Water Air Soil Pollut (2015) 226:274 DOI 10.1007/s11270-015-2533-0

Removal of Copper(II) from Aqueous Solutions by Biosorption-Flotation Ligia Stoica & Ana-Maria Stanescu & Carolina Constantin & Ovidiu Oprea & Gabriela Bacioiu

Received: 22 January 2015 / Accepted: 7 July 2015 # Springer International Publishing Switzerland 2015

Abstract This study investigates the removal of Cu(II) from aqueous solutions by biosorption-flotation as a function of several parameters, such as collector type, pH, molar ratio, air pressure, time, and initial metal concentration. Dissolved air flotation was applied as a polishing technique for the additional purification of the effluent resulted after biosorption. The obtained results were supported by the physicochemical characteristics of the surfactants used as flotation reagents and suggested that cetylpyridinium bromide (CPB) was the optimum collector for Cu(II) ion removal. Cu(II) removal efficiency exhibited a maximum of 97.09 % in the following operating conditions: biosorption pH 4.5, Cu(II) initial concentration 250 mg/L, biosorbent dosage 0.5 % w/v, agitation rate 200 rpm, temperature 20 °C, biosorption time 30 min, flotation pH 9, air pressure 4.5×105 Pa, dilution ratio 3:1, flotation time 10 min, collector CPB 0.01 M, and molar ratio collector/

Electronic supplementary material The online version of this article (doi:10.1007/s11270-015-2533-0) contains supplementary material, which is available to authorized users. L. Stoica : A.
Cu(II) 5×10−1:1. The experimental data confirmed that the flotation stage contributed to the optimization of the overall separation process. Keywords Biosorption . Copper(II) . Dissolved air flotation . Surfactants

1 Introduction Due to the progressive development of the industry, increasing amounts of toxic heavy metals are being released into the environment in bioavailable form, endangering natural ecosystems and human health worldwide. Moreover, because of their high mobility, heavy metal ions are being concentrated and accumulated throughout the food chain (Naja et al. 2010; Naja and Volesky 2009; Chojnacka 2009; Kotrba et al. 2011). Consequently, controlling heavy metal discharges and removing the toxic heavy metals from aqueous solutions have become a challenge for the field of research and development (Volesky 2001). The toxicity of heavy metals is highly associated with metal ion speciation (i.e., dissolved forms are more toxic than particulate forms, copper is such a case) (Trivunac et al. 2012). Copper is a natural microelement, essential for many biochemical pathways, but its excess leads to important toxicological concerns (Peng et al. 2010; Hanafiah and Ngah 2009). Copper contamination is generally being caused by a variety of industrial processes, such as the following: electroplating, electronic and electrical industries, paint and pigments

