In this study, column experiments in the laboratory were set up to examine how the concentrations of nitrate nitrogen, nitrite nitrogen, and ammonia nitrogen changed when a nitrate-rich solution was passed through a medium comprising zero-valent iron, activated carbon, zeolite, and coarse sand. We varied the proportions of the components of the medium to determine how it influenced the nitrate removal and nitrogen fractions. Three different scenarios were used, with: (1) iron, activated carbon, and coarse sand at a ratio of 3:1:6; (2) iron, activated carbon, and zeolite at a ratio of 3:1:6; and (3) iron, activated carbon, and zeolite at a ratio of 3:3:4. The nitrate nitrogen concentration decreased from 25 mg/L to 2 mg/L in the first scenario. Removal was better when zeolite was added to the medium as most of the nitrate nitrogen broke down to ammonia nitrogen, with nitrite nitrogen as an intermediate product. The results of the tests showed that nitrate removal was best when the medium was iron, activated carbon, and zeolite, mixed at a ratio of 3:1:6. This study provides a scientific reference for in situ remediation of nitrate pollution in groundwater.
Nitrate-contaminated groundwater is harmful to human health (Ganesan et al. 2013; Goudarzi et al. 2017). Methods such as ion exchange, biological deoxygenation, permeable reactive barriers (PRB), reverse osmosis, and electrodialysis have been used to reduce the nitrate concentrations in groundwater (Lu et al. 2014; Ghaeminia & Mokhtarani 2018; Peng et al. 2018). Of these, PRB, considered environmentally friendly and non-intrusive, have become an increasingly popular method for in situ remediation of groundwater in recent years.
The reaction media used in in situ remediation include sepiolite, activated carbon, zero-valent iron (ZVI), zeolite, bimetallic materials, solid carbon, and denitrifying bacteria. The main types of ZVI used include nanometre iron and sponge iron. Shi et al. (2016) found that nanometre iron performed well when removing nitrate nitrogen from water. However, applications of nanometre iron are limited because the particles are very small, they oxidize easily and aggregate in water, and are influenced by the pH (Shi et al. 2013). An et al. (2013) reported that ammonia nitrogen accounted for between 74.3% and 98.6% of the amount of NO3-N removed, so ammonia nitrogen was the main by-product of the reaction between sponge iron and NO3-N. Because nanometre iron is small and aggregates easily in water, the specific surface area, reactions, and fluidity all decrease. Nanometre iron oxidizes easily and the surface passivation layer is not conducive to reducing pollutants (Babaei et al. 2015; Jiang et al. 2015). And the physical and chemical properties of the particles' surfaces need to be modified, so that stable uniform particles are dispersed in the system.
ZVI is used instead of either nanometre or sponge iron to remove nitrate from groundwater because it is not expensive and is easy to use. Many recent studies have reported how ZVI has been used to remediate nitrate pollution (Suzuki et al. 2012; Fu et al. 2014). Tang et al. (2012), from their studies of PRB, found that as well as producing nitrate nitrogen, reactions could occur in alkaline conditions with ammonia nitrogen as the main product, and the corroding products of Fe2+ and Fe3+ strengthened the denitrification. Zeolite can be used to reduce the ammonia nitrogen concentrations. Natural zeolite is a kind of porous silicate mineral with a frame structure. Thanks to its well-developed pores, channel-like structure, and exchangeable cations, the physical adsorption and cation exchange capacities of zeolite are high (Gennaro et al. 2016). Given its limited ability to absorb ammonia and nitrogen, many researchers have tried to improve the adsorption capacity of natural zeolite. Natural zeolite can be activated by roasting at a high temperature, treating with acid (alkali), or immersing in a salt solution and baking (Soh et al. 2017). Once activated, impurities, such as organic matter, can be removed and retained in the zeolite pores or in the channel network, and the adsorption performance of zeolite is improved (Arzumanov et al. 2014). Therefore, nitrate nitrogen, nitrite nitrogen, and ammonia nitrogen, pH, the overall nitrogen balance, and soluble iron in nitrate-rich water were monitored to facilitate the detection of changes that occurred when activated zeolite was added to the test medium. Changes in the dielectric permittivity of the medium when the activated zeolite was added were also explored.
