Removal of Lead and Nitrate from Simulated Lead- and Nitrate-Containing Wastewater via Hydroxide Precipitation

Abstract

Lead and nitrate are pollutants that are commonly found in wastewater, and these pollutants pose significant risks to humans, animals, plants, and the environment. Therefore, it is essential to treat the wastewater to remove these toxic substances. This study utilized hydroxide precipitation for the removal of lead and nitrate from simulated lead- and nitrate-containing wastewater through jar testing. The effects of pH, lead nitrate (Pb(NO₃)₂) concentration, and precipitant-to-metal ([P]/[M]) ratio were examined. The hydroxide precipitation effectively removed lead and nitrate by forming basic lead nitrate precipitates, such as lead hydroxide nitrates and lead oxide hydroxide nitrates, and operated efficiently at a pH of around 8.0. Lead and nitrate removal was highly effective and primarily influenced by the [P]/[M] ratio, with [P]/[M] of 1.0 as the optimum condition. Varying the lead nitrate concentrations resulted in a higher sludge volume compared to other parameters; however, it was only significant in nitrate removal with an optimum concentration of 0.07 M.

1. Introduction

The utilization of lead nitrate might result in the presence of lead and nitrate in wastewater, indicating potential contamination from industrial effluents [1]. The toxicity of lead and nitrate in aquatic environments has garnered significant attention due to its adverse impact on the ecosystem and human health. Today, Russia, Canada, and other major gold-producing countries often employ a combination of these two pollutants, called lead nitrate, to extract gold to improve the leaching efficiency and recovery [2]. According to Soltani et al., the use of 500 g/t of lead nitrate significantly enhances gold leaching efficiency, representing a relatively high dosage [3]. Consequently, this high level of lead nitrate can result in elevated concentrations of lead and nitrate in the wastewater produced from this process. Although Taiwan and the Philippines are not among the world’s top gold producers, the Philippines has a significant gold mining industry that plays an important role in its economy. Employing lead nitrate in the extraction of gold from ore gives rise to potential risks for the ecosystem, human health, and livestock production. Lead nitrate is also considered to have an anthropogenic origin because it has been used in producing explosives and pigments, resulting in the emission of wastewater containing lead nitrate from the production process [4].

Lead pollution is primarily caused by industrial effluents from metal plating, battery manufacturing, and mining sectors, posing a severe threat due to its toxic nature and contributing to health risks, particularly neurological damage and developmental issues [5]. Moreover, the common sources of nitrate pollution include agricultural runoff, sewage discharge, and the use of nitrogen-based fertilizers. Excessive nitrate in drinking water is harmful as it can lead to methemoglobinemia, a condition that interferes with the oxygen-carrying capacity of the blood, posing health risks, especially to infants [6]. Due to the use of lead nitrate in various industries, the World Health Organization set the lead and nitrate limits in drinking water to 0.01 mg/L and 10 mg-N/L, respectively [7]. Since then, researchers have been working to decrease the levels of lead and nitrate in the wastewater produced by industries.

Numerous methods have been created to mitigate lead and nitrate pollution, including precipitation, adsorption, ion exchange, reverse osmosis, and electrochemical reduction [2]. A study by University of Bologna researchers showcased the efficacy of reverse osmosis, employing a polyamide membrane, in removing over 99% of lead from industrial wastewater contaminated with heavy metals [8]. Additionally, another study utilizes dried water hyacinth as a biosorbent for metal ions, and the removal obtained was more than 90% for lead [9]. On the other hand, the study by Elazzouzi et al. employed aluminum electrode plates to treat urban wastewater through an electrocoagulation process, achieving a 70% removal of nitrate under operating conditions with a pH of 7 and a current density of 2 mA/cm² [10].

Among the existing techniques, chemical precipitation is the most frequently used method in the industry because it offers straightforward process control, effectiveness across a broad temperature range, and relatively low operational costs. The chemical precipitation mechanism involves dissolved metals reacting with a precipitating agent in the solution, leading to the formation of insoluble metal precipitates. These precipitates typically form as hydroxides, phosphates, sulfides, and carbonates. During this process, fine particles are generated and enlarged using chemical precipitants, coagulants, and flocculation, resulting in their removal as sludge and releasing solutions with low metal concentrations after metals precipitate into solids. Furthermore, hydroxide precipitation removes heavy metals from wastewater by adding alkalis to adjust the pH to the metal’s minimum solubility, offering easy operation, ambient conditions, suitability for automation, and low cost [11]. The optimal removal of metal ions is achieved by adjusting parameters like pH, temperature, initial metal concentration, and ion charge [12].

