(279f) Lead and Sulfate Ions Removal from Aqueous Solutions with Electrodialysis Process

The pollution incurred by heavy metals is a serious and escalating environmental problem, as a result of the continuous increase of industrial activities and technological development. Lead exists in wastewater from various sources, including processes for the production of batteries, pigments, plastics, and electronics, whereas its removal has been a major concern to scientists until this day. Sulfuric acid is also contained in the effluents of battery production processes and efficient treatment is required to avoid the discharge of water with low pH. Conventional treatment of wastewater with the abovementioned properties includes ion exchange (Petruzzelli et al., 1999), electrochemical reduction (Lin et al., 1999), adsorption (Runtti et al., 2016), and ion precipitation (Vu et al., 2019). However, these technologies may exhibit incomplete ion removal, high startup costs and operation time and so forth. Electrodialysis (ED) is a promising process as it requires little pretreatment and produces clean and reusable streams. ED is an electrochemical separation process in which ions are transferred through ion exchange membranes by means of a direct current (DC) voltage. The solution to be treated is pumped and circulated through the ED unit, that consists of a series of alternating parallel anion exchange membranes (AEM) and cation exchange membranes (CEM) fixed between two electrically charged electrodes, the anode and the cathode (Fig. 1a).

In published literature, results are available regarding the separation of lead ions from wastewater using ED, but in small scale systems of rather low active membrane area and small volumes of treated wastewater. The group of Gherasim (2014) used a batch ED system of 10 cell pairs to remove lead ions from aqueous solutions at 10 Volts, 70 L/h flow rate and 1000 mg/L feed concentration. In another study (Mohammadi et al., 2004), the effect of process parameters on lead separation from wastewater was examined with the use of a 1-cell-pair system, at 0.25 L/h, 30 Volts and 1000 mg/L. For sulfate ions removal by ED, research is rather limited. Amor and coworkers (1998) investigated the removal of fluoride and other ions, including sulfate, from brackish water with electrodialysis, with the aid of a system composed of 10 cell pairs, at 15 Volts, 180 L/h and 730 mg/L sulfate ions concentration. In another work (Lee et al., 2003), the recovery of ammonium and sulfate ions from waste was investigated with an ED system of 10 cell pairs, at 180 L/h, 30 Volts and 730 mg/L ammonium sulfate concentration. To the authors’ knowledge, there is no work in literature concerning the removal of sulfate or the simultaneous removal of sulfate and lead ions by ED at larger scale under realistic operating conditions.

The experimental results of a pilot-scale investigation of sulfate ions removal from aqueous solutions by electrodialysis are presented in this study. For the scope of the work, an ED pilot plant composed of 68 cell pairs of cation- and anion-exchange membranes of 5.17 m² effective membrane area and maximum nominal feed flow rate 350 L/h, operating in batch mode, was used. The capability of the ED plant to remove sulfate ions was initially optimized using an experimental design based on central composite design (CCD) coupled with response surface methodology (RSM). This aimed to evaluate the effect of key process parameters like the applied voltage in the stack (U_st), the product (diluate) to concentrate ratio (r in percentage), and the sulphate concentration ([SO₄^2-]). The best operating conditions are applied for the pilot testing of the simultaneous removal of lead and sulfates from simulated battery manufacturing industry effluents.

The pilot unit (Fig. 1b) is equipped with an electrodialysis four-chamber ED stack and is designed to operate also as a salt splitting (EDBM) and salt metathesis (EDM) unit. For the scope of the standard ED tests, only two out of the four circuits of the cell are in use with one of the cell systems being the diluate (where the ions are removed) and the other one being the concentrate (in which the ions are collected). Feed solutions were prepared by dissolving Η₂SO₄ in demineralized water. The electrolyte rinse solution was Na₂SO₄ 60 g/L. The experiments were carried in batch recirculation mode at ambient temperature with diluate/concentrate tank volumes of 35 L. Samples from dilute and concentrate compartments were obtained at specific time intervals and the sulfate concentrations were measured by means of Ion Chromatography (IC). Three optimization criteria were considered: a) maximization of the sulphate separation (SP in percentage), b) minimization of the energy consumption (EC) (not including the energy for pumping), and c) maximization of current efficiency (CE in percentage), defined as the ratio between the electricity consumed in the cell for the separation of the sulfate ions and the total electricity supplied.

Based on the CCD, a total of seventeen (17) experiments were conducted to determine the optimum values of the three independent variables. The experiments were conducted by recirculating synthetic sulfate solutions at flow rate 150 L/h, and they were randomly performed to minimize the effect of systematic errors. The three independent variables varied over three levels (maximum, minimum and midpoint levels) at determined ranges chosen based on preliminary experiments (data not shown here) and background knowledge (U_st: 5, 10 or 15 Volts, r: 40%, 65% or 90%, [SO₄^2-]: 1000, 1500 or 2000 mg/L). It is noted that the flow rate did not have any significant effect in the sulfate ions separation, whereas the value of 150 L/h was proven the optimum due to the lower current density monitored in the system and therefore the lower energy consumption.

The obtained contour (Fig. 1c) and response surface plots (Fig. 1d) assisted in identifying the interrelationship between the variables U_st, r and [SO₄^2-] and providing a three-dimensional view of the changes in the three performance responses (SP, EC, CE) for different combinations of independent variables. These RSM plots were functions of two independent variables by maintaining the third variable at a constant value and helped determine the optimum experimental conditions for maximum response. As verified by the ANOVA analysis, SP is linearly correlated with the variable r, with U_st and [SO₄^2-] having marginal effect on the sulfate ions separation. As expected, the higher the U_st the larger the EC. A similar conclusion is drawn for variable r and the interaction factor U_st•r, that correlates positively to EV values. On the contrary, r correlated negatively to the current efficiency, since CE was higher at lower diluate recoveries. A number of 99 solutions of optimum U_st, r and [SO₄^2-] values were proposed by the statistical software yielding maximum SP, CE and minimum EC. Among the solutions proposed, the first solution, presenting a rather good desirability of 0.712, was selected as the optimum one. This solution required U_st=15 V, r=90% for [SO₄^2-]=2000 mg/L which is representative of the sulfate ions content in lead-acid battery wastewater effluents. Under these conditions, a set of experiments was conducted aiming to validate the correlation models (Fig. 1e). Under the optimum operating conditions, sulphate ions were removed by approx. 65% within 30 min, with a current efficiency of 33% and a recorded 0.5 kW/m³ consumption. High efficiency is also achieved for experiments that involved the simultaneous removal of lead and sulfate ions.

Acknowledgements

This research has been co‐financed by the European Union and Greek national funds through the Operational Program Competitiveness, Entrepreneurship and Innovation, under the call RESEARCH–CREATE–INNOVATE (project code: T1EDK-02677).

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