Adsorptive removal of Pb(II) ions from groundwater samples in Oman using carbonized Phoenix dactylifera seed (Date stone)

2.1 Apparatus

Atomic absorption spectrometer (Thermo-Scientific, iCE 3500 series, UK) was used to determine Pb(II) ions in test and real samples. Hanna pH meter (USA) was used to measure pH of solutions. Electrical linear plate shaker (SM-30, Edmund Bühler GmbH, Germany) was used to stir solution at 120 revolutions per minute (rpm). Electrical vacuum pump (Rocker 300, Taiwan) was used for filtration.

2.2 Standard and samples solutions

A standard aqueous solution of 100 mg L⁻¹ lead(II) was prepared in 250 mL demineralized water and maintained at pH 5. The ground water samples were collected in new pre-cleaned polypropylene bottles (450 mL capacity) from different locations of Oman such as Al-Jarda well (Ash Sharqiyah region), Wadi Andem and Falaj Samail (Ad Dakhilliyah region) in the morning time between 7:00 to 9:00 AM. Sampling procedure was adopted as per the grab method (Canadian Council of Ministers of the Environment, 2011). Water samples from Al-Jarda well and Wadi Andem were collected from 2 directions (North and East). The properties of water samples are given in Table 1.

All ground water samples were stored at 4 °C and the pH was maintained at 5 with HNO₃ prior to the determination of lead(II) ions by atomic absorption spectrophotometry. Neghal (a productive date palm tree in Oman) date stones were collected from Nizwa, Oman.

2.3 Method for analysis of lead in ground water samples

The absorbance of solutions containing varying concentration of Pb(II) ions was measured at 217 nm using atomic absorption spectrometer (AAS) against reagent blank. The linear regression equation was generated using OriginPro 6.1 Software (USA). The absorbance of ground water samples was recorded (pH maintained to 5). The amount of lead ions in ground water samples was computed using linear regression equation.

2.4 Method for preparation of activated carbonized date stones

Date stones of Neghal were taken and washed properly with distilled water. The date stones were dried at room temperature. The dried date stones were soaked in hot water for at least 1 h to remove soft materials and impurities from date stones. The date stones were cleaned with hot water, then with doubly distilled water and kept under sun light for 3 h. Dried date stones were transferred into crucible and heated under fume hood with Bunsen burner flame until all of the white fumes came out. Roasted date stones were kept inside the furnace at 600 °C for 15 min and thereafter, powdered using mortar and pestle. The powdered carbonized date stones were sieved using 120 mesh sieve (particle size < 125 μm). The filtered powder was transferred into a sample tube and kept in desiccator. Carbonized date stone (5 g) was taken and washed with 5 × 50 mL of doubly distilled water. The material was filtered using vacuum pump with glass crucible (Duran No. 4) and washed with distilled water until the pH of the filtrate became 7.

2.5 Experimental design

In Box-Behnken design (BBD) (Rahman and Nasir, 2018), three factors were selected: pH (A; 2.5–7.5), adsorbent dose (B; 0.10–0.40 g) and initial concentration (C; 1.5–8.5 mg L⁻¹).Design Expert software (free trial 11.1.0.1 version) was used to generate the BBD matrix and experiments were conducted accordingly to evaluate the effect of three main independent variables on the Pb (II) removal efficiency. For response surface methodology, the experimental data were fitted to the second order quadratic model using the following equation:$$y={\beta }_0+\sum _{i=1}^k{\beta }_i{X}_i+\sum _{i=1}^k\sum _{j=1}^k{\beta }_{\mathit{ij}}{X}_i{X}_j+\sum _{i=1}^k{\beta }_{\mathit{ii}}{X}_i^2+\in$$where y represents the predicted response, β₀ is the constant coefficient. β_i, β_ii and β_ij are the, linear, quadratic and interaction coefficients of input variables, respectively. Analysis of variance (ANOVA) was performed to determine the significance of model terms.

2.6 Adsorption procedure

0.3 g of carbonized date stone was transferred into different dried 100 mL stoppered conical flasks. To each flask, 50 mL of each Pb(II) ions solution (concentration range: 1.5 to 5.0 mg L⁻¹; pH maintained to 5) was added and placed on electrical linear shaker for 2 min at 120 rpm at 25 °C. The solution was filtered using vacuum pump with glass crucible (Duran No.4) and concentration of Pb(II) in the filtrate was determined by AAS. The uptake of Pb(II) ions by carbonized date stone was calculated using the following equation:$$q_e=\frac{(C_i-C_e)V}{m}$$where q_e is the amount of Pb(II) ions adsorbed (mg g⁻¹), V and m are the volume of solution (litre) and mass of adsorbent (g), respectively.

