Enhanced method for the removal of U (VI) and Th (IV) from aqueous solutions using chemically modified kaolinite

ABSTRACT Jordanian kaolinite was treated with two phosphate modifiers: sodium polyphosphate (MK 1) and sodium hexametaphosphate (MK 2). Both raw and modified kaolinites were characterised by XRD and FTIR techniques. The adsorption behaviour of Th(IV) and U(VI) ions onto the two modified kaolinites were studied as a function of dosage, contact time, pH and temperature. The adsorption isotherm of both metal ions onto modified kaolinite was investigated using Langmuir, Freundlich and Dubinin–Radushkevich (D-R) models, which fits better to the Langmuir model. MK 1 kaolinite showed higher metal ion uptake than MK 2 kaolinite for both ions. The metal ion uptake of Th(IV) ions on both modified kaolinites was significantly higher than that of U(VI) ions. The % adsorption efficiency (73.8 and 69.0%) of Th(IV) ions on MK 1 and MK 2 was significantly higher than that of U(VI) ions (35.0 and 13.3%). The adsorption capacities for Th(IV) (188.7 and 9.6 (mg/g)) and U(VI) (53.5 and 15.3(mg/g)) were improved when MK 1 and MK 2 kaolinites were used at 25°C. The thermodynamic studies indicated that the removal of U(VI) and Th(IV) ions using both modified kaolinites is spontaneous and favourable at higher temperatures. The positive entropy values indicated that the adsorption process increased the disorder of the system. The kinetic study had indicated that the pseudo-second-order kinetic model fit the adsorption data, suggesting a chemisorption nature of the adsorption process. Binary system experiments under optimised conditions showed higher removal of Th(IV) ions compared to U(VI) ions. The desorption study showed that the elution of both metal ions increased as the concentration of HNO3 increased. Both the modified kaolinites were regenerated three times, a slight reduction of their adsorption efficiency was observed, and the presence of interferences has the same effect.


Introduction
Nuclear energy has been introduced as a new, highlyefficient and clean source of energy to replace the limited fossil fuel resources.Nuclear energy is always associated with many environmental concerns, especially from mining and the waste produced after nuclear power production.Thorium(IV) and uranium(VI) are examples of radioactive elements, which are widely used in nuclear plants.Mining ores containing Th(IV) and U(VI) have serious environmental issues that should be taken into consideration.The mining processes might produce a wastewater having harmful amounts of these two elements, which can pollute both surface and underground water.The presence of these radioactive elements in the ecological system can be fatal for human and environment, causing serious impacts [1,2].Consequently, the treatment of wastewater containing Th(VI) and U(VI) is an important environmental issue and there are developing research studies concerning this aspect.
Adsorption of metal ions from aqueous solution to the solid phase is the most conventional method for the removal of these ions from their solutions, especially for metal ions with low concentrations.It was proposed that the adsorption is a reversible process where an equilibrium is established between the ion concentration in solution and adsorbent [3][4][5][6][7][8][9][10].
Natural clays had been introduced as superior adsorbent due to their low cost and high availability [11].Kaolinite, naturally occurring clay, has different applications in ceramics, paper, ink, paintings, polymers, etc. [12][13][14][15].Kaolinite, Al 2 Si 2 O 5 (OH) 4 , is a type 1:1 clay consisting of octahedral AlO 2 (OH) 4 and tetrahedral SiO 4 , forming double layers of the aluminosilicate structure [16].The kaolinite structure produces an uncharged surface, at which the double layers are bonded by hydrogen bonds, which are formed by the intermolecular forces between AlO 2 (OH) 4 and SiO 4 .Three hydroxyl groups of octahedral AlO 2 (OH) 4 are present in the internal surface, whilst the remaining OH is located inside the layer [17].The strong hydrogen bonds between the two layers are formed when the inner hydroxyl groups in the octahedral sheet are perpendicular to OSi present in the tetrahedral sheet, producing a kaolinite with a non-expendable structure [18].The superior sorption properties of kaolinite [19,20] lead to a developing increase in their applications in heavy metal and radionuclide adsorption [21,22].Due to the low cation exchange capacity of raw kaolinite, its efficiency in the removal of cations and organic pollutants from their solutions is low, suggesting that modification of its surface can be carried out to increase its ability to adsorb different species [23].
Adsorption isotherms are widely used to describe the adsorption process of metal ions from their aqueous solutions by a solid sorbent.In these isotherms, the capacity of adsorbent, which indicates the amount of adsorbent required to remove a unit mass of metal ions, is described and can be quantified [24][25][26].In this study, Langmuir [27], Freundlich and Dubinin-Radushkevich isotherms are used to describe the adsorption process of uranium(VI) and thorium(IV) onto kaolinites modified by polyphosphate and hexametaphosphate.In the Langmuir isotherm model, the interactions between metal ions in a solution are negligible, indicating a monolayer or homogenous sorption process.This suggests that the heat of adsorption of the metal uptake on the sorbent surface is independent of the metal ion concentration [28].In contrast, the Freundlich isotherm is related to a heterogeneous adsorption process, suggesting that the heat of adsorption is strongly related to the sorbent active sites [24].This means that the Freundlich model proposes an adsorbent with multilayers in which the amount of metal ions adsorbed increases upon increasing their concentrations [29].The Dubinin-Radushkevich (D-R) adsorption isotherm is used to describe the physical and chemical characteristics of the adsorption process.It is widely used to describe the adsorption process of each metal ion independently, but it suggests a heterogeneous surface of the adsorbent with multilayers [30].
A negatively changed kaolinite surface will be formed when Al substituted Si in the tetrahedral sheets, creating a surface that is able to remove cations from their aqueous solutions [31,32].It was reported that the low adsorption capacity of raw kaolinite can be improved by modifying the surface with organic ligands, which attached on the surface of the kaolinite and improved its ability to adsorb different organic species [33,34].The formation of a hydride organic-inorganic surface has potential application in nanotechnology, environmental applications and clay sciences [35][36][37][38].The introduction of organic ligands on the surface of the clay can be achieved by different methods.The organic ligand may displace a metal on the surface of the clay, causing the organic species to be impregnated with the surface of the clay.The organic modifier can also be introduced by the formation of covalent bonds between the clay surface and the organic ligand, a method called covalent grafting.This could occur via organosilane [39][40][41][42][43], alcohols, such as saturated alcohols, diols, long-chain glycol, monoethers, etc. [44].In addition, the organic molecules can displace the exchangeable cations on the surface of the clay, creating a modified surface with different physical and chemical properties [45].
Adsorption is a temperature-dependent process and it is widely described as a function of change in free Gibbs energy (∆G o ), standard change in enthalpy (∆H o ) and standard change in entropy (∆S o ).These thermodynamic values can explain the nature of the adsorption process, endothermic or exothermic or whether the process is spontaneous or not [46,47].
This paper focuses on the characteristics and properties of a modified Jordanian kaolinite using polyphosphate (MK 1) and hexametaphosphate (MK 2), for the removal of U(VI) and Th(IV) from aqueous solutions.The modified kaolinite was characterised for its physiochemical properties using different analytical techniques such as FTIR and X-ray diffraction.The modified kaolinites were tested as sorbents for the uptake from aqueous solutions of U(VI), Th(IV) and a binary system of both.The adsorption characteristics of the modified kaolinite were investigated as a function of the clay dosage, contact time, pH and temperature with the objective of optimising the removal process [48,49].The adsorption data were studied using Langmuir, Freundlich and Dubinin-Radushkevich (DR) isotherm models.The parameters derived from these isotherms were used to investigate the properties of the adsorption process.The dependency of the adsorption process on changing temperatures was analysed by calculating ΔH°, ΔS° and ΔG° thermodynamic parameters [50].The effect of the contact time on the adsorption process was explored by the application of pseudo-first and pseudo-second order kinetic models [51].

