• In-situ remediation of arsenic contaminated ground water using iron filings permeable walls,
• In-situ remediation of arsenic contaminated sandy soils by incorporating iron filings into the sand,
• In-situ treatment of contaminated lake and wetland sediments,
• Treatment of domestic drinking water supply, and
• Ex-situ treatment of groundwater.

Arsenic is a metalloid element that has been notorious for its toxicity. Naturally, arsenic is found as a main component of several minerals such as arsenopyrite. Throughout the ages, arsenic has been used in medicine, cosmetics industry and agriculture. It has been used as an insecticide, and it still is used as a desiccant, rodenticide, and herbicide. Industrial uses of arsenic include doping of solid state devices, laser material, bronzing etc. Traditional arsenic contaminated sites include areas of mining activities and smelters. Arsenic can be also found in coal and coal combustion by-products. Inorganic arsenic species in contaminated industrial sites exist in the arsenate (oxidation form = V), arsenite (oxidation form = III), arsenic sulfide (HAsS₂), elemental arsenic (As^o) and arsine (oxidation form = - III) gas (AsH₃) forms. Arsenate forms include H₃AsO₄, H₂AsO₄^-, HAsO₄^-2, and AsO₄^-3. Arsenite forms are the reduced inorganic arsenic species and include H₃AsO₃, H₂AsO₃^-, HAsO₃^-2, and AsO₃^-3. Elemental iron on the other hand, in the presence of aqueous solution, can be oxidized both aerobically and anaerobically providing electrons for the reduction of other redox sensitive chemical species such as arsenate and sulfate. This invention presents a new method for the immobilization of inorganic arsenic species, such as arsenates and arsenites, by iron filings. The method uses iron filings (zero valent iron) and sand to reduce inorganic arsenic species to iron co-precipitates, mixed precipitates and, in conjunction with sulfates, to arsenopyrites. Other constituents may be added to the mixture to control porosity or chemistry. The method may be employed, for example, as part of an in-situ funnel-and-gate groundwater treatment system or ex-situ as part of a groundwater extraction and treatment system (pump and treat).

Inorganic arsenic contaminated water, spiked with equivalent sulfate concentrations passes through an iron filings/sand "filter". This results in removing most of the arsenic from the solution. The chemical process that takes place is as follows: Elemental iron will oxidize to ferrous iron (Fe(II)). If there is oxygen present in solution (aerobic), it will be consumed according to the reaction:

2Fe^o + O₂ + 4H⁺ = 2Fe⁺² + 2H₂

This reaction will utilize all oxygen in solution and it will cause a temporal reduction in the pH of the solution. When the solution becomes anaerobic, iron oxidation will be coupled with the hydrolysis of the water. When this occurs, the sulfate and arsenate reduction is as follows:

Fe^o = Fe⁺² + 2e^- Iron Oxidation

Fe⁺²= Fe⁺³ + e^- Iron Oxidation

8e^- + 9H⁺ + SO₄^-2 = HS^- + 4H₂O

Sulfate reduction

2e^- + 2H⁺ = H₂(g) Hydrolysis of water

2e^- + 4H⁺ + HAsO₄^-2= H₂O + H₃AsO₃

Arsenate reduction

Further on, the products of these reactions can form precipitates that would include the formation of Fe(OH)₃, FeAsO₄, FeAsS and the like. Inorganic arsenic species could also be removed from the solution through the formation of co-precipitates, mixed precipitates and by adsorbing onto the ferric hydroxide solids. A schematic of the AsRT technology is presented in Figure 1.

Figure 1. Schematic representation of the AsRT technology

Zero valence metals have been shown in the past to effectively remediate halogenated organic compounds and selected metals such as UO₂²⁺, MoO₄^2-, TcO₄^-, and CrO₄^2-. This new technology consists of two novel features. First, this is the first time that it has been shown that zero valence metals can effectively remediate inorganic arsenic species. Second, in addition to showing that inorganic arsenic species can be remediated by creating iron-arsenic co-precipitates, we have enhanced the remediation process by introducing sulfate via barite (barium sulfate) in the solution, thus providing the opportunity for arsenopyrite precipitation to occur.

