Remediation of LNAPL-contaminated sand by using Humic acid as a surfactant.
Underground storage tanks commonly contain liquid fuels and chemicals, and many have leaked through the years. Prior to 1970, most underground storage tanks were made of steel, which tends to corrode in a wet subsurface environment (Fitts, 2002). Some of these liquids were immiscible organic compounds when released tend to form and distribute as a separate non-aqueous liquid phase in the subsurface porous medium. These released liquids or LNAPLs would partition themselves into the air, water, or soil phases based on their physiochemical characteristics. By partitioning, the LNAPL could impact an ever larger environment where humans, other fauna, and flora live.
Remediation of this contaminated subsurface could involve the recovery of the LNAPL by pumping the porous medium. It is well known that some of the organic liquid remains behind as a residual LNAPL during the pumping effort. To increase the efficiency of the recovery, surfactants are injected into the subsurface to mobilize the LNAPL and to make more available for extraction. This study will examine the use of a natural Humic acid (HA) as a surfactant.
Residual LNAPL typically occupies 10-50% of the available pore space (Chatzis et al, 1986). Commercial surfactants commonly remove up to 90% of the residual LNAPL in laboratory tests (Abdul et al., 1990). Two mechanisms can be involved in this improved recovery--the surfactant can: (1) increase the LNAPL mobility, and (2) increase LNAPL solubility in water.
Humic acid is part of the humic substances that are extracted as a by-product from peat processing. The generation of humic substances is an inexpensive, large volume source that could provide a cost effective source of needed surfactant. Using a naturally occurring humic substance such as HA may eliminate unwanted environmental consequences of using a surfactant in the remediation of a contaminated soil. The use of HA as a surfactant is an untested technique in the remediation of contaminated soil.
A process has been developed for the production of low-cost HA from peat processing. HA appears to have proper surfactant capabilities and soil stabilization properties that could augment remediation of hydrocarbon contaminated sites (Steffy et al., 2001). The testing and quantification of HA characteristics and applications in remediation are needed so that the potential of these surfactants can be evaluated.
Humic substances are nonvolatile, semi-polar polymers composed of a "chicken wire" pattern of aromatic carbon rings that are [10.sup-3] to [10.sup.-6] mm (colloidal to molecular dimension) in length, with a molecular weight ranging up to 200,000 (Ghassemi and Christman, 1968). Surface activities of humic substances have been observed to be inversely related to the acidity of the humate and the pH of a humic aqueous solution (Chen and Schnitzer, 1978). Hence, some humic substances are readily soluble in dilute alkaline media (HA), but some are precipitated upon acidification (fulvic acids). Humic solutions are described as having anionic-surfactant characteristics and contributing to soil aggregation stability (Piccolo and Mbagwu, 1989). Tschapek and Wasowski (1976) found that the alkali-soluble fraction of humics are surface active; that is, they lower the surface tension of water, and appear to be dependent on ionic strength and on the extraction method of the HA. This dependency appears to be a function of the polydipsersive (heterogeneous) nature of the alkali-soluble fraction.
To date, HA has had only limited application as a remediation tool for contaminated soils (Bickerton, et al., 2004). Bickerton et al (2004) used a HA solution at a concentration of 3.66 mg/ml on petroleum-contaminated sands and silts in a controlled remediation effort. The results were inconclusive due to the lack of data. An earlier laboratory study by the same research group, found that HA solubilizes diesel contamination and also promotes the in-situ biodegradation process (Van Stempvoort et al., 2002). These promising results warrant further investigation of HA and its use in remediating contaminated soils. Information that needs to be acquired includes: its dispersive capabilities, flocculation behavior, interfacial tensions, viscosities, saturation-pressure relationships, toxicity, and effects on the mobilization of residual contaminants.