Materials
Analytical grade chemicals were used in this study as received without further purification.All working solutions were prepared using deionised water.The following materials were used for the preparation of the modified kaolinite and purchased from Sigma-Aldrich: sodium polyphosphate (Na 3 O 4 P, purity 96%) and sodium hexametaphosphate (65-70% P 2 O 5 basis).1000 mg/L stock solutions of each U(VI) and Th(IV) were prepared by dissolving an exact weight of uranyl nitrate hexahydrate (UO 2 (NO 3 ) 2 • 6H 2 O, purity >99%, BDH chemicals Ltd Poole-England) and thorium tetranitrate pentahydrate (Th(NO 3 ) 4 • 5H 2 O, purity >99%, BDH chemicals Ltd Poole-England) in deionised water.Further dilution of the stock solutions of U(VI) and Th(IV) was necessary to obtain a working solution for adsorption experiments.

Synthesis of modified kaolinite
The kaolinite sample was obtained from Natural Resources Authority -Jordan from Batn El-Ghoul deposit in the southern of Jordan.A representative sample of kaolinite was ground to less than 0.15 mm.The raw kaolinite was treated with different reagents of different concentrations ranging from 0.5 to 5.0% w/w.The raw kaolinite was treated with HCl, NaOH, Na 2 S 2 O 3 and H 2 O 2 before the impregnation of phosphate modifiers.Based on the adsorption efficiency towards the studied metal ions, the reagent that had been used for kaolinite treatment was HCl with a concentration of 1.0% w/w.After the acidic treatment, the sample was washed several times with deionised water and then dried at 105°C for 3 h in a flow of N 2 to a constant weight.Phosphate-modified kaolinite, with different phosphate modifier loadings (1-30% w/w), was synthesised by impregnation of sodium phosphate (MK 1) or sodium hexametaphosphate (MK 2) into acid-treated kaolinite.The optimum concentration for both modifiers was 10% w/w.After that, 150 mL of the aqueous solutions of sodium phosphate or hexametaphosphate of optimum concentration was prepared by dissolving a specific amount of its salt in the deionised water at moderate temperature (~ 50°C) whilst stirring at 140 rpm.Treated kaolinite (10 g) was added to this solution with continuous stirring for 24 h.The solid particles were then separated by decantation followed by washing several times with deionised water to remove unreacted components of modifiers.Finally, the solid particles were separated by filtration and dried at 60°C for 24 h.The prepared modified kaolinites, MK 1 and MK 2, were ground and stored in a desiccator until further use.

Characterisation of raw and modified kaolinite
The X-ray diffraction (XRD) profile of raw and modified kaolinite, MK 1 and MK 2, was recorded on a Philips X pert PW 3060, operated at a voltage of 45 kV,a current of 40mA anda scanning rate of 1°/min in a temperature range of 1-60°.The Fourier transform infrared (FT-IR) spectrum was obtained using a Thermo Nicolet NEXUS 670FT-IR spectrometer using KBr discs in the range of 4000-600 cm −1 at a spectral resolution of 4cm −1 .

Adsorption of metal ions
The adsorption of both Th(IV) and U(VI) on modified kaolinites was studied by the determination of both the metal ions' concentration remaining in the adsorbate solution using á UV-visible spectrometer after building up analytical calibration curves for each metal ion.Linear calibration curves were built for U(VI) and Th(IV) using blank solution and prepared working standard solutions of each metal ion [52].The concentration of Th(IV) and U(VI) ions was determined using a UV-Vis spectrophotometer of METASH model V-5100 and a 1.0 cm quartz cell.The standard solutions of these metal ions were introduced into the UV-Visible instrument and their analytical signals were recorded after the correction of the blank signal.
In order to study the effect of MK 1 and MK 2 dosage on the adsorption of Th(IV) and U(VI) ions, 50.0 mL of 50 ppm standard solutions of both metal ions were introduced to different masses of the modified kaolintes at specific pH, temperature and shaking time.The temperature of the incubator was adjusted to 25°C and the contact time was set at 5 h.The amount of modified kaolinite was varied from 0.1 to 0.5 g for Th(IV) and U(VI) experiments.The pH of the solution was adjusted at 3.0.
The effect of pH on Th(IV) and U(VI) adsorption on MK 1 and MK 2 kaolinites was studied as follows: 50 mL solution of both metal ions of concentration 50 ppm in each Th(IV) and U(VI) at 25°C was shaken for 5 h using a certain amount of modified kaolinite in different pH ranges (2)(3)(4)(5)(6)(7).The pH was adjusted using either diluted nitric acid (HNO 3 ) or a solution of sodium hydroxide (NaOH).
The effect of the contact time on the adsorption of both metal ions was investigated by adding 50.0 mL of 50 ppm standard solutions of Th(IV) and U(VI) metal ions, at a pH value of 3, to a constant weight of the clays and the solution was then shaken for different time periods (0.5-24 h) at a temperature of 25°C .

