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Abstract

A composite of rice husk (RH), caustic soda and aluminium oxide was synthesized at 500°C. The activated carbon and amorphous silica dispersed over the aluminium oxide selectively adsorbed uranium in the presence of other elements. At equilibrium time 1 h, phase ratio S/L (0.1 g/10 ml), pH = 5 and uranium initial concentration 120.6 mg/l uranium adsorption efficiency was 96.35%. The uranium stripping efficiency from the load RHA–alumina composite fulfilled 99.9% at 1 h equilibrium time, a phase ratio (S/A) of 0.05 g/10 ml and 0.5 mol/l HNO₃. The scanning electron microscopy photos revealed that the rice husk ash (RHA)–alumina composite has vacant or regular cavities before the adsorption, and the cavities are fully occupied by uranium after adsorption. The Fourier transform infrared spectroscopy shows a more broadening of the band υ = 3526 and 3462 cm⁻¹ which was ascribed to the uranium adsorption. The composite adsorbed 93.75% of uranium from a waste sample. The uranium adsorption exhibited a Langmuir isotherm.

Keywords

Rice husk ash uranium adsorption activated carbon amorphous silica

Introduction

Rice husk (RH) is a by-product of rice cultivation and harvesting that causes hazardous environmental problems and pollution in Egypt. The cultivated rice region in Egypt encompasses approximately about 810,000 hectares. The Egyptian production of rice is nearly 4.5 million tons per year, according to the Egyptian Ministry of Agriculture. Thus, immense quantities of RH yielded from rice cultivation must be utilized and innovative chemical methods for elimination are needed. RH can be exploited for many applications including transition metal adsorption (Franco et al., 2017), refractory ceramic developments (Sobrosa et al., 2017), cement (Sinyoung et al., 2017), removing chemical impurities (Lee et al., 2017) and catalysts (Setthaya et al., 2017; Shi et al., 2017) and catalysis for biodiesel production (Guo et al., 2017). Two goals must be fulfilled: eliminating the pollutants that are associated with rice cultivation in Egypt and using the chemically modified pollutants to remove some of the other environmentally hazardous pollutants such as heavy elements, i.e. dispose of pollutants using chemically modified pollutants.

The major contents of RH are silica and organic matter (cellulose) (Mohamed et al., 2015). The synthesis of a composite with a novel matrix from RH and inorganic compounds (alumina and caustic soda) via ignition results in activated carbon (AC) (produced from organic matter which is the active adsorbent). The silica that is converted to amorphous silica after ignition has a high surface area permitting and enhancing the removing of heavy elements. The AC derived from RH is a porous carbonaceous solid material with a large surface area and high porosity (Hu and Srinivasan, 1999) which allows sorption of wastes from gas and liquid phases (Jankowska et al., 1991). The raw material type used and the activation routes influence the pore characteristics and adsorption capability of AC. AC is normally created through carbonization and activation of the RH precursor. These procedures are essential for the porous structure and overabundant pore volume of RH-derived AC. The carbonization process is typically executed at temperatures ranging from 500 to 900°C to eliminate the non-carbon elements present in RH, such as oxygen, hydrogen and nitrogen as volatile gaseous products (Daud et al., 2000). The formation of free interstitial spaces and the remnant carbon atoms combining into aromatic sheets (cross-linked in a random style) lead to the formation of pores. The pores created in carbonized RH can be further developed via an activation process. Activation widens the existing pores by burning off the walls of the adjacent pores and removing the disorganized carbon that blocks the pores in carbonized RH. Physical, chemical and physiochemical (a combination of two preprocesses, i.e. physical and chemical) activation processes can be used to prepare RH-derived AC (Hameed et al., 2007). Chemical activation of RH-derived AC involves carbonization and impregnation in a single step using chemical agents.

