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Abstract

Goethite is a stable and widespread mineral present in soil with many uses, and it affects the transportation and immobilization of heavy metals in solution. Nanogoethite was synthesized by a chemical precipitation method and used to batch adsorb U(VI) in solution. Adsorption experiments were used to understand the role of nanogoethite in controlling the U(VI) adsorption behavior in soil. The morphology and the crystallinity of nanogoethite were characterized by scanning electron microscopy and wide-angle X-ray powder diffractometry, respectively. The results showed that the crystallinity of nanogoethite after the adsorption of uranium did not change, but small particles appeared on the surface of the scales. The surface area was determined from N₂ adsorption–desorption experiments using the Brunauer–Emmett–Teller to be 81.86 m²/g. The effects of factors such as the contact time, pH, adsorbent dosage, and the initial concentration of uranium on the adsorption of U(VI) were investigated. The experimental results showed that nanogoethite removed over 85% of the U(VI) in an aqueous 5.0 mg/L U(VI) solution at pH 4.0 and at 298 K. The pseudo-second-order model was used to simulate the adsorption process. The results show that chemisorption plays a major role in the adsorption process. The results of this study suggest that nanogoethite may play a significant role in controlling the migration and transfer of U(VI) in the soil, thus controlling the presence of U(VI) in soil.

Keywords

Uranium chemical precipitation synthesis nanogoethite adsorption iron oxide

Introduction

Uranium is an important raw material for the development and utilization of nuclear energy. Uranium nuclear fuel is produced through mining, purification, and enrichment processes. In these processes, the uranium is transformed between various chemical species, such as U₃O₈ and UO₂. As a result, a large amount of uranium-containing wastewater is produced. Uranium is harmful to the environment because of its radioactive nature (Zong et al., 2013). The predominant nature valence states of uranium are U(IV) and U(VI) depending on the redox environment (Sheng et al., 2014). The oxidized U(VI) state is highly migratory and more soluble (Sun et al., 2014). Thus, the discharge of U(VI) is particularly hazardous to human health. For example, U(VI) can reach the top of the food chain where it is consumed by humans, thus causing harmful effects on the human body, such as kidney damage, liver damage, and even death (Sethy et al., 2015; Yang et al., 2011). U(VI) is, thus, a significant threat to human health, particularly because of its mutagenic and carcinogenic characteristics (Konietzka, 2015; Njinga et al., 2016; Saini and Melo, 2015). The most effective method of U(VI) removal from aqueous solution is adsorption or immobilization by reduction. Therefore, it is vital to remove U(VI) before it is released into the environment.

At present, common methods for the removal of U(VI) include chemical precipitation (Vigier et al., 2018), ion exchange method (Amphlett et al., 2018), the solvent extraction (Tsaoulidis et al., 2018), membrane method (Torkabad et al., 2017), and adsorption (Pooley et al., 2018). Of these methods, adsorption is the most effective method for the removal of U(VI) ions from solution because of its high efficiency, easy operation, wide range of adsorbents, and economic feasibility. Many different adsorbents have been used to remove and recover U(VI) from water, including zeolites (Bakatula et al., 2017), carbon materials (Shao et al., 2009), magnetic composite (Das et al., 2010), polymer and metal organic frameworks (Ma et al., 2015), goethite (Lee et al., 2016), and nanomaterials (Shao et al., 2010). However, these materials have some inherent defects such as low adsorption capacities, poor stabilities, high costs, and low surface areas. Therefore, significant research to find alternatives has been carried out.

