Heavy Metal Sorption by Sludge-Derived Biochar with Focus on Pb2+ Sorption Capacity at μg/L Concentrations

Ida Sylwan,Henrik Romar,Lena Johansson Westholm,Hanna Runtti,Eva Thorin

doi:10.3390/pr8121559

Abstract

Municipal wastewater management causes metal exposure to humans and the environment. Targeted metal removal is suggested to reduce metal loads during sludge reuse and release of effluent to receiving waters. Biochar is considered a low-cost sorbent with high sorption capacity for heavy metals. In this study, heavy metal sorption to sludge-derived biochar (SDBC) was investigated through batch experiments and modeling and compared to that of wood-derived biochar (WDBC) and activated carbon (AC). The aim was to investigate the sorption efficiency at metal concentrations comparable to those in municipal wastewater (<1 mg/L), for which experimental data are lacking and isotherm models have not been verified in previous works. Pb2+ removal of up to 83% was demonstrated at concentrations comparable to those in municipal wastewater, at pH 2. SDBC showed superior Pb2+ sorption capacity (maximum ~2 mg/g at pH 2) compared to WDBC and AC (<0 and (3.5 ± 0.4) × 10−3 mg/g, respectively); however, at the lowest concentration investigated (0.005 mg/L), SDBC released Pb2+. The potential risk of release of other heavy metals (i.e., Ni, Cd, Cu, and Zn) needs to be further examined. The sorption capacity of SDBC over a metal concentration span of 0.005–150 mg Pb2+/L could be predicted with the Redlich–Peterson model. It was shown that experimental data at concentrations comparable to those in municipal wastewater are necessary to accurately model and predict the sorption capacity of SDBC at these concentrations.

Highlights

Current municipal wastewater management causes exposure of humans and the environment to heavy metals
It was shown that experimental data at concentrations comparable to those in municipal wastewater are necessary to accurately model and predict the sorption capacity of sludge-derived biochar (SDBC) at these concentrations
The average pore size and pore volume of SDBC were in the same order of magnitude as that of Chen et al [14], where SDBC was produced by pyrolysis at 500–900 ◦ C

Summary

Introduction

Current municipal wastewater management causes exposure of humans and the environment to heavy metals. Municipal wastewater treatment does not target heavy metals [1]; instead, the heavy metals entering municipal wastewater treatment plants (WWTPs) partly associate with sludge and may enter agricultural land through sludge reuse, while the remainder is released to receiving waters. In the EU, roughly 40% of sludge is reused; plant nutrients and organic matter from wastewater can be utilized, benefitting circularity. An additional concern is that heavy metals that enter receiving waters through the release of wastewater effluent may cause harm to aquatic organisms [3]. Regulations for limiting the heavy metal release into water and the heavy metal load from sludge into agriculture are necessary.

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