Aqueous Lead Removal under Optimized Conditions Using Phosphoric Acid Activated Coconut Coir Activated Carbon

The global burden of heavy metal environmental pollution remains one of the most challenging issues to be addressed urgently. Lead (Pb) has been well recognized as a toxic environmental pollutant. The main objective of this study was to examine the adsorption efficiency of phosphoric activated coconut coir activated carbon for lead (II) removal from an aqueous solution. Synthesized activated carbon was characterized before and after the adsorption of Pb(II) by powder X-Ray diffraction, Fourier transforms infrared spectroscopy and scanning electron microscopy coupled with energy dispersive X-Ray. Furthermore, the removal efficiency of Pb(II) of synthesized activated carbon was tested with different concentrations of Pb(II) solutions, pH levels, adsorbent dosages, and contact time. Atomic absorption spectroscopy was used to analyze the Pb(II) concentrations in water samples. The maximum Pb(II) removal percentage of 100% was obtained with 50 mL of 5 mg/L Pd(II) ion solution and 0.20 g of the synthesized activated carbon. Adsorption data were well fitted with the Freundlich adsorption isotherm model, and adsorption kinetics were fitted with the pseudo-second-order kinetic model with R of 0.99. These results conclude that the synthesized activated carbon can be used as a potential sorbent for the removal of lead from wastewaters.


Introduction
Industrialization and the increase in population have boosted the growth of different industries which lead to the discharge of pollutants into the water bodies.
As a result, more and more heavy metals are being continually discharged into the environment. Therefore there is a necessity to take more concern of these heavy metals as they have significant effects on human health [1]. The most concerning toxic metals are Cd, Zn, Pb, and Ni. They enter the water bodies through wastewater from metal plating industries, mining, pigments, dezincification, and other industrial wastes.
Currently, lead pollution is becoming a global burden due to its spreading over the earth and groundwater. Lead is a heavy, soft, malleable, bluish-gray metal present as the form of lead(II) ions in an aqueous medium [2]. Also, lead is one of the most toxic heavy metals existing in wastewater, which comes from both natural sources and industrial effluents [3]. Exposure to high levels of lead in the environment may cause long-term health risks to humans and ecosystems by acute and chronic effects. According to the World Health Organization (WHO), the maximum permissible limit (MPL) of lead in drinking water is 0.05 mg/L and in wastewater is 0.05 mg/L as given by Environmental Protection Agency (EPA) [4].
Hence, there is an urgent need for efficient and cost-effective treatment of industrial wastewater to protect human, aquatic life, and land from lead toxicity.
Removal of lead from wastewater can be accomplished through a range of chemical and physical processes. The traditional methods used for wastewater treatment are precipitation, adsorption, and coagulation. Although these processes are costly to hold, they are not efficient.
Over the last few decades, adsorption is used as one of the most feasible methods and has gained much attraction in industrial-scale purification, separation, and recovery process [5]. Recent research approaches are focused on the use of novel adsorbents for the removal of heavy metal ions from an aqueous solution that is low in cost and efficient compared to conventional adsorbents [6].
Adsorption is a widely used method for the efficient removal of lead ions [2] due to its high intensified metal binding capacity to prevent lead pollution and it can protect public health by improving the quality of the treated effluent [7].
However, the use of activated carbon as an adsorbent has been diminished due to its less adsorption capacity [8].
Activated carbon (AC) has been extensively utilized as an adsorbent [9] for environmental applications such as the removal of organic and inorganic pollutants from air and water streams over the past few decades. Most common applications of activated carbon include air and water purification, water filtration, groundwater remediation, adsorption of poisons, and drug overdoses [10]. Activated carbon is a carbonaceous material with a characteristic high surface area with microporosity consists of 87% to 97% carbon with other trace elements [10]. Most of the low-cost materials with high carbon content and low inorganic D. D. T. T. D. Senarathna et al. American Journal of Analytical Chemistry elements can be used as a raw material for the mass production of activated carbon [5]. The demand for activated carbon will benefit from the continuing intensification of the global environmental movement with industrialization.
There are several methods available for the production of activated carbon.
However, for specific industrial applications, through chemical activation, a solid adsorbent with wide pore size distribution can be preferably obtained. In recent years, agricultural wastes are notably utilized as the raw material for the preparation of activated carbon and, also previous studies have reported the removal of heavy metal ions with many agricultural byproducts [11]. This bio-char application is one of the most attractive alternatives for environmental remediation because of the abundance of cheap biomass that can be used for biochar production. Furthermore, the cost of biochar is six times lower than the cost of activated carbon because of the lower energy requirement of biochar production.
It can be used without further chemical or physical activation [12].
Coconut coir is one of the low-cost agricultural wastes with a higher abundance in Sri Lanka that can act as an ideal carbon precursor [13]. Coconut coir is a soft biomass residue that is separated during the processing of coconut (B Ash) and it is rich in lignin (16% -45%), hemicellulose (24% -47%), and pectin (2%) content [14]. The physical and chemical properties of activated carbon depend mainly on the activation methods used [11]. Commonly there are two different types of processes for the preparation of activated carbon. They are physical activation and chemical activation, and they increase the adsorption capacity by combining with other adsorbents, modification with acids, grafting with various functional groups, and combining mineral and metal adsorbents with some polymers [2].
Chemical activation is considered the best activation method over physical activation due to its ability to develop a better porous structure. Furthermore, the chemicals added for activation can be easily recovered for reuse. The activation of carbon by phosphoric acid is largely used in industrial-scale applications to impregnate lignocellulose materials, such as coir and wood [15]. hosphoric acid increases the yield by making important changes in the pyrolytic decomposition of the lignocellulosic materials and promotes depolymerization, dehydration, and redistribution of constituents [8]. Nowadays, phosphoric acid has become more popular due to the improvements introduced in the process of acid recovery.
As mentioned earlier, there is understandably a great effort to find low-cost material to produce activated carbon. In this work, we have described our efforts to remove lead ions from contaminated aqueous solution by using the activated carbon produced from coconut coir along with chemical activation by phosphoric acid. The ultimate goal of this work is to develop and utilize a microporous activated carbon from coconut coir for the efficient removal of lead ions from the aqueous phase. This study provides an ideal platform to convert coconut coir, which is highly abundant in Sri Lanka, into activated carbon by phos-American Journal of Analytical Chemistry phoric acid that can be commercialized for the removal of lead (II) ion contaminants from the aqueous phase.

