A novel MnOx-impregnated on peanut shells derived biochar for high adsorption performance of Pb(II) and Cd(II): Behavior and mechanism
Introduction
Heavy metal pollution (e.g., Hg, Pb, Cr, Cd) that has a great threat to human health is a global concern, because of their great toxicity (e.g., carcinogenicity and mutagenicity) [1,2]. Their entry into the environment is mainly due to some industrial activities, such as metal plating, discharges from mining, battery, and paper industries. For example, China's law allows the maximum concentrations of lead and cadmium in drinking water to be 10 μg L−1 and 5 μg L−1 respectively. Once they exceed the maximum concentration, they will have adverse effects on the skin, organs and nervous system [3], [4], [5]. So, it is necessary to find an economical and efficient technology to solve these problems.
In order to treat heavy metal wastewater, various technologies have been applied to remove heavy metals from water including ion exchange, electrochemical treatment, chemical precipitation and membrane technologies [6]. However, these technologies have some disadvantages, such as high cost, long reaction time, low efficiency and secondary pollution, which greatly limits their popularization and application [1,[7], [8], [9]]. In these cases, adsorption seems to be one of the most suitable and economical methods to treat heavy metal wastewater, which possesses many advantages such as availability and efficiency, less operation cost and investment cost, fast, ease of operation. [10], [11], [12].
Biochar (BC) refers to C-rich materials produced by heating of biomass with a limited or without oxygen supply [13]. Because of its charge density, negative surface charge, high degree of porosity, extensive surface area and rich functional groups, biochar is considered as a low-cost material with sufficient applicability and selectivity, which can be used to remove toxic heavy metal ions in various water environments [14]. Nevertheless, the adsorption performance of original biochar can not meet the demand for removal of heavy metal ions in wastewater, because it has less adsorption sites on the surface [15].Thus it is important to modify raw biochar for high adsorption performance.
Many nanosized inorganic metallic oxides or hydroxides (e.g., ferric oxide, manganese oxide, activated alumina oxides, zirconium oxide, and cerium hydroxide) can form specific inner-sphere complex (i.e., two atoms share a pair of electrons, with one atom providing the lone electron pair and the other one providing the empty orbit) with heavy metals [16]. However, nanosized inorganic metallic oxides readily aggregate and thus lose sorption capacity [16], [17], [18]. So many researchers attempt different biochar as supporters to stabilize nanosized metallic oxides, which can fully combine the advantages of both materials [19,20]. Manganese oxides were reported to have stronger binding with heavy metals (e.g., Pb2+) than iron oxides with similar surface area [21]. So, manganese oxides are usually loaded on biochar to remove heavy metals from industrial wastewater. [22], [23], [24], [25]. Conventional preparation methods of MnOx-BC include NaClONaOH methods [16,26] and KMnO4 methods [27,28], but these methods often need a long time and high temperature in the synthesis, and the energy consumption is relatively large, which limits its application to a certain extent.
In this study, we firstly synthesized HMBC (MnOX-OBC) with H2O2-NaOH method. H2O2 enhance the valence state of Mn in the alkaline environment, leading to formation of a high-valence state manganese oxide, the high-valence state manganese oxide and hydrogen peroxide produce a Fenton-like reaction, resulting OH· further oxidizes BC (biochar). Eventually, HMBC (MnOX-OBC) with high adsorption performance was formed. The entire modification process only needs to last for 10 min at room temperature, and the synthesis method is simple and convenient, and the energy consumption is low. In addition, we also explored the effect of the amount of H2O2 and NaOH on the adsorption capacity of HMBC and the mechanism of adsorption of Pb and Cd, in order to provide better technology for treating Pb and Cd wastewater.
Section snippets
Materials
Peanut shells were collected from the countryside of Guangzhou, Guangdong Province. H2O2 (30%), MnCl2·4H2O, Pb(NO3)2 and Cd(NO3)2 were purchased from Aladdin. NaOH and HCl was purchased from Tianjin Zhiyuan Chemical Reagent Co., Ltd (Tianjin, China). We dissolve Pb(NO3)2 and Cd(NO3)2 in deionized water to prepare different concentrations of Pb(Ⅱ) and Cd(Ⅱ). All chemicals were analytical grade or better. Peanut shells were milled and sieved (10 mesh) for subsequent use.
Preparation of HMBC
MBC was prepared by
Characterizations of adsorbents
The Mn content of HMBC was determined to 10% in Mn mass by chemical digestion (HNO3). As can be seen from the (a), (b) and (c) of Fig. 1, HMBC has obvious hydrogen peroxide etching, the surface becomes very smooth, and the large pieces of biochar are also washed into small pieces by a sharp gas(O2), and there are two different morphologies of manganese oxides (spherical and rod-shaped) on the biochar, and there does not seem to be such a significant crystal morphology in other studies [30], [31]
Conclusion
In general, the manganese oxide is supported on biochar using the KMnO4 methods and the NaClO-NaOH methods. We have successfully loaded manganese oxides on biochar with H2O2-NaOH, and our method is modified for only 10 min at room temperature compared to the KMnO4 and NaClO-NaOH methods. The maximum adsorption capacities of Pb2+ and Cd2+ by HMBC were 164.59 mg g − 1 and 36.77 mg g − 1, respectively, and the main adsorption mechanisms were complexation, precipitation, cation interaction with Π
CRediT authorship contribution statement
Xueqiang Liu: Methodology, Data curation, Writing – original draft, Investigation, Formal analysis. Diandi Li: Investigation, Formal analysis, Supervision. Jiguang Li: Methodology, Writing – review & editing, Project administration, Funding acquisition. Jianle Wang: Data curation, Formal analysis. Shujie Liang: Investigation, Formal analysis. Hong Deng: Methodology, Writing – review & editing, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Key Research and Development Program of China [Nos.2019YFC1805902, 2019YFA0210402]; Guangdong Science and Technology Program [2020B121201003]; and the Guangxi Science and Technology Base and Talent Special Project (No. 2019AC20054); the Guangxi Natural Science Foundation (Nos. 2018JJA130034, 2018JJB130018); the Guangxi Key Laboratory of Beibu Gulf Marine Biodiversity Conservation, Beibu Gulf University (Nos. 2021KA01, 2022ZB01); the High-level Talents
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