A functionalized carbonaceous material was prepared via hydrothermal carbonization (HTC) of biogas
residues and denoted as hydrochar (HC). The obtained HC was used to synthesize a hybrid (Ni/Fe-HC)
to conduct Pb(II) removal from aqueous solution. SEM images show that Ni0/Fe0 was anchored tightly
to HC particles due to a cellular structure of HC formed during HTC. Complete Pb(II) removal (>99.5%)
was achieved within 1.5 h using Ni/Fe-HC whereas more than 85% of Pb(II) remained in bulk solution
after 10 h with bare HC. SEP results indicate that Pb(II) was mainly associated with the fraction bound
to carbonates and Fe–Mn oxides in the spent Ni/Fe-HC particles. Consequently, the adsorptive precipitation
and specific adsorption dominated Pb(II) removal by Ni/Fe-HC despite minor reduction of Pb(II) to
Pb0. However, the reduction of Pb(II) to Pb0 was conducted by the catalytically generated atomic hydrogen
rather than Ni/Fe nanoparticles as electron donor. Ni/Fe-HC not only acted as an effective adsorbent
for Pb(II) but also played a role of catalyst to generate atomic hydrogen. HC prepared with biogas residues
can be used as an excellent supporting material to prepare a hybrid of Ni/Fe-HC for Pb(II) removal.
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