1 Introduction

Water is polluted in many ways, such as wastewater from the leather and chemical industries, electroplating and paint industries [1]. Industrially polluted water often contains significant amounts of toxic heavy metals. Thus, water bodies polluted with Pb, Cr, Hg, Cu, Cd, Zn, and Ni, pose an important environmental problem [2,3,4]. Removal of the ions of these metals from industrial wastewater has become imperious in many research efforts [5,6,7].

Most industries are encouraged to minimize the metal content in wastewater. Applied methods include precipitation, adsorption [8], ion exchange [9], chemical oxidation [10], coagulation [11], flocculation [12], reverse osmosis, and membrane filtration [13,14,15].

Lead (Pb+2) is one of the commonest hazardous heavy metal [16]. Human organs and tissues, bones, liver, kidney, and brain, accumulate of Pb(II) and this leads to serious health disorders, such as encephalopathy, seizures, mental retardation, anaemia, and nephropathy [14].

Chitosan, cellulose, starch, and alginate are polysaccharide homopolymers. They are abundant in nature. Therefore, their use in the removal of heavy metal ions is comparably cost-effective. Their adsorption capacity, mechanical stability, and hydrophilicity can be improved in composite forms [17,18,19].

Chitosan (poly-β-(1 → 4)-2-amino-2-deoxy-D-glucose) is a nitrogen-containing (amino-based) polysaccharide produced in large quantities by N-deacetylation of chitin [20, 21]. Because of its non-toxic, biocompatible, and biodegradable properties, it is also exploited in medicine, cosmetics, food industry, and in the production of biological membranes [22]. The biggest advantage of chitosan is the availability of substitutable positions in its chemical structure. It has very powerful heavy-metal chelating moieties, amino and hydroxyl groups [23]. And nanofibers containing chitosan have been thoroughly investigated via a number of heavy-metal adsorption experiments [24].

Starch, another versatile natural polysaccharide, is also used in food, cosmetics, and pharmaceuticals [25, 26]. Starch is a mechanically stable adsorbent with its rich hydroxyl groups, and high surface area. It has recently received significant attention in wastewater treatment efforts [27,28,29].

This research focuses on the preparation of a chitosan-starch composite and its use in Pb(II) removal from aqueous media, as metals are mostly ionized in an aqueous environment. Langmuir and Freundlich isotherms were used to fit the adsorption equilibrium data. Adsorption kinetics were calculated by pseudo-first-order, and pseudo-second-order models. The negative ∆G and ∆H values indicated that the Pb adsorption reaction of the chitosan-starch composite was exothermic and spontaneous, respectively. The binding energy for the interaction between adsorbent and adsorbate was 11.416 eV. This high interaction energy can be explained by the Hard and Soft Acid–Base Principle. Pb2+, thanks to -NH2 groups in the structure of the chitosan, could act as a soft base. The reaction mechanism was closely related to the chemical hardness values of the adsorbent and adsorbate.

2 Materials and methods

2.1 Chemicals used

Starch, epichlorohydrin (ECH), erythrosine B, Ethanol (EtOH), hydrochloric acid (HCl), chitosan, sodium hydroxide (NaOH), and sodium tripolyphosphate (TPP) were of analytical grade and were obtained from Merck and Sigma-Aldrich companies.

2.2 Preparation of chitosan-starch composites

Double cross-linked chitosan-starch composite beads were prepared by mixing chitosan and starch at room temperature (25 °C): 4 g chitosan and 4 g starch in 160 mL (5% v/v) acetic acid (Fig. 1). The suspension was stirred vigorously for 2 h. Then, 160 mL (0.01 M) epichlorohydrin (ECH) pH 10.0 was added to the suspension and stirred vigorously for an additional 1 h. The suspension was added dropwise to the solution containing 500 mL (0.05 M) sodium tripolyphosphate (NaTPP). The mixture was stirred at 100 rpm for 3 h. The beads formed were filtered, and washed with distilled water to remove excess NaTPP, and were then dried at 37 °C. Before use, the beads were ground [30, 31].