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manufacturing, fertilizers, refineries, and tannery industries (Hanafiah and Ngah 2009; Jaman et al. 2009). Copper tends to accumulate in the human body when exposed through air, water, or food sources causing harmful effects to the kidneys, liver, and nervous system, and it is probably best known for its association with Wilson’s disease. In addition, there is some evidence to suggest that copper may be carcinogenic (Wase and Forster 2003). United States Environmental Protection Agency (US EPA) recommends a maximum acceptable concentration of copper in industrial effluents and drinking water of 5 and 1.3 mg/L, respectively (http://www.epa.gov/). Compared to the conventional treatment methods used for heavy metal decontamination from waters and/or wastewaters (i.e., ion exchange, solvent extraction, reverse osmosis, membrane technologies, chemical precipitation, electrochemical technologies, and evaporation) that have several drawbacks (i.e., incomplete metal removal, high operating costs, high reagent and/ or energy requirements, and generation of toxic sludge), biosorption is regarded as a cost-effective, eco-friendly, and easy to operate alternative technology (Naja et al. 2010; Chojnacka 2009; Kotrba et al. 2011; Volesky 2001; Peng et al. 2010; Hanafiah and Ngah 2009; Jaman et al. 2009; Wase and Forster 2003; Wang and Chen 2006; Zan et al. 2012; Chen and Wang 2008; Wang and Chen 2008; Zhang et al. 2010b; Das 2012; El-Sayed and El-Sayed 2014). Biosorption relies on the property of living and/or nonliving biosorbents (i.e., bacteria, fungi, yeast, algae, food industry/ agricultural waste, plants, and animal origin by-products) to rapidly bind/adsorb/extract and/or concentrate toxic metals, radionuclides, light metals, and rare earth elements even from very diluted aqueous solutions (<100 mg/L) by physicochemical mechanisms (Naja et al. 2010; Naja and Volesky 2009; Chojnacka 2009; Peng et al. 2010; Hanafiah and Ngah 2009; Jaman et al. 2009; Wase and Forster 2003; Meneghel et al. 2013; Altun and Pehlivan 2007; Sarkar et al. 2010; Yeddou and Bensmaili 2007; Volesky et al. 1993; Stanescu et al. 2014). Since it is not limited to only one mechanism or to a specific type of contaminant, biosorption can find applications in pollution prevention, pollution control, environment restoration, element and biomass recycle, and/or recovery (El-Sayed and El-Sayed 2014; Chojnacka 2009). Previous research indicates that biosorption can be successfully combined with other separation and/or

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decontamination techniques, namely, ultrafiltration, nanofiltration, reverse osmosis, incineration, and flotation, in order to develop an optimum technological process (Tsibranska and Saykova 2013; Won et al. 2010; Feris et al. 2004; Matis et al. 2003; Zouboulis et al. 2001). According to the data available in the literature, it was noticed that the flotation techniques coupled with biosorption, also known as biosorptive flotation, were mainly used as solid/liquid separation methods (Zamboulis et al. 2004; Matis et al. 1996; Matis et al. 2003; Yenial et al. 2014; Ghazy and Ragab 2011; Zouboulis et al. 2010; Mohammed et al. 2013; Lazaridis et al. 2001; Lazaridis et al. 2004; Zouboulis et al. 2001). However, in this study, we applied dissolved air flotation (DAF) as a polishing technique for the additional purification of the effluent resulted after biosorption and not as a solid/liquid separation method. Flotation had its beginnings in mineral processing, and as such, it has been used for a long time, although for a number of years, the process has found wide applications to selectively separate different microparticles (ions and molecules) (Rubio et al. 2002; Stoica 1997; Shammas and Bennett 2010). Furthermore, flotation is also practiced in other fields, such as analytical chemistry; protein separation; harvesting and/or removal of algae; separation or harvesting of microorganisms; and clarification of fruit juices (Rubio et al. 2002; Stoica 1997). DAF is by far the most widely used flotation method for the treatment of industrial effluents, due to its rapidity, efficiency, selectivity, as well as for its technical and economical advantages (Lazaridis et al. 2004; Feris et al. 2004; Rubio et al. 2002; Stoica 1997; Shammas and Bennett 2010). The process consists of the following basic steps: (i) bubble generation into the wastewater; (ii) contact between the gas bubble and the suspended matter; (iii) attachment of fine bubbles to the surface of the suspended matter; (iv) collision between gas-attached suspended particles with the formation of agglomerates; (v) entrapment of more gas bubbles in the agglomerates; (vi) upward rise of floc structures in a sweeping action (Stoica 1997; Shammas and Bennett 2010). The objective of this study was to investigate Cu(II) removal from aqueous systems by combining biosorption onto inactive dry baker’s yeast Saccharomyces cerevisiae with flotation. The influence of several factors (collector—characteristics and type, pH, molar ratio, air

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pressure, flotation time, and initial metal concentration) on Cu(II) removal efficiency was investigated. The importance of the physicochemical properties of Cu(II) ions (i.e., speciation), as well as of the collectors in the biosorption-flotation process, was also discussed.