Batch adsorption experiments
Adsorption experiments of U(VI) and Th(IV) ions onto MK 1 and MK 2 kaolinites were conducted by the batch equilibrium method at different temperatures of 25, 35 and 45°C in a shakerincubator at a shaking speed of 140 rpm.Equilibrium adsorption isotherms were obtained by bringing 0.1 g of MK 1 and MK 2 kaolinites into contact with 50.0 mL solution of U(VI) and Th(IV) with initial concentrations of 10-120 ppm.The pH of the solution was adjusted at 3.0, whilst the contact time was fixed at 5 h.The adsorptions of both metal ions on the surface of modified kaolinites were carried out in duplicates.The solid-liquid mixture was centrifuged for 5 minutes at a speed of 4500 rpm and then the mixture was filtered.Arsenazo (III) solution was added as a complexation agent in order to determine the concentrations of both metal ions remaining in the solution using a UV-Vis spectrophotometer.For this purpose, 4 mL of U(VI) solution and 1 mL of Th(IV) solution were acidified by 20 mL of 0.01 M HCl and then 1 mL of Arsenazo (III) was added.The resulting solution was diluted to 50 mL and then measured spectrophotometrically after one hour at a wavelength of 650 nm for U(VI) and 660 nm for Th(IV).The concentrations of both metal ions were determined using their calibration curves, previously established by measuring the absorbance of standard solutions of both metal ions in the range of 5 to 50 mg/L.The analysis of the results had shown that the reproducibility of the measurements occurred within the range of ±1.5%.The metal ion uptake, expressed as mg metal ion per gram kaolinite, was calculated according to the following equation: where q e represents the amount (mg) of metal ions adsorbed by one gram of kaolinite.
C o represents the initial concentration (mg/L) of the metal ions mixed initially with the kaolinite.
C e represents the final concentration (mg/L) of the metal ions after equilibrium has been reached.
V represents the metal ion solution volumes, expressed in L. m represents the mass of kaolinites (in grams).

Thermodynamic studies
The thermodynamic functions were studied by mixing 50 mg/L solution of Th(IV) and U(VI) ion solution with 0.2 g of kaolinites at temperatures of 25, 35, and 45°C for 5 h.

Kinetic studies
Adsorption kinetics were investigated at different initial adsorbate concentrations 10-120 ppm for both U(VI) and Th(IV) using 0.1 g of both modified kaolinites per 50 mL of both metal ion solution.At predetermined time intervals (0.5-24 h) for both U(VI) and Th(IV), samples were taken for analysis.The kinetic experiments were performed at 25°C in order to find the kinetic parameters, which modelled the experimental data.

Binary system experiment
In order to study the competitive adsorption behaviour of the two radioactive metals on both modified kaolinites, a mixture of 50 ppm of both metal ions were mixed with 0.3 g of MK 1 and MK 2 adsorbents, the removal of each metal ions was investigated at pH of 3 and 25°C and the mixture was shaken for 5 h.

Desorption and reusability experiments
The desorption study of Th(IV) and U(VI), previously loaded on both modified kaolinites, was carried out to investigate the capability of the modified kaolinites to be regenerated for further use.The optimum mass of each modified kaolinite was loaded with U(VI) and Th(IV) using 50 mL of 50 ppm of each metal ion, which was incubated for 24 h.The bound ions were washed with 50 mL of 0.1 M HNO 3 and 1.0 M HNO 3 to remove the loaded metal ions.After the desorption process, the metal ions recovered by the acids were determined using a UV-Vis spectrophotometer and expressed as percent recovery.After the desorption process, both modified kaolinites were regenerated by washing with distilled water until neutral pH was obtained.The adsorption/desorption and regeneration processes were repeated three times.

Effect of interferences
The effect of interferences containing 0.5 M concentration of FeCl 3 , MgCl 2 and NaCl on the adsorption efficiency for 50 ppm of each Th(IV) and U(VI) onto both modified kaolinites was studied.The experiments were carried out using the batch technique under the optimised conditions.The concentration of U(VI) and Th(IV) remaining in the concentration was determined using a UV-Vis spectrophotometer.The adsorption efficiency of both the modified kaolinites towards both the metal ions in the presence of these interferences was calculated according to equation 1.