The % removal of lead(II) ions was calculated using the equation:$$\%removaloflead=\frac{C_i-C_e}{C_i}\times 100$$where C_i and Ce are initial and equilibrium concentrations in mg L⁻¹ at t = 0 and t = equilibrium time, respectively.

3.1 Effects of contact time and revolutions per minute

The effect of contact time on % removal of Pb(II) by carbonized date stone was studied and maximum % removal of Pb(II) was achieved in 1 min. Therefore, 2 min of contact time was recommended as the optimum contact time for further studies. The effect of rpm on percent Pb(II) removal was studied using 5 mg L⁻¹ Pb(II) solution and 0.3 g of adsorbent. The maximum removal was obtained at 100 rpm, beyond this up to 150 rpm, the % removal was constant Hence 120 rpm was selected for further adsorption studies.

3.2 Box-Behnken statistical analysis

The proposed BBD requires 17 experimental runs to model the response surface. The experiments were conducted with different sets of input parameters and the percent removal of Pb(II) was measured for each set of independent variables. The percent removal of Pb(II) is considered as response in the model. The removal of Pb(II) with different sets of input parameters varies in the range of 54.87–88.50%.

The experimental data were fitted to second order quadratic models to obtain the empirical relationship between the predicted response and the independent variables. The second order quadratic equation in terms of coded factors is given below:

Removal = +88.5 + 5.46 A + 3.10B −1.28C + 9.00 AB − 1.20 AC − 8.26 BC − 15.43 A² − 6.85 B² − 6.08 C²

The results of ANOVA for the second order quadratic model showed that F-value for the model was very high (8.11 × 10⁶) which indicated that most of the variations in the response were explained well by the model. Moreover, all the model terms have very high F-values and lower p-values (<0.0001), suggested that all model terms have significant effect on the response. Additionally, the effects of interaction terms (AB, AC and BC) and quadratic terms (A², B² and C²) on the precent removal of Pb(II) are also significant at 95% confidence level.

Fig. 3 (a-c) showed the three dimensional surface plots of the quadratic model. Fig. 3 (a) showed the interaction effects of adsorbent dose and pH, keeping initial concentration constant on the removal efficiency of Pb(II). It was observed that the removal efficiency increased with increasing pH and adsorbent dose. The uptake of lead ions decreased as the acidity of the contact solution increases. The pH of the aqueous solution governs the adsorption process as it affects the surface charge of adsorbents and influences the degree of ionization of metal ions in solution. The maximum removal efficiency was achieved with 0.30 g of adsorbent at pH 5.0. The combined effect of pH and initial concentration on the present removal of Pb(II) is shown in Fig. 3 (b). As can be seen in Fig. 3 b that the removal efficiency increased with increasing concentration up to 5 mg L⁻¹. The interactive effect of adsorbent dose and initial concentration on the removal efficiency is displaced in Fig. 3 (c). The removal efficiency increased with increasing both adsorbent dose and initial concentration and maximum percent removal was obtained with 0.3 g adsorbent using initial concentration of 5 mg L⁻¹.

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Fig 3

Response surface plots showing the effects of (a) pH and adsorbent dose, (b) pH and initial concentration and (c) adsorbent dose and initial concentration on the percent removal of Pb(II).

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For the numerical optimization, the input variables were given specific ranged values while response was set at maximum. Under these conditions, the optimum values of each independent variable were obtained using Design Expert software. The maximum removal was 88.50% at an initial pH of 5.0, Pb(II) ions concentration of 5 mg L⁻¹ and adsorbent dose of 0.3 g. Experiments were conducted under these conditions. The results are very close to the predicted values.

3.3 Adsorption isotherms

Equilibrium data were analysed by Langmuir and Freundlich isotherm models.

The Langmuir model is described by the following equation (Langmuir, 1918):$$\frac{1}{q_e}=\frac{1}{q_m}+\frac{1}{K_L{q}_m}\times \frac{1}{\mathit{Ce}}$$where q_m is the amount adsorbed in mg g⁻¹ (maximum adsorption capacity); K_L is the Langmuir constant in L mg⁻¹. The graph between 1/q_e and 1/C_e was plotted (Fig. 4) which yielded q_m and K_L values of 9.03 mg g⁻¹ and 0.157 L mg⁻¹, respectively with R² = 0.9999. Carbonized date stone has the advantage of removing lead(II) ions in 1.0–2.0 min whereas reported methods (Mishra and Patel, 2009; Abudaia et al., 2013) required longer time to remove lead(II) ions.

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Fig. 4

Langmuir adsorption isotherm for adsorption of Pb(II) ions at 298 K.