Uranium is used in the nuclear fuel industry (Hore-Lacy, 2016) and it can be applied in electronic industries, semiconductors (Adamska et al., 2015; Bacci et al., 1989), catalysis (Amrute et al., 2013; Dong et al., 2015) and alloys (Ahn et al., 2016; Ghoshal et al., 2014). Given the importance of uranium, many chemical methods have been utilized for the recovery and separation of uranium such as solvent extraction (Ahn et al., 2016; Dartiguelongue et al., 2016; Zhu et al., 2016; Ghoshal et al., 2014), ionic exchange resins, (Chen et al., 2016; Ogden et al., 2017), adsorption over modified adsorbent (Gajowiak et al., 2013; Grabias et al., 2013), liquid emulsion membrane (Biswas et al., 2012; El Sayed, 2003), chelating resin (Donia et al., 2009; Ilaiyaraja et al., 2017), ion inclusion membranes (Kolev et al., 2013; St John et al., 2012) and AC sorption (Afsari et al., 2012; Yakout et al., 2013).

In this article, a composite consisting of RH, caustic soda and alumina was synthesized by mixing and igniting at 500°C and the adsorption potential of the composite for uranium was studied. The ignition of RH with alumina and caustic soda resulted in the dispersion of active sites on the adsorbent surface, which was reflected in enhanced, selective uranium sorption in the presence of impurities. The adsorption of uranium by the RHA–alumina composite was elucidated with infrared (IR) spectroscopy and scanning electron microscope (SEM).

Experimental

Preparation of RH–alumina composite

Typically, 10 g of RH was obtained from a local farm in the Sharkia governorate. Table 1 shows the chemical compositions of RH. The RH was added to 10 g of Al–carbonate and 20 g of NaOH in a porcelain crucible, and the mixture was polymerized at 500°C for 3 h. A black powder was obtained, washed several times with deionized water to neutralize the powder and remove excess chemicals, and desiccated at 120°C to complete dryness to obtain the RHA–alumina composite.

Table 1.

The chemical composition of the studied rice husk sample.

Constituent	wt%	Constituent	wt%
N	0.3	H	6
P	0.35	O	4.5
K	0.25	S	0.05
Si	11.3	Moisture	10.5
C	35	Lignin	30
Bulk; density kg/m³	0.99	Hardness Mohr’s scale	5.7

Material characterization

The characteristic main structure of the RHA–alumina composite was determined using different analytical techniques such as:

Fourier transform infrared spectroscopy (FTIR)

A Thermo Scientific Nicolet IS10, model instrument Germany.

SEM

SEM model Philips XL 30 ESEM (25–30 keV accelerating voltage, 1–2 mm beam diameter and 60–120 s counting time). The minimum detectable weight concentration ranged from 0.1 to 1 wt% with a realized precision less than 1%.

Preparation of standard solutions

All chemical reagents used were analytical grade. Stock solutions of 1000 mg/l of thorium and copper were prepared from standard stock solutions and the uranium, calcium and iron solutions were prepared from uranium acetate, dehydrated calcium chloride and ferric chloride, respectively, by dissolving the specified weight of the salt in deionized water. Uranium was determined in all the different working aqueous solutions using Arsenazo III dye complexation method (Marczenko, 1976). The uranium and Arsenazo III dye complex that formed was measured at a wavelength of approximately 655 nm against proper standard solutions using a double beam Shimadzu, 1401 UV/VIS spectrophotometer (Japan). The copper and iron concentrations were determined using an atomic absorption spectrometer apparatus. The concentration of Th(IV) in the aqueous solution phase was determined spectrophotometrically using Thorn method. The absorption of Th(IV) was measured for Th(IV) at 540 nm.

Batch experiments (adsorption and elution studies)

The effects of different factors on the adsorption process were studied. The studied factors included the solution pH, equilibrium time, uranium concentration and solid/liquid ratio. The adsorption experiments were performed by shaking 0.1 g samples of the prepared RHA–alumina composite with 10 ml of a uranium solution (42.4–3500 mg/l), and the pH was changed from 2 to 10. The effect of the presence of co-ions was demonstrated and calculated using different concentrations (≤120 mg/l) of iron, copper, calcium and thorium. The flasks were placed on a shaker at different temperatures (25–60°C), and aqueous samples were taken at time periods of 15–120 min. After treatment, the solid phases were separated using Whatman filter paper (no. 40), and the uranium concentration in the filtrate was chemically determined. The quantity of uranium loaded on the composite was calculated using equation (1), i.e. by taking the difference between the initial and residual concentrations of uranium in solution and dividing it by the weight of the adsorbent. The removal or adsorption efficiency (R_e) was defined as the U sorption percentage relative to the initial concentration (equation (2))

q_{e} = \frac{C_{o} - C_{e}}{M} * V

(1)