Few studies have explored the use of chemical precipitation methods to treat wastewater co-contaminated with heavy metals such as lead and pollutants like nitrate. Therefore, this study aims to examine the impact of different parameters such as pH, concentration, and molar ratio using hydroxide precipitation to remove lead and nitrate by using simulated lead- and nitrate-containing wastewater where these pollutants coexist. The presence and interference of other ions are not addressed in this initial investigation, as this approach was taken to simplify the study and focus on the specific interaction between lead and nitrate, serving as a preliminary step in understanding the removal process of these two pollutants together.

2. Materials and Methods

2.1. Chemicals

All reagents were of analytical grade and used without further purification. Lead nitrate (Pb(NO₃)₂, 99%) and sodium hydroxide (NaOH, 95%) were purchased from Union Chemical Works Ltd., Taipei, Taiwan and used for the simulated wastewater and precipitating agent, respectively. Acid and base buffer solutions were prepared from concentrated sulfuric acid (H₂SO₄, 95% v/v) and liquid sodium hydroxide (NaOH, 35% solution), respectively, to regulate the pH of the effluent. All solutions were prepared using ultrapure water with a resistivity greater than 18.2 MΩ-cm at 25 °C, obtained from the Millipore Milli-Q system, Merck KGaA, Darmstadt, Germany.

2.2. Batch Experiments

Batch experiments of chemical precipitation were conducted through jar tests at different pH levels of 6.0, 7.0, 8.0, 9.0, and 10.0, lead nitrate concentrations of 0.05, 0.07, 0.09, 0.11, and 0.13 M, and molar ratios of 0.5, 1.0, 1.5, 2.0, and 2.5 [13,14,15]. Figure 1 shows the experimental setup for this study. All the experimental runs were carried out at room temperature (25 °C). Lead nitrate and sodium hydroxide were used as the simulated wastewater and hydroxide precipitant, respectively, to determine the lead and nitrate removal efficiency. The mixtures were stirred at a speed of 100 revolutions per min (rpm) for a duration of 10 min, followed by a reduced speed of 30 rpm for 50 min, as described in the referenced literature [16]. Subsequently, the mixtures were left to settle for 12 h to draw liquid samples for residual lead and nitrate analysis. For the lead and nitrate residue, the supernatant liquid was filtered with a 0.45 μm syringe filter and digested with 50 μL H₂SO₄ to terminate precipitation [17].

2.3. Analytical Methods

The volumetric method was used to determine the estimated volume of the decanted sludge. The sludge volume at 30 min (SV₃₀) was determined using an Imhof cone (Kartell Labware, Noviglio, Italy). The sludge settling rate (SSR, cm³ min⁻¹) was determined in the reaction system given by Equation (1) [12]. Lead and nitrate residues were measured by using atomic absorption spectroscopy (FAAS PerkinElmer AAnalyst 500, PerkinElmer, Inc., Waltham, MA, USA) and ion chromatography (Thermo Scientific Dionex Easion, Thermo Fisher Scientific Inc., Waltham, MA, USA), respectively.

$$\mathrm{S}\mathrm{l}\mathrm{u}\mathrm{d}\mathrm{g}\mathrm{e} \mathrm{s}\mathrm{e}\mathrm{t}\mathrm{t}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e},\mathrm{S}\mathrm{S}\mathrm{R}\left({\displaystyle \frac{\mathrm{c}{\mathrm{m}}^3}{\mathrm{m}\mathrm{i}\mathrm{n}}}\right)={\displaystyle \frac{{\mathrm{V}}_{\mathrm{s}\mathrm{o}{\mathrm{l}}^{\mathrm{’}}\mathrm{n}}-\mathrm{S}{\mathrm{V}}_{30}}{{\mathrm{t}}_{\mathrm{s}}}}$$