Materials
All the chemicals were of analytical grade. Pd(NO 3 ) (98% assay) and H 3 PO 4 (85% assay) were purchased from Sigma-Aldrich, and were used without further purification. Water used was distilled.

Preparation of Activated Carbon (ACC)
Coconut coir was washed using hot distilled water and oven dried at 110˚C for 12 hours. Then, cleaned coconut coir was sieved through a 1 mm mesh. About 25.0 g of sieved coir was treated with 500 ml of 50% (V/V) phosphoric acid and boiled for few hours. Next, it was oven dried at 100˚C for 48 hours followed by pyrolysis at 450˚C for 1 hour to obtain phosphoric activated ACC. The sample was allowed to become room temperature and washed thoroughly with distilled water followed by low concentrated KOH washing to remove any traces of remaining phosphoric acid until the pH of water reaches pH of 6.0 -7.0 range.
Then, ACC was dried for 4 h at 90˚C.

Characterization of ACC
Morphological analysis of the synthesized ACC was done using Hitachi SU6600 Scanning Electron Microscope (SEM) depositing gold on prepared ACC to an optimum 5 nm thick layer. The crystallographic analysis of the ACC was performed using Siemens D5000 powder X-ray Diffractometer (XRD). Fourier transform infrared (FTIR) spectroscopy for identifying chemical composition of the ACC was done using Shinmadzu IR Prestige-21 spectrometer using the KBr pellet method. The sample was mixed with dry KBr solid in 1:40 mass ratio for pellet preparation.

Methylene Blue (MB) Adsorption Test
First, (0.500 ± 0.001 g) of synthesized ACC was added to a 250 ml beaker. Next, about 50 ml of methylene blue solution (1000 ppm) was added into it followed by stirring with a mechanical stirrer for 1 hour at a speed of 500 rpm. Then the mixture was filtered through a filter paper in a funnel. Then the absorption of the sample was measured at wavelength of 664.5 nm using a UV-VIS spectrophotometer.

