REMOVAL OF ARSENIC, CHROMIUM AND LEAD FROM SIMULATED GROUNDWATER WITH REACTIVE NANOSCALE IRON PARTICLES

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1 REMOVAL OF ARSENIC, CHROMIUM AND LEAD FROM SIMULATED GROUNDWATER WITH REACTIVE NANOSCALE IRON PARTICLES Kenji Okinaka (Toda Kogyo Corporation, Yamaguchi, Japan) Andreas D. Jazdanian (BioManagement Services, Inc., Crown Point, Indiana, U.S.A.) Junichi Nakano (Toda America Incorporated, Schaumburg, Illinois, U.S.A.) Koji Kakuya (Toda Kogyo Corporation, Yamaguchi, Japan) Tomoko Okita (Toda Kogyo Corporation, Hiroshima, Japan) ABSTRACT: This study investigated the removal of trivalent arsenic (As(III)), hexavalent chromium (Cr(VI)), and divalent lead (Pb(II)) from simulated groundwater using Reactive Nanoscale Iron Particles (RNIP). A series of static batch experiments were conducted with varying ionic strength, alkalinity, RNIP, As(III), Cr(VI) and Pb(II) concentrations. The simulated groundwater was prepared with sodium sulfate (Na 2 SO 4 ) and sodium bicarbonate (NaHCO 3 ). The removal rates (k obs ) for As(III), Cr(VI) and Pb(II) were determined. The X-ray diffraction (XRD) pattern of RNIP reacted with As-, Cr-, and Pb-containing solutions were recorded. The average particle size of RNIP was 70 nm (0.07 µm). The average reactive surface area of RNIP was 28.8 m 2 /g. The removal of As(III) and Cr(VI) from solution increased with increasing RNIP concentration. Ionic strength did not have an influence on sorption of Cr(VI) for the investigated RNIP and Cr(VI) concentration range. At low RNIP concentrations (0.5 and 1 g/l), the removal of As(III) from solution decreased with increasing ionic strength. Higher RNIP concentrations provided a multitude of sorption sites that obviated a noticeable competition of arsenite oxyanions with sulfate and bicarbonate anions for sorption sites. The reaction of RNIP with water raised the solution ph thereby diminishing the Pbsolubility. Low-temperature As-containing phases were not identified with XRD methods. Peak patterns for Pb-phases were observed for RNIP reacted with a lead-nitrate solution. However, distinct patterns for Pb(NO 3 ) 2 or PbFe 2 O 4 were not identified. The presence of chromite (FeCr 2 O 4 ) in the XRD pattern of RNIP reacted with a potassiumchromate (K 2 Cr 2 O 4 ) solution confirmed the reduction of Cr(VI) to Cr(III). INTRODUCTION Arsenic chemistry in the environment involves redox transformations, precipitation reactions and adsorption to mineral surfaces. The dominant oxidation states of arsenic (As) in aqueous environments are As(III) and As(V). In oxidizing aqueous environments, As(V) is favored over As(III) and exists as arsenic acids (H 3 AsO 4, H 2 AsO 4 -, HAsO 4 2- ) and as the arsenate oxyanion (AsO 4 3- ). Under reducing conditions, the As(III) species present in aqueous environments are arsenious acids (H 3 AsO 3, H 2 AsO 3 - and HAsO 3 2- ) and the arsenite oxyanion (AsO 3 3- ), depending on solution ph. The As(III) species are more toxic and have a greater mobility in the environment than the As(V) species. The dominant oxidation states of chromium (Cr) in natural waters are Cr(III) and Cr(VI). The Cr(VI) species occurs as the mobile anion chromate (CrO 4 2- ). Cr(III) species form an insoluble hydroxide mineral (Cr(OH) 3 ). Cr(VI) adsorbs to soil less strongly than

2 Cr(III). Cr(VI) minerals are more soluble than Cr(III) minerals in aquatic environments. For these reasons, Cr(VI) species are more toxic than Cr(III) species. Adsorption to minerals and soil organic matter and precipitation are the primary mechanisms that remove lead (Pb) from groundwater. The dominant Pb species in dilute solutions up to a ph of about ph 7 is Pb 2+. In alkaline solutions (ph >7), PbCO 3 and PbOH + are the dominant species. Above ph 10, insoluble Pb(OH) 2 is formed (Moore and 2- Ramamoorthy, 1983). A significant amount of PbSO 4 forms at SO 4 concentrations greater about 10-3 mol/l (Deutsch, 1997). Lead strongly sorbs to Fe-Mn oxides and soil organic matter. In this study the removal of As, Cr and Pb from aqueous solutions was investigated as a function of ionic strength, alkalinity, RNIP, As(III), Cr(VI) and Pb(II) concentrations. MATERIALS AND METHODS An aqueous suspension of Reactive Nanoscale Iron Particles (RNIP) was produced using the method described in the European and United States patents by Uegami et al. (2003). Arsenic (As), chromium (Cr) and lead (Pb) standard solutions with 1,000 mg/l As, Cr and Pb, respectively, were obtained and used as received. The standard solutions were made with arsenite (As 2 O 3 ), potassium-chromate (K 2 Cr 2 O 7 ) and lead-oxide (PbO) dissolved in hydrochloric (0.1 M) and nitric acid solutions (0.1 M). Further, sodium sulfate (Na 2 SO 4 ) and sodium bicarbonate (NaHCO 3 ) were obtained in 95 % purity, and were used as received. The sulfur content of RNIP was determined with an element analyzer by complete combustion and thermal conductivity detection. X-ray powder diffraction (XRD) patterns were recorded with a diffractometer using a monochromatic CuKα radiation in the angular range from 10º to 90º (2θ). The scanning electron microscope (SEM) analysis was performed with an operating voltage of 10 kv. The specific surface area was determined with the BET gas adsorption method using nitrogen gas. Stock solutions with ionic strength of 0.05 M and 0.1 M were prepared with NaHCO 3 and Na 2 SO 4. Stock solution I with 0.05 M ionic strength was prepared with 2.0 mm NaHCO 3 and 16.0 mm Na 2 SO 4. Stock solution II with 0.1 M ionic strength was prepared with 8.0 mm NaHCO 3 and 30.7 mm Na 2 SO 4. The total dissolved solids (TDS) concentration of stock solution I and II was calculated to be 2,440 mg/l and 5,027 mg/l, respectively, based on the concentration of the dissolved ionic species (HCO 3 -, SO 4 2- and Na + ). The ph of the stock solutions was measured with a calibrated ph meter and was ph 8.1 ± 0.05 and 8.4 ± 0.05 for stock solutions I and II, respectively. The CaCO 3 alkalinity of stock solution I and II was calculated to be about 100 mg/l and 400 mg/l, respectively, based on the solution ph and concentration of bicarbonate (Deutsch, 1997). A series of batch experiments were prepared with deionized water (DIW), solutions I and II (Sol I and II), RNIP, As, Cr or Pb standard solution. The experiments were prepared by first adding dry RNIP, followed by DIW or Sol I or Sol II, followed by standard solution to the reaction bottles. The initial ph of the solution was determined with a calibrated ph-meter. The 68 ml reaction bottles were crimp-sealed with PTFE/rubber septa. All experiments were static (no shaking) and were kept in the isothermal room (24 ºC). The total volume of liquids in the reaction bottles was 30 ml. The RNIP concentrations ranged from 0.5 to 12 g/l. The initial aqueous (aq.) 2

3 concentrations of As(III), Cr(VI), and Pb(II) are presented in Tables 1 through 3. The experiments were sampled after seven different reaction times by withdrawing a maximum of 100 µl supernatant solution for analysis with ICP. The experiments were terminated at reaction times ranging from 192 h to 504 h. At the end of the experiment the solution was filtered with a glass fiber filter (0.6 µm pore size) using an aspirator pump. The final solution concentrations were determined with an ICP. The detection limit for As, Cr and Pb was mg/l. The final solution ph was determined using a calibrated ph-meter. The temporal concentration data points (T 1 through T 7 ) were used to generate semi-logarithmic plots. The removal rates (k obs ) were derived by calculating the slope of linear regressions. The formation of As, Cr and Pb-phases was investigated by preparing batch tests with RNIP : As, RNIP : Cr and RNIP : Pb mole ratios of 1:1. These tests were prepared with arsenite, lead-nitrate (Pb(NO 3 ) 2 ) and potassium-chromate solutions. The reaction times for these tests were 14 days with As-solution, 21 days with the Cr-solution and 35 days with the Pb-solution. The reacted RNIP was dried and XRD patterns were recorded with a diffractometer using a monochromatic CuKα radiation in the angular range from 10º to 90º (2θ). RESULTS AND DISCUSSION Toda Kogyo s Reactive Nanoscale Iron Particles (RNIP) consisted of an elemental iron core (α-fe) and a magnetite shell (Fe 3 O 4 ) determined with x-ray diffraction methods. The approximate composition of RNIP is 50 wt.% α-fe core and 50 wt.% Fe 3 O 4. The density of the aqueous RNIP suspension was 1.27 g/ml, and its solids concentration was 25.6 wt.