R_{e} = \frac{C_{o} - C_{e}}{C_{o}} * 100

(2)

where q_e (mg/g) is the amount of uranium loaded per unit weight of the adsorbent; C_o and C_e are the initial and equilibrium (or at any time) uranium or interfering ions concentration (mg/l), respectively; V is the volume per litre of solution and M is the weight (g) of the adsorbent (RHA–alumina composite). To strip uranium from the RHA–alumina composite adsorbent, a variety of stripping agents were examined, including HCl, H₂SO₄, Na₂CO₃, HNO₃ and NaCl.

Liquid waste properties

One litre of a waste solution with a pH of 1.3 was obtained from an ore processing unit (Inshass) which later adjusted to working optimum pH 5. The concentrations of some cations and anions in the solution are shown in Table 2.

Table 2.

Concentration of some cations and anions of waste sample.

Element	ppm
SO₄⁻²	53
CO₃⁻²	80
Cl	16
UO₂	100
SiO₂	800
Fe	1600
Zr	158
Zn	10
La	596
Mn	230
Mg	1540
Ce	190
Cu	2
Pb	270
Ca	1000

Results and discussion

Characterization

FTIR

The loading capacity of the RHA–alumina composite depends on the porosity, pore content and functional group reactivity on the surface. IR spectrum was used to qualitatively interpret the functional groups on the RHA–alumina composite (Figure 1).The IR spectra were similar to the spectrum of some AC and silica. The band at υ = 3620 cm⁻¹ disappeared, which was attributed to the sorption of uranium. The absorption bands at 1022 cm⁻¹ belong to Si–O–Si stretching modes which change significantly before and after uranium adsorption of the absorption bands at 3526 and 3462 cm⁻¹ became wider, which was also attributed to the sorption of uranium. The change in the absorption bands can be explained as follows, the adsorption of uranium on the surface of the adsorbent leads to change in the energies needed for vibrational motion and angle bending for the bonds before adsorption and after.

Figure 1.

FTIR spectra of the RHA–alumina composite before and after uranium sorption.

Surface morphology studies

The SEM images of the RHA–alumina composite macrostructure before and after the sorption of uranium (Figure 2(a) and (b)) show the rudimentary RHA–alumina composite surface that formed and the surface structure is attributed to the volatility and the decomposition of the hydrothermal treatment. After the adsorption, the pores and crevices were occupied by uranium ions (Figure 2(b)).

Figure 2.

SEM images of the RHA–alumina composite (a) before and (b) after the adsorption of uranium.

Controlling factors on the adsorption process

Effect of pH

The pH can greatly influence the uranium adsorption efficiency (A%) and uranium uptake (q_e) (mg/g). Synthetic uranium solution pH intervals in the range 2–10 were shaken with RHA–alumina composite and the other experimental conditions were S/A phase ratio 0.1 g/10 ml, 134 mg/l U(VI) concentration, 1 h equilibrium time and room temperature 25 ± 2°C. Figure 3 reveals that the efficiency of the uranium adsorption (A%), uranium uptake (q_e) increased from 18 to 96% and a maximum value of 13 mg/g, respectively, at pH value 5. At pH 10, uranium uptake decreases to 3.8 mg/g. Thus, pH 5 was used for the synthetic uranium solutions in the next experiments.

Figure 3.

Effect of pH on the uranium adsorption efficiency (A%) and uptake (mg/g).

At a higher acidity, the uranium (VI) uptake decreased due to the increasing H⁺ ion concentrations, which caused the surface of the adsorbent to be increasingly more positive. Then, competition occurs between the H⁺ ions, which are small and fast, and the uranium species, which are also positive, and adsorption is not favoured. Thus, the presence of a higher concentration of H⁺ ions in the reaction mixture caused a reduction in the uranium uptake. In contrast, increasing the pH of the synthetic uranium solutions resulted in the adsorbent surface becoming more negatively charged, which resulted in the more favourable adsorption of positively charged species (Han et al., 2007).

