media. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00082. Specifications of paraffin wax (Table S1); schematic illustration of variations in the uniformity of particle spatial distribution within the mixture of SPS and wax (Figure S1); apparatus of a one-dimensional column SPS SR-rod release experiment (Figure S2); variations of SPS concentrations with time in the mixture of paraffin wax submerged in different concentrations of SPS solutions (Figure S3) (PDF)

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Figure 5. SPS minimum release time as a function of radius of the SPS SR-rod. Curves drawn based upon a range of De determined according to the matrix boundary diffusion model. Inset shows SPS release from a SPS SR-rod (r = 2.1 and 13 cm in length) in the column with water flushing rate of 1 mL min−1.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +886-4-22856610. Fax: +886-4-22862587. E-mail: [email protected]. ORCID

Chenju Liang: 0000-0002-5028-4668 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of Taiwan under Project No. NSC 100-2622-E005-016-CC3. F

DOI: 10.1021/acs.iecr.7b00082 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX