The optimization of Cr(VI) reduction and removal by electrocoagulation using response surface methodology
Introduction
Chromium is used in many industrial processes such as tanning, electroplating, printed circuit board manufacturing, metal processing and metal finishing [1], [2], [3], [4], [5]. Chromium usually exists in both trivalent and hexavalent forms in aqueous systems that are characterized by different chemical behavior, bioavailability and toxicity. Hexavalent chromium (Cr(VI)) is known to be toxic to humans, animals, plants and microorganisms. Cr(VI) is also carcinogenic and has a high solubility in aqueous medium [6], [7], [8], [9], [10]. In contrast, trivalent chromium (Cr(III)) is much less toxic, has a low solubility in aqueous solutions and readily precipitates as Cr(OH)3 at pH values greater than 4.0 [11], [12], [13]. Therefore, chromium reduction is an important phenomenon since it converts toxic mobile Cr(VI) into less toxic immobile Cr(III) [14].
Cr(VI) can be removed from aqueous waste by a variety of techniques, such as chemical reduction followed by precipitation, ion exchange, reverse osmosis and adsorption. The conventional treatment application currently used to remove Cr(VI), is its reduction to Cr(III) by chemical means followed by precipitation of Cr(OH)3 [10], [11], [13], [14], [15], [16], [17], [18]. The reducing agents commonly used are ferrous iron, sulfites and zerovalent metals such as FeSO4, NaHSO3 and metalic iron. In order to remove Cr(VI) by chemical means, it is necessary to perform reduction of Cr(VI) to Cr(III) at acidic pH (2.0–4.0) and precipitation of formed Cr(III) at alkaline pH (7.0–10.0) conditions, in a two-step process.
Cr(VI), which can be found in aqueous waste as either chromate (CrO42−), or dichromate (Cr2O72−) species, can be reduced to Cr(III) by dissolved Fe(II) according to equations given below:CrO42− + 3Fe2+ + 8H+ → Cr3+ + 3Fe3+ + 4H2OCr2O72− + 6Fe2+ + 14H+ → 2Cr3+ + 6Fe3+ + 7H2OAs is evident from Eqs. (1) and (2), reduction from Cr(VI) to Cr(III) requires acidic media and a Fe(II) source in order to shift the equilibrium to the right-hand side; therefore, continuous proton and Fe(II) sources have to be supplied.
In the electrocoagulation (EC) process both reduction and precipitation take place in the same reactor. The electrochemical process involves the liberation of ions into the solution due to the anodic polarization of electrodes [18]. In an EC cell the following simultaneous reactions occur when iron electrodes are employed:
During these reactions, the Fe(II) ions released at the anode causes reduction of Cr(VI) species to Cr(III). Oxidized iron (Fe(III)) combines with the hydroxyl ions produced at the cathode to form the precursor of the insoluble ferric hydroxide (Fe(OH)3) or geothite (FeOOH) matrix, necessary for the precipitation of Cr(III) species [19]. Cr(III) may be removed through the precipitation of Cr(OH)3, adsorption onto goethite or substitution with Fe(III) in the Fe(OH)3 [11], [19], [20], [21], [22]. A great deal of work performed in the last decades has proven that electrochemical treatment using iron and aluminum electrodes is an effective method for the reduction of Cr(VI) [18], [19], [23], [24]. However, these studies have mostly been conducted on synthetic solutions containing low concentrations of Cr(VI) [5], [18], [23], [24].
Dissolved oxygen may interfere with the reaction between Cr(VI) and Fe(II) by its own ability to oxidize Fe(II). It is well known that Fe(II) oxidation by dissolved oxygen is primarily dependent on the pH and the dissolved oxygen concentration of an aqueous solution [25]. Schlautman and Han [16] concluded that the effect of dissolved oxygen on Fe(II)–Cr(VI) reaction will be minor, particularly for lower pH values. Previous researchers also stated that the presence of dissolved oxygen is expected to be important only at pH values greater than 8.0 [15], [26], [27].
In the EC process, many factors such as pH, applied electric current, the electrolyte concentration and the application time influence the process efficiency. The process efficiency may be increased by the optimization of these factors. In conventional multifactor experiments, optimization is usually carried out by varying a single factor while keeping all the other factors fixed at a specific set of conditions [28]. This method is time consuming and incapable of effective optimization. Recently, Response Surface Methodology (RSM) has been employed to optimize and understand the performance of complex systems [28], [29], [30], [31], [32], [33], [34]. By application of RSM it is possible to evaluate the interactions of possible influencing factors on treatment efficiency with a limited number of planned experiments.
In the present study, RSM, trial version of Design Expert 7.1.3, was employed for the optimization of Cr(VI) reduction and removal by EC with stainless steel electrodes. The main objectives were to optimize the process and investigate the factors that influence the removal efficiency. A sample of metal finishing industry wastewater having a high Cr(VI) concentration (1470 mg/L) was used in the experimental study. Cr(VI) abatement efficiency was chosen as the response parameter and the applied electric current, the electrolyte concentration and the application time were selected as process variables. The optimal conditions for complete Cr(VI) removal were also determined from the model obtained via experimental data.
Section snippets
Wastewater source and character
The studied metal finishing industry employs hard chromium coating. Hard chromium coating process produces a heavy coating (2.5–500 μm) which provides engineering properties such as wear resistance, heat resistance, corrosion resistance, hardness and lubricity. The hard chrome coating bath is operated with a chromic acid concentration of 250–400 g/L at elevated temperatures (55 °C). The coating time can range from 1 to 10 h. The wastewater samples were taken from rinse tanks which follow the hard
Experimental results
In optimizing a response surface, an adequate fit of the model should be obtained to avoid poor or ambiguous results [39]. This is important to ensure the adequacy of the employed model. Table 3 shows the analysis of variance (ANOVA) of regression parameters of the predicted response surface quadratic model for Cr(VI) removal by EC process using the results of all experiments performed. As it can be seen from the table, the model F-value of 4.66 and a low probability value (Pr > F = 0.0373)
Conclusions
In the present study, the performance of electrochemical treatment of hard chromium plating process wastewater was studied focusing on the influence of operating parameters such as applied electric current, electrolyte concentration and application time by using RSM with CCD. The results obtained from the present study revealed that RSM was a suitable method to optimize the operating conditions of EC for Cr(VI) removal. The response surface models developed in this study for predicting Cr(VI)
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