(1)

where V_sol’n is the total volume of the solution (cm³), SV₃₀ is the sludge volume at 30 min (cm³), and t_s is the settling time (30 min). The obtained precipitates were subjected to comprehensive analyses and characterization techniques to better understand the reaction mechanism undergone by the recovered particles. The surface morphology and elemental composition were determined by the scanning electron microscope (FE-SEM, Japan JEOL JSM-7800F, JEOL Ltd., Tokyo, Japan) and energy dispersive spectrometry (EDS, Japan JEOL JSM-7800F, JEOL Ltd., Tokyo, Japan), respectively. The functional groups present in the compound were determined using Fourier transform infrared spectroscopy (JASCO FT/IR-4700, JASCO International Co., Ltd., Tokyo, Japan). In addition, the crystalline structure of recovered solids was identified using a high-resolution X-ray diffractometer (HR XRD Bruker D8 DISCOVER, Bruker Corp., Billerica, MA, USA).

3. Results and Discussion

3.1. Lead Ion Speciation

The acidic constant for nitric acid is very large:

$$\mathrm{H}\mathrm{N}{\mathrm{O}}_3={\mathrm{H}}^{+}+\mathrm{N}{\mathrm{O}}_3^{-}{\mathrm{K}}_{\mathrm{a}}=2.4\times {10}^1$$

(2)

Thus, the concentration of $\mathrm{N}{\mathrm{O}}_3^{-}$ anions for the formation of lead nitrate salt is related to the soluble carbonate concentration ([$\mathrm{N}{\mathrm{O}}_3^{-}$]) by the ratio 1/${\mathsf{\alpha }}_{\mathrm{N}\mathrm{O}}$,

$${\left[\mathrm{N}{\mathrm{O}}_3\right]}_{\mathrm{s}}=\left[\mathrm{H}\mathrm{N}{\mathrm{O}}_3\right]+\left[\mathrm{N}{\mathrm{O}}_3^{-}\right]=\left[\mathrm{N}{\mathrm{O}}_3^{-}\right]\left(1+{\displaystyle \frac{\left[{\mathrm{H}}^{+}\right]}{{\mathrm{K}}_{\mathrm{a}}}}\right)=\left[\mathrm{N}{\mathrm{O}}_3^{-}\right]{\mathsf{\alpha }}_{\mathrm{N}\mathrm{O}}$$

(3)

The hydrolysis of lead ions also requires an equilibrium with the protons in water molecules, as follows:

$$\mathrm{P}{\mathrm{b}}^{2+}+{\mathrm{H}}_2\mathrm{O}=\mathrm{P}\mathrm{b}\mathrm{O}{\mathrm{H}}^{+}+{\mathrm{H}}^{+} {\mathrm{K}}_1={10}^{-8.2}$$

(4)

$$\mathrm{P}{\mathrm{b}}^{2+}+{2\mathrm{H}}_2\mathrm{O}=\mathrm{P}\mathrm{b}{\left(\mathrm{O}\mathrm{H}\right)}_{2\left(\mathrm{a}\mathrm{q}\right)}+{2\mathrm{H}}^{+} {\mathsf{\beta }}_2={10}^{-17.2}$$

(5)

$$\mathrm{P}{\mathrm{b}}^{2+}+{3\mathrm{H}}_2\mathrm{O}=\mathrm{P}\mathrm{b}{\left(\mathrm{O}\mathrm{H}\right)}_3^{-}+{3\mathrm{H}}^{+} {\mathsf{\beta }}_3={10}^{-28.1}$$

(6)

Another ratio, 1/${\mathsf{\alpha }}_{\mathrm{P}\mathrm{b}}$, is obtained from the pH-dependent concentration of dissolved lead ions [Pb²⁺],

$${\left[\mathrm{P}\mathrm{b}\right]}_{\mathrm{s}}=\left[\mathrm{P}{\mathrm{b}}^{2+}\right]\left(1+{\displaystyle \frac{{\mathrm{K}}_1}{\left[{\mathrm{H}}^{+}\right]}}+{\displaystyle \frac{{\mathsf{\beta }}_2}{{\left[{\mathrm{H}}^{+}\right]}^2}}+{\displaystyle \frac{{\mathsf{\beta }}_3}{{\left[{\mathrm{H}}^{+}\right]}^3}}\right)=\left[\mathrm{P}{\mathrm{b}}^{2+}\right]{\mathsf{\alpha }}_{\mathrm{P}\mathrm{b}}$$

