Introduction

Heavy metals (e.g., Cu2+, Ni2+, and Zn2+) in the contaminated water are persistent toxins and may accumulate in living organisms, causing different diseases and disorders1. The main sources of heavy metals are, e.g., electronic and cables industry (Cu), nickel alloy production (Ni), and brass coating (Zn)2. Levels of heavy metals in wastewater vary with the sources and locations3, e.g., concentration of Zn is between 0.16–56.7 mg L−14. According to World Health Organization—Guidelines for drinking-water quality, the permissible limits of heavy metal ions in drinking water are e.g., 1.5, 0.1, and 5 mg L−1 for Cu, Ni, and Zn5.Several techniques such as coagulation6, ion exchange7, membrane filtration8, and precipitation9, for the treatment of heavy metals in solution have been developed. However, they either show low removal efficiency or are expensive in particular for treating heavy metals with a low concentration (e.g., < 100 ppm)10. Alternatively, adsorption has been found superior to the above techniques for the removal of heavy metals in terms of flexibility of design, initial investment cost, ease of operation, and low maintenance cost11. A variety of adsorbents (e.g., charcoal12 and activated carbon13) have been widely studied in literature and applied in the polluted water treatment plants.

In the latest development, the bio-adsorption technology using bio-based materials as the adsorbents (bio-adsorbents) has emerged and has become significant in the field14, including the removal of heavy metals in solution15. Different from adsorption, biosorption applies biological materials as the adsorbents16. A few bio-adsorbents (e.g., raw orange peel15, modified sugarcane bagasse17, modified oak sawdust18, modified Lignin19, modified sunflower stalks20, peanut hull21, dehydrated wheat bran22, maize leaf11, hazelnut shells23, Pinus bark24, banana and orange peels25, modified sugar beet pulp26, modified corncob27, Schleichera oleosa bark28, and bottlebrush plant seeds29) show the adorability for a wide range of heavy metals. Lignocellulosic biomass is a type of low-cost and abundantly available bio-adsorbent30 and shows the promise for the removal of heavy metals31. One of the examples could be the work by Rowell et al., who studied the bio-adsorption of Cu2+, Ni2+, and Zn2+ from aqueous solution using several lignin-containing agricultural materials32.

Date pits (DP) is an interesting lignocellulosic biomass33, which is a by-product of food processing and Jam production34 and has a large annual production of e.g., 600,000 tons in 2019, 750,000 tons in 2020, and 900,000 tons in 2021 in Iraq35. The potential of DP as a bio-adsorbent has been demonstrated by many research groups for the adsorption of heavy metals such as Au3+36, Cd2+33, and Al2+37. Two examples for the adsorption of Cu2+, Zn2+, and Ni2+ (that are interested in this study) could be the early work by Al-Ghouti et al. (showing the adsorption capacities of 0.15 mmol g−1 DP without any pretreatment for the removal of Cu2+)33 and the recent work by the authors (showing the adsorption capacities of 0.21, 0.15, and 0.13 mmol g−1 Al-Zahdi Iraqi DP for the removal of Cu2+, Zn2+, and Ni2+)38. Nevertheless, these bio-adsorption capacities of DP are lower compared to those of the other types of bio-adsorbents reported in literature39. Therefore, improving the adsorption capacity of DP-based bio-adsorbent remains the research interest.

In literature, modification (e.g., by coating40) of the raw bio-adsorbent towards a more effective bio-adsorbent has been widely studied19,41. Coating with ZnO is of significant research interest considering that ZnO is a comparatively inexpensive and environmentally benign material, and contains functional groups (e.g., -OH) in an aqueous medium that renders the absorption of metal ions42. ZnO, such as ZnO nanoparticles43 and nanorods44, could be used alone for the adsorption of heavy metals (e.g., Cd2+, Cu2+, Ni2+, Pb2+, Zn2+, Cd2+, Hg2+, and As3+) from solution, however, is often used in combination with other adsorbents (e.g., mesoporous carbon45 and graphene oxide nanocomposites46) for an improved removal of heavy metals from solution.

