1 Introduction

Water contamination with heavy metals is one of the most serious environmental issues [1,2,3]. In order to remove heavy metals from aquatic phase, various technologies such as sorption, ion exchange and precipitation have been applied [4,5,6]. Among these technologies, sorption has been considered as a promising technology [7, 8]. Many different sorbents such as biomass, clay minerals, and metal oxides have been proposed for removal of heavy metals from water and wastewater [7, 9, 10].

Raw biomass materials and their carbonized products have been intensively studied for the removal of heavy metals from water and wastewater [11,12,13]. For example, peatmoss was found to be a low-cost sorbent for the removal of cadmium and nickel. Bark was reported to show a high removal efficiency for several heavy metals such as copper and lead. Biochars produced from different crop straws showed a high sorption affinity for copper. Also, various modification methods such as metal ion doping and acid surface treatment have been found to improve sorption capacity of the biomass derived materials [14, 15]. Even though sorption of heavy metals by biomass-derived materials was investigated in many studies, there are only a few studies on their sorption properties under different conditions of pH, ionic strength, and competitive sorption.

The aim of this study was to examine sorption properties of bark and treated bark materials with copper and cadmium. For this purpose, sorption capacities of the sorbents were investigated under different pH values and ionic strengths. Also, competitive sorption test was performed in bi-solute system with copper and cadmium. Materials characterization of the above sorbents was carried out for better understanding of their sorption properties. In addition, sorption isotherm models were applied to understand their sorption characteristics.

2 Materials and methods

2.1 Preparation and characterization of bark-derived sorbents

Different bark-derived materials were prepared as low-cost sorbents and their heavy metal sorption characteristics were investigated. The bark-derived sorbents were prepared using bark as precursor. Raw pine bark chips (~ 70 mm) were purchased from a local vendor (Gyeongju, Republic of Korea). The bark chips were washed with deionized (DI) water three times and dried at room temperature. The prepared bark chips were used as untreated bark sorbents without further treatment. Acid treated biomass sorbents such as acid treated bark was prepared by a chemical treatment using 0.1 M HCl solution. Also, alkali treated bark was prepared by a chemical treatment using 0.1 M NaOH solution. High-resolution field emission scanning electron microscopy (HR FE-SEM) (Merlin, Carl Zeiss, Germany) was used to examine the morphology of bark derived sorbents. Energy dispersive spectroscopy (EDS) (Merlin, Carl Zeiss, Germany) was performed to examine the chemical composition of bark derived sorbents. In addition, the specific surface areas and pore size distributions of the bark derived sorbents were determined using a Micromeritics ASAP 2020 surface area and porosity analyzer (Micro-meritics Instrument Corp).

2.2 Sorption isotherm test

To determine equilibrium time, sorption of heavy metals with different bark-derived sorbents was conducted. Two grams of each sorbent was added to 100 mL of solution with an initial concentration of 30 mg/L of Cu2+ or Cd2+. The concentration of Cu2+ or Cd2+ remaining in solution after their uptake was measured as a function of time (5 min, 10 min, 15 min, 30 min, 1 h, 8 h, 12 h and 24 h). Sorption isotherm experiments were also conducted to examine the sorption properties of the bark-derived sorbents for Cu2+ or Cd2+. Two g of each sorbent was added to 100 mL of solutions with the different initial concentrations of Cu2+ or Cd2+ (10, 15, 20, 25, and 30 mg/L) in a single solute system. The equilibration was performed in a shaking incubator for 24 h at 25 °C. After the sorption reached equilibrium, the solid and liquid phases were separated by centrifugation (Mega21R, Hanil Corp.) at 4000 rpm for 5 min. To examine the effect of pH and ionic strength on the sorption of different bark derived sorbents for Cu2+ and Cd2+, the sorption isotherm experiments were carried out under different pH (4.5, 5.5, and 7) conditions and ionic strength values (0 mg/L, 3000 mg/L and 10,000 mg/L as NaCl). The concentration of heavy metals such as Cu2+ and Cd2+ in aqueous phase were determined using atomic absorption spectroscopy (AAS) (Thermo M series-AAS, USA). Also, to examine the competition between Cu2+ and Cd2+ for sorption sites on three different bark-derived sorbents, sorption isotherm tests were performed in bi-solute system with both Cu2+ and Cd2+ ions. For the competitive sorption test, 2 g of each sorbent was added to 100 mL of solutions containing equivalent amounts of Cu2+ and Cd2+ ions. The different initial concentrations of each metal (Cu2+ or Cd2+) were 10, 15, 20, 25, and 30 mg/L. Like the single solute system, the concentrations of Cu2+ and Cd2+ in aqueous phase were determined using atomic absorption spectroscopy.