2 Materials and Methods 2.1 Materials Commercial instant dry baker’s yeast (S. cerevisiae) purchased from local commercial company was prepared as inactive biomass by oven-drying at 105 °C for 24 h. Subsequently, the inactive instant dry baker’s yeast biomass was stored in desiccators till further use. The average size of the granular biosorbent particles was 425 μm (Stanescu et al. 2014). Copper stock solution of 1000 mg/L was prepared by dissolving CuSO4 ·5H2O (Merck, Germany) of analytical reagent grade into distilled water. Copper test solutions of different concentrations (10, 25, 50, 100, 150, 200, and 250 mg/L) were obtained by diluting the stock solution. The pH of the solutions was adjusted by adding 0.1 M H2SO4 or 0.1 M NaOH solutions if needed. Cationic surfactants: dodecylamine (DDA) (C12H27N), cetylpyridinium bromide (CPB) (C21H38BrN), and cetyltrimethylammonium bromide (CTAB) (C19H42BrN) of 0.01 M (Sigma-Aldrich, UK) were used as collectors. The collector solutions were prepared by dissolving the proper amount of surfactant in an ethanol/water mixture (volume ratio 1:1). 2.2 Biosorption-Flotation Experiments The biosorption-flotation tests were conducted at laboratory scale. The biosorption experiments were performed under batch conditions with continuous stirring (200 rpm), at room temperature (20 °C) and pH 4.5 by adding a constant dose of inactive instant dry baker’s yeast biomass of 0.5 %w/v per 100 mL sample of different initial metal concentrations, for 30 min contact time. The biosorption studies were carried out at the optimal operating parameters previously investigated and reported (Stanescu et al. 2014). The metal loaded biomass was separated from the metal solutions by decantation, and the

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liquid phase was subsequently subjected to a flotation stage for additional purification. The flotation experiments were conducted by using a dissolved air flotation unit (Stoica 1997). Prior to the flotation tests, the pH of the sample was adjusted. The pH values were measured with an ORION 290 A pH meter. The desired quantity of the collector solution was added to the sample, which was maintained under continuous stirring for 5 min at 200 rpm (HEILDORPH VIBRAMAX 100 shaker); then, the mixture was transferred to the flotation column. The flotation column consists of a cylindrical glass tube of 4.4 cm inner diameter and 25.5 cm length fitted with a stopcock at the bottom. A water stream presaturated with air kept under the pressure of 4.5×105 Pa was introduced to the cell base. Fine air bubbles necessary for an appropriate separation were generated by reducing the pressure (Stoica 1997). The flotation tests were performed for 10 min. The effluents obtained after flotation were analyzed for Cu(II) final concentration by atomic absorption spectrometry (UNICAM PAY SP9). All biosorption-flotation experiments were performed in duplicate. The removal efficiency (Y) was calculated according to the following equation (Zhang et al. 2010a):   Cf Y ð%Þ ¼ 1−  100 Ci

ð1Þ

where Ci and Cf are the initial and final metal concentration in solution, respectively (mg/L). In order to obtain a complete description of the surfactants used as collector reagents in the flotation tests, their main characteristics were evaluated. The molecular structure simulation was performed with the HyperChem version 8.0 software. The surface tension of their diluted aqueous solutions held at 20 °C was determined with a Kruss tensiometer by using the ring method. A distilled water sample was used as reference. Experiments to evaluate the influence of the surfactants on the surface charge of the system were carried out by measuring the electrophoretic mobility of the aqueous dispersions in an electric field with a Zetasizer Nano ZS analyzer (Malvern Instruments Ltd., U.K.). The speciation of Cu(II) ions in solution at 20 °C, pressure 1.01325×105 Pa, and metal concentration of 50 mg/L was calculated using pH-REdox-Equilibrium (PHREEQC) program (Parkhurst and Appelo 2013).