There are problems associated with the use of surfactants, however. The following issues need to be addressed when using HA or any surfactant. Short-column tests of surfactant application in a two-phase system (oil, surfactant) resulted in a non-uniform distribution of residual LNAPL after treatment (Ang and Abdul, 1991). Non-uniform distribution may result in channeling, which reduces the surfactant's effectiveness (Hornof and Morrow, 1987). Another concern is that certain surfactants hydrolyze to flocs which can combine and disperse soil colloids, which in turn could lead to aquifer clogging (Abdul et al., 1990). Surfactants act at liquid-liquid interfaces, but also at solid-liquid interfaces, where they may adsorb to the solid (Rosen, 1989). Alternatively, they may precipitate under certain conditions (Stellner and Scamehorn, 1989; Jafvert and Heath, 1991). Both sorption and precipitation will reduce surfactant availability. Temperature reduction can reduce surfactant effectiveness, critically so below the Krafft point (West and Harwell, 1992). Surfactants can partition into the LNAPL if their solubility in LNAPL is high enough. They can also separate chromatographically.
Finally, surfactants must be acceptable environmentally. Laboratory studies reveal that recovery of LNAPL could be improved if HA were continuously pumped through the contaminated porous medium. Because of the large percentage of HA used, a toxicity/teratogencity test was used to determine the effects of HA. The bioassay test was carried out independently of the surfactant testing. The frog embryo teratogenesis assay-Xenopus (FETAX) was used to assess the developmental toxicity of HA. This assay has been used to evaluate the developmental toxicity of chemicals and mixtures for both human health and environmental health (Bantle et al. 1994; Rayburn et al. 1991).
Clearly, surfactant selection is a multifaceted issue (Vigon and Rubin, 1989), although guidelines for proper selection are readily available (Rosen, 1989; West and Harwell, 1992).
This study is directed towards characterizing the physical and chemical properties of HA developed from peat processing in terms of its surfactant capabilities and flocculent behavior. We also measured by laboratory column studies the effectiveness of utilizing HA as a surfactant in the remediation of hydrocarbon contaminated soil.. This study also investigated the dispersion of clays caused by the presence of increasing HA concentrations in the porous medium. Finally, we quantified the environmental acceptability of HA in terms of its toxicity.
MATERIALS AND METHODS
Humic Acid Extraction:
HA was derived from shredded, dry peat that was harvested in Bemidji County in northern Minnesota. Production of the HA from the peat for the laboratory tests was a simple batch process of acid/base extraction. For bioassay work HA, the pH was adjusted to approximately 8 with an addition of a sodium hydroxide solution. HA was stored at 4[degrees]C until use. HA concentration was determined by gravimetric analysis.
Critical Micelle Concentration Measurement:
The critical micelle concentration (CMC) is the concentration of the HA solution at which the surface tension is the minimized (Lowe, 1999). Surface tensions of various HA solution concentrations were measured using a du-Nouy interfacial tensiometer (CSC Scientific Co., Fairfax, Virginia). A plot of HA concentration versus surface tension provides an estimate of the CMC.
LNAPL Recovery Tests:
Recovery of LNAPL was measured by a series of short-column tests. These test determined the relative performance of surfactant removal efficiencies. A contaminated soil with a known level of LNAPL saturation was flushed with a HA solution. The concentration of the solution was at the CMC . The efficiency of removal was measured by determing the amount of LNAPL that remained in the soil after flushing. Under the proper conditions, HA acts as a surfactant when flushed through unsaturated porous medium containing residual amounts of water and mineral oil. These tests followed the procedures of Ang and Abdul (1991) which provided guidelines for initial testing.
The laboratory tests were conducted with homogenous fine-grained sand, packed in a 54-cm borosilicate tube with a diameter of 3 cm (Table 1). The resulting packed column had sand bulk densities of ~ 1.00 g/[cm.sup.3] and porosities ranging from 21.5 to 24.7% (Table 1a). The LNAPL was a mineral oil dyed with Sudan IV (Sciencelab.com, Houston, Texas). The tests were initiated by establishing a water table condition in a vertical sand-packed column that becomes contaminated by the LNAPL.
Three glass columns packed with silica sand were initially saturated with water; the water was then allowed to gravity drain to establish the specific retention of the porous medium. Then 50 ml of mineral oil was allowed to infiltrate from the top while the bottom of the column freely drained into a graduated cylinder. After the mineral oil was drained, an initial level of residual saturation was measured ranging from 10.6 to 39.9%. The column was then rotated horizontally and pumped with the surfactant at a constant rate. The amount and rate of LNAPL displaced was measured. One hundred (100) ml of a 10% HA solution (3.4 mg/ml) was flushed through the column from top to bottom. The amount of mineral oil flushed (recovered) was measured.
