Mechanistic Study of Pb²⁺ Removal from Aqueous Solutions Using Eggshells

Abstract

This study investigates the impact of eggshell particle size and solid-to-water (s/w) ratio on lead (Pb²⁺) removal from aqueous solution. Collected raw eggshells were washed, crushed, and sieved into two particle sizes (<150 and 150–500 µm). Batch Pb²⁺ removal experiments were conducted at different s/w ratios with initial Pb²⁺ concentrations of up to 70 mg/L. The contribution of precipitation to Pb²⁺ removal was simulated by quantifying removal using eggshell water, whereas sorbed Pb²⁺ was quantified by acid digestion. Results indicated that eggshell particle sizes did not affect Pb²⁺ removal. High removal (up to 99%) of Pb²⁺ was achieved for low initial Pb²⁺ concentrations (<30 mg/L) across all s/w ratios studied. However, higher removal capacity was observed at lower s/w ratios. In addition, results confirmed that precipitation played a major role in the removal of Pb²⁺ by eggshells. Yet, this role decreased as the s/w ratio and initial concentration of Pb²⁺ increased. A predictive relationship that relates the normalized removal capacity of eggshells to the s/w ratio was developed to potentially facilitate the design of the reactor.

1. Introduction

The presence of lead (Pb²⁺) in industrial wastewater has become a noteworthy source of water pollution and poses a significant health hazard to humans [1]. Physiological damage to human kidneys, liver, brain, and nervous system can happen as a result of ingesting levels of Pb²⁺ higher than the body’s tolerance [2]. In addition, elevated levels of Pb²⁺ in the human body have been shown to have negative health effects including fatigue, a decreased reproductive ability, problems in the digestive tract, and anemia [1,3]. In fact, sterility, stillbirths, and neonatal deaths are associated with constant exposure to Pb²⁺ [1]. Pb²⁺ poisoning is especially dangerous for children, as high levels of Pb²⁺ in the blood stream are associated with depression of brain development and cognitive skills [4,5,6]. Pb²⁺ poisoning can be contracted by humans through polluted drinking water, ingestion from food, inhalation of contaminated dust, or occupational exposure [1,2,4,6,7,8,9]. It can infiltrate and contaminate the water sources, and from there, contaminate every level of the food chain [1,2,10]. It has been difficult to scientifically specify a level under which Pb²⁺ is no longer associated with negative health effects, therefore many jurisdictions focus on the ability of treatment technologies to completely remove Pb²⁺ from water. The maximum acceptable concentration of Pb²⁺ in drinking water is becoming more stringent, with some jurisdictions assigning a level as low as 5 µg/L [11], while the US EPA has set the maximum contaminant level goal for Pb²⁺ in drinking water at zero but an action level at 15 µg/L [12].

Sources of Pb²⁺ in water originate mainly from industrial activities such as mining, smelting, printing, and metal plating. In addition, manufacturing of batteries, paints, alloys, ceramic glass, and plastics contribute to increased Pb²⁺ pollution in water bodies [1]. Wastewater from battery manufacturing contains, on average, a Pb²⁺ content of 0.5–25 mg/L [13,14,15,16]. Regulations for industrial effluent discharge or reuse vary considerably between different jurisdictions. Certain jurisdictions specify a maximum Pb²⁺ level for discharge of effluent wastewater into the environment; this level can be as low as 0.1 mg/L (Saudi Arabia and Oman) or as high as 1 mg/L (Tunisia) [17].

Several methods were proposed and investigated for the removal of Pb²⁺ from industrial wastewater including ion exchange, filtration and membrane processes, electro-dialysis, chemical precipitation, solvent extraction, and chemical coagulation [1,2]. In addition, electrocoagulation, membrane electrodialysis, and biosorption were also employed for Pb²⁺ removal [14]. A widely used method for Pb²⁺ removal from industrial wastewater is to precipitate Pb²⁺ with caustic soda at a pH 8.5–9.2 in the presence of Fe (III) salts, followed by the addition of a polyelectrolyte, which facilitates the flocculation of the Pb²⁺ precipitate. The effluent is further treated with sedimentation and filtration, after which the Pb²⁺ concentration is reduced to the legal allowable limit [16]. Sorption by activated carbon is also a convenient and efficient treatment method. However, the use of commercial activated carbon for sorption can be costly [18]. Therefore, there is an increasing need to re-use biowaste in wastewater treatment [19]. Many studies have focused their efforts on deriving substitute sorbents from waste materials, particularly agricultural waste [20]. This not only helps to reduce the cost of sorbents, but it also helps recycle agricultural waste. Waste materials that have been used for Pb²⁺ removal include: Chlorella vulgaris, chicken feathers reinforced with chitosan, cow bone, leather, coconut shell, peach and apricot stones, Alocacée shell, Mimosaceae husk, and Burseraceae sawdust [1,21,22,23,24,25].