Fig. 1
figure 1

Structure of cross-linked chitosan-starch complex

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2.3 Characterization of the biosorbent

The synthesized chitosan-starch composite was analyzed by Fourier Transform Infrared (FTIR) Spectrometer (ATR, Bruker, Tensor II), Scanning Electron Microscope (SEM), Energy Dissipative X-Ray (EDX) at CUTAM Central Laboratory (Sivas Cumhuriyet). University, Turkey) and ultraviolet–visible spectroscopy (TESCAN MIRA3 XMU, T-60, China).

2.4 Effect of PZC on the biosorbent

Biosorbent, 0.1 g, was dissolved in a series of 0.1 mol L−1 KNO3 solutions, pH1.0 and 12.0. For the adjustment of pH, HCl or NaOH (0.1 mol L-1) were used. PZC values were obtained by plotting initial pH values against ΔpH. Final pH was recorded after 24 h [32].

2.5 Batch adsorption experiments

2.5.1 Pb2+ adsorption from aqueous solution

Batch technique was used in all the adsorption studies, and adsorbed Pb2+ concentrations were determined by spectrophotometry (Perkin Elmer UV/Vis Spectrophotometer Lambda 25) using PAR [4-(2-pyridylazo) resorcinol)]: equilibrium solution, 50 µL, was added to 3.5 × 10–3 mol L−1 PAR solution, prepared in 0.7 M Tris/HCl (pH 8.5) buffer. The absorbance was read at 518.5 nm against the PAR reference [33]. The adjustment curve was created for the concentration range where the concentration absorbance change was linear [34].

Change in Adsorption with pH

The adsorbent, 0.05 g, was used within the pH in the range between 1.0 and 5.5, prepared with HCl or NaOH. This solution was interacted with 10 mL of 50 mg L−1 Pb2+ solutions.

Adsorption Variation with Concentration

After the completion of adsorption reaction, the ion concentration was measured. The adsorption pH was chosen as 5.8. Experimental isotherms showing the variation of adsorption with concentration were created, and the relevant parameters were derived using their fit with Langmuir, Freundlich and Dubinin-Radushkevich (DR) isotherms.

Change in Adsorption with Adsorbent Mass

Chitosan-starch composite, 30, 50, 100, or 250 mg, was added to 10 mL of 50 mg L−1 Pb2+ solution and the ion concentrations at equilibrium were measured after 24 h.

Change in Adsorption with Time

Sixty millilitres of 50 mg L−1 Pb2+ were added to 0.05 g adsorbent, and ion concentration change in the equilibrium solutions over time was detected at time intervals, using 1 mL aliquots for 24 h, and kinetic parameters were obtained.

Change in Adsorption with Temperature

Ten millilitres of 50 mg L−1 constant Pb2+ concentration solutions were added on 0.05 g adsorbent and the variation of adsorption with temperature was investigated at 5, 25, 40, and 50 °C. Thermodynamic parameters were derived from the variation of adsorption with temperature.

2.6 Details of density functional theory calculations

All DFT-calculations were performed using the B3LYP exchange–correlation functional [35, 36] and hybrid electronic basis set combining 6-31G(d) [37] for elements H, C, N, O and the lanl2dz [38] for Pb. The dispersion corrections D3 proposed by Grimme [39] were also included to take into account the weak non-covalent interactions, using the graphics processor-based TeraChem software [40,41,42,43]. Geometry optimization was carried out with the efficient geomeTRIC energy minimizer [44].

3 Results and discussion

3.1 Characterization studies

3.1.1 FTIR analysis

The chemical structure of the synthesized chitosan-starch composite, before and after Pb+2 adsorption, was investigated by FTIR spectroscopy (Fig. 2a and b). The broad peak occurring at 3248 cm−1 is attributable to a stretch O–H of the composite. It may also indicate N–H stretching. Peaks at 2929, 1633, 999–1013 cm−1 can be attributed to C-H, O–H, and C-O stretchings, respectively [45]. The peak seen at 2102 cm−1 represented minor starch components (lipid and protein) [46]. The small peak at 1632 cm−1 can be attributed to the C = O double bond stretch of the amide group, and the peak at 898 cm−1 to the C-H stretch of the cross-linked chitosan molecule, respectively [47]. After Pb2+adsorption, slight shifts were observed in the position of the functional groups (Fig. 2b). Some electrostatic or chelation interactions between Pb2+ and functional groups of the composite could be responsible for these changes [48]. An overall decrease in peak intensity was also obvious.

