Presently, the large amount of industrial leaded wastewater creates a great challenge to both environmental governance and wastewater recycling. Lead complexes in washed water must be removed mostly before the washed water can be recycled. This paper reports the mechanism and factors of removing Pb complexes in simulated washed water by the sulfide precipitation method. The reaction time, sodium sulfide dosage, pH, and polymeric aluminum chloride (PAC) dosage were analyzed and the optimal conditions were explored. The composition of the reaction products was also verified by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Additionally, the kinetics of the precipitation reaction of sodium sulfide and Pb-EDTA were studied. These results showed that the Pb removal efficiency reached 91.7% under the optimal process conditions which were as follows: the dosages of Na2S and PAC were 188 mg/L (Na2S/Pb2+ molar ratio of 5:1) and 30 mg/L, respectively, the reaction time was 40 min, and the pH was 9. It was demonstrated using SEM and XRD that the reaction product in the separation process was PbS and the precipitation process was fitted to the following first-order reaction kinetics equation: Ct = 89.1e−0.1047t + 10.1 (R2 = 0.9929; Ct is Pb concentration at reaction time t).
Lead is widely used in modern industrial production including machinery, metallurgy, automobile manufacturing, and other industries. This is accompanied by a large amount of lead wastewater generated during the production process (Singha & Das 2012; Hamza et al. 2013; Khosravihaftkhany et al. 2013). Generally, the composition of leaded wastewater is relatively complex, and the heavy metal in the wastewater is often not a single form of heavy metal ion, but one complexed with some ligand, such as ethylenediaminetetraacetic acid (EDTA), citric acid, and tartaric acid (Baek et al. 2005; Wang et al. 2010). Since EDTA exhibits high stability with heavy metal, soil washing with EDTA has been extensively applied to remediation of Pb-contaminated soils. Washed water with highly concentrated complexed lead has also been produced in the soil washing process (Palma et al. 2003; Leštan et al. 2008; Pociecha & Lestan 2012). The washed water containing lead complexes must be treated and recycled in the soil washing processes. Of late, several methods have been proposed for the treatment of metal-EDTA washed water and recycling of EDTA. Washed water containing Pb-EDTA has been successfully treated using the advanced oxidation process and the electrochemical advanced oxidation process, in which EDTA was degraded and a precipitation agent or adsorbent was added to remove lead (Andreozzi et al. 1999; Pociecha et al. 2009). However, the oxidative decomposition of EDTA resulted in waste byproducts. The energy generated during EDTA decomposition was also wasted. The second method involved the metal-EDTA complex reacting with zero-valent metals (Mg0-Pb0, Mg0-Ag0), which resulted in precipitation of the metallic contaminants while liberating EDTA (Lee & Marshall 2002). This method found limited application due to its high cost. The third method adopted a TiO2-assisted photocatalytic technique to treat wastewater containing Pb-EDTA, in which the Pb was adsorbed on TiO2 and then released as Pb2+ after a photocatalytic process (Pociecha & Lestan 2012). This method was restricted by the low adsorption of heavy metal ions and the narrow light spectrum used by TiO2 for photocatalysis. The sulfide precipitation method involved destabilizing the metal complex followed by precipitating the liberated metals by adding Na2S. Compared with other treatment technologies, the sulfide precipitation treatment process was promising, cost-effective, and potentially feasible. Some researchers studied the removal of Pb-EDTA from wastewater by the sulfide precipitation method and showed that this method was highly efficient for Pb removal (Hong et al. 1999; Zeng et al. 2005; Pociecha & Lestan 2012). Although sulfide precipitation reduced Pb concentration to acceptable discharge levels, the kinetics of the precipitation reaction and the specific composition of the precipitation products were not demonstrated.
In this research, the removal of lead was determined under different conditions (pH, the dosage of both Na2S and polymeric aluminum chloride (PAC), precipitation reaction time, etc.), and the composition of the products was investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD) in order to analyze the reaction mechanism of Pb removal by the sulfide precipitation method. In addition, the kinetics characteristics of the sulfide precipitation reaction were studied. This study aimed to lay a foundation for the treatment of washed water containing Pb complexes by the sulfide precipitation method.
