NANOIRON S ACTIVITY MEASUREMENT TECHNIQUE AND SOIL ENRICHMENT POSSIBILITIES Petr BENEŠ, María de Marco RODRIGO, Pavel Mašín, Martin KUBAL Vysoká škola chemicko technologická v Praze, Technická 5, Praha 6, 166 28, e-mail: Petr.Benes@vscht.cz Summary Use of nanotechnologies is currently in the center of attention in many scientific fields. Because of lower prices of nanomaterials, one of new fields where the nanotechnologies are starting to play an important role is decontamination of polluted sites. A suspension of nanoiron was used as a decontamination medium on some sites in the Czech Republic in a pilot scale. The application seems to be very successful in means of destructing pollutants but there are still many chemical and/or technical problems with nanoiron application into a subsurface. Two important problems are enrichment possibilities of the soil and nanoiron activity that have been studied in this work. It has been proved by column tests that it is possible to enrich soil with nanoiron to obtain very active mixture for destructing present contaminants such as chlorinated ethylenes without stopping the underground water flow. It is well known that quality (activity) of nanoiron suspension varies (depending on the producer) and is decreasing with time. The main contribution of this work is designing and verifying of fast and simple technique for measuring the activity of nanoiron suspension. This technique is based on the measurement of hydrogen volume produced by reaction of zerovalent iron with sulphuric acid which can be used not only in the laboratory but also on the locality of decontamination. By this way the activity of nanoiron can be investigated simultaneously with the suspension infiltration on the field. Thanks to this new measurement technique companies carrying out decontamination processes can save a lot of money, time and technical troubles. 1. INTRODUCTION In environmental applications, nano-based iron materials are becoming to be remarkably effective tools for cleaning up contaminated soil and groundwater. Because of their small size, nano-based iron materials are much more reactive than conventional iron powders and they can be pumped straight into the contaminated site easily in form of water suspension [1]. Elemental iron itself has no published toxic effects, considering it is one of the most abundant metals on Earth. When exposed to air, elemental iron oxidizes to brick-red iron oxide. When metallic iron oxidizes in the presence of organic contaminants such as trichloroethane (TCA), trichloroethene (TCE), tetrachloroethene (PCE), or carbon tetrachloride, these organic components can be broken down into simple carbon compounds that are less or non toxic [2]. Moreover, oxidizing iron can reduce heavy metals such as lead, nickel, or mercury to an insoluble form that tends to stay fixed in the soil. Compared to microparticles, nanoscale iron-based particles have higher rates of reactivity due to their high specific surface area (see Figure 2) and more reactive surface sites. In addition, due to their ability to remain in suspension, nano-iron particles can be injected into contaminated soils, sediments, and aquifers [3,4]. But because of the aggregation of nanoiron particles it is difficult to prepare a stable suspension. It has been demonstrated that different stabilizers significantly inhibit aggregation and increase transport of the nano-iron particles. Many reports indicate that nano-iron has been accepted as a versatile tool for the remediation of groundwater, soil, and air on both the experimental and field scales.
2. THEORY Nanoscale zerovalent iron (nzvi) is an effective reagent for treatment of toxic and hazardous chemicals. As a strong reductant, nzvi can degrade a wide range of pollutants by adsorption and chemical reduction. Both organic (e.g., chlorinated hydrocarbons) and inorganic (nitrate, chromate, perchlorate, metal ions) pollutants in the environment can be treated with nzvi. Favorable chemical and structural factors contribute to the increasing environmental applications of nzvi. Chemically, zerovalent iron serves as a cost-effective and environmentally friendly reductant. Structurally, the small size of nanoparticles provides a high surface-tovolume ratio, which promotes mass transfer to and from the solid surface and increases the adsorption and reaction capacity for contaminant removal and degradation. In engineering practice, the small size offers a combined advantage of easy mixing and potential mobility in groundwater which will be mentioned in this study. Fig.1: Core-shell structure of nanoiron particle Fig.2: Surface area as a function of particle diameter It is generally accepted that nzvi has a core-shell [3] structure with a zerovalent iron core surrounded by an iron oxide/hydroxide shell (see Figure 1) which grows thicker with the progress of iron oxidation. However, it is difficult to measure the exact thickness of the shell due to the high reactivity of iron which reacts very rapidly with both oxygen and water and oxidizes in the air. The shell thickness is estimated on the basis of measurement of the zerovalent iron content which is determined by its corrosion rate and as in our case by production of hydrogen gas. The measurement technique of the zerovalent iron content is the aim of this project. The surface morphology of nzvi is shown in Figure 3. The fresh nzvi particles are generally spherical in shape with the majority of them in the size range of 50-100 nm. Some images reveal that the particles are connected in chains due to magnetic dipole interactions and chemical aggregation. 2
