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Remediation potential of mulch for removing lead a
A. Jang & P. L. Bishop
b
a
School of Civil and Environmental Engineering, College of Engineering , Sungkyunkwan University , Chunchun-dong 300, Suwon , Kyonggi-do , 440-746 , South Korea b
Department of Civil and Environmental Engineering , University of Cincinnati , Cincinnati , Ohio , USA Accepted author version posted online: 09 Jun 2011.Published online: 21 Oct 2011.
To cite this article: A. Jang & P. L. Bishop (2012) Remediation potential of mulch for removing lead, Environmental Technology, 33:6, 623-630, DOI: 10.1080/09593330.2011.586730 To link to this article: http://dx.doi.org/10.1080/09593330.2011.586730
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Environmental Technology Vol. 33, No. 6, March 2012, 623–630
Remediation potential of mulch for removing lead A. Janga∗ and P.L. Bishopb a School
of Civil and Environmental Engineering, College of Engineering, Sungkyunkwan University, Chunchun-dong 300, Suwon, Kyonggi-do 440-746, South Korea; b Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, Ohio, USA
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(Received 6 February 2011; final version received 4 May 2011 ) Hardwood bark mulch has good physicochemical properties for the adsorption of lead (Pb(II)). Batch tests were conducted to obtain the sorption coefficient of Pb(II) in mulch. The results of the Freundlich model were not in as good agreement as for the case of the Langmuir model. In addition, a laboratory-scale mulch permeable reactive barrier (PRB) system was designed for the treatment of Pb(II)-contaminated groundwater. The mulch PRB system, using a mulch layer, can potentially be used in the subsurface for cost-effective and in situ transformation of the Pb(II) into environmentally acceptable forms. From the Pb(II) breakthrough curve, the mulch becomes saturated more quickly at higher flow rates. Keywords: batch test; groundwater; lead (Pb(II)); mulch; permeable reactive barrier
1. Introduction Environmental concentrations of heavy metals have frequently exceeded the US Environmental Protection Agency’s (EPA’s) water quality criteria/standards because, while they do occur naturally, they also may be found as a result of a variety of anthropogenic activities, such as mining, smelting, metal plating/finishing operations, automobile battery production, application of pesticides and inorganic fertilizers and industrial applications of metals [1–3]. Heavy metals are among the most insidious pollutants because of their non-biodegradable nature and ability to persist for long periods [3,4]. Their progressive accumulation and resultant toxicity disrupt natural ecosystems and affect the food chain, leading to health problems in humans and animals [5]. In particular, lead (Pb(II)) is found in a wide variety of chemical forms (galena ore, PbS; cerussite, Pb(CO3 )2 ; hydrocerussite, Pb(CO3 )2 (OH)2 ; anglesite, PbSO4 ; crocoites, PbCrO4 ; and lead oxide, PbO) [4] throughout soil and groundwater systems and can be readily transformed by changes in geochemical conditions and other environmental processes. Dissolved lead is of specific concern due to its known potential to cause adverse health impacts [6,7]. Pb(II) is ranked number two in the US Agency for Toxic Substances and Disease Registry (ATSDR) Priority List of Hazardous Substances due to its association with cancer [8]. There are various mechanisms to explain how the groundwater becomes contaminated with toxic lead ions. It is likely that, within soil and groundwater systems, soils can be a reservoir for and possible future source of pollutants. ∗ Corresponding
author. Email:
[email protected]
ISSN 0959-3330 print/ISSN 1479-487X online © 2012 Taylor & Francis http://dx.doi.org/10.1080/09593330.2011.586730 http://www.tandfonline.com