Therefore, based on the above promising adsorption performance of ZnO-containing adsorbents and the DP-based bio-adsorbents, ZnO-modified DP (MDP) could be an alternative bio-adsorbent for the bio-adsorption of heavy metals, which to the best of our knowledge, has not been considered in literature. In this contribution, the Al-zahdi Iraqi Date pits (DP) were modified by loading ZnO followed by thermal treatment. The MDP bio-adsorbent was characterized in detail and the isotherms, kinetics, and thermodynamics of the bio-adsorption of heavy metals (Cu2+, Zn2+, and Ni2+) in solution were studied systematically, focusing on the surface properties of the bio-adsorbent and the consequent effect of bio-adsorption conditions (adsorbent concentration, adsorbent particle size, shaking speed, pH of the solution, initial concentration of heavy metals, adsorption time, and adsorption temperature) on the bio-adsorption performance. The present work shows the significance of the modification of date pits (DP)-based bio-adsorbent by loading of ZnO and the followed thermal treatment, which remarkably improved bio-adsorption efficiency for the removal of heavy metals (Cu2+, Ni2+, and Zn2+) in the solution compared to those in literature.

Experimental

Preparation of the bio-adsorbents

The steps for the preparation of ZnO-modified date pits (MDP) from date pits (DP) are shown in Fig. 1. Iraqi Zahedi date was bought from the local market in Baghdad. The obtained date pits (DP, Fig. 1) were cleaned and washed 3 times with deionized water and were dried in air for 7 days. The pretreated DP were further dried in an oven at 120 °C for 24 h, followed by crushing, milling, and sieving to obtain the roasted DP with particle sizes of 1500, 1000, 500, and 200 µm. The roasted DP (10 g) were then added to aqueous zinc chloride solution (2 M, 100 ml) in a 500-mL flask and the mixture was stirred (150 rpm) at 80 °C for 4 h, followed by filtration-washing for 5 cycles with deionized water and drying in an oven at 120 °C for 12 h. The dried samples were further added to aqueous HCl solution (0.1 M, 50 ml) and the mixture was stirred (50 rpm) for 2 h, followed by decanting, filtration, and washing the precipitate with deionized water for 5 times. After drying in an oven at 120 °C for 24 h, the solid samples were heated to 350 °C at a heating rate of 5 °C min−1 and were calcined for 4 h in a furnace. The obtained samples were denoted as modified date pits (MDP, Fig. 1) and stored in a desiccator. All procedures were conducted in accordance with the guidelines.

Figure 1
figure 1

Photos of date pits (DP) and ZnO-modified date pits (MDP) (left) and the steps for the preparation of MDP from DP (right).

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Characterization of the bio-adsorbents

The fresh and spent MDP were characterized to study the characteristics of adsorbents for an understanding of the process and mechanisms involved in the bio-adsorption. Fourier transform infrared (FT-IR) spectra were obtained on an FT-IR spectrometer (Perkin Elmer Spectrum 2000). N2 physisorption was performed on a Surfer Gas Adsorption Porosimeter (Thermo Scientific). The specific surface area was determined using the Brunauer–Emmett–Teller (BET) method according to ASTM D1993. Scanning electron microscopy (SEM) was performed on a JSM-6090LV scanning electron microscope (JEOL). X-Ray Diffraction (XRD) patterns were obtained on a Magix Pro MPD X-ray diffractometer (Philips/Panalytical). The pH of the point of zero charge (pHpzc) was determined on a pH meter (inoLab 7110, WTW) according to the method described in Ref47.

Bio-adsorption of heavy metals from aqueous solution and real wastewater

Bio-adsorption of heavy metals (including Cu2+, Ni2+, and Zn2+) from aqueous solution was performed in a batch system using MDP as the adsorbents. The metal salts namely copper sulfate pentahydrate (CuSO4·5H2O), cobalt(II) nitrate hexahydrate (Ni (NO3)2·6H2O), and zinc sulphate heptahydrate (ZnSO4·7H2O) were dissolved in 50 mL water in a 100-mL flask, followed by adding the adsorbents under shaking. The adsorption conditions were: pH of 2–8, adsorbent dosing of 0.4–6 g L−1solution, adsorbent particle size of 200–1500 µm, shaking speed of 150–400 rpm, adsorption time of 10–180 min, initial concentration of heavy metals of 10–90 mg L−1, and adsorption temperature of 25–55 °C. The liquid samples were taken at different adsorption time intervals and were analyzed on a GBC 932 atomic absorption spectrometer after the filtration using cellulose nitrate membrane with pore size of 0.45 µm.