Fig. 2
figure 2

FTIR spectra of chitosan-starch before (a) adsorption, and after (b) the adsorption

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3.1.2 XRD characterization

The peaks at 2θ = 23.39° and 18.45° were characteristic of starch (Fig. 3), and those of chitosan was located at 23° [49]. After the adsorption, the density decreased in all composite peaks, due to Pb2+ adsorption.

Fig. 3
figure 3

XRD spectrums of chitosan/starch and Pb2+ loaded chitosan-starch

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3.1.3 Thermal analysis

The thermal stability of the composite was investigated in a temperature range between 22 and 890 °C, and was examined by TGA (Fig. 4a and b). Weight loss below 100 °C represented the evaporation of water and volatile substances. Antioxidants may also be degraded [50, 51]. In the range between 260 °C and 375 °C, the decomposition step accelerated. Chitosan chain may be split [52]. At this stage, chitosan amino moieties were expected to decompose. Starch can decompose into glycerol at about 290 °C [51]. Glucose units of starch undergo depolymerisation [46]. Finally, chitosan decomposes by pyrolysis up to approximately 600 °C. The cyclic structures of starch are degraded [50, 51]. Decomposition process caused a dramatic mass loss, 70%. After adsorption although the degradation stages were similar, the mass loss was 67% (Fig. 4b).

Fig. 4
figure 4

TG curves of chitosan/starch and Pb2+ loaded chitosan-starch

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3.1.4 SEM–EDX characterization

Composite morphology and elemental analysis were investigated by SEM and EDX (Fig. 5a, b, c and d). It can be said that the chitosan-starch composite had a compact structure (Fig. 5a). This may be due to the strong interaction between cross-linked chitosan, starch and NaTPP [47]. After Pb2+ adsorption, a flattening and smoothing of the surfaces occurred due to swelling.

Fig. 5
figure 5

SEM images chitosan/starch and Pb2+ loaded chitosan-starch (a and b) and EDX data (c and d)

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3.2 Data of adsorption

3.2.1 Effect of pH

The pH value of the adsorbate solution is very important in the adsorption process. The surface charge and degree of ionization of the adsorbent depend on the pH of the solution [53]. In understanding pH effect on adsorption, pHpzc was investigated first [54]. pHpzc is the pH value at which the net surface charge of the adsorbent is zero, and it is important for electrostatic interactions. It has been reported that cationic ions are better adsorbed when the solution pH is higher than pHpzc, whereas anionic ions are better adsorbed when the solution pH is lower than pHpzc [55]. Chitosan-starch composite pHpzc was 5.12 (Fig. 6a). The adsorbent surface becomes protonated with H+ ions when the pH is less than 5.12. The surface becomes anionic, with OH− ions, when pHpzc is greater than 5.12 (Fig. 6b). As Pb precipitates at high pH values, adsorption efficiency and Q decreases [56]. This may be because when pH is greater than 5, Pb hydrolyzes and therefore precipitates, making adsorption difficult [57].

Fig. 6
figure 6

a Point Zero Charge (pHpzc) for adsorbent, b The effect of pH on the adsorption

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The results showed that with the increase of pH, the adsorption efficiency and adsorption capacity firstly increased and then fluctuations occured. The highest adsorption efficiency and Q value were reached at pH 3.46 as 78% and 0.038 mol/kg, respectively. As the pH increases, the OH− ion in the environment would also increase, and some Pb salts might form [58].

3.2.2 Effect of adsorbent amount

The effect of different adsorbent amounts (30, 50, 100, 200 mg) on adsorption was studied (Fig. 7). It was determined that Q decreased as the amount of adsorbent increased. This may be due to the fact that as the amount of adsorbent increased, the active sites on the surface became saturated and agglomeration occurred [59]. At high adsorbent dose, the surface area might begin to decrease due to the overlap of the adsorbent’s active sites [60, 61]. The highest adsorption efficiency was reached with 100 mg, and the adsorption capacity (Q) reached its highest value with 30 mg adsorbent.