The effect of the initial uranium concentration

The effect of the initial uranium(VI) concentration on the adsorption was studied. The RHA–alumina composite (0.1 g) was shaken with approximately 10 ml of the uranium solutions at different concentrations ranging from 42.4 to 3500 mg/l, and the other parameters were a pH of 5, 1 h shaking time and ambient temperature 25 ± 2°C. The relationship between the different concentrations and the uranium adsorption efficiency (%A) and uranium uptake (q_e) is illustrated in Figure 4. The uranium adsorption efficiency decreased with the increasing uranium concentration because the uranyl ions (UO₂²⁺) have a higher mobility in diluted solutions, which permits more interactions between the adsorbent and uranyl ions. However, increasing the uranium concentration over 120.6 mg/l results in increasing competition of the UO₂²⁺ ions for the free active sites, which negatively affects the adsorption efficiency, not the burden capacity. The amount of adsorbed uranium and uranium uptake, q_e (mg/g), increased with the increasing uranium concentration, and the largest adsorption capacity was at a uranium concentration of 1032.35 mg/l. Uranium concentrations above this level did not negatively or positively influence the uranium adsorption or uptake. Therefore, the maximum uranium loading capacity of the RHA–alumina composite is 85 mg/g.

Figure 4.

The effect of the initial uranium concentration on the uranium adsorption efficiency (A%) and uptake (mg/g).

Adsorption isotherms

Several frequently used adsorption isotherm models were used to model the isotherm data from the equipoise adsorption of the RHA–alumina composite. Three of these models are Langmuir, Freundlich and Dubinin isotherms.

A. Langmuir isotherm:

In the Langmuir model, adsorption uniformly occurs on the active sites of the absorbent, and once a sorbate occupies a site, no further sorption can occur at this site. Thus, the Langmuir model is represented by the following equation (Chegrouche et al., 1997; Mellah and Chegrouche, 1997)

C_{e} / q_{e} = 1 / {bQ}_{o} + C_{e} / Q_{o}

(3)

where Q_o and b are the Langmuir constants for the sorption capacity of the saturated monolayer and the sorption equilibrium constant, respectively. A graph of C_e/q_e versus C_e will result in a straight line with a slope of (1/Q_o) and an intercept of 1/bQ_o, as seen in Figure 5. The Langmuir specifications given in Table 3 can be used to anticipate the tendency of the sorbate and sorbent using the dimensionless separation factor R_L (Bhatnagar and Jain, 2005; Parab et al., 2005)

R_{L} = 1 / (1 + {bC}_{o})

(4)

Figure 5.

Langmuir isotherm for uranium adsorption on the RHA–alumina composite.

Table 3.

Langmuir and Freundlich parameters for uranium adsorption on the RHA–alumina composite.

Metal	Adsorbent	Langmuir model parameters			Freundlich model parameters			Dubinin–Radushkevich model
Metal	Adsorbent	Q_o (mg/g)	b (l/mg)	R ₂	1/n	K f (mg/g)	R ₂	E	R ₂
Uranium	RHA–alumina composite	85	0.029	0.99	0.696	1.33	0.952	4.22	0.847

The R_L value indicates whether the isotherm is irreversible (R_L=0), proper (0 < R_L<1), linear (R_L=1) or improper (R_L>1) (Fan et al., 2011; Ho and McKay, 1999).

The values of RL for the sorption of uranium (VI) onto the RHA–alumina composite are presented in Figure 6, and the values indicate that the adsorption of uranium (VI) is higher at higher initial uranium (VI) concentrations and the opposite was true at lower concentrations.

Figure 6.

The separation factor, RL, for uranium(VI) adsorbed on the RHA–alumina composite.