Iodine Adsorption
First, three samples of synthesized ACC of masses (1.000 ± 0.100 g), (1.500 ± 0.100 g) and (2.000 ± 0.100 g) were placed in three reagent bottles equipped with stoppers. Then 10.00 ml of 5% (wt./v) hydrochloric acid solution was added into each reagent bottle containing activated coir. After that, bottles were stoppered D. D. T. T. D. Senarathna et al. and swirled the solution gently until the activated coir get completely wetted.
Then the stoppers were loosened, and the reagent bottles were placed on a hot plate in the fume cupboard. The contents were brought to boil for 30 s to remove any Sulphur which may interfere during the reactions. Thereafter the bottles containing ACC were removed from the hot plate and cooled to room temperature. After that, 50.00 ml of iodine solution (0.100 N) was added to the reagent bottle. The bottle was vigorously shaken for 1 min. Then the content was quickly filtered, and the filtrate was collected. Moreover, the pipette was rinsed using the first 10 ml of the filtrate. The remaining filtrate was collected on a clean reagent bottle. Then 50.00 ml of the filtrate was pipetted out into a clean reagent bottle and the titrated with standard sodium thiosulphate solution (0.100 M) until the pale yellow colour appeared. Then a few drops of starch were added, and the titration was continued until the blue solution turned colourless. The volume of the Sodium thiosulphate was recorded and the iodine number was calculated.

Batch Adsorption Study
To prepare lead(II) (Pb 2+ ) ion solution, lead nitrate (98%) solid was dissolved in 1.0 L of deionized water in a volumetric flask to prepare a 1000 mg/L of stock Pb 2+ solution. This stock solution was used to prepare the 10-ppm Pb 2+ solution and other concentrations ranges used in isotherm studies and initial concentration optimization. Optimization studies were done using a 50.0 mL volume of synthetic Pb 2+ solution at room temperature (25˚C ± 2˚C) with a 150-rpm shaking speed for optimizing the parameters affecting for lead adsorption such as contact time, initial Pb 2+ concentration, pH of the solution and the ACC dosages. Table 1 shows the conditions used in parameter optimization studies.
All treatments were done as triplicates and at the end of each optimization, solutions were allowed 60 min for settling and filtered using filter papers. Finally, the remaining Pb 2+ concentration of each solution was measured using Thermo Fisher Scientific iCE 3500 AAS Atomic Absorption Spectrometer.

Kinetic and Adsorption Isotherm Studies
Kinetic studies were carried out as follows. First, 10 mg/L of Pb 2+ solution was prepared using 1000 mg/L stock solution. Then, 1 L of 10 mg/L solution was added into a 2 L beaker and 4.00 g of ACC was added into the solution. The mixture was stirred using the magnetic stirrer at 150 rpm. After the addition of ACC, 10 ml of Pb 2+ solution was withdrawn at desired time intervals. It was filtered using filter papers. Then, the remaining concentration of Pb 2+ was deter-

Desorption Studies
A 1000 mg/L Pd 2+ ion solution was treated with 2.000 (±0.001) g of synthesized ACC for 10 minutes and it was filtered using filter paper. Then the treated ACC was washed gently using deionized water to remove excess lead ions which were not adsorbed onto the ACC surface. Then it was dries at 100 C for 24 hours. After that, a concentration series of HCl was prepared as 0.025, 0.050, 0.100 and 0.200 M and 0.100 (±0.001) g of treated ACC was added into each solution.
Then the solutions were agitated for 30 minutes and filtered solutions were tested for lead(II) ions using the Atomic Adsorption Spectroscopy (AAS).

Comparison Study
Commercial activated charcoal, raw coconut coir and KOH activated ACC were treated with lead solutions at the optimized conditions. Then the percentage removal of lead ions was compared with the percentage removal of lead obtained from phosphoric activated ACC.

Bed Column Filtration
About 0.0100 (±0.001) g of ACC was placed on a funnel with sintered glass disc and 25 mL of 96.54 mg/L lead solution was added into it. The filtrate was collected, and it was passed again through a new set up with 0.100 (±0.001) g ACC on the funnel with sintered glass disc. The process was repeated three times and the concentration of the filtrate was measured using the Atomic Adsorption spectrometer (AAS).