Fig.3: Electron microscopy image of nanoiron particles with core-shell structure visible in the right picture [5]. 2.1. Migration of nanoiron particles in porous soil environment. Mobility of nanoiron particles is one of the key steps leading to the successful decontamination of the sites [6,7,8,9]. This depends on the suspension characteristics (nanoiron size, ph and aggregate stability of the suspension), properties of soil (porosity and permeability) and ground water (flow rate, the total mineralization, etc.) [8,9]. There are a few different phenomenas that happen when nanoiron particles are transported into porous saturated environment containing DNAPL (dense non-aqueous phase liquid) layer. Each particle can aggregate and be captured on the soil grains or DNAPL molecules (see Figure 4) which leads to clogging the pores in the soil. This restricts further mobility of nanoiron suspension. In practice, it is desirable to effectively capture nanoiron particles only on the surface of DNAPL [10]. Particles of soil are Fig. 4: Model transportation suspension of nanoparticles Fe showing the filtration, adhesion of the grains of soil and attachment to the DNAPL molecule [10]. treated as active filters which significantly reduces the mobility of injected suspension of nanoiron particles. Filtration of colloidal solutions into porous environment involves three main mechanisms [8]: Brownian molecular movement and diffusion which causes irregular movement of colloidal particles. Convection movement of particles in the current of groundwater. Gravity causing vertical movement of particles. Brownian movement applies for very small particles under 100 nm and gravitational sedimentation works mainly on particles over 1000 nm [8]. Between these size limits there are areas where the colloidal particles move mostly in the direction of groundwater current and therefore Brownian movement and gravity sedimentation can be partly neglected. The optimal interval, however, cannot be considered as constant because it depends on the particle type and size distribution, used stabilizer, geological environment, ground-water flow rate, temperature etc. It is not entirely clear what the optimum dimension of nanoiron particles in terms of its ideal transport in 3
porous environment should be. Elliott and Zhang [8] have published that under typical conditions of groundwater flow velocity in the range of 0.1-10 m/day the optimal size of Fe nanoparticles in suspension is 100-200 nm. Schrick et al. [7] believe that for the passage through a layer of sand or soil the appropriate size of Fe particles is in the range of 400 500 nm. Larger particles may be easier to aggregate, have less specific surface area and are therefore less reactive. The suspension of nanoiron colloidal particles is unstable and, moreover, the particles contain zerovalent iron and oxides with a positive charge, which show great affinity to the surface of soil grains. Nanoiron particles can be attracted to the environment which contributes to the filtration and immobilization [11]. For this reason, their transport in the environment is very difficult [10]. Nanoiron particles transport can be improved by reducing their affinity to surfaces of soil grains and increasing their colloidal stability using appropriate modifiers. 2.2. Determination of zerovalent iron activity by reaction with sulphuric acid This method determining the activity of iron (zerovalent iron ratio) is based on the following reaction: Fe 0 + H 2 SO 4 FeSO 4 + H 2 (1) When the iron is in contact with the H 2 SO 4 three different reactions can happen. Each reaction appears under different conditions. The reaction (1) works best in the laboratory temperature (approx 21 C, and H 2 SO 4 concentration about 50%). By measuring the amount of produced H 2 it is possible to know the amount of zerovalent iron from the stoichiometry of the reaction (1) (with one mole of hydrogen produced one mole of iron is consumed). 3. EXPERIMENTAL 3.1. Determination of nanoiron s activity by Mašín s tube The amount of produced H 2 is measured by a volumetric U-tube connected into apparatus (Figure 5). The sample of nanoiron or nanoiron/soil mixture is inserted into reaction flask which is placed inside temperated ultrasound bath and connected to the U-tube filled with distilled water. After adding the H 2 SO 4 to the sample the reaction (1) is occurring and the H 2 is produced. The volume of H 2 is measured in the U shaped tube with scale. Hydrogen volume changes caused by pressure difference are corrected mathematically. We named this apparatus with the tube in U shape Mašín s tube because the initial idea to use measurement of hydrogen volume to get information about nanoiron s activity comes from our very talented PhD. student Pavel Mašín. 4
Fig.5: Scheme of nanoiron s activity measuring apparatus called Mašín s tube. By this technique the activity of TODA nanoiron commercial suspension used for following experiments has been measured. 3.2. Column experiments Soil definition: For the column experiments sandy soil mined in a sandpit near Libcice u Prahy has been used. Position (GPS) 50 o 11 35.97 N, 14 o 21 23.122 E. Mined in depth 0.5m - 1.5m. Sieve analysis is visible in the Figure 6. It has been proved that we are able to measure the nanoiron s activity in water suspension. The main goal of the following soil column experiment was to answer these questions: Is it possible to measure the activity of nanoiron in soil mixture? How much can be our model soil enriched by nanoiron? Is the nanoiron s activity changing during the contact with the soil? In our recent studies [12] it has been proved that it is possible to enrich our model soil by nanoiron to approx. 25g/kg by flowing nanoiron suspension with concentration about 1 g/l through the soil filled in the vertical experimental column. For the following activity tests the soil was prepared directly by mixing the soil with concentrated nanoiron suspension (c=100g/l) because the nanoiron s activity is decreasing with time and the infiltration of diluted nanoiron suspension into the soil takes a long time. 5