Eggshells are an example of an agricultural waste that could be used for Pb²⁺ removal. In fact, the per capita consumption of eggs in the United Arab Emirates (UAE) was 8.3 kg in 2013 [26]. Previous research reported that eggshells consist of (weight %) calcium carbonate (94%), magnesium carbonate (1%), calcium phosphate (1%), and organic matter (4%) [27]. Eggshells were studied for removal of various pollutants from water, including heavy metals such as copper, iron, cadmium, and Pb²⁺ [28,29,30,31,32,33]. Other work investigated the removal of dyes such as C.I. Reactive Yellow 205, methylene blue, and malachite green using eggshells [34,35,36]. However, few of these studies provided a detailed investigation of the mechanisms responsible for contaminant removal. It is plausible that, in addition to sorption, other removal mechanisms could be responsible for contaminant removal when using a naturally occurring material as a sorbent. Such mechanisms include ion exchange, as reported by Andersson et al. [37] for the removal of heavy metals from water onto the calcite surface, precipitation [38,39,40], or surface reactions [41]. However, only few studies investigated the impact of operation conditions (eggshell particle size and dose) on the mechanisms of Pb²⁺ removal. Nonetheless, existing literature on the use of eggshells for Pb²⁺ removal acknowledged that precipitation and sorption occurred [13,28,30,42]. However, the reviewed studies offer no quantification of the contribution of each mechanism, i.e., precipitation and sorption, which is an important aspect of process design.

Therefore, the objectives of this study were twofold: first, to identify the mechanisms responsible for the removal of Pb²⁺ from aqueous solutions using eggshells and to quantify the contribution of each of these mechanisms towards total removal. The authors were specifically interested in isolating the role of precipitation from surface attachment. While surface attachment may take different forms (i.e., sorption, ion exchange, complexation, etc.), for simplicity, it is referred to as sorption from here on. The second objective was to investigate the impact of the applied eggshells mass-to-solution volume ratio on Pb²⁺ removal. Results of this study provide novel and beneficial information for the design of eggshell reactors for optimal removal of Pb²⁺, while also contributing to sustainable waste management by recycling eggshell material to treat wastewater.

2. Materials and Methods

A general framework for investigating and characterizing the use of biowaste materials for treatment and contaminant removal was previously published [19]. In this study, the framework is applied to carry out a mechanistic study of the removal of Pb²⁺ from aqueous solutions using eggshells (Figure 1).

2.1. Materials

Hanna Instruments 0.1M standard Pb²⁺ solution (HI4012-01) was used to prepare all Pb²⁺ solutions throughout this study. The required amount of standard Pb²⁺ solution was diluted in ultrapure water (Milli-Q^® IQ 7000 Ultrapure, Merck, Darmstadt, Germany) as required to prepare the needed Pb²⁺ concentrations. In addition, H₂SO₄ (95–97% assay), ethanol (95% purity), phenolphthalein (Riedel-de Haën^TM, indicator grade, Honeywell International Inc., Charlotte, NC, US), and methyl orange powder (Fluka^TM, indicator grade, Honeywell International Inc., Charlotte, NC, US) were used for alkalinity measurements.

2.2. Preparation of Eggshells

Eggshells were collected from a local restaurant in the city of Al Ain, UAE. Two batches were collected from the restaurant across a period of two weeks; each batch contained the eggshells collected by the restaurant for the week. All batches of collected eggshells were then mixed and rinsed several times with deionized water to wash away any impurities. They were then dried at 100 °C for 24 h in a conventional drying oven (ULE 400, Memmert, Schwabach, Germany). The eggshells were then ground with a pulverizer (LC-53, Gilson Company Inc., Lewis Center, OH, USA). The distance between the blades was set at about 0.7 mm to obtain particles in the range of 0.8–1.0 mm. This particle size was required to help physically remove the inner membranes. The eggshell inner membranes were then removed by soaking the eggshells in deionized water and mixing them, after which the inner membranes floated to the top and were physically removed. The eggshells were then rinsed several times to remove any residual membranes, dried again in the oven for 24 h at 100 °C, and further ground in the pulverizer. Subsequently, the obtained particles were sieved using stainless steel ASTM test sieves (Gilson Company Inc., Lewis Center, OH, USA). The resulting eggshells were classified into 4 groups depending on size, namely <150 µm, 150–500 µm, 500–800 µm, and >800 µm. The ground eggshells were stored in plastic sealable bags at room temperature until use.

2.3. Eggshells Characterization

The powdered eggshells were characterized to evaluate their potential performance for Pb²⁺ removal. Eggshell particles underwent nitrogen gas adsorption for characterization of their surface area and porosity. This was carried out with a Quantachrome Autosorb-1-C volumetric gas sorption instrument at 77K. Before measurements, samples were degassed at 150 °C for one hour. Further, Brunauer–Emmett–Teller (BET) theory was used to calculate surface area, and pore size distributions were determined by the Barett–Joyner–Halenda (BJH) model based on the desorption branch of the N₂ isotherms. Fourier-transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) analyses were used to investigate the changes to the microstructure of eggshells as a result of Pb²⁺ removal, as will be discussed in detail in Section 2.8. Based on the results of eggshell particles characterization, it was decided to use only two particle sizes (<150 µm of d₅₀ = 75 µm and 150–500 µm of d₅₀ = 300 µm) in the Pb²⁺ removal experiments, as they had the highest surface area. In addition, the study focused on particle size 150–500 µm, which was thought to be more practical and feasible to scale to column studies.

