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# Optimal ranges of variables for an effective adsorption of lead(II) by the agricultural waste pomelo (Citrus grandis) peels using Doehlert designs

## Introduction

Removing toxic heavy metal ions from aqueous solutions is still a key issue in the management of environmental pollution. Mercury (Hg), cadmium (Cd), chromium (Cr), arsenic (As), lead (Pb) and nickel (Ni) are major contributors to poisonousness in wastewater, while among these ions, lead(II) is more likely to cause people’s attention because of its cumulative toxicity, extensively existence and various sources1,2,3. According to World Health Organization (WHO), lead exposure is estimated to account for 0.6% of the global burden of disease, with the highest burden in developing regions4, which makes the removal of lead(II) from wastewater of great urgency and importance.

According to current studies, the adsorption rate of bio sorbents is affected by a variety of factors, for example, the pH of the solution, temperature and time of the adsorption, the amount of bio sorbents and initial concentration of toxic heavy metal ions. In comparison with Doehlert designs24, one of response surface methods which are more efficient and offer advantages in relation to central composite13,14 and Box–Behnken designs25, one-variable-at-a-time, the method that mostly applied for the optimization of factors11,12,18,19 in bio sorbents’ adsorptions loses sight of the potential interactions between experimental variables as well as needs more time and experiments to optimize the whole factors. Besides, different from one-variable-at-a-time, Doehlert designs give the prediction model of the adsorption rate, through which the adjustment of some variables with others remaining the same is accessible and makes the optimized variables could be in a range rather than a single point, facilitating the industrial use of agricultural wastes as bio sorbents.

To investigate the capacity of pomelo peels’ adsorption on lead(II) removal without modification or carbonization, to explore the potential interactions between four experimental variables, which were pH, temperature, time and initial concentration of lead(II), and to provide quantitative effects, optimal ranges as well as the prediction model at the same time, Doehlert designs were selected and conducted in this study. Adsorption kinetic and isotherm models were fitted to discuss the mechanism of pomelo peel’s adsorption. Then, interfering study was carried out to test the selectivity of pomelo peels. The dominant functional groups were discovered by Fourier transform-infrared (FT-IR).

## Results and Discussion

#### Significant variables

Analytical results demonstrated that the linear model of lead(II) removal by pomelo peels as well as the whole four variables were significant, seen in Table 1, without significant interactions.

Quantitative effects and optimal ranges of these four significant variables were illustrated in Fig. 1. Dotted red crosses in Fig. 1 provided a choice for the adsorption rate approximately reaching 100%, that was, 20 mg/L for initial lead(II) concentration, 3 for pH, 50 °C for temperature and 210 min for time, respectively.

It was obvious that in the range of this study, the adsorption rate of lead(II) on pomelo peels increased as the pH, temperature and time increased, while as the initial lead(II) concentration decreased. Trends of temperature, time and initial lead(II) concentration were consistent with existing researches7,8,12,26,27,28; the adsorption process through pomelo peels required time to arrive at equilibrium and a higher temperature allowed the process to reach equilibrium more rapidly; also, the binding sites in pomelo peels for lead(II) were limited so that when the initial lead(II) concentration was at a certain value, pomelo peels were going to reach saturation and would no longer absorb lead(II) from aqueous solutions.

As for pH, situation was not similar. Due to the upper limit of pH in this study was 3.5, the optimum of pH for lead(II) removal with the value of 5.0 or 6.0 proposed by other researches29 did not occur; neither did the tendency with an ascent at first followed with a decline afterwards. Nevertheless, based on the trend of pH in Fig. 1, it was appropriate to infer that the optimal value of 5.0 or 6.0 in other researches was in the optimal ranges of pH in this study. The upper limit of pH with the value of 3.5 was selected for the following two reasons: i) to investigate the feasibility of a lower pH for lead(II) removal through pomelo peels; ii) to make the optimal pH for lead(II) removal closer to other heavy metal ions’ optimums, which was benefit to the simultaneous adsorption.