(7)

Eventually, [Pb]_s reaches equilibrium with lead hydroxide nitrate and lead oxide hydroxide nitrate at a background [NO₃]_s.

$$\mathrm{P}\mathrm{b}\left(\mathrm{N}{\mathrm{O}}_3\right)=\mathrm{P}{\mathrm{b}}^{2+}+\mathrm{N}{\mathrm{O}}_3^{-}$$

$${\mathrm{K}}_{\mathrm{s}\mathrm{p}1}=\left[\mathrm{P}{\mathrm{b}}^{2+}\right]\left[\mathrm{N}{\mathrm{O}}_3^{-}\right]={\displaystyle \frac{{\left[\mathrm{P}\mathrm{b}\right]}_{\mathrm{s}}{\left[\mathrm{N}{\mathrm{O}}_3\right]}_{\mathrm{s}}}{{\mathsf{\alpha }}_{\mathrm{P}\mathrm{b}}{\mathsf{\alpha }}_{\mathrm{N}\mathrm{O}}}}$$

(8)

$${\left[\mathrm{P}\mathrm{b}\right]}_{\mathrm{s}}={\displaystyle \frac{{\mathrm{K}}_{\mathrm{s}\mathrm{p}1}{\mathsf{\alpha }}_{\mathrm{P}\mathrm{b}}{\mathsf{\alpha }}_{\mathrm{N}\mathrm{O}}}{{\left[\mathrm{N}{\mathrm{O}}_3\right]}_{\mathrm{s}}}}$$

(9)

$$\mathrm{P}{\mathrm{b}}_2{\left(\mathrm{O}\mathrm{H}\right)}_3\left(\mathrm{N}{\mathrm{O}}_3\right)=2\mathrm{P}{\mathrm{b}}^{2+}+3\mathrm{O}{\mathrm{H}}^{-}+\mathrm{N}{\mathrm{O}}_3$$

$${\mathrm{K}}_{\mathrm{s}\mathrm{p}2}={\left[\mathrm{P}{\mathrm{b}}^{2+}\right]}^2{\left[\mathrm{O}{\mathrm{H}}^{-}\right]}^3\left[\mathrm{N}{\mathrm{O}}_3^{-}\right]={\displaystyle \frac{{\left[\mathrm{P}\mathrm{b}\right]}_{\mathrm{s}}^3{\left[\mathrm{N}{\mathrm{O}}_3\right]}_{\mathrm{s}}{\left[\mathrm{O}{\mathrm{H}}^{-}\right]}^3}{{\mathsf{\alpha }}_{\mathrm{P}\mathrm{b}}^3{\mathsf{\alpha }}_{\mathrm{N}\mathrm{O}}}}$$

(10)

$${\left[\mathrm{P}\mathrm{b}\right]}_{\mathrm{s}}={\left({\displaystyle \frac{{\mathrm{K}}_{\mathrm{s}\mathrm{p}2}{\mathsf{\alpha }}_{\mathrm{P}\mathrm{b}}^3{\mathsf{\alpha }}_{\mathrm{N}\mathrm{O}}}{{\left[\mathrm{N}{\mathrm{O}}_3\right]}_{\mathrm{s}}{\left[\mathrm{O}{\mathrm{H}}^{-}\right]}^3}}\right)}^{{\displaystyle \frac{1}{3}}}$$

(11)

Additionally, one study performed an experiment about the basic lead nitrates using this chemical reaction, wherein the basic nitrates were prepared by the reaction of sodium hydroxide with lead nitrate according to Equation (12) [15].

$$\left(\mathrm{x}+1\right)\mathrm{P}\mathrm{b}{\left(\mathrm{N}{\mathrm{O}}_3\right)}_2+2\mathrm{x}\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}\to \mathrm{P}\mathrm{b}{\left(\mathrm{N}{\mathrm{O}}_3\right)}_2·\mathrm{x}\mathrm{P}\mathrm{b}{\left(\mathrm{O}\mathrm{H}\right)}_2+2\mathrm{x}\mathrm{N}\mathrm{a}\mathrm{N}{\mathrm{O}}_3$$

