The screening of various biochars for Cd 2+ removal at relevant soil pH

Fourteen biochars from seven biomass sources were investigated on their long-term Cd removal. The experiments consisted of a ten-day batch Cd adsorption in a pH-buffered solution (pH = 6) to minimise pH effects. Insect frass, spent peat and chicken manure-derived biochars are promising Cd adsorbents. Pyrolysis temperature was crucial for optimising Cd removal by insect frass and spent peat-derived biochars. For these biochars, a pyrolysis temperature of 450 °C was optimal. In contrast, the Cd removal by chicken manure biochars was independent of pyrolysis temperature. The Cd removal by insect-frass and spent peat-derived biochars was associated with chemisorption on surface functionalities, while using chicken manure biochars was more associated with Cd precipitation. The kinetics of Cd removal over the course of ten days showed that insect frass biochar (450 °C) showed a gradual increase from 36 to 75 % Cd removal, while chicken manure and spent peat-derived biochar (450 °C) already showed a higher Cd removal (72 - 89 %) after day 1. This evidences that a long-term Cd removal effect can be expected for some biochars. This should certainly be taken into consideration in future soil-based experiments.