Nanomaterials are promising adsorbents for the removal of various dissolved contaminants from solution because of theirs large specific surface areas and other appropriate properties. Furthermore, nanomaterials can be used for the solid-phase extraction and measurement of samples at a trace level. Additionally, the size of particles may affect the adsorption of radioactive contaminants on iron oxides/hydroxides because of the large specific surface areas of the nanoparticles and increased proportion of edge sites, surface tension, surface plasmon resonance, dispersion ability, and intrinsic reactivity (Waychunas et al., 2005). The structure and morphology of nanopowder surfaces are dramatically different from those of larger particles, which can affect both the physical affinity and chemical capacity of radioactive contaminants. Zeng et al. (2009) investigated the adsorption of U(VI) on nanohematite particles and found that the adsorption affinity decreased as the particle size increased from 12 to 125 nm. Goethite is an abundant and nontoxic mineral. It has an excellent adsorption capacity, high catalytic activity, stability to weak acid or bases, and in addition, it is environmentally friendly. In addition, goethite (α-FeOOH) is a principal component in soil and iron ore. Furthermore, it is a chemically stabilized iron oxide in many soils and sediments. In summary, goethite is an effective adsorbent of contaminants from solution (Ni et al., 2017), because it is highly insoluble and has a net positive surface charge at environmental pH levels. Concerning the surface chemistry, the synthetic α-FeOOH particles are often used as ideal adsorbents in fundamental studies of the adsorption/desorption phenomena. Moreover, goethite is a promising adsorbent for contaminants because of high specific surface area, stable chemical properties, well-defined crystalline structure, and other important characteristics and is widely used in the treatment of contaminants in solution (Cui et al., 2017; Singh et al., 2012; Tinnacher et al., 2011; Um et al., 2011). Recently, papers (Gu et al., 2011) have been reported on the mechanism of the adsorption of norfloxacin by goethite. The results indicated that this process is an exothermic process and its adsorption mechanism is mainly ion exchange. Later, Sherman et al. (2008) reported the adsorption of U(VI) on goethite by batch adsorption experiments and the use of density functional theory (DFT) to calculate the energetics and structures of possible surface complexes. Similarly, an experiment by Tang et al. (2010) showed the adsorption of main pollutants As(V) on goethite in the presence of the fluoride, and it was found that the affinity of As(V) for goethite was greater than that for fluoride. In addition, some researchers have studied the adsorption of U(VI) on goethite (Bao, 2013; Liu et al., 2014) . However, the influencing factors, properties, and mechanism of uranium adsorption on nanogoethite are poorly understood. In this study, we synthesized nanogoethite by chemical precipitation to explore the mechanism of the adsorption of U(VI) on nanogoethite.

First, we prepared the nanogoethite by chemical precipitation and characterized the crystallinity and morphology using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. To investigate the effects of factors, such as the contact time, pH, adsorbent dosage, and the initial concentration of U(VI) on the adsorption of U(VI) on nanogoethite, batch experiments were carried out. In addition, the adsorption of U(VI) on nanogoethite was studied. The results will help the management of the uranium-contaminated soil and establish a model for uranium migration in groundwater.

Experimental

Main reagents

All chemical reagents in the experiments were of analytical grade or higher and used without further purification. A stock solution of U(VI) (1 g/L) was prepared by dissolved U₃O₈ (purity 99.99%) in 1 mg/L hydrogen nitrate, which was then diluted to the required concentration for the experiments. Solution of 1 mol/L FeC1₃, 0.6 mol/L NaOH, 0.01 mol/L HCl, 0.01 mol/L NaOH, and 0.01 mol/L HNO₃ solution were prepared. All the stock solutions were prepared with deionized water for both material preparation and adsorption experiments.

Synthesis of nanogoethite

The nanogoethite was synthesized using a previously reported chemical precipitation method (Siyuan, 2012). Briefly, 1 mol/L FeC1₃ and 0.6 mol/L NaOH were mixed and stirred for 10 min, and then NaOH (0.01 mol/L) solution and HCl (0.01 mol/L) solution were added to the mixed solution to adjust the pH. The mixed solution was heated at 450°C for a period, and the mixture was centrifuged. The precipitate was washed with deionized water and 0.01 mol/L HNO₃ several times and finally dried at 70°C in an oven for 24 h.

Characterization

The crystal structures of the treated samples were determined using a powder X-ray diffractometer (Bruker, Bruker D8) operating with Cu-K_α radiation (λ = 1.5418 Å) generated at an accelerating voltage of 40 kV and applied current of 30 mA. The XRD patterns were collected from 10° to 70° in 2θ at a scan rate of 4°/min at room temperature. The surface morphology of the treated samples was observed by using an SEM (JEOL, JSM-7500F) operating at 12.5 kV. The specific surface area and pore size distribution were determined from adsorption–desorption experiments (Micromeritics, tristar2020) using the Brunauer–Emmett–Teller (BET) isotherm.

