Preparation of MnOx-loaded biochar for Pb2+ removal: Adsorption performance and possible mechanism
Graphical abstract
Introduction
Agriculture wastes such as rice husk are available in excess in various zones all over the world. Normally, agriculture waste has been used as a low-grade energy resource and sometime burned directly in the agriculture field, causing serious air pollution problem. Agriculture wastes such as rice husk mainly consist of lignin, cellulose, hemi-cellulose and silica [1]. After thermal and chemical modification, active binding sites can be provided by these compositions for the adsorption of various organic and inorganic pollutants. Therefore, increasing attention has been paid on the thermal or chemical modification of agriculture wastes for their efficient reuse.
Biochar is often derived from agriculture wastes through pyrolysis process in which the biomass is subjected to thermochemical conversion under oxygen-limited condition [2]. Fast pyrolysis technique is a superior way for the valorization of agriculture wastes to generate biochar. The oxygen-limited pyrolysis of carbon rich biomass such as coconut shell, maize cob, rice husk, grape stalk and peanut shell, at temperature range of 300–1000 °C, often yields biochar possessing large surface area and porous structure [2]. However, blank biochar often exhibits relatively low adsorption efficiency. Rich husk ash has been applied for Pb2+ removal but its maximum adsorption capacity was low as 12.4 mg g−1 [3]. Therefore, chemical modification has been suggested in order to improve adsorption capacity of biochar [4].
Recent studies have revealed that nanoparticles, such as aluminum oxide, nickel oxide, zero valent ion, MnOx, titanium oxide, cerium oxide and magnesium oxide, could be used as efficient adsorbents for heavy metals removal from polluted environment [5], [6]. However, these nanoparticles were prone to agglomeration due to their nano-scale size, which limited their direct application as adsorbents [7]. This imperfection could be solved by the development of biochar based composite adsorbents through the loading of these metal oxides onto biochar surface [8]. Han et al. [9] confirmed that nano-sized MnOx particles coated on sand surface provide higher surface area due to their polymorphic structure, which resulted in adsorption superiority. MnOx loaded biochar (MOLBC) has been prepared, and the reported maximum adsorption capacities for Pb2+ were higher than that of blank biochar [10], [11]. Thus, it gave us an idea to use composite adsorbent consist of nano MnOx and porous biochar for the removal of heavy metals from wastewater.
Selection of MnOx here was due to its relatively higher sorption attraction for many heavy metals than Fe and Al oxides [12]. Wang et al. [10] indicated that Pb2+ removal by MOLBC could be attributed to precipitation mechanism, complex formation with surface functional groups and cation replacement. Moreover, cation-π interaction could be another important contributor to heavy metal adsorption [13]. Ding et al. [14] attributed Pb2+ sorption by baggase biochar to ion exchange, intraparticle diffusion, complex formation as well as P-induced precipitation. Furthermore, ion exchange mechanism was suggested when amorphous MnO2 functionalized porous diatomaceous adsorbent was applied for Pb2+ removal [15]. Especially, during the preparation of MOLBC, besides the formation of surface dentate complexes, oxidation at higher temperature often resulted in the change of biochar surface chemistry such as cation-π interaction and formation of vacancy defects on biochar surface. Therefore, adsorption mechanism and interaction type between the adsorbent and adsorbate are still doubtful as far.
Herein, MOLBC with different Mn loading was synthesized. The structure of the as-prepared adsorbents was studied by FT-IR, TEM, SEM-EDS, XPS and BET analysis. The adsorption performance of the as-prepared MOLBC, the adsorption kinetics and isotherms of 40%MOLBC were investigated in detail. In addition, the possible mechanism involved in the Pb2+ removal by MOLBC was proposed.
Section snippets
Preparation of blank biochar
The rice husk was washed with distilled water to remove adhering impurities. Then it was subjected to drying process at 90 °C and finally passed through sieves to obtain the particles with the size less than 0.6 mm. The obtained rice husk was then carbonized in muffle furnace at 800 °C for 3 h under nitrogen atmosphere to obtain blank biochar [2].
Synthesis of MOLBC
For the synthesis of MOLBC, 5 g blank biochar was immersed in 50 mL KMnO4 solution at desired concentrations. Then, these suspensions were subjected to
Characterization of MnOx-loaded biochar
In this study, blank biochar was prepared by carbonization and then was immersed in KMnO4 solution under ultrasonically treatment. Further heat treatment exposure (800 °C) for 20 min in N2 environment accelerated KMnO4 conversion into MnOx nanoparticles. Fig. 1 showed the SEM and TEM images of blank biochar and 40%MOLBC. The SEM image showed rough surface of 40%MOLBC as compared to smooth surface of blank biochar due to deposition of MnOx nanoparticles (Fig. 1a and 1b). Comparing with blank
Conclusion
The present work focused on the adsorption performance improvement and possible removal mechanism of Pb2+ by MOLBC. In comparison to the blank biochar, high adsorption capacity for Pb2+ was observed for MOLBC. The enhanced Pb2+ adsorption by MOLBC was attributed to the loaded manganese oxide nanoparticles onto biochar. 40%MOLBC exhibited almost five times higher adsorption capacity than blank biochar. XPS and FT-IR analyses revealed that the adsorption was not limited to the involvement of nano
Acknowledgments
This research is financed by Fundamental Research Funds for Jiangsu Province Biomass Energy and Materials Laboratory (No. JSBEM201408) and Fundamental Research Funds for the Central Universities (No. 30916011312).
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