After the bio-adsorption, the sludge (namely the spent MDP bio-adsorbent) was regenerated and reused to evaluate the reusability of the MDP. The spent MDP containing the adsorbed heavy metals was added to 14 ml aqueous solution containing 50 mM H2SO4. The slurry was stirred at a speed of 350 rpm and at a desorption temperature of 25 °C continuously for 48 h. After the filtration and washing with water, the MDP was reused for the bio-adsorption for 4 times.

In addition, a real wastewater, which was collected from an electroplating company in Baghdad and contained 21.6 mg L−1 of Cu2+, 13.9 mg L−1 of Ni2+, and 18.9 mg L−1 of Zn2+, was applied to evaluate the bio-adsorption performance.

The removal efficiencies of heavy metals (R, %) and the adsorption capacity of the bio-adsorbent (qe, mg g−1)47,48 were calculated using Eqs. (1)48 and (2)49, in which C0, Ct, and Ce are the initial concentration, the concentration at adsorption time (t, min), and the equilibrium concentration of heavy metals in the aqueous solution (mg L−1), respectively; v is the volume of the solution (L); and w is the weight of the bio-adsorbent (g). All the experiments were triplicated and the averaged data are reported.

$$R=\frac{{C}_{0}-{C}_{t}}{{C}_{0}}\times 100$$
(1)
$${q}_{e}=\frac{\left({C}_{0}-{C}_{e}\right)\times v}{w}$$
(2)

Results and discussion

Characterization of the bio-adsorbents

The specific surface area of Date pits (DP) is 82.4 m2 g−1. It consists of cellulose, hemicellulose, and lignin50. The FT-IR spectrum of DP (Fig. 2a) shows the typical adsorption bands of these biomass components, e.g., at 3500–3400 cm−1 (vibration of inter- or intramolecular hydrogen bonding (O–H) in cellulose51), 3000–2800 cm−1 (stretching of aliphatic C-H bonding52), 1745–1600 cm−1 (stretching of unconjugated C=O bonding in hemicellulose53), 1604.77 cm−1 (stretching of aromatic C=C bonding53), 1400–800 cm−1 (deformation of C-H bonding in lignin54), 1365 cm−1 (deformation of C-H bonding in cellulose and hemicellulose52), 1246 cm−1 (stretch of C–O bonding in lignin and xylan52), 1083.99 cm−1 (stretch of C–O bonding in hemicellulose52), and 869 cm−1 (stretch of C–O bonding in cellulose52). Besides, the XRD pattern of DP (Fig. 3a) also shows the representative diffraction peaks of crystalline cellulose (2θ = 16.2 and 23.9°55), amorphous hemicellulose (2θ between 30 and 50°56), and lignin (2θ = 64.4°57).

Figure 2
figure 2

FT-IR spectra of (a) DP, (b) fresh MDP, and (c) spent MDP after simultaneous adsorption of heavy metals (at bio-adsorption conditions: adsorption temperature 25 °C, initial concentration of 90 mg L−1, MDP dosing of 2 g L−1solution, adsorption time of 180 min, particle size of 200 µm, shaking speed of 300 rpm, and pH of 5).

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Figure 3
figure 3

XRD patterns of (a) DP, (b) fresh MDP, and (c) spent MDP after simultaneous adsorption of heavy metals (at bio-adsorption conditions: adsorption temperature 25 °C, initial concentration of 90 mg L−1, MDP dosing of 2 g L−1solution, adsorption time of 180 min, particle size of 200 µm, shaking speed of 300 rpm, and pH of 5).