2.4. Pb²⁺ Removal Experiments

For all Pb²⁺ removal batch experiments, Pb²⁺ solutions at the desired concentrations were prepared as stock solutions from which sample volumes of 50 or 100 mL were obtained. Specific amounts of crushed eggshell powder were added to the samples to achieve the desired solid-to-water (s/w) ratio. Samples were then subject to shaking in a water bath shaker (MaXturdy 18, Daihan Scientific Co., GANG-WON -DO, South Korea) at a temperature of 25 °C and rotational speed of 150 rpm. If required, they were filtered using syringe filters (Thermo Scientific^TM, PTFE, 0.45 µm, 25 mm). Measurements of Pb²⁺, Mg²⁺, and Ca²⁺ concentration in solution were taken by Varian ICP-OES 720 ES with standards by Chem-Lab^TM, Belgium. A Thermo Scientific^TM Orion^TM 3-star pH meter was used to carry out pH measurements. Alkalinity was measured by titration, according to method 2320 [43].

For all the experiments explained in the sections below, samples were analyzed in triplicate, and the coefficient of variation for Pb²⁺ concentration ranged from 0.3% to 8.4%. Average readings for the initial and final Pb²⁺ concentrations were used to calculate the capacity of removal (m in mg/g), regardless of the mechanism of removal, as shown in Equation (1): where V is the volume of solution used (L), M is the mass of eggshells used (g), C₀ is the initial Pb²⁺ concentration (mg/L), and C_e is the final (equilibrium) Pb²⁺ concentration (mg/L). The removal percentage of Pb²⁺ was also calculated using Equation (2).

$$m = \frac{\left(C_0-C_e\right)V}{M}$$

(1)

$$\mathrm{Pb}2+ \mathrm{removal} (\%) = \frac{\left(C_0-C_e\right)}{C_0} \times 100$$

(2)

2.5. Preliminary Investigation of Pb²⁺ Removal

A screening experiment was conducted to determine the adequate contact time (until equilibrium) and to demonstrate the potential of eggshells for Pb²⁺ removal. At an s/w ratio of 1:200, eggshells were added to Pb²⁺ solutions of concentrations 0 to 70 mg/L for a contact time of 2 h. To help theorize the removal mechanisms involved, other parameters besides Pb²⁺ were monitored including pH, alkalinity, Ca²⁺, and Mg²⁺.

To investigate the potential precipitation of Pb²⁺ due to the presence of dissolved carbonates and bicarbonates, stock solutions of 1000 mg/L of Na₂CO₃ and NaHCO₃ were prepared and then left to react with Pb²⁺ in solution. One milliliter of the 1000 mg/L Na₂CO₃ stock solution was added to the 20 mg/L (0.19 meq/L) of Pb²⁺ solution and filled to the 100 mL mark in a volumetric flask. The resulting solution had a concentration of 10.07 mg/L (0.19 meq/L) Na₂CO₃. Furthermore, 1.62 mL of the 1000 mg/L Na₂CO₃ solution was added to the 20 mg/L Pb²⁺ solution and filled to the 100 mL mark in a volumetric flask, leading to a solution of 16.2 mg/L (0.19 meq/L) NaHCO₃. These solutions were then left for 2 h to observe any precipitation of Pb²⁺ by CO₃²⁻ or HCO₃⁻. Resulting solutions were then filtered and the Pb²⁺ concentration, pH, and total alkalinity were measured before and after the addition of Na₂CO₃ and NaHCO₃.

2.6. Effect of Particle Size and s/w Ratio on Removal of Pb²⁺

In order to investigate the effect of eggshell particle size and s/w ratio on the removal of Pb²⁺, 500 mL of Pb²⁺ solutions of 0 to 70 mg/L were prepared and their initial pH, Ca²⁺, Mg²⁺, and Pb²⁺ concentrations were measured. Then, 100 mL of each solution was added to 0.5 g of eggshells of particle size 150–500 µm, giving an s/w ratio of 1:200. The samples were shaken for 2 h then filtered. The filtrate was then analyzed for pH, alkalinity, Pb²⁺, Ca²⁺, and Mg²⁺ concentration. This process was also repeated for an s/w ratio of 1:100, 1:1000, and 1:2000 for eggshells of particle sizes 150–500 µm and <150 µm. This process is detailed in Figure 2.

2.7. Differentiation between Removal of Pb²⁺ by Sorption and Precipitation

To elucidate the role of sorption versus precipitation in Pb²⁺ removal by eggshells, these mechanisms have to be isolated. To do so, eggshells used in the previous experiment (Figure 2) with an s/w ratio of 1:2000 and Pb²⁺ concentrations 0 to 60 mg/L were collected and separated from the solution with a sieve (number 120 mesh size 125 µm). The solution passing through the sieve was filtered through a 0.45 µm membrane, and a portion of the filtrate was used to rinse the eggshell particles to remove any Pb²⁺ precipitate on the surface. The eggshells were then dried in an oven at 60 °C for 16 h, and then subjected to ICP analysis after undergoing acid digestion according to method 3050B [44]. This was carried out to directly measure the amount of Pb²⁺ sorbed on the surface of the eggshells. The exact amount of Pb²⁺ sorbed was calculated using mass balance after accounting for the amount present in the water film originally surrounding the wet eggshells.