2.3 Sorption isotherms

To estimate the sorption affinity of heavy metals for different bark-derived sorbents, the following linear sorption isotherm was applied [16].

$$q_{e} \, = \,K_{d} C_{e}$$
(1)

where qe is the amount of heavy metal ion (Cu2+ or Cd2+) sorbed on the different bark-derived sorbents in equilibrium (mg/g). Sorption distribution coefficient, Kd (L/g) and Ce is the equilibrium concentration of heavy metal ion in the solution (mg/L), respectively.

For understanding the sorption characteristics of heavy metals with different bark-derived sorbents under different conditions, Langmuir and Freundlich models were used. The following Eqs. 2 and 3 are Langmuir and Freundlich isotherms, respectively [17,18,19].

$$q_{e} \, = \,\frac{{q_{\max } K_{L} C_{e} }}{{\left( {1 + K_{L} C_{e} } \right)}}$$
(2)
$$q_{e} \, = \,K_{f} C_{e}^{1/n}$$
(3)

where qe is the amount of heavy metal ion (Cu2+ or Cd2+) sorbed on the bark-derived sorbents in equilibrium (mg/g), qmax is the maximum sorption capacity (mg/g), KL is Langmuir constant, respectively. Kf, and 1/n are Freundlich empirical constant and Freundlich exponent, respectively. In addition, Ce is the equilibrium concentration of heavy metal ion in the solution (mg/L).

3 Results and discussions

3.1 Characterization of different bark-derived sorbents

Elemental analysis of three different bark derived sorbents is summarized in Table 1. Major elements found in all three sorbents were carbon and oxygen, as expected. There was no significant difference in carbon composition of all three sorbents and it is around 60%. In the case of oxygen content, untreated bark was a little bit lower than the other sorbents. In the case of minor elements, both Ca and Fe contents in acid treated bark were relatively lower than the other sorbents. This result implies that replacement of some cations on the bark surface by hydrogen ions is likely to occur during acid treatment [20].

Table 1 Elemental composition of different bark derived sorbents
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SEM images show morphologies of different bark derived sorbents (Fig. 1). However, there is no significant difference in morphology among bark-derived sorbents. The specific surface area of untreated bark was measured to be 1.47 m2/g. For acid and alkali treated barks, the specific surface areas were found to be 0.44 and 0.67 m2/g, respectively. The decrease in specific surface area after acid treatment may be due to pore blocking or erosion of surface structure [21]. The pore size distributions of different bark derived sorbents are shown in Fig. 2. The pore size distribution of untreated bark shows a high peak of the pore size range of 2 to 4 nm, which is in the mesopore size region of 2 to 50 nm (Fig. 2a). For acid treated bark, the pore size distribution shows several narrow and small peaks in the mesopore region (Fig. 2b). Similar to acid treated bark, the pore size distribution of alkali treated bark shows several peaks with pore diameters of around 2.6, 4.3, 5.6, and 9.2 nm in the mesopore region (Fig. 2c). This result indicates that chemical treatments with acid and alkali of bark resulted in relatively different mesopore diameters.

Fig. 1
figure 1

SEM images of three different bark-derived sorbents: a untreated bark, b acid treated bark, and c alkali treated bark

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Fig. 2
figure 2

Pore-size distribution curves: a untreated bark, b acid treated bark, and c alkali treated bark

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3.2 Sorption of copper and cadmium with different bark-derived sorbents