(12)

The solubilities of lead compounds at various pH values of the suspensions were investigated as shown in Figure 2, wherein the solid line uses 0.5 M lead nitrate and the dashed line uses 0.05 M lead nitrate, both at 25 °C [14]. It can be observed that the basicity in the chemical composition of the precipitated lead hydroxide nitrates increased with the increasing pH of the suspension. However, as the pH range 10.0–11.0 was reached, the solubility decreased, and a pH greater than 11.0 will lead to the formation of lead oxides such as massicot and litharge. Furthermore, this study also determined that lowering the concentration of lead nitrate shifts the pH range of formation of these lead hydroxide nitrates to a much lower value. Also, the formation of the massicot was broadened, implying that the stable phase is easily formed by decreasing the lead nitrate concentration.

3.2. Zeta Potential

In colloid science and electrochemistry, the concept of zeta potential holds significant importance. This potential represents the electric charge present at the slipping plane of a particle within a solution. The slipping plane is generally situated at the interface between the particle’s surface and the adjacent liquid phase. The value of the zeta potential provides insight into the likely stability of a colloidal system. Among the various factors influencing zeta potential, pH is the most critical.

Figure 3a presents the measurement results for the zeta potential and electromobility of the HA suspension at varying pH levels. The isoelectric point (IEP) is identified at the pH where the zeta potential equals zero. At this juncture, colloid particles can easily penetrate the electric double layer (EDL) surrounding them, leading to potential coagulation due to the minimal repulsive force, ensuring kinetic stability. The EDL primarily provides kinetic stability to colloids. Only if collisions are sufficiently energetic to disrupt the ion and solvation layers, or if thermal motion disperses the surface charge, will particles penetrate the EDL and coalesce. At the isoelectric point, the zeta potential is zero, significantly reducing resistance to disruption of the EDL, rendering the colloid or suspension unstable and prone to particle agglomeration.

In this study, the lead nitrate system exhibited an isoelectric point at pH 7.99, where the plot intersects zero zeta potential. This pH is crucial from a practical standpoint as the colloidal system is the least stable, allowing particles to aggregate and flocculate, thus making it ideal for precipitation. Consequently, pH 8.0 is found to be more effective for the precipitation of lead nitrate polymorphs compared to other pH levels studied. As illustrated in Figure 3b, the largest amount of sludge particles was formed at pH 8.0. However, a notable decrease in sludge was observed at pH 7.0, likely due to the proximity of another isoelectric point around pH 6.0, which favors flocculation over the neutral condition. Additionally, as the pH increases, the sludge amount diminishes because the zeta potential becomes more negative, moving away from zero. The study by Wang et al. revealed similar findings, demonstrating that the yield percentage of the product is optimal at pH 8.0 but that the yield is notably lower at pH levels between 6.0 and 7.0, and beyond pH 8.0, the yield percentage declines consistently [18]. Furthermore, zeta potential does not significantly impact concentration and molar ratio. The results indicate that precipitate formation increases with higher concentrations and molar ratios up to 2.0, after which it starts to decline.

3.3. Effect of pH

The effect of pH on the removal of lead and nitrate in co-contaminated systems was investigated across pH levels ranging from 6.0 to 10.0. The study treated a 0.05 M lead nitrate pollutant concentration with a precipitant-to-metal ratio of 1.0 over a reaction time of 1 h in a batch system. The pH measures the concentration of hydrogen ions (H⁺) in a solution and affects the net electric charge and mobility of substances in water. Heavy metals with low mobility and solubility have a negative net charge at high pH, leading to precipitation known as alkaline precipitation. Metals with high mobility and solubility have a positive net charge in acidic conditions, resulting in dissolution. The pH also affects hydroxide ion concentration, influencing the nucleation and growth of crystalline precipitates. Thus, pH is a critical factor in the treatment process.

