The screening of various biochars for Cd²⁺ removal at relevant soil pH

Fourteen biochars from seven biomass sources were investigated on their long-term Cd²⁺ removal. The experiments consisted of a ten-day batch Cd²⁺ adsorption in a pH-buffered solution (pH = 6) to minimise pH effects. Insect frass, spent peat and chicken manure-derived biochars are promising Cd²⁺ adsorbents. Pyrolysis temperature was crucial for optimising Cd²⁺ removal by insect frass and spent peat-derived biochars. For these biochars, a pyrolysis temperature of 450 °C was optimal. In contrast, the Cd²⁺ removal by chicken manure biochars was independent of pyrolysis temperature. The Cd²⁺ 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 Cd²⁺ precipitation. The kinetics of Cd²⁺ removal over the course of ten days showed that insect frass biochar (450 °C) showed a gradual increase from 36 to 75 % Cd²⁺ removal, while chicken manure and spent peat-derived biochar (450 °C) already showed a higher Cd²⁺ removal (72 - 89 %) after day 1. This evidences that a long-term Cd²⁺ removal effect can be expected for some biochars. This should certainly be taken into consideration in future soil-based experiments.