Enhanced separation capacity of carbonaceous materials (hydrochar, biochar, and activated carbon) toward potential toxic metals through grafting copolymerization

Oxygen-containing groups on the surfaces of materials (i.e., –COOH and –OH) are vital to the adsorption of toxic metals. Acrylic acid is used as a green grafter to enhance the density of these groups in carbonaceous materials, whereas ammonium cerium nitrate is used as an initiator. Hydrochar (prepared via hydrothermal carbonization at 190 °C), biochar (generated via pyrolysis at 800 °C), and activated carbon (AC; generated via chemical activation with K2CO3 at 800 °C) derived from ginger residues are used as feedstock materials for the grafting process. Carbonaceous materials are characterized using scanning electron microscopy, X-ray photoelectron spectrometry, Brunauer–Emmett–Teller analysis, Fourier transform infrared spectroscopy, and zeta potential measurements. Cu2+, Cd2+, and Pb2+ are selected as adsorbates. Equilibrium adsorption experiments for the three metal ions on ungrafted and grafted carbonaceous materials are conducted at 25 °C and pH 5.0. Results indicated that the oxygen content in the grafted material is higher than that in the ungrafted material. Grafting can increase the number of OH and COOH functional groups on the carbonaceous materials, thus resulting in more metal ions being absorbed. The Langmuir maximum adsorption capacity (Qmax) of the carbonaceous materials for the toxic metal ions (mol/kg) is ranked in the order Cu2+ > Cd2+ > Pb2+. The amount of potentially toxic metal ions adsorbed on the ungrafted and grafted (abbreviated as G) materials are in the order hydrochar > AC > biochar. The Qmax values of hydrochar (72.8, 74.7, and 109.3 mg/g), biochar (53.6, 69.1, and 53.9 mg/g), AC (54.4, 73.1, and 101.8 mg/g), G-hydrochar (130.9, 146.0, and 271.4 mg/g), G-biochar (92.6, 143.6, and 153.6 mg/g), and G-AC (96.3, 146.1, and 259.1 mg/g) for the adsorbing metals (Cu2+, Cd2+, and Pb2+) are higher than that of commercial activated carbon (21.83, 22.13, and 25.15 mg/g, respectively). The adsorption capacity of the grafted hydrochar is restored after five adsorption–desorption cycles. The primary adsorption mechanisms are complexation and ion exchange.