Results and Discussion
In this study, the Pb 2+ adsorption efficiency of activated carbon was studied. Coconut coir was selected as the low-cost precursor for ACC production and the chemical activation process was followed using the phosphoric acid in order to increase the adsorption efficacy of activated carbon. Coconut coir is mainly composed of lignin (46.4%), cellulose (21.6%) and

Characterization of ACC
hemicellulose (17.3%) [16]. According to the XRD pattern of Figure 1(a), it clearly shows the contribution of highly amorphous lignin for the crystalline structure while indicating the cellulose crystalline plane at broad peak around 22˚ -23˚ [17]. After the activation process (Figure 1  as same as in the raw coconut coir but it is apparent that the transmittance has been increased. This may be due to the removal of lignin in the activation American Journal of Analytical Chemistry  process and the dryness of the ACC. Peak intensity at 2891 cm −1 which was corresponded to C-H group stretching has been reduced in the ACC. The peak (1630 cm) corresponding to alkenyl C=C stretching also reduced the intensity which was predominant in raw coir. Peaks at 1389 cm −1 and 1061 cm −1 can be attributed to the carboxylate group and primary or secondary alcohol C-O stretching respectively. The peak at wavenumber 499 cm −1 represents the C-Cl bond stretching [18]. When compare the FT-IR spectrum of raw coir with the phosphoric acid activated ACC, there are more peaks observed in ACC than in raw coir which was formed during the activation process. Therefore, the adsorption capacity of ACC is higher than the raw coir.
The surface morphology of the raw coir and phosphoric acid-treated were analyzed using scanning electron microscopy (SEM). SEM images present the wide varieties of pores present in the compounds with various structures. The surface of the raw coconut coir (Figure 3(a)) appears clean and smooth while the treated ACC (Figure 3(b)) shows the availability of mesoporous of diameter less than 50 nm and the many sites of macropores with a diameter of above the 50 nm. Furthermore, there are many defects and imperfections available on the phosphoric acid-treated ACC surface. Therefore, it can be assumed that the synthesized ACC has a great surface area compared to the raw coir increasing the adsorption capacity.  For the elemental characterization of the surface EDX was done for the prepared N-TR-ACC. Table 2 illustrates the C and O elemental nature and presence of P as impurities which was the residue of PA in the sample of N-TR-ACC.

Methylene Blue (MB) Adsorption and Iodine Adsorption
Methylene blue and iodine value tests were performed to estimate the surface area and porosity of the prepared ACC compared to the commercially available ACC. Usually, Iodine numbers may be used as an approximation to identify micro porosity while MB is used to get an idea about mesoporosity. Related data for this study (MB value and iodine value) are shown in Table 3. According to the data, phosphoric acid activated ACC has a higher adsorption capacity than commercially available ACC.

Batch Adsorption Study
To optimize the conditions used in lead(II) removal, it was studied as a function of contact time, pH of the medium, ACC dosage and initial lead(II) ion concentration used. Results are depicted as graphs in Figure 4 from (a) to (d), respectively. Figure 5(a) shows that the lead(II) removal percentage has been dramatically increased from 1 min to 10 min and reached a constant value of ~ 82%.
The reason for this phenomenon is the high active site concentration of the ACC at the beginning of the adsorption process and saturation after 10 minutes of shaking time. Under the conditions used 82% of lead(II) ions from 50.0 mL of 10 mg/L solution has been adsorbed by 0.100 g of phosphoric activate ACC at pH 8.
Therefore, 10 min shaking time was considered to be the optimum contact time.

Kinetic Studies
Heterogeneous kinetics of adsorption of solution species on solid surfaces usually follows pseudo-first order proposed by Lagergren or pseudo-second order kinetics and the linearized form of pseudo-first order is given in Equation (1) as you can see in Figure 6 [20] [21].
The linearized form of the pseudo-second-order expression is given in Equation (2).
Here, q t and q e are considered as the amount of solute adsorbed per mass of sorbent (mg·g −1 ) at any time (t) and equilibrium (e), respectively and K 1 p is the rate constant of the pseudo first-order adsorption (1/min) and K 2 p is the rate constant of the (g·g −1 ·min −1 ) pseudo second-order adsorption.
According to the kinetic results show in Figure 5, the data obtained from lead(II) uptake with respect to the time, well fitted with the pseudo-second-order kinetic model which shows a 0.99 of R 2 value. This indicates that the rate of occupation of adsorption sites is proportional to the square number of unoccupied sites, because the pseudo second order model is based on a second order mechanism. It proves the adsorption process of lead ions onto the synthesized Figure 6. Adsorption kinetic study of Pb 2+ adsorption on phosphoric activated ACC (a) pseudo first order kinetic model (b) pseudo second order kinetic model. ACC happens through the formation of covalent bonds or the mechanism is more relevant towards the chemisorption than the physisorption described by pseudo-first-order kinetic model (Table 4). However, it doesn't confirm that the only way of adsorption is happening through the chemisorption as the pseudo second order says only that the probability of being a chemisorption is high.
Therefore, the isotherm studies may give a clear idea about the process.