Sorption of Cu2+ with the three different bark derived sorbents as a function of reaction time at pH 7 is shown in Fig. 3. Each sorption experiment was performed in triplicate, and each data point represents the average of three test runs. The sorption of Cu2+ with untreated bark and alkaline treated bark sorbents appeared to reach equilibrium after a reaction time of around 8 and 1 h, respectively. The sorption capacities of untreated bark and alkali treated bark were 1.336 and 1.437 mg/g, respectively. For acid treated bark, the sorption equilibrium appeared to reach after a reaction time of around 12 h. The sorption capacity of acid treated bark was 1.220 mg/g. In the case of Cd2+ sorption, alkali treated bark appeared to reach equilibrium after a reaction time of around 1 h, which is similar to Cu2+ sorption. For untreated and acid treated barks, equilibrium reached after around 8 h and 20 h, respectively. The sorption capacity of untreated bark and alkali treated bark were 1.439 and 1.450 mg/g, respectively (Fig. 3).

Fig. 3
figure 3

The amount of Cu2+ (a) and Cd2+ (b) sorbed on different bark derived sorbents as a function of time

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To investigate the effect of pH on sorption affinity of Cu2+ and Cd2+ for three different bark derived sorbents, sorption isotherm experiments were carried out under different pH conditions (Fig. 4). In addition, their linear sorption distribution coefficients (Kd) were estimated to better understand the sorption affinity of heavy metals (Table 2). For Cu2+ sorption with untreated bark, sorption affinity increased with increasing pH (Fig. 4a and Table 2). Based on Kd values, the order of sorption affinity of Cu2+ is as follows: pH 7.0 > pH 5.5 > pH 4.5. However, in case of both acid and alkaline treated barks, the effect of pH on Cu2+ sorption affinity did not appear to be significant (Fig. 4b, c and Table 2). For Cd2+ with untreated bark, there was a clear increasing trend in sorption affinity with the increase of pH (Fig. 4d and Table 2). When compared to the sorption of Cu2+ with untreated bark, pH had more influence on the sorption affinity for Cd2+. In the case of Cd2+ sorption with alkali treated bark, sorption affinity increased with the increase of pH (Fig. 4f and Table 2), whereas, for acid treated bark, removal of Cd2+ seemed to be less sensitive to the initial pH range 4.5–7. This result may be due to the degree of surface protonation and electrostatic repulsion between the positively charged surface and cations.

Fig. 4
figure 4

Effect of pH on sorption of Cu2+ and Cd2+ with three different bark-derived sorbents: a Cu2+ with untreated bark, b Cu2+ with acid treated bark, c Cu2+ with alkali treated bark, d Cd2+ with untreated bark, e Cd2+ with acid treated bark, and f Cd2+ with alkali treated bark

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Table 2 Linear sorption coefficients (Kd) of Cu2+ and Cd2+ sorption with three different bark-derived sorbents under different pH conditions in single solute system
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On the other hand, the effect of ionic strength on the sorption affinity of Cu2+ and Cd2+ with three different bark-derived sorbents was examined (Fig. 5 and Table 3). For Cu2+ sorption with both untreated bark and acid treated bark, there was a decreasing trend in copper uptake with increasing ionic strength (Fig. 5a, b and Table 3). This result would suggest the high concentration of Na+ ions would compete with Cu2+ ions and sorb on the available sorption sites of sorbents. However, Cu2+ sorption with alkali treated bark, the effect of ionic strength on sorption affinity was not clearly observed (Fig. 5c and Table 3). In case of Cd2+ sorption with three different bark-derived sorbents, the effect of ionic strength on sorption affinity is obvious as can be observed from Fig. 5.

Fig. 5
figure 5

Effect of ionic strength on sorption of Cu2+ and Cd2+ with three different bark-derived sorbents: a Cu2+ with untreated bark, b Cu2+ with acid treated bark, c Cu2+ with alkali treated bark, d Cd2+ with untreated bark, e Cd2+ with acid treated bark, and f Cd2+ with alkali treated bark

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Table 3 Linear sorption coefficients (Kd) of Cu2+ and Cd2+ sorption with three different bark-derived sorbents under different ionic strength conditions in single solute system
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To study competitive sorption of Cu2+ and Cd2+ by different bark-derived sorbents, sorption isotherms were constructed at pH 7 as shown in Fig. 6. For Cu2+ sorption isotherms in single solute and bi-solute system, sorption affinities for different sorbents in bi-solute system were smaller than those corresponding to the single solute system (Fig. 6a, b). In case of Cd2+ sorption isotherms a similar trend was observed (Fig. 6c, d). In addition, to estimate the effect of competitive sorption of Cu2+ and Cd2+ ions, linear sorption distribution coefficients (Kd) of both heavy metal ions are summarized in Table 4. Compared to Kd values in the single solute system, their corresponding Kd values in bi-solute system are relatively small. The Kd values in bi-solute system except for Cu2+ sorption with acid treated bark are approximately less than half of the corresponding values in single solute system (Table 4). These results imply that Cu2+ and Cd2+ ions may compete with each other for sorption sites [22,23,24].