%. The average particle size was determined with a scanning electron microscope (SEM) and was 70 nm (0.07 µm). The average BET surface area of RNIP was 28.8 m 2 /g. RNIP s sulfur content was 4,580 mg/kg. The sulfur originates from the ferrous sulfate starting material that is used for the production of RNIP. The investigated RNIP masses ranged from 0.15 g to 0.36 g (for 0.5 to 12 g/l), which is equivalent to total reactive surface areas ranging from 0.43 m 2 to m 2 (about 4.7 ft 2 to ft 2 ). ARSENIC REMOVAL In general, the removal of As(III) from solution increased with increasing RNIP concentration. The results of the removal of As(III) from solution as a function of RNIP concentration and solution composition are presented in Table 1. The initial solution ph ranged from ph 5.09 to ph 5.25, ph 7.85 to ph 8.23, and ph 8.16 to ph 8.73 for the tested RNIP and As(III) concentration ranges for tests with DIW, Sol. I and Sol. II, respectively. The final solution ph ranged from ph 7 to ph 9.57, ph 8.34 to ph 11.06, and ph 8.53 to ph for the tested RNIP and As(III) concentration ranges for tests with DIW, Sol. I and Sol. II, respectively. The increase in solution ph is due to the reactions of RNIP with water. The reaction of zero-valent iron with water yields hydrogen gas and hydroxyl ions (see Eq. 1). Fe H 2 O Fe 2+ + H 2 +2 OH - (1) For the experiments with low RNIP concentration (0.5 and 1 g/l), the removal of As(III) from solution decreased with increasing ionic strength. For experiments with 3

4 RNIP concentrations greater 1 g/l, the solution composition had a negligible effect on As(III) removal (see Table 1). The removal rates ranged from to h -1 for the investigated As(III) and RNIP concentration ranges (data not shown). The removal rate (k obs ) generally increased with increasing RNIP concentration, and decreased with increasing As(III) concentration. At low RNIP concentrations (0.5 and 1 g/l), k obs decreased with increasing ionic strength. At RNIP concentrations greater 1 g/l, k obs seemed more or less unaffected by ionic strength. The influence of ionic strength on the removal of As(III) is due to the competition of the arsenite oxyanion (AsO 3-3 ) with the anions sulfate (SO 2-4 ) and bicarbonate (HCO - 3 ) for sorption sites on the RNIP surfaces (Su and Puls, 2003; Jazdanian et. al, 2004). At RNIP concentrations greater 1 g/l, a plethora of sorption sites are available resulting in a negligible effect of anion activity on As(III) sorption (see Table 1). As containing phases were not identified with XRD methods. TABLE 1. Arsenic Removal as a Function of RNIP Concentration and Solution Composition Concentration Final As Concentration As Sorption As(III) RNIP DIW Sol. I Sol. II DIW Sol. I Sol. II (mg/l) (g/l) (mg/l) (mg/g) < CHROMIUM REMOVAL In general, the removal of Cr(VI) from solution increased with increasing RNIP concentration. The results of the removal of Cr(VI) from solution as a function of RNIP concentration and solution composition are presented in Table 2. The initial solution ph ranged from ph 3.35 to ph 4.09, ph 7.19 to ph 8.1, and ph 7.79 to ph 8.56 for the tested RNIP and Cr(VI) concentration ranges for tests with DIW, Sol. I and Sol. II, respectively. The final solution ph ranged from ph 9.46 to ph 10.92, ph 9.02 to ph 10.9, and ph 8.28 to ph for the tested RNIP and Cr(VI) concentration ranges for tests with DIW, Sol. I and Sol. II, respectively. The ionic strength had an insignificant effect on the sorption of Cr(VI) by the investigated RNIP concentration range (see Table 2). The removal rates ranged from to h -1 for the investigated Cr(VI) and RNIP concentration ranges (data not shown). The removal rate (k obs ) generally increased with increasing RNIP concentration and decreased with increasing Cr(VI) concentration. The XRD pattern of RNIP reacted with a potassium-chromate solution showed the formation of chromite (FeCr 2 O 4 ). The reduction of Cr(VI) to Cr(III) was evidenced by the formation of chromite. The uptake of Cr(VI) may be limited by the rate of chromite formation on RNIP surfaces. The chromite formation rate depends on the corrosion rate of RNIP that provides the ferrous iron (Fe 2+ ) for chromite. In solutions with high Cr(VI) concentration these rate limiting steps may 4

