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Abstract. Hydrothermal carbonization has been proven as one of the simplest and most efficient technologies to produce a variety of carbon-based hydrochars with various applications. Like other types of biochars, hydrochar has reactive, functionalized/aromatic surfaces. Those properties make hydrochar a potential low-cost adsorbent for water purification and soil remediation. In order to increase its ability to remove heavy metal contaminants from aqueous solutions, chemical modification/activation of hydrochar surface using KOH has been suggested. The presented FT-IR spectroscopy analysis of hydrochars obtained from grape pomace and miscanthus (Miscanthus×giganteus) showed that KOH modification increased the oxygen-containing functional groups on hydrochar surfaces and its aromaticity. As a result, activated hydrochars may demonstrate enhanced sorption efficiency.

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In this study, hydrothermal carbonization (HTC) at five temperatures (180, 200, 220, 240, and 260 °C) was applied to transform Paulownia leaves (PL) into a carbonaceous sorbent of Pb(II) from aqueous solutions. To enhance the adsorption efficiency of the obtained hydrochar (PH), subsequent alkali activation was performed using NaOH. Preliminary results of the Pb(II) adsorption (CPb = 200 mg/L) showed removal coefficients after 48 h of 73.44 mg/g, 82.37 mg/g, and 110.9 mg/g for PL, PH-220, and MPH-220, respectively. The selected hydrochar (PH-220) and modified hydrochar (MPH-220) were further investigated by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The results revealed that alkali treatment changed the hydrochar structure and, thus, improved its adsorption performance. The kinetic parameters showed that the Pb(II) sorption onto MPH-220 followed a pseudo-second-order model, while the intra-particle diffusion went through two simultaneous stages. The Langmuir isotherm model best described the experimental data and indicated the value of 174.75 mg Pb(II)/g as the maximum adsorption capacity. The two possible mechanisms of Pb(II) binding were complexation and/or Pb-π electron interaction. The obtained results indicate the great potential of MPH-220 for Pb(II) removal from aqueous media and its potential utilization as an effective adsorbent for wastewater purification.