B. Freundlich isotherm

The Freundlich model postulates that the ratio of the adsorbed solute to the solute concentration is a function of the solution. The empirical model was shown to be consistent with exponential distribution of active centres, characteristic of heterogeneous surfaces. The quantity of adsorbed solute at equipoise q_e, the concentration of the uranium in the solution at equilibrium C_e, is represented in the following equations (Chegrouche et al., 1997; Mellah and Chegrouche, 1997)

q_{e} = K_{F} C_{e} 1 / n

(5)

This expression can be linearized to obtain

Log q_{e} = {logK}_{F} + (1 / n) log C_{e}

(6)

where K_F and n are the Freundlich constants related to the adsorption capacity and adsorption intensity, respectively. A plot of logq_e versus log C_e results in a straight line with a slope of 1/n and an intercept of logK_F as shown in Figure 7. Table 3 presents the Freundlich constants.

Figure 7.

Freundlich isotherm for uranium adsorption on the RHA–alumina composite.

Figure 8.

Dubinin–Radushkevich isotherm for uranium adsorption on the RHA–alumina composite.

The experimental results indicate that uranium adsorption on the RHA–alumina composite was better fit by a Langmuir isotherm than Freundlich.

C. Dubinin–Radushkevich isotherm model

The Dubinin–Radushkevich isotherm model is used to denote the adsorption mechanism with the Gaussian energy distribution on a heterogeneous surface. The model has often successfully fitted with high solute activities and the intermediate range of concentration data well. The Dubinin–Radushkevich isotherm equation is linearly represented as follows

{Inq}_{e} = {Inq}_{s} - B_{d} {(ε)}^{2}

(7)

ε = R T ln [1 + 1 / C_{e}]

(8)

where q_e (mg/g) is the adsorbed value of the uranyl ion at equilibrium concentration, q_s is the theoretical isotherm saturation capacity (mg/g), B_d is the Dubinin–Radushkevich isotherm constant (mol²/kJ²), ε is the polanyl potential, C_e (mg/l) is U(VI) concentration in leach liquor at equilibrium, R (J/mol K) is the gas constant and T (K) is the absolute temperature. A plot of the logarithm of amount adsorbed In q_e versus the square of potential energy ε² where q_d and B_d are calculated from the slope and intercept of the linear plots. The mean adsorption energy E (J/mol) of adsorption of uranyl ions to the RHA–alumina composite from infinity in the standard solutions can be estimated from the following equation

E^{2} = 1 / 2 B_{d}

(9)

If the magnitude of E is between 8 and 16 kJ/mol, the sorption process is supposed to proceed via chemisorption, but if E is less than 8 kJ/mol, the sorption process is physisorption.

The mean adsorption energy E (J/mol) of adsorption of uranyl ions to the RHA–alumina composite is calculated to be 4.22 kJ/mol (Dabrowski, 2001; Dubinin, 1960).

The effect of the equilibrium time

The effect of the equilibrium time on the uranium adsorption efficiency (A%) from a 10 ml solution of uranium (120.6 mg/l) using the RHA–alumina composite was studied. The time was varied from 15 to 120 min at ambient temperature (25 ± 2°C) with a pH of 5 and approximately 0.1 g of the RHA–alumina composite. The data are plotted in Figure 9, and a gradual increase in the uranium adsorption efficiency was observed with the increasing equilibrium time. A maximum value of 96% was attained at 60 min, and then the adsorption remained constant. Hence, the adsorption equilibrium time used for the subsequent experiments was 60 min.

Figure 9.

Equilibrium time versus the uranium adsorption efficiency (A%) on the RHA–alumina composite.

The effect of the solid/liquid ratio(S/a)

The influence of the solid/liquid ratio, g/ml, was studied in the range varied from 2.5 to 15 g/ml on the adsorption efficiency of uranium (VI) from a solution assaying 120.6 mg U/l onto RHA–alumina composite. The other parameters were fixed on shaking time 60 min, ambient temperature 25± 2°C and a pH of 5. The results plotted in Figure 10 show the relation between the uranium adsorption efficiency, A%, and uranium uptake quantity and the solid/liquid ratio. An increase in the solid/liquid ratio from 2.5 to 10 g/l clearly caused an increase in the adsorption efficiency from 66.83 to 96.35%, but the uranium uptake decreased from 32.24 to 11.66 mg/g. Therefore, a liquid/solid ratio, L/g used for the other experiments was 10 g/l.