Isotherm Studies
Linear forms of the isotherms models are also widely adopted to determine the isotherm parameters or the most fitted model for the adsorption system due to the mathematical simplicity. The Langmuir isotherm model assumes that the adsorption is occurring as a homogeneous monolayer adsorption onto a surface containing a limited number of adsorption sites of uniform strategies, adsorbed molecules do not interact with no transmigration of adsorbate in the plane adsorbent surface [22].
where, e q , m q , e C and L K are corresponding adsorption capacity (mg/g), maximum adsorption capacity (mg/g), equilibrium concentration (mg/L) and adsorption equilibrium constant called the Langmuir constant (L/mg) [23].
The Freundlich isotherm model describes that the adsorption is a multilayer heterogeneous adsorption and its linear form can be expressed as Equation (4): where, F K , n , e C and e q are Freundlich constant (mg/g), a constant, equilibrium concentration (mg/L) and corresponding adsorption capacity (mg/g), respectively.
According to the graph given in Figure 7, the data were well fitted with the phosphoric activated ACC is a heterogeneous multilayer adsorption. Additionally, the "n" value of the Freundlich isotherm shows that the process is a physisorption as it is a value above 1 (Table 5). Because, it is considered n = 1 as a linear adsorption, n < 1 as a chemical process and n > 1 as a physical process [24].
The pysisorption process was assumed to be happened due to the formation of van der waals bonds. According to the kinetics study, it shows the adsorption process much weighted towards the chemisorption. But still the adsorption mechanism follows the Freundlich isotherm model with 2.21 n value which proves that the adsorption is done by physisorption. However, it can be assumed that the phosphoric activated ACC adsorb lead ions through van der waal bonds and covalent bonds with remaining phosphate ions or the first adsorption layer might be occurred through covelent bonds and then other layers may occur through van der waal bonds.

Desorption Studies
Desorption study was carried out to investigate desorption of Pb 2+ from the ACC. This study also gives an insight into the possibility of regeneration of the ACC. If used ACC can be regenerated and reused, the same adsorbent can be used several times in many cycles in the removal process and also lead can be extracted from the concentrated solution. A lead(II) solution of 1000 mg/L was used in the initial treatment of ACC to saturate the adsorption sites. Obtained desorption percentages are tabulated in Table 6.

Comparison Study
The comparison study was done to examine the removal efficiency of lead from the aqueous solutions by commercially available activated charcoal and compare it with the phosphoric activated ACC. Also, the raw coir and the KOH activated carbon was used in this study. Table 7 tabulates the percentage removal of Pd(II) with the respective compound.
According to the data obtained, the Phosphoric activated ACC has a higher percentage removal of Pb 2+ than the commercially available activated charcoal at the optimized conditions. It is significant that the KOH activated ACC and Raw coir also show a comparatively higher removal percentage of Pb 2+ than the commercially available ACC.

Bed Column Filtration
This experiment was carried out to determine the efficiency of removal without agitation. In this experiment, a 25 ml of lead solution was passed through 5 funnels with sintered glass disc which contained 0.100 (+ 0.001) g of synthesized ACC. The solution was passed through ACC by gravitational pull and the solution was not agitated. Table 8 consists of the data used to calculate the Pb 2+ removal percentage of this method.  96.84% removal was observed without agitation. This confirms that the phosphoric activated ACC can be used to remove heavy metals without using mechanical agitators with high removal efficiency. This is an important observation giving a potential solution for industrial waste water treatment to remove heavy metals.

Conclusion
In this study, Pb(II) removal behavior was studied using chemically activated coconut coir activated carbon. Activated carbon was synthesized using phosphoric acid as the activation agent and characterized using XRD, FT-IR, SEM, and EDAX. The maximum Pb(II) removal behavior of 100% removal was observed under 10 min of contact time, 0.20 g of activated carbon dosage, under pH 8, and 5 mg/L of initial lead(II) concentration. The data were well fitted with the pseudo-second-order kinetic model and the Freundlich isotherm model which described the adsorption mechanism as heterogeneous multilayer adsorption. According to the study, it can be concluded that the synthesized phosphoric activated coconut coir activated carbon is a potential candidate application for removing Pb(II) ions in wastewater to prevent environmental contamination and associated health risk.

Conflicts of Interest
The authors declare no conflicts of interest regarding the publication of this paper.