#### Prediction model

The prediction model given by Doehlert designs was presented as the following:

$${\rm{adsorption}}\,{\rm{rate}}=-38.78+21.86\,\times {\rm{pH}}+1.02\times {\rm{temperature}}+0.27\times \text{time}\,-1.36\,\times {\rm{Pb}}({\rm{II}}){\rm{concentration}}$$

Figure 2 demonstrated the kinetics of the adsorption of 20 mg/L lead(II) by pomelo peels at 45 °C with pH of 3. Sharply increased at the initial stage (0–30 min) of adsorption, the amount of lead(II) adsorpted by pomelo peels gradually increased at the middle stage (30–120 min) and reached an equilibrium value in approximately 210 min. Thus, the adsorption time was fixed at 210 min for the rest experiments to assure the equilibrium was arrived at.

From Table 2, parameters presented that the pseudo-first-order model was followed better than the pseudo-second-order model not only the R2, but also the consistence between the qe (cal.) and qe (exp.), which was in accordance with the existed research8 using the peel of Citrus reticulata without modification as the bio sorbent.

In isotherm models, Langmuir model fitted better than Freundlich model and model parameters were listed in Table 3 with plots given in Fig. 3. Valid for monolayer sorption onto a surface with a finite number of identical sites, which were homogeneously distributed over the adsorbent surface, Langmuir model indicated the adsorption mechanism of pomelo peels.

The value of R L pointed out the type of Langmuir isotherm to be irreversible (R L  = 0), favourable (0 < R L  < 1), linear (R L  = 1) or unfavourable (R L  > 1). In this study, R L was found to less than 0.3 for lead(II) concentration of 5 mg/L and less than 0.015 for 100 mg/L, which presented that pomelo peel was an efficient adsorbent with satisfactory adsorption effect in the removal of lead(II) from aqueous solutions, even without modifications or carbonizations.

### Interfering study

Compared the result of interfering study, absorption rate (int.), with adsorption rate (exp.) obtained from the kinetic experiment which shared the same experiment condition, seen in Table 4, it was concluded that the presence of calcium(II) and magnesium(II) at 100 times concentration and the presence of copper(II) and zinc(II) at the same concentration had no interference with the adsorption of lead(II) by pomelo peels.

### SEM analysis

Before adsorption, plenty of even, developed and well pronounced pores were over the surface of pomelo peels, as Fig. 4(a) demonstrated, forming an orderly pore structure. However, after adsorption, the orderly pore structure of pomelo peels was destroyed or disappeared to a certain extent, illustrated in Fig. 4(b). SEM micrographs of pomelo peels before and after lead(II) adsorption suggested that adsorption might be conducted by physical adsorption due to pomelo peels’ pore structure, though, interfering study’s result also indicated the possibility of chemical bonding, which was evaluated and confirmed by FT-IR analysis.

### FT-IR analysis

The pattern of sorption of metals onto plant materials was attributable to the active groups and bonds present on them32. FT-IR spectroscopy was, therefore, carried out for preliminary qualitative analysis of major functional groups worked in the adsorption of lead(II) by pomelo peels. From Fig. 5, differences of spectrum (a) and spectrum (b) were classified into two categories: i) changes of peaks intensities and ii) generations of peaks. As Fig. 5 showed, peak 3412 cm−1, 2920 cm−1, 2850 cm−1, 1637 cm−1, 1384 cm−1, 1262 cm−1, 1067 cm−1 and 538 cm−1 belonged to the first category, while peak 1617 cm−1, 990 cm−1, 618 cm−1 and 514 cm−1 were classified to the second category. The broad absorption peak at 3412 cm−1 might be correspond to the O–H stretching vibrations of cellulose, pectin and lignin18,33. Peaks of 2920 cm−1 and 2850 cm−1 were assigned to symmetric vibration of CHn especially C–CH2 bonds18, peak 1637 cm−1, 1384 cm−1, 1262 cm−1, 1067 cm−1 and 538 cm−1 could be related to ketones, -CH3 in alcohols and phenols, secondary amides, -C-O stretching vibrations of primary alcohols and C-Br34. New generated peaks 1617 cm−1, 990 cm−1, 618 cm−1 and 514 cm−1 might be regarded as intermolecular hydrogen bonds in ketones, alkene mono-substitution and C-Cl according to current literatures34. Hydroxyl groups in biopolymers were considered as proton donors, hence, deprotonated hydroxyl groups may be involved in coordination with metal ions35, so did in this research.