MATERIALS AND METHODS
Simulated washed water
The sulfide precipitation steps for removing lead were tested using a prepared solution of Pb-EDTA. A stock solution containing approximately 100 mg/L of Pb was prepared by dissolving an adequate mass of lead nitrate in 20 L of deionized water. An equimolar amount of sodium EDTA crystals was also added to the solution.
Pb precipitation studies
The simulated solution of Pb-EDTA and reagents was added into a 1,000-mL beaker with the aid of a mechanical stirrer operating at 120 rpm. After a certain reaction and settling time, 20 mL of the supernatant was extracted randomly using plastic syringes. The supernatant was filtered through 0.22-μm filter paper and saved in a plastic bottle. Then the Pb concentration of the filtrate was tested.
The optimal conditions for the precipitation of Pb compounds for treatment of washed water with the Pb-EDTA complex were determined by conducting a series of experiments considering of four different factors: pH, the dosage of both Na2S and PAC, and reaction time. Only one of these factors was varied at a time, with the rest held constant. Mechanical stirring was used for all precipitation experiments. During the experiment, the pH of the solution was adjusted by addition of HCl (5%) or NaOH (5%). Deionized water was used in all dilutions and solution preparations. All tests were performed in triplicate and results are presented as averages of triplicate extracts.
Under the optimal operation conditions, the reaction kinetics of Na2S and Pb-EDTA varying over the time of precipitation were investigated. Like the operation steps given in the ‘Pb precipitation studies’ section, 20 mL of the supernatant was extracted randomly using plastic syringes. The supernatant was filtered through 0.22-μm filter paper and saved in a plastic bottle. Then the Pb concentration of the filtrate was tested with sampling at 10 minute intervals from 0 to 60 minutes. Tests were carried out in duplicate.
The dissolved Pb concentrations in the solution were determined in bulk samples by inductively coupled plasma atomic emission spectroscopy (Lee et al. 2006).
Characterization of precipitation products
The deposition of Na2S and Pb-EDTA in simulated washed water was analyzed by SEM and XRD to obtain the sediment composition, surface features, and particle size. The XRD spectrum of the precipitate was analyzed and compared with the standard spectrum using the Jade 5.0 software package.
RESULTS AND DISCUSSION
Effects of different factors on lead removal efficiency
Effect of pH on lead removal efficiency
To evaluate the effect of the initial pH on Pb removal efficiency, the pH of the washed water was adjusted from 2 to 12. The remaining Pb concentration in the aqueous solution after sulfide precipitation was determined at different pH values, and then Pb removal efficiency was calculated and plotted accordingly. As shown in Figure 1, Pb removal increased slowly as the pH was increased from 2 to 7. Incomplete Pb removal (50%) was observed at pH 2, maximum removal (88.7%) at pH 9, and a relatively stable range (88 ± 0.7%) of removal at pH 9–12 when only the solution pH was adjusted. When the concentration of hydrogen ions in the solution was relatively high, Pb removal efficiency was low (∼50%), similar to the study by Alekseeva et al. (2007). This might be related to the hydrolysis of S2− (i.e., Equations (1)–(3)). This means that S2− reacted with H+ in solution, generating HS−, followed by further hydrolysis to generate H2S causing S2− loss. The Pb removal efficiency increased rapidly (>85%) when the solution pH was between 8 and 10. It appeared possible that more OH− neutralized H+, causing the equilibrium to advantageously shift to the generated S2− direction, increasing the S2− concentration in the solution (i.e., Equations (1) and (2)). Furthermore, higher concentrations of S2− are more likely to compete with EDTA for Pb2+, thereby generating PbS (i.e., Equation (3)). Hence, it was easy to enhance Pb removal efficiency at an alkaline pH. In addition, the reason for the decrease in efficiency when pH exceeded 10 is that Pb(OH)2 may be produced when pH exceeded 10. Because the solubility of Pb(OH)2 is stronger than PbS, the removal efficiency of Pb was decreased, which agrees with the study by Lewis (2010). Considering the alkalization cost and the need to produce as little H2S gas as possible, the optimal pH in simulated washed water was held at 9.