Figure 4a illustrates the impact of pH on the behavior of lead and nitrate. For lead removal, the data show that all pH levels achieve over 99.9% efficiency, indicating nearly complete lead removal, primarily due to the formation of lead oxide hydroxide nitrates and lead oxide (PbO). Studies by Wang et al. and Narita et al. confirm that within the pH range of 6.0 to 10.0, various lead oxide hydroxide nitrates (e.g., Pb₃O₂(OH)NO₃, Pb₂₀O₆(OH)₁₆(NO₃)₁₂, Pb₂₀(OH)NO₃) and lead hydroxide nitrates (e.g., Pb₂(OH)₃NO₃) are produced [14,18]. Conversely, the optimal pH for nitrate removal is pH 8, achieving a 55.77% removal efficiency. Previous studies have achieved optimal conditions at pH 7 using electrocoagulation and electrochemical adsorption processes, with removal efficiencies of 70% and 67%, respectively [10,19]. Although this study found that a pH of 7.0 resulted in an efficiency of 51.95%, a pH of 8.0 is still considered optimal as it is 3.82% higher than the former. The initial and reaction pH was consistently maintained within the desired range with a ±0.3 tolerance.

The volume of sludge generated at 30 min was measured to assess its settleability at different pH levels under consistent operating conditions, as shown in Figure 4b. For lead and nitrate removal through hydroxide precipitation, the sludge volume increased with higher pH levels but significantly decreased at pH 7.0. This decrease is likely due to the neutral pH, where the balance of positive and negative ions does not promote effective particle coagulation. Additionally, the sludge settling rate is inversely proportional to the sludge volume, and therefore the SSR consistently decreases at higher pH levels but suddenly increases at neutral pH.

3.4. Effect of Concentration

The coagulation process can also be affected by the presence and concentrations of ions or ionic strength in general. These factors influence the destabilization of impurities and the solubility of salts and may involve certain ions in coagulant precipitation [20]. The concentration of metal ions in a solution significantly impacts hydroxide precipitation through the solubility product constant (K_sp) and precipitation rate. When metal ion concentration exceeds the K_sp, hydroxide ions form insoluble metal hydroxide precipitates. Higher metal concentrations accelerate this process by providing more ions for reaction, resulting in faster precipitate formation.

The influence of concentrations on the behavior of lead and nitrate are presented in Figure 5a. For lead removal, it can be observed that the concentration produces more than 99.9% efficiency, implying that almost all lead is removed despite the varying concentrations and owing to the basic lead nitrates and lead oxide formed, as mentioned previously. Additionally, nitrate removal efficiency increased from 0.05 M to 0.07 M, peaking at around 56.24% before decreasing to 46.33% and lower at higher concentrations. Thus, the optimal concentration for nitrate removal is 0.07 M of lead nitrate, achieving a maximum removal efficiency of 56.24%. In addition, the initial and reaction pH was also maintained within the desired pH with ±0.3 tolerance.

The generated sludge volume at 30 min was observed to determine the settleability of the sludge at different concentrations under identical operating conditions, as depicted in Figure 5b. The sludge volume increased with higher concentrations in the lead and nitrate removal process via hydroxide precipitation. This is because higher metal concentrations significantly enhance the precipitation rate, resulting in a faster formation of more insoluble precipitates. Additionally, this may be due to the higher total metal concentration and the greater dosage of precipitant used in treating the lead and nitrate in the system. As the total metal concentration ([M] = [Pb] + [NO₃]) increases, the precipitant concentration will also increase as the system maintains a molar ratio of 1.0. However, the sludge settling rate continues to decrease at higher concentrations, due to the possible trapping of water molecules in the formed precipitates [21]. A prior study by Quiton et al. similarly observed that higher total metal concentrations require increased precipitant dosages, resulting in the slower settling of precipitates and greater sludge production [12].

3.5. Effect of Molar Ratio

According to crystallization theory, particle nucleation and growth depend on the solution’s supersaturation level, with higher saturation increasing the nucleation rate [22]. Precipitant concentration plays a crucial role in chemical precipitation. Low precipitant concentrations facilitate particle growth on existing solids, whereas high concentrations promote the rapid formation of new particles, increasing their number but reducing their size [23]. To determine the optimal ratio for removing lead and nitrate in co-contaminated systems, experiments were conducted with 0.5 to 2.5 molar ratios under identical conditions.

