Background

Iron is widely used in industry as plumbing, gear wheel, and roofing. However, excessive iron can be harmful for the environment. Furthermore, in humans, it will cause stomach upset and ulcer, mental retardation, liver, and brain damage. Excessive amounts of Fe(III) in public water supplies caused turbidity, unpleasant taste, and odor[1–3]. Therefore, removal of iron from effluents is essential.

Various methods have been reported regarding the removal of heavy metals including thermal, biological, physical, and chemical treatments[4, 5]. However, these methods are expensive and difficult; therefore, there is a serious need for the development of the easy and low cost methods. In recent years, there has been an increase in the use of biological materials as adsorbents for the removal of heavy metals from aqueous solutions. Agricultural by-products, such as banana pith, rice bran, peanut hull, sunflower, sugar beet pulp, orange peel have been demonstrated to remove heavy metal ions from wastewater[6–16].

The adsorption behavior of Cu(II), Pb(II), Cd(II), Ni(II) and methylene blue[17–19] from aqueous solutions on the hazelnut hull had previously been investigated. This paper presents the study of biosorption characteristics of powdered hazelnut hull for removing Fe(III) from aqueous solutions. The adsorption of hazelnut hull for Fe(III) was shown as a function of initial pH, adsorbent and adsorbate concentrations, temperature, and contact time. The adsorbent-adsorbate equilibrium behavior has been investigated using Langmuir and Freundlich isotherms.

Results and discussion

Effect of variables

The effect of Fe(III) concentration on the sorption by the hazelnut hull was investigated by varying the Fe(III) concentration from 20 to 60g/mL (Figure1). At low concentrations, Fe(III) cations in the solution were adsorbed almost 100%. However, at higher concentrations, cations were unadsorbed due to the saturation of the adsorption sites.

Figure 1
figure 1

Effect of initial concentration on the removal of Fe(III). Conditions: pH, 2; amount of adsorbent, 0.5g; contact time, 60min; temperature, 25C; solution volume, 100mL.

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The effect of pH on Fe(III) adsorption onto hazelnut hull was studied over pH range 1 to 5. The results are in Figure2. The optimum uptake occurred at pH 3.0 when 81.0% of the Fe(III) was eliminated from the solution. At very low pH, the surface of sorbent would be surrounded by the hydrogen ions, which compete with Fe(III) ions during the binding of the biosorbent sites. When the pH is increased (pH>4), the Fe(OH)3 can be produced and the removal was decreased.

Figure 2
figure 2

Effect of the solution on initial pH upon removal of Fe(III). Conditions: initial concentration, 50g/mL; adsorbent amount, 0.5g; contact time, 60min; temperature, 25C; solution volume, 100mL.

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The temperature of the solution-sorbent mixture was investigated in a range of 10C to 70C. Figure3 shows the effect temperature on the of adsorption. Maximum adsorption was obtained at 30C.

Figure 3
figure 3

Effect of the temperature on the removal of Fe(III). Conditions: initial concentration, 50g/mL; pH, 3; adsorbent amount, 0.5g; contact time, 60min; solution volume, 100mL.

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Figure4 shows the effect of contact time on adsorption. According to this figure, the uptake of Fe(III) by hazelnut hull shows an increase from 40 to 60min. The maximum uptake occurred when the contact time was 60min. In this situation, 82.3% of the Fe(III) was removed from the solution.

Figure 4
figure 4

Effect of contact time on the removal of Fe(III). Conditions: initial concentration, 50g/mL; pH, 3; adsorbent amount, 0.5g; temperature, 30C; solution volume, 100mL.

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The dependence of the Fe(III) adsorption on the adsorbent amount was studied by varying over range of 0.1 to 0.7g, at other variables constant (Figure5). With increasing adsorbent amount, the removal of Fe(III) increased and became almost constant at 0.6g of adsorbent. Therefore, this amount was selected as the optimum value.

Figure 5
figure 5

Effect of the adsorbent amount on the removal of Fe(III). Conditions: initial concentration, 50g/mL; pH, 3; contact time, 70min; temperature, 30C; solution volume, 100mL.

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Adsorption isotherms

Langmuir and Freundlich isotherms were employed to study the biosorption process. These are most commonly used to describe the biosorption characteristics of adsorbents used in water and wastewater treatment.

The linear form of Langmuir isotherm is given by the following equation:

C e / q e = C e / q m + C e / b q m
(1)

where qe is the amount adsorbed at equilibrium (mg/g), Ce is the equilibrium concentration (mg/L), and q m and b (L/mg) are the Langmuir constants related to maximum adsorption capacity and energy of adsorption, respectively. The Langmuir constants for adsorption of Fe(III) using hazelnut hull are given in Table1. The capacity of the hazelnut hull for adsorption of Fe(III) in the optimized conditions was 13.59mg/g.

Table 1 Results of Fe(III) biosorption isotherms at 30 C
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The adsorption data were also analyzed by Freundlich equation. The logarithmic form of Freundlich equation is given in the following equation:

l n q e = l n K f + 1 / n l n C e
(2)

where K f (mg/g) and n are constants related to adsorption capacity and adsorption intensity, respectively. The Freundlich constants are also given in Table1. The good correlation coefficients showed that the two models are suitable for studying the adsorption equilibrium of Fe(III).

Experimental

Hazelnut hull and chemicals

Hazelnut hull was cleaned and dried to a constant weight at room temperature and was then ground to pass through a 40-mesh sieve for experiments. Fourier transform infrared spectroscopy spectrum of the hazelnut hull was shown in Bulut and Tez[19]. All solutions were prepared with double distilled water. Chemicals used were of analytical grade. Stock solution of Fe(III), 1,000mg/L, was prepared by dissolving 0.4321g of Fe(NO3)3 (Merck & Co., Inc., Whitehouse Station, NJ, USA) in water in a 1,000mL volumetric flask. The Fe(III) concentration in the solution was determined by an atomic absorption spectrometer (Varian-AA220 Spectrometer, Salt Lake City, UT, USA). Temperature control and shaking were performed by the water bath shaker (NB-303 Orbital, Merton, London, UK). A pH meter (ISTEK Inc., Kuro-ku, Seoul, South Korea) was used to determine the pH. Solutions of NaOH and HNO3, 0.1mol/L, were used to adjust the solution pH).

Conclusions

This study shows that hazelnut hull is a suitable adsorbent for the removal of Fe(III) from aqueous solution due to its high capacity adsorption, availability, and low cost. The Fe(III) removal was pH-, adsorbent amount-, and contact time-dependent. Under optimum conditions, the maximum removal of Fe(III) and the capacity of adsorbent (based on Langmuir equation) were found to be 83.5% and 13.59mg/g, respectively.