High-efficiency removal of lead from wastewater by biochar derived from anaerobic digestion sludge
Introduction
Waste activated sludge (WAS) is a main byproduct from biological wastewater treatment, and its transport and disposal are generally expensive (Wang et al., 2015a). Anaerobic digestion (AD) of WAS, during which organic matters are converted into methane through microbial fermentation, is an efficient method to reduce sludge volume and avoid environmental pollution (Zahedi et al., 2016). Though AD can reduce the sludge volume, the residual sludge still consists of significant amount of various heavy metals, organic micro-pollutants and pathogens, which can cause severe environmental safety issues. Conventional WAS disposal methods such as landfill disposal, incineration and use in agriculture are limited by environmental, legal, economic and social constraints (Yang et al., 2013). Therefore, development of an environmentally responsible method for the sludge disposal is critical.
Pyrolysis, which is a cost-effective clean technology to treat sludge, reduces the risks of releasing heavy metals, organic micropollutants and pathogens (Chen et al., 2014a). In addition, the sludge can be simultaneously converted to biofuels such as bio-oils and pyrolytic biogas, and biochar. Notably, biochar can efficiently remove both organic and inorganic environmental contaminants due to its high surface area, stable structure, high ion exchange capacity and the presence of various value-added surface functional groups, for e.g. OH, COOH, CO/CO (Inyang et al., 2010).
Heavy metals can cause adverse human and ecological health impacts, due to their toxic and non-biodegradable characteristics. Significant amounts of heavy metals in industrial or agricultural effluents are discharged into surface water, and consequently contaminate ground water. Toxic and carcinogenic heavy metals such as lead, copper, cadmium, nickel, zinc and chromium may cause serious human health problems, and should be removed from the environment (Kılıç et al., 2013). In particular, dairy manure biochar (DMBC) and waste sludge biochar (WSBC) have been found to be high-efficient sorbents for the removal of heavy metals from wastewater (Liu et al., 2017, Xu et al., 2013b). From cost point of view, using the biochar produced from the anaerobic digestion sludge (ADS) for heavy metal removal is appropriate because the sludge can be utilized to produce considerable amount of methane or hydrogen. However, only limited information is available about the effect of biochar derived from ADS on heavy metal removal.
The aim of this work was to investigate whether ADS can be efficiently used for functional biochar production, along with the identification of the surface chemical behavior and definition of heavy metal sorption mechanisms of anaerobic digestion sludge biochar (ADSBC). In this study, WSBC and ADSBC were initially obtained from WAS and ADS through pyrolysis, respectively. The removal capacities of WSBC and ADSBC for various heavy metals such as Pb2+, Cd2+, Cu2+, Ni2+, Zn2+ and Cr6+ were investigated under different pyrolysis temperatures. The adsorption kinetics, sorption isotherm and corresponding Pb2+ removal mechanisms were studied under optimal conditions. In addition, the physicochemical properties and adsorption mechanisms of the as-prepared biochar before and after adsorption were also studied through analyzing the change in surface chemical behavior through elemental analyzer (EA), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), X-ray diffractometer (XRD) and X-ray photoelectron spectroscopy (XPS). The knowledge created from this study can be used to conclude whether the biochar produced from ADS can be utilized as efficient adsorbents to remove heavy metals, and the biogas released during the anaerobic digestion can be converted to bioenergy, which makes the proposed method a promising way to treat sludge.
Section snippets
Sludge sources and batch dewatering tests for anaerobic digestion
The WAS sample was obtained from the secondary sedimentation tank at Taiping Municipal Wastewater Treatment Plant (Harbin City, China), which has the anaerobic-aerobic biological treatment (A/O) capacity to treat 345,000 m3 wastewater per day (Xie et al., 2016). The WAS sample was concentrated by settling for 24 h at 4 °C. The WAS was anaerobically digested for seven days to produce methane at mesophilic temperature of 37 °C. Then, the ADS sample was collected.
Preparation and characterization of as-prepared biochar
WSBC and ADSBC were prepared by
Elemental composition and surface morphology of as-prepared biochar
The main characteristics of as-prepared biochar produced under different temperatures are shown in Table 1. The percentage of biochar yield for WSBC and ADSBC declined from 68.5% to 62.7% and 70.3% to 63.4%, respectively, when the pyrolysis temperature was increased from 400 °C to 800 °C. The percentage of C was lower in ADSBC compared with WSBC, suggesting that higher yield and lower C content obtained from ADSBC were due to the massive conversion of organic matters to biomass energy during
Conclusion
In this study, physicochemical properties of WSBC and ADSBC under different pyrolysis temperatures were characterized through analyzing their surface chemical behavior. Among them, ADSBC600 was found to be the most efficient adsorbent for heavy metals, especially for Pb2+. The Pb2+ adsorption kinetics and isotherms on ADSBC600 are well-fitted to the pseudo-second-order model and Langmuir isotherm, respectively. This suggests that the dominant mechanisms for Pb2+ removal by ADSBC600 include
Acknowledgements
This work was supported by the State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology – China) (No. 2016TS07). This work was also supported by the Project of Thousand Youth Talents.
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