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SEM image of DP is shown in Fig. 4a, indicating its rough surface with macropores58. The morphological characteristics of DP are changed after the incorporation of ZnO on DP (Fig. 4b), showing additional particles on the DP surface. EDX analysis of ZnO-modified date pits (MDP, Fig. S2) confirms the presence Zn and O, which are related to the ZnO particles. According to the EDX results, the ZnO content on MDP is about 35 wt.%. MDP contains ZnO particles with a particle size distribution of 0.1–1.2 μm (centered at 0.64 μm, Fig. S1, estimated by using the Digimizer program) and have a coarse (or unsmooth) surface (Fig. 4b). Compared to DP, MDP has a higher surface area of 195.89 m2 g−1, which is likely related to the loaded ZnO particles on the surface and is advantageous for the bio-adsorption (vide infra)59. It needs to be noted that MDP was treated at 350 °C, at which temperature the main components of DP could be partially converted, e.g., lignin starts to degrade at 200 °C, hemicellulose starts to decompose at 220 °C, and cellulose starts to depolymerize at 310 °C60. Since the thermal treatment temperature is higher than biomass torrefaction (e.g., 250–320 °C) but lower than the pyrolysis temperature (e.g., 450–600 °C), a bio-char-type material was likely obtained (Fig. 1). This is well reflected by the FT-IR spectrum of MDP (Fig. 2b, showing the disappearance and shifting of the adsorption bands for several functional groups on DP) and the XRD pattern of MDP (Fig. 3b, showing the disappearance of the diffraction peaks for the crystalline components in DP). The incorporation of ZnO onto DP is evidenced by the two characteristic adsorption bands of ZnO at 594 and 478 cm−1 (corresponding to the inorganic ZnO stretching61) in the FT-IR spectrum of MDP (Fig. 2b) and the diffraction peaks of ZnO at 2θ = 33.2°, 33.8°, 38.2°, 58.7°, and 69.1° (corresponding to the lattice planes of [100], [004], [101], [110], and [112], JCPDS card No. 00-036-1451) in the XRD pattern of MDP (Fig. 3b). ZnO crystal size was calculated using Scherrer equation (Eq. 3)62, in which K (= 0.9) is the Scherrer constant, λ (= 0.15406 nm) is the wavelength of the X-ray beam used, β is the full width at half maximum (FWHM) of the peak, and θ is the Bragg angle. The estimated ZnO crystal size is 7.0 nm, which is by far lower than the that of the particle size observed by SEM (vide supra). This indicates that the ZnO nanocrystals agglomerated on the surface of the date pits.

Figure 4
figure 4

SEM images of (a) DP, (b) fresh MDP, and (c) spent MDP after simultaneous adsorption of heavy metals (at bio-adsorption conditions: adsorption temperature 25 °C, initial concentration of 90 mg L−1, MDP dosing of 2 g L−1solution, adsorption time of 180 min, particle size of 200 µm, shaking speed of 300 rpm, and pH of 5).

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$$D=\frac{K\times \lambda }{\beta \times {\text{cos}}\theta }$$
(3)

After simultaneous bio-adsorption of heavy metals with an initial concentration of 90 mg L−1 at 25 °C and pH of 5 for 180 min, the morphological characteristics of the spent MDP (Fig. 4-) show distinct change to those of the MDP (Fig. 4-b), most likely related to deposition of the adsorbed heavy metal ions (including Cu2+, Ni2+, and Zn2+) on the spent MDP surface. ZnO is retained on the spent MDP according to the characteristic adsorption bands of ZnO in the FT-IR spectrum of the spent MDP (Fig. 2c) and diffraction peaks of ZnO in the XRD pattern of the spent MDP (Fig. 3c). The average ZnO particles on the spent MDP (Fig. 4c) is about 0.64 μm, comparable to that on the fresh MDP (vide supra). These characteristics indicate a good stability of MDP for the bio-adsorption of heavy metals.

Bio-adsorption of Cu 2+ , Ni 2+ , and Zn 2+ using MDP

Optimization of bio-adsorption parameters

Four bio-adsorption parameters, namely the pH of the solution, the particle size of the bio-adsorbent, MDP dosing, and the shaking speed, were screened to optimize the bio-adsorption performance. For this, the bio-adsorption experiments were performed for 180 min, assuming that the bio-adsorption reached equilibrium and a maximum removal of heavy metals from the solution was obtained.