Since soil has a finite capacity to retain lead ions, inherently these lead ions trapped in the soil matrix will leach and migrate deep into the ground water by advection, mechanical dispersion, molecular diffusion and chemical mass transfer, resulting in increased groundwater pollution. From the environmental protection point of view, it is highly desirable to apply cost-effective and suitable remediation techniques to such sites. Pump-and-treat approaches are well-established ex situ remediation technologies for heavy metal-contaminated groundwaters; however, they have the shortcoming of high pumping and re-injection costs [9,10]. Of the various in situ techniques available for the removal of lead ions from the contaminated groundwater, however, passive treatment technologies have begun to receive attention among researchers, mainly due to their lower long-term operation and maintenance costs as compared with pumpand-treat techniques [11,12]. In a permeable reactive barrier (PRB) system, the contaminated groundwater is allowed to pass through the reactive wall where physicochemical or biological or combined reactions, such as filtration, precipitation, complexation, ion exchange or sorption/biosorption, occur [13]. Among these, it has long been known that adsorption is quite selective and effective, and is able to remove very low levels of heavy metals from the contaminated water [14]. When considering contaminant remediation through the use of a PRB system, one of the important parameters is to select a relatively inexpensive barrier material that can be operated effectively for a long period [15]. So, finding low cost and abundantly available materials becomes a challenge to researchers for groundwater
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purification. Examples of materials widely studied for heavy metal adsorption include tannin and chitosan-rich materials [16,17], zeolite [18], clay [19], apatite [20,21], fly ash [22,23] and waste slurry [24]. Wood-based mulch could also be a feasible alternative as an efficient adsorbent in PRB systems for Pb(II)-removal processes due to several advantages it possesses: it is plentiful, long lasting, inexpensive (approximately 2.4 cents per kilogram) and readily available, and can be a relatively simple, effective and economical means for contaminated groundwater treatment. In spite of their obvious benefits, few attempts have been made to study the application of a PRB system with mulch for toxic metal-contaminated groundwater remediation. Therefore, the aim of this research was to study the sorption capacity of mulch for Pb(II) by determining the equilibrium isotherm, and to demonstrate the removal capacity and efficiency of Pb(II) when mulch was used as a barrier. 2.
Materials and methods
2.1. Materials Synthetic solutions of Pb(II) were prepared from analytical grade Pb (NO3 )2 (Fisher Scientific, USA) for the batch adsorption testing and the mulch PRB system. The required Pb(II) concentrations were prepared by diluting the stock solution with deionized (DI) water (17 M). Cresol redthymol blue (LabChem Inc., PA, USA) was used as the tracer in the tracer test. Hardwood bark, a natural material mulch, was used as a sorbent to adsorb Pb(II) in solution. The raw mulches were screened and sieved to obtain a 1–3 mm mulch particle size range by using a #10 mesh (2 mm) sieve. The screened mulches were washed three times with DI water to remove impurities, subsequently decanted and dried at an oven temperature of 60◦ C overnight. The homogenized mulch particles were used for both the adsorption batch tests and the mulch PRB system. In order to suppress biotic sorption during the experiments, the mulch was treated with sodium azide (98% purity, EM Science, USA). The aqua regia extraction procedure was carried out according to the International Organization for Standardization (ISO) Standard 11466 method (1995). More details on the experimental conditions can be found elsewhere [25]. Sand, purchased from Sigma-Aldrich (No. 274739), was used without further size processing; it was reported to contain >99% of sand particles between 297 μm (#50 mesh) and 210 μm (#70 mesh), with an average size of 250 μm (#60 mesh). Sand was also rinsed with DI water and dried in an oven at 60◦ C overnight. 2.2. Sorption tests Batch isothermal sorption tests were carried out by mixing a constant amount of the mulch (2 g) with known volumes of metal solutions (200 ml) containing the desired initial concentrations of 0.5–1.75 Pb(II) mM. All of the