Characterisation of raw and modified kaolinite
The XRD patterns for raw and both modified kaolinites are shown in Figure S1 (Supplementary Data).All the samples consist of strong peaks of kaolinite (K) as major clay mineral (at 12.4°, 20.9°, 24.8° and 26.6°), whereas quartz (Q), feldspar (F), haematite (H) and illite (I) are also present.These patterns match with those reported in the literature for the typical raw kaolinite [53].In Figure S1 (Supplementary Data), two weak diffraction peaks at 2° values of 9° and 30° represent the illite (I) and feldspar (F) and haematite (H), respectively.Those peaks disappeared in MK 1 and MK 2 samples, which might be due to the treatment of the raw kaolinite with hydrochloric acid.These minerals become water soluble after the acid treatment and they were washed out with water after modification.
In addition, the acid treatment caused a decrease in peak intensities, which is a result of a decrease in structural order of kaolinite sheets.This treatment opened the edges of the clay layers and increased the diameters of the pore.Moreover, the acid activation process caused the exchangeable cations to be replaced with protons, followed by partial dissolution of tetrahedral and octahedral sheets, which results in increasing new spaces.The acid sites formed at the layer edges contribute to the fixing of phosphate modifiers in the kaolinite structure [54,55].Figure S2 (Supplementary Data) illustrates the FTIR spectrum for raw and modified kaolinites, which gives better insight into the surface characteristics of raw and treated kaolinites.Two strong absorption bands around 3690 and 3650 cm −1 are assigned to the hydroxyl stretching mode of the silanol groups positioned inside the octahedral and tetrahedral sheets of the kaolinite structure.The modified kaolinites showed a noticeable broadband at 3433 cm −1 assigned to O-H-O, which might be due to the oxidation of the structure results from the phosphate modifiers that are rich in oxygen [56].In MK 1 and MK 2, a noticeable decrease in the band intensities at 3690 and 3650 cm −1 and 1600 cm −1 compared to the raw kaolinite suggests the formation of additional acidic sites, due to the hydroxyl group, resulting from incorporation of phosphate on the kaolinite [57].The absorption bands at 967 and 793 cm −1 are responsible for the stretching vibration of Si-O along with the intense peak at 1035 cm −1 , which is attributed to the symmetric stretching vibration of Si-O-Si [58].
It was reported that the natural clays are widely used for the adsorption of metal ions from water as they are economic solution for this purpose.However, the adsorption capacity of these natural materials is low and could be enhanced by modification with different chemicals [59].This enhancement of the adsorption capacity mainly depends on the type of modifier, the activation of the clay and the type of modified sorbent.The modification of kaolinite had enhanced the ion exchange capacity of the kaolinite as it changed the composition of the kaolinite surface [22].In a previous study, the effective cation-exchange capacity of the kaolinite towards the adsorption of Pb +2 was higher when it was modified by potassium hydrogen phosphate [60].The modification of kaolinite with phosphate modifiers can affect adsorption of U(VI) and Th(IV) on the surface of modified kaolinite due to the interaction between these modifiers with both metal ions [60].It should be noted that the modification of kaolinite with phosphate modifiers had a significant effect on the surface of kaolinite rather than the crystal structure of the clay mineral [61].Regarding the mechanism of the attachment of phosphate modifiers on the surface of kaolinite, it was reported that a chemical reaction between these modifiers and the alumina on the surface of the kaolinite can be developed, causing a formation of new hexagonal crystals, which create a larger size and multilayer growth on the surface of the kaolinite.This will have the effect to increase the adsorption capacity [62].Another study had compared the FTIR of raw and phosphate-modified kaolinites and it stated that the modification occurred via Si-O bond linkage [22].The shape of unmodified kaolinite was in the form of dense sheet and this structure had been changed to the fractional acerose structure after the modification.The layers of the unmodified kaolinite had been broken, thus forming more micro-sized particles with a larger surface area [22].