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R_L, the separation factor was calculated with K_L value using the following expression:$${\text{R}}_{\text{L}}=\frac{1}{1+{\text{K}}_{\text{L}}{\text{C}}_{\text{i}}}$$where C_i is the initial concentration in mg L⁻¹. The value of R_L predicted the nature of adsorption. Here, R_L values were in the range of 0.515–0.809 at 298 K demonstrated 0 < R_L < 1, hence the adsorption was favourable at room temperature.

Freundlich isotherm model was also investigated using the following equation (Rahman and Nasir, 2019):$$\text{ln}{\text{q}}_{\text{e}}=\text{ln}{\text{K}}_{\text{f}}+1/\text{n}\text{ln}{\text{C}}_{\text{e}}$$where K_f and n are related to adsorption capacity and intensity of adsorption, respectively. The graph between ln qe and ln Ce was plotted (Fig. 5), yielded K_f and n values of 1.25 and 1.06, respectively with R² = 0.9998 which demonstrated the favourable adsorption.

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Fig. 5

Freundlich adsorption isotherm for Pb(II) adsorption at 298 K.

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The values of R² ˃ 0.9999 were found for both Langmuir and Freundlich isotherm models. Therefore, both the models were able to describe the adsorption process very well.

3.4 Adsorption thermodynamics

The orientation and feasibility of the adsorption of lead(II) ions onto carbonized date stone was evaluated at room temperature by Gibb’s free energy, ΔG° using following expressions:$$\mathrm{\Delta }{G}^0=-RTln{K}_c$$

K_c = q_e × 1000/C_ewhere K_c is the equilibrium constant, R is universal gas constant (8.314 JK⁻¹mol⁻¹), T is the absolute temperature in Kelvin. If ΔG ° reaches to −20 kJ mol⁻¹, would be the basis for electrostatic interaction between adsorbent sites and metal ions (Yao et al., 2010). Here, ΔG° was found to be −17.91 kJ mol⁻¹. Hence, the adsorption of lead(II) ions on to carbonized date stone was spontaneous, feasible and resulted in physical adsorption process. As the temperature for adsorption of lead(II) ions increased (greater than 298 K), the sorption decreased. Hence, the sorption of lead(II) ions is endothermal in nature.

3.5 Adsorption kinetics

The Lagergren’s first-order kinetic model (Rahman and Nasir, 2019) was applied using following equation:$$\text{ln}({\text{q}}_{\text{e}}-{\text{q}}_{\text{t}})=\text{ln}{\text{q}}_{\text{e}}-{\text{k}}_1\text{t}$$where q_t and k₁ refer to metal adsorbed per gram of adsorbent at time, t in min and rate constant in min⁻¹, respectively. ln (q_e -q_t) was plotted against t (Fig. 6), resulted with slope and intercept of −2.06 and −1.51, respectively. q_e was calculated and found to be 0.22 which is much smaller than the experimental q_e (0.737 mg g⁻¹). Hence, the adsorption of Pb(II) ions on to carbonized date stone did not obey Lagergren first-order kinetic model.

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Fig. 6

Pseudo-first-order reaction plot for Pb(II) adsorption onto carbonized date stone.

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The adsorption data were then analysed by a pseudo-second order kinetic model (Rahman and Nasir, 2019) using the equation:$$\frac{t}{q_t}=\frac{1}{k_2{q}_e^2}+\frac{1}{q_e}\times$$where k₂ is the pseudo-second-order rate constant in g mg⁻¹ min⁻¹.

The adsorption rate, h_o in mg g⁻¹ min⁻¹ at t → 0 was calculated by:$$h_o=k_2\times q_e$$

A curve between t/q_t and t was plotted (Fig. 7) which provided slope and intercept of 1.33 and 0.077, respectively. q_e, k₂, and h_o were calculated and found to be 0.752 mg g⁻¹, 22.85 g mg⁻¹ min⁻¹ and 12.98 mg g⁻¹ min⁻¹, respectively. The calculated q_e (0.752 mg g⁻¹) was close to q_{e, exp} (0.737 mg g⁻¹) and hence the adsorption of Pb(II) ions onto carbonized date stone was best described by pseudo second-order kinetic model. This suggested that the multilayer adsorption occurred onto the surface of adsorbent.

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Fig. 7

Pseudo-second-order reaction plot for Pb(II) adsorption onto carbonized date stone.

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3.6 Application to environmental water samples

The carbonized date stone was applied for remediation of lead(II) ions in groundwater samples. The removal percentage of lead(II) ions in well, falaj and wadi water samples were about 78% (Table 2).

The presence of other ions in real water samples slightly interfered with the adsorption of lead(II) ions under the optimized conditions. Hence, the % removal of lead(II) ions in ground water samples was found to be less than that obtained using Box-Behnken statistical analysis.