Erlenmeyer flasks were sealed with foil caps to minimize evaporation and then were mixed at room temperature (23 ± 2◦ C) for the predetermined time interval (3–120 min) with a magnetic stirrer at 150 rpm. The initial pH of the metals salt solution was adjusted to the desired value (pH 5.5) with 0.1 M HCl or NaOH. The small sample volumes were filtered so that the total volume of the batch test did not change by more than 10% in order to minimize the influence of changing mulch/solution ratio. Filtered samples were stored in polyethylene bottles to which 0.1 ml of HCl (70%) was added for sample preservation. The initial and equilibrium concentrations of Pb(II) in the aqueous phase were determined, either directly or after dilution (if necessary), by atomic absorption spectrophotometry (AAS) using an acetylene-air flame. The error in the determination of the Pb(II) equilibrium concentrations was within 3%. In order to dissolve possible metal complexes formed on the walls of the bottles, all glassware was washed in nitric acid (1N HNO3 ) overnight before use and was rinsed with DI water. 2.3. Mulch PRB test bed setup In order to simulate the groundwater-mulch PRB system, a laboratory-scale mulch PRB reactor was constructed of polyacrylic plate with a total volume of 12,000 cm3 . As shown in Figure 1, the mulch PRB reactor included the following: sand chamber (5.2 kg of sand, 4350 cm3 of working volume), permeable mulch barrier (0.1 kg of mulch, 480 cm3 of working volume), influent tank (2700 cm3 ) and effluent tank (1800 cm3 ). One side of both the influent and effluent tanks had multiple small size holes in a vertical line at regular intervals, which were used to equally pass water into/from the sand container. To permit a groundwater flux across the mulch PRB, there was a stainless steel sieve with a #70 mesh (0.5 mm) at the front and back of the mulch barrier. The different hydraulic levels of the two tanks generated the passive water flow. A steady-state flow rate was established in a mulch PRB reactor within 90 minutes. The mulch PRB reactor was run for 2 hours before starting injection experiments. In order to keep constant hydraulic head in the mulch PRB reactor, Masterflex drive pumps with an easy-load pump heads were installed at the influent and effluent tanks, respectively. Sampling ports allowed access to fluids both before and after the mulch PRB reactor during the test bed experiment. In order to quantify lead ions, samples could be withdrawn using a syringe inserted into the port. After filtering with a disposable syringe filter (0.45 μm pore size nylon filter membrane, Whatman Inc., NJ, USA), the filtrate was directly analysed to estimate Pb(II) concentration levels using AAS. Previous adsorption batch experiments in our laboratory (results not shown) had indicated that the adsorption of Pb(II) on the sand used in the mulch PRB reactor was negligible. In order to obtain the mean values or typical results, the experiments were carried out in duplicate. Results of
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Environmental Technology
Figure 1.
The mulch PRB reactor to simulate contaminated groundwater.
Pb(II) analyses corresponded to within ±5%. The size of the error was similar to those of the symbols used in all figures presented, so the addition of error bars was disregarded. 2.4. Tracer test After establishing steady-state flow conditions, 20 ml of cresol red-thymol blue was injected into the porous sand to characterize the Pb(II) transport in the mulch PRB reactor. It took less than 20 seconds to inject 20 ml of indicator (redbrown colour). The movement of the tracer was recorded by taking high-resolution digital pictures at regular time intervals. 3.
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Results and discussion
3.1. Mulch properties Although they differ in appearance and function, most mulches are applied to the soil surface in order to retain moisture, retard weeds, maintain soil temperatures, reduce water runoff and soil erosion, improve landscape aesthetics and keep plants healthy. Recently, research has shown that mulch consists primarily of carbon, predominantly in the form of complex biopolymers such as lignin (>53% by mass) and cellulose (>33% by mass) [10]. It was also reported that mulch has metal-binding function groups, such as carboxyl, hydroxyl, sulfate, phosphate and amino groups [26]. It is generally possible that such multi-components play an important role in the overall heavy metals sorption process and, consequently, influence their mobility. Due to the pH dependence of heavy metal mobility, the slightly alkaline pH (greater than 7) of typical hardwood bark mulch might be effective in increasing the sorption capacity for heavy metal ions. It is generally accepted that oxidation-reduction potential (ORP) is also a key parameter controlling Pb(II) mobility [27]. The values ranged from 10 to −50 mV.