Adsorption study of Th(IV) and U(VI) on modified kaolinite
As shown in Table 1, the metal ion uptake (mg of metal ions adsorbed per g of modified kaolinites) decreased as the mass of both modified kaolinite increased.The decrease of both metal ion uptake with the increasing modified kaolinite dosage can be explained by the formation of a low concentration gradient between the metal ions in aqueous solution and their concentration on the surface of kaolinites, causing a mass transfer in the solid-liquid interface system [63].In addition, the adsorption efficiency increased as the mass of modified kaolinite increased and then it remained relatively constant at high dosages.As the mass of modified kaolinites increases, more available sites for adsorption are present, thus increasing the adsorption efficiency [64].It was reported that the increases in adsorption efficiency with the increasing adsorbent amount is due to the higher proportions of the sites available for exchanging cations [65].The adsorption remained constant at a high level of masses since the competition between the modified kaolinite solid particles is high, causing a decrease in the exchangeable surface sites with the increasing solid content [66].Generally, the optimum weight that achieved higher adsorption for Th(IV) was 0.3 g for MK 1 kaolinite and 0.5 g for MK 2 kaolinite.In the case of U(VI), the optimum weight that achieved higher adsorption was 0.5 g for both modified kaolinites.In summary, the mg of U(VI) and Th(IV) ions adsorbed per one gram of modified kaolinites had the trend to decrease as the mass of the solids increased, but the overall adsorption efficiency had been increased.This can be attributed to the dependency of the adsorption capacity and efficiency on the adsorption sites available on the surface of the modified kaolinites [67].
In order to study the effect of pH of solution on the removal of U(VI) and Th(IV) using both modified kaolinites, the removal efficiency of these metal ions from their aqueous solution was investigated as a function of pH in the range of 2-7. Figure 1 shows that the adsorption of both metal ions on the surface of modified kaolinites increased as the pH increased until it reached a maximum at pH = 7, at which no precipitation of metal ions was observed.The pH of the solution determines the mechanism of the adsorption of both metal ions on the surface of the modified kaolinites.It was reported that at low pH values, the ion exchange mechanism is predominate, whilst the precipitation and innersphere complex formations can occur at high pH values [66].
The effect of pH can be attributed to the charge that is developed on the surface of the adsorbent and the forms of both metal ions at different pH values [68].The ability of the modified kaolinite to adsorb both metal ions is highly affected by the pH of the solution.At low pH values, the surface of the modified kaolinites is positively charged since it will be protonated by the excess H 3 O + ions [69].As the pH of the metal ion solution increases, the surface becomes deprotonated and more negative charges can be developed on the surface of both modified kaolinites, which increases the electrostatic interaction between the surface and both metal ions [69,70].In addition, in acidic solution, there will be a competition between both metal ions and H 3 O + on the active sites of the modified kaolinites available for the binding process [71,72].As pH increases, the concentration of H 3 O + decreases and higher negative charges on the surface are developed, causing an increase in the adsorption process [72].The change in aqueous solution pH has a significant effect on the form of both metal ions developed in the solution.The most abundant form of uranyl is UO 2 +2 (hexavalent state).At low pH values, UO 2 (OH) + , (UO 2 ) 3 (OH) 5 + , (UO 2 ) 2 (OH) 2 +2 and (UO 2 ) 3 (OH) 4 +2 complex ions are formed [66,73,74].This indicates that at low pH values, the electrostatic repletion between the positively charged surface and metal ions caused low adsorption efficiency.These cationic forms of uranyl species can undergo hydrolysis and polymerisation, causing a decrease in the adsorption process [75].At pH values higher than 7, uranium can exist in the solution as UO 2 (OH) −3 , UO 2 (OH) 4 −2 , (UO 2 ) 2 (OH) −3 and (UO 2 ) 3 (OH) 7 − complex ions.When the pH is extremely higher (above 9), uranium will be precipitated as UO 2 (OH) 2 [74].
The most abundant form of thorium in acidic solution is Th +4 .In the acidic solution, thorium will be in the form of Th(OH 2 ) +2 and as the pH values increased, thorium will exist as Th 2 (OH) 2 +6 .The mechanism of thorium adsorption on the modified kaolinite can be explained as follows: hydration of Th +4 ions first occurred, forming Th(OH) +3 , which then binds to the negatively charged surface.In addition, Th +4 can directly bind to the negatively charged surface of the modified kaolinite [76].At high pH values, thorium will be precipitated as hydroxides.
Studying the thermal stability of an adsorbent is essential to evaluate its practical applications in radioactive environments.Therefore, the thermal stability of both modifiers was investigated based on the removal efficiency of both radioactive metals at different temperatures.The effect of increasing temperature on the adsorption of Th(IV) and U(VI) on both types of modified kaolinites is shown in Figure 2. It was reported that when the adsorption increases at low temperatures, the process is exothermic and when the adsorption increases at high temperatures, the process is endothermic [3,46].As shown in Figure 2, the adsorption capacity increases as the temperature increases from 25 to 45°C, indicating an endothermic process.This is because the energy required for the dehydration of both the metal ions in their aqueous solutions exceeds the energy released when these metal ions attached on the surface of both modified kaolinites [64,66].MK 2 showed higher removal efficiency for both radioactive metals at all temperatures studied, suggesting that this modified kaolinite has a more uniform surface dominated by the negatively charged oxygen edge, favouring the removal of both metal ions.
As shown in Figure 3, at an initial concentration of 50 ppm of both metal ions at constant pH of 3 and a temperature of 25°C, high removal of metal ions was achieved quickly and then the increase in the contact time has little effect on increasing both metal ion removal from their aqueous solution.The adsorption process occurred rapidly at first 30 minutes and increased with the increasing mixing time before slowing down until equilibrium is reached within the time period of 0.5-24 h, at which the adsorption of both metal ions remained constant with the increasing time.In the first 30 min, there was high affinity for both metal ions to be adsorbed on the active sites on both modified kaolinites due to the presence of a large number of these active sites available for binding metal ions.Then, the adsorption process became unchanged as these active sites became filled so that no vacant sites are available for the adsorption.This will cause a saturation on the surface of the modified kaolinites [64,72].Furthermore, the reduction of the adsorption process at higher contact times can be attributed to the tendency of metal ions to diffuse into the active sites located in the deeper mesoporous structure of both modified kaolinites, which is a slow process [64].

Adsorption isotherms on modified kaolinites
The adsorption process can be evaluated by adsorption isotherms, which are mathematical models that are used to evaluate the relationship between the concentrations of both metal ions that are adsorbed on the surface of modified kaolinites at constant temperature and the concentrations of these metal ions remained residual [59].This distribution of metal ions between the liquid solution and the surface of modified kaolinites is based on the homogeneity and heterogeneity of the surface of the adsorbent, the type of coverage (monolayer or multi-layer), and the interaction between metal ions in the solution [69].The adsorption isotherm models (Langmuir, Freundlich and Dubinin-Radushkevich (D-R)) were used to analyse the experimental data in order to investigate the nature of the adsorption process.As the correlation coefficient (R 2 ) is closer to one, the isotherm model is more suitable to describe the adsorption process [59].The adsorption isotherms for Th(IV) and U(VI) were determined at a pH value of 3.0 and at three different temperatures (25, 35 and 45°C) in the range of concentrations from 10 to 100 ppm for both radioactive metals.Figure 4 presents the adsorption isotherms for these metal ions at pH = 3 and at a temperature of 25°C.For MK 1, Figure 4 shows that increasing the concentration has little effect on the removal of Th(IV).However, an opposite trend was observed for U(VI).For MK 2, the results demonstrated that the removal of Th(IV) is significantly dependent on the concentration, but has no effect on the removal efficiency of U(VI).The characteristic parameters obtained using the three adsorption isotherms are presented in Table 2.The maximum adsorption capacity for Th(IV) and U(VI) at 25°C onto MK 1 was found to be 20 mg/g at an initial concentration of 90 ppm and 43 mg/g at an initial concentration of 80 ppm for both radioactive metals, respectively.The maximum adsorption capacity for Th(IV) and U(VI) at 25°C onto MK 2 was found to be 17 mg/g at an initial concentration of 80 ppm and 5 mg/g at an initial concentration of 90 ppm.