Another experiment was conducted to quantify the removal of Pb²⁺ by precipitation using eggshell extracts at different s/w ratios. A volume of 600 mL of eggshell water (ESW) with an s/w ratio of 1:200 was prepared using 150–500 µm eggshells. The mixture was mixed for 2 h in a water bath shaker at 150 rpm and 25 °C. The mixture was then decanted and filtered. The filtrate gave an ESW sample, which was used in the preparation of Pb²⁺ solutions. The pH, alkalinity, Ca²⁺, and Mg²⁺ concentration of the ESW were measured. The ESW was then used to prepare 50 mL solutions of concentrations 0 to 70 mg/L Pb²⁺. The solutions were mixed again for 2 h. Subsequently, they were filtered and analyzed for alkalinity, pH, Ca²⁺, Mg²⁺, and Pb²⁺ concentration. This process was repeated for the same eggshell size using an s/w ratio of 1:2000. This process is detailed in Figure 3.

2.8. Microstructure Characterization of Pb²⁺ Removal

FTIR and SEM analyses were used to investigate the changes in functional groups and morphology of selected eggshells post-exposure to Pb²⁺ and examine the removal mechanisms. FTIR spectroscopy was carried out on raw eggshells using a Shimadzu, IR-Prestige-21 spectrometer in transmittance mode at a resolution of 4 cm⁻¹ from 500 to 4000 cm⁻¹. Similarly, it was performed on typical used eggshells after Pb²⁺ removal in a solution of 40 mg/L Pb²⁺ and an s/w ratio of 1:200. Samples were formulated by pulverizing eggshells into a powder, which was then sieved through an 80-micron sieve to remove coarse particles. Obtained powder samples were mixed with potassium bromide at a 3:1 ratio, by mass, and made into pellets for FTIR testing.

A JEOL-JSM 6390A scanning electron microscope was used to examine the morphology of eggshells with a particle size of 150–500 µm before and after Pb²⁺ removal. Pb²⁺ solutions of 20 mg/L at s/w ratios of 1:200 and 1:2000 were utilized. To separate the eggshells from the solution, the mixture was passed through a 125-micron sieve, rinsed with deionized water to remove any precipitated solids on the eggshell surface, and dried in the oven at 100 °C for 24 h. Samples were sputter-coated with a thin gold layer and examined in high-vacuum mode at 15 kV and magnification ranging from 100–1000.

3. Results and Discussion

3.1. Raw Eggshells Characterization

Removal of contaminants from solution by a biomass material depends on the material’s surface area and accessibility to active sites [45]. Nitrogen adsorption curves were used to evaluate the surface area according to BET theory, while N₂ desorption curves were used to evaluate the pore size distributions based on the BJH model. The results are presented in

Table 1. BET and Langmuir surface area calculations of biowaste material at various particle size ranges.

Material	Size (µm)	Surface Area (m2/g)	Pore Volume (103 cm3/g)	Pore Size (nm)	Reference
Eggshells	<150	0.977	3.544	14.8	This work
	150–500	0.056	-	-	This work
Eggshells	77.9	1.023	6.5	-	[35]
Olive stones	<1000	0.163	1.84	45.302	[46]
Tomato husks	75–150	0.68	0.0015	-	[47]

and Figure 4. As expected, the surface area was found to decrease with increasing eggshell particle size. It is worth noting that the BET surface area for eggshells was found to be comparable to the surface areas of other types of biomass used for sorption, as olive stones, tomato husks, and Bacillus badius (Table 1). In addition, the surface area of eggshells found in this study for particle size <150 µm, 0.977 m²/g, was similar to that reported by Tsai et al. [35] for eggshells of particle size 77 µm, 1.023 m²/g. Figure 4 shows a representative curve for the nitrogen adsorption of eggshells of particle size <150 µm; the eggshells follow a type II isotherm with negligible hysteresis. This indicates a lack of affinity between eggshells and N₂ molecules [35]. The N₂ adsorption isotherm signifies poor pore properties, owing possibly to a non-porous surface. Tsai et al. [35] also reached a similar conclusion when analyzing eggshells.

The thermogravimetric analysis of raw eggshells has been examined in previous studies [48,49,50]. Such analysis highlights the change in eggshell powder mass with temperature and gives an indication of its composition. There is general agreement that the decomposition of raw eggshells with temperature has two regions of mass loss. In the first region, a minor mass loss (around 3%) indicates the decomposition of organic matter and occurs from 100 to 550 °C. Meanwhile, in the second region, a major mass loss (around 40%) takes place at higher temperatures up to 850 °C, signifying the decomposition of calcium carbonate [48,49,50].

3.2. Preliminary Pb²⁺ Removal Experiments

A screening experiment of removal with time indicated that Pb²⁺ removal happened over a short time period. The experiment was done using an initial Pb²⁺ concentration of 100 mg/L and s/w ratio of 1:100; measurements of Pb²⁺ concentration were taken at 30 min and every hour for 6 h. Little differences in removal were observed after 30 min of contact time. Therefore, to ensure equilibrium status is achieved, 2 h was chosen as the contact time for all further experiments. Another screening experiment was conducted to investigate the capacity of eggshells for Pb²⁺ removal at an s/w ratio of 1:200. The results indicated that eggshells showed a good potential for Pb²⁺ removal compared to previously investigated biowaste materials [1,23,46]. A removal capacity (m) of 8.18 mg/g (pH = 5.9) was found for an initial Pb²⁺ concentration of 70 mg/L, at which an equilibrium aqueous concentration of 29.5 mg/L was reached. During the analysis of Pb²⁺ solution after contact with eggshells, it was noticed that Ca²⁺ and Mg²⁺ were released into the solution by eggshells. Therefore, it was hypothesized that there could be other removal mechanisms, in addition to sorption, which could be responsible for the removal of Pb²⁺. Previous studies have noted that Pb²⁺ precipitation due to reaction with carbonates could play a role in Pb²⁺ removal [51,52].