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3 Results and Discussion 3.1 Characterization of the Collectors The collector plays an important role in the flotation process, since it affects the hydrophobicity of the system. The collector or the so-called depressing reagent is a surface-active substance (anionic or cationic) with long linear carbon chain used in order to change the surface tension at the gas–solid interface, changing the specific attachment of bubble and solids (i.e., increasing or decreasing the affinity and/or favoring the foaming process) (Ghazy and Ragab 2011; Stoica 1997; Shammas and Bennett 2010). In order to obtain a complete description of the surfactants used as collectors (DDA, CPB, and CTAB) in the flotation experiments, their main characteristics were evaluated. Therefore, the molecular structures of the collectors, the surface tension of their diluted aqueous solutions, as well as their influence on the zeta potential of the biosorbent were disscused. The partial charges of the constituting atoms, the electrostatic potential, the 3D structure, as well as the dipole moment, surface area, and volume for each collector molecule, modeled/simulated with HyperChem Software are presented in Figs. 1, 2, 3 and Table 1, respectively. As it can be noticed from Fig. 1b, the electrostatic potential of the DDA molecule consists of a collection of negative and positive charges. Figures 2b and 3b show that the charge distribution of CTAB, respectively, CPB is positive. As shown in Table 1, the molecule of CPB presents the largest surface area and volume of 718 Å2 and 1178 Å3, respectively, compared with the other two surfactants. The presence of the collector reagent in the system decreases the dimension of the air bubbles, increasing thereby the efficiency of the DAF process (Stoica 1997). We estimate that the properties of the collector (i.e., charge distribution, surface area, and volume) may influence the removal efficiency of Cu(II). Therefore, it is possible that the efficiency of the biosorption-flotation process may increase with the increase in the surface area and volume of the collector molecule. The surface properties of DDA, CPB, and CTAB were assessed by drawing the surface tension isotherms of their diluted aqueous collector solutions. A distilled water sample was used as reference (σH2O =71.31× 103 N/m). The surface tension isotherms for each collector are presented in Fig. 4.

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From Fig. 4, it can be seen that the surface tension decreases with the increase in the collector concentration. This trend was similar for all the surfactants. The descending surface tension isotherms (Fig. 4) suggest that DDA, CPB, and CTAB present surface-active properties as concerns the surface forces and the coordinating power, and therefore, these surfactants could be suitable for Cu(II) removal by biosorption-flotation (Zouboulis et al. 2010; Stoica 1997). The zeta potential is a measure of the surface charge of the biosorbent and was used to study the behavior (Zhang et al. 2010b) of the system after Cu(II) ion biosorption and cationic collector addition. In our previous study, we investigated and reported the zeta potential of the biosorbent (heat pretreated instant dry baker’s yeast) before and after Cu(II) biosorption (Stanescu et al. 2014). The zeta potential of the heat pretreated instant dry baker’s yeast before Cu(II) ion biosorption as a function of pH (3–6) was negative and varied from −12.1 to −29.1 mV. The zeta potential of the biosorbent increased to −9.75 to −22.4 mV, after Cu(II) ion (Ci 50 mg/L) biosorption (Stanescu et al. 2014). The zeta potential of the biosorbent as a function of pH, after Cu(II) ion (Ci 200 mg/L) biosorption and cationic collector addition, is illustrated in Fig. 5. From Fig. 5, it can be observed that the zeta potential of the biosorbent after Cu(II), 200 mg/L biosorption was still negative, but increased and varied in the range of −7.96 to −10.53 mV due to the increase of the amount of positive charges of Cu(II) neutralizing the negative charge of the biosorbent (Zhang et al. 2010b). The negative charge of the system after Cu(II) biosorption justifies the use of cationic collectors. The influence of the flotation reagents on the zeta potential was studied in order to increase and bring the charge of the biosorbent/ system in the range of −5 to 5 mV, given that the flotation process works properly under these conditions. From Fig. 5, it can be seen that the addition of the collectors increases the zeta potential of the biosorbent/ system to −4.77 to −8.36 mV when DDA was added, to −1.01 to −6.19 mV when CTAB was used and, respectively, to 2.3 to −1.09 mV when CPB was added. Therefore, the obtained results suggest that DDA, CPB, and CTAB can be used as collectors in the flotation experiments. Similar findings were reported by Zouboulis et al. (2001), who also found that the presence of the metal mixture and/or surfactant increased the values of the zeta potentials of the yeast suspension used in the biosorptive flotation experiments.