Fig. 7
figure 7

Adsorbent dependency of adsorption

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3.2.3 Isotherms

Lead adsorption of chitosan-starch composite was investigated by Langmuir, Freundlich, Temkin and Dubinin-Radushkevich (D-R) isotherm models. The Langmuir isotherm (Eq. 1) indicates that the adsorbent surface is homogeneous and the presence of monolayer adsorption. Here, Qm refers to the maximum adsorption capacity and b refers to the Langmuir constant. RL in Eq. 2 gives information on the suitability of the isotherm adsorption process. If RL is equal to 1, it means that the adsorption is linear, between 0 and 1 it is favourable, if it is greater than 1, it is unfavourable [56]. Freundlich isotherm (Eq. 3) expresses a heterogeneous adsorbent surface and multilayered adsorption [62]. The Temkin isotherm (Eq. 4) is used to determine the heat of adsorption between adsorbent and adsorbate. Here At, BT, and B are constants. The D-R isotherm calculates (Eq. 5) and characterizes the free energy in the adsorption process. KDR is the D-R isotherm constant, and \(E\) (Eq. 7) is the free energy constant [63].

$$\frac{1}{qe}=\frac{1}{qmax}+\left(\frac{1}{b.qmax}\right)\left(\frac{1}{Ce}\right)$$
(1)
$${R}_{L}=\frac{1}{1+KL.Co}$$
(2)
$$Lnqe=LnKf+\left(\frac{1}{n}\right)LnCe$$
(3)
$${q}_{e}= \beta .Ln{K}_{T} + \beta .Ln Ce$$
(4)
$$Ln{q}_{e}=Ln{Q}_{DR}-{K}_{DR}({\varepsilon }^{2})$$
(5)
$$\varepsilon =RTLn\left(1+\frac{1}{{C}_{e}}\right)$$
(6)
$$E=\frac{1}{\sqrt{2{K}_{DR}}}$$
(7)

Non-linear (Fig. 8) and linear (Fig. 9) forms of isotherm models were applied. The D-R isotherm showed the best fit to the q and C curves (Table 1) (Fig. 8). In Langmuir isotherm, Qm was 99.01. RL values resided between 0 and 1, indicating a favourable adsorption. The highest R2 value (0.99) was found with the D-R isotherm. The E value was 500 kJ/mol (Table 1), indicating a chemical adsorption [64]. The regression values delineated the models as D-R > Freundlich > Langmuir > Temkin.

Fig. 8
figure 8

Nonlinear adsorption isotherms: Experimental, Langmuir, Freundlich, Temkin and D-R models

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Fig. 9
figure 9

Linear adsorption isotherms a) Langmuir, b) Freundlich, c) Temkin, d) D-R

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Table 1 Adsorption isotherm constants
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3.2.4 Kinetics

Examination of kinetic models is important for understanding the adsorption mechanism [56]. To investigate the adsorption mechanism, pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models were used. PFO and PSO were expressed in Eqs. 8 and 9, respectively. k1 is the kinetic model constant for PFO, k2 is the kinetic model constant for PSO.

$${\text{Ln}}\left({q}_{e}-{q}_{t}\right)=Ln{q}_{e}-\frac{{k}_{1}}{2.303}{\text{t}}$$
(8)
$$\frac{t}{{q}_{t}}=\frac{1}{{k}_{2}.{q}_{e}2}+\frac{1}{{q}_{e}}t$$
(9)

Regression values were used for the the suitability of the kinetic models (Figs. 10 and 11). Pb2+ adsorption on the chitosan-starch composite suited to PSO with an R2 value of 0.99 (Table 2). Nonlinear regression analysis (graphs for PFO and PSO) (Fig. 10), indicated that the experimental qt data was in a better agreement with the qt values. The highest R2 value of linear regression was 0.99. This result indicated a chemisorption, taken place between the chitosan-starch composite and Pb2+ [56, 60].

Fig. 10
figure 10

Non-linear PFO and PSO kinetics

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Fig. 11
figure 11

Linear kinetic models a) PFO, b) PSO

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Table 2 PFO and PSO kinetic data at 298 K
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3.2.5 Adsorption thermodynamics

Thermodynamic parameters enthalpy (∆H), Gibbs free energy (∆G) and change in entropy (∆S) were investigated between 288 and 323 K (Eqs. 10-12). Ln Kc vs. 1/T values were plotted (Van't Hoff equation, Eq. 11; Fig. 12) (Table 3). The ideal gas constant R was 8.314 J/mol.K. Negative ∆H indicated that Pb2+ adsorption on chitosan-starch composite was exothermic. Negative ∆G values also point out that adsorption was spontaneous and favourable [65]. ∆G decreased as the temperature increased (Table 3). This result showed that a spontaneous adsorption process took place [65]. As the temperature increased, the spontaneous reaction also increased [46]. The positive ∆S value was brought about an increased randomness between the solid/solution interface [66].

