Elsevier

Bioresource Technology

Volume 246, December 2017, Pages 142-149
Bioresource Technology

High-efficiency removal of lead from wastewater by biochar derived from anaerobic digestion sludge

https://doi.org/10.1016/j.biortech.2017.08.025Get rights and content

Highlights

  • Anaerobic digestion sludge (ADS) is promising material for producing biochar.

  • Biochar derived from ADS is a promising adsorbent for heavy metal removal.

  • ADSBC600 can effectively adsorb Pb2+ with a adsorption capacity of 51.20 mg/g.

  • Possible Pb2+ adsorption mechanisms are revealed.

Abstract

The properties of biochar derived from waste activated sludge and anaerobic digestion sludge under pyrolysis temperature varying from 400 °C to 800 °C were investigated. The heavy metals adsorption efficiency of the sludge-derived biochar was also examined. Among the biochar samples tested, ADSBC600 possessing highly porous structure, special surface chemical behaviors and high thermal stability was found to remove Pb2+ from aqueous solutions efficiently with an adsorption capacity of 51.20 mg/g. The Pb2+ adsorption kinetics and isotherm for ADSBC600 can be described using the pseudo second-order model and Langmuir isotherm, respectively. Analysis of the characteristics of biochar before and after metal treatment suggests that electrostatic attraction, precipitation, surface complexation and ion exchange are the possible Pb2+ removal mechanisms. This study demonstrates a successful example of waste refinery by converting anaerobic digestion sludge to feasible heavy metal adsorbents to implement the concept of circular economy.

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. single bondOH, single bondCOOH, Csingle bondO/Cdouble bondO (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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