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Fig. 1 Molecular structure of DDA modeled with HyperChem. a Partial charges of the constituting atoms. b The electrostatic potential mapped as a 3D charge surface; c 3D structure

3.2 Biosorption-Flotation Tests Previous investigations indicated that the heat pretreated commercial instant dry baker’s yeast, S. cerevisiae, presents a good adsorption capacity for Cu(II) ions. Consequently, the biosorption experiments were performed under batch conditions in accordance to our previous study (Stanescu et al. 2014).

As shown in Table 2, the removal efficiency of Cu(II) by flotation increased in the order: CTAB (87.85 %)< DDA (91.55 %)
3.2.1 Influence of Collector Type In order to select the appropriate collector necessary for Cu(II) removal after biosorption, the three aforementioned flotation reagents were tested: DDA (0.01 M), CPB (0.01 M), and CTAB (0.01 M). The influence of the collector type on Cu(II) removal by biosorptionflotation is presented in Table 2.

3.2.2 Influence of pH and Molar Ratio The pH of the solution represents an important controlling factor of the process, since it affects both metal (i.e., speciation) and biosorbent (i.e., surface charge) properties (Peng et al. 2010; Das 2012). Therefore, before we

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Fig. 2 Molecular structure of CPB modeled with HyperChem. a Partial charges of the constituting atoms; b The electrostatic potential mapped as a 3D charge surface; c 3D structure

determined the optimum pH for the biosorptionflotation process, we calculated the speciation of Cu(II) ions in solution using PHREEQC program

(Parkhurst and Appelo 2013). It was observed that the speciation distribution of Cu(II) ions calculated with PHREEQC program was in aggrement with the

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Fig. 3 Molecular structure of CTAB modeled with HyperChem. a Partial charges of the constituting atoms; b The electrostatic potential mapped as a 3D charge surface; c 3D structure

literature reported data (Zhang et al. 2010b; Doyle and Liu 2003; Aksu and Doyle 2002). Data regarding the speciation distribution of Cu(II) ions calculated with PHREEQC program can be found in Online Resource 1. Although precipitation may contribute to the overall removal of the metal (Naja et al. 2010), it was reported that the suitable pH range for Cu(II) biosorption is 1–6, because within this range, the only stable existing species are represented by hydrated copper ions [Cu(H2O)4]2+ (Naja et al. 2010; Peng et al. 2010;

Zhang et al. 2010b). At higher pH values, Cu(II) ions start to precipitate resulting hydroxides (Cu(OH)+ and

Table 1 Calculated dipole moment, surface area and volume for each collector molecule Collector

DDA

CPB

CTAB

1.325

37.6

38.82

Surface area, Å

503

718

699

Volume, Å3

791

1178

1152

Dipole moment, Debyes 2

Fig. 4 Surface tension isotherms of the diluted collector solutions (temperature, 20 °C, σH2 O =71.31×103 N/m)

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Table 3 Influence of pH and molar ratio on Cu(II) removal efficiency by biosorption-flotation

Fig. 5 Comparison between the zeta potential of the biosorbent before and after collector addition (Ci Cu(II) 200 mg/L, biosorbent dosage 0.5 %w/v, agitation rate 200 rpm, biosorption time 30 min, temperature 20 °C, collector concentration 0.01 M)