5 slow the removal rate of Cr(VI) from solution. At high initial Cr(VI) concentration (50 mg/l), the removal rate declined with increasing ionic strength. Cr oxyanions may be competing with sulfate and carbonate for Fe 2+ species. TABLE 2. Chromium Removal as a Function of RNIP Concentration and Solution Composition Concentration Final Cr Concentration Cr Sorption Cr(VI) RNIP DIW Sol. I Sol. II DIW Sol. I Sol. II (mg/l) (g/l) (mg/l) (mg/g) < < LEAD REMOVAL The results of the removal of Pb(II) from solution as a function of RNIP concentration and solution composition are presented in Table 3. The initial solution ph ranged from ph 3.01 to ph 3.26, and from ph 7.43 to ph 8.22 for the tested RNIP and Pb concentration ranges for tests with DIW and Sol. II, respectively. The final solution ph ranged from ph 9.2 to ph 9.31 and from ph 8.43 to ph 9.06 for the tested RNIP and Pb concentration ranges for tests with DIW and Sol. II, respectively. The XRD pattern of RNIP reacted with lead-nitrate solution showed a peak pattern that could either be identified as a lead containing ferrite (PbFe 2 O 4 ), or Pb(NO 3 ) 2, or both. A distinction between the two phases was not possible due to the generally low intensity of reflections, and in particular the very low intensity of lower order reflections. In addition, the distinct peak pattern of the iron oxyhydroxide (FeO(OH)), lepidocrocite, was identified. TABLE 3. Lead Removal as a Function of RNIP Concentration and Solution Composition Concentration Final Pb Concentration Pb Sorption Pb(II) RNIP DIW Sol. I Sol. II DIW Sol. I Sol. II (mg/l) (g/l) (mg/l) (mg/g) < < The reaction of RNIP with water raised the solution ph (see Eq. 1). The increase in ph diminished the Pb solubility. Therefore, Pb removal due to precipitation could not be differentiated from Pb removal due to sorption. The removal rates of selected experiments with As(III), Cr(VI) and Pb(II) containing solutions are presented in Table 4. The Pb experiments with the lower RNIP concentration than the As and Cr experiments had removal rates that were about 4 to 20 times greater than those for the As and Cr experiments. The higher removal rates seem to be indicative of diminished Pb solubility. 5

6 TABLE 4. Removal Rates (k obs ) for As(III), Cr(VI) and Pb(II) RNIP DIW Sol. 1 Sol. 2 As(III), 5 mg/l 2 g/l Cr(VI), 10 mg/l 2 g/l Pb(II), 5 mg/l 0.5 g/l Notes: Unit for k obs is h -1. CONCLUSIONS The removal of As from aqueous solutions with ionic strength and alkalinity similar to natural groundwater is primarily controlled by the adsorption of As oxyanions to RNIP or Fe-phases (e.g. lepidocrocite) that form as a result of RNIP corrosion. RNIP reduces Cr(VI) to Cr(III) which adsorbs to RNIP surfaces and also forms chromite. The reactions of RNIP with water result in physico-chemical conditions (increased ph and decreased oxidation-reduction potential) that diminish Pb solubility. Pb removal rates indicate rapid adsorption/precipitation. The occurrence of Pb and Cr containing Fe-phases as well as Fe-oxyhydroxides (e.g., lepidocrocite) evidences both the corrosion of RNIP and fixation of metals (including Fe) in solid phases. REFERENCES Deutsch, W.J Groundwater Geochemistry. Fundamentals and Application to Contamination. CRC Press, Boca Raton, FL, 221 p. Jazdanian, A.D., C.O. Stevens, J. Tang, S.R. Munsch, K.L. Lowe, J.J. Kilbane In- Situ Treatment of Arsenic Impacted Groundwater Containing Elevated Orthophosphate Concentrations. Proceedings of the Fourth International Conference on Remediation of Chlorinated and Recalcitrant Compounds, Monterey, California Moore, J.W. and S. Ramamoorthy Heavy Metals in Natural Waters. Springer- Verlag, New York, 268 p. Su, C., and R.W. Puls In Situ Remediation of Arsenic in Simulated Groundwater Using Zerovalent Iron: Laboratory Column Tests on Combined Effects of Phosphate and Silicate. Environmental Science and Technology 37: Uegami et al Iron Particles for Purifying Contaminated Soil or Ground Water, Process for Producing the Iron Particles, Purifying Agent Comprising the Iron Particles, Process for Producing the Purifying Agent and Method of Purifying Contaminated Soil or Ground Water U.S. Patent: US2003/ A1 Uegami et al Iron Particles for Purifying Contaminated Soil or Ground Water. European Patent Application: EP A2 6