To test the viability of the new technology for the removal of arsenic from aqueous solutions, we conducted both laboratory and field experiments. The laboratory experiments were conducted to demonstrate the validity of the concept of the AsRT technology. The field experiments demonstrated the efficiency of the technology under real-life conditions. The validation steps of the technology are summarized as follows:

Table 1. AsRT Validation Tests

As can be seen in below, the AsRT technology performed as expected and has been sufficiently demonstrated.

Laboratory Experiment #1: Arsenate adsorption isotherm experiments were conducted using saturated zone sand taken from the vicinity of a landfill in Maine USA and iron filings to examine arsenate immobilization. The equilibrium arsenate removal experiment was carried out by a standard bottle point technique. The experiment was conducted as follows: 5.0 g of sand and 5.0 g of iron filings were mixed with 100 mL of 0.01 M NaNO₃ in a series of polyethylene bottles having different concentrations of arsenate. The mixtures were allowed to reach equilibrium at room temperature. The solutions of different arsenate concentrations were prepared by adding an appropriate quantity of sodium arsenate stock solution to the constant ionic strength solution. The bottles were then placed on a rotary shaker until equilibrium was reached. The supernatant was filtered through 0.45-micron nylon filter and analyzed for residual metals by GFAAS. Based on our past experience with a similar work, a contact time of seven days was deemed to be adequate to attain equilibrium. The pH was allowed to drift throughout the experiment.

To evaluate the impact of iron filings on the immobilization the arsenate alone, we conducted a control experiment that was exactly the same as the one described above, only excluding the iron filings. The initial pH of the solution was adjusted to 6.7 and was allowed to drift throughout the experiment. Figure 2 presents the results of the first experiment. The sand used was capable of removing between 50 and 94% of the arsenic in solution, the initial arsenic concentration ranged from 42 to 4299 ppb. On the other hand, the combination of sand and iron filings (1:1 by weight) removed greater than 97% of the arsenic in solution for all concentrations (45 to 8600 ppb initial concentration). In fact, for concentrations less than 5000 ppb in solution, the results indicate that the final dissolved concentration will be less than the groundwater protection criteria for arsenic, which is 50 ppb.

Figure 2. Arsenate adsorption experiment using Baker Co. iron filings and Maine sand.

Laboratory Experiment #2. This experiment is similar to experiment 1. The only variant was the type of sand used. In this experiment, we used a sand obtained from Iowa that contained calcareous material. The pH of the sand was 9.0. In this experiment, we used 10 g of sand, 10 g of iron filings and 200 mL of NaNO₃. The results are presented in Figure 3.

Figure 3. Arsenate adsorption experiment using Baker Co. iron filings and Iowa sand.

The Iowa sand was not capable of removing any amount of arsenic from the solution. The initial arsenic concentration ranged from 0 to 20,000 ppb. On the other hand, the combination of sand and iron filing (1:1 by weight) removed greater than 81% of the arsenic in solution for all concentrations (0 to 20,000ppb initial concentration). In fact, for concentrations less than 2000 ppb in solution, the results indicate that final dissolved concentration will be less than groundwater protection criteria for arsenic, which is 50 ppb.

Laboratory Experiment #3. The aim of this experiment was to evaluate the efficiency of iron filings immobilization of inorganic arsenic species under conditions that are more closely related to subsurface environments. A column experiment was conducted using iron filings and silica sand. The eluent solution contained 470 ppb of arsenate in 0.01 M NaNO₃ solution. The flow rate of the eluent was 1.0 mL/min. The results indicate that the effluent arsenate concentration did not exceed 27 ppb throughout the course of the experiment. Figure 4 presents the results of this experiment. It is important to notice that the pore volume of the column was 6 mL, which indicates that the column was flushed with more than 800 pore volumes of eluent. To compare the efficiency of iron filings removal of arsenic, we conducted a similar experiment using an uncontaminated Maine USA lake sediment. The results indicate that lake sediment column reached breakthrough within 200 pore volumes of eluent.