Fig. 6
figure 6

Sorption isotherms of Cu2+ and Cd2+ with three different bark-derived sorbents in single solute and bi-solute systems: a Cu2+ sorption isotherm in single solute system, b Cu2+ sorption isotherm in bi-solute system, c Cd2+ sorption isotherm in single solute system d Cd2+ sorption isotherm in bi-solute system

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Table 4 Comparison of linear sorption coefficients with single solute and bi-solute systems at pH 7
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3.3 Sorption models of different bark-derived sorbents

For understanding sorption of Cu2+ or Cd2+ ion by different bark-derived sorbents in the single solute system, Langmuir and Freundlich models were applied. The sorption parameters of the two models were obtained by fitting their isotherm data (Table 5). Based on the coefficients of determination (R2), the sorption isotherm data were well described by both Langmuir and Freundlich models. In case of Langmuir model, the qmax value of Cu2+ ions with alkali treated bark is relatively greater than those of both untreated and acid treated barks. Similarly, the qmax value of Cd2+ ions with alkaline treated bark is relatively greater than those of both untreated and acid treated barks. For Freundlich model, Kf values of Cu2+ and Cd2+ ions with acid treated bark are greater than those for untreated bark, respectively.

Table 5 Langmiur and Freundlich parameters for the sorption of Cu2+ or Cd2+ by three different bark derived sorbents in single solute system at pH 7
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Like single solute system, the sorption isotherm data in the bi-solute system are well described by the two models (Table 6). In case of Langmuir, the qmax value of Cu2+ ion by alkaline treated bark is relatively greater than those of both untreated and acid treated barks. Also, the qmax value of Cd2+ ion by alkaline treated bark is relatively greater than those of untreated and acid treated barks. On the other hand, comparison of the qmax values of Cu2+ ion by different bark-derived sorbents between single solute and bi-solute systems showed that the qmax values of Cu2+ and Cd2+ ions by different bark-derived sorbents in bi-solute system are relatively smaller than those in single solute system. The results are likely to be due to a competition for the same active sorption sites.

Table 6 Langmiur and Freundlich parameters for the sorption of Cu2+ and Cd2+ by three different bark derived sorbents in the bi-solute system at pH 7
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4 Conclusions

In this study heavy metal sorption properties of three different bark derived sorbents were investigated. For single solute system, the removal efficiency of Cu2+ by alkaline treated bark was around 95% (ca. 1.43 mg/g) at pH 7.0, which was higher than those by untreated bark (~ 90.3%) and acid treated bark (~ 83.4%). Similarly, for Cd2+ sorption at pH 7.0, the removal efficiency of alkali treated bark (~ 98.5%) was higher than those by untreated and acid treated barks. Sorption affinity of untreated bark for both Cu2+ and Cd2+ ions increased with the increase of pH. For Cd2+ sorption with three different bark-derived sorbents, sorption affinity decreased with increase of ionic strength. For bi-solute system, the removal efficiencies of Cu2+ and Cd2+ ions by different bark derived sorbents were relatively lower than the corresponding removal efficiencies in single solute system. This result suggests that there was competition between Cu2+ and Cd2+ ions for the active sorption sites. The sorption of Cu2+ and Cd2+ ions with the three bark-derived sorbents was well described by both Langmuir and Freundlich models. The order of the maximum sorption capacity of Langmuir model was in good agreement with the order of sorption affinity obtained from sorption isotherm experiments. Based on the results, alkali treated bark could be used as cost effective sorbents for removal of heavy metals from aqueous phase. In practical terms, the chemical treated bark can be used as a filtration material for artificial recharge of groundwater with stormwater.