Batch adsorption tests

U(VI) solution used in all batch adsorption experiments was prepared by a diluting U(VI) standard concentration solution with deionized water. In subsequent adsorption experiments, 100 mg of α-FeOOH nanoparticles was added to a 250 mL conical flask along with 100 mL of uranium solution. The initial pH of the solution was adjusted with HCl (0.01 mol/L) and NaOH (0.01 mol/L) solutions. After the conical flasks had been shaken at the desired temperature (T, K) for a given time (t, min), the samples were centrifuged and the U(VI) concentrations were analyzed by titania reduction/ammonium vanadate oxidation titration (Wei-Dong et al., 2010) after adsorption. All assays were carried out in triplicate, and the mean values are presented. For the kinetic studies, 100 mg of nanogoethite was added to 100 mL of a 5 mg/L uranium solution. The suspensions were withdrawn at appropriate time intervals. The mixtures were shaken for 120 min, which was confirmed to be sufficient for adsorption equilibrium. The adsorption amount Q_e (mg/g) and removal percentage were calculated according to equations (1) and (2).

Q e = \frac{(C_{0} - C_{e}) V}{m}

(1)

R = \frac{C_{0} - C e}{C_{0}} \times 100 %

(2)

where C₀ (mg/L) and C_e (mg/L) are the initial and equilibrium concentrations of uranium, respectively, V (L) is the volume of the uranium solution, and m (g) is the amount of adsorbent.

Results and discussion

Characterization of nanogoethite

XRD measurements were used to determine the crystalline phases of the material, and XRD patterns of nanogoethite before and after adsorption of U(VI) are shown in Figure 1. For the XRD pattern of nanogoethite, the reflections at 2θ = 21.19°, 26.37°, 33.17°, 36.68°, and 41.12° were indexed to the α-FeOOH goethite phase (JCPDS No. 029-713), and similar peaks were observed in nanogoethite after adsorption, although the intensity of peaks changed slightly, indicating that the crystallinity of the nanogoethite did not clearly change after adsorption of U(VI). Similar results have been obtained previously (Pablo, 2008). The surface morphology was studied by SEM. The SEM images of nanogoethite before and after the adsorption of uranium are presented in Figure 2(a) and (b). The SEM images shows that nanogoethite have a 3D hierarchical self-assembled flake-like morphology, composed of numerous nanorods. The formation of these flakes is due to the formation of nuclei at the initial stage of the reaction. These nuclei have an unstable surface energy and tend to aggregate to reduce the interfacial free energy (Cornell and Schwertmann, 1996). The nanogoethite grain surface is positively charged, and the hydrolysis of urea produces OH⁻ to neutralize the positive charge on the grain surface, leading to a reduction in the surface charge. Thus, the forces between the particles changed from repulsive to attractive, and the resulting particles aggregate, significantly reducing the particle surface charge. Many short rods are petal-shaped and overlapped, growing from the center outwards and self-assembling into the 3D flake-like structure (Krehula and Musić, 2008). Moreover, there were many pores in the nanogoethite, which provided more channels and space for the adsorption of U(VI), facilitating the diffusion of uranium to the interior of the particles.

Figure 1.

XRD pattern of nanogoethite: (a) initial and (b) after adsorption.

Figure 2.

SEM images of nanogoethite: (a) initial and (b) after adsorption.

As shown in Figure 2(b), the edges of the nanorods become rougher, and some of the pores became blocked. SEM measurements reveal that the particles measure 2–100 nm in diameter. The specific surface area of nanogoethite is 81.86 m²/g according to the multipoint N₂-BET analysis. The pore size distribution of nanogoethite is shown in Figure 3, and the pore size is mainly distributed at 2.5 nm.

Figure 3.

Pore size distribution from BET analysis (t = 2 h, T = 120°C).