The above discussion of recent literature shows that different media can influence the removal of nitrate, and that ammonia nitrogen is frequently produced when nitrate is removed with ZVI. This study demonstrates how the performance of PRB technology was improved using ZVI; the nitrate-removal ability of a medium comprising ZVI, activated carbon, coarse sand and activated zeolite, combined at different ratios, was investigated.
MATERIALS AND METHODS
The main test unit included a Markov bottle with a capacity of 18 L, an acrylic circular tube (10 cm inner diameter, length 100 cm), and a total of four horizontal sampling tubes, spaced 20 cm apart along the acrylic circular tube (Figure 1). These four tubes were used for the sampling and also to calculate the permeability coefficient. A glass tube with an inner diameter of 5 mm was placed in the Markov bottle, and the glass tube was inserted below the liquid level in the bottle to keep the head difference constant. The constant flow rate of 0.005 m3/d was controlled by a flow meter. The bottom of the pipe was supported by a porous plate and a nylon mesh. A water buffer of about 5 cm was reserved to prevent water from washing the reaction material directly.
The test materials were granular activated carbon, iron powder, homogeneous coarse sand, and activated zeolite. The particles of the granular activated carbon were between 1 and 2 mm; the iron, pre-treated with water, had an average diameter of 40 mesh, specific surface area of 30–35 m2/g as reported by the manufacturer (see also http://www.bjyxch.cn/); the homogeneous uncontaminated coarse sand (from Hefei Huaguang Quartz Sand Company) was between 0.25 and 2 mm with measured bulk density of 1.27 g/cm3 and porosity n = 0.34, and the particles of zeolite were between 0.5 and 1 mm.
Three sets of test protocols were set up to compare and examine the ability of zeolite to remove ammonia nitrogen during the test. Other studies have shown that ZVI has an important role in the removal of nitrate, and that activated carbon can accelerate the removal of nitrate. Coarse sand can prevent clumping of the iron powder, maintain good hydraulic conditions, and extend the useful life of PRB. The medium in scenario I was iron, activated carbon, and coarse sand, mixed at a volume ratio of 3:1:6. After receiving the test results of the first scenario, the medium was changed, and the coarse sand was replaced with zeolite so the excess ammonia nitrogen was adsorbed, and called scenario II. The volume ratio remained unchanged, with iron, activated carbon, and zeolite at a ratio of 3:1:6. To look more closely at the role of activated carbon in the reaction process, the ratio of the activated carbon in scenario II was adjusted, so that the ratio of iron, activated carbon, and zeolite was 3:3:4, and this was called scenario III. The hydraulic conductivity of the mixed media of scenarios I, II, and III is 298 m/d, 287 m/d and 234 m/d, respectively; the corresponding porosity is 0.30, 0.27 and 0.22, respectively.
Different nations have different threshold concentrations for NO3-N. For example, the World Health Organization drinking water quality standards state that the maximum allowable NO3-N concentration is 10 mg/L, while the thresholds for the United States and China are 11.3 and 20 mg/L, respectively. In this test, because the groundwater is polluted by nitrate, the initial concentration should be greater than 20 mg/L, and we set it at 25 mg/L. The test was conducted at an environmental temperature of 22 °C–24 °C.
Iron powder and sand were pre-treated by washing with water. The sand was washed to remove some impurities and the iron powder was washed to increase the activity and speed of the reaction.
The test columns were set up. Depending on the required ratio of the test materials, the materials were evenly mixed and filled into the test columns. The materials had the same dry density and were loaded into the columns in layers. The air was discharged from the dielectric materials as the columns became gradually saturated. The experiment was set up so that a saturated flow state could be maintained.
Nitrate solution with a concentration of 25 mg/L was prepared with KNO3 and distilled water.
The prepared nitrate solution was put into the Markov bottle, and was supplied to the reaction columns from the Markov bottle.
When the outflow stabilized, samples were collected once every day at the outlet. The test was stopped when the total nitrogen concentrations stabilized.
The samples were tested spectrophotometrically. The concentrations of nitrate nitrogen, nitrite nitrogen, and ammonia nitrogen were determined by UV spectrophotometry, double nitrogen coupled spectrophotometry, and Nessler reagent spectrophotometry.
