Heavy metal adsorption onto functionalized amorphous biochar: a DFT study

Biochar is a highly porous carbonaceous material commonly produced by the thermal decomposition of lignocellulosic material. It is an excellent, low cost adsorbent and is of great interest in the remediation of heavy metals from water. The surface properties of Biochar can be modified to enhance the efficiency of adsorption of specific compounds. However, experimental studies into the ad- sorbate - adsorbent interactions remain difficult and tedious. Although several in-silico studies have been carried out into these interactions using Density Func- tional Theory (DFT), these studies use graphitic models and single molecules in -lieu of biochar, which are not accurate representations of amorphous biochar. In this work, an amorphous Biochar model was developed through a classi- cal molecular dynamics simulation using a ReaxFF interatomic potential in LAMMPS. The resulting structure was then validated visually as well as by analyzing the calculated FTIR spectrum of the model. The FTIR spectrum was obtained by Fourier transforming the dipole auto-correlation function cal- culated using LAMMPS. The calculated spectrum of the biochar model was compared with similarly calculated spectra of several common moieties found in biochar. The in-silico biochar model was deemed to be an accurate represen- tation of amorphous biochar. The resulting model was then used to investigate the adsorption of Pb and Cd onto a non-functionalized as well as functionalized biochar surface using DFT. DFT calculations were carried out using the Quan- tum Espresso software package. The investigation showed that the adsorption of the Pb and Cd onto the non-functionalized surface was more favourable than the functionalized surface. The amorphous BC structure that we have developed in this study is a very close representation to the BC found in nature and hence the adsorption studies that were subsequently carried out is also more accurate than existing studies on the adsorption of various chemical species onto BC.