Co-Al layered double oxide activated carbon composite for eliminating lead ions from water

In the current study, preparation of cobalt-aluminum layered double oxide doped activated carbon (Co-Al LDO/AC) was achieved by the co-precipitation technique and utilized for the remediation of lead (Pb2+) from water. Various methods were employed to examine the properties of the composite material, including BET, XRD, FTIR, SEM, and EDS analysis. The material characterization outcomes indicated that the LDO structure was successfully incorporated into the AC matrices with a surface area of 189.4 m2/g. The influence of adsorption parameters including Co-Al LDO/AC dosage, period of contact, initial Pb2+ loading, and initial solution pH were investigated. Moreover, the isotherm and kinetic models were investigated to provide a deeper understanding of the elimination mechanism of Pb2+ ions. The adsorption results illustrated that pH has a substantial influence on Pb2+ removal with a highest removal effectiveness at pH = 6 and a fast adsorption rate within 7 h. The kinetic data were well aligned with the pseudo-second-order model while the isotherm data obeyed the Sips model (R2>0.966). The highest adsorption uptake, estimated by the Sips model was 25.09 mg/g. Considering the modeling and characterization of the spent Co-Al LDO/AC, a chemical interaction process was involved in the elimination process and mainly controlled by ion exchange, electrostatic interactions, and surface complexation mechanisms. Accordingly, the Co-Al LDO/AC could have great potential as a promising hybrid for the purification of toxic Pb2+ ions from contaminated water streams.


Introduction:
In the water environment, various contaminants have been detected and reported to cause pollution and serious human diseases [1].Heavy metals, organic wastes, radioactive wastes, and pharmaceuticals are among the major pollutants that gained considerable attention due to their severe impact on the human body and the ecological system [2].Lately, heavy metals attracted numerous attention due to their toxicity, nonbiodegradability, and tendency to persist in the environment [3].Although some minerals are beneficial for human, animal, and plant health such as iron (Fe) and zinc (Zn); other elements such as lead (Pb), chromium (Cr), cadmium (Cd), and mercury (Hg), are extremely harmful in trace concentrations can lead to determinantal impacts [4].
Lead exists mainly in an ionic form of Pb 2+ in an aqueous solution which was found to be among the major heavy metals that lead to high intoxication in humans and other organisms.Pb 2+ is a well-known neurotoxin that damages the central nervous system, affects the nerves' development in both aquatic creatures and humans, and causes renal and hepatic disorders [5].Based on the World Health Organization (WHO) report, 143,000 deaths worldwide are attributed to lead exposure, where children are among the most affected categories [6].Although Pb 2+ can exist naturally in the environment, human activities played a major role in increasing its concentration.Leaded-petroleum spills, leaded pipes for water distribution, and wastewater discharge from lead-based industries such as paints, dyes, electroplating, and battery manufacturing are the major sources of Pb 2+ exposure in water [7], [8].Therefore, according to the WHO standards, potable water must not exceed 10 μg/L of Pb 2+ [9].
To remediate such contamination, several treatment technologies have been developed including electro-coagulation [10], ion replacement [11], chemical precipitation [12], and adsorption [13] techniques.However, adsorption is viewed to be a promising treatment method for Pb 2+ ions removal due to its simplicity, low cost, efficiency in removing trace concentrations, facile regeneration of its adsorbent material, and ability to recover the pollutant [14].Numerous adsorbents have been introduced to eliminate Pb 2+ from contaminated water including natural clay [15], activated carbon (AC) [16], fly ash [17], layered double hydroxide/oxide (LDH/LDO) [18], zeolite [19], biochar [20], and sugarcane bagasse [21].ACs are recently attracting more research for heavy metal removal because of their advantageous features such as large SA, renewability, extraordinary adsorption efficiency, synthesis using environmentally friendly materials, chemical stability, and resistance to strong acids and bases [22].However, attention has been directed to improve the AC and enhance its adsorption capacity and its removal rate towards Pb 2+ ions by modifying its surface.Examples are iron oxide-doped AC [23], polypyrrole-based AC [24], and nanoscale zero-valent iron/AC composite [25].Despite the successful modifications presented by these studies, there has been a growing emphasis on conducting further research to investigate the viability of using LDH/LDObased AC composite for Pb 2+ removal.
Layered double hydroxides/oxides (LDH/LDO) were proposed as unique materials for the elimination of various pollutants including heavy metals, mainly accredited to their special layered structure, high ion exchange capacity, and chemical stability [26].Zhao et.al. [27] investigated the use of Mg-Al LDH and illustrated a Pb 2+ adsorption uptake of 75.18 mg/g.While Rahmanian et.al. [28] revealed the highest adsorption uptake of 94.3 mg/g using citric intercalated Zn-Fe LDH.Whereas Liang et.al., [29] showed a remarkable adsorption uptake of 170.09 mg/g using diethylenetriaminepentaacetic acid (DTPA) intercalated Mg-Al LDH.Due to the high capacity of the LDHs for Pb 2+ adsorption, several research studies were undertaken to evaluate the potential doping of LDH/LDOs onto the surface of different supports for Pb 2+ removal such as carbon nanofiber loaded with Co-Al LDH and hematite [30], Mg-Al LDH modified palygorskite clay [31], and Mg-Fe LDH/biochar composite [32].AC as a cheap material, has plenty of functional groups with high surface area (SA) that could be an effective carrier of LDO/LDH structure and contribute to better distribution of these particles and consequently might provide better removal efficiency toward contaminants such as Pb 2+ ions [33].To the best of the authors' knowledge, the remediation of Pb 2+ ions by Co-Al LDO-doped AC has not been explored in the literature yet.
Accordingly, this paper aims to investigate the use of Co-Al LDO-doped AC for the removal of Pb 2+ ions.The main objectives are (I) synthesizing Co-Al LDO/AC following the co-precipitation method, (II) characterizing the LDO/AC composite for its surface and morphological structure, and (III) investigating the impact of LDO/AC dosage, period of contact, and initial solution pH (pHi) on Pb 2+ adsorption in addition to the kinetic and isotherm models.

