Use of packed scrap iron anodes for continuous electrochemical Cr(VI) reduction process in electroplating wastewater treatment

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Highlights

  • Cr(VI) removal using continuous electrochemical reduction was investigated.

  • HRT had negligible impact on Cr(VI) removal efficacy.

  • Current supply rate of 90% was enough to achieve 100% removal of Cr(VI).

  • Cr(VI) reduction was well obtained with ORP control.

  • Continuous treatment saved 40–57% of the cost required for batch treatment.

Abstract

The performance of an electrochemical reduction process was investigated for the treatment Cr(VI)-containing electroplating wastewater using a continuous-flow scrap iron packed column as an sacrificial anode. The effects of the key operating parameters including hydraulic retention time (HRT), current supply ratio (CSR), and inlet Cr(VI) concentration were systematically studied. With a fixed CSR of 106%, the HRT has no impact on Cr(VI) removal, but energy consumption increased with decreasing HRT. A complete Cr(VI) removal was obtained with a final ORP of approximately −100 mV. Increasing the inlet Cr(VI) concentration caused an inefficient removal efficiency of Cr(VI) with an increases in ORP. In contrast, Cr(VI) removal remained virtually unchanged, but ORP was dramatically decreased, when the influent Cr(VI) concentration decreased. Continuous and complete Cr(VI) reduction via electrochemical reactions can be achieved by the ORP range of −200 to −100 mV. The operating cost using continuous flow treatments with HRT of 60 min required less than batch reactor treatment around 40–57%.

Introduction

Hexavalent chromium (Cr(VI)) is mainly generated from electroplating, metal finishing, chromate preparation, tannery and fertilizer industries and is well known for its toxicity. The discharge of Cr(VI) is strictly regulated worldwide; in Taiwan, the discharge standard for total Cr and Cr(VI) are 1.5 and 0.35 mg/L, respectively. The conventional methods to treat Cr(VI) include chemical reduction, biological degradation, ion-exchange, and electrochemical reduction [1,2]. However, these conventional methods come with various drawbacks. For example, a high chemical cost is associated with the chemical reduction process for reduction and precipitation of Cr(VI) [3]. A continuous-flow scrap iron packed column was utilized to treat Cr(VI)-containing wastewater because of the simplicity of the process and use of low-cost waste materials in the process [[4], [5], [6], [7], [8]]. However, the Cr(VI) removal efficiencies by the waste cast iron packed in columns is highly pH-dependent [[4], [5], [6], [7], [8]]. Specifically, the influent pH level is a key parameter in determining the requirement of hydraulic retention time (HRT) for complete Cr(VI) reduction and the column breakthrough time [[4], [5], [6], [7]]. Chen et al. [4] investigated the fluidized zero valent iron (ZVI) to reduce Cr(VI) from electroplating wastewater and reported that the column just operated around 335 bed volumes (BVs) of breakthrough time even at the highly acidic pH level of 1.3, showing that only 15% of scrap iron was utilized. The life-time and the packed iron consumed dramatically decreased to 115 BVs and 5%, respectively, when the feed water pH was 1.5. Gheju et al. [8] packed different types of scrap iron, including large spiral fibers, small spiral fibers, iron shaving and fine iron powder resulting from the mechanic processing of steel, in columns to treat the synthesized Cr(VI)-containing wastewater. The author elucidated that 1-g scrap iron just reduced around 19.2 mg Cr(VI), corresponding to 2% of the packed iron consumed. Being dependent on the Cr(VI) concentration and reaction time, the estimation scrap iron capacity for Cr(VI) reduction was only 4.56 mg Cr(VI)/g Fe consumed at pH 3.0, meaning that only 0.5% of the scrap iron actually used in a study by Wang et al. [6]. The forming passive coating layers caused the inefficient use of iron, eventually causing the activity loss of iron left under the coating layer [4,[8], [9], [10]].