To confirm the potential precipitation of Pb²⁺ due to reaction with carbonates, an investigation was conducted to determine the extent of Pb²⁺ precipitation by reaction with CO₃²⁻ and HCO₃⁻. NaHCO₃ and Na₂CO₃ were added to 20 mg/L Pb²⁺ solution. As expected, the pH and alkalinity of Na₂CO₃ and NaHCO₃ solutions decreased when added to a Pb²⁺ solution. Indeed, immediately after the addition of Na₂CO₃, the pH was 8.29 and then decreased to 6.09, and alkalinity dropped from 14 to 11 mg/L as CaCO_3. Similarly, after the addition of NaHCO₃, the pH decreased from 7.31 to 5.94 and the alkalinity dropped from 16 to 5.6 mg/L as CaCO₃. The drop in alkalinity was accompanied by a drop in Pb²⁺ concentration from 23.66 to 0.75 and 2.53 mg/L for Na₂CO₃ and NaHCO₃ solutions, respectively. This indicates that alkalinity was consumed and Pb²⁺ was precipitated upon contact with CO₃²⁻ and HCO₃⁻. These findings provide evidence that Pb²⁺ is precipitated by carbonates that are released by eggshells in solution.

3.3. Effect of Particle Size and s/w Ratio on Removal of Pb²⁺

The method outlined in Section 2.6 was used to investigate the effect of s/w ratio and particle size on the eggshell capacity for Pb²⁺ removal. The results, presented in Figure 5, show that eggshell particle size generally did not have a measurable effect on the removal capacity, but a higher removal capacity was achieved with the smaller particle size (<150 µm) at high initial Pb²⁺ concentrations (>50 mg/L). This effect at a higher initial concentration of Pb²⁺ was observed in previous studies where m decreased with the increase in eggshell particle size [42], or with the increase in the particle size of calcite and aragonite [51]. Despite the large difference in the surface area of the two eggshell sizes used in this study (Table 1), a rather insignificant difference in the eggshell capacity for Pb²⁺ removal was noticed between the two sizes for initial Pb²⁺ concentrations below 50 mg/L (Figure 5). However, eggshell particle size could result in a significantly lower removal capacity for larger particles and higher initial Pb²⁺ concentration.

Furthermore, the removal capacity of eggshells decreases as the s/w ratio increases; at s/w ratios 1:1000 and 1:2000, the removal capacity is higher than that for ratios 1:100 and 1:200 (Figure 6). If the removal of Pb²⁺ is due to sorption, then a decrease in the eggshell removal capacity with the increase in the s/w ratio may be due to less effective mixing at a higher density of eggshells in the solution causing reduced accessibility to sorption sites. On the other hand, if the removal of Pb²⁺ is due to precipitation, then a decrease in the eggshell capacity with the increase in the s/w ratio could be due to a decreased dissolution of CaCO₃ per unit mass of eggshells at higher s/w ratios. In addition, if precipitation occurs on the eggshell surface (microprecipitation) then less sites become available for sorption. In this batch experiment mode, the highest removal capacity was found to be about 70 mg/g which occurred for particle size <150 µm at an s/w ratio of 1:2000 and an initial Pb²⁺ concentration of about 60 mg/L. Furthermore, high percentage removals (up to 99%) were achieved for low Pb²⁺ concentrations (<30 mg/L) across all s/w ratios studied (not shown here). The low concentration of solute ensured high removal.

Previous studies (

Table 2. Eggshell capacity for removal of Pb²⁺ as reported by other studies.

Study	Eggshell Size (µm)	s/w Ratio	pH	C0 (mg/L)	m (mg/g)
[42] a	750	1:500	5.0	523, 1045	156
[28]	<1000	1:40	5.5	10–150	2.24
[30] b	<500	1:100	5.0	100–1435	12.9
[29]	210–400	1:40	NA	3	0.12

^a Only the results for the 750 µm particle size are considered here. ^b Considered values represent those before a plateau for removal capacity was reached.

) showed variations in the capacity of eggshells employed for Pb²⁺ removal, with values as low as 0.12 mg/g [29] to as high as 156 mg/g [42]. Such variations could be due to differences in the experimental conditions employed, including differences in the initial concentration, particle size, and s/w ratio. However, these studies only included one s/w ratio in their investigation. As demonstrated in this study, the s/w ratio has a significant impact on the removal capacity (m). To gain insight into the effect of s/w ratio, normalized removal capacity (m/C₀) is plotted versus s/w ratio as shown in Figure 7. Normalized capacity values for each s/w ratio used in this study were obtained from the slope of the best fit line with zero intercept of the corresponding data set presented in Figure 6. Data from previous studies [28,29,30,42] were superimposed for comparison purposes. Since these previous studies only included one s/w ratio in their investigation, an average value of the slope (m/C₀) was taken for each study. One study by Soares et al. [30] showed a decreasing trend of (m/C₀) with the increase in C₀ and therefore the selected data points (Figure 7) represent those before a plateau for removal capacity (m) was reached. The data of Figure 8 show a linear trend on a log–log scale.