Knowing the exact content of heavy metals in mulch is also important for determining the adsorption capacity of the mulch. Despite the fact that the determination of total metal concentration is insufficient to properly assess information about the origin, solubility, bioavailability, mobility and transport of metals in natural environments, the aqua regia digestion method is still the most widely employed tool to study the total heavy metal content in many solid materials, such as soil, sludge, sediment and solid waste, due to its simplicity, low cost and adaptability as a routine procedure [28]. The aqua regia digestion method, using a heated concentrated acid mixture (HNO3 /HCl; 1:3 v/v), was used to determine whether the total heavy metals content in mulch is within the range of background levels. As an alternative to assess the toxicity of pollutants in the environment, the Toxicity Characteristic Leaching Procedure (TCLP) extraction (US method 1311) method was used [29]. In general, two types of TCLP buffered acidic solutions are available for extracting metals, depending on the pH and buffering capacity of the mulch. Since the pH of mulch was higher than 5, solution #2 with a pH of 2.88 ± 0.05 was used. Analytical results for both aqua regia and TCLP extractions are presented in Table 1. From Table 1, the leaching capacity of heavy metals for the aqua regia method was higher than for the TCLP extraction. This indicates that the total heavy metal concentrations may be determined Table 1. Average concentrations of heavy metals extracted by aqua regia and TCLP using AAS for their quantifications (n = 3, ND means not detected, (detection limit: 0.22 mg Cu(II)/l; 0.17 mg Pb(II)/l; 0.02 mg Zn(II)/l). Heavy metal ions (metals mg/mulch kg) Extraction method
Cu(II)
Pb(II)
Zn(II)
Aqua regia TCLP
9 ± 1.5 ND
7 ± 1.2 ND
18 ± 2.5 1.5 ± 0.5
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more precisely using more acidic solutions. It could be also observed that copper and lead were detected in the aqua regia leaching solution, while they were below analytical detection limits by the TCLP extraction method, indicating that most Cu(II) and Pb(II) was present in a not easily extractable form. The results also indicated that, although small amounts of Zn(II) were extracted by the TCLP method from mulch, the total concentration of Zn(II) was lower than that of the background soil (58 mg Zn(II)/kg) [30]. Thus, it can be concluded that mulch is as an environmentally friendly adsorbent for the treatment of heavy metals.
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3.2. Batch adsorption test In order to apply mulch in a PRB system as an effective adsorbent, the parameters and mechanisms affecting metal ion capture by mulch must be identified. Batch adsorption studies are an essential prerequisite to a mulch PRB test bed experiment because they yield valuable information on the adsorption capacity of the mulch for the metal ions, and the rate of uptake. Since the sorption of metals is a competitive and complex process between ions in solution and those sorbed onto the mulch surface, it is not easy to separate all phenomena at the solid–solution interface, including sorption, charge transfer and ion and ligand exchanges. Thus, the sorption used in this paper refers to all reactions at the interface layers. In order to investigate the effect of metal ion concentration on the adsorption capacity of the hardwood bark mulch, an equilibrium experiment was performed using a dosage of hardwood bark mulch (2 g) that was subjected to various initial concentrations of Pb(II) ranging from 0.5 to 1.5 mM. Batch isothermal sorption tests were performed until equilibrium was reached for Pb(II) between the mulch and the metal solution. The physicochemical data described in a previous paper [31] indicates that the water content of the hardwood bark mulch ranged from 49% to 62%, organic carbon was less than 15% and the cation exchange capacity (CEC) values ranged from 39 to 45 meq/100 g. Since the CEC value of mulch was lower than that of either typical organic-rich agricultural soil (50–100 meq/100 g) or compost (100–300 meq/100 g) [26], the removal of Pb(II) from solution in the presence of mulch may be due to adsorption on surfaces and pores, and to complexation with metal-binding functional groups. Preliminary study of the time to equilibrium showed relatively fast kinetics, with approximately 90% equilibrium being attained within the first 30 min. Thus, to ensure complete equilibrium, samples were left shaking for about 2 hours, which was enough time to reach an equilibrium state, since there was little variation between before and after 2 hour analyses. It appears from Figure 2 that, under the given experimental conditions, results generally follow the expected metal adsorption behaviour. This means that
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Figure 2. (a) Adsorption capacities of mulch adsorbent at room temperature and pH 5 (•) and 6 (). (b) Langmuir adsorption isotherms of Pb(II) on mulch adsorbent (Ce : final equilibrium concentration, Qe : equilibrium adsorption capacity).
