Comparison of sorption capacities for recovery of U (VI) from aqueous solutions

Comparison of Sorption Capacities

for Recovery of U(VI) from Aqueous

Solutions by Akaganeite

SABRİYE YUŞAN, Sema AKYIL

Ege University

Institute of Nuclear Sciences 35100 Bornova-Izmir/TURKEY

In this study removal of U(VI) ions from aqueous solutions was investigated using synthetic akaganeite-type nanocrystals. Nanocrystals of iron oxihydroxides were synthesized with three different methods and then compared their adsorption capacities.

The product materials were examined by powder X-ray diffraction for crystalline phase identification, scanning electron microscope (SEM) and Brunauer-Emmett-Teller (BET) .

Uranium is a toxic heavy metal arising from the nuclear industry as well as from anthropogenic activities. It is usually found in the environment in the hexavalent form, as the mobile, aqueous uranyl ion, UO₂+2.

Various techniques are employed for the removal of uranium ions from wastewaters and radioactive wastes. Precipitation, membrane processes, ion exchange, solvent extraction and adsorption are the most common used methods.

Iron oxides and oxyhydroxides are of technological importance as catalysts, sorbents, pigments, flocculents, coating, gas sensors and size-selective anion-exchange materials. Metal oxides or hydroxides such as iron, aluminum and manganese play a very important role in the sorption of contaminants in wastewater systems. Iron – based adsorbents are usefull due to their economic and safety merits. Iron-oxyhydroxide is known to occur commonly as goethite (-FeOOH), akaganeite (-FeOOH), and lepidocrocite (-FeOOH). These minerals have high sorption capacities for metal and anionic contaminats such as arsenic, chromium, lead, mercury, selenium, phosphate and uranium.

Akaganeite (-FeOOH), a natural product of the corrosion of iron in chloride-containing environments, has a tetragonal structure consisting of double chains of edge shared octahedral that share corners with adjacent chains to form channels running parallel to the c- axis. Among the iron compounds, the iron oxyhydroxide phase akaganeite, -type FeOOH, has a large tunnel-type structure where iron atoms are strongly bonded to framework. This tunnel structure makes  -FeOOH an especially interesting material in the areas of catalysis and ion exchange.

Synthesis of adsorbents Synthesis of Akaganeite (I)

0.1 M FeCl₃.6H₂O +

NaOH

Strirring at pH 10 for 1 hour Centrifugation

Washing with deionized water +

Washing with absolute ethanol

Centrifugation Dried in air 0.506 M FeCl₃.6H₂O + (NH4 )₂CO₃ pH 8 Membrane filtration

Washing with deionized water

Freeze-dried

Synthesis of Akaganeite III 0.25 M FeCl₃.6H₂O Suspension + Urea Magnetic stringing at 85o_{C’de for 9 hours}

Filtration + Washing

Drying at 60o_C

The uranium adsorption capacities were determined. 0.01 g of the adsorbents contacted with 10 mL of 2380 mg/L of standard uranium solution at 30oC for 24 hours.

The adsorption capacity for UO₂2+ was determined

spectrophotometrically using salicylic acid method as complexing agent at 468 nm against reagent blank.

According to this method, uranium adsorption capacities of AK-1, AK-2 and AK-3 were calculated as 0.49 mmol/g adsorbent, 0.38 mmol/g adsorbent, 0.29 mmol/g adsorbent, respectively.

. Characterization of the materials

XRD pattern of AK-1 _{XRD pattern of AK-2}

Reference of literature for beta type of Akaganeite (I) -FeOOH nanorod; (II) hexagram

SEM image of AK-1

SEM image of AK-3

The produced materials presented the following physical characteristics:

AK-1: surface area 109.055 m2/g

AK-2: surface area 75.400 m2/ g

Batch adsorption experiments were carried in a thermostated shaker bath, GFL-1083 model. AK-1, AK-2 and AK-3 (0.01 g) were added to 10 mL solution containing various uranium concentrations at different temperatures for various contact time. The pH was adjusted by adding HNO₃ and Na₂CO₃ to the solutions at the each experiment. The suspension was filtreted by using Whatman filter paper No:44. A simple and sensitive spectrophotometric method was used in the experiments to determine uranium in solution. The uranium remained in solution was analyzed with the DBM-TOPO as complexing agent at 405 nm against reagent blank employing spectrophotometric method on Shimadzu UV-1601 UV-VIS spectrophotometer. The amount of adsorbed uranium was estimated from the difference of the uranium concentrations in the aqueous phase before and after the adsorption.