Langmuir isotherm on modified kaolinites
The highest values of R 2 of the Langmuir adsorption isotherm (Table 2) indicated that this model fit for assessing the adsorption process.The Langmuir adsorption isotherm indicates that the surface of the modified kaolinites has a finite identical adsorption capacity (monolayer) as each active site on its surface can only bind a single ion and all the exchangeable sites are independent of the metal ion amount adsorbed on the surface [28,73].In this work, the Langmuir adsorption isotherm fit the adsorption process and consequently, it was chosen to determine the maximum adsorption capacity of both metal ions on the modified kaolinites.There are three linear forms for the Langmuir isotherm: form (I), form (II) and form (III).
The experimental data showed that the adsorption process fits the three linear forms of the Langmuir adsorption isotherm.Since the Langmuir form (II) fits better, it will be discussed in detail whilst evaluating the adsorption process.
Langmuir isotherm form (II) can be expressed by the following linear relationship:  (Figure S3 (Supplementary Data)), where c and q represent the equilibrium concentration and metal ion uptake, respectively.Plotting 1/q versus 1/c will give a linear curve and the resulting equation can be analysed to determine Langmuir parameters.The slope and intercept of the linear equation give 1/q m K L and 1/(q m ), respectively.q m is a parameter that corresponds to the adsorption capacity, whereas K L is related to the adsorption energy of the process.Langmuir isotherms were determined for Th(IV) and U(VI) on modified kaolinites at pH = 3 and at different temperatures in the range of concentrations from 10 to 120 mg/L.
According to Table 2, q m values for MK 1 kaolinite are higher than those for MK 2 kaolinite.No higher values of q m were obtained since both metal ions had filled the available sites on both modified kaolinites.Generally, the adsorption capacity (q m ) and adsorption energy (K L ) increased with increasing temperature.All the three forms of Langmuir show strong linear correlation (R 2 > 0.9).It was observed from Table 2 that the q m obtained from the Langmuir adsorption isotherm decreased as temperature increased.In a previous study on the adsorption of Pb 2+ and Cd 2+ from aqueous solution by a biomass, the experimental data were fit with Langmuir at all studied temperatures and it was reported that the adsorption capacity of these ions decreased with increasing temperature and adsorption capacity was improved at lower temperatures.This is probably due to the reduction in the boundary layer thickness at high temperatures, causing the metal ions to move from the surface of the adsorbent to the aqueous solution [77].
The values of q m and K L for kaolinite modified with sodium phosphate (MK 1) are considerably higher than those forr the kaolinite modified with sodium hexametaphosphate (MK 2), suggesting that the incorporation of sodium hexametaphosphate may have the effect of blocking the adsorption sites of the raw kaolinites and decrease the efficiency of metal ion uptake.These ions seem to reach saturation, which indicates that the metal ions had filled all available sites of the modified kaolinite surface, which caused a decrease in the adsorption process.

Freundlich and Dubinin-Radushkevich isotherms on modified kaolinites
The data were also processed using the Freundlich adsorption isotherm, which indicated that the active sites on the surface of the modified kaolinites are distributed in heterogeneous pattern with the multilayered surface and are involved for the adsorption process [59].In this model, the adsorption efficiency increases as the concentration of metal ions increases due to the presence of different kinds of adsorption sites on the surface of modified kaolinites [69].
The Freundlich adsorption isotherm can be expressed by the following linear relationship: (Figure S4 (Supplementary Data)).
The plot of log q versus c produces a linear curve and can be used to determine the adsorption capacity (K F ) and the adsorption intensity (n).The slope of the linear curve is 1/ n, whilst the intercept represents (log K F ).
The linear form for the Dubinin-Radushkevich isotherm has the following expression: (Figure S5 (Supplementary Data)), where R represents the gas constant (8.314J.mol −1 .K −1 ), whilst the temperature should be expressed in Kelvin.When lnq is plotted versus ε 2 , a linear curve should be obtained and the linear equation can be used to determine q max (adsorption capacity) and β (adsorption energy) with the slope and intercept of the linear equation.The adsorption energy (β) can be used to determine the free energy (E), which is required to adsorb one mole of the adsorbate onto the surface of the adsorbent, and this energy can be calculated using the following formula: Freundlich and Dubinin-Radushkevich isotherm's parameters, K F , n, β, qm/ and E, were determined for Th(IV) and U(VI) on modified kaolinites at pH = 3 and different temperatures in the range of concentrations from 10 to 120 mg/L (Table 2).K F and n are the parameters that describe the Freundlich adsorption isotherm, in which K F indicates the relative adsorption capacity of the adsorbent and n indicates the extent of ion adsorption of the surface of the adsorbent [69].According to Table 2, the values of n are less than 1.0, which indicates that the system is less heterogeneous and more efficient at low concentrations [59,64,69,78].The results showed that for Th(IV) adsorption on both modified kaolinites, the values of n increased as the temperature increased at a specific pH value, indicating that the adsorption process is favourable at high temperatures.The values of n remained slightly constant or decreased when U(VI) was adsorbed onto the surface of both modified kaolinites, which might be referred to the small extent of the adsorption of U(VI) onto the surface of the sorbent.The increase in the values of K F indicates higher adsorption capacity [78].The results shows that Th(IV) adsorption on both modified kaolinites has higher adsorption capacity as temperature increased.This trend is the same for the adsorption of U(VI) on the surface of MK 1 kaolinite but to a smaller extent.For MK 2 kaolinite, the values of K F slightly decreased as temperature increased, which indicates lowering of the adsorption capacity for U(VI) at higher temperatures.For the D-R isotherm, q m and E are increase with increasing temperature, indicating that the adsorption is more favourable at high temperatures (Figure S4 and Figure S5 (Supplementary Data)).There are differences in q m values derived from the Langmuir and Dubinin-Radushkevich model, which can be attributed to the difference in the definition of q m in each model.In the Langmuir model, q m represents the maximum adsorption capacity of both metal ions on a monolayer surface, whilst q m in the Dubinin-Radushkevich model represents the maximum adsorption of metal ions at a total specific microporous volume of sorbent [72].