Effect of Na2S dosage on lead removal efficiency
In the sulfide precipitation reaction, Na2S, which provided HS− and S2− anions, was used as an anionic precipitant to compete with EDTA for the chelated Pb; thus the dosage of Na2S was the key variable for Pb removal. The appropriate amount of Na2S had a direct influence on Pb removal, the cost of treatment and reuse, and secondary pollution. For example, a lower dosage would not reach the desired Pb removal efficiency, accordingly resulting in lower recycling efficiency of EDTA, while an excessive dosage probably led to pollutant concentrations of sulfur ions in water as well as the extra cost of processing washed water (Alekseeva et al. 2007; Atonen et al. 2010).
Effect of reaction time on lead removal efficiency
The reaction time in the sulfide precipitation process was one important factor affecting the Pb removal efficiency. A long reaction time resulted in high operating costs, while Pb2+ was not completely precipitated by S2− in a short time. As shown in Figure 3, the Pb removal efficiency rapidly increased initially, gradually stabilizing after 40 min, at which time it reached 88.6% (Figure 3). Thus, the optimal reaction time was held at 40 min.
Effect of PAC dosage on lead removal efficiency
The particle diameter of the sulfide precipitation products was very small making it difficult to naturally settle. According to the previous research and preliminary experiments, PAC is an efficient flocculent which could contribute to Pb removal. Adding the proper amount of PAC to the reaction solution while stirring generated a large number of aggregates with obvious stratification observed after 4 settling hours. The supernatant was transparent indicating that the sedimentation rate of the precipitant had rapidly increased. As shown in Figure 4, with an increasing concentration of PAC, Pb removal efficiency increased from 88.6% to 91.7%, reaching a maximum of 91.7% at 30 mg/L, and decreased after 30 mg/L. Thus, the optimal dosage of PAC was controlled at 30 mg/L, where sediment was flocculated and deposited efficiently. This was helpful for Na2S precipitation in order to remove Pb from washed water with Pb-EDTA.
The study of precipitation and kinetics equations
Analysis of precipitation
In order to lay a good foundation for a follow-up study, the deposition of Na2S and Pb-EDTA in simulated washed water was analyzed by SEM and XRD to obtain the sediment composition, surface features, and particle size.
As shown in Figure 5, the SEM results indicated that the black deposition was an irregularly shaped crystalline substance. By magnifying to 1,000 times and 10,000 times, it was observed that the precipitate was dense flocs with agglutinated particles. In addition, the crystal particle size was very small (submicron size).
The XRD spectrum of the precipitate was analyzed and compared with the standard spectrum of PbS using the Jade 5.0 software package. The sample spectra were consistent with the standard spectrum of PbS (Figure 6). Therefore, it was determined that the main component of the precipitate was crystalline PbS. As a result, the reaction scheme was deduced: Pb-EDTA + Na2S → PbS + Na2EDTA.
Precipitation reaction kinetics of Na2S and Pb-EDTA
Based on the experimental results, it was shown that the optimal conditions for Pb removal in simulated washed water containing Pb-EDTA by the sulfide precipitation process were pH 9, Na2S/Pb2+ molar ratio of 5:1, 40 min reaction time, and PAC dosage of 30 mg/L, with a maximum Pb removal efficiency of 91.7%.
The composition of the precipitates was investigated by SEM and XRD. These results indicated that the principal reaction product was PbS and that the main reaction scheme of the precipitation was Pb-EDTA + Na2S → PbS + Na2EDTA. The reaction process of Pb-EDTA and S2− in solution was fitted well to a first-order reaction kinetics equation and the kinetic equation of Pb2+ precipitation was Ct = 89.1e−0.1047t + 10.1 (R2 = 0.9929).
This research was supported by Beijing Natural Science Foundation (8162026). The Key Laboratory for Water and Sediment Sciences of Ministry of Education, School of Environment, Beijing Normal University, is gratefully acknowledged.