From Figure 7, it is clear that the impact of the s/w ratio is significant. The best fit relationship for the data in Figure 7 is given in Equation (3). The relation shows that the slope (m/C₀) is almost inversely proportional to the s/w ratio. Equation (3) is valid only in the range of the s/w ratio presented in Figure 8 and before the removal capacity of eggshells (m) reaches a plateau, and for particle size <1 mm.

$$\log \left(\raisebox{1ex}{$ m$}\!\left/ \!\raisebox{-1ex}{$C_0 $}\right.\right) = -0.933 \log (\mathrm{s}/\mathrm{w} \mathrm{ratio})-2.9\hspace{1em}\hspace{1em}\hspace{1em}{\mathrm{R}}^2 = 0.99$$

(3)

The predictive relationship (Equation (3)) could be used in designing batch reactors utilizing eggshells for Pb²⁺ removal from wastewater. Specifically, this relationship could be used to optimize the design conditions and configuration of the batch reactors (single versus sequential batch reactors). For example, based on Equation (3), 250 kg of eggshells would be required to reduce Pb²⁺ concentration from 55 to 1 mg/L with a reaction time of 2 h for 10 m³ of contaminated water if one batch reactor is used. However, if two or three batch reactors in series were to be used, the mass of eggshells would be reduced to 80 and 12 kg, respectively, with a reaction time of 2 h for each reactor. Thus, using multiple reactors in series reduces the required mass of eggshells and consequently reduces the mass of generated waste. However, the use of multiple reactors as compared to a single reactor would entail higher construction and operation costs. As such, Equation (3) could serve as a guide to reach an optimal system design subject to economic and environmental constraints.

3.4. Differentiation between Removal of Pb²⁺ by Sorption and Precipitation

To study the removal of Pb²⁺ by precipitation, the method outlined in Section 2.7 was followed. The percentage of Pb²⁺ removal as a function of the initial Pb²⁺ in solution (C₀) is presented in Figure 8. The figure compares the total Pb²⁺ removal obtained when eggshells were physically in solution and Pb²⁺ removal due to precipitation with eggshell water (ESW). Clearly, precipitation plays a major role in the removal of Pb²⁺ by eggshells. At a low initial concentration, Pb²⁺ removal could entirely be attributed to precipitation due to the presence of carbonates in the solution. However, the role of precipitation is reduced as the s/w ratio and initial concentration of Pb²⁺ become higher (Figure 8a). The difference between the removal in the two cases (eggshells and ESW) could be attributed to sorption. However, it was observed that aqueous Ca²⁺ in equilibrium with eggshells increased with the increase in initial Pb²⁺ concentration, while alkalinity remained almost constant (Figure 8a). The level of Ca²⁺, in the case of ESW, remained the same at 8.3 and 7.5 mg/L for s/w ratios 1:200 and 1:2000, respectively, at all initial Pb²⁺ concentrations. Thus, comparison between Pb²⁺ removal with ESW versus that of eggshells may provide an underestimation of the extent of precipitation. Further, the increase in Ca²⁺ in the presence of eggshells was more pronounced at the high s/w ratio (Figure 8a) compared to that for the low s/w ratio (Figure 8b). Such a finding was accompanied by an almost constant alkalinity due to the continuous release of carbonates by eggshells. This could explain the higher difference between the removal of Pb²⁺ by eggshells and ESW at the high s/w ratio (Figure 8a).

To provide a better elucidation of the roles of different removal mechanisms. The authors resorted to direct quantification of the amount of sorbed Pb²⁺ by acid digestion. Precipitation was then calculated as the difference between total removal and percentage of Pb²⁺ sorbed on the eggshells. This quantification is presented in Figure 9, which distinguishes Pb²⁺ removed by sorption from that removed by precipitation at an s/w ratio of 1:2000. Though the percentage of Pb²⁺ removal declined with increasing initial Pb²⁺ concentration, precipitation continued to be more prominent than sorption up to an initial Pb²⁺ concentration of 60 mg/L, after which removal by precipitation significantly dropped (Figure 9a) and became almost similar to removal by sorption. On the other hand, the contribution of sorption to the percent removal was the same at different initial concentrations but slightly decreased after 60 mg/L. These findings are consistent with those of Ahmad et al. [28], who concluded that precipitation may become important for the removal of Pb²⁺ and Cu²⁺ by waste eggshells. It should be noted that some studies suggest that microprecipitation of Pb²⁺ by carbonates present in eggshells was followed by adsorption of the metal carbonates on the eggshell surface [30,42]. Thus, it is difficult to confirm that all the sorbed Pb was in the form of Pb²⁺, as there could be some PbCO₃ remaining on the surface, which could result in an underestimation of the role of precipitation in Pb removal.

Figure 9b shows the changes in removal capacity of eggshells as a function of initial Pb²⁺ concentration. The contribution of both sorption and precipitation to the removal capacity increased with the increase in the initial Pb²⁺, but then both dropped after 60 mg/L. This behavior was also observed for the total removal capacity at this s/w ratio (1:2000) and particle size (150–500 µm) as shown in Figure 5b and Figure 6. The expectation is for the removal capacity (m) to reach a plateau value after a certain initial concentration, as shown for Pb²⁺ removal by eggshells in the work of Soares et al. (2016) and Cu²⁺ removal by eggshells in the work of Ahmad et al. (2012), in which the solution pH was maintained at a fixed value (at 5 and 5.5, respectively). However, in this study, the observed decline, rather than a plateau, in the removal capacity (m) could be attributed to a decrease in the available carbonates needed for the precipitation reaction, coupled with a decrease in the solution pH. The latter could be the reason why the observed removal capacity by sorption dropped as well.

