Relative distribution of Pb2+ sorption mechanisms by sludge-derived biochar

Lead sorption capacity and mechanisms by sludge-derived biochar (SDBC) were investigated to determine if treatment of acid mine drainage (AMD) containing metals with SDBC is feasible. It was found that the biochar derived from pyrolysis treatment of sewage sludge could effectively remove Pb2+ from acidic solution with the capacities of 16.11, 20.11, 24.80, and 30.88 mg g−1 at initial pH 2, 3, 4 and 5, respectively. Lead sorption processes were pseudo-second order kinetic and faster at a higher pH. Furthermore, the relative contribution of both inorganic mineral composition and organic functional groups of SDBC for Pb2+ removal mechanisms, was quantitatively studied at pH 2–5. The results showed that Pb sorption primarily involved the coordination with organic hydroxyl and carboxyl functional groups, which was 38.2–42.3% of the total sorbed Pb varying with pH, as well as the coprecipitation or complex on mineral surfaces, which accounted for 57.7–61.8% and led to a bulk of Ca2+ and Mg2+ release during sorption process. A new precipitate was solely observed on Pb-loaded SDBC as 5PbO·P2O5·SiO2(lead phosphate silicate) at initial pH 5, confirmed by XRD and SEM–EDX. The coordination of Pb2+ with carboxyl and hydroxyl functional groups was demonstrated by FT-IR, and the contribution of free carboxyl was significant, ranging from 26.1% to 35.5%. Results from this study may suggest that the application of SDBC is a feasible strategy for removing metal contaminants from acid solutions.

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► SDBC has a Pb2+ sorption capacity comparable to high-efficient sorbants at low pH.
► Pb(II) sorption is primarily irreversible.
► Sorption involves the co-precipitation and coordination, thus causing Ca/Mg release.
► Lead phosphate silicate is the main form of precipitates at pH 5.