Synthesis of Co-Al LDO/AC composite
Co-Al LDO/AC was fabricated by the co-precipitation approach.In brief, 43.65 g of Co(NO3)2• 6H2O and 18.76 g of Al(NO3)3.9H2O (Co/Al ratio = 3) were dissolved in 300 mL DI water and agitated for 30 min to obtain a homogenous metal solution.Next, 5 g of raw AC was added to the metal solution and stirred for 1 h.Adjustment of the solution at pH 11 was conducted using 1 M NaOH solution and kept for 2 h under agitation at 70 ℃.Afterward, the heat was turned off and the Co-Al/AC solution was stirred overnight at room temperature (RT).Separation and DI washing of the precipitant were then carried out using a centrifuging machine (Hitachi CR22N) where the resultant composite material was oven-dried at 100 ℃ for a day.The dried Co-Al LDH/AC was then subjected to calcination at 500 ℃ for 4 h and using a rate of heating of 5℃/min.The composite synthesized was labeled as Co-Al LDO/AC.

Characterization of Co-Al LDO/AC composite
For the raw AC and Co-Al LDO/AC composite, the average pore size, pore size distribution, and BET-SA were determined following Barrett-Joyner-Halenda (BJH) and N2 adsorption-desorption isotherm methods using NOVAtouch LX2 analyzer where the samples were degassed at 300 ℃ for 4 h.Scanning Electron Microscopy (EDS) (Thermo scientific Apreo C) linked with Energy dispersed X-ray spectroscopy (EDS) were utilized to assess the morphology and the chemical composition of the raw and LDO/AC composite.Fourier-Transform Infrared (FT-IR) Spectroscopy (JASCO 6300) was further utilized to investigate the chemical functional groups of the composite surface (KBr pellet method).The determination of the crystalline structure of the adsorbents was achieved through X-ray diffraction (XRD) analysis (D8 Advance instrument, Burker, Germany).Finally, the PZC was measured following the pH drift method for the raw and LDO/AC composite.The PZC was determined by dispersing 30 mg of the raw AC and Co-Al LDO/AC in 30 mL of 0.1 M NaCl solution.The PZC was obtained by measuring the pH before and after agitation of the materials for 72 h at 150 rpm.

Batch adsorption experiments
Batch adsorption tests were conducted to determine the adsorption uptake and the elimination efficiency of Pb 2+ using Co-Al LDH/AC composite.Table 1 illustrates the conditions set for each adsorption study where all the experiments were conducted using conical flasks and test tubes and at fixed Co-Al LDO/AC dosage (2 g/L).Pb 2+ adsorption capacity and removal efficiency were determined using eq. 1 and 2.
where q is the Pb 2+ adsorption capacity of Co-Al LDO/AC (mg/g), ‫ܥ‬ is the original Pb 2+ loading and ‫ܥ‬ is the exit loading (mg/L), ܵ is the Co-Al LDO/AC dosage (g/L), and ܴ% is the removal efficiency of Pb 2+ .The residual Pb 2+ concentration was determined by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) (Thermo Scientific iCAP 7000 Series) where calibration standards were prepared between 5 to 50 mg/L of Pb 2+ .]i = 50 mg/L, So = 2 g/L, t = 24 h, RT.