Chuang et al. [3] studied an electrochemical reduction (ECR) process for reducing Cr(VI) from synthetic wastewater using the sacrificing iron plate anode, showing that the Cr(VI) reduction efficiency was highly depended on the electrogenerated ferrous ions. Assuming that there have been no other side reaction, Cr(VI) is reduced to Cr(III) by the electrogenerated Fe(II), which in turn are oxidized to ferric, and both Cr(III) and Fe(III) are thus precipitated out. The overall reaction is shown in Eq. (1). Two moles of hydronium ions and three moles of Fe are utilized to reduce 1-mole Cr(VI), accompanying a pH decline.2H++CrO42+3Fe+8H2O3H2+CrOH3s+3FeOH3s

At the beginning reaction stage, decreasing pH level causes the increase of Cr(VI) removal rate, but overall Cr(VI) removal efficiency is initial pH-independent due to the increasing pH value with reaction time [3]. Under the acidic pH levels, the additional Cr(VI) removal was spontaneously attributed by both the direct reduction of Cr (VI) by Fe0 and the adsorption of Cr(VI) onto Cr(OH)3/Fe(OH)3 flocs. Additionally, the dissolved oxygen (DO) level had an little impact on Cr(VI) reduction. Ya et al. [11] developed a novel sacrificing anode by rolling titanium meshes as a cage and packing the scrap iron inside to treat the Cr(VI)-containing wastewater. The system was able to operate at very low current densities due to the large surface area and open structure of the sacrificing anode, resulting that the operating cost was 72–77% less than a conventional plate electrode when operating with intermittent current supplies. However, the system was only operated in a batch mode.

Since the electrochemical Cr(VI) reduction reaction is the redox reactions between Cr(VI) and Fe(II), the oxidation-reduction potential (ORP) can influence the removal efficacy of Cr(VI) [3,12]. Indeed, Chuang et al. [3] reported that the ORP of less than −700 mV achieved complete Cr(VI) removal through its electrochemical reduction in a batch reactor.

Based on the discussion above, the current study investigated a continuous flow-through column reactor for electro-chemical Cr(V) reduction to avoid the inefficient use of iron by the formation of passive coating layers and to take advantage of the simple process control using ORP as a control parameter. Scrap iron anodes were packed in the column reactor, which was designated as continuous electrochemical Cr(VI) reduction (CECR). Since the generation of Fe(II) ions greatly affects Cr(VI) reduction greatly, the effects of HRT was primarily investigated at a fixed current supply rate (CSR). Then, the effects of CSR and inlet Cr(VI) concentration were explored at a fixed HRT. The variation of ORP during the Cr(VI) removal was monitored as a key parameter to adjust the electrical current intensity. Finally, the operating costs were estimated and compared to that of a batch system.

Section snippets

Chemicals

Cylindrical scrap iron materials with a diameter of ~30 mm, which were produced during steel punching process for automobile parts, were collected from Fong Fong Enterprise Company, Northern Taiwan (see Fig. S1),. Before use, the scrap iron was cleaned in a series solutions (acidified de-ionized water with pH of 1, acetone, and dionized water (DI)) [11].

Concentrates liquid samples containing Cr(VI) were collected from an electroplating bath in a local electroplating factory with solution

Effects of HRT

With the Cr(VI) concentration (200–250 mg/L) and CSR 106%, the supplied current intensities were calculated at different HRTs using Eq. (4). The current intensities were inversely proportional to the HRTs, decreasing from 2.97 to 0.5 A, when the HRT increased from 10 to 60 min (see inset in Fig. 2A). As shown in Fig. 2A, the system was capable of completely reducing Cr(VI) for all the HRTs tested. Since the current intensity applied was 106% in CSR, the applied Fe dose calculated based on the

Conclusions

A CECR reactor was designed and the effects of key operational parameters including HRT, CSR, and inlet Cr(VI) concentration on electroplating wastewater treatment were investigated. The HRT had no impact on the Cr(VI) removal efficacy, but increase the energy consumption. The CSR of 90% was sufficient enough to achieve complete Cr(VI) removal with ORP values stabilized at around −100 mV. Thus, controlling the ORP value at ~ −100 mV was implemented to maintain the Cr(VI) removal efficiency.

Declaration of competing interest

We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

Acknowledgement

The study has been supported by the Ministry of Science and Technology of Taiwan under Grant Number 107-2221-E-032-001-MY3.

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