the initial adsorption capacity of the hardwood bark mulch increased with increasing initial Pb(II) concentration, but after saturation the adsorption capacity of the mulch was stable, with an increase in initial metal concentration. It is a reasonable assumption that the number of binding sites to Pb(II) on mulch is proportional to the amount of mulch added to the tests. Comparing the adsorption capacities at pH 5.0 and 6.0, it can be seen that an increase of pH in the range of 5.0–6.0 promotes the Pb(II) adsorption by mulch. In order to better understand the adsorption capacity of the hardwood bark mulch for removal of Pb(II), experimental data were fitted to the Freundlich and Langmuir isotherms. The results of the Freundlich model, represented by plotting log(Qe ) versus log(Ce ) (not presented here), were not in as good agreement as for the case of the Langmuir model (Figure 2(b)). The maximum adsorption capacity (Qm ) of the mulch, in terms of mmoles of solute adsorbed per gram of sorbent, can be evaluated from
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the linear regression analysis of the Langmuir isotherm equation. The values of Qm were calculated as 0.3 (pH 5), and 0.4 (pH 6), respectively. In addition, the values of distribution coefficient (Kd (l/g)) were also calculated as 47 (pH 5) and 55 (pH 6), respectively. The high correlation coefficients for the isotherm of Pb(II) removal indicate that there is a strong positive relationship for the data and that all experimental sorption data are fitted to the Langmuir model. The data fitting of the model yielded a determination coefficient (R2 ) of greater than 0.95. This implies that Pb(II) sorption by mulch is largely a monolayer adsorption or one-directional process. Accordingly, it is seen that mulch is a good sorbent for Pb(II) removal, because of the high Pb(II)-removal rate and removal capacity. 3.3.
Mulch PRB test bed
Although isotherm batch reactors provide good and more convenient information about the effectiveness of metalsorbent systems, they cannot provide the design parameters needed for pilot-scale mulch PRB systems. On the other hand, a laboratory-scale mulch PRB test bed operating at steady state would be representative of the pilot-scale mulch PRB operation at the site, and could provide critical design parameters, such as bed dimensions, flow rate of metal solution into the system and initial concentrations for the field application of the mulch PRB system. Conceptually, in situ mulch PRB processes are based on the binding of dissolved heavy metals in the groundwater to trapping materials. This concentrated metal-laden mulch barrier would eventually need to be removed from the PRB system to prevent immobilized metals from potentially being desorbed to groundwater, due to the aqueous chemistry conditions and acidity. Once the mulch is saturated with metal ions in the mulch PRB system, efforts could be made to regenerate the mulch for recovery of metal ions from the metal-laden mulch by separation technologies. However, due to the relatively cheap price of the raw adsorbent, as well as the high level of impurities (low economic feasibility), most of metal-laden mulch is transferred to a confined space or treatment location where no leachate escapes into the environment. The distribution of pollutants among specific geochemical fractions is highly dependent on the chemical properties of the individual pollutant and the characteristics of the soil chemical environment. The chemical composition of groundwater is often a complex mixture of different species.
Table 2.
