A study of Pb and Cd removal from a drinking water source using pristine and modified cocoa shell and miscanthus biochars

Ensuring the provision of safe water for all is a challenging task that requires continuous research effort as nations seek to actualise Goals 6 and 11 (clean water and sanitation, and sustainable cities and communities respectively) of the Sustainable Development Goals (SDGs) by 2030. Commercial adsorbents like granular activated carbon have been used for heavy metals removal from drinking water for many decades. However, due to cost effectiveness, this treatment method is not feasible for low- and middle-income countries (LMIC). Biochar has emerged as a nature-based alternative that is effective, cheap, and can be made from available materials in a local context. In this study, the Pb and Cd removal efficiency (RE) of cocoa shell- and miscanthus-derived biochars produced via low temperature gasification technology were assessed using laboratory-scale batch adsorption experiments. The physico-chemical characteristics of miscanthus biochar produced at 400℃ (MCB 400) and 650℃ (MCB 650), and cocoa shell biochar produced with modification (+ 4% ferrous sulphate) and without modification (MCSB & PCSB) both obtained at the same temperature - 650℃ were also investigated. The results show that all the materials had the capacity to reduce Pb concentration to acceptable drinking water standards (RE > 92%) at the initial concentration studied (100 μg/L), and except MCB 650, the other materials could reduce Cd concentration to acceptable limits. Nevertheless, leaching of dissolved organic carbon (DOC) and salts (e.g. phosphates) is of concern. The physico-chemical characteristics of the biochars were more influenced by the feedstock type and to a lesser extent, by the production temperature. Under the study conditions, biochar modification had no major influence on Pb and Cd removal. Competitive adsorption studies of both metals revealed that Pb is more favourably adsorbed on PCSB over Cd. Cd adsorption on PCSB fitted well with the Freundlich isotherm and Pseudo-second-order kinetic model indicating a probable dominance of chemisorption in the rate limiting step and favourable heterogeneous surface adsorption. The Langmuir maximum removal capacity (qmax) for Cd on PCSB was 476.2 μg/g. Based on PhreeqC simulation, metal precipitation, mineral precipitation, complexation, and ion exchange are the most probable mechanisms involved in Pb and Cd adsorption on the biochars. Overall, the results demonstrated that the biochars studied can be used for Pb and Cd removal from a drinking water source, however, there may be need to perform a post-treatment for pH correction and DOC reduction before the water is fit for drinking purposes