Cu2(OH)22+ at pH≥6, Cu(OH)2 and Cu(OH)3− at pH≥ 10, Cu(OH)42− at pH≥12), and the precipitation is complete around pH 12. In order to establish the optimum pH necessary for Cu(II) removal by flotation with CPB as collector, the pH was varied from 9 to 11. Within this pH range, copper is almost completely under precipitated form. The influence of pH on Cu(II) removal is summarized in Table 3. Table 3 also lists the influence of molar ratio on Cu(II) removal by biosorption-flotation. From Table 3, it can be seen that the maximum removal efficiency, 96.15 %, was obtained at pH 9 for a molar ratio CPB/Cu(II) of 5×10−1:1. It can be noticed that at pH values higher then 9, the percentage removal efficiency slightly decreased. These results are

Ci Cu(II)

Molar ratio collector/Cu(II)

pH

Cf Cu(II)

Y, %±SD

200

10−1:1

9

12.4

93.80±0.3

200

10−1:1

9.5

12.9

93.55±0.2

200

10−1:1

10.5

12.6

93.70±0.1

200

10−1:1

11

12.8

93.60±0.2

200

5×10−1:1

9

7.7

96.15±0.1 95.55±0.1

−1

200

5×10 :1

9.5

8.9

200

5×10−1:1

10.5

8.6

95.70±0.2

200

5×10−1:1

11

9.2

95.40±0.2

The biosorption-flotation experiments were conducted at Ci Cu(II) 200 mg/L; biosorption pH 4.5; biosorbent dose 0.5 %w/v; agitation rate 200 rpm; temperature 20 °C; biosorption time 30 min; pressure 4.5×105 Pa; dilution ratio 3:1; flotation time 10 min, collector CPB, 0.01 M

comparable to those obtained by Zouboulis et al. (2001) for zinc, copper, and nickel removal from simulated wastewater by biosorptive flotation, using dodecylamine as collector reagent. Table 3 also shows that the removal efficiency of Cu(II) increased with the increase of the molar ratio. Therefore, the pH 9 and the molar ratio CPB/Cu(II) 5×10−1:1 were selected as the optimum parameters for Cu(II) removal and were used for the following experiments. 3.2.3 Influence of Air Pressure The air pressure necessary for the generation of the bubbles is the major parameter controlling air solubility in a DAF unit and is an important factor in flotation

Table 2 Influence of collector type on Cu(II) removal efficiency by biosorption-flotation Ci Cu(II)

Collector

Molar ratio Collector/Cu(II)

pH

Cf Cu(II)

Y, %±SD

200

DDA

10−1:1

9–9.5

20.3

89.85±0.2

200

DDA

5×10−1:1

9–9.5

16.9

91.55±0.1

200

CPB

10−1:1

9

12.4

93.80±0.1

200

CPB

5×10−1:1

9

7.7

96.15±0.1

200

CTAB

10−1:1

7–7.2

26.5

86.75±0.1

200

CTAB

5×10−1:1

7–7.2

24.3

87.85±0.2

The biosorption-flotation experiments were conducted at Ci Cu(II) 200 mg/L; biosorption pH 4.5; biosorbent dose 0.5 %w/v; agitation rate 200 rpm; temperature 20 °C; biosorption time 30 min; pressure 4.5×105 Pa; dilution ratio 3:1; flotation time10 min

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Table 4 Influence of air pressure on Cu(II) removal efficiency by biosorption-flotation Ci Cu(II)

Pressure×105, Pa

Cf Cu(II)

Y, %±SD

200

3

10.83

94.58±0.3

200

2.3

12.11

93.94±0.4

200

4

15.81

92.09±0.3

200

4.5

200

5

7.7

96.15±0.1

12.23

93.88±0.2

The biosorption-flotation experiments were conducted at Ci Cu(II) 200 mg/L; biosorption pH 4.5; biosorbent dose 0.5 %w/v; agitation rate 200 rpm; temperature 20 °C; biosorption time 30 min; dilution ratio 3:1; flotation time 10 min, collector CPB, 0.01 M; molar ratio CPB/Cu(II) of 5×10−1 :1