As groundwater migrates through the permeable materials, the composition may be changed due to interactions with the solid matrix. Thus, the hydraulic conductivity (K) of an aquifer is of great importance for groundwater flow and contaminant transport. Prior to initiating breakthrough testing, the mulch PRB test bed was slowly filled with distilled water from an influent reservoir. In order to reach the steady-state flow conditions, the water levels and the head difference between the influent and effluent chambers were kept constant. The flowrate (Q) of influent and effluent was equal, measured as 0.379 ml/s (using the maxium pump rate at 10 rpm). As shown in Table 2, at this flowrate, the head (h) difference between the two chambers was observed as 5 cm. The cross-sectional area (A) was 80 cm2 . Accordingly, the hydraulic conductivity, K = Q/(I × A), was calculated as 0.011 cm/s. It was in the range of the empirical K value (10−4 –10−1 cm/s) for sand. In this case, the velocity of transport in sand was calculated as 4.09 m/d. Tracer tests are commonly conducted in porous media aquifers in a variety of forms [32], because the tracer does not react with organic matter or minerals. So, tracer tests in the mulch PRB test bed are the most reliable means available for determining Pb(II) transport. In order to characterize the Pb(II) transport in the sand soil, tracer tests were conducted using cresol red-thymol blue indicator solution. Twenty ml of indicator solution was injected quickly into the pinhole into the system (at the rate of 2 rpm). The brownish red colour made it easy to observe the dye’s movement. The transport of the tracer was recorded by taking highresolution digital pictures at regular time intervals. Figure 3 shows that, although some vertical transport was observed, most of the coloured tracer was dispersed and moved horizontally. It took approximately 10.5 hours to reach the mulch PRB. A continuous-flow experiment was carried out to study the mulch adsorption capacity for the lead ions and to estimate their effective lifetime. As already mentioned, the mulch barrier allows the removal of Pb(II) from the groundwater by the process of sorption. Most of lead ions are positively charged and therefore tend to adhere to negatively charged metal-binding functional groups, which eventually lead to accumulation to the mulch. It is worth noting that, since the sorption process is kinetically much faster than precipitation [33], it can be predicted that the predominant removal mechanisms for Pb(II) are attributed to selective sorption processes. Significant precipitation within the test bed may lead to operational problems due to increased
Velocity parameters under steady-state flow conditions at different pump rates.
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Figure 3.
A. Jang and P.L. Bishop
Observed tracer profiles at various time intervals.
the mulch gets saturated more quickly at higher flow rates (20 ml/min).
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pressure drop and channelling, but no pressure drop was observed within the test bed prior to Pb(II) breakthrough. Adsorption of Pb(II) by mulch was represented in the sharp S-shape of the breakthrough curves, because mass transfer rates were finite. In these curves, the concentration ratio (C/C0 ) of Pb(II) in the effluent was plotted against time. Figure 4 shows the breakthrough profile of lead adsorption for different flow rates. Results generally followed the expected metal adsorption behaviour, with greater adsorption occurring at lower flow rates (5 ml/min), meaning that
Conclusions
Groundwater is often preferably considered as one of the fresh water sources because, under most conditions, it tends to be less contaminated directly by waste products or other substances that can change water properties. Despite significant efforts, however, some groundwater aquifers have been contaminated with hazardous metals. For this reason, numerous approaches, including in situ and ex situ processes, have been suggested to decontaminate polluted subsurface waters. Conventional metal removal via soil washing can be expensive and is often appropriate only for small areas where rapid, complete decontamination is required. However, the mulch PRB system has many advantages over conventional pump-and-treat systems in that it is in situ, natural, potentially cost effective, visually unobtrusive, simple and is a low-impact plant- and soil-based treatment/infiltration technology; it offers site restoration, partial decontamination and maintenance of the biological activity and physical structure of the soil. Mulch possesses several characteristics that make it a potentially effective sorbent for the removal of dissolved metals from groundwater. In addition, the equilibrium adsorption data for hardwood mulch follows the Langmuir isotherm model, indicating that once a metal ion occupies a site, no further sorption can take place at that site. The mechanism of Pb(II)
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binding to mulch is an area of great debate and the heterogeneity of mulch makes comparing study results difficult. Mulch PRB represents a promising approach for removing hazardous metals in groundwater, due to its easy operation, low cost and fast remediation effect. Mulch PRB systems are constructed by building a subsurface permeable barrier of mulch in a trench that intercepts the groundwater flow. However, heavy metals cannot be immobilized in mulch PRB systems forever. With the variation of the physicochemical characteristics of water conditions, part of these fixed metals could release into aquatic environment again. So, in order to avoid a possible pollution of the aquatic environment, replacing technologies to remove the concentrated metal-laden mulch barrier from the PRB system should be considered.
[12]
[13]
[14] [15]
[16]
Acknowledgements This research was supported by a grant from the National Institute of Environmental Health Sciences (NIEHS) under the Superfund Basic Research Program (SBRP) Individual Research Projects (R01) (1R01ES015446-01). We would also like to thank Linxi Chen for her technical support with reactor operation.
[17]
[18]
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