Mg-Fe LDH-coated biochars for metal(loid) removal: Surface complexation modeling and structural change investigations

Although an increasing number of studies involving layered double hydroxide/biochar composites (LDH/BC) have been published recently, development of advanced modeling approaches of adsorption onto the composites and/or their testing and use in real soils and waters are rare. In this study, Zn(II) and As(V) sorption onto Mg-Fe LDH/BC composites was quantified using bulk sorption experiments, and the observed adsorption behavior was modeled using a non-electrostatic surface complexation model (NEM). In general, it was found that LDH is the main phase causing the removal of metal(loid)s from solution. In contrast, the contribution of BC was evident only in systems containing the most effective type of BC. Inner-sphere surface complexation was found to be the dominant adsorption mechanism at low Zn(II) and As(V) concentrations. In contrast, precipitation of Zn and formation of amorphous ferric arsenates contributed to the total metal removal at higher metal(loid) loadings. Surprisingly, increasing As(V) concentrations led to enhanced Mg leaching from the LDH/BC composites and to the destruction of the LDH structure. Nevertheless, all composite materials were stable at environmentally relevant metal(loid) concentrations. Our study is the first to use a surface complexation component additivity (CA) modeling approach to account for metal(loid) adsorption behavior onto LDH/BC composites, and we find that although site blocking does occur to some extent, the CA approach provides a reasonable estimate of the adsorption behavior across a wide range of pH conditions. Comprehensive studies such as this one are necessary in order to understand and optimize the behavior of novel composite materials before their subsequent use in environmental technologies.