.The percentage adsorption of uranium from aqueous

solution was computed as follows:

Adsorption % = C_int - C_fin / C_int x 100

where C_int and C_fin are the initial and final uranium concentration, respectively.

Fig. 1. The effect of pH on the uptake of U(VI) by AK-1, AK-2 and AK-3 (m:0.010 g, c:50 mg/ L, v:10 mL, t:2 h)

Fig. 2. The effect of concentration on the uptake of U(VI) by AK-1, AK-2 and AK-3.

(AK-1; m=0.01 g,v=10 mL, t=2 h, pH=4); (AK-2; m=0.01g, v=10 mL, t=2 h, pH=6); (AK-3; m=0.01 g, v=10 mL, t=2 h, pH=5).

Fig. 3. The effect of contact time on the uptake of U(VI) by AK-1, AK-2 and AK-3.

(AK-1:m=0.010 g, c=100 mg/L, v=10 mL, pH=4); (AK-2: m=0.01g, c=150 mg/L, v=10 mL, pH=6); (AK-3: m=0.01g, c=35 mg/L, v=10 mL, pH=5)

Fig. 4. The effect of temperature on the uptake of U(VI) by AK-1, AK-2 and AK-3.

(AK-1; m=0.01g, c=100 mg/L, v=10 mL, pH=4, t=2 saat); (AK-2: m=0.01g, c=150 mg/L, v=10 mL, pH=6, t=1 saat); (AK-3: m=0.01g, c=35 mg/L, v=10 mL, pH=5, t=1 saat)

Fig. 5. A plot against lnKd to 1/T for removal of U(VI) from AK-1, AK-2 and AK-3, respectively

Table 1.

Table 1. Thermodynamic parameters for U(VI) sorption on Akaganeite as a function Thermodynamic parameters for U(VI) sorption on Akaganeite as a function of temperature

of temperature

Adsor

Adsorbanban ΔH°ΔH° (kJ/mol)

(kJ/mol) (kJ/mol)(kJ/mol)ΔS°ΔS° (kJ/mol)(kJ/mol)ΔG° ΔG° 293 K 293 K 303 K303 K 313 K313 K 323 K323 K AK-1 AK-1 40.40.6060 0.210.21 -20.93-20.93 -23.03-23.03 -25.13-25.13 -27.23-27.23 AK-2 AK-2 24.824.888 0.10.166 -21.4-21.422 -2-23.003.00 -24.5-24.588 -26.1-26.166 AK-3 AK-3 -27.60-27.60 -0.03-0.03 --18.5918.59 --18.5118.51 --18.2118.21 --17.9117.91

Fig. 6. Langmuir, Freundlich and D-R isotherm for AK-1 Isotherms of Adsorption

Fig 7. Langmuir, Freundlich and D-R isotherm for AK-2 Isotherms of Adsorption

Fig. 8. Langmuir, Freundlich and D-R isotherm for AK-3 Isotherms of Adsorption

Adsorbe Adsorbe nt nt (mmol/(mmol/QQ g) g) B B (L/g) (L/g) _{(mmol/g(mmol/g}KKF F ) ) n n Xm Xm (mmol/g (mmol/g ) ) E E (kj/mol) (kj/mol) AK-1 AK-1 00..5533 4444..6464 00..0088 44..6622 00..6666 11..3434 AK-2 AK-2 00..7474 2828..9898 00..0077 44..1177 11..4242 00.80.80 AK-3 AK-3 00..0909 235x10235x10-6-6 _44..5x105x10-6-6 _{22..8888 00..1199 22..5858} Table 2. Values of Langmuir, Freundlich and Dubinin-Radushkeviche (D-R) constants

The experimental studies showed that akaganeite could be used as an economic, effective and low-risk sorbent material to remove toxic and radioactive U(VI) ions from wastewaters.

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