Elsevier

Chemical Engineering Journal

Volume 351, 1 November 2018, Pages 22-30
Chemical Engineering Journal

Promotion effects of potassium permanganate on removal of Pb(II), Ni(II) and Cd(II) from hydrous manganese dioxide

https://doi.org/10.1016/j.cej.2018.06.042Get rights and content

Highlights

  • The Zeta potential of HMO was taken to assess the surface properties and performance of heavy metal removal.

  • Adding KMnO4 to alkaline solution could improve the removal effectiveness of heavy metal.

  • HMO acts as a catalyst to prompt reaction between Pb(II) and KMnO4.

Abstract

Surface properties of Hydrous manganese dioxide (HMO) can be described by Zeta potential (ZP), which is used to determine the performance of Pb(II), Ni(II) and Cd(II) removal. HMO with the highest ZP value (−51 mV) has been proved to have superior heavy metals removal ability. A novel discovery is that KMnO4 can facilitate HMO removing heavy metal. The results showed that residual Pb(II), Ni(II) and Cd(II) in pH∼8 were 0.01 mg/L, 0.02 mg/L and 0.05 mg/L after depth reaction, respectively. The mechanism of oxidation of Pb(II) in alkaline solution by KMnO4 with HMO catalyst was studied. The XPS results demonstrated that HMO improved oxidation efficiency of KMnO4 and the products Pb(IV) content increased by 50 percent approximately. The mechanism of Ni(II) removal was similar to Pb(II). Cd(II) could be absorbed but not be oxidized, while KMnO4 accelerated Cd(II) removal effect of HMO as well. KMnO4 promotion was still existing in the acidic conditions, which showed KMnO4 could promote HMO adsorption of heavy metal, and reaction mechanism is required for further study.

Introduction

The treatment and removal of heavy metals in the soils, sediments and aquatic environment have attracted considerable attention because of their extreme toxicity towards biology and environments. Heavy metals such as Pb(II), Ni(II) and Cd(II) are introduced into the aquatic environment from plating and other industrial operations [1], [2], [3]. At present there are more effective methods for removing pollutants include photocatalysis [4], [5], [6], membrane [7], [8], etc. According to the China Environmental Protection Bureau, allowable Pb concentrations in surface water are divided into five classes from class I to class V corresponding to 0.01, 0.01, 0.05, 0.05 and 0.1 mg/L, respectively. Similarly, Cd concentration standards in surface water from class I to class V are 0.001, 0.005, 0.005, 0.005 and 0.01 mg/L respectively. And Ni concentration is 0.02 mg/L in concentrated drinking water standard [9]. To minimize their health risks, it is necessary to get rid of heavy metals from aquatic environment to reach extremely low concentration. The depth removal is a way of advanced waste treatment that converts wastewater into clean water which can meet the surface water criterion.

Various functional inorganic nano-oxides showed potential characteristics of catalysis and adsorption by its unique surface structures and internal formations [10], [11]. For example, the oxides Co3O4 incorporated with Cu, Ni, Fe and Mn are then tested for electrochemical water oxidation, and Cu, Ni, and Fe incorporations show beneficial effect on the catalytic activity of Co3O4 [19]. The amine functionalized mesoporous silica materials are suitable for adsorption of toxic metal ions and show complete removal efficiency when the initial concentration of As (V), Cr(VI), Pb (II) and Hg (II) metal ions were less than 1.5 ppm, 7.5 ppm, 2.5 ppm and 4.0 ppm respectively [20]. However, the complicated synthesis methods and expensive cost might be unsatisfactory in wastewater industries. Hydrous manganese dioxide (HMO) is a well-known adsorbent with high potential for metal ions binding due to its active nanoparticles surface, which usually produce a large adsorption capacity [12], [13], [14]. The adsorption abilities of HMO for removing Pb(II), Ni(II), and Cd(II) from aqueous solutions have been reported [15], [16], [17]. For instance, Gohari et al. demonstrated Pb uptake capacity of HMO was 204.1 mg/g and Fan et al. reported HMO nanoparticles adsorption capacities was 324 mg/g for moving Pb(II) at pH 3.5 [14], [18]. Its conventional porous materials provide effective sites for many adsorption processes, and their irregular and wide pore size distribution will be beneficial to adsorption. The properties of the HMO and the interface between manganese dioxide and aqueous solution are of broad interest in science and engineering. There is a superior active surface of HMO than that of the dry manganese dioxide particle [21]. The HMO nanoparticles surface properties can be characterized by zeta potential (ZP). For suspended particles, ZP is related to the charge state of surfaces, it is used to infer and inform many important physical and chemical properties of interfacial systems and the higher ZP value of shape-controlled particles delivers a better stability of dispersions of particles around neutral pH due to its larger repulsive electrostatic interaction.[22]. ZP also provides a measurable factor to monitor optimal water clarification capabilities and it is approximately equal to surface charge when suspended particles were placed in distilled water[23]. The shape-controlling process could result in different adsorption and affinity of protons on the surface, its difference might be also due to changes in surface area and the adsorptive abilities to hydroxyl ions on surfaces of nanoparticles [21]. ZP value of HMO prepared by different methods can exhibit various surface properties and be selected as experimental condition to get rid of pollution. In order to evaluate relationship between the zeta potential of HMO and its heavy metal removal ability, a detailed study of the HMO under different preparation conditions has been undertaken.