Effect of pH of solution

The amount of removed heavy metals (Cu2+, Ni2+, and Zn2+) by the bio-adsorption on MDP increases with the increase of the pH value from 2 to 5, followed by a decrease when the pH value is further increased, e.g., to 8 (Fig. 5a). This is likely due to that the varied pH of the solution of the adsorption system changes the solubilities of heavy metal ions, the concentrations of the counter ions on the adsorbent, and the degree of ionization of the adsorbent63, affecting the electrostatic interactions between positively charged heavy metal ions and negatively charged functional groups of the bio-adsorbent (such as −OH, −COOH, –O–, and −CO–NH–), which affects the adsorption capacity. At a low pH value, MDP has a positive surface charge. However, the bio-adsorbent is surrounded by hydronium ions (H3O+) with a high concentration in solution, leading to a repulsive force22 that hinders the access of the heavy metal ions to the adsorption sites of the bio-adsorbent. With the increase of pH, the surface of bio-adsorbent becomes less positive charge64 and may become neutral at a pHpzc (point of zero charge) of ca. 6.1, which is estimated according to the experimental data shown in Fig. 5a. At a high pH value (e.g., > pHpzc), the heavy metal ions may react with the OH− ions with a high concentration in solution17, which is detrimental for the bio-adsorption performance.

Figure 5
figure 5

Effect of (a) pH, (b) particle size, and (c) shaking speed on the removal of heavy metals by the bio-adsorption on MDP. (Bio-adsorption conditions: adsorption temperature 25 °C, initial concentration of 50 mg L−1, MDP dosing of 2 g L−1solution, adsorption time of 180 min; (a) particle size of 200 µm and shaking speed of 300 rpm; (b) pH of 5 and shaking speed of 300 rpm; (c) pH of 5 and particle size of 200 µm.)

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Effect of particle size of MDP

Removal of the three heavy metals by the bio-adsorption on MDP increases with the decrease of the bio-adsorbent particle size from 1500 to 200 μm (Fig. 5b). This could be related to the more amount of accessible adsorption sites on MDP, which were inside the big bio-adsorbent particles but are exposed to the external surface of the bio-adsorbent with a small particle size after crushing and grinding38. A further decreased particle size (< 200 μm) was not investigated, considering the significant difficulty of the separation of the fine bio-adsorbent particles from the solution.

Effect of shaking speed

Bio-adsorption of the three heavy metals on MDP increases with the increase of the shaking speed from 150 to 300 rpm (Fig. 5c), likely related to the enhanced external mass transfer of the metal ions from the solution to the active adsorption sites. Nevertheless, a further increasing the shaking speed from 300 to 400 rpm leads to a slight drop in the bio-adsorption performance. This might be attributed to the desorption of those heavy metals with a weak adsorption on the bio-adsorbent65.

Effect of MDP dose

It can be seen from Fig. 6 that the bio-adsorption efficiencies for the removal of Cu2+, Zn2+, and Ni2+ increase proportionately with the increase of MDP dose from 0.4 to 2 g L−1solution. This could be rationalized by the increased number of adsorption sites on the bio-adsorbents with a higher concentration in the solution. However, a further increasing the MDP dose, e.g., from 2 to 6 g L−1solution, does not show visible increase in the removal of the three metals (Fig. 6). On one hand, increasing the adsorbent concentration could cause the aggregation/agglomeration of adsorbent particles, leading to the dead points between the adsorbent particles that are not accessible for the adsorption59. On the other hand, the interaction/collision of the MDP particles with a high concentration in solution could lead to the desorption of the metal ions that weakly adsorbed on the adsorbent surface25. As such, an optimal MDP dose of 2 g L−1solution is determined for the present study, under which conditions the removal percentages of 97.4%, 96.7%, and 90.2%, respectively, were obtained for the bio-adsorption of Cu2+, Ni2+, and Zn2+using MDP (Fig. 6).

Figure 6
figure 6

Effect of MDP dosing on the removal of heavy metals by the bio-adsorption on MDP. (Bio-adsorption conditions: adsorption temperature 25 °C, initial concentration of 90 mg L−1, MDP dosing of 0.4–6 g L−1solution, particle size of 200 µm, pH of 5, shaking speed of 300 rpm, and adsorption time of 60 min.)

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Effect of the adsorption time

In a batch bio-adsorption system, the removal percentage of heavy metals (Cu2+, Ni2+, and Zn2+) on MDP increases with the adsorption time (in the initial 10–60 min, after which plateau adsorption was reached, Fig. 7). This could be related to the fewer adsorption sites available on MDP and the decreased concentrations of heavy metals in solution with the adsorption time66. The latter is also evidenced by the higher efficiencies for removing heavy metals with higher concentrations shown in Fig. 7.