Adsorption experiments

Effect of pH on uranium adsorption

The effect of the pH on the adsorption of U(VI) on nanogoethite was investigated using 100 mg of adsorbents and 100 mL of 5.0 mg/L U(VI) at pH values from 3.0 to 8.0 at 298 K. The flasks were agitated on a shaker for 120 min to ensure equilibrium was achieved. After adsorption equilibrium had been obtained, the concentrations of U(VI) were quantified by titania reduction/ammonium vanadate oxidation titration. The experimental results are presented in Figure 4(a). Clearly, the pH had a strong influence on the U(VI) adsorption of nanogoethite in the range from 3.0 to 8.0. The adsorption rate and capacity of U(VI) increased rapidly from pH 3.0 to 4.0, reaching relative equilibrium at pH from 4.0 to 7.0, and a maximum at a pH of 8.0. The pH may have a significant effect on the surface charge distribution and active sites of nanogoethite as well as the U(VI) species in aqueous solution (Wang et al., 2017). At low pH, the dominant species is UO₂²⁺ and there is a high concentration of H⁺ ions, which compete with UO₂²⁺ for the binding sites on the surface of the adsorbent. On the other hand, the surface of nanogoethite is positively charged by the protonation reaction and repels U(VI), which is positively charged. Therefore, the low adsorption efficiency is due to the electrostatic repulsion between UO₂²⁺ and the positively charged binding groups of nanogoethite. With an increase in pH, the U(VI) species are transformed from UO₂²⁺ to multi-nuclear hydroxide complexes, such as (UO₂)₂(OH)₂²⁺ and (UO₂)₃(OH)₅⁺, and the repulsion between multi-nuclear hydroxide complexes and nanogoethite is weakened by the deprotonation of the functional groups. Similar trends have been reported by other researchers (Wang et al., 2015). The pH value of U(VI)-containing wastewater was less than 5 in general, and, at a pH of 4.0, the U(VI) is in a diffuse state, which is more conducive to removal. Thus, the subsequent experiments were conducted at a pH of 4.0.

Figure 4.

Adsorption of U(VI) on nanogoethite at different conditions: ((a) U(VI): 5 mg/L, dosage: 100 mg, T: 25°C, and t: 120 min; (b) T: 25°C, t: 120 min, dosage: 100 mg, and pH: 4; (c) U(VI): 5 mg/L, T: 25°C, t: 120 min, and pH: 4; (d) U(VI): 5 mg/L, dosage: 100 mg, T: 25°C, and pH: 4).

Effect of the initial uranium concentration on uranium adsorption

The effect of the initial uranium concentration on the adsorption of U(VI) on nanogoethite was investigated at a pH of 4.0 and at 298 K. Again, 100 mg nanogoethite was added to 100 mL of U(VI) solutions at initial U(VI) concentrations from 5 to 200 mg/L. The flasks were agitated on a shaker for 120 min to ensure equilibrium was reached. After adsorption reached equilibrium, the concentrations of U(VI) were quantified by titania reduction/ammonium vanadate oxidation titration. From the results presented in Figure 4(b), we can see that the adsorption capacity of U(VI) substantially increased with an increase in U(VI) concentration and reached a maximum value at 104.22 mg/g in the 200 mg/L U(VI) solution. The effective collision probability between the uranyl ions and adsorbents increased with increase in initial uranium concentration, which is beneficial for the uranium adsorption on nanogoethite and, thus, increase the adsorption capacity. However, the removal rate of U(VI) decreased with an increase in the U(VI) concentration, which is similar to the trend observed for the adsorption of uranium on polypyrrole (Abdi et al., 2017), hydrous ferric oxide-modified zeolite (Nekhunguni et al., 2017), and a phosphate-modified flower-like α-FeOOH composite (Zhang et al., 2018). The concentration of most uranium mine wastewater is less than 5 mg/L, so, we chose a U(VI) concentration 5 mg/L as the initial concentration in the subsequent experiments.

Effect of adsorbent dosage on uranium adsorption

The effect of adsorbent dosage on the adsorption of U(VI) on nanogoethite was studied using a 5.0 mg/L U(VI) solution at a pH of 4.0 and at 298 K. Again, the nanogoethite was added to 100 mL U(VI) solutions for 20–1000 mg dosage, respectively. The flasks were agitated on a shaker for 120 min to ensure equilibrium was reached. After reaching adsorption equilibrium, the concentration of U(VI) was quantified by titania reduction/ammonium vanadate oxidation titration. To probe the optimal dosage of the adsorbents, the effect of the solid content (20–1000 mg) on the U(VI) adsorption was analyzed. The results are shown in Figure 4(c). The adsorption rate of U(VI) increased from 56% to 90% with an increase in nanogoethite from 20 to 100 mg, respectively; however, the adsorption rate only changed by 6% when the dosage increased from 100 to 1000 mg. That is, adsorption reached 96% when the dosage was 1000 mg. These results indicate that the nanogoethite shows good performance for U(VI) adsorption. At low adsorbent dosage, the high adsorption rate is ascribed to the plentiful active sites on the adsorbent and high particle activity. However, at high adsorbent dosages, the adsorption rate increases slowly with an increase in adsorbent dose because of the high adsorbent accumulation effect and because almost all of the U(VI) has been adsorbed on the active sites of the adsorbent. This result is consistent with previous findings (Zou et al., 2017). From an economic point of view, an adsorbent dosage of 100 mg was chosen as the most suitable dosage for the further experiments.