The molar ratio of precipitant to metal ions significantly influences both the kinetics of the chemical reaction and the properties of the resulting precipitate. This ratio determines the availability of hydroxide ions necessary for the precipitation process. A higher molar ratio generally increases the reaction rate by providing an excess of hydroxide ions, which accelerates precipitation. Conversely, a lower molar ratio may slow the reaction or cause incomplete precipitation. Additionally, the molar ratio affects the purity, particle size, and morphology of the precipitate. Optimal ratios produce high-purity precipitates with consistent and desirable characteristics, while deviations can result in impurities, varying particle sizes, and irregular morphologies.

Figure 6a depicts the effect of molar ratios on the removal efficiencies of lead and nitrate. The optimal molar ratio for both lead and nitrate removal is 1.0, achieving efficiencies of 99.90% and 53.61%, respectively. The efficiency decreases at a molar ratio of 0.5 due to insufficient sodium hydroxide to react with all lead and nitrate ions, leaving some ions dissolved and not precipitating. Conversely, higher molar ratios introduce excess sodium hydroxide, which can cause problems such as the formation of soluble complexes like Na₂PbO₂ or Pb(OH)₄²⁻ or basic lead compounds like PbO or Pb(OH)₂, particularly at higher natural reaction pH levels of around 12.0. These issues reduce the overall removal efficiency. Figure 6a also shows that increasing the molar ratio decreases the lead removal efficiency from 99.90% to 97.48% and the nitrate removal efficiency from 53.61% to 34.74%. These findings are supported by the study conducted by Kwestroo et al., which indicates that a high precipitant concentration achieved through a high molar ratio leads to the formation of PbO, thereby preventing nitrate removal from the effluent [15]. The initial and reaction pH were maintained within a ± 0.3 tolerance of the desired range.

Figure 6b shows the generated sludge volume at 30 min, used to assess sludge settleability at various concentrations under consistent operating conditions. For lead and nitrate removal via hydroxide precipitation, the sludge volume increased with higher molar ratios due to the faster precipitation rate and increased formation of insoluble precipitates. However, when the molar ratio exceeded 2.0, the sludge volume decreased because of solubility effects, leading to the formation of more soluble lead species or the re-dissolution of previously formed precipitates as the pH exceeded 12 [14]. Conversely, the sludge settling rate decreased with increasing molar ratios but started to rise again at a molar ratio of 2.5, as the sludge volume began to decrease at this point.

3.6. Solids Characterization

The crystals obtained at all the studied effluent pH values were subjected to X-ray diffraction (XRD) analysis to identify the crystalline structure of recovered solids and confirm the presence of lead- and nitrate-containing precipitates at these respective pH values. Raw XRD peaks were compared with the existing literature data and the Crystallography Open Database website. Across the pH range studied, the dominant peaks in the XRD patterns corresponded to lead nitrate polymorphs, namely lead hydroxide nitrate and lead oxide hydroxide nitrate, as shown in Figure 7. Major peaks of lead hydroxide nitrate (Pb₂(OH)₃NO₃) were observed at 2θ = 10.5, 11.6, 20.6, 13.2, 29.4, 29.9, 31.8, 36.6, 41.9, 47.4, and 48.6 [4]. Additionally, different lead oxide hydroxide nitrates were also found with the chemical formula of Pb₃O₂(OH)(NO₃) at peaks 2θ = 21.4, 23.8, 35.2, and 44.6, Pb₂₀O₆(OH)₁₆(NO₃)₁₂ at peaks 2θ = 26.7, 27.2, and 54.7, and Pb₂₀(OH)(NO₃) at peaks 2θ = 12.3, 21.9, 28.2, 37.4, and 52.5 [18]. Furthermore, lead oxide can also be found in the precipitate at peaks 2θ = 41.0, 53.4, 60.5, and 62.1 [24,25]. The strong peaks observed at higher pH levels, like 9.0 and 10.0, were the ones containing oxide because as the pH of a system becomes higher, it is more prone to producing oxide, confirming the formation of lead oxide hydroxide nitrate instead of only lead hydroxide nitrate. These findings correspond to the study by Wang et al. [18].