Figure 10.

The effect of the solid/liquid ratio on the uranium adsorption efficiency, A%, and uptake (mg/g) by the RHA–alumina composite.

The effect of the temperature

The effect of the temperature on the uranium adsorption efficiency with 0.1 g of the RHA–alumina composite in a 10 ml solution with 120.6 mg/l of standard uranyl nitrate was studied in the temperature range from 25 to 60°C. The other factors were held constant, i.e. a pH of 5 and 60 min shaking time. The results in Figure 11 show that the uranium adsorption efficiency of the RHA–alumina composite decreased with the increasing temperature, which indicated that the U(VI) adsorption was favoured at ambient temperature.

Figure 11.

The effect of the temperature on the uranium adsorption efficiency (A%) of the RHA–alumina composite.

The effect of competing ions

The influence of other interfering metal ions, such as copper, thorium and calcium, which may be present along with uranium ions in synthetic aqueous solutions, was studied by separately adding different cations to a uranium solution under the following conditions: 0.1 g of the adsorbent was contacted with 10 ml of a 120.6 mg/l uranium solution with different concentrations of the desired elements at 25 ± 2°C for 1 h, and the solution pH was maintained at 5. Figure 12 shows that in the presence of co-ions, the uranium uptake per cent on the RHA–alumina composite slightly decreased and exhibited the trend Th⁺⁴> Fe⁺³> Cu⁺²> Ca⁺². This trend may explain why the adsorption sites present on the RHA–alumina composite are better for thorium, iron and copper ions than calcium.

Figure 12.

The effect of the presence of co-ions on the uranium adsorption efficiency, A% by RH–alumina composite.

Elution or desorption of uranium

The expediency of using the AC doped with aluminium oxide for the preconcentration–separation of uranium was assessed by elution studies. For these studies, an amount of the adsorbent was shaken under the conditions previously determined to load the adsorbent. The desorption of the uranyl ions into the RHA–alumina composite was performed in a batch mode. The factors examined included the eluent agent, stripping agent concentration, phase ratio and shaking time.

The effect of the eluent type

The following salt and acid solutions, Na₂CO₃, NaCl, H₂SO₄, HCl and HNO₃, were used to desorb uranium from the loaded adsorbent. The elution experiments were conducted by shaking 0.05 g of the loaded adsorbent for 15 min with 10 ml of the elution agents (0.1 mol/l). The results are shown in Figure 13. HNO₃ was the optimum solution for the elution of uranium from the loaded adsorbent. The other factors controlling the elution process that were studied were the shaking time, phase ratio and concentration of the elution agent.

Figure 13.

The effect of the eluent agent on the desorption of uranium from the loaded RHA–alumina composite.

The effect of the shaking time

To study the influence of the equilibrium time on the uranium elution from the loaded adsorbent, a series of experiments were performed by contacting 0.05 g of the loaded adsorbent with 10 ml of 0.1 mol/l HNO₃ for interval periods ranging from 15 to 120 min. The results are shown in Figure 14. The stripping efficiency increases with the increasing time from 15 to 60 min, but it did not further increase as the shaking time increased above 60 min. Therefore, 60 min is the optimum shaking time.

Figure 14.

The effect of the equilibrium time on the desorption of uranium from the loaded RHA–alumina composite.

The effect of the phase ratio (S/a)

The influence of the solid–liquid ratio on the desorption of U(VI) from the loaded RHA–alumina composite was varied (0.05 g/5 ml, 0.05 g/7.5 ml, 0.05 g/10 ml and 0.025 g/10 ml) at 298 K with 1 h shaking time and 0.1 mol/l HNO₃. The results are illustrated in Figure 15, and they revealed that the uranium desorption or elution efficiency decreased due to dilution.

Figure 15.

The effect of the phase ratio on the desorption of uranium from the loaded RHA–alumina composite.