Kinetic and isotherm modeling
To determine the rate-limiting step of Pb 2+ removal and the adsorption uptake of Co-Al LDO/AC, the adsorption kinetics were examined.This was conducted by analyzing the experimental data using the linearized intra-particle diffusion (IPD), Elovich, pseudo-first-order (PFO), and pseudo-second-order (PSO) models (Table 2).While to evaluate the adsorption isotherm, the experimental data were optimized using nonlinearized Langmuir, Redlich-Peterson (R-P), Freundlich, and Dubinin-Radushkevich (D-R) models (Table 3).The presented isotherm data were compared by computing the determination factor (R 2 ) and the sum of square errors (SSE) using an Excel solver.Calculation of SEE was carried out using eq. 3 [14].
Table 2: Adsorption kinetic models using linearized equations.

Kinetic models
Linearized equation x vs. y Table 3: Non-linearized equations of adsorption isotherm models.

Composite characterization 3.1.1. Structural properties
The SA of raw AC and Co-Al LDO/AC composite was analyzed and estimated using the N2 adsorptiondesorption method.According to the categorization of the IUPAC, and as depicted in Figure 1(a), the raw AC exhibited type IV isotherm with an H4 hysteresis loop usually obtained for mesoporous materials [34].While the Co-Al LDO/AC composite, shown in Figure 1(b), demonstrates type II adsorption isotherm with an H3 hysteresis loop representing a microporous structure material [33].While in the BET-SA result, a reduction in the SA was noticed where the raw AC and Co-Al LDO/AC exhibited 829.6 and 189.4 m 2 /g, respectively.This implies that the agglomeration of Co and Al led to AC pore blocking and the combustion of the carbon during the calcination process may have contributed to the reduction in the BET-SA.[33].

Surface functional groups
FT-IR assessment was attained to explore the functional groups of the raw and Co-Al LDO/AC composite's surface as depicted in Figure 3(a).Based on the IR spectrum, peaks were noticed at 3446 cm −1 and 1636 cm −1 which related to the stretching vibrations of the -OH functional group [35].These peaks were indicative of the presence of the absorbed water on the AC's surface.The peak at 2922 cm −1 was ascribed to the C-H stretching vibration and the peak at 2359 cm −1 was ascribed to the stretching vibration of the atmospheric CO2 [36].Moreover, peaks observed after 1200 cm −1 were attributed to the C=C vibrations and C-O stretching [14].For the Co-Al LDO/AC composite, the sharpness of the 3446 cm −1 band was dramatically decreased when the Co and Al were incorporated, and the composite was calcined at 500 ℃.In addition, a characteristic peak at 950 cm −1 was observed due to the C-O-C group.While distinct peaks were observed too at 673 cm −1 and 568 cm −1 which were recognized to the vibrational modes of M-O-H (Co-O-H and Al-O-H) and M-O-M (Co-O-Al) lattice vibrations, respectively [30].This was an indication of the successful doping of Co-Al LDH onto the AC matrix.

PZC Determination
The PZC for the raw AC and Co-Al LDO/AC composite were determined via the pH drift method and the results were illustrated in Figure 3(c).The raw AC demonstrated a PZC value at pH 7.1, After loading Co-Al LDO onto the raw AC, the PZC slightly increased to 7.5.These outcomes imply that the metal oxides have increased the positivity of the AC.Generally, the material holds a positive charge at a pH lower than the PZC and a negative charge at a pH higher than the PZC.Although, increasing the positivity of the material might not be favorable for the removal of positively charged contaminants such as the Pb 2+ ions, the affinity of Co and Al oxides toward Pb 2+ ions is expected to significantly increase the removal efficiency as will be investigated in the coming sections.