3.2.4 Influence of Flotation Time The flotation time is a significant factor to be considered for practical applications. Figure 6 shows the influence of time on Cu(II) removal efficiency by biosorptionflotation. As seen from Fig. 6, Cu(II) removal efficiency increased rapidly and reached maximum after only 10 min. It can be noticed that further increase in the flotation time did not caused any significant changes in the removal efficiency. Thereby, 10 min was selected as optimum flotation time for Cu(II) removal. 3.2.5 Influence of Initial Metal Concentration

operation. Thus, the amount of air dissolved in the solution and consequently the amount of air released upon reducing of the pressure are both direct functions of the initial air pressure (Stoica 1997; Shammas and Bennett 2010). The influence of air pressure on Cu(II) removal efficiency is presented in Table 4. As shown in Table 4, Cu(II) removal efficiency exhibited a maximum of 96.15 % at 4.5×105 Pa. Thus, 4.5×105 Pa was selected as the optimum pressure necessary for Cu(II) removal by biosorption-flotation and was used for the following experiments.

The influence of initial metal concentration on Cu(II) removal by biosorption-flotation is illustrated in Fig. 7. The initial metal concentration was varied in the range of 10–250 mg/L. From Fig. 7, it can be noticed that Cu(II) removal efficiency increased with increasing initial metal concentration. As the initial concentration of Cu(II) increased from 10 to 250 mg/L, the removal efficiency increased from 66.7 to 97.09 %, respectively. As we previously reported (Stanescu et al. 2014), the biosorption process was effective (62.30–71.12 %) at low Cu(II) concentrations (10–50 mg/L). Therefore, the

Fig. 6 Influence of flotation time on Cu(II) removal efficiency by biosorption-flotation (Ci Cu(II) 200 mg/L; biosorption pH 4.5; biosorbent dose 0.5 %w/v; agitation rate 200 rpm; temperature 20 °C; biosorption time 30 min; pressure 4.5×105 Pa; dilution ratio 3:1; collector CPB, 0.01 M; molar ratio CPB/Cu(II) of 5× 10−1:1)

Fig. 7 Influence of initial metal concentration on Cu(II) removal efficiency by biosorption flotation (biosorption pH 4.5; biosorbent dose 0.5 %w/v; agitation rate 200 rpm; temperature 20 °C; biosorption time 30 min; pressure 4.5×105 Pa; dilution ratio 3:1; flotation time 10 min, collector CPB, 0.01 M; molar ratio CPB/ Cu(II) of 5×10−1:1)

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results presented herein (Fig. 7) show that the flotation process contributed to the increase of Cu(II) removal efficiency throughout the whole range of concentration. Furthermore, these results demonstrate that DAF can be successfully applied after biosorption for the additional purification of the resulted effluent and the optimization of the overall separation process, respectively. The results reported by Mohammed et al. (2013) for removal of lead ions from wastewater using SDS as surfactant and barley husk as biosorbent also indicated that the sorptive flotation process is more efficient than the flotation process.

4 Conclusions This study was focused on the removal of Cu(II) ions from aqueous solutions by biosorption-flotation. The influence of different operating parameters, such as collector type, pH, molar ratio, air pressure, time, and initial metal concentration, was investigated, and it was noticed that the separation process is directly related to these controlling factors. The experimental results were supported by the physicochemical characteristics of the surfactants used as collector reagents. The maximum removal efficiency of Cu(II) by biosorption-flotation, 97.09 %, confirmed that the flotation stage contributed to the optimization of the overall separation process. Moreover, these results demonstrate that DAF can be successfully applied after biosorption resulting thereby a rapid, effective, and relatively low-cost process. Acknowledgments This work was supported by the Sectoral Operational Programme Human Resources Development 20072013 of the Romanian Ministry of Labour, Family and Social Protection through the Financial Agreement POSDRU/107/1.5/ S/76903.

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