The effect of the elution agent (HNO₃) concentration

To study the effect of the elution agent concentration (HNO₃) on the uranium elution efficiency, various concentrations of HNO₃ were prepared. Then, 10 ml of the HNO₃ and 0.05 g of the loaded adsorbent were combined and shaken for 60 min. The results are shown in Figure 16. Obviously, 0.5 mol/l HNO₃ can elute approximately 100% of the loaded uranium, and this was chosen as the optimum concentration.

Figure 16.

The effect of nitric acid concentration on desorption of uranium from loaded RHA–alumina composite.

Case study

A case study was performed based on the sorption results using 500 ml of a waste solution with a uranium concentration of 100 mg/l. The solution was contacted with 5 g of the RHA–alumina composite for 60 min at room temperature 25 ± 2°C and a pH of 5. After equilibration, the solution (case study) was filtered and analysed to determine the uranium concentration. The overall adsorption capacity of the RHA–alumina composite was 99%. The uranium desorbed with 0.5 M HNO₃, and the uranium was precipitated by H₂O₂ after adjusting the pH to 2.5–3.

The experimental capacity of RHA compared with the desorption capacities of some chelate-modified solid-phase extraction procedures

Table 4 shows different chemically modified adsorbents that are used for uranium adsorption. The sorption capacities (mg/g) were in the following order: RHA–alumina composite > AC> Coir pith> palm shell> orange peels> powdered corncob> sunflower> succinic acid-impregnated Amberlite XAD-4> date pits> natural clay> natural clinoptilolite zeolite (85 > 28.5 > 28 > 25 > 15.91 > 14.21 > 13.45 > 10 > 3.53 > 0.7 mg/g, respectively).

Table 4.

Desorption capacities of some chelate-modified solid-phase extraction procedures for U(VI) uptake from aqueous solutions.

Adsorbent	Adsorption capacity (mg/g)	pH	Shaking time	Adsorption %	References
RHA–aluminium composite	85	5	1 h	96.35	Present study
Gel-amide	28.98	6	6 h	30%	Venkatesan et al. (2004)
AC	28.5	–	–	–	Abbasi and Streat (1994)
Coir pith	28	4.3	0.5 h	–	Parab et al. (2005)
Palm shell	200	1	3 h	–	Kushwaha and Sudhakar (2013)
Orange peels	15.91	4	1	–	Mahmoud (2013)
Powdered corncob	14.21	5	1	–	Mahmoud (2016)
Sunflower	13.45	4		–	Roongtanakiat et al. (2010)
Succinic acid impregnated amberlite XAD-4	12.33	4.5	0.75	80–90%	Ahmad et al. (2015)
Date pits	10	6–7	0.25	98	Saad et al. (2008)
Natural clay in order	3.53	6	2		Eba et al. (2013)
Natural clinoptilolite zeolite	0.7	6	0.75	95.6	Camacho et al. (2010)

When preparing an adsorbent of aluminium carbonate with silica and caustic soda (AS composite) in the same conditions as RHA–alumina composite prepared and applying optimum conditions for adsorption of uranium, the uptake was found to be 35 mg/g.

Conclusions

RHA–alumina composite having a high surface area and large pore volume was synthesized. The structure of the new sorbent was characterized by FTIR spectroscopy and SEM. The RHA–alumina composite, which was synthesized by the ignition of RH with alumina and caustic soda at 500°C to have activation sites on the adsorbent surface, was used for the removal of U(VI) ions from a synthetic solution and a case sample study using a batch system, and it has been established as an effective adsorbent. The binding preference for uranium ions is attributed to its atomic properties and the chemistry of the solution, e.g. the pH. The proper pH value for the adsorption of the studied ions onto the RHA–alumina composite from an aqueous solution was 5. A higher adsorption capacity is obtained at ambient temperature 25 ± 2°C. A Langmuir isotherm fits the equilibrium data better than the Freundlich isotherm, and the uranium adsorption capacity was 68 mg/l. In addition, the adsorption equilibrium data fit very well to both models in the studied concentration range. The results of the study indicate that the investigated RHA–alumina composite can effectively remove uranium anionic species from aqueous solutions.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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Synthesis,characterization and application of composite derived from rice husk ash with aluminium oxide for sorption of uranium