Clay Dispersion Assay:
An assay of HA's ability to disperse clay in solution was evaluated. A disadvantage of using an anionic surfactant is that promotes clay dispersion, thus increasing the potential for aquifer clogging, and reduces the delivery of the surfactant to all areas of porous medium that contain residual LNAPL.
The procedure of this assessment was to fill a glass vial with approximately 5 grams of kaolinite, a non-swelling clay. The clay was mixed with de-ionized, distilled water for 2 hours, after which the mixture was allowed to settle for 2 hours. The turbidity was then measured using a HACH 2100P Turbidimeter (Hach Co., Loveland, Colorado). The procedure was done 7 times for each fluid tested. Various concentrations of HA were used. The turbidity versus HA concentration was then plotted to depict their relationship.
The Frog Embryo Teratogenesis Assay-Xenopus (FETAX) is a 96-h in vitro assay used to determine the developmental toxicity of compounds and mixtures (American Society for Testing and Materials, 1992). This assay uses embryos of the South African clawed frog, Xenopus laevis. This assay exposes to embryos from the small cell blastula stage to a free living larvae to chemicals and mixtures to determine potential developmental toxicity. Adult Xenopus were purchased from Xenopus I (Ann Arbor, MI) and kept in glass aquaria with recycled filtered water and kept on a 12 h: 12 h light-dark cycle. They were fed high protein fish pellets with vitamins added. Adults were bred using human chronic gonadotrophin (Sigma, St. Louis, MO), injected in to their dorsal lymph sacs; 200 and 500 units for males and females respectively. Adults were placed in a false bottom breeding chamber as described by (McCallum and Rayburn, 2006). Embryos were collected the next morning and the jelly coat removed with 2% L cystiene (Sigma, St. Louis, MO). Embryos were double sorted and randomly placed into plastic Petri dishes (Fisher, Pittsburgh, PA). (60mm X 15mm) filled with control or test solutions. Three different experiments were performed with three different clutches of embryos.
The experimental unit was 20 embryos per plastic Petri dish (8 ml of solution; 2 replicates per dose; 4 control dishes) for each experiment. Three experiments were performed with 8 to 11 concentrations used for each experiment approximately 520 embryos were required for each test. A single HA extraction was prepared and used for all three experiments. The embryos were then placed in an incubator at 24[degrees]C with static renewal of solutions every 24 h. The test duration was 96 h. The dead embryos were removed and counted every 24 h.
At the end of 96 h, survivors were counted and scored for malformations, and lengths were measured. Statistical analysis began with a two way ANOVA using experiment as one factor, and dose as the second factor for length comparsions, followed by Bonferroni t-test multiple comparisons
Tox Tools, a software for dose-response modeling, benchmark dose estimation and risk assessment was used to calculate LC50 (lethal concentration to induce 50% mortality) and EC50 (effective concentration to induce 50% malformation) with standard errors (ToxTools, 2001). ToxTools was chosen because it has a Developmental Toxicity model that incorporates mortality, malformation and growth. ToxTools also analyzed all of the results together for each of the three experiments. The additive model was used for all calculations in this paper. Teratogenic index (TI) is calculated by dividing the 96h LC50/96 h EC50. A TI ratio of greater than 1.5 indicates an increase of teratogenic risk (ASTM, 1991). A Bonferroni t-test was used to determine significant differences from controls for embryo length comparisons.
Measurement of the CMC:
Systematic measurement of the interfacial surface tension as a function of the concentration of HA solution reveals a break in its linear relationship. Generally, as the HA solution concentration increases, the interfacial tension decreases. A break in this relationship occurs at a concentration of 3.4 mg/ml (Fig. 1). The break provides an estimate of the CMC of the HA solution (Lowe et al., 1999).
[FIGURE 1 OMITTED]
Recovery of LNAPL:
HA was observed to quickly mobilize the residual oil by reducing the interfacial tensions of the water-oil system. Residual oil in unsaturated silicate sand with an approximate bulk density of 1.0 g/[cm.sup.3] ranged from 30 to 42% (Table 2).
