Introduction

Thorium is not only the main energy material for nuclear industry, but also the toxic radioactivity element [1–3]. The toxic nature of this radionuclide has been a public health problem for many years. When thorium(IV) ion enter into the living organisms, it is accumulated usually in lung, liver, spleen and marrow [4, 5]. At present, thorium(IV) in the aqueous solution is not only from the nuclear industry, but also from other anthropogenic activities like lignite burning in power stations, ore processing and use of fertilizers [6]. Conventional physiochemical methods for removing thorium(IV) include chemical precipitation, electro floatation, ion exchange, reverse osmosis and so on [7]. However, these methods have several disadvantages, for example, incomplete metal removal, high reagent requirements, high energy requirements, and generation of toxic sludge or other waste products [8, 9]. Therefore, the above methods have limitation in application.

Biosorption is famous for the ability of biological materials to accumulate heavy metals from wastewater through metabolically mediated or physicochemical pathways [10], and it has been considered as a useful approach for remediation of heavy metals ions from the aqueous medium [11–14]. Mangrove endophytic fungus Fusarium sp. #ZZF51 (the tested fungus) was collected from South China Sea (Zhanjiang sea area). It is well known that marine environment has lots of special natures, like high salt, high pressure, low temperature, weak light and poor nutrition, which result marine microorganisms have theirs own special species and metabolic approaches. The initial work showed that the tested fungus could produce special metabolite bis(5-butyl-2-pyridinecarboxylato-N1,O2)-copper and had strong ability of uptaking copper(II) with 82.14 % of removal efficiency under the optimized conditions of pH 6.5, adsorption time 90 min, and initial copper(II) concentration 50 mg L−1 [15, 16]. The following research announced uranium(VI) biosorption process onto the tested fungus powders was optimized at pH 4.0, adsorption time 60 min, and initial uranium(VI) concentration 50 mg L−1 with 61.89 % of removal efficiency [17]. In order to understand the tested fungus more clearly and develop it as an environmental fungus strain, this paper studies its ability to remove thorium(IV) from aqueous solution, and the effects of different parameters such as pH, contact time, initial thorium(IV) concentration and biomass dose on biosorption are investigated. Further various kinetics and equilibrium models are also applied to understand the uptake process.

Materials and methods

Materials

All chemical reagents were of analytical grade. 1,000 mg L−1 of thorium(IV) stock solution was prepared by dissolving 2.3795 g Th(NO3)4·4H2O in 20 mL of 7 mol L−1 HCl and being diluted with distilled water to 1 L. Subsequently, different working solutions of required concentration were prepared by proper dilution. Arsenazo-III solution was prepared by dissolving 0.1250 g arsenazo-III in 5 mL nitric acid solution with pH 3.0 and being diluted with distilled water to 250 mL. The pH value of the tested solution was adjusted by 0.1 mol L−1 HCI and 0.1 mol L−1 NaOH.

Preparation of the biosorption materials

The tested fungus Fusarium sp. #ZZF51 was obtained from the South China Sea (Zhanjiang sea area) and desosited in school of chemistry and chemical engineering, Sun Yat-sen (zhongshan) university, Guangzhou, China. It was grown at 25 °C in liquid medium that was consisted of glucose 10 g L−1, peptone 2 g L−1, yeast extract 1 g L−1 and sea salt 2 g L−1. 22 days latter, the fungus mycelium was harvested and washed thoroughly by distilled water, then it was dried at 120 °C for 20 h, ground into fine powder, sieved through 100 mesh and reserved for later use.

Biosorption experiments

Batch experiments were carried out at room temperature in a series of Erlenmeyer flasks. In order to optimize thorium(IV) removal conditions, the effects of pH, contact time, initial thorium(IV) concentration and biomass dosage were comprehensively tested. In the batch adsorption experiments, 0.2 g fine mycelium powder was added to the 50 mL thorium(IV) ions solution in 250 mL flask which concentration was controlled within 20–60 mg L−1 and pH value 1.0–5.0. The flask was shaken at different time (0–60 min) in the rate of 150 rpm, then the mycelium powder was separated from solution in the high rotary rate of 4,000 rpm. The thorium(IV) concentration in solution was determined by UV visible spectrophotometer at 660 nm by the thorium(IV) arsenazo(III) complex method [18]. All experiments were run in triplicate. The biosorption efficiency (R) and biosorption quantity (q e ) were calculated by using the following equations:

$$ R\left( \% \right) = \frac{{C_{0} - C_{e} }}{{C_{0} }} \times 100\,\% \,\,\,\,\,\,q_{e} \left( {{\text{mg}}\,{\text{g}}^{ - 1} } \right) = \,\frac{{(C_{0} - C_{e} )V}}{m} $$

Where q e (mg g−1) is the equilibrium biosorption capacity. C 0 and C e are the initial and equilibrium thorium(IV) concentrations, V (L) is the volume of solution and m (g) is the dry mass of fine mycelium powder.