Kinetic study of the adsorption process
In order to predict the adsorption rate and mechanism of both metal ions on the surface of modified kaolinites, the adsorption kinetics were used to process the data obtained by measuring the adsorption at different time intervals [79].The adsorption data were modelled using pseudo first-order and second order models.The pseudo-first and second order kinetics were obtained by plotting the amounts of both metal ions adsorbed on both modified kaolinites at different time intervals.The linear form of the pseudo-first-order equation is expressed as follows: The linear forms of the pseudo-second-order of kinetics are described by the following equation: where q t is the amount (mg/g) of U(VI) and Th(IV) ions adsorbed at time t.q e is the amount (mg/g) of U(VI) and Th(IV) ions per unit mass adsorbed at equilibrium.K 1 and K 2 are the pseudo-first-order rate constant and pseudo-second-order rate constants, respectively (g.mg −1 .min−1 ).
The parameters of kinetic models and the regression correlation coefficient (R 2 ) are shown in Table 3.According to Table 3, the pseudo-second order model fit more than the pseudo-first order model as it has a higher regression coefficient (R 2 ), indicating that the adsorption of both metal ions is a chemosorption process rather than a physiosorption process [70,73].The chemosorption nature of adsorption indicates that the overall adsorption process depends on the amount of both metal ions in the solution and the number of adsorption active sites on the surface of modified kaolinites [64].The compatibility of kinetic data with the second order model also indicates that the theoretical and experimental adsorption capacities of both metal ions were close to each other, suggesting a high correlation with the experimental data for the adsorption of U(VI) and Th(IV) from the aqueous solution on the modified kaolinites [72].

Thermodynamics study of adsorption process
In the thermodynamic study of the adsorption process, changes in Gibbs free energy (ΔG o ), change in the enthalpy of adsorption (ΔH o ) and changes in entropy of adsorption (ΔS o ) were calculated using the following equation: where K d and R represent the equilibrium and gas constant, respectively.The temperature (T) is used in Kelvin.When lnK d was plotted versus 1/T, the resulting curve will be linear and the curve obtained can be used to determine the thermodynamic parameters.The slope and intercept of the linear equation were used to calculate ΔH o and ΔS o , respectively.
For each temperature studied, ΔG o was calculated as follows: Figure S6 (Supplementary Data) shows the plots of lnK d versus 1/T.ΔG o , ΔH o and ΔS o thermodynamic functions for the adsorption process are presented in Table 4.The higher negative values of ΔG o were found at higher temperatures, indicating that the process of adsorption is more favourable at high temperatures, and it is a spontaneous process.The negative value of ∆G o indicates that the energy required for both metal ions to bind with the active site on both modified kaolinites is stronger than metal ion-water intermolecular forces [64].The high values of ∆G o indicate that the adsorption process is chemical sorption [64].According to previous studies, it was reported that when the values of ΔG o increase as temperature increases with values greater than 40 kJ/mol, the change in the adsorbent surface will transfer or share the metal ions to form a coordinate bond [47].
In addition, the positive value of ∆S o indicates that the adsorption process increased the randomness of the liquid-solid system due to the release of more water molecules when the metal ions are bonded to the surface of the modified kaolinites, causing an increase in the number of particles in the system [59,64,71].The positive values of ∆S o are always favourable in the adsorption process since it indicates that the metal ions undergo a dehydration process, which enhances their attachment on the surface of the adsorbent [64,72].The positive values of ∆H o indicate that the process of adsorption of both metal ions on the surface of modified kaolinites is an endothermic process, suggesting that the adsorption system required external energy to occur [59,64,72].As mentioned above, the adsorption of U(VI) and Th(IV) required an energy to overcome the ion-dipole intermolecular forces between the metal ions and water molecules to allow the metal ions to be adsorbed on the surface of both modified kaolinites [64,72].

Binary system experiment
Solutions of U(VI) and Th(IV) were prepared and then used to study the selectivity of both themodified kaolinites towards these two metal ions.When the two metal ions were mixed with each other, a competition occurs at the modified kaolinite surface.It is expected that the presence of one metal ions affects the adsorption of the other, and consequently, a difference between single and binary systems will be noticed on both modified kaolinites.When the two metal ions of 50 ppm were mixed with MK 1, the adsorption efficiencies of Th(IV) and U(VI) were 93.1 and 83.5%, respectively.When these two metal ions were mixed with MK 2, the adsorption efficiencies of Th(IV) and U(VI) were 69.1 and 18.2%, respectively.The results showed that the modified kaolinites are more selective to adsorb Th(IV), even in the presence of U(VI).This indicates that the binding of Th(IV) on the surface of modified kaolinites requires less energy than for the binding of U(VI).The modified kaolinites had more affinity to adsorb Th +4 ions in the presence of UO 2 +2 due to the effect of the ionic charge and atomic radius.Th +4 is more favourable since its effective ionic charge (radii/ionic charge) is lower (94 ppm/4) than UO 2 +2 (253 ppm/2) [76].The adsorption of Th(IV) and U(VI) metal ions on both modified kaolinites decreases when going from pure solution (single system) to a mixture solution (binary system).The competition between the two metal ions on the available sites on the surfaces of both modified kaolinites reduced the adsorption of these two metal ions compared to their pure solution.

Desorption and reusability experiments
To investigate the efficiency of the adsorption process and the ability of the both modified kaolinites to be regenerated for further use, the desorption/regeneration experiments were carried out as described in section 2.8.The high dependence of the adsorption process on pH indicates that both the metal ions can be recovered by acidic solutions.The percent recovery of Th(IV) from MK 1 and MK 2 kaolinites using 0.1 M HNO 3 was 76.8 and 69.0%, respectively, and when 1.0 M HNO 3 was used, the percent recovery of Th(IV) from MK 1 and MK 2 kaolinites increased to 79.1 and 95.1%, respectively.For U(VI) ions, the percent recovery of this metal ion from MK 1 and MK 2 using 0.1 M HNO 3 was 46.7 and 38.6%, respectively, and the percent recovery had been increased to 53.4 and 41.3% when 1.0 M HNO 3 was used.These results show that the elution of both metal ions from the modified kaolinites loaded with these metal ions has been increased by increasing the concentrations of HNO 3 , suggesting the effect of H 3 O + to recover these metal ions adsorbed on the surface of both modified kaolinites.The percent recovery results showed that Th(IV) was easier than U(VI) to be recovered, indicating that U(VI) is strongly bonded on the surface of modified kaolinites compared with Th(IV).The reusability of the modified kaolinites for U(VI) and Th(IV) was tested three times and the resulting adsorption percentages were calculated as described in equation 1.The adsorption averages of Th(IV) of the three regenerations cycles were 65.4% for MK 1 and 57.7% for MK 2, respectively.In the case of U(VI), the adsorption averages of the three regeneration cycles were 20.8% for MK 1 and 55.9% for MK 2, respectively.