Variations in both the residual water and oil saturations before flushing and the amount of recovered oil are probably due to uneven packing throughout the column which in turn could cause instabilities in fluid fronts and result in channeling.
Figure 2 depicts the overall effectiveness of pumping a HA solution to mobilize LNAPL in comparison to water. At equal pumping rates, the final amount of oil recovered is 81% for the HA solution and 60% for water. This represents a 35% improvement in oil recovery
[FIGURE 2 OMITTED]
Visual observations indicate that the HA solution mobilized the LNAPL (mineral oil) by decreasing the interfacial tension between the LNAPL and water phases. The change in interfacial tension is promoting the movement within the LNAPL continuum. Apparently, when the 10% HA solution was added to the column, the HA distributed itself as part of the water continuum rather than LNAPL changing its physical properties by dissolving the HA solution. Two tables 1 and 2 give the results of the recovery tests. Table 1 shows the basic characteristics of the columns used in the recovery assay. Table 2 shows the recovery of mineral oil with HA solution at 3.4 mg/ml. These results showed the proportion of mineral oil flushed was between 58% and 70% (Table 2).
Table 1. Physical characteristics of columns. Test Length of Column (cm) Bulk Density (g/cm3) Porosity (%) 1 52.0 1.00 21.5 2 52.3 1.02 24.5 3 53.1 1.00 24.7 Table 2. Results of flushing tests. Test Pumping Initial Residual Recovered Efficiency Rate Water Oil (%) Oil (%) (%/cm3/min) (cm3/min) Volumetric Saturation(%) 1 3.55 21.5 42.0 58.0 16.3 2 2.13 24.3 30.0 70.0 32.9 3 1.41 24.7 40.0 60.0 42.6
Laboratory tests found that the pump rates showed no relationship to the amount of LNAPL recovered (displaced) (Fig. 3). However, in terms of pumping efficiency (amount of LNAPL recovered / amount of fluid pumped)--a low pump rate of 1.41 [cm.sup.3]./min was the most efficient. HA at its critical micelle concentration of 3.4 mg/ml was then used to increase LNAPL mobilization. When the HA solution was used, recovery was increased from 60% to 81% (Fig. 2), and efficiency was improved by over 180%.
[FIGURE 3 OMITTED]
Clay Dispersion Assessment:
The HA acts as an anionic surfactant, and promotes the suspension of clay particles in solution. There is a rapid increase in the dispersive capability of the HA measured as turbidity up to a concentration of 10 % HA, after which the rate of increase in the dispersive capability drops of with increasing HA concentration (Fig. 4). This change in the dispersive capability of the HA solution occurs near the CMC concentration (3.4 mg/ml).
[FIGURE 4 OMITTED]
Results of Toxicity Testing:
A total of 1280 embryos were used for the three experiments. Of these, 240 were control embryos with ASTM acceptable control rate of 6.24% for mortality and 7.62% for malformation. The 96 h LC50 was 3.729 mg/ml (Table 3). The 96 h EC50 (malformation) was 6.499 (Table 3). The LC10 (to cause increase of risk of 10% mortality) was 1.440 mg/ml and an EC10 malformation of 2.060 mg/ml. The probability estimation curve for malformation (Fig. 5) indicates risk estimation reached 50% at the highest HA concentration used in this study. The average mean growth of control embryos over the 96 hr test duration was 9.45 mm (Fig. 6). There were only two concentrations with means significantly different from controls, 1 and 5 mg/ml (Fig. 6). Because means of the 2-4 mg/ml concentrations were not significantly the different than controls, result for 1 mg/ml is most likely an anomaly. Only the mean of the highest concentration differed significantly from the control, indicating that the chemical did not cause significant growth reduction at concentrations that do not affect mortality. Tox-tools estimated a maximum risk of <0.01 (<1%) for the highest concentration of HA tested (data not shown). The Teratogenic Index (TI) is the 96h LC50/ 96h EC50 which is 0.574 (Table 3) which also indicates that HA is not a weak teratogen. Few malformations were seen except at extremely high concentrations (concentrations greater than the LC50 value). These malformations included muscular kinking of the tail, reduced head, and gut malformations typical of non-teratogenic compounds (Fig. 7).