Fig. 12
figure 12

The van’t Hoff plot of lnKD vs. 1/T for the estimation of thermodynamic parameters

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Table 3 Thermodynamic parameters for lead adsorption onto chitosan-starch
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$${K}_{D}={C}_{a}/{C}_{e}$$
(10)
$$In\;K_D=\frac{\Delta\text{S}}{\text{R}}-\frac{\Delta H}R.\frac1T$$
(11)
$$\Delta G=\Delta H- T\Delta S$$
(12)

3.2.6 Recovery

Reusability of adsorbents is important in terms of sustainability. The reusability of the chitosan-starch composite was tested thrice in a NaOH solution and 65% recovery of the activity was achieved (Fig. 13). Desorption was executed at different concentrations of HCl, NaOH and ethanol eluents, and the best result was obtained with 0.1 M in NaOH (Fig. 14).

Fig. 13
figure 13

Reusability of chitosan-starch (0,1 mol L.−1 10 mL NaOH)

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Fig. 14
figure 14

Effect of type, concentration, and volume of eluent solution on recovery of Pb

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After the third round of adsorption/desorption, adsorbent activity was 65%. This result can be attributed to the dissolution or degradation of the adsorbent [46]. The alkaline environment might adversely affect the structure of starch, causing partial degradation by breaking its hydrogen bonds [67].

3.2.7 Results of DFT calculations

Possible effects of Pb2+ on the geometry and electronic structure of the chitosan-starch composite was investigated. Two-ring simple fragments were used both as chitosan- and starch models (Figs. 15 and 16, respectively). The binding energy of chitosan-starch composite (Fig. 17) was determined by using the equation below:

Fig. 15
figure 15

Chitosan: atomic structure (a), HOMO (b), and LUMO (c)

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Fig. 16
figure 16

Starch: atomic structure (a), HOMO (b), and LUMO (c)

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Fig. 17
figure 17

Chitosan-starch composite: atomic structure (a), HOMO (b), and LUMO (c)

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$${E}_{b}=E\left({\text{chitosan}}\right)+E\left({\text{starch}}\right)-E\left({\text{chitosan}}-{\text{starch}}\right)$$
(13)

The binding energy was 2.034 eV (Table 4). It can be seen that Pb2+ is geometrically located near the aromatic oxygen atoms on the chitosan and starch rings (Fig. 18). The presence of the ion significantly changed the electronic structure of the chitosan-starch composite, facilitating charge transfer, and reducing the gap between the frontier orbitals (Table 4). The adsorption energy, 11.416 eV, was obtained using the formula below:

Table 4 Calculated electronic characteristics of chitosan, starch, their composite, and their complex with ion Pb2+
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Fig. 18
figure 18

Chitosan-starch composite/Pb.2+: atomic structure (a), HOMO (b), and LUMO (c)

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$${E}_{ads}=E\left({\text{chitosan}}-{\text{starch}}\right)+E\left({{\text{Pb}}}^{2+}\right)-E\left({\text{chitosan}}-{\text{starch}}/{{\text{Pb}}}^{2+}\right)$$
(14)

Chemical hardness is defined as the resistance towards electron cloud polarization or deformation of chemical species. Hard acids prefer to coordinate to hard bases, and soft acids prefer the coordinate to soft bases. In CDFT, chemical hardness was calculated as \(\eta ={E}_{LUMO}-{E}_{HOMO}\) based on frontier orbital energies.

4 Conclusion

In this study, chitosan-starch composite was synthesized and used for the adosrption of Pb2+ in aqueous solutions. The synthesized composite was characterized by FTIR, XRD, TG and SEM techniques and its structure was investigated. By optimizing the parameters affecting the adsorption process, 94% lead removal was achieved. It was determined that the adsorption process followed the PSO kinetic model with the D-R isotherm, in which it emerged with a chemical reaction. The maximum adsorption capacity was calculated as 99.01 mg/g. With these results, it is thought that the environmentally friendly composite obtained by using an important biopolymer, chitosan, and starch can be used effectively in water treatment.