In this paper, combination of KMnO4 and HMO shows a novel and excellent performance in removal of Pb(II), Ni(II) and Cd(II), its efficiency is superior to HMO adsorption merely. Pb(II) and Ni(II) at basic pH could be oxidized by KMnO4 to Pb(IV) and Ni(III), respectively:2MnO4-+3Pb2++4OH-=2MnO2+3PbO2+2H2O2MnO4-+6Ni2++10OH-=2MnO2+3Ni2O3+5H2O

Their oxidation efficiency was influenced by original concentration and environmental condition. In-situ MnO2 produced from Pb(II) and Ni(II) oxidation had higher activity and removed heavy metals efficiently. Cronan reported that Pb in the marine Mn nodules is in the Pb(IV) oxidation state [24]. Indeed, even though the free energy change of the reactionMnO2(s)+Pb2+=Mn2++PbO2(s)is unfavorable, the existence of a surface redox process cannot be excluded [25]. Gadde and Laitinen suggested that the high affinity of Pb for HMO over a wide pH range was attributed to oxidation of Pb(II) to Pb(IV) by MnO2 [26]. But infinitesimal product content of Pb(IV) couldn’t be measured by general analytical equipments. Manceau et al. presented the first XAS evidence for Pb reacted with birnessite [27]. This study aims to use HMO as adsorbent and catalyst for depth removal of Pb(II), Ni(II) and Cd(II), and the treated wastewater could meet the national discharged standard of sewage. In this work, Zeta potential was selected as part as experimental conditions to explore surface properties of HMO and guide contaminants removal. The role of KMnO4 were investigated by contrast experiments. And HMO catalysis mechanisms were discussed. KMnO4 could promote HMO to remove Cd(II) as well, even Cd(II) couldn’t be oxidized by KMnO4. This combination of KMnO4 and HMO applied in depth removal displayed better effectiveness than simple adsorption, and saved adsorbents as well.

Section snippets

Materials

The manganese dioxide used in these experiments was prepared by the redox reaction of KMnO4 and MnSO4:3Mn2++2MnO4-+2H2O=5MnO2+4H+.

All the chemicals are of analytic grade and used without any further purification obtained from Guangzhou Chemical Reagent Co., Ltd.. HMO was prepared by slow titration of MnSO4·H2O and KMnO4 at room temperature and meanwhile the entire solution was well mixed with a magnetic stirrer (Re > 10r/s) for 30 min. The synthetic suspended MnO2 was allowed to settle and the

The zeta potential of HMO

Zeta potential of HMO nanoparticles in aqueous suspensions can be affected by a number of factors including pH, ion concentrations and charges of ions. The HMO nanoparticles were washed with distilled water to neutral environment that could clear away the complicated factors and make ZP represent surface property. The effect of the KMnO4/MnSO4 (adding KMnO4 into MnSO4) molar ratio on the zeta potential was illustrated in Fig. 1. A unique discovery is that at HMO synthesis, zeta potential value

Conclusion

ZP of HMO was taken as a basic requirement to distinguish the HMO performance. The maximum ZP value of HMO was −51 mV, which displayed excellent spherical surface and large surface area formed rods structure. The residual concentrations of Pb(II) and Ni(II) were 0.01 mg/L and 0.02 mg/L respectively and entirely reached sewage discharge standard after depth removal. However, Cd(II) concentration needed to be further reduced to 0.01 mg/L by optimizing adsorption and removal process.

KMnO4 can

Acknowledgements

The present work has been financially supported by Science Technology Foundation (B2152990).

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