## Materials and Methods

### Chemicals

All chemicals and reagents used in this research were of analytical grade and bought from Sinopharm Chemical Reagent Co., Ltd, Shanghai, P. R. China. Deionized water (≥18.2 MΩ) was applied for the preparation of aqueous solutions.

Lead(II) solutions with various concentrations were prepared by mixing appropriate amounts of Pb(NO3)2 and deionized water. 0.1 M HNO3 or NaOH was employed to adjust the pH of lead(II) solutions with the help of Sartorius PB-10 pH meter.

Pomelos were acquired from Yuhuan County, Zhejiang Province, P. R. China, and their peels were washed with distilled water several times and immersed in deionized water for 24 hours to eliminate the interference effect of other particles. Then pomelo peels were dried in a hot air oven at 60 °C18 until they were in constant weights and stored in the desiccator.

The amount of pomelo peels in this study was 0.5 g in the dry state through cutting and weighting by the balance. The volume of lead(II) solutions was 50 mL per experiment.

#### Doehlert designs

A Doehlert matrix for four-variables25 was selected and details were shown in Table 5. Ranges of variables were designed on the basis of current literatures.

The concentration of the residual lead(II) in solutions was determined by atomic absorption spectrophotometry (AAS; PE AAnalyst 800) and the adsorption rate (Y) was calculated as the equation (1):

$$Y=\,\frac{{C}_{i}-{C}_{r}}{{C}_{i}}\times 100 \%$$
(1)

where C i was the initial lead(II) concentration in mg/L and C r was the residual lead(II) concentration determined by AAS in mg/L.

Data acquired from Doehlert designs were analyzed by Design-Expert 10 (Stat-Ease, Inc., U.S.A.) and 0.05 was applied as the significant level.

#### Kinetic studies

A good correlation of the kinetic data explained the adsorption mechanism of the metal ion on the solid phase36. To evaluate the kinetic mechanism, kinetic studies were conducted in the temperature of 45 °C with the pH of 3. The initial lead(II) concentration was 20 mg/L and adsorption time was the variable, from 10 min, 20 min, 30 min, 60 min to 120 min, 180 min, 240 min and 300 min. The pseudo-first-order and pseudo-second-order models were applied for the kinetic studies.

The pseudo-first-order37 model was shown in equation (2):

$$\mathrm{log}({q}_{e}-{q}_{t})=\,\mathrm{log}\,{q}_{e}-\frac{{k}_{1}}{2.303}t$$
(2)

where qe and qt (mg/g) were the amount of adsorbed lead(II) at equilibrium and at time t; k1 (min−1) was the rate constant of the pseudo-first-order model. Through drawing the plot of $$\mathrm{log}({q}_{e}-{q}_{t})$$ vs. t, qe and k1 were calculated as the slope and intercept and it was required that the calculated equilibrium adsorption capacity value, qe (cal.), should be in accordance with the experimental qe (exp.) value38.

Equation (3) gave the model of the pseudo-second-order39:

$$\frac{t}{{q}_{t}}=\frac{1}{{k}_{2}{q}_{e}^{2}}+\frac{t}{{q}_{e}}$$
(3)

where k2 (g mg−1 min−1) was the rate constant of pseudo-second-order adsorption. From the slope and intercept of the plots $$\frac{t}{{q}_{t}}$$ vs. t, the pseudo-second-order rate constant k2 and qe values were acquired.