FTIR spectroscopy analysis

Fourier transforms infrared (FTIR) spectra were used to probe the functional groups on the surface of the fungus Fusarium sp. #ZZF51. The spectra were recorded with a spectrometer within the range 400–4,000 cm−1 by using a KBr window. The KBr background was automatically subtracted from the sample spectra.

Results and discussion

Effect of pH on the sorption efficiency

The influence of pH value on thorium(IV) adsorption is given in Fig. 1. It can be seen in Fig. 1 that the thorium(IV) percentage removal and the tested fungus biosorption capacity are greatly affected by the pH value of solution. The sorption efficiency increases with pH from 1.0 to 3.0, then decreases from 3.0 to 5.0. The maximum adsorption efficiency reaches 91 ± 1 % at pH 3.0. This can be explained as below: at lower pH values, the surface charge of biosorbent is positive, which is not favourable to thorium(IV) ions biosorption. Meanwhile, hydrogen ions compete strongly with metal thorium(IV) ions for the active sites, which results in less biosorption. When solution pH is from 1.0 to 3.0, the competing effect of hydrogen ions decreases, instead, thorium(IV) ions biosorption increases. When pH is 3.0, a large number of active adsorption sites are released, which provide more opportunities for thorium(IV) ions attack, so there are the maximum thorium(IV) biosorption efficiency at pH 3.0. When pH is over 3.0, thorium(IV) ions present in the other forms such as Th(OH)3+, Th(OH) 2+2 , Th2(OH) 6+2 and Th6(OH) 9+15 , which results in the decline of thorium(IV) percentage removal and biosorption efficiency. With further increase in pH, insoluble thorium hydroxide starts precipitating, which also results in lower amount of thorium(IV) ions removal at equilibrium [19]. Compared with pH values of the maximum metal ions biosorption efficiency (4.0 and 6.5 pH values for uranium(VI) and copper(II) respectively), pH 3.0 for thorium(IV) is less, which largely relates to the hydrolysis products of thorium(IV) ions.

Fig. 1
figure 1

Effect of initial solution pH on thorium sorption

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Effect of biosorbent dosage on the sorption efficiency

The effect of sorbent dosage on the sorption efficiency of thorium(IV) is presented in Fig. 2. When biomass dose increases from 0.05 to 0.20 g, the biosorption efficiency of thorium(IV) increases significantly. The increase of adsorption percentage is attributed to the increase of active sites in biomass, therefore, the metal ions can easily penetrate into the adsorption sites. When the adsorbent dose exceeds 0.20 g, the biosorption capacity of thorium(IV) ions decreases, and this change can be explained as below: the high dosage produces a‘screen effect’on the cell wall, and protects the binding sites [20].

Fig. 2
figure 2

Effect of biomass dose on thorium sorption

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Effect of contact time on the sorption efficiency

Biosorption of thorium(IV) at different contact time is studied under the same conditions of 50 mg L−1 initial thorium(IV) concentration, pH 3.0 and adsorbent dose 0.2 g. The results are presented in Fig. 3. It is clear that all sorbent sites are vacant and the solute concentration gradient is high at the beginning of biosorption. After 20 min, the change of sorption efficiency for thorium(IV) does not show notable effects, and this may be because of the reason of metal binding sites saturation. Therefore, it can be concluded that the appropriate equilibrium reach at 20 min. In previous studies, the uranium(VI) biosorption process was optimized at 60 min. These shows that the active sites on tested fungus have different affinity for uranium(VI) and thorium(IV).

Fig. 3
figure 3

Effect of contact time on thorium sorption

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Effect of initial thorium(IV) concentration on the sorption efficiency

The adsorption experiments are studied under the conditions of 0.2 g biomass, pH 3.0, contact time 20 min, and initial thorium(IV) concentrations varing from 20 to 60 mg L−1. The results are presented in Fig. 4. It is clear that the sorption capacity increases when the initial thorium(IV) concentration increases from 20 to 50 mg L−1, and the sorption capacity slightly decreases when the initial concentration was over 50 mg L−1. The change may be due to the fact the available free sites (metal binding sites) approach saturation at higher thorium(IV) concentration [21].

Fig. 4
figure 4

Effect of initial thorium concentration on thorium sorption

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Adsorption kinetics

In order to obtain the controlling mechanism of adsorption process, several kinetic models like pseudo-first-order, pseudo-second-order, and Elovich models are used to the experimental data [22].

$$ {\text{Pseudo-first-order}}:In(q_{e} - q_{t} ) = Inq_{e} - \,k_{1} t $$
$$ {\text{Pseudo-second-order}}:\frac{t}{{q_{t} }} = \frac{1}{{k_{2} (q_{e} )^{2} }} + \frac{t}{{q_{e} }} $$

Where k 1 and k 2 are the rate constant of adsorption and q e denotes the amount of adsorption at equilibrium.

$$ {\text{Elovich equation}}:q_{t} = \frac{In(\alpha \beta )}{\beta } + \frac{Int}{\beta } $$

Where α is the initial sorption rate (mg g−1 min−1), and β is the activation energy for chemisorption (g mg−1).