Effect of interferences
The effect of the presence of chloride counterions and cations (Fe +3 , Mg +2 , Na + ) on the adsorption of U(VI) and Th(IV) on the modified kaolinites was investigated.It was shown that the presence of these interferences in the mixture had an effect on the adsorption of U(VI) and Th(IV) on the modified kaolinites.The adsorption efficiency of Th(IV) on MK 1 and MK 2 had decreased by 40.3 and 32.2%, respectively, and the adsorption efficiency of U(VI) on MK 1 and MK 2 had decreased by 18.2 and 22.8%, respectively.Previous results had indicated that high concentrations of nitrate, chloride and sulphate anions had little effect on U(VI) and Th(IV) adsorption, depending on the characteristics of the adsorbent surface [73].Other studies had indicated that the adsorption of U(VI) in the presence of sulphates and phosphates had been enhanced, whilst the adsorption decreased when chloride ions were in the solution [66].Generally, the effect of the interferences on the adsorption of metal ions on the surface of the adsorbent mainly depends on the type of counterion or cation and the characteristics of the adsorbent surface.

Conclusion
Raw kaolinite was modified by two types of phosphate salts: sodium polyphosphate (MK 1) and sodium hexametaphosphate (MK 2) and their applications in Th(IV) and U(VI) removal from their solutions were investigated.The results showed that the impregnation of phosphates' modifiers on the surface of kaolinite enhanced the removal of these metal ions from their solutions.Raw and modified kaolinites were characterised using XRD and FTIR in order to study the effect of phosphate modifiers on the surface of kaolinites.XRD patterns showed that the illites, feldspar and haematite disappeared after the modification process, suggesting a successful treatment and modification of the raw kaolinite surface.The FTIR spectrum showed a decrease in the intensities of 3690 and 3650 cm −1 , which indicates the formation of additional acidic sites caused by the phosphates' modifier incorporation.The adsorption process was studied as a function of dosage, contact time, pH and temperatures.The percentage of adsorption of both metal ions on modified kaolinites increased as the dosage increased.The optimum weight, which achieved higher Th(IV) removal, was 0.3 g for MK 1 and 0.5 g for MK 2, whilst 0.5 g for both modified kaolinites achieved higher adsorption efficiency for U(VI).The effect of pH was studied in the range of 2-7 and the results had revealed that the percent of Th(IV) adsorption increased at pH higher than 3 and percent of U(VI) adsorption increased at pH higher than 5 for both modified kaolinites.The adsorption of both metal ions on the modified kaolinites increased with increasing temperature, suggesting an endothermic adsorption process.High metal ion removal was achieved quickly and little effect was observed with the increasing contact time.The optimised adsorption conditions for U(VI) into both modified kaolinites include the following: solution pH = 3, adsorbent dose 0.5 g, initial concentration = 50 ppm, contact time = 5 h and temperatures of 35°C (MK 1) and 25°C (MK 2), corresponding to uptake of about 102 (MK 1) and 25 (MK 2) mg/g.The optimised adsorption conditions for Th(IV) into both modified kaolinites include the following: solution pH = 3, adsorbent dose (0.3 g MK 1, 0.5 g MK 2), initial concentration = 50 ppm, contact time = 5 h and a temperature of 25°C, corresponding to uptake of about 188 (MK 1) and 9 (MK 2) mg/g.The adsorption isotherm fit Langmuir better than Freundlich and Dubinin-Radushkevich isotherms.The value of q m and K L increased as the temperature increased.MK 1 showed higher values of q m and K L than MK 2 for both metal ions.Kinetic studies indicated that the adsorption process on both modified kaolinites follow pseudo-second-order.Thermodynamic studies imply that U(VI) and Th(IV) adsorption on both modified kaolinites is an endothermic and spontaneous process.The desorption of both metal ions from the modified kaolinites, previously loaded with these metal ions, showed a higher percent recovery of both metal ions when 0.1 M and 1.0 M HNO 3 was used for this purpose.The percent recovery increased as the concentration of the acid increased due to the high concentration of hydronium ions.The recovery of Th(IV) ions was easier than that of U(VI) ions, indicating a high bonding of U(VI) ions to the active sites present in both modified kaolinites.Both the modified kaolinites were regenerated three times and the results indicated a little effect on the efficiency of the modified kaolinites to adsorb the metal ions after the desorption/regeneration process.The presence of anionic and cationic interferences had the effect to reduce the adsorption of Th(IV) and U(VI) on the modified kaolinites, probably due to the competition of these interferences with the studied metal ions on the available sites on both modified kaolinites.

Table 1 .
Th(IV) and U(VI) ion uptake, expressed as mg/g and % adsorption efficiency, by MK 1 and MK 2 kaolinite as a function of dosage at a concentration of 50 ppm (50 mL volume), 25°C, pH = 3.0 and a shaking time of 5 h.

Figure 1 .
Figure 1.Effect of pH on Th(IV) and U(VI) adsorption on both modified kaolinites.

Figure 2 .
Figure 2. Effect of temperature on Th(IV) and U(VI) adsorption on both modified kaolinites.

Figure 3 .
Figure 3.Effect of the contact time on Th(IV) and U(VI) adsorption on modified kaolinites.

Figure 4 .Table 2 .
Figure 4.The impact of the feeding solution isotherm of U(VI) and Th(IV) on uptake onto modified kaolinite.

Table 4 .
Thermodynamic functions for Th(IV) and U(VI) adsorption on both types of modified kaolinite at different temperatures.