Table 1: Used soil specification. Fig.6: Soil sieve analysis Characteristic Value Dry matter (%) 97.73 Bulk density (kg/m 3 ) 1482 Density (kg/m 3 ) 2580 Porosity (%) 42.55 Total organic carbon (%) 0.28 Column setup: Experimental setup with prepared soil/nanoiron mixture (25g/kg) was following: The soil was filled into a vertical experimental column to the thickness of 15cm. After filling the column demineralized water (with gases removed) was flowing through the column. The speed of water flow and the concentration and activity of nanoiron in the outflowing leachate was measured. Finally, when water without nanoiron was flowing from the column, the water was stopped, soil from the column was extracted and the amount and activity of the rest of nanoiron in the soil was measured in two profiles (top and bottom). 4. RESULTS AND DISCUSSION 4.1. Results of nanoiron s activity measurements By measuring the activity of commercial nanoiron suspension TODA we got a value of approximately 20% w/w of zerovalent (active) part of the suspension. This result is in agreement with another measurement [6] and the measured activity is high enough to provide reduction power to destruct target contaminants. This suspension with defined activity was used for tests with the soil in our experimental column. 4.2. Results of column experiments The concentration of nanoiron and its activity in leachate flowing out from the experimental column set up as described in chapter 4.2 was measured. The Figure 7 shows that the iron concentration is decreasing with time (sample 4 was taken 20 min after start of the experiment, sample 9 two hours after start). But the percentage of nanoiron s active form stays almost the same all the time. This proves that nanoiron flowing through porous medium in the subsurface does not loose its activity quickly and can be transported to a significant distance in almost the same active condition as it was infiltrated. 6
1200 1000 Passive Active Active form % 100 80 Fe [mg/l] 800 600 400 60 40 Active form % 200 20 0 4 5 6 7 8 9 Sample 0 Fig.7: Concentration of active and passive form of iron in the liquid samples from the column Moreover, the result measured on the top and bottom profiles of the soil extracted from the column after the end of the experiment shows that a lot of nanoiron stays fixed in the porous media. 17 g/kg (bottom) resp. 22 g/kg (top) stays in the soil with activity of 13% (bottom) resp. 10% (top). 25 20 Passive Active Active form % 100 80 Fe [g/kg] 15 10 60 40 Active form % 5 20 0 Top Bottom 0 Fig.8: Concentration of active and passive form of iron in the soil samples (top and bottom) from the column. 7
C 2 Cl 4 + 4Fe 0 + 4H + C 2 H 4 + 4Fe 2+ + 4Cl ( 2 ) The concentration and activity of nanoiron is high enough to destroy contaminants present in usual concentrations in the contaminated subsurface and also other contaminants coming to the field with underground water. If we make a simple calculation based on the reaction (2) one kilogram of the soil enriched by 20g of nanoiron with active part of 10% w/w can theoretically clean up 15L of water contaminated with 100mg/L PCE. 5. CONCLUSION The soil enrichment experiments using different concentrations of nanoiron in the column showed that it is possible to work with a relative high concentration of nanoiron in the soil - final concentration in the soil was approx. 25 g/kg [12]. Not only the concentration of total iron but also its activity is important to describe the system studied. In this work the way of measuring the iron activity in the different nanoiron suspensions was measured. The successful method using H 2 SO 4 based on the measurement of produced hydrogen volume by the reaction between zerovalent iron and sulfuric acid gave the best and reliable results. By this method using Mašín s tube it is possible to find out the exact amount of zerovalent iron in each nanoiron suspension and also in the mixture with soil. The percentage of Fe 0 in our sample of this nanoiron TODA is almost 20% w/w which is high enough to use TODA as a decontamination nanoiron suspension for polluted soils. By investigating the nanoiron behaviour and activity changes in the column the following facts have been proved: The water going through the column filled with soil/nanoiron mixture (25g/kg) washes out part of the nanoiron but the outflowing iron concentration is decreasing with time. The percentage of active nanoiron part in the outflowing water remains almost constant (15%w/w - 19%w/w) so it can continue destroying the contaminants in the surrounding subsurface. The percentage of active nanoiron in the soil that stays in the column is high enough (10%w/w - 13%w/w) which means that the pollutants can still be destroyed by the nanoiron left in the soil. During this work a simple technique for measuring the nanoiron activity has been designed and verified. By this method it is possible to measure the Fe 0 percentage in the nanoiron suspension and nanoiron/soil mixture not only in the laboratory conditions but also on the field of application. 8
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