Figure 4. Column experiment of arsenate immobilization using Baker Co. iron filings and silica sand. Eluent solution contained 470 ppb of arsenate in 0.01 NaNO3

Laboratory Experiment #4. This experiment aimed at improving the efficiency of iron filings immobilization of inorganic arsenic species by introducing sulfate into the eluent solution and creating the opportunity for the formation and possible precipitation of arsenopyrites. The experiment was conducted using two columns in series. The first column was packed with barite (barium sulfate) and silica sand. The eluent of the first column was introduced to the second column that was packed with iron filings and silica sand. The eluent solution contained 1000 ppb of As(V) in 0.01 M NaNO₃ solution. The flow rate of the eluent was 1.0 mL/min. Sulfate was provided through the dissolution of barite from the first column. This iron filings column had about 40% less iron and sand in the column than the column in experiment #3. The results are presented in Figure 5. This experiment showed that the combination of barite and iron filings improved the removal of arsenic from the solution. The removal efficiency was about 90% for more than 1000 pore volumes of eluent. The overall removal efficiency was approximately the same as in experiment #3. However, this experiment was conducted with higher arsenic concentrations and less iron filings and sand. Additional mass of iron filings will improve the removal efficiency of arsenic.

Figure 5. Column experiment of the immobilization of arsenate with barite, iron filings, and silica sand. Eluent solution contained 1000 ppb of arsenate in 0.01 NaNO₃.

Laboratory Experiment #5. This experiment pertains to the desorption of arsenic from the iron filings. A column experiment was conducted using iron filings and silica sand. The eluent solution contained 1000 ppb of arsenite and 50 ppm of sulfate in 0.01 M NaNO₃ solution. The flow rate of the eluent was 1.0 mL/min. The results indicate that the effluent arsenite concentration increased to 70% of the incoming eluent concentration within 400 pore volumes, and it stayed constant until the eluent was changed to 0.01 M NaNO₃ solution (no arsenic) that represented the desorption eluent. The concentration of arsenic desorbing from the column dropped by 500 ppb within 30 pore volumes, and to less than 10 ppb within 700 pore volumes. Overall, we desorbed less than 12% of the total arsenic adsorbed in the column. Figure 6 presents the results of this experiment. The results indicate that arsenic on iron filings will not pose a significant long-term disposal problem.

Figure 6. Adsorption and desorption of arsenite from the AsRT technology.

The five laboratory experiments clearly demonstrate the validity of the AsRT technology. With proper design, iron filings can effectively remove both arsenate and arsenite from aqueous solutions. In addition, preliminary results indicate that arsenic will be tightly bound to the iron filings, making their disposal easier.

Field Experiment #1: To validate the AsRT technology under natural conditions, a field demonstration was conducted at an arsenic contaminated site in Maine. Figure 7 presents the concentration of arsenic coming out of the columns as a function of the pore volumes of water through the column (this relates to time; the experiment lasted 2 months). We evaluated two types of iron filings (J.T. Baker Co. and Connelley Co.) and a control (Ledyard sand). We also evaluated the amount of iron filings in the column using the Baker iron filings (Baker full column was 54 cm length and Baker Half was 27 cm length).

Figure 7. Aqueous arsenic concentrations (mg/l).

The results indicate that the Connelley iron filings removed all arsenic from the solution. The Baker columns initially removed about 60% of the arsenic and after 350 pore volumes removed all arsenic from the solution. The reason that the Baker iron filings did not remove all arsenic from the solution in the first 350 pore volumes is due to low surface area (20 times less than the Connelley one). As corrosion was taking place the Baker iron filings increased in

Figure 8. View of the AsRT filter device.

surface area and the arsenic removal improved. Figure 9 presents the accumulated concentration of arsenic on the Connelley column iron filings and sand. The results indicate that all arsenic removal occurred in the first 10 cm of the column.

Figure 9. Arsenic concentration on the Connelley column iron filings and sand (mg/g).

There are numerous sites around the world with arsenic impacted ground water, soils and wetland media that can be remediated using a variation of the demonstrated technology. For drinking water applications, a post AsRT sand filter is recommended.

These systems can be design optimized using the Heavy Metal Model, which simulates the multi-component transport of heavy metals and radionuclides in porous media.

Dr. Nikolaos P. Nikolaidis

Associate Professor

Department of Civil and Environmental Engineering

University of Connecticut

261 Glennbrook Road, U-37

Storrs, CT 06269-2037

Telephone: (860) 486-5648

Fax: (860) 486-2298

E-mail: nikos@eng2.uconn.edu

http://www.eng2.uconn.edu/cee/bio/Nikolaidis.html