3.5. Thermodynamic and Stiochiometric Analysis

Several Pb²⁺ solid phases could occur in the Pb-CaCO₃-H₂O system including PbO, Pb(OH)₂, PbCO₃ (cerussite), Pb₃(CO₃)₂(OH)₂ (hydrocerussite), and Pb₁₀(OH)₆(CO₃)₆ (plumbonacrite). PbO and plumbonacrite are thermodynamically stable at very high pH values (pH > 12) [53]. Pure Pb(OH)₂(s) is believed not to exist, but the basic salt, used to prepare the Pb solution, combined with aqueous Pb(OH)₂ could exist as a solid form at high pH [54]. In this study, the solution pH started at around pH 5.5 (only Pb²⁺ in the water) and increased to less than 9.0 after two hours from the addition of eggshells, making lead carbonate, in the form of cerussite or/and hydrocerussite, the more probable solid that could be formed. The chemical reactions describing the formation of cerussite and hydrocerussite along with their solubility product (K_sp) values [52] are

Pb²⁺(aq) + CO₃²⁻(aq) = PbCO₃(s)            K_sp = 7.9 × 10⁻¹⁴

(4)

3Pb²⁺(aq) + 2CO₃²⁻(aq) + 2OH⁻(aq) = Pb₃(CO₃)₂(OH)₂(s)  K_sp = 3.16 × 10⁻⁴⁶

(5)

Thermodynamic analysis was conducted to investigate the possible formation of cerussite or hydrocerussite using the range of values of Pb²⁺, CO₃²⁻, and pH encountered in this study. Pb²⁺ ranged from 10 to 70 mg/L (4.8 × 10⁻⁵ to 3.3 × 10⁻⁴ mol/L) and alkalinity ranged from 10 to 40 mg/L as CaCO₃. If all alkalinity is due to the release of CO₃²⁻ from the eggshells, then the molar concentration of CO₃²⁻ would range from 1 × 10⁻⁴ to 4.2 × 10⁻⁴ mol/L. The activity coefficients of Pb²⁺ and CO₃²⁻ were determined using the Debye–Huckel law as described by Snoeyink and Jenkins [55] and fell in the range of 0.77 to 0.89 (an average value of 0.83 was considered here for Pb²⁺ and CO₃²⁻). Based on that, the values of the ion activity product for cerussite ({Pb}{CO₃}, where { } refers to ion activity = activity coefficient × molar concentration) range from 3.3 × 10⁻⁹ to 9.7 × 10⁻⁸. These values are higher than the K_sp of cerussite (7.9 × 10⁻¹⁴), indicating favorable conditions for the formation of this solid. Analogous calculations of the ion activity product for hydrocerussite ({Pb}³{CO₃}²{OH}²) resulted in values that range from 4.4 × 10⁻³⁹ to 2.6 × 10⁻²⁸, which are higher than its K_sp (3.16 × 10⁻⁴⁶), and hence the conditions also favor the formation of hydrocerussite.

The above analysis indicates that the precipitate formed in the Pb-eggshell system of this study could be cerussite or/and hydrocerussite. There is no agreement in the literature about the relative stability of cerussite and hydrocerussite. For aqueous Pb²⁺ in contact with CaCO₃ in open systems, Bilinski and Schindler [56] concluded that hydrocerussite is the most stable solid phase, whereas Taylor and Lopata [53] indicated that cerussite is thermodynamically more stable than hydrocerussite at all pH values. On the other hand, for a closed aqueous system (i.e., with no exchange of atmospheric CO₂), hydrocerussite becomes more stable at pH > 7 [51].

Stoichiometric analysis was carried out to investigate the extent to which the experimental data of Pb²⁺ precipitation with eggshells are aligned with the formation of cerussite and hydrocerussite. Figure 10 plots the concentration of reacted Pb²⁺ (expressed as the difference between the initial and final Pb²⁺ concentration in solution) versus the drop in the carbonate level estimated from the alkalinity measurement of the control solution (with eggshells but without Pb²⁺) and of the Pb²⁺ solution for two s/w ratios. The dotted line in Figure 10 represents the stoichiometry of the cerussite formation. This line has a slope of 1.0 since 1 mol of Pb²⁺ reacts with 1 mol carbonate to form cerussite. The solid line represents the stoichiometry for hydrocerussite formation and has a slope of 1.5 (3 mol of Pb²⁺ reacts with 2 mol of carbonate to form hydrocerussite). Most of the experimental data lie within the two lines, which is consistent with the notion that the removal of Pb²⁺ from the eggshell solution is due to the formation of lead carbonate. As shown in Figure 10, some of the experimental data are above the lines of cerussite and hydrocerussite formation. This is expected and could be attributed to two reasons: (1) not all Pb²⁺ precipitated but some was sorbed on the eggshells which resulted in a higher reacted Pb²⁺ than what could be predicted by stoichiometry, and (2) the eggshells could release additional carbonate during the reaction to reach a new equilibrium state and this could result in an apparent lower CO₃²⁻ change than that estimated based on the reaction stoichiometry. Nonetheless, the observed drop in Pb²⁺ versus the drop in alkalinity is justified based on the stoichiometry for the formation of lead carbonate.