Figure 7
figure 7

Effect of the initial concentration on the removal of Cu2+ (a), Ni2+ (b), and Zn2+ (c) by the bio-adsorption on MDP. (Bio-adsorption conditions: adsorption temperature 25 °C, MDP dosing of 2 g L−1solution, particle size of 200 µm, pH of 5, shaking speed of 300 rpm, and adsorption time of 10–180 min.)

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Effect of initial concentrations of heavy metals

The effect of initial concentrations of the three heavy metals (Cu2+, Ni2+, and Zn2+, Fig. 7) indicates that the removal percentage increases with increased initial concentration. This is very likely related to the increased driving force at a higher concentration’s gradient67.

Effect of adsorption temperature

Increasing the bio-adsorption temperature (e.g., from 25 to 55 °C) has a negative effect on the removal of heavy metals (Fig. 8). One of the reasons could be due to the decreased viscosity of the aqueous solution at a higher temperature, resulting in an enhanced diffusion resistance to the bulk (external) and pore (internal) borders of the MDP particles41. Another reason might be related to the desorption of heavy metals from the adsorbent surface, which is favored at a higher temperature68.

Figure 8
figure 8

Effect of adsorption temperature on the removal of Cu2+ (a), Ni2+ (b), and Zn2+ (c) by the bio-adsorption on MDP. (Bio-adsorption conditions: initial concentration of 90 mg L−1, MDP dosing of 2 g L−1solution, particle size of 200 µm, pH of 5, shaking speed of 300 rpm, and adsorption time of 60 min.)

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Isotherm study

In literature, Langmuir69 and Freundlich70 adsorption models have been widely used for modelling the adsorption of heavy metals on various adsorbents40. In this study, the experimental data of the isothermal bio-adsorption equilibrium of the three heavy metals (Cu2+, Ni2+, and Zn2+) on MDP were also simulated using the above two models (Eqs. 4–671,72) and the non-linear fitting curves are shown in Fig. S3. The constants of the two models and the correlation coefficients were calculated using nonlinear regression and are shown in Table 1.

Table 1 Parameters of the two isotherm models for the bio-adsorption of heavy metals on MDP.
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$${q}_{e}=\frac{{q}_{m}\times {K}_{Lan}\times {C}_{e}}{1+{K}_{Lan}\times {C}_{e}}$$
(4)
$${q}_{e}={K}_{F}\times {C}_{e}^\frac{1}{n}$$
(5)
$${R}_{Lan}=\frac{1}{1+{K}_{Lan}\times {C}_{0}}$$
(6)

The fitting curves (Fig. S3) and parameters (R2 and SSR, Table 1) indicate that the bio-adsorption of Cu2+, Ni2+, and Zn2+ on MDP adjust better to the Freundlich isotherm model compared to the Langmuir isotherm model. This suggests73 that these heavy metals were adsorbed on the multi-layer and heterogeneous adsorption sites on the MDP surface. The maximum equilibrium adsorption amounts of Cu2+, Ni2+, and Zn2+ on a complete monolayer of MDP (qm) are 82.4, 71.9, and 66.3 mg g−1, respectively, according to the Langmuir isotherm model (Table 1). These qm values are considerably higher than those reported in literature for the bio-adsorption of heavy metals on the date pits (DP)-based bio-adsorbents (Table 2,38,74,75) and also those for the roasted DP (without modification of ZnO, Tables 2), likely attributed to the modified characteristics of the DP upon loading of ZnO and the followed thermal treatment (vide infra).

Table 2 The adsorption capacity of heavy metals on the date pits (DP)-based bio-adsorbent.
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Kinetic study

The experimental kinetic data (Sections "Effect of the adsorption time" and "Effect of initial concentrations of heavy metals") were modeled to study the bio-adsorption kinetics, considering the practical implementation of a bio-adsorption system and also aiming to obtain insights into the types and mechanisms of the bio-adsorption of heavy metals on MDP. Two kinetic models, namely pseudo-first-order (Eq. (7) 76) and pseudo-second-order (Eq. (8) 77) that were widely used to model the adsorption of inorganic and organic matters from aqueous solution in literature78,79, were applied in this study. The parameters and the regression coefficients were calculated using the nonlinear fitting algorithms in the MATLAB program and are shown in Table 3. The good R2 values for the pseudo-second-order model indicate that pseudo-second-order model fits the bio-adsorption kinetics better than pseudo-first-order model.

