Treatment of dye wastewater with permanganate oxidation and in situ formed manganese dioxides adsorption: Cation blue as model pollutant

Abstract

This study investigated the process of potassium permanganate (KMnO₄) oxidation and in situ formed hydrous manganese dioxides (δMnO₂) (i.e., KMnO₄ oxidation and δMnO₂ adsorption) for the treatment of dye wastewater. The effectiveness of decolorization, removing dissolved organic carbon (DOC), and increasing biodegradable oxygen demand (BOD) were compared among these processes of KMnO₄ oxidation, δMnO₂ adsorption, and KMnO₄ oxidation and δMnO₂ adsorption. δMnO₂ adsorption contributed to the maximum DOC removal of 65.0%, but exhibited limited capabilities of decolorizing and increasing biodegradability. KMnO₄ oxidation alone at pH 0.5 showed satisfactory decrease of UV–vis absorption peaks, and the maximum BOD₅/DOC value of 1.67 was achieved. Unfortunately, the DOC removal was as low as 27.4%. Additionally, the great amount of acid for pH adjustment and the much too low pH levels limited its application in practice. KMnO₄ oxidation and δMnO₂ adsorption at pH 2.0 was the best strategy prior to biological process, in balancing the objectives of decolorization, DOC removal, and BOD increase. The optimum ratio of KMnO₄ dosage to X-GRL concentration ($R_{\text{KMn}{\text{O}}_4\text{/X-GRL}}$) was determined to be 2.5, at which KMnO₄ oxidation and δMnO₂ adsorption contributed to the maximal DOC removal of 53.4%. Additionally, the optimum pH for X-GRL treatment was observed to be near 3.0.

Introduction

Potassium permanganate (KMnO₄) is often used for Fe²⁺ and Mn²⁺ oxidation [1], arsenite oxidation [2], taste and odor control [3], disinfection by-products formation control [4], algae removal [5], and organic chemicals degradation (e.g., TCE and MTBE) [6] during water treatment and underground water rehabilitation. In strong acid conditions, KMnO₄ exhibits high oxidative reactivity with oxidation potential (E^o) of +1.51 V, and its reductive product is Mn²⁺ (Eq. (1)). However, KMnO₄ respectively shows E^o values of +1.70 V and +0.59 V in acidic-neutral pH and alkaline conditions, and results in the formation of hydrous manganese dioxide (δMnO₂) (Eqs. (2), (3)).

In strong acid condition:

MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O +1.51 V

In alkaline condition:

MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂(s) + 4OH⁻ +0.59 V

In acidic-neutral pH:

MnO₄⁻ + 4H⁺ + 3e⁻ → MnO₂(s) + 2H₂O +1.70 V

The δMnO₂ exhibited promising adsorptive activity due to its high surface area, and the active surface hydroxyl groups (i.e., Mn–OH). The adsorption of heavy metals, humic acid, and anions (e.g., arsenate, phosphate) has been investigated [7], [8], [9], [10].

KMnO₄ contributes to the degradation and transformation of organics species. Simultaneously, KMnO₄ itself is reduced to δMnO₂, which shows adsorptive reactivity towards the intermediates. However, rare studies focus on the effect of KMnO₄ oxidation on the subsequent adsorption of oxidative intermediates onto in situ formed δMnO₂.

Dye wastewater, which receives great concern due to its difficulty to treat, contains high intensity of color, high chemical oxygen demand (COD); and perhaps more serious, they are toxic and carcinogenic to aquatic living organisms [11], [12]. Biological process is unlikely to achieve good performance if the biodegradability is not satisfactorily increased by chemical oxidation. Adsorption could not minimize the risk to environments, due to its inability in degrading dye molecules. The advanced oxidation processes (AOPs), such as O₃, H₂O₂, Fenton's reagent, photo-Fenton reagent, photo-catalysis reaction, and UV-H₂O₂, contribute to satisfactory decolorization, degradation, and mineralization of dyes [13], [14], [15], [16]. However, AOPs are rarely used in practice, owing to the complicate reactors and high costs. The development of effective, cheap, and easy-to-handle process is of crucial importance for the treatment of dye wastewater. KMnO₄ has been proposed for the treatment of dye wastewater [17], [18]. Xu et al. reported that KMnO₄ at pH 0.5 contributed to satisfactory decolorization of different dyes and the increase of BOD₅/COD [17]. However, KMnO₄ oxidation resulted in limited degradation of total organic carbon (TOC), and the pH level as low as 0.5 is often prohibited in engineering.

The study employed the azo dye of cationic blue (X-GRL) as model pollutant, and aimed: (1) to compare the variation of UV–vis spectrometry and DOC removal, and transformation of Mn species among processes of KMnO₄ oxidation, δMnO₂ adsorption, and KMnO₄ oxidation with in situ formed δMnO₂ adsorption (i.e., KMnO₄ oxidation and δMnO₂ adsorption); (2) to investigate the effect of oxidizing X-GRL by KMnO₄ on the adsorption of intermediates onto in situ formed δMnO₂; (3) to optimize the process of KMnO₄ oxidation and δMnO₂ adsorption for the treatment of dye wastewater.