3.6. FTIR Analysis

The change in functional groups of eggshells before and after Pb²⁺ removal was examined using FTIR spectroscopy. Figure 11 shows the typical FTIR spectrum of raw and used eggshells with 40 mg/L Pb²⁺ solution. The infrared bands at 712 and 872 cm⁻¹ are associated with the respective in-plane and out-of-plane bending vibrations of calcium carbonate (CaCO₃) in its polymorph form of calcite [35,57]. A broad band in the range of 1300–1500 cm⁻¹, centered at 1422 cm⁻¹, is recognized as a C–O stretching mode vibration of CaCO₃ [57]. Such bands have also been identified as v₄, v₂, and v₃ vibration modes of carbonate ion, respectively [58]. These bands are observed in both samples, providing evidence to the stability and inertness of calcite when exposed to the Pb²⁺ solution. Furthermore, raw eggshells were characterized by a major absorption peak at 2350 cm⁻¹, corresponding to a C=O stretching vibration [59]. However, such a peak was not detected in the used eggshells sample, indicating that this carbonate polymorph was less stable and was released into the Pb²⁺ solution to cause removal by precipitation.

3.7. SEM Analysis

SEM micrographs of Figure 12, Figure 13 and Figure 14 illustrate the eggshells with a particle size of 150–500 µm before and after Pb²⁺ removal. Figure 12 of raw eggshells (processed based on the procedure in Section 2.2) shows the agglomeration of small particles, with the absence of connecting channels. This supports the BET results, suggesting eggshells to be a non-porous material. This conclusion is further supported by the N₂ isotherm type II curve (Figure 4), corresponding to non-porous materials. Similar findings have been reported for raw eggshells in other work [29,35]. In addition, the micrographs show that the surface texture of raw eggshells is rough and uneven.

The removal process induced changes to the morphology of the eggshells. Figure 13 presents the microstructure of eggshells post-exposure to Pb²⁺ with an s/w ratio of 1:200. It is clear that the surface of the eggshells is different from that of the raw eggshells. In fact, the surface is denser and less porous, owing to the deposition of Pb²⁺ and formation of lead carbonate (PbCO₃). At a lower s/w ratio of 1:2000 (Figure 14), more Pb²⁺ has been deposited on the eggshells surface, leading to the growth of needle-like PbCO₃ crystals. The morphology of PbCO₃ illustrated herein is consistent with that reported in previous studies [60,61]_. This is also consistent with the trend of a higher removal capacity of eggshells (m) at lower s/w ratios (Figure 6). Nevertheless, SEM micrographs presented herein cannot be used to differentiate the mechanism of Pb²⁺ deposition onto the eggshells, i.e., microprecipitation or sorption.

4. Conclusions

Eggshells are a common biowaste that could be recycled for the removal of Pb²⁺ from wastewater, thus promoting the use of nature-based sustainable solutions for both waste management and wastewater treatment. In this study, the influence of the particle size of ground eggshells and s/w ratio on removal of Pb²⁺ from aqueous solutions was investigated. Results show that the percent Pb²⁺ removal was not significantly different for particle sizes 150–500 µm and <150 µm despite the large difference in the surface area of the two eggshell sizes. A high percentage removal (up to 99%) of Pb²⁺ by eggshells was achieved for low initial Pb²⁺ concentrations (<30 mg/L) across all s/w ratios studied. SEM images confirmed that the surface of eggshells used for Pb²⁺ removal became denser and less porous due to Pb²⁺ deposition and lead carbonate formation. Needle-like PbCO₃ crystals were observed in samples with a lower s/w ratio, which coincides with the trend where eggshells exhibited higher removal capacity at lower s/w ratios. A developed predictive relationship suggests that the removal capacity of eggshells normalized to the initial Pb²⁺ concentration is almost inversely proportional to the s/w ratio. The relationship explains differences in the Pb²⁺ removal capacities of eggshells reported by others. The developed relationship could be utilized to design batch reactors for Pb²⁺ removal by eggshells subject to economic and environmental constraints.

The study also attempted to quantify the role of different removal mechanisms in the total removal of Pb²⁺. Results confirmed that precipitation played a major role in the removal of Pb²⁺ by eggshells. However, this role was reduced as the s/w ratio and initial concentration of Pb²⁺ became higher. The difference between the extent of Pb²⁺ removal by eggshells and eggshell water suggested that sorption also played a role in Pb²⁺ removal. This was confirmed by the direct quantification of the concentration of Pb²⁺ sorbed on the eggshells and by the results from FTIR spectroscopy. Indeed, FTIR spectroscopy of eggshells highlighted the presence of high- and low-stability calcium carbonate polymorphs that existed in raw eggshells. While the former polymorph was detected in the used eggshells sample, the latter was not, signifying its release into the Pb²⁺ solution to cause removal by precipitation. It was also observed that the contribution of both sorption and precipitation dropped at the high concentration. It was speculated that such decline was due to a decrease in the available carbonates needed for precipitation coupled with a decrease in the solution pH.