Table 3 Parameters of various kinetic models for the bio-adsorption of heavy metals on MDP.
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$${q}_{t}= {{\text{q}}}_{e} \times (1- {{\text{e}}}^{{k}_{1}t} )$$
(7)
$${q}_{t}={k}_{2}\times {{q}_{e}}^{2}\times \mathrm{ t}/(1+{k}_{2}\times {q}_{e}\times \mathrm{t })$$
(8)

In addition, the above experimental kinetic data were also analyzed using the intra-particle diffusion model (Eq. (9)80), which is often used to decide whether intra-particle diffusion is the rate-limiting step. It can be seen from Fig. 9 that the good linear fitting plots could be obtained, however, all the C values (in Eq. (9)) are greater than zero. These results suggest that the intra-particle diffusion mechanism is not dominant and very likely, there are also other mechanisms such as film, surface, or pore diffusion regulating the adsorption kinetics81.

Figure 9
figure 9

Intraparticle diffusion on the removal of Cu2+ (a), Ni2+ (b), and Zn2+ (c) by the bio-adsorption on MDP. (Bio-adsorption conditions: adsorption temperature 25 °C, MDP dosing of 2 g L−1solution, particle size of 200 µm, pH of 5, shaking speed of 300 rpm, and adsorption time of 10–180 min.)

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$${q}_{e}={k}_{ip}\times {t}^{0.5}+C$$
(9)

Thermodynamic study

The experimental data of the effect of adsorption temperature (Section "Effect of adsorption temperature") were used to study the thermodynamics of the bio-adsorption of heavy metals on MDP. The thermodynamic parameters, namely Gibbs free energy change (ΔG , kJ mol−1), surface adsorption of entropy change (ΔS, kJ mol−1 K−1), and enthalpy change (ΔH, kJ mol−1) were calculated using Eqs. (10) 40 and (11)82 and the results are shown in Table 4. The three thermodynamic parameters (ΔG, ΔH, and ΔS) for the bio-adsorption at four temperatures (25–55 °C) are all negative values (Table 4), indicating that the adsorption of Cu2+, Ni2+, and Zn2+ on MDP is spontaneous (negative value of ΔG)83, exothermic (negative value of ΔH)84, and the randomness between the solid/liquid interfaces at the liquid–solid interface is decreased (negative value of ΔS)85.

Table 4 Thermodynamic analysis for the bio-adsorption of heavy metals on MDP.
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$$\Delta G=-RT\times {\text{ln}}({k}_{d})$$
(10)
$${\text{ln}}\left({k}_{d}\right)=\frac{-\Delta H}{RT}+\frac{\Delta S}{R}$$
(11)

Bio-adsorption mechanisms

The adsorption process forms a layer of adsorbate (metal ions) on the surface of adsorbents. Bio-adsorbents, often have a porous structure with various cavities and surface sites (Fig. 4), on which the metal ions can be bonded. The improved pores and cavities on the MDP lead to an increased surface area for adsorption, promoting interaction between the metal ions with the bio-adsorbent86. Adsorption of the pollutant onto the adsorbent often includes three steps, namely transportation of the pollutant from aqueous solution to the adsorbent surface, adsorption onto the solid surface, and transport within the adsorbent particle. Kinetic study (Section "Kinetic study") indicates that both the intra-particle diffusion mechanism and the intra-particle diffusion mechanism are not dominant. The heavy metals have a vigorous affinity to the functional groups on the surfaces of the adsorbent87. On the surface of agricultural waste, functional groups such as −OH, −COOH, –O–, and −CO–NH–, react with heavy metal ions for their removal from aqueous solution88. Therefore, it is very likely that the electrostatic attractions between positively charged metal ions and negatively charged functional groups of bio-adsorbents promote the adsorption capacity89. Dispersion of ZnO on the DP surface improves the surface for electro-interaction. ZnO particles are generally found to have a positive value of the zeta potential90, which however, can be turned into a negative value because of the appearance of negatively charged centers, e.g., OH − groups on ZnO surface91. An illustration of the proposed bio-adsorption mechanisms is shown in Fig. 10.