Figure 8 shows the FTIR spectra of the lead nitrate precipitates under hydroxide precipitation at different pH levels. As depicted in the FTIR spectrum, the visible peak centered at 3385 cm⁻¹ and 1330 cm⁻¹ pertains to the stretching and bending vibrations of the O-H band [26,27]. The O-H band indicates that hydroxyl groups are integrated into the structure of the precipitates, suggesting that the precipitates may be lead hydroxide nitrate or lead oxide hydroxide nitrate. The stretching vibration modes at 1025 cm⁻¹ correspond to C-O, which indicates a minor contribution of CO₂ dissolution from the air since the atmospheric CO₂ has slightly interacted with the sample [26]. The asymmetric stretching vibration of the Pb-O bond is represented by a peak around 584 cm⁻¹ [26,28]. The presence of this peak is indicative of lead oxide species within the precipitate, confirming that lead oxides are part of the formed compounds, and that the presence of lead oxide hydroxide nitrate is possible.

However, the comparison between precipitate compounds consisting of hydroxides and metal content from EDS to theoretical metal content is imprecise due to carbon presence and hydrogen absence. Carbon is detected regardless of its initial absence in the system, and hydrogen is not detected because EDS lacks accuracy for low-atomic-number elements. The EDS analysis in

Table 1. Atomic percentage of precipitates at different pH levels from EDS analysis.

Varying pH	Pb (%)	N (%)	O (%)
6.0	15.88	17.40	66.72
7.0	11.84	22.72	65.44
8.0	19.29	13.63	67.08
9.0	22.41	13.22	64.38
10.0	15.87	20.72	63.42

also showed and confirmed the existence of lead, nitrogen, and oxygen in the precipitates, indicating successful precipitation and the removal of lead and nitrate from the simulated co-contaminated wastewater. Furthermore, from the SEM analyses, it can be observed that the precipitates are formed together by coagulation except for pH 7.0, which looks like it does not flocculate together, confirming that this system does not favor neutral conditions, as also observed from the sludge volume from the jar test experiment. The formed precipitates in Figure 9 show the same structure of particles as in the study by Narita et al., wherein the compound contains Pb(NO₃)₂ · Pb(OH)₃, confirming that the precipitate formed is a basic lead nitrate [14].

4. Conclusions

Lead and nitrate were removed from the co-contaminated system through the formation of basic lead nitrate particles, such as lead hydroxide nitrate and lead oxide hydroxide nitrates. The lead removal efficiency exceeded 99.9% across pH values 6.0 to 10.0 due to the low solubility of lead compounds. Lead likely forms initial lead-bearing precipitates, and any remaining lead in the simulated wastewater stream adsorbs onto the surface of these precipitates, resulting in complete removal. However, nitrate removal was more challenging because pH significantly affects its removal efficiency, with a maximum efficiency of 55.77% at pH 8.0. The concentration of lead nitrate had no significant effect on lead removal because all removal efficiencies were greater than 99.94%, but it did influence nitrate removal, peaking at 56.24% efficiency at 0.07 M. Higher concentrations beyond 0.07 M cause the system to reach a saturation point, at which the removal process becomes less efficient. The precipitant-to-metal ratio also plays a significant role in the removal of lead and nitrate. For lead removal, a [P]/[M] of 1.0 is optimal, achieving 99.9% efficiency. In contrast, nitrate removal is significantly influenced by varying [P]/[M] ratios, with 53.61% as the maximum removal efficiency at a [P]/[M] of 1.0. A lower ratio results in incomplete reactions, while ratios higher than 1.0 can lead to supersaturation and the potential re-dissolution of precipitates. Hydroxide precipitation is an effective method for treating pollutants in simulated co-contaminated wastewater, resulting in high removal efficiency. One notable advantage of this system is its ability to operate optimally at lower pH values, around 8.0.

Additionally, further tests on wastewater containing these two pollutants, like actual gold cyanidation effluent, are recommended to validate the findings of this research and assess the practical applicability of the lead and nitrate removal process under actual industrial conditions. This research can also serve as a foundation for further advancements in the simultaneous removal of lead and nitrate. Employing advanced treatment technologies, such as fluidized bed homogeneous crystallization (FBHC), could significantly enhance the efficiency and effectiveness of the removal process. Exploring these advanced methods makes enhancing industrial wastewater treatment more feasible, leading to sustainable and eco-friendly solutions.