Batch adsorption analyses 3.2.1. Adsorbent dosage impact
The influence of Co-Al LDO/AC dosage on Pb 2+ ions adsorption efficiency was examined, and outcomes were displayed in Figure 4(a).It was clearly noted that by elevating the composite dosage from 0.7 to 3 g/L, the Pb 2+ elimination efficiency was significantly enhanced from 61.4% to 97.3%.A higher dosage of the adsorbent implies more vacant active adsorption sites, therefore; more Pb 2+ ions are exposed to the Co-Al LDO/AC surface leading to higher removal efficiency.However, since there was no dramatic increase in the Pb 2+ removal efficiency from dosage 2 g/L (84.9%) to 3 g/L (97.3%), 2 g/L Co-Al LDO/AC dosage was recommended for the remaining experiments.This trend was in agreement with a recently reported study that investigated the use of CoAl-LDH@biochar composite for various heavy metals removal [39].

Contact time effect
The influence of contact time on the elimination of Pb 2+ ions by Co-Al LDO/AC was demonstrated in Figure 4(b).The outcomes showed that more than 40 % of the Pb 2+ were removed after 40 min of contact.This removal is mainly attributed to the external adsorption of Pb 2+ ions on the surface of the adsorbent.Next, a slower rate of removal was noticed where the removal efficiency increased to 72% after 440 min.This is mainly ascribed to the diffusion of Pb2+ into the internal pores of the adsorbent.After that, the adsorption process started to reach equilibrium, and no more changes in the removal efficiency were obtained after 7 h of contact.These outcomes suggest that 7 h is the saturation time of our adsorption process.

Impact of pHi
Since the pHi has a substantial influence on the uptake capacity of a pollutant, the influence of this parameter was evaluated at a pH range of 2 to 6.As depicted in Figure 4(c), it was clearly shown that by improving the pHi from 2.2 to 5.9, the Pb 2+ elimination efficiency has dramatically increased from 9.1% to 99.2% with an upsurge in the adsorption uptake from 2.15 mg/g to 23.52 mg/g.After pH 6, some studies reported an improvement in the elimination efficiency of Pb 2+ ions which was ascribed to the formation of Pb 2+ hydroxide precipitation [23], [40].At pH 2.2, the adsorbent holds a highly positive charge explaining the low adsorption capacity at this pH.However, increasing the pH to 5.9 suggests less electrostatic interactions and hence improved adsorption capacity.Higher pH values were not considered, a white precipitate was noticed during raising the pH more than 7 before adding the adsorbent, this is due to the precipitation of Pb 2+ hydroxides.As precipitation is occurring, the adsorption process cannot be considered as a removal mechanism and therefore pH higher than 7 was not investigated in our study.

Adsorption isotherm
Langmuir, Freundlich, Sips, and Redlich-Peterson (R-P) nonlinearized models were utilized to examine the equilibrium experimental data of Pb 2+ adsorption.The fitted data were depicted in Figure 4(d) and their modeling factors and numerical coefficients were presented in Table 4.The Sips model was the best model describing the experimental data with the greatest R 2 (> 0.966) and the lowest SSE (0.076), where the highest obtained adsorption uptake was 25.09 mg/g, which is a considerable capacity compared with other materials (see Table 5).Freundlich (R 2 > 0.962, SSE = 0.084) and R-P models (R 2 > 0.963, SSE = 0.083) were also aligned with the experimental data whereas Langmuir has the lowest R 2 (> 0.902) and highest SSE (0.218).This suggests that the binding sites of the composite surface were more likely to have heterogenous and unequal energy distribution than homogenous [30], [41].adsorption isotherm and the fitted models.So = 2 g/L, pHi =6.1-6.5, t = 24 h, and at RT.

Adsorption kinetic:
The determination of Pb 2+ adsorption kinetic has a major influence on assessing the adsorption rate and the mechanism of Co-Al LDO/AC in Pb 2+ uptake.Hence, the experimental data of 7 h adsorption were fitted into four linearized kinetic models, i.e., PFO, PSO, Elovich, and IPD model, as depicted in Figure 5, and their factors were presented in Table 6.Based on the obtained results, the experimental data exhibited strong conformity with PFO and PSO models with an R 2 of 0.986 and 0.987, respectively.Nonetheless, the theoretical adsorption capacity of the PFO model (13.8 mg/g) was not compatible with the experimentally obtained equilibrium adsorption capacity (19.8 mg/g).While the PSO model presented a better fitting with a high determination coefficient (R 2 > 0.987) and a theoretical adsorption uptake of 18.1 mg/g, indicating that the experimental data was best presented by the PSO model.Therefore, it can be anticipated that both chemisorption and physisorption processes between Pb 2+ and Co-Al LDO/AC play a role in the adsorption of Pb 2+ ions.Elovich and inter-particle diffusion models were examined too where both models complied with the experimental data, with an R 2 > 0.928 and 0.985, respectively.This could suggest the presence of heterogeneous surfaces on Co-Al LDO/AC with a chemical adsorption process and internal diffusion mechanism dominating.0.985