Figure 10
figure 10

An illustration of the proposed Bio-adsorption mechanisms.

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Reuse of the spent MDP

The reusability of the MDP bio-adsorbent was evaluated by recycling the regenerated MDP for additional 4 times. The used MDP, which was saturated with heavy metals during the bio-adsorption, was regenerated by following a desorption protocol described in Section "Bio-adsorption of heavy metals from aqueous solution and real wastewater". It can be seen from Fig. 11 that the removal percentage is decreased after each recycling. This indicates the irreversible deactivation of the MDP after the adsorption-regeneration cycle, of which the factors are under investigation and will be reported in due course.

Figure 11
figure 11

The removal percentages of Cu2+ (a), Ni2+ (b), and Zn2+ (c) by the bio-adsorption on the fresh and regenerated MDP. (Bio-adsorption conditions: initial concentration of 90 mg L−1, MDP dosing of 2 g L−1solution, particle size of 200 µm, pH of 5, shaking speed of 300 rpm, and adsorption time of 60 min.)

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Bio-adsorption of a real wastewater using MDP

A real wastewater which was collected from a local electroplating company in Baghdad was applied to evaluate the bio-adsorption performance using the optimized adsorption conditions above, namely MDP particle size of 200 μm, MDP dosing of 2 g L−1solution, the shaking speed of 300 rpm, adsorption time of 1 h, and the adsorption temperature of 25 °C. The initial and final concentrations of the heavy metals before and after bio-adsorption process, the removal percentage, and the permissible limits are shown in Table 5. It can be seen that the removal efficiencies are higher than 90%, however, the concentration of the heavy metals in the treated water are still higher than the permissible limits5 for using as the drinking water. This is likely due to the presence of the other heavy metals (such as cadmium and lead, out of this study), which leads to an adsorption competition on the MDF surface. As such, the treated water after the bio-adsorption could only be used for non-drinking water, e.g., agriculture drain.

Table 5 Bio-adsorption of real wastewater using MDP.
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Conclusion

The present work has shown the significantly improved bio-adsorption efficiency of the date pits (DP)-based bio-adsorbent after modification by loading of ZnO and the followed thermal treatment for the removal of heavy metals (Cu2+, Ni2+, and Zn2+) in the solution. This is rationalized by the changes in the characteristics (morphology, crystallinity, surface functional groups, and the specific surface area) of the ZnO-modified DP (MDP), compared to those of the raw date pits. At an optimized bio-adsorption condition (e.g., pH of 5, the particle size of 200 μm, the shaking speed of 300 rpm, initial concentration of 90 mg L−1, MDP dosing of 2 g L−1solution, adsorption time of 60 min, and the adsorption temperature of 25 °C), a high bio-adsorption efficiency of MDP for the removal of Cu2+ (97.4%), Ni2+ (96.7%), and Zn2+ (90.2%) in the solution was obtained. Irreversible deactivation was observed during the 5 cycles of adsorption-regeneration. Bio-adsorption of a real wastewater showed that the treated water can only be used as for non-drinking water. The isothermal analysis by the Langmuir isotherm model and the Freundlich isotherm model showed that both heterogeneous and homogeneous adsorption sites on MDP surface were involved for the bio-adsorption of heavy metals. According to the Langmuir isotherm model, the maximum bio-adsorption amount of the three heavy metals on MDP follows the order of Cu2+ (82.4 mg g−1) > Ni2+ (71.9 mg g−1) > Zn2+ (66.3 mg g−1). The kinetic study using the pseudo-first-order model, the pseudo-second-order model, and the intra-particle diffusion model suggested that the nature of kinetic adsorption is chemical and the intra-particle diffusion mechanism is not dominant, indicating the multiple bio-adsorption mechanisms. The thermodynamic parameters (ΔG, ΔH, and ΔS) pointed out that the bio-adsorption of heavy metals on MDP was spontaneous and exothermic, and the randomness between the solid/liquid interfaces at the liquid–solid interface is decreased. The modification protocol developed in this work is facile and could be a universal treatment of the bio-adsorbents for an improved bio-adsorption performance in terms of the purification of polluted water containing heavy metals.