Table 3. Ninety-six (96) h LC 10, 30, 50 and EC 10, 30, 50, values for HA 96 h Mortality Mortality Malformation Malformation Teratogenic Risk (mg/ml) SE (mg/ml) SE Index * only for LC50 value LC10 1.440 0.420 2.060 0.624 LC30 2.748 0.444 4.506 1.393 LC50 3.729 0.427 6.499 2.033 0.574 Teratogenic Index = 96 h LC50 / 96 h EC50.
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
Laboratory studies show HA to have potential utility as a surfactant for use in the displacement of LNAPL in sand aquifers. At concentrations of [greater than or equal to]3.4 mg/ml, a critical HA micelle concentration occurs, disrupting the interfacial tension between the air-HA solution. When a HA solution at this concentration is delivered to the water phase of an LNAPL-contaminated sand, the interfacial tension is quickly reduced, allowing the movement of residual LNAPL globules to occur. Laboratory-scale pumping tests demonstrated that LNAPL recovery increased from 60 to 81% with addition of HA, a 35% improvement over water as the displacing fluid. The efficiency of this recovery also improved 180%. Therefore, the use of HA solution as a surfactant could improve the remediation effort both in terms of effectiveness and economics. Field-scale applications of HA in the remediation of hydrocarbon-contaminated sand aquifers have shown some promising results as well (Bickerton, et al., 2004; Van Stempvoort, et al., 2002). These studies indicate that recovery of LNAPL is enhanced by HA solubilizing the residual LNAPL globules; however, our visual observations found that the majority of the recovery was enhanced by the HA changing the interfacial tension between oil and water (HA solution).
HA and fulvic acid are part of the naturally occurring dissolved organic carbon (DOC) component in water, and collectively are called humic substances. DOC commonly occurs at concentrations < 0.05 mg/ml in surface water areas such as wetlands, and 0.002-0.015 mg/ml in rivers and lakes (Drever, 1997). The DOC diminishes in concentration to < 0.002 mg/ml in groundwater systems because of degradation (Fitts, 2002). HA accounts for ~5% of the DOC (Drever, 1997). Therefore, the CMC HA concentration of 3.4 mg/ml used in this study is ~7,000 higher than what is naturally occurring in water systems. As such, toxicity assessment of HA is warranted.
Overall HA did not indicate an increase in teratogenic risk. The FETAX bioassay showed that general cytotoxicity was observed with an LC50 of 3.73. It is interesting that the LC50 is very close to the CMC of 3.4 mg/ml. This would indicate that toxicity of HA may be due to surfactant changes of water induced by HA.
HA has many attributes that make it a promising surfactant to enhance the mobilization of trapped LNAPL in a sand aquifer. The CMC of HA occurs at a relatively low concentration of 3.4 mg/ml, although this is ~7,000 times higher than is found in natural water concentrations. Applying HA at its CMC concentration insures that optimal surfactant effectiveness is realized in the remediation process. When applied through the aqueous phase, the HA quickly mobilizes the NAPL by reducing the interfacial tension in the LNAPL-water system. Laboratory testing of fine-sand material indicates that a simple continuous flushing recovered up to 81% of LNAPL, and that the higher pumping rates produced faster and larger oil recovery rates. However, recovery efficiency was optimized at a low pumping rate. Other advantages of using HA as a surfactant in the remediation of a sand aquifer are that HA is easy and inexpensive to produce, and places low oxygen demand on the natural aquatic system.
A disadvantage of using HA as a surfactant is that it readily disperses clays that may promote pore clogging. Generally, the dispersive effect increases as the concentration of HA increases. In addition, the CMC concentration of HA is near the LC50 as determined by FETAX tests.
The authors would like to thank the following JSU students for their work in data collection: Cody St. John, Melissa Bandy, and Daniel Grogan. The authors also would like to thank Jacksonville State University and the Faculty Development Grants that supported this research project.
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David Steffy (1) and James Rayburn (2)
(1) Department of Physical and Earth Sciences, (2) Department of Biology, Jacksonville State University, 700 Pelham Rd. North, Jacksonville, AL 36265-1602
Correspondence: Steffy, David (firstname.lastname@example.org)