#### Isotherm studies

The adsorption isotherm provided the relationship between the amounts adsorbed by a unit weight of adsorbent and the amount of adsorbate remaining in aqueous solutions at equilibrium12.

In this research, isotherm studies were carried out in lead(II) concentrations from 10 mg/L to 30 mg/L as the initial concentration, and pH, adsorption temperature along with adsorption time were selected accordingly with values of 2.5, 30 °C and 210 min. Langmuir model and Freundlich model were tested to figure out the better one.

Langmuir model40 was calculated as the equation (4):

$${q}_{e}=\frac{{q}_{max}b{C}_{e}}{1+b{C}_{e}}$$
(4)

where q e (mg/g) was the amount of lead(II) bound to per gram of the pomelo peel at equilibrium, C e (mg/L) was the residual (equilibrium) lead(II) concentration left in the solution after binding, q max (mg/g) was the maximum amount of lead(II) per unit weight of the pomelo peel to form a complete monolayer on the surface and b (L/mg) was the constant related to the affinity of the binding sites12.

The essential characteristics of Langmuir model was explained in terms of dimensionless constant separation factor (R L ), defined by equation (5):

$${R}_{L}=\frac{1}{1+b{C}_{0}}$$
(5)

where b (L/mg) was the Langmuir constant and C0 (mg/L) was the initial concentration of lead(II).

Freundlich model was expressed as equation (6)41:

$$\mathrm{log}\,{q}_{e}=\,\mathrm{log}\,{K}_{F}+\frac{1}{n}\,\mathrm{log}\,{C}_{e}$$
(6)

where K F and n were Freundlich isotherm constants related to biosorption capacity and intensity of adsorption. If the equation (6) worked, a plot of logqe versus logCe would give a straight line of slope $$\frac{1}{n}$$ and intercept KF.

#### Interfering study

10 times concentration of Ca2+ and Mg2+ with the value of 200 mg/L and the same concentration of Cu2+ and Zn2+ as Pb2+ with the value of 20 mg/L were designed for interfering study to test the selectivity of pomelo peels on absorbing lead(II). Temperature and pH were 45 °C and 3, respectively. The adsorption time was set according to the result of kinetic studies when the solution reached equilibrium.

Interfering study was carried out for three times and the average adsorption rate was chosen to compare and evaluate.

#### SEM analysis

Micrographs of pomelo peels before and after lead(II) adsorption were acquired with a E-1010 Ion Sputter scanning electron microscopy (Hitachi) for the characterization of the adsorbent as well as the elucidation of the probable mechanism of adsorption12.

#### FT-IR analysis

Spectra of pomelo peels before and after lead(II) adsorption were recorded with a Nicolet™ iS™ 10 FT-IR spectrophotometer (Thermo Scientific) using KBr pellets methods to discover the functional groups.

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## Acknowledgements

This work was supported by 863 National High-Tech Research and Development Plan [grant number 2013AA102301] and National key point research and invention program of the thirteenth [grant number 2016YFD0700304].

## Author information

Authors

### Contributions

Xiao-Lan Yu designed and carried out the experiments. Xiao-Lan Yu managed the data processing and wrote this manuscript. Yong He reviewed the initial design of the experiments and made guidance for the writing of the manuscript. All authors reviewed the manuscript.

### Corresponding author

Correspondence to Yong He.

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The authors declare that they have no competing interests.

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Yu, XL., He, Y. Optimal ranges of variables for an effective adsorption of lead(II) by the agricultural waste pomelo (Citrus grandis) peels using Doehlert designs. Sci Rep 8, 729 (2018). https://doi.org/10.1038/s41598-018-19227-y

• Accepted:

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• DOI: https://doi.org/10.1038/s41598-018-19227-y

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