Effect of contact time on the adsorption of uranium by nanogoethite and adsorption kinetics

The effect of the contact time on the adsorption rate and capacity of U(VI) on nanogoethite was investigated with 5.0 mg/L U(VI) and 100 mg adsorbent. The temperature was controlled by a water bath set at 298 K, and the pH used was 4.0. The results are presented in Figure 4(d). The adsorption rate of U(VI) on nanogoethite increased with an increase in contact time. The adsorption rate of U(VI) on nanogoethite increased rapidly in the initially, and almost 82% U(VI) adsorption was attained in only 5 min, which is rapid compared to those of a cucurbit[6 (the number of structural units of Glycoside)] uril/graphene oxide/Fe₃O₄ system (Shao et al., 2016) and a magnetic cobalt ferrite/multiwalled carbon nanotube (MWCNT) composite (Tan et al., 2015): 10 min and 180 min, respectively. The adsorption process reached equilibrium after 120 min. The rapid adsorption rate in the initial stages may be caused by two reasons. On the one hand, the high concentration of U(VI) in the initial stages results in rapid diffusion into the surface of nanogoethite and combination with a large number of adsorption sites on the surface. On the other hand, the strong chelation of U(VI) by hydroxyl groups, the nanosize, and 3D structure of nanogoethite contribute to rapid diffusion. After 5 min, with the decrease in U(VI) concentration and the reduction in number of surface sites, the adsorption rate slightly increased, and the adsorption process attained equilibrium after 120 min.

A study of the adsorption kinetics was carried out to provide information about the adsorption mechanism. Two common kinetic models (Ho and Mckay, 1999), pseudo-first-order and pseudo-second-order models, were used to simulate the kinetic adsorption process, as shown in equations (3) and (4).

Pseudo-first-order model

q_{t} = q_{e} [1 - e x p (- k_{1} t)]

(3)

Pseudo-second-order model

q_{t} = (k_{2} q_{e}^{2} t) / (1 + k_{2} q_{e} t)

(4)

where q_e (mg/g) and q_t (mg/g) are the amounts of U(VI) adsorbed at equilibrium and time t (min), respectively, and k₁ (min⁻¹) and k₂ (g/mg·min⁻¹) are the pseudo-first-order and pseudo-second-order adsorption rate constants, respectively. The pseudo-first-order and pseudo-second-order models are macroscopic dynamic models commonly used to model the adsorption process. The pseudo-first-order model supposes that adsorption is controlled by the diffusion step. The pseudo-second-order model assumes that the adsorption rate is determined by the amount of adsorbed vacant sites unoccupied at the surface of the adsorbent. The adsorption process is controlled by the chemical adsorption mechanism, which involves electron sharing or electron transfer between the adsorbent and the adsorbate (Namasivayam and Sureshkumar, 2007).

The curves of the above equations are shown in Figures 5(a) and (b), and the kinetic constants were calculated and are listed in Table 1. Based on the obtained determination coefficient (R²), which was found to be 0.998, the experimental data fit the pseudo-second-order model better than with the pseudo-first-order model. Moreover, in the pseudo-second-order model, the calculated q_e (4.22 mg/g) values were nearly same as the experimental values (4.25 mg/g), which further indicates that the pseudo-second-order model simulates the kinetic data well. Furthermore, the average relatives error of the pseudo-second-order model was 0.7% and smaller than that of the pseudo-first-order model, which confirms that U(VI) ion adsorption on nanogoethite can be described by the pseudo-second-order model. The results suggest that the predominant adsorption mechanism is chemical adsorption. They also indicate that the rate-determining step might be chemical adsorption, and the adsorption behavior might involve interactions via electron exchange between U(VI) ions and nanogoethite (Bhattacharya et al., 2008). This can be represented by the following reaction: ≡FeOH + UO₂²⁺ ↔≡FeOUO₂⁺+H⁺ or 2≡FeOH +UO₂²⁺ ↔(≡FeO)₂UO₂ + 2H⁺. That is, the main adsorption site is oxygen in the hydroxyl groups on the surface, and similar results have been obtained previously (Budnyak et al., 2016; Gładysz-Płaska et al., 2018).