Adsorption mechanism
In order to examine the mechanism of Pb 2+ uptake onto Co-Al LDO/AC composite, the surface functional groups of the Co-Al LDO/AC were examined after Pb 2+ adsorption, and the FTIR spectra were elucidated in Figure 3(a).As depicted in the figure, after the Pb 2+ elimination process, the broad band at the 3446 cm -1 region was noticed which corresponds to the stretching vibration of OH groups as well as interlayer and surface H2O molecules.The stretching frequency of this was more intense after Pb 2+ adsorption indicating the presence of hydroxyl groups that were intercalated into the Co-Al LDO/AC interlayers [45].While the intensity of the characteristic peaks at 673 cm −1 and 568 cm −1 was noticeably intensified which may be ascribed to the vibrational modes of M-O-H (Pb-O-H) and M-O-M (Al-O-Pb) [32].This suggests that successful adsorption of the Pb 2+ ions might occur through either physical adsorption into the AC porous material or an ion exchange mechanism involving Pb 2+ and H + [18].Additionally, it is suggested that there is a chemical binding between Pb 2+ ions and some of the OH -groups on the LDO surface, resulting in an inner-sphere complexes formation or through electrostatic interactions [27].These processes can be expressed by eq. 4 and eq. 5 [46].

Conclusion
In the current study, the synthesis of Co-Al LDO/AC composite was achieved following the co-precipitation method for the elimination of Pb 2+ from an aqueous solution.The surface and structural characteristics of the hybrid were thoroughly investigated and the results revealed a successful incorporation of Co and Al onto the AC matrix with a SA of 189.4 m 2 /g.The batch adsorption experiments demonstrated that by comparing Co-Al LDO/AC with the raw AC, Pb 2+ removal efficiency was enhanced by approximately 50%.
The optimum dosage of Co-Al LDO/AC was 2 g/L.The outcomes further revealed that equilibrium was achieved within 7 h and an optimum pH of 5.9.The kinetic experimental data were well-fitted with the PSO model (R 2 > 0.987) while the isotherm data were best explained by the Sips model (R 2 > 0.966).The maximum Sips adsorption capacity was 25.09 mg/g.These modeling and characterization outcomes proposed that the removal of Pb 2+ could be controlled by ion exchange, electrostatic interaction, and surface complexationadsorption mechanisms.Our outcomes suggested that Co-Al LDO/AC could be a potential material for the remediation of Pb 2+ ions from water.

Figure 1 :
Figure 1: N2 adsorption-desorption isotherm data of (a) raw AC and (b) Co-Al LDO/AC composite.The structural morphology and elemental mapping of the raw AC and Co-Al LDO/AC composite were investigated by SEM-EDS and their related monographs were illustrated in Figure 2. As seen in Figure 2(a), the raw AC exhibited a porous material and the EDS images (Figure 2(c)) confirmed that the carbon was the main component of the adsorbent (97.4 Wt%) with a low percentage of Fe (0.7 Wt.%), Al (0.5 Wt.%), and Si (0.7 Wt.%).Figure 2(b) displayed that the material maintained its porosity and roughness with irregular shapes after loading the Co-Al LDO.Moreover, the density of shiny particulates looks more in the modified material confirming the successful doping of the Co-Al oxides.The AC pores look covered by the metal oxides suggesting a reduction in the SA confirming the BET analysis.The EDS mapping, demonstrated in Figure 2(d), clearly illustrated the homogeneous distribution of the doped elements with

Figure 2 (
b) displayed that the material maintained its porosity and roughness with irregular shapes after loading the Co-Al LDO.Moreover, the density of shiny particulates looks more in the modified material confirming the successful doping of the Co-Al oxides.The AC pores look covered by the metal oxides suggesting a reduction in the SA confirming the BET analysis.The EDS mapping, demonstrated in Figure2(d), clearly illustrated the homogeneous distribution of the doped elements with significant improvement in oxygen element onto the surface of the AC implying the successful doping of the Co and Al elements onto the AC matrix.

Table 1 :
Adsorption kinetic models using linearized equations.

Table 4 :
Determined factors of isotherm study for Pb 2+ adsorption on Co-Al LDO/AC composite.

Table 6 :
Parameters and determination coefficient of the four kinetic models for Pb 2+ adsorption onto Co-Al LDO/AC.