The values of q e , k 1, k 2, α, β are calculated from the slopes and the intercepts of the straight lines which are shown in Figs. 5, 6, 7 and Table 1.

Fig. 5
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Pseudo-first-order for the biosorption kinetics

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Fig. 6
figure 6

Pseudo-second-order for the biosorption kinetics

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Fig. 7
figure 7

Elovich equation for the biosorption kinetics

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Table 1 Kinetic parameters for the adsorption
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The calculated q e value from pseudo-second-order kinetic equation agreed very well with the experimental values. The kinetic experiments show that the adsorption follows a pseudo-second-order kinetic model (R 2 = 0.9999), and the model q e (11.35 mg g−1) is close to the theoretical q e (exp) 11.356 mg g−1.

Biosorption isotherms

Thorium(IV) uptake by the fungal biomass is modeled by Langmuir, Freundlich and Temkin type isotherm [23].

$$ {\text{Langmuir adsorption isotherm}}:\frac{1}{{q_{e} }} = \frac{1}{Q} + \,\frac{1}{{bQC_{e} }} $$

Where Q (mg g−1) represents the saturate monolayer sorption capacity and b (L mg−1) represents the sorption equilibrium constant.

$$ {\text{Freundlich adsorption isotherm}}:Inq_{e} = Ink + \,\frac{{InC_{e} }}{n} $$

Where k represents the constant of sorption capacity, and 1/n represents the constant of sorption intensity.

$$ {\text{Temkin adsorption isotherm}}:q_{e} = BInA + \,BInC_{e} $$

Where A and B are Temkin isotherm constants. B = (RT)/b, R is the universal gas constant (8.314 J mol−1 K−1), T is absolute temperature, C e is the residual equilibrium concentration (mg L−1), and q e is the amount of adsorbent (mg g−1) at equilibrium.

The biosorption isotherm models for thorium(IV) are shown in Figs. 8, 9 and 10. The values of isotherm constants and correlation coefficients are listed in Table 2. Temkin adsorption isotherm describs better with high correlation coefficient (R 2 = 0.9907) in comparison with those of Freundlich (R 2 = 0.9867) and Langmuir isotherm model (R 2 = 0.8884).

Fig. 8
figure 8

Freundlich adsorption isotherm of thorium(IV)

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Fig. 9
figure 9

Langmuir adsorption isotherm of thorium(IV)

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Fig. 10
figure 10

Temkin adsorption isotherm of thorium(IV)

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Table 2 Isotherm constant parameters for the adsorption
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FTIR spectra characterization of Fusarium sp. #ZZF51 biomass

FTIR spectra are useful to identify functional groups in compounds [24]. The FTIR spectra of the virgin and thorium(IV)-loaded biosorbents of tested fungus powders are respectively shown in Fig. 11a, b. As seen in Fig. 11a, the broad and strong band ranging from 3,200 to 3,600 cm−1 may be attributed to –OH groups and –NH2 groups, and the band at 2,924 cm−1 is due to C–H stretching vibrations. The strong peak at 1,639 cm−1 is produced by –C=O stretching vibration. The region from 1,000 cm−1 to 1,400 cm−1 contains adsorption bands of phosphate group or C–O stretching vibration. Compared with FTIR spectrum of virgin roots, FTIR spectrum of thorium(IV)-loaded roots displays significant shift in some peaks. The FTIR spectroscopic analysis of thorium(IV)-loaded biosorbents indicates intensity shifted strong asymmetrical stretching bands at 3,278 cm−1 (indicative of –OH and –NH groups) when compared with that of unloaded biomass which shows the same absorption at 3,462 cm−1. The shift of the peak from 1,639 to 1,654 cm−1 reflects the changes in the stretching frequency of carboxylate upon binding of thorium(IV). These observations indicate the involvement of these functional groups in the biosorption process [25].

Fig. 11
figure 11

FTIR spectra for fungus Fusarium sp. #ZZF51 biomass before and after loaded with thorium(IV)

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Conclusion

Batch adsorption experiments show that the fungus Fusarium sp. #ZZF51 has good biosorption ability for thorium(IV). Thorium(IV) biosorption process onto the tested fungus powders is optimized at pH 3.0, equilibrium time 20 min, thorium(IV) initial concentration 50 mg L−1 and non-living biomass dose 4.0 g L−1 with 91 ± 1 % of removal efficiency and 11.35 mg g−1 of sorption capacity. The experimental datas are analyzed by using parameter and kinetic models, and it is obtained that Temkin isotherm model (R 2 = 0.9907) and pseudo-second-order kinetic model (R 2 = 0.9999) provide better correlation with the experimental data for adsorption of thorium(IV). By comparing with Fourier transform infrared spectra for the tested fungus before and after loaded with thorium(IV), hydroxyl (peak shift from 3,462 to 3,278 cm−1) and carboxyl groups (peak shift from 1,639 to 1,654 cm−1) have an important contribution to the adsorption process.