Figure 5.

Adsorption kinetics of U(VI) on nanogoethite: (a) pseudo-first-order fitted curve and (b) pseudo-second-order fitted curve.

Table 1.

Relative parameters base on the pseudo-first-order and pseudo-second-order models.

Condition	Pseudo-first-order kinetic model			Pseudo-second-order kinetic model
Condition	k₁/min⁻¹	q_e/(mg/g)	R²	k₂/min⁻¹	q_e/(mg/g)	R²
5 mg/L	0.724	4.18	0.996	0.997	4.22	0.998

Comparison of nanogoethite with other adsorbents

The U(VI) adsorption capacity (mg/g) of nanogoethite was compared with those of other similar adsorbents reported in literature, as shown in Table 2. In this work, the maximum adsorption capacity for nanogoethite was 104.22 mg/g at an initial U(VI) concentration of 200 mg/L. This is higher than other nanomaterials or goethites, demonstrating the high adsorption capacity of nanogoethite. This can be explained by the large number of pores and adsorption sites of nanogoethite.

Table 2.

A summary of the maximum adsorption capacity of some commonly used adsorbents for U(VI) removal.

Adsorbent	Qe (mg/g)	pH	References
Magnetite nanoparticles	5	7	Das et al. (2010)
Fe₃O₄	5.3	6	Singhal et al. (2017)
Multi-walled carbon nanotubes	26.18	5.0	Shao et al. (2009)
Chitosan grafted MW-CNTS	39.2	5.0	Shao et al. (2010)
Goethite	34.2	6.0	Yusan and Erenturk (2011)
Flower-goethite	48.24	4.0	Zhang et al. (2018)
Nanogoethite	104.22	4.0	This work

Conclusions

In summary, α-FeOOH nanoparticles were successfully synthesized by chemical precipitation and used for U(VI) adsorption from water containing a low concentration of U(VI). SEM, BET and XRD analyses were used to identify the morphology and structure of nanogoethite. The results show that this material has a 3D porous structure with a specific surface area of 81.86 m²/g. The U(VI) adsorption behavior was investigated under various conditions by batch experiments, and the results showed that this adsorbent could quickly and efficiently adsorb U(VI) from solution. In particular, the adsorption capacity of U(VI) on nanogoethite particles was strongly dependent on the pH. The removal rate of uranium was more than 85% after a contact time of 120 min with the 5.0 mg/L U(VI) solution at pH 4.0. The adsorption kinetic data fitted the pseudo-second-order model well, suggesting that the process of U(VI) adsorption is mainly controlled by chemical adsorption, and that nanogoethite is an excellent material for the removal of U(VI) from water having a flexible preparation process and outstanding adsorption performance. The results are useful for understanding the interactions of U(VI) with nanogoethite, as well as the migration and fate of U(VI) in soil and sediment under natural conditions.

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) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This paper is supported by the double first class construct program of USC (2017SYL05), and Hunan Provincial Natural Science Youth Foundation (2018JJ3437).

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Adsorption of U(VI) ions from aqueous solution using nanogoethite powder

Abstract

Keywords

Introduction

Experimental

Main reagents

Synthesis of nanogoethite

Characterization

Batch adsorption tests

Results and discussion

Characterization of nanogoethite

Adsorption experiments

Effect of pH on uranium adsorption

Effect of the initial uranium concentration on uranium adsorption

Effect of adsorbent dosage on uranium adsorption

Effect of contact time on the adsorption of uranium by nanogoethite and adsorption kinetics

Comparison of nanogoethite with other adsorbents

Conclusions

Footnotes

Declaration of Conflicting Interests

Funding

References