RESULTS AND DISCUSSION
Changes in the concentrations of the different nitrogen forms
When the ratio of the medium (iron, activated carbon, and sand) was 3:1:6, the concentrations of NO3-N, NO2-N, and NH3-N changed as shown in Figure 2. After 1 day, the NO3-N concentrations had rapidly dropped to 1.5 mg/L, after which, because of the reaction rate, the concentration stabilized and fluctuated between 1.5 and 2 mg/L. The concentration reduction represented a removal rate of 95%. The concentration of the intermediate product of the reaction, NO2-N, remained low, and was below 0.1 mg/L. As the experiment progressed, the NH3-N concentration increased, and stabilized after 6 days at around 22 mg/L, and about 95% of the NO3-N was converted to NH3-N. The concentration was higher than the safety limit, which indicates that, for this medium and at this ratio, ammonia nitrogen was the main product from the nitrate reaction. Therefore, when iron, activated carbon, and coarse sand were used as the reaction materials, the nitrate concentration decreased, but ammonia nitrogen was generated at high concentrations, which is not ideal. As reported by Suzuki et al. (2012), who analyzed the crystal structure of the zeolite, the volume of the holes in the zeolite crystal structure can account for more than 50% of the total volume of the zeolite crystals. The zeolite pores are uniformly distributed but have a small pore size. The inner surface of the zeolite crystals is greater than the inner surface of the average particle; solid particles generally have a surface area of only a few square metres per gram, but the inner surface area per gram of zeolite can reach more than 1,000 square metres, so the adsorption of zeolite is particularly large, and the zeolite can effectively adsorb ammonia nitrogen. Because of these properties, zeolite was used instead of sand in the second scenario, but the iron, activated carbon, and zeolite ratio was maintained at 3:1:6. The changes in the concentrations of NO3-N, NO2-N, and NH3-N in scenario II are shown in Figure 2. The concentration of NO3-N remained at around 2.0 mg/L for the 30 days of the test, representing a removal rate of more than 90%. The concentration of NH3-N was less than 0.5 mg/L, and was lower than the safety limit. Once the zeolite became saturated, the concentration of NH3-N at the outlet increased rapidly, and finally stabilized at around 20 mg/L, and accounted for about 93% of the decrease in the NO3-N concentration. The results show that when iron powder is the main component of the medium for removing nitrates from groundwater, ammonia nitrogen is the main by-product, and NO2-N is produced at low concentrations as an intermediate product (around 0.2 mg/L). When the iron, activated carbon, and zeolite were used at a ratio of 3:1:6, the NO3-N removal rate was over 90%, and the ammonia nitrogen concentration was lower than the safety limit for 30 days. This shows therefore that when the medium comprises iron, activated carbon and zeolite at a ratio of 3:1:6, the nitrate concentrations can be maintained within the required standards for 30 days under laboratory conditions.
Activated carbon has a large specific surface area, which means that it can improve the permeability of the reaction column and promote nitrate reactions. Therefore, the ratio of iron, activated carbon, and zeolite in the medium in scenario II was adjusted to 3:3:4 (scenario III). The changes in the concentrations of NO3-N, NO2-N, and NH3-N in scenario III are shown in Figure 2. After 25 days, the NO3-N concentration was 1.5 mg/L, representing a decrease of about 98%, which was higher than the concentration decrease for scenarios I and II. When the medium in the column was ZVI, activated carbon, and zeolite, the activated carbon and iron powder formed many tiny battery systems during the nitrate reaction process that accelerated the electron transfer, promoted the reaction, and improved the reaction efficiency. However, because the volume of the zeolite was lower than in scenario II, the adsorption was lower and the time to saturation was less, so that the concentration of NH3-N was significantly higher on day 35 than earlier in the experiment, and finally stabilized at about 20 mg/L.
Total nitrogen concentrations
The changes in the total nitrogen concentrations over time were plotted to facilitate further analysis of the transformations and time taken to achieve equilibrium between NO3-N, NH3-N, and NO2-N during the reaction (Figure 3). The concentration curves in Figure 3 show that the total nitrogen concentrations decreased rapidly at the beginning of scenarios I, II and III, and then increased, and gradually stabilized. The changes in the total nitrogen concentrations mainly reflect the adsorption of NO3-N or NH4+-N, which was facilitated by the activated carbon. The iron powder and the compounds produced in the iron powder reactions, Fe3O4 and Fe2O3, will also affect the adsorption of NO3-N and NH4-N. There was no gaseous escape of nitrogen from the experimental set-up.
The difference observed between scenarios I and II reflects the difference in the medium. The coarse sand used in scenario I had almost no adsorption capacity, so the total nitrogen concentrations began to increase rapidly on day 2 of the experiment. Because the zeolite had not reached its saturation capacity, the total nitrogen concentration in scenario III first decreased, and then changed gradually from day 2 until day 30, after which the total nitrogen concentrations increased rapidly when the zeolite became saturated. This shows that the removal efficiency of the medium is directed controlled by the zeolite adsorption capacity. The total nitrogen and NH3-N concentrations in the effluent of scenarios II and III followed similar patterns (Figure 3); the NO3-N concentration was relatively stable during the experiment, and NO2-N made a very small contribution to the overall total nitrogen content. In the later stage of the reaction, the NH3-N concentrations increased rapidly as the zeolite became saturated. The three main forms of nitrogen basically reached equilibrium during the test period, which once again demonstrates that nitrogen was in equilibrium when nitrate nitrogen was being removed by the industrial iron powder. Because there was more zeolite in scenario II than in scenario III, the sum of the concentrations of the main nitrogen components was lower in scenario II than in scenario III from day 10 until day 35, which highlights the large contribution of ammonia nitrogen to the total nitrogen balance.
Changes in the pH
The remediation of nitrate in groundwater using iron powder as a reducing agent is mainly controlled by the pH. The pH in scenario I had stabilized at about 8.5 by the end of the test period. Scenarios II and III consumed more H+, and the pH gradually stabilized at about 10.
The hydraulic conductivity K is frequently used to characterize the permeability of rock and soil, and is calculated with a fixed water head test. According to Darcy's Law, before and after the tests, seepage tests were carried out in the experimental device with a one-dimensional steady flow water supply.
The permeability coefficient of the media for the three different tests decreased dramatically after the test. The permeability coefficient of scenarios I, II, and III decreased from 298 to 73 m/d, from 287 to 68 m/d, and from 234 to 59 m/d, respectively. In scenario II, the coarse sand was replaced by zeolite. Zeolite was more rounded and absorbent, and had a longer operating time, so the results of the hydraulic conductivity test for scenario II were lower than those for scenario I. The lower permeability coefficient in scenario III corresponds with the lower volume of zeolite used.
The permeability coefficient can reflect the permeability of a geotechnical soil or medium, and the permeability coefficient of the medium was lower after the test. Insoluble in water, Fe3O4 and Fe2O3 block the device, and impede the progress of the reaction so that the removal rate gradually decreases. This problem must be considered when promoting and applying ZVI to reduce nitrate in groundwater.
In this study, iron powder, activated carbon, zeolite, and coarse sand were used in the reaction medium. Using a laboratory column experiment, three tests were carried out with the medium components at different ratios to treat the nitrate pollution. In our tests, the NO3-N concentration decreased from 25 to 2 mg/L when iron powder was the main component of the medium. Most of the nitrate nitrogen was transformed into ammonia nitrogen, nitrite nitrogen was the intermediate product, and the treatment effect decreased gradually as the reaction time increased. Zeolite is a good medium for treating ammonia nitrogen. Therefore, so that the concentrations of NO3-N, NO2-N, and NH3-N can be maintained within the quality thresholds for groundwater quality, we suggest using the medium tested in scenario II, with iron, activated carbon, and zeolite at a ratio of 3:1:6.
The results from this study will be useful when selecting the ratio of the components for a medium to remediate nitrate pollution in groundwater by PRB technology. However, because Fe3O4 and Fe2O3, which are insoluble in water, were generated in the reaction, the device blocked, which caused a decrease in the permeability coefficient of the medium. And as this is a laboratory-scale study, there may need to be added more data about the long-term effects of using the materials (e.g. ion exchange and biological denitrification) at the full scale. These two problems will be explored in the next step of our research.
This work was supported in part by the National Natural Science Foundation of China Grants